Koki Ikemoto

X-ray snapshot observation of palladium-mediated aromatic bromination in a porous complex.

Pd-mediated aromatic bromination is intriguing to synthetic and organometallic chemists due to both its synthetic utility and, more importantly, a proposed mechanism involving an uncommon Pd(IV)/Pd(II) catalytic cycle. Here, we report an X-ray snapshot observation of a Pd reaction center during a Pd-mediated aromatic bromination in a single crystal of a porous coordination network crystalline scaffold. Upon treatment of a single crystal with N-bromosuccinimide, sequential X-ray snapshots revealed that the aryl-Pd(II)-L species embedded in the network pores was converted to the brominated aryl product through a transient aryl-Pd(II)-Br species, which is normally unobservable because of its rapid dimerization into insoluble Pd2(μ-Br)2 species. Though the reaction pathway may be biased by the crystalline state, the new X-ray snapshot method relies on crystalline flasks to provide important mechanistic insight.

Diels–Alder via Molecular Recognition in a Crystalline Molecular Flask

In the pore of a porous coordination network, Diels-Alder reactants, a diene and a dienophile, are recognized by donor-acceptor and multiple H-bond interactions, respectively, and fixed at ideal positions for the reaction. Heating the crystals promoted the Diels-Alder reactions with enhanced reactivity and controlled regioselectivity as clearly monitored by in situ X-ray crystallography.

The Reaction of Organozinc Compounds with an Aldehyde within a Crystalline Molecular Flask

When bulky substrates and reagents exhibit pseudo solutionstate mobility and reactivity within porous coordination networks, the networks can be employed as “crystalline molecular flasks”. The robust crystallinity of the networks facilitates the use of X-ray crystallography, the ultimate method of structural determination, for the in-situ observation of various organic transformations. Unstable products and even a transient reaction intermediate were protected within the crystalline flasks and were directly observed by Xray crystallography. Until now, reactions have been limited to mild conditions (typically, neutral pH at room temperature), but herein we show that crystalline flasks can tolerate the inclusion of reactive organozinc reagents and that the subsequent reaction with an embedded aromatic aldehyde showed enhanced reactivity and selectivity. Recently, the addition of organozinc reagents to aldehydes in organic solvents was catalyzed in the presence of a binaphtholcontaining porous network, but it was unclear where the reaction occurred. In contrast, direct X-ray analysis of crystalline flasks revealed the precise reaction location and underscores the potential applications of crystalline flasks toward organometallic transformations. Our porous network crystallized from ZnI2, tris(4-pyridyl)triazine (2) and 2-formyltriphenylene (3) in nitrobenzenemethanol solvent mixture (Scheme 1). Elemental analysis elucidated the molecular formula of {[(ZnI2)3(2)2(3)]· x(solvent)}n (1), with nitrobenzene as cocrystallized solvent (x = 4.5). Triphenylene guest 3 is an integral part of the network structure, which tolerates a variety of functional groups. Analogous networks containing 3 have been reported, 2c] but herein, the procedure was scaled up and network complex 1 prepared in large quantities (205 mg) in 21% yield (see the Supporting Information). The porous structure 1, determined by X-ray crystallographic analysis, exhibits two pores, A and B, which are characteristic of this type of coordination network (Figure 1a). Guest 3 is embedded in columnar stacks of ligand 2, Scheme 1. a) Preparation of the network 1, and b) addition reaction of organozinc compounds to 3 within crystals of 1.

Cyclo-meta-phenylene Revisited: Nickel-Mediated Synthesis, Molecular Structures, and Device Applications

From a one-pot nickel-mediated Yamamoto-type coupling reaction of m-dibromobenzene, five congeners of [n]cyclo-meta-phenylenes were synthesized and fully characterized. The [n]cyclo-meta-phenylenes possessed a commonly shared arylene unit and intermolecular contacts but varied in packing structures in the crystalline solid state. Columnar assembly of larger congeners yielded nanoporous crystals with carbonaceous walls to capture minor protic or aliphatic solvent molecules. The concise and scalable synthesis allowed exploration of the macrocyclic hydrocarbons as bipolar charge carrier transport materials in organic light-emitting diode devices.

