Methylxanthines for halogen bonded cocrystals with 1,4-diiodotetrafluorobenzene: green synthesis, structure, photophysics and DFT studies

Abstract

Four new halogen-bonded cocrystals of biological methylxanthines, named caffeine, theophylline and theobromine, have been prepared with 1,4-diiodotetrafluorobenzene as a halogen bond donor by mechanochemical and solution processes. For theophylline, N⋯I and N⋯O interactions were observed, while for caffeine and theobromine, only N⋯I was detected. The solids were characterized by PXRD, SC-XRD, FTIR and thermal methods (TGA-DSC analyses). In addition, the solid-state photoluminescence properties of the methylxanthines and their respective cocrystals have been studied and quantum chemistry calculations have been performed to rationalise and understand the electronic and optical properties of all compounds. This work provides a triad of natural methylxanthines capable of forming halogen-bonded multicomponent systems that can give rise to a cocrystal-to-crystal transformation with off–on luminescence activation.