Excited state dynamics and photophysics of aggregated dye chromophores in solution

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Abstract
We have studied the excited state relaxation processes of monomeric and aggregated diethylthiadicarbocyanine iodide (DTDCI) by means of picosecond absorption spectroscopy. The strongly interacting chromophores of dimers and trimers are found to have very fast and efficient radiationless processes, whose rates are independent of temperature and viscosity, but strongly dependent on solvent dielectric constant. These results and similar ones for the covalently linked bis-DTDCI dimer allow us to identify the relaxation mechanism and suggest a theoretical model which accounts for the observations. We suggest that the radiationless processes are induced by the very strong dipole–dipole interactions between the units of the aggregate; analogies to the so called medium induced radiationless processes are also made. Estimates of the relaxation rates in the aggregated dye yields values which are in order-of-magnitude agreement with the measured values.