Theoretical studies on the electronic structures and optical properties of the thiophene oligomer containing 2-(trifluoromethyl) thieno [3, 4-b] thiophene moiety and the CF _3 end-caps

摘要

Structural, electronic, and optical properties of a series of φ-conjugated thiophene oligomers P1-P3 and CF _3P1-CF _3P3 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 CF _3P1-CF _3P3 with the CF _3 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 lowestlying absorptions of all the oligomers can be characterized as φ-φ ~* 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 CF _3P1 to CF _3P3. To compare the P- and corresponding CF _3P- oligomers, the end-cap CF _3 group causes the slight blue shifts in absorption and emission spectra. The ionization potentials (IPS), electron affinities (EAs), and reorganization energies (l) 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 CF _3P-polymer, indicating that the P-polymer is more suitable for hole transport than CF _3P-polymer, while the higher EA and EEP for CF _3P-polymer suggest the better electron transfer property. For CF _3 end-caps, the CF _3P-polymer exhibits the equal reorganization energy between electron and hole, which is a precondition for the charge transfer balance.