While thermal electrocyclic reactions occur via the ground state HOMO, the photochemical analogs proceed via the excited state HOMO.

The stereochemistry of the products formed under photochemical activation depends on the number of π  electron pairs and the mode of ring closure.

Let's consider a system with an odd number of π  electron pairs like a conjugated triene.

Photochemical excitation promotes an electron from the ground state HOMO to the LUMO of the triene, transforming the LUMO into an excited state HOMO.

Notice that the terminal lobes in the excited state HOMO are antisymmetric. A constructive overlap is possible only if both lobes rotate in the same directions, resulting in a conrotatory ring closure. Hence, the product is trans.

Next, consider the photochemical electrocyclization of a diene.

Unlike the triene, here, the terminal lobes of the excited state HOMO are symmetric. So, the ring closure proceeds via a disrotatory pathway, yielding a cis product.

In summary, photochemical electrocyclic reactions are conrotatory if the number of π electron pairs is odd and disrotatory if it is even.