In 2001, he began studies at Boston University, earning a B.A. in Chemistry while working in the laboratory of Prof. John P. Caradonna. In the fall of 2005, he began graduate school in the laboratory of Prof. Daniel G. Nocera at the Massachusetts Institute of Technology, focusing on molecular approaches for solar energy conversion, specifically photochemical halogen production. Tim began a postdoctoral appointment at the University of Utah in the laboratory of Prof. Peter J. Stang in the summer of 2010. Dilek K. Dogutan, was born and grew up in Erenko ¨y/Istanbul, Turkey.

No abstract

The efficient storage of solar energy in chemical fuels, such as hydrogen, is essential for the large-scale utilisation of solar energy systems. Recent advances in the photocatalytic production of H(2) are highlighted. Two general approaches for the photocatalytic hydrogen generation by homogeneous catalysts are considered: HX (X = Cl, Br) splitting involving both proton reduction and halide oxidation via an inner-sphere mechanism with a single-component catalyst; and sensitized H(2) production, employing sacrificial electron donors to regenerate the active catalyst. Future directions and challenges in photocatalytic H(2) generation are enumerated.

The construction of a new class of compounds--the hangman corroles--is provided efficiently by the modification of macrocyclic forming reactions from bilanes. Hangman cobalt corroles are furnished in good yields from a one-pot condensation of dipyrromethane with the aldehyde of a xanthene spacer followed by metal insertion using microwave irradiation. In high oxidation states, X-band EPR spectra and DFT calculations of cobalt corrole axially ligated by chloride are consistent with the description of a Co(III) center residing in the one-electron oxidized corrole macrocycle. These high oxidation states are likely accessed in the activation of O-O bonds. Along these lines, we show that the proton-donating group of the hangman platform works in concert with the redox properties of the corrole to enhance the catalytic activity of O-O bond activation. The hangman corroles show enhanced activity for the selective reduction of oxygen to water as compared to their unmodified counterparts. The oxygen adduct, prior to oxygen reduction, is characterized by EPR and absorption spectroscopy.

Bis-cyclometalated iridium complexes with enhanced phosphorescence quantum yields in the red region of the visible spectrum are described. Here, we demonstrate that incorporating strongly π-donating, nitrogen-containing β-ketoiminate (acNac), β-diketiminate (NacNac), and N, N'-diisopropylbenzamidinate (dipba) ancillary ligands can demonstrably perturb the excited-state kinetics, leading to enhanced photoluminescence quantum yields (Φ) for red-emitting compounds. A comprehensive study of the quantum yields and lifetimes for these complexes reveals that for the compounds with the highest quantum yields, the radiative rate constant ( k) is significantly higher than that of related complexes, and contributes substantially to the increase in Φ. Experimental and computational evidence is consistent with the notion that an increase in spin-orbit coupling, caused by an enhancement of the metal-to-ligand charge transfer (MLCT) character of the excited state via destabilization of the HOMO, is mainly responsible for the faster radiative rates. One of the compounds was shown to be effective as the emissive dopant in an organic light-emitting diode device.

Monomeric complexes of the type Au(III)(PR(3))X(3) and bimetallic complexes of the type Au(2)(I,III)[mu-CH(2)(R(2)P)(2)]X(4) and Au(2)(III,III)[mu-CH(2)(R(2)P)(2)]X(6) (R = Ph, Cy, X = Cl(-), Br(-)) undergo facile photoelimination of halogen. M-X bond activation and halogen elimination is achieved upon LMCT excitation of solutions of Au(III) complexes in the presence of olefin chemical traps. As opposed to the typical one-electron redox transformations of LMCT photochemistry, the LMCT photochemistry of the Au(III) centers allows for the unprecedented (i) two-electron photoelimination of X(2) from a monomeric center and (ii) four-electron photoelimination of X(2) from a bimetalllic center. The quantum yields for X(2) photoproduction, in general, are between 10% and 20% for all species, showing only minimal dependence on the identity of the ligands about gold, or the nuclearity of the complex. Efficient X(2) photoelimination is observed in the absence of a chemical trap, providing a rare example of authentic, trap-free halogen elimination from a transition metal center.

