We report the first examples of deep-red Ir(iii) photochemistry using [Ir(ppy)2(p-biphe)]PF6 (ppy = 2-phenylpyridine; p-biphe = 6,6',7,7'-biphenanthridine). Red light (740 nm) directly excites into the triplet manifold, populating a long-lived excited state (54 ± 3 ns) with mixed triplet metal-to-ligand charge-transfer/ligand-centered character capable of energy-transfer and electron-transfer photocatalysis.

A Ru(II) complex supported by trans-amido donors is presented. Ru(t2g)-N(2p) dπ-pπ mixing generates highly covalent frontier MOs and strong absorption of visible/NIR light with π-anti-bonding-to-ligand CT character, covering a much wider range than is typical of Ru(II) chromophores. Low-energy irradiation generates a 33 ns excited-state capable of mediating electron-transfer photochemistry.

The development of phosphorescent complexes of second-row transition metals remains a goal of molecular design, driven by the generally lower cost, higher abundance and thus improved sustainability with respect to their applications. In this work, the synthesis, structures, and photophysical characterization of a mononuclear cyclometalallated palladium complex LPdOAc and its acetate-bridged dinuclear analogue [(LPd)2(kappa 2-OAc)]+ are presented, where L represents an NCN-coordinated, cyclometallated 1,3-bis(2-quinolinyl)benzene. The benzannulated ligand stabilizes the dinuclear derivative through ligand-ligand interactions between pi-systems, which is further facilitated by the acetate adopting a kappa 2-binding mode bridging the two Pd(II) centres. The mononuclear (but not the dinuclear) complex is weakly phosphorescent in the deep red region of the spectrum (lambda max = 676 nm) in solution at room temperature, while both compounds phosphoresce in frozen media at 77 K with essentially identical profiles, emission maxima, and lifetimes of 180 mu s. Analysis of the photophysical properties and electronic structures of both monomer and dimer are supported by density functional theory (DFT) and time-dependent DFT (TDDFT) simulations.

The manuscript reports results of syntheses and characterizations of Schiff base, (E)-(5-chloro-2-((2-hydroxy-3methoxy-benzylidene)amino)phenyl)(phenyl)methanone (HL) and its copper(II) complex, bis[(E)-2-(((2-benzoyl-4-chlorophenyl)imino)methyl)-6-methoxyphenolate-kappa 2N,O]copper(II) (1). Molecular structure determination exposes the Schiff base containing two symmetry-independent but opposite configured molecules (A and B) in the unit cell, showing both molecules to adopt a usual O-H & sdot;& sdot;& sdot;N(imine) form via intramolecular hydrogen-bond. Interestingly, microcrystals of 1 provide a set of polymorphs 1a and 1b following two separate crystallization protocols. There is a single molecule in the unit cell of 1a, while two symmetry-independent molecules (Cu1 and Cu2) are present in the unit cell of 1b. Further, the asymmetric unit contains two formula units in HL, a half formula unit in 1a and two half formula units in 1b. Molecular structures for the polymorphs 1a and 1b demonstrate that the two N,O-chelates coordinate to the copper(II) with trans-N,N' and -O,O' arrangements in a square-planar geometry, with significant intramolecular hydrogen bonds. Supramolecular studies disclose intermolecular C-H & sdot;& sdot;& sdot;& sdot;O contacts among symmetry related molecules both in the Schiff base and complexes. The Schiff base structure is further assembled by two-dimensional intermolecular C-H & sdot;& sdot;& sdot;& sdot;O contacts, forming a onedimensional double strand among symmetry related molecule B, while by a long chain via C21-H21A & sdot;& sdot;& sdot;Cl1 contacts in A. Quantitative analyses by Hirshfeld surface support these supramolecular contacts. Experimental PXRD profile for microcrystals 1 matches well with the simulated profile for 1b (from X-ray structure), while different from the simulated profile for 1a, suggesting a distinct structural phase for the polymorph 1a from that of 1b (or 1), alike thermal analyses and X-ray structures. Thermal analyses further feature an irreversible phase transition from crystalline-solid to isotropic-liquid except for an intermediate liquid-crystal phase (mesomorph) for HL, this latter feature is also proven by optical texture observations via polarizing light microscope (PLM). Cyclic voltammetry explores a quasi-reversible redox profile with two consecutive one electron transfer processes in DMF. The simulated UV-vis. spectra and MOs calculations aligned well with experimental spectra as well as the optimized structures with X-ray molecular structures.

