The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.

Selection Rules: Photochemical Activation

Conjugated systems containing an even number of π-electron pairs undergo a disrotatory ring closure. For example, photochemical activation of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives cis-3,4-dimethylcyclobutene.

Conjugated systems with an odd number of π-electron pairs undergo a conrotatory ring closure. For example, (2E,4Z,6E)-2,4,6-octatriene, a conjugated diene containing three π-electron pairs, forms trans-5,6-dimethyl-1,3-cyclohexadiene.