Amino-anthraquinone chromophores functionalised with 3-picolyl units: structures, luminescence, DFT and their coordination chemistry with cationic Re(I) di-imine complexes

The syntheses of four new ligands based upon 3-picolyl functionalised amino anthraquinone (AQ) chromophores are described via a one-pot reductive amination procedure giving the desired ligands L1-L4 (L1, 1-(3-picolylamino)anthracene-9,10-dione; L2, 1-hydroxy-4-(3-picolylamino)anthracene-9,10-dione; L3, 1,4-bis(3-picolylamino)anthracene-9,10-dione; L4, 1,5-bis(3-picolylamino)anthracene-9,10-dione). Each ligand was characterised in solution via H-1 and C-13{H-1} NMR, with three examples giving single crystal X-ray diffraction data. The structures confirmed the proposed formulations and also revealed the presence of intramolecular H-bonding between the quinone and secondary amine units. The electronic characteristics of the ligands were investigated using a combined experimental/theoretical approach, revealing that in each case absorption in the visible region constitutes significant charge transfer (CT) character, originating from N-(amine)-to-quinone transitions, and is solvent sensitive. Density functional theory (DFT) calculations also suggest that the position of amino-substitution at the AQ core influences the wavelength of the lowest energy feature, by modulation of the HOMO, rather than the LUMO energy. The coordination chemistry of the ligands was probed through reaction with fac-[ Re(CO)(3)(di-imine)(MeCN)](BF4) where di-imine = 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10-phenanthroline (dmp). Combined structural and spectroscopic studies confirmed that the ligands coordinate to Re(I) exclusively via the pyridyl units, however in the case of L3 only monometallic complexes were isolated. The optical properties of the complexes are dominated by AQ-centred (> 425 nm) absorptions superimposed upon (MLCT)-M-1 features, as well as diimine-based intra-ligand (< 350 nm) transitions. The luminescence properties of the complexes generally display dual emission, which was dependent upon the wavelength of sensitisation, with short-lived AQ fluorescence superimposed upon long-lived (MLCT)-M-3 phosphorescence.