Photoinduced Metastable State Research Articles

Large and ultrafast photoinduced reflectivity change in the charge separated phase ofEt2Me2Sb[Pd(1,3−dithiol−2−thione−4,5−dithiolate)2]2

A large photoinduced change in reflectivity has been observed in the low-temperature charge separated (CS) phase of a dimeric radical anion salt, ${\text{Et}}_{2}{\text{Me}}_{2}\text{Sb}{[\text{Pd}{(\text{dmit})}_{2}]}_{2}$ ($\text{dmit}=1,3$-dithiol-2-thione-4,5-dithiolate). Just after the photoexcitation, the reflectivity abruptly changed reflecting the appearance of a photoinduced metastable state, indicating occurrence of recrystallizing of the CS phase by intradimer photoexcitation within a picosecond. Quantitative analysis considering the linear combination of the dielectric functions of the CS and the dimer-Mott state suggests a rather high efficiency of the photoinduced phase transition. One photon can change the valence of about five dimers. This photoinduced metastable state relaxed to the initial CS state via two successive types of relaxation processes, a fast and a slow one. The relaxation time $(\ensuremath{\tau})$ and the reflectivity of the fast process showed a clear excitation intensity and temperature dependence. In particular, $\ensuremath{\tau}$ and the estimated domain size were enhanced up to the transition temperature $({T}_{\text{c}})$ with increasing temperature. This phenomenon, a sort of critical slowing down around ${T}_{\text{c}}$, suggests that the density of the photoinduced state as well as the external temperature plays an important role in determining the relaxation dynamics of the photoinduced state. The results obtained indicate that this photoinduced phenomenon can be classified as a tuning of the charge in crystals via cooperative interaction between the degrees of freedom of ``charge'' and ``molecular orbital'' of the constituents, i.e., as a type of photoinduced phase transition.

Relaxation process from photoinduced states of double-step spin-crossover systems using a kinetic two-sublattice Ising-like model including intra-site coupling

A two-sublattice kinetic mean-field Ising-like model is proposed to describe the relaxation process from photo-induced metastable states in double-step spin-crossover complexes. Its originality lies in the rigorous treatment of intra-site coupling that yields dynamical equations by using additionally the Glauber stochastic treatment of master equations. The numerical resolution of these equations permits one to extract not only the high-spin fraction but also the intra-site correlation and the symmetry-breaking parameter which are hidden order parameters in these systems. Our simulation can demonstrate interesting relaxation behavior. As the result of synergy of inter- and intra-site interactions, double-step relaxation curves can be obtained in most cases. When decreasing the magnitude of interactions, one-step sigmoidal relaxation or stretched exponential relaxations appear. Asymmetry between $A$ and $B$ sublattices may even induce curious dynamics such as triple steplike behavior that is also observed for weak fluctuations in the initial configuration. For stronger fluctuations the system relaxes to an ordered metastable state instead of reaching the ground state. This model is finally applied to genuine systems, namely, the binuclear spin-crossover complex ${[\text{Fe}(\text{bt}){(\text{NCS})}_{2}]}_{2}(\text{bpym})$ and the polymeric spin-crossover compound ${\text{Fe}(\text{pmd})[\text{Ag}{(\text{CN})}_{2}]{\text{Ag}}_{2}{(\text{CN})}_{3}}$ that exhibit both a two steps thermal spin transition. The relaxation curves are then conveniently reproduced considering weak-interaction parameters whose possible origins will be highlighted.

Bulk and Surface Switching in Mn−Fe-Based Prussian Blue Analogues

Many Prussian Blue Analogues are known to show a thermally induced phase transition close to room temperature and a reversible, photo-induced phase transition at low temperatures. This work reports on magnetic measurements, X-ray photoemission and Raman spectroscopy on a particular class of these molecular heterobimetallic systems, specifically on Rb0.81Mn[Fe(CN)6]0.95_1.24H2O, Rb0.97Mn[Fe(CN)6]0.98_1.03H2O and Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.86_2.05H2O, to investigate these transition phenomena both in the bulk of the material and at the sample surface. Results indicate a high degree of charge transfer in the bulk, while a substantially reduced conversion is found at the sample surface, even in case of a near perfect (Rb:Mn:Fe=1:1:1) stoichiometry. Thus, the intrinsic incompleteness of the charge transfer transition in these materials is found to be primarily due to surface reconstruction. Substitution of a large fraction of charge transfer active Mn ions by charge transfer inactive Cu ions leads to a proportional conversion reduction with respect to the maximum conversion that is still stoichiometrically possible and shows the charge transfer capability of metal centers to be quite robust upon inclusion of a neighboring impurity. Additionally, a 532 nm photo-induced metastable state, reminiscent of the high temperature Fe(III)Mn(II) ground state, is found at temperatures 50-100 K. The efficiency of photo-excitation to the metastable state is found to be maximized around 90 K. The photo-induced state is observed to relax to the low temperature Fe(II)Mn(III) ground state at a temperature of approximately 123 K.

