Tuning the circularly polarized luminescence in homoleptic and heteroleptic chiral CrIII complexes

A series of highly emissive inert and chiral Cr^III complexes displaying positive and negative circularly polarized luminescence (CPL) within the near-infrared (NIR) region at room temperature have been prepared and characterized to decipher the effect of ligand substitution on the photophysical properties, more specifically on the chiroptical properties. The helical homoleptic [Cr(dqp-R)₂]³⁺ (dqp = 2,6-di(quinolin-8-yl)pyridine; R = Ph, ≡-Ph, DMA, ≡-DMA (DMA = N,N-dimethylaniline)) and heteroleptic [Cr(dqp)(L)]³⁺ (L = 4-methoxy-2,6-di(quinolin-8-yl)pyridine (dqp-OMe) or L = N²,N⁶-dimethyl-N²,N⁶-di(pyridin-2-yl)pyridine-2,6-diamine (ddpd)) molecular rubies were synthesized as racemic mixtures and then resolved and isolated into their respective pure PP and MM enantiomeric forms by chiral stationary phase HPLC. The corresponding enantiomers show two opposite polarized emission bands within the 700–780 nm range corresponding to the characteristic metal-centered Cr(²E’→⁴A₂) and Cr(²T₁^’→⁴A₂) transitions with large g_lum ranging from 0.14 to 0.20 for the former transition. In summary, this study reports the rational use of different ligands on Cr^III and their effect on the chiroptical properties of the complexes.

1 Introduction

Chiral chromophores displaying intense circularly polarized luminescence (CPL) signal and high CPL brightness (B_CPL) are up-and-coming candidates for applications as molecular probes in biological systems (Carr et al., 2012; Staszak et al., 2019), in bio-imaging (Heffern et al., 2014), in light-emitting devices (e.g., CP-OLEDs) (Brandt et al., 2017; Zinna et al., 2017; Furlan et al., 2024), and in counterfeiting agents (e.g., security inks) (MacKenzie and Pal, 2021). The dissymmetry factor g_lum is used to estimate the excess of emitted right- or left-circularly polarized light in an isotropic solution. It is deduced from I_L and I_R, which represent the emission intensities of left and right circularly polarized light, respectively (Equation 1 center, Richardson, 1979; Arrico et al., 2020). This parameter is directly related to the electric dipole ($\overrightarrow{{\mu }_{ij}}$), the magnetic dipole ($\overrightarrow{m_{ij}}$) transition moment, and the angle between these two vectors (θ) (Equation 1 right).

$\left|g_{\text{lum}}\right|=\frac{2\times \left(I_{\mathrm{L}}-I_{\mathrm{R}}\right)}{\left(I_{\mathrm{L}}+I_{\mathrm{R}}\right)}=4\times \frac{\left(\overrightarrow{{\mu }_{ij}}/\overrightarrow{m_{ij}}\right)\times \cos \theta }{{\left(\overrightarrow{{\mu }_{ij}}/\overrightarrow{m_{ij}}\right)}^2+1}.(1)$

An important number of photons must be detected to apply chiral chromophores in CPL materials. To account for that, Zinna and coworkers introduced the CPL brightness (B_CPL), which combines the molar absorption extinction coefficient (ε_λexc), the photoluminescence quantum yield (ϕ_PL), and the dissymmetry factor in Equation 2 (Arrico et al., 2020).

$B_{\text{CPL}}={\epsilon }_{\lambda \text{exc}}\times {\varphi }_{\text{PL}}\times \raisebox{1ex}{$\left|g_{\text{lum}}\right|$}\!\left/ \!\raisebox{-1ex}{$2.$}\right.(2)$

