Unexpected Reduction of a Coordinated Diazapyridinophane Ligand Bound to Chromium(III) Ion Leading to Delocalization of the Unpaired Electron across Two Isolated Pyridine Units

In the tetraazamacrocyclic ligand N,N'-dimethyl-2,11-diaza-[3.3](2,6)pyridinophane (L-N4 Me2 ), the two pyridine units are separated from each other by sp3 -hybridized triatomic bridges. Such electronically isolated pyridine moieties are considerably less prone to reductions than di- or triimines. A detailed structural, magnetic, and spectroscopic investigation of the complexes [Cr(L-N4 Me2 )(OAc)2 ] and [Cr(L-N4 Me2 )(OAc)2 ](PF6 ), in combination with theoretical calculations, reveals that the reduced complex must be described as a chromium(III) ion coordinated to the anionic radical ligand (L-N4 Me2 )⋅- rather than a low-spin chromium(II) ion bound to closed-shell ligands. Thus, it is, to the best of our knowledge, only the second example of a stable and structurally characterized metal complex containing a reduced isolated pyridine unit. The stability is attributed to the delocalization of the unpaired electron across the two pyridine units, mediated by their interaction to the metal ion.

Intense Photoinduced Intervalence Charge Transfer in High-Valent Iron Mixed Phenolate/Carbene Complexes

We report high-valent iron complexes supported by N-heterocyclic carbene (NHC)-anchored, bis-phenolate pincer ligands that undergo ligand-to-metal charge transfer (LMCT) upon photoexcitation. The resulting excited states - with a lifetime in the picosecond range - feature a ligand-based, mixed-valence system and intense intervalence charge transfer bands in the near-infrared region. Upon oxidation of the complex, corresponding intervalence charge transfer absorptions are also observed in the ground state. We suggest that the spectroscopic hallmarks of such LMCT states provide useful tools to decipher excited-state decay mechanisms in high-valent NHC complexes. Our observations further indicate that NHC-anchored, bis-phenolate pincer ligands are not sufficiently strong donors to prevent the population of excited metal-centered states in high-valent iron complexes.

Efficient Triplet-Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first-row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)2 ]3+ (bpmp=2,6-bis(2-pyridylmethyl)pyridine) containing earth-abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10-diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet-triplet energy transfer from excited [Cr(bpmp)2 ]3+ to DPA gives 3 DPA with close-to-unity quantum yield. TTA of 3 DPA furnishes green-to-blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less-hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)2 ]3+ and green light.

Synthesis and Characterisation of Luminescent [CrIII 2 L(μ-carboxylato)]3+ Complexes with High-Spin S=3 Ground States (L=N6 S2 donor ligand)

The synthesis, structure, magnetic, and photophysical properties of two dinuclear, luminescent, mixed-ligand [CrIII 2 L(O2 CR)]3+ complexes (R=CH3 (1), Ph (2)) of a 24-membered binucleating hexa-aza-dithiophenolate macrocycle (L)2- are presented. X-ray crystallographic analysis reveals an edge-sharing bioctahedral N3 Cr(μ-SR)2 (μ1,3 -O2 CR)CrN3 core structure with μ1,3 -bridging carboxylate groups. A ferromagnetic superexchange interaction between the electron spins of the Cr3+ ions leads to a high-spin (S=3) ground state. The coupling constants (J=+24.2(1) cm-1 (1), +34.8(4) cm-1 (2), H=-2JS1 S2 ) are significantly larger than in related bis-μ-alkoxido-μ-carboxylato structures. DFT calculations performed on both complexes reproduce both the sign and strength of the exchange interactions found experimentally. Frozen methanol-dichloromethane 1 : 1 solutions of 1 and 2 luminesce at 750 nm when excited into the 4 LMCT state on the 4 A2 → 2 T1 (ν2 ) bands (λexc =405 nm). The absolute quantum yields (ΦL ) for 1 and 2 were found to be strongly temperature dependent. At 77 K in frozen MeOH/CH2 Cl2 glasses, ΦL =0.44±0.02  (for 1), ΦL =0.45±0.02  (for 2).

