Annulenes DOI: 10.1002/anie.201202324 A Tetrathiafulvalene-Functionalized Radiaannulene with Multiple Redox States** Kasper Lincke, Anders Floor Frellsen, Christian Richard Parker, Andrew D. Bond, Ole Hammerich, and Mogens Brøndsted Nielsen* Radiaannulenes (RAs) are cyclic molecules having both exo- and endocyclic double bonds, [1] and therefore structurally lie between radialenes [2] and annulenes. [3] As they possess a quinoid-like structure (Figure 1), we decided to investigate the possibility for employing them as Wurster-type [4] two- stage redox systems. A benzannelated derivative of octade- hydro[14]annulene (1) has previously been shown to exhibit 14p aromaticity as asserted from a diatropic ring current. [5] This same core could formally be generated from the expanded RA 2 by reduction. Herein, we have combined 2 with tetrathiafulvalene (TTF, Figure 1), a two-stage Weitz- type [4] redox system which has found wide interest in materials and supramolecular chemistry. [6] By undergoing two one-electron oxidations, TTF achieves two 6p-aromatic 1,3-dithiolium rings, [7] and fusing a TTF unit to each of the endocyclic double bonds of 2 should thus provide a TTF/RA molecule that could potentially exist in seven or more redox states. Each of these are expected to exhibit characteristic electronic excitations in the UV/Vis region and maybe even in the NIR/IR region. Molecules reversibly changing color upon charging/decharging are particularly attractive for the devel- opment of electrochromic materials. Subjecting the tetraethynylethene (TEE) 3 [8] (after desi- lylation) and the diiodo TTF 4 [9] to fourfold Sonogashira couplings gave the TTF/RA 5 in a yield of 10 %, thus corresponding to 56 % yield for each step in the cyclization (Scheme 1). The compound was dark-green in solution and as crystals, but was red when isolated as a solid film. The structure was confirmed by X-ray crystallographic analysis (Figure 2), [10] which showed some bending of the TTF units and triisopropylsilylethynyl groups away from the almost planar RA core. Compound 5 comprises two TEE units, which are themselves known to be electron acceptors. [11] For a compar- ison of properties, we also prepared the related TTF/TEE 6 [9] (Figure 3). Both 5 and 6 are strong chromophores with broad charge- transfer (CT) transitions in CH 2 Cl 2 at approximately l = 644 and 522 nm, respectively (Figure 4). This band extended to l = 800 nm in the case of 5, thus suggesting a particularly strong acceptor character of the RA core. The CT character of the absorption was supported by DFT calculations (B3LYP/6-31G(d) using Gaussian 09 [12] ) on the related molecule 5(4H) with the silyl groups replaced by H atoms. The HOMO and HOMO1 reside mainly on the two TTFs (and partially on the external diethynylethenes), while the LUMO resides mainly on the cyclic core (Figure 5). Figure 1. Structures of 1, 2, and TTF. Scheme 1. Synthesis of TTF/RA scaffold. THF = tetrahydrofuran. [*] Dr. K. Lincke, A. FloorFrellsen, Dr. C. R. Parker, Prof. Dr. O. Hammerich, Prof. Dr. M. Brøndsted Nielsen Department of Chemistry, University of Copenhagen Universitetsparken 5, 2100 Copenhagen Ø (Denmark) E-mail: mbn@kiku.dk Prof. Dr. A. D. Bond Department of Physics, Chemistry and Pharmacy University of Southern Denmark, 5230 Odense M (Denmark) [**] The Danish Council for Independent Research / Natural Sciences (no. 10-082088), The Carlsberg Foundation, and the European Union 7th Framework Programme (FP7/2007–2013) under the grant agreement no. 270369 (“ELFOS”) are acknowledged. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201202324. A ngewandte Chemi e 6099 Angew. Chem. Int. Ed. 2012, 51, 6099 –6102  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim