Ratiometric Turn-On Fluorophore Displacement Ensembles for Nitroaromatic Explosives Detection

There
is a recognized need in the area of explosives detection
for fluorescence-based sensing systems that are capable of not only
producing a turn-on response but also generating a distinctive spectral
signature for a given analyte. Here, we report several supramolecular
ensembles displaying efficient fluorophore displacement that give
rise to an increase in fluorescence intensity upon exposure to various
nitroaromatic compounds. The synthetic supramolecular constructs in
question consist of a tetrathiafulvalene (TTF)-based pyrrolic macrocycle,
benzo-TTF-calix­[4]­pyrrole (Bz-TTF-C4P), and fluorescent dyes, monomeric
or dimeric naphthalenediimide (NDI) and perylenediimide (PDI) derivatives,
as well as chloride or hexafluorophosphate (PF6–) salts of rhodamine 6G (Rh-6G). In chloroform solution, these assemblies
exist in the form of discrete supramolecular complexes or oligomeric
aggregates depending on the specific dye combinations in question.
Each ensemble was tested as a potential explosive-responsive fluorescence
indicator displacement assay (FIDA) by challenging it with a series
of di- and trinitroaromatic compounds and examining the change in
fluorescence spectral characteristics. Upon addition of nitroaromatic
compounds (NACs), either a “turn-on” or a “turn-off”
fluorescent response was observed depending on the nature of the constituent
fluorophore and, where applicable, the counteranion. The FIDAs based
on the PDI derivatives were found to display not only a ratiometric
fluorescence enhancement but also analyte-dependent spectral changes
when treated with NACs. The NAC-induced fluorescence spectral response
of each ensemble was rationalized on the basis of various solution-phase
spectroscopic studies, as well as single-crystal X-ray diffraction
analyses.