CHLOROPHYLLS IN POLYMERS. II. PHEOPHYTIN a IN POLYMERS and THE INFLUENCE OF STRETCHING ON THE STATE OF CHLOROPHYLLS IN ANHYDROUS POLYMER FILMS

Abstract— In the first part of this study the spectral properties of pheophytin a in rigid, unstretched anhydrous polyvinyl alcohol and nitrocellulose films have been studied in order to establish the influence of the central magnesium atom on the state of chlorophylls in polymer systems. The absorption, fluorescence, excitation spectra and fluorescence intensity decays in the polymer films and in the solutions from which they are cast are reported. It is shown that pheophytin a aggregate formation is influenced by the nature of the polymer system. An aggregate of pheophytin a is found in polyvinyl alcohol films over a wide concentration range. On the other hand, pheophytin a exists in the monomeric form in unstretched nitrocellulose films at concentrations below 6 × 10^-6 mol/g.
In the second part of this work, the influence of stretching of the films on the state and distribution of embedded chlorophyll pigments, is described. Here we show that the chlorophyll a molecules are found to undertake a heterogenous distribution in polyvinylalcohol matrices, since stretching partially disrupts the pocket-like structures present in unstretched films. In contrast, chlorophyll a and pheophytin a molecules can be embedded in a monomeric state in nitrocellulose matrices and moreover they remain homogeneously distributed upon stretching. The chlorophyll/nitrocellulose system is concluded to be a useful model system for studies of donor-donor energy transfer processes.     

CHLOROPHYLLS IN POLYMERS. I. STATE OF CHLOROPHYLL a IN UNSTRETCHED POLYMER SYSTEMS

Abstract— Model systems for the study of energy transfer processes are useful for the elucidation of the various factors governing the mechanism of energy transfer in photosynthetic systems. Here we describe the characterization of two systems, consisting of chlorophyll a incorporated in anhydrous nitrocellulose and polyvinylalcohol films. First, optical spectroscopy and time-resolved fluorescence techniques are used to characterize the state of the chlorophyll molecules in the films. We find that in nitrocellulose films the state of chlorophyll a depends strongly on the ratio of nitrocellulose to dimeth-ylsulfoxide in the solutions from which the films are cast. The state of chlorophyll a in polyvinylalcohol films does not depend on the amount of polymer originally dissolved in dimethylsulfoxide. In these films the pigment is monomeric at low concentrations of chlorophyll a, but aggregates are formed at much lower concentrations than in nitrocellulose. The latter fact is explained by the existence of pockets in polyvinylalcohol, leading to high local concentrations.
To further test the suitability of the nitrocellulose polymer films as model systems for energy transfer processes, time-resolved fluorescence anisotropy profiles are measured in dependence of the concentration of pigments in the matrix. Fits of the observed decay profiles to the predicted decay show good correspondence, as long as no traps are present. Furthermore, the fitted decay times yield the correct value of the Forster radius R₀ as compared to the value obtained spectroscopically. We thus conclude that the chlorophyll a-nitrocellulose system can be very appropriate for the study of energy transfer processes between photosynthetic pigment, since the pigments are uniformally distributed in the matrix.     

SPECTROSCOPIC STUDY OF CHLOROPHYLL a IN ORGANIC SOLVENTS AND POLYMERIZED ANHYDROUS POLYVINYL MATRIX

In this paper we present a spectroscopic study of chlorophyll a in solutions and in anhydrous polyvinyl alcohol films. Absorption, excitation and emission spectra, combined with fluorescence lifetime and time-resolved anisotropy measurements show that chlorophyll a in anhydrous polyvinyl alcohol films exists in a purely monomeric state. Furthermore, it appears that the monomeric chlorophyll a exhibits an efficient excitation energy transfer in this polyvinyl alcohol matrix. These results are rationalized in terms of a model in which the chlorophyll a molecules are located within pockets, formed by the polymer chains. It is concluded that the chlorophyll a-anhydrous polyvinyl alcohol film is a suitable system for studying energy transfer processes, especially because the factors governing energy transfer such as mutual orientation and separation of the molecules can easily be controlled.     

