Varying Nonadiabaticity in the Excited-State Intramolecular Proton Transfer Tautomers of Bispyrroleindigo

The nonadiabatic nature of the excited-state intramolecular single- and double-proton-transfer processes in photoexcited bipyrroleindigo is studied using model vibronic Hamiltonians and wavepacket simulations. The S₁ and S₂ states of three tautomers, normal (N), single- and double-proton-transferred (SPT and DPT), are (near-) degenerate at their respective Franck-Condon (FC) geometries. Electronic structure calculations show that S₁ and S₂ states are dipole-allowed (ππ*) and dipole-forbidden (<i>n</i>π*) in N and SPT tautomers, while both states are ππ* in the DPT tautomer. The accessible S₁-S₂ conical intersection within the FC zone enables the electronic population exchange between these states within a few tens of femtoseconds after initiating the wavepacket on the dipole-allowed state. These features suggest the nonadiabatic behavior in the early events of excited-state dynamics. Analysis of the population exchange trends and low proton transfer barrier energies shows that the two-state proton transfer would be more dominant in N than the SPT tautomer.