For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Physical mechanisms of generation and deactivation of singlet oxygen.

2. Activatable Photosensitizer Design Considerations 2842 2.1. Activation Strategy 2842 2.2. Photosensitizer Selection 2844 2.3. Photosensitizer Conjugation 2845 3. Examples of Activatable Photosensitizers 2845 3.1. Environment-Activated Photosensitizers 2845 3.2. Enzyme-Activated Photosensitizers 2846 3.3. Nucleic Acid-Activated Photosensitizers 2852 3.4. Other Activation Mechanisms 2853 4. Conclusion and Outlook 2855 5. Abbreviations 2855 6. Acknowledgments 2855 7. References 2855

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Activatable photosensitizers for imaging and therapy

Singlet molecular oxygen, 02(’Ag) , has been intensively investigated over the last few decades, primarily in relation to photosensitization reactions. In combination with the increasing awareness and interest in singlet oxygen has been the development of various methodologies for the detection and quantification of singlet oxygen production. A large body of literature now exists concerning the singlet oxygengenerating abilities of a large variety of endogenous and exogenous compounds that can be photoactivated in the UV and visible regions of the electromagnetic spectrum. The purpose of this review was to compile data from many different sources to provide a convenient resource for biological scientists with interest in this area. This compilation is essentially an updated subset of the excellent comprehensive review on singlet oxygen yields by Wilkinson et af. (l), which, unlike this compilation, was not limited to biological molecules. The data are current up to the early months of 1999. The following tables include various related parameters in addition to the central parameter of the quantum yield of singlet oxygen generation (QA) for a given photosensitizer. The methodology or technique used to measure the QA value for each entry is given and ranges from direct detection of the luminescence produced on relaxation of singlet oxygen (time-resolved [TRILIT or steady-state [SSIL]), calorimetric

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-1097.1999.tb08240.x

A Compilation of Singlet Oxygen Yields from Biologically Relevant Molecules

Novel boron(III) subphthalocyanines (SubPcs) soluble in organic solvents containing a variety of donor and acceptor substituent groups have been synthesized by boron trihalide-induced cyclotrimerization of adequately substituted derivatives of phthalonitrile in 1-chloronaphthalene. The choice of the substituents on the 1,2-dicyanobenzene derivatives has been made taking into account the high reactivity of the Lewis acid BCl3 toward many functional groups. Considering this limitation, we set out to synthesize phthalodinitriles equipped with iodo, nitro, alkyl- or arylthio, alkyl- or arylsulfonyl groups that are sufficiently stable under the required reaction conditions and also provide an easily accessible set of acceptor/donor substituents. The quadratic and cubic hyperpolarizabilities of these compounds as well as their linear optical and electrochemical properties have been measured by several techniques, including EFISH (at two wavelengths), HRS, and THG, steady-state and time-resolved absorption and f...

Synthesis and Nonlinear Optical, Photophysical, and Electrochemical Properties of Subphthalocyanines

Quenching of singlet oxygen (1O2) in D2O-ethanol by the amino acids tryptophan, tyrosine, histidine, methionine, cysteine and their derivatives was measured by exciting the sensitizers rose bengal or meso-tetra (N-methyl-4-pyridyl)porphyrin tetratosylate in the presence of oxygen and the above quenchers in solution. In our polar solvent, containing 75% D2O on a molar basis it was found that (1) substitution of the aromatic ring in indole, phenol and imidazole by the electron-donating methyl group increases the total (i.e. nonreactive and reactive) quenching rate constant by a factor of five to eight. Free or blocked amino and carboxyl groups removed by two methylene groups from the ring counteract the above increase in the rate constant. The reactive quenching of singlet oxygen, which leads to oxidative destruction of the aromatic ring, correlates with the above substitution effects. It has been proposed that the quenching process takes place by formation of an exciplex between 1O2 and the quencher. Thus our results indicate that the better an electron donor the amino acid residue is the more pronounced is the charge transfer contribution in the exciplex formed with 1O2 and the more likely it is to lead to charge separation and hence to a chemical reaction. (2) Oligopeptides in solution or peptide bonds linked to the amino acid residue have only a minor effect on singlet oxygen. It can therefore be expected that the polypeptide chains per se in the protein network will not interact significantly with the single oxygen molecules present.(ABSTRACT TRUNCATED AT 250 WORDS)

