Bipyridinium Radical Research Articles

Radical Pairing Interactions and Donor-Acceptor Interactions in Cyclobis(paraquat-p-phenylene) Inclusion Complexes.

Understanding molecular interactions in mechanically interlocked molecules (MIMs) is challenging because they can be either donor-acceptor interactions or radical pairing interactions, depending on the charge states and multiplicities in the different components of the MIMs. In this work, for the first time, the interactions between cyclobis(paraquat-p-phenylene) (abbreviated as CBPQTn+ (n = 0-4)) and a series of recognition units (RUs) were investigated using the energy decomposition analysis approach (EDA). These RUs include bipyridinium radical cation (BIPY•+), naphthalene-1,8:4,5-bis(dicarboximide) radical anion (NDI•-), their oxidized states (BIPY2+ and NDI), neutral electron-rich tetrathiafulvalene (TTF) and neutral bis-dithiazolyl radical (BTA•). The results of generalized Kohn-Sham energy decomposition analysis (GKS-EDA) reveal that for the CBPQTn+···RU interactions, correlation/dispersion terms always have large contributions, while electrostatic and desolvation terms are sensitive to the variation in charge states in CBPQTn+ and RU. For all the CBPQTn+···RU interactions, desolvation terms always tend to overcome the repulsive electrostatic interactions between the CBPQT cation and RU cation. Electrostatic interaction is important when RU has the negative charge. Moreover, the different physical origins of donor-acceptor interactions and radical pairing interactions are compared and discussed. Compared to donor-acceptor interactions, in radical pairing interactions, the polarization term is always small, while the correlation/dispersion term is important. With regard to donor-acceptor interactions, in some cases, polarization terms could be quite large due to the electron transfer between the CBPQT ring and RU, which responds to the large geometrical relaxation of the whole systems.

Role of N, N′‐Diboryl‐4, 4′‐bipyridinylidene in the Transition‐Metal‐Free Borylation of Aryl Halides and Direct C−H Arylation of Unactivated Benzene

AbstractN,N′‐Diboryl‐4,4′‐bipyridinylidene (BBiPy) has been utilized in sub‐stoichiometric amounts along with anionic base as one electron donor species for both transition metal‐free borylation of aryl halides and arene C−H functionalization with aryl iodides for biaryl syntheses. Reaction between BBiPy and potassium tert‐butoxide or methoxide has led to the formation of 4,4′‐bipyridine radical anion (BiPy⋅−) with the release of an electron. The BiPy⋅− has been structurally characterized. The single electron released is transferred to aryl halides generating aryl radicals, which then react with diboron in the presence of methoxide to form aryl boronate and with unactivated benzene in the presence of tert‐butoxide to form biaryls. Substrate scopes studies showed that aryl iodides and bromides undergo borylation (1B–12B), while only aryl iodides undergo C−C bond formation with arene C−H (1C–8C) through radical chain reaction. Mechanisms for C−B and C−C bond formation have been proposed based on the experimental findings.

Radically Enhanced Dual Recognition

AbstractComplexation between a viologen radical cation (V.+) and cyclobis(paraquat‐p‐phenylene) diradical dication (CBPQT2(.+)) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V.+ using radical‐pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+) in a size‐matched bisradical dicationic host — namely, cyclobis(paraquat‐2,6‐naphthalene)2(.+), i.e., CBPQN2(.+). Central to this dual recognition process was the choice of 2,6‐bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN2(.+) suitable for binding two MV.+ with relatively short (3.05–3.25 Å) radical‐pairing distances. The size‐matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex — namely, [(MV)2⊂CBPQN]4(.+) – in MeCN was confirmed by VT 1H NMR, as well as by EPR spectroscopy. The solid‐state superstructure of [(MV)2⊂CBPQN]4(.+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V.+, suggesting that localization of the radical‐pairing interactions has a strong influence on the packing of the two MV.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host‐guest radical‐pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox‐switchable recognition motif for the construction of MIMs and AMMs.

Radically Enhanced Dual Recognition.

