Third-order nonlinear optical properties of dendritic molecular aggregates: Effects of fractal architecture

Abstract

We investigate the microscopic third-order nonlinear optical properties, i.e., the second hyperpolarizabilities (γ), of two different sizes of molecular aggregates with a dendritic, i.e., Bethe-lattice, structure. One possesses a nonfractal structure, while the other has a fractal structure. The aggregate is treated in a two-exciton model composed of two-state monomers coupled to each other by the dipole–dipole interaction. The off-resonant γ of the aggregates are calculated by the numerical Liouville approach, including relaxation effects. The total γ value is partitioned into the contribution of virtual exciton generation, and its spatial contribution to γ is analyzed in relation to the virtual excitation processes in the perturbation theory. It is found that the intermolecular-interaction effect enhances both one- and two-exciton-generation contributions, while the relaxation effect reduces those, although the one- and two-exciton-generation contributions have mutually opposite signs. From the comparison of spatial contributions to γ between the nonfractal and fractal aggregates, an enhancement of the contribution to γ from the periphery to the core is observed in the fractal structure, while such a feature is not observed in the nonfractal structure. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001