Spin Conversion Process Research Articles

Two‐Photon Excited Magneto‐Photoluminescence in Exciplex Material for 3D Mapping of Magnetic Field

AbstractTwo‐photon excited fluorescence materials with sufficient responsiveness to magnetic field are essential for applications such as 3D mapping of magnetic field distributions based on magneto‐optical effect. In this study, a two‐photon excited magneto‐photoluminescence (MPL) in the intermolecular exciplex of 1,1‐bis[(di‐4‐tolylamino)phenyl]cyclohexane and tris(2,4,6‐trimethyl‐3‐(pyridin‐3‐yl)phenyl)borane (TAPC:3TPYMB) is observed. This MPL can be attributed to the promotion of delocalization of excitons in the exciplex under two‐photon excitation, resulting in a weakened spin‐exchange interaction and thereby enhancing the magnetic field‐sensitive spin conversion process. The temperature‐dependent MPL further validates the strong correlation between the delocalization of excitons in the exciplex and the enhanced MPL observed during temperature variation. Combining the penetration capability and magneto‐optical effect of two‐photon excited fluorescence in TAPC:3TPYMB exciplex, a novel magnetic sensor suitable for 3D mapping of magnetic field distributions is further developed. The present work not only enhances the understanding of the magneto‐optical coupling mechanism but also opens up opportunities for the application of molecular materials in opto‐spintronic devices.

Efficient Spin Interconversion by Molecular Conformation Dynamics of a Triplet Pair for Photon Up-Conversion in an Amorphous Solid.

Solid-state materials with improved light-to-energy conversions in organic photovoltaics and in optoelectronics are expected to be developed by realizing efficient triplet-triplet annihilation (TTA) by manipulating the spin conversion processes to the singlet state. In this study, we elucidate the spin conversion mechanism for delayed fluorescence by TTA from a microscopic view of the molecular conformations. We examine the time evolution of the electron spin polarization of the triplet-pair state (TT state) in an amorphous solid-state system exhibiting highly efficient up-conversion emission by using time-resolved electron paramagnetic resonance. We clarified that the spin-state population of the singlet TT increased through the spin interconversion from triplet and quintet TT states during exciton diffusion with random orientation dynamics between the two triplets for the modulation of the exchange interaction, achieving a high quantum yield of up-conversion emission. This understanding provides us with a guide for the development of efficient light-to-energy conversion devices utilizing TTA.

Anthracene derivatives with strong spin-orbit coupling and efficient high-lying reverse intersystem crossing beyond the El-Sayed rule.

The attention to materials with hot exciton channel and triplet-triplet fusion (TTF) mediated high-lying reverse intersystem crossing (hRISC) has been raised for their ability to convert non-emissive 'dark' triplets into radiative singlet excitons. This spin conversion process results in high exciton utilization efficiency (EUE) that exceeds the theoretical limits. Notably, it is known that such spin conversion processes from the high-lying excited triplet to the singlet state are facilitated by the orthogonal orbital transition effect governed by the El-Sayed's rule. In this study, an anthracene derivative with indenoquinoline substituent 7,7-dimethyl-9-(10-(4-(naphthalen-1-yl)phenyl)anthracen-9-yl)-7H-indeno[1,2-f]quinoline (2MIQ-NPA) was synthesized and analyzed to investigate whether the hRISC process occurs in these molecules, even when the El-Sayed's rule is not followed. The hRISC channels of the emitter were fully unraveled through DFT calculations and experiments, which were quantitatively subdivided using transient electroluminescence measurements. The results showed that 2MIQ-NPA, which does not follow the El-Sayed's rule and has a relatively strong spin-orbit coupling matrix element of 0.116 cm-1 between the high-lying triplet state of T4 and the lowest singlet state of S1, effectively converted triplet excitons into singlet excitons with an EUE of 64.3%, contributed by a direct hot exciton channel of 19.2% and a TTF-mediated hot exciton channel of 15.1%. Despite the low outcoupling efficiency, the non-doped device with 2MIQ-NPA achieved an excellent device performance with an external quantum efficiency of 7.0%.

Spin-vibronic coherence drives singlet-triplet conversion.

