Linear and nonlinear optical properties of trans- and cis-poly(1-ethynylpyrene) based sonogel hybrid materials

The synthesis of sol-gel materials induced by ultrasonic action (sonolysis) is implemented as an alternative method for the fabrication of highly pure organic-inorganic composites with good monolithic and optical properties. The resulting SiO₂ glass exhibits high porosity and allows the inclusion of organic compounds in the colloidal sol-state. In this work, optical properties of trans-poly(1-ethynylpyrene) (trans-PEP) and cis-poly(1-ethynylpyrene) (cis-PEP) (M_w=24,000 g/mol) incorporated in this kind of gels were studied by absorption and fluorescence spectroscopies. Absorption spectra of the polymers showed that trans-PEP possesses a higher degree of conjugation than its homologue cis-PEP in sol-gel. Intramolecular interactions occur between adjacent pendant pyrene units (associated pyrenes) present in each polymer, giving rise to static excimer emissions, strongest in cis-PEP because of the shorter distances between aromatic rings. The results were compared to those previously reported for these polymers in solution. Besides, trans- and cis-PEP exhibited nonlinear optical properties like third harmonic generation (THG), which were measured in sol-gel phase for spin-coated film samples.     

Novel polyacetylenes containing pendant 1-pyrenyl groups: synthesis, characterization, and thermal and optical properties

We report the synthesis as well as the thermal, optical and photophysical properties of four different polyacetylenes with pendant 1-pyrenyl groups: poly(1-ethynylpyrene) (PEP), poly(1-(trimethylsilanylethynyl)pyrene) (PTMSEP), poly(1-(4-(trimethylsilanyl-buta-1,3-diynyl)pyrene) (PTMSBDP) and poly(1-buta-1,3-diynylpyrene) (PBDP). Polymerizations were carried out with W and Ta catalysts, respectively, for mono- and disubstituted monomers. Soluble poly(1-ethynylpyrene) with high molecular weights (up to 4×10⁵) and extended conjugation of the main chain was obtained with good yields. Lower molecular weights (up to 6×10³) were obtained with the other polymers. Oligomers and polymers displayed high thermal stability. From the absorption spectra of the various polymers, it is found that PEP possesses a higher degree of conjugation than the other polyacetylenes. Molecular interactions occur between pyrene units present in each polymer giving rise to an emission due to associated pyrenes. These interactions are affected by the steric hindrance present in the polymer backbone. Excitation spectra combined with fluorescence decay profiles show that these interactions occur in the ground state (excited complex).     

On the existence of cis/trans peptide mixtures in poly(N-methylglycine)

Unperturbed dimensions have been computed for poly(trans-N-methylglycine), poly(cis-N-methylglycine) and poly(cis/trans-N-methylglycine) by a Monte Carlo simulation technique using potential energy calculations. Computed characteristic ratios for the above polypeptides vary within a narrow range supporting the view that both cis and trans units could be present in the polymer in different proportions under different solvent conditions.     

Photochemistry and photophysics of copolymers of (−)menthyl acrylate with 4-hydroxystilbene acrylate

Copolymers of trans-4-hydroxystilbene with (—)menthyl acrylate exhibit upon irradiation transåcis isomerization of the side-chain stilbene chromophores analogously to trans-4-hydroxystilbene-2-methylpropanoate, chosen as low molecular weight model compound. U.v. measurements on polymer samples indicate a deviation from first-order kinetics. In the fluorescence spectra of all-trans polymer samples both excimer and monomer emissions are observed. On irradiation the excimeric component is quenched much faster than the monomeric one. Fluorescence polarization experiments indicate a higher conformational rigidity of the stilbene chromophore when inserted in polymeric systems. Comparison of chiroptical properties of both unirradiated and irradiated copolymers suggests that on irradiation a local conformational rearrangement of macromolecules takes place.     

