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

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

A series of four-coordinate zinc(II) complexes is presented in which the amido vs. 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.

2-(Thienyl)quinoxaline derivatives and their application in Ir(III) complexes yielding tuneable deep red emitters

The synthesis and characterisation of eleven different 2-(thienyl)quinoxaline species that incorporate different points of functionality, including at the thiophene or quinoxaline rings, are described. These species display variable fluorescence properties in the visible region (λem = 401-491 nm) depending upon the molecular structures and extent of conjugation. The series of 2-(thienyl)quinoxaline species were then investigated as cyclometalating agents for Ir(III) to yield [Ir(C^N)2(bipy)]PF6 (where C^N = the cyclometalated ligand; bipy = 2,2'-bipyridine). Eight complexes were successfully isolated and fully characterised by an array of spectroscopic and analytical techniques. Two Ir(III) examples were structurally characterised in the solid state using single crystal X-ray diffraction; both structures confirmed the proposed formulations and coordination spheres in each case showing that the thiophene coordinates via a Ir-C bond. The photophysical properties of the complexes revealed that each complex is luminescent under ambient conditions with a range of emission wavelengths observed (665-751 nm) indicating that electronic tuning can be achieved via both the thienyl and quinoxaline moieties.

8-Phenylquinoline-Based Tetradentate 6/6/6 Platinum(II) Complexes for Near-Infrared Emitters

A series of novel tetradentate 6/6/6 Pt(II) complexes containing an 8-phenylquinoline-benzo[d]imidazole-carbazole ligand was designed; the Pt(II) complexes could be synthesized by metalizing the corresponding ligand with K2PtCl4 in high isolated yields of 60-90%. Experimental and theoretical studies suggested that the ligand modification of the quinoline moieties of the Pt(II) complexes could tune their electrochemical, photophysical, and excited-state properties. Notably, all the Pt(II) complexes exhibited highly electrochemical stabilities with reversible redox processes except the quasi-reversible reduction of PtYL3. The large π-conjugation of the ligand together with increased metal-to-ligand charge-transfer (3MLCT) characters in T1 states enabled the Pt(II) complexes to show broad Gaussian-type NIR emission spectra with high photoluminescence quantum efficiencies of 1.2-1.5% and short τ of 0.8-1.5 μs in dichloromethane at room temperature. This work should provide a valuable reference for the design and development of monomer NIR emitters.

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. The free, uncoordinated organic molecule displays blue fluorescence in solution. It can be singly protonated with strong acids, and the protonated form displays more intense yellow emission. The effect of acid on the excited states is interpreted with the aid of TDDFT calculations. Two Ru(II) coordination complexes, tris(6,6'-biphenanthridine)ruthenium(II) dichloride, [Ru(biphe)₃]Cl₂, and bis(2,2'-bipyridine)(6,6'-biphenanthridine)ruthenium(II) tetraphenylborate, [Ru(bpy)₂(biphe)](BPh₄)₂, are also reported and their structures determined in the solid state by X-ray diffraction. Both complexes display emission at 77 K that is strongly bathochromically shifted by almost 200 nm compared to that of the archetypal ³MLCT emitter [Ru(bpy)₃]²⁺. Such a red shift is consistent with the more extended conjugation and lower-energy π* orbitals associated with the biphe ligand, lowering the energy of the ³MLCT excited state, as revealed by TDDFT calculations. The efficient non-radiative decay that is typical of such low-energy emitters renders the phosphorescence extremely weak and short-lived at ambient temperature, and rapid ligand photodissociation also competes with radiative decay, especially in the heteroleptic complex. Electrochemical analysis illustrates the effect of biphe's stabilized vacant π* manifold, with multiple reversible reductions evident at much less negative potentials than those observed for [Ru(bpy)₃]²⁺.

Platinum(ii) complexes of benzannulated N∧N−∧O-amido ligands: bright orange phosphors with long-lived excited states

Benzannulated N∧N−∧O-chelating ligands enable facile formation of remarkably bright orange-emitting Pt(ii) phosphors with very long-lived excited states.

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

A series of intramolecular, donor-stabilized BF₂ 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 ¹⁹F-¹H NOE NMR spectroscopy. The neutral compounds are nonetheless only weakly luminescent in fluid solution, ascribed to nonradiative decay pathways enabled by rotation of the N-heterocyclic unit. Methylation of the phenanthridinyl nitrogen restricts this rotation, "switching on" comparably strong emission in solution. Modeling by density functional theory (DFT) and time-dependent DFT (TDDFT) indicates that the character of the lowest energy excitation changes upon methylation, with shallow calculated potential energy surfaces of the neutral complexes consistent with their lack of significant radiative decay.

Efficiently luminescent heteroleptic neutral platinum(II) complexes based on N^O and N^P benzimidazole ligands

A series of new luminescent cycloplatinated(II) complexes (5a-8a and 5b-8b) with formulas Pt(bt)(N^O) and Pt(bt)(N^P) have been synthesized [bt = phenylbenzothiazole, N^O = (2-(1H-benzimidazole)-phenyl)diphenylphosphine oxide derivatives for 1a-4a and N^P = (2-(1H-benzimidazole)-phenyl)diphenylphosphine derivatives for 1b-4b]. The crystal structures of the complexes show distorted square planar geometries around the platinum centers. There are no obvious π-π and Pt-Pt intermolecular interactions in the crystal lattice due to the presence of sterically bulky ancillary ligands. Consequently, these complexes exhibit structured monomeric emissions in the range of 527-540 nm in CH₂Cl₂ solution. The photoluminescent quantum yields of Pt(bt)(N^O) (5a-8a) in CH₂Cl₂ solution at room temperature are higher than those of Pt(bt)(N^P) (5b-8b). The above result is well consistent with the crystal structural characteristics of the complexes. The structured emission with microsecond radiative lifetimes and the result of TD-DFT calculations indicate that the emissions of these complexes are mainly attributed to a mixed ³LC-MLCT state.

Brightly Luminescent Platinum Complexes of N∧C-∧N Ligands Forming Six-Membered Chelate Rings: Offsetting Deleterious Ring Size Effects Using Site-Selective Benzannulation

Brightly emissive platinum(II) complexes (λ_emission,max = 607-612 nm) of the type ^RLPtCl are reported, where ^RL is a cyclometalated N^∧C^-∧N-coordinating ligand derived from 1,3-di(2-trifluoromethyl-4-phenanthridinyl)benzene (^CF3LH) or 1,3-di(2-tert-butyl-4-phenanthridinyl)benzene (^tBuLH). Metathesis of the chlorido ligand can be achieved under mild conditions, enabling isolation of ionic compounds with the formula [^CF3LPtL']PF₆ where L' = pyridine or (4-dimethylamino)pyridine (DMAP), as well as the charge-neutral species ^tBuLPt(C≡C─C₆H₄─tBu) (C≡C─C₆H₄─tBu = 4-tert-butylphenylacetylido). Compared with N^∧N^∧N-ligated Pt(II) complexes that form 5-membered chelates, these compounds all contain 6-membered rings. Expanding the chelate ring size from 5 to 6 has been previously demonstrated to enhance emission in some N^∧N^∧N-coordinated Pt(II) species─for example, in complexes of 2,6-di(8-quinolinyl)pyridine vs those of 2,2':6',2″-terpyridine─but in related N^∧C^-∧N-coordinated species, luminescence quantum yields are significantly lower for the 6-membered chelate ring complexes. Here, we demonstrate that site-selective benzannulation of the quinolinyl side-arms can offset the deleterious effect of changing the chelate ring-size and boost photophysical properties such as the quantum yield. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations suggest that benzannulation counterintuitively destabilizes the emissive triplet states compared to the smaller π-system, with the "imine-bridged biphenyl" form of the phenanthridinyl arm helping to buffer against larger molecular distortions, enhancing photoluminescence quantum yields up to 0.09 ± 0.02. The spontaneous formation under aerated conditions of a Pt(IV) derivative (^CF3LPtCl₃) is also reported, together with its molecular structure in the solid state.

