Spin-correlated radical pair

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

Buckley C D, Hunger D A, Here P J and McLauchlan K A 1987 Electron spin resonance of spin-correlated radical pairs Chem. Phys. Lett. 135 307-12... 

Avdievich N I and Forbes M D E 1995 Dynamic effects in spin-correlated radical pair theory J modulation and a new look at the phenomenon of alternating line widths in the EPR spectra of flexible biradicals J. Phys. Chem. 99 9660-7... 

Forbes M D E, Avdievich N I, Schulz G R and Ball J D 1996 Chain dynamics cause the disappearance of spin-correlated radical pair polarization in flexible biradicals J. Phys. Chem. 100 13 887-91... 

O.G. Poluektov, L.M. Utschig, A.A. Dubinskij and M. Thurnauer, ENDOR of spin-correlated radical pairs in photosynthesis at high magnetic field A tool for mapping electron transfer pathways, J. Am. Chem. Soc., 2004, 126, 1644. 

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... 

Bagryansky, V. A., Borovkov, V. I., Molin, Yu. N. (2002) Singlet-triplet oscillations of spin-correlated radical pairs caused by the Larmor precession in weak magnetic fields, Doklady Physical Chemistry 382, 62-65. 

Salikhov, K.M., Kandrashkin, Yu.E., and Salikhov, A.K. (1992) Peculiarities of free induction and primary spin echo signals for spin-correlated radical pairs, Appl. Magnet. Reson. 3, 199-217. 

Tang, J., Thurnauer, M.C., and Norris, J.R. (1994) Electron spin echo envelope modulation due to exchange and dipolar interactions in a spin correlated radical pair, Chem. Phys. Lett., 219, 283-290. 

Extensive studies of the sensitizer dependence and the solvent dependence of the polarization patterns led to the identihcation of two parallel pathways of that deprotonation. One is a proton transfer within the spin-correlated radical pairs, with the radical anion A acting as the base. The other is a deprotonation of free radicals, in which case the proton is taken up by surplus starting amine DH. Furthermore, evidence was obtained from these experiments that even in those situations where the polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6 these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by deprotonation of the radical cation by either of the described two routes. The whole mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited for an analysis is the product V. 

Spin correlated radical pairs, SCRPs, have been observed in micellar solutions and their origin was elucidated by Eorbes et al. These SCRP have been widely studied under pho tor eduction of benzophenone and other electron/hydrogen acceptors mostly... 

SCRP - spin-correlated radical pair TM - triplet mechanism RPM-radical pair mechanism... 

A detailed description of CIDEP mechanisms is outside the scope of this chapter. Several monographs and reviews are available that describe the spin physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from singlet-triplet electron spin wave function evolution during the first few nanoseconds of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism (TM) is a net polarization arising from anisotropic intersystem crossing in the molecular excited states. For the polymers under study here, the TM is net E in all cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a later section. Other CIDEP mechanisms, such as the radical-triplet pair mechanism and spin-correlated radical pair mechanism, are excluded from this discussion, as they do not appear in any of the systems presented here. 

The findings of such peculiar phase patterns attracted considerable attention because they could not explained by the ordinary CIDEP theories (the TM, STo mixing, and ST i mixing). Buckley et al.[22] and Closs et al.[23] independently interpreted the these peculiar patterns in terms of spin-correlated radical pairs. It is noteworthy that ESR signals due to radical pairs were found to be directly observable in solution even at room temperature. Since then, CIDEP due to this spin-correlated mechanism (SCM) has often been obtained not only in micellar solutions but also in other confined systems such as in viscous solutions [24], in linked systems [25], and in bacterial photosynthesis systems [26]. 

Since the STo mixing occurs through Q in this model, the eigenstates ( f)) and eigenvalues ( p)of a spin-correlated radical pair become as follows ... 

Fig. 5-11 also shows the CIDEP spectra calculated with this model (a) 7 > 0 J (w > 0 J) and (b) 7 < 0 J (w < 0 J). As clearly seen in this figure, each spectrum has two anti-phase doublets with an A/E (E/A) pattern for 7 > 0 J (7 < 0 J). Similarly, one can show that it has an E/A (A/E) one for a radical pair generated from an S-precursor for 7 > 0 J (7 < 0 J). The splitting of each anti-phase doublet corresponds to 27. This is a novel method to determine the 7 values of radical pairs and biradicals in solution. Let us consider the conditions for detecting the CIDEP of spin correlated radical pairs. When 3 (= 7/ ) is much larger than Q, little polarization can be obtained because Ipg becomes very small from Eq. (5-43). When 3 (= 7 ti ) is smaller than the line width of each ESR line, the CIDEP signals also vanishes because the anti-phase components of each doublet cancel out with each other. For intermediate 3 (= 7/ft ), CIDEP due to the SCM becomes intense. Each of the anti-phase... 

The low MW power levels commonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the full time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable technique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [JJ] and nuclear modulations [18], that have been useful to interpret EPR data on light-induced spin-correlated radical pairs. 

Analysis of the dephasing of the out-of-phase echo for radical pairs produced by photo-excitation can give the interspin distance. This technique has been applied predominantly in studies related to photosynthesis. Following creation of the radical pair, a two-pulse spin echo is created. Crucial to the interpretation is that there is phase coherence between eigenstates present in the photo-induced spin-correlated radical pair (84). The spin-spin interaction causes deep modulation in the out-of-phase echo, whereas the normal, in-phase, echo vanishes (84). Distances in the range of 25 - 34 A have been measured to define distances between donor and acceptors in the photosystem (84-96). 

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