Cyclometalated NHCs Pt(II) Compounds with Chelating P^P and S^S Ligands: From Blue to White Luminescence

Abstract

Ionic [Pt(C^C* A/B )(P^P)]PF 6 and neutral [Pt(C^C* A/B )(S^S)] complexes were designed and synthesized containing cyclometalated N-heterocyclic carbenes (C^C* A/B ) and diphosphines (P^P: dpfppe, dcypm) or dithiocarbamates (S^S: dmdtc, pdtc) as chelating ligands. Their structural and spectroscopic properties were investigated and found to be dependent on both cyclometalated (C^C* A/B ) and ancillary (P^P, S^S) ligands. The photophysical and computational studies for the [Pt(C^C* A/B )(P^P)] + complexes disclose the nature of the low-lying electronic transitions to be mainly intraligand charge transfer [ILCT (C^C* A/B )] with some contribution of ligand-to-ligand charge transfer (LL'CT) or ligand-to metal charge transfer (LMCT) for the dpfppe or dcypm derivatives, respectively. The blue and cyan emissions of PMMA films doped with the [Pt(C^C* A/B )(P^P)] + complexes exhibited very high quantum yields (QYs) reaching up to ∼90%. However, the low-energy absorptions and emissions of the [Pt(C^C* A/B )(S^S)] complexes in solution (rt or 77 K) arise from mixed ILCT [C^C* A/B ]/MLCT [dπ(Pt) → π*(C^C* A/B )] excited states, showing no change with the different S^S ligands. In solid-state and in doped films, these dithiocarbamate complexes, excluding 8B, display dual emissions with the high-energy vibronic band ( 3 ILCT / 3 MLCT) appearing together with an additional low-energy structureless band. The latter is attributed to 3 ππ* transitions originated from π-stacked aggregates, as reported in the X-ray structure of 7A. Thus, white light emissions can be obtained with photo-and colorimetric values lying inside the stipulated limits for general lighting applications; yet, they display low QY.

Phosphorescent Tris-cyclometalated Pt(IV) Complexes with Mesoionic N-Heterocyclic Carbene and 2-Arylpyridine Ligands

“…Bidentate cyclometalated aryl-substituted NHC ligands (aryl-NHCs, C∧C*) have been extraordinarily successful with the Ir­(III) − and Pt­(II) − ions as a replacement of cyclometalated 2-arylpyridines (C∧N), enabling better photostabilities, wider color tunability, and higher emission efficiencies. These enhancements are brought about by the larger ligand-field splitting induced by the NHC moiety with respect to the pyridine and, consequently, the reduced thermal accessibility of MC states from the triplet, mixed ligand-centered/metal-to-ligand charge-transfer ( 3 LC/MLCT) emissive state of Ir­(III) and Pt­(II) complexes.…”

Strongly Luminescent Pt(IV) Complexes with a Mesoionic N-Heterocyclic Carbene Ligand: Tuning Their Photophysical Properties

“…N -Heterocyclic carbenes (NHCs) have emerged as very valuable ligands for the design of highly efficient luminescent complexes of late-transition-metal ions, such as Ir­(III), ,,,− Pt­(II), ,,,− or Au­(III) . The beneficial effects exerted by these ligands can be attributed to their exceptional σ-donor capabilities, , which lead to strong ligand–field splittings and an increased energy of nonemissive excited states that arise from electronic transitions to dσ* orbitals, which could otherwise become thermally populated and cause nonradiative deactivation or even degradation via ligand–metal σ-bond labilization.…”

“…Consequently, the use of NHCs brings about improved stabilities and emission efficiencies, which are particularly important for the development of blue emitters. Diverse types of NHC ligands have been used to synthesize luminescent complexes, including chelating dicarbenes (C* ∧ C*), − pyridyl-NHCs (N ∧ C*), , and cyclometalated aryl-NHCs (C ∧ C*). ,, Most incorporate normal Arduengo-type NHC moieties, whereas the use of mesoionic NHCs is rather infrequent. ,,− Although cyclometalating aryl-NHCs have been demonstrated as chromophoric ligands in homoleptic Ir­(III) emitters, − mixed-ligand systems have also been developed in which they act as supporting ligands while other chelating heteroaromatic ligands, such as arylpyridines , or bipyridines, are responsible for the emission.…”

A Carbazole-Bridged Biscyclometalated Diplatinum Complex: Synthesis, Characterization, and Dual-Mode Aggregation-Enhanced Phosphorescence

“…Square planar d 8 platinum complexes have recently drawn considerable interest because of their intriguing light-emitting and assembly properties. − Platinum complexes often display phosphorescent emissions due to the strong spin–orbit coupling of the platinum atom. By tuning the ligand structure and coordination mode, full-color emissive platinum complexes have been synthesized and investigated. − The assembly properties of platinum complexes are mostly associated with their tendency to form π–π and Pt–Pt interactions upon aggregation. − In this context, a great number of platinum complexes have been reported to display appealing assembly behavior with tunable emission properties. − …”