Hans U. Güdel

Upconversion Processes in Transition Metal and Rare Earth Metal Systems

The design and characterization of new luminescent materials is an active area of research. Here we present several current topics in the area of upconversion by transition-metal and rare-earth-metal doped halide lattices. Following introduction to the necessary background material related to upconversion mechanisms and kinetics, a series of topics are discussed which illustrate some key areas of developing interest in the field. These include the use of unconventional experimental and theoretical techniques for gaining insight into upconversion processes in rare-earth-doped lattices (e.g., power-dependence measurements, two-color laser excitation schemes, and correlated crystal field calculations), as well as several specific examples of exciting and unusual upconversion behavior in both transition-metal and rareearth-metal systems. Finally, we discuss the variety of interesting effects host-lattice variation can have on the upconversion processes of a dopant ion, ranging from multiphonon relaxation properties to exchange interactions.

Electronic spectra of M(bipy)2+3 complexions (M = Fe, Ru and Os)

Abstract Linearly polarized absorption spectra of the complex ions M(bipy) 2+ 3 (M = Fe, Ru, Os) have been determined, together with the CD of the corresponding optical isomers at low temperature. The visible electronic spectra have been assigned to spin-allowed and spin-forbidden charge transfer as well as ligand-localized singlet to triplet transitions. The structure of the intense visible absorption band is due to two electronic transitions.

Optical spectroscopy of exchange-coupled transition metal complexes

A. Introduction B. Exchange effects in optical spectra. (i) Energy splittings (a) Effects which contribute to spectral splittings (b) Experimental manifestation of exchange effects (c) Theoretical models of exchange splittings (1) Ground state (2)Excitedstates (ii) Intensitymechanismsandselectionrules (a) Example (b) Mechanisms (iii) Orbital exchange parameters and exchange pathways (iv) Doubleexcitations (v) “Spin clusters’l: C.Spectros&pictechniques (9 Single-crystal polarized absorption spectroscopy (ii) High resolution luminescence and excitation spectroscopy (iii) Site-selective spectroscopy and luminescence line narrowing (iv) Time-resolvedspectroscopy (v) Zeeman,MCDandMCPLspeetroscopy (vi) Pressure dependence of exchange effects D.Specificsystems (0 Pairspectraindiamagnetichostlattices (a) Oxide lattices (b)Halidelattices (ii) A3M2X9compounds (iii) Polynuclear chromium(II1) complexes (iv) Dinuclear copper(I1) complexes 69

Competition between ligand centered and charge transfer lowest excited states in bis cyclometalated Rh3+ and Ir3+ complexes

In this article the results of a detailed optical spectroscopic investigation of a series of related bis-cyclometalated Rh3+ and Ir3+ complexes of the general formula [M(C∩N)2N∩N]+, M=Rh3+, Ir3+ (HC∩N=2-phenylpyridine or 2-(2-thienyl) pyridine; N∩N-2,2′-bipyridine or ethylenediamine) are summarized. The nature of the lowest excited states of the compounds is discussed on the basis of their absorption, luminescence and luminescence line narrowing spectra in solutions and glasses at temperatures from 10 K to 300 K, whereas for the characterization of higher excited states single crystal absorption spectra and excitation spectra of polycrystalline samples are used. In the Rh3+ complexes the lowest excited states correspond to a 3π-π* transition localized on the cyclometalating ligands, whereas in the Ir3+ complexes, depending on the environment, either a 3π-π* or a metal to ligand charge transfer (3MLCT) excited state is lowest in energy. The pronounced solvatochromic and rigidochromic effects of the Ir3+ compounds are responsible for the reversal of the order of the lowest excited states. Mixing between the π-π* and MLCT excited states is reflected in the oscillator strengths, luminescence lifetimes, vibrational structure and zero field splittings. The phenomenon of dual luminescence is attributed to a large inhomogeneous distribution of sites in solution and glasses.

New tri- and tetranuclear transition metal spin clusters incorporating a versatile polydentate Schiff base ligand

Abstract A trinuclear Fe(III) complex [Fe3(OAc)3L3] (1), a tetranuclear Mn(III) complex [Mn4Cl4L4] (2) and a tetranuclear Ni(II) complex [Ni4(MeOH)4L4] (3) have been synthesized from the reaction of simple metal salts with ligands derived from salicylidene-2-ethanolamine (H2L). Structural characterization of the three complexes reveals distinctly different topologies, despite the similar coordination modes of the ligand L2−. Preliminary magnetochemical studies have been performed and reveal spin ground states of S=1/2,8 and 4 for complexes 1, 2 and 3, respectively.

