Electronic properties of diastereomeric Möbius shaped cyclotris[5]helicenes

Abstract

The in-depth analysis of the configurational, (chir)optical and aromatic properties of two diastereomeric singly and triply twisted Möbius-shaped macrocycles, the cyclotris[5]helicenes, in which three [5]helicene fragments with like and unlike helicity are connected by three C(sp²)–C(sp²) single bonds, led to the conclusion that global π electronic delocalization exists in these molecules. This was analyzed as a consequence of the reduced torsion angle (≤30°) at the C(sp²)–C(sp²) single bonds connecting the [5]helicene fragments. The enantiomers of both diastereomeric cyclotris[5]helicenes were thoroughly analyzed experimentally by unpolarized and circularly polarized vibrational, absorption and luminescence (fluorescence and phosphorescence) spectroscopies, all correlated with advanced DFT models, which provided insightful data on their S₀, S₁ and T₁ electronic states, and on their chirality. The theoretical analyses of aromaticity in both diastereomeric cyclotris[5]helicene macrocycles, as compared with recently reported analogues, confirmed their Möbius aromaticity.

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Introduction

With progress in organic synthesis, methods are now available for the property-driven design and reproducible precision synthesis of a variety of chiral polycyclic aromatic hydrocarbons (PAH). For instance, carbo[5]helicene units are now thought as elementary fragments to elaborate complex chiral shape-persistent π-conjugated chiral molecules, typically multi[5]helicenes¹ and macrocyclic cyclo[5]helicenes.² In this direction, Bock and Durola have developed the Perkin synthetic strategy,³ which enabled the synthesis of many carboxyl-substituted polycyclic aromatic compounds with diverse shapes, and notably various [5]helicene-containing macrocyclic molecules: the figure-eight cyclobis[5]helicene 1,⁴ the triply[5]helicene-bridged(1,3,5)cyclophane 2,⁵ and the diastereomeric, triply and singly twisted Möbius-shaped⁶ cyclotris[5]helicenes D₃-3 and C₂-3 (Fig. 1).⁷ In these macrocycles, two or three [5]helicene fragments are linked by C(sp²)–C(sp²) biphenyl-like single bonds (with a phenyl tether for 2). In principle, because there is no substituent at any ortho position around these biphenyl-like single bonds, free rotation around these bounds may be expected at 298 K. However, the mechanical constraint induced by the spatial arrangement of the [5]helicene fragments impedes free rotation around the biphenyl-like single bonds, which make these molecule shape-persistent. The direct consequence of this is that some of these biphenyl-like single bonds are stereogenic due to atropisomerism. For instance, it was previously analysed in 1 having two (M)-configured [5]helicene fragments that both single bonds are blocked in the (aS)-configuration.⁸ Thus, a complete stereochemical description of 1 according to Cahn–Ingold–Prelog (CIP) rules is (M,aS,M,aS)-1 for the depicted enantiomer in Fig. 1 [and (P,aR,P,aR)-1 for the other enantiomer]. Similarly, cyclotris[5]helicenes D₃-3 embeds six stereogenic elements: three helices and three axes, and the complete stereochemical description of its depicted enantiomer in Fig. 1 is (M,aS,M,aS,M,aS)-D₃-3 [and (P,aR,P,aR,P,aR)-D₃-3 for the other enantiomer]. For its diastereomer C₂-3, application of the CIP nomenclature for the depicted enantiomer in Fig. 1 results in (M,aS,M,aR,P,aR)-C₂-3 [and (P,aR,P,aS,M,aS)-C₂-3 for the other enantiomer]. The single bond in the triply[5]helicene-bridged(1,3,5)cyclophane 2 are not stereogenic due to CIP equivalence at both sides of the central phenyl ring, a stereochemical fact not yet revealed about this molecule.

Fig. 1 Macrocyclic cyclo[5]helicenes synthesized through the Perkin strategy by the Durola group.

The existence of torsion angles at the single bonds between the [5]helicenyl moieties in 1 and 3, and between the phenyl and the [5]helicenyl moieties in 2, raises the question: are these molecules globally π-conjugated, with ring-to-ring π electrons delocalization across the single bonds? There is no simple yes or no answer to this question because: (i) it depends largely on the torsion angles at each single bond, (ii) multiple delocalization circuits, with local, semi-local and global character, can develop in these molecules, and (iii) strictly speaking, pure σ and π systems no longer exist in these contorted molecules as they overlap (and overlap differently on both faces of each ring), and it may be preferable to invoke π-type conjugation rather than just π conjugation for contorted molecules. For clarity, only π conjugation is referred to as herein, with the understanding it is an (acceptable) approximation. We and others have previously attempted to answer the question of global π conjugation for molecules 1 and 2: figure-eight 1 was found not a globally π conjugated molecule, i.e., with two separated 22-electron π systems, while a 78-electron π global delocalization circuit was found to exist in cyclophane 2, albeit of minor intensity compared to other delocalization circuits operating in this molecule.^5,9 For the macrocyclic triply twisted Möbius-shaped molecule D₃-3, previous investigations indicated that electronic delocalisation circuits exist at its edges: a 48-electron Möbius aromatic system located at the external edge with mainly a π character, and a 24-electron anti-aromatic circuit located at the internal edge with a pronounced σ character.⁷ In this article, we provide complementary analyses of the electronic π delocalization in both diastereomers of the Möbius-shaped molecules 3, now using an isotropic magnetic criterion, the 3DIMS (three-dimensional isotropic magnetic shielding) analysis,¹⁰ and an electronic criterion, the EDDB (electron density of delocalized bonds) analysis,¹¹ which confirmed their Möbius aromaticity.

