In principle, the optical configuration for measuring CPL relies on a similar process whether using a home-made or commercial spectrometer: photoexcitation of the chiral sample with a non-polarized light and detection of the emitted CPL using a suitable light modulator, further translated in a time-modulated electric current, the a.c. and d.c. components of which correspond to the circularly polarized light intensity and the total light intensity, respectively. With these values in hand, one can directly estimate the intensity of the polarized emission of the chiral sample (g) using the formula given above. As will be detailed later in this Perspective, the sensitivity is highly dependent on the total emission that can be detected and analysed by the optics. While home-made CPL instruments have relied on different elements, the use of an electro-optical modulator or photoelastic light modulator (PEM) offers significant advantages in emission collection because of its specific features (wide spectral range, large aperture, wide acceptance angle and high precision of phase modulation). Accordingly, g values as small as 10 have been detected using these devices. Such small values have not been reproduced more recently. Although the recent commercialization of CPL instruments based on PEMs has contributed to the measurement of low-intensity CPL (below 10), additional contributions (such as linear birefringence and fluorescence anisotropy) to the measured CPL signals other than the 'real' CPL emitted by the chiral system cannot be ruled out. The accurate measurement of g is clearly a critical issue in this field, as poor characterization of this parameter can lead to incorrect interpretation and rationalization of the processes involved in CPL emission, potentially resulting in biases within the literature and in applications of CPL. Similar problems arise when characterizing the circularly polarized electroluminescence dissymmetry factor of circularly polarized organic light-emitting diodes, g, which is also complicated by varying device architectures. The CPL brightness (B) has been proposed as a complementary metric, and allows a straightforward comparison of various CPL emitters belonging to different chemical classes. Defined as B = ε × φ × g, it takes into account both the molar extinction (ε) and the luminescence quantum yield (φ). In this Perspective our aim is to provide advice and guidelines for the measurement of CPL from chiral emitters to standardize their characterization, which is crucial for this area of research. We aim to focus on the different levels of CPL intensity and on the challenges posed by (small) chiral organic molecules, perovskites and lanthanide complexes, as well as other types of chiral emitters.
The most critical requirement for characterizing chiral luminescent materials is a sensitivity high enough to accurately detect the difference in the left and right polarization of the emitted light, which can often be less than one part per thousand. In this regard, the historical interest in lanthanide complexes is clearly linked to their ability to afford intense g values, which can reach up to 1.5 for some complexes. When the light emission is stable and g is large enough, reliable CPL spectra may be measured even with standard optics for luminescence measurements using a quarter-wave plate and a linear polarizer between the sample and the emission photodetector, as described in the supplementary information of ref. . With this set-up, CPL lines with g values as small as 0.07 could be quantified. With smaller |g| values, as are classically encountered with organic and organometallic CPL emitters (which usually display |g| values in the range of 10), one must use an electro-optical modulator or a PEM to accurately characterize weak CPL signals. A recent review has highlighted the different approaches used to measure CPL and the related optics and excitation geometries, which offer distinct advantages and disadvantages for mitigating measurement artefacts, depending on the sample type. Regardless of the instrument used, it would be relevant to test the measurement sensitivity with a standard reference. Unfortunately, no real standard exists for CPL experiments. However, it is important to first reproduce literature data before measuring the CPL of new chiral systems and materials, as careful measurements are needed to not only compare one study with another, but also (or perhaps primarily) to adequately understand the nature of the circular polarization that these materials exhibit. Chiral lanthanide complexes or camphor derivatives such as europium tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (Eu(facam); ref. ), and (R)- and (S)-camphorquinone (ref. ) are commercially available and can be used as standards. Requesting CPL samples from research groups already experienced in this field could be another valuable option.
As for any fluorescence measurement, precautions regarding temperature, concentration and photostability need to be taken when measuring CPL in solution. One challenging aspect of such measurements is the small difference between I and I in comparison with the total luminescence of the chiral compounds. Accordingly, we recommend that researchers:
This advice is, of course, most applicable to the ideal case where one can access the two enantiomers of the chiral compound and the molecule displays sufficient luminescence to be measured in solution in an ambient atmosphere at room temperature. Apart from these basic concepts, each type of molecular material has its own specific characteristics, which are detailed below for chiral lanthanide complexes, 'small' chiral molecular systems (CMMs) and chiral perovskites, and illustrated with experimetnal CPL measurements in Fig. 1. These chiral materials represent the CPL emitters that have been investigated most intensively so far. We then discuss solid-state CPL measurements, which could be subject to additional sources of uncertainty, and finally highlight CPL from chiral perovskites and other semiconductor materials, which represent an emerging class of chiral emitters.