Large Spin-orbit Coupling Constant Research Articles

Single component long-lived dynamic room temperature phosphorescence with large-scale preparation and visible light excitation

A pure organic room temperature phosphorescence (PORTP) luminogen named as DCP was successfully prepared and showed long room temperature phosphorescence (RTP) and afterglow lifetimes (3 ms and 4 s in sequence). Meanwhile, DCP showed high fluorescence (0.14) and RTP (0.39) quantum yields in crystalline state, simultaneously presented visible light excitation and dynamic RTP. By host-guest doping and optimization, Urea/DCP doping system emitted blue RTP, with RTP and afterglow lifetimes of 341 ms of 5 s in turn. Crystal analysis and theoretical calculations indicated H stacking, large spin orbit coupling constant, and small energy gap between the lowest singlet and triplet states result in long RTP lifetime and high quantum yield for DCP. Based on long phosphorescence emission, an advanced anti-counterfeiting pattern was constructed. As far as we know, it is the first report that such small molecules containing 3, 5-dicyanopyridine show long-lived RTP in both crystalline state and doping system. In this paper, not only ultralong and blue PORTP materials with the advantage of large-scale preparation, but also single component dynamic RTP, and RTP of visible light excitation were achieved.

Triplet-triplet energy transfer between host and guest induced strong phosphorescence in the organic doped system

Recently, purely organic room-temperature phosphorescence (RTP) from guest-host-doped systems has grown exponentially due to their high modulation flexibility. Benzophenone has been found as a preferred host matrix thanks to its low melting point, minimal biological toxicity, remarkable stability, and cost-effectiveness. However, the precise role of benzophenone remains elusive and has been debated within the scientific community concerning mechanistic elucidations. Here, five indole derivatives were designed and synthesized as guest molecules while retaining benzophenone as the host to construct doped materials exhibiting outstanding RTP properties. Our experimental findings unequivocally establish that the host benzophenone initially absorbs excitation energy, subsequently intersystem crossing into triplet exciton, facilitating the exciton transfer to guest molecule through the triplet energy transfer and culminating in guest RTP emission. Mechanistic investigation corroborates the superiority of benzophenone in possessing a large spin-orbit coupling constant and quantitatively triplet energy transfer. Collectively, this work provides compelling evidence deciphering energy transfer between host and guest entities for the manifestation of phosphorescence performance in benzophenone-hosted doped materials.

Modulating excited state properties of thermally activated delayed fluorescence molecules by hybrid long-range and short-range charge transfer strategy

Balancing the rapid radiative decay process and the fast reverse intersystem crossing (RISC) process of thermally activated delayed fluorescence (TADF) molecule remains a great challenge and efficient molecular design strategies are highly desired. Herein, from a theoretical perspective, excited state properties of three reported TADF molecules (1TICz, 1BOICz and 2BOICz) are investigated based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the thermal vibration correlation function (TVCF) method. Results indicate that, by introducing the multi-resonance (MR) acceptor, 1BOICz possesses hybrid long-range and short-range charge transfer features, balanced small energy gap (ΔEST) and large oscillator strength (f) is obtained. Furthermore, by incorporating double equivalent MR acceptors in 2BOICz, largely enhanced f with slightly changed ΔEST is achieved, inner mechanism for remarkable photophysical property is illustrated. Keep this strategy, seven new TADF molecules (2pDBA-bICz-1, 2pDBA-bICz-2, 2OSBA-bICz, 2DQAO-bICz, 2QAO-bICz, 2SQAO-bICz and 2OQAO-bICz) are theoretically designed, detailed physical parameters are analyzed and excited state energy consumption process is studied. Strong electrophilicity on acceptor is determined and the strength of nucleophilic sites on the bridge-phenyl of 2DQAO-bICz, 2QAO-bICz, 2SQAO-bICz and 2OQAO-bICz is increased, this promotes the short-range charge transfer property. In addition, the excitation processes for all studied molecules are dominated by long-range charge transfer from donor to acceptors, and supplemented by the short-range charge transfer on the bridge-phenyl with MR effect. Compromise energy gap and oscillator strength as well as large spin orbit coupling (SOC) constant are obtained for designed molecules. Thus, by regulating the long-range and short-range charge transfer ratios, excited state properties are successfully modulated and new efficient TADF molecules are proposed. Our research aims to provide deeper insight into long-range and short-range charge transfer features in balancing small ΔEST and large f, which could facilitate the development of novel efficient TADF molecules.

