Electron-spin polarization

All the above EPR spectroscopy was carried out in the steady state. With the use of fast-response spectrometers, however, it was discovered a decade ago that when measured early (at room temperature a few /u,s) after a light flash, the EPR spectra of primary reactants in PS I [105,106] and in bacterial RC [107] showed EPR lines characteristic of systems out of Boltzmann equilibrium (Table 2). Part or all of these so-called spin-polarized lines may then either be in emission or show absorption that is enhanced or decreased compared to the equilibrium absorption (Fig. 7). Electron spin polarization occurs through magnetic interactions between two simultaneously induced donor-acceptor radicals (reviewed in Ref. R14). Thus, a study of spin-polarized EPR lines yields information on these magnetic interactions and therefore on the configuration (distance, relative orientation, etc.) of the radicals (see e.g. Ref. R14). 

From EPR studies on oriented chloroplasts and PS I particles [108-110] and on perdeuterated bacterial RCs [35] it was concluded that anisotropic (dipolar) interactions played a major role, at least in oriented samples. Applying a theoretical 

Time evolution of the spin-polarized EPR signal of prereduced RCs (P I Q ) treated as in Fig. 7. Note the inversion of the 3 ms spectrum with respect to the unpolarized 40 ms spectrum. The shoulder at low g value in the 50 /its spectrum is due to magnetic interaction with P. From Ref. 128. 

When the primary acceptor is prereduced, electron spin polarization can transfer by exchange interaction from BPh to Q, leading to an inversion of the EPR line of in RCs where was magnetically uncoupled from Fe [112] (Fig. 8). From a phenomenological treatment [112-114] it was concluded that the exchange interaction /(BPh QA) was 3 - 5 G, whereas /(P BPh ) was between 1 and 5 G. A more sophisticated treatment of the three-spin system P BPh QX [115] led to 7(P BPh ) between 0 and +8 G. (Note that for / = 0 polarization may develop if D = 0.) A positive value of / for a biradical state is unusual it might be explained by some form of superexchange via an intermediate (possibly one of the accessory bacteriochlorophylls). 

Regardless of the precise value of /(P BPh ) it is clear that the values obtained are much (some three orders of magnitude) smaller than expected from the rate of charge separation (2.8 ps [116]) when simple tunneling theory is applied [50,57,113,117,118]. This also seems to indicate the need for another intermediate (which need not function as a true electron acceptor but could act as a transmitting medium via a superexchange mechanism). 

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While each of die previous examples illustrated just one of the electron spin polarization iiiechanisms, the spectra of many systems involve polarizations from multiple iiiechanisms or a change in meclianism with delay time. 

Atkins P W and Evans G T 1974 Electron spin polarization in a rotating triplet Mol. Phys. 27 1633—44... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Goudsmit G-H, Paul H and Shushin A I 1993 Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion J. Phys. Chem. 97 13 243-9... 

Closs G L and Forbes M D E 1991 EPR spectroscopy of electron spin polarized biradicals in liquid solutions. Technique, spectral simulation, scope and limitations J. Phys. Chem. 95 1924-33... 

Norris J R, Morris A L, Thurnauer M C and Tang J 1990 A general model of electron spin polarization arising from the interactions within radical pairs J. Chem. Phys. 92 4239—49... 

We also plotted the electron spin polarization, by itself (top row) and projected onto the electron density isosurface for the molecules containing the CH2, O and Be substituents (the orientation of the atoms in the plots is indicated at the left) ... 

VI. Chemically Induced Dynamic Electron Spin Polarization (CIDBP)... 

Hence, provided that I g is known and that R has been determined by means of an independent experiment, provides the cross-relaxation rate ct. This enhancement is called nuclear Overhauser effect (nOe) (17,19) from Overhauser (20) who was the first to recognize that, by a related method, electron spin polarization could be transferred to nuclear spins (such a method can be worked out whenever EPR lines are relatively sharp it is presently known as DNP for Dynamic Nuclear Polarization). This effect is usually quantified by the so-called nOe factor p... 

