Cis/trans-isomerizations in 1,2-bis(2-thienyl)ethene derivatives: a joint experimental and computational approach

Two substituted 1,2-bis(2-thienyl)ethenes were synthesized and their Z/E-is omerization was studied in solution. Despite their structural similarities, they behaved very differently concerning the position of the equilibrium a nd the rate with which it was established: 1,2-bis(3-methoxy-2-thienyl)ethe ne (BMTE) was found to exist completely in the E-form and the transformatio n from Z to E was extremely fast, while 1-cyano-1-(2-thienyl)-2-(3-methoxy- 2-thienyl)ethene (CTMTE) displayed an equilibrium mixture composed of 97% Z - and 3% E-CTMTE, which was reached far slower. In an attempt to rationaliz e these differences ab initio calculations were performed which enabled us to correlate relative energy differences of starting and end conformations and of estimated intermediary states on the one hand with the experimentall y observed isomerizational behaviour on the other. Furthermore, the data pr esent insight into the effects of the methoxy and nitrile group on E/Z stab ility and conjugation length. (C) 1999 Elsevier Science B.V. All rights res erved.