Radical Chromophores Research Articles

Achieving Maximal Enhancement of Electron Spin Polarization in Stable Nitroxyl Radicals at Room Temperature.

Enhancing the polarization of spin levels at room temperature is one of the active research areas in magnetic resonance. Generation of electron spin hyperpolarization involves a complex interplay of electronic and spin processes. In this work, the optimization of crucial electron spin polarization (ESP) generating parameters and synthesis of a radical-chromophore adduct are described. The ESP of the synthesized adduct is about 550 times the equilibrium polarization at room temperature, which is possibly the maximal value for a chromophore-nitroxyl system. The present work highlights the crucial role of the photophysical quenching process toward the generation of a large ESP. Additionally, a chromophore-diradical adduct is synthesized, and the effects of the additional radical in the ESP generation process are discussed. Enhanced photochemical stability is demonstrated for the diradical adducts, thereby suggesting a potential route toward the generation of photostable radical-chromophore adducts for future studies. The large ESP in these molecules should enable a wide range of applications, such as in DNP, spintronics, and magnetometers.

Design and Photo-Induced Dynamics of Radical-Chromophore Adducts with One- or Two-Atom Separation: Toward Potential Probes for High Field Optical DNP Experiments.

Breaking the maximum enhancement barrier of 660 at room temperature in a conventional dynamic nuclear polarization (DNP) experiment has the immense potential of practical applications. Optical DNP experiments with radical-chromophore (RC) adducts, which harnesses hyperpolarized radicals, instead of thermalized radicals, offers a powerful way to achieve this. Typical DNP and NMR experiments, however, are carried out at high magnetic fields of about 5-10 T, whereas the large electron spin hyperpolarization (ESP) demonstrated in the RC adducts so far are at a much low field of 0.3 T. Thus, in order to realize a successful optical DNP experiment, it is imperative to ask whether the RC adducts, which are currently available, can achieve a large ESP even at high fields. The present work poses this question and shows that the current RC adducts would not generate a large ESP at high fields unless the separation between the chromophore and nitroxyl moiety is reduced to less than four bonds. Two serious bottlenecks in this direction are the near impossibility of synthesizing such RC adducts using the common nitroxyl radicals and the absence of any photophysical studies on RC adducts with such short spacer groups. In this regard, the present work exploits the spin trapping methodology to synthesize one- and two-atom separated naphthalene-nitroxyl RC adducts. Good yields and excellent stability of the adducts have been demonstrated. Furthermore, the present work presents their detailed photophysical and photochemical studies by transient optical and time-resolved EPR studies. On the basis of the present results, a potential RC adduct is proposed for the high field optical DNP experiments. Finally, the prospect of exploiting the large EPR signal enhancement due to ESP in the field of spin trapping studies has been discussed.

Exploiting exciton coupling of ligand radical intervalence charge transfer transitions to tune NIR absorption.

We detail the rational design of a series of bimetallic bis-ligand radical Ni salen complexes in which the relative orientation of the ligand radical chromophores provides a mechanism to tune the energy of intense intervalence charge transfer (IVCT) bands in the near infrared (NIR) region. Through a suite of experimental (electrochemistry, electron paramagnetic resonance spectroscopy, UV-vis-NIR spectroscopy) and theoretical (density functional theory) techniques, we demonstrate that bimetallic Ni salen complexes form bis-ligand radicals upon two-electron oxidation, whose NIR absorption energies depend on the geometry imposed in the bis-ligand radical complex. Relative to the oxidized monomer [1˙]+ (E = 4500 cm-1, ε = 27 700 M-1 cm-1), oxidation of the cofacially constrained analogue 2 to [2˙˙]2+ results in a blue-shifted NIR band (E = 4830 cm-1, ε = 42 900 M-1 cm-1), while oxidation of 5 to [5˙˙]2+, with parallel arrangement of chromophores, results in a red-shifted NIR band (E = 4150 cm-1, ε = 46 600 M-1 cm-1); the NIR bands exhibit double the intensity in comparison to the monomer. Oxidation of the intermediate orientations results in band splitting for [3˙˙]2+ (E = 4890 and 4200 cm-1; ε = 26 500 and 21 100 M-1 cm-1), and a red-shift for [4˙˙]2+ using ortho- and meta-phenylene linkers, respectively. This study demonstrates for the first time, the applicability of exciton coupling to ligand radical systems absorbing in the NIR region and shows that by simple geometry changes, it is possible to tune the energy of intense low energy absorption by nearly 400 nm.

