Independent rotation design on ESIPT-scaffold for precise fluorescence modulation and trace hydrazine sensing

Hydrazine (N 2 H 4 ) is a highly toxic and carcinogenic chemical extensively used in pharmaceutical manufacturing, agrochemicals, and aerospace industries, posing significant threats to environmental safety and human health due to its persistence and volatility. Consequently, the development of reliable methods for trace-level detection of hydrazine in both aqueous and gaseous environments is urgently needed. Here, we report a fluorescence turn-on probe based on an excited-state intramolecular proton transfer (ESIPT) platform, employing a unique rotation-induced complete energy dissipation strategy with the advantage of effective conversion between the turn-on/off mode. The probe is rationally designed by embedding the dual isolated rotors onto the ESIPT-scaffold, which introduces substantial rotation freedom in molecules and large dihedral angular distortion, resulting in the dissipation of the original radiative transition and the Emission-OFF state. Upon the specific binding of analyte and the reactive rotor, the rigid hexagonal ring could efficiently form and the π-conjugation plane could further expand, leading to the disruption of ESIPT and the Emission-ON state. This mechanism enables efficient switching from a non-emissive to a highly emissive state with near-zero background interference. Thus, a boosted turn-on detection towards trace hydrazine was achieved in both aqueous and vapor phases with theoretical limit of detection (LOD) of 11.64 nM and 26 ppb, as well as desirable anti-interference capability against 18 soluble and 11 volatile substances, respectively. The present strategy is expected to greatly expand the near-zero background fluorescence turn-on molecular design strategies and open avenues for high-performance volatile threats sensing.