Theoretical study on the structure of the ground state and photo-induced metastable states of [M(CN)5NO]2− (M = Ru, Fe), and mechanism of the photo-rearrangement among them

Abstract In this note, we reviewed theoretical investigations of the photo-induced metastable states and discussed possible photo-rearrangement paths in [M(CN)5NO]2− (M = Fe and Ru). Using calculated results of ab initio multi-reference single and double excitation configuration interaction calculations with Davidson type quadruple correction (MRSDCI + Q) and those of density fuctional theory (DFT), we showed that the two metastable states (MS1 and MS2) were local minima of the electronic ground state potential surface. The theoretical calculations supported the experimental observation of the structure of the three isomers (GS, MS1 and MS2), relative stability of MS1 and MS2. We discussed the character of the lower electronic excited states and excitation energies of the three isomers of both compounds. Based on these results and excited state adiabatic potential energy surfaces, possible mechanism of photo-rearrangement among GS, MS1, and MS2 were discussed with the aid of our results on [Mn(CN)5NO]3− of which no photo-rearrangement has been reported.

Effect of temperature and illumination intensity on the formation of metastable states in a-Si:H

Nonmonotonic variation in the concentration of slow photoinduced metastable states and in the half-width of the distribution function for these states concerning their annealing times is observed in a-Si:H as temperature is increased in the range from 400 to 480 K. These nonmonotonic variations in the parameters of ensembles are determined by the temperature-dependent ratio between the rates of formation and annealing of the metastable states under study. It is also established that, as the film’s illumination intensity is decreased, a decrease in the concentration of metastable states and in the half-width of their annealing-time distribution sets in at higher temperatures. This behavior can be accounted for, in particular, by a substantial decrease in the rate of annealing of slow metastable states in comparison with a decrease in the rate of photoinduced formation of these states; according to the three-level model, this formation also involves a thermal process.

Valence tautomerism interconversion triggers transition to stable charge distribution in solid polymeric cobalt–polyoxolene complexes

The reaction of 3,3,3',3'-tetramethyl-5,5',6,6'-tetrahydroxy-1,1'spiro-bis(indane), L, with cobalt salts in the presence of diazine ligands affords polymeric derivatives of general formula [CoL(diazine)]infinity . nH2O, L being coordinated as a mixed-valence SQ-Cat species. The bipiridyl and o-phenantroline derivatives have been characterised by means of magnetic, EPR and calorimetric measurements. The characterisation showed that both systems are obtained in a metastable charge distribution, presumably trapped due to the class I character of the mixed valence form of the ligand. The occurrence of entropy-driven valence tautomeric interconversion induces the transition to the most stable charge distribution. The photomagnetic characterisation evidenced that valence tautomeric process can be photoinduced and that the lifetime of the photoinduced metastable state, obtained in low yield, is of ca. 2 x 10(5) s at 9 K.

Photo-induced Metastable State of S8Clusters in Liquid Sulfur

AbstractIt is well known that liquid sulfur undergoes a polymerization transition from S8 rings to the mixture of S8 rings and polymeric chains at Tp=159°C. In addition to the transition, it has been known that there is a transformation from S8ring to a molecular species other than S8ring and polymeric chain below Tpsince 19thcentury. Such molecular species can also be obtained by the illumination of light. Several models of the molecular form have been presented. However, the conclusive model which explains all experimental results has not been settled yet. Recently we measured transient photo-induced optical absorption in liquid sulfur and Munejiri et al. performed first-principles molecular-dynamics simulations to clarify the microscopic origin of the experimental results. The results of the simulations presented a plausible molecular form as the molecular species, which is a tadpole-shaped S8 cluster. We show that the molecular form clearly explains the previous experimental results. A possibility of the formation of an intimate valence alternation pair in the small cluster is also discussed.

