Chiral molecular nanosilicas

Abstract

Molecular nanoparticles including polyoxometalates, proteins, fullerenes and polyhedral oligosiloxane (POSS) are nanosized objects with atomic precision, among which POSS derivatives are the smallest nanosilicas. Incorporation of molecular nanoparticles into chiral aggregates either by chiral matrices or self-assembly allows for the transfer of supramolecular chirality, yet the construction of intrinsic chirality with atomic precision in discrete molecules remains a great challenge. In this work, we present a molecular folding strategy to construct giant POSS molecules with inherent chirality. Ferrocenyl diamino acids are conjugated by two or four POSS segments. Hydrogen bonding-driven folding of diamino acid arms into parallel β-sheets facilitates the chirality transfer from amino acids to ferrocene and POSS respectively, disregarding the flexible alkyl spacers. Single crystal X-ray structures, density functional theory (DFT) calculations, circular dichroism and vibrational circular dichroism spectroscopy clearly verify the preferential formation of one enantiomer containing chiral molecular nanosilicas. The chiral orientation and chiroptical properties of POSS show pronounced dependence on the substituents of α-amino acids, affording an alternative way to control the folding behavior and POSS chirality in addition to the absolute configuration of amino acids. Through the kinetic nanoprecipitation protocol, one-dimensional aggregation enables chirality transfer from the molecular scale to the micrometer scale, self-assembling into helices in accordance with the packing propensity of POSS in a crystal phase. This work, by illustrating the construction of chiral molecular nanosilicas, paves a new way to obtain discrete chiral molecular nanoparticles for potential chiroptical applications.

Scheme 1. Molecular structures of ferrocenyl POSS conjugates as well as the schematic representation of the intrinsic chirality of POSS by intramolecular folding, and chirality evolution across hierarchical levels via self-assembly.

Fig. 1. Geometry variation in the solid-state X-ray structures after conjugating the POSS segment to Val. Intramolecular folded POSS segments are highlighted in the CPK mode. In the packing mode, POSS segments grow into helices.

Fig. 2. (a) Single crystal structure of Phe and Phe-POSS which features the helical folded conformation and packing. (b) Hetero-chiral structure of Met-POSS and the packing mode in the single crystal structure. The POSS segment was highlighted in CPK. (c) DFT optimized structure of Glu-POSS in different views.

Fig. 3. (a) CD spectra of Val-POSS in CHCl₃ (1 mM) with different configurations. (b) Dissymmetry g-factors of different building units, “POSS” was omitted for clarity. (c–e) Temperature-variable CD spectra with heating and cooling processes of Val-POSS (1 mM in dioxane). (f) Partial ¹H NMR spectra of Val-POSS in CDCl₃ at different temperatures.

Fig. 4. (a–c) VCD spectra of Val-POSS, Phe-POSS and Glu-POSS in CCl₄ (50 mg mL⁻¹) respectively. Insets display the corresponding intramolecular folding of POSS segments. (d–f) Calculated VCD spectra of Val-POSS, Phe-POSS and Glu-POSS respectively via time-dependent DFT (TDDFT) at B3LYP/6-31g(d) level of theory (full width at half maximum = 15 cm⁻¹).

Fig. 5. (a) TEM image of l-Val-POSS self-assembly (1 mM) in a THF/ACN mixture (1/9, v/v). (b–e) Corresponding SEM images of l-Val-POSS self-assembly at different magnifications. (f) SEM image of d-Val-POSS under the same conditions. (g and h) AFM (840 × 840 nm) and height profile of Val-POSS self-assembly. (i) Corresponding 3D AFM image. (j) Powder XRD pattern comparison as well as the speculated 1D growth of Val-POSS directed by hydrogen bonds.

Fig. 6. (a and b) TEM images of PGly-POSS (1 mM) self-assemblies in the THF/ACN mixture (1/9, v/v). (c) Corresponding SEM image of PGly-POSS (1 mM) self-assemblies. (d) TEM of Glu-POSS self-assembly under the same conditions. (e and f) TEM images of Pro-POSS plate assemblies under the same conditions. (g and h) XRD patterns and the plane assignment of different self-assemblies.