Solvatofluorochromic Contrast with Supramolecular Stereoisomers Using Linked Rotaxane Structures to Investigate Local Solvation in Excited Donor-Bridge-Acceptor Systems

To optimize the photophysical properties of donor-bridge-acceptor (D-B-A) systems, the elucidation of the excited state of each component (i.e., D, B, and A) plays a crucial role in controlling the forward and back electron transfer rates. However, the direct evaluation of solvation spheres and their rearrangement on the excited bridge is still challenging. In this work, we report a method that exploits a linked rotaxane structure to uncover the solvation effects on an excited bridge by leveraging solvatofluorochromic contrast. We synthesize a D-insB-A system, in which a diphenylacetylene-based bridging moiety is insulated by a covalently linked permethylated α-cyclodextrin (PM α-CD), as a structural isomer of the uninsulated counterpart (D-uninsB-A). In the D-insB-A system, PM α-CD selectively covers the bridge while the donor and acceptor remain exposed. In this way, a comparison of the fluorescence wavelengths of D-insB-A and D-uninsB-A can reveal the local solvation effects on the excited bridge moiety. The investigation of the fluorescence properties in various solvents highlights a weak solvent interaction with the excited bridge in protic solvents and strong one in chlorinated polar solvents. These results provide crucial insight into the properties of excited D-B-A systems from the perspective of external environments, including solvent interactions.