# Curcuminoid Derivatives with a Donor-Acceptor-Donor Architecture: an Outstanding Platform for Highly-Efficient Near-Infrared Electroluminescence and Amplified Spontaneous Emission

Anthony d'Aléo, Xun Tang, Dae-hyeon Kim, Danillo Valverde, Elena Zaborova, Gabriel Canard, Arnaud Brosseau, Loic Mager, Gilles Clavier,

Chihaya Adachi, et al.

► To cite this version:

Anthony d'Aléo, Xun Tang, Dae-hyeon Kim, Danillo Valverde, Elena Zaborova, et al. Curcuminoid Derivatives with a Donor-Acceptor-Donor Architecture: an Outstanding Platform for Highly-Efficient Near-Infrared Electroluminescence and Amplified Spontaneous Emission. Advanced Optical Materials, In press, 10.1002/adom.202500338. [hal-05031373](https://hal.science/hal-05031373v1)

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# Curcuminoid Derivatives with a Donor-Acceptor-Donor Architecture: an Outstanding Platform for Highly-Efficient Near-Infrared Electroluminescence and Amplified Spontaneous Emission

Anthony D'Aléo,* Xun Tang, Dae-Hyeon Kim, Danillo Valverde, Elena Zaborova, Gabriel Canard, Arnaud Brosseau, Loïc Mager, Gilles Clavier, Chihaya Adachi, Yoann Olivier,* and Jean-Charles Ribierre*

The development of high-efficiency near-infrared (NIR) emitters for organic light-emitting diodes (OLEDs) and organic semiconductor lasers has become an important target in organic photonics. Herein, it is demonstrated that donor-acceptor-donor borondifluoride curcuminoid derivatives represent a versatile and simple platform for the molecular engineering of high-efficiency NIR emitters combining thermally-activated delayed fluorescence (TADF) with excellent electroluminescence properties and amplified spontaneous emission (ASE) activity. A series of donor-acceptor-donor curcuminoid compounds containing triphenylamino-substituents for the donor side groups and various acceptor units in the meso position were designed and synthesized. The investigation of the effects of the molecular structure on the TADF properties show that the nature of the substituents enables a fine tuning of the emission wavelengths while maintaining high photoluminescence quantum yield values. These NIR TADF dyes were used in OLEDs with an external quantum efficiency of almost 1% for a maximum emission wavelength of 797 nm. They also show a low threshold tuneable amplified spontaneous emission between 725 and 900 nm. Overall, this study provides new essential insights to rationalize the TADF activity of this family of NIR emitters and offers important prospects for designing the next generation of NIR TADF-OLEDs and organic semiconductor laser materials.

## 1. Introduction

Since the pioneering works in the near-infrared (NIR) organic light-emitting diodes (OLEDs) based on lanthanide complexes,[1] there has been a growing interest in the design and the development of novel organic materials and devices that emit light efficiently at wavelengths longer than 700 nm. The current intense research efforts to demonstrate high-performance NIR OLEDs and organic semiconductor lasers have been stimulated by their potential applications in important areas such as the telecommunication, defense, biometric authentication, and biomedical sectors.[2-4] In fact, the performances of NIR OLEDs have improved significantly when lanthanide and conventional fluorescent emitters have been replaced by phosphorescent heavy-metal complexes. For instance, a recent study has reported on a new family of phosphorescent platinum (II) complexes that were used in

A. D'Aléo, L. Mager
Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS)
Université de Strasbourg
CNRS UMR 7504
Strasbourg Cedex 2 67034, France
E-mail: anthony.daleo@ipcms.unistra.fr

X. Tang, D.-H. Kim, C. Adachi, J.-C. Ribierre
Center for Organic Photonics and Electronics Research (OPERA)
Kyushu University
Motooka 744, Nishi, Fukuoka 819-039, Japan
E-mail: jr43@st-andrews.ac.uk

<figure><img src="image_3.png" /></figure>

ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202500338
© 2025 The Author(s). Advanced Optical Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adom.202500338

D. Valverde, Y. Olivier
Univ Namur
Namur Inst Struct Matter
Lab Computat Modeling Funct Mat
Rue Bruxelles 61, Namur B-5000, Belgium
E-mail: yoann.olivier@unamur.be

E. Zaborova, G. Canard
Centre Interdisciplinaire de Nanoscience de Marseille (CINaM)
Aix Marseille Univ
CNRS UMR 7325, Campus de Luminy, Case 913, Marseille 13288, France

A. Brosseau, Clavier
Univ Paris Saclay, CNRS
PPSM, ENS Paris, Gif Sur Yvette, Saclay F-91190, France

C. Adachi, J.-C. Ribierre
Japan Science and Technology Agency (JST)
ERATO
Adachi Molecular Exciton Engineering Project
Fukuoka 819-0395, Japan
NIR OLEDs with a maximum external quantum efficiency (EQE) value of over 24%.[5,6] Such phosphorescent heavy-metal complexes offer the possibility to promote radiative decay from spin-forbidden triplets and produce electroluminescence with 100% internal quantum efficiency due to their ability to convert both singlet and triplet excitons into light. However, the high cost and rarity of the heavy metals incorporated into the dopants limit their long-term potential for mass production and practical applications. As an alternative, purely organic thermally-activated delayed fluorescent (TADF) emitters have been identified as the third generation of OLED materials and can be used in electroluminescent (EL) devices that harvest both singlet and triplet excitons via efficient reverse intersystem crossing (RISC) at room-temperature.[7] For instance, several studies have already reported TADF OLEDs operating in the visible spectral region with 100% internal quantum efficiency.[7] The molecular architecture of most high-efficiency intramolecular charge-transfer (ICT) TADF OLED emitters is based on a combination of electron donor (D) and acceptor (A) units, which is specifically designed to give rise to a small spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This molecular engineering strategy is advantageous in most TADF systems for minimizing their singlet-triplet energy splitting (ΔE<sub>ST</sub>) and accelerating the RISC process from the first triplet excited state (T<sub>1</sub>) to the first singlet excited state (S<sub>1</sub>) while maintaining a moderate oscillator strength. Following such guideline rules, a significant number of twisted ICT-type D-A TADF emitters with a substantial steric hindrance has been developed and successfully used in blue and green TADF OLEDs, with EQE values of 36% for sky-blue[8] and exceeding 30% for green,[9] 29% for yellow[10] and orange.[11] Although deep-red and NIR TADF OLEDs are also particularly attractive for a variety of applications, their performance still remains lower and needs to be substantially improved via the development of novel high-efficiency TADF emitters.

