This project explors the boundary between the individual molecule and the bulk solid in the context of polymeric organic semiconductors by constructing and studying molecular aggregates from the single molecule level upwards. Using time-resolved and steady-state spectroscopies at elevated and at cryogenic temperatures, the interaction of individual molecular units will be revealed. For example, the question arises as to how large a molecular aggregate can become to still behave as an individual quantum-mechanical entity, emitting just one photon at a time. How far can photoexcitations migrate in self-organized mesoscopic aggregates, and what is the interaction length with quenching species such as charges? Under which conditions does the coupling between molecular units weaken to become incoherent and irreversible? The work program combines routes to controlling self-assembly in-situ and monitoring conformational dynamics of the polymer chain as well as aggregation effects in real-time. Superresolution microscopic techniques will be applied to spatially localize excitations on a polymer chain and watch their migration. Single-molecule fluorescence will be combined with spin-resonance techniques to study charge formation und unravel radical-based material breakdown processes. Besides this bottom-up control of spectroscopic features, a top-down approach to device engineering will be explored with the goal of identifying the smallest-possible device features below which the effects of discreteness dominate leading to single-electron and single-photon devices. Breakthroughs with implications beyond organic electronics are anticipated, since the materials provide models for polymer physics, quantum optics and solid-state mesoscopics. Sensory functions are expected to derive from the control and understanding of light-matter interactions on super-molecular sub-ensemble length scales.