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Photosensitized Singlet Oxygen and Its Applications

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Photosensitized Singlet Oxygen and Its Applications Coordination Chemistry Reviews 233Á/234 (2002) 351Á/371 www.elsevier.com/locate/ccr Photosensitized singlet oxygen and its applications Maria C. DeRosa, Robert J. Crutchley * Department of Chemistry, Ottawa-Carleton Chemistry Institute, Carleton University, 2240 Herzberg Building, 1125 Colonel By Drive, Ottawa, ON, Canada K1S 5B6 Received 18 September 2001; accepted 15 February 2002 Contents Abstract . ....................................................................... 351 1. Introduction ................................................................... 352 2. Properties of singlet oxygen ........................................................... 352 2.1 Electronic structure of singlet oxygen .................................................. 352 1 2.2 Quenching of O2 ............................................................. 352 2.3 Generation of singlet oxygen . ..................................................... 353 3. Types of photosensitizers ............................................................ 354 3.1 Organic dyes and aromatic hydrocarbons ............................................... 355 3.2 Porphyrins, phthalocyanines, and related tetrapyrroles ........................................ 355 3.3 Transition metal complexes . ..................................................... 357 3.4 Photobleaching of organic and metal complex photosensitizers ................................... 359 3.5 Semiconductors .............................................................. 359 3.6 Immobilized photosensitizers . ..................................................... 359 1 4. Applications of photosensitized O2 ....................................................... 361 4.1 Fine chemicals synthesis and wastewater treatment .......................................... 361 4.2 Singlet oxygen in photodynamic processes ............................................... 363 4.2.1 Blood sterilization . ..................................................... 363 4.2.2 Photodynamic therapy of cancer (PDT) ........................................... 363 4.2.2.1 Porphryins .................................................... 364 4.2.2.2 Other macrocyclic systems ........................................... 366 4.2.2.3 Third generation photosensitizers ....................................... 366 4.2.3 Insecticides and herbicides ................................................... 368 5. Future studies ................................................................... 369 Acknowledgements ................................................................... 369 References . ....................................................................... 369 Abstract The study of singlet molecular oxygen production and reactivity has emerged as a rich and diverse area with implications in fields ranging from polymer science to cancer therapy. In this review, we address the photophysical properties of singlet oxygen and of the photosensitizers used in its generation. Photosensitizers based on organic molecules and coordination compounds are examined and compared. Recent advances in the photosensitized production of singlet oxygen and its uses in photochemistry and photobiology are highlighted, with particular focus on its role in wastewater treatment, fine chemical synthesis, and photodynamic therapy (PDT). Future directions in photosensitizer development and singlet oxygen applications are also explored. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Singlet oxygen; Photosensitizers; Photodynamic therapy * Corresponding author. Tel.: 1-613-520-2600; fax: 1-613-520-2659. E-mail address: [email protected] (R.J. Crutchley). 0010-8545/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 0 - 8 5 4 5 ( 0 2 ) 0 0 0 3 4 - 6 352 M.C. DeRosa, R.J. Crutchley / Coordination Chemistry Reviews 233Á/234 (2002) 351Á/371 1. Introduction Despite being discovered in 1924, singlet molecular oxygen only became the focus of intense laboratory study after 1963 when Khan and Kasha interpreted the chemiluminescence of the hypochlorite-peroxide reac- tion as caused by liberated singlet oxygen [1]. Since then, the physical, chemical, and biological properties of this energetically rich form of molecular oxygen have garnered serious attention. In particular, the photosen- sitized production of singlet oxygen has significance in a range of areas from photooxidation, DNA damage, photodynamic therapy (PDT) of cancer, to polymer science. Recently, important treatises by Wasserman and Murray [2], and Ra˚nby and Rabek [3] have Fig. 2. Primitive representations of molecular oxygen lowest singlet provided the impetus for further study of this field. and triplet states. This review will survey the literature regarding the photosensitized generation of singlet oxygen and its 2. Properties of singlet oxygen applications, focusing mainly on the latest results from 1995 to early 2001. It will begin with an introduction to 2.1. Electronic structure of singlet oxygen the electronic structure of singlet oxygen and its reactivity, followed by the sources of singlet oxygen Molecular oxygen has two low-lying singlet excited with particular attention to photosensitized reactions. 