The design of deep-red and NIR TADF emitters with high pho-
toluminescence quantum yield (PLQY) still represents a chal-
lenge for several reasons. First, the PLQY generally tends to de-
crease when the emission is shifted toward a longer wavelength
due to the competition between the larger nonradiative decay
rates and the radiative fluorescence decay rate (k<sub>f</sub>), in accordance
with the energy gap law. Another issue is that in far red/NIR light-
emitting organic thin films, there is often greater aggregation-
caused-quenching of fluorescence. Further, previous works have
also shown that a simple extension of the π-conjugation of
twisted ICT-type D-A molecules generally leads to molecules
with large ΔE<sub>ST</sub> and k<sub>f</sub>, which are not suitable for achieving
high-efficiency far-red and NIR TADF emission.[12] However,
some examples of increasing the distance between the D and
A units (introducing phenyl bridges in ICT D-A-D molecules)
were found to simultaneously enhance k<sub>f</sub> and lower ΔE<sub>ST</sub>. As
such, Zhang et al. synthesized highly efficient anthraquinone-

based ICT TADF emitters and could demonstrate red OLEDs
with a maximum EQE value of over 12% for a maximum emis-
sion wavelength of ≈624 nm.[13] Another critical aspect of de-
signing NIR TADF emitters seems to be the incorporation of
strong electron-withdrawing A moieties with deep LUMOs such
as anthraquinone, 2,3-dicyanopyrazino phenanthrene (DCPP),
or dibenzo[a,c]phenazine-11,12-dicarbonitrile (DBPzDCN). For
instance, TADF emitters containing the DCPP and DBPzDCN
A units were used in OLEDs with the maximum EQE val-
ues of 9.8% (for a maximum emission wavelength of 668 nm)
and 7.68% (for a maximum emission wavelength of 698 nm),
respectively.[14,15] In addition, when this strategy was achievable,
amplified spontaneous emission (ASE) was not efficient due
to the low oscillator strength induced by the HOMO-LUMO
disconnection.[16] Therefore, recently, new TADF dyes, where
the TADF mechanism is based on a multiple resonance (MR)
scheme, have been developed, allowing them to reach high os-
cillator strength that is due to the planarity of the dyes[17] but
the design of such dyes, based on the insertion of boron and ni-
trogen atoms, to emit in the red/NIR region[18] is less common
and more complicated.[19] In this context, another molecular de-
sign to achieve highly efficient NIR TADF electroluminescence,
with very high oscillator strength, was reported based on a D-A-
D borondifluoride curcuminoid backbone CurcBF<sub>2</sub>.[20,21] In con-
trast to the conventional D-A twisted TADF and MR-TADF emit-
ter architectures, the curcuminoid derivative exhibits a larger spa-
tial overlap between the hole and electron wavefunctions involved
in the radiative transition together with a large oscillator strength
and a high radiative decay rate. This remarkable behavior for a
TADF emitter was rationalized using a spin-vibronic model ac-
counting for non-adiabatic coupling between the T<sub>1</sub> and T<sub>2</sub> as
well as S<sub>1</sub> and S<sub>2</sub>.[20] The simultaneous observation of an effective
upconversion from triplets to singlets via RISC driven by spin-
orbit coupling with an excellent ASE was explained by the vibra-
tion and spin-orbit-coupling-assisted TADF mechanism and the
large oscillator strength of this NIR emitter. Importantly, this ini-
tial report on the borondifluoride curcuminoid derivatives con-
taining two triphenylamine (TPA) donor moieties and one acety-
lacetonate borondifluoride acceptor group demonstrated a NIR
OLED with a maximum EQE value of ≈10% for a maximum
emission wavelength of 721 nm.[20] Shortly after, a study came
out suggesting the possibility that this TADF mechanism occurs
through intermolecular CT states driven by hyperfine interaction
due to the formation of dimers in the film.[22]

Another recent study showed the possibility of appropriate
engineering of the molecular design to shift the NIR emission
of borondifluoride curcuminoid dyes toward longer wavelengths
while maintaining high PLQY and maximum EQE values.[21]
These pioneering works have provided clear evidence of the
strong potential of curcuminoid derivatives for organic NIR
light-emitting devices and applications. However, the structure-
property relationship for this family of NIR-emitting dyes still
needs to be examined in detail. This is essential to fully under-
stand their detailed mechanism and lasing properties to develop
the next generation of NIR TADF dyes with improved EL and las-
ing performances.

Herein, we report on the design and synthesis of a series of
novel D-A-D type solution-processable curcuminoid derivatives
containing various types of triphenylamino donor D and A units.

<footnote>J.-C. Ribierre
Organic Semiconductor Centre, SUPA, School of Physics and Astronomy
University of St Andrews
North Haugh, St Andrews, Fife KY16 9SS, UK</footnote>
<figure><img src="image_4.png" /></figure>

1: X= H, Y= H

2: X= Bu, Y= H

3: X= COOEt, Y= H

4: X= Cl, Y= H

6: X= COOEt, Y= OMe

<caption>**Scheme 1.** Chemical structures of the investigated dyes.</caption>

The characterization of their photophysical properties in solution and thin films, together with quantum chemistry calculations, are carried out to clarify the impact of the molecular architecture on their TADF and electronic properties. In addition, this systematic study provides evidence that a fine-tuning of the highly efficient NIR OLED between 690 and 800 nm and ASE properties in the spectral range between 725 and 900 nm can be achieved with simple chemical modifications of the curcuminoid dyes. For example, a curcuminoid derivative containing dimethoxyphenylamino as a donor and an ester function in *meso* position (CurcBF₂ 6) as D and A units were used as NIR emitter in an OLED with a maximum EQE value of 0.99% for a maximum emission wavelength of 797 nm. In addition, we show that the CurcBF₂ 6 also exhibits an ASE that can reach 892 nm even if the threshold is as high as 123 µJ cm⁻². Overall, this study not only clarifies the TADF mechanism for this family of dyes but also demonstrates the outstanding potential of curcuminoid derivatives as a versatile platform for high-efficiency NIR OLEDs and organic semiconductor lasers.

## 2. Results and Discussion

### 2.1. Design, Synthesis, and Characterization

CurcBF₂ dyes with triphenylamino-donor groups have been designed by mainly changing the strength of the accepting group, especially by varying the pendant group (Scheme 1, dyes 1–5) at the *meso* position considering non-accepting hydrogen (1) or butyl (2) groups to substituents increasing the accepting character such as ester or chlorine (3 and 4, respectively). Note that the presence of another CurcBF₂ group in *meso* position of 5 allows to increase the acceptor strength but it also benefits from the excitonic coupling between the two CurcBF₂ units.[21] Finally, a CurcBF₂ dye (6) containing methoxy groups in the para-position of the nitrogen of the triphenylamine was prepared to increase the donor strength.

<figure><img src="image_5.png" /><figcaption>5</figcaption></figure>

The synthesis was conducted as previously reported[23] (Scheme S1, Supporting Information) using a strong base (morpholine) to increase the reaction yield.[20,21] Briefly, the acetylacetonate derivative was first complexed to boron difluoride moiety by warming up a solution of boron trifluoride etherate in ethyl acetate. Subsequently, a solution of the proper aldehyde activated by tributyl borate in ethyl acetate was added and the reaction was refluxed overnight in the presence of morpholine as catalyst. All dyes were characterized by NMR and mass spectrometry to assess the purity of the molecules (see Figures S1 and S2, Supporting Information).

