Optical Nuclear Polarization in the Excited State Through Cross-Relaxation and Its Use in the Study of the Carbon-13 Hyperfine Coupling in the Lowest Triplet State of 1-13C-p-Benzoquinone

In this paper the phenomenon of optical nuclear polarization in the excited state through cross-relaxation is described. It is shown that when the populating and depopulating rates of the triplet spin sublevels are known the absolute nuclear polarizations can be calculated and that optical detection of the nuclear alignment in at least one electron spin state is quite often feasible. The paper further deals with the magnetic resonance properties of the lowest triplet state in 1-13C-p-benzoquinone whereof the zero-field splitting parameters and carbon-13 hyperfine data are reported. The carbon-13 hyperfine data, obtained by optical detection of ENDOR and CRENDOR, indicate that in the lowest nπ* triplet state of p-benzoquinone the π-electron spin density at the carbonyl carbon and oxygen are about equal while the remaining carbon atoms carry about half this spin density.