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. 2025 Nov 12;10(46):56411–56417. doi: 10.1021/acsomega.5c08440

Synthesis and Media-Dependent Photochromic Properties of Diarylindene Derivatives

Baojun Shen †, Tao Ou ‡, Lingxu Kong §, Changsen Song †, Zhonglei Qiu §, Wei Li †, Ruiji Li †,*
PMCID: PMC12658646  PMID: 41322516

Abstract

We herein report photochromic 1,1′-(1,1-dimethyl-1H-indene-2,3-diyl)­bis­(2-methylnaphthalene) (1o), 3,3′-(1,1-dimethyl-1H-indene-2,3-diyl)­bis­(2-methyl-5-phenylthiophene) (2o), and 4,4′-(1,1-dimethyl-1H-indene-2,3-diyl)­bis­(5-methyl-2-phenylthiazole) (3o) possessing 1,1-dimethyl-1H-indene as the ethene bridge of diarylethene. All three compounds were synthesized from 2,3-dibromo-1,1-dimethyl-1H-indene and characterized by NMR and HRMS. Investigation of their photochromism revealed that compound 1o exhibits photochromic behavior exclusively in solution. In contrast, compounds 2o and 3o undergo photoinduced cyclization reactions in solution, PMMA films and neat films. All three compounds demonstrate relatively high photocyclization quantum yields and excellent fatigue resistance in solution. Additionally, compounds 2o and 3o also exhibit high durability in PMMA films upon repeated coloration/bleaching cycles without distinct photodegradation. Compared with numerous solution-exclusive photochromic compounds, compounds 2o and 3o hold potential for optoelectronic devices and enable solid-state modulation of other properties via photoinduced reversible molecular transformations.

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Introduction

The development of organic multifunctional materials represents one of the most compelling frontiers in contemporary chemistry and materials science. − Specifically, chemiluminescent materials, , fluorescent probes, , photochromic materials, and other related materials have garnered significant interest due to their unique properties. Within this domain, photochromic materials have garnered substantial research interest owing to their capacity for reversible optical transformations under specific light irradiation. − Over recent decades, numerous classes of organic photochromic compounds–including diarylethenes, − azobenzenes, − donor–acceptor Stenhouse adducts (DASAs), − spiropyrans, − and fulgides − –have been extensively synthesized and characterized. Among these, diarylethene derivatives stand out as particularly promising candidates, distinguished by their synthetic accessibility, efficient photoinduced isomerization reaction, high photocyclization quantum yields, and exceptional fatigue resistance. ,

Building upon approximately four decades of foundational research on diarylethenes, diverse ethene bridges–such as perfluorocyclopentene, − benzo­[b]­thiophene, , phenylthiazole, − maleimide, , and related structures , –have been strategically employed in molecular design. Critically, heteroatom-containing ethene bridges facilitate intramolecular noncovalent interactions between the central ethene unit and peripheral aryl groups. These interactions are pivotal for preorganizing the photoreactive antiparallel conformation, thereby enhancing photochemical performance and yielding superior cyclization quantum yields in solution. Moreover, structural variations endow diarylethene compounds with unique propertiessuch as gated photochromism, fluorochromism, and fast T-type photochromismas well as enable the construction of functional derivatives like multibranched diarylethene systems. ,

Indenes − constitute a significant class of carbocyclic frameworks whose derivatives exhibit notable biological activities , –including antitumor, antihypercholesterolemic, antiallergic, anticonvulsant, herbicidal, fungicidal, and antimicrobial effects. Beyond pharmaceutical applications, indene scaffolds have also demonstrated utility in materials science contexts. , However, to the best of our knowledge, only a single precedent exists for incorporating an indene unit within photochromic diarylethene systems, where it served exclusively as a peripheral side-chain substituent. Notably, no studies have explored diarylethene architectures utilizing this conformationally rigid moiety as the central ethene bridge, despite its structurally critical role in photoisomerization. The absence of such designs is particularly surprising given indene’s demonstrated potential for achieving high-performance photochromic behavior through steric and electronic modulation.

