Mono- and di-nuclear photoluminescent complexes of zinc(ii), cadmium(ii) and mercury(ii) of a chiral diimine ligand

Abstract

Reaction of α-pyridoin and N-phenyl-o-phenylenediamine affords 2-(2-(phenylamino)phenylimino)-1,2-di(pyridin-2-yl)ethanol (L) which undergoes cyclization to a chiral diimine, 2-methoxy-1-phenyl-2,3-di(pyridin-2-yl)-1,2-dihydroquinoxaline, L^OMeOMe (conjugated 14πe system) in the presence of zinc(II), cadmium(II) and mercury(II) ions affording [Zn(L^OMe)Cl₂] (1), [Cd₂(L^OMe)₂Cl₄] (2) and [Hg₂(L^OMe)₂Cl₄] (3) complexes. Ligand L and complexes 1–3 are substantiated by elemental analyses, mass, IR, ¹H NMR and UV-vis spectra including the single-crystal X-ray structures of 1 and 3. The possibility of the atropisomerism of L is restricted in cyclic L^OMeOMe. L and complexes 1–3 are fluorescent in fluid solutions at 298 K (CH₂Cl₂: 1, λ_ex = 470 nm, λ_em = 627 nm, Φ = 0.014, τ_avg = 2.5 ns; 2, λ_ex = 430 nm, λ_em = 599 nm, Φ = 0.08, τ_avg = 7.6 ns; 3, λ_ex = 415 nm, λ_em = 600 nm, Φ = 0.021, τ_avg = 2.8 ns). Time-resolved emission spectra (TRES) established that the two-component lifetimes of 1–3 are due to the existence of two conformers. Density functional theory (DFT) and time dependent (TD) DFT calculations authenticated that 1–3 complexes are fluorescent due to intra-ligand charge transfer (ILCT) to the π_diimines* orbital.