Chalcogen-doped, (seco)-hexabenzocoronene-based nanographenes: synthesis, properties, and chalcogen extrusion conversion

Abstract

A series of chalcogen-doped nanographenes (NGs) and their oxides are described. Their molecular design is conceptually based on the insertion of different chalcogens into the hexa-peri-hexabenzocoronene (HBC) backbone. All the NGs adopt nonplanar conformations, which would show better solubility compared to planar HBC. Except for the oxygen-doped, saddle-shaped NG, the insertion of large chalcogens like sulfur and selenium leads to a seco-HBC-based, helical geometry. All the three-dimensional structures are unambiguously confirmed by single-crystal X-ray diffractometry. Their photophysical properties including UV-vis absorption, fluorescence, chiroptical, charge distribution, and orbital gaps are investigated experimentally or theoretically. The properties of each structure are significantly affected by the doped chalcogen and its related oxidative state. Notably, upon heating or adding an acid, the selenium-doped NG or its oxide undergoes a selenium extrusion reaction to afford seco-HBC or HBC quantitatively, which can be treated as precursors of hydrocarbon HBCs.

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Introduction

Polycyclic aromatic hydrocarbons (PAHs) have fascinated chemists for a long time due to their wide utility in materials science,¹ optoelectronic devices,² biological probes,³ and supramolecular chemistry.⁴ To exploit three-dimensional aesthetic structures and modulate optoelectronic or photophysical properties, an increasing number of contorted polycyclic aromatic hydrocarbons (PAHs) or nanographenes (NGs) have been developed in the past decades.⁵ Such nonplanar PAHs or NGs have been constructed with a diversity of shapes, including bowls,⁶ saddles,⁷ helixes,⁸ belts,⁹ rings,¹⁰ and hybrid conformations¹¹ by “bottom-up” strategies. Compared to their planar counterparts, nonplanar PAHs or NGs have shown intriguing properties including high solubility, different aggregation patterns, tunable optoelectronic properties, and specific supramolecular interactions.^3a,12 Strategically, the introduction of main-group elements into hydrocarbon PAHs is another powerful method to not only modulate their three-dimensional structures but tune their intrinsic properties including chemical reactivity, electronic energy gap, and photophysical behavior.¹³

As a planar NG unit, hexa-peri-hexabenzocoronene (HBC), a D_6h – symmetric planar π scaffold (Fig. 1a), and its derivative have been intensively studied.¹⁴ Owing to the nature of its planar structure, HBC generally exhibits low solubility and a high aggregation tendency in solvents, which commonly hampers its application, particularly in solution-based device fabrication.^3a,15 Thus, numerous efforts have been made to engineer the structure of HBC. For example, by incorporation of a non-hexagonal ring like a seven-,^7a eight-,¹⁶ or nine-membered¹⁷ ring into the HBC skeleton, saddle-, or helical-, seco-HBC-based (Fig. 1a) structures were revealed, and these novel nonplanar structures exhibit distinct photophysical properties and aromaticity compared to the parent HBC. Recently, by insertion of nitrogen into the HBC backbone in the fjord region, our group reported nitrogen-doped HBC- or seco-HBC-based NGs^7b,18 (Fig. 1b), and the optical properties can be mediated by introducing versatile substituents at the peripheral nitrogen. In this context, we focused our attention on the insertion of chalcogens into the HBC backbone. We envisioned that varied chalcogens in the new backbone of NGs would significantly affect the properties of the entire π-system in the following aspects: (1) specific atomic radii of doped chalcogens may lead to diverse three-dimensional structures; (2) distinct electronegativities and multiple oxidation states of chalcogens would be beneficial to sophisticatedly tune the electronic properties; (3) by means of the unique chalcogen-extrusion reaction,¹⁹ a specific reactivity toward atom-extrusion conversion would offer dynamic control of ring-reconstruction.

Fig. 1 (a) Structures of planar HBC and partially cyclized seco-HBC. (b) Structures of nitrogen-doped, (seco)-HBC-based NGs. (c) Insertion of chalcogens to design heteroatom-doped HBC or seco-HBCs.

