Quenching, radical formation, and disproportionation in the photoreduction of 4-carboxybenzophenone by 4-carboxybenzhydrol, hydrazine, and hydrazinium ion

Abstract

Laser flash and steady irradiation studies have been carried out on the 4-carboxybenzophenone (CB)-4-carboxybenzhydrol (CBH2) and CB-hydrazine systems over the pH range 5.8-12. In reaction with CBH2, CB triplet is quenched in part by the carboxylate, Aq 4 x 106 M"1 s"1; total reaction, A4 = 3.0 X 107 M"1 s"1, leads to the ketyl radical CBH•, = 1.7. Absorption spectra were recorded for CB triplet, Xma, = 535 nm (e = 6600 M"1 cm"1), for CBH•, Xmal = 570 nm (e = 5500), and for the radical anion CB"•, X^, = 660 nm (e = 8100). The pAa of CBH• was determined, 8.2. Rate constants were determined: for combination of CBH• to pinacol, 2 k6 = 1.8 X 109 M"1 s'1; for reactions of CBH• with CB", k1 = 1.1X 109 M'1 s'1; for disproportionation of 2CB"•, 2Ag = 9.8 X 106 M"1 s"1. Factors to convert observed yields of photoreduction by hydrazine to yields of ketyl radical leading to pinacol and hydrol were determined from effects of pH on quantum yields of reduction of CB by CBH2. Reduction of CB triplet by neutral hydrazine occurs with kir = 6.8 X 108 M"1 s"1, forming ketyl radical with = 1.0, measured relative to the CBH2 reaction. The reaction with protonated hydrazine is slower, k'ir = 4.4 X 106 M"1 s"1. Computer simulation, using the measured rate constants, pKa and primary yields, and literature constants

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

“…Such experiments were performed with 4-carboxybenzophenone (4-CB-COOH), with the acidity of the corresponding ketyl radical (pK a = 8.2) [47] being significantly higher than that of its unsubstituted congener (pK a = 9.2). [48] In ACN-H 2 O (1:4 v/v, pH 10) solutions of this BP derivative, the rapid triplet-state quenching is actually accompanied by efficient formation of the direct ET products, as can be read from the characteristic absorption of the respective ketyl radical anions with l max = 650 nm and a shoulder at 430 nm that corresponds to the radical cation of anisole ( Figure 7).…”

One‐Step Photochemical Green Synthesis of Water‐Dispersible Ag, Au, and Au@Ag Core–Shell Nanoparticles

“…These spectral and kinetic features are attributable to the ketyl radical derived by intramolecular H‐abstraction of the BP moiety . Interestingly, no evidence for the excited triplet state of BP, the absorption maximum of which is typically expected at 535 nm for BP carboxy derivatives, was noted. These results suggest that the triplet state of the BP moiety decays too fast (<50 ns) to be observed with nanosecond laser flash photolysis, due to the very efficient H‐abstraction process.…”

Kinetics of Photoinduced Electron Transfer between DNA Bases and Triplet 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution of Different pH's: Proton-Coupled Electron Transfer?

“…They are very similar to the triplet spectra reported for benzophenone and 4-BC. 7 Triplet BPTC shows a strong absorption peak at λ max = 590 nm in water at pH = 2.0, shifting to λ max = 550 nm at pH = 12.0. Because of these results, we have chosen 590 and 550 nm as observation wavelengths for kinetic traces of all quenching experiments in the acidic media and in the basic media, respectively.…”

“…BPTC •– absorbs at λ max = 630 nm, and the decay monitored at λ obs = 630 nm obeys a second-order kinetics ( k 2 /ε BPTC •– = 3 × 10 5 cm s –1 ). In addition, Inbar et al 7 reported that the pinacol yield of the 4-BC •– + 4-BC •– reaction is minor. These lead to the postulation that after an initial electron transfer followed by proton transfer, ketyl radicals (BPTCH·) are formed, which dissociate into free radical ions and thereafter randomly encounter to undergo the pinacol reaction (BPTCH· + BPTCH·).…”