Host-guest strategy for full-visible-spectrum piezochromism in halogen-bonded organic frameworks

Domain: physics
Published: 2026-01-13
Editorial outcome: minor_revision

Abstract
Full-visible-spectrum piezochromism is vital for advanced anti-counterfeiting and storage device applications, yet remains challenging. Polycyclic aromatic hydrocarbons represent a promising class of piezochromic materials. However, pressure-induced structural disorder severely limit their performance by causing photoluminescence (PL) quenching through energy dissipation. In framework materials, host-guest modes offer a promising avenue to overcome the limitations. Herein, we demonstrate that the halogen-bonded organic framework host incorporating acridine guests (denoted as XOF@AD) enables full-visible-spectrum piezochromism modulation with the PL red-shift of 237 nm during compression. Both experiments and calculations reveal that strengthened \pi–hole···\pi interactions lower its interaction energy to preserve the structural rigidity of XOF host. This suppresses the structural disorder of AD guests and maintains their enhanced \pi–\pi stacking interactions, which leads to a decreased band gap. This work establishes host-guest 3D-XOF as a superior platform that achieves full-visible-spectrum piezochromism and presents promising potential for pressure-responsive sensing.

Introduction
Full-visible-spectrum piezochromic materials have aroused considerable attention in the field of anti-counterfeiting, storage devices and intelligent responsive, because of its tunable molecular stacking structures under high pressure 1–6. Purely organic polycyclic aromatic hydrocarbons (PAHs) emerged as promising candidates for full-visible-spectrum piezochromic materials due to their intrinsic \pi–\pi stacking arrangement 7,8. However, most PAHs are prone to undergo pressure-induced amorphization. It is challenging to observe their full-visible-spectrum emission behavior as this disordered structure results in a loose molecular packing mode, which subsequently quenches the photoluminescence (PL) through energy dissipation caused by vibration 9–12. Tremendous advances have been made to achieve a wide range of tunable PL wavelengths by constructing a rigid environment through designing crystalline porous materials (CPMs) based on PAHs 13–15. Yet, the piezochromic performance of most CPMs are severely hampered owing to the low crystallinity and inferior structural reversibility. In light of this, how to implement full-visible-spectrum piezochromism accompanied by PL wavelength shifts exceeding 190 nm within the visible range has been the main focus in piezochromic materials.

To tackle the aforementioned limitation, taking advantage of host-guest strategy to encapsulate guest in CPMs can maintain the structural integrity of guest molecules, holding the promise of achieving full-visible-spectrum piezochromic behavior 16–18. Among them, three-dimensional halogen-bonded organic frameworks (3D-XOFs) constitute a compelling category of CPMs, characterized by directional halogen bonds as structural nodes and high crystallinity 19–21. These 3D-XOFs establish unique platforms for the host-guest strategy by providing nanopores to encapsulate PAH guests. Due to the synergistic combination of \pi–hole···\pi interactions and robust halogen bonds, 3D-XOF is endowed with exceptional structural rigidity to resist collapse. This enables the preservation of PAH molecular alignment under high pressure and effectively suppressing amorphization (Fig. 1). Furthermore, 3D-XOFs serve as exceptional hosts that facilitate the formation of parallel-offset \pi–\pi stacking configuration in PAH guests to drive their optimal and considerable piezochromic behavior. Therefore, this class of 3D-XOF@PAH (3D-XOF host combined with PAH guests) has emerged as a highly promising candidate for light-emitting materials owing to its superior optoelectronic properties and pressure responsiveness 22–25. Given that the PL emission variations are predominantly influenced by pressure-induced reductions in intermolecular distances and enhanced molecular interactions, the utilization of 3D-XOF@PAH presents an effective strategy to realize full-visible-spectrum piezochromism 26,27.

Herein, we demonstrate that the host-guest XOF@AD (3D-XOF host combined with acridine guests) achieves full-visible-spectrum piezochromic behavior, accompanied by a PL emission red-shift of 237 nm under hydrostatic compression. This considerable PL emission tunability spans from deep blue (2.81 eV) to red (1.83 eV), exceeding the performance of all previously reported visible-range piezochromic materials. Studies on the mechanisms of pressure-induced structural evolution have elucidated that intensified \pi–hole···\pi interactions rigidify the 3D-XOF host by lowering the interaction energy under high pressure. This structural reinforcement effectively inhibits the amorphous phase transition in AD guests while maintaining their enhanced \pi–\pi stacking arrangement, which serves as the critical factor enabling full-visible-spectrum piezochromism. In addition, pressure-induced emission enhancement (PIEE) behavior was observed at a mild pressure of 1.2 GPa, attributed to the restriction of chemical bond vibrations-mediated suppression of non-radiative decay pathways.