A Near-Infrared-II Emissive Chromium(III) Complex

The combination of π-donating amido with π-accepting pyridine coordination units in a tridentate chelate ligand causes a strong nephelauxetic effect in a homoleptic CrIII complex, which shifts its luminescence to the NIR-II spectral range. Previously explored CrIII polypyridine complexes typically emit between 727 and 778 nm (in the red to NIR-I spectral region), and ligand design strategies have so far concentrated on optimizing the ligand field strength. The present work takes a fundamentally different approach and focusses on increasing metal-ligand bond covalence to shift the ruby-like 2 E emission of CrIII to 1067 nm at 77 K.

Pyrene-Decoration of a Chromium(0) Tris(diisocyanide) Enhances Excited State Delocalization: A Strategy to Improve the Photoluminescence of 3d6 Metal Complexes

There is a long-standing interest in iron(II) complexes that emit from metal-to-ligand charge transfer (MLCT) excited states, analogous to ruthenium(II) polypyridines. The 3d⁶ electrons of iron(II) are exposed to a relatively weak ligand field, rendering nonradiative relaxation of MLCT states via metal-centered excited states undesirably efficient. For isoelectronic chromium(0), chelating diisocyanide ligands recently provided access to very weak MLCT emission in solution at room temperature. Here, we present a concept that boosts the luminescence quantum yield of a chromium(0) isocyanide complex by nearly 2 orders of magnitude, accompanied by a significant increase of the MLCT lifetime. Pyrene units in the diisocyanide ligand backbone lead to an enlarged π-conjugation system and to a strongly delocalized MLCT state, from which nonradiative relaxation is less dominant despite a sizable redshift of the emission. While the pyrene moiety is electronically coupled to the core of the chromium(0) complex in the excited state, UV-vis absorption and 2D NMR spectroscopy show that this is not the case in the ground state. Luminescence lifetimes and quantum yields for our pyrenyl-decorated chromium(0) complex exhibit an unusual bell-shaped dependence on solvent polarity, indicative of two counteracting effects governing the MLCT deactivation. These two effects are identified as predominant deactivation either through an energetically nearby lying metal-centered state in the most apolar solvents, or alternatively via direct nonradiative relaxation to the ground state following the energy gap law in more polar solvents. This is the first example of a 3d⁶ MLCT emitter to benefit from an increased π-conjugation network.

An Ultra-Long-Lived Triplet Excited State in Water at Room Temperature: Insights on the Molecular Design of Tridecafullerenes

Suitably engineered molecular systems exhibiting triplet excited states with very long lifetimes are important for high-end applications in nonlinear optics, photocatalysis, or biomedicine. We report the finding of an ultra-long-lived triplet state with a mean lifetime of 93 ms in an aqueous phase at room temperature, measured for a globular tridecafullerene with a highly compact glycodendrimeric structure. A series of three tridecafullerenes bearing different glycodendrons and spacers to the C60 units have been synthesized and characterized. UV/Vis spectra and DLS experiments confirm their aggregation in water. Steady-state and time-resolved fluorescence experiments suggest a different degree of inner solvation of the multifullerenes depending on their molecular design. Efficient quenching of the triplet states by O2 but not by waterborne azide anions has been observed. Molecular modelling reveals dissimilar access of the aqueous phase to the internal structure of the tridecafullerenes, differently shielded by the glycodendrimeric shell.

Investigation of Luminescent Triplet States in Tetranuclear CuI Complexes: Thermochromism and Structural Characterization

