Humidity visualization through a simple thermally activated delayed fluorescent emitter: The role of hydrogen bonding

Humidity monitoring is of great importance in our daily life and industrial process. However, humidity sensors based on fluorescent materials are usually hybrid system, which inevitably causes poor stability and bad repeatability. Herein, a conceptually new class of humidity-responsive thermally activated delayed fluorescent (TADF) material, AcHPM, is presented. AcHPM displays significant emission color changes from deep blue (446 nm) to green (510 nm) under different relative humidity (RH) from 1 % to 95 %. In addition to the high contrast of fluorescent emission (>60 nm), the AcHPM-sensor also shows ultrafast response time (about 20 ms) and good reversibility (>10 cycles). In view of the excellent humidity-responsive performance of AcHPM, the potential application of real-time RH monitoring, anti-counterfeiting, and fingerprint identification were presented by using an AcHPM-equipped test paper. To disclose the mechanism behind the humidity-dependent fluorescence, we further carry out the Fourier transform infrared spectroscopy, powder X-ray diffraction, and theoretical simulation, which jointly verify the critical role of hydrogen bond between water and AcHPM molecule. This work not only extends an avenue to develop humidity sensor with single component for various applications in our daily life, but also provides a new sight to explore the mechanism for the fluorescent-responsive processes.