RICERCA ITALIANA     

Research program

Nanoscale self-assembled porphyrin based complexes: properties and technological applications

University Co-ordinator

Università degli Studi di BARI - CHIMICA - ()

Research Unit Leader

Angela Agostiano

Description

The Bari research unit (RU) will be interested in the characterization of supramolecular complexes based on photosynthetic pigments and proteins. It is well known as most of the photosynthetic molecular machines show a hierarchical organization of components at the nanoscopic level which can be used as model for new materials and functional systems which can be suitably addressed toward energy transformation and data processing applications. The study will be developed through two principal research lines. On one hand the nature, intensity and specificity of intra-molecular interactions of natural and synthetic porphyrins will be studied. On the other hand the properties of these systems of self-assembling in supramolecular structures with progressively hierarchic organization will be verified in order to address them toward specific application in the field of energy conversion, sensor devices and biomedicine.
1) STUDY OF MOLECULAR SELF-ASSEMBLY PROCESSES OF PORPHYRINS AND PHTHALOCYANINES.
Chlorophyll is the molecule that presides over processes both of capture and harvesting of light, energy transfer and charge separation. The chlorophyll dual functionality is due to its capability of modulating its molecular organisation, therefore the mutual orientation and distance between the chromofores inside the micro and macro environment in which it is inserted. From this point of view supramolecular structures generated by self-aggregation processes of natural and chemically modified chlorophylls in solution or in different systems (thin films onto electrode) can supply useful information for understanding the pigment molecular organization in photosynthetic organisms and for developing new photoactive artificial systems able to combine molecules with specific functionality in devices able to work in the capture and conversion of light energy. In the Bari RU laboratories, characterized by a well-established experience in photosynthetic systems, a study will be performed on the molecular organization of natural chlorophylls, extracted from photosynthetic organisms, both in the Water-Rich-Region and in lipid-based microheterogeneous systems. The investigation will be extended to Zn-Chloropyll and Zn-pheoporbide, synthetized in the laboratories of the Prof. Hugo Scheer. The results obtained with chlorophyll c, basis pigment in chlorosomial antenna systems, will be particularly interesting for its potential utilization in artificial antenna system. Particular attention will be paid to aggregation processes of phthalocyanines, model molecules of natural pigments. It is well known that, in solution, phthalocyanines spontaneously form J- or H-aggregates, with peculiar spectroscopic and electronic properties, which make these species exploitable as molecular devices. In this RU phthalocyanines aggregation processes will be investigated in different solvents with the aim of evidencing the influence of chemical-physical parameters like dipole moments, dielectric constant, solvent number and the solvent ability of coordination to the central metal. Free base and different metal (Pd, Zn, Mg, Cu, V and Fe) phthalocyanines will be considered. Furthermore, phthalocyanines with a wider extension of the conjugation obtained by the insertion of different moieties in the macrocycle, as ester groups, alkylic chains with different lengths or saturation degree, will be examined. Finally, the influence of symmetry and molecule charge state on aggregation properties will be considered. The nature of pigment interactions will be investigated by UV-Vis-NIR Spectrophotometry, Fluorescence, Circular Dichroism and NMR. Dynamic (DLS) and Resonance Light Scattering (RLS) will allow the evaluation of the aggregate dimensions. In collaboration with the research unit of Catania, time resolved spectroscopy will be used to study the photochemical properties of pigment aggregates. The pigments in the systems under investigation can be deposited as monolayer at the water/air interface by Langmuir-Blodgett (LB) tecnique on different substrates. The pigment fluorescence or the addition of appropriate fluorescent probes will allow the use of Fluorescence microscopy and Brewster Angle Microscopy. Both techniques are sensitive to aggregation processes, to the formation of different domain in the monolayer and to photochemical processes. The pigment-pigment or pigment-lipids monolayers formation will be monitored also by reflection UV-Vis spectroscopy in the laboratorie of the Lecce RU. Redox properties of pigment aggregates will be also investigated by electrochemical and spectroelectrochemical techniques. The photoelectrochemical properties will be tested by means of modified electrodes. The deposition on electrode will be performed by spin coating or LB technique in the laboratory of the RU of Lecce. On these systems additional thermodynamic studies will be carried out in order to investigate the effects of varying pigment concentration and solvent properties. Biomimetic systems and classical solvents with different polarity will be used. The studies will be based on measures of volume and enthalpy, besides heat capacity, performed by the differential scanning calorimetry.
Thanks to the presence of a research unit of Padova University, the two-photon absorption (TPA) properties of the characterized J-aggregates will be investigated. These studies can experimentally be used to verify the theoretically predicted optic non-linear response increment in these systems, for their potential use as optical limiting devices. The measurement of the TPA coefficient will be performed, together with pump and probe experiments (excitation between 400 and 800 nm, detection between 450 and 1100 nm) with a 150 fs time resolution and a 1 ns time interval. These measurements will be done with the Z-scan technique and will supply additional information about the potential application of these systems as photosensitizers in the Photodynamic therapy (PDT). Data on the pigment aggregation will be analysed by theoretical model based on stochastic simulation of the kinetics of such processes. These simulations will be performed using algorithms and programs developed ad–hoc also thanks to the collaboration with the reaserch unit of Naples CNR, present in our RU.
2) ORGANIZATION OF PIGMENTS IN HIERARCHIC SUPRAMOLECULAR COMPLEXES
-hetero-junctions of pigments/inorganic nanocrystal semiconductor in solar cells.
In this research thread, hetero-junctions of inorganic nanostructured semiconductor/organic molecules, as photosynthetic pigments, porphyrins and phthalocyanines, will be projected. In particular in this study the photophysical and photoelectrochemical processes on charge transfer and separation at the interface between the two materials will be characterized. Great interest has been drawn toward the study of hetero-junctions of wide band-gap semiconductor materials sensitized with suitable pigment. The use of sensitizer modifies the surface of semiconductor materials making them suitable for photoelectrochemical application in the visible range. Semiconductor nanocrystals, intermediate state between macroscopic solid and atomic or molecular system, are characterized by optical and electrochemical properties that can be modulated by nanocrystal size. Further, the reduced dimension results in a high surface extension of nanosized semiconductor film. The nanosizes TiO2, ZnO, CdS and CdSe semiconductors will be utilized. These large band-gap semiconductors need to be associated with molecules that absorb in visible range of the solar spectrum and give rise to a charge transfer at the surface. The study will be carried on by using modified chlorophylls with different functional groups for modulating their electronic properties and hydrophobic character, an important parameter for regulating the interaction with the semiconductor surface. Further as in the photosynthetic light-harvesting complex the high efficiency of absorption and light energy transfer processes is related to the presence of non covalent bonds between the different pigments, we intend to study the condition in which these pigments can self-assembly forming supramolecular complexes on the nanosized semiconductor surface. The following charge transfer processes between pigments should induce the formation of charge-separated states of longer lifetime compared to those of the single pigment, thus enhancing the efficiency of the sensitization process. These considerations will be supported by theoretical studies. In addition to photosynthetic pigments, particular attention will be paid to their synthetic model, the phthalocyanines. These compounds result to be more stable at thermal treatments and are characterized by stable charge-separated states. We intend to study the phthalocyanine/nanocrystalline semiconductor hetero-junctions varying both the solvent and the deposition techniques, and changing also the thermal treatment modalities which give different aggregation characteristics to the deposited dye film. Photocurrents and photopotentials will also be measured. Finally porhyrins, ad hoc synthesized in order to shift the molecule absorption maximum toward red region of spectrum as much as possible, will be tested. At the beginning pigment mixtures will be studied in solution by means of UV-Vis-NIR and FTIR spectroscopy, allowing the evaluation of the interaction between pigments; therewith steady-state and time-resolved Fluorescence will be used with the aim of following the radiative and nonradiative decay of excited states and the energy and charge transfer processes. Photocurrents and photopotentials will be measured both using an “ad hoc” photoelectrochemical cell (sandwich cell) and a classical three electrode polarographic cell. Further, fluorescence anisotropy measurements will be carried out in collaboration with the Messina UR in order to highlight the pigment molecule preferential orientations on semiconductor surface. At last, the collaboration with the Lecce UR will allow the transfer of pigment films on different substrates, as for example flexible polymers, and the study of their optical and morphological properties by means of Fluorescence microscopy, Brewster Angle Microscopy. High resolution TEM and AFM measurements will allow the morphological characterization of the film.
-Pigment-protein complexes of PSII on substrate for biosensing application.
