Hexaphenylethane Derivatives Exhibiting Novel Electrochromic Behavior

摘要

9,9,10,10-Tetraphenyl-9,10-dihydrophenanthrene (3) is a stable molecuie whose central C-C bond is predicted to be the longest among "clamped" hexaphenylethane derivatives. Although this weakened bond is resistant to homolytic rupture, the activation energy for its scission can be lowered by electron transfer, especially when suitable substituents are introduced. Bearing this in mind, we have now designed a new redox system in which reversible C-C bond breaking and bond making occur upon two-electrontransfer (Scheme 1). This system has the following interesting features: 1) the four dimethylamino and methoxy substituents in 1 a and 1 b, respectively, should facilitate the removal of electrons from these molecules, thereby activating the scissile bond; 2) the central bond in 1 like that in 3 is still expected to be -sufficiently stable against homolysis taking into consideration that the aromatic substituents have little effect on the Gibbs activation energy of the homolytic cleavage DELTA G_(homolysis)~(not =) in 1,2-diaryiethanes; 3) the dicatioris 2~(2+) formed by the oxidation of 1 should be stabilized by delocalization of each positive charge over half of the molecule and due to the presence of the triarylmethy'enium chrornophores these cations are intensely colored; 4) the interconversions between 1 and 2~(2+) should proceed very cleanly because they are intramolecular processes. Here we report the preparation, properties, and structures of these novel redox couples.