Improving B_CPL is a current challenge as commonly bright luminescent molecules (displaying high ε_λexc and ϕ_PL) are usually accompanied with weak g_lum, as rationalized by Equation 1 because intense ED-allowed transitions (ligand-to-metal charge transfer, metal-to-ligand charge transfer, or π* → π transitions) imply $\overrightarrow{{\mu }_{ij}}\gg \overrightarrow{m_{ij}}$, typically in the range of ${\left|\overrightarrow{{\mu }_{ij}}\right|}^2\approx 75000\cdot {\left|\overrightarrow{m_{ij}}\right|}^2$ (Reiné et al., 2018; Albano et al., 2020). Consequently, organic dyes and 4d/5d complexes, which display large ϕ_PL and ε_λexc, are very challenging systems for improving the B_CPL due to the limiting weak g_lum (Saleh et al., 2014; Reiné et al., 2018; Gauthier et al., 2020; Song et al., 2022; Yoshida et al., 2024). Many strategies have been put in place to counter this limitation and to improve the dissymmetry factor, such as (i) the use of supramolecular assemblies (Sang et al., 2020) or (ii) enclosing chiral molecules into liquid crystals (Albano et al., 2020), into excimers (Nakanishi et al., 2016; Homberg et al., 2018; Hara et al., 2019; Mori, 2020), or into cyclic aromatic structures (Sato et al., 2017). The competitor to this type of molecules is lanthanide-based luminescent complexes as, thanks to the primogenic effect, the intrashell ED forbidden/MD allowed character of some emissive f-f transitions brings $\left|\overrightarrow{m_{ij}}\right|$ in the range of $\left|\overrightarrow{{\mu }_{ij}}\right|$, thus maximizing the dissymmetry factor (Equation 1). A large dissymmetry factor often comes with weak luminescence through direct metal center excitation (Lunkley et al., 2008; Nagata and Mori, 2020). For that purpose, intense activity in the field is dedicated to improving ϕ_PL and ε_λexc using approaches such as the antenna effect, which avoids the unfavorable direct excitation of the emissive metallic centers (Ward, 2010). Limited by their high cost and, more importantly, by their intrinsic kinetic lability, 4f metal ions could be difficult to handle due to the challenges of maintaining their molecular structures for applications. Chiral 3d metal ions are drawing more attention as an alternative due to their low cost and also because they benefit from the primogenic effect. However, they are limited by (i) their usual kinetic lability, (ii) the lack of efficient Laporte-forbidden d–d emissive states due to weak ligand-field strength, (iii) the mixing of the energy states with ligand-to-metal charge transfers (LMCT)/MLCT, and (iv) the important non-radiative deexcitation pathways that correspond to major handicaps. Cr^III complexes in octahedral geometry are exceptions because the large crystal-field stabilization energy (CFSE) found for the 3d³ electronic configuration (CSFE = 1.2 Δ_oct) makes these complexes kinetically inert (Helm and Merbach, 2005; Richens, 2005). In addition, the linear correlation of the Cr(⁴T₂) energy level with ligand-field splitting (LFS) limits damaging back intersystem crossing (BISC) from the low-lying excited state when strong donor atoms are bound to Cr³⁺ (Figure 1). As a result, this kind of Cr^III complex is known to display characteristic long-lived ED-forbidden/MD-allowed metal-centered spin-flip Cr(²E→⁴A₂) and Cr(²T₁→⁴A₂) NIR emission (Maiman, 1960; Kirk, 1999; Otto et al., 2015; Jimenez et al., 2019; Jiménez et al., 2020; Jimenez et al., 2021; Sinha et al., 2021).

Figure 1

Figure 1. Simplified Tanabe–Sugano diagram for a d³ electronic configuration in an octahedral field with C/B = 4.5. Two deexcitation pathways are shown: radiative (k_r, red arrow) and back intersystem crossing (BISC, blue domain). A strong LFS prevents the non-radiative deexcitation of the excited state through BISC.

Despite uniting the searched intrinsic kinetic inertness with emissive spin-flip transitions, the design of chiral Cr^III complexes with adapted absorption and emission properties remains scarce. Recently, the meridional coordination of bis-terdentate six-membered chelates rings to Cr^III has been demonstrated to be an interesting approach for inducing intense spin-flip chiroptical responses (Jimenez et al., 2019; Jimenez et al., 2023). In contrast, introducing chiral centers in the ligand scaffold did not lead to strong CPL emission in related complexes (Poncet et al., 2021; Cheng et al., 2023). In this context, the recent chiral resolution of the inert emissive complexes [Cr(dqp)₂]³⁺ (dqp = 2,6-di(quinolin-8-yl)pyridine) (Jimenez et al., 2019) and [Cr(ddpd)₂]³⁺ (ddpd = N²,N⁶-dimethyl-N²,N⁶-di(pyridin-2-yl)pyridine-2,6-diamine) (Dee et al., 2019) displaying large dissymmetry factors reactivated the interest for this topic. Concomitant high quantum yields could be obtained, making cheap and earth-abundant chromium assemblies promising candidates for CPL applications.

