Chiral π-extended diindenoperylenes featuring dithia[7]helicenes

Abstract

Helically chiral π-expanded diindenoperylenes were synthesized and their chiroptical properties were characterized. The enantiopure synthesis of the perylene framework was achieved by two-fold Yamamoto coupling of π-expanded dibromofluoranthenes, each comprising one dithia[7]helicene unit. Oxidation of the thiophene units to the corresponding sulfones allowed late-stage modification of the chiroptical and electrochemical properties.

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Introduction

The synthesis of non-planar, chiral polycyclic aromatic hydrocarbons (PAHs) and nanographenes has sparked widespread interest due to their unique optoelectronic and physical properties.^1–7 Potential applications include spin-selective electron transport^4,8 as well as circularly polarized light (CPL) detection⁹ and emission.¹⁰ CPL emitters in the near-infrared region are of particular interest, as, compared to visible light, near-infrared radiation is less absorbed in biological tissues and optical fibres, allowing deeper tissue penetration and far-reaching signal transfer, which are crucial for novel technological and bioimaging applications.¹¹

Helicenes are a class of inherently chiral PAHs, where nonplanarity is achieved through steric strain preventing a planar geometry.^3,12 The inherent chirality of their π-system makes helicenes useful building blocks for chiral optical and electronic materials. [4]Helicene is considered the smallest helicene. It is only slightly helical due to the steric interactions of the hydrogen atoms in the bay region, and the two enantiomeric forms rapidly interconvert through a C_2v symmetric, planar transition state.¹³ The barrier of racemization increases with the length of the helicene and, for example, enantiomers of [5]helicene are isolable but racemize within days at room temperature (rt),¹⁴ while [6]helicene is considered configurationally stable under ambient conditions.¹⁵

Examples of helicenes comprising perylene and related polycyclic units are shown in Fig. 1. The [6]helicene 1 recently synthesized by Nuckolls and coworkers features two perylene diimide units at its termini (Fig. 1A). Interactions of the π-systems of both perylene diimide units were demonstrated and attributed to their spatial proximity.¹⁶ Regarding its chiroptical properties, compound 1 exhibits an absorption maximum of λ_max,abs = 516 nm and a dissymmetry factor g_abs = 1.5 × 10⁻³. In compound 2, recently synthesized by Wang and coworkers,¹⁷ a perylene unit is embedded in the center of two [6]helicenes, offering an outstanding fluorescence quantum yield of 93% (λ_abs,max = 538 nm, λ_em,max = 562 nm) and a dissymmetry factor of g_abs = 7.0 × 10⁻³ at 360 nm. Compound 3 comprises a peropyrene unit as the centerpiece of an X-type double [7]helicene, with each [7]helicene unit featuring two thiophene units. In compound 4, these thiophene units were oxidized to the respective sulfones, demonstrating late-stage tuning of the structural and optoelectronic properties.¹⁸ Further noteworthy examples include a perylene-comprising double [5]helicene by Mastalerz and coworkers,¹⁹ showing excellent fluorescence quantum yields of 70% (λ_abs,max = 488 nm, λ_em,max = 526 nm), and heptagon-embedded saddle-shaped nanographenes featuring thia[6]helicene units by Hu, Chen, and coworkers,²⁰ further exemplifying the utility of oxidative late-stage functionalization of the thiophene units.

Fig. 1 Examples of configurationally stable π-extended helicenes. (A) [6]Helicene featuring two perylene diimide units.¹⁶ (B) π-Extended perylene comprising two [6]helicene units.¹⁷ (C) Peropyrene-based X-type double dithia[7]helicene and disulfone[7]helicene, demonstrating post-modification of thiophene units.¹⁸ (D) Diindenoperylene-embedded double dithia[7]helicene. Perylene units have been highlighted in blue color. mCPBA = m-chloroperoxybenzoic acid; Pr = n-propyl.

