Theoretical studies on the electronic structures and optical properties of the thiophene oligomer containing 2-(trifluoromethyl) thieno [3, 4-b] thiophene moiety and the CF3 end-caps

Abstract

Structural, electronic, and optical properties of a series of π-conjugated thiophene oligomers P1-P3 and CF3P1-CF3P3 have been theoretically investigated. P1-P3 contain the 2- (trifluoromethyl) thieno [3, 4-b] thiophene moiety as the centre and 1–3 repeating thiophene units adjacent to its two sides respectively, while their corresponding derivatives CF3P1-CF3P3 with the CF3 as end-caps. The geometric structures of the oligomers in the ground and excited state were optimized by PBE1PBE and CIS methods with 6–31G (d) basis sets, respectively. All the oligomers exhibit zigzag arrangements. The absorptions and emissions were calculated by the time-dependent density functional theory method (TD-PBE1PBE). The lowest-lying absorptions of all the oligomers can be characterized as π-π* electron transition. For each series of oligomers, there is a progressive lowering in HOMO-LUMO gap with the increase of the repeating unit, being consistent with the red-shifted trend in the lowest-lying absorption and fluorescence from P1 to P3 and CF3P1 to CF3P3. To compare the P- and corresponding CF3P-oligomers, the end-cap CF3 group causes the slight blue shifts in absorption and emission spectra. The ionization potentials (IPS), electron affinities (EAs), and reorganization energies (λ) as well as the hole/electron extraction energies (HEP/EEP) of the oligomers were explored and those of the corresponding polymer were obtained by extrapolation method. The IP and HEP of P-polymer are lower than those of CF3P-polymer, indicating that the P-polymer is more suitable for hole transport than CF3P-polymer, while the higher EA and EEP for CF3P-polymer suggest the better electron transfer property. For CF3 end-caps, the CF3P-polymer exhibits the equal reorganization energy between electron and hole, which is a precondition for the charge transfer balance.