Photoinduced electron-transfer processes involving substituted stilbene oxides

Das, P. K.; Müller, Adolf; Griffin, G. W.

doi:10.1021/jo00185a029

Cited by 26 publications

(24 citation statements)

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“…This assignment is based on the similarity of the absorption spectral features, decay kinetic behavior, and oxygen sensitivity with those reported in the literature 23,24 for the radical anion.…”

Section: Transient Absorption Spectra and Kineticsmentioning

confidence: 74%

Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

Cyr

Das

2014

J. Phys. Chem. A

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Employing nanosecond laser flash photolysis, we determined the relative importance of two fragmentation modes, namely, C-C bond cleavage and deprotonation, for the radical cation of 1,1,2,2-tetraphenylethane photogenerated by electron transfer to cyanoaromatic singlet excited states in acetonitrile at room temperature. Analysis of the kinetic data for this phenyl alkane suggests that the C-C bond cleavage dominates over the deprotonation by a ratio of about 2:1. In addition, the deprotonation kinetics of diphenylmethane, 1,1-diphenylethane, triphenylmethane, and several phenyl-substituted alcohols have been investigated. To aid identification and characterization, experiments based on two laser pulses in tandem (308 and 337.1 nm) were performed to probe fluorescence and photochemistry of the transient radicals formed as products of radical ion fragmentation. The first-order rate constants for growth of transient absorptions due to fragmentation-derived radicals were measured to be ≥1 × 10(6) s(-1). Activation parameters, with activation enthalpies in the range 10-18 kJ/mol and activation entropies between -60 and -91 J/(mol.K), are also reported for fragmentation kinetics of radical cations of several systems under study.

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Section: Transient Absorption Spectra and Kineticsmentioning

confidence: 74%

Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

Cyr

Das

2014

J. Phys. Chem. A

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“…at 565 and 610 nm [8,42] or TMB radical cation (TMB . ) at 470 nm [44] was observed after the 355 nm pulse, and an inset records the growth of TMPD . at 565 nm.…”

Section: Electron Transfer From Lipss and Dhla To The Triplet Riboflavinmentioning

confidence: 99%

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B₂)

Bucher

Sander

2004

ChemPhysChem

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As a powerful natural antioxidant, lipoic acid (LipSS) and its reduced form dihydrolipoic acid (DHLA) exert significant antioxidant activities in vivo and in vitro by deactivation of reactive oxygen and nitrogen species (ROS and RNS). In this study the riboflavin (RF, vitamin B2) sensitized UVA and visible-light irradiation of LipSS and DHLA was studied employing continuous irradiation, fluorescence spectroscopy, and laser flash photolysis (LFP). Our results indicate that LipSS and DHLA quench both the singlet state (1RF*) and the triplet state (3RF*) of RF by electron transfer to produce the riboflavin semiquinone radical (RFH.) and the radical cation of LipSS and DHLA, respectively. The radical cation of DHLA is rapidly deprotonated twice to yield a reducing species, the radical anion of LipSS (LipSS.-). When D2O was used as solvent, it was confirmed that the reaction of LipSS with 3RF* consists of a simple electron-transfer step, while loss of hydrogen occurs in the case of DHLA oxidation. Due to the strong absorption of RFH. and the riboflavin ground state, the absorption of the radical cation and the radical anion of LipSS can not be observed directly by LFP. N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and N,N,N',N'-tetramethyl benzidine (TMB) were added as probes to the system. In the case of LipSS, the addition resulted in the formation of the radical cation of TMPD or TMB by quenching of the LipSS radical cation. If DHLA is the reducing substrate, no formation of probe radical cation is observed. This confirms that LipSS.- is produced by riboflavin photosensitization, and that there is no oxidizing species produced after DHLA oxidization.

