Photo-excited triplet state DNP
The oxygen molecule O2 is an ubiqitous example of a (stable) triplet molecule. Two of its 16 electrons are in orbitally distinct (2-fold degenerate) ∏2p* orbitals and the total electron spin S = 1.
The molecule is paramagnetic and was used by Curie to derive his law:
P. Curie, Lois expérimentales du magnétisme. Propriétés magnétiques des corps à diverses températures, Ann. Chim. Phys. 5, 289 (1895).
The spin polarization is proportional to B/T (in the temperature range accessible to Curie). There is no polarization in zero field, because the Sz = -1 and Sz = +1 levels are equally populated (and Sz = 0 does not contribute anyway). Optically created triplets have the interesting property that these two Sz levels may be unequally populated by the optical process: the spin polarization may be considerable, even in low fields, and at relatively high temperatures. They "may" be unequally populated, but are not necessarily so: for practical purposes the polarization is an experimental property of a given molecule. Only a few molecules are known that yield optically created triplets with interesting polarizations and suitable lifetimes (in the end, these triplets must decay into the original singlet ground state). The most studied systems for DNP are single crystals of naphtalene (the polarized protons of which will form the target) doped with pentacene (which will provide the triplets). Suitably cut single crystals improve the efficiency of the process, but a melt could also be used. Actual transfer of polarization from the electron spin system to the proton system is done by suitable microwave irradiation in a static magnetic field. Two irradiation schemes have been used: frequency-swept CW ("Integrated Solid Effect", ISE); and pulsed spin-locking (NOVEL). For ISE: Henstra A, Derksen P, Wenckebach WTh. Enhanced dynamic nuclear polarization by the integrated solid effect. Phys Lett A 134, 134-136 (1988).
For NOVEL: Henstra A, Wenckebach WTh.The theory of nuclear orientation via electron spin locking (NOVEL). Molecular Physics 106, 859 - 871 (2008).