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Supramolecular Control of Singlet Oxygen Generation

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Supramolecular Control of Singlet Oxygen Generation molecules Review Supramolecular Control of Singlet Oxygen Generation Akshay Kashyap 1, Elamparuthi Ramasamy 2, Vijayakumar Ramalingam 3 and Mahesh Pattabiraman 1,* 1 Department of Chemistry, University of Nebraska Kearney, Kearney, NE 68849, USA; [email protected] 2 Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX 76019, USA; [email protected] 3 Department of Biology and Chemistry, SUNY Polytechnic Institute, Utica, NY 13502, USA; [email protected] * Correspondence: [email protected] 1 Abstract: Singlet oxygen ( O2) is the excited state electronic isomer and a reactive form of molecu- lar 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 syn- thetic 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 1 PSs as efficient O2 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 refer- ence for future research in this direction. The aim of this review is to highlight the value in the Citation: Kashyap, A.; Ramasamy, E.; supramolecular photochemistry approach to tapping the unexploited technological potential within Ramalingam, V.; Pattabiraman, M. this historic reaction. Supramolecular Control of Singlet Oxygen Generation. Molecules 2021, Keywords: singlet oxygen; supramolecular chemistry; cavitands; cucurbituril; cyclodextrin; cal- 26, 2673. https://doi.org/10.3390/ ixarene; singlet oxygen generation (SOG); photosensitizer (PS); oxidation; photodynamic therapy molecules26092673 (PDT); near IR (NIR) Academic Editor: Daniel Escudero Received: 17 March 2021 Accepted: 27 April 2021 1. Introduction 1 Published: 2 May 2021 Singlet oxygen ( O2) is the common name of an electronically excited state of molecular 3 oxygen, which is less stable than the ubiquitous triplet molecular oxygen ( O2) in its ground Publisher’s Note: MDPI stays neutral state that is found (and referred to) as ambient oxygen [1,2]. Though chemists understood with regard to jurisdictional claims in that the combination of light and organic dyes generated a highly reactive oxygen species published maps and institutional affil- in the 1930s, a clear picture of its electronic state and intermediary nature did not emerge iations. 1 until the late 1960s [3,4]. As the excited state electronic isomer of ambient oxygen, O2 possesses higher oxidative power and reactivity than ambient oxygen [5]. These features confer a great deal of biological and chemical significance on this reactive oxygen species. Singlet oxygen can be produced as a product in a (ground state) reaction between sodium Copyright: © 2021 by the authors. hypochlorite and hydrogen peroxide [6]. It can also be generated through direct laser 3 Licensee MDPI, Basel, Switzerland. excitation of O2 at ~1064 nm [7]. However, the most convenient way of accessing this This article is an open access article species is through sensitization, wherein a photosensitizer in the excited triplet state relaxes 3 1 distributed under the terms and to its ground state by triplet energy transfer to ambient oxygen, resulting in O2 ! O2 [8]. conditions of the Creative Commons The biological relevance of this molecular species can be deduced from the works Attribution (CC BY) license (https:// published on its role as reactive oxygen species (ROS) in botany [9], atmospheric chem- creativecommons.org/licenses/by/ istry [10], cellular signaling [11], and physiopathology [12]. In medicine, photo-dynamic 4.0/). Molecules 2021, 26, 2673. https://doi.org/10.3390/molecules26092673 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2673 2 of 23 Molecules 2021, 26Molecules, x FOR PEER 2021, REVIEW26, x FOR PEER REVIEW 2 of 21 2 of 21 therapy (PDT,therapytherapy Figure (PDT,1) is a FigureFigure singlet1 )1) isoxygen-derived is a singleta singlet oxygen-derived oxygen-derived treatment modality treatment treatment that modality modality utilizes that itsthat utilizes utilizes its cy-its cytotoxicity tocytotoxicitytotoxicity achieve toselective achieveto achieve apoptosis selective selective apoptosis or apoptosisnecrosis or necrosisthrough or necrosis through its localized through its localized generationits localized generation generation [13,14]. [13,14]. Due to[13,14].Due its target to Due its specificity target to its target specificity and specificity minimal and minimal