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

Abstract

The electronic structure, spectra and linear and second-order polarizabilities of two symmetric ketocyanines, which are prototypic examples of D–A–D chromophores, have been investigated with two different toolsets: (i) the so-called ‘essential-state model’, here comprising three states, the ground and two lowest excited ¹ππ* states, has been adapted for these non-centrosymmetric, yet symmetric compounds to determine their permanent electric dipole moments, polarizabilities and first hyperpolarizabilities making use of experimental transition energies and moments; (ii) extensive TDDFT calculations have provided ground-state conformational results consistent with NMR-derived structural information, energies and dipole moments of up to 20 lowest-lying electronic states as well as, within the sum-over-states (SOS) scheme, the most relevant components of the polarizabilities and first hyperpolarizabilities. The two levels of description form consistent pictures of the ketocyanine excited states that provide the most relevant contributions to hyperpolarizabilities: extension of the SOS set beyond the three states of the basic model left unchanged (within ∼10%) the calculated vector component of the second-order polarizability tensor along the direction of the ground-state dipole moment (β_y). Both approaches indicate that these D–A–D compounds, in spite of their quasi-linear structure, reminiscent of that of centrosymmetric quadrupolar chromophores, feature significant second-order molecular polarizabilities. These rapidly increase with the length of the polyenic bridges in the chromophores. About half of the total value of β_y is predicted to come from the three-level-term part, β_y,3, most of which derives from the contribution involving the three electronic states of the essential-state model.