title: Tris(2,2'-bipyridine)ruthenium Derivatives with Multiple Viologen Acceptors: Quadratic Dependence of Photocatalytic H2 Evolution Rate on the Local Concentration of the Acceptor Site.
abstract: Three Ru(bpy)3 (2+) derivatives tethered to multiple viologen acceptors, [Ru(bpy)2 (4,4'-MV2)](6+) , [Ru(bpy)2 (4,4'-MV4)](10+) , and [Ru(bpy)(4,4'-MV4)2 ](18+) [bpy=2,2'-bipyridine, 4,4'-MV2=4-ethoxycarbonyl-4'-(N-G1 -carbamoyl)-2,2'-bipyridine, and 4,4'-MV4=4,4'-bis(N-G1 -carbamoyl)-2,2'-bipyridine, where G1 =Asp(NHG2 )-NHG2 and G2 =-(CH2 )2 -N(+) C5 H4 -C5 H4 N(+) -CH3 ] were prepared as "photo-charge separators (PCSs)". Photoirradiation of these complexes in the presence of a sacrificial electron donor (EDTA) results in storage of electrons per PCS values of 1.3, 2.7, and 4.6, respectively. Their applications in the photochemical H2 evolution from water in the presence of a colloidal Pt H2 -evolving catalyst were investigated, and are discussed along with those reported for [Ru(bpy)2 (5,5'-MV4)](10+) , [Ru(4,4'-MV4)3 ](26+) , and [Ru(5,5'-MV4)3 ](26+) (Inorg. Chem. Front. 2016, 3, 671-680). The PCSs with high dimerization constants (Kd =10(5) -10(6)  m(-1) ) are superior in driving H2 evolution at pH 5.0, whereas those with lower Kd values (10(3) -10(4)  m(-1) ) are superior at pH 7.0, where Kd =[(MV(+) )2 ]/[MV(+) (.) ](2) . The (MV(+) )2 site can drive H2 evolution only at pH 5.0 as a result of its 0.15 eV lower driving force for H2 evolution relative to MV(+) (.) , whereas the PCSs with lower Kd values exhibit higher performance at pH 7.0 owing to the higher population of free MV(+) (.) . Importantly, the rate of electron charging over the PCSs is linear to the apparent H2 evolution rate, and shows an intriguing quadratic dependence on the number of MV(2+) units per PCS.
authors.affiliation: Department of Chemistry, Faculty of Science, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan.|International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan.
authors.forename: Kyoji
authors.initials: K
authors.lastname: Kitamoto
authors.affiliation: Department of Chemistry, Faculty of Science, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan. ksakai@chem.kyushu-univ.jp.|International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan. ksakai@chem.kyushu-univ.jp.|Center for Molecular Systems (CMS), Kyushu University, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan. ksakai@chem.kyushu-univ.jp.
authors.forename: Ken
authors.initials: K
authors.lastname: Sakai
keywords: electron storage; electron transfer; photochemisty; ruthenium complexes; viologen