Extreme Conformational Constraints in π-Extended Tetrathiafulvalenes: Unusual Topologies and Redox Behavior of Doubly and Triply Bridged Cyclophanes

摘要

Doubly and triply bridged 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (ex-TTF) derivatives have been synthesized. Key steps are the generation and macrocyclization reactions of ex-TTF-dithiolate reagents. The X-ray crystal structures of the doubly bridged cyclophanes 15 and 16 and the triply bridged system 23 show that the saddle-like conformation of the ex-TTF framework is enhanced by the short bridges between the dithiole rings. Unlike all previous ex-TTF derivatives (which display a single quasi-reversible two-electron oxidation wave, D~0 → D~(2+)), cyclic voltammetry of the cyclophanes reveals two reversible, one-electron oxidation steps (D~0 → D~(·+) → D~(2+)), with differences between the half-wave potentials (E_2~(1/2) - E_1~(1/2)) of 0.22-0.26 V. The conformational changes and gain in aromaticity which drive the second oxidation process in unrestricted ex-TTF systems (including singly bridged cyclophanes) have been prevented by multiple bridging. The radical cation species gives rise to a very broad, low-energy band (λ_(max) = 2175 and 2040 nm for 15 and 21, respectively), assigned to an intramolecular interaction. The steric constraints imposed by multiple bridging have become so extreme that the π-framework of 15, 16, 21, and 23 exhibits remarkable optical and redox behavior which is not characteristic of ex-TTF systems.