While the restriction of intramolecular motion (RIM) is widely recognized as the core mechanism of aggregation-induced emission (AIE), achieving effective RIM in the nonaggregated state remains a significant challenge. Constructing such a system with improved dispersion and intensified fluorescence is crucial for expanding the applications of AIE luminogens (AIEgens) in scenarios that call for high dispersion stability, such as physiological environments and high-sensitivity sensors. In this study, we report a strategy to attain intense fluorescence in the nonaggregated state through electrostatic-interaction-mediated RIM, establishing a dual-mode RIM system that switches between conventional aggregation-driven RIM and electrostatically confined nonaggregated RIM. A tetra-amidine-functionalized AIEgen, TPE-4amidine, was designed and synthesized. It exhibits reversible fluorescence switching between the aggregated state (blue emission, λem ≈ 475 nm) and the nonaggregated state (quenched) in aqueous solution via pH variation. Crucially, the introduction of a small amount of poly(acrylic acid) (PAA) into a dilute TPE-4amidine solution successfully induced intense blue emission, providing direct evidence for achieving RIM primarily via electrostatic interactions assisted by directional hydrogen bonding without aggregation. Furthermore, upon incorporating TPE-4amidine into a hydrogen-bonded smectic liquid crystal polymer (SLCP) network, a remarkable fluorescence color shift to yellow-green was observed. More importantly, adjusting the pH enables reversible emission color switching of the SLCP film between yellow-green and blue. This system exhibits reversible pH-dependent color changes, highlighting its potential for anticounterfeiting applications. The underlying mechanism, highlighting the pivotal role of electrostatic interactions in governing RIM across different states, was unequivocally corroborated by comprehensive spectroscopic analyses and density functional theory (DFT) calculations. This work underscores the great potential of achieving restricted intramolecular motion in a nonaggregated electrostatically confined state for the development of advanced stimulus-responsive luminescent materials.
Mai et al. (Thu,) studied this question.