In situ studies of phase transitions in thin discotic films

The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.     

Structural evolution of hexa-peri-hexabenzocoronene adlayers in heteroepitaxy on n-pentacontane template monolayers

The growth and structure of self-assembled adlayers of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C12) adsorbed on highly ordered pyrolitic graphite (HOPG) decorated by an n-pentacontane (n-C50H102) monolayer have been investigated by scanning tunneling microscopy (STM). Whereas on HOPG the HBC-C12 molecules readily self-assemble into a unique stable 2D structure, on the [n-C50H102 monolayer/graphite] system we observe morphological phase transitions with formation of time dependent alpha, beta, and gamma phases (alpha-->beta-->gamma). The initial alpha-phase is similar to that obtained on bare graphite, while intermediate beta- and final gamma-structures present molecular dimers and rows, respectively. The observed two-dimensional polymorphism is due to weak interaction between HBC-C12 molecules and n-C50H102-modified graphite substrate. Our results constitute an important step toward the control of the growth and structure of highly ordered monolayers of functional conjugated molecules by modifying the graphite surface with an n-alkane monolayer of appropriate chain length.     

Clustering of polycyclic aromatic hydrocarbons in matrix-assisted laser desorption/ionization and laser desorption mass spectrometry

The desorption/ionization behaviour of polycyclic aromatic hydrocarbons (PAHs) in matrix-assisted laser desorption/ionization (MALDI) and laser desorption (LD) mass spectrometry was studied by the solvent-free sample preparation method. As the understanding of the desorption/ionization mechanism in MALDI is normally hampered by the different ionization and desorption efficiencies of the analytes, this work was focused on the analyses of a homologous series of four hexabenzocoronenes (HBCs) possessing virtually the same ionization efficiency: HBC parent, hexamethyl-hexabenzocoronene (HBC-C1), hexapropyl-hexabenzocoronene (HBC-C3) and hexakis(dodecyl)-hexabenzocoronene (HBC-C12). The different signal intensities obtained in their mass spectra can be related to differences in their desorption efficiencies, which are attributed to the different strengths of the intermolecular interactions between unsubstituted and alkylated HBCs in the solid state. The influence of the aromatic structure of PAHs on their photoionization/desorption probability was investigated. As a model system, an equimolar mixture composed of HBC-C12 and hexakis(dodecyl)-hexaphenylbenzene (HPB-C12) was chosen. The aromatic structures of both molecules and thus their absorption coefficients at the laser wavelength differ substantially and have a huge influence on their photoionization efficiency. The combined effect of laser light absorption and intermolecular interactions on the desorption/ionization behaviour of giant PAHs was further studied by using an equimolar mixture composed of a larger PAH (C(222)H(42)) and its dendritic precursor (C(222)H(150)). This mixture shows the opposite behaviour to that of the former example, because the balance between desorption and ionization efficiency has changed significantly. The present investigation should be of interest for providing a better understanding of MALDI and LD spectra obtained from natural PAH-containing samples, such as heavy oils, asphaltenes or pitches, for which our artificial mixtures represent suitable model systems.     

Characterization of Langmuir-Blodgett films of a ferroelectric liquid crystal

Molecular orientation, structure, and phase transition behaviors in Langmuir-Blodgett (LB) and cast films of a ferroelectric liquid crystal of sec-butyl 6-(4-(nonyloxy)benzoyloxy)-2-naphthoate (FLC-1) are determined by ultraviolet (UV) spectroscopy, X-ray diffraction, and infrared (IR) spectroscopy. It is found that the orientation angle of chromophores theta in LB films is 41 degrees from the surface normal. The tilt angle of the chromophore changes at 56, 70, and 88 degrees C, respectively, which denotes the presence of phase transitions. Two kinds of layered or isomeric crystal structures of the LB films with layer spacings of 3 and 3.5 nm at room temperature have been found while the latter disappears above 45 degrees C, as confirmed by measurement of temperature-dependent IR spectra.     

