N,C,N-Pincers in Platinum Bimetallic Complexes: Influence of the Pincer and Bridging Ligands on the Metal-Metal Bond and the Photophysical Properties

Precursors PtCl{κ3-N,C,N-[py-C6HMe2-py]} (1), PtCl{κ3-N,C,N-[py-O-C6H3-O-py]} (2), Pt(OH){κ3-N,C,N-[py-C6HMe2-py]} (3), and Pt(OH){κ3-N,C,N-[py-O-C6H3-O-py]} (4) were used to prepare d8-platinum bimetallic complexes. Precursors 1 and 2 react with AgBF4 and 7-azaindole (Haz) to give [Pt{κ3-N,C,N-[py-C6HMe2-py]}{κ1-N-[Haz]}]BF4 (5) and [Pt{κ3-N,C,N-[py-O-C6H3-O-py]}{κ1-N-[Haz]}]BF4 (6) and 3 and 4 with indolo[2,3-b]indole (H2ii) to generate Pt{κ1-N-[Hii]}{κ3-N,C,N-[py-C6HMe2-py]} (7) and Pt{κ1-N-[Hii]}{κ3-N,C,N-[py-O-C6H3-O-py]} (8). Subsequent addition of 3 and 4 to 5-7 affords bimetallic derivatives [{Pt[κ3-N,C,N-(py-C6HMe2-py)]}2{μ-N,N-[az]}]BF4 (9), [{Pt[κ3-N,C,N-(py-O-C6H3-O-py)]}2{μ-N,N-[az]}]BF4 (10), and {Pt[κ3-N,C,N-(py-C6HMe2-py)]}2{μ-N,N-[ii]} (11). X-ray structures of 9-11 reveal separations between the metals in sequence 9 (3.0515(4) Å) < 10 (3.2689(9) Å) < 11 (3.2949(2) Å). DFT calculations support σ overlap of the dz2 orbitals of platinum atoms, for 9 and 10. Accordingly, their absorption spectra show a MMLCT transition. Complex 9 is a red emitter. The excited state has 3MMLCT characteristics and a Pt-Pt separation of 2.763 Å. Complex 11 is a dual emitter in the red and NIR regions, in solid. Both excited states have a 3LC/LMCT characteristic and platinum-platinum separations of 3.290 and 3.202 Å. Intermediate 5 is a green emitter that achieves quantum yields close to unity, when diluted in PMMA and 1,2-dichloroethane at low concentrations.

Cyclometalated luminescent platinum(II) complexes of dissymmetrical 2,2':4',2''-terpyridine and its self-assembled dimer presenting Pt-Ag dative bonds

A series of cyclometalated (N^C^N) Pt(II)-platinum complexes featuring a terpyridine ligand with a non-coordinating nitrogen atom and a Pt-C bond was synthesized. In the presence of Ag+, the bis(isonitrile)Pt(II) complex formed a remarkable self-assembled helicoidal dimer stabilized by coordination of Ag(I) and metallophilic Pt-Ag interactions. Its assembly was observed in the solid state and maintained in solution. All complexes show strong luminescence and multiple emitting states, which could be rationalized based on solid state X-ray structures and coordinating environment.

Antiprotozoal Pt(II) Complexes as Luminophores Bearing Monodentate P/As/Sb-Based Donors: An X-ray Diffractometric, Photoluminescence, and 121Sb-Mössbauer Spectroscopic Study with TD-DFT-Guided Interpretation and Predictive Extrapolation toward Bi

In this study, it is demonstrated that the radiative rate constant of phosphorescent metal complexes can be substantially enhanced using monodentate ancillary ligands containing heavy donor atoms. Thus, the chlorido coligand from a Pt(II) complex bearing a monoanionic tridentate C^N*N luminophore ([PtLCl]) was replaced by triphenylphosphane (PPh3) and its heavier pnictogen congeners (i.e., PnPh3 to yield [PtL(PnPh3)]). Due to the high tridentate-ligand-centered character of the excited states, the P-related radiative rate is rather low while showing a significant boost upon replacement of the P donor by heavier As- and Sb-based units. The syntheses of the three complexes containing PPh3, AsPh3, and SbPh3 were completed by unambiguous characterization of the clean products using exact mass spectrometry, X-ray diffractometry, bidimensional NMR, and 121Sb-Mössbauer spectroscopy (for [PtL(SbPh3)]) as well as steady state and time-resolved photoluminescence spectroscopies. Hence, it was shown that the hybridization defects of the Vth main-group atoms can be overcome by complexation with the Pt center. Notably, the enhancement of the radiative rate constants mediated by heavier coligands was achieved without significantly influencing the character of the excited states. A rationalization of the results was achieved by TD-DFT. Even though the Bi-based homologue was not accessible due to phenylation side reactions, the experimental data allowed a reasonable extrapolation of the structural features whereas the hybridization defects and the excited state properties related to the Bi-species and its phosphorescence rate can be predicted by theory. The three complexes showed an interesting antiprotozoal activity, which was unexpectedly notorious for the P-containing complex. This work could pave the road toward new efficient materials for optoelectronics and novel antiparasitic drugs.

