Excited Trapping Research Articles

Control of Iron(II)-Tris(2,2'-Bipyridine) Light-Induced Excited-State Trapping via External Electromagnetic Fields.

Light-induced excited spin-state trapping reactions in iron pyridinic complexes allow the iron's low-to-high spin transition in a sub-picosecond timescale. Employing a recently developed model for [Fe(2,2'-bipyridine)3]2+ photochemical spin-crossover reaction in conjunction with quantum wavepacket dynamics, we explore the possibility of controlling the reaction through external electromagnetic fields, aiming at stabilizing the initial metal-to-ligand charge transfer states. We show that simple Gaussian-shaped electromagnetic fields have a minor effect on the population kinetics. However, introducing vibrationally excited initial wavepacket representations allows for maintaining the population trapped in the metal-to-ligand charge transfer states. Using optimal control theory, we propose an electromagnetic field shape that increases the lifetime of metal-to-ligand charge transfer states. These results open the route for controlling the iron photochemistry through the action of external electric fields.

A cyanide-bridged FeII/FeIII mixed-valence chain exhibiting spin transition in the Fe(II) sites

A cyanide-bridged {FeIII2FeII}n one-dimensional chain {[Fe(Tp)(CN)3]2Fe(btb)}n (1) (Tp = hydrotris(pyrazolyl)borate, btb = 1,4-(1H-1,2,4-triazole)butane) has been synthesized via the hydrothermal reaction of Li[Fe(Tp)(CN)3], btb, and FeII(ClO4)2·6H2O at 120 °C. Single-crystal X-ray analysis reveals that compound 1 is an isolated neutral mixed-valence {FeIII2FeII}n double-zigzag chain with each [FeIII(Tp)(CN)3]− unit connecting with two Fe(II) centres via two of its three CN− groups in the cis-positions, and two triazole groups from btb coordinated with Fe(II) ion in its axial positions. Single-crystal data obtained at different temperature together with magnetic data indicate that complex 1 exhibits thermal induced spin transition in the Fe(II) sites with T1/2 ≈ 165 K. Additionally, light-induced excited spin-state trapping (LIESST) of 1 was also observed at low temperature.

Correlating Valence and 2p3d RIXS Spectroscopies: A Ligand-Field Study of Spin-Crossover Iron(II).

The molecular spin-crossover phenomenon between high-spin (HS) and low-spin (LS) states is a promising route to next-generation information storage, sensing applications, and molecular spintronics. Spin-crossover complexes also provide a unique opportunity to study the ligand field (LF) properties of a system in both HS and LS states while maintaining the same ligand environment. Presently, we employ complementing valence and core-level spectroscopic methods to probe the electronic excited-state manifolds of the spin-crossover complex [FeII(H2B(pz)2)2phen]0. Light-induced excited spin-state trapping (LIESST) at liquid He temperatures is exploited to characterize magnetic and spectroscopic properties of the photoinduced HS state using SQUID magnetometry and magnetic circular dichroism spectroscopy. In parallel, Fe 2p3d RIXS spectroscopy is employed to examine the ΔS = 0, 1 excited LF states. These experimental studies are combined with state-of-the-art CASSCF/NEVPT2 and CASCI/NEVPT2 calculations characterizing the ground and LF excited states. Analysis of the acquired LF information further supports the notion that the spin-crossover of [FeII(H2B(pz)2)2phen]0 is asymmetric, evidenced by a decrease in eπ in the LS state. The results demonstrate the power of cross-correlating spectroscopic techniques with high and low LF information content to make accurate excited-state assignments, as well as the current capabilities of ab initio theory in interpreting these electronic properties.

Bidirectional photomagnetism, exciplex fluorescence and dielectric anomalies in a spin crossover Hofmann-type coordination polymer.

