Time-resolved total internal reflection fluorescence spectroscopy. Part I. Photophysics of Coumarin 343 at liquid/liquid interface.

Pico-second time-resolved time-correlated single photon counting (TCSPC) technique under the total internal reflection (TIR) condition has been used to study the photophysical properties of Coumarin 343 (C343) dye molecules adsorbed at the water/1,2-dichloroethane (DCE) interface. The fluorescence decay profile of C343 under TIR condition at the water/DCE interface was non-exponential and fitted to the double exponential decay function with the fluorescence lifetimes 0.3 and 3.6 ns, which proved the existence of two different forms of C343 species having largely different lifetimes at the interface. The longer fluorescence lifetime component of C343 at the interface is attributed to the emission from the monomeric form of the dye molecules and the shorter lifetime component is due to the aggregation of dye molecules. The penetration depth dependence of decay curves indicated no change in the fluorescence lifetime components, however, the amplitude corresponding to the lifetime of aggregate increased and the amplitude corresponding to the lifetime of monomer decreased with the decrease in penetration depth of the aqueous phase from the interface. Aggregation is significant in the interfacial layer. The decrease in monomer lifetime at the interface compared to that in the bulk solution is interpreted in terms of excitation energy migration between the dye molecules.