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Johnson, C., Schwarz, J., Deegbey, M., Prakash, O., Sharma, K., Huang, P., . . . Lomoth, R. (2023). Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited. Chemical Science, 14(37), 10129-10139
Open this publication in new window or tab >>Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited
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2023 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 14, no 37, p. 10129-10139Article in journal (Refereed) Published
Abstract [en]

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [Fe-II(ImP)(2)] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its (MLCT)-M-3 (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(II) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(II) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher ((MLCT)-M-2) state of its Fe(III) analogue [Fe-III(ImP)(2)](+), which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe-III(ImP)(2)](+), we confirmed however that higher ((MLCT)-M-2) states of [Fe-III(ImP)(2)](+) are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [Fe-III(ImP)(2)](+) prepared by us only shows red emission from its lower (LMCT)-L-2 state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe-III(ImP)(2)](+) is instead attributed to an impurity, most probably a photoproduct of the Fe(II) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [Fe-III(ImP)(2)](+) itself.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2023
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-514757 (URN)10.1039/d3sc02806b (DOI)001062236300001 ()37772113 (PubMedID)
Funder
Swedish Foundation for Strategic Research, EM16- 0067Knut and Alice Wallenberg Foundation, KAW, 2018.0074Swedish Research Council, VR, 2020-03207Swedish Energy Agency, P48747-1Sten K Johnson FoundationRoyal Physiographic Society in LundSwedish Research Council, VR, 2020-05058
Available from: 2023-10-24 Created: 2023-10-24 Last updated: 2024-02-23Bibliographically approved
Zhang, M., Johnson, C., Ilic, A., Schwarz, J., Johansson, M. B. & Lomoth, R. (2023). High-Efficiency Photoinduced Charge Separation in Fe(III)carbene Thin Films. Journal of the American Chemical Society, 145(35), 19171-19176
Open this publication in new window or tab >>High-Efficiency Photoinduced Charge Separation in Fe(III)carbene Thin Films
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2023 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 145, no 35, p. 19171-19176Article in journal (Refereed) Published
Abstract [en]

Symmetry-breaking charge separation in molecular materials has attracted increasing attention for optoelectronics based on single-material active layers. To this end, Fe(III) complexes with particularly electron-donating N-heterocyclic carbene ligands offer interesting properties with a (LMCT)-L-2 excited state capable of oxidizing or reducing the complex in its ground state. In this Communication, we show that the corresponding symmetry-breaking charge separation occurs in amorphous films of pristine [Fe(III)L-2]PF6 (L = [phenyl(tris(3-methylimidazol-2-ylidene))borate](-)). Excitation of the solid material with visible light leads to ultrafast electron transfer quenching of the (LMCT)-L-2 excited state, generating Fe(II) and Fe(IV) products with high efficiency. Sub-picosecond charge separation followed by recombination in about 1 ns could be monitored by transient absorption spectroscopy. Photoconductivity measurements of films deposited on microelectrode arrays demonstrated that photogenerated charge carriers can be collected at external contacts.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2023
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-517379 (URN)10.1021/jacs.3c05404 (DOI)001161617600001 ()37616472 (PubMedID)
Funder
Swedish Research Council, 2020-05058Swedish Energy Agency, P48747-1Knut and Alice Wallenberg Foundation, 2018.0074Swedish Foundation for Strategic Research, EM16-0067
Available from: 2023-12-07 Created: 2023-12-07 Last updated: 2024-03-13Bibliographically approved
Ahmed, M. E., Nayek, A., Krizan, A., Coutard, N., Morozan, A., Dey, S. G., . . . Dey, A. (2022). A Bidirectional Bioinspired [FeFe]-Hydrogenase Model. Journal of the American Chemical Society, 144(8), 3614-3625
Open this publication in new window or tab >>A Bidirectional Bioinspired [FeFe]-Hydrogenase Model
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2022 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 144, no 8, p. 3614-3625Article in journal (Refereed) Published
Abstract [en]

With the price-competitiveness of solar and wind power, hydrogen technologies may be game changers for a cleaner, defossilized, and sustainable energy future. H-2 can indeed be produced in electrolyzers from water, stored for long periods, and converted back into power, on demand, in fuel cells. The feasibility of the latter process critically depends on the discovery of cheap and efficient catalysts able to replace platinum group metals at the anode and cathode of fuel cells. Bioinspiration can be key for designing such alternative catalysts. Here we show that a novel class of iron-based catalysts inspired from the active site of [FeFe]-hydrogenase behave as unprecedented bidirectional electrocatalysts for interconverting H-2 and protons efficiently under near-neutral aqueous conditions. Such bioinspired catalysts have been implemented at the anode of a functional membrane-less H-2/O-2 fuel cell device.

