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.

Field-Induced Single-Ion Magnet Behavior in Nickel(II) Complexes with Functionalized 2,2':6'-2″-Terpyridine Derivatives: Preparation and Magneto-Structural Study

Two mononuclear nickel(II) complexes of the formula [Ni(terpyCOOH)₂](ClO₄)₂∙4H₂O (1) and [Ni(terpyepy)₂](ClO₄)₂ MeOH (2) [terpyCOOH = 4'-carboxyl-2,2':6',2″-terpyridine and terpyepy = 4'-[(2-pyridin-4-yl)ethynyl]-2,2':6',2″-terpyridine] have been prepared and their structures determined by single-crystal X-ray diffraction. Complexes 1 and 2 are mononuclear compounds, where the nickel(II) ions are six-coordinate by the six nitrogen atoms from two tridentate terpy moieties. The mean values of the equatorial Ni-N bond distances [2.11(1) and 2.12(1) Å for Ni(1) at 1 and 2, respectively, are somewhat longer than the axial ones [2.008(6) and 2.003(6) Å (1)/2.000(1) and 1.999(1) Å (2)]. The values of the shortest intermolecular nickel-nickel separation are 9.422(1) (1) and 8.901(1) Å (2). Variable-temperature (1.9-200 K) direct current (dc) magnetic susceptibility measurements on polycrystalline samples of 1 and 2 reveal a Curie law behavior in the high-temperature range, which corresponds to magnetically isolated spin triplets, the downturn of the χ_MT product at lower temperatures being due to zero-field splitting effects (D). Values of D equal to -6.0 (1) and -4.7 cm^-1 (2) were obtained through the joint analysis of the magnetic susceptibility data and the field dependence of the magnetization. These results from magnetometry were supported by theoretical calculations. Alternating current (ac) magnetic susceptibility measurements of 1 and 2 in the temperature range 2.0-5.5 K show the occurrence of incipient out-phase signals under applied dc fields, a phenomenon that is characteristic of field-induced Single-Molecule Magnet (SMM) behavior, which herein concerns the 2 mononuclear nickel(II) complexes. This slow relaxation of the magnetization in 1 and 2 has its origin in the axial compression of the octahedral surrounding at their nickel(II) ions that leads to negative values of D. A combination of an Orbach and a direct mechanism accounts for the field-dependent relation phenomena in 1 and 2.

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.

Investigation of second-order nonlinear optical responses in a series of V-shaped binuclear platinum(ii) complexes

A series of four D-(π-Pt-π-A)₂ new V-shaped binuclear platinum(ii) complexes bearing a diphenylpyranylidene ligand as a pro-aromatic donor group (D) and various electron-attracting groups (A) separated by platinum bis-acetylide fragments have been synthesized, characterized and studied for their electrochemical, photophysical and second-order nonlinear optical (NLO) properties. The nonlinear optical properties of these complexes have been determined by using the Electric-Field-Induced Second Harmonic (EFISH) generation technique, and their optical properties have been rationalized by Time-Dependent Density Functional Theory (TD-DFT) calculations relying on a range-separated hybrid. All complexes display positive μβ₀ values. In addition, the second-order NLO responses of the complexes could be easily modulated by incorporating various end-capped electron-attracting groups, namely malononitrile, indane-1,3-dione, pyrimidine and pyrimidinium. Remarkably, complex 7 bearing a pyrimidinium fragment displays the highest μβ₀ value among all the complexes of this series. Its NLO response is twice as high as that of the mononuclear analogue complex RD2, which has been confirmed both experimentally and theoretically.

Computational Analysis of Mesomerism in para-Substituted mer-NCN Pincer Platinum(II) Complexes, Including its Relationships with Hammett σp Substituent Parameters

Density Functional Theory studies of square-planar Pt^II pincer structures, (4-Z-NCN)PtCl ([4-Z-NCN]^- =[4-Z-2,6-(Me₂ NCH₂ )₂ C₆ H₂ -N,C,N]^- , Z=H, NO₂ , CF₃ , CO₂ H, CHO, Cl, Br, I, F, SMe, SiMe₃ , tBu, OH, NH₂ , NMe₂ ), enable characterisation of mesomerism for the pincer-Pt interaction. Relationships between Hammett σ_p substituent parameters of Z and DFT data obtained from NBO6 and AOMix computation are used to probe the interaction of the 5d_yz orbital of platinum with π-orbitals of the arene ring. Analogous computation for 2,6-(Me₂ CH₂ )₂ C₆ H₃ Z (Z=H, CF₃ , CHO, Cl, Br, I, F, SMe, SiMe₃ , tBu, OH, NH₂ ) and (4-H-NCN)PtZ allows an estimation of the relative substituent effects of "(CH₂ NMe₂ )₂ PtZ" on π-delocalisation in the pincer system.

