Step photoisomerization of dicarbocyanine dyes

Abstract

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 γβ-di-cis-isomer (a two-step process). It was proved that the γβ-di-cis-isomer is formed mainly from a γ-mono-cis-isomer. The relaxation of the ground state of the γβ-di-cis-isomer to the ground state of a stable all-trans-isomer occurs in steps via two channels, in which γ-and β-mono-cis-isomers arise as intermediate products. The relaxation rates of both channels are different. The shift of the wavelength 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.