Transient phenomena in the photochemistry of trans-azocumene

Boate, D. R.; Scaiano, J. C.

doi:10.1016/s0040-4039(01)80761-9

Cited by 14 publications

(16 citation statements)

References 29 publications

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“…Concerning our work we are particularly interested to know the thermolysis rate of cis-azocumene (cAC). The values 5 μs and 12 μs were reported in cyclopentane [25] and in heptane [26], respectively. This time interferes with time range of our study (10 -7 -10 -5 s).…”

Section: Direct Excitation Photochemistrymentioning

confidence: 98%

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

Milikisiyants

Steiner

Paul³

2011

Appl Magn Reson

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The triplet-sensitized photodecomposition of azocumene into nitrogen and cumyl radicals is investigated by time-resolved electron paramagnetic resonance and absorption spectroscopy. The radicals are found to be created spin polarized with a yield depending on the strength of the applied magnetic field. The phenomenon arises because in triplet azocumene, the decay into radicals competes with a fast triplet-sublevel selective intersystem crossing back to the azocumene ground state. The size of the initial spin polarization of the radicals and the magnetic field effect on their yield are determined in solvents of different viscosities. Data analysis yields rate constants for the intersystem crossing and the cleavage reaction of triplet azocumene as well as its zero-field splitting D ZFS. At room temperature in nonpolar solvents, the most probable values are: k x = k y = 1.2 × 1011 s−1 and k z = 1.9 × 1010 s−1 for the intersystem crossing from the energetically lower and upper triplet substates, respectively, k p = 1.6 × 109 s−1 for the cleavage reaction and for the zero-field splitting D ZFS = −3.4 × 1010 s−1 (0.18 cm−1).

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Section: Direct Excitation Photochemistrymentioning

confidence: 98%

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

Milikisiyants

Steiner

Paul³

2011

Appl Magn Reson

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“…A rate constant k 1 ~ 2" 10 5 s -1 has been estimated for the decomposition of cis-azocumene in cyclohexane [21]. Thus, k 1 can only be varied around this value.…”

Section: Reaction Rate Constants and Initial Transient Concentrationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…The cis-isomer (cAC) decays by a kinetics of first order (steps b and d) with a mean lifetime of 5 ixs at room temperature in cyclohexane solution [21]. The lifetime of the cumyldiazenyl radical (step e) is not exactly known, but an upper limit of less than 20 ns has been suggested [21]. In inert solvents the resulting cumyl radicals terminate in in-cage and out-of-cage processes to combination and disproportionation products [23,24].…”

Section: Introductionmentioning

confidence: 99%

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Quantitative time-resolved EPR CIDEP study of the photodecomposition oftrans-azocumene in solution

Savitsky

Paul

1997

Appl. Magn. Reson.

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The cumyl radical system, which is created after laser flash irradiation of trans-azocumene in benzene solution at room temperature, is investigated using time-resolved EPR spectroscopy. From the quantitative analysis of EPR time-profiles at different microwave powers the spin relaxation times T~ = 3.5+0.3 ~ts and T 2 = 2.5_+0.1 ~ts are evaluated as well as the magnitude of the chemically induced electron pola¡ (CIDEP), which is generated by the radical pair mechanism (RPM). The geminate RPM polarization is found to be considerably smaller than the F-pair one, 32_+2 and 48_+5 in units of the Boltzmann pola¡ respectively. This is attributed to ah initial radical separation in the geminate pair, caused by the cleavage reaction. Besides cleavage, the photoexcited transazocumene also decays via isomerization to the thermally unstable cis-isomer, the lifetime of which is found to be 14_+3 ~ts at 293 K in benzene, three times longer than in cyclohexane. The quantum yield of free radicals, escaping from the primary cage, is determined as 0.28_+0.06 for the decay of the excited trans-azocumene and 0.18_+0.04 for the thermal cleavage of the cis-isomer. The self-termination of cumyl radicals proceeds with a rate constant 2k t = (7_+1). 10 s M-Xs -l in benzene at RT.

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“…By using nanosecond-resolved laser flash photolysis (LFP) and the N-methylquinolinium tetrafluoroborate (NMQ + BF 4 À )/biphenyl (BP) co-sensitization technique, the absorption maximum (l ob ) of 2 dC + was observed at 476 nm [8] in dichloromethane ( Figure 1a). Interestingly, the l ob of 2 dC + appears at an unusually long wavelength in comparison with those of its chromophoric components, the cumyl radical (3 dC, l ob = 322 nm in cyclohexane, Figure 1b) [9,10] and the cumyl cation (3 d + , l ob = 322 nm in HSO 3 F, Figure 1c). [11,12] Williams reported earlier that the parent cyclohexane-1,4-diyl radical cation (4C + ) in its chair conformation has a l ob at 685 nm in CFCl 4 .…”

Section: Introductionmentioning

confidence: 99%

Evidence for Significant Through‐Space and Through‐Bond Electronic Coupling in the 1,4‐Diphenylcyclohexane‐1,4‐diyl Radical Cation Gained by Absorption Spectroscopy and DFT Calculations

Ikeda

Hoshi

Namai

et al. 2007

Chemistry A European J

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Photoinduced single-electron-transfer promoted oxidation of 2,5-diphenyl-1,5-hexadiene by using N-methylquinolinium tetrafluoroborate/biphenyl co-sensitization takes place with the formation of an intense electronic absorption band at 476 nm, which is attributed to the 1,4-diphenylcyclohexane-1,4-diyl radical cation. The absorption maximum (lambda(ob)) of this transient occurs at a longer wavelength than is expected for either the cumyl radical or the cumyl cation components. Substitution at the para positions of the phenyl groups in this radical cation by CH(3)O, CH(3), F, Cl, and Br leads to an increasingly larger redshift of lambda(ob). A comparison of the rho value, which was obtained from a Hammett plot of the electronic transition energies of the radical cations versus sigma(+), with that for the cumyl cation shows that the substituent effects on the transition energies for the 1,4-diarylcyclohexane-1,4-diyl radical cations are approximately one half of the substituent effects on the transition energies of the cumyl cation. The observed substituent-induced redshifts of lambda(ob) and the reduced sensitivity of lambda(ob) to substituent changes are in accordance with the proposal that significant through-space and -bond electronic interactions exist between the cumyl radical and the cumyl cation moieties of the 1,4-diphenylcyclohexane-1,4-diyl radical cation. This proposal gains strong support from the results of density functional theory (DFT) calculations. Moreover, the results of time-dependent DFT calculations indicate that the absorption band at 476 nm for the 1,4-diphenylcyclohexane-1,4-diyl radical cation corresponds to a SOMO-3 --> SOMO transition.

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Transient phenomena in the photochemistry of trans-azocumene

Cited by 14 publications

References 29 publications

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

Quantitative time-resolved EPR CIDEP study of the photodecomposition oftrans-azocumene in solution

Evidence for Significant Through‐Space and Through‐Bond Electronic Coupling in the 1,4‐Diphenylcyclohexane‐1,4‐diyl Radical Cation Gained by Absorption Spectroscopy and DFT Calculations

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