Results
Crystal structure and PL properties of XOF@AD
The synthesis of XOF@AD was modified from co-crystal engineering method according to the literature 28. At ambient conditions, 3D-XOF host provides rigid hexagonal-channels to accommodate AD guest molecules, thereby forming the XOF@AD assembly. Adjacent AD molecules are stabilized by intermolecular parallel-offset \pi–\pi stacking, while the host-guest assembly is further reinforced by weak C–H···C/F hydrogen bonds (Fig. 2a and Supplementary Fig. 1). Due to \pi–hole···\pi interactions between alternately reversed 4-phenylpyridine-N-oxide (PPNO) molecules from adjacent chains, the channel units propagate infinitely along the c-axis to generate the 3D-XOF. The non-coplanar conformation of PPNO molecules within the 3D-XOF directly induces self-quenching in the extended network. As a consequence, XOF@AD exhibits a deep blue emission centered at 442 nm under 355 nm excitation, which predominantly originates from the PL of the encapsulated AD guests at ambient conditions (Supplementary Fig. 2). Then, we loaded XOF@AD samples with a culet size of 400 \mu m and 150 \mu m diameter cavity in asymmetric diamond anvil cell (DAC) to pressurize. The normalized PL spectrum of XOF@AD displays a remarkable and considerable red-shift up to 237 nm from 1 atm to 22.8 GPa (Fig. 2b and Supplementary Fig. 3). This shift corresponds to a chromatic transition from deep blue (2.81 eV) to red (1.83 eV), spanning nearly the entire visible range. A series of optical photomicrographs unambiguously demonstrated the progressive color evolution (Fig. 2c). Complementing these observations, the pressure-dependent shift in Commission Internationale de l'Éclairage (CIE) coordinates from (0.16, 0.12) at 1 atm to (0.51, 0.34) at 22.8 GPa confirms the polychromatic variation of the PL emission (Fig. 2d). As shown in Fig. 2e, the pressure-dependent wavelength of XOF@AD exhibits an early linear correlation with applied pressure (R2=0.99) with a sensitivity of 9.82 nm·GPa−1. This high linearity enables precise mapping of the wide color gamut to specific pressures and demonstrates the potential pressure-sensing applications of XOF@AD. Furthermore, the pronounced piezochromic behavior retains robustness over ten consecutive pressure-release cycles, thereby confirming that the optical reversibility of XOF@AD remains largely preserved under extreme conditions (Supplementary Figs. 4, 5). The first compression cycle exhibits a gradual PIEE that reaches the maximum intensity at 1.2 GPa before progressively attenuating at higher pressures (Fig. 2f). This is the first report of PIEE in host-guest 3D-XOF systems. To quantitatively assess the pressure-dependent PL enhancement, the PL quantum yield (PLQY) was evaluated using a previously established protocol, which increases from 10.8% at 1 atm to 22.3% at 1.2 GPa (Supplementary Fig. 6) 29,30. To further validate the PIEE phenomenon under 1.2 GPa, we performed time-resolved photoluminescence (TRPL) measurements to systematically investigate the pressure-dependent PL decay kinetics (Supplementary Fig. 7). Analysis of the PL decay dynamics revealed that non-radiative decay rates (\kappa_nr) significantly exceeded radiative decay rates (\kappa_r) across during compression. This dominance of \kappa_nr over \kappa_r fundamentally governs the pressure-dependent evolution of PLQY. Crucially, the \kappa_nr values for XOF@AD remained suppressed under 1.2 GPa, which directly accounts for the enhanced PL emission intensity and improved PLQY 31. Notably, XOF@AD exhibits a record-breaking piezochromic shift of 237 nm within the visible range, representing the largest chromatic modulation documented to date under hydrostatic compression (Supplementary Fig. 8 and Table 1). This performance outperforms all known pressure-responsive materials in visible chromatic range (Fig. 2g) 8,13–15,32–41.

Given that the PL of XOF@AD predominantly originates from the encapsulated AD guest molecules, we investigated the high-pressure PL behavior of AD (Supplementary Figs. 9, 10). At ambient conditions, AD exhibits bright blue emission centered at 462 nm with a 20 nm red-shift compared to XOF@AD. Analogous to XOF@AD, a PIEE phenomenon was also observed in AD up to 1.2 GPa. However, the PL emission of AD exhibits a lower pressure quenching threshold in contrast to XOF@AD under continuous compression. A series of optical photomicrographs unambiguously capture the PL behavior of AD. The distinct PL behavior observed between XOF@AD and AD presumably originates from synergistic C–I···O–N+ halogen bond and \pi–hole···\pi interactions within the 3D-XOF. These interactions stabilize rigid hexagonal-channels that spatially confine and protect AD guest molecules.

