Controlling circularly polarized luminescence of a pyrene modified chiral Zn(II) complex based on a temperature-dependent diastereomer equilibrium and solid-state excimer formation

Abstract

A pyrene modified chiral Schiff-base Zn(II) complex was synthesized and its circularly polarized luminescence (CPL) properties were examined. Single crystal X-ray diffraction analysis of the (S,S)-isomer revealed its distorted tetrahedral coordination geometry with Λ coordination chirality. An equilibrium of the two diastereomers with different coordination chiralities, Λ-(S,S) and Δ-(S,S), was observed in solution, and the ratio of the isomers was found to change with temperature. The complex showed negligible CPL in CHCl₃ at room temperature, whereas a clear mirror-image signal was obtained when the solution was cooled to 253 K. Interestingly, excimer-derived long wavelength emission was also observed in the solid state, resulting in CPL sign inversion compared to the solution state. The relationship between their photophysical properties and coordination geometries was theoretically investigated using density functional theory (DFT) calculations.

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Introduction

Circularly polarized luminescence (CPL) is a phenomenon in which chiral luminescent materials exhibit left- or right-handed biased CP light.¹ In the last decade, various CPL materials such as liquid crystals,² polymers³ and small organic molecules⁴ have been developed for application in three-dimensional displays,⁵ biological probes,⁶ and anti-counterfeiting materials.⁷ In particular, recently there has been interest in developing methodologies to control CPL properties using various external stimuli.⁸ Metal complexes are promising stimuli-responsive materials because various coordination conformations can be designed by adjusting metal ions and chelating ligands.⁹ A number of chiral metal complexes have been developed and methods to control their CPL properties have been broadly explored.^10–14 Zinc(II) complexes are known to adopt various coordination geometries, such as tetrahedral,^15–18 trigonal bipyramidal,^19–21 square pyramidal,²² and octahedral,^23,24 depending on the design of the chelating ligands, enabling unique CPL properties based on their three-dimensional structures. More recently, application of chiral Zn(II) complexes to circularly polarized organic light-emitting diodes (CP-OLEDs) has also been achieved.²⁵

Pyrene is an aromatic hydrocarbon with four fused benzene rings, and its derivatives have a wide range of applications in organic electronics,²⁶ biological probes,²⁷ and optical materials.²⁸ One of the most important characteristics of pyrene derivatives is the formation of excimers in the excited state. It is known that when two pyrene rings are in a chiral orientation, they exhibit CPL properties that are distinct from those of the monomeric state.²⁹ Therefore, the pyrene backbone is considerably useful as a CPL switching component, and a variety of derivatives responsive to various external environments and external stimuli have been reported.^21,30–34

Our research group is interested in chiral metal complexes with Schiff-base ligands. In the course of our studies, we have succeeded in controlling their CPL properties by inducing chiral distortions in the coordination structure through appropriate ligand design.^35–38 Janiak's group has investigated the Zn(II) complex of the salicylaldiminato ligand derived from chiral 1-phenylethylamine [Zn(O-N)₂] and established an equilibrium of diastereomers, differing only in the configuration at the Zn center.³⁹ We prepared and characterized the corresponding naphthalene and phenanthrene complexes, in which CPL inversion was successfully achieved by controlling Λ and Δ coordination chirality.¹⁷ In the present work, we studied the pyrene-modified Zn(II) complex 1 (Fig. 1), introducing the new aspect of excimer formation through stacking of the pyrene rings. The complex showed a diastereomer equilibrium between Λ-(S,S) and Δ-(S,S) in solution, and the ratio of the diastereomers was found to change depending on temperature. Temperature-dependent CPL measurements strongly reflect the diastereomer ratio changes, and CPL enhancement was observed at low temperature. Moreover, excimer-derived CPL was observed in the solid state, and the signal was inverted compared with that of the solution state. Density functional theory (DFT) calculations were performed to further understand their chiroptical properties.

Fig. 1 Chemical structures and the diastereomer equilibrium of chiral zinc(II) complexes (S,S)-1.

Results and discussion

Synthesis and structures

The synthetic route to the pyrene modified chiral Zn(II) complex 1 is shown in Scheme 1. 1-Hydroxy-2-pyrenecarboxaldehyde was prepared according to a published procedure.⁴⁰ Schiff-base ligand (S)-L1 was synthesized by condensation of 1-hydroxy-2-pyrenecarboxaldehyde with commercially available (S)-1-phenylethylamine. The pyrene-modified Zn(II) complex (S,S)-1 was prepared by the reaction of ZnCl₂ with (S)-L1 in the presence of ^tBuOK. The enantiomer, (R,R)-1, was also synthesized by the same method using (R)-1-phenylethylamine as the starting material. Complex 1 was characterized by nuclear magnetic resonance (NMR) spectroscopy (Fig. S1 and S2, ESI†) and high-resolution mass spectrometry (HRMS).

