Circularly polarized room-temperature phosphorescence in microcrystals via aggregation-induced chirality transfer

Abstract

Circularly polarized room-temperature phosphorescence (CPRTP) materials integrating room-temperature phosphorescence (RTP) and circularly polarized luminescence (CPL) show great promise for frontier applications like optoelectronics. Herein, we report a chiral luminophore Ben-2Chol, which can self-assemble into micrometer-scale sheets in the aggregated state of solution and spin-coated films and achieve circularly polarized fluorescence (CPF) through aggregation-induced chirality transfer, with the maximum g_lum reaching −1.1 × 10⁻³. Notably, its liquid-phase-diffused fibrous microcrystals exhibit CPRTP with inverted polarization relative to the sheets, featuring g_lum values of +6.0 × 10⁻³ (blue) and +1.0 × 10⁻³ (yellow-green) with a 41.7 ms of luminescence lifetime. Mechanical grinding eliminates RTP/CPL, confirming crystallization-induced properties. This study provides a simple strategy for constructing CPRTP materials through aggregation-induced chirality transfer in microcrystals, offering new insights for the design of chiral luminescent materials with dual functionalities.

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Introduction

Pure organic persistent room-temperature phosphorescence (OPRTP) materials based on long-lived triplet states have garnered significant attention due to their potential applications in organic light-emitting diodes, chemical sensing, bioimaging, and information encryption.¹ Additionally, compared with transition-metal-based phosphorescent materials, OPRTP materials offer advantages such as low cost, easy synthesis, low toxicity, and superior flexibility, which are conducive to practical applications.² Currently, the construction of metal-free OPRTP materials typically involves introducing heavy atoms or aromatic carbonyl groups to enhance the spin–orbit coupling (SOC) effects and promote the generation of triplet excitons.³ Meanwhile, strategies such as crystal engineering, host–guest systems, and polymer doping are employed to provide a rigid environment, thereby suppressing non-radiative decay and stabilizing triplet excitons.⁴ Therefore, constructing OPRTP with long-lived triplet states under ambient atmospheric conditions remains highly challenging.

Concurrently, circularly polarized luminescence (CPL) materials have attracted extensive interest from researchers due to their potential applications in frontier scientific fields such as 3D display, chiral recognition, information encryption, and asymmetric photocatalysis.⁵ The luminescence dissymmetry factor (g_lum) is one of the key parameters for measuring the performance of CPL materials, and practical applications require CPL materials to have a large g_lum.⁶ However, in pure organic chiral molecular systems, it is common that chiral molecules have flexible skeletons or chiral centers that are far from the luminescent units, making such chiral luminescent molecules either hardly exhibit CPL activity or produce extremely weak CPL signals.⁷ Through aggregation-driven supramolecular self-assembly, effective chiral transfer occurs, whereby chiral information is transmitted from single molecules to supramolecular levels, forming CPL-active assemblies with markedly enhanced g_lum.⁸ On the other hand, most practical applications of CPL materials rely on aggregated states such as thin films and crystals.⁹ Therefore, constructing efficient CPL aggregated-state materials is of great significance.

Circularly polarized room-temperature phosphorescence (CPRTP) materials, formed by the perfect combination of room-temperature phosphorescence (RTP) and chirality, exhibit promising application prospects in various emerging frontier fields.¹⁰ However, current research on CPL materials primarily focuses on singlet excited states, namely, circularly polarized fluorescence (CPF). Achieving CPRTP remains challenging due to the inherent instability of triplet excitons.¹¹ For metal-free organic small molecules, a simple and effective strategy to realize long-lived CPRTP is a direct covalent linkage of chiral units with RTP emitters.¹² For example, in 2020, Chen et al. developed a pair of reversible stimuli-responsive CPRTP materials with high photo/thermal stability based on achiral carbazole and chiral ester chains.^12a In 2024, Li et al. achieved a CPRTP material by bonding chiral binaphthol with a phenoselenazine derivative, whose polystyrene thin films exhibited a high g_lum and long-lived CPRTP after photoactivation.^12b

