Time-resolved resonance raman spectra and decay kinetics of triplet anthracene in fluid media

Reported here are the time-resolved resonance Raman spectra and decay kinetics of the lowest triplet state (³B_2u⁺) of anthracene-h₁₀ and anthracene-d₁₀ molecules in fluid media at room temperature. The triplet population (≈3 × 10^?5 M) is observed to decay at microsecond times by triplet—triplet annihilation. Vibrational assignments for the observed Raman bands are proposed.     

Triplet state dynamics in polycyclic aromatic molecules

Utilizing the technique of triplet absorption detection of magnetic resonance (TADMR), we have determined the rates of intersystem crossing for the individual spin levels of the anthracene-h₁₀ and -d₁₀ and 3,4-benzpyrene triplet states in Shpolskii solvents. The anthracene results are in agreement with other experimental and theoretical work on intersystem crossing in aromatic hydrocarbons. The benzpyrene results, however, are unique, showing a large contribution from the out-of-plane spin level. Possible explanations of this result are discussed.     

Delayed fluorescence from the lowest 1B+3u state of anthracene,due to hetero-triplet—triplet annihilation of 3anthracene* and 3xanthone*

The P-type delayed fluorescence (DF) S_i→S_o of aromatic compounds results from the population of excited singlet states S_i by triplet—triplet annihillation (TTA) of molecules in their lowest and metastable triplet state T₁ : T₁ + T₁

S_i + S_o; S_i may be any excited singlet state whose excitation energy E(S_i ? 2 E(T₁). TTA of unlike molecules A and B (hetero-TTA) may lead to excited singlet states either of A or of B. In particular, if E(T_A¹) < E(T₁^B), hetero-TTA may lead to excited singlet states S_k^A which are not accessible by TTA of 2 T₁^A. In the present paper we report the first example of the detection of the DF from a very short-lived upper excited singlet state S_k^A which has been populated by hetero-TTA. The systems investigated are liquid solutions of A = anthracene-h₁₀ or anthracene-d₁₀ or 9,10-dimethylanthracene and B = xanthone in 1,1,2-trichlorotrifluoroethane at 243 K. S_k^A is the lowest ¹B_3U⁺ state (B_b state) of anthracene.     

Triplet-state ESR spectra of aromatic molecules oriented in stretched polyethylene films

Triplet-state ESR spectra show that when the aromatic molecules anthracene, anthracene-d₁₀, terphenyl, and acridine are dissolved in stretched, low-density polyethylene films, a fraction of these solute molecules become oriented with their X axis along the stretch direction but with their XY planes oriented randomly to the stretch direction. The fraction of molecules involved in the orientation process is largest for terphenyl.     

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Coherent neutron inelastic scattering intensifies for two low-frequency Γ₄ internal modes in anthracene-d₁₀ have been measured at several different Brillouin zone centres. These intensifies have been used to determine the eigenvectors of the two modes involved. It is shown that the motion in both cases is a combination of a butterfly motion (B) and a twisting motion (T). The energy proportions in these two modes are 0.97B and 0.03T (±0.01) for the mode at 3.16 THz and 0.94T and 0.06B (±0.04) for the mode at 4.60 THz. This experimental result strengthens our reliance on model calculations for the assignment of phonon frequencies measured by neutron coherent inelastic scattering.     

Fluorescence spectra of the mixed crystal system anthracene-perdeuteroanthracene. Mixed exciton band

The fluorescence spectra of the mixed crystal system anthracene (A-h₁₀)-perdeuteroanthracene (Ad₁₀) have been studied over the full concentration range at temperatures between 1.6 and 77°K. There exists a collective (mixed) A-h₁₀-A-d₁₀ S₁ exciton band at all concentrations, the lower edge of which represents the emitting level for the intrinsic fluorescence at low temperatures. This edge shifts from 25097 cm^?1 in the neat A-h₁₀ crystal to 25156 cm^?1 in the neat A-d₁₀ crystal. The edge position is a non-linear function of the mixed crystal concentration. From that one gets the “critical” distances U_cr⁺ = 92 cm^?1 and U_cr^? = 131 cm^?1 between the molecular S₁ levels of a guest and the host anthracene (non-deuterated or deuterated), using a formula given by Lifshitz. For the critical distance U_cr⁺ one finds just a bound guest state above the host band, and for U_cr^?1 a bound guest state below the host band.The transition from the S₁ band edge to the S₀ ground state is always forbidden, in the same way as in the neat A-h₁₀ crystal. Only transitions to levels of intramolecular and lattice vibrations of the ground state have been observed. At all concentrations the fluorescence transitions from the lower S₁ band edge of the mixed crystal take place to the vibronic levels of both A-h₁₀ and A-d₁₀. The difference in the frequencies of equivalent intramolecular vibrations of A-h₁₀ and A-d₁₀ results in the appearance of vibronic doublets in the intrinsic fluorescence. The relative intensities of the two doublet components depend strongly on both concentration and temperature. This is due to the influence of quasi-resonance and exciton-superexchange upon the guest states inside the mixed exciton band.For all concentrations one observes two transitions originating from levels inside the S₁ exciton band.     

