Dimethylsilyl- and methylene-bridged aromatic groups : II. Phosphorescent triplet states

The zero-field splitting parameters D and E, or D, were measured by electron spin spectroscopy for the triplet states of bis(4-biphenylyl)dimethylsilane, 4,4′-bis-(4-biphenylyldimethylsilyl)biphenyl, 1,2-diphenyltetramethyldisilane, several monosilanes, bis(4-biphenylyl)methane, 4,4′-bis(4-phenylbenzyl)biphenyl, and dimethyldiphenylmethane. The zero-field splitting values of compounds having biphenyl subunits bridged with either SiMe₂- or CH₂-groups, compared with those of mono- and disubstituted biphenyls, show that the triplet electrons are localized mainly on one biphenyl group in each of the bridged compounds on the time scale of the ESR observations.     

Optical activity in the biaryl series

The relationship between the CD spectra of the chiral biaryls and their stereochemical configuration, as a function of the dihedral angle between the molecular planes of the aromatic moieties, has been investigated for biphenyl, 1,1′-binaphthyl, 1,1′-bianthryl and 9,9′-bianthryl in the exciton approximation and, for the 1,1′-binaphthyls, in the π-SCF approximation. Both methods provide unambiguous assignments of absolute configuration except for biaryls with a critical dihedral angle of π/2 in those with effective D_data2 chromophoric symmetry, or 100–110° in the case of the 1,1′-binaphthyls.     

Synthesis and characterization of liquid–crystalline polyesters with chiral and achiral twin spacers

A series of novel thermotropic main-chain chiral liquid–crystalline random copolyesters consisting of spacers of two different types—chiral and achiral—was synthesized. Polyesters (BmTa) with tartaric acid as the chiral spacer (Ta), aliphatic diols (with ‘m’ = 2–10 methylene groups) as the achiral spacers, and 4,4′-dihydroxy biphenyl (B) as the mesogen were synthesized via condensation polymerization in solution after duly protecting the 2,3-dihydroxy groups of tartaric acid by acetylation. The copolymers were characterized by Fourier transform infrared spectroscopy, ¹H and ¹³C NMR spectra, gel permeation chromatography, and thermogravimetric analysis. Transition temperatures for phase changes recorded by DSC were corroborated with the textures observed by a hot-stage optical polarizing microscope. The wide-angle X-ray diffraction (WAXD) profiles indicated a SmE phase at room temperature. The lower angle region at 2θ = 0.5–2.45 covered by WAXD indicated a layer of thickness of 161 Å, less than the molecular length for B0Ta. The [α_D] values were recorded on a digital polarimeter. The birefringence was lost at higher mesophase temperatures in lower members with m < 5, a behavior found in certain chiral systems, and the higher members with m > 5 showed a lesser number of phase transitions. On cooling, the polyesters produced a texture with the formation of transition bars. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1743–1752, 2001     

Binaphthyl-based chiral macrocyclophanes with variously sized cavities: Dn-symmetrical structure constructed from unidirectionally-inclined rod segments

As new and chiral macrocyclophanes with unique structures, variously sized Pn and Mn (n=2–7=number of ‘rod’ segments) with D₂–D₇ symmetry were constructed by alternating connection of axially chiral binaphthyls and linear biphenyls via –CH₂O– moieties, so that the macrocycle consists of multiple rod-like naphthalene–biphenyl–naphthalene units linked together at the binaphthyl bonds. The dihedral angle of the two naphthalene rings of binaphthyl is restricted to around 90°, and the calculated values of strain energy difference per naphthalene–biphenyl unit in P2–P7 are almost independent of the macrocycle size, presumably owing to the flexibility of the –CH₂O– connectors.     

Spectres de vibration de composes organiques des elements de la colonne IVB : XI. Discussion de la structure des composes (CH3)3MN(CH3)2 (M = Si,Ge, Sn) a partir des spectres et du calcul des modes normaux de vibration

The normal modes of (CH₃)₃MN(CH₃)₂ (M= Si, Ge or Sn) compounds are discussed using infrared and Raman spectra analysis. A valence force field model has been utilized to calculate the frequencies and potentiel energy distribution for each molecule in different structural hypotheses.Experimental results and calculated values are in good agreement when the dihedral angle ψ between the two MNC planes decreases from silicon (144°) to germanium (130°) to tin (120°).The differences in the molecular geometries may be related to the basicity and to the P_π → d_π interactions of these compounds.     

