Long-lived Radical Cation Research Articles

Generation and direct observation of a triplet arylnitrenium ion

Nitrenium ions are important reactive intermediates in both chemistry and biology. Although singlet nitrenium ions are well-characterized by direct methods, the triplet states of nitrenium ions have never been directly detected. Here, we find that the excited state of the photoprecursor partitions between heterolysis to generate the singlet nitrenium ion and intersystem crossing (ISC) followed by a spontaneous heterolysis process to generate the triplet p-iodophenylnitrenium ion (np). The triplet nitrenium ion undergoes ISC to generate the ground singlet state, which ultimately undergoes proton and electron transfer to generate a long-lived radical cation that further generates the reduced p-iodoaniline. Ab Initio calculations were performed to map out the potential energy surfaces to better understand the excited state reactivity channels show that an energetically-accessible singlet-triplet crossing lies along the N-N stretch coordinate and that the excited triplet state is unbound and spontaneously eliminates ammonia to generate the triplet nitrenium ion. These results give a clearer picture of the photophysical properties and reactivity of two different spin states of a phenylnitrenium ion and provide the first direct glimpse of a triplet nitrenium ion.

Completing a Charge Transport Chain for Artificial Photosynthesis.

A ruthenium polypyridyl chromophore with electronically isolated triarylamine substituents has been synthesized that models the role of tyrosine in the electron transport chain in photosystem II. When bound to the surface of a TiO2 electrode, electron injection from a Ru(II) Metal-to-Ligand Charge Transfer (MLCT) excited state occurs from the complex to the electrode to give Ru(III). Subsequent rapid electron transfer from the pendant triarylamine to Ru(III) occurs with an observed rate constant of ∼1010 s-1, which is limited by the rate of electron injection into the semiconductor. Transfer of the oxidative equivalent away from the semiconductor surface results in dramatically reduced rates of back electron transfer, and a long-lived (τ = ∼165 μs) triarylamine radical cation that has been used to oxidize hydroquinone to quinone in solution.

Combined static and dynamic quenching in micellar systems-closed-form integrated rate laws verified using a versatile probe.

We demonstrate that the 3-aminoperylene radical cation is a near-ideal probe for investigating kinetic and transport processes in SDS micellar systems. Its isolated generation by two-photon ionization at a wavelength where most quenchers are transparent (532 nm) is free from side reactions; no exit from the micelles is detectable on a millisecond timescale; and its unquenched lifetime is as long as 350 ms, thus allowing the study of quenching processes over a time frame spanning at least 7 orders of magnitude. The lipophilic antioxidant ascorbyl palmitate reconverts it to its parent compound through the interplay of static and fast dynamic intramicellar quenching as well as through subsequent slow intermicellar migration. Using this radical-cation probe, we have successfully validated closed-form expressions which we derived for the probe decay in all these situations. From these functions, we also obtained an exact and closed-form analytical result for Stern-Volmer experiments with combined static and dynamic intramicellar quenching.

Generation of long-lived methylviologen radical cation in the triplet-state mediated electron transfer in a β-cyclodextrin based supramolecular triad

A novel tris(bipyridyl)ruthenium–pyrene–methylviologen supramolecular triad was assembled through inclusion complexation of adamantane-linked Ru(II)–Py dyad in MV2+-linked β-cyclodextrin. Excitation of the Ru(II) chromophore populated its 3MLCT which upon energy transfer gave 3Py, which donates an electron to MV2+ to give Ru(II)–Py+–MV+. A second electron transfer then occurs from Ru(II) to Py+ to give the supramolecular Ru(III)–Py–MV+ charge separated state. Laser flash photolysis experiments confirmed formation of MV+ which exhibited 100μs lifetime. Steady state irradiation of the self-assembled system in the presence of sacrificial donor also led to formation of long-lived MV+.

