Hexyl Chains Research Articles

Ionic Liquid Based Aqueous Biphasic Systems with Controlled pH: The Ionic Liquid Cation Effect

This work addresses the evaluation of the ability of ionic liquid cations on the formation of aqueous biphasic systems (ABSʼs), with K2HPO4 or a mixture of inorganic salts, K2HPO4/KH2PO4, aiming at controlling the pH values of the coexisting aqueous phases. Using chloride-based ionic liquids, the effects of the cation core, the length of the alkyl side chain, and the positional isomerism on ABS formation ability were investigated. All binodal curves were determined by the cloud-point titration method at 298 K. From the obtained phase diagrams it is shown that the biphasic area increases with the cation side chain length, from ethyl to hexyl chains, although for longer chains, an inversion on the binodal curves sequence appears due to the self-aggregation of the longer chain ionic liquids in aqueous solutions. The influence of the cation core and the positional isomerism of the ionic liquids on their ability to form ABS closely correlates with the ionic fluid affinity for water.

Mechanistic Considerations of Bending‐Strain Effects within Organic Semiconductors on Polymer Dielectrics

AbstractThe development of organic transistors for flexible electronics requires the understanding of device behavior upon the application of strain. Here, a comprehensive study of the effect of polymer‐dielectric and semiconductor chemical structure on the device performance under applied strain is reported. The systematic change of the polymer dielectric results in the modulation of the effects of strain on the mobility of organic field‐effect transistor devices. A general method is demonstrated to lower the effects of strain in devices by covalent substitution of the dielectric surface. Additionally, the introduction of a hexyl chain at the peripheries of the organic semiconductor structure results in an inversion of the effects of strain on device mobility. This novel behavior may be explained by the capacitative coupling of the surface energy variations during applied strain.

Evidence for Fully Conjugated Double-Stranded Cycles

A chain of circumstantial evidence for the existence of the first fully conjugated, double-stranded cycles is presented. The products have the structure of the belt-region of fullerene C(84)(D(2)) and carry either four hexyl chains or four phenyl groups. The unsubstituted parent cycle is also presented. The chain of evidence is mainly based on mass spectrometric analysis and trapping reactions, the latter being supported by quantum mechanical calculations. It is also of importance that the phenyl-substituted and unsubstituted products cannot undergo a [1,5] hydrogen shift, the only reasonable side-reaction that recently could not be excluded for the alkyl-substituted analogue. It is concluded that the fully aromatic targets truly exist in the gas phase. Whether they can be generated in solution under the applied conditions cannot yet be firmly decided; theoretical evidence speaks against.

Effects on Membrane Lateral Pressure Suggest Permeation Mechanisms for Bacterial Quorum Signaling Molecules

Quorum sensing is an intricate example of "social" behavior in microbial communities mediated by small secreted molecules (autoinducers). The mechanisms of membrane permeation remain elusive for many of them. Here we present the assessment of membrane permeability for three natural autoinducers and four synthetic analogues based on their polarity, surface activity, affinity for lipid monolayers, and ability to induce lateral pressure changes in the inner E. coli membrane sensed by the bacterial tension-activated channel MscS. AI-1 (N-(3-oxodecanoyl)-l-homoserine lactone) is surface-active, and it robustly inserts into lipid monolayers, indicating strong propensity toward membranes. When presented to membrane patches from the cytoplasmic side, AI-1 transiently shifts MscS's activation curve toward higher tensions due to intercalation into the cytoplasmic leaflet followed by redistribution to the opposite side. Indole showed no detectable surface activity at the air-water interface but produced a moderate increase of lateral pressure in monolayers and was potent at shifting activation curves of MscS, demonstrating transients on sequential additions. AI-2 (4,5-dihydroxy-2,3-pentanedione, DPD) showed little activity at the interfaces, correspondingly with no effect on MscS activation. After chemical modification with isobutyl, hexyl, or heptyl chains, AI-2 displayed strong surface activity. Hexyl and especially heptyl AI-2 induced robust transient shifts of MscS activation curves. The data strongly suggest that both AI-1 and indole are directly permeable through the membrane. AI-2, more hydrophilic, shows low affinity toward lipids and thus requires a transport system, whereas alkyl analogues of AI-2 should permeate the membrane directly.

