Circularly polarised luminescence (CPL) control of oligopeptide–Eu(III) hybridized luminophores by interaction with peptide side chains

Abstract

Chiral oligopeptide-naphthalene/Eu(III) hybridized luminophores emit strong circularly polarised solution-state luminescence (CPL) from Eu(III) at 592 and 614 nm (|g_CPL| ≤ 2.1 × 10⁻²). Although the peptide ligands have matching absolute configurations, the CPL sign is controllable by varying the number of naphthalene units and peptide/Eu(III) coordination ratio.

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Photoluminescent organic and inorganic materials are used in high-performance electroluminescent and optoelectronic devices. Recently, optically active luminophores that emit circularly polarised luminescence (CPL) with high quantum yield (Φ_F) have received a great deal of interest.¹ When coordinated by chiral organic ligands, f–f transition metal ions, such as Eu(III), create strong CPL-emitting luminophores due to high CPL dissymmetry ratio (g_CPL), very narrow full-width at half-maximum (FWHM) bandwidth, and highly tuneable CPL intensity, spanning the visible to near-infrared (NIR) region.²

Generally, two enantiomeric organometallic luminophores bearing opposite chirality are required for the selective emission of left-handed and right-handed CPL; however, such chiral organic ligands are not always synthetically tractable. Therefore, it is highly desirable to develop a novel method for controlling the CPL sign of chiral organometallic luminophores without the need to synthesise their respective enantiomers.

Recently, we reported a series of chiral oligopeptide-pyrene luminophores in which the CPL signs of the bipyrenyl-based excimer were successfully controlled by tuning either the distance between the two pyrenyl fluorophores or the solvent.³ In addition, the CPL signs of binaphthyl-Eu(III) hybridized luminophores were successfully inverted by altering the distance between the axial chiral point and the Eu(III) ion.⁴ Nevertheless, it remains difficult to control the chiroptical properties of metal-based luminophores coordinated to stereoisomeric ligands of the same absolute configuration owing to the metal's multiple coordination sites and the large distance between the metal and the ligand's chiral centre.

Herein, we report oligopeptide–Eu(III) hybridized luminophores, in which the signs of the Eu(III) CPL signals at the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions were subtly inverted in accordance with the number of naphthalene rings in the peptide and the coordination ratio of Eu(III) to peptide, despite the oligopeptide ligand having the same stereocentres (L or D). Notably, the CPL signals were inverted even though the corresponding chiroptical signs in the circular dichroism (CD) signals of all oligopeptides with L- or D-stereocentres were identical.

Using conventional solid-phase peptide synthesis, we prepared eight compounds comprised of the L- and D-isomers of four chiral oligopeptide–naphthalene organic ligands having different numbers of aromatic naphthalene groups: H-Sp6-L-Ala(Nap)-Sp6-NH₂ (L-1), H-Sp6-L-Ala(Nap)-L-Ala(Nap)-Sp6-NH₂ (L-2), H-Sp6-L-Ala(Nap)-L-Ala(Nap)-L-Ala(Nap)-Sp6-NH₂ (L-3), and H-Sp6-L-Ala(Nap)-L-Ala(Nap)-L-Ala(Nap)-L-Ala(Nap)-Sp6-NH₂ (L-4), and the corresponding D-isomers, D-1–D-4 (Chart 1; ESI, Fig. S1–S4†). Optically inactive Eu(III)(hfa)₃(H₂O)₂ [hfa; 1,1,1,5,5,5-hexafluoropentane-1,4-dione] was prepared as previously described (Chart 1).⁴

Chart 1 Chiral oligopeptide-naphthalene ligands L-1–L-4 (and D-1–D-4) and Eu(III)(hfa)₃.

Four hybridized luminophores, L-1/Eu–L-4/Eu (and D-1/Eu–D-4/Eu), were obtained by mixing the corresponding 1–4 with Eu(III)(hfa)₃(H₂O)₂ in the desired molar ratio in chloroform (CHCl₃) at room temperature. We assume that, in the ground state, Eu(III)(hfa)₃ exists as a mixture of Δ- and Λ-isomers of D₃-symmetrical facial geometry.