Reactivity of singlet oxygen toward amino acids and peptides

Time‐resolved thermal‐lensing was used to measure the absolute quantum yield (φΔ) of singlet molecular oxygen, O2(1Δg), produced by hematoporphyrin photosensitization in ethanol. Deuteration of the solvent did not affect the value of φΔ. The value of φΔ= 0.53 was then used as reference to evaluate φΔ in O2 (1Δg) phosphorescence experiments with the related porphyrins, monohydroxyethylvinyl deuteroporphyrin and dihematoporphyrin ether. The φΔ values, in conjunction with the respective quantum yields of intersystem crossing (measured using a nanosecond laser flash photolysis technique) served to evaluate efficiencies, SΔ, of O2 (1Δg) production from the porphyrin triplet states. The lifetime TΔ in monodeuterated ethanol was measured as 29 ± 3 μs and 30 ± 1 (xs by time‐resolved thermal lensing and phosphorescence detection, respectively. TΔ in ethanol and fully deuterated ethanol were in good agreement with values reported in the literature.

Thermal-lensing and phosphorescence studies of the quantum yield and lifetime of singlet molecular oxygen (1 delta g) sensitized by hematoporphyrin and related porphyrins in deuterated and non-deuterated ethanols.

An approach was developed to determine triplet quantum yields, Φ T , which overcomes most of the difficulties usually encountered in flash photolysis studies of triplet states. The combined application of near-IR emission and optoacoustics yields Φ T values which are independent of the absorption properties of the triplets. Thus, the triplet energies derived from near-IR phosphorescence spectra together with the values of the heat stored by the triplets determined from optoacoustic experiments afforded Φ T =0.3±0.1 for porphycene (PO) and 0.4±0.1 for its tetra-n-propyl derivatiue (TPrPO)

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Laser-induced optoacoustics combined with near-infrared emission: an alternative approach for the determination of intersystem crossing quantum yields applied to porphycenes

Study of 124-kilodalton oat phytochrome photoconversions in vitro with laser-induced optoacoustic spectroscopy

Triplet lifetime determination by laser-induced optoacoustic spectroscopy. benzophenone/iodide revisited

The quantum yield of triplet formation, phi T, and that of the photosensitized formation of singlet molecular oxygen, phi delta, were determined for a rare nucleoside, 4-thiouridine (4t-Urd), in water and in acetonitrile, using singlet molecular oxygen phosphorescence, laser-induced optoacoustics and time-resolved thermal lensing. These yields, phi T and phi delta, the latter in aerated solutions, were found to be, respectively, in water: 0.67 +/- 0.17 and 0.18 +/- 0.04 and in acetonitrile: 0.61 +/- 0.15 and 0.50 +/- 0.20. The fraction of the 4t-Urd triplet molecules quenched by oxygen leading to singlet molecular oxygen, S delta, was calculated to be between 0.7 and unity in both solvents, this value being indicative of a pi pi*character for the lowest triplet state of 4t-Urd.

Klaus Heihoff

Papers

Thermal-lensing and phosphorescence studies of the quantum yield and lifetime of singlet molecular oxygen (1 delta g) sensitized by hematoporphyrin and related porphyrins in deuterated and non-deuterated ethanols.

Laser-induced optoacoustics combined with near-infrared emission: an alternative approach for the determination of intersystem crossing quantum yields applied to porphycenes

Study of 124-kilodalton oat phytochrome photoconversions in vitro with laser-induced optoacoustic spectroscopy

Triplet lifetime determination by laser-induced optoacoustic spectroscopy. benzophenone/iodide revisited

Quantum yields of triplet and O2(1 delta g) formation of 4-thiouridine in water and acetonitrile.