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Published: 4 June 2021

Last updated: 4 February 2022

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1

Yuan, Ningning, Wenqing Wang, Ziye Wu, Sheng Chen, Gengwen Tan, Yunxia Sui, Xinping Wang, Jun Jiang, and Philip P. Power. "A boron-centered radical: a potassium-crown ether stabilized boryl radical anion." Chemical Communications 52, no. 86 (2016): 12714–16. http://dx.doi.org/10.1039/c6cc06918e.

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2

Lim, Gary N., Whitney A. Webre, and Francis D'Souza. "Charge separation in supramolecular ferrocene(s)-zinc porphyrin-fullerene triads: A femtosecond transient absorption study." Journal of Porphyrins and Phthalocyanines 19, no. 01-03 (January 2015): 270–80. http://dx.doi.org/10.1142/s108842461550008x.

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Mechanistic aspects of photoinduced charge separation in supramolecular triads, constructed using covalently linked zinc porphyrin-ferrocene(s) dyads — self-assembled via axial coordination to either pyridine or phenylimidazole appended fulleropyrrolidine ( Fc x- ZnP : PyC 60 or Fc x- ZnP : ImC 60; x = 1 or 2), has been investigated using femtosecond pump-probe transient spectroscopy. Upon photoexcitation of ZnP , charge separation from ferrocene to 1 ZnP * to yield the initial Fc +- ZnP •-: C 60 radical ion-pair or charge separation from 1 ZnP * to C 60 to yield the initial Fc - ZnP •+: C 60•- radical ion-pair, depending upon the ferrocene-zinc porphyrin intermolecular distance, was observed. These radical ion-pairs resulted in the formation of ultimate distantly separated Fc +- ZnP : C 60•- radical ion-pairs either via an electron migration (former case) or hole shift (latter case) process. Kinetics of charge separation as a function of spacer connecting the ferrocene and porphyrin, and spacer between the porphyrin and fullerene is reported. In agreement with our earlier study (J. Phys. Chem. B 2004; 108: 11333–11343), the Fc +- ZnP : C 60•- radical ion-pair persisted beyond the monitoring time window of our instrument, suggesting charge stabilization in these supramolecular triads.

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3

Jones II, Guilford, Xiaochun Wang, and Jingqiu Hu. "Photochemistry of rhodamine dye salts involving intra-ion-pair electron transfer." Canadian Journal of Chemistry 81, no. 6 (June 1, 2003): 789–98. http://dx.doi.org/10.1139/v03-074.

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The electron-transfer photochemistry of rhodamine 6G thiocyanate ion pairs has been investigated. For dye in a low polarity solvent, such as ethyl acetate, the emission of rhodamine 6G is significantly quenched by thiocyanate counterions. Laser photolysis of rhodamine 6G and thiocyanate in ethyl acetate was studied in detail with the identification of the reduced rhodamine 6G radical species (λmax = 410 nm). The growth and decay of the R6G radical could be accounted for in part by a mechanism involving initial formation of dye triplet followed by electron transfer which provides a triplet radical-pair state on a µs timescale.Key words: electron transfer, ion pair, rhodamine 6G, triplet state.

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4

Kim, J. M., and J. A. Pincock. "Internal return in the photochemistry of ring-substituted 1-(1-naphthyl)ethyl esters of phenylacetic acid." Canadian Journal of Chemistry 73, no. 6 (June 1, 1995): 885–95. http://dx.doi.org/10.1139/v95-111.

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The photochemistry in methanol of the esters 12a–d has been studied in order to determine the importance of internal return of both ion pair and radical pair intermediates. The efficiency of internal return, determined by monitoring 18O exchange between the alcohol and carbonyl oxygens, was shown to be substituent dependent, varying from approximately 10% for the 4,7-dimethoxy substrate to nearly 50% for the 4-cyano case. The corresponding ground state solvolysis reactions gave about 10% internal return and, within experimental error, were substituent independent. Internal return was also examined by racemization of the chiral center in 12a and 12d. In summary, these combined results reveal that internal return probably occurs mainly through a contact (not solvent-separated) radical pair. More important, internal return has little effect on previously calculated electron transfer rate constants for converting the radical pair to the ion pair. Therefore, the previously reported Marcus' correlations are valid. Keywords: photochemistry of benzylic esters, internal return, photosolvolysis, electron transfer, radical pairs.

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5

Morlet-Savary, F., J. P. Fouassier, Hideo Tomioka, Iwao Sumiyoshi, and Yasuyuki Takimoto. "Ion and Radical Pair Generation in Tribromoacetophenone." Chemistry Letters 26, no. 12 (December 1997): 1267–68. http://dx.doi.org/10.1246/cl.1997.1267.

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6

Aramaki, Yoshitaka, Naoki Imaizumi, Mao Hotta, Jun Kumagai, and Takashi Ooi. "Exploiting single-electron transfer in Lewis pairs for catalytic bond-forming reactions." Chemical Science 11, no. 17 (2020): 4305–11. http://dx.doi.org/10.1039/d0sc01159b.

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7

Goez, Martin, Isabell Frisch, and Ingo Sartorius. "Electron and hydrogen self-exchange of free radicals of sterically hindered tertiary aliphatic amines investigated by photo-CIDNP." Beilstein Journal of Organic Chemistry 9 (February 26, 2013): 437–46. http://dx.doi.org/10.3762/bjoc.9.46.