Thermal transitions in the bilayer assembly of dimyristoylphosphatidylglycerol sodium salt dispersion in water: a new phase transition through an intermediate state

The thermal properties of the dispersion of sodium salt of dimyristoylphosphatidylglycerol (NaDMPG) in water have been investigated as functions of incubation temperature and aging time by DSC, XRD, sodium ion activity, pH, zeta-potential, and IR measurements. The DSC charts for NaDMPG dispersions incubated below 30 degrees C showed an endothermic peak at 31.7 degrees C with a small shoulder peak at Tm (gel-liquid crystal transition temperature: 23.5 degrees C). The temperature of 31.7 degrees C coincides with the T* temperature at which a high-order transition in the NaDMPG bilayer assembly has been found to occur in our previous studies. However, no peak was observed for the dispersions incubated above 32 degrees C. These results indicate that thermal properties of NaDMPG bilayers definitely differ below and above the T* temperature. The dispersion which had been once incubated at 40 degrees C for 24 h never showed the endothermic peak at T* even after the further aging at 3 degrees C for 12-day. Namely, the NaDMPG bilayer assembly exhibits an intensive thermohysteresis. The XRD charts for the NaDMPG dispersions incubated at 25 degrees C showed a sharp X-ray diffraction pattern corresponding to the repeat distance of d = 4.75 nm regardless of their aging time, while the dispersions incubated at 40 degrees C had no diffraction peak until 9-day elapsed. After 10-day aging at 40 degrees C, however, a diffraction peak corresponding to d = 5.55 nm clearly appeared. In the DSC measurements for the dispersion incubated at 40 degrees C, a few endothermic peaks began to appear between Tm and T* after approximately 7-day aging. Then, they shifted toward higher temperatures and finally converged into a single peak at 40-42 degrees C after 14-day aging. These XRD and DSC peaks observed after a long period of aging time above T* suggest that conformations of the hydrophilic groups and the hydrocarbon chains in the NaDMPG bilayers take a more tight and closer arrangement very slowly via an intermediate state above T*, and a new gel phase of the bilayers is consequently formed, the transition temperature (T(I) temperature) of which is 40-42 degrees C. A molecular interpretation for such transition processes in the bilayer assembly of NaDMPG dispersions has been proposed on the basis of pH, sodium ion activity, zeta-potential, IR data, etc.     

四氯合锌酸二(正十一烷基铵)晶体相变的Raman光谱

用Raman光谱研究了[n-C~11H~23NH~3]~2ZnCl~4(简记为C~nZn)配合物的固-固相变。结果表明, 配合物产生的固-固相变主要与烷烃链的堆积结构和分子构象变化有关, 在T~cl=25℃的相变是由于烷烃链的侧向堆积和分子构象的有序到部分无序变化。在中间相, 分子链局部产生旁式构象。在T~c2=87℃的相变主要来源于烷烃链从部分构象有序到完全无序的变化, 高温相形成了构象完全无序相, 相应于烷烃链的"熔化"。     

Structure and phase transitions of alkyl chains on mica

We use molecular dynamics as a tool to understand the structure and phase transitions [Osman, M. A.; et al. J. Phys. Chem. B 2000, 104, 4433-4439. Osman, M. A.; et al. J. Phys. Chem. B 2002, 106, 653-662] in alkylammonium micas. The consistent force field 91 is extended for accurate simulation of mica and related minerals. We investigate mica sheets with 12 octadecyltrimethylammonium (C(18)) ions or 12 dioctadecyldimethylammonium (2C(18)) ions, respectively, as single and layered structures at different temperatures with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside preferably above the cavities in the mica surface with an aluminum-rich boundary. The nitrogen atoms are 380-390 pm away from the superficial silicon-aluminum plane. With increasing temperature, rearrangements of C(18) ions on the mica surface are found, while 2C(18) ions remain tethered due to geometric restraints. We present basal-plane spacings in the duplicate structures, tilt angles of the alkyl chains, and gauche-trans ratios to analyze the chain conformation. Agreement with experimental data, where available, is quantitative. In C(18)-mica with less than 100% alkali-ion exchange, the disordered C(18) rods in the island structures [Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913-5917] break at 40 degrees C. At 60 degrees C, the headgroups of the C(18) alkyl chains rearrange on the mica surface, and the broken chain backbones assume a coillike structure. The C(18)-mica obtained on fast cooling of this phase is metastable due to slow reverse rearrangements of the headgroups. In 2C(18)-mica with 70-80% ion exchange, the alkali ions are interspersed between the alkyl chains, corresponding to a single phase on the surface. The observed phase transition at approximately 53 degrees C involves an increase of chain disorder (partial melting) of the 2C(18) ions without significant rearrangements on the mica surface. We propose a geometric parameter lambda for the saturation of the surface with alkyl chains, which determines the preferred self-assembly pattern, that is, islands, intermediate, or continuous. lambda allows the calculation of tilt angles in continuous layers on mica or other surfaces. The thermal decomposition seems to be a Hofmann elimination with mica as a base-template.     

