Dependence on Local Polarizability

The vibronic development of the Sq to Si spectra is dominated by the C-C and C=C stretch modes. This means that the difference between the relaxed equilibrium geometries in the Sq and Si states is almost completely restricted to changes in the carbon-carbon bond lengths. 

TABLE 4. Si(2lAg) and S2 (l By) 0-0 excitation energies for isomers of octatetraene in n-hexane and n-octane matrixes. 

All available evidence supports a picture of the S (2 Ag) excited state that has the order of double and single bonds reversed from the order in the ground state. This follows from high quality ab initio quantum chemical calculations for octatetraene [59], and is strongly supported by direct measurements of the barrier in the octatetraene Si state to rotation about 

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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Dependence on Local Polarizability. Tlie solvent shift behavior of the S2 excitation energy for diphenylpolyenes with 1-6 and 8 double bonds in the polyene chain [65,75] are... 

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