Highly Efficient Blue Phosphorescence from Pillar-Layer MOFs by Ligand Functionalization

Room temperature phosphorescence (RTP) has been extensively researched in heavy-metal containing complexes and purely organic systems. Despite the rapid blossom of RTP materials, it is still a tremendous challenge to develop highly efficient blue RTP materials with long-lived lifetimes. Taking the metal-organic framework (MOF) as a model, herein, a feasible strategy of ligand functionalization is proposed, including two essential elements, to develop blue phosphorescence materials with high efficiency and long-lived lifetimes simultaneously under ambient conditions. One is isolation of the chromophores with assistance of another predefined co-ligands, the other is restriction of the chromophores' motions through coordination and host-guest interactions. Remarkably, it renders the MOFs with highly efficient blue phosphorescence up to 80.6% and a lifetime of 169.7 ms under ambient conditions. Moreover, a demo of the crown is fabricated with MOFs ink by 3D printing technique. The potential applications for anti-counterfeiting and fingerprint visualization have been also demonstrated. This finding not only outlines a universal principle to design and synthesize highly efficient RTP materials, but also endows traditional MOFs with fresh vitality for potential applications.

Efficient Deep Blue Platinum Acetylide Phosphors with Acyclic Diaminocarbene Ligands

Here we report five blue-phosphorescent platinum bis-phenylacetylide complexes with an investigation of their photophysical and electrochemical attributes. Three of the complexes (1-3) are of the general formula cis-Pt(CNR)₂ (C≡CPh)₂ , in which CNR is a variably substituted isocyanide and C≡CPh is phenylacetylide. These isocyanide complexes serve as precursors for complexes of the general formula cis-Pt(CNR)(ADC)(C≡CPh)₂ (4 and 5), in which ADC is an acyclic diaminocarbene installed by amine nucleophilic addition to one of the isocyanides. All of the complexes exhibit deep blue phosphorescence with λ_max ∼430 nm in poly(methyl methacrylate) (PMMA) thin films. Whereas isocyanide complexes 1-3 exhibit modest photoluminescence quantum yields (Φ_PL ), incorporation of one acyclic diaminocarbene ligand results in a three-fold to 16-fold increase in Φ_PL while still maintaining an identical deep blue color profile.

Multiresponsive Tetradentate Phosphorescent Metal Complexes as Highly Sensitive and Robust Luminescent Oxygen Sensors: Pd(II) Versus Pt(II) and 1,2,3-Triazolyl Versus 1,2,4-Triazolyl

Two Pd(II) complexes based on tetradentate chelate ligands with either a 1,2,4-triazolyl (Pd1) or 1,2,3-triazolyl (Pd2) unit were synthesized, and their structure-property relationships were studied. Both Pd1 and Pd2 are rare bright deep blue Pd(II) phosphors with contrasting properties. Pd1 displays stimuli-responsive luminescence in response to UV irradiation, concentration, or temperature change, which is ascribed to the facile switching of monomer to excimer emission. In contrast, a similar stimuli-responsive luminescence was not observed for Pd2. Crystal structures and time-dependent density functional theory computational studies established that the excimer formation of Pd1 is caused by electronically favored intermolecular π-π interactions and less steric protection of the Pd core because of the position of its alkyl chains, compared to Pd2. In solution, the excimer emission of Pd1 shows a much greater sensitivity toward oxygen than the monomer emission with a very large Stern-Volmer constant ( K_sv) that is more than twice that of the monomer emission. Both Pd(II) complexes are found to be outstanding oxygen sensors in ethyl cellulose films with superior sensitivity ( K_sv^app = 0.228-0.346 Torr^-1) over their Pt(II) equivalents ( K_sv^app = 0.00674-0.0110 Torr^-1), owing to their long phosphorescence decay lifetimes. Furthermore, Pd1 shows an excellent photostability, compared to the Pt(II) analogue, making it one of the best and highly robust oxygen sensors based on cyclometalated metal complexes.

Pyridine-Based Electron-Transport Materials with High Solubility, Excellent Film-Forming Ability, and Wettability for Inkjet-Printed OLEDs

Film morphology has predominant influence on the performance of multilayered organic light-emitting diodes (OLEDs), whereas there is little reported literature from the angle of the molecular level to investigate the impact on film-forming ability and device performance. In this work, four isomeric cross-linkable electron-transport materials constructed with pyridine, 1,2,4-triazole, and vinylbenzyl ether groups were developed for inkjet-printed OLEDs. Their lowest unoccupied molecular orbital (∼3.20 eV) and highest occupied molecular orbital (∼6.50 eV) levels are similar, which are mainly determined by the 1,2,4-triazole groups. The triplet energies of these compounds can be tuned from 2.51 to 2.82 eV by different coupling modes with the core of pyridine, where the 2,6-pyridine-based compound has the highest value of 2.82 eV. Film formation and solubility of the compounds were investigated. It was found that the 2,6-pyridine-based compound outperformed the 2,4-pyridine, 2,5-pyridine, and 3,5-pyridine-based compounds. The spin-coated blue OLEDs based on the four compounds have achieved over 14.0% external quantum efficiencies (EQEs) at the luminance of 100 cd m^-2, and a maximum EQE of 12.1% was obtained for the inkjet-printed device with 2,6-pyridine-based compound.

High Power Efficiency Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Using Exciplex-Type Host with a Turn-on Voltage Approaching the Theoretical Limit

Three solution-processable exciplex-type host materials were successfully designed and characterized by equal molar blending hole transporting molecules with a newly synthesized electron transporting material, which possesses high thermal stability and good film-forming ability through a spin-coating technique. The excited-state dynamics and the structure-property relationships were systematically investigated. By gradually deepening the highest occupied molecular orbital (HOMO) level of electron-donating components, the triplet energy of exciplex hosts were increased from 2.64 to 3.10 eV. Low temperature phosphorescence spectra demonstrated that the excessively high triplet energy of exciplex would induce a serious energy leakage from the complex state to the constituting molecule. Furthermore, the low energy electromer state, which only exists under the electroexcitation, was found as another possible channel for energy loss in exciplex-based phosphorescent organic light-emitting diodes (OLEDs). In particular, as quenching of the exciplex-state and the triplet exciton were largely eliminated, solution-processed blue phosphorescence OLEDs using the exciplex-type host achieved an extremely low turn-on voltage of 2.7 eV and record-high power efficiency of 22.5 lm W(-1), which were among the highest values in the devices with identical structure.

Low-driving-voltage blue phosphorescent organic light-emitting devices with external quantum efficiency of 30%

A homoleptic iridium (iii) tris(pheny-limidazolinate) complex realizes a high EQE of 30%, a low turn-on voltage of 2.5 V, and a small efficiency roll-off in a blue organic light-emitting device (OLED). This device also shows high power efficiencies over 75 lm W(-1) and an ideal light distribution pattern at 100 cd m(-2).

Using an organic molecule with low triplet energy as a host in a highly efficient blue electrophosphorescent device

To achieve high efficiencies in blue phosphorescent organic light-emitting diodes (PhOLEDs), the triplet energies (T1) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S1) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6-difluorophenylpyridinato-N,C(2'))iridium(III)] tetrakis(1-pyrazolyl)borate (FIr6; T1=2.73 eV). In both the photo- and electro-excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic-layer device, the maximum current efficiency of 37 cd A(-1) and power efficiency of 40 Lm W(-1) were achieved for the FIr6-based blue PhOLEDs.