  Excited state normal modes analysis is systematically applied to investigate
and compare relaxation and internal conversion dynamics of a free-base
porphyrin with a novel functional porphyrin derivative. We discuss strenghts
and limitation of the method, and employ it to predict very different dynamical
behaviours in the two compounds and to clarify the role of high reorganization
energy modes in driving the system towards critical regions of the potential
energy landscape. For the functionalized porphyrin, we identify modes of
vibrations along which the energy gap between different excited state potential
energy surfaces within the Q band manifold may vanish, or be significantly
reduced, with respect to the one observed in the bare porphyrin.
