Ultrafast Formation of a Photoinduced Hidden State Driven by a Bond-Order Wave in a Metal-Organic Framework.

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 ≈30  fs timescale, signaling the ultrafast formation of the PH state. Time-frequency analysis captures spectral-weight transfer between phonons at ≈500  cm^{-1} and 600  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.