1 1 1 states, Dg and ag ; 95 (22.5 kcal mol ) and 158 kJ The groups of photosensitizers that will be examined mol1 (31.5 kcal mol1)above the triplet state, are: (1) the organic dyes and aromatics; (2) the respectively, as shown in Fig. 1 [4]. porphyrins, phthalocyanines, and related macrocycles; Electronic configurations of these states differ only by (3) semiconductors; and (4) transition metal complexes. the structure of the p-antibonding orbitals. The config- The effect of immobilizing photosensitzers in a polymer uration of the molecular orbitals of the first excited 1 2 2 2 4 matrix will also be discussed. The section on applica- state, Dg, is as follows: O2KK(2sg) (2su) (3sg) (1pu) - tions will explore the recent literature regarding the use 1 (/1pg )/(/1pg )]: In the second excited state, ag ; the of singlet oxygen in several main areas: wastewater electronic¯ configuration is identical to that of the treatment, fine chemical synthesis, and photodynamic ground state, except that the last two electrons have applications such as blood sterilization, sunlight-acti- antiparallel spins (see Fig. 2). 1 3 vated herbicides and insecticides, as well as photody- The transition from the Dg state to the ag state is 1 namic cancer therapy. Finally, the section on future spin forbidden, thus the Dg O2 is a relatively long-lived studies will summarize the research that is needed to species. The second excited state of oxygen, on the other expand the current understanding of photosensitized hand, is short-lived due to a spin-allowed transition to 1 singlet oxygen generation and the application of this the Dg state. This difference in stability is confirmed by 1 1 research. the radiative lifetimes of O2( Dg) and O2(/ ag ); which 6 are 45 min and 7Á/12 s in the gas phase [5] and 10 Á/ 3 11 9 10 s, and 10 Á/10 s in solution [6], respectively. 1 1 3 Due to the metastability of the Dg state, the Dg l ag transitions at 1268.7 nm are observed in absorption and emission spectra despite being spin and symmetry forbidden. For a more detailed treatise on the spectral 1 properties of O2, see Ref. [7]. 1 2.2. Quenching of O2 Once dioxygen is in its singlet excited state, it can be deactivated by other species to return to its ground state. This quenching can take place in two major ways [8]: k 1 p 3 Fig. 1. Potential energy curves for the three low-lying electronic states 1) Physical quenching: O2 A 0 O2 A; in which of molecular oxygen. interaction leads only to deactivation of singlet M.C. DeRosa, R.J. Crutchley / Coordination Chemistry Reviews 233Á/234 (2002) 351Á/371 353 oxygen with no O2 consumption or product forma- Many metal complexes have been shown to possess tion, and singlet oxygen quenching ability, particularily Ni(II) 1 kc 1 2) Chemical quenching: O2 A 0 P; where the chelates [18]. Reactions of O2 to form energetic and quencher reacts with singlet oxygen to give a new reactive metalÁ/dioxygen complexes have also been product. explored. Foote and Selke made the preliminary report that singlet oxygen reacts with trans- Early work with singlet oxygen found that this active Ir(CO)X(PPH3)2, where X/Cl, Br, or I to form an species could oxidize substrates that were unaffected by Ir(III) peroxo complex at ca. 109 times faster rate than oxygen in its normal energy state. Oxygen is ca. 1 V when reacted with triplet oxygen [19]. Singlet oxygen has more oxidizing in its singlet excited state and is therefore also been used to produce the rhodium complex trans- significantly more electrophilic, reacting rapidly with Rh(CO)Cl(PPh3)2O2 from the Rh analogue of the unsaturated carbonÃ/carbon bonds, neutral nucleophiles Vaska’s complex [20]. More recently, cationic complexes such as sulfides and amines, and as well as with anions. [M(PPh3)2(CO)S] , where S/CH3CN and M/Ir, Rh Consequently, organic chemists have found utility in can also quench singlet oxygen to form the correspond- singlet oxygen as a versatile synthetic reagent [9]. The ing peroxo complex, [M(CO)S(PPh3)2O2] [21]. common reactions of singlet oxygen will be described While excited triplet state molecules can be efficient below. The application of these reactions to fine sensitizers of singlet oxygen (see Section 2.3), in many chemical synthesis and wastewater treatment can be cases they are also efficient
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