A highly sought-after property of shape-persistent chiral π-conjugated molecules is their emission of circularly polarized luminescence (CPL) with potentially large dissymmetry factors |g_lum|.¹² Maximal dissymmetry factors in molecules can be expected when their electric and magnetic transition moments are parallel or antiparallel one to another. For instance, the D₂ point group symmetric figure-eight compound 1 shows a relatively high dissymmetry factor |g_lum| = 5 × 10⁻³ (at its maximum of fluorescence at 447 nm) for a molecular organic CPL emitter, ca. a 35 fold enhancement when compared to carbo[6]helicene.⁸ More recently, some figure-eight arylophane analogs of 1, i.e., with small aryl tethers (1,4-phenyl, 2,6-naphthyl, 2,6-anthracenyl, 2,7-pyrenyl or 3,10-perylenyl) between the two [5]helicene fragments, exhibited a range of dissymmetry factors |g_lum| = 0.19–38 × 10⁻³.^2c,f Lately, some analogs of Möbius-shaped cyclotris[5]helicenes D₃-3 and C₂-3 embedding small aryl tethers (1,4-phenyl, 2,6-naphthyl or 2,7-pyrenyl) between the three [5]helicene fragments have shown comparable |g_lum| in the range 0.75–1.2 × 10⁻³ for the D₃ point group symmetric diastereomers, and 0.20–1.3 × 10⁻³ for the C₂ point group symmetric diastereomers.^2f These molecular dissymmetry factors, however, remain modest when compared to the nearly maximum dissymmetry factors that can be obtained from chiral luminescent materials, i.e., 2 for a total left or right CPL.¹³ In this article, we report the resolution of the enantiomers of both diastereomers of the Möbius-shaped cyclotris[5]helicenes D₃-3 and C₂-3 by semi-preparative HPLC on chiral stationary phases, their chiroptical properties including optical rotation dispersion at discrete wavelengths, VCD (vibrational circular dichroism), ECD (electronic circular dichroism) and CPL spectroscopies, and the examination of their conformational behavior by experimental and computational methods.

Results and discussion

Racemic samples of the Möbius-shaped cyclotris[5]helicenes D₃-3 and C₂-3 were obtained in equivalent amounts from the photocyclization of a macrocyclic precursor as previously described.⁷ The resolution of the enantiomers of D₃-3 and C₂-3 was achieved through semi-preparative HPLC (see details in section S1 in the SI). From a 46 mg sample of racemic D₃-3 contaminated by minor products using a Chiralpak IE column (250 × 10 mm) eluted with heptane/ethanol/dichloromethane (5 : 3 : 2), 8 mg of the first eluted enantiomer (rt = 7.44 min) and 11 mg of the second eluted enantiomer (rt = 9.30 min) were obtained (note: +27 mg recovered mixture) both with >99.5% ee. Similarly, now using a Chiralpak ID (250 × 10 mm) column eluted with heptane/ethanol/dichloromethane (5 : 3 : 2), 47 mg of material containing essentially racemic C₂-3 afforded 12 mg of the first eluted enantiomer (rt = 5.57 min) with >98% ee and 14 mg of the second eluted enantiomer (rt = 7.86 min) with >98.5% ee (note: +21 mg recovered mixture). The absolute configurations of the enantiomers of D₃-3 and C₂-3 were determined by VCD and ECD spectroscopies (vide infra), comparing their experimental and simulated spectra (Fig. 3 and 4). The enantiomers of both compounds displayed mirror-image spectra, and it could be safely concluded that the first eluted enantiomers are (P,aR,P,aR,P,aR)-D₃-3 (rt = 7.44 min) and (M,aS,M,aR,P,aR)-C₂-3 (rt = 5.57 min), respectively.

