Ortho Dimers Research Articles

Synthesis and optoelectronic properties of bis-BODIPY-meso-phenyleneethynylene dimers

Three different bis-BODIPY-meso-phenyleneethynylene dimers were prepared in a straightforward manner following a sequence Liebeskind-Srogl- Stille cross-couplings, which allowed us to control the architecture of the final products. The phenyleneethynylene (PE) bridged bis-meso-BODIPYs, indistinctly of the ortho, meta or para substitution exhibit in dichloromethane, a main excitonic (S0–S1 electronic transition) peak at 503 ± 1 nm, similar to that of the monomer meso-phenyl BODIPY, evidencing the poor electronic communication between the two BODIPYs. This is supported by theoretical study where modest energy splitting of the S0–S1 electronic transition was found and by voltammetry where voltammograms of para and meta dimers show similar redox process to that of the monomer. The fluorescence spectra are also quite similar, but the ortho dimer presents two emission bands. The fluorescence quantum yield (φ) is much lower for para (0.6 %) and meta (2.9 %) dimers with respect to the monomer (4.3 %) and to the ortho homologous (15.1 %). This is likely due to non-radiative losses derived from the free rotation of the two BODIPYs in meso that is restricted in the ortho substitution. Intramolecular charge transfer (ICT) through photoinduced symmetry breaking can also occur in this latter dimer according to the Weller equation using the voltammetric oxidation/reduction potentials, and to the fluorescence quenching in polar solvents. Noncovalent interactions studies indicate significant π-π interactions in the excited state that could promote excimer formation. For which, the two emission bands in this compound could be also due to a monomer-like ICT state and excimers.

Effect of BN dimers on the stability, electronic, and thermal properties of monolayer graphene

We theoretically investigate structural stability, electronic and thermal characteristic of boron and nitrogen codoped monolayer graphene using density functional theory and Boltzmann transport equation. Three types of BN dimers, ortho, meta, and para dimers, are identified at different concentration ratios of B and N atoms. Our DFT calculations suggest that the BN ortho dimers are structurally favorable configurations due to the lowest required formation energy. At low doping ratio, large bandgap for BN para dimer is predicted leading to high Seebeck coefficient and figure of merit. In addition, a large deviation in the Wiedemann–Franz ratio is also seen, and a maximum value of the Lorenz number is thus found. In contrast, at high doping ratio, high Seebeck coefficient and figure of merit are found for BN ortho dimer and a low Seebeck coefficient for BN para dimer is noticed. Furthermore, a small deviation in Lorenz number is found for high doping ratio where the distance between BN pair is large.

Ortho and para hydrogen dimers on G/SiC(0001): combined STM and DFT study.

The hydrogen (H) dimer structures formed upon room-temperature H adsorption on single layer graphene (SLG) grown on SiC(0001) are addressed using a combined theoretical-experimental approach. Our study includes density functional theory (DFT) calculations for the full (6√3 × 6√3)R30° unit cell of the SLG/SiC(0001) substrate and atomically resolved scanning tunneling microscopy images determining simultaneously the graphene lattice and the internal structure of the H adsorbates. We show that H atoms normally group in chemisorbed coupled structures of different sizes and orientations. We make an atomic scale determination of the most stable experimental geometries, the small dimers and ellipsoid-shaped features, and we assign them to hydrogen adsorbed in para dimers and ortho dimers configuration, respectively, through comparison with the theory.

Antidepressant‐like effect of bis‐eugenol in the mice forced swimming test: evidence for the involvement of the monoaminergic system

Dehydrodieugenol, known as bis-eugenol, is a eugenol ortho dimer, and both compounds were able to exhibit anti-inflammatory and antioxidant activities in previous studies. Furthermore, eugenol showed antidepressant-like effect; however, the biological actions of bis-eugenol on experimental models for screening antidepressant activity are still unknown. The present study investigated a possible antidepressant-like activity of bis-eugenol in the forced swimming test (FST) and tail suspension test (TST) in mice and the involvement in the monoaminergic system in this effect. In addition, a neurochemical analysis on brain monoamines of mice acutely treated with bis-eugenol was also conducted. Bis-eugenol decreased the immobility time in the FST and TST without accompanying changes in ambulation in the open field test at 10 mg/kg, i.p.. Nevertheless, it induced ambulation at 25 and 50 mg/kg doses. The anti-immobility effect of bis-eugenol (10 and 50 mg/kg, i.p.) was prevented by pretreatment of mice with p-chlorophenylalanine (PCPA, 100 mg/kg, i.p., an inhibitor of serotonin synthesis, for four consecutive days), yohimbine (1 mg/kg, i.p., an α2-adrenoceptor antagonist), SCH23390 (15 μg/kg, s.c., a dopamine D1 receptor antagonist) and sulpiride (50 mg/kg, i.p., a dopamine D2 receptor antagonist). Monoamines analysis using high-performance liquid chromatograph revealed significant increase in the 5-HT, NE and DA levels in brain striatum. The present study indicates that bis-eugenol possesses antidepressant-like activity in FST and TST by altering dopaminergic, serotonergic and noradrenergic systems function.