In this work, we aimed to embed a diindenoperylene between two dithia[7]helicenes, unifying the appealing optoelectronic properties of diindenoperylene^21–23 with the chirality provided by the helicenes and the potential of the thiophene units for late-stage modification^18,24–27 to enable tailored chiroptical characteristics.

Results and discussion

Synthesis

As shown in Fig. 2, the synthesis started from diarylnaphthylenecyclopentadienones 5 and 6, which were obtained from commercially available methyl 2-(3,4-dihydroxyphenyl)acetate using established methodology (see the SI).²⁸ A Diels–Alder reaction with bis(benzothiophen-3-yl)ethyne²⁹ (7) yielded the precursors 8 and 9. Subsequent oxidative cyclization using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and trifluoromethanesulfonic acid (TfOH) yielded dithia[7]helicenes rac-HT and rac-HT-Br₂. While an oxidative dimerization of rac-HT was not achieved, a reductive Yamamoto dimerization of rac-HT-Br₂ yielded an inseparable mixture of stereoisomers. Therefore, the two enantiomers of HT-Br₂ were separated by chiral HPLC (for details, see the SI). The subsequent Yamamoto coupling allowed dimerization to the enantiopure double dithia[7]helicenes (P,P)-DT (76%) and (M,M)-DT (81%) under mild conditions, avoiding racemization.

Fig. 2 Synthesis of diindenoperylene-embedded double dithia[7]helicene DT and double [7]helicenedisulfone DS, and of monohelicenes HT, HT-Br₂, HS, and HS-Br₂. (a) 200–220 °C, 14–18 h. (b) DDQ, TfOH, CH₂Cl₂, 0 °C, 30 min. (c) [Ni(COD)₂], COD, DMAP, THF, 60 °C, 2 h. (d) m-Chloroperoxybenzoic acid, CH₂Cl₂, 0 °C, 5–24 h. COD = 1,5-cyclooctadiene; DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; DMAP = 4-dimethylaminopyridine; Pr = n-propyl; THF = tetrahydrofuran; TfOH = trifluoromethanesulfonic acid.

Thiahelicenes rac-HT-Br₂ and rac-HT were oxidized with m-chloroperoxybenzoic acid (mCPBA) to the corresponding S,S,S′,S′-tetroxides rac-HS-Br₂ (49%) and rac-HS (43%).¹⁸ The subsequent Yamamoto dimerization employing rac-HS-Br₂ yielded double [7]helicenedisulfone DS as an inseparable mixture of diastereomers, albeit in a low yield (14%). To avoid the low-yielding Yamamoto dimerization of HS-Br₂, double [7]helicenedisulfones (P,P)/(M,M)-DS were synthesized by the oxidation of (P,P)/(M,M)-DT with mCPBA in yields of 35% and 23%, respectively. For reference, enantiopure (P)/(M)-HS were obtained by oxidation of the respective enantiopure (P)/(M)-HT.

Structure and aromaticity

Single crystals of racemic helicenes HT, suitable for X-ray diffraction analysis, were obtained (gas phase diffusion from CDCl₃/MeOH), HT-Br₂ (gas phase diffusion, 1,2-dichloroethane/MeCN) and HS-Br₂ (gas phase diffusion, o-dichlorobenzene/MeOH) (Fig. 3A and Table 1). The impact of the oxidation of the thiophene units can be observed by comparison of HT-Br₂ with HS-Br₂. The most notable changes concern the bond lengths in the thiophene unit. The average C–S bond length increases by 0.05 Å, while the C–C bond length in the 5-membered ring opposite to the sulfur atom increases by 0.05 Å. Both changes can be explained through the loss of aromaticity in the thiophene ring upon oxidation, as indicated by the HOMA^30,31 indices and NICS(1)_zz,avg values (GIAO^32–38-CAM-B3LYP³⁹/D3BJ⁴⁰/def2-TZVP^41,42/SMD(CH₂Cl₂)⁴³)⁴⁴ (Table 1 and Fig. 3B). The increased helical pitch may result from the increased bond lengths. The steric demand of the sulfone oxygen atoms should not directly influence the geometric properties of the helicene unit due to their considerable distance. Apart from the thiophene ring, DT and DS exhibit comparable aromaticity with rings A, A′, D, and D′ of the helicene subunits being highly aromatic (Fig. 3B). In the diindenoperylene unit, HOMA and NICS(1)_zz,avg values indicate the low aromaticity of the 5-membered rings (F) and the central 6-membered ring (H), while the other rings (E, G, G′) are indicated as aromatic.

