Hironori Kaji

AbstractMolecular orientation in amorphous organic semiconducting thin-film devices is an important issue affecting device performance. However, to date it has not been possible to analyze the “distribution” of the orientations. Although solid-state NMR (ssNMR) spectroscopy can provide information on the “distribution” of molecular orientations, the technique is limited because of the small amount of sample in the device and the low sensitivity of ssNMR. Here, we report the first application of dynamic nuclear polarization enhanced ssNMR (DNP-ssNMR) spectroscopy for the orientational analysis of amorphous phenyldi(pyren-1-yl)phosphine oxide (POPy2). The 31P DNP-ssNMR spectra exhibited a sufficient signal-to-noise ratio to quantify the distribution of molecular orientations in amorphous films: the P=O axis of the vacuum-deposited and drop-cast POPy2 shows anisotropic and isotropic distribution, respectively. The different molecular orientations reflect the molecular origin of the different charge transport behaviors.

Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy

Co-reporter:Dr. Katsuaki Suzuki;Shosei Kubo;Dr. Fabien Aussenac;Dr. Frank Engelke;Dr. Tatsuya Fukushima; Dr. Hironori Kaji

Angewandte Chemie 2017 Volume 129(Issue 47) pp:15038-15042

Publication Date(Web):2017/11/20

DOI:10.1002/ange.201707208

Combined Inter- and Intramolecular Charge-Transfer Processes for Highly Efficient Fluorescent Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching

Co-reporter:Chang-Ki Moon;Katsuaki Suzuki;Katsuyuki Shizu;Chihaya Adachi;Jang-Joo Kim

Advanced Materials 2017 Volume 29(Issue 17) pp:

Publication Date(Web):2017/05/01

DOI:10.1002/adma.201606448

Inter- and intramolecular charge-transfer processes are combined using an exciplex-forming host and a thermally activated delayed fluorescent dopant, for fabricating efficient fluorescent organic light-emitting diodes along with the reduced efficiency roll-off at high current densities. Extra conversion on the host from triplet exciplexes to singlet exciplexes followed by energy transfer to the dopant reduces population of triplet excitons on dopant molecules, thereby reducing the triplet exciton annihilations at high current densities.

Photocontrolled Organocatalyzed Living Radical Polymerization Feasible over a Wide Range of Wavelengths

Co-reporter:Akimichi Ohtsuki; Lin Lei; Miho Tanishima; Atsushi Goto

Journal of the American Chemical Society 2015 Volume 137(Issue 16) pp:5610-5617

Publication Date(Web):April 16, 2015

DOI:10.1021/jacs.5b02617

Photocontrolled organocatalyzed living radical polymerization was conducted over a wide range of irradiation wavelengths (350–750 nm). The polymerization was induced and controlled at the desired wavelengths by exploiting suitable organic catalysts. This system was finely responsive to the irradiation wavelength; the polymerization was instantly switched on and off, and the polymerization rate was sensitively modulated by altering the irradiation wavelength. The polymer molecular weight and its distribution (Mw/Mn = 1.1–1.4) were well controlled for methacrylate monomers up to fairly high conversions in many cases. The monomer scope encompassed various functional methacrylates, and their block copolymers were obtained. The feasibility of such a wide range of wavelengths and the fine response to the wavelength are unprecedented features. As a unique application of the wavelength-responsive nature of this system, we demonstrated “one-pot” selective regulation of living radical polymerization and another type of polymerization (ring opening polymerization), where the regulation was achieved by simply altering the irradiation wavelength. Facile operation and applicability to a wide range of polymer designs are advantages of this polymerization.

Multiscale simulation of charge transport in a host material, N,N′-dicarbazole-3,5-benzene (mCP), for organic light-emitting diodes

Co-reporter:Furitsu Suzuki, Katsuyuki Shizu, Hisafumi Kawaguchi, Shinya Furukawa, Tohru Sato, Kazuyoshi Tanaka and Hironori Kaji