Evolution of crystal structure in XOF@AD under pressure
To elucidate the piezochromic behavior and pressure-dependent structure evolution of XOF@AD, in situ high-pressure angle-dispersive synchrotron X-ray diffraction (ADXRD) measurements were conducted from 1 atm to 22.8 GPa (Fig. 3a and Supplementary Fig. 11a). The ADXRD profiles reveal gradual shifts of Bragg reflections toward higher 2\theta angles, consistent with uniform lattice contraction under hydrostatic compression. Notably, no abrupt peak splitting or emergence of new reflections is observed throughout the compression cycle, indicating the absence of phase transitions. We used the General Structure Analysis System (GSAS) software to characterize the structural variations of XOF@AD via Rietveld refinement. At ambient conditions, the refinement of ADXRD pattern matches well with the experimental results (R_wp=0.84%, R_p=0.46%) through validation of the initial C2/c space group assignment.

We utilized the three-order Birch-Murnaghan equation of state to fit the pressure-volume data obtained in the experiment under high pressure 42,43. The continuous volumetric contraction suggests no abrupt crystal or electronic structural phase transitions in XOF@AD from 1 atm to 22.8 GPa (Fig. 3b). Figure 3c illustrates the anisotropic compressibility of lattice parameters under pressure, where distinct axial compression rates reflect the structural anisotropy of XOF@AD. The c-axis exhibits the most compressibility compared to the a-axis and b-axis, evidenced by the progressive decrease in (001) plane d-spacings that reflects its greater pressure sensitivity (Fig. 3d). The continuous compression of the (001) planes led to progressive strengthening of \pi–\pi stacking interactions and \pi–hole···\pi interactions. Simultaneously, the enhancement of C–I···O–N+ halogen bond correlates with the gradual contraction in the d-spacings of the (020) planes during compression. Under hydrostatic compression, the \pi–hole···\pi distance along the c-axis exhibited a significant reduction, synergistically cooperating with the progressively enhanced halogen bond to collectively maintain the structural rigidity of 3D-XOF (Fig. 3e, f). This structural evolution likely suppresses the amorphous transition of AD guests within 3D-XOF to enable the observation of the PL emission under high pressure. More notably, the \pi–\pi stacking distance between AD guests decreased nearly linearly, accompanied by a progressive increase in stacking overlaps, which is identified as the critical determinant enabling the realize of full-visible-spectrum piezochromism (Fig. 3g). To further assess the protective role of the 3D-XOF host on AD guests under high pressure, ADXRD measurements were conducted on AD under pressure conditions (Supplementary Fig. 11b). At ambient conditions, the Rietveld refinement of the ADXRD patterns agreed well with the experimental data (R_wp=0.57%, R_p=0.35%). In stark contrast to XOF@AD, AD exhibited severe pressure-induced amorphization, directly correlating with the PL quenching under high pressure. This pronounced structural contrast provides definitive evidence that the 3D-XOF effectively suppresses configurational disorder of the encapsulated AD guests during compression. Supported by enhanced \pi–hole···\pi interactions, the \pi–\pi stacking distance of AD guests decreases nearly linearly from 3.88 to 3.08 Å during compression (Fig. 3h) 44–46. The \pi–\pi stacking overlaps ratio increases from 64.2% at ambient pressure to 68.6% at 22.8 GPa. These coordinated \pi–interactions directly correlate with the full-visible-spectrum piezochromic behavior observed in XOF@AD during compression 47.

High-pressure IR and Hirshfeld surface analysis of XOF@AD under pressure
To further validate the pressure-induced evolution of molecular interactions underlying the pronounced piezochromism, we carried out in situ high-pressure infrared (IR) spectroscopy on XOF@AD (Fig. 4a and Supplementary Fig. 12). The vibrational bands at 850 cm−1 and 750 cm−1 are respectively assigned to the out-of-plane \gamma(sp2 C–H) bending and \phi(sp2 C–C) ring deformation modes 39. Both modes exhibit negligible frequency variation below 1.2 GPa, contrary to the conventional pressure-dependent blue-shift trend. This anomalous behavior implies significantly strengthened intermolecular interactions within XOF@AD. Such interactions rigidify molecular conformations and suppress non-radiative vibrational dissipation, thereby amplifying radiative recombination efficiency and driving the PIEE phenomenon. Moreover, the spectral region between 1400 and 1600 cm−1 corresponds to the \nu(C–C) aromatic ring breathing modes. The typical broadening and splitting of \nu(C–C) directly confirm the enhancement of \pi–\pi stacking interactions 48,49. The absorption band at 1358 cm−1 is assigned to the in-plane \beta(C–H) bending mode of the benzene ring at ambient conditions (Fig. 4b) 50. This continuous red-shift vibrational mode of ~40 cm−1 represents progressive strengthening of C–H···C hydrogen bond interaction during hydrostatic compression. The accelerated red-shift of the \beta(C–H) mode from 1 atm to 1.2 GPa potentially correlates with the PIEE phenomenon. Most IR vibrational modes show blue-shift accompanied by splitting and broadening under further compression, which indicates that \pi–\pi stacking interactions become more pronounced. This enhancement promotes substantial increases in non-radiative decay that leads to a gradual decline in PL emission until complete quenching occurs.

