RICERCA ITALIANA     

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

Università degli Studi di Bari

Abstract

The present research project is primarily addressed to use the strong competencies developed inside the research units in the field of porphyrinic macromolecular systems in order to evaluate their potentiality for application in environmental, biological and material science. Three of the four proponent research units (RU), more precisely Bari, Lecce and Messina, thanks to a previously financed project (PRIN2002), developed integrated and/or complementary strategies of investigation crosswise to the specific interest fields, which contributed to develop new common research subjects and to coordinate their own approach to the study of the functional aspects of the studied systems toward the design of macromolecular complex of high modeling and technological value. Today this allows the individuation, among all the possible applications, of three areas on which the different RUs' scientific interests converge, and precisely the development of new material for energy conversion, optoelectronic and sensing, the environment and the biomedicine. These four units will share a great number of techniques, from steady-state and time-resolved spectroscopies, to calorimetry, electrochemistry, photoelectrochemistry and microscopy. In this field theoretical and computational support to the experimental activities of the RUs will be introduced in correlation with the prevision of molecular structures, the prevision and interpretation of spectroscopic parameters and the process modelling. The studies will be carried out on natural and synthetic porphyrin-based systems in solution or on substrate; for sake of simplificity, they will be organized within the three great applicative areas previously identified, even if, as the research description will clarify, most of the systems under investigation evidence transverse applicability. The most relevant objectives of the present research project are:
-structural and kinetic investigation on formation of porphyrin nanoaggregate also deposited by means of UV radiation.
-interactions of water soluble porphyrin with model proteins.
-heterojunctions of natural and synthetic pophyrins with nanosized semiconductors
-thin films of charge transfer complexes in molecular electronics.
-pigment-protein complexes on substrate for biosensing application.
-LB films of substituted bis-porphyrins selectively sensible to gas.
-porphyrinic systems for biochemical and surface stress-based sensors.
-chlorophyll and synthetic porphyrin water delivery through cyclodextrins.
-interactions of porphyrins with amphiphilic cyclodextrins.
-development of chemical-physical models for molecular and supramolecular systems of porphyrins.

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Principal Investigator

Angela Agostiano Università degli Studi di BARI

Research Objectives

The spontaneous organization of molecules in supramolecular structures is object of an enormous interest for the possibility of modulating mesoscopic structure and chemical-physic properties, choosing suitable basic molecular components. In this ambit natural and synthetic porphyrins are a particularly important class of compounds. These substances are involved in a huge amount of systems in which they are active and play very different roles depending on their interactions with the physic, micro or macroscopic, environment.
The present research project is mainly devoted to develop the well-established competencies of the proponent research units in the field of macromolecular porphyrin-based systems towards application in different fields of technological interest. Three of the four proponent research units (RUs), Bari, Lecce e Messina, in a previous PRIN project (PRIN2002), financed by MIUR, had built-up a national-wide research team to study the nature, intensity and specificity of inter- and intra-molecular interactions of porphyrins involved in energy transduction processes. This study has provided a huge amount of information at molecular level, which, in this project, will be used to orient the research toward the design of macromolecular complexes having high modelling and technological value, with a more applicative interest. From this point of view the insertion of the Catania RU introduces, in the project, specific photochemical competencies about the use of light in controlling molecular organization and/or in activating specific functions of porphyrins, with a well-established collaboration with the STmicroelectronics of Catania. The study will be organized within two main subjects. One will study the nature, intensity and specificity of intra-molecular interactions of natural and synthetic porphyrins. The other will verify their properties of organizing in supramolecular structures with progressively hierarchic organization in order to address them toward specific application in the two field in which the different RU interests converge: energy conversion, optoelectronic, sensoristic and biomedical delivery of biological molecules. The investigation strategies and integrated and/or complementary techniques of the four units will allow the sharing of a great number of techniques, going from steady-state and time-resolved spectroscopies to calorimetry, from electrochemistry and photoelectrochemistry to several microscopies. The whole set of techniques that will be used is summarized in the following scheme:

