J. Am. Chem. Soc., ASAP Article 10.1021/ja0565774 S0002-7863(05)06577-7
Web Release Date: March 9, 2006
CP/MAS 13C NMR Characterization of the Isomeric States and Intermolecular Packing in Tris(8-hydroxyquinoline) Aluminum(III) (Alq3)
Hironori Kaji,* Yasunari Kusaka, Goro Onoyama, and Fumitaka Horii
Contribution from the Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan, and PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
The isomeric states and intermolecular packing of tris(8-hydroxyquinoline) aluminum(III) (Alq3) in the -, -, and -crystalline forms and in the amorphous state, which are important for understanding the light-emitting and electron-transport properties, have been analyzed by CP/MAS 13C NMR. This simple NMR experiment shows that the isomeric state of - and amorphous Alq3 is meridional, whereas that of - and -Alq3 is facial. In the amorphous Alq3, the inclusion of facial isomers has been under debate. Our experiments show that meridional isomers are dominant in the amorphous Alq3, although the existence of facial isomers cannot be completely denied. The local structure of amorphous Alq3 is similar to that of -Alq3 and is significantly different from those of - and -Alq3. Among these Alq3 samples, the effect of intermolecular interaction is not found only for -Alq3. This finding can explain the good solvent solubility of -Alq3, compared with the other crystalline forms. It is also shown that the structures are locally disordered not only for amorphous Alq3 but also for -Alq3, although clear X-ray diffraction peaks are observed for -Alq3. In contrast, the local structures of - and -Alq3 are well defined. A clear relation is found between the spectral patterns of CP/MAS 13C NMR and the fluorescence wavelengths; the samples, which consist of facial isomers, show blue-shifted fluorescence compared with those of meridionals.