Factors Controlling Cage Escape Yields of Closed- and Open-Shell Metal Complexes in Bimolecular Photoinduced Electron Transfer.

The cage escape yield, i.e., the separation of the geminate radical pair formed immediately after bimolecular excited-state electron transfer, was studied in 11 solvents using six Fe(III), Ru(II), and Ir(III) photosensitizers and tri-p-tolylamine as the electron donor. Among all complexes, the largest cage escape yields (0.67-1) were recorded for the Ir(III) photosensitizer, showing the highest potential as a photocatalyst in photoredox catalysis. These yields dropped to values around 0.65 for both Ru(II) photosensitizers and to values around 0.38 for the Os(II) photosensitizer. Interestingly, for both open-shell Fe(III) complexes, the yields were small (<0.1) in solvents with dielectric constant greater than 20 but were shown to reach values up to 0.58 in solvents with low dielectric constants. The results presented herein on closed-shell photosensitizers suggest that the low rate of triplet-singlet intersystem crossing within the manifold of states of the geminate radical pair implies that charge recombination toward the ground state is a spin-forbidden process, favoring large cage escape yields that are not influenced by dielectric effects. Geminate charge recombination in open-shell metal complexes, such as the two Fe(III) photosensitizers studied herein, is no longer a spin-forbidden process and becomes highly sensitive to solvent effects. Altogether, this study provides general guidelines for factors influencing bimolecular excited-state reactivity using prototypical photosensitizers but also allows one to foresee a great development of Fe(III) photosensitizers with the 2LMCT excited state in photoredox catalysis, providing that solvents with low dielectric constants are used.

Synthesis of Ru(II) and Os(II) photosensitizers bearing one 9,10-diamino-1,4,5,8-tetraazaphenanthrene scaffold.

The synthesis of eight Ru(II) and Os(II) photosensitizers bearing a common 9,10-disubstituted-1,4,5,8-tetraazaphenanthrene backbone is reported. With Os(II) photosensitizers, the 9,10-diNH2-1,4,5,8-tetraazaphenanthrene could be directly chelated onto the metal center via the heteroaromatic moiety, whereas similar conditions using Ru(II) resulted in the formation of an o-quinonediimine derivative. Hence, an alternative route, proceeding via the chelation of 9-NH2-10-NO2-1,4,5,8-tetraazaphenanthrene and subsequent ligand reduction of the corresponding photosensitizers was developed. Photosensitizers chelated via the polypyridyl-type moiety exhibited classical photophysical properties whereas the o-quinonediimine chelated Ru(II) analogues exhibited red-shifted absorption (520 nm) and no photoluminescence at room temperature in acetonitrile. The most promising photosensitizers were investigated for excited-state quenching with guanosine-5'-monophosphate in aqueous buffered conditions where reductive excited-state electron transfer was observed by nanosecond transient absorption spectroscopy.

Factors that Impact Photochemical Cage Escape Yields.

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Structure-Activity Relationship of Benzophenazine Derivatives for Homogeneous and Heterogenized Photooxygenation Catalysis.

Singlet oxygen is a powerful oxidant used in various applications, such as organic synthesis, medicine, and environmental remediation. Organic and inorganic photosensitizers are commonly used to generate this reactive species through energy transfer with the triplet ground state of oxygen. We describe here a series of novel benzophenazine derivatives as a promising class of photosensitizers for singlet oxygen photosensitization. In this study, we investigated the structure-activity relationship of these benzophenazine derivatives. Akin to a molecular compass, the southern fragment was first functionalized with either aromatic tertiary amines, alkyl tertiary amines, aromatic sulfur groups, alkyl sulfur groups, or cyclic ethers. Enhanced photophysical properties (in terms of triplet excited-state lifetime, absorption wavelength, triplet state energy, and O2 quenching capabilities) were obtained with cyclic ether and sulfur groups. Conversely, the presence of an amine moiety was detrimental to the photocatalysts. The western and northern fragments were also investigated and slightly undesirable to negligible changes in photophysical properties were observed. The most promising candidate was then immobilized on silica nanoparticles and its photoactivity was evaluated in the citronellol photooxidation reaction. A high NMR yield of 97 % in desired product was obtained, with only a slight decrease over several recycling runs (85 % in the fourth run). These results provide insights into the design of efficient photosensitizers for singlet oxygen generation and the development of heterogeneous systems.