The collaboration with Prof. Bozio’s research groups of Padova University, realized by the presence of two units as external personnel, will allow the access to the two-photon absorption technique (TPA) permitting the investigation of non-linear optical properties of porphyrinic aggregates. Thanks also to the presence of an external unit from CNR of Napoli, a suitable theoretical and computational support to the experimental activities of the RUs will be introduced. The most important contributes of this unit will deal with the prevision and interpretation of spectroscopic parameters and the process modelling. The studies will be conducted on natural and synthetic porphyrin-based systems, in solution and on substrate, and, for sake of simplicity, will be organized inside of the two main applicative areas previously reported, even if, as the section devoted to the description of the research activity will clarify, the studied systems show quite transverse applicative potentialities.
The peculiarity of the Bari RU will be the pigment and photosynthetic protein characterization, studying porphyrin and phthalocyanine self-assembly processes together with their organization in hierarchical supramolecular complexes. Hetero-junctions of inorganic nanostructured semiconductors with photosynthetic pigments and related model compounds, as phthalocyanines, will be designed in view of their use in solar cells. Always in the same RU, higher plant Photosystem II immobilization on substrates for biosensing purposes will be optimized to maximize the electrical signal changes due to the presence of herbicides or other inhibitors of photosynthetic activity. Furthermore Bari will investigate porphyrin and chlorophyll solubilization in aqueous solution by means of cyclodextrins to develop photosensitizer systems in medical field, such as Photodynamic Therapy. At the same time Messina RU will study the interaction of porphyrins with amphiphilic cyclodextrins, which can delivery porphyrins inside tumoral cells, and on the formation of aggregates of porphyrins with albumin, one of the greater carriers of porphyrins in blood flow. In addition, Messina will conduct structural and kinetic investigation on J-type metallo-porphyrin nanoaggregate in presence of polyamines and spermine to prepare efficient antenna systems with dendritic morphology.
The photochemical properties of heterotopic nanoparticles of a sulfonated tetraphenyl porphyrin (TPPS) entangled within cationic amphiphilic cyclodextrins, the use of porphyrin-cyclodextrin conjugates to study induced chirality effects on the porphyrin units through the presence of the oligosaccharide and energy and/or electron transfer processes between cyclodextrin included guest molecule and conjugated porphyrin unit, will be studied by RU of Catania. This RU is also aimed to investigate dimeric Zn porphyrins in the presence of suitable photoactive ligands reversibly controlled by photons of different energy, for application as optical switches. Finally another goal of the Catania RU, in strong collaboration with the ST Microelectronics, will be the self-assembling of porphyrinic systems on the surface of ultrathin platinum films in order to obtain photo- and electrochromic self- assembled monolayer (SAM). The RU in Lecce, will be involved in the use of charge transfer biological systems in molecular electronics. Thin films containing mixtures of chlorophyll a and a strong electron-acceptor, C60, will be studied directly at the air/water interface with perfusion-induced ATR-FTIR Spectroscopy, Brewster Angle Microscopy and UV-Vis Reflection Spectroscopy. LB films of opportunely substituted bis-porphyrins, selectively sensitive to low gas level or analytes in solution, will be prepared for an envisaged use as optical or piezoelectric transduction sensors. At the same time this RU will study the chirality induction on the investigated bis-porphyrin directly at the air-water interface in order to obtain supramolecular tweezer complexes. 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.
The participation and the interactions among the research groups in the different application fields are summarized in the following scheme:
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Timescale