Microwave spectrum,conformational preference,intramolecular hydrogen bond,barrier to internal rotation,dipole moment,and centrifugal distortion of 1-fluoro-2-propanol

The microwave spectra of 1-fluoro-2-propanol, CH ₃CH(OH)CH ₂F, and one deuterated species, CH₃,CH(OD)CH₂F, have been investigated in the 18–30 GHz spectral region. Only one rotamer with an intramolecular hydrogen bond formed between the fluorine atom and the hydroxyl group was assigned. This conformation is also characterized by having the C-F bond approximately anti to the methyl group. The FCCO dihedral angle is 59 ± 2° and the HOCC dihedral angle is 58 ± 3°. Further conformations, if they exist, are at least 0.75 kcal mol^?1 less stable. Five vibrationally excited states belonging to four different normal modes were assigned and their fundamental frequencies determined. The barrier to internal rotation of the methyl group was found to be 2796 ± 50 cal mol^?1. The dipole moment is μ_a = 0.510 ± 0.009 D, μ_b = 1.496 t 0.026 D, μ_c = 0.298 ± 0.014 D, and μ_tot = 1.608 ± 0.030 D. Extensive centrifugal distortion analyses were carried out for the ground and the first excited state of the heavy-atom torsional mode and accurate values were determined for all quartic and two sextic coefficients.     

Optimized structures and vibration frequencies of the ether–water complex: a DFT and FTIR study

The infrared spectrum of ether was studied using Fourier transform infrared spectroscopy in conjunction with the density functional theory (DFT). The optimized structures and vibrational frequencies of the ether·(H₂O) _n (n = 1–3) complexes were obtained at B3LYP/6-31G(d) theory levels. Compared to those of free-form ether, the C–O stretching vibrational frequencies of the ether–water complexes are found to shift to red by up to 39 cm^?1 with an increase in the C–O length of 0.016 Å. Meanwhile, the frequency of the O–H stretching modes of water in the complexes appears significantly redshifted to a varying degree. The DFT calculations suggest that these shifts are caused by the hydrogen bonding between ether and water.     

An electron diffraction study of the molecular structure of 1,2-difluoroethane

The molecular structure of 1,2-difluoroethane in the gas phase has been determined by electron diffraction at room temperature. Only the gauche conformation was found, the dihedral angle F-C-C-F is 74.5°. The bond lengths r_g(1) are: r(C-C) = 1.535 Å, r(C-F) = 1.394 Å, r(C-H) = 1.13 Å. The valency angles are: α(C-C-F) = 108.3, α(C-C-H) = 108.3. The dihedral angle between the C-C-F and C-C-H planes is 113.6°.     

Refined ab initio 6-31G split-valence basis set optimization of the molecular structures of biphenyl in twisted,planar, and perpendicular conformations

Improved full ab initio optimizations of the molecular structure of biphenyl in twisted minimum energy, coplanar, and perpendicular conformations by use of Poles's GAUSSIAN 82 program have been performed in the 6-31G basis set. These lead to geometries and energies of much higher reliability than our earlier STO-3G results. The torsional angle Φ_min obtained now is 45.41° in close agreement with the recent experimental value of 44.4° ± 1.2°. Calculated CC distances may be converted to experimental ED r_g-values by means of independently determined linear regression correlations with very high statistical confidence, although they agree better with experimental x ray data for coplanar biphenyl without this correction. Calculated intramolecular angles are very similar for both STO-3G and 6-31G basis sets. The calculated torsional energy barrier towards Φ = 90° (ΔE⁹⁰) is 6.76 kJ/mol in close agreement with the experimental-31G value of 6.5 ± 2.0 kJ/mol. For coplanar biphenyl with D_2h-symmetry the calculated torsional energy barrier ΔE⁰ is 13.26 kJ/mol which is surprisingly much higher than the experimental value of 6.0 ± 2.1 kJ/mol. This discrepancy could not be resolved by optimizations assumed for two kinds of distortions of planarity of orthohydrogens from the molecular plane of the coplanar carbon atoms. But for the twisted minimum energy conformation asymmetric bending of ortho-H atoms lead to a torsional angle Φ_min = 44.74° together with a dihedral angle towards ortho-H of 1.22°, and consequently even to an increase of torsional energy barriers to ΔE⁰ = 13.51 and ΔE⁹⁰ = 6.91 kJ/mol.     

