UV-Light-Induced Vibrational Coherences: The Key to Understand Kasha Rule Violation in trans -Azobenzene

We combine sub-20 fs transient absorption spectroscopy with state-of-the-art computations to study the ultrafast photoinduced dynamics of trans-azobenzene (AB). We are able to resolve the lifetime of the ππ∗ state, whose decay within ca. 50 fs is correlated to the buildup of the nπ∗ population and to the emergence of coherences in the dynamics, to date unobserved. Nonlinear spectroscopy simulations call for the CNN in-plane bendings as the active modes in the subps photoinduced coherent dynamics out of the ππ∗ state. Radiative to kinetic energy transfer into these modes drives the system to a high-energy planar nπ∗/ground state conical intersection, inaccessible upon direct excitation of the nπ∗ state, that triggers an ultrafast (0.45 ps) nonproductive decay of the nπ∗ state and is thus responsible for the observed Kasha rule violation in UV excited trans-AB. On the other hand, cis-AB is built only after intramolecular vibrational energy redistribution and population of the NN torsional mode.