The electrochemical properties of dyes 2 and 6 were investigated in DCM solution containing 0.1 M of (n-Bu)₄NPF₆ and compared to the ones obtained for the others CurcBF₂. The cyclic voltammograms (CV) are given in Figure S3 (Supporting Information) and the oxidation and reduction half-wave potential values (E₁/₂ vs ferrocene/ferrocenium) are listed in Table 1. All CurcBF₂ exhibit one-electron oxidation and reduction waves that are attributed to the triphenylamino-donor end-groups and the dioxaborine ring acceptor substituted in the *meso* position,

<table><caption>**Table 1.** Oxidation and reduction half-wave potential values for the studied CurcBF₂ derivatives (0.5 mM) in dichloromethane solution versus Fc/Fc⁺ using tetrabutylammonium hexafluorophosphate as electrolyte (100 mM), the values are given in volt; HOMO, LUMO energies (data in eV) with E<sup>HOMO</sup> = -(E<sub>Oox</sub> (vs Fc/Fc⁺) + 5.13) eV and E<sup>LUMO</sup> = E<sup>HOMO</sup> + ΔE<sub>g</sub> with HOMO-LUMO (i.e., electrochemical) gap ΔE<sub>g</sub> = (E<sub>1/2</sub><sup>ox</sup>)₁ - E<sub>1/2</sub><sup>red</sup>, in V.</caption><thead><tr><th></th><th>E<sub>1/2</sub><sup>red</sup></th><th>(E<sub>1/2</sub><sup>ox</sup>)<sub>1</sub></th><th>(E<sub>1/2</sub><sup>ox</sup>)<sub>2</sub></th><th>E<sup>HOMO</sup></th><th>E<sup>LUMO</sup></th><th>ΔE<sub>g</sub></th></tr></thead><tbody><tr><td>1</td><td>-1.27</td><td>0.58</td><td>0.68</td><td>-5.71</td><td>-3.86</td><td>1.85</td></tr><tr><td>2</td><td>-1.30</td><td>0.56</td><td>0.68</td><td>-5.69</td><td>-3.83</td><td>1.86</td></tr><tr><td>3</td><td>-1.16</td><td>0.61</td><td>0.72</td><td>-5.74</td><td>-3.97</td><td>1.77</td></tr><tr><td>4</td><td>-1.09</td><td>0.60</td><td>0.74</td><td>-5.74</td><td>-4.04</td><td>1.70</td></tr><tr><td>5</td><td>-1.14</td><td>0.66</td><td>1.02</td><td>-5.79</td><td>-3.99</td><td>1.80</td></tr><tr><td>6</td><td>-1.24</td><td>0.35</td><td>0.43</td><td>-5.48</td><td>-3.89</td><td>1.59</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 1.** a) Overlaid normalized electronic absorption spectra and b) overlaid corrected normalized fluorescence emission spectra of 1 (–), 2 (–), 3 (–), 4 (–), 5 (–), and 6 (–) recorded in toluene at room-temperature at concentrations below $2 \times 10^{-6}$ mol.L$^{-1}$.</figcaption></figure>

respectively. The first oxidation potential decreases when ap-
pending methoxy-groups on the diphenylamino-moieties (shift-
ing from 0.56-0.66 V for the TPA donor to 0.35 V, Table 1). In
the meantime, increasing the acceptor strength results in a de-
crease in the reduction potential (from -1.30 V for 2 to -1.09 V
for 4, Table 1). The HOMO-LUMO of the six dyes, calculated from
the oxidation and reduction potentials measured by CV,[24] are
reported in Table 1 and the comparison of their energy level is
represented in Figure S4 (Supporting Information). For 1-5, the
HOMOs, based on the diphenylamino-donor, are almost at the
same energy (i.e., between -5.69 and -5.79 eV). On the other
hand, the variation on the LUMOs energies is larger since only
the group in meso position changes (going from -3.83 eV for the
poor butyl-acceptor 2 to -4.04 for the good chloro-acceptor 4).
On the opposite, the CurcBF₂ 6 has its LUMO at the same en-
ergy as the ones of 1-5 (-3.89 eV for 6 vs -3.83 to -4.04 eV),
while its HOMO is far higher by 0.2-0.3 eV (HOMO at -5.48 eV,
Table 1). Finally, the HOMO-LUMO electrochemical gap is de-
creased when going to the best acceptor (from 1.86 eV for 2 to
1.7 eV for 4, Table 1) and to a better donor (from 1.77 eV for 3 to
1.59 eV for 6, Table 1).

The thermogravimetric analysis (TGA) measurements of the
CurcBF₂ in vacuum (at 1 Pa) in Figure S5 (Supporting Informa-
tion) showed the decomposition of the dyes at temperatures be-
tween 242 and 267 °C. As it can be seen, no thermal evapora-
tion can occur since decomposition occurs at first on the con-
trary of what has been observed for HemiCurcBF₂ containing
TPA.[24] These results indicate the difficulty of thermally evap-
orating CurcBF₂ dyes to prepare thin films for OLEDs and this is
why we opted for the preparation of spin-coated thin films. How-
ever, since the decomposition temperatures are above 240 °C,
these CurcBF₂ dyes show thermal stability that is high enough
for OLEDs' operation.

## 2.2. Photophysical Properties

The photophysical properties of the dyes were measured in di-
luted toluene solution at a concentration below 2 10⁻⁶ mol L⁻¹.
The electronic absorption spectra of all CurcBF₂ in toluene are
displayed in Figure 1a. The spectra consist mainly of one intense

transition band at low energy (550–650 nm), which is attributed
to a strongly allowed π–π* transition. This transition is rather
broad and a shoulder is apparent at 540 nm for 6. This transi-
tion belongs to the two-photon allowed transition and has a dipo-
lar character as evidenced previously.[25,26] Additional electronic
transition bands of much lower intensity appear at higher energy
(below 400 nm). It can also be seen that the absorption band of
CurcBF₂ 5 is the most redshifted, while the electrochemical gap
is not the lowest. This likely stems from intramolecular excitonic
coupling between the two curcuminoid chromophores. It is in-
teresting to note that the molar absorption coefficient for those
CurcBF₂ dyes varies from 100 000 to 105 000 M⁻¹ cm⁻¹, which
represents largely allowed values obtained in twisted D-A TADF
OLED emitters.[7]

