Förster and Dexter energy-transfer processes in fluorescent BAlq thin films doped with phosphorescent Ir(ppy)3 molecules

We studied the photoluminescence proprieties of bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum (BAlq) thin films doped with fac tris(2-phenylpyridine) iridium [Ir(ppy)3] over a wide temperature range from 8to295K. The efficient Förster and Dexter energy transfers between the fluorescent BAlq molecule and the phosphorescent Ir(ppy)3 one enabled us to observe strong yellow phosphorescence from BAlq, where Ir(ppy)3 plays an important role as a phosphorescent sensitizer for BAlq. It was found that BAlq acts not only as a donor but also as an acceptor in the energy transfer between BAlq and Ir(ppy)3. The triplet energy of BAlq was estimated to be 2.18eV from the highest energy peak of the phosphorescence spectra, and the phosphorescence lifetime was 16ms at 8K. Furthermore, it was revealed from the temperature dependence of the photoluminescence intensity that the triplet-excited states in BAlq are in thermal equilibrium with those in Ir(ppy)3 at temperatures above ∼200K in the BAlq:Ir(ppy)3 system.