The effect of the molecular structure of organic material on the properties of solid-state fluorescence and electroluminescence

We have designed, synthesized and characterized a novel compound comprising red fluorescent material, (2Z,2′Z)-3,3′-[4,4″-bis(dimethylamino)-1,1′:4′,1″-terphenyl-2′,5′-diyl]bis(2-phenylacrylonitrile) (ABCV-P), which prevented concentration quenching in non-doped, organic light-emitting diodes (OLEDs). 4-(Dicyanomethylene)-2-methyl-6-(julolidine-4-yl-vinyl)-4H-pyran (DCM2), which is used as a red dopant in OLEDs, is vulnerable to concentration quenching in the solid state due to its planar and polar molecular structure. To investigate the effect of the molecular structure on the surface morphology of thin solid films, which affects solid-state fluorescence and electroluminescence (EL), a thin solid film of ABCV-P was compared with that of DCM2 by atomic force microscopy (AFM) analysis. The AFM analysis showed that the thin solid film of ABCV-P exhibited badly crystalline or amorphous-like surface morphology, in contrast to that of DCM2. The photoluminescence (PL) of the thin solid film of ABCV-P peaked at 596 nm, while no such peak was observed for DCM2. A device with the structure of ITO/NPB/ABCV-P/BCP/Alq3/Liq/Al showed an EL spectrum which peaked at 586.5 nm at 8 V, with a maximum luminance of 3320 cd/m2 at 687 mA/cm2 and a maximum luminous efficiency of 0.703 cd/A at 7.7 mA/cm2. In comparison, the OLED device with DCM2 as a host-emitter did not exhibit any red EL due to concentration quenching in the solid state.