Indolo[3,2-b]carbazoles (ICZs) and their analogs are excellent organic semiconductor materials that have been widely applied in the field of organic optoelectronics.¹³ One of the ICZ analogs, 6,12-diphenyl-5,6,11,12-tetrahydroindolo[3,2-b]carbazole (Ben-2H), has been reported for constructing efficient RTP materials with long triplet state lifetimes.¹⁴ For instance, in 2019, Huang et al. developed a series of dual-emission OPRTP materials based on charge transfer interactions using Ben-2H as the skeleton, with lifetimes ranging from 2 to 759 ms.^14a In 2022, Zhao et al. regulated the RTP and thermally activated delayed fluorescence (TADF) emissions of Ben-2H derivatives by introducing aromatic and aliphatic bromine atoms, achieving dual-persistent emissions with a long lifetime of 0.26 s and a persistent luminescence quantum yield of 10%.^14b These findings inspired our hypothesis that introducing chiral groups into Ben-2H may help in fabricating CPRTP materials with long emission lifetimes.

As a natural chiral structure, a cholesterol unit exhibits excellent chiral stability and assembly properties, making it widely used in the development of high-performance chiral materials.¹⁵ Herein, we synthesized a novel chiral luminophore Ben-2Chol by covalently linking two flexible chiral cholesterol groups to the centrosymmetric sites of a Ben-2H skeleton. The compound showed no chiral optical signals in the single-molecule state, but in the aggregated state of solution or spin-coated films, it enabled effective chiral transfer from the cholesterol side chains to the chromophore, self-assembling into micrometer-scale sheet structures and emitting CPF. Additionally, we unexpectedly obtained fibrous microcrystals of Ben-2Chol, which exhibited RTP with a yellow-green afterglow and a high RTP lifetime of 41.7 ms. Interestingly, compared with the micrometer-sheet aggregates, the microcrystals showed chiral optical properties with opposite polarization directions—displaying significantly enhanced CPL in the blue emission region and achieving CPRTP in the yellow-green emission region (Fig. 1).

Fig. 1 Schematic of CPL of Ben-2Chol in various states.

Results and discussion

The target compound Ben-2Chol was synthesized via a simple one-pot nucleophilic substitution reaction of Ben-2H with Chol-Br in the presence of KOH, affording a yield of 30%. The synthetic route is described in Scheme S1. The structure of the target compound was fully characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS), and the results are shown in Fig. S1–S3.

The UV-vis absorption spectra and photoluminescence (PL) spectra of Ben-2Chol in dilute THF solution were studied (Fig. 2a). Due to the spin-allowed π–π* transition in the aromatic ring, strong absorption bands were exhibited at 264–315 nm, with a single fluorescence emission peak at 422 nm, similar to the previously reported Ben-2H.^14a In addition, persistent luminescence of Ben-2Chol was observed in a THF solution at 77 K, as shown in Fig. 2b and c. After applying a gating delay time of 1 ms, the phosphorescence spectrum of Ben-2Chol showed dual phosphorescence emissions caused by different triplet states, with average luminescence lifetimes of 2789.5 ms and 2417.0 ms at 503 nm and 531 nm, respectively (Table S1).

Fig. 2 (a) UV-vis absorption (black line) and PL (blue line) spectra of Ben-2Chol (excitation wavelength = 340 nm) in THF at 293 K. Inset: Photographs taken under a 365 nm UV radiation. (b) Normalized steady-state (293 K, black line; 77 K, red line) and delayed (77 K, blue line) emission spectra of Ben-2Chol (excitation wavelength = 340 nm) in THF. (c) Lifetime decay profiles of Ben-2Chol (excitation wavelength = 340 nm) in THF at 77 K. (d) Fluorescence spectra of Ben-2Chol (excitation wavelength = 340 nm) in THF/water mixed solvents. (e) Plot of the relative emission peak intensity versus f_w at 432 nm for the THF/water mixed solvents, where I = emission intensity and I₀ = emission intensity in the THF solution. (f) CD spectra and (g) CPL spectra of Ben-2Chol (excitation wavelength = 300 nm) in THF/water mixed solvents with various f_w values. SEM images of the assemblies formed by Ben-2Chol in THF/H₂O mixed solvents with (h) f_w = 60% and (i) f_w = 70%. Concentration: 1.0 × 10⁻⁵ mol L⁻¹.

