1,3‐Alternate calix[4]arene‐bonded silica stationary phases. Effect of calixarene skeleton substituents on the retention mechanism and column selectivity

Four novel 1,3‐alternate calix[4]arene‐bonded silica gel stationary phases possessing different aromatic and aliphatic substituents at the upper rim (CalixNph, CalixBph, CalixHex, and CalixDdc) were prepared and structurally characterized. The comparison and selectivity of these phases were done by using alkylbenzenes, fatty acid p‐bromophenacyl esters, aromatic positional isomers, and polynuclear aromatic hydrocarbons as analytes. Quantum chemistry calculations have also been performed (using an ab initio method) to support the experimental findings. The effect of the type and content of organic modifier on the retention and selectivity of the alkylbenzenes was studied. The retention mechanism is also discussed. The results indicate that the stationary phases behave like RP packings. However, inclusion complex formation and hydrophobic and π–π interactions seem to be involved in the separation process.     

An Experimental Study on the Effect of Substituents on Aromatic–Aromatic Interactions in Dithia[3,3]‐metaparacyclophanes

Simple model systems based on the 2,11‐dithia[3,3]‐metaparacyclophane skeleton were synthesized to study the effects of substituents on the intramolecular aromatic–aromatic interactions between benzene rings. X‐ray crystallography established that, in their more stable conformations, these metaparacyclophanes featured partially overlapping aromatic rings (interplanar distances of about 3.5 Å), with the planes of the aromatic systems arranged in a slightly tilted disposition (interplanar angles in the range 5–19°). Calculations showed that these derivatives underwent topomerization by flipping of the meta‐substituted ring over the para‐substituted one, a process in which the two rings adopted a continuum of edge‐to‐face dispositions, including an orthogonal one, which were less stable than the starting face‐to‐face arrangement. The energy barriers to the isomerization process were experimentally determined by variable‐temperature NMR spectroscopy, by using an internal temperature standard to assess even minor differences in energy (relative experimental error: (±0.1 kJ mol^?1). The variation in the barriers as a function of the different substituents on the interacting ring was small and apparently unrelated to the effect of the substituents on the polarity of the π‐systems. An explanation based on the charge‐penetration effect seemed more‐suitable to rationalize the observed trends in the barriers.     

Sterically Crowded Trianglimines—Synthesis,Structure, Solid‐State Self‐Assembly,and Unexpected Chiroptical Properties

The chiral, triangular‐shape hexaimine macrocycles (trianglimines), bearing bulky alkynyl or aryl substituents were synthesized and studied by means of experimental and theoretical methods. The macrocyclization reactions are driven by the extraordinary stability of the trianglimine ring and provided products with high yields. Electrostatic repulsion between imine nitrogen atoms and the substituents forced an anti conformation of the aromatic linkers. Although the DFT‐optimized structure of 7 is D₃ symmetrical, in the crystal, the macrocycle adopts a bowl‐like molecular shape. The macrocycle self‐assembles into tail‐to‐tail dimers by mutual interdigitation of aromatic moieties. In contrast, macrocycle 8 adopts a rigid pillararene‐like conformation. The nature of the substituent significantly affects the electronic properties of the linker. As a result, unexpectedly high exciton Cotton effects are observed in the electronic circular dichroism (ECD) spectra. The origin of these effects was subject of an in‐depth study.     

Are water–aromatic complexes always stabilized due to π–H interactions? LMP2 study

