Self-Assembled Hydrazone-Derived Macrocycles Show Cucurbit [8]Uril-Assisted Tunable Excimer-Like Emission

Abstract

[Abstract] Enforced chromophore proximity provides a powerful molecular design principle to reshape excited-state interactions and tune emission. In this work, we report the optical properties of two self-assembled cyclophanes in which the interplanar distance between the longitudinal bispyridinium chromophores is sufficiently short to promote intramolecular interactions. Both cyclophanes were efficiently synthesized through acid-catalyzed hydrazone condensation in water without the need for a templating effect, highlighting their high preorganization for cyclization. Photophysical studies in acetonitrile revealed clear excited-state interchromophoric interactions in both systems. The cyclophane containing the more conjugated bispyridinium unit (o-REx 4+) displayed a pronounced red-shifted emission (91 nm) and a nearly ninefold increase in emission efficiency relative to its monomeric counterpart. In contrast, chromophoric interaction in the less conjugated cyclophane (o-R4+) induced fluorescence of an otherwise non-emissive bispyridinium chromophore. The emissions are strongly quenched in water, however, complexation with cucurbit[8]uril (CB[8]) restores the fluorescence of both cyclophanes through the formation of stabl ring-in-ring host–guest complexes. These results highlight an efficient synthetic approach to highly preorganized cycl phanes and demonstrate how solvent effects and CB[8] complexation can be exploited to modulate and recover the fluorescence of intramolecular chromophore dimers.