Investigation of excited‐state properties of fluorene–thiophene oligomers by the SAC‐CI theoretical approach

Abstract

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.

Photophysical Properties and Photochemistry ofEE-,EZ-, andZZ-1,4-Dimethoxy-2,5-bis[2-(thien-2-yl)ethenyl] Benzene in Solution: Theory and Experiment

“…By the SAC–CI method, we have investigated the photophysical properties of numerous π-conjugated molecules. They include the absorption/emission spectra and molecular geometry in excited states of OLED molecules − and the photochemistry of the ultraviolet B blocking molecules . In those works, the electronic spectra were well reproduced with the reliable assignments proposed, and the color tuning by introducing substituents was also examined satisfactorily.…”

Synthesis and photophysical properties of hyperbranched polyfluorenes containing 2,4,6-tris(thiophen-2-yl)-1,3,5-triazine as the core

“…Indeed, it has been shown that in fluorene-thiophene oligomers the formation the charge transfer state is strongly dependent on the torsional angle between the electron donor and acceptor moieties being favored by torsional angles around 901. 102 In addition, the emission decay curves of all polymers measured at 520 nm with excitation at 420 and 360 nm superimpose, without any detectable rise-time component for excitation at 360 nm. This indicates that the energy transfer to the charge-transfer state is very fast.…”

“…101 The existence of more than one emitting state can be understood in terms of the conformational flexibility associated with the torsional angle dependence of the nature of the excited state and the corresponding oscillator strength. 102 The incomplete quenching of the blue emission can be understood if: (i) the solution contains a distribution of molecules composed of species that fluoresce in the blue while others emit in the red, and the solution is dilute enough to avoid intermolecular energy transfer processes; (ii) there is only one molecular species in solution but the intramolecular energy transfer from the upper excited states to the low lying energy state is insufficient to allow emission from the higher energy state. The first possibility is excluded because the synthetic methodology excludes polymers composed of only fluorene, and the purification procedures exclude the presence of low molecular weight fluorene species.…”

Theoretical studies of the effect of electron-withdrawing dicyanovinyl group on the electronic and charge-transport properties of fluorene-thiophene oligomers

“…There are little discrepancies among the bond lengths and bond angles for FT obtained from different functionals, and they are in good agreement with the experimental values except for the largest error of 4.5% for C-S bond evaluated from BP86. Considering the rationality of B3LYP functional reported in evaluation of geometric structures and spectra properties for oligofluorene-thiophene oligomers [19,20,[45][46][47][48][49], B3LYP hybrid functional combined with the 6-31G* basis set was employed to optimize geometries for the other two molecules in this article (see Table S2). The basis set dependency of molecular geometry is described in supporting information (see Table S3).…”