S. Karabunarliev et al., Rigorous Franck-Condon absorption and emission spectra of conjugated oligomers from quantum chemistry, J CHEM PHYS, 113(24), 2000, pp. 11372-11381
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].