Alternating Copolymers Based on 2,1,3‐Benzothiadiazole and Hexylthiophene: Positioning Effect of Hexyl Chains on the Photophysical and Electrochemical Properties

AbstractA series of donor–acceptor alternating π‐conjugated copolymers based on 2,1,3‐benzothiadiazole and hexylthiophene units has been synthesized by the palladium‐catalyzed Stille cross‐coupling method. Various precursory monomers possessing dibromo and bis(tributylstannyl) functionalities were readily prepared in high yields. Microwave‐assisted polymerization proved efficient for the production of high‐molecular‐weight copolymers, ranging from 13550 to 52490 g mol–1. All copolymers exhibited excellent solubilities in most common organic solvents. The thermal properties of these copolymers were investigated by thermogravimetric analysis and differential scanning calorimetry, and the polymers showed high thermal stabilities. Incorporation of the benzothiadiazole unit into polyhexylthiophene chains affected the photophysical and electrochemical properties. The thin‐film absorption spectra of all polymers are significantly red‐shifted relative to the corresponding absorption bands in solution and exhibit broader absorption bands. The optical band gaps were estimated to be in the range of 2.02–1.74 eV. The highest occupied and lowest unoccupied molecular orbital energy levels are in the ranges of –5.37 to –5.66 eV and –3.33 to –3.44 eV, respectively. In the X‐ray diffraction analysis of the deposited film of the copolymer P3, strong diffraction peaks were observed at 2θ = 5.72° (15.43 Å) and 23.12° (3.84 Å). These values relate to the distances between chains with interdigitated hexyl chains and to π–π stacking between the conjugated chains.

Synthesis and characterization of block copolythiophene with hexyl and triethylene glycol side chains

A well-defined diblock copolythiophene, poly(3-hexylthiophene)- b-poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene) (P3HT- b-P3TEGT) was successfully synthesized by a Grignard metathesis (GRIM) polymerization. The structure of the block copolymer was confirmed by size exclusion chromatography (SEC), 1H NMR spectroscopy, differential scanning calorimetry (DSC), ultraviolet-visible (UV-vis) spectroscopy and fluorescence spectroscopy. The self-assembled structure of the block copolythiophene was investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and synchrotron grazing incidence X-ray scattering (GIXS). These analyses found that the block components in films form vertically oriented lamellar structure via phase separation. Furthermore, both the phases were found to consist of molecular multi-layers respectively, in which the layers stack in the out-of-plane of the film. The P3HT phase exhibited crystalline, which is originated from the π−π stacked thiophene backbones. In contrast, the poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene) (P3TEGT) phase revealed amorphous. Overall, the amphiphilic nature of P3HT- b-P3TEGT successfully demonstrated to lead to well-defined self-assembly structure in films.

Synthesis, Characterization, and Field Effect Transistor Properties of Regioregular Poly(3-alkyl-2,5-selenylenevinylene)

Here we report the synthesis of novel poly(3-alkyl-2,5-selenylenevinylene)s (PSV) by the polymerization of 2,5-dibromo-3-alkylselenophene and (E)-1,2-bis(tributylstannyl)ethylene under microwave-assisted Stille coupling conditions. Polymers containing hexyl and decyl side chains were prepared. The decyl-substituted polymer exhibited good solubility in common organic solvents, and 1H NMR analysis indicated a side-chain regioregularity in excess of 90%. The polymer exhibits an absorption maxima at 621 nm in solution, with a significant red shift in absorption upon film formation. Thin film transistors fabricated from PSV exhibited hole carrier mobilities up to 0.01 cm2/(V s). The polymer displayed balanced ambipolar performance when hydroxyl free organic dielectrics were used as the gate insulator.

