Revisiting the Robin-Day Classification through Switchable Electronic States in Multimetallic Vanadium Oxides

Polyoxovanadate-alkoxide clusters are redox-active molecular oxides offering profound electronic tunability. The oxygen-deficient species [(V₆O₅)(μ₆-O)(μ₂-OCH₃)₁₂] is an ideal platform for probing the multisite localization and delocalization of redox states. Here, we introduce the redox topology modulation which is governed by the position of the central μ₆-O oxygen and ligand coordination at the oxygen-deficient site as the mechanism controlling the stability of different electromers. The noncoordinated cluster exhibits a localized, Robin-Day class I/II hybrid ground state, featuring a V(III) center at the vacancy defect. We demonstrate computationally that ligand-field tuning inverts this behavior; coordination of a strong donor destabilizes the localized topology, stabilizing an electromer with all V(IV) topology as the new ground state. Time-dependent density functional theory calculations show that photoexcitation of species where centers are V(IV) triggers photoinduced intervalence charge transfer regenerating a valence-trapped class II excited state. This work establishes the redox topology modulation as a rational design principle for molecular switches, where the fundamental electronic topology can be toggled by chemical stimulus and/or by light. Furthermore, our results suggest that the Robin-Day classification should be revised and extended for multicenter systems, where valence behavior is better understood as excitation-specific rather than molecular-specific.