Cyclization regulated fluorescent emission on multiple conjugate acceptors: mechanistic studies and protein labeling applications in living cells

With this research, we introduce and validate the cyclization-regulated fluorescence emission (CRFE) as a photophysical mechanism that activates fluorescence through chemically triggered cyclization. This mechanism, developed from traditional fluorescence principles, generates fluorescence emission only when the rigidity of the lower part of the molecule increases through cyclization, while still satisfying the conditions of other fluorescence mechanisms. The universality of this mechanism was verified in multiple conjugate acceptors derived by integrating different fluorophores into a single conjugated acceptor via Suzuki coupling. The photophysical properties of these molecules were characterized, with density functional theory (DFT) calculations providing insight into the conformational and electronic changes occurring during excitation. It is concluded that cyclization stabilizes the conjugated double bond during excitation, facilitating fluorescence emission. Guided by the mechanism and chemical reactivity, a typical fluorescent probe was exploited for the selective labeling of vicinal thiol groups on target proteins, both in vitro and in living cells. This work presents a new pathway for the design of fluorescent probes with activable photoluminescence, highlighting the potential applications in bioimaging, protein dynamics tracking, and biosensing.