Pi Complexes between Organic Free Radicals

Hausser, Karl H.; Murrell, J. N.

doi:10.1063/1.1743757

Cited by 139 publications

(50 citation statements)

References 7 publications

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“…Immediately after the laser irradiation, an absorption band appeared at 700 nm, which is ascribed to the triplet excited state of C 60 G2. At 500 ns after the laser irradiation, new absorption bands appeared at 620 and 1000 nm: The 620 nm band is assigned to the radical cation of TMPD, 30 while the 1000 nm band can be attributed to the radical anion of C 60 G2; this absorption is located at the wavelength similar to those reported for the radical anion of mono-functionalized fullerene derivatives. 31,32 The generation of radical ions with the decrease in the triplet state absorption indicates that the photoinduced electron transfer occurred via the triplet excited state of C 60 Gn as shown in eq 5:…”

Section: Resultsmentioning

confidence: 81%

Photochemical and Photophysical Properties of C₆₀ Dendrimers Studied by Laser Flash Photolysis

et al. 2002

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Photochemical and photophysical properties of fullerene dendrimers, in which the fullerene-moiety was connected via an acetylene bond with benzyl-ether type dendrons from the second to fourth generation, have been investigated by time-resolved fluorescence and time-resolved absorption methods, in addition to steadystate spectra. The photophysical properties of the dendrimers such as lifetimes of the singlet and triplet excited states were essentially the same, regardless of the dendrimer generation. However, the rate constants of intermolecular processes such as triplet-triplet annihilation, triplet energy transfer, and electron transfer via the triplet states decreased with the increase in dendrimer generation. The relation between the free energy changes and the quenching rate constants revealed long-range electron transfer processes due to steric hindrance of the dendron groups. Furthermore, it was revealed that the solvation of radical ion pairs is also affected by size of the dendron groups. Back electron transfer kinetics became first-order for the electron transfer system of the smaller donor and/or smaller dendron groups, while second-order kinetics was observed for larger donor and/or larger dendron groups. The dendron groups were also found to be effective to keep unstable species such as the chemically generated fullerenyl anion persistent.

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Section: Resultsmentioning

confidence: 81%

Photochemical and Photophysical Properties of C₆₀ Dendrimers Studied by Laser Flash Photolysis

et al. 2002

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“…4B) (4,(28)(29)(30)(31)(32)(33). We term the former case with an odd number of spin/charge as type I CR and the latter case with an even number as type II CR.…”

Section: Resultsmentioning

confidence: 99%

Tryptophan-mediated charge-resonance stabilization in the bis -Fe(IV) redox state of MauG

Geng

Dornevil

Davidson

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

102

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The diheme enzyme MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor protein to generate a tryptophan tryptophylquinone cofactor. The MauG-catalyzed reaction proceeds via a bis -Fe(IV) intermediate in which one heme is present as Fe(IV)=O and the other as Fe(IV) with axial histidine and tyrosine ligation. Herein, a unique near-infrared absorption feature exhibited specifically in bis -Fe(IV) MauG is described, and evidence is presented that it results from a charge-resonance-transition phenomenon. As the two hemes are physically separated by 14.5 Å, a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes is reversibly oxidized and reduced to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis -Fe(IV) MauG. Analysis of the MauG structure reveals that electron transfer via this mechanism is rapid enough to enable a charge-resonance stabilization of the bis -Fe(IV) state without direct contact between the hemes. The finding of the charge-resonance-transition phenomenon explains why the bis -Fe(IV) intermediate is stabilized in MauG and does not permanently oxidize its own aromatic residues.

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“…The π-dimerization of planar radical species was first observed in the middle of the last century [1][2][3][4][5][6] in both the solid state and in concentrated solutions at low temperatures. Under these conditions, equilibria are established between two paramagnetic free radical species and a diamagnetic dimer, most often with a particular equilibrium favoring the dissociated, non-dimer state.…”

Section: Early Examples Of Radical Dimersmentioning

confidence: 99%

Molecular recognition and switching via radical dimerization

Spruell

2010

Pure and Applied Chemistry

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This article highlights the emerging use of the interactions of radical π-dimers to drive both molecular recognition and switching processes within supramolecular systems and mechanically interlocked molecular architectures. The enhanced stability experienced by dimers of radical cation species when encapsulated, as compared to when they are free in solution, is driving their useful incorporation into functional systems. The redox stimulation used in the production of radical cation species provides the ideal trigger for molecular switching events. Moreover, the nature and strength of the radical dimerization events introduces a completely novel recognition motif within supramolecular and mechanically interlocked molecular systems, complementing well-established techniques and enabling new research opportunities to blossom.atom-centered radical dimerization and, thereafter, I will highlight the recent advances that have led to the use of this unusual class of bonding as a practical and fully orthogonal molecular recognition motif. EARLY EXAMPLES OF RADICAL DIMERSThe π-dimerization of planar radical species was first observed in the middle of the last century [1][2][3][4][5][6] in both the solid state and in concentrated solutions at low temperatures. Under these conditions, equilibria are established between two paramagnetic free radical species and a diamagnetic dimer, most often with a particular equilibrium favoring the dissociated, non-dimer state. These types of dimeric associations have been observed for a whole range of different types of planar radical species, varying in both the size of the planar systems and the charges of the dimer components. Both radical anions and cations, as well as neutral free radicals, have been observed to associate with one another in this manner. Some of the most common radical π-dimeric species are illustrated in Fig. 1, including the associated radical anions of singly reduced tetracyanoethylene (TCNE) [7], and the neutral radical pairs of phenalenyl [8], as well as the radical cation π-dimers of singly oxidized tetrathiafulvalene (TTF) [9,10] and singly reduced bipyridium (BIPY 2+ ) units [11,12].The π-association of phenalenyl has received much attention [8,13] as a model for the radical dimerization of large planar π-systems. The persistent (i.e., stable) neutral radical is readily generated during the oxidation of phenylene. Its stability can be attributed to the delocalization of the radical throughout the fused polycyclic structure, its character residing predominately on 6 of the 12 peripheral carbons [14,15]. When substituted with bulky side groups to prevent σ-binding (the radical-radical coupling to form a traditional covalent sigma (σ) bond), π-dimer formation is observed in concentrated deoxygenated solutions. Moreover, this π-dimer is comprised of two perfectly overlapping delocalized singly occupied molecular orbitals (SOMOs) on each phenalenyl unit in such a way that the two formal free radical electrons pair to form what has been described as a two-electron...

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Pi Complexes between Organic Free Radicals

Cited by 139 publications

References 7 publications

Photochemical and Photophysical Properties of C₆₀ Dendrimers Studied by Laser Flash Photolysis

Photochemical and Photophysical Properties of C₆₀ Dendrimers Studied by Laser Flash Photolysis

Tryptophan-mediated charge-resonance stabilization in the bis -Fe(IV) redox state of MauG

Molecular recognition and switching via radical dimerization

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Pi Complexes between Organic Free Radicals

Cited by 139 publications

References 7 publications

Photochemical and Photophysical Properties of C60 Dendrimers Studied by Laser Flash Photolysis

Photochemical and Photophysical Properties of C60 Dendrimers Studied by Laser Flash Photolysis

Tryptophan-mediated charge-resonance stabilization in the bis -Fe(IV) redox state of MauG

Molecular recognition and switching via radical dimerization

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Photochemical and Photophysical Properties of C₆₀ Dendrimers Studied by Laser Flash Photolysis

Photochemical and Photophysical Properties of C₆₀ Dendrimers Studied by Laser Flash Photolysis