Stepped spin crossover (SCO) complexes with three or more spin states have promising applications in high-order data storage, multi-switches and multi-sensors. Further synergy with other functionalities, such as luminescence and dielectric properties, will provide a good chance to develop novel multifunctional SCO materials. Here, a bent pillar ligand and luminescent pyrene guest are integrated into a three-dimensional (3D) Hofmann-type metal-organic framework (MOF) [Fe(dpoda){Au(CN)2}2]·pyrene (dpoda = 2,5-di-(pyridyl)-1,3,4-oxadiazole). The magnetic data show an incomplete and two-step SCO behavior with the sequence of 1 ↔ 1/2 ↔ 1/4. The rare bi-directional light-induced excited spin-state trapping (LIESST) effect and light-induced stepped thermal relaxation after LIESST are observed. The pyrene guests interact with dpoda ligands via offset face-to-face π⋯π interactions to form intermolecular exciplex emissions. The competition between thermal quenching and stepped SCO properties results in a complicated and stepped exciplex fluorescence. Moreover, the stepped dielectric property with higher dielectric permittivity at lower temperature may be related to the more frustrated octahedral distortion parameters in the intermediate spin states. Hence, a 3D Hofmann-type MOF with bent pillar ligands and fluorescent guests illustrates an effective way for the development of multifunctional switching materials.

Simultaneous observation of semi-continuous and Ramsey-coherent population trapping resonances using the Ramsey pulse sequence with drive current modulation

In this study, we proposed a method to observe the semi-cw excited coherent population trapping (SCW-CPT) and the Ramsey-CPT resonance during a Ramsey pulse sequence. By simultaneously observing the Ramsey-CPT and SCW-CPT resonances, which have distinct light shift coefficients, it is easy to detect light shift from the resonance frequency ratio of these two atomic resonances. Experimental results demonstrated the capability of the proposed method to simultaneously observe both the Ramsey-CPT and SCW-CPT resonances. The frequency difference attributable to the light shift was 308.9 Hz, indicating the successful detection of the light shift. Long-term measurements revealed that the frequency drift rates of the two atomic resonances differ by a factor of 3.5, and the variation in the frequency ratio enabled the detection of fluctuations in these two resonance frequencies.

Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(II) complexes.

Spin-state switching in iron(II) complexes composed of ligands featuring moderate ligand-field strength-for example, 2,6-bi(1H-pyrazol-1-yl)pyridine (BPP)-is dependent on many factors. Herein, we show that spin-state switching in isomeric iron(II) complexes composed of BPP-based ligands-ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt, L1) and (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methylacetate (BPP-CH2OCOMe, L2)-is dependent on the nature of the substituent at the BPP skeleton. Bi-stable spin-state switching-with a thermal hysteresis width (ΔT1/2) of 44 K and switching temperature (T1/2) = 298 K in the first cycle-is observed for complex 1·CH3CN composed of L1 and BF4- counter anions. Conversely, the solvent-free isomeric counterpart of 1·CH3CN-complex 2a, composed of L2 and BF4- counter anions-was trapped in the high-spin (HS) state. For one of the polymorphs of complex 2b·CH3CN-2b·CH3CN-Y, Y denotes yellow colour of the crystals-composed of L2 and ClO4- counter anions, a gradual and non-hysteretic SCO is observed with T1/2 = 234 K. Complexes 1·CH3CN and 2b·CH3CN-Y also underwent light-induced spin-state switching at 5 K due to the light-induced excited spin-state trapping (LIESST) effect. Structures of the low-spin (LS) and HS forms of complex 1·CH3CN revealed that spin-state switching goes hand-in-hand with pronounced distortion of the trans-N{pyridyl}-Fe-N{pyridyl} angle (ϕ), whereas such distortion is not observed for 2b·CH3CN-Y. This observation points that distortion is one of the factors making the spin-state switching of 1·CH3CN hysteretic in the solid state. The observation of bi-stable spin-state switching with T1/2 centred at room temperature for 1·CH3CN indicates that technologically relevant spin-state switching profiles based on mononuclear iron(II) complexes can be obtained.