Journal of Materials Chemistry A 2015 vol. 3(Issue 21) pp:5549-5555

Publication Date(Web):20 Apr 2015

DOI:10.1039/C5TC00543D

We have performed multiscale charge transport simulations in organic amorphous thin films by explicitly considering organic molecules. The simulations were based on quantum chemical and Monte Carlo calculations. The amorphous layer was composed of N,N′-dicarbazole-3,5-benzene, which is a widely used host material in the emissive layer of blue-emitting organic light-emitting diodes. The hole mobility was calculated to be three to four times larger than the electron mobility. This trend was consistent with the experimentally obtained mobility ratio. It was also found that the charges are transported dominantly by a diffusion-type character at low applied electric fields and the contribution of the drift-type character increases as the applied electric field is increased. The difference between the number of hops in the forward and the backward directions contributes to the actual charge transport. From the detailed molecular level analysis, it was turned out that the molecular pairs with a large electronic coupling do not necessarily have large contributions to the charge transport, rather they can temporarily trap charges. We found important molecular pairs, which form effective charge-transfer paths, although the electronic coupling was not substantially large.

Distribution ratio of carbon black in polyisobutyrenepolyisoprene rubber blends using high-resolution solid-state 13C NMR

Co-reporter:Marina Kotani, Hidehiko Dohi, Hideaki Kimura, Kiyoshige Muraoka and Hironori Kaji

Polymer Journal 2015 47(6) pp:422-427

Publication Date(Web):February 18, 2015

DOI:10.1038/pj.2015.3

We attempted to estimate the distribution ratio of carbon black (CB) in blends of polyisobutyrene rubber and polyisoprene rubber using high-resolution solid-state 13C nuclear magnetic resonance (NMR). Our NMR analysis revealed that, in polyisobutyrene rubber/polyisoprene rubber/CB composites, CB more easily distributes in polyisoprene rubber than in polyisobutyrene rubber. We found that more than 70% of CB is distributed into the polyisoprene rubber phase. The sum of the amounts of CB in both the polyisobutyrene rubber and polyisoprene rubber phases estimated by our method using 13C NMR closely corresponds with the amount of CB originally added to the compound, verifying the validity of the NMR method. Further, we observed the distribution of CB in polyisobutyrene/polyisoprene rubber blends using transmission electron microscopy and the Carbon-Black-Gel method, which can only be used for unvulcanized rubber blends, for comparison. The results obtained using these three methods show similar tendencies, which confirms the accuracy of this method of using 13C NMR to determine CB distribution in rubber blends.

Triarylboron-Based Fluorescent Organic Light-Emitting Diodes with External Quantum Efficiencies Exceeding 20%

Co-reporter:Dr. Katsuaki Suzuki;Shosei Kubo;Dr. Katsuyuki Shizu;Dr. Tatsuya Fukushima;Dr. Atsushi Wakamiya;Dr. Yasujiro Murata;Dr. Chihaya Adachi;Dr. Hironori Kaji

Angewandte Chemie International Edition 2015 Volume 54( Issue 50) pp:15231-15235

Publication Date(Web):

DOI:10.1002/anie.201508270

Abstract

Triarylboron compounds have attracted much attention, and found wide use as functional materials because of their electron-accepting properties arising from the vacant p orbitals on the boron atoms. In this study, we design and synthesize new donor–acceptor triarylboron emitters that show thermally activated delayed fluorescence. These emitters display sky-blue to green emission and high photoluminescence quantum yields of 87–100 % in host matrices. Organic light-emitting diodes using these emitting molecules as dopants exhibit high external quantum efficiencies of 14.0–22.8 %, which originate from efficient up-conversion from triplet to singlet states and subsequent efficient radiative decay from singlet to ground states.

Triarylboron-Based Fluorescent Organic Light-Emitting Diodes with External Quantum Efficiencies Exceeding 20%

Co-reporter:Dr. Katsuaki Suzuki;Shosei Kubo;Dr. Katsuyuki Shizu;Dr. Tatsuya Fukushima;Dr. Atsushi Wakamiya;Dr. Yasujiro Murata;Dr. Chihaya Adachi;Dr. Hironori Kaji

Angewandte Chemie 2015 Volume 127( Issue 50) pp:15446-15450

Publication Date(Web):

DOI:10.1002/ange.201508270

Abstract

A designed fluorescent anthracene derivative: Theory, calculation, synthesis, and characterization