Thus, a series of chalcogen-doped NG analogs were designed and the influences of the doped chalcogens or oxidized chalcogens on the overall synthesis, structural geometry, electronic properties, and chalcogen extrusion reaction of the π-systems are demonstrated (Fig. 1c). Single-crystal X-ray analysis unequivocally reveals the saddle- and saddle–helix-shaped conformations of these novel chalcogen-doped NGs. The photophysical properties as well as the chiroptical properties of the helical, sulfur- and selenium-doped NGs were investigated. Meanwhile, thermal- or acid-induced chalcogen extrusion reactions for these NGs were studied, and the heteroatom-dependent chalcogen-extrusion processes were observed. Particularly, the selenium-doped NGs exhibit controllable, fast, and quantitative selenium extrusion conversion to form HBC under acidic conditions at room temperature. For the first time, the saddle–helix precursors of HBC, with over 100-fold higher solubility than HBC, were identified. The details of these results will be elucidated in this article.

Results and discussion

Synthesis of chalcogen-doped nanographenes

To obtain a series of chalcogen-doped NGs, we used a universal synthetic route to synthesize NGs with different alkyl side chains for single crystal cultivation (Scheme 1). In brief, the Diels–Alder reaction of dibenzo[b,f]oxepine (DBO) 1 with tetrabromothiophene-S,S-dioxide 2 in toluene followed by oxidative aromatization in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) afforded tetrabrominated aromatics 3 in 75% yield. Subsequently, fourfold Suzuki–Miyaura cross-coupling reactions of compound 3 with (4-ethylphenyl) boronic acid in the presence of 15 mol% PdCl₂(CH₃CN)₂, 30 mol% SPhos and excess K₃PO₄ then furnished the hexaphenylbenzene 4 in 81% yield. The treatment of compound 4 with DDQ (10.0 equiv.) in the presence of triflic acid (13.0 equiv.) in DCM in a water-ice bath resulted in the fully fused oxa-NG 5_O within 5 minutes in 65% yield with the formation of five new C–C bonds. For sulfur- or selenium-doped analogs, thiepine 8 and selenepine 9 were prepared in a two-step reaction with overall 58% and 24% yields respectively: the copper-catalyzed oxidative coupling of (2-formalphenyl) boronic acid with the sulfur or selenium followed by intramolecular McMurry olefin formation. Then Diels–Alder reactions of thiepine 8 and selenepine 9 with compound 2 followed by oxidative aromatization with DDQ furnished compounds 10 and 11 in 90% and 56% yields respectively. Notably, a higher temperature (140 °C in xylene) and longer reaction time are needed for selenepine 9 compared to that for oxepine 1 and thiepine 8 in the Diels–Alder reaction (synthetic details in the ESI†). Then arylations for compounds 10 and 11 were performed using Suzuki–Miyaura cross-coupling conditions in the presence of 15 mol% PdCl₂(CH₃CN)₂, 30 mol% SPhos, excess boronic acid, and K₃PO₄, affording hexaphenylbenzene 12–15 in 54–78% yields. When compounds 12–15 were subjected to Scholl reactions under the conditions of DDQ/CF₃SO₃H in DCM at 0 °C, the seco-HBC-based structures 16_S, 17_S, 18_Se, and 19_Se were obtained with the formation of four continuous C–C bonds. Instead of the fully cyclized saddle of oxygen-embedded NG, helical structures were formed for sulfur- and selenium-doped NGs presumably due to the large strain caused by atom size increasing. In any attempts of increasing the temperature, adding more equivalent reagents, or using other oxidative cyclization conditions, no fully fivefold cyclization product was observed (Scheme S6†). Meanwhile, the synthesis of tellurium-doped NGs was also investigated (Scheme S4†). In order to construct a tellurium-doped analog, we synthesized dibenzo[b,f]tellurepine 21 from dibromide 20 after failed attempts by using the same synthetic procedure as compounds 6 and 7 from boronic acid. Through two-fold lithium-halide exchange with n-butyl lithium followed by tellurium insertion, compound 20 was successfully transformed to dibenzo[b,f]tellurepine 21 in 58% yield. The mono-tellurium-doped aromatics 21 was unambiguously confirmed by X-ray crystallography analysis.²⁰ Subsequently, we subjected compound 21 to similar Diels–Alder reaction conditions by reacting with tetrabromide 2. Consequently, no tellurium-insertion product 22 was detected by gradually raising the heating temperature. Instead, we observed phenanthrene formed as the dominant product through tellurium extrusion of compound 21 upon heating to 140 °C, and some amount of tetrabromo 23 was observed when the heating temperature raised up to 180 °C. These results indicated that telluride 21 is prone to undergo a tellurium extrusion reaction prior to the Diels–Alder reaction, and compound 23 was most likely formed by [4 + 2] cycloaddition of phenanthrene and compound 2. Structure 23 was also confirmed by X-ray crystallography analysis.²⁰ Hence, the tellurium extrusion process suggested a higher possibility of heteroatom-extrusion by doping a larger-size chalcogen.

