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

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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. The microwave heating technique has been widely used in various organic and inorganic syntheses, and it has been reported that this technique provides the desired compounds quickly and easily. In this work, we applied microwave heating to synthesize redox-active organic nanoparticles consisting of hyperbranched viologen units. By the precise temperature control afforded by use of microwave irradiation, we succeeded in controlling the growth and termination reactions kinetically and preparing viologen nanoparticles in one-pot. We also demonstrated that the size of the nanoparticles can be controlled by varying the temperature program.