Based on the Rietveld refinements, we conducted complementary Hirshfeld surface calculations to systematically investigate non-covalent interactions in XOF@AD (Fig. 4c). Hirshfeld surfaces analysis at 22.8 GPa revealed intensified intermolecular interactions, manifested through expanding red regions that contrasted with ambient-pressure blue zones characteristic of weaker interactions. We established a quantitative model by using the aromatic fused ring of AD as the reference plane to calibrate critical intermolecular distances (Supplementary Fig. 13). d1 corresponds to \pi–\pi stacking interactions of guests, d2 and d3 to C–H···C and C–H···F hydrogen bonds, while d4 and d5 represent \pi–hole···\pi interactions and halogen bonding, respectively. Consistent with the behavior of \nu(C–C) in IR measurements, the progressive reduction of d1 from 3.88 Å to 3.08 Å under pressure correlates with a marked enhancement of \pi–\pi stacking interactions, as evidenced by their increased contribution to finger-print plot from 11.1 to 18.3% (Fig. 4d, e). The systematic decrease in d2 and d3 distances drives the increase in hydrogen bond contribution from 34.2 to 36.3%, which correlates with the red-shift in-plane \beta(C–H) bending and enhances these non-covalent interactions. Notably, the weaker enhancement of chemical bond interaction compared to \pi–\pi stacking interactions establishes a competitive interplay between these mechanisms. The initial PIEE is rationalized by the restriction of chemical bond vibrations-mediated suppression of non-radiative decay, whereas the eventual PL quenching at elevated pressures is dominated by \pi–\pi stacking interactions. The pressure-dependent evolution of distances demonstrated progressive reductions in d4 from 4.06 to 3.27 Å and in d5 from 2.70 to 1.94 Å to strongly support the formation of a more rigid 3D-XOF. Concurrently, the relative contributions of \pi–hole···\pi interactions and halogen bond increased significantly from 24.8 to 30.1% and from 9.4 to 11.8%, respectively (Supplementary Figs. 14, 15). These molecular interactions evolution directly provide evidence for the pressure-induced suppression of guest amorphization within rigid 3D-XOF, which suggest the maintenance of the parallel-offset \pi–\pi stacking arrangement of guest molecules during compression.

Full-visible-spectrum piezochromism and PIEE mechanism of XOF@AD
To directly elucidate the mechanism underlying the full-visible-spectrum piezochromism phenomenon, in situ pressure-dependent absorption spectroscopy was conducted on XOF@AD (Fig. 5a and Supplementary Fig. 16). At ambient conditions, XOF@AD exhibited an absorption edge at 426 nm and appeared nearly colorless. Under progressive compression, the absorption edge undergoes continuous red-shift to ~693 nm, accompanied by a chromatic transition from nearly colorless through orange to deep red (Fig. 5b, c). This spectral evolution displays a systematic strengthening of intermolecular interactions coupled with reduced intermolecular distances. By constructing Tauc plots of (\alpha h\nu)2 versus h\nu plots under high pressure, the progressive narrowing of band gap from 2.98 eV to 1.98 eV was revealed (Supplementary Fig. 17). This directly demonstrates that the pressure-dependent electronic structure modulation is dominated by the enhanced \pi–\pi stacking interactions (Fig. 5d) 51,52. For comparison, we also carried out in situ pressure-dependent absorption spectroscopy on AD (Supplementary Fig. 18). The absorption edge of AD displays a gradual red-shift up to 15.6 GPa during compression, and the band gap derived from the Tauc plot analysis exhibits a steady decrease from 2.75 to 1.76 eV (Supplementary Fig. 19). Nevertheless, a clear turning point emerges beyond 15.6 GPa, followed by an increase to 1.83 eV at 22.8 GPa. This indicates that an electronic structural transformation has occurred, which can also be correlated with the increased degree of structural disorder under pressure. These results provide compelling evidence that the rigid 3D-XOF host effectively maintains the \pi–\pi stacking configuration of AD guests, thereby confirming that the enhanced \pi–\pi stacking interactions under high pressure are the origin of the observed full-visible-spectrum piezochromic behavior. In addition, we carried out geometry optimization and calculated the band gap with the density of states (DOS) in XOF@AD based on density functional theory (DFT) 53. At ambient conditions, both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized exclusively on guests, indicating that PL emission originates predominantly from AD (Fig. 5e). This initial deep-blue PL emission arises from the \pi\pi* electronic transition within AD, which is a critical prerequisite for achieving full-visible-spectrum piezochromism under high pressure. To further elucidate the mechanism of PIEE, we performed time-dependent DFT (TDDFT) calculations at the M06-2X/Def2-TZVP level for pressures below 1.2 GPa to investigate the excited-state electronic transitions in XOF@AD (Supplementary Fig. 20). The oscillator strength increased from 0.1105 at 1 atm to 0.1339 at 1.2 GPa, reflecting the strengthened transition dipole moment under high pressure (Supplementary Fig. 21). This provides further evidence for the continuous increase of the \kappa_r (Supplementary Fig. 7). Therefore, the restriction of chemical bond vibrations significantly suppresses non-radiative decay, which effectively accounts for the PIEE phenomenon in XOF@AD. Figure 5f and Supplementary Fig. 22 illustrate the pressure-dependent evolution of simulated band gap under progressive compression. The calculated band gap narrowing trend aligns with experimental absorption data confirms the consistency between theoretical and experimental results, and the enhancement in the DOS dominated by C-p orbital contributions further corroborates the band gap narrowing (Fig. 5g). These synergistic electronic effects of intensified DOS contributions and reduced band gap collectively underscore the pivotal role of pressure-induced \pi–\pi stacking interactions enhancement in governing electronic modulation 54. To investigate the pressure-induced suppression of guest amorphization within 3D-XOF, we analyzed calculations of electrostatic surface potentials (ESP) for PPNO through preferential c-axis compression at the level of M06-2X/6-311G(d) (Fig. 5h). The elevated ESP values correlated with strengthened \pi–hole···\pi interactions drive tighter PPNO stacking that amplifies non-covalent intermolecular forces, thereby lowering the interaction energy of the 3D-XOF host to enhance stability 55–57. Further quantitative analysis of the \pi–hole···\pi interaction energy showed a decrease from −3.00 kcal·mol−1 at 1 atm to −3.23 kcal·mol−1 at 22.8 GPa, directly demonstrating that this strengthening interaction effectively stabilizes the structure (Supplementary Fig. 23) 58,59. This preserves the parallel-offset \pi–\pi stacking alignment of AD guests and prevents their disordering by maintaining the rigidity of the 3D-XOF structure. This stabilization mechanism critically accounts for the observation of piezochromic behavior under high pressure. Both experiments and calculations conclusively validate that the host-guest strategy effectively maintains the structural rigidity, which enables manipulating full-visible-spectrum piezochromism through pressure-modulated \pi–interactions.