A theoretical study on the second-order nonlinear optical properties of Pt(II) bis-acetylide complexes: substituent and redox effects

Density functional theory studies on the geometric and electronic structures, UV-vis absorption spectra, and second-order nonlinear optical (NLO) properties of four-coordinate Pt(II) bis-acetylide complexes, cis-[Pt(CNtBu)(ADC)(CCR)2] , have been employed. The effects of ligand variation and the single electron redox process on the structures and NLO response of complexes have also been investigated. It shows that the variations of the ligand and electron have little effect on the geometries of the complexes, but there is a significant effect on their electronic structures and NLO responses. The introduction of a single -NO2 group in acetylide ligands increases the first hyperpolarizability of complex 12 times, while one electron lost in five complexes enhances the first hyperpolarizability 496 times at the most. Both methods are considered effective ways for improving the NLO response of Pt(II) bis-acetylide complexes. Based on the analysis of the electronic and optical properties of fifteen studied complexes, the increase of NLO response is mainly ascribed to strong oscillator strengths, lower electron transition energy, and well-directed effective charge transfer. This work reveals some underlying relationships between the NLO responses and electronic structures of complexes, which is helpful for the design and synthesis of high-performance NLO materials of Pt(II) bis-acetylide complexes.

Unusual Excimer/Dimer Behavior of a Highly Soluble C,N Platinum(II) Complex with a Spiro-Fluorene Motif

In this work, we introduce a spiro-fluorene unit into a phenylpyridine (CN)-type ligand as a simple way to deplanarize the structure and increase the solubility of the final platinum(II)···complex. Using a spiro-fluorene unit, orthogonal to the main coordination plane of the complex, reduces intermolecular interactions, leading to increased solubility but without significantly affecting the ability of the complex to form Pt···Pt dimers and excimers. This approach is highly important in the design of platinum(II) complexes, which often suffer from low solubility due to their mainly planar structure, and offers an alternative to the use of bulky alkyl groups. The nonplanar structure is also beneficial for vacuum-deposition techniques as it lowers the sublimation temperature. Importantly, there are no sp3 hybridized carbon atoms in the cyclometalating ligand that contain hydrogens, the undesired feature that is associated with the low stability of the materials in OLEDs. The complex displays high solubility in toluene, ∼10 mg mL-1, at room temperature, which allows producing solution-processed OLEDs in a wide range of doping concentrations, 5-100%, and EQE up to 5.9%, with a maximum luminance of 7400 cd m-2. Concurrently, we have also produced vacuum-deposited OLEDs, which display luminance up to 32 500 cd m-2 and a maximum EQE of 11.8%.

Platinum(II) Complexes of Nonsymmetrical NCN-Coordinating Ligands: Unimolecular and Excimeric Luminescence Properties and Comparison with Symmetrical Analogues

A series of seven new platinum(II) complexes PtLnCl have been prepared, where Ln is an NCN-coordinating ligand comprising a benzene ring 1,3-disubstituted with two different azaheterocycles. In PtL1-5Cl, one heterocycle is a simple pyridine ring, while the other is an isoquinoline, a quinoline, a pyrimidine (L1, L2, L3), or a p-CF3- or p-OMe-substituted pyridine (L4 and L5). PtL6Cl incorporates both a p-CF3 and a p-OMe-substituted pyridine. The synthesis of the requisite proligands HLn is achieved using Pd-catalyzed cross-coupling methodology. The molecular structures of six of the Pt(II) complexes have been determined by X-ray diffraction. All the complexes are brightly luminescent in deoxygenated solution at room temperature. The absorption and emission properties are compared with those of the corresponding symmetrical complexes featuring two identical heterocycles, PtLnsymCl, and of the parent Pt(dpyb)Cl containing two unsubstituted pyridines [dpybH = 1,3-di(2-pyridyl)benzene]. While the absorption spectra of the nonsymmetrical complexes show features of both PtLnsymCl and Pt(dpyb)Cl, the emission generally resembles that of whichever of the corresponding symmetrical complexes has the lower-energy emission. PtL1Cl differs in that─at room temperature but not at 77 K─it displays emission bands that can be attributed to excited states involving both the pyridine and the isoquinoline rings, despite the latter being unequivocally lower in energy. This unusual behavior is attributed to thermally activated repopulation of the former excited state from the latter, facilitated by the very long-lived nature of the isoquinoline-based excited state. At elevated concentrations, all the complexes show an additional red-shifted emission band attributable to excimers. For PtL1Cl, the excimer strikingly dominates the emission spectra at all but the lowest concentrations (<10-5 M). Trends in the energies of the excimers and their propensity to form are compared with those of the symmetrical analogues.

