Multi-Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods.

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1. What are the contributions mentioned in the paper "Multi-photon excitation in photoredox catalysis: concepts, applications, methods" ?

2. What is the key limitation in this concept?

3. What is the reductive potential of the aryl radical anions?

4. What is the direct and informative way to elucidate a photochemical mechanism?

Accepted Answer

Herein, the authors have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging

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The need for catalytic species with photoactive excited states in two different oxidation states is a key limitation in this concept.

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Secondary excitation is then thought to form *PC- with a reduction potential of ca. -2.4 V vs. SCE,[19] sufficient for reductive debromination of aryl bromides and thereby leading to short-lived aryl radicals that can be intercepted by N-methylpyrrole.

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The most direct and informative way of elucidating a photochemical mechanism is to detect reaction intermediates by time-resolved methods such as transient absorption spectroscopy, and to identify the species causing the observable spectroscopic signatures by a complementary technique such as spectroelectrochemistry.

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An emerging field in multi-photon chemistry is the use of a catalyst at different stages of a chemical transformation to perform two-photon driven cascade reactions.

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In some cases, transient infrared spectroscopy can be much better suited than UV-Vis detection, because IR signatures can be more diagnostic, allowing better discrimination between photoproducts resulting from single and double excitation.[83]

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OsII polypyridines represent another attractive option which allow direct excitation into 3MLCT states,[65] and this can minimize the energy loss associated with TTA UC and maximize achievable anti-Stokes shifts.

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When Asc2- is employed as reductant, the ascorbate monoanion HAsc- is always present in noticeable concentrations, and the latter can act as an efficient H-atom donor.[45]

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Key observables are the UV-Vis spectral changes occurring after photoexcitation, resulting in transient difference spectra that essentially represent a subtraction of absorption spectra recorded before and after the excitation laser pulses (upper right of Fig. 9a).

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As noted above, excitation of PC- in Fig. 2 can induce photoionization in selected cases, leading to solvated electrons that can have much longer lifetimes than photoexcited organic radical ion species.

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stronger electron donors are needed for this purpose than in the case of the mechanism in Fig. 1, because *PC is usually a weaker oxidant than PC+.

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As a rule of thumb, that energy loss amounts to at least 25 % (more typically about 50%) compared to the initial photon energy, which is due to the accumulated energy losses caused by internal conversion, intersystem crossing, and oxidation or reduction of the catalyst itself.

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The observation time window in the continuous irradiation approaches in Fig. 10 is typically up to 10 orders of magnitude longer than the timescale of the laser experiments in Fig. 9, and the latter usually fail to provide information concerning changes of the photochemical system over long irradiation times.[23c]

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Long explored by pulse radiolysis and later with pulsed laser methods, the hydrated electron only recently made the important step into the reaction flask for lab-scale photoredox catalysis under continuous irradiation.

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PAH radical anions have strong reducing power already in their electronic ground state, and hence the idea of exploiting A- as reductant is attractive.[50]

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photoredox catalysis was not performed with this system; the need for UV excitation and the low ionization quantum yield would represent significant disadvantages.

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Recording a photo-action spectrum is a somewhat laborious experiment, which might become easier in the future with wavelength tunable LEDs and on-line photoproduct detection.[34]

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General methods for mechanistic studies in photocatalysis have been reviewed recently,[80] and therefore the authors focus here exclusively on techniques that are particularly relevant for photoredox catalysis involving multi-photon excitation.

Accepted Answer

By this strategy, the thermodynamic hurdles are overcome by combining the energy of two (or more) photons per catalytic turnover.

Multi-Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods.

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Most frequently asked questions

1. What are the contributions mentioned in the paper "Multi-photon excitation in photoredox catalysis: concepts, applications, methods" ?

2. What is the key limitation in this concept?

3. What is the reductive potential of the aryl radical anions?

4. What is the direct and informative way to elucidate a photochemical mechanism?

5. What is the definition of a multi-photon chemistry?

6. What is the way to detect photoproducts?

7. What is the common method of achieving anti-Stokes shifts?

8. What is the effect of the reductive h-atom donor?

9. What are the key observables of the photochemical reactions?

10. What is the effect of excitation of PC- in Fig. 2?

11. Why is the mechanism in Fig. 1 better suited to the photoactive compound?

12. How much energy loss is due to the accumulated losses caused by the catalyst itself?

13. What is the timescale of the continuous irradiation approaches in Fig. 10?

14. What is the important step in the process of photoredox catalysis?

15. What is the main reason for the use of A- as a reductant?

16. What are the disadvantages of the photoredox catalysis system?

17. What is the way to measure the photo-action spectrum?

18. What are the main methods for mechanistic studies in photocatalysis?

19. What is the common way to overcome the thermodynamic hurdles?

Figures

Citations

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis.

Non-innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides.

Organocatalyzed Birch Reduction Driven by Visible Light

Unveiling Extreme Photoreduction Potentials of Donor-Acceptor Cyanoarenes to Access Aryl Radicals from Aryl Chlorides

Rational Design of Photocatalysts for Controlled Polymerization: Effect of Structures on Photocatalytic Activities.

References

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

Organic Photoredox Catalysis

Upconversion and Anti-Stokes Processes with f and d Ions in Solids

Visible light photoredox catalysis: applications in organic synthesis

Dual Catalysis Strategies in Photochemical Synthesis

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