Optical spectroscopy of the dimer system Cs3Yb2Br9

Single crystals of Cs3Yb2Br9 were grown using the Bridgman technique. Well‐resolved absorption, luminescence, excitation, and Raman measurements down to 5 K are reported and analyzed. The crystal‐field splitting of Yb3+ in Cs3Yb2Br9 was determined and can be rationalized in terms of a trigonally distorted YbBr63− octahedron. Under near‐infrared (NIR) cw‐laser excitation of the 2F7/2→2F5/2 transitions, Cs3Yb2Br9 shows up‐conversion luminescence in the green spectral region. Conversely, NIR 2F5/2→2F7/2 luminescence can be excited at twice the energy of the 2F7/2→2F5/2 transitions in the green spectral region. These effects are shown to result from cooperative processes within the Yb2Br93− dimers. The cooperative dimer‐absorption oscillator strength of 4.0×10−12 corresponds to the square of the NIR absorption oscillator strength of 1.9×10−6, as expected for weak coupling within the Yb2Br93− dimer. By a numerical deconvolution the cooperative dimer luminescence spectrum can be quantitatively correlated with t...

Broadband near‐infrared luminescence of Cr+3 in the elpasolite lattices Cs2NaInCl6, Cs2NaYCl6, and Cs2NaYBr6

The broadband luminescence of isolated Cr+3 ions in the title lattices is investigated. The 4T2g lowest excited state is found to undergo Jahn–Teller distortion in the eg coordinate, in addition to expanding along the a1g. Quantitative determination of these displacements is made by analysis of extremely rich fine structure, the Ham quenched spin‐orbit splitting of the electronic origins, and the Stokes shifts. The Jahn–Teller effect is found to influence the temperature dependence of lifetimes at less than 50 K. In the yttrium containing hosts, nonradiative deactivation of the 4T2g is observed above 200 K. Analysis of lifetime data with four models for nonradiative decay indicates that the active vibrations are again the a1g and eg. The trends observed among the host materials are rationalized on the basis of the freedom of the 4T2g to relax along the a1g and eg directions.

Near-infrared to visible photon upconversion processes in lanthanide doped chloride, bromide and iodide lattices

The near-infrared to visible upconversion behavior of some selected Er3+, Tm3+, Dy3+ and Ho3+ doped chloride, bromide and iodide host lattices is summarized and discussed. The step from the well studied oxide and fluoride materials to the heavier halides can lead to dramatic changes in the upconversion behavior. This is mainly due to the lower vibrational energies which reduce the efficiency of multiphonon relaxation processes. As a consequence, new upconversion, cross-relaxation and luminescence processes become competitive. The relevance of these new materials lies in their potential for upconversion laser and phosphor applications.

COOPERATIVE OPTICAL BISTABILITY IN THE DIMER SYSTEM CS3Y2BR9:10% YB3+

In single crystals of the dimer compound Cs3Y2Br9:10% Yb3+ below 31 K, both visible (VIS) and near‐infrared (NIR) luminescence intensities were found to exhibit hysteresis as a function of incident NIR intensity and temperature. The optical bistability is intrinsic to Cs3Y2Br9:10% Yb3+ and not a result of an external feedback. Lowering the temperature to 11 K strongly enhances the all‐optical switching behavior. The switching on VIS cooperative upconversion and NIR luminescence transitions occurs simultaneously and with opposite polarity reflecting the competition of both emission processes. On/Off switching ratios of up to 4.8 and 1.7 were observed for VIS and NIR luminescence intensities. Using NIR luminescence spectroscopy, differences in the internal sample temperature of up to 7 K were found between the upper and lower branches of the hystereses. A two‐level density‐matrix model is developed which includes ground‐ and excited‐state interactions and shows that the intrinsic bistability due to a local ...

Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering

Understanding the spin dynamics in magnetic nanostructures is important for spintronics, but so far it has been impossible to probe the spin dynamics directly. A neutron-scattering technique providing direct information about dynamical two-spin correlations in a molecular nanomagnet has now been demonstrated.