Photoisomerization mechanism

A further type of photoisomerization mechanism for oxepin (7) was found by using material specifically labelled with 2H at the 3- and 6-positions (74JA5578). The location of the 2H atoms in the finally isolated sample of (7) was consistent with the migration of the oxirane ring to a neighboring position. The latter process (Scheme 13) has been described... 

Fig. 1. Idealized photoisomerization mechanisms. These can lead to different final products as shown. However, one should realize that hot ground state reactions could also occur, obscuring a simple relationship between products and excited state dynamics.

We have presented the first published results of a QM/MM wavepacket dynamics study of a photochemical reaction. The photoisomerization mechanism for the GFP chromophore that we observe has the signatures of HT motion, even in the complete absence of an environment. The HT mechanism is aborted in both the gas phase and solution, but the... 

Despite great interest in azobenzene photophysics, the basic photoisomerization mechanism remains disputed [173] in contrast to the expectations of Kasha s rule, the isomerization quantum yield decreases rather than increases with increasing photon energy. In Fig. 22, the two possible isomerization channels, proceeding via either a planar pathway (inversion) or a nonplanar, twisted pathway (torsion) are shown. Previous studies determined that isomerization in the first excited state S1 state proceeds along the inversion coordinate [171]. The second excited state is generally thought to be... 

The photoisomerization mechanism in aminoazobenzene- and pseudo-stilbene-type compounds has attracted far less attention than the mechanism for azobenzenes. In pseudo-stilbenes, the (n,7t ) state is buried under the intense it —> Jt band and cannot be populated selectively. No state-specific quantum yields are available because the yields are independent of the exciting wavelength.There is only a very narrow experimental basis for a discussion of these two mechanisms. However, this may change when pseudo-stilbenes are subjected to ultrashort-time experiments. 

Three different photoisomerization mechanisms of singlet excited polyenes have been proposed (1) one-bond twist, a typical process observed in fluid solutions but also in glassy media,525 553 565 (2) the bicycle-pedal mechanism involving simultaneous rotation about two original double bonds, assumed to occur in a constraining environment,566-568 and (3) a volume-conserving two-bond hula twist 531 569 510 (Scheme 6.7). In some cases, the existence of the last mechanism has been ruled... 

Sandros and co-workers [126] prepared substituted p-styrylstilbene and studied the cis-to-trans photoisomerization process. They found that excitation of cis, cw-p-styrylstilbene gave both isomers cis.trans and trans,trans. The isomerization process was found to be from the singlet excited state and also adiabatic in nature (Scheme 7). Support for the proposed singlet excited state adiabatic photoisomerization process stems from the quantum yield of isomerization and time-resolved fluorescence measurements. Furthermore, they have performed quantum mechanical calculations to generate surface profiles to support the proposed adiabatic photoisomerization mechanism. This reaction is an interesting example showing twofold adiabatic photoisomerization from the singlet excited state. 

The photoisomerization of cis-and trans-stilbenes has been studied in ionic liquids and evidence is presented that the mechanism depends on the particular ionic liquid used. In basic N-butylpyridinium chloride/AlCb the photoequilibration involves stilbene radical cations, whereas in basic l-ethyl-3-methyl-imidazolium chloride/AlCb the process occurs via the standard singlet state photoisomerization mechanism. It has been found that the tetra-O-acetylribo-flavin (126) sensitized dehydrogenation of substituted benzyl alcohols, giving the... 

Stilbene and Related compounds. As a result of calculations on the isomerization of ethene, it has been suggested that there is a need for the reconsideration and refinement of the photoisomerization mechanism of stilbene . Both a semi-classical approachand simulations have been employed to study the cisprans-isomQnzdiiion of stilbene. Simulations have examined the photoisomerization of stilbene, and ab initio quantum chemistry has also been utilized to study this system. 

The effed of thienyl groups on the photoisomerization and rotamerism of symmetric and asymmetric stUbenes has been studied [73]. Stationary and pulsed fluorimetric techniques, laser flash photolysis, and conventional photochemical methods and theoretical calculations were used for investigating photochemistry of flve symmetric (bis-substituted) and asymmetric (mono-substituted) analogues of -stilbene, where one or both side aryls are 2 -thienyl or 3 -thienyl groups. It was shown that the presence of one or two thienyl groups and their positional isomerism afled the spectral behavior, the relaxation properties (radiative/reactive competition), the photoisomerization mechanism (singlet/triplet), and the ground-state rotamerism. 

Ponterini, G. and MomicchioK, F., Trans-cis photoisomerization mechanism of carbocyanines experimental check of theoretical models, Chem. Phys., 151, 111, 1991. 

Mizukami, T., Kandori, H., Shichida, Y, Chen, A.H., Derguini, E, Caldwell, C.G., Biffe, C.F., Nakanishi, K., and Yoshizawa, T., Photoisomerization mechanism of the rhodopsin chromophore picosecond photolysis of pigment containing 11 -cis-locked eight-membered ring retinal, Proc. Natl. Acad. Sci. USA, 90, 4072, 1993. 

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