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.

Department of Chemistry University of California Irvine California 92697 2025 United States

Dipartimento di Chimica Industriale Università degli Studi di Bologna Viale del Risorgimento 4 1 40136 Bologna Italy

European Center for Theoretical Studies in Nuclear Physics and Related Areas 38123 Trento Italy

Faculty of Mathematics and Physics Charles University Praha 8 180 00 Prague Czech Republic

IFN CNR Dipartimento di Fisica Politecnico di Milano Piazza Leonardo da Vinci 32 1 20133 Milano Italy

Institute for Photon Science and Technology University of Tokyo 7 3 1 Hongo Bunkyo ku Tokyo 113 0033 Japan

Laboratoire de Chimie UMR 5182 Université Lyon ENS de Lyon CNRS Université Lyon 1 46 Allée d'Italie FR 69342 Lyon France

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