The development of inexpensive electrocatalysts that can promote the reduction of CO2 to CO with high selectivity, efficiency, and large current densities is an important step on the path to renewable production of liquid carbon-based fuels. While precious metals such as gold and silver have historically been the most active cathode materials for CO2 reduction, the price of these materials precludes their use on the scale required for fuel production. Bismuth, by comparison, is an affordable and environmentally benign metal that shows promise for CO2 conversion applications. In this work, we show that a bismuth–carbon monoxide evolving catalyst (Bi-CMEC) can be formed under either aqueous or nonaqueous conditions using versatile electrodeposition methods. In situ formation of this thin-film catalyst on an inexpensive carbon electrode using an organic soluble Bi3+ precursor streamlines preparation of this material and generates a robust catalyst for CO2 reduction. In the presence of appropriate imidazolium based ionic liquid promoters, the Bi-CMEC platform can selectively catalyze conversion of CO2 to CO without the need for a costly supporting electrolyte. This inexpensive system can catalyze evolution of CO with current densities as high as jCO = 25–30 mA/cm2 and attendant energy efficiencies of ΦCO ≈ 80% for the cathodic half reaction. These metrics highlight the efficiency of Bi-CMEC, since only noble metals have been previously shown to promote this fuel forming half reaction with such high energy efficiency. Moreover, the rate of CO production by Bi-CMEC ranges from approximately 0.1–0.5 mmol·cm−2·h−1 at an applied overpotential of η ≈ 250 mV for a cathode with surface area equal to 1.0 cm2. This CO evolution activity is much higher than that afforded by other non-noble metal cathode materials and distinguishes Bi-CMEC as a superior and inexpensive platform for electrochemical conversion of CO2 to fuel.
5,5-dimethylphlorin derivative (3H(PhlCF3)) was prepared and studied through a combination of
redox, photophysical, and computational experiments. The phlorin macrocycle
is significantly distorted from planarity compared to more traditional
tetrapyrrole architectures and displays solvatochroism in the soret
region of the UV–vis spectrum (∼370–420 nm).
DFT calculations indicate that this solvatochromic behavior stems
from the polarized nature of the frontier orbital (LUMO+1) that is
most heavily involved in these transitions. Compound 3H(PhlCF3) also displays an intriguing supramolecular
chemistry with certain anions; this phlorin can cooperatively hydrogen-bond
two equivalents of fluoride to form 3H(PhlCF3)·2F– but does not bind larger
halides such as Cl– or Br–. Analogous
studies revealed that the phlorin can hydrogen-bond with carboxylate
anions such as acetate to form 1:1 complexes such as 3H(PhlCF3)·OAc–. These supramolecular
assemblies are robust and form even in relatively polar solvents such
as MeCN. Hydrogen-bonding of fluoride and acetate anions to the phlorin
N–H residues significantly attenuates the redox and photophysical
properties of the phlorin. Moreover, The ability to independently
vary the size and pKa of a series of carboxylate
hydrogen-bond acceptors has allowed us to probe how phlorin–anion
association is controlled by the anion’s size and/or basicity.
These studies elucidate the physical properties and the electronic
effects that shape the supramolecular chemistry displayed by the phlorin
The development of inexpensive electrocatalysts
that can promote
the reduction of CO2 to CO with high selectivity, efficiency,
and large current densities is an important step on the path to renewable
production of liquid carbon-based fuels. While precious metals such
as gold and silver have historically been the most active cathode
materials for CO2 reduction, the price of these materials
precludes their use on the scale required for fuel production. Bismuth,
by comparison, is an affordable and environmentally benign metal that
shows promise for CO2 conversion applications. In this
work, we show that a bismuth–carbon monoxide evolving catalyst
(Bi-CMEC) can be formed under either aqueous or nonaqueous conditions
using versatile electrodeposition methods. In situ formation of this
thin-film catalyst on an inexpensive carbon electrode using an organic
soluble Bi3+ precursor streamlines preparation of this
material and generates a robust catalyst for CO2 reduction.