Photoelectrochemical study on photosynthetic pigments-sensitized nanocrystalline ZnO films

Hetero-structures formed by quantum-sized ZnO nanocrystals and photosynthetic pigments were prepared by adsorbing either chlorophyll a, carotenoids or their mixture onto a film of organic-capped ZnO nanoparticles. Photoelectrochemical measurements were comparatively performed on both bulk and nanocrystalline ZnO films after dye-covering in order to probe the photosensitization process occurring at the hetero-junction. The photoconversion process was found to be greatly enhanced at the nanocrystalline electrodes upon sensitization with a dye mixture. The sensitization process is discussed on the basis of the aggregation state of chlorophyll a, and of the specific photoprotective action played by carotenoids.     

Self-assembly of discotic liquid crystal porphyrin into more controllable ordered nanostructure mediated by fluorophobic effect

The novel nanoscale discotic liquid crystal porphyrin with partial chain perfluorination, which has the same basic structure as the best photoreceptor in nature (chlorophyll), shows an exceptionally enhanced tendency to self-assemble into ordered nanostructure. Defect-free homeotropically aligned fluorinated porphyrin thin films were, for the first time, fabricated and characterised. The ability to self-assemble large π-conjugated discotic molecules into highly ordered nanostructure via partial chain perfluorination provides new insight for the bottom-up nanofabrication of molecular devices. The controllable ordered porphyrin nanostructure with directed molecular arrangement holds great promise for use in high-performance electronic devices.     

LIQUID-CRYSTALLINE PHASE TRANSITION OF MONOMOLECULAR LAYERS OF CHLOROPHYLL a*

Monomolecular layers of chlorophyll a at the air-water interface were investigated by absorption spectroscopy and simultaneous thermodynamic measurements. Whereas at temperatures near 20°C at all pressures, only a liquid phase exists, at a temperature of 4°C, a liquid-crystalline phase transition is observed at a surface pressure of 5 dynes/cm. Pressure-induced changes in the chlorophyll arrangement become evident from a drastic change in the absorption spectra, accompanying the phase transition. The crystalline phase exhibits an extremely narrow absorption band (halfwidth below 9 nm) centered at 698 nm, indicative of a coplanar chlorophyll arrangement in a well-ordered environment. It is highly probable that in these model membranes a chlorophyll arrrangement could be established that is equivalent to the one proposed for the reaction center P700.     

Structural, optical and photoelectric properties of ZnO:In and Mg x Zn1 −x O nanofilms prepared by sol-gel method

The photochromic sol-gel hybrid materials containing cyanoazobenzene chromophores were described. These materials were obtained by copolycondensation of the functionalized triethoxysilane and tetraethoxysilane precursor. They were deposited on glass substrates via spin coating and casting techniques to provide thin transparent films. The UV-vis spectroscopy showed reversibility of the trans-cis photoisomerization of the chromophoric fragments. The reversible change of refractive index of the films on illumination with white light was determined by ellipsometry. The difference of real part of the refractive index of the sample was in the range 0.0053–0.0075. Formation of diffraction grating was achieved by two beam coupling arrangement using a 532 nm laser. The diffraction efficiency for the first order diffraction was in the range of 2–3.5%. The kinetics of photochromic grating recording and erasing was described by biexponential function approach.     

Radiation effect of a high energy electron beam on thin films of saturated and unsaturated fatty acids

Studies have been carried out on electron beam induced polymerization of LB films of saturated and unsaturated long chain aliphatic acids, in an attempt to obtain information about the dose dependence of conversion in relation to molecular arrangement. The results indicate that polymerization of unsaturated aliphatic acid LB films occurred when irradiation was carried out in a nitrogen atmosphere and that the LB films of a disordered state are more sensitive to radiation than tightly packed LB films.     

