Min-Jie Huang

A New Molecular Design Based on Thermally Activated Delayed Fluorescence for Highly Efficient Organic Light Emitting Diodes.

Two benzoylpyridine-carbazole based fluorescence materials DCBPy and DTCBPy, bearing two carbazolyl and 4-(t-butyl)carbazolyl groups, respectively, at the meta and ortho carbons of the benzoyl ring, were synthesized. These molecules show very small ΔEST of 0.03 and 0.04 eV and transient PL characteristics indicating that they are thermally activated delayed fluorescence (TADF) materials. In addition, they show extremely different photoluminescent quantum yields in solution and in the solid state: in cyclohexane the value are 14 and 36%, but in the thin films, the value increase to 88.0 and 91.4%, respectively. The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light with EQEs of 24.0 and 27.2%, respectively, and with low efficiency roll-off at practical brightness level. The crystal structure of DTCBPy reveals a substantial interaction between the ortho donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. This interaction between donor and acceptor substituents likely play a key role to achieve very small ΔEST with high photoluminescence quantum yield.

Superior Contact for Single-Molecule Conductance: Electronic Coupling of Thiolate and Isothiocyanate on Pt, Pd, and Au

One of the critical issues for the realization of molecular electronics is the development of ideal molecule-electrode contacts that render efficient charge transportation and thus attenuate the unwanted voltage drop and power loss. The conductance at the single-molecule level has long been expected to be correlated strongly with the electrode materials. However, other than gold, systematic studies of a homologous series of molecules to extract the headgroup-metal contact conductance (G(n=0)) have not been reported. Carefully examined herein are the conductances of alkanedithiols anchored onto electrode materials of Au and Pt as well as the conductances of alkanediisothiocyanates on Au, Pd, and Pt by utilizing the method of STM-BJ (scanning tunneling microscopy break junction). In comparison with Au substrate, Pd and Pt are group 10 elements with stronger d-orbital characteristics, and larger local density of states near the Fermi level. The model compounds, SCN(CH(2))(n)NCS (n = 4, 6, and 8), are studied because the isothiocyanate (-NCS) headgroup is a versatile ligand for organometallics, an emerging class of molecular wires, and can bind to substrates of noble metals to complete a metal-molecule-metal configuration for external I-V measurements. Also studied include alkanedithiols, one of the most scrutinized systems in the field of single-molecule conductance. The results show that the conductance for single molecules bridged between a pair of Pt electrodes is about 3.5-fold superior to those between Au electrodes. On all electrode materials, observed are two sets of conductance values, with the smaller set being 1 order of magnitude less conductive. These findings are ascribed to the degree of electronic coupling between the headgroup and the electrode.

Highly efficient orange and deep-red organic light emitting diodes with long operational lifetimes using carbazole–quinoline based bipolar host materials

Orange and deep red-emitting phosphorescent organic light-emitting diodes (PhOLEDs) are important for OLED displays and lighting; however, high-performance with long operational lifetime hosts designed for orange/deep red PhOLEDs are very rare. Three new carbazole–quinoline hybrids are synthesized and used as the host materials for orange and deep-red PhOLEDs. These bipolar hosts show high glass transition temperatures of 90–146 °C and triplet energy gaps of 2.51–2.95 eV. The optimized orange PhOLEDs using 9-(4-(4-phenylquinolin-2-yl)phenyl)-9H-carbazole (CzPPQ) as the host show the highest external quantum efficiency (EQE) of 25.6% and a power efficiency of 68.1 lm W−1, which are the highest values for orange PhOLEDs. More importantly, the efficiency roll-off is extremely small for both the orange and deep-red devices. For example, an orange device showed an EQE of 25.1% at 100 cd m−2 and 23.6% at 1000 cd m−2; the result appears to be the lowest efficiency roll-off for orange PhOLEDs to date. Additionally, the operational lifetime of both the orange and deep-red devices gave a T50 of more than 26412 and 11450 h, respectively, at an initial luminance of 500 cd m−2. The values are 12 times (orange) and 6 times (red) longer than those of the corresponding devices using CBP as the host.

On the tuning of electric conductance of extended metal atom chains via axial ligands for [Ru3(μ3-dpa)4(X)2]0/+ (X = NCS−, CN−)

The influence of a pi-acid cyanide axial ligand on the metal-metal interactions of [Ru(3)(mu(3)-dpa)(4)(X)(2)](0/+) (X = NCS(-), CN(-)) is manifested by the measurements of single-molecule conductance coupled with in situ electrochemical control.

Shear-Induced Long-Range Uniaxial Assembly of Polyaromatic Monolayers at Molecular Resolution

The control of spatial arrangements of molecular building blocks on surfaces opens the foundational step of the bottom-up approach toward future nanotechnologies. Contemporarily, the domain size of monolayers exhibiting crystallinity falls in the submicrometer scale. Developed herein is a method that allows the alignment of polyaromatics with one-single domain for as long as 7 mm. Even more exciting is the fact that the method is applicable to every laboratory and costs practically nothing. The monolayers are prepared simply by placing a piece of folded lens paper against the substrate and the deposition solution containing the compound of interest. The preparation scheme is similar to the Couette flow where the laminar flow takes place between two concentric walls, one of which rotates and creates viscous drag proven useful to align macromolecules. The method can induce an edge-on orientation for 3,6,11,14-tetradodecyloxydibenzo[g,p]chrysene (DBC-OC12), 3,6,12,15-tetrakis(dodecyloxy)tetrabenz[a,c,h,j]anthracene (TBA-OC12), and hexakis(4-dodecyl)-peri-hexabenzocoronene (HBC-C12) and unsubstituted coronene which would otherwise adopt the face-on arrangement on graphite. This finding will be useful to the research and industry that demands high quality alignment of polyaromatics such as OTFTs, optical polarizers, and nanodevices associated with molecular self-assembly.