Methods
Materials
All chemicals were purchased from Shanghai Titan Scientific Co., Ltd and used directly without further purification. 1,4 diiodotetrafluorobenzene (1,4-DITFB, Adamas-beta, 99%), 4-Phenylpyridine-N-oxide (PPNO, Adamas-beta, 98%), Acridine (AD, Adamas-beta, 98%), Acetone (GENERAL-REAGENT, 99.5%), Ethanol (GENERAL-REAGENT, 99.7%).

Growth of XOF@AD and 3D-XOF Cocrystals
XOF@AD were synthesized referring to the “HAD” in the literature method by cocrystal engineering with further improvements. 1,4-DITFB (40.2 mg, 0.1 mmol), PPNO (17.1 mg, 0.1 mmol) and AD (17.9 mg, 0.1 mmol) were absolutely dissolved in 10 mL acetone with a molar ratio of 1:1:1 in a glass culture dish, then sealed the culture dish with Parafilm and quickly poked some small holes with the needle. The acetone would evaporate completely within 5 days. The light yellow cocrystals at the bottom were collected. Ethanol was used to remove unreacted impurities from the cocrystals surface to obtain XOF@AD cocrystals (yield: ~64%). A bright deep blue emission can be seen under 365 nm UV light. The samples were directly stored in sample vials under ambient conditions without requiring any special storage precautions.

The 3D-XOF was synthesized following a procedure analogous to that described above. Specifically, 1,4-DITFB (40.2 mg, 0.1 mmol) and PPNO (17.1 mg, 0.1 mmol) were completely dissolved in 8 mL of acetone at a 1:1 molar ratio in a glass culture dish. The dish was then sealed with Parafilm, and several small holes were carefully punctured in the sealing film using a needle. The acetone was allowed to evaporate completely over a period of 3 days. Subsequently, the resulting cocrystals were washed with ethanol to remove any unreacted impurities, yielding the final 3D-XOF cocrystals (yield: ~57%).

In situ high pressure experiments
In all experiments, we used the same symmetric DAC which has a pair of type IIa ultra-low fluorescence diamonds with a diameter of 400 \mu m to generate high pressure at room temperature. We loaded the sample into a T301 steel gasket featuring a 150 \mu m diameter cavity and a 45 \mu m pre-indentation thickness. The co-crystallized XOF@AD single crystals of millimeter scale were ground to fit the 150 \mu m sample chamber for all in situ high-pressure experiments. The pressure in DAC was calibrated using the ruby fluorescence technology. Silicon oil (Aldrich) was used as the pressure transmitting medium for PL, UV-visible absorption, angle-dispersive X-ray diffraction (ADXRD) and TRPL measurements; whereas KBr (Adamas-beta, 99.9%) was employed for the FT-IR measurements.

