Structural, electronic and optical properties of a series of oligofluorene-thiophene oligomers and polymers

A significant drawback of thiophene oligomers are their low fluorescence and poor stability, especially in solid state. To overcome this problem, end-capped oligofluorene-thiophenes were synthesized. By adjusting the conjugation length of the thiophene units between the two fluorenes, it is possible to fine-tune the energy level and emission color of the resulting materials. In this contribution, we apply quantum-chemical techniques to investigate a series of oligomers and polymers based on end-capped oligofluorene-thiophenes. The optimized structures, the characterization of frontier molecular orbitals, HOMO-LUMO gaps (ΔH-L), in addition to the ionic potentials (IP) and electron affinity (EA) were obtained by B3LYP/6-31G density functional theory (DFT) calculations. The lowest excitation energies (Eg) and the maximal absorption wavelength λabs of the oligomers are studied employing the time dependent density functional theory (TD-DFT). The ΔH-Ls, Egs, IPs and EAs of the polymers were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n=0). The outcomes show that both the hole and electron accepting and transporting properties in polymers are better than that in oligomers. As the conjugation lengths increase the energy gaps decrease and thus the absorption spectra exhibit bathochromic shift.