Synthesis and properties of polyimides derived from cis- and trans-1,2,3,4-cyclohexanetetracarboxylic dianhydrides

Cis-1,2,3,4-cyclohexanetetracarboxylic dianhydride (cis-1,2,3,4-CHDA) was synthesized. It was found that under such conditions as heating or boiling in acetic anhydride, cis-1,2,3,4-CHDA could be converted to its trans-isomer. The process of thermal isomerization was monitored by ¹H NMR spectra and the mechanism of conversion was proposed. Their absolute structures of cis- and trans-1,2,3,4-CHDAs were elucidated by single crystal X-ray diffraction. The polycondensations of cis- and trans-1,2,3,4-CHDAs with aromatic diamines such as 4,4′-oxydianiline (ODA), 4,4′-methylenedianiline (MDA), 4,4′-diamino-3,3′-dimethyldiphenylmethane (DMMDA), 4,4′-bis(4-aminophenoxy)benzene (TPEQ), 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) were studied. It is easy to obtain higher molecular weight polyimides from trans-1,2,3,4-CHDA using conventional one-step or two-step methods. However, higher molecular weight polyimides derived from cis-1,2,3,4-CHDA could not be prepared by the usual methods (solid content ca. 10%) owing to the trend of forming cyclic oligomers. Increasing the concentration of monomers could give higher molecular weight cis-polymers. All of the cis-polyimides were soluble at room temperature in aprotic polar solvents and phenolic solvents and some of them even soluble in chloroform and tetrahydrofuran, while the corresponding trans-polymers showed poor solubility as compared to cis-polymers. All of the polyimides showed good thermal stability with the 5% weight loss temperatures in air over 415 °C. Furthermore, polyimides derived from cis-1,2,3,4-CHDA have higher glass transition temperatures (T_gs) than corresponding trans-polyimides. The flexible polyimide films possessed a tensile modulus range of 2.1-3.6 GPa, a tensile strength range of 42-116 MPa, an elongation at break of 4-18%. These polyimides exhibited cutoffs at wavelengths around 270 nm and were entirely colorless. All the polyimides showed amorphous pattern according to Wide angle X-ray diffraction measurements. The differences of polymerization and properties were explained by the structural changes resulted from isomerism.     

Cationic phenyl-substituted poly(p-phenylenevinylene) related copolymers with efficient photoluminescence and synthetically tunable emissive colors

A series of amino-functionalized phenyl-substituted poly(p-phenylenevinylene) (PPV) related copolymers were synthesized by Wittig reaction. Their corresponding cationic conjugated polymers were successfully obtained via a post-polymerization approach. On the basis of FT-IR and ¹H NMR spectra, it was found that phenyl-substituted PPV related copolymers containing alkoxylated benzene (neutral polymer P1 and quaternized polymer P1′), phenylated benzene (neutral polymer P2 and quaternized polymer P2′) and fluorene (neutral polymer P3 and quaternized polymer P3′) moieties are of 55, 80, and 45% cis-vinylic linkage respectively while the polymer containing thiophene moiety (neutral polymer P4 and quaternized polymer P4′) is primarily of trans-vinylic linkage. Their photoluminescence (PL) were conveniently tuned from blue color to yellow color by introducing units with different optoelectronic properties into the PPV backbones. The polymer with fluorene unit and bulky phenylene-substituted benzene unit in the backbone exhibited the highest PL efficiency among these neutral and quaternized PPVs. P4′ containing little cis-vinylic linkage showed complete quenching while P1′-P3′ containing much more cis-vinylic linkage showed incomplete quenching, indicating that the quenching behavior of these cationic PPVs may be highly influenced by the content of cis-vinylic linkage in the PPV backbones.     

On the packing properties of poly(acetylene) chains

Packing calculations relevant to the crystal structure of trans-poly(acetylene) are described. Three classes of chain arrangements have been studied: pseudomonoclinic structures with one chain per unit cell and orthorhombic and monoclinic structures containing two symmetry related chains. The results are compared with experimental data on trans-poly(acetylene) reported in the literature and structure analyses of octatriene(2,4,6)-acid and 1,10-dicyanooctatetraene(1,3,5,7) which are reported as well. The same packing program has been used to recalculate the structure of cis-poly(acetylene). The results indicate the possibility for polymorphism in poly(acetylene) because several packing modes can be derived which differ very little with regard to their packing energy. However, literature data on the assignment of lattice parameters from poorly resolved X-ray, electron and neutron diffraction data can now be critically discussed.     