Complexation between a viologen radical cation (V.+ ) and cyclobis(paraquat-p-phenylene) diradical dication (CBPQT2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V.+ using radical-pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+ ) in a size-matched bisradical dicationic host - namely, cyclobis(paraquat-2,6-naphthalene)2(.+) , i.e., CBPQN2(.+) . Central to this dual recognition process was the choice of 2,6-bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN2(.+) suitable for binding two MV.+ with relatively short (3.05-3.25 Å) radical-pairing distances. The size-matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex - namely, [(MV)2 ⊂CBPQN]4(.+) - in MeCN was confirmed by VT 1 H NMR, as well as by EPR spectroscopy. The solid-state superstructure of [(MV)2 ⊂CBPQN]4(.+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 Å) between the three different V.+ , suggesting that localization of the radical-pairing interactions has a strong influence on the packing of the two MV.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host-guest radical-pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox-switchable recognition motif for the construction of MIMs and AMMs.

A Donor-Acceptor [2]Catenane for Visible Light Photocatalysis.

Colored charge-transfer complexes can be formed by the association between electron-rich donor and electron-deficient acceptor molecules, bringing about the narrowing of HOMO-LUMO energy gaps so that they become capable of harnessing visible light. In an effort to facilitate the use of these widespread, but nonetheless weak, interactions for visible light photocatalysis, it is important to render the interactions strong and robust. Herein, we employ a well-known donor-acceptor [2]catenane-formed by the mechanical interlocking of cyclobis(paraquat-p-phenylene) and 1,5-dinaphtho[38]crown-10-in which the charge-transfer interactions between two 4,4'-bipyridinium and two 1,5-dioxynaphthalene units are enhanced by mechanical bonding, leading to increased absorption of visible light, even at low concentrations in solution. As a result, since this [2]catenane can generate persistent bipyridinium radical cations under continuous visible-light irradiation without the need for additional photosensitizers, it can display good catalytic activity in both photo-reductions and -oxidations, as demonstrated by hydrogen production-in the presence of platinum nanoparticles-and aerobic oxidation of organic sulfides, such as l-methionine, respectively. This research, which highlights the usefulness of nanoconfinement present in mechanically interlocked molecules for the reinforcement of weak interactions, can not only expand the potential of charge-transfer interactions in solar energy conversion and synthetic photocatalysis but also open up new possibilities for the development of active artificial molecular shuttles, switches, and machines.

Tuning radical interactions in trisradical tricationic complexes by varying host-cavity sizes.

Although host-guest pairing interactions between bisradical dicationic cyclobis(paraquat-p-phenylene) (BB2(˙+) ) and the bipyridinium radical cation (BIPY˙+ ) have been studied extensively, host molecules other than BB2(˙+) are few and far between. Herein, four bisradical dicationic cyclophanes with tunable cavity sizes are investigated as new bisradical dicationic hosts for accommodating the methyl viologen radical cation (MV˙+ ) to form trisradical tricationic complexes. The structure-property relationships between cavity sizes and binding affinities have been established by comprehensive solution and solid-state characterizations as well as DFT calculations. The association constants of the four new trisradical tricationic complexes are found to range between 7400 and 170 000 M-1, with the strongest one being 4.3 times higher than that for [MV⊂BB]3(˙+) . The facile accessibility and tunable stability of these new trisradical tricationic complexes make them attractive redox-controlled recognition motifs for further use in supramolecular chemistry and mechanostereochemistry.

An Efficient Viologen-Based Electron Donor to Nitrogenase.