Theoretical perspective for structural isomerism effect on photoelectric properties of organic room temperature phosphorescence molecules

Purely organic room temperature phosphorescence (RTP) materials have garnered extensive attentions in anti-counterfeiting and encryption, information display, biological imaging and organic light emitting diodes (OLEDs) due to the features of long lifetimes, low toxicity and good biocompatibility. Nevertheless, both the species and amounts of the efficient RTP molecules are far from meeting the requirements for practical applications, and the quantitative relationship between molecular structures and optoelectronic properties needs full elucidated. Herein, the photophysical properties and properties of three isomers (o-BA, m-BA and p-BA) are explored based on the first-principles calculations coupled with thermal vibration correlation function (TVCF) method and kinetic Monte Carlo simulations. The substituent effects on the molecular structures, intermolecular interactions, transition properties, Huang-Rhys factors and reorganization energies, excited state dynamics as well as transfer integrals and charge carrier mobilities are investigated in detail to clarify the structure-property relationships. Results show that o-BA achieves fast radiative rate on account of impressive transition dipole moment and oscillator strength, but equally large non-radiative decay rate leads to low RTP efficiency. Promisingly, the highest holes mobility is determined for o-BA due to the largest transfer integrals of holes originating from the strong intermolecular interactions. In addition, compared with o-BA and p-BA, m-BA could achieve fast intersystem crossing (ISC) rate with large spin orbit coupling constant and low non-radiative decay rate with small geometric structure changes. Relatively balanced holes and electrons mobilities of m-BA have been demonstrated, which contributes to the recombination of excitons and the improvement of luminescence efficiency. For p-BA, the strong intermolecular hydrogen bonding restricts the free rotation of single bonds and promotes the molecular coplanarity, which contributes to efficient emission. Furthermore, the charge mobility is calculated and the temperature dependence is investigated, the hopping mechanisms for studied isomers are illustrated. This work reveals the relationship among molecular structures, crystal packing modes, photophysical properties and charge transport properties of isomers, and provides an insight for molecular design and property prediction.

A Bipolar Delayed Fluorescence Luminogen with Fast Reverse Intersystem Crossing and High Horizontal Dipole Orientation for High‐Performance Sky‐Blue and White OLEDs

AbstractThe development of robust luminescent materials plays a key role for organic light‐emitting diodes (OLEDs). Herein, a tailor‐made sky‐blue delayed fluorescence luminogen (DCPC‐BP‐SFAC) containing a carbonyl acceptor and spiro[acridine‐9,9″‐fluorene] and 9,9″‐(cyclohexane‐1,1‐diylbis(4,1‐phenylene))bis(9H‐carbazole) donors is explored, which holds merits of excellent thermal stability, strong photoluminescence, balanced carrier transport, and horizontal dipole orientation. Owing to a charge‐transfer dominated singlet state and a charge‐transfer and local‐excitation hybrid triplet state, DCPC‐BP‐SFAC has a small energy gap between singlet and triplet as well as a large spin–orbit coupling constant, which result in a fast reverse intersystem crossing process and thus efficient delayed fluorescence. High‐performance sky‐blue OLEDs with outstanding external quantum efficiencies (ηext) reaching 32.6% are achieved based on DCPC‐BP‐SFAC, benefiting from high exciton utilization and large light out‐coupling efficiency. Moreover, using DCPC‐BP‐SFAC as blue emitter and host material simultaneously, efficient all‐fluorescence out‐of‐phase sensitizing white OLEDs without interlayer are fabricated, providing high ηext of up to 21.6% and stable warm‐white light with a high color rendering index of 84. The outstanding electroluminescence performances demonstrate the great potentials of DCPC‐BP‐SFAC in practical display and lighting devices.

Role of Halide Ions in the Nature of the Magnetic Anisotropy in Tetrahedral CoII Complexes.

