Journal of Physical Chemistry, Vol.98, No.50, 13157-13161, 1994
Modeling of the Franck-Condon Structure of the Electronic-Transitions of an Oligomer of Polydiacetylene
The Franck-Condon structures of the absorption and emission spectra of C14H8 (HC=C-CH=CH-C=C-CH=CH-C=C-CH=CH-C=CH) are investigated through model calculations. Because of the relatively low resolution of the spectra available and to avoid a possible bias in the simulation, the Franck-Condon parameters are calculated by an ab initio procedure and by a more extensive set of semiempirical QCFF/PI calculations. In the process, the QCFF/PI model is upgraded to include the treatment of linear bendings. The calculated vibro-electronic spectra agree well with the experiment. In particular, the modeling reproduces the lack of mirror image between absorption and emission and the different behavior of the 0-0 band in the two spectra. Because of the success in the Franck-Condon simulation, the calculations are used to discuss the different nature of the excitation in the two lowest-lying electronically excited states of this prototypical molecule.