Toward Understanding Ligand-Directed Excited-State Electron Flow in Bridged Ruthenium(II) Complexes

Effectively and efficiently capturing and directing light to drive chemical reactions in molecular systems remains a challenge. A series of tpphz (tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine)-bridged bimetallic Ru complexes was prepared using substituted polypyridyl distal ligands with a range of electron-withdrawing (EW) and electron-donating (ED) substituents to investigate the flow of electrons within the complex. The excited state is localized on the distal ligand with the most EW substituent, while charge transfer to the tpphz bridge is promoted by ED substituents. Density functional theory and time-dependent density functional theory were employed to identify the electronic transitions in the visible region of the spectrum and are supported by electrochemical and spectroscopic measurements. Cyclic voltammetry showed a two-electron oxidation of the ruthenium metal centers that correlates with the EW or ED nature of the substituents. Transient absorption spectroscopy revealed a trend in the excited-state lifetimes based on the EW or ED nature of the distal ligands. Computational and experimental results were used to propose an excited-state mechanism involving electron transfer to the tpphz bridge in all complexes except with the most EW substituent. The results provide molecular-level understanding of how excited-state electron transfer can be controlled in bridged transition metal complexes through choice of distal ligands.