Bromo induced reversible distinct color switching of a structurally simple donor–acceptor molecule by vapo, piezo and thermal stimuli

Altering the luminescence properties of a material through external factors is an attractive feature that has the potential for various luminescence-related applications. Here, we report the synthesis and luminescence properties of two anthracene-based donor acceptor compounds, N,N-di-p-tolylanthracen-9-amine (TAA) and 10-bromo-N,N-di-p-tolylanthracen-9-amine (TAAB). In the solid state, the bromo-substituted compound TAAB shows reversible visible switching of the emission by external stimuli such as solvent, mechanical grinding and temperature. Single crystal X-ray studies, powder-XRD analysis and theoretical calculations reveal that switchable emission originated from the different stacking modes of TAAB. Furthermore, we have found that the bromo group interaction in the solid state plays a crucial role in this tunable emission. The other anthracene-based compound TAA does not show such switching of the emission by external stimuli. The observed changes in the luminescence of TAAB by external stimuli suggest potential applications in rewritable optical media, sensors, and optoelectronic devices.

Lysophosphatidic acid up-regulates vascular endothelial growth factor-C and lymphatic marker expressions in human endothelial cells

Abstract.Lysophosphatidic acid (LPA) is a low-molecular-weight lipid growth factor, which binds to G-protein-coupled receptors. Previous studies have shown that LPA enhances vascular endothelial growth factor-A (VEGF-A) expression in cancer cells and promotes angiogenesis process. However, the roles of LPA in lymphatic vessel formation and lymphangiogenesis have not been investigated. Here, we demonstrated that LPA up-regulated VEGF-C mRNA and protein expressions in human umbilical vein endothelial cells (HUVECs). Furthermore, the expression levels of lymphatic markers, including Prox-1, LYVE-1 and podoplanin, were enhanced in LPA-stimulated tube forming endothelial cells in vitro and in vivo. Moreover, we showed that pretreatment with MAZ51, a VEGFR-3 kinase inhibitor, and introduction of VEGFR-3 siRNA suppressed LPA-induced HUVEC tube formation and lymphatic marker expressions. These results demonstrated that LPA enhances expression of lymphatic markers through activating VEGF-C receptors in endothelial cells. This study provides basic information that LPA might be a target for therapeutics against lymphangiogenesis and tumor metastasis.

Conductance of Tailored Molecular Segments: A Rudimentary Assessment by Landauer Formulation

One of the strengths of molecular electronics is the synthetic ability of tuning the electric properties by the derivatization and reshaping of the functional moieties. However, after the quantitative measurements of single-molecule resistance became available, it was soon apparent that the assumption of negligible influence of the headgroup-electrode contact on the molecular resistance was oversimplified. Due to the measurement scheme of the metal--molecule-metal configuration, the contact resistance is always involved in the reported values. Consequently the electrical behavior of the tailored molecular moiety can only be conceptually inferred by the tunneling decay constant (βn in Rmeasured = R(n=0)e(βnN), where N is the number of repeated units), available only for compounds with a homologous series. This limitation hampers the exploration of novel structures for molecular devices. Based on the Landauer formula, we propose that the single-molecule resistance of the molecular backbones can be extracted. This simplified evaluation scheme is cross-examined by electrode materials of Au, Pd, and Pt and by anchoring groups of thiol (-SH), nitrile (-CN), and isothiocyanate (-NCS). The resistance values of molecular backbones for polymethylenes (n = 4, 6, 8, and 10) and phenyl (-C6H4-) moieties are found independent of the anchoring groups and electrode materials. The finding justifies the proposed approach that the resistance of functional moieties can be quantitatively evaluated from the measured values even for compounds without repeated units.

The First Heteropentanuclear Extended Metal‐Atom Chain: [Ni+Ru25+Ni2+Ni2+(tripyridyldiamido)4(NCS)2]

This study develops the first heteropentametal extended metal atom chain (EMAC) in which a string of nickel cores is incorporated with a diruthenium unit to tune the molecular properties. Spectroscopic, crystallographic, and magnetic characterizations show the formation of a fully delocalized Ru2(5+) unit. This [Ru2]-containing EMAC exhibits single-molecule conductance four-fold superior to that of the pentanickel complex and results in features of negative differential resistance (NDR), which are unobserved in analogues of pentanickel and pentaruthenium EMACs. A plausible mechanism for the NDR behavior is proposed for this diruthenium-modulated EMAC.

Energy‐Level Alignment for Single‐Molecule Conductance of Extended Metal‐Atom Chains

The use of single-molecule junctions for various functions constitutes a central goal of molecular electronics. The functional features and the efficiency of electron transport are dictated by the degree of energy-level alignment (ELA), that is, the offset potential between the electrode Fermi level and the frontier molecular orbitals. Examples manifesting ELA are rare owing to experimental challenges and the large energy barriers of typical model compounds. In this work, single-molecule junctions of organometallic compounds with five metal centers joined in a collinear fashion were analyzed. The single-molecule i-V scans could be conducted in a reliable manner, and the EFMO levels were electrochemically accessible. When the electrode Fermi level (EF ) is close to the frontier orbitals (EFMO ) of the bridging molecule, larger conductance was observed. The smaller |EF -EFMO | gap was also derived quantitatively, unambiguously confirming the ELA. The mechanism is described in terms of a two-level model involving co-tunneling and sequential tunneling processes.