To characterise the photophysical properties of the L-1–L-4/Eu (1/1 molar ratio) luminophores as a function of naphthalene unit number, the steady-state unpolarised photoluminescence (PL) and CPL spectra of dilute L-1–L-4/Eu and L-1–L-4 solutions in CHCl₃ were obtained (Fig. 1 and ESI, Fig. S5–S9,† and Table 1). Although aggregation-induced quenching regularly limits the use of luminophores, L-1–L-4/Eu remarkably exhibited Eu(III) PL in CHCl₃ (Fig. 1, lower panel). The photoluminescence maxima (λ_em) for L-1–L-4/Eu are ∼592 nm (⁵D₀ → ⁷F₁), ∼614 nm (⁵D₀ → ⁷F₂), ∼651 nm (⁵D₀ → ⁷F₃), and ∼698 nm (⁵D₀ → ⁷F₄). The values of λ_em for L-1–L-4/Eu subtly differ from one another by approximately 1−2 nm. The Φ_F values for L-1–L-4/Eu, mainly arising from the electric-dipole-allowed ⁵D₀ → ⁷F₂ transitions, are also similar (0.21, 0.27, 0.34 and 0.32, respectively), owing to the ligands' identical peptide backbone unit.

Fig. 1 CPL (upper traces) and PL (lower traces) spectra of L-1/Eu (orange lines), L-2/Eu (blue lines), L-3/Eu (red lines), and L-4/Eu (green lines). Conditions: 1.0 × 10⁻⁴ M L/Eu (1/1 molar ratio) in CHCl₃, path length = 10 mm, ex: 300 nm. Arrows indicates y-axis of respective traces.

Table 1 Chiroptical properties of L-1–L-4/Eu (1/1 molar ratio) in CHCl₃ solution

	CPL				ΦF	CD
	λCPL (nm)	gCPL (×10^−2)	λCPL (nm)	gCPL (×10^−2)		λCD (nm)	gCD (×10^−4)
L-1/Eu	592	−0.93	615	+0.067	0.21	330	+3.1
L-2/Eu	592	−2.1	615	+0.15	0.27	329	+5.3
L-3/Eu	593	+0.39	611	−0.083	0.34	333	+4.9
L-4/Eu	597	+0.24	614	−0.074	0.32	334	+0.8

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Luminophores L-1–L-4/Eu exhibit clear CPL signals characteristic of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions (Fig. 1, upper panel). The chiral oligopeptides are thus powerful ligands for Eu(III)(hfa)₃ luminescence, affording CPL at 4f–4f transitions. Interestingly, although L-1–L-4 have the same chirality centre, the signs of their CPL spectra for the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions changed in accordance with the number of naphthalene units in the peptide; specifically, −/+ signs for L-1–L-4/Eu having one or two naphthalene units and −/+/−/+ signs for L-1–L-4/Eu having three or four naphthalene units are clearly observed. As expected for stereoisomers, the CPL spectra of L-1–L-4/Eu and D-1–D-4/Eu almost mirror one another (ESI, Fig. S5b–S8b†). The g_CPL values in the photoexcited state allow us to quantitatively describe the degree of CPL inducibility provided by L-1–L-4/Eu, defined as g_CPL = (I_L – I_R)/[1/2 (I_L + I_R)] where I_L and I_R denote the intensities of the left- and right-handed CPL, respectively, following excitation with unpolarised light. The g_CPL values for L-1/Eu and L-2/Eu are −0.93 × 10⁻² at 592 nm and +0.067 × 10⁻² at 615 nm, and −2.1 × 10⁻² at 592 nm and +0.15 × 10⁻² at 615 nm, respectively. The g_CPL values for L-3/Eu and L-4/Eu are +0.39 × 10⁻² at 593 nm and −0.083 × 10⁻² at 611 nm, and +0.24 × 10⁻² at 597 nm and −0.074 × 10⁻² at 614 nm, respectively. Overall, L-2/Eu delivers the highest |g_CPL| values among the luminophores. These results suggest that the chirality, L or D, of the ligand's peptide backbone does not determine the CPL signs of Eu(III) ions at 4f–4f transitions; rather, the CPL signs of peptide ligand-bound Eu(III) are guided by the number of naphthalene units in the peptide.

