Synthesis and characterization of a cyano-substituted electroluminescent polymer with well-defined conjugation length

A novel copolymer, poly[1,6-hexanedioxy-1,4-phenylene-1,2-ethenylene-(2,5-dicyano-1,4- phenylene)-1,2-ethenylene-1,4-phenylene] (HCN-PPV), with alternating conjugated and non-conjugated blocks is synthesized by the Wittig reaction. The conjugated block containing one unit of 2,5-dicyano-1,4-phenylene vinylene is linked to a flexible linear hexyl chain spacer by an ether group in the backbone. The polymer is soluble in common organic solvents such as CHCl₃ and CH₂Cl₂. The number average molecular weight (M_n) is 2517 with a polydispersity index of 1.75. The optical bandgap of HCN-PPV is 2.48 eV. λ_max of the PL spectrum of the polymer film sample lies at 533 nm, corresponding to green light emission. The polymer also shows good thermal stability with a decomposition temperature of 304 °C under air atmosphere. Cyclic voltammetric investigation of the polymer reveals a reversible n-doping process and an irreversible p-doping process. The onset for the n-doping process occurs at -1.48 V (vs. SCE), while the onset for the p-doping process occurs at 1.19 V (vs. SCE). The results imply that HCN-PPV is an n-dope type electroluminescent polymer.