To better understand the excited-state properties of these
compounds, singlet, and triplet vertical excitation energies
were calculated using the spin-component scaling second-order
algebraic-diagrammatic construction (SCS-ADC(2))[27] in com-
bination with the def2-SVP basis sets[28] based on the ground
state geometry computed optimized at the density functional
theory level (DFT) using the LC-ωPBE functional. This level of
theory was selected since the predicted ΔE<sub>ST</sub> demonstrated a
good agreement with the ΔE<sub>ST</sub> measured experimentally using
low-temperature steady-state fluorescence and phosphorescence
spectroscopy in multi-resonance TADF emitters.[29,30] Our results
suggest that the first singlet excited state (S<sub>1</sub>) is a bright state car-
rying a large oscillator strength (f > 1) and is described by a domi-
nant HOMO to LUMO transition (≈60% weight), as displayed in
Table S1 and Figure S6 (Supporting Information). With respect
to molecule 1, the S<sub>1</sub> excitation energy does not change much for
compounds 2 and 3 (≈0.05 eV), while there is a larger redshift
of 0.2 eV for compounds 4, 5, and 6, which agrees well with our
experimentally measured absorption spectra (Figure 1). As can
be seen from the difference density plots (Figure S7, Support-
ing Information), the first triplet excited state (T<sub>1</sub>) has an elec-
tronic nature similar to that observed for the S<sub>1</sub> state, leading
to a small spin-orbit coupling (SOC) between these states (Table
S2, Supporting Information), in agreement with El-Sayed rule.
In addition, except for compounds 4 and 5, the vertical singlet-
triplet energy gap between the S<sub>1</sub> and the T<sub>1</sub> states is relatively
large (≈0.3 eV), which limits a direct TADF conversion involving
$S_1$ and $T_1$ states. Interestingly, on the other hand, the $T_2$ excited state is close in energy with the $S_1$ excited state for compounds **1–6**, especially for system **4** in which $T_2$ is 0.06 eV below the $S_1$ excited state. As this excited state has a different electronic character than the $S_1$ state, a SOC value of one order of magnitude larger than that of the $S_1 \rightarrow T_1$ is obtained (Table S2, Supporting Information). Potentially, the $T_2$ could act as an intermediate state triggering RISC and the whole TADF process in these systems. By using a fragment-based analysis using the one-electron transition density matrix (1TDM), as implemented in TheoDORE 3.2,[31] we identified that the electronic nature of the $S_1$ and $T_1$ excited states is composed by localized transitions on $BF_2$ moiety (roughly 30–40%), while the amount of the CT-like transition is ≈30–40% (Figure S8, Supporting Information) so that these excitations are a mixed CT-LE (Local Excited state). In turn, the CT transitions from TPA to $BF_2$ groups are the main contributions for the $T_2$ excited state for compounds **1–4** and **6** (≈30% for **1–4** and 40% for **6**). For compound **5**, the excited state $S_2$ is predicted to be nearly degenerate in energy with the $S_1$ state and has a strong oscillator strength too (Table S1, Supporting Information). Similarly, $T_1$ and $T_2$, and $T_3$ and $T_4$ excited states are close in energy. Based on these findings and the fact that the SOC is high between states of different electronic nature, the RISC process involving higher triplet states ($T_3$ and $T_4$ excited states) can take place and thus repopulate $S_1$ and $S_2$ excited states.

As shown in Figure 1b, $CurcBF_2$ **1–6** in toluene solutions are fluorescent in the visible region (630–725 nm) upon excitation of the low-energy transition band and exhibit high PLQY in apolar solvents. In agreement with electronic absorption data, an increase in the acceptor or donor strength causes a redshift of the fluorescence emission from 627 nm (**1**) to 712 nm (**6**). Overall, these results show that the absorption and steady-state emission spectra in the toluene solution are bathochromically shifted as a function of the electrochemical gap.

The solvent dependence of absorption and emission properties was studied for the six borondifluoride complexes (Figure S9 and Tables S3, and S4, Supporting Information). Both electronic UV/visible absorption and fluorescence emission spectra undergo a bathochromic shift and lose their vibronic structures when solvent polarity increases (Figure S9, Supporting Information). While the bathochromic shift of the absorption spectra of all the compounds remains moderate (energy difference of ≈0.15 eV between the absorption maximum measured in cyclohexane and dichloromethane, the least and the highest polar solvents, respectively), the emission spectra exhibit very large redshift as well as an increase of its width upon increasing the polarity of the solvent.

Furthermore, the positive slope of the Lippert–Mataga plots (Figure S10, Supporting Information) reveals that, in those systems, the relaxation occurs toward a solvent-equilibrated charge-transfer (CT) singlet excited state along with an increase of dipole moment with respect to the ground state. Quantum chemical calculations also corroborate these findings with the $S_1$ state having a larger dipole moment than the ground state (Table S5, Supporting Information). The vibronically-resolved absorption and emission spectra of compounds **1–4** were simulated within the Franck–Condon (FC) approximation employing the vertical Hesian (VH) approach to compute the overlap between the vibrational normal modes obtained at the $LC-\omega PBE/def2-TZVP$ level

in the ground and first excited states, as implemented in FC-Classes 3.3[32] (see Supporting Information for further computational details). Our predictions show that the low-energy absorption band is dominated by the $S_1$ state (Figure S12, Supporting Information). In addition, our calculations capture well the trend in the evolution in the energy of the maximum absorption peak as compared to the experimental spectrum, that is, the absorption peak for compounds **1–4** are close to each other, while the absorption peak for compound **4** is shifted by ≈70 nm (≈0.4 eV). This good agreement is also verified in the shape of the spectrum since the experimental and theoretical FWHM are close (Theor. [Exp]: 0.48 eV [0.37 eV] for **1**, 0.49 eV [0.32 eV] for **2**, 0.44 eV [0.31 eV] for **3**, 0.44 eV [0.30 eV] for **4**). The simulation of the emission spectra reveals that their broadening arises from the strong coupling of the $S_1$ transition with a low-frequency vibrational normal mode (frequency of roughly $10^{-1}$ cm⁻¹) (Figure S13, Supporting Information), being predominantly described by the twisting motion of the phenyl groups (Figure S14, Supporting Information). The theoretical FWHM values for the emission spectra are considerably larger than the experimental ones for the emission spectra (Theor. [Exp]: 0.45 eV [0.25 eV] for **1**, 0.46 eV [0.24 eV] for **2**, 0.44 eV [0.22 eV] for **3**, 0.49 eV [0.21 eV] for **4**). The larger FWHM values arise from the large Huang–Rhys factor (values between 1 and 2.5) of this low-frequency mode which unrealistically broadens the emission spectrum. It is important to recall that these calculations were performed considering the isolated molecule in a gas-phase. Surrounding effects may have a strong impact on the position of the absorption and fluorescence peaks as well as on the broadening of the emission spectra. Besides, a good agreement is reached for the Stokes shift (Theor. [Exp]: 0.24 eV [0.18 eV] for **1**, 0.26 eV [0.16 eV] for **2**, 0.28 eV [0.20 eV] for **3**, 0.16 eV [0.16 eV] for **4**), which was obtained by the energy difference between the absorption and emission peaks.

