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

Abstract

A series of seven new platinum(II) complexes PtL n Cl have been prepared, where L n 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 HL n 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, PtL nsymCl, 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 PtL nsymCl 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.

Perimidocarbene-Based Tetradentate Platinum(II) Complexes with an Unexpectedly Negligible 3MLCT Character

“…The results of these analyses were in good agreement with the broad emission spectra, the almost unchanged variable-temperature PL spectra, the absence of significant solvatochromism, and the low quantum efficiencies of PtYK1 and PtYK2. In addition, many other Pt­(II) complexes reported in the literature showed a short vibrational progression involving ligand-centered characters, as typically observed for the tridentate Pt­(II) complexes with large π-conjugated ligands developed by the Williams, and Yersin groups. − These results also reveal that the NHC carbene moiety plays a critical role in their photophysical properties, and the emission spectra of the Pt­(II) complexes are strongly dependent on the excited-state properties and ligand structures.…”

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

“…Such effects have been seen in a range of platinum complexes and metallo-gels. 19 This points to ground state aggregation even at micromolar concentration at low temperature (before freezing the matrix). However, this was notably absent in the case of dimer 3 , which may in part be due to the less planar adopted conformation which is less conducive to stacking.…”

Luminescent Platinum(II) Complexes with Terdentate N∧C∧C Ligands

“…An aspect of great importance to the development of Pt­(II) emitters is the possibility to modulate their luminescence through the formation of aggregates or molecular assemblies, ,,− ,− which can occur via π interactions between the aromatic systems of the ligands and/or metallophilic Pt···Pt contacts, that usually fall in the range 3.0–3.5 Å. , These interactions, either separately or in combination, can cause significant modifications of frontier orbitals or favor the formation of supramolecular entities, such as aggregates or excimers, which usually produce emissions that are red-shifted with respect to those from the monomeric complexes. Several environmental factors can affect the formation of these entities, such as the solvent or solid matrix, the concentration, and the presence of other molecules that may interact with the complex.…”