Induction of J Aggregate-like Optical Transitions in Dihydroxyquinone by Coordination with Al(III)

The metal-complex formation of 1,4-dihydroxyanthraquinone (quinizarin, QNZ) and 6,11-dihydroxy-5,12-naphthacenedione (DHN) with Al(III) ions is investigated by stationary and time-resolved emission spectroscopy combined with quantum chemical calculations of optical properties. UV-vis and fluorescence spectra revealed small red-shifts of 200 and 60 meV for the QNZ and DHN metal complexes, respectively. The fluorescence quantum yield increases from 0.08 to 0.23 for QNZ, while for DHN it changes from 0.24 to 0.79 upon complexation, suggesting the presence of J-aggregate-like exciton coupling within the coordination structure. The average fluorescence lifetime of QNZ varies from 0.65 ns of the free ligand to 2.77 ns, and in the case of DHN it goes from 1.57 to 2.61 ns after Al(III) complexation. These results are consistent with formation of a more rigid molecular structure which effectively decreases the nonradiative rate constant. Confocal fluorescence microscopy images of Al(III) complexes adsorbed into the μmZeolite L structure gave similar red-shifted J type emission. Density functional theory, at the B3LYP/def2-TZVP level of theory, and the analysis of the electronic transition dipole moment, calculated with TDDFT at the CAM-B3LYP/def2-TZVP level, supports a near head-to-tail chromophore arrangement containing two metal centers coordinated with two chromophores. The Al(III)₂DHN₂ complex exhibits the stronger transition dipole coupling and a more pronounced J-type character when compared with Al(III)₂QNZ₂ complex. The radiative rate constant of Al(III)₂DHN₂ is twice that of the single DHN chromophore.