Luminescent copolyethers containing isolated 1,4-distyrylbenzene derivatives backbone and 7-oxy-4-methylcoumarin side group: Synthesis and characterization

Four new copolyethers (PI-P4) consisting of two isolated emitting chromophores [2,5-dihexyloxy-1,4-distyrylbenzene (HODSB) and 2,5-dihexyloxy-1,4-di(4methylenestyryl)benzene (HOMDSB) for P1 and P2, 2,5-dihexyl-1,4-distyrylbenzene (HDSB) and HOMDSB for P3 and P4] in the backbone, in which P2 and P4 further contain electron-transporting chromophores [7-oxy-4-methylcoumarin (OMC)] in the side chain, were successfully prepared by the Heck coupling reaction. The photoluminescence spectra and quantum yields of the copolymers depended mainly on compositions of the isolated fluorophores. Their highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels were estimated from their cyclic voltammograms. Electrochemical investigations proved that the oxidation started at holetransporting DSB segments, whereas reduction began at electron-transporting OMC groups in P2 and P4. The electron affinity of P2 and P4 was enhanced by introducing electron-transporting OMC chromophores. Double-layer light-emitting diodes (ITO/PEDOT:PSS/polymer/Al) of P1 and P2 revealed green electroluminescence, and those of P3 and P4 emitted blue light. Moreover, incorporation of OMC side groups effectively reduced turn-on electric field and enhanced luminance efficiency of the EL devices due to increased electron affinity.