Poly(p-phenylenevinylene) derivatives containing electron-transporting aromatic triazole or oxadiazole segments

We report the synthesis, optical and electrochemical details, and properties of copolymers P1-P3 consisting of alternate hole-transporting 1,4-bis(hexyloxy)-2,5-distyrylbenzene (HDB) and electron-transporting 4-(4-(hexyloxy)phenyl)-3,5-diphenyl- 4H-1,2,4-triazole (EDT) or 2,5-diphenyl-1,3,4-oxadiazole (EDO) segments linked via an ether spacer or a twisted σ-bond (biphenyl). These copolymers are soluble in common organic solvents such as chloroform, NMP, and 1,1,2,2-tetrachloroethane and exhibit good thermal stability with decomposition temperatures higher than 375°C. P1-P3 show efficient energy transfer from EDT or EDO to EDO fluorophores when photoexcited. Optical and electrochemical properties of P1-P3 are also investigated in detail by comparing with P4 and P5 containing similar chromophores. From the cyclic voltammograms the onset oxidation and reduction potentials for isolated P1 and conjugated P2 are comparable, indicating that the effect of the twisted σ-bond in P2 is similar to that of the ether spacer in PI. The optimized geometries of P2 and P3 show that the torsion angle between HDB and EDT or EDO are 83.6° or 89.6°, respectively, based on MNDO semiempirical calculations. The large torsion angle in P2 and P3 significantly limits delocalization of charges between hole- and electron-transporting segments. Accordingly, in P2 and P3 the oxidation and reduction starts at the hole- and the electron-transporting, respectively, like those in isolated PI. The HOMO and LUMO energy levels of P1, P2, and P3, estimated from electrochemical data, are -5.16, -5.12, and -5.19 eV and -3.35, -3.38, and -3.23 eV, respectively. Single-layer light-emitting diodes (A1/P1-P3/ITO) have been successfully fabricated, and they reveal blue or yellow electroluminescence.