Microscopic structures of adsorbed cationic porphyrins on clay surfaces: molecular alignment in artificial light-harvesting systems

The intercalation behavior of cationic porphyrin derivatives within the interlayer spaces of nano-layered clay minerals has been investigated. The porphyrins were successfully intercalated by the newly adopted method of repeated freeze-thaw cycles. The absorption spectra of the porphyrins were compared in the solution phase, adsorbed onto the exfoliated clay nano-sheets, intercalated within the interlayer spaces of clay sheets dispersed in water and intercalated in dry films. Substantial red shifts of the λ_max values in the absorption spectra of the porphyrins were observed on the exfoliated clay sheets, and further red shifts were induced within the interlayer space. The dry films of the intercalated samples exhibited the largest red shifts. X-ray diffraction studies revealed that the clearance space between the layers in these intercalated hybrid compounds is only large enough for the porphyrins to be rigidly packed parallel to the clay layer. For the exfoliated clay nano-sheets, theoretical calculations were carried out on the correlation between the dihedral angle of the meso-substituted pyridiniumyl plane vs. the porphyrin ring and the λ_max of the porphyrin Soret band. An extrapolation of the experimental λ_max value to the correlation curve, afforded the dihedral angle to be 61.6°. The microscopic structure of the adsorbed state of the cationic porphyrins on the exfoliated clay nano-sheets was, thus, proposed to involve an orientation parallel to the clay surface, with a distance of 0.15 nm from the surface, which implies the expulsion of the solvent water molecules.     

Intramolecular disorder and its relation to mesophase structure in lyotropic liquid crystals

NMR SPDE measurements are reported for the lamellar (dispersions and multibilayer stacks) and hexagonal phases of sodium octanoate/octanol/D₂O mixtures. In the lamellar L_β and L_γ (gel) phases the octyl chains are rigid and perfectly ordered, while in the lamellar L_α and hexagonal phases they are flexible and disordered. In particular, the measurements show that in the fluid lamellar L_α phase, there is a marked discontinuity in the octyl chain flexibility at the C₅-C₆ segment; this behaviour is identical to that previously reported for the alkyl end-chains in smectic 4,4′-di-n-octyloxyazoxybenzene. In contrast, in the hexagonal phase, there is an effectively continuous flexibility gradient along the whole length of the octyl chain as in nematic 4,4′-di-n-octyloxyazoxybenzene. The behaviour in the lamellar phase is attributed to interference between cooperative conformational modes and localized random thermal fluctuations.     

Intricacies of ligand coordination in tricarbonylchromium(0) complexes with ortho‐ and para‐fluorobiphenyls

The steric and electronic factors that influence which of the two rings of a substituted biphenyl ligand coordinates to chromium are of interest and it has been suggested that haptotropic rearrangements within these molecules may be limited if the arene–arene dihedral angle is too large. Two tricarbonylchromium(0) complexes and their respective free ligands have been characterized by single‐crystal X‐ray diffraction. In the solid state, tricarbonyl[(1′,2′,3′,4′,5′,6′‐η)‐2‐fluoro‐1,1′‐biphenyl]chromium(0), [Cr(C₁₂H₉F)(CO)₃], (I), exists as the more stable isomer with the nonhalogenated arene ring ligated to the metal center. Similarly, tricarbonyl[(1′,2′,3′,4′,5′,6′‐η)‐4‐fluoro‐1,1′‐biphenyl]chromium(0) crystallizes as the more stable isomer with the phenyl ring bonded to the Cr⁰ center. The arene–arene dihedral angles in these complexes are 55.77 (4) and 52.4 (5)°, respectively. Structural features of these complexes are compared to those of the DFT‐optimized geometries of ten tricarbonyl[(η⁶‐C₆H₅)(4‐F‐C₆H₄)]chromium model complexes. The solid‐state structures of the free ligands 2‐fluoro‐1,1′‐biphenyl and 4‐fluoro‐1,1′‐biphenyl, both C₁₂H₉F, exhibit arene–arene dihedral angles of 54.83 (7) and 0.71 (8)°, respectively. The molecules of the free ligands occupy crystallographic twofold axes and exhibit positional disorder. Weak intermolecular C—H…F interactions are observed in all four structures.     