In a dilute THF solution, the isolated molecules of Ben-2Chol showed no CD or CPL signals due to their small chiral structures and the absence of effective chiral transfer. Considering that alkyl chains facilitate molecular self-assembly, we promoted the self-assembly of Ben-2Chol by adding water as a poor solvent, aiming to achieve amplified chirality. The fluorescence spectra of Ben-2Chol in THF/water mixtures with different water fractions (f_w) are shown in Fig. 2d. With an increase in the water content, the fluorescence intensity showed an increasing trend, reaching its peak when f_w reached 60% (Fig. 2e). The fluorescence quantum efficiency increased from 39.9% (THF solution) to 45.8% (f_w = 60%), indicating that Ben-2Chol exhibited typical aggregation-induced emission enhancement (AIEE) characteristics in the THF/water mixed system (Table S2).

We further measured the CD signals of Ben-2Chol at different f_w (Fig. 2f and Fig. S5). A weak positive CD peak emerged at 303 nm when the f_w reached 40%. As the f_w increased to 60%, a significantly enhanced negative CD signal appeared at approximately 243 nm. Upon further increasing f_w to 70%, the negative CD peak exhibited a slight intensification and blue-shifted to 233 nm, accompanied by the emergence of a positive CD signal at 245 nm. Correspondingly, in Fig. 2f, the absorption peak underwent distinct shape changes with increasing water content, accompanied by a notable tailing effect in the visible light region, which was attributed to the light scattering caused by nanoparticle formation.¹⁶ Using CPL spectroscopy, we probed the excited-state chirality of the aggregated solutions. Weak negative CPF signals were detected in the aggregated states at f_w of 60% and 70%, with fluorescence lifetimes of 6.1 ns and 6.2 ns, and g_lum values of −4.8 × 10⁻⁴ and −2.3 × 10⁻⁴, respectively (Fig. 2g and Fig. S4, S6). These results demonstrate that aggregation induces the transfer of chirality from the chiral carbon center to the chromophore, thereby enhancing the chiroptical activity of the molecules. In addition, the morphology of the aggregates in solution was visualized by scanning electron microscopy (SEM) (Fig. 2h, i and Fig. S7). The SEM images revealed that Ben-2Chol molecules self-assembled into two-dimensional (2D) thin sheets with sizes of tens of micrometers. The sheet-like aggregates presented a planar morphology, with the lateral dimensions ranging from tens to hundreds of micrometers and a relatively uniform thickness, indicating that there was a high degree of structural order during the self-assembly process, which facilitated the realization of chirality amplification from single molecules to supramolecules.

By manipulating the volume ratio of the THF/water mixed solvent, the above-mentioned chiral amplification preliminarily demonstrated its characteristic of chiral transfer through aggregation induction. We further prepared PMMA-doped films and pure films of Ben-2Chol to study their chiral optical properties in the thin-film state. Compared with the PMMA-doped films, the pure film exhibited a slight red-shift in its emission peak, while the PMMA-doped films showed emission characteristics similar to those in the THF solution state (Fig. 3a). In addition, both films showed fluorescent emission. The PMMA-doped films (8.3 ns) exhibited a longer luminescence lifetime than the pure film (3.0 ns) and a higher fluorescence quantum yield (Table S2 and Fig. S8).

Fig. 3 (a) PL spectra and (b) CD spectra of the PMMA films doped with Ben-2Chol (0.1 wt%, black line; 1.0 wt%, red line) and the neat film of Ben-2Chol (the film was prepared from a Ben-2Chol DCE solution with a concentration of 10 mg mL⁻¹, blue line). (c) CPL spectra (excitation wavelength = 300 nm) and (d) SEM image of the neat film of Ben-2Chol.