Scheme 1 Synthesis of the chiral Zn(II) complex (S,S)-1.

Single crystals of (S)-L1, (S,S)-1 and (R,R)-1 were obtained by recrystallization from CHCl₃/EtOH, and the molecular structures were elucidated by single crystal X-ray diffraction (XRD) analysis. The crystallographic data are presented in Table S1.† ORTEP drawings of (S)-L1 and (S,S)-1 are shown in Fig. 2. The ligand (S)-L1 crystallized in the tetragonal, chiral space group P4₃2₁2. Because ligand (S)-L1 does not contain any heavy atoms, the Flack parameter (−0.5(10)) is not sufficient to determine the absolute configuration. However, since optically pure (S)-1-phenylethylamine was used as the starting material, the absolute configuration of S is considered correct (Fig. 2a). In contrast, complex (S,S)-1 crystallized in the orthorhombic, chiral space group P2₁2₁2₁ with a sufficiently low Flack parameter (−0.006(5)). The chirality angle of O(1)–N(1)–O(2)–N(2) (φ) of (S,S)-1 has the positive value 72.02° (Fig. 2b). Hence, (S,S)-1 has the Λ configuration in the crystal form.^17,41 In addition, XRD measurements confirm that the (R,R)-1 configuration is a perfect mirror image of (S,S)-1 (Fig. S4, ESI†). The absolute configuration was assigned as Δ-(R,R) from the negative value of φ (−67.98°). The packing structure in the crystal of (S,S)-1 is shown in Fig. S5,† where non-covalent interactions such as CH–π and π–π stacking were observed between the pyrene rings.

Fig. 2 ORTEP drawings of (a) (S)-L1 and (b) Λ-(S,S)-1. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. The dihedral angle of the tetrahedral geometry O(1)–N(1)–O(2)–N(2) (φ) is given under the structure in (b).

Diastereomer equilibrium in solution

Although the complexes crystallized with single coordination chirality (Λ or Δ) in the crystalline state, a diastereomer equilibrium was observed in solution. The ¹H NMR spectra of (S,S)-1 in CDCl₃ showed a diastereomer equilibrium based on different signals associated with Λ-(S,S)-1 and Δ-(S,S)-1 (Fig. 3). All proton signals of each diastereomer were successfully assigned using gCOSY and NOESY spectra (Fig. S6 and S7, ESI†). The diastereomer ratio was estimated to be Λ : Δ = 1 : 0.38 (diastereomeric excess (de) is 45%) at 293 K from integration values of the ¹H NMR spectrum. Upon cooling, the signals of the Δ-isomer gradually increased, and the de reached 36% at 253 K. Such temperature-dependent changes were similar to those of a previous report of our group.¹⁷

Fig. 3 Variable-temperature (VT)-¹H NMR spectra (500 MHz) of (S,S)-1 in CDCl₃. The peak with an asterisk symbol corresponds to the free ligand in the sample.

Photophysical properties

The photophysical properties of complexes (S,S)- and (R,R)-1 in CHCl₃ and in the KBr-dispersed pellet state were investigated, and the data of (S,S)-1 are summarized in Table 1. The UV-vis spectrum of (S,S)-1 showed a broad π–π* transition band at approximately 400–550 nm in CHCl₃ (Fig. 4 bottom). Compared to previously reported Zn(II) complexes with naphthalene and phenanthrene frameworks,¹⁷ a bathochromic shift was observed due to the extension of the π-conjugated system. The circular dichroism (CD) spectra of the two enantiomers, (S,S)-1 and (R,R)-1, were recorded under the same conditions (Fig. 4 top). The two enantiomers showed completely mirror-image signals, with positive Cotton effects for (S,S)-1 and negative Cotton effects for (R,R)-1 in the low energy region attributed to the S₀-to-S₁ transition. The g_abs (=Δε/ε) value of the transition was calculated to be 4.5 × 10⁻⁴.

Fig. 4 CD (upper plot) and UV-vis absorption (lower plot) spectra of (R,R)- and (S,S)-1 in CHCl₃ at 293 K.