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The photoreactions of diazabicyclo[2,2,2]octane (DABCO) and triisopropylamine (TIPA) with the sensitizers anthraquinone (AQ) and xanthone (XA) or benzophenone (BP) were investigated by time-resolved photo-CIDNP (photochemically induced dynamic nuclear polarization) experiments. By varying the radical-pair concentration, it was ensured that these measurements respond only to self-exchange reactions of the free amine-derived radicals (radical cations DH • + or α-amino alkyl radicals D • ) with the parent amine DH; the acid–base equilibrium between DH • + and D • also plays no role. Although the sensitizer does not at all participate in the observed processes, it has a pronounced influence on the CIDNP kinetics because the reaction occurs through successive radical pairs. With AQ, the polarizations stem from the initially formed radical-ion pairs, and escaping DH • + then undergoes electron self-exchange with DH. In the reaction sensitized with XA (or BP), the polarizations arise in a secondary pair of neutral radicals that is rapidly produced by in-cage proton transfer, and the CIDNP kinetics are due to hydrogen self-exchange between escaping D • and DH. For TIPA, the activation parameters of both self-exchange reactions were determined. Outer-sphere reorganization energies obtained with the Marcus theory gave very good agreement between experimental and calculated values of ∆G ‡ 298.

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8

Murai, Hisao, Yoshinori Yamamoto, and Yasumasa J. I'Haya. "Time-resolved ESR study on photochemical formation of radical pair in cyclodextrin cavities." Canadian Journal of Chemistry 69, no. 11 (November 1, 1991): 1643–48. http://dx.doi.org/10.1139/v91-241.

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The photoreduction of xanthone with diethylaniline in cyclodextrin cavities was studied at 77 K and room temperature by a time-resolved ESR technique. The radical pair observed in β- and γ-cyclodextrins showed inverted spin polarization compared to that of precursor excited triplet xanthone. This result is rationalized by taking account of the fixed orientation of the radical ion pair in the cyclodextrins. Frozen aqueous solutions and dried powder-like samples provided similar results. The spectrum of the radical pair was also detected in an aqueous solution of β-cyclodextrin at room temperature. Key words: cyclodextrins, xanthone, spin polarization, radical ion-pair, time-resolved ESR.

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9

John, Andreas, and Hans Bock. "Elektronentransfer und Kontaktionen-Bildung, 42 [1,2] Cyclovoltammetrische und ESR / ENDOR-Untersuchungen der Einelektronen-Reduktion von Diphenochinonen / Electron Transfer and Contact Ion Pair Formation, 42 [1,2] Cyclovoltammetric and ESR / ENDOR Investigations of the One-Electron Reduction of Diphenoquinones." Zeitschrift für Naturforschung B 50, no. 11 (November 1, 1995): 1699–716. http://dx.doi.org/10.1515/znb-1995-1118.

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Semiquinone radical anions are prototype compounds for contact ion pair formation with metal counter cations. In order to investigate the still open question whether bulky alkyl groups can sterically interfere, diphenoquinone derivatives O=C(RC=CH)2C=C(HC=CR)2C=O with R = C(CH3)3, CH(CH3)2 and CH3 have been selected and the following ESR/ENDOR results are obtained for the alkaline metal cations: The tetrakis(tert-butyl)-substituted radical anion only adds Li⊕ and Na⊕, while K⊕ forms no ion pair. The 3,3ʹ,5,5ʹ-tetra(isopropyl)diphenoquinone radical anion is accessible to all cations Me⊕, although Rb⊕ and Cs⊕ seem to be present solvent-separated in solution. The tetramethyl-substituted radical anion unfortunately polymerizes rapidly. Additional information concerns the ESR/ENDOR proof for ion triple radical cation formation [Li⊕ M•⊖Li⊕]•⊕, or the difference in the coupling constants upon Me⊕ docking at one δ⊖O=C group, suggesting that about 87% of the spin density is located in the cation-free molecular half of the diphenoquinone radical anion. Based on the wealth of ESR/ENDOR information, crystallization of the contact ion pairs and their structural characterization should be attempted.

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10

Morack, Tobias, Christian Mück‐Lichtenfeld, and Ryan Gilmour. "Bioinspired Radical Stetter Reaction: Radical Umpolung Enabled by Ion‐Pair Photocatalysis." Angewandte Chemie International Edition 58, no. 4 (January 21, 2019): 1208–12. http://dx.doi.org/10.1002/anie.201809601.

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11

Bock, H., and P. Hänel. "Elektronentransfer und Ionenpaar-Bildung, 27 [1, 2] Darstellung von Semichinon-Ionenpaaren durch Reduktion von Chinonen mit Tetraalkylammonium-boranat in aprotischen Lösungen: 1,10-Phenanthrolin-5,6-dion / Electron Transfer and Ion Pairing, 27 [1, 2] Preparation of Semiquinone Ion Pairs by Reduction of Quinones Using Tetraalkylammonium Boranate in Aprotic Salt Solutions: 1,10-Phenanthrolin-5,6-dione." Zeitschrift für Naturforschung B 47, no. 2 (February 1, 1992): 288–300. http://dx.doi.org/10.1515/znb-1992-0222.

