Acid Responsive Hydrogen-Bonded Organic Frameworks

A porous
hydrogen-bonded organic framework (HOF) responsive to
acid was constructed from a hexaazatrinaphthylene derivative with
carboxyphenyl groups (CPHATN). Precise structures of
both 1,2,4-trichlorobenzene solvate [CPHATN-1­(TCB)] and
activated HOF with permanent porosity (CPHATN-1a) were
successfully determined by single-crystalline X-ray diffraction analysis.
Permanent porosity of CPHATN-1a was evaluated by gas
sorption experiments at low temperature. CPHATN-1a also
shows significant thermal stability up to 633 K. Its crystals exhibit
a rich photochemistry thanks to intramolecular charge-transfer and
interunit proton-transfer reactions. Femtosecond (fs) experiments
on crystals demonstrate that these events occur in ≤200 fs
and 1.2 ps, respectively. Moreover, single-crystal fluorescence microscopy
reveals a shift of the emission spectra most probably as a result
of defects and a high anisotropic behavior, reflecting an ordered
crystalline structure with a preferential orientation of the molecular
dipole moments. Remarkably, CPHATN-1a, as a result of
the protonation of pyradyl nitrogen atoms embedded in its π-conjugated
core, shows reversible vapor acid-induced color changes from yellow
to reddish-brown, which can be also followed by an ON/OFF of its emission.
To the best of our knowledge, this is the first HOF that exhibits
acid-responsive color changes. The present work provides new findings
for developing stimuli responsive HOFs.