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 ∼ 1 µm and a height of ∼ 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, ∼ 20 nm) than that in polyvinyl sulphate (PVS, ∼ 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 ∼ 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.