Initial triplet spin polarization

The utility of CIDEP in photochemistry was greatly enhanced when it was realized (131) that the radical-pair mechanism is not the exclusive spin polarization mechanism. Initial triplet spin polarization produced by the different intersystem-crossing rates to the excited triplet sublevels can be "transferred" to radicals formed by the photochemical reaction of the polarized triplet. 

The situation is quite different with S-T -type CIDNP because nuclear spins are flipped in that case. Owing to the coupling of nuclear spin motion and electron spin motion, not only the electron spin state oscillates in such a system but also the nuclear spin state. Since, however, one-half of the pairs or biradicals cannot participate in this because their nuclear spin state does not allow an electron-nuclear flip-flop transition, the oscillation is not symmetrical. Its turning points are zero nuclear spin polarization and 100% nuclear spin polarization of one sign only. In contrast, the distribution of nuclear spin polarizations between singlet and triplet members of the ensemble is symmetrical. As an example, consider an ensemble of biradicals, where each biradical contains a single proton. Let the ensemble be created in the state T >, and without initial nuclear spin polarization. Half of the pairs, namely those that have nuclear spin /J>, cannot undergo flip-flop transitions. The others oscillate between T a> and S/3>. When all of those happen to be in S/ >, every nuclear spin of the triplet biradicals and every... 

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. 

CIDEP Initial Polarization. The establishment and the development of the photoexcited triplet mechanism in CIDEP of transient radicals in solution had been rather controversial, if not as turbulent and exciting as the photoexcitation process itself. The early objections centered around two very important questions. The first one concerns the uncertainty of whether the spin polarization in the molecular frame can be effectively transferred to the laboratory frame for triplet systems in liquid solution. The second related question involves the fact that the polarized triplet molecules are rotating rapidly with respect to the laboratory axes and the triplet spin lattice relaxation time T x (normally between 10 and lO-- - s) would be too short for the polarization to be retained in the radical pair. The earlier photoexcited triplet mechanism developed by Wong et al. (136,137) is based on a "static model" with the excited triplet molecules being randomly oriented. Such a static model cannot deal satisfac-... 

In the triplet model the spin polarization is with respect to the internal molecular states, TjJ>, Ty>, and T > of the triplet and evolves with time according to the time-dependent Schrodinger equation into a spin polarization with respect to the electron spin Zeeman levels Ti>, Tq>, and T i> in an external magnetic field Bq. Consider a simple case of axially symmetric zero-field splitting (i.e., D y 0 and E = 0 D and E are the usual zero-field parameters). Tx>, [Ty>, and TZ> are the eigenstates of the zero-field interaction Hzfs, where Z is the major principal axis. The initial polarization arising from the population differences among these states can be expressed as... 

At first, such CIDEP was considered to be due to the polarization transfer from an excited triplet molecule to a radical. The initial spin polarization of triplet benzophenone is emissive, but that of triplet pyruvic acid is absorptive. Similarly, the spin polarization of the radicals was always emissive regardless of the spin polarization of the triplet states [8]. Thus, such emissive signals were attributed to the T-D quenching as shown by Processes (13-8)... 

A normal emission/absorption/emission/absorption pattern that did not vary in the time interval 0.7-5 ps was observed for the radical pairs from 35 and 36, while the spectrum from 34 was initially totally absorptive (0.4-0.6ps), then rapidly changed to emissive/absorptive (1.9-2.1 ps), and eventually became totally emissive (4.6-5 ps).37 It is suggested that for ZP4V, due to the smaller spacer chain, the time elapsed between laser excitation and radical-pair formation was shorter than for ZP6V and ZP8V and the spin polarization in the porphyrin triplet was retained to a larger extent before electron transfer took place.37... 

Figure Bl.16.6. An example of CIDNP net effect for a radical pair with two hyperfme interactions. Part A shows the spin levels and schematic NMR spectrum for unpolarized product. Part B shows the spin levels and schematic NMR spectrum for polarized product. Populations are indicated on each level. Initial conditions gj > g2, > 0 a2 > 0 spins on different radicals the RP is initially triplet.

The first steps of the plant photosystem I (PSI) primary reactions are believed to be P q AqA —hiL i p AqA P qqAqAj P700 and Aq are chlorophyll species and Aj is a quinone-like molecule. (1) The initial suggestions that Aq and A act as electron acceptors in sequence prior to the acceptor X were based on EPR experiments which showed that two electron acceptors are photoaccumulated at low redox potential. (2,3) Support for this suggestion came from following the triplet state found in PSI as a function of the photoaccumulation of the proposed Aq and A, and interpretations of the transient electron spin polarized (esp) signal believed to be due to PyqqAJ which has been observed by time resolved EPR experiments. 

Takamori et al The CIDEP spectra observed in the case of 1,4-naphthoquinone in ot- and y-cyclodextrins show the main formation of naph-thaquinone radical-anions whereas in the case of p-cyclodextrins the neutral naphthosemiquinone radicals dominate the EPR spectra. The carbon-centred radicals from the cyclodextrins are also identified in all cases. The spin polarization patterns of all spectra prove that the reaction takes place via the excited triplet state of naphthaquinone, and the hydrogen abstraction reaction from the inside of the framework of the cyclodextrins is the initial step of the photoreduction of 1,4-naphthoquinone. Similar results were obtained for 2-methyl-1,4-naphthaquinone. 

The triplet-sensitized photo-decomposition of azocumene into nitrogen and cumyl radicals was investigated by TR EPR and optical spectroscopy. " The cumyl radicals observed carry an initial spin polarization and are formed with a yield which depends on both the solvent viscosity and the strength of the external magnetic field. This phenomenon is interpreted in terms of a depopulation-type triplet mechanism, i.e. competition between decay into radicals and fast triplet sub-level intersystem crossing" back to the azocumene ground state. 

Based upon the current theories of CIDEP and CIDNP, we propose that in many photochemical systems the primary photochemical reaction of the excited triplet state contributes to magnetic polarization via the triplet mechanism. The secondary reaction of the polarized primary radicals may transfer their initial polarization to the "secondary radicals" provided that the radical reactions can compete with the radical spin-lattice relaxation process (59,97). On the other hand, secondary reactions of the primary radical pair or the uncorrelated F pair contribute to polarization by the radical-pair mechanism. A general scheme showing the possible and simultaneous operations of both the... 

In principle, initial polarization (triplet mechanism) develops immediately following excitation reaction, whereas radical-pair polarization evolves within a time period slightly behind the development of the initial polarization. Time-resolved experiments, therefore, can separate to certain extent the simultaneous contributions of the two different mechanisms. The key problem for the experimentalist who is doing quantitative measurements of the two different types of polarization boils down to how to beat spin relaxation. 

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