The use of light is one of the most promising ways to reversibly control various physical properties of organic and metal-organic compounds and opens up the possibility for the generation of new information storage devices. [1,2] Among these materials, iron(ii) spin-crossover complexes are particularly interesting as they are known to exhibit a light-induced low-spin (LS) to high-spin (HS) transition.[3] However, this light-induced excited spin state trapping (LIESST) phenomenon is only efficient at cryogenic temperatures (typically below 50 K) because at higher temperatures the photoinduced HS state relaxes rapidly to the ground state. This is a serious limitation for the development of optical switches based on spin-crossover materials.A possible approach to overcome this problem consists of working within the thermal hysteresis loop, in which the metastable HS or LS states have "infinite" lifetimes.[4] Such hysteresis phenomena have been found in many spin-crossover solids and can be related to long-range elastic interactions between the spin-state-changing molecules.[5] It should be emphasized that the hysteresis loop can be finetuned by chemical methods to situate it around room temperature.[6] The first light-induced LS!HS transition in the hysteresis region was recently observed by means of optical reflectivity when an 8-ns laser pulse was applied at 170 K to [Fe(pm-bia) 2 (NCS) 2 ] (pm-bia = N-2'-pyridylmethylene-4-aminobiphenyl), but the reverse phenomenon could not be detected.[2a] Herein, we report spectroscopic evidence for a bi-directional, light-induced spin transition at room temperature in [Fe(pyrazine){Pt(CN) 4 }] (Figure 1) by applying a one-shot laser pulse of irradiation.The synthesis of a hydrated form of the above-mentioned compound has been reported elsewhere.[7] Subsequent research has demonstrated that the spin-crossover behavior of this hydrated form ([Fe(pyrazine){Pt(CN) 4 }]·n H 2 O; n % 2-3) depends dramatically on the water content. Thermogravimetric analysis revealed that heating the sample at 420 K for 30 minutes is necessary to remove the water of crystallization. The dehydrated form [Fe(pyrazine){Pt(CN) 4 }] exhibits "improved" spin-crossover behavior in that the transition becomes complete and is centered at room temperature. Moreover, the hysteresis loop becomes wider, square-shaped, and reproducible over several cycles. This compound preserves its spin state (HS or LS) for several months if it is kept in a dry atmosphere at room temperature (295 K). Figure 2 shows the temperature dependence of the c M T value (c M is the molar magnetic susceptibility) for

Single crystals of the {Fe (II)(pyrazine)[Pt(CN) 4]} spin crossover complex were synthesized by a slow diffusion method. The crystals exhibit a thermal spin transition around room temperature (298 K), which is accompanied by a 14 K wide hysteresis loop. X-ray single-crystal analysis confirms that this compound crystallizes in the tetragonal P4/ mmm space group in both spin states. Within the thermal hysteresis region a complete bidirectional photoconversion was induced between the two phases (high spin right arrow over left arrow low spin) when a short single laser pulse (4 ns, 532 nm) was shined on the sample.

Magnets derived from inorganic materials (e.g., oxides, rare-earth–based, and intermetallic compounds) are key components of modern technological applications. Despite considerable success in a broad range of applications, these inorganic magnets suffer several drawbacks, including energetically expensive fabrication, limited availability of certain constituent elements, high density, and poor scope for chemical tunability. A promising design strategy for next-generation magnets relies on the versatile coordination chemistry of abundant metal ions and inexpensive organic ligands. Following this approach, we report the general, simple, and efficient synthesis of lightweight, molecule-based magnets by postsynthetic reduction of preassembled coordination networks that incorporate chromium metal ions and pyrazine building blocks. The resulting metal-organic ferrimagnets feature critical temperatures up to 242°C and a 7500-oersted room-temperature coercivity.

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb(0.8)Mn[Fe(CN)(6)](0.93).1.62H(2)O and Co(3)[W(CN)(8)](2)(pyrimidine)(4).6H(2)O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices.

Aqueous ions are central to catalysis and biological function and play an important role in radiation biology as sources of damage-inducing electrons. Detailed knowledge of solute-solvent interactions is therefore crucial. For transition-metal ions, soft X-ray L-edge spectroscopy allows access to d orbitals, which are involved in chemical bonding. Using this technique, we show that the fluorescence-yield spectra of aqueous ionic species exhibit additional features compared with those of non-aqueous solvents. Some features dip below the fluorescence background of the solvent and this is rationalized by the competition between the fluorescence yields of the solute and solvent species, and between the solute radiative (fluorescence) and non-radiative channels; in particular, electron transfer to the water molecules. This method allows us to determine the nature, directionality and timescale of the electron transfer. Remarkably, we observe such features even for fully ligated metal atoms, which indicates a direct interaction with the water molecules.

