title: Nonadiabatic dynamics of a truncated indigo model.
abstract: Indigo (1) is stable when exposed to ultraviolet light. We employ electronic structure calculations and nonadiabatic trajectory surface-hopping dynamics simulations to study the photoinduced processes and the photoprotection mechanism of an indigo model, bispyrroleindigo (2). Consistent with recent static ab initio calculations on 1 and 2 (Phys. Chem. Chem. Phys., 2011, 13, 1618), we find an efficient deactivation process that proceeds as follows. After vertical photoexcitation, the S(1)(ππ*) state undergoes an essentially barrierless intramolecular single proton transfer and relaxes to the minimum of an S(1) tautomer, which is structurally and energetically close to a nearby conical intersection that acts as a funnel to the S(0) state; after this internal conversion, a reverse single hydrogen transfer leads back to the equilibrium structure of the most stable S(0) tautomer. This deactivation process is completely dominant in our semiempirical OM2/MRCI nonadiabatic dynamics simulations. The other two mechanisms considered previously, namely excited-state intramolecular double proton transfer and trans-cis double bond isomerization, are not seen in any of the 325 trajectories of the present surface-hopping simulations. On the basis of the computed time-dependent populations of the S(1) state, we estimate an S(1) lifetime of about 700 fs for 2 in the gas phase.
authors.affiliation: Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.
authors.forename: Ganglong
authors.initials: G
authors.lastname: Cui
authors.forename: Walter
authors.initials: W
authors.lastname: Thiel