As expected, when the doping concentration of Ben-2Chol in the PMMA films was 0.1 wt%, its CD signal was too weak to be noticeable. However, when the concentration was increased to 1.0 wt%, two enhanced negative CD signals were observed at 326 nm and 251 nm. Similarly, the pure films exhibited negative CD signals at 247 nm and 344 nm, with intensities comparable to those of the PMMA film at high doping concentrations (Fig. 3b). Notably, the CPF signal of the pure films showed a significantly enhanced negative signal compared with that of the aggregated state in solution, with the g_lum value reaching −1.1 × 10⁻³ (Fig. 3c and Fig. S9). SEM observations revealed the formation of self-assembled micrometer-scale sheet structures in the films, which were highly similar to the nanosheet structures in the solution-aggregated state (Fig. 3d and Fig. S10). However, the higher degree of molecular aggregation in the film state led to a significant enhancement of the CPF signal. These results fully demonstrated that the molecular aggregation behavior in the Ben-2Chol system played a crucial role in the amplification of the chiral optical signals.

We slowly added ethanol to a concentrated dichloromethane solution of Ben-2Chol and obtained a microcrystalline sample with RTP property through liquid-phase diffusion. As shown in the SEM image of Fig. 4a and Fig. S11, the microcrystals exhibited regular elongated fibrous structures, with lengths ranging from tens to hundreds of micrometers, which were significantly different from the plate-like structures of the solution-aggregated state and pure films. Additionally, powder X-ray diffraction (XRD) results showed sharp diffraction peaks of the microcrystals, confirming their crystalline phase structure (Fig. 4b).

Fig. 4 (a) SEM image of Ben-2Chol in microcrystalline state. (b) XRD patterns of Ben-2Chol in microcrystalline state before (blue line) and after (black line) grinding. (c) Normalized steady-state (293 K, blue solid line; 77 K, blue dashed line) and delayed (77 K, green solid line; 77 K, green dashed line) emission spectra of Ben-2Chol (excitation wavelength = 340 nm) in microcrystalline state. (d) Lifetime decay profile of Ben-2Chol in microcrystalline state at 541 nm under room temperature. (e) Luminescence photographs of Ben-2Chol in microcrystalline state and amorphous state before and after a 365 nm UV excitation.

The phosphorescence properties of Ben-2Chol microcrystals were further studied by PL spectroscopy (Fig. 4c and Table S3). At room temperature, the maximum emission peak of the microcrystals was located at 431 nm with an emission lifetime of 4.4 ns, exhibiting a blue light emission. After applying a 1 ms delay time-gated technique, the phosphorescence spectrum showed dual phosphorescence emissions at 439 nm and 541 nm. At 77 K, the maximum emission peak was located at 425 nm, with an emission lifetime of 1.8 ns; this peak was slightly blue-shifted compared to room temperature, and the relative intensity ratio of emissions in the blue and yellow-green spectral regions decreased. Using the 1 ms delay time-gated technique, the phosphorescence spectrum showed a broad emission peak with the maximum emission at 497 nm across the 400–700 nm range. The room-temperature phosphorescence lifetimes of the microcrystals at 439 nm and 541 nm were 15.2 ms and 41.7 ms, respectively (Fig. 4d and Fig. S13). Significantly prolonged phosphorescence lifetimes were observed at 77 K with the phosphorescence lifetime at 497 nm reaching up to 222.9 ms (Fig. S14). It was worth noting that the blue-emission region exhibited both nanosecond and millisecond luminescence lifetimes, indicating that the emission in the blue light portion was composed of both fluorescence and phosphorescence.

