Tuning the photochromism of indeno-fused 2H-naphthopyrans using steric spirocyclic groups†

Ruiqi Wei^a, Ruiyuan Zhou^b, Ripei Shen^b and Jie Han*^ab
aCollege of Chemistry and Environmental Science, Kashi University, Kashi 844008, P. R. China. E-mail: hanjie@nankai.edu.cn
^bKey Laboratory of Advanced Energy Material Chemistry, College of Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, P. R. China

First published on 7th August 2024

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Introduction

Photochromic 2H-naphthopyrans have been widely used in ophthalmic lenses,¹ intelligent textiles,² anti-counterfeiting,³ and, more recently, dye-sensitized solar cells⁴ due to the advantages such as high sensitivity to sunlight and excellent fatigue resistance, and drawn continuous research interest.⁵ In addition, the photophysical properties of such photo-functional materials, especially the photochromic mechanism⁶ and the corresponding structure–property relationship,⁷ are still very active scientific issues to be explored. 2,2-Bis(4-methoxyphenyl)-2H-benzo[h]chromene (NP) is a typical 2H-naphthopyran derivative and taken as an example to describe the photochromic mechanism of 2H-naphthopyrans. As shown in Fig. 1a, the closed form (CF) of NP undergoes UV-induced photoisomerization, resulting in the generation of two colored photomerocyanine isomers: the transoid-cis (TC) form and the transoid-trans (TT) form.⁸ The TC form rapidly reverts to the original CF under thermal drive, while the thermal conversion from TT to CF is significantly slower. This is attributed to the stability of the TT form, which necessitates overcoming a relatively large potential energy barrier to isomerize into the TC form. Thus, the thermal fading step often follows a biexponential kinetics with two different rate constants, meaning that there is initially a fast TC decay followed by a slow TT decay. The slow thermal fading rate has seriously limited the practical applications of 2H-naphthopyrans and become a challenging issue.⁹ Several effective strategies have been proposed to accelerate the thermal fading rate of 2H-naphthopyrans, which was well summarized in a review paper.¹⁰ The main recent progress in developing naphthopyrans with a fast thermal fating rate is listed as follows. In 2017, Abe et al. found a new method to substantially reduce the amount of the undesirable long-lived colored TT form by the C–H⋯O intramolecular hydrogen bonding between the oxygen atom of the alkoxy group and the olefinic proton in the TC form of the azino-fused chromenes,¹¹ and this method can also be used for naphthopyran derivatives.¹² In 2021, our research group synthesized a new photochromic supramolecular system containing both the pillar[5]arene subunit and naphthopyran moiety, and found that the fading rate can be effectively tuned by the supramolecular host–guest interactions of the pillar[5]arene/nitrile molecular recognition motif.¹³ The thermal fading rate of naphthopyrans could also be controlled through restriction of the double bond isomerization in the TT form. Recently, Coelho et al. constructed a series of polycyclic fused naphthopyrans to prevent the formation of the TT isomer and thus to suppress the undesired residual color.¹⁴ The introduction of fused rings to the naphthopyran skeleton is another important strategy to improve the photochromic performance, and the indene-fused ring is the most often used among various fused rings.¹⁵ The fluorene moiety incorporates a methylene bridge connecting two benzene rings and keeps a planar orientation, thereby enhancing their orbital overlap and electronic conjugation. Such a structural feature may significantly mitigate the limitations commonly associated with traditional color-enhancing heteroatoms and also imparts relatively high thermal and photochemical stability to the molecule.¹⁶ In addition, the steric spiro-annulated fluorene ring could control the fading rate of 2H-naphthopyrans to 14 seconds with ideal absorption intensity.¹⁷

As stated above, how to tune the decoloration speed of 2H-naphthopyrans within about 10 seconds while maintaining good coloring ability has become a challenging issue and drawn continuous attention, but it still remains unsolved to date. Herein, two spirocyclic indeno-fused 2H-naphthopyrans NP-a and NP-b, shown in Fig. 1b, have been synthesized and their photochromic properties have been investigated in detail. Compared to the parent compound 2,2-bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran (NP), both NP-a and NP-b exhibit dramatically fast thermal fading rates and higher colorability. The effect of the steric spirocyclic unit on photochromic properties has been discussed briefly in this work.