A solution of (M,aS,M,aR,P,aR)-C₂-3 (>98% ee) was heated in boiling ethanol (bp = 78.4 °C) and the enantiomers ratio was monitored by chiral HPLC overtime (see details in section S2 in the SI). A clean enantiomerization was observed, which allowed to determine experimentally the barrier to enantiomerization of C₂-3 at ΔG^‡_enant = 114.4 kJ mol⁻¹ (at 78.4 °C in ethanol). This ΔG^‡_enant value indicates that enantiopure C₂-3 is chemically stable at 78 °C, and that it can racemize at a significant rate at temperatures above room temperature. In contrast, heating a 1,2,4-trichlorobenzene solution of (P,aR,P,aR,P,aR)-D₃-3 (>99.5% ee) at 214 °C for 24 h did not allow to evidence any change by chiral HPLC, indicating both its chemical and conformational stability at 214 °C, and that the barrier to diastereomers interconversion D₃-3 → C₂-3 is higher than 183 kJ mol⁻¹.

Using DFT methods at the SMD(ethanol)-D3(BJ)-B3LYP/6-311G(d,p)//SMD(ethanol)-B3LYP/6-311G(d,p) level of theory at 351.5 K, it was determined that C₂-3 is the more stable diastereomer, with a relative Gibbs energy of D₃-3 found at ΔG = 25.8 kJ mol⁻¹ (Fig. 2). Notably, as previously identified,⁷ the inclusion of a dispersion correction term, namely D3(BJ),¹⁴ is crucial to analyze the relative stability of C₂-3 and D₃-3 (see details in section S3 in the SI, Fig. S1 and S2). The conversion of the less stable diastereomer D₃-3 into the thermodynamic diastereomer C₂-3 would require to surmount a barrier of ΔG^‡_dia > 200 kJ mol⁻¹ (via TS₁), indicating it is shape-persistent at temperatures below 200 °C. Examination of the enantiomerization of C₂-3 confirmed it is a single step process occurring via TS₂, requiring the inversion of configuration of only one [5]helicene fragment, with a barrier to enantiomerization computed at ΔG^‡_enant = 137.4 kJ mol⁻¹ (at 78.4 °C). Yet consistent with the experimental value, the computed value of ΔG^‡_enant for C₂-3 is somewhat over-estimated (by 23.0 kJ mol⁻¹). A plausible reason for this deviation (20% over-estimation) is it was not possible to identify the corresponding transition state using the D3(BJ) dispersion correction scheme in the calculations (see discussion in section S3 in the SI). A noticeable fact about the enantiomerization of C₂-3, a chiral molecule that contains three stereogenic [5]helicene fragments, is that the inversion of configuration of a single of its [5]helicene fragments leads to a net enantiomerization of the entire molecule. Comparing the experimentally determined enantiomerization barriers of C₂-3 (ΔG^‡_enant = 114.4 kJ mol⁻¹ at 351.5 K) and [5]helicene itself (ΔG^‡_enant = 98.3 kJ mol⁻¹ at 304–320 K (ref. 15)), notwithstanding the ester substituents, indicated that the rigid chiral environment in C₂-3 surprisingly leads to only a modest increase in the barrier.

Fig. 2 Computational conformational analysis of cyclotris[5]helicenes D₃-3 and C₂-3 using DFT. Gibbs energies in kJ mol⁻¹ computed at 351.52 K at the SMD(ethanol)-B3LYP-D3(BJ)/6-311G(d,p)//SMD(ethanol)-B3LYP/6-311G(d,p) level of theory.

As a first chiroptical analysis, the optical rotation dispersions (ORD) of the enantiomers of D₃-3 and C₂-3 were measured at several discrete wavelengths as summarized in Table 1. The increasing magnitude of the optical rotations at increasingly energetic wavelengths, from 589 nm to 546 nm, is a standard phenomenon for comparable molecules,^4,5 and no attempt was made to rationalize the sign and amplitude of these values at this stage. For (P,aR,P,aR,P,aR)-D₃-3: [α]²⁵_D = +1850° mL g⁻¹ (dichloromethane, c = 0.114), and for (P,aR,P,aS,M,aS)-C₂-3: [α]²⁵_D = ° mL g⁻¹ dm⁻¹ −900 (dichloromethane, c = 0.105).

Table 1 Optical rotation dispersions of the enantiomers of D₃-3 and C₂-3 measured at several discrete wavelengths in dichloromethane (c = 0.114, 0.135, 0.105, and 0.150 from top to bottom). Values are given in ° mL g⁻¹ dm⁻¹

	[α]^25589	[α]^25578	[α]^25546
(P,aR,P,aR,P,aR)-D3-3	+1850	+2000	+2500
(M,aS,M,aS,M,aS)-D3-3	−1850	−2000	−2500
(P,aR,P,aR,M,aS)-C2-3	−900	−975	−1250
(M,aS,M,aS,P,aR)-C2-3	+900	+975	+1250

