SET and HAT/PCET acid-mediated oxidation processes in helical shaped fused bis-phenothiazines

Helical shaped fused bis-phenothiazines 1 – 9 have been prepared and their red-ox behaviour quantitatively studied. Helicene radical cations (Hel^.+) can be obtained either by UV-irradiation in the presence of PhCl or by chemical oxidation. The latter process is extremely sensitive to the presence of acids in the medium with molecular oxygen becoming a good single electron transfer (SET) oxidant. The reaction of hydroxy substituted helicenes 5 – 9 with peroxyl radicals (ROO^.) occurs with a ‘classical’ HAT process giving HelO^. radicals with kinetics depending upon the substitution pattern of the aromatic rings. In the presence of acetic acid, a fast medium-promoted proton-coupled electron transfer (PCET) process takes place with formation of HelO^. radicals possibly also via a helicene radical cation intermediate. Remarkably, also helicenes 1 – 4 , lacking phenoxyl groups, in the presence of acetic acid react with peroxyl radicals through a medium-promoted PCET mechanism with formation of the radical cations Hel^.+. Along with the synthesis, EPR studies of radicals and radical cations, BDE of Hel-OH group (BDE_OH), and kinetic constants (k_inh) of the reactions with ROO^. species of helicenes 1 – 9 have been measured and calculated to afford a complete rationalization of the redox behaviour of these appealing chiral compounds.