The effect of the inversion of one pyrrole ring in porphyrin macrocycle on thermodynamic characteristics of hysteresis process of floating layers of two porphyrins, namely 5,10,15,20-tetraphenylporphyrin (I) and 2-aza-21-carba-5,10,15,20-tetraphenylporphyrin (II), was studied. The impact of this inversion on the morphology and spectral properties of their thin films transferred onto a solid substrate by Langmuir-Schaefer (LS-film) technique was also investigated. Based on the analysis of hysteresis loops in three successive compression-decompression cycles of the floating layers, the free energy of hysteresis ∆Ghys, configurational entropy T∆Shys and configurational enthalpy of hysteresis ∆Hhys were calculated. It was found for both porphyrins that ∆Ghys increases with each successive cycle. The T∆Shys values indicate that with each successive cycle, the floating layers become denser with more stable layer architecture. The ∆Hhys value in the first compression-decompression cycle of porphyrin II is approximately 20 times smaller than that found for porphyrin I (-15.274 kcal/mol and −0.804 kcal/mol, respectively). Negative values of ∆Hhys indicate that during floating layer compression, enthalpically favorable (exothermic) molecular interactions are established. LS-films of porphyrins I and II were transferred onto solid substrates prior to the compression of floating layers. Scanning electron microscopy, polarization optical microscopy, as well as absorption and fluorescence spectroscopies demonstrated that the structural elements of the obtained LS-films are J-type aggregates. A detailed analysis of the thermodynamic characteristics and aggregation ability of the studied porphyrins allows us to understand the nature of the processes occurring in floating layers of tetrapyrrole macrocyclic compounds of different structures at the air/water interface. The results obtained are of interest to researchers working in the field of thin-film material design, with a wide range of potential applications, such as sensor and receptor systems for the detection and selective extraction of substrates of various natures from solutions and gas phase.
