Picosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophores

The particle-like nature of light becomes evident in the photon statistics of fluorescence from single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occur, also yielding photon antibunching but confounding interpretation. Here we develop picosecond time-resolved antibunching (psTRAB) to identify and disentangle such processes. We use psTRAB to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures and to determine distance-dependent exciton–exciton annihilation rates. We further quantify exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates, distinguishing one-dimensional intra-chain from three-dimensional inter-chain diffusion at different delay times and extracting disorder-dependent diffusion lengths. This single-particle method enables precise studies of excitons, informing the design of ultra-bright probes and optoelectronic materials.