Selective and sensitive
moisture sensors have attracted immense
attention due to their ability to monitor the humidity content in
industrial solvents, food products, etc., for regulating
industrial safety management. Herein, a hydroxy naphthaldehyde-based
piezochromic luminogen, namely, 1-{[(2-hydroxyphenyl)imino]methyl}naphthalen-2-ol
(NAP-1), has been synthesized and its photophysical and
molecular sensing properties have been investigated by means of various
spectroscopic tools. Owing to the synergistic effect of both aggregation-induced
emission (AIE) and excited-state intramolecular proton transfer (ESIPT)
along with the restriction of CN isomerization, the probe
shows bright yellowish-green-colored keto emission with high quantum
yield after the interaction with a trace amount of water. This makes NAP-1 a potential sensor for monitoring water content in the
industrial solvents with very low detection limits of 0.033, 0.032,
0.034, and 0.033% (v/v) from tetrahydrofuran (THF), acetone, dimethyl
sulfoxide (DMSO), and methanol, respectively. The probe could be used
in the food industry to detect trace moisture in the raw food samples.
The reversible switching behavior of NAP-1 makes it suitable
for designing an INHIBIT logic gate with an additional application
in inkless writing. In addition, an Internet of Things-(IoT) based
prototype device has been proposed for on-site monitoring of the moisture
content by a smartphone via Bluetooth or Wi-Fi. The
aggregated probe also has the ability to recognize Cu2+ from a purely aqueous medium via the chelation-enhanced
quenching (CHEQ) mechanism, leading to ∼84% fluorescence quenching
with a Stern–Volmer quenching constant of 1.46 × 104 M–1 and with an appreciably low detection
threshold of 57.2 ppb, far below than recommended by the World Health
Organization (WHO) and the United States Environmental Protection
Agency (U.S. EPA). The spectroscopic and theoretical calculations
(density functional theory (DFT), time-dependent DFT (TD-DFT), and
natural bond orbital (NBO) analysis) further empower the understanding
of the mechanistic course of the interaction of the host–guest
recognition event.