DOI: 10.1002/cphc.200900140 Electronic Studies on Oligothienylenevinylenes : Understanding the Nature of Their Ground and Excited Electronic States Rocío Ponce Ortiz , [a, c] Sandra R. Gonzµlez, [a] Juan Casado, [a] Juan T. López Navarrete,* [a] David L. Officer, [b] Pawel Wagner, [b] John C. Earles, [c] and Keith C. Gordon* [c] 1. Introduction The drive for inexpensive renewable energy sources has direct- ed research toward the development of low-cost organic pho- tovoltaic devices. These materials offer the possibility of cheap and easy methods to produce energy from light. [1] The main advantages of organic semiconductors are: 1) their low-cost synthesis, and 2) easy manufacture of thin-film devices by vacuum evaporation or by solution-based techniques. One type of cell that has proven successful is the dye-sensitised solar cell, pioneered by Graetzel et al. [2] A number of sensitisers have been incorporated into this architecture, including por- phyrin species. [3] The use of appropriate anchoring groups has been very successful in developing and improving the efficien- cy of these types of cells. [4] This is, in part, because the anchor- ing group directs the electronic structure of the frontier molec- ular orbitals. Such a strategy may also be applied to thio- phenes, as these have utility in solar cells. This study examines a series of thiophene–vinylene oligo- ACHTUNGTRENNUNGmers with backbone conjugational positions substituted with acceptor groups, either cyano moieties at the internal vinylene or malonic acid at the alpha termini. A variety of oligo(thio- phene–vinylene)s have been the subject of intensive studies, mainly by Roncali et al., in which long oligomers (up to 16 repeat units) were characterised. [5, 6] Thus, most of the existing electronic-structure calculations have pursued characterization of the dependence of the optical band gap on chain and con- jugation lengths. [7] This is justified since these oligomers are one of the best alternatives for large and efficient molecular wires. [5] Their emission properties, however, have been much less studied, likely due to their low fluorescence efficiency in connection with their small optical S 1 –S 0 band gap. [8] Experi- mental work on donor–acceptor-substituted derivatives mainly comes from Roncali et al. [9] as well, but little work has centred on their emission or fluorescence data. [10] More recently, a few works have dealt with the theoretical characterization of excit- ed states of these conjugated compounds, with emphasis on the singlet and triplet states. [10] In combination with acceptors, these oligomers are now being exploited as solar-cell dyes. [11] A major issue in understanding the electronic nature of the series of compounds presented here (see Figure 1) is to deter- mine the nature of the conformers that are present. If these materials are ever to be used in devices, then the solid-state structures and electronic properties need to be established. We have determined the conformer of each species using den- sity functional theory in concert with the vibrational spectra of solid-state samples. Having determined the conformer, we then examine other properties using density functional theory and spectroscopic methods. In addition, we study how CN groups along the ethylene–thiophene backbone can alter the electronic properties of these compounds with consequences for the optical properties of interest. The electronic and molecular structures of a family of oligo- thienylenevinylenes for organic solar cells are studied by means of UV/Vis, fluorescence and Raman spectroscopy, aided by quantum chemical calculations. By using different anchor- ing groups, the alteration of the electronic properties upon in- serting electron-withdrawing groups into different positions on the oligothienylenevinylene backbone is determined. In addi- tion, a thorough study of the photophysical properties is car- ried out to understand their potential use in optoelectronic de- vices. The charge defect of one of these systems is analysed to elucidate the possible charge carriers photogenerated during device operation. [a] Dr. R. Ponce Ortiz , S. R. Gonzµlez, Dr. J. Casado, Prof. Dr. J. T. López Navarrete Department of Physical Chemistry University of Mµlaga, 29071-Mµlaga (Spain) Fax: (+ 34) 952-132000 E-mail : teodomiro@uma.es [b] Prof. Dr. D.L. Officer, Dr. P. Wagner Intelligent Polymer Research Institute ARC Centre of Excellence for Electromaterials Science & Department of Chemistry, University of Wollongong Wollongong, NSW (Australia) [c] Dr. R. Ponce Ortiz, J. C. Earles, Prof. Dr. K. C. Gordon Department of Chemistry, MacDiarmid Institute for Advanced Materials and Nanotechnology University of Otago, Dunedin (New Zealand) Fax: (+ 64) 34797906 E-mail: kgordon@chemistry.otago.ac.nz ChemPhysChem 2009, 10, 1901 – 1910  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1901