Effect of Diethyl Phosphonate Substituents on the Supramolecular Organization and Optical Response of Porphyrin Langmuir–Blodgett and Langmuir–Schaefer Films

A great amount of attention is currently being paid to the development of low-dimensional nanoarchitectonics, such as functional films, that exhibit highly sensitive and selective optical responses to analytes. Porphyrins are particularly attractive for this field due to their outstanding optical and coordination properties. In this work, systematic comparative studies were carried out on Langmuir monolayers and Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films of sterically hindered, hydrophobic meso-tetra(mesityl)porphyrin (H₂TMP) and its phosphonate-substituted analogue H₂DMDPP, in which two mesityl groups are replaced by bulky PO₃Et₂ substituents. Both porphyrins form densely packed LB and LS nanoscale films on hydrophilic and hydrophobic solid substrates. The introduction of two hydrophilic phosphonate groups is critical for controlling monolayer organization at the air-water interface and on solid supports. The structural differences of the films translate to pronounced changes in optical properties, film stability, and pH sensing performance. Importantly, the transfer mode of the Langmuir monolayer (perpendicular (LB) vs parallel (LS)) is as decisive for sensor performance as the molecular structure of the porphyrin itself. All studied single-layer films exhibited pH sensitivity, allowing the spectral detection of HCl vapors. However, a visual fluorescence response toward this analyte was observed exclusively for LB films of H₂DMDPP, likely due to the high emissivity of these films.