Green circularly polarized luminescence with a high dissymmetry factor emitted by a [2.2]fluorenonophane

Abstract

We report the chiroptical properties of enantiopure [2.2]fluorenonophane measured using electronic circular dichroism and circularly polarized luminescence spectroscopy. The target compound exhibited excellent chiroptical properties with high dissymmetry factors of g_abs = ±1.3 × 10⁻² and g_lum = +2.5 × 10⁻²/−2.6 × 10⁻² in a wide range of solvents, accompanied by strong indications of excimer formation. This study emphasizes the outstanding role of [2.2]paracyclophanes as model systems for small molecules with relatively large CPL and expands this compound class by using the easily accessible and modifiable fluorenone derivative.

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The intriguing photophysical phenomenon of circularly polarized luminescence (CPL) is currently attracting the attention of researchers due to its potential applications in advanced technologies such as 3D displays,^1–3 chiral sensing^4–6 or information storage devices.^7–9 CPL refers to the differential emission of photons in the left and right circularly polarized states, which is usually quantified by the dissymmetry factor of luminescence: g_lum = 2ΔI(λ)/I(λ), where ΔI(λ) = I_L(λ) − I_R(λ) and I(λ) = I_L(λ) + I_R(λ).¹⁰ By definition, the g_lum values can range from −2 to +2, corresponding to fully left or right circularly polarized light, respectively. One strategy to achieve CPL is the use of non-racemic mixtures of chiral luminophores that preferentially emit in one polarization state.^11–13 Small chiral organic molecules typically display g_lum values in the range of 10⁻⁵–10⁻³, whereas examples with g_lum > 10⁻² or even 10⁻¹ are only rarely reported.^11,14–18 Hence, designing and synthesizing molecules with high g_lum values is still a key challenge in CPL research.¹⁹

The favorable optoelectronic properties^20,21 combined with the prochiral nature of the [2.2]paracyclophane (PCP) scaffold make it an ideal platform for designing CPL emitters.^22–25 By substituting or extending the π system of [2.2]PCP, not only can planar chirality be induced, but the optoelectronic properties can also be adjusted. Morisaki and coworkers, who have been very active in the field of [2.2]PCP based CPL emitters in recent years, successfully used this strategy to design potent CPL emitters with exceptional g_lum values of up to 2.3 × 10⁻².^26–31 Other groups also contributed significantly to this field of research by synthesizing [2.2]PCP derivatives that can undergo thermally activated delayed fluorescence (TADF) or show solvent induced CPL sign switching.^32,33

The dissymmetry factor of luminescence, g_lum, can be predicted accurately by computational chemistry, making it a powerful tool in guiding the discovery of highly efficient CPL active materials.^34–43 Following our previous strategy,³⁵ we computationally screened for potential CPL emitters and identified [2.2]fluorenonophane 1 as an interesting lead structure. 1 was synthesized first in 2021 by Wu et al.⁴⁴ and was never investigated for its chiroptical properties. We herein report that the readily accessible 1 has a remarkably high g_lum value, demonstrating the power of a synergistic approach to CPL materials.

The synthesis of the target compound (R_p)-/(S_p)-1 is based on previous work by Wu et al. (Scheme 1).⁴⁴ The synthesis and optical resolution of the enantiopure starting material (R_p)-/(S_p)-2 were performed according to the established protocol of Morisaki et al., starting from the commercially available pseudo-meta-dibromo[2.2]paracyclophane.²⁶ (R_p)- and (S_p)-2 were obtained with yields of 26% and 19%, respectively, and an ee ≥ 99.9% (estimated by HPLC analysis of the diastereomeric camphanic ester intermediates, Fig. S7, SI). The enantiopure (R_p)-/(S_p)-2 was then transformed into (R_p)-/(S_p)-3 via Suzuki cross coupling with a yield of 77%. In contrast to Wu et al., we utilized the commercially available ethyl 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzoate instead of ethyl-2-(5,5-dimethyl-1,3,2-dioxaborinan-2-yl)benzoate as the boronic acid component in the Suzuki cross coupling. In the last step, (R_p)-/(S_p)-3 was cyclized to the target molecule (R_p)-/(S_p)-1 via an intramolecular Friedel–Crafts reaction, catalyzed by polyphosphoric acid, with a yield of 47%. The target molecule (R_p)-/(S_p)-1 was obtained as a yellow solid with an overall yield of 36% (over the two steps) and was characterized by ¹H, ¹³C, and correlated NMR, high-resolution mass spectrometry, single-crystal X-ray crystallography and by (chir)optical methods.

Scheme 1 Synthesis of (R_p)-/(S_p)-1. Reagents and conditions: (a) ethyl 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzoate (3.00 eq.), SPHOS (20 mol%), Pd(PPh₃)₄ (20 mol%), tetrabutylammonium bromide (2.00 eq.), K₂CO₃ (6.00 eq.), xylene (isomeric mixture)/EtOH/H₂O (2 : 1 : 1 v/v), 120 °C, and 18 h, 77%; (b) polyphosphoric acid, 120 °C, 18 h, and 47%.

