Spin-crossover is observed for the first time in a FeII(py)3(NC⋯)3 coordination environment which arises in a two-fold interpenetrated 3-D Hofmann-like framework.

A detailed study of the two-dimensional (2-D) Hofmannlike framework [Fe(furpy) 2 Pd(CN) 4 ]•nG (furpy: N) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environ-ments�this is a strategic approach that we achieve by the inclusion of a ligand with multiple host−host and host−guest interaction sites. Variabletemperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A•H 2 O,Et and an abrupt single-step SCO for A•H 2 O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A•H 2 O,Et characterized by a unique Fe II center�the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A•H 2 O shows a triclinic symmetry with two inequivalent Fe II centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure−function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Two analogous 2-D Hofmann-type frameworks, which incorporate the novel ligand N-(pyridin-4-yl)benzamide (benpy) [FeII(benpy)2M(CN)4]·2H2O (M = Pd (Pd(benpy)) and Pt (Pt(benpy))) are reported. The benpy ligand was explored to facilitate spin-crossover (SCO) cooperativity via amide group hydrogen bonding. Structural analyses of the 2-D Hofmann frameworks revealed benpy-guest hydrogen bonding and benpy-benpy aromatic contacts. Both analogues exhibited single-step hysteretic spin-crossover (SCO) transitions, with the metal-cyanide linker (M = Pd or Pt) impacting the SCO spin-state transition temperature and hysteresis loop width (Pd(benpy): T½↓↑: 201, 218 K, ∆T: 17 K and Pt(benpy): T½↓↑: 206, 226 K, ∆T: 20 K). The parallel structural and SCO changes over the high-spin to low-spin transition were investigated using variable-temperature, single-crystal, and powder X-ray diffraction, Raman spectroscopy, and differential scanning calorimetry. These studies indicated that the ligand–guest interactions facilitated by the amide group acted to support the cooperative spin-state transitions displayed by these two Hofmann-type frameworks, providing further insight into cooperativity and structure–property relationships.

Foremost, practical applications of spin‐crossover (SCO) materials require control of the nature of the spin‐state coupling. In existing SCO materials, there is a single, well‐defined dimensionality relevant to the switching behavior. A new material, consisting of 1,2,4‐triazole‐based trimers coordinated into 1D chains by [Au(CN)2]− and spaced by anions and exchangeable guests, underwent SCO defined by elastic coupling across multiple dimensional hierarchies. Detailed structural, vibrational, and theoretical studies conclusively confirmed that intra‐trimer coupling was an order of magnitude greater than the intramolecular coupling, which was an order of magnitude greater than intermolecular coupling. As such, a clear hierarchy on the nature of elastic coupling in SCO materials was ascertained for the first time, which is a necessary step for the technological development of molecular switching materials.

Nanoconfinement offers opportunities to tune physical properties of molecular entities by altering their assembled structures. This also applies to acene-based molecules with potentially rich π–π interactions. Unlike most of the previous cases with acene-based guests directly incorporated into hosts, we take a further step by oligomerizing a fluorescent anthryl monomer, 9-vinylanthracene, inside nanochannels of a metal–organic framework, which is a pillared three-dimensional kagome net of [Zn2(bdc)2(dabco)] (bdc2– = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]­octane). The fluorescence emission of the guest can be significantly enhanced after oligomerization, which is likely due to the suppressed nonemissive interaction between the oligomerized molecules in the nanospace and the MOF wall. The case we have demonstrated for fluorescence enhancement via confined oligomerization provides inspiration for the design of luminescent composites and is encouraging for further exploration of molecules in a nanoconfined space.