Two-Photon Absorption by Fluorene Derivatives: Systematic Molecular Design

Abstract

In this article, we employ a systematic approach to the computational quantum chemical study of the two-photon absorption (2PA) properties of 161 representative molecules containing a symmetrically substituted fluorene unit. The molecules studied contain meta- or para-substituted phenyl groups, five- and six-membered heterocycles, and benzo derivatives of five-membered heterocycles. The computational procedure employed to calculate the 2PA parameters was previously described [Chem. Mater. 2008, 20, 4142] and is based on semiempirical electronic structure methods: the RM1 model to optimize the molecular geometry and the INDO/S method to calculate the spectroscopic properties of the molecules. We further advance a new, simplified expression employed to calculate an approximate three-level contribution of the imaginary part of the negative component of the second hyperpolarizability. We then show that, in order to rationalize the 2PA cross sections for the substituted fluorenes, the three-level approximation...