Photoisomerization of 3,3'-diethyloxacarbocyanine (DOC) and 3,3'-diethylthiacarbocyanine (DTC): CS INDO CI potential energy curves for the free and solvated molecules

The photochemical trans-cis isomerizations of the two title carbocyanines (DOG and DTC) have been investigated theoretically with a dual objective: 1) to verify the applicability to more complex cyanines of a previously proposed theoretical model derived fr om calculations on a prototype compound (pentamethinecyanine) formally neglecting the solvent; 2) to try to establish the structure of the photoisomers. The potential energy curves of the ground (S-o) and lowest excited singlet (S-1) states were calculated by the CS INDO CI method along the paths leading to the two distinct mono-cis isomers, and the electrostatic solute-solvent interaction energy was evaluated for the two states according to the simple virtual charge (solvaton) model. It was found that solvation energy must be explicitly taken into account for the calculated potential energy curves to be consistent with the early model and the general photochemical behaviour of carbocyanines. From a thorough comparison between the calculated potential curves and the available experimental data the photoisomers appeared to be identifiable as the C(8)-C(9) cis for DOC and C(2)-C(8) cis for DTC.