Harnessing Photovoltage: Effects of Film Thickness, TiO₂ Nanoparticle Size, MgO and Surface Capping with DSCs

High photovoltage dye-sensitized solar cells (DSCs) offer an exceptional opportunity to power electrocatalysts for the production of hydrogen from water and the reduction of CO₂ to usable fuels with a relatively cost-effective, low-toxicity solar cell. Competitive recombination pathways such as electron transfer from TiO₂ films to the redox shuttle or oxidized dye must be minimized to achieve the maximum possible photovoltage (<i>V</i>_oc) from DSC devices. A high <i>V</i>_oc of 882 mV was achieved with the iodide/triiodide redox shuttle and a ruthenium NCS-ligated dye, <b>HD-2-mono</b>, by utilizing a combined approach of (1) modulating the TiO₂ surface area through film thickness and nanoparticle size selection, (2) addition of a MgO insulating layer, and (3) capping available TiO₂ film surface sites post film sensitization with an F-SAM (fluorinated self-assembled monolayer) treatment. The exceptional <i>V</i>_oc of 882 mV observed is the highest achieved for the popular NCS containing ruthenium sensitizers with >5% PCE and compares favorably to the 769 mV value observed under common device preparation conditions

Sequential Hydrocarbon Functionalization: Allylic C−H Oxidation/Vinylic C−H Arylation

A Pd(II)/sulfoxide-catalyzed sequential allylic C−H oxidation/vinylic C−H arylation of α-olefins to furnish E-arylated allylic esters in high regio- and E:Z selectivities (>20:1) is reported. The broad scope of this method with respect to the α-olefin, carboxylic acid, and aryl boronic acid enables the rapid assembly of densely functionalized fragments for complex molecule synthesis from cheap, abundant hydrocarbon starting materials. The Pd(II)/sulfoxide-catalyzed vinylic C−H arylation of electronically unbiased olefins with aryl boronic acids proceeds under oxidative, acidic conditions and mild temperatures (room temperature to 45 °C)

Sequential Hydrocarbon Functionalization: Allylic C−H Oxidation/Vinylic C−H Arylation

A Pd(II)/sulfoxide-catalyzed sequential allylic C−H oxidation/vinylic C−H arylation of α-olefins to furnish E-arylated allylic esters in high regio- and E:Z selectivities (>20:1) is reported. The broad scope of this method with respect to the α-olefin, carboxylic acid, and aryl boronic acid enables the rapid assembly of densely functionalized fragments for complex molecule synthesis from cheap, abundant hydrocarbon starting materials. The Pd(II)/sulfoxide-catalyzed vinylic C−H arylation of electronically unbiased olefins with aryl boronic acids proceeds under oxidative, acidic conditions and mild temperatures (room temperature to 45 °C)

A General and Highly Selective Chelate-Controlled Intermolecular Oxidative Heck Reaction

A novel chelate-controlled intermolecular oxidative Heck reaction is reported that proceeds with a wide range of nonresonance stabilized α-olefin substrates and organoboron reagents to afford internal olefin products in good yields and outstanding regio- and E:Z stereoselectivities. Pd−H isomerization, common in many Heck reactions, is not observed under these mild, oxidative conditions. This is evidenced by outstanding E:Z selectivities (>20:1 in all cases examined), no erosion in optical purity for proximal stereogenic centers, and a tolerance for unprotected alcohols. Remarkably, a single metal/ligand combination, Pd/bis-sulfoxide complex 1, catalyzes this reaction over a broad range of coupling partners. Given the high selectivities and broad scope, we anticipate this intermolecular Heck reaction will find heightened use in complex molecule synthesis

Synthesis of Complex Allylic Esters via C−H Oxidation vs C−C Bond Formation

A highly general, predictably selective C−H oxidation method for the direct, catalytic synthesis of complex allylic esters is introduced. This Pd(II)/sulfoxide-catalyzed method allows a wide range of complex aryl and alkyl carboxylic acids to couple directly with terminal olefins to furnish (E)-allylic esters in synthetically useful yields and selectivities (16 examples, E/Z ≥ 10:1) and without the use of stoichiometric coupling reagents or unstable intermediates. Strategic advantages of constructing allylic esters via C−H oxidation vs C−C bond-forming methods are evaluated and discussed in four “case studies”

