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A series of 2,1,3-benzothiadiazole–Au(I)–L complexes have been synthesised, structurally characterised and investigated for their photophysical properties. These are the first organometallic Au(I) complexes containing a C–Au bond on the highly electron-deficient benzothiadiazole unit. The complexes exhibit solution-phase phosphorescence at room temperature, assigned to the intrinsic triplet state of the benzothiadiazole unit that is efficently populated through its attachment to gold. Comparison with routinely reported Au(I) complexes, which include intervening alkenyl linkers, suggests that previous assignments of their phosphorescence as ¹π → π*(CCR) might be incomplete. Our observations affirm that, in addition to the heavy atom effect, breaking symmetry in the involved aryl motif may be of importance in controlling the luminescence properties.

Detailed photophysical investigation of a Mn(IV)-carbene complex has revealed that excitation into its lowest-energy absorption band (∼500 nm) results in the formation of an energetic ligand-to-metal charge-transfer (LMCT) state with a lifetime of 15 ns. To the best of our knowledge, this is the longest lifetime reported for charge-transfer states of first-row-based transition metal complexes in solution, barring those based on Cu, with a d¹⁰ configuration. A so-called superoxidant, Mn(IV)-carbene exhibits an excited state potential typically only harnessed via excited states of reactive organic radical species. Furthermore, the long-lived excited state in this case is found to be a dark doublet, with its transition to the quartet ground state being spin-forbidden, a contrast to most first-row literature examples, and a possible cause of the long lifetime. Showcasing excited state properties which in some cases exceed those of complexes based on precious metals, these findings not only advance the library of earth-abundant photosensitizers but also shed general insight into the photophysics of d³ and related Mn complexes.

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1′-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic Fe^III-hexa-NHC [Fe(mbmi)₃](PF₆)₃ and heteroleptic Fe^II-tetra-NHC [Fe(mbmi)₂(bpy)](PF₆)₂ (bpy = 2,2′-bipyridine) complexes. They are compared to the reported Fe^III-hexa-NHC [Fe(btz)₃](PF₆)₃ and Fe^II-tetra-NHC [Fe(btz)₂(bpy)](PF₆)₂ complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)₃](PF₆)₃ and [Fe(mbmi)₂(bpy)](PF₆)₂ are evaluated through L–Fe–L bond angles and ligand planarity and compared to those of [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. The Fe^II/Fe^III redox couples of [Fe(mbmi)₃](PF₆)₃ (−0.38 V) and [Fe(mbmi)₂(bpy)](PF₆)₂ (−0.057 V, both vs Fc^+/0) are less reducing than [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. The two complexes show intense absorption bands in the visible region: [Fe(mbmi)₃](PF₆)₃ at 502 nm (ligand-to-metal charge transfer, ²LMCT) and [Fe(mbmi)₂(bpy)](PF₆)₂ at 410 and 616 nm (metal-to-ligand charge transfer, ³MLCT). Lifetimes of 57.3 ps (²LMCT) for [Fe(mbmi)₃](PF₆)₃ and 7.6 ps (³MLCT) for [Fe(mbmi)₂(bpy)](PF₆)₂ were probed and are somewhat shorter than those for [Fe(btz)₃](PF₆)₃ and [Fe(btz)₂(bpy)](PF₆)₂. [Fe(mbmi)₃](PF₆)₃ exhibits photoluminescence at 686 nm (²LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10^–4, compared to (3 ± 0.5) × 10^–4 for [Fe(btz)₃](PF₆)₃.

Direct excitation of aromatic compounds grants access to high-energy intermediates that can be utilised in organic synthesis. Understanding and predicting the substituent effects at the excited state for aromatic molecules remains challenging for the synthetic photochemist. In this work, we present an experimental and computational investigation of the excited state of the isomeric chloroanilines, which promptly react by losing the chloride when the amino group is in para position, but are non-reactive and non-emissive in the meta and ortho isomers. XMS-CASPT2//CASSCF computations explain this apparent contradiction of the meta-ortho selectivity rule of Zimmerman, which originates from the substituent effects lowering to a different extent the barrier to populate the prefulvenic conical intersection that deactivates non-radiatively the singlet excited state of the chloroanilines. Computational chemistry allows to elucidate the observed selectivity in the photochemistry of chloroanilines. A meta-ortho effect of the substituents favours the population of the prefulvenic conical intersection which leads to rapid deactivation of the m- and o-isomers of chloroaniline, while the para derivative lives long enough to emit and populate the reactive triplet state which leads to C-Cl dissociation.+image

We show here that soap films-typically expected to host symmetric molecular arrangements-can be constructed with differing opposite surfaces, breaking their symmetry, and making them reminiscent of functional biological motifs found in nature. Using fluorescent molecular probes as dopants on different sides of the film, resonance energy transfer could be employed to confirm the lack of symmetry, which was found to persist on timescales of several minutes. Further, a theoretical analysis of the main transport phenomena involved yielded good agreement with the experimental observations.

