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 1-4 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 1-4 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 R(0)=37 Å for the BODIPY-dye FRET pair and an average cavitand opening angle α=16° in the vase conformation.