Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes.

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.

Switching on emission in Zn(II) coordination complexes by tempering N character.

A series of four-coordinate zinc(II) complexes is presented in which the amido . imino character of a ligated nitrogen donor correlates to the luminescence intensity. DFT analysis points to a distinct mechanism for this trend wherein emission can be switched on by restricting non-radiative decay pathways through the resonance-induced delocalization of amido ligand lone-pairs.

Photoactive monofunctional platinum(II) anticancer complexes of multidentate phenanthridine-containing ligands: photocytotoxicity and evidence for interaction with DNA.

Pt(II) complexes supported by chelating, multidentate ligands containing π-extended, planar phenanthridine (benzo[c]quinoline) donors (LPtCl) exhibit a promising in vitro therapeutic index compared with phenanthriplatin, a leading preclinical anticancer complex containing a monodentate phenanthridine ligand. Here, we report evidence for non-specific interactions of LPtCl with DNA through intercalation-mediated turn-on luminescence in O-saturated aqueous buffer. Brief irradiation with visible light (490 nm) was also found to drastically increase the activity of LPtCl, with photocytotoxicity increased up to 87% against a variety of human cancer cell lines.

Yellow-Emitting, Pseudo-Octahedral Zinc Complexes of Benzannulated N^N^O Pincer-Type Ligands.

A series of yellow-emitting, pseudo-octahedral Zn(II) complexes supported by monoanionic, tridentate acetylacetone-derived N^N^O ligands incorporating phenanthridine (benzo[]quinoline) units is presented. These species emit weakly in solution but exhibit extended millisecond luminescence lifetimes in the solid state at room temperature, and in a frozen glass at 77 K, indicative of phosphorescence from low-lying triplet excited states. Excitation spectra indicate a role for aggregation in enhancing emission in the solid state.

Synthesis and Coordination Chemistry of a Benzannulated Bipyridine: 6,6'-Biphenanthridine.

The synthesis, characterization, and coordination chemistry of a doubly π-extended bipyridine analogue, 6,6'-biphenanthridine (biphe), is presented. The structure of the molecule has been determined in the solid state by X-ray diffraction, showing an angle of 72.6° between the phenanthridine planes.

Donor-Acceptor Boron-Ketoiminate Complexes with Pendent -Heterocyclic Arms: Switched-on Luminescence through -Heterocycle Methylation.

A series of intramolecular, donor-stabilized BF complexes supported by phenanthridinyl-decorated, β-ketoiminate chelating ligand scaffolds is described, along with their characterization by spectroscopy and X-ray diffraction. In solution, the relative orientation of the pendent phenanthridinyl arm is fixed despite not coordinating to the boron center, and a well-resolved through-space interaction between a phenanthridinyl C-H and a single fluorine atom can be observed by F-H NOE NMR spectroscopy. The neutral compounds are nonetheless only weakly luminescent in fluid solution, ascribed to nonradiative decay pathways enabled by rotation of the -heterocyclic unit.

Reduction of Electron Repulsion in Highly Covalent Fe-Amido Complexes Counteracts the Impact of a Weak Ligand Field on Excited-State Ordering.

The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(), with panchromatic absorption and a 3 ns excited-state lifetime.

Pseudo-octahedral nickel(ii) complexes of strongly absorbing benzannulated pincer-type amido ligands: ligand-based redox and non-Aufbau electronic behaviour.

The synthesis, structures and electronic characterization of three strongly coloured, pseudo-octahedral Ni(ii) complexes supported by redox-active diarylamido ligands featuring benzannulated -heterocyclic donor arms are reported. The = 1 paramagnets each present two singly occupied molecular orbitals (SOMOs) identified as metal-based by density functional theory (DFT), consistent with solid-state and solution magnetism measurements. Upon applying oxidative potentials, non-Aufbau behaviour leads to the appearance of intense and well-defined absorption features extending into the near IR (NIR).

In Pursuit of Panchromatic Absorption in Metal Coordination Complexes: Experimental Delineation of the HOMO Inversion Model Using Pseudo-Octahedral Complexes of Diarylamido Ligands.

The ability of a compound to broadly absorb light across the incident solar spectrum is an important design target in the development of molecular photosensitizers. The 'HOMO inversion' model predicts that for [(tpy)Fe] (tpy = 2,2':6',2″-terpyridine) compounds, adjusting the character of the highest occupied molecular orbital (HOMO) from metal-centered to ligand-centered can drastically improve photophysical properties by broadening absorption in the visible and increasing molar extinction coefficients. In an effort to experimentally realize strong, panchromatic absorption, a tridentate diarylamido ligand bearing flanking benzannulated -heterocyclic donors () was used to prepare deeply colored, pseudo-octahedral coordination complexes of a range of first-row transition and main-group metals [()M; M = Fe, Co, Ni, Zn, Ga].

