PHOTOISOMERIZATION OF ASYMMETRIC INDOBENZIMIDAZOLO CYANINE DYES

The isomerization kinetics of three indobenzimidazolo polymethine cyanines have been investigated and compared with the corresponding properties of the parent symmetric dyes. A significant alternation of the pi bond orders in the polymethine chain was found in the ground states (S-0) of the asymmetric dyes. A similarly alternant behaviour was exhibited by the calculated S-0 potential energy barriers for twisting around the polymethine chain bonds of the asymmetric carbocyanine, whereas uniform barriers were found for the symmetric parent compounds. The experimentally observed sequence of back-isomerization activation energies was interpreted on the basis of these theoretical results. It was suggested by some spectral and kinetic fluorescence properties, and it was confirmed by the calculated polymethine-chain pi bond orders of the lowest singlet excited slates (S-1) that the electronic asymmetry induced by the different terminal heterocycles was strongly reduced upon excitation of these dyes to their S-1 states. In spite of this, the trans-cis photoisomerization of asymmetric cyanines occurred invariably around those bonds having the highest pi bond orders in the ground state. A tentative theoretical explanation of this systematic behaviour is reported.