Excited singlet state of oxygen

Another interesting question is whether excited singlet states of oxygen are released during the photochemical decomposition of the Co-0-0-Co5+ chromophore. We explored this question by decomposing 2-p in the presence of 1,3-cyclohexadiene which is known to react with excited singlet states of 02 to give the endo peroxide.82 Some endo peroxide may have been formed, and other peroxidic products were formed, but the nature of this reaction is not yet clear. 

The proposals that singlet oxygen is involved in heterogeneous catalytic reactions have not yet been explored fully. The methods used for the detection of the excited singlet state of oxygen need to be improved, and an approach based on the detection of the emission from ( Ag)02 as observed by Khan (23b) will present a significant advance if it can be applied to heterogeneous systems. It is clear that more quantitative work is required and in this respect the evidence that 02 can react with water to form ( Ag)02 (23b) could have considerable mechanistic interest. 

At present, however, the excited singlet states of oxygen are mainly of interest in connection with experimental work. Any reaction involving molecular oxygen could, in principle, lead to the formation of excited... 

State> S excited singlet state, T excited triplet state, 1 A excited singlet state of oxygen n (n, n )transition, p ( n, n )transition, l first excited state, 2 second excited state ++ supposed transition route (not yet clear)... 

Figure 22.1 Higher occupied moiecuiar orbitais of (a) ground-state nitrogen ( Nj, 2g F (b) ground-state oxygen ( Oj, 2 ), and the (c) and (d) two lowest excited singlet states of oxygen ( Oj. Ag) and ( Oj, 2+).

Reaction with sensitizers, such as ketones and certain dyes, excited by radiation to their triplet states, results in the excited singlet state of oxygen, a very reactive form of the molecule. Singlet oxygen is responsible for the rapid deterioration of many materials, particularly those with conjugated unsaturation such as natural rubber and synthetic elastomers ... 

The excited singlet state of dioxygen (O2). The term singlet oxygen should not be used since it is unclear... 

In 1963, E. J. Bowen published his classic review The Photochemistry of Aromatic Hydrocarbon Solutions, in which he described two major reaction pathways for PAHs irradiated in organic solvents photodimerization and photooxidation mediated by the addition of singlet molecular oxygen, 02 ) (or simply 102), to a PAH (e.g., anthracene). For details on the spectroscopy and photochemistry of this lowest electronically excited singlet state of molecular oxygen, see Chapter 4.A, the monograph by Wayne (1988), and his review article (1994). For compilations of quantum yields of formation and of rate constants for the decay and reactions of 02( A), see Wilkinson et al., 1993 and 1995, respectively. 

Wilkinson, F., W. P. Helman, and A. B. Ross, Quantum Yields for the Photosensitized Formation of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution, J. Phys. Chem. Ref. Data, 22, 113-262 (1993). 

Evidence for adiabatic photolytic cycloreversions at room temperature has been obtained more frequently in recent years [121,122], The adiabatic generation of singlet oxygen by photochemical cycloreversion of the anthracene and 9,10-dimethylanthracene endoperoxides 105 and 106 proceeds with wavelength-dependent quantum yields of 0.22 and 0.35, respectively, and involves the second excited singlet state of the endoperoxides [123]. Photodissociation of the 1,4-endoperoxide from l,4-dimethyl-9,10-diphenylanthracene was found to yield both fragments, i.e., molecular oxygen and l,4-dimethyl-9,10-diphenylanthracene, in their electronically excited state [124]. 

Wilkinson F, Helman WP, Ross AB (1995) Rate constants for the decay and reactions of the lowest electronically excited singlet state of molecular oxygen in solution. An expanded and revised compilation. J Phys Chem Ref Data 24 663-1021... 

Scheme 1 Major photochemical processes of coloured dissolved natural organic matter (CDOM) that may lead to the transformation of organic contaminants.1X1 Excited singlet state of the species X, 3X excited triplet state of the species X, P an organic contaminant, Pox oxidation product(s) formed upon reaction of P with singlet oxygen, full arrow heads chemical reactions, open arrow heads energy transfer/loss processes, continuous arrows radiative processes, dashed arrows radiationless processes...

At the moment of their birth (i.e., when the excited singlet states of the aryl esters undergo lysis), the geminal radical pairs mustbe in positions that make their shape similar to that of their precursor ester the radical center of the acyl part is very near the oxygen atom of the aryloxy part, and all subsequent diffusion of the two species starts from this orientation. As a result, addition of an acyl radical to its aryloxy partner is favored spatially at the nearer adj acent ortho) position(s) than at the more distant meta and para positions. However, the ability of the acyl radical to add to each of the positions of an aryloxy radical is expected to depend on the energies of the adducts" ° (which are keto intermediates that enolize thermally with time to the eventual products Equation 13.5 shows an example of the keto intermediate for acyl addition to the ortho position of phenoxy). The spin densities at the 2-, 4-, and oxy-positions of 1-naphthoxy (and of phenoxy) from ESR measurements and HF/6-31G level calculations (in parentheses) are shown in Fig. 13.1. Those for the 2-naphthoxy radical by ESR measurements " and MNDO-UHF calculations are collected in Table 13.1. These considerations explain why the yields of products from addition at the meta positions are very low. 

Control over the course of the rearrangements of aryl esters can be achieved when the host matrix can interact with the ester and its intermediates through electrostatic or H-bonding interactions. Zeolites and Nafion membranes are two polymeric matrices in which such studies have been conducted. As mentioned above, the positions of the radical centers of the acyl radicals are known to be very near the oxygen atoms of their aryloxy partners at the moment of birth of the radical pairs from the excited singlet state of an aryl ester in all media all subsequent diffusion... 

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