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The experimental IR and VCD spectra of both D₃-3 and C₂-3, together with the simulated spectra for (P,aR,P,aR,P,aR)-D₃-3 and (P,aR,P,aR,M,aS)-C₂-3 computed at the SMD(CH₂Cl₂)-B3LYP/6-311G(d,p) level of theory, are shown in Fig. 3 (see also section S4 in the SI, Fig. S4 and S5). The calculated spectra reproduce well the main features of the experimental data for the first eluted enantiomer of D₃-3 and the second eluted enantiomer of C₂-3, respectively. In particular, the VCD spectra exhibit clear differences at 1300 and 1267 cm⁻¹, corresponding to negative and positive C_ar–H deformation bands, respectively, which are accurately reproduced by the simulations. These modes are delocalized over the three cyclotris[5]helicene units rather than localized on a single subunit. For D₃-3, which contains three [5]helicene fragments of like helicity, these deformation bands are significantly more intense than in C₂-3, where unlike helicities reduce their amplitude. In both (P,aR,P,aR,P,aR)-D₃-3 and (P,aR,P,aS,M,aS)-C₂-3, the VCD band associated with the C=O stretching vibration of the ethyl ester groups exhibits a negative sign, indicating that this mode is largely insensitive to the [5]helicene stereogenicity. The consistent agreement between experimental and computed IR/VCD spectra, including the sign and relative intensity of diagnostic bands, unambiguously supports the assignment of the absolute configurations as (P,aR,P,aR,P,aR)-D₃-3 for the first-eluted enantiomer and (P,aR,P,aS,M,aS)-C₂-3 for the second-eluted enantiomer.

Fig. 3 VCD spectra of (a) D₃-3 and (b) C₂-3 recorded in CD₂Cl₂. Green (light and dark): first-eluted enantiomers; red (light and dark): second-eluted enantiomers; blue (light and dark): calculated for (P,aR,P,aR,P,aR)-D₃-3 and (P,aR,P,aS,M,aS)-C₂-3, using the SMD(CH₂Cl₂)-B3LYP/6-311G(d,p) level of theory. (c) Overlay of experimental VCD spectra of first-eluted D₃-3 and second-eluted C₂-3, and calculated VCD spectra of (P,aR,P,aR,P,aR)-D₃-3 and (P,aR,P,aS,M,aS)-C₂-3.

An intriguing feature of D₃-3 and C₂-3 lies in their extended π-electron delocalization, potentially expanding across the three [5]helicene units, which governs their photophysical and chiroptical electronic responses in the UV-vis region of the electromagnetic spectrum. These properties are determined by the balance between the electric and magnetic transition dipole moments, and thus depend critically on the electronic coupling between the helicenic subunits and their relative (M) or (P) configurations.¹⁶ In the present systems, this structure–property relationship can be rationalized both experimentally and theoretically, as D₃-3 contains three [5]helicene fragments of identical configuration (like relationship) while C₂-3 contains one [5]helicene fragment of opposite configuration relative to the two others (unlike relationship). The UV-vis absorption spectra of both macrocyclic compounds have been previously investigated, and compared to the reference tetraethyl ester [5]helicene fragment.⁷ This first comparative characterization of D₃-3 and C₂-3 clearly evidences an electronic interaction between the helicene subunits, which is highlighted by a 75 nm bathochromic-shift in the onset of their UV–vis spectra (onset at 450 nm) relative to the reference tetraethyl ester [5]helicene fragment (onset at 375 nm). Further insights into the effect of helicene configuration within these macrocycles were obtained from electronic circular dichroism (ECD) measurements (Fig. 4, see also Fig. S6 and S7 in the SI). As expected, the enantiomers of both D₃-3 and C₂-3 display mirror-image ECD spectra, although with marked differences between the diastereomers. For instance, the ECD response of (P,aR,P,aR,P,aR)-D₃-3 comprises: (i) a positive band at 200 nm (Δε ≈ +180 M⁻¹ cm⁻¹), (ii) a strong negative band at 230 nm (Δε ≈ −300 M⁻¹ cm⁻¹), (iii) an intense bisignate signal with extrema at 270 and 300 nm (|Δε| ≈ 410 M⁻¹ cm⁻¹), (iv) a broad positive band at 360 nm (Δε ≈ +400 M⁻¹ cm⁻¹) with a shoulder at 340 nm, and (v) a weak negative band at 405 nm (Δε ≈ −50 M⁻¹ cm⁻¹). In contrast, (P,aR,P,aS,M,aS)-C₂-3 exhibits a weaker overall intensity, along with additional positive/negative features between 220 and 340 nm. This spectral region typically reflects the ¹B_a and ¹B_b transitions characteristic of carbohelicenes,¹⁷ highlighting the strong influence of the (M)/(P) relative configuration in the chiroptical response. The simulation of the UV-vis and ECD spectra of both D₃-3 and C₂-3 by TD-DFT methods well reproduced their experimental spectra, which confirmed the absolute configurations of the samples as deduced from their VCD spectroscopy analyses. In line with these observations, the absorption dissymmetry factors (|g_abs|) at 405 nm reach 4.0 × 10⁻³ for D₃-3 and 3.3 × 10⁻³ for C₂-3, values comparable with those reported for other [5]helicene-based Möbius-shaped macrocycles.^2f

