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), 2-phenylpyridine (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–·2H2O] (1) and [(2,2-BPY-2H)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 (ϕ = 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 (ϕ = 29.3) and very dim in water (ϕ = 10.1)