Blue Luminescent Organoaluminum Compounds: Al(CH₃)₂(dpa), Al₂(CH₃)₅(dpa)₂, Al₄(O)₂(CH₃)₆(dpa)₂, and Al(pfap)₃, dpa = Deprotonated Di-2-pyridylamine, pfap = Deprotonated 2-Pentafluoroanilinopyridine

Abstract

The reactions of Al(CH3)3 with di-2-pyridylamine (dpaH) yielded a mononuclear zwitterion complex, Al(CH3)2(dpa) (1), a dinuclear complex, Al2(CH3)5(dpa) (2), and a tetranuclear complex, Al4(O)2(CH3)6(dpa)2 (3), depending on the stoichiometry and conditions of the reactions. The crystal structures of all three compounds were determined by single-crystal X-ray diffraction analyses. All three compounds emit an intense blue color in solution and the solid state at λmax = 475, 445, and 450 nm, respectively, when irradiated by an UV light. Molecular orbital calculations using ab initio methods (Gaussian 94) on dpa, dpaH, and compound 1 were performed which revealed that the aluminum ion in compounds 1−3 plays a key role in stabilizing the complexes and promoting the blue luminescence. The reaction of Al(CH3)3 with 2-pentafluoroanilinopyridine (pfapH) resulted in the formation of a mononuclear octahedral complex, Al(pfap)3 (4). Variable-temperature and COSY 1H NMR established that compound 4 is highly fluxional in solution due to a rapid interconversion of mer and fac isomers, with the mer isomer dominating at low temperature. The structure of the fac isomer of 4 was determined by X-ray diffraction. Compound 4 emits at λmax = 409 nm when irradiated by an UV light.