Molecular-based selection of porphyrins towards the sensing of explosives in the gas phase

Abstract

In this work, we have used thin films of the multiporphyrin, 1,3-di[5-(3-hydroxyphenyl)-10,15,20-(triphenyl)porphyrin]-2-(5-(3-hydroxy-phenyl)-10,15,20-(triphenyl)porphyrin)-2-methylpropane (Triad), its monomer 5-(3-hydroxyphenyl)-10,15,20-triphenylporphyrin (TPPOH) and the simple porphyrin 5,10,15,20-tetraphenyl-porphyrin (TPP), to detect explosive vapors through fluorescence spectroscopy. UV–vis spectroscopy has been used to analyze the degree of molecular aggregation and stability of the porphyrins in the films. The comparison between the three macrocycles has shown that even though the porphyrin Triad can reduce the molecular aggregation and gain stability in films prepared by spin coating in comparison with TPP films, TPPOH forms the least aggregated and most stable films over the porphyrins used in this work. Absorbance response of porphyrins films towards explosive vapors has been analyzed through their exposure to 2,4-dinitrotoluene (DNT) as testing vapor, featuring relatively small and slow spectral changes. Moreover, these absorbance responses have made evident the detrimental effect of molecular aggregation over the sensing capabilities of the porphyrins. However, exposure of porphyrins films to DNT, 1,3-dinitrobenzene (DNB) and 2,4,6-trinitrotoluene (TNT) has resulted in a fast, intense, reversible and repetitive quenching of the fluorescence in all cases, with the monomeric TPPOH exhibiting the best sensing performance among the three compounds. Surprisingly, TPP has shown a relatively large and fast fluorescence response that indicates that aggregation may not affect much the sensing capabilities of porphyrins in terms of fluorescence responses, in contrast to the absorbance responses. This fact has been explained by the lack of emission of H and J-aggregates in the porphyrin films. Finally, although the Triad has featured the slowest quenching response compared to the other two porphyrins, its quenching percentage is in some extent dependent on the type of nitroaromatic, which could be interpreted as a possible ability of Triad to respond more selectively towards explosive vapors.