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.

Heptalene, a nonaromatic, bicyclic 12 π-electron system with a twisted structure, is of great interest with regard to its potential Hückel aromaticity in the two-electron oxidized or reduced forms. The synthesis of thiophene-fused heptalene 5 from the reductive transannular cyclization of bisdehydro[12]annulene 4, and its solid-state structure, which was confirmed by X-ray crystallographic analysis, is presented. Chemical reduction of 5 readily generated the corresponding dianion, which was successfully isolated as [(K[2.2.2]cryptand)⁺]₂5²⁻. The X-ray crystallographic analysis of the dianion revealed a shallower saddle structure for the heptalene moiety and a lesser degree of bond alternation relative to 5. ¹H NMR spectroscopy exposed the effect of a diamagnetic ring current on dianion 5²⁻, which was corroborated by nucleus-independent chemical shift (NICS) calculations. These results demonstrate that the heptalene dianion, containing 14 π-electrons, does indeed exhibit pronounced degrees of Hückel aromaticity.

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 Φ_F>0.7) with a large Stokes shift (circa 5900 cm⁻¹). Even after irradiation with a Xe lamp (300 W, λ_ex=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.

Heptalene, a nonaromatic, bicyclic 12 π-electron system with a twisted structure, is of great interest with regard to its potential Hückel aromaticity in the two-electron oxidized or reduced forms. The synthesis of thiophene-fused heptalene 5 from the reductive transannular cyclization of bisdehydro[12]annulene 4, and its solid-state structure, which was confirmed by X-ray crystallographic analysis, is presented. Chemical reduction of 5 readily generated the corresponding dianion, which was successfully isolated as [(K[2.2.2]cryptand)⁺]₂5²⁻. The X-ray crystallographic analysis of the dianion revealed a shallower saddle structure for the heptalene moiety and a lesser degree of bond alternation relative to 5. ¹H NMR spectroscopy exposed the effect of a diamagnetic ring current on dianion 5²⁻, which was corroborated by nucleus-independent chemical shift (NICS) calculations. These results demonstrate that the heptalene dianion, containing 14 π-electrons, does indeed exhibit pronounced degrees of Hückel aromaticity.

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.

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 Φ_F>0.7) with a large Stokes shift (circa 5900 cm⁻¹). Even after irradiation with a Xe lamp (300 W, λ_ex=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.

Aiming at a molecule that has distinct fluorescence properties dependent on the environment of the sample, we designed and synthesized 2-(o-hydroxyphenyl)-substituted benzophosphole oxides, which can form an intramolecular hydrogen bond between the hydroxy group and the phosphine oxide moiety. A 3-phenylated derivative fluoresces in several specific solvents that have hydrogen-bond-accepting abilities, whereas it is virtually non-emissive in toluene, CH₂Cl₂, and MeCN. Experimental and theoretical studies indicate that this trend likely results from the switching of the excited-state character. When in weak hydrogen-bonding solvents, the compound forms a non-emissive excited state that results from excited-state intramolecular proton transfer (ESIPT). In hydrogen-bonding solvents the compound forms an emissive excited state with an intermolecular hydrogen bond between the hydroxy group and the solvent. Switching between the intramolecular and intermolecular hydrogen bonds can be a mechanism for controlling the fluorescence properties of the phosphole-oxide-based π-electron systems.