Photoelectrochemical C−H Activation Through a Quinacridone Dye Enabling Proton-Coupled Electron Transfer

Dye-sensitizedphotoanodesfor CH activationin organicsubstratesare assembledby vacuumsublimationof a commer-ciallyavailablequinacridone(QNC)dye in the form of nanosizedrods onto fluorine-dopedtin oxide(FTO),TiO2, and SnO2slides.The photoanodesdisplayextendedabsorptionin the visiblerange(450–600nm)andultrafastphotoinducedelectroninjection(<1 ps, as revealedby transientabsorptionspectro-scopy)of the QNCdye into the semiconductor.The proton-coupledelectron-transferreactivityof QNCis exploitedforgeneratinga nitrogen-basedradicalas its oxidizedform,whichis competentin CH bondactivation.The key reactivity parameteris the bond-dissociationfree energy(BDFE)associ-atedwiththe N·/NH couplein QNCof 80.5�2.3 kcalmol1,whichenableshydrogenatomabstractionfromallylicorbenzylicCH moieties.A photoelectrochemicalresponseisindeedobservedfor organicsubstratescharacterizedby CHbondswith BDFEbelowthe 80.5 kcalmol1threshold,such asγ-terpinene,xanthene,or dihydroanthracene.This workprovidesa rational,mechanisticallyorientedrouteto the designof dye-sensitizedphotoelectrodesfor selectiveorganictransforma-tions.