Optically detected electron—nuclear double resonance and electron—nuclear—nuclear triple resonance study of 17O hyperfine coupling in the lowest nπ* triplet state of benzil

Optically detected ENDOR and electron—nuclear—nuclear triple resonance of ¹⁷O were measured via phosphorescence from ³(nπ^*) benzil in benzophenone-d₁₀ crystals at high magnetic field. The n and π^* spin densities on the oxygen atom are 0.201 and 0.092, respectively, the angle between the two CO bonds being 150°.     

Study of Integer Spin S = 1 in the Polar Magnet β-Ni(IO3)2

Polar magnetic materials exhibiting appreciable asymmetric exchange interactions can potentially host new topological states of matter such as vortex-like spin textures; however, realizations have been mostly limited to half-integer spins due to rare numbers of integer spin systems with broken spatial inversion lattice symmetries. Here, we studied the structure and magnetic properties of the S = 1 integer spin polar magnet β-Ni(IO₃)₂ (Ni²⁺, d⁸, ³F). We synthesized single crystals and bulk polycrystalline samples of β-Ni(IO₃)₂ by combining low-temperature chemistry techniques and thermal analysis and characterized its crystal structure and physical properties. Single crystal X-ray and powder X-ray diffraction measurements demonstrated that β-Ni(IO₃)₂ crystallizes in the noncentrosymmetric polar monoclinic structure with space group P2₁. The combination of the macroscopic electric polarization driven by the coalignment of the (IO₃)⁻ trigonal pyramids along the b axis and the S = 1 state of the Ni²⁺ cation was chosen to investigate integer spin and lattice dynamics in magnetism. The effective magnetic moment of Ni²⁺ was extracted from magnetization measurements to be 3.2(1) µ_B, confirming the S = 1 integer spin state of Ni²⁺ with some orbital contribution. β-Ni(IO₃)₂ undergoes a magnetic ordering at T = 3 K at a low magnetic field, μ₀H = 0.1 T; the phase transition, nevertheless, is suppressed at a higher field, μ₀H = 3 T. An anomaly resembling a phase transition is observed at T ≈ 2.7 K in the C_p/T vs. T plot, which is the approximate temperature of the magnetic phase transition of the material, indicating that the transition is magnetically driven. This work offers a useful route for exploring integer spin noncentrosymmetric materials, broadening the phase space of polar magnet candidates, which can harbor new topological spin physics.     

Observation and interpretation of 157.5 T internal magnetic field in Fe[C(SiMe3)3]2 coordination compound

The compound Fe[C(SiMe₃)₃]₂ has been prepared and investigated by the means of powder X-ray diffractometry and ⁵⁷Fe Mössbauer spectroscopy. The compound’s unique geometry, in which iron is linearly coordinated by the two C(SiMe₃)₃ ligands, results in a unusual electronic structure of iron, which is visualized as an extreme high hyperfine magnetic field of 157.5(8) T as sensed by the ⁵⁷Fe nucleus at T = 20 K. In order to obtain information on the electronic structure of iron and on the bonds to the ligands, DFT (density functional theory) calculations were carried out on Fe[C(SiMe₃)₃]₂. The high-spin state of iron was found to be energetically favored: an Fe(II) electron configuration of 3d^5.83 4s^0.72 is predicted, where the 4s electron density is only slightly polarized, and most of the unpaired electrons have 3d character. By assuming a linear crystal field, and associated 3d level scheme as a starting point, it is suggested that the extreme high hyperfine magnetic field, observed along with an apparently negative quadrupole splitting, is perpendicular to the C–Fe(II)–C bond axis, and can be decomposed mainly into contact (B _c ≈ 44 T), dipolar (B _d ≈ 14 T), and orbital (B _L ≈ 99 T) hyperfine magnetic field contributions.     

