Carborane radical anions: spectroscopic and electronic properties of a carborane radical anion with a 2n + 3 skeletal electron count

One-electron reduction of the well-known carborane 1,2-Ph2-1,2-C2B10H10 (1) gives rise to a stable carborane radical anion ([1]-) with a true 2n + 3 cluster electron count; the geometry of ([1]-) features an elongated C...C cage distance but no significant pi-bonding interactions between the cage and the phenyl substituents.     

Spectroscopic identification of a number of radiation—produced alkane radical cations

Absorption bands were recorded after irradiation of protiated and deuterated octane and of different tridecane isomers in pentane, 3-methylpentane, 1,1,2-trichlorotrifluoroethane and 1,1-difluorotetrachloroethane at 77 K with the aim of gaining information concerning the identity of the trapped ions. Maxima of absorption bands obtained after irradiation of protiated and deuterated octane in these matrices are all situated between 575 nm and 605 nm — (deuteration induces a blue shift of about 20 nm) —, whereas band maxima lie between 770 and 975 nm in the case of the tridecane isomers. Similar results were obtained in fluorotrichloromethane and 1,1,1-trichlorotrifluoroethane which were used as matrix in some cases. Effects of branching appear to be quite specific for the alkane additives, irrespective of the matrix employed. Overall, results are not consistent with an attribution of these absorption bands to condensation ions. On the basis of these and earlier results, the observed absorption bands may thus conclusively be attributed to radical cations of the alkane additives.     

Radiation-induced hydrogen fluoride formation in fluorine-containing ethanes and ethenes

Several fluorine-containing ethanes (monofluoro, 1,1-difluoro, 1,1,1-trifluoro, 1,1,2-trifluoro, 1,1,2,2-tetrafluoro, and pentafluoro) and ethenes (1,1-difluoro and trifluoro) form hydrogen fluoride when irradiated with gamma rays in the gas phase at 25°C. Hydrogen fluoride is apparently formed from fluoroethanes by a mechanism which involves formation of an intermediate semiion pair. We observed identical HF yields both in the absence and presence of molecular oxygen, except for monofluoroethane. A reduction of G(HF) with increasing sample pressure, for example, of 1,1,2,2-tetrafluoroethane, indicates that collisional stabilization of excited fluoroethane molecules competes with the process of HF elimination. High G values for HF and CO₂ in mixtures of CF₂?CFH and O₂ reveal the occurrence of a chain reaction.     

The use of freons and perfluoro (cyclo) alkanes as matrix in spectroscopic studies of radiation-produced radical cations

A study is made of the suitability of different freons (fluorotrichloromethane, 1,1,1- and 1,1,2-trichlorotrifluoroethane and 1,1-difluorotetrachloroethane) and perfluoro (cyclo) alkanes (perfluorohexane, perfluoromethylcyclohexane and perfluorodecaline) as matrix in spectroscopic investigations of radiation-produced radical cations. After gamma irradiation of the freon matrices, containing small amounts of alkane additives, complex optical absorption spectra are obtained. Deconvolution of these spectra is possible by successive illuminations with suitable wavelengths. Details of such bleaching procedures, which yield pure spectra of the alkane radical cations studied, are described. Spectra and bleaching procedures in the perfluoro (cyclo) alkane matrices are less complex, because the absorption of the irradiated pure matrices is much less pronounced and situated primarily at short wavelengths.     

Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations : The diphenylmethyl system

The photosensitized (electron transfer) reaction of methyl 2,2-diphenylethyl ether (1), 1,1,2,2-tetraphenylethane (5), 2-methyl-1,1,2-triphenylpropane (6), and 2-methoxy-2-diphenylmethylnorbornane (11 endo and exo) with 1,4-dicyanobenzene (4) in acetonitrile-methanol leads to products indicating cleavage of an intermediate radical cation to give the diphenylmethyl radical and a carbocation. The diphenylmethyl radical is then reduced by the radical anion of the photosensitizer and protonated to yield diphenylmethane. The carbocation fragment reacts with methanol to yield ether and/or acetals. The effect of temperature on the efficiency of cleavage of 5 and 6 has been analyzed. The increase in efficiency observed at higher temperatures reflects an activation energy for the cleavage of the radical cations. In cases where no cleavage is observed, the activation energy for cleavage may be so high that back electron transfer from the radical anion of the pbotosensitizer is the dominant reaction. The C—C bond dissociation energies of the radical cations of 5 and 6 were estimated by analysis of the thermochemical cycle using the bond dissociation energies and the oxidation potentials of the neutral molecules and the oxidation potential of the diphenylmethyl and cumyl radicals. The direction of cleavage of the radical cation is explained in terms of the relative oxidation potentials of the two possible radicals.     

