Transient Intermediates in the Laser Flash Photolysis of Ketoprofen in Aqueous Solutions:  Unusual Photochemistry for the Benzophenone Chromophore

Abstract

The transient intermediates in the nanosecond laser flash photolysis of ketoprofen, an aryl propionic acid, show the formation of a carbanion in aqueous solutions at pH 7.1. This carbanion incorporates spectroscopic properties from both a ketyl radical anion and a benzylic radical. The ketoprofen carboxylate undergoes biphotonic photoionization, a process that contributes less than 10% to its photodecomposition and leads to a benzylic-type radical after decarboxylation with a rate constant ≥1 × 107 s-1. On the other hand, the carbanion forms monophotonically and the unsuccessful attempts to sensitize the formation of the ketoprofen triplet excited state in aqueous solutions suggest that the carbanion precursor is either an excited singlet state or an extremely short-lived triplet. In organic solvents of lower polarity, the excited triplet state is readily detectable.

Fast transient absorption spectroscopy of the early events in photoexcited chiral benzophenone–naphthalene dyads

“…The 530 nm band is attributed to the ketoprofen triplet-triplet absorption, which has been previously reported by e.g. Scaiano [20].…”

Water‐ and Acid‐Mediated Excited‐State Intramolecular Proton Transfer and Decarboxylation Reactions of Ketoprofen in Water‐Rich and Acidic Aqueous Solutions

“…[21] The decay of the KP carbanion is due to the protonation of water and esterification of KP, which Abstract: We present an investigation of the decarboxylation reaction of ketoprofen (KP) induced by triplet excited-state intramolecular proton transfer in water-rich and acidic solutions. Nanosecond time-resolved resonance Raman spectroscopy results show that the decarboxylation reaction is facile in aqueous solutions with high water ratios (water/acetonitrile !…”

“…[17,21] Our previous study proposed that the rate of an apparent hydrogen abstraction or other process that forms ketyl radicals from the triplet state KP ( 3 [KP]) exhibits an intriguing dependence on the ratio of water in less than 50 % water concentration solutions. [22] However, further increasing the ratio of water may lead KP to exhibit a significant decarboxylation process, especially in solutions with a very high ratio of water (such as water/acetonitrile !…”

Diastereomeric Differentiation in the Quenching of Excited States by Hydrogen Donors

“…1.6:1) lead to an unambiguous assignment of this transient to the triplet-triplet absorption (the triplet state) of the ketoprofen chromophore. [29][30][31] The ketyl radical of ketoprofen has similar absorption maxima (330 and 540 nm), but with a much larger ratio of the intensities of the two bands. The kinetics of the transient was measured at l obs = 525 nm and shown to consist of a major fast component and a minor slow component (lifetime ca.…”

“…Ketoprofen is a known photosensitizer similar to benzophenone which is photophysically well-characterized, [29] with special emphasis devoted to its photobiological importance. [30] Since benzophenone can be considered as the classical carbonyl chromophore, our investigations regarding diastereomeric differentiation during hydrogen-abstraction reactions are expected to provide deeper insights into a photochemical mechanism of general interest.…”