Comparison of the Electronic and Vibrational Optical Activity of a Europium(III) Complex

Abstract

The geometry and the electronic structure of chiral lanthanide(III) complexes are traditionally probed by electronic methods, such as circularly polarised luminescence (CPL) and electronic circular dichroism (ECD) spectroscopy. The vibrational phenomena are much weaker. In the present study, however, significant enhancements of vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectral intensities were observed during the formation of a chiral bipyridine-Eu(III) complex. The ten-fold enhancement of the vibrational absorption and VCD intensities was explained by a charge-transfer process and the dominant effect of the nitrate ion on the spectra. A much larger enhancement of the ROA and Raman intensities and a hundred-fold increase of the circular intensity difference (CID) ratio were explained by the resonance of the λ = 532 nm laser light with the (7)F0 → (5)D0 transitions. This phenomenon is combined with a chirality transfer, and mixing of the Raman and luminescence effects involving low-energy (7)F states of europium. The results thus indicate that the vibrational optical activity (VOA) may be a very sensitive tool for chirality detection and probing of the electronic structure of Eu(III) and other coordination compounds.

Recognition of Oligosaccharides by Chirality Induced in Europium (III) Compounds

“…In addition, the usually positive 5 D 0 → 7 F 0 band (1550 cm −1 ) is negative for β‐ and γ‐CD. The 2043 cm −1 band also gives the largest CID ( CID =( I R − I L )/( I R + I L ), where I R and I L are intensities of the right‐ and left circularly‐polarized light, respectively) of −2.2×10 −2 (Table S2), very close to a similar value (1.7×10 −2 ) previously found for an inherently chiral bipyridine‐Eu III complex . This suggests a stable, rigid geometry of the resultant β‐CD and EuEDTA associate.…”

“…The biggerr ings of band g-CDs give larger CPL, where the EuEDTAp robe is extreme,w ith as ignal of about two orders (! [18] This suggestsastable, rigid geometry of the resultant b-CD and EuEDTAa ssociate. For b-CD, we can see three 5 D 0 !…”

“…The 2043 cm À1 band also gives the largest CID (CID = (I R ÀI L )/(I R + I L ), where I R and I L are intensities of the rightand left circularly-polarized light,r espectively)o fÀ2.2 10 À2 (Table S2), very closet oasimilarv alue (1.7 10 À2 )p reviously found for an inherently chiral bipyridine-Eu III complex. [18] This suggestsastable, rigid geometry of the resultant b-CD and EuEDTAa ssociate. Outside the displayed region weaker bands at 619, 659, and 685 cm À1 assignable to the 5 D 1 !…”

Detection of Circularly Polarized Luminescence of a Cs‐EuIII Complex in Raman Optical Activity Experiments

“…13 This luminescence appears at 1989 cm −1 in the Raman spectrum, resulting in CID=0.71 for the Λ enantiomer. As typical for similar europium complexes,10–12 the Raman signal is dominated by europium fluorescent transitions at 1722 and 1989 cm −1 . Pure vibrational Raman spectrum of the ligand (Supporting Information, Figure S2) is not visible in the scale of Figure 2.…”

“…The CPL component in scattered radiation was previously proposed for sensitive chirality detection of optically active alcohols and ketones chelated with an achiral Eu(fod) 3 complex, although the phenomenon was rather incorrectly referred to as induced resonance ROA (IRROA) 11. Recently, we also reported a comparative study of electronic and vibrational optical activity in a chiral bipyridine Eu III complex,12 where a very high CID of approximately 10 −2 was observed in its ROA spectrum.…”