RICERCA ITALIANA     

Research program

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

University Co-ordinator

Università degli Studi del SALENTO - INGEGNERIA DELL'INNOVAZIONE - ()

Research Unit Leader

Ludovico Valli

Description

The Unit in Lecce will follow four different, although strictly correlated, subjects permitting thorough interactions with the other Units involved in the project and STMicroelectronics.
i) Most of electron transport systems in biology are involved with bioenergetics, the photosynthetic system being the best characterised. The fundamental event is light-induced primary charge separation. There are special complexes, known as Light Harvesting Complexes, consisting of proteins and tightly associated porphyrins, chlorophylls, which absorb light energy and transfer it to a special pair of chlorophyll a molecules located in the Photosynthetic Reaction Centres (PRC). An electron is promoted into an excited state and can then pass to an acceptor of lower redox potential called pheophytin and hence to a secondary acceptor, a quinone. Hence electrons can flow down through the protein-bound pigments and be coupled through a tyrosine residue and Manganese atoms to the splitting of water.
Surface modification through these molecules enables the electron transfer to be directed to external electrodes. There is considerable scope for exploiting biological electron transfer systems in the context of molecular electronics. A great effort has been recently dedicated to the construction of thin films containing chlorophyll a: we shall use the LB method to fabricate films of pure chlorophyll a and its mixtures with a strong electron-acceptor, C60. It is surprising to check that the reports concerning the C60-porphyrin dyads are very numerous, while the system chlorophyll a/fullerene is almost absent. It is interesting to study photoinduced phenomena in such a blend: the powerful artificial acceptor C60 (in solution capable of capturing up to six electrons with a negligible rearrangement energy) and the natural system of chlorophyll a. Photophysical properties of chlorophyll a are strictly dependent on its supramolecular organization, so that it is extremely important to investigate its aggregation in thin films. FTIR spectroscopy can be effectively employed for identifying the formation of oxygen- or water-coordinated aggregates, which involve peripheral carbonyl groups. The characterization of chlorophyll a/C60 systems in terms of aggregation would allow relating supramolecular organization to photoelectric properties. Different types of chlorophyll molecules other than chlorophyll a, natural or artificial, could be tested for improving photophysical properties. The presence of gases and solutes may affect the aggregation states of chlorophyll, so that it can be interesting to investigate the molecular changes induced by these species by perfusion-induced ATR-FTIR spectroscopy. This innovative technique has been employed with success for probing redox changes of porphyrin cofactors in membrane proteins.(vide infra) The attraction of the method lies in the ability to exchange the medium above the surface film onto the ATR crystal so that the properties of the buried surface seen by the IR beam are modified. Thus, ATR-FTIR difference spectra of chlorophyll may be recorded as aggregation changes are induced by change of perfusant that was flowed continuously above the surface of the hydrophobic film allowing to identify functional groups involved in structural modifications.
All other Units in the project have previously acquired noteworthy competences in the investigations of chlorophyll assemblies and aggregates.
Various artificial molecular devices have been fabricated by mimicking the electron transport function of biological photosynthesis. We have just begun to incorporate meso-tetra(4-sulfonyl)porphyrin into multilayer assemblies by a mixed method using the Langmuir-Schäfer and ionic self-assembly technique, depositing a fulleropyrrolidine derivative bearing a positive charge and the anionic porphyrin. This technique can be used for oligo-charged species, such as disk-shaped porphyrins containing substituents other than sulfonic groups, such as phosphonic pendants. Assembly of these molecules into multilayer assemblies can find application in catalysis, voltaics, electrochromics, non-linear optics, and so on. In particular, in this approach a solution of the C60 derivative was spread on the water surface, while the porphyrin bearing peripheral anionic groups was dissolved into the subphase. Evidences of the interactions between the two moieties at the interface have been obtained from the analysis of the floating layers by pressure/area curves. Brewster Angle Microscopy and UV-Vis Reflection Spectroscopy directly at the air-water interface will be carried out to gain further confirmation of the interaction between the two different constituents. The characterization of the deposited films by UV-Vis spectroscopy reveals that the two constituents behave as discrete and weakly interacting -systems.
The use of polarized light suggests the existence of a preferential direction of macrocycle rings with an edge-on arrangement with respect to the substrate surface. Finally, preliminary photoaction spectra recorded from films deposited by only one horizontal lifting onto ITO electrodes evidenced the generation of photocurrent; its dependence on transfer surface pressure now needs to be clarified.
Finally IR spectroscopy can provide information about the electronic and vibrational properties of both donor and acceptor moieties and therefore about the electronic interactions between the chemical units. Upon charge transfer, in a donor-acceptor system there is a variation in the electronic levels of both materials and consequently a reorganisation of their structures; this can give a different IR pattern.
The research subject of item i) implies deep collaboration with the RU in Bari and STMicroelectronics.