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Ion pairs of 1,10-phenanthrolin-5,6-dione radical anion [M · ⊖Me⊕n] ·⊕(n−1) with Me⊕n = Mg⊕⊕, Ca⊕⊕, Sr⊕⊕, Zn⊕⊕, Cd⊕⊕, Pb⊕⊕ and La⊕⊕⊕ are advantageously prepared in aprotic DMF solution containing appropriate metal salts Me⊕nX⊖ by using the ‘mild’ single-electron reducing agent tetra(n-butyl)ammonium-boranate R4N⊕BH4⊖ . For comparison, the ‘naked’ radical anion with the largely interaction-free [K⊕(2.2.2)-cryptand]⊕ counter cation is chosen, which is formed on reduction with potassium in THF solution of (2.2.2)-cryptand. Addition of excess Na⊕[B(C6H5)4]⊖ to the reduction solution only yields a solvent-separated ion pair (M · ⊖)DMF ··· (Na⊕)DMF, whereas in the presence of multiply charged counter cations Me⊕n the respective contact ion pair radical cations [M · ⊖Me⊕n] · ⊕(n−1) are formed. Their g values decrease with increasing nuclear charge of Me⊕n and their metal-s-spin densities increase with the effective counter cation charge n⊕/rMe⊕n. The ESR /ENDOR data recorded suggest Me⊕n complexation by the δ⊖OC -COδ⊖ chelate tongs and the ion pair stability, which is modified by the dielectric properties of the solvent used, may be rationalized by the Coulombic attraction between the radical anion M · ⊖ and the counter cations Me⊕n.

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12

Crich, David, and Dae-Hwan Suk. "The β-(acyloxy)alkyl radical rearrangement revisited." Canadian Journal of Chemistry 82, no. 2 (February 1, 2004): 75–79. http://dx.doi.org/10.1139/v03-148.

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A β-(acyloxy)alkyl radical precursor, containing a carboxylate residue suitably placed for the trapping of any intermediate alkene radical cations, has been constructed. In nonpolar solutions the probe, in the form of either the free acid or its tetrabutylammonium salt, undergoes the typical rearrangement reaction with no evidence of trapping, leading to the conclusion that the reaction is either concerted or that collapse of any intermediate contact ion pair is so rapid as to preclude the possibility of trapping.Key words: radical, rearrangement, contact ion pair.

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13

Pincock, James A. "Photochemistry of Arylmethyl Esters in Nucleophilic Solvents: Radical Pair and Ion Pair Intermediates." Accounts of Chemical Research 30, no. 1 (January 1997): 43–49. http://dx.doi.org/10.1021/ar960177r.

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14

Bascal, H. A., K. J. Jordan, and R. H. Lipson. "Ion-pair state spectroscopy of HgCl." Canadian Journal of Chemistry 71, no. 10 (October 1, 1993): 1615–21. http://dx.doi.org/10.1139/v93-201.

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Fluorescence excitation spectra of HgCl produced and cooled in a corona-excited supersonic jet discharge are presented. The dominant feature between ≈ 368 and 267 nm is an extensive vibrationally and isotopically resolved band system which is assigned to the B2Σ+(ν′) ← X2Σ+(ν″ = 0, 1, and 2) valence-to-ion-pair state transition of the radical. Transition wavenumbers for vibrational bands with 75 ≥ ν′ ≥ 21 and 2 ≥ ν″ ≥ 0 were measured, and B-state Dunham parameters were derived which are valid between 75 ≥ ν′ ≥ 0. The equilibrium bondlength for the B-state was determined to be 2.960(2) Å by Franck–Condon calculations. B–X Franck–Condon factors are tabulated which are expected to help in the interpretation of HgCl2 photodissociation and HgCl laser spectra.

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15

Peters, Kevin S., and Joseph Lee. "Picosecond dynamics of stilbene-olefin contact and solvent-separated radical ion pairs: role of electron transfer and radical ion pair diffusion." Journal of the American Chemical Society 115, no. 9 (May 1993): 3643–46. http://dx.doi.org/10.1021/ja00062a031.

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16

Hilborn, James W., and James A. Pincock. "Photolysis of the 1-naphthylmethyl ester of substituted phenylacetic acids: intramolecular charge transfer and rates of decarboxylation of arylacyloxy radicals." Canadian Journal of Chemistry 70, no. 3 (March 1, 1992): 992–99. http://dx.doi.org/10.1139/v92-131.

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The photolysis of esters 6 and 8 in methanol leads to products resulting from both naphthylmethyl cations and radicals. The product distribution is nearly independent of X for the esters 6 except when X equals methoxy. A mechanism involving initial homolytic cleavage of the carbon–oxygen bond in the excited singlet state of the ester is proposed. Competition between electron transfer in the radical pair to form the ion pair and decarboxylation of the arylacyloxy radical allows calculations of the rates for this decarboxylation process. The ρ value versus σ is close to zero. When X equals methoxy, intramolecular electron transfer occurs with the naphthalene ring serving as the acceptor and the methoxyaromatic as the donor. This exciplex fragments to carbon dioxide and 1-(1-naphthyl)-2-arylethane. Keywords: acyloxy radical, decarboxylation, photolysis of benzylic esters.

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17

Jie, Jialong, Kunhui Liu, Lidan Wu, Hongmei Zhao, Di Song, and Hongmei Su. "Capturing the radical ion-pair intermediate in DNA guanine oxidation." Science Advances 3, no. 6 (June 2017): e1700171. http://dx.doi.org/10.1126/sciadv.1700171.