Competition between N,C,N-Pincer and N,N-Chelate Ligands in Platinum(II)

Replacement of the chloride ligand of PtCl{κ3-N,C,N-[py-C6HR2-py]} (R = H (1), Me (2)) and PtCl{κ3-N,C,N-[py-O-C6H3-O-py]} (3) by hydroxido gives Pt(OH){κ3-N,C,N-[py-C6HR2-py]} (R = H (4), Me (5)) and Pt(OH){κ3-N,C,N-[py-O-C6H3-O-py]} (6). These compounds promote deprotonation of 3-(2-pyridyl)pyrazole, 3-(2-pyridyl)-5-methylpyrazole, 3-(2-pyridyl)-5-trifluoromethylpyrazole, and 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole. The coordination of the anions generates square-planar derivatives, which in solution exist as a unique species or equilibria between isomers. Reactions of 4 and 5 with 3-(2-pyridyl)pyrazole and 3-(2-pyridyl)-5-methylpyrazole provide Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[R'pz-py]} (R = H; R' = H (7), Me (8). R = Me; R' = H (9), Me (10)), displaying κ1-N1-pyridylpyrazolate coordination. A 5-trifluoromethyl substituent causes N1-to-N2 slide. Thus, 3-(2-pyridyl)-5-trifluoromethylpyrazole affords equilibria between Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[CF3pz-py]} (R = H (11a), Me (12a)) and Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N2-[CF3pz-py]} (R = H (11b), Me (12b)). 1,3-Bis(2-pyridyloxy)phenyl allows the chelating coordination of the incoming anions. Deprotonations of 3-(2-pyridyl)pyrazole and its substituted 5-methyl counterpart promoted by 6 lead to equilibria between Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[R'pz-py]} (R' = H (13a), Me (14a)) with a κ-N1-pyridylpyrazolate anion, keeping the pincer coordination of the di(pyridyloxy)aryl ligand, and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[R'pz-py]} (R' = H (13c), Me (14c)) with two chelates. Under the same conditions, 3-(2-pyridyl)-5-trifluoromethylpyrazole generates the three possible isomers: Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[CF3pz-py]} (15a), Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N2-[CF3pz-py]} (15b), and Pt{κ2-N,C-[pyO-C6H3(Opy)]}{κ2-N,N-[CF3pz-py]} (15c). The N1-pyrazolate atom produces a remote stabilizing effect on the chelating form, pyridylpyrazolates being better chelate ligands than pyridylpyrrolates. Accordingly, reactions of 4-6 with 2-(2-pyridyl)-3,5-bis(trifluoromethyl)pyrrole yield Pt{κ3-N,C,N-[py-C6HR2-py]}{κ1-N1-[(CF3)2C4(py)HN]} (R = H (16), Me (17)) or Pt{κ3-N,C,N-[pyO-C6H3-Opy]}{κ1-N1-[(CF3)2C4(py)HN]} (18), displaying κ1-N1-pyrrolate coordination. Complexes 7-10 are efficient green phosphorescent emitters (488-576 nm). In poly(methyl methacrylate) (PMMA) films and in dichloromethane, they experience self-quenching, due to molecular stacking. Aggregation occurs through aromatic π-π interactions, reinforced by weak platinum-platinum interactions.

