An accessible visible-light actinometer for the determination of photon flux and optical pathlength in flow photo microreactors

Abstract

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.

Principles of co‐axial illumination for photochemical reactions: Part 1. Model development

“…[14][15][16] Simplified models, as used in photochemical and actinometric studies, implicitly consider plug flow and homogeneous illumination, and these models yield a very good match to experimental results when used in micro-and millichannel reactors, and are considered in this work. [17][18][19][20][21] In the following section, three different models are derived: cross-current, co-current, and counter-current illumination related to the relation between the light and flow direction. Cross-current illumination is the current state-of-the-art in photochemical reactor operation and will be treated first, see Figure 1.…”

“…[15] The model admits an effective path length, measured experimentally by actinometry or ray tracing. [17] The mass balance in terms of the reagent A (see Equation 1) is derived assuming plug flow. In this model, we explicitly assume plug flow, which is implicitly using in most articles and justified by experimental validation.…”

“…In this model, we explicitly assume plug flow, which is implicitly using in most articles and justified by experimental validation. [17][18][19][20][21] The residence time distribution (RTD) factor will be added to the model in a further step. The mass balance over a small volume element (dV) can be described as:…”

“…The concentration c i,0 (mol m −3 ) is easily determined when preparing the solution. The average path length, ℓ (m), can be determined via actinometry [17] or via ray tracing simulations. Based on the modelling results, absorbance has to be sufficiently high for decent conversion, see Figure 5 till Figure 10.…”

Principles of coaxial illumination for photochemical reactions: Part 2. Model validation

“…A typical wavelength distribution of the LEDs is found in Figure S1. This normalized distribution is later used to determine the average specific absorbance (ε avg ) of the reagent using the strategy as explained by Roibu et al [19] and Meir et al [6] for λ [330 400] nm. The normalized version of the spectral irradiance of the light sources is shown in Figure S1 and is used to determine g(λ).…”