Nonadiabatic dynamics of a truncated indigo model

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(pi pi*) 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.