Control of Charge Transfer in Donor/Acceptor Metal-Organic Frameworks

Charge transfer (CT) of D(0)A(0)<-> D(delta+)A delta(-) not only involves an electron transfer from D to A, but also generates a new spin set of S = 1/2 spins with an exchange interaction. Therefore, the control of CT in multidimensional frameworks could be an efficient way to design electronically/magnetically functional materials. The use of redox-active metal complexes as D and/or A building blocks expands the variety of such D/A frameworks with the formulation of D(m)A(n) (m, n >= 1), permitting the design of donor/acceptor metalorganic frameworks (D/A-MOFs). This Account summarizes our ongoing research on the design of D/A-MOFs and on the systematic control of CT in such D/A-MOFs toward the discovery of unique electronic/magnetic materials exhibiting nontrivial phenomena. For this purpose, the D/A combinations of carboxylate-bridged paddlewheel-type diruthenium(II,II) complexes ([Ru-2(II,II)]) that act as one-electron (1e(-)) donors and polycyanoorganic acceptors such as 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and N,N'-dicyanoquinodiimine (DCNQI) have been chosen. Even in the covalently bonded motif, the CT in this system is systematically dependent on the intrinsic ionization potential (I-D) and electron affinity (E-A) of the D and A units, respectively, which is controllable by chemical modification of the D/A units. As we consider the energy difference between the HOMO of D and the LUMO of A (Delta EH-L(DA)) instead of h nu(CT) proportional to vertical bar I-D - E-A vertical bar, the neutral (N) and ionic (I) states can be defined as follows: (i) the D/A materials with Delta EH-L(DA) > 0 (i.e., the LUMO level of A is higher than the HOMO level of D) should be neutral, and (ii) complexes adopted when Delta EH-LL(DA) < 0 are, meanwhile, ionic. Materials located near Delta EH-L(DA) approximate to 0, that is, at the boundary between the N and I phases, are candidates for the N-I transition driven by external stimuli such as temperature, pressure, and photoirradiation. Even in the ionic state, two distinct states could be isolated for the D(2)A type: (ii-1) the 1e(-) transferred D(2)A-MOFs provide mixed-valence systems of D(+)D(0)A(-) possibly involving intervalence CT, which produce magnetic correlations via radical A units, and (ii-2) when the 2e(-) reduced form of A (e.g., TCNQ(2-)) is energetically favored beyond the on-site Coulomb repulsion on A, the oxidation state of D+ (2)A(2) is produced, for which magnetic measurements reveal a paramagnetic state attributed to the isolated D+ units. The interspatial Coulombic interaction is another factor in determining the charge distribution in materials, which is related to the spatial Coulombic stability of D/A packing and possibly yields a mixture of N and I domains when it is more advantageous to get Coulombic gain than in the uniform N or I phase. Such a phase could be observed at the boundary between N and I phases involving the N-I transition. These charge-distributed states/phases are systematically demonstrated in a D/A-MOF system made by the combination of [Ru-2(II,II)] and TCNQ/DCNQI; however, we immediately recognize the charge distribution of D/A-MOF only by understanding the nature of the starting D/A units. The present D/A-MOF system should be an intriguing platform to look for new functionalities with synergistic correlations among charge, spin, and lattice.