Tuning Solid-State Luminescence in Conjugated Organic Materials: Control of Excitonic and Excimeric Contributions through π Stacking and Halogen Bond Driven Self-Assembly

Two polymorphs with distinctly different fluorescence emission (green and yellow; G, Y) emanating from excitonic and excimeric contributions were prepared from solution as well as by using physical vapour transport. Based on crystal structure investigations, the vibrationally-resolved excitonic emission is found to originate from a beta-Sheet arrangement (G), whereas a sandwich herringbone structure is responsible for the excimer emission (Y). The intermolecular interactions and energies were quantified to have a complete picture of the decisive factors that controls the self-assembly. Halogen-bond directed self-assembly was explored to fine-tune the intermolecular interactions through co-crystallization as well as a commercially viable liquid assisted grinding method. A smooth fluorescence shift from G to Y was achieved by co-assembly due to substantial differences in the pi orbital overlap in the molecular packing. Our investigation provides a comprehensive understanding of the origin of excitonic and excimeric contributions of emission behaviour in conjunction with the molecular packing and pi-pi orbital overlap, and might provide a directive towards the engineering of fluorescent functional molecular materials.