Circularly polarized emission, probe

The first two chapters of this work cover theoretical and practical aspects of the emission process, the spectroscopic techniques and the equipment used to characterize the emission. Chapter 3 introduces and reviews the property of circularly polarized emission, while Chapter 4 reviews the use of lanthanide ion complexes in bioimaging and fluorescence microscopy. Chapter 5 covers the phenomenon of two-photon absorption, its theory as well as applications in imaging, while Chapter 6 reviews the use of lanthanide ions as chemo-sensors. Chapter 7 introduces the basic principles of nanoparticle upconversion luminescence and its use for bioimaging and Chapter 8 reviews direct excitation of the lanthanide ions and the use of the excitation spectra to probe the metal ion s coordination environment in eoordination compounds and biopolymers. Finally, Chapter 9 describes the formation of heterobimetallic complexes, in whieh the lanthanide ion emission is promoted through the hetero-metal. 

Circularly polarized luminescence (CPL) from chiral molecular systems is the emission analog of circular dichroism (CD) and as such reflects the chirality of the excited state in the same maimer as CD probes reflect the chirality of the ground state (Riehl and Muller, 2005). For lanthanide ions, laige CPL (and/or CD) signals are expected for f-f transitions obeying magnetic dipole selection rules, in particular A J = 0, 1 Eu(5Do —> 7Fi), Tb(5D4 7F4, 5D4 7F5), Dy(4F9/2 6Hn/2), Yb(2Fs/2 2F7/2) emissions are typical examples. Recent... 

Fig. 5.15 Normal emission photoelectron spectra of a Co island structure on W(llO) for the two magnetization states M and M . The spectra were taken with left (LCP) and right circularly polarized (RCP) synchrotron radiation with a photon energy of hv = 150 eV thus probing the MCDAD effect. From [52], used with permission...

A number of lanthanide complexes have been shown to exhibit circularly polarized luminescence (CPL—the differential spontaneous emission of left- and right-circularly polarized light). In the absence of any externally applied fields, CPL is exhibited only by systems that have net chirality in their structures or are subject to chiral perturbations by their environment. CPL exhibited by the Af-Af transitions of chiral lanthanide systems provides a sensitive probe of coordination and structure in solution. Applications are limited to systems which possess some element of chirality, but in many cases this merely requires that > 1 ligand of interest has a chiral atom or carries a chiral label (such as a chiral substituent group). ... 

The measurement of the time dependence of gium may be used to probe various chiral aspects of excited state energetics, molecular dynamics, and reaction kinetics. Although there are some time-dependent circular polarization effects due to molecular reorientations that parallel time-dependent linear polarization measurements, the most interesting studies are those that involve the time-dependence of intrinsic molecular chirality. For a sample containing one chiral luminescent lanthanide chromophore, it might be the case that there are processes that affect chirality occurring on the same time scale as emission that could be probed by time-dependent CPL. To date, however, there have been no reports of such studies, and all of the time-dependent CPL measurements have involved racemic mixtures. 

There is an impressive battery of spectroscopic techniques available for probing interactions between metal complexes and DNA. The oldest of these, UV/vis spectroscopy, is still one of the most sensitive ways to analyze dye-DNA interactions. For chiral metal complexes, circular dichroism is an invaluable tool. Fluorescence spectroscopy has in particular made great strides in recent years with respect to these applications, and aside from the measurement of simple emission from an excited metal complex, one can utilize emission polarization, luminescence lifetimes, and differential fluorescence quenching to obtain still more information about the environment about a metal complex. The application of ruthenium complexes, in particular, to developing probes for DNA, has been initiated in our laboratory and we focus here on some of its applications. 

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