does not provide any substantial electronic interaction, explaining its relative instability compared to all the other (partly) stacked conformers. In fact, for the latter structures 1-8, the frontier orbitals correspond to linear combinations of the HOMO and LUMO of each monomer. The excimer's S1 state thus contains a significant contribution from the superposition of localized La excitations. As the intermolecular distance gets shorter, excitonic coupling between these (strongly) dipole-allowed transitions, together with charge transfer interactions, steer the resulting delocalized state at the bottom of the excited state manifold, overcompensating for the initial Lb -La gap. The associated change of diabatic character along the adiabatic S1 PES has been observed in all previous scans of the stacking coordinate for the excimer of pyrene and other PAHs due to the similarities in the electronic structures of the constituent species. Moreover, a simplified excitonic model provides a good approximation to the brightness of the S0→S1 transition, which correlates well with the twist angle between the monomers, affecting fluorescence rate and probability. Indeed, an increased twisting avoids mutual cancellation of the transition dipoles for the excitonically coupled localized excitations.