Scheme 1 Synthesis of oxygen-doped NG 5_O, sulfur-doped NGs 16_S, and 17_S, and selenium-doped NGs 18_Se and 19_Se.

With the sulfur- and selenium-doped NGs in hand, their oxidative analogs were prepared (Scheme 2). The selective oxidation of sulfur-NGs 16_S and 17_S and selenium-NGs 18_Se and 19_Se was performed by treating them with oxone at 0 °C for 2 hours to afford the corresponding mono-oxides 24_SO and 25_SO, and 26_SeO and 27_SeO in nearly quantitative conversion. Meanwhile, the S,S-dioxide 28_SO2 was obtained by oxidation of compound 25_SO at room temperature in the presence of an excess amount of meta-chloroperoxybenzoic acid (m-CPBA), while 27_SeO cannot be oxidized under these conditions. Due to the highly distorted skeletons, all the chalcogen-doped NGs as well as the corresponding oxides show good solubility in organic solvents; therefore, they could be unequivocally confirmed by means of ¹H- and ¹³C NMR spectroscopy (Fig. S29–S50†) and high-resolution mass spectrometry (see the ESI†). Meanwhile, the seco-HBC-type molecules including 16_S, 18_Se, and 24_SO were further characterized by 2D ¹H–¹H ROESY and COSY (Fig. S1–S3†).

Scheme 2 Synthesis of oxidative derivatives of S-, Se-doped NGs.

Structural analysis of NGs

We next try to grow single crystals for all the NGs with different alkyl chains, and as a result, single crystals of 5_O, 17_S, 18_Se, and 24_SO suitable for X-ray diffraction analysis were obtained by slowly evaporating their solutions at room temperature (Tables S1 and S2†).²⁰ As shown in Fig. 2a–d, all the molecular backbones are distorted due to the incorporation of a heteroatom-doped, seven-membered ring. All the seven-membered rings exhibit different degrees of antiaromaticity suggested by nucleus-independent chemical shift (NICS)²¹ by DFT calculations at the B3LYP/6-311+G(d,p) level of theory (Fig. S27†). Positive NICS (0) values between 2.79 and 6.49 were observed for all chalcogen-doped, seven-membered rings. The localized orbital locator (LOL)-π electron distribution calculation indicated that the electrons were localized on the seven hexagonal rings with strong π-conjugation (Fig. S27†). In detail, the X-ray crystallographic analysis of 5_O reveals a saddle-shaped, negatively curved structure in which the oxygen atom protrudes from the π surface. Such a distorted conformation resembles that of a previously reported N–Me doped analog.^7b The depths of the saddle, defined as the perpendicular distance from the centre of ring A to the line across C14 and C3 is 0.91 A (Fig. 2e), slightly shallower than the N–Me doped analogue. Meanwhile, the structures of 17_S, 18_Se, and 24_SO exhibited saddle–helix geometry with a protruding chalcogen out of the π surface (Fig. 2b–d). The end-to-end dihedral angle of [5]helicene in NG 17_S defined as the angle formed by the two benzene rings located at its terminal edges is 71.4°, and the torsion angle (C8–C14–C12–C38) is 34.9° (Fig. 2b). Meanwhile the dihedral angle and torsion angle in NG 18_Se are 73.0° and 32.2° (C5–C4–C3–C2) respectively (Fig. 2c). Longer Se1–C1 (1.917 Å) and Se1–C35 (1.920 Å) bonds in 18_Se were observed compared to C–S bonds (1.766 and 1.778 Å) in 17_S due to the larger atomic radii of selenium. Both helicenes 17_S and 18_Se exhibited a larger end-to-end dihedral angle and torsion angle than the nitrogen-doped analogs (64.4° and 31.1° respectively)^18b due to the increased atom size of sulfur and selenium. As helicenes, the crystallography of both 17_S and 18_Se, shown in Fig. 2f as a representative example, displays P/M enantiomers in single crystal structures (Fig. S5 and S6†). The S-oxide 24_SO was also unequivocally confirmed by its X-ray structure (Fig. 2d). The formation of the SO bond decreased the end-to-end dihedral angle (58° in 24_SO) and torsion angle (29.8°, C23–C22–C21–C29). Due to the electronegativity of sulfoxide, the intermolecular hydrogen bond interaction between sulfoxide and dichloromethane was observed in the crystal packing besides the π–π stacking (Fig. S7†). The electron-static potential (ESP) map of 17_S, 24_SO, and 28_SO2 obtained through DFT calculations at the CAM-B3LYP/6-31+(d,p) level of theory (Fig. 2g) clearly showed the influence of different sulfur-oxidation states on the charge of the NG molecules. As expected, the electrons were distributed almost evenly on the surface of sulfur-doped NG 17_S, and aggregated at the oxygen once the sulfur was oxidized in 24_SO, and 28_SO2. The HOMO and LUMO orbitals²² of seco-HBC-based NGs 17_S, 19_Se, 24_SO, 26_SeO, and 28_SO2 calculated at the PBE0/6-311G(d,p) level do not differ a lot from each other: each HOMO is found to be delocalized on the entire molecule, while the LUMOs are delocalized on the coplanar benzenoid rings (Fig. 2h).

