ChemPhysChem 2004, 5, 529 ± 539 DOI: 10.1002/cphc.200300963 ¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 529 SpectroscopicandTheoreticalStudyofthe MolecularandElectronicStructuresofaTerthio- phene-Based Quinodimethane Juan Casado, [a] Ted M. Pappenfus, [b] Kent R. Mann, [b] Enrique OrtÌ,* [c] Pedro M. Viruela, [c] Begon ƒ a Milia ¬n, [c] VÌctor Herna ¬ndez, [a] and Juan T. Lo ¬ pez Navarrete* [a] The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5''-bis(dicyanomethylene)-5,5''-dihydro-2,2':5',2''-ter- thiophene 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 p ± p* 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 p-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. 1.Introduction Conjugated oligothiophenes are currently receiving much attention as organic semiconductors for electronic devices such as field-effect transistors (FETs). [1, 2] Oligothiophenes typically behave as p-type conductors or hole-transporting materials. n-Type or electron-transporting oligothiophenes, however, are also of great interest for the fabrication of p ± n heterojunctions and complementary circuits. The operating properties of these organic-based devices depend on the charge-transport proper- ties, which are closely related to the energies of the HOMO and the LUMO that govern charge injection, and to the splitting of these levels in the solid state (due to intermolecular interac- tions), which determines charge mobility. [3, 4] The strength of the electronic coupling between the HOMO levels controls the hole transport and that between the LUMO levels modulates the electron transport. Therefore, charge-transfer processes and intermolecular interactions are key topics to be investigated for the development of these materials. Recently, thin-film transistors (TFTs) based on 3',4'-dibutyl- 5,5''-bis(dicyanomethylene)-5,5''-dihydro-2,2':5',2''-terthiophene [Bu 2 (DCM) 2 Tth, Figure 1] have been reported. [5±7] The initial report shows that this molecule works very efficiently as n-channel semiconductor, which, as mentioned above, is some- what rare for this class of compounds. [6] Quinoid oligothio- phenes have a HOMO ± LUMO energy gap inherently smaller than that of conventional aromatic oligothiophenes, which makes ambipolar (both p- and n-type) transport more likely because smaller gate voltages are required to populate the Figure 1. Chemical structure of Bu 2 (DCM) 2 Tth. HOMO and LUMO levels (valence and conduction bands) with charge carriers. In fact, within the course of this investigation, the group at the University of Minnesota successfully achieved ambipolar transport in a TFT using Bu 2 (DCM) 2 Tth as the semi- conducting layer. [7] Ambipolar behavior in organic-based tran- sistors is rare and, to the best of our knowledge, the case of Bu 2 (DCM) 2 Tth represents the first example of ambipolar trans- port in a TFT device based on a single conjugated organic [a] Dr. J. Casado, Prof. Dr. V. Herna ¬ndez, Prof. Dr. J. T. Lo ¬pez Navarrete Departamento de QuÌmica FÌsica, Universidad de Ma ¬laga 29071 Ma ¬laga (Spain) E-mail: teodomiro@uma.es [b] Dr. T. M. Pappenfus, Prof. Dr. K. R. Mann Department of Chemistry, University of Minnesota Minneapolis, MN 55455 (USA) [c] Prof. Dr. E. OrtÌ, Prof. Dr. P.M. Viruela, B. Milia ¬n Institut de Cie ¡ncia Molecular, Universitat de Vale ¡ncia 46100 Burjassot (Vale ¡ncia) (Spain) Fax:  (34)-963543156 E-mail: enrique.orti@uv.es