Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials.

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (ηext) of 26% using an emission layer with horizontal orientation ratio (Θ) of 79%, which is higher than that of an almost randomly oriented emission layer with Θ of 62% (ηext = 22%).

Room-Temperature Phosphorescence from a Series of 3-Pyridylcarbazole Derivatives.

Exploration of pure metal-free organic molecules that exhibit strong room-temperature phosphorescence (RTP) is an emerging research topic. In this regard, unveiling the design principles for an efficient RTP molecule is an essential, but challenging, task. A small molecule is an ideal platform to precisely understand the fundamental role of each functional component because the parent molecule can be easily derivatized. Here, the RTP behaviors of a series of 3-pyridylcarbazole derivatives are presented. Experimental studies in combination with theoretical calculations reveal the crucial role of the n orbital on the central pyridine ring in the dramatic enhancement of the intersystem crossing between the charge-transfer-excited singlet state and the locally excited triplet states. Single-crystal X-ray crystallographic studies apparently indicate that both the pyridine ring and fluorine atom contribute to the enhancement of the RTP because of the restricted motion owing to weak C-H⋅⋅⋅N and H⋅⋅⋅F hydrogen-bonding interactions. The single crystal of the fluorine-substituted derivative shows an ultra-long phosphorescent lifetime (τP ) of 1.1 s and a phosphorescence quantum yield (ΦP ) of 1.2 %, whereas the bromine-substituted derivative exhibits τP of 0.15 s with a ΦP of 7.9 %. We believe that this work provides a fundamental and universal guideline for the generation of pure organic molecules exhibiting strong RTP.

Electrochemical determination with a long-length carbon nanotube electrode of quercetin glucosides in onion, apple peel, and tartary buckwheat.

Since the intake of quercetin glucosides has healthy benefits, the analysis of quercetin glucosides in food is useful. The electrochemical determination of individual quercetin glucosides (quercetin-3-glucoside (Q3G), quercetin-4'-glucoside (Q4'G), and quercetin-3,4'-diglucoside (Q34'G)) in food is carried out. For the detection of quercetin glucosides, a long-length carbon nanotube electrode offers attractive properties such as well-defined current peaks, high sensitivity, and high reproducibility. Cyclic voltammetry (CV) demonstrates distinct and specific peak currents: the oxidation peaks at +0.37, +0.45, and +0.78 V are assigned to the catechol group in the B-ring of Q3G, the 3-hydroxy group in the C-ring of Q4'G, and the resorcinol group in the A-ring of both Q4'G and Q34'G, respectively. Currents, which are determined by CV, of individual quercetin glucosides at the peak potential are proportional to the concentrations of onion, apple peel, and tartary buckwheat, which show good agreement with those obtained by high-performance liquid chromatography.

Polyphenol Analysis in Black Tea with a Carbon Nanotube Electrode.

An electrochemical analysis of polyphenols (theaflavin (TF1), theaflavin-3-gallate (TF2A), theaflavin-3'-gallate (TF2B), theaflavin-3,3'-digallate (TF3), and epigallocatechingallate (EGCG)) in a black tea infusion is demonstrated. The characterization of each polyphenol in a solution containing only a single type of polyphenol for a redox reaction at the CNT electrode with cyclic voltammetry (CV) was conducted. The oxidation peak at around +0.30 V for TF1 is assigned to catechol group in a benzotropolone ring. The oxidation peak at around +0.35 V for TF2A, TF2B, and TF3 is assigned to both of the catechol groups in the benzotropolone ring and the pyrogallol group in the gallate ring. The oxidation peak at around +0.35 V for EGCG is assigned to a pyrogallol group in the gallate ring. Current changes of those individual polyphenols at the peak potential are proportional to their concentrations (linear range 0.28 - 94 µM; detection limit 0.11 µM). The CV curve for real black tea, which is mainly composed of a mixture of the mentioned five compounds, is produced by the sum of those. The current change of the mixture solution of polyphenols is also proportional to the mass concentration of the total polyphenols and the sensitivity defined as the slope of current vs. concentration plot is independent of the ratio of the individual polyphenols. This indicates that the peak current at around +0.35 V can quantify the total amount of polyphenols in a black tea. Additionally, the shape of the CV curve can roughly estimate the ratio of [catechins]/[theaflavins]. The values for real samples determined from CVs show good agreement with that obtained by high-performance liquid chromatography.