Fig. 2 X-ray crystallographic structures of 5_O (a), 17_S (b), 18_Se (c), and 24_SO (d) with 50% probability of thermal ellipsoids. Side view of the saddle-shaped NG 5_O with the saddle-depth marked (e) and a pair of enantiomers shown in the single crystal 18_Se (f). Electrostatic potential (ESP) maps and the energy colour bar (a.u.) of S-, SO-, and SO₂-doped NGs (g), in which the alkyl chains were replaced by methyl groups. TD-DFT calculation of HOMO and LUMO orbitals with energies (eV) of 5_O, 17_S, 18_Se, 24_SO, 26_SeO, and 28_SO2 at the PBE0/6-311G(d,p) level of theory (h). The alkyl chains were replaced by methyl groups.

Photophysical and electrochemical properties

The optical properties of chalcogen-doped NGs were investigated by UV-vis absorption and fluorescence spectroscopy. The UV-vis spectra of 5_O, 17_S, 19_Se, 25_SO, and 27_SeO in DCM show that the absorption maxima are nearly identical (351–353 nm) together with multiple shoulder peaks, while sulfone 28_SO2 exhibits a red-shifted absorption band with the maximum peak at 360 nm (Fig. 3a). The weak absorption peaks of 5_O, 24_SO, and 26_SeO with ethyl substituents at approximately 450 nm were observed rather than the tert-butyl substituted analogs probably due to the formation of an exciplex by intermolecular π–π interaction (Fig. S8†). To obtain insights into the origin of the absorptions, we conducted time-dependent density-functional theory (TD-DFT) (Fig. S28, Tables S10–S15†). The DFT calculations indicated that the lower-energy bands at around 430 and 400 nm are mainly contributed by the HOMO–LUMO transition. The high-energy bands at approximately 375 and 350 nm can be assigned to either HOMO–LUMO+1 or HOMO−1–LUMO and HOMO−1–LUMO+1 respectively, which exhibit larger oscillator strength. The fluorescence maximum of these NGs is similarly located at around 475 nm with shoulder emission peaks for 5_O, and a bathochromic-shifted peak at 490 nm for 28_SO2 was observed (Fig. 3b). These compounds exhibit a large Stokes shift of 120 nm (over 7200 cm⁻¹). Meanwhile, these chalcogen-doped NGs exhibit fluorescence intensity that depends on the doped heteroatoms and their oxidative state. With a similar helical conformation, 17_S doped with sulfur exhibited 10-fold higher fluorescence intensity and nearly 20-fold higher fluorescence quantum yield (Table S3†) than the selenium-doped analog 19_Se. The fluorescence efficiency of NGs can be significantly enhanced by changing the doping heteroatoms S or Se to S-oxide or Se-oxide (17_Svs.25_SO; 19_Sevs.26_SeO, Fig. 3b, Table S3†). As a result, the S-oxide 25_SO was highly fluorescent with the highest fluorescence quantum yield of 0.30, which is significantly higher than that of the corresponding HBC 30_HBC and seco-HBC 29_seco-HBC (Table S3†). Notably, the difference in the magnitude of the fluorescence decay was observed by doping different heteroatoms. As shown in Fig. 3c, the oxygen- and sulfur-doped candidates 5_O, 17_S, 25_SO, and 28_SO2 exhibit very close decay rates with fluorescence lifetimes over 17 ns, whereas, for selenium-doped compounds 19_Se and 27_SeO, the fluorescence lifetimes are less than 4 ns. Electrochemical properties were investigated by cyclic voltammetry in CH₂Cl₂ in the presence of Bu₄NPF₆ as a supporting electrolyte (Fig. S11†).

