The energy input on bulk powder triggers the exfoliation of few-layer covalent organic nanosheets (CONs), which reduces the ACQ and generates an emitting dispersion of BDP-TFP-CONs. Upon irradiation of a BDP-TFP-COF dispersion (0.1 mg mL -1 in Millipore H2O) at λexc = 420 nm, a steady-state emission spectrum (Figure ) shows a broad emission band with a maximum located at 646 nm. The rather large width might be explained by the coexistence of agglomerated COF layers with varying thickness. In continuation with the water remediation investigations in the previous section, these emissive aqueous dispersions of BDP-TFP-COF raises hope for potential applications in sensing of selected pollutant molecules in contaminated waters. MB sensing experiments were carried out by monitoring the emission maximum upon the addition of MB to an aqueous BDP-TFP-COF dispersion (0.1 mg mL -1 in Millipore H2O) (Figure ). Excitation at 420 nm was deliberately chosen to avoid an inner filter effect caused by reabsorption of MB (Figure ). Already after the addition of 1 ppm of analyte, the emission was almost completely quenched (Figure ), while the decrease exhibited excellent linearity up to 200 ppb (Figure ). In contrast, no quenching was observed upon the addition of BPA (Figure S21c,d). One reasonable explanation for the quenching mechanism is the wide spectral overlap between the emission of BDP-TFP-COF and the absorption of MB, which might facilitate a Förster resonance energy transfer (FRET) (Figure ). In fact, the previously shown preferred adsorption sites of the dye within the COF pores prearrange the energy donor and acceptor at an adequate distance for the FRET process. Simulations with the adsorption tools of Materials Studio indicated intermolecular distances in the range of 2.6-3.8 Å between the guest and the framework, which are stabilized by CH-π interactions between the methyl groups of the BODIPYs and the aromatic core of MB (Figure ). On the contrary, the incremental addition of NEt3 to a BDP-TFP-COF dispersion (0.1 mg mL -1 in THF) provoked an enhancement of the fluorescence. Presumably, the deprotonation of the enaminone creates a negative charge on the aromatic backbone, which enhances the emission by decreasing the photoinduced electron transfer (PET) between building blocks (Figure and Figure ). This remarkable dual behavior of the fluorescence for BDP-TFP-COF reveals the high potential for selective sensing and the application of this crystalline polymer as a dual sensor is currently underway in our laboratories.