PhotoFlow Reactors

Photoflow reactors are continuous flow devices specifically designed to enable photochemical reactions — chemical transformations driven by the absorption of photons from an external light source rather than by thermal energy. Photochemistry provides access to unique reaction pathways, excited states, and reactive intermediates that are inaccessible through conventional thermal chemistry, including [2+2] cycloadditions, Paterno-Büchi reactions, singlet oxygen oxidations, radical photoinitiation, and a rapidly expanding library of photoredox catalysis reactions using ruthenium, iridium, or organic photocatalysts. In batch, photochemical reactions are severely limited by the Beer-Lambert law: light penetration into a solution decreases exponentially with path length and concentration, meaning that beyond approximately 2–5 mm depth, inner filter effects cause negligible reaction in the reactor bulk. Photoflow reactors address the light penetration problem fundamentally: by processing the reaction stream in a thin film or narrow-channel geometry (typically 0.5–3 mm path length), every molecule in the flow stream is exposed to near-uniform irradiation throughout its entire residence time in the irradiation zone. This thin-geometry approach allows the use of higher reagent concentrations than batch photochemistry, shortens reaction times by orders of magnitude (from hours to minutes or seconds), and provides far more reproducible photon dosing than batch lamp systems subject to lamp degradation and hot spots.

Light sources in photoflow reactors are predominantly high-output LEDs (wavelength-selectable across 365–730 nm, with green, blue, and violet LEDs covering the primary photoredox catalysis absorption windows) or UV lamps (254 nm, 302 nm, 365 nm) for direct substrate excitation. LED light sources have eliminated many of the practical disadvantages of older mercury arc lamp systems — LEDs produce narrow-band emission at the precise wavelength required, consume a fraction of the electrical power, have lifetimes exceeding 50,000 hours, and produce no UV-C or ozone. The reactor channel is typically fabricated in transparent fluoropolymer (FEP, PFA) tubing or glass, wrapped around or sandwiched between the LED arrays to maximise irradiance across the full reactor volume.