Humidity Sensing through Reversible Isomerization of a Covalent Organic Framework

Here we report that a covalent organic framework (COF),
which contains
2,5-di­(imine)-substituted 1,4-dihydroxybenzene (diiminol) moieties,
undergoes color changes in the presence of solvents or solvent vapor
that are rapid, passive, reversible, and easily detectable by the
naked eye. A new visible absorption band appears in the presence of
polar solvents, especially water, suggesting reversible conversion
to another species. This reversibility is attributed to the ability
of the diiminol to rapidly tautomerize to an iminol/cis-ketoenamine and its inability to doubly tautomerize to a diketoenamine.
Density functional theory (DFT) calculations suggest similar energies
for the two tautomers in the presence of water, but the diiminol is
much more stable in its absence. Time-dependent DFT calculations confirm
that the iminol/cis-ketoenamine absorbs at longer
wavelength than the diiminol and indicate that this absorption has
significant charge-transfer character. A colorimetric humidity sensing
device constructed from an oriented thin film of the COF responded
quickly to water vapor and was stable for months. These results suggest
that tautomerization-induced electronic structure changes can be exploited
in COF platforms to give rapid, reversible sensing in systems that
exhibit long-term stability.