Perylene Bisimide Dimer Aggregates: Fundamental Insights into Self‐Assembly by NMR and UV/Vis Spectroscopy

A novel perylene bisimide (PBI) dye bearing one solubilizing dialkoxybenzyl and one bulky 2,5‐di‐tert‐butylphenyl substituent was synthesized and its aggregation behavior was analyzed by NMR and UV/Vis spectroscopy in various chloroform/methylcyclohexane (MCH) solvent mixtures. In the presence of no less than 10 vol % chloroform, exclusive self‐assembly of this PBI dye into π‐stacked dimers was unambiguously confirmed by means of both concentration‐dependent ¹H NMR and UV/Vis spectroscopic experiments. Based on ROESY NMR, a well‐defined π‐stacked dimer structure was determined and further corroborated by molecular modeling studies. By varying the solvent composition of chloroform and MCH, the solvent effects on the Gibbs free energy of PBI dimerization were elucidated and showed a pronounced nonlinearity between lower and higher MCH contents. This observation could be related to a further growth process of dimers into larger aggregates that occurs in the absence of chloroform, which is required to solvate the aromatic π surfaces. With the help of a single‐crystal structure analysis for a related PBI dye, a structural model could be derived for the extended aggregates that are still composed of defined π–π‐stacked PBI dimer entities.     

INTERCONVERSION OF BACTERIOCHLOROPHYLL c AGGREGATES IN SOLID FILMS UPON ORGANIC VAPOR TREATMENT

Bacteriochlorophyll c (BChl c) solid films were prepared from a carbon tetrachloride solution on CaF₂ plates as artificial aggregates. Effects of organic vapor such as acetone and tetrahydrofuran (THF) on the BChl c films were studied by absorption and Fourier-transform infrared spectroscopy. Two major homologs (R[E,E]BChl c_F and R[P,E]BChl c_F) and one minor homolog (S[I,E]BChl c) isolated from the green photosynthetic bacterium Chlorobium limicola strain 6230 were examined for the experiments. The BChl c polymeric aggregates absorbing at739–753 nm similar to those in the chlorosome were induced for all homologs upon the treatment of BChl c solid film with acetone vapor. The 13¹-keto C=O stretching band in the R[E,E]BChl c_F solid film showed a downward shift from 1651 cm^?1to 1643 cm^?1 with a concomitant shift of the 3¹-OH stretching bands from 3337 and 3238 cm^?1 to 3163 cm^?1. It was suggested that the lower aggregates brought about by Mg…O=C(13¹) and (3¹)O…O=C(13¹) bonds were transformed into the higher aggregates strongly hydrogen-bonded in a Mg…(3¹)O-H…O=C(13^l) interaction. They were transformed to a monomer-like form absorbing at 667 nm upon exposure to THF vapor and were reversibly converted to the higher aggregates upon removal of THF molecules in vacuo.     

pi-Electron interaction of PAHs with anion-exchange silica gels modified with anionic metal-porphine and -phthalocyanine derivatives as HPLC stationary phase for preparative column in organic solvents

To develop easy-to-prepare stationary phases for HPLC, we investigated anion-exchange silica gels, Nucleosil 5SB (Nuc), modified with metal-porphines and -phthalocyanines (M-P). The modified silica gels (M-P_N) were evaluated for the availability as a stationary phase of HPLC for the separation of π-electron-rich polyaromatic hydrocarbons (PAHs) in polar and non-polar eluents. Separation ability of silica gels modified with Cu-phthalocyanine derivative (Cu-PCS_N) was comparable to that of the silica gels binding Cu-PCS through sulfonamide bonds; however, the latter requires troublesome procedures for the preparation. The PAHs tested interact with Cu-PCS_N in non-polar organic eluents through their π-electrons similarly as in the case of the PYE column^®, in which interaction with PAHs was reported to be only the π-π-electron interaction.     

Evaluation of selectivity for l-glutamide-derived highly ordered assemblies in reversed-phase high-performance liquid chromatography

