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The origin of the photochromic properties of diarylethenes is a conical intersection (which we have located computationally), but we show that dynamics calculations are necessary to explain why the conical intersection is accessible, because the excited-state reaction path is not contained in the branching space defining the intersection. Four different systems have been studied: 1,2-di(3-furyl)ethene, 1,2-di(3-thienyl)ethene, 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene, and a model hydrocarbon system. Critical points on the ground- and excited-state potential energy surfaces were calculated using complete active space self-consistent field (CASSCF) theory; dynamics calculations were carried out using the molecular mechanics-valence bond (MMVB) method. The main experimental observations (i.e., picosecond time domain, quantum yield, temperature dependence, and fluorescence) can be interpreted on the basis of our results

Knowledge about the electronic motion in molecules is essential for our understanding of chemical reactions and biological processes. The advent of attosecond techniques opens up the possibility to induce electronic motion, observe it in real time, and potentially steer it. A fundamental question remains the factors influencing electronic decoherence and the role played by nuclear motion in this process. Here, we simulate the dynamics upon ionization of the polyatomic molecules paraxylene and modified bismethylene-adamantane, with a quantum mechanical treatment of both electron and nuclear dynamics using the direct dynamics variational multiconfigurational Gaussian method. Our simulations give new important physical insights about the expected decoherence process. We have shown that the decoherence of electron dynamics happens on the time scale of a few femtoseconds, with the interplay of different mechanisms: the dephasing is responsible for the fast decoherence while the nuclear overlap decay may actually help maintain it and is responsible for small revivals. DOI: 10.1103/PhysRevLett.118.083001 Electronic motion initiates specific rearrangements of atoms in molecules that are responsible for chemical reactions and biological processes. Because of the advent of attosecond techniques [1,2], it is possible to induce electron dynamics in molecules. Observing and potentially steering electronic motion on its natural time scale may provide novel pathways towards controlling chemical processes [3][4][5][6][7][8]. Since the electron distribution is usually considered to be changing much faster than the nuclear geometry, many theoretical studies treat molecular electron dynamics upon ionization as a purely electronic process, at a single static nuclear geometry [9][10][11][12]: long-lived oscillatory charge migration is then predicted. The fixed-nuclei and single-geometry approximations have however limited validity [13][14][15][16][17][18][19]. The fundamental challenge is to understand to what extent the electronic wave packet retains its coherence, i.e., how long the oscillations in the electronic density survive, in the presence of interactions with the nuclear degrees of freedom.Using a semiclassical description for the coupled systembath evolution, Fiete and Heller identified three processes that contribute to decoherence of the quantum system [20]: (i) system wave packet displacement, (ii) bath overlap decay, and (iii) phase jitter. In the context of molecular electron dynamics, the "system" consists of the electrons and the "bath" of the nuclei. The three mechanisms above can respectively be interpreted as (i) change in the electronic state populations, (ii) decrease of the overlap between the nuclear wave packets on different electronic states, and (iii) dephasing of the different wave packet components. The importance of these mechanisms on the coherent electron dynamics upon molecular ionization remains an outstanding question, that we aim to address in the present Letter.Previous works showed that the n...

An algorithm for the computation of initial relaxation directions (IRD) from the tip of a conical intersection is discussed. The steepest descent paths that can be computed starting from these IRD provide a description of the ground state relaxation of the "cold" excited state species that occur in organic photochemistry where slow motion and/or thermal equilibration is possible (such as in cool jet, in matrices, and in solution). Under such conditions we show that the central conclusions drawn from a search for IRD and those obtained from semiclassical trajectory computations are the same. In this paper, IRD computations are used to investigate the mechanism of photoproduct formation and distribution in the photolysis of cyclohexadiene (CHD) and cZc-hexatriene (cZc-HT). A systematic search for the IRD in the region of the 2A 1 /1A 1 conical intersection (see Celani, P.; Ottani, S.; Olivucci, M.; Bernardi, F.; Robb, M. A. J. Am. Chem. Soc. 1994, 116, 10141-10151) located on the 2A 1 potential energy surface of these systems yields three relaxation paths. The first two paths, which start in the strict vicinity of the intersection, are nearly equivalent energetically and lead to production of CHD and cZc-HT, respectively. The third path, which begins at a much larger distance, lies higher in energy and ends at a methylenecyclopentene diradical (MCPD) minimum. Further, while the first two paths define directions that form a 60°angle with the excited state entry channel (i.e. the direction along where the conical intersection region is entered), the third path is orthogonal. It is shown that these findings are consistent with the experimental observations which show nearly equivalent quantum yields for CHD and cZc-HT and no production of MCPD. The results of the IRD computations have been validated by investigating the decay dynamics of trajectories starting from a "circle" of points around the conical intersection, with the initial kinetic energy distributed in randomly sampled vibrational modes. These computations have been carried out using a trajectory-surface-hopping (TSH) method and a hybrid molecular mechanics valence bond (MM-VB) force field to model the ab initio potentials.