Eight complexes of various aromatic molecules with water have been studied theoretically at the local Møller–Plesset 2nd order theory (LMP2)/aug‐cc‐pVTZ(‐f)//LMP2/6‐31+G* level of theory. Two types of complexes can be formed, depending on the electronic structure of aromatic molecules. Donor hydrocarbons form A‐type complexes, while aromatics bearing electron‐withdrawing substituents form B‐type complexes. A‐type complexes are stabilized due to π–H interactions with the OH bond pointing to the aromatic molecule plane, while B‐type complexes have geometry with the oxygen atom pointing to the aromatic molecule plane stabilized by the interaction of highest occupied molecular orbital (HOMO) of water molecule with π* orbitals of the aromatics. It has been found that a (? HOMO–lowest unoccupied molecular orbital (LUMO)/2 value of aromatic molecule, which can be called “molecular electronegativity,” is useful to predict the type of complex formed by aromatic molecule and water. Aromatic hydrocarbons with “molecular electronegativity” of <0.15 tend to form A‐type complexes, while aromatic molecules with “molecular electronegativity” of <0.15 a.u. form B‐type complexes. The binding energy of water–aromatic complexes undergoes a minimum in the area of switching from A‐type to B type complexes, which can be rationalize in terms of frontier orbital interactions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Reactions of Verbenol Epoxide with Aromatic Aldehydes Containing Hydroxy or Methoxy Groups in the Presence of Montmorillonite Clay

The reactions of (?)‐cis‐verbenol epoxide with a number of aromatic aldehydes containing OH and/or MeO groups in the presence of montmorillonite K10 clay have been studied. Several new O‐containing heterocyclic compounds with different frameworks, including compounds with a previously unknown octahydro‐2H‐4,6‐(epoxymethano)chromene framework, have been synthesized. Introduction of one donor substituent in the benzaldehyde molecule led to a decrease in the total yield of intermolecular by formed products, while the introduction of two and more substituents led to an increase in the yield of these products.     

Fluorinated benzoxazines and the structure‐property relationship of resulting polybenzoxazines

Three fluorinated benzoxazines ( 14–16 ), which cannot be synthesized by the traditional one‐step approaches, were synthesized by a three‐step procedure using fluorinated aromatic diamines ( 2–4 ) as starting materials. The structures of the monomers were confirmed by ¹H NMR, IR, and high‐resolution mass spectra. The low dielectric thermosets, P( 14–16 ), were prepared by ring‐opening of ( 14–16 ). IR analysis was utilized to monitor the ring‐opening reaction of ( 14–16 ) and to propose the structures of P( 14–16 ). The thermal and dielectric properties of P( 14–16 ) were studied and compared with a nonfluorinated polybenzoxazine P( 13 ), which is derived form the ring‐opening of 2,2‐bis(4‐aminophenoxy)phenyl)propane ( 1 ). Besides, the structure–property relationship of the P( 13–16 ) is discussed. According to T_g measurement, the ortho‐positioned CF₃ substituents impart greater steric hindrance for ring‐opening of benzoxazines than CF₃ substituents of hexafluoropropane. Incorporating a biphenol F‐based benzoxazine, ( F‐a ), into fluorinated benzoxazines ( 15–16 ) can dilute the effect of ortho‐positioned CF₃ substituents on steric hindrance, leading to a higher crosslinking density and consequently a higher Tg. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4970–4983, 2008     

Specific Noncovalent Association of Chiral Large‐Ring Hexaimines: Ion Mobility Mass Spectrometry and PM7 Study

Ion mobility mass spectrometry and PM7 semiempirical calculations are effective complementary methods to study gas phase formation of noncovalent complexes from vaselike macrocycles. The specific association of large‐ring chiral hexaimines, derived from enantiomerically pure trans‐1,2‐diaminocyclohexane and various isophthaldehydes, is driven mostly by CH–π and π–π stacking interactions. The isotrianglimine macrocycles are prone to form two types of aggregates: tail‐to‐tail and head‐to‐head (capsule) dimers. The stability of the tail‐to‐tail dimers is affected by the size and electronic properties of the substituents at the C‐5 position of the aromatic ring. Electron‐withdrawing groups stabilize the aggregate, whereas bulky or electron‐donating groups destabilize the complexes.     

