Performance of Density Functional Approximations in Calculations of Electronic Two-Photon Transition Strengths of Fluorescent Dyes

The performance of B3LYP, PBE0, and CAM-B3LYP functionals in the prediction of the two-photon transition strengths (for low-energy transitions) of 18 dipolar donor-acceptor systems containing a difluoroborate moiety was evaluated against results obtained using the resolution-of-identity implementation of the coupled-cluster CC2 model. The generalized few-state model approach, in which the two-photon transition strength is expressed in terms of electronic structure parameters, i.e., excitation energies, dipole moments, and transition dipole moments, was applied to gain deeper insight into the behavior of selected exchange-correlation functionals. The obtained results show that all three functionals provide two-photon transition strengths that differ significantly from the reference strengths, especially in the case of molecules exhibiting the highest 2PA strength.