Hetero-Aggregation-Induced Tunable Emission in Multicomponent Crystals

Crystal engineering is a green and convenient approach to designing desirable materials through rational manipulation of intermolecular interactions. We have reported the lesser reported sulfonate-pyridinium intermolecular interaction for the design and synthesis of organic co-crystals with improved features. Here in we report the utilization of the interaction to tune the solid-state luminescence of organic precursor naphthalene disulfonic acid (NDSA-2H). Organic salts of NDSA-2H are synthesized and characterized with three isostructural bipyridyl co-formers: 4-phenylpyridine (4-PhPy), 2phenylpyridine (2-PhPy) and 2,2 '-bipyridine (2,2-bpy). Structural investigation validates aggregation of organic acid and base co formers through sulfonate-pyridinium synthon and proton transfer between them. Compared to NDSA-2H, the molecular salts [(4-PhPy-H)2+(NDSA)2-center dot 2H2O] (1) and [(2,2-BPY2H)2+(NDSA)2-] (3) undergo a blue and red shift, respectively, while solid-state emission of [(2-PhPy-H)2+(NDSA)2-] (2) remains unaltered. This solid-state emission tuning is attributed to the different modes of crystal packing and arises from monomer emissions in 1 and 2 and excimer emission in 3.3 also exhibits a relatively longer lifetime of 20.7 ns while 2 exhibits better quantum yields (0 = 22.7). Solution-phase photophysical behavior has been investigated for representative co-crystal system 3, after validation of its solution-phase integrity. 3 undergoes a hetero-aggregation-induced tunable emission (HAITE) phenomenon in the water- acetone system to exhibit maximum emission intensity at 90% volume fraction of acetone, which is further validated by the diffuse light scattering (DLS) and scanning electron microscopy (SEM) studies. 3 also exhibits solvatochromism in terms of emission intensity change with the nature of the solvent, being brightest in methanol (0 = 29.3) and very dim in water (0 = 10.1).

Automated summary

The interaction between sulfonate and pyridinium can be used to control the solid-state luminescence of organic salts, and the emission properties are influenced by the crystal packing and solvent nature.