Asymmetric anionic polymerizations of 7‐(o‐substituted phenyl)‐2,6‐dimethyl‐1,4‐benzoquinone methides: Electrostatic interaction and steric,inductive, and resonance effects of the ortho‐substituent on the optical activity

7‐(o‐Substituted phenyl)‐2,6‐dimethyl‐1,4‐benzoquinone methides which have an electron‐donating methoxy‐(o‐OMe, 2a ) and methyl‐ (o‐Me, 2b ) substituents or an electron‐withdrawing cyano‐ (o‐CN, 2c ) and trifluoromethyl‐ (o‐CF₃, 2d ) substituents at the ortho‐position of the aromatic ring and 7‐(m‐substituted phenyl)‐2,6‐dimethyl‐1,4‐benzoquinone methide with an electron‐withdrawing trifluoromethyl‐ (m‐CF₃, 2e ) substituent at the meta‐position of the aromatic ring were synthesized, and their asymmetric anionic polymerizations using the complex of lithium 4‐isopropylphenoxide with (?)‐sparteine were carried out in toluene at 0 °C. The polymers with negative optical activity were obtained for all of five monomers, and their specific rotation values largely changed depending upon the substituents of the monomers. On the basis of the comparison of various substituents effects, it was found that the specific rotation of obtained polymers is significantly affected by the electronic effects such as inductive and resonance effects rather than the steric and electrostatic effects of the substituent. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 1048–1058     

Stabilizing Fluorine–π Interactions

A series of N ‐arylimide molecular balances were designed to study and measure fluorine–aromatic (F–π) interactions. Fluorine substituents gave rise to increasingly more stabilizing interactions with more electron‐deficient aromatic surfaces. The attractive F–π interaction is electrostatically driven and is stronger than other halogen–π interactions.     

Effects of Substituents on Metastable‐State Photoacids: Design,Synthesis, and Evaluation of their Photochemical Properties

Recently, metastable‐state photoacids have been widely used to control proton transfer in numerous chemical and biological processes as well as applications with visible light. Generally, substituents have a great influence on the photochemical properties of molecules, which will further affect their applications. Yet, the effects of substituents on metastable‐state photoacids have not been studied systematically. In this work, 16 metastable‐state photoacid derivatives were designed and synthesized on the basis of substituents having a large range of σ–π electron–donor–acceptor capabilities. The effects of substituents on the color display [or maximum absorption band(s)], solubility, pK_a values, dark/photoacidity, photosensitivity, and relaxation kinetic(s) were investigated in detail. This study will be helpful for the targeted design and synthesis of promising photoacids and the application of their photocontrolled proton‐release processes in functional materials/devices.     

Synthesis,kinetics, and mechanism of bromophenols by N‐bromophthalimide in aqueous acetic acid

The kinetics and mechanism of bromination of phenol and its substituents, viz. 4‐chlorophenol, 4‐bromophenol, 4‐methylphenol, and 4‐methoxyphenol by N‐bromophthalimide (NBP) in the presence of mercuric acetate in the temperature range of 303–318 K in aqueous acetic acid medium have been investigated. The reaction follows first‐order dependence on [NBP] and fractional order dependence of rate on [Phenol]. The activation parameters have been evaluated, and based on the observed kinetic results the probable mechanism has been proposed. Observed kinetic features and Hammett's reaction constant (ρ) suggests that bromination occurs through electrophilic substitution of bromonium ion (Br⁺) into the aromatic ring in the transition state. Large negative entropy of activation values probably suggests the rigid nature of transition state.     

Synthesis of New Porphyrin–Fullerene Dyads Capable of Forming Charge‐Separated States on a Microsecond Lifetime Scale

A series of covalently linked axially symmetric porphyrin–fullerene dyads with a rigid pyrrolo[3,4‐c]pyrrolic linker enabling a fixed and orthogonal arrangement of the chromophores has been synthesized and studied by means of transient absorption spectroscopy and cyclic voltammetry. The lifetime of the charge‐separated state has been found to depend on the substituents on the porphyrin core, reaching up to 4 μs for a species with meso‐(p‐MeOC₆H₄) substituents. The ground and excited electronic states of model compounds have been calculated at the DFT and TD‐DFT B3LYP(6‐31G(d)) levels of theory and analyzed with regard to the effect of the substituent on the stabilization of the charge‐separated state in the porphyrin–fullerene ensemble with a view to explaining the observed dependence.     