Carbon-13 NMR Chemical Shifts in Columnar Liquid Crystals

In this article, we present quantum chemical density functional theory (DFT) calculations of the NMR (13)C chemical shift (CS) tensors in 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT). The DFT calculations are performed on a smaller model molecule where the hexyl chains were reduced to methyl groups (HMTT). These tensors are compared with our previously reported experimental results carried out under magic-angle spinning (MAS) conditions. The phase diagram of HHTT is K ↔ H ↔ D(hd) ↔ I, where H is a helical phase and D(hd) is a columnar liquid crystal. The motivation for the present study was to explain experimentally observed and puzzling thermal history effects, which resulted in different behavior in the helical phase upon cooling and heating. In particular, the CS tensors for the aromatic carbons measured in the helical phase upon heating from the solid phase were essentially unaffected, while the cooling from the columnar liquid crystal resulted in a significant averaging. We investigate the effect on the CS tensors of (i) conformational transitions, and (ii) relative molecular orientations within the columns for dimer and trimer configurations. Finally a motional wobbling (PIZZA) model for the dynamic averaging of the CS tensor in the helical phase is suggested.

N,N-Dihexyl-4-[2-(4-nitro-phen-yl)vin-yl]aniline.

The title compound, C26H36N2O2, was prepared by Horner olefination of p-dihexyl­amino­benzaldehyde and diethyl p-nitro­benzyl­phospho­nate. It crystallizes with two independent mol­ecules in the asymmetric unit. Both have similar geometries of the π-systems but the conformations of all hexyl chains are different. Whereas one hexyl chain of the first mol­ecule shows the typical all-anti conformation, the second is arranged in a gauche-anti-gauche-anti conformation with N—C—C—C, C—C—C—C, C—C—C—C and C—C—C—C torsion angles of −65.1 (4), 167.3 (3), 63.3 (4), and 179.4 (3)°. One of the hexyl chains in the other mol­ecule has an anti-anti-gauche-anti conformation [N—C—C—C, C—C—C—C, C—C—C—C and C—C—C—C torsion angles = 179.6 (3), −179.8 (3), −68.7 (5) and −178.8 (4)°], the other starts with an anti-gauche-gauche sequence. Molecules A and B are composed of five planar subunits. The angle sums around the N atoms are in the range 356 (2)–360.0 (2)°. Torsion angles between these segments do not exceed 4.9 (4)°, except for one of the alkyl chains each [molecule A = 26.2 (4)°; molecule B = −6.0 (4)°]. The high planarity of the molecules and the short aniline C—N bonds [1.385 (3) Å in molecule A and 1.378 (3) Å in molecule B] indicate a strong electronic coupling through the stilbene unit. One methylene group is disordered over two positions with an occupancy ratio of 0.72:0.28.

Ionic Liquid-Induced Changes in Properties of Aqueous Cetyltrimethylammonium Bromide: A Comparative Study of Two Protic Ionic Liquids with Different Anions

