CPL spectroscopy

CPL spectroscopy may also be used to signal the nature of the ternary anion adduct. In the series of phospho-anion complexes of [Eu.la]3+, Figure 5, complexes of HP042 and glucose-6-phosphate give identical CPL spectra - consistent with their very similar NMR spectral profile. Adducts with phospho-tyrosine, N-acetyl phospho-Tyr and a short peptide (phosphorylated at the Pyr residue) are also near-identical, consistent with chemoselective binding of the Tyr-OP phospho-anion. 

In CPL spectroscopy, one analyzes the luminescence from a chiral sample and determines the differential emission intensity at wavelength X, AI(X). This quantity is related to the intensity of left (II) and right (Ir) circularly polarized light as follows [7,8]... 

Because of the difficulty in measuring absolute emission intensities, in CPL spectroscopy one commonly reports the ratio of AI(X) to the total intensity I(X). This ratio, glum(X), is referred to as the luminescence (or emission) dissymmetry ratio, and is explicitly defined at wavelength, X, as... 

Before discussing some of specific uses of CPL spectroscopy, it is relevant to review some of the more important aspects of luminescence techniques, since it is the combination of the attributes of optical activity and luminescence... 

For purposes of discussion, we divide applications of CPL spectroscopy into three categories (1) efforts to develop reliable CPL "sector rules", (2) use of comparative CD and CPL studies to probe excited state geometry changes, and (3) the specific use of the selectivity and sensitivity of CPL to probe details of molecular and electronic structure, and dynamics. Since in this book we are primarily concerned with "analytical" applications of these chiroptical methods, we will emphasize here the last of these categories. 

Rather than presenting a review of all the possible applications of CPL spectroscopy as a selective probe of chiral structure, we will focus our discussion in this section on three specific experiments that, we believe, illustrate the kinds of unique information that may be obtained from this technique. These three studies will all be concerned with CPL from optically active lanthanide complexes of approximate D3 symmetry. Almost all of these particular CPL measurements have... 

There have been several reviews published on the general use of CPL to study chiral molecular systems. Steinberg wrote the first review on CPL spectroscopy emphasizing applications to biochemical systems (Steinberg, 1975), and Richardson and Riehl published a review of CPL in 1977 (Richardson and Riehl, 1977) and an updated review in 1986 (Riehl and Richardson, 1986). Brittain published a review of the use of CPL to study chiral lanthanide complexes in 1989 (Brittain, 1989). Several other recent articles describing various general aspects of CPL measurements and CPL theoretical principles are also available (Riehl and Richardson, 1993 Riehl, 1993, 2000 Maupin and Riehl, 2000). In this article we will review the various applications of CPL to the study of lanthanide complexes, as well as provide an up-to-date assessment of the state of theory and instramentatiorL We will emphasize both the qrralitative artd qrrarrtitative molecular information that has so far been obtained from this technique, and disctrss the future of CPL as a reliable probe of the molecular stereocherrristry of lanthanide complexes. 

In CPL spectroscopy one is interested in measuring the difference in the emission intensity AI) of left circularly polarized light (7l) versus right circularly polarized light Ir). By convention this difference is defined as follows... 

Since the last complete review of CPL spectroscopy (Riehl and Richardson, 1986) there have been approximately 150 articles pnblished involving this technique with the vast majority being concerned with lanthanide (m) ions. In this section we will review the varions types of applications of CPL to the stndy of the molecnlar and electronic stracture of chiral systems containing lanthanide ions. We will attempt to be as complete as possible concerning work appearing since this last full review, but we will also include some of the earlier work in order to put some of the more recent resnlts in context. 

Lanthanide complexes with macrocyclic ligands such as those based on 1,4,7,10-tetraazacy-clododecane or polyaminocarboxylates have been studied extensively by CPL spectroscopy. These complexes often possess large stability constants in aqueous solution and, moreover, present interesting chiroptical properties. There have been a number of reports of CPL... 

For the first ten to fifteen years of CPL spectroscopy, a fairly large number of papers were published in which the main purpose of the work was to determine whether or not the solution species formed exhibited circularly polarized luminescence, and was therefore, chiral... 

