Hans-Herbert Schmidtke

Low temperature luminescence spectra of the d10s2 complexes Cs2MX6 (M = Se, Te and X = Cl, Br). The Jahn—Teller effect in the Γ−4(3T1u) excited state

Abstract The emission spectra of the title compounds in microcrystalline form have been measured at 10 K. The extensive vibrational progression in the e g mode is indicative of a tetragonal Jahn—Teller distortion in the Γ − 4 ( 3 T 1u ) excited state. The vibronic coupling of a threefold electronic state with a doubly degenerate e g mode (T—e coupling), linear in the nuclear coordinates, has been reinvestigated considering spin—orbit coupling up to second order perturbation on energy levels which result from an a 1 1g t 1 1u electron configuration. For an estimation of Jahn—Teller coupling constants, the intensity distributions in the progressions were compared with the theoretical line shape functions which were obtained from a model which also permits the determination of potential energy minima and vibrational fundamentals of the excited state. The unusually large increase in the e g vibrational frequency compared to the ground state is due to Jahn-Teller forces which distort the potential surface, yielding steeper excited state energy curves.

The luminescence spectrum of ReCl6 2- doped in various host lattices with antifluorite-type structure

The vibrational structure derived from the Γ7(2 T 2g ) → Γ8(4 A 2g ) phosphorescence spectrum at liquid nitrogen temperature of a number of cubic host crystals A2BX6 (A = K, Rb, Cs; B = Pt, Sn, Pb, Te; X = Cl, Br) doped with ReCl6 2- has been investigated. The best resolved spectra were obtained for those host crystals absorbing appreciably more of the exciting light than ReCl6 2-. This suggests an energy transfer mechanism from host molecules to the chromophore. The spectral bands are assigned to normal frequencies of the octahedral chromophore, to lattice vibrations and to various combinations. For a complete assignment, vibrational states must be considered which belong to special points in the Brillouin zone other than the zone centre. The zero phonon transitions vary in a definite pattern for host crystals which have closed electron shells. The shifts, which depend more on the cation A+ than on the host complex BX6 2-, are discussed by means of ligand field theory.

Intensity distribution in the progressions of vibronic transitions of metal-ion complexes

The intensity distribution of the vibrational progressions t 1u + na 1g and t 2u + na 1g in a vibronically induced transition has been investigated using ReCl6 2- doped in Rb2TeCl6 and Cs2TeCl6 as an example. It has been shown that this distribution can be expressed in condensed form by an intra-molecular distribution function I 0 (n, ΔΓ, βΓ) where n denotes the member in the progression and ΔΓ and βΓ represent changes in equilibrium position and frequency, respectively, of the mode Γ following an electronic transition. For Rb2TeCl6 : Re4+ the experimental data are best fitted to the theory with the parameters Δ = 0·37 and β = 0·995, and for Cs2TeCl6 : Re4+ Δ = 0·32 and β = 0·995. The Δ-parameters correspond to a change in the nuclear equilibrium configuration of the metal-ligand distances by 0·015–0·020 A on excitation of the molecule to 2 T 2g . The detection of an eg vibronic line and its combinations eg + na 1g is explained by a Jahn-Teller distortion in the 4 T 2g excited state.

The angular overlap model for transition metal compounds forming extended structures

Abstract A straightforward application of the angular overlap model (AOM) calculating electronic structures of infinite chain transition metal ion compounds is presented using Bloch functions and the “fragment within the solid” method. Starting from Lowdins perturbation formulas derived from a basis set which is partitioned into metal ion and ligand orbital subsets, the common AOM parameters e λ and angular functions F λ (θ,φ) depending on the geometry of structural units are introduced.Translational symmetry restrictions do not allow the ligand orbitals to be optimally aligned for maximum overlap which leads to energy band orders different from orbital sequences as obtained for the isolated molecules. These effects are enhanced if metal-metal (through-space)interactions become important. Applications are presented on one- to three-dimensional chain compounds with one and two bridging atoms forming linear or zigzag and edge-sharing square planar or tetrahedral molecular chains, respectively. The latter types of compound are realized in some (MS 2 ) ∞ systems which recently obtained some interest. The energy bands calculated, in part, from analytic expressions are compared with extended Huckel results reported in the literature for tetrahedral chain compounds for which a much larger number of model parameters has been used. A discussion of the results on the basis of AOM formulas can be carried out with high transparency.

