“…[2+2] Photocycloaddition is one of the best known reactions of olefins in solid-state organic photochemistry [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. By using the reaction, cyclobutane molecules, almost inaccessible by other synthetic methods, are easily obtained in high or at least reasonably good yields.…”