On-chip synthesis of ruthenium complex by microwave-induced reaction in a microchannel coupled with post-wall waveguide

We demonstrate the on-chip synthesis of a ruthenium complex based on a microwave-induced reaction in a microchannel coupled with a post-wall 24.15 GHz waveguide. The chip structure for continuous microwave irradiation of solvents and reactants comprises a post-wall waveguide and a microchannel that enters and exits the waveguide by passing between the metallic posts. To miniaturize the waveguide, it is designed to contain the 24.125 GHz industrial, scientific, and medical (ISM) band instead of the commonly used 2.45 GHz ISM band. Polytetrafluoroethylene (PTFE; εr = 2.1) is used as a dielectric material through which the microwaves propagate because PTFE is suitable as chemical container owing to its high resistance to heat load (up to 300 °C) and its chemical inertness. Various microfluidic configurations such as curved channels and reservoirs may be integrated into the waveguide cavity between the post walls. For a microwave input power of 3.0 W, the water temperature in the microfluidic channel increases to 75.9 °C. We used such a chip to synthesize a ruthenium complex via a microwave-induced reaction in the microchannel inside the post-wall waveguide. For this synthesis, the microwave frequency and power in the waveguide were 24.15 GHz and 3.0 W, the irradiation time was 120.0-600.0 s, and the solvent temperature was 70 °C. After microwave irradiation of the microfluidic channel, the specific fluorescence emission spectrum of Tris (2,2′-bipyridyl) ruthenium(II) is observed. This work thus demonstrates the on-chip synthesis of a ruthenium complex with a high reaction rate and yield by using a microwave-induced reaction in a microchannel inside a post-wall waveguide designed to contain 24.15 GHz microwave radiation.