Under excitation with a 365 nm ultraviolet lamp at room temperature, Ben-2Chol microcrystals exhibited bright blue light emission. Due to the presence of components with long emission lifetimes, a yellow-green afterglow lasting approximately 2.5 s was observed after turning off the UV lamp. Interestingly, this afterglow emission disappeared after mechanical grinding (Fig. 4e). PL spectroscopy revealed a slight blue-shift in the emission peak after grinding, with no obvious change in the peak profile (Fig. S15). However, the long-lived component vanished, and the ground sample exhibited nanosecond-scale fluorescence emission with a lifetime of 3.2 ns (Table S2 and Fig. S16). SEM images showed that the crystalline fibers disappeared after intense grinding, with the morphology transforming into disordered bulk structures (Fig. S17). XRD patterns also indicated that the ground sample lacked any sharp diffraction signals, confirming its amorphous (non-crystalline) state (Fig. 4b). These results indicated that the RTP phenomenon of Ben-2Chol was crystallization-induced. The underlying mechanism might be related to the facts that the weak intermolecular interactions in the crystal promoted charge transfer between molecules and the rigid environment stabilized the triplet excited state.^14a

Further studies on the chiral properties of Ben-2Chol microcrystals were conducted. The CD spectrum showed that the microcrystals exhibited positive CD signals in the range of 210–400 nm (Fig. 5a). As shown in Fig. 5c of the CPL spectrum, the microcrystals displayed a significantly stronger positive CPL signal in the range of 380–500 nm, and the CPL emission was extended all the way to 700 nm. Based on the RTP emission property of the microcrystals, it was proposed that both CPF and CPRTP coexisted in the blue emission region, while the weaker signals at longer wavelengths were ascribed to CPRTP.¹⁷ The g_lum values at 448 nm and 541 nm were +6.0 × 10⁻³ and +1.0 × 10⁻³, respectively (Fig. 5d). Interestingly, the microcrystals exhibited a supramolecular chirality that was completely different from other aggregates, with opposite chiral information in the ground state and excited state and significantly enhanced CPL emission. Notably, after mechanical grinding, the CD signal of the microcrystals weakened and reversed, and no CPL emission was detected (Fig. 5a, b and Fig. S18). These results indicated that the CPRTP of Ben-2Chol microcrystals were dependent on the rigid environment of the crystalline phase, which significantly stabilized triplet excitons. This characteristic enabled the CPRTP of the microcrystals to respond to mechanical stimuli.

Fig. 5 (a) CD spectra of Ben-2Chol microcrystals before (blue line) and after (black line) grinding. CPL spectra (excitation wavelength = 300 nm) of Ben-2Chol microcrystals (b) after and (c) before grinding. (d) g_lum spectra of Ben-2Chol microcrystals.

Conclusions

In summary, we synthesized a novel chiral luminophore Ben-2Chol, which exhibited no chiroptical signals in dilute THF solution but self-assembles into micrometer-scale sheet structures in aggregated states, enabling aggregation-induced chirality transfer and CPF. Notably, the fibrous microcrystals of Ben-2Chol prepared via a liquid-phase diffusion displayed CPRTP with the polarization directions opposite to those of the aggregated sheets, featuring g_lum values of +6.0 × 10⁻³ (blue region) and +1.0 × 10⁻³ (yellow-green region), along with an RTP lifetime of 41.7 ms at 541 nm. Mechanical grinding disrupted the crystalline structure, eliminating both the RTP and CPL signals, confirming the crystallization-induced nature of CPRTP. This study presents a simple strategy for constructing CPRTP materials through aggregation-induced chirality transfer, offering new insights for the design of chiral luminescent materials with dual RTP and CPL functionalities.

Conflicts of interest

There are no conflicts to declare.

Data availability

All supporting data are included in the article and its SI.

Detailed experimental sections, NMR spectroscopy, and comprehensive information on thin film preparation. See DOI: https://doi.org/10.1039/d5qm00447k

Acknowledgements

We acknowledge financial support from the National Natural Science Foundation of China (22175069), and the Fundamental Research Funds for the Central Universities (CCNU25ZZ281). The authors gratefully acknowledge the technical contributions of Dr. Zhengping Fang with structural analysis and characterization.

Notes and references

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