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Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.ORCID iD: 0000-0001-9975-6577
Lund Univ, Dept Chem, Ctr Anal & Synth, Box 124, SE-22100 Lund, Sweden..
North Carolina State Univ, Dept Chem, Raleigh, NC 27695 USA..
Lund Univ, Dept Chem, Ctr Anal & Synth, Box 124, SE-22100 Lund, Sweden..
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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. Vol. 14, no 37, p. 10129-10139
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-514757DOI: 10.1039/d3sc02806bISI: 001062236300001PubMedID: 37772113OAI: oai:DiVA.org:uu-514757DiVA, id: diva2:1806837
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, 2020-05058Available from: 2023-10-24 Created: 2023-10-24 Last updated: 2024-12-07Bibliographically approved
In thesis
1. Ferrous and Ferric N-Heterocyclic Carbene Complexes: Characterization and Applications in Photoredox Catalysis
Open this publication in new window or tab >>Ferrous and Ferric N-Heterocyclic Carbene Complexes: Characterization and Applications in Photoredox Catalysis
2025 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Iron complexes are emerging as favourable substitutes to noble metal complexes as photocatalysts due to iron being earth-abundant and inexpensive. Much of the recent progress has been enabled by the strong electron-donating character of N-heterocyclic carbene (NHC) ligands that strongly destabilizes metal-centred (MC) states of FeNHC complexes and thereby greatly extends the lifetimes of their charge transfer (CT) states that are otherwise rapidly deactivated via low-lying MC states.

The first part of this thesis successfully employed FeNHC complexes in different photoredox catalysis (PRC) reactions and an example of high-turnover catalytic hydrogen production. The latter was accomplished with the benchmark ferric bis-tridentate scorpionate complex [FeIII(phtmeimb)2]+ (phtmeimb = phenyl(tris(3-methylimidazol-1-ylidene))borate) which has a 2LMCT state lifetime of two nanoseconds and excellent photostability. It was further employed in two PRC reactions that yielded synthetically-useful organic compounds, where fast and efficient reductive quenching of the 2LMCT state by various amine donors with cage escape yields between 2 and 22 % were observed. A tris-bidentate complex with favourable excited-state (ES) redox properties and lifetimes in both oxidation states, [FeII,III(btz)3]2+,3+ (btz = 3,3’-dimethyl-1,1’-bis(p-tolyl)-4,4’-bis(1,2,3-triazol-5-ylidene)), was employed in a two-photon PRC reaction utilizing both oxidative and reductive quenching steps, making the PRC reaction overall more efficient. 

The second part of this thesis describes the electrochemical and photophysical characterization of novel FeNHC complexes with three different motifs in view of their potential suitability as photocatalysts. (i) For a series of ferric bis-tridentate complexes with cyclometalating ligands, not only were their emissive 2LMCT states with lifetimes of hundreds of picoseconds approaching values previously obtained with the [FeIII(phtmeimb)2]+ motif, their electrochemical and ES properties were more tunable by substituent effects. (ii) For the (NHC)4(bpy)2 bis-tridentate complexes [Fe(btz)2bpy]2+,3+ (bpy = 2,2'-bipyridyl) and [Fe(btz)2mbpy]2+,3+ (mbpy = 4,4'-dimethyl-2,2'-bipyridyl), both ferrous and ferric analogues offered insufficient ES lifetimes on the order of ten ps. The ferrous mbpy variant featured however a more long-lived, presumably MC state that deserves further characterization, also in regard to its potential reactivity. (iii) For the ferrous analogue of [FeIII(phtmeimb)2]+, the strikingly-short picosecond 3MLCT state lifetime concludes that even the phtmeimb- ligand with superior σ-donating ability cannot sufficiently prevent the relatively high-energy 3MLCT state from rapid deactivation. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2025. p. 156
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2482
Keywords
Iron, Earth-Abundant, N-Heterocyclic Carbene, Photoredox Catalysis, Excited State Electron Transfer, Characterization, Photophysics
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-544629 (URN)978-91-513-2331-2 (ISBN)
Public defence
2025-02-06, Polhemsalen, Room 10134, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
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Supervisors
Available from: 2025-01-15 Created: 2024-12-07 Last updated: 2025-01-15

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Johnson, CatherineHuang, PingEricsson, ToreHäggström, LennartLomoth, Reiner

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