FULL PAPER DOI: 10.1002/ejoc.200900850 Synthesis and Optical and Electronic Properties of Thiophene Derivatives Raphael P. Jimenez, [a] Masood Parvez, [a] Todd C. Sutherland,* [a] and Joel Viccars [a] Keywords: Structure-activity relationships / Heterocycles / Chromophores / Electrochemistry / Density functional calculations Using the Hinsberg synthesis of thiophenes, a versatile method to prepare fully derivatized, π-extended thiophenes is reported. Functionalized thiophenes were divergently syn- thesized to create three classes of compounds – electron-de- ficient, extended conjugation and electron-rich – to assess substituent effects on optical and electrochemical properties. Properties were assessed by solution absorption spec- troscopy, solution- and solid-state fluorescence spectroscopy, cyclic voltammetry and density functional theory calcula- tions. Tetracyano derivatives, prepared through Knoevena- gel condensations of malononitrile with thiophene-2,5-dicar- baldehydes, were used as electron-poor analogs. These de- rivatives showed quasireversible reduction reactions and very low-lying calculated LUMO energies (–0.55 V reduction potentials vs. Fc/Fc + ). The effect of extending π conjugation Introduction Interest in organic electronics has grown significantly over the past several decades. Research has focused on im- proving material processability, stability and perform- ance. [1–7] Today, thiophene-based materials show applica- tions in organic field-effect transistors (OFETs), [8–12] or- ganic light-emitting diodes (OLEDs), [13,14] conducting poly- mers [15–17] and organic photovoltaic devices. [18–20] More re- cently, efforts in developing and optimizing materials useful for optoelectronics has increased due to the need for solar energy conversion materials. [21] Near-IR absorption [22,23] as well as band-gap tunability [6,19,24–28] are key features needed for such materials. The ubiquitous use of thiophene in or- ganic materials is in part due to its electronic properties, ease of derivatisation and commercial availability. The synthesis of new molecules that have small HOMO– LUMO energy differences is paramount to advancing or- ganic materials applications. Moreover, the synthesis should also be scalable, efficient, modular and divergent to allow tuning of the electronic structure and solubility characteris- tics required for different materials applications. Two strate- [a] Department of Chemistry, University of Calgary, 2500 University Drive NW, Canada Fax: +1-403-289-9488 E-mail: todd.sutherland@ucalgary.ca Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.200900850 or from the author. Eur. J. Org. Chem. 2009, 5635–5646 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5635 on the optical and electrochemical properties was investi- gated by the installation of bis(2-thienylacryoni- trile) groups onto three thiophene cores. The extended conju- gation led to compounds that demonstrated both solution- and solid-state fluorescence and moderate, irreversible re- duction potentials (–1.2 V versus Fc/Fc + ). The Lewis-acid- catalyzed coupling of four indoles to thiophene-2,5-dicarbal- dehydes was studied to assess the effects of both electron- donating substituents and a quinoidal thiophene. The tetra- indolic thiophenes possessed low HOMO–LUMO energy gaps of 1.91 eV, high-lying HOMO energy levels and tunable LUMO energy levels, attributed to the thiophene quinoidal ground-state structure. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) gies are generally employed to decrease the HOMO– LUMO energy gap. The first strategy involves increasing the degree of conjugation by the introduction of more double bonds. The second strategy involves the incorpora- tion of both an electron-donating group (EDG) and elec- tron-withdrawing group (EWG) within the same mole- cule. [29] Small-molecule tuning, as opposed to a polymeric approach, permits well-defined structure-property relation- ships to be built in the absence of aggregate or supramolec- ular structures. Low-band-gap organics have been synthe- sized by Osuka and co-workers [30,31] using fused porphyrin tapes that demonstrate electronic transitions at energies as low as 0.45eV. Unfortunately, due to low yields and poor solubility, these materials do not lend themselves to practi- cal materials applications. Roncali and co-workers have syn- thesized a wide variety of thiophenes for use as organic semiconductors with tunable HOMO–LUMO ener- gies. [32–34] Molecules with small HOMO–LUMO energy dif- ferences can also be obtained by the installation of an elec- tron-donating (increasing the HOMO energy) moiety and an electron-accepting (lowering the LUMO energy) moiety into a single molecule. [29] However, the frontier molecular orbitals (FMOs) of these donor-acceptor (D-A) dyads are localized on the respective electron-rich and electron-poor regions of the molecule, often resulting in poor overlap and weaker electronic transitions. Nevertheless, small molecules with low HOMO–LUMO energy differences have been syn- thesized using the D-A approach. [35]