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry1-3. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings4,5. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes6-9 to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.

Electrochemical Characterization of Ionic Dynamics Resulting from Spin Conversion of Water Isomers

Para- and ortho-isomers of water have different chemical and physical properties. Excitations by magnetic field, laser emission or hydrodynamic cavitation are reported to change energetic levels and spin configurations of water molecules that in turn change macroscopically measurable properties of aqueous solutions. Similar scheme is also explored for dissolved molecular oxygen, where physical excitations form singlet oxygen with different spin configurations and generate a long chain of ionic and free-radical reactions. This work utilizes electrochemical impedance spectroscopy (EIS) to characterize ionic dynamics of proposed spin conversion methods applied to dissolving of carbon dioxide CO2 and hydrogen peroxide H2O2 in pure water excited by fluctuating weak magnetic field in μT range. Measurement results demonstrate different ionic reactivities and surface tension effects triggered by excitations at 10−8 J/mL. The CO2- and O2-related reaction pathways are well distinguishable by EIS. Control experiments without CO2/H2O2 input show no significant effects. Dynamics of electrochemical impedances and temperature of fluids indicates anomalous quasi-periodical fluctuations pointing to possible carbonate-induced cyclic reactions or cyclical spin conversion processes. This approach can underlie the development of affordable electrochemical sensors operating with spin conversion technologies with applications in quantum biology, biophysics, and material science.

Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogs: Diamond-like Carbon and Graphite

Abstract Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear spin conversion (NSC) processes of H2 molecules on diamond-like carbon and graphite surfaces are investigated experimentally by employing temperature-programmed desorption and resonance-enhanced multiphoton ionization methods. For the diamond-like carbon surface, the NSC time constants were determined at temperatures of 10–18 K and from 3900 ± 800 s at 10 K to 750 ± 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp3 or sp2 hybridization) have little effect on the NSC rates.

Unraveling the Important Role of High-Lying Triplet-Lowest Excited Singlet Transitions in Achieving Highly Efficient Deep-Blue AIE-Based OLEDs.

Aggregation-induced emission (AIE) materials are attractive for achieving highly efficient nondoped organic light-emitting diodes (OLEDs) owing to their strong luminescence in the solid state. However, the electroluminescence efficiency of most AIE-based OLEDs remains low owing to the waste of triplet excitons. Here, using theoretical calculations, photophysical dynamics, and magnetoluminescence measurements, the spin conversion process is demonstrated between the high-lying triplet state (Tn ) and the lowest excited singlet state (S1 ) in AIE materials. Moreover, the relative positions of Tn (n<4) and S1 are shown to have a significant impact on the spin-conversion efficiency, thus influencing the harvesting of triplet excitons and the device efficiency. Finally, by selecting an upconversion material with an appropriate energy level for further utilizing the triplet excitons, a deep-blue fluorescent OLED with CIE coordinates of (0.15, 0.08), amaximum external quantum efficiency of 10.2%, low efficiency roll-off, and a high brightness of 16817cdm-2 is developed. This is one of the most efficient deep-blue OLEDs based on AIE materials reported so far. These findings also provide new insights into the design of more efficient AIE molecules and corresponding OLEDs by managing high-lying triplet excitons.

Singlet to triplet conversion in molecular hydrogen and its role in parahydrogen induced polarization.

Detailed experimental and comprehensive theoretical analysis of singlet-triplet conversion in molecular hydrogen dissolved in a solution together with organometallic complexes used in experiments with parahydrogen (the H2 molecule in its nuclear singlet spin state) is reported. We demonstrate that this conversion, which gives rise to formation of orthohydrogen (the H2 molecule in its nuclear triplet spin state), is a remarkably efficient process that strongly reduces the resulting NMR (nuclear magnetic resonance) signal enhancement, here of 15N nuclei polarized at high fields using suitable NMR pulse sequences. We make use of a simple improvement of traditional pulse sequences, utilizing a single pulse on the proton channel that gives rise to an additional strong increase of the signal. Furthermore, analysis of the enhancement as a function of the pulse length allows one to estimate the actual population of the spin states of H2. We are also able to demonstrate that the spin conversion process in H2 is strongly affected by the concentration of 15N nuclei. This observation allows us to explain the dependence of the 15N signal enhancement on the abundance of 15N isotopes.

