Supramolecular Sensing of 246-Trinitrophenol bya Tetrapyrenyl Conjugate of Calix4arene: Applicability in Solutionin Solid State and on the Strips of Cellulose and Silica Gel andthe Image Processing by a Cellular Phone

摘要

A calix[4]arene conjugate possessing a tetrapyrenyl moiety at its upper rim (>R) is designed as a receptor for sensing trinitrophenol (TNP). To understand the role of the calix[4]arene platform and that of pyrenyl moieties in >R, two other control molecules were synthesized. These are as follows: the one possessing a tetraphenyl moiety in place of tetrapyrenyl (>R>1) and the other one is a p-pyrenyl-hydroxy benzene (>R>2) that is devoid of the calix[4]arene platform. The >R shows high sensitivity toward TNP in tetrahydrofuran (THF) over eleven other nitroaromatic compounds (NACs) studied by exhibiting large fluorescence enhancement and hence is selective to TNP over the other NACs studied. However, the control molecules >R>1 and >R>2 showed only marginal fluorescence enhancement, supporting the need of a calixarene platform and the presence of a tetrapyrenyl moiety in the receptor system for the selective sensing of TNP. Further, >R>1 and >R>2 are not suitable for sensing, since these exhibit similar fluorescence response over several NACs studied.The binding of TNP by >R has been addressed by fluorescencetitration and isothermal titration calorimetry. The nature of thecomplexation of TNP by >R has been revealed by the computationalcalculations, wherein the data showed the entrapment of TNP by twoadjacent pyrene moieties via π–π stacking interactions.Such host–guest complexation is expected to restrict the mobilityof the pyrene moieties present in >R. The reduction ofthe flexibility of the pyrenyl moieties of >R upon TNPbinding is evidenced by the ¹H NMR spectral study, whereinthis acts as an additional evidence for the complexation. In the presentstudy, the sensing of TNP by >R has been shown in THFsolution, on the surface of silica gel and the cellulose paper toresult in lowest detection limits (LODs) of 1.5, 3.5, and 6.5 μM,respectively. Even the solid mixture of >R and TNP showedLOD of 2.1 μmol. Since >R is expected to show supramolecularaggregation that is dependent on the guest species, the correspondingdetails were probed by microscopy techniques, using scanning electronmicroscopy, atomic force microscopy, and transmission electron microscopymethods, and significant changes in the aggregation of >R upon interaction with TNP were found. Such aggregation is responsiblefor the observed fluorescence enhancement. Thus, the tetrapyrenylcalix[4]arene conjugate (>R) acts as a sensitive and robustplatform for selective detection of TNP from a mixture of nitroaromaticcompounds (NACs) wherein the fluorescence intensities can be imagedand managed by a cellular phone.