Influence of ion pair formation on the photochemistry of asymmetric cationic indobenzimidazolo cyanines

The influence of counterions in ion pairs of asymmetric cationic indobenzimidazolo cyanines formed in low polarity solvents on the main photochemical processes of the cyanine cations (fluorescence emission, photoisomerization and thermal ground state back isomerization, S-1 --> T and T --> S-0 intersystem crossings) was experimentally studied. The previously found structural similarity of the ion pairs of symmetric benzimidazolo cyanines and those of asymmetric indobenzimidazolo cyanines is reflected in the similarity of the effects of ion pairing on the dynamics of isomerization of these dyes in the ground and lowest excited singlet states. Indirect evidence based on fluorescence steady-state and time-resolved measurements indicates a weak catalytic counterion effect on the S-1-state isomerization dynamics of the investigated benzimidazolo and indobenzimidazolo cyanines. The I- counterion in ion pairs strongly affects the S-1 state of the asymmetric cyanines by inducing S-1 --> T and T-1 --> S-0 intersystem crossings (heavy atom effect). (C) 1998 Elsevier Science S.A. All rights reserved.