Horseradish peroxidase in ionic liquids: Reactions with water insoluble phenolic substrates

The reactivity of horseradish peroxidase (HRP) with water insoluble phenolic compounds has been studied in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4])/water mixtures. The enzyme retained some catalytic activity up to 90% ionic liquid in water at 25 °C only at pH values higher than 9.0. Activity steadily decreased towards neutral and acidic conditions, as judged by 4-aminoantypirin/phenol activity tests. Inhibition of horseradish peroxidase under neutral acidic condition was due to the binding of fluoride anions released from tetrafluoroborate anion to the heme iron as demonstrated by the sharp UV–visible absorption transition diagnostic of the conversion from a five coordinated to a six coordinated high spin ferric heme iron. Thus, reactions with water insoluble phenols were carried out under alkaline reaction conditions and 75% [BMIM][BF 4]/water mixture. Under these conditions, the distribution of the reaction products was much narrower with respect to that observed in aqueous buffers or water/dimethylformamide or water/dimethylsulfoxide mixtures, and polymeric species other than dimers were not observed. Technical scale preparations of a novel 4-phenylphenol ortho dimer [2,2′-bi-(4-phenylphenol)] with a high yield of the desired product were obtained.

An ortho dimer of butylated hydroxyanisole inhibits nuclear factor kappa B activation and gene expression of inflammatory cytokines in macrophages stimulated by Porphyromonas gingivalis fimbriae

Butylated hydroxyanisole, BHA, is widely used as a potent antioxidant, but its adverse effects such as carcinogenesis and proinflammatory activity have been reported, which are possibly due to the prooxidant property of this compound. We recently demonstrated that the dimer of 2-methoxyphenols exhibits cyclooxygenase-2 inhibition, because of lessening of its prooxidant property caused by the dimerization. In the present study, toward our goal of developing a chemopreventive agent for chronic periodontal diseases, we examined whether 2-BHA (2- tert-butyl-4-methoxyphenol) and its synthetic ortho dimer, bis-BHA (3,3′-di- tert-butyl-5,5′-dimethoxy-1,1′-biphenyl-2,2′-diol) could inhibit the Porphyromonas gingivalis fimbria-stimulated inflammatory reaction. The fimbria-induced expression of interleukin-1β and neutrophil chemoattractant KC genes in RAW264.7 murine macrophages was strongly inhibited by bis-BHA. In contrast, 2-BHA showed only slight inhibition. bis-BHA also significantly inhibited the fimbria-stimulated phosphorylation-dependent degradation of the alpha inhibitor of nuclear factor-κB and the transcriptional activity of this factor in the cells. These findings suggest that bis-BHA possesses anti-inflammatory activity against chronic periodontal diseases.

Microwave photochemistry III: Photochemistry of 4- tert-butylphenol

Influence of UV and combined MW–UV irradiation on 4- tert-butylphenol (4TBP) transformation was investigated in the presence and in the absence of sensitizers with different value of singlet and triplet energy and in the presence of solvents with different polarity. Irradiation by UV and also by MW–UV gave C O and C C products: 4′,5-di- tert-butyl-2-hydroxydiphenyl ether ( 1) ( ortho-O), 5,5′-di- tert-butylbiphenyl-2,2′-diol ( 2) ( ortho– ortho) and 2TBP as an isomerization product. Their ratio depends upon the nature of solvents and sensitizers used. In non-polar solvents (hexane, heptane, toluene), direct 4TBP radiation led to the formation of products 1, 2 and 2TBP where ortho– ortho dimer 2 is predominant over ortho-O ether 1. In acetonitrile ortho– ortho dimer 2 was selectively formed under UV and MW–UV irradiation. In 1,4-dioxane and in methanol, no product formation was observed. Influence of solvent polarity (acetone, hexane) on the relative products distribution of 4TBP photoreaction was also observed in the presence of sensitizers: acetone, naphthalene, acetophenone and benzophenone. The acetone-sensitized reaction was found to give all three products: ortho-O ether 1, ortho– ortho dimer 2 and 2TBP where ortho-O ether 1 is predominant. In the presence of singlet sensitizer naphthalene, the reaction of 4TBP in hexane gave mainly ortho-O ether 1 and 2TBP, whereas ortho– ortho dimer 2 was not found. Triplet-sensitized reaction with acetophenone in hexane led selectively to ortho-O ether 1, while with benzophenone gave moreover 2TBP and its ortho– ortho 3 and ortho– para 4 dimers.

