Isomer design unlocks rainbow phosphorescence

Domain: biology
Published: 2026-03-17
Editorial outcome: major_revision

Abstract
Achieving predictable color-tunable organic room-temperature phosphorescence (RTP) remains
challenging due to limited understanding of triplet-state regulation in heteroaromatic systems.
Carbazole and benzindole isomers provide an ideal platform to clarify how nitrogen positional
isomerism governs triplet exciton behavior and emission energetics. Here, we establish a unified
comparative framework to systematically investigate carbazole together with Bd[f], Bd[e], and
Bd[g]. Nitrogen-site modulation within the fused tricyclic skeleton generates distinct red, yellow,
green, and blue phosphorescence, while mechanochemical solvent-free synthesis enables scalable
preparation of previously inaccessible benzindole isomers. Photophysical measurements
combined with DFT/TD-DFT calculations, single-crystal analysis, and interaction region indicator
theory reveal that positional isomerism controls exciton localization, triplet stabilization, and
nonradiative decay independent of the host matrix. Here, we show that isomer-regulated triplet
dynamics enable full-spectrum RTP, ultralong lifetimes up to 4.23 s, TSFRET behavior, and
matrix-universal multifunctionality, establishing a general molecular design principle for rainbow-
like organic phosphorescent materials.

Introduction
Ultralong organic phosphorescent (UOP) materials have garnered significant interest due to their
unique long-lived emission characteristics and broad application prospects in optoelectronics,
bioimaging, information encryption, and anti-counterfeiting1-10. However, achieving persistent
intense phosphorescence under ambient conditions remains a formidable challenge. This limitation
primarily arises from the intrinsically weak spin-orbit coupling (SOC) in purely organic systems
and the rapid nonradiative decay of triplet excitons. The spin-forbidden nature of intersystem
crossing (ISC) between singlet (S ) and triplet (T ) excited states further impedes the generation
n n
and utilization of triplet states, thereby compromising RTP efficiency11-18. To address these issues,
extensive efforts have been devoted to developing molecular design principles and materials
engineering strategies aimed at enhancing room-temperature phosphorescence (RTP).
Representative approaches include crystal engineering, polymer matrix confinement, host-guest
doping systems, molecular self-assembly, and the construction of supramolecular architectures19-
23. These strategies generally operate by rigidifying the molecular environment, suppressing
intramolecular vibrational and rotational motions, minimizing nonradiative decay pathways, and
stabilizing the triplet state to prolong exciton lifetimes24-29. However, despite these advancements,
the precise and predictable modulation of phosphorescence emission wavelengths within a unified
molecular framework remains a significant challenge30-33. In this context, structural isomerism
represents a powerful yet underexplored approach. By altering the connectivity or spatial
arrangement of key atoms within the same molecular backbone, isomeric molecules can exhibit
distinct electronic structures, intermolecular packing modes, and excited-state dynamics, thereby
enabling systematic tuning of energy levels, exciton diffusion behavior, and emission
characteristics.
Among the various molecular frameworks employed in RTP materials, carbazole and its
derivatives have attracted sustained attention due to their rigid coplanar structures, high triplet
energy levels, and excellent photostability34-38. As a structural isomer of carbazole, benzindole (Bd)
offers additional molecular tunability through variation of the nitrogen atom position within the
fused tricyclic aromatic system, giving rise to several regioisomers (Bd[g], Bd[e], and Bd[f]) with
distinct electronic structures. Despite their close structural similarity, subtle positional differences
between benzindole and carbazole result in markedly different triplet-state behaviors and RTP
properties, while benzindole-based RTP systems have received comparatively little attention. It
was not until 2020 that Liu39 reported that trace Bd[f] impurities in commercial carbazoles could
significantly influence RTP behavior, which subsequently stimulated growing interest in Bd[f]-
based systems40-42. Nevertheless, most existing studies remain focused on the Bd[f]
configuration8,43,44, while other isomers such as Bd[e] and Bd[g] have received far less attention,
and systematic structure property comparisons across the carbazole-benzindole isomer family are
still lacking. Although previous studies have demonstrated that altering the nitrogen atom position
can modulate phosphorescence color45, these investigations were limited to individual molecules
rather than a complete isomeric series. In contrast, the present work systematically investigates the
full carbazole-benzindole family (Cz, Bd[f], Bd[e], and Bd[g]), enabling unified structure property
correlations and full-spectrum RTP color tuning in polymer matrices. Notably, unlike conventional
strategies relying on extensive substitution, heavy-atom effects, or multicomponent host-guest
systems, full-spectrum RTP is achieved here through minimalist backbone isomerization within a
single heteroaromatic framework, offering a structurally economical and scalable design approach.
A major challenge in this field lies in the synthesis of certain isomeric forms-particularly
Bd[f]-which typically require multi-step solution-based protocols characterized by low selectivity,
prolonged reaction times, and cumbersome purification steps. These factors have significantly
impeded direct comparative studies among Bd isomers. Recent findings demonstrate that the rarely
explored Bd[g] isomer can be selectively synthesized via a one-step, solvent-free
mechanochemical approach under ball-milling conditions, offering a greener and more efficient
alternative to traditional methods46. Building on recent advances in solvent-free mechanochemical
synthesis, Bd[g] and Bd[e] derivatives were efficiently obtained via a one-step grinding strategy,
while Bd[f]-based analogues were prepared through optimized multistep solution synthesis. This
synthetic capability enables the construction of a complete Cz/Bd isomeric platform, including Cz,
Bd[f], Bd[e], and Bd[g] (Fig. 1a).
Here, we show that this unified molecular family establishes systematic correlations between
isomeric structure and key RTP parameters, including emission color, triplet-state energetics, and
lifetime. When dispersed in polymer matrices such as poly(vinyl alcohol) (PVA), these
luminophores collectively span the full visible spectrum from blue to red under ambient conditions.
Notably, Bd[g] and Bd[e], as isomeric analogs of carbazole, exhibited RTP performance not only
in PVA but also in other matrices such as polyvinylpyrrolidone (PVP), poly(vinyl butyral) (PVB),
and poly(methyl methacrylate) (PMMA), underscoring the universal nature of their
phosphorescent behavior (Fig. 1b). The resulting UOP materials, when embedded in PVB and
PMMA matrices, exhibit excellent chemical stability, long-lasting afterglow, and seawater
tolerance, enabling practical applications in underwater imaging, durable anti-counterfeiting, and
marine bio-interaction studies. Their visible phosphorescence under UV or sunlight activation
offers opportunities for optical tagging and remote sensing in complex ocean environments. (Fig.
1c). The spectral tunability demonstrated by this heterogeneous platform provides a good
foundation for the development of next-generation RTP technologies.

Results
Synthesis of Isomers
The carbazole model compound (Ph-Cz) and the benzindole[f] isomer (Ph-Bd[f]) were synthesized
using a conventional solution-phase thermal method, a widely adopted approach that typically
involves multi-step reactions, extended purification processes, and substantial solvent
consumption. The overall yields for Ph-Cz and Ph-Bd[f] were 70% and 16%, respectively. In
contrast (Fig. 2a), the less explored isomers Ph-Bd[g] and Ph-Bd[e] were successfully synthesized
under solvent-free conditions using a one-step mechanochemical ball-milling strategy, achieving
high efficiency and yields of approximately 76% and 46%. This solid-state approach not only
simplifies the synthetic process but also adheres to the principles of green chemistry by eliminating
hazardous solvents and minimizing energy input. Bd[g] and Bd[e], representing previously
underutilized isomeric cores within the benzindole family, can be accessed via a sustainable and
scalable route and exhibit favorable optical properties compared to the extensively studied Bd[f]
isomer. These features render Bd[g] and Bd[e] promising candidates for the development of next-
generation, heavy metal-free RTP materials. The incorporation of Bd[g] and Bd[e] into the existing
carbazole-Bd[f] framework enables a complete and systematic comparison across the isomeric
series. The chemical structures and purities of the synthesized compounds were confirmed by
nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction (SCXRD), high-
resolution mass spectrometry (HR-MS), and high-performance liquid chromatography (HPLC).
Detailed synthetic procedures and characterization data are provided in the Supporting Information
(Supplementary Figs. 1-4; Supplementary Figs. 5-22). The resulting carbazole and benzindole-
based isomers were physically doped into various polymer matrices at a concentration of 1 wt%,
and homogeneous thin films were obtained via drop-casting, as detailed in the Materials
Preparation section of the Methods.