Iron(II) Complexes of 2,6-Di[4-(ethylcarboxy)pyrazol-1-yl]pyridine with Reversible Guest-Modulated Spin-Crossover Behavior.

Three solvatomorphs of the iron(II) complex of 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (bpCOOEt2p) of formulas [Fe(bpCOOEt2p)2](ClO4)2·1.5MeNO2 (1), [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2), and [Fe(bpCOOEt2p)2](ClO4)2·2MeNO2 (3) have been prepared and characterized. They show interesting spin-crossover (SCO) properties ranging from partial to complete thermal spin transitions and a light-induced excited spin-state trapping (LIESST) effect. In solvatomorph 2, a robust structure is formed with channels that enable the entrance or removal of solvent molecules by vapor diffusion without losing the crystallinity. Thus, solvent-exchanged samples [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2·MeNO 2 ), [Fe(bpCOOEt2p)2](ClO4)2·MeCN (2·MeCN), [Fe(bpCOOEt2p)2](ClO4)2·0.5Me2CO (2·Me 2 CO), and [Fe(bpCOOEt2p)2](ClO4)2·MeCOOH (2·MeCOOH) were prepared by vapor diffusion of the solvents in a crystal of the compound previously heated to 400 K in a single-crystal to single-crystal (SCSC) fashion. Interestingly, this causes a change of spin state with a stabilization of the low-spin state in 2·Me 2 CO and the high-spin state in 2·MeCN. Therefore, the SCO properties of 2 can be tuned in a reversible way by exposure to different solvents.

Interface versus Bulk Light-Induced Switching in Spin-Crossover Molecular Ultrathin Films Adsorbed on a Metallic Surface.

Spin-crossover molecules present the unique property of having two spin states that can be controlled by light excitation at low temperature. Here, we report on the photoexcitation of [FeII((3, 5-(CH3)2Pz)3BH)2] (Pz = pyrazolyl) ultrathin films, with thicknesses ranging from 0.9 to 5.3 monolayers, adsorbed on Cu(111) substrate. Using X-ray absorption spectroscopy measurements, we confirm the anomalous light-induced spin-state switching observed for sub-monolayer coverage and demonstrate that it is confined to the first molecular layer in contact with the metallic substrate. For higher coverages, the well-known light-induced excited spin-state trapping effect is recovered. Combining continuous light excitation with thermal cycling, we demonstrate that at low temperature light-induced thermal hysteresis is measured for the thicker films, while for sub-monolayer coverage, the light enables extension of the thermal conversion over a large temperature range. Mechanoelastic simulations underline that, due to the intermolecular interactions, opposite behaviors are observed in the different layers composing the films.

Two-Dimensional Coordination Polymer Showing Spin-Crossover Behavior with a 64 K Wide Hysteresis Loop.

A two-dimensional grid-like coordination polymer, [Fe(NCBH3)2(Py2ttz)2]·4CHCl3 (1·4CHCl3, Py2ttz = 2,5-di(pyridin-4-yl)thiazolo[5,4-d]thiazole), showed one-step complete spin crossover with unexpectedly large hysteresis loop of 64 K wide and temperature-induced excited spin-state trapping effect below 91 K.

Spin-Crossover in a New Iron(II)/Di(pyrazolyl)pyridine Complex with a Terpyridine Embrace Lattice. Thermally Induced Excited Spin State Trapping and Clarification of a Structure–Function Correlation

