Insights into reactivity trends for electrochemical C-N bond formations.

Electrochemical synthesis techniques are currently of great interest due to the possibility of synthesizing products while limiting reactant and energy input and providing potentially unique selectivity. Our group has previously reported the development of the "anion pool" synthesis method. As this is a new method for organic synthesis and the coupling of C-N bonds, it is important to understand the reactivity trends and limitations this method provides. In this report we explore the reactivity trends of a series of nitrogen-containing heterocycles under reductive electrochemical conditions. The results show that anionic nitrogen heterocycles are stable at room temperature in acetonitrile/electrolyte solutions up to a parent N-H pKa value up to 23. Addition of carbon electrophiles to solutions containing the electrochemically generated anionic nitrogen heterocycles led to the C-N cross-coupling reactivity. Product yields tracked linearly with the pKa value of the N-H bond of the heterocycles over 4 orders of acidity magnitude. Both benzylic halides and perfluorinated aromatics were found suitable for undergoing C-N cross-coupling with the anionic nitrogen heterocycles with product yields as high as 90%. It is also shown that the stability and reactivity of the anions are affected by the choice of electrolyte and temperature. Additionally, this procedure compares well to green chemistry processes in atom economy and PMI values.

Molecular Z-Scheme for Solar Fuel Production via Dual Photocatalytic Cycles.

Natural photosynthesis uses an array of molecular structures in a multiphoton Z-scheme for the conversion of light energy into chemical bonds (i.e., solar fuels). Here, we show that upon excitation of both a molecular photocatalyst (PC) and a substituted naphthol (ROH) in the presence of a sacrificial electron donor and proton source, we achieve photocatalytic synthesis of H2. Data support a multiphoton mechanism that is catalytic with respect to both PC and ROH. The use of a naphthol molecule as both a light absorber and H2 producing catalyst is a unique motif for Z-scheme systems. This molecular Z-scheme can drive a reaction that is uphill by 511 kJ mol-1 and circumvents the high-energy constraints associated with the reduction of weak acids in their ground state, thus offering a new paradigm for the production of solar fuels.

Stimuli-Modulated Metal Oxidation States in Photochromic MOFs.

Tuning metal oxidation states in metal-organic framework (MOF) nodes by switching between two discrete linker photoisomers via an external stimulus was probed for the first time. On the examples of three novel photochromic copper-based frameworks, we demonstrated the capability of switching between +2 and +1 oxidation states, on demand. In addition to crystallographic methods used for material characterization, the role of the photochromic moieties for tuning the oxidation state was probed via conductivity measurements, cyclic voltammetry, and electron paramagnetic resonance, X-ray photoelectron, and diffuse reflectance spectroscopies. We confirmed the reversible photoswitching activity including photoisomerization rate determination of spiropyran- and diarylethene-containing linkers in extended frameworks, resulting in changes in metal oxidation states as a function of alternating excitation wavelengths. To elucidate the switching process between two states, the photoisomerization quantum yield of photochromic MOFs was determined for the first time. Overall, the introduced noninvasive concept of metal oxidation state modulation on the examples of stimuli-responsive MOFs foreshadows a new pathway for alternation of material properties toward targeted applications.

Assembled triphenylamine bis-urea macrocycles: exploring photodriven electron transfer from host to guests.

Absorption of electronic acceptors in the accessible channels of an assembled triphenylamine (TPA) bis-urea macrocycle 1 enabled the study of electron transfer from the walls of the TPA framework to the encapsulated guests. The TPA host is isoskeletal in all host-guest structures analyzed with guests 2,1,3-benzothiadiazole, 2,5-dichlorobenzoquinone and I2 loading in single-crystal-to-single-crystal transformations. Analysis of the crystal structures highlights how the spatial proximity and orientation of the TPA host and the entrapped guests influence their resulting photophysical properties and allow direct comparison of the different donor-acceptor complexes. Diffuse reflectance spectroscopy shows that upon complex formation 1·2,5-dichlorobenzoquinone exhibits a charge transfer (CT) transition. Whereas, the 1·2,1,3-benzothiadiazole complex undergoes a photoinduced electron transfer (PET) upon irradiation with 365 nm LEDs. The CT absorptions were also identified with the aid of time dependent density functional theory (TD-DFT) calculations. Cyclic voltammetry experiments show that 2,1,3-benzothiadiazole undergoes reversible reduction within the host-guest complex. Moreover, the optical band gaps of the host 1·2,5-dichlorobenzoquinone (1.66 eV), and host 1·2,1,3-benzothiadiazole (2.15 eV) complexes are significantly smaller as compared to the free host 1 material (3.19 eV). Overall, understanding this supramolecular electron transfer strategy should pave the way towards designing lower band gap inclusion complexes.