Single crystals of enantiopure (R_p)-1 suitable for X-ray crystallography were obtained by slow evaporation of a solution of the compound in a mixture of n-hexane and dichloromethane at 5 °C. (R_p)-1 crystallizes in the triclinic space group P1, in contrast to the racemic mixture of 1, for which Wu et al. reported the monoclinic P2₁/c space group.⁴⁴ The compound adopts a π-stacked packing in the crystal with a distance of 3.37 Å between the molecules, indicating pronounced π–π interactions in the solid state (Fig. 1).

Fig. 1 Molecular structure of (R_p)-1 in the solid state (CCDC: 2457695): (a) top view and (b) side view. (c) Packing of (R_p)-1 in the solid state. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level.

Compound (R_p)-/(S_p)-1 dissolved in CH₂Cl₂ appears as a yellow solution and exhibits green fluorescence. The corresponding absorption and emission spectra (Fig. 2a) resemble the shape of previously reported spectra in toluene by Wu et al.⁴⁴ The absorption spectrum shows four features with band maxima at 269 nm (ε_{269 nm} = 47.3 × 10³ M⁻¹ cm⁻¹), 317 nm (ε_{317 nm} = 6.9 × 10³ M⁻¹ cm⁻¹), 366 nm (ε_{366 nm} = 3.1 × 10³ M⁻¹ cm⁻¹) and 416 nm (ε_{416 nm} = 1.5 × 10³ M⁻¹ cm⁻¹). The emission spectrum shows a broad structureless band with a maximum at 544 nm and a Stokes shift of 5660 cm⁻¹ (Fig. 1a). The emission band shape, accompanied by a large Stokes shift, is characteristic of paracyclophanes and points to intramolecular excimer formation.^45,46 This interpretation is also supported by the difference between the ground and excited state dipole moments of Δµ = 8.09 D (determined via the Lippert–Mataga method; Fig. S11, SI), which is indicative of at least partial charge transfer during excitation.^47,48 In CH₂Cl₂, the quantum yield was absolutely determined to be Φ_PL(1) = 4% and the measured monoexponential lifetime τ was 7.67 ns (Fig. S10, SI). The quantum yield can be increased to 7% by changing the solvent to DMF (Table S1, SI). The compound also features strong positive solvatochromism where the emission maximum can be tuned from 509 nm in n-hexane up to 584 nm in methanol (Fig. S9, SI).

Fig. 2 (a) Absorption (solid line) and emission (dotted line, λ_ex = 405 nm) spectra of rac-1 in CH₂Cl₂. (b) ECD (solid line) and CPL (dotted line, λ_ex = 405 nm) spectra of (R_p)-/(S_p)-1 in CH₂Cl₂.

Enantiopure samples of (R_p)-/(S_p)-1 were investigated for their chiroptical properties using electronic circular dichroism (ECD) and CPL spectroscopy (Fig. 2b). The ECD spectra of the enantiomers are perfectly mirrored and exhibit pronounced Cotton effects. The sign of circular dichroism is oscillating rapidly in the UV region between 250 and 280 nm, which probably originates from exciton coupling between the two fluorenone chromophores.⁴⁹ The dissymmetry factor of absorption, g_abs, derived from the lowest energy band at 433 nm of the ECD spectra is +1.3 × 10⁻²/−1.3 × 10⁻² for the R_p and S_p enantiomers. The CPL spectra are characterized by one band with a maximum at around 530 nm corresponding to the emission maximum. The enantiomers exhibit strong CPL with dissymmetry factors (g_lum) of +2.5 × 10⁻²/−2.6 × 10⁻² for the R_p and S_p enantiomers, respectively, which are unusually large for organic small molecules. In particular, when compared to other PCP based CPL emitters containing fused rings or carbonyl groups, 1 emerges as superior in terms of the g_lum value.^26,50–52 For example, it surpasses the coumarin system of Benedetti et al., which displays a g_lum value of ±5 × 10⁻³ at a comparable quantum yield.⁵¹ The chiral circularly polarized brightness B_CPL = 1/2ε_λ × Φ_lum × |g_lum| of 1 results in B_CPL = 0.8 M⁻¹ cm⁻¹ at an excitation wavelength of 405 nm. The g_lum value remains constant throughout the entire emission band (Fig. S15, SI), as expected for a single transition responsible for the emission. The dependence of g_lum on the solvent was also investigated by recording the CPL spectra in n-hexane, acetonitrile, chloroform, dimethylformamide, ethyl acetate and ethanol, but no significant difference in sign or magnitude of g_lum could be observed (Fig. S17–S22, SI). This indicates that the geometry of the emissive excited state does not depend significantly on the polarity of the solvent. It is also remarkable that the g_lum/g_abs ratio is greater than 1, which is in contradiction with the findings of Mori et al., who reported an empirical correlation of g_lum = 0.8 × g_abs for π–π* transitions of small organic molecules.⁵³ Enhanced dissymmetry factors for the luminescence compared to the absorption can indicate intramolecular excimer formation, which is consistent with the previously discussed interpretation of the absorption and emission spectra.⁵⁴ In the literature, the amplification of g_lum through excimer formation has mainly been studied in systems containing pyrene as an excimer forming luminophore, where the g_lum values of the best performing representatives also exceeded 10⁻².^55–58 Hence, the reported [2.2]fluorenonophane provides a complementary system for the exploitation of excimer formation, enabling high g_lum values. Although the broad excimer emission band could be a drawback for actual application in devices, the reported emission spectrum of 1 is comparable to that of materials used to study circularly polarized electroluminescence.^32,59–62 Therefore 1, with its simple structure but strong chiroptical response, has the potential to serve as a model system for application-oriented CPL research.