Ultrafast X-ray spectroscopy and modelling were used to characterize distortions in the photo-generated quintet state of Fe sensitizers bearing hybrid N-heterocycle/amido ligands. Photoexcitation induces a 0.10-0.35 Å Fe-N elongation that is smaller for d(Fe-Namido) compared to d(Fe-Nheterocycle), suggesting that the partial expansion of the Fe-N bonds involving the N-heterocyclic ligands is critical to the rapid population of ligand-field states.

A detailed understanding of excited-state evolution is critical to realizing the full potential of abundant-metal coordination complex photosensitizers. Here, we show how wide-band optical transient absorption spectroscopy (oTA) can delineate the complete energy relaxation pathway of the photoexcited state of Fe(ii) polypyridyl complexes supported by benzannulated diarylamido ligands. By covering a broader spectral region from 370 to 1200 nm, we resolve consecutive evolution of a photoexcited Fe-amido chromophore from an initially generated singlet 'π-antibonding-to-ligand' charge transfer (1PALCT) excited state to a long-lived metal-centred quintet (5MC) via both a 3PALCT and what we assign as a 3MC state. Notably, we identify spin-parity transformations by observing photogeneration of the 1PALCT followed by its conversion into a 3PALCT state, and the subsequent 3MC-to-5MC transformation via observation of an isosbestic point in the oTA spectral dynamics. The state-to-state transformations are accompanied by coherent oscillations which are impulsive Raman-induced, originating in the ground state. Combining high-resolution, wide-band oTA experiments with the unique absorptive properties of diarylamido ligand-metal complexes, we are thus able, for the first time, to trace the complete deactivation trajectory of an iron(ii) polypyridyl sensitizer using optical spectroscopy.

Reaction of 2-hydroxy-acetophenone/benzophenone (HL/HL′) with nickel( ii ) acetate provides ferromagnetically coupled tetranuclear [Ni(L)(μ 3 -CH 3 O)(CH 3 OH)] 4 (1) or [Ni(L′)(μ 3 -CH 3 O)(H 2 O)] 4 ·H 2 O (2) complexes.

The coordination chemistry of the planar, doubly π-extended bipyridine analog, 6,6',7,7'-biphenanthridine (p-biphe), is presented. The phenanthridine units in p-biphe are fused together at the 6- and 7- positions, and the resulting rigid ligand is compared with the more flexible parent "biphe" fused only at the 6-positions. p-Biphe is intensely fluorescent in solution with a much higher quantum yield, but, unlike biphe, at 77 K the fluorescence is not accompanied by any significant phosphorescence. Two four-coordinate Cu(I) complexes and pseudo-octahedral Ru(II) and Ir(III) complexes are described: [Cu(p-biphe)2]+, [(P^P)Cu(p-biphe)]+, [Ru(bpy)2(p-biphe)]2+, and [Ir(ppy)2(p-biphe)]+, isolated as PF6 salts (P^P  =  4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; bpy = 2,2'-bipyridine; ppy = 2-phenylpyridine). The complexes are strongly coloured due to intense low-energy absorption to charge-transfer excited states based on TDDFT calculations. The acceptor nature of the p-biphe ligand is clear from multiple reduction events observed electrochemically. The Ir(III) complex displays remarkably low-energy phosphorescence (λmax = 812 nm) in solution at room temperature with a vibrational progression evident extending into the near IR, with no contamination from visible light emission. While the heteroleptic Cu(I) complex is phosphorescent at 77 K, no emission is detectable from [Cu(p-biphe)2]+ or [Ru(bpy)2(p-biphe)]2+, likely due to competitive nonradiative decay.

The ability to manipulate excited-state decay cascades using molecular structure is essential to the application of abundant-metal photosensitizers and chromophores. Ligand design has yielded some spectacular results elongating charge-transfer excited state lifetimes of Fe(II) coordination complexes, but triplet metal-centered (3MC) excited states─recently demonstrated to be critical to the photoactivity of isoelectronic Co(III) polypyridyls─have to date remained elusive, with temporally isolable examples limited to the picosecond regime. With this report, we show how strong-field donors and intramolecular π-stacking can conspire to stabilize a long-lived 3MC excited state for a remarkable 4.1 ± 0.3 ns in fluid solution at ambient temperature. Analysis of variable-temperature time-resolved absorption data using theoretical models ranging from Arrhenius to semiclassical Marcus theory, combined with computational modeling and X-ray crystallography, reveal a Jahn-Teller stabilized excited state with a high activation barrier for ground-state recovery. The net result is a chromophore with a 3MC excited-state lifetime that is orders of magnitude longer than anything yet observed for an Fe(II) complex.