Fig. 3 Photophysical and chiroptical properties of chalcogen-doped NGs. (a) UV-vis absorption spectra, (b) fluorescence emission spectra, and (c) fluorescence decay profiles of 5_O, 17_S, 19_Se, 25_SO, 27_SeO, and 28_SO2. The spectra were collected in DCM solution with 5 μM concentration and λ_exc. = 350 nm for fluorescence excitation. (d) CD spectra of enantiomeric 17_S-P and 17_S-M, enantiomeric 19_Se-P and 19_Se-M, and enantiomeric 25_SO-P and 25_SO-M in hexane. (e) CPL and direct current (DC) spectra of 17_S-P/17_S-M and 25_SO-P/25_SO-M in dichloromethane (λ_exc. = 350 nm, the measurement was carried out in air).

Notably, the redox properties can be modulated by controlling the oxidative states of the doped chalcogens. The oxidized structures 25_SO, 27_SeO, and 28_SO2 show more oxidation waves than their reduced analogs 17_S and 19_Se.

Chiroptical properties and determination of the racemization barrier

Helicene-based NGs have triggered much interest in the past decades due to their intrinsic chirality.²³ We attempt to resolve the two sets of enantiomers for both 17_S and 19_Se and study their chiroptical properties. The enantiomers of 17_S and 19_Se were successfully separated by chiral high-performance liquid chromatography (HPLC) at 25–30 °C with ethanol as the mobile phase (Fig. S12 and S13†). The electronic circular dichroism (ECD) spectra of the enantiomers of 17_S and 19_Se in hexane demonstrated a perfect mirror image relationship with multiple opposite Cotton effects in the UV/vis region from 200 to 450 nm (Fig. 3d). Due to the homologous chiral structures, similar profiles of enantiomers from 17_S and 19_Se were displayed. Each enantiomer of 17_S or 19_Se was confirmed by comparing each ECD spectrum with the calculated spectrum (Fig. S26†). Since sulfoxide 25_SO shows high emission efficiency, we prepared the optically pure enantiomers 25_SO-P/25_SO-M by oxidation of enantiomers 17_S-P and 17_S-M (Fig. S14†), and the ECD spectra of 25_SO-P/25_SO-M indicated a perfect enantiomer relationship (Fig. 3d). Next, we carried out circularly polarized luminescence (CPL) measurements for each enantiomer. The CPL spectra of 17_S-P/17_S-M and 25_SO-P/25_SO-M exhibited a mirror-imaging relationship in dichloromethane under ambient conditions (Fig. 3e), which suggested that 17_S and 25_SO show CPL activity.

The luminescence dissymmetry ratio (g_lum), evaluating the degree of CPL, was determined to be 1.1 × 10⁻³ and 5.6 × 10⁻⁴ for 17_S and 25_SO respectively at 480 nm (Fig. S15†). Due to the low fluorescence quantum yield (Φ_F < 0.01), the CPL of 19_Se-P and 19_Se-M was feeble to be detected. Meanwhile, due to the high extinction coefficient and Φ_F, the enantiomers of 17_S and 25_SO exhibit B_CPL up to 13 M⁻¹ cm⁻¹ and 12 M⁻¹ cm⁻¹ respectively. The successful resolution of two enantiomers of 17_S or 19_Se indicated relatively high enantiomerization barriers for both helicenes.

