Charge Manipulation in Metal–Organic Frameworks: Toward Designer Functional Molecular Materials

Multi-dimensional coordination frameworks whose charge states are controllable by the sophisticated chemical modification of the components or by the application of stimuli are fascinating targets for the design of electronic/magnetic functional materials. A simple way to design such frameworks is to assemble electron donor (D) and electron acceptor (A) units in a DmAn ratio with electronically conjugated linkages; we call this type of framework a D/A metal–organic framework (D/A-MOF). In this account article, our previous studies on D/A-MOFs composed of carboxylate-bridged paddlewheel-type diruthenium units ([Ru2]) and polycyano organic molecules such as N,N′-dicyanoquinodiimine (DCNQI) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) as the D and A subunits, respectively, are summarized. In this family of D/A-MOFs, the charge distribution between the internal D and A subunits can be precisely tuned by varying their electronic structure, i.e., depending on what kind of D and A we choose. Crucially, the diverse charge states, as well as anisotropic framework and often porous nature, of D/A-MOFs are well correlated with their bulk electronic and magnetic properties. How do we design functional frameworks that are sensitive to stimuli? We have focused on the design of electron-conjugated charge-transfer (CT) frameworks comprising electron donor (D) and electron acceptor (A) sites or subunits having a DmAn formula; we denote these frameworks D/A-MOFs, which allow their charge distribution to be tuned by varying the intralattice CT between the D and A components.