CV report - Electrochemical and Photophysical study of a...

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Electrochemical and Photophysical study of a hydrogen evolving, photoactive Iridium complex and Cobalt electron relay. David C Grauer*, Jae W. Hammett, Jennifer M. Hart and Neal D. McDaniel. David Grauer, Department of Chemistry, Princeton University, Princeton, NJ 08544. January 15, 2008; [email protected] In response to a recent rise in global temperatures as a result of steadily increasing, apparently anthropogenic, greenhouse gas concentrations, much attention has been directed towards the effective production and consumption alternative fuels. Of these many and diverse possible resources, molecular hydrogen has emerged as one of the most promising. While hydrogen is an extremely abundant element on earth, it is almost entirely found bound to oxygen in water; molecular hydrogen, however, is comparatively rare. To efficiently release hydrogen from oxygen’s hold in water, a variety of methods can be used from electrolysis to enzymatic catalysis. This paper examines one of those methods, photocatalysis. In this process, an iridium photosensitizer (PS), [Ir(ppy 2 )bpy] + , is coupled to one of a variety of cobalt electron relays (ER), [Co(bpy) 3 ] 2+ , [Co(dtb-bpy) 3 ] 2+ and [Co(trpy) 2 ] 2+ (figure 1) 1-4 . This general combination of materials has been well studied as a photocataltyic method for the production of hydrogen 5 . a. b. c. The mechanism of this complex system relies on the excitation of the Ir PS using UV radiation. The excited {[Ir(ppy 2 )bpy] + }* can then follow one of two paths: oxidative or reductive quenching via a sacrificial reductant (SR). Both pathways end with an electron transfer to the cobalt complex ER. A second electron is transferred to the Co complex bringing the oxidation state of the Co to Co[I]. During this process, after one of the two electron transfers, it is believed that one of the three bipyridine ligands falls off forcing the other two ligands to adopt a square planar structure thereby opening up the dz 2 orbitals up. Two protons can then coordinate at those two sites and receive two electrons from the Co complex (figure 2). In response to this mechanism, two terpyridine groups are used in the third ER as a control since the difficulty of removing only one trpy and leaving the complex lopsided stymies the formation of a square planar structure and, consequently, the production of hydrogen. 1 Using a combination of cyclic voltammetry, photoreaction and photophysical studies we have attempted to measure the efficiency of each relay, determine whether reductive or oxidative quenching dominates, as well as propose mechanisms for the two different electron transfers and reactions between the PS and three ERs. 6,7 Reductive quenching by SR. Oxidative quenching by ER. Materials and Methods Chemicals In running these studies it was necessary to synthesize the three Co ERs as well as the Ir PS. To synthesize the three Co ERs, 100 mg (or 35 mg in the case of trpy) of Co(Cl 2 )(H 2 O) 6 was added to each of three flasks each of which contained 20 mL EtOH. Each of the three flasks received a molar excess (207 mg bpy, 340 mg dtb-bpy and 80.5 mg trpy) and--- mg trpy), of one of
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