Iron’s abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facialtris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}−, exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (2LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the2LMCT state of [Fe(phtmeimb)2]+in bimolecular quenching studies with methylviologen and diphenylamine.
The photofunctionality of the cobalt−hexacarbene complex [Co(III)(PhB(MeIm) 3 ) 2 ] + (PhB(MeIm) 3 = tris(3methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ ∼ 10 −4 ) but remarkably long-lived (τ ∼ 1 μs) orange photoluminescence at 690 nm in solution at room temperature following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chemical calculations as a rare case of luminescence from a metal-centered state in a 3d 6 complex. Singlet oxygen quenching supports the assignment of the emitting state as a triplet metal-centered state and underlines its capability of driving excitation energy transfer processes.
Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.
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-methylimidazoline-2-ylidine)borate]–, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazoline-2-ylidine)borate]–, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb
= [(4-carboxyphenyl)tris(3-methylimidazoline-2-ylidine)borate]–. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazoline-2-ylidine)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.
Iron-based photosensitizers for dye-sensitized solar cells with a rod-like push–pull design. Solar cell performance was limited by ultrafast (sub-ps) recombination, but yielded better performance than the homoleptic parent photosensitizer.
A combination of ultrafast spectroscopy and DFT/TD-DFT calculations of a recently synthesised iron carbene complex elucidates the ultrafast excited state evolution processes in these systems.
Investigating the optical properties of various chemical
compounds
using UV–vis spectrophotometers is an essential part of education
in chemistry. However, commercial spectrophotometers are usually treated
as “magic black boxes”, where the dominant majority
of optical elements are hidden “under the hood”. This
often limits understanding of the mechanisms behind the generation
of spectral curves, which in turn may impede the ability to understand
the limitations of the applied method and, in some cases, interpret
the acquired data. In addition, the study of optical emission phenomena
using fluorescence spectrophotometers is seldom implemented in educational
laboratories due to the practical challenges and costs of the devices,
which severely limit pedagogic access to this topic. For students
to be more confident with these two basic spectroscopy techniques,
we have developed a laboratory kit that provides a multifaceted learning
experience. Starting with a basic exploration of an instrument assembly,
it teaches, for example, such technical concepts as spectral resolution
and detection sensitivity. More fundamentally, it enables deeper learning
of the Beer–Lambert law and the notion of Stokes shift. The
spectrophotometer is built from cost-efficient materials and is easily
scalable, making it affordable for many educational laboratories.
Due to a modular design, it is adaptable to various levels of education
and has been successfully applied during high school-, undergraduate-,
and graduate-level classes.
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