Approaching the Shortest Intermetallic Distance of Half‐Lantern Diplatinum(II) Complexes for Efficient and Stable Deep‐Red Organic Light‐Emitting Diodes

Abstract

A fast radiative decay process for long-wavelength molecular light-emitters is vital to achieving a high emission efficiency by outcompeting the nonradiative decay imposed by the energy-gap law. An ensuing short emission lifetime is also beneficial for fabricating high-performance organic light-emitting diodes. Herein a series of half-lantern dinuclear platinum(II) complexes is reported, which shows high-efficiency deep red phosphorescence (𝝀 em > 660 nm). The molecules are designed to have a cofacially aligned structure featuring short Pt-Pt distances of 2.80-2.83 Å by using 10H-pyrido[3,2-b][1,4]benzoxazine (PyXZ) as the rigid bridges, which are revealed by single crystal X-ray diffraction analysis. Together with the strong electron-donating property of PyXZ bridges, the metallophilic interaction endows low-energy triplet excited states with mixed metal-metal-to-ligand charge-transfer ( 3 MMLCT) and ligand-to-ligand charge-transfer ( 3 LLCT) characters. The deep red devices based on the diplatinum(II) complexes show maximum external quantum efficiencies (EQEs) up to 21.8%. The EQE of 19.4% and operational lifetime (LT 85 ) of 334 h (the operational time after which the luminance drops to 85% of the initial value) at a luminance of 1000 cd m −2 promise the pratical use of these complexes.

NIR-II Emission from Cyclometalated Dinuclear Pt(III) Complexes

“…Thus, they could be measured in different solvents (10 −4 M), showing a small negative solvatochromism, which is characteristic of charge transfer transitions (Figure S14 [dσ*(Pt−Pt) → π*(C ∧ N)]. [18][19][20][21][22][23][24][25][26][27][28][29][30]32,33,35,36 Those for metal− metal bond Pt 2 (III,III) derivatives are originated from an admixture of axial ligand (X) to metal−metal charge transfer, ( 1 XMMCT) and 1 MC [dσ−dσ*] transitions 47,61 or from 1 LMMCT [π(S ∧ N) → dσ*(Pt−Pt)] transitions. 44 To help with the UV−vis assignments, DFT and TD-DFT calculations were performed on 1, 2, 1-Cl, and 1-I (see Tables S2, S3 and Figures 4, 5, S15).…”

“…In this regard, half-lantern platinum complexes with C ∧ N cyclometalated ligands and strong intermetallic interactions ( d Pt–Pt ≤ 3 Å) are some of the most representative cases of 3 MMLCT [dσ*­(Pt) 2 → π*­(C ∧ N)] emitters. Among them, mercapto- − or hydroxy- ,, substituted N-heterocycles have been typically used as 4-bond bridging ligands to prepare compounds with bright red emissions that, in some cases, have been successfully implemented in organic light-emitting diodes. − , Recently, α-carboline or 10 H -pyrido­[3,2- b ]­[1,4]­benzoxazine has been used instead so as to push further the emission toward the deep red or NIR spectral region because the use of these rigid bridges confine close Pt···Pt contacts. Besides, comprehensive studies on these and also on related complexes have shown that changes on the chromophoric C ∧ N ligands can push the emission to red as well.…”

“…By these means, not only the lowest unoccupied molecular orbital (LUMO) energy level (π*­(C ∧ N)) is modified but also that of the highest occupied molecular orbital (HOMO) (dσ*­(Pt) 2 ). The π-backdonation from the Pt center to the C ∧ N aromatic system provokes a shortening of the Pt center distance, which leads to a reduced H–L energy gap and consequently a red-shifted emission. ,,− …”

Efficient radical-based near-infrared organic light-emitting diodes with an emission peak exceeding 800 nm

“…Triplet excitons are generally dark due to the spin-forbidden transition. 6,7 Different technologies have been exploited to harvest triplet excitons, such as phosphorescent, 8–14 thermally activated delayed fluorescence (TADF) 5,15–21 and hybridized local charge transfer (HLCT) emission. 22–27 Unlike closed-shell emitters, mono-radicals possess a single unpaired electron.…”

Rigidly linked dinuclear platinum(ii) complexes showing intense, excimer-like, near-infrared luminescence

“…32–36 Excimer-forming Pt( ii ) complexes are attracting more and more interest as potential deep-red or NIR emitters for OLEDs. 21,37–45…”