Luminescent Zn(ii) and Cd(ii) complexes with chiral 2,2'-bipyridine ligands bearing natural monoterpene groups: synthesis, speciation in solution and photophysics.

Mononuclear zinc(ii) and cadmium(ii) complexes, ZnLCl2(1), CdLCl2(2), (ZnLCl2)-Cl-1 center dot 2H(2)O (3), and (CdLCl2)-Cl-1 center dot 2H(2)O (4), with chiral ligands containing a 2,2 '-bipyridine moiety and natural terpene (+)-limonene (L) or (+)-3-carene (L-1) moieties were synthesized. In these complexes theLandL(1)ligands are shown to coordinate Zn(2+)and Cd(2+)ions through the 2,2 '-bipyridine moiety. The acetamide group of the ligands interacts with M(2+)ions by forming NMIDLINE HORIZONTAL ELLIPSISM(2+)and C& xe001;OMIDLINE HORIZONTAL ELLIPSISM(2+)contacts and N-HMIDLINE HORIZONTAL ELLIPSISCl hydrogen bonds with coordinated Cl(-)ions. In solutions the complexes have several conformers differing by the degree of the turn of the acetamide moiety relative to the ligand core and the type of its interaction with the coordination core. The ligands and complexes exhibit luminescence with the quantum yield increasing in the order: ligand < cadmium(ii) complex < zinc(ii) complex. The complexes3and4demostrate excitation wavelength independent single-channel fluorescence. As opposed to3and4, the complexes1and2demonstrate excitation wavelength dependent emission with nanosecond and microsecond lifetimes of the excited states. According to our TD-DFT calculations, an interplay of ligand centered and halide to ligand transitions facilitates two deactivation channels in1and2: S-1-S(0)and T-1-S-0.