H-bond engineering as a general strategy for inhibiting twisted intramolecular charge transfer in donor–acceptor fluorescent probes: Reshaping the pre-twisting method

Twisted intramolecular charge transfer (TICT) is a fluorescence quenching mechanism that occurs in donor–acceptor (D‒A) molecules. Chemical engineering research into TICT regulation over the past 50 years has primarily focused on manipulating steric factors by introducing alkyl groups at the D–A junction (pre-twisting). Herein, we report a significant advance in TICT-based probes through the introducing of H-bond as an efficient strategy for suppressing TICT. Accordingly, ortho-Cl installation in the N-phenylpyrazine-2-carboxamide (PPC) platform can achieve complete reversal from the quenching mode to the light-up mode. This specific H-bonding (N–H⋯Cl) effectively blocks N–C(Ar) bond rotation, leading to fluorescence-ON. This suggested that TICT inhibition may be involved. Therefore, in a sharp contrast to the general nature of the pre-twisting method in rotor molecules, which involves incorporating steric hindrance at either the donor or acceptor moiety to enhance intramolecular rotation (promotion TICT), the ortho-H bonding strategy completely freezes D‒A bond twisting (suppression TICT), resulting in improved fluorescent intensity. Furthermore, the fluorophores were evaluated for Hg2+ detection and in vivo bio-imaging. Notably, Hg-complexation induced another fluorescence inversion (OFF-ON) by imposing spatial constraints on twisting freedom in 3,4-Cl-PPC. Taken together, this work provides a valid and generalizable tactic for the development of high-performance sensing fluorophores through inhibition of TICT.