Features of exciton dynamics in molecular nanoclusters (J-aggregates): Exciton self-trapping (review article)

In this review experimental results have been presented and analyzed, that are demonstrated an anomalous manifestation of the well-known for bulk crystals effect of exciton self-trapping in ordered molecular nanoclusters, so called J-aggregates. The main structure motive of the J-aggregates are weakly coupled 1D molecular chains in which electron excitations are existed as 1D Frenkel excitons. According to continuum theory of Rashba-Toyozawa only self-trapped excitons should exist in J-aggragtes because 1D excitons have to be barrierless self-trapped at any exciton-phonon coupling constant g = ϵLR/2β, where ϵLR - the lattice relaxation energy, 2β - the half-width of exciton band. By contrast, in experiments on the J-aggregates only emission of mobile free excitons was often observed. Using the Urbach rule for low-frequency edges analysis of exciton absorption spectra at low temperatures it was found that both weak (g 1) cases of exciton-phonon coupling could be realized in the J-aggregates. Moreover, it was experimentally demonstrated coexistence of free and self-trapped excitons in excited state of the J-aggregates at certain conditions, i.e. existence of the self-trapping barrier for the 1D Frenkel excitons. In the review it has been shown and analyzed reasons for the anomalous coexistence of the free and self-trapped excitons in the J-aggregates and demonstrated a control of exciton self-trapping efficiency in the J-aggregates through the constant g varying, which is principally impossible for the bulk molecular crystals. Using the exciton self-trapping phenomenon it has been discussed an alternative interpretation of wide band emission for some J-aggregates, which was explained in the framework of strongly localized exciton model. PACS: 71.35.Aa Frenkel excitons and self-trapped excitons; 78.67.Sc Nanoaggregates; nanocomposites.