The complex salts [FeL2]X2 (1X2; L = 2,6-di{4-fluoropyrazol-1-yl}pyridine; X– = BF4– or ClO4–) exhibit abrupt spin-transitions with narrow thermal hysteresis, at T1/2 = 164 K (X– = BF4–) and 148 K (X– = ClO4–). The transition in 1[ClO4]2 is complicated by efficient thermally induced excited spin-state trapping (TIESST) of its high-spin state below ca. 120 K, and the fully low-spin state was achieved only inside the magnetometer at a scan rate of 0.5 K min–1. Crystals of 1[BF4]2 are tetragonal (P421c, Z = 2; phase 1) at 300 K but transform to a highly twinned monoclinic phase 2 (P21, Z = 2) at 285 ± 5 K. These are forms of the “terpyridine embrace” crystal lattice, which often affords cooperative spin-transitions in iron/di(pyrazolyl)pyridine complexes. Phase 2 of high-spin 1[BF4]2 shows a significant temperature dependence by powder diffraction, which reflects increased canting of the monoclinic unit cell as the temperature is lowered. In contrast, 1[ClO4]2 retains phase 2 between 100 and 300 K, and was crystallographically characterized in its thermally trapped metastable high-spin state at 100 K, as well as its thermodynamic high- and low-spin forms at higher temperatures. The spin-crossover transition temperature in 1[ClO4]2 and related compounds correlates well with a parameter describing angular changes to the metal coordination sphere during the transition but not with other commonly used structural indices. The TIESST metastable high-spin state of 1[ClO4]2 shows no single molecule magnet properties at 2 K.

Native defect-related broadband ultrafast photocarrier dynamics in n-type β-Ga2O3

We report two-photon excited ultrafast carrier trapping and recombination in n-type β-Ga2O3 crystals by using femtosecond transient absorption spectroscopy. The broadband absorption spectra arising from the defect are polarization dependent, especially, two absorption peaks can be observed by subtracting the absorption transients under two probe polarizations. We attribute these observed defect-related absorption features to optical transitions from the valence band to different charge states of a native defect (such as gallium vacancies). A model for carrier capture by multilevel of a single defect is proposed to interpret the data, wherein holes are captured more efficiently than electrons by the defects, and the absorption cross sections for the defects are at least ten times larger than that for free carriers. Our results reveal the potential applications of β-Ga2O3 in ultrafast and broadband optoelectronic devices.

Light-induced thermal hysteresis and high-spin low-spin domain formation evidenced by optical microscopy in a spin-crossover single crystal

The low-temperature photoinduced effects of the spin-crossover $[{\mathrm{Fe}{(2\text{\ensuremath{-}}\mathrm{pytrz})}_{2}[\mathrm{Pd}{(\mathrm{CN})}_{4}]}].3{\mathrm{H}}_{2}\mathrm{O}$ single crystal have been investigated by means of a cryogenic optical microscopy technique down to 10 K from which the imaging and quantitative analysis of the spatiotemporal transformation are derived. The magnetic investigations revealed that this compound exhibits an incomplete spin transition between a full high-spin (HS) state at high temperature and an intermediate HS and low-spin (LS) state, where HS and LS species coexist, as a result of the existence of an elastic frustration at the molecular scale, most likely caused by the rigidity of the interconnected $[\mathrm{Pd}{(\mathrm{CN})}_{4}$] [Fe(II)/Pd(II)] two-dimensional network. At low temperature (10 K), thanks to reverse light-induced excited spin-state trapping effect, we could switch the system from the intermediate HS-LS state to the fully photoinduced LS state by irradiating the sample in near-infrared region, revealed by photomagnetic and optical microscopy studies. Optical microscopy images showed monotonous and homogeneous transformation of the crystal color along this process, corresponding to a gradual change of the spin state under light. In contrast, the thermal relaxation in the dark of this photoinduced LS-LS state shows a transition to the intermediate HS-LS state at $\ensuremath{\sim}90$ K with domain formation, which is characteristic of a first-order transition at equilibrium. Interestingly, the same behavior is also obtained during the heating process of the reverse light-induced thermal hysteresis cycle with a heating branch almost unchanged, confirming that light does not act on this transition. It is then concluded that the transition from the full LS to the intermediate HS-LS transition is of first order and therefore the LS state reached by light is a hidden stable state of the system until $\ensuremath{\sim}90$ K. These experimental results are modeled using an adapted version of the electroelastic model including photoexcitation effects and solved by Monte Carlo method. Both thermal equilibrium and light-induced thermal hysteresis are reproduced, in fair qualitative agreement with the experimental data of photomagnetism and optical microscopy.

