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. Schwoerer1 — 1Experimentalphysik 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 (320–380∘C), were identified. We confirmed the conversion from one phase into the other by different thermal treatment cycles: sublimation, annealing (380–390∘C), annealing and fast quenching (77 K) and melting (419∘C), 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.