Single-molecule spectroscopy has provided unprecedented access to underlying structure-function relations of highly disordered conjugated polymers such as P3HT , but is often considered of only limited utility in illuminating molecular characteristics in the solid state. This perception has changed with the introduction of single-molecule solvent-vapor annealing techniques, which allow the deterministic growth of multichain aggregates – mesoscopic nanoparticles – to mimic bulk material characteristics while retaining the low degree of heterogeneity characteristic of a sub-ensemble experiment. Such single aggregates reveal that exciton generation in bulk films is an inherently intermolecular process, involving scores of polymer chains because of the prevalent process of singlet-singlet annihilation. As a consequence, even large aggregates containing many polymer chains exhibit deterministic single-photon emission in the form of photon antibunching . A corollary of this effect is strong triplet-polaron quenching in large multichain aggregates, which leads to the disappearance of triplet-exciton photon correlation signatures in particles grown by controlled solvent-vapor annealing .
Most recently, it has become possible to directly image the growth and arrangement of multichain aggregates during annealing. Annealing promotes chain ordering, which strengthens the intrinsic J-aggregate character of polymer chains. Once the solvent molecules are removed by drying, cofacial stacking of these carefully ordered chains is encouraged, which gives rise to pronounced H-aggregate spectral characteristics. This drying process, on the level of single nanoparticles, is highly reversible and can be repeated many times to show a controlled switching between J- and H-type characteristics. Single-particle annealing therefore offers a unique window to monitoring the dynamics of solvent removal from polymer films, in real time, during processing .
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