Two dioctadecyl l-glutamic acid derivatives with amide and ester type bondings have been synthesized and immobilized from 3-aminopropyltrimethoxysilane (APS) grafted silica (Sil-APS) to be used in reversed-phase high-performance liquid chromatography (RP-HPLC). Subsequent studies showed that dioctadecyl-l-glutamide derivative (GLN) can self-assemble into highly ordered structures by forming three-dimensional fibrillar aggregates as observed in scanning and transmission electron microscopes (SEM and TEM). Variable temperature ¹H NMR and FT-IR spectra of organogel revealed that the special aggregation morphology shown by GLN was stabilized by inter and or intra molecular hydrogen bonding among amide moieties. However, such ordered aggregated or self-assembled structures were not observed for the dioctadecyl-l-glutamate (GLU) derivative. The stationary phases Sil-GLN and Sil-GLU were characterized by DRIFT, elemental analysis, TGA, and ¹³C and ²⁹Si CP-MAS NMR spectroscopic measurements. The chromatographic selectivity for both stationary phases was evaluated from the retention studies of different size and shape polycyclic aromatic hydrocarbons (PAHs). The chromatographic experiment for PAHs and geometrical isomers in RP-HPLC showed that Sil-GLN demonstrated extremely enhanced selectivity than Sil-GLU. The higher selectivity attributed by Sil-GLN has been brought by multiple π-π interactions among the π-electrons of the grafted organic phase and π-electrons of the guest PAHs molecules. Thermodynamic studies for linear and nonlinear PAHs revealed that the retention behavior does not change over a temperature range from 10 to 60 °C for both stationary phases.     

Complexation of trivalent lanthanoid ions with 4-benzoyl-3-phenyl-5-isoxazolone and p-tert-butylcalix[4]arene fitted with phosphinoyl pendant arms in solution during synergistic solvent extraction and structural study of solid complexes by IR and NMR

The liquid extraction of 14 lanthanoids with a 4-benzoyl-3-phenyl-5-isoxazolone (HPBI) alone in CHCl₃ as a diluent from perchlorate medium at constant ionic strength μ = 0.1 is investigated. The synergistic solvent extraction of five selected lanthanoid ions (La³⁺, Nd³⁺, Eu³⁺, Ho³⁺ and Lu³⁺) with 4-benzoyl-3-phenyl-5-isoxazolone (HPBI) and 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(dimethylphosphinoylmethoxy)calix[4]arene, (S) in CHCl₃ has been studied too. The stoichiometry of the extracted species was characterised by a classical log–log plot analysis. It was found that the composition of the extracted species with HPBI are Ln(PBI)₃ and in the presence of the phosphorus-containing calix[4]arene the lanthanoid ions have been extracted as [Ln(PBI)₃S₂]. The values of the equilibrium constants and the separation factors have been calculated. The influence of the synergistic agent on the extraction process has been discussed.     

The π-Orbital Sequence in Norbornadiene and Related Hydrocarbons

A method outlined previously [1] is used to show that in norbornadiene ( 3 ) the b ₂(π) orbital lies above a ₁(π), as predicted by theory. This indicates that in 3 through-space interaction between the two basis π-orbitals π_a and π_b is more important than through-bond interaction. Analysis of the PE.-spectra of 8-isopropylidene-tricyclo[3.2.1.0^2,4]-octane ( 13 ) and the corresponding octene ( 15 ) confirms that the π-orbital π_c of the exocyclic double bond conjugates more strongly with the symmetric Walsh-orbital e _s of the cyclopropano moiety than with the π-orbital π_a of a double bond in the same position.     

Studies of molecular aggregation of a polybenzimidazole in solution by fluorescence spectroscopy

A purified, well characterized sample of polybenzimidazole (PBI) has been studied by fluorescence spectroscopy in order to obtain information concerning molecular aggregation in solution. Fluorescence and excitation spectra of PBI and a model compound have been measured in N,N-dimethylacetamide (DMA) and in formic acid (FA). The major emission band for PBI solutions is attributed an excited ¹L_b state, which consists of 0-0 and 0-1 vibrational energy bands. Concentration quenching is attributed to reabsorption of the emitted light, and self-quenching due to the formation of molecular aggregates. Therefore, the extent of molecular aggregation in solution is estimated from the self-quenching constant. In the course of the investigation, a new fluorescence band was found for the PBI-DMA solution; its intensity increased markedly due to overlapping of polymer coils. The formation of polyelectrolytes in PBI-FA solutions was confirmed by viscometry. The major fluorescence band was found to originate from the excited ¹L_b state and to shift to longer wavelengths upon addition of lithium bromide.     

Thermo- and light-triggered reversible interconversion of dysprosium–anthracene complexes and their responsive optical,magnetic and dielectric properties

Artificial smart materials with switchable multifunctionality are of immense interest owing to their wide application in sensors, displays and memory devices. Lanthanide complexes are promising multifunctional materials integrating optical and magnetic characteristics. However, synergistic manipulation of different physical properties in lanthanide systems is still challenging. Herein we designed and synthesized a mononuclear complex [Dy^III(SCN)₃(depma)₂(4-hpy)₂] (1), which incorporates 9-diethylphosphonomethylanthracene (depma) as a photo-active component and 4-hydroxypyridine (4-hpy) as a polar component. This compound shows several unusual features: (a) reversible thermo-responsive phase transition associated with the order–disorder transition of 4-hpy and SCN⁻, which leads to thermochromic behavior and dielectric anomaly; (b) reversible photo-induced dimerization of anthracene groups, which leads to synergistic switching of luminescence, magnetic and dielectric properties. To our knowledge, compound 1 is the first example of lanthanide complexes that show stimuli-triggered synergistic and reversible switching of luminescence, magnetic and dielectric properties.

