Exciton-Exciton Annihilation as a Mechanism for Uphill Transfer in a Molecular Excitonic System

Exciton dynamics in a HJ-aggregate of cyanine dye TTBC are investigated by transient absorption with a time resolution of <60 fs and power-dependent emission spectroscopies. Both measurements are consistent with an exciton delocalization length of similar to 28 monomers. A model assuming diffusive exciton motion reveals that the exciton mobility is at least bimodal and restricted to one spatial dimension. J-band diffusion rates of 2.69 and 2.79e(-3) cm(2)s(-1) are found, leading to maximal diffusion lengths of 449 and 14.5 nm. The findings indicate that exciton-exciton annihilation is the origin of effective uphill transfer. This mechanism, if present under solar radiation, maybe useful for organic photovoltaic systems.