Circularly Polarized Luminescence (CPL) of Proteins and Protein Complexes

Watching Excited-State Symmetry Breaking in Multibranched Push-Pull Molecules

The emissive properties of symmetric molecules containing several donor-acceptor branches are often similar to those of the single-branched analogues. This is due to the at least partial localization of the excitation on one branch. Detailed understanding of this excited-state symmetry breaking (ES-SB) requires the ability to monitor this process in real time. Over the past few years, several spectroscopic approaches were shown to enable visualization of ES-SB and of its dynamics. They include the detection of new vibrational or electronic absorption bands associated with transitions that are forbidden in the symmetric excited state. Alternatively, ES-SB can be detected by observing transitions that become weaker or vanish upon localization of the excitation. Herein, we discuss these different approaches as well as their merits and weaknesses.

Circularly polarized lanthanide luminescence for advanced security inks

Authenticating products and documents with security inks is vital to global commerce, security and health. Lanthanide complexes are widely used in luminescent security inks owing to their unique and robust photophysical properties. Lanthanide complexes can also be engineered to undergo circularly polarized luminescence (CPL), which encodes chiral molecular fingerprints in luminescence spectra that cannot be decoded by conventional optical measurements. However, chiral CPL signals have not yet been exploited as an extra security layer in advanced security inks. This Review introduces CPL and related concepts that are necessary to appreciate the challenges and potential of lanthanide-based, CPL-active security inks. We describe recent advances in CPL analysis and read-out technologies that have expedited CPL-active security ink applications. Further, we provide a systematic meta-analysis of strongly CPL-active Euⁱⁱⁱ, Tbⁱⁱⁱ, Smⁱⁱⁱ, Ybⁱⁱⁱ, Cmⁱⁱⁱ, Dyⁱⁱⁱ and Crⁱⁱⁱ complexes, discussing the suitability of their photophysical properties and highlighting promising candidates. We conclude by providing key recommendations for the development and advancement of the field.

Rapid time-resolved Circular Polarization Luminescence (CPL) emission spectroscopy

Circular polarisation luminescence (CPL) emission spectroscopy is a powerful tool for probing the fundamental chiroptical features of optically emissive chiral molecular systems. However, uptake of CPL spectroscopy has been impeded by the limitations of conventional scanning monochromator (SM) CPL spectrometers, which are costly to acquire and maintain, and typically require tens of minutes to acquire a typical CPL spectrum. Here, we demonstrate a design of CPL spectrometer which uses rapid readout solid state (SS) spectrometer detectors and a dual channel optical layout to acquire CPL spectra in as little as 10 milliseconds. We validate and demonstrate equivalent CPL measurement by measuring CPL spectra of two reference europium(III) complexes. Further, we demonstrate time-gated CPL acquisition, enabling long-lived CPL luminescence to be distinguished from short-lived emission of other fluorescent species. We anticipate that SS-CPL spectrometers will enable flexible, rapid, and relatively low-cost CPL spectroscopy for diverse applications.

Circular polarization luminescence (CPL) spectroscopy is a tool to study chiroptical systems, but the measurement process is generally very slow. The authors introduce a CPL technique with much faster acquisition, demonstrating meaningful time-dependent measurements and enabling new applications.

Chirality in fluorescence and energy transfer

Optical polarization features associated with the fundamental processes of molecular fluorescence and resonance energy transfer are in general studied with reference to plane polarizations. When any of the species involved is chiral, the associated emission processes may exhibit an element of circular polarization-a degree of optical helicity. Although usually a minor effect, some systems can exhibit a sizeable component of circularly polarized luminescence, whose helicity correlates with the enantiomeric form. In studies of multi-component systems, in which initial excitation of a donor species-followed by energy transfer-leads to emission from an acceptor molecule, the handedness of both donor and acceptor may influence output circularity. In systems with an achiral acceptor, a degree of fluorescence circularity may be influenced by the handedness of a chiral donor, but this should not be construed in terms of 'conveying' chirality. Chiral molecules may also play a passive role by inducing helicity in the fluorescence from achiral neighbours, and further tiers of complexity arise if the initial excitation is itself of circular polarization. In all such processes, symmetry principles play a major role in determining a sensitivity to molecular handedness, and their detailed consideration enables a range of new experimental procedures to be identified. Casting the fundamental theory in terms of formal photon-molecule couplings enables the quantum mechanisms involved in all such phenomena to be clearly resolved. The results provide fresh physical insights, and establish connections across a range of indirectly related chiroptical phenomena including induced circular dichroism.