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18

Sinks, Louise E., Emily A. Weiss, Jovan M. Giaimo, and Michael R. Wasielewski. "Effect of charge delocalization on radical ion pair electronic coupling." Chemical Physics Letters 404, no. 4-6 (March 2005): 244–49. http://dx.doi.org/10.1016/j.cplett.2005.01.093.

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19

Kuno, Shinichi, Hiroshi Akeno, Hiroyuki Ohtani, and Hideya Yuasa. "Visible room-temperature phosphorescence of pure organic crystals via a radical-ion-pair mechanism." Physical Chemistry Chemical Physics 17, no. 24 (2015): 15989–95. http://dx.doi.org/10.1039/c5cp01203a.

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20

PETERS, K. S., and J. LEE. "ChemInform Abstract: Picosecond Dynamics of Stilbene-Olefin Contact and Solvent-Separated Radical Ion Pairs: Role of Electron Transfer and Radical Ion Pair Diffusion." ChemInform 24, no. 36 (August 20, 2010): no. http://dx.doi.org/10.1002/chin.199336089.

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21

PINCOCK, J. A. "ChemInform Abstract: Photochemistry of Arylmethyl Esters in Nucleophilic Solvents: Radical Pair and Ion Pair Intermediates." ChemInform 28, no. 19 (August 4, 2010): no. http://dx.doi.org/10.1002/chin.199719283.

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22

Ohkita, Hideo, Steffan Cook, Yeni Astuti, Warren Duffy, Martin Heeney, Steve Tierney, Iain McCulloch, Donal D. C. Bradley, and James R. Durrant. "Radical ion pair mediated triplet formation in polymer–fullerene blend films." Chem. Commun., no. 37 (2006): 3939–41. http://dx.doi.org/10.1039/b608832e.

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23

DeCosta, D. P., and J. A. Pincock. "Intramolecular electron transfer in the photochemistry of substituted 1-naphthylmethyl esters of benzoic acids." Canadian Journal of Chemistry 70, no. 7 (July 1, 1992): 1879–85. http://dx.doi.org/10.1139/v92-235.

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Direct excitation of the esters 5 in methanol solvent leads to rapid intramolecular exciplex formation (kex = 1010 s−1 for X = CH3O, Y = CN) with electron transfer from the naphthalene to the benzoate ring. This process dominates the usual fluorescence and reaction of the excited singlet state. The rate of this process can be varied over 103 by suitable change in the substituents X and Y. The electron-transfer rates can be correlated with the two-parameter Hammett equation: log kex = 8.48 − 1.5σ+ + 0.77σ. For cases where the rate of exciplex formation is slow, the usual homolytic carbon–oxygen bond cleavage occurs from the excited singlet state. The eventual products result from the ion pair since the rate of electron transfer in the radical pair to form the ion pair is considerably faster than the rate of decarboxylation of the benzoyloxy radical.

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24

Sánchez-Palacios, Angela, and Rut Delgado. "ESR Study of the Radical Cations 1,1′-Dimethylene and 1,1′-Trimethylene 2,2′-Bipyridinium: Charge-Transfer Complexes between Their Dications and the Donor Cysteine." Applied Spectroscopy 48, no. 8 (August 1994): 926–32. http://dx.doi.org/10.1366/0003702944029659.

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In order to compare the acceptor properties of 1,1′-dimethylene and 1,1′-trimethylene 2,2′-bipyridinium dications opposite to a donor cysteine and the stabilities of their cation radicals, UV-visible spectrophotometric and ESR studies were carried out. The diquat dication (DQ2+) forms a red charge-transfer (CT) complex with cysteine (Cys2–) of the ion-pair type. The complex presents two CT bands at 348 and 494 nm, respectively. By use of the Benesi-Hildebrand and other treatments (Scott, Foster, and Scatchard), the association constant for the ion association and ελ were determined. The results obtained by all the procedures are in fair agreement. The triquat dication (TQ2+) does not form a CT complex with cysteine (Cys2–). The triquat radical (TQ+) was generated by its reduction with alkaline sodium dithionite, and it was detected by electron spin resonance spectroscopy at room temperature. The ESR spectrum of the diquat radical (DQ+) was reinvestigated. The corresponding hyperfine coupling and g factors of the radicals DQ+ and TQ+ were given.

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25

Parman, T., J. A. Pincock, and P. J. Wedge. "The photochemistry of 1-naphthylmethyl carbonates and carbamates." Canadian Journal of Chemistry 72, no. 5 (May 1, 1994): 1254–61. http://dx.doi.org/10.1139/v94-159.

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The photochemistry in methanol of 1-naphthylmethyl phenyl carbonate (3) and 1-naphthylmethyl benzyl carbonate (4) has been studied. Products resulting from both the 1-naphthylmethyl cation and the 1-naphthylmethyl radical are obtained for 3, but only from the cation for 4. Similar results were obtained for the corresponding 1-naphthylmethyl derivatives 5 and 6 of N-phenyl and N-benzyl carbamic acids. The product yields for all four compounds can be explained by a mechanism of initial homolytic cleavage of the 1-naphthylmethyl carbon–oxygen bond from the excited singlet state. The radical pair generated then partitions between the two pathways: electron transfer to form the ion pair or decarboxylation. For PhO-CO-O• and PhNH-CO-O•, decarboxylation is rapid and competitive with electron transfer. For PhCH2O-CO-O• and PhCH2NH-CO-O•, decarboxylation is slower, electron transfer dominates, and only products from the ion pair are obtained.

