Steric Control of Electron Transfer. Changeover from Outer-Sphere to Inner-Sphere Mechanisms in Arene/Quinone Redox Pairs

Abstract

The various aromatic hydrocarbons (Chart ) constitute a sharply graded series of sterically encumbered (unhindered, partially hindered, and heavily hindered) donors in electron transfer (ET) to quinones (Chart ). As such, steric effects provide the quantitative basis to modulate (and differentiate) outer-sphere and inner-sphere pathways provided by matched pairs of hindered and unhindered donors with otherwise identical electron-transfer properties. Thus the hindered donors are characterized by (a) bimolecular rate constants (k 2) that are temperature dependent and well correlated by Marcus theory, (b) no evidence for the formation of (discrete) encounter complexes, (c) high dependency on solvent polarity, and (d) enhanced sensitivity to kinetic salt effectsall diagnostic of outer-sphere electron-transfer mechanisms. Contrastingly, the analogous unhindered donors are characterized by (a) temperature-independent rate constants (k 2) that are 102 times faster and rather poorly correlated by Marcus theory, (b) weak dependency on solvent polarity, and (c) low sensitivity to kinetic salt effectsall symptomatic of inner-sphere ET mechanisms arising from the preequilibrium formation of encounter complexes with charge-transfer (inner-sphere) character. Steric encumbrances which inhibit strong electronic (charge-transfer) coupling between the benzenoid and quinonoid π systems are critical for the mechanistic changeover. Thus, the classical outer-sphere/inner-sphere distinction (historically based on coordination complexes) is retained in a modified form to provide a common terminology for inorganic as well as organic (and biochemical) redox systems.

Spin dynamics and zero-field splitting constants of the triplet exciplex generated by photoinduced electron transfer reaction between erythrosin B and duroquinone

“…The solid lines are obtained from the nonlinear least squares fits based on the modified Table 1 Kinetic parameters and intrinsic enhancement factors of DQ Bloch equation (discussed later). The observation of the unusual net-A polarization suggests the production of the triplet exciplex as the reaction intermediate, because the SOC interaction due to heavy atoms is a short-range interaction [1][2][3][4][5][6][7][8][9]. The intermediate would be a nearly pure charge transfer complex or contact RP in the polar solvent.…”

“…We examined the acceptor concentration dependence of the k 1st obs values because a nonlinear relationship is the diagnostic of a pre-equilibrium intermediate between the donor and acceptor [7][8][9]27]. For concentrations up to 0:05 mol dm À3 in 1-PrOH at room temperature, the kinetics plots revealed the linear dependence of the k 1st obs on the DQ concentration.…”

“…The dynamics and reaction mechanisms of triplet exciplexes have received much attention in the field of photochemistry for a long time [1][2][3][4][5][6][7][8][9]. Significant progress has recently been made in understanding the character of triplet exciplexes.…”

“…The MFE is attributed to the sublevel selective back electron transfer (ET) reaction due to the SOC interaction from the triplet exciplex to the ground state. Recently, transient absorption bands observed in the infrared region were assigned to triplet exciplexes of quinones and aromatic molecules [7][8][9]. The rate constants of the exciplex formation and ET in the exciplexes were determined from the buildup and decay rates of the bands.…”

The Chemical Problem of Energy Change: Multi-Electron Processes

“…Marcus Theory was initially formulated to deal with weak donor-acceptor interactions and quasi-equilibrium situations (outer-sphere electron transfer) and has successfully been applied to many electron transfer problems in chemistry and biology. There continues to be sophisticated extensions and generalizations of the theory to deal with stronger resonance interactions including strong-overlap (inner-sphere) electron transfer, [7][8][9] and vibronic coupling [9,10] where electron and nuclear motions cannot be separated; recently this theme has incorporated hydrogen atom, proton, and hydride transfer. [11] However, even with its great success, some electron transfer problems have struggled to reach a consensus through advancements of the Marcus Theory framework.…”