Two quinonoid bis(dicyanomethylene) oligothiophenes, terthiophene and quaterthiophene analogues of TCNQ, have been investigated by spectroelectrochemical experiments and density functional theory calculations. Electrochemical data show that the molecules can be both reduced and oxidized at relatively low potentials, and that the quaterthiophene derivative forms four stable redox species, the dianion, neutral, cation radical, and dication. The neutral oligomers are characterized by a strong electronic absorption in the red or near-infrared region and can be viewed as structural and electronic analogues of aromatic oligothiophenes in the dication or bipolaron state. Upon reduction, dianions, not anion radicals, are formed which absorb in the visible region. The theoretical calculations show that the dianions have aromatic oligothiophene moieties with two anionic dicyanomethylene groups. The transition from a quinonoid to an aromatic structure is fully supported by UV-vis-near-IR and vibrational spectroscopic data. Oxidation, generating cation radicals and dications, occurs at rather low potentials similar to those reported for oligothiophenes. The electronic spectra of these cations are understood from the calculations, which suggest that the oxidized species are stabilized by the partial aromatization of the oligothiophene backbone. IR spectra of the species, especially the CN stretching frequencies, confirm the structural conclusions and allow comparison with TCNQ and the TCNQ dianion.
The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)2 groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)2 groups. The molecule is characterized by a strong absorption in the red, due to the HOMO-->LUMO pi-pi* electronic transition of the terthiophene backbone that shifts hypsochromically on passing from the solid state to solution and with the polarity of the solvent. The analysis of the vibrational spectra confirms the structural conclusions and supports the existence of an intramolecular charge transfer. Vibrational spectra in several solvents and as a function of temperature have also been studied. Significant frequency upshifts of the vibrations involved in the pi-electron-conjugated pathway have been noticed upon solution in polar solvents and with the lowering of the temperature. Finally, we propose a quinoid molecule as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doped polythiophene.
Donor-acceptor chromophores containing three different types of thiophene-based electron spacers and the same donor (1,3-dithiol-2-ylidene) and acceptor (dicyanomethylene) end groups have been investigated by infrared and vis-near-IR absorption spectroscopies with the aim of elucidating the ability of the heteroquinonoid spacers as electron transmitters. Density functional theory calculations have been carried out, both within the standard and the time-dependent formalisms, to assign the most relevant electronic and infrared features of these chromophores and to assess useful information about their molecular structures. Both theoretical calculations and vibrational spectra demonstrate the occurrence of a sizable intramolecular charge transfer from the electron-donor unit to the electron-acceptor group in the ground state. The optical properties of the chromophores are strongly influenced by the electron spacer. The intense optical absorptions recorded in the visible mainly correspond to the π-π* excitation of the oligothienoquinonoid bridge. As an additional merit of these molecular materials, their infrared spectra recorded at various temperatures between -170 and +180 °C reveal, at the molecular level, their high thermal stability, what has importance for their potential use in molecular electronics and optoelectronic devices.
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