Color-Tunable RTP from Isomeric Guests
The molecular structures of the four guest molecules are presented in Fig. 2b. Differential scanning
calorimetry (DSC) measurements indicated that the doped PVA films exhibited similar glass
transition and melting temperatures, suggesting that the low doping concentration had negligible
influence on the interactions between PVA chain segments, thereby preserving the intrinsic semi-
crystalline nature of the polymer matrix (Supplementary Fig. 23b). Thermogravimetric analysis
(TGA) further confirmed the excellent thermal stability of all doped films, with decomposition
temperatures exceeding 240 °C, highlighting their suitability for practical applications
(Supplementary Fig. 23a). Upon cessation of UV excitation, the doped films Ph-Bd[f]@PVA, Ph-
Bd[e]@PVA, Ph-Bd[g]@PVA, and Ph-Cz@PVA exhibited rainbow-like persistent room-
temperature phosphorescence (RTP) (Supplementary Fig. 28), featuring distinct red, yellow,
green, and blue emissions across the visible spectrum. Their phosphorescence emission maxima were
observed at 611 nm, 527 nm, 502 nm, and 467 nm, respectively (Fig. 2c), closely matching the
low-temperature phosphorescence profiles of the corresponding guest molecules measured at 77
K (Supplementary Fig. 25). No room-temperature phosphorescence was observed for the guest
molecules in their pristine states. (Supplementary Fig. 26) The CIE chromaticity coordinates of
the RTP emissions were determined to be (0.62, 0.35), (0.36, 0.60), (0.29, 0.58), and (0.16, 0.23),
respectively (Fig. 2e). As illustrated in Fig. 2f, photographic images captured after switching off
the UV irradiation revealed distinct blue, green, yellow, and red emissions from Ph-Cz@PVA, Ph-
Bd[g]@PVA, Ph-Bd[e]@PVA, and Ph-Bd[f]@PVA, respectively-corresponding to the four
primary colors essential for display and full-color applications. To date, this represents the
demonstration of full-spectrum RYGB phosphorescence emission from purely organic phosphors
through a simple molecular backbone isomerization strategy.
In order to gain insight into the phosphorescence behavior of these RTP films containing
carbazole- and benzindolyl-guest molecules with different configurations, their photophysical
parameters, including phosphorescence lifetimes and quantum yields, were systematically
investigated as shown in Fig. 2d and Supplementary Fig. 27. Notably, the Ph-Cz@PVA film
exhibits RTP performance, featuring both a high phosphorescence quantum yield (Φ
P
= 16.6%)
and an ultralong lifetime (τ = 4.23 s). In comparison, although the Ph-Bd-based systems do not
reach the same level of RTP efficiency and lifetime as Ph-Cz, they offer unique advantages as
structural isomers. These isomers enable tunable phosphorescence emissions across a wide
spectral range, from green, yellow to red, via precise control over molecular configuration.
Specifically, Ph-Bd[g]@PVA and Ph-Bd[e]@PVA demonstrate bright green and yellow RTP
emissions, respectively (τ = 0.93 s, Φ = 13.4%; τ = 0.74 s, Φ = 12.9%). Despite their relatively
P P P P
shorter lifetimes, these isomers are synthetically more accessible and cost-effective, making them
attractive candidates for scalable applications in areas such as anti-counterfeiting and optical
tagging. On the other hand, Ph-Bd[f]@PVA displays a red afterglow emission with similar
quantum efficiency (Φ = 10.7%) but a modest lifetime (τ = 0.2 s) which agree well with other
P P
Bd[f] phosphor8,39,47 . However, the synthesis of the Bd[f] isomer remains more challenging and
less economically viable, limiting its practical applicability in large-scale material development.
The contrasting performance profiles of Ph-Cz and its benzindole isomers provide a valuable
framework for understanding the structure-property relationships governing organic
phosphorescence. Further mechanistic studies are warranted to elucidate the underlying
photophysical pathways, particularly the factors contributing to the long-lived RTP of Ph-Cz and
the color tunability observed across the Bd-series.

Mechanism of Isomer RTP
To gain deeper insight into the room-temperature phosphorescence (RTP) emission mechanisms
of carbazole and its benzindole derivatives, systematic density functional theory (DFT) and time-
dependent DFT (TD-DFT) calculations were performed at the B3LYP/6-31G level
(Supplementary Data. 1). As shown in Fig. 3a, the calculated HOMO-LUMO energy gaps exhibit
a progressively decreasing trend with structural isomerism, namely Ph-Cz (4.56 eV), Ph-Bd[e]
(4.34 eV), Ph-Bd[g] (4.11 eV), and Ph-Bd[f] (3.65 eV), which correlates well with the
experimentally observed red-shifts in the absorption (Supplementary Fig. 29) and
phosphorescence spectra (Fig. 2b). This trend can be attributed to the enhanced π-conjugation
introduced by the benzindole moieties48, which effectively reduces the electronic band gap and
lowers the triple energy level (T ) (Supplementary Fig. 30a), as evidenced by the cavity/electron
1
composition analysis, where Ph-Cz exhibits the smallest charge density difference during the T
1
transition (Supplementary Figs. 39-42). To further evaluate the efficiency of the intersystem
crossing (ISC) process, the singlet and triplet energy level distributions and spin-orbit coupling
(SOC) constants within a ±0.3 eV energy window around the S state were analyzed. Considering
1
the realistic operational environment of the materials, a polyvinyl alcohol (PVA) segment was
introduced as a model system to optimize the conformations of the guest molecules embedded
within the polymer matrix (Fig. 3b). All four compounds exhibit small S -T energy gaps and
1 n
relatively high SOC constants, favoring efficient ISC transitions (Supplementary Tables. 2-5).
Notably, the calculated results in both gas-phase and crystal-state structures are highly consistent
(Supplementary Fig. 30), further confirming the generality and structure-governed nature of the
observed electronic evolution. Furthermore, Hole-Electron analyses were performed between the
S state and the representative T states with the highest transition contributions (Supplementary
1 n
Fig. 43-44) to quantitatively elucidate the excited-state charge-transfer characteristics. The S
1
states of all four systems are predominantly governed by HOMO-LUMO π-π* transitions, with
contributions of 91.0% for Ph-Cz, 71.4% for Ph-Bd[e], 96.0% for Ph-Bd[g], and 97.3% for Ph-
Bd[f]. According to El-Sayed’s rule49, the presence of triplet states with significant n-π* character
can facilitate spin-flip processes. Correspondingly, the T state of Ph-Cz (54.4%), T of Ph-Bd[g]
5 6
(95.5%), T of Ph-Bd[e] (95.8%), and T of Ph-Bd[f] (63.4%) all exhibit prominent n-π*
5 2
characteristics, theoretically supporting efficient S
1
-T
n
ISC channels. To account for dynamic host-
guest effects, full molecular dynamics (MD) simulations were performed for all mixed systems,
followed by electronic structure analysis of the MD-sampled conformational ensembles. Across
all systems, the T energy levels extracted from the MD trajectories consistently follow a
1
decreasing order from Cz to Bd[e], Bd[g], and Bd[f], in agreement with the static DFT results,
indicating that the triplet-state energetic evolution is primarily governed by backbone isomerism
(Supplementary Figs. 31-38).
Despite theoretically favorable spin-orbit coupling (SOC) constants and efficient intersystem
crossing (ISC) channels predicted for the isomeric systems, the experimentally observed room-
temperature phosphorescence (RTP) lifetimes of the benzindole derivatives are significantly
shorter than that of Ph-Cz. In practical systems, molecular rigidity, matrix-imposed restriction of
intramolecular motions, and synergistic non-covalent interactions-particularly hydrogen bonding-
are essential for suppressing nonradiative decay and stabilizing triplet excitons. Detailed
electrostatic potential (ESP) surface analyses50 were performed based on density functional theory
(DFT) and Multiwfn51-53 calculations to investigate the interactions between the phosphorescent
guest molecules and the PVA matrix (Fig. 3c). All four luminophores exhibit distinct ESP
distributions, with negative potential localized on their aromatic cores and positive potential near
the N-H groups. Notably, while the negative ESP values for all guests are comparable (~ -20
kcal/mol), their positive ESP values vary significantly, with Ph-Cz exhibiting the highest value
(+45.15 kcal/mol). This gradient corresponds to a progressive reduction in intramolecular charge
separation across the series from Ph-Cz, Ph-Bd[e], Ph-Bd[g] to Ph-Bd[f]. These spatial charge
distributions promote electrostatic complementarity with the hydroxyl-rich PVA matrix, enhancing
host-guest interactions that facilitate microenvironmental rigidification. The pronounced
electrostatic polarity of Ph-Cz leads to stronger binding with the polymer host, effectively
restricting molecular motions and suppressing nonradiative deactivation pathways. This
theoretical insight aligns well with the experimental RTP lifetime data (Fig. 2c), where Ph-
Cz@PVA displays the longest phosphorescence lifetime among the series, underscoring the pivotal
role of electrostatic compatibility in stabilizing long-lived triplet states. Collectively, these
multiscale investigations highlight that efficient RTP arises from a synergistic interplay among
electronic structure.

Mechanistic Understanding of Carbazole-Based RTP
To further understand the origin of the RTP performance of Ph-Cz, we conducted structural-level
analyses focusing on its intrinsic molecular geometry and solid-state packing behavior. Single
crystals of guest were obtained via slow evaporation from a dichloromethane/n-hexane solvent
mixture under ambient conditions (Supplementary Figs. 20-22). Subsequent single-crystal X-ray
diffraction analysis revealed that Ph-Cz crystallizes in the orthorhombic P2
1
2
1
2
1
space group
(Supplementary Table 1), adopting a distinct parallel-stacked dislocation motif. The crystal lattice
features a dense network of non-covalent interactions, including N-H…C hydrogen bonds (2.80-
2.88 Å) and pronounced π…π stacking (~2.93 Å) between adjacent carbazole units (Fig. 4a), which
collectively promote exciton confinement. The asymmetric unit contains four independent
molecules, providing multiple rigidified conformations (Supplementary Fig. 20). A large dihedral
angle (51.6°) between the carbazole core and the pendant phenyl ring induces a twisted, non-planar
geometry, which suppresses nonradiative decay while maintaining favorable spin-orbit coupling
in the solid state. To further elucidate the mechanism underlying the RTP performance of Ph-Cz
in polymer matrices54, interaction region indicator (IRI) analysis was performed using
Multiwfn52,53,55 to visualize the non-covalent interactions between Ph-Cz and the PVA host (Fig.
4b). In addition to extensive van der Waals contacts, Ph-Cz forms strong directional hydrogen
bonds between its N-H group and the hydroxyl groups of PVA, with a calculated interaction energy
of -6.43 kcal mol-1. These interactions effectively rigidify the local microenvironment, suppress
nonradiative deactivation, and stabilize triplet excitons. In contrast, the benzindole derivatives Ph-
Bd[g] and Ph-Bd[e] interact with PVA predominantly through weak van der Waals forces, while
Ph-Bd[f] exhibits only limited hydrogen-bonding capability (Supplementary Fig. 45), rationalizing
their comparatively lower RTP efficiencies.
Excitation-phosphorescence mapping of Ph-Cz@PVA shows excitation-independent
emission over a broad range (Fig. 4c), indicating homogeneous dispersion and a single emissive
species. To assess matrix dependence, Ph-Cz was further embedded in PVP, PVB, and PMMA,
polymers with distinct rigidity, polarity, and oxygen permeability. Persistent RTP is observed in
all matrices (Fig. 4d, Supplementary Fig. 46, Supplementary Table 6), confirming the intrinsic
photophysical robustness of Ph-Cz. However, the exceptionally prolonged lifetime is unique to
PVA, highlighting the critical role of hydrogen-bond-assisted confinement in triplet stabilization.

