Chiral zinc(II) and cadmium(II) complexes with a dihydrophenanthroline ligand bearing (–)-α-pinene fragments: Synthesis, crystal structures and photophysical properties

Abstract

Mononuclear and dinuclear zinc(II) and cadmium(II) complexes, [ZnLCl2] (1) and [Cd2L2Cl4] (2), with a chiral pentacyclic dihydrophenanthroline ligand L bearing a natural monoterpene (–)-α-pinene fragment were synthesized. DFT calculations of the specific rotation [α] and an NMR study show that the molecules of ligand L, complex 1 and the monomeric form of complex 2 exist in CDCl3 solutions with the P-helix conformation for the dihydrophenanthroline moiety. The dihydrophenanthroline fragments of L and mononuclear complex 1 also exists as a P-helix in the solid state, whereas the crystalline dinuclear complex 2 exists as an asymmetric dimer, [Cd2Cl4LPLM], where LP and LM are the ligand with P-helix and M-helix conformations of the dihydrophenanthroline fragment respectively. Full sets of photophysical parameters for L and complexes 1 and 2 have been determined. Compounds L, 1 and 2 display blue luminescence (λmax=427, 405 and 415nm in the solid state, respectively). The ligand L and complex 2 show moderate-to high quantum yields of 44% and 24%, respectively. The quantum yield for complex 1 is 6%. The emission lifetimes increase on going along the spectrum of luminescence, and in the deep red region, a rise of luminescence with times of 200–500ps is observed. These processes can be explained by the migration of excitons and the capture of excitation by “defective” centers.