To experientially determine the isomerization barrier (ΔG^‡), thermal racemization of optically pure 17_S-P was performed followed by chiral HPLC analysis (Fig. S16†). HPLC traces of the samples after heating at 100 °C exhibited a clean transformation from the 17_S-P to the 17_S-M isomer. The half-life (τ_1/2) for loss of enantiomeric excess was determined to be 5.5 hours at 100 °C. It gives a racemization barrier ΔG^‡ (373 K) of 29.6 kcal mol⁻¹ using the Eyring equation, which demonstrated a relatively stable helicene enantiomer and comparable with reported [5]helicene containing NGs.²⁴ In addition, thermal racemization of optically pure 19_Se-P was performed similarly. However, 19_Se-P stayed stable after heating at 100 °C for hours, which suggested a higher isomerization barrier of 19_Se than that of 17_S. Interestingly, the selenium-extrusion reaction was observed together with enantiomerization when the heating temperature was raised to 130 °C (Fig. S17†). Nevertheless, excluding other chemical reactivity, the racemization barrier ΔG^‡ of 19_Se was evaluated to be 31.2 kcal mol⁻¹ at 403 K.

Chalcogen-extrusion processes

After getting a hint from the observation of the selenium-extrusion process during the synthesis, we then investigated the chalcogen-extrusion reactions of these heteroatom-doped NGs. Several organosulfur compounds are susceptible to undergoing photo- or thermal-induced sulfur extrusion reactions.²⁵ Initially, we examined whether the extrusion reactions could be triggered by UV light. Each compound was treated in DCM solution under an atmosphere of N₂, followed by photoirradiation with 254 or 365 nm UV light. Unfortunately, no distinct change was observed for all the NGs upon irradiation for a short period of time (<10 min) and compounds gradually degraded over a long time. Alternatively, we performed the thermal chalcogen extrusion for compounds 5_O, 17_S, 19_Se, 25_SO, 27_SeO, and 28_SO2 (Fig. 4a). The experiments were conducted by heating milligrams of solid of each sample in an oven for a specific time, and monitoring the changes in the absorption spectra after re-dissolving the preheated sample in DCM. To our delight, for selenium-doped candidates, 19_Se and 27_SeO, identical de-selenium products were detected by observing the same absorption profiles formed after heating at 200 °C for only 5 min. These new species were confirmed as selenium-extrusion product 29_seco-HBC in comparison with the UV-absorption of standard product 29_seco-HBC (Fig. 4be). The HPLC analyses of the brown samples formed upon heating (Fig. 4b, inset) indicated a quantitative formation of 29_seco-HBC (Fig. S20 and S21†). In contrast, sulfur 17_S and sulfone 28_SO2 as well as the oxygen-doped saddle 5_O showed no change in the absorption spectra upon heating up to 250 °C for 2 hours, which suggests relatively thermally stable structures (Fig. 4c, f, and S18†). Meanwhile, sulfoxide 25_SO exhibited an inert sulfur extrusion process by heating over 250 °C compared to selenium-doped analogs (Fig. 4d and S19†). These results suggested the accessibility and rate of chalcogen extrusion reaction are highly dependent on the sort and oxidative state of the embedded chalcogens.

Fig. 4 (a) Chalcogen extrusion reactions of S-, Se-doped NGs. UV-vis absorption spectra of 19_Se (b), 17_S (c), 25_SO (d), 27_SeO (e), and 28_SO2 (f) before and after heating. The spectra were collected by dissolving the solid (before and after heating) in DCM and the heating time of each compound was shown in the corresponding spectrum. For comparison, the UV-vis spectra of the chalcogen-extrusion product 29_seco-HBC were shown together with 19_Se (b), 25_SO (d), and 27_SeO (e). The inset in (b) shows the photographic images of the 19_Se solid before and after heating, and the dissolved brown heated solid in DCM.

