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.