Electron Spin Polarization and Time-Resolved Electron Paramagnetic Resonance: Applications to the Paradigms of Molecular and Supramolecular Photochemistry

Turro, Nicholas J.; Kleinman, Mark H.; Karatekin, Erdem

doi:10.1002/1521-3773(20001215)39:24<4436::aid-anie4436>3.0.co;2-x

Cited by 70 publications

(35 citation statements)

References 43 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Figure 2d presents the ESE ESR spectrum of 4 (2.8 mM) in THF recorded 80 ns after laser flash photolysis, which agrees with similar time-resolved ESR spectra observed earlier. 48,74,107,110 The septet of doublets of the 2-hydroxy-2-propyl (dimethyl ketyl) radicals 4a are emissively polarized by the triplet mechanism as reported earlier 74,110 with coupling constants of a(6H ) ) 1.933 ( 0.005 mT [a(6H ) ) 1.93-1.98 mT 111 ] and a(OH) ) 0.022 ( 0.002 mT [a(OH) ) 0.03-0.07 mT 111 ]. A weak broad emissive singlet can be detected near the centerline of radical 4a that corresponds to the benzoyl radicals (4b) with a slightly lower g-factor (g j ) 2.0007 111,112 ) than the 2-hydroxy-2-propyl radical (g j ) 2.0031 111,112 ).…”

Section: S(τ)mentioning

confidence: 64%

“…Similar spectra were observed 48,74,79,87,[91][92][93] and analyzed earlier. 92,93 The spectrum is polarized in net absorption due to the triplet mechanism 45,[94][95][96][97][98] and shows a doublet of singlets due to the diphenylphosphinoyl radical 1a.…”

Section: S(τ)mentioning

confidence: 99%

“…A weak broad emissive singlet can be detected near the centerline of radical 4a that corresponds to the benzoyl radicals (4b) with a slightly lower g-factor (g j ) 2.0007 111,112 ) than the 2-hydroxy-2-propyl radical (g j ) 2.0031 111,112 ). Similar to the other photoinitiators 1-3, the signal intensity ratio of the benzoyl 4b to 4a is much lower than in the TR CW ESR spectra 48,74,107,110 due to the different relaxation times of these radicals. In the absence of alkenes, the phase memory T M of the benzoyl radical 4b was determined to be T M ) 142 ( 4 ns in THF and T M ) 139 ( 8 ns in toluene, whereas the dimethyl ketyl radicals 4a have higher values by a factor of 6 of T M ) 890 ( 50 ns in THF and T M ) 800 ( 35 ns in toluene.…”

Section: S(τ)mentioning

confidence: 99%

“…[42][43][44][45][46][47][48] The latter can be achieved by choosing photoinitiators that generate strong polarized radicals after laser flash excitation of the photoinitiator. However, the extraction of rate constants from intensity-time profiles is tedious and time-consuming.…”

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,

Weber

Turro

2003

J. Phys. Chem. A

Self Cite

View full text Add to dashboard Cite

A time-resolved Fourier transform electron spin resonance (TR FT ESR) method for determining absolute rate constants of reactive, free radical reactions is described. The phase memory times T M of a family of reacting radicals were measured precisely by electron spin-echo (ESE) detection. In all cases the ESE signal decays show a simple pseudo-first-order behavior. Rate constants of the reactions were extracted from the linear relationship between the reciprocal of the T M values and the alkene concentrations. The validity of the method is demonstrated for the measurement of the rate constant for addition of different phosphinoyl and 2-hydroxy-2-propyl radicals to several acrylates, methyl methacrylate, and vinyl pivalate in organic solvents at room temperature. The results are in excellent agreement with those obtained by other kinetic investigations employing time-resolved steady-state electron spin resonance (TR CW ESR) and optical absorption spectroscopy. The technique is shown to be applicable to systems with rate constants between 10 4 and 10 8 M -1 s -1 , a range covered by the alkenes examined.

show abstract

Section: S(τ)mentioning

confidence: 64%

Section: S(τ)mentioning

confidence: 99%

Section: S(τ)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,

Weber

Turro

2003

J. Phys. Chem. A

Self Cite

View full text Add to dashboard Cite

show abstract

“…Time-resolved ESR offers a method for direct detection and characterization of radical structure and reactivity, but requires strong spin polarization of the radicals. [19][20][21][22][23][24][25] Strong polarization of radical adducts is assured if the photoinitiators produce strong polarized radicals, which in turn, undergo fast addition to alkenes. Upon photolysis (2,4,6-trimethylbenzoyl)diphenylphosphine oxide [26][27][28] and 2-hydroxy-2-methyl-1-phenyl-propan-1-one, [29][30][31][32][33] which are widely used as photoinitiators for free radical polymerization, produce strongly polarized radicals that undergo fast addition to acrylate and, therefore, were chosen as photoinitiators for our studies.…”

