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

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.