Abstract
The pulsed laser photolysis at 308 nm of the benzophenone‐triethylamine system in acetonitrile solution has been studied at room temperature. In the absence of triethylamine, phosphorescence emission of triplet benzophenone centred at 460 nm and with a lifetime of τ = 27 ± 1 μs is observed. Addition of triethylamine results in quenching of the benzophenone phosphorescence. Quenching rate constants KQ have been determined from emission intensifies and from experimental decay rate constants. The values KQ = (2.6 ± 0.3) × 109 M-1s-1 and KQ = (4.0 ± 0.5) × 109 M-1s-1, respectively, are obtained. On the addition of triethylamine, benzophenone phosphorescence diminishes and simultaneously a new emission with a fast decay rate and centred at 575 nm develops. This signal has been identified as emission from the excited benzophenone ketyl radical. A single pulse two‐photon absorption mechanism responsible of the emission at 575 nm has been identified and a kinetic model based on this mechanism has been developed. The model gives a satisfactory quantitative account of both our experimental results and those reported in the literature (quantum yield of radical formation and behaviour of the system as photoinitiator).
Highlights
The present interest in tailor-made photoinitiators for speci.fic applications requires the prediction of the relationship between molecular structure and properties
Aromatic carbonyl compounds have been proposed as photoinitiators because of the ability of their triplet state, formed at high yield on ultraviolet irradiation, to abstract hydrogen atoms and produce radicals. ,2 In addition, photoinitiators containing aromatic carbonyl functions as chromophores have been widely studied in order to determine the behaviour of intermediaries which can play a significant role in free
In this work we have simultaneously followed the time integrated intensity of both benzophenone phosphorescence centred at 460 nm and fluorescence of the excited benzophenone ketyl radical at 575 nm
Summary
The present interest in tailor-made photoinitiators for speci.fic applications requires the prediction of the relationship between molecular structure and properties. In this work we have simultaneously followed the time integrated intensity of both benzophenone phosphorescence centred at 460 nm and fluorescence of the excited benzophenone ketyl radical at 575 nm. Both emissions have been readily identified in this work from their spectra and lifetimes. The data obtained have allowed us to report a case in which a single 308 nm laser pulse of modest energy (a few millijoules) is able to generate an excited benzophenone ketyl radical, in a solution of benzophenone and triethylamine in acetonitrile, through a process that may be defined as a two-photon single pulse assisted reaction. Photo or laser-assisted reactive collisions is an extremely active field of research (see ref. 28 for a review), we are not aware of any well documented report of single pulse two-photon laser-assisted reaction in the liquid phase
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