Influence of Molecular Organization of Asymmetrically Substituted Porphyrins on Their Response to NO₂ Gas

Abstract

Two asymmetrically substituted porphyrins (5,15-bis(4-nitrophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)-21H,23H-porphine, CAH3; 5,15-bis(4-aminophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)-21H,23H-porphine, CAH4) have been used in this work. Good Langmuir monolayers of these compounds have been prepared on water and transferred successfully to glass slides at much faster than conventional deposition rates (500 mm min -1 ). The monolayer behavior has been investigated by measuring surface pressure-area isotherms as well as by Brewster angle microscopy (BAM) and reflection spectroscopy. For both porphyrins, the intermolecular π-π interaction is sufficiently strong that significant preaggregation occurs prior to compression of the film. However, as the monolayers are compressed, changes in the molecular association and orientation are observed for CAH4 while no molecular reorganization is appreciated for CAH3. The different molecular packing of the Langmuir monolayers is maintained during the transfer. The UV-vis spectra of CAH3 and CAH4 solutions, respectively, were found to be sensitive to NO2 gas. However, only the Langmuir-Blodgett (LB) assemblies of CAH4 showed a response in the presence of the toxic gas. This different behavior has been explained in terms of the different molecular organization of the two porphyrins.

A transparent TMPyP/TiO2 composite thin film as an HCl sensitive optochemical gas sensor

“…The first one is a faster response and recovery because of the high porosity and stability of the TiO 2 as host material, and the second one is the extremely low concentration of TMPyP necessary to obtain a sensing film with a good response to the analyte. These two features lead to a very little aggregation of the porphyrin in the solid film which is of great importance when designing dye-based optical gas sensors [15].…”

Anchoring effect on (tetra)carboxyphenyl porphyrin/TiO2composite films for VOC optical detection

“…Techniques such as Langmuir-Blodgett (LB), spin-coating or casting can be used to obtain solid lms of the dyes, but the organization of these lms in terms of molecular aggregation is sometimes unsatisfying, mainly because of the strong p-p interaction between porphyrins, which can strongly prevent a proper response to the analytes. 10 Several methods have been studied in order to avoid aggregation. In LB lms, the addition of host molecules such as calixarenes has been found to reduce aggregates and hence improve the sensing properties of the lms.…”

“…However, it is known that the lack of aggregation enhances the sensing properties of porphyrins, allowing the gas molecules a better access to their coordination sites. 10 Therefore, the sensing system based on composite porphyrin/TiO 2 thin lms may be improved by avoiding dye aggregation.…”

Preparation and Optimization of Fluorescent Thin Films of Rosamine-SiO2/TiO2 Composites for NO2 Sensing

“…For instance, the decrement of 23% observed for the most diluted solution is only 9% when the RosB concentration is quintupled (data not shown). This difference can be clearly explained in terms of the different aggregation state of the RosB molecules in these films: while low aggregation facilitates the analyte penetration and interaction, high molecular aggregation prevents this process [68]. The situation is even clearer when the more sensible fluorescence is tested as a sensing parameter, while, in the selected film with low aggregation state, the intensity falls off up to 84% and this drop is only of about 37% for a film infiltrated with a 36 $\mathsf{\mu }$M solution.…”