alkene photocycloaddition

Photochemical process leading to formation of a cyclobutane upon addition of an electronically excited alkene to another ground-state alkene or of an electronically excited α,β-unsaturated carbonyl compound to a ground-state alkene. The latter is called a de Mayo reaction.
Note:
A special case of photocycloaddition is a photodimerization.
Source:
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 301