Table 1 Photophysical data for complex (S,S)-1

Medium	λ abs [nm]	λ max [nm]	Φ ^,	τ [ns]	k r × 10^7 ^e [s^−1]	k nr × 10^8 ^f [s^−1]	g lum	CIE [x, y]^b
a Data were obtained from 2.0 × 10^−4 M solutions at 293 K. b λ ex = 450 nm. c Luminescence quantum efficiencies measured using the absolute method with an integrating sphere. d λ ex = 366 nm. e k r = Φ298 K/τ. f k nr = (1 − Φ298 K)/τ. g The 2ΔI/I values around emission peak maxima (λmax) are listed. h The data measured at 253 K.
CHCl3^a	288, 360, 381, 472	549 (547)^h	0.08	2.8	2.9	3.3	(2 × 10^−3)^h	0.45, 0.55
KBr pellet	—	565, 621	0.03	82 (65%), 3.7 (35%)	0.055	0.18	−2 × 10^−3	0.49, 0.51

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Complex 1 exhibited yellow and orange photoluminescence in CHCl₃ and in the KBr-dispersed state, respectively (Fig. S8, ESI†). The red-shifted emission observed in the KBr-dispersed state is attributed to excimer formation induced by molecular aggregation. This interpretation is further supported by the emission lifetime measurements in the KBr pellet, which revealed a long-lived component characteristic of excimer emission (Fig. S10, ESI†). Although well-defined π–π stacking interactions were not observed in the X-ray crystallographic analysis, it is considered that the mechanical grinding during the preparation of the KBr pellet facilitated excimer formation by promoting intermolecular interactions. In the excitation spectra, a red-shift was observed in the KBr-dispersed pellet compared to the solution, suggesting the formation of static excimers due to enhanced intermolecular interactions in the solid state (Fig. S11, ESI†).^42,43 Emission quantum efficiencies (Φ) were determined as 8% for solution and 3% for solid samples. The CPL and total emission spectra were measured for CHCl₃ solution and KBr-dispersed pellet samples (Fig. 5). While complex 1 showed only weak CPL signals in solution at room temperature (Fig. S12, ESI†), clear signals were observed when the solutions were cooled to 253 K (Fig. 5a, top). Such temperature-dependent CPL enhancement can be attributed to the variation in the diastereomer ratio of complex 1 and will be discussed based on theoretical calculations in a later section. The sign of CPL in the solution state corresponded to the S₀-to-S₁ transition in CD, which was positive in (S,S)-1 and negative in (R,R)-1 (Fig. 4, top). More interestingly, sign-inverted CPL signals were observed in the KBr-dispersed pellet state compared to the solution state (Fig. 5b, top). Pyrene derivatives are known to exhibit CPL sign inversion depending on the stacking orientation during excimer formation.²⁹ Therefore, the interaction between the pyrene rings of complex 1 could have caused the excimer-derived CPL in the solid state. To investigate excimer formation in the aggregated state of complex 1, polymer-dispersed films with varying concentrations were prepared, and their photoluminescence properties were examined (Fig. S13, ESI†). Green luminescence was observed in a polymethyl methacrylate (PMMA) film doped with 5 wt% of complex 1. At higher concentrations (≥10 wt%), an additional orange emission band appeared in the low-energy region (>600 nm), which is attributed to excimer emission. The emission peaks around 540–570 nm are assigned to monomer emission arising from different conformations of the complex. In the excitation spectra, a red shift was observed with increasing doping concentration of the complex (Fig. S14, ESI†), suggesting the formation of static excimers, similar to the behavior seen in the KBr-dispersed pellet as shown in Fig. S11, ESI.†

Fig. 5 CPL (upper plot) and total emission (lower plot) spectra of (R,R)- and (S,S)-1. (a) In CHCl₃ (2.0 × 10⁻⁴ M) at 253 K. (b) In the KBr-dispersed pellet state at 293 K.

DFT calculations

DFT calculations were performed for (S,S)-1 at the B3LYP/6-31+G(d,p) level of theory to reveal the correlation between coordination chirality and photophysical properties. Since NMR measurements confirmed that the two diastereomers, Λ-(S,S)-1 and Δ-(S,S)-1, are mixed in solution, structural optimization and estimation of their energies were performed for each diastereomer. Fig. 6 shows the optimized structures of Λ-(S,S)-1 and Δ-(S,S)-1 in the S₀ state. The energy difference between the two diastereomers was estimated to be 0.65 kcal mol⁻¹, with the Λ-(S,S)-form being slightly more stable than the Δ-(S,S)-form. This result is consistent with the fact that Λ-(S,S)-1 was the main product in solution as confirmed by ¹H NMR spectra (Fig. 3). The frontier orbitals of Λ-(S,S)-1 and Δ-(S,S)-1 and their eigenvalues are given in Fig. S16† (S₀) and S17 (S₁). The HOMOs of both diastereomers are principally π orbitals of the pyrene framework, whereas the LUMOs are the π* orbitals of both the S₀ and S₁ states.

