Electronic spectra and (hyper)polarizabilities of non-centrosymmetric D–A–D chromophores. An experimentally based three-state model and a theoretical TDDFT study of ketocyanines

The electronic structure, spectra and linear and second-order polarizabilities of two symmetricketocyanines, which are prototypic examples of D–A–D chromophores, have been investigatedwith two different toolsets: (i) the so-called ‘essential-state model’, here comprising three states,the ground and two lowest excited 1pp* states, has been adapted for these non-centrosymmetric,yet symmetric compounds to determine their permanent electric dipole moments, polarizabilitiesand first hyperpolarizabilities making use of experimental transition energies and moments; (ii)extensive TDDFT calculations have provided ground-state conformational results consistent withNMR-derived structural information, energies and dipole moments of up to 20 lowest-lyingelectronic states as well as, within the sum-over-states (SOS) scheme, the most relevantcomponents of the polarizabilities and first hyperpolarizabilities. The two levels of descriptionform consistent pictures of the ketocyanine excited states that provide the most relevantcontributions to hyperpolarizabilities: extension of the SOS set beyond the three states of thebasic model left unchanged (within B10%) the calculated vector component of the second-orderpolarizability tensor along the direction of the ground-state dipole moment (by). Both approachesindicate that these D–A–D compounds, in spite of their quasi-linear structure, reminiscent of thatof centrosymmetric quadrupolar chromophores, feature significant second-order molecularpolarizabilities. These rapidly increase with the length of the polyenic bridges in thechromophores. About half of the total value of by is predicted to come from the three-level-termpart, by,3, most of which derives from the contribution involving the three electronic states of theessential-state model.