Regulation of Multistep Spin Crossover Across Multiple Stimuli in a 2-D Framework Material.

We investigate the effects of a broad array of external stimuli on the structural, spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)2Pd(CN)4]·guest (cintrz = N-cinnamalidene 4-amino-1,2,4-triazole; A·guest; guest = 3H2O, 2H2O, and Ø). This framework exhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we show that manipulation of the pore contents across guests = 3H2O, 2H2O, and Ø can be exploited to regulate this balance. In A·3H2O, the dominant antiferroelastic interaction character between neighboring FeII sites sees the low-temperature persistence of the mixed spin-state species {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin). Elastic interaction strain is responsible for stabilizing the {HS-LS} state and can be overcome by three mechanisms: (1) partial (2H2O) or complete (Ø) guest removal, (2) irradiation via the reverse light-induced excited spin-state trapping (LIESST) effect (λ = 830 nm), and (3) the application of external hydrostatic pressure. Combining experimental data with elastic models presents a clear interpretation that while guest molecules cause a negative chemical pressure, they also have consequences for the elastic interactions between metals beyond the simple chemical pressure picture typically proposed.

Spin Crossover in a Cobalt Complex on Ag(111).

The Co‐based complex [Co(H2B(pz)(pypz))2] (py=pyridine, pz=pyrazole) deposited on Ag(111) was investigated with scanning tunneling microscopy at ≈5 K. Due to a bis(tridentate) coordination sphere the molecules aggregate mainly into tetramers. Individual complexes in these tetramers undergo reversible transitions between two states with characteristic image contrasts when current is passed through them or one of their neighbors. Two molecules exhibit this bistability while the other two molecules are stable. The transition rates vary linearly with the tunneling current and exhibit an intriguing dependence on the bias voltage and its polarity. We interpret the states as being due to S=1 / 2 and 3 / 2 spin states of the Co2+ complex. The image contrast and the orders‐of‐magnitude variations of the switching yields can be tentatively understood from the calculated orbital structures of the two spin states, thus providing first insights into the mechanism of electron‐induced excited spin‐state trapping (ELIESST).

Shifting photo-stationary light-induced excited spin state trapping equilibrium towards higher temperature by increasing light fluence

The Light-Induced Excited Spin-State Trapping (LIESST) process allows tuning the electronic state of spin-crossover materials from low (LS) to high spin (HS) states. The photoinduced HS state is long-lived, up to a characteristic temperature, T(LIESST), above which the system relaxes rapidly to the LS state. We study the effect of light irradiance on competing LS → HS up-conversion and HS → LS thermal relaxation on the [Fe(phen)2(NCS)2] system. Raman spectroscopy and magnetic measurements are used to investigate this phenomenon. An empiric model describes the competition between up conversion and thermal relaxation mechanisms and the stabilization of photo-stationary state towards higher temperature.

Light-induced hidden multistability in a spin crossover metal–organic framework

An extremely rare bidirectional light-induced excited spin-state trapping effect is firstly observed in a 3D Hofmann-type metal–organic framework with 1-fluoronaphthalene guests.

Trend in light-induced excited-state spin trapping in Fe(II)-based spin crossover systems.

A computational study of the light-induced excited spin-state trapping (LIESST) in a number of Fe(II) spin crossover complexes, coordinated by monodentate, bidentate and multidentate ligands is carried out, with the goal to uncover the trend in the low temperature relaxation rate. A nine order of magnitude change in low temperature relaxation rate is observed among the complexes. The trend is rationalized in terms of the change in metal-ligand covalency, numerically estimated by the crystal orbital Hamiltonian population, thus influencing the back donation or delocalization of the electrons from the low-lying Fe(II)-centered molecular orbital to the empty low-lying ligand-centered π* antibonding molecular orbitals.

