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 (ϕ_f) and lifetime (τ_f) of R1 decrease monotonically up to a rather high molar ratio of water (${\chi }_{{\mathrm{H}}_2\mathrm{O}}$). 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 ϕ_f beyond a critical ${\chi }_{{\mathrm{H}}_2\mathrm{O}}$ 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 ${\chi }_{{\mathrm{H}}_2\mathrm{O}}$, is assigned to the breakdown of aggregates into smaller ones. A qualitative correlation between the size of aggregates and τ_f is obtained by using fluorescence lifetime imaging microscopy.