Current Inorganic Chemistry (Discontinued) - Volume 6, Issue 1, 2016

One-dimensional Fe(II) chains with 1,2,4-triazole as bridging ligands present the LIESST effect; i.e. their spin state switched from low-spin to high-spin after light irradiation at low temperature. This account summarizes the findings in this area of photomagnetism where 57Fe Mössbauer spectroscopy was used as a primary detection tool of the LIESST effect.

Background: Recent progress in the study of photoinduced phase transition phenomena in cyano-bridged metal complexes called Prussian blue analog by means of CN vibrational spectroscopy is reviewed. The material discussed in this article is one of the most attractive and well-studied material, RbxMn[Fe(CN)6]y·nH2O, which shows a thermal phase transition accompanying a charge transfer and a change in magnetization around room temperature. Owing to a unique opportunity in this material to investigate the domains as well as the initial and final phases, by using the CN stretching vibration modes, domain growth dynamics and the nuclear formation processes are investigated in detail. Raman Spectroscopy: Raman spectroscopy is used to distinguish ion pairs corresponding to the low and high temperature phases and the boundary. The resonant Raman spectroscopy reveals the coupling of the CN ions with the local electronic states, and gives some information about the local lattice structure. Domain Growth: Time dependence of the phase fractions and the amount of boundary under irradiation of light is investigated. Continuous frequency shift suggests relaxation of the local strain during the growth of the domain. The domain growth process is found to be dependent on the excitation power density and the behavior is described by a kinetic model based on mean field approximation. Ultrafast Dynamics: A picosecond infrared spectroscopy is utilized to observe the ultrafast phenomena following the femtosecond pulse excitation. In addition to the decrease of the initial phases, creation of a large number of boundary component is found for photoinduced phase transition in both directions.

Background: The design, synthesis and physicochemical characterization of new switchable magnetic materials is among the most important tasks in the search of multifunctional compounds. Along these lines, intense efforts are focused on the investigation of cyanido–bridged coordination polymers: Prussian Blue Analogs and bimetallic coordination networks constructed of octacyanidometallates. Within this family, the CuII–[Mo(CN)8]4- assemblies have been extensively studied for their photomagnetic properties. The main aim of this work was the design, synthesis, as well as structural, spectroscopic and magnetic characterization of novel molecular material based on copper(II) complex with aliphatic bidentate polyamine and octacyanidometallate( IV) which shows a high number of cyanido bridges. The consequent objective was the study of potential photomagnetic properties of this new material. Methods: A newly synthesized compound [CuII(Me2en)][CuII(Me2en)2][MoIV(CN)8]·6H2O (1), Me2en = N,N– dimethylethylenediamine, was investigated structurally using single crystal X-ray diffraction, spectroscopically by studying IR and UV/Vis spectra, and photo- and magnetically characterized by studying its magnetic response in the dark and under irradiation in a variety of magnetic fields and temperatures. Results: Incorporation of N,N–dimethylethylenediamine (Me2en) into the CuII–[MoIV(CN)8]4- system leads to the construction of a novel coordination polymer [CuII(Me2en)][CuII(Me2en)2][MoIV(CN)8]·6H2O (1). The assembly reveals the unique structure consisting of 1–D chains constructed of {[Cu(Me2en)][Mo(CN)8]}n 2n- ladders with {[Cu(Me2en)2]2n+ pendant arms connected through cyanido–bridges to the Mo(IV) centers. Structural parameters of 1 were compared with other Cu(II)–Mo(IV) compounds with bidentate aliphatic polyamine ligands. Magnetic studies of 1 indicate paramagnetic behavior with weak antiferromagnetic interactions at low temperature. Photomagnetic studies with the application of 405, 473, 532 and 650 nm light sources and white light show no detectable photomagnetic effect. Conclusion: Novel 1–D ladder with pendant arms [CuII(Me2en)][CuII(Me2en)2][MoIV(CN)8]·6H2O (1) coordination polymer reveals unprecedented topological pattern and shows one of the highest complexities among 1–D octacyanidometallate– based chains. The local environments of metal centers were compared with other Cu(II)–Mo(IV) assemblies containing bidentate chelating polyamine ligands. In accordance with this analysis, two types of copper(II) moieties were distinguished among this kind of assemblies. Moreover, the analysis of [Mo(CN)8]4- for systems with various dimensionalities shows the same geometry and structural parameters of the moieties, differing in the number of bridging cyanides. Despite similar structural, spectroscopic and magnetic properties of 1 in comparison with other copper(II) complexes with aliphatic polyamines and octacyanidomolybdate(IV) revealing photomagnetic effect, no light induced increase of magnetization was observed. This leads to the observation that a large number of Cu(II)–NC–Mo(IV) linkages does not guarantee the presence of photomagnetic behavior.

