Crystals as Electroluminescent Materials

Green Electroluminescent, Low-Molar-Mass Organic Materials 

The materials collated in Tables 5.7 and 5.8 may be used either as discrete, combined electron-transport-and-emission layers or as electroluminescent dopants at low concentration in a non-emissive, electron-transporting matrix. Many of these materials are self-quenching, so that the luminescent efficiency of doped layers of these materials in an inert matrix, even at very low concentrations ( 0.5% c 5 %) can exhibit higher quantum efficiencies than those exhibited by thin layers of the pure material itself. 

The molecular structures 38-43 shown in Table 5.9 are typical of red, low-molar-mass electroluminescent materials. The number of efficient low-molar-mass electroluminescent materials emitting in the red is relatively limited. The europium complexes (38 and 39) exhibit narrow emission spectra in the red part of the visible spectrum. The first europium complexes synthesised, such as compound 38, were not stable enough to be deposited by physical vapour 

The molecular structures (50-54) collated in Table 5.11 are typical of blue electroluminescent metal chelates. The azomethin-zinc compound (50) exhibits bright blue electroluminescence. The aluminium metal chelates (51-53) illustrate attempts to blue-shift the green emission of Alqs (3) by modifying the number and nature of the ligands. However, the unfilled co-odination sites of the aluminium compound (52) render this material intrinsically unstable. The presence of the methyl substituents on compound 52 shields this material from nucleophilic attack. This substantially increases the stability of this compound. 

The presence of the electron-withdrawing nitrogen atom at position 5 of the compound (53) results in a significant hypsochromic shift to the blue (90 nm) compared to the emission maximum of Alqs (3). The metal chelate (54) with the group II magnesium ion emits bright blue light.  

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