This work follows this strategy with the synthesis of four novel homoleptic complexes [Cr(dqp-R)₂]³⁺ (R = Ph (1), ≡-Ph (2), DMA (3), and ≡-DMA (4), DMA = N,N-dimethylaniline). The photophysics of these complexes can be compared with those of the heteroleptic complexes [Cr(dqp)(dqp-OMe)]³⁺ (5, dqp-OMe = 4-methoxy-2,6-di(quinolin-8-yl)pyridine) and [Cr(dqp)(ddpd)]³⁺ (6) (Jimenez et al., 2020) The enantiomeric resolutions proved to be successful for all complexes, which paved the way for addressing their chiroptical properties.

2 Results and discussion

The ligands dqp-OMe, ddpd, and dqp; the salt Cr(CF₃SO₃)₂·2H₂O; and the heteroleptic complexes 5 and 6 were prepared according to published methods (Supplementary Appendix 1; Cantuel et al., 2002; Otto et al., 2015; Jimenez et al., 2019; Jiménez et al., 2020). The preparation of the homoleptic complexes [Cr(dqp-R)₂]³⁺ is achieved by mixing one equivalent of Cr^II precursor and two equivalents of the corresponding ligands dqp-R (dqp-Ph for 1, dqp-≡-Ph for 2, dqp-DMA for 3, and dqp-≡-DMA for 4) under anaerobic conditions at room temperature. The formed rac-[Cr^II(dqp-R)₂]²⁺ are oxidized to the kinetically inert rac-[Cr^III(dqp-R)₂]³⁺ using AgSO₃CF₃, affording the desired complexes in good-to-excellent yields (74%–96%, Figure 2).

Figure 2

Figure 2. Synthesis of homoleptic complexes rac-[Cr(dqp-R)₂]³⁺. Racemic mixtures of the PP and MM enantiomers are isolated for both homoleptic and heteroleptic complexes.

Single crystals suitable for X-ray diffraction analysis (Supplementary Tables S1–S13; Supplementary Figures S1–S7 in the Supplementary Material) were obtained through slow diffusion of diethyl ether into a concentrated methanol solution for 1, 2, 3 (with triflate counterions), and 4 (with chlorine counterions). The dqp derivatives systematically adopt the thermodynamically more stable meridional binding around the kinetic labile Cr^II intermediate, leading to meridional arrangement in the final Cr^III complexes after oxidation. Under acidic conditions, the tertiary amine groups in 3 and 4 can be further protonated to yield [Cr(dqp-DMAH)₂]⁵⁺ (H₂-3, DMAH = N,N-dimethylanilininum) and [Cr(dqp-≡-DMAH)₂]⁵⁺ (H₂-4). Single crystals suitable for X-ray diffraction of both protonated complexes could also be successfully isolated by crystallization. The semi-flexible nature of the dqp backbone upon mer binding joined with the kinetic inertness of Cr^III results in the formation of racemic mixtures of helical PP and MM enantiomers of D₂-symmetry in the crystals (Figure 2). As previously reported, the two instances of intramolecular, interligand π-stacking between the aromatic quinolines produce stabilizing interactions compatible with the exclusive formation of the PP and MM enantiomers (Jimenez et al., 2021). No meso PM complex could be observed, likely due to steric hindrance, as simulated in the parent [Cr(dqp)₂]³⁺ (Jimenez et al., 2021). To better appreciate the intramolecular interligand π-stacking, the interplanar angle is calculated as the angle between the mean plane of the 10 atoms of each quinoline (Supplementary Figures S8–S15). The closer the angle is to 0, the more parallel to each other the quinolines are. The calculated values for the complexes 1–5, [H₂-3] and [H₂-4] lie within the 15.54°–16.68° range. In the heteroleptic complex [Cr(dqp)(ddpd)]³⁺ (6), the corresponding quinoline–pyridine interplanar angle reaches 31.48°, far larger than in the other compounds, but yet smaller than the parent [Cr(ddpd)₂]³⁺ (46.52°) (Otto et al., 2015). The transoid bite angles N-Cr-N are in the 176.0(1)°–177.7(8)° range for the homoleptic complexes (1–4, H₂-3, and H₂-4) and in the 175.5(9)°–176.1(10)° range for the heteroleptic complexes (5 and 6). In addition, to evaluate the structural distortion with respect to a perfect octahedron, the following parameter $\mathrm{\Sigma }={\sum }_{i=1}^{12}\left|90-{\phi }_i\right|$ is computed with φ_i being the cisoid bite angles N-Cr-N. The distortion in complexes 1–6, H₂-3, and H₂-4 ranges from 22.73° ≤ Σ ≤ 31.49°, with the largest distortion in the heteroleptic complex 6.

