Triplet mechanism polarization

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. 

Koga T, Ohara K, Kuwata K and Mural H 1997 Anomalous triplet mechanism spin polarization... 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

CIDEP (Chemically Induced Dynamic Electron Polarization) Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscope... 

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 current theories of chemically induced magnetic polarization can therefore be summarized into the two basically different mechanisms the photoexcited triplet mechanism (PTM) responsible for the initial electron polarization and the observed Overhauser effect in nuclear polarization, and the radical-pair mechanism which, to date, accounts for almost the remaining bulk of the known polarization systems. We proceed to describe the simple physical models of these two mechanisms by beginning with the more sophisticated radical-pair theory. 

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-... 

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. 

A detailed consideration of all these steps in the Overhauser triplet mechanism operating in the photoreduction of qulnone systems has been given by Adrian et al. (10). With some reasonable approximations (a pseudo-first-order reaction of the polarized radicals and steady-state conditions), it is possible to show that the rate of production of nuclear spin polarized products is... 

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. 

In a time-resolved experiment, the intrinsic polarization of the system is measured with very little correction except for power saturation (98,99). To measure the initial polarization, the signal heights S and Sq (hence the enhancement factor V and the polarization factor P = /So) can be obtained directly from the time profile. For a photo-CIDEP system induced only by the triplet mechanism and the triplet reaction involves a substrate SH, a kinetic analysis yields the following relationship ... 

The introduction of the photochemically excited triplet mechanism leading to CIDEP of the resulting radicals has added a new dimension to the potentials of the CIDEP techniques in photochemistry. In liquid photochemical systems, very little is known experimentally about the exact nature of the intersystem crossing process, but the rate or efficiency of such ISC process can sometimes be estimated by chemical (86) and optical methods (51,105). The treatment of the phototriplet mechanism in CIDEP of radicals in liquid solution is consistent with the following conclusions (1) ISC occurs mainly by the spin-orbit coupling mechanism in carbonyl compounds, (2) spin polarization of the triplet sub-levels is obtained via the selective ISC processes, and (3) the chemical reaction rate of the triplet is at least comparable to its depolarization rate via spin-lattice relaxation. 

Polarization Transfers and Reaction Mechanisms. Polarization transfers include the previously mentioned electron-nuclear Over-hauser effect and the nuclear-nuclear Overhauser effect. In this section we will discuss only electron-electron polarization transfer via a secondary chemical reaction involving a primary polarized radical. Again we shall use the photoreduction of quinone (t-butyl-p-benzoquinone) as an example. In solvent containing isopropanol, reaction of triplet quinone by phenols leads to two structural isomers, radicals I and II ... 

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. 

From a mechanistic point of view Fischer s classification may be modified in the following manner For stilbenes of group 1, trans - cis photoisomerization occurs mainly via the singlet mechanism (Section VI.A.2). Contribution of the upper excited triplet mechanism has been determined in cases where < isc is substantial (e.g., 4-chloro- and 4-bromostilbene). Nitrostilbenes constitute a class of stilbenes in which trans -> cis photoisomerization occurs mainly via the lowest triplet state (Section VI.B.2). For polar nitrostilbenes a radiationless deactivation pathway not leading to the cis isomer is operative (Section VI.C.3). 

In solution at ambient temperature the 3t 3p equilibrium is established within a few nanoseconds or even faster [31,180]. Rotation into the twisted configuration is a prerequisite for a triplet mechanism the occurrence of the back step 3p - 3t is based on the effects of temperature, viscosity, polarity, and added quenchers on rT (and 4>T). At room temperature the rate constant for the step 3p -> p is kp = (1-3) x 107 s-, that is, similar to that of stilbene. The equilibrium constant K = [3p ]/[3t ] depends on the substituents, the solvent polarity, and the temperature it shows the general trend to be shifted to the trans side with increasing polarity and decreasing temperature [32],... 

Because orbital momentum, and thus orbital shape and orientation, are involved in intersystem crossing driven by spin-orbit coupling, molecular symmetry and electronic configuration come into play. Consequently, the efficiency of intersystem crossing is usually different for the sublevels of a given multiplicity. This effect is the basis of the so-called triplet mechanism of electron spin polarizations as far as CIDNP is concerned, Hso only plays a role for systems with restricted diffusion (biradicals, radical pairs in micelles). 

The question of whether there are other mechanisms leading to CIDNP besides the radical pair mechanism is of central importance because chemical conclusions that are drawn from CIDNP results on the basis of the latter mechanism might of course be entirely wrong with another mechanism being the source of the polarizations. There has been some evidence [67-72] that cross-relaxation in radicals, by which electron spin polarization (C1DEP) is converted into CIDNP, could provide such a mechanism. Depending on whether cross-relaxation occurs by flip-flop transitions (Am = 0) or by double spin flips (Am = 2), opposite or equal phases of CIDEP and CIDNP would result. Since the origin of the electron spin polarizations is usually the triplet mechanism, this cross-relaxational mechanism is sometimes referred to as the triplet mechanism of CIDNP. 

CIDNP and CIDEP Studies.—Two mechanisms have been proposed to account for the observation of electron spin polarization in radical reactions (CIDEP), the first being termed the radical pair mechanism, in which the polarization results from the mixing of the singlet and triplet states of the radical pair by the magnetic interactions within the radicals, and the second the triplet mechanism, in which the polarization originates in the triplet as a result of spin-selective intersystem crossing from the photoexcited singlet state, as is known to occur in several systems from ODMR measurements (see above). It has recently been pointed out 480 that, for the latter mechanism, unequal population of the triplet sub-levels will depend upon zero-field D and E terms and Zeeman terms, but also... 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

It consists of thioxanthone or 2-isopropylthioxanthone in combination with (2,4,6-trimethylbenzoyl)-diphenyl-phosphine oxide or bis(2,4,6-trimethyl-benzoyl)-phenylphosphine oxide. The mechanism of sensitization was reported to involve triplet-triplet energy transfer from the thioxanthones to die phosphine oxides. That is followed by formation of radicals through a-cleavage of the photoinitiators. Direct photolysis of the phosphine oxides results in an absorptive, chemically induced, d)mamic electron polarization due to the triplet mechanism of polarization of the substituted benzoyl. The phosphoras-centered radicals produced by a-cleavage of the photoinitiators are the same radicals that... 

Quinones. The method of triplet-sensitized electron transfer which produces spin-polarized radicals via the CIDEP triplet mechanism with enhanced line intensities allows their detection on the nanosecond time scale at room temperature by FT EPR. The reaction scheme shown in Figure 4 describes the generation of the radicals by the CIDEP triplet mechanism. In the laser photolysis of the acceptor A, the triplet state Aji is generated by efficient intersystem crossing (isc) from the excited singlet state Asi ( denotes spin-polarized transi-... 

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