A series of cyclometalated iridium complexes with β-ketoiminate and β-diketiminate ligands are described. Two different cyclometalating (C^N) ligands-2-phenylpridine (ppy) and 2-phenylbenzothiazole (bt)-are used in concert with three different ancillary (LX) ligands-a phenyl-substituted β-ketoiminate (acNac(Me)), a phenyl-substituted β-diketiminate (NacNac(Me)), and a fluorinated version of the β-diketiminate (NacNac(CF3))-to furnish a suite of six complexes. The complexes are prepared by metathesis reactions of chloro-bridged dimers [Ir(C^N)2(μ-Cl)]2 with potassium or lithium salts of the ancillary LX ligand. Four of the complexes are characterized by X-ray crystallography, and all six were subjected to in-depth optical and electrochemical interrogation. Cyclic voltammetry shows both reduction and oxidation waves, with the latter strongly dependent on the identity of the LX ligand. The complexes are all luminescent, with the nature of the emissive excited state and the quantum yield (Φ) dependent on the identity of both the C^N and LX ligands. Whereas the complexes Ir(ppy)2(NacNac(Me)) and Ir(ppy)2(acNac(Me)) are weakly luminescent (Φ ≈ 0.01), the complexes Ir(bt)2(NacNac(Me)) and Ir(bt)2(acNac(Me)) are strongly luminescent, with the latter's quantum efficiency (Φ = 0.82) among the highest ever observed for cyclometalated iridium complexes. Fluorination of the NacNac ligand gives rise to completely disparate emission behavior suggestive of a NacNac-centered emissive state. The results described here, in comparison with previous groups' studies on acetylacetonate (acac) analogues, suggest that the weaker-field NacNac and acNac ligands raise the energy of the metal-centered HOMO, with energy of the HOMO increasing in the order NacNac(CF3) < acNac(Me) < NacNac(Me).

Photochemical radical initiation is a powerful tool for studying radical initiation and transport in biology. Ribonucleotide reductases (RNRs), which catalyze the conversion of nucleotides to deoxynucleotides in all organisms, are an exemplar of radical mediated transformations in biology. Class Ia RNRs are composed of two subunits: α2 and β2. As a method to initiate radical formation photochemically within β2, a single surface-exposed cysteine of the β2 subunit of Escherichia coli Class Ia RNR has been labeled (98%) with a photooxidant ( Re tricarbonyl 1,10-phenanthroline methylpyridyl rhenium I ). The labeling was achieved by incubation of S355C-β2 with the 4-(bromomethyl) pyridyl derivative of [Re] to yield the labeled species, [Re]-S355C-β2. Steady-state and time-resolved emission experiments reveal that the metal-to-ligand charge transfer (MLCT) excited-state 3 Re is not significantly perturbed after bioconjugation and is available as a phototrigger of tyrosine radical at position 356 in the β2 subunit; transient absorption spectroscopy reveals that the radical lives for microseconds. The work described herein provides a platform for photochemical radical initiation and study of protoncoupled electron transfer (PCET) in the β2 subunit of RNR, from which radical initiation and transport for this enzyme originates.proton-coupled electron transfer | radical generation | radical transport T he initiation and transport of many amino acid radicals occurs by proton-coupled electron transfer (PCET) (1-4). Accordingly, the importance of radicals in biological function (5) provides an imperative for the description of PCET in natural systems whose functions derive from radical-based chemistry (6). The PCET activity of amino acid radicals originates from the dependence of the reduction potential on the pK a of the amino acid in its oxidized and reduced states as well as the intrinsic redox potentials of the amino acid in its protonated and deprotonated states, and its association with hydrogen bonded partners as has been shown in proteins (7,8), β-hairpin peptides with interstrand dipolar contacts (9,10), and other de novo designed protein maquettes (11).Ribonucleotide reductases (RNRs) are essential enzymes of all organisms (12) that demonstrate exquisite control of radical transport for their function (13). RNRs catalyze the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs) and are therefore largely responsible for maintaining the cellular pool of monomeric DNA precursors. E. coli class Ia RNR consists of two homodimeric subunits, α2 and β2 (14,15). The α2 subunit contains the enzyme active site as well as two allosteric regulation sites, whereas β2 contains a diferric tyrosyl cofactor (•Y122), which is where the radical resides in the resting state of RNR. Nucleoside reduction requires formation of an α2∶β2 complex. Substrate turnover occurs by a radical mechanism mediated by an active site cysteine thiyl radical in α2 (•C439). A docking model (Fig. S1) based on crystal structure...