Formation of intramolecular poly(4-hydroxystyrene) dimer radical cation

Poly(4-hydroxystyrene) (PHS) has been used in lithography as a backbone polymer and is also a promising material for extreme-ultraviolet or electron beam lithography. The dynamics of PHS radical cations generated upon exposure to electron beam were investigated. The transient absorption of PHS was observed in the near-infrared region in p-dioxane solutions by pulse radiolysis. Charge resonance (CR) bands that represent pi-pi interaction between the two chromophores of the intramolecular PHS dimer radical cation were observed, whereas p-cresol shows no distinct CR band. Although the radical cations of phenol derivatives are known to be easily deprotonated, it was found that the dimer radical cation formation leads to less deprotonation by its charge resonance stabilization.     

Electroreduction of 9-fluoro-10-cyanoanthracene

The electroreduction of 9-fluoro-10-cyanoanthracene (1) was voltammetrically studied. In contrast to expectations, no halide elimination was observed at the radical anion level but after a radical ion coupling the dimer 9,9^′-bianthryl-10,10^′-dicarbonitrile (2) was formed in quantitative yield. The mechanism was studied using digital simulation. Thermodynamic and kinetic parameters were determined. The low activation energy of the dimerization step indicates a diffusion-controlled radical ion coupling, which produces a σ-dimer but not a π-dimer as often suggested in the literature.     

Dihydroxylation-based approach for the asymmetric syntheses of hydroxy-γ-butyrolactones

A method of preparing enantiopure hydroxy-γ-butyrolactones containing multiple contiguous stereocenters in high yield with good diastereoselectivity has been developed. Osmium tetroxide mediated dihydroxylation of a range of β-alkenyl-β-hydroxy-N-acyloxazolidin-2-ones results in formation of triols that undergo spontaneous intramolecular 5-exo-trig cyclization reactions to provide hydroxy-γ-butyrolactones. The stereochemistry of these hydroxy-γ-butyrolactones has been established using NOE spectroscopy, which revealed that 1-substituted, 1,1-disubstituted, (E)-1,2-disubstituted, (Z)-1,2-disubstituted, and 1,1,2-trisubstituted alkenes undergo dihydroxylation with anti-diastereoselectivity, while 1,2,2-trisubstituted systems afford syn-diastereoisomers. The synthetic utility of this methodology has been demonstrated for the asymmetric synthesis of the natural product 2-deoxy-D-ribonolactone.     

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.     

Selective Boryl‐Anion Migration in a Vinyl sp2−sp3 Diborane Induced by Soft Borane Lewis Acids

An intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B₂Pin₂)]^? using “soft” BR₃ electrophiles (BR₃=BPh₃ or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B?O activation. The BR₃‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B₂Pin₂ and BR₃ and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B₂Pin₂, 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}^? transfer to BPh₃, thus enabling simple access to unsymmetrical sp²?sp³ diboranes.     