Addressing this unexplored chemical space, we designed and synthesized three novel diarylindene derivatives (1o–3o) utilizing 1,1-dimethyl-1H-indene as the central ethene bridge. The photochromic behavior of these compounds was systematically investigated through comprehensive analytical methods. Remarkably, these compounds exhibit efficient photochromism not only in solution and poly­(methyl methacrylate) (PMMA) films but also in the solid state. This rare solid-state photochromism demonstrates significant potential for controlling optoelectronic properties within condensed phases, thereby broadening their applicability in advanced devices such as optical memory systems, light-driven switches, and next-generation information storage technologies.

Experimental Section

General Information

Chemicals were purchased from Sun Chemical Technology (Shanghai) Co., Ltd., Bide Pharmatech Ltd., or Tokyo Chemical Industry Co., Ltd. (TCI) and used as received without further purification. Silica gel column chromatography was performed using Silica gel (pH 6.0–7.0, particle size 200–300 mesh or pH 6.0–7.0, particle size 300–400 mesh). 1H NMR and 13C NMR spectra were measured with Bruker Avance NEO 400 M NMR (400 MHz) spectrometers. High–resolution mass spectrometry (HRMS) analyses were performed with mass spectrometers (Xevo G2-XS QTof). Single–crystal diffraction data were collected using a Rigaku XtaLAB mini II diffractometer with Mo Kα radiation.

Synthesis

Three target diarylethene compounds 1o–3o featuring a 1,1-dimethyl-1H-indene unit as the ethene bridge were synthesized as shown in Scheme . The newly developed diarylethene derivatives 1o–3o were characterized by 1H and 13C NMR spectroscopy, high-resolution mass spectrometry (HRMS), and X-ray crystallography.

1. Synthetic Route of Diarylindene Derivatives 1o–3o .

1

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Compounds 1o–3o were successfully prepared via Suzuki-Miyaura cross-coupling of 2,3-dibromo-1,1-dimethyl-1H-indene with respective boronic acid pinacol esters in moderate to good yields. All coupling reactions were carried out under identical experimental conditions, with tetrakis­(triphenylphosphine)­palladium(0) [Pd­(PPh3)4] employed as the catalyst and tripotassium phosphate (K3PO4) used as the base. Specifically, when 2,3-dibromo-1,1-dimethyl-1H-indene underwent cross-coupling with 4,4,5,5-tetramethyl-2-(2-methylnaphthalen-1-yl)-1,3,2-dioxaborolane, diarylethene 1o was obtained with a yield of 60%. Compounds 2o and 3o were synthesized following an analogous protocol, using their respective matched boronic acid pinacol esters as reactants, and their isolated yields were 71% and 59%, respectively. Prior to the cross-coupling reactions, both the 2,3-dibromo-1,1-dimethyl-1H-indene precursor and the series of boronic acid pinacol ester reagents , were synthesized in accordance with well-established procedures from the existing literature. Additionally, comprehensive details regarding the synthetic procedures for all target compounds (1o–3o) are provided in the Supporting Information.

Results and Discussion

Geometry Analysis

The Woodward–Hoffmann rules state that the photocyclization activity of diarylethene photoswitches is directly related to their geometric structures. Specifically, the 6π-conrotatory photocyclization reaction of diarylethenes has strict conformational requirements: it must adopt an antiparallel conformation, and the distance between the reactive carbons must be less than 0.4 nm; otherwise, the reaction cannot occur. To clarify the conformational preferences of diarylethene compounds 1o–3o, the study employed a combination of DFT calculations, 1H NMR spectroscopy, and X-ray crystallography for confirmation.

