1,3,5,7-Octatetraene excited singlet states

Allan M, Neuhaus L and Haselbach E (1984) (All-E)-1,3,5,7-octatetraene Electron-energy loss and electron-transmission spectra. Helv Chim Acta 67 1776-1782 Andersson PO andGiUbro T (1995) Photophysics and dynamics of the lowest excited singlet state in long substituted polyenes with implications for the very long chain limit. J Chem Phys 103 2509-2519... 

The triple bond of 6-phenyl-2-hexane [75] adds intramolecular ly to the phenyl ring to a cyclo-octatetraene derivative. The cycloaddition is a hundred times less efficient than in [74] and is found to be a singlet state reaction of the excited phenyl moeity. 

In the condensed phase singlet excitation relaxes to a thermodynamic distribution over the vibronic levels of Si on the picosecond timescale, independent of the state initially excited. Because the expectation value of a 1-electron operator vanishes between configurations that differ in two spin-orbitals, the transition dipole between Si and the ground state is very small even when the symmetry is lowered as, for example, in a cis isomer. In consequence, the lifetime of the Si state is relatively long (225 nsec for octatetraene in n-octane) and usually is determined by non-radiative rather than radiative processes [56]. 

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