Tetraarylborates {[Ar]4B-}: estimation of oxidation potentials and reorganization energies from electron-transfer rates

Abstract

Photoinduced electron transfer in ion pairs where the anion is a tetrasubstituted borate gives a boranyl radical. This species is reactive. When the boranyl radical contains an alkyl group, the carbon-boron bond cleaves in ca. 250 fs. The lifetime of tetraarylboranyl radicals is greater than 40 ps. The short lifetime of boranyl radicals prohibits measurement of the oxidation potentials of borates by electrochemical methods. These quantities were estimated by application of Marcus electron-transfer theory, which relates the energetics to the kinetics of the reaction. We find that the use of the empirical Rehm-Weller equation for this purpose is not appropriate and leads to errors in estimation of both the reorganization energy of the reaction and the oxidation potential of the borates. The oxidation potentials were determined in acetonitrile solution by this approach for two series of borates, (tolyl)n(Ph)knB-, where the tolyl group might be ortho or para substituted and n = 0-4. The data reveal that A, the reorganization energy, is 1 .O eV. The oxidation potentials of the borates vary systematically in the p-tolyl series from 0.8 1 to 1.03 V vs SCE with decreasing number of tolyl groups on the borate. The o-tolyl series shows higher than expected oxidation potentials for the tetra-and tritolyl cases, which may be due to greater steric hindrance to the acceptor for these two borates.

Reinvestigation of the Mechanism of the Free Radical Polymerization Photoinitiation Process by Camphorquinone–Coinitiator Systems: New Results

“…The decomposition of the alkyltriphenylborate radical occurs with the rate approximately 10 12 s À1 and eliminates the possibility of proton transfer from an oxidized electron donor to an acceptor radical ion. [21] In other words, the butyltriphenylborate anion should behave similarly to PTAA AS. The evidences of these similarities are shown in Figure 1 and 2.…”

Left recurrent laryngeal nerve palsy secondary to left pulmonary artery stent in a child

“…The thermodynamically meaningful oxidation potential can be established as it was mentioned earlier, using indirect, kinetic method described by Murphy and Schuster 20b. The oxidation potentials measured electrochemically and kinetically differ by about 0.3 V. Therefore, the obtained electrochemical values for both components of photoredox pairs may have only approximate thermodynamic meaning yet, allows to estimate roughly Δ G el for the PET process.…”

“…This makes these systems attractive for investigation of electron transfer theories. In an attempt to obtain accurate and thermodynamically meaningful values for the oxidation potentials of tetraarylborates, Schuster and coworkers developed a kinetic method20 and found that the application of the Rehm–Weller relationship leads to an overestimation of the oxidation potentials when compared with those obtained from the classical Marcus relationship.…”

Electron Transfer Photoinitiating Systems. The Effect of the Co‐Initiator Structure on the Photoinitiation Ability of a Photoredox Pair Containing Neutral Hemicyanine Dyes as Sensitizers

“…The photochemistry of the dye/aromatic thiocarboxylic acid photoredox pair is also very complex 20,28. In the case of alkyltriphenylborate salts as electron donors, the alkyl radical is formed as a result of the rapid cleavage of the alkyl boron bond of the boranyl radical obtained as a result of the photoinduced electron transfer from a borate anion to the excited state of an electron acceptor 22,23,29. The initiation of the polymerization for a dye/ N , N′ ‐dialkoxy‐4,4′‐bipyridinium dication systems results from the cleavage of the nitrogen‐oxygen bond of a N , N′ ‐dialkoxy‐4,4 ′ ‐bipyridine radical cation obtained after electron transfer from the excited dye molecule to the ground state of a N , N′ ‐dialkoxy‐4,4 ′ ‐bipyridinium salt.…”