Correlation between Excited-State Intramolecular Proton-Transfer and Singlet-Oxygen Quenching Activities in 1‑(Acylamino)anthraquinones

Excited-state intramolecular proton-transfer (ESIPT) and singlet-oxygen (¹O₂) quenching activities of intramolecularly hydrogen-bonded 1-(acylamino)­anthraquinones have been studied by means of static and laser spectroscopies. The ESIPT shows a substituent effect, which can be explained in terms of the nodal-plane model. The ESIPT activity positively and linearly correlates with their ¹O₂ quenching activity. The reason for this correlation can be understood by considering ESIPT-induced distortion of their ground-state potential surface and their encounter complex formation with ¹O₂. Intramolecularly hydrogen-bonded hydroxyanthraquinones found in aloe also show a similar positive and linear correlation, which can be understood in the same way

Correlation among Singlet-Oxygen Quenching, Free-Radical Scavenging, and Excited-State Intramolecular-Proton-Transfer Activities in Hydroxyflavones, Anthocyanidins, and 1‑Hydroxyanthraquinones

Singlet-oxygen (¹O₂) quenching, free-radical scavenging, and excited-state intramolecular proton-transfer (ESIPT) activities of hydroxyflavones, anthocyanidins, and 1-hydroxyanthraquinones were studied by means of laser, stopped-flow, and steady-state spectroscopies. In hydroxyflavones and anthocyanidins, the ¹O₂ quenching activity positively correlates to the free-radical scavenging activity. The reason for this correlation can be understood by considering that an early step of each reaction involves electron transfer from the unfused phenyl ring (B-ring), which is singly bonded to the bicyclic chromen or chromenylium moiety (A- and C-rings). Substitution of an electron-donating OH group at B-ring enhances the electron transfer leading to activation of the ¹O₂ quenching and free-radical scavenging. In 3-hydroxyflavones, the OH substitution at B-ring reduces the activity of ESIPT within C-ring, which can be explained in terms of the nodal-plane model. As a result, the ¹O₂ quenching and free-radical scavenging activities negatively correlate to the ESIPT activity. A catechol structure at B-ring is another factor that enhances the free-radical scavenging in hydroxyflavones. In contrast to these hydroxyflavones, 1-hydroxyanthraquinones having an electron-donating OH substituent adjacent to the O–H---OC moiety susceptible to ESIPT do not show a simple correlation between their ¹O₂ quenching and ESIPT activities, because the OH substitution modulates these reactions

Synthesis, Structures, and Properties of Core-Expanded Azacoronene Analogue: A Twisted π‑System with Two N‑Doped Heptagons

A core-expanded, pyrrole-fused azacoronene analogue containing two unusual N-doped heptagons was obtained from commercially available octafluoronaphthalene and 3,4-diethylpyrrole in two steps as a heteroatom-doped nonplanar nanographene. Full fusion with the formation of the tetraazadipleiadiene framework and the longitudinally twisted structure was unambiguously confirmed by single-crystal X-ray diffraction analysis. The edge-to-edge dihedral angle along the acene moiety was 63°. This electron-rich π-system showed four reversible oxidation peaks. Despite the nonplanar structure, the Hückel aromaticity owing to a peripheral π-conjugation in the dicationic state was concluded from the bond-length alternation and nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations

Synthesis, Structures, and Properties of Core-Expanded Azacoronene Analogue: A Twisted π‑System with Two N‑Doped Heptagons

A core-expanded, pyrrole-fused azacoronene analogue containing two unusual N-doped heptagons was obtained from commercially available octafluoronaphthalene and 3,4-diethylpyrrole in two steps as a heteroatom-doped nonplanar nanographene. Full fusion with the formation of the tetraazadipleiadiene framework and the longitudinally twisted structure was unambiguously confirmed by single-crystal X-ray diffraction analysis. The edge-to-edge dihedral angle along the acene moiety was 63°. This electron-rich π-system showed four reversible oxidation peaks. Despite the nonplanar structure, the Hückel aromaticity owing to a peripheral π-conjugation in the dicationic state was concluded from the bond-length alternation and nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations

Synthesis, Structures, and Properties of Core-Expanded Azacoronene Analogue: A Twisted π‑System with Two N‑Doped Heptagons

A core-expanded, pyrrole-fused azacoronene analogue containing two unusual N-doped heptagons was obtained from commercially available octafluoronaphthalene and 3,4-diethylpyrrole in two steps as a heteroatom-doped nonplanar nanographene. Full fusion with the formation of the tetraazadipleiadiene framework and the longitudinally twisted structure was unambiguously confirmed by single-crystal X-ray diffraction analysis. The edge-to-edge dihedral angle along the acene moiety was 63°. This electron-rich π-system showed four reversible oxidation peaks. Despite the nonplanar structure, the Hückel aromaticity owing to a peripheral π-conjugation in the dicationic state was concluded from the bond-length alternation and nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations

