Multilevel chirality transfer and second harmonic generation in mesoscopic double helical supramolecular self-assemblies of fullerene enantiomers

Abstract

Inducing chirality in the highly symmetric fullerene C₆₀ is an effective approach that leads to chiroptical responses over a wide range. However, the optically active expression of C₆₀ is a challenging research topic and multiscale modulation of fullerene chirality is still in its infancy. Herein, we synthesized a pair of fulleropyrrolidine enantiomers named R/S-MBA-C₆₀ with a monoclinic P2₁ chiral space group in the solid-state. Rare mesoscopic double helical supramolecular self-assemblies with distinctly opposite chirality have been obtained. Substantial π–π stacking and non-covalent interactions realize the chirality transfer and amplification across different scales, from the chiral carbon atom to the mesoscopic helix. Such fullerene-based double helical structures exhibit intense CD responses over an extremely wide range, from the UV-visible region and even up to the NIR region, and a SHG effect due the non-centrosymmetric molecular structure. This research develops an efficient approach to precisely modulate fullerene chirality and highlights the potential of chirality-based applications in fullerene science.

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Introduction

The demand for chiral nanomaterials with excellent chiroptical properties has been increasing, driven by their potential applications in chiral bio-imaging, spintronics and optoelectronic devices.^1,2 Of great interest are π-conjugated systems with photo- and electroactivities, and diverse topological structures.³ By modulating the supramolecular interactions of π-conjugated molecules possessing steric elements with other nano-motifs,^4,5 it offers a flexible strategy to tune their chiroptical responses on different scales. Nevertheless, most of them exhibit chiroptical activities mainly in the ultraviolet (UV)-visible region,⁶ which may impact practical applications to a certain extent. Thus, chiral nanomaterials possessing chiroptical activity in the far red and possibly NIR regions is highly desired.

Fullerene C₆₀ (denoted as C₆₀ hereafter) is a unique spherical π-conjugated system, with molar absorption coefficients over an extremely wide wavelength range.⁷ Since its discovery in 1985, C₆₀ has shown remarkable optical and electrical properties and has become a crucial supramolecular building block.⁸ Nevertheless, due to the lack of inherent chirality, the chiral science of C₆₀ is still under development.⁹ Drawing on the rich chemically modifiable features of fullerenes, even achiral C₆₀ can acquire chirality depending on the addends and addition patterns (Table S1),^10–15 exhibiting an excellent chiroptical response.^16–18 For instance, Murata et al. reported that an open-cage C₆₀ derivative synthesized through a multistep process has a high g_abs factor covering the NIR region.¹⁹ Fuchter and co-workers found that bis [60]PCBM shows a fast and high photocurrent response in CPL-detecting devices.^20,21 Moreover, the achiral pristine C₆₀ can acquire induced chirality within chiral supramolecular hosts (Table S2),^22–25 resulting in an intense chiroptical signal in the visible region.

Undoubtedly, C₆₀-based π-conjugated structures might constitute a class of chiral optical materials with outstanding performance which are still less explored. Compared to conventional organic chiral molecules, our knowledge on the strategies to hierarchically modulate the chiral response of C₆₀ is still limited.^26–28 Very recently, we have found that supramolecular chirality transfer has an impact on the chiroptical properties of pristine C₆₀ in the NIR range.²⁸ Under these circumstances, we maintain scientific skepticism about the relevant behavior of chiral C₆₀ derivatives in a similar situation. Considering that second harmonic generation (SHG) is highly sensitive to structural symmetries, we were also interested in exploring the second-order nonlinear optical (NLO) effect of non-centrosymmetric C₆₀ derivatives in addition to its chiroptical response.

In the current work, we showcase the synthesis and multilevel chirality transfer in the self-assembly process of fulleropyrrolidine enantiomers. Helical structures with well-defined opposite chirality were obtained by employing the liquid/liquid interfacial precipitation (LLIP) strategy. During this process, chirality transfer and amplification occurred at multiple length scales, which induced intense chiroptical response over a broadband range. The pure enantiomeric assemblies also exhibit efficient SHG responses. Our strategy presented herein features the simple synthesis of chiral C₆₀ building blocks, leading to multi-channel responses.

