Electron-Transfer Mechanisms with Photoactivated Quinones. The Encounter Complex versus the Rehm−Weller Paradigm

Abstract

Photoexcited quinones (Q*) are efficiently quenched by polymethylbenzenes (ArH) via electron transfer (ET). However, the second-order rate constants (k 2) exhibit Rehm−Weller (outer-sphere) dependence on the free energy (ΔG ET), despite our new findings that the quenching occurs via a series of rather strong encounter complexes [Q*, ArH] with substantial (charge-transfer) bonding. The relatively high formation constants (K EC) of the encounter complexes indicate that any mechanistic interpretation of the driving-force dependence of the observed rate constants is highly ambiguous since k 2 must be a composite of K EC and the intrinsic rate constant (k ET) for electron transfer within the intermediate (inner-sphere) complex. As such, the reorganization energies extracted from Rehm−Weller plots lack thermodynamic significance. On the other hand, the unambiguous driving-force dependence of k ET represents a unique example for the “normal” Marcus behavior of the endergonic electron transfer between the donor/acceptor pair in van der Waals contact as extant in the encounter complex.

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.…”

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

“…[8] Such radical-ion pairs may or may not separate into free ions by diffusional processes, with the efficiency of free radical formation being defined, among others, by the permittivity of the medium, [9] the free energy for the back-electron transfer, [10] and spin state interconversion. [11] Cases where the triplet-quenching is not accompanied by a net chemical change have been ascribed to the formation of intermediates with only partial charge-transfer character [Eq.…”