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The Raman spectra of a series of push-pull molecules containing probenzenoid or quinoid spacers which are substituted with 1,3-dithiol-2-ylidene as donor and dicyano-methylene or barbituric acid as acceptors have been analyzed. The experimental spectra have been assigned and interpreted according to density functional theory calculations. Correlations between the Raman spectra of the isolated spacers and of the substituted molecules have been done. Raman bands in the 1620–1560 cm-1 interval provide vibrational markers of the quinoid↔aromatic structural evolution. This finding is supported by a careful inspection of geometrical parameters, namely, bond length alteration data and particular bond distances. As a result, the peak positions and relative intensities of these Raman features can be used to evaluate the benzenoid character of the spacer as a function of the donor/acceptor substitution pattern. This paper shows that Raman spectroscopy is a powerful spectroscopic tool for the analysis of the conjugational properties (i.e., intramolecular donor→acceptor charge transfer) of new organic materials. [ABSTRACT FROM AUTHOR]

4H-Pyranylidene-containing push-pull chromophores built around a bithiophene (BT) p relay or a rigidified thiophene-based unit, namely cyclopenta[1,2-b:3,4-b']dithiophene (CPDT) or dithieno[3,2-b:2',3'-d]pyrrole (DTP), have been synthesized and characterized. The effect of these different relays on the polarization and the second-order nonlinear optical (NLO) properties has been studied. For the sake of comparison, the corresponding reported dithieno[ 3,2-b:2',3'-d]thiophene (DTT) derivatives have also been included in the discussion. Replacement of the BT core by a rigidified unit (CPDT, DTP) leads to more polarized systems. Calculated NBO charges and electrochemical measurements show that dithienopyrrole has a remarkable donor character that allows an important charge transfer between the donor and the acceptor. The influence of the rigidification of the BT relay on the NLO responses depends on the acceptor strength. For the weakest acceptor used (thiobarbituric acid), passing from the BT relay to the rigidified units always involves an increase in the μβo figure of merit. Nevertheless, for the strongest acceptor (2-dicyanomethylene-3-cyano- 4,5,5-trimethyl-2,5-dihydrofuran (TCF)), a slight increase in μβo with respect to the BT chromophore is only observed for the DTP derivative. Thus, rigidification of the BT core is not enough to improve the second-order nonlinearity and the incorporation of a DTP moiety has proven to be the most efficient approach for this purpose. [ABSTRACT FROM AUTHOR]

New complexes with six ferrocenyl (Fc) groups connected to ZnII or CdII tris(2,2′-bipyridyl) cores are described. A thorough characterisation of their BPh4− salts includes two single-crystal X-ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(FeII)→π* metal-to-ligand charge-transfer (MLCT) bands accompany the π→π* intraligand charge-transfer absorptions in the near UV region. Each complex shows a single, fully reversible FeIII/II wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper-Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities β0 reaching as high as approximately 10−27 esu and generally increasing with π-conjugation extension. Z-scan cubic NLO measurements reveal high two-photon absorption cross-sections σ2 of up to 5400 GM in one case. DFT calculations reproduce the π-conjugation dependence of β0, and TD-DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature. [ABSTRACT FROM AUTHOR]

The effect of alkylthio substituents on the redox and nonlinear optical behavior of tetrathiafulvalenes has been studied by Density Functional Theory Calculations. [ABSTRACT FROM AUTHOR]

Discusses research which describes the synthesis and study of chemical structures, where a quinoid spacer links a 1,3-dithiole donor fragment and an electron acceptor. Factors affecting the nonlinear optical response of the compounds; Result of the progressive benzoannelation of the spacer; Information on the contribution of the neutral and zwitterionic limiting forms to the description of the molecules.