The HBC reveals wide application in materials science but is detrimental to the fabrication of material devices due to aggregation-caused low solubility. Apparently, having a soluble precursor of HBC would significantly facilitate the material fabrication of HBC. We envisioned whether HBC structures could be obtained by spontaneous oxidative cyclodehydrogenation of the chalcogen-extrusion products. By screening commonly used acids like acetic acid, HCl, and trifluoroacetic acid, it was found that selenium oxide 27_SeO could slowly convert to 30_HBC in a trifluoroacetic acid/DCM (1/2, v/v) solution (Fig. 5a) and the formation of target product 30_HBC was unequivocally confirmed by X-ray crystallography (Fig. 5b, Scheme S5†).²⁰ As monitored by UV-vis absorption (Fig. 5c), through selenium extrusion and spontaneous oxidation in air, the characteristic peak of 27_SeO at 353 nm gradually shifted to 358 nm over 20 hours in the trifluoroacetic acid/DCM (1/2, v/v) solution, and the absorption spectrum after a 20 hour reaction was almost identical to that of 30_HBC, whereas the other NGs were stable under these conditions (Fig. S23†). Together with the HPLC analyses (Fig. S22†), a nearly quantitative conversion was obtained for 27_SeO. Moreover, this transformation could be accelerated by the addition of stronger acid methanesulfonic acid instead of trifluoroacetic acid (Fig. S24†). To evaluate the difference in soluble properties between HBC and this precursor, we measured the solubility of 27_SeO and 30_HBC in dichloromethane and ethanol (Fig. S25†). The precursor 27_SeO exhibits more than 100-fold higher solubility in either dichloromethane or polar solvent ethanol.

Fig. 5 (a) Acid-catalyzed HBC formation from HBC precursor selenium-doped NG 27_SeO. (b) X-ray crystallographic structure of 30_HBC with 50% probability of thermal ellipsoids. (c) Changes in the UV-vis absorption spectra of 27_SeO upon dissolving it in trifluoroacetic acid/DCM (1/2, v/v) with 3.3 μM concentration under ambient conditions.

Conclusions

We have developed a new class of chalcogen-doped NGs, in which a chalcogen is conceptually embedded into the HBC or seco-HBC backbone in the fjord region. As a eesult of the size of chalcogens, two types of structures were obtained toward the final step of the oxidative ring closure reaction: the oxygen-doped saddle and the sulfur- or selenium-doped helix. The saddle and helical three-dimensional structures of the chalcogen-doped NGs and the sulfur-oxide were revealed by single-crystal X-ray diffraction analysis, and both P- and M-enantiomers of helicene enantiomers cocrystallize in a single crystal. Enantiomers of sulfur- and selenium-doped helical NGs were isolated by chiral HPLC. The chiroptical properties including ECD and CPL were investigated and the sulfur-embedded NG exhibited CPL activity with a g_lum of 1.1 × 10⁻³ at 480 nm and B_CPL of 13 M⁻¹ cm⁻¹. The racemic barriers of S-NG and Se-NG were determined to be 29.6 and 31.2 kcal mol⁻¹ respectively by HPLC-based thermal isomerization. The experimental and computational study including UV-vis absorption, fluorescence emission, efficiency, decay, HOMO–LUMO gaps, and charge distribution indicated that the chalcogen and its oxidative state in the NG significantly affect the electronic properties. The stability measurements of these NGs upon heating suggested that the Se-doped NG and its oxide undergo quantitative selenium-extrusion reactions, which offers dynamic control of the structural reconstruction. Notably, the highly distorted helical Se-oxide NG can transform to planar HBC under acidic conditions. The planar HBC generally exhibits low solubility and a high aggregation tendency in solvents. This HBS precursor would significantly facilitate the application of HBC in material fabrication.

Data availability

The full experimental details, synthetic procedures, characterization data, HPLC traces, UV-vis, fluorescence, IR and NMR spectra, computational details, and supplementary discussions associated with this article are provided in the ESI.†

Author contributions

Ranran Li performed most of the synthesis work and property studies. Bin Ma started the initial synthesis of compound 5_O. Shengtao Li helped with NMR, HPLC, and photophysical property measurements. P. An performed the DFT calculation and conceived the concept. P. An and C. Lu prepared and revised the manuscript. All the authors analyzed and interpreted the results.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support provided by the National Natural Science Foundation of China (21901226 and 22061046). We thank the advanced analysis and measurement center of Yunnan University for assistance with instrumentation. Particularly, we thank Dr Jie Zhou from the Advanced Analysis and Measurement Center of Yunnan University for the X-ray analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2238626–2238632. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc02595k

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