THE ROLE OF INTERMOLECULAR INTERACTIONS IN MOLECULAR ELECTRONICS - COMPUTATIONAL DESIGN OF ARCHITECTURES WITH LARGE 2ND-ORDER OPTICAL NONLINEARITIES

The role of intermolecular interactions in determining quadratic nonlinear optical macroscopic hyperpolarizabilities is investigated using the intermediate neglect of differential overlap (INDO/S) (ZINDO) sum-over-excited particle-hole-states (SOS) formalism on clusters (dimers and trimers) of archetypical donor-acceptor pi-electron chromophores. The calculated aggregate hyperpolarizability strongly depends on relative molecular orientations, exhibiting the largest values in slipped cofacial arrangements, where the donor substituent of one unit is in close proximity to the acceptor substituent of the nearest neighbor. These results argue that cofacial assembly of chromophores having low dipole moments should maximize molecular contributions to the macroscopic susceptibility. The classical ''two-level'' model is generally a good approximation for estimating hyperpolarizabilities in such systems. The second-order response of model molecular 1:1 and asymmetric 2:1 electron donor-acceptor (EDA) complexes is also investigated using the INDO/S formalism. Intermolecular charge-transfer (CT) transitions in EDA complexes represent a promising approach to achieving sizable second-order optical nonlinearities. Calculated hyperpolarizabilities reflect the strength of the donor-acceptor interaction in the complex, affording for a given acceptor, the largest values in the case of the strongest donors. The large change in dipole moment accompanying intermolecular CT transitions and low-lying CT excitation frequencies are major sources of the large calculated second-order nonlinearities. The two-level model is again a useful first approximation for predicting the nonlinear response of such complexes.