For density functional theory (DFT) calculations, the B3LYP functional with the 6–31+G­(d) basis set was employed using the Gaussian 6.1 suite (Figure ). The most stable optimized geometries of compounds 1o–3o all exhibit a photoreactive antiparallel conformation, with the two methyl groups on the reactive carbon atoms oriented in opposite directions. However, the distance between the reactive carbon atoms in compound 1o exceeds 0.4 nm, rendering it photochemically nonreactive. In contrast, this distance is less than 0.4 nm in both compounds 2o and 3o, enabling photochromic behavior in solution. Notably, close heteroatomic contacts in compound 3o (N60–H42:0.253 nm; N61–H34:0.266 nm) likely play a significant role in stabilizing its antiparallel conformation, further reducing the distance between the reactive carbon atoms to 0.366 nm. These intramolecular heteroatomic interactions in compound 3o may cause the hydrogen atoms of one methyl group to be situated in a lower magnetic field, thereby accounting for the larger difference in the 1H NMR chemical shifts of the indene methyl groupswhen comparing 3o (δ = 1.76 ppm, 1.59 ppm; Δ = 0.17 ppm) with compounds 1o (δ = 1.53 ppm, 1.47 ppm; Δ = 0.06 ppm) and 2o (δ = 1.95 ppm; Δ = 0 ppm). DFT calculations also suggest that the parallel (1o- p , 2o- p and 3o- p ) and antiparallel (1o- ap , 2o- ap and 3o- ap ) conformations of 1o–3o can interconvert freely at ambient temperature due to low energy barriers between them. Therefore, both conformations likely coexist in solution for these three compounds.

1.

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Optimized structures of the open form and closed form of 1–3 by DFT (B3LYP/6–31+G­(d)) calculations.

Meanwhile, the unsymmetrical molecular structures of compounds 1o–3o, along with the presence of four methyl group signals in their 1H NMR (Figures S1, S4 and S7) and 13C NMR spectra (Figures S2, S5 and S8), indicate that only one conformation (either antiparallel or parallel) of their corresponding open-form isomers exists in solution.

Single crystals of compound 1o suitable for X-ray diffraction were successfully obtained via slow evaporation of a hexane/dichloromethane mixture. In contrast to the theoretical calculations, the crystal structure of compound 1o adopts a parallel conformation, where the two methyl groups on the reactive carbons are oriented in the same direction (Figure and Table S1). This specific conformational preference may arise from significant steric hindrance between the two methyl groups on the indene bridge and the naphthalene side chains. This result suggests that compound 1o is unlikely to undergo photoinduced isomerization in the crystalline or solid state.

2.

2

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ORTEP representation of the molecular structures of 1o with thermal ellipsoids (50% probability).

Photochromism in Solution

Diarylethene compounds 1o–3o exhibited distinct photochromic behaviors under UV light irradiation in different media.

When irradiated with UV light, compounds 1o–3o underwent efficient photochromic reactions in solution, accompanied by significant absorption spectral and solution color changes (Figures and S11). During UV light irradiation, the UV–vis absorption spectra of compound 1o in n-hexane solution showed a gradual decrease in absorption at 290 nm, along with the emergence of new absorption bands ranging from 380 to 520 nm. The presence of an isosbestic point at 302 nm confirms the photoinduced isomerization, corresponding to the transformation from open isomer (1o) to closed isomer (1c). Concomitantly, the solution color changed from colorless to pale yellow. For compound 2o, its UV–vis absorption spectra in n-hexane solution exhibited a gradual decrease in absorption at 288 nm, with new absorption bands appearing at 365, 382, and 550 nm. An isosbestic point at 326 nm indicates the conversion from open (2o) to closed (2c) isomers via photoinduced isomerization, and the solution color shifted from colorless to purple. In the case of compound 3o, its UV–vis absorption spectra in n-hexane solution featured a gradual decrease in absorption at 293 nm, accompanied by new absorption bands at 346, 362, and 522 nm. Two isosbestic points (309 and 332 nm) confirm the photoinduced transformation from open (3o) to closed (3c) isomers, with the solution color changing from colorless to red.

3.

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(a) Photochromic reaction schemes of diarylethene compounds 1o–3o upon light irradiation. UV–vis absorption spectral changes of (b) 1o (c = 6 × 10–5 M), (c) 2o (c = 4 × 10–5 M) and (d) 3o (c = 5 × 10–5 M) in n-hexane upon UV light irradiation (λirr. = 254 nm; hand-held UV lamp) at room temperature (298 K).

