RICERCA ITALIANA     

Research program

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

University Co-ordinator

Università degli Studi di CATANIA - SCIENZE CHIMICHE - ()

Research Unit Leader

Salvatore Sortino

Description

Light-initiated processes in porphyrin compounds represent nowadays a topic involving interdisciplinary areas of the modern scientific research spanning from chemistry and physics to biology and medicine. The thread of the research program of the RU of Catania is the combination “porphyrins-light”. The RU has gained a considerable expertise in the field of photoinduced processes in a variety of molecular and supramolecular systems as well as in nanoparticles and thin films. In particular, the RU intends to exploit the multifunctionality and the multiplexicity of the UV/Vis radiation both as investigation tool able to provide insights into the self-organization processes of porphyrin compounds and appropriate external stimulus to control the molecular organization as well as to activate specific functions.
The research program aim is addressed at investigating the organization of porphyrin-based compounds of potential interest in molecular electronics and optoelectronics, biomedicine and sensing, 1) in solution, 2) in nanoparticles, 3) in thin films.

1) Photoinduced processes in solution

In this activity the RU will be taken into account porphyrin derivatives able to self-organize in supramolecular complexes characterized by tunable optical properties. In particular, the attention will be mainly paid on Zn-porphyrin dimers and porphyrin-cyclodextrin conjugates. Although different, both systems represent suitable models for studying interchromophoric interactions through energy and electron transfer processes important for the development of nanodevices for optoelectronics and biomedical applications.
Very promising preliminary results recently obtained in collaboration with the RU of Lecce have shown that dimeric Zn porphyrins are able to change their molecular arrangement (syn to anti) in the presence of appropriate ligand species (i.e. alcohols, amines, pyridines) able to coordinate the metal center. Such conformational switching is controllable even at the water-air interface [1], and it is accompanied by remarkable changes of the steady-state and transient absorption and emission of the porphyrin units. In view of these results, the idea to pursue is to investigate the properties of these derivatives in the presence of suitable photoactive ligands whose coordinating properties can be reversibly controlled by photons of different energy. Such strategy is addressed at obtaining supramolecular complexes characterized by new spectroscopic and photophysical properties that can be controlled by external light stimuli. Such investigations will represent a crucial starting point for the development, in collaboration with the RU of Lecce, of mono and multilayer Langmuir-Blodgett films based on the same chromophoric units tailored for their deposition on solid surfaces.
The use of porphyrin-cyclodextrin conjugates is an appropriate strategy to confer certain solubility in aqueous medium of poor hydrosoluble porphyrins as well as to introduce induced chirality effects on the porphyrin units through the presence of the chiral oligosaccharide. On the basis of preliminary results already obtained in collaboration with other research groups, the RU of Catania aims to investigate a three-fold aspect of these systems. The first is related to their potential to self-assembly at nanoscopic level, originating structures with peculiar photophysical properties that, in turn, can be controlled by parameters such as temperature, pH, solvents, etc.); the second is related to their to incorporate within the cyclodextrin units suitable guest molecule able not only to trigger new molecular arrangements of the nanostructures but also to communicate with the porphyrin units through energy and/or electron transfer processes induced by light; the third aspect is related to the potential of these porphyrin-cyclodextrin conjugates to photosensitize the formation of “singlet oxygen”, the main active species in photodynamic cancer therapy (see, base di partenza for details).
Finally, the RU aims also to shed light into the self-organization processes of the above porphyrin compounds together with new derivatives in biological mimicking systems such as micelle and vescicles with different size and charge.
In order to carry out this research activity the combination of several steady-state and time-resolved spectroscopic techniques such as UV-Vis absorption and emission, polarized fluorescence, induced circular dichroism, nuclear magnetic resonance, laser flash photolysis with nanosecond time resolution, time-resolved UV-Vis phosphorescence with microsecond time-resolution, time-resolved IR luminescence with sub-microsecond time-resolution will be used. Furthermore, calorimetric measurements, particularly appropriated for the study of the organization phenomena, will be performed in collaboration with the RU of Bari.

2) Photoinduced processes in nanoparticles

This research activity aims to develop aspects strictly related to the formation, characterization and activation of systems based on nanoparticles and addressed to potential applications in the field of photodynamic cancer therapy, optoelectronics and biosensing. Also in this case, the RU intends to exploit the light both as a tool to investigate the properties of pre-organized porphyrins and as external trigger to promote the self-organization and activation of porphyrin-based nanoparticles.
A recent study carried out with the research unity of Messina [2], which represent a stimulating starting point for the development of part of the present project, has highlighed the promising photodynamic properties of heterotopic nanoparticles of a sulfonated tetraphenyl porphyrin entangled within a novel class of cationic amphiphilic cyclodextrins. Such nanoparticles systems are characterized by specific organization and related photochemical properties depending on the content of porphyrin within the cyclodextrin network. The complete photochemical and photobiological characterization of this “non-covalent” system has revealed that, under appropriated experimental conditions, it present the ideal for a “carrier-photosensitizer” systems, such as effective production of singlet oxygen, good delivery properties within the cell compartment and remarkable cell death upon visible light excitation. On the basis of these very promising results the RU of Catania, in collaboration with the RU of Messina, aims to extend such investigation to novel types of porphyrin-containing nanoparticles characterized by the presence of galactose units in the structure of the amphiphilic cyclodextrins with the goal to further improve the carrier properties of the whole systems preserving the photodynamic activity.
Another goal of the RU is the self-assembling of thiol-terminated porphyrins on the surface of metal nanoparticles in order to obtain photoactivable metal protected clusters (MPC). To this aim, the RU will exploit a recently developed, straightforward, effective and cheap methodology allowing the obtainment of platinum nanoparticles from either organic or aqueous solution, through a “one-step” process involving a platinum complex and the light as the sole reactants [3]. This well-tested protocol is extendible also to the preparation of other metal nanoparticles and will facilitate the self-organization of the porphyrins on the metal surface simultaneously to its photostimulated formation. The final goal is to fabricate MPC which, in contrast to those based on gold nanoparticles, exhibit effective response to light excitations in terms of fluorescence, triplet and singlet oxygen quantum yield in view the expected suppression of the porphyrin to metal energy transfer processes due to the high value of the resonance plasmon absorption band of the platinum. Furthermore, the RU aims at extending such investigations to “semiconductor-porphyrin” systems with the goal to fabricate “bichromophoric” nanoparticles also with potential optoelectronics applications. In fact, in view of the absorption properties of the semiconductor nanoparticles, it will be possible to activate energy and electron transfer processes through the selective excitation of either the porphyrin or the semiconductor unit. For a complete and detailed characterization of these nanoparticles, together to the variety of spectroscopic techniques illustrated at the point 1, the RU may rely on of the consolidated collaboration with STMicroelectronics will allow to perform SEM and TEM analyses aimed at obtaining structural and dimensional information.


3) Photoinduced processes in thin films

This research activity is addressed to investigate the spectroscopic, photochemical and structural properties of porphyrin derivatives self-organized in thin films, prepared by Langmuir-Blodgett (LB) and Self-Assembling (SA) techniques, of potential applications in the fields of optoelectronics and sensing. Recent studies carried out in collaboration with the RU of Lecce on LB films of the dimeric Zn-porphyrins [4], already described at the point 1, have shown that it is possible to control the molecular arrangement and the consequent steady and transient spectroscopic properties of the porphyrin units through the appropriate choice of the sub-phase. On these grounds, the collaboration between the two RU, will aim to extend these investigations to chiral and/or photoactive films through the insertion in the subphase of suitable chiral and/or photoactive ligands able to coordinate to the metal center and to originate LB films characterized by dichroic and/or photocontrollable properties. The development of this activity will also allow the interaction with the RU of Messina. The systems based on cationic amphiphilic cyclodextrins prepared by this RU (see point 2) may in fact represent fascinating “building blocks” for the fabrication of photoactive LB multilayers through the self-assembling of anionic porphyrins encouraged by electrostatic interactions.
The organization of porphyrinic systems on transparent metal substrates is an intriguing goal in the perspective to develop electro-optical nanodevices for differentiated applications. At this regard, the RU of Catania, in collaboration with STMicroelectronics, has recently published a simple, effective and cheap methodology for the preparation of ultrathin platinum films (ca. 10 nm) exhibiting, for the first time, the combination of high optical transparency, electrical conductivity and robustness and homogeneity [5]. Such substrates have shown ideal for the fabrication of photo- and electro-chromic self-assembled.monolayers (SAM) [6,7]. The RU has in fact shown the first examples of molecular switches whose optical properties can be reversibly controlled by electrical or light stimuli and easily monitored through conventional spectrophotometers in the transmission mode. In this context, another objective of the RU is the self-organization of porphyrinic systems though both LB and SA techniques on these platinum electrodes. The possibility to incorporate on these electrodes porphyrin-based systems with peculiar optical and photochemical properties as well as sensitive to specific analites, will offer a great advantage with respect to the gold semitransparent electrodes previously reported. The absence of the plasmon absorption band in the case of the ultrathin platinum films in the whole visible region and the high molar absorption coefficient of the porphyrins in the same region represent the appropriate combination in order to facilitate the monitoring of both the self-organizing process and the modification of the optical properties through conventional transmission spectroscopy, even at low percentage of surface coverage. For the spectroscopic characterization of the above systems, the RU, besides the conventional spectroscopic techniques, will avail of: a fluorimeter equipped with a double monochromator and front face apparatus, appropriated for measurements on solid samples, a laser flash photolysis systems adaptable also for measurements on transparent solid films, a spectroelectrochemical systems adapted for measurements on the above-mentioned ultrathin transparent platinum electrodes and, in collaboration with the RU of Bari, of AFM analysis.


[1] V. Borokov, S. Casilli, S. Conoci, Y. Inoue, S. Sortino and L. Valli, Surf. Sci, 2004, 572- 66-76.
[2] S. Sortino, A. Mazzaglia, L. Monsù Scolaro, F. Marino Merlo, V. Valveri and M. T. Sciortino, Biomaterials, 2006 in press.
[3] S. Giuffrida, G. Ventimiglia, S. Petralia, S. Conoci and S. Sortino Inorg. Chem. 2006 (45) 508-510
[4] L. Valli, S. Casilli, L. Giotta, B. Pignataro, S. Conoci, V. V. Borovkov, Y. Inoue, and S. Sortino, J. Phys. Chem. B, 2006( 110) 4691-4698.
[5] S. Conoci, S. Petralia, P. Samorì, F. M. Raymo, S. Di Bella and S. Sortino, Adv. Func. Mater. 2006, in press
[6] S. Sortino, S. Petralia, S. Conoci and S. Di Bella, J. Am. Chem. Soc. 2003 (125) 1122.
[7] S. Sortino, S. Di Bella, S. Conoci, S. Petralia, M. Tomasulo, E. J. Pacsial and F. M. Raymo, Adv. Mater. 2005 (17) 1790.