Radially π-Extended Pyrrole-Fused Azacoronene: A Series of Crystal Structures of HPHAC with Various Oxidation States.

Radially π-extended hexapyrrolohexaazacoronene (HPHAC 2) was synthesized in two steps with 2,5-di-tert-butyl-acenaphtho[1,2-c]pyrrole and hexafluorobenzene. Like its parent HPHAC 1a, 2 exhibited reversible oxidation behavior. Crystal structures of not only the neutral and dicationic compounds but also the radical cation were all revealed. While analyzing its peculiar structure, the formation of a 1:1 complex with C60 was observed in solution by NMR and absorption analyses. The spectroscopic analyses of 2 and 1a were performed based on magnetic circular dichroism and theoretical 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.

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.

Template Synthesis of Decaphyrin without Meso-Bridges: Cyclo[10]pyrrole.

An acenaphthylene-fused cyclo[10]pyrrole 1b was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2'-bipyrrole with the appropriate dianion template, croconate anion. The structure of 1b as the isolated largest cyclo[n]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.

Scavenging rate constants of hydrophilic antioxidants against multiple reactive oxygen species.

Scavenging rate constants of eight hydrophilic antioxidants, including caffeic acid, chlorogenic acid, genistein, glutathione, N-acetylcysteine, rutin, trolox, and uric acid against multiple ROS, namely superoxide anion, hydroxyl radical, singlet oxygen, and alkoxyl radical were determined with the electron spin resonance method. Direct flash photolysis measurement of the second-order rate constant in the reaction of alkoxyl radical plus the spin trap 5,5-dimethyl-pyrroline N-oxide made it possible to evaluate scavenging rate constants in antioxidants. The magnitudes of scavenging rate constants were notably dependent on the character of each ROS and the overall rate constants were highest in hydroxyl radical scavenging and the lowest in superoxide anion. The highest scavenging rate constant against superoxide anion was obtained by chlorogenic acid (2.9 × 10(5) M(-1) s(-1)) and the lowest was by N-acetylcysteine (5.0 × 10(2) M(-1) s(-1)). For singlet oxygen, the highest was by glutathione (9.4 × 10(8) M(-1) s(-1)) and the lowest was by uric acid (2.3 × 10(6) M(-1) s(-1)). All other numbers are listed and illustrated. Redox potential measurements of the antioxidants indicated that the antioxidants are likely to react with superoxide anion and singlet oxygen through electron transfer processes.

Acenaphthylene-fused cyclo[8]pyrroles with intense near-IR-region absorption bands.

An acenaphthylene-fused cyclo[8]pyrrole was synthesized by using an oxidative coupling reaction of the corresponding 2,2'-bipyrrole. Two conformational isomers 1 a and 1 b were isolated, and their molecular structures were elucidated by X-ray crystallographic studies. The less-polar and lower-symmetry 1 b isomer can be converted into the 1 a isomer through a thermal ring flip. Application of the perimeter model developed by Michl to magnetic circular dichroism spectroscopic data and theoretical calculations demonstrate that there is a marked redshift of the near-IR absorption maxima relative to cyclo[8]isoindole because there is a significant stabilization of the LUMO due to the differing effects of a fused ring expansion with acenaphthylene and benzene moieties on the frontier π molecular orbitals.

Molecular photoconductor with simultaneously photocontrollable localized spins.

UV irradiation reversibly switches a new insulating and nonmagnetic molecular crystal, BPY[Ni(dmit)(2)](2) (BPY = N,N'-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 (BPY(2+)) 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 (BPY(2+)/[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.

Comparison between the free-radical-scavenging activities with vitamin E and ubiquinol in biological systems based on their reaction rates: a research account.

Detailed kinetic studies have been performed for the reaction of aroxyl (ArO.) radical with vitamin E (alpha-, beta-, gamma-, delta-tocopherol, TocH), ubiquinol-10, and related antioxidants in micellar solution, using a stopped-flow spectrophotometer. The second-order reaction rates (ks) obtained increased in the order of hydroquinone < tocol

Structure-activity relationship of the free-radical-scavenging reaction by vitamin E (alpha-, beta-, gamma-, delta-Tocopherols) and ubiquinol-10: pH dependence of the reaction rates.