A series of mononuclear tetrahedral CoII complexes with a general molecular formula [CoL2 X2 ] [L=thiourea and X=Cl (1), Br (2) and I (3)] were synthesized and their structures were characterized by single-crystal X-ray diffraction. Direct-current (dc) magnetic susceptibility [χM T(T) and M(H)] and its slow relaxation of magnetization were measured for all three complexes. The experimental dc magnetic data are excellently reproduced by fitting both χM T(T) and M(H) simultaneously with the parameters D=+10.8 cm-1 , g1 =2.2, g2 =2.2, and g3 =2.4 for 1; D=-18.7 cm-1 , giso =2.21 for 2; and D=-19.3 cm-1 , giso =2.3 for 3. The replacement of chloride in 1 by bromide or iodide (in 2 and 3, respectively) was accompanied by a change in both sign and magnitude of the magnetic anisotropy D. Field-induced out-of-phase susceptibility signals observed in 10 % diluted samples of 1-3 imply slow relaxation of magnetization of molecular origin. To better understand the magnetization relaxation dynamics of complexes 1-3, detailed ab initio CASSCF/NEVPT2 calculations were performed. The computed spin Hamiltonian parameters are in good agreement with experimental data. In particular, the calculations unveil the role of halide ions in switching the sign of D on moving from Cl- to I- . The large spin-orbit coupling constant associated with the heavier halide ion and weaker π donation reduces the ground state-excited state gap, which leads to a larger contribution to negative D for complex 3 compared to complex 1. Further magnetostructural D correlations were developed to understand the role of structural distortion in the sign and magnitude of D values in this family of complexes.

Electronic Structure and Energy Transfer in Europium(III)-Ciprofloxacin Complexes: A Theoretical Study.

The structure and ligand-localized excited states of [Eu(cfqH) (cfq)(H2O)4]Cl2 (cfqH is ciprofloxacin) are studied by XMCQDPT2/CASSCF with full geometry optimization. The complex includes one anionic and one zwitterionic ligand. Two low-lying triplet states, both localized on the anionic ligand, are found. One of them has sufficient energy to transfer to the (5)D1 sublevel of Eu(3+), because its T-S0 vertical transition energy is equal (or very close) to the (7)F0-(5)D1 Eu(3+) excitation energy. The other triplet state has a very small S0-T1 gap, which favors fast nonradiative relaxation. Two other triplet states are localized on the zwitterionic ligand. One low-lying excited singlet state (S1) is localized on the anionic ligand; the other excited singlet is localized on the zwitterionic one. Spin-orbit coupling constants were calculated for the relaxed geometry of each state (ground state, two low-lying triplets, and one low-lying excited singlet) by spin-orbit configuration interaction (CI) with Pauli-Breit Hamiltonian. Large spin-orbit coupling constants between S1 and both triplets together with small energy gaps are indicative of fast intersystem crossing (ISC) from the excited singlet state to the triplet manifold. This ISC process is followed by energy transfer from the ligand-localized triplet states to the (5)D1 sublevel of Eu(3+). However, relatively large spin-orbit coupling constants between S0 and one of the triplet states together with the small T-S0 energy gap shows that this state can decay without transferring its energy to Eu(3+). This mechanism is expected to be common for other Ln(3+)-fluoroquinolone complexes.

Theoretical investigation of [Ru(tpy)2]2+, [Ru(tpy)(bpy)(H2O)]2+ and [Ru(tpy)(bpy)(Cl)]+ complexes in acetone revisited: Inclusion of strong spin–orbit couplings to quantum chemistry calculations