Section: Introductionmentioning

confidence: 99%

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

2001

Self Cite

View full text Add to dashboard Cite

Addition of two radicals (diphenyl phosphinoyl and 2-hydroxy-2-propyl) to the bifunctional alkene, vinyl acrylate, was studied by both time-resolved (TR) and steady-state (SS) ESR and laser flash photolysis (LFP). The adduct radicals are predominately a result of tail addition (addition to the unsubstituted carbon atom) of the acrylate double bond. Chemical structures of the adducts were established by comparison of the observed ESR spectra with those of adducts of the same reactive radicals to structurally related alkenes, tert-butyl acrylate and vinyl pivalate, which have only one type of double bond. Adducts of bulky phosphinoyl radicals to the acrylates demonstrate hindered rotation and a cis-trans isomerization at room temperature. The structure of the adduct radicals and the reactivity of the two radicals are discussed. Absolute rate constants for the addition of the phosphinoyl radical to the alkenes were measured by LFP in ethyl acetate at 296 K. A rate constant of k add ) (33 ( 1) × 10 6 M -1 s -1 was found for vinyl acrylate. The latter value is ∼1.5 times higher than that for tert-butyl acrylate (k add ) (22 ( 1) × 10 6 M -1 s -1 ) and ∼17 times higher than that for vinyl pivalate (k add ) (2.0 ( 0.1) × 10 6 M -1 s -1 ). These rate constants are consistent with conclusions derived from the ESR data. The results provide some insights into free radical polymerization of vinyl acrylate and vinyl ethers.

show abstract

Direct Observation of a Distant Ion Pair Generated in a Photoconductive Poly(N-vinylcarbazole) Film with Dopant

et al. 2001

View full text Add to dashboard Cite

Poly(N-vinylcarbazole) (PVCz) has received much attention as an organic photoconductor, since the PVCz film is one of the best functional materials for photodetection systems, electrophotographic imaging, etc. [1] Over the past few decades, a considerable number of studies on the PVCz film have been made to clarify the photoconduction mechanism in the amorphous organic solid state. [2] A simplified mechanism for the carrier generation prior to the carrier transport is proposed for the photoexcitation of charge-transfer (CT) complexes in acceptor-doped films in which the CT complexation takes place efficiently. The geminate ion pair is a key intermediate in the generation of the free ions (Scheme 1). Scheme 1. Carrier generation process.The interionic distance (r 0 ) of the ion pair was reported to be around 20 ± 30 on the basis of the external electric field effects on the steady-state fluorescence from the exciplex [3] and on the generation yield of the carriers (free ions). [4] On the other hand, the dichroism relaxation measurements using picosecond optical spectroscopy gave a negative response to the presence of the hypothetical distant ion pair developed during a cooling process of the exciplex. [5] In the present study, we detected a spin-correlated radical pair (SCRP) and determined the distance between the transient pair radicals in 1,2,4,5-tetracyanobenzene (TCNB)-doped PVCz film [6] by using time-resolved electron paramagnetic resonance (EPR) spectroscopy. [7] The spin-polarized EPR spectrum was clearly observed, as shown in Figure 1 a, when the CT complex in the PVCz film was selectively excited by the laser-pulse irradiation at 532 nm. The spectral phase is enhanced absorption (A) in the low-field half and emission (E) in the high-field half. It is noteworthy that the signals are detected over a relatively wide region of more than 8 mT. The spectrum intensity decreased exponentially with the delay time after the laser flash, in Figure 1. a) Time-resolved EPR spectrum of TCNB-doped PVCz film observed at 500 ns after the pulse excitation at the CT band at room temperature. b) Continuous wave (Cw)-EPR spectra obtained by numerical integration of the first-derivative signals of PVCz in concentrated H 2 SO 4 and TCNB À in MTHF at 8 K recorded using a lock-in detection.keeping the same broad spectral shape with the A/E pattern during the decay. To assign the observed transient paramagnetic species, we measured separately the conventional steady-state EPR spectra for the TCNB anion [8] and the PVCz cation [9] in rigid glassy solvents (Figure 1 b). Both free ions showed EPR signals at the magnetic field corresponding to g 2.0028, but the spectral widths were quite different from each other, mainly because of the difference of the hyperfine interactions. The central field of the time-resolved EPR spectrum for the TCNB-doped PVCz film is also in agreement with g 2.0028, which indicates that the transient species are associated with the TCNB À and PVCz radicals. However, the spectral width of the transient ...

show abstract

Electron Spin Polarization and Time-Resolved Electron Paramagnetic Resonance: Applications to the Paradigms of Molecular and Supramolecular Photochemistry

Cited by 70 publications

References 43 publications

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

Direct Observation of a Distant Ion Pair Generated in a Photoconductive Poly(N-vinylcarbazole) Film with Dopant

Contact Info

Product

Resources

About

Electron Spin Polarization and Time-Resolved Electron Paramagnetic Resonance: Applications to the Paradigms of Molecular and Supramolecular Photochemistry

Cited by 70 publications

References 43 publications

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

Electron Spin Resonance and Laser Flash Photolysis Study of Radical Addition to Vinyl Acrylate and Related Alkenes

Direct Observation of a Distant Ion Pair Generated in a Photoconductive Poly(N-vinylcarbazole) Film with Dopant

Contact Info

Product

Resources

About

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes^,