Networked-cage microcrystals for evaluation of host-guest interactions.

We have developed a new synthetic protocol for the preparation of a microcrystalline powder (median size: X50 = 25 μm) of networked M6L4 cages 1a for the stationary phase of an affinity column on a greater than 50 g scale. Analogously to large single crystals 1b (X50 ≈ 0.5 mm), microcrystals 1a accommodate guest molecules tetrathiafulvalene (TTF) and fullerene (C60) at up to 32 and 35 wt %, respectively. Importantly, the host-guest interactions within networked cages could be evaluated in terms of the retention time from HPLC analysis by using microcrystals 1a as the stationary phase. In this way, favorable guests for networked cages 1 and even solution M6L4 cage 2 could easily be assessed by HPLC.

Modular Synthesis of Aromatic Hydrocarbon Macrocycles for Simplified, Single-Layer Organic Light-Emitting Devices

A method for the modular synthesis of aromatic hydrocarbon macrocycles has been developed for base materials in single-layer organic light-emitting devices. The method with Ir-catalyzed direct C-H borylation and Suzuki-Miyaura coupling was concise and scalable, which allowed for a gram-scale preparation of aromatic hydrocarbon macrocycles that have bulky substituents at the periphery. The new arylated hydrocarbon macrocycles enabled a quantitative electro-optical conversion in organic light-emitting devices with a phosphorescent emitter, which is, notably, in a single-layer architecture consisting of two regions of doped and undoped materials. The highest external quantum efficiencies reached 24.8%, surpassing those of previous hydrocarbon base materials.

Entropy-Driven Ball-in-Bowl Assembly of Fullerene and Geodesic Phenylene Bowl

Complexation of C60 at a conical region of a nanometer-sized geodesic phenylene bowl has been demonstrated. Proton NMR spectroscopy showed formation of a 1:1 complex that was driven by entropy gains for the assembly. Crystallographic analyses revealed its unique ball-in-bowl structure, and the presence of smoothly curved surfaces was unveiled at their interfaces.

Efficient Blue Electroluminescence from a Single-layer Organic Device Composed Solely of Hydrocarbons

An interesting physical phenomenon, electroluminescence, that was originally observed with a hydrocarbon molecule has recently been developed into highly efficient organic light-emitting devices. These modern devices have evolved through the development of multi-element molecular materials for specific roles, and hydrocarbon devices have been left unexplored. In this study, we report an efficient organic light-emitting device composed solely of hydrocarbon materials. The electroluminescence was achieved in the blue region by efficient fluorescence and charge recombination within a simple single-layer architecture of macrocyclic aromatic hydrocarbons. This study may stimulate further studies on hydrocarbons to uncover their full potential as electronic materials.

[n]Cyclo‐3,6‐phenanthrenylenes: Synthesis, Structure, and Fluorescence

Five congeners of [n]cyclo-3,6-phenanthrenylene with 3, 4, 5, 7, and 8 panels were obtained from one-pot macrocyclization of dibromophenanthrene, and their crystal structures with diverse molecular shapes were revealed by X-ray crystallography. The compounds, except the four-panel congener, were highly fluorescent in solution, with quantum yields up to 85 %. The least fluorescent four-panel congener showed the smallest change in its absorption spectrum from that of monomeric phenanthrene, which provided an interesting structure-activity relationship for fluorescent macrocycles to guide future studies.

Magneto-electroluminescence effects in the single-layer organic light-emitting devices with macrocyclic aromatic hydrocarbons

Magneto-electroluminescence (MEL) effects are observed in single-layer organic light-emitting devices (OLEDs) comprising only macrocyclic aromatic hydrocarbons (MAHs). The fluorescence devices were prepared using synthesized MAHs, namely, [n]cyclo-meta-phenylene ([n]CMP, n = 5, 6). The MEL ratio of the resulting OLED is 1%–2% in the spectral wavelength range of 400-500 nm, whereas it becomes negative (−1.5% to −2%) in the range from 650 to 700 nm. The possible physical origins of the sign change in the MEL are discussed. This wavelength-dependent sign change in the MEL ratio could be a unique function for future single-layer OLEDs capable of magnetic-field-induced color changes.