Ancillary\nLigand-Promoted Charge Transfer in Bis-indole\nPyridine Ligand-Based Nickel Complexes

There is growing demand for the utilization of first-row\ntransition\nmetal complexes in light-driven processes instead of their conventional\nnoble metal counterparts due to the greater sustainability of first-row\ntransition metal complexes. However, their major drawback is the ultrafast\nlifetime of the electronic excited states of these first-row transition\nmetal complexes, particularly those of d<sup>8</sup> square-planar\nsystems such as Ni(II) complexes, wherein low-lying metal-centered\n(MC) states provide the deactivation pathway. To increase the excited-state\nlifetime and broaden their applications, it is important to develop\nsterically bulky, strong field ligands with low-lying π* orbitals\nand a highly σ-donating nature to augment the energy of MC states.\nThe current strategy relies on synthetically carbene-based ligands,\nwhich are substitutionally cumbersome and act as σ-donors only.\nIn this work, we introduce a bis-indole pyridine (H<sub>2</sub>BIP)\nligand framework, whose dianionic congener (BIP) demonstrates the\nability to form stronger covalent bonds with a Ni(II) center compared\nto neutral donors like carbene and its effect on the complex to produce\na less distorted excited-state structure. When conjoined with ancillary\nligands such as pyridine or lutidine, the BIP ligand orchestrates\nthe formation of low-energy <sup>3</sup>CT states, which decay in\n∼40 ps.