Article pubs.acs.org/IC
Induction of Circularly Polarized Luminescence from Europium by Amino Acid Based Ionic Liquids Ben Zercher and Todd A. Hopkins* Department of Chemistry, Butler University, 4600 Sunset Avenue, Indianapolis, Indiana 46208, United States S Supporting Information *
ABSTRACT: Materials that emit circularly polarized light have application in several important industries. Because they show large optical activity and emit sharp visible light transitions, europium complexes are often exploited in applications that require circularly polarized luminescence (CPL). Chiral and coordinating ionic liquids based on prolinate, valinate, and aspartate anions are used to induce CPL from a simple achiral europium triflate salt. The sign of the induced CPL is dependent on the handedness (L vs D) of the amino acid anion. Comparison of the CPL spectra in ionic liquid with proline and valine vs aspartate shows that the number of carboxylate groups in the amino acid anion influences the europium coordination environment. DFT calculations predict a chiral eight-coordinate Eu(Pro)4− structure in the prolinate ionic liquid and a chiral seven- or eight-coordinate Eu(Asp)33− structure in the aspartate ionic liquid.
■
INTRODUCTION Materials that emit circularly polarized light have uses in many applications across science, engineering, and entertainment. Three-dimensional display technologies and the motion picture industry use circularly polarized light to give the appearance of a third dimension.1 The polarization states of left and right circularly polarized light can also be used for information storage in quantum computing.2,3 Circularly polarized light is used in circular dichroism to measure and understand the structure and chirality of biomolecules.4,5 Given the utility, it is not surprising that there has been a recent resurgence in research efforts aimed at developing chiroptical luminescent molecules including organic dyes,6−11 transition metals,12−14 and many that involve a chiral lanthanide complex.15−19 The properties of luminescent lanthanide ions have been exploited in the development of circularly polarized luminescent materials for decades.20,21 Circularly polarized luminescence (CPL) is the differential emission of left vs right circularly polarized light. In order to compare CPL emission across systems with different overall emission efficiencies, it is common to use the emission dissymmetry factor, gem, shown in eq 1. gem =
2(IL − IR ) IL + IR
The luminescence properties of 4f−4f transitions within tripositive europium ions are favorable for the development of luminescent ionic liquids. The Eu3+ emitting state (5D0) is longlived (100−1000s μs) and gives a characteristic narrow wavelength red/orange emission bands due to the 5D0 → 7 F1,2 transition regions. The theoretical basis for 4f−4f chiroptical spectra, both circular dichroism and CPL, was developed by Richardson and Faulkner22 and has been used in other studies of CPL spectra−structure relationships.16,21,23 The emission dissymmetry factor for a transition from state 1 to 2 within the 4f electronic configuration can be represented by gem =
(2)
where D12 is the dipole strength of the transition and R12 is the rotatory strength of the transition.23 The dipole strength of a transition within the 4f6 configuration of Eu3+ can be written as D12 = |⟨1|μ|̂ 2⟩| 2 + |⟨1|m̂ |2⟩|2 = |μ12 |2 + |m12|2
(3)
where m12 and μ12 are the magnetic and electric dipole transition moment vectors. Transitions within the 4f 6 configuration of Eu3+ are parity (LaPorte) forbidden in the electric dipole approximation. However, μ12 (and to a large extent D12) is nonzero, because the portion of the ligand field Hamiltonian that transforms ungerade under inversion (i.e., the chiral part) promotes interconfigurational mixing of opposite parity states with the f-electron states of the metal.22 On the
(1)
where IL and IR are the emission intensities of left and right circularly polarized light, IL − IR is the CPL, and IL + IR is the total luminescence. Therefore, the value of gem can vary from +2 to −2. Many of the 4f−4f transitions in chiral lanthanide complexes have larger dissymmetry factors, gem, than organic dyes or transition metals, which are typically gem ≤ 10−3.6,8,12 © XXXX American Chemical Society
4R12 |D12|
Received: June 2, 2016
A
DOI: 10.1021/acs.inorgchem.6b01343 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry other hand, magnetic dipole transitions, m12, are parity allowed within the 4f6 configuration. The rotatory strength in eq 2 for a 4f−4f transition can be written as22 R12 = |m12|·|μ12 |cos τ12
Figure 1. Cation and anions in the ionic liquids.
