Excitation relaxation and structure of TPPS4 J-aggregates

The energy relaxation kinetics and the structure of the J-aggregates of water-soluble porphyrin 5,10,15,20-tetrasulphonatophenyl porphine (TPPS4) were investigated in aqueous medium by means of time-resolved fluorescence spectroscopy and confocal laser-scanning fluorescence microscopy. The excitation of the J-aggregates, at excitation intensities higher than ∼1015 photons/cm2 per pulse, results in a remarkable decrease of the fluorescence quantum yield and in the appearance of an additional, non-exponential energy relaxation channel with a decay constant that depends on the excitation intensity. This relaxation mechanism was attributed to the exciton single–singlet annihilation. The exciton lifetime in the absence of the annihilation was calculated to be ∼150 ps. Using exciton annihilation theory, the exciton migration within the J-aggregates could be characterized by determining the exciton diffusion constant (1.8±0.9) 10−3 cm2/s and the hopping time (1.2±0.6) ps. Using the experimental data, the size of the J-aggregate could be evaluated and was seen to yield at least 20 TPPS4 molecules per aggregate. It was shown by means of confocal fluorescence laser scanning microscopy that TPPS4 does self-associate in polyvinyl alcohol (PVA) at acidic pH forming molecular macro-assemblies on a scale of ∼1 μm in PVA matrices.