THEORETICAL AND PHOTOPHYSICAL STUDY OF PHOTOISOMERISM OF CYANINE DYES- BISPHENYLAMINOPENTAMETHINE CYANINE (BPPC)

The trans-cis photoisomerism of BPPC was investigated as part of a com prehensive, both theoretical and experimental, study on the photophysi cal and photochemical properties of cyanines having pentamethine cyani ne as a common chromophore. In this work the determination of the tran s --> cis photoisomerization and thermal back isomerization kinetics i n alcohols was combined with CS INDO CI calculations of S-0 and S-1 po tential 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 chromop hore (BMPC), and an additional isomer, N(1)-C(2) cis, peculiar to BPPC . The role of the solute-solvent interactions is discussed with refere nce to the purely intramolecular model previously proposed for the cya nine photoisomerization.