Effects of substituents in the polymethine chain on the photoprocesses in indodicarbocyanine dyes

1,1',3,3,3',3'-Hexamethylindodicarbocyanine perchlorate (1) and iodide (1') and five indodicarbocyanine derivatives (with substituents in the meso-position of the polymethine chain; R = CH3 2, Cl 3, Br 4, CN 5 and 1,3,3-trimethylindoline-2-ethenyl 6) were studied by time-resolved spectroscopy in slightly and strongly polar solvents, where ion pairs with ClO4- are present and absent, respectively. The quantum yield of fluorescence at 24 degrees C is Phi(f) = 0.012 for 6 in ethanol and larger for 1-5. Phi(f) increases markedly with decreasing temperature, approaching 0.7-0.9 at -196 degrees C. The fluorescence lifetime, e.g. typically tau(f) = 3 ns in glassy ethanol, is correlated. trans --> cis Photoisomerization occurs in virtually all cases. The quantum yield depends significantly on the medium and meso substituent; Phi(t-->c) is substantial for 2 and 3 in ethanol at 24 degrees C and decreases with decreasing temperature. The initial rotation around one of the central double bonds in the excited singlet state is activated. At room temperature the lifetime of the observed cis form is in the ms range. No discernible effect of ion-pairing on the kinetic parameters of the thermal cis --> trans reversion could be found in toluene. The triplet state of the six dyes was characterized by sensitization; it plays a (minor) role on direct excitation only for 4 or in the presence of a heavy-atom solvent.