Exciton relaxation in nanotubular TPPS4 aggregates in water solution and in polymeric matrix

Exciton properties in meso-Tetra(4-sulphonatophenyl)porphine (TPPS4) aggregates in water solution and in PVA matrix were investigated at different temperatures by means of transient absorption and fluorescence spectroscopy. Analysis of the steady state and transient absorption spectra shows that excitons delocalized over about 10 TPPS molecules are formed under the aggregate excitation to the higher energy absorption band, however they localize on one or two molecules after relaxation to the lowest energy excited state. The transient absorption and time resolved fluorescence show different relaxation kinetics, which is interpreted in terms of competition between the exciton relaxation to the nonradiative state and quenching by the quenching centers reached during thermally activated exciton diffusion. Exciton-exciton annihilation starts at high excitation intensity when more than 1 exciton per 20 molecules is created indicating that excitations visit about 20 molecules during their lifetime. The exciton relaxation becomes faster in optically annealed samples, when optically destroyed molecules create additional quenching centers.