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26

Holmes, John L., Nick A. van Huizen, and Peter C. Burgers. "Proton affinities and ion enthalpies." European Journal of Mass Spectrometry 23, no. 6 (August 30, 2017): 341–50. http://dx.doi.org/10.1177/1469066717728451.

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Proton affinities of a number of alkyl acetates (CH3–C(=O)–OR) and of methyl alkanoates (R–C(=O)–OCH3, R=H, alkyl) have been assembled from the literature or measured using the kinetic method. It was observed that the proton affinities for the isomeric species CH3–C(=O)–OR and R–C(=O)–OCH3 are almost identical, an unexpected result as the charge in these protonated ester molecules is largely at the keto carbon atom and so this site should be more sensitive to alkyl substitution. Analysis of the data, including those from lone pair ionisation and core-electron ionisation experiments available from the literature, indicate that after protonation, extensive charge relaxation (or polarisation) takes place (as is also the case, according to the literature, after core-electron ionisation). By contrast, after lone pair ionisation, which results in radical cations, such relaxation processes are relatively less extensive. As a consequence, changes in ion enthalpies of these protonated molecules follow more closely the changes in neutral enthalpies, compared with changes in enthalpies of the corresponding radical cations, formed by electron detachment. Preliminary analyses of published energetic data indicate that the above finding for organic esters may well be another example of a more general phenomenon.

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27

Bock, Hans, and Markus Kleine. "Elektronentransfer und Ionenpaar-Bildung, 48 [1] Titrationen von Tetraphenyl-p-benzochinon mit Natrium- und Kaliummetall in aprotischen Lösungen / Electron Transfer and Ion Pair Formation, 48 [ 1 ] Titrations of Tetraphenyl-p-benzoquinone with Sodium and Potassium Metals in Aprotic Solutions." Zeitschrift für Naturforschung B 51, no. 9 (September 1, 1996): 1222–28. http://dx.doi.org/10.1515/znb-1996-0902.

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UV/VIS and ESR spectra of electron transfer reaction products in aprotic (cH⊕ < 0,1 ppm) solution can be measured in an especially designed and sealed glass apparatus and provide information on unknown facets of the microscopic pathway through the network of interdependent equilibria. For tetraphenyl-p-benzoquinone in tetrahydrofuran, single-electron reduction by a sodium metal mirror produces a red solution and, unexpectedly, after addition of 2.2.2. cryptand, contact with a potassium metal mirror generates a green (!) one. For both, ESR/ENDOR spectra prove the presence of tetraphenyl-p-benzoquinone radical anion. UV/VIS measurements provide the clue: In the equilibrium revealed by repetetive spectra recording, M·⊖solv + Me⊕solv ⇄ [M·⊖···Me⊖]solv, the radical anion is green (vm = 16900 cm-1) and the contact ion pair red (vm=18900 cm-1 ). On ion pair formation, therefore, the excitation energy of the radical anion increases by 0.25 eV.

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28

Nevill, S. M., and J. A. Pincock. "The design of radical clocks to probe the reactivity of the intermediates in arylmethyl ester photochemistry." Canadian Journal of Chemistry 75, no. 2 (February 1, 1997): 232–47. http://dx.doi.org/10.1139/v97-027.

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The photochemistry in methanol of the esters 1–6was examined. These reactions normally proceed through radical pairs that result from homolytic cleavage of the carbon–oxygen bond in the excited singlet state. Each of the esters was designed to probe the intervention and reactivity of the substituted arylmethyl radical by incorporating a potential radical clock at the carbon of the reactive bond. For esters 1–5, the products isolated indicated that the radical clock was not reactive enough to compete with the very rapid alternate processes of the radical pair, namely, electron transfer to form the corresponding ion pair and decarboxylation of the phenylacyloxy radical (k = 4.6 × 109 s−1). Ester 6, which incorporates the extremely rapid fluorenylcyclopropylcarbinyl clock, showed very unusual reactivity. On thermal solvolysis in methanol, 6 rearranged quantitatively to the ester 20. No methyl ethers were detected. In contrast, photolysis of 6 in benzene resulted in an alternate rearrangement to the cyclobutyl ester, 22, resulting from the aryl version of the cyclopropyl-π-methane photochemical rearrangement. No ester cleavage occurred on excitation. A rationale for the latter conversion was based on stereoelectronic arguments provided by a crystal structure of 6. Keywords: photochemistry of arylmethyl esters, radical clocks.

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29

Subedi, Dili R., Habtom B. Gobeze, Yuri E. Kandrashkin, Prashanth K. Poddutoori, Art van der Est, and Francis D'Souza. "Exclusive triplet electron transfer leading to long-lived radical ion-pair formation in an electron rich platinum porphyrin covalently linked to fullerene dyad." Chemical Communications 56, no. 45 (2020): 6058–61. http://dx.doi.org/10.1039/d0cc02007a.