We next measured the CD properties of L-1–L-4/Eu (1/1 molar ratio) and L-1–L-4 in CHCl₃ (Fig. 2 and ESI, Fig. S10–S12,† upper panel, and Table 1). In contrast to the variation observed for the CPL signs, L-1–L-4/Eu complexes commonly exhibit negative (−)-CD spectra at 297 nm, characteristic of the π–π* transitions of the chiral naphthalene units, while the positive (+)-CD spectra at 331 nm arise from the π–π* transitions of both the chiral naphthalene units and the hfa ligands. The CD spectra for L-1–L-4/Eu and D-1–D-4/Eu are mirror images of each other (ESI, Fig. S10†). The dimensionless Kuhn's anisotropy factors for the ground state chirality, g_CD, for 1–4/Eu were ∼+3.1 × 10⁻⁴ at 330 nm (λ_CD), ∼+5.3 × 10⁻⁴ at 329 nm (λ_CD), ∼+4.9 × 10⁻⁴ at 333 nm (λ_CD), and ∼+0.8 × 10⁻⁴ at 334 nm (λ_CD), respectively.

Fig. 2 CD (upper traces) and UV-Vis absorption (lower traces) spectra of L-1/Eu (orange lines), L-2/Eu (blue lines), L-3/Eu (red lines), and L-4/Eu (green lines). Conditions: 1.0 × 10⁻⁴ M L/Eu (1/1 molar ratio) in CHCl₃, path length = 1 mm. Arrows indicate y-axis of respective traces.

Upon comparing the CPL and CD signs of 1–4/Eu, we concluded that the S₀ ground state chiralities of the luminophores are the same, but the S₁ photoexcited state chiralities are opposite. In solution, optically inactive Eu(hfa)₃ exists as a mixture of interconvertible D₃-symmetric Δ- and Λ-isomers, owing to the low barrier height between the two forms. The CPL sign inversion in 1–4/Eu may therefore result from the dynamic switching between the Δ- and Λ-forms of Eu(III)(hfa)₃ owing to a dynamic change in the chiral peptide chain, which occurs only in the photo-excited states in the CHCl₃ solution.

To further expand the CPL control produced by oligopeptide–Eu(III) hybridized luminophores, we sought to modulate the CPL by varying the peptide/Eu(III) molar ratio. Concentration-dependent CPL and PL spectra of L-1–L-4/Eu (1/0.5, 1/1, 1/2, 1/3, and 1/4 molar ratios) were obtained using a fixed concentration of [L-2–4] (1.0 × 10⁻⁴ M in CHCl₃) (Fig. 3; ESI, Fig. S6–S8†). The CPL intensities at ∼592 nm and ∼614 nm are similar for (L-2/Eu) ratios of 1/0.5 and 1/1. However, when the molar ratio of L-2 to Eu varies from 1/1 to 1/4, these intensities decreased. These results suggest that L-2 coordinates to Eu(III) in a 1/1 ratio in CHCl₃. Unfortunately, despite the intensity changes, the CPL sign remained constant.

Fig. 3 CPL (upper trace) and PL (lower trace) spectra excited at 300 nm of (a) L-2/Eu, (b) L-3/Eu, and (c) L-4/Eu as a function of the molar ratio of Eu to L-2, L-3, and L-4 [L-2–4/Eu = 1/0.5 for black lines, = 1/1 for orange lines, = 1/2 for blue lines, = 1/3 for red lines, and = 1/4 for green lines] in CHCl₃. Conditions: [L-2–L-4] = 1.0 × 10⁻⁴ M, path length = 10 mm, ex: 300 nm. Arrows indicate y-axis of respective traces.

