Viscosity responsiveness of excited-state dynamics in aggregated-induced emission luminogens

Aggregation-induced emission luminogens (AIEgens) exhibit viscosity-responsive behavior resembling those of molecular rotors; however, their response mechanisms are more complex and cannot be adequately described using simple rotational models. AIEgens demonstrate intricate dynamics that are highly dependent on their molecular structures. In this study, we synthesized water-soluble derivatives of representative AIEgens, including tetraphenylethene (TPE), bis(N,N-dialkylamino)anthracene (BDAA), and bridged stilbene, and systematically investigated the dependence of their photophysical properties in water/glycerol mixed solvents on temperature and viscosity. To elucidate the origin of their viscosity responsiveness, quantum chemical calculations were conducted to analyze their potential energy surfaces (PESs). The results revealed that compared to typical molecular rotors, these AIEgens exhibit significantly higher sensitivity to viscosity in low-viscosity regions. Notably, for TPE and BDAA derivatives, the viscosity responsiveness was found to be governed not by the activation energy barrier (ΔE_a) based on the PES, but rather by the viscosity-dependent constraints on molecular structural changes. Furthermore, molecules possessing multiple aromatic rings or large, flexible, rotatable moieties were found to exhibit enhanced sensitivity to viscosity due to increased frictional interactions in solutions. This study provides critical insights into the mechanistic origins of the viscosity responsiveness of AIEgens, thereby advancing the fundamental understanding of their behavior and expanding their potential application as viscosity-sensitive probes.