PHOTOPHYSICS OF A THIACARBOCYANINE DYE IN ORGANIC-SOLVENTS

The photophysical properties of the triethylammonium salt of sulfoprop yl)-5,5'-dichloro-9-ethylthiacarbocyanine (THIATS) and other related d yes have been studied by means of steady-state and time-resolved spect roscopic techniques. Absorption and fluorescence maxima and bandwidths suggest, together with NMR data, the existence of more than one groun d-state conformation for this dye. Analysis of the fluorescence decays by time-correlated single-photon counting and time-resolved fluoresce nce spectroscopy further supports the steady-state spectroscopy result s. An additional short-lived fluorescent species is observed in some p olar solvents such as methanol and I-octanol. The dependence of the sp ectral features and the fluorescence kinetics on the nature of the sol vent is explained by a model invoking ion-pairing and conformational c hanges in the ground state of the dye. It is shown that polarity and p olarizability of the solvent control the photophysical properties of T HIATS.