Photochromism and photofluorochromism of arylvinylene phenanthridines

Abstract

Novel phenanthridine-based light-induced, multicolour photochromic π-conjugated compounds (PC₁–PC₃) have been developed. PC₁–PC₃ were synthesised via Knoevenagel condensation on phenanthridine using various aromatic aldehydes [triphenylaminyl (TPA) and pyrenyl (PY)] in the presence of benzoic acid and benzoic anhydride in 63–70% yields. The compounds absorb strongly in the visible region (λ_max = 360–408 nm for PC₁, λ_max = 360 nm for PC₂, λ_max = 385 nm for PC₃). Interestingly, the solutions of these compounds undergo photo-transformation (under 254 nm UV light) with a colour change from yellow to purple, orange, and brown, respectively, for PC₁–PC₃, accompanied by a redshift of 120–140 nm in the absorption maxima. This transformation is rapid (within 60 seconds) and reverts to its original state by heating the solution at 35 °C for 30 minutes. In addition, PC₁–PC₃ also exhibit variations in fluorescence, with PC₁ and PC₂ red-shifting by 150 nm (yellow to red emission) and PC₃ undergoing complete quenching. We attribute this transformation to a photoexcited partial proton transfer, leading to a metastable hydrogen bonding state, as supported by ¹H NMR and DFT calculations.

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Introduction

Chromic materials are smart materials that undergo a reversible change in response to heat, light, electricity, chemicals, and mechanical force. As a result, they alter the optical properties; these materials are widely used in sensors, displays, and smart coatings, among other applications.^1–8 Among the various chromic materials, photochromic systems offer significant advantages due to activation by light, which is a non-invasive, environmentally friendly, and sustainable energy source.⁹ These systems exhibit reversible colour changes upon UV or visible light exposure. This phenomenon occurs due to the structural transformation between two forms, resulting from a change in the optical properties of the compounds.¹⁰ The photochromic materials have diverse applications in optical and photonics,^11,12 display technology,¹³ data storage,^14,15 biological sensing,¹⁶ smart materials,^17,18 etc. Photochromism occurs in both inorganic and organic materials. Inorganic photochromic materials, such as transition metal oxides (WO₃, TiO₂, V₂O₅, Nb₂O₅, and MoO₃), have gained attention in the photochromic field due to their high thermal stability, long cycling life, and thermal/chemical resistance. However, these materials suffer from tunability (difficult to modify structure) and a complex synthetic process.^19–22 In contrast, organic photochromic materials exhibit outstanding structural flexibility and can be easily modified, making them more versatile for specific applications.^23–27 Several organic photoswitch derivatives have been reported. Among them, azobenzene, which undergoes cis–trans isomerisation, and diarylethene and spiropyran, which undergo ring-opening/closing upon shining light, are highly explored.^14,28–30 On the other hand, photochromic systems that undergo reversible colour changes based on changes in intramolecular hydrogen bonding interactions have found significant interest due to their high rate of conversion, precise colour tuning, and enhanced stability (Chart 1).

Chart 1 Reported photochromic systems based on hydrogen-bonding.

Ivan Ap et al.³¹ reported the quinoline-substituted hydrazone (E) photoswitch in the solution state; after shining the 442 nm light, the Z isomer converted to an E isomer with a colour change from yellow to colourless, and the Z isomer completely reversed back to Z after shining the 340 nm light. Newhouse et. al.³² and Castellano et al.³³ developed hemithioindigo- or dicyanorhodanine-pyrrole-based photoswitches, where upon shining the UV-light (405 nm), the system converts from Z to E with a considerable redshift in the absorption spectra from 503/436 nm to 567/466 nm. The E-isomer develops an intramolecular H-bonding pyrrole N–H with the adjacent carbonyl group. Besides, excited-state intramolecular hydrogen bonding (ESIPT) inspired photochromism has also been studied. For example, Tang et al.,³⁴ Yoon et al.³⁵ and Li et al.³⁶ developed salicylaldehyde-containing photoswitches that, upon photoexcitation, undergo enol to keto conversion, resulting in a change of colour and fluorescence. The keto form reverts to the enol form when exposed to visible light. Along similar lines, Bernard M. et al.,³⁷ reported a triaryl hydrazone-based ESIPT photoswitch. The compound under light turns the hydrazo (=N–NH–) to azo (–N=N–) with an associated colour change from yellow to red. Overall, to the best of our knowledge, there are only a few examples of H-bonding-promoted photochromic systems known.

