Synthesis, Spectral Stability, and Electroluminescent Properties of Random Poly(2,7-fluorenylenevinylene-co-3,6-carbazolylenevinylene) Obtained by a Suzuki-Heck Cascade Reaction

The synthesis of a series of new random poly(9,9-dioctyl-2,7-fluorenylenevinylene-co-N-octyl- 3,6-carbazolylenevinylene) copolymers at 20-80% mol/mol of carbazole (P2-P6) was achieved by a versatile Suzuki-Heck reaction cascade between the suitable dibromoaryl derivatives at different monomer ratios and potassium vinyltrifluoroborate. Their properties have been investigated by 1H NMR, IR, TGA, DSC, cyclic voltammetry, UV-vis, and photoluminescence and compared to those of poly(9,9-dioctyl-2,7-fluorenylenevinylene) (P1) obtained by the same method. The control of the ð-conjugation extension and the inductive effects, determined by the amount of carbazolyl-3,6-ene moieties, permitted the modulation of the HOMO and LUMO energy levels of these materials without remarkably modifying their emission properties. Furthermore, different from poly- (9,9-dioctylfluorene), a good spectral stability in the solid state was proven for P1 after a thermal annealing under air at 130 °C. The electroluminescence properties of the materials were tested by constructing OLED devices of ITO/PEDOT-PSS/P1-P6/Ca/Al configuration. The improvement of the device performances for P2-P4 (20-50 mol/mol of carbazole) with respect to P1 was attributed to the better hole transport in the material because of the presence of carbazole units. The better performances of P5 (450 cd/m2 at 22 V, 0.0225 cd/A at 22 V) were ascribed to the easier injection of electrons in the device with respect to the other copolymers, as a consequence of its higher electron affinity. These figures of merit could be improved by inserting a suitable electron injecting layer (Alq3) between the cathode and P5 (890 cd/m2 at 18 V, 0.0435 cd/A at 10 V).