Induced alignment of a solution-cast discotic hexabenzocoronene derivative for electronic devices investigated by surface X-ray diffraction

A surface X-ray diffraction study is presented showing that highly ordered and uniaxially aligned hexa(3,7-dimethyl-octanyl)hexa-peri-hexabenzocoronene (HBC-C8,2) films can be fabricated by crystallization from solution onto friction-transferred poly(tetrafluoroethylene) (PTFE) layers. Three crystalline HBC-C8,2 majority phases result. In all three phases, the HBC-C8,2 molecules self-organize into columns which are uniaxially aligned along the direction defined by the PTFE macromolecules of the substrate. The three phases are quite similar, the major difference being their orientation with respect to the substrate. A quasi-2D epitaxial growth mechanism with a grapho-epitaxial component for one of the three phases explains the formation of the three rotational HBC-C8,2 variants. A method to obtain a thin film with only one phase is proposed. The results show that standard THETAV;-2THETAV; X-ray diffraction and transmission electron diffraction can be very misleading tools to estimate the crystalline quality in a thin film of complex structure.     

Correlation of the physicochemical properties of symmetric 1,3-dialkoylamidopropane-based cationic lipids containing single primary and tertiary amine polar head groups with in vitro transfection activity

The physicochemical properties of a novel series of symmetric 1,3-dialkylamidopropane-based cationic amphiphiles [M. Sheikh, J. Feig, B. Gee, S. Li, M. Savva, In vitro lipofection with novel series of symmetric 1,3-dialkoylamidopropane-based cationic surfactants containing single primary and tertiary amine polar head groups, Chem. Phys. Lipids 124 (2003) 49-61] were studied by several techniques, in an effort to correlate cationic lipid structure with transfection efficacy. It was found that only the unsubstituted amine and tertiary amine dioleoyl derivatives 1,3lmp5 and 1,3lmt5, respectively, mediated in vitro transfection activity in the absence of helper lipids. This activity pattern was consistent with ethidium bromide fluorescence quenching studies, which indicated that only these two derivatives bound to and efficiently condense plasmid DNA at physiological pH. Dynamic light scattering indicated that lipoplexes made by these two cationic lipids were relatively small particles below 1 microm, in sharp contrast to lipoplexes bigger than 3 microm composed of saturated cationic derivatives. Transmission electron microscopy studies clearly indicated that cationic lipid dispersions made by saturated derivatives form multilamellar tubules at physiological pH. Calorimetric studies showed that cationic amphiphiles with saturated acyl chains longer than 12 carbons exhibit solid-to-liquid crystalline phase transitions above 37 degrees C. In agreement with the microscopy and calorimetry studies, Langmuir film balance experiments indicated that saturated derivatives with hydrophobic chains longer that 12 carbons are not well hydrated and exist at a chain-ordered state at ambient temperature. Calculation of compressibility moduli from monolayer compression isotherms at 23 degrees C suggested that monolayers made by cationic lipids bearing saturated acyl chains are less compressible relative to those of the dioleoyl derivatives 1,3lmp5 and 1,3lmt5. In conclusion, high hydration, increased fluidity and high elasticity of cationic lipid assemblies in isolation, all correlate with high in vitro transfection activity.     

