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

The locus of the photodimerization reaction of 9-methylanthracene in the crystal was examined by high-resolution solid-state NMR techniques. Examination of the 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 T₁ relaxation curves for the monomer and the dimer at various exposure times were separately observed via well-resolved 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 μm. The heterogeneous domain structure generated by the photodimerization indicates that the reaction takes place at defects of the crystal in the monomer.