The absorption spectra of the homoleptic complexes (1–4) were recorded in water at room temperature. Complexes H₂-3 and H₂-4 were recorded in acidic media (aqueous HCl 1 M) to ensure full protonation of the terminal tertiary amine (Figure 3).

Figure 3

Figure 3. Absorption spectra of complexes 1–4 in H₂O and H₂-3 and H₂-4 in aqueous HCl (1 M) at 293 K.

The maxima observed within the 345–250 nm range (29,000–40,000 cm⁻¹) are associated with π*←π transitions located on the ligands. Ligand-to-metal charge transfers (LMCTs) are observed at lower energies from 435 nm to 345 nm (23,000–29,000 cm⁻¹). Additionally, a shoulder is observed within the 420–400 nm range (23,800–25,000 cm⁻¹), which has been assigned to the spin-allowed, parity-forbidden metal-centered (MC) Cr(⁴T₂←⁴A₂) transition according to TD-DFT calculations performed on the parent [Cr(dqp)₂]³⁺ complex (Jimenez et al., 2019). Because Δ = E(Cr(⁴T₂←⁴A₂)) in octahedral complexes, the extracted energy values point to similar ligand-field splitting for all complexes within the 24,272–24,876 cm⁻¹ range (Supplementary Table S14). Complexes 3 and 4 display an additional intense broad absorption band in the visible range of the electromagnetic spectrum 800–435 nm (12,500–23,000 cm⁻¹) assigned to the intraligand charge transfer (ILCT) from the terminal nitrogen of the aniline to the trivalent chromium center, as observed in related terpyridine-based complexes (Barbour et al., 2017). Ensuring full protonation using acidic conditions (aqueous 1 M HCl) removes the CT band in H₂-3 and H₂-4 (Figure 3). Moving down in energy and using an increased concentration of complexes, the spin-forbidden/parity-forbidden spin-flip (SF) transitions Cr(²T₁,²E←⁴A₂) can be observed with low molar extinction coefficients ranging from 0.1 M⁻¹ cm⁻¹ to 0.6 M⁻¹ cm⁻¹.

Because of the slightly distorted geometry going from O_h to D₂, a splitting of the two expected bands produces five distinct excited energy levels. The Cr(²T₁) splits into three non-degenerated energy levels, and the Cr(²E) splits into two (Figure 4; Supplementary Figures S16–S19) (Jimenez et al., 2023). Recording the SF bands was impossible in complexes 3 and 4 because of the overlap with the intense charge transfer absorption band in the visible region of the spectrum. Individual assignments of the absorption bands together with experimental radiative rate constants k_rad, radiative lifetimes τ_rad, oscillator strengths f_exp, and dipole strengths D_exp are compiled in Supplementary Table S14. From the absorption spectra and the calculated energies of the accessible excited levels, the ligand-field parameter Δ and the Racah parameters B and C can be estimated using Equations 3–6 (Supplementary Table S15) (Jorgensen, 1963; Witzke, 1971; Chong et al., 2022).

$E\left({}^4{\mathrm{T}}_2\right)=∆.(3)$

$E\left({}^2{\mathrm{T}}_1\right)=9B+3C-24\left(\frac{B^2}{∆}\right).(4)$

$E\left({}^2\mathrm{E}\right)=9B+3C-50\left(\frac{B^2}{∆}\right).(5)$

$E\left({}^2{\mathrm{T}}_2\right)=15B+5C-176\left(\frac{B^2}{∆}\right).(6)$

$E\left({}^4{\mathrm{T}}_1\right)=1.5\Delta +7.5B-0.5\sqrt{\left(225B^2+{\Delta }^2-18\Delta B\right).}(7)$

Figure 4

Figure 4. Perrin–Jablonski diagram for a Cr^III in O geometry. Excitation (blue arrows), internal conversion (dashed arrows), ILCT quenching in 3 and 4 (black arrow), and expected emission in a D₂ distorted geometry (red arrow) are represented. The splitting of the ²T₁ and ²E energy levels leads to five expected optically active energy levels in D₂ symmetry.

For all studied complexes, B ranges from 611 cm⁻¹ to 655 cm⁻¹, and C ranges from 2,743 cm^–−1 to 2,885 cm⁻¹, which implies a weak impact of the extension of the π-delocalized conjugated system contrariwise to a previously reported substitution of methoxy groups in the same position in the homoleptic [Cr(dqp-OMe)₂]³⁺ (Jimenez et al., 2021). We note that for the [Cr(dqp)₂]³⁺, the ratio C/B equals 4.7, but the ratio is only 3.1 for [Cr(dqp-OMe)₂]³⁺. Typical values are in the range of 4.2–4.9 and are sometimes assumed to be 4.7 (Adachi, 2024).