In the presence of appropriate imidazolium based ionic liquid promoters,
the Bi-CMEC platform can selectively catalyze conversion of CO2 to CO without the need for a costly supporting electrolyte.
This inexpensive system can catalyze evolution of CO with current
densities as high as jCO = 25–30
mA/cm2 and attendant energy efficiencies of ΦCO ≈ 80% for the cathodic half reaction. These metrics
highlight the efficiency of Bi-CMEC, since only noble metals have
been previously shown to promote this fuel forming half reaction with
such high energy efficiency. Moreover, the rate of CO production by
Bi-CMEC ranges from approximately 0.1–0.5 mmol·cm–2·h–1 at an applied overpotential
of η ≈ 250 mV for a cathode with surface area equal to
1.0 cm2. This CO evolution activity is much higher than
that afforded by other non-noble metal cathode materials and distinguishes
Bi-CMEC as a superior and inexpensive platform for electrochemical
conversion of CO2 to fuel.
The reduction of carbon dioxide to chemical fuels such as carbon monoxide is an important challenge in the field of renewable energy conversion. Given the thermodynamic stability of carbon dioxide, it is difficult to efficiently activate this substrate in a selective fashion and the development of new electrocatalysts for CO2 reduction is of prime importance. To this end, we have prepared and studied a new fac-ReI(CO)3 complex supported by a bipyridine ligand containing ancillary BODIPY moieties ([Re(BB2)(CO)3Cl]). Voltammetry experiments revealed that this system displays a rich redox chemistry under N2, as [Re(BB2)(CO)3Cl] can be reduced by up to four electrons at modest potentials. These redox events have been characterized as the ReI/0 couple, and three ligand based reductions – two of which are localized on the BODIPY units. The ability of the BB2 ligand to serve as a non-innocent redox reservoir is manifest in an enhanced electrocatalysis with CO2 as compared to an unsubstituted Re-bipyridine complex lacking BODIPY units ([Re(bpy)(CO)3Cl]). The second order rate constant for reduction of CO2 by [Re(BB2)(CO)3Cl] was measured to be k = 3400 M−1s−1 at an applied potential of −2.0 V versus SCE, which is roughly three times greater than the corresponding unsubstituted Re-bipyridine homologue. Photophysical and photochemical studies were also carried out to determine if [Re(BB2)(CO)3Cl] was a competent platform for CO2 reduction using visible light. These experiments showed that this complex supports unusual excited state dynamics that precludes efficient CO2 reduction and are distinct from those that are typically observed for fac-ReI(CO)3 complexes.
BODIPY; carbon dioxide; catalysis; electrochemistry; photochemistry; rhenium bipyridine derivatives
Controlling the molecular topology of electrode–catalyst interfaces is a critical factor in engineering devices with specific electron transport kinetics and catalytic efficiencies. As such, the development of rational methods for the modular construction of tailorable electrode surfaces with robust molecular wires (MWs) exhibiting well-defined molecular topologies, conductivities and morphologies is critical to the evolution and implementation of electrochemical arrays for sensing and catalysis. In response to this need, we have established modular on-surface Sonogashira and Glaser cross-coupling processes to synthetically install arrays of ferrocene-capped MWs onto electrochemically functionalized surfaces. These methods are of comparable convenience and efficiency to more commonly employed Huisgen methods. Furthermore, unlike the Huisgen reaction, this new surface functionalization chemistry generates modified electrodes that do not contain unwanted ancillary metal binding sites, while allowing the bridge between the ferrocenyl moiety and electrode surface to be synthetically tailored. Electrochemical and surface analytical characterization of these platforms demonstrate that the linker topology and connectivity influences the ferrocene redox potential and the kinetics of charge transport at the interface.