Synthesis, characterization and phase transitions of the inorganic-organic layered perovskite-type hybrids [(C(n)H(2n+1)NH3)2PbI4], n = 7, 8, 9 and 10

Four inorganic-organic hybrid materials that consist of 2-D layers of corner-sharing lead(II) iodide octahedra separated by alkylammonium chains have been crystallized and characterized via single-crystal XRD (SCXRD). The four hybrids, represented by the general formula [(C(n)H(2n+1)NH(3))(2)PbI(4)] and abbreviated C(n)PbI, exhibit multiple reversible phase transitions for a narrow temperature range. The transition temperatures were determined with differential scanning calorimetry experiments. The number of transitions and the transition temperatures are dependant on the chain length; for n = 7 and 10, there are three transitions, and for n = 8 and 9, there are two transitions. Regardless of the number of transitions, all four compounds have identical lowest temperature phases, which have inorganic layers that are eclipsed, non-planar conformations of the alkyl ammonium chains and yellow-coloured crystals. The next highest temperature phase for three of the compounds (C(10)PbI goes through an intermediate phase first), has staggered inorganic layers, all-trans planar conformations of the chains and orange coloured crystals. The highest temperature phase for n = 8 and 10 has red-coloured crystals and shows a disordering of the alkylammonium chains over two positions and staggered inorganic layers. The high temperature phase of C(7)PbI retains its orange colour and has only increased thermal motion of its alkylammonium chain. The structure of the high temperature phase of C(9)PbI was not determined. The SCXRD structures of the various phases give clues to the structural changes that the compounds undergo at the phase transitions, which will now enable future studies of their optical and electronic properties to be better understood.     

Cross-strand coupling of a beta-hairpin peptide stabilized with an Aib-Gly turn studied using isotope-edited IR spectroscopy

Isotope-edited IR spectroscopy was used to study a series of singly and doubly 13C=O-labeled beta-hairpin peptides stabilized by an Aib-Gly turn sequence. The double-labeled peptides have amide I' IR spectra that show different degrees of vibrational coupling between the 13C-labeled amides due to variations in the local geometry of the peptide structure. The single-labeled peptides provide controls to determine frequencies characteristic of the diagonal force field (FF) contributions at each position for the uncoupled 13C=O modes. Separation of diagonal FF and coupling effects on the spectra are used to explain the cross-strand labeled spectral patterns. DFT calculations based on an idealized model beta-hairpin peptide correctly predict the vibrational coupling patterns. Extending these model results by consideration of frayed ends and the hairpin conformational flexibility yields an alternate interpretation of details of the spectra. Temperature-dependent isotopically labeled IR spectra reveal differences in the thermal stabilities of the individual isotopically labeled sites. This is the first example of using an IR-based isotopic labeling technique to differentiate structural transitions at specific sites along the peptide backbone in model beta-hairpin peptides.     

Temperature-reversible ultrathin films of N-isopropylacrylamide terpolymer adsorbed at the solid-liquid interface

This article describes the stability and reversibility of ultrathin films of N-isopropylacrylamide (NIPA)-vinylimidazole (VI)-poly(ethylene glycol) (PEG) graft terpolymer adsorbed at the solid-liquid interface upon temperature cycling from below to above its phase transition temperature. The coil-to-globule and globule-to-coil phase transitions were captured by in situ fluid tapping atomic force microscopy (AFM). The film thickness of 1 nm was determined by AFM, X-ray photoelectron spectroscopy, and X-ray reflectivity. The concentration required to reach full coverage was found to be higher when adsorption occurred below the phase transition temperature. From 23 to 42 degrees C, the adsorbed NIPA terpolymer film was observed to be molecularly smooth, corresponding to the close-packed structure of flexible polymer coils. Particles containing between one and a few globules appeared abruptly at the interface at 42-43 degrees C, the same temperature as the solution phase transition temperature, which was determined by dynamic light scattering. The size of the particles did not change with temperature, whereas the number of particles increased with increasing temperature up to 60 degrees C. The particles correspond to the collapsed and associated state of the globules. The film morphological changes were found to be reversible upon temperature cycling. Subtle differences were observed between dip-coated and spin-coated films that are consistent with a higher degree of molecular freedom for spin-coated films. The study contributes to the fundamental understanding and applications of smart ultrathin films and coatings.     

