Photoisomerization of 2-styrylquinoline in neutral and protonated forms

The quantum yields of the trans-cis and cis-trans photoisomerization of 2-styrylquinoline (2SQ) and its several derivatives were measured in neutral, protonated, and quaternized forms. It was shown that electron-donor substituents in the styryl moiety increase the quantum yield of trans-cis photoisometization ?_tc in the neutral form as a result of stabilization of the intermediate zwitterionic perpendicular conformer. On passing from the neutral to the positively charged forms (protonated or quaternized), an increase in the quantum yields to ?_tc > 0.5 was observed, thus suggesting in terms of the classical diabatic mechanism of photoisomerization via the perpendicular conformer the shift of the minimum on the potential energy surface (PES) of the S ₁ state relative to the maximum of the S ₀ state PES to the cis-isomer or a possible contribution of the adiabatic route to the photoisomerization of the 2SQ cations.     

Spectral and photochemical properties of hydroxylated 2-styrylquinoline derivatives

The spectral and photochemical properties of 2-(4-hydroxystyryl)quinoline 1 and 2-(2-hydroxystyryl)quinoline 2 have been studied. The trans-cis photoisomerization quantum yield for 1 and 2 in the neutral form is 0.2–0.5, and it is reduced by a factor of 6–7 by protonation, presumably, owing to the competitive process of hydroxyl group deprotonation in the excited state and the formation of quinoid tautomers. The quantum yield of the back cis-trans photoisomerization in the protonated form varies insignificantly. The neutral, protonated, and deprotonated forms of 1 have been used for to simulate a molecular logic gate, the half-adder with chemical inputs and absorbance as outputs.     

Photoisomerization of naphthylquinolylethylenes in neutral and protonated forms

The quantum yield of the trans-cis photoisomerization of 1-(1-naphthyl)-2-(2-quinolyl)ethylene and 1-(2-naphthyl)-2-(2-quinolyl)ethylene is close to the theoretical limit (0.5) for diabatic photoisomerization and does not change on passing from the neutral to the protonated form. The data obtained indicate the absence of the α-effect for the test compounds, which consists in an increase in the trans-cis photoisomerization quantum yield to values of >0.5 upon protonation of some azadiarylethylenes with the nitrogen atom in the α-position to the ethylene group.     

Photochemical properties of 1-(9-Anthryl)-2-(2-Quinolyl)ethylene

Spectral and photochemical properties of 1-(9-anthryl)-2-(2-quinolyl)ethylene (9A2QE) in neutral and protonated forms have been studied experimentally and by quantum-chemical methods. It has been found that the quantum yield of trans-cis photoisomerization (φ_tc) has values of φ_tc < 0.5 typical of the diabatic photoisomerization for both forms of 9A2QE. A comparison of this data with the results of the study of other aza-diarylethylenes containing the 2-styrylquinoline (2SQ) moiety has led to the general conclusion that the increase in the π-system in 2SQ upon fusion of the benzene rings results in the disappearance of the α-effect, which lies in the fact that the quantum yield increases upon going from the neutral to protonated form up to the values φ_tc > 0.5, which exceeds the limiting value for the diabatic photoisomerization.     

Photoisomerization and photocyclization of 4-styrylquinoline derivatives

The photochemical properties of 4-styrylquinoline and its derivatives with fluorine and chlorine atoms and nitro group in the para-position of the styryl moiety were studied. Under irradiation in neutral alcohol solutions, these compounds undergo reversible trans-cis photoisomerization with quantum yields close to 0.5, and the cis-isomers undergo cyclization to give benz[i]phenanthridines with quantum yields that do not exceed 0.01. For several compounds, the primary product of photocyclization, dihydrobenz[i]phenanthridine, was spectrally detected. On passing from the neutral to protonated forms, the photoisomerization quantum yields of these styrylquinolines changed insignificantly, and the photocyclization did not occur at all.     

