light can also cause the double bonds to form a ring. cycloadditions in which 4n π electrons participate can also occur as a result of photochemical activation. here, one component makes an electron move from the highest occupied molecular orbital (homo) (π bonding) to the lowest unoccupied molecular orbital (lumo) (π* antibonding). after the electron is promoted to the higher orbital, orbital symmetry allows the reaction to proceed in a suprafacial-suprafacial manner. an example is the demayo reaction. another example is shown below, the photochemical dimerization of cinnamic acid.