Obtaining single crystals containing cationic porphyrins from aqueous solutions: a systematic approach including nano-crystallization, organic modifiers and gel crystallization

Obtaining high-quality single crystals containing charged molecules from aqueous solutions can sometimes be challenging. In this publication, we studied the crystallization of polycationic porphyrins. Crystals of three polycationic porphyrins, [5,10,15,20-tetrakis(1-methylpyridin-1-ium-4-yl)porphyrin, 5-(4-cyanophenyl)-10,15,20-tris(1-methylpyridin-1-ium-4-yl)porphyrin and 5-(4-carboxyphenyl)-10,15,20-tris(1-methylpyridin-1-ium-4-yl)porphyrin], were obtained with different anions. We started initially with `nano-crystallization' screening in order to identify promising combinations of the cations with suitable anions, followed by optimizing the ratio of anions to cations. Both steps yielded crystals, leading to the determination of one porphyrin-containing crystal structure from each step. In the next step, five different organic solvents were trialled as additives in an aqueous hanging-drop crystallization setup, which led to the determination of three more crystal structures. It was shown that it was possible to improve the quality of the crystals without the organic additive being incorporated into the crystal structure. Finally, crystals of one porphyrin salt were obtained by gel crystallization. These crystals were higher quality than the crystal grown by vapour diffusion, leading to a high-quality and higher-resolution X-ray structure determination. This is the first porphyrin structure to be determined from a crystal grown by gel crystallizaton.