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

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Abstract
Proceeding from quantum-chemical potential energy surfaces, we compute the absorption and fluorescence spectra of conventional and ladder-type para-phenylene oligomers (OPP and OLPP) with up to 7 benzene rings. Electronically excited states are addressed by means of extended configuration interaction within a standard molecular all-valence-electron semiempirical Hamiltonian. Adiabatic excitation energies, interstate distortions and normal modes are used to compute Franck–Condon band shapes with rigorous consideration of vibrational structure. Theoretical spectra agree with the experiment and rationalize the striking disparities in the linear optical response of OPP and 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-frequency libration motions. The results highlight the importance of ring-torsional flexibility in conjugated polymers.