Place, publisher, year, edition, pages
American Chemical Society (ACS)American Chemical Society (ACS), 2022
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:uu:diva-472759 (URN)10.1021/jacs.1c12605 (DOI)000773646200033 ()35184564 (PubMedID)
Available from: 2022-04-14 Created: 2022-04-14 Last updated: 2024-01-15Bibliographically approved
Schwarz, J., Ilic, A., Johnson, C., Lomoth, R. & Warnmark, K. (2022). High turnover photocatalytic hydrogen formation with an Fe(iii) N-heterocyclic carbene photosensitiser. Chemical Communications, 58(35), 5351-5354
Open this publication in new window or tab >>High turnover photocatalytic hydrogen formation with an Fe(iii) N-heterocyclic carbene photosensitiser
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2022 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 58, no 35, p. 5351-5354Article in journal (Refereed) Published
Abstract [en]

Herein we report the first high turnover photocatalytic hydrogen formation reaction based on an earth-abundant Fe-III-NHC photosensitiser. The reaction occurs via reductive quenching of the (LMCT)-L-2 excited state that can be directly excited with green light and employs either Pt-colloids or [Co(dmgH)(2)pyCl] as proton reduction catalysts and [HNEt3][BF4] and triethanolamine/triethylamine as proton and electron donors. The outstanding photostability of the Fe-III-NHC complex enables turnover numbers >1000 without degradation.

Place, publisher, year, edition, pages
Royal Society of Chemistry (RSC), 2022
National Category
Physical Chemistry Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-483680 (URN)10.1039/d2cc01016j (DOI)000777682500001 ()35373799 (PubMedID)
Funder
Swedish Foundation for Strategic ResearchKnut and Alice Wallenberg FoundationSwedish Research CouncilSwedish Energy Agency
Available from: 2022-09-05 Created: 2022-09-05 Last updated: 2022-09-05Bibliographically approved
Prakash, O., Lindh, L., Kaul, N., Rosemann, N. W., Losada, I. B., Johnson, C., . . . Wärnmark, K. (2022). Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)-NHC Complexes with Long-Lived 2LMCT Excited States. Inorganic Chemistry, 61(44), 17515-17526
Open this publication in new window or tab >>Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)-NHC Complexes with Long-Lived 2LMCT Excited States
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2022 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 61, no 44, p. 17515-17526Article in journal (Refereed) Published
Abstract [en]

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)2]PF6 (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]–, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]–, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]–. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]– by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (2LMCT) excited states feature nanosecond lifetimes (1.6–1.7 ns) and sizable emission quantum yields (1.7–1.9%) through spin-allowed transition to the doublet ground state (2GS), completely in line with the parent complex [Fe(phtmeimb)2]PF6 (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2022
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-494098 (URN)10.1021/acs.inorgchem.2c02410 (DOI)
Funder
Swedish Research Council, 2020-03207Swedish Research Council, 2020-05058Swedish Research Council, 2021-05313Swedish Energy Agency, P48747-1Knut and Alice Wallenberg Foundation, 2018.0074Carl Tryggers foundation
Available from: 2023-01-13 Created: 2023-01-13 Last updated: 2023-07-03Bibliographically approved
Ilic, A., Schwarz, J., Johnson, C., de Groot, L. H. M., Kaufhold, S., Lomoth, R. & Warnmark, K. (2022). Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)–N-heterocyclic carbene complex. Chemical Science, 13(32), 9165-9175
Open this publication in new window or tab >>Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)–N-heterocyclic carbene complex
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2022 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 13, no 32, p. 9165-9175Article in journal (Refereed) Published
Abstract [en]

