Photochemistry of 1‐pyrenyldiazomethane*

Silva, Salom; Olea, Andreas F.; Thomas, J. K.

doi:10.1111/j.1751-1097.1991.tb02050.x

Cited by 9 publications

(14 citation statements)

References 15 publications

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“…Scheme 2 Photophysical and photochemical pathways for reactions of electronically excited PDAM. 23 PyCH: = pyren-1-ylcarbene, PyCH 2 ؒ = pyren-1-ylmethyl radical, and R ؒ is a polymeric radical from a polymer chain R-H. The concentration of oxygen in the solid films during our experiments (performed under N 2 atmospheres) is very low.…”

Section: Reaction Mechanism and Selectivity Of Attachmentmentioning

confidence: 94%

“…The smaller spectral overlap between the unreactive π,π* state and the reactive n,π* excited state in less polar media leads to greater reaction efficiencies. 23 The probable mechanism for loss of nitrogen by electronically excited PDAM and subsequent insertion of pyren-1-ylcarbene into C-H bonds of polymer chains is shown in Scheme 2. 22,23 PDAM should react in polyethylene films like it does in liquid aliphatic hydrocarbons (Fig.…”

Section: Reaction Mechanism and Selectivity Of Attachmentmentioning

confidence: 99%

“…The π,π* and n,π* excited states of PDAM in PVAc are mixed to a greater extent than in the less polar polyethylene, and deactivation of the excited singlet state may involve a larger fraction of non-reactive pathways, including intersystem crossing that can result in initial formation of triplet carbene. 23 Insertion of triplet carbene into a C-H bond of a polymer chain is a stepwise process (see steps 6-11 of Scheme 2). 23 Regardless, the same set of attachment products, but perhaps with a different distribution, should be formed from insertions of singlet and triplet pyren-1-ylcarbene.…”

Section: Reaction Mechanism and Selectivity Of Attachmentmentioning

confidence: 99%

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Selective attachment of pyrenyl groups to ethylene-co-vinyl acetate copolymers: dynamic and static fluorescence studies

Yamaki

Atvars

Weiss

2002

Photochem Photobiol Sci

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Micromorphology is an important factor in determining polymer properties and uses. Here, steady-state and dynamic fluorescence from covalently attached 1-alkylpyrenyl groups are used to investigate the micromorphology of several random ethylene-co-vinyl acetate (EVA) copolymers with defined compositions of vinyl acetate monomer. The results are compared with those from homopolymers of high- and low-density polyethylenes and poly(vinyl acetate). Selective attachment of pyren-1-yl groups to polymer chains was accomplished by irradiation of pyren-1-yldiazomethane sorbed into polymer films. Steady-state fluorescence spectra and fluorescence decay rates of attached pyrenyl groups in films of the polymers have been compared with those from films with sorbed pyrene. I1/I3 intensity ratios from vibrational bands in the fluorescence spectra and the fluorescence decay rates of attached 1-alkylpyrenyl groups are much less sensitive to changes in the copolymer composition than are those of pyrene. These observations suggest that attachment occurs selectively to the olefinic segments (rather than to the acetate-rich regions) of polymer chains of EVA copolymers. This conclusion is consistent with the known preferences for reaction by pyren-1-ylcarbene in solution.

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Section: Reaction Mechanism and Selectivity Of Attachmentmentioning

confidence: 94%

Section: Reaction Mechanism and Selectivity Of Attachmentmentioning

confidence: 99%

Section: Reaction Mechanism and Selectivity Of Attachmentmentioning

confidence: 99%

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Selective attachment of pyrenyl groups to ethylene-co-vinyl acetate copolymers: dynamic and static fluorescence studies

Yamaki

Atvars

Weiss

2002

Photochem Photobiol Sci

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“…In general, attachment to polymer chains facilitates analyses of luminescence results because translocation of lumophores upon macroscopic perturbations such as temperature changes or film stretching are avoided; doped lumophores can (and frequently do) change their locations within a polymer as a perturbation is applied. The PyCH 2 -attachment technique, irradiation of 1-pyrenyldiazomethane [46,52] is more selective and yields fewer side products than our more commonly used method to introduce covalently-attached Py-groups, direct UV/Vis irradiation [26,27] or bombardment with MeV range particles [28,29] of pyrene molecules.…”

