Theoretical and photophysical study of photoisomerism of cyanine dyes : bisphenylaminopentamethine cyanine (BPPC)

The trans-cis photoisomerism of BPPC was investigated as part of a comprehensive, both theoretical and experimental, study on the photophysical and photochemical properties of cyanines having pentamethine cyanine as a common chromophore. In this work the determination of the trans → cis photoisomerization and thermal back isomerizatkm kinetics in alcohols was combined with CS INDO CI calculations of S0 and S1 potential energy curves for the paths leading to mono-cis isomers, where the dielectric solvent effects were taken into account by the simple virtual charge model. In agreement with the indications of a previous steady-state spectroscopy study, it was concluded that irradiation of BPPC into the visible absorption region gives rise to formation of the C(3)–C(4) cis planar isomer, already observed with the parent chromophore (BMPC), and an additional isomer, N(1)-C(2) cis, peculiar to BPPC. The role of the solute-solvent interactions is discussed with reference to the purely intramolecular model previously proposed for the cyanine photoisomerization.