Ground and Excited-Electronic-State Dissociations of Hydrogen-Rich and Hydrogen-Deficient Tyrosine Peptide Cation Radicals.

We report a comprehensive study of collision-induced dissociation (CID) and near-UV photodissociation (UVPD) of a series of tyrosine-containing peptide cation radicals of the hydrogen-rich and hydrogen-deficient types. Stable, long-lived, hydrogen-rich peptide cation radicals, such as [AAAYR + 2H](+●) and several of its sequence and homology variants, were generated by electron transfer dissociation (ETD) of peptide-crown-ether complexes, and their CID-MS(3) dissociations were found to be dramatically different from those upon ETD of the respective peptide dications. All of the hydrogen-rich peptide cation radicals contained major (77%-94%) fractions of species having radical chromophores created by ETD that underwent photodissociation at 355 nm. Analysis of the CID and UVPD spectra pointed to arginine guanidinium radicals as the major components of the hydrogen-rich peptide cation radical population. Hydrogen-deficient peptide cation radicals were generated by intramolecular electron transfer in Cu(II)(2,2':6',2″-terpyridine) complexes and shown to contain chromophores absorbing at 355 nm and undergoing photodissociation. The CID and UVPD spectra showed major differences in fragmentation for [AAAYR](+●) that diminished as the Tyr residue was moved along the peptide chain. UVPD was found to be superior to CID in localizing Cα-radical positions in peptide cation radical intermediates. Graphical Abstract ᅟ.

Benchmarking Electronic Excitation Energies and Transitions in Peptide Radicals.

Excited electronic states in several radical chromophores representing photochemically active groups in peptide and protein radicals and cation radicals were investigated computationally using equation-of-motion coupled cluster (EOM-CCSD) and time-dependent density functional theory (TD-DFT) methods. The calculations identified the main transitions responsible for photodissociations of gas-phase peptide cation radicals in the near-UV region of the spectrum. Analysis of the EOM-CCSD benchmarks showed that no TD-DFT method was universally accurate across the various radical motifs that included Cα-amide, aminoketyl, formamidyl, guanidyl, carbamyl, benzyl, phenoxy, and tautomeric dihydrophenyl and imidazolyl radicals. Overall, the ωB97XD, M06-2X, and LC-BLYP hybrid functionals showed acceptable performance when benchmarked against EOM-CCSD calculations. However, the performance of these TD-DFT methods depended on the nature of the radical chromophore, emphasizing the need for benchmarking and careful analysis.

Probing Peptide Cation–Radicals by Near-UV Photodissociation in the Gas Phase. Structure Elucidation of Histidine Radical Chromophores Formed by Electron Transfer Reduction

Electron transfer reduction of gas-phase ions generated from histidine-containing peptides forms stable cation-radicals that absorb light at 355 nm, as studied for AAHAR, AAHAK, DSHAK, FHEK, HHGYK, and HHSHR. Laser photodissociation of mass-selected cation-radicals chiefly resulted in loss of H atoms, contrasting dissociations induced by slow collisional heating. The 355 nm absorption was due to new chromophores created by electron transfer and radical rearrangements in the cation-radicals. The chromophores were identified by time-dependent density functional theory calculations as 2H,3H-imidazoline and 2H-dihydrophenol radicals, formed by hydrogen atom transfer to the histidine and tyrosine side chain groups, respectively. These radicals undergo facile C-H bond dissociations upon photon absorption. In contrast, dissociations of histidine peptide cation-radicals containing the 1H,3H-imidazoline ring prefer loss of 4-methylimidazole via a multistep reaction pathway. The isomeric cation-radicals can be distinguished by a combination of collision-induced dissociation and near-UV photodissociation. The TD-DFT excitation energies in model imidazoline radicals were benchmarked on EOM-CCSD energies, and a satisfactory agreement was found for the M06-2X and ωB97XD functionals. The combination of electron transfer, photodissociation, collisional activation, and theory is presented as a powerful tool for studying structures and electronic properties of peptide cation-radicals in the gas phase.