ii) Another fruitful research area is the study of conformational equilibria directly on the water surface for bis-porphyrins in which the two macrocycles are connected by a sature bridge –C2H4-. In particular a dimer of Zn-octaethylporphyrin has been investigated; it exists as the conformers syn and anti which exhibit different spectral characteristics (Fig. 1). The corresponding conformational switching involves that B band intensity of syn conformer, centred at 397 nm, gradually diminishes when equilibrium is shifted towards the anti conformer; at the same time a splitted band centred at 420 nm appears. For the first time we have transferred such an investigation from solution to the air-water interface and on LB or LS films; we also succeeded in shifting the equilibrium towards one of the two conformers by adding in the spreading solution ligands of different strength (amines, alcohols, fatty acids).


Figure 1


A further development has been made through fluorescence and transient absorption measurements carried out in collaboration with the group in Catania. Fluorescence has been observed in LB films containing both bis-porphyrin and arachidic acid only when a high molar ratio (1:30, respectively) has been used. In other cases when an equimolar ratio has been used fluorescence has been quenched; the same effect has been detected with alternate films of the same two componenents. The rationale of such phenomenon in the last cases is that fluorescence quenching is a consequence of intermolecular aggregation of porphyrin moieties. Therefore, in the film obtained with a large excess of fatty acid, the arachidic acid molecules act as spacers thus avoiding (or at least substantially limiting) aggregation of macrocycles. Fig. 2 illustrates the two different cases: fluorescence quenching in the upper part and fluorescence in the case of bis-porphyrin molecules dispersed in the fatty acid matrix.


Figure 2
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A natural improvement of this research is now the study of chirality induction on the investigated bis-porphyrin directly at the air-water interface. A supramolecular tweezer is defined as a specially shaped type of host-guest complex arising from the insertion of a guest molecule between two binding sites of a molecular host through various noncovalent interactions, which lead to a fascinating spatial architecture, resembling a pincer holding an object (Fig. 3). Although only a few bis-porphyrin based chiral tweezers have been reported thus far, they are of prime interest not only due to their sophisticated molecular design, but also due to diverse practical applications (molecular recognition, asymmetric catalysis, chemical sensors, chirality memories).


Figure 3


We shall use (1S,2S)-(+)-diaminocyclohexane as guest molecule inside our bisporphyrins. Preliminary investigations performed on films at the air-water interface and immobilised on solid supports are very promising; we want to modulate the properties and capabilities (molecular recognition, chemical sensing, chirality) of Langmuir and Langmuir-Schaefer films by varying the diamine concentration in the water subphase and also the surface pressure.
Moreover the presence of H- or J-aggregates is typical in LB films. With the RU in Bari, the two-photon absorption (TPA) properties of the characterized J-aggregates will be investigated in order to verify the theoretically predicted optic non-linear response increment in these organised systems, for their potential use as optical limiting devices.