Results and discussion

Synthesis and characterization

The synthetic route to NP-a is shown in Scheme 1a. The starting compound 5-hydroxy-7H-benzo[c]fluoren-7-one 1 was synthesized according the procedure reported in the literature.¹⁷ The nucleophilic substitution reaction between 1 and iodomethane provided 2 in high yield, which was reacted with triphenylvinyl magnesium bromide – prepared in situ from bromotriphenylethylene and magnesium – to yield 3. Compound 4 was synthesized through a stannous chloride-catalyzed cyclization reaction of 3, followed by deprotection using tribromo borane to obtain 5 in good yield. Then, 5 underwent a classic coupling reaction with propargyl alcohol, providing the final product NP-a in 54% yield. For comparison, compound NP-b is synthesized by a three-step reaction route with 4 as the starting material (Scheme 1b). The catalytic oxidation and cyclization reaction of 4 to form 6 is the key step, which is carried out using potassium iodide under 365 nm UV irradiation. Initially, we have tried to obtain NP-b from NP-a under the same reaction conditions. However, we failed because the physical behaviors of NP-a and NP-b are so similar that it is difficult to separate and purify them.

All the intermediate compounds and the final products were characterized by means of solution proton nuclear magnetic resonance (¹H NMR), carbon-13 nuclear magnetic resonance (¹³C NMR) (Fig. S1–S18, ESI†), and high-resolution mass spectrometry (HRMS) (Fig. S19–S25, ESI†). The ¹H NMR spectrum of NP-a is depicted in Fig. 2. The coupling constant (J = 9.8 Hz) between Ha and Hb provides evidence for the formation of naphthopyrans. Due to the deshielding effect of the spirocyclic ring, the chemical shift of Hb noticeably moves towards a lower field, whereas the effect on the chemical shift of Ha is relatively slight. The molecular structure of NP-a was further confirmed by single-crystal X-ray diffraction¹⁸ (Scheme 1 and Table S1, ESI†).

Photochromism of NP, NP-a and NP-b in solution

The absorbance at the photostationary state (PSS) is a critical factor in evaluating the performance of photochromic materials. The optical density and absorption wavelength of these dyes may vary with different solvents and concentrations. In the visible region, the maximum absorption wavelengths of NP-a in several common solvents such as tetrahydrofuran, acetonitrile, acetone, chloroform, and toluene were observed to be approximately 551–562 nm at a concentration of 5 × 10⁻⁵ mol L⁻¹ (Fig. 3a and Table S1, ESI†), when exposed to UV light (365 nm, 260 mW) to achieve the PSS. Notably, both absorbance and molar absorptivity were the highest in chloroform among the above solvents. Thus, chloroform was chosen as the optimal solvent for the following UV testing. Then, the effect of the concentration of NP-a in chloroform on the absorption was investigated (Fig. S26, ESI†), and 8 × 10⁻⁵ mol L⁻¹ was chosen as the optimal concentration for the following UV testing.

The time-resolved absorption spectra of NP-a, NP-b and NP in chloroform (8 × 10⁻⁵ mol L⁻¹) at 298 K were investigated. Before UV irradiation, both NP-a and NP-b in chloroform solution exhibited a strong absorption in the UV region at around 350 nm, and nearly no absorption was observed in the visible region (Fig. 3b), indicating they all were colorless for NP-a or pale yellow for NP-b (Fig. S27, ESI†). In contrast, the spectra of NP showed a relatively strong absorption in the visible region due to the partial ring opening during the experimental process, and the solution showed a pale red color consequently. After exposure to UV light, all of these solutions exhibited a strong broad absorption band in the visible region of the spectrum, with the maximum absorption peak at 491 nm for NP and 560 nm for NP-a and NP-b (Fig. 3c), suggesting the formation of coloured TC and TT. Both NP-a and NP-b have similar maximum absorption wavelengths and exhibit a redshift of 70 nm compared to NP, and the absorption intensity is increased significantly due to the extended conjugated structure of the indene in the naphthopyran skeleton.

The solution of NP-a in chloroform (8 × 10⁻⁵ mol L⁻¹) was irradiated with a 365 nm UV lamp (260 mW cm⁻²) for different time periods of 2, 4, 6, 8, 10, 12, 14, 16 s, and so on, respectively, then the UV-vis spectra were recorded immediately until the absorbance values of the solutions basically ceased to change. The maximum absorbance of each solution was fitted to the light exposure time by nonlinearity to afford the fit curve of photochromic response. It is noted that the fit curve is not a straight line as the photochromic reactions do not conform to a first order reaction, which may explain reasonably the inflection points on the curve. As shown in Fig. 3d, NP-a displayed a quick photochromic process, then gradually reached the PSS in the absorbance at λ_max after UV light exposure of about 18 seconds, meaning the photo-responsive speed is quite fast. As expected, the photo-responsive speed of NP-b is almost the same as that of NP-a, as they have similar molecular structures. In contrast, NP showed a quite slow photo-responsive speed as shown in Fig. S28 (ESI†).