In the present paper, we theoretically reinvestigate structural properties, and photo-physical and chemical characteristics and electronic absorption spectra of three kinds of ruthenium polypyridyl complexes [Ru(tpy)[Formula: see text], [Ru(tpy)(bpy)(H2O)][Formula: see text], and [Ru(tpy)(bpy)(Cl)][Formula: see text] complexes in acetone (tpy[Formula: see text]2,2[Formula: see text],2[Formula: see text]-terpyridine and bpy[Formula: see text]2,2[Formula: see text]-bipyridine). In particular, the experimental absorption spectra of these complexes are revisited theoretically in detail and are simulated using the first-order perturbation theory based on time-dependent density functional theory (TD-DFT) where the first-order perturbation term is the spin–orbit (SO) coupling Hamiltonian, and quantum chemistry calculations based on various functionals and basis sets. It was found that in general the theory including SO coupling can reproduce experimental data better than the simple quantum chemistry calculation neglecting SO coupling, which indicates that SO coupling is very important to understand the optical features of these complexes and that therefore the mixing between singlet and triplet states is strong due to the large SO coupling constant of Ru atom involved in these complexes. This suggests the fact that the disagreement between the experimental and calculated absorption spectra was found in TDB3LYP/(SDD with triple-[Formula: see text] for Ru and 6-31G* for others) [Jakubikova EJ et al., Inorg Chem 48:10720, 2009] can be tracked down to the neglect of SO couplings. It was also found that the choice of the DFT functionals and basis sets is crucial for a good theoretical reproduction of experimental data.

Synthesis and photophysical characterization of a bis-pincer osmium complex

The neutral bis-pincer transition metal complex Os(PCP)2 (PCP=2,6-(CH2PPh2)2C6H3, Ph=C6H5) was prepared via two synthetic routes with up to 42% yield. The lemon-yellow complex is air stable as a neat solid and in solution, and is thermally stable as can be purified by high vacuum zone sublimation at 280-240-200°C under 1×10−6torr. The pincer ligands coordinate in a pseudo-octahedral arrangement around the metal center. X-ray crystallography reveals that the complex is in pseudo-D2 geometry symmetry due to the twist caused by the methylenes. NMR studies at varied temperatures between 223 and 343K along with low temperature inversion recovery studies show the fluxional behavior and exchanging process between the two enantiomers (Ea=8.6kcal/mol). The Os(PCP)2 shows no emission in solution at room temperature, but emits at 77K in 2-MeTHF glass (Φ=60%). At room temperature, the neat solid of Os(PCP)2 shows yellow emission of 3% quantum yield. TDDFT predicted S0→S1 transition shows that the lowest singlet state (S1) is metal-ligand (the Os-Xylyl fragments) to ligand’ (the PPh2 fragments) charge transfer (ML-L’CT) in character with a small contribution from the higher lying metal centered (1MC) states. The large spin–orbit coupling constant of osmium leads to effective intersystem crossing to a triplet state with increased MC character. The DFT predicted triplet spin surface of Os(PCP)2 indicates that the emission is dominated by 3MC state. Significant Os–P bonds elongation at the lowest triplet excited state also strongly supports the conclusion.

Photophysics of Soret-Excited Tetrapyrroles in Solution. I. Metalloporphyrins: MgTPP, ZnTPP, and CdTPP

The photophysical behavior of three Soret-excited diamagnetic meso-substituted tetraphenylmetalloporphyrins, MgTPP, ZnTPP, and CdTPP, have been examined in a wide variety of solvents using both steady-state and femtosecond fluorescence upconversion methods. The S 2 population of MgTPP decays to S 1 on the time scale of a few picoseconds with unit S 2-S 1 internal conversion efficiency, and the decay rates conform to the weak coupling case of radiationless transition theory. The energy gap law parameters characterizing the coupling of the S 2 and S 1 states of MgTPP have been obtained. The most important accepting vibrational modes in the S 1 state are multiple in-plane C-C and C-N stretches in the 1200-1500 cm (-1) range. Net S 2-S 1 decay is the dominant decay path for ZnTPP and CdTPP as well, but the process occurs at rates that exceed (in the case of CdTPP, they vastly exceed) those predicted by weak interstate coupling. Alternate mechanisms for the radiationless decay of the S 2 states of ZnTPP and CdTPP have been explored. Large spin-orbit coupling constants and the presence of multiple, near-equiergic triplet states suggest that S 2-T n intersystem crossing might occur at rates competitive with internal conversion. However, the measured efficiencies of S 2-S 1 internal conversion show that, at most, only a few percent of the S 2 population of ZnTPP and no more than about 30% of the S 2 population of CdTPP can decay by a "dark" path such as intersystem crossing.

