 The study of one-dimensional periline bisimide aggregates using ultrafast transient absorption spectroscopy and kinetic Monte Carlo simulations reveals that at low temperatures, the dynamics is dominated by diffusion-driven exciton-exiton annihilation in infinite chains, while at higher temperatures, the aggregates dissociate into small fragments consisting of few monomers, supported by time-dependent anisotropy measurements. This article was authored by Stefan Walter, Janice Azazes, Francis Cofenall, and others.