Nitrogenase catalyzes the reduction of N2 to NH3, supporting all biological nitrogen fixation. Electron donors to this enzyme are ferredoxin or flavodoxin (in vivo) and sodium dithionite (in vitro). Features of these electron donors put a limit on spectrophotometric studies and electrocatalytic applications of nitrogenase. Although it is common to use methyl viologen as an electron donor for many low-potential oxidoreductases, decreased nitrogenase activity is observed with an increasing concentration of methyl viologen, limiting its utility under many circumstances. In this work, we suggest that this concentration-dependent decrease in activity can be explained by the formation of a dimer of the radical cation of methyl viologen (Me2V•+)2 at higher methyl viologen concentrations. In addition, viologens functionalized with positively and negatively charged groups were synthesized and studied using spectroscopy and cyclic voltammetry. A sulfonated viologen derivative, 1,1'-bis(3-sulfonatopropyl)-4,4'-bipyridinium radical {[(SPr)2V•]-}, was found to support full nitrogenase activity up to a mediator concentration of 3 mM, while the positively charged viologen derivative was not an efficient reductant of nitrogenase due to the high standard redox potential. The utility of [(SPr)2V•]- as an electron donor for nitrogenase was demonstrated by a simple, sensitive spectrophotometric assay for nitrogenase activity that can provide accurate values for the specific activity and turnover rate constant under argon. Under N2, the formation of ammonia was confirmed. Because of the observed full activity of nitrogenase and low overpotential, [(SPr)2V•]- should also prove to be valuable for nitrogenase electrocatalysis, including bioelectrosynthetic N2 reduction.

Cyclotris(paraquat-p-phenylenes).

Reported here is the synthesis, solid-state characterization, and redox properties of new triangular, threefold symmetric, viologen-containing macrocycles. Cyclotris(paraquat-p-phenylene) (CTPQT6+ ) and cyclotris(paraquat-p-1,4-dimethoxyphenylene) (MCTPQT6+ ) were prepared and their X-ray single-crystal (super)structures reveal intricate three-dimensional packing. MCTPQT6+ results in nanometer-sized channels, in contrast with its parent counterpart CTPQT6+ which crystallizes as a couple of polymorphs in the form of intercalated assemblies. In the solid state, MCTPQT3(.+) exhibits stacks between the 1,4-dimethoxyphenylene and bipyridinium radical cations, providing new opportunities for the manipulation and control of the recognition motif associated with viologen radical cations. These redox-active cyclophanes demonstrate that geometry-matching and weak intermolecular interactions are of paramount importance in dictating the formation of their intricate solid-state superstructures.

Cyclotris(paraquat‐p‐phenylenes)

AbstractReported here is the synthesis, solid‐state characterization, and redox properties of new triangular, threefold symmetric, viologen‐containing macrocycles. Cyclotris(paraquat‐p‐phenylene) (CTPQT6+) and cyclotris(paraquat‐p‐1,4‐dimethoxyphenylene) (MCTPQT6+) were prepared and their X‐ray single‐crystal (super)structures reveal intricate three‐dimensional packing. MCTPQT6+ results in nanometer‐sized channels, in contrast with its parent counterpart CTPQT6+ which crystallizes as a couple of polymorphs in the form of intercalated assemblies. In the solid state, MCTPQT3(.+) exhibits stacks between the 1,4‐dimethoxyphenylene and bipyridinium radical cations, providing new opportunities for the manipulation and control of the recognition motif associated with viologen radical cations. These redox‐active cyclophanes demonstrate that geometry‐matching and weak intermolecular interactions are of paramount importance in dictating the formation of their intricate solid‐state superstructures.

The Role of H-Bonding in Proton-Coupled Electron Transfer: The Reduction of N-Methyl-4,4’-Bipyridine Radical in Acetonitrile in the Presence of Acidic Alcohols

Proton-coupled electron transfer reactions (PCET) play a significant role in many chemical and biological mechanisms. However, although these reactions have been known and studied for over a century, it is only within the last few decades that the importance of H-bonding intermediates in these reactions has come to be appreciated. In previous studies, we examined how explicit consideration of electron transfer and proton transfer within a H-bond intermediate (the “wedge” scheme) could explain the voltammetry of oxidation reactions involving phenylenediamines and bases in acetonitrile. The focus of this new study is to see if the wedge scheme can also explain the chemically reversible PCET reaction due to addition of acidic alcohols to the N-methyl-4,4’-bipyridinium redox couple (commonly called “monoquat” or MQ) in acetonitrile. MQ, which starts out as a +1 cation, undergoes two reversible reductions in acetonitrile, first to an uncharged radical and then to a quinoidal anion. For this study, different types of acidic alcohols, such as trifluoromethanol, methanol, and dichloroethanol, were added at different concentrations. The cyclic voltammetry shows a positive shift in the E1/2 of the second reduction after adding the acidic alcohol as a guest. This is likely due to protonation of the monoquat anion by the alcohol. The overall reaction was expected to be MQ + ROH + e- = MQH + RO-, which would show a 60 mV per log[ROH] change in E1/2. However, analysis of the collected data contradicts this hypothesis by giving a 180 mV per log[ROH] E1/2 shift, indicating that three alcohols are involved in the overall reaction. Since it is highly unlikely that the MQ anion would be triply protonated, our current hypothesis is that the product conjugate base of the alcohol forms a strongly H-bonded complex with two other alcohols, making the overall reaction, MQ + 3ROH + e- = MQH + (RO-)(ROH)2. Results of on-going research to either disprove or provide further support for this hypothesis will be reported.