Multicolor Afterglow via TS-FRET
Guided by the mechanistic benchmark established by Ph-Cz, and considering both the
synthetic challenges associated with the Bd[f] isomer and the absence of markedly improved
phosphorescent performance in the Ph-Bd[f]@PVA system, attention was directed toward the more
synthetically accessible and functionally advantageous Ph-Bd[g] and Ph-Bd[e] derivatives. In
benzindole isomers, nitrogen-position isomerization intrinsically reshapes triplet-state energies,
providing an effective handle for tuning RTP emission wavelengths without relying on substituents
or matrix effects. Ph-Bd[g] and Ph-Bd[e] were doped into various polymer matrices, including
PVP, PVB, and PMMA, to achieve guest-induced organic room-temperature phosphorescence (Fig.
5a), with their corresponding quantum yields and lifetimes summarized in Supplementary Figs.
47-48. The resulting doped systems exhibit distinct and persistent RTP emissions (Supplementary
Table 6), establishing Ph-Bd[g] and Ph-Bd[e] as robust phosphorescent scaffolds for wavelength-
tunable organic afterglow.
These systems show robust room-temperature phosphorescence, indicating their promising
potential for tunable afterglow emission via triplet-to-singlet Förster resonance energy transfer
(TSFRET). To this end, two commercially available fluorescent dyes-fluorescein sodium (FLS)
and rhodamine B (RHB)-were selected as energy acceptors (Fig. 5f). Their absorption spectra show
substantial overlap with the phosphorescence emission spectra of Ph-Bd[g]@PVA and Ph-
Bd[e]@PVA (Fig. 5b), fulfilling the spectral matching requirements for efficient TSFRET. Upon
doping FLS and RHB into these RTP matrices, ternary composite films were prepared. Delayed
emission spectra of the resulting materials display distinct emission peaks at 555 nm and 620 nm
(Fig. 5c), confirming successful energy transfer. Further evidence is provided by CIE chromaticity
analysis (Fig. 5e), which indicates a significant change in the afterglow color after energy transfer.
Even after the 365-nm ultraviolet excitation was stopped, persistent yellow and red luminescence
could still be observed with the naked eye (Supplementary Fig. 49). Time-resolved decay curves
(Fig. 5d) show that the afterglow lifetimes of Bd[g]-FLS, Bd[g]-RHB, Bd[e]-FLS and Bd[e]-RHB,
are 281 ms, 184 ms, 225 ms, and 103 ms, respectively, highlighting the effective energy collection
and transfer processes of these materials. Overall, these results underscore the advantages of the
Bd[g] and Bd[e] isomers-both overlooked structural analogs of carbazole-over the Bd[f] isomer.
While the FRET process has been reported for RTP systems56, here the Bd[g] and Bd[e] isomers
enable more synthetically accessible and tunable multicolor afterglow, providing opportunities for
designing ternary energy-transfer systems with controlled emission. Their benefits extend beyond
synthetic accessibility to compatibility with time-resolved Förster resonance energy transfer
(TSFRET)-based strategies, enabling more efficient and tunable multicolor organic afterglow
emissions.

Functional Applications
All four guest molecules demonstrated excellent phosphorescent emission across various polymer
matrices. By leveraging the distinct physicochemical properties of each host polymer, their
application potential in functional RTP systems was systematically investigated. Notably, when
doped into a polyvinylpyrrolidone (PVP) matrix, the guest molecules retained stable room-
temperature phosphorescence even under elevated temperatures (up to 100 °C) (Supplementary
Fig. 50). Utilizing this property, a high-temperature phosphorescence-based anti-counterfeiting
display was constructed by arranging the digits “2025” using different RTP composites (Fig. 6a),
including Ph-Cz@PVP for the digit “2”, Ph-Bd[g]@PVP for “0”, and Ph-Bd[e]@PVP for “5”.
Under 365 nm UV irradiation at 100 °C, the assembled pattern displayed uniform blue
fluorescence. After removal of the excitation source, the digits exhibited distinct persistent
phosphorescence-blue for “2”, green for “0”, and yellow for “5”. This proof-of-concept
demonstration highlights the potential of thermally stable organic phosphorescent materials in
advanced anti-counterfeiting technologies requiring high-temperature tolerance and multicolor
emission encoding. In addition, the Ph-Bd[g]@PVA composite exhibited sunlight-activated RTP
behavior (Fig. 6b), offering practical advantages for low-power emergency signage. Following
exposure to solar irradiation, the material was transferred into a dark enclosure, where a bright and
long-lived green afterglow was observed. The emitted light was sufficiently intense to clearly
illuminate an emergency exit sign, ensuring visibility under low ambient lighting conditions. These
results underscore the applicability of sunlight-responsive organic RTP materials in energy-
independent and self-sustaining safety signage systems. Further demonstrating the practical
versatility of these RTP materials, phosphorescent emblems of Ocean University of China and its
School of Materials Science and Engineering were fabricated using Ph-Cz@PVA and Ph-
Bd[e]@PVA, respectively (Fig. 6c). Under ambient fluorescent lighting, the films remained highly
transparent, causing minimal optical interference. Upon cessation of UV excitation, the emblems
emitted distinct afterglow-blue for Ph-Cz@PVA and yellow for Ph-Bd[e]@PVA-clearly
delineating the institutional logos. This demonstration highlights not only the aesthetic adaptability
of the materials but also their potential in multifunctional display technologies, signage
applications, and low-energy identity lighting systems.
After immersion in natural seawater for one month, the phosphorescent guest molecule-doped
PVB and PMMA composites exhibited excellent structural and optical stability. Although some
salt crystallization was observed due to water evaporation (Supplementary Figs. 51-53), no
significant degradation in phosphorescence performance was detected, indicating chemical
robustness and corrosion resistance (Supplementary Figs. 54-55). These properties suggest strong
suitability for long-term deployment in marine environments. As illustrated in Fig. 6d, coatings of
Ph-Bd[g]@PVB and Ph-Bd[e]@PVB applied to shell surfaces maintained clearly visible
phosphorescence under UV excitation, even in seawater. This observation confirms the feasibility
of utilizing such RTP materials for visually identifiable and persistent luminescent tagging systems
on marine equipment, vessels, and offshore platforms. In addition to the demonstrated material
stability, the potential relevance of the emission characteristics of these RTP materials to marine
biological systems was considered from a photophysical perspective. Many marine organisms
exhibit wavelength dependent visual sensitivity; accordingly, the photoluminescence profiles of
PMMA-based RTP films were examined under UV excitation (Fig. 6e). The materials display
stable and persistent afterglow across the visible region, encompassing wavelength ranges
commonly involved in marine visual perception. Notably, the green-to-yellow emission band
(approximately 500-580 nm) overlaps with reported sensitivity windows of certain reef-associated
fish and benthic invertebrates. This spectral overlap suggests that such RTP materials could
provide a useful optical basis for future investigations into light-organism interactions in marine
environments.

Methods
Materials
1,4-Dibromo-2-methyl-5-nitrobenzene (98%), Palladium (II) Acetate (98%) and N-(Naphthalen-
2-yl)acetamide (99.9%) were purchased from Adamas Co., Ltd. Silver hexafluoroantimonate
(99.9%), 1-Phenyl-1-propyne (99.9%), Dichloro(p-cymene)ruthenium(II) dimer (98%), Oxalic
acid (99.58%), 1,3-Dimethylurea (99.96%), Pivalic acid (99.72%) and 1-Naphthylhydrazine
hydrochloride (98%) were purchased from Bide Pharmatech Co., Ltd. 4'-Methylpropiophenone
(97%) and 2-Naphthylhydrazine hydrochloride (98%) were purchased from Energy Chemical.
[Bis(trifluoroacetoxy)iodo]benzene (98%) was purchased from Leyan.com. Triphenylphosphine
was purchased from Franklin C. McLean.

Physical measurements and instrumentation
1H NMR and 13C NMR were recorded on a Bruker AVANCE III 600 NMR instrument with
solvents CDCl and DMSO-d , internal standard tetramethylsilane (TMS), and room temperature.
3 6
ESI high-resolution mass spectrometry analyses were performed on a Waters Xevo G2 Qt mass
spectrometer. Fluorescence and phosphorescence photoluminescence spectra were measured on a
Hitachi F-4700 fluorescence spectrometer. Lifetime curves were measured on a FLS1000
photoluminescence spectrometer in Edinburgh. Absolute PL quantum yield (PLQY) was measured
on a C11347 integrating sphere spectrometer (Hamamatsu, Japan). UV-visible absorption spectra
were measured by a UV-3600 Shimadzu spectrophotometer. Thermogravimetric (TGA) and
Differential Scanning Calorimetry (DSC) analyses on the Thermal Analyser STA 8000. High
performance liquid chromatography (HPLC) tests were performed on a chromatograph with
instrument model SDANA-PC, using a column Shim-pack GIST C18, column temperature of
40 °C, injection volume of 10 ul and flow rate of 1 mPl/min. The time interval for data acquisition
was set to 500 ms, and the analysis start and end times were from 0 to 20 min, the intensity unit
was millivolt (mV), and the intensity multiplier was 0.001. Mechanical ball milling equipment
model is XQMCM-2.

Single Crystal Analysis
Single Crystal X-ray diffraction data were collected using a Bruker D8 Quest diffractometer (Cu
Kα, λ= 1.54178 Å). Indexing and data integration were performed using APEX4 (Difference
Vectors method). Absorption correction was performed by multiscan method implemented in
SADABS. Space groups were determined using XPREP implemented in APEX4. Structures were
solved using SHELXL-2014 (direct methods) and refined using SHELXL-2014 (full-matrix least-
squares on F2) with anisotropic displacement contained in APEX4 program packages. Hydrogen
atoms on carbon and nitrogen were calculated in ideal positions with isotropic placement
parameters set to 1.2 × U of the attached atoms.
eq

Density Functional Theory (DFT) calculations and time-dependent DFT calculations
All chemical structures were optimized at the level of B3LYP/6-31G and the excited energies were
calculated by the time-dependent density functional theory (TD-DFT) method at the level of
B3LYP at the 6-31G (d, p). All the calculations were performed within Gaussian 09 software
package. Hole/Particle analyses, atomic charge distributions and electron density surfaces for
electrostatic potential (ESP) residency mapping were calculated using Multiwfn software and
visualised using the VMD program.

