Cyclic Interconversion among Molecular Salts via Neat Grinding and Related Photoluminescence Properties

We report three molecular salts formed
by 5-sulfosalicylic acid
dihydrate (A·2H2O) and 4,4′-diaminodiphenylmethane (B) in different
stoichiometric ratios. The A1B1·H2O, A1B2 and A2B1·H2O molecular salts can be accessed
via mechanochemical grinding and from solution. Structural aspects
were analyzed using single crystal and X-ray powder diffraction (XRPD).
The reversible solid state ionic interconversion between A1B1·H2O, A1B2,
and A2B1·H2O upon neat grinding by adding an appropriate amount of A·2H2O or B is described. In the ionic interconversion water molecules
are included or excluded upon grinding, which suggests that water
plays an important role in the proton transfer. The solid-state fluorescence
spectra of A1B1·H2O, A1B2, and A2B1·H2O showed that the three molecular salts
exhibit emission features and are red-shifted compared with the starting
materials A·2H2O and B. The interparticle (CT) interactions and the
degree of anion deprotonation are important in the solid-sate fluorescence
of the described molecular salts. The photoluminescence experimental
results have been corroborated using time-dependent density functional
theory (TD-DFT), which has been shown to provide reasonable results
for the excited states of molecules involved in the formation of the
molecular salts.