Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
Thermally-induced [2 + 2] cycloadditions are symmetry forbidden. This is because the ground state HOMO of one ethylene molecule and the LUMO of the other ethylene are out of phase, preventing a concerted suprafacial-suprafacial overlap.
Absorption of UV light by one of the ethylene molecules promotes an electron from the ground state HOMO to the LUMO. The excited state HOMO now has the required symmetry to interact with the LUMO of the other ethylene via a concerted symmetry-allowed pathway.
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