Redox Reaction of Benzyl Radicals with Aromatic Radical Ions Photogenerated. The Marcus Inverted Region and the Selective Formation of Carbocations or Carbanions

Abstract

Efficient redox reactions of benzyl-type radicals were achieved by irradiating an aromatic donor/acceptor pair with substituted dibenzyl ketones as a radical precursor in MeOH−MeCN. In this system, the aromatic radical ion pair was generated by photoinduced electron transfer followed by one-electron oxidation and reduction of photogenerated benzyl radicals (R•) by the radical ions to afford benzyl cations (R+) and anions (R-). The cations and anions were trapped by MeOH to yield ROMe and RH, respectively. The relative product ratios were determined for a variety of donor−acceptor pairs, reflecting the relative efficiencies of the redox reaction of benzyl radicals with a steady-state concentration of radical ions. The selective formation of carbocations or carbanions was achieved in some sets of donor/acceptor pairs. Assuming that the radical ions exist in a 1:1 ratio in the steady state, the product ratios are equal to the relative electron transfer rates, which are analyzed in terms of the free energy changes of the processes. The present results indicated that the rates became maximal at the energy gap of ca. −0.5 eV, representing a novel example of the Marcus inverted region. This interpretation is discussed in comparison with other related cases and in relation to recent theories on electron transfer rates.

Photoinduced Processes in Fluorene‐Bridged Rhenium–Phenothiazine Dyads – Comparison of Electron Transfer Across Fluorene, Phenylene, and Xylene Bridges

“…The relevant bridge potential for hole transfer is the oxidation potential of fluorene, which has been reported as 1.58 V vs. SCE 4,16b. For reference, biphenyl is oxidized at 1.96 V vs. SCE 30. With an excited state oxidation potential of 1.42 V vs. SCE for the rhenium hole donor,21a one thus arrives at estimates for Δ ϵ of 0.16 eV for the fluorene bridge and 0.54 eV for the biphenyl spacer (Scheme ) 21b.…”

Conformational Effects on Long‐Range Electron Transfer: Comparison of Oligo‐p‐phenylene and Oligo‐p‐xylene Bridges

“…As illustrated in Scheme , in the Ru‐ph n ‐PTZ molecules there is hole tunneling from PTZ · + to Ru(bpy) 3 + , whereas in the Ru‐xy n ‐PTZ systems there is Ru III to PTZ hole tunneling. The energy levels in Scheme were estimated from the redox potentials of all relevant species (Table 2); the bridge potential comes from a prior electrochemical study of the 4,4′‐dimethyldiphenyl (dmdp) molecule 44. It has not been possible to mesure the oligo‐ p ‐phenylene and oligo‐ p ‐xylene bridge redox potentials directly by cyclic voltammetry, but dmdp represents a reasonable approximation to the real bridges in our dyads.…”

“…It has not been possible to mesure the oligo‐ p ‐phenylene and oligo‐ p ‐xylene bridge redox potentials directly by cyclic voltammetry, but dmdp represents a reasonable approximation to the real bridges in our dyads. The dmdp molecule is oxidized at +1.67 V vs. SCE and reduced at –2.60 eV,44 hence the preference for hole over electron tunneling.…”

Tuning the Rates of Long‐Range Charge Transfer across Phenylene Wires

“…At any rate, the redox potentials of their monomers are known: p-xylene is oxidized at 1.7 V vs SCE, whereas 1,4-dimethoxybenzene is oxidized at the much lower potential of 1.35 V vs SCE. [20] On this basis, we estimate that De % 0.5 eV in PTZÀxy 4 ÀRu [7] and De % 0.1 eV for PTZÀ dmb 4 ÀRu (Scheme 2).…”