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Berlin 2005 – scientific programme

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CPP: Chemische Physik und Polymerphysik

CPP 33: Novel materials I

CPP 33.2: Talk

Wednesday, March 9, 2005, 10:00–10:15, TU C230

Different phases of electroluminescent Alq3: triplet and quartet states studied by EPR — •A. Mirea1, M. N. Grecu2, M. Cölle3, J. Gmeiner1, and M. Schwoerer11Experimentalphysik II, Universität Bayreuth, D-95440 Bayreuth — 2National Institute for Materials Physics, R-76900 Bucharest, Romania — 3Philips Research, NL-5656 AA Eindhoven, The Netherlands

The metal chelate complex tris-(8-hydroxyquinoline)aluminium (Alq3), which is one of the most frequently used low-molecular weight material for organic light-emitting devices (OLED’s), was studied by means of EPR. Different crystalline phases, obtained by drain sublimation (320–380C), were identified. We confirmed the conversion from one phase into the other by different thermal treatment cycles: sublimation, annealing (380–390C), annealing and fast quenching (77 K) and melting (419C), and melting and fast quenching. X-band EPR spectra of the sublimated powders show two types of signals centered at g = 2.00 and g = 4.33 whose intensities are dependent on the phase structure and exhibit different temperature dependencies. The Q-band spectrum of annealed Alq3 consist of overlapping of randomly oriented triplet and quartets. Depending on the thermal cycle their appearence could be separated. Their simulation and analysis based on a spin Hamiltonian appropriate to a high spin system (S = 1, S = 3/2) indicates the existence of triplet and quartet states in annealed Alq3. The D and E zero-field parameters are evaluated. The temperature dependence of the line intensity shows an antiferomagnetic like behaviour (TN = 90 K). Around 50 K a reversible phase transition takes place and the line intensity increases rapidly.

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