Red emitters

Eluorescent lamps for showing plants use a blue-white phosphor blended with a deep red-emitting phosphor. This more closely corresponds to the action spectmm for plant growth because there is Htfle green in the spectmm, African violets, for example, have leaves which appear more purple in color. The deep red emitter which is commonly used is magnesium fluorogermanate activated by Mn. ... 

P-22 ZnS Ag+ + ZnCdS2 Ag+ + Y202S Eu3+ Mix of phosphors, each phosphor prepared and applied separately. ZnS, (Zno.88-Ago.i2)S2> MgCl2 LiS04 ZnS, (Zn0.715.Ag0.285)S2, CdS MgCl2.NaCl Y203 Euo.65 S Color television (blue, green, and red emitters)... 

Q. Hou, Q. Zhou, Y. Zhang, W. Yang, R. Yang, and Y. Cao, Synthesis and electroluminescent properties of high-efficiency saturated red emitter based on copolymers from fluorene and 4,7-di(4-hexylthien- 2-yl)-2,l,3-benzothiadiazole, Macromolecules, 37 6299-6305, 2004. 

While the DCJTB series replaced the active methyl group with tert-butyl or iso-propyl substituents to avoid bis-Knovenagel condensation reactions during the synthesis of DCM or DCJ series, Zhang et al. came up with the idea of using substituted cyclohexane rings to block the reactive site of the pyran ring. They then synthesized a series of 4H-benzopyran-based red emitters (158-160) as shown in Scheme 3.49 [210]. 

The merit of these chromene dopants is their relatively long emission wavelength peaks compared to DCM or DCJTB materials due to the more conjugated chromene moiety, and this contributes to the more saturated red emission. In fact the EL spectra of OLED devices of ITO/TPD/Alq3 chromene-dopants/Alq3/Mg Ag exhibited satisfactory red emission color, especially for Chromene-1 and Chromene-2 dopants. However, these chromene-based red emitters showed lower fluorescent quantum yield (18%, 15%, and 54% for Chro-... 

SCHEME 3.49 Chemical structures of 4H-benzopyran-based red emitters. 

SCHEME 3.50 Chemical structures of chromene-based red emitters. 

SCHEME 3.51 Chemical structures of isophorone-based red emitters. 

Knoevenagel condensation of 1 2.1 stoichiometric ratio of the pyran and aldehyde generates a pure bis-condensation product. Due to the extended ir-conjugated system, these bis-condensed red emitters are about 40-50 nm red-shifted compared with their respective mono-substituted DCM analogues. 

Red emission chomophores having a long wavelength emission band are usually polar, such as the above DCM series. The push-pull red emitters are normally prone to aggregation in the solid state owing to dipole dipole interactions or through intermolecular tt-tt stacking, especially when the molecules are flat as is the case for DCM. As a consequence, the push-pull red... 

SCHEME 3.53 Chemical structures of push-pull red emitters. 

SCHEME 3.55 Chemical structures of red emitters of metal chelates. 

White emission can also be achieved by directly combining a blue emitter and an orange-red emitter as codopants. The combination of blue and orange-red emission generates white emission. 

A leading material reported as a red emitter is the fluorescent dye material typified by 4-(dicyanomethylene)-2-tert-butyl-6-(l,l,7,7-tetramethyljulolidyl-9-enyl)-4f/-pyran (DCJTB) [359]. This material is typically doped into an electron transporting host matrix such as Alq3 and delivers good chromaticity with CIE (0.646, 0.351) and a reasonable EL efficiency up to 4.4 cd/A and a power efficiency of 2.09 lm/W at 20 mA/cm2 and 6.8 V. The operational stability of the DCJTB-doped EL device has a projected half-life of over 33,800 h driven at an initial brightness of 100 cd/m2 (Scheme 3.95) [360]. 

The highest efficiency red emitters belong to the class of phosphorescent materials and are based on iridium organometallic complexes. The best performance achieved in guest-host systems, for example using a carbazole host and an Ir emitter (Ir(piq-F)2acac, has a maximum power efficiency and luminescent efficiency up to 4.73 lm/W and 13.7 cd/A, respectively. An EQE of 6.7% at 20 mA/cm2 with CIE (0.61, 0.36) has been demonstrated (Scheme 3.96) [361]. 

Other work by Tsuboyama et al. reported a very highly efficient red PHOLED with power efficiency of 8.0 lm/W at 100 cd/m2 using Ir(piq)3 as a dopant [362], Most exciting, however, is the relatively recent demonstration of exceptional lifetimes for these materials in OLED devices where work from UDC has claimed a 14 cd/A red CIE (0.65, 0.35) with a lifetime of 25,000 h at 500 nit. Such performance promises much for phosphorescent red emitters in commercial devices and even higher efficiencies have been realized in systems that compromise the chromaticity toward the deep red with CIE (0.67, 0.33) and lifetimes >100,000 h at 500 cd/m2 [363],... 

T. Liu, C. Iou, S. Wen, and C.H. Chen, 4-(Dicyanomethylene)-2- -butyl-6-( 1,1,7,7-tetramethyl-julolidyl-9enyl)-4//-pyran doped red emitters in organic light-emitting devices, Thin Solid Films, 441 223-227 (2003). 

Fig. 3b). The conversion of red emitters into green emitters with time was explained as an oxidation reaction confirmed by the addition of extra reductant, which leads to the transformation of green emitters into red ones [48]. 

Fig. 4 Steady-state excitation and emission spectra for live distinct ssDNA encapsulated Ag clusters, (a) Blue emitters created in 5 -CCCTTTAACCCC-3, (b) green emitters created in 5 -CCCTCTTAACCC-3, (c) yellow emitters created in 5 -CCCTTAATCCCC-3, (d) red emitters created in 5 -CCTCCTTCCTCC-3, and (e) near-IR emitters created in 5 -CCCTAACTCCCC-3. (f) Pictures of emissive solutions in (a)-(d) [46]...

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