Unusual intermolecular charge transfer enables supramolecular fluorescent viscosity sensors

We propose and demonstrate a novel molecular design strategy to develop supramolecular fluorescent viscosity probes. Our new probes are constructed based on host-guest chemistry, in which the guest fluorophore itself is not a viscosity sensor. However, upon complexation with the host molecule, the resulted supramolecule exhibits unusual intermolecular charge transfer between the host and guest molecules. This intermolecular charge transfer drives molecular rotations and suppresses fluorescence output. As solvent viscosity increases from 1.1 cP to 612.6 cP, hindering molecular rotations leads to considerable fluorescence intensification by up to 21 times in our probes. Our design strategy opens a new avenue for developing fluorescent viscosity sensors.