Results and discussion

Synthesis and structural analysis of R/S-MBA-C₆₀

Enantiomers of mono-substituted fulleropyrrolidines named R/S-MBA-C₆₀ were synthesized through a one-step thermal reaction (Fig. 1a).³⁰ Their structures were confirmed by multiple characterization studies (Fig. S1–S5). Single crystals of R/S-MBA-C₆₀ were obtained by slow diffusion of MeOH into a stock CS₂ solution at 4 °C. Single crystallographic X-ray diffraction (SCXRD) analysis revealed that both R- and S-MBA-C₆₀ crystallized in the P2₁ chiral space group with a monoclinic lattice, where the pyrrole-ring attaches to the C₆₀ cage at a single [6,6]-bond junction (C1–C2) with a length of 1.60 Å (Fig. 1b).³¹ Meanwhile, the purity and homogeneity of R/S-MBA-C₆₀ crystals were confirmed by powder X-ray diffraction (PXRD) patterns (Fig. 1c). Thermogravimetric analysis (TGA) showed both enantiomers possess high thermal stabilities up to 174 °C and 180 °C, respectively (Fig. S6). The UV-vis absorption, circular dichroism (CD), and dissymmetry factor g_abs spectra of the enantiomers in o-xylene are shown in Fig. 1d. The characteristic absorption peak at 430 nm can be attributed to the [6,6]-C₆₀ monoadducts (Fig. S7). Obvious mirror-image Cotton effects can be observed over a wide range covering the visible to far-red region (300–750 nm), indicating chiral induction across the whole C₆₀ spherical skeleton through its functionalization with a chiral R/S-MBA precursor (Fig. S8 and S9).

Fig. 1 (a) Reaction of C₆₀ with paraformaldehyde and (R/S)-(±)-α-methylbenzylamine (R/S-MBA). (b) Crystal structures of R-MBA-C₆₀ (left) and S-MBA-C₆₀ (right), with grey representing C, cyan representing N, and white representing H. (c) Experimental and simulated powder XRD patterns of R/S-MBA-C₆₀. (d) CD, g_abs, and UV-vis spectra of the R/S-MBA-C₆₀ enantiomers in o-xylene (0.5 mg mL⁻¹).

Chirality amplification and supramolecular assembly of mesoscopic helices

Next, we tried to amplify the chirality of individual molecules by triggering the self-assembly of R/S-MBA-C₆₀ through a well-known LLIP method in fullerene science.^29,32,33 By placing a poor solvent onto a solution of R/S-MBA-C₆₀ in a good solvent, self-assemblies with diverse morphologies could be obtained. By screening various kinds of solvent systems (Fig. S10 and S11), the combination of o-xylene/isopropyl alcohol (IPA) was screened. Optimization of the conditions yielded a feed concentration of 1 mg mL⁻¹ of R/S-MBA-C₆₀ and an incubation time of 72 h, which led to the formation of double helixes with definite opposite supramolecular chirality (Fig. 2a and S12). Obviously, the chirality exhibited by individual C₆₀ derivatives was successfully transferred to the supramolecular structure. It should be noted that the use of pure enantiomers is necessary to obtain the helixes, as in a control experiment, only flower-like self-assemblies lacking supramolecular chirality were observed when a racemic mixture (rac-MBA-C₆₀) was used (Fig. S13). This observation highlights the role played by the chiral carbon center during self-assembly.³⁴

Fig. 2 (a) Micrographs and SEM images of R-MBA-C₆₀ (i–iii) and S-MBA-C₆₀ (iv–vi) obtained in a solvent system of o-xylene/IPA (v/v = 1 : 5, 1 mg mL⁻¹, incubating for 72 h at 25 °C). (b) The molecular packing mode of R/S-MBA-C₆₀ in the helix. The π–π interactions between C₆₀–C₆₀ cages are omitted for presentation purposes. The circles denote SEM images of the original R/S-MBA-C₆₀ powder. Powder XRD patterns (c), and FT-IR spectra (d) of the R/S-MBA-C₆₀ helix. For comparison, curves of pristine powder of R/S-MBA-C₆₀ without the LLIP process are also given.