For compounds **1–6**, the fluorescence intensity continuously decreases with increasing solvent polarity in agreement with the energy gap law, as the $S_1$ energy continuously decreases with increasing solvent polarity, which agrees with the previously published results for **3** and **5**.[20,21] We also observed that the fluorescence lifetimes also decrease with increasing solvent polarity leading to almost constant values of the radiative rate constants $k_f$ for the six $CurcBF_2$. Concomitantly, the non-radiative rate constants $k_{nr}$ tremendously increase in polar solvents. Both solvatochromism and the positive solvatokinetic behavior can be explained by the energy gap law.[33] The ground state (GS) and excited state (ES) dipole moments were determined by the solvatochromic shift method using the Bakshiev polarity function (Figure S11a, Supporting Information) and Bilot–Kawasaki,[34,35] Kawasaki–Chamma–Viallet[36] (Figure S11, Supporting Information) and Lippert–Mataga.[37,38] The obtained values are reported in Table S5 (Supporting Information). In the meantime, the Lippert–Mataga formalism was used to approximate the excited state dipole moment of those $CurcBF_2$ plotting the Stokes' shift as a function of the polarizability function ($\Delta f'$, Figure S10, Supporting Information). The positive slope of the Lippert–Mataga plots reveals that, in those systems, the relaxation occurs toward a solvent-equilibrated CT singlet excited state along with an increase of dipole moment with respect to the ground state.[39] For instance, the ground state dipole moment was determined to be between 5.5 and 6.6 D for those dyes which is in good
<figure><img src="image_7.png" /><figcaption>**Figure 2.** The transient absorption spectrum of 3 with its steady-state absorption (−) in a/ toluene and b/THF solutions (with an optical density of ≈ 0.7–0.8).</figcaption></figure>

agreement with the values that have been determined for BF₂ derivatives.[40,41] The excited state dipole moment is therefore estimated to be between 14.5–17.3 D. Both ground- and excited-state dipole moments are rather high for those dyes. Such values are due, in the first place, to the AcacBF₂ moiety, and the excited state dipole moments reveal the occurrence of a strong CT state within those dyes.

To better understand the excited state dynamics, transient absorption (TA) spectroscopy at the sub-microsecond time scale was measured for those dyes in solution. The TA spectra of 3 in toluene and in tetrahydrofuran (THF) are displayed in **Figure 2**. Excited state absorption (ESA) bands are observed at the wavelength shorter than 450 and above 650 nm, while bleaching due to the absorption of 3 can be seen between 450 and 650 nm. The intensity of the ESA at a longer wavelength compared to the bleaching is less intense than the one at a shorter wavelength. It is interesting to notice that both ESA and bleaching present the same lifetime and that those features (ESA and bleaching) were present for a long time (260 ns in toluene and 440 ns in THF). Such lifetimes can therefore only be attributed to the triplet excited state lifetime. This clearly proves the formation of the triplet excited state via intersystem crossing in both solvents despite the high PLQY in toluene or the low PLQY in polar solvents such as THF.

To confirm that the measured lifetimes were originating from the triplet excited state, the same measurements were made after degassing the sample by bubbling argon (Figure S15, Supporting Information). While the spectrum in degassed toluene fits well with the non-degassed one (Figure S15, Supporting Information), the lifetime of the kinetics in THF (Figure S15b, Supporting Information) becomes much longer due to the strong reduction of quenching of the triplets by oxygen, indicating that the ESA corresponds to the T₁-Tn transitions.

It should be noted that the same features have been obtained for the other CurcBF₂ with the bleaching part at the absorption wavelength and ESA at higher and lower energy (Figure S16, Supporting Information). As for 3, the ESA at the longer wavelength is less intense than the one at the shorter wavelength. Conse-

quently, the triplet-triplet absorption of the dyes at their emission wavelength is weak.

The photophysical properties of the NIR emitting CurcBF₂ were further investigated in blend films with different doping concentrations in a 4,4'-bis(N-carbazolyl)-1,10-biphenyl (CBP) host and the data are reported in Figures S17–S20 (Supporting Information) and **Table 2**. A CBP host was selected because its HOMO and LUMO fit well with the ones of the TPA-containing CurcBF₂. Furthermore, the triplet excited state of CBP is at high energy. As for the electronic absorption spectra in solution, the electronic absorption spectra of the blends are very similar to those measured in solution, except for the presence of an intense transition in the UV part of the spectrum (at 325 nm), which can be attributed to the π−π* transition of the CBP host (Figure S17, Supporting Information). However, as previously reported,[20] a redshift of the absorption of the film is observed when compared to the absorption in solution (in apolar and polar solution, Figure S18, Supporting Information). The shift can be interpreted in terms of dimerization in the film, due to the large ground state dipole moment of the dyes. There is no shift in the absorption spectrum as the concentration increases, indicating that no larger aggregates can be seen (up to 8 wt.%, Figure S17, Supporting Information). The emission spectra were also measured in the blended films (Figure S17, Supporting Information). For all the dyes, the emission is redshifted with increasing the concentration which is due to the high ground state dipole moment. Furthermore, as reported for 3, biexponential decays were observed when measuring the blend films of different concentrations.[20] This was attributed to the presence of monomeric and dimeric species in the spin-coated films. It should be noted that the NIR emission of these dyes benefits from such aggregation. This is illustrated by the difference in emission between monomer and mixed monomer-dimer emission (Figure S19, Supporting Information).

