In the present study, three LED lamps from Vapourtec (365 nm, 420 nm, and 470 nm) were tested. Based on preliminary data, the highest yield was achieved with 470 nm lamp under fixed conditions. Based on the UV-Vis absorption spectra (Figure , ESI) measured from 200 nm to 800 nm, the fac-Ir(ppy)3 catalyst absorbs under all three lamps with a measured excitation coefficient of 1413.5 (mol/L) -1 cm -1 at 470 nm. Although both reactants absorb around 310- 320 nm, and hence do not affect light penetration, HE absorbs at 425 nm, affecting light penetration under the LEDs with the wavelength of 365 nm and 420 nm. Moreover, using lower energy lamps could avoid potential side reactions as it is less likely that other reaction species will absorb at longer wavelengths. Combining the preliminary data with the UV-Vis absorption spectra, a 470 nm LED lamp was used for the optimisation. Under irradiation with a 470 nm lamp, 1 mmol/L of photocatalyst in 1 mm ID tubing results in an absorbance of 0.14. This for a theoretical scalability to 10 mm ID (i.e., 100x volume increase from 10 mL reactor to 1 L scale) or an increase of catalyst concentration to 14 mM (Abs = 2) whilst maintaining a clear solution without sacrificing uniform light absorption.