The thermal decay curves of the transient absorbance at the maximum absorption wavelength (λ_max) encompass the isomerization processes of TC to CF and TT to CF, meaning an initial rapid decay and a subsequent slow decay. The thermal decoloration of naphthopyrans can be described using a double exponential equation: f(t) = A₁e^−k₁t + A₂e^−k₂t + A_th.¹⁹ In this equation, f(t) represents the optical density at λ_max of the opened forms, A₁ and A₂ are the contributions to the initial optical density, and A_th is the residual coloration at the end of the testing period. The parameters k₁ and k₂ are the kinetic constants. The t_1/2 value, which is the time taken for the sample to fade to half of the initial absorbance value, is used to compare overall kinetics.

The solution of NP-a, NP-b, or NP in chloroform (8 × 10⁻⁵ mol L⁻¹) was irradiated with a 365 nm UV lamp (260 mW cm⁻²) until the absorbance value of the solution basically ceased to change. Then, the UV-visible absorption spectra were measured at 10 s intervals in a light-avoiding environment, and the fading processes of the photochromic compounds were measured until the absorbance of the solution no longer changed. The curves for the fading process were fitted by nonlinearity using the maximum absorbance versus the fading time, and a double exponential equation was afforded. The constants A₁ and A₂, and the kinetics parameters k₁ and k₂ were derived from the double exponential equation (Fig. S29, ESI†). Table 1 summarizes the spectral and kinetic characteristics of NP-a, NP-b and NP. Compared to NP (λ_max = 491 nm, t_1/2 > 500 s),^5a both NP-a and NP-b fade more rapidly due to the introduction of the indeno-fused spirocyclic group. NP-a shows a thermal fating rate with a half-time of 16 seconds, while the half-time of NP-b is further reduced to 11 seconds. The kinetic constants k₁ and k₂ represent the rate of fading; the larger the k₁ and k₂, the faster the rate of fading, which is consistent with the half-time. In addition, the higher the relative content of A_th, the greater the residual color. Overall, the A_th content for NP-a and NP-b is much lower than that for NP. Therefore, incorporating a steric spirocyclic group into the naphthopyran system proves to be an effective strategy to control the thermal back-reaction rate with low residual color.

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Photochromism in solid film

Usually, due to the compact stacking and limited free volume, naphthopyrans are unable to exhibit photochromism in the solid state. To enhance the photochromic behavior of solid-state materials for daily use, naphthopyran molecules are often added to polymer matrices to increase the required free volume.²⁰ Herein, we prepared PMMA films doped with NP-a to investigate the photochromic properties. The absorption spectra of the PMMA film doped with NP-a are illustrated in Fig. 4a. There is no absorption in the visible region in the spectra of the colorless film before 365 nm UV light irradiation. In contrast, the film changed to purple after 120 seconds of 365 nm UV irradiation, and a strong broad absorption band appeared between 400 and 700 nm in the absorption spectra due to the formation of two colored photomerocyanine isomers. The time to reach PSS for the PMMA film doped with NP-a is about 110 s (Fig. 4b), which is much longer that of NP-a in solution. The thermal fading rate of NP-a in the PMMA film is also slightly lower compared to that in solution (Fig. 4c). Both the slower photo-responsive rate and thermal fading rate are due to the restricted conformational changes of the photochromic compound within the rigid polymer matrices. For practical applications, it is often necessary for photochromic materials to exhibit excellent fatigue resistance. After 10 cycles of UV irradiation and exposure to darkness, there is almost no noticeable loss in absorbance for the photochromic PMMA film of NP-a, suggesting that the film shows high fatigue resistance (Fig. 4d). As shown in Fig. S30 and S31f (ESI†), the photochromic properties of the PMMA film doped with NP-b are similar to those of the PMMA film doped with NP-a. The ideal photochromic properties of NP-a and NP-b make them desirable candidates for optical information storage, photo-switching lenses and smart windows.

The photochromic film doped with NP-a was taken as an example to demonstrate the application of the products in optical information storage. A plastic plate with a hollow pattern was placed above the film and irradiated with ultraviolet light at 365 nm (260 mW) for 120 seconds, and the areas protected by the plate remain clear, while the exposed areas turn purple (Fig. 5). Thus, the same pattern as the hollow part of the plastic plate can be accurately inscribed on the film, and the writing process is finished. When the film is shaded for 10 minutes, the pattern printed on it will gradually disappear and return to the original colorless state, which is the erasing process. The film can accurately replicate the pattern from the plastic plate, allowing for easy identification with the naked eye. Additionally, the photochromic PMMA film can undergo multiple cycles of writing and erasing information without noticeable fatigue.