Long-Lived Radical Cation Salts Obtained by Interaction of Monocyclic Arenes with Niobium and Tantalum Pentahalides at Room Temperature: EPR and DFT Studies

The 1:3 reactions of the alkoxy arenes 1,4-(MeO)2 C6 H4 and 1,4-F2 -2,5-(MeO)2 C6 H2 with TaF5 in chloroform at 40-50 °C resulted in formation in about 35 % yield of the long-lived radical cation salts [1,4-(MeO)2 C6 H4 ][Ta2 F11 ] (2 a) and [1,4-F2 -2,5-(MeO)2 C6 H2 ][Ta2 F11 ] (2 b), respectively. The non-alkoxy-substituted [arene][M2 X11 ] [M=Ta, X=F: arene=C6 H5 Me (2 c), 1,4-C6 H4 Me2 (2 d), C6 H5 F (2 e), C6 H5 NO2 (2 f); M=Nb, X=F: arene=C6 H5 Me (4 a), 1,4-C6 H4 Me2 (4 b), C6 H5 F (4 c), C6 H5 NO2 (4 d); M=Ta, X=Cl: arene=1,4-C6 H4 Me2 (5)] were obtained from the 3:1 reactions of MX5 with the appropriate arene in chloroform at temperatures in the range 40-90 °C. Compounds 2-5 were detected by EPR spectroscopy (in CHCl3 ) at room temperature, and their gas-phase structures were optimized by DFT calculations. Formation of the M(IV) species [MX4 (NCMe)2 ] [M=Ta, X=F (3 a); M=Nb, X=F (3 b); M=Ta, X=Cl (3 c)] was ascertained by EPR spectroscopy on solutions obtained by treatment of the reaction mixtures with acetonitrile. Non-selective reactions occurred upon combination of 1,4-F2 -2,5-(MeO)2 C6 H2 with AgNbF6 (in CH2 Cl2 ) and 1,4-(MeO)2 C6 H4 with SbF5 .

Efficient Oxidative Cycloreversion Reaction of Photochromic Dithiazolythiazole

Electrochemical response of photochromic tearylenes was surveyed by means of cyclic voltammetry, DFT calculations, and spectroelectrochemistry. 4,5-Bis(2-phenyl-5-methylthiazolyl)-2-phenylthiazole was found to show electrochemical oxidative ring-cycloreversion reaction. The net current efficiency of the cycloreversion reaction under constant potential electrolysis was as high as 900%, which is ascribed to an electrochemical local-cell mechanism and a chain reaction mechanism. Electron transfer stopped-flow study using a chemical oxidant successfully identified radical cation intermediates of both closed- and open-ring isomers, involved in the oxidative cycloreversion process. The significantly long-lived radical cation of open-ring isomer with the lifetime of 33 s takes part in the indirect electron transfer process from the neutral closed-ring isomer to the radical cation of open-ring isomer in the chain reaction manner.

Coumarin 314 Free Radical Cation: Formation, Properties, and Reactivity toward Phenolic Antioxidants

We have explored the photogeneration of the coumarin 314 radical cation by using nanosecond laser excitation at wavelengths longer than 400 nm in benzene, acetonitrile, dichloromethane, and aqueous media. In addition, time-resolved absorption spectroscopy measurements allowed detection of the triplet excited state of coumarin 314 (C(314)) with a maximum absorption at 550 nm in benzene. The triplet excited state has a lifetime of 90 μs in benzene. It is readily quenched by oxygen (k(q) = 5.0 × 10(9) M(-1) s(-1)). From triplet-triplet energy transfer quenching experiments, it is shown that the energy of this triplet excited state is higher than 35 kcal/mol, in accord with the relatively large singlet oxygen quantum yield (Φ(Δ) = 0.25). However, in aqueous media, the coumarin triplet was no longer observed, and instead of that, a long-lived (160 μs in air-equilibrated solutions) free radical cation with a maximum absorbance at 370 nm was detected. The free radical cation generation, which has a quantum yield of 0.2, occurs by electron photoejection. Moreover, density functional theory (DFT) calculations indicate that at least 40% of the electronic density is placed on the nitrogen atom in aqueous media, which explains its lack of reactivity toward oxygen. On the other hand, rate constant values close to the diffusion rate limit in water (>10(9) M(-1) s(-1)) were found for the quenching of the C(314) free radical cation by phenolic antioxidants. The results have been interpreted by an electron-transfer reaction between the phenolic antioxidant and the radical cation where ion pair formation could be involved.

Study on Reactions of Long-Lived Phenoxathiin Radical Cation with Aliphatic Alcohols, Phenol and Phenyl Halides in Ambient Condition by Fused-Droplet Electrospray Ionization Mass Spectrometry

he reactions of phenoxathiin radical cations with diverse organic compounds in ambient conditions were realized by using fused-droplet electrospray ionization mass spectrometry. In the investigation, the phenoxathiin radical cation was prepared by electrospray ionization. The reactants included aliphatic alcohols, phenol and phenyl halides and the reaction studies showed the unique reactivity the of phenoxathiin radical cation towards neutral organic compounds in ambient conditions, which has not been revealed in previous studies.

