Synthesis, crystal structure, and nonlinear optical behavior of beta-unsubstituted meso-meso E-vinylene-linked porphyrin dimers

[structure: see text] A vinylene-linked porphyrin dimer, with no substituents at the beta-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the crystal structure of this dimer, the C(2)H(2) bridge is twisted by 45 degrees relative to the plane of the porphyrins. The absorption, emission spectra, and electrochemistry reveal substantial porphyrin-porphyrin pi-conjugation. The triplet excited-state absorption spectrum of this dimer makes it suitable for reverse saturable absorption at 710-900 nm.     

H-NMR studies of duplex DNA decamer containing a uracil cyclobutane dimer: implications regarding the high UV mutagenecity of CC photolesions

To determine the origin of the UV-specific CC to TT tandem mutation at the CC site, we made a duplex DNA decamer containing a uracil cis-syn cyclobutane dimer (CBD) as the deaminated model of a cytosine dimer. Two-dimensional 1H-NMR spectroscopy studies were performed on this sequence where two adenines (Ade) were opposite to the uracil dimer. Two imino protons of the uracil dimer were found to retain Watson-Crick hydrogen bonding with the opposite Ade, although the 5'-U(NH) of the dimer site showed unusual upfield shift like that of the 5'-T(NH) of the TT dimer, which seemed to be associated with deshielding by the flanking base rather than with reduced hydrogen bonding. (McAteer et al. 1998, J. Mol. Biol. 282:1013-1032). Hydrogen bondings at the dimer site were also supported by detecting typical strong nuclear Overhauser effects (NOE) between two imino protons and the opposite Ade H2 or NH2. But sequential NOE interactions of base protons with sugar protons were absent at the two flanking nucleotides of the 5' side of the uracil dimer and at the intradimer site, contrasting with its thymine analog where sequential NOE was absent only at the A4-T5 step. In addition, NOE cross peak for U5(NH) <--> A4(H2) was detected, although the NOE interactions of U6(NH) with A7(H2) and A17(H2) were not observed in contrast to the thymine dimer duplex. This different local structural alteration may be affected by the induced right-hand twisted puckering mode of cis-syn cyclobutane ring of the uracil dimer in the B-DNA duplex, even though the isolated uracil dimer had left-hand twisted puckering rigidly. In parallel, these observations may be correlated with observed differences in mutagenic properties between cis-syn UU dimer and cis-syn TT dimer.     

Synthesis and characterisation of dimeric eight-coordinate lanthanide(III) complexes of a macrocyclic tribenzylphosphinate ligand

The macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7-triyl(methylenebenzyl-phosphinic acid) H3L3, has been prepared and its complexes with Eu, Gd and Tb(III) studied by NMR, relaxometry, luminescence and single crystal X-ray crystallography. In solution and in the crystal, the complexes have eight-coordinate metal centres with bridging phosphinate groups linking the two twisted square antiprismatic coordination polyhedra. A single stereoisomer crystallises from solution with an RRR and SSS configuration at the P centres in each sub-unit. The relaxivity of [GdL3]2 is low (1.9 mM-1 s-1, 298 K, 20 MHz), consistent with the absence of any proximate water molecules. The terbium dimer possesses a relatively long excited state lifetime (2.47 ms, 298 K).     

Vibrational and electronic spectra of benzophenone in different phase states: Ab initio calculations and experiment

An ab initio method has been used to perform quantum mechanical calculations of the formation energy of different conformers of benzophenone: planar molecule, twisted molecule, planar molecule dimer, twisted molecule dimer; electronic and vibrational spectra of these conformers were also obtained. An assessment of the medium (solvent) influence on the optimal geometry, dipole moment and stability of different forms of benzophenone was performed in the self-consistent reaction field approximation. It is shown that the twisted conformer is more stable than the planar one (the difference of free energies is 32 kJ/mol for free molecules) and it becomes even more stable with the increase in solvent polarity. The calculated electronic and vibrational spectra agree well with the experimental data and properly reflect the complication of the vibrational spectrum when passing from the gaseous phase to the condensed state of benzophenone. The difference between spectral properties of the two dimer forms allows their identification from the spectra and qualitative explanation of the observed peculiarities of phosphorescence of the amorphous phase of benzophenone by the stabilization of different conformers.     