Here we report phenanthridine-based H-bonding-based photochromic systems (PC₁–PC₃) for the first time. The compounds (PC₁–PC₃) have been developed through the Knoevenagel condensation of triphenylaminyl (TPA) and pyrinyl (PY) aldehydes with an active-methyl-containing phenanthridine (6 and 7) in yields of 63–70%. PC₁–PC₃ exhibit donor–acceptor interactions and, thus, absorb strongly in the visible region (λ_abs = 300–410 nm). PC₁–PC₃ display photochromic behaviour upon exposure to UV light, changing their colours from yellow to purple, orange, and brown, respectively, and reverting to their original state upon heating. Similarly, the fluorescence of PC₁–PC₃ has also been influenced by light exposure, which turns yellow to red for PC₁–PC₂, while it is quenched for PC₃. The photo-transformation has been attributed to photoexcited partial proton transfer leading to a metastable hydrogen bonding state upon light irradiation, as supported by ¹H NMR and DFT calculations.

Results and discussion

The photochromic phenanthridine systems were synthesised in several steps, as depicted in Scheme 1. The synthesis begins with the Suzuki–Miyaura coupling of 2-bromobenzene with aryl boronic acids (a and b), yielding 2-arylbenzenes (2 and 3). The subsequent N-acylation of the amine can be accomplished using acetyl chloride in dichloromethane to obtain compounds 4 and 5. Furthermore, 4 and 5 were subjected to a Bischler–Napieralski cyclization using POCl₃ under reflux conditions, rendering the key building blocks, 2-methyl-phenanthridine derivatives (6 and 7). Finally, the target compounds (PC₁, PC₂, and PC₃) have been achieved by performing a Knoevenagel condensation on 6 and 7 with triphenylaminyl (TPA) and pyrenyl (PY) aldehydes in the presence of benzoic acid and benzoic anhydride. The compounds have been obtained in moderate to good yields (63–70%) and characterised with NMR and HRMS.

Scheme 1 Synthesis of aryl-vinylene phenanthridine-based photoswitches; crystal structure of compound PC₁ (CCDC: 2351276): (a) top view and (b) side view.

Crystal structure and density functional theory

The crystal structure of PC₁ indicates that the entire molecular backbone adopts a planar conformation, which supports extended π-delocalisation. In addition, the imine-N is found to exert bonding interactions with the β-carbon of the vinylene (2.44 Å) and thus the substituents remain in the trans-conformation (Scheme 1a, b and Fig. S1 and Tables S1–S5). The compounds have also been studied by Density Functional Theory (DFT: B3LYP/6-311G(d) level of theory) to evaluate the geometry-optimised structures and electronic properties (Fig. 1). The optimized structures of PC₁–PC₃ exhibit a planar conformation, with the torsional angle between the vinyl and phenanthridine of 0.17°, matching closely that of the crystal structure of PC₁ (0.11°). The Highest Occupied Molecular Orbitals (HOMOs) of PC₁ and PC₂ are mainly localized on the aryl units due to their electron-donating nature. In contrast, the HOMO of PC₃ is delocalized across the entire molecule, due to the strong electron-donating diphenylamine unit connected to the phenanthridine core. On the other hand, the Lowest Unoccupied Molecular Orbital (LUMO) of PC₁ is mainly localised on the phenanthridine unit, whereas in PC₂ and PC₃, it is delocalized over the aryl and phenanthridine units. The HOMO and LUMO energies of PC₁, PC₂ and PC₃ have been found to be −5.13/−2.00, −5.37/−2.32 and −5.23/−2.27 eV, and the band gaps are 3.13, 3.05, and 2.96 eV, respectively. This indicates that the aryl substitution significantly affects the HOMO and LUMO energy levels: compared to PC₁, the LUMO energies of PC₂ and PC₃ are lowered by 0.3 eV and 0.2 eV, while the HOMO energy of PC₁ is higher than that of PC₂ and PC₃ by 0.2 eV and 0.1 eV, respectively. In addition, the band gap of PC₃ is found to be smaller than those of PC₁ and PC₂ by 0.04 eV and 0.1 eV, respectively, due to the strong donor–acceptor (D–A) interactions between the diphenylamine and phenanthridine units.

Fig. 1 DFT-optimised energy level diagram of the HOMOs and LUMOs of PC₁–PC₃ and their photo-transformed products .

