Molecular Design Principles for Magneto-Electric Materials: All-Electric Susceptibilities Relevant to Optimal Molecular Chromophores

Magneto-electric (M-E) response at the molecular level arises from the interaction of matter with the electric and magnetic fields of light, and can manifest itself as nonlinear M-E magnetization (M_NL) or M-E rectification (P_NL). However, there is presently a limited understanding of how molecular material properties impact M-E response. Here we investigate the relationship between M-E nonlinear coefficients and the third-order electric susceptibility, χ⁽³⁾, finding that M_NL is proportional to χ_xxxx ⁽³⁾ while P_NL scales with χ_zzxx ⁽³⁾ due to a cascaded nonlinearity. By applying a sum-over-states (SOS) expression for the elements of χ⁽³⁾ to valence-bond charge-transfer (VB-CT) models, we formulate practical guidelines for the design of materials expected to exhibit enhanced M-E properties. On this basis, we predict that many conventional nonlinear optical chromophores with large values of χ_xxxx ⁽³⁾ may be suitable for generating optical magnetism at low intensities. In the case of M-E rectification, analysis of Λ-shaped, X-shaped, and octupolar VB-CT models suggests that their molecular structures can be tuned to enhance the response by maximizing χ_zzxx ⁽³⁾. In particular, octupolar molecules with a predominantly CT character ground state and Λ-shaped chromophores with weakly conjugated bridges between donor and acceptor moieties should promote off-diagonal nonlinearity and M-E rectification.