Investigations into the thermotrofic phase behaviour of natural membranes extracted from gram-negative bacteria and artificial membrane systems made from lipopolysaccharides and free lipid A

It is reported on investigations into the thermotropic phase behaviour of outer, cytoplasmic, and cell envelope preparations of Gram-negative bacteria, and their amphiphatic compounds - lipopolysaccherides and phospholipids - applying calorimetric, infrared spectroscopic, and 90°-light scattering techniques. Except for free lipid A, the lipid moiety of lipopolysaccharide, all investigated compounds exhibited gel-liquid crystalline phase transitions of the hydrocarbon chains in the temperature range 10 to 42 °C. The transition regions of the outer membrane end lipopolysaccharide preparations are narrower and appear at significantly higher temperatures (around 37 °C) as those from the cytoplasmic membrane and phospholipid preparations (around 26 °C). For both amphiphatic compounds the phase transitions are shifted to lower values at reduced growth temperature of the cells.     

Order-order and order-disorder transitions in thin films of an amphiphilic liquid crystalline diblock copolymer

In this study, we quantitatively investigated the temperature-dependent phase transition behaviors of thin films of an interesting amphiphilic diblock copolymer, poly(ethylene oxide)-b-poly(11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate) (p(EO)-b-p(MAAZ)) and the resulting morphological structures by using synchrotron grazing incidence X-ray scattering (GIXS) and differential scanning calorimetry. The quantitative GIXS analysis showed that the diblock copolymer in the homogeneous, isotropic melt state undergoes phase-separation near 190 degrees C and then forms a body-centered cubic (BCC) structure of spherical p(EO) domains in the p(MAAZ) matrix, at which point the p(EO) domains and the p(MAAZ) matrix are both in amorphous, liquid states. The BCC structure of spherical p(EO) domains is converted to a hexagonal cylinder structure near 120 degrees C, which is induced by the transformation of the isotropic phase of the p(MAAZ) matrix to the smectic A phase, which is composed of a laterally ordered structure of p(MAAZ) blocks with fully extended side groups. The resulting hexagonal cylinder structure is very stable below 120 degrees C. This microscopic hexagonal cylinder structure is retained as the smectic A phase of the p(MAAZ) matrix undergoes further transitions to smectic C near 104 degrees C and to a smectic X phase near 76 degrees C, while the amorphous, liquid phase of the p(EO) cylinders undergoes crystallization near -15 degrees C. These complicated temperature-dependent disorder-order and order-order phase transitions in the films were found to take place reversibly during the heating run. A face-centered orthorhombic structure of p(EO) domains was also found during the heating run and is an intermediate structure in the hexagonal cylinder structure to BCC structure transformation. We use these structural analysis results to propose molecular structure models at various temperatures for thin films of the diblock polymer.     

KNa(3)In(9): a Zintl network phase built of layered indium icosahedra and zigzag chains. Synthesis,structure, bonding,and properties

This phase was discovered following direct fusion of the elements in welded Nb tubes at 550 degrees C and equilibration at 300 degrees C for 1 week. Single-crystal X-ray diffraction analysis reveals that KNa(3)In(9) crystallizes in an orthorhombic system (Cmca, Z = 8, a = 9.960(1) A, b =16.564(2) A, c = 17.530(2) A, 23 degrees C). The structure contains a three-dimensional indium network built of layers of empty In(12) icosahedra that are each 12-bonded and interconnected by 4-bonded indium atoms that also form zigzag chains. All cations bridge between cluster faces or edges, and their mixed sizes appear critical to the stability of this particular structure, which does not occur in either binary system. Both empirical electron counting and EHTB band structure calculations on the macroanion indicate that the bonding in this structure is closed-shell, whereas resistivity and magnetic susceptibility measures show that the compound is a moderately poor metal.     

Thermotropic phase behaviour of long-chain alkylmaltosides

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C14G2) and n-hexadecyl-beta-D-maltoside (C16G2) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C14G2 has the same melting point as that of C16G2, namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C14G2 crystallises from water at room temperature, whereas the hemihydrate of C14G2 crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.     

