High-Triplet-Energy Dendrons Enhancing the Luminescence

High-Triplet-Energy Dendrons Enhancing the Luminescence -...

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High-Triplet-Energy Dendrons: Enhancing the Luminescence of Deep Blue Phosphorescent Iridium(III) Complexes Shih-Chun Lo, Ruth E. Harding, Christopher P. Shipley, § Stuart G. Stevenson, Paul L. Burn,* ,† and Ifor D. W. Samuel* ,‡ Centre for Organic Photonics & Electronics, The Uni V ersity of Queensland, School of Chemistry & Molecular Biosciences, QLD 4072, Australia, Organic Semiconductor Centre, SUPA, School of Physics & Astronomy, Uni V ersity of St Andrews, North Haugh, Fife, KY16 9SS, U.K., and Chemistry Research Laboratory, Department of Chemistry, Uni V ersity of Oxford, Mans±eld Road, Oxford, OX1 3TA, U.K. Received April 20, 2009; E-mail: p.burn2@uq.edu.au; idws@st-andrews.ac.uk Abstract: Solution-processable blue phosphorescent emitters with high luminescence efFciency are highly desirable for large-area displays and lighting applications. This report shows that when a fac -tris[1-methyl- 5-(4-±uorophenyl)-3- n -propyl-1 H -[1,2,4]triazolyl]iridium(III) complex core is encapsulated by rigid high-triplet- energy dendrons, both the physical and photophysical properties can be optimized. The high-triplet-energy and rigid dendrons were composed of twisted biphenyl dendrons with the twisting arising from the use of tetrasubstituted branching phenyl rings. The blue phosphorescent dendrimer was synthesized using a convergent approach and was found to be solution-processable and to possess a high glass transition temperature of 148 ° C. The dendrimer had an exceptionally high solution photoluminescence quantum yield (PLQY) of 94%, which was more than three times that of the simple parent core complex (27%). The rigid and high-triplet-energy dendrons were also found to control the intermolecular interactions that lead to the quenching of the luminescence in the solid state, and the Flm PLQY was found to be 60% with the emission having Commission Internationale de l’Eclairage coordinates of (0.16, 0.16). The results demonstrate that dendronization of simple chromophores can enhance their properties. Single layer neat dendrimer organic light-emitting diodes (OLEDs) had an external quantum efFciency (EQE) of 0.4% at 100 cd/m 2 . Bilayer devices with an electron transport layer gave improved EQEs of up to 3.9%. Time-resolved luminescence measurements suggest that quenching of triplets by the electron transport layer used in the bilayer OLEDs limits performance. Introduction Organic light-emitting diodes (OLEDs) 1 based on luminescent heavy metal complexes are playing a key role in next generation fat panel displays and solid-state lighting. Phosphorescence From osmium(II), platinum(II), and in particular iridium(III) gives an advantage over fuorescent materials in that both the singlet and triplet excitons that Form in a device can be captured and decay radiatively thus leading to higher eF±ciencies.
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This note was uploaded on 03/27/2011 for the course CHEM 2211L taught by Professor T.a. during the Spring '08 term at University of Georgia Athens.

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High-Triplet-Energy Dendrons Enhancing the Luminescence -...

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