The CPL spectra of L-3/Eu in CHCl₃ differ greatly from those of L-2/Eu (Fig. 3(b), ESI, Fig. S7†). Surprisingly, the CPL sign could be controlled by varying the molar ratio of Eu(III) to L-3. The ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions have decisively opposite CPL signs: −/− signs for 1/0.5 (L-3/Eu) molar ratios, +/− signs for 1/1 (L-3/Eu) molar ratios and −/+ signs for 1/2–4 (L-3/Eu) molar ratios. Additionally, 1/2–4 (L-3/Eu) molar ratios display similar CPL intensities, indicating that L-3 coordinates to Eu(III) at a maximum 1/2 (L-3/Eu) ratio in CHCl₃. The CPL sign could also be controlled by varying the molar ratio in the L-4/Eu system (Fig. 3(c), ESI, Fig S8†). While the CPL signs for the ⁵D₀ → ⁷F₂ transitions were unchanged, decisively opposite CPL signs for the ⁵D₀ → ⁷F₁ transition were observed: − signs for 1/0.5–1 (L-4/Eu) molar ratios and + signs for 1/2–4 (L-4/Eu) molar ratios. These results are consistent with the idea that the CPL sign can be tailored by tuning both the molar ratio of Eu(III) to peptide and the configurational chirality (L or D) of the oligopeptide.

To investigate the ground-state chirality of different L-3/Eu molar ratios, CD and UV-Vis absorption spectra of 1/0.5–4 L-3/Eu in CHCl₃ were obtained (Fig. 4; ESI, Fig S10†). Despite the L-3/Eu molar ratio, several characteristic CD bands arising from the π–π* transitions of the naphthalene units and the hfa ligands were observed. There were no CD signal sign changes observed. These results further confirm that there are no differences between the S₀ ground state chiralities of the four luminophores.

Fig. 4 CD (upper trace) and UV-Vis absorption (lower trace) spectra of L-3/Eu as a function of the molar ratio of Eu to L-3 [L-3/Eu = 1/0.5 for black lines, = 1/1 for orange lines, = 1/2 for blue lines, = 1/3 for red lines, and = 1/4 for green lines] in CHCl₃. Conditions: [L-3] = 1.0 × 10⁻⁴ M, path length = 1 mm. Arrows indicate y-axis of respective traces.

For a fixed concentration of L-2–L-4 (1.0 × 10⁻⁴ M), the g_CPL value at the two CPL bands (592 and 614 nm) in CHCl₃ slightly depended on the molar ratio when changed from 1/0.5 to 1/4 (L-2–4/Eu) (Fig. 5; ESI, Fig. S6–S8†). In L-2, although the |g_CPL| values decreased as the molar ratio of Eu(III) to L-2 increased from 1/0.5 to 1/4, the slope of the curve drastically changes between L-2/Eu molar ratios of 1/1 and 1/2 (Fig. 5(a); ESI, Fig. S6†). On the other hand, for L-3, the |g_CPL| values increased with an increasing L-3/Eu molar ratio from 1/0.5 to 1/2 (Fig. 5(b); ESI, Fig. S7†). A dramatic slope change was observed between 1/2 and 1/3 (L-3/Eu) molar ratios. Complex changes in the |g_CPL| values were observed for L-4 (Fig. 5(c); ESI, Fig. S8†). These results further confirm that L-2 and L-3 coordinate to Eu(III) in CHCl₃ in 1/1 and 1/2 ratios, respectively.

Fig. 5 The |g_CPL| values of ⁵D₀ → ⁷F₁ (592 nm) (blue lines) and ⁵D₀ → ⁷F₂ (614 nm) (red lines) transitions as a function of the molar ratio of Eu to (a) L-2, (b) L-3, and (c) L-4 in CHCl₃. [L-2–L-4] = 1.0 × 10⁻⁴ M.

