Ultrafast exciton dynamics of J- and H-aggregates of the porphyrin-catechol in aqueous solution.

The aggregation behavior of 5,10,15-trisphenyl-20-(3,4-dihydroxy phenyl) porphyrin (L) in aqueous solution has been studied as a function of pH and concentration with the help of steady state absorption and emission spectroscopy. Our studies revealed that, for a particular concentration range, molecules of L undergo a reversible aggregation process and form two different aggregates at two different pH ranges, namely, J- and H-aggregates. We have monitored the excited state lifetimes of different aggregates by picosecond time-resolved emission spectroscopy and found that the emission lifetime of L reduces drastically with the formation of these aggregates. To study the dynamics associated with the excited state of both non-aggregated and aggregated (both J and H) porphyrins in an ultrafast time domain, we have carried out femtosecond transient absorption spectroscopy in the visible region following excitation of the respective samples at 400 nm. In addition to other de-excitation channels, another extra 100 fs (major) component for H-aggregates and 200 fs (major) component for J-aggregates has been observed and attributed to the exciton decay components of the respective aggregates.