Singlet excitation energies of thiophene derivatives of fluorene: TD‐DFT study

Abstract

Fluorene-thiophene (FT)-based oligomers and polymers and their derivatives are good candidates for organic blue light-emitting diodes. In this work, the intrinsic properties of the ground and excited states of FT monomer and its derivatives are studied. The ground-state optimized structures and energies are obtained using molecular orbital theory and density functional theory (DFT). The ground-state potential energy curves or surfaces of FT and its derivatives are also obtained. All derivatives are nonplanar in their electronic ground states. The character and energy of the first 20 singlet-singlet electronic transitions are investigated by applying the timedependent density functional theory (TD-DFT) approximations to the correspondingly optimized ground-state geometries. The lowest singlet state is studied with the configuration interaction (singles) approach (CIS). Excitation energies are red shifted when the FT unit or its derivatives are extended longitudinally. CIS results suggest geometry relaxation in the first singlet excited state. When available, a comparison is made with experimental results.

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).…”

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

“…The emission energy is 3.43 eV or k E 361 nm (expt: 3.46 eV or 359 nm) 11 for FEDOT, and 2.50 eV or 496 nm for FTSO2. A comparison with ZINDO and TDDFT results shows that SAC-CI provided more accurate emission energy of FEDOT than did ZINDO (390 nm) 37 or TDDFT (354 nm 37 and 376 nm 36 for B3LYP/6-31G(d) basis set and 374 nm 36 for B3PW91/ 6-31G(d)). In addition, the simulated spectra provided a fwhm E of the band which agreed well with the experimental band.…”

“…FEDOT gave dr 5 20.039 at the ground state. Gong and Lagowski 36 predicted dr \ 0 (20.034), which represents an aromatic structure. In contrast, there was a quinoid structure at the excited state due to dr 5 0.009 (0.013).…”