Synthesis of Paramagnetic Mechanically Interlocked Molecules

Mechanically interlocked molecular (MIM) architectures are examples of molecules where each molecular component is connected not through traditional bonds, but instead as a consequence of their topology.

An important example of MIM is represented by rotaxanes where a "dumbbell shaped molecule" is threaded through a "macrocycle". The introduction of one or more radical centers in rotaxane structures allows the use of EPR techniques to study their properties and in particular the relative movement and the distance of the molecular components. 
By making using of azide-alchine click chemistry which is compatible with the presence of persistent radicals we have developed synthetic methods for the introduction of sterically hindered nitroxide radicals like 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) both in the wheel and/or in the axle of rotaxanes. With these procedures rotaxanes containing both cyclodextrins or cucurbiturils as the macrocycles can be prepared and characterized by EPR spectroscopy. 
The synthesis of new classes of spin labels containing persistent radicals different from TEMPO is another active research field of the group.

Selected publications

• P. Franchi, V. Bleve, E. Mezzina, C. Schafer, G. Ragazzon, M. Albertini, D. Carbonera, A. Credi, M. Di Valentin, M. Lucarini, Chem. Eur. J. 2016, 22, 8745.
• R. Pievo, C. Casati, P. Franchi, E. Mezzina, M. Bennati, M. Lucarini, ChemPhysChem 2012, 13, 2659
• C. Casati, P. Franchi, R. Pievo, E. Mezzina, M. Lucarini, J. Am. Chem. Soc. 2012, 134, 19108.

Characterization of Host-Guest Complexes

A subject we are currently investigating concerns the application of radical spin probes for studying host-guest inclusion complexes.

In the last years we have shown that specifically designed spin probes provide, through the use of EPR spectroscopy, accurate descriptions of the supramolecular architecture of non-covalent complexes with structural details that are complementary to those obtained with traditional techniques. This is due to the fact that the formation-dissociation of supramolecular complexes takes place within a frequency range that is accessible only in very peculiar situations either to the optical techniques or to NMR spectroscopy. These processes, characterized by a time scale between 10-5 and 10-9 sec, can be instead conveniently investigated by conventional EPR spectroscopy. In particular it has been found that benzyl tert-butyl nitroxide and related dialkyl nitroxides are very suitable probes to investigate host-guest interactions. Evidence for the formation of paramagnetic complexes between these radicals and host systems are obtained from the considerable spectra changes induced by the different environment experienced by the radical upon complexation. This technique has been applied successfully to different type of host molecules like cyclodextrins, cucurbiturils and calixarenes.

Selected publications

• M. Lucarini, E. Mezzina “EPR investigations of organic non-covalent assemblies with spin labels and spin probes” in “Electron Paramagnetic Resonance” Eds G. Gilbert & D. Murphy, RSCPublishing, 2011

Monolayer Protected Nanoparticles

Monolayer protected gold nanoparticles are aggregates made by a metallic gold core protected on the surface by a layer of organic thiols.

Depending on the operative conditions it is possible to obtain MPN with the size of the gold core varying from 1.5 to 8.0 nm and protected by different type of thiols. Functionalized thiols may be introduced in the protecting layer thus obtaining MPN decorated on the surface with different chemical moieties able to perform different functions.
EPR technique coupled with specifically designed spin probes is one of the few methodologies available to investigate the partition of an organic substrate in the organic monolayer bound to the nanoparticles surface. By using this technique it is possible to investigate the factors (nanoparticles size, nature of the thiol monolayer protecting the gold nanoparticles surface, organization of the monolayer, lipophilicity of the organic substrate, media composition, etc.) which controls the partition of hydrophobic molecules in the organic monolayer bound the gold nanoparticles surface. With analogous methodologies and using spin probes sensitive to the lipophiliticty of the environment such as, for example, the benzyl tert-butyl nitroxide substituted on the aromatic ring with alkyl chains of different lengths, it is also possible to investigate the spatial organization of mixed monolayer formed by different thiols.

The work is carried out in collaboration with the group of Lucia Pasquato at the Univeristy of Trieste.

Selected publications

• M. Lucarini, L. Pasquato, “ESR spectroscopy as a tool to investigate the properties of self-assembled monolayers protecting gold nanoparticles”, Nanoscale, 2010, 2, 668