First study of the photo-induced metastable (paramagnetic) high-spin state in a [Fe(ptz)6](BF4)2 single crystal by polarised neutron diffraction

Some spin-crossover solids can show photo-convertibility at low temperatures, which generally makes this class of compounds interesting for studies towards application as optical switches and memories in numerical displays. The mechanism of the optical switching is the light-induced excited spin state trapping (LIESST) and the reverse-LIESST, as reported before [Inorg. Chem. 24 (1985) 2174; J. Phys. Chem. 94 (1991) 2741]. These processes can be combined with the relaxation in order to cycle the excited state, which involves temperature scanning of the title system [Polyhedron 20 (2001) 1599]. In this paper, we report the spin density map in the [Fe(ptz)6](BF4)2 single crystal in its undercooled R3i space group in the photo-induced metastable high-spin state, which is paramagnetic, at T=2 K. The experiment was carried out on the 5C1 polarised neutron diffractometer at LLB newly equipped with an optical fiber for the light-coupled measurements. The photo-excitation reaches a steady state within an hour and a half. The lifetime of the photo-induced high-spin state at 40 K exceeds 10 h, as in accordance with previous spectroscopic, magnetic and Mössbauer measurements. The first results obtained on [Fe(ptz)6](BF4)2 can serve as a base to future investigations of other photo-convertible materials.

Photoinduced Metastable States in Amorphous Organic Polysilanes Studied by the Transient Photocurrent Technique

Ultra-violet-light-induced states in amorphous polymethylphenylsilane have been investigated by a time-of-flight technique for transient photocurrent measurements. It is found that the photo-induced change is reversible below 320 K, while irreversible above 320 K. The finding is attributable to the formation of two photocreated dangling bond states having different metastabilities. It is shown that the experimental results are successfully explained in terms of the configurational-coordinate potential-energy diagram representing the ground-state and two different metastable-dangling-bond-state configurations. Origins of the two metastable dangling bond states are discussed.

Photo-induced reverse valence tautomerism in a metastable Co compound

Abstract In our preceding paper, we have reported that a Co complex, [CoIII-LS(tmeda)(3,5-DBSQ)(3,5-DBCat)]·0.5C6H5CH3, exhibits a photo-induced valence tautomerism. The photo-induced metastable state, [CoII-HS(tmeda)(3,5-DBSQ)2]·0.5C6H5CH3, has a back electron transfer band from CoII-HS to 3,5-DBSQ at around 800 nm, indicating that the valence tautomerism should be photo-reversible. In fact, we have found that the metastable state could be photo-pumped back to the original state by exciting the back electron transfer band. The process can be expressed as [ Co II-HS ( tmeda )(3,5 -DBSQ ) 2 ]·0.5 C 6 H 5 CH 3 ( metastable state )→[ Co III-LS ( tmeda )(3,5 -DBSQ )(3,5 -DBCat )]·0.5 C 6 H 5 CH 3 (ground state).

Photoinduced ferrimagnetic systems in Prussian blue analogues C(I)xCo4[Fe(CN)6]y (C(I) = alkali cation). 4. Characterization of the ferrimagnetism of the photoinduced metastable state in Rb1.8Co4[Fe(CN)6]3.3-13H2O by K edges X-ray magnetic circular dichroism.

In Part 2 of this work, the electronic and local structure of the photoinduced metastable magnetic state of the Prussian blue analogue Rb1.8Co4[Fe(CN)6]3.3-13H2O were characterized. To determine directly the relative orientation of the magnetic moments of Co(II) and Fe(III) ions in the metastable state, and the nature of the exchange interaction between them, we performed X-ray magnetic circular dichroism (XMCD) experiments at the cobalt and iron K edges. We present the first direct experimental evidence of the antiferromagnetic interaction between the cobalt and the iron ions, leading to the ferrimagnetism of the photoinduced metastable state.

Photo-Induced Long-Lived Intramolecular Electron Transfer in a Co Valence Tautomeric Complex

Abstract A photo-induced valence tautomerism was observed in a Co complex, [CoIII-LS(phen)(3,5-DBSQ)(3,5-DBCat)]. In this system, visible light illumination induces an electron transfer from 3,5-DBCat to CoIII-LS, producing [CoII-HS(phen)(3,5-DBSQ)2]. The resultant photo-induced metastable state could be trapped more than 1 h at 5 K.