Co-reporter:Motoyuki Uejima, Tohru Sato, Masahiro Detani, Atsushi Wakamiya, Furitsu Suzuki, Hajime Suzuki, Tatsuya Fukushima, Kazuyoshi Tanaka, Yasujiro Murata, Chihaya Adachi, Hironori Kaji

Chemical Physics Letters 2014 Volume 602() pp:80-83

Publication Date(Web):20 May 2014

DOI:10.1016/j.cplett.2014.04.017

•Anthracene derivatives with small vibronic coupling constants and large transition dipole moment are theoretically designed.•The designed molecule was synthesized. The molecule exhibited a high quantum yield of 96%.•The observed absorption and photoluminescence spectra are consistent with calculations.•The molecules are the first examples of designed fluorescent molecules from the view of vibronic couplings.•The present approach can contribute to the development of light-emitting materials.5,11-Bis(phenylethynyl)benzo[1,2-f:4,5-f′]diisoindole-1,3,7,9(2H,8H)-tetraone 1H was designed as an application of the theoretical design principle for fluorescent molecules which is derived from the vibronic coupling density analysis. For solubility reasons, tertiary-butylated 1H, 2,8-di-tert-butyl-5,11-bis(phenylethynyl)benzo[1,2-f:4,5-f′]diisoindole-1,3,7,9(2H,8H)-tetraone 1 was synthesized and its fluorescence properties were measured. It is found that the photoluminescence quantum yield of 1 was 96%. We discuss the rationale for designing 1H as a highly efficient fluorescent molecule, and compare the theoretical calculations for 1 with the observed absorption and photoluminescence spectra.

Clarification of isomeric structures and the effect of intermolecular interactions in blue-emitting aluminum complex Alq3 using first-principles 27Al NMR calculations

Co-reporter:Furitsu Suzuki, Yusuke Nishiyama, Hironori Kaji

Chemical Physics Letters 2014 Volumes 605–606() pp:1-4

Publication Date(Web):17 June 2014

DOI:10.1016/j.cplett.2014.05.015

Highlights

•: Blue emission of Alq₃ is caused not by intermolecular packing but by facial isomerization.
•: ²⁷Al NMR is a very sensitive probe for the intramolecular structure of Alq₃.
•: ¹³C NMR is a good measure for intermolecular interactions.
•: GIPAW analyses of ¹³C and ²⁷Al NMR are complementary to each other.

Systematic Study on Alkyl Iodide Initiators in Living Radical Polymerization with Organic Catalysts

Co-reporter:Lin Lei, Miho Tanishima, Atsushi Goto, Hironori Kaji, Yu Yamaguchi, Hiroto Komatsu, Takuya Jitsukawa, and Michihiko Miyamoto

Macromolecules 2014 Volume 47(Issue 19) pp:6610-6618

Publication Date(Web):September 16, 2014

DOI:10.1021/ma501569j

Several low-molar-mass alkyl iodides were studied as initiating dormant species in living radical polymerization with organic catalysts. Primary, secondary, and tertiary alkyl iodides with different stabilizing groups (ester, phenyl, and cyano groups) were systematically studied for the rational design of initiating alkyl iodides. The activation rate constants of these alkyl iodides were experimentally determined for quantitative comparison. These alkyl iodides were used in the polymerizations of methyl methacrylate and butyl acrylate to examine their initiation ability in these polymerizations. A telechelic polymer was prepared using an alkyl iodide with a functional group. Alkyl iodides with multi-initiating sites were also studied.

On-Top -Stacking of Quasiplanar Molecules in Hole-Transporting Materials: Inducing Anisotropic Carrier Mobility in Amorphous Films

Co-reporter:Dr. Atsushi Wakamiya;Hidetaka Nishimura;Dr. Tatsuya Fukushima;Furitsu Suzuki;Dr. Akinori Saeki;Dr. Shu Seki;Dr. Itaru Osaka;Dr. Takahiro Sasamori;Dr. Michihisa Murata;Dr. Yasujiro Murata;Dr. Hironori Kaji

Angewandte Chemie International Edition 2014 Volume 53( Issue 23) pp:5800-5804

Publication Date(Web):

DOI:10.1002/anie.201400068

Abstract

Dimers of partially oxygen-bridged triarylamines were designed and synthesized as hole-transporting materials. X-ray structural analyses revealed that these compounds form on-top π-stacking aggregates in the crystalline state. TRMC measurements showed that high levels of anisotropic charge transport were induced in the direction of the π-stacking. Surprisingly, even in vacuum-deposited amorphous films, these compounds retained some of the face-on π-stacking, thus facilitating an out-of-plane carrier mobility.