Optical, Electrochemical, and Photovoltaic Effects of an Electron-Withdrawing Tetrafluorophenylene Bridge in a Push–Pull Porphyrin Sensitizer Used for Dye-Sensitized Solar Cells

We have prepared a novel push–pull porphyrin with an electron-donating diarylamino group at the meso-position and an electron-withdrawing 4-carboxy-2,3,5,6-tetrafluorophenylethynyl anchoring group at the opposite meso-position to address the substitution effects of the electron-withdrawing fluorine atoms on photovoltaic properties for the first time. The fluoro-substituted porphyrin showed slightly improved light-harvesting properties when compared to the reference porphyrin in solution, due to the enhancement of charge-transfer character. The fluoro-substituted porphyrin-sensitized TiO2 cell exhibited a moderate power conversion efficiency of 4.6% but was lower than the corresponding value (6.9%) of the reference porphyrin-sensitized TiO2 cell. The carboxylic group in the reference porphyrin binds to the TiO2 surface with bidentate coordination, leading to formation of the densely packed monolayer on the TiO2 surface, whereas both of the electron-withdrawing fluorine atoms and the carboxylic group of the fluoro-substituted porphyrin interact with the TiO2 surface, adopting relatively parallel orientation to the TiO2 surface with monodentate binding of the carboxylic acid to the TiO2, eventually yielding to the small surface coverage and low cell performance due to the fast charge recombination and small amount of the long-lived porphyrin radical cation. Such fundamental information may be useful for the molecular design of highly efficient dye-sensitized solar cells based on push–pull porphyrins.

Electron-Transfer-Induced Intermolecular [2 + 2] Cycloaddition Reactions Based on the Aromatic “Redox Tag” Strategy

Novel electron-transfer-induced intermolecular [2+2] cycloaddition reactions between an aliphatic cyclic enol ether and several unactivated olefins have been demonstrated on the basis of the aromatic "redox tag" strategy. The aromatic "redox tag" was oxidized during the formation of the cyclobutane ring, affording the relatively long-lived aromatic radical cation, which was then reduced to complete the overall reaction that constructed the corresponding [2+2] cycloadducts. The aromatic "redox tag" was also found to facilitate electron-transfer-induced cycloreversion reactions of cyclobutane rings.

Room-temperature long-lived [Nb 2F 11] − salts of radical cations of simple arenes: EPR, UV–Vis and DFT results

The computed structures of the long-lived radical cation salts [Arene][Nb 2F 11] [Arene = 1,4-F 2-2,5-(OMe) 2C 6H 2, 2; 1,4-(OMe) 2C 6H 4, 3; 2,5-(OEt) 2(Me)C 6H 3, 4; C 6H 6, 5] and that of the transient [1,3-(OMe) 2C 6H 4][Nb 2F 11], 6, obtained for the gas-phase by DFT at the B3LYP/6-31G∗∗ level, are presented. The degree of inertness observed in chloroform solution seems to increase on decreasing the steric demand of the ring substituents, and may be correlated to the calculated distance between the cation-centroid and the niobium atoms. The room-temperature EPR spectra of 2– 4, in CHCl 3, are described in detail; the spectrum of 3 is compared to those of analogous 1,4-dimethoxybenzene radical species reported previously. The EPR spectra display a hyperfine structure due to coupling of the unpaired electron with nuclei belonging to both the cation (H and F in the case of 2) and the anion (F and eventually Nb). The UV–Vis spectra of 2– 4 exhibit one strong absorption attributed to the cation and one anion-to-cation charge-transfer band (e.g. for 3 at 398 and 589 nm, respectively). Thermodynamic calculations indicate that the low yield formation of the benzene radical salt 5 occurs with Gibbs free energy variation significantly higher than those involved in the synthesis of 2– 4 and 6.

