Emission Tuning of Heteroleptic Arylborane–Ruthenium(II) Complexes by Ancillary Ligands: Observation of Strickler–Berg-Type Relation

Novel heteroleptic arylborane–ruthenium(II) complexes having a series of ancillary ligands L′ ([Ru(B2bpy)L′2]2+) in CH3CN showed low-energy/intense metal-to-ligand charge transfer (MLCT)-type absorption and intense/long-lived emission compared to the reference complexes. The spectroscopic and photophysical properties of [Ru(B2bpy)L′2]2+ were shown to be manipulated synthetically by the electron-donating ability of the ancillary ligand(s). The intense and long-lived emission observed for [Ru(B2bpy)L′2]2+ in CH3CN at 298 K is responsible for the accelerated radiative and decelerated nonradiative decay processes, which are controllable through the electronic structures of the ancillary ligand(s). On the basis of the present systematic study, furthermore, we succeeded in demonstrating the Strickler–Berg-type relation between the molar absorption coefficients of the MLCT bands and the radiative rate constants of the complexes.