Time-resolved ESR spectra

Figure 11 shows the t,-resolved ESR spectra of the irradiated PMMA before and after the photobleaching at 77 K. The broad singlet line at the center of the spectrum in Fig. 11 (A) is due to the anion radical. The anion radical signal becomes comparatively less intense with the increase of tx, and a doublet signal with a coupling constant of 2 mT becomes apparent. This doublet can be attributed to the main-chain radical, -CH- [39,42], and is evidenced for the first time to exist immediately after the irradiation by using the ESE method. The tt-resolved ESR spectra after the photobleaching (Fig. 11(B)) show clearly the doublet due to the main-chain radical and the triplet due to the side-chain radical, because the spectrum of the methyl radical decays away very quickly due to its fast phase relaxation. On the other hand, the spectrum of the main-chain radical is not observed by the conventional cw ESR method, because it was masked by the strong spectral lines of the methyl radical.

Analysis of a CIDEP pattern with time-resolved ESR (TR ESR) spectra provides a solid conclusion to be made on the spin multiplicity of molecular precursors of polarized free radicals (a singlet or a triplet excited molecule) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry. Continuous wave TR ESR (CW TR ESR) devices are widely used for detection of photogenerated radicals. They usually consist of a pulsed ns laser with detection of transients by their ESR spectra with a X band ESR spectrometer in the direct detection mode (no hied modulation).Time-resolved Eourier transform ESR (FT ESR) has some advantages and drawbacks with respect to CW TR ESR. Rather sophisticated FT ESR devices have become available, and FT ESR studies become more common. 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

Carbonyl groups are most significant in triplet state photochemistry. Spectroscopic studies include time resolved ESR studies of enols of -hydroxybenzaldehyde 2 energy transfer from acetophenones to 9, 1 0-d ibr omoa n t hr a ce ne (S.,) which involves higher triplet states of the donor283, two photon excitation of substituted triplets of acetophenone , transient resonance Raman spectra of deuterated benzophenones , and diffuse reflectance of... 

The radical pairs in the diradical and dicarbene reaction intermediates have been extensively studied spectroscopically and lead to a consistent picture of the reaction mechanisms and of the ir-bond and radical electron structure of the intermediates. A new field of research has been opened by time resolved ESR spectroscopy of transient triplet states on the polymer chains. Spectroscopic worlc concerning triplet excited states is just at the very beginning. The correlation of the optical spectra (Figure 8), the transient absorption data well... 

The inclusion of xanthone, 90, in the three cyclodextrins was studied by time-resolved ESR spectroscopy at 77 K [262,263]. The spectra in the... 

First, a mixture of oligomers containing 2-7 monomer units (P = 2-7) was prepared by ATRP and model radicals with short-chain lengths were generated from the mixture without any further separation. Well-resolved ESR spectra of the model radicals were observed at various temperatures. The 12-line ESR spectrum observed at 150°C is shown in Fig. 7a the two p-methylene protons are almost equivalent in small radicals at such high temperature. This finding indicates that rotation of the radical chain end is too fast to detect differences in methylene protons on the time scale of ESR spectroscopy. In order to estimate the critical chain length that would show the 16-line spectrum, model radical precursors with... 

An electrochemical and ESR study of 2,7-disubstituted phenazines has appeared <1996CPB1448>. The electrochemically generated radical cation of phenazine A, A -dioxide was investigated by ESR electrolysis and cyclic voltammetry <2002MI4245>. Time-resolved and steady-state ESR spectra were observed for the lowest excited triplet (Ti) states of phenazine and its monoprotonated cation (phenazinium) in sulfuric acid-ethanol mixture at 77 K <2005SAA1147>. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

The ESE method was used to identify the overlapping ESR spectra of the irradiated PMMA more definitely. The measurements of the ESE-detected ESR were made at 77 K with the 90° — t — 180 two-pulse sequence, at various fixed times of longitudinal relaxation, tl5 while the external magnetic field was swept slowly. The t( -resolved ESR spectrum was obtained from the difference between the echo intensity at a fixed x of 0.5 ps with and without a 90° saturation pulse... 

In solid-state studies, ESR spectroscopy is the best detection method for studying radical intermediates in radiolysis. It is, however, difficult to apply to liquid-phase studies, and generally, optical methods are favoured. In solid-state work, radicals are trapped (matrix-isolated) and can be studied by any spectroscopic technique at leisure. However, for liquid-phase studies, time-resolved methods are often necessary because the intermediates are usually very short lived. In the technique of pulse radiolysis, short pulses of radiation, followed by pulses of light which explore the UV spectrum, are used. The spectra help to identify the species, but also their kinetic behaviour can be accurately monitored over very short time-scales (from picoseconds to milliseconds). This technique is discussed in Section 3.3. 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

Multi-resonance and pulsed ESR techniques provide better spectral resolution than conventional ESR, although usually with a lower sensitivity. Multi-resonance involves ENDOR, TRIPLE and ELDOR in CW and pulsed modes. ENDOR is mainly used to increase the spectral resolution, so that overlapping or unresolved hyperfine structure in the ESR spectra can be resolved. The techniques are applicable both to liquid and solid samples. CW X-band spectrometers with an ENDOR attachment that also allows TRIPLE experiments have been commercially available for a long time. In a TRIPLE experiment two RE fields are applied. A theoretical application is to determine the relative signs of two hyperfine couplings. Accessories... 

Molecular order and dynamics in thylakoid lipids obtained form measurement of time-resolved fluorescence anisotropy (FA) of TMA-DPH in lipid vesicles, ESR spectra of cholestane in liposomes and AFD of DPH in planar oriented membranes. 

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