Fig. 4 ECD spectra of (a) D₃-3 and (b) C₂-3 recorded in acetonitrile. Green (light and dark): first-eluted enantiomers; red (light and dark): second-eluted enantiomers; blue (light and dark) calculated for (P,aR,P,aR,P,aR)-D₃-3 and (P,aR,P,aS,M,aS)-C₂-3, using the SMD(acetonitrile)-CAM-B3LYP/Def2SVP//B3LYP/6-311G(d,p) level of theory. (c) Overlay of experimental ECD spectra of first-eluted D₃-3 and second-eluted C₂-3, and calculated ECD spectra of (P,aR,P,aR,P,aR)-D₃-3 and (P,aR,P,aS,M,aS)-C₂-3.

Interestingly, the sign of the lowest-energy ECD band (390–430 nm) is inverted between (P,aR,P,aR,P,aR)-D₃-3 and (P,aR,P,aS,M,aS)-C₂-3, despite comparable intensity. This inversion suggests that all helicene units contribute significantly to the frontier molecular orbitals (FMO) in both macrocycles. Examination of the FMO revealed that the HOMO and HOMO−1, and the LUMO and LUMO+1 of D₃-3 are degenerated, with equivalent electronic density of each [5]helicene fragments, which is expected form the molecular symmetry (Fig. 5a). In contrast, the FMO of C₂-3 are, as expected, close in energy but not degenerated (Fig. 5b): the HOMO−1 is mostly localized over the [5]helicene fragment of unlike configuration, the HOMO is predominately delocalized on the two [5]helicene fragments of like configuration, the LUMO is delocalized over two [5]helicene fragments of unlike configurations, and the LUMO+1 is mostly localized over one [5]helicene fragment of like configuration. The lowest-energy band in the ECD spectrum of (P,aR,P,aR,P,aR)-D₃-3, corresponding to the S₀ → S₁ transition, is negatively signed. In contrast, the lowest-energy band in the ECD spectrum of (P,aR,P,aS,M,aS)-C₂-3 is positively signed. The FMO distribution shown in Fig. 5 helps rationalizing these opposite signs: in both macrocycles the S₀ → S₁ transitions arise mainly from HOMO−1 → LUMO and HOMO → LUMO+1 contributions. However, in C₂-3 it is dominated by the [5]helicene fragment of unlike configuration, leading to the inversion of the sign of the lowest ECD band.

Fig. 5 Frontier molecular orbitals (FMO) of (a) D₃-3 and (b) C₂-3, calculated at the SMD(acetonitrile)-CAM-B3LYP/Def2SVP//B3LYP/6-311G(d,p) level of theory.

The influence of the relative configurations of the [5]helicene fragments in D₃-3 and C₂-3 is modestly reflected in their emissive properties (Fig. 6, see section S5 in the SI for details). In solution in three solvents of different polarity (toluene, dichloromethane and acetonitrile), both compounds show broad emission with maxima at 465 nm (for C₂-3) and 450 nm (for D₃-3), comparable to the parent tetraethyl ester [5]helicene (with maximum at 457 nm, see Fig. S8 in the SI). The photoluminescence quantum yields are modest (Φ_PL = 14% for D₃-3 and Φ_PL = 17% for C₂-3) with fluorescence lifetimes of 5.5 ns for both compounds, consistent with the efficient intersystem crossing generally observed in carbohelicenes and other distorted aromatic molecules.¹⁸ At 77 K in frozen 2-MeTHF matrix, both compounds exhibit dual emission: a structured fluorescence between 420–515 nm, and a lower-energy phosphorescence between 560–760 nm, with a long lifetime of ca. 0.5–0.6 s. The time-gated measurements (middle of Fig. 6c, see also Fig. S12 in the SI) unambiguously isolated the phosphorescence, which is red-shifted relative to the parent tetraethyl ester [5]helicene fragment (onset at 525 nm), indicating that the D₃-3 and C₂-3 multihelicenic frameworks stabilizes the triplet states more efficiently than the singlet ones (onsets at 550 nm). The luminescence properties of D₃-3, C₂-3 and the parent [5]helicene were simulated using time-dependent DFT and a path integral approach for the dynamics¹⁹ as implemented in ORCA 6.1.²⁰ However, to avoid prohibitive calculation costs, it was judged necessary to reduce the size of the systems, i.e., using their stripped-down versions without the ester substituents (with ‘only’ 102 atoms vs. 210 atoms for the full systems). The calculated spectra reproduce very well the two peaked vibronic bands at 440 and 470 nm as well as the shoulder at 500 nm, originating from the S₁ vibronic states. The phosphorescence spectra display two bands at 590 and 645 nm, originating from the T₁ vibronic states. Notably, it was necessary to include the D4 dispersion correction scheme²¹ in the calculations for the calculated energies to match the experimental ones. In both fluorescence and phosphorescence spectra, it is interesting to note that more than 99% of the band intensities are due to the vibronic coupling (Herzberg-Teller or HT effect), the Franck–Condon intensities being at most four orders of magnitude smaller than the HT intensities for the fluorescence and two orders of magnitude smaller than the HT intensities for the phosphorescence. The adiabatic energy differences (ZPE corrected) between the S₀, S₁ and T₁ states for D₃-3, C₂-3 and the parent [5]helicene fragment are reported in Table 2. Both the S₀–S₁ and S₀–T₁ excitation energies decrease from the parent [5]helicene fragment to the cyclotris[5]helicenes D₃-3 and C₂-3, which aligns with the red shift observed in the experimental fluorescence and phosphorescence spectra.

