Inherent photon energy recycling effects in the up-converted delayed luminescence dynamics of poly(fluorene)-Pt(II)octaethyl porphyrin blends.

We present results of steady-state and transient photoluminescence studies of molecularly doped poly(fluorene) films. We study blends with increasing content of the triplet emitter (2,3,7,8,12,13,17,18-octaethyl-porphyrinato)Pt(II) (PtOEP) when dispersed in the polymeric poly(fluorene) matrix of the poly[9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl] (PF26) derivative. We carry out a unified study of the photophysical reactions that are involved in the energy transfer processes in this system by probing the three luminescence processes of a) PF26 fluorescence, b) triplet-triplet annihilation (TTA) induced up-converted PF26 delayed fluorescence and c) PtOEP phosphorescence. With increasing PtOEP content, the process of photon energy recycling in the PF26:PtOEP system is manifested from the quenching of the TTA-induced up-converted PF26 delayed fluorescence and it is rationalized with the use of Forster theory of resonant energy transfer. Based on the combined results of the photophysical and the transmission electron microscopy characterization of the as-spun PF26:PtOEP films, we determine the onset of PtOEP aggregation at 2-3 wt % PtOEP content. The analysis of the photophysical data is based on the use of modified Stern-Volmer photokinetic models that are appropriate for the solid state. A static component in the PL quenching of PF26 is revealed for PtOEP contents below 2 wt %. The modified Stern-Volmer kinetic scheme further suggests that co-aggregation effects between PF26 and PtOEP are operative with an association constant of ground state complex formation k(bind) approximately 15-17 M(-1). The involvement of the ground state heterospecies in the TTA-mediated PF26 up-converted luminescence is discussed. The participation of an electron-exchange step, in the excited state energy transfer pathway between PtOEP and PF26, is proposed for the activation mechanism of the PF26 up-converted fluorescence.

Up-conversion fluorescence: noncoherent excitation by sunlight.

We demonstrate up-conversion of noncoherent sunlight realized by ultralow excitation intensity. The bimolecular up-conversion process in our systems relies on the presence of a metastable triplet excited state, and thus has dramatically different photophysical characteristics relative to the other known methods for photon up-conversion (two-photon absorption, parametric processes, second harmonic generation, sequential multiphoton absorption, etc.).

Static and dynamic solution properties of monodisperse oligofluorenes.

Dynamic light scattering techniques were employed for the study of molecular and collective properties of newly synthesized oligomers (n = 3-7) of the polyfluorene in toluene solutions. At low concentrations, the molar mass, interaction parameter, optical anisotropy and the transport coefficients for translational and rotational diffusion provide a consistent description using a rigid rod model with thickness b = 0.6 nm and length L = nl (l = 0.75 nm). This description is in agreement with the persistence length (6-7 nm) reported for the fluorene polymers. In the non-dilute regime, the estimated concentration for the lyotropic transition further supports the proposed molecular shape and the second slow process in the intermediate scattering function relates to the formation of almost spherical clusters above about 30%.

Orientation dynamics in isotropic phases of model oligofluorenes: glass or liquid crystal.

Orientation molecular dynamics were investigated in a series of "defect-free" oligofluorenes by depolarized dynamic light scattering and dynamic NMR spectroscopy. Typical liquid crystalline pretransitional dynamics were observed upon cooling the isotropic phase to the liquid crystalline phase with strong increase of the scattered intensity and slowing down of the characteristic time of the probed collective relaxation. This is well accounted for by the Landau-de Gennes theory, however, with a strong temperature dependence of the viscosity coefficient, reflecting the proximity of the glass transition. For the trimer the two transitions almost overlap and the molecular orientation coincide with the alpha-relaxation associated with the glass transition. The NMR measurements confirm that the time scale of the dynamics is completely governed by the glass process, yet the geometry of the motion is anisotropic, yielding order parameters ranging from 0.15 to 0.25 for the long axis in the liquid crystalline phase. The glass transition is therefore geometrically restricted with poorly ordered mesophase which is consistent with the weak transverse phonons in the light scattering experiment down to Tg+20 K.

Zinc phosphate as versatile material for potential biomedical applications Part 1.