Fe-N-heterocyclic carbene (NHC) complexes attract increasing attention as photosensitisers and photoredox catalysts. Such applications generally rely on sufficiently long excited state lifetimes and efficient bimolecular quenching, which leads to there being few examples of successful usage of Fe-NHC complexes to date. Here, we have employed [Fe(iii)(btz)(3)](3+) (btz = (3,3 '-dimethyl-1,1 '-bis(p-tolyl)-4,4 '-bis(1,2,3-triazol-5-ylidene))) in the addition of alkyl halides to alkenes and alkynes via visible light-mediated atom transfer radical addition (ATRA). Unlike other Fe-NHC complexes, [Fe(iii/ii)(btz)(3)](3+/2+) benefits from sizable charge transfer excited state lifetimes >= 0.1 ns in both oxidation states, and the Fe(iii) (LMCT)-L-2 and Fe(ii) (MLCT)-M-3 states are strong oxidants and reductants, respectively. The combined reactivity of both excited states enables efficient one-electron reduction of the alkyl halide substrate under green light irradiation. The two-photon mechanism proceeds via reductive quenching of the Fe(iii) (LMCT)-L-2 state by a sacrificial electron donor and subsequent excitation of the Fe(ii) product to its highly reducing (MLCT)-M-3 state. This route is shown to be more efficient than the alternative, where oxidative quenching of the less reducing Fe(iii) (LMCT)-L-2 state by the alkyl halide drives the reaction, in the absence of a sacrificial electron donor.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2022
National Category
Theoretical Chemistry Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-483786 (URN)10.1039/d2sc02122f (DOI)000834974500001 ()
Funder
Swedish Foundation for Strategic Research, EM16-0067Knut and Alice Wallenberg Foundation, 2018.0074Swedish Research Council, 2020-03207Swedish Research Council, 2020-05058Swedish Energy AgencyWenner-Gren Foundations
Note

Title in Web in Science: Photoredox catalysis via consecutive (LMCT)-L-2- and (MLCT)-M-3-excitation of an Fe(iii/ii)-N-heterocyclic carbene complex

Available from: 2022-09-06 Created: 2022-09-06 Last updated: 2022-09-06Bibliographically approved
Lindh, L., Gordivska, O., Persson, S., Michaels, H., Fan, H., Chabera, P., . . . Warnmark, K. (2021). Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality. Chemical Science, 12(48), 16035-16053
Open this publication in new window or tab >>Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality
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2021 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 12, no 48, p. 16035-16053Article in journal (Refereed) Published
Abstract [en]

A new generation of octahedral iron(ii)-N-heterocyclic carbene (NHC) complexes, employing different tridentate C<^>N<^>C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push-pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(ii) centre, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochemistry, and quantum chemical calculations demonstrate the improved molecular excited state properties in terms of a broader absorption spectrum compared to the reference complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push-pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I-/I-3(-) redox mediator and standard operating conditions, outcompeting the corresponding DSSC based on the homoleptic reference complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO2. Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO2. However, an ultrafast recombination process results in undesirable fast charge recombination from TiO2 back to the oxidized dye, leaving only 5-10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 mu s) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6-8 mu s). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2021
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-468899 (URN)10.1039/d1sc02963k (DOI)000724271000001 ()35024126 (PubMedID)
Funder
Swedish Foundation for Strategic ResearchKnut and Alice Wallenberg FoundationSwedish Research CouncilSwedish Energy AgencySwedish National Infrastructure for Computing (SNIC)StandUp
Available from: 2022-03-15 Created: 2022-03-15 Last updated: 2023-06-27Bibliographically approved
Kaul, N. & Lomoth, R. (2021). The Carbene Cannibal: Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene. Journal of the American Chemical Society, 143(29), 10816-10821
Open this publication in new window or tab >>The Carbene Cannibal: Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene
2021 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 143, no 29, p. 10816-10821Article in journal (Refereed) Published
Abstract [en]

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. Here, we present the first case of direct observation of bimolecular SB-CS in a transition metal complex, [(FeL2)-L-III](PF6) (L = [phenyl(tris(3-methylimidazol-1-ylidene))borate](-)). Photoexcitation of the complex in the visible region results in the formation of a doublet ligand-to-metal charge transfer ((LMCT)-L-2) excited state (E0-0 = 2.13 eV), which readily reacts with the doublet ground state to generate charge separated products, [(FeL2)-L-II] and [(FeL2)-L-IV](2+), with a measurable cage escape yield. Known spectral signatures allow for unambiguous identification of the products, whose formation and recombination are monitored with transient absorption spectroscopy. The unusual energetic landscape of [(FeL2)-L-III](+), as reflected in its ground and excited state reduction potentials, results in SB-CS being intrinsically exergonic (Delta G(CS)degrees similar to -0.7 eV). This is in contrast to most systems investigated in the literature, where Delta C-CS degrees is close to zero, and the charge transfer driven primarily by solvation effects. The study is therefore illustrative for the utilization of the rich redox chemistry accessible in transition metal complexes for the realization of SB-CS.