Combining UV photodissociation with electron transfer for peptide structure analysis.

The combination of near-UV photodissociation with electron transfer and collisional activation provides a new tool for structure investigation of isolated peptide ions and reactive intermediates. Two new types of pulse experiments are reported. In the first one called UV/Vis photodissociation-electron transfer dissociation (UVPD-ETD), diazirine-labeled peptide ions are shown to undergo photodissociation in the gas phase to form new covalent bonds, guided by the ion conformation, and the products are analyzed by electron transfer dissociation. In the second experiment, called ETD-UVPD wherein synthetic labels are not necessary, electron transfer forms new cation-peptide radical chromophores that absorb at 355 nm and undergo specific backbone photodissociation reactions. The new method is applied to distinguish isomeric ions produced by ETD of arginine containing peptides.

Reversible Photogeneration of a Stable Chiral Radical-Pair from a Fast Photochromic Molecule

The photochromic naphthalene-bridged imidazole dimer containing a naphthyl moiety that tethers two triarylimidazole units shows instantaneous coloration upon exposure to UV light and rapid fading in the dark. In this work, we demonstrate the formation of a stable chiral radical-pair that exhibits no photoracemization even by repeated photochromic cycles. The photogenerated radical-pair from the imidazole dimer exhibits the Cotton effect in the visible light region, indicating the retention of the enantiomeric conformation of the radical-pair. This result suggests that the chirality resulting from the binaphthyl moiety induces exciton coupling between the two radical chromophores by through-space interaction.

Excitation and Emission Spectra of Jet-Cooled Naphthylmethyl Radicals

Gas phase excitation and emission spectra of three naphthylmethyl radical chromophores are presented. These resonance-stabilized species, 1-naphthylmethyl, 2-naphthylmethyl, and α-acenaphthenyl, each possessing an sp(2) carbon adjacent to a naphthalene moiety, are studied by resonant two-color two-photon ionization, laser induced fluorescence, and dispersed fluorescence spectroscopy. Identification of the radicals is made through a combination of dispersed fluorescence and density functional theory calculations. All three species possess spectra in the 580 nm region. The possible relevance to unidentified spectroscopic features such as the diffuse interstellar bands and emission from the Red Rectangle nebula is discussed.

The spectroscopy and thermochemistry of phenylallyl radical chromophores

The resonant two-color two-photon ionization and laser induced fluorescence excitation spectra of the 1-phenylallyl (cinnamyl) and inden-2-ylmethyl radicals are reported. The 1-phenylallyl radical is found to fluoresce with low yield, permitting only a coarse dispersed fluorescence spectrum, while the inden-2-ylmethyl radical yields sufficient fluorescence to obtain ground-state vibrational frequencies and two-dimensional fluorescence spectra. Computed ionization energies and thermochemical properties including radical stabilization energies are reported for a range of resonance-stabilized radicals, including the phenylpropargyl, vinylpropargyl and phenylallyl radicals.

Application of zeolites as matrices for pigments

Zeolites A mixed with various sodium polysulfides form ultramarine analogs of different colors upon thermal treatment. The resulting colors reflect the contribution of sulfur radical chromophores (mostly S 3 − and S 2 −) and can be modified by choosing the polysulfide of adequate S chain length and by the conditions of thermal treatment. In some products the original LTA structure is retained. In the other cases (particularly when alkalinity of the mixture is increased) the transformation towards SOD takes place. Some natural zeolites (e.g. chabazite) have been used as starting material for the synthesis of ultramarine analogs of the SOD structure.