Toward Near-Infrared Emission in Pt(II)-Cyclometallated Compounds: From Excimers' Formation to Aggregation-Induced Emission

Two series of Pt(II)-cyclometallated compounds containing N^C^N tridentate and alkynyl-chromophore ligands have been synthesized and structurally characterized. The N^C^N ligands differ on the presence of R₁ = H or F in the central aromatic ring, while six different chromophores have been introduced to the alkynyl moiety. Single-crystal X-ray structures for some of the compounds reveal the presence of weak intermolecular contacts responsible for the formation of some dimers or aggregates. The photophysical characterization shows the presence of two emission bands in solution assigned to the ³π-π* transition from the N^C^N ligands mixed with ³MLCT/³ILCT transitions (higher energy band) in deaerated samples. The formation of excimers has also been identified as a broad band at longer wavelengths [near-infrared (NIR) emission] that becomes the main emission band for compounds containing phenanthrene as the chromophore. NIR emission behavior has also been explored using acetonitrile/water mixtures, and the presence of aggregates that emit at ca. 650 nm has also been detected.

Polymorphism and Mechanochromism in 2-Phenylbenzothiazole Cyclometalated PtII Complexes with Chelating N∧O Ligands

New cyclometalated Pt^II complexes with 2-phenylbenzothiazole (pbt) and two different picolinate ligands [Pt(pbt)(R-pic-κN,O)] (R = H (1), OH (2)) were prepared. In contrast to 1, the OH substituent group on 2 allows modulation of the packing in the solid state through donor-acceptor H-bonding interactions with the CH₂Cl₂ solvent. Thus, three pseudopolymorphs of 2 with different aggregation degrees were isolated, including yellow 2-Y, orange-red 2-R (2·0.5CH₂Cl₂) and black 2-B (2·0.75CH₂Cl₂) with emissions at 540, 656, and 740 nm, respectively, in the solid state at 298 K. 2-R and 2-B can be transformed to the pristine solid 2. Studies of their crystal structures show that 1 and 2-Y stack in columns with only π···π stacking interactions, whereas 2-R displays strong aggregated 1D infinite chains based on Pt···Pt and π···π stacking interactions, consistent with the colors and the photophysical properties, measured in several media. Interestingly, 1 and 2 exhibit reversible mechanochromic behavior with high contrast in the color and color emission upon mechanical grinding due to a phase transition between a crystalline and an amorphous state, as confirmed by powder X-ray diffraction (PXRD) studies. Theoretical calculations indicate that Pt···Pt contacts are more relevant in the trimers and tetramers than in the dimers, particularly in their T₁ states, associated with a change from a ³IL/³MLCT transition in the monomer to ³MM(L+L')CT in the oligomers. Noncovalent interaction (NCI) theoretical studies indicate that the π···π stacking among chelates also exerts a strong influence in the metal-metal-to-ligand charge transfer transition character.

Near-infrared electroluminescence beyond 940 nm in Pt(N^C^N)X complexes: influencing aggregation with the ancillary ligand X

We present a study of aggregate excited states formed by complexes of the type Pt(N^C^N)X, where N^C^N represents a tridentate cyclometallating ligand, and X = SCN or I. These materials display near-infrared (NIR) photoluminescence in film and electroluminescence in NIR OLEDs with λ max EL = 720-944 nm. We demonstrate that the use of X = SCN or I modulates aggregate formation compared to the parent complexes where X = Cl. While the identity of the monodentate ligand affects the energy of Pt-Pt excimers in solution in only a subtle way, it strongly influences aggregation in film. Detailed calculations on aggregates of different sizes support the experimental conclusions from steady-state and time-resolved luminescence studies at variable temperatures. The use of X = I appears to limit aggregation to the formation of dimers, while X = SCN promotes the formation of larger aggregates, such as tetramers and pentamers, leading in turn to NIR photo- and electroluminescence > 850 nm. A possible explanation for the contrasting influence of the monodentate ligands is the lesser steric hindrance associated with the SCN group compared to the bulkier I ligand. By exploiting the propensity of the SCN complexes to form extended aggregates, we have prepared an NIR-emitting OLED that shows very long wavelength electroluminescence, with λ max EL = 944 nm and a maximum EQE = 0.3 ± 0.1%. Such data appear to be unprecedented for a device relying on a Pt(ii) complex aggregate as the emitter.