Fig. 6 (a) Fluorescence and CPL profiles of (P,aR,P,aR,P,aR)-D₃-3 (solid lines) and (M,aS,M,aS,M,aS)-D₃-3 (dashed lines) in toluene, dichloromethane and acetonitrile (λ_ex = 395 nm for fluorescence, λ_ex = 350 nm for CPL). (b) Fluorescence and CPL profiles of (M,aS,M,aR,P,aR)-C₂-3 (solid lines) and (P,aR,P,aS,M,aS)-C₂-3 (dashed lines) in toluene, dichloromethane and acetonitrile (λ_ex = 395 nm for fluorescence, λ_ex = 350 nm for CPL). (c) Normalized fluorescence and phosphorescence spectra of D₃-3 and C₂-3 in a frozen 2-MeTHF matrix at 77 K; the calculated spectra were obtained using the B3LYP-D4/def2-TZVP level of theory; the time-gated experimental measurements isolating phosphorescence are incrusted.

Table 2 S₁ and T₁ ZPE corrected adiabatic energy differences relative to the S₀ state calculated in eV (B3LYP-D4/Def2-SVP level) and experimental luminescence onset energies for D₃-3, C₂-3 and the parent tetraethyl ester [5]helicene

	S1 (eV)	T1 (eV)
D3-3 calculated	2.98	2.24
C2-3 calculated	2.86	2.17
D3-3 and C2-3 experimental	2.93	2.36
Parent [5]helicene calculated	3.15	2.39
Parent [5]helicene experimental	3.02	2.19

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The spin density maps and the 3DIMS maps¹⁰ of the T₁ states of both D₃-3 and C₂-3 are shown in Fig. 7. Interestingly, the excess of α spin densities corresponds to regions where the 3DIMS maps of the respective T₁ states display negative values of the magnetic shielding, corresponding to semi-local paratropic currents, i.e., semi-local antiaromaticity, but of different intensities: strong for D₃-3 and weak for C₂-3. In the case of the T₁ state of D₃-3, the excess of α spin density is localized on a single [5]helicene fragment in which a semi-local 22 π electrons antiaromatic paratropic current can easily develop. This is consistent with Baird's rule which states that the T₁ state of a 4n + 2π electron system should be antiaromatic. In contrast, in the T₁ state of C₂-3, the excess of α spin density is localized over two halves of two [5]helicene fragments connected by a single bond, which impedes the formation of a large and intense semi-local antiaromatic paratropic current, resulting in a semi-local non aromatic (actually just slightly antiaromatic) character visible on the corresponding 3DIMS map.

Fig. 7 Spin density maps (left) and 3DIMS maps (right) of the T₁ states at the geometry of the S₀ state for (a) D₃-3 and (b) C₂-3. In the spin density maps, the red color corresponds to an excess of α spin density, the blue color corresponds to an excess of β spin density.

