High-pressure single-crystal X-ray structural analyses of isostructural MFM-133(M) (M = Zr, Hf) of flu topology and incorporating the tetracarboxylate ligand TCHB [HTCHB = 3,3',5,5'-tetrakis(4-carboxyphenyl)-2,2',4,4',6,6'-hexamethyl-1,1'-biphenyl] and {M(μ-OH)(OH)(COO)} clusters confirm negative linear compressibility (NLC) behavior along the c axis. This occurs via a three-dimensional wine-rack NLC mechanism leading to distortion of the octahedral cage toward a more elongated polyhedron under static compression. Despite the isomorphous nature of these two structures, MFM-133(Hf) shows a higher degree of NLC than the Zr(IV) analogue. Thus, for the first time, we demonstrate here that the NLC property can be effectively tuned in a framework material by simply varying the inorganic component of the frameworks without changing the network topology and structure.

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The surface-enhanced Raman (SER) spectra of the aliphatic amine l,4-diazabicyclo[2.2.2]octane (DABCO) adsorbed on the surface of silver electrodes have been studied in detail as a function of the electrode potential and the excitation energy. Variations in the SERS intensity resulting from the variation in these two parameters and the supporting electrolyte have been characterized and attributed to the existence of a photon-driven charge-transfer (PDCT) enhancement of 1-2 orders of magnitude, of the adsorbed DABCO molecule, which operates in addition to an electromagnetic enhancement. The direction of the charge transfer is from molecule to metal. Variations in the band frequencies of the adsorbed DABCO with electrode potential and with the supporting electrolyte have also been measured and ascribed to the existence of a ground-state charge transfer between adsorbed DABCO and the silver electrode. The Raman spectrum of a charge-transfer complex between DABCO and iodine is presented for comparison.