Appearance of a Photoinduced Hidden State in the Electron Donor-Acceptor Type Metal-Organic Framework (NPr₄)₂[Fe₂(Cl₂An)_3]

Electron donor-acceptor (DA)-type metal-organic frameworks (MOFs) with valence instability are promising molecular materials for the design of photo-responsive and electronic/magnetic functional materials. Here, ultrafast photoinduced dynamics in a DA-type layered MOF that exhibits a charge-transfer (CT)-type phase transition are reported: (NPr4)(2)[Fe-2(Cl(2)An)(3)], where NPr4+ = tetra-n-propylammonium and Cl(2)An(2-) = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinonate. At room temperature (300 K: RT), ultrafast photoinduced CT between the Fe and Cl(2)An ions induces a sensitive change in the state associated with the valence instability from a single-chain, electron-correlated state to a new photoinduced, structurally modulated state. In the photoinduced state, two absorption bands are observed, one on the higher-energy side of the CT band and the other in the mid-IR range. This strongly implies that the local inversion center on the Cl(2)An ion that exists in the initial state disappears instantly upon photoexcitation, causing an ultrafast change in the lattice structure due to the softening of rigid bonds. This has never been realized in thermal excitation. These findings demonstrate that a new electronic state with a unique lattice structure-i.e., a photoinduced hidden state-appears in this MOF system at ultra-high speed (within 110 fs) upon photoexcitation at RT.