Sub-10-fs pump-probe reflectivity measurements combined with model calculations reveal the emergence of a photoinduced hidden (PH) state in a donor-acceptor-type metal-organic framework. Transient spectra exhibit an additional absorption band developing on a $\approx 30\mathrm{fs}$ timescale, signaling the ultrafast formation of the PH state. Time-frequency analysis captures spectral-weight transfer between phonons at $\approx 500{\mathrm{cm}}^{-1}$ and $600{\mathrm{cm}}^{-1}$, reflecting coherent oscillatory dynamics that accompany the formation of the PH state. Model calculations indicate that the PH state is a metastable polar state stabilized by bond-order-wave-driven lattice modulation, suggesting electronic ferroelectricity. These findings establish metal-organic frameworks as a versatile platform for exploring nonequilibrium quantum states.

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• Charge density waves
• Charge dynamics
• Charge order
• Electronic structure
• Electronic transitions
• Lattice dynamics
• Phase transitions
• Symmetry breaking states
• Ultrafast phenomena
• 1-dimensional systems
• Photoexcitation
• Time-resolved reflection spectroscopy