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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.

Publication costs assisted by Consigtio Nazionale delle Ricerche (Roma) Intramolecular reasons were sought for the deviation of absorption and emission spectra of polyphenyls from the mirror image relation. As a working hypothesis it was supposed that the differences between absorption and fluorescence spectra originate in the excitation of the twisting mode of interannular bonds. Curves of the potential energy as a function of the dihedral angle between phenyl rings were obtained for the ground state and the lowest singlet excited states by using semiempirical calculation procedures. A polynomial approximation was adopted for the potential functions so that vibrational energies and wave functions could be calculated simply by expanding the wave functions in a harmonic oscillator basis set. By this technique, Franck-Condon distributions were obtained for both absorption and emission. The contours of the distributions enabled us to explain both the fine structure of fluorescence spectra and the structureless long-wavelength uv bands in polyphenyls.