In this work we have shown a comparative study of changes in physicochemical properties of an aqueous solution of a common cationic surfactant cetyltrimethylammonium bromide (CTAB) upon addition of two protic ionic liquids N,N-dimethylethanolammonium [corrected] hexanoate (DAH) and N,N-dimethylethanolammonium [corrected] formate (DAF). The aim of this manuscript is to offer a comparative study and establish the role of the alkyl chain length of the anion of the added protic ionic liquids on the physicochemical properties of aqueous solution of CTAB. At lower concentration (i.e., ≤ 30 mM) both ionic liquids show the same trend in modifying the properties of aqueous CTAB solution, but DAH as an additive shows a more dramatic increase in aggregation number and size of the CTAB micelle compared to that of DAF as an additive. At higher concentrations of additives (DAF and DAH), the properties of aqueous CTAB solution change in an entirely different way. The size of the CTAB micelle was found to be 0.9 nm. With the addition of 215 mM DAH, the size of the CTAB micelle increases to 25.0 nm, whereas with the addition of 215 mM DAF it increases to only 5.6 nm. Zeta potential, electrical conductance, microviscosity, and dipolarity measurements were performed to gain insight into this abrupt size change in the case of DAH. It is proposed that the formate and hexanoate anions undergo Coulombic attractive interaction with cationic head groups of the CTAB micelle at all concentrations. In the case of DAH, the presence of a hexyl chain on the hexanoate ion allows it to align with the tail part of CTAB, whereas in the case of DAF the absence of an alkyl chain in the formate ion is apparently unable to align the formate anion with the tail part of CTAB. So this difference in the location of the anions of DAF and DAH is responsible for the different size changes and different behaviors of the two ionic liquids.

Insights into the stability and thermal degradation of P3HT:C60 blended films for solar cell applications

This paper demonstrates the changes in the nanoscale morphology of the blended films induced by a diffusion of C60 molecules and degradation during longer thermal treatment above the glass transition temperature (130 °C). The results showed that the film morphology, including the size and population of poly(3-hexylthiophene) (P3HT) crystallites, rapidly reduced with annealing time. A large-scale (>1μm) C60 aggregation, demonstrating a bulky phase separation between the polymer and C60, was identified after 5 h annealing, which resulted in a degradation of charge carrier mobility and conductivity. X-ray diffraction verifies that the interchain packing of P3HT within the crystallized phase improved with an increasing in annealing time, but the volume fraction of the P3HT (100) phase normal to substrate increased up to 3 h and decreased at longer annealing times resulting in the ageing of the films. Changes in the infrared spectra of the extended annealed samples were recorded and the oxidation products were identified. A degradation mechanism that accounted for the modifications in the infrared spectra and a detachment of the hexyl chain from P3HT was demonstrated, resulting in chain cutting, conjugation loss and a reduction in the UV–vis absorbance. The morphology change with the annealing time resulted in an abrupt decrease in the PCE of P3HT:C60 solar cells. These findings signify that the stability of P3HT:C60 solar cells cannot be secured for longer annealing period owing to the unsettled morphology.

Hydrophobic 1-allyl-3-alkylimidazolium dicyanamide ionic liquids with low densities

Nine 1-allyl-3-alkylimidazolium ([ACnIm]) or 1-vinyl-3-alkylimidazolium ([VCnIm]) dicyanamides (DCA) were prepared and characterized, and their physicochemical properties were studied in detail. Except for [AVIm]DCA and [AC4Im]DCA, the other dicyanamides with hexyl or longer alkyl chains on the cation exhibited the characteristic of hydrophobicity. Among them, [AC8Im]DCA and [AC10Im]DCA were found to possess similar densities to water, i.e. 1.007 g cm−3 and 0.988 g cm−3 at 25 °C, respectively. Interestingly, a reversible phase reversion of a [AC8Im]DCA/H2O mixture was observed as the temperature varied. These environmentally-friendly liquid materials have potential applications as precursors for syntheses of conducting polymeric materials.

U‐Shaped Conformation of Alkyl Chains Bound to a Synthetic Receptor Cucurbit[8]uril

