Investigation of excited-state properties of fluorene-thiophene oligomers by the SAC-CI theoretical approach.

Excited states of fluorene-ethylenedioxythiophene (FEDOT) and fluorene-S,S-dioxide-thiophene (FTSO2) monomers and dimers were studied by the symmetry-adapted cluster (SAC)-configuration interaction (CI) method. The absorption and emission peaks observed in the experimental spectra were theoretically assigned. The first three excited states of the optimized conformers, and the conformers of several torsional angles, were computed by SAC-CI/D95(d). Accurate absorption spectra were simulated by taking the thermal average for the conformers of torsional angles from 0 degrees to 90 degrees. The conformers of torsional angles 0 degrees, 15 degrees, and 30 degrees mainly contributed to the absorption spectra. The full width at half-maximum of the FEDOT absorption band is 0.60 eV (4839 cm(-1)), which agrees very well with the experimental value of 0.61 eV (4900 cm(-1)). The maximum absorption wavelength is located at 303 nm, which is close to those of the experimental band (327 nm). The calculated absorption spectrum of FTSO2 showed two bands in the range of 225-450 nm. This agrees very well with the available experimental spectrum of a polymer of FTSO2, where two bands are detected. The excited-state geometries were investigated by CIS/6-31G(d). These showed a quinoid-type structure which exhibited a shortening of the inter-ring distance (0.06 A for FEDOT and 0.04 A for FTSO2). The calculated emission energy of FEDOT is 3.43 eV, which agrees very well with the available experimental data (3.46 eV). The fwhm(E) is about 0.49 eV (3952 cm(-1)), while the experimental fwhm is 0.43 eV (3500 cm(-1)). For FTSO2, two bands were also found in the emission spectrum.