A tetrathiafulvalene–tetracyanoanthraquinodimethane (TTF–TCNAQ) diad with a chemically tunable HOMO–LUMO gap Jincai Wu, a,y Shi-Xia Liu, a, * Antonia Neels, b Franck Le Derf, c Marc Sall  e c and Silvio Decurtins a a Departement f € ur Chemie und Biochemie, Universit € at Bern, Freiestrasse 3, CH-3012 Bern, Switzerland b Institut de Microtechnique, Universite´ de Neuch^ atel, Rue Jaquet Droz 1, CH-2002 Neuch^ atel, Switzerland c Laboratoire de Chimie, Inge´nierie Mole´culaire et Mate´riaux d’Angers (CIMMA), Universite´ d’Angers, CNRS UMR 6200, 2 Bd Lavoisier, 49045 Angers, France Received 10 July 2007; revised 21 August 2007; accepted 28 August 2007 Available online 1 September 2007 Abstract—An imidazole-incorporated TTF–TCNAQ diad was synthesised, which shows remarkable optical and electrochemical responses as a function of pH. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Molecules with energetically low HOMO–LUMO gaps are of prime importance for the good performance of organic electronic devices. 1,2 As a consequence, much effort has been devoted to the preparation of a variety of p-conjugated oligo- or polymers 3 as well as to covalent linkage of a p-electron donor (D) to a p-electron acceptor (A) through a s-bonded molecular bridge. 4–6 The latter has received a great deal of attentions due to their potential application in fields such as molecular electronics, nonlinear optics, pho- tovoltaics and artificial photosynthetic models. 7 Among all kind of electron donors and acceptors, tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) derivatives have been widely used for the study of photoin- duced intramolecular charge transfer processes in such D–A systems. 5–7 It remains a challenge to tune the HOMO– LUMO gap by applying external stimuli, and therefore to control the performance of the corresponding electronic de- vices. Obviously, the development of ‘responsive’ D–A sys- tems is very appealing. Such responsive systems are also highly desirable in redox sensor materials with valuable optical and electronic properties. In this context, we have fo- cused our efforts on systems offering a good electrochemical amphotericity and being prone to exhibit an efficient switch- ing behaviour. The imidazole unit possesses two heteroatoms, which not only retain a versatile reactivity, for example, towards alkyl- ation, deprotonation and protonation, but also give rise to hydrogen bonding. These unique structural features offer a possibility of tuning the HOMO–LUMO gap via chemical stimuli. Therefore, an efficient synthetic route to an imida- zole-incorporated TTF–TCNAQ diad 1 has been developed. This new compound shows remarkable optical and electro- chemical responses as a function of pH as well as to the nature of substituents of the imidazole ring. Particularly, the addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) significantly improves the electron-donating ability of the TTF and decreases the HOMO–LUMO gap. 2. Result and discussion The target compound 1 was prepared via the direct conden- sation reaction of the diamine TTF precursor 5d and a formyl- functionalised tetracyanoanthraquinodimethane 8 in 85% isolated yield (Scheme 1). Further alkylation with 4- substituted phenylsulfonyl chloride affords compounds 2 and 3 quantitatively. Single crystals suitable for X-ray structural determination were obtained by slow evaporation of a saturated solution of 1 in CH 3 CN. The compound crystallises in the centro- symmetric triclinic space group with two TTF–TCNAQ Keywords: Tetrathiafulvalene; 7,7,8,8-Tetracyano-p-quinodimethane; HOMO–LUMO gap; Intramolecular charge transfer; Cyclic voltammetry. * Corresponding author. Tel.: +41 31 6314296; fax: +41 31 6314399; e-mail: liu@iac.unibe.ch y Present address: State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, PR China. 0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2007.08.091 Tetrahedron 63 (2007) 11282–11286