Upon UV–VIS excitation (λ_exc = 350–435 nm) at room temperature, the typical sharp NIR dual emissions (FWHM ≈200 cm⁻¹) observed in complexes 1 and 2 (Figure 5A) can be assigned to the radiative relaxation of the Cr(²E) and Cr(²T₁) excited state levels to the Cr(⁴A₂) ground state level in approximate O symmetry (Jimenez et al., 2019; Jimenez et al., 2021). The most intense band is attributed to the Cr(²T₁’→⁴A₂) SF transition (maxima 13,227 cm⁻¹ (756 nm) in 1 and 2) and the less intense to Cr(²E’→⁴A₂) (maxima at 13,698 cm⁻¹ (730 nm) for 1 and 13,661 cm⁻¹ (732 nm) for 2). Contrariwise, the luminescence of complexes 3 and 4 is completely quenched, likely due to energy back transfer into the non-emissive CT levels (Figure 4). Upon protonation of the anilines (H₂-3 and H₂-4), a weak luminescence is retrieved with an overall photoluminescence quantum yield of Φ_PL ≤ 0.0011% (Supplementary Table S16; Figure 5A), in contrast to the non-luminescent terpyridine analog (Barbour et al., 2017).

Figure 5

Figure 5. Near-infrared emission spectra with λ_exc = 435 nm of (A) Complexes 1–4 in H₂O and H₂-3 and H₂-4 in aqueous 1 M HCl at 293 K and (B) Complexes 1–4 in DMSO/H₂O 1:1 and H₂-3 and H₂-4 in aqueous 1 M HCl at 77 K.

Upon changing the temperature, the Boltzmann distribution of the thermally equilibrated Cr(²E’) and Cr(²T₁’) levels is modified, and 77 K measurements result in a close to single emission assigned to Cr(²T₁’→⁴A₂) (Figure 5B). Interestingly, the total (3 and 4) and partial (H₂-3 and H₂-4) quenching pathways of the luminescence happening at 293 K vanish at 77 K, and strong luminescence is recovered (Figure 5B). For both H₂-3 and H₂-4 at 77 K, the main emission band is blue-shifted compared to the non-protonated 3 and 4 complexes. The room temperature Φ_PL values determined in aerobic and anaerobic conditions (CH₃CN, λ_exc = 435 nm) are gathered in Supplementary Table S16. The obtained values in oxygen-free solutions are 12.4%, making complexes 1 and 2 in the same range as the previously reported record-holding deuterium-free Cr^III complexes (Jimenez et al., 2019; Jimenez et al., 2021). The origin of the high Φ_PL observed is attributed to (i) the weak trigonal distortion forming the octahedral geometry, preventing the non-radiative deexcitation pathways (Kitzmann et al., 2022) and (ii) the strong LFS induced by the dqp-type ligands, avoiding BISC to the Cr(⁴T₂) excited state level (Figure 1).

Time-resolved experiments were conducted and displayed mono-exponential decays, resulting in ${\tau }_{\text{Cr},\text{obs}}^{{{}^2\mathrm{E}}^{\prime },{{}^2\mathrm{T}}_1^{\prime }}$ > 1 ms in the deaerated solution at 293 K and up to 2.62(2) ms at 77 K (2, Supplementary Table S16; Supplementary Figures S20–S25). Air-equilibrated experiments demonstrated the extreme dependence on dissolved dioxygen, as previously reported for Cr^III chromophores (${\tau }_{\text{Cr},\text{obs}}^{{{}^2\mathrm{E}}^{\prime },{{}^2\mathrm{T}}_1^{\prime }}$ < 65 µs) (Kirk, 1999). For more insight, the reader is referred to the in-depth analysis of the mechanism discussed by Alazaly et al. (2023).

Additionally, the rate of energy transfer from Cr(²E’/²T₁’) to O₂, k_q, can be estimated in 1 and 2 with the relationship k_q = 1/[O₂]⋅(1/τ_air −1/τ_Ar) (Burgin et al., 2022), in which [O₂] is the oxygen concentration in the solvent at the experimental temperature (2.42 mM in CH₃CN at 293 K), τ_air is the Cr(²E’/²T₁’) lifetime under air-equilibrated conditions, and τ_Ar is the respective lifetime under deaerated conditions. The obtained values of 6.23∙10⁶ s⁻¹ (1) and 7.59∙10⁶ s⁻¹ (2) demonstrate the effectiveness of O₂ quenching (Supplementary Table S16).

