Valency-Controlled Multiphotochromism: Gated Switching and Photoswitchable Lewis Superacidity at Silicon.

Multiphotochromic systems promise multistate molecular information storage, yet strategies to control their switching-state distributions and interchromophore communication remain scarce. Here, we demonstrate that silicon valency can serve as a molecular control element to gate multiphotochromism. Two silicon-based diarylethene Lewis acids undergo stepwise twofold cyclization, with interconversion between tetra- and pentacoordinate states modulating electronic coupling between ligands. Donor coordination enables selective monocyclization, wavelength-gated activation, or complete suppression of photoreactivity, depending on the bound base. In the donor-free state, stepwise cyclization progressively enhances Lewis acidity and culminates in the first example of photoswitchable Lewis superacidity. In addition, dynamic Si─N/Si─O bond metathesis provides thermodynamic access to switching-state distributions beyond statistical photochemical limits. These findings establish valency control as a design principle for multiphotochromic architectures.