Two crystal forms of mesogenic bis(4′‐cyanobiphenyl‐4‐yl) butanedioate

The title compound, C₃₀H₂₀N₂O₄, exhibits a nematic phase in the wide temperature range between 498.5 and 538.6 K, in spite of the short linker moiety. Two crystal forms have been found. In both forms, the molecule is centrosymmetric. Form I has a planar biphenyl group, while form II has a twisted biphenyl group with a twist angle of 34.75 (6)°. The packing modes are also different. In form I the long molecular axes are tilted with respect to each other at about 30°, while in form II the long molecular axes have an almost parallel arrangement.     

Reactions of methyltris(triarylphosphine)cobalt : II. Rearrangement of triarylphosphine and oxidative additions of aryl halides

Solutions of [(C₆H₅)₃P]₃CoCH₃ (I) in C₆H₅Cl yield biphenyl, triphenylphosphine, methyldiphenylphosphine and diphenylphosphine. In 4-ClC₆H₄CH₃, 4-methylbiphenyl and 4,4′-bitolyl form as well. Solutions of I in C₆H₆, C₆D₆, C₆H₅CH₃, C₆H₅Br yield only triphenylphosphine and biphenyl, while in 4-FC₆H₄I 4,4′-difluorobiphenyl is formed but no biphenyl. The cobalt compound is recovered as (Ph₃P)_nCoX or as CoX₂ (XCl, Br, I, n = 3 or 2) from reactions with arylhalides. The results are rationalized in terms of the very strong tendency for I to undergo oxidative addition reactions.     

Polyazapodands Derived from Biphenyl. Study of their Behaviour as Conformationally Regulated Fluorescent Sensors

Eight new polyazapodands containing a 4,4′-substituted biphenyl moiety have been synthesised. Four (7, 8, 9 and 11) are functionalised on positions 4 and 4′ with a nitro group and four (1, 2, 3 and 10) with a dimethylamino substituent. Comparison of the emission behaviour of 1, 2, 3 with that of the reference compounds 10 and tetramthylbenzidine, clearly suggests that a modification in the dihedral angle between the biphenyl rings is an important factor in determining the fluorescent response of the molecule. The fluorescence is pH dependent, due to the formation of intramolecular hydrogen bonds between protonated aliphatic nitrogens and a carbonyl oxygen, which influences the aforementioned dihedral angle. A crystal structure resolved by X-ray diffraction of 7·2HCl has been determined, and confirms the dependence of the angle and the rigidity on the hydrogen bonding. The complexation properties of these ligands have been studied with Zn²⁺, Cd²⁺, Ni²⁺, Cu²⁺ and Pb²⁺, which show that the number of amino groups within the pendants has a strong influence on the nature of the complexation and the fluorescent response of each ligand.     

Synthesis,crystal structure and photoluminescence of 3-(4-(anthracen-10-yl)phenyl)-benzo[5,6]coumarin

A new coumarin derivative, 3-(4-(anthracen-10-yl)phenyl)-benzo[5,6]coumarin, was synthesized and characterized by FT-IR, ¹H NMR, element analysis and single crystal X-ray crystallography. The dihedral angle of benzo[5,6]coumarin ring and phenyl group is 36.15°, and the dihedral angle of phenyl group and anthracene skeleton is 89.37°. The UV–vis absorption and photoluminescence of the compound were discussed. The result shown that the compound exhibits high fluorescence quantum yield (Φ_F), large Stokes shift and green emission (508 nm). The molecular structure of the compound was optimized using density functional theory (DFT) at B3LYP/6-31G(d) level, and the HOMO and LUMO levels of the compound were deduced.     

Absolute Configuration Assignment from Optical Rotation Data by Means of Biphenyl Chiroptical Probes

A non-empirical approach for the assignment of the absolute configuration of chiral 2-alkyl-substituted carboxylic acids and primary amines by [α]_D measurements has been developed. The method requires the conversion of the chiral acids or amines into the corresponding 4,4′-disubstituted biphenylamides or biphenylazepines, respectively. In these derivatives a central-to-axial chirality transfer induces a preferred torsion in the biphenyl moiety revealed by the sign of the biphenyl A band in the ECD spectrum. By 4,4′-substitution on the biphenyl moiety a redshift of the A band is obtained, leading to an increase of its relative contribution to optical rotation. This allows to reliably establish a direct correlation between the [α]_D sign, the biphenyl twist and, then, the substrate absolute configuration. This approach thus constitutes a really practical and reliable method to assign the absolute configuration of chiral carboxylic acids and primary amines by simple and straightforward [α]_D measurement, readily obtainable by a routine instrumentation like the polarimeter.     