In the solid state, the as-synthesized R/S-MBA-C₆₀ powder shows only random structures (Fig. S14). The path dependence of the helixes indicates that their formation is kinetically controlled. To gain insights into the intermolecular interaction dictating the chirality transfer process, a detailed survey was performed on the composition and inner structure of the self-assemblies. As shown in Fig. 2b and S15, in the crystal packing mode of R/S-MBA-C₆₀, the observed π–π stacking interactions occur between MBA and fullerene units, with vertical plane-to-cage distances of 3.62 Å and 3.64 Å, corresponding to the dihedral angles of 13.58° and 13.12°, respectively.

As illustrated in Fig. 2c, the XRD patterns of such R/S-MBA-C₆₀ helixes are subtly different from that of the original powder, although all of them display a monoclinic phase. Substantial π–π interactions and the solvophobic effect induce oriented growth of R/S-MBA-C₆₀ in (001) and (010) planes during the self-assembly process. The difference in crystal facet growth rates is precisely the decisive factor in the formation of mesoscale double helical structures, thus realizing the transfer from molecular chirality to supramolecular chirality.^35,36 Combined with the Fourier-transform infrared (FTIR) spectrum (Fig. 2d and S16), both the C=C stretching vibration (1630 cm⁻¹) peak and the characteristic bands of the mono-substituted benzene ring (695 cm⁻¹, 759 cm⁻¹) are red-shifted compared to the pristine powder, again strongly demonstrating the presence of supramolecular interactions in the assemblies, which could be the main reason for their improved thermal stability since they are not solvates, as suggested by thermogravimetric analysis (TGA, Fig. S17).

So far, the mechanism for the apparent chiral amplification trend of the CD signals of the R/S-MBA-C₆₀ double helix over the broad wavelength range of 200–800 nm in DRCD spectra (Fig. 3a) has become quite clear. Due to the complexity of the structural features, we further employed dark-field scattering and circular differential scattering (CDS) to measure the optical and chiroptical properties of chiral R/S-MBA-C₆₀ helices at the single-particle level. The helix was spin-coated onto clean ITO glass and the corresponding scattering spectra were collected under light excitation using a halogen lamp (Fig. S18). Left/right-handed circular polarized (LCP/RCP) light was generated by adding a quarter-wave plate and polarizer in the optical path. Hyperspectral imaging (Fig. S19) and scattering spectra (Fig. 3b and c) indicate that the R/S-MBA-C₆₀ helices tend to scatter light of longer wavelengths and exhibit significant scattering signals between 700 and 790 nm, due to their large size up to several hundred microns.

Fig. 3 Chiroptical response of the R/S-MBA-C₆₀ helix. (a) Diffuse reflectance circular dichroism (DRCD) spectra of R/S-MBA-C₆₀ helices; for comparison, curves of pristine powder of R/S-MBA-C₆₀ without the LLIP process are also given. Single-particle scattering spectra of R-MBA-C₆₀ (b) and S-MBA-C₆₀ (c) helices under LCP and RCP excitation. The insets show the corresponding geometric models. LCP/RCP: left/right-handed circularly polarized light. (d) Typical CDS spectra of an individual R/S-MBA-C₆₀ helix.

Meanwhile, the R/S-MBA-C₆₀ helices have a higher tendency to scatter chiral light, i.e., the LCP/RCP light. R-MBA-C₆₀ helix has a stronger scattering ability for LCP between 700 and 750 nm whereas the S-MBA-C₆₀ helix exhibits the opposite behavior. The CDS spectra (Fig. 3d) of R/S-MBA-C₆₀ helices show nearly perfect mirror-symmetry, which can be obtained by calculating the scattering dissymmetry factor (g-factor, see details in the SI).^37–39 While each individual chiral helix exhibits a different spectral line shape, peak position, and intensity of CDS spectra (Fig. S20), averaging more than 20 particles results in CDS spectra that are similar to the ensemble DRCD spectra.