Synthesis of Complex Allylic Esters via C−H Oxidation vs C−C Bond Formation

A highly general, predictably selective C−H oxidation method for the direct, catalytic synthesis of complex allylic esters is introduced. This Pd(II)/sulfoxide-catalyzed method allows a wide range of complex aryl and alkyl carboxylic acids to couple directly with terminal olefins to furnish (E)-allylic esters in synthetically useful yields and selectivities (16 examples, E/Z ≥ 10:1) and without the use of stoichiometric coupling reagents or unstable intermediates. Strategic advantages of constructing allylic esters via C−H oxidation vs C−C bond-forming methods are evaluated and discussed in four “case studies”

Oxidative Heck Vinylation for the Synthesis of Complex Dienes and Polyenes

We introduce an oxidative Heck reaction for selective complex diene and polyene formation. The reaction proceeds via oxidative Pd­(II)/sulfoxide catalysis that retards palladium-hydride isomerizations which previously limited the Heck manifold’s capacity for furnishing stereodefined conjugated dienes. Limiting quantities of nonactivated terminal olefins (1 equiv) and slight excesses of vinyl boronic esters (1.5 equiv) that feature diverse functionality can be used to furnish complex dienes and polyenes in good yields and excellent selectivities (generally E:Z = >20:1; internal:terminal = >20:1). Because this reaction only requires prior activation of a single vinylic carbon, improvements in efficiency are observed for synthetic sequences relative to ones featuring reactions that require activation of both coupling partners

A General and Highly Selective Chelate-Controlled Intermolecular Oxidative Heck Reaction

A novel chelate-controlled intermolecular oxidative Heck reaction is reported that proceeds with a wide range of nonresonance stabilized α-olefin substrates and organoboron reagents to afford internal olefin products in good yields and outstanding regio- and E:Z stereoselectivities. Pd−H isomerization, common in many Heck reactions, is not observed under these mild, oxidative conditions. This is evidenced by outstanding E:Z selectivities (>20:1 in all cases examined), no erosion in optical purity for proximal stereogenic centers, and a tolerance for unprotected alcohols. Remarkably, a single metal/ligand combination, Pd/bis-sulfoxide complex 1, catalyzes this reaction over a broad range of coupling partners. Given the high selectivities and broad scope, we anticipate this intermolecular Heck reaction will find heightened use in complex molecule synthesis

Oxidative Heck Vinylation for the Synthesis of Complex Dienes and Polyenes

We introduce an oxidative Heck reaction for selective complex diene and polyene formation. The reaction proceeds via oxidative Pd­(II)/sulfoxide catalysis that retards palladium-hydride isomerizations which previously limited the Heck manifold’s capacity for furnishing stereodefined conjugated dienes. Limiting quantities of nonactivated terminal olefins (1 equiv) and slight excesses of vinyl boronic esters (1.5 equiv) that feature diverse functionality can be used to furnish complex dienes and polyenes in good yields and excellent selectivities (generally <i>E</i>:<i>Z</i> = >20:1; internal:terminal = >20:1). Because this reaction only requires prior activation of a single vinylic carbon, improvements in efficiency are observed for synthetic sequences relative to ones featuring reactions that require activation of both coupling partners

Photocatalytic Reduction of CO₂ with Re-Pyridyl-NHCs

A series of Re­(I) pyridyl N-heterocyclic carbene (NHC) complexes have been synthesized and examined in the photocatalytic reduction of CO₂ using a simulated solar spectrum. The catalysts were characterized through NMR, UV–vis, cyclic voltammetry under nitrogen, and cyclic voltammetry under carbon dioxide. The complexes were compared directly with a known benchmark catalyst, Re­(bpy) (CO)₃Br. An electron-deficient NHC substituent (PhCF₃) was found to promote catalytic activity when compared with electron-neutral and -rich substituents. Re­(PyNHC-PhCF₃) (CO)₃Br was found to exceed the CO production of the benchmark Re­(bpy) (CO)₃Br catalyst (51 vs 33 TON) in the presence of electron donor <b>BIH</b> and photosensitizer <i>fac</i>-Ir­(ppy)₃. Importantly, Re­(PyNHC-PhCF₃) (CO)₃Br was found to function without a photosensitizer (32 TON) at substantially higher turnovers than the benchmark catalyst Re­(bpy) (CO)₃Br (14 TON) under a solar simulated spectrum