| Ir(III)(dF-CF<\/span>3<\/span>-ppy)<\/span>2<\/span>(dtbpy)<\/span><\/td> | Ir(III)*\/Ir(II)<\/span><\/td> | 1.21 V<\/span><\/td> | Ir(III)*\/Ir(IV)<\/span><\/td> | -0.89 V<\/span><\/td><\/tr>| Ru(II)(phen)<\/span>3<\/span><\/td> | Ru(II)*\/Ru(I)<\/span><\/td> | 0.82 V<\/span><\/td> | Ru(II)*\/Ru(III)<\/span><\/td> | -0.87 V<\/span><\/td><\/tr>| Ru(II)(bpy)<\/span>3<\/span><\/td> | Ru(II)*\/Ru(I)<\/span><\/td> | 0.77 V<\/span><\/td> | Ru(II)*\/Ru(III)<\/span><\/td> | -0.81 V<\/span><\/td><\/tr>| Ir(III)(ppy)<\/span>2<\/span>(dtbpy)<\/span><\/td> | Ir(III)*\/Ir(II)<\/span><\/td> | 0.66 V<\/span><\/td> | Ir(III)*\/Ir(IV)<\/span><\/td> | -0.96 V<\/span><\/td><\/tr><\/tbody><\/table>
To overcome these limitations, Wenger and coworkers recently reviewed multi-photon synthetic concepts and approaches looking to exploit multi-excitation processes for synthetic applications (Ref 1).\u00a0 Spectroscopists routinely study multi-photon reactions with expensive pulsed lasers, short-lived excited intermediates and low micromolar concentrations.\u00a0 However, for synthetic purposes the utility of multiphoton excitation is a recent development with the availability of high-powered LEDs.<\/span><\/p>Consecutive Photo-Excitation as a Multi-Photon Approach to Synthetic Photochemistry<\/b><\/p> The simplest scenario to utilize two photons of energy involves consecutive excitations to produce a highly reducing species (Figure 1).\u00a0 The ground state photocatalyst (PC) absorbs the first photon of light generating an excited triplet state.\u00a0 If the resulting excited photocatalyst has a sufficient lifetime and a suitable excitation band, a second photon excitation can result in a higher excited state with ultimate release of the superreductant hydrated electron (e<\/span>\u00b7-<\/span><\/sup>aq<\/span><\/sub> (-2.9 V vs. NHE).\u00a0 The system can be made catalytic in the presence of a sacrificial donor to return the photocatalyst.\u00a0\u00a0<\/span><\/p>Figure 1:\u00a0 <\/b>Consecutive photoexcitation of photocatalyst<\/span><\/p>![\"Consecutive](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-11.49.53-AM.png\") <\/p>
Wenger demonstrated the generation of hydrated electrons in two recent examples (Ref 3,4).\u00a0 Using the photocatalyst Ir(sppy)<\/span>3 <\/span><\/sub>(Figure 2A), triethanolamine (\"TEAO\") and a blue continuous wave laser (447 nm) afforded conditions suitable for several powerful reductions (Ref 3).\u00a0 The consecutive two photon excitation mechanism was demonstrated with a pulsed laser system.\u00a0 The initial laser pulse generates a triplet excited Ir(sppy)<\/span>3<\/span><\/sub> species with a lifetime of 1.6 \u00b5s and a strong absorbance between 460 nm and 570 nm.\u00a0 The second laser pulse at 532 nm promotes a higher excited state with photoionization to give the hydrated election. Both ground and excited state excitation of Ir(sppy)<\/span>3<\/span><\/sub> occurred with a single continuous wave 447 nm diode laser.\u00a0 This powerful reduction with a low energy (inexpensive) laser and water-soluble catalyst represents an opportunity for the degradation of pollutants.\u00a0 For example, the CF<\/span>3<\/span><\/sub> functional group is a common inert group in many pharmaceuticals and pesticides while the benzylammonium cation is a common wastewater pollutant (Figure 2B).<\/span><\/p>Figure 2<\/b>:\u00a0 A). Ir(sspy)3<\/sub> catalyst\u00a0 B.)\u00a0 Applications of hydrated electron reductions<\/span><\/p>![\"A).](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-11.58.35-AM.png\") <\/span><\/p>With the same system, Wenger and coworkers demonstrated reaction control through the modification of the light source intensity(Ref 4).