Resonant Raman scattering and photoinduced metastability in oxygen-deficient YBa 2Cu 3O x

We studied the photoinduced metastable states in oxygen-deficient YBa 2Cu 3O x (Y123) ( x=6.35–6.87). The states are identified by the intensity dependences of CuO-chain resonance on photo irradiation and temperature. Below room temperature (RT) the CuO-chain resonance can be completely suppressed by irradiation with visible light polarized along the CuO chains, indicating photo-assisted oxygen ordering. Above RT, the resonance intensity returns and saturates at T>100°C. This behavior exactly reflects the temperature dependence of chain oxygen ordering in the same crystals observed by hard X-ray diffraction. The stability of the photo-induced state is in agreement with reported persistent photo-induced superconductivity effects.

Photoinduced metastability in YBa 2Cu 3O x studied by in situ Raman scattering

We study the photoinduced metastable states in oxygen deficient YBa 2Cu 3O x ( x=6.35 – 6.87). The states are identified by the intensity dependences of the CuO-chain resonance with photo irradiation and temperature. Below room temperature (RT) the CuO-chain resonance can be completely suppressed by irradiation with visible light polarized along the CuO chains, indicating photo-assisted oxygen ordering. Above RT the resonance intensity returns and saturates at T>100°C. This behavior exactly mirrors the temperature dependence of chain oxygen ordering observed by hard X-ray diffraction. The stability of the photo-induced state is in line with the reported persistent photo-induced superconductivity effects.

Molecular-Orbital Calculation on the Neutral Mixed-Valent Platinum Complex [PtIICl2(en)][PtIVCl4(en)

The character of the photo-induced metastable state is investigated via molecular-orbital calculation. The anisotropic g-factor and the hyperfine coupling constants are simulated based on the extended-Hückel calculation. The most plausible environment around the metastable PtIII is found to be a spin-soliton like state, whose spin-orbital mainly comprises one platinum and one chlorine atoms. The vibration mode of the elemental unit of the complex is calculated for both high- and low-temperature structures employing a density-functional method.

Special features of relaxation of metastable states induced thermally and by photoexcitation in (a-Si:H):P films

Kinetics of relaxation of metastable states that are induced thermally or by photoexcitation and cause an increase in dark electrical conductivity of (a-Si:H):P films is discussed. It is established that the relaxation is described by expanded exponential functions with the parameters τ and β depending differently on temperature in the cases of thermal excitation and photoexcitation. Thus, the relaxation of photo-induced states is characterized by a decrease in β with temperature, whereas the parameter β is almost temperature-independent for thermally induced states. It is shown that these dissimilar temperature dependences of β correlate with temperature variations of the half-width of annealing-energy distribution for these states. The observed features of relaxation of thermally induced and photo-induced metastable states are caused by different mechanisms of their formation. The origin of these states can be the same and related to activation of hydrogen-passivated phosphorus atoms.

The First Crystallographic Evidence for Side-On Coordination of N2 to a Single Metal Center in a Photoinduced Metastable State

The First Crystallographic Evidence for Side-On Coordination of N₂ to a Single Metal Center in a Photoinduced Metastable State

Relaxation of photoinduced metastable states in a-Si:H films deposited at high temperatures

The results of an experimental study of dark-conductivity kinetics in a-Si:H after short-term and long-term illumination, are presented. The films were deposited at temperatures in the range Ts=300–390 °C. Data on relaxation of the photoinduced metastable states were found to correlate with the Fermi-level position.

Photo-induced structural change in amorphous organic polysilanes: comparison with hydrogenated amorphous silicon

Photo-induced metastable states of transient hole transport in amorphous polymethylphenylsilane (PMPS) have been investigated with the time-of-flight technique, and are compared with those in hydrogenated amorphous silicon (a-Si:H). The photo-induced states of both materials can essentially be explained by the photo-created dangling bond mechanism, but in contrast to a-Si:H, the photo-induced structural states in PMPS become more disordered. The origins of the difference and of the photo-induced structural change in PMPS are discussed.

Structural and photophysical investigations of polyacetylene prepared by different precursor routes

We report here structural and photophysical studies on polyacetylene prepared by various precursor routes. We find that control of stereoregularity, molecular weight and molecular weight distribution of the precursor allows improvements in ordering parameters, such as molecular orientation and crystallographic order of the final polymer. The photophysical properties of polyacetylene also depend on the particular precursor preparation route. The peak in the π–π∗ absorption lies between 2.15 and 2.65 eV, depending on the precursor; also, photoinduced metastable band-gap states vary in position, shape and strength.