Cyclic Interconversion among Molecular Salts via Neat rinding 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.