The in situ high-pressure PL spectra measurements of XOF@AD were recorded by an optical fiber spectrometer (Ocean Optics, QE65000) with the data-collection time of 2000 ms. A 355 nm line of a UV DPSS laser with the power of 10 mW was used for PL measurements. PL images of XOF@AD were obtained using a camera (Canon EOS 5D Mark II) equipped on a microscope (Ecilipse TI-U, Nikon). The complete procedure, including compression, calibration, and spectral collection, required ~30 s, with no deliberate pressure-holding step incorporated into the measurement cycle.

The in situ high-pressure UV-visible absorption measurements of XOF@AD were similarly recorded by an optical fiber spectrometer (Ocean Optics, QE65000) with the data-collection time of 300 ms. A deuterium-halogen light was used for the UV-visible absorption measurements. The corresponding images of XOF@AD were obtained using a camera (Canon Eos 5D mark II).

The in situ high-pressure FT-IR absorption measurements of XOF@AD were carried out by using a Bruker Vertex 70V FT-IR spectrometer (BRUKER OPTIK GMBH, Germany) equipped with a nitrogen-cooled mercury-cadmium-telluride detector.

The in situ high-pressure TRPL measurements of XOF@AD which are performed to investigate the PL decay kinetics were measured using a 405 nm pulsed diode laser as excitation source with the focal length grating spectrograph (HRS-300MS, PI).

In situ X-ray diffraction (XRD) measurements
The powder XRD (P-XRD) data at ambient condition was measured by using Cu as the target, then the experimental data was obtained from 4 to 50 degrees (R-AXIS RAPID II, Rigaku). The corresponding in situ high-pressure ADXRD experiments were carried out at Shanghai Synchrotron Radiation Facility (SSRF), beamline 15U1 (BL15U1). X-ray beam with a diameter of about 5 \mu m and wavelengths of 0.6199 Å was used in the diffraction experiment. CeO2 was employed for the standard sample to calibrate. The image obtained by experiment were comprehensively analyzed using the Dioptas program, exporting as the 1-dimensional intensity versus 2-theta diffraction angle pattern.

Method of calculations
Based on the Rietveld refinements, we obtained the high-pressure structures of XOF@AD during compression. We got the symmetrical crystal structure model by occupying half an atom of N in AD. By using the Crystal Explorer 17.5 program, we analyzed and visualized the interactions in XOF@AD. The corresponding 3D Hirshfeld surfaces and finger-print maps during compression were obtained.

The first-principles calculations of geometry optimizations and band gaps were carried out by using the Cambridge Sequential Total Energy Package (CASTEP), with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation framework of DFT. Grimme’s approach and the DFT-D3 correction method were employed to describe long-range van der Waals interactions.

The Charge density difference diagrams and interaction energy were described by using the Vienna Ab initio Simulation Package with the PBE as the exchange-correlation function and the frozen-core projector-augmented wave pseudopotentials as parameterization with a plane-wave cutoff energy of 500 eV. The long-range van der Waals interactions were also accounted for using Grimme’s DFT-D3 method with Becke-Johnson damping.

