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

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 (-)-alpha-pinene fragment were synthesized. DFT calculations of the specific rotation [alpha] 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, [(Cd2Cl4LLM)-L-P], where L-P and L-M 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 (lambda(max) = 427, 405 and 415 nm 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 200500 ps is observed. These processes can be explained by the migration of excitons and the capture of excitation by defective centers. (C) 2016 Elsevier Ltd. All rights reserved.