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

Abstract

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 ≈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.