THE ORIENTATION OF THE TRANSITION DIPOLE MOMENTS OF CHLOROPHYLL a and PHEOPHYTIN a IN THEIR MOLECULAR FRAME

Abstract— In this paper we describe the determination of the orientation of the absorption and emission transition dipoles of chlorophyll a and pheophytin a in their molecular frame. For this purpose we have embedded the pigments in anhydrous nitrocellulose films with a concentration of 2 × 10^-7 mol/g. We have shown previously that under these conditions the pigments are in a purely monomeric state, are distributed uniformly both before and after stretching and that no intermolecular energy transfer among the molecules takes place.
Using a combination of steady-state anisotropy experiments on unstretched films and angle-resolved fluorescence depolarization measurements on stretched films, we obtain the orientation of the transition dipole moments of both pigments in their molecular frame and the orientational distribution function of the molecules relative to the stretching direction of the film.
The steady-state anisotropy measurements indicate that chlorophyll a has two distinct emission dipole moments and that excitation in the Soret-region results in simultaneous excitation of two or more absorption transition dipole moments. On the other hand, excitation in the Q_Y-band involves only a single dipole moment. The directions of the transition dipole moments in the molecular frame are obtained from the angle-resolved measurements. Pheophytin a also exhibits two emission dipole moments, but the angle between them is much smaller than that between the corresponding dipoles for chlorophyll a . As a consequence the dipole moments contributing to the Soret-region could not be resolved and only an effective absorption transition dipole moment in the Soret-region is extracted.     

Luminescence-spectral characteristics and conformational transformations in the electronically excited state of the dimethylaminophenyl derivatives of pyridine and pyridinium and pyrylium cations

The electronic absorption spectra and luminescence-spectral characteristics of the 4-dimethylaminophenyl derivatives of pyridine and pyridinium and pyrylium cations in solutions and polymeric films were studied. The obtained data make it possible to suppose that conformations with the orthogonal arrangement of the donating and accepting fragments (TICT structure) both in the cations and in the neutral molecules arc formed as a result of structural relaxation in the excited S₁ state.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, 625–630, May, 1992     

Kinetic Effects on Self‐Assembly and Function of Protein–Polymer Bioconjugates in Thin Films Prepared by Flow Coating

The self‐assembly of nanostructured globular protein arrays in thin films is demonstrated using protein–polymer block copolymers based on a model protein mCherry and the polymer poly(oligoethylene glycol acrylate) (POEGA). Conjugates are flow coated into thin films on a poly(ethylene oxide) grafted Si surface, forming self‐assembled cylindrical nanostructures with POEGA domains selectively segregating to the air–film interface. Long‐range order and preferential arrangement of parallel cylinders templated by selective surfaces are demonstrated by controlling relative humidity. Long‐range order increases with coating speed when the film thicknesses are kept constant, due to reduced nucleation per unit area of drying film. Fluorescence emission spectra of mCherry in films prepared at <25% relative humidity shows a small shift suggesting that proteins are more perturbed at low humidity than high humidity or the solution state.

     

Synthesis and nonlinear optical properties of side chain liquid crystalline polymers containing azobenzene push–pull chromophores

Azobenzene monomeric precursors bearing piperazine as donor moiety with different withdrawing groups and derived side chain polymethacrylates have been prepared and characterized. Monomers having terminal cyano or nitro groups, and the corresponding polymers, exhibited smectic A phases. Linear and nonlinear optical properties of every monomer and thin films of the cyano polymer ( pol‐PZ‐CN ) have been also studied. UV‐vis spectroscopy revealed out‐of‐plane orientation in the as prepared films, as confirmed by waveguide refractive index measurements. Moreover, absorption spectra indicated the presence of azo aggregates in these films. The initial molecular arrangement has been modified by applying thermal annealing within the mesophase range and UV‐blue irradiation. Although thermal annealing resulted in a significant amplification of the out‐of‐plane optical anisotropy due to thermotropic self‐organization of side chain azo moieties, irradiation with 440 nm light induced some disruption of aggregates. The nonlinear optical response of Corona poled films has been studied by second harmonic generation measurements, and the influence of the molecular arrangement on the nonlinear d_ij coefficients has been analyzed. The more efficient poling corresponded to preirradiated films. In any case, a noticeable degree of polar order (70% of the initial d₃₃ value) remained for several months after the poling in films kept at RT. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 232–242, 2010     

