SAC-CI theoretical investigation on electronic structure of fluorene-thiophene oligomers

The excited states of the fluorene-thiophene compounds whose polymers are useful as a light-emitting diode were studied by SAC-CI method. The effect of the torsional angle on the excited state was examined for the FT and FMT monomers in details. The first three excited states were calculated for the conformers of several torsional angles. These three excited states were found to change their characters by varying the torsional angle from 0 to 90°. The accurate absorption spectra were simulated by taking the thermal average for the conformers of torsional angle from 0 to 90°. The absorption spectra of dimer and trimer were also calculated at the equilibrium structure. It was found that the oscillator strength of S1 states of the dimer and trimer was very high and the red shift occurs due to the π-conjugation. The equilibrium structures are planar for both FT and FMT, and the calculated emission energies are in consistent agreement with the experiment. The effect of applying the electric fields to these molecules was examined.