Anthracene derivatives with several TEMPO radicals (2-4, 10) were prepared, and each photodimerization reaction was investigated. Although the photodimerization was unsuccessful in obtaining the dimers of anthracenes 2 and 3, which could be alternatively prepared in a stepwise manner, the photodimers of anthracenes 4 and 10 were available by the direct photoreaction. The dissociation reaction of the dimers proceeded well by heating them in solution to give the corresponding monomers in each case, and thus the reversible system could be constructed in the latter two systems. While no large difference was observed in their magnetic behaviors between the monomer/dimer pair of 4 and 8, an intriguing difference was found in the magnetic behaviors for the pair of 10 and 11 from ferromagnetic interactions in 10 to the variable magnetic interactions in 11 depending on the solvent molecules incorporated in the crystals.