Ground and excited state properties of the newly prepared complexes Re(CO) 3 (2,2′-biquinoline)L S + , L S ) pyrazine or 4,4′-bipyridine, and Re(CO) 3 (2,2′-bipy)(2-pyrazinecarboxylate) were investigated by steady state and time-resolved spectroscopy. The X-ray structure of the latter compound showed that the spectator ligand 2-pyrazinecarboxylate is coordinated through the carboxylate group to Re(I). A component of the complexes' luminescence was associated with long-lived Re to ligand, 2,2′-biquinoline or 2,2′-bipy, charge transfer, while a fast component of the emission was associated with intraligand excited states. Quenching of the luminescence by CuCl 2 involved energy transfer via dynamic and static mechanisms. The complexes in their excited states were reduced by 2,2′,2′′-nitrilotriethanol with the formation of Re(I) ligand-radical species. Similar products were generated by the pulse radiolytic reduction of the complexes. The photochemical properties of the 2,2′-biquinoline complexes and Re(CO) 3 (1,10-phen)4-nitrobenzoate are compared, and the mechanisms of their photochemical reactions are discussed.

A mechanistic study of the photoinduced reduction of carbon dioxide to carbon monoxide in the triethanolamine /dimethylformamide/ReBr(CO)3(bpy) (bpy is 2,2'-bipyridine) system is described. Continuous photolysis at 436 nm results in the highly specific formation of CO with a quantum yield, that reaches 0.15. The value of €0 decreases with increasing triethanolamine concentration in the range 0.75-3.8 M; addition of 10% water to the solvent medium also lowers 0co-Luminescence measurements reveal that triethanolamine reductively quenches the Re-to-bpy charge-transfer excited state of ReBr(CO)3(bpy) with a rate constant of 6 X 107 M"1 2s'1, whereas C02 undergoes no discernible interaction with the photoexcited complex. Formation and decay of the initial reduction product [ReBr(CO)3(bpy)]~h ave been observed in flash photolysis experiments. The amount of [ReBr(CO)3(bpy)]' produced correlates with the value of 0CO, thereby implicating this 19e complex in the mechanism of C02 reduction. Evidence that [ReBr-(CO)3(bpy)]' reacts directly with C02 has been obtained, although the identity of the resulting product is unknown at present.

To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H2 generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS with a maximum H2 generation rate of 58.06 ± 3.59 μmol h(-1) g(-1) for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H2 generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes.

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The photochemical and photophysical properties of a polymer containing nearly 200 pendant groups Re(CO)3(1,10-phenanthroline)+ bonded to poly-(vynilpyridine)600 and the related monomer pyRe(CO)3(1,10-phenanthroline)+ were investigated in solution phase. The yield of formation and the kinetics of decay of the MLCT excited state were found to be dependent on medium and laser power. MLCT excited states in the polymer undergo a more efficient annihilation and/or secondary photolysis than in the monomer. In the polymer, redox quenching of MLCT excited states by methyl viologen and by 2,2‘,2‘ ‘-nitrilotriethanol revealed the presence of intrastrand electron-transfer processes. These processes exhibited a complex kinetics. Mechanisms of the excited-state annihilation and electron-transfer processes in the polymer are proposed.

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Reported are mechanistic studies of the photoinduced reduction of carbon dioxide to carbon monoxide mediated by ReBr(CO),(bpy) (bpy is 2,2'-bipyridine). Water, ethanol, and perchlorate ion enhance the intensity of luminescence from the Re-to-bpy charge-transfer excited state, whereas bromide ion and triethylamine (TEA) quench this emission. In the case of the amine, the quenching process leads to prompt formation of [ReBr(CO),(bpy)]-. Additional, slower production of this 19e species by reduction of the parent complex by a TEA-derived radical is unimportant. Water and ethanol lower the quantum efficiency of CO formation, whereas Br-has little effect. Reduction of bicarbonate ion to CO can be excluded as an important pathway in the system. Patirradiation spectral changes occur over several hours, but the chemistry responsible for this behavior is unrelated to CO formation.