Porphyrin Self-Assembly in Water: a Versatile Approach to Realize Chiral and Hybrid Materials Based on Melanin-Like Polymers

Over the last decades, chemists have been fascinated in the construction of nanomaterials from small building blocks by self-assembly of porphyrinoids.1 However, these molecules hide manifold trouble; even if the peripheral charged groups make these macrocycles water-soluble, nevertheless, porphyrins remain mainly hydrophobic molecules, preserving the well-known tendency to self-aggregate in aqueous solution. This “dichotomy” is worthy of special attention because (i) it plays a central role in the non-covalent syntheses and (ii) in conjunction with inorganic -or polymeric- structures to arrange quite complex hybrid composite materials for various technological applications. For instance, inspired by adhesive proteins secreted by mussels for attachment to wet surfaces, melanin-like polymers, obtained from the spontaneous oxidative polymerization in water of the L-(or D-)3,4-dihydroxyphenylalanine, L-(or D-)DOPA, have been successfully employed to provide highly resistant and adhesive biomaterials for the deposition of multifunctional films.2 However, although detailed studies on DOPA polymerization mechanism have been carried out, scant knowledge on the intrinsic chiroptical features of melanin polymers is reported to date. Thus, in the attempt to asymmetrically drive the oxidation of DOPA to melanin-like polymers in aqueous solution, we employed a “from-Imprinted-to-Template” approach by exploiting the well-known inertia of porphyrin hetero-aggregates.3 Our supramolecular approach revealed the possibility to chirally guide the polymerization of DOPA with the surprising generation of asymmetric melaninic structures in water which can subsequently self-assemble onto a commercial glass substrate to realize chiral surfaces.3 Lastly, we will show as such supramolecular strategy can be also exploited to obtain hybrid porphyrin/DOPA-melanin-like@glass films in water.4 In particular, by using a synthetic porphyrin–spermine derivative we were able to obtain new hybrid DOPA-melanin films, self-assembled onto a ordinary glass substrate, through a viable synthesis in aqueous medium at room temperature.5 Our hybrid films displayed increased adsorption capacity towards aqueous solutions containing methylene blue as a dye pollutant. Interestingly, the possibility to reuse our prototype for continuous adsorption/desorption cycles enabled us to have a smart device for water remediation technology on a larger scale.