Calculation of two-photon absorption spectra of donor-pi-acceptor compounds in solution using quadratic response time-dependent density functional theory

Linear and quadratic response time-dependent density functional theories have been applied to calculate the photophysical properties of donor-pi-acceptor molecules which are known to have large nonlinear absorption. The linear absorption and two-photon absorption spectra predicted using hybrid functionals, including the Coulomb-attenuated model, with continuum solvation models are reported and compared to experiment and to previous theoretical predictions. While the quadratic response with these functionals overestimated the TPA cross sections relative to experiment when a Gaussian linewidth function was used, a fairly good agreement was obtained when a Lorentzian linewidth function was applied. In addition, the comparison of the TPA cross sections calculated by the sum over states with those calculated by the two-state approximation indicates the importance of the higher energy states in TPA, particularly in nondegenerate experiments.