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30

Ali, Sameh Saad, Günter Grampp, Stephan Landgraf, and Michael Sacher. "Magnetic field modulation of the delayed fluorescence yield in the photoionization reaction of N, N, N', N'-tetramethyl-p-phenylenediamine in water." International Journal of Photoenergy 1, no. 3 (1999): 177–81. http://dx.doi.org/10.1155/s1110662x99000318.

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External magnetic field effects on the recombination fluorescence (MARY effect) in the photoionization reaction ofN,N,N′,N′-Tetramethyl-p-phenylenediamine (TMPPD) in water and DMSO/water mixture are studied. Relatively large magnetic field effects (MFE),∼2–4%, on the fluorescence yield are observed in the extremely polar water solvent under magnetic fields as small as 3 mT. Such MFE is hardly expected in water due to instability and very fast escape of the solvated electron from the solvent cage. Enhancement in the signal-to-noise ratio and superior time resolution characterizing the technique of field modulation allowed the detection of a very short lived radical ion pair (about 1 ns). The observed MARY spectra illustrate that the singlet radical ion pair is more reactive than the triplet one.

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31

Cooper, Martin J., Trinidad Diez-Rojo, Leon J. Rogers, Colin M. Western, Michael N. R. Ashfold, and Jeffrey W. Hudgens. "Ion-pair states of the ClO radical observed by multiphoton ionisation spectroscopy." Chemical Physics Letters 272, no. 3-4 (June 1997): 232–38. http://dx.doi.org/10.1016/s0009-2614(97)88014-1.

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32

Ji, Chang, Mohamed Ahmida, M'hamed Chahma, and Abdelaziz Houmam. "Radical/Ion Pair Formation in the Electrochemical Reduction of Arene Sulfenyl Chlorides." Journal of the American Chemical Society 128, no. 48 (December 2006): 15423–31. http://dx.doi.org/10.1021/ja062796t.

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33

Murai, Hisao, Hidekazu Honma, and Keiji Kuwata. "CIDEP Studies of Transient Radical-Ion Pair: Photooxidation of TMPD and Carbazoles*." Zeitschrift für Physikalische Chemie 1, Part_2 (January 1992): 31–40. http://dx.doi.org/10.1524/zpch.1992.1.part_2.031.

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34

Murai, Hisao, Hidekazu Honma, and Keiji Kuwata. "CIDEP Studies of Transient Radical-Ion Pair: Photooxidation of TMPD and Carbazoles*." Zeitschrift für Physikalische Chemie 182, Part_1_2 (January 1993): 31–40. http://dx.doi.org/10.1524/zpch.1993.182.part_1_2.031.

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35

Pincock, J. A., and I. S. Young. "The photochemistry of indenyl alcohols and esters: Substituent effects on the competition between ion- and radical-derived products." Canadian Journal of Chemistry 81, no. 10 (October 1, 2003): 1083–95. http://dx.doi.org/10.1139/v03-131.

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The photochemistry of the indenyl acetates 1 and pivalates 2, substituted with X = H, 5-CH3O, and 6-CH3O, have been examined in both methanol and cyclohexane. The precursor alcohols 3 were also found to be photoreactive. Although only radical-derived products were obtained in cyclohexane, both ion- and radical-derived products were formed in methanol. The absence of significant fluorescence emission from any of the substrates 1, 2, and 3 indicates that the excited singlet states are highly reactive. A mechanism is proposed for the ion-derived products that proceeds through direct heterolytic cleavage to give an indenyl cation carboxylate anion pair. The indenyl cations generated are anti-aromatic in the ground state and their efficient generation by this photochemical solvolysis is in sharp contrast to the very low reactivity of related ground-state substrates. For the pivalate esters 2, an excited-state migratory decarboxylation is proposed for the formation of tert-butyl derived products.Key words: ester photochemistry, indenyl cations, indenyl radicals.

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36

Takano, Yuta, Shota Obuchi, Naomi Mizorogi, Raúl García, M. Ángeles Herranz, Marc Rudolf, Silke Wolfrum, et al. "Stabilizing Ion and Radical Ion Pair States in a Paramagnetic Endohedral Metallofullerene/π-Extended Tetrathiafulvalene Conjugate." Journal of the American Chemical Society 134, no. 39 (September 24, 2012): 16103–6. http://dx.doi.org/10.1021/ja3055386.

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37

Horwitz, Noah E., Brian T. Phelan, Jordan N. Nelson, Catherine M. Mauck, Matthew D. Krzyaniak, and Michael R. Wasielewski. "Spin Polarization Transfer from a Photogenerated Radical Ion Pair to a Stable Radical Controlled by Charge Recombination." Journal of Physical Chemistry A 121, no. 23 (June 2, 2017): 4455–63. http://dx.doi.org/10.1021/acs.jpca.7b03468.

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38

Yamanaka, Ken-ichi, Mamoru Fujitsuka, Yasuyuki Araki, Kentaro Tashiro, Azumi Sato, Tetsuro Yuzawa, and Takuzo Aida. "Intramolecular photoinduced electron-transfer processes in buta-1,3-diynyl-benzene-linked porphyrin-fullerene dyad." Journal of Porphyrins and Phthalocyanines 11, no. 06 (June 2007): 397–405. http://dx.doi.org/10.1142/s108842460700045x.

