Spectroscopic and Theoretical Study of the Molecular and Electronic Structures of a Terthiophene-Based Quinodimethane

• Published in: Chemphyschem Vol. 5; no. 4; pp. 529 - 539
• Main Authors: Casado, Juan, Pappenfus, Ted M., Mann, Kent R., Ortí, Enrique, Viruela, Pedro M., Milián, Begoña, Hernández, Víctor, López Navarrete, Juan T.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 19.04.2004; WILEY‐VCH Verlag; Wiley
• Subjects: Atomic and molecular physics; Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations); Crystallography, X-Ray - methods; density functional calculation; Density-functional theory; electronic spectroscopy; Electronic structure of atoms, molecules and their ions: theory; Electronics; Exact sciences and technology; Infrared spectra; Models, Molecular; Molecular properties and interactions with photons; Molecular spectra; oligothiophenes; Physics; Quinoxalines - chemistry; Spectrophotometry; thin films; Thiophenes - chemistry; Ultraviolet spectra; vibrational spectroscopy; Visible spectra; density functional calculation; electronic spectroscopy; Raman spectra; Fourier transform spectroscopy; Molecular structure; Molecular crystals; Theoretical study; Infrared spectra; Experimental study; Bond lengths; thin films; Intermolecular interaction; Vibrational transition; Thiophene derivatives; Electronic structure; Density functional method; Nitriles; Ultraviolet visible spectrum; vibrational spectroscopy; Absorption spectra; Electron density; oligothiophenes; Organic compounds
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200300963

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 π–π* 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 π‐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. Quinoid terthiophene: Bu2(DCM)2TTH has a quinoid molecular structure, shows a strong optical absorption in the red (see spectrum), and exhibits ambipolar (p‐ and n‐type) transport in organic TFT devices. Furthermore, Bu2(DCM)2TTH mimics the structural and electronic properties of dication species (bipolarons) in doped oligothiophenes (polythiophenes).