Chemistry Reference
In-Depth Information
SCHEME 5.29
Diphenylcyclopentaphenanthrenone
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and polytriphenylene dendrimer
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.
pyrene would follow that demand, while the perylene tetracarboxydiimides usually
exhibit red emission. Stable blue light-emitting polymers for commercial use are still
under development [96].
Therefore, new conjugated dendrons were considered which are emissive them-
selves, but still do not undergo quenching effects following a new design principle.
Accordingly, blue light emission should be caused by the presence of electronically
decoupled polycyclic aromatic hydrocarbon (PAH) units, which should adopt ste-
rically defined positions and disallow intra-dendrimer chromophore-chromophore
interactions, thus forming amorphous films. Finally the amount of “useless” sub-
stituents and coupling units should be kept at a minimum. This led to the design of
diphenylcyclopentaphenanthrenone
, which is the precursor of a fluorescent
triphenylene. With this building block the polytriphenylene dendrimer
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up to third
generation became available (Scheme 5.29) [97]. Most importantly, the twisted
triphenylene units not only function as chromophores but also effectively prevent
the inter- or intramolecular fluorescence quenching, as proved by increased PL
quantum yield with increasing generation. These dendrimers exhibited stable and
pure-blue emission in both PL and EL spectra. While the triphenylene itself has a low
PL quantum yield (
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4%) the quantum yield increased considerably for the emission
of higher generation up to 35% for G3. These dendrimers could provide an avenue for
dendritic emitters with the optimized EL efficiency/color purity trade-offs needed for
pure blue light emission.
The fluorescent polymers and dendrimers are limited to 25% internal quantum
yield due to the singlet:triplet ratio [98]. Therefore, the phosphorescent dopants
recently raised a lot of interest because of their quantum yield up to unity [98-100]. In
this way, iridium(III) complexes are particularly promising phosphorescent materials
because they can emit light from both singlet and triplet excitons with extremely high