Recent advancements in the development of molecular organic photocatalysts.

Research in the development of molecular organic photocatalysts for applications in chemical syntheses has burgeoned in recent years. While organic photosensitizers have been known for over a century, tuning the properties of these molecules to increase photocatalytic efficiencies is now of growing importance. The properties that help improve the performance of organic photocatalysts include: a wider range of redox potentials, increased molar absorptivity (ε) in the visible spectrum, increased quantum yields (Φ), long-lived excited-state lifetimes (ns to µs), and increased chemical stability. This review examines some of the recent advancements in the development of molecular organic photocatalysts, specifically cyanoarenes, acridinium dyes, phenazines, thiazines, oxazines, and xanthenes, with respect to these properties and examines the chemical synthesis routes now achieved by organic photocatalysts.

Photocatalytic Oxidative Coupling of Arylamines for the Synthesis of Azoaromatics and the Role of O2 in the Mechanism.

The photocatalytic oxidative coupling of aryl amines to selectively synthesize azoaromatic compounds has been realized. Multiple different photocatalysts can be used to perform the general reaction; however, Ir(dF-CF3-ppy)2(dtbpy)+, where dF-CF3-ppy is 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine and dtpby is 4,4'-tert-butyl-2,2'-bipyridine, showed the greatest range of reactivity with various amine substrates. Both electron-rich and -deficient amines can be coupled with yields up to 95% under an ambient air atmosphere. Oxygen was deemed to be essential for the reaction and is utilized in the regeneration of the photocatalyst. Fluorescence quenching and radical trap experiments indicate an amine radical coupling mechanism that proceeds through a hydrazoaromatic intermediate before further oxidation occurs to form the desired azoaromatic products.

Bis-Cyclometalated Iridium Complexes Containing 4,4'-Bis(phosphonomethyl)-2,2'-bipyridine Ligands: Photophysics, Electrochemistry, and High-Voltage Dye-Sensitized Solar Cells.

In this report, the synthesis and characterization of two bis-cyclometalated iridium(III) complexes are presented. Single-crystal X-ray diffraction shows that [Ir(ppy)2(4,4'-bis(diethylphosphonomethyl)-2,2'-bipyridine)]PF6 adopts a pseudooctahedral geometry. The complexes have an absorption feature in the near-visible-UV region and emit green light with excited-state lifetimes in hundreds of nanoseconds. The redox properties of these complexes show reversible behavior for both oxidative and reductive events. [Ir(ppy)2(4,4'-bis(phosphonomethyl)-2,2'-bipyridine)]PF6 readily binds to metal oxide supports, like nanostructured SnIV-doped In2O3 and TiO2, while still retaining reversible redox chemistry. When incorporated as the photoanode in dye-sensitized solar cells, the devices exhibit open-circuit voltages of >1 V, which is a testament to their strength of these iridium(III) complexes as photochemical oxidants.

A Dual Threat: Redox-Activity and Electronic Structures of Well-Defined Donor-Acceptor Fulleretic Covalent-Organic Materials.

The effect of donor (D)-acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two- and three-dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight-fold conductivity enhancement. The first evaluation of redox behavior of buckyball- or tetracyanoquinodimethane-integrated crystalline was conducted. In parallel with tailoring the D-A alignment responsible for "static" changes in materials properties, an external stimulus was applied for "dynamic" control of the electronic profiles. Overall, the presented D-A strategic design, with stimuli-controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.

Guest Inclusion Modulates Concentration and Persistence of Photogenerated Radicals in Assembled Triphenylamine Macrocycles.

Substituted triphenylamine (TPA) radical cations show great potential as oxidants and as spin-containing units in polymer magnets. Their properties can be further tuned by supramolecular assembly. Here, we examine how the properties of photogenerated radical cations, intrinsic to TPA macrocycles, are altered upon their self-assembly into one-dimensional columns. These macrocycles consist of two TPAs and two methylene ureas, which drive the assembly into porous organic materials. Advantageously, upon activation the crystals can undergo guest exchange in a single-crystal-to-single-crystal transformation generating a series of isoskeletal host-guest complexes whose properties can be directly compared. Photoinduced electron transfer, initiated using 365 nm light-emitting diodes, affords radicals at room temperature as observed by electron paramagnetic resonance (EPR) spectroscopy. The line shape of the EPR spectra and the quantity of radicals can be modulated by both polarity and heavy atom inclusion of the encapsulated guest. These photogenerated radicals are persistent, with half-lives between 1 and 7 d and display no degradation upon radical decay. Re-irradiation of the samples can restore the radical concentration back to a similar maximum concentration, a feature that is reproducible over several cycles. EPR simulations of a representative spectrum indicate two species, one containing two N hyperfine interactions and an additional broad signal with no resolvable hyperfine interaction. Intriguingly, TPA analogues without bromine substitution also exhibit similar quantities of photogenerated radicals, suggesting that supramolecular strategies can enable more flexibility in stable TPA radical structures. These studies will help guide the development of new photoactive materials.

