Luminescent properties: light yield

Polystyrenes have also been used to support chromophores useful in organic light-emitting diodes (OLEDs). Week and coworkers have attached tris(2-phenylpyridine) iridium complexes to aminomethylated polystyrene using a Schiff base reaction, 4 [21]. There was no major diminution of the desirable luminescence properties of the iridium complexes (high emission quantum yields of 0.23 and lifetimes of about a microsecond). Similar results have been reported for aluminum and boron 8-hydroxy quinoline complexes tethered to polystyrene using Schiff base condensation [22]. 

Fluorescent semiconductor nanocrystals (CdSe, CdTe, PbSe, and others), otherwise included in the term quantum dots (QDs), have attracted much attention in various research fields for more than 20 yeais owing to their chemical and physical properties, which differ markedly fi om those of the bulk solid (quantum size effect). Quantum dots have size-tuneable light emission (usually with a narrow emission band), bright luminescence (high quantum yield), long stability (photobleaching resistance), and broad absorption spectra for simultaneous excitation of multiple fluorescence colors compared with classical organic fluorescent dyes. 

Many inorganic nanocrystals, including CdSe/ZnS core/shell quantum dots and nanocrystals doped with Ce as luminescent centers (e.g., YaOarCe, LaP04 Ce), were synthesized as described above. With particles sizes below 5 nm, narrow size distribution, and high crystallinity, they demonstrated high light yields, a desirable property for efficient scintillators. 

The luminescence properties of the composites, however, can be improved by photocorrosion. In photocorrosion, quanmm dots are illuminated with light above their band gap, typically near ultraviolet light. Oxidation processes are triggered by illumination (19.13) [44—47]. Since surface defects are photooxidized preferentially, photoactivation is a convenient technique to remove such defects and improve the photoluminescence quantum yield. 

Lanthanide p-diketonates are amongst the best smdied rare-earth luminescent complexes [58]. They are brightly luminescent and volatile so that incorporation into various electroluminescent materials is simple. Moreover their photophysical properties are easily tuned by a judicious choice of ancillary ligands. Indeed, conventional synthesis usually yields bis(hydrated) lanthanide tris(P-diketonates), but the two solvent molecules can be substituted by either a fourth diketonate anion or a donor ligand with adequate functionalisation as to provide convenient light harvesting and subsequent energy transfer onto the metal ion. It is noteworthy that not only visible but also near-infrared luminescence [59,60] is efficiently sensitised in lanthanide p-diketonates. In the case of Eu , some ternary complexes have quantum yields up to 85% [8] and the main asset of their luminescent properties is an emission essentially concentrated in the hypersensitive Dq transition... 

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