The thermal equilibrium of the Cr(²T₁’) and Cr(²E’) levels was confirmed by the recording of identical excited state lifetimes at both maxima. The sensitization efficiency for transferring the energy from the ligand to the Cr(²E’/²T₁’) excited state ${\eta }_{\text{sens}}^{\mathrm{L}\to \text{Cr}}$ is found to be above 71% (Supplementary Table S16). The measurement of certain photophysical properties in 3, 4, H₂-3, and H₂-4 was limited by (i) the weak emitted signals (H₂-3 and H₂-4) that prevented reliable measurements of the excited state lifetimes and (ii) the lack of emissive properties in 3 and 4. Nevertheless, the retrieval of the luminescence at 77 K allowed time-resolved measurements on all complexes. Excitation spectra were recorded in dilute solutions and closely match the absorption spectra of the corresponding compounds (Supplementary Figures S26–S29).

Chiral stationary phase high-performance liquid chromatography (CSP-HPLC) was proven to be effective, straightforward, and quick in the separation of the racemic mixture of d-block complexes such as Ru^III, Cr^III, and Co^III (Yoshida et al., 2013; Cortijo et al., 2017; Dee et al., 2019). Isocratic elution using a CH₂Cl₂/CH₃CH₃OH/triethylamine/trifluoroacetic acid 50/49.2/0.5/0.3 (v:v) mixture resulted in the separation of the complexes 1–6 (Figures 6A–F). For complexes 1, 2, and 5, the elutions of MM enantiomers tail and slightly overlap with the elution peaks of the PP enantiomers. As a result, a small amount of contamination was observed upon reinjection in the analytical column (Supplementary Figures S30–S32). Nevertheless, thorough integrations of the chromatogram reveal that ≤0.5% mol was present in the sample. Therefore, the chiroptical studies were carried out while considering these contaminations as negligible. The obtention of a mirror image of the circular dichroism (CD) and CPL signals confirmed the adequacy of the latter assumption. Note that the efficient separation of 3 and 4 would allow a larger scale separation.

Figure 6

Figure 6. CSP-HPLC conditions required for the enantiomeric resolution of complexes (A) 1, (B) 2, (C) 3, (D) 4, (E) 5, and (F) 6.

The CD was recorded, and mirror images were systematically obtained for all complexes 1–6, H₂-3, and H₂-4 (Figures 7, 8). The study of the analogous crystallized MM-[Cr(dqp)₂]³⁺ in CD allowed the assignment of the measured complexes (Jimenez et al., 2019). In all complexes except for 6 (Figure 8D), a strong Cotton effect could be observed in the 410–430 nm range, corresponding to the MC Cr(⁴T₂←⁴A₂) transition. The UV range is also dominated by considerable Cotton effect matching with π*←π transitions located on the ligands. Despite the significant absorption of the CT bands in complexes 3 and 4 (ε = 25,000–32000 M⁻¹cm⁻¹), little to no Cotton effect was observed, confirming the low g_abs for these specific transitions (Figures 7A, C). Because 3 and 4 could be protonated in solution, their conversion to the respective protonated species H₂-3 and H₂-4 could be followed by CD upon successive addition of hydrochloric acid (0.05 M) in the aqueous solution (Figures 7E, F). The MC ligand-field Cr(⁴T₂←⁴A₂) transition is less affected by the protonation of the ligands than the π*←π transitions, which display important changes in the 250–400 nm range.

Figure 7

Figure 7. Crystalline structures of PP and MM enantiomers and the corresponding absorption (gray traces) and CD spectra (red and green traces) for (A) 3, (B) H₂-3, (C) 4, and (D) H₂-4. CD titration of complexes (E) 3 to H₂-3 and (F) 4 to H₂-4 in aqueous media.

Figure 8

Figure 8. Crystalline structure of the PP and MM enantiomers and the corresponding CD (middle, red, and green traces) and CPL (λ_exc = 340 nm, right, (red and green traces)) spectra for (A) 1, (B) 2, (C) 5, and (D) 6. The experimental bandwidth (EBW) for the CPL spectra in A–C is 0.5 nm and 5 nm for (D). The gray traces show the absorption (middle) and emission (right) spectra.