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Intramolecular electron-transfer process of porphyrin-fullerene dyad linked by phenyl buta-1,3-diynyl-phenyl unit ( ZnP - sp - C 60) was studied by laser flash photolysis. Picosecond fluorescence lifetime and transient absorption measurements revealed that photoinduced charge-separation takes place via the excited singlet state (1 ZnP *) with the rate constant of (1-2) × 1010 s −1. For the charge recombination, about a half of the radical-ion pair decayed quickly with 2.9 × 109 s −1 as evaluated from picosecond transient absorption measurements, whereas the remaining half was long-lived with slow decay (1.6 × 106 s −1) as estimated from nanosecond transient absorption measurements. The lifetime of the radical-ion pair of ZnP - sp - C 60 was longer than those of directly connected dyads with a buta-1,3-diynyl bridge and buta-1,3-diynyl-phenyl bridge by the insertion of an extra phenyl group in addition to the pyrrodino ring.

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39

Warrier, M., Lakshmi S. Kaanumalle, and V. Ramamurthy. "Alkali metal ion controlled product selectivity during photorearrangements of 1-naphthyl phenyl acylates and dibenzyl ketones within zeolites." Canadian Journal of Chemistry 81, no. 6 (June 1, 2003): 620–31. http://dx.doi.org/10.1139/v03-041.

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Photochemical behaviors of 1-naphthyl phenyl acylates and dibenzyl ketones included in zeolites have been compared. 1-Naphthyl phenyl acylates while in solution produce eight photoproducts; within NaY it gives a single product. The selectivity is attributed to the restriction brought on the mobility of the primary radical pair by the alkali metal ions present in zeolites. Photochemistry of dibenzyl ketones within NaY reveals that the intersystem crossing in caged radical pairs could be influenced by the heavy alkali metal ions. Structures of complexes among Li+ ion and the guest 1-naphthyl phenyl acetates and dibenzyl ketone computed at the B3LYP level have been useful to understand the origin of the observed product selectivity within zeolites.Key words: photo-Fries reaction, zeolites, cationπ interaction, spin-orbit coupling, heavy atom effect.

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40

Herr, Jonathan D., and Ryan P. Steele. "Ion–Radical Pair Separation in Larger Oxidized Water Clusters, (H2O)+n=6–21." Journal of Physical Chemistry A 120, no. 36 (September 2016): 7225–39. http://dx.doi.org/10.1021/acs.jpca.6b07465.

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41

Murai, Hisao, Akihito Matsuyama, Tateki Ishida, Yohei Iwasaki, Kiminori Maeda, and Tohru Azumi. "Conrolling of radical-ion pair reactions by microwave radiation: photoconductivity-detected magnetic resonance." Chemical Physics Letters 264, no. 6 (January 1997): 619–22. http://dx.doi.org/10.1016/s0009-2614(96)01383-8.

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42

Pal, Kunal, Daniel R. Kattnig, Günter Grampp, and Stephan Landgraf. "Experimental observation of preferential solvation on a radical ion pair using MARY spectroscopy." Physical Chemistry Chemical Physics 14, no. 9 (2012): 3155. http://dx.doi.org/10.1039/c2cp23858f.

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43

Milanovsky, G. E., V. V. Ptushenko, D. A. Cherepanov, and A. Yu Semenov. "Mechanism of primary and secondary ion-radical pair formation in photosystem I complexes." Biochemistry (Moscow) 79, no. 3 (March 2014): 221–26. http://dx.doi.org/10.1134/s0006297914030079.

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44

Kobori, Y., T. Yago, and S. Tero-Kubota. "Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair." Applied Magnetic Resonance 23, no. 3-4 (September 2003): 269–87. http://dx.doi.org/10.1007/bf03166621.

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45

Sankararaman, S., W. A. Haney, and J. K. Kochi. "Annihilation of aromatic cation radicals by ion-pair and radical pair collapse. Unusual solvent and salt effects in the competition for aromatic substitution." Journal of the American Chemical Society 109, no. 25 (December 1987): 7824–38. http://dx.doi.org/10.1021/ja00259a035.

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46

Ueda, Mitsuhiro, Miho Hayama, and Hiroyuki Hashishita. "[2+2] Photocycloaddition of 3-Alkoxycoumarins with C60." Synlett 30, no. 18 (October 7, 2019): 2068–72. http://dx.doi.org/10.1055/s-0039-1690698.

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The [2+2] cycloaddition of 3-alkoxycoumarins with C60 proceeded stereoselectively under photoirradiation conditions to produce a new class of fullerene fused oligocyclic cyclobutane products. This reaction seems to progress via radical ion pair intermediates that arise from the SET processing of 3-alkoxycoumarins with the excited form of C60.

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47

Quast, Helmut, Georg Gescheidt, and Martin Spichty. "Topological Dynamics of a Radical Ion Pair: Experimental and Computational Assessment at the Relevant Nanosecond Timescale." Chemistry 2, no. 2 (March 31, 2020): 219–30. http://dx.doi.org/10.3390/chemistry2020014.