Low symmetry splittings due to phase transitions in the vibronic spectrum of ReCl62− and ReBr62− doped in K2PtCl6‐type crystals

The vibronic structure of ReCl62− and ReBr62− doped in A2MX6 (A=Na, K; M=Pt, Sn, Te; X=Cl, Br) as obtained from phosphorescence spectra of microcrystals at 10 K has been investigated. Phase transitions of the host crystals to lower symmetry cause splittings of the Γ8(4A2g) electronic ground state into Kramers doublets, which are detected by splittings of the zero phonon transition and the vibronic side band. The crystal distortions also split the internal vibrational modes of the complex molecule, giving rise to a splitting pattern which is higher in symmetry than the corresponding site symmetry of the host lattice would predict. The assignment of phosphorescence bands to internal and lattice vibrations as well as to combinations and progressions is discussed.

Vibrational assignment of sharp line luminescence bands from a higher excited state of OsCl2−6 doped in K2PtCl6

Abstract The micro crystalline luminescence spectrum of OsCl 2− 6 doped in K 2 PtCl 6 originating from Γ 1 ( 1 A 1g ) → Γ i 3 T 1g ), i = 1,3,4,5, electronic transitions has been measured at 10 K. The bands are assigned to internal complex and lattice vibrational modes using experimental data (FIR and Raman) and theoretical results (dispersion curves). The only zero-phonon band observed is due to the group theoretical allowed Γ 1 ( 1 A 1g ) → Γ 4 ( 3 T 1g ) transition.

Optical properties of Ni[tbu2P(O)NR]2: A paramagnetic d8 complex with planar structure

Abstract The title complexes of Ni(II) with trans-planar structure are paramagnetic because they have a triplet ground state (R = i -propyl, green isomer of c -hexyl) or there is another electronic level closely located above the singlet ground state which is available by thermal energy kT (R = t -butyl). Angular overlap model (AOM) calculations are able to interpret these ground states as well as the ligand field spectra measured from the crystals in the near-IR and visible region. For this the model must be extended considering metal ion s-d orbital mixing for a NiN 2 O 2 ; chromophore. A fitting of absorption peaks to theoretical transition energies supplies AOM parameters e σ , e π and e sdσ of ligands with oxygen and nitrogen donor atoms which compare well to parameter values of other ligands of similar nature, as far as these are known.

Temperature-dependent luminescence spectra and lifetime measurements of octahedral Se(IV) and Te(IV) hexahalogeno coordination compounds

Abstract Emission spectra and lifetime measurements of microcrystalline A 2 SeCl 6 , A=Cs, Rb and Cs 2 TeX 6 , X=Cl, Br excited in the lowest electronic transition are reported for the temperature interval from T =1.9 to 240 K. The results are explained by a model which considers spin-orbit coupling, electron-electron interaction and the electron-phonon coupling (Jahn-Teller effect) of the complex molecule MX 6 2− . Evaluation of the measured data indicates that the lowest excited states Γ 1 and Γ 4 resulting from the sp electron configuration differ by only about 110 and 95 cm −1 for the Se(IV) and Te(IV) compounds, respectively. For the SeCl 6 2− complex, emission only from Γ 1 is observed at temperatures below 2.1 K. The emission properties of octahedrally coordinated s 2 ions can be explained by simple models including kinetic aspects. The relative orders of magnitude calculated for the decay rates correspond to predictions obtained from selection rules.

Vibrational assignments from sharp line luminescence of a higher excited state of ReCl62− in cubic host crystals

Abstract The micro crystalline luminescence spectrum of [ReCl 6 ] 2− doped in K 2 [SnCl 6 ] and K 2 [PtCl 6 ] due to the Γ 7 ( 2 T 2g , t 2g 3 ) → Γ 8 ( 4 A 2g , t 2g 3 ) transition has been measured at liquid nitrogen temperature. The bands are assigned to internal complex and lattice vibrational modes using also far IR and Raman data. A pair of bands located at zero phonon position and a couple of satellite bands near fundamental peaks indicate that the electronic ground state is split into two Kramers doublets by about 10 cm −1 due to a second order Jahn-Teller distortion.

The emission spectrum of OsCl2−6doped in various cubic host lattices

Abstract The incorporation of OsCl 2− 6 ions as impurity into several cubic host crystals of A 2 MCl 6 type by simultaneous precipitation from solutions has been investigated. Due to varying solubilities, only for a few compounds could sufficient quantities of the chromophore be doped into the host lattices to obtain well-resolved emission spectra. For this purpose the rate of cooling during precipitation has been found to be of importance. The host lattice effects on the emission spectra become particularly apparent in the zero-phonon transition energies and stretching frequencies involved in the vibronic transitions. The experimental results are explained by ligand field theory and by simple vibrational models.