The search for hidden phase using photo-induced phase transition is an important challenge in multistable materials. In this work we report the study of Prussian blue analogs of formula RbxMn[Fe(CN)6](x+2)/3. zH2O. For composition of Rb+ above 65 %, a thermal hysteresis loop is observed as a reflect of the electron-transfer phase transition from MnIII (S=5/2)-NC-FeIII (S=1/2) at high temperature (HT phase) to MnIII (S=2)-NC-FeII (S=0) at low temperature (LT phase). At low temperature the HT phase can be thermally quenched up to the T(TIESST) temperature (TIESST = Thermally-Induced Excited Spin-State Trapping) above which it relaxes back to the LT phase. Upon the decrease of Rb+ amount, the thermal hysteresis and the T(TIESST) are getting closer and overlap for values of x below 65 %. For the latter compounds, the phase transition is inhibited and the hysteresis loop is hidden. By this simple consideration we explain the observation of Light-Induced Phase Collapse already reported giving also some trends to obtain new hidden phases.

Background: Spin crossover (SC) compounds are among the most fascinating molecular solids exhibiting thermal and photo-switching. The spin crossover phenomenon manifests, generally, in iron (II) complexes in octahedral symmetry, surrounded by nitrogen atoms. The splitting of the energy of the d orbitals into the t2g (ground state) and eg (excited state) allows the existence of two possible electronic states, namely high-spin (HS, S=2) and low-spin (LS, S=0) for weak and strong ligand-field configurations, respectively. At the solid state, the switching between these two electronic states is possible with temperature, light, pressure, etc. and may lead to gradual or first-order transitions, accompanied with a thermal hysteresis loop. In addition, SC materials exhibit interesting non-equilibrium phenomena, like photo-induced metastable states at low-temperature and their non-linear thermally-induced relaxation, which will be investigated in the current work. Methods: Optical microscopy technique is used here to visualize the first-order thermal spin transition in the single crystal [{Fe(NCSe)(py)2}2 (m-bpypz)], where py = pyridine and bpypz= 3,5-bis(2-pyridyl)-pyrazolate, hereafter abbreviated Fe(NCSe). The experimental set-up was also adapted to include a photo-excitation through the objective of the microscope. We then succeeded in imaging, the spatiotemporal transformation of the SC single crystal under light at very low temperature, ~20K. Image processing allowed to furnish quantitative data on this transformation. Results: This report focusses on the first direct visualization by optical microscopy of the photo-transformation of the SC single crystal under light. The phenomenon behind this process, called LIESST effect (Light-induced Excited Spin State Trapping) is well known in the SC topic, and was studied in the past using magnetic and optical absorption techniques, but not yet visualized. Our results show that the transformation from LS to HS under light proceeds in a homogeneous way, without any presence of domain coarsening (at the resolution of the optical microscopy, ~500 nm), in good agreement with the general admitted idea that the LIESST effect is a non-cooperative process. We have also followed the spatiotemporal transformation (at higher temperature, T~60K) of the crystal during its thermal relaxation from the photo-induced metastable HS state to the stable LS state. We could derive the time dependence of the HS fraction which was found to follow the usual sigmoidal shape, in excellent agreement with previous magnetic data, thus confirming the cooperative character of this relaxation. As for the photo-excitation, our data demonstrated that the relaxation process from HS to LS proceeds without any domain formation: the crystal remains in the homogeneous state during the overall relaxation process. From the theoretical view point, the photoexcitation and the relaxation processes are described by the kinetic mean field model whose all parameters were derived from the experiment. Conclusion: We presented optical microscopy studies on the single crystal Fe(NCSe) which included the visualization of the first-order thermal transition and also the photo-excitation and the relaxation of one single crystal Fe(NCSe) under light. The thermal equilibrium transition, that is the first-order transition observed at ~112K, proceeds via the formation of a single domain which propagates overall the crystal. In contrast, the photoexcitation process and the subsequent thermal relaxation of the photo-induced HS state, did not show any domain formation. These two last processes take place through homogenous transformations of the crystal. The analysis of our optical microscopy data of photoexcitation/relaxation, in the frame of a kinetic mean-field description, allowed to derive all thermodynamic parameters of the model, such as the interaction parameter and the effective energy barriers. The current observations underline the role of the lattice dynamics and its crucial temperature dependence, in the conditions of occurrence of domains formation. In general, the existence of macroscopic domains requires the presence of an instability.