The CPL spectra of the enantiopure (>98% ee) samples of D₃-3 and C₂-3 were recorded in three solvents of different polarity (toluene, dichloromethane and acetonitrile). Negligible solvent effects were observed, indicating the absence of charge-transfer character in the low-energy transitions, which is consistent with the calculated π → π* transitions of the FMO. For both D₃-3 and C₂-3, mirror-image responses were observed with maxima corresponding with those of the non-polarized spectra. The |g_lum| values reach 2.0 × 10⁻³ for D₃-3 and 1.5 × 10⁻³ for C₂-3, in line with typical good organic CPL emitters, higher than the recently reported |g_lum| values for some [5]helicene-based Möbius arylophanes.^2f A notable constitutional difference between Möbius-shaped D₃-3 and C₂-3 discussed herein and other [5]helicene-based Möbius-shaped arylophanes is that in D₃-3 and C₂-3 no or very limited free rotation is allowed at the C(sp²)–C(sp²) biphenyl-like single bonds, unlike in other comparable molecules. Our findings suggest that restricting internal rotation around the C(sp²)–C(sp²) biphenyl-like linkages with torsion angles in the magnitude of 30° or less is a promising strategy to enhance CPL intensity in semi-condensed multihelicenic architectures.²² The CPL brightness (B_CPL = ε × Φ_PL × |g_lum|/2)²³ for D₃-3 and C₂-3 were determined to be 3.6 and 2.4 M⁻¹ cm⁻¹, respectively, as a result of their modest molar extinction coefficients and modest emission quantum yields; nevertheless these values of B_CPL are state-of-the-art values for multicarbo[n]helicenes (typically <1 M⁻¹ cm⁻¹).²⁴

Analyzing magnetically-induced current densities (MICD, an anisotropic vectorial criterion of aromaticity) and their anisotropy (AICD, an isotropic scalar criterion of aromaticity) with the ACID program²⁵ in cyclotris[5]helicene D₃-3, some of us previously showed that this macrocycle is, to some extent globally π-conjugated and exhibits triply twisted Möbius aromaticity.⁷ Global Möbius aromaticity can a priori exist in both D₃-3 (triply twisted) and C₂-3 (singly twisted) as a consequence of the reduced torsion angle at the single bonds between the [5]helicene fragments, which allow ring-to-ring π delocalization.^5,9b This can be verified in the DFT optimized geometries discussed above (see Tables S1 and S4 in the SI), and from single crystal X-ray diffraction data (SCXRD, see previously reported⁷ data in CCDC 1826373 and 1826374): experimentally, in the monocrystalline racemate solid state, the average torsion angle is 30.1° (23.6–35.2°) in D₃-3, and 28.1° (15.2–40.2°) in C₂-3; it is significantly higher for biphenyl in the gas phase (44.4°).²⁶ The 3DIMS maps¹⁰ of both D₃-3 and C₂-3 (Fig. 8) show only blue color, indicating an aromatic character of both molecules. The dark blue areas indicate intense delocalization, which is visibly occurring in each 22-electron semi-local circuits at each [5]helicene fragments. Now looking specially at the single bonds connecting the [5]helicene fragments, both maps allow to visualize the uninterrupted π conjugation across these single bonds: at all single bonds in D₃-3 and C₂-3, and from any 3D perspective, a continuum of light blue color (and even medium blue color as revealed by close inspection, see the SI) between the C atoms is visible. The 3DIMS maps of D₃-3 and C₂-3 capture the chirality of their aromatic π systems and their twists. Detailed quantitative inspection revealed maximum IMS values over these bonds of 9.7 and 11.6 ppm (internal and external faces, respectively) for D₃-3, and 9.3 and 12.2 ppm (internal and external faces, respectively) for C₂-3, comparable with the value for the single bond in s-cis-butadiene (11.0 ppm),¹⁰ the archetype of a π conjugated molecule. The EDDB_H(r) analysis¹¹ of both D₃-3 and C₂-3 (Fig. 8) also confirmed their global Möbius aromatic character, with the delocalization of their π electrons extending to 76.0%, a value comparable to the one found in large kata-condensed isomeric [n]fibonacene tetraesters (n = 9–14).²⁷

Fig. 8 (a) 3DIMS maps and EDDB_H(r) plots (isovalue = 0.02) of (M,M,M)-D₃-3. (b) 3DIMS maps and EDDB_H(r) plots (isovalue = 0.02) (P,P,M)-C₂-3. (c) 3DIMS maps and EDDB_H(r) plot (isovalue = 0.02) of (M,M,P)-C₂-4. (d) 3DIMS maps and EDDB_H(r) plot (isovalue = 0.02) of (P,P,M)-C₂-5. The calculated fraction of e per atom shown between parentheses corresponds to the % of delocalized π electrons in the molecule.