The coordination capacity of the amide units of 1–4 to Eu(III) was supported by positive mode LCMS-IT-TOF analysis (Table 2 and ESI, Fig. S13 to S18†). This work confirmed that 1–4 coordinated to Eu(III): [Eu(L-1)(hfa)₂]⁺ (m/z 1451.4020) for L-1, [Eu(L-2)(hfa)₂]⁺ (m/z 1648.4832) for L-2, [Eu(L-3)(hfa)₃HNa]²⁺ (m/z 1037.2530) for L-3, and [Eu(L-4)(hfa)₂]⁺ (m/z 2042.6524) for L-4. Although three hfa ligands were observed for L-3/Eu, only two hfa ligands were observed for L-1, L-2, and L-4 complexes. In addition, the molar ratio of Eu(III) complex to peptide ligand for L-3 changed from 1/1 to 1/3. Unfortunately, in the case of 1/2 and 1/3 (L-3/Eu) ratios, only the signal corresponding to a 1/1 L-3/Eu ratio was observed; i.e., [Eu(L-3)(hfa)₂]⁺ (m/z 1845.5684) for L-3/Eu = 1/2 and [Eu(L-3)(hfa)₂]⁺ (m/z 1845.5662) for L-3/Eu = 1/3. The coordination of Eu(III) to O (from the peptide) is rather weak; therefore, the Eu(III)–O bonds of the hybrid luminophores may easily dissociate under laser-assisted ionisation energy.

Table 2 LCMS-IT-TOF analyses of L-1–L-4/Eu

	m/z	Found Eu complex
L-1	1451.4020	[Eu(L-1)(hfa)2]^+
L-2	1648.4832	[Eu(L-2)(hfa)2]^+
L-3 (L-3/Eu = 1/1)	1037.2530	[Eu(L-3)(hfa)3HNa]^2+
L-3 (L-3/Eu = 1/2)	1845.5684	[Eu(L-3)(hfa)2]^+
L-3 (L-3/Eu = 1/3)	1845.5662	[Eu(L-3)(hfa)2]^+
L-4	2042.6524	[Eu(L-4)(hfa)2]^+

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¹⁹F-NMR spectra of Eu(III)(hfa)₃ with 1–4 (1/1) were measured in CDCl₃ (ESI, Fig. S19–S24†). The ¹⁹F-NMR spectra of L-1–L-4/Eu in CDCl₃ indicates that L-1–L-4/Eu exhibit strong resonance signals at −75.23 ppm (L-1), −79.53 ppm (L-2), −79.46 ppm (L-3), and −79.59 ppm (L-4), respectively, and weak resonance signals at −78.85 ppm and −79.59 ppm (L-1), −75.14 ppm (L-2), −75.08 ppm (L-3), −75.23 ppm and −78.85 ppm (L-4), respectively. These clear chemical shift changes were attributed to the dynamic coordination of 1–4 to Eu(III) in solution. The eighteen fluorine atoms of Eu(hfa)₃ are no longer magnetically equivalent due to a lowering of the molecular symmetry from D₃ to C₁ upon interaction with L-1–L-4, as the chirality inducing ligands in the ground state and possibly in the photoexcited states. A significant reorganisation of the three strong hfa ligands along with weaker 1–4 ligands to induce chirality towards Eu(III) ions might occur in the photoexcited state.

The reason behind the CPL sign inversion has not yet been elucidated. The rotational freedom of the peptide backbone in ligands 1–4 is likely the key to imparting the CPL sign-swapping capability in the photoexcited state, even though 1–4/Eu have the same CD sign in the ground state. It is possible that the twists of the peptide chain, naphthalene units, and hfa ligands in these luminophores are opposite only in the photo-excited states in chloroform.

Chiral 1–4/Eu luminophores were easily prepared by mixing the oligopeptide ligands with Eu(III)(hfa)₃ in CHCl₃. By varying the number of naphthalene units in the ligand and the molar ratio of peptide ligands/Eu(III), the CPL signs of the luminophores could be controlled. Our findings offer a novel method to control the CPL sign of CPL-functionalised Eu(III)-based luminophores and have obvious applications to other chiral peptide ligands and, potentially, other lanthanide(III) ions. In addition, these results provide a deeper understanding of how photoexcited, chiral organometallic luminophores can relax to their ground states with minimum to maximum reorganisation, as revealed by the conservation and breaking of signs between the CD and CPL signals.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by a Grant-in-Aid for Scientific Research (no. 18K05094 and 19H04600) from MEXT/Japan Society for the Promotion of Science and the KDDI and Futaba Foundations.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra09708b

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