Ligand phosphorescence

Rh(bpyL3+ is an example of a complex that exhibits an almost pure n-n phosphorescence and demonstrates one of the limitations of nearly pure ligand localized emissions. At 77K, the complex is highly emissive with a beautifully structured blue ligand phosphorescence (Amax = 446 nmfor the first peak) having at in the tens of msec,(17) but it has no detectable room temperature emission. It is this very long radiative lifetime that causes the absence of room temperature emission. The radiative decay is so slow that it cannot compete effectively against inter- and intramolecular radiationless decay at room temperature. 

Absorption by the chromophore ligand is commonly (6) followed by inter-system crossing (ISC) to the triplet state of the ligand. This state can decay either by emission (ligand phosphorescence) or by... 

Keywords Iridium complexes Cyclometalating ligands Phosphorescence Electrochemistry Multicomponent arrays OLEDs Photoinduced processes... 

A synthesis of [Ir(bipy)3]3 + has been reported by Flynn and Demas.256 I3C and proton NMR spectra have confirmed the tris-complex assignment. [Ir(bipy)3]3+ exhibits a phosphorescence at 22.3 kK with a mean lifetime of 80 / s in MeOH/EtOH glass at 77 K luminescence was assigned as predominantly n-n ligand phosphorescence. 

As the ligand phosphorescence is, in general, completely quenched at room temperature, for these coordination compounds , it is necessary to record the spectra at low temperature (77 K), otherwise the T states are deactivated by non-radiative processes. In this case, trivalent gadolinium complexes are generally used due to the intrinsic spectroscopic characteristics of the Gd + ion (Figure 2). 

Low-temperature studies of solid tris-bipyridine europium, terbium, lanthanum(III), and sodium clathrochelates were performed on UV excitation [390]. The lanthanum (III) and sodium clathrochelates were examined to get more information on the luminescent properties of the ligand. These clathrochelates showed luminescence at temperatures below 100 K upon longwave UV excitation corresponding to ligand-centred absorption. The quantum yield of the ligand phosphorescence is co 0.02. An increase of temperature results in a drastic decrease in the luminescence intensity, and at 100 K it becomes equal to zero (Fig. 69). 

The changes in the physical decay properties of the phen complex which are brought about by ligand substituents are rather different (Table 9), and are inconsistent with predictions based on the spin-orbit coupling mechanism (171). The proximity of the emission bands of the substituted phen complexes to the respective free ligand phosphorescence bands, and the relatively long emission lifetimes of these complexes led Crosby et al. to propose that the emitting levels have contributions from both IL and MLCT excited states (171,172). 

To create pure red phosphorescent emission, a systematic study of the ligand structure and the emission properties was carried out by Tsuboyama et al. (Scheme 3.85) [304], It was found that the red-shift of the phosphorescence is attributable to introduction of more conjugated ligands. 

Chen and coworkers have developed two new phosphorescent blue emitters, which have two identical 2-(2,4-difluorophenyl)pyridine ligands and are derivatives of the Firpic compound, iridium(III) bis(4,6-difluorophenylpyridinato)-3-(trifluoromethyl)-5-(pyridin-2-yl)-1, 2,4-triazolate (Firtaz) and iridium(III) bis(4,6-difluorophenylpyridinato)-5-(pyridin-2-yl)-l//-tetrazolate (FirN4) (Scheme 3.90) [314]. Both these two blue emitters show a 10-nm blue-shift of the emission compared with Firpic. Unfortunately, the efficiency of such blue emitters is inferior to those of Firpic and Fir6. There is no lifetime data reported for such devices. 

The progress with phosphorescent blue emitters suggests that it may be quite possible to achieve high-efficiency blue phosphorescent candidates by carefully designing the proper ligands coupled with appropriate selection of auxiliary ligands. 

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