Design of Chromophoric Molecular Assemblies with Large Second-Order Optical Nonlinearities. A Theoretical Analysis of the Role of Intermolecular Interactions

The role and nature of intermolecular interactions in determining quadratic nonlinear optical macroscopic hyperpolarizabilities are investigated using the INDO/S (ZINDO) sum-over excited particle-hole-states formalism on clusters (dimers and trimers) of archetypical donor/acceptor organic π-electron chromophore molecules. It is found that the calculated aggregate hyperpolarizability depends strongly on relative molecular orientations, exhibiting the largest values in slipped cofacial arrangements, where the donor substituent of one molecular unit is in close spatial proximity to the acceptor substituent of the nearest neighbor. These results convey important suggestions for the design of multichromophore assemblies having optimum x⁽²⁾ values. For example, cofacial assembly of chromophores having low ground-state dipole moments should maximize molecular contributions to the macroscopic susceptibility. The classical “two-level” model is a good approximation for estimating the hyperpolarizability in such cluster systems, although at larger distances it yields overestimated β_ijk values. Other cases where the two-level model breaks down more significantly are also identified.