The UV–vis absorption spectra and solution colors of all three compounds in n-hexane could be restored to their initial states upon irradiation with visible light (λirr. > 430 nm). Moreover, the characteristic UV–vis spectral changes and solution color transitions were reversibly switched by alternating irradiation with UV light (λirr. = 254 nm) and visible light (λirr. > 430 nm), not only in nonpolar solvents but also in polar solvents (Figure S11). These results indicate that the presence of two methyl groups on the indene bridge does not force the compounds into exclusively parallel conformations, nor does it suppress their photochromism in solution. Moreover, unlike its crystalline state, compound 1o exhibits a photoactive antiparallel conformation in solution and can undergo a photochromic reaction upon specific light irradiation. This is also a well-documented phenomenon for diarylethene compounds.

The presence of clear isosbestic points and reversible coloration-bleaching switching during repeated irradiation cycles indicates that the photochromic reactions of 1o–3o proceed as clean 1-to-1 transformations without significant side reactions. This high durability, reflecting their excellent fatigue resistance, was confirmed by subjecting each compound to at least 10 alternating cycles of UV and visible light irradiation (Figure S12). No detectable degradation or side-product formation was observed. The exceptional photostability of 1o–3o positions them as promising candidates for future molecular electronic devices.

Table summarized the optical properties of 1o–3o including the cyclization quantum yields. The photochemical quantum yields for photocyclization reactions of compounds 1o–3o in n-hexane solution were determined to be 0.48, 0.63, and 0.82, respectively. The significantly higher quantum yield of compound 3o in n-hexane is presumably attributed to intramolecular CH/N interactions between the indene-ethene bridge and thiazole side units, as confirmed by DFT calculations (Figure ). These interactions promote the adoption of an antiparallel conformation by 3o in both solution and crystalline states. Notably, the quantum yields of 1o–3o decrease gradually with increasing solvent polarity, a trend likely arising from twisted intramolecular charge transfer (TICT) effects.

1. Absorption Characteristics of Diarylethene Derivatives 1–3 in Solution.

      Φo‑c
Compds. λmax ε (M–1·cm–1) n-hexane MeCN MeOH
1o 285 nm 1.70 × 104 0.48 0.32 0.26
1c 455 nm -      
2o 290 nm 1.64 × 104 0.63 0.46 0.38
2c 550 nm -      
3o 308 nm 1.67 × 104 0.82 0.65 0.54
3c 522 nm -      
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Thermal Stability

The thermal stability of the closed isomers 1c–3c was evaluated in toluene solution under different temperature gradients. All closed isomers 1c–3c of these three compounds exhibited significant fading with increasing temperature, indicating a remarkably fast thermal back reactions for the closed isomers (Figure S13). The decay kinetics followed first-order bleaching kinetics. Using the Arrhenius plot, the activation energies (E a) for the cycloreversion of 1c, 2c and 3c were estimated to be 83, 67, and 45 kJ·mol–1, respectively. These activation energies are notably lower than those of typical terarylenes and diarylethenes. In particular, the frequency factors of 1c, 2c and 3c are 1.28 × 1010, 2.30 × 108 and 1.08 × 105 s–1, which are considerably larger than those of diarylethenes. The relatively small activation energies, large frequency factors and steric hindrance between methyl groups and side chains in the closed isomers likely contribute to the rapid bleaching of 1c, 2c and 3c, with their respective half-lifetimes (t 1/2) calculated to be 11.6, 1.26, and 0.32 h at 20 °C.

Photochromism in Films

We further investigated the photochromic properties of compounds 1o–3o in poly­(methyl methacrylate) (PMMA) films. Transparent, colorless films were prepared by drop-casting and drying a CHCl3 solution containing 1o–3o (12 mM) and PMMA (10 wt %) onto a quartz plate. In contrast to their behavior in solution, the PMMA film of 1o (1o/PMMA = ca. 5 wt %) exhibited no photochromism upon specific light irradiation (Figure a). This phenomenon is likely attributed to the presence of a photochemically inactive parallel conformation in the solid state, which is consistent with the X-ray crystallographic analysis.

