Blue-emitting materials

In the effort to make pure blue-emitting materials Shim and coworkers [146] synthesized a series of PPV-based copolymers containing carbazole (polymers 95 and 96) and fluorene (polymers 97 and 98) units via Wittig polycondensation. The use of trimethylsilyl substituents, instead of alkoxy groups, eliminates the electron donor influence of the latter and leads to chain distortion that bathochromically shifts the emission (Amax = 480 nm for 95 and 495 nm for 97). In addition, a very high PLQY was found for these polymers in the solid state (64 and 81%, respectively). Single-layer PLEDs fabricated with 95 and 97 (ITO/polymer/Al) showed EL efficiencies of 13 and 32 times higher than MEH-PPV, respectively (see also Ref. [147] for synthesis and PLED studies of polymers 99 and 100) (Chart 2.20). 

H. Ogawa, K. Ohnishi, and Y. Shirota, Tri(p-terphenyl-4-yl)amine as a novel blue-emitting material for organic electroluminescent devices, Synth. Met., 91 243-245 (1997). 

These generally blue-emitting materials were studied extensively by Hosokawa and coworkers.73 Among them, 4,4 -bis(2,2 -diphenylvinyl)-l,l -biphenyl (DPVBi) (see Figs. 1.1 and 1.3) has proven to be a particularly promising material for blue OLEDs. The degradation of OLEDs based on this material is apparently due to its crystallization, which results from its relatively low Tg 64° C. Indeed, the related spiro-DPVBi, with Tg 100° C, yields considerably more stable devices.70... 

There are several properties of luminescent materials that need to be controlled in order to make efficient LEDs and lasers. The first is the colour of the emission, which is primarily determined by the energy difference (band-gap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), but in the solid state is also affected by interactions between the molecules or polymer chains which can lead to red-shifts in the emission due to formation of aggregates. This can be controlled by manipulating both the polymer backbone and the substituents. Polyphenylenes are intrinsically blue-emitting materials with large HOMO-LUMO gaps, but as we will show, by copolymerisation with other materials it is possible to tune the emission colour across the entire visible spectrum. Even without the incorporation of comonomers it is possible to tune the... 

Besides, replacing the carbon atom at C-9 position of fluorenes with other atoms (such as silicon, germanium, and phosphorus) may produce excellent blue emitting materials, which originates from the particular interactions between the heteroatom and the 7i-conjugated backbone. 

Lo, M.Y. and SeUinger, A. (2006) Highly fluorescent blue-emitting materials from the Heck reaction of triphenylvinylsUane with conjugated dibromoaromatics. Synlett, 3009-12. 

Polyfluorene-based blue-emitting materials. Macromol. Chem. Phys., 210, 1580-1590. 

Phenylene-based polymers have been extensively investigated as blue-emitting materials for LEDs. Here, three areas of... 

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