Preparation of RTP materials
All solvents were purchased from the market and used as is without further purification.
Tetrahydrofuran solution (1 ml) of guest molecule (1 mg) was thoroughly mixed with aqueous
PVA solution (100 mg PVA1799 (polymerization 17, alcohol solubility 99) and 9 ml water)
respectively, then the well mixed solution was dropped into a dye cartridge and baked for 2-3 hours
at 60 ℃ to obtain the films. A dichloromethane solution (1 ml) of the guest molecule (1 mg) was
thoroughly mixed with a dichloromethane solution (4 ml) of PVP, PVB and PMMA (100 mg),
respectively, and then the well-mixed solution was dropped into a dye cartridge and evaporated to
dryness at room temperature to obtain the films. All films were then dried in a vacuum oven at 65 ℃
for 12 hours to remove residual moisture for subsequent testing.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Room-temperature phosphorescence (RTP) is a key aspect and frontier in photophysics. Herein, the authors report an isomeric design strategy for achieving full-color, rainbow-like organic phosphorescence. Precise nitrogen-site isomerization species are of interest for developing state-of-the-art models to address the thorny challenges in the RTP field. However, the manuscript fails to address the key aspect of deep structure-photophysical relationships. Up to date, larger strategies are proposed to tune the emission color of organic RTP materials, which is not the challenge in this area. The intrinsic exciton behaviors of the isomers require further investigation, rather than focusing solely on doping in matrices such as PVA—these matrices are not new for RTP and rely on basic host-guest systems. Additionally, carbazole (Cz), as a foundational reference, should be included for detailed discussion of RTP emission mechanisms. While the four isomers represent a valuable set, their utility is limited to tuning emission wavelength, with only modest, unsystematic changes in RTP efficiency. Critically, this study does not clarify the significant differences between the three benzindole isomers and Cz. I recommend rejection. Specific detailed concerns are as follows:\n1. Lack of deep structure-photophysical relationships, alongside the absence of quantitative analysis and generalizable rules for doping matrices. The quantitative effects of isomerism on RTP performance must be further elaborated in the study.\n2. The theoretical framework should incorporate molecular dynamics simulations of mixed samples, followed by calculations of electronic structures to better reflect real-world system dynamics.\n3. the key role of non-novel matrices (e.g., PVA) and incomplete analysis of the host-guest system, which fails to advance beyond established RTP matrix design paradigms.\nGiven these fundamental limitations, I recommend rejecting the manuscript. To strengthen the work, the authors should: (1) dissect the intrinsic exciton behaviors of the isomers (e.g., via comparative studies of neat isomer films vs. polymer-doped systems); (2) integrate molecular dynamics (MD) simulations into the theoretical analysis to model dynamic isomer-matrix interactions; (3) establish quantitative structure-RTP relationships (e.g., linking nitrogen position to triplet energy levels or intersystem crossing efficiency); and (4) frame Cz as a mechanistic benchmark to derive clear rules for distinguishing benzindole isomers. A revised version of this work may be more suitable for submission to specialized journals.", "reviewer_2": "The paper presents a comprehensive and systematic study of the room-temperature phosphorescence (RTP) properties of carbazole (Cz) and its benzindole isomers (Bd[f], Bd[e] and Bd[g]). By precisely controlling the nitrogen atom position within the tricyclic backbone, the authors achieve full-spectrum, tunable RTP spanning red, yellow, green, and blue emissions. Two new isomers, Bd[e] and Bd[g] were synthesized by a one-pot solvent-free mechanical method, which is greener and more efficient than the traditional multi-step solution method (Bd[f]). The material exhibits excellent stability and long afterglow emission in matrices such as PVA, PVP, PVB, and PMMA, and can be used in applications such as anti-counterfeiting, Marine identification, and biological simulation.\nThe experimental content is innovative and has a complete experimental system. However, there are numerous issues in writing and formatting. I recommend the manuscript requires a thorough revision before it can be considered for publication.\n1. The paper reads overly complicated and repetitive in many parts, suggesting that the authors may have relied heavily on AI-assisted writing. I recommend a substantial revision to improve clarity and authenticity of expression.\n2. The NMR spectra don’t indicate the solvent reference peaks, and why CDCl₃ was chosen as the NMR solvent, as some product signals appear close to the solvent peak.\n3. Page numbers are missing, the display errors are found on page 22 of the main text and page 1 of the supplementary information.\n4. the reference format is incorrect (refs. 28 and 44).\n5. I think the final \"discussion\" should be changed to \"conclusion\". I think the title \"Discussion\" of section should be changed to \"Conclusion\".\n6. It is unnecessary to label the figure as “Scheme 1”. Just using the conventional term “Figure 1” would be sufficient.\n7. “Supplementary Fig.” is lengthy. It is recommended to use the abbreviation such as “Fig. S1”.\n8. the “Results and Discussion” section is rather lengthy and would benefit from being divided into several subsections for better readability.\n9. Some pictures in the supplementary information have a light color. It is recommended to darken them.\n10. It is recommended to include a comparative summary table in the main text.", "reviewer_3": "The manuscript “Isomer design unlocks rainbow phosphorescence” presents a systematic study on carbazole and benzindole isomers as a unified molecular platform for achieving full-spectrum organic room-temperature phosphorescence (RTP). By integrating isomeric backbone engineering, solvent-free mechanochemical synthesis, polymer matrix confinement, and multiscale photophysical/theoretical analyses, the authors demonstrate red-yellow-green-blue (RYGB) persistent phosphorescence and showcase a series of application-oriented demonstrations. The work is clearly written, experimentally thorough, and conceptually appealing. In particular, the completion of the carbazole-benzindole isomer family and the scalable access to Bd[g] and Bd[e] via mechanochemistry are noteworthy advances. The manuscript has the potential to meet the publication standard of Nature Communications. Nevertheless, several minor points require careful clarification and strengthening before acceptance.\n1. The manuscript convincingly demonstrates that subtle backbone isomerization can simultaneously modulate emission color, triplet-state energetics, and RTP lifetime. While these results are well supported, the underlying design principle is currently distributed across several sections. It would benefit the broader Nature Communications readership if the authors explicitly summarize, in the Discussion, a concise and transferable design rule, for example, how nitrogen-site isomerization governs both electronic structure (triplet energy) and host-guest interaction strength, thereby defining the accessible RTP color range and lifetime window within a single molecular framework. This would help readers readily apply the concept to other heteroaromatic RTP systems.\n2. One notable strength of this work is that full-spectrum RTP is achieved without complex substitution, heavy atoms, or multicomponent crystalline systems, relying instead on minimalist backbone isomerization. This conceptual simplicity is highly attractive but not yet fully highlighted. The authors are encouraged to more explicitly contrast this “isomeric minimalism” strategy with conventional approaches (e.g., substituent engineering, heavy-atom effects, or host-guest energy-level matching), thereby underscoring the conceptual elegance and scalability of the present design.\n3. Although multiple polymer matrices (PVA, PVP, PVB, PMMA) are explored, the mechanistic framework is strongly anchored in PVA-specific interactions. The authors should clarify: To what extent the isomer-dependent color sequence and energy-level trends are preserved across different hosts. Whether the superiority of Ph-Cz in lifetime is universal or matrix-dependent.\n4. The marine and biological application discussions are intriguing but somewhat speculative. The authors should more clearly distinguish demonstrated material stability (e.g., seawater immersion, optical retention) from hypothesized biological regulation effects. Claims related to “behavior modulation” of marine organisms should be softened or explicitly framed as future prospects unless direct biological evidence is provided.\n5. The manuscript is generally well written, but minor language polishing is recommended to reduce repetition in the Introduction and Discussion.\n6. The term “triple excitons” should be corrected to “triplet excitons” where applicable."}, "author_response": "Dear editor and referees:\nWe sincerely thank the editor and all the reviewers for their valuable comments,\nwhich helped us to improve the quality of our manuscript. We have tried our best to\nimprove the manuscript and have made some changes in the manuscript. These\nchanges do not affect the content and scope of the paper. And here, the reviewers’\ncomments are listed in normal font and specific questions are numbered, our changes\nand additions to the manuscript are listed in red font. We sincerely thank the editors\nand reviewers for their hard work and hope that the corrections will be accepted.\nYours sincerely,\nMingliang Sun\nSchool of Materials Science and Engineering, Ocean University of China, Qingdao\n266100, China.\nE-mail: mlsun@ouc.edu.cn\nReviewers' comments:\nReviewer 1:\nComments:\nRoom-temperature phosphorescence (RTP) is a key aspect and frontier in\nphotophysics. Herein, the authors report an isomeric design strategy for achieving\nfull-color, rainbow-like organic phosphorescence. Precise nitrogen-site isomerization\nspecies are of interest for developing state-of-the-art models to address the thorny\nchallenges in the RTP field. However, the manuscript fails to address the key aspect of\ndeep structure-photophysical relationships. Up to date, larger strategies are proposed\nto tune the emission color of organic RTP materials, which is not the challenge in this\narea. The intrinsic exciton behaviors of the isomers require further investigation,\nrather than focusing solely on doping in matrices such as PVA—these matrices are not\nnew for RTP and rely on basic host-guest systems. Additionally, carbazole (Cz), as a\nfoundational reference, should be included for detailed discussion of RTP emission\nmechanisms. While the four isomers represent a valuable set, their utility is limited to\ntuning emission wavelength, with only modest, unsystematic changes in RTP\nefficiency. Critically, this study does not clarify the significant differences between\nthe three benzindole isomers and Cz. I recommend rejection. Specific detailed\nconcerns are as follows:\nResponse: We sincerely thank the reviewer for the thorough and critical evaluation of\nour manuscript. We fully appreciate the reviewer’s concerns regarding the depth of\nstructure-photophysical relationship analysis and the need for clearer differentiation\nbetween the benzindole isomers and carbazole (Cz). We acknowledge that these\naspects were not sufficiently articulated in the original version and regret any\nambiguity this may have caused.