Fabrication of Langmuir-Blodgett chiral films from cationic (L)-proline-porphyrin derivatives

Stable porphyrin films expressing specific chirality at a supramolecular level have been achieved by Langmuir-Blodgett (LB) deposition of two inherently chiral (L)-proline porphyrin derivatives. Spectroscopic measurements (UV-vis, CD and fluorescence) evidenced that the stereogenic center stored on the peripheral proline residue dictates the specific supramolecular organization of the macrocycles at the air/water interface, which is found to be significantly different from that observed in solution (i.e. EtOH/H2O 25/75, v/v solvent mixture). In the case of free-base porphyrin, the firmness as well as the homogeneity of the corresponding chiral LB films are not optimal for a number of layers below 10. For the zinc derivative, thermal annealing helps to make the films more oriented and produces an amplification of the chirality of the treated films. The results described are of relevance, for example, for the development of stereoselective sensors, where the fabrication of chiral surfaces with specific and reproducible stereochemistry represents a crucial issue.