Aiko Fukazawa

Ladder π-Conjugated Materials Containing Main-Group Elements

Ladder pi-conjugated compounds, which have fully ring-fused polycyclic skeletons, are an important class of materials possessing significant potentials for application in organic electronics. The incorporation of main-group elements, such as B, Si, P, S, and Se, into the ladder skeletons as bridging moieties is a powerful strategy to endow unusual electronic structures as well as suitable molecular arrangements in the solid state, giving rise to attractive photophysical and electronic properties. Recent efforts have produced a number of fascinating ladder materials, some of which indeed showed high performance as light-emitting materials and charge carrier transporting materials. This Focus Review is an overview of the progress in this chemistry, focusing on several important pi-conjugated skeletons.

Bis-Phosphoryl-Bridged Stilbenes Synthesized by an Intramolecular Cascade Cyclization

Bis-phosphoryl-bridged stilbenes have been synthesized using an intramolecular cascade cyclization. They show intense blue fluorescences at longer wavelengths with higher quantum yields compared to those of the known element-bridged stilbenes. In addition, they have much lower reduction potentials due to the inductive effect of phosphoryl groups. The incorporation of the phosphoryl moiety is an effective way for the construction of highly electron-accepting pi-conjugated systems.

Reaction of pentaarylboroles with carbon monoxide: an isolable organoboron carbonyl complex

The highly Lewis acidic perfluoropentaphenylborole forms a stable, isolable adduct with the weak Lewis base carbon monoxide. A similar adduct with the unfluorinated borole is observed at low temperature, but this adduct undergoes reaction involving insertion into the B–C bonds due to the greater nucleophilicity of the α-carbons. Together these observations provide concrete chemical evidence for long held presumptions regarding the observed reactivity of organoboranes with carbon monoxide.

Environment-Sensitive Fluorescent Probe: A Benzophosphole Oxide with an Electron-Donating Substituent†

Electron-donating aryl groups were attached to electron-accepting benzophosphole skeletons. Among several derivatives thus prepared, one benzophosphole oxide was particularly interesting, as it retained high fluorescence quantum yields even in polar and protic solvents. This phosphole-based compound exhibited a drastic color change of its fluorescence spectrum as a function of the solvent polarity, while the absorption spectra remained virtually unchanged. Capitalizing on these features, this phosphole-based compound was used to stain adipocytes, in which the polarity of subcellular compartments could then be discriminated on the basis of the color change of the fluorescence emission.

Bis(phosphoryl)-Bridged Biphenyls by Radical Phosphanylation: Synthesis and Photophysical and Electrochemical Properties†

The design of new electron-accepting p-conjugated frameworks is of particular significance for the development of n-type semiconducting materials and narrow-band gap polymers, which show great potential as components in organic electronics, such as thin-film transistors and photovoltaic cells. Biphenyls containing electron-accepting moieties might be simple and viable scaffolds for the design of such p-electron systems. However, by simply introducing electronwithdrawing groups as substituents renders the biphenyl framework to appear in a twist conformation, resulting in a decrease of p-conjugation. This problem can be elegantly solved by incorporating an electron-accepting unit as the bridging moiety. In this regard, main group elements are attractive as the bridging moieties, since they not only fix the biaryl framework in a planar geometry, but also allow the electronic structure to be modified through the choice of element. Accordingly, various electron-accepting dibenzoheteroles 1 featuring Si, P, and S as bridging elements have been reported and used in various applications. In particular, P-containing p-electron systems have so far attracted considerable attention, because of their rich follow-up chemistry, which is attributed to easy transformations to oxides, sulfides, and metal/Lewis acid complexes. Among the various functionalities derived from phosphanes, phosphine oxides or sulfides are of particular interest due to their highly electron-accepting character. As a novel electron-accepting biphenyl, we designed bis(phosphoryl)-bridged biphenyl (BPB) 2, which displayed the following characteristics: a) compact and planar structure enabling effective orbital overlap and b) high electronaccepting ability owing to two phosphine oxide units. Related biphenyls 3 containing Si, S, Se, and C as the bridging moieties have already been reported (Figure 1). Herein, we disclose the synthesis of this novel skeleton and discuss its potential as the electron-accepting unit. All our initial attempts to access BPB 2 by fourfold lithiation of 2,2,2’,2’-tetrabromobiphenyl (4) followed by trapping with dichlorophenylphosphane and P-oxidation failed. The problem was associated with multiple lithiation of 4. We have recently shown that radical phosphanylation of reactive aryl radicals is a highly efficient approach for the synthesis of arylphosphanes. The reactions occur under rather mild conditions, and expensive transition-metal catalysts are not necessary. Stannylated and silylated phosphanes have been used as reagents, and reactions occurred in high yields. We envisioned that 2 should be accessible by multiple radical phosphanylation of biphenyl 4 with bis(trimethylstannyl)phenylphosphane and subsequent oxidation. We first tried radical phosphanylation of 4 with readily prepared (Me3Sn)2PPh using a,a’-azobisisobutyronitrile (AIBN) as initiator at 80 8C in benzene. Disappointingly, after 24 h little conversion of the starting material had occurred and after H2O2 oxidation none of the targeted BPB 2 was identified (Table 1, entry 1). The same result was obtained by performing the reaction at 125 8C (Table 1, entry 2). To our delight, switching to 1,1’-azobis(cyclohexane-1-carbonitrile) (V-40) as initiator afforded traces of the desired BPB after 24 h (Table 1, entry 3). Prolonged reaction time led to higher conversion, and after oxidation with H2O2 trans-2 and cis-2 were isolated in good yields (Table 1, entry 4). As expected, oxidation did not occur diastereoselectively and both isomers were isolated in similar yields. By running the reaction in benzotrifluoride the reaction time could be shortened to two days without decreasing the yield (Table 1, entry 5). The success of this transformation is an impressive demonstration of the efficiency of the radical phosphanylation: four highly reactive aryl radicals are trapped sequentially and although severe ring strain is generated in the formation of the second five-membered Figure 1. Electron-accepting dibenzoheteroles 1 and doubly bridged biphenyls 2 and 3.