Iron complexes supported by phosphine and pyridine-based ligands are widely seen as efficient molecular catalysts for catalytic transformations, but examples of these complexes as molecular electrocatalysts for the hydrogen evolution reaction (HER) are limited. In this work, we present a ligand design based on a 6,6'-fused dimer of a 2,4-substituted (phosphino)phenanthridine (benzo[c]quinoline). Starting from the monomeric P^N precursor, we show how the P^N^N^P dimer can be selectively formed and report the synthesis and characterization of an Fe(II) complex of this unique scaffold. We demonstrate the potential of such frameworks in the electrocatalytic reduction of Brønsted acids in acetonitrile solution.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) and its derivatives are highly useful fluorescent dyes employed in myriad applications in chemistry and biology. Here, we revisit a series of dihalogenated (Cl, Br, I) BODIPY derivatives with rare 1,7-regiochemistry. In addition to their synthesis and structural characterization, we fill in a missing piece of the current literature by delineating their photophysical behavior, including the light-driven generation of singlet oxygen (1O2) which is mediated with particularly high efficiency by the heavier diiodinated congener.

ABSTRACTCatalytic hydrogenation refers to the (often) metal‐mediated addition of dihydrogen (H2) equivalents to unsaturated compounds to form new element‐hydrogen bonds. This conceptually simple reaction is ubiquitous in the production of a vast number of essential chemicals. Despite a growing recognition of the importance of sustainability in manufacturing, the use of fossil‐derived hydrogen gas and precious metal catalysts in hydrogenation remains widespread. Electrochemical variants of these processes are an appealing alternative, especially those that can make use of sustainable Brønsted acids, more abundant electrode materials and renewable electricity. In this mini‐review, we give a selective overview of electrochemical hydrogenation methodologies for N‐heterocycles and some related substrates from the specific perspective of the synthetic chemistry made possible by this increasingly popular approach.

A systematic search for novel singlet fission materials based on the recently synthesized 6,6′-biphenanthridine (biphe) framework is reported, utilizing a straightforward computational approach.

Ligands containing phenanthridine (benzo[c]quinoline) have presented notable exceptions to the conventional logic that increasing ligand benzannulation leads to bathochromic (red) shifts in the absorption and emission of their coordination complexes. The counterintuitive blue shifts have been attributed to the peculiar structure of phenanthridines, whose ground states are dominated by imine-bridged biphenyl resonance contributors. These serve to isolate the C=N unit electronically from the rest of the ligand framework and allow the C=N moiety to act as a 'shock-absorber', buffering against larger molecular distortions in a molecule's excited state, and reducing the observed pseudo-Stokes' shift. Here, we provide experimental evidence for this assertion in the form of a counterfactual that reverses this trend: substitution at the phenanthridine 6-position (i. e., at the C=N sub-unit) breaks the phenanthridine's tendency to cause hypsochromic luminescence shifts. The synthesis, full characterization, and comparison of 2-quinolinyl and 6-phenanthridinyl exemplars is provided, supported by a detailed theoretical treatment.

Dehydrogenative coupling (DC) is an attractive approach to constructing new C-N bonds using alcohols as electrophiles. In 'hydrogen-borrowing' variants of DC, the H2 liberated can be used to re-hydrogenate unsaturated intermediates to produce saturated products. Here, we show how so-generated H2 can also be used to replace fluorine atoms with hydrogens in CF3 groups in a tandem dehydrogenative coupling/hydrodefluorination process. + image

To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

Two new tetradentate, dianionic N"N-"N-"N ligands combining amido and pi-extended phenanthridine (benzo[c] quinoline) donors are presented. The coordination chemistry of the ligands was explored via construction of complexes of divalent transition metal Ni(II), Cu(II), and Pd(II) ions. Single -crystal X-ray diffraction confirmed the expected distorted square -planar geometry for examples of each metal, consistent with both ligand-field expectations and the geometric restrictions of the ligand framework. The complexes absorb strongly in the visible region of the electromagnetic spectrum, with steady-state electronic absorption spectra containing a prominent low energy band attributed by time -dependent density functional theory (TD-DFT) to transitions of mixed ILCT/MLCT character, with contributions from d -> d transitions. Cyclic voltammetry and spectroelectrochemical experiments were used to probe the ability of the new ligand frameworks to support electrontransfer.