## 2 Eosin Y as Organocatalyst in Photochemical Flow Reactions  
According to the _Beer-Lambert-Bouguer Law_[21], a photochemical transformation performed in a traditional batch reactor will be exposed to a non-uniform radiation distribution due to the attenuation effect of photon transport. The middle of the reaction mixture is subjected to a lower intensity irradiation compared to the mixture located close to the reactor walls simply because it is further away from the irradiating source. This phenomenon causes the formation of different reactor zones characterized by different reaction rates. Due to this non-uniform situation, the use of large photochemical reactors should be discouraged in order to prevent the over-irradiation of external zones, which can facilitate the formation of undesired byproducts. Photochemical transformations conducted in micro- or meso-reactors under fluidic conditions can overcome this problem, since the narrow channels allow the entire reaction mixture to be irradiated in a homogeneous and controlled manner. For this reason, continuous flow photochemical reactions show shorter reaction times, lower catalyst loadings, significantly less byproduct formation and increased productivity if compared to traditional batch processes [22]. These advantages are in addition to those commonly associated with flow chemistry processes such as an increased mass and heat transfer, enhanced safety, the possibility of continuous monitoring the synthesis with integrated analysis devices and the possibility to work with gaseous reactants and hazardous intermediates in a safe manner [23; 24]. Although micro- and meso-reactors are not suitable for the production of the large amounts of materials required by an industrial company, the scalability of continuous flow synthesis can be achieved through two essential strategies: by increasing the flow rate and the length of the reactor to allow a higher throughput or by a numbering-up approach, which involves the use of multiple reactors in a parallel manner. In the last decade, important progress has been made in photochemistry in developing novel synthetic strategies using microflow technology [22]. Furthermore, organic dyes such as Eosin Y have been widely studied and proved to be economically and ecologically superior to metal-based catalysts in photocatalyzed organic synthesis. One of the first examples of photochemical transformation performedunder fluidic conditions was reported in 2012 by [PERSON], who reported a photocatalytic reductive \(\alpha\)-dehalogenation of carbonyl compounds catalyzed by Eosin Y and promoted by a Hantzsch ester [25]. Eosin Y was successfully employed in the dehalogenation of activated halogenides using a microreactor setup under fluidic conditions. A premixed \(N\),\(N\)-dimethylformamide (DMF) solution of halogenated compound (1) or (2), Eosin Y and iPr\({}_{2}\)Net was fed into a commercially available glass microreactor (100 \(\upmu\)L reaction volume, rectangular channel shape, 500 \(\times\) 600 \(\upmu\)m) equipped with two green LEDs (operating at 530 nm). Under these conditions, a tremendous acceleration of the dehalogenation process was observed compared to that in the traditional batch approach, and in the case of bromoacetophenone (1), the desired halogenated ketone was obtained in 40 s of residence time only (Scheme 3). Interestingly, when compound (2) was used, no aromatic bromide dehalogenation was observed. In the same paper, [PERSON] reported also the \(\alpha\)-alkylation of aliphatic aldehydes promoted by Eosin Y (0.5 mol%) in the presence of a catalytic amount of the second-generation MacMillan imidazolidinone triflate salt chiral catalyst (3). At the moment, this is the only example known related to the use of an organo-dye catalyst in a stereoselective transformation performed under fluidic conditions. Bromo-malonate (4) reacts with octanal (5) in the presence of catalyst (3), Eosin Y and 2,6-lutidine (6) to afford the corresponding \(\alpha\)-alkylated aldehyde (7) under fluidic conditions (Scheme 4).