Synthesis, structure, and photophysical properties of luminescent zinc and cadmium complexes containing 2-pyridyl-substituted pyrrolo[2,3-b]quinoxaline

Abstract

New [N,N,N]-tridentate ligand containing 2-aminomethylpyridyl group at the enamino-pyrrolo[2,3-b]quinoxalin-2-one system (2) was synthesized and characterized. The reaction of ligand with zinc acetate and zinc acetylacetonate resulted in five-coordinated zinc complexes: coordination polymer (E)-2-Zn(µ2-η1:η1-OAc) and (E)-2-ZnPyacac. Cadmium acetate and also acetylacetonate with the ligand both formed six-coordinated complexes with binding mode 2:1. The new complexes were characterized by NMR, elemental analysis, UV–Vis, and fluorescence spectroscopy. X-ray structural analysis of the single crystals of [(E)-2]2Cd, (E)-2-Zn(µ2-η1:η1-OAc), and (E)-2-ZnPyacac showed the orientation of nitrogen and oxygen donor groups, the distortion of the fluorophore conformations, CH-π and π-π non-covalent interactions and formation of hydrogen bonds were responsible for the fluorescence enhancement in the solid state. The effect of an alkyl linker (CH2)n, n=0, 1 or 2 length, between the enamine-pyrrolo[2,3-b]quinoxaline fluorophore and an ionophore pyridyl group, on the coordination geometry of fluorescent zinc and cadmium complexes and on the fluorescence quantum yields was discussed. The increase of zinc coordination numbers from 4 to 6 is accompanied by a substantial decrease of the fluorescence quantum yield from 0.109 to 0.006 for the complexes in acetonitrile as well as the decrease of dipole moments in ground state from 9.87D to 6.04D. Comparing the local potential energy density to charge density ratio, G(r)/ρ(r), calculated using a density functional theory (DFT), indicated that the zinc complexes were more stable when the coordination number of the zinc ion is lower than 6.