Deeply coloured and highly fluorescent dipolar merocyanines based on tricyanofuran

TCF as the acceptor group of merocyanines, having its own extended π-system, gives access to deeply coloured dyes exhibiting good fluorescence efficiency in the red and near-IR spectral range.

Novel cyclometallated 5-π-delocalized donor-1,3-di(2-pyridyl)benzene platinum(ii) complexes with good second-order nonlinear optical properties

For platinum complexes bearing a cyclometallated 5-thiophene-1,3-di(2-pyridyl)benzene, β_EFISH increases following the order of the LUMO stabilization.

An investigation on the second-order nonlinear optical response of cationic bipyridine or phenanthroline iridium(iii) complexes bearing cyclometallated 2-phenylpyridines with a triphenylamine substituent

The synthesis and characterisation of six new cationic iridium(iii) complexes bearing either 4,4'-di-tert-butyl-2,2'-bipyridine (dtBubpy) or 5-NO2-1,10-phenanthroline along with two cyclometallated 2-phenylpyridine derivative ligands, decorated with triphenylamine groups either meta or para to the Ir-CC^N bond or para to the Ir-NC^N bond, are reported. The second-order nonlinear optical (NLO) properties of all the compounds have been determined by the electric field induced second harmonic generation technique and show that the μβEFISH value can be tuned by the nature of the iridium coordination sphere. The dipole moment, μ, necessary to evaluate the quadratic hyperpolarizability βEFISH, was theoretically determined. The linear optical properties of the complexes are also presented and rationalised by quantum-chemical calculations. One of the prepared iridium compounds was incorporated into a polystyrene film, affording the first example of a second-order NLO active polymeric film based on a cationic organometallic complex.

Tuning the dipolar second-order nonlinear optical properties of 5-π-delocalized-donor-1,3-di(2-pyridyl)benzenes, related cyclometallated platinum(ii) complexes and methylated salts

Increase of the μβ_EFISH absolute value by methylation or cyclometallation of novel substituted dipyridylbenzenes.

Synthesis, photovoltaic performances and TD-DFT modeling of push-pull diacetylide platinum complexes in TiO2 based dye-sensitized solar cells

In this joint experimental-theoretical work, we present the synthesis and optical and electrochemical characterization of five new bis-acetylide platinum complex dyes end capped with diphenylpyranylidene moieties, as well as their performances in dye-sensitized solar cells (DSCs). Theoretical calculations relying on Time-Dependent Density Functional Theory (TD-DFT) and a range-separated hybrid show a very good match with experimental data and allow us to quantify the charge-transfer character of each compound. The photoconversion efficiency obtained reaches 4.7% for 8e (see TOC Graphic) with the tri-thiophene segment, which is among the highest efficiencies reported for platinum complexes in DSCs.

Second-order NLO switches from molecules to polymer films based on photochromic cyclometalated platinum(II) complexes

Novel photochromic dithienylethene-based platinum(II) complexes (C^N^N)Pt(C≡C-DTE-C6H4-D) ((C^N^N) = 4,4'-di(n-hexyl)-6-phenyl-2,2'-bipyridine; D = H, NMe2) were prepared and characterized. Their excellent photochromic properties allow the photoinduced switching of their second-order nonlinear optical properties in solution, as measured by the EFISH technique, due to formation of an extended π-conjugated ligand upon suitable electromagnetic radiation. Insights into the electronic structures of the complexes and the nature of their excited states have been obtained by DFT and TD-DFT calculations. These novel Pt(II) complexes were nanoorganized in polymer films which were poled, affording new materials characterized by a good second-order NLO response that can be easily switched, with an excellent NLO contrast. To the best of our knowledge, our compounds allowed designing the very first examples of switchable NLO polymer films based on metal complexes.