The Role of H-Bonding in PCET: The Chemically Reversible One Electron, Three Acid Reduction of N-Methyl-4,4'-Bipyridine Radical in Acetonitrile

Oxidation and reduction of organic redox-couples typically leads to huge changes in their acidity/basicity, with the result that proton transfer (PT) often accompanies electron transfer (ET). These types of reactions are known as proton-coupled-electron-transfer or PCET reactions. While the prevalence of PCET reactions in reversible organic redox transformations is something that has been recognized for over 100 years, it is only much more recently that it has become apparent that H-bonding intermediates often play a significant role in the mechanism of these reactions. One way in which H-bonding can do so is through what we have termed a “wedge-scheme” mechanism in which ET and PT occur through the H-bond intermediate formed between the redox-active acid or base and the added base or acid.1 This can provide an easier path for ET-PT in either direction and thus increase the reversibility of the overall reaction. Another means through which H-bonding can affect PCET is by stabilizing the products. In the PCET reaction to be discussed in this presentation, reduction of N-methyl-4,4'-bipyridinium (commonly called “monoquat” or MQ) in acetonitrile in the presence of acidic alcohols, it appears that both H-bond effects are present. MQ undergoes two reversible one electron reductions in acetonitrile, the first corresponding to reduction of the starting cation, MQ+, to an uncharged radical, MQ. This is followed by a second reduction at significantly more negative potentials of the radical to the anion, MQ−, which formally contains a negatively-charged nitrogen and is thus quite basic. Not surprisingly, CV studies show that addition of trifluoroethanol or dichloroethanol, both fairly acidic alcohols, result in no change in E1/2 of the first reduction but a significant positive shift in the E1/2 of the second reduction, most likely due to protonation of the anion. The second reduction wave is slightly broad, but continues to shift positive and remain chemically reversible up to at least 400 equivalents of added alcohol, at which point it has begun to merge with the first reduction. What is surprising, in addition to the chemical reversibility, is that analysis of the observed E1/2 as a function of added alcohol concentration is consistent with a single 1e− transformation involving three alcohols throughout the entire range. Since it is highly unlikely that the MQ anion would be triply protonated, our current hypothesis is that the product conjugate base of the alcohol forms a strongly H-bonded complex with two other alcohols, making the overall reaction MQ + 3ROH + e− = MQH + (RO−)(ROH)2. The mechanism would involve initial 1 e−, 1H+ transfer occurring through the H-bond intermediate (wedge scheme), followed by rapid formation of the homoconjugate complex (RO−)(ROH)2. The results of current and future work on this system to further elucidate the mechanism, and either support or refute the wedge-scheme/homoconjugate hypothesis, will be presented. 1 L. A. Clare, A. T. Pham, F. Magdaleno, J. Acosta, J. E. Woods and D. K. Smith, Journal of the American Chemical Society 2013, 135, 18930-18941.

