Microwave-assisted and conventional synthesis, photophysics and electroluminescence of poly(9,9-dihexadecylfluorene-2,7-diyl-alt-2,2′-bithiophene-5,5′-diyl)

Alternating electroluminescent copolymers, poly(9,9-dihexadecylfluorene-2,7-diyl-alt-2,2′-bithiophene-5,5′-diyl)s (PFC16BTs), were synthesized using two different routes of Suzuki coupling reaction in which either dibromo derivative of fluorene was reacted with diboronic acid ester of bithiophene or vice versa, with microwave-assisted (MA) or conventional (C) heating in high boiling point solvents, chlorobenzene (CB) and xylene (Xy). PFC16BT copolymers with various molecular weights (MW) were obtained depending on the synthetic conditions, and the influence on MW is discussed. The photophysical and electrochemical properties and the electroluminescence (EL) of the PFC16BT copolymers were studied and compared with those of poly(9,9-dioctylfluorene-2,7-diyl-alt-2,2′-bithiophene-5,5′-diyl)s (PFC8BTs) to reveal the impact of MW and the length of side chains on their properties. A slight dependence on the MW was only observed in the absorption spectra in solutions. Photoluminescence (PL) of PFC16BTs solutions emitted green light with a maximum of approximately 500 nm, whereas the PL emission spectra of thin films were redshifted with the maxima at 540 nm. The ionization potential (HOMO level) and electron affinity (LUMO level) values of 5.4 eV and 2.6 eV, respectively, were evaluated for PFC16BTs, which are similar to those for PFC8BTs. The electrochemical bandgap value of 2.8 eV was higher than the value of the optical bandgap of 2.4 eV. The PFC16BT copolymers with various MW were tested as an active layer in light-emitting devices (LEDs), and the results are compared with those of PFC8BT LEDs. The PFC16BT LEDs exhibited a luminance higher than 3500 cd m–2 and low onset voltages of approximately 2 V. The shape of the EL spectra of LEDs is similar to that of PL spectra of the thin films. PL emission of PFC16BT thin films and EL emission were blue shifted compared with those of PFC8BT, indicating a more pronounced aggregation in PFC8BT.