Gas-Phase Fragmentation of Long-Lived Cysteine Radical Cations Formed Via NO Loss from Protonated S-Nitrosocysteine

In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with S-nitrosoglutathione followed by transfer of the resulting S-nitrosocysteine into the gas phase by electrospray ionization mass spectrometry (ESI-MS). Independent of the way it was formed, protonated S-nitrosocysteine readily fragments via bond homolysis to form a long-lived radical cation of cysteine (Cys(*+)), which fragments under collision-induced dissociation (CID) conditions via losses in the following relative abundance order: *COOH CH(2)S >> *CH(2)SH approximately = H(2)S. Deuterium labeling experiments were performed to study the mechanisms leading to these pathways. DFT calculations were also used to probe aspects of the fragmentation of protonated S-nitrosocysteine and the radical cation of cysteine. NO loss is found to be the lowest energy channel for the former ion, while the initially formed distonic Cys(*+) with a sulfur radical site undergoes proton and/or H atom transfer reactions that precede the losses of CH(2)S, *COOH, *CH(2)SH, and H(2)S.

Response: Why had long-lived electron-transfer states of donor-substituted 10-methylacridinium ions been overlooked? Formation of the dimer radical cations detected in the near-IR region

Photoexcitation of an acetonitrile solution of the 9-(1-naphthyl)-10-methylacridinium ion results in formation of the electron-transfer state that forms the long-lived dimer radical cation with the acridinium ion, which was clearly detected as the transient absorption spectrum in the near-IR region.

Long-Lived Radical Cation−Electron Pairs Generated by Anthracene Sorption in Non Brønsted Acidic Zeolites

The sorption of anthracene (ANT) in non Brønsted acidic ZSM-5 zeolite through the mere exposure at room temperature of solid ANT and dehydrated zeolite crystals with Li(3.4)(AlO2)n(SiO2)(96-n) chemical formulas per unit cell generates spontaneous ionization of ANT (IP 7.44 eV in the gas phase). In contrast, ANT was found to be sorbed as an intact molecule in M(3.4)ZSM-5 with M = Na+, K+, Rb+, and Cs+. The radical cation (ANT*+) of the long-lived ANT*+@Li(3.4)ZSM-5*- pair was characterized by conventional diffuse reflectance UV-visible and resonance Raman spectrometry. In contrast, the X-band continuous wave (CW) EPR signal was found to be typical of a weakly coupled spin correlated ion pair. The two-dimension hyperfine sublevel correlation (2D-HYSCORE) spectra provide a detailed description of the microenvironment of the trapped electron of the ANT*+@Li(n)ZSM-5*- pair. The trapped electron appears localized in close proximity of occluded ANT*+, Li+, and the Si-O-Al nearest group of the zeolite framework.

Selective solid state photooxidant.

Irradiation of biphenyl encapsulated in the cavities of a NaZSM-5 zeolite framework has been reported to result in the formation of an extremely long-lived radical cation. Here, we show that such zeolite encapsulated radical cations can act as irreversible one-electron oxidants for simple alkenes and dienes, in a solid-state analogue to solution-phase cosensitization. Compared to the well-known semiconductor photooxidizers, such as titanium dioxide, the NaZSM-5 zeolite-based solid photooxidants exhibit enhanced selectivity based on oxidation potential, molecular size and shape, and Lewis base character.

Use of N, N -dimethyl- p -phenylenediamine to Evaluate the Oxidative Status of Human Plasma

Oxidative stress has been clearly implicated in human disease by a growing body of scientific evidences. There is no ideal method for the measurement of this parameter. A possible strategy would be to measure simultaneously several biomarkers representing damage to different cellular components or, alternatively, a method able to evaluate the hydroperoxides, intermediate products of oxidation originating from different classes of molecules, such as lipids, peptides, amino acids, etc. can be used. We are introducing a simple, rapid and inexpensive assay to measure the oxidative status of human plasma. It is based on the properties of N, N -dimethyl- p -phenylenediamine (DMPD), a compound able to produce a fairly long-lived radical cation. The absorbance at 505 nm of a DMPD solution in the presence of plasma, which is proportional to the amount of hydroperoxyl compounds, is related to the oxidative status of the sample and could be expressed as hydrogen peroxide equivalents (HPE). This assay was not influenced by freezing-thawing and storage time of the plasma samples. The assay can be automated, performed in a kinetic mode, and used for routine analyses. The DMPD assay alone or in combination with analytical methods for assessing antioxidant capacity is suggested as a reliable tool to obtain information in pathologies related to oxidative stress.