Tailoring the Energy Storage Properties of the Lu2(Ge,Si)O5:Pr Phosphors Varying the Ge:Si Ratio

Lu2(Gex,Si1−x)O5:Pr phosphors were recently reported as very good multimodal luminescence thermometers. However, their luminescence decay traces exhibited profound background at some temperatures, which was detrimental for their thermometric performance. We set out to reveal the possible reasons for such properties. Thermoluminescence glow curves recorded observing either the total emission generated by the Pr3+ activator or only one of its three components resulting from the 5d → 4f, 3P0 → 3H4, and 1D2 → 3H4 transitions, allowed us to understand that the phosphors show a very strong capability of excited carriers trapping. These are released while the sample temperature reaches the value when the traps become thermally unstable. Consequently, at some temperature ranges, the luminescence arises both from photoluminescence and persistent luminescence. This is what deteriorates the thermometric properties of the phosphors family. For the luminescence intensity ratio mode or the decay time mode, such properties are evidently unfavorable and should not be neglected in critical analysis of the data.

Asymmetric Design of Spin-Crossover Complexes to Increase the Volatility for Surface Deposition.

A mononuclear complex [Fe(tBu2qsal)2] has been obtained by a reaction between an Fe(II) precursor salt and a tridentate ligand 2,4-di(tert-butyl)-6-((quinoline-8-ylimino)methyl)phenol (tBu2qsalH) in the presence of triethylamine. The complex exhibits a hysteretic spin transition at 117 K upon cooling and 129 K upon warming, as well as light-induced excited spin-state trapping at lower temperatures. Although the strongly cooperative spin transition suggests substantial intermolecular interactions, the complex is readily sublimable, as evidenced by the growth of its single crystals by sublimation at 573 → 373 K and ∼10-3 mbar. This seemingly antagonistic behavior is explained by the asymmetric coordination environment, in which the tBu substituents and quinoline moieties appear on opposite sides of the complex. As a result, the structure is partitioned in well-defined layers separated by van der Waals interactions between the tBu groups, while the efficient cooperative interactions within the layer are provided by the quinoline-based moieties. The abrupt spin transition is preserved in a 20 nm thin film prepared by sublimation, as evidenced by abrupt and hysteretic changes in the dielectric properties in the temperature range comparable to the one around which the spin transition is observed for the bulk material. The changes in the dielectric response are in excellent agreement with differences in the dielectric tensor of the low-spin and high-spin crystal structures evaluated by density functional theory calculations. The substantially higher volatility of [Fe(tBu2qsal)2], as compared to a similar complex without tBu substituents, suggests that asymmetric molecular shapes offer an efficient design strategy to achieve sublimable complexes with strongly cooperative spin transitions.

Spectral and dynamic analysis of CsPbBr3 perovskite nanocrystals with enhanced water stability using sodium passivation

Sodium (Na) passivated CsPbBr3 nanocrystals (NCs) are synthesized using a modified ligand-assisted reprecipitation method at room temperature. With Na element introducing, small blue-shift absorption and photoluminescence (PL) are observed due to the surface passivation of NCs. The Na passivated NCs with the PL quantum yield of 66% have the improvement of water and thermal stability, with the reduction of 20% comparing with initial PL intensity in water/n-hexane after 12 h and a higher exciton binding energy of 150.8 meV. The spectral and dynamic excitonic recombination for Na: CsPbBr3 NCs are detected by time-resolved PL and transient absorption spectroscopy. The dynamics indicate that the coupling between the lowest excited state and exciton trapping state has significantly enhanced exciton recombination efficiency for Na: CsPbBr3 NCs. It can be observed that faster PL lifetime exists for CsPbBr3 NCs than that of Na: CsPbBr3 NCs in water/n-hexane, which indicates surface defects passivated through sodium incorporation. Na: CsPbBr3 NCs dispersing in NaBr aqueous/nonpolar solution remain bright after 60 days. The obtained results demonstrate that the effective strategies via sodium passivation can improve emission efficiency and stability of perovskite NCs, making them superior potential materials in optoelectronic applications.