Solvatochromic and Viscosity‐Dependent Aggregation‐Induced Emission in a Fluorene‐Based Schiff Base

ABSTRACT

A fluorene‐based Schiff base chromophore 8 incorporating an azomethine (─C═N─) linkage was synthesized, and its photophysical properties were systematically investigated. The chromophore exhibits pronounced positive solvatochromism, with the fluorescence emission maximum shifting from 390 nm in toluene to 428 nm in acetonitrile, indicating the formation of an intramolecular charge‐transfer (ICT) excited state. In dilute solution, the fluorophore displays weak fluorescence, whereas a significant enhancement in emission intensity is observed in both aggregated and highly viscous media. Dynamic light scattering measurements confirmed the formation of nanoscale aggregates in water‐rich mixtures, providing direct evidence for its aggregation‐induced emission (AIE) behavior. Furthermore, the fluorescence quantum yield increased markedly from 0.09 in methanol to 0.56 in a 90% glycerol medium, consistent with the restriction of intramolecular motion (RIM) mechanism. Time‐dependent density functional theory (TD‐DFT) calculations revealed a donor‐π‐acceptor electronic structure with a dominant HOMO→LUMO ICT transition, in excellent agreement with the experimental results. The combined solvatochromic, AIE, and viscosity‐responsive fluorescence properties highlight this fluorene‐based Schiff base as a promising environmentally sensitive chromophore for advanced sensing and optoelectronic applications.