The Important Role of Coordination Geometry on Photophysical Properties of Blue-Green Emitting Ruthenium(II) Diisocyano Complexes Bearing 2-Benzoxazol-2-ylphenolate

A series of blue-green emitting Ru^II diisocyano complexes containing 2-benzoxazol-2-ylphenolate (PBO) have been prepared. The complexes were isolated under varied reaction conditions in two isomeric forms, i.e., trans,trans,trans- (1) and cis,trans,cis- (2), with varied ligand coordination geometry above the Ru^II center. The photoluminescence of the isomeric complexes has been compared and tuned by the systematic variation of the electronic properties of the isocyanides. The cis,trans,cis- isomers exhibit structureless emission in the blue-green region (471-517 nm) upon excitation at λ_ex > 400 nm in dichloromethane solution at room temperature. Both isomeric forms show similarly structured greenish emission at 499-523 nm on excitation at λ_ex > 355 nm in a methanol/ethanol (4:1) glassy medium at 77 K. On careful comparison with the corresponding absorption and electrochemical data, it is suggested that the solution emission of the cis,trans,cis- isomers (2) at room temperature is originated from the metal-to-ligand charge transfer (MLCT), while a ligand-centered (LC) parentage is assigned for the emission in a glassy state for both isomeric forms. In line with the above experimental results, DFT calculation demonstrates the change in the nature and relative energy of the HOMOs and LUMOs with respect to the varied ligand coordination geometry and π-accepting ability of the isocyanides.