Effect of temperature on the surface phase behavior of n-hexadecyl dihydrogen phosphate in adsorption layers at the air-water interface

We present the adsorption kinetics and the surface phase behavior of n-hexadecyl dihydrogen phosphate (n-HDP) at the air-water interface by film balance and Brewster angle microscopy (BAM). A phase diagram, which shows a triple point at about 25.8 degrees C, is constructed by measuring the surface pressure (pi)-time (t) adsorption isotherms. Below 25.8 degrees C, each of the pi-t curves shows a plateau at about zero surface pressure indicating the existence of a first-order phase transition. The BAM observation confirms the order of this phase transition by presenting two-surface phases during this plateau. However, the BAM observation also shows clearly another second-order phase transition from an isotropic phase to a mosaic-textured liquid condensed (LC) phase. The initial phase is a gas (G) phase. Considering the peculiarity of the middle phase, we suggest this phase as an intermediate (I) phase. Above the triple point, the pi-t curves predict the existence of two-step first-order phase transitions. Similar to the results at lower temperatures, the BAM images show two-surface phases during these first-order phase transitions together with a second-order phase transition from an isotropic phase to an LC phase. These transitions are classified as a first-order G-LE (liquid expanded) phase transition, which is followed by another first-order LE-I phase transition. The second-order phase transition is an I-LC phase transition. Contrary to these results, at 36 degrees C both the pi-t measurements and the BAM observation present only two first-order phase transitions, which are G-LE at zero surface pressure and LE-LC transition at higher surface pressure. The shape of the domains during the main transitions shows a peculiar change from a circular at 20 degrees C to an elongated at 24 degrees C and finally to a circular shape at 36 degrees C. Such a change in the domain shapes has been explained considering the dehydration effect at higher temperatures as well as the nature of phases.     

Vibrational study on structural transitions of potassium pelargonate CH3(CH2)7CO2K

Anhydrous potassium pelargonate (KC9) undergoes four thermal transitions from room temperature to 450 degrees C. A normal mode analysis was made for the molecule in phase I at room temperature and the molecule was considered to have an all-trains conformation. With increasing temperature, partial melting of the alkyl chains occurred in phase II and complete melting was observed in phase III, as confirmed by vibrational spectra. In addition, orientational disorder of the carboxylate groups was suggested in phase II. This transition behavior of potassium pelargonate was compared with the behavior of nonane and pelargonic acid.     

PAMAM修饰的六苯并蔻衍生物的合成及其光物理研究

设计合成了外围以0.5、1.5、2.5代聚酰胺-胺(PAMAM)树枝形聚合物修饰的六苯并蔻衍生物HBC-C₆-PAMAM-D_m(m=0.5, 1.5, 2.5), 化合物结构通过了¹HNMR、¹³CNMR、IR和MS的表征.测定了HBC-C₆-PAMAM-D_m在四氢呋喃、乙腈和二氯甲烷中的吸收和荧光发射光谱, 三种溶剂中不同代数的HBC-C₆-PAMAM-D_m均以聚集态形式存在,外围树枝形聚合物骨架与核心HBC发色团的振动耦合作用随代数增加而增强,导致HBC振动能级不均化,引起吸收和荧光光谱发生变化.     

Thermotropic mesomorphic behavior of surfactant-encapsulated polyoxometalate hybrids

We investigate in detail novel organic-inorganic hybrid liquid crystalline materials, the complexes of surfactant-encapsulated polyoxometalate clusters (SECs), using thermal, X-ray diffraction, and FT-IR spectroscopic analyses. The differential scanning calorimetry measurements reveal four phase transitions under heating processes. We employ FT-IR spectroscopy to understand these phase behaviors. On the basis of vibration spectral assignments, the evidence suggests that the first two phase transitions are associated with the increase of gauche conformers and the disruption of alkyl chains packing in the heating run; the third phase transition is due to the full conformational disorder of alkyl chains covered on the polyoxometalates (PMs); no significant C-H stretching or wagging vibrations are observed with the fourth transition. We find that the fourth endothermic peak is sensitive to the charges of the PMs, and the transition temperature decreases from 185, 177, to 164 degrees C with decreasing PM charges from 13, 11, to 9, respectively. Interestingly, the temperatures of the first three phase transitions of SECs are essentially independent of the PM charges.