Manifestation of Size Heterogeneity of Molecular Aggregates in the Distribution of Their Fluorescence Lifetimes and Diffusion Coefficients

Microscopic insights into molecular aggregation have been obtained using R1, a model solvatochromic fluorophore in DMF–water mixtures. Fluorescence quantum yield (ϕf) and lifetime (τf) of R1 decrease monotonically up to a rather high molar ratio of water (χH2O). Near invariance of the hydrodynamic radius, estimated using fluorescence correlation spectroscopy, assigns this to the progressive increase in polarity of the medium and not aggregation. An abrupt increase in ϕf beyond a critical χH2O marks the onset of aggregation of R1. Concomitantly, fluorescence decays become bimodal, with the emergence of a longer component associated with the aggregates. Significant heterogeneity is observed in the distribution of lifetimes, indicating the existence of aggregates of different kinds. Fluorescence quenching, observed upon further increase in χH2O, is assigned to the breakdown of aggregates into smaller ones. A qualitative correlation between the size of aggregates and τf is obtained by using fluorescence lifetime imaging microscopy.