Intermolecular hydrogen bonds induce restriction of access to the dark state for triggering aggregation-induced emission

Since the concept of aggregation-induced emission (AIE) was proposed, along with the development of new AIE molecules and broadening of its application field, the exploration of its mechanism has attracted more and more attention. During the past years, various mechanisms have been proposed to reveal this novel phenomenon, including restriction of intramolecular motion (RIM) and restriction of access to the dark state (RADS). However, most of the studies have been focused on the effect of molecular structures and their stacking mode on their AIE behavior, and little effort has been made to reveal the role of the interaction between solvent and AIE aggregates on it. Herein, a series of AIE molecules have been designed and synthesized by introducing the carbonyl group into different aromatic rings, since it possesses a good spin-orbit coupling (SOC) effect and hydrogen bond acceptor, which could allow the molecules to enter the dark state. The solvent effect on their fluorescence properties and excited state electronic structures was investigated by using transient and steady state spectra, X-ray single crystal diffraction spectra and density functional theory (DFT) calculations. It was found that the water molecules not only acted as the poor solvent to induce the significant aggregation and inhibit the nonradiative transition but also as the hydrogen bond donor to induce the RADS process for triggering fluorescence. Moreover, owing to the unique intermolecular hydrogen bond effect on its fluorescence behavior, the 4-(quinolin-8-yl) benzaldehyde (QLB-8) molecule had been applied to the detection of aspartic acid. These results not only clarify the role of hydrogen bonding in AIE properties, but also provide new ideas to further design AIE molecules.