Circularly polarized luminescence (CPL) measurements were recorded on each enantiomer of the emissive complexes 1, 2, 5, and 6 (Figures 8A–D). The low photoluminescence quantum yield of H₂-3 and H₂-4 and the non-emissive character of 3 and 4 prevented CPL measurements. To ensure a correct determination of the dissymmetry factor, the highest resolution of the CPL spectra is ensured with the smallest emission slit aperture settings available at the cost of the signal intensity (Sickinger et al., 2024). The experimental bandwidth (EBW) used in these experiments is 0.5 nm for 1, 2, and 5 and 5 nm for 6. A better signal-to-noise ratio can be obtained by opening the slits (larger EBW), but the resolution of the spectra must be sacrificed, skewing the results and the value of g_lum. It is, therefore, more correct to measure the sample at the smallest slit aperture available and divide the CPL spectra by the maximum read value of the corresponding emission. Under unpolarized excitation (λ_exc = 340 nm), complexes 1, 2, 5, and 6 (Figures 8A–D) displayed strong circularly polarized emission within the 720–780 nm range. The dissymmetry factors |g_lum| obtained for the Cr(²E’→⁴A₂) transition reach 0.07–0.08 for all four complexes (Supplementary Figures S33–S36). Interestingly, for the Cr(²T₁’→⁴A₂) transition, complexes 1, 2, and 5 display |g_lum| = 0.20, whereas complex 6 reaches only |g_lum| = 0.14.

Putting these results in perspective with those of the previously reported homoleptic complexes of [Cr(ddpd)₂]³⁺ and [Cr(dqp-R)₂]³⁺ (R = H, Br, OMe, C≡CH, Table 1) (Otto et al., 2015; Jimenez et al., 2019; Jimenez et al., 2021), a trend emerges for the Cr(²T₁’→⁴A₂) transitions. Indeed, all complexes containing two dqp-based ligands display the same dissymmetry factor of 0.2, whereas the more flexible [Cr(ddpd)₂]³⁺ culminated at 0.093 (Dee et al., 2019). By exploiting the chemical inertness of Cr^III, one of each ligand could be implemented in the same complex to give the heteroleptic complex [Cr(dqp)(ddpd)]³⁺ (6), for which |g_lum| = 0.14 corresponds to the average of the dissymmetry factors of the two parent homoleptic complexes. As an attempt to rationalize this result, a hypothetical “ligand dissymmetry factor” $g_{\text{lum}}^{\mathrm{L}}$ could be imagined for a given transition defined as Equation 8.

$\left|g_{\text{lum}}^{\mathrm{L}}\right|=\frac{{\eta }^{\mathrm{L}}}{n}\cdot \left|g_{\text{lum}}^{\text{total},\text{homoleptic}}\right|.(8)$

${\eta }^{\mathrm{L}}$ is the denticity of the ligand, $n$ the number of available coordination sites around the metal center (6 for octahedral symmetry), and $g_{\text{lum}}^{\text{total},\text{homoleptic}}$ is the dissymmetry factor of the corresponding homoleptic complex of a specific transition. Therefore, in the cases of [Cr(dqp)₂]³⁺ and [Cr(ddpd)₂]³⁺, the following values are obtained: $g_{\text{lum}}^{\text{dqp}}=0.1$ and $g_{\text{lum}}^{\text{ddpd}}=0.0465$. From these values, the dissymmetry factor of a heteroleptic complex could be estimated by applying Equation 9.

$\left|g_{\text{lum}}^{\text{total},\text{heteroleptic}}\right|=\sum \left|g_{\text{lum}}^{\mathrm{L}}\right|.(9)$

Table 1

Table 1. CPL brightness (B_CPL) calculation for each emissive transition in complexes 1, 2, 5, and 6.

For 2, one obtains $g_{\text{lum}}^{\text{dqp}}+g_{\text{lum}}^{\text{ddpd}}=0.1+0.0465=0.1465$, which corresponds closely to the experimental value of 0.14, verifying the hypothesis. Unfortunately, the number of available compounds is limited; thus, to validate or invalidate the statement, more flexible tridentate ligands must be studied. As a general conclusion to the chiroptic luminescence, an extension of the organic π-delocalized electronic cloud in the para position of the pyridine does not influence the dissymmetry factor, while the implementation of ddpd ligands results in an attenuated g_lum. A plausible explanation for this observation is that the rigidification of the complex is key in the obtention of a large dissymmetry factor, and the rigidity must be maximized.

