A Green‐LED Driven Source of Hydrated Electrons Characterized from Microseconds to Hours and Applied to Cross‐Couplings

Abstract

We present a novel photoredox catalytic system that delivers synthetically usable concentrations of hydrated electrons when illuminated with a green light-emitting diode (LED). The catalyst is a ruthenium complex protected by an anionic micelle, and the urate dianion serves as a sacrificial donor confined to the aqueous bulk. By virtue of its chemical properties, this donor not only suppresses charge recombination that would limit the electron yield, but also enables this system to perform cross-couplings through the action of hydrated electrons, the first examples of which are reported here. We have investigated the kinetics of all the steps involving the electron and its direct precursor in a comparative study by means of laser flash photolysis and by monitoring product formation during LED photolysis. Despite the differences in timescales, each approach on its own already gives a complete picture of the reaction over a temporal range spanning ten orders of magnitude. Discrepancies between the kinetic parameters obtained with the two complementary techniques can be rationalized with the slow secondary chemistry of the system; they reveal that the product-based method provides a more accurate description because it also responds to the changes of the system composition during a synthesis; hence, they demonstrate that in complex systems the timescale of the experimental observation should be matched to that of the actual application.

Micellized Tris(bipyridine)ruthenium Catalysts Affording Preparative Amounts of Hydrated Electrons with a Green Light‐Emitting Diode

“…The clear second‐order kinetics during the first ms—as established by the linearizations in the inset, which directly give the values of k rec in Table —naturally suggest interpreting this decay as the expected recombination of OER and the quenching by‐product Asc .− . This identification as a reaction between unlike species formed in equal amounts is validated by different decay rate constants k rec when OER of the same catalyst is accessed with different quenchers, for example, Asc 2− vs. the urate dianion . As a secondary effect that only becomes visible in wider observation windows, we found an apparent decrease of the recombination rate constant over time.…”

“…First, secondary reactions such as a disproportionation of the quencher‐derived radicals can cause deviations; and there is little chance of extrapolating such effects from the timescale of laser flash photolysis to that of preparative photolysis. Ascorbate is not very critical in this respect, although the inset of Figure a indicates noticeable discrepancies, but with urate the disproportionation participates to such an extent that the square‐root dependence no longer holds . Second, the stability of our catalysts cannot be captured by laser flash photolysis because the decomposition quantum yields are too small to be measurable in pulsed experiments but become important through accumulating over the hours of the LED illumination.…”

“…On the basis of the relationships that we have derived theoretically and demonstrated experimentally for the mechanism of Figure , the influence of the chemical, spectroscopic, and photokinetic parameters on TOF( ) is given by Equation . …”

“…Their trend reflects the interplay of capture and radical‐ion stability. Concentrating on the chlorine loss rather than on the stable end product benzoate is justified because it is equivalent to quantifying the formation of the highly reactive carboxyphenyl radicals, which can be used for cross coupling (see also below, Section 2.2.3) . The trapping of these radicals by a coupling product or by a hydrogen donor is strongly dependent on the concentration of the intercepting agent and on the structure of the substrate.…”

“…In a recent communication, we have presented the first photoredox catalytic system that generates on a laboratory scale while being both user‐friendly and sustainable through operating with a green light‐emitting diode (LED) instead of a high‐power laser and consuming only a bioavailable sacrificial species. After an attempt at replacing the latter to accommodate a specific class of synthetic applications, here we focus on improving the catalyst.…”