Engineering the solid-state luminescence of organic crystals and cocrystals

Fine-tuning the solid-state emission of organic materials is a topic of immense commercial significance and academic interest. Of the various approaches employed to achieve emission-tuning, the co-crystal approach is less reported. In this study, we integrate the effect of functionalization and co-crystallization to investigate the optical properties. Three functionalized pyridyl-hydrazone molecules 1a (NO2), 2a (CN) and 3a (Br) have been synthesized and further utilized for cocrystal development with 5-sulfosalicylic acid (5-SSA-2H) to obtain organic salts 1-3, respectively. 1a-3a exhibit varied aggregation-induced emission (AIE) behavior, which is further tuned through co-crystallization. Emission quenching in 1a is attributed to long-range pi-pi stacking between the organic molecules while no pi-pi stacking interactions are observed in 2a (464 nm) and 3a (442 nm), resulting in their emissive behavior. Co-crystallization leads to more regulation of the emission wavelengths as 1a undergoes emission turn-on in the organic salt 1 (467 nm), and exhibits brownish luminescence; meanwhile, solid-state emission of 2a and 3a is moderately and significantly red-shifted in 2 (472 nm) and 3 (484 nm), respectively. The emission turn-on in 1 and red-shift in 2 is attributed to J-aggregate formation in their solid state and a significant red-shift of 3 is attributed to the formation of isolated head-to-tail dimers or excimers in the crystal lattice. The results are further supported with powder-X-ray diffraction, AIE and Hirshfeld studies. A crystal engineering approach has been used to investigate the integrated impact of functionalization and co-crystallization on the photophysical properties of organic solids.