Neighboring Component Effect in a Tri-stable [2]Rotaxane

The redox properties of cyclobis(paraquat- p-phenylene)cyclophane (CBPQT4+) render it a uniquely variable source of recognition in the context of mechanically interlocked molecules, through aromatic donor-acceptor interactions in its fully oxidized state (CPBQT4+) and radical-pairing interactions in its partially reduced state (CBPQT2(•+)). Although it is expected that the fully reduced neutral state (CBPQT(0)) might behave as a π-donating recognition unit, resulting in a dramatic change in its binding properties when compared with the other two redox states, its role in rotaxanes has not yet been investigated. To address this challenge, we report herein the synthesis of a tri-stable [2]rotaxane in which a CBPQT4+ ring is mechanically interlocked with a dumbbell component containing five recognition sites-(i) a bipyridinium radical cation (BIPY(•+)) located centrally along the axis of the dumbbell, straddled by (ii) two tetrafluorophenylene units linked to (iii) two triazole rings. In addition to the selective recognition between (iv) the CBPQT4+ ring and the triazole units, and (v) the CBPQT2(•+) ring and the reduced BIPY(•+) unit in the dumbbell component, investigations in solution have now confirmed the presence of additional non-covalent bonding interactions between the CBPQT(0) ring, acting as a donor in its neutral state, and the two tetrafluorophenylene acceptors in the dumbbell component. The unveiling of this piece of molecular recognition in a [2]rotaxane is reminiscent of the existence in much simpler, covalently linked, organic molecules of neighboring group participation (anchimeric assistance giving way to transannular interactions) in small-, medium-, and large-membered rings.

Interfacial electron transfer yields in dye-sensitized NiO photocathodes correlated to excited-state dipole orientation of ruthenium chromophores

Interface dynamics of nanocrystalline NiO thin films sensitized with two ruthenium polypyridyl chromophores have been investigated to examine the influence that excited-state dipole orientation and the position of the bipyridine radical formed in the charge-separated state have on interfacial electron transfer yields. In ultrafast transient absorption experiments, the charge separated state is observed on the nanosecond timescale for the trifluoromethyl-substituted chromophore, [Ru(flpy)2(dcb)]2+ (flpy = 4,4′-bis(trifluoromethyl)-2,2′-bipyridine, dcb = 4,4′-dicarboxy-2,2′-bipyridine), but not for [Ru(bpy)2(dcb)]2+ (bpy = 2,2′-bipyridine). Differences are attributed to the positioning of the bipyridine radical formed in the charge separated state; for [Ru(flpy)2(dcb)]2+, the electron is localized on the flpy ligand distal to the surface, whereas for [Ru(bpy)2(dcb)]2+, the electron is localized on the dcb ligand, proximal to the NiO surface. Enhanced photovoltaic performance is observed for dye-sensitized solar cell devices prepared with [Ru(flpy)2(dcb)]2+, demonstrating that enhanced charge separation can be correlated with device efficiency.

Densely Charged Dodecacationic [3]- and Tetracosacationic Radial [5]Catenanes

Summary Here, we report the efficient syntheses of a dodecacationic [3]catenane, which consists of three mechanically interlocked 4+ charged rings despite their mutual Coulombic repulsion, and a tetracosacationic radial [5]catenane, which consists of four 4+ charged rings mechanically interlocked around an 8+ charged ring and bears up to 24 positive charges in its co-constitution. The solid-state structures, determined by single-crystal X-ray crystallography, show that the two mechanical bonds in the [3]catenane result in the nano-confinement of 12 positive charges within a volume of less than 1.65 nm3, corresponding to 7.3 positive charges per nm3, whereas the radial [5]catenane has a charge density of 6.0 positive charges per nm3 and a molecular length of up to 3.7 nm. The template-directed strategy for constructing mechanically interlocked molecules, consisting of multiply charged rings, represents a departure in chemistry tantamount to producing a blueprint for exploring a novel class of organic molecules that boast large numbers and high densities of like charges.

The Role of H-Bonding in Pcet: The Chemically Reversible One Electron, Two Proton Reduction of N-Methyl-4,4'-Bipyridine Radical in Acetonitrile

