Elsevier

Coordination Chemistry Reviews

Volumes 233–234, 1 November 2002, Pages 351-371
Coordination Chemistry Reviews

Photosensitized singlet oxygen and its applications

https://doi.org/10.1016/S0010-8545(02)00034-6Get rights and content

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.

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 reaction 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 photosensitized 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 Rånby and Rabek [3] have provided the impetus for further study of this field.

This review will survey the literature regarding the photosensitized generation of singlet oxygen and its applications, focusing mainly on the latest results from 1995 to early 2001. It will begin with an introduction to the electronic structure of singlet oxygen and its reactivity, followed by the sources of singlet oxygen with particular attention to photosensitized reactions. The groups of photosensitizers that will be examined are: (1) the organic dyes and aromatics; (2) the porphyrins, phthalocyanines, and related macrocycles; (3) semiconductors; and (4) transition metal complexes. The effect of immobilizing photosensitzers in a polymer matrix will also be discussed. The section on applications will explore the recent literature regarding the use of singlet oxygen in several main areas: wastewater treatment, fine chemical synthesis, and photodynamic applications such as blood sterilization, sunlight-activated herbicides and insecticides, as well as photodynamic cancer therapy. Finally, the section on future studies will summarize the research that is needed to expand the current understanding of photosensitized singlet oxygen generation and the application of this research.

Section snippets

Electronic structure of singlet oxygen

Molecular oxygen has two low-lying singlet excited states, 1Δg and 1g+, 95 (22.5 kcal mol−1) and 158 kJ mol−1 (31.5 kcal mol−1) above the triplet state, respectively, as shown in Fig. 1 [4].

Electronic configurations of these states differ only by the structure of the π-antibonding orbitals. The configuration of the molecular orbitals of the first excited state, 1Δg, is as follows: O2KK(2σg)2(2σu)2(3σg)2(1πu)4(1πg+)(1π̄g+)]. In the second excited state, 1g+, the electronic configuration is

Types of photosensitizers

There are several groups of UV–vis absorbing molecules that have shown singlet oxygen generating ability. Photosensitizers should exhibit the following properties: (1) high absorption coefficient in the spectral region of the excitation light; (2) a triplet state of appropriate energy (ET≥95 kJ mol−1) to allow for efficient energy transfer to ground state oxygen; (3) high quantum yield of the triplet state (ΦT>0.4) and long triplet state lifetimes (τT>1 μs), since the efficiency of the

Applications of photosensitized 1O2

After 1O2 is generated, it can either lose its energy through a radiative process, a non-radiative process, i.e. heat, or it can react with a substrate. The reactivity of singlet oxygen can be detrimental, as is the case in the photodegradation of polymers, but can also be beneficial as is illustrated in this section.

Future studies

The chemistry of singlet oxygen is rich and we have only begun to realize its potential uses. Much remains to be done to modify and improve existing photosensitizers to better suit their properties to desired applications. This, of course, requires an improved understanding of mechanisms of photosensitizer quenching, photobleaching, and localization in tissues. The investigation of new photosensitizers is also crucial to further development in this field.

Semiconductors with band gaps in the

Acknowledgements

The authors wish to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support. M.C.D. thanks NSERC for the award of a postgraduate scholarship.

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