Fragmentation behavior of Si2Cln+ cations (n = 1—6)

Sector-field mass spectrometry was used to probe the fragmentation patterns of the cationic silicon chlorides Si2Cln+ (n = 1—6). For almost all Si2Cln+ ions, Si—Si fragmentation predominates the Si—Cl bond cleavage both in the metastable ion and collisional activation mass spectra. Analysis of the fragmentation patterns indicates that the long-lived radical cation Si2Cl6·+ corresponds to a complex [SiCl2·SiCl4]·+ rather than the intact molecular ion of hexachlorodisilane. The behavior of Si2Cl5+ is consistent with the formation of the (trichlorosilyl)dichlorosilyl cation Cl3SiSICl2+. Structural aspects are also discussed for the other Si2Cln+ species. A semi-quantitative analysis of the fragmentation patterns in conjunction with the literature thermochemistry data was used to estimate some thermochemical properties of the Si2Cln+ cations.

Definitive evidence for the existence of a long-lived vinylidene radical cation, H2C=C+.

Charge inversion mass spectra of C2H2+ ions produced by electron impact from HC≡CH and CH2=CCl2 were measured using K and Cs targets. Clear differences in the charge inversion spectra between HC≡CH and CH2=CCl2 indicate that the C2H2+ ion formed from CH2CCl2 is H2C=C+⋅. The lifetime of H2C=C+⋅ is found to be longer than 8.5 μs, and the state is proposed to be the 2B1 ground-state rather than the 4A2 state.

The radical cation of trimethylphosphine oxide

The structure of the gaseous long-lived radical cation generated upon electron ionization of trimethylphosphine oxide, (CH 3) 3PO, has been investigated by using ion-molecule reactions in a Fourier transform ion cyclotron resonance mass spectrometer. A radical cation with the connectivity of trimethylphosphine oxide is expected to react by facile electron transfer with triethylamine, pyridine and dimethyl disulfide since all these reactions are highly exothermic. However, no electron transfer reactions were observed. Instead, the radical cation transfers a proton to triethylamine and to pyridine, i.e., acts as a Brønsted acid. Further, the radical cation abstracts CH 3S from dimethyl disulfide and hence demonstrates behavior characteristic of a distonic ion with a carbon radical center. This reactivity is unprecedented for a radical cation such as (CH 3) 3P +-O with the odd spin located at an oxygen atom. These experimental results indicate that the initially generated radical cation (CH 3) 3P +-O undergoes unimolecular isomerization to (CH 3) 2P +(OH)CH 2 within a millisecond time frame. Ab initio molecular orbital calculations carried out at the unrestricted second-order Møller-Plesset (UMP2/6-31G** + ZPVE) level of theory support this conclusion by predicting that (CH 3) 2P +(OH)CH 2 lies 23 kcal mol −1 lower in energy than (CH 3) 3P +-O. The energy barrier for unimolecular [1,3]-hydrogen atom migration in (CH 3) 3P +-O is estimated to be 24 kcal mol −1. This study demonstrates that the PO moiety provides a very strong driving force for hydrogen shifts in phosphorus containing radical cations.

Bond dissociation energy of the radical cation dimers of diethyl sulfide, di- n-propyl sulfide and di- n-butyl sulfide

Long-lived radical cation dimers of diethyl sulfide, di- n-propyl sulfide and di- n-butyl sulfide have been prepared via an association reaction of the dialkyl sulfide radical cation with the dialkyl sulfide in the cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Under low energy collisionally induced dissociation (CID) conditions the studied radical cation dimer dialkyl sulfides, bonded via a two-center three-electron (2c/3e) S-S bond, are found to dissociate exclusively back to the monomer reactants resulting in the detection of the product radical cations of the corresponding dialkyl sulfides. Translational energy resolved CID has led to the determination of S-S bond dissociation energies (BDE) for (Et 2S) 2 +, ( n-Pr 2S) 2 +, and ( n-Bu 2S) 2 + of 1.19 ± 0.10 eV, 1.01 ± 0.10 eV, and 0.92 ± 0.10 eV, respectively. These BDEs clearly show a significant trend which indicates that the 2c/3e S-S bond becomes weaker with larger alkyl groups. Weakening of the 2c/3e S-S bond indicates a decrease of the σ/σ * orbital energy splitting of the 2c/3e S-S bond, which evidently is induced by the enhanced electron donation from the larger sulfide alkyl groups. Based on the determined bond dissociation energies, heats of formation of 530, 453 and 373 kJ mol −1 have been estimated for (Et 2S) 2 +, ( n-Pr 2S) 2 +, and ( n-Bu 2S) 2 +, respectively.