Two-Photon Absorption Cross-Sections from Electronic Structure Methods: Mesoionic Compounds

Abstract

Absorption of two photons by organic molecules has many important technological applications, making theoretical modeling and synthesis of new molecules with large values of cross-sections, δ(ω), much needed activities. In this article, we present a computational procedure based on semiempirical electronic structure methods, which allows us to rank a series of homologous compounds in terms of δ(ω), as well as estimate single- and two-photon absorption wavelengths. This procedure uses a truncated version of the expression for the two-photon absorption peak, which retains only 4 of the 24 terms of the nontruncated equation and results in two-photon absorption peaks that are nearly identical to the ones obtained using all 24 terms, provided the incident photon energy is far from one-photon resonances. When the incident photon energy gets closer to one-photon resonances, the negative component of the hyperpolarizability becomes dominant and we advance a simplified, but still exact, form of this negative compo...