Reversible Generation of a Carbon-Centered Radical from Alkyl Iodide Using Organic Salts and Their Application as Organic Catalysts in Living Radical Polymerization

Co-reporter:Atsushi Goto ; Akimichi Ohtsuki ; Haruki Ohfuji ; Miho Tanishima

Journal of the American Chemical Society 2013 Volume 135(Issue 30) pp:11131-11139

Publication Date(Web):July 19, 2013

DOI:10.1021/ja4036016

A new method of producing carbon-centered radicals was discovered through the reaction of an alkyl iodide (R-I) with organic salts to reversibly generate the corresponding alkyl radical (R•). Via this new reaction, the organic salts were used as new and highly efficient organic catalysts in living radical polymerization. The catalysts included common and inexpensive compounds such as tetrabutylammonium iodide and methyltributylphosphonium iodide. Notably, the catalysts were highly reactive. They enabled the synthesis of high-molecular-weight polymers (up to Mn = 140 000) and the control of acrylate polymerization, which had been difficult with other organic catalysts. The organic salt catalysts were highly versatile, reacting with methacrylate, acrylate, styrene, acrylonitrile, and functional methacrylate monomers. Well-defined block copolymers were also prepared by using this method. A kinetic study quantitatively confirmed the high reactivity of these catalysts. Attractive features of this system include its low cost, its ease of operation, and its ability to access a wide range of polymer designs.

π-Extended Planarized Triphenylboranes with Thiophene Spacers

Co-reporter:Ayumi Shuto, Tomokatsu Kushida, Tatsuya Fukushima, Hironori Kaji, and Shigehiro Yamaguchi

Organic Letters 2013 Volume 15(Issue 24) pp:6234-6237

Publication Date(Web):November 25, 2013

DOI:10.1021/ol403084x

Planarized triphenylboranes extended with thiophene or bithiophene spacers were synthesized, which showed intense fluorescences in solution and reversible redox waves for reduction in cyclic voltammetry. Organic light-emitting diodes (OLEDs) using these compounds as an electron-transporting material were fabricated.

Sensitivity boosting in solid-state NMR of thin organic semiconductors by a paramagnetic dopant of copper phthalocyanine

Co-reporter:Yusuke Nishiyama, Tatsuya Fukushima, Masashi Fukuchi, Subaru Fujimura, Hironori Kaji

Chemical Physics Letters 2013 Volume 556() pp:195-199

Publication Date(Web):29 January 2013

DOI:10.1016/j.cplett.2012.11.043

Sensitivity enhancements in cross-polarization magic-angle-spinning (CPMAS) solid-state NMR spectra of organic thin-film semiconductors are reported. A vacuum-deposited paramagnetic dopant on the semiconductor films reduces 1H T1 relaxation time, enabling faster repetition of NMR measurements. When the thickness of the films is between 50 and 100 nm, 1H spins are well polarized by 1H–1H spin diffusion, giving the maximum sensitivity without harmful effects such as paramagnetic shift, broadening, and quenching of CPMAS signals. The binary films of copper phthalocyanine (CuPc) and phenyldipyrenylphosphine oxide (POPy2) show a threefold reduction of the NMR measurement time.Graphical abstractHighlights► Paramagnetic doping enhances 31P CPMAS intensities of thin films by a factor of 1.7. ► Most 1H spins in the 50 to 100 nm organic films are polarized by 1H–1H spin diffusion. ► No harmful effect of paramagnetic doping is observed. ► 1H magnetizations are quickly produced by paramagnetic relaxations. ► 1H magnetizations are distributed among the organic films by 1H–1H spin diffusion.

