General discussion

Kornfeld, G.; Farkas, L.; Rabinowitsch, E.

doi:10.1039/tf9343000130

Cited by 5 publications

(4 citation statements)

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“…The concept of a solvent cage was first expressed in 1934 by Franck and Rabinowitsch while studying the photochemical generation of radicals in solution. , At that time, lower reaction yields measured in the solution phase relative to the gas phase were not understood. This photochemistry did not involve photosensitizers but rather light absorption by stable diatomic molecules to populate dissociative excited states that resulted in homolytic bond cleavage.…”

Section: Introduction Background and Motivationmentioning

confidence: 99%

Factors that Impact Photochemical Cage Escape Yields

Goodwin,

Dickenson,

Ripak

et al. 2024

Chem. Rev.

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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.

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Section: Introduction Background and Motivationmentioning

confidence: 99%

Factors that Impact Photochemical Cage Escape Yields

Goodwin,

Dickenson,

Ripak

et al. 2024

Chem. Rev.

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“…Molecules that populate dissociative excited states were used, leading to a corresponding geminate radical pair upon homolytic bond cleavage. [1][2] Franck and Rabinowitsch initially proposed that the geminate products had to possess sufficient kinetic energy to "find their way through the surrounding 'walls' of the solvent and to put more molecular layers between them before coming to rest". 2 Experimental evidence of this cage effect was provided by Lyon and Levy, who investigated the decomposition of mixtures of azomethane and per-deuterated azomethane in the gas phase and in isooctane solutions.…”

Section: Introductionmentioning

confidence: 99%

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

Ripak,

Vega Salgado,

Valverde

et al. 2024

Preprint

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The cage escape yield, i.e. the separation of the geminate radical pair formed immediately after bimolecular excited-state electron transfer was studied in eleven solvents using six Fe(III), Ru(II), Os(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 constants greater than 20 but were shown to reach values up to 0.58 in solvents with low dielectric constant. The results presented herein on closed shell pho-tosensitizers suggest that the low rate of triplet-singlet intersystem crossing within the manifold of states of the geminate radical pair implies that charge recombination towards 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 to foresee a great development of Fe(III) photosensitizers with 2LMCT excited state in photoredox catalysis, providing that solvent with low dielectric constants are used.

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“…1 Historically, the cage escape concept was first introduced in 1934 by Franck and Rabinowitsch who proposed that the geminate products had to possess sufficient kinetic energy to "find their way through the surrounding "walls" of the solvent and to put more molecular layers between them before coming to rest". 2,3 Experimental evidence of this cage effect was first provided by Lyon and Levy, using classical crossover experiments of CH 3 CH 3 and CD 3 CD 3 . 4 Cage escape yields have been extensively studied with organic photosensitizers such as cyano-substituted anthracene, as well as pyrylium, xanthene, and perylene derivatives.…”

Section: ■ Introductionmentioning

confidence: 99%

“…The cage escape yield (Φ CE ) has attracted much interest in the field of photoredox catalysis where it is used to describe the separation of the geminate radical pair formed upon the bimolecular electron transfer reactivity between an excited-state photosensitizer and a quencher . Historically, the cage escape concept was first introduced in 1934 by Franck and Rabinowitsch who proposed that the geminate products had to possess sufficient kinetic energy to “find their way through the surrounding “walls” of the solvent and to put more molecular layers between them before coming to rest”. , Experimental evidence of this cage effect was first provided by Lyon and Levy, using classical crossover experiments of CH 3 CH 3 and CD 3 CD 3 …”

Section: Introductionmentioning

confidence: 99%

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

Ripak,

Vega Salgado,

Valverde

et al. 2024

J. Am. Chem. Soc.

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The cage escape yield, i.e., the separation of the geminate radical pair formed immediately after bimolecular excitedstate 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 2 LMCT excited state in photoredox catalysis, providing that solvents with low dielectric constants are used.

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General discussion

Cited by 5 publications

References 0 publications

Factors that Impact Photochemical Cage Escape Yields

Factors that Impact Photochemical Cage Escape Yields

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

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

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