andinvasiveness, minimal invasiveness, invasiveness,PDT is the PDT preferred PDT is the is the preferredmode preferred mode mode of of treatment foroftreatment treatmentconditions for for conditionssuch conditions as psoriasis such such as [15,16], as psoriasis psoriasis certain [15 [15,16],,16 forms], certain certain of cancers forms forms of(skin, cancersof cancers blad- (skin, (skin, bladder, blad- der, lung) [17–19],der,lung) lung) macular [17– 19[17–19],], degeneration macular macular degeneration degeneration[20]. As a [20 reactive]. [20]. As a As reactiveoxygen a reactive species oxygen oxygen (ROS), species species its (ROS), bio- (ROS), its biological its bio- logical oxidizinglogicaloxidizing propensity oxidizing propensity holds propensity holdspromise promise holds as a disinfectionpromise as a disinfection as a technology disinfection technology to technology neutralize to neutralize bac-to neutralize bacteria bac- and teria and virusesteriaviruses [21,22]. and [21 viruses, 22]. [21,22]. Figure 1. BiologicalFigure and 1.1. Biologicaltherapeutic and appl therapeuticications of applicationsappl singletications oxygen. ofof singletsinglet oxygen.oxygen. 1 1 2 The increasedTheThe oxidative increased power oxidative and electronic power and configuration electronic configurationconfiguration of 1O2 has given of Orise2 hashas to given rise to its use as a syntheticitsits use use as tool as a a synthetic (Figure synthetic 2) tool to tool acce (Figure (Figuress numerous 2)2 )to to acce access organicss numerous numerous intermediates organic organic intermediates that intermediates are not that that are arenot easily achievedeasilynot through easily achieved achieved classical through throughorganic classical tran classicalsformations organic organic tran [23–25].sformations transformations Reactions [23–25]. [ 23such– Reactions25 as]. Reactionssin- such suchas sin- as glet-ene [26,27],glet-enesinglet-ene cycloaddition [26,27], [26,27 cycloaddition ],(4 cycloaddition + 2 and 2 + (42) (4 +[28], 2 + and 2 and 2 2+heteroatom +2) 2) [28], [28 ],and and oxygenations heteroatom heteroatom oxygenations oxygenations[29,30] [29,30] [29,30] have been usefulhavehave in been been fine useful usefulchemical in in fine fineand chemical large-scale and and synthesis large- large-scalescale of numerous synthesis ofnatural numerous products natural products and drugs (includeandand drugs pictures (include of reactions). pictures of A reactions).recently fast-growing A recently fast-growinginterdisciplinary interdisciplinary area area of research involving singlet oxygenation is designing functional systems such as self- of research involvingof research singlet involving oxygenation singlet isoxygenation designing functionalis designing systems functional such systems as self- such as self- cleaning [31] and disinfecting materials [22,32], and lasers for solar energy conversion [33]. cleaning [31] andcleaning disinfecting [31] and materials disinfecting [22,32] materials, and lasers [22,32] for, andsolar lasers energy for conversion solar energy [33]. conversion [33]. Figure 2. ChemicalFigure reactions 2. Chemical affected reactions by singlet affected oxygen. by singlet oxygen. As outlined above,AsAs outlined outlined there hasabove, above, not therebeen there anyhas has notshor not beentage been anyof anyscientific shor shortagetage exploration of scientific of scientific into exploration this exploration into intothis ROS or its centralROSthis in ROSor role its orcentral in itsmodern central in role technology. inin rolemodern in modernIt technology. is predicted technology. It that is predicted there It iswill predicted that be a there growing thatwill therebe a growing will be a growing demand for newer inventions for medicinal, pharmacological, and chemical demand for newerdemand inventions for newer for inventions medicinal, for ph medicinal,armacological, pharmacological, and chemical and applications chemical applications applications based on this chemical entity. While much is known about the electronic based on this basedchemical on thisentity. chemical While entity.much isWhile known much about is known the electronic about the nature electronic of singlet nature of singlet nature of singlet oxygen, its excited state dynamics, and physical properties, there are also oxygen, its excitedoxygen, state its dynamicsexcited state, and dynamics
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