Two-dimensional characteristics of the second-order nonlinear optical response in dipolar donor-acceptor coordination complexes

The two-dimensional (2D) nonlinear optical (NLO) response of a series of dipolar donor-acceptor bis(salicylaldiminato)Ni(II) Schiff base planar coordination complexes was investigated using the INDO/SCI-SOS formalism. The in-plane 2D NLO character of this class of materials can be related to various low-energy charge-transfer (CT) states and is achieved by an accurate design of substitution pattern of the unsubstituted bis(salicylaldiminato) framework. While z-polarised electronic transitions, parallel with respect the molecular dipolar (z) axis, contribute to the diagonal hyperpolarisability tensor component, perpendicular x-polarised transitions contribute to off-diagonal hyperpolarisability tensors. Unlike non-dipolar multidimensional NLO structures, whose nonlinearity within the framework of the sum-over-states formalism is necessarily due only to three-level terms, in this case the 2D nonlinearity is generally dominated by two-level terms. It necessarily requires the existence of a strong, lowest energy, perpendicular CT transition in order to provide a substantial two-level contribution to off diagonal tensors.