Size-controlled one-pot synthesis of viologen nanoparticles via a microwave heating technique

Abstract

We report a size-controlled one-pot synthesis of redox-active organic nanoparticles consisting of viologen units via precise temperature control afforded by the use of microwave heating. A DMF solution of 4,4′-bipyridine and 1,3,5-tris(bromomethyl)benzene (TBMB) corresponding to monomers and ethyl bromide corresponding to a terminator was subjected to microwave heating at 80 °C for 20 min as the first stage corresponding to the growth reaction, which was then followed by heating at 150 °C for 5 min as the second stage corresponding to the termination reaction. ¹H NMR analysis of the reaction mixture at each step confirmed that at 80 °C, 4,4′-bipyridine primarily reacts with TBMB, leading to an increase in molecular weight and that a termination reaction with ethyl bromide occurs due to the increase in temperature to 150 °C. Moreover, the size of the nanoparticles can be controlled by varying the first stage reaction time; the nanoparticle diameter was determined to be 142 ± 15 nm for the first stage reaction time of 10 min, which increased to 269 ± 67 nm at 20 min, as evidenced by dynamic light scattering measurements.