Synthesis, characterization and photophysical properties of phthalimide-pyrazole compounds: Effect of the substituent in the pyrazole ring on the crystal structures

• L1 and the corresponding three phthalimide-pyrazole derivatives, L2, L3 and L4 have been synthesized with high yields and their detailed characterization including single-crystal X-ray analysis are reported in this paper. • The crystallization was tuned by the pyrazole ring substituents and introduction of nitro or halogen groups significantly changed molecular conformation and supramolecular arrangements. • The compounds exhibits fluorescence in DCM, MeCN, and DMSO, with the greatest intensity observed in DMSO due to HOMO-LUMO transitions in the phthalimide ring. • The phthalimide ring on the photophysical properties have also been justified by theoretical calculations where heteroatoms (N and O) are involved in the HOMO, while carbon in the LUMO orbitals. Four phthalimide-pyrazole derivative compounds ( L1 = H, L2 = NO 2 , L3 = Br, and L4 = I) were synthesized and fully characterized. Single crystals of each compound were obtained, and their conformational and supramolecular structures were analyzed. The four crystal structures (L1 to L4) are stabilized by different intermolecular interactions and therefore have different characteristics from each other. L1 is dominated by long-range Van der Waals forces and not by short hydrogen bonds. On the other hand, L2 packs into a molecular sheet along the b and c axes. L3 and L4 have a common monoclinic structure ( P 2 1 / c ) and thus allow stabilization of the respective compounds by halogen-halogen interactions (Br–Br, I–I) and σ-hole effects. In all compounds, these interactions, whether Van der Waals forces, π‧‧‧‧π stacking or halogen bonds, are responsible for their packing and molecular stability. Therefore, it was concluded that the substituent on the pyrazole ring is the determining factor in their crystallization. In addition, photophysical and theoretical studies showed that the compounds exhibit emission in the presence of DCM, acetonitrile (MeCN), and DMSO, primarily due to the HOMO-LUMO orbitals in the phthalimide ring.