Comparison of the 3DIMS maps of C₂-3 with the 3DIMS maps of its phenyl-tethered and alkyne-tethered analogues C₂-4 ^2f (Fig. 8c) and C₂-5 ^2d (Fig. 8d), respectively, is informative. As for C₂-3, the 3DIMS maps of C₂-4 and C₂-5 show only blue color, indicating they all are aromatic molecules. However, while the global conjugation in C₂-3 and C₂-4 can be visualized by a continuum of light blue or medium blue color over its single bonds in their 3DIMS map (indicating IMS ≥ 5.5 ppm over these bonds), the 3DIMS map of C₂-5 shows visible areas of the neutral color over the alkynyl–[5]helicenyl single bonds (indicating IMS < 5.5 ppm over these bonds, see red arrows in Fig. 8d). This indicates that global π conjugation is significantly reduced at the alkyne tethers in C₂-5, when compared to the biphenyl-type single bonds in its analogs C₂-3 and C₂-4. Just by a qualitative visual inspection, the 3DIMS maps in Fig. 8 indicate (i) that intense semi-local 22-electron circuits exist at each [5]helicene fragment in D₃-3, C₂-3, C₂-4 and C₂-5, (ii) that significant yet less intense global π delocalization circuit exists in D₃-3, C₂-3 and C₂-4, and (iii) that global π delocalization is weaker in C₂-5. In agreement with the 3DIMS analysis, the EDDB_H(r) analysis of C₂-5 indicated that only 70.9% of its π electrons are delocalized, while this fraction raises to 76.0% in both D₃-3 and C₂-3, and is up to 80.3% in C₂-4 (see Fig. S13 in the SI for the complementary analysis of D₃-4).

Conclusions

This article summarizes the comprehensive studies performed in our laboratories on the properties of the structurally appealing, yet stereochemically different, diastereomeric triply and singly twisted Möbius-shaped cyclotris[5]helicenes D₃-3 and C₂-3. Their vibrational and electronic properties have been comprehensively examined experimentally by precision spectroscopies, with a focus on the tiny differences observed in their circularly polarized infrared, absorption and luminescence spectra. All experimentally observed spectroscopic responses (using or not circularly polarized light) could be suitably modeled and rationalized using DFT calculations. Additionally, DFT calculations allowed to examine comparatively their aromaticity (not an experimentally measurable property), including at the T₁ excited states in correlation with their phosphorescence. Detailed information could be obtained on the S₀, S₁ and T₁ electronic states of both diastereomeric molecules. This in-depth study on the properties of two stereochemically different semi-condensed π-conjugated macrocycles made of [5]helicene fragments shows that the differences in these diastereomeric molecules can be evidenced by several experimental and theoretical precision techniques.

Some highlights of this comprehensive study are: (i) the understanding that the enantiomerization process of C₂-3 requires the inversion of configuration of a single [5]helicene fragment in a molecule that contains three [5]helicene fragments, and the demonstrated persistent chirality of D₃-3; (ii) the necessity to include dispersion correction schemes [D3(BJ), preferably D4] in the DFT calculations to simulate accurately some properties (conformational and electronic) in this series; (iii) the demonstration of existing global π-electrons delocalization in these molecules as deduced from their experimental absorption and emission spectroscopies (including phosphorescence) and the theoretical evaluation of their aromaticity by advanced methods, thereby confirming their Möbius aromatic character; (iv) the relatively high fluorescence dissymmetry factor (|g_lum| ≥ 1.5 × 10⁻³) and CPL brightness (B_CPL ≥ 2.4 M⁻¹ cm⁻¹) for both macrocycles.

Overall, from the two examples thoroughly analyzed in this study and the previously analyzed examples, it becomes clear that semi-condensed π conjugated molecules made of [5]helicene fragments can exhibit global π electronic delocalization despite the presence of C(sp²)–C(sp²) single bonds connecting their polycyclic π-conjugated systems, providing the torsion around these single bonds is restricted to ≤30°.

Author contributions

HB and FD performed the syntheses. NV performed the analytical and preparative resolution of the enantiomers by HPLC, the measurement of the barrier to enantiomerization of C₂-3, and the optical rotation dispersion measurements. JVN and YCo performed the theoretical conformational analyses. JVN performed the VCD and ECD analyses and related calculations. MR and LF performed the luminescence measurements, and DHR and YCa performed the corresponding calculations. AA performed the aromaticity analyses. FD and YCo conceived the study and obtained the research funds to realize it. YCo drafted the manuscript with revisions from all authors.

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). All analytical raw data (spectroscopy data and the corresponding DFT simulations) are available upon reasonable request to the authors (FD and YCo).

Supplementary information: all details of the studies and complementary data (PDF), all 3DIMS maps discussed herein (VTK). See DOI: https://doi.org/10.1039/d5qo01741f.

Acknowledgements

This work was funded in part by the French Agence Nationale de la Recherche—ANR (ANR-13-JS07-0009 and ANR-19-CE07-0041) and the German Research Foundation (SFB677). This work was supported by the computing facilities of the Centre Régional de Compétences en Modélisation Moléculaire de Marseille—CRCMM. Institutional financial support from Aix Marseille University, University of Bordeaux, University of Rennes, Centrale Méditerranée and the Centre National de la Recherche Scientifique—CNRS is gratefully acknowledged.

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Footnotes

† Current address: Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona (UdG), Facultat de Ciències, Girona, Catalunya 17003, Spain.

‡ Current address: Univ Angers, CNRS, MOLTECH-Anjou, SFR MATRIX, 49000 Angers, France.

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