Emergence of ligand-to-metal charge transfer in homogeneous photocatalysis and photosensitization

Li, Chenfei; Kong, Xin Ying; Tan, Zheng; Yang, Crystal Ting; Soo, Han Sen

doi:10.1063/5.0086718

Cited by 21 publications

(13 citation statements)

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“…11−17 Furthermore, there is a poor understanding of the orbital structure of ligands that can participate in LMCT transitions. 17,18 Thus, the switch between an MLCT excited state to an LMCT excited state upon oxidation of [Mn-(CNPh) 6 ] + suggests rich orbital structures of isocyanide ligands. 8 Hexacyano complexes [M(CN) 6 ] (n+1)− and their oxidized forms [M(CN) 6 ] n− [where M = Fe (n = 3) and Mn (n = 4)] exhibit charge transfer inversion from MLCT to LMCT upon oxidation, similar to the manganese hexakisphenylisocyanide complex introduced above.…”

Section: ■ Introductionmentioning

confidence: 99%

Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex

et al. 2023

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A metal-to-ligand charge transfer with mixed intraligand character is observed for the rhenium hexakisarylisocyanide complex [Re(CNAr)6]PF6 (CNAr = 2,6-dimethylphenylisocyanide, λmax = 300 nm). Upon oxidation to [Re(CNAr)6](PF6)2, the dominant low energy optical transition is a ligand-to-metal charge transfer (LMCT) mixed with intraligand transitions (λmax = 650 nm). TD-DFT was used to identify the participating ligand-based orbitals in the LMCT transition, revealing that the majority of the donor orbital is based on the aryl ring (85%) as opposed to the CN bond (14%). For both [Re(CNAr)6]+ and [Re(CNAr)6]2+, structural characterization by X-ray diffraction reveals deviations from Oh geometry at the central Re ion, with larger reduction in symmetry observed for Re(II). For [Re(CNAr)6]+, these structural changes lead to a broadening of the strong ν(CN) stretch (2065 cm–1), as the degeneracy of the T1u IR-active mode is broken. Furthermore, a shoulder is observed for this ν(CN) stretch, resulting from deviation of the C–N–Ar bond from linearity. By contrast, [Re(CNAr)6]2+ has two weak bands in the ν(CN) region (2065 and 2121 cm–1). DFT calculations indicate that reduction of symmetry at the central rhenium ion manifests in the decrease in intensity and the observed split of the ν(CN) band. Stability of both complexes are limited by light-induced decomposition where Re(I) dissociates a isocyanide ligand upon irradiation and Re(II) absorbance decays under ambient light. These data provide new insights to the electronic structure of [Re(CNAr)6]2+, enhancing our understanding of LMCT excited states and the versatility of isocyanide ligands.

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Section: ■ Introductionmentioning

confidence: 99%

Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex

et al. 2023

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“…For instance, there have been a growing number of papers on the use of molecular catalysts comprising V, 54−59 Cr, 60−62 Fe, 63 Co, 64−69 and Cu 70−74 for photoredox reactions. 75,76 However, examples of Mn in photocatalysis are scant. Some notable examples are Nam and Fukuzumi's seminal work on a high valent chiral Mn-oxo complex generated by photooxidation using [Ru(bpy) 3 ] 2+ , 77 and a number of cases where CO complexes of Mn were photoactivated to mediate C−C bond formation reactions are reported (Figure 2a).…”

Section: ■ Introductionmentioning

confidence: 99%

“…Over the past decade, there has been a renaissance in the use of light as the energy source instead of heat during photoredox catalysis to initiate reactions for organic synthesis, pioneered by the work of MacMillan, Fukuzumi, Yoon, and others. − Majority of the previous work has relied on photosensitizers based on expensive Pt group metals such as Ru and Ir because of their efficacy, but there has been growing interest in the application of the more earth-abundant and potentially less-toxic first-row transition metals. For instance, there have been a growing number of papers on the use of molecular catalysts comprising V, − Cr, − Fe, Co, − and Cu − for photoredox reactions. , However, examples of Mn in photocatalysis are scant. Some notable examples are Nam and Fukuzumi’s seminal work on a high valent chiral Mn-oxo complex generated by photooxidation using [Ru(bpy) 3 ] 2+ , and a number of cases where CO complexes of Mn were photoactivated to mediate C–C bond formation reactions are reported (Figure a). ,− This is despite the fact that oxygen-evolving Mn clusters form the backbone of redox catalysis in natural and artificial photosynthesis.…”

Section: Introductionmentioning

confidence: 99%

Understanding the Visible-Light-Initiated Manganese-Catalyzed Synthesis of Quinolines and Naphthyridines under Ambient and Aerobic Conditions

Patra

Bhattacherya

et al. 2022

ACS Catal.