Two new 2,2’-bipyridine (bpy) derivatives containing ancillary BODIPY chromophores attached at the 5- and 5’-positions (BB3) or 6- and 6’-positions (BB4) were prepared and characterized. In this work, the basic photophysics, electrochemistry and electrogenerated chemiluminescence (ECL) of BB3 and BB4 are compared with those previously reported for a related bpy-BODIPY derivative (BB2) (J. Phys. Chem. C
2011, 115, 17993–18001). Cyclic voltammetry revealed that BB3 and BB4 display reversible 2e− oxidation and reduction waves, which consist of two closely spaced (50 – 70 mV) 1e− events. This redox behavior is consistent with the frontier molecular orbitals calculated for BB3 and BB4 and indicates that the 2,2’-bipyridine spacer of each bpy- BODIPY homologue does not facilitate efficient electronic communication between the tethered indacene units. In the presence of a coreactant such as tri-n-propylamine (TPA) or benzoyl peroxide (BPO), BB3 and BB4 exhibit strong ECL and produce spectra that are very similar to their corresponding photoluminescence profiles. The ECL signal obtained under annihilation conditions, however, is significantly different and is characterized by two distinct bands. One of these bands is centered at ~570 nm and is attributed to emission via an S- or T-route. The second band, occurs at longer wavelengths and is centered around ~740 nm. The shape and concentration dependence of this long-wavelength ECL signal is not indicative of emission from an excimer or aggregate, but rather is suggests that a new emissive species is formed from the bpy-BODIPY luminophores during the annihilation process.
Electrochemistry; Electrogenerated Chemiluminescence; Long-Wavelength Emission; BODIPY; Bipyridine Derivatives
Harnessing of solar energy to drive the reduction of carbon dioxide to fuels requires the development of efficient catalysts that absorb sunlight. In this work, we detail the synthesis, electrochemistry and photophysical properties of a set of homologous fac-ReI(CO)3 complexes containing either an ancillary phenyl (8) or BODIPY (12) substituent. These studies demonstrate that both the electronic properties of the rhenium center and BODIPY chromophore are maintained for these complexes. Photolysis studies demonstrate that both assemblies 8 and 12 are competent catalysts for the photochemical reduction of CO2 to CO in DMF using triethanolamine (TEOA) as a sacrificial reductant. Both compounds 8 and 12 display TOFs for photocatalytic CO production upon irradiation with light (λex ≥ 400 nm) of ~5 hr−1 with TON values of approximately 20. Although structural and photophysical measurements demonstrate that electronic coupling between the BODIPY and fac-ReI(CO)3 units is limited for complex 12, this work clearly shows that the photoactive BODIPY moiety is tolerated during catalysis and does not interfere with the observed photochemistry. When taken together, these results provide a clear roadmap for the development of advanced rhenium bipyridine complexes bearing ancillary BODIPY groups for the efficient photocatalytic reduction of CO2 using visible light.
BODIPY; carbon dioxide; catalysis; electrochemistry; photochemistry; rhenium bipyridine derivatives
The wide-scale implementation of solar and other renewable sources of electricity requires improved means for energy storage. An intriguing strategy in this regard is the reduction of CO2 to CO, which generates an energy rich commodity chemical that can be coupled to liquid fuel production. In this work, we report an inexpensive Bismuth Carbon Monoxide Evolving Catalyst (Bi-CMEC) that can be formed upon cathodic polarization of an inert glassy carbon electrode in acidic solutions containing Bi3+ ions. This catalyst can be used in conjunction with ionic liquids to effect the electrocatalytic conversion of CO2 to CO with appreciable current density at overpotentials below 0.2 V. Bi-CMEC is selective for production of CO, operating with a Faradaic efficiency of approximately 95%. When taken together these correspond to a high energy efficiency for CO production, on par with that which has historically only been observed using expensive silver and gold cathodes.
