DCM self-trapping by the host deformation in flexible host–guest molecules

Abstract

Host–guest molecular crystals are classical types of molecular materials widely applied for fluorescent sensors, absorption, separation, etc. Their significance lies in deciphering the intermolecular interactions in crystal engineering and expanding the scope of pharmaceuticals and chemical-responsive smart materials. Here, we report three flexible host–guest molecular solvate crystals, 1·CH₂Cl₂, 1·dioxane, and 1·DMF, that crystallize with a much lower boiling point guest dichloromethane (DCM, 313 K), and high boiling points guest dioxane (374 K) and dimethylformamide (DMF, 426 K). Surprisingly, thermogravimetric and crystallographic characterizations show that 1·CH₂Cl₂ has a relatively high desolvation onset temperature of 390 K, which is much higher than that of 1·dioxane (305 K) and 1·DMF (307 K). Crystal structure analysis shows that this high stability originates from the confinement and steric effects of the trapped guest solvent molecules in the crystal cavities. Powder X-ray diffraction shows that the three compounds' desolvated powder can self-trap the CH₂Cl₂ (DCM) molecules, returning to the 1·CH₂Cl₂ state. Moreover, spectroscopic characterization presents an emission redshift of about 30 nm for 1·dioxane and 1·DMF compared to that of the former. These results are attributed to the flexible host plate's dihedral angle that undergoes a drastic change of about 50° which leads to the greatly enhanced conjugation degree of the host plate.