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Polyaromatic N-heterocycles are some of the most common building blocks in natural products and active pharmaceutical ingredients. Significant efforts have been devoted to developing catalytic protocols, including those which use an acceptorless dehydrogenation strategy at elevated temperatures, to produce polyaromatic N-heterocycles like quinolines and naphthyridines. However, photoinitiated catalysis driven by visible light offers a milder and often more selective protocol as an alternative to thermal reactions. Here, we present the catalytic syntheses of quinolines and naphthyridines from ortho-aminobenzyl alcohols and ketones using the photocatalyst [Mn(L 1 H)(CO) 3 Br] (L 1 H = 7-hydroxy-2-methyl-1,8-naphthyridine-N-oxide), bearing a phenolic unit on a 1,8-naphthyridine-N-oxide scaffold, under ambient and aerobic conditions with visible light illumination. We describe a broad, functional group-tolerant substrate scope of >30 examples under modest reaction conditions. A variety of 2-aminobenzyl alcohols containing electron-donating and electron-deficient groups and (2-aminopyridin-3-yl)methanol are converted to the corresponding quinolines and naphthyridines using ambient air as an oxidant in the presence of KOH. We synthesized a wide range of derivatives, including some of the bioactive antimalarial drug chloroquine and the steroids progesterone and pregnenolone to highlight the value-added applications of this catalytic protocol for pharmaceutical ingredient and natural product syntheses. We performed substrate viability, ultraviolet−visible, electron paramagnetic resonance, and X-ray photoelectron spectroscopy studies, as well as density functional theory calculations to gain mechanistic insights to the reaction pathway. The catalytic cycle involves condensation of the amino group in the ortho-aminobenzyl alcohol with the ketone initially, which is followed by aerobic oxidation of the benzyl alcohol to the corresponding benzaldehyde catalyzed by the photoinitiator [Mn(L 1 H)(CO) 3 Br] in the presence of visible light, and finally, a KOH-promoted condensation and cyclization to afford quinolines as the final products.

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“…Photoexcited electron transfer in organic dyes and transition metal complexes has been studied by several research groups because of their photophysical properties. − In these systems, electron transfer either from the organic dye or from the metal center in both directions has been studied. − Different families of organic dyes, including perylene diimides (PDIs), have been studied to explore PET. PDIs offer a high absorption coefficient, photostability, and ease of synthetic manipulation, making them suitable for various applications involving electron transfer. − PDI derivatives attached to a heteroleptic iridium complex were used to show single-electron oxidation of a catalyst precursor for water oxidation, and a D–A system involving PDI was demonstrated to show excellent power conversion efficiency in organic solar cells .…”

Section: Introductionmentioning

confidence: 99%

Ultrafast Dynamics of Photoinduced Electron Transfer in Bay-Aryl-Substituted Perylene Diimide Derivatives

et al. 2022

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Blends of donors and acceptors have been widely used in bulk-heterojunction solar cells to have exciton formation and charge separation by photoinduced electron transfer (PET). In this work, we have synthesized perylene diimide (PDI)-based materials having different aryl substituents at the bay positions (4-Anisyl-PDI, CBZ-N-Ph-PDI, and 4-Pyridyl-PDI) to understand the excited-state dynamics of electron transfer. The detailed photophysics was studied using steady-state as well as ultrafast dynamics of the excited states in different solvents. CBZ-N-Ph-PDI showed tremendous effects of the solvent on the electronic properties compared with the other two derivatives. The emission quantum yield of CBZ-N-Ph-PDI decreases drastically in dichloromethane and other polar solvents, indicating strong electron transfer. DFT calculations showed that in CBZ-N-Ph-PDI the HOMO is centered mostly on the N-phenylcarbazole and the LUMO is on the electron-poor PDI moieties. In addition, the energy levels of the HOMO and HOMO−1 in CBZ-N-Ph-PDI are estimated to be identical. The free energy change for charge separation (ΔG CS ) was calculated using electrochemical and photophysical data and found to be negative for CBZ-N-Ph-PDI. The ground-and excitedstate dipole moment ratios suggest that the excited state of 4-Pyridyl-PDI (1.90) is less polar than that of 4-Anisyl-PDI (3.67), which provides an idea of the lower possibility of charge separation in 4-Anisyl-PDI and 4-Pyridyl-PDI. Ultrafast photodynamics studies of 4-Anisyl-PDI, CBZ-N-Ph-PDI, and 4-Pyridyl-PDI showed fast electron transfer only in CBZ-N-Ph-PDI and not in the other PDI derivatives. It was also observed that electron transfer is faster in DCM and THF than in toluene. Ultrafast dynamics studies showed the presence of an equilibrium between electron transfer and decay from the singlet excited state. Ultrafast studies also showed the features of the N-phenylcarbazole cation and PDI anion, further confirming the intramolecular electron transfer in CBZ-N-Ph-PDI.

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Emergence of ligand-to-metal charge transfer in homogeneous photocatalysis and photosensitization

Cited by 21 publications

References 135 publications

Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex

Isocyanide Ligands Promote Ligand-to-Metal Charge Transfer Excited States in a Rhenium(II) Complex

Understanding the Visible-Light-Initiated Manganese-Catalyzed Synthesis of Quinolines and Naphthyridines under Ambient and Aerobic Conditions

Ultrafast Dynamics of Photoinduced Electron Transfer in Bay-Aryl-Substituted Perylene Diimide Derivatives

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