Diacetylenes and mesophases

During the past five years, over 600 papers have been published on research work carried out with diacetylenes. Far fewer papers can be found on diacetylene mesophase studies, and these are reviewed in this paper. Some new findings by the authors are also presented.     

Two-dimensional self-assembly of linear molecular rods at the liquid/solid interface

We report on the synthesis and scanning tunneling microscopy (STM) studies of a series of linear molecular rods (1-5) comprising different numbers and/or spatial arrangements of perfluorinated benzene and benzene subunits interlinked with diacetylenes in the para position and decorated with or without terminal dodecyl chains. The molecules organize themselves into well-ordered 2D crystal structures at the liquid/solid interface through intermolecular and molecule-substrate interactions. Whereas the molecules substituted by dodecyl chains form the lamellar structures with alternating rigid core rows and alkyl chain rows, the unsubstituted ones change the orientation of the rigid backbones with respect to the lamellar axis. The molecular arrangement is not influenced by fluoro substituents on any phenyl ring of the backbone, which suggests that the interactions between the π-conjugated backbones are dominated by close packing rather than by the dipole moments of the rods or fluorine-based intermolecular interactions.     

Thermal and optical studies on hexa-2,4-diyne-1,6-diyl-bis(4-n-hexoxybenzoate)

As part of a programme investigating mesogenic diacetylenes a symmetrically disubstituted diacetylene has been synthesized and polymerized. Thermal, thermo-optical and IR studies have been carried out to investigate the different crystalline forms of the diacetylene.     

Thermotropic diacetylenic liquid crystals

Further studies are reported on polar diacetylenes with conjugated side chains which show liquid-crystalline behaviour. Results obtained using differential scanning calorimetry, thermogravimetry, and hot stage microscopy are reported. One group of materials forms a smectic liquid crystal phase and all polymerize from a nematic liquid crystal phase.     

Theoretical investigation of two-photon absorption allowed excited states in symmetrically substituted diacetylenes by ab initio molecular-orbital method

Symmetrically substituted diacetylene compounds, which shows large two-photon absorption (TPA) cross sections, have been theoretically investigated by the ab initio molecular-orbital method employing several theoretical models including the configuration interaction with single excitation (CIS), random phase approximation (RPA), and time-dependent density-functional theory (TDDFT) methods. The calculated excited energies are overestimated by CIS or RPA, whereas underestimated by TDDFT with the B3LYP parametrization for both one-photon absorption (OPA) and TPA allowed states. The lowest OPA state is well described by the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transition. On the other hand, lower TPA allowed states can be represented as the superposition of the HOMO-LUMO+1 and HOMO-1-LUMO transitions, giving rise to two TPA allowed states. The absorption intensity for the lower TPA state of the diacetylenes molecules is discussed in terms of the alternancy symmetry and its breaking. The symmetry property is differently manifested for neutral and dicationic diacetylenes. Introduction of charges breaks the alternancy symmetry, which gives rise to an increase in the TPA cross sections at the lower frequency. The upper TPA state is calculated to show huge TPA cross sections, which reproduces the enhancement of the TPA cross section experimentally observed for one of the diacetylenes at the higher-frequency region. The enhancement is discussed employing an index defined as the ratio of the transition polarizability and its static limit, which represents the degree of influence of one-photon resonance on the TPA intensity. The huge TPA cross sections are found to be due to a near-resonance effect. The present theoretical calculation approves the previously proposed assumption based on the four-state (dual three-state) model, which consists of the ground, one OPA allowed, and two TPA allowed states.     

Synthesis and cytotoxic activity of a series of diacetylenic compounds related to falcarindiol

The synthesis of a series of diacetylenic compounds related to the natural product falcarindiol has been carried out. Unsymmetrical diacetylenes were prepared by a modification of the Cadiot-Chodkiewicz coupling reaction, while a Glaser coupling was used to prepare symmetrical diacetylenes. These compounds have been tested for in vitro cytotoxic activity against Hep-G2, and H-4-II-E cell lines. Diacetylenes with additional unsaturation at C-1, 2, appended with hydroxyl groups at C-3 and C-8, or with long hydrophobic chains, exhibited IC50 values in the micromolar range.     

