Abstract
The aggregation of a free base porphyrin, meso-tetrakis(4-carboxyphenyl)porphyrin and its Zn(II) derivative have been studied at the air/water interface in the presence of a p-tert-butylcalyx[8]arene matrix. The mixed Langmuir films were obtained either by premixing the compounds (cospreading) or by sequential addition. The negative deviation from the additivity rule of the cospread films is indicative of a comparatively good miscibility that was further confirmed by Brewster angle microscopy. The images of the cospread mixed films showed a more homogeneous morphology in comparison with those of pure porphyrin that is attributed to a deeper and earlier self-aggregation state at the interface of the latter. These results were similar for both porphyrins and revealed the disaggregating effect of the calixarene matrix. The orientation and association of the porphyrins were studied by UV–visible reflection spectroscopy at the interface. A different aggregation behavior can be inferred from the resulting spectra, and a higher orientational freedom was observed when the molecules were less aggregated in mixed cospreaded films. The disaggregating effect was retained when the films were transferred to solid supports as demonstrated by UV–visible spectroscopy. Finally, the potential use of these Langmuir–Blodgett films as optical gas sensors was tested against ammonia and amine vapors. The changes in the spectrum in the presence of the volatile compounds are higher for the Zn-porphyrin. The presence of calixarene enhances the sensor response due to the higher accessibility of volatiles to disaggregated porphyrins in the mixed films. The resulting changes were mapped into a numerical matrix that can be transformed into a color pattern to easily discriminate among these gases.
Highlights
Research into porphyrins and derivatives deposited on Langmuir−Blodgett (LB) films for optical gas sensing continues attracting attention in defense and environmental applications.[1−12] The reason behind this interest relies on the modification of their electronic structure and, the UV−visible (UV−vis) spectrum in the presence of the analyte.[4]
Among the possibilities established in the literature, there are several studies aiming at minimizing the aggregation of the porphyrin film by adding a calixarene matrix to form mixed films and enhance the porphyrin organization and uniformity of the LB films.[3,4,7,12,24,25]
It has been shown that the calixarene promotes the disaggregation of the associated structures of the carboxyphenyl porphyrins
Summary
Research into porphyrins and derivatives deposited on Langmuir−Blodgett (LB) films for optical gas sensing continues attracting attention in defense and environmental applications.[1−12] The reason behind this interest relies on the modification of their electronic structure and, the UV−visible (UV−vis) spectrum in the presence of the analyte.[4]. P-TCPP (Figure 5a) shows wider Soret bands with a shoulder at longer wavelengths, while p-ZnTCPP exhibits the most shifted ones (Figure 5c).[3,41] The bathochromic shift of the Soret band is ascribed to aggregates of type J (in which the molecular planes are displaced with respect to the stacking direction), while the increment of the spectra width can be attributed to aggregates of a higher order These results clearly demonstrate the disaggregating effect of the calixarene matrix in the mixed films whose corresponding spectra (Figure 5b,d) show a maximum of the Soret band very near to that of the solution spectrum. Future studies will be focused on the analysis of the sensing capability of mixed LB films including other TCPP derivatives, more analytes, and the kinetics of the response and recovery after the gas exposure
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