[Dy^III(SCN)₃(depma)₂(4-hpy)₂] (1) shows reversible thermo-induced phase transition associated with thermochromism and dielectric anomaly and photo-induced dimerization with synergistic switching of luminescence, magnetic and dielectric properties.     

Heptagon-Containing Nanographene Embedded into [10]Cycloparaphenylene

We report the synthesis and characterization of a novel type of nanohoop, consisting of a cycloparaphenylene derivative incorporating a curved heptagon-containing π-extended polycyclic aromatic hydrocarbon (PAH) unit. We demonstrate that this new macrocycle behaves as a supramolecular receptor of curved π-systems such as fullerenes C₆₀ and C₇₀, with remarkably large binding constants (ca. 10⁷ M⁻¹), as estimated by fluorescence measurements. Nanosecond and femtosecond spectroscopic analysis show that these host-guest complexes are capable of quasi-instantaneous charge separation upon photoexcitation, due to the ultrafast charge transfer from the macrocycle to the complexed fullerene. These results demonstrate saddle-shaped PAHs with dibenzocycloheptatrienone motifs as structural components for new macrocycles displaying molecular receptor abilities and versatile photochemical responses with promising electron-donor properties in host-guest complexes.     

A new analytical methodology for a fast evaluation of semi-volatile polycyclic aromatic hydrocarbons in the vapor phase downstream of a diesel engine particulate filter

A new sampling method was developed to collect vapor-phase polycyclic aromatic compounds (PAHs) downstream of a diesel engine equipped with a diesel particulate filter (DPF). This configuration allowed us to collect separately the particulate phase, which was trapped inside the DPF, and the vapor phase, which was sampled downstream of the DPF. PAHs, which were not predominantly absorbed into the poor organic fraction of the diesel soot, but were rather physically sorbed on high energetic adsorption sites, should be extracted using very drastic extraction conditions Microwave-assisted extraction using solvent mixtures composed of pyridine and diethylamine were used to desorb particulate PAHs, and the total PAH amounts corresponded to a very low value, i.e., 8 μg g⁻¹ or 0.24 μg km⁻¹, with a predominance of low weight PAHs. For collection of the vapor phase, gas bubbling in an aqueous medium was preferred to conventional methods, e.g., trapping on solid sorbents, for several reasons: aqueous trapping allowed us to use a solid phase enrichment process (SPE) that permitted PAH sampling at the sub-picogram levels. Consequently, low volume sampling was possible even if the sampling duration was very short (20 min). Additionally, the amount of time saved for the analysis was considerable when coupling SPE to the analytical system (liquid chromatography with fluorimetric detection). Solvent consumption for the overall sampling and analytical processes was also drastically reduced. Experiments on a diesel engine showed that vapor phase samples collected downstream of the DPF contained all of the 15 target priority PAHs, even the heaviest ones. The total vapor-phase PAH amount was 6.88 μg N m⁻³ or 10.02 μg km⁻¹, which showed that the gaseous fraction contains more PAHs than the particulate fraction. Partitioning coefficients (K_p) were estimated showing the predominance in the vapor phase of all the PAHs. However, the DPF technology effects a considerable decrease in the total PAH emission when compared to non-equipped diesel vehicles.     

Phanes with Three- and Four-Membered Rings as Building Elements

The systematic integration of the small 2π- and 4π-electron systems cyclopropenylium ion, cyclopropenone, oxo-cyclobutenylium ion, and cyclobutadiene into phane chemistry was initiated only a few years ago. [n₂]Cyclopropenylophanes, [n₂]cyclopropenonophanes, metal-capped [n₄]cyclobutadienosuperphanes, and other new families of double-decker phane species became available from cycloalkydiynes through special methods of double [2+1]cycloaddition with carbenes and metal complex induced dimerization by double [2+2]cycloaddition. Phane-specific structural features were elucidated by X-ray structural analyses. Cyclic voltammetry and PE spectroscopy as well as MO calculations reealed considerable interactions between closely spaced π-electron systems. Decapping cyclobutadienosuperphanes formally extended the synthetic principle to threefold [2+2]cycloadditions resulting in [n₄]-bridged tricyclo-[4.2.0.0^{3, 5}]octa-3,7-dienes, which represent a new type of cage compounds. Moreover, completion to fourfold [2+2]addition was achieved with the photoinduced transformation of [3₄]bridged tricyclo[4.2.0.0^{3, 5}]octa-3,7-diene into propella[3₄]cubane.     

