Kelvin probe investigation of the thickness effects in Langmuir–Blodgett films of pyrrolic macrocycles sensitive to volatile compounds in gas phase

Langmuir–Blodgett (LB) films are utilized as sensing materials for many different chemical sensors. This technique allows the fabrication of very thin films, down to one single monolayer. As is well known from classical material science, there is a minimum number of elementary blocks (atoms or molecules) that have to be aggregated before the material properties can be assessed. To this regard, molecular films tend to behave in a different way; since the basic block is a complex molecule, most of the properties are found in the single molecule. This opens the possibility of obtaining materials for which the surface to volume ratio can be maximized in order to increase, for instance, the sensor sensitivity. In this paper, some aspects related to the electronic structure of LB films of porphyrinoids are studied by the Kelvin probe technique as a function of the number of monolayers. Results show that the material properties are, in the case of corrole, confined to the single molecule, while in the case of sapphyrin, a build-up process seems to take place, so that many monolayers are necessary to establish the bulk material properties.