Fig. 3 (A) ORTEP representation of the single crystal X-ray structures of HT, HT-Br₂, and HS-Br₂. For clarity, hydrogen atoms are omitted and alkyl chains are represented as wireframes. Color code: (C: grey; O: red; S: yellow; Br: orange). Shown are average torsion angles of cove region C–C–C–C bonds (ϕ_avg), dihedral angles between terminal rings A, A′ (Θ_A,A′), and helical pitch (for detailed definition, see the SI). (B) Calculated NICS(1)_zz,avg (color coded) and HOMA (written) indices at the GIAO-CAM-B3LYP/def2-TZVP/D3BJ level of theory for compounds DT and DS.

Table 1 Aromaticity indices of thiophene units of the investigated helicenes. HOMA indices are derived from single crystal X-ray structures, unless noted otherwise. NICS(1)_zz,avg values computed at the GIAO-CAM-B3LYP/def2-TZVP/D3BJ level of theory⁴⁴

Compound	Avg. HOMA index of thiophene units	Avg. NICS(1)zz,avg index of thiophene units^b
a Computational geometry (CAM-B3LYP/def2-TZVP/D3BJ) was utilized.b For simplicity, average NICS(1)zz,avg of NICS(1)zz and NICS(−1)zz of all thiophene units in the molecules was calculated (for specific values, see the SI).
HT	0.55	−13.8
HT-Br2	0.62	−13.9
HS	0.23^a	11.3
HS-Br2	−0.17	11.2
DT	0.63^a	−13.8
DS	0.24^a	11.2

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The strongly helical structure and considerable spatial overlap between the terminal benzene rings suggest configurational stability. In this study, the enantiomers were indeed successfully separated without special precautions using chiral HPLC and no indication of racemization was observed. To verify the configurational stability, the process of racemization was investigated computationally (r²SCAN-3c,⁴⁵ SI) using HT as a model compound. The calculated C_s-symmetric transition state lies approximately 159 kJ mol⁻¹ above the minima, consistent with negligible racemization and excellent configurational stability at rt. In contrast, the [5]helicene units, formed by rings D, C, E, F, G, and D’, C’, E, F, G’, respectively, have a considerably lower calculated barrier of racemization of only 51 kJ mol⁻¹. The expected rapid interconversion under ambient conditions is consistent with the apparent C₂ symmetry in NMR studies. The heterochiral diastereomer of the [5]helicene units was found to be energetically favored over the homochiral diastereomer by 2 kJ mol⁻¹, consistent with the observed crystal structures.