Oxidation and reduction of organic redox-couples typically leads to huge changes in their acidity/basicity, with the result that proton transfer (PT) often accompanies electron transfer (ET). These types of reactions have come to be known as proton-coupled-electron-transfer or PCET reactions. While the prevalence of PCET reactions in reversible organic redox transformations is something that has been recognized for over 100 years, it is only much more recently that it has become apparent that H-bonding intermediates often play a significant role in the mechanism of these reactions. One way in which H-bonding can do so is through what we have termed a “wedge-scheme” mechanism in which ET and PT occur through the H-bond intermediate formed between the redox-active acid or base and the added base or acid.1 This can provide an easier path for ET-PT in either direction and thus increase the reversibility of the overall reaction. Previously we have demonstrated the utility of the wedge scheme in oxidation reactions involving phenylenediamines and bases.1,2 In new work, we are exploring the utility of the wedge scheme in explaining the chemically reversible PCET reaction occurring upon addition of acidic alcohols to the N-methyl-4,4'-bipyridinium redox couple (commonly called “monoquat” or MQ) in acetonitrile. This compound undergoes two reversible one electron reductions in acetonitrile, the first corresponding to reduction of the starting cation, MQ+, to an uncharged radical, MQ. This is followed by a second reduction at significantly more negative potentials of the radical to the anion, MQ-, which formally contains a negatively-charged nitrogen and is thus quite basic. Not surprisingly, cyclic voltammetry (CV) studies show that addition of trifluoroethanol (TFE), a fairly acidic alcohol, results in no change in E1/2 of the first reduction but a significant positive shift in the E1/2 of the second reduction, most likely due to protonation of the anion. The second reduction wave is slightly broad, but continues to shift positive and remain chemically reversible up to at least 400 equivalents of added alcohol, at which point it has shifted over 0.6 V and almost merged with the first reduction wave. What is perhaps surprising, in addition to the chemical reversibility, is that analysis of the observed E1/2 as a function of added alcohol concentration is consistent with a single 1e-, 2 H+ transformation (MQ + 2 HA + 1e- = MQH2+ + 2 A-) throughout the entire range. Qualitatively similar behavior observed with addition of alcohols to quinones in acetonitrile has commonly been attributed to strong H-bonding between the alcohol and the quinone dianion. However, a H-bonding only mechanism is not likely in this case both because of the expected basicity of MQ- and the uniformity of the reaction stoichiometry over the entire concentration range. (With H-bonding only, the stoichiometry would likely start 1:1, then go to 2:1 at higher concentrations.) On the other hand, a classic step-wise PCET mechanism, ET-PT-PT, can not explain the observed reversibility over the entire concentration range. What we believe can explain all of the above is initial 1 e-, 1H+ transfer occurring through the H-bond intermediate (wedge scheme), followed by the second 1H+ transfer. The results of current and future work on this system to further elucidate the mechanism, and either support or refute the wedge-scheme hypothesis, will be described in this presentation. These will include studies with other acids, CV simulations and analysis of deuterium isotope effects 1 L. A. Clare, A. T. Pham, F. Magdaleno, J. Acosta, J. E. Woods and D. K. Smith, Journal of the American Chemical Society 2013, 135, 18930-18941. 2B. T. Tamashiro, M. R. Cedano, A. T. Pham and D. K. Smith, Journal of Physical Chemistry C 2015, 119, 12865-12874.

Coulombic-enhanced hetero radical pairing interactions

Spin–spin interactions between two identical aromatic radicals have been studied extensively and utilized to establish supramolecular recognition. Here we report that spin-pairing interactions could also take place between two different π-electron radicals, namely a bipyridinium radical cation (BPY+•) and a naphthalene-1,8:4,5-bis(dicarboximide) radical anion (NDI─•). The occurrence of this type of previously unreported hetero radical-pairing interactions is attributed to enhancement effect of Coulombic attraction between these two radicals bearing opposite charges. The Coulombic-enhanced hetero radical pairing interactions are employed to drive host–guest recognition, as well as the reversible switching of a bistable [2]rotaxane.

Shuttling Rates, Electronic States, and Hysteresis in a Ring-in-Ring Rotaxane.