The reaction rates (ks) of vitamin E (alpha-, beta-, gamma-, delta-tocopherols, TocH), ubiquinol-10, and related antioxidants (tocol, ubiquinol-0, and hydroquinone) with aroxyl (ArO(.-)) radical have been measured in micellar solution by stopped-flow spectrophotometer. The ks values increased in the order of hydroquinone < tocol < delta-TocH < ubiquinol-0 < gamma-TocH approximately beta-TocH < ubiquinol-10 < alpha-TocH at pH 4 approximately 8. The antioxidants which have lower oxidation potentials showed higher reactivities. The ks values of alpha-, beta-, gamma-, delta-tocopherol, and tocol remained constant between pH 4 and 10, and decreased rapidly at pH 11 approximately 12 by increasing pH value. From the pH dependence of ks values, the pKa values (= 13.1 approximately 12.6) have been determined for these tocopherols. The ks values of ubiquinol-10 also remained constant between pH 4 and 9, and increased rapidly at pH 9.5. Ubiquinol-10 is dibasic acid and can exist in three different molecular forms, depending on the pH value. By comparing the ks values with the mole fraction of each molecular form of ubiquinol-10, the reaction rate ks1 (= 1.21 x 10(5) M(-1)s(-1)) for the undissociated form, ks2 (= 1.04 x 10(6) M(-1)s(-1)) for monoanion and ks3 (= 0 M(-1)s(-1)) for dianion, and the pKa1 and pKa2 values (= 11.4 and 12.7) were determined. The ks2 value is 8.6 times as large as the ks1 value. Similar analyses were performed for ubiquinol-0 and hydroquinone. It was found that the relative ratio of ks values (100:21:20:2.9) of alpha-, beta-, gamma-, delta-tocopherols in micellar dispersion has good correlation with the relative biopotency ratios for rat fetal resorption, rat hemeolysis, and chicken muscle dystrophy. The relative antioxidant activities of alpha-tocopherol and ubiquinol-10 have been discussed based on the ks values obtained and their concentrations in serum and several tissues (heart, muscle, liver, kidney, and brain).

UV protection and singlet oxygen quenching activity of aloesaponarin I.

The UV protection and singlet oxygen quenching of aloesaponarin I have been studied by means of laser spectroscopy. The excited-state intramolecular proton transfer that provides the UV protection takes place along only one of the molecule's two intramolecular hydrogen bonds, and this can be understood by considering the nodal pattern of the wave function. The functional groups participating in the excited-state intramolecular proton transfer also play important roles in the singlet oxygen quenching. Aloesaponarin I has a quenching rate constant larger than that of vitamin E and has a long duration of action due to its resistance to UV degradation and chemical attacks by singlet oxygen and free radicals.

Kinetics of the reaction by which natural vitamin E is regenerated by vitamin C.

The rate constant and activation energy of the regeneration reaction of natural vitamin E by vitamin C were determined with a double-mixing stopped-flow spectrophotometer. The formation of vitamin C radical was observed in the absorption spectrum. The kinetic effect of methyl substitution on the aromatic ring of vitamin E radical indicates that partial charge-transfer plays a role in the reaction. Since a substantial deuterium kinetic isotope effect was not found, the tunneling effect may not play an important role under the present experimental conditions.

Kinetic study of the quenching reaction of singlet oxygen by tea catechins in ethanol solution.

A kinetic study of the quenching reaction of singlet oxygen ((1)O(2)) with catechins (catechin (CA), epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), epigallocatechin gallate (EGCG)) and related compounds (5-methoxyresorcinol (MR), 4-methylcatechol (MC), and n-propyl gallate (PG)) was performed in ethanol at 35 degrees C. MR, MC, and PG are considered to be a model of resorcinol (A)-, catechol (B)-, and gallate (G)-rings in catechins, respectively. The overall rate constants, k(Q) (= k(q) + k(r), physical quenching + chemical reaction), for the reaction of catechins with (1)O(2) increased in the order of PG < MR < MC < CA < EC < EGC < ECG < EGCG. In a comparison of the rate constants, the relationship between quenching rates and chemical structures is discussed. The catechins which have lower peak oxidation potentials, E(P), show higher reactivities. It was observed that the chemical reaction (k(r)) is almost negligible in the quenching reaction of (1)O(2) by catechins. The k(Q) values of EGCG (1.47 x 10(8) M(-1) s(-1)) and ECG (7.81 x 10(7)) were found to be larger than those of lipids (1.3 x 10(5)-1.9 x 10(5) M(-1) s(-1)), amino acids (<3.7 x 10(7)), and DNA (5.1 x 10(5)). Further, these values are similar to those (1.15 x 10(8)-2.06 x 10(8) M(-1) s(-1)) of alpha- and gamma-tocopherol, ubiquinol-10, and gamma-tocopherol hydroquinone (plastoquinol model). The result suggests that catechins may contribute to the protection of oxidative damage in biological systems, by quenching (1)O(2).