Finally, it should be noted that the poor overlap between the emission of the CBP host compared to the S₀-S₁ and S₀-S₂ absorption results in an inefficient energy transfer. This is illustrated by the requirement of 6 wt.% of all dyes in CBP to obtain complete energy transfer (Figure S20, Supporting Information).
<table><caption>**Table 2.** Photoluminescence and electroluminescence data of the CurcBF₂ 1, 2, 4 & 6 in CBP host with Φf<sup>CBP</sup> the residual fluorescence quantum yield of CBP, Φf<sup>Curc</sup> the fluorescence quantum yield of the CurcBF₂, η<sup>EQE</sup> the external quantum efficiency (taken at 10<sup>-2</sup> mA cm<sup>-2</sup>) and turn-on the turn-on voltage of the electroluminescence.</caption><thead><tr><th rowspan="2">CurcBF<sub>2</sub></th><th rowspan="2">CBP: CurcBF<sub>2</sub> (x wt.%)</th><th colspan="4">Photoluminescence</th><th colspan="4">Electroluminescence</th></tr><tr><th>λ<sup>abs</sup>/nm</th><th>λ<sup>em</sup>/nm</th><th>Φ<sub>f</sub><sup>CBP</sup></th><th>Φ<sub>f</sub><sup>Curc</sup></th><th>λ<sup>em</sup>/nm</th><th>η<sup>EQE</sup>/%</th><th>Turn-on/V</th></tr></thead><tbody><tr><td rowspan="5">1</td><td>1 wt.%</td><td>599</td><td>680</td><td>0.027</td><td>0.313</td><td>679</td><td>3.45</td><td>8.1</td></tr><tr><td>2 wt.%</td><td>600</td><td>691</td><td>0.013</td><td>0.396</td><td>692</td><td>3.50</td><td>8.2</td></tr><tr><td>4 wt.%</td><td>601</td><td>703</td><td>0.007</td><td>0.393</td><td>701</td><td>4.00</td><td>8.5</td></tr><tr><td>6 wt.%</td><td>601</td><td>709</td><td>0.001</td><td>0.387</td><td>710</td><td>3.63</td><td>8.2</td></tr><tr><td>8 wt.%</td><td>601</td><td>714</td><td>&lt;0.001</td><td>0.333</td><td>714</td><td>2.89</td><td>7.8</td></tr><tr><td rowspan="5">2</td><td>1 wt.%</td><td>609</td><td>686</td><td>0.022</td><td>0.575</td><td>687</td><td>2.80</td><td>9.3</td></tr><tr><td>2 wt.%</td><td>609</td><td>693</td><td>0.011</td><td>0.604</td><td>693</td><td>2.76</td><td>11.3</td></tr><tr><td>4 wt.%</td><td>610</td><td>699</td><td>0.005</td><td>0.510</td><td>700</td><td>3.71</td><td>10.3</td></tr><tr><td>6 wt.%</td><td>611</td><td>707</td><td>&lt;0.001</td><td>0.409</td><td>708</td><td>3.41</td><td>11.6</td></tr><tr><td>8 wt.%</td><td>612</td><td>713</td><td>&lt;0.001</td><td>0.351</td><td>713</td><td>2.90</td><td>9.6</td></tr><tr><td rowspan="5">4</td><td>1 wt.%</td><td>651</td><td>730</td><td>0.038</td><td>0.422</td><td>736</td><td>3.55</td><td>8.0</td></tr><tr><td>2 wt.%</td><td>651</td><td>737</td><td>0.025</td><td>0.416</td><td>743</td><td>3.89</td><td>8.5</td></tr><tr><td>4 wt.%</td><td>650</td><td>744</td><td>0.012</td><td>0.268</td><td>749</td><td>3.65</td><td>8.6</td></tr><tr><td>6 wt.%</td><td>649</td><td>748</td><td>0.004</td><td>0.222</td><td>752</td><td>4.14</td><td>8.4</td></tr><tr><td>8 wt.%</td><td>649</td><td>752</td><td>0.002</td><td>0.185</td><td>760</td><td>3.46</td><td>7.7</td></tr><tr><td rowspan="5">6</td><td>10 wt.%</td><td>649</td><td>760</td><td>0.001</td><td>0.137</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>15 wt.%</td><td>649</td><td>762</td><td>&lt;0.001</td><td>0.124</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>20 wt.%</td><td>649</td><td>770</td><td>&lt;0.001</td><td>0.076</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>25 wt.%</td><td>650</td><td>775</td><td>&lt;0.001</td><td>0.061</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>40 wt.%</td><td>649</td><td>779</td><td>&lt;0.001</td><td>0.034</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td rowspan="5">6</td><td>1 wt.%</td><td>640</td><td>748</td><td>0.033</td><td>0.116</td><td>747</td><td>3.66</td><td>8.9</td></tr><tr><td>2 wt.%</td><td>639</td><td>758</td><td>0.019</td><td>0.091</td><td>758</td><td>3.51</td><td>6.3</td></tr><tr><td>4 wt.%</td><td>639</td><td>770</td><td>0.008</td><td>0.067</td><td>769</td><td>3.51</td><td>6.3</td></tr><tr><td>6 wt.%</td><td>639</td><td>782</td><td>0.0025</td><td>0.045</td><td>781</td><td>1.87</td><td>12.0</td></tr><tr><td>8 wt.%</td><td>639</td><td>790</td><td>0.0015</td><td>0.033</td><td>791</td><td>1.33</td><td>6.1</td></tr><tr><td rowspan="5">6</td><td>10 wt.%</td><td>638</td><td>795</td><td>0.001</td><td>0.024</td><td>795</td><td>1.05</td><td>11.6</td></tr><tr><td>15 wt.%</td><td>638</td><td>797</td><td>&lt;0.001</td><td>0.020</td><td>797</td><td>0.99</td><td>6.3</td></tr><tr><td>20 wt.%</td><td>639</td><td>800</td><td>&lt;0.001</td><td>0.010</td><td>800</td><td>0.59</td><td>6.3</td></tr><tr><td>25 wt.%</td><td>640</td><td>810</td><td>&lt;0.001</td><td>0.006</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>40 wt.%</td><td>635</td><td>823</td><td>&lt;0.001</td><td>0.004</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr></tbody></table>

The PLQYs of all CurcBF₂ dyes at different doping concentrations were then characterized and the results are reported in Table 2 except for 3<sup>[20]</sup> and 5<sup>[21]</sup> that have been reported elsewhere. As can be seen, the values of PLQYs reach the maximum for 1 or 2 wt.% concentrations and then decrease with increasing the dye concentration which is likely due to the formation of aggregates. When comparing 2 with 4, it is interesting to note that the largest PLQY values are observed for the more blue-shifted emitter of these compounds. However, when comparing 1 with 2 or 4, it is evident that another explanation is needed to explain how to obtain high PLQY. Moreover, all these PLQYs are lower than those of 3 and 5 suggesting that the non-radiative decay mechanism is more complex than a simple energy gap law. However, it is worth noting that high PLQY in the NIR region (797 and 800 nm) can be achieved by preparing thin films using 15 and 20 wt.% of 6 in CBP, giving PLQY of 2% and 1%, respectively.

The time-resolved photoluminescence of all the blend films was measured and it was found that all CurcBF₂ at

2 wt.% (Figure 3; Figure S21, Supporting Information) and 6 wt.% (Figure S22, Supporting Information) blend in CBP exhibits prompt and delayed fluorescence (i.e., TADF) originating from a singlet excited state. All the measurements are shown to be biexponential which can be attributed to the TADF properties of the monomeric and dimeric species (with one lifetime one order of magnitude larger than the other one, Figure S21, Supporting Information). It should be noted that another mechanism involving hyperfine coupling with a dimer could be considered to explain the TADF.[22] The concentration dependence of the TADF properties was further investigated using dyes 3 and 4. As can be seen in Figure S23 (Supporting Information), the RISC signal decreases with increasing concentration, which follows the PLQY of the dyes in the mixture (which also decreases with concentration).

In conclusion, all dyes show TADF properties in a film state with interesting PLQY that can be further exploited in OLEDs.
<figure><img src="image_8.png" /><figcaption>**Figure 3.** Luminescence lifetime of film containing 2 wt.% in CBP of 1 (—), 2 (—), 3 (—), 4 (—), 5 (—), and 6 (—) in deaerated conditions proving the presence of thermally activated delayed fluorescence of the six dyes.</figcaption></figure>

## 2.3. OLED Characterization

The EL properties of all CurcBF₂ derivatives were examined using the OLED architecture described thereafter: indium tin oxide (ITO) (100 nm)/PEDOT:PSS (45 nm)/EML (≈80 nm)/DPEPO (10 nm)/TPBi (55 nm)/LiF (1 nm)/Al (100 nm). PEDOT:PSS, DPEPO, and TPBi play the role of hole injection, hole blocking, and electron transport layers, respectively. The emissive layers (EML) were composed of the blends in CBP spanning from 1 to 8 wt.% of 1–5 except for 6 for which were prepared up to 20 wt.% of 6 to reach EL at 800 nm.