Optical properties

The UV-Vis absorption spectra of PC₁, PC₂, and PC₃ in CHCl₃ solution exhibit one or two strong π → π* transitions in the visible region (300 to 410 nm) (Fig. 2) with absorption maxima at 308 and 410 nm (ε = 16 250 and 29 390 = M⁻¹ cm⁻¹) for PC₁, 364 nm (29 390 M⁻¹·cm⁻¹) for PC₂, and broad band 300–500 (29 390 M⁻¹ cm⁻¹) for PC₃. At the same time, a blue shift of 25–46 nm is observed for PC₂ compared to PC₁, due to strong donor–acceptor interaction, while PC₃ exhibits a broad absorption band. The broad absorption spectrum of PC₃ could be attributed to several signals resulting from (a) vibronic transitions of pyrene and (b) strong intramolecular charge transfer (ICT) transitions between triphenylamine (donor) and phenanthridine (acceptor). The ICT in PC₃ will be significantly higher due to greater π–π communication between the donor and acceptor.

Fig. 2 (a) Absorption spectra (CHCl₃, 1 × 10⁻⁵ M) and (b) emission spectra (CHCl₃, 1 × 10⁻⁶ M) of PC₁–PC₃.

Recording the absorption spectra of PC₁–PC₃ in different solvents resulted in no change in the peak positions (only a variation of 3–5 nm), indicating minimal influence of solvent polarity (Fig. S2). PC₁–PC₃ have been found to be fluorescent in the solution state. The chloroform solutions of PC₁–PC₃ display green, cyan and yellow fluorescence with emission maxima at 518 (fluorescence quantum yields, ϕ_f = 45%), 500 (ϕ_f = 15%) and 550 nm (ϕ_f = 55%), upon excitation (λ_ex) at 405 nm, 365 nm and 420 nm, respectively (Fig. 2b). The emission spectra in different solvents vary significantly, with a redshift of 90–100 nm in polar solvents, emphasising the strong D–A interactions (Fig. S3).

Photochemistry

PC₁–PC₃ exhibited photochromic properties upon irradiation with a 254 nm light source. All the compounds exhibit positive photochromism, displaying a bathochromic shift in their absorption bands (324 to 546 nm). Upon shining the light for 30 secs, the photo excited systems displayed new absorption bands in the red region: 324, 546 nm (ε = 19110, 36160 M⁻¹ cm⁻¹); 346, 512 nm (ε = 24670, 14710 M⁻¹ cm⁻¹) and 345, 515 nm (ε = 37820, 14850 M⁻¹ cm⁻¹), respectively (Fig. 3 and Fig. S4). When the light was irradiated at an interval of 5 seconds, the spectrum gradually changed from PC to PC* and reached a final state, characterised by two isosbestic points at 356 nm and 455 nm for PC₁, and the colour of the solution changed from yellow to purple (Fig. 3a and Fig. S4a). A similar observation was found in PC₂ and PC₃; PC₂ also shows two absorption bands at 284 and 364 nm, which disappear, and new bands at 346 and 512 nm appear, with isosbestic points at 306 and 396 nm, upon light irradiation. The colour of the solution also changes from light yellow to orange (Fig. 3b and Fig. S4b). PC₃ exhibits a broad absorption band from 300 to 500 nm, which, upon UV radiation, splits into three new bands at 278, 345, and 520 nm, accompanied by three isosbestic points at 280, 262, and 454 nm. The colour changed from yellow to brown (Fig. 3c and Fig. S4c). The significant redshift in the absorption can be attributed to the increased donor–acceptor interactions within the molecule upon photoexcitation. This could be possible if the photo-excited state promotes photoinduced partial proton transfer, leading to the formation of a phenanthridine carbocation. Such molecular transformation enhances the donor–acceptor interactions within the molecule, altering its optical properties (Fig. 3g). Photochromic kinetics studies of PC₁–PC₃ in the solution state have been conducted. The rate constant for the forward photochromic step in PC₁–PC₃ was found in the range of 3 × 10⁻² – 12 × 10⁻² sec⁻¹. The photochromic change was found to be faster in the case of PC₃, followed by PC₂ and PC₁, which is reflected in their rate constants. The quantum efficiency of the PC₁–PC₃ colouring process was found to be 16–27%. On the other hand, the rate constants for the thermal reversibility of PC₁–PC₃ are in the range of 10 × 10⁻² to 76 × 10⁻² min⁻¹ with the order of PC₂ > PC₃ > PC₁ (Fig. S37 and S38 and Table S6).

Fig. 3 Absorption spectra [(a)–(c) (CHCl₃, 1 × 10⁻⁵ M)]; emission spectra [(d)–(f) (CHCl₃, 1 × 10⁻⁶ M)] of PC₁–PC₃ (before irradiation (red line) and after irradiation at 254 nm (blue line); (g) schematic representation of photoexcited partial proton transfer forming a metastable hydrogen-bonded state.

