Consequences of H-dimerization on the photophysics and photochemistry of oxacarbocyanines

The photophysical/photochemical behaviour of the monomers and the H dimers of four oxacarbocyanines (dyes D-G in the scheme) was investigated in water. In contrast with the usually observed effect of H dimerization, the dimers of dyes D-G were found to fluoresce with efficiencies comparable to or larger than those of the corresponding monomers. Analysis of the decay paths of the lowest excited singlet state showed, however, that dimerization causes a decrease of the radiative rate constants and an enhancement of intersystem crossing to the triplet manifold, as expected from application of exciton theory to a model H dimer. Twisting about one of the polymethine bonds contributes to the decay from the spectroscopic minimum of monomers, though yielding a rather small amount of a distorted cis isomer. The process is inhibited in dimers, likely due to a pronounced increase of activation energy connected with a loss of van der Waals attractive energy at the twisted geometry.