Luminescent properties: excitonic emission

Ley and Schanze have also examined the luminescence properties of the polymers Pq, Pio> P25> and P50 in solution at 298 K, and in a 2-methyltetrahydro-furan solvent glass at 77 K. These spectroscopic studies reveal that fluorescence from the 71,71" exciton state is observed at Amax=443 nm, 2.80 eV in the polymers P0-P50 at 298 and 77 K, but the intensity and lifetime of the fluorescence is quenched as the mole fraction of Re in the polymers is increased. This indicates that the metal chromophore quenches the 71,71" state. The quenching is inefficient even when the mole fraction is large, suggesting that interchain diffusion of the 71,71" exciton is slow compared to its lifetime [70]. Phosphorescence from the 71,71" state of the conjugated polymer backbone is observed at > max=b43 nm, 1.93 eV in P10-P50 at 77 K, and emission at Amax=690 nm, 1.8 eV is assigned to the d7i(Re) 7i oiy MLCT transition. 

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an cxciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Titus, for example, an electron trap in the ETL will capture and hold an election until a hole is injected nearby from the HTL. In this case, the dopant is the recombination mmo.-... 

The electronic properties of RGS have been under investigation since seventies [3-7] and now the overall picture of creation and trapping of electronic excitations is basically complete. Because of strong interaction with phonons the excitons and holes in RGS are self-trapped, and a wide range of electronic excitations are created in samples free excitons (FE), atomic-like (A-STE) and molecular-like self-trapped excitons (M-STE), molecular-like self-trapped holes (STH) and electrons trapped at lattice imperfections. The coexistence of free and trapped excitations and, as a result, the presence of a wide range of luminescence bands in the emission spectra enable one to reveal the energy relaxation channels and to detect the elementary steps in lattice rearrangement. 

In the recent years doped semiconductor nanocrystals are widely investigated including their intrinsic properties and the properties modified by inqjurities [1, 2], Quantum confinement effects are well known to modify the electronic properties of nanocrystals when their diameter is comparable to or smaller than the diameter of the bulk exciton [1-3], Moreover, early results onZnS.Mn nanocrystals [4] show that the position of the Mn emission band is slightly shifted from that of the bulk material. The authors of Ref. [4] also claimed that the Ti level lifetime of manganese ions in nanocrystals reduced by five orders of magnitude as compared to the bulk material. However, recent reports do not confirm this statement [5], Fvuther studies of luminescence in doped nanocrystals are necessary. 

The general picture which emerges from the optical properties of the alkali azides is that they are wide-band-gap ( 8.5 eV) materials which exhibit two regions of exciton absorption spectra. Absorption characteristic of an intraanionic excitation of the azide molecular ion appears at the same energy ( 5.6 eV) in all the alkali azides. Luminescent emission characteristic of the azide ion has been observed. 

---