Interestingly, we have also found that the fluorescence of the photo-transformed products of PC₁–PC₃ changes significantly upon light irradiation (Fig. 3d–f and Fig. S5). Upon light irradiation, the fluorescence signal of PC₁ shifted from 518 nm to 665 nm, exhibiting a redshift of 147 nm, accompanied by an increase in emission intensity. Moreover, the fluorescence colour has changed from green to orange-red (Fig. 3d and Fig. S5a). The same observation was found for PC₂, which showed a redshift of 154 nm in the emission maximum from 520 nm to 672 nm (Fig. 3e and Fig. S5b). On the other hand, the fluorescence of PC₃ (536 nm) was completely quenched after applying the UV light (Fig. 3f and Fig. S5c). The fluorescence quantum yields of light-irradiated samples and were increased to ϕ_f = 58% and 65%, respectively. We have also tested the photo-transformation of PC₁ under sunlight, which showed similar spectral and colour changes as the UV lamp; however, the complete conversion takes about 2 h (Fig. 5a). The reversible process of the photoexcited compounds can be triggered by heating the samples at 35 °C for 30 min (Fig. 4). Moreover, the system exhibits excellent stability with little degradation for PC₁ in absorbance over five cycles of photoexcitation and heating (Fig. 5b). The solid-state photochromic behaviour was also tested by dispersing PC₁ in a polymer matrix. The blend of PC₁ and PMMA (polymethyl methacrylate) exhibits photochromism when illuminated for 5 minutes, changing the colour (colourless to pink) and fluorescence (cyan to red) of the coated film. The process is also found to be reversible, as evidenced by heating the film at 50 °C for 10 minutes (Fig. 6a).

Fig. 4 Reversibility analysis of (a) PC₁, (b) PC₂, and (c) PC₃ after heating at 35 °C for 30 minutes.

Fig. 5 (a) Absorption spectra of compound PC₁ under sunlight, (b) photo-transformation cycles of PC₁ (1 × 10⁻⁵ M) in CHCl₃; the absorbance change at λ_max = 540 nm was monitored under irradiation at 254 nm and during heating at 35 °C; ¹H NMR spectra (400 MHz, CDCl₃) of compound PC₁ before (c) and after (d) exposure to UV light.

Fig. 6 (a) The usage of the PC₁ and PMMA blend in the application of anticounterfeiting and (b) the usage of PC₁ solution in chloroform and showing its application in signboards.

The performance of PC₁–PC₃ has been compared to the reported ESIPT systems (Table S6). PC₁–PC₃ are comparable to the reported systems in terms of stability, switching time, and reversibility. Moreover, PC₁–PC₃ possess several unique features, such as (a) tunability of the photochromic colours by changing the substituents; (b) tunable photofluorochromic behaviour, such as turn-on fluorescence along with emissive colour modulation for PC₁ and PC₂ and turn-off fluorescence for PC₃; and (c) large red shift of 154 nm in the emission signal, which helps to avoid the overlap of colours, and obtain pure colours.

Time-resolved fluorescence decay measurements of PC₁–PC₃ were carried out in chloroform (Fig. S6). Before irradiation, the fluorescence decay profiles of PC₁ and PC₃ were best fitted with a single exponential function, yielding lifetimes of τ = 1.18 ns (100%) for PC₁ and τ = 6.35 ns (100%) for PC₃. In contrast, PC₂ exhibited a biexponential decay with components τ₁ = 0.30 ns (97%) and τ₂ = 3.0 ns (3%). Upon photoirradiation, both PC₁ and PC₂ showed biexponential decay behaviour. The photo-irradiated PC₁ exhibited lifetimes of τ₁ = 1.2 ns (95.45%) and τ₂ = 5.0 ns (4.55%), while photo-irradiated PC₂ showed τ₁ = 1.5 ns (92.04%) and τ₂ = 5.0 ns (7.96%). The longer lifetimes of and indicate that these species possess stable charge–transfer states.

To understand the mechanism of the photo-transformation, PC₁ has been studied using ¹H NMR spectroscopy before and after photo-irradiation (Fig. 5c and d). Interestingly, the ¹H NMR spectra of PC₁, before and after light irradiation, match closely (with a variation of 0.01–0.02 ppm) with each other except for the two protons corresponding to the β-proton of vinylene and the j-proton of phenanthridine. These proton signals have been broadened and experienced a downfield shift from 8.06 ppm to 8.15 ppm for the β-proton of the vinylene and from 8.20 ppm to 8.30 ppm for the j-proton of the phenanthridine. The ¹H NMR studies suggest that PC₁, upon light irradiation, (1) undergoes minimal structural changes, indicating that there is no isomerisation; (2) the β-proton of vinylene and j-proton of phenanthridine possess a labile nature; (3) sharp signals indicate that there is no radical formation.