Separationless and Adsorptionless Quantification of Individual Catechins in Green Tea with a Carbon Nanotube-Carboxymethylcellulose Electrode.

We demonstrate the electrochemical quantification of individual catechins (epicatechin, EC; epigallocatechin, EGC; epicatechingallate, ECG; and epigallocatechingallate, EGCG) in a green tea infusion without a separation process nor any adsorption complication. In the detection of catechins, long-length carbon nanotube (CNT)-carboxymethylcellulose (CMC) thin-film electrodes have attractive properties, such as well-defined current peaks, high reproducibility from sample to sample, high repeatability, and low background current. Cyclic voltammograms (CVs) for real green tea, which is mainly composed of a mixture of the four catechins, are produced by the sum of those catechins. A set of three specific peaks in the CVs of the real green tea samples, as catechin-mixture solutions, was used for quantification of the individual catechins. The CVs of the real samples are similar to the CVs of intentionally prepared mixture solutions with the catechin-component ratios determined by high-performance liquid chromatography (HPLC). The values for the real samples determined from the CVs show good agreement with those obtained by HPLC. The novelty of the work is the demonstration of the usefulness of the CNT-CMC electrode and the separationless quantification of individual catechins in green tea for the first time.

A Series of Lithium Pyridyl Phenolate Complexes with a Pendant Pyridyl Group for Electron-Injection Layers in Organic Light-Emitting Devices.

We report a new series of lithium pyridyl phenolate complexes with a pendant pyridyl group, Li2BPP, Li3BPP, and Li4BPP, in which the pendant pyridines are substituted at the 2-, 3-, and 4-positions, respectively. The most important difference between these complexes is their molecular planarity; Li3BPP and Li4BPP adopt twisted bipyridine structures, whereas Li2BPP adopts a planar structure owing to the steric hindrance and chelating effect of bipyridine on the Li core. The planar structure leads to crystallization through π-π stacking interactions, and the small differences in the molecular structures of the pendant pyridine rings cause drastic differences in the physical properties of thin solid films of these complexes. We applied these complexes as electron-injection layers (EILs) in Ir(ppy)3-based organic light-emitting devices. When thin EILs were used, Li3BPP and Li4BPP afforded lower driving voltages than Li2BPP; the order of the driving voltages followed the order of their electron affinity values. Moreover, the dependence of driving voltage on the EIL thickness was investigated for each complex. Among the three LiBPP derivatives, Li2BPP-based devices showed almost negligible EIL thickness dependence, which may be attributable to the high crystallinity of Li2BPP. All LiBPP-based devices also showed higher stability than conventional 8-quinolinolato lithium-based devices.

Manipulating the Electronic Excited State Energies of Pyrimidine-Based Thermally Activated Delayed Fluorescence Emitters To Realize Efficient Deep-Blue Emission.

The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m-2, most likely because of the long delayed fluorescent lifetime (τd). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τd. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac-XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (ES) and triplet state (ET) energies. Among them, Ac-3MHPM, with a high ET of 2.95 eV, exhibited a high external quantum efficiency (ηext,max) of 18% and an ηext of 10% at 100 cd m-2 with Commission Internationale de l'Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters.

Simultaneous Electrochemical Determination of Nicotinamide Adenine Dinucleotide and Ascorbic Acid at Carbon Nanotube Electrode.

Simultaneous electrochemical determination of nicotinamide adenine dinucleotide (NADH) and ascorbic acid (AA) at a carbon nanotube electrode is presented. The discrimination of NADH and AA is conducted with the difference of peak potential by differential pulse voltammetry. Two well-distinguished anodic peaks, +0.56 and +0.26 V, due to NADH and AA are observed. The characteristics of those peaks were independent from each other. The attained characteristics for simultaneous determination of NADH and AA are (i) NADH measurement at the concentration range of 0.030 - 2.0 mM in the presence of 1.2 mM AA, and (ii) AA measurement at the concentration range of 0.030 - 2.0 mM in the presence of 2.0 mM NADH.