In this part of the project the Photosystem II (PSII) will be studied. The PSII is the pigment-protein complexe directly involved in the water splitting in higher plants. This photosystem (PS) is constituted by more than 20 polypeptides, some of them functioning as antennae. This research will be carried on by purifying and characterizing small PSII complexes, with particular attention to the content of associated lipids in order to study their influence on the dimerization process. The characterization will be done through Mass spectroscopy and NMR, whereas the aggregate formation will be followed by means of DLS measurements, carried out in collaboration with the Messina UR, and Time resolved Fluorescence. Again in collaboration with the Messina UR further factors influencing aggregation processes of these proteic complexes will be evaluated, for example temperature and irradiation effects on the aggregate growth. The acquired information from these studies will be used for PS immobilization procedures on suitable substrates for biosensing purposes. In fact PS are nanoscopic molecular machines able to perform an efficient charge separation which can be transformed in electric signal in properly designed devices. The stabilization of the charge separation is due to a photoinduced vectorial electron transfer among photosystem cofactors. Thus the change of the interactions between cofactors and binding sites due to herbicides or other toxic pollutants, can be measured by means of a proportional change of the electric signal. In addition to electrical techniques, in collaboration with the Lecce UR,the signal will be followed by using the Surface Plasmon Resonance (SPR), which is more sensitive to the binding site occupancy by molecules at higher molecular weight of the native. In order to PSII immobilization encouraging attempts have been effectuated by our research group onto Au and suitably functionalized dextran. Positive SPR signals, in fact, have confirmed the protein binding on the chip, allowing, in addition, to follow the protein dimerization processes on the substrate. The research will be carried on starting with small molecular complexes and realizing their functional aggregation on substrate also by means of the insertion in lipids which, as cardiolipin, are extremely important for the in vivo molecular organization. These assemblies will be characterized both at a structural level by using surface techniques (Lecce) and at a functional level through spectroscopic and electrochemical techniques. The ability of these systems of acting as biosensors for molecules altering photosynthetic activity will be verified in photoelectrochemical cells and suitably projected flow cells.
-chlorophylls and synthetic porphyrins/cyclodextrins complexes for Photodynamic Therapy.
In the last years the problem of water solubilization of hydrophobic solutes, as porphyrins and chlorophylls, has been object of several studies for the development of photosensitizing systems in medical field, as PDT. Recently in our laboratories a systematic study on the characterization in aqueous solution of synthetic and natural porphyrin has been carried out. The attention has been focused on the behaviour of three pigments: Chlorophyll a (Chl a), Chlorophyllide a (Chlide a) and meso-tetrakis-(4-pyridyl)-porphine (H2TPyP) using cyclodextrins (CDs) as delivery systems. CDs are cyclic oligosaccharides in a conical shape with a hydrophobic chiral central cavity, and a hydrophylic external surface, able to include substrates having suitable shape and size, modifying their chemical-physical properties and reactivity. On these inclusion complexes a study will be performed in order to evaluate stacking and hydrophobic interactions responsible of molecular aggregates. In detail inclusion complexes of the above mentioned three pigments with three different b-cyclodextrin (heptakis(2,3,6-tri-O-methyl)- -cyclodextrin (TRIMEB), heptakis(2,6-di-O-methyl)- -cyclodextrin (DIMEB) and hydroxypropyl- -cyclodextrin ( -HP)), characterized by equal external diameter, but differently functionalized external surface, will be studied. Fluorescence and absorption spectroscopic measurements will allow the determination of both complex stoichiometry and binding constants. The thermodynamics of the porphyrin delivery process in water will be studied by means of ITC (Isothermal Titration Calorimetry). Circular Dichroism and fluorescence quenching experiments will give information on the formation of aggregated specie, whereas RLS measures will indicate the presence of excitonic coupling. Both steady state and dynamic fluorescence anisotropy will allow the eventual presence of complexes with different geometry and stoichiometry to be revealed. These experiments will be done in collaboration with the Messina UR. The inclusion complex formation will be evidenced also by means of FTIR, NMR relaxation times, Cyclic Voltammetry and Spectroelectrochemistry. The singlet oxygen presence will be highlighted through phosphorescence decay measure effectuated at Catania UR. We intend also studying inclusion complexes and cells interaction with the aim of verifying compatibility both with pigment and CD concentration, by means of fluorescence and visible microscopy. Further singlet oxygen measures will be conducted in presence of cells in collaboration with Catania UR using a pump and probe technique for evaluating the inclusion complexes cytotoxicity in presence of light.