Synthesis and characterization of triphenylamine-3-hexylthiophene oligomer hybrids: A triphenylamine core carrying three terthiophene branches and triphenylamine end-capped quaterthiophene

Two different triphenylamine-3-hexylthiophene (TPA-HT) oligomer hybrids were synthesized via multi-steps: a TPA core carrying three terthiophene branches (TPA-3HT₃) and a TPA end-capped quaterthiophene (2TPA-4HT). The compounds are amorphous in nature, and thermally stable up to about 400 °C under a nitrogen atmosphere. The UV-vis absorption and photoluminescence emission peaks of TPA-3HT₃ in chloroform were observed at 408 and 523 nm, while those of 2TPA-4HT were observed at 304/398 and 494/512 nm, respectively. The lower energy onsets of the absorption band and the onset oxidation potentials for the two compounds were very similar, indicating that the conjugation lengths of TPA-3HT₃ and 2TPA-4HT must be similar. The unexpected result relating to the conjugation length was ascribed to the position of the hexyl substituent on the HT which is directly attached to the phenyl group of TPA in 2TPA-4HT, leading to repulsion between the α-methylene hydrogens on the alkyl group and the hydrogen atom at ortho position in the phenyl ring of TPA. The estimated highest occupied molecular orbital (HOMO) energy levels of the two compounds were very close to those of the well-known hole-transporting materials, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) and poly(3,4-ethylenedioxy thiophene) (PEDOT).