Fluorescent properties of NP-a

As both NP-a and NP-b have a fluorescent fluorene unit in the molecular naphthopyran skeleton, they may display fluorescent properties. Herein, NP-a was chosen to briefly investigate the fluorescent properties. As shown in Fig. 6a, the solution of NP-a in chloroform (8 × 10⁻⁵ mol L⁻¹) exhibited an excitation spectrum with the maximum wavelength of 337 nm and displayed a strong emission spectrum with a maximum wavelength of 410 nm (Fig. 6a). In contrast, upon irradiation with UV light (365 nm, 260 nm) for 18 seconds, the as-formed colored solution did not display fluorescent behaviour as expected. The UV-vis absorption spectra of the open forms of NP-a are overlapped well with the emission spectra of the closed form of NP-a, and the fluorescence was quenched consequently. Thus, the fluorescence can be tuned by UV light, and NP-a might function as a potential fluorescent switch. Similar results were also reported in the literature.^3c After 10 cycles of alternate UV irradiation to achieve the PSS and then exposure to darkness to restore the closed form, there was almost no noticeable loss in the fluorescence intensity of NP-a (Fig. 6b), also suggesting that this compound exhibited excellent fatigue resistance.

Experimental

General information on the synthesis, characterizations and methods

5-Hydroxy-7H-benzo[c]fluoren-7-one^15a and 1,1-bis(4-methoxyphenyl)-2-propyn-1-ol²¹ were prepared according to the procedures in the literature. The photochromic composite film was prepared according to a similar procedure reported in the literature.^9b,22 All the other reagents were purchased commercially and used without further purification. All air- and moisture-sensitive manipulations were carried out under a nitrogen atmosphere with a standard Schlenk technique. Column chromatography was performed by using 200–300 mesh silica gels.

NMR spectra were recorded with a Bruker Avance III HD 400 spectrometer. MS spectra were recorded using a Varian 7.0 T FTMS. Single crystal X-ray diffraction data were obtained using a Super Nova single crystal diffractometer equipped with graphite-monochromated Cu-K_α radiation (λ = 1.54184 Å). UV-vis spectra were recorded using an Analytik Jena Specord 210 Plus UV-vis spectrophotometer. The UV light source was a UV-LED (UV-400, Keyence) equipped with a UV-L6 lens unit (365 nm, 260 mW). Fluorescence spectra were recorded using an Edinburgh-FS5 spectrofluorometer (Edinburgh Instruments). The melting points were collected using X-4 melting point apparatus (Beijing Tech Instrument Co., Ltd).

All absorption spectra of photochromic films were measured by means of the transmission method using the PMMA film of the undoped sample as a reference, which was prepared in the same way as that for the photochromic film. For the fatigue resistance test, the film was irradiated with 365 nm UV light to reach the maximum absorption (Max-Abs) value, then the film was placed in the dark for 45 min to make it fade to colourless, and the above test is repeated for 10 cycles. For the time-dependent thermal bleaching curve test, all films are irradiated with 365 nm UV light until the absorbance value of the film basically ceased to change. Then, the UV-visible absorption spectra were recorded immediately at a 10 s interval within a period in a light-avoiding environment. A curve was fitted by nonlinearity using the maximum absorbance of the film doped with NPs versus the fading time. All tests were carried out at 298 K.

Conclusions

A new type of photochromic indeno-fused 2H-naphthopyrans with a steric spirocyclic moiety have been devised and synthesized in this work, and the photochromism, especially the thermal fading rate, has been investigated in detail. The results demonstrate that the spirocyclic indeno-fused moiety can effectively suppress the TT form and tune the thermal fading rate with a half-life (t_1/2) of about 10 seconds. The introduction of a fused indene ring may expand the conjugated system and increase the molar extinction coefficient of the products. Moreover, the target compound also exhibits fluorescent properties with excellent fatigue resistance. Furthermore, the photochromic PMMA film displayed the ability to undergo multiple cycles of information writing and erasing without apparent fatigue, suggesting potential applications in photochromic lenses, smart windows, and dynamic holographic materials.

Author contributions

J. Han designed the project. R. Wei performed the experiments. R. Wei and J. Han wrote the original manuscript. J. Han revised and finalized the manuscript. All authors discussed the results and contributed to the data interpretation.

Data availability

All data obtained during the study are presented in the submitted article and the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the Innovation Program for Novel Photochromic Materials from Tianjin Forsheen Optical Ltd (No. F1035631).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Full experimental details, characterization data including copies of NMR and HRMS spectroscopy of the products, UV-vis absorption spectroscopy, and single X-ray diffraction data. CCDC 2360939. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj02706j

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