24 months

National and international background

Both naturally occurring and synthetic porphyrins are a large class of compounds, almost ubiquitous in nature and they perform several roles acting as prostetic group or cofactors in different proteic complexes (for example myoglobin and hemoglobin, chlorophylls and structurally related compounds in enzymatic complexes of photosynthetic organisms) essential for energetic metabolism. Porphyrins can act as catalyst and undergo reversible redox reactions in which the site of the electron transfer may be localized on the macrocycle or on the central metal ion eventually present. Both reactions are important for natural processes [1]. They also participate to energy and electron transfers [2,3] and can act as artificial light-harvesting systems [4], and so on. All these capabilities depend on the porphyrins ability of mutate their physico-chemical properties as a function of their molecular organization, which can be used in different application fields from photocatalysis [5] to optoelectronic [6,7]. In particular in photosynthetic organisms the interest has been devoted to the comprehension of chlorophylls supramolecular organization focusing on the self-aggregation modalities relative to the different functions carried out in the photosynthetic proteic complexes [8,9]. From this point of view recent studies have been conducted about the coordination state of the central metal with the aim of characterizing the pigment coordination state in antenna systems of photosynthetic bacterial organisms [10]. The molecular interactions with the proteic environment involve both central atom and porphyrinic macrocycle substituents, and has been widely studied in different model environments [8,11]. From these studies several information on the role of macrocyclic functional groups and on the influence of hydrophobic effect on the pigment aggregation have been obtained. Actually the interest in this field is addressed on one hand to the comprehension of supramolecular organization of light-harvesting pigment-protein complexes in photosynthetic organisms, on the other to the characteristics of typical self-aggregation processes in chlorosomes, where a great number of chlorophylls organizes themselves forming rod-like aggregates without protein support [12]. The attention was devoted also toward the use of hetero-junctions formed by nanostructured semiconductor films sensitized by organic molecules [13] and to biosensors for herbicides which make use of higher plant Photosystem II proteins [14]. Moreover great interest is addressed to the potential use of photosensitizers based on porphyrinic compounds in photochemotherapy. Natural porphyrinic compounds as bacteriochlorophylls, chlorophylls and substituted porphyrins, characterized by a different electronic conjugation, show noteworthy differences both in spectroscopic properties and red-ox properties, highlighting a strong red shift of the absorption bands which result to be very intense [15]. These peculiarity have stimulated the development of a new class of antitumoural drug based on chlorin and bacteriochlorin correlated compounds [16]. Particular interest in this field resides on the use of cyclodextrin (CD) as photosensitizer biodelivery systems. Various experiments have been conducted about the comprehension of the interaction between Chlorophyll a and CD by varying the structural characteristics of cyclodextrin [17,18]. The modulation of the porphyrin and phthalocynine structure, varying the central metal or introducing different moieties in the macrocycle, extending its conjugation, evidenced their optical non-linear properties [19]. Recently the two-photon absorption of porphyrinic aggregates has been used for studying the potentiality of these systems in the field of optical limiting devices [20] besides in the PDT[21].
The shift of modern technology towards molecular dimensions has various rationales. The signal propagation times of molecular gates are due in large extent to their small dimensions. If the gate is designed to operate via electron transfer or electron tunnelling a decrease in dimension will generate a consequent increase in speed. This consequence is ascribable to the fact that almost all gates are activated by the shift in the position of a charge carrier, and all charge carriers have mass. This mass implies the existence of a limit on how rapidly the variations can occur. Therefore, dimension and speed are intimately related [22]. In Nature, many eukaryotic and prokaryotic organisms employ the biological process of photosynthesis to convert light into chemical energy [23]. The early events have near 100% quantum efficiency, as energy-wasting charge recombination is reduced spatially by having the electron shuttled away a long distance from its point of origin, and kinetically by the low reorganisation energy. The replication of these two key properties has been the focus of research on artificial donor-acceptor systems designed to mimic the PRC [24]. The identification of the donor and acceptor identities is a fundamental task. The choice of the nature of the donor moiety is quite immediate, since porphyrinic derivatives have been generally employed. On the other side, concerning the acceptor characteristics, the peculiar properties of the "young" fullerene buckyball, connected with the monotonically growing availability of C60, have all contributed to identify it [25]. In fact, the remarkable electron accepting character of fullerenes makes them attractive moieties in several areas of chemistry [26,27]. The singlet state is in part deactivated to the ground state via a radiative process giving rise to a faint fluorescence band centred around 700 nm [28]. But the main deactivation pathway is intersystem crossing to give the triplet state with high yields. This triplet state can be monitored by means of a variety of techniques [29]. The fact that fullerenes are good electron acceptors and, at the same time, exhibit low-lying electronic states have noteworthy implications in photochemistry. Excitation of a porphyrin molecule in the presence of fullerene (the two moieties can be chemically linked or not) may result, in principle, both in an energy and an electron transfer quenching process. The large majority of last applications of such substances requires the immobilisation of the active layers (containing also porphyrin derivatives) in the form of coatings onto suitable substrates. Nowadays much effort has been directed towards considering how specific molecules with suitable functional groups may be manipulated and organised in molecular assemblies, with a high degree of compatibility with the existing planar technologies associated with micro- and opto-electronics industries. Thin film (20 nm- 10 mm) preparation requires the deposition of an active layer onto a substrate, that can be also flexible, such as mylar, without any consideration for possible defects and crystal periodicity matching. Multilayers of porphyrins, in typical supramolecular assemblies, can be deposited by a variety of methods [30], strictly connected to the applications of such moieties in the field of opto-electronics, controlling the distribution, orientation, and density of functional groups [32,33]. Besides their interaction with applied electric, magnetic or electromagnetic fields, porphyrins and metallo-porphyrins can also interact with other chemical species [34]. One might view such interaction as chemo-responsive rather then field-responsive. One example of such applications is that porphyrin solids, being highly porous, are involved in the current development of molecularly based molecular sieves or shape-selective solid catalists. Porphyrins and metallo-porphyrins have also been examined for a variety of sensor applications, further proving their importance as a class of chemo-responsive materials [34], owing to their environment-sensitive physical properties [35,36]. For example, LB films of such materials have been studied mainly as potential candidates for NOx detection [37].