PHOTO-CIDNP STUDY OF PYRIMIDINE DIMER SPLITTING II: REACTIONS INVOLVING PYRIMIDINE RADICAL ANION INTERMEDIATES

A series of photo-CIDNP (chemically induced dynamic nuclear polarization) experiments were performed on pyrimidine monomers and dimers, using the electron-donor Nα-acetyltryptophan (AcTrp) as a photosensitizer. The CIDNP spectra give evidence for the existence of both the dimer radical anion, which is formed by electron transfer from the excited AcTrp* to the dimer, and its dissociation product, the monomer radical anion. The AcTrp spectra are completely different from those obtained with an oxidizing sensitizer like anthraquinone-2-sulfonate, because of different unpaired electron spin density distributions in pyrimidine radical anion and cation. In the spectra of the anti (1,3-dimethyluracil) dimers, polarization is detected that originates from a spin-sorting process in the dimer radical pair, pointing to a relatively long lifetime of the dimer radical anions involved. Although the dimer radical anions of the 1,1′-trimethylene-bridged pyrimidines may have a relatively long lifetime as well, their protons have only very weak hyperfine interaction, which explains why no polarization originating from the dimer radical pair is detected. In the spectra of the bridged pyrimidines, polarized dimer protons are observed as a result of spin sorting in the monomer radical pair, from which it follows that the dissociation of dimer radical anion into monomer radical anion is reversible. A study of CIDNP intensities as a function of pH shows that a pH between 3 and 4 is optimal for observing monomer polarization that originates from spin-sorting in the monomer radical pair. At higher pH the geminate recombination polarization is partly cancelled by escape polarization arising in the same product.     

PHOTOCHEMISTRY, PHOTOPHYSICS, AND MECHANISM OF PYRIMIDINE DIMER REPAIR BY DNA PHOTOLYASE

Abstract— DNA photolyases photorepair pyrimidine dimers (PyroPyr) in DNA as well as RNA and thus reverse the harmful effects of UV-A (320–400 nm) and UV-B (280–320 nm) radiations. Photolyases from various organisms have been found to contain two noncovalently bound cofactors; one is a fully reduced flavin adenine dinucleotide (FADH^-) and the other, commonly known as second chromophore, is either methenyltetrahydrofolate (MTHF) or 8-hydroxydeazaflavin (8-HDF). The second chromophore in photolyase is a light-harvesting molecule that absorbs mostly in the near-UV and visible wavelengths (300–500 nm) with its high extinction coefficient. The second chromophore then transfers its excitation energy to the FADH^-. Subsequently, the photoexcited FADH^- transfers an electron to the Pyr<>Pyr generating a dimer radical anion (Pyr<>Pyr^-) and a neutral flavin radical (FADH^-). The Pyr<>Pyr^- is very unstable and undergoes spontaneous splitting followed by a back electron transfer to the FADH^-. In addition to the main catalytic cofactor FADH^-, a Trp (Trp277 in Escherichia coli ) in apophotolyase, independent of other chromophores, also functions as a sensitizer to repair Pyr <> Pyr by direct electron transfer.     

Stepwise charge separation and charge recombination in ferrocene-meso,meso-linked porphyrin dimer-fullerene triad

A meso,meso-linked porphyrin dimer [(ZnP)(2)] as a light-harvesting chromophore has been incorporated into a photosynthetic multistep electron-transfer model for the first time, including ferrocene (Fc), as an electron donor and fullerene (C(60)) as an electron acceptor to construct the ferrocene-meso,meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)(2)-C(60)). Photoirradiation of Fc-(ZnP)(2)-C(60) results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [(1)(ZnP)(2)] to the C(60) moiety to produce the porphyrin dimer radical cation-C(60) radical anion pair, Fc-(ZnP)(2)(*+)-C(60)(*-). In competition with the back electron transfer from C(60)(*-) to (ZnP)(2)(*+) to the ground state, an electron transfer from Fc to (ZnP)(2)(*+) occurs to give the final charge-separated (CS) state, that is, Fc(+)-(ZnP)(2)-C(60)(*-), which is detected as the transient absorption spectra by the laser flash photolysis. The quantum yield of formation of the final CS state is determined as 0.80 in benzonitrile. The final CS state decays obeying first-order kinetics with a lifetime of 19 micros in benzonitrile at 295 K. The activation energy for the charge recombination (CR) process is determined as 0.15 eV in benzonitrile, which is much larger than the value expected from the direct CR process to the ground state. This value is rather comparable to the energy difference between the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)) and the final CS state (Fc(+)-(ZnP)(2)-C(60)(*-)). This indicates that the back electron transfer to the ground state occurs via the reversed stepwise processes,that is, a rate-limiting electron transfer from (ZnP)(2) to Fc(+) to give the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)), followed by a fast electron transfer from C(60)(*-) to (ZnP)(2)(*+) to regenerate the ground state, Fc-(ZnP)(2)-C(60). This is in sharp contrast with the extremely slow direct CR process of bacteriochlorophyll dimer radical cation-quinone radical anion pair in bacterial reaction centers.     

