Eriko Yamaguchi

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.

Synthesis of pyridine N-oxide-BF2CF3 complexes and their fluorescence properties

Pyridine N-oxide-BF2CF3 and -BF2C2F5 complexes and their derivatives were synthesized. Most of the complexes show fluorescence both in solution and in the solid state. By expanding the π-conjugated skeleton, the color of the fluorescence could be changed dramatically. A fluorophore with a high solvent dependency could also be produced. Since such compounds can be synthesized on a gram scale in high yield, and are stable to oxygen, water, and heat, the complexes hold great potential as organic functional materials.

Synthesis and Physical Properties of Strained Doubly Phosphorus-Bridged Biaryls and Viologens

New P/N-containing π-electron systems comprising fully planar biaryl arrays are synthesized by multiple radical phosphanylation. The biaryl moiety in these highly strained planar π-systems is rigidified by double P-bridging. The electronic properties of the core biaryl entity are varied by introducing N-donor substituents or by installing N-atoms within the π-system, thereby moving to the viologen core structure. The electrochemical and photophysical properties of these compounds are discussed and compared with those of related systems.