Luminescent properties: cross-luminescence

The luminescent centers require a range of properties that include a large cross-section for the collision excitation to occur, an ionic radius and valency to fit the lattice and be stable under the applied high electronic fields, and the capability to display high luminous efficiency when excited.11 Metal ions suitable for EL devices include Mn, Tb, Sm3+, Tm3+, Pr3+, Eu2+, and Ce3+.12-17 ZnS lattices doped with Mn2+ (yellow-orange emission at ca. 585 nm) have proved to be one of the best phosphors for EL devices. 

A new class of photochromic diarylethenes containing thieno[3,2-b] thiophenes, dithieno[3,2-b 2, 3 -d]thiophene (08CC5203), and dithieno [3,2-fc 2, 3 -d]pyrroles (09CEJ10005) with photoswitchable luminescence properties were synthesized by Suzuki cross-coupling reactions (Scheme 48). 

Figure 2. Schematic representation of some relevant ground and excited-state properties of Ru(bpy)j. MLCT and MLCT are the spin-allowed and spin-forbidden metal-to-ligand charge transfer excited states, responsible for the high intensity absorption band with = 450 nm and the luminescence band with = 615 nm, respectively. The other quantities shown are intersystem crossing efficiency energy (E°°) and lifetime (x) of the MLCT state luminescence quantum yield ( ) quantum yield for ligand detachment (O,). The reduction potentials of couples involving the ground and the MLCT excited states are also indicated.

The excited state lifetimes and luminescence properties of metal complexes are related to the relative positions of the potential energy wells shown in Figure 4.77. On the left we have a lowest excited state which resembles geometrically the ground state (the internuclear distances, r, are similar). The crossing between these states requires a high activation barrier E (in a classical picture) and the excited state lifetime is therefore relatively long. The Stoke s shift between the absorption band (a) and the emission band (e)... 

It allows the existence of the scintillating properties of BaF2 crystals a wide forbidden band is necessary for avoiding reabsorption of the ultraviolet 2pF —> 5pBa emission produced when holes are formed in the 5pBa core levels by X-rays or y-rays (cross-luminescence) [3],... 

Multicolored luminescence is the most attractive property of rare earth-based compounds. Lanthanide ions possess many sharp emission lines that cover the visible and near infrared (NIR) region due fo fhe abundanf fransifions of f-orbital configurations. However, the forbidden f-f fransi-fions induce narrow excitation lines for mosf rare earfh ions. This low absorbency cross-section is the bottleneck in practical application, so host-sensitized emission mode is commonly employed by rare earth phosphors. The vanadate matrix is one of fhe candidafes, which excifes lanthanide ions via charge-transfer energy migration. 

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