Poly(ortho‐phenylene ethynylene)s: Synthetic accessibility and optical properties

Poly(ortho‐phenylene ethynylene)s (PoPEs) have been synthesized via an in situ activation/coupling AB′ polycondensation protocol. The resulting polymers have been characterized by several analytical methods and are shown to have no structural defects. Although the Sonogashira–Hagihara polycondensation reaction is less efficient than for the preparation of the corresponding meta‐ and para‐linked polymers, presumably because of steric hindrance caused by the ortho substituents, the process can be accelerated by the use of microwave irradiation. Optical spectroscopy indicates solvent‐dependent conformational changes between extended transoid and helical cisoid conformations, providing the first experimental evidence for solvophobically driven folding of the PoPE backbone. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1619–1627, 2006     

High refractive index materials: A structural property comparison of sulfide‐ and sulfoxide‐containing polyamides

High‐refractive‐index polyamides (PAs) are developed by incorporation of sulfide‐ or sulfoxide linkages and chlorine substituents. The PAs are synthesized through the polycondensation of two novel diamine monomers, 2,2′‐sulfide‐bis(4‐chloro‐1‐(4‐aminophenoxy) phenyl ether (3a) and 2,2′‐sulfoxide‐bis(4‐chloro‐1‐(4‐aminophenoxy) phenyl ether (3b), with various aromatic diacids (a–e). The ortho‐sulfide or sulfoxide units, pendant chlorine groups, and flexible ether linkages in the diamine monomers endowed the obtained PAs with excellent solubilities in organic solvents. The resulting PAs showed high thermal stability, with 10% weight loss temperatures exceeding 415 °C under nitrogen and 399 °C in air atmosphere. The combination of chlorine substituents, sulfide or sulfoxide linkages, and ortho‐catenated structures provided polymers with high transparency along with high refractive index values of up to 1.7401 at 632.8 nm and low birefringences (<0.0075). The structure–property relationships of the analogous PAs containing sulfide or sulfoxide linkages were also studied in detail by comparing the results. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2867–2877     

Studies on gas-phase cyclometalations of [ArNi(PPh3)n]+ (n = 1 or 2) by electrospray ionization tandem mass spectrometry

Gas-phase cyclometalation of [ArNi(PPh₃)n]⁺ (n = 1, 2) complexes have been studied by ESI-MS/MS. The electron-donating substituents of aromatic iodides in the para position were found to inhibit the cyclometalation process of losing ArH, while the electron-withdrawing substituents in the para position were found to enhance it. These results indicate that the cyclometalation process of losing ArH is favored by electron-deficient aromatic groups. In addition, the detailed dissociation pathways of the cationic nickel complexes were studied, and among these pathways, the process of aryl-aryl interchange was also found to proceed in ESI-MS/MS.     

Kinetics of the Reaction between Aromatic Aldehydes and 1,5‐Diamino‐4,8‐Dihydroxypyridazino[4,5‐d]Pyridazine

Kinetic studies were performed to investigate the mechanism of Schiff base formation in the reaction between aromatic aldehydes and 1,5‐diamino‐4,8‐dihydroxypyridazino[4,5‐d]pyridazine (DDPP). The studies were conducted at different temperatures with ethanol as solvent and acetic acid as catalyst. It is proposed that the first step involves the reaction of the aldehyde with a solvated proton (i.e. specific acid catalysis). Depending on the acidity of the medium, free or mono‐protonated (DDPP) at tacks the carbonyl group to form a carbinol‐amine inter mediate which then de hydrates to form the product. Plot of ΔH^# versus ΔS^# for the reaction gave a good straight line with isokinetic temperature of 343.15 K. Good linear relationship was obtained from the plot of log k against σ° values. The rate law derived from the proposed mechanism is in agreement with the experimental data and observations. The effects of meta and para substituents of benzaldehyde toward reactivity have been studied.     

