FRET Studies on a Series of BODIPY-Dye-Labeled Switchable Resorcin[4]arene Cavitands



A series of borondipyrromethane (BODIPY)-dye-labeled resorcin[4]arene cavitands 14 with different lengths of oligo(phenylene–ethynylene) spacers between the dyes and the macrocyclic rim has been synthesized. Their switching behavior from the “vase” to “kite” conformations in bulk solution was examined by both variable-temperature (VT) NMR and fluorescence spectroscopy. Both VT-NMR and VT fluorescence resonance energy transfer (FRET) experiments showed that cavitands 14 undergo vase-to-kite switching at low temperatures. Acid-triggered switching to the kite conformation was observed by fluorescence spectroscopy. Quantitative evaluation of the FRET data led to the determination of the Förster radius R0=37 Å for the BODIPY-dye FRET pair and an average cavitand opening angle α=16° in the vase conformation.