Spin-polarized radical pairs

Analytical expressions were derived for the CW EPR line-shape for spin-polarized radical pairs in the limit where the combined dipolar and exchange interaction is weak relative to the energy differences between the resonances of the two spins.19 The equations were applied to the case of charge-separated sites in Ti02 nanoparticles. This approach simplifies the analysis of the distributions of interspin distances. 

In radical-pair states, the average distance between the two unpaired electron spins is typically much larger than in triplet states. Hence, TREPR spectra of photogenerated (and electron-spin polarized) radical pairs are narrower due to the reduced mutual dipolar and exchange interactions as compared to flavin triplets. This is shown in Fig. 7b, where the TREPR signal of a flavin-based radical pair in photolyase is depicted [19]. Analysis of the spectral shapes of TREPR signals yields information on the chemical nature of the individual radicals of the radical-pair state, and the interaction of the radicals with each other and with their immediate surroundings. 

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... 

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... 

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. 

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. 

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... 

Wasielewski, M. R., Gaines, HI, G. L., Wiederrecht, G. R, Svec, W. A., Niemczyk, M. R (1993). Biomimetic modeling of photosynthetic reaction center function Long-lived, spin-polarized radical ion pair formation in chlorophyll-porphyrin-quinone triads, 7. Am. Chem. Soc., 115 10442. 

Requirements for Charge Separation and Spin-Polarized Radical Ion Pair Formation... 

To observe the important structure-dependent anisotropic spin-spin interactions, such as the dipolar interaction, D, within radical pairs and to prevent spin lattice relaxation from destroying the spin polarization, it is necessary to examine the radical pairs in the solid state at low temperatures. Thus, our work on achieving high quantum yield charge separation in low temperature solids gives us the necessary tools to produce the appropriate supermolecule. We recently reported preliminary results on 2-TAPD-ZP-2-NQ in Figure 3, the first molecule to exhibit spin-polarized radical ion pair formation in the solid state.[27]... 

The EPR spectra of both 2-TAPD -ZP-2-NQ and 2-TAPD -ZP-l-NQ can be attributed to 2 radicals, TAPD with a broad linewidth at lower g-factor and NQ with a narrow linewidth at higher g-factor. Spin-polarization is observed on a millisecond time scale because the spin-lattice relaxation times of the radicals are long at 5 K. Spin polarization in these radical pairs can result from two mechanisms. The first mechanism is the usual radical pair mechanism, RPM, of CIDEP.[35] S-Tq mixing in TAPD-ZP" -NQ is followed by polarization transfer to a non-interacting radical pair TAPD -ZP-NQ, i.e. = 0. The second mechanism assumes that TAPD -ZP-NQ is itself an interacting spin correlated radical pair, i.e. 2J 0.1 -10 G and D 1 - 5 G.[36-38] In this case S-Tq mixing in TAPD -ZP-NQ can also produce polarized spectra. 

Figure Bl.16.5. An example of the CIDNP net effect for a radical pair with one hyperfme interaction. Initial conditions g > g2, negative and the RP is initially singlet. Polarized nuclear spin states and schematic NMR spectra are shown for the recombination and scavenging products in the boxes.

In the early 1990s, a new spin polarization mechanism was posPilated by Paul and co-workers to explain how polarization can be developed m transient radicals in the presence of excited triplet state molecules (Blattler et al [43], Blattler and Paul [44], Goudsmit et al [45]). While the earliest examples of the radical-triplet pair mechanism (RTPM) mvolved emissive polarizations similar in appearance to triplet mechanism polarizations, cases have since been discovered m which absorptive and multiplet polarizations are also generated by RTPM. 

Closs G L and Trifunac A D 1969 Chemically Induced nuclear spin polarization as a tool for determination of spin multiplicities of radical-pair precursors J. Am. Chem. Soc. 91 4554-5... 

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... 

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