Photoisomerization of arylstilbazolium ligands in the presence of DNA

The ability of the DNA duplex to behave as an efficient organized medium for trans-cis photoisomerization has been explored. The presence of DNA affected isomerization in a variety of ways depending on the aryl moiety properties of the ligand and its DNA-binding mode. Contrary to intercalating ligands, 9-[2-(N-methylpyridinium-4-yl)vinyl]phenanthrene (2) and 9-[2-(N-methylpyridinium-4-yl)vinyl]anthracene (3), which gave only cis and trans isomers, the additional products--cyclobutane photodimers--were detected for 2-[2-(N-methylpyridinium-4-yl)vinyl]naphthalene (1), which binds cooperatively to the minor groove of DNA. Photostationary states (pss) for all ligands were seriously affected by the presence of DNA. A trans isomer-rich pss and restriction of trans --> cis process, observed for ligands 1 and 2, were explained in terms of a different binding affinity of DNA toward particular isomers. On the contrary, 9-anthryl derivative 3 isomerized against the isomer-binding preferences, showing cis isomer-rich pss and enhanced quantum yield of isomerization. The unique behavior of ligand 3-DNA complex was attributed to different isomerization mechanism that consists in quantum chain isomerization from an excited singlet state possessing a charge transfer character. This is the first example of ligand, which undergoes DNA-mediated cis-trans isomerization in the opposite direction than expected from DNA-binding preferences. The analysis of pss data based on two alternative pathways of photoisomerization showed that investigated ligands follow the model that allows isomerization of both free and DNA-bound ligands.     

Photochemistry and photophysics of osazones I: photochemical and thermal isomerization of D-“arabino”-hexulose phenylosazone

The photochemical and thermal N-chelate—O-chelate isomerization of D-“arabino”-hexulose phenylosazone (PH) in solution was studied. The rate constants and activation parameters for thermal isomerization were calculated. An obvious wavelength effect on the photoisomerization quantum yield and on the position of the photostationary state was observed. Direct photoisomerization is believed to occur via a common excited singlet state. PH was found to have photochromic properties. The photoisomerization reaction was proved to be a fast efficient method of synthesizing the O-chelate isomer of osazone.     

Proton-controlled photoisomerization of 1-(2-pyridyl)-2-(2-quinolyl)ethylene

Quantum-chemical calculations (B3LYP/6-31G*) predict the formation of intramolecular hydrogen bond (IMHB) in the monoprotonated Z-isomer of 1-(2-pyridyl)-2-(2-quinolyl)ethylene (2P2Q), with this bond stabilizing the isomer relative to its E-counterpart. An experimentally observed increase in the quantum yield of trans-cis photoisomerization (φ_tc) by more than an order of magnitude (from 0.033 to 0.42 in acetonitrile) on passing from the neutral to the monoprotonated form of 2P2Q can be associated with IMHB, which manifested itself in the spectral properties of the Z-isomer. The IMHB breaks in the diprotonated form, and the value of φ_tc decreases back to the initial value. In addition to the photoisomerization, the photoreduction and photoaddition reactions of solvent molecules have been observed in an ethanol solution of 2P2Q.     

A Multi‐Addressable Switch Based on the Dimethyldihydropyrene Photochrome with Remarkable Proton‐Triggered Photo‐opening Efficiency

A series of photochromic derivatives based on the trans‐10b,10c‐dimethyl‐10b,10c‐dihydropyrene (DHP, “closed form”) skeleton has been synthesized and their photoisomerization leading to the corresponding cyclophanediene (CPD, “open form”) isomers has been investigated by UV/Vis and ¹H NMR spectroscopies. Substitution of the DHP core with electron‐withdrawing pyridinium groups was found to have major effects on the photoisomerization efficiency, the most remarkable examples being to enhance the quantum yield of the opening reaction and to allow fast and quantitative conversions at much lower radiant energies. This effect was rationalized by theoretical calculations. We also show that the reverse reaction, that is, going from the open form to the closed form, can be electrochemically triggered by oxidation of the CPD unit and that the photo‐opening properties of pyridine‐substituted DHPs can be efficiently tuned by protonation, the system behaving as a multi‐addressable molecular switch. These multi‐addressable photochromes show promise for the development of responsive materials.     

苯基-[雌(甾)-17β-醇-3]-偶氮化合物(PED)在聚甲基丙烯酸甲酯薄膜中的光化学异构化反应研究

本文研究了PED偶氮化合物在聚甲基丙烯酸甲酯固体介质中的光化学顺反异构化反应。对此种体系在π,π^*和n,π^*激发条件下光化学顺反异构化的量子产率、光稳态组成以及所在聚合物体系中的自由体积分布等的测定方法和原理进行了详细的介绍。这些方法对其它类似的光致变色体系同样也是适用的。     

Photoisomerization of the Graft Copolymer of Starch and Azo-dye

Optical data storage materials are mainly based on organic molecules. To be suited for as an optical data storage material, molecules have to be photochromatic, they must be capable of being reversibly switched be. tween two spectroscopically distinct forms by use of light[1].Photoisome rization (trans ～cis) of azoaromatic compounds has been investigated extensively using UV-Vis spectroscopy. In 1972[2], the isomerization of various azobenzenes bound on the side chain or within the main chain of poly (methyl methacrylate), poly(ethyl methacrylate),polystyrene, a polyamide, and polyester had. been demonstrated. There are three processes: a trans-cis photochemical isomerization, a cis-trans photochemical isomerization, and a cis-trans thermal isomerization, which occurs due to the higher stability of the trans configuration, since that pioneering study, two main directions of research developed and are very active in the eurrent literature.     