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)(2)](PF6)(4) (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the pi-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)(2)](PF6)(2) (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)(2)](PF6)(2) (cpbmi = 1,1 '-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)(2)](PF6)(4) is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)(2)](PF6)(4) a promising pH-insensitive analogue of [Fe(cpbmi)(2)](PF6)(2). Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)(2)](PF6)(4) was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

This thesis explores the photophysics of some transition-metal complexes (TMCs) which utilize N-heterocyclic carbenes as ligands. After a historical interlude which traces the development of the broader field of transition metal complexes and their photophysical investigations, there is an overview of theoretical concepts and the spectroscopic methods employed. The focus thereafter is placed on complexes of the type [ML₂]ⁿ⁺ (where M=Fe and Mn) which feature the tripodal ‘Scorpionate’ motif, i.e. L=[phenyl(tris(3-methylimidazol-1-ylidene))borate]^–. L, like many carbenes, is an exceptional sigma-donor, and by some metrics is the strongest tripodal donor known. It is therefore able to sufficiently destabilize metal-centred states in conjunction with several 3d metals, allowing for the realization of long-lived charge-transfer states on the nanosecond timescale, in sharp contrast to many complexes based on polypyridyl ligand motifs previously investigated.

[Fe^IIIL₂]⁺ features a 2 ns doublet ligand-to-metal charge transfer (LMCT) excited state, which is substantially energetic and is shown to be capable of engaging in photoinduced electron transfer reactions with both donors and acceptors. The strong ligand field imposed on the iron centre furthermore makes possible the occurrence of two metal-centred redox events before the ligand oxidation – this translates to the unusual situation of the LMCT excited state of [Fe^IIIL₂]⁺ being able to oxidize or reduce its own ground state: a phenomenon called photoinduced symmetry-breaking charge separation. The finding is the first documented case with direct evidence for a transition-metal complex, and the only one which proceeds with a substantial driving force generally. [Mn^IVL₂]²⁺ features a long-lived LMCT excited state, which is found to be a potent photo-oxidant, capable of oxidizing a range of substrates including solvents such as methanol. Its excited state lifetime of 16 ns also presents a near order of magnitude improvement over the iron counterpart. One possible cause is traced to the spin-forbidden nature of the transition back to the ground state, highlighting the importance of such a design principle for the realization of longer lifetimes, as has been the case previously for excited states based on precious metals. The last half of the thesis features benzothiadiazole-Au-carbene (and phosphine) chromophores that are bright phosphors in room temperature solution – it is found that the carbene is inconsequential to the photophysics in this case, which is instead contingent on the direct linkage of the gold atom to a heteroarene moiety, causing an efficient population of its triplet manifold. Concluding remarks are furnished.

Photoredox catalysis's relevance in organic synthesis research and innovation will increase in the coming decades. However, the processes rely almost exclusively on expensive noble metal complexes, most notably iridium complexes, to absorb light and transfer a single charge to a substrate or a catalyst to initiate cascade transformations. Light-triggered plasmon resonances generate a non-Fermi-Dirac energy distribution with many hot carriers that decay in similar to 1 ps. Their ultrafast relaxation makes performing single electron transfer (SET) transformations challenging. Herein, a novel photosystem is proposed based on surface-modified gold nanoparticles (aka plasmon "molecularization"), which improved charge separation and, more importantly, enabled SET reactions, expanding the portfolio of photocatalysts available for photoredox catalysis. The photosystem was made into an electrode, permitting its use in photoelectrochemical arrangements that leverage electro- and photo-chemical approaches' benefits and chemical engineering solutions, helping the synthetic chemistry efforts towards greener synthesis and synthesis of more complex structures on a scale.

The majority of visible light-active plasmonic catalysts are often limited to Au, Ag, Cu, Al, etc., which have considerations in terms of costs, accessibility, and instability. Here, we show hydroxy-terminated nickel nitride (Ni3N) nanosheets as an alternative to these metals. The Ni3N nanosheets catalyze CO2 hydrogenation with a high CO production rate (1212 mmol g(-1) h(-1)) and selectivity (99%) using visible light. Reaction rate shows super-linear power law dependence on the light intensity, while quantum efficiencies increase with an increase in light intensity and reaction temperature. The transient absorption experiments reveal that the hydroxyl groups increase the number of hot electrons available for photocatalysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy shows that the CO2 hydrogenation proceeds via the direct dissociation pathway. The excellent photocatalytic performance of these Ni3N nanosheets (without co-catalysts or sacrificial agents) is suggestive of the use of metal nitrides instead of conventional plasmonic metal nanoparticles.

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.