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SummaryandConclusions Wehavestudiedthetwo-photonabsorptionpropertiesoftwoacceptor-p-acceptor systems with dioxaborine acceptor unitsusing highly correlated quantum-mechanical simulations, andfoundexcellentagreementwithnanosecondandfemtosecondspectroscopicdata.Intheaccessiblespectralregion,therearetwo significant TPA features. The calculations show that thelarge difference in the nonlinear absorption response of thetwo states involved is a consequence of correlation-inducedoscillatorstrengthredistribution;thisemphasizesthatelectroncorrelation is the key to understanding the factors governingtheTPAresponseinp-conjugatedorganicchromophores.Con-figuration mixing also affects the lowest one-photon excitedstate and, in this way, reduces the overall TPA response thatcanbeachieved.Moreover,wefindstrongindicationsthatTPAintotheone-photonallowedstate(S 1 )ismediatedbychannelsinvolving vibrational sublevels of the ground state, therebysupportingthetheoreticalpredictionsinref.[18] Methodology Detailsregardingthesynthesisofthecompoundsandtheirspec-troscopic investigations are contained in the Supporting Informa-tion.Thespecificvaluesofthefluorescencequantumyieldsdeter-minedforIandIIare68and72%,respectively.Quantum-Chemical Modeling: To calculate the TPA cross-sections,westartfrommoleculargeometriesoptimizedwiththesemiempir-icalAM1Hamiltonian.

We report spectroscopic and quantum-chemical investigations comparing the two-photon absorption (TPA) properties of a bis(dioxaborine)-substituted derivative of biphenyl with those of a bis(dioxaborine) carbazole derivative. The former molecule is close to linear and centrosymmetric, while the dioxaborine groups of the latter are in a V-shaped arrangement, due to their linkage to the 3 and 6 positions of the bridging group. For both systems, we find sizable TPA cross sections (on the order of 360−530 × 10-50 cm4 s/photon). Interestingly, while the TPA response in the biphenyl-based system can be well described on the basis of the traditional three-state model, a significantly larger number of excited states needs to be considered for the carbazole derivative. We present a detailed comparison of the convergence of the theoretical approaches and an analysis of the various channels that contribute to the TPA response in molecules with low effective symmetries.