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

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, D0 → D2+), cyclic voltammetry of the cyclophanes reveals
two reversible, one-electron oxidation steps (D0 → D•+ → D2+), with differences between the half-wave
potentials (E21/2 − E11/2) 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 (λmax = 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.