A Novel Class of Cyclometalated Platinum(II) Complexes for Solution-Processable OLEDs

Substitution of the chlorido ligand of cyclometalated [Pt (5-R-1,3-di(2-pyridyl) benzene)Cl] (R = methyl, mesityl, 2-thienyl, or 4-diphenylamino-phenyl) by 4-phenylthiazole-2-thiolate leads to related thiolato complexes, which were fully characterized. Their photophysical properties were determined in degassed dichloromethane solution. The emission color of the novel complexes can be easily tuned by the nature of the substituents on the terdentate ligand, as is the case for the parent chlorido complexes. Their luminescence Quantum Yield is high, with that of the compounds with the 2-thienyl or 4-diphenylamino-phenyl substituents being much higher than that of the related chloride complexes. The platinum complex with the cyclometalated 5-(2-thienyl)-1,3-di(2-pyridyl) benzene was used as the emitter for the fabrication of a yellow solution-processable OLED.

Multistimuli-Responsive Properties of Aggregated Isocyanide Cycloplatinated(II) Complexes

Here, we describe the neutral cyclometalated tert-butylisocyanide Pt^II complexes, [Pt(C^∧N)Cl(CNBu^t)] 1, the double salts [Pt(C^∧N)(CNBu^t)₂][Pt(C^∧N)Cl₂] 2, and the cationic complexes [Pt(C^∧N)(CNBu^t)₂]ClO₄ 3 [C∧N = difluorophenylpyridine (dfppy, a), 4-(2-pyridyl)benzaldehyde (ppy-CHO, b)]. A comparative study of the pseudopolymorphs 1a, 1a·CHCl₃, 1b, 1b·0.5Toluene, 1b·0.5PhF, and 3a·0.25CH₂Cl₂ reveals strong aggregation through Pt···Pt and/or π···π stacking interactions to give a variety of distinctive one-dimensional (1D) infinite chains, which modulate the photoluminescent properties. This intermolecular long-range aggregate formation is the main origin of the photoluminescent behavior of 1a and 1b complexes, which exhibit highly sensitive and reversible responses to multiple external stimuli including different volatile organic compounds (VOCs), solvents, temperatures, and pressures, with distinct color and phosphorescent color switching from green to red. Furthermore, complex 1b undergoes supramolecular self-assembly via Pt···Pt and/or π···π interactions into a polymer thin polystyrene (PS) film 10 wt % in response to toluene vapors, and 3a exhibits vapochromic and vapoluminescent behavior. Theoretical simulations on the dimer, trimer, and tetramer models of 1a and 1b have been carried out to get insight into the photophysical properties in the aggregated solid state.

Platinum(II)-Substituted Phenylacetylide Complexes Supported by Acyclic Diaminocarbene Ligands

We introduce phosphorescent platinum aryl acetylide complexes supported by tert-butyl-isocyanide and strongly σ-donating acyclic diaminocarbene (ADC) ligands. The precursor complexes cis-[Pt(CNtBu)₂(C≡CAr)₂] (4a-4f) are treated with diethylamine, which undergoes nucleophilic addition with one of the isocyanides to form the cis-[Pt(CNtBu)(ADC)(C≡CAr)₂] complexes (5a-5f). The new compounds incorporate either electron-donating groups (4-OMe and 4-NMe₂) or electron-withdrawing groups [3,5-(OMe)₂, 3,5-(CF₃)₂, 4-CN, and 4-NO₂] on the aryl acetylide. Experimental HOMO-LUMO gaps, estimated from cyclic voltammetry, span the range of 2.68-3.61 eV and are in most cases smaller than the unsubstituted parent complex, as corroborated by DFT. In the ADC complexes, peak photoluminescence wavelengths span the range of 428 nm (2a, unsubstituted phenylacetylide) to 525 nm (5f, 4-NO₂-substituted), with the substituents inducing a red shift in all cases. The phosphorescence E_0,0 values and electrochemical HOMO-LUMO gaps are loosely correlated, showing that both can be reduced by either electron-donating or electron-withdrawing substituents on the aryl acetylides. The photoluminescence quantum yields in the ADC complexes are between 0.044 and 0.31 and the lifetimes are between 4.8 and 14 μs, a factor of 1.8-10× higher (for Φ_PL) and 1.2-3.6× longer (for τ) than the respective isocyanide precursor (Φ_PL = 0.014-0.12, τ = 2.8-8.2 μs).

