Single- and Multiexciton Dynamics in Porphyrin Nanorings

Exciton transport in molecular assemblies is typically studied in linear chains and polydisperse aggregates, where chain ends perturb the electronic structure and structural inhomogeneity can obscure size-dependent trends. Molecular nanorings overcome both limitations. They are end-group-free macrocycles with well-defined sizes, while their dimensions can be synthetically tuned from a few to several tens of porphyrin units. Here, we use intensity-cycled femtosecond transient absorption spectroscopy to disentangle single-exciton and multiexciton dynamics in a series of fully conjugated molecular nanorings containing 6–40 Zn-porphyrin units. By isolating the third-, fifth-, and seventh-order nonlinear responses, we independently track single-, two-, and three-exciton dynamics, respectively, enabling the ring-size dependence of exciton–exciton annihilation (EEA) to be determined without relying on a priori kinetic assumptions. The EEA rate decreases systematically with an increase in ring size, accompanied by a crossover from annihilation-limited to diffusion-limited EEA between 20 and 30 porphyrin units. The smallest nanoring exhibits no resolvable multiexciton dynamics, consistent with exciton delocalization across the entire ring or equivalent subinstrument response EEA, whereas larger rings exhibit increasingly diffusive exciton motion as static disorder limits delocalization. The retrieved exciton diffusion coefficients are 2–3 orders of magnitude higher than those we previously reported, indicating that the assumptions inherent to conventional intensity-dependent fitting can substantially underestimate exciton mobility.