Refined Structure Determination of Blue-Emitting Tris(8-hydroxyquinoline) Aluminum(III) (Alq3) by the Combined Use of Cross-Polarization/Magic-Angle Spinning 13C Solid-State NMR and First-Principles Calculation

Co-reporter:Furitsu Suzuki, Tatsuya Fukushima, Masashi Fukuchi, and Hironori Kaji

The Journal of Physical Chemistry C 2013 Volume 117(Issue 37) pp:18809-18817

Publication Date(Web):August 23, 2013

DOI:10.1021/jp404430v

The combined use of cross-polarization/magic-angle spinning (CP/MAS) 13C NMR experiments and gauge-including projector-augmented wave (GIPAW) isotropic chemical shift calculations is an easy and useful method for the structural refinement of organic aggregates. In this study, the method is applied to an important material for organic light-emitting diodes, tris(8-hydroxyquinoline) aluminum(III) (Alq3). CP/MAS 13C NMR spectra include precise structural information of not only the conformation of the molecules but also the intermolecular packing. First, the structural refinements were performed for the Alq3 in the γ- and δ-crystalline forms employing the combined method. Second, information on intramolecular structures and intermolecular structures was distinguished by comparing GIPAW calculations for crystals under periodic boundary conditions and those for isolated molecules extracted from the crystals. It was found from the analysis that the γ-Alq3 and δ-Alq3 crystals have similar intramolecular structures both in the facial isomeric state, whereas their intermolecular packing is significantly different. Both the γ-Alq3 and δ-Alq3 crystals exhibit unusual blue emission, which is different from conventional green emission, and the origin of the difference has been debated. This investigation shows that the origin of the blue-shifted emission is the isomeric states of Alq3, not the intermolecular packing.

Green- and blue-emitting tris(8-hydroxyquinoline) aluminum(III) (Alq3) crystalline polymorphs: Preparation and application to organic light-emitting diodes

Co-reporter:Tatsuya Fukushima, Hironori Kaji

Organic Electronics 2012 Volume 13(Issue 12) pp:2985-2990

Publication Date(Web):December 2012

DOI:10.1016/j.orgel.2012.08.036

Tris(8-hydroxyquinoline) aluminum(III) (Alq3) is known to have two isomeric states, namely meridional and facial isomers. Typical Alq3 crystals composed of meridional and facial isomers are α- and δ-form crystals, respectively. First, we investigate the temperature change in the crystalline forms of α-Alq3 and δ-Alq3 in X-ray diffraction experiments in a vacuum. α-Alq3 remains in α-form up to 300 °C, immediately before sublimation. In contrast, δ-Alq3 is found to transform into γ-form at ∼180 °C, and remain in γ-form immediately before sublimation. Both γ-Alq3 and δ-Alq3 are composed of facial isomers and emit blue luminescence, which is different from the typical green emissions of α-Alq3. Second, we fabricate organic light-emitting diodes (OLEDs) from different crystals as source powders; i.e., from (1) α-Alq3, (2) δ-Alq3, and (3) a mixture of α-, γ-, and δ-Alq3. All the OLEDs exhibit green electroluminescence with almost the same maximum wavelength, suggesting that some facial isomers become meridional while Alq3 is in the gas phase. In contrast, electroluminescence efficiency depends on the Alq3 crystalline polymorph; the OLED fabricated from the mixture of α-, γ-, and δ-Alq3 has up to 1.4 times the efficiency of the OLED fabricated from α-Alq3 for the same device structure.Graphical abstractHighlights► Pure δ-Alq3 is obtained by vacuum-encapsulation and thermal annealing. ► Thermal changes of crystalline forms for α-Alq3 and δ-Alq3 are investigated. ► Effects of Alq3 polymorphs as source powders on OLED performance are examined. ► An OLED from αγδ-Alq3 has 1.4 times higher EL efficiency than that from α-Alq3.

Enhanced hole injection in organic light-emitting diodes by optimized synthesis of self-assembled monolayer