Circularly polarized luminescence spectroscopy (CPLS) is a measure of the chirality of a luminescent excited state. The excitation source can be either a laser or an arc lamp, but it is important that the source of excitation be unpolarized to avoid possible photoselection artifacts. The CPLS experiment produces two measurable quantities, which are obtained in arbitrary units and related to the circular polarization condition of the luminescence. It is appropriate to consider CPLS spectroscopy as a technique that combines the selectivity of CD with the sensitivity of luminescence. The major limitation associated with CPLS spectroscopy is that it is confined to emissive molecules only. 

As described above, CPL spectroscopy is particularly suited for the study of forbidden transitions, so the potential for applying this technique to intraconfigu-rational transitions in metal complexes is very high. In the case of transition metals, however, applications of CPL have been quite limited. The reasons for this are many. Certainly, the preparation of resolved chiral transition metal complexes is quite difficult. 

By far the most common class of inorganic compounds that have been studied by CPL spectroscopy are luminescent lanthanide complexes. There are several reasons for this special interest. Complexes of... 

Some of the very early applications of CPL spectroscopy involved chiral polymeric systems. In particular, the CPL from achiral dye molecules dissolved in cholesteric liquid crystals has been used to probe chirality changes. CPL from chromophores attached to a chiral poly-amino acid may also be used to study exciton coupling between aromatic chromophores. In these polymeric systems, it is often observed that gijjjjj is quite large. In some cases it is so large, in fact, that detection using static quarter-wave plates is possible. 

The differential emission of left and right circularly polarized light from luminescent molecular systems is called circularly polarized luminescence (CPL), and is at the basis of the corresponding spectroscopic technique (CPL spectroscopy) [23-25]. CPL spectroscopy should not be confused with fluorescence detected circular dichroism (see Sect. 6.1.6) in the latter technique the differential absorption of the circularly polarized components is detected through fluorescence measurements, owing to the different extent of photoexcitation that left- and right-handed light can produce on a chiral molecule. 

As noted above, CPL spectroscopy can be considered as the emission equivalent of circular dichroism spectroscopy. In summary, the CPL phenomenon can occur in the following cases, even upon excitation with unpolarized or linearly polarized light ... 

The measured quantities in CPL spectroscopy are the difference of intensity (A/) of the left handed (/l) and right handed (7r) circularly polarized components of the emitted light, and the total luminescence intensity (I) emitted by the sample ... 

CPL spectroscopy is not as popular as luminescence or circular dichroism spectroscopies, and is much less diffused in research laboratories. This is the reason why stand-alone instruments for CPL measurements are not commercially available. The scheme of a CPL spectrofluorimeter, however, is not very complicated. In analogy with the spectropolarimeter, which is essentially a spectrophotometer capable of detecting circular dichroism, a CPL spectrofluorimeter is in fact a normal spectrofluorimeter equipped with a circular polarization analyzer (Fig. 6.12). 

The analysis of the CPL spectra constitutes a straightforward method for the study of the chirality of molecules in their luminescent excited states. By means of comparative CD/CPL measurements one can investigate the geometrical differences between the ground and excited states. The observation of CPL has the problems and limitations already described in the previous sections. In particular, the molecular or supramolecular species must contain a luminophore exhibiting a sufficiently high emission quantum yield. CPL spectroscopy, however, has a number of advantages in terms of specificity and selectivity that can be extremely useful in supramolecular chemistry, namely ... 

As mentioned above, CPL spectroscopy can be used to distinguish a racemic mixture of a chiral species from an achiral one, through the photoselection effect brought about by exciting the sample with circularly polarized hghL Such an experiment requires that the examined species is luminescent, and that any racemization process is slower than the decay time of the excited state under investigation. 

In many cases, the chemical species studied with CPL spectroscopy are complexes of lanthanide ions, particularly those of Eu(III) and Tb(III) [27-30]. In fact the f-f electronic transitions of these ions possess a strong magnetic dipole character and a modest electric dipole moment, and give rise to very narrow emission bands. Such transitions were exploited to investigate the interaction between these metal ions (or their complexes) and other species, for examples molecules of biological interest [31, 32]. Figure 6.13 shows the CPL band corresponding to the Fs D4 transition of Tb " induced by the interaction with D-(+)-mannose in aqueous solution. 

Consequently, an attractive complementary tool is the use of Ln(III) luminescence spectroscopy, and especially circularly polarised luminescence (CPL) spectroscopy, the... 

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