Photodimerization Reactions of Aromatic Compounds

Anthracene and 9-substituted anthracene undergo photodimerization via a singlet excimer, which collapses to dimer in a symmetry allowed [jt s + jt j-cycloaddition process. Substituted anthracenes with different substituents at C-9 position give dimers having head-to-head geometry of the substituents [51, 52]. 

The kinetic study indicated that the reaction takes place in the following steps  

9-Substituted anthracene 52 gives dimer 53 of head-to-tail regiochemistry [51]. 

Similarly, 2-methoxynaphthalene 54 undergoes photodimerization to give two isomeric products 55 and 56 [53, 54]. 

Methyl naphthalene 2-carboxylate and 2-cyanonaphthalene 57 undergo photodimerization followed by [2 + 2]-cycloaddition to give cage compounds 58 [55]. 

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