Pure organic molecules based thermally activated delayed fluorescence (TADF) emitters have been successfully developed in recent years for their propitious application in highly efficient organic light emitting diodes (OLEDs). In the case of orange red emitters, the non-radiative process is known to be a serious issue due to its lower lying singlet energy level. However, recent studies indicate that there are tremendous efforts put to develop efficient orange red TADF emitters. In addition, the external quantum efficiency (EQE) of heteroaromatic based orange red TADF OLEDs surpassed 30 %. Such heteroaromatic type emitters showed wide emission spectra; therefore, more attention is being paid to develop highly efficient orange red TADF emitters along with good color purity. Herein, the recent progress of orange red TADF emitters based on molecular structures, such as cyanobenzene, heteroaromatic, naphthalimide, and boronbased acceptors, are reviewed. Further, our insight on these acceptors has been provided by their photophysical studies and device performances. Future perspectives of orange red TADF emitters for real practical applications are discussed.[a] K.

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The difference in reaction rates for compounds 1 and 2 is probably due to the variation in the Au-P bond energies. It is also significant that |/( 031 )| appears to be greater for compound 2 than for compound 1, confirming a stronger Au-P interaction in the former. Finally, in ir spectra reported by Coates, et al.f the Au-P stretching frequencies show a higher value for Aum than for Au1 compounds. According to molecular models, steric factors are not likely to play a decisive role.(8) C. H. Langford and . B. Gray, "Ligand Substitution Processes,"

. Can. J. Chem. 55,2728Chem. 55, (1977. The phosphorescence emission and electron spin resonance spectra of methyl mesitoate (1) and mesitonitrile (2) have been studied in order to assess the stericeffect (inhibition of resonance) in the former. The triplet energy of 1 (80.5 kcal mol-') is considerably higher than that of 2 (74.2 kcal mol-') while the zero-field splitting parameters are very similar (1, Dlhc = 0.127 cm-', Elhc = 0.013 cm-' ; 2, Dlhc = 0.127 cm-l, Elhc = 0.006 cm-'). The triplet energy and zero-field splitting parameters of o-, m-, and p-methyl toluate and o-, m-, and p-tolunitrile are also reported.

The photochemistry of the title compound 5 in a number of solvents is described and compared with that of the parent dienone 1 originally studied by Schuster and Patel. The effects on the photochemistry produced by introduction of the additional methyl group are varied and pronounced. In tert-butyl alcohol, both epimeric 6-methyl-6-trichloromethylbicyclo[3.1.0]hexenones (lumiketones) 7 and 8 are formed from 5. The stereochemistry, assigned on the basis of spectral data, hydrogenation experiments, and an X-ray crystal analysis, shows the major product has the CC13 group in the exo orientation. It is concluded that the sole lumiketone formed from the parent dienone 1 also has an exo CC13 group. These results are rationalized mechanistically on the basis of a competition of steric effects, favoring an endo orientation in the intermediate and derived lumiketone of the bulkier dienone C-4 substituent (CCI3 in these systems), and coulombic effects favoring an exo orientation of CC13 and other electron-rich groups. The electronic effects clearly dominate the reaction course in these systems. Other photoproducts include 2-chloro-3,4-dimethyl-4-trichloromethylcyclohex-2-en-l-one (10), a bicyclo[3.2.Ojhexenone (13), 3,4-dimethylphenol ( 14), 3,5-dimethylphenol (15), and hydrogen chloride. Compounds 13 and 15 are formed by secondary photolyses of the initially formed lumiketones. Mechanisms for formation of all products are suggested. Noteworthy is the novel formation of 10, which is proposed to involve trapping of zwitterion 37, derived from 5, by HC1 generated in situ, a new mode of zwitterion trapping. The surprising formation of phenol 14 in good yield in benzene, r-BuOH, and other poor H-donor solvents, in contrast to the lack of phenol formation from 1 in such solvents, was shown in benzene to involve H abstraction not from the solvent but from ground state dienone, presumably from the allylic methyl group. Quantum yield data are presented. The triplet yield for 5 in benzene was found to be close to unity. Sensitization and quenching experiments in several solvents confirm that all products are triplet derived. As in earlier studies, quenching by 1,3-cyclohexadiene is somewhat more efficient than by //•tm-piperylene. The Stern-Volmer quenching slopes in 2-propanol for formation of lumiketone 7 decreased as dienone concentration was increased, while the slopes for formation of phenol 14 simultaneously increased. These results require two different triplets as precursors to these products, at least in 2-propanol. The triplet leading to lumiketones is suggested to be a , * triplet, showing a typical self-quenching effect. The triplet which abstracts hydrogen from the solvent, leading to phenol 14, is suggested to be the lowest , * triplet. The increased quenching slopes in the latter case are tentatively attributed to trapping by ground state dienone of radicals, produced in the reaction, which could act as triplet quenchers. The "two triplet" mechanism required by the results is compared to previously proposed mechanism...