A Phototautomeric 3D Covalent Organic Framework for Ratiometric Fluorescence Humidity Sensing

Photoinduced
proton transfer is an essential photochemical process
for designing photocatalysts, white light emitters, bioimaging, and
fluorescence sensing materials. However, deliberate control of the
excited/ground states and meticulous manipulation of the excited state
intramolecular proton transfer (ESIPT) pathway constitute a significant
challenge in liquids and dense solids. Here, we present the integration
of a hydronaphthoquinone fluorophore into a crystalline, porous, phototautomeric
dynamic 3D covalent organic framework (COF) to show guest-induced
fluorescence turn-on, emission redshift enhancement, and shortened
lifetimes for ratiometric fluorescence humidity sensing. Theoretical
and spectroscopic studies provide mechanistic insights into the conformational
dynamics, charge transfer coupled with local excitation, and ground-state
uphill regulation for the multiple tautomers. We illustrate the sensitive,
rapid, steady, and self-calibrated ratiometric fluorescence sensing
for a wide range of humidity benefiting from the architectural and
chemical robustness and crystallinity of such a phototautomeric 3D
COF. These findings provide molecular insights into the design of
functional porous materials that integrate host–guest mutual
recognition and photoelectronic response for multiplex molecular sensing
for environmental monitoring and biomedical diagnostics applications.