Pure-Red Dye for Organic Electroluminescent Devices: Bis-Condensed DCM Derivatives

In this paper, the bis‐condensed 4‐(dicyanomethylene)‐2‐methyl‐6‐[p‐(dimethylamino)styryl]‐4H‐pyran (DCM) derivatives are introduced as a new class of red dye for organic light‐emitting devices (OLEDs). They showed more red‐shifted emission than the mono‐substituted DCM derivatives and the emission maxima increased as the electron‐donating ability of the aromatic donor group increased. On the basis of these results, red light‐emitting devices were fabricated with bis‐condensed DCM derivatives as red dopants. For a device of configuration ITO/TPD/Alq3 + DADB (5.2 wt.‐%)/Alq3/Al (where ITO is indium tin oxide, TPD is N,N′‐diphenyl‐N,N′‐bis(3‐methylphenyl)‐1,1′‐biphenyl‐4,4′‐diamine, Alq3 is tris(8‐hydroxyquinoline) aluminum, and DADB is [2,6‐bis[2‐[5‐(dibutylamino)phenyl]vinyl]‐4H‐pyran‐4‐ylidene]propanedinitrile), pure red emission was observed with Commission Internationale de l’Eclairage (CIE 1931) coordinates of (0.658, 0.337) at 25 mA/cm2. The synthesis and properties of high‐quality red dyes for use in organic light‐emitting diodes (OLEDs) are described. The dyes are formed by 1:2 condensation of 2,6‐dimethyl‐4‐dicyanomethylene‐4H‐pyran with para‐donor (mostly NR2) substituted benzaldehydes (see Figure), rather than 1:1 condensation as for conventional DCM. Devices doped with the bis‐condensed DCM derivatives show good luminance and outstanding color purity.