Mesoscopic structures and dynamics of merocyanine J-aggregate studied by time-resolved fluorescence SNOM

Time-resolved fluorescence SNOM is used to probe the mesoscopic structure and dynamics of long-chain merocyanine (C18MC) J-aggregates on glass plates prepared by spin coating, casting, and casting of water-soluble polymer films. A globular structure with an average diameter of approximately 1 microm and a height of approximately 50 nm was attributed to the J-aggregate of C18MC in the spin-coating film. In polymer films, the bandwidth of the absorption of J-aggregate is much narrower in polyvinyl alcohol (PVA, approximately 20 nm) than that in polyvinyl sulphate (PVS, approximately 60 nm). We have demonstrated that the large bandwidth of the spectrum is due to the inhomogeneous distribution of the J-aggregate. The fluorescence image of the J-aggregate in PVA film was rather uniform, whereas non-uniform distribution of the fluorescence was observed in PVS film. The fluorescence of C18MC J-aggregate in a small domain of PVA film was a single exponential decay with a lifetime as short as 19 ps, which was shorter than that in PVS film with a two-exponential decay (average lifetime of approximately 25 ps). The fluorescence lifetime of the J-aggregate and its single exponential behaviour are considered to be indicators of the uniform distribution of the J-aggregate. The non-uniform distribution of the J-aggregate in PVS film was interpreted in terms of electrostatic interaction between PVS and merocyanine.