Rigorous Franck-Condon absorption and emission spectra of conjugated oligomers from quantum chemistry

A harmonic Condon approach is used to calculate excitation and emission ban d shapes for the lowest dipole-allowed electronic transitions in conjugated oligomers: polyenes, oligorylenes, and para-phenylenevinylenes. Ground- an d excited-state adiabatic energies, equilibrium structures, and vibrational modes are obtained within standard all-valence-electron molecular Hamilton ian incorporating extended configuration interaction. The interstate distor tion is cast in normal coordinates and used to calculate transition probabi lities from the zero-phonon initial state to the vibrational manifold of th e final state. Spectral profiles are obtained as a superposition of Lorentz ian line shapes. Theoretical band shapes reproduce prominent features in th e absorption and fluorescence spectra of the oligomers in question. The str ength of the bond-stretching vibronic progression increases with oligomeric length in polyenes, but decreases in para-phenylenevinylenes. In line with experiment, absorption and emission band shapes of para-phenylenevinylenes are obtained intrinsically nonsymmetric due to stiffening of the accepting vibrational modes in the excited state. The Stokes shifts of the apparent 0-0 features in the latter are reproduced and traced back to relaxations in slow, ring-torsional motions. (C) 2000 American Institute of Physics. [S00 21-9606(00)51648-1].