Franck-Condon spectra and electron-libration coupling in para-polyphenyls

Proceeding from quantum-chemical potential energy surfaces, we compute the absorption and fluorescence spectra of conventional and ladder-type para-ph enylene oligomers (OPP and OLPP) with up to 7 benzene rings. Electronically excited states are addressed by means of extended configuration interactio n within a standard molecular all-valence-electron semiempirical Hamiltonia n. Adiabatic excitation energies, interstate distortions and normal modes a re used to compute Franck-Condon band shapes with rigorous consideration of vibrational structure. Theoretical spectra agree with the experiment and r ationalize the striking disparities in the linear optical response of OPP a nd OLPP. Whereas electron-phonon coupling in OLPP is essentially restricted to the carbon-carbon bond-stretching modes, photoexcitation, and emission processes in OPP are followed by significant relaxations in ring-torsional degrees of freedom. The broadening of spectra of OPP, especially pronounced in absorption, and the large Stokes shift between absorption and emission are traced back to the strong coupling of electronic excitations and low-fr equency libration motions. The results highlight the importance of ring-tor sional flexibility in conjugated polymers. (C) 2001 American Institute of P hysics.