Symmetry-Breaking Triplet Excited State Enhances Red Afterglow Enabling Ubiquitous Afterglow Readout

Molecular vibrations are often factors that deactivate luminescence. However, if there are molecular motion elements that enhance luminescence, it may be possible to utilize molecular movement as a design guideline to enhance luminescence. Here, the authors report a large contribution of symmetry-breaking molecular motion that enhances red persistent room-temperature phosphorescence (RTP) in donor-π-donor conjugated chromophores. The deuterated form of the donor-π-donor chromophore exhibits efficient red persistent RTP with a yield of 21% and a lifetime of 1.6 s. A dynamic calculation of the phosphorescence rate constant (kp) indicates that the symmetry-breaking movement has a crucial role in selectively facilitating kp without increasing nonradiative transition from the lowest triplet excited state. Molecules exhibiting efficient red persistent RTP enable long-wavelength excitation, indicating the suitability of observing afterglow readout in a bright indoor environment with a white-light-emitting diode flashlight, greatly expanding the range of anti-counterfeiting applications that use afterglow.

Effects of Ancillary Ligands on Deep Red to Near-Infrared Cyclometalated Iridium Complexes

The design of organometallic compounds with efficient phosphorescence in the deep red to near-infrared portions of the spectrum is a long-standing fundamental challenge. Here we describe a series of heteroleptic bis-cyclometalated iridium complexes with phosphorescence in these low-energy regions of the spectrum. The cyclometalating ligands in this study feature a metalated benzothiophene aryl group substituted with a quinoline, isoquinoline, or phenanthridine heterocycle. Increasing the conjugation on the heterocycle stabilizes the ligand-centered LUMO, decreases the HOMO-LUMO gap, and enables phosphorescence to occur at long wavelengths. These cyclometalating ligands are paired with a variety of electron-rich ancillary ligands, such as dithiocarbamate (dipdtc), β-ketoiminate (acNac), β-diketiminate (NacNac), amidinate (dipba), and hexahydropyrimidopyrimidine (hpp), some of which have significant influences on the phosphorescence wavelength and excited-state dynamics. The syntheses of seven compounds in this series are described, three of which are structurally validated by single-crystal X-ray diffraction. Cyclic voltammetry reveals the effects of ligand modification on the frontier orbital energies. The photophysical properties of all compounds are thoroughly characterized by UV-vis absorption spectroscopy and steady-state photoluminescence at room-temperature and 77 K. Photoluminescence quantum yields and lifetimes of all compounds are reported.

Spirobifluorene Core-Based Novel Hole Transporting Materials for Red Phosphorescence OLEDs

Two new hole transporting materials, named HTM 1A and HTM 1B, were designed and synthesized in significant yields using the well-known Buchwald Hartwig and Suzuki cross- coupling reactions. Both materials showed higher decomposition temperatures (over 450 °C) at 5% weight reduction and HTM 1B exhibited a higher glass transition temperature of 180 °C. Red phosphorescence-based OLED devices were fabricated to analyze the device performances compared to Spiro-NPB and NPB as reference hole transporting materials. Devices consist of hole transporting material as HTM 1B showed better maximum current and power efficiencies of 16.16 cd/A and 11.17 lm/W, at the same time it revealed an improved external quantum efficiency of 13.64%. This efficiency is considerably higher than that of Spiro-NPB and NPB-based reference devices.