Luminescent cyclometalated alkynylplatinum(II) complexes with 1,3-di(pyrimidin-2-yl)benzene ligands: synthesis, electrochemistry, photophysics and computational studies

In this article, we report on a series of cyclometalated chloro- and alkynyl-platinum(II) complexes bearing various tridentate N^C^N-cyclometalated ligands derived from 1,3-bis(pyrimidin-2-yl)benzene. The X-ray crystal structures of two alkynyl-platinum(II) complexes were determined and other structures were DFT-calculated. Electrochemical and DFT-computational studies suggest a ligand-centred reduction on the R¹-substituted N^C^N ligand, whereas oxidation likely occurs either on the Pt-phenylacetylide moiety and/or the cyclometalated ligand. In CH₂Cl₂ solution at room temperature, the complexes show phosphorescent emissions ranging from green to orange, depending on the R¹ and R² substituents on the ligands. In KBr solid state matrix, excluding complexes bearing a trifluoromethyl substituted ligand, all compounds exhibit red emission. The presence of an alkynyl ancillary ligand has limited influence on absorption and emission spectra except in the case of the complex with the strongly electron-donating diphenylamino R² substituent on the alkynyl ligand, for which a significant red-shift was observed. The alkynyl Pt(II) complex with OMe groups as both R¹ and R² substituents shows the best emission quantum yield (0.81 in CH₂Cl₂ solution) in this series. The full series of DFT calculated band gaps correlated generally well with the electrochemical and absorption data and reasonably model the impact of the substituents on the electronics of these complexes.

Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (g_PL). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including (S)-methyl, (R)- and (S)-isopropyl, and (S)-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the (S)-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and g_PL values of the MMLCT phosphorescence emission of the (S)-isopropyl Pt(II) complex are 0.49 and 8.4 × 10^-4, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; g_PL, 2.4 × 10^-4). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (g_EL, 1.2 × 10^-4) over the unimolecular emission devices (EQE, 5.8%; g_EL, 0.3 × 10^-4). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.

Role of Substituents at 3-position of Thienylethynyl Spacer on Electronic Properties in Diruthenium(II) Organometallic Wire-like Complexes

A series of organometallic complexes [Cl(dppe)₂ Ru-C≡C-(3-R-C₄ H₂ S)-C≡C-Ru(dppe)₂ Cl] (3-R-C₄ H₂ S=3-substituted thienyl moiety; R=-H, -C₂ H₅ , -C₃ H₇ , -C₄ H₉ , -C₆ H₁₃ , -OMe, -CN in 5 a-5 g respectively) have been synthesized by systematic variation of 3-substituents at the thienylethynyl bridging unit. The diruthenum(II) wire-like complexes (5 a-5 g) have been achieved by the reaction of thienylethynyl bridging units, HC≡C-(3-R-C₄ H₂ S)-C≡CH (4 a-4 g) with cis-[Ru(dppe)₂ Cl₂ ]. The wire-like diruthenium(II) complexes undergo two consecutive electrochemical oxidation processes in the potential range of 0.0 - 0.8 V. Interestingly, the wave separation between the two redox waves is greatly influenced by the substituents at the 3-position of the thienylethynyl. Thus, the substitution on 3-position of the thienylethynyl bridging unit plays a pivotal role for tuning the electronic properties. To understand the electronic behavior, density functional theory (DFT) calculations of the selected diruthenium wire-like complexes (5 a-5 e) with different alkyl appendages are performed. The theoretical data demonstrate that incorporation of alkyl groups to the thienylethynyl entity leaves unsymmetrical spin densities, thus affecting the electronic properties. The voltammetric features of the other two Ru(II) alkynyl complexes 5 f and 5 g (with -OMe and -CN group respectively) show an apparent dependence on the electronic properties. The electronic properties in the redox conjugate, (5 a⁺ ) with K_c of 3.9×10⁶ are further examined by UV-Vis-NIR and FTIR studies, showing optical responses in NIR region along with changes in "-Ru-C≡C-" vibrational stretching frequency. The origin of the observed electronic transition has been assigned based on time-dependent DFT (TDDFT) calculations.

Room-Temperature Phosphorescent Platinum(II) Alkynyls with Microsecond Lifetimes Bearing a Strong-Field Pincer Ligand

The use of organometallic triplet emitters in organic light emitting diodes (OLEDs) is motivated by the premise of efficient intersystem crossing leading to unit internal quantum efficiencies. However, since most devices are based on solid-state components, an inherent limitation to square-planar platinum(II) phosphors is their tendency toward aggregation-based quenching. Here, a new class of emissive, four-coordinate Pt^II species based on the bisimidazolyl carbazolide (BIMCA) ligand is introduced, which displays highly efficient, long-lived solid-state phosphorescence at room temperature. A set of four BIMCAPt phenyl acetylides were synthesized that emit in the green (λ_max =507-540 nm) with >60 % quantum yield and millisecond lifetimes. The structures of the resulting species reveal a non-planar structure imposed by steric clashes between BIMCA and the iodo or alkynyl co-ligand. Ground-state and photophysical characterization are presented. Density functional theory calculations indicate that the BIMCA ligand dominates the frontier orbitals along with the first Franck-Condon singlet and triplet excited states.

