Direct Evidence for Excited Ligand Field State-based Oxidative Photoredox Chemistry of a Cobalt(III) Polypyridyl Photosensitizer

Alowakennu, Micheal M.; Ghosh, Atanu; McCusker, James K.

doi:10.1021/jacs.3c09374

Cited by 16 publications

(10 citation statements)

References 52 publications

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“…With classical polypyridine ligands, the comparison of photoactive d 6 metal complexes with exact identical coordination environment is typically limited to two different oxidation states, for example between [Fe(bpy) 3 ] 2+ and [Co(bpy) 3 ] 3+ , or between [Os(tpy) 3 ] 2+ and [Ir(tpy) 3 ] 3+ . ,,− Owing to their combined σ-donor and π-acceptor properties, the isocyanide ligands used herein are able to accommodate a broader range of metal oxidation states, and the chelating nature of our ligands gives particularly robust homoleptic tris(bidentate) complexes of Cr 0 , Mn I and Fe II . The combined NMR, theoretical and photophysical studies enabled by this special molecular design provide unusually clear insight into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT, MC and IL excited states (Figure and Table ), and how these individual electronically excited states relax to the ground state (Figure ).…”

Section: Discussionmentioning

confidence: 99%

“…Recent work compared isostructural low-spin d 6 complexes of iron(II) and cobalt(III) in polypyridine coordination environments, yielding important insights regarding the MC excited states within the low-spin d 6 valence electron configuration. ,− Here, in the present study, the focus is on MLCT excited states and on lower-valent d 6 complexes in isocyanide ligand fields, which are substantially stronger than the ligand fields provided by polypyridines. Following early studies of group 6 metal complexes with isocyanide ligands , and more recent reports on luminescent tungsten(0) complexes with monodentate isocyanide, − our group discovered that chelating diisocyanide ligands facilitate access to luminescent chromium(0), molybdenum(0) − and manganese(I) complexes.…”

Section: Introductionmentioning

confidence: 99%

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Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II

Wegeberg,

Häussinger,

Kupfer

et al. 2024

J. Am. Chem. Soc.

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Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II

Wegeberg,

Häussinger,

Kupfer

et al. 2024

J. Am. Chem. Soc.

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“…The spontaneous recoordination of a photodissociated ligand subunit in [Cr(L tri ) 2 ] (found to occur up to 4200 times per photodissociated complex) can be seen as a case of self-healing, in which a species exhibiting MLCT luminescence and competent for photocatalysis is reinstated after photolysis. The reversible nature of the photosubstitution reaction, corroborated by relative actinometry and quantitative photodegradation studies, furthermore opens perspectives for switching applications, similar to the photoinduced linkage isomerism in Ru II sulfoxide and nitroprusside complexes. − , In the bigger picture, the insights gained herein complement other recent key conceptual advances concerning photoactive first-row transition metal complexes, for example, the exploitation of the Marcus inverted region for photocatalysis, − the nephelauxetic effect to tune photophysical behavior, − or the discovery of higher excited-state photoredox activity …”

Section: Discussionmentioning

confidence: 56%

Reversible Photoinduced Ligand Substitution in a Luminescent Chromium(0) Complex

Sinha,

Wellauer,

Maisuradze

et al. 2024

J. Am. Chem. Soc.

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Light-triggered dissociation of ligands forms the basis for many compounds of interest for photoactivated chemotherapy (PACT), in which medicinally active substances are released or "uncaged" from metal complexes upon illumination. Photoinduced ligand dissociation is usually irreversible, and many recent studies performed in the context of PACT focused on ruthenium(II) polypyridines and related heavy metal complexes. Herein, we report a first-row transition metal complex, in which photoinduced dissociation and spontaneous recoordination of a ligand unit occurs. Two scorpionate-type tridentate chelates provide an overall six-coordinate arylisocyanide environment for chromium(0). Photoexcitation causes decoordination of one of these six ligating units and coordination of a solvent molecule, at least in tetrahydrofuran and 1,4-dioxane solvents, but far less in toluene, and below detection limit in cyclohexane. Transient UV−vis absorption spectroscopy and quantum chemical simulations point to photoinduced ligand dissociation directly from an excited metal-to-ligand charge-transfer state. Owing to the tridentate chelate design and the substitution lability of the first-row transition metal, recoordination of the photodissociated arylisocyanide ligand unit can occur spontaneously on a millisecond time scale. This work provides insight into possible self-healing mechanisms counteracting unwanted photodegradation processes and seems furthermore relevant in the contexts of photoswitching and (photo)chemical information storage.

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“…We carried out a series of measurements following the approach of Meyer and co-workers ( 57 ), which are designed to bracket the excited-state oxidation potential of [Co(4,4′-Br 2 bpy) 3 ](PF 6 ) 3, which was determined to be E 0 *[Co(III)/Co(II)] ~ +1.65 V versus SCE in MeCN. This cobalt complex is more oxidizing than the highly oxidizing iridium photocatalyst [Ir(dF(CF 3 )-ppy) 2 (dtbbpy)](PF 6 ) ( E 0 *[Ir(III)/Ir(II)] = +1.21 V versus SCE in MeCN) ( 58 ).…”

Section: Mechanistic Investigationmentioning

confidence: 99%

Exploiting the Marcus inverted region for first-row transition metal–based photoredox catalysis

Chan,

Ghosh,

Yarranton

et al. 2023

Science

Self Cite

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Second- and third-row transition metal complexes are widely employed in photocatalysis, whereas earth-abundant first-row transition metals have found only limited use because of the prohibitively fast decay of their excited states. We report an unforeseen reactivity mode for productive photocatalysis that uses cobalt polypyridyl complexes as photocatalysts by exploiting Marcus inverted region behavior that couples increases in excited-state energies with increased excited-state lifetimes. These cobalt (III) complexes can engage in bimolecular reactivity by virtue of their strong redox potentials and sufficiently long excited-state lifetimes, catalyzing oxidative C(sp 2 )–N coupling of aryl amides with challenging sterically hindered aryl boronic acids. More generally, the results imply that chromophores can be designed to increase excited-state lifetimes while simultaneously increasing excited-state energies, providing a pathway for the use of relatively abundant metals as photoredox catalysts.

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Direct Evidence for Excited Ligand Field State-based Oxidative Photoredox Chemistry of a Cobalt(III) Polypyridyl Photosensitizer

Cited by 16 publications

References 52 publications

Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II

Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II

Reversible Photoinduced Ligand Substitution in a Luminescent Chromium(0) Complex

Exploiting the Marcus inverted region for first-row transition metal–based photoredox catalysis

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Direct Evidence for Excited Ligand Field State-based Oxidative Photoredox Chemistry of a Cobalt(III) Polypyridyl Photosensitizer

Cited by 16 publications

References 52 publications

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII

Reversible Photoinduced Ligand Substitution in a Luminescent Chromium(0) Complex

Exploiting the Marcus inverted region for first-row transition metal–based photoredox catalysis

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Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II

Controlling the Photophysical Properties of a Series of Isostructural d⁶ Complexes Based on Cr⁰, Mn^I, and Fe^II