Section: Discussionmentioning

confidence: 99%

“…1-Pyrenyldiazomethane (UV/Vis: λ max 366, 387 nm in methanol) was prepared by a literature procedure [52] from the respective hydrazone, followed by pyrolysis in ethylene glycol. Typically, a 1 cm × 3.5 cm piece of polyethylene film was immersed in a 0.01 M 1-pyrenyldiazomethane/diethyl ether solution at −5 to −10 • C overnight in a dark environment, air-dried, rinsed with methanol to remove surfaceaccessible 1-pyrenyldiazomethane molecules and irradiated for 10 min at 365 nm (450 W Hanovia medium pressure mercury lamp; Ultra-Achromatic-Takumar filter, Asahi Optical, Japan) in a Pyrex tube purged with nitrogen.…”

Section: Covalent Attachment Of 1-pyrenylmethyl Lumophoresmentioning

confidence: 99%

Covalently-attached 1-pyrenylmethyl groups as a probe of temperature-dependent relaxation processes in polyethylene films

Xu¹,

Luo²,

Atvars³

et al. 2004

Res. Chem. Intermed.

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1-Pyrenylmethyl lumophores have been attached covalently to chains of several types of polyethylene (crystallinities 37-74%). The temperature dependence of their fluorescence between 40 and 400 K, as well as that of ultra-high-molecular-weight polyethylene (85% crystallinity) containing covalently-attached 1-pyrenyl groups, has been combined with data from DSC measurements and information concerning the properties of films to determine the utility and limitations of the fluorescence changes to detect the onset temperatures of α-, β-and γ -relaxation processes in the polyethylenes. The relative sensitivities of fluorescence from covalently-attached pyrenyl and anthryl (with a much shorter excited singlet-state lifetime) probes to each of the relaxation processes are compared in the same films. The probe whose excited singlet-state lifetime better matches the rate associated with a relaxation process is better able to detect it.

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Solvent Reorganization Controls the Rate of Proton Transfer from Neat Alcohol Solvents to Singlet Diphenylcarbene

2002

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Femtosecond transient absorption spectroscopy was used to study singlet diphenylcarbene generated by photodissociation of diphenyldiazomethane with a UV pulse at 266 nm. Absorption by singlet diphenylcarbene was detected and characterized for the first time. Similar band shapes were observed in acetonitrile and in cyclohexane with lambda(max) approximately 370 nm. The singlet absorption decays by intersystem crossing to triplet diphenylcarbene at rates that agree with previous measurements. The singlet absorption band is completely formed 1 ps after the pump pulse. It is preceded by a strong and broad absorption band, which is tentatively assigned to excited-state absorption by a singlet diazo excited state. In neat alcohol solvents the growth and decay of the diphenylmethyl cation was observed. This species is formed by proton transfer from an alcohol molecule to singlet diphenylcarbene. Since a shell of solvent molecules surrounds each nascent carbene, the intrinsic rate of protonation in the absence of diffusion could be measured. In methanol, proton transfer occurs with a time constant of 9.0 ps, making this the fastest known intermolecular proton-transfer reaction to carbon. In O-deuterated methanol proton transfer occurs in 15.0 ps. Slower rates were observed in the longer alcohols. The protonation times correlate reasonably well with solvation times in these alcohols, suggesting that solvent fluctuations are the rate-limiting step. In all alcohols studied, the carbocations decay on a somewhat slower time scale to yield diphenylalkyl ethers. In methanol and ethanol the rate of decay is determined by reaction with neutral solvent nucleophiles. There is evidence in 2-propanol that geminate reaction within the initial ion pair is faster than reaction with solvent. No isotope effect was observed for the reaction of the diphenylmethyl carbocation in methanol. Using comparative actinometry the quantum yield of protonation was measured. In methanol, the quantum yield of carbocations reaches a maximum value of 0.18 approximately 18 ps after the pump pulse. According to our analysis, 30% of the photoexcited diazo precursor molecules are eventually protonated. Somewhat lower protonation efficiencies are observed in the other alcohols. Because the primary quantum yield for formation of singlet diphenylcarbene is still unknown, the importance of reaction channels that might exist in addition to protonation cannot be determined at present. Singlet carbenes are powerful, photogenerated bases that open new possibilities for fundamental studies of proton transfer in solution.

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Photochemistry of 1‐pyrenyldiazomethane*

Cited by 9 publications

References 15 publications

Selective attachment of pyrenyl groups to ethylene-co-vinyl acetate copolymers: dynamic and static fluorescence studies

Selective attachment of pyrenyl groups to ethylene-co-vinyl acetate copolymers: dynamic and static fluorescence studies

Covalently-attached 1-pyrenylmethyl groups as a probe of temperature-dependent relaxation processes in polyethylene films

Solvent Reorganization Controls the Rate of Proton Transfer from Neat Alcohol Solvents to Singlet Diphenylcarbene

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