Photoinduced One-Electron Chloride Oxidation in Water Using a Pentacationic Ir(III) Photosensitizer.

A novel iridium(III) photosensitizer containing pyridinium-decorated terpyridines has been used for the photo-oxidation of chloride in water. Despite its abundance, the very positive one-electron reduction potential (E° Cl•/- = 2.1-2.4 V vs NHE) restricted its use in energy conversion schemes and artificial photosynthesis. The kinetics of the photoinduced electron transfer process were investigated through Stern-Volmer quenching experiments and nanosecond transient absorption spectroscopy, which provided unambiguous evidence that photoinduced chloride oxidation occurred with a quenching rate constant kq = 5.0 × 1010 M-1 s-1. Complementary spectroelectrochemistry and photolysis experiments confirmed the formation of the reduced photosensitizer and showcased the redox and photostability of the Ir(III) photosensitizer that holds great promise for the HX splitting approach.

Investigation of the Excited-State Electron Transfer and Cage Escape Yields Between Halides and a Fe(III) Photosensitizer.

Excited-state quenching and reduction of [Fe(phtmeimb)2]+, where phtmeimb is phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate, with iodide, bromide, and chloride were studied in dichloromethane, acetonitrile, and acetonitrile/water 1:1 mixture by means of steady-state and time-resolved spectroscopic techniques. Quenching rate constants were almost diffusion-limited in dichloromethane and acetonitrile and followed the expected periodic trend, i.e., I- > Br- > Cl-. Confirmation of excited-state reductive electron transfer was only unambiguously obtained when iodide was used as a quencher. The cage escape yields, i.e., the separation of the geminate radical pair formed upon bimolecular excited-state electron transfer, were determined. These yields were larger in dichloromethane (0.079) than in acetonitrile (0.017), and no photoproduct could be observed in acetonitrile/water 1:1. This study further emphasizes that solvents with low dielectric constant are more suited for productive excited-state electron transfer using Fe(III) photosensitizers with 2LMCT excited state.

pH-Activatable Ruthenium(II) Fluorescein Salphen Schiff Base Photosensitizers for Theranostic Applications.

Ruthenium(II) polypyridyl complexes exhibit a lack of selectivity toward cancer tissues despite extensive studies as photosensitizers for photodynamic therapy (PDT). Here, we report pH-activatable RuII photosensitizers for molecularly targeted PDT by exploiting the higher acidity of tumoral tissue. The fluorescein moiety, well known for its high pH sensitivity, was connected to a RuII center to yield novel photosensitizers for pH-sensitive 1O2 photogeneration. Their ability to photosensitize molecular dioxygen was studied at various pHs and revealed a drastic enhancement from 0.07 to 0.66 of the 1O2 quantum yield under acidic conditions (pH 7.5 to pH 5.5). Their photocytotoxicity against U2OS osteosarcoma cells was also investigated at pH 5.5 and 7.5 through IC50 determination. A strong enhancement of the photocytotoxicity reaching 930 nM was observed at pH 5.5, which showed the potential of such photosensitizers for pH-activatable PDT.

Photocatalytic conversion of aryl diazonium salts to sulfonyl fluorides.

Sulfonyl fluorides have emerged as powerful tools in chemical biology for the selective labelling of proteins. A photocatalytic method is described for the conversion of aryl diazonium salts to aryl sulfonyl fluorides. The diazonium substrates are easily obtained in one step from functionalized anilines. We present the optimization of this mild method for the synthesis of sulfonyl fluorides, the scope of the transformation with a series of functionalized diazonium salts, and we discuss photophysical measurements that provide detailed information about the mechanism of the photochemical process.