Optical anisotropy and gas sensing properties of ordered porphyrin films

Abstract

Ordered films of porphyrins are routinely obtained by Langmuir-Blodgett technique, with a thickness control at the level of a single monolayer. The study of their optical anisotropy by reflectance anisotropy spectroscopy (RAS) has evidenced an interesting modification of the spectral line shape around the Soret band of the porphyrin layer after a certain critical thickness has been deposited onto the substrate. This finding has been connected to a structural modification of the film, although a detailed model was lacking until now. Recently, a new theoretical study based on the local field interaction has proposed to explain the RAS spectra in terms of the orientation of the molecules in a layer, that would change from layer to layer as the system is assembled. In order to test the model, we have collected new experimental data studying the modification of the optical anisotropy during exposure of the film to a gas (ethanol), mimicking the application for gas sensing analysis. The results show that the model is adequate to describe the real system: reordering of the layer during or after growth originates the anomalous spectra

Alteration of optical anisotropy by adsorption of volatile molecules on ordered metalloporphyrins layers

“…In our previous experiments with ethanol, we have shown that this change is not permanent: the original RAS signal can be recovered after purging the layer in nitrogen flux for several minutes [10]. But in other cases (an example is reported here), the modification of the RAS spectrum is permanent.…”

“…It has been already demonstrated that LB and LS porphyrin thin films show evident changes in their optical anisotropy spectra when exposed to VOCs [10]. Moreover, the interaction of porphyrins with some specific compounds (ethanol, in Ref.…”

“…Quite recently, we demonstrated that is possible to follow the analyte binding equilibria of a Langmuir-Blodgett (LB) porphyrin thin film by using Reflectance Anisotropy Spectroscopy (RAS) [10], an optical technique usually applied in surface physics investigation, assessing a good sensitivity to gas. However, the interpretation of the measured RAS line shape as well as of its evolution during the exposure to VOCs has been not definitively explained.…”

Unveiling the robustness of porphyrin crystalline nanowires toward aggressive chemicals

“…Exposure to alcohol has been already demonstrated to lead to detectable variations of the anisotropy signal in the visible-UV region in porphyrins [37]. Monitoring the RAS response of the H 2 TPP and ZnTPP nanowires during exposure to ethanol and butanol permits checking the strength and characteristics of the interaction resulting with these porphyrins when they are in the form of crystalline nanowires with the specific structure.…”

Ultra-thin film sensors based on porphyrin-5-ylphosphonate diesters for selective and sensitive dual-channel optical detection of mercury(II) ions

“…The influence of metal ions and peripheral substituents on the organization of porphyrin supramolecular assemblies at the air-water interface and their transfer onto solid supports have been thoroughly investigated [75][76][77][78][79][80]. The ultra-thin films prepared by these methods were also widely used for detection of gases or volatile organic compounds (VOCs) in gaseous phase [72,79,[81][82][83][84][85][86]. It is somewhat surprising however, that this great effort devoted to the preparation of sensing monolayers has generated only a few examples of porphyrin-based sensors operating in aqueous solutions [87].…”