Generation of polyacetylene sulfoxide radicals through spin migration from the main-chain to the sulfoxide moiety in the side chain of poly[p-(n-butylsulfoxide)phenylacetylene] prepared with a [Rh(norbornadiene)Cl]2 catalyst

A phenylacetylene bearing an n-butylsulfoxide group, i.e., p-(n-butylsulfoxide)phenylacetylene (1) was prepared in high yields using the [Rh(norbornadiene)Cl]₂-NEt₃ catalyst in the presence of various solvents under mild conditions. The resulting polymer, poly[p-(n-butylsulfoxide)phenylacetylene] (poly(1)), was characterized in detail by ¹H NMR, ESR, laser Raman, and diffuse reflective UV-vis methods. The data clearly showed that cis-to-trans isomerization of the polymer can be induced when pressure is imposed to the polymer at room temperature, rotationally breaking the cis CC bonds to generate the cis and trans radicals. Further, the spin density in the cis radical was migrated from the main-chain to the sulfoxide moiety as the side chain of the phenyl ring to magnetically interact with the first two methylene protons in the n-butyl group giving a triplet line ESR spectrum with an extremely large g value, g = 2.0081.     

N.m.r. spectroscopy of polyesters from bridged bicyclic lactones

The ¹³C n.m.r. spectra of some polyesters produced by anionic ring-opening polymerization of 2-oxabicyclo[2,2,2] octan-3-one are reported and discussed. From a comparison with the spectrum of the monomer and by hydrolysis of the polymer, it was possible to assign to the polymer, obtained using n-butyllithium as a catalyst, a microstructure in which the ester groups on the 1 and 4 positions of the cyclohexane unit are cis to each other. The polymers obtained both by sodium tert-butoxide and sodium-potassium alloy as initiators, showed spectral differences associated with the presence of another isomeric structural unit. Decreasing of the relaxation times (T₁) of cis unit by increasing the concentration of this new unit was reasonably explained by assuming for this latter a trans structure linked with the cis unit to form a stereocopolymer.     

Sequence distribution of cis-1,4- and trans-1,4-units in polyisoprenes

The ¹³C n.m.r. spectra of chicle polyisoprene and cis-trans isomerized 1, 4-polyisoprenes were studied. The splittings of signals were observed in the C₁, C₂, and C₄ carbon signals of the isomerized polyisoprenes. The newly appearing signals were assigned to the carbon atoms in cis-trans linkages. The fractions of the diad sequences (trans-trans, trans-cis, cis-trans, and cis-cis) were determined by using the four signals of C₁ carbon. It was found that the cis-1, 4- and trans-1, 4-units were randomly distributed in the isomerized polyisoprenes and it was confirmed that the chicle polyisoprene was a mixture of cis-1, 4- and trans-1, 4-polyisoprenes.     

On the structure of polybutadiene: 4. 13C n.m.r. spectrum of polybutadienes with cis-1,4-, trans-1,4- and 1,2-units

¹³C n.m.r. spectra of polybutadienes with different contents of cis-1,4- and 1,2-units are assigned in the resonance region of the cis and trans carbon double bond. The observed signals are assigned to configurational triads.     

13C n.m.r. spectra of polymers made by ring-opening polymerization of (±)- and (+)-exo-5-methylbicyclo [2.2.1] hept-2-ene using metathesis catalysts

A range of olefin metathesis catalysts has been used to prepare ring-opened polymers of (±)- and (+)-exo-5-methylbicyclo [2.2.1] hept-2-ene, (l), having cis double bond contents of 11–100%. The ¹³C n.m.r. spectra of these polymers are interpreted in terms of TH, TT, HH, HT and tt, tc, ct, cc structures (T = tail, H = head, referring to methyl groups; t = trans, c = cis, referring to double bonds). The all-cis polymer has a fully-syndiotactic ring sequence, but polymers with less than 55% cis double bonds have an atactic ring sequence. The substitution shift parameters indicate that the cyclopentane rings in the polymer chain adopt the puckered conformation which minimizes non-bonded repulsion between the cis-1,3-olefinic substituents.     

