Crystals of the 2,4,6-tri-tert-butylphenoxyl radical have been isolated and characterized by X-ray diffraction, and calculations have been performed that give the distribution of spin density in the radical.

Reported herein are thermochemical studies of hydrogen atom transfer (HAT) 2+ , is surprisingly well predicted by the trends for electron transfer half-reaction entropies, ΔS o ET , in aprotic solvents. This is because both ΔS o ET and ΔS o HAT have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between ΔS o HAT and ΔS o ET provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

We have constructed and studied photoelectrochemical solar cells (PECs) consisting of a photoanode prepared by direct deposition of independently synthesized CdSe nanocrystal quantum dots (NQDs) onto a nanocrystalline TiO(2) film (NQD/TiO(2)), aqueous Na(2)S or Li(2)S electrolyte, and a Pt counter electrode. We show that light harvesting efficiency (LHE) of the NQD/TiO(2) photoanode is significantly enhanced when the NQD surface passivation is changed from tri-n-octylphosphine oxide (TOPO) to 4-butylamine (BA). In the PEC the use of NQDs with a shorter passivating ligand, BA, leads to a significant enhancement in both the electron injection efficiency at the NQD/TiO(2) interface and charge collection efficiency at the NQD/electrolyte interface, with the latter attributed mostly to a more efficient diffusion of the electrolyte through the pores of the photoanode. We show that by utilizing BA-capped NQDs and aqueous Li(2)S as an electrolyte, it is possible to achieve ∼100% internal quantum efficiency of photon-to-electron conversion, matching the performance of dye-sensitized solar cells.

In order to understand how the separation between the electron and proton-accepting sites affects proton-coupled electron transfer (PCET) reactivity, we have prepared ruthenium complexes with 4′-(4-carboxyphenyl)terpyridine ligands, and studied reactivity with hydrogen atom donors (H-X). Ru II (pydic)(tpy-PhCOOH) (Ru II PhCOOH), was synthesized in one pot from [(p-cymene) RuCl 2 ] 2 , sodium 4′-(4-carboxyphenyl)-2,2′:6′,2″-terpyridine ([Na + ]tpy-PhCOO − ), and disodium pyridine-2,6-dicarboxylate (Na 2 pydic). Ru II PhCOOH plus n Bu 4 NOH in DMF yields the deprotonated Ru(II) complex, n Bu 4 N[Ru II (pydic)(tpy-PhCOO)] (Ru II PhCOO − ). The Ru(III) complex (Ru III PhCOO) has been isolated by one-electron oxidation of Ru II PhCOO − with triarylaminium radical cations (NAr 3 •+ ). The bond dissociation free energy (BDFE) of the O-H bond in Ru II PhCOOH is calculated from pK a and E 1/2 measurements as 87 kcal mol -1 , making Ru III PhCOO a strong hydrogen atom acceptor. There are 10 bonds and ca. 11.2 Å separating the metal from the carboxylate basic site in Ru III PhCOO. Even with this separation, Ru III PhCOO oxidizes the hydrogen atom donor TEMPOH (BDFE = 66.5 kcal mol -1 , ΔG°r xn = -21 kcal mol -1 ) by removal of an electron and a proton to form Ru II PhCOOH and TEMPO radical in a concerted proton-electron transfer (CPET) process. The second order rate constant for this reaction is (1.1 ± 0.1) × 10 5 M -1 s -1 with k H /k D = 2.1 ± 0.2, similar to the observed kinetics in an analogous system without the phenyl spacer, Ru III (pydic)(tpy-COO) (Ru III COO − ). In contrast, hydrogen transfer from 2,6-di-tert-butyl-p-methoxyphenol [ t Bu 2 (OMe)ArOH] to Ru III PhCOO is several orders of magnitude slower than the analogous reaction with Ru III COO.Coupling electron transfer to proton transfer is key to a wide range of chemical and biochemical processes, such as converting solar energy to chemical fuels. 1 While many of the fundamentals of electron transfer (ET) are well understood, the principles of proton-coupled electron transfer (PCET) are still being developed. The effects of increasing the distance between reaction centers has been much studied for ET, 2 and we have started to explore PCET systems in which the electron-and proton-accepting sites are increasingly separated. 3 PCET processes with large separations appear to be important in a number of biological systems, such as ribonucleotide reductases and photosystem II. 4 They may also be involved in charge injection into oxide semiconductors from ruthenium polypyridyl-carboxylate complexes. 5 In our previously reported ruthenium terpyridine-4′-carboxylate complex Ru III COO (Scheme 1), the Ru is six bonds and 6.9 Å removed from the basic carboxylate oxygen atoms. 3 Despite this separation, the reported reactions occur with H + and e − transferring in the same kinetic step, by concerted proton-electron transfer (CPET). 1,3,[6][7][8] In this report, the distance between the metal and basic Email: mayer@chem.washington.edu. Supporting Information A...