Tuning Solvation Dynamics by Modulating the Shape of Water-Soluble Cationic Nanocages

Understanding how nanoconfinement geometry modulates solvent dynamics is critical for controlling the photochemical reactivity inside supramolecular hosts. Here, we investigate how altering the shape of a cationic metal-organic nanocavity from an octahedral-shaped full-cage to a square-pyramidal-shaped half-cage influences the nonequilibrium solvation dynamics around photoexcited host-guest charge-transfer (H-G CT) states in water. Femtosecond broadband transient absorption spectroscopy reveals that photoexcitation of the host-guest CT complex formed by entrapment of aromatic aldehydes inside both the full-cage and half-cage produces highly dipolar states, which relax via water-reorientation around the whole complex. The octahedral-shaped full-cage exhibits rapid solvation with an average time scale of ∼4 ps, whereas the square-pyramidal-shaped half-cage displays markedly slower solvation with an average time scale of ∼40 ps. This pronounced modulation is attributed to geometry-induced restriction of solvent access, host-guest complex population heterogeneity, and enhanced hydrophobic shielding within the half cage, establishing nanocavity shape as one of the critical parameters for tuning solvent-mediated excited-state stabilization in water-soluble nanocages.