Random poly(fluorenylene-vinylene)s containing 3,7-dibenzothiophene-5,5-dioxide units: synthesis, photophysical, and electroluminescence properties

The synthesis of new random poly(arylene-vinylene)s containing the electron withdrawing 3,7-dibenzothiophene-5,5-dioxide unit was achieved by the Suzuki- Heck cascade polymerization reaction. The properties of poly[9,9-bis(2-ethylhexyl)- 2,7-fluorenylene-vinylene-co-3,7-dibenzothiophene-5,5-dioxide-vinylene] (50/50 mol/ mol, P1) and poly[1,4-bis(2-ethylhexyloxy)-2,5-phenylene-vinylene-co-3,7-dibenzothiophene- 5,5-dioxide-vinylene] (50/50 mol/mol, P2) were compared with those of terpolymers obtained by combining the fluorene, dibenzothiophene, and 1,4-bis(2-ethylexyloxy) benzene in 20/40/40 (P3), 50/25/25 (P4), and 80/10/10 (P5) molar ratios. The polymers were characterized by 1H NMR and IR, whereas their thermal properties were investigated by TGA and DSC. Polymers P1-5 are blue-green emitters in solution (kem between 481 and 521 nm) whereas a profound red shift observed in the solid state is emission (kem from 578 to 608 nm) that can be attributed both to the charge transfer stabilization exerted by the polar medium and to intermolecular interactions occurring in the solid state. Cyclic voltammetry permitted the evaluation of the ionization potentials and also revealed a quasi-reversible behavior in the reduction scans for the polymers (P1-4) containing the higher amounts of 3,7-dibenzothiophene- 5,5-dioxide units. Electroluminescent devices with both ITO/PEDOT-PSS/P1-5/Ca/Al (Type I) and ITO/PEDOT-PSS/P1-5/Alq3/Ca/Al (Type II) configuration were fabricated showing a yellow to yellow-green emission. In the case of P4, a luminance of 1835 cd/m2 and an efficiency of 0.25 cd/A at 14 V were obtained for the Type II devices