Rational Synthesis of Metallo-Cations Toward Redox- and Alkaline-Stable Metallo-Polyelectrolytes.

Cations are crucial components in emerging functional polyelectrolytes for a myriad of applications. Rapid development in this area necessitates the exploration of new cations with advanced properties. Herein we describe a combination of computational and experimental design of cobaltocene metallo-cations that have distinct electronic and redox properties. One of the direct outcomes on the first synthesis of a complete set of cation derivatives is to discover highly stable cations, which are further integrated to construct metallo-polyelectrolytes as anion-exchange membranes in solid-state alkaline fuel cells. The device performance of these polyelectrolytes under highly basic and oxidative environments is competitive with many organo-polyelectrolytes.

UV-irradiation of self-assembled triphenylamines affords persistent and regenerable radicals.

UV-irradiation of assembled urea-tethered triphenylamine dimers results in the formation of persistent radicals, whereas radicals generated in solution are reactive and quickly degrade. In the solid-state, high quantities of radicals (approximately 1 in 150 molecules) are formed with a half-life of one week with no significant change in the single crystal X-ray diffraction. Remarkably, after decay, re-irradiation of the solid sample regenerates the radicals to their original concentration. The photophysics upon radical generation are also altered. Both the absorption and emission are significantly quenched without external oxidation likely due to the delocalization of the radicals within the crystals. The factors that influence radical stability and generation are correlated to the rigid supramolecular framework formed by the urea tether of the triphenylamine dimer. Electrochemical evidence demonstrates that these compounds can be oxidized in solution at 1.0 V vs. SCE to generate radical cations, whose EPR spectra were compared with spectra of the solid-state photogenerated radicals. Additionally, these compounds display changes in emission due to solvent effects from fluorescence to phosphorescence. Understanding how solid-state assembly alters the photophysical properties of triphenylamines could lead to further applications of these compounds for magnetic and conductive materials.

Selective N1-Acylation of Indazoles with Acid Anhydrides Using an Electrochemical Approach.

An electrochemical synthesis method for the selective N1-acylation of indazoles has been developed. This "anion pool" approach electrochemically reduces indazole molecules generating indazole anions and H2. Acid anhydrides are then introduced to the solution resulting in selective acylation of the N1-position of the indazoles. This procedure can also be applied to the acylation of benzimidazoles and indoles. The reaction can also be performed using a 9 V battery without loss of reaction efficiency.

Mild synthesis of silyl ethers via potassium carbonate catalyzed reactions between alcohols and hydrosilanes.

A method has been developed for the silanolysis of alcohols using an abundant and non-corrosive base K2CO3 as a catalyst. Reactions between a variety of alcohols and hydrosilanes generate silyl ethers under mild conditions. The use of hydrosilanes leads to the formation of H2 as the only byproduct thus avoiding the formation of stoichiometric strong acids. The mild conditions lead to a wide scope of possible alcohol substrates and good functional group tolerance. Selective alcohol silanolysis is also observed in the presence of reactive C-H bonds, lending this method for extensive use in protection group chemistry.

Hierarchical Corannulene-Based Materials: Energy Transfer and Solid-State Photophysics.

We report the first example of a donor-acceptor corannulene-containing hybrid material with rapid ligand-to-ligand energy transfer (ET). Additionally, we provide the first time-resolved photoluminescence (PL) data for any corannulene-based compounds in the solid state. Comprehensive analysis of PL data in combination with theoretical calculations of donor-acceptor exciton coupling was employed to estimate ET rate and efficiency in the prepared material. The ligand-to-ligand ET rate calculated using two models is comparable with that observed in fullerene-containing materials, which are generally considered for molecular electronics development. Thus, the presented studies not only demonstrate the possibility of merging the intrinsic properties of π-bowls, specifically corannulene derivatives, with the versatility of crystalline hybrid scaffolds, but could also foreshadow the engineering of a novel class of hierarchical corannulene-based hybrid materials for optoelectronic devices.

Photoredox-Assisted Reductive Cross-Coupling: Mechanistic Insight into Catalytic Aryl-Alkyl Cross-Couplings.

Here, we describe a photoredox-assisted catalytic system for the direct reductive coupling of two carbon electrophiles. Recent advances have shown that nickel catalysts are active toward the coupling of sp3-carbon electrophiles and that well-controlled, light-driven coupling systems are possible. Our system, composed of a nickel catalyst, an iridium photosensitizer, and an amine electron donor, is capable of coupling halocarbons with high yields. Spectroscopic studies support a mechanism where under visible light irradiation the Ir photosensitizer in conjunction with triethanolamine are capable of reducing a nickel catalyst and activating the catalyst toward cross-coupling of carbon electrophiles. The synthetic methodology developed here operates at low 1 mol % catalyst and photosensitizer loadings. The catalytic system also operates without reaction additives such as inorganic salts or bases. A general and effective sp2-sp3 cross-coupling scheme has been achieved that exhibits tolerance to a wide array of functional groups.

