Synthesis and characterization of 3,4-diphenylmaleimide copolymers that exhibit orange to red photoluminescence and electroluminescence

A series of newly designed 3,4-diphenylmaleimide-based T-conjugated copolymers that exhibit red fluorescence in both solution and solid state were synthesized and characterized. Bright and efficient red fluorescence in the solid state was achieved by varying the structural combination of thiophene and/or fluorene with 3,4-diphenylmaleimide fluorophore. The range of thiophene- and fluorene-derived 3,4-diphenylmateimide model compounds revealed that thiophene moieties were effective in extending the emission wavelength to the red region, and fluorene moieties significantly enhanced fluorescence intensity. Polymer light-emitting diodes (PLEDs) fabricated from 3,4-diphenylmaleimide-terthiophene copolymer (PTTML) exhibited saturated red electroluminescence (EL) with emission lambda(EL)(max) of 676 nm, although the diode was inefficient and dim. TPBI (2,2',2"-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole)) as the electron-transporting layer in PLEDs was found to improve the performance of 3,4-diphenylmaleimide-bithiophene copolymer (PBTML). PBTML PLED yielded reddish EL (emission lambda(EL)(max) of 620 nm) with peak efficiencies of 0.61% (0.89 cd/A) and a maximum electroluminance of 129 cd/m2. A rare fluorene copolymer of a 3,4-diphenylmaleimide-thiophene-fluorene triad (PFTML) emitted orange-red photoluminescence (PL) at a lambda(fl)(max) of 598 nm in solution and lambda(fl)(max) of 601 nm in a solid thin film, with relatively high fluorescence quantum yields of 37 and 22%, respectively. Its PLEDs yielded bright orange-red EL (emission lambda(EL)(max) of 614 nm) with a high intensity of over 2000 cd/m(2) and an enhanced peak luminous efficiency of 1.25 cd/A or an external quantum efficiency of 0.74%.