(4)
where τ12 is the angle between the electric and magnetic dipole transition moment vectors. Substituting eqs 3 and 4 into eq 2 and assuming that the transition is predominantly electric dipole (|μ12|2 ≫ |m12|2), such as the 5D0 → 7F2 transition in Eu3+, the dissymmetry factor is written as22 gem = 4
|m12|cos τ12 |μ12 |
excellent potential ligands for coordinating tripositive lanthanide ions (i.e., Eu3+).39,40,39 Historically, amino acids have been used to induce CPL in lanthanide complexes through the Pfieffer effect, where interaction with the amino acid generates a nonracemic population of lanthanide complexes,41−44 or the association of the amino acids induces a conformational change to an otherwise achiral europium complex.45 In this study, amino acid anions of the ionic liquids are the only source of chirality and are used as ligands to induce CPL signal in the europium ion. The influence of the amino acid handedness on CPL is probed by studying both the L- and D-enantiomers. This means that there is a total of six ILs, [TBA][L-Val], [TBA][DVal], [TBA][L-Pro], [TBA][D-Pro], [TBA]2[L-Asp], and [TBA]2[D-Asp], in this study. Comparison of [TBA]2[Asp] vs [TBA][Pro] and [TBA][Val] probes the impact of two vs one carboxylate moiety on the ability of the ILs to induce CPL from europium. The principal objective of this study is to demonstrate that CPL is induced when a simple achiral europium salt is dissolved into amino acid based ionic liquids. A secondary objective is to understand how structural changes in the amino acids affect the induced CPL (e.g., two vs one carboxylate). Because of the complexity in measuring the structure of the europium complex in the ILs, DFT calculations are used to help predict the number of amino acid anion ligands and the structure(s) of the europium species.
(5)
However, if we assume the transition mechanism is predominantly magnetic dipole (|m12|2 ≫ |μ12|2), such as the 5 D0 → 7F1 transition in Eu3+, the dissymmetry factor is written as22
gem = 4
|μ12 |cos τ12 |m12|
(6)
As eqs 5 and 6 show, the magnitude and sign of the dissymmetry factor for 4f−4f transitions depend on each of m12, μ12, and τ12. The sign and magnitude of μ12 is dependent on the chiral part of the ligand field Hamiltonian and is directly related to the coordination around the europium ion. The angle, τ12, between electric and magnetic dipole transition vectors is also dictated by the structure of the europium complex. In 4f−4f transitions that are not magnetic dipole allowed (where ΔJ ≠ 0, ±1), m12 can only be nonzero because of J-level mixing by the achiral part of the ligand field. Therefore, the rotatory strengths and observed dissymmetry factors for 4f−4f transitions in Eu3+ are entirely a consequence of the chirality of the structural environment of the ligands about the europium ion. This paper presents a study demonstrating that a coordinating chiral ionic liquid can generate the chiral environment around Eu3+ and induce CPL. Ionic liquids are low melting point salts with properties that differentiate them from molecular liquids and make them suitable for many applications, including as phosphors or “soft” luminescent materials.24−28 Ionic liquids offer a significant advantage as a solvent for luminescent lanthanides, because they do not significantly quench the lanthanide luminescence.29,30 This is not the case in many molecular solvents, especially water, which quenches lanthanide luminescence through nonradiative decay pathways. Another advantage to using ionic liquids (ILs) in luminescent soft materials is the ability to exercise synthetic control over their properties through choice of cation and/or anion. For example, using a chiral cation or anion adds chirality to the IL, which has been exploited in asymmetric catalysis and enantiomeric separation applications.31−35 Addition of a coordinating anion to the IL can impact the coordination environment of metals added to the IL.36 Figure 1 shows the structure of the cation and anions used to make the ILs in this study. The physical properties, such as melting point and viscosity, for all of the IL combinations shown in Figure 1 have been characterized in the literature.37,38 In each IL, the cation is a tetrabutylammonium ion and the anion is one of three different deprotonated amino acids, valinate, prolinate, or aspartate. The amino acids are intrinsically chiral and have carboxylate moieties, which make them
■
EXPERIMENTAL SECTION
L-Proline, D-proline, L-valine, D-valine, L-aspartic
acid, D-aspartic acid, and 55% tetrabutylammonium hydroxide solution were purchased from Sigma-Aldrich and used without further purification. Ionic liquids, [TBA][Pro], [TBA][Val], and [TBA]2[Asp], were prepared by mixing stoichiometric amounts of the tetrabutylammonium hydroxide with the amino acid. The solution was then heated to approximately 50 °C for several hours and stored under vacuum in a vacuum desiccator to remove excess water until the IL had