Geometries and Terahertz Motions Driving Quintet Multiexcitons and Ultimate Triplet–Triplet Dissociations via the Intramolecular Singlet Fissions

Importance of vibronic effects has been highlighted for the singlet-fission (SF) that converts one high-energy singlet exciton into doubled triplet excitons, as strongly coupled multiexcitons. However, molecular mechanisms of spin conversion processes and ultimate decouplings in the multiexcitons are poorly understood. We have analyzed geometries and exchange couplings (singlet-quintet energy gaps: 6J) of the photoinduced multiexcitons in the pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)2 and m-(Pc)2] by simulations of the time-resolved electron paramagnetic resonance spectra. We clarified that terahertz molecular conformation dynamics play roles on the spin conversion from the singlet strongly coupled multiexcitons 1(TT) to the quintet multiexcitons 5(TT) and on the intramolecular decouplings in the 6J to form spin correlated triplet pairs (T+T). The strongly coupled 5(TT) multiexcitons are revealed to possess entirely planar conformations stabilized by mutually delocalized spin distributions, while the intramolecular decoupled spin-correlated triplet pairs generated at 1 μs are also stabilized by distorted conformations resulting in two separately localized biradical characters.

Spin-phonon coupling and dynamic zero-field splitting contributions to spin conversion processes in iron(II) complexes.

Magnetization dynamics of transition metal complexes manifest in properties and phenomena of fundamental and applied interest [e.g., slow magnetic relaxation in single molecule magnets, quantum coherence in quantum bits (qubits), and intersystem crossing (ISC) rates in photophysics]. While spin-phonon coupling is recognized as an important determinant of these dynamics, additional fundamental studies are required to unravel the nature of the coupling and, thus, leverage it in molecular engineering approaches. To this end, we describe here a combined ligand field theory and multireference ab initio model to define spin-phonon coupling terms in S = 2 transition metal complexes and demonstrate how couplings originate from both the static and dynamic properties of ground and excited states. By extending concepts to spin conversion processes, ligand field dynamics manifest in the evolution of the excited state origins of zero-field splitting (ZFS) along specific normal mode potential energy surfaces. Dynamic ZFSs provide a powerful means to independently evaluate contributions from spin-allowed and/or spin-forbidden excited states to spin-phonon coupling terms. Furthermore, ratios between various intramolecular coupling terms for a given mode drive spin conversion processes in transition metal complexes and can be used to analyze the mechanisms of ISC. Variations in geometric structure strongly influence the relative intramolecular linear spin-phonon coupling terms and will define the overall spin state dynamics. While the findings of this study are of general importance for understanding magnetization dynamics, they also link the phenomenon of spin-phonon coupling across fields of single molecule magnetism, quantum materials/qubits, and transition metal photophysics.

Magnetic field effects in rigidly linked D-A dyads: Extreme on-resonance quantum coherence effect on charge recombination

Charge recombination in the photoinduced charge separated (CS) state of a rigidly linked donor/bridge/acceptor triad with a triarylamine (TAA) donor, a 1,3-diethynyl-2,5-dimethoxy benzene bridge (OMe), and a perylenediimide (PDI) unit as an acceptor, represents a spin chemical paradigm case of a rigid radical ion pair formed with singlet spin and recombining almost exclusively to the locally excited PDI triplet state (3PDI). The magnetic field dependence of the CS state decay and 3PDI formation kinetics are investigated from 0 to 1800 mT by nanosecond laser flash spectroscopy. The time-resolved magnetic field affected reaction yields spectra of the CS state population and 3PDI population exhibit a sharp and deep resonance at 18.9 mT, indicating level crossing of the S and T+ levels separated by an exchange interaction of J = 18.9/2 mT at zero field. The kinetics are biexponential around the resonance field and monoexponential outside that range. The monoexponential behavior can be simulated by a classical kinetic model assuming a single field dependent double Lorentzian function for the energy gap dependence of all spin conversion processes. The full field dependence of the kinetics has been simulated quantum theoretically. It has been shown that incoherent and coherent hyperfine coupling contribute to S/T+ spin conversion at all fields and that the biexponentiality of the kinetics at resonance is due to a partitioning of the overall kinetics into 2/3 of the singlet hyperfine states exhibiting strong isotropic coupling to T+ and 1/3 of the singlet hyperfine states that do not or only weakly couple isotropically to T+.