Quantification of energy transfer in bimetallic Pt(ii)-Ln(iii) complexes featuring an N^C^N-cyclometallating ligand

Cyclometallated Pt(ii) complexes with arylpolypyridyl ligands have impressive photophysical properties (high quantum yields, long lifetimes and tuneable emission) which can be readily tuned by modification of the organic ligand. Despite this, few examples of cyclometallated Pt(ii) complexes as sensitisers for Ln(iii) emission have been reported. Herein, we report the photophysical properties for a series of bimetallic complexes incorporating an N^C^N-coordinated Pt(ii) bearing an alkynyl terpyridine as a metalloligand for a Ln(iii) ion (where Ln = Nd, Gd, Er, Yb and Lu). Using a combination of steady state, time-resolved, and transient absorption experiments, the influence on the photophysical properties of the metalloligand exerted by the different Ln(iii) cations has been investigated, together with the energy transfer efficiency from the metalloligand to the Ln(iii) 4f* excited state.

Cyclometallated tridentate platinum(ii) arylacetylide complexes: old wine in new bottles

Platinum(ii) cyclometallated pincer complexes with an alkynyl ligand in the fourth coordination site display excellent luminescent properties. By manipulation of the pincer and the alkynyl ligand their luminescence can be fine-tuned for opto-electronic applications.

A family of readily synthesised phosphorescent platinum(ii) complexes based on tridentate N^N^O-coordinating Schiff-base ligands

Tridentate ligands, easily synthesised by condensation reactions of simple starting materials, can be used to prepare Pt(ii) complexes that are luminescent in solution, emitting in the red or deep-red spectral region, according to the substituents.

Diruthenium(ii)-capped oligothienylethynyl bridged highly soluble organometallic wires exhibiting long-range electronic coupling

Organometallic molecular wires with π-conjugation along their molecular backbones are of considerable interest for application in molecular-scale electronics. In this regard, thienylethynyl-based π-conjugated oligomers of three, five and seven thienylethynyl units with -C[triple bond, length as m-dash]C-H termini have been successfully synthesized through stepwise Pd(0)/Cu(i)-catalyzed Sonogashira coupling. The corresponding highly soluble diruthenium(ii) diacetylide complexes (O1-Ru2, O3-Ru2, O5-Ru2 and O7-Ru2, respectively) have been prepared by the reaction of cis-Ru(dppe)2Cl2 and NaPF6 in DCM with the corresponding rigid rod-like thienylethynyl oligomers with one, three, five and seven thienylethynyl π-conjugated segments containing alkynyl termini (O1, O3, O5 and O7). These Ru(ii)-Cl capped diacetylide complexes have been further functionalized by incorporating a phenylacetynyl moiety to afford [Ru(ii)-C[triple bond, length as m-dash]C-Ph]-capped diacetylide organometallic wires (O1-Ru2-Ph, O3-Ru2-Ph, O5-Ru2-Ph and O7-Ru2-Ph). The photophysical properties of the highly soluble thienylethynyl-based oligomers and Ru(ii)-organometallic wires have been explored to understand their electronic properties. Electrochemical studies of the binuclear ruthenium(ii)-alkynyl complexes showed highly interesting results, revealing long-range electrochemical communication between the two remote Ru(ii) termini connected even with five and seven thienylethynyl units. DFT computational studies further support the long range electrochemical communication between the redox active metal termini through heavy participation of the thienylethynyl bridge in the corresponding mono-oxidized mixed valence species of the organometallic wire-like complexes.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

Controlling the emission in flexibly-linked (N^C^N)platinum dyads

Comparative luminescence studies of the mononuclear complex Pt-1 and the differently linked dinuclear complexes Pt-2 and Pt-3 reveal distinctive luminescent properties. The emission of Pt-2 can be readily modulated over a wide range of the visible part of the spectrum.

How do ligands influence the quantum yields of cyclometalated platinum(ii) complexes, a theoretical research study

Quantum yield differences were explained by the cyclometalated ligand, molecular rigidity and ligand-field strength.

