ALBERT

Description: We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J -aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J -aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J -aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ε LR /2β, wherein ε LR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J -aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J -aggregates can have both a weak ( g 〈 1) and a strong ( g 〉 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J -aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J -aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J -aggregates by varying the values of g , which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J -aggregates, which has thus far been explained by the strongly localized exciton model.