Synthesis of ascorbate and urate in the ovary of water buffalo

Blood flow interruption is associated with oxygen depletion and loss of factors for function and survival in downstream tissues or cells. Hypoxia and absence of gonadotropins trigger apoptosis and atresia in the ovary. We studied the antioxidant response of follicular cells to plasma deprivation in ovaries dissected from water buffalo. Aliquots of follicular fluid were aspirated from each antral follicle, before and during incubation of the ovaries at 39°C. Urate, ascorbate, retinol and α-tocopherol in the fluid were, titrated by High Performance Liquid Chromatography (HPLC) with spectrophotometric or spectrofluorimetric detection. The total antioxidant capacity of follicular fluid was determined as absorbance decrease, following addition of a source of radical chromophores. The more the incubation progressed, the higher levels of urate, ascorbate and total antioxidant capacity were found. Conversely, changes in concentration of the liposoluble antioxidants were not observed. Ascorbate synthesizing activity in the follicle was demonstrated by detecting the enzyme L-gulono-γ-lactone oxidase in microsomes prepared from granulosa cells. These cells were also analyzed for the expression of the enzyme CPP32. The enzyme level, measured as DEVD-p-nitroanilide cleaving activity, was found related with the immunoreactivity to anti-CPP32 antibodies. Negative correlation between the enzyme activity (which is known to be induced by peroxynitrite) and the follicular level of urate (which scavenges peroxynitrite) was also observed. The amount of nitrotyrosine, a product of peroxynitrite attack on proteins, was measured in follicular fluids by Enzyme Linked ImmunoSorbent Assay (ELISA). This amount was found positively correlated with the CPP32 activity, and negatively correlated with the urate level in follicular fluid. Alterations in concentrations of ascorbate or urate may be associated with oxidative stress during follicular atresia.

Aryl substituent effects and solvent effects on the decarbonylation of phenacetyl radicals

Five aryl-substituted phenacetyl radicals (X = p-MeO, p-Me, H, p-Cl, p-CF3) were generated by laser photolysis of the corresponding dibenzyl ketones in n-hexane and acetonitrile. The decarbonylation reaction was monitored through the rise in time-resolved absorption of the benzyl radical chromophore at 317 nm. The decarbonylation rate constants were obtained by a numerical integration procedure, where second-order radical reactions were explicitly taken into account. Values of (2–3) × 106 s−1 in acetonitrile and (6–10) × 106 s−1 in n-hexane revealed a large solvent effect for all derivatives (by a factor of ∼3). The electronic substituent effect indicates that both electron-withdrawing and electron-donating para substituents accelerate the decarbonylation slightly. The rate constants followed the order MeO > Me, Cl, CF3 > H. The substituent effects are interpreted in terms of the ability of the para substituent to stabilize the benzyl radical resulting from decarbonylation. Evidence for a polar effect was not obtained. The underlying reasons for the observed solvent effect are discussed. Copyright © 2000 John Wiley & Sons, Ltd.

Estimation of average orientation of surface-confined chromophores on electrode surfaces using electroreflectance spectroscopy

A novel method was proposed to estimate the orientation of chromophores confined on a planar electrode surface using electroreflectance (ER) spectroscopy (potential-modulated UV-vis reflectance spectroscopy). The intensity ratio of the ER response to p-polarized and s-polarized incident light was formulated in an equation as a function of incident angle and the orientation of the chromophore by adopting approximations. The method was applied to estimate the orientation of viologen cation radical chromophore in a monolayer of N-butyl- N′-(4-mercaptobutyl)-4,4′-bipyridinium dihexafluorophosphate adsorbed on a gold electrode. The orientation of the long axis of the viologen cation radical was found to be approximately 70° with respect to the surface normal. Both two-phase and three-phase models were examined and the validity of these models was discussed.

Hydrogen bonding in systems containing imidazoline nitroxide radicals

Hydrogen bonding between imidazoline nitroxide free radicals and hydrogen donors of varying strength [acetone, chloroform, trifluoroacetic acid (TFA)] has been examined using magnetic resonance techniques. Low-field dynamic nuclear polarization and spin–lattice relaxation measurements have been used to determine absolute scalar and dipolar coupling strengths. Dipolar coupling was found to increase in the order acetone <CHCl3<TFA, while only TFA systems showed substantial scalar coupling. For acetone, observed dipolar relaxation rates are consistent with translational modulation and little, if any, complexation. With CHCl3, both translational and rotational modulation are required; the data suggest that CHCl3 forms weak hydrogen bonds with the radical chromophore with little spin density transferred to the bridge proton. In contrast, the protons of TFA show strong scalar coupling with the various radicals which results in positive proton NMR enhancements or strong hyperfine coupling (AH?5 G). The 1H scalar relaxation rates are consistent with strong hydrogen bond formation at the imine site and complex lifetimes of ∼10−9–10−8 sec. The 19F nuclei of TFA show much weaker, radical-independent scalar coupling and little rotational dipolar modulation consistent with, at best, weak, nonstereospecific complex formation.