Joachim Degen

Calculation of vibronic coupling constants for tetrahedral and octahedral d electron systems via dynamic ligand field theory and application to optical spectra

A perturbative model for the calculation of electron-phonon coupling constants of tetrahedral transition metal complexes based on the electrostatic ligand field theory is presented. Orbital vibronic coupling constants are expressed in terms of common ligand field parameters, and linear many-electron coupling constants for all d2-d9 strong field states in tetrahedral and octahedral symmetry are reduced to orbital coupling constants, enabling comparisons of coupling strength and directions of nuclear distortions over a wide range of transition metal complexes in a general way. The usefulness of the model is demonstrated for well resolved optical spectra of tetrahedral [MnO4]2-, [CrO4]3-, [CoCl4]2- and octahedral [CrF6]3-, [Cr(NH3)6]3+, [VO6]8- complexes, revealing good correspondence. The method is especially valuable for the interpretation of such spectra because it is easy to apply and the prediction of band shapes does not require additional parameters to be fitted to experiment. Nuclear distortions from...

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.

The vibrational fine structure of the 1A2g ← 1A1g polarized absorption band of trans-[Co(NH3)4(CN)2]+. The excited state geometry

Abstract The low temperature polarized absorption, far-infrared, and Raman spectra of trans-[Co(NH 3 ) 4 (CN) 2 ]Cl (H) and of the deuterated (D) compound

A dynamic ligand field theory for vibronic structures rationalizing electronic spectra of transition metal complex compounds

A ligand field model which considers the dynamics of vibrating ligands of transition metal ions is formulated, maintaining fundamental assumptions and model parameters of common (static) ligand field theory. It allows a survey to be obtained on corresponding theoretical approaches available on various occasions in the literature which interpret vibrational fine structures of band progressions in electronic absorption and emission spectra including the entanglements caused by Jahn-Teller effects. In the unified model the electronic d-states are vibronically coupled to the nuclei moving in a harmonic force field applying first order perturbation terms in the Herzberg-Teller approximation. The linear vibronic coupling constants, stabilization energies and geometric distortions due to excitation can be calculated from appropriate ligand field parameters of the ground state which are obtained by comparison with the measured spectra and from metal-ligand atomic equilibrium distances. The model is applied to systems of octahedral symmetry representing d-electron coupling with α1, ɛ and/or τ2 vibrations for which well resolved vibronic spectra have been reported.

On the use of the thermal lens effect for measuring absolute luminescence quantum yields of transition metal complexes

Abstract The thermal lens effect or thermal blooming of a laser beam passing through an absorbing medium is used to determine the fraction of absorbed laser power which is converted into heat. By this photocaloric method absolute luminescence quantum yields Φ can be evaluated covering the full range of possible Φ values. A check with organic standards for which quantum yields of 1, 0.52 and 0 are reported, supplies values of 0.99, 0.52 and 0.04, respectively. The sample of compounds [Ru(bipy) 3 ]X 2 , X  Cl, ClO 4 , and bipy  bipyridine, were studied using different concentrations in water and methanol solution at room temperature. The results strongly depend on the counter ion: for the Cl − - and (ClO 4 ) − -salts quantum yields of Φ = 0.31 and 0.79, respectively, are obtained, which may be explained by different polarization conditions. The yields are, on the other hand, independent from the solvent and from the concentration, which was considered ranging from 10 −4 to 2.5 × 10 −5 M. Thermal blooming was also observed from [Ru(bipy) 3 ]Cl 2 contained in KBr pellets, measuring at various temperatures.

Time-resolved luminescence of octahedral selenium (IV) : complexes using different excitation pulse widths

Low temperature (T = 10 K) luminescence decay curves of A2SeCl6 (A = Rb, Cs) are recorded which follow optical excitation at λexc = 458 nm applying different pulse lengths between 90 ns and 100 μs. For short excitation pulses a biexponential decay is observed, for pulse widths t exc > 10 μs a single exponential decay occurs. The experimental results can be explained from two rate equations which describe the (de)population of the lowest excited states (of octahedral s2 systems) within the kinetics in a three-level system. The present compounds are obviously examples where thermally nonequilibrated populations of close-lying excited states can be observed within moderate time scales.

Time-resolved luminescence spectra of mixed halogeno tellur(IV) compounds (C4v) and characterization of excited electronic states

Abstract Time-resolved emission spectra and temperature-dependent lifetime measurements of microcrystalline [TeCl 5 X] 2- , X = Br, I, have been performed for the temperature range between 2 K and 340 K. The results are explained in the molecular approximation by considering all possible interactions on a complex of C 4v point symmetry in an excited sp electron configuration of the Te(IV) central ion. From selection rules as derived from group theory and using kinetic results the lowest excited states are characterized supplying energy levels of several emitting and non-emitting states and nonradiative transition rates. In the case of [TeCl 5 I] 2- a total of seven excited states has been identified.

Observation of dynamically induced fluctuations of the photon flux and resolution of the broad luminescence band of Cs2TeBr6

A frequency-selective photon correlation experiment is carried out using the 10 K emission of microcrystalline Cs2TeBr6 as an example. The recording of dynamically inducedD-fluctuations allows to resolve the vibrational fine structure of an electronic transition which is not obtained by usual emission spectroscopy at this temperature. The experimental findings support the physical relevance of Prigogines star-unitary transformations.

Different regional changes of fluorescence spectra of clear human lenses and nuclear cataracts

Fluorescence spectra are recorded from the cortex and nucleus of the same human lenses [clear and cataracta brunescens (nigra) with colorless cortex]. When comparing clear cortices with either the harder nucleus of a clear lens, or a cataracta brunescens for a given excitation wavelength, a shift of the fluorescence maxima of the nucleus to longer wavelengths is observed. The shift appears to be independent of the degree of coloring since it is very similar for different nuclei, and it is not increased in cataracta nigra. The fluorescence intensities are similar when comparing the clear cortex of clear lenses and cataracta brunescens. For the nuclei, however, the intensity increases by up to four to six times with increasing coloring. For constant excitation wavelength, the fluorescence band maximum of the nucleus (of clear lenses and of cataracta brunescens) exhibits roughly the same shift to longer wavelengths as that of the cortex. Upon 320 nm excitation the fluorescence intensity of a cataracta nigra is about twice that of a clear lens of juvenile age. Upon 380 nm excitation the factor increases to four. Therefore in older and colored lens nuclei a red shift of the fluorescence maximum with increasing excitation wavelength is observed. We discuss whether or not the changes in the molecular proteins, in addition to advanced glycolization end products, may be responsible for the different fluorescence properties (and the brown color) with increasing age.

Temperature-dependent luminescence spectra of [ReCl6]2− doped in K2PtCl6-type crystals

Abstract Luminescence spectra of [ReCl 6 ] 2− doped in various cubic Rb 2 MCl 6 (M = Te, Sn) host crystals have been measured at temperatures between 20 and 240 K. Excitation was obtained at different energy using various laser lines. The intensities of vibronic fundamentals belonging to the Γ 7 ( 2 T 2g ) → Γ 8 ( 4 A 2g ) transition are compared to theoretical models which describe temperature-dependent transition rates giving rise to vibronic sidebands.