The B_CPL of the emissive complexes can be calculated using Equation 2. Because the g_lum values associated with the two observed emissions Cr(²T₁’→⁴A₂) and Cr(²E’→⁴A₂) are of opposite signs in the same enantiomer, B_CPL must be similarly split into two components. Accordingly, the ϕ_PL must be split relative to the intensity of each band at the temperature measurement (293 K). The emission spectrum of the corresponding complex is approximated as two Gaussian curves, deconvoluted as such, and the ratio between them is calculated (idealized Cr(²T₁→⁴A₂) and Cr(²E→⁴A₂) transitions, Supplementary Figures S37–S40). The calculated values of B_CPL are compiled in Table 1 and range from 299 M⁻¹cm⁻¹ to 122 M⁻¹cm⁻¹ for the Cr(²T₁’→⁴A₂) transition and 60–14 M⁻¹cm⁻¹ for the Cr(²E’→⁴A₂) transition, which are among the highest reported values for CPL active compounds (Arrico et al., 2020). A higher B_CPL can be achieved by exciting the maxima of the absorption band to increase the value of ε in the B_CPL calculation. The ϕ_PL is considered invariant with the excitation wavelength.

3 Conclusion

A series of new chiral homoleptic and heteroleptic Cr^III chromophores with dqp-based ligands with para functionalization of the central pyridine have been synthesized and characterized. The addition of N,N-dimethylaniline to the complex resulted in a large increase in absorbance in the VIS region (CT) accompanied by quenching of the luminescence. Interestingly, weak luminescence is retrieved upon protonation of the aniline (Φ_PL ≤ 0.0011%). The highly luminescent complexes [Cr(dqp-Ph)₂]³⁺ and [Cr(dqp-≡-Ph)₂]³⁺ are promising candidates for use as chiral luminescent probes. Enantiomeric resolution of all six racemic complexes could be achieved by CSP-HPLC. Implementing N,N-dimethylaniline as a substituent to the complex resulted in a baseline separation of the two enantiomers, allowing a potential large-scale separation. Moreover, the evolution of the circular dichroism from the non-protonated to the protonated species in [Cr(dqp-DMA)₂]³⁺/[Cr(dqp-DMAH)₂]⁵⁺ and [Cr(dqp-≡-DMA)₂]³⁺/[Cr(dqp-≡-DMAH)₂]⁵⁺ could be followed. Near-perfect octahedral geometries are obtained with the help of the six-membered chelate rings, providing long excited state lifetime and high overall photoluminescence quantum yields at room temperature. Dual circularly polarized luminescence arises from the Cr(²T₁’) and Cr(²E’) excited level to the Cr(⁴A₂) ground state within the 720–780 nm range. The observed |g_lum| for the emissive complexes amounts to 0.2 except for [Cr(dqp)(ddpd)]³⁺, for which the |g_lum| was measured to be in between the two corresponding parent homoleptic complexes [Cr(dqp)₂]³⁺ (|g_lum| = 0.2) and [Cr(ddpd)₂]³⁺ (|g_lum| = 0.093). High B_CPL values, ranging from 299 ⁻¹cm⁻¹ to 122 M⁻¹cm⁻¹ for the Cr(²T₁’→⁴A₂) transition and 60–14 M⁻¹cm⁻¹ for the Cr(²E’→⁴A₂) transition, were obtained, reaching the typical range of 4f-based chiral chromophores with the added value of the inertness and low cost of trivalent chromium.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

MP: conceptualization, formal analysis, investigation, and writing–original draft. CB: formal analysis and writing–review and editing. LG: formal analysis and writing–review and editing. J-RJ: conceptualization, supervision, validation, and writing–review and editing. CP: conceptualization, funding acquisition, supervision, validation, and writing–review and editing.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work is supported through grants from the Swiss National Science Foundation (grant 200020_207313). Financial support from the Swiss National Science Foundation is gratefully acknowledged. J-RJ acknowledges the funding received from the grant TED2021.129598A.I00 funded by MCIN/AEI/10.13039/501100011033 and by European Union NextGenerationEU/PRTR and is grateful to the Ministerio de Ciencia Innovación y Universidades for a Ramón y Cajal contract (grant RYC 2022-037255-I) funded by MCIN/AEI/10.13039/501100011033 and FSE+.

Acknowledgments

K. L. Buchwalder is acknowledged for performing elemental analysis.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2024.1472943/full#supplementary-material

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