Immobilization of coordinatively unsaturated metal centers (UMCs) into porous frameworks is a very attractive area of research because the porous framework can induce regioselectivity or shape/size selectivity by creating an appropriate environment around the metal center in the restricted available space. Furthermore, porous frameworks can stabilize the catalytic center by efficiently isolating the sites in a manner similar to the peptide architecture of enzymes in biological systems. Immobilization of UMCs into porous hosts has been attempted with zeolites, polymeric matrices, and clays through ion exchange, impregnation, and isomorphous substitution. [1,2] However, in these cases, the isolation and uniformity of the UMCs is not sufficient and the environment around the UMCs is not clear. Completely isolated and uniform catalytic centers can be realized if the UMCs are directly incorporated into channel walls of crystalline microporous coordination polymers constructed from transition-metal ions and organic bridging ligands. [3,4] Such a situation would lead to novel highly selective catalysts and sensors.In spite of the importance, the incorporation of UMCs into porous frameworks is still rare because of difficulties associated with the formation of UMCs in channel walls by a self-assembly process.[5-7] Therefore, we focused on the establishment of a rational synthetic method to immobilize various UMCs in the pore walls of microporous coordination polymers. Ligands based on metallo Schiff bases are known to be useful for the generation of supramolecular systems for homogeneous catalyses and so might be suitable for incorporation into infinite porous frameworks for applications in heterogeneous catalyses and sensors. [8,9] Here, we report a novel three-dimensional (3D) microporous coordination framework obtained from Schiff base type ligands prepared from the reaction of N,N'-phenylenebis (salicylideneimine)

The isomeric states and intermolecular packing of tris(8-hydroxyquinoline) aluminum(III) (Alq(3)) in the alpha-, gamma-, and delta-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 (13)C NMR. This simple NMR experiment shows that the isomeric state of alpha- and amorphous Alq(3) is meridional, whereas that of gamma- and delta-Alq(3) is facial. In the amorphous Alq(3), the inclusion of facial isomers has been under debate. Our experiments show that meridional isomers are dominant in the amorphous Alq(3), although the existence of facial isomers cannot be completely denied. The local structure of amorphous Alq(3) is similar to that of alpha-Alq(3) and is significantly different from those of gamma- and delta-Alq(3). Among these Alq(3) samples, the effect of intermolecular interaction is not found only for gamma-Alq(3). This finding can explain the good solvent solubility of gamma-Alq(3), compared with the other crystalline forms. It is also shown that the structures are locally disordered not only for amorphous Alq(3) but also for alpha-Alq(3), although clear X-ray diffraction peaks are observed for alpha-Alq(3). In contrast, the local structures of gamma- and delta-Alq(3) are well defined. A clear relation is found between the spectral patterns of CP/MAS (13)C NMR and the fluorescence wavelengths; the samples, which consist of facial isomers, show blue-shifted fluorescence compared with those of meridionals.

The structures of tris(8-hydroxyquinoline) aluminum(III) (Alq3) in the different polymorphs, α-, γ-, and δ-Alq3, and in the amorphous state, amo-Alq3, have been analyzed by solid-state 27Al nuclear magnetic resonance (NMR). The local structures of α- 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 γ- and δ-Alq3 are facial. The 27Al NMR spectrum of δ-Alq3 is influenced by intermolecular interactions, whereas that of γ-Alq3 is determined only by a single facial Alq3 molecule, suggesting that intermolecular interactions are negligible for γ-Alq3. This result is closely related to the experimentally observed good solubility of γ-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.