\u00a0 At low intensity, the catalyst acts as a photoredox catalyst (E<\/span>1\/2<\/span><\/sub> = -1.6 V) for one electron reduction.\u00a0 At higher intensity light focused with a lens, the two-photon process generates the superreductant e\u00b7-<\/sup><\/span>aq<\/sub><\/span> (E<\/span>1\/2<\/sub><\/span> = -2.9 V) nearly doubling the available redox energy compared to the one photon system. This selectively was demonstrated with the dehalogenation of 4-bromo-2-chloro-5-fluorobenzoic acid (Figure 3).\u00a0 At low intensity light, the primary pathway involves reductive activation of the weaker C-Br bond.\u00a0 Under the two-photon process, the reduction potential was sufficient for activation of the C-Cl bond.\u00a0 Neither system activated the strongest C-F bond.<\/span><\/p>Figure 3.<\/b>\u00a0 Control over light intensity allows selectivity in the reaction. Irridiation of Ir(sspy) catalyst, TEOA in water.<\/span><\/p>![\"Control](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-11.58.45-AM.png\") <\/p>
Consecutive Photoinduced Electron Transfer (ConPET) <\/b>as a Multi-Photon Approach to Synthetic Photochemistry<\/b><\/p> In an alternate approach, classified as a Consecutive Photoinduced Electron Transfer (ConPET), the two photon excitation steps are separated by reduction of the photocatalyst (Figure 4).\u00a0 Here, the first photon results in an excited photocatalyst, which upon reduction by a sacrificial reductant generates a photoactive radical anion species suitable for excitation with a second electron. \u00a0 It is then the radical anion of the PC that is excited by the second photon generating the excited triplet photocatalyst photoionizing to the solvated electron. \u00a0 The scheme was first demonstrated by Konig and coworkers (Ref 5) using the PDI photocatalyst (Figure 5A) and blue LEDs for the activation of the carbon-halogen bonds (Figure 5B).\u00a0 Konig <\/span>et. al.<\/span><\/i> use this system to activate aromatic chlorides demonstrating the dehalogenation and successful cross-coupling with multiple coupling agents.\u00a0 Alternative approaches have used an electrode to replace the sacrificial donor in this scheme (Ref 6).\u00a0\u00a0<\/span><\/p>
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![\"Multi-photon](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-12.00.44-PM-300x234.png\")
Multi-photon approaches to synthetic photochemistry expand the potential pathways to create more efficient and potentially safer reaction conditions.<\/span>Wenger and coworkers recently published a fascinating in-depth review on multi-photon excitation in photoredox catalysis<\/a> (Ref 1). The general paradigm of most modern synthetic photoredox catalysis involves absorption of one photon of visible light by a photoactive catalyst to give an oxidative\/reductive species that can give or receive an electron as needed. Thus, the majority of preparative scale photoredox reactions reported to date invoke catalytic cycles where 1 photon productively absorbed by the photocatalyst is required for 1 equivalent of product. Two factors limit the range of reactivity available via traditional photoredox:<\/span><\/p>\n![\"Scheme](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-11.51.10-AM.png\")
<\/p>\n![\"\"](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/figure5b.gif\")
<\/p>\n![\"Classical](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-11.59.52-AM.png\")
<\/p>\n![\"Visible](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-12.00.12-PM.png\")
<\/p>\n![\"Modified](\"https:\/\/hepatochem.com\/wp-content\/uploads\/2020\/08\/Screen-Shot-2020-08-12-at-12.00.22-PM.png\")