Christopher Foote's Discovery of the Role of Singlet Oxygen [¹O₂ (¹Δ_g)] in Photosensitized Oxidation Reactions

Abstract

The chemistry of singlet molecular oxygen [1O2 (1Delta g)], its importance in atmospheric, biological, and therapeutic processes, and its use as a reagent in organic synthesis have been of considerable interest. Many aspects of singlet oxygen chemistry have emanated from the work of Christopher S. Foote and co-workers. Singlet oxygen is a historically interesting molecule with an unusual story connected with its discovery. Foote and Wexler conducted experiments in the 1960s where evidence was obtained supporting 1O2 generation via two independent routes: (1) a photochemical reaction (dye-sensitized photooxidation) and (2) a chemical reaction (NaOCl with H2O2). An important factor in the discovery of 1O2 as the critical reaction intermediate in dye-sensitized photooxygenations was Foote's reassessment of the chemical literature of the 1930s, when 1O2 was suggested to be a viable intermediate in dye-sensitized photooxidation reactions. Experiments that used silica gel beads provided evidence for a volatile diffusible oxidant such as 1O2. However, a contemporaneous quarrel surrounded this early work, and the possible existence of solution-phase 1O2 was ignored for over 2 decades. Not long after Foote's initial studies were published in 1964, the idea of singlet oxygen as an intermediate in photooxidation chemistry gained increasing recognition and verification in organic, gas phase, and biological processes. There are many documented impacts that 1O2 has had and continues to have on biology and medicine, for example, photodynamic therapy and plant defenses.

Macroscopic singlet oxygen modeling for dosimetry of Photofrin-mediated photodynamic therapy: an in-vivo study

“…ROS interact with substrates to cause biological damage. [8][9][10] The photosensitizer triplet may react directly with a biological substrate under hypoxic conditions. In type II interactions, the excited triplet will transfer energy to 3 O 2 to produce excited-state singlet oxygen ( 1 O 2 ), the major cytotoxic species causing biological damage.…”

Photodynamic therapy with fullerenes

“…32 In a similar fashion to the tetrapyrrole PS used for photodynamic therapy, 33,34 illumination of fullerenes dissolved in organic solvents in the presence of oxygen leads to the efficient generation of highly reactive singlet oxygen via energy transfer from the excited triplet state of the fullerene.35 However, some recent reports have shown that in polar solvents, especially those containing reducing agents (such as NADH at concentrations found in cells), illumination of various fullerenes will generate different reactive oxygen derivatives, such as superoxide anion. 36 These two pathways (singlet oxygen and superoxide anion) are analogous to the Type II and Type I photochemical mechanisms frequently discussed in PDT with tetrapyrroles.…”

Citation analysis of the scientific publications of Britton Chance in ISI citation indexes

“…He and coworkers probably provided one of the early experimental evidences for the generation of singlet oxygen in biological processes. [41][42][43] The third source of new publications was the studies [44][45][46][47][48] by Chance and Power et al using synchrotron radiation X-ray spectroscopy to illustrate structure-function relation for biomolecules including cytochrome oxidase, myoglobin, glyoxalase, etc.…”