Tunable emission in the visible range from a single organic fluorophore through time-controlled morphological evolution

For the vast majority of solid organic luminescent compounds, the emission color depends critically on the molecular packing in their crystal structure, giving rise to different emission colors for different polymorphs. Since the number of possible polymorphs is limited, it is very unusual to obtain materials with emissions covering the entire visible range for the same molecule. In this work, we report a new organic compound containing pyrene and thiophene fragments that shows an extremely rare feature in which the tunable solid-state emission is wholly reliant on its particle size and surface morphological evolution, which can be controlled by adjusting the reaction time. This allowed us to obtain compounds of the same material emitting at different regions of the visible spectrum, from blue to orange-red. High-resolution transmission electron microscopy (HR-TEM) images and field emission scanning electron microscopy (FE-SEM) images of the compounds isolated at different reaction times reveal a distinct change in the aggregate size and morphology (1D assembly, which is slowly turned into 2D layers, that is further converted to 3D hierarchical flowery architectures) that can be unambiguously associated with an observed gradual red shift in the PL emission. Time-correlated single-photon counting (TCSPC), dynamic light scattering, Raman spectroscopy, powder X-ray diffraction and J-aggregates formation are used to complement the explanation of the observed phenomenon.