Formation of highly stabilized intramolecular dimer radical cation and pi-complex of [3n]cyclophanes (n = 3, 5, 6) during pulse radiolysis

Formation of radical cation and charge-transfer complex of [3n]cyclophanes (n = 3, 5, 6) was investigated by transient absorption spectroscopy during pulse radiolysis. Radical cations of [3n]cyclophanes showed the charge resonance band around 700 nm which exhibited a blue-shift as the number of trimethylene bridges increased, indicating formation of highly stabilized intramolecular dimer radical cation of [3n]cyclophanes. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accord with the oxidation potential of [3n]cyclophanes.     

Preparation and emission spectra of polymeric 1,2-diketones

Monomers of the methacrylate type, viz. 1-[4-(2-methacroyloxyethoxy)phenyl] propandione-1,2 (7a) and 1-phenyl-2-[4-(2-methacroyloxyethoxy)phenyl] ethandione-1,2 (7b) having the 1,2-dicarbonyl chromophore in the side-chain, were synthesized. The soluble homopolymer of monomer 76 and copolymers of both monomers 7a and 7b with styrene and methyl methacrylate were prepared by radical polymerization in solution. The absorption and emission spectra of a model compound and the homopolymer showed that the 1,2-dicarbonyl chromophore behaved as an isolated unit. No fluorescence was observed for the model compound or the homopolymer in emission spectra of poly(methyl methacrylate)-doped films. Phosphorescence of low- and high-molecular carbonyls was quenched by ferrocene in solution. Comparison of Stern-Volmer constants indicates partial steric hindrance of energy transfer for high-molecular donor.     

Pyrimidine dimer splitting in covalently linked dimer-arylamine systems.

Cyclobutadipyrimidines (pyrimidine dimers) undergo photosplitting which is sensitized by electron donors. We prepared a series of compounds in which a dimer is directly linked to an arylamine, which acts as sensitizer for dimer splitting. Two diastereomers of the dimer-arylamine exhibited very different splitting efficiencies. Also studied were N-methyl, ring methoxy, as well as deuterated derivatives of the sensitizer. These dimer-arylamines had an absorption band with lambda max approximately 300 nm. In each case intramolecular photosensitization of dimer splitting was highly dependent on the solvent, ranging in one instance from phi spl = 0.02 in water to a high value of 0.31 in the least polar solvent mixture examined (1,4-dioxane: isopentane, 1:99). A mechanism is proposed which involves photoinduced electron transfer from arylamine to dimer and splitting of the dimer radical anion. The dependence of splitting on the solvent was rationalized on the basis of retardation of back electron transfer due to Marcus inverted behavior of the charge-separated species. Photolyases might achieve their high efficiency of dimer splitting in part by employing a hydrophobic active site to slow back electron transfer in a similar manner.     

Dimer radical cations of indole and indole-3-carbinol: localized and delocalized radical cations of diindolylmethane

Extending our previous study on the title species (J. Phys. Chem. A2010, 114, 6787), we investigated the dimer cations that are formed on oxidation of the glucobrassin derivatives indole-3-carbinol (I3C) and diindolylmethane (DIM) and of parent indole (I). Radiolysis in ionic liquid and Ar matrices shows that, at sufficiently high concentrations and/or on annealing the solid glasses, intense intermolecular charge-resonance (CR) absorption bands in the NIR herald the formation of sandwich-type dimer cations. The molecular and electronic structure of these species is modeled by calculations with the double-hybrid B2-PLYP-D density functional method which yields predictions in good accord with experiment. The radical cation of DIM also shows a CR band, but unlike in the case of I and I3C, its occurrence is not dependent on the concentration but instead on the solvent: in ionic liquid the CR band is initially absent and arises only on annealing, whereas in Ar matrices it is present from the outset and undergoes blue shifting and sharpening on annealing. These puzzling findings are rationalized on the basis of B2-PLYP-D calculations which predict that neutral DIM exists in the form of two conformers, present in different relative amounts in the two experiments, which on vertical ionization form distinct radical cations, a nonsymmetric one where the odd electron is largely localized on one of the two indole moieties and one with C(2) symmetry where charge and spin are completely delocalized over both halves of the molecule, thus giving rise to an intramolecular CR transition. On annealing, the nonsymmetric cation relaxes to a similarly delocalized structure with C(s) symmetry, thus explaining the observed increase and the shift of the CR band. We believe that DIM(?+) represents the first example of a radical cation which can exist under the same conditions as a localized and a delocalized complex cation.     

