Crystals of Charge-Transfer Complexes with Reorienting Polar Molecules: Dielectric Properties and Order-Disorder Phase Transitions

摘要

Crystals of charge-transfer (CT) complexes were synthesized using the polar molecule tetrachlorophthalonitrile (TCPN) as the electron acceptor and nonpolar aromatic hydrocarbons, such as perylene, coronene, chrysene, and pyrene, as the electron donors. Variable-temperature X-ray crystal structure analyses revealed that the TCPN molecules in all the CT crystals exhibit orientational disorder at room temperature. Some of the CT crystals undergo an order-disorder-type phase transition upon cooling, where the orientation of the TCPN molecules becomes ordered at low temperature. The CT crystals show large dielectric constants at room temperature arising from in-plane reorientation of the polar TCPN molecules. The order-disorder phase transition results in a drastic reduction of the dielectric constant of the CT crystals upon cooling. This study demonstrates that the formation of CT crystals from polar molecules represents a promising method for the development of dielectric crystalline materials, whose properties vary significantly in response to phase transitions.