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The second-order nonlinear optical response of model molecular 1:1 and asymmetric 2:1 organic pi electron donor-acceptor (EDA) complexes is investigated using the INDO/S sum-over-''cited particle-hole-states formalism. It is found that intermolecular charge-transfer transitions in EDA complexes represent a promising approach to achieving sizablc second-order optical nonlinearities. Calculated hyperpolarizabilities may be generally related to the strength of the donor-acceptor interaction in the complex, affording for a given acceptor, the largest values in the case of aminoarene donors. The large change in dipole moment that accompanies intermolecular charge-transfer transitions and the relatively low-lying charge-transfer excitation energies are the major sources of the large calculated second-order nonlinearities. The relative orientation of donor and acceptor components is also an important feature, leading to stabilization of the ground state as well as to maximization of the oscillator strength of the lowest energy charge-transfer excitation and, in tum, the NLO response. In the case of asymmetric 2:1 EDA complexes, calculated hyperpolarizability enhancements over the 1:1 complexes can be related to the red-shift of the charge-transfer excitation as well as to an increase in dipole moment change between ground and excited states. The perturbation theoretical ''two-level'' model is a useful first approximation for predicting the second-order nonlinear response of such complexes.

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