Ru-induced spin rotation in La 1.2Sr 1.8Mn 1.9Ru 0.1O 7 bilayered manganite

We have studied the magnetic properties of Ru-doped La 1.2Sr 1.8Mn 1.9Ru 0.1O 7 bilayered manganite using neutron diffraction (ND) technique. The ND measurements show that the compound exhibits ferromagnetic ordering with the spins direction parallel to the c-axis. This is in sharp contrast to the parent compound where the spins are oriented along the a– b plane. The large spin–orbit coupling constant of Ru +4 ion ( λ = ∼ 900 cm - 1 ) seems to play an important role on lifting of the orbital degeneracy of the e g state by which the e g electron prefers to occupy the d 3 z 2 - r 2 orbital and therefore, the spins align along the c-axis.

Quantum transport analysis and narrow-gap heterojunction growth for Rashba-type spintronics devices

Research results of spintronics based on spin-orbit (SO) interaction in non-magnetic semiconductor hetero-junctions obtained recently have been described. Works are based on the two-dimensional electron gases (2DEGs) confined at compound semiconductor narrow bandgap hetero-interface. Due to the electric field originated from the confining potential asymmetry, the 2DEG often yields strong SO interaction which could reveal under no magnetic field. This type of SO interaction (Rashba interaction) can be controlled by the applied gate voltage and hence the field effect transistor (FET) utilizing this principle has so far been proposed and discussed extensively. We describe two recent results in this paper: First is molecular beam epitaxy (MBE) growth of novel narrow-gap modulation-doped heterojunction, InGaSb/InAlSb material system which possibly reveals high quality electronic properties as well as very strong Rashba SO coupling. Recently we indeed obtained the sample with a very large SO coupling constant of ∼40 × 10–12 eVm which is almost comparable to the best value obtained in the former InGaAs/InAlAs systems. Second is relating to the control of Rashba SO interaction in long wires with side gates. Asa result of careful analysis about the dependencies of the SO coupling constant on the gate voltage, we confirmed the side-gate control of the Rashba effect for the first time, which could be a promising result to develop the spin-FET based quantum-bit devices.

Spin-related transport in one-dimensional conductors made at high-in content InGaAs/InA1As hetero-junctions

Results of spin-related transport observed in various one-dimensional (1D) samples made at high In-content InGaAs/InAlAs modulationdoped hetero-junctions are reported. This type of material is interesting especially from the view point of non-magnetic semiconductor spintronics, since it contains two-dimensional electron gas (2DEG) which reveals large spin–orbit coupling constant (α) as well as very high electron mobility (μe) at low temperatures. 1D structures studied here are diffusive narrow wires (width, w) and quantum point contacts (QPCs) as typical example of ballistic transport regime. In the long diffusive wires with a variety of width, αis found to remain unchanged when w > 2 μm. But in the diffusive wires with side-gate (the width of the wire can thus be controlled by the side-gate voltage), α is enhanced by applying negative voltage to the side gate. This is probably due to that the side-gate voltage has enhanced asymmetric lateral electric field perpendicular to the moving direction of 2DEG. In the QPC samples defined by the wire and the finger side-gate, conductance quantization in unit of 0:5(2e2/h) is confirmed. This might be caused by the formation of ‘spin-related Tomonaga–Luttinger wire’, which encourages spin-polarized transport in such an ideal conductor. Finally, we propose several quantum information processing devices based on the spin-FET, that could construct a solid-state qubit and quantum computation devices and circuits.

Intense fluorescence of metal-to-ligand charge transfer in [Pt(0)(binap)(2)] [binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl

[Pt(0)(binap)(2)] (binap = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) is found to exhibit a luminescence from metal-to-ligand charge transfer state (MLCT) with a quantum yield of 0.12 and a lifetime of 1.2 micros in toluene at an ambient temperature. Prompt fluorescence with a quantum yield of 1.6 x 10(-)(4) is observed by means of a picosecond time-correlated single photon counting technique. The spectrum of the steady-state luminescence is almost identical to that of the prompt fluorescence, indicating that the intense luminescence is mainly delayed fluorescence from thermally activated (1)MLCT. The analysis of the temperature-dependent emission indicates that the energy difference between the (1)MLCT and (3)MLCT is 1.15 x 10(3) cm(-)(1). The lifetime of the prompt fluorescence is determined to be 3.2 ps from the decay of stimulated emission overlapped on subpicosecond transient absorption spectra. The lifetime of the (1)MLCT is much longer than expected from the large spin-orbit coupling constant of 5d (Pt) electrons (4000 cm(-)(1)). Theoretical analysis based on density functional theory reveals that structural distortion in the MLCT states causes large energy splitting between HOMO and HOMO - 1, which prevents a very fast ISC induced by strong spin-orbit interactions between these orbitals. The relatively slow ISC is therefore induced by weak spin-orbit interactions (ca. 50 cm(-)(1)) between ligand-centered molecular orbitals. Theoretical calculations indicate that the phosphorescence observed at lower temperatures is due to intensity borrowing from 4(1)B(2) --> GS transition. However, the large energy difference between HOMO and HOMO - 2 reduces the extent of mixing between the lowest (3)MLCT and 4(1)B(2) due to spin-orbit interaction, thereby decreasing the radiative rate of the phosphorescence.

Structure-Dependent Photophysical Properties of Singlet and Triplet Metal-to-Ligand Charge Transfer States in Copper(I) Bis(diimine) Compounds

The photophysical properties of singlet and triplet metal-to-ligand charge transfer (MLCT) states of [Cu(I)(diimine)(2)](+), where diimine is 2,9-dimethyl-1,10-phenanthroline (dmphen), 2,9-dibutyl-1,10-phenanthroline (dbphen), or 6,6'-dimethyl-2,2'-bipyridine (dmbpy), were studied. On 400 nm laser excitation of [Cu(dmphen)(2)](+) in CH(2)Cl(2) solution, prompt (1)MLCT fluorescence with a quantum yield of (2.8 +/- 0.8) x 10(-5) was observed using a picosecond time-correlated single photon counting technique. The quantum yield was dependent on the excitation wavelength, suggesting that relaxation of the Franck-Condon state to the lowest (1)MLCT competes with rapid intersystem crossing (ISC). The fluorescence lifetime of the copper(I) compound was 13-16 ps, unexpectedly long despite a large spin-orbit coupling constant of 3d electrons in copper (829 cm(-1) ). Quantum chemical calculations using a density functional theory revealed that the structure of the lowest (1)MLCT in [Cu(dmphen)(2)](+) (1(1)B(1)) was flattened due to the Jahn-Teller effect in 3d(9) electronic configuration, and the dihedral angle between the two phenanthroline planes (dha) was about 75 degrees with the dha around 90 degrees in the ground state. Intramolecular reorganization energy for the radiative transition of 1(1)B(1) was calculated as 2.1 x 10(3) cm(-1), which is responsible for the large Stokes shift of the fluorescence observed (5.4 x 10(3) cm(-1)). To understand the sluggishness of the intersystem crossing (ISC) of (1)MLCT of the copper(I) compounds, the strength of the spin-orbit interaction between the lowest (1)MLCT (1(1)B(1)) and all (3)MLCT states was calculated. The ISC channels induced by strong spin-orbit interactions (ca. 300 cm(-1)) between the metal-centered HOMO and HOMO - 1 were shown to be energetically unfavorable in the copper(I) compounds because the flattening distortion caused large splitting (6.9 x 10(3) cm(-1)) between these orbitals. The possible ISC is therefore induced by weak spin-orbit interactions (ca. 30 cm(-1)) between ligand-centered molecular orbitals. Further quantum mechanical study on the spin-orbit interaction between the lowest (3)MLCT (1(3)A) and all (1)MLCT states indicated that the phosphorescence borrows intensity from 2(1)B(1). The radiative rate of the phosphorescence was also structure-sensitive. The flattening distortion reduced the transition dipole moment of 2(1)B(1) --> the ground state, and decreased the extent of mixing between 1(3)A and 2(1)B(1), thereby considerably reducing the phosphorescence radiative rate at the MLCT geometry compared to that at the ground state geometry. The theoretical calculation satisfactorily reproduced the radiative rate of ca. 10(3) s(-1) and accounted for the structure-sensitive phosphorescence intensities of copper(I) bis(diimine) compounds recently demonstrated by Felder et al. (Felder, D.; Nierengarten, J. F.; Barigelletti, F.; Ventura, B.; Armaroli, N. J. Am. Chem. Soc. 2001, 123, 6291).

