Effect of the Counterion on Circularly Polarized Luminescence of Europium(III) and Samarium(III) Complexes.

Lorenzo Arrico,Andrea Melchior,Lorenzo Di Bari,Fabio Piccinelli,Chiara De Rosa

doi:10.1021/acs.inorgchem.0c00280

Abstract

Each enantiopure europium(III) and samarium(III) nitrate and triflate complex of the ligand L, with L = N,N′-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine ([LnL(tta)2]·NO3 and [LnL(tta)2(H2O)]·CF3SO3, where tta = 2-thenoyltrifluoroacetylacetonate) has been synthesized and characterized from a spectroscopic point of view, using a chiroptical technique such as electronic circular dichroism (ECD) and circularly polarized luminescence (CPL). In all cases, both ligands are capable of sensitizing the luminescence of both metal ions upon absorption of light around 280 and 350 nm. Despite small differences in the total luminescence (TL) and ECD spectra, the CPL activity of the complexes is strongly influenced by a concurrent effect of the solvent and counterion. This particularly applies to europium(III) complexes where the CPL spectra in acetonitrile can be described as a weighed linear combination of the CPL spectra in dichloromethane and methanol, which show nearly opposite signatures when their ligand stereochemistries are the same. This phenomenon could be related to the presence of equilibria interconverting solvated, anion-coordinated complexes and isomers differing by the relative orientation of the tta ligands. The difference between some bond lengths (M–N bonds, in particular) in the different species could be at the basis of such an unusual CPL activity.

Highlights

Polarized luminescence (CPL) is a chiroptical phenomenon by which a luminescent compound or material emits different intensities of left and right circularly polarized light at a specific wavelength after excitation with unpolarized light.[1−6] In order to define quantitatively the importance of this phenomenon, the luminescence dissymmetry factor glum is calculated, which is defined as follows: glum = 2(IL − IR)/(IL + IR), with IL and IR being the left and right polarized intensity, respectively
Small differences in both the absorption and Electronic Circular Dichroism (ECD) spectra are detected between samarium and europium, upon changing the solvent from AN to MeOH, by using nitrate instead of triflate as a counteranion (Figures S2 and S3)
Unlike the analogue europium(III) triflate complexes, where one water molecule was detected in the inner coordination sphere, when the complex was dissolved in AN, in the case of samarium(III) triflate complexes, no water molecule should be present in close proximity of the cation because the lifetimes observed in this solvent are relatively high

Summary

■ INTRODUCTION

Unlike the analogue europium(III) triflate complexes, where one water molecule was detected in the inner coordination sphere, when the complex was dissolved in AN, in the case of samarium(III) triflate (and nitrate) complexes, no water molecule should be present in close proximity of the cation because the lifetimes observed in this solvent are relatively high (at least higher than those in the case of a MeOH solution). This conclusion is supported by the D2O/H2O exchange experiments in AN, described above for europium(III) complexes. Further investigation on the anion binding as a function of the solvent and lanthanide is in progress

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES