Spectra Of Radical Anions Research Articles

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex

Dynamics of metal to ligand charge transfer in the excited states of ruthenium polypyridyl complexes, which have shown promise as materials for artificial solar energy harvesting, has been of immense interest recently. Mixed ligand complexes are especially important for broader absorption in the visible region. Dynamics of ultrafast vibrational energy relaxation and inter-ligand charge transfer processes in the excited states of a heteroleptic ruthenium complex, [Ru(bpy)2(pap)](ClO4)2 (where bpy is 2,2'-bipyridine and pap is 2-(phenylazo)pyridine) have been investigated using femtosecond to nanosecond time-resolved transient absorption spectroscopic techniques. A good agreement between the TA spectrum of the lowest excited (3)MLCT state of [Ru(bpy)2(pap)](ClO4)2 complex and the anion radical spectrum of the pap ligand, which has been generated using the pulse radiolysis technique, confirmed the charge localization at the pap ligand. While the lifetime of the inter-ligand charge transfer from the bpy to the pap ligand in the (3)MLCT state is about 2.5 ps, vibrational cooling of the pap-localized(3)MLCT state occurs over a much longer time scale with a lifetime of about 35 ps. Ultrafast charge localization dynamics observed here may have important consequences in artificial solar energy harvesting systems, which employ heteroleptic ruthenium complexes.

On the Radical Anion Spectra of Fullerenes C60 and C70

The radical anion spectra of C60 and C70 can be easily generated and studied in primary aliphatic amines. In these conditions the C60 _• and C70 _• species are accompanied by the dianions C60 2− and C70 2−. In the near infrared region the electronic transitions at 1075, 1058 and 991 nm were assigned to C60 _• while those at 1020–1030 nm and at 933 nm were assigned to the fullerene dianion C60 2−. The electronic transition of C70 _• occurs at 1370 nm and that of C60 2− at 1170 nm, but in primary aliphatic amines C70 also display a broad absorption band at 814–820 nm with other features certainly due to a specific interaction of the amine with C70. The behavior of C60 was studied with primary aromatic amines, but no interaction was found. Furthermore, solutions of C60 and C70 were studied with the secondary amine dioctylamine and with the tertiary amines triethylamine and N,N,N',N'-tetramethylethylenediamine (TMEDA). FT-IR spectra of selected fullerene-amine adducts were reported.

Electronic excitation spectra of radical anions of cyanoethylenes and cyanobenzenes: Symmetry adapted cluster–configuration interaction study

Electronic excitation spectra of the radical anions of cyanoethylenes (trans-dicyanoethylene and tetracyanoethylene) and cyanobenzenes (1,2-dicyanobenzene: o-DCNB, 1,3-dicyanobenzene: m-DCNB, and 1,4-dicyanobenzene: p-DCNB) were studied by the symmetry adapted cluster-configuration interaction (SAC-CI) method. Theoretical calculations predicted positive electron affinities for all the molecules in good agreement with the experimental observations. Electronic excitation spectra of open-shell radicals is a topic that has not been studied as much as such spectra of closed-shell molecules, but this can be easily addressed using SAC-CI theory. The present paper systematically describes the calculation procedures for radical anions by investigating several basis sets, including anion diffuse and Rydberg functions. The calculated excitation energies were in good agreement with the experimental UV∕NIR (near infrared region) spectra, which had been observed by one of the present authors in 2-methyltetrahydrofuran matrix frozen to transparent glassy solids at 77 K. For p-DCNB, the SAC-CI theoretical spectrum agreed particularly well with the experimental spectrum. An extremely weak π*(SOMO) - π* excitation at 1.41 eV predicted in the present work, but had been overlooked in the previous experimental spectrum published in 1988, was confirmed to be real by a careful re-examination of the old spectrum.

Unraveling the Electron Spin Resonance Pattern of Nonsymmetric Radicals with 30 Fluorine Atoms: Electron Spin Resonance and Vis−Near-Infrared Spectroelectrochemistry of the Anion Radicals and Dianions of C60(CF3)2n (2n = 2−10) Derivatives and Density Functional Theory-Assisted Assignment

The charged states of C(60)(CF(3))(2n) (2n = 2-10) derivatives have been studied by electron spin resonance (ESR) and vis-near-infrared (NIR) spectroelectrochemistry. The anion radicals and diamagnetic dianions were furthermore described by theoretical calculations. The ESR spectra of anion radicals exhibit complex patterns due to multiple CF(3) groups. Their interpretation is accomplished by DFT calculations with B3LYP functional. It is shown that calculations provide reliable results when the extended aug-cc-pCVTZ basis set is used for fluorine atoms; however, specially tailored basis sets such as EPR-III also give very similar results with only a fraction of the computational cost. Absorption spectra of the anions exhibit NIR absorption bands, whose assignment is provided by time-dependent DFT calculations.

Mono- and biphotonic photochemistry in glass matrices

Photochemistry in hard glassy solvent matrices gives different results than in gas matrices. It is performed at 83, 77, and ≥10 K by continuous irradiation and by pulsed multi MW cm −2 peak intensity excitation for those systems that do not react monophotonically. The highly structured matrix spectra should be taken as a basis for the interpretation of transient spectra to avoid ambiguities. Numerous [2.2]paracyclophanes are photolyzed. Most of them give stable diradical and quinodimethane spectra in addition to fluorescence and phosphorescence. Some benzylic diradicals undergo chemiluminescence after their photochemical generation. Matrix isolation spectroscopy is at variance with common interpretations in the lepidopterene case. A [2+4]-photocycloreversion of a substituted cyclohexene at 83 K leads to diene stereoisomers/rotamers that isomerize upon further irradiation. E/Z-photoequilibria are obtained in MTHF matrix from both sides with ω-nitrostyrene and α-benzylidene-γ-butyrolactone at 83 K, the latter stereoisomerization was also successfully studied at 10 K. Pulsed irradiation of technical photostabilizers at 10 K leads to stable zwitterion formation by proton migration that cannot be seen by continuous excitation. Inter- and intramolecular donor acceptor systems provide stable charge separation at 15 or 77 K upon pulsed laser irradiation and radicalanion spectra are recorded. Biphotonic photochemistry at ≥10 K allows for the formation of new ring systems such as dioxathiirane ( cyclo-SO 2), several aryldioxaziridines, and an electron rich triaziridine, compounds that revert upon thawing and could not be obtained by continuous irradiation, except cyclo-SO 2 that can also be formed after absorption of the long lived SO 2 triplet by another two-photon process.

Photoinduced electron transfer between C 70 fullerene and 3,3′,5,5′-tetramethylbenzidine studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) in situ spectroscopy was applied in the study of photoinduced electron transfer between 3,3′,5,5′-tetramethylbenzidine (TMB) and C 70 in different solvent systems. The changes found in UV-vis spectra pointed at ground state charge transfer complex formation {C 70–TMB} in benzonitrile. Upon selective excitation of C 70 using steady-state monochromatic irradiation with a wavelength of 546 nm, two EPR singlets were observed, which were assigned to C 70 mono- and di-anion. In the photochemical and cathodic in situ reductions, identical EPR spectra of anion radicals were obtained. C 70 mono-anion was investigated also in frozen 1,2-dichlorobenzene solutions within the temperature range from 110 to 210 K, and at 110 K the anisotropic EPR spectrum of C 70 mono-anion was simulated assuming an axially symmetric g-matrix with g ‖− g ⊥=0.00165.

Effects of Lowering Symmetry on the ESR Spectra of Radical Anions of Fullerene Derivatives and the Reduction Potentials

Radical anions of dibenzyl adducts of C60, 1,4-(C6H5CH2)2C60•- and 1,2-(C6H5CH2)2C60•-, give ESR spectra which have larger g values (2.0004 and 2.0001, respectively) and much smaller line widths (ΔHmsl = 2.5 and 3.3 G, respectively, at 213 K) than the g value (1.9984) and the ΔHmsl value (30.9 G at 213 K) of C60•-. An even smaller ΔHmsl value (0.17 G) and a larger g value (2.0011) are observed for the tetrabenzyl C60 adduct radical anion, 1,4,10,24-(C6H5CH2)4C60•- and this is ascribed to a large splitting of the degenerate t1u orbitals caused by introduction of four benzyl groups to C60. In this case, a hyperfine structure, due to two nonequivalent protons of only one benzyl group (aH1 = 0.31 G, aH2 = 0.11 G), is observed and this is consistent with the predicted localized spin density at the C2 position next to the C1 carbon to which a benzyl group is attached. The radical anions of the mono- and bisadducts formed in the Diels−Alder cycloaddition reaction of C60 with 9,10-dimethylanthracene show ESR signals at different g values (2.0003 for the monoadduct and 2.0009 for the bisadduct). The relationship between lower symmetry and the ESR spectra of radical anions of various C60 derivatives is discussed in terms of the g values and the line widths. The energy gap (δ) between the singly occupied orbital and the two other orbitals which had a t1u symmetry prior to introduction of addends to C60 is derived from differences in the g values of various C60•- derivatives from the free spin value (2.0023). A linear correlation is shown to exist between log ΔHmsl and −δ. The δ values are also obtained from Arrhenius plots of ln ΔHmsl vs T-1 and they agree well with the values derived from the g values and the reduction potentials.

Electronic spectra of anion radicals of C 60 and its derivative in γ-irradiated MTHF solid solutions

The γ-dose dependence of the absorbance of the electronic absorption bands in the near-IR region of the anion radicals produced in MTHF solid solution containing C 60 at 77 K was investigated. The bands at λ max=1075 and 950 nm were ascribed to C 60 − and C 60 2− anion radicals, respectively. In the case of the solid solution containing [5,6]-bis(4-methoxyphenyl)methano[60]fullerene, which was used as a C 60 derivative, the bands at λ max=1075 and 1140 nm were ascribed to the mono-anion, and the band at λ max=1025 nm to the di-anion radical of the C 60 derivative. It is inferred that the unpaired electron density is delocalized on the C 60 moiety with a slight perturbation from the [5,6]-bis(4-methoxyphenyl)-methane functionality.

Cyclovoltammetric and EPR study of arylazophosphonates

Monofunctional arylazophosphonates p-X–C6H4N12N2PO(OCH3)21a–f with X=H (1a, 1b), Cl (1c), CH3O 2a–c with X=–(2a), O (2b) and CO (2c) were prepared. They showed relatively high photochemical and thermal stability. Monofunctional compounds 1a–f were reversibly reduced at the cathode in the region -0.85 to -1.35 V (vs. SCE) and the corresponding peak potentials correlated well with Hammett constants of the substituents. Bifunctional compounds 2a–c were reversibly reduced at -0.68 to -1.34 V. The first peak was reversible, and the second peak, following closely, was quasireversible. Generally, the anodic oxidation was irreversible. Using EPR spectroscopy, stable anion radicals were generated from the monomeric arylazophosphonates 1a–f. The unpaired spin density was centred on the azo and phosphonate group, with average splitting constants aN10.80, aN20.37 and aP0.88 mT, and nonequivalent ortho and meta protons aHo,o0.31 and 0.39 mT, aHm,m0.11 and 0.12 mT. Splitting constants aN1, aN2 and aP correlated well with the Hammett constants of the substituents X with a positive slope for aN2 and aP and a negative one for aN1. The EPR spectra of anion radicals from the bifunctional compounds 2a–c are more complex and indicate a consecutive rearrangement of primary anion radicals.

Photogenerated radical intermediates of vitamin K 1: a time-resolved resonance Raman study

Quinones play a vital role in the process of electron transfer in bacterial photosynthetic reaction centers. It is of interest to investigate the photochemical reactions involving quinones with a view to elucidating the structure–function relationships in the biological processes. Resonance Raman spectra of radical anions and the time-resolved resonance Raman spectra of vitamin K 1 (model compound for Q A in Rhodopseudomonas viridis, a bacterial photosynthetic reception center) are presented. The photochemical intermediates of vitamin K 1, viz. radical anion, ketyl radical and o-quinone methide have been identified. The vibrational assignments of all these intermediates are made on the basis of comparison with our earlier TR3 studies on radical anions of naphthoquinone and menaquinone.

ESR study of radicals formed by the reduction of α-diketones C(O)C(O)CF3 (R=(CF3)2CF, C6F5, (CF3)3C) with several group I–III metals

The ESR spectra of radical anions formed by reduction of α-diketones RC(O)C(O)CF3 (R=(CF3)2CF, C6F5, (CF3)3C) with metals (Li, Na, K, Mg, Cd, Zn, Hg, In, and TI) in THF were studied. For R=(CF3)2CF and C6F5, the radical anions are formed ascis-isomers, whereas for R=(CF3)3C,trans-isomers are obtained. Line broadening due to solvation and desolvation of the cation is observed in the latter case. The reduction of α-diketone (CF3)2CFC(O)C(O)CF3 with Group II metals (Mg, Cd, Zn) results in the formation of radical pairs.

EPR studies of pyrazoline radical ions that are potential precursors to non-Kekulé polyene radical ions

EPR spectra are reported for radical anion and (for the first time) radical cation derivatives of pyrazolines. The radical anion spectra are consistent with the unpaired electron being localized principally in the NN π*-orbital and this agrees both with theoretical calculations and literature precedents. For the radical canons of 4-alkylidenepyrazolines, the near degeneracy of the NN n–-and CC π-orbitals makes the distribution of the unpaired electron difficult to predict. The spectra obtained indicate that the unpaired electron is chiefly associated with the NN orbital. There is also significant density in the CC bond. This is underestimated by UHF-INDO calculations. In principle, an elimination of (neutral) nitrogen from these radical ions would generate non-Kekule polyene radical ions but so far this has not been achieved.