Insight into the Properties of Heteroleptic Metal Dithiolenes: Multistimuli Responsive Luminescence, Chromism, and Nonlinear Optics

A comprehensive investigation of the functional properties of heteroleptic donor-M-acceptor dithiolene complexes Bu₄N[M^II(L1)(L2)] is presented (M = Pd, Pt). The acceptor L1 consists of the chiral (R)-(+)α-methylbenzyldithiooxamidate ((R)-α-MBAdto), the donor L2 is 2-thioxo-1,3-dithiole-4,5-dithiolato (dmit) in 1 (Pd) and 2 (Pt), 1,2-dicarbomethoxyethylenedithiolate (ddmet) in 3 (Pd) and 4 (Pt), or [4',5':5,6][1,4]dithiino[2,3-b]quinoxaline-1',3'-dithiolato (quinoxdt) in 5 (Pd) and 6 (Pt). L1 is capable of undergoing proton exchange and promoting crystal formation in noncentrosymmetric space groups. L2 has different molecular structures while it maintains similar electron-donating capabilities. Thanks to the synergy of the ligands, 1-6 behave as H⁺ and Ag⁺ switchable linear chromophores. Moreover, the compounds exhibit a H⁺-switchable second-order NLO response in solution, which is maintained in the bulk for 1, 3, and 4 when they are embedded into a PMMA poled matrix. 5 and 6 show unique anti-Kasha H⁺ and Ag⁺ tunable colored emission originating from the quinoxdt ligand. A correlation between the electronic structure and properties is shown through density functional theory (DFT) and time-dependent DFT calculations.

Red to near-infrared phosphorescent Ir(iii) complexes with electron-rich chelating ligands

The design of molecular phosphors with near-unity photoluminescence quantum yields in the low-energy regions of the spectrum, red to near-infrared, is a long-standing challenge. Because of the energy gap law and the quantum mechanical dependence of radiative decay rate on the excited-state energy, compounds which luminesce in this region of the spectrum typically suffer from low quantum yields. In this article, we highlight our group's advances in the design of top-performing cyclometalated iridium complexes which phosphoresce in red to near-infrared regions. The compounds we have introduced in this body of work have the general formula Ir(C^N)₂(L^X), where C^N is a cyclometalating ligand that controls the photoluminescence color and L^X is a monoanionic chelating ancillary ligand. The Ir(C^N)₂(L^X) structure type is among the most widely studied and technologically successful classes of molecular phosphors, particularly when L^X = acetylacetonate (acac). In our work we have pioneered the use of electron-rich, nitrogen containing ancillary (L^X) ligands as a means of controlling the excited-state dynamics and optimizing them to give record-breaking phosphorescence quantum yields. This paper progresses through our work in three distinct regions of the spectrum - red, deep-red, and near-infrared - and summarizes the many insights we have gained on the relationships between molecular structure, frontier orbital energies, and excited-state dynamics.

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 N-heterocyclic donor arms are reported. The S = 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). The attribution of these features to the onset of aminyl radical character through ligand-based redox is corroborated by exceptionally strong intervalence charge-transfer (IVCT) transitions ascribed to electronic communication between two Namido moieties across a Ni(ii) bridge.

Group VIII coordination complexes of bidentate P^N ligands bearing π-extended quinoline or phenanthridine N-heterocycles

Group 8 complexes of π-extended N-heterocyclic ligands present some unexpected results. Benzannulation shifts the lowest energy excitation to the blue by impacting the character of the transition rather than affecting frontier orbital energies.

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)₂Fe]²⁺ (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 N^N^-^N diarylamido ligand bearing flanking benzannulated N-heterocyclic donors (^tBuL) was used to prepare deeply colored, pseudo-octahedral coordination complexes of a range of first-row transition and main-group metals [(^tBuL)₂M^0/+; M = Fe, Co, Ni, Zn, Ga]. While the Fe(II) congener exhibits the sought-after broad absorption, isostructural and isoelectronic complexes of other first-row transition and main-group metals show vastly different absorption and redox properties. Density functional theory (DFT) calculations point toward the relative energies of the metal d orbitals and ligand orbitals as the source of major changes in electronic structure, confirming aspects and limitations of the predictive 'HOMO inversion' model in experimentally realized systems with implications for the design of abundant transition-metal sensitizers with broad, panchromatic absorptive properties.