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Chemical processes mostly happen in fluid environments where reaction partners encounter via diffusion. The bimolecular encounters take place at a nanosecond time scale. The chemical environment (e.g., solvent molecules, (counter)ions) has a decisive influence on the reactivity as it determines the contact time between two molecules and affects the energetics. For understanding reactivity at an atomic level and at the appropriate dynamic time scale, it is crucial to combine matching experimental and theoretical data. Here, we have utilized all-atom molecular-dynamics simulations for accessing the key time scale (nanoseconds) using a QM/MM-Hamiltonian. Ion pairs consisting of a radical ion and its counterion are ideal systems to assess the theoretical predictions because they reflect dynamics at an appropriate time scale when studied by temperature-dependent EPR spectroscopy. We have investigated a diketone radical anion with its tetra-ethylammonium counterion. We have established a funnel-like transition path connecting two (equivalent) complexation sites. The agreement between the molecular-dynamics simulation and the experimental data presents a new paradigm for ion–ion interactions. This study exemplarily demonstrates the impact of the molecular environment on the topological states of reaction intermediates and how these states can be consistently elucidated through the combination of theory and experiment. We anticipate that our findings will contribute to the prediction of bimolecular transformations in the condensed phase with relevance to chemical synthesis, polymers, and biological activity.

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48

Mazzocchi, Paul H., and Gregory Fritz. "Photolysis of N-(2-methyl-2-propenyl)phthalimide in methanol. Evidence supporting radical-radical coupling of a photochemically generated radical ion pair." Journal of the American Chemical Society 108, no. 17 (August 1986): 5362–64. http://dx.doi.org/10.1021/ja00277a061.

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49

El-Khouly, Mohamed E., Jun Hasegawa, Atsuya Momotake, Mikio Sasaki, Yasuyuki Araki, Osamu Ito, and Tatsuo Arai. "Intramolecular photoinduced processes of newly synthesized dual zinc porphyrin-fullerene triad with flexible linkers." Journal of Porphyrins and Phthalocyanines 10, no. 12 (December 2006): 1380–91. http://dx.doi.org/10.1142/s1088424606000752.

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Zinc porphyrin-fullerene-zinc porphyrin triad, in which two zinc porphyrin ( ZnP ) moieties and a fullerene ( C 60) moiety are linked by flexible bonds and which is intended to be a working model of the photosynthetic antenna-reaction centre, has been newly synthesized. Its photophysical properties have been investigated by both time-resolved emission and transient absorption techniques. Excitation of the zinc porphyrin moiety of the triad induced charge separation, generating the radical ion pair, in which the electron localizes on the C 60 moiety and the hole localizes on the zinc porphyrin moiety. In polar solvents, the charge-separated states decayed with lifetimes of 300-600 ns returning to the ground state. Compared with ZnP - C 60 dyad, ZnP - C 60- ZnP triad showed longer lifetimes for the radical ion pair due to the conformation of the two ZnP moieties. The effects of the coordinating reagents on the zinc atom have been studied, with the expectation of conformational change of the two ZnP moieties with respect to C 60.

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50

Bock, Hans, Andreas John, Markus Kleine, Christian Näther, and Jan W. Bats. "Elektronentransfer und Ionenpaar-Bildung, 33 [1, 2]. Die Einelektronen-Reduktion von Tetraphenyl-p-benzochinon mit Alkalimetallen: ENDOR-Spektren von Kontaktionen-Paaren sowie Tripelionen in Lösung und Einkristallstrukturen der Neutralverbindung und ihres Natrium-Salzes / Electron Transfer and Ion Pair Formation, 33 [1,2]. The Single Electron Reduction of Tetraphenyl-p-benzoquinone by Alkali Metals: ENDOR Spectra of Contact Ion Pairs as well as Triple Ions in Solution and Single Crystal Structures of Both the Neutral Compound and its Sodium Salt." Zeitschrift für Naturforschung B 49, no. 4 (April 1, 1994): 529–41. http://dx.doi.org/10.1515/znb-1994-0416.

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Tetraphenyl-p-benzoquinone, according to its single crystal structure, shows some steric congestion: its quinone ring is distorted by 7° to a chair conformation, and its phenyl substituents are twisted around their CC axes between 46° and 72°. The half-wave reduction potentials of -0.57 and -1.25 V in acetonitrile confirm negligible π interaction of the phenyl substituents. Addition of alkalimetal tetraphenylborate salts lowers the second reduction potential due to contact ion formation, which can be confirmed by UV/VIS spectra recorded under aprotic conditions. Extensive ESR/ENDOR investigations prove the formation of the following species in THF solution: Tetraphenyl-p-benzosemiquinone radical anion contact ion pairs [M·⊖ Me⊕solv]' (Me⊕: Li⊕, Na⊕, Rb⊕, Cs⊕) and contact triple ion radical cations both with identical cations [M·⊖ (Me⊕solv)2]·⊕ (Me⊕: Li⊕, Na⊕, Cs⊕) and different cations [M·⊖ (Li⊕solv)(Me⊕solv)]·⊕ (Me⊕: Na⊕, Cs⊕). Addition of crown ethers can lead to external solvation of the Me⊕ counter cations, whereas cryptands form internal solvation complexes. The radical anion of 2,6-diphenyl-p-benzosemiquinone adds cations at its phenyl-free molecular half. The radical anion salt [tetraphenyl-p-benzosemiquinone·⊖ (Na⊕(tetrahydropyrane) 2)] could be crystallized and its structure determined at 200 K. In agreement with the Hirota sign rules for contact radicals in solution, the Na⊕ ion is found 62 pm above the π plane and 29° outside the axis of the CO bound, which is elongated due to one-electron reduction by 5 pm to 127 pm.

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