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“…The fourth term is the solvation energy for the contact ion pair (the dipole moment (13) Table 2. The value of IP for TMB in the gas phase has been reported to be 6.1-6.8 eV,'J and the energy of the triplet TMB* is 2.7 eVS4 Assuming here that Thus the CIP formation from the excited triplet state of TMB is energetically less favorable, by about 0.5 eV, than that of TMPD for a system involving the same AX.…”

Section: Rxmentioning

confidence: 99%

Mechanism of Contact Ion Pair Formation in Photolyzed Mixtures of N,N,N',N'-Tetramethylbenzidine with Halogen-Containing Compounds in Nonpolar Solvent

Shimamori¹,

Okuda²

1994

J. Phys. Chem.

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Time variation of microwave dielectric absorption has been examined for photolyzed solutions of N,N,N',N'-tetramethylbenzidine (TMB) admixed with a halogen-containing compound AX (AX = CC4, CZHSI, CzHSBr, CaH51, and C6HsBr) in benzene. In each solution the dielectric absorption signals show rapid growth after photoirradiation, indicating formation of a contact ion pair (CIP) TMB+X-(X denotes a halogen atom). The amplitudes of the signals are dependent on the AX concentration and are large for CC14 but relatively small in other molecules when compared at the same concentration. The growth rate corresponds to the response time of the apparatus and are not dependent on the concentration of the solute molecule, which excludes the possibility of electron transfer from an isolated excited triplet state of TMB to the AX molecule, as opposed to the case of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). A mechanism has been suggested that the CIPs are formed from direct excitation of complexes (TMBsAX) present in the solution. The difference in the efficiencies of the CIP formation among the electron-acceptor molecules can be explained by assuming nonadiabatic transitions for the electron-transfer processes. IntroductionA recent study1 with the time-resolved microwave dielectric absorption technique has shown that an electronically excited state, probably in a singlet state, of N,N,N',N'-tetramethyl-pphenylenediamine (TMPD) reacts with solvent CC14 to form a contact ion pair (CIP) TMPDTl-with near unit quantum yield. It has also been found2 that t'he excited triplet state of TMPD can produce a similar CIP, TMPD+X-(X = halogen atom), in the presence of a halogenated compound AX (CC4, CZH~I, C&I, C&Br, and C6HsCl) in benzene solvent. These processes occur one-photonically, and the rates and the efficiencies of CIP formation in these systems correlate well with the nature of dissociative electron attachment to the molecule AX. The observed drastic variation of the efficiency of CIP formation on the electron-acceptor molecule implies that the ionization via the excited triplet state of TMPD is a result of just crossing over a border of ionization threshold. Most of the energy required for the ionization appears to be brought from the energy gained by forming the CIP. A more recent study has shown that similar ion pairs can be formed in photolyzed N,N,N',N'-tetramethylbenzidine (TMB) in solvent cc14,3 and the properties associated with the contact ion pair well resemble those in TMPD+Cl-. The TMB molecule involved in this case may be in an excited singlet state. Then a question arises as to whether the excited triplet state of TMB can also produce the CIP, as in the TMPD case. The energy of the lowest triplet state of TMB is 2.7 eV4 which is slightly lower than that of TMPD (=2.9 e V 9 , whereas the ionization potential of TMB (6.1-6.8 eV in the gas phase'**) may be higher than that of TMPD (5.9-6.6 eV9-11). The latter can be supported by the fact that the oxidation potential of TMB vs SCE is 0.53 eV higher than TMPD.12 From...

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Photoinduced electron-transfer processes involving substituted stilbene oxides

Cited by 26 publications

References 0 publications

Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B₂)

Mechanism of Contact Ion Pair Formation in Photolyzed Mixtures of N,N,N',N'-Tetramethylbenzidine with Halogen-Containing Compounds in Nonpolar Solvent

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Photoinduced electron-transfer processes involving substituted stilbene oxides

Cited by 26 publications

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Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

Kinetics of C–C and C–H Bond Cleavage in Phenyl Alkane Radical Cations Generated by Photoinduced Electron Transfer

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B2)

Mechanism of Contact Ion Pair Formation in Photolyzed Mixtures of N,N,N',N'-Tetramethylbenzidine with Halogen-Containing Compounds in Nonpolar Solvent

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Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B₂)