Radical pair fluorescence detection

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. ... 

A related technique, fluorescence detected magnetic resonance (FDMR), is suitable for the observation of short-lived radical ion pairs with lifetime in the range of... 

Brocklehurst et al. found that the MFE on the fluorescence intensity at 200 ns after the pulse increase with increasing B from 0 T to 0.1 T, but that the MFE shows a saturated value (40 % increase) with increasing B from 0.1 T to 0.5 T as shown in Fig. 6-3(c). Such a MFE on the singlet yield can be explained by the HFC from an S-precursor as shown in Fig. 6-2(b). According to the HFCM, the triplet yield should be increased by the fields of 0.1 - 0.5 T, but such a MFE on the triplet yield was not clear in this reaction. Later, such MFEs on the triplet yield were found in photochemical reactions as shown in section 6.6. Similar results were also found in cyclohexane, but the observed MFEs were less than those observed in squalane. In benzene, there was no detectable MP on the fluorescence intensity. This solvent effect can be explained by the effect on the lifetime of the generated ion-radical pairs. This means that the more viscous the solvent is the longer the radical pair lifetime becomes. 

Two different types of pulsed EPR experiments are possible a spectrum can be measured at a fixed time after the pulse by variation of the field strength B (Eq. 72), or the time profile of a particular spectral line can be measured at constant B to give kinetic information. One vziriation of this kinetic method is to detect the recombination of singlet-state radical ion pairs in liquid hydrocarbons by the fluorescence of the product excited state [142]. This technique is known as fluorescence-detected magnetic resonance (FDMR) and provides information on the spin dynamics of the radical ion pair as well as the chemical kinetics. 

Two papers have been presented on the photochemistry of 5-methylphena-zinium salts in aqueous solution. Fluorescence, optical flash photolysis, and electron paramagnetic resonance (e.p.r.) techniques have been used to elucidate various aspects of product formation and quantum yield. Two products have been identified, namely the 5-methyl-10-hydrophenazinium cation radical (MPH ) and the pyocyanine (l-hydroxy-5-methyl-phenozinium) cation (PyH ) in a stoicheiometric ratio of 2 1. The quantum yield of formation of (MPH ) was found to be 0.29 0.03 at pH 7.0 and 1.1 0.1 at pH 3.0. The triplet state of MP (Ti) has also been detected by triplet-triplet absorption and is found to have a lifetime of 0.5 ns. Flash photolysis and e.p.r. have also been used to study a geminate triplet radical pair obtained from hydrogen abstraction by excited triplet acetone from propan-2-ol. The authors demonstrate that the geminate pairs contribute most of the polarization in photochemically-induced dynamic electron polarization (CIDEP) as compared with free random-phase pairs. 

Reactions which have occurred at zero time are replaced by their reactive products (if applicable). For all surviving species reactions times are generated from a model distribution conditioned on the radical pair separation distance. The fluorescence intensity I (t) (which is the experimentally observable quantity) is detected as D D [reaction (10)] and D [reaction (7)] to allow better statistics to be obtained. The simulation is therefore run twice, one with zero field parameters and one using high field parameters the ratio of I sit)/hit) is then obtained to observe the magnetic field effect. In the simulation, no T or relaxation mechanism was assumed to take place at high fields (unless otherwise stated). For zero field calculations Tq was assumed to be equal to T2 with the spin-spin relaxation time set to a value of 30 and 9 ns for S+ and D+ respectively. These values are based on the analysis by Borovkov on the rate of electron self-exchange [34, 41, 42] for S+ and D+, with... 

St /G AG-0.14 V fluorescence quenching stilbene radical anion detected in transient absorption no Gs near injection site or in intervening sequence k = 1012-108 s 1 for 0-4 intervening A-T base-pairs (-3.4-17 A) exponential distance dependence of CT rate constant (3 0.6-0.7 A"1 small variations in k depending on whether G is in the A or T arm of the hairpin... 

Increasing the solvent polarity results in a red shift in the -t -amine exciplex fluorescence and a decrease in its lifetime and intensity (113), no fluorescence being detected in solvents more polar than tetrahydrofuran (e = 7.6). The decrease in fluorescence intensity is accompanied by ionic dissociation to yield the t-17 and the R3N" free radical ions (116) and proton transfer leading to product formation (see Section IV-B). The formation and decay of t-17 have been investigated by means of time resolved resonance Raman (TR ) spectroscopy (116). Both the TR spectrum and its excitation spectrum are similar to those obtained under steady state conditions. The initial yield of t-1 is dependent upon the amine structure due to competition between ionic dissociation and other radical ion pair processes (proton transfer, intersystem crossing, and quenching by ground state amine), which are dependent upon amine structure. However, the second order decay of t-1" is independent of amine structure... 

Because of their high intensity. X-ray tubes were commonly used as laboratory radiation sources for radiation chemistry experiments until they were superceded by particle accelerators during the middle part ofthe 20th Century. They still retain specialized uses in research applications such as being used as the radiation source for MARY (MAgnetic field effect on Reaction Yield) spectroscopy studies of radical cation lifetimes and reactivity in alkane solvents [14,18]. MARY spectroscopy uses fluorescence to detect variations in singlet-triplet dynamics in radical ion pairs as a function of magnetic field. It is particularly useful for short-lived transients that are difficult to study by ESR. 

An illustration of an intramolecular PET from an electron-rich aromatic ring to a benzoate by irradiation at 2 4 nm in methanol as the solvent is given in Scheme II. The radical ion pair is then transformed to the observed fragmentation and reduction products. Interestingly, no reaction was detected at 300 nm in methanol, even though ethylbenzoate was found to quench the fluorescence of p-fV,A-dimethyltoluidinc. 

MARY spectroscopy uses fluorescence from the exciplex to detect the variation in the singlet-triplet spin dynamics of a radical ion pair as a function of the magnetic field. 

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