The behavior of a series of alkanes bound to the molecular host cucurbit[8]uril (CB[8]) has been systematically studied by 2D (1)H NMR spectroscopy and isothermal titration calorimetry (ITC). CB[8] and alkyltrimethylammonium (C(m) TA(+), (CH(3))(3)N(+)C(m)H(2m+1), m=6-16) form 1:1 host-guest complexes with a high binding constant (K≈10(6) m(-1)). The shortest hexyl chain of C(6)TA(+) can be fully encapsulated in an extended conformation inside the CB[8] cavity, which is driven by both enthalpy and entropy. However, for the longer aliphatic chains, C(8)-C(16), the long alkyl tails take a U-shaped conformation inside the cavity, and their complexation is dominantly or almost exclusively enthalpy-driven, owing to the increased van der Waals contact between the folded aliphatic chain and the inner wall of the host cavity. As the chain length increases from C(8) to C(16), the ammonium head group of the guests moves away from the portal of CB[8] while the long aliphatic tails maintain the U-shaped conformation inside the cavity. The complexation of C(m)TA(+) with CB[8] follows the enthalpy-entropy compensation rule commonly observed in molecular recognition systems. For example, among the guest molecules, C(12)TA(+) shows the highest enthalpic gain (most favorable), owing to the large van der Waals contact between the guest and the host cavity, and at the same time the most unfavorable entropic contribution, owing to the severe conformational restriction of the U-shaped alkyl chain inside the host. The enthalpy-entropy compensation plot for the complexation suggests large conformational changes of the long alkyl chains and extensive dehydration associated with the inclusion complex formation.

3-(6-Bromohexyl)-1,5-dimethyl-1H-1,5-benzodiazepine-2,4(3H,5H)-dione

The seven-membered ring in the title compound, C17H23BrN2O2, adopts a boat-shaped conformation (with the C atoms of the fused-ring as the stern and the methine C atom as the prow). The bromo­hexyl substituent occupies an equatorial position, with the hexyl chain exhibiting an extended conformation. Weak inter­molecular C—H⋯O hydrogen bonding is present in the crystal structure.

Structure of the Room-Temperature Ionic Liquid 1-Hexyl-3-methylimidazolium Hydrogen Sulfate: Conformational Isomerism

The acidic room-temperature ionic liquid 1-hexyl-3-methylimidazolium hydrogen sulfate has recently been identified to have beneficial properties for practical applications in catalysis and electrochemistry. In the present work, the conformational isomerism of this ionic liquid is studied by means of density functional theory calculations and experiments in terms of infrared absorption and Raman scattering spectroscopy. For the hydrogen sulfate anion, the trans conformer is found to be the favored isomer in the ionic liquid. For the 1-hexyl-3-methylimidazolium cation, three different low-energy conformations were obtained, differing only in the orientation of the hexyl chain. The comparison of vibrational frequencies with IR and Raman data showed good agreement for all three conformations, indicating their presence in the ionic liquid. Beyond revealing the conformational information, the experimental spectra indicate strong interionic interactions. Vibrations of sulfuric acid could be observed, indicating possible proton transfer from the cation to the anion. This is further supported by the appearance of modes around 2000 cm(-1) in the IR spectrum, which could tentatively be assigned to C2-H stretching vibrations red-shifted as a result of strong interionic hydrogen bonds as a prerequisite of proton transfer.

Low Band Gap Polymers Based on Benzo[1,2-b:4,5-b′]dithiophene: Rational Design of Polymers Leads to High Photovoltaic Performance

This study centers upon the summarized four design criteria for conjugated polymer photovoltaic materials: high oxidation potential, low band gap, a planar symmetrical structure, and high molecular weight. Two polymer materials were synthesized to fulfill these design criteria, both based upon copolymers of benzo[1,2-b:4,5-b′]dithiophene (BnDT) and 4,7-di(2-thienyl)-2,1,3-benzothiadiazole (DTBT). Additional hexyl chains were added to one polymer on the 4 position of the DTBT unit, which increased the molecular weight of the polymer without detracting from its electrochemical or optical properties. Both polymers perform exceptionally well in preliminary bulk heterojunction solar cells, reaching power conversion efficiencies greater than 4%.

