Substituents make a difference: 6,6″-modified terpyridine complexes with helix configuration and enhanced emission

A series of complexes L2 2 -M (L2: 6,6″-bis(4-methoxyphenyl)-4′-phenyl-2,2′:6′,2″-terpyridine, M: Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) were synthesized by coordinating p -methoxyphenyl 6,6″-substituted terpyridine ligand with first-row transition metal ions and characterized by NMR, ESI-MS, and X-ray single crystal diffraction techniques. Single-crystal structures demonstrated that the steric hindrance of p -methoxyphenyl substituents endowed complexes L2 2 -M with obvious longer coordination bond lengths and larger bond angles and dihedral angles compared with unmodified L1 2 -M (L1: 4′-phenyl-2,2′:6′,2″-terpyridine). The chiral helix geometry was observed for L2 2 -M , in which 2,2′:6′,2″-terpyridine moiety dramatically twisted to a spiral form in comparison to the nearly coplanar structure of the parent L1 2 -M , resulting in plentiful intramolecular and intermolecular π-π interactions. Also, the appealing racemic (P and M) double helix packed structure for 6,6″-modified bisterpyridine complex L2 2 -Cu was formed in the crystal. The consequent appealing charge transfer (CT) emission for L2 2 -Zn in the solution and solid were investigated via UV-vis and fluorescence spectroscopy techniques and time-dependent density functional theory (TD-DFT) calculations. This work afforded a new method to achieve intriguing chiral geometry and CT optical properties via the subtle design and modification of terpyridine ligands. The novel terpyridine complexes show chiral helical geometry and intriguing CT optical properties via subtle 6,6″ modification, distinctly different from the parent bis(2,2′:6′,2″-terpyridine) complexes.