Water motion-controlled reversible phase transition and de/absorption-controlled reversible phase transformation in the hydrate crystal (BEDABCO)ClO4·H2O and its analogs

Abstract

A reversible phase transition and phase transformation double-adjustable compound (BEDABCO)ClO₄·H₂O (1) and its analogs (BEDABCO)ClO₄·D₂O (2), (CEDABCO)ClO₄·H₂O (3) and (BEDABCO)BF₄·H₂O (4) were synthesised and studied (BEDABCO = 1-(2-bromoethyl)-1,4-diazabicyclo[2.2.2]octane, CE = chloroethyl group). It was found that all the compounds exhibit reversible phase transitions owing to the swing-like movement of the water molecules. Strength change of the anion-based hydrogen bonds controls the phase transition temperature point, e.g. the temperature point of 4 shows a decrease of about 30 K compared to 1. Upon heating–cooling cycles, the water molecules partially escape from the crystals, which can transform into dehydrated unsteady states and recover the rehydrated states at room humidity (30–40% RH) by reversible de/absorption of the water molecules. The phase transition and transformation properties based on the water quantity in the crystal lattices can be used to develop time switches and water catchers or new water sensor devices.