A Strap Strategy for Construction of an Excited‐State Intramolecular Proton Transfer (ESIPT) System with Dual Fluorescence

An amine-embedded flexible alkyl strap has been incorporated into an emissive boryl-substituted dithienylpyrrole skeleton as a new entity of excited-state intramolecular proton transfer (ESIPT) chromophores. The π-electron system shows a dual emission, which covers a wide range of the visible region depending on the solvent polarity. The incorporation of the aminoalkyl strap as well as the terminal boryl groups efficiently stabilize the zwitterionic excited-state species resulting from the ESIPT even in an aqueous medium.

Electron-donating tetrathienyl-substituted borole.

Borole is a boron-containing unsaturated five-membered ring with 4 p electrons, and is isoelectronic to a cyclopentadienyl cation (C5H5 ). The unusual electronic structures of borolebased p-electron-containing materials have been predicted by theoretical studies. After the first synthesis of a borole derivative, 1,2,3,4,5-pentaphenylborole (1a, Scheme 1), by Eisch and coworkers, 5] only few reports on boroles have appeared for almost four decades, and the crystal structure of 1a has only quite recently been determined by X-ray crystallography. 7] Thereupon, the interest in this chemistry has been dramatically revived. Various intriguing borole derivatives have been reported within the past several years, including fused boroles, ferrocenylborole, haloboroles, borole–metal complexes, carbene-coordinated boroles, and the highly Lewis-acidic perfluorinated boroles. The most notable electronic feature of the borole skeleton in comparison to other heteroles is its significantly low-lying LUMO. 2] Therefore, borole-based p-conjugated compounds have been widely recognized as electron-accepting systems. The low-lying LUMOs of these compounds in combination with their antiaromaticity allow them to undergo various reactions, such as the formation of adducts with Lewis acids and bases, B C bond cleavage, Diels–Alder reactions with alkynes, 16c] and reduction to a radical anion or dianion. 12, 13b] Recently, the activation of H2 without any transition metal has also been demonstrated by the use of borole derivatives. The Lewis acidity of the perarylated boroles 1 can be significantly enhanced by modification of the peripheral aryl groups by introducing electron-withdrawing substituents, as exemplified by perfluorophenylborole 2 described by Piers and co-workers. The utilization of the borole skeleton as an electron-accepting unit in the extended p-conjugated system has been also demonstrated. In our studies, we noticed the other important feature of the borole ring, its relatively high-lying HOMO compared with other electron-accepting heteroles, such as silole and phosphole. This difference might be due to the inductive effect that results from the s-donating character of the boron atom, because boron is a more electron-positive element than carbon. In order to highlight this feature, we designed a thienyl-substituted borole 3 (Scheme 1) with a pronounced electron-donating character as a new example of borole-