Microwave photochemistry: II. Photochemistry of 2- tert-butylphenol

Influence of UV and combined MW–UV irradiation on 2- tert-butylphenol (2TBP) transformation was investigated in the presence and in the absence of sensitizers with different value of singlet and triplet energy and in the presence of solvents with different polarity. Irradiation by UV and also by combination MW–UV gave three CC dimers: 3,3′-di- tert-butylbiphenyl-2,2′-diol ( 1) ( ortho–ortho), 3,3′-di- tert-butylbiphenyl-2,4′-diol ( 2) ( ortho–para) a 3,3′-di- tert-butylbiphenyl-4,4′-diol ( 3) ( para–para). Their ratio depends upon the nature of solvents and sensitizers used. In non-polar solvents (hexane, heptane, benzene, toluene), direct 2TBP radiation led to the formation of two products 1 and 2 where ortho–para 2 is predominant over ortho–ortho dimer 1. Irradiation in dipolar aprotic solvent 1,4-dioxane led only to ortho–ortho 1 and in case of acetonitrile only to ortho–para dimer 2. In polar solvents (acetic acid, water, methanol/water = 1:1 v/v), it led to the formation of all three products. In methanol, the reaction did not proceed. Influence of solvent polarity (hexane, acetonitrile) to relative product distribution of 2TBP photoreaction was also observed in the presence of sensitizers: acetophenone, benzophenone, naphthalene, acridine and anthracene. Reaction in hexane gave ortho–ortho 1 and ortho–para 2 dimers with all tested sensitizers. The acridine-sensitized reaction in acetonitrile was found to give ortho–para 2 and a greater amount of para–para 3 dimer. Singlet-sensitized reaction (naphthalene, anthracene) in acetonitrile led to ortho–para dimer 2. Nonsensitized as well as singlet-sensitized reaction of 2TBP proceeded from singlet state situated under singlet state energy of anthracene (320 kJ mol −1). Triplet state of 2TBP was found to be under triplet state of benzophenone (290 kJ mol −1).

Evidence for π−π Interactions in the S1 State of Zinc Porphyrin Dimers Revealed by Picosecond Time-Resolved Resonance Raman Spectroscopy

The S1 states of Zn(II)−porphyrin dimers have been investigated with picosecond time-resolved resonance Raman spectroscopy. The transient absorption and Raman spectra of porphyrin dimers, in which two Zn(II)−porphyrins are covalently linked at the ortho or meta positions of phenylene spacers, are compared with those of their component monomer unit. Although the Q-band of the ortho dimer was definitely different from those of the meta dimer and reference monomer, the ground-state Raman spectra of the ortho and meta dimers are nearly the same as that of the monomer, suggesting that the porphyrin π−π interactions strongly appear in the excited state but little in the ground state in the ortho dimer. Several characteristic Raman bands were observed for the S1 excited state at 2 ps after photoexcitation. The monomer S1 state gave the marker bands at slightly lower frequencies (by 3−4 cm-1) than the corresponding ground state, and they showed no frequency shifts with time between 2 and 300 ps. On the contrary, in the case of the ortho dimer S1 state, two characteristic bands (ν2, ν4) appeared at frequencies significantly lower (by 10−13 cm-1) than the corresponding ground-state bands, and in addition, the frequency of the ν4 band exhibited an upshift around 10−20 ps following photoexcitation. This frequency shift of the ortho dimer was appreciably perturbed by steric hindrance between the two porphyrins groups when bulky tert-butyl groups at the para position were incorporated. The behaviors of transient Raman bands of the meta dimer appeared intermediate between the monomer and the ortho dimer. These observations give the first clear evidence for the presence of π−π interactions in the S1 excited state of porphyrin dimers with phenylene spacer and the occurrence of geometric relaxation toward the monomer-type structure in several tens of picoseconds.

Coherent scattering of the hydrogen dimer from a LiF crystal

A molecular beam of hydrogen dimers is scattered from a LiF(001) crystal at 48 meV. About 5% of the dimers are found to scatter elastically and appear in well-defined coherent first-order diffraction peaks. Debye–Waller experiments are used to estimate the amount of thermal phonon interaction and, together with elastic scattering, accounts for about 20% of the scattering. The remaining 80% are believed to be fragmented and lost due to exchange of translational-internal energy within the dimer bond in an elastic collision with the surface. Para enriched hydrogen beams are used to show that para dimers, despite having a larger binding energy, are more likely to fragment during surface scattering than ortho dimers. The survival of trimer and higher clusters is at least an order of magnitude less than the dimer so that the diffractive scattering provides a filter to form well-defined pure dimer beams.