Tuning crystal structure and absorption properties of 4-hydroxyisophthalic acid co-crystals using pyrazine derivatives

Abstract

Co-crystallization of 4-hydroxyisophthalic acid (4-HIPA) with 2-methylpyrazine (MP), 2,3-dimethylpyrazine (DMP), and 2,3,5,6-tetramethylpyrazine (TMP) has been performed, resulting in the formation of three co-crystals 1 (MP:2(4-HIPA)), 2 (DMP:2(4-HIPA)), and 3 (TMP:2(4-HIPA)), respectively. The co-crystals were analyzed using single-crystal X-ray diffraction, infrared spectroscopy, thermogravimetric analysis (TGA), fluorescence spectroscopy, and UV–vis spectroscopy. Hirshfeld surface analysis revealed that the proportion of O–H⋯N interactions in the co-crystals decreased with increasing number of methyl groups on the pyrazine derivative. In comparison with 4-HIPA, the maximum absorption peaks in the UV–vis absorption spectra of co-crystals 1–3 exhibited a red shift, which was caused by the presence of the corresponding pyrazine derivative in each case. As the number of methyl groups on the pyrazine derivatives increased, a weak blue shift was observed in UV–vis spectrum (Fig. 5). The fluorescence emission spectra peaks of co-crystal 1–3 were nearly at same position, 440 nm for solution state, 396 nm for solid state, irrespective of the number of methyl groups.