Electron transfer in photoexcited pyrrole dimers

Following on from previous experimental and theoretical work [Neville et al., Nat. Commun. 7, 11357 (2016)], we report the results of a combined electronic structure theory and quantum dynamics study of the excited state dynamics of the pyrrole dimer following excitation to its first two excited states. Employing an exciton-based analysis of the Ã(π3s/σ^*) and B̃(π3s/3p/σ^*) states, we identify an excited-state electron transfer pathway involving the coupling of the Ã(π3s/σ^*) and B̃(π3s/3p/σ^*) states and driven by N-H dissociation in the B̃(π3s/3p/σ^*) state. This electron transfer mechanism is found to be mediated by vibronic coupling of the B̃ state, which has a mixed π3s/3p Rydberg character at the Franck-Condon point, to a high-lying charge transfer state of the πσ^* character by the N-H stretch coordinate. Motivated by these results, quantum dynamics simulations of the excited-state dynamics of the pyrrole dimer are performed using the multiconfigurational time-dependent Hartree method and a newly developed model Hamiltonian. It is predicted that the newly identified electron transfer pathway will be open following excitation to both the Ã(π3s/σ^*) and B̃(π3s/3p/σ^*) states and may be the dominant relaxation pathway in the latter case.