CPL Measurements

Unlike CD, the measurement of CPL is still mainly dependent on the use of custom-made instruments that have been designed, developed and improved by a limited number of research groups around the world over the last three decades [36,38,41,42,45,52,59-63]. However, the growing interest in developing chiral luminescent probes, and in particular Ln(in)-based systems, has resulted in the advertising and some availability of commercial instrumentation. The first commercial CPL spectrometer, which essentially consists of two CD spectrometers with the second one used as the emission spectrometer, was manufactured by JASCO Inc., the JASCO CPL-200. More recently, the company OLIS Inc. developed its Polarisation Toolbox to support fluorescence, polarisation of fluorescence, anisotropy, CPL, CD, and FDCD (fluorescence detected circular dichroism) measurements for its CD instrumentation. As of Febmary 2012, only CPL-related studies using the JASCO CPL-200 instrument have appeared in the hterature [64-67]. 

Although there is still a limited use of commercial CPL instruments, the technique has continued to be developed to a point where the detection of CPL can be performed with a 

To conclude, it must be emphasised that it is important to minimise the sources of depolarisation. As a result, no optical elements should be placed between the sample compartment and the PEM. Doing so ensures that no linearly polarised luminescence would be detected, since this latter is 10 to 100 times more intense than circularly polarised emitted light. Additional information is provided in Section 3.3.3. In addition to the use of a 

Since the CPL technique is based on a photon-counting method, it is possible to calculate the standard deviation tjj in the measurement of the giu , directly from the total number of photon counts, N. 

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In these equations, B is an instrumental constant <)> is the probability that an absorbed photon leads to fluorescence and A (X) and A (X) are, respectively, the absorbance of the fluorophore only, and the total absorbance under left circularly polarized excitation at wavelength X. Note that we have assumed that < > is independent of excitation polarization and wavelength. The form of eqs. (26) and (27) display one of the problems in simple interpretation of FDCD results in terms of ordinary CD spectroscopy. On the front surface of the sample cell the intensity of the alternating circular polarizations will be equal, but if Ar does not equal A then the intensities will change due to differential absorption. Just as in CPL measurements, one is concerned in this case with measurement of the differential signal and the total fluorescence intensity, F(X)... 

CPL and CD are based upon similar aspects of molecular structure. It is important to realize, however, that, even if the same states are involved, these measurements do not usually supply redundant information. From the Franck-Condon principle, CPL is a probe of excited state geometry, and CD is a probe of ground state geometry. CPL measurements have some advantages over the measurement of CD, as well as some inherent limitations. The most serious limitation is, quite obviously, that the optically active molecule under study must contain a luminescent chromophore with a reasonable quantum yield. Although this severely limits the range of possible applications of CPL, it does result in a specificity and selectivity that is not present in CD or absorption experiments. 

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. 

Just as in ordinary luminescence measurements, the determination of absolute emission intensities is quite difficult, so it is customary to report CPL measurements in terms of the ratio of the difference in intensity, divided by the average total luminescence intensity... 

Note that we have a superscript, n, to indicate the polarization of the excitation beam, and also added a subscript to the lineshape function. As indicated above all CPL measurements are made relative to the total luminescence intensity which we may express using similar formalism as... 

One final simplification is the assiunption that the lineshapes for total luminescence and circularly polarized luminescence are identical. This is appropriate for the usually sharp isolated pure electronic transitions that are often the target of CPL measurements. In this case we rewrite eq. (21) as follows... 

It is unfortunate that there is to date no generally applicable spectra-structure correlations for CPL measurements from lanthanide (HI) complexes. However, the number of chiral lanthanide complexes with well-understood geometry and solution dynamics is increasing, al-... 

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. 

The use of photon-coimting techniques in combination with sinnsoidal variation of stress-induced polarization modulation imposes quantifiable lirtritatiorrs on the accnracy and precision of CPL measurements. We consider first the optical characteristics of the PEM. The phase difference q>) between the two orthogonal crystal axes of the PEM is related to the sinusoidally varying periodic stress (sin at) thronghthe following Bessel fnnction... 

List of the racemic lanthanide (III) complexes for which CPL measurements have been reported, following circularly polarized excitation and/or perturbation of... 

Racemization kinetics from CPL measurements. Experiments concerned with the enantiomer interconversion processes were carried out on the [Eu(TTHA)] compound in neutral aqueous solution (Mondry et al., 1994). It has been demonstrated that the CPL spectrum of this system shows only a very slight temperature dependence from 283 to 353 K,... 

In a previous review article, CPL measurements from mixed-ligand lanthanide complexes appearing in the literature prior to 1985 have been tabulated. In table 12 we list the publications involving CPL from these types of systems that have appeared since this last review. 

As described throughout this chapter, CPL is becoming increasingly useful as a probe of the existence of chiral lanthanide structures, and as an indicator of changes in chiral stmcture. However, there are currently no reliable correlations relating specific aspects of chiral stmcture to CPL measurements. The development of such spectra-stmcture correlations is key to the advancement of this technique as a useful probe of the stereochemistry of chiral lanthanide systems. 

For the simple model enantiopure systems described above, it was concluded that the time dependence of the CPL and total luminescence were identical, and, therefore, the dissymmetry ratio contained no dynamic molecular information. This, of course, would not be the case if intramolecular geometry changes, that would effect the chirality of the molecular transitions, were occurring on the same time scale as emission. However, no such examples of this type of study have yet appeared. Time-resolved CPL measurements have been useful in the study of racemic mixtures of lanthanide complexes in which racemi-zation or excited state quenching is occurring on the same time scale as emission. 

The incident (absorption) beam is wavelength-selected by an excitation monochromator and then converted to alternately left and right circular polarization by use of a linear polarizer and quarter-wave modulator. The quarter-wave modulator may be a PEM as described above for CPL measurement, or a Pockels cell. This luminescence in phase with the incident polarization modulation is collected by two photomultiplier tubes (PMTl and PMT2) oriented at right angles to each other and the incident beam. If the excitation is linearly polarized, then the sum of... 

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 ... 

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