Co-reporter:Yasuaki Tokudome, Tatsuya Fukushima, Atsushi Goto, Hironori Kaji

Organic Electronics 2011 Volume 12(Issue 10) pp:1600-1605

Publication Date(Web):October 2011

DOI:10.1016/j.orgel.2011.06.022

The purpose of the present study is to demonstrate that formation conditions for a self-assembled monolayer (SAM) on indium tin oxide (ITO) highly influence the device performance of organic light-emitting diodes (OLEDs). An ITO substrate was modified with a silane modifier under systematically controlled conditions. Pentyltriethoxysilane (PTES) and tetrahydrofuran (THF) were used as a surface modifier and a solvent, respectively. The immobilization of PTES on the ITO substrate was performed under both the acidic and basic conditions with various H2O/PTES ratios, r. The relationship between the resultant SAM structure and the hole-injection property was investigated by using hole only devices (HODs) fabricated on the SAM-modified ITO substrate. It was found that the catalytic condition, the value of r, and the concentration of PTES highly influence the structure of obtained SAM, and thus affect the hole-injection property of HOD. The SAM formation under the acidic conditions allows homogeneous coverage of a silane layer on the ITO surface, which leads to the improved hole-injection from the anode. On the other hand, the use of basic catalysts results in the inhomogeneous coverage of a silane layer on the ITO surface and the decreased hole-injection from the anode. The choice of r and the concentration of PTES also affect the kinetics of hydrolysis and condensation reactions of PTES, and hence affects the resultant SAM structure and hole-injection property. The results obtained here show that the device performance can be improved drastically by choosing the SAM formation conditions appropriately, even in the case that particular and novel modifiers are not employed.Graphical abstractOptical microscope images of the OLEDs under the applied voltage of 9 V. The SAM layers were formed with (a) acidic (0.01 M HNO3) and (b) basic (0.01 M NH3) conditions.Highlights► SAM formation condition for enhanced OLED performances was systematically studied. ► Acidic conditions lead to improved hole-injection properties from the anode. ► Concentration of the modifier and content of water also influence the device performances. ► Appropriate choice of SAM formation conditions improved the device performance.

Revealing bipolar charge-transport property of 4,4′-N,N′-dicarbazolylbiphenyl (CBP) by quantum chemical calculations

Co-reporter:Tomonori Yamada, Furitsu Suzuki, Atsushi Goto, Tohru Sato, Kazuyoshi Tanaka, Hironori Kaji

Organic Electronics 2011 Volume 12(Issue 1) pp:169-178

Publication Date(Web):January 2011

DOI:10.1016/j.orgel.2010.10.022

4,4′-N,N′-Dicarbazolylbiphenyl (CBP) has both the hole- and electron-transport ability, that is, bipolar charge-transport property, and has been widely used for organic light-emitting diodes (OLEDs). In this study, we explain the bipolar charge-transport property of CBP by quantum chemical calculations for the crystal. Both the reorganization energies and the charge transfer integrals were investigated, and charge-transfer rate constants were calculated based on Marcus theory. The hole- and electron-transfer rate constants thus calculated were found to be similar in magnitude. This is in sharp contrast with the case of a structurally similar but a poor electron-transport material, N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD), for which the hole-transfer rate constants were calculated to be much larger than the electron-transfer rate constants. From the detailed analysis of the charge transfer integrals for CBP, it was found that holes transfer through all the segments of the molecules, using the delocalized HOMO over the whole molecule as in the case of TPD. On the other hand, electrons transfer advantageously through the delocalized LUMO on the carbazole moieties of CBP, which have close intermolecular contacts. The LUMO of TPD localizes on the central biphenylene moiety, resulting in small electron transfer integrals. The results clearly show the difference of the electron-transport property between CBP and TPD.Graphical abstractResearch highlights► Different charge-transport properties between CBP and TPD are explained. ► The electron-transport property (ETP) of CBP is superior to that of TPD. ► The LUMO of TPD is localized on the central biphenylene moiety (BM). ► The BM of TPD do not have close intermolecular contacts, resulting in inferior ETP. ► In contrast, electrons transfer through the carbazole moieties in CBP.

Reversible Complexation Mediated Living Radical Polymerization (RCMP) Using Organic Catalysts

Co-reporter:Atsushi Goto, Tomohiro Suzuki, Haruki Ohfuji, Miho Tanishima, Takeshi Fukuda, Yoshinobu Tsujii, and Hironori Kaji

Macromolecules 2011 Volume 44(Issue 22) pp:8709-8715

Publication Date(Web):October 19, 2011

DOI:10.1021/ma2014589

A novel class of living radical polymerization using amines as organic catalysts was developed. It is based on a new reversible activation mechanism, reversible complexation (RC). The polymer molecular weight and its distribution (Mw/Mn = 1.1–1.4) were well controlled in the polymerizations of methyl methacrylate (MMA), styrene, acrylonitrile, and some functional methacrylates with a fairly high conversion in hours in many cases. The catalysts include such common amines as triethylamine and tetramethylethylenediamine (TMEDA). Their low cost, good environmental safety, and ease of handling may be attractive for possible applications. Kinetic studies supported the RC mechanism. The activation rate constant for the MMA/TMEDA system was large enough to explain why the system provides low-polydispersity polymers from an early stage of polymerization.