Fig. 6 Optimized structures of Λ-(S,S)-1 and Δ-(S,S)-1 in the S₀ ground state estimated by DFT calculations (B3LYP/6-31+G(d,p)). The values in parentheses are relative Gibbs free energies in kcal mol⁻¹. Hydrogen atoms are omitted for clarity.

The energy levels and electronic configurations of the singlet states of 1 were estimated from time-dependent (TD)-DFT calculations (B3LYP/6-31+G(d,p)) (Tables S2 and S3†). The major electronic configurations of the upward S₀-to-S₁ and downward S₁-to-S₀ transitions are determined to be the HOMO-to-LUMO transition, which suggests that emission from 1 is mainly attributable to π–π* transitions. The transition energies of the upward S₀-to-S₁ transition were calculated to be 2.65 eV (467 nm) for Λ-(S,S)-1 and 2.60 eV (477 nm) for Δ-(S,S)-1, which are consistent with the absorption wavelength of the π–π* transition band observed in CHCl₃ (472 nm; Table 1). The downward S₁-to-S₀ transition energies were calculated to be 2.28 eV (544 nm) for Λ-(S,S)-1 and 2.21 eV (560 nm) for Δ-(S,S)-1, which also satisfactory reproduced the experimental emission spectrum in CHCl₃ (549 nm; Table 1).

To gain further insight into the chiroptical properties of (S,S)-1, we subsequently analysed the transition dipole moments for the S₀-to-S₁ and S₁-to-S₀ transitions by using TD-DFT calculations (Fig. 7 and S18†). The magnitude and sign of CD and CPL can be estimated from the theoretical g value defined as g = 4(|μ_e||μ_m|cos θ_e,m)/(|μ_e|² + |μ_m|²). Here, |μ_e| and |μ_m| are the electric and magnetic transition dipole moments, respectively, and θ_e,m is the angle between the two vectors.⁴⁴ For the upward S₀-to-S₁ transition, the two diastereomers were found to exhibit opposite CD signs, positive for Λ-(S,S)-1 and negative for Δ-(S,S)-1, with g_abs values on the order of 10⁻² (Fig. S16†). In solution, these two transitions cancel each other, resulting in a weak Cotton effect of the order of g_abs = 10⁻⁴ (Fig. 4 top). In the downward S₁-to-S₀ transition, both diastereomers were estimated to have a positive CPL (Fig. 7). Furthermore, the g_lum values were found to differ six-fold between the diastereomers, and a comparably low value of the order of 10⁻⁴ was calculated for Λ-(S,S)-1, which was the main product in CHCl₃. Combined with the VT-¹H NMR results, the enhancement of CPL at low temperatures can be attributed to a shift of the diastereomer equilibrium to Δ-(S,S)-1, which is estimated to have a large g_lum value. Because both diastereomers were expected to show the same sign of CPL as monomers, the inversion of the CPL signal in the KBr pellet state is considered to be due to excimer formation caused by intermolecular stacking interactions between the pyrene rings.

Fig. 7 Electric (μ_e, orange) and magnetic (μ_m, green) dipole moments of the S₁-to-S₀ transition for (a) Λ-(S,S)-1 and (b) Δ-(S,S)-1 calculated at the B3LYP/6-31+G (d,p) level. Calculated values of transition dipole moments (|μ_e|, |μ_m| and θ_e,m) and g_lum are given under each structure.

Conclusions

In summary, we have synthesized the pyrene-modified Schiff-base zinc(II) complex 1. The complex crystallized in a single configuration, Λ-(S,S)-1, whereas it exhibits a diastereomer equilibrium in solution based on coordination chirality rearrangement. The diastereomer ratio in solution depended on the temperature, and it was a key factor in improving the CPL properties of the complexes at low temperatures. Interestingly, complex 1 exhibits excimer-derived CPL in the KBr pellet state, and the opposite sign of the CPL signal was obtained compared to that in solution. DFT and TD-DFT calculations of the structures and electronic configurations of (S,S)-1 support the experimental photophysical properties. We believe that the CPL control based on the coordination chirality rearrangements demonstrated in this study will provide important insights for the design of future CPL switching materials.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

This work was supported by JSPS KAKENHI (grant numbers JP23K13768 (M. I.), JP23H02040 (Y. I.) and JP21K05234 (T. T.)), Nihon University Industrial Technology Fund for Supporting Young Scholars (M. I.) and Iketani Science and Technology Foundation (grant number 0361227-A (M. I.)). We gratefully acknowledge Prof. Dr Henri Brunner (Universität Regensburg) for helpful discussion and comments. We also acknowledge Prof. Dr Shuichi Suzuki (Osaka University) for HRMS measurements. Elemental analysis was performed at the Center for Creative Materials Research, CST, Nihon University.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2426213–2426215. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi01086a

‡ These authors contributed equally to this work.

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