Luminescence transition metal elements

The aim of this chapter is to review the chemistry of chalcogenolates in the last 10 years. The more recent reviews in this field included chalcogenolates of the s-block elements,13,14 early transition metal thiolates,15 metal complexes with selenolate and tellurolate ligands,16 copper(I), lithium and magnesium thiolates,17 functionalized thiolate complexes,18 19 pentafluorobenzenethiolate platinum group compounds,20 tellurium derivatives,21 luminescent gold compounds,22 and complexes with lanthanide or actinide.23... 

Molecules that contain heavy elements (in particular 5d transition metals) play an important role in the photochemistry and photophysics of coordination compounds for their luminescent properties as well as for their implication in catalysis and energy/electron transfer processes. Whereas molecular properties and electronic spectroscopy of light molecules can be studied in a non-relativistic quantum chemical model, one has to consider the theory of relativity when dealing with elements that belong to the lower region of the periodic table. As far as transition metal complexes are concerned one has to distinguish between different manifestations of relativity. Important but not directly observable manifestations of relativity are the mass velocity correction and the Darwin correction. These terms lead to the so-called relativistic contraction of the s- and p- shells and to the relativistic expansion of the d- and f- shells. A chemical consequence of this is for instance a destabilisation of the 5d shells with respect to the 3d shells in transition metals. 

James (16), Kitigawa and co-workers (17-19), Wuest (20), and Chen and coworkers (21) explore the chemistry, structures, and properties of MOFs and CPs from a variety of perspectives. Indeed, a recent special issue of the Journal of Solid State Chemistry was dedicated to these very topics (22). One wiU notice almost immediately that this field is dominated by materials based on block transition metal compositions. Lanthanide (Ln)-containing materials have been much scarcer, perhaps for reasons to be discussed herein (e.g., a tendency to exhibit higher coordination numbers) (23). With this in mind, however, recent advances in polymeric Ln-containing materials suggest that these compounds are as structurally diverse and that the unique luminescence behavior of the /-elements may be harnessed for applications, such as sensing and molecular recognition (23-30). Such inherent properties may extend the applications of framework materials beyond those introduced above. 

A brief review of Ln coordination chemistry is warranted as a precursor to appreciating the structural systematics of extended solids comprised thereof. In addition, this section aims to exemplify a number of Ln-specific properties and coordination preferences with the goal of distinguishing these materials from transition metal MOFs and CPs. It is felt that there are a number of attractive features of the/-elements (specifically their luminescent behavior) that make Ln-MOFs potentially more appropriate for certain applications (e.g., sensing). 

Luminescence has been observed from a large number of later transition element complexes and a rich array of excited states have been observed. Related sections from CCC (1987) include Chapter 16.5 on Pt, Rh, and Ir complexes, 36.3 on Mo halide clusters, 43 on Re complexes, 45.4 on Ru polypyridyls, 46.4 on Os polypyridyls, 48.6 on Rh complexes, and sections of Chapter 52 on Pt complexes. Several recent reviews have been published on polynuclear complexes, the photophysics of gold complexes,and platinum diimine complexes. Many other more narrowly focused review articles have been published on transition metal complex luminescence a significant number are published in the journal Coordination Chemistry Reviews and some of these reviews are cited in this chapter. 

Luminescence centers of transition-metal ions The general outer electronic configuration of transition-metal ions (cl-block elements) is (n - ns. ... 

A UC phosphor consists of a host and dopant (activator). The dopant acts as luminescent centers, and the host provides a matrix to bring these centers into optimal position. A large number of suitable hosts doped with actinide [36, 37] and transition-metal ions—such as Cm ", U ", Mo ", Os ", Ni ", Ti ", and Re" —have been reported to show upconversion luminescence [38]. However, this occurs mainly in the RE elements due to its special inner shell configurations,... 

Semiconductors in nano-crystallized form exhibit markedly different electrical, optical and structural properties as compared to those in the bulk form [1-10]. Out of these, the ones suited as phosphor host material show considerable size dependent luminescence properties when an impurity is doped in a quantum-confined structure. The impurity incorporation transfers the dominant recombination route from the surface states to impurity states. If the impurity-induced transition can be localized as in the case of the transition metals or the rare earth elements, the radiative efficiency of the impurity- induced emission increases significantly. The emission and decay characteristics of the phosphors are, therefore, modified in nanocrystallized form. Also, the continuous shift of the absorption edge to higher energy due to quantum confinement effect, imparts these materials a degree of tailorability. Obviously, all these attributes of a doped nanocrystalline phosphor material are very attractive for optoelectronic device applications. 

In another example, weak metal-metal interactions that involve other heavy transition elements, such as Au, have recently been used to help guide the self-assembly of metallocydic building blocks 6.29 into luminescent, superhelical fibers based on stacked structures (Fig. 6.16) [73]. 

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