Dielectric and pyroelectric properties of disubstituted diacetylenes

The dielectric and pyroelectric properties of monomer and polymer single crystals of substituted diacetylenes carrying polar side groups are presented for four examples: NP/4-MPU, TS/FBS, DNP and IPUDO. Large and anisotropic permittivities are generally observed. The symmetrically disubstituted diacetylenes IPUDO and DNP show spontaneous electric polarization despite of their symmetric molecular unit in polymer and/or monomer crystals at room or low temperature, respectively. Polar crystals of the unsymmetrically substituted diacetylenes NP/4-MPU and TS/FBS can be grown from appropriate solvents. However, polymorphism is an obstacle to a systematic tailoring of dielectric properties of diacetylene single crystals.     

Synthese, 13C-NMR-Spektren und räumliche Struktur von ‘Push-Pull’-Diacetylenen

Synthesis, ¹³C-NMR Spectra, and X-Ray Investigation of ‘Push-Pull’ Diacetylenes Phenyl-substituted ‘push-pull’ diacetylenes 1f and 1g have been prepared by acetylation and benzoylation of the appropriate lithiodiynylamines 4 (Scheme 2). ¹³C-NMR spectra of diacetylenes 1a–g (Table 1) are discussed with respect to the expected polarisation of the diacetylene unit by ‘push’ and ‘pull’ substituents. X-Ray investigations of 1c , 1e , and 1f have been performed in view of the planned solid-state polymerisation of ‘push-pull’ diacetylenes. In the crystalline state, diacetylenes 1c and 1f are stacked, however, the stacking parameters do not allow a solid-state polymerisation.     

Versuche zur Synthese von ‘Push-Pull’-Diacetylenen

Attempted Synthesis of Push-Pull Diacetylenes Two alternative synthesis of push-pull diacetylenes of type 1 (5-amino-2,4-alkadiynals) are investigated. A bromination-dehydrobromination sequence starting with 5-dimethylamino-2,4-pentadienal ( 2 ) as well as the application of the well-known Cadiot-Chodkiewicz coupling reaction give new intermediates 3–5 , and 7 and 8 , respectively, but fail to give the target molecules 1 .     

Reactivity and flammability of conjugated acetylenes

The thermal reactivities of model phenyleneacetylenes and diacetylenes and of poly(butadiynylene-1,3-phenylene) have been examined by differential scanning calorimetry and compared with the oxygen indices of the materials. No direct relationship was found between the heat of reaction and the oxygen index in these compounds. However, the heat of reaction, in general, does decrease with increasing molecular weight and is lower for rigid linear molecules than for related angular structures. These observations appear to be due to a lack of solid-state reactivity in the crystalline phase and retardation in the melt due to viscosity effects related to molecular size and rigidity. The synthesis of model compounds is described.     

Effects of aromatic diacetylenes on polyurethane degradation by gamma irradiation

The effects of four aromatic diacetylenes on the gamma-irradiation-induced degradation of a commercial polyurethane were studied. Addition of 1 wt% of diphenylbutadiyne, which is homogeneously distributed in the polymer, effectively suppressed the polymer chain degradation. The dose required to decrease tensile strength by 50% was found to be 582 kGy for the polyurethane with 1 wt% of diphenylbutadiyne, while a dose of only 310 kGy for the polyurethane itself. The Young's modulus of the polyurethane alone decreased with dose; meanwhile the films with diphenylbutadiyne did not change. The films with p,p′-dinitrodiphenylbutadiyne behaved differently from others due to their inhomogeneous composition. The amide substituted diacetylenes also showed protecting effect, but in less extent due to the steric effect.     

Changes in Electron Structure of the Triple Bond in Substituted Acetylene and Diacetylene Derivatives

The substituent effect is usually considered by means of various Hammett-like substituent constants and is most often related to aromatic systems. Unlike this, we present results of our research on the influence of 27 substituents spanning a wide range of electronic properties, from strongly electron-withdrawing to strongly electron-donating, on the electron structure of X-substituted acetylenes and diacetylenes – thus the systems which until now have practically not been subject of any deeper studies. It is shown that the interaction through triple bond(s) is associated with a significant advantage of resonance effects and that the substituent effect transmitted by the C≡C−C≡C unit is about half of that transmitted by the C≡C unit alone. Substituent X mainly affects the closest carbon atom by means of proximity effect, hence changes of charge on this atom do not follow any substituent constants. The effect on further carbon atoms is much smaller. The presence of the C≡C−C≡C unit withdraws more charge from X than a triple bond alone, and hinders communication between X and the terminal H atom. Comparison of substituent effects to those present in X-substituted benzene derivatives shows that the electronic properties of the terminal hydrogen atom in acetylenes and diacetylenes are most similar to the electronic properties of ortho and para hydrogen atoms in X-substituted benzene derivatives.     

