Symmetrical and asymmetrical end capped 9,9 -spirobifluorene with electron donor or/and acceptor groups are calculated to investigate the influence of the end-capping groups by using DFT/B3LYP method at 6-31G(d) level. We fully optimize all the structures concerned. Analyzing the HOMO and LUMO energy levels and HOMO-LUMO gaps of all species, we find that the LUMO level drops while the HOMO level slightly increases and therefore the HOMO-LUMO gap shrinks when 9,9 -spirobifluorene is asymmetrically end-capped by an electron donor group oxadiazole (O) and an electron acceptor group thiophene (T). Full optimizations, at the same B3LYP/6-31G(d) level, on the structures of the oligo-9,9 -spirobifluorene end-capped with thiophene and oxadiazole [T(SBF)_nO, n＝1～4] show that the ratio of the partial charge of the donor group over that of the acceptor group grows larger with increasing chain length n. The donor and acceptor moieties exhibit intramolecular charge transfer (ICT) and the charge transfer gets enhanced in oligomers. The ionization potentials (IPs), electron affinities (EAs), hole extraction potentials (HEP), electron extraction potentials (EEP) and the reorganization energies (l) are calculated to show that the charge carrier injection from donor to acceptor group further facilitates the transport of both electrons and holes. Absorption spectra of T(SBF)_nO (n＝1～4) are obtained by employing TD-DFT and ZINDO calculations. The results show that, with increasing chain length n, the excitation energy becomes smaller, the spectra are red-shifted, and the oscillator strength is enhanced. The singlet excited geometry of T(SBF)₁O is calculated using the single-excitation configuration interaction CIS method at the same 6-31G(d) level. Compared to the ground state, the excited state tends to move toward a planar geometry.

Key words: 9,9-spirobifluorene, thiophene, oxadiazole, density functional theory, end-cap