Place, publisher, year, edition, pages
American Chemical Society (ACS)American Chemical Society (ACS), 2021
National Category
Physical Chemistry Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-451419 (URN)10.1021/jacs.1c03770 (DOI)000679913600002 ()34264638 (PubMedID)
Funder
Swedish Foundation for Strategic Research , EM16-0067Knut and Alice Wallenberg Foundation, KAW 2018.0074Swedish Research Council, 2020-05058
Available from: 2021-09-07 Created: 2021-09-07 Last updated: 2024-01-15Bibliographically approved
Prakash, O., Chabera, P., Rosemann, N. W., Huang, P., Häggström, L., Ericsson, T., . . . Warnmark, K. (2020). A Stable Homoleptic Organometallic Iron(IV) Complex. Chemistry - A European Journal, 26(56), 12728-12732
Open this publication in new window or tab >>A Stable Homoleptic Organometallic Iron(IV) Complex
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2020 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 26, no 56, p. 12728-12732Article in journal (Refereed) Published
Abstract [en]

A homoleptic organometallic Fe(IV)complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb)(2)](PF6)(2)(phtmeimb=[phenyl(tris(3-methylimidazolin-2-ylidene))borate](-)). This (FeN)-N-IV-heterocyclic carbene (NHC) complex was characterized by(1)H NMR, HR-MS, elemental analysis, scXRD analysis, electrochemistry, Mossbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb)(2)](PF6)(2)is a triplet Fe(IV)low-spinS=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb)(2)](PF6)(2)in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand-to-metal charge transfer) excitation. For the first time the excited state dynamics of a Fe(IV)complex was studied and revealed a approximate to 0.8 ps lifetime of the(3)LMCT excited state of [Fe(phtmeimb)(2)](PF6)(2)in acetonitrile.

Place, publisher, year, edition, pages
WILEY-V C H VERLAG GMBH, 2020
Keywords
high-valent iron, N-heterocyclic carbenes, organometallic complexes, photophysics, transient absorption spectroscopy
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-439172 (URN)10.1002/chem.202002158 (DOI)000567890800001 ()32369645 (PubMedID)
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Knut and Alice Wallenberg FoundationSwedish Energy AgencySwedish National Infrastructure for Computing (SNIC)Swedish National Infrastructure for Computing (SNIC)
Available from: 2021-03-30 Created: 2021-03-30 Last updated: 2021-03-30Bibliographically approved
Materna, K. L., Lalaoui, N., Laureanti, J. A., Walsh, A. P., Pettersson-Rimgard, B., Lomoth, R., . . . Hammarström, L. (2020). Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications. ACS Applied Materials and Interfaces, 12(4), 4501-4509
Open this publication in new window or tab >>Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications
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2020 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 12, no 4, p. 4501-4509Article in journal (Refereed) Published
Abstract [en]

A facile surface amide-coupling method was examined to attach dye and catalyst molecules to silatrane-decorated NiO electrodes. Using this method, electrodes with a push-pull dye were assembled and characterized by photoelectrochemistry and transient absorption spectroscopy. The dye-sensitized electrodes exhibited hole injection into NiO and good photoelectrochemical stability in water, highlighting the stability of the silatrane anchoring group and the amide linkage. The amide-coupling protocol was further applied to electrodes that contain a molecular proton reduction catalyst for use in photocathode architectures. Evidence for catalyst reduction was observed during photoelectrochemical measurements and via photocathodes.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2020
Keywords
solar fuels, photocathode, nickel oxide, silatrane, amide coupling
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-406495 (URN)10.1021/acsami.9b19003 (DOI)000510532000032 ()31872996 (PubMedID)
Funder
Swedish Energy Agency, 11674-8
Available from: 2020-03-11 Created: 2020-03-11 Last updated: 2023-10-31Bibliographically approved
Projects
Photochemistry of iron carbene complexes [2020-05058_VR]; Uppsala University
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-2246-1863

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