4.

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UV–vis absorption spectral changes of (a) 1o, (b) 2o and (c) 3o in PMMA upon light irradiation.

However, the PMMA films of 2o (2o/PMMA = ca. 5 wt %) and 3o (3o/PMMA = ca. 5 wt %) displayed photochromic reactions similar to those observed in solution. Upon UV light irradiation, distinct absorption bands appeared at 576 nm for 2c (Figure b) and 531 nm for 3c (Figure c). These bands subsequently reverted to the initial states under visible light irradiation (Figure ). Compared to their behavior in solution, the maximum absorption wavelengths (λmax) of the colored closed forms in PMMA were red-shifted by approximately 25 nm for 2c and 10 nm for 3c, respectively. This red-shift could primarily be attributed to the restricted molecular motion, which enforces greater planarity and enhanced π-conjugation, thereby reducing the HOMO–LUMO bandgap. The durability of the films for 2o and 3o was confirmed by repeating the coloration and bleaching cycles 10 times without significant photodegradation (Figure S14). The photochromic activity of 2o and 3o in PMMA films indicates that both compounds adopt reactive antiparallel conformations in this matrix.

Based on the efficient photochromic behaviors of 2o and 3o in PMMA, photopatterning experiments were performed using their PMMA films. Both compounds exhibited photoresponsive color-changing film patterns (Figure S15).

Additionally, neat films of 2o and 3o, prepared by drop-casting their CHCl3 solutions, were also assessed (Figure S16). Both compounds showed changes in their UV–vis absorption spectra under UV light irradiation, accompanied by color transformations: from colorless to purple for 2o and from colorless to red for 3o. Similarly, the UV–vis absorption spectra and colors of the neat films could be restored upon visible light irradiation.

Compared with the numerous photochromic compounds that operate exclusively in solution, solid-state organic photochromic materials are of particular significance. Therefore, the development of compounds 2o and 3o can broaden the application scope of diarylethene compounds.

Conclusion

In conclusion, we herein synthesized novel diarylethene derivatives 1o–3o featuring an indene-ethene bridge and investigated their photochromic behaviors. All three compounds exhibited high photochromic efficiency in both polar and nonpolar solutions, with conventional molar absorption coefficients. The comparatively large quantum yield of the open–to–closed photoreaction for compound 3 (Φo‑c = 0.82) in n-hexane relative to 2 and 3 may originate from intramolecular heteroatomic interactions. The relatively small activation energies and large frequency factors may contribute to rapid bleaching of 1c–3c, yielding half-lifetimes (t 1/2) of 11.6, 1.26, and 0.32 h at 20 °C, respectively. Compounds 2 and 3 additionally demonstrated photochromism in PMMA films and solid states, exhibiting reversible absorption spectral and color changes. The relatively high photosensitivity in different media, elevated solution-phase quantum yields, and low thermal stability of these compounds highlight their promising candidate characteristics for optoelectronic applications, directly attributable to their tailored molecular structures.

Supplementary Material

ao5c08440_si_001.pdf (1.4MB, pdf)
ao5c08440_si_002.cif (654.3KB, cif)

Acknowledgments

This research was funded by Shandong Provincial Natural Science Foundation, grant number ZR2021QB124, Outstanding Youth Foundation (Overseas) Project of Rizhao City Natural Science Foundation, grant number RZ2021ZR1.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08440.

  • Synthesis of 1o–3o and additional experimental data (Figures S1–S16) (PDF)

  • X-ray crystal analysis of 1o (CIF)

∥.

B.S. and T.O. contributed equally to this work. B.S. and T.O.: validation, methodology, investigation, writingoriginal draft preparation; L.K.: validation, investigation; C.S., Z.Q. and W.L.: investigation; R.L.: conceptualization, methodology, validation, investigation, writingoriginal draft preparation, writingreview and editing, supervision, funding.

The authors declare no competing financial interest.

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