For the series, all NIR electroluminescence spectra are fully consistent with the photoluminescence spectra and shift toward lower energy with increasing the doping concentration. For example, the maximum electroluminescence and photoluminescence wavelength of the 2 wt.% CBP:6 blend was ≈758 nm, while the 20 wt.% blend CBP:6 is at ≈800 nm. The external quantum efficiency ($η^{EQE}$) values were then measured as a function of current density in all devices. The spectra and results are reported in Figures 4 and S24 and Table S2 (Supporting Information). Those data clearly show a decrease of the maximum $η^{EQE}$ values as the doping concentrations increase from 1 to 8 wt.% for

1, 2, and 4, which is in total agreement with the behavior of the doping concentration dependence of the PLQYs. It is noteworthy that the most efficient devices based on the 4 wt.% blend exhibit a maximum $η^{EQE}$ value of 4.00% and 3.71% for 1 and 2 (with emission wavelengths of 679 and 687 nm, respectively), while the 6 wt.% blend shows an $η^{EQE}$ value of 4.14% for 4 with emission wavelength of 736 nm. Therefore, considering these PLQY values of 39.3%, 51.0%, and 22.2% for 1, 2, and 4, respectively, and since there is a random orientation of the CurcBF₂ dyes as demonstrated with 3,[20] these efficiencies are much higher than the theoretical upper limits of 1.97%, 2.55%, and 1.11% of a conventional fluorescent emitter (first generation OLEDs). Such a calculation indicates that there is harvesting of the triplet excited states (i.e., by TADF activity) of these CurcBF₂ dyes. For 6, the best OLED was obtained for a 1 wt.% blend with $η^{EQE}$ of 3.66 (emission wavelength of 747 nm), and the emissions reaching 797 and 800 nm could be obtained with $η^{EQE}$ values of 0.99 and 0.59 (for 15 and 20 wt.% blends in CBP, respectively). It can be observed that all these devices show pronounced roll-off at high current densities which is typical for TADF OLEDs and presumably due to singlet–triplet and triplet–triplet annihilation.[42]

## 2.4. ASE Characterization

NIR ASE and lasing above 800 nm in organic solid-state thin films have been scarcely published. Recently, benzobisthiadiazole containing fluorene derivative[43] graphene-like molecule[44–46] and curcuminoid boron difluoride[20,21,47] have shown good efficiency/stability above such wavelength. The first two families are purely fluorescent and do not harvest the triplet excited state, limiting the EL efficiency to 5% EQE. On the other hand, CurcBF₂ derivatives show TADF properties that allow an increase in the EQE under electrical pumping, which could be a great advantage for the realization of an electrically pumped organic laser diode.[48] Therefore, the development of NIR lasing in TADF organic thin films should be of interest to this field of research and could impact the development of organic laser diodes. Consequently, we evaluated the potential of these CurcBF₂ dyes for organic lasers by investigating their ASE properties in CBP-mixed films with different doping concentrations. For this purpose, the samples were spin-coated on

<figure><img src="image_9.png" /><figcaption>**Figure 4.** a) Electroluminescence spectra, b) Current density–voltage (J–V) curves, c/ $η^{EQE}$ as a function of current density measured in the OLEDs based on CBP blends with different doping concentrations for CurcBF₂ 2.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 5.** Spectra showing the ASE spectra for 2, 4, 6, and 8 wt.% in CBP for 1, 2, 3, 4, 5, and 6. Note that 10, 15, and 20 wt.% for 4 and 6 are added.</figcaption></figure>

fused silica substrates and photo-excited by a nitrogen laser (ex-
citation wavelength of 337 nm, repetition rate of 8 Hz, and
pulse width of 800 ps). An optical fiber connected to a charge-
coupled device spectrometer was used to measure the emis-
sion spectra from the edge of the organic layers.[16] **Figure 5**
shows the ASE spectra of all CurcBF₂ derivatives above the
threshold at different doping concentrations. The emission spec-
trum of the CurcBF₂ in CBP blends at different excitation densi-
ties is reported in Figures S21–S24 (Supporting Information).

At high pumping intensities, spectral line narrowing is ob-
served, which is attributed to spontaneously emitted photons
that are waveguided in the organic layer and amplified by stimu-
lated emission. Above the ASE thresholds, the full-width at half-
maximum (FWHM) values were found to decrease to 15–20 nm.
Plots of the output intensity emitted from the edge of the films as
a function of pumping intensity were then used to determine the
ASE threshold for each doping concentration (Figures S25–S28,
Supporting Information).

The ASE wavelength and the ASE threshold FWHM values
measured in the CBP blends are summarized in Table S6 (Sup-
porting Information). It can be seen that the ASE thresholds
decrease with the decrease in doping concentration, which is
mainly due to the reduction of concentration quenching. It is
noteworthy that the lowest ASE thresholds were obtained for
the 2 wt.% samples (at lower concentrations where the PLQY is
higher) since no ASE was found at 1 wt.% in CBP. Similar to
what was observed for the PL and EL spectra, the ASE peak wave-
length can be tuned from 725 to 745 nm (for 1 and 2), from 775
to 820 nm (for 4), from 800 to 860 nm (for 5) and from 790 to
890 nm (for 6). Although the measured ASE thresholds (Table
S6, Supporting Information) are higher than the lowest values
reported for organic thin films emitting in the visible (≈0.1–0.4
µJ cm⁻²),[49] the values reported here, together with the broad
PL spectra in the NIR region, suggest the possibility of realiz-
ing efficient and tunable organic semiconductor lasers in the
NIR.

**3. Conclusion**

We have reported the synthesis of curcuminoid borondifluoride derivatives terminated with triphenylamino-groups that exhibit TADF properties in the NIR region of the spectrum. While the process is not common and cannot be attributed to the disconnection between the HOMO and LUMO nor to the multi-resonance process, efficient NIR emitters could be obtained. The process is more related to strong spin-orbit coupling between singlet and triplet excited states. Within those molecules, we showed that the HOMO-LUMO gap could be tuned by tuning the group in the *meso* position (adding electrowithdrawing group). The increase of the donor strength by adding methoxy groups in the *para* position of the diphenylamino-groups (for 6) also affects the emission wavelength by shifting it toward 800 nm. Using this strategy, we have successfully prepared CurcBF₂ that shows emission in OLEDs slightly at 797 nm with EQE as high as 0.99%. However, these devices show a disadvantageous efficient roll-off, which is mainly due to the slow RISC processes within the devices. Such performances are interesting to fill the need for efficient NIR emitters. Since our first work on CurcBF₂ emitters, only a few ones have appeared, such as dibenzo phenazine derivatives emitting at 734 nm with an EQE of 13.4%[50] or mono-substituted phenazine allowing to reach 17.1 and 24.0% at 708 and 716 nm, respectively.[51] Furthermore, these dyes have been shown to be rather efficient lasing ASE/lasing dyes with efficient gain for NIR emitters which is due to the high molar absorption coefficient and rather low threshold which is certainly due to the limited T₁-Tn absorption in the ASE/lasing region. Furthermore, ASE up to 892 nm is obtained with a threshold of 123 µJ cm⁻². Finally, it should be stressed that these CurcBF₂ dyes are interesting because they exhibit TADF properties and ASE, two properties that are highly sought after for the realization of electrically pumped organic lasers.

