PHOTOPHYSICS OF A THIACARBOCYANINE DYE IN ORGANIC-SOLVENTS

The photophysical properties of the triethylammonium salt of 3,3'-bis(sulfopropyl)-5,5'-dichloro-9-ethylthiacarbocyanine (THIATS) and other related dyes have been studied by means of steady-state and time-resolved spectroscopic techniques. Absorption and fluorescence maxima and bandwidths suggest, together with NMR data, the existence of more than one ground-state conformation for this dye. Analysis of the fluorescence decays by time-correlated single-photon counting and time-resolved fluorescence spectroscopy further supports the steady-state spectroscopy results. An additional short-lived fluorescent species is observed in some polar solvents such as methanol and I-octanol. The dependence of the spectral features and the fluorescence kinetics on the nature of the solvent is explained by a model invoking ion-pairing and conformational changes in the ground state of the dye. It is shown that polarity and polarizability of the solvent control the photophysical properties of THIATS.