Transition Metal and Rare Earth Compounds

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.

The ground and excited state geometry of the six-coordinate copper(II) ion is examined in detail using the CuF ₆ ⁴⁻ and Cu(H₂O) ₆ ²⁺ complexes as examples. A variety of spectroscopic techniques are used to illustrate the relations between the geometric and electronic properties of these complexes through the characterization of their potential energy surfaces.

Two homologous organometallic compounds, Pd(2-thpy)₂ and Pt(2-thpy)₂ (with 2-thpy-=2-(2-thienyl)-pyridinate, structure formulae in Fig. 1), are chosen for case studies of photophysical properties of the lowest excited states. The triplets of these two representative compounds are marked by differences of nearly two orders of magnitude in metal/MLCT (metalto-ligand charge transfer) character. Determination of detailed photophysical properties of both compounds is possible, since highly resolved spectra are obtained when the compounds are dissolved in an n-octane matrix (Shpol’skii matrix), when the measurements are carried out at low temperature (typically at T = 1.3 K), and when modern techniques of laser spectroscopy are applied. In addition, methods of time-resolution and of microwave double-resonance, such as optically detected magnetic resonance (ODMR), microwave recovery, and phosphorescence microwave-double resonance (PMDR) are used. In particular, it is shown that with increasing metal character of the triplet, that is when Pd(2-thpy)₂ is compared to Pt(2-thpy)₂, many properties change characteristically and in part by orders of magnitude. For example, the following properties will be addressed: Transition probabilities, emission decay times, zero-field splittings (zfs), processes of spin-lattice relaxation (slr), intersystem crossing rates, intrastate relaxation rates, excited state binding properties as compared to those of the electronic ground state, anharmonicity effects, metal-mediated ligand-ligand coupling or spatial extensions of the excited state wavefunctions. Moreover, we focus on spin-selectivity in the vibrational satellite structures of the emission spectra as identified by the complementary methods of time-resolved emission and PMDR spectroscopy. We also discuss radiative deactivation processes, such as spin-vibronic Herzberg-Teller and Frank-Condon activities. Further, we specify sub-picosecond relaxation paths on the basis of micro-second time resolution by applying for the first time the method of time-resolved excitation spectroscopy to transition metal complexes. It is further demonstrated that the size of zero-field splitting of the triplet state can be used as an ordering parameter, that reflects the metal participation in the lowest triplet. Thus, one can relate Pd(2-thpy)₂ and Pt(2-thpy)₂ to a larger number of other compounds, such as [Rh(bpy)₃]³⁺, [Pt(bpy)₂]²⁺, Pt(qol)₂, [Pt(mnt)₂]^2-, [Ru(bpy)₃]²⁺, [Os(phen)₃]²⁺, [Os(bpy)₃]²⁺, etc. (compare Fig. 1 and Table 11) and one obtains a series that demonstrates chemical tunability of photophysical properties. — For several specific subjects, we present the basic background information in order to make the paper more easily readable, also for non-specialists.