Percolation paths for charge transports in N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD)

Co-reporter:Tomonori Yamada, Tohru Sato, Kazuyoshi Tanaka, Hironori Kaji

Organic Electronics 2010 Volume 11(Issue 2) pp:255-265

Publication Date(Web):February 2010

DOI:10.1016/j.orgel.2009.11.003

We investigated charge transports in N,N′-diphenyl-N,N′-di(m -tolyl)benzidine (TPD), a well-known hole-transport material for organic light-emitting diodes (OLEDs), in the orthorhombic and monoclinic polymorphs. Based on Marcus theory, charge-transfer rate constants in both the polymorphs were investigated by calculating the reorganization energies and charge transfer integrals. The rate constants for hole transfers, kCT+, were calculated to be two to three orders of magnitude larger than those for electron transfers, kCT-, for both the polymorphs. The small kCT- values result not only from the large reorganization energies for electron transfers but also from the small electron transfer integrals. To investigate the charge transfer integrals in more detail, the contributions of respective moieties of TPD molecules, nitrogen, central biphenylene, outer phenyl, and outer tolyl moieties, to charge transfer integrals were analyzed separately. From the analysis, the small electron transfer integrals compared to hole transfer integrals were found to originate from little contribution of LUMO to the outer phenyl and outer tolyl moieties, which has close intermolecular contacts. In contrast, HOMO spreads over these moieties, which results in relatively large hole transfer integrals. Holes can transport through these moieties with close contacts. Percolated hole-transport paths, consisting of consecutive molecular pairs with large hole transfer integrals, and therefore, large kCT+, exist in both the polymorphs, showing the good hole-transport property.

Planarity of triphenylamine moieties of a typical hole-transport material for OLEDs, N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD), in the amorphous state

Co-reporter:Tomonori Yamada, Hironori Kaji

Journal of Molecular Structure 2009 Volume 927(1–3) pp:82-87

Publication Date(Web):18 June 2009

DOI:10.1016/j.molstruc.2009.02.026

N,N′-Diphenyl-N,N′-di(m-tolyl)benzidine (TPD) is a well-known hole-transport material for organic light-emitting diodes (OLEDs). Here, we studied the planarity of triphenylamine moieties of TPD in the amorphous state, by the combined use of solid-state 15N NMR experiments and density functional theory (DFT) calculations. It was shown that the planarity is of crucial importance for hole-transport performance of TPD, because of the strong effects for the shape of HOMO and for the intermolecular repulsion energy. It was also found that the molecular structure around the nitrogen in its amorphous state is not pyramidal, but is instead planar, which is favorable for hole-transport. Natural bond orbital calculation indicates that the nitrogen atom is sp2 hybridized. This is unlike most amine compounds, which have pyramidal structures with sp3 hybridized nitrogens.

Characterization of local structures in amorphous and crystalline tris(8-hydroxyquinoline) aluminum(III) (Alq3) by solid-state 27Al MQMAS NMR spectroscopy

Co-reporter:Yusuke Nishiyama, Tatsuya Fukushima, Kousuke Takami, Yasunari Kusaka, Toshio Yamazaki, Hironori Kaji

Chemical Physics Letters 2009 Volume 471(1–3) pp:80-84

Publication Date(Web):16 March 2009

DOI:10.1016/j.cplett.2009.02.012

Abstract

The local structure of tris(8-hydroxyquinoline) aluminum(III) (Alq₃) in three different crystalline polymorphs, α-, γ-, and δ-Alq₃, and amorphous Alq₃, are investigated by ²⁷Al NMR experiments, which include two-dimensional multi-quantum magic-angle spinning (MQMAS) experiments and one-dimensional variable B₀ field MAS experiments. The quadrupolar and chemical shift parameters of these Alq₃ samples are determined, including the distributions. These parameters reflect the local structure of respective samples and it is found that the γ- and δ-Alq₃ consist of the facial isomer with well-defined crystal structure, whereas the α- and amorphous Alq₃ consist of the meridional isomer with distributions of quadrupolar coupling constants. It is also found that two distinct components exist in α-Alq₃.