Chiroptical and electrochemical properties

The optoelectronic and chiroptical properties of mono- and double helicenes are summarized in Fig. 4 and Table 2. The substitution and oxidation state of the monohelicenes impacts their UV/vis absorption properties (Fig. 4A). The bromination leads to a bathochromic shift of the lowest-energy absorption maximum from 493 nm (HT) to 513 nm (HT-Br₂) and from 513 nm (HS) to 526 nm (HS-Br₂). Oxidation of the sulfur atoms leads to red-shifts of 13 nm (HS-Br₂ vs. HT-Br₂) or 20 nm (HS vs. HT), as well as an increase in the molar extinction coefficient of the lowest-energy absorption maximum (ε = 1.68 × 10⁴ M⁻¹ cm⁻¹ (HS-Br₂) vs. 6.86 × 10³ M⁻¹ cm⁻¹ (HT-Br₂); ε = 1.42 × 10⁴ M⁻¹ cm⁻¹ (HS) vs. 8.11 × 10³ M⁻¹ cm⁻¹ (HT)). Unsubstituted helicene HT shows fluorescence (λ_em,max = 614 nm), albeit with a low quantum yield (1.9%). HS shows fluorescence at λ_em,max = 610 nm with a higher quantum yield (9.0%). After dimerization to the perylene DT, the lowest-energy absorption maximum drastically shifts to 660 nm (ε = 5.69 × 10⁴ M⁻¹ cm⁻¹, Fig. 4B). Despite its oxidized thiophene units, DS shows nearly the same low-energy absorption bands (λ_abs,max = 657 nm, ε = 5.58 × 10⁴ M⁻¹ cm⁻¹). Time-dependent density functional theory (TD-DFT) calculations (CAM-B3LYP(D3BJ)/def2-TZVP/SMD(CH₂Cl₂), for details, see the SI) indicate that the lowest-energy absorption band is dominated by a HOMO–LUMO transition, with both molecular orbitals being localized mostly on the diindenoperylene unit.

Fig. 4 UV/Vis absorption and emission spectra of monohelicenes (A) and double helicenes (B). CD spectra of monohelicenes (C) and double helicenes (D). All measurements in CH₂Cl₂, rt, approx. 10⁻⁵ M.

Table 2 Experimental optoelectronic, chiroptical, and electrochemical data of the title compounds. UV/Vis absorption and CD spectra recorded in CH₂Cl₂ at rt. Reduction and oxidation potentials were measured by cyclic voltammetry in CH₂Cl₂ at rt with n-Bu₄NPF₆ as the supporting electrolyte and are referenced versus Fc/Fc⁺

Compound	λmax [nm] (ε(λmax) [M^−1 cm^−1])	λΔεmax [nm] (Δεmax [M^−1 cm^−1])	λgabs,max [nm] (gabs,max)	Ered,1 [V]	Eox,1 [V]
HT	493 (8.11 × 10^3)	332 (139)	331 (3.62 × 10^−3)	−1.86	+0.52
HT-Br2	513 (6.86 × 10^3)	321 (83.2)	321 (3.33 × 10^−3)	−1.77	+0.58
HS	513 (1.42 × 10^4)	250 (212)	334 (2.80 × 10^−3)	−1.37	+1.00
HS-Br2	526 (1.68 × 10^4)	321 (123)	319 (3.26 × 10^−3)	−1.34	+0.98
DT	660 (5.96 × 10^4)	310 (376)	311 (5.68 × 10^−3)	−1.24	+0.44
DS	657 (5.58 × 10^4)	340 (132)	343 (2.13 × 10^−3)	−1.01	+0.78

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Neither DT nor DS shows any detectable photoluminescence between 600 and 1600 nm. In general, the extended structures comprising highly flexible [5]helicene units may facilitate non-radiative relaxation processes, outcompeting fluorescence.⁴⁶

All enantiopure helicenes showed circular dichroism (CD) (Fig. 4C and D and Table 2). The absolute configurations were assigned based on TD-DFT (CAM-B3LYP(D3BJ)/def2-TZVP/SMD(CH₂Cl₂)) simulated CD spectra (for details, see the SI). Out of the monomeric helicenes, the unsubstituted HS has the highest absolute Δε (212 M⁻¹ cm⁻¹ at 250 nm), while HT, HT-Br₂ and HS-Br₂ show their highest Δε in the range of 321 nm–332 nm. HT offers the highest g_abs,max (3.62 × 10⁻³ at 331 nm). The double helicene DT shows even higher Δε (376 M⁻¹ cm⁻¹ at 310 nm) and g_abs,max (5.68 × 10⁻³ at 311 nm), values significantly higher than those of DS (Δε = 132 M⁻¹ cm⁻¹ at 340 nm and g_abs,max = 2.13 × 10⁻³ at 343 nm). These values are comparable to those reported for [6]helicene 1 (g_abs,max = 1.5 × 10⁻³)¹⁶ and double [6]helicene 2 (g_abs = 7.0 × 10⁻³).¹⁷