Catalytic Synthesis of Luminescent Pyrimidines via Acceptor-less Dehydrogenative Coupling.

A simple catalytic synthesis of luminescent pyrimidines from benzamidines and alcohols is reported. These one-pot, acceptor-less dehydrogenative coupling reactions are catalyzed by a ruthenium hydrido chloride complex (), supported by a chelating ligand () bearing a benzannulated phenanthridine donor arm. The pyrimidines thus produced are emissive in solution, with photoluminescence quantum yields reaching 72%.

(8-Amino)quinoline and (4-Amino)phenanthridine Complexes of Re(CO) Halides.

In this report, we present a study on the synthesis, structure, and electronics of a series of (8-amino)quinoline and (4-amino)phenanthridine complexes of Re(CO)X, where X = Cl and Br. In all cases, the (amino)heterocycles bind as bidentate ligands, with surprisingly symmetric modes of binding based on Re-N bond lengths. Between the complexes of (8-amino)quinolines and (4-amino)phenanthridines studied in this report, we do not observe much structural variation, and remarkably similar UV-visible absorption spectra.

Deep-Red Luminescence from Platinum(II) Complexes of ^^-Amido Ligands with Benzannulated -Heterocyclic Donor Arms.

A synthetic methodology for accessing narrow-band, deep-red phosphorescence from mononuclear Pt(II) complexes is presented. These charge-neutral complexes have the general structure ()PtCl, in which the Pt(II) centers are supported by benzannulated diarylamido ligand scaffolds bearing substituted quinolinyl and/or phenanthridinyl arms. Emission maxima ranging from 683 to 745 nm are observed, with lifetimes spanning from 850 to 4500 ns.

Monofunctional platinum(ii) anticancer complexes based on multidentate phenanthridine-containing ligand frameworks.

Phenanthriplatin is a leading preclinical anticancer Pt complex distinguished by a phenanthridine ligand that facilitates DNA-targeted covalent binding via intercalation. We report here that Pt(ii) complexes incorporating phenanthridine into a chelating, multidentate ligand scaffold exhibit a superior in vitro therapeutic index compared with phenanthriplatin and cisplatin.

Iron(II) coordination complexes with panchromatic absorption and nanosecond charge-transfer excited state lifetimes.

Replacing current benchmark rare-element photosensitizers with ones based on abundant and low-cost metals such as iron would help facilitate the large-scale implementation of solar energy conversion. To do so, the ability to extend the lifetimes of photogenerated excited states of iron complexes is critical. Here, we present a sensitizer design in which iron(II) centres are supported by frameworks containing benzannulated phenanthridine and quinoline heterocycles paired with amido donors.

Site-Selective Benzannulation of N-Heterocycles in Bidentate Ligands Leads to Blue-Shifted Emission from [( P^N)Cu](μ-X) Dimers.

Benzannulated bidentate pyridine/phosphine ( P^N) ligands bearing quinoline or phenanthridine (3,4-benzoquinoline) units have been prepared, along with their halide-bridged, dimeric Cu(I) complexes of the form [( P^N)Cu](μ-X). The copper complexes are phosphorescent in the orange-red region of the spectrum in the solid-state under ambient conditions. Structural characterization in solution and the solid-state reveals a flexible conformational landscape, with both diamond-like and butterfly motifs available to the CuX cores.

Fourier Transform Microwave Spectroscopic and ab Initio Study of the Rotamers of 2-Fluorobenzaldehyde and 3-Fluorobenzaldehyde.

The rotational spectra of 2-fluorobenzaldehyde (2-FBD) and 3-fluorobenzaldehyde (3-FBD) were recorded using Fourier transform microwave (FTMW) spectroscopy from 4 to 26 GHz. Two planar rotamers were observed for each species that correspond to structures in which the carbonyl bond is directed toward (O-cis) or away from (O-trans) the C1-C2 bond. Observation of transitions due to heavy atom isotopes (C, O) in natural abundance allowed derivation of the ground state effective (r) structures and mass dependence (r) structures for the lowest energy rotamer of 2-FBD (O-trans) and both rotamers of 3-FBD, which compare favorably with ab initio estimates of the equilibrium (r) geometries at the MP2/aug-cc-pVTZ level.