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

We report three molecular salts formed by 5-sulfosalicylic acid dihydrate (<b>A·2H</b>_<b>2</b><b>O</b>) and 4,4′-diaminodiphenylmethane (<b>B</b>) in different stoichiometric ratios. The <b>A</b>_<b>1</b><b>B</b>_<b>1</b><b>·H</b>_<b>2</b><b>O</b>, <b>A</b>_<b>1</b><b>B</b>_<b>2</b> and <b>A</b>_<b>2</b><b>B</b>_<b>1</b><b>·H</b>_<b>2</b><b>O</b> 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 <b>A</b>_<b>1</b><b>B</b>_<b>1</b><b>·H</b>_<b>2</b><b>O</b>, <b>A</b>_<b>1</b><b>B</b>_<b>2</b>, and <b>A</b>_<b>2</b><b>B</b>_<b>1</b><b>·H</b>_<b>2</b><b>O</b> upon neat grinding by adding an appropriate amount of <b>A·2H</b>_<b>2</b><b>O</b> or <b>B</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 <b>A</b>_<b>1</b><b>B</b>_<b>1</b><b>·H</b>_<b>2</b><b>O</b>, <b>A</b>_<b>1</b><b>B</b>_<b>2</b>, and <b>A</b>_<b>2</b><b>B</b>_<b>1</b><b>·H</b>_<b>2</b><b>O</b> showed that the three molecular salts exhibit emission features and are red-shifted compared with the starting materials <b>A·2H</b>_<b>2</b><b>O</b> and <b>B</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