The calculations of excited state electronic transitions of XOF@AD were performed using Gaussian09 program by TDDFT method at the M06-2X/Def2-TZVP level. The maps (electron density 0.001 e·Bohr−3) of ESP of PPNO were calculated under the level of M06-2X/6-311G(d), the color range maps from red to blue corresponds to ESP from negative to positive.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript reports a host–guest complex between acridine and a halogen-bonded organic framework (XOF), which exhibits full-visible-spectrum piezochromic luminescence under pressure. The authors demonstrate that the rigidity of the XOF matrix suppresses amorphization of the guest acridine under high pressure, leading to a pronounced emission shift from blue to red. Such a pronounced shift would be the largest shift reported to date. The experimental characterization is thorough and includes in situ X-ray diffraction, UV-vis absorption, photoluminescence, and IR spectroscopy under varying pressure, along with DFT-based theoretical analysis. While the data is of high quality and the piezochromic behavior is clearly demonstrated, I regret to conclude that the manuscript does not meet the novelty and conceptual impact required for publication in Nature Communications. Several materials exhibiting large piezochromic shifts have already been reported, and while this particular complex may achieve a record shift, such quantitative superiority alone does not constitute a sufficiently novel conceptual advance. The design strategy, utilizing rigid porous frameworks to stabilize luminescent guests, is not so unprecedented. One potentially interesting aspect is the observation of pressure-induced emission enhancement (PIEE). However, the mechanistic discussion of this phenomenon remains speculative. For instance, the change in IR spectral features near 1.2 GPa is used as suggestive evidence, but this alone is not convincing or sufficient to support the claimed mechanism. Given these considerations, I recommend that the manuscript is not suitable for publication in Nature Communications in its current form. However, the work could be of interest to a more specialized journal after appropriate revision and clarification of the following points:\n1. To demonstrate potential sensing applications, only two pressure-release cycles is insufficient. At least ten cycles should be conducted to evaluate the reproducibility and stability of the piezochromic response.\n2. To include the absorption spectral change of acridine alone under pressure in the Supporting Information would be important.\n3. The manuscript does not explain why acridine was chosen as the guest molecule. A brief discussion of the advantages in photophysical properties and why acridine is suited for the current study would strengthen the conceptual framing.", "reviewer_2": "This manuscript presents a comprehensive investigation of full-visible-spectrum piezochromism achieved through a host-guest halogen-bonded organic framework (3D-XOF) encapsulating acridine (AD) molecules. The authors demonstrate a record-breaking photoluminescence (PL) red-shift of 237 nm under hydrostatic pressure, supported by an extensive suite of experimental characterizations and theoretical calculations. The topic is highly relevant to the fields of piezochromic materials, high-pressure physics, and optical sensing. The manuscript is well-organized, and the experimental data convincingly support the proposed mechanism. The authors effectively integrate structural analyses (ADXRD, Hirshfeld surfaces), spectroscopic measurements (PL, UV-vis, IR), and DFT calculations to elucidate the pressure-dependent behavior of the system.\nHowever, the points raised below need to be addressed before the manuscript can be considered for publication.\n1) The correlation between the pressure-dependent chromic behavior of 3D-XOF and molecular aggregation phenomena is unclear.\n2) The co-crystallization protocol is briefly outlined. Please provide information on the reproducibility and yield of the synthesis. Additionally, are the cocrystals stable under ambient humidity and temperature over time? Please clarify whether the XOF@AD samples can be handled in ambient atmosphere or whether special storage conditions are required.\n3) A more detailed structural visualization in Figure 2a is necessary to unambiguously demonstrate 3D-XOF, including its elemental distribution and specific halogen bonding coordination environments.\n4) The reversibility of the piezochromic response is a key advantage. While two pressurization cycles are shown, more detailed information on potential degradation is needed. Does the PL quantum yield fully recover after each cycle? Is there any observable structural damage after multiple pressure cycles?\n5) The DFT methodology employed is standard and appropriate. However, the text should explicitly state whether dispersion corrections were employed in the band gap calculations. Given the critical role of π–π interactions, the omission of dispersion corrections could impact the accuracy of the results. Please clarify this point.\nPlease give the functional(s) and basis set(s) employed in the DFT calculations of XOF@AD to facilitate better evaluation of the computational methodology.\n6) The proposed mechanism, centered on the synergistic reinforcement of π–hole···π interactions and halogen bonding maintaining structural rigidity and suppressing amorphization, is well-supported by Hirshfeld surface analyses and DFT calculations. Nevertheless, the manuscript would benefit from a more quantitative discussion. For example, could the authors provide explicit interaction energies or quantify the magnitude of the energy barriers preventing guest disorder?\n7) The font sizes in the Supporting Information figures should be increased to improve readability.", "reviewer_3": "In this study, Yang et al. introduce an innovative host–guest strategy to tackle the challenge of achieving a broad and controllable piezochromic response. By encapsulating readily amorphizable AD guest molecules within a rigid three-dimensional, halogen-bonded organic framework, they preserve structural integrity under high pressure. The confinement provided by the host framework effectively restricts molecular displacement and suppresses irreversible phase transitions, thereby maintaining the guests’ piezochromic activity. Combining detailed experiments with DFT calculations, the authors show that the host–guest architecture substantially suppresses amorphization of the AD guests under compression. This mechanism yields a wide piezochromic tuning range spanning the entire visible spectrum. Moreover, the material exhibits excellent reversibility, which is critical for practical applications. This work establishes the host–guest system as a robust platform for full spectrum piezochromism and opens new avenues for developing smart, visible range pressure sensing materials. The conclusions are supported by comprehensive data and a clear mechanistic rationale, representing a notable advance in stimuli responsive optical materials. I strongly recommend publication in Nature Communications after addressing the following minor concerns.