Theoretical study on the structures,force field,and vibrational spectra of cyclooctatetraene and cyclooctatetraene-d8

The structures and force field of 1,3,5,7-cyclooctatetraene (COT) have been studied using ab initio theory at the SCF level with the 4-21G basis set. The quadratic force field of the D_2d structure obtained by systematic scaling of the ab initio force constants successfully reproduces the observed frequencies of COT and COT-d₈ with a mean deviation of less than 10 cm⁻¹ for non-CH stretching modes. On the basis of the calculated results, assignments of the fundamental vibrations are examined. The normal mode υ₅ is reassigned to a weak band at 758 cm⁻¹ in the Raman spectrum of COT and to a weak band at 591 cm⁻¹ in the Raman spectrum of COT-d₈. The calculations favor the assignment of υ₂₆ given by Lippincott et al. [J. Am. Chem. Soc. 73, 3370 (1951)] over the revised assignment of Perec [Spectrochim. Acta 47A, 799 (1991)]. The calculations also furnish reliable prediction for the inactive A₂ fundamentals of COT and COT-d₈. The fundamental frequencies and IR and Raman intensities of ¹³CC₇H₈, which constitutes about 9% of COT in natural abundance, are also calculated. Only ν₁₀ (calculated at 908 cm⁻¹) of the formal inactive A₂ modes has appreciable Raman intensity (0.23 Å⁴/amu). A spectral feature due to this fundametal is identified in the liquid Raman spectrum of Tabacik and Blaise [C. R. Acad. Sci. Ser. II 303, 539 (1986)] as a weak peak at 908 cm⁻¹.     

Magnetic circular dichroism of manganese pentacarbonyl bromide

The absorption and magnetic circular dichroism (MCD) spectra of manganese pentacarbonyl bromide Mn(CO)₅Br in a solution of 2-methyl tetrahydrofuran were measured at various temperatures over the 3500–4500 Å region. Within experimental error the MCD spectra are not temperature dependent and in addition to a negative B term exhibit a positive A term, indicative for a degenerate excited state. Following the irreducible tensor method we conclude that the transition involved will be mainly e[π, Br] → a₁[σ^*, 3d₂²] and for the excited state we calculate a magnetic moment which agrees quite closely with the experimental value.     

Sign of the electric field gradient tensor and the nature of the low temperature phase transition in Fe(pyridine)2Cl2

Mossbauer spectra of Fe(pyridine)₂Cl₂ have been determined in large external magnetic fields at room temperature and 78 K. The principal component of the electric field gradient tensor is positive and consistent with an electron in the d_xy orbital. The results indicate no change in orbital ground state for the low temperature phase transition of this material.     

Microwave spectrum of 1,2,4-trifluorobenzene

The microwave spectrum of 1,2,4-trifluorobenzene has been observed in the range 12.5–18.0 GHz and 21.5–25.3 GHz at dry-ice temperature and assigned up to angular momentum state J = 39. The ground state rotational constants and the five quartic centrifugal distortion constants thus obtained are (in MHz): à = 3084.0037 ± 0.0108, B? = 1278.3614 ± 0.0062, C? = 903.6989 ± 0.0108, d_j = ( ?4.599 ± 0.621) · 10^?4, d_jk = (5.9757 ± 1.1586) · 10^?3, d_k = (11.4923 ± 2.0886) · 10^?3, d_wj = (4.0 ± 1.0) · 10^?7, d_wk=(?5.8± 1.1) · 10^?6.The small value of Δ = 0.029 (amu Ų) shows that the molecule is planar and an r₀ - structure using a regular hexagonal benzene ring with the bond lengths C-C = 1.397 Å, C-H = 1.084 Å and C-F = 1.312 Å, reproduces the rotational constants.     

Zeeman experiments on the 3Eu α 1A1g transition of platinum porphein in an n-alkane single crystal at 4.2

We report absorption spectra from the ground state to the photoexcited triplet state of platinum porphin (PtP) in single crystals of n-octane (C₈) and n-decane (C₁₀) at 4.2 K, with and without a magnetic field. For PtP in C₁₀ the same transition was studied in emission. From the experiments, values are derived of the spin-orbit coupling parameter Z, the crystal field splitting δ and the orbital angular momentum A for PtP in the two hosts: Z = 76 ± 2 cm^?1 (C₈, C₁₀), δ = 71 ± 1 cm^?1 (C₈), 55 ± 1 cm^?1 (C₁₀) and A = 1.6 ± 0.1 (C₈, C₁₀). For the ratio of the in-plane and the z-polarized electric dipole transition moments we obtain ¦Mx,y¦/¦Mz¦=76± 0.3 (C₈).     

