Solid-state photodimerization of 9-methylanthracene as studied by solid-state 13C NMR.

The locus of the photodimerization reaction of 9-methylanthracene in the crystal was examined by high-resolution solid-state 13C NMR techniques. Examination of the 13C spectra of the products showed that only the trans dimer is formed by the solid state photodimerization, while both trans and cis dimers are formed by the photodimerization in benzene solution. The 1H T1 relaxation curves for the monomer and the dimer at various exposure times were separately observed via well-resolved 13C peaks. The two T1 curves exhibit characteristic features for relaxation of a weakly coupled two-spin system. By analyzing the T1 curves, the spin diffusion rates between the monomer and the dimer and the fraction of the dimer were obtained for various exposure times. From the result, the maximum domain size of the minor component during the photodimerization process was estimated to be ca. 0.3 microm. The heterogeneous domain structure generated by the photodimerization indicates that the reaction takes place at defects of the crystal in the monomer.