Porphyrin Nanocrystal Synthesized via Chemical Reaction Route: pH-Sensitive Reversible Transformation between Nanocrystals and Bulk Single Crystal

Abstract

Crystalline nanostructures with octahedral morphology have been prepared by self-assembling of cationic porphyrin (H₆TPyP)⁴⁺·4Cl^– produced through chemical reaction route in aqueous solution depending on the synergistic interactions among hydrogen-bonding, π–π stacking, and ion pairing. Unexpectedly, the gradual decrease in pH by the slow evaporation of solvent in the nano-octahedron aqueous suspension obtained in situ led to the selective etching of the original nanocrystal and the isolation of (H₆TPyP)⁴⁺·4Cl^– bulk single crystals in the last stage. More interestingly, the increase in pH by adding water again into this bulk single-crystal-containing system led to the regeneration of nano-octahedrons, indicating the reversible transformation between porphyrin nano-octahedrons and bulk single crystals triggered by pH. Mechanistic investigations through powder and single-crystal X-ray diffraction analyses together with the electron microscopic, in particular, HRTEM, clearly reveal that the unique surface effect and anisotropic character of the nanomaterials differing from the bulk organic materials are responsible for such pH-sensitive reversible transformation of the two crystalline materials by controlling the dissolution or aggregation of (H₆TPyP)⁴⁺·4Cl^–, which actually induces the reversible formation and breaking of the (pyridine)­N⁺–H···Cl^–···H–O­(H₂O)···H–N⁺(pyridine) hydrogen bonds among cationic porphyrin building blocks at different pH. This result, to control the crystallinity and the unprecedented reversible transformation between nanocrystal and bulk single crystals just by tuning the pH of the synthesis process, as well as the use of the peculiar nanoeffect such as surface effect to adjust the self-assembling process, provides useful a tool for the controllable synthesis of crystalline materials and is expected to be helpful for further research and application of organic nanomaterials