\nIn response, we have substantially revised the manuscript with a particular\nfocus on strengthening the intrinsic structure-property correlations of the isomeric\nsystem, rather than emphasizing emission color tuning alone. Specifically, we have\nreorganized the Results and Discussion sections to explicitly correlate nitrogen-site\nisomerization with exciton localization, triplet-state energetics, and nonradiative\ndecay suppression, thereby providing a more mechanistic interpretation of the\nobserved RTP behaviors. These revisions aim to clarify that the isomeric design does\nnot merely modulate emission wavelength, but systematically alters the photophysical\nlandscape of triplet excitons.\nRegarding the reviewer’s concern that matrix-assisted RTP systems are not\nconceptually new, we fully agree that polymer hosts such as PVA represent a\nwell-established platform. Accordingly, we have revised the text to clarify that the\npolymer matrix is employed as a controlled environment to suppress oxygen\nquenching and molecular motion, enabling a fair comparison of intrinsic exciton\nbehaviors among different isomers. The emphasis of the revised manuscript has been\nshifted away from the matrix itself and toward the inherent photophysical differences\narising from molecular isomerism.\nIn line with the reviewer’s suggestion, carbazole (Cz) has now been more\nexplicitly positioned as a foundational reference. We have expanded the discussion to\ninclude a direct comparison between Cz and the three benzindole isomers,\nhighlighting how nitrogen-site variation modulate spin-orbit coupling and triplet-state\nstabilization. This comparative analysis clarifies the origin of the distinct RTP\nresponses and addresses the previously insufficient differentiation among the isomers.\nAll revisions are highlighted in red in the revised manuscript. We believe that\nthese substantial clarifications and structural improvements directly address the\nreviewer’s concerns and significantly strengthen the mechanistic depth and scientific\nrigor of the study.\n1. Lack of deep structure-photophysical relationships, alongside the absence of\nquantitative analysis and generalizable rules for doping matrices. The quantitative\neffects of isomerism on RTP performance must be further elaborated in the study.\nResponse: We sincerely thank the reviewer for this important and constructive\ncomment. We fully agree that a central objective of the present work is to clarify the\nstructure-photophysical relationships governing RTP behavior, particularly within an\nisomeric molecular framework.\nIn the revised manuscript, we have carefully refined and expanded the discussion\nto more clearly and quantitatively describe the influence of benzindole isomerism on\nRTP performance. Specifically, our analysis focuses on systematically comparable\nphotophysical parameters, including phosphorescence emission wavelengths,\ntriplet-state energy levels, and lifetime trends, all examined within the same molecular\nbackbone and under identical doping conditions. By adopting this unified platform,\nwe aim to establish a direct and internally consistent correlation between\nnitrogen-position isomerization and triplet-state reorganization, while minimizing\npotential confounding contributions from substituent effects or variations in the host\nmatrix. With respect to the choice of doping matrices, we would like to clarify that the\nintention of this study is not to derive a universally applicable empirical rule across all\npossible host systems. Rather, our goal is to demonstrate that the RTP modulation\ninduced by molecular isomerism is consistently preserved across several\nrepresentative polymer matrices. While absolute phosphorescence lifetimes and\nintensities understandably vary with host rigidity and local microenvironment, the\nrelative emission trends associated with isomerism remain unchanged. This\ncross-matrix consistency supports the general validity of the isomer-driven\nmodulation mechanism at the molecular level.\nAccordingly, we have revised the manuscript to more explicitly articulate these\nquantitative relationships and their underlying physical origins, with the\ncorresponding changes highlighted in red. We are grateful to the reviewer for this\nthoughtful comment, which has prompted us to sharpen both the mechanistic focus\nand the quantitative clarity of the discussion. Within the benzindole isomeric family,\nwe propose that the impact of molecular isomerism on RTP behavior can be\nquantitatively rationalized in terms of triplet-state electronic reorganization.\nSystematic variation of the nitrogen position leads to predictable shifts in\nphosphorescence emission wavelengths and corresponding T energy levels, while\n1\npreserving an identical conjugated backbone and substituent environment. Importantly,\nthe observed RTP modulation primarily originates from intrinsic changes in\ntriplet-state localization and spin-orbit coupling efficiency, rather than from external\nsubstituent effects or host-specific interactions. In this context, matrix effects mainly\ninfluence the magnitude of RTP responses, whereas backbone-level isomerization\ngoverns their direction and energetic trends.\nWe hope that this revised presentation more clearly conveys the scope, intent,\nand mechanistic significance of the study.\n2. The theoretical framework should incorporate molecular dynamics simulations of\nmixed samples, followed by calculations of electronic structures to better reflect\nreal-world system dynamics.\nResponse: We sincerely thank the reviewer for this valuable suggestion. Following\nthis recommendation, we have incorporated molecular dynamics (MD) simulations of\nthe mixed host-guest systems and subsequently performed electronic-structure\ncalculations on representative configurations extracted from the equilibrated MD\ntrajectories, in order to better reflect realistic structural heterogeneity and dynamic\nenvironments.\nThe MD simulations confirm that, despite local conformational fluctuations and\nhost-guest interactions, the doped guest molecules remain structurally well-confined\nwithin the polymer matrix. Importantly, electronic-structure calculations based on\nMD-sampled geometries reveal that the relative ordering and energy separation of the\ntriplet states among different benzindole isomers are well preserved. This result\nindicates that the isomer-dependent modulation of RTP emission energies is robust\nagainst dynamic environmental variations. These findings support our central\nconclusion that backbone-level isomerism governs the intrinsic triplet-state energetics,\nwhile dynamic host-guest effects primarily influence the magnitude of RTP\nperformance rather than the qualitative emission trends. The MD methodology,\ncomputational details, and corresponding results have been added to the revised\nmanuscript and are highlighted in red.\nWe appreciate the reviewer’s suggestion, as the inclusion of MD simulations\nsignificantly strengthens the physical realism and mechanistic robustness of the\ntheoretical framework.\nSupplementary Fig. 27. The simulation model of Ph-Cz (a), Ph-Bd[g] (b), Ph-Bd[e] (c), Ph-Bd[f]\n(d), in PVA film.\nSupplementary Fig. 28. Calculated energy levels and SOC constants of Ph-Cz, Ph-Bd[g], Ph-Bd[e],\nPh-Bd[f], RTP systems. Guest molecules were optimized in PVA matrices based on geometries\nextracted from MD simulations. SOC constants of S -T transitions available for ISC processes\n1 n\nwere highlighted in red.\nSupplementary Fig. 29. The simulation model of Ph-Cz (a), Ph-Bd[g] (b), Ph-Bd[e]\n(c), Ph-Bd[f] (d), in PVP film.\nSupplementary Fig. 30. Calculated energy levels and SOC constants of Ph-Cz,\nPh-Bd[g], Ph-Bd[e], Ph-Bd[f], RTP systems. Guest molecules were optimized in PVP\nmatrices based on geometries extracted from MD simulations. SOC constants of S -T\n1 n\ntransitions available for ISC processes were highlighted in red.\nSupplementary Fig. 31. The simulation model of Ph-Cz (a), Ph-Bd[g] (b), Ph-Bd[e]\n(c), Ph-Bd[f] (d), in PVB film.\nSupplementary Fig. 32. Calculated energy levels and SOC constants of Ph-Cz,\nPh-Bd[g], Ph-Bd[e], Ph-Bd[f], RTP systems. Guest molecules were optimized in PVB\nmatrices based on geometries extracted from MD simulations. SOC constants of S -T\n1 n\ntransitions available for ISC processes were highlighted in red.\nSupplementary Fig. 33. The simulation model of Ph-Cz (a), Ph-Bd[g] (b), Ph-Bd[e]\n(c), Ph-Bd[f] (d), in PMMA film.\nSupplementary Fig. 34. Calculated energy levels and SOC constants of Ph-Cz,\nPh-Bd[g], Ph-Bd[e], Ph-Bd[f], RTP systems. Guest molecules were optimized in\nPMMA matrices based on geometries extracted from MD simulations. SOC constants\nof S -T transitions available for ISC processes were highlighted in red.\n1 n\n3. the key role of non-novel matrices (e.g., PVA) and incomplete analysis of the\nhost-guest system, which fails to advance beyond established RTP matrix design\nparadigms.\nGiven these fundamental limitations, I recommend rejecting the manuscript. To\nstrengthen the work, the authors should: (1) dissect the intrinsic exciton behaviors of\nthe isomers (e.g., via comparative studies of neat isomer films vs. polymer-doped\nsystems); (2) integrate molecular dynamics (MD) simulations into the theoretical\nanalysis to model dynamic isomer-matrix interactions; (3) establish quantitative\nstructure-RTP relationships (e.g., linking nitrogen position to triplet energy levels or\nintersystem crossing efficiency); and (4) frame Cz as a mechanistic benchmark to\nderive clear rules for distinguishing benzindole isomers. A revised version of this\nwork may be more suitable for submission to specialized journals.\nResponse: We sincerely thank the reviewer for the detailed and technically insightful\nevaluation. We greatly appreciate the time and effort invested in these careful\ncomments. Below, we respond to each point with the aim of further clarifying the\nscope, underlying rationale, and scientific contributions of the present work.\n(1) Role of non-novel matrices and advancement beyond established RTP paradigms.\nResponse: We acknowledge that commonly used polymer matrices such as PVA or\nPMMA are not structurally novel. However, the primary objective of this study is not\nmatrix innovation, but rather to employ representative and well-understood host\nsystems as controlled environments to isolate and interrogate the intrinsic\nphotophysical effects arising from molecular isomerism. The use of established\nmatrices is therefore deliberate, enabling us to minimize confounding variables and to\ndemonstrate that the observed RTP modulation originates from the guest molecular\nbackbone rather than from host-specific design strategies. In this context, the work\nadvances beyond conventional matrix-centered paradigms by shifting the design focus\ntoward intrinsic isomer-driven triplet-state regulation.\n(2) dissect the intrinsic exciton behaviors of the isomers (e.g., via comparative studies\nof neat isomer films vs. polymer-doped systems).