Dipole orientation effects on nonlinear optical properties of organic molecular aggregates

We consider a few dipolar organic molecules in several of their packing arrangements to understand the aggregation effect. We have performed an extensive semiempirical calculations based on multireference doubles configuration interactions for the dimer arrangements. This is coupled with a simple theory based on dipole-dipole interactions and hydrogen-bonding effects. We find that the best dimer configuration is the in-line head-to-tail arrangement of the monomer molecules, which gives rise to an enormous increase in nonlinear optical properties compared to its monomer counterparts, at small distances. We have also shown that such dimer configurations have an appreciable absorption intensity, and for an aggregate, the absorption appears deep in the IR region. These excitations are excitonic in character and are associated with a large dipole moment change, along the long axis of the dimer configurations. We suggest the experimental methods by which such tunings can be realized