ACQ to AIE transformation of quinoline derivatives: modulating substituent electronic effects to alter excited-state reorganization energy distribution

Abstract

Since the discovery of aggregation-induced emission (AIE), the development of new AIE fluorogens (AIEgens) has become a significant field owing to their versatile application in diverse fields. To achieve aggregation-caused quenching (ACQ) to AIE conversion, considerable attention is paid to the strategy of changing the stacking pattern of molecules, and several researchers are focused on modulating the electronic structure of the excited state of molecules. In this study, acetoxy and methoxy electron-donating groups (EDG) and carboxyl and ester electron-withdrawing groups (EWG) were introduced onto quinoline through a facile Suzuki coupling reaction. The experimental results reveal that the behavior of quinoline derivatives can be converted from ACQ to AIE by altering substituents from EDG to EWG. Single-crystal X-ray diffraction (SCXRD) and transient and steady-state spectra were employed to reveal that their AIE mechanism involved the restriction of intramolecular motion (RIM). In addition, density functional theory (DFT) calculations showed that the ACQ to AIE transition can be achieved by changing the reorganization energy distribution of the molecule with different substituents in the excited state. Furthermore, the coordination of quinoline nitrogen atoms to Fe³⁺ facilitated the production of larger nanofibers that possessed enhanced AIE properties. This unique characteristic was utilized to leverage the capabilities of methyl 4-(quinolin-5-yl)benzoate (5-MQB) as a ratiometric fluorescent probe for detecting Fe³⁺ in aqueous solution.