Radiation-induced grafting of styrene vapor to polyethylene

The radiation-induced grafting of styrene vapor to low-density polyethylene film of 0.063 mm thickness was studied at 23°C at a dose rate of 1.98 × 10⁴ rad/hr. The concentration C of monomer in the film was measured as a function of pre-irradiation exposure time to monomer vapor. The concentration-dependent diffusion coefficient of styrene in polyethylene was calculated to be 4.9 × 10^?9 exp {2.0C/C₀} cm²/sec, where C₀ is the saturation concentration of styrene in the film, and a linear boundary diffusion coefficient for styrene vapor into polyethylene film was found to be 2.0 × 10^?7 cm/sec. The rate of grafting was determined as a function of the concentration of styrene absorbed in the film. The maximum graft yield was obtained with an initial styrene concentration in the film of 4 wt-%. Under conditions of low initial monomer concentration, the grafting rate increases with irradiation time. The results are compared with previously published data on grafting of polyethylene from methanol–styrene solutions. They are explained in terms of the viscosity of the amorphous region as a function of styrene content and the resistance to the diffusion of monomer at the film–vapor interface.     

In Situ Synthesis of Polybenzimidazole on the Surface of Single Wall Carbon Nanotubes

Single wall carbon nanotube (SWNT) was carboxylated to introduce –OH, –COOH, and –NH₂ on the surfaces, which made it possible for polybenzimidazole (PBI) to be in situ synthesized. The resulting SWNT doped PBI (SWNT/PBI) was characterized by Fourier transform infrared spectroscopy thermogravimetric analysis. The conductivity of membrane casting from PBI/N,N-dimethylacetamide (DMAc) and SWNT/PBI/DMAc solution was also investigated. Owing to the incorporation of SWNT, the SWNT/PBI membranes achieve a maximum conductivity of 3.70 × 10^?4 Ω.cm^?1 at 200°C.     

Bay区取代苝二酰亚胺分子构型及聚集调控:邻位甲基位阻效应

设计并合成了2种苝二酰亚胺分子PBI1和PBI2,研究了bay区的苯氧基团邻位甲基取代对分子构型及分子聚集的影响.通过对单晶结构的分析,发现邻位甲基的引入明显影响苝二酰亚胺分子构型,使得4个苯氧基呈中心对称分布.由于甲基的空间位阻效应,有效地减弱了分子间π-π相互作用,从而提高了分子的溶解性与溶液加工成膜性.研究结果表明,在π共轭分子结构中的关键位置引入小的甲基取代基能够显著调控分子的聚集行为,有效减少光电材料分子中非光电活性(增溶性基团)的含量,对光电材料分子的设计合成具有重要的指导意义.     

Perylene Bisimide and Naphthyl-Based Molecular Dyads: Hydrogen Bonds Driving Co-planarization and Anomalous Temperature-Response Fluorescence

The origin of the positive temperature effect in fluorescence emission of a newly designed perylene bisimide (PBI) derivative with two naphthyl units containing ortho-methoxy group (NM) at its bay positions (PBI-2NM) was elucidated. A key point is the finding of a weak hydrogen bond (<5.0 kcal mol⁻¹) between the methoxy group of the NM unit and a nearby hydrogen atom of the PBI core. It is the bonding that drives co-planarization of the different aromatic units, resulting in delocalization of the π-electrons of the compound as synthesized, inducing fluorescence quenching via intramolecular charge transfer (ICT). With increasing temperature, the co-planar structure could be distorted in part, resulting in a decreased degree of ICT, and hence leading to enhanced fluorescence emission. The unique positive temperature effect in emission induced by H-bond-driven co-planarization may pave a new avenue in designing functional molecular systems complementary to conventional methods.     

Gold nanoparticle-templated assembly of oligothiophenes: preparation and film properties

Terthiophene-appended gold nanoparticles were prepared by the reduction of AuCl₄⁻(C₈H₁₇)₄N⁺ with NaBH₄ in the presence of bis[2,5-di(3-hexylthiophen-2-yl)thiophene-3-carboxyloxyhexanyl]disulfide. A hexagonal self-assembly of particles with gold core diameters (1.9±0.1 nm) was detected by high-angle annular dark-field scanning transmission electron microscopy. The electric conductivity of the iodine-doped film was 9.1×10⁻⁶ S cm⁻¹, which was ascribable to the terthiophene-based inter-ligand π-π interactions. The Au/terthiophene hybrid spin-coated film consisted of a highly three-dimensional assembled structure of terthiophenes, as inferred from grazing-incidence small-angle X-ray scattering, indicating that such monodispersed and small-sized gold nanoparticles can serve as a template for this organization. In this study, a gold nanoparticle-templated assembly of oligothiophenes has been fabricated for proposing a method to develop tailor-made organizations of π-conjugated oligomers.     