iii) A great deal of research into the behaviour of LB films is concerned with the measurements of changes in their physical properties in response to the presence of low concentrations of gases or analytes in solution. LB chemiresistors are simple devices to produce and sometimes show enormous conductivity changes of the active layer upon exposure to a few ppm of analytes. Resistive sensors exhibit a voltage or current output and in this circumstances in order to obtain a digital output signal or even an output frequency, a voltage to frequency converter must be used. This adds some complications in these sensors in terms of post-processing circuits. This problem can be avoided making use of optical or piezoelectric transduction methods. Even though our RU has gained significant expertise in the field of optical sensing, we are moving our interest also towards the use of QCM apparatus. The resonant frequency change (?f) of a piezoelectric crystal induced by the interaction with an analyte in gas phase can be expressed according to the Sauerbrey equation. From this equation it is possible to infer that for a crystal able to oscillate at about 10 MHz, a mass sensitivity ?f/?m of the order of 0.5 Hz/ng/cm2 can be considered possible. The resolution limit is estimated to be around 10-11 g.
But in a suitable oscillator configuration piezocrystals also can oscillate in liquids. In this case the frequency of oscillation is dependent on other parameters such as viscosity, density and conductivity of the solution. We want to make use of piezoelectric quartz crystal oscillators resonating in aqueous solutions containing phenols derivatives, already reported by USEPA as priority pollutants. The crystal will be coated with sensitive macrocycle (phthalocyanines and porphyrins from the RU in Messina) layers which have the ability to adsorb solute molecules from the aqueous phase, thus gaining mass and causing a shift in the oscillating frequency of the quartz device.
The thorough interaction with the RUs in Bari and Messina will involve the check of immobilisation of Photosystem II onto QCM crystals and of the subsequent interaction of immobilised PSII with analytes of environmental interest, such as herbicides and phenols.

iv) The set-up of a suitably up-graded FT-IR spectrophotometer will be employed in order to acquire spectra in differential modality. The horizontal sampling system, where the sample is deposited on a few millimeter diameter microprism, the internal reflection element (IRE, sealed in a stainless steel plate), is particularly versatile and is shown in Fig. 4.


Figure 4


The sample is then within the reach of the scientist who can operate on its external surface without perturbing the interaction with the IR beam. The evanescent wave indeed penetrates 1-2 µm within the sample deposited on the IRE, thus interacting only with the thin layer in contact with the prism. Our ATR accessory has been suitably modified and improved by the construction of microcells These cells allow obtaining micro-chambers near the IRE, where it is possible performing different chemical and physical treatments on the sample and simultaneously monitoring the effect of the treatment by the acquisition of IR spectra. Our system is equipped with two kind of cells: flow cells and electrochemical cells. The flow cells (Fig. 5) present two holes to allow suitable solutions to flow within the chamber. A peristaltic pump permits to set the fluid flow rate and to keep it constant within the cell.


Figure 5


The chemical composition of the solution can be chosen and modified as required by the experiment. Moreover, by means of the pump and a computer-based switching system, it is possible to send alternatively two solutions onto the sample for several cycles, in order to register direct and inverse difference spectra and to evaluate the reversibility of sample transitions. Through this technique it will be possible to successfully investigate molecular recognition phenomena between porphyrin macrocycles and small soluble molecules, biologically or environmentally relevant. The analysis of difference bands allows in this case to evaluate both the affinity constant and the macromolecule portion involved actively in the recognition process. Porphyrin and phthalocyanine materials, presenting signals also in the visible region, can be studied in addition by means of an analogue cell, equipped with a fiber optics reflection probe, that sends visible light towards the sample and collects the reflected photons sending them to a suitable detector, as illustrated in Fig. 6.


Figure 6


Hence it is possible to drive out simultaneously infrared and visible spectroscopy increasing the range of data obtainable. Redox transitions in macrocycle films can be studied either chemically, using suitable solutions containing oxidant and reductant agents, or electrochemically by means of the apparatus shown in the Fig. 7.


Figure 7


In this configuration the upper part of the chamber is represented by a glassy carbon disc, which acts as working electrode and which is positioned close to the sample deposited on the prism. Thus the sample can rapidly equilibrates with the redox potential of the working electrode. To poise the potential, the working electrode, a platinum counter electrode and an Ag/AgCl reference electrode are connected to a conventional potentiostat.
It is clear that the ATR apparatus is suitable for various applications and operating conditions. As far as porphyrins systems are concerned, photo-induced processes are particularly interesting. The investigation of these processes will be carried out by means of the apparatus shown in the Fig. 8. Here a visible beam, even coming from a laser source, is sent perpendicularly on the sample, triggering photochemical changes, which can be studied analysing the resulting IR difference bands.


Figure 8