Impact of configuration and fluorination on the solubility of octyl ester benzoate dimers in CO2

Data are reported on the phase behavior of hydrocarbon and semifluoroinated octyl ester benzoate dimers in CO₂ to temperatures of 100 °C and pressures of 1 600 bar. The experimental data at 75 °C demonstrate that the non-fluorinated head-to-head (H–H) dimer dissolves in CO₂ at ∼400 bar lower pressures than the non-fluorinated tail-to-tail (T–T) dimer. Even though partially fluorinating the octyl tails of the H–H and T–T dimers renders them soluble in CO₂ at pressures near 200 bar, it still takes ∼40 bar more pressure to dissolve the fluorinated T–T dimer as compared to the H–H dimer. The difference in pressures needed to dissolve these dimers is attributed to steric constraints on the coplanarity of the benzene rings imposed by the H–H regiochemistry that do not exist with T–T dimers. Semi-empirical quantum mechanics calculations suggest that the H–H dimer has a twisted, non-coplanar conformation due to the steric effect of the octyl ester groups while the T–T dimer has a less twisted conformation. Steric hindrance in the H–H dimer reduces considerably resonance or conjugation between the π electrons of the aromatic groups which also reduces the dipole moments of the H–H dimers compared to those of the T–T dimers.     

Molecular recognition: preorganization of a bis(pyrrole) Schiff base derivative for tight dimerization by hydrogen bonding

Multiple techniques have been used to delineate the self-assembly of a bis(pyrrole) Schiff base derivative (compound 4, C(16)H(14)N(4)), which forms an unusual dimer through complementary N-H...N=C hydrogen bonds between twisted, C2-symmetric monomer units. The asymmetric unit of the crystal structure comprises one and a half dimer units, with one dimer exhibiting approximate D2 point-group symmetry and the other exact D2 symmetry (space group C2/c). The dimers pack into columns whose axes are collinear with the a axis of the unit cell. The columns assemble into discrete layers with two distinct types of hydrogen-sized voids residing between the layers. Despite the promising architecture of the voids within the lattice of 4, the absence of genuine channels to interconnect the voids precludes the uptake of hydrogen gas, even at elevated pressures (10 bar). AM1 calculations of the structure of dimeric 4 indicate that self-recognition through hydrogen bonding depends primarily on favorable electrostatic interactions. The potential-energy surface for monomeric 4 mapped by counter-rotation of an adjacent pair of C=C-N=C torsion angles indicates that the X-ray structures of the four monomeric units are global minima with highly nonplanar conformations that are preorganized for self-recognition by hydrogen bonding. The in vacuo enthalpy of association for the dimer was calculated to be significantly exergonic (DeltaG(assoc)=-21.9 kJ mol(-1), 298 K) and in excellent agreement with that determined by 1H NMR spectroscopy in CDCl3 (DeltaG(assoc)=-16.6(4) kJ mol(-1), 298 K). Using population and bond order analyses, in conjunction with the conformation dependence of the frontier MO energies, we have been able to show that pi-electron delocalization is only marginally diminished in the nonplanar conformers of 4 and that the electronic structures of the constituent monomers of the dimer are well mixed.     

Inclusion Complexes of Dyes and Cyclodextrins: Modeling Supermolecules by Rigorous Quantum Mechanics

The results of structure optimization and molecular dynamics simulationof host-guest -cyclodextrin-pinacyanol dye inclusion complexesare obtained by applying a density functional based tight-binding code.The results attempt to correlate UV/Vis and circular dichroism spectraldata with calculated aggregate structures of the sandwich dimer, withthe monomers twisted slightly against each other. The sense of twist ispredetermined by the chirality of the complexing host. The UV/Vis-spectralare interpreted using the exciton model. Within this model, each excitedstate of the monomer generates two excitonic states in the dimer. Theinteraction between the two monomers results then in a Davydov splittingof the two dimer states. The opposite signs of the two dimer states can beattributed to the twist of the monomers when they interact.     

Twisted perylene stereodimers reveal chiral molecular assembly codes

Unique perylene diastereomeric linear and cyclic dimers were synthesized from twisted perylene monomers, revealing that pi-stacking stereoisomerism imparted specific intermolecular self-assembly and intramolecular folding. Only the homochiral twisted tetrachloroperylene monomers cyclized via a cooperative reaction, forming the homochiral diastereomers. The heterochiral tetrachloroperylene monomers proceeded through a stepwise reaction and yielded a linear heterochiral dimer, which equilibrated with the linear homochiral dimers. The linear homochiral dimers cyclized to produce the same cyclic homochiral diastereomers. These results demonstrated that homochiral and heterochiral self-assemblies were two distinct molecular codes, directing two specific chemical pathways. The homochiral cyclic dimers remain isomerically pure at -20 degrees C but can be interconverted to the heterochiral cyclic dimer meso compound at room temperature. The diastereomers were readily separated by HPLC. While driven by solvophobic forces, foldable linear dimers synthesized from the same twisted monomers using phosphoramidite chemistry folded into homodimer and heterodimer, confirming the inherent molecular codes, which were dictated by the perylene chirality, ultimately gauged the weak pi-stack forces, and directed self-assembly and folding.     

Supramolecular probe for bicarbonate exhibiting anomalous pyrene fluorescence in aqueous media

Highly selective HCO3- sensing is realized in a pH 8.6 aqueous solution on the basis of the HCO3--induced anomalous pyrene fluorescence of an association dimer that consists of pyrene-appended gamma-cyclodextrins having a triamine linker. The anomalous fluorescence that is produced by the HCO3--induced twisted conformation of the pyrene residues in the association dimer is insensitive to the presence of other anions.     

Theoretical and experimental investigations of ligand exchange in guanidinate ligand systems for group 13 metals

The ligand exchange of guanidinate ligands between metal centres can play an important role in guanidinate chemistry, and ligand exchange between aluminium centres will form a dimeric intermediate. The synthesis and characterization of the dimer [Me(2)NC(N(i)Pr)(2)](2)Al(2)Cl(4) is reported here: compound crystallizes with a twisted boat conformation of its dimer ring. This compound decomposes to monomers at room temperature over four days, or within 18 hours at 90 degrees C. We undertook a detailed computational characterization of the reaction pathway, which supported the dimer structure and subsequent monomer formation. The ligand exchange route was also exploited for the synthesis of [MeC(N(i)Pr)(2)](2)AlCl, [EtC(N(i)Pr)(2)](2)AlCl, [MeC(N(i)Pr)(2)](2)GaCl, and [Me(2)NC(N(i)Pr)(2)](2)GaCl.     

Crystal structure of 4,5-bis(4-dimethylaminophenyl)-2-(4-nitrophenyl)imidazole.

A nonlinear optical chromophore, 4,5-bis(4-dimethylaminophenyl)-2-(4-nitrophenyl)imidazole, was investigated by X-ray crystallography. The study focused on coplanarity among several aromatic rings, including phenyls and imidazole. Two phenyl rings with NMe2 groups are twisted by 46.39(3) degrees from each other. However, they are twisted by 23.05(5) degrees and 46.84(3) degrees from the imidazole, respectively. These unequal twists were elucidated by different conjugation pathways from the donors to the acceptor. The phenyl ring with the NO2 group is twisted by only 6.76(6) degrees from the imidazole.     

X射线单晶衍射研究系列功能七元杂环桥联联苯构象

含有七元杂环桥联联苯的π-共轭聚合物是重要的宽禁带光电材料, 其发光性质决定于两苯环间扭曲角. 利用X射线单晶衍射仪确定了系列功能的七元杂环桥联联苯化合物的晶体结构, 比较了桥联原子, 6,6位取代基及分子间弱氢键对这类桥联联苯扭曲构象的影响. 桥联键是—CH2—O—CH2—和—CH2—S—CH2—的七元杂环桥联联苯两苯环间的扭曲角分别为39°和51°, 在6,6位上含有不同取代基的双氧桥联七元杂环桥联联苯的扭曲角存在5°的差异. 在双氧桥联的七元杂环桥联联苯的二聚物中, 桥联的氧原子与非桥联的苯亚基-苯亚基2,2’位置上的两个氢原子之间形成的氢键网络结构导致中心联苯呈现平面构象, 但氢键并没有改变桥联联苯的构象. 七元杂环桥联联苯的构象主要由桥联原子决定, 同时分子间相互作用亦能轻微调控这类桥联联苯的扭曲角.     