**4. Experimental Section**

**Materials Syntheses:** Details on the synthesis, purification, and characterization of the chemical properties of molecules **2** and **6** are provided (Figures S1 and S2, Supporting Information). Synthetic preparation of molecules **1**, [52] **3**, [20] **4**, [52] and **5**[21] were reported elsewhere.

**Photophysical Measurements:** All solvents used for these photophysical measurements were of spectroscopic grade. For the characterization of the photophysical and ASE properties in thin films, the films were deposited onto precleaned fused silica substrates by spin-coating from chloroform solution. The thicknesses of these films were measured using a Dektak profilometer.

Absorption spectra in both solution and film were recorded using a Varian Cary 5000 spectrometer and a UV-2550 spectrometer (Shimadzu), respectively. Steady-state PL spectra were obtained using a Fluoromax-4 spectrophotometer (Horiba Scientific) and corrected following the procedure reported by Parker et al.[53] to take into account the response of the detector.[54] The PLQY values were measured using an integrating sphere system coupled with a photonic multichannel analyzer (Hamamatsu Photonics C9920-02, PMA-11).

For the time-resolved PL measurements at room-temperature, thin films were placed in a nitrogen atmosphere. A nitrogen laser (Ken-X, Usho Optical systems) delivering 500 ps excitation pulses with a wavelength of 337 nm and a repetition rate of 20 Hz was used for photo-excitation. The transient PL emission was recorded using a streak camera (Hamamatsu Photonics C4334).
To characterize the ASE properties, the samples in a nitrogen atmosphere were photo-excited by a nitrogen laser (excitation wavelength of 337 nm, pulse width of 800 ps, and repetition rate of 8 Hz). A set of neutral density filters were used to vary the energy of the excitation pulses. The excitation beam was focused using a cylindrical lens into a stripe of dimension 0.5 cm x 0.08 cm. The emission was detected from the edge of the organic film using an optical fiber connected to a charge-coupled device spectrometer (PMA-11, Hamamatsu Photonics).

Transient absorption decay curves were recorded with a 1 cm optical path length using an Edinburgh instrument LP920 laser flash photolysis spectrometer combined with a nanosecond (10 ns pulse) Nd:YAG laser (Continuum) tripled at 355 nm via nonlinear crystals. This third harmonic was optimized to pump an OPO that could generate a 570 nm signal (directly or after a doubler stage) to pump the sample (8 ns pulse duration). The energy was reduced to 5 mj per pulse from Continuum (Surelite II-10). A 450 W Xe lamp was used to probe the sample at 90° of the pump. A filter was used to remove the residual excitation light at 570 nm. The transmitted light was separated by a monochromator and detected with the integrated Hamamatsu R928 photomultiplier. Absorption difference was measured at multiple wavelengths to obtain transient spectra. The Levenberg-Marquardt algorithm was used for non-linear least square fit (tail fit) of the decays as implemented in the L900 software (Edinburgh instrument). In order to estimate the quality of the fit, the weighted residuals were calculated.

**Electrochemistry:** Cyclic voltammetric (CV) data were acquired using a BAS 100 Potentiostat (Bioanalytical Systems) and a PC computer containing BAS100 W software (v2.3). A three-electrode system with a Pt working electrode (diameter 1.6 mm), a Pt counter electrode, and an Ag/AgCl (with 3 M NaCl filling solution) reference electrode was used. [(nBu)₄N]PF₆ (0.1 m in CH₂Cl₂) served as an inert electrolyte. Cyclic voltammograms were recorded at a scan rate of 100 mV·s⁻¹. Ferrocene was used as the internal standard (+0.46 V vs SCE).[⁵⁵]

**Device Fabrication and Measurements:** For the fabrication of OLEDs, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) was purchased from Heraeus CleviosTM while 4,4'-bis(N-carbazolyl)-1,10-biphenyl (CBP), 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and bis[2-[di(phenyl)phosphino]-phenyl ether oxide (DPEPO) were supplied by TCI chemicals. The PEDOT:PSS layers were spin-coated on top of cleaned pre-patterned ITO glass substrates and then annealed at 180 °C for 30 min. Blends with different doping concentrations were spin-coated on top of the PEDOT:PSS layer from a chloroform solution. DPEPO and TPBi films were then thermally evaporated and cathodes consisting of 1 nm thick LiF and 100 nm thick Al were finally deposited by thermal evaporation to complete the devices. The active area of the devices was designed to be 4 mm². Importantly, the devices were encapsulated in a glove box filled with nitrogen to avoid contamination by oxygen and moisture. For OLED characterization, current (I)-voltage (V)-luminance (L) characteristics were obtained under direct current (DC) driving using a source meter (Keithley 2400, Keithley Instruments Inc.) and external quantum efficiency measurements were performed using a C9920-12 Hamamatsu Photonics detector. The EL spectra were recorded using an optical fiber connected to a spectrometer (PMA-12, Hamamatsu Photonics).

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work was supported by Agence Nationale de la Recherche through the project Despot-Dyes (ANR-23-CE30-0003), through the framework of the French Investments for the Future Program Idex, UNISTRA (ANR-10-IDEX-0002) and SFRI STRAT'US project (ANR-20-SFRI-0012), financing the Interdisciplinary Thematic Institute HiFunMat (part of the ITI 2021-2028 of the UNISTRA, CNRS, and INSERM UMR 1121) and by Institut

Carnot MICA. We also acknowledge JST CREST (grant no. JPMJCR22B3) and JSPS KAKENHI International Leading Research (ILR) (23K20039). Y.O. and D.V. acknowledge funding by the Fonds de la Recherche Scientifique-FNRS under Grant n°T.0226.24 (OMLED). Computational resources were also provided by the "Consortium des Équipements de Calcul Intensif" (CÉCI), funded by the "Fonds de la Recherche Scientifiques de Belgique" (FRS-FNRS) under Grant No. 2.5020.11. J.C.R. acknowledges the Engineering and Physical Sciences Research Council of the UK for the financial support from grant EP/Y021495/1. The authors want to acknowledge Arnaud Marquette and Damien Cianfarani (OPTIC' characterization facility of IPCMS) and Emilie Couzigné, Marie-Noëlle Lalzol-Vogel, Nicolas Beyer, for the quality of their work and their willingness all along this project.

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

amplified spontaneous emission, curcuminoid borondifluoride, OLED

Received: January 31, 2025

Revised: March 17, 2025

Published online:

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