Deep Blue Phosphorescence from Platinum Complexes Featuring Cyclometalated N-Pyridyl Carbazole Ligands with Monocarborane Clusters (CB₁₁H₁₂^–)

Abstract

The utilization of deep blue phosphorescent materials in high-performance displays and solid-state lighting requires high quantum efficiencies and color purities. Here, we describe the preparation and luminescent properties of novel platinum triplet emitters featuring cyclometalated N-pyridyl-carbazole ligands functionalized with closo-monocarborane clusters [CB 11 H 12 ] − . All reported complexes were fully characterized by using standard small molecule techniques (UV−vis, cyclic voltammetry, nuclear magnetic resonance (NMR), high-resolution mass spectrometry (HRMS)), and their solid-state structures were elucidated by X-ray diffraction. These platinum phosphors emit in the blue region of the visible wavelength spectrum in both the solid and solution states. Complex 4a exhibits the highest luminous efficiency at λ em = 439 nm with a photoluminescent quantum yield (PLQY) of 60% by dispersing in a PMMA matrix. Electrochemical and computational studies of complexes 4a and 4b revealed that the blue phosphorescence originates mainly from intraligand 3 π → π* ( 3 ILCT) transitions with relatively small 3 MLCT mixing. A deep-blue OLED containing 4a as the light-emitting dopant was successfully fabricated using a solution-processed method, and the device exhibited blue photoluminescence with CIE coordinates of (0.17, 0.15) and a maximum external quantum efficiency (EQE max ) value of 6.2%. This article represents the pioneering study of a deep blue PhOLED using a Pt complex bearing a closo-monocarborane anion substituent, providing a new avenue into the preparation of novel triplet emitters based on boron-rich cluster anions.

Platinum(ii) diimine complexes containing phenylpyridine ligands decorated with anioniccloso-monocarborane clusters [CB11H12]−

“…In another project, the monocarborane clusters combined with the N-pyridyl carbazole ligand improved the bis-NHC Pt(II) complexes' emission quantum yield to 60% at the blue region of 439 nm. 35 Inspired by these results, we concluded that the three-dimensional monocarborane clusters can surely balance the complexes' net charge, and also can improve the triplet emitters' photophysical properties by perturbing the electronic energy levels in the excited states.…”

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

“…S4, ESI †) supported the 3 LLCT and 3 MLCT nature of their phosphorescence, which will be discussed in the next paragraph. The radiative rate constants (k r ) and nonradiative rate constants (k nr ) were calculated from the quantum yield and lifetime using the following mathematical equations: 13,27 k r = F PL /t…”

“…1). 22 The first type is Pt with four a Key Laboratory of Chemistry and Engineering of Forest Products, State Ethnic monodentate ligands, 23 the second type is Pt with one bidentate ligand and two monodentate ligands, 24,25 the third group is Pt with two bidentate ligands, 26,27 the fourth group is Pt with one tridentate ligand and one monodentate ligand, [28][29][30][31][32][33][34] and the last case is Pt embraced with one tetradentate ligand. 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. Additionally, acetylacetonate and its derivatives are another group of good bidentate ligands that can also form six-membered ring Pt complexes and further improve the photophysical properties of Pt emitters.…”

Synthesis of phosphorescent syn, anti-isomeric clamshell platinum(ii) dimers for OLED applications

“…14,15,23 Hence, higher overall internal quantum efficiency can be achieved. Meanwhile, Pt(II) normally adopts a square planar coordination geometry, 24 leaving two vacant sites in the perpendicular z-axis. Therefore, it is possible to form intermolecular p-p stacking and Pt-Pt coupling in the solid state.…”