Double-stranded (ds) DNA of a λ-phage virus are combed on octadecyltrichlorosilane (OTS)-modified borosilicate glass substrates and investigated by means of tip-enhanced Raman spectroscopy (TERS) using tips coated with an Ag/Au bilayer. Owing to an enhancement factor higher than 6 × 10 2 and a lateral spatial resolution better than 9 nm (which is below the size of the tip apex radius), cross-sections of nanowire-shaped thin DNA bundles can be spatially resolved. TER spectra reveal vibrational modes typical of DNA nucleobases and backbone, as confirmed by confocal Raman measurements carried out on dense stacks of DNA strands. While the TER signature of nucleobases is congruent with observations in single-stranded (ss) DNA, additional modes tied to the DNA backbone can be discerned in ds DNA. TERS enables ss and ds DNA samples to be distinguished from each other and hence can be exploited for the detection of DNA hybridization. Moreover, no TER contribution of the OTS layer appears, suggesting that functionalized DNA strands could be studied without spectral perturbation from the substrate. This work paves the way toward the nanoscale spectral study of organized DNA-based nanostructures.

The interest in bistable molecular materials for information processing has been discussed by several authors. [1] Iron(ii) spin-crossover (SCO) compounds [2] are particularly promising in this respect, because the conversion between the highspin (HS) (S = 2) and low-spin (LS) (S = 0) states can be triggered not only by temperature, pressure, [3] and pulsed magnetic fields, [4] but also by irradiation with light. [5] This latter phenomenon-usually called light-induced excitedspin-state trapping (LIESST)-was discovered by Decurtins et al., [6] who observed a light-induced LS!HS transition involving quantitative trapping of the molecules in the excited HS state at low temperatures (typically below 50 K). Our recent investigations demonstrated that it was feasible to trigger SCO by light even at room temperature by using a nanosecond laser pulse, [7] and potential applications of SCO compounds as active elements in optical devices were also discussed. [8] Furthermore, we also observed hysteretic bistability in the dielectric properties of SCO materials. [9] The real part of the dielectric constant was found to be significantly different in the two spin states, which suggests that capacitance measurements can be used to "read" the information stored in the bistable system. Taking advantage of this property, we have constructed and patented a prototype of a thermal molecular memory device. [10] Within this context, the existence of a correlation between photomagnetic and dielectric properties would open interesting prospects because switchable dielectric properties and optical addressing are two physical principles widely used for information storage and processing. The possible advantages of using SCO materials for these aims include: a) the short addressing times (picosecond scale on the molecular level), [11] b) photostability over successive cycles, c) low addressing power (on the order of mW cm À2 ), and d) high storage densities (because the LIESST effect is purely molecular). [8,12] Herein, we report on the observation of the change in dielectric constant upon SCO induced by irradiation with light. A preliminary theoretical approach, based on density functional theory (DFT), to interpret the variation of the dielectric constant with the SCO is also presented. For this study, we selected the SCO complex [Fe(L)(CN) 2 ]·H 2 O [13,14] in which L is a Schiff base macrocyclic ligand derived from the condensation of 2,6-diacetylpyridine with 3,6-dioxaoctane-1,8-diamine (L = 2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene). This molecule was chosen because its critical temperature (T LIESST ), defined as the temperature at which the lightinduced HS information is erased, [8] is the highest (130 K) ever obtained for a SCO compound. [13] Dynamic dielectric spectroscopy was used to measure the light-induced and thermal variations of the complex dielectric permittivity (e* = e'Ài e'') in the frequency range 10 2 -10 6 Hz. The imaginary part e'' represents the dielectric losses and ...

For the first time, natural Aβ fibrils (WT) implicated in Alzheimer's disease, as well as two synthetic mutants forming less toxic amyloid fibrils (L34T) and highly toxic oligomers (oG37C), are chemically characterized at the scale of a single structure using tip-enhanced Raman spectroscopy (TERS). While the proportion of TERS features associated with amino acid residues is similar for the three peptides, a careful examination of amide I and amide III bands allows us to clearly distinguish WT and L34T fibers organized in parallel β-sheets from the small and more toxic oG37C oligomers organized in anti-parallel β-sheets.