Excited-state normal-modes analysis: the case of porphyrins

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.