Synthesis and Photophysical Properties of Monometallic C^C* Platinum(II) Formamidinate Complexes using Sterically Demanding Ligands

Abstract

A novel class of cyclometalated platinum(II) complexes-previously considered to be inaccessiblewas synthesized by an improved synthetic route utilizing ligands predicted by density functional theory calculations. Based on a concise quantum chemical screening three model ligands with varying steric demand were chosen and a series of six photoluminescent C^C* cyclometalated platinum(II) formamidinate complexes was obtained. The least sterically demanding ligand led to a bimetallic complex in two isomeric forms, which could be separated and confirmed by the corresponding solid-state structures. Sterically more hindered amidinate ligands gave the monometallic complexes supporting the theoretical predictions. The monometallic complexes show a significant hypsochromic shift of the emission wavelength, explained by the loss of the metalmetal interactions. Depending on the cyclometalating ligand quantum yields up to 87 % with short decay times were found for this new class of phosphorescent greenblue to pure blue platinum(II) emitters.

C^C* Platinum(II) Complexes with Bis(pyridyl)borate Ligands: Synthesis, Crystal Structures, and Properties

“…We as well as other research groups have established a class of such ligands in emissive transition metal complexes in the past decade, namely cyclometalated N -heterocyclic carbene (C^C*) ligands. ,− As derivatives of the more common 2-phenylpyridines (C^N), the C^C* ligands help to facilitate more energy-rich emissions due to the stronger donor effect of the carbene motif. In addition to the extensive variation of the cyclometalated NHC ligands, − we investigated the influences of different auxiliary ligands on the emission properties of emissive platinum(II) complexes in several studies. − More recently, we successfully used bis(pyrazolyl)borate ligands as auxiliary ligands, resulting in excellent phosphorescent platinum emitters. , To further explore the applicability of borate ligands in photophysically active complexes, we chose to use bis(pyridyl)borate ligands. Transition metal complexes with bis(pyridyl)borates − and tris(pyridyl)borates − have been known for more than 20 years.…”

Phosphorescent Pt(ii) acetylacetonate complexes bearing 9-(pyrimidin-2-yl)-9H-carbazole ligand: syntheses, photophysical properties and OLED applications

“…9,35,36 Among these five coordination pattern types, Pt complexes with four monodentate ligands and those bearing one tridentate and one monodentate ligand are often used for chemical probing, 24,[37][38][39] while the remaining three groups are explored for the OLED applications because of their better thermal stability and photophysical properties. Phenylpyridine [40][41][42][43][44][45] and N-phenylimidazolium 7,[46][47][48][49][50][51][52][53][54] are two groups of bidentate ligands regularly used to prepare five-membered ring cyclometalated Pt triplet emitters because of their strong ligand field strength, which can push the nonradiative d-d transition to a higher energy level, therefore facilitating metal-to-ligand charge transfer (MLCT). Six-membered ring cyclometalated Pt emitters with C^N ligands have rarely been reported; 8,9,27 9-(pyrimidin-2-yl)-9H-carbazole, which is a carbazole modified with a pyrimidine, is a good bidentate ligand that can generate binuclear cyclometalated Pt complexes with the formation of two sixmembered rings.…”