The photoreactor setup, featuring the widely used Kessil light source (PR160L, 370 nm), incorporates a capillary reactor (4.1 mL, 1.6 mm outer diameter (OD), 0.8 mm inner diameter (ID)) coiled to span the full height of the holder. Considering the effects of refraction and the curvature of the capillary surface, the reactor's effective collection area can be extended to cover the entire height of the capillary, simplifying the determination of photon flux. Ray-tracing simulations confirmed that the total photon flux on the capillary is 1.9 µmol/s, a value validated experimentally (see Supplementary Information (SI) section S3.4). Using this data, the effective optical path length of the system, in the absence of gas, was determined by fitting the kinetic curve shown in Figure , yielding a value of 2.6 mm.