Strong Emission Enhancement in pH-Responsive 2:2 Cucurbituril Complexes
S. Schoder, H. V. Schröder, L. Cera, R. Puttreddy, A. Güttler, U. Resch‐Genger, K. Rissanen, C. A. Schalley – 2019
Organic fluorophores, particularly stimuli-responsive molecules, are very interesting for biological and material sciences applications, but frequently limited by aggregation- and rotation caused photoluminescence quenching. A series of easily accessible bipyridinium fluorophores is reported, whose emission is quenched by a twisted intramolecular charge transfer (TICT) mechanism. Encapsulation in a cucurbituril host yields a 1:1 complex exhibiting a moderate emission increase due to destabilization of the TICT state inside the apolar cucurbituril cavity. A much stronger is the fluorescence enhancement is observed in 2:2 complexes with the larger cucurbituril, which is caused by additional conformational restriction of rotations around the aryl-aryl bonds. As the cucurbituril complexes are pH switchable, this system represents an efficient supramolecular ON/OFF fluorescence switch.