\nResponse: Following the reviewer’s helpful suggestion, we have carefully\nstrengthened the discussion of intrinsic excitonic behavior by explicitly comparing the\nphotophysical responses of neat isomer films with those of polymer-doped systems.\nWe appreciate this comment, as it allowed us to more clearly delineate\nmolecular-level effects from matrix-assisted phenomena.\nSpecifically, we have added room-temperature phosphorescence emission spectra\nof the neat guest molecular films. These additional data show that, under ambient\nconditions, the neat isomer films do not exhibit detectable phosphorescence. This\nobservation is consistent with our original description that all four molecular\nskeletons require polymer matrices to effectively suppress non-radiative decay\nchannels and enable room-temperature, and even elevated-temperature,\nphosphorescence emission.\nAt the same time, the comparative analysis reveals that although polymer\nmatrices play a crucial role in enhancing RTP observability by restricting molecular\nmotion, the relative ordering of emission wavelengths and triplet-state energy levels is\nalready encoded at the molecular level. Notably, these intrinsic trends are consistently\npreserved upon incorporation into polymer hosts. Taken together, these results support\nthe conclusion that molecular isomerism governs the fundamental exciton energetics,\nwhile the matrix primarily modulates the intensity and lifetime of RTP performance.\nWe believe that this revised discussion and the newly added experimental data\nprovide a clearer and more balanced understanding of the respective roles of\nmolecular design and matrix effects, and we are grateful to the reviewer for prompting\nthis clarification.\nSupplementary Fig. 22. Room-temperature phosphorescence (RTP) emission spectra of the four\nguest molecules.\n(3) integrate molecular dynamics (MD) simulations into the theoretical analysis to\nmodel dynamic isomer-matrix interactions\nResponse: We fully agree that dynamic host-guest interactions are important for\nunderstanding realistic mixed systems. Accordingly, we have incorporated molecular\ndynamics (MD) simulations of representative host-guest models, followed by\nelectronic-structure calculations based on MD-sampled geometries. The results\ndemonstrate that, despite local structural fluctuations, the relative triplet-state energy\nordering among different benzindole isomers remains robust. This finding reinforces\nthe conclusion that isomer-dependent RTP regulation is intrinsically molecular in\norigin, while dynamic environmental effects play a secondary, modulatory role.\n(4) establish quantitative structure-RTP relationships (e.g., linking nitrogen position to\ntriplet energy levels or intersystem crossing efficiency)\nResponse: We have revised the manuscript to more explicitly articulate quantitative\ncorrelations between nitrogen-position isomerization and RTP-relevant parameters,\nincluding phosphorescence emission energies and triplet-state levels. Within a unified\nmolecular backbone and under identical doping conditions, nitrogen-position\nvariation provides a means to tune triplet energetics. While absolute RTP lifetimes and\nintensities remain matrix-dependent, the isomer-dependent energy trends are\nquantitatively preserved, establishing a meaningful structure RTP relationship at the\nplatform level.\n(5) frame Cz as a mechanistic benchmark to derive clear rules for distinguishing\nbenzindole isomers\nResponse: We appreciate the reviewer’s suggestion regarding carbazole (Cz). In the\nrevised manuscript, Cz is explicitly framed as a mechanistic reference rather than a\ndirect performance comparator. Its role is to provide a conceptual baseline for\nunderstanding how backbone modification and nitrogen relocation alter triplet-state\nelectronic structures. Once the design focus shifts to the benzindole isomeric family,\nthe RTP behavior is governed by intrinsic electronic reorganization within the Bd\nframework itself, enabling clear differentiation among Bd isomers without relying on\nCz-derived performance metrics.\nIn summary, we respectfully submit that the present work seeks to establish a coherent\nand quantitative isomer-based framework for regulating organic RTP, supported by\nsystematic experimental, comparative, and theoretical analyses. We fully recognize\nthat this study does not attempt to introduce matrix innovation per se, and we do not\nintend to position it as such. Rather, we hope that the emphasis on intrinsic molecular\nskeleton design may provide a complementary and mechanistically distinct viewpoint\nalongside existing matrix- and substituent-centered strategies in the field. By\nelucidating how nitrogen-position isomerism within a unified molecular framework\ninfluences emission wavelength and triplet-state behavior, we aim to offer a\npotentially generalizable design concept that could be further explored in organic\noptoelectronic materials. In particular, we envision different molecular skeletons\nmight be judiciously selected and subsequently modified according to targeted\nemission requirements, thereby enabling more application-oriented molecular\ntailoring. From this perspective, the present framework may also contribute, in a\nmodest way, to expanding the range of molecular backbones considered for RTP and\nrelated optoelectronic applications beyond the extensively studied carbazole motif.\nWe sincerely thank the reviewer for the insightful and constructive comments, which\nhave been invaluable in helping us more clearly define the scope, positioning, and\nbroader implications of this work.\nReviewer 2:\nThe paper presents a comprehensive and systematic study of the room-temperature\nphosphorescence (RTP) properties of carbazole (Cz) and its benzindole isomers (Bd[f],\nBd[e] and Bd[g]). By precisely controlling the nitrogen atom position within the\ntricyclic backbone, the authors achieve full-spectrum, tunable RTP spanning red,\nyellow, green, and blue emissions. Two new isomers, Bd[e] and Bd[g] were\nsynthesized by a one-pot solvent-free mechanical method, which is greener and more\nefficient than the traditional multi-step solution method (Bd[f]). The material exhibits\nexcellent stability and long afterglow emission in matrices such as PVA, PVP, PVB,\nand PMMA, and can be used in applications such as anti-counterfeiting, Marine\nidentification, and biological simulation.\nThe experimental content is innovative and has a complete experimental system.\nHowever, there are numerous issues in writing and formatting. I recommend the\nmanuscript requires a thorough revision before it can be considered for publication.\nResponse: We sincerely thank the reviewer for the positive and encouraging\nevaluation of our work, as well as for the careful assessment of its strengths and\nlimitations. We greatly appreciate the reviewer’s recognition of the systematic nature\nof our study, the isomeric nitrogen-site design strategy, the green mechanochemical\nsynthesis, and the demonstrated RTP performance and stability across different\npolymer matrices.\nWe fully acknowledge the reviewer’s concern regarding the issues in writing and\nformatting. We agree that, while the experimental framework is complete and the\nscientific content is innovative, the current presentation does not yet meet the high\nstandards of clarity, conciseness, and organization required for publication in Nature\nCommunications. We sincerely apologize for this shortcoming. In response, we have\ncarried out a thorough and careful revision of the entire manuscript. Specifically, we\nhave comprehensively improve the language quality, refine the logical flow of the text,\nand restructure key sections to better highlight the central scientific message and\nconceptual advances of this work. We also have carefully revised the formatting,\nfigures, and captions to ensure consistency, readability, and alignment with the\njournal’s style and expectations.\nWe highly value the opportunity to revise our manuscript and firmly believe that,\nwith substantial improvements in writing and presentation, this work can more clearly\nconvey its scientific significance and meet the standards of Nature Communications.\nWe sincerely thank the reviewer again for the constructive comments and the time\ninvested in evaluating our manuscript, which we believe will greatly help us improve\nits overall quality.\n1. The paper reads overly complicated and repetitive in many parts, suggesting that\nthe authors may have relied heavily on AI-assisted writing. I recommend a substantial\nrevision to improve clarity and authenticity of expression.\nResponse: We sincerely thank the reviewer for this important and candid comment.\nWe fully acknowledge that the current manuscript reads overly complicated and\ncontains repetitive descriptions in several sections, which compromises clarity and\nweakens the overall readability. We take the reviewer’s concern very seriously and\nagree that the expression should be more concise, direct, and authentic.\nIn response, we have undertaken a substantial, author-driven revision of the entire\nmanuscript. This revision involve a careful restructuring of the narrative, elimination\nof redundant descriptions, and a thorough rewriting of sentences and paragraphs to\nensure clarity, coherence, and a more natural scientific tone. Particular attentions have\nbeen paid to simplifying complex expressions and ensuring that each paragraph\nconveys a clear and distinct message. We sincerely appreciate the reviewer’s\ncomment, which highlights a critical aspect of the manuscript that requires\nimprovement.\n2. The NMR spectra don’t indicate the solvent reference peaks, and why CDCl was\n3\nchosen as the NMR solvent, as some product signals appear close to the solvent peak.\nResponse: We sincerely thank the reviewer for this careful observation. We apologize\nfor not clearly indicating the solvent reference peaks in the original presentation.\nThe residual solvent signals of CDCl (δ = 7.26 ppm for 1H and δ = 77.16 ppm\n3\nfor 13C) are indicated in the NMR spectra; In the revised manuscript, we have clearly\nannotated the solvent peaks and added explicit descriptions to avoid any ambiguity.\nCDCl was chosen due to the good solubility of the compounds and the overall quality\n3\nof the spectra. Although some signals appear close to the solvent peak, they are well\nresolved and reliably assigned. This point has now been clarified in the revised text.\nWe thank the reviewer for helping us improve the clarity of the characterization data.\n3. Page numbers are missing, the display errors are found on page 22 of the main text\nand page 1 of the supplementary information.\nResponse: We sincerely thank the reviewer for pointing out these issues. We\napologize for our oversight regarding the missing page numbers and the display errors\nin the main text and Supplementary Information. These issues have now been\ncarefully corrected in the revised manuscript. We appreciate the reviewer’s careful\nattention to detail, which has helped us improve the overall quality and presentation of\nthe manuscript.\n4. the reference format is incorrect (refs. 28 and 44).\nResponse: We sincerely thank the reviewer for pointing out this issue. We apologize\nfor the incorrect reference formatting in refs. 28 and 44. The reference formats have\nnow been carefully corrected, and the corresponding changes have been highlighted in\nred in the revised manuscript for clarity.