The trisradical recognition motif between a 4,4′-bipyridinium radical cation and a cyclo-bis-4,4′-bipyridinium diradical dication has been employed previously in rotaxanes to control their nanomechanical and electronic properties. Herein, we describe the synthesis and characterization of a redox-active ring-in-ring [2]rotaxane BBR·8PF6 that employs a tetraradical variant of this recognition motif. A square-shaped bis-4,4′-bipyridinium cyclophane is mechanically interlocked around the dumbbell component of this rotaxane, and the dumbbell itself incorporates a smaller bis-4,4′-bipyridinium cyclophane into its covalently bonded structure. This small cyclophane serves as a significant impediment to the shuttling of the larger ring across the dumbbell component of BBR8+, whereas reduction to the tetraradical tetracationic state BBR4(+•) results in strong association of the two cyclophanes driven by two radical-pairing interactions. In these respects, BBR·8PF6 exhibits qualitatively similar behavior to its predecessors that interconvert between hexacationic and trisradical tricationic states. The rigid preorganization of two bipyridinium groups within the dumbbell of BBR·8PF6 confers, however, two distinct properties upon this rotaxane: (1) the rate of shuttling is reduced significantly relative to those of its predecessors, resulting in marked electrochemical hysteresis observed by cyclic voltammetry for switching between the BBR8+/BBR4(+•) states, and (2) the formally tetraradical form of the rotaxane, BBR4(+•), exhibits a diamagnetic ground state, which, as a result of the slow shuttling motions within BBR4(+•), has a long enough lifetime to be characterized by 1H NMR spectroscopy.

Observation of Single Molecule Plasmon-Driven Electron Transfer in Isotopically Edited 4,4'-Bipyridine Gold Nanosphere Oligomers.

We clarify mechanistic questions regarding plasmon-driven chemistry and nanoscale photocatalysis within optically confined near-field plasmonic systems. Using surface-enhanced Raman scattering (SERS), we directly monitor the photoinduced reaction dynamics of 4,4'-bipyridine molecules, localized in plasmonic hot spots within individual gold nanosphere oligomers. Our experiment generates surface electrons from the gold nanoparticle using an intense off-molecular resonance continuous wave pump field, and detects radical anion products via SERS. This is done by adopting a dual-wavelength spectroscopic approach. Empirical evidence of plasmon-driven electron transfer is provided for the first time by direct detection of the 4,4'-bipyridine radical anion species localized in the plasmonic hot spots of individual gold nanosphere oligomers, corroborated by open-shell density functional theory calculations. An isotopologue approach using both protonated and deuterated 4,4'-bipyridine molecules demonstrates the single molecule response of plasmon-driven electron transfer occurring in single nanosphere oligomer systems with a 3% yield, a phenomenon unobserved in ensemble measurements under analogous experimental conditions. This mechanism has broad applicability to using nanoscale chemical reactors for surface redox reactions on the subnanometer scale.

Mechanical-Bond-Protected, Air-Stable Radicals.

Radical templation centered around a heterotrisradical tricationic inclusion complex DB•+⊂DAPQT2(•+), assembled from an equimolar mixture of a disubstituted 4,4'-bipyridinium radical cation (DB•+) and an asymmetric cyclophane bisradical dication (DAPQT2(•+)), affords a symmetric [2]catenane (SC·7PF6) and an asymmetric [2]catenane (AC·7PF6) on reaction of the 1:1 complex with diazapyrene and bipyridine, respectively. Both these highly charged [2]catenanes have been isolated as air-stable monoradicals and characterized by EPR spectroscopy. X-ray crystallography suggests that the unpaired electrons are delocalized in each case across two inner 4,4'-bipyridinium (BIPY2+) units forming a mixed-valence (BIPY2)•3+ state inside both [2]catenanes, an observation which is in good agreement with spin-density calculations using density functional theory. Electrochemical studies indicate that by replacing the BIPY2+ units in homo[2]catenane HC•7+-composed of two mechanically interlocked cyclobis(paraquat-p-phenylene) rings-with "zero", one, and two more highly conjugated diazapyrenium dication (DAP2+) units, respectively, a consecutive series of five, six, and seven redox states can be accessed in the resulting SC·7PF6 (0, 4+, 6+, 7+, and 8+), HC·7PF6 (0, 2+, 4+, 6+, 7+, and 8+), and AC·7PF6 (0, 1+, 2+, 4+, 6+, 7+, and 8+), respectively. These unique [2]catenanes present a promising prototype for the fabrication of high-density data memories.

Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2

We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2′-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2′-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2− in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6–2N]2N-4, where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.