Reversible ON-OFF switching of FRET effect in the functionalized CB[6]-guest complex via photoisomerization

Abstract

Host-guest complexation of amino-substituted styryl dye in basic (ASP) and protonated (ASPH+) forms with cucurbit[6]uril functionalized with naphthalene fluorophore (CB[6]-Nap) was used to obtained photoresponsive supramolecular systems with fluorescence resonance energy transfer (FRET) effect. Energy transfer from the naphthalene group of CB[6]-Nap (energy donor) to ASP or ASPH+ (energy acceptor) in inclusion complexes takes place with an efficiency of 75% and 92%, respectively. E-Z photoisomerization of ASPH+ in the presence and in the absence of CB[6]-Nap has been thoroughly investigated by UV/Vis absorption and fluorescence spectroscopy and NMR spectroscopy. The bending of ASPH+ structure upon E-to Z-isomerization leads to dissociation of the host-guest complex and, as a consequence, to the disappearance of energy transfer, while the back photoisomerization returns the system to its original state. Thus, light-controlled encapsulation and release of dye molecule in the ASPH+@CB[6]-Nap complex allow the FRET process to be reversibly switched. The results obtained demonstrated a novel example of the spatiotemporal control of complexation and fluorescence response in the supramolecular system by light.