\n1. How about the chemical stability of the studied XOF@AD? It will be important for future applications in piezochromic applications.\n2. In Fig. 2b, the 3D PL spectra make it difficult to compare peak shifts. It is suggested that corresponding 2D PL spectra be provided in the Supplementary Information for clearer comparison. Additionally, for the ADXRD patterns shown in Fig. 3a, the authors should draw dotted lines through the peaks to highlight the changes for better understanding.\n3. The experimental details section should be expanded to include more comprehensive information. For example, what type of samples were used in the measurements, single crystals or powder? What is the synthesis method for the 3D-XOF material in Supplementary Fig. 7? Is the PL transition during compression instantaneous, or does it require a pressure-holding duration to stabilize?\n4. The PL intensity is lower than its initial state upon complete pressure release from 22.8 GPa. Would the PL intensity be higher than the initial state if the pressure is released from 1.2 GPa (the pressure at which PL is strongest)?\n5. The PL measurements were performed using the 355 nm laser line. However, the authors do not discuss whether any changes occur in the optical properties under high pressure during irradiation. Please provide additional relevant PL experiments.\n6. In Fig. 5d, the authors have presented the evolution of the absorption spectra during pressurization. Corresponding Tauc plots should be provided in the Supplementary Information.\n7. There are some formatting and grammatical errors in the manuscript that need to be checked carefully. To name a few:\n(1) the subscripts of “non-radiative decay rates”, “radiative decay rates”, and “distance”.\n(2) The x-axis label of “Pressure (GPa)” in Supplementary Fig. 8 is incorrect.\n(3) In Page 14, Line 20, “is critically accounts for” should be revised to “critically accounts for”."}, "author_response": "We acknowledge your comments and suggestions very much, which are valuable in improving the quality of our Manuscript. For clarity, we would like to emphasize the highlights of the present work as follows:\nFirst, we firstly demonstrate full-visible-spectrum piezochromism in XOF@AD from deep blue (442 nm) to red (679 nm), which exhibits a record red-shift of 237 nm. It is true that previous researchers have achieved piezochromic shifts in materials such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), perovskites, and quantum dots. However, the color changes in reported piezochromic materials are largely confined to limited shift range, such as from green to red (187 nm), yellow to red (154 nm), or from sky blue to yellow (123 nm).1-10 This severe limitation in spectral coverage and chromaticity shift poses a long-standing challenge for achieving full-visible-spectrum piezochromism. In contrast, this work features full-visible-spectrum fluorescence modulation (237 nm) and a continuous linear PL response (R2=0.99). Notably, it also exhibits excellent compressive reversibility, as evidenced by stable emission over ten pressure-release cycles. These characteristics identify it as an excellent candidate for smart luminescent materials in sensing.\nSecond, we firstly introduce the host-guest strategy to high-pressure conditions to maintain structural rigidity, thereby achieving the wide-range PL modulation. In fact, the existing studies on the host-guest strategy under high pressure focus on achieving white-light emission. Specifically, by regulating the dual emission through host-guest interactions, white-light emission has been achieved in RhB@HNU-49 under high pressure.11 To date, no research has been reported on the improvement of fluorescence chromic properties utilizing the host-guest strategy under high pressure. Furthermore, at ambient conditions, the multi-color PL via the host-guest strategy is mainly realized by incorporating various PAH guests into the same rigid host to synthesize distinct materials.12-15 Therefore, the design strategy of utilizing host-guest strategy to stabilize luminescent guests of a single material under high pressure remains challenging. Herein, we establish a high-pressure design strategy for piezochromic materials, addressing the inherent pressure-induced structural disorder in PAH guests and holding great potential for optical pressure-responsive detection.\nTo further clarify the mechanism of the observed pressure-induced emission enhancement (PIEE) under high pressure, we conducted additional analysis and supplemented with corresponding density functional theory (DFT) calculations.16-17 We analyzed chemical bond vibrations and intermolecular interactions using in situ high-pressure IR measurements. Intriguingly, we found that the out-of-plane γ(sp2 C–H) bending, φ(sp2 C–C) ring deformation modes, and in-plane β(C–H) bending mode remained basically unchanged or exhibited an anomalous red shift below 1.2 GPa (Revised Fig. 4). This indicates that the restriction of the corresponding bond vibrations suppressed the non-radiative energy dissipation, thereby facilitating the PIEE phenomenon in XOF@AD.\nWe further performed calculations of the excited-state electronic transitions on XOF@AD. These computations were carried out with the Gaussian09 program, employing the time-dependent DFT (TDDFT) method at the M06-2X/Def2-TZVP level (Added Supplementary Fig. 20).18-20 We found that the oscillator strength increased from 0.1105 at 1 atm to 0.1339 at 1.2 GPa, reflecting the strengthened transition dipole moment under high pressure (Added Supplementary Fig. 21). Therefore, the pressure-enhanced transition dipole moments contribute significantly to the improved PL intensity. As evidenced from the above, the restriction of chemical bond vibrations significantly suppresses the non-radiative energy dissipation to increase oscillator strength, which effectively accounts for the PIEE phenomenon in XOF@AD.\nThe revised details are highlighted in red and could be found in Lines 17-19, Page 4; Lines 16-24, Page 14; Lines 5-7, Page 17 of the revised manuscript, as well as added Supplementary Figs. 20-21 of the revised Supporting Information.\n[... Author responses to specific comments 1-3 for Reviewer 1, 1-7 for Reviewer 2, and 1-7 for Reviewer 3 follow ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have thoroughly revised their manuscript in accordance with the reviewers’ comments and suggestions. The comprehensive additional data set and theoretical calculations strongly support their conclusions and further enhance the novelty of the work. I am now willing to recommend the revised manuscript for publication in Nature Communications.", "reviewer_2": "All the issues have been resolved. It can be accepted now.", "reviewer_3": "I would recommend the publication of this manuscript in Nature Communications."}, "author_response": ""}]

Decision letter