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 (1)ππ* 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.
The study is devoted to investigation of a number of ketocyanine dyes (polyenic ω, ωbisaminoketones) with a central pyranone or pyridone fragment and respective polymethine salts by UV-VIS absorption and NMR spectroscopies. It was found that replacement of bridging O atom with NMe group suppresses the chromophore interaction in the cross-conjugated ketone and corresponding tridecamethine salt. Furthermore, incorporation of meso-amino substituent into the polymethine chain of the salts with the central pyrylium fragment sharply decreases the chromophore interaction. In spite of dramatic differences between the absorption spectra of the ketocyanines containing O-or NMe-bridging fragment, 13 C NMR spectra of these dyes are similar. A comparison of the differences between the chemical shifts of adjacent polymethine carbon atoms in the 13 C NMR spectra for these dyes point to stronger alternation of charges along the polymethine chain and, consequently, stronger chromophore interaction for the dye with the central pyrylium cycle. Flash photolysis experiments showed photoisomerization of the dyes with NMe bridging group to form long-lived isomeric form.
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