Furthermore, we have also conducted FT-IR and cyclic voltametric analysis on PC₁ and . In FT-IR, the stretching frequency of C=N in PC₁ changes from 1589 cm⁻¹ to 1575 cm⁻¹ for . On the other hand, in cyclic voltammetry, the oxidation potential of PC₁ shifts anodically from 1.06 V (vs. Ag/AgCl) to 1.23 V (vs. Ag/AgCl) for . FT-IR reveals that C=N is more polarised towards a higher single bond character, while cyclic voltammetry shows a reduction in the electron density on the nitrogen atom of the phenanthridine unit.

Based on the results obtained from the above experiments, we hypothesise that a light-induced hydrogen bonding interaction might exist between the β-proton of vinylene and the j-proton of phenanthridine with the phenanthridine nitrogen, leading to partial proton transfer to the phenanthridine moiety (Fig. 5c, d and Fig. 3g). Further studies are necessary to prove this hypothesis.

To validate our hypothesis, DFT-optimisation has been carried out on . The photoexcited systems exhibit no structural changes with respect to PC₁–PC₃, which supports the minimal changes observed in the ¹H NMR. The HOMO energy of compared to their pristine samples has been found to increase by 0.09–0.14 eV, while the LUMO energy decreases by 0.03–0.10 eV, resulting in a significant reduction in the band gaps (0.12–0.20 eV). , compared to PC₁–PC₃, have also been found to be less stable by 56.2, 55.2 and 55.3 Kcal/mol than the , and , respectively. These changes indicate the formation of a metastable state involving partial proton transfer upon exposure to light. This process enhances donor–acceptor (D–A) interactions, resulting in band gap reductions of 0.20 eV for PC₁, 0.17 eV for PC₂ and 0.12 eV for PC₃ (Fig. 1). The absorption spectra of PC₁–PC₃ and their photoexcited systems optimised using TD-DFT have also supported the experimental data (Fig. S7).

The solid-state photochromic behaviour of PC₁ can be used as an anticounterfeiting ink. A dichloromethane solution of a blend of PMMA and PC₁ was used to write the MRR lab word on a glass slide, which remained invisible after writing. Upon exposure to light, the PC₁ changed its colour to pink, and the word became visible. The process was found to be reversible upon heating the glass slide at 35 °C (Fig. 6a). Under long UV, the fluorescence of the word changed from cyan to red, which can be used for signboards. A similar behaviour was also observed in the solution (Fig. 6b).

Conclusion

In conclusion, three photochromic systems based on phenanthridine have been developed by condensing active methylene-substituted phenanthridine units (6 and 7) with several aromatic aldehydes [triphenylaminyl (TPA), and pyrenyl (PY)] in the presence of benzoic acid and benzoic anhydride. The compounds have been thoroughly characterised using ¹H NMR, HRMS, and single-crystal XRD (for PC₁). PC₁–PC₃ have been studied to exhibit photochromic behaviour under UV light (254 nm). The photo-transformation changes the absorption maxima of PC₁–PC₃ from 330–350 nm to 550–600 nm and the colour from yellow to purple/orange/brown. Moreover, PC₁–PC₃ also exhibit a fluorescence change, turning on red emission (λ_em = 665–675 nm). The photo-transformation has been characterised by UV-Vis absorption, fluorescence and ¹H NMR spectroscopy. The studies indicated that the compounds undergo a photoexcited partial proton transfer, leading to a metastable hydrogen bonding state.

Conflicts of interest

There are no conflicts to declare.

Data availability

The supporting data for this article have been included in the supplementary information (SI). Supplementary information contains synthetic experimental procedures, NMR spectral data, cyclic voltammograms, solvatochromic studies of UV-Vis absorption and fluorescence emission, crystal data, FT-IR, life time data. See DOI: https://doi.org/10.1039/d5tc02868j.

CCDC 2351276 contains the supplementary crystallographic data for this paper.³⁸

Acknowledgements

Rajeswara Rao thanks SERB, India, and IIT Dharwad for partially supporting this research through a Core Research Grant (CRG/2023/002129). The authors are grateful to the Sophisticated Central Instrumentation Facility (SCIF), IIT Dharwad, and all its staff members for letting them use the facilities and assisting them with the material characterisation studies.

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