To develop new and flexible CuI containing luminescent substances, we extend our previous investigations on two metal-centered species to four metal-centered complexes. These complexes could be a basis for designing new organic light-emitting diode (OLED) relevant species. Both the synthesis and in-depth spectroscopic analysis, combined with high-level theoretical calculations are presented on a series of tetranuclear CuI complexes with a halide containing Cu4 X4 core (X=iodide, bromide or chloride) and two 2-(diphenylphosphino)pyridine bridging ligands with a methyl group in para (4-Me) or ortho (6-Me) position of the pyridine ring. The structure of the electronic ground state is characterized by X-ray diffraction, NMR, and IR spectroscopy with the support of theoretical calculations. In contrast to the para system, the complexes with ortho-substituted bridging ligands show a remarkable and reversible temperature-dependent dual phosphorescence. Here, we combine for the first time the luminescence thermochromism with time-resolved FTIR spectroscopy. Thus, we receive experimental data on the structures of the two triplet states involved in the luminescence thermochromism. The transient IR spectra of the underlying triplet metal/halide-to-ligand charge transfer (3 M/XLCT) and cluster-centered (3 CC) states were obtained and interpreted by comparison with calculated vibrational spectra. The systematic and significant dependence of the bridging halides was analyzed.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Near-IR to Near-IR Upconversion Luminescence in Molecular Chromium Ytterbium Salts

Upconversion photoluminescence in hetero-oligonuclear metal complex architectures featuring organic ligands is an interesting but still rarely observed phenomenon, despite its great potential from a basic research and application perspective. In this context, a new photonic material consisting of molecular chromium(III) and ytterbium(III) complex ions was developed that exhibits excitation-power density-dependent cooperative sensitization of the chromium-centered 2 E/2 T1 phosphorescence at approximately 775 nm after excitation of the ytterbium band 2 F7/2 →2 F5/2 at approximately 980 nm in the solid state at ambient temperature. The upconversion process is insensitive to atmospheric oxygen and can be observed in the presence of water molecules in the crystal lattice.

Structural Characterization and Lifetimes of Triple-Stranded Helical Coinage Metal Complexes: Synthesis, Spectroscopy and Quantum Chemical Calculations

This work reports on a series of polynuclear complexes containing a trinuclear Cu, Ag, or Au core in combination with the fac-isomer of the metalloligand [Ru(pypzH)3 ](PF6 )2 (pypzH=3-(pyridin-2-yl)pyrazole). These (in case of the Ag and Au containing species) newly synthesized compounds of the general formula [{Ru(pypz)3 }2 M3 ](PF6 ) (2: M=Cu; 3: M=Ag; 4: M=Au) contain triple-stranded helical structures in which two ruthenium moieties are connected by three N-M-N (M=Cu, Ag, Au) bridges. In order to obtain a detailed description of the structure both in the electronic ground and excited states, extensive spectroscopic and quantum chemical calculations are applied. The equilateral coinage metal core triangle in the electronic ground state of 2-4 is distorted in the triplet state. Furthermore, the analyses offer a detailed description of electronic excitations. By using time-resolved IR spectroscopy from the microsecond down to the nanosecond regime, both the vibrational spectra and the lifetime of the lowest lying electronically excited triplet state can be determined. The lifetimes of these almost only non-radiative triplet states of 2-4 show an unusual effect in a way that the Au-containing complex 4 has a lifetime which is by more than a factor of five longer than in case of the Cu complex 2. Thus, the coinage metals have a significant effect on the electronically excited state, which is localized on a pypz ligand coordinated to the Ru atom indicating an unusual cooperative effect between two moieties of the complex.

Spin Crossover and Long-Lived Excited States in a Reduced Molecular Ruby

The chromium(III) complex [CrIII (ddpd)2 ]3+ (molecular ruby; ddpd=N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine) is reduced to the genuine chromium(II) complex [CrII (ddpd)2 ]2+ with d4 electron configuration. This reduced molecular ruby represents one of the very few chromium(II) complexes showing spin crossover (SCO). The reversible SCO is gradual with T1/2 around room temperature. The low-spin and high-spin chromium(II) isomers exhibit distinct spectroscopic and structural properties (UV/Vis/NIR, IR, EPR spectroscopies, single-crystal XRD). Excitation of [CrII (ddpd)2 ]2+ with UV light at 20 and 290 K generates electronically excited states with microsecond lifetimes. This initial study on the unique reduced molecular ruby paves the way for thermally and photochemically switchable magnetic systems based on chromium complexes complementing the well-established iron(II) SCO systems.

Multi-Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible-light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near-IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low-energy photons can address both challenges simultaneously, and whilst multi-photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi-photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.