QM/MM Studies on Photoisomerization Dynamics of Azobenzene Chromophore Tethered to a DNA Duplex: Local Unpaired Nucleobase Plays a Crucial Role

The photoresponsive azobenzene‐tethered DNAs have received growing experimental attention because of their potential applications in biotechnology and nanotechnology; however, little is known about the initial photoisomerization of azobenzene in these systems. Herein we have employed quantum mechanics/molecular mechanics (QM/MM) methods to explore the photoisomerization dynamics of an azobenzene‐tethered DNA duplex. We find that in the S₁ state the trans–cis photoisomerization path is much steeper in DNA than in vacuo, which makes the photoisomerization much faster in the DNA environment. This acceleration is primarily caused by complex steric interactions between azobenzene and the nearby unpaired thymine nucleobase, which also change the photoisomerization mechanism of azobenzene in the DNA duplex.     

Photochemical properties of amino and nitro derivatives of 2-and 4-styrylquinolines and their hydrochlorides

The trans-cis and cis-trans photoisomerization reactions of a double bond in nitro and amino derivatives of 2-and 4-styrylquinolines in the neutral and protonated (hydrochloride) forms were studied. Protonation in nitrostyrylquinolines was shown to have no effect on the photoisomerization quantum yields. In aminostyrylquinolines, the photoisomerization reaction is “switched off” for the monocationic form as a result of the competitive process of intramolecular charge transfer and is “switched on” again for the dication. In the latter case, the quantum yield of trans-cis photoisomerization decreases by a factor of 2–2.5 and the quantum yield of cis-trans photoisomerization remains practically unchanged as compared to that of the neutral compound. Upon long-term irradiation of 4-(4′-nitrostyryl)quinoline, the photocyclization reaction of the cis-isomer was observed.     

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.     

IR125和HDITCP在不同烷基链长阳离子离子液体中的光异构化动力学

利用稳态吸收和荧光光谱以及时间相关单光子计数实验,分别测得近红外花菁分子IR125和HDITCP在不同烷基链长阳离子离子液体中的荧光量子产率和荧光寿命,并通过计算获得了它们各自在相应离子液体中的光异构化速率.发现IR125和HDITCP在不同离子液体中的光异构化速率没有随着离子液体粘度的增大而产生明显变化.与IR125和HDITCP在与离子液体具有相同粘度的甘油水溶液中的光异构化速率对比,发现IR125和HDITCP在离子液体中的光异构化能垒比它们在甘油水溶液中的光异构化能垒增大约2 kJ?mol-1,这表明在高粘度的离子液体中IR125或HDITCP与离子液体之间特殊的相互作用会阻碍它们各自的光异构化过程.     

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.     

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.     

Photochromic aziridines. A novel photoisomerization and the geometry of colored intermediates in solution phase as compared to solid state

Cis-trans photoisomerization was observed in solution-phase photolysis of some photochromic aziridines, indicating a common planar intermddiate. However, in the solid state, each isomer produced a different color, and no photoisomerization occurred, indicating separate and noninterconvertible structures.     

CO2 laser induced isomerization and photodissociation of 1,2-dichloroethylene

Infrared photoisomerization of both trans- and cis-1,2-dichloroethylene molecules sensitized by SF₆ has been observed by using a CO₂ laser. The direct infrared photoisomerization has also been observed for the trans molecule. The reaction rate of the sensitized photoisomerization increases by 6 times as compared with the direct process. The dielectric breakdown induced by an intense laser pulse enhances a dissociative reaction.     

Transient Raman studies of the cis—trans photoisomerization processes in stilbene and retinal

This article reviews the present status of the application of transient or time-resolved Raman spectroscopy to the mechanistic studies of the cis—trans photoisomerization in conjugated molecules. Attention is focused on the vibrational information about the molecular structure of the electronically excited intermediates which are involved in the process of photoisomerization. Two examples, the trans to cis photoisomerization of stilbene through the lower excited singlet state and the cist to trans isomerization of retinal through the lowest excited triplet state, are included.     

MO Study of the photochemical behavior of the imine bond

The photoisomerization of imine compounds is studied in terms of an ab initio MO CI calculation. The potential curves of the syn-anti isomerization via the rotation and the inversion are examined for benzaldimine. It is suggested that the photoisomerization is initiated through the rotation around the C? N bond in both singlet and triplet states. The ease of the photoisomerization is found to be determined by both the conformation of phenyl ring in the ground state and the energy difference of vertically excited states between two isomers.     

Photoisomerization locking of azobenzene by formation of a self-assembled macrocycle

Reaction of an azobenzene-linked biscatecholate ligand with boron and titanium sources gave ring- and cage-shaped complexes in a self-assembly fashion, respectively. These complexes were inert to photoisomerization though the ligand itself was isomerized upon photoirradiation. The self-assembled macrocyclization caused inhibition of the photoisomerization.     

Polymer micelles as building blocks for the incorporation of azobenzene: enhancing the photochromic properties in layer-by-layer films

We described the use of block copolymer micelles as building blocks for the incorporation of water-insoluble photochromic species of azobenzene and the fabrication of multilayer films by alternating the deposition of the block copolymer micelles of poly(styrene-b-acrylic acid), incorporating azobenzene and poly(diallyl-dimethylammonium chloride). The azobenzene incorporated into the block copolymer micelles can undergo a reversible photoisomerization under the irradiation of UV and visible light sources. An interesting finding is that the photoisomerization of the azobenzene in the multilayer film is faster than it is in its normal solid film, but very similar to that in its diluted solution. Furthermore, the amount of azobenzene incorporated into the micelles can influence the photoisomerization rates in the films. Therefore, we expect that the block copolymer micelles may provide a proper microenvironment for the photoisomerization of azobenzene and the as-prepared polyelectrolyte/block copolymer micelle thin films will be useful for photoswitching materials.     

