## 2 Eosin Y as Organocatalyst in Photochemical Flow Reactions
### Continuous Flow Photocatalytic Oxidation Mediated by Eosin Y  
In 2016, it was found that Eosin Y could also operate in aerobic conditions; [PERSON] and co-workers demonstrated that thiols could be easily oxidized to disulfides under fluidic conditions by a convenient numbering-up strategy combining up to eight parallel reactors [30]. Even if this work was focused on the evaluation of the feasibility of the numbering-up approach rather than the chemical application, it was possible to synthetize diphenyl sulfide (12) with a high yield operating with 1 mol% of Eosin Y as a photocatalyst in the presence of tetramethylethylenediamine (TMEDA) and pure oxygen (as oxidant) using a multicapillary photomicroreactor system (Scheme 8).  
The multicapillary microreactor system employed by [PERSON] was assembled starting from commercially available parts, and it is composed of two units: the first one guarantees the equal distribution of reagents to all parallel microreactors, whereas the second module consists of eight micro-photoreactors, each one irradiated by white LEDs. The substrate solution and the catalyst were filled into the device using a series of T-micromixers and combined with pure oxygen controlled by a gas mass flow controller, resulting in a two-phase flow. The flow rate ratio between the oxygen and the liquid phase was kept at 3:1, and different flow rates were investigated. The best results were obtained with 0.3 mL/min, leading to the formation of the desired product with up to 90% yield. Even if a small standard deviation in the flow distribution performance was observed (SD \(<\) 10%), the global yield of the numbered-up photomicroreactors was comparable to the yield obtained when a single fluidic device was used.  
Another photocatalytic oxidation mediated by Eosin Y was reported in 2019 by [PERSON] and co-workers [PERSON] and co-workers [PERSON] and co-workers [PERSON] (2019). In this example, the authors reported the fabrication of a new fluidic device based on the combination of luminescent solar concentrators (LSC) and a photomicroreactor (PM). LCS are inexpensive slabs of luminophore-doped polymeric materials that are able to harvest and concentrate photons. LSC-PM devices couple sunlight modulation/concentration with microflow chemistry; under this system, the solar photons reaching the device are absorbed by the fluorescent dye and re-emitted. However, due to the nature of the system, emitted photons remain trapped in the polymeric material, which acts as a waveguide for the delivery of the down-converted photon to the reaction channels. In this example, perfluoroalkoxy alkane capillaries containing DFSB-K160 dye (emitting at 530 nm) were embedded in poly(methyl methacrylate) lightguides and were employed in the photocatalytic oxidation of benzylamine (13) to dibenzylimine (14) mediated by the presence of Eosin Y (Scheme 9).  
As the results, the desired product was obtained with a 78% yield after 18 s only (productivity: 30 mmol m\({}^{2}\) h\({}^{-1}\)). The Eosin Y solution and the benzylamine solution were fed into the a 20 \(\times\) 20 cm  
Figure 8: Aerobic photocatalytic oxidation of thiophenol to diphenyl disulfide catalyzed by Eosin Y.  
LSC-PM device using a syringe pump and mixed with pure oxygen with a 5:1 gas/liquid ratio with a flowrate of 2.02 mL/min for O\({}_{2}\) and 405 \(\upmu\)L/min for the liquid stream. The mixture was then irradiated under solar-simulated conditions, and the presence of the desired product was investigated by gas chromatography (GC).