Structure-activity relationship of the tocopherol-regeneration reaction by catechins.

The reaction rates (k(r)) of 5,7-diisopropyl-tocopheroxyl radical (Toc) with catechins (epicatechin (EC), epicatechin gallate (ECG), epigallocatechin (EGC), epigallocatechin gallate (EGCG)) and related compounds (methyl gallate (MG), 4-methylcatechol (MC), and 5-methoxyresorcinol (MR)) have been measured by stopped-flow spectrophotometer. The k(r) values increased in the order of MR < < MG < EC < MC approximately ECG < EGC < EGCG in ethanol and 2-propanol/H(2)O (5/1, v/v) solutions, indicating that the reactivity of the OH groups in catechins increased in the order of resorcinol A-ring < < gallate G-ring < catechol B-ring < pyrogallol B-ring. The catechins which have lower oxidation potentials show higher reactivities. The rate constants for catechins in micellar solution showed notable pH dependence with one or two peaks around pH 9-11, because of the dissociation of various phenolic hydroxyl protons in catechins. The structure-activity relationship in the free-radical-scavenging reaction by catechins has been clarified by the detailed analyses of the pH dependence of k(r) values. The reaction rates increased remarkably with increasing the anionic character of catechins, that is, the electron-donating capacity of catechins. The mono anion form at catechol B-and resorcinol A-rings and dianion form at pyrogallol B-and gallate G-rings show the highest activity for free-radical-scavenging. It was found that catechins (EC, ECG, EGC, and EGCG) have activity similar to or higher than that of vitamin C in vitamin E regeneration at pH 7-12 in micellar solution.

Kinetic study of the quenching reaction of singlet oxygen by flavonoids in ethanol solution.

The quenching rate of singlet oxygen (1O2) by seven kinds of flavonoids (flavone, flavonol, chrysin, apigenin, rutin, quercetin, and myricetin) with 2,3-double bonds has been measured spectrophotometrically in ethanol at 35 degrees C. The overall rate constants kQ (= kq + kr, physical quenching + chemical reaction) increased as the number of OH groups substituted to the flavone skeleton (that is, the total electron-donating capacity of flavonoids) increases. The existence of catechol or pyrogallol structure in the B-ring is essential for the 1O2 quenching of flavonoids. Log kQ was found to correlate with their peak oxidation potentials, E(P); the flavonoids that have smaller E(P) values show higher reactivities. Similarly, log kQ values of flavonoids correlate with the energy level of the highest occupied molecular orbital (E(HOMO)), calculated by the PM3 MO method, and the longest wavelength pi pi* excitation energy (E(ex)). The contribution of the chemical reaction (kr) was found to be negligible in these flavonoids. The kQ values of rutin, quercetin, and myricetin [(1.21 approximately 5.12) x 10(8) M(-1) s(-1)] were found to be larger than those of lipids [(0.9 approximately 6.4) x 10(4) M(-1) s(-1)], amino acids (<3.7 x 10(7) M(-1) s(-1)), and DNA (5.1 x 10(5) M(-1) s(-1)). The result suggests that these flavonoids may contribute to the protection of oxidative damage in foods and plants, by quenching 1O2.

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 ((1)O2) 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 (1)O2 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 (1)O2. Intramolecularly hydrogen-bonded hydroxyanthraquinones found in aloe also show a similar positive and linear correlation, which can be understood in the same way.

Simultaneous control of carriers and localized spins with light in organic materials.

An organic insulating crystal reversibly becomes a magnetic conductor under UV irradiation. The rapid and qualitative change in the physical properties is wavelength selective and explained by charge transfer between donor and photochemically active acceptor molecules. The photochemical redox reaction in the crystal produces a partially filled band and localized spins simultaneously.

Synthesis, crystal structure, and photodynamics of π-expanded porphyrin-fullerene dyads synthesized by Diels-Alder reaction.