Studies on the odd‐even effect of methylene spacers in poly(pyromellitimide‐ester)s

Three series of poly(pyromellitimide‐ester)s were synthesized from various N,N′‐bis(ω‐hydroxyalkyl)pyromellitimides (HAPMIs) by melt condensation with dicarboxylic acids, including terephthalic acid (TPA), 4,4′‐biphenyldicarboxylic acid (BPDA), and 4,4′‐azobenzenedicarboxylic acid (ABDA). Polymers were characterized by elemental analysis, solubility, inherent viscosity, spectra (IR, ¹H‐NMR, ¹³C‐NMR), and X‐ray diffraction (XRD). Thermal stability and phase transition behaviour were evaluated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and hot‐stage optical polarized microscopy (HOPM). The d‐spacings, calculated from XRD data, showed an odd‐even effect with varying numbers of methylene spacers. Crystallinity of polymers decreased in the following order: azobenzene > biphenyl > phenyl polymers. Similarly, DSC‐obtained melting temperatures (T_m's) showed an odd‐even effect, and glass transition temperatures (T_g's) decreased with increasing numbers of methylene spacers. Thermal stability decreased as methylene chain length increased. Thermal stability of polymers occurred in the following order: phenyl > biphenyl > azobenzene polymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1755–1761, 1999     

A conformational study of aromatic imide compounds. Part 2. Compounds containing diphenyl sulfide,diphenyl sulfone,and diphenylmethane moieties

An attempt was made to estimate the dihedral angles, φ, ψ, ω₁, and ω₂, of bis(4-hydroxyphthalimide)s (BHPI) and bis(phenylphthalimide)s (BPI) having diphenyl sulfide, diphenyl sulfone, or diphenylmethane linkages at the center of molecules using solid–state ¹³C CP/MAS NMR and ab initio nuclear shielding calculations. The TOSS and TOSS & DD pulse sequences were performed in the NMR measurements to obtain exact chemical shifts of each carbon. Total energies were calculated using the B3LYP/6-31G(d) level of theory, and shielding constants were calculated using the RHF/6-31G(d) level of theory for diphenyl sulfide, diphenyl sulfone, diphenylmethane with varying angles of φ, ψ from 0 to 180° at intervals of 10°. It was clarified that the –S– and –SO₂– linkages lead asymmetrical conformations with different ω₁ and ω₂ or with different φ and ψ for BHPIs and BPIs. In contrast, the compounds having –CH₂– linkages have symmetrical conformations. The dihedral angle of imide ring and phenylene ring (ω) are in the range of 40–90°, and the dihedral angles (φ,ψ) distribute in the stable regions of the energy surfaces ranging from 40 to 90°.     

Diamondoids approach to electronic,structural, and vibrational properties of GeSi superlattice nanocrystals: a first-principles study

Germanium silicide diamondoids are used to determine electronic, structural, and vibrational properties of GeSi superlattice nanocrystals and bulk as their building block limit. Density functional theory at the generalized gradient approximation level of Perdew, Burke, and Ernzerhof (PBE) with 6-31G(d) basis including polarization functions is used to investigate the electronic structure of these diamondoids. The investigated molecules and diamondoids range from GeSiH₆ to Ge₆₃Si₆₃H₉₂. The variation of the energy gap is shown from nearly 7 eV toward bulk value which is slightly higher than the average of Si and Ge energy gaps. Variations of bond lengths, tetrahedral, and dihedral angles as the number of atoms increases are shown taking into account the effect of shape fluctuations. Localized and delocalized electronic charge distribution and bonds for these molecules are discussed. Vibrational radial breathing mode (RBM) converges from its initial molecular value at 332 cm^?1 to its bulk limit at 0 cm^?1 (blue shift). Longitudinal optical-highest reduced mass mode (HRMM) converges from its initial molecular value 332 cm^?1 to experimental bulk limit at 420.7 cm^?1 (red shift). Hydrogen vibrational modes are nearly constant in their frequencies as the size of diamondoids increases in contrast with lower frequency Ge–Si vibrational modes. GeSi diamondoids can be identified from surface hydrogen vibrational modes fingerprint, while the size of these diamondoids can be identified from Ge–Si vibrational modes.