Composite Langmuir–Blodgett (LB) films of polyaniline and cadmium stearate

Composite monolayers of cadmium stearate and polyaniline processed with camphor sulphonic acid have been obtained at the air–water interface and subsequently transferred onto substrates as uniform Langmuir–Blodgett multilayers. Pressure–area (Π-A) isotherm studies indicated that polyaniline molecules are not squeezed out of the cadmium stearate matrix during compression or in the compressed state. Monolayer stability is seen to decrease when the polymer content is increased beyond 60% (in weight) which is probably associated with the formation of microaggregates that also affected the monolayer surface potential at large areas per molecule. With increasing amount of polyaniline in the mixture, a transition from Y-type to Z-type deposition has been observed. Transferred multilayer LB films were characterized by UV-vis, FTIR, XRD, surface potential, and dc electrical conductivity measurements. UV-vis results indicated that though the polyaniline was in the doped state in the spreading solution, the transferred films were in the emeraldine base state. FTIR studies revealed that the transferred films mainly contained cadmium stearate salt in addition to polyaniline. XRD results indicated that the stacking order is dependent on the polyaniline content in the composite films, the order was found to decrease upon increasing the polymer amount. The uniformity of transferred films was confirmed by surface potential studies. A possible packing arrangement in these composite Langmuir and Langmuir–Blodgett films has been proposed.     

Temperature-dependence of mechanical and morphological properties of ultra-high molecular weight polyethylene cross-linked by electron beam irradiation

Cross-linking of ultra-high molecular weight polyethylene was performed with electron-beam irradiation in the range of radiation dose from 12 to 96 Mrad under nitrogen. Dry gel films and melt films were used as specimens. Two kinds of cross-linked specimens could be kept at 200°C for a prolonged time in an undeformed state and this tendency was independent of radiation dose. The elongation of the gel films hampered the heat-resistant effect and the drawn specimens were broken at temperatures lower than 175 °C. The elongation of the melt films could not be realized, because of a marked fixation of chains in the fiber network, even at a dose of 12 Mrad.     

How a Small Structural Difference Can Turn Optical Properties of π‐Extended Coumarins Upside Down: The Role of Non‐Innocent Saturated Rings

The fluorescence properties of two new families of heterocycles possessing either a seven‐ or five‐membered ring attached at the core molecule are entirely different in solution and in the solid state. Crystallization has the effect of inhibiting non‐radiative excited‐state deactivation pathways, operative in solution for the seven‐membered ring compounds, thus leading to significant fluorescence efficiency in the solid state, with quantum yields ranging from 0.10 to 0.36. Conversely, the five‐membered ring derivatives, which display notable emission properties in solution, are almost non‐emissive in the crystalline state, characterized by a long‐range π‐stacked arrangement. When embedded in polymeric films, both series show fluorescence features similar to the solution case, with remarkable fluorescence quantum yields ranging from 0.09 to 0.41. According to quantum chemical calculations, 3H‐chromeno[3,4‐c]pyridine‐4,5‐diones show the specific mechanism of fluorescence quenching. The derivatives bearing the seven‐membered ring undergo, in solution, a significant structural deformation in the excited state, resulting in a large decrease of the energy gap between S₁ and S₀ and hence to a substantial contribution of the internal conversion in the relaxation process. The fluorescence quenching of the five‐membered ring derivatives is in turn related to the intermolecular interaction between adjacent molecules prevailing to a greater extent in the crystal lattice.     

Orientation studies on hot-drawn tubular blown films of isotactic polypropene

Orientation studies of hot-drawn polypropene films were performed using small-angle light scattering and wide-angle and small-angle x-ray scattering over a wide range of deformation. At low deformation, the classical scheme of affine deformation, as several authors have pointed out, seems to be quite realistic. However, for high deformations, i.e., up to 500%, a new conception of the microfibrillar state is proposed. The microfibrils are formed of fully extended crystals which have a high degree of parallel orientation, but deviations of the units from their ideal position occur. “Wrong” chains are embedded in the lattice, long-range order is lost, and paracrystallinity appears. A nematic arrangement develops gradually, and we suppose that the “needlelike entities” have some cylindrical symmetry with a crystalline core. The true amorphous component acts as a connection between these zones.     