Second-order nonlinear optical effects in mesoscopic helices

Owing to the non-centrosymmetric space group and the chiroptical activity of R/S-MBA-C₆₀, second-order NLO properties are expected. As shown in Fig. S21, a home-built micro-area femtosecond laser system (refer to the SI) was employed to investigate the second harmonic generation (SHG) of the helical assemblies. The wavelength-dependent SHG spectra of both R- and S-MBA-C₆₀ helixes showed a strong SHG response over a wide wavelength range from 800 to 960 nm, with the strongest SHG intensity at 430 nm under 860 nm excitation (Fig. 4a). Additionally, the results of power-dependent measurements in Fig. 4b confirmed a quadratic dependence of the SHG intensity on the laser power (Fig. 4c) with a slope of about 2, indicating the two-photon nature of the SHG process. To gain further insight into the second-order NLO properties of these chiral helices, polarization dependent measurements were recorded and plotted in a dipolar profile (Fig. 4d), which coincides with a cos4 θ function, showing a dumbbell-shaped fitted curve. The crystal symmetry of R/S-MBA-C₆₀ determines the SHG intensity maxima at polarization angles of 160 and 340 deg for R-MBA-C₆₀, and of 100 and 280 deg for S-MBA-C₆₀, parallel to the dipolar axis. The polarization ratio, defined as ρ = (I_max − I_min)/(I_max + I_min), was 87.3% and 72.5% for R- and S-MBA-C₆₀ helices, respectively, showing high anisotropy with linear polarization. Given that R/S-MBA-C₆₀ also shows a photocurrent response (Fig. S22), such multiple-channel properties pave the way for applications in polarization-sensitive optoelectronic prototypes, CPL-sensitive devices and chiral photonics in fullerene science.

Fig. 4 NLO properties of the R/S-MBA-C₆₀ helix. (a) The SHG intensity of R/S-MBA-C₆₀ helices pumped with light from 800 nm to 960 nm with a power of 10 mW. (b) SHG spectra of R/S-MBA-C₆₀ helices under 860 nm excitation at various powers. (c) Logarithmic plot of the power-dependent SHG intensity under 860 nm excitation. (d) Polarization dependence of the SHG intensity as a function of the linear polarization angle. The solid lines indicate the fitting results.

Conclusions

In summary, we synthesized a pair of optically active fullerene enantiomers via [6,6] addition of chiral functional groups. Mesoscopic double helical assemblies of fullerene with distinctly opposite chirality have been successfully obtained. Substantial π–π stacking and solvophobic effects mediate this rare assembly process, which realizes the chirality transfer and amplification across different scales from the chiral molecule to the mesoscopic helix. Such fullerene-based double helical structures exhibit multiple prominent responses, including chiroptical activity over a wide range, extending even to the NIR region, SHG NLO effects, and polarization-sensitive optoelectronic responses. This work develops an effective approach for precisely modulating fullerene chirality and promotes chirality-based applications in fullerene science.

Author contributions

JRL performed most of the analysis, and led the writing of the manuscript (formal analysis/investigation/methodology/writing). KPZ and YLT contributed to the interpretation of the data (data curation/visualization). QFZ, HGL and JCH improved the text (reviewing/editing).

Conflicts of interest

There are no conflicts to declare.

Data availability

All relevant data are either in the main text or in the SI. Additional data are also available from the authors on request.

CCDC 2414209 (for R-MBA-C₆₀) and 2414210 (for S-MBA-C₆₀) contain the supplementary crystallographic data for this paper.^40a,b

Supplementary information: additional experimental details on the synthesis of the chemicals, instrumentation, characterizations, and Fig. S1–S22. See DOI: https://doi.org/10.1039/d5sc05354d.

Acknowledgements

This research was made possible by a generous grant from the National Natural Science Foundation of China (Grant Number 21875129 and 22174104). The authors acknowledge the assistance of the Center for Structural and Property Analysis, Core Facilities Sharing Platform, Shandong University.

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