ESR spectra of radical anions ? Products of the reaction of fullerene with alkali metals

The ESR spectra of the products of the reaction of fullerene C60 with lithium, sodium, and potassium, including doping in time, were described. The formation of a radical anion with homogeneous reduction of fullerene by samarium iodide in solution was demonstrated.

Localization and migration of excess electron in aromatic substituted polyacetylenes in solution

Optical absorption spectra of radical anions of poly(1-phenyl-1-propyne)(PPPr) and poly(phenylacetylene)(PPA) were studied in 2-methyltetrahydrofuran (MTHF). The radical anions of the polymers were obtained by γ-radiolysis at 77 K (and 4 K) and pulse radiolysis at ambient temperature and 140 K. There were two different trapping sites for electrons in PPPr dissolved in MTHF. The electron produced by ionizing radiation was initially trapped by a site to form the radical anion which gave rise to an absorption spectrum with λ max at 390 and 560 nm and a shoulder at ca 800 nm. In liquids or warmed matrices, the spectrum relaxed to a new one which was similar to that of the radical anion of trans-β-methylstyrene, a model compound of the polymer. The result suggested that an electron trapped by a shallow site was ultimately localised on a deeper site of the size of a monomer unit. In contrast, the absorption spectrum of PPA - showed a strong IR band which was attributed to a dimer-type radical anion. It was concluded therefore that the excess electron was delocalized over two monomer units in PPA. The difference was interpreted in terms of the twisted main chain structure of PPPr arising from the repulsive interactions between bulky phenyl groups and methyl groups of the polymer.

Electron paramagnetic resonance spectra of aliphatic thioketyl radical anions in fluid solution

The first EPR spectra of the radical anions of simple aliphatic thioaldehydes and thioketones have been observed by photolysis of solutions of the corresponding potassium thiolate in tert-butyl alcohol.

Electron paramagnetic resonance spectra of aliphatic ketyl radical anions in fluid solution

The first EPR spectra of the radical anions of simple aliphatic thioaldehydes and thioketones have been observed by photolysis of solutions of the corresponding potassium thiolate in tert-butyl alcohol.

Electrochemistry and near-infrared spectra of anion radicals containing several imide or quinone groups

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectrochemistry and near-infrared spectra of anion radicals containing several imide or quinone groupsStanton F. Rak, Thomas H. Jozefiak, and Larry L. MillerCite this: J. Org. Chem. 1990, 55, 16, 4794–4801Publication Date (Print):August 1, 1990Publication History Published online1 May 2002Published inissue 1 August 1990https://doi.org/10.1021/jo00303a008RIGHTS & PERMISSIONSArticle Views922Altmetric-Citations53LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Absorption spectra of radical anions and anions of leuco forms of aminoanthraquinone dyes

Absorption spectra of radical anions and anions of leuco forms of aminoanthraquinone dyes

Investigation of EPR spectra of anion radicals of alkylenediphosphine dioxides and diaryl (carbamoylmethyl)phosphine oxides

It has been found that the length of the alkylene bridge in anion radicals of alkylenediphosphine dioxides influences the EPR spectra — an effect that is related to differences in structure of the ion pair. Temperature-dependent changes in the EPR spectra of anion radicals of diaryl (carbamoylmethyl)phosphine oxides are related to retarded rotation of the fragments of the anion radicals around the C-C bridge bond.

ChemInform Abstract: Electronic Absorption Spectra and Geometry of Anion Radicals Generated from (E)‐Stilbene and ′Stiff′ Stilbenes.

The electronic absorption spectra of anion radicals produced by γ-ray irradiation of (E)-stilbene (1), indeno[2,1-a]indene (2), 5,6,11,12-tetrahydrochrysene (3), (E)-1,1′-bi-indanylidene (4), and (E)-1,1′,2,2′,3,3′,4,4′-octahydro-1,1′-binaphthylidene (5) in glassy 2-methyltetrahydrofuran solution at 77 K have been measured. Examination of the spectra leads to the following conclusions. (a) The (E)-stilbene skeleton in the anion radical of (1) as well as of (2), (3), and (4) is planar or nearly planar; (b) in the anion radical of (5) immediately after its production, the ethylene bond and the C–Ph bonds are substantially twisted, and on controlled warming or on illumination, the twisting of the ethylene bond is increased.