C‐oriented and {010} Facets Exposed BiVO4 Nanowall Films: Template‐Free Fabrication and their Enhanced Photoelectrochemical Properties

Vertically aligned BiVO₄ nanowall films on indium tin oxide (ITO) glass have been fabricated through a template‐free hydrothermal method for the first time. Based on the structural understanding of both BiVO₄ and ITO, the lattice matches ({020}_BiVO4 and {040}_ITO, {200}_BiVO4 and {004}_ITO, respectively) and the similarity of metal atomic arrangement parallel to {001} planes turn out to be crucial for the fabrication of the nanowalls. Consequently, the growth of a BiVO₄ film begins from heteroepitaxy and undergoes an Ostwald ripening process to form an extended network, resulting in a c‐orientation and exposing {010} facets. Through this process, it is much easier to obtain a range of nanowall films with different packing densities, as the surface state of ITO glass is alterable by adjusting the concentration of acid. The films can be directly used as an electrode, which exhibits an excellent response to visible light, especially light with low intensity, allowing for the electrical interconnection, highly active surface, appropriate orientation, and a good contact with the substrate. There are great benefits in improving the technique for detecting the weak light source signals.     

A study of coated-wire potassium-valinomycin and sodium-monensin ion-sensing systems by use of a conventional field-effect transistor

Polymer films containing a sodium or potassium ion-selective exchanger were coated onto platinum wire and incorporated into a potentiometric arrangement. Comparative results obtained by utilizing different measuring devices, one a conventional pH-meter and the other a field-effect transistor (FET) in series with an electrometer, are discussed. The linear range of either system is comparable with that of other electrochemical techniques. Possible applications of such a device are described.     

Self-organization of ZnO wrinkles oriented by patterned PMMA templates

In this study, well-ordered ZnO wrinkles were fabricated by sol–gel derived ZnO films via heat treatment at 400 °C for 2 h with the assistance of polymethymethacrylate (PMMA) templates. PMMA templates with various patterns were employed to investigate the effects of template geometries on arrangements of the formed ZnO wrinkles. The influence of thickness on the transformation of the entire resulting structures of the spin-coated ZnO sol–gel films is discussed. With 2-dimensional arrangement of PMMA patterns, more than two orientations of wrinkled ZnO can be induced in one template. By controlling the arrangement of PMMA patterns and thickness of ZnO sol–gel films, specific circular, hexagonal, triangular and triangular-star complex structures were produced due to the constrained shrinkage force of the ZnO films applied toward the PMMA side walls during drying. By this technique, we can obtain 3-dimensional ZnO structures that conventional photolithography and nano-imprinting lithography are hard to achieve in few process steps.     

Sensitively probing the cofactor redox species and photo-induced electron transfer of wild-type and pheophytin-replaced photosynthetic proteins reconstituted in self-assembled monolayers

An ultrathin, ordered, and packed protein film, consisting of the 2-mercaptoacetic acid (MAA), polydimethyldiallylammonium chloride (PDDA), and wild-type (WT) photosynthetic reaction center (RC; termed as WT-RC) or its pheophytin (Phe)-replaced counterpart (termed as Phe-RC), was fabricated by self-assembling technique onto gold electrode for facilitating the electron transfer (ET) between RC and the electrode surface. Near-infrared (NIR)-visible (Vis) absorption and fluorescence (FL) emission spectra revealed the influence of pigment substitution on the cofactors arrangement and excitation relaxation of the proteins, respectively. Square wave voltammetry (SWV) and photoelectric tests were employed to systematically address the differences between the WT-RC films and mutant ones on the direct and photo-induced ET. The electrochemical results demonstrated that ET initiated by the oxidation of the primary donor (P) was obviously slowed down, and the formed P⁺ had more population as well as more positive redox potential in the Phe-RC films compared with those in the WT ones. The photoelectrochemical results displayed the dramatically enhanced photoelectric performances of the mutant ones, further suggesting the slow-down formation of final charge-separated state in Phe-RC. The functionalized protein films introduced in this paper provided an efficient approach to sensitively probe the redox cofactors and ET differences resulting from only minor changes in pigment arrangement in the pigment–protein complex. The favored ET process observed for the membrane proteins RC was potentially valuable for a deep understanding of the multi-step biological ET process and development of versatile bioelectronic devices.