A microstructured falling film reactor was applied to the dye-sensitized photochemical conversion of 1,5-dihydroxynaphthalene to juglone. This continuous-flow microreactor enables the efficient contacting of a gas and a liquid phase in combination with external irradiation by high-power LED arrays offering various wavelengths. Two sensitizers were used for the photochemical in situ generation of singlet oxygen as key step in the synthesis of the natural product juglone. The photochemical process was investigated according LED wavelength, LED power, oxygen partial pressure, reactor architecture, substrate concentration and flow rate, and optimized to a conversion of X = 97% with 99% selectivity. Based on the experimental results process parameters like quantum efficiency, productivity and space time yield were calculated and used for the evaluation of the photochemically catalyzed synthesis of juglone in continuous-flow mode.