A homologous set of 5,5-dimethylphlorin macrocycles in which the identity of one aryl ring is systematically varied has been prepared. These derivatives contain ancillary pentaflurophenyl (3H(PhlF)), mesityl (3H(PhlMes)), 2,6-bismethoxyphenyl (3H(PhlOMe)), 4-nitrophenyl (3H(PhlNO2)) or 4-tert-butylcarboxyphenyl (3H(PhlCO2tBu)) groups at the 15-meso-position. These porphyrinoids were prepared in good yields (35 – 50%) and display unusual multielectron redox and photochemical properties. Each phlorin can be oxidized up to three times at modest potentials and can be reduced twice. The electron-donating and releasing properties of the ancillary aryl substituent attenuate the potentials of these redox events; phlorins containing electron-donating aryl groups are easier to oxidize and harder to reduce, while the opposite trend is observed for phlorins containing electron-withdrawing functionalities. Phlorin substitution also has a pronounced effect on the observed photophysics, as introduction of electron-releasing aryl groups on the periphery of the macrocycle is manifest in larger emission quantum yields and longer fluorescence lifetimes. Each phlorin displays an intriguing supramolecular chemistry and can bind 2 equivalents of fluoride. This binding is allosteric in nature and the strength of halide binding correlates with the ability of the phlorin to stabilize the buildup of charge. Moreover, fluoride binding to generate complexes of the form 3H(PhlR)·2F− modulates the redox potentials of the parent phlorin. As such, titration of phlorin with a source of fluoride represents a facile method to tune the ability of this class of porphyrinoid to absorb light and engage in redox chemistry.
A set polyethylene glycol (PEG) appended BODIPY architectures (BOPEG1 – BOPEG3) have been prepared and studied in CH2Cl2, H2O:CH3CN (1:1) and aqueous solutions. BOPEG1 and BOPEG2 both contain a short PEG chain and differ in substitution about the BODIPY framework. BOPEG3 is comprised of a fully substituted BODIPY moiety linked to a PEG polymer that is roughly 13 units in length. The photophysics and electrochemical properties of these compounds have been thoroughly characterized in CH2Cl2 and aqueous CH3CN solutions. The behavior of BOPEG1 – BOPEG3 correlates with established rules of BODIPY stability based on substitution about the BODIPY moiety. ECL for each of these compounds was also monitored. BOPEG1, which is unsubstituted at the 2- and 6-positions dimerized upon electrochemical oxidation while BOPEG2, which contains ethyl groups at the 2- and 6-positions, was much more robust and served as an excellent ECL luminophore. BOPEG3 is highly soluble in water due to the long PEG tether and demonstrated modest ECL activity in aqueous solutions using tri-n-propylamine (TPrA) as a coreactant. As such, BOPEG3 represents the first BODIPY derivative that has been shown to display ECL in water without the need for an organic cosolvent, and marks an important step in the development of BODIPY based ECL probes for various biosensing applications.
ECL; Electrochemistry; Emission; Sensing
Amidine-appended ferrocene derivatives form a supramolecular assembly with Ru(ii)(bpy-COOH) (L)22+ complexes (bpy-COOH is 4-CO2H-4′-CH3-bpy and L = bpy, 2,2′-bipyridine or btfmbpy, 4,4′-bis (trifluoromethyl)-2,2′-bipyridine). Steady-state, time-resolved spectroscopy and kinetic isotope effects establish that the metal-to-ligand charge transfer excited states of the Ru(ii) complexes are quenched by proton-coupled energy transfer (PCEnT). These results show that proton motion can be effective in mediating not only electron transfer (ET) but energy transfer (EnT) as well.
Porphyrins are attractive chromophores for incorporation into light harvesting devices. Some of the most efficient porphyrin derivatives in this regard are synthetically complex platforms with specially tailored electronic properties. This work details the unique geometric and electronic structure of the phlorin framework. X-ray crystallography and NMR spectroscopy demonstrate that unlike typical tetrapyrrole macrocycles, the phlorin is not aromatic. These unusual electronics are manifest in distinct photophysical and redox properties, as the phlorin displays a rich multielectron redox chemistry. The phlorin also displays an intriguing supramolecular chemistry and can reversibly bind up to two equivalents of fluoride in cooperative fashion. Accordingly, this synthetically accessible sensitizer displays a rich multielectron redox chemistry, excellent spectral coverage and an intriguing anion binding chemistry that distinguishes this system from more commonly studied porphyrinoids.
