Oxygen excited, from flash photolysis

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

The transient absorption formed from laser flash photolysis of 15 was not quenched in oxygen-saturated solutions, presumably because the photoenolization to form 16 takes place from a singlet excited state of 15. Furthermore, Schworer and Wirz did not observe any transient absorption that can be attributed to the formation of the triplet excited state of 15 from direct irradiation. In contrast. Hurley and Testa used energy transfer to estimate that the triplet excited state of 15 is formed in 67% yield,whereas Takezaki et al. have measured the yields for forming the triplet excited state of 15 to be slightly higher or above 80%. They estimated the lifetime of the lowest triplet excited state of 15 to be 350 ps in ethanol. 

In fact, the addition of 1,4-dimethoxybenzene (DMB) and/or several similar compounds, at concentrations as low as 10 4 M, to a mixture of aryl-olefins and DCA almost completely inhibits the reactions. Concentration dependence and flash photolysis studies confirm that the primary electron-transfer process occurs between the singlet excited sensitizer and DMB (E01 = 1.34 V vs SCE) with the generation of the corresponding radical ion pair. As a consequence, quantum yields lower than 1, even at infinite substrate concentration, are measured [95]. In this regard, valuable confirmations came from the cyanoaromatic photoinduced electron-transfer oxygenation of alkynes [99], Farid and Mattes reported that the photooxygenation of diphenylacetylene DPA (E° = 1.85 V vs SCE) 25, leading to a mixture of benzil 26 and benzoic acid 27, was efficiently sensitized by DCA ( = 0.15), but poorly by TCA ( > < 0.001) [Eq. (12)] [99]. 

Scheme III is not, however, the only possible electron-transfer mechanism. In the presence of oxygen, an electron may be transferred directly from the excited polymer repeating unit to oxygen, generating the radical cation and superoxide without the intermediate radical anion (Scheme VIII). Our flash photolysis experiments cannot rule out this possibility because the transients formed...

Quantitative photophysical, photochemical and photopolymerisation data are presented on five novel water soluble benzophenone photoinitiators. Phosphorescence quantum yields, triplet lifetimes and transient formation on conventional flash photolysis correlate with the ability of the initiators to photoinduce the polymerisation of 2-hydroxyethylmethacrylate and a commercial monoacrylate resin in aqueous media. The results indicate that the lowest excited triplet state of the initiator is abstracting an electron from the tertiary amine cosynergist probably via a triplet exciplex followed by hydrogen atom abstraction. This is confirmed by a detailed analysis on the effect of oxygen, pH and the ionisation potential of the amine on transient formation and photopolymerisation. Using photocalorimetry a linear correlation is found between the photopolymerisation quantum yields of the initiators and their photoreduction quantum yields in aqueous media. 

F. 6.9 Nanosecond las flash photolysis (see Fig. 6.7) and femtosecond pump-probe experiments (see Fig. 6.8) allow to determine the spectra and the reaction rate of diiodoBODIPY (10) excited states and their reaction with oxygen. Reprinted with permission from [24]... 

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