The electrochemical properties of all compounds were investigated by cyclic voltammetry measurements in CH₂Cl₂ with n-Bu₄NPF₆ as the supporting electrolyte versus ferrocene/ferrocenium (vs. Fc/Fc⁺). The redox potentials of the dimeric helicenes were impacted by oxidation of the thiophene units (Table 2 and Fig. 5). Sulfone DS is both oxidized and reduced at higher potentials than DT with the thiophene units. The anodic shift is in agreement with the electron-withdrawing nature of the sulfone moieties. Hence, the first oxidation is shifted by +340 mV (+0.44 V (DT) vs. +0.78 V (DS) (vs. Fc/Fc⁺)) and the first reduction by +230 mV (−1.24 V (DT) vs. −1.01 V (DS)). In the monomeric helicenes HT and HS, the effect of oxidation of the thiophene units is even more pronounced. The first oxidation potential occurs anodically shifted by +480 mV (0.52 V (HT) vs. 1.00 V (HS)) and the first reduction by +490 mV (−1.86 V (HT) vs. −1.37 V (HS)). In contrast, the bromo substitution in the monomeric helicenes HT-Br₂ and HS-Br₂ has only a negligible impact on the redox potentials, all of which occur within a range of ±0.1 V compared to parent HT and HS (for details, see the SI). All discussed redox events for helicenes and double helicenes were reversible or quasi-reversible with the exception of HT-Br₂, where the reduction is irreversible (see the SI).

Fig. 5 Cyclic voltammetry (CV, middle), differential pulse voltammetry (DPV, top) and square wave voltammetry (SWV, bottom) measurements of single and double helicenes in CH₂Cl₂ at rt (approx. 2 mM, n-Bu₄NPF₆ as the supporting electrolyte, CV: scan rate 149 mV s⁻¹. DPV: step potential 20 mV, pulse width 50 ms, pulse period 200 ms, and pulse amplitude 50 mV. SWV: step potential 10 mV, square-wave amplitude 25 mV and square-wave frequency 15 Hz).

Conclusions

π-Extended double dithia[7]helicenes DT and DS with a diindenoperylene core were synthesized and investigated for their structural, chiroptical and electrochemical properties. Our newly developed synthetic route involving Diels–Alder and Scholl reactions followed by Yamamoto coupling as the key step provides versatile access to a broad range of functionalized helicenes. The oxidation of the thiophene units to the corresponding sulfone acceptors further modulates the photophysical and redox properties of the compounds. While no fluorescence was detected for both DT and DS, intense bathochromically shifted UV/vis absorption maxima up to 660 nm and considerable dissymmetry factors reaching 5.68 × 10⁻³ were observed for DT. Computational analysis revealed excellent configurational stability of the [7]helicene unit (ΔE^‡ = 159 kJ mol⁻¹), potentially enabling long-term applications of our π-extended [7]helicenes without risk of racemization.

Author contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Conceptualization: J. B. and M. K; methodology: G. B. and J. B.; investigation: G. B.; computational analysis: G. B.; X-ray crystallographic analysis: F. R.; writing – original draft: G. B.; writing – review and editing: J. B. and M. K.

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: synthetic and computational details and characterization data. See DOI: https://doi.org/10.1039/d5qo01701g.

CCDC 2516116–2516118 contain the supplementary crystallographic data for this paper.^47a–c

Acknowledgements

The generous funding from Deutsche Forschungsgemeinschaft (DFG), project number 281029004-SFB 1249, is acknowledged. 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).

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