Free base and zinc porphyrins are linked with fullerene (C(60)) through ß,ß'-pyrrolic positions rather than meso-positions by Diels-Alder reaction of monoanthraporphyrins (H(2)P-mA and ZnP-mA) and C(60) to afford a π-expanded free base porphyrin-fullerene dyad (H(2)P-C(60)) and its zinc porphyrin-fullerene dyad (ZnP-C(60)) in 86 and 51% yield, respectively. The X-ray crystallographic analysis of ZnP-C(60) showed two comma-shaped swirls-like structure in a unit cell. The intramolecular center-to-center and edge-to-edge distances between the porphyrin and fullerene moieties were 11.5 and 2.5 Å, respectively. The porphyrins and fullerenes were packed in layer-by-layer structure and the porphyrins lied down in parallel. The intermolecular center-to-center distances between the neighboring fullerenes were 10.252, 10.028, and 10.129 Å, which were less than typical van der Waals distance and the π-π interaction spread two-dimensionally. The energy of the charge-separated (CS) state of ZnP-C(60) (1.11 eV) determined by differential-pulse voltammetry measurements in benzonitrile is significantly lower than those of zinc porphyrin-C(60) dyads linked at meso positions because of the low oxidation potential of the π-expanded ZnP moiety. The CS energy of ZnP-C(60) in a nonpolar solvent such as toluene (1.40 V) is lower than the triplet excited state of C(60) ((3)C(60)*), enabling us to attain a long lived triplet CS state (8.1 µs) in toluene, detected by nanosecond laser flash photolysis experiments. The distance between unpaired electrons in the triplet CS state was determined by the EPR spectrum to be 9.1 Å, which agreed with the distance between a zinc atom of the porphyrin and a carbon edge of C(60) in the crystal structure of ZnP-C(60). The rate constants of photoinduced electron transfer and back electron transfer of H(2)P-C(60) and ZnP-C(60), which were determined by femtosecond and nanosecond laser flash photolysis measurements, were analyzed in light of the Marcus theory of electron transfer.

Singlet oxygen quenching by trolox C in aqueous micelle solutions.

Singlet oxygen ((1)O(2)) quenching by trolox C (TC, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), which is a water-soluble vitamin E analogue, in ethanol-water solutions and in aqueous SDS and CTAC micelles was studied by measuring the time-profiles of (1)O(2) phosphorescence at 1274 nm. The second-order rate-constant for (1)O(2) quenching by TC was determined in the ethanol-water solution to be 1.03 x 10(8), 6.22 x 10(7) and 6.23 x 10(7) M(-1) s(-1) at pH 2.0, 7.0, and 8.4, respectively. These values mean that the non-dissociated form of TC under acidic conditions has superior activity to the mono-anion form. In aqueous micelle systems, the decay rate of (1)O(2) at first decreased and then increased with increase of the concentration of TC. This behavior is explained in terms of the (1)O(2) quenching by TC in the bulk phase and in terms of shifting the environment surrounding (1)O(2) to lipophilic by dissolving TC in the hydrophobic region inside the micelle. The present investigation on (1)O(2) emission dynamics in inhomogeneous solutions made it possible to detect a little change in the solutions, which affects the environment around (1)O(2), such as the micelle formation and dissolving solute in the micelle.

Photochemical synthesis of pentacene and its derivatives.

A novel alpha-diketone precursor of pentacene, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was prepared and converted successfully to pentacene in 74 % yield by photolysis of the precursor in toluene: Irradiation of the diketone solution in toluene with light of 460 nm under an Ar atmosphere caused the solution to change from yellow to fluorescent orange-pink within a few minutes, after which, purple precipitates appeared. After 35 min, the solution changed to colorless and the purple precipitates were filtered to give pentacene in 74 % yield. By contrast, in the presence of oxygen, the color of the solution changed from yellow to pale yellow, and only 6,13-endoperoxide of pentacene was quantitatively obtained. The rate of the reaction upon photolysis was measured by observing the decay of n-pi* absorption of the precursor at 460 nm, and was found to be similar in both the presence and absence of oxygen. Therefore, the photoreaction of the alpha-diketone precursor seemed to occur via the singlet excited state. Because the T-T absorption of pentacene was observed upon photolysis of the precursor in the nanosecond transient absorption measurement under an Ar atmosphere, the excited triplet state of the pentacene generated singlet oxygen by sensitization, and it reacted with the ground-state pentacene to give the 6,13-endoperoxide. The alpha-diketone deposited on glass was also converted successfully to pentacene film by photoirradiation. In addition, diketone precursors of a mixture of 2,8- and 2,9-dibromopentacene and 2,6-trianthrylene were also prepared and their photoconversion was performed.