A Library of Multipurpose Supramolecular Supergelators: Fabrication of Structured Silica,Porous Plastics,and Fluorescent Gels

Supramolecular gels find applications in various fields. Usually, a specific gelator is useful only for a specific application. This one‐gelator‐one‐application format is one factor that limits the usefulness of supramolecular gels. We report the synthesis of a library of gelators from a common core by using a click‐chemistry approach. Thus, the click reaction of β‐azido‐4,6‐O‐benzylidene–galactopyranoside ( 1 ) with various alkynes gave 11 different gelators having varying gelation abilities. Whereas gelators having alkyl‐chain substituents congealed alkanes and tetraethylorthosilicate (TEOS), the gelators having aromatic substituents congealed aromatic solvents. We exploited this difference in gelling behavior in the templated synthesis of silica rods and porous plastics. The styrene gel of gelator 2 j was polymerized, and the gelator was removed by washing to obtain porous polystyrene. The TEOS gel of gelator 2 b was polymerized to silica, and the gelator template was removed by calcination to give microstructured silica rods. We also developed fluorescent gelator 2 f by this method, which might find applications by virtue of its fluorescence in the assembled state.     

Nitroxide‐mediated polymerization of 1,3‐butadiene in the presence of diphenylamine with hydrogen peroxide as initiator

Living behavior of a nitroxide‐mediated polymerization of 1,3‐butadiene in the presence of diphenyl nitroxide (DPN) as counter radical and hydrogen peroxide (H₂O₂) as initiator has been studied to elucidate the bimodal molar mass distribution of the product by four approaches such as hydrolyzation of the oligomeric product, peak separation technique, which defines the oligobutadiene molar mass dependency in relation to α as a ratio of [nitroxide] to [H₂O₂], variation of hydroxyl value with respect to α, and para‐substituted DPN. It is demonstrated that the synthesized DPN shows efficient control of the polymerization of 1,3‐butadiene, but because of the formation of difunctional nitroxide during the oxidation process of aromatic diphenylamine, a bimodal distribution in the final product is observed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

A novel sorbent for chromatographic separations: A silica matrix modified with non‐covalently bonded tetrakis(β‐cyclodextrin)–porphyrin conjugates

We prepared new phases for LC that consisted of silica modified with non‐covalently bonded tetrakis(β‐cyclodextrin)–porphyrin (where cyclodextrin is CD) conjugates. The effects of the porphyrin core, type of spacer and β‐CD moieties on the behaviours of the modified phases for the separation of aromatic compounds (benzene, toluene, ethylbenzene, propylbenzene, butylbenzene, pentylbenzene, o‐terphenyl, triphenylene, phenol and caffeine) and fluorinated aromatic compounds (pentafluorobenzonitrile, pentafluoronitrobenzene and hexafluorobenzene) were studied using the Tanaka test. The results indicate that the non‐covalent substitution of silica with CD‐based macromolecules that have a porphyrin core can be a very effective method for preparing novel sorbents with specific chromatographic properties for applications in LC.     

Magnetic Circular Dichroism of the Long‐Wavelength π‐π* Transitions in Nitrogen‐Containing Heteroconjugated Aromatic Compounds and in Selected Ruthenium Complexes

The long‐wavelength magnetic circular dichroism (MCD) spectra of some nitrogen‐containing heteroconjugated aromatic compounds have been recorded and interpreted on the basis of quantum‐chemical model calculations. In particular, the dependence of the MCD bands on the positions of the N‐atoms inside the aromatic rings and on substituents has been investigated. Some of the compounds considered form Ru²⁺ complexes. The influence of complexation on the long‐wavelength π‐π* bands of the ligands has also been recorded and discussed.