Comment on the applicability of Urbach's rule to molecular crystals

The extent to which the absorption profile below the first exciton band can be predicted without recourse to resonance transfer and strong exciton—phonon coupling is evaluated for a crystal transition derived from a moderately intense free molecule transition. It is argued, on the basis of a simple model, that Urbach rule behavior can arise from the same class of weak exciton—phonon interactions that determine hot band absorption shapes.