Chemical processes initiated by light absorption (photoinduced processes) are strictly related to the human evolution, its activities and its natural environment. Porphyrin-based systems are suitable candidates for the development of a variety of light-controlled systems of technological interest, by virtue of their unique photophysical and photochemical properties. Photons-activated and /or -controlled processes in porphyrinic systems find, in fact, multifaceted applications in different fields spanning from molecular electronics and optoelectronics to sensing, catalysis and biomedicine [38,39]. Supramolecular structures of porphyrin compounds organized at nanoscopic level in solution, the understanding of the factors governing their formation and organization, and the study of their spectroscopic and structural properties, represent nowadays topics of great interest among the scientific community [40,41]. Specific attention has been recently paid to dimeric metal porphyrins as effective models for the study of photoinduced energy and electron transfer [42,43] in the perspective of the development of photoconductors [44] and chemical sensors [45]. Several methodologies addressed to the selective control of the molecular arrangement of these compounds in solution [46] and at the water-air interface [47] have been recently proposed. In the case of porphyrinic systems devoted to biotechnological applications, the solubility in aqueous environment is a highly desiderable requisite. A recent strategy is based on the use of porphyrin-cyclodextrin conjugates in which the two units are linked together by covalent bonds [48,49] and such systems are able of self-organizing in a specific fashion [49,50]. Porphyrin complexes find also large applications in photodynamic therapy as photosensitizers [51]. However, to be photochemically active, the porphyrin has indeed to be localized in close proximity of the bio-target. This issue has inspired an intense research activity devoted to develop bio-compatible delivery vehicles able not only to delivery the photosensitizer into the cell compartment but also to preserve its photodynamic activity [52]. Recent works have reported that nanoparticles in which the porphyrin is entangled in a network of amphiphilic cyclodextrins [53] represents very promising “non-covalent” systems for photodynamic therapy as they combine good delivery characteristics with preservation of the photodynamic properties. “Covalent” systems in which the porphyrin is bound to the surface of either metal or semiconductor nanoparticles by means of thiolic endings, constitue a valid option [18,54,55]. These three-dimensional nanoarchitectures, commonly referred as metal-protected clusters (MPC) are also well-suited for potential application in optoelectronics and sensing [56]. The dimensional control of the metal core is, of course, of great importance in the fabrication of MPC [57]. The self-organization of porphyrins in thin films is also a topic of increasing interest in the material nanoscience [58]. Langmuir-Blodgett (LB) and Self-Assembling (SA) techniques are the most exploited to this end [36]. Organization of porhyrins in self-assembled-monolayers (SAM) on noble metal surface requires their specific functionalization with thiol terminations obtaining structures characterized by some order degree by a S-metal linkage [59]. Actually it is particularly interesting the development of SAM of pophyrins on transparent or semitrasparent metal electrodes [60,61] being the simple transmission spectroscopy a powerful tool to shed light in the self-assembling process, to investigate the binding with small molecules [60] as well as to monitor the molecular switching induced by electrical stimuli [61]. In this context, platinum electrodes are a valid alternative to gold as the platinum plasmon absorption is located only in the far UV region [62,63].
Supramolecular chemistry has raised an enormous interest in the scientific community. Inspired by many examples coming from biology, supramolecular approach is based on a recognition process of different molecular entities through the interplay of a series of mainly non-covalent interactions. The resulting structures, even if maintaining the basic properties of the parent building blocks, can exhibit new features, both in terms of structure and reactivity, and consequently in terms of functions [64]. In this framework, porphyrins have been extensively exploited as building blocks, due to their rather facile functionalization, their favorable photophysical properties and their ability to act as macrocyclic ligands toward many metal ions. This latter feature provides an access for these ligands to the rich and mani-fold coordination chemistry, allowing an extended modulation of the electronic and steric characteristics and introducing magnetic and redox properties. All these properties are strongly influenced by the well-known tendency of porphyrins to self-aggregate. The extent of this phenomenon is governed by the presence of a large aromatic system, the hydrophobicity, the net charge and the steric hindrance [65]. The behavior of water-soluble porphyrins has been studied, due to the possibility of exerting a better control of the aggregation extent through a fine tuning of thermodynamic parameters, showing that some charged porphyrins are able to self-aggregate into large clusters having mesoscopic sizes, which can be described in terms of fractal geometry [66,67]. The growth dynamics of these systems have been investigated and explained in the frame of the theories developed for colloidal suspensions [68]. Many examples of extended porphyrin arrays are reported in literature, exploiting covalent bonds between porphyrin units and scaffold systems [69]. In this ambit supramolecular approach seems to be a promising methodology to access in a simple and quantitative way to complex structures [70]. On these bases, the interaction of charged porphyrins with polyelectrolytes gives the opportunity to enlarge the complexity of the aggregated structures and to get a higher control on the size and the degree of organization of the supramolecular aggregates. From this point of view, the interaction with charged biopolymers allows for a possibile tuning of orientation and chirality of the resulting species. Charged porphyrins interact with biological polyelectrolytes leading to aggregates, which are stabilized through a combination of electrostatic and dispersive (van der Waals) forces. The binding of porphyrins with nucleic acids [71,72] and various proteins [73] has been widely studied. The interaction with albumin, considered the principal endogenous carrier for porphyrins as photosensitizers in the PDT, is particularly interesting [74]. The above mentioned properties are the starting point for the application of porphyrins in biomedicine as spectroscopic probes for DNA conformations, as markers for tumors (TCPP,Biomoda Inc.) or as drugs in various diseases [75,76]. <<<