Holstein–Peirls–Hubbard trimer as a model for quadrupolar two-photon absorbing dyes

Abstract

The linear and nonlinear optical properties of a Donor–Acceptor–Donor system have been investigated by using a two-electron three-point-site model system. Some basic features of electron correlations are included in the model by means of a bi-electronic density matrix. The polarizabilities and second hyperpolarizabilities have been computed with a modified version of the Collective Electronic Oscillators (CEO) method which allowed us to include the electron–phonon coupling. Both singly- and doubly-excited states are taken into account in the computation of (hyper-)polarizabilities. The effects of electron–phonon coupling on the two-photon absorption and on the third harmonic generation in the infrared region are discussed.