Exciton Dynamics in Anthracene Nanoaggregates

Anthracene nanoaggregates (NA), synthesized using a well-known reprecipitation method, have shapes of nanodisks with the average diameter and height of about 260 and 50 nm, respectively. Following photoexcitation of the anthracene nanoaggregate, we have observed population of two different types of singlet excitonic states, namely, free exciton and self-trapped exciton or excimeric state. Our investigation reveals that the presence of defect conditions in the molecular aggregates reduces the free exciton lifetime and slows the exciton diffusion process marginally. Both the exciton diffusion coefficient and exciton diffusion length in the nanoaggregate and in crystals are comparable. Comparable values of the diffusion lengths in the case of the singlet exciton in the nanoaggregate and crystals suggest that the nanoaggreagates may also be considered as materials for efficient optoelectronic and photovoltaic devices. However, drastic reduction in triplet exciton diffusion coefficient in the nanoaggregate as compared to that in the crystalline form is explained by considering the triplet energy transfer mechanism via spin exchange, which is a slow process