Barriers in the excited 2 1A g state for the c i s,t r a n s isomerization of c i s,c i s-octatetraene: General features of the excited state potential

The lifetime of the excited 2 1Ag state of cis,cis-octatetraene (the eight carbon oligomer of cis-polyacetylene) in a n-octane host crystal has been measured as a function of temperature in the region 8<T<116 K. The lifetime is relatively long at 8 K (130 ns), drops to a lower value at approximately 30 K, and then begins to decrease rapidly with increasing temperature in the neighborhood of 100 K. The highly resolved optical spectra show unambiguously that at 30 K the decrease in lifetime is associated with thermal isomerization to cis,trans-octatetrane in the excited 2 1Ag state. These data are combined with other spectroscopic and kinetic data to obtain estimates for the energies of all three double bond isomers in ground and excited states and the activation energies for double bond isomerization in the excited state.