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Visible-Light-Driven Two-Way Photoisomerization of 1-(1-Pyrenyl)-2-(2-Quinolyl) Ethylene in Neutral and Protonated Forms

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Visible-Light-Driven Two-Way Photoisomerization of 1-(1-Pyrenyl)-2-(2-Quinolyl) Ethylene in Neutral and Protonated Forms Photochemical & Photobiological Sciences View Article Online PAPER View Journal | View Issue Visible-light-driven two-way photoisomerization of 1-(1-pyrenyl)-2-(2-quinolyl)ethylene in neutral Cite this: Photochem. Photobiol. Sci., 2018, 17,213 and protonated forms† Mikhail F. Budyka * and Vitalii M. Li Diarylethylenes with large π-systems often lose their photochemical activity (the size effect). 1-(1- Pyrenyl)-2-(2-quinolyl)ethylene (1P2QE), despite having a large conjugated π-system of 28 electrons, undergoes two-way reversible trans–cis photoisomerization both in the neutral and protonated forms with quantum yields as high as 0.13–0.83. For the neutral 1P2QE, experimental data and quantum-chemi- Received 26th September 2017, cal calculations indicate a diabatic (nonadiabatic) reaction mechanism. Due to high photoisomerization Accepted 8th December 2017 quantum yields and the long-wavelength absorption band at 340–460 nm and 390–560 nm for the DOI: 10.1039/c7pp00359e neutral and protonated compounds, respectively, 1P2QE can be used as a molecular photoswitch that is rsc.li/pps sensitive to visible light. Creative Commons Attribution 3.0 Unported Licence. 1. Introduction region is more suitable, since it reduces the damage from UV irradiation of the material.9 Therefore, the possibility of Photoisomerization is one of the fundamental reactions in switching the photochrome using visible light has attracted – photochemistry. Photoisomerization of retinal is a primary increasing attention.10 13 photochemical step in the signal transduction cascade of There are several mechanisms of spectral tuning. In nature, vision.1,2 Photoisomerization of stilbene and azobenzene the spectral sensitivity of the retinal photochrome is tuned This article is licensed under a derivatives, which are the most widespread photochromes, is a over a broad wavelength range by environmental variation functioning mechanism of many photonic molecular switches since the retinal absorption spectrum depends on interaction – and logic gates.3 6 For example, by photoisomerization of with the opsin protein, to which retinal is bound (on the different diarylethylenes (DAEs), it is possible to perform the polarity of the retinal binding pocket of rhodopsin, on proxi- Open Access Article. Published on 08 December 2017. Downloaded 9/27/2021 12:56:19 AM. functions of different molecular logic gates.7,8 mity of the counter ion).1,2 Owing to this dependence, the A molecular photonic switch is defined as a compound that same retinal photochrome provides color vision across the can reversibly transform between two (or more) states under visible spectrum. light irradiation. There are two main parameters that charac- In azobenzene-based photoswitches, the spectrum is tuned terize the photochemical properties of a photonic switch: the by introducing electron-donating and/or electron-withdrawing region of spectral sensitivity and the photoisomerization substituents, which can significantly affect the π–π* and n–π* quantum yield. Spectral sensitivity is defined by the absorp- absorption bands (and simultaneously, the thermal stability).9 tion spectrum of a photochrome that is used as a photonic The most general mechanism of spectral tuning, which is switch. Commonly, photoswitches, being rather small mole- used for designing visible-light photoswitches, is a decrease in cules, absorb light in the UV region (at least, one of the photo- the photochrome HOMO–LUMO (highest occupied – lowest chrome isomers); however, for many applications, the visible unoccupied molecular orbital) gap that is achieved by extend- ing the conjugated π-system of the photochrome.9 A smaller HOMO–LUMO gap produces a red-shifted absorption spec- Institute of Problems of Chemical Physics, Russian Academy of Sciences, trum. However, photochrome, with a large π-system, often pr. Akademika Semenova 1, Chernogolovka, Moscow region, 142432, Russian Federation. E-mail: [email protected] loses photochemical activity that prevents its exploitation as a †Electronic supplementary information (ESI) available: Synthetic details, experi- photoswitch despite possessing the necessary absorption mental and calculated absorption spectra, fluorescence decay profile, depen- spectrum. dence of fluorescence spectra on excitation wavelength, principal component For example, on going from stilbene to 2-styrylnaphthalene analysis, transition energies, nature and oscillator strengths of the lowest excited (SN), the DAE π-system increases from 14 to 18 π-electrons and singlet states calculated by TDDFT/B3LYP, calculated optimized geometries in the trans → cis photoisomerization