A General Twisted Intramolecular Charge Transfer Triggering Strategy by Protonation for Zero-Background Fluorescent Turn-On Sensing.

The exploration of organic fluorescent sensing materials and mechanisms is of great significance, especially for the deep understanding of twisted intramolecular charge transfer (TICT). Here, the electron-donating ability of a chemically protonated amino group and the corresponding excitation primarily ensure the occurrence of excited-state intramolecular proton transfer. Due to the hybridization of the amino group from sp to sp, the steric hindrance effect and conjugative effect together boost the rotation efficiency of the TICT process and the complete elimination of the background fluorescent signal. Furthermore, a sharp turn-on fluorescent detection of trace nitrite particulate with a diameter of 0.44 μm was realized. In addition, this protonation-induced change in the amino group configuration was verified through around nine categories of compounds. We expect this modulation of the photochemical activity path of the TICT process would greatly facilitate the exploration of novel fluorescent sensing mechanisms.