Phlorin; Porphyrin; Corrole; DSC; Solar Energy
The reactivity of a dimethyl palladium complex supported by a dicarbene chelate (MDCMes)PdMe2 towards CO2 has been investigated. In the presence of trace H2O, this reaction yields the corresponding methyl bicarbonate complex (MDCMes)PdMe(O2COH), which goes on to give the corresponding κ2-carbonato complex upon crystallization (MDCMes)Pd(CO3). This chemistry, as well as related protonolysis by acetic acid was monitored by a combination of techniques including React-IR spectroscopy.
Two new 2,2′-bipyridine (bpy) based ligands with ancillary BODIPY chromophores attached at the 4 and 4′-positions were prepared and characterized, which vary in the substitution pattern about the BODIPY periphery by either excluding (BB1) or including (BB2) a β-alkyl substituent. Both absorb strongly throughout the visible region and are strongly emissive. The basic photophysics and electrochemical properties of BB1 and BB2 are comparable to those of the BODIPY monomers on which they are based. The solid-state structures and electronic structure calculations both indicate that there is negligible electronic communication between the BODIPY moieties and the intervening bpy spacers. Electrogenerated chemiluminescence spectra of the two Bpy-BODIPY derivatives are similar to their recorded fluorescence profiles and are strongly influenced by substituents on the BODIPY chromophores. These 2,2′-bipyridine derivatives represent a new set of ligands that should find utility in applications including light-harvesting, photocatalysis, and molecular electronics.
BODIPY; bipyridine; electrochemistry; photophysics; electrogenerated chemiluminescence
A Zn(II) porphyrin-amidinium is the excited state electron donor (D) to a naphthalene diimide acceptor (A) appended with either a carboxylate or sulfonate functionality. The two-point hydrogen bond (---[H+]---) formed between the amidinium and carboxylate or sulfonate establishes a proton-coupled electron transfer (PCET) pathway for charge transfer. The two D---[H+]---A assemblies differ only by the proton configuration within the hydrogen bonding interface. Specifically, the amidinium transfers a proton to the carboxylate to form a non-ionized amidine-carboxylic acid two-point hydrogen network whereas the amidinium maintains both protons when bound to the sulfonate functionality forming an ionized amidinium-sulfonate two-point hydrogen network. These two interface configurations within the dyads thus allow for a direct comparison of PCET kinetics for the same donor and acceptor juxtaposed by an ionized and non-ionized hydrogen-bonded interface. Analysis of PCET kinetics ascertained from transient absorption and transient emission spectroscopy reveal that the ionized interface is more strongly impacted by the local solvent environment, thus establishing that the initial static configuration of the proton interface is a critical determinant to the kinetics of PCET.
Viral antigens were fixed to the surface of microtiter wells, and serial dilutions of antiviral antibody were added. The amount of antiviral antibody bound to viral antigens was determined by measuring the extent to which the antiviral antibody either inhibited the specific binding of 125I-labeled antiviral immunoglobulin G (IgG) (direct technique) or enhanced the specific binding of 125I-labeled anti-IgG (indirect technique). Immune complexes composed of viral antigens and antiviral antibody (human) could be detected by the binding of 125I-labeled rheumatoid factor. Specific binding was influenced by the concentration of protein in the diluents used during the different steps of the procedure. A high concentration of protein in the diluent used with the viral antigens decreased specific binding, whereas a high concentration of protein in the diluent used with 125I-labeled anti-IgG increased specific binding by decreasing nonspecific attachment of the labeled anti-IgG. Under the conditions employed, the titer of a given antiviral serum was several hundredfold greater by the indirect than by the direct technique.