A combined experimental and theoretical study of the conformation of N,N′-diphenyl-N,N′-di(m-tolyl)benzidine using solid-state 15N NMR and DFT calculations

Co-reporter:Hironori Kaji, Tomonori Yamada, Naoki Tsukamoto, Fumitaka Horii

Chemical Physics Letters 2005 Volume 401(1–3) pp:246-253

Publication Date(Web):1 January 2005

DOI:10.1016/j.cplett.2004.11.052

Abstract

The conformation of N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD) is studied by solid-state ¹⁵N NMR and density functional theory (DFT) calculations. The results of the ¹⁵N NMR for amorphous TPD agree with those of the DFT calculations, confirming that the DFT-optimized TPD single molecule reflects the structure in the condensed amorphous state. Various stable conformers are considered to exist in the amorphous state. Torsion angles change the shape of molecules and the state of the electron clouds around the nitrogens, and therefore significantly affect the intermolecular electron coupling. It suggests that the torsion angle is a crucial factor for the carrier transport properties.

Sine-modulated two-dimensional pure exchange solid-state NMR as a tool for characterizing dynamics in solids

Co-reporter:Hironori Kaji, Fumitaka Horii

Chemical Physics Letters 2003 Volume 377(3–4) pp:322-328

Publication Date(Web):15 August 2003

DOI:10.1016/S0009-2614(03)01186-2

Abstract

We present a simple two-dimensional (2D) solid-state exchange NMR method to suppress significant undesirable diagonal signals. Two delays, τ, incorporated in standard 2D exchange experiments modulate the 2D spectra with , where Ω₁ and Ω₂ are precession frequencies of a given site before and after the mixing time, respectively, yielding the spectra free of diagonal signals. The suppression of diagonal signals is demonstrated for dimethyl sulfone. This technique is combined with magic angle spinning and is applied to polyethylene. Not only the exchange signals between the crystalline and noncrystalline components but also those among the noncrystalline components in different states are clearly observed.

Visible-Light-Induced Reversible Complexation Mediated Living Radical Polymerization of Methacrylates with Organic Catalysts

Co-reporter:Akimichi Ohtsuki ; Atsushi Goto

Macromolecules () pp:

Publication Date(Web):December 18, 2012

DOI:10.1021/ma302244j

Photoinduced reversible complexation mediated polymerization (photo-RCMP) was developed as a new photoinduced living radical polymerization (LRP). It consisted of an alkyl iodide as a dormant species and an amine as a catalyst, using visible light at 350–600 nm. The amine catalysts include such common compounds as tributylamine. Mechanistically, the polymerization is induced by the photolysis of the dormant species and dormant species/catalyst complex, which frequently occurs as the main activation process. The polymer molecular weight and its distribution (Mw/Mn = 1.1–1.4) were well controlled in the polymerizations of methyl methacrylate and some functional methacrylates up to fairly high conversions in many cases. Perfectly no polymerization took place without photoirradiation, meaning that the system is an ideal photo “on”–“off” switchable system. The polymerization rate was also finely tunable by the external irradiation power. Attractive features of photo-RCMP include the uses of inexpensive compounds and visible light, good polydispersity control, good tolerance to functional groups, and fine response to external photoirradiation.

Multiscale simulation of charge transport in a host material, N,N′-dicarbazole-3,5-benzene (mCP), for organic light-emitting diodes

Co-reporter:Furitsu Suzuki, Katsuyuki Shizu, Hisafumi Kawaguchi, Shinya Furukawa, Tohru Sato, Kazuyoshi Tanaka and Hironori Kaji

Journal of Materials Chemistry A 2015 - vol. 3(Issue 21) pp:NaN5555-5555

Publication Date(Web):2015/04/20

DOI:10.1039/C5TC00543D

CAS:1395881-58-9

CAS:1395881-55-6