Franckndash;Condon modeling of the structure of theS0rarr;S2transition oftrans,transhyphen;,cis,transhyphen;, andcis,cishyphen;octatetraene

摘要

The Franckndash;Condon structure of the lowest lying intense polyenic electronic transition oftrans,transhyphen;,cis,transhyphen;, andcis,cishyphen;octatetraene is investigated through model calculations. To avoid a possible bias in the parameters of the model, the starting inputs are obtainedabinitio. The molecular orbital (MO) procedure consists in first optimizing the structures ofS0andS2and then calculating the vibrational frequencies at the stationary points on the potential energy surface of the three isomers. Together with the minima associated with the three isomers, we find one more saddle point inS0and two more inS2. These three saddle points correspond to planarS0cis,cishyphen;octatetraene, planarS2cis,cishyphen;octatetraene, and planarS2cis,transhyphen;octatetraene. The displacement, between the surfaces, of the harmonic oscillators associated with the normal modes, are obtained and used to simulate the Franckndash;Condon activity of theS0rarr;S2transitions of the three isomers. Such displacements are calculated in two schemes, the first uses the variation of the equilibrium position of the vibrational oscillators in the two states involved in the transition and requires full geometry optimization of all the geometrical parameters of the two states; the second, approximate, scheme requires only a single point calculation on the excited state surface and is therefore far less demanding. A simple scaling procedure, proposed before for hexatriene, is used to improve the agreement between theory and experiment. The model calculations of the Franckndash;Condon structure simulate very well theS0rarr;S2absorption spectra oftrans,transhyphen; andcis,transhyphen;octatetraene. It is further proposed that the large homogeneous linewidth in theS0rarr;S2transitions of polyenic systems is a function of the nonplanarity of these molecules inS2.