Self-termination and electronic spectra of substituted benzyl radicals in solution

Claridge, R.F.C.; Fischer, Hanns

doi:10.1021/j100234a026

Cited by 94 publications

(93 citation statements)

References 10 publications

(12 reference statements)

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“…Thus, the band at 320 nm can be assigned to the p-(chloromethyl)benzyl radical (2b) by comparison with the band of the p-methylbenzyl radical in solution. 7 On the other hand, the band at 290 nm is assigned to p-xylylene in accordance with literature data. 8 At first glimpse, the formation of p-xylylene from 1b appeared to be more efficient than the generation of o-xylylene from 1a, even if both dihalides were photolyzed under the same conditions [compare curve (b) in Fig.…”

supporting

confidence: 87%

Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α′-dichloroxylenes

Miranda¹,

Font‐Sanchis²,

Pérez‐Prieto³

et al. 1998

Chem. Commun.

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Two-colour two-laser techniques have unambiguously proved that photolysis of the o-/p-(chloromethyl)benzyl radical leads to the sequential two-photon generation of o-/pxylylene from a,aA-dichloro-o-/p-xylene.Xylylenes are molecules of considerable theoretical and synthetic interest, which have been detected and characterized by different spectroscopic methods. 1 Although benzylic dichlorides have been used as very simple and readily accessible precursors, the involved mechanisms are not completely understood. 2 It has been recently reported 3 that 266 nm laser irradiation of a,aA-dichloro-o-xylene (1a) produces the dissociation of both C-X bonds via a two-photon process; however, the nature of the intermediate species which absorbs the second photon is uncertain. Three possibilities have been considered: the S 1 and T 1 states of 1a and the o-(chloromethyl)benzyl radical (2a). Based on the short lifetime of the singlet state of the dichloride and the apparent relative yields of the observed transients, the undetected T 1 state of the dichloride has been suggested as the photochemical precursor of o-xylylene.Our aim was to explore the photochemistry of the benzyl radical 2a produced after dissociation of one C-X bond of a,aAdichloro-o-xylene, using two-laser two-colour techniques, in order to obtain new data to support or reject the intermediacy of 2a in the formation of o-xylylene. We report here our findings and compare them with those obtained in the photolysis of a,aAdichloro-p-xylene. Based on the obtained results, a mechanism for the formation of o-and p-xylylenes from their corresponding dichloro precursors is presented.Laser flash photolysis of deaerated 1.5 mm solutions of 1a in cyclohexane at 266 nm (Nd: YAG laser, fourth harmonic, < 10 ns, @20 mJ pulse 21 ) yielded the transient absorption spectra shown in Fig. 1. Two transients with different lifetimes were obtained. According to the literature, 3 they were assigned to 2a (with a maximum at 330 nm) and o-xylylene (3a) (maximum at 360 nm, e max = 3 3 10 3 dm 3 mol 21 cm 21 ), 4 generated through one-and two-photon processes, respectively.When irradiating with a 266 nm laser it is useful to place a beam diffuser (which eliminates 'hot' spots in the laser beam) close to the sample in order to get a better observation of monophotonic transients. 5 Actually, a new spectrum obtained under these conditions showed a higher ratio of radical 2a to the two photon intermediate, 3a (Fig. 1).To confirm the radical nature of the transient at 330 nm, oxygenated samples were examined, showing that, while the band at 330 nm was quenched at close to the diffusion controlled limit (see insert Fig. 1), the lifetime of the transient at 360 nm appeared insensitive to the presence or absence of oxygen (data not shown).Furthermore, to study the photobehaviour of 2a, two-colour two-laser flash photolysis experiments 6 were carried out using 266 nm laser pulses (20 mJ pulse 21 ) to photolyze 1a and a 308 nm excimer laser (90 mJ pulse 21 ) to irradiate 2a. The two pulses were ty...

