ChemInform Abstract: ELECTRON‐TRANSFER FLUORESCENCE QUENCHING OF RADICAL IONS

How Radical Are “Radical” Photocatalysts? A Closed-Shell Meisenheimer Complex Is Identified as a Super-Reducing Photoreagent

“…Therefore, the measured excited state lifetimes of nearly all radical doublet species are orders of magnitude too short to result in productive photochemistry. Indeed, excited state spectral features with lifetimes longer than 1 ns measured for several presumptive radical anions 23,24 were later shown to instead arise from longer-lived closed shell impurities. For instance, the radical anions of both 9,10-dicyanoanthracene and anthraquinone were later found to contain the emissive byproducts cyanoanthrolate or bianthrolate, respectively.…”

Reductive Electrophotocatalysis: Merging Electricity and Light To Achieve Extreme Reduction Potentials

“…For example, cathodic reduction of dicyanoanthracene (DCA; E 1/2 = −0.82 V) results in the corresponding radical anion DCA •− , which absorbs visible light and exhibits a strong fluorescence emission (excitation energy E 0,0 = 2.38 eV; see Scheme 1). 9 The photoexcited DCA (DCA •− *) is estimated to display an exceptionally high reducing potential of −3.2 V (vs SCE), 10 which TD-DFT calculations suggest arises from a SOMO-HOMO level inversion featuring a very unstable electronic structure with a half-filled bonding orbital (ψ 1 ) and a filled antibonding orbital (ψ 2 ) (see Figure S3 in Supporting Information). This value is on par with some of the most reducing elemental metals such as Li (E = −3.3 V).…”

“…This value is on par with some of the most reducing elemental metals such as Li (E = −3.3 V). Moreover, DCA •− * has been estimated to have a lifetime of 13.5 ns, 9 sufficiently long to engage in reductive activation of suitable substrates. Importantly, the tandem electrochemical activation and photoexcitation allows the formation of this highly reducing catalyst in a very low concentration in a controllable fashion, thus providing an avenue to circumvent the chemoselectivity issues frequently associated with strongly reducing conditions using chemical reductants or direct electrolysis.…”

“…The UV−vis absorption spectrum of this solution is identical to that of DCA •− reported in the literature. 9 We next set out to investigate our proposed electrophotocatalytic system in the context of the reductive functionalization of aryl halides. In recent years, the generation of aryl radicals from aryl halides has been shown to be a productive synthetic strategy.…”

“…However, the reaction was prematurely terminated due to the formation of a zinc bridge between the cathode and anode that short-circuited the electrochemical setup (entry 8). Anthraquinone (AQ), which exhibits similar light-induced redox reactivities, 9 was also an effective catalyst, albeit with diminished yield (entry 9). Importantly, this reaction was also applicable to the reductive borylation of 4-chloroanisole (3), which has a highly negative reduction potential (E red = −2.90 V vs SCE) 15 (entry 10).…”