Microscopic model for superexchange interactions and photomagnetism in binuclear transition metal complexes

Recent experiments show that the superexchange interaction in molecular clusters containing transition metal ions A = Ni-II and B = W-V, Nb-IV or Mo-V in some cases is antiferromagnetic, contrary to the conventional superexchange rules. To understand this anomaly, we develop a quantum many-body model Hamiltonian and solve it exactly using a valence bond (VB) approach. We identify the various model parameters which control the ground state spin in different clusters of the A-B system. We present quantum phase diagrams that delineate the high and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region of intermediate spin ground state in the parameter space, in clusters of larger size. The spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The above microscopic model is generic and can also be employed to explain photomagnetism in the MoCu6 system. We solve the model for MoCu6 and find that ground state is degenerate and is spanned by the S 0, 1, 2 and 3 manifolds with doubly occupied Mo site corresponding to Mo(IV) and singly occupied Cu sites corresponding to Cu(II) configurations. In each of these spin spaces, we observe that there exist charge-transfer (CT) states at approximate to 3 eV above the ground state which are dipole coupled to the ground state. The transition dipole in the S = 3 manifold is the largest for the CT excitations. Coupled with the fact that the density of states of the S = 3 manifold is sparse, compared to other spin manifolds, we expect that the S = 3 CT excited state to be long-lived, thereby explaining the experimentally observed photomagnetism in the MoCu6 system.