RELAXATION DYNAMICS OF EXCITONS IN J-AGGREGATES REVEALING A 2-COMPONENT DAVYDOV SPLITTING

The J-aggregates of the thiacarbocyanine dye THIATS revealing a clear- cut two-component Davydov splitting have been investigated by differen t optical methods. The presence of two molecules per unit cell was pro ved both by polarised fluorescence and linear dichroism in the absorpt ion of aligned aggregates in a rotating cell. The fluorescence excitat ion spectrum of the aggregates and their quantum yield (QY) were measu red across the entire exciton band. The QY was found to be 0.1 and 0.4 for excitation via the upper and lower Davydov components respectivel y. The radiative lifetime of emission of the J-band was estimated to b e about 15 ns combining fluorescence decay data with the fluorescence quantum yield. The excitation intensity dependence of the fluorescence decays suggests the occurrence of exciton-exciton annihilation (EEA). EEA occurs even at an excitation photon fluxes below 10(10) photons/( pulse cm(2)) corresponding to less than one absorbed photon per 10(6) monomer units. (C) 1998 Elsevier Science B.V.