Molecular Photoconductor with Simultaneously Photocontrollable Localized Spins

UV irradiation reversibly switches a new insulating and nonmagnetic molecular crystal, BPY­[Ni­(dmit)₂]₂ (BPY = <i>N</i>,<i>N</i>′-ethylene-2,2′-bipyridinium; Ni­(dmit)₂ = bis­(1,3-dithiole-2-thione-4,5-dithiolato)­nickelate­(III)), into a magnetic conductor. This is possible because the bipyridyl derivative cations (BPY²⁺) trigger a photochemical redox reaction in the crystal to produce a change of ∼10% in the filling of the Ni­(dmit)₂ valence band, leaving localized spins on the BPY themselves. In the dark, almost all of the BPY molecules are closed-shell cations, and most of the Ni­(dmit)₂ radical anions form spin-singlet pairs; thus, this material is a diamagnetic semiconductor. Under UV irradiation, a photocurrent is observed, which enhances the conductivity by 1 order of magnitude. Electron spin resonance measurements indicate that the UV irradiation reversibly generates carriers and localized spins on the Ni­(dmit)₂ and the BPY, respectively. This high photoconductivity can be explained by charge transfer (CT) transitions between Ni­(dmit)₂ and BPY in the UV region. In other words, the photoconduction and “photomagnetism” can be described as reversible optical control of the electronic states between an ionic salt (BPY²⁺/[Ni­(dmit)₂]⁻, nonmagnetic insulator) and a CT complex (BPY^2(1−δ)+/[Ni­(dmit)₂]^(1−δ)– (δ ≈ 0.1), magnetic conductor) in the solid state

Molecular Photoconductor with Simultaneously Photocontrollable Localized Spins

UV irradiation reversibly switches a new insulating and nonmagnetic molecular crystal, BPY­[Ni­(dmit)₂]₂ (BPY = <i>N</i>,<i>N</i>′-ethylene-2,2′-bipyridinium; Ni­(dmit)₂ = bis­(1,3-dithiole-2-thione-4,5-dithiolato)­nickelate­(III)), into a magnetic conductor. This is possible because the bipyridyl derivative cations (BPY²⁺) trigger a photochemical redox reaction in the crystal to produce a change of ∼10% in the filling of the Ni­(dmit)₂ valence band, leaving localized spins on the BPY themselves. In the dark, almost all of the BPY molecules are closed-shell cations, and most of the Ni­(dmit)₂ radical anions form spin-singlet pairs; thus, this material is a diamagnetic semiconductor. Under UV irradiation, a photocurrent is observed, which enhances the conductivity by 1 order of magnitude. Electron spin resonance measurements indicate that the UV irradiation reversibly generates carriers and localized spins on the Ni­(dmit)₂ and the BPY, respectively. This high photoconductivity can be explained by charge transfer (CT) transitions between Ni­(dmit)₂ and BPY in the UV region. In other words, the photoconduction and “photomagnetism” can be described as reversible optical control of the electronic states between an ionic salt (BPY²⁺/[Ni­(dmit)₂]⁻, nonmagnetic insulator) and a CT complex (BPY^2(1−δ)+/[Ni­(dmit)₂]^(1−δ)– (δ ≈ 0.1), magnetic conductor) in the solid state

Template Synthesis of Decaphyrin without <i>Meso</i>-Bridges: Cyclo[10]pyrrole

An acenaphthylene-fused cyclo[10]­pyrrole <b>1b</b> was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2′-bipyrrole with the appropriate dianion template, croconate anion. The structure of <b>1b</b> as the isolated largest cyclo­[<i>n</i>]­pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl’s perimeter and Gouterman’s 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations

Template Synthesis of Decaphyrin without <i>Meso</i>-Bridges: Cyclo[10]pyrrole

An acenaphthylene-fused cyclo[10]­pyrrole <b>1b</b> was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2′-bipyrrole with the appropriate dianion template, croconate anion. The structure of <b>1b</b> as the isolated largest cyclo­[<i>n</i>]­pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl’s perimeter and Gouterman’s 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations

Template Synthesis of Decaphyrin without <i>Meso</i>-Bridges: Cyclo[10]pyrrole

An acenaphthylene-fused cyclo[10]­pyrrole <b>1b</b> was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2′-bipyrrole with the appropriate dianion template, croconate anion. The structure of <b>1b</b> as the isolated largest cyclo­[<i>n</i>]­pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl’s perimeter and Gouterman’s 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations