Chiral Porphyrin Assemblies: From Solution to Solid State

Abstract

Chirality has increasingly nurtured scientific interests because, beyond the concepts related with the emergence and evolution of Life, it has remarkable implications in practical fields, where chiral materials, coupled with nanotechnology, can be exploited in chiral sensing, optical displays, information storage, asymmetric catalysis and so on.Porphyrins have been extensively used as building blocks to assembling organized systems featuring chirality at supramolecular level, mostly in solution. Indeed, the fine tuning of the properties of these chiral systems, is made possible by a florilegium of either internal or external stimuli, such as solvent polarity, pH, ionic strength, and templates.1 Although the achievement in solution is now well-established, the transfer of the chiral assemblies from solution to thin-film onto inorganic surfaces is not so obvious. Indeed, outside factors as hydrophilic/hydrophobic character of the substrates or the solvent evaporation process can deeply influence film formation and, consequently, its chirality. Furthermore, multiple limitations concerning, among others, robustness and versatility of the chiral nanostructures has still to be overcome for their efficient real-life implementation in a device.Over the years, we investigated the construction of chiral aggregates based on prolinated porphyrin derivatives in hydroalcoholic mixtures, whose formation was steered by hydrophobic effect. We demonstrated the influence of structural parameters (metal coordinated, charge/stereochemistry of the proline moiety) as well as solvent bulk properties (composition, concentration, ionic strength, presence of co-solutes) on both the overall aggregation process and the final morphologies observed.2-4 The implementation on a device often requires the fabrication of solid film by varied controlled deposition techniques able to maintain/amplify the chiral features of the aggregates formed in solution or to create ordered chiral layers by symmetry breaking events (i.e. Langmuir Blodgett or Langmuir-Schaefer techniques). Porphyrin derivatives bearing an appended (L)- or (D)- functionality on a meso phenyl position offered multiple options for realizing chiral surfaces, which are illustrated in Figure 1. Indeed, chiral films can be obtained by drop casting the aggregates formed in EtOH/H2O mixtures on glass, as well as a toluene solution of the porphyrin monomers. Alternatively, the carboxylic group placed on the proline unit can be exploited to anchor the macrocycle onto different inorganic nanostructures, as ZnO or silica nanoparticles, giving chiral layers on glass or quartz slides. In this regard, we also recently found that hybrid systems constituted by chiral Zn-porphyrin capped ZnO nanostructures are able to selectively detect the different enantiomers of chiral analytes, such as limonene vapours, when layered on quartz microbalance surface.5 The potential use of these chiral systems as sensitive materials in stereoselective sensors is indeed particularly appealing, since the most of new emergent pollutants released into the environment by pharmaceutical or agrochemical industries are chiral compounds, whose biological impact strictly depends on the specific enantiomer.In this contribution, an overview of the different preparation protocols of chiral porphyrin-based films will be provided, thoroughly illustrating the characterization of the obtained surfaces by several microscopies and spectroscopic investigations. Furthermore, studies on the chiral discrimination potential featured by the developed surfaces will be also presented. Figure 1