Singlet Oxygen: Discovery, Chemistry, C₆₀‐Sensitization^†

Abstract

This review article refers to the discovery of excited molecular oxygen, in particular on its lower singlet excited state ( 1 D g , 1 O 2 ). After a short report on singlet oxygen generation, the review is focused on the chemistry of this reactive species. Specifically, the three major reactions of 1 O 2 with unsaturated organic substrates, namely the [4 + 2] and [2 +2] cycloadditions as well as the ene reaction, are reviewed. The proposed mechanisms of these reactions, through the years, based on experimental and computational work, have been presented. Selected examples of singlet oxygen-synthetic applications are also mentioned. The [60]fullerene and fullereno-materials photosensitized oxidations in homogeneous, as well as in heterogeneous conditions, are also comprehensively discussed.Finally, the self-sensitized photooxidation of open cage fullerenes as well as fullerenes bearing oxidizable groups is reported.

Luminescence enhancement by deuterium

“…These include the formation of unstable dioxetanes, which themselves can decompose with chemiluminescence emission. 77 One of the curious properties of singlet oxygen noted by early investigator David Kearns was its extreme variation in lifetime depending upon its solvent environment. In particular, Kearns discovered that the lifetime of singlet oxygen increased by a significant factor in D 2 O compared with water and proposed that this chemiluminescence enhancement could be used as part of a confirmatory test for the putative existence of singlet oxygen intermediacy in an oxidation reaction.…”

“…Once singlet oxygen has been elevated to the excited state, it can return to its ground state by a characteristic red chemiluminescence emission or by engaging in a number of chemical reactions. These include the formation of unstable dioxetanes, which themselves can decompose with chemiluminescence emission 77 . One of the curious properties of singlet oxygen noted by early investigator David Kearns was its extreme variation in lifetime depending upon its solvent environment.…”

Use of Singlet Oxygen in the Generation of a Mononuclear Nonheme Iron(IV)-Oxo Complex

“…In contrast to ground-state triplet oxygen ( 3 O 2 ), the lowest excited state of triplet oxygen (i.e., singlet oxygen ( 1 O 2 : 1 Δ g )) is by 0.98 eV higher in energy than 3 O 2 and the electron-transfer reactivity of 1 O 2 is also enhanced significantly, − due to the much more positive one-electron reduction potential ( E red 0* vs SCE = 0.11 V) and the smaller reorganization energy (λ = 1.73 eV) as compared with those of 3 O 2 ( E red 0 vs SCE = −0.87 V, λ = 1.97 eV) . If 1 O 2 is employed instead of 3 O 2 , electron transfer from [Fe II (TMC)] 2+ to 1 O 2 (reaction b ) would be energetically much more favorable by 0.98 eV (= 23 kcal mol –1 ), as compared to the electron transfer from [Fe II (TMC)] 2+ to 3 O 2 (reaction a ).…”

A Photosensitized Singlet Oxygen (1O2) Toolbox for Bio-Organic Applications: Tailoring 1O2 Generation for DNA and Protein Labelling, Targeting and Biosensing

“…In the 1960s, it was proposed by C. S. Foote and S. Wexler that 1 O 2 , the excited state of molecular oxygen with the lowest energy, was the reactive intermediate in photosensitized oxidation reactions. This proved to be a milestone in 1 O 2 mediated research, paving the way for its use in various (bio)organic reactions and photodynamic therapy [ 13 , 14 , 15 , 16 ]. A simple and visual explanation for 1 O 2 generation can be provided by following the so-called Jablonski diagram ( Figure 1 ).…”