Identification of the Photoisomers of 2 Carbocyanines by H-1-NMR Spectroscopy

The H-1 NMR spectra of the labile photoisomers of 3,3'-diethyloxa- and 3,3'-diethylthiacarbocyanines have been recorded for the first time. Analysis of these spectra leads to the identification of the photoisomer of the first dye as a mono-cis form obtained by twisting about the central polymethine bond, and to the suggestion that the photoisomerization of the second dye takes place around the external polymethine bond. These assignments are shown to be generally consistent with the results of earlier experimental and theoretical studies of the ground- and excited-state behaviour of these compounds.