Structural, electrochemical and photophysical properties of an exocyclic di-ruthenium complex and its application as a photosensitizer.

The reaction of cis-bis(2,2'-bipyridine)dichlororuthenium(ii) hydrate with a conformationally mobile bipyridyl macrocycle afforded [(bpy)2Ru(µ-L)Ru(bpy)2]Cl4·6H2O, a bridged di-Ru complex. Single crystal X-ray diffraction showed the macrocyclic ligand adopting a bowl-like structure with the exo-coordinated Ru(ii) centers separated by 7.29 Å. Photophysical characterization showed that the complex absorbs in the visible region (λmax = 451 nm) with an emission maximum at 610 nm (τ = 706 ns, ΦPL = 0.021). Electrochemical studies indicate the di-Ru complex undergoes three one-electron reversible reductions and a reversible one-electron oxidation process. This electrochemical reversibility is a key characteristic for its use as an electron transfer agents. The complex was evaluated as a photocatalyst for the electronically mismatched Diels-Alder reaction of isoprene and trans-anethole using visible light. It afforded the expected product in good conversion (69%) and selectivity (dr > 10 : 1) at low loadings (0.5-5.0 mol%) and the sensitizer/catalyst was readily recycled. These results suggest that the bipyridyl macrocycle could be widely applied as a bridging ligand for the generation of chromophore-catalyst assemblies.

Redox-Active Corannulene Buckybowls in a Crystalline Hybrid Scaffold.

A porous crystalline corannulene-containing scaffold, which combines the periodicity, dimensionality, and structural modularity of hybrid frameworks with the intrinsic properties of redox-active π-bowls, has been prepared. Single-crystal and powder X-ray diffraction, ab initio density functional theory computations, gas sorption analysis, fluorescence spectroscopy, and cyclic voltammetry were employed to study the properties of the novel corannulene derivatives and the buckybowl-based hybrid materials. X-ray diffraction studies revealed the preservation of the corannulene bowl inside the prepared rigid matrix, which offers the unique opportunity to extend the scaffold dimensionality through the buckybowl curvature. Merging the inherent properties of hybrid frameworks with the intrinsic properties of π-bowls opens a new avenue for preparing redox-active materials and potentially improving charge transport in the scaffold.

Base-enhanced catalytic water oxidation by a carboxylate-bipyridine Ru(II) complex.

In aqueous solution above pH 2.4 with 4% (vol/vol) CH3CN, the complex [Ru(II)(bda)(isoq)2] (bda is 2,2'-bipyridine-6,6'-dicarboxylate; isoq is isoquinoline) exists as the open-arm chelate, [Ru(II)(CO2-bpy-CO2(-))(isoq)2(NCCH3)], as shown by (1)H and (13)C-NMR, X-ray crystallography, and pH titrations. Rates of water oxidation with the open-arm chelate are remarkably enhanced by added proton acceptor bases, as measured by cyclic voltammetry (CV). In 1.0 M PO4(3-), the calculated half-time for water oxidation is ∼7 µs. The key to the rate accelerations with added bases is direct involvement of the buffer base in either atom-proton transfer (APT) or concerted electron-proton transfer (EPT) pathways.

Electrochemical Instability of Phosphonate-Derivatized, Ruthenium(III) Polypyridyl Complexes on Metal Oxide Surfaces.

The oxidative stability of the molecular components of dye-sensitized photoelectrosynthesis cells for solar water splitting remains to be explored systematically. We report here the results of an electrochemical study on the oxidative stability of ruthenium(II) polypyridyl complexes surface-bound to fluorine-doped tin oxide electrodes in acidic solutions and, to a lesser extent, as a function of pH and solvent with electrochemical monitoring. Desorption occurs for the Ru(II) forms of the surface-bound complexes with oxidation to Ru(III) enhancing both desorption and decomposition. Based on the results of long-term potential hold experiments with cyclic voltammetry monitoring, electrochemical oxidation to Ru(III) results in slow decomposition of the complex by 2,2'-bipyridine ligand loss and aquation and/or anation. A similar pattern of ligand loss was also observed for a known chromophore-catalyst assembly for both electrochemical water oxidation and photoelectrochemical water splitting. Our results are significant in identifying the importance of enhancing chromophore stability, or at least transient stability, in oxidized forms in order to achieve stable performance in aqueous environments in photoelectrochemical devices.