A Phosphole Oxide Based Fluorescent Dye with Exceptional Resistance to Photobleaching: A Practical Tool for Continuous Imaging in STED Microscopy

The development of stimulated emission depletion (STED) microscopy represented a major breakthrough in cellular and molecular biology. However, the intense laser beams required for both excitation and STED usually provoke rapid photobleaching of fluorescent molecular probes, which significantly limits the performance and practical utility of STED microscopy. We herein developed a photoresistant fluorescent dye C-Naphox as a practical tool for STED imaging. With excitation using either a λ=405 or 488 nm laser in protic solvents, C-Naphox exhibited an intense red/orange fluorescence (quantum yield ΦF >0.7) with a large Stokes shift (circa 5900 cm(-1) ). Even after irradiation with a Xe lamp (300 W, λex =460 nm, full width at half maximum (FWHM)=11 nm) for 12 hours, 99.5 % of C-Naphox remained intact. The high photoresistance of C-Naphox allowed repeated STED imaging of HeLa cells. Even after recording 50 STED images, 83 % of the initial fluorescence intensity persisted.

Intense fluorescence of 1-aryl-2,3,4,5-tetraphenylphosphole oxides in the crystalline state

A series of 2,3,4,5-tetraphenylphosphole oxides with various aryl groups on the phosphorus atoms were synthesized and their fluorescence properties in the crystalline state were investigated. Some of these compounds showed an intense fluorescence with the quantum yields of 0.75–0.91. These intense emissions are attributable not only to the restricted rotation of the peripheral phenyl rings, but also to the long intermolecular distances between the adjacent molecules in the crystal packing.

Zwitterionic Ladder Bis(arylethenyl)benzenes with Large Two‐Photon Absorption Cross Sections

The two-photon absorption (TPA) properties of organic chromophores have recently attracted much attention owing to a variety of potential applications, such as 3D microfabrication, ultra-high-density optical data storage, optical-limiting devices, up-converted lasing, and photodynamic therapy. Considerable effort has been devoted to developing an excellent organic TPA material with a large TPA cross section (d). Several TPA materials with d values of several hundreds to thousands GM (1 GM=10 50 cm s photon ) have already been reported, including dipolar or quadrupolar p-conjugated chromophores, oligomeric porphyrin derivatives, and giant macrocycles. These studies have provided important guidelines for the molecular design of excellent TPA materials. Namely, 1) a sufficiently long pconjugation length, 2) introduction of strong donor and acceptor moieties into a p-conjugated skeleton, and 3) a planar and rigid molecular structure are important structural requirements for large d values. Regarding these guidelines, one promising class of TPA materials may be the laddertype p-conjugated compounds with polar electronic structures. We have recently succeeded in synthesizing such compounds categorized into this class, i.e. , phosphoniumand borate-bridged distyrylbenzenes (DSBs) 1 and 2 (Figure 1). In these compounds, the zwitterionic bridges not only fix the DSB framework in a rigid and coplanar fashion, but also provide a highly polar electronic structure. The phosphonium and borate functionalities serve as strong electron-accepting and -donating moieties, respectively. All these features seem to well satisfy the foregoing requirements for excellent TPA properties. Whereas it has already been reported that several p-electron systems with zwitterionic groups have excellent nonlinear optical properties, our current interest lies in the effects of the zwitterionic bridges in the completely planar p skeleton. We now report the TPA characteristics of a series of phosphoniumand borate-bridged ladder p-electron systems. In particular, we newly synthesized dipolar PB PB -DSB 3 (A-D-A-D type, Figure 1) and compared its photophysical properties with those of the quadrupolar PB B P-DSB 1 (A-D-A type) and B PPB -DSB 2 (D-A-D type) to address the effect of the bridging patterns (D and A denote the donor and acceptor, respectively). Furthermore, we designed and synthesized a thiophene-terminated analogue, the B PP [a] Dr. A. Fukazawa, Dr. H. Yamada, Prof. Dr. S. Yamaguchi Department of Chemistry, Graduate School of Science Nagoya University Furo, Chikusa, Nagoya 464-8602 (Japan) Fax: (+81) 52-789-5947 E-mail : yamaguchi.shigehiro@b.mbox.nagoya-u.ac.jp [b] Y. Sasaki, Dr. S. Akiyama Mitsubishi Chemical Group Science and Technology Research Center, Inc. 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.200900517. Figure 1. Zwitterion-bridged ladder p-conjugated compounds.