ELECTRONIC-SPECTRA AND TRANS-CIS ISOMERISM OF STREPTOPOLYMETHINE CYANINES - A CS INDO CI STUDY

The understanding of the role played by cyanine dyes in various fields of application calls for a thorough knowledge of the excited state properties of their parent chromophores, i.e. cationic streptopolymethyne cyanines. For this aim we performed a detailed CS INDO CI investigation on the electronic spectra of tri-, penta- and heptamethine cyanines, both unsubstituted (TC, PC, HC) and terminally substituted by methyl groups (BMTC, BMPC, BMHC). The study comprised S0Sn, and S0Tn transitions of the trans and all mono-cis isomers. CI expansions involved solely π orbitals of the polymethine chains and pseudo-π orbitals of the substituents and included all singly-excited and the most important doubly- and triply-excited configurations. The spectroscopic effects of methyl substitution are fairly well described and are shown to be especially important in the upper excited states. The identification of the photochemically formed stereoisomers is attempted in terms of mono-cis isomers. For BMPC, in particular, we report the absorption spectrum of the phototropic form and show that the photoisomer is identifiable as the 3-4 cis form. © 1990.