Gold(I)-dithioether supramolecular polymers: synthesis, characterization, and luminescence

A series of discrete compounds and supramolecular polymers were synthesized by self-assembly of dithioether building blocks and HAuCl4.3H2O. In complexes 1 {[AuL(1-Me)Cl], where L(1-Me) is bis(methylthio)methane} and 2 {[Au2L(2-Ph)Cl2], where L(2-Ph) is 1,2-bis(phenylthio)ethane}, adjacent units are connected via aurophilic interactions. Complex 1, a one-dimensional (1D) supramolecular polymer, and complex 2, a two-dimensional supramolecular network, both feature nearly linear [Au-Au-](infinity) chains. Complexes 4a, 4b, and 4c, all of which contain 1,3-bis(phenylthio)propane (L(3-Ph)), are polymorphs having the composition [Au2L(3-Ph)Cl2]. Complex 3 {[Au2L(1-Ph)Cl2], where L(1-Ph) is bis(phenylthio)methane}and complexes 4a and 4b consist of nearly identical 1D supramolecular polymers formed through Au-Au interactions. The third polymorph, 4c, is a molecular complex, as it does not have metal-metal interactions. Complex 5 {[Au2L(4-Ph)Cl2], where L(4-Ph) is 1,4-bis(phenylthio)butane} is also molecular. UV-vis spectra showed that the absorption bands of these complexes are allowed ligand-centered transitions between 230 and 260 nm. Complexes 1, 2, and 6 {[AuL(3-Me)Cl], where L(3-Me) is 1,3-bis(methylthio)propane} exhibited solid-state luminescence at 5 K with vibronic progressions and band maxima at approximately 570 nm. It is suggested that complex 6 contains [Au-Au-](infinity) chains.     

Neutral and charged excited states in polar organic films: origin of unusual electroluminescence in tri-p-tolylamine-based hole conductors

The photoluminescence (PL) and electroluminescence (EL) of thin films of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) are remarkably different. Similar PL and EL are instead observed in films of the closely related donors tri- p-tolylamine (TTA) and N, N'-diphenyl- N, N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD). Such films show a wide range of hole transport that depends on the morphology and on external parameters such as temperature and electric field. Restricted configuration-interaction calculations performed on TTA, TAPC, TPD, and radical ions of TTA indicate that the unusual EL of TAPC films is due to direct recombination from a low-lying charge-transfer (CT) state. The CT state is strongly stabilized by electrostatic interactions with the polar environment. Theory confirms that TAPC can be considered a dimer of TTA. The charge distributions of TTA (+) and TTA (-) indicate charge localization in the anion that rationalizes low electron mobility as well as a strong charge-induced-dipole stabilization of the CT state.     

Photophysics of the galvinoxyl free radical revisited

The photophysical properties of the free neutral radical galvinoxyl were studied by a combination of femtosecond time-resolved spectroscopy and quantum chemical calculations. The electronic absorption spectrum is dominated by an intense band at 430 nm that is ascribed to the D(9,10)← D(0) transitions. Upon photoexcitation at 400 nm, the population of the D(9,10) states decays within less than 200 fs to the electronic ground state. This ultrafast internal conversion does not involve intramolecular modes with large amplitude motion as the measured dynamics does not show any significant dependence on the environment, but is most probably facilitated by a high density of electronic states of different character. Depending on the solvent, a weak transient band due to the galvinoxylate anion is also observed. This closed-shell species, which is fluorescent although its deactivation is also dominated by non-radiative decay, is generated upon biphotonic ionization of the solvent and electron capture. The ultrashort excited-state lifetime of the galvinoxyl radical precludes photoinduced disproportionation previously claimed to be at the origin of the formation of both anion and cation.