Photochemical properties of the naphthol-styrylquinoline dyad in neutral and ionic forms

The photochemical properties of the naphthol-styrylquinoline dyad 2-(E)-{4-[4-(3-hydroxynaphthalen-2-yloxy)butoxy]styryl}quinoline (SQ4Np) have been investigated. It has been found that the excited-state acidity of the styrylquinoline (SQ) moiety is reduced by six orders of magnitude and that of the naphthol (Np) moiety increases by four orders of magnitude. As part of the dyad in the neutral and protonated forms, the SQ moiety retains a high photoisomerization quantum yield characteristic of model styrylquinoline. Deprotonation of the Np moiety of the dyad reduces the SQ photoisomerization quantum yields, presumably because of the formation of intramolecular complexes (exciplexes) or energy transfer to the Np anion.     

电子效应调控丁烯二腈光分子开关的CASSCF和MS-CASPT2理论研究

采用多组态CASSCF方法和MS-CASPT2方法研究了丁烯二腈中性分子及阳离子和阴离子的顺-反异构化机理.结果表明,中性分子和离子态的光顺-反异构化反应经历不同的非绝热跃迁途径:中性丁烯二腈受光激发至S1态后,需克服一个不低于19.7 k J/mol的能垒才有机会到达基态和激发态间的圆锥交叉(S_1/S_0-CI),随后经非辐射跃迁回到基态,S_1/S_0-CI在结构上偏离C=C双键旋转路径,且能量较高,因此会降低旋转速度,阻碍旋转的单向性;丁烯二腈阳离子和阴离子自由基的D_0态和D_1态旋转势能面在90°处相交,优化的D_1/D_0-CI与D_1态中间体的结构和能量均相近,因此从D1态经由D_1/D_0-C_I无辐射跃迁到D_0态的过程无势垒,在此过程中C=C旋转方向性得到最大限度的保持.研究结果证实了电子诱导不仅能降低基态热旋转势垒,而且能够调控光旋转的非绝热跃迁机理.     

Medium effects on the direct cis-trans photoisomerization of 1,4-diphenyl-1,3-butadiene in solution

Quantum yields for the photoisomerization of trans,trans-1,4-diphenyl-1,3-butadiene (tt-DPB), determined in benzene, cyclohexane, methylcyclohexane, hexane, and perfluorohexane, confirm the low values reported earlier for benzene and cyclohexane and reveal even lower values in the last two solvents. In contrast to trans-stilbene (t-St), fluorescence and torsional relaxation leading to photoisomerization do not account exclusively for S(1)tt-DPB decay. Competing radiationless singlet excited-state decay pathways exist in tt-DPB, which do not lead to photoisomerization and may not involve large-amplitude torsional motions. Our results invalidate analyses of tt-DPB fluorescence quantum yields and lifetimes that assign all radiationless decay to the isomerization channel. Gas-phase chromatography analysis of tt-DPB photoisomerization in hexane shows the reaction to be concentration-independent and reveals, for the first time, a significant, two-bond photoisomerization pathway, φ(tt→tc) = 0.092 and φ(tt→cc) = 0.020. The dominant one-bond-twist (OBT) process is accompanied by a bicycle pedal (BP) process that accounts for almost 20% of tt-DPB photoisomerization. The OBT tt-DPB photoisomerization quantum yield is largest in benzene (Bz) and smallest in perfluorohexane (PFH). Contrary to expectations, reduction in medium friction in PFH is accompanied by a decrease in φ(tt→tc). The 1(1)B(u)/2(1)A(g) order and energy gap appear to control the contribution of torsional relaxation to radiationless decay. Lowering the 1(1)B(u) energy as in Bz favors photoisomerization. Reversal of the 1(1)B(u)/2(1)A(g) order in PFH is accompanied by short τ(f) and small φ(f) and φ(tt→tc) values that suggest the presence of competing 2(1)A(g) → 1(1)Ag relaxation paths that are unproductive with respect to photoisomerization. We conclude that the Birks extension to diphenylpolyenes of the Orlandi-Siebrand cis-trans photoisomerization mechanism is not valid. Photoisomerization appears to occur in the 1(I)B(u) state, and we argue that this applies to t-St as well.     

