Excitonically coupled oligomers and dendrimers for two-photon absorption

Abstract

The first part of this review presents the state of the art on two-photon absorbing molecules. Early works concerned the optimization of dipolar push-pull molecules. Recently, the synthesis of linear and branched centrosymmetrical quadrupolar molecules led to higher nonlinearities, but still far from reaching the fundamental limits of molecular two-photon cross-sections. The second part of this review summarizes our theoretical and experimental results on fluorene-based oligomers, and branched oligomers (V-shape molecules and dendrimers). They are model molecules to investigate an alternative approach based on spatial assemblies of nonsubstituted π-electron systems that are coupled by dipole-dipole interactions. In all geometries, two-photon absorption cross-sections have superlinear dependencies that depend on coupling energies and oligomer sizes. These results are well rationalized by an excitonic model based on N interacting three-level systems. Analytical expressions for one-photon and two-photon energies and absorption strengths are derived for linear oligomers. An accurate calculation of large exitonic systems is obtained by diagonalizing the Hamiltonian operator on a reduced basis set. © 2008 Springer-Verlag.