Spectroscopic properties of meso-substituted cyanine dyes: Evidences for intramolecular charge transfer from a julolidine moiety as a meso-substituent to the cyanine chromophore

Synthesis and spectral properties of cyanine dyes 1 and 2, having julolidinylethenyl and α-methyl julolidinylethenyl moieties respectively at the meso position, are described. In comparison with planer 1, the methyl group of the julolidinylethenyl group in 2 prevents the coplanarity of the whole dye system. Their characteristic electronic absorption spectra are analyzed based on the molecular orbital calculations, and the calculations well reproduce the observed data. Analysis of ¹H NMR spectrum of 1 indicates that the π-electrons in the julolidinylethenyl group shift toward the cyanine main chromophore through the π-conjugation. Intramolecular charge transfer from the julolidine moiety to the cyanine main chromophore contributes to lightfastness of dyes. These findings make it possible to design a new type of stable cyanine dyes against photoirradiation with new functionalities such as intramolecular charge transfer system.