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[Fe2(Cl2An)3], where NPr4+ = tetra‐n‐propylammonium and Cl2An2− = 2,5‐dichloro‐3,6‐dihydroxo‐1,4‐benzoquinonate. At room temperature (300 K: RT), ultrafast photoinduced CT between the Fe and Cl2An 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 Cl2An 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. Ultrafast and efficient photoinduced charge‐transfer dynamics, along with a unique lattice‐structure modulation, occur within 110 fs in the donor–acceptor type metal–organic framework (NPr4)2[Fe2(Cl2An)3] at room temperature (300 K). The present study provides an essential first step toward realizing flexible photo‐controlled MOFs, opening the way for developing magnetically active photo‐switching in such robust structures.