ECD exciton chirality method today: a modern tool for determining absolute configurations

Abstract

The application of the exciton chirality method (ECM) to interpret electronic circular dichroism (ECD) spectra is a well-established and still popular approach to assign the absolute configuration (AC) of natural products, chiral organic compounds, and organometallic species. The method applies to compounds containing at least two chromophores with electric dipole allowed transitions (e.g., π-π* transitions). The exciton chirality rule correlates the sign of an exciton couplet (two ECD bands with opposite sign and similar intensity) with the overall molecular stereochemistry, including the AC. A correct application of the ECM requires three main prerequisites: (a) the knowledge of the molecular conformation, (b) the knowledge of the directions of the electric transition moments (TDMs), and (c) the assumption that the exciton coupling mechanism must be the major source of the observed ECD signals. All these prerequisites can be easily verified by means of quantum-mechanical (QM) calculations. In the present review, we shortly introduce the general principles that underpin the use of the ECM for configurational assignments and survey its applications, both classic ones and some reported in the recent literature. Based on these examples, we will stress the advantages of the ECM but also the key requisites for its correct application. Additionally, we will discuss the dependence of the couplet sign on geometrical parameters (angles α,β,γ between TDMs), which can be helpful for discerning the sign of exciton chirality in ambiguous situations. Finally, we will present a molecular orbital (MO) description of the exciton coupling phenomenon.

Sulfobutylation of Beta-Cyclodextrin Enhances the Complex Formation with Mitragynine: An NMR and Chiroptical Study

“…It is also worthy of note that intramolecular exciton coupling may occur between the π-π* dipole moments of the methoxyacrylate and the indole chromophore ( 1 L a and 1 B b ), giving rise to additional CD activity in the 230–250 nm wavelength range. Due to the asymmetric centers of the quinolizidine ring, these π-systems might be chirally oriented relative to each other, allowing the interaction of their local transition dipoles, which results in CEs in the respective spectral regions [ 37 ]. Interestingly, magnitude of the 240 nm CE doubled in ethanol ( Figure 6 A).…”

“…Consequently, the bathochromic shift obtained by addition of SBEβCyD is indicative of the inclusion of the indole moiety into the apolar cavity of the host. Taking into consideration the molecular exciton model [ 37 , 39 ], this situation enables dipole-dipole coupling between the π-π* transition moments of the encapsulated indole ring and the weak electronic moments induced by the oscillating electric field of light in the σ-bonds of the chiral glucose units. According to the empirical sector rule elaborated by Kajtár, the sign of the induced CE is determined by the relative steric orientation of the host-guest dipole moments [ 39 ].…”

Specific chiroptical sensing of cysteine via ultrasound-assisted formation of disulfide bonds in aqueous solution

“…As three prerequisites for the application of ECM were satisfied, the helicity of transition dipole moments between NBD-R groups resulted in an ECCD spectrum with two bisignate CEs ( Fig. 4 e-4g) [39] , [59] , [60] . According to the positive CE at longer wavelengths (498 nm), a clockwise screw of two transition dipole moments was deduced ( Fig.…”

“…The construction of products with active ECD signals via molecular probes is an effective and convenient method [7] , [38] , [39] . The 7-nitro-1,2,3-benzoxadiazole (NBD) group is electron-deficient and can emit strong fluorescence in typical push–pull systems via photo-induced electron transfer or intramolecular charge transfer pathways [40] , [41] .…”

“…NBD-Cl ( 1 ) can distinguish Cys from other amino acids or biothiols in which less active amino group is the only reaction site [42] , [43] , [44] , [45] , [46] , [47] . It is deduced that an efficient exciton coupling will be generated between the NBD groups in the final products and the exciton chirality method (ECM) can be applied [39] . In this study, NBD-Cl was tested for the first time in the chiral sensing of Cys.…”