Optical origin of efficient and selective trans <-> cis isomerization in photoexcited polyenes: An ab initio calculation

An ab initio calculation is carried out to investigate the optical origin o f highly-efficient and selective trans <----> cis isomerizations in a short polyene. It is found that the highly-efficient isomerization is closely re lated to the highly-selective optical transitions. These dipole-allowed tra nsitions occur selectively at two configurations with the dihedral angles t heta(11,12)=60 degrees and 120 degrees, where the system can be excited or de-excited and is also able to undergo cis or trans isomerization effective ly. For many configurations which are not favorable to the isomerization, t he transition matrix elements are basically zero. This rules out many unfav orable transitions at these configurations, which is why photoinduced isome rization is so efficient. (C) 1999 American Institute of Physics. [S0021-96 06(99)51217-8].