Tetrathiafulvalene (TTF)-Bridged Resorcin[4]arene Cavitands: Towards New Electrochemical Molecular Switches



We report the synthesis of novel resorcin[4]arene-based cavitands featuring two extended bridges consisting of quinoxaline-fused TTF (tetrathiafulvalene) moieties. In the neutral form, these cavitands were expected to adopt the vase form, whereas, upon oxidation, the open kite geometry should be preferred due to Coulombic repulsion between the two TTF radical cations (Scheme 2). The key step in the preparation of these novel molecular switches was the P(OEt)3-mediated coupling between a macrocyclic bis(1,3-dithiol-2-thione) and 2 equiv. of a suitable 1,3-dithiol-2-one. Following the successful application of this strategy to the preparation of mono-TTF-cavitand 3 (Scheme 3), the synthesis of the bis-TTF derivatives 2 (Scheme 4) and 19 (Scheme 5) was pursued; however, the target compounds could not be isolated due to their insolubility. Upon decorating both the octol bowl and the TTF cavity rims with long alkyl chains, the soluble bis-TTF cavitand 23 was finally obtained, besides a minor amount of the novel cage compound 25a featuring a highly distorted TTF bridge (Scheme 6). In contrast to 25a, the deep cavitand 23 undergoes reversible vase → kite switching upon lowering the temperature from 293 to 193 K (Fig. 1). Electrochemical studies by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) provided preliminary evidence for successful vase → kite switching of 23 induced by the oxidation of the TTF cavity walls.