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A combination of Raman spectroscopy and density functional theory calculations is used to describe the structural and spectroscopic properties of the different isomeric cations of diferrocenyl quaterthiophenes. Isomerisation of the thienyl β-positions provides site selective oxidation, which gives rise to species that can interconvert by moving the charge over the bridge. The spectroscopic study allows us to describe a sequence of stationary trapped cationic, either ferrocenyl or thienyl, states which constitutes an energy cascade of accessible sites through which the charge transfer can proceed. [ABSTRACT FROM AUTHOR]

The vibrational Raman spectra of several series of aromatic and quinoidal compounds have been analyzed considering the downshifts and upshifts of the frequencies of the relevant Raman bands as a function of the number of repeating units. Oligothiophenes, oligophenylene-vinylenes, and oligoperylenes (oligophenyls) derivatives are studied in a common context. These shifts are taken as spectroscopic fingerprints of the changes in π-conjugation. For a given family, aromatic and quinoidal oligomers have been studied together, and according to their Raman frequency shifts located in the two-well BLA-energy curve of their ground electronic state as a function of the bond-length-alternation pattern (BLA). The connection among BLA values, π-conjugation, and Raman frequencies is taken here as the basis of the study. These Raman shifts/BLA changes have been related to important electronic properties of these one-dimensional linear π-electron delocalized systems such as quinoidal (polyene) and aromatic characters. [ABSTRACT FROM AUTHOR]

A general description of the use of the Raman spectra of polyconjugated molecules to infer their electronic and molecular properties and how these change in a number of situations is illustrated with several examples, with particular emphasis on oligothiophene molecules. The most noticeable features of the Raman spectra of these molecules and the main theoretical models developed for their understanding have been revised and explained in terms related to π-electron delocalization intimately connected with the aromatic and quinoidal characters. [ABSTRACT FROM AUTHOR]

In the present paper, the synthesis and characterization of 2,2′-bis{bi[2,2’-(3,4-ethylenedioxy)thiophen-5-yl]}-3,3′-bithianaphthene, nicknamed BT 2 E 4 , is reported. We show that electrooligomerization of BT 2 E 4 leads to electroactive films which are evaluated by cyclic voltammetry (CV), UV/vis spectroelectrochemistry and CV coupled with in-situ conductance measurements. Direct comparisons to the properties of the oligomers obtained from the atropisomeric all-thiophene analogue BT 2 T 4 show that the introduction of EDOT leads to a strong reduction of the band-gap, an overall red-shifted absorption spectrum and a rigidification of the structure. Finally, DFT and TD-DFT calculations were performed in parallel to investigate and to compare the electronic and molecular structures of neutral and charged monomer and oligomer species. [ABSTRACT FROM AUTHOR]

We have used Raman spectroscopy to study the molecular and electronic structures of the radical cations and dications of carbon-bridged oligo(para-phenylenevinylene)s (COPVn, n = 1-6) possessing consecutive fused pentagons and hexagons, up to 19, along with COPV derivatives having electron-donating and -withdrawing groups. This study was made possible by the outstanding stability of the charged states of COPVs. We could untangle the effects of π-conjugation in the planar structure on the Raman frequency by distinguishing it from other structural effects, such as strain in the vinylene groups shared by the two pentagons. The analyses showed that the radical cations have benzo-quinoidal structures confined in the center of the molecule, as well as benzo-aromatic rings at the terminal sites. In contrast, dications of COPVn longer than n = 3 exhibit a biradicaloid character because of the recovery of aromaticity in the central rings and quinoidal rings at the terminal positions. These biradicaloids favor a singlet nature in their ground electronic states because of the double spin polarization. The introduction of electron-donating and -withdrawing groups on the termini of a COPV core affords, upon oxidation or reduction, a fully delocalized class III mixed valence system because of the high degree of conjugation of the COPV platform, which favors extensive charge delocalization. [ABSTRACT FROM AUTHOR]

A series of push — pull chromophores built around thiophene-based π-conjugating spacers and bearing various types of amino donors and cyanovinyl acceptors have been analyzed by means of UV— vis — NIR, IR, and Raman spectroscopic measurements in the solid state as well as in solution. The intramolecular charge transfer (ICT) of these π-conjugated systems has also been tested by analyzing the ability of the solute molecules to undergo shifts in their fluorescence emission maxima with increasing solvent polarity. These push—pull oligomers also display an attractive electrochemical behavior since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the electron-rich aminooligothienyl half-part of the molecule and leads to the formation of stable cations. On the other hand, reduction to radical anions and dianions is mainly cyanovinyl-centered but also affects the π-conjugated electron relay. Density functional theory (DFT) calculations have been carried out to help the assignment of the most relevant electronic and vibrational features and to derive useful information about the molecular structure of these NLO-phores. [ABSTRACT FROM AUTHOR]

The Raman spectrum of a quaterthiophene oligomer incorporating a central vinylene spacer has been investigated using density functional theory B3LYP/6-31G** calculations. The spectrum has been fully assigned with the aid of the calculations and in comparison with unsubstituted quaterthiophene. The spectrum preserves most of the vibrational features of linear oligothiophenes. The vibrations of the vinylene spacer are clearly differentiated from those of the rest of the chain. The vinylene spacer increases the conjugation length of the molecule and induces a frequency downshift of ≈20 cm−1 for the normal modes associated with the νas(C&z.dbnd6;C) and νs(C&z.dbnd6;C) vibrations of the thiophene rings. The molecule is predicted to have a structure in which both the thiophene rings and the vinylene spacer are in an all-anti conformation. [Copyright &y& Elsevier]