Unusual Magnetic Field Responsive Circularly Polarized Luminescence Probes with Highly Emissive Chiral Europium(III) Complexes

Abstract

Chirality is ubiquitous within biological systems where many of the roles and functions are still undetermined. Given this, there is a clear need to design and develop sensitive chiral optical probes that can function within a biological setting. Here we report the design and synthesis of magnetically responsive Circularly Polarized Luminescence (CPL) complexes displaying exceptional photophysical properties (quantum yield up to 31 % and |g_lum | up to 0.240) by introducing chiral substituents onto the macrocyclic scaffolds. Magnetic CPL responses are observed in these chiral Eu^III complexes, promoting an exciting development to the field of magneto-optics. The |g_lum | of the ⁵ D₀ → ⁷ F₁ transition increases by 20 % from 0.222 (0 T) to 0.266 (1.4 T) displaying a linear relationship between the Δg_lum and the magnetic field strength. These Eu^III complexes with magnetic CPL responses, provides potential development to be used in CPL imaging applications due to improved sensitivity and resolution.

Keywords: chirality; circularly polarized luminescence; lanthanide; magnetic properties.

Scheme 1

Molecular structures of EuL1–6 (top). Peptoid used for conjugation for EuL7 (bottom), A) piperidine/DMF (1:5, v/v); B) DMF, (R)(SAP)EuL3, NMM, PyBop; C) 95 % TFA, 3 % water, 2 % TIPS.

Figure 1

A) Front view of SAP (left) and TSAP (right); B) Top‐down view of SAP (left) and TSAP (right), atoms are represented as spheres with different color: europium (red), oxygen (blue), nitrogen (black); C) Emission spectra of (SAP)EuL3 (top) and (TSAP)EuL3 (bottom), excited at 350 nm, in 0.1 M HEPES buffer, pH 7.3, with 380 nm long pass filter.

Figure 2

CPL spectra (upper curves) and total luminescence (lower curves) of (SAP)EuL3 isomers in 0.1 M HEPES, pH 7.3, λ _exc=340 nm.

Figure 3

A) CPL spectra of (SAP)EuL3 in 0.1 M HEPES (purple), MeOH (cyan) and DMSO (red), λ _ex=340 nm. B) CPL spectra & C) g_lum spectra of (SAP)EuL3 titrated with L‐threonine (purple), L‐histidine (blue), L‐glutamic acid (cyan), L‐ascorbate (yellow) and HSA (red) in 0.1 M HEPES, pH 7.3.

Figure 4

A) Magnetic field dependent g_lum spectra of (R)&(S)(SAP)EuL3 in 0.1 M HEPES buffer, pH 7.3, excited at 350 nm at room temperature, arrows indicate the trends of g_lum changes, inserted diagrams show the direction of applied magnetic field; B) g_lum and C) Δg_lum as a function of magnetic field strength at ⁵D₀ → ⁷F₁ transition.