Why is the [2 + 2] cycloaddition between two but-2-ene molecules photochemically allowed but thermally forbidden?

• A. The [2 + 2] cycloaddition of but-2-ene is thermally forbidden due to the trans geometry of the double bond while their photochemical cycloaddition is allowed due to the change in the geometry of the double bond.
• B. The [2 + 2] cycloaddition of but-2-ene is thermally forbidden due to the antibonding interaction of the pi orbitals while their photochemical cycloaddition is allowed as the HOMO is changed due to the excitation of an electron.
• C. The [2 + 2] cycloaddition of but-2-ene is thermally allowed and photochemically forbidden
• D. The [2 + 2] cycloaddition of but-2-ene is thermally forbidden due to the bonding interaction of the pi orbitals while their photochemical cycloaddition is allowed due to the antibonding interaction of the HOMO and LUMO.

Answer: (B)