Evolution of HJ Coupling in Nanoscale Molecular Self-Assemblies

We investigated the evolution of combined Frenkel exciton and charge-transfer chromophore coupling in nanoscale self-assembled clusters of 7,8,15,16-tetraazaterrylene. Using spatially and wavelength-resolved fluorescence imaging, we observed significant changes in the relative intensities of vibronic transitions (J-like to H-like) in isolated clusters with increasing size (along the crystal growth axis), suggestive of a change in the interference of the combined short- and long-range interactions upon assembly. Large clusters and nanowires showed spectral signatures that appear superficially like H-aggregates with a diminished origin (0-0) intensity relative to the higher vibronic sidebands, whereas small nanoclusters-appearing as diffraction limited spots-had distinct J-aggregate spectral signatures (enhancement of the origin intensity) suggestive of different packing structures in small clusters. The isolation of the different nanoscale assemblies allows for the opportunity to understand changes in molecular registration upon solution-phase crystallization.