Objective: We review the dynamical behavior of the charge transfer phase transition in aniron mixed valence system, (CnH2n+1)4N[FeIIFeIII(dto)3] (dto = C2O2S2). Background: In the case of assembled metal complex systems whose spin states are situated in the spin crossover region, synergetic phenomena coupled with spin and charge have been expected. Based on this viewpoint, we have developed a ferromagnetic organic-inorganic hybrid system, (CnH2n+1)4N[MIIFeIII(dto)3], and discovered the charge transferphase transition (CTPT), where the thermally induced electron transfer between FeII and FeIII occurs reversibly. However, the dynamics of the CTPT has not well been elucidated by the measurements of magnetic susceptibility, EPR and 57Fe Mössbauer spectroscopy. Methods: In order to investigate the dynamics of CTPT for (CnH2n+1)4N[MIIFeIII(dto)3], we performed muon spin relaxation spectroscopy at Rutherford-Appleton Laboratory (UK) and the Paul Scherrer Institut (Switzerland), and dielectric constant measurement. Results: From the analysis of muon spin relaxation, we revealed the hopping rate of electrons between the FeII and FeIII sites at the CTPT. Moreover, we observed an anomalous enhancement of dielectric constant due to the valence fluctuation at the CTPT. This anomaly has a tendency to be divergent as the measuring frequency is lowered to 1 Hz, which is quite similar to the dielectric relaxation in relaxor ferroelectrics. Conclusion: In a mixed-valence system, (CnH2n+1)4N[MIIFeIII(dto)3]], we have investigated the CTPT due to the synergetic effect coupled with spin and charge, and revealed the valence fluctuation at the CTPT by means of muon spin relaxation spectroscopy, and dielectric constant measurement techniques.

Background: Photoinduced phase transitions represent a new way to control physical properties of materials by light. Achieving macroscopic and complete switching with a single laser pulse is of great interest for various applications, especially in the bistable hysteresis domain. Spin-crossover materials are prototype photoactive systems and various physical properties (magnetic, chromic, dielectric...) switch by light between low spin and high spin states. Objective: It is of fundamental interest to understand how the photoswitching initiated by a single ns laser pulse occurs inside the thermal hysteresis. For this purpose, we investigate the photoresponse of the spincrossover {FeII(pz)[Pt(CN)4]} material. Method: The photoinduced transformation inside the thermal hysteresis is investigated by combining the complementary optical microscopy and x-ray diffraction techniques, which are sensitive electronic and structural reorganizations. Results: These single-crystal studies show that a complete conversion from low spin to high spin states can be reached with a single laser shot above a threshold excitation density, as previously reported by Bousseksou by Raman spectroscopy. The structural reorganization after a single laser pulse is similar to the one observed during the complete thermal conversion from low spin to high spin states. Partial conversions, obtained with weaker excitation densities, are associated with the formation of high spin domains, evidenced by x-ray diffraction. Conclusion: Our results show that the photoinduced phase transition is stabilized by an important volume change. In addition, the non-linear response to light excitation density indicates that the process is mainly driven by the temperature jump of the crystal following laser excitation.