13C N.m.r. Spectra of ring-opened polymers of 1-methylbicyclo[2.2.1]hept-2-ene and their hydrogenated products

The ¹³C n.m.r. spectra of the title polymers have been completely assigned for a comprehensive range of cis contents (σ_c = 0–1) and head-tail bias (HT/(HH + TT) = 1 −∞). In the unsaturated polymers (II), cis HH dyads are never found, so that all-cis polymers are also all-Ht, but cis TT dyads are formed in cis-trans polymers, equal to the difference between trans HH and trans TT. The olefinic region consists of seven triplets; two each from cis HT and trans HT, and one each from cis TT, trans TT and trans HH. In the case of trans each triplet consists of a main centre peak due to ttt and ctc sequences and two wing peaks due to ttc and ctt sequences, as defined in the text. The upfield peaks also show cc/ct or tt/tc splittings as well as HH/HT or TT/TH splittings in most cases. In the saturated polymers C⁶ is sensitive to m/r ring dyad sequences. A corrected assignment is given for the hydrogenated polymer of norbornene.     

Synthesis and Characterization of Cis- and Trans-Trimethyltriphenylcyclotrisiloxane

2,4,6-Trimethyl-2,4,6-triphenylcyclotrisiloxane (P₃) exists as cis and trans-stereoisomers. Herein, we present the detailed synthesis and characterization of the cis- and trans-P₃ isomers. After preparation of the mixed cyclic methylphenylsiloxanes, it was found that vacuum distillation was the best method for separating P₃ from the mixture of P₃ and 2,4,6,8-tetramethyl-2,4,6,8-tetraphenylcyclotetrasiloxane (P₄) on a large scale and that pure cis-P₃ can be isolated by washing the mixed P₃ isomers with methanol. It is also shown that a selective crystallization method was successful for isolating cis-P₃ from the mixture of P₃ and P₄ directly and that large crystals may be obtained. Using gas chromatography and nuclear magnetic resonance (NMR) spectroscopy, the two different isomers and four different isomers in P₃ and P₄, respectively, are easily identified. The nuclear magnetic resonance (NMR) spectra, differential scanning calorimeter (DSC) thermograms, and infrared (IR) spectra with various ratios of cis-P₃ and trans-P₃ were also analyzed. A melting point depression of the cis-P₃ was observed with an increased amount of trans-P₃ for the mixed isomers, as expected. These P₃ stereochemical isomers provide valuable starting materials for the preparation of stereoregular siloxanes using ring-opening polymerization.     

Cis and trans radicals generated in helical poly(propargyl acetate)s prepared using a [Rh(norbornadiene)Cl]2 catalyst

The poly(propargyl acetate) (A) having a helical cis-transoid structure was stereospecifically prepared using the Rh complex catalyst, [Rh(norbornadiene)Cl]₂, in MeOH or NEt₃ solvent at 0 and 40 °C in moderate yield. Electron spin resonance (ESR) analysis of the polymer revealed the formation of the cis (B) and trans (C) radicals which were produced through the thermal rotational scission of the helical cis CC bonds in the main-chain during the polymerization. The spatial and geometrical structure was successfully deduced using the two analogues’ polymers in which either methyl or methylene group is deuterated, by the aide of computer simulation of the observed ESR spectra together with the calculation of spin density of the two radicals.     

Direct evidence for chain orientation in poly(acetylene)

Two conflicting structural models (platelet and fibril) have been proposed in the literature for pristine cis-poly(acetylene). The respective models have separate, distinct implications for the chemical, physical and electrical properties of the material, in particular, the cis-trans isomerization, doping and conduction mechanisms. Luttinger-type cis-poly(acetylene) was prepared and studied in the transmission electron microscope (TEM) by low-dose conventional imaging, in dark-field mode and by electron diffraction. Thin ‘films’ of the material were shown to consist of fibrillar nets, having fibrils of diameter 5–25 nm. Combined electron diffraction and dark-field observations in the TEM indicated the structural arrangement within the fibrils as that having polymer molecular chains aligned parallel to fibril axes.     

