On the photoisomerization coordinate of tetraphenylethylene

The existence of two energetically distinct minima on the biradicaloid triplet state surface of tetraphenylethylene (TPE) was detected via AM1 and 3–21G∗ UHF ab initio calculations. The interconversion between the minima proceeds through a large amplitude rotation of the interlocking phenyl rings. The importance of the presence of two excited state minima and the possibility of conformational trapping are discussed in context of the recent spectroscopic and opto-calorimetric measurements on TPE. The most favorable pathway for the combined cis-trans photoisomerization and enantiomerization of TPE is postulated.