VOC Triggered Optical Signaling: C–N-Fused Azophenine Tetramer as an Exogenous Imaging Agent, Molecular Traffic Light, and R-S Latch Circuit

Abstract Herein, for the first time, an azophenine-based organic functional material, N1,N2,N4-tris(2-methoxyphenyl)benzene-1,2,4,5-tetraamine (MMCT), was rationally synthesized by the solvent-free tetramerization of o-anisidine using low Lewis-acid catalyst loading. SC-XRD, HR-MS, UV–vis, fluorescence, FT-IR, and 1H NMR spectroscopy studies were used for the full-fledged characterization of MMCT. The optical probe, MMCT, selectively and rapidly recognized the toxic industrial VOC pollutant, formaldehyde (FA), exhibiting intense green emission within 10 min through formimine adduct formation, attenuating photoinduced electron transfer (PET), which operated in the free sensor from the amino functional group to the molecular scaffold. In the presence of FA, the emission band of MMCT showed a dramatic ∼7-fold fluorescence enhancement with ∼50 nm bathochromic shifting from 465 to 515 nm due to the prohibition of the intramolecular charge transfer process throughout the molecular skeleton, leading to a low detection threshold of 5.6 μM (168.2 ppb), which lies below the physiological FA level (50–500 μM). After interaction with FA, the quantum yield of MMCT was enhanced from 0.04 to 0.53 with pseudo-first-order reaction kinetics of 0.23 min–1. A reduced HOMO–LUMO energy gap, obtained from theoretical DFT studies, was found to thoroughly validate the experimentally obtained data. MMCT has also been successfully employed for the detection of FA in the vapor phase. Inspired by the good sensitivity and fast response time, the application of MMCT was expanded toward FA detection in fetal bovine serum specimens, from which FA was recovered with a quality rate (∼93%). Interestingly, MMCT was also exploited for in vitro cellular imaging of exogenous FA in the oral cancer KB cell line. Furthermore, the reversible OFF–ON response of NaHSO3 can efficiently formulate the potential molecular traffic signals and R-S latch logic circuitry-functionalized memory elements, highlighting its potential as a multifunctional optical switching platform.