\n5. I think the final \"discussion\" should be changed to \"conclusion\". I think the title\n\"Discussion\" of section should be changed to \"Conclusion\".\nResponse: We thank the reviewer for this helpful and constructive suggestion. We\nagree that “Conclusion” is a more appropriate title for the final section. Accordingly,\nthe section heading has been revised from “Discussion” to “Conclusion” in the\nrevised manuscript, and the modification has been highlighted in red for clarity.\n6. It is unnecessary to label the figure as “Scheme 1”. Just using the conventional term\n“Figure 1” would be sufficient.\nResponse: We sincerely thank the reviewer for this helpful suggestion. The figure\noriginally labeled as “Scheme 1” has been renamed “Figure 1” in the revised\nmanuscript. The corresponding change has been highlighted in red for clarity. We\nappreciate the reviewer’s careful reading and valuable suggestion.\n7. “Supplementary Fig.” is lengthy. It is recommended to use the abbreviation such as\n“Fig. S1”.\nResponse: We sincerely thank the reviewer for this helpful suggestion. We agree that\nusing abbreviated figure labels improves clarity and readability. Accordingly,\n“Supplementary Fig.” has been replaced with the abbreviated format “Fig. S1”, “Fig.\nS2”, etc., throughout the manuscript and Supporting Information. All corresponding\nchanges have been highlighted in red in the revised version for clarity. We appreciate\nthe reviewer’s careful attention to presentation details.\n8. the “Results and Discussion” section is rather lengthy and would benefit from being\ndivided into several subsections for better readability.\nResponse: We sincerely thank the reviewer for this constructive suggestion. We agree\nthat the current “Results and Discussion” section is relatively lengthy and could\nbenefit from improved organization. In the revised manuscript, we have divided this\nsection into several clearly defined subsections to enhance readability and better guide\nthe reader through the results and discussion. All corresponding changes have been\nimplemented accordingly. We appreciate the reviewer’s helpful suggestion, which has\nimproved the clarity and structure of the manuscript.\n9. Some pictures in the supplementary information have a light color. It is\nrecommended to darken them.\nResponse: We sincerely thank the reviewer for this helpful suggestion. We agree that\nsome figures in the Supplementary Information were too light and could affect\nreadability. Accordingly, these figures have been adjusted to darker colors to improve\nclarity and visibility. The corresponding changes have been implemented in the\nrevised version. We appreciate the reviewer’s careful attention to presentation details,\nwhich has helped us improve the quality of the Supplementary Information.\n10. It is recommended to include a comparative summary table in the main text.\nResponse: We thank the reviewer for this helpful suggestion. In response, a\ncomparative summary table has been added to the Supporting Information (Table S6),\nwhere the relevant data can be presented in a more detailed and systematic manner.\nKey comparative trends are now explicitly discussed in the main text.\nSupplementary Table 6. RTP performance of Ph-Cz, Ph-Bd[g], Ph-Bd[e], and Ph-Bd[f] doped\nsystems utilized in matrix universality.\nMolecule matrix RTP color T (eV) τ (s) Φ (%)\nPh-Cz PVA Blue 2.70 4.23 16.6\nPh-Bd[g] PVA Green 2.29 0.93 13.4\nPh-Bd[e] PVA Yellow 2.05 0.74 12.9\nPh-Bd[f] PVA Red 1.65 0.20 10.7\nPh-Cz PVP Blue 2.54 0.63 16.2\nPh-Bd[g] PVP Green 2.26 0.56 13.7\nPh-Bd[e] PVP Yellow 2.11 0.40 13.1\nPh-Bd[f] PVP - 1.65 - -\nPh-Cz PVB Blue 2.68 0.91 9.2\nPh-Bd[g] PVB Green 2.43 0.38 11.3\nPh-Bd[e] PVB Yellow 2.01 0.26 10.9\nPh-Bd[f] PVB - 1.69 - -\nPh-Cz PMMA Blue 3.60 0.41 7.1\nPh-Bd[g] PMMA Green 2.23 0.25 9.7\nPh-Bd[e] PMMA Yellow 2.05 0.17 8.6\nPh-Bd[f] PMMA - 1.65 - -\nReviewer 3:\nThe manuscript “Isomer design unlocks rainbow phosphorescence” presents a\nsystematic study on carbazole and benzindole isomers as a unified molecular platform\nfor achieving full-spectrum organic room-temperature phosphorescence (RTP).\nBy integrating isomeric backbone engineering, solvent-free mechanochemical synthesis,\npolymer matrix confinement, and multiscale photophysical/theoretical analyses, the\nauthors demonstrate red-yellow-green-blue (RYGB) persistent phosphorescence and\nshowcase a series of application-oriented demonstrations. The work is clearly written,\nexperimentally thorough, and conceptually appealing. In particular, the completion of\nthe carbazole-benzindole isomer family and the scalable access to Bd[g] and Bd[e]\nvia mechanochemistry are noteworthy advances. The manuscript has the potential to meet\nthe publication standard of Nature Communications. Nevertheless, several minor\npoints require careful clarification and strengthening before acceptance.\nResponse: We sincerely thank the reviewer for the thorough, positive, and\nencouraging assessment of our manuscript. We greatly appreciate the reviewer’s\nrecognition of the systematic nature of our study, the completion of the\ncarbazole-benzindole isomer family, the mechanochemical access to Bd[g] and Bd[e],\nand the potential relevance of this work to the field of organic room-temperature\nphosphorescence. We are particularly grateful for the reviewer’s assessment that the\nmanuscript has the potential to meet the publication standard of Nature\nCommunications. We take this opportunity very seriously and have carefully\naddressed all the minor points raised by the reviewer. Each comment has been\nconsidered in detail, and corresponding revisions and clarifications have been\nimplemented in the manuscript to further strengthen clarity, rigor, and presentation.\nWe sincerely thank the reviewer for the constructive suggestions and the time invested\nin evaluating our work, which have significantly improved the quality of the\nmanuscript.\n1. The manuscript convincingly demonstrates that subtle backbone isomerization can\nsimultaneously modulate emission color, triplet-state energetics, and RTP lifetime.\nWhile these results are well supported, the underlying design principle is currently\ndistributed across several sections. It would benefit the broader Nature\nCommunications readership if the authors explicitly summarize, in the Discussion, a\nconcise and transferable design rule, for example, how nitrogen-site isomerization\ngoverns both electronic structure (triplet energy) and host-guest interaction strength,\nthereby defining the accessible RTP color range and lifetime window within a single\nmolecular framework. This would help readers readily apply the concept to other\nheteroaromatic RTP systems.\nResponse: We sincerely thank the reviewer for this insightful and constructive\nsuggestion, which encouraged us to more carefully reflect on how the central design\nconcept of this work should be articulated for a broad Nature Communications\nreadership.\nAfter careful consideration, and in light of the comprehensive revisions already made\nthroughout the manuscript, we have revised the Discussion section to present the\ndesign principle in a more cautious, precise, and physically well-defined manner.\nImportantly, we now avoid framing the concept as a predictive rule for RTP lifetime\nor host–guest interaction strength. Instead, we explicitly delimit its applicability to the\nintrinsic electronic structure of the molecular backbone.\nIn the revised discussion, we emphasize that nitrogen-site isomerization within the\nbenzindole framework provides a systematic means to modulate triplet-state energy\nlevels and their relative ordering. This intrinsic energetic regulation defines the\naccessible phosphorescence emission energy (and thus emission color) within a single\nmolecular family. By contrast, we now clearly state that RTP lifetimes and absolute\nperformance metrics are highly sensitive to external factors, including matrix rigidity,\nlocal microenvironment, and the efficiency of nonradiative decay suppression.\nAccordingly, the present work does not aim to establish a universal or transferable\nrule for predicting RTP lifetimes based solely on molecular isomerism.\nAdditional comparisons across different polymer matrices further support this\nclarification. While the absolute phosphorescence lifetimes vary substantially\ndepending on the host environment, the emission energy trends imposed by\nnitrogen-site isomerization remain consistent, underscoring that the proposed design\nprinciple operates at the level of triplet-state energetics rather than excited-state\ndynamics.\nBy reformulating the design rule in this restrained and mechanism-focused manner,\nwe aim to provide readers with a reliable and transferable conceptual framework:\nheteroatom site isomerization can be used to rationally tune triplet-state energy\nlandscapes and phosphorescence color in organic RTP systems, while lifetime\noptimization and host-guest interactions must be addressed separately in\nsystem-specific implementations. We are grateful to the reviewer for this valuable\ncomment, which has significantly improved the conceptual clarity and scientific rigor\nof the revised manuscript.\n2. One notable strength of this work is that full-spectrum RTP is achieved without\ncomplex substitution, heavy atoms, or multicomponent crystalline systems, relying\ninstead on minimalist backbone isomerization. This conceptual simplicity is highly\nattractive but not yet fully highlighted. The authors are encouraged to more explicitly\ncontrast this “isomeric minimalism” strategy with conventional approaches (e.g.,\nsubstituent engineering, heavy-atom effects, or host-guest energy-level matching),\nthereby underscoring the conceptual elegance and scalability of the present design.\nResponse: We sincerely thank the reviewer for this positive and insightful comment.\nWe agree that the conceptual simplicity of achieving full-spectrum RTP through\nbackbone isomerization alone represents an important strength of this work and merits\nclearer emphasis.\nIn response, we have revised the Introduction and Discussion to more explicitly\ncontrast the present backbone isomerization based minimalist strategy with commonly\nused approaches, including extensive substituent engineering, heavy-atom\nincorporation, and multicomponent host-guest or crystalline systems. We now clarify\nthat, in contrast to these strategies which often rely on increased molecular\ncomplexity or precise compositional control the current approach achieves color\nregulation within a single molecular framework through subtle positional\nisomerization.\nWe emphasize that this comparison is intended to highlight the conceptual\neconomy and potential scalability of the isomeric design, rather than to discount the\neffectiveness of conventional methods. The corresponding revisions have been\nhighlighted in red in the revised manuscript.\n3. Although multiple polymer matrices (PVA, PVP, PVB, PMMA) are explored, the\nmechanistic framework is strongly anchored in PVA-specific interactions. The authors\nshould clarify: To what extent the isomer-dependent color sequence and energy-level\ntrends are preserved across different hosts. Whether the superiority of Ph-Cz in\nlifetime is universal or matrix-dependent.\nResponse: We sincerely thank the reviewer for this insightful comment, which raises\nan important point regarding the generality and matrix dependence of the proposed\nmechanism.