Flattened 1D fragments of fullerene C₆₀ that exhibit robustness toward multi-electron reduction

フラーレンに迫る電子受容能をもつ平坦な一次元π共役炭化水素の開発. 京都大学プレスリリース. 2023-05-15.Flat fullerene fragments attractive to electrons. 京都大学プレスリリース. 2023-06-01.Fullerenes are compelling molecular materials owing to their exceptional robustness toward multi-electron reduction. Although scientists have attempted to address this feature by synthesizing various fragment molecules, the origin of this electron affinity remains unclear. Several structural factors have been suggested, including high symmetry, pyramidalized carbon atoms, and five-membered ring substructures. To elucidate the role of the five-membered ring substructures without the influence of high symmetry and pyramidalized carbon atoms, we herein report the synthesis and electron-accepting properties of oligo(biindenylidene)s, a flattened one-dimensional fragment of fullerene C₆₀. Electrochemical studies corroborated that oligo(biindenylidene)s can accept electrons up to equal to the number of five-membered rings in their main chains. Moreover, ultraviolet/visible/near-infrared absorption spectroscopy revealed that oligo(biindenylidene)s exhibit enhanced absorption covering the entire visible region relative to C₆₀. These results highlight the significance of the pentagonal substructure for attaining stability toward multi-electron reduction and provide a strategy for the molecular design of electron-accepting π-conjugated hydrocarbons even without electron-withdrawing groups

A Near‐Infrared Dye That Undergoes Multiple Interconversions through Acid–Base Equilibrium and Reversible Redox Processes

AbstractA near‐infrared (NIR) polymethine dye (1), consisting of a cyclohepta[1,2‐b;4,3‐b′]dithiophene and two phenol moieties, was synthesized. This dye exhibited pH‐responsive changes in its photophysical properties due to a two‐step acid–base equilibrium that produced a protonated cation (1H+) and an anion (1−). While 1H+ showed an intense fluorescence in the red region of the visible spectrum, 1− exhibited a strong absorption in the NIR region. The tropylium ion character in 1H+ induces high pKa1 and pKa2 values for 1. Moreover, a stable radical (1.) was prepared, which showed a NIR absorption band with a maximum at circa 1600 nm. The cyclic voltammogram of 1. revealed a two‐step reversible redox process that produced 1− and the cation 1+, which is different from 1H+. These redox processes accompany drastic electrochromic changes in the vis–NIR region. Overall, 1 is susceptible to multiple interconversions between five forms, due to the multifaceted character of the cycloheptadithiophene skeleton

Negative fluorescence anisotropy of phosphole oxide-based dyes in nematic liquid crystals

The alignment of dyes within liquid crystals has significant consequences for their applications in microscopy and LCDs. Here the negative fluorescence anisotropy of a series of phosphole oxide-based dyes in liquid crystals is rationalised by theoretical analysis of their structures

One-dimensional fragments of fullerene C60 that exhibit robustness toward multi-electron reduction and pronounced light absorption

Fullerenes are compelling molecular materials owing to their exceptional robustness toward multi-electron reduction. Although scientists have attempted to address this feature by synthesizing various fragment molecules, the origin of this electron affinity remains unclear. Several structural factors have been suggested, including high symmetry, curved structures, and five-membered ring substructures. To elucidate the role of the five-membered ring substructures without the influence of high symmetry and curved structure, we herein report the synthesis and electron-accepting properties of oligo(biindenylidene)s, a one-dimensional fragment of fullerene C60. Electrochemical studies corroborated that oligo(biindenylidene)s accept electrons equal to the number of five-membered rings in their main chains. Moreover, ultraviolet/visible/near-infrared absorption spectroscopy revealed that oligo(biindenylidene)s exhibit significantly enhanced absorption covering the entire visible region in relation to C60. These results highlight the significance of the pentagonal substructure for attaining stability toward multi-electron reduction and provide a new strategy for the molecular design of electron-accepting -conjugated hydrocarbons

Tuning the Photophysical Properties of Photostable Benzo[<i>b</i>]phosphole <i>P</i>‑Oxide-Based Fluorophores

We previously reported that constrained 2-phenylbenzo­[<i>b</i>]­phosphole <i>P</i>-oxides bearing a diphenylamino group show high photostability and thus are promising dyes for fluorescence imaging. Herein we investigated the impact of the bridging moieties on their photophysical properties. A series of benzo­[<i>b</i>]­phosphole <i>P</i>-oxides constrained with various carbon or silicon bridges were synthesized. All of these compounds showed significant solvatochromism in fluorescence due to the intramolecular charge-transfer character in the excited state. The dipole moments in the excited state for the carbon-bridged derivatives are slightly larger than the silicon-bridged counterparts. Nevertheless, the latter compounds showed orange-red fluorescence in polar solvents with ca. 30 nm red-shifted maxima compared to the carbon analogues. Most importantly, the assessment of their photobleaching resistance revealed that the photostability of this compound series highly relies on the steric bulkiness of the bridging moiety, and even the silicon-bridged derivative can show outstanding photostability, as far as the silicon-bridging moiety has sufficient bulkiness

Dearomatization-Induced Transannular Cyclization: Synthesis of Electron-Accepting Thiophene‑<i>S</i>,<i>S</i>‑Dioxide-Fused Biphenylene

The transannular cyclization of dehydroannulenes bearing several alkyne moieties in close proximity is a powerful synthetic method for producing polycyclic aromatic hydrocarbons. We report that the reactivity can be switched by the aromaticity of the ring skeletons fused with the dehydroannulene core. Thus, while thiophene-fused bisdehydro[12]­annulene <b>1</b> was handled as a stable compound in the air at room temperature, the oxidation with <i>m</i>-chloroperbenzoic acid from the aromatic thiophene rings to the nonaromatic thiophene-<i>S</i>,<i>S</i>-dioxides induced the transannular cyclization, even at room temperature, which was completed within 1 day to produce the formal [2 + 2] cycloadduct <b>3</b>. This is in stark contrast to the fact that the thermal cyclization of <b>1</b> itself required heating at 80 °C for 9 days for completion. Experimental and theoretical studies indicate that the oxidation of even one thiophene ring in <b>1</b> sufficiently decreases the activation barrier for the transannular cyclization that proceeds through the 8π and 4π electrocyclic reaction sequence. The thiophene-<i>S</i>,<i>S</i>-dioxide-fused biphenylene <b>3</b> thus produced exhibits a set of intriguing properties, such as a higher electron affinity (<i>E</i>_1/2 = −1.17 V vs Fc and Fc⁺) and a stronger fluorescence (Φ_F = 0.20) than the other relevant biphenylene derivatives, which have electron-donating and nonfluorescent characteristics