In this communication, a very simple method is proposed to calculate the potential energy profile of a reaction step for a system in its first electronic excited state. It is based on an extension of the Grochala-Albrecht-Hoffmann rule. Theoretical arguments supporting this extension are given. The proposed method is validated on two examples that are interesting from a biological point of view: photocycloadditions [2+2] involved in the formation of pyrimidic dimers under DNA exposition to UV radiation. (C) 2009 Elsevier B.V. All rights reserved.