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supporting

confidence: 87%

Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α′-dichloroxylenes

Miranda¹,

Font‐Sanchis²,

Pérez‐Prieto³

et al. 1998

Chem. Commun.

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“…[1][2][3][4][5][6][7][8][9] The quantum yield for diphenylethane formation is about 0.6-0.8 for most dibenzyl ketones, 1,3,12 except for derivatives with strongly electron-withdrawing groups (p-CN and p-NO 2 ), which do not undergo this photoreaction efficiently (quantum yield`2%). This is known for the p-CN derivative 1 and was confirmed in this work for the p-NO 2 derivative; higher energy excitation (248 nm) was also employed in the present work, but did not produce significant yields of the p-CN and p-NO 2 substituted benzyl radicals.…”

Section: Resultsmentioning

confidence: 99%

“…1 Upon excitation with a 308 nm laser pulse, the dibenzyl ketone precursors gave transient absorptions around 317 nm, the 2 2 A 2 2À X 2 B 2 absorption band of the benzyl radical chromophore. 6,17 Dissociation of dibenzyl ketone triplets is known to be fast 3 and generally occurs within the duration of the laser pulse, producing one equivalent of benzyl radical. 4,5 This results in an initial 'step' feature in the decay traces ( Fig.…”

Section: Resultsmentioning

confidence: 99%

“…This is attributed to the decarbonylation reaction, which produces another equivalent of benzyl radicals. 4,5 On longer time-scales, the benzyl radical absorption depletes with second-order kinetics, 6,9,20 when oxygen is excluded. 11 This reaction corresponds to benzyl radical coupling, 2 which is known to be a diffusion-controlled reaction.…”

Section: Resultsmentioning

confidence: 99%

“…for the development of radical clocks. [1][2][3][4][5][6][7][8][9][10] Photocleavage of dibenzyl ketones, for example, proceeds through phenacetyl radicals (Scheme 1). Photolysis produces a singlet-excited state with a lifetime of ca 3 ns, 11 which undergoes intersystem crossing to the triplet state, along with fluorescence and some cleavage.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

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

Zhang

Nau

2000

J. Phys. Org. Chem.

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Control of the Termination Mechanism in Radical Polymerization by Viscosity: Selective Disproportionation in Viscous Media

Nakamura

Ogihara

Hatano

et al. 2016

Chemistry A European J

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The termination mechanism of radical polymerization, that is, disproportionation (Disp) versus combination (Comb), determines the chain length and end-group structure of the resulting polymer as well as polymer properties, and yet factors governing the mechanism are still unclear. Furthermore, no attempts have been made to control the mechanism. Here, the effects of temperature and viscosity on the termination of methyl methacrylate (MMA) and styrene (St) polymerization were elucidated by using small molecular model-radicals and the corresponding polymer radicals in various solvents. The results showed that Disp was preferred over Comb if the temperature was decreased and the viscosity of the media was increased for all the radicals examined. Although the temperature effect on the Disp/Comb selectivity is counterintuitive because Disp should be favored entropically over Comb considering the decrease in the number of polymer chains in Comb, the results clearly showed that the observed inverse temperature effect was a result of the viscosity effect. Disp was favored over Comb at lower temperatures and in more viscous solvents because the transition state leading to Disp is more flexible than that for Comb. Because of the significant viscosity effect, Disp selectively occurred in highly viscous solvents; the Disp/Comb selectivity was 97/3 in both MMA and St termination. For the first time, the termination mechanism was intentionally controlled and such a high Disp selectivity was observed. In particular, the termination mechanism in St is described as Comb in textbooks, but nearly complete inversion of the selectivity from Comb to Disp is realized by simply changing the viscosity of the media.

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Self-termination and electronic spectra of substituted benzyl radicals in solution

Cited by 94 publications

References 10 publications

Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α′-dichloroxylenes

Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α′-dichloroxylenes

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

Control of the Termination Mechanism in Radical Polymerization by Viscosity: Selective Disproportionation in Viscous Media

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