Exciton Dynamics in Pyrene and Perylene Nanoaggregates

Photophysical and diffusion properties of excitons generated in perylene and pyrene nanoaggregates have been investigated using steady state as well as time-resolved absorption and emission spectroscopic techniques. The average sizes and heights of the nanoaggregates prepared by the reprecipitation method have been estimated as 120 +/- 20 and 60 +/- 10 nm, respectively. X-ray diffraction spectra reveal that molecular packing in these nanoaggregates is very similar to that in the alpha-form (face-to-face pair or dimeric structure) of the crystals, but with possibilities of a large number of disordered regions consisting of monomer molecules. Therefore, following photoexcitation of nanoaggregates, both monomeric and dimeric exciton states are populated. This work, for the first time, could reveal the dynamics of interactions between these two kinds of exciton states, because of which the photophysical and diffusion properties of the excitons are significantly different from those in single crystals. Population yield of the dimeric self-trapped exciton or E state, which is the lowest energy exciton state, is negligibly small via self-trapping of dimeric excitons (dimeric channel) in the <200 ps time domain because of efficient energy transfer from dimeric excitons to monomers. However, the monomeric excitons, which are populated either independently through direct photoexcitation or by energy transfer from the dimeric excitons, contribute to an additional (monomeric) channel populating the E state via energy transfer processes occurring in the nanosecond (ns) time domain. Diffusion coefficients and diffusion lengths of the monomeric excitons estimated in both these nanoaggregates are comparable to those in the beta-form of the crystals but much larger than those values reported for the alpha-form and hence ensure a better or comparable efficiency of energy migration in nanoaggregates as compared to that of single crystals.