Effects of sonication on the size and crystallinity of stable zwitterionic organic nanoparticles formed by reprecipitation in water

Nanoparticles of a novel organic zwitterionic Meisenheimer complex, N',N'',N"'-tri(isopropyl)-4-oxo-6(isopropyliminio)-2-s-(2H)triazinespiro-1'-2',4',6'-trinitrocyclohexadienylide, were synthesized by reprecipitation in water under different conditions. While reprecipitation alone resulted in a suspension of amorphous particles that fell out of solution within hours, sonication for different periods of time resulted in the formation of crystalline particles that were stable in solution over the course of weeks. The disk-shaped particles had an average diameter of 140 nm and a thickness of 70 nm. Comparison of the optical spectroscopy of these particles with the monomer indicates that they possess delocalized excitonic states and enhanced radiative decay rates. The use of zwitterionic molecules in conjunction with sonication provides a way to exert some level of control over particle size and morphology, as well as increased colloidal stability.