13C nuclear magnetic resonance studies of the adducts of bis-(π-crotyl-D7-nickel iodide) and 2-alkylbutadienes as related to mechanism of polymerization

By ¹³C NMR spectroscopy, the structures of (C₄D₇Nil)₂ adducts with isoprene, 2-ethyl-, 2-isopropyl- and 2-tert. butylbutadienes have been investigated. The nature of the 2-alkyl substituent in the anti- and syn-1,2-disubstituted π-allylic complexes influences the character of electron charge densities distribution on the carbon atoms of a π-allylic ligand. Mutual arrangement of substituents in the π-allylic active sites has been shown to remain in the double bonds of elementary units of a polymer chain.     

Tuning the selectivity of polymeric ionic liquid sorbent coatings for the extraction of polycyclic aromatic hydrocarbons using solid-phase microextraction

A new generation polymeric ionic liquid (PIL), poly(1-4-vinylbenzyl)-3-hexadecylimidazolium bis[(trifluoromethyl)sulfonyl]imide (poly(VBHDIm⁺ NTf₂⁻)), was synthesized and is shown to exhibit impressive selectivity towards the extraction of 12 polycyclic aromatic hydrocarbons (PAHs) from aqueous samples when used as a sorbent coating in direct-immersion solid-phase microextraction (SPME) coupled to gas chromatography (GC). The PIL was imparted with aromatic character to enhance π–π interactions between the analytes and the sorbent coating. For comparison purposes, a PIL with similar structure but lacking the π–π interaction capability, poly(1-vinyl-3-hexadecylimidazolium bis[(trifluoromethyl)sulfonyl]imide) (poly(HDIm⁺ NTf₂⁻)), as well as a commercial polydimethylsiloxane (PDMS) sorbent coating were evaluated and exhibited much lower extraction efficiencies. Extraction parameters, including stir rate and extraction time, were studied and optimized. The detection limits of poly(VBHDIm⁺ NTf₂⁻), poly(HDIm⁺ NTf₂⁻), and PDMS coatings varied between 0.003–0.07 μg L⁻¹, 0.02–0.6 μg L⁻¹, and 0.1–6 μg L⁻¹, respectively. The partition coefficients (log K_fs) of eight PAHs to the three studied fiber coatings were estimated using a static SPME approach. This study represents the first report of analyte partition coefficients to any PIL-based material.     

Designing Heteroatom-Codoped Iron Metal–Organic Framework for Promotional Photoreduction of Carbon Dioxide to Ethylene

Rational engineering active sites and vantage defects of catalysts are promising but grand challenging task to enhance photoreduction CO₂ to high value-added C2 products. In this study, we designed an N,S-codoped Fe-based MIL-88B catalyst with well-defined bipyramidal hexagonal prism morphology via a facile and effective process, which was synthesized by addition of appropriate 1,2-benzisothiazolin-3-one (BIT) and acetic acid to the reaction solution. Under simulated solar irradiation, the designed catalyst exhibits high C₂H₄ evolution yield of 17.7 μmol g⁻¹⋅h, which has been rarely achieved in photocatalytic CO₂ reduction process. The synergistic effect of Fe-N coordinated sites and reasonable defects in the N,S-codoped photocatalyst can accelerate the migration of photogenerated carriers, resulting in high electron density, and this in turn helps to facilitate the formation and dimerization of C−C coupling intermediates for C₂H₄ effectively.     

IR observation of adsorption and initial reactions of olefins on Brønsted acid sites of a deuterated ZSM-5

Adsorption of linear olefins (C₂?C₄) on a deuterated H-ZSM-5 (D-ZSM-5) was studied by infrared (IR) spectroscopy. The initial interaction of the olefins with Brønsted acidic OD groups was hydrogen-bonding to form π-complexes at low temperatures. The adsorbed ethene and propene desorbed by heating under evacuation, while various reactions took place for adsorbed 1-butene; double bond migration (DBM) to 2-butene below 230 K followed by dimerization at room temperature. An unusual reaction path was deduced for DBM of 1-butene, where proton transfer from Brønsted acid sites (BAS) to the adsorbed 1-butene was not essential.