Analysis of deuterium NMR spectra of probe chains diffusing in a stretched polybutadiene network

Polybutadiene network was swollen with short perdeuterated network-like chains (PBD). Deuterium NMR spectra of these probe chains are measured as the rubber matrix is uniaxially elongated. Due to the presence of two kinds of deuterons (methylene and ethylene) and two possible configurations of monomer units (cis and trans) in PBD chains, composite NMR spectra are observed and analysed. They evidence for different orientational behaviour of cis and trans units. ‘Rod-like’ motional behaviour of the trans units leads to higher degree of orientational order than for the cis units.     

Icosahedron cage occupancy of structure-H hydrate helped by Xe -1,1-, cis -1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes

Four mixtures of 1,1-, cis-1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H₂O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis-1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans-1,2-DMCH nor cis-1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis-1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis-1,4-DMCH and methane generates the structure-H hydrate.     

The infra-red spectrum and molecular structure of poly(cyclohexane 1,4-dimethylene terephthalate)

Films of poly(cyclohexane 1,4-dimethylene terephthalate) (pcht) that had been drawn and crystallized in various ways were examined by normal and polarized infra-red radiation in the range 5000 cm⁻¹ to 20 cm⁻¹. The transpcht isomer is centro symmetric in the crystalline phase. The space group of the monomeric unit is P, 1 isomorphous with C_i symmetry. The cis-polymer is not centro symmetric, the space group is probably P1 isomorphous with C₁ symmetry. A number of model compounds were also studied. The terephthalate framework is expected to behave similarly to that in polyethylene terephthalate, the terephthalate conformation being more affected by changes in molecular orientation and crystallization than by those in the conformation of the diol fragment. In the same way the terephthalate framework is expected to be centro-symmetric and planar (symmetry V_h  D_2h) only when highly crystallized and oriented. In amorphous regions, out of plane bending of CO groups causes lowering to C_2v symmetry. The spectral changes in cis and transpcht, as would be expected from the above, are very similar to those of pet. There is little difference between cis and trans isomers in the same state, apart from a strong band at 630 cm⁻¹ in both Raman and infrared spectra of the cis isomer, that is practically absent in the trans. On change of state, however, trans substituted isomers do show significant intensity changes, in contrast to cis isomers.     

Synthesis and Characterization of Cis- and Trans-Trimethyltriphenylcyclotrisiloxane

2,4,6-Trimethyl-2,4,6-triphenylcyclotrisiloxane (P₃) exists as cis and trans-stereoisomers. Herein, we present the detailed synthesis and characterization of the cis- and trans-P₃ isomers. After preparation of the mixed cyclic methylphenylsiloxanes, it was found that vacuum distillation was the best method for separating P₃ from the mixture of P₃ and 2,4,6,8-tetramethyl-2,4,6,8-tetraphenylcyclotetrasiloxane (P₄) on a large scale and that pure cis-P₃ can be isolated by washing the mixed P₃ isomers with methanol. It is also shown that a selective crystallization method was successful for isolating cis-P₃ from the mixture of P₃ and P₄ directly and that large crystals may be obtained. Using gas chromatography and nuclear magnetic resonance (NMR) spectroscopy, the two different isomers and four different isomers in P₃ and P₄, respectively, are easily identified. The nuclear magnetic resonance (NMR) spectra, differential scanning calorimeter (DSC) thermograms, and infrared (IR) spectra with various ratios of cis-P₃ and trans-P₃ were also analyzed. A melting point depression of the cis-P₃ was observed with an increased amount of trans-P₃ for the mixed isomers, as expected. These P₃ stereochemical isomers provide valuable starting materials for the preparation of stereoregular siloxanes using ring-opening polymerization.