Coupling photochemistry with flow microreactors enables novel synthesis strategies with higher efficiencies compared to batch systems. Improving the reproducibility and understanding of the photochemical reaction mechanisms requires quantitative tools such as chemical actinometry. However, the choice of actinometric systems which can be applied in microreactors is limited, due to their short optical pathlength in combination with a large received photon flux. Furthermore, actinometers for the characterization of reactions driven by visible light between 500 and 600 nm (e.g. photosensitized oxidations) are largely missing. In this paper, we propose a new visible-light actinometer which can be applied in flow microreactors between 480 and 620 nm. This actinometric system is based on the photoisomerization reaction of a diarylethene derivative from its closed to the open form. The experimental protocol for actinometric measurements is facile and characterized by excellent reproducibility and we also present an analytical estimation to calculate the photon flux. Furthermore, we propose an experimental methodology to determine the average pathlength in microreactors using actinometric measurements. In the context of a growing research interest on using flow microreactors for photochemical reactions, the proposed visible-light actinometer facilitates the determination of the received photon flux and average pathlength in confined geometries.

Photochemical reactors with conventional homogeneous illumination suffer from a light efficiency problem, which is inherent to their design: Dark zones arise near the reagent-rich inlet whereas the reagent depleted outlet is overilluminated. Any attempt to mitigate dark zones at the inlet will only increase photon losses further downstream. This study reports the principles and model equations for co-and counter-current illumination in photochemical reactors, along with an optimization study to determine the most efficient and productive operating point. This work proves that the use of co-and counter-current illuminated reactors increases the energy efficiency while easing scalability by implementing larger path lengths, without altering the reactor's geometry. We report a simple model to determine the conversion obtained by such novel illumination techniques and compare it to the current state-of-the-art. Two nondimensional groups where derived that describe all possible reactor configurations, these are the initial absorbance (A) and the quantum photon balance (ρϕ). Variation of both parameters leads for noncompetitive photochemical reactions to an optimal point for the current state-of-the-art as well as the novel co-axial illumination. Ultimately, we recommend the use of an initial absorbance value (A) of at least 1, and a quantum photon balance (ρϕ) equal to 1 to introduce sufficient light and enable near complete absorption of light.

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