Controlling narrowband phosphorescence and delayed fluorescence by aggregate size.

Exciton coherence is a phenomenon involving collective electronic transition dipole moments of molecular aggregates with interesting photophysical behaviors such as superradiance and subradiance in J- and H-type aggregates, respectively. Although singlet aggregate excitons have been extensively studied, the understanding of triplet exciton coherence in terms of specific conditions is still lacking due to limited experimental observations. Here, by synthesizing model organic compounds of fluorene monomer, dimer, trimer, and polymer, we systematically studied their photoluminescence in both solution and aggregation states. Strong triplet exciton coherence is present in polyfluorene aggregates, including nanoparticles, microparticles, and thin films, and is manifested either as HJ-aggregate phosphorescence or delayed fluorescence depending on the size of these aggregates. Notably, in the phosphorescence state, the polyfluorene polymer exhibits an unusually sharp atomic-spectrum-like emission band with a narrow full width at half maximum (FWHM) of only 0.05 eV (14 nm) and the longest lifetime of 0.63 s. This study shows that the size of organic aggregates is essential in dictating organic exciton dynamics in the solid state and holds importance for the development of advanced optoelectronic technologies.