The Dependence of Chemical Quantum Yields of Visible Light Photoredox Catalysis on the Irradiation Power

Abstract

Despite the great achievements of advanced photoredox catalysis for organic‐synthetic reactions, the literature is rather vague with respect to reaction quantum yields – the number of product molecules per absorbed photon. This stands in contrast to the clear and commonly used chemical yield as standard parameter to quantify the efficiency of chemical reactions. We applied an opto‐electronic device to measure the reaction quantum yields of a reference reaction in a rapid and facile way, which revealed that this parameter cannot be regarded as a single, isolated value. A so far undescribed strong dependence of the reaction quantum yield on the incident light power and the irradiation time was revealed. The light input even decides on the interplay of the closed photoredox catalytic cycle and the radical chain propagation. The reaction kinetics were modelled in full detail to obtain important insight into the general description of photoredox catalytic mechanisms.

The forgotten reagent of photoredox catalysis

“…3b). 48 As suggested by previous work, the same chemistry could occur via two different mechanisms; high power irradiation, common in most experimental photoreactors, enabled a conventional monophotonic cycle, with a quantum yield Φ < 1. Low intensity instead revealed a prominent radical chain pathway ( Φ ≫ 1), after a brief induction period.…”

“…The complexity of measuring photochemical quantum yields often results in different values reported for the same reaction; accurate values require standardized light sources with a well-quantified photon flux. Reported quantum yields of the photoredox α-alkylation of 1-octanal by bromomalonate range between 0.09–20, 46–50 but using [Ru(bpy) 3 ]Cl 2 as the photocatalyst irradiated with blue light ( λ = 432 nm, 30 μW–28.3 mW) saw the quantum yield surprisingly decrease with increasing light intensity (Fig. 3b).…”

Light as a Tool in Organic Photocatalysis: Multi‐Photon Excitation and Chromoselective Reactions

“…However, in photocatalysis, this correlation is not always simple and the incident light can influence the reaction mechanism and accordingly the quantum yield of reaction. [11] In the next section, we will discuss how the intensity of light can be used to change reactivity of the photocatalyst. More precisely, how absorption of several photons sequentially offers new reactive states of the excited photocatalyst and how they can be used to run chemical reaction (Figure 1c).…”

“…Thus, the quantum yield of the reaction depends on the power of used LED, transmittance of the reaction vessel and the time of irradiation. However, in photocatalysis, this correlation is not always simple and the incident light can influence the reaction mechanism and accordingly the quantum yield of reaction [11] . In the next section, we will discuss how the intensity of light can be used to change reactivity of the photocatalyst.…”

Dynamically triggering photoreactions for high performance and efficiency

“…The demand for reliable actinometers that can be applied for various spectral ranges, most importantly for the UVA and visible region, has recently triggered research on new chemical actinometer systems as well as on reliable measurement procedures. [46] Radiometric methods were proposed as complementary methods that can also provide spatial insights into the radiation field. [47] 3 Research demands Contrary to former times, the increasing research activities in light-driven reactions are picked-up by chemical industries now.…”