Photoisomerization of PtII Complexes Containing Two Different Photochromic Chromophores: Boron Chromophore versus Dithienylethene Chromophore

In order to examine competitive photoisomerization, a series of novel photochromic Pt^II molecules that contain both dithienylethene (DTE) and B(ppy)Mes₂ units (ppy=2-phenylpyridine, Mes=mesityl) were successfully synthesized and fully structurally characterized. Their photochromic properties were examined by UV/Vis, emission and NMR spectroscopy. It was found that the DTE unit in all three compounds is the preferred photoisomerization site, exhibiting reversible photochromism with irradiation. The B(ppy)Mes₂ unit does not undergo photoisomerization in these molecules, but likely enhances the photoisomerization quantum efficiency of the DTE moiety through the antenna effect. Extended irradiation with UV light leads to the rearrangement of the ring-closed isomers of DTE. TD-DFT computational studies indicate that the DTE photocyclization proceeds via a triplet pathway through an efficient energy transfer process.     

侧基含偶氮基的硅氧烷梳状聚合物的光致变色性

本文研究了对十一烯酰胺基偶氮苯及其硅氧烷均聚、共聚的梳状聚合物在溶液或薄膜状态下的光致变色性和热回复性。它们的反、顺异构体组分比随光照时间趋于某稳定值, 该稳定的组分比与照射光波长有关。光异构反应是可逆的。比较了异构反应速率和异构转换率, 以聚硅氧烷共聚物PSII3A[侧基比(M1/M2)=1/9, M1是对十一烯酰基偶氮苯, M2是对十一烯酰氧对氧基苯酯]为最佳。升高温度能增进热回复异构。     

Steric effects in hula-twist photoisomerization. 1,4-dimethyl- and 2,3-dimethyl-1,4-diphenylbutadienes

1,4-Dimethyl-1,4-diphenylbutadiene was shown to be able to execute regiospecific HT-1 photoisomerization around a methyl group when irradiated in a low-temperature organic glass, albeit at reduced efficiency. 2,3-Dimethyl-1,4-diphenylbutadiene exhibited a different type of steric effect, causing the E,E-isomer to undergo regiospecific HT-1 photoisomerization.     

Photoisomerizable DNA Ligands. A Simple Method to Probe the Regions of AT-Rich Sequences

The dramatic effect of the microenvironment on the mechanism and quantum yield of photoisomerization have been widely evidenced; therefore, one can expect that the DNA matrix can modulate photoisomerization of properly designed ligands, and it may have an analytical importance.     

Counterion controlled photoisomerization of retinal chromophore models: a computational investigation

CASPT2//CASSCF photoisomerization path computations have been used to unveil the effects of an acetate counterion on the photochemistry of two retinal protonated Schiff base (PSB) models: the 2-cis-penta-2,4-dieniminium and the all-trans-epta-2,4,6-trieniminium cations. Different positions/orientations of the counterion have been investigated and related to (i) the spectral tuning and relative stability of the S0, S1, and S2 singlet states; (ii) the selection of the photochemically relevant excited state; (iii) the control of the radiationless decay and photoisomerization rates; and, finally, (iv) the control of the photoisomerization stereospecificity. A rationale for the results is given on the basis of a simple (electrostatic) qualitative model. We show that the model readily explains the computational results providing a qualitative explanation for different aspects of the experimentally observed "environment" dependent PSB photochemistry. Electrostatic effects likely involved in controlling retinal photoisomerization stereoselectivity in the protein are also discussed under the light of these results, and clues for a stereocontrolled electrostatically driven photochemical process are presented. These computations provide a rational basis for the formulation of a mechanistic model for photoisomerization electrostatic catalysis.     

硅胶固载光敏剂的合成及其在反式维生素D3光敏异构化为顺式维生素D3中的应用

采用丙二酸作为连接体,将9-蒽甲醇连接到硅胶上生成硅胶固载的光敏剂.利用此固相光敏剂分别在乙醇和甲苯溶液中敏化反式维生素D₃.实验结果表明,此固相光敏剂能够有效地敏化反式维生素D₃为顺式维生素D₃的反应,并且非常容易从反应体系中分离出来.     

Synthesis of Silica-anchored Sensitizer and its Application in the Preparation of Previtamin D3 from Tachysterol

A silica gel-anchored photosensitizer was synthesized with anthracene as sensitizer moiety. The photoisomerization of tachysterol to previtamin D3 was carried out with this heterogeneous photosensitizer in ethanol. The experiment results demonstrate that this solid phase photosensitizer is efficient for the photoisomerization and can be simply separated from the reaction mixture by filtration.     

The photochemistry of 3,4,5-trideuteriopyridine

Irradiation of 3,4,5-trideuteriopyridine at 254 nm in the vapor phase results in the formation of a mixture of 2,3,4-trideuteriopyridine and 2,3,6-trideuteriopyridine. The formation of these products is consistent with a photoisomerization mechanism involving equilibrating azaprefulvene intermediates. This is the first direct evidence that pyridine vapor undergoes photoisomerization resulting in transposition of the pyridine ring atoms.