Step photoisomerization of dicarbocyanine dyes

Step photostereoisomerization of solutions of dicarbocyanine dyes was studied by laser photolysis. It is shown that, due to excitation, primary mono-cis-photoisomers are formed (a one-step process), whose excitation results in the formation of a gammabeta'-di-cis-isomer (a two-step process). It was proved that the gammabeta'-di-cis-isomer is formed mainly from a gamma-mono-cis-isomer. The relaxation of the ground state of the gammabeta'-di-cis-isomer to the ground state of a stable all-trans-isomer occurs in steps via two channels, in which gamma- and beta-mono-cis-isomers arise as intermediate products. The relaxation rates of both channels are different. The shift of the wave-length related to the maximum of absorption in the spectra of the primary and secondary isomers relative to that of the all-trans-isomer depends on the electron-donor ability of terminal groups.' Quantum-chemical calculations of changes in the energies of the ground and first excited singlet states of a molecule, caused by the rotation around several bonds in the methine chain, were carried out. (C) 2002 MAIK "Nauka/Interperiodica".