Approximate D4d symmetry of the cation site in hexahydrated trichlorides of rare earths and actinides

Many spectroscopic and magnetic properties of the monoclinic hexahydrated chlorides of rare earths have been shown to approximate those of a uniaxial crystal. We explain this approximate symmetry by the shape of the coordination polyhedron of the rare-earth ions which is approximately that of a square antiprism of D_4d symmetry, with an inverse eightfold axis. The directions of this axis and of the spectroscopically or magnetically determined axes agree well. The absence of M_J mixing for some levels, as observed in the Zeeman effect, can be explained by an eightfold axis. As a first approximation, we propose a study of the magnetic ions using a D_4d crystal field hamiltonian with the limited set of even parameters B₀², B₀⁴ and B₀⁶; polarization and intensity spectral data, Zeeman effect, electron paramagnetic resonance and susceptibility measurements can be interpreted in this symmetry. Such remarks also apply to actinide hexahydrated chlorides.     

Parametric fits of the atomic fine structure of ZrI and MoI

Least squares fits including effective two- and three-body interactions have been performed for the mixed configurations (4d+5s)⁴ in ZrI and (4d+5s)⁶ in MoI. A test of the eigenvectors in intermediate coupling derived from the fits has been accomplished by comparing calculatedg _J values andg _J values, which have been measured with high precision by atomic beam magnetic resonance. The agreement found is typically better than a few parts in 10⁴ except for some strongly mixed high-lying states in Mo.     

J-dependence of the isotope shift in Gd I 4f 7 5d 2 6s(a 11 F)

TheJ-dependence of the isotope shift in the terma ¹¹ F of 4f ⁷ 5d ² 6s between six stable Gd isotopes was found to be represented by these parameter values (in MHz):z _5d (160?158)=19.4(1.8),z _5d (160?157)=37.2(1.0),z _5d (160?156)=42.4(1.7),z _5d (160?155)=49.1(2.0),z _5d (160?154)=59.0(2.0). The normalization with the corresponding changes in the mean-square nuclear charge-radiiδ〈r ²〉 yields values which are almost constant, mean value:z _5d /δ〈r ²〉=134(14) MHZ/fm². This indicates a second order IS interaction of the magnetic and the field shift operator.     

Thermal low spin-high spin equilibrium of Fe(II) in thiospinels CuFe0.5(Sn(1−x)Tix)1.5S4 (0≤x≤1)

A series of spinel compounds with composition CuFe_0.5(Sn_(1−x)Ti_x)_1.5S₄ (0≤x≤1) is analysed by X-ray diffraction, measurements of magnetic susceptibilities and ⁵⁷Fe Mössbauer spectroscopy. All samples show a temperature-dependent equilibrium between an electronic low spin 3d(t_2g)⁶(e_g)⁰ and a high spin 3d(t_2g)⁴(e_g)² state of the Fe(II) ions. The spin crossover is of the continuous type and extends over several hundred degrees in all samples. The Sn/Ti ratio influences the thermal equilibrium between the two spin states. Substitution of Sn(IV) by the smaller Ti(IV) ions leads to a more compact crystal lattice, which, in contrast to many metal-organic Fe(II) complexes, does not stabilise the low spin state, but increases the residual high spin fraction for T→0 K. The role played by antiferromagnetic spin coupling in the stabilisation of the high spin state is discussed. The results are compared with model calculations treating the effect of magnetic interactions on spin state equilibria.     

Studies on radiative lifetimes in the 4d 10 ns2 S and 4d 10 nd 2 D sequences of neutral silver

We have performed lifetime measurements in the sequences of² S _1/2 and² D _3/2 states for the alkali-like 4d ¹⁰ ns,nd configurations of neutral silver. Selective stepwise excitations from the 4d ¹⁰ 5s ² S _1/2 ground state were performed using two pulsed dye lasers. Optical transients were recorded and evaluated with regard to the decay time. The lifetime results are compared with those of a theoretical study in which a model potential has been used to describe the polarisation of the 4d shell.     

Phosphorescence and its characteristics in pyridine vapor

The phosphorescence emission of pyridine-d₀ and -d₅ has been observed in the vapor phase by means of time-resolved spectroscopy. The results of the experiments, which are described in full detail in this paper, on the phosphorescence spectrum, the excitation spectrum, the phosphorescence decay and sensitization of biacetyl phosphorescence indicate that the emission concerned is the genuine phosphorescence of the pyridines. For pyridine-d₀ (-d₅), the wavelength of the phosphorescence maximum is 450 nm (440 nm), the phosphorescence quantum yield 1.5 × 10^?6) (1.7 × 10^?6) and the phosphorescence lifetime is 1.2 μs (2.1 μs), the values for pyridine-d₅ being given in parentheses. The phosphorescence characteristics of pyridine are compared with those of other related molecules. The nature of the phosphorescent triplet state of pyridine is discussed with particular regard to the exceptionally fast non-radiative decay from that state.