Platinum and Gold Complexes for OLEDs

Encouraging efforts on the design of high-performance organic materials and smart architecture during the past two decades have made organic light-emitting device (OLED) technology an important competitor for the existing liquid crystal displays. Particularly, the development of phosphorescent materials based on transition metals plays a crucial role for this success. Apart from the extensively studied iridium(III) complexes with d(6) electronic configuration and octahedral geometry, the coordination-unsaturated nature of d(8) transition metal complexes with square-planar structures has been found to provide intriguing spectroscopic and luminescence properties. This article briefly summarizes the development of d(8) platinum(II) and gold(III) complexes and their application studies in the fabrication of phosphorescent OLEDs. An in-depth understanding of the nature of the excited states has offered a great opportunity to fine-tune the emission colors covering the entire visible spectrum as well as to improve their photophysical properties. With good device engineering, high performance vacuum-deposited OLEDs with external quantum efficiencies (EQEs) of up to 30 % and solution-processable OLEDs with EQEs of up to 10 % have been realized by modifying the cyclometalated or pincer ligands of these metal complexes. These impressive demonstrations reveal that d(8) metal complexes are promising candidates as phosphorescent materials for OLED applications in displays as well as in solid-state lighting in the future.

Copper(I) Complexes for Thermally Activated Delayed Fluorescence: From Photophysical to Device Properties

Molecules that exhibit thermally activated delayed fluorescence (TADF) represent a very promising emitter class for application in electroluminescent devices since all electrically generated excitons can be transferred into light according to the singlet harvesting mechanism. Cu(I) compounds are an important class of TADF emitters. In this contribution, we want to give a deeper insight into the photophysical properties of this material class and demonstrate how the emission properties depend on molecular and host rigidity. Moreover, we show that with molecular optimization a significant improvement of selected emission properties can be achieved. From the discussed materials, we select one specific dinuclear complex, for which the two Cu(I) centers are four-fold bridged to fabricate an organic light emitting diode (OLED). This device shows the highest efficiency (of 23 % external quantum efficiency) reported so far for OLEDs based on Cu(I) emitters.

C^C* cyclometalated platinum(II) N-heterocyclic carbene complexes with a sterically demanding β-diketonato ligand – synthesis, characterization and photophysical properties

Neutral cyclometalated platinum(ii) N-heterocyclic carbene complexes [Pt(C^C*)(O^O)] with C^C* ligands based on 1-phenyl-1,2,4-triazol-5-ylidene and 4-phenyl-1,2,4-triazol-5-ylidene, as well as acetylacetonato (O^O = acac) and 1,3-bis(2,4,6-trimethylphenyl)propan-1,3-dionato (O^O = mesacac) ancillary ligands were synthesized and characterized. All complexes are emissive at room temperature in a poly(methyl methacrylate) (PMMA) matrix with emission maxima in the blue region of the spectrum. High quantum efficiencies and short decay times were observed for all complexes with mesacac ancillary ligands. The sterically demanding mesityl groups of the mesacac ligand effectively prevent molecular stacking. The emission behavior of these emitters is in general independent of the position of the nitrogen in the backbone of the N-heterocyclic carbene (NHC) unit and a variety of substituents in 4-position of the phenyl unit, meta to the cyclometalating bond.

Luminescent zinc(ii) and copper(i) complexes for high-performance solution-processed monochromic and white organic light-emitting devices

The synthesis and spectroscopic properties of luminescent tetranuclear zinc(ii) complexes of substituted 7-azaindoles and a series of luminescent copper(i) complexes containing 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate ligand are described. These complexes are stable towards air and moisture. Thin film samples of the luminescent copper(i) complexes in 2,6-dicarbazolo-1,5-pyridine and zinc(ii) complexes in poly(methyl methacrylate) showed emission quantum yields of up to 0.60 (for Cu-3) and 0.96 (for Zn-1), respectively. Their photophysical properties were examined by ultrafast time-resolved emission spectroscopy, temperature dependent emission lifetime measurements and density functional theory calculations. Monochromic blue and orange solution-processed OLEDs with these Zn(ii) and Cu(i) complexes as light-emitting dopants have been fabricated, respectively. Maximum external quantum efficiency (EQE) of 5.55% and Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.19) were accomplished with the optimized Zn-1-OLED while these values were, respectively 15.64% and (0.48, 0.51) for the optimized Cu-3-OLED. Solution-processed white OLEDs having maximum EQE of 6.88%, CIE coordinates of (0.42, 0.44), and colour rendering index of 81 were fabricated by using these luminescent Zn(ii) and Cu(i) complexes as blue and orange light-emitting dopant materials, respectively.