Dynamic Effects in Electron Scattering in the 3d-Subshell of Lanthanum

The work reports the calculation of cross sections of inelastic electron scattering on metallic La, exciting the inner 3d-shell. The calculations are based on an atomic physics description of the triply ionized free atom. The large spin-orbit coupling constant of the 3d-hole leads to an approximate jK-coupling. The resulting excitation strength is formulated in terms of the Born amplitude including resonant contributions. The intensity is folded with a Lorentzian broadening with full width at half maximum (FWHM) parameter Γ. This parameter is considered to be dependent on the projectile electron energy so that the dynamic effects found in experimental EELS may be addressed. The systematic variation of the relative intensities of the spin-orbit doublet structures with excitation energy found in experiment is partly accounted for as Γ is varied from the experimentally measured values of 0.7 eV–0.5 eV and beyond.

Ligand-field model for the electronic structure of the fluorescent levels of d 5 ions in II–VI compounds

An analysis of the first-order spin-orbit (SO) interaction for the fluorescent levels of Mn 2+ in the common cation series ZnS, ZnSe, ZnS and also in CdTe shows that, with respect to the results of the crystal-field model, the SO interaction is strongly reduced for the fluorescent state of Mn in ZnS and strongly enhanced and of opposite sign for ZnSe, ZnS and CdTe. It is shown that, for ligands Se and Te, the molecular SO interaction is primarily controlled by the large SO coupling constants of the electrons 4p(Se) and 5p(Te) of the ligands.

Difference-frequency laser spectroscopy of the 3ν2+ν3 band of C2H

A spectrum of C2H 2Π–2Σ+ transition centered at 2928 cm−1 has been measured in the gas phase using a high resolution difference-frequency infrared laser spectrometer. The C2H molecules are produced in a hollow cathode discharge through a mixture of C2H2 (∼20 mTorr) and H2 (∼500 mTorr). The discharge amplitude modulation technique is used to enhance sensitivity and selectivity. About 125 lines are rotationally analyzed and accurate molecular constants are determined through a least-squares fit. The band is assigned to the 3ν2+ν3 combination band. The assignment is consistent with the observed 2Π–2Σ+ symmetry and is in good agreement with the recent theoretical calculation of the vibrational frequency and the spin–orbit coupling constant. The relatively large spin–orbit coupling constant of the (0 3 1) state is indicative of strong vibronic interaction between the à and X̃ states. A small perturbation appears near the end of observed Q-branch series, which is probably caused by a Σ vibronic state. New assignments of the progressions (0 v2 0) and (0 v2 1) toward higher v2 are proposed based on the existing gas phase and rare gas matrix data with the help of recent ab initio theoretical predictions.

Evidence for strong mixing between the LC and MLCT excited states in bis(2-phenylpyridinato-C2,N')(2,2'-bipyridine)iridium(III)

The well-resolved absorption, excitation, and luminescence spectra of [Ir(ppy)2bpy]+ (ppyH = 2-phenylpyridine, bpy = 2,2'-bipyridine) in different media at cryogenic temperatures are presented. In solutions and glasses at ambient temperature the lowest energy excited state corresponds to an Ir - bpy charge-transfer excitation whereas in the crystalline host lattice [Rh(ppy)2bpy]PF6 the lowest excited state at 21 450 cm-1 is assigned to a 37r-r* excitation localized on the cyclometalating ppy- ligands. The next higher excited Ir - bpy charge-transfer state has shifted to 21 820 cm-', only 300 cm-I above the 3LC excited state. The close proximity of the 3LC and 3MLCT excited states and the large spin-orbit coupling constant of Ir3+ induce a strong mixing of charge-transfer character into the 3LC lowest excited states, resulting in increased oscillator strengths, reduced lifetimes, short axis polarized transitions, and a large zero-field splitting of 10-15 cm-1.

Magnetic field effect on reactions of radical pairs possessing large spin-orbit coupling constants

In this communication we report that application of an external magnetic field during photolysis has a dramatic effect on the measured quantum yield for the solid state reaction of chlorine with methylcyclopropane.