ELSEVIER Synthetic Metals 89 ( 1997) 159-160 A theoretical investigation of a,a’-dimethyl end-capped oligothiophenes: structures, vibrational spectra and conjugation defects J. Casado, V. Hernhdez, F.J. Rarnfrez, J.T. L6pez Navarrete Departammto de t&iinica Fisica, Universidad de Milaga, 29071 Mdlaga, Spain Abstract Ab initio quantum-chemical methods are used in the study of the effect of ionization on the geometries and vibrational spectra of a series of a,oc’-dimethyl end-capped oligothiophenes (from dimer to hexamer). For thatpurpose we determine thegeometric structures, vibrational frequencies, and infrared and Raman intensities of neutral, radical cationic and dicationic oligomers. These theoretical data, when combined with the results of the experimental studies, provide a complete picture of the vibrational structure of the neutral state of doped oligomers. 0 1997 Elsevier Science S.A. Kej~ords: Ab initio quantum chemical methods and calculations; Polythiophene and derivatives 1. Introduction Conjugatedpolymers have attractedconsiderable attention since the discovery of possible insulator-to-metal transitions following chemical or electrochemicaldoping. In order to get a deeperinsight into the complex propertiesof the polymers, the study of oligomer analogshas emergedin the past few years asa very useful tool. Polythiophene and its oligomers have attracted much interestdue to their good environmental stability in both the neutral and doped states. The large amount of studies dedicatedto these compounds has opened the way to major developmentsin the fabrication of devices [ 11. Moreover, the ol,cy’-dimethyl substitution has recently proved very effective in enhancing chemical stability and crystallinity without damagingthe electronic response of the resulting material [ 21. In this communication, the geometricandvibrationalprop- erties of a series of a,a’-dimethyl end-capped oligothiophe- nesaretheoretically investigated in both neutral andoxidized states; the results are then compared to the experimentaldata. 2. Methodology The methyl end-cappedoligothiophenes investigated in this work contain from 2 to 6 thiopheneunits (from the dimer to the hexamer). The geometry optimizations were carried out using wave functions of the Hartree-Fock type: restricted (RI-IF) for closed-shell systems (the neutral and the dica- tionic systems), spin unrestricted (UHF) and spin restricted 0379-6779/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved PUSO379-6779(97)03916-7 (ROHF) for open-shell systems (the radical cation systems). The basis set used in all calculationswas the 6-31G”” which is split valence and includes a setof d-polarization functions for heavy atomsand p-polarization functions for the hydro- gens. We have initially limited our theoretical investigation to the all-anti planar conformations. 3. Results and discussion 3.1. Geometric stmctwes The 6-3 lG*” optimized geometricstructures of the neutral oligomers show that all the inner thiophene rings present almost the same geometry, which slightly differs from that found in the outer units due to the methyl-end substitution. Ab initio calculations are indicative of a degree of bond- length alternation of about 0.07 A in good correspondence with the experimental data. The 6-31G*” optimized geometry of the dicationic c~,ol’- dimethyl end-capped oligothiophenes indicates the formation of a positive bipolaron defect localized in the middle of the molecule and extending over the adjacent repeat units. The charged species are characterized by a reversal of the single- double C-C bond pattern, the geometry relaxation process thus induces the appearance of a strong quinoid character within the molecule. In the case of the radical cations, there occurs the appear- ance of a positive polaron defect localized in the middle of the molecule.ROHF and UHF geometries (seeFig. 1 for the hexamer) of the radical cations exhibit weaker, structural