Rotational Motion and Nuclear Spin Interconversion of H2O Encapsulated in C60 Appearing in the Low-Temperature Heat Capacity

The heat capacity of H2O encapsulated in fullerene C60 is determined for the first time at temperatures between 0.6 and 200 K. The water molecule in H2O@C60 undergoes quantum rotation at low temperature, and the ortho-H2O and para-H2O isomers are identified by labeling the rotational energy levels with the nuclear spin states. A rounded heat capacity maximum is observed at ∼2 K after rapid cooling due to splitting of the rotational J KaKc = 101 ground state of ortho-H2O. This anomalous feature decreases in magnitude over time, reflecting the conversion of ortho-H2O to para-H2O. Time-dependent heat capacity measurements at constant temperature reveal three nuclear spin conversion processes: a thermally activated transition with Ea ≈ 3.2 meV and two temperature-independent tunneling processes with time constants of τ1 ≈ 1.5 h and τ2 ≈ 11 h.

J-Resonance Line Shape of Magnetic Field-Affected Reaction Yield Spectrum from Charge Recombination in a Linked Donor–Acceptor Dyad

Magnetic field effects (MFEs) allow detailed insight into spin conversion processes of radical pairs that are formed, for example, in all charge separation processes, and are supposed to play the key role in avian navigation. In this work, the MFE of charge recombination in the charge-separated state of a rigid donor–bridge–acceptor dyad was analyzed by a classical and a quantum theoretical model and represents a paradigm case of understanding spin chemistry with unprecedented detail. The MFE is represented by magnetic field-affected reaction yield (MARY) spectra that exhibit a sharp resonance, resulting from S/T level crossing as the Zeeman splitting equals twice the exchange interaction. Although in the classical kinetic model, the spin conversion processes between the four singlet and triplet substates are shown for the first time to obey an identical generalized energy dependence, quantum theory proves that the MARY resonance line is composed of relaxation, coherent hyperfine induced spin mixing, and S/T dephasing contributions.

Temperature- and Pressure-Induced Spin Crossover in Co1+xCr2-xSe4 (x = 0.24): A Diffraction Study.

The Co(2+) ions of the Co1+xCr2-xSe4 phase (Co1.24Cr1.76Se4 composition with x = 0.24, C2/m space group, Cr3S4-type structure) undergo a high- to low-spin-state transition around 230 K, as concluded from the temperature-dependent single-crystal synchrotron radiation diffraction experiments and the previously reported physical property studies. The change of the spin state is not instantaneous and goes through a wide spin-crossover (SCO) region of 75 K. A similar Co(2+) high- to low-spin-state transition is suggested at a pressure of 14.5 GPa, as is evident from the pressure-dependent single-crystal synchrotron radiation diffraction experiments. The corresponding SCO region is equal to 5 GPa, and the structural behavior is different from the one observed during the temperature-dependent transition. Coupling between the spin-conversion process in Co1+xCr2-xSe4 and the concomitant changes in the physical properties opens a way for a controlled tuning of the observed physical response through compositional and structural modifications.

Ortho-para hydrogen conversion characteristics of amorphous and mesoporous Cr2O3 powders at a temperature of 77 K

In this study, mesoporous Cr2O3 catalysts are prepared by use of the hard template method using mesoporous silica (SBA-15) as template, the prepared catalysts were characterized, and investigated for ortho to para hydrogen. The Cr2O3 catalysts at post-treated temperature at 150 °C have the largest BET surface area of 230 m2 g−1, and the length and diameter of the particles are 2 μm and 5 nm, respectively. The highest spin conversion from ortho to para hydrogen was observed for the mesoporous Cr2O3 catalysts with the largest surface area post-treated at 150 °C in the range from 150 to 300 °C. The nanowire array morphology of the powders was maintained at the post-treatment temperature ranges of 150 and 200 °C. However, when temperature increased up to 300 °C, the nanowire array morphology disappeared. It was showed that the spin conversion process was strongly dependent on various factors such as BET surface area, calcination temperature and morphology of mesoporous Cr2O3 catalysts.