Abstract 3785: Isothiocyanates vs. isoselenocyanates: Reactivity toward glutathione and cysteinyl residues in cell and cell-free systems

Abstract Isothiocyanates (ITCs) are naturally occurring compounds found in cruciferous vegetables. Both naturally occurring phenylalkyl ITCs (BITC, PEITC) and longer chain synthetic ITCs (PBITC, PHITC) have been studied extensively and are shown to inhibit carcinogenesis due to inhibition of CYP450 enzymes, as well as induction of Phase II detoxification enzymes. These actions are thought to be mediated by the reactivity of the ITC functional group (N=C=S) with sulfhydryl groups of cell thiols, mainly glutathione and proteins (cysteinyl residues). Our lab has modified both the naturally occurring and synthetic phenyl alkyl ITCs by isosterically replacing sulfur with selenium to make isoselenocyanates (ISCs): ISC1, ISC2, ISC4, and ISC6. We have found these compounds to be more potent in cancer cell viability assays and in xenograft mouse model at inhibiting tumor growth than corresponding ITCs. Our hypothesis is the better potency of ISC compounds may be due to their modified reactivity rates toward cell thiols. To test this, A549 lung cancer cells were treated with our panel of ITC compounds or corresponding ISC compounds from 500 nM to 15 µM for 1 hour. GSH depletion appeared to be dose dependent for all compounds except for hexyl chain compounds PHITC and ISC6. For the remaining compounds, ISC compounds depleted GSH in cells to a greater extent than corresponding ITCs. 15 µM of ISC1, ISC2, and ISC4 depleted GSH levels to 58%, 53%, and 63% of control, respectively, whereas BITC, PEITC, and PBITC only depleted GSH to 69%, 86%, and 86%, respectively. These results may be related to redox cycling capabilities which measures the reactivity of compounds with GSH and O2 to form superoxide. In a chemiluminesence assay measuring redox cycling, ISC compounds showed a much greater ability to redox cycle compared to corresponding ITCs. The redox cycling capabilities were as follows: ISC2≫PEITC>ISC1=ISC4=ISC6>PHITC>PBITC>BITC. In a cell free system, overall binding of ITC and ISC compounds were analyzed for GSH using the Ellman assay. In this cell free system both ITCs and ISCs showed a trend of increasing binding with decreasing chain length: BITC>PEITC>PBITC>PHITC and ISC1>ISC2>ISC4>ISC6. However, corresponding ITC and ISC compounds bound similarly to GSH. Similar experiment using tubulin protein exhibited the same trend of reactivity with ITCs, as with GSH. However, among ISC compounds, ISC2 showed the greatest binding followed by ISC1=ISC4>ISC6. These results suggest that cell environment is playing a role in the reactivity of these compounds with glutathione and proteins. In conclusion, ISC compounds depleted GSH in cells to a greater extent than corresponding ITC compounds and were also better able to redox cycle. Future studies will analyze the exact rate of reactivity of ISC compounds toward GSH, identifying protein targets of these compounds, and binding preferences of different proteins. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3785.

Synthesis of 9,9‐Dialkyl‐4,5‐diazafluorene Derivatives and Their Structure–Activity Relationships Toward Human Carcinoma Cell Lines

A homologous set of 9,9-dialkyl-4,5-diazafluorene compounds were prepared by alkylation of 4,5-diazafluorene with the appropriate alkyl bromide and under basic conditions. The structures of these simple organic compounds were confirmed by spectroscopic techniques (FTIR, NMR, and FABMS). Their biological effects toward a panel of human carcinoma cells, including Hep3B hepatocellular carcinoma, MDAMB-231 breast carcinoma, and SKHep-1 hepatoma cells, were investigated; a structure-activity correlation was established with respect to the length of the alkyl chain and the fluorene ring structure. The relationship between the mean potency [log(1/IC(50))] and alkyl chain length was systematically studied. The results show that compounds with butyl, hexyl, and octyl chains exhibit good growth inhibitory effects toward these three human carcinoma cell lines, and the 9,9-dihexyl-4,5-diazafluorene further exhibits antitumor activity in athymic nude mice Hep3B xenograft models. For the structurally related dialkylfluorenes that lack the diaza functionality, in vitro cytotoxicity was not observed at clinically relevant concentrations.

