Molecular Rotors as Guest Fluorophores Probing the Local Environment inside Host G4 Supramolecular Hydrogels

Fluorescent molecular rotors with a high binding affinity toward the guanosine quartet (G4) were incorporated as guest fluorophores into host supramolecular hydrogels based on the self-assembly of G4 units, to probe the local environment. Torsional dynamics of the rotors were severely inhibited inside the hydrogels in comparison with aqueous solutions, although the hydrogels were composed of >95% water. Moreover, even though all the gels were rigid bodies with no spontaneous deformation or flow property at room temperature, torsional dynamics in G4 borate gels was found to be consistently several orders of magnitude slower than those in the other G4 gels, irrespective of the identity of the molecular rotor probe. This clear difference in the molecular mobilities of the guest fluorophore could be attributed to systematic differences in the internal structure between the two categories of host G4 hydrogels. In specific terms, the borate groups in G4 borate hydrogels serve as bridging units between separate G4 quadruplex strands, generating additional cross-links that reinforce the network structure of the gel. The results demonstrate that molecular rotors act as efficient fluorescent probes for the quantitative assessment of the molecular-level environment and dynamics inside the hydrogels, an aspect that is missed out by most other analytical methods that are routinely employed for studying them.