Supramolecular assembly of phosphonate-substituted porphyrins in Langmuir layers and Langmuir–Schäfer films: structural studies and selective sensing of pyridine vapors†

The fabrication of porphyrin-based 2D materials is of great interest because they are valuable in molecular electronics, photovoltaic cells, thermoelectric energy harvesting, gas sensory and medicine. In this work, interfacial behavior of porphyrins bearing two, four and eight diethyl phosphonate (P(O)(OEt)₂) groups at the periphery of the tetrapyrrolic macrocycle was investigated to demonstrate the influence of the number of these hydrophilic and voluminous groups bearing three Lewis basic centers on the structure of porphyrin monolayers and films. The morphology and optical properties of the Langmuir layers formed by these porphyrins on the water surface has been studied by in situ UV-vis and fluorescence spectroscopies and Brewster angle microscopy (BAM). Monolayers were successfully transferred onto both hydrophilic and hydrophobic solid substrates (mica and polyvinyl chloride (PVC)) by the Langmuir–Schäfer (LS) technique and the films thus obtained were studied by absorption and fluorescence spectroscopies and atomic force microscopy (AFM). These systematic studies demonstrated that the P(O)(OEt)₂ substituent plays a key role in the organization of Langmuir monolayers and their transfer onto solid surfaces. The replacement of traditionally investigated meso-substituted porphyrins with β-octa-functionalized derivatives allows for decreasing π–π stacking interactions in the monolayers without significantly reducing their stability and the efficiency of their transfer onto solid supports. The optical properties of H₂OPPP are tuned by lateral compression at the air–water interface but are preserved after transfer of Langmuir layers onto PVC. In contrast to many porphyrin films reported previously, LB films of MOPPP (M = H₂, Zn, Ni) transferred at 18 mN m⁻¹ onto a mica substrate are composed of discrete and closely packed monolayer fibrils being homogeneous at the mesoscopic scale. DFT computations were used to gain insight into the structural organization of fibrils by modeling different dimers of ZnOPPP. The monolayer films of H₂OPPP are acidochromic and those of ZnOPPP on PVC allow for selective visual sensing of pyridine vapors. This is an interesting example of a supramolecular sensor in which the selective visual response was achieved by careful optimization of the structure of the monolayer film and sensing procedure.