Synthese von ‘Push-Pull’-Diacetylenen

Synthesis of ‘Push-Pull’ Diacetylenes The first synthesis of push-pull diacetylenes of type 1 is described. Reaction of perchlorobutenyne ( 8 ) with two equivalents of dialkylamine, followed by dechlorination using two equivalents of butyllithium gives lithio-dialkylamino-diynes 7 . Final acylation of these intermediates leads to push-pull diacetylenes 1b–1e in good yields. The method allows the introduction of both push and pull substituents in a simple one-pot-procedure. In addition, 1a is prepared by hydroxymethylation of lithio-morpholino-diyne 7c , followed by oxidation with manganese dioxide in acetone.     

Practical,high-yield synthesis of thiol-terminated diacetylenes for formation of conductive monolayers

Self-assembled monolayers of thiol terminated conjugated diacetylenes can be cross-linked using ultraviolet light to form highly conjugated polydiacetylenic conductive monolayers [1]; however, the reported syntheses of the diacetylene monomers present numerous problems that prevent the wide spread application of these in functional materials. We report a redesigned four-step synthesis that proceeds in 75–80% overall yields and allows gram scale production of an array of thiol terminated conjugated diacetylenes, thereby allowing examination and application of these low-dimensional conductive materials.     

Liquid crystal properties of novel diacetylenes having a pyridine end group

Asymmetrical diacetylenes with a pyridine ring bonded directly to one end of the diacetylene unit and an aryl system containing a flexible alkoxy chain at the opposite end, have been synthesized and show liquid crystalline behaviour. The mesophase behaviour is shown to be dependent on the length of the flexible alkoxy chain. Incorporation of one of these diacetylenes in a polymer 'guest-host' system at a 15 wt% concentration gave a high X⁽²⁾ non-linear optical susceptibility of 4.27 × 10^-9e.s.u.     

Synthesis of oligoenynes and oligomeric conjugated diacetylenes

Novel oligoenynes and oligomeric conjugated diacetylenes have been synthesized via a new Pd/Cu-catalyzed reaction scheme. This Sonogashira-style preparation of oligoenynes has complementary properties to previously published procedures via ionic intermediates.     

Synthesis of phenols from metal-carbynes and diynes

Metal-carbynes RCM(CO)₄Br with diacetylenes give specific phenols in good yield. The reactions take only a few minutes at or below room temperature.     

Synthesis of substituted pyrroles in the glaser reaction

The substituted pyrroles and dipyrroles along with diacetylenes and cumulenes have been synthesized in high yields using a new synthetic method under mild reaction conditions using the Glaser coupling reaction. Although diacetylenes are formed from 2‐propargyl‐1,3‐dicarbonyl compounds having electron‐donors substituents such as Ph or OEt, only polyfunctional substituted cumulenes are formed from those compounds under the modified conditions. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:66–73, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20184     

Polydiacetylene–inorganic clay nanocomposites

Two new diacetylenes, 14-amino-10,12-tetradiynoic acid (a diacetylenic aminoacid) and 10,12-docosadiyndiamine (diacetylenic diamine), were successfully synthesized via Eglinton and Cadiot–Chodkiewicz coupling. These two diacetylenes and 10,12-pentacosadiynyl amine (monoamine) were intercalated in mica-type layered silicates, montmorillonite and vermiculite, via a cationic exchange reaction. The intercalation process was confirmed by X-ray diffraction and Fourier transform infrared spectroscopy. The interlayer spacing and polymerizability of the intercalated diacetylene were found to depend upon the length of the diacetylene molecule, the layer charge density of the clay and the solvent treatment. The X-ray diffraction pattern of vermiculite–diacetylene showed evidence of the in-plane packing of the diacetylene. The polymerization of the intercalated diacetylene was confirmed by Raman spectra. © 1998 John Wiley & Sons, Ltd.