Synthetic alpha - and beta -Hopeite, two polymorphs of zinc phosphate tetrahydrates (ZPT) have been synthesized by hydrothermal crystallization from aqueous solution at 20 degrees C and 90 degrees C respectively. Aside from their sub-title crystallographic differences originating from a unique hydrogen bonding pattern, their thermodynamic interrelation has been thoroughfully investigated by means of X-Ray diffraction (XRD) and differential scanning calorimetry (DSC), combined with thermogravimetry (TGA-MS). Using a new heterogeneous step-reaction approach, the kinetics of dehydration of the two forms of ZPT was studied and their corresponding transition temperature determined. Low temperature DRIFT, FT-Raman and (1)H, (31)P MAS-NMR reveal an oriented distortion of the zinc phosphate tetrahedra, due to a characteristic hydrogen bonding pattern and in accordance with the molecular tetrahedral linkage scheme of the phosphate groups. Biogenic Hydroxyapatite (HAP) and one of its metastable precursors, a calcium dihydrogen phosphate dihydrate (DCPD) or Brushite were also obtained and used to underline the resulting variations of chemical reactivity in zinc phosphates.

Zinc phosphate as versatile material for potential biomedical applications Part II.

Surface chemical reactivity of two modifications of synthetic zinc phosphate tetrahydrate (alpha - and beta -form of Hopeite, alpha -,beta -ZPT) has been studied by selective chemical and e-beam etching in presence of diluted phosphoric acid and ammonia by Scanning Electron Microscopy (SEM) and microelectrophoresis (zeta potential measurements) in correlation with the corresponding bulk properties and crystal size distributions. The subtitle crystallographic differences between alpha -and beta -ZPT originating from a unique hydrogen bonding pattern, induce drastic variations of both surface potential and surface charge. Biogenic Hydroxyapatite (HAP) and one of its metastable precursors, a calcium dihydrogen phosphate dihydrate (DCPD) or Brushite were used to underline this resulting variation of chemical reactivity in zinc phosphates. In-situ monitoring of the transformation of Brushite in Hydroxyapatite is also reported.

Molecular dynamics of oligofluorenes: a dielectric spectroscopy investigation.

The molecular dynamics were investigated in a series of "defect-free" oligofluorenes up to the polymer by dielectric spectroscopy (DS). The method is very sensitive to the presence of keto "defects" that when incorporated on the backbone give rise to poor optical and electronic properties. Two dielectrically active processes were found (beta and alpha process). The latter process (alpha) displays strongly temperature dependent relaxation times and temperature- and molecular weight-dependent spectral broadening associated with intramolecular correlations. The glass temperature (Tg) obeys the Fox-Flory equation and the polymer Tg is obtained by DS at 332 K. The effective dipole moment associated with the alpha process is 0.27 +/- 0.03 D.

Effects of temperature and pressure on the stability and mobility of phases in rigid rod poly (p-phenylenes).

The structure and the associated dynamics have been investigated in melts of hairy-rod macromolecules composed from a poly(p-phenylene) backbone with sulfonate ester and dodecyl side chains. For the structure investigation, polarizing optical microscopy, differential scanning calorimetry, pressure-volume-temperature, and wide-angle x-ray scattering have been employed whereas for the dynamics dielectric spectroscopy as a function of temperature and pressure was used. Based on the combined information from structure and dynamics the relaxation mechanisms were identified and the origin of the glass transition has been discussed in terms of insufficient thermal energy rather than insufficient free volume. The relevant phase diagram has been constructed and the stability and mobility of phases is discussed.

On the origin of the heterogeneous orientation dynamics of semiflexible polymers in solutions.

Collective rotational motion in nondilute isotropic solutions of semirigid chains is sensitively probed by depolarized light scattering over a broad time range. The bimodal shape and the peculiar dependence of the orientational relaxation function on the scattering angle might arise from the coupling between orientational and shear modes of molecular motion (P.G. de Gennes, Mol. Cryst. Liq. Cryst. 12, 193 (1971)). The dynamic heterogeneity, i.e. the separation of the time scales and the shape of the relaxation functions appears to be system specific.

Membrane-based gas separation of ethylene/ethylene oxide mixtures for product enrichment in microreactor technology.

For the off-gas product separation in an ethylene oxide microreactor, new concepts need to be developed. Instead of using the technical process of low-temperature distillation, the performance of gas separation through a dense polymeric membrane was investigated. Different noncrosslinked and crosslinked 6FDA-based polyimides and copolyimides (6FDA=4,4'-hexafluoroisopropylidene diphthalic anhydride) were screened as potential membrane materials. The polymer structure of the membrane material was found to influence the solubility of ethylene oxide and ethylene. The introduction of polar substituents in the polymer backbone increases the solubility selectivity drastically. For permeation experiments in the same pressure range, a novel setup was developed using microstructured membrane supports. With this device, different polymer membranes were tested with respect to the pure gas permeability of ethylene oxide and ethylene as a function of the feed pressure, so that plasticisation effects could be discussed. For different polyimides and copolyimides, ideal ethylene oxide/ethylene selectivities between 3 and 4.5 were found combined with permeabilities between 1.8 and 80.8 barrer.

