Donor-Acceptor Stacks in Flexible Porous Coordination Polymers Toward Room-Temperature Ferroelectricity

Ferroelectricity has long been of great scientific interest due to its tremendous potential for information storage technologies and wide-ranging applications. Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs), with their structural tunability and dynamic host-guest chemistry, offer a powerful platform for engineering next-generation ferroelectrics. However, progress in ferroelectric MOFs/PCPs has been limited by their intrinsically weak polarization, restricting the realization of non-trivial switchable ferroelectricity feasible at room temperature. We address this limitation, reporting a series of flexible PCPs with mixed donor-acceptor (D-A) stacks designed to achieve room-temperature ferroelectricity via strong charge-transfer interactions. The 3D Zn-based PCP, {[Zn(o-phen)(2,6-ndc)]·DMF}_n, having a supramolecular dynamic nano-space for D-A stacking, exhibits intrinsic flexibility that endows structural adaptation to the guest molecules of varying shape, size, and chemical nature. The encapsulation of electron-rich aromatic amine guests like N,N'-dimethyl aniline and N,N'-dimethyl-p-toluidine results in the formation of extended D-A stacks with acceptor motifs of the framework along a particular crystallographic axis. Peierls-like distortion of these 1D extended D-A stacks gives rise to spontaneous polarization, ultimately resulting in room-temperature ferroelectricity. We furthermore show that the ferroelectric features of the PCP are closely related to the packing and geometry of the guest molecules incorporated.