We present a benchmark study of density functional approximation (DFA) performances in predicting the two-photon-absorption strengths in π-conjugated molecules containing electron-donating/-accepting moieties. A set of 48 organic molecules is chosen for this purpose, for which the two-photon-absorption (2PA) parameters are evaluated using different DFAs, including BLYP, PBE, B3LYP, PBE0, CAM-B3LYP, LC-BLYP, and optimally tuned LC-BLYP. Minnesota functionals and ωB97X-D are also used, applying the two-state approximation, for a subset of molecules. The efficient resolution-of-identity implementation of the coupled-cluster CC2 model (RI-CC2) is used as a reference for the assessment of the DFAs. Two-state models within the framework of both DFAs and RI-CC2 are used to gain a deeper insight into the performance of different DFAs. Our results give a clear picture of the performance of the density functionals in describing the two-photon activity in dipolar π-conjugated systems. The results show that global hybrids are best suited to reproduce the absolute values of 2PA strengths of donor–acceptor molecules. The range-separated functionals CAM-B3LYP and optimally tuned LC-BLYP, however, show the highest linear correlations with the reference RI-CC2 results. Hence, we recommend the latter DFAs for structure–property studies across large series of dipolar compounds.

Femtosecond Z-scan measurements of aggregates of a quadrupolar D-π-D dye, 9,10-bis(N-hexylcarbazol-2-yl-vinyl-2)anthracene (BHCVA), provide direct evidence that aggregation of this AIE (aggregation-induced emission) chromophore leads to a sizeable enhancement of the intrinsic two-photon absorption cross section. Aggregation-induced improvement of nonlinear response is explained by theoretical analysis of electronic structure and geometrical parameters of aggregates.

The quality of the static electronic dipole (hyper)polarizabilities (α and γ) calculated using the coupled cluster with singles and doubles (CCSD) method and various basis sets for the isolated hydrogen anion (H⁻) was verified against highly accurate results obtained within the variation-perturbation scheme with the optimized explicitly correlated Gaussian functions. Moreover, the influence of spatial confinement on α and γ of H⁻ was analyzed. The effect of confinement (orbital compression) was modeled by the spherically symmetric harmonic oscillator potential. Our results demonstrate that in order to obtain the value of α with a relative error less than 10% and correct order of magnitude of γ for the unconfined H⁻ at the CCSD level of theory, at least triply augmented correlation consistent basis set (t-aug-cc-pVXZ, X = D,T,Q,5,6) has to be used. Furthermore, it was found that the spatial confinement significantly affects α and γ of H⁻.