The dependence of the dissociation constant kj) and the recombination constants kjt in the reaction (a) on the optical polarization of the atoms A was predicted by Bernheim [65] and Kastler [225] and later demonstrated experimentally [7, 363]. The dependence can be understood from the obvious fact that only hydrogen-like atoms with opposite electronic spins may recombine and form a molecule A2(X1E+). Hence we have kR = kft(l — S2), where S is the degree of electron spin polarization of the atoms. A convenient indicator of dimer formation is provided by the kinetics of the laser-induced molecular fluorescence after switching on magnetic resonance which destroys the polarization of the atoms, as performed by Huber and Weber [201] for a Na — Na2 mixture. The se-... 

A subset of electron-hole radical pairs exhibits features of Spin Correlated Radical Pair (CRRP) electron spin polarization mechanism [101] which can be observed at somewhat longer times via light/field modulated (LFM) EPR measurements. This technique is only sensitive to the light dependent part of the EPR spectrum on the time scale of the light modulation frequency (millisecond regime, insert Fig. 1.15). Using LFM EPR it was observed that both the transitions of the holes localized on the surface modifier and electrons localized on the Ti02... 

Tero-Kubota S, Katsuki A, Kobori Y. Spin-orbit coupling induced electron spin polarization in photoinduced electron transfer reactions. J Photochem Photobiol C Photochem Rev 2001 2 17-33. 

Second, it is conceivable that amines quench triplet ketones before spin-lattice relaxation takes place within the three triplet sublevels 159>, which are populated unevenly 160>. In that event, radicals can be produced with their electron spins polarized. The CIDEP phenomenon 161>, whereby EPR emission is observed upon irradiation of ketones and very reactive substrates, may involve this mechanism. In fact, certain CIDNP observations may depend on rapid quenching of spin-polarized triplets 162>. 

The phenomenon of electron spin polarization in photoexcited triplet molecules in solids has been known for quite some time (39,51,52). The mechanism is associated with the unequal populations of the triplet sublevels induced by the spin-selective nature of the spin-orbit coupling interactions which couple the excited singlet and triplet states during the intersystem crossing (ISC) process. In the presence of an external magnetic field the spin polarization in the molecular frame can be transferred to the laboratory frame for esr observation. Kim and Weissman (83,84) have recently demonstrated beautifully that the initial polarization following photoexcitation of the triplet molecules such as pentacene in dilute solid solution can be readily observed up to a temperature of 275°K. 

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

Tr. To date, all the photoexcited triplet CIDEP observed satisfy these conditions with k-p having a value of 10 -10 s-- -and D < coz. While almost all of the photoexcited triplet CIDEP in solution involved the carbonyl chromophore, the singular case of solid-state photochemical decomposition of diphenyldiazo-methane at 1.2°K giving a ground-state triplet diphenylmethylene with electron spin polarization was reported by Doetschman et al. in 1976 (44). There is a genuine potential that the photoexcited triplet mechanism can operate in systems containing chromophores other than the carbonyl and in solid-state photochemical systems. 

The first step in the mechanism is the photoexcited triplet CIDEP process with the different quantum efficiencies of producing radicals in the upper and lower spin states being denoted as Q+ and Q, respectively. The corresponding nuclear spin states are n+ and n . The second step is the partial transfer of the electron spin polarization to the nuclear spin states by electron-nuclear cross relaxation, Wq and W2 Finally, the radicals with nuclear spin polarization must react to form diamagnetic products for CIDNP observation. These chemical reactions must compete... 

Decweeu i ne pruaucis in i ne n ana tne n st.ai.es. c,n lepiesetiLs the fraction of the electron spin polarization which is transferred to the nuclear states, and it is given by the expression... 

Electron Spin Polarization Transfer from Radicals of Photoinitiators to Stable Nitroxyl Polyradicals... 

Clarke (326) has studied the optical electron spin polarization in triplet anthracene and has observed ESR emission at 1.5°K which was attributed to a non-Boltzman distribution over the triplet spin levels at low temperature. The dynamics of optical spin polarization in triplet naphthalene at 1.6°K was also reported by Sixl and Schwoerer (327a) and van der Waals et al. (327b). have used a general method to study dynamics of populating and depopulating triplet spin levels by microwave-induced delayed phosphorescence. These experiments enable measurements of the lifetimes of each triplet spin state and thus can provide important information about intramolecular decay processes and intermolecular triplet energy transfer. 

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