Photochemical electrocyclic reactions follow similar principles as those detailed in thermal
reactions with one important difference. In photochemical reactions, we must consider the
orbitals of the HOMO of the excited state to determine the course of the reaction. The
excited state HOMO has the opposite orientation of the outermost p orbitals compared to
the HOMO of the ground state. As a result, the method of ring closure of a photochemical
electrocyclic reaction is opposite to that of a thermal electrocyclic reaction for the same
number of " bonds