The in vivo efficacy of phthalocyanine–nanoparticle conjugates for the photodynamic therapy of amelanotic melanoma

The efficiency of a Zn(II)-phthalocyanine disulphide (C11Pc), a compound with both phthalocyanine units bearing seven hexyl chains and a sulphur terminated C11 chain, as a photodynamic therapy (PDT) agent was investigated in C57 mice bearing a sub-cutaneously transplanted amelanotic melanoma. The phthalocyanine was intravenously injected at a dose of 1.5 μmol/kg body weight either free or bound to gold nanoparticles, using a Cremophor emulsion as a delivery vehicle. Biodistribution studies at selected post-injection times showed that the nanoparticle-associated C11Pc was recovered in significantly larger amounts from all the examined tissues and the serum and yielded a greater selectivity of tumour targeting: thus, the ratio between the amount of phthalocyanine recovered from the amelanotic melanoma and the skin (peritumoural tissue) increased from 2.3 to 5.5 from the free to the gold nanoparticle-bound C11Pc at 24 h after injection. PDT studies with the C11Pc-loaded amelanotic melanoma showed a markedly more significant response of the tumour in the mice that had received the nanoparticle-bound photosensitiser; the PDT effect was especially extensive if the irradiation was performed at 3 h after C11Pc injection when large phthalocyanine amounts were still present in the serum. This suggests that the PDT promoted by C11Pc predominantly acts via vascular damage at least in this specific animal model. This hypothesis was fully confirmed by electron microscopy observations of tumour specimens obtained at different times after the end of PDT, showing an extensive damage of the blood capillaries and the endothelial cells.

Interface Formation between Calcium and Electron-Irradiated Poly(3-hexylthiophene)

The adsorption of Ca on electron-irradiated poly(3-hexylthiophene) (P3HT) surfaces at 300 K (E(kin) = 100 eV) has been studied by adsorption microcalorimetry, atomic beam/surface scattering, X-ray photoelectron spectroscopy (XPS), and low-energy He(+) ion scattering spectroscopy (LEIS). The results are compared to previous studies of Ca adsorption on pristine P3HT. The major structural effect of electron irradiation is a substantial increase in the fraction of unsaturated carbon atoms, probably a result of electron-induced hydrogen abstraction from the hexyl chains and formation of new C=C double bonds. No loss of sulfur was observed. The combined XPS, LEIS, and calorimetry data indicate that the reaction and growth behavior of Ca on P3HT surfaces is not significantly affected by this electron damage, apart from an increased sticking probability at low coverages. The sticking probability of Ca on the irradiated P3HT is initially 0.63, compared to 0.36 on the pristine surface. It increases with coverage, approaching unity between 4 and 5 ML. The heat of adsorption stays nearly constant at 405 kJ/mol up to a coverage of 0.6 ML, which is ascribed to Ca diffusing below the surface and forming CaS clusters by abstraction of sulfur from the thiophene rings, based on XPS and LEIS data. The heat of adsorption then decreases gradually until it reaches the heat of sublimation of bulk Ca, 178 kJ/mol, by 4 ML; this is attributed to the formation of 3D Ca islands on top of the polymer, which eventually coalesce into a continuous Ca film by 11 ML. The heat of reaction versus coverage and the ultimate depth up to which the Ca atoms react with the polymer thiophene groups (approximately 3 nm) are nearly independent of electron damage, except for a difference in the heat of adsorption below 0.1 ML associated with defects or impurities. The increase in initial sticking probability caused by electron damage is attributed to stronger bonding of Ca adatoms to unsaturated versus saturated hydrocarbons. These very weakly held Ca adatoms are transient precursors to the two reactions which dominate the measured heat of adsorption (reaction with thiophene units and Ca cluster formation), but they can also desorb in this three-path kinetic competition. Mass spectrometer data show that these precursors have longer surface residence times on the electron-damaged surface.