Crystalline self-assembly into monolayers of folded oligomers at the air-water interface

Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 A. induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystalline state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 A separation between alkyl chains, yields the conventional herringbone arrangement.

Ultrathin films of a polyelectrolyte with layered architecture

Posttransfer modification of preformed Langmuir-Blodgett films of poly(tert-butyl methacrylate) and poly(tert-butyl acrylate) by gaseous hydrochloric acid yields films with layered architecture of poly(methacrylic acid) and poly(acrylic acid), respectively. X-ray reflectivity and infrared spectroscopy confirm monolayer by monolayer transfer of the source polymers and their transformation to acid multilayer assemblies with retention of low surface roughnesses. The incorporation of cross-linking groups into the system offers the possibility for further chemical modification to produce ultrathin films of model networks desirable for bioadsorption studies and as hydrophilic spacing layers for tethered membranes.

Electromechanical properties of an ultrathin layer of directionally aligned helical polypeptides.

The electromechanical properties of a monomolecular film of poly-gamma-benzyl-L-glutamate (PBLG) 15 nanometers thick grafted at the carboxyl-terminal end to a flat aluminum surface were measured. The field-induced change in film thickness, dominated by a large inverse-piezoelectric effect, demonstrates that the "grafting-from" technique forces the chains into a parallel arrangement. The mechanical plate modulus of the film as determined by electrostriction agrees with the theoretical prediction for a single PBLG molecule along the chain axis. The experiments show that ultrathin polypeptide layers with large persistent polarization can be fabricated by the grafting approach.

Studies of Structure and Local Wetting Properties on Heterogeneous, Micropatterned Solid Surfaces by Microinterferometry.

The microstructure and the local wettability of functionalized patterns is reported. The patterns are formed by the photolithographic structuring of multilayers of hairy rod polymers on glass/MgF2/SiO2 substrates. By application of quantitative reflection interference contrast microscopy (RICM), the RICM image analysis was improved by accounting for (i) multiple reflections and the finite illumination aperture in stratified systems and (ii) geometrical effects in wedge-shaped systems and by (iii) optimizing the contrast enhancement through adjustment of the MgF2 coating. The surface free energy of hairy rod films were determined by analysing the contact angle of partially wetting liquid droplets as a function of the surface tension of the liquid. Periodic lattices of hydrophobic domains separated by hydrophilic domains were applied to check the theory of Joanny and de Gennes [J. Phys. Chem. 81, 552-562 (1984)] of contact angle hysteresis at periodic arrays of pinning centers. Measurements of the shape of the triple line and the contact angle hysteresis by RICM confirmed the theoretical predictions. Copyright 1997 Academic Press.

Assembly of polymer/lipid composite films on solids based on hairy rod LB-films.

The present work deals with the assembly of multilayers or rod-like polymers with hydrophobic side chains (called hairy rods) and their potential application as ultrathin polymer cushions for the build-up of self healing supported membranes on various solids (Si/SiO(2)-wafer, gold covered substrates). Three types of hairy rods were studied: Isopentyl cellulose (IPC), phtalocyaniatopolysiloxane with mixed alkane side chains (PCPS) and trimethylsilane cellulose (TMCS). Detailed analysis of the thickness of supported multilayers as a function of the number of deposited monolayers with ellipsometry, near infrared surface plasmon resonance (NIR-SPR), a quartz crystal microbalance (QCM) and reflection interference contrast microscopy (RICM), show that the basic building blocks of hairy rod multilayers are bilayers with the hydrophobic surfaces of the monolayers facing each other. Continuous and stable firms of hairy rods can be deposited if the hydrophobicities of the solid surface and the monolayer are matched. It is demonstrated by lateral diffusion measurements (using photobleaching techniques) that continuous phospholipid bilayers can be deposited onto multilayers of rigid rods of TMCS after hydrophilization by cleavage of trimethylsilane side chains in HCl-vapour, while stable lipid monolayers can be deposited onto hydrophobic surfaces of rigid rod layers. NIR-SPR allows the observation of double band reflectivity curves at interfaces separating different surface layers and thus offers the possibility of differential detection of ligand binding at the interface of differently functionalized domains.