quantum yield (ϕ ) drops the S0 and S1 states, minimal energy paths calculated by PM3 and PM3-CI tc 14–16 (2 × 2). See DOI: 10.1039/c7pp00359e from ∼0.5 to 0.12. The further increase in the π-system by This journal is © The Royal Society of Chemistry and Owner Societies 2018 Photochem. Photobiol. Sci.,2018,17,213–220 | 213 View Article Online Paper Photochemical & Photobiological Sciences 4 electrons gives rise to 2-styrylanthracene (SA, 22 electrons), at corresponding s-trans and s-cis conformers relative to the ϕ 17,18 which the tc value reduces practically to zero. Since the vinyl-aryl single bond can also exist). In this series, the cis-isomer remains reactive (the cis → trans photoisomerization π-system size increases from 18 π-electrons (SQ) to 26 quantum yield ϕct>0), the photochemical properties of SA are π-electrons (9A2QE). In accordance with this, the long-wave- characterized as a ‘one-way’ cis → trans photoisomerization length absorption band (LWAB) in the row of SQ–2N2QE– reaction. The sharp ϕtc decrease when the π-system exceeds 9A2QE is bathochromically shifted in the sequence of some threshold size, which is the so-called ‘size effect’,is 340–348–393 nm for the trans-isomers and 272–327–390 nm explained by the change in the reaction mechanism from for the cis-isomers. In the same row, the photoisomerization diabatic (nonadiabatic) to adiabatic: upon excitation of the cis- quantum yield changes as 0.27–0.39–0.33 for the ϕtc and isomer, the system ‘slides’ along the excited-state potential 0.38–0.54–0.40 for the ϕct, i.e., all these compounds are photo- energy surface (PES) to the trans-isomer (adiabatic mechanism) active and undergo two-way photoisomerization. Therefore, in passing the perp-conformer (phantom state), where it could this series of aza-DAEs, the threshold size of the π-system is branch between trans- and cis-isomers (diabatic mechanism, more than 26 electrons. see below). When the trans-isomer is excited, the energy is The next compound in this series where the size effect localized on the large aromatic nucleus, which increases the could manifest itself is 1-(1-pyrenyl)-2-(2-quinolyl)ethylene barrier on the excited state PES and prevents twisting to the (1P2QE), which has 28 π-electrons, Scheme 1. This compound perp-conformer (and further to the cis-isomer).19 One-way adia- was previously synthesized,24 however, its photochemical pro- batic cis–trans photoisomerization (in the singlet-excited state) perties have not been studied. The lack of the size effect is also characteristic for 1-styrylpyrene (SP), which has 24 observed previously makes the pyrene-substituted system a π-electrons.20 good candidate for pushing the spectral response into the According to the functioning mechanism, azobenzene visible region. derivatives and DAEs are E/Z (trans/cis) photoswitches. In con- An additional advantage of aza-DAEs in comparison to all- Creative Commons Attribution 3.0 Unported Licence. trast to azobenzene derivatives, DAEs are rarely used as photo- carbon analogues is that they have an extra channel for spec- switches despite their advantages, for example, they have ther- tral tuning: under protonation, their absorption spectra are mally stable isomers separated by a large barrier in the ground red-shifted, widening the spectral sensitivity region.22,23 The ff (S0) state that results in fully photo-driven switching. e ect of the nitrogen heteroatom on the photochemical pro- X. Guo et al.11 have noted that surprisingly little effort has perties of 1,2-diarylethenes has been previously studied.25 been devoted to developing visible-light-triggered “non-azo” Considering the above discussion, in the present paper, we E/Z photoswitches. They have designed bis(dithienyl)dicya- studied the photochemical properties of 1P2QE. In contrast to noethene (4TCE) with absorption bands in the visible region, SP, 1P2QE proved to be photoactive and underwent two-way which is capable of isomerizing under visible light.11,21 An photoisomerization in the neutral and protonated forms with This article is licensed under a interesting property of 4TCE is that E/Z photoswitching oper- quantum yields of 0.13–0.83 under visible light irradiation. ates by multiplicity-exclusive pathways: trans-to-cis isomeriza- tion only occurs in the S1 state, whereas cis-to-trans isomeriza- tion occurs in the T1 state. Moreover, the photoisomerization Open Access Article. Published on 08 December 2017. Downloaded 9/27/2021 12:56:19 AM. 2. Experimental section quantum yields vary with excitation wavelength; for example, 21 ϕct is negligible at 530 nm and increases to 0.05 at 420 nm. 1P2QE was prepared from quinaldine and 1-pyrenecarboxalde- Earlier, we have studied the size effect in a series of aza- hyde by slightly modifying the previously described micro- DAEs – derivatives of 2-styrylquinoline (SQ). Upon stepwise wave-assisted solvent-free technique26 (see details in the ESI†).
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