Helical Oligophenylene Linked with [2.2]Paracyclophane: Stereogenic π-Conjugated Dye for Highly Emissive Chiroptical Properties

A stereogenic π-system based on dimer ( 2 ) and trimer ( 3 ) of [2.2]paracyclophane (PC) and biphenyl was prepared and its structural, photophysical, and chiroptical properties were investigated. X-ray analysis revealed that the quaterphenyl moieties in 2 adopt a double helical structure anchoring [2.2]PC from both sides. Furthermore, 3 forms a isosceles triangle structure with a large chiral cavity. A homodesmotic reaction using DFT calculations revealed that 2 has a larger strain energy than 3 owing to its highly twisted phenylene linkers. Electronic and circular dichroic (CD) spectra were recorded in CH₂Cl₂ solution. The spectra of both 2 and 3 are similar, and their longest absorption band accompanying a remarkable Cotton effect is attributed to the transition from HOMO to LUMO, which is delocalized to the quaterphenyl moiety. These compounds exhibit fairly high fluorescence quantum yields (ϕ=0.70–0.83) and moderate dissymmetry factor (|g_CPL|=1.6×10⁻³) in circularly polarized luminescence (CPL).     

Magnetic interaction of tri- and di-oxytriphenylamine radical cation FeCl4 salts

The tetrachloroferrates of the 2,2':6',2':6',6-trioxytriphenylamine (TOT.+.FeCl4-) and 2,2':6',2'-dioxytriphenylamine (DOT.+.FeCl4-) radical cations were prepared, and their structures, magnetic properties, and the relationship between them were investigated. The TOT.+ moiety had a highly planar structure and packed as a dimer surrounded by tetrachroferrates, which also formed a dimer structure. The magnetic properties of TOT.+.FeCl4- were characterized by strong (2J/kB=approximately -1.3x10(3) K, H=-2JS1/2.S1/2) and weak (2J/kB=-1.76 K, H=-2JS5/2.S5/2) antiferromagnetic interactions due to the (TOT.+)2 and (FeCl4-)2 structures, respectively. DOT.+ had a twisted form and no dimer formation was observed between the DOT.+'s and FeCl4-'s. Instead, short contacts between the DOT.+ and chlorine atoms and between the DOT.+'s producing a DOT.+ chain were observed. The magnetic properties of DOT.+.FeCl4- were characterized by a 3D magnetic phase transition to an antiferromagnet with TN=approximately 8 K.     

Unique contrast patterns from resonance-enhanced chiral SHG of cell membranes

Chirality can produce novel nonlinear optical effects that may form the basis for new imaging contrast agents. In this paper, we developed a new chiral chromophore 2, which is the dimer of a known voltage sensitive dye, monomer 1, with the chirality originating from the twisted orientation between two subunits. Racemic dimer and monomer 1 were used as the references to study the effect of chirality in SHG microscopy of live cells. All these dyes selectively stain the outer leaflets of cell membranes, producing strong resonance-enhanced SHG images. At the symmetric junction between two adherent cells, monomer or racemic dimer SHG is forbidden due to centrosymmetry, and indeed little SHG was observed (10 +/- 1% relative to nonjunction). When stained with the chiral dimer, the junction is no longer centrosymmetric and much stronger SHG was observed (39 +/- 4% relative to nonjunction). Plane polarized light produces highly polarized images of spherical cells stained with racemic dye, but for the chiral dye, the polarized pattern is largely eliminated by the chiral SHG emanating from the subresolution membrane convolutions.     

Highly functionalized dimeric tetraethynylethenes and expanded radialenes: strong evidence for macrocyclic cross-conjugation

A selection of dimeric tetraethynylethenes (TEEs) and perethynylated expanded radialenes, containing different donor/acceptor substitution patterns, have been prepared and fully characterized. The first X-ray crystal structure of an expanded [6]radialene, with twelve peripheral 3,5-di(tert-butyl)phenyl substituents, is presented. This macrocycle, the all-carbon core of which is isomeric with fullerene C60, adopts a non-planar, "chair-like" conformation. Also a TEE dimer, carrying N,N-dimethylaniline donor substituents, has been subjected to an X-ray crystallographic analysis. The electronic properties were studied by UV/Vis spectroscopy and electrochemistry, providing fundamental insight into mechanisms of pi-electron delocalization in the acyclic and macrocyclic chromophores. Donor or donor-acceptor-substituted dimeric TEE derivatives show very strong absorptions extending over the entire UV/Vis region; their longest wavelength absorption bands have high charge-transfer character. Macrocyclic cross-conjugation in the expanded radialenes becomes increasingly efficient with increasing donor-acceptor polarization. A dual, strongly solvent-polarity-dependent fluorescence was observed for a tetrakis(N,N-dimethylaniline)-substituted dimeric TEE; this interesting emission behavior is explained by the twisted intramolecular charge-transfer (TICT) state model. Donor-substituted expanded radialenes display huge resonance-enhanced third-order nonlinear optical coefficients.     

Reductive coupling of metal triangles in sandwich complexes

The one-electron reduction of [Pd3(C7H7)2(CH3CN)3][BF4]2 in acetonitrile resulted in the formation of the dimer dication [Pd6(C7H7)4(CH3CN)4][BF4]2, whose structure containing a novel bitriangle hexapalladium skeleton was determined by X-ray crystallographic analysis. The dimer is stable in CD3CN at ambient temperature for several days but is highly air-sensitive. Similarly, the cycloheptatriene tripalladium complex [Pd3(C7H7R)2(CH3CN)3][BF4]2 (R = H, t-Bu) dimerized upon one-electron reduction. Both monomer and dimer of cycloheptatriene complexes were structurally determined by X-ray crystallographic analyses.     

从中药佛手中分离鉴定两个柠檬油素二聚体

用 IR、NMR和X-射线晶体衍射分析了从佛手中分离出来的两个柠檬油素二聚体,证明它们的化学结构分别是顺式头-尾-3,3′,4,4′-柠檬油素二聚体(1)和顺式头-头-3,3′,4,4′-柠檬油素二聚体(2)。通过光反应从柠檬油素制备了1和反式头-头-3,3′,4,4′-柠檬油素二聚体(3)。     

Supramolecular dimeric structures of pyrazole-containing meso-oxo carbaphlorin analogues

Synthesis and properties of a novel meso-oxo carbaphlorin analogue embedded with an N-free pyrazole moiety are described. The N-benzyl precursor was prepared by a [3 + 1]-MacDonald condensation of N-benzyl pyrazole dialdehyde and β-alkyl-substituted tripyrrane dicarboxylic acid and subsequent oxidation by ferric chloride. Upon deprotection of the benzyl group, the resulting N-free oxophlorin analogue formed a unique supramolecular dimer through mutual hydrogen bonding interactions between the pyrazole NH and meso-carbonyl group. The assembled behaviour was characterised by various spectroscopies, X-ray crystallographic analysis and vapour pressure osmometry. Under the similar reaction conditions, the condensation of meso-phenyl-substituted tripyrrane derivative afforded an unprecedented tetrapyrrolic macrocycle fused with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone unit at their α-pyrrolic positions. The X-ray crystallographic analysis of the macrocycle revealed a twisted core structure, and nonaromatic nature was elucidated.     

Gallium(III) protoporphyrin IX]2: a soluble diamagnetic model for malaria pigment

Gallium(III) protoporphyrin IX forms a dimeric propionate-bridged dimer, 2, that is a soluble diamagnetic analogue of hematin anhydride. The single-crystal structure of 2 corresponds to a nondisordered inversion-symmetric dimer similar to malaria pigment but, unlike it, has a six-coordinate metal and an intraporphyrin rather than an interporphyrin hydrogen bond. NMR NOE correlations demonstrate the presence of the propionate linkage in solutions with pyridine. Taken together, this is the first single-crystal X-ray diffraction study of a propionate-linked dimer as found in malaria pigment and the first evidence for its presence in solution.