Triplet pairing

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. 

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

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... 

Here (TiTi) is the encounter complex of two triplet molecules or, in other words, a triplet pair with energy 2Ejj capable of dissociating back into two triplet molecules or which may undergo intracomplex relaxation (spin exchange, vibrational relaxation, electron transfer) creating, for instance, either singlet, or triplet, or quintet dimers (8,9), or ion-radical pairs (18). 

The equations (11) - (13) Indicate that relaxation of the triplet pair (TjTi) through an Intermediate state X, I.e. bypassing MS2S0), Is the Important process but the efficiency of molecule creation from X Is low. The equation (12) makes It possible to estimate the (T Tj) pair lifetime. 

The eigenstates of are symmetric spin-free two-electron functions corresponding to singlet pairs, those of F are antisymmetric spin-free two-electron functions corresponding to triplet pairs. Tr(F+) is the probability that two electrons are coupled to a singlet pair, while Tr(F ) is the probability that two electrons are coupled to a triplet pair. 

However, what is not immediately obvious from this information alone is whether these spins are associated with a triplet pair of free radicals or with a true molecular triplet. For this purpose, the D value is indispensable, because from it, one can determine the average separation of the spins. 

The property of the spin-triplet components fi,2,z un) = — /i,2,3(—w ) means that their presence is not easy to observe. For example, the order parameter A is related to the sum X n=-oo /(wn) in which all contributions of the odd functions f 1,2,3 cancel. However, there are phenomena where the presence of the spin-triplet pairing plays a crucial role. One of them is the effect of the Tc dependence on the mutual orientation of magnetizations in the F/S/F structure. Another one is the predicted long-range proximity effect based on the spin-triplet component, which should lead to a Josephson current in F/S/F/S structures with anomalously thick F-layers.[10] The latter is relevant for experimental results of Ref. [5]. 

The theory underlying this effect depends critically on two selection principles the nuclear spin dependence of intersystem crossing in a radical pair, and the electron spin dependence of the rates of radical pair reactions. Combined, these selection principles cause a sorting of nuclear spin states into different products and result in characteristic nonequilibrium populations in the nuclear spin levels of geminate reaction products (whose formation is allowed for singlet pairs but spin forbidden for triplet pairs) and in complementary nonequilibrium populations in the spin levels of free-radicals ( escape ) products (whose formation is electron spin independent). The transitions between these levels will be in the direction towards restoring the normal Boltzmann population their intensities will depend on the extent of nonequilibrium population. The observed effects are... 

In this mechanistic scheme, the CIDNP intensities of reactant and product are determined by the competition of key steps at each stage of the reaction. For the system discussed here, the qualitative features of the observed polarization suggest that nuclear spin lattice relaxation during the lifetime of the olefin triplet state is negligible, that singlet and triplet pairs recombine with similar efficiencies, and that the triplet state decays to each of the isomers with equal efficiency. 

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