Abstract
Singlet oxygen (1O2) is the excited state electronic isomer and a reactive form of molecular oxygen, which is most efficiently produced through the photosensitized excitation of ambient triplet oxygen. Photochemical singlet oxygen generation (SOG) has received tremendous attention historically, both for its practical application as well as for the fundamental aspects of its reactivity. Applications of singlet oxygen in medicine, wastewater treatment, microbial disinfection, and synthetic chemistry are the direct results of active past research into this reaction. Such advancements were achieved through design factors focused predominantly on the photosensitizer (PS), whose photoactivity is relegated to self-regulated structure and energetics in ground and excited states. However, the relatively new supramolecular approach of dictating molecular structure through non-bonding interactions has allowed photochemists to render otherwise inactive or less effective PSs as efficient 1O2 generators. This concise and first of its kind review aims to compile progress in SOG research achieved through supramolecular photochemistry in an effort to serve as a reference for future research in this direction. The aim of this review is to highlight the value in the supramolecular photochemistry approach to tapping the unexploited technological potential within this historic reaction.
Highlights
Singlet oxygen (1O2) is the common name of an electronically excited state of molecular oxygen, which is less stable than the ubiquitous triplet molecular oxygen (3O2) in its ground state that is found as ambient oxygen [1,2]
The instances of singlet oxygen generation (SOG) systems outlined in this paper presented various approaches to influencing the photophysics of photosensitizers and energy-transfer events that influence the efficiency of this process
It was evident that the non-covalent interaction afforded by cavitands and other supramolecular components are very useful in manipulating the photo-dynamics of singlet oxygen generation
Summary
Singlet oxygen (1O2) is the common name of an electronically excited state of molecular oxygen, which is less stable than the ubiquitous triplet molecular oxygen (3O2) in its ground state that is found (and referred to) as ambient oxygen [1,2]. Singlet oxygen can be produced as a product in a (ground state) reaction between sodium hypochlorite and hydrogen peroxide [6] It can be generated through direct laser excitation of 3O2 at ~1064 nm [7]. The unique NIR phosphorescence of singlet oxygen is unambiguous, convenient, and direct; as a result, its direct observation is highly advantageous whenever possible This method often suffers from weak signal intensity due to competing non-radiative pathways that are generally more efficient. Practically useful sensitizers should possess additional features such as high energy-transfer efficiency to 3Σg−, resulting in high quantum yield of singlet oxygen generation (1ΦO2). This requires that the PS possesses the ability to maintain long triplet lifetimes as this increases the probability of energy transfer (and greater 1ΦO2)
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