Relationships between Light-Emitting Properties and Different Isomers in Polymorphs of Tris(8-hydroxyquinoline) Aluminum(III) (Alq3) Analyzed by Solid-State 27Al NMR and Density Functional Theory (DFT) Calculations

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The structures of tris(8-hydroxyquinoline) aluminum(III) (Alq3) in the different polymorphs, $\alpha$-, $\gamma$-, and $\delta$-Alq3, and in the amorphous state, amo-Alq3, have been analyzed by solid-state 27Al nuclear magnetic resonance (NMR). The local structures of $\alpha$- and amo-Alq3 are found to be similar; both samples are composed of the meridional isomer and are locally disordered. No evidence of the existence of the facial isomer is found even for amo-Alq3. In contrast, the isomeric states of $\gamma$- and $\delta$-Alq3 are facial. The 27Al NMR spectrum of $\delta$-Alq3 is influenced by intermolecular interactions, whereas that of $\gamma$-Alq3 is determined only by a single facial Alq3 molecule, suggesting that intermolecular interactions are negligible for $\gamma$-Alq3. This result is closely related to the experimentally observed good solubility of $\gamma$-Alq3. Density functional theory (DFT) calculations support the identification of the isomeric state and the effect of the intermolecular interactions. A clear correlation between the isomeric state and the fluorescence wavelength is found, indicating that the isomeric state of Alq3 is a crucial factor for the light-emitting properties.
Published by the Japan Society of Applied Physics through the Institute of Pure and Applied Physicsの論文
2005-06-15