\nIn the revised manuscript, we have clarified that the isomer-dependent\nphosphorescence color sequence and associated energy-level trends are intrinsic to the\nmolecular backbone design and are consistently preserved across different polymer\nhosts (PVA, PVP, PVB, and PMMA). This robustness reflects that the emissive color\nis primarily governed by the molecular electronic structure rather than host-specific\ninteractions.\nBy contrast, we now explicitly emphasize that the exceptionally long\nphosphorescence lifetime observed for Ph-Cz is not universal across all matrices but\nis strongly matrix-dependent. In particular, in the PVA matrix, strong intermolecular\nhydrogen-bonding interactions between PVA and carbazole units provide effective\ntriplet-state stabilization and nonradiative suppression, leading to markedly prolonged\nphosphorescence lifetimes. Such specific hydrogen-bond-assisted confinement is\nabsent in other polymer matrices (e.g., PVP, PVB, PMMA), and consequently, Ph-Cz\ndoes not exhibit comparably superior lifetime performance in those hosts.\nTo avoid potential ambiguity, we have revised the mechanistic discussion to\nclearly distinguish molecular-intrinsic color regulation from host-dependent lifetime\nenhancement, and the corresponding text has been carefully rephrased and highlighted\nin red in the revised manuscript.\n4. The marine and biological application discussions are intriguing but somewhat\nspeculative. The authors should more clearly distinguish demonstrated material\nstability (e.g., seawater immersion, optical retention) from hypothesized biological\nregulation effects. Claims related to “behavior modulation” of marine organisms\nshould be softened or explicitly framed as future prospects unless direct biological\nevidence is provided.\nResponse: We sincerely thank the reviewer for this thoughtful and constructive\ncomment. We agree that it is important to clearly distinguish experimentally\ndemonstrated material properties from forward-looking application perspectives.\nIn response, we have further strengthened the discussion of material stability by\nsupplementing additional phosphorescence emission spectra after seawater immersion,\nwhich directly support the retention of optical performance under marine-relevant\nconditions. These results provide concrete experimental evidence for the\nphotophysical robustness of the RTP composites, and the corresponding data have\nbeen added to the revised manuscript. Meanwhile, following the reviewer’s\nsuggestion, we have carefully revised the application-related discussion to avoid\noverinterpretation. Statements concerning potential biological regulation or behavioral\neffects have been softened and are now explicitly framed as future prospects rather\nthan experimentally validated outcomes.\nAll modifications have been incorporated into the manuscript and are highlighted\nin red. We appreciate the reviewer’s guidance, which has helped improve the\nprecision and balance of the application discussion.\n“In addition to the demonstrated material stability, the potential relevance of the\nemission characteristics of these RTP materials to marine biological systems was\nconsidered from a photophysical perspective. Many marine organisms exhibit\nwavelength dependent visual sensitivity; accordingly, the photoluminescence profiles\nof PMMA-based RTP films were examined under UV excitation (Fig. 6e). The\nmaterials display stable and persistent afterglow across the visible region,\nencompassing wavelength ranges commonly involved in marine visual perception.\nNotably, the green-to-yellow emission band (approximately 500-580 nm) overlaps\nwith reported sensitivity windows of certain reef-associated fish and benthic\ninvertebrates. This spectral overlap suggests that such RTP materials could provide a\nuseful optical basis for future investigations into light-organism interactions in marine\nenvironments.”\nSupplementary Fig. 50. Phosphorescence emission spectra of Ph-Cz@PVB (a), Ph-Bd[g]@PVB\n(b), and Ph-Bd[e]@PVB (c) films recorded before and after seawater immersion.\nSupplementary Fig. 51. Phosphorescence emission spectra of Ph-Cz@PMMA (a),\nPh-Bd[g]@PMMA (b), and Ph-Bd[e]@PMMA (c) films recorded before and after seawater\nimmersion.\n5. The manuscript is generally well written, but minor language polishing is\nrecommended to reduce repetition in the Introduction and Discussion.\nResponse: We sincerely thank the reviewer for this helpful comment. In response, we\nhave carefully polished the language in the Introduction and Discussion sections, with\nparticular attention to reducing repetitive phrasing and improving clarity. All revisions\nhave been highlighted in red in the revised manuscript.\n6. The term “triple excitons” should be corrected to “triplet excitons” where\napplicable.\nResponse: We sincerely thank the reviewer for pointing out this terminology error.\nWe apologize for the inaccurate use of the term “triple excitons” in the original\nmanuscript. This has now been carefully corrected to “triplet excitons” throughout the\nrevised text, and the corresponding descriptions have been rephrased accordingly to\nensure scientific accuracy and clarity."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The manuscript describes the preparation of a series of carbazole and benzindole positional isomers and investigates their room-temperature phosphorescence (RTP) properties. By modulating the nitrogen substitution site, the authors demonstrate emission color tuning spanning the visible region and correlate these variations with changes in triplet-state characteristics through experimental measurements and theoretical calculations.\nThe study is systematically conducted and the data are generally convincing. The structure-property relationships are presented in a clear manner, and the photophysical analyses are adequate to support the reported phenomena. Nevertheless, in my assessment, the advance is primarily incremental rather than transformative. While the positional isomer strategy provides a useful structure-emission correlation, similar approaches have been widely explored in organic RTP systems, and the mechanistic interpretation remains largely within established theoretical frameworks. The work does not substantially deepen the fundamental understanding of triplet-state regulation nor introduce a broadly enabling design paradigm. In view of the scope and selectivity of Nature Communications, which typically requires a clear conceptual breakthrough or a field-defining advance, the present study appears more suitable for a specialized journal in organic optoelectronic or photophysical materials rather than for publication in Nature Communications.", "reviewer_2": "The revised paper can be accepted.", "reviewer_3": "The present manuscript had been revised in an appropriate and disciplined manner. The revision increased the quality of the manuscript and hence the manuscript may be suitable for its publication in its present form."}, "author_response": "Reviewers' comments:\nReviewer 1:\nComments:\nThe manuscript describes the preparation of a series of carbazole and benzindole\npositional isomers and investigates their room-temperature phosphorescence (RTP)\nproperties. By modulating the nitrogen substitution site, the authors demonstrate\nemission color tuning spanning the visible region and correlate these variations with\nchanges in triplet-state characteristics through experimental measurements and\ntheoretical calculations.\nThe study is systematically conducted and the data are generally convincing. The\nstructure-property relationships are presented in a clear manner, and the photophysical\nanalyses are adequate to support the reported phenomena. Nevertheless, in my\nassessment, the advance is primarily incremental rather than transformative. While the\npositional isomer strategy provides a useful structure-emission correlation, similar\napproaches have been widely explored in organic RTP systems, and the mechanistic\ninterpretation remains largely within established theoretical frameworks. The work\ndoes not substantially deepen the fundamental understanding of triplet-state regulation\nnor introduce a broadly enabling design paradigm. In view of the scope and selectivity\nof Nature Communications, which typically requires a clear conceptual breakthrough\nor a field-defining advance, the present study appears more suitable for a specialized\njournal in organic optoelectronic or photophysical materials rather than for publication\nin Nature Communications.\nResponse: We sincerely thank the reviewer for the careful reading of our manuscript\nand for the constructive and balanced assessment. We particularly appreciate the\nreviewer’s positive comments regarding the systematic experimental design, the clarity\nof the structure-property relationships, and the reliability of the photophysical\ncharacterization. We fully understand the reviewer’s concern that positional isomer\nstrategies have been explored in organic RTP systems and that the mechanistic\nframework is largely consistent with established triplet-state photophysics. We\nrespectfully acknowledge this perspective.\nOur intention in this work was not to claim an entirely new mechanistic framework,\nbut rather to provide a unified and systematic molecular platform that enables direct\nand controlled evaluation of how nitrogen substitution topology influences triplet-state\nenergetics and emission behavior. By constructing a complete carbazole/benzindole\nisomer family and minimizing confounding structural variables, we aimed to reveal\nsubtle yet reproducible correlations between heteroatom positioning, intermolecular\norganization, and RTP color evolution.\nWe agree that individual elements of the strategy have precedents in the literature.\nHowever, we hope that the combination of a structurally coherent isomer library,\nexperimentally supported triplet-state regulation, and the demonstration of continuous\nemission tuning through intrinsic molecular topology may offer useful design insight\nfor metal-free RTP materials. In this sense, the work is intended to contribute\nincremental but systematic understanding that may facilitate more predictable\nmolecular design.\nWe are very grateful for the reviewer’s thoughtful comments, which have helped\nus further clarify the positioning and significance of the study in the revised manuscript.\nReviewer 2:\nThe revised paper can be accepted.\nResponse: We are very grateful to the reviewer for the careful assessment of the revised\nmanuscript and for the recommendation for acceptance. We truly appreciate the\nreviewer’s valuable comments and support, which have significantly helped strengthen\nthe manuscript.\nReviewer 3:\nThe present manuscript had been revised in an appropriate and disciplined manner. The\nrevision increased the quality of the manuscript and hence the manuscript may be\nsuitable for its publication in its present form.\nResponse: We sincerely thank the reviewer for the positive and encouraging\nassessment of the revised manuscript. We are grateful for the reviewer’s recognition of\nthe improvements made during revision and for the recommendation that the\nmanuscript is suitable for publication. The reviewer’s constructive comments have been\ninvaluable in helping us enhance the quality and clarity of the work."}]

Decision letter