Cis-trans photoisomerization of fluorescent-protein chromophores

Photochromic variants of fluorescent proteins are opening the way to a number of opportunities for high-sensitivity regioselective studies in the cellular environment and may even lead to applications in information and communication technology. Yet, the detailed photophysical processes at the basis of photoswitching have not been fully clarified. In this paper, we used synthetic FP chromophores to clarify the photophysical processes associated with the photochromic behavior. In particular, we investigated the spectral modification of synthetic chromophore analogues of wild-type green fluorescent protein (GFP), Y66F GFP (BFPF), and Y66W GFP (CFP) upon irradiation in solutions of different polarities. We found that the cis-trans photoisomerization mechanism can be induced in all the chromophores. The structural assignments were carried out both by NMR measurements and DFT calculations. Remarkably, we determined for the first time the spectra of neutral trans isomers in different solvents. Finally, we calculated the photoconversion quantum yields by absorption measurements under continuous illumination at different times and by a nanosecond laser-flash photolysis method. Our results indicate that cis-trans photoisomerization is a general mechanism of FP chromophores whose efficiency is modulated by the detailed mutant-specific protein environment.     

Influence of solvent on the photochemistry of 4,4′-diaminostilbene

Studies on the fluorescence and direct trans-cis photoisomerization of 4,4′-diaminostilbene in a variety of solvents, indicate a marked effect of solvent polarity. With increasing solvent polarity there is a decrease in fluorescence yield and lifetime and an increase in the rate of photoisomerization. The results are discussed in terms of a polarity effect on the energy barrier to photoisomerization.     

Coordination-synchronized trans-cis photoisomerization of bipyridylazobenzene driven by a Cu(II)/Cu(I) redox change

The trans-cis photoisomerization behavior of azobenzene-bipyridine ligand (dmpAB) was synchronized with coordination of the bipyridine moiety to copper. The coordination reaction can be reversibly controlled with reversible redox reaction of copper, to afford [Cu(dmpAB)(2)](+) in Cu(I) state and free dmpAB in Cu(II) state. UV irradiations to Cu(I) and Cu(II) samples form trans-rich and cis-rich compositions, respectively. The results enable us to control the trans-cis isomerization reversibly through Cu(II)/Cu(I) redox and a single UV light.     

联有含氮艹/氐的2-吡唑啉化合物的光物理行为

合成了一种在(5)位上联有含氮的2-吡唑啉化合物，研究了该化合物分子所呈现的光敏顺-反异构化现象，并对其机理进行了考察，研究结果表明：具电子转移性质的该化合物顺-反异构化反应速率强烈的依赖于溶剂的极性。同时，光敏异构化还导致了该吡唑啉化合物荧光发射强度随光照时间而增大的有趣结果。     

In vitro cis-trans photoisomerization of palythene and usujirene. Implications on the in vivo transformation of mycosporine-like amino acids

The in vitro photoinduced reactions of the mycosporine-like amino acids (MAA) usujirene and palythene were studied by monochromatic stationary irradiation at 366 nm. High-performance liquid chromatography analysis of the irradiated aqueous solution of usujirene indicated a low photoreactivity on the basis of the observed photodecomposition quantum yield of phi(-U) = (2.86 +/- 0.80) x 10(-5), which can be partially accounted for by the cis-trans photoisomerization of usujirene to palythene (phi(U-->P) = [1.71 +/- 0.13] x 10(-5)). However, palythene in aqueous solution showed a higher photostability than did usujirene under equivalent conditions, establishing a photostationary mixture of cis-trans isomers with a relative composition of palythene-usujirene (11:1). These results may explain the preferential in vivo accumulation of palythene relative to that of usujirene observed in several dinoflagellate species.     

Ab initio excited-state dynamics of the photoactive yellow protein chromophore

The photoisomerization mechanism of the neutral form of the photoactive yellow protein (PYP) chromophore is investigated using ab initio quantum chemistry and first-principles nonadiabatic molecular dynamics (ab initio multiple spawning or AIMS). We identify the nature of the two lowest-lying excited states, characterize the short-time behavior of molecules excited directly to S2, and explain the origin of the experimentally observed wavelength-dependent photoisomerization quantum yield.