In order to gain more insight into the (chir)optical properties of 1, the compound was studied computationally using TD-DFT (for more details, see the SI). The calculated absorption and ECD spectra obtained from single point TD-MN15/def2-TZVP (SMD: CH₂Cl₂) calculations are in good agreement with the measured ones (Fig. S23 and S24, SI).^63–65 The lowest energy transition from S₀ to S₁ is computed to be at 416 nm with a small oscillator strength (f = 0.0306). Hence, the observed absorption band with λ_max = 416 nm can be assigned to the S₀ → S₁ transition, as it is in perfect agreement with the computed value. NTO analysis reveals that the nature of the S₀ → S₁ transition is dominated by ππ* character (occupation = 0.85), where the π- and π*-orbitals are both delocalized over the whole molecule (Fig. S25, SI).

The parameters characterizing the emission properties were obtained by optimizing the emitting S₁ state at the TD-MN15/def2-TZVP level of theory. The calculated emission maximum of λ_Em.,calcd. = 501 nm agrees reasonably well with the experimentally observed emission maximum of λ_Em.,max. = 544 nm in dichloromethane. Also, the predicted g_lum,_calcd. value of 2.1 × 10⁻² calculated from the electric ([small mu, Greek, vector]) and magnetic (m⃑) transition dipole moments reproduce the observed g_lum value of +2.5 × 10⁻²/−2.6 × 10⁻² well. As g_lum is proportional to cos(θ_μ,m) (θ_μ,m is the angle between [small mu, Greek, vector] and m⃑), the almost perfectly antiparallel alignment between the electric and magnetic transition dipole moment vectors [small mu, Greek, vector] and m⃑ (θ_μ,m = 178.4°) is highly favorable and gives rise to the observed high g_lum value.¹⁹ According to the calculations, only the HOMO→LUMO transition is responsible for the S₁→S₀ transition, which therefore possesses pure ππ* character (Fig. 3). While the LUMO (representing the emissive orbital in this context) is delocalized over the whole molecule with a significant transannular overlap between the two π-systems, the HOMO is localized on the central [2.2]paracyclophane unit with no transannular overlap between the π-systems. This supports the previously proposed interpretation of the spectroscopic data that the emissive state exhibits excimer character and that the electronic transition features partial charge transfer.

Fig. 3 Frontier molecular orbitals (visualized at an isovalue of 0.03) in the S₁ state, representing the S₁ → S₀ transition with the computed values of the electronic transition properties in the S₁ state at the TD-MN15/def2-TZVP level of theory. f is the oscillator strength, [small mu, Greek, vector] and m⃑ are the electric and magnetic transition dipole moments, respectively, and θ_μ,m is the angle between [small mu, Greek, vector] and m⃑.

In summary, we report the first enantiopure synthesis and chiroptical investigations of (R_p)-/(S_p)-1. Despite its simple structure, enantiopure samples of (R_p)-/(S_p)-1 exhibit a strong chiral response in ECD and CPL spectroscopy, with high dissymmetry factors of g_abs = ±1.3 × 10⁻² and g_lum = +2.5 × 10⁻²/−2.6 × 10⁻². The enhanced dissymmetry factor for luminescence compared to absorption is probably attributed to intramolecular excimer formation. The chiroptical properties were accurately reproduced by TD-DFT calculations, highlighting the potential of computational chemistry in guiding the discovery of highly efficient CPL-active materials.

We gratefully acknowledge financial support from the Volkswagen Foundation under project 0072510-00. The computations were performed on bwForCluster JUSTUS2. The authors acknowledge support from the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no. INST 40/575-1 FUGG (JUSTUS 2 cluster) for computational facilities.

Author contributions

A. Loleit: synthesis, analytics, computations, spectroscopy measurements (absorption and ECD), visualization, writing – original draft. L. Eckhardt: spectroscopy measurements (absorption, emission and CPL), visualization, writing – review & editing. M. Ströbele: X-ray crystallography. J. L. Hiller: construction of the CPL spectrometer setup. M. Scheele: conceptualization, funding acquisition, project administration, resources, writing – review & editing, supervision. H. F. Bettinger: conceptualization, funding acquisition, project administration, resources, writing – review & editing, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cp00273k.

CCDC 2457695 ((R_p)-1) contains the supplementary crystallographic data for this paper.⁶⁶

Notes and references

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