Spin conversion of hydrogen using supported iron catalysts at cryogenic temperature

Alumina-supported iron oxides have been prepared by incipient wetness impregnation method and employed for orthohydrogen to parahydrogen spin conversion at cryogenic temperature. These materials were characterized using a series of characterization techniques such as SEM, XRD, Raman and in situ FTIR spectroscopy. The spin conversion was investigated at low temperature by a batch mode of operation. The in situ FTIR spectra were collected in a transmission mode to obtain the spin conversion. While the iron oxide was highly dispersed over alumina support at low loading percent, a rodlike crystallite of iron oxide was formed at high loading percent. The 10 and 20wt% iron oxides on alumina were proved to be the most active catalysts. The spin conversion process was very slow and time-dependent. It was concluded that the spin conversion was a function of various factors including the iron oxide loading percent, calcination temperature, and different supports.

Nuclear spin conversion of water inside fullerene cages detected by low-temperature nuclear magnetic resonance.

The water-endofullerene H2O@C60 provides a unique chemical system in which freely rotating water molecules are confined inside homogeneous and symmetrical carbon cages. The spin conversion between the ortho and para species of the endohedral H2O was studied in the solid phase by low-temperature nuclear magnetic resonance. The experimental data are consistent with a second-order kinetics, indicating a bimolecular spin conversion process. Numerical simulations suggest the simultaneous presence of a spin diffusion process allowing neighbouring ortho and para molecules to exchange their angular momenta. Cross-polarization experiments found no evidence that the spin conversion of the endohedral H2O molecules is catalysed by (13)C nuclei present in the cages.

Preparation, isolation, storage, and spectroscopic characterization of water vapor enriched in the ortho-H2O nuclear spin isomer

Using magnetic focusing in a supersonic jet, a beam of ``normal'' H${}_{2}$O molecules seeded in a krypton carrier gas is shown to provide a source of water molecules that is highly enhanced in the ortho-H${}_{2}$O ($o$-H${}_{2}$O) nuclear spin isomer over the high-temperature equilibrium 3:1 ortho:para ratio. Water from the magnetically focused beam is then isolated and stored within a Kr matrix at 13 K whereby the amplitude and lifetime of this strong nuclear spin polarization are quantified spectroscopically. Attempts to store the polarization in a colder Kr matrix reveal complex nuclear spin conversion processes.

Molecular spin conversion in solid deuterated methane

The spin conversion of methane molecules in pure deuterated methane crystals and CD(4)-Kr solid solution for a wide range of concentrations of krypton was investigated in the temperature range 1.5-10 K. The experiment was performed by use of a steady-state heat flow experimental setup for determination of the thermal conductivity, utilized in an unconventional way. The obtained results were discussed in the frame of the spin conversion model taking into account direct one-phonon processes and indirect librationally-activated processes. It was found that the conversion, both for pure and krypton doped crystals, is dominated by the one-phonon mechanism. However, the importance of the indirect processes increases rapidly with the temperature. The obtained results indicate that the krypton admixture does not change the values of energy levels of the spin-librational (spin-rotational) spectrum of the crystal. The presence of Kr in the structure of CD(4) enhances the intensity of the direct one-phonon spin conversion processes and weakens the indirect librationally-activated ones.

Femtosecond X-ray Absorption Spectroscopy οf a Light-Driven Spin-Crossover Process

Understanding the initial steps during ultrafast molecular reactions involving large spin state changes is a vital goal in structural dynamics research. These involve knowledge of the geometric structure of the system. Ultrafast X-ray absorption spectroscopy with 50-100 picosecond time resolution establishes the geometric structure of the short-lived (tau = 0.6 ns) high spin state. Here we focus on time-resolved X-ray absorption studies with 160-200 fs temporal resolution to monitor the structural evolution in this spin-conversion process.