H-bond engineering as a general strategy for inhibiting twisted intramolecular charge transfer in donor-acceptor...

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.