Complete photochromic structural changes in ruthenium(II)-diimine complexes, based on control of the excited states by metalation

The thermal and photochemical reactions of a newly synthesized complex, [Ru^II(TPA)(tpphz)]²⁺ (1; TPA=tris(2-pyridylmethyl)amine, tpphz=tetrapyrido[3,2-a:2,3-c:3′,2′-h: 2′,3′-j] phenazine), and its derivatives have been investigated. Heating a solution of complex 1 (closed form) and its derivatives in MeCN caused the partial dissociation of one pyridylmethyl moiety of the TPA ligand and the resulting vacant site on the Ru^II center was occupied by a molecule of MeCN from the solvent to give a dissociated complex, [Ru^II(η ³-TPA)(tpphz)(MeCN)]²⁺ (1, open form), and its derivatives, respectively, in quantitative yields. The thermal dissociation reactions were investigated on the basis of kinetics analysis, which indicated that the reactions proceeded through a seven-coordinate transition state. Although the backwards reaction was induced by photoirradiation of the MLCT absorption bands, the photoreaction of complex 1 reached a photostationary state between complexes 1 and 1 and, hence, the recovery of complex 1 from complex 1 was 67 %. Upon protonation of complex 1 at the vacant site of the tpphz ligand, the efficiency of the photoinduced recovery of complex 1+H⁺ from complex 1+H⁺ improved to 83 %. In contrast, dinuclear μ-tpphz complexes 2 and 3, which contained the Ru^II(TPA)(tpphz) unit and either a Ru^II(bpy)₂ or Pd^IICl₂ moiety on the other coordination edge of the tpphz ligand, exhibited 100 % photoconversion from their open forms into their closed forms (2→2 and 3→3). These results are the first examples of the complete photochromic structural change of a transition-metal complex, as represented by complete interconversion between its open and closed forms. Scrutinization by performing optical and electrochemical measurements allowed us to propose a rationale for how metal coordination at the vacant site of the tpphz ligand improves the efficiency of photoconversion from the open form into the closed form. It is essential to lower the energy level of the triplet metal-to-ligand charge-transfer excited state (³MLCT) of the closed form relative to that of the triplet metal-centered excited state (³MC) by metal coordination. This energy-level manipulation hinders the transition from the ³MLCT* state into the ³MC* state in the closed form to block the partial photodissociation of the TPA ligand. Ru being served? Photochromic structural changes of Ru^II/TPA (TPA=tris(2- pyridylmethyl)amine) complexes that contain a diimine ligand are reported. Further metal coordination to the diimine ligand allows interconversion between its open/closed forms (see scheme).