My guess is, having the node of the HOMO (giggles) intersect an atom raises its energy, while the LUMO has an energy density maximum over that atom and is unaffected. The result is, the HOMO-LUMO gap gets smaller, and can be excited by blue range light.
Most photocatalytic molecules have 2-fold or 3-fold symmetry. Splicing the electrons of an atom across this axis of symmetry is probably how they get the effect.
Blue light converts the molecule to an excited state, which is usually just a diradical, and the radical reacts with the substrate>TL;DR it's the atom spliced across an axis of symmetry