Monitoring the Hierarchical Evolution from a Double-Stranded Helix to a Well-Defined Microscopic Morphology Based on a Turbine-like Aromatic Molecule.

1H-Indazolo[1,2-b]phthalazine-5,10-dione IPDD with an approximate turbine-like spatial structure, primary assembled double-stranded helices at the first level, was predicted by quantum chemical calculations and confirmed by atomic force microscopy. The higher-dimensional hierarchical architectures including fibrils, helical fibers, spherical shells, and porous prismatic structures were observed in sequence by the scanning electron microscopy technique. The final porous prismatic structures sensitive to NH3 vapors have the potential to be applied in gas sensing and absorbing materials.

Introduction

Helical architectures, which present the elaborate and unique functions, such as genetic information storage, molecular programming, specific molecular recognition, and catalysis, are one of the most important primary building blocks in a living system. These helical units can undergo further evolution from lower-dimensional structures to higher-dimensional architectures to achieve more precision, selectivity, and efficiency in their functions.[1] As exemplified by various natural proteins and chromatins, their distinctive superstructures arise from the precise secondary arrays of α-helices and DNA double-stranded helices with other biological units in space, respectively.[2] In addition, the evolution of hierarchical architectures is related to the inconsistency of chirality between initial helices and final superstructures. Therefore, tracing the formation of hierarchical superstructures from helices is of paramount importance. To mimic the delicate biological functions involving natural helices and decipher the transition process of hierarchical architectures, recently, effort has been devoted to explore the functionalized and morphological processes via developing the straightforward approaches toward reconstructive natural helical systems.[3] For instance, α-helices in proteins have been well modified and successfully used as building blocks in the controllable construction of hierarchical superstructures, such as micelles, vesicles, hydrogels, and organic–inorganic hybrids,[4] which have been further applied for controlled release, biological simulation, and tissue engineering.[5] However, it is still an important challenge in the case of synthetic helical systems because of their elusive assembly mechanism based on the complex multiple driving forces between helices.[6]

In the recent few years, π–π interaction-induced hierarchical self-assembly has developed to be a highly efficient strategy for the precise fabrication of one-dimensional (1D) nanostructures with well-defined shapes and sizes, such as helical fibers.[7] This is owing to the intrinsic tropism of π–π stacking in architectures[8] and the supplement of other driving forces such as hydrogen bonds,[9] hydrophobic and coordination interactions,[10] and electrostatic interactions[11] between their alterable substituents. These π conjugated systems are of particular interest not only for the predictability of their self-assembled nanostructures but also because of the tunability of self-assembled nanostructures as a result of the stimuli-responsiveness of substituents.[12] Moreover, the features of π-conjugated building blocks provide wide potential applications such as optoelectronic devices, organic semiconductors, photocatalysis, and sensors for their different dimensional nanostructures.[13] Because of the above-mentioned advantages, branched aromatic molecules with immobilized conformation provide a potential platform for the construction of supramolecular helical configuration and further exploration of the secondary assembly behavior of obtained helices based on theoretical simulation and imaging technique.[14]

Results and Discussion

We previously reported the synthesis of a new type of turbine-like aromatic molecule 1H-indazolo[1,2-b]phthalazine-5,10-dione derivative (IPDD) (see Figure a).[15] The reaction proceeded in a highly diastereoselective fashion. Although the products contained four stereocenters, only one diastereoisomer was obtained in high dr (>20:1) after silica gel column chromatography. We obtained X-ray single-crystal diffraction displayed that it is of enantiomers (see Figures b and S1). This is in good agreement with the quantum chemical calculations (see the Supporting Information for computational details), where the two enantiomers have the same electronic energy (see Figure S2). The spatial structures showed that four aromatic parts as fan blades are approximately perpendicular to the hexatomic ring in the center, while the fifth fan blade of 1H-indazolo[1,2-b]phthalazine-5,10-dione (IPD) bind the centronucleus in nearly a flat plane (see Figure b).

Figure 1

(a) Molecular structure of IPDD. (b) Enantiomers of IPDD in the crystal structure. (c) Arrangement of coupled enantiomorphism isomers of IPDD in the crystal structure. (d) Geometry and energy (in kcal/mol) values of the most stable IPDD dimer at the BP86+D3/def2-SVP//BP86/def2-SVP level. Color code: N, blue; O, red; C, green; H, white. (e) Optimized two-layer IPDD tetramer.

Moreover, the crystal diffraction data also revealed that the IPD groups of coupled enantiomers arrange side by side reversely in a horizontal direction while they are in parallel at a stagger angle in a direction along the axis as well (see Figures c and S1), indicating that the π–π stacking interactions and intermolecular hydrogen bonding interactions play an essential role in stabilizing the overall conformation of permutation. This is supported by DFT calculations at the RI-BP86+(D3)/def2-SVP level, and Figure d shows the optimized geometry, which is in excellent agreement with the experimental structure. The two central heterocyclic moieties in coupled enantiomorphism isomers are completely on the same plane. As shown by a short H-bond distance of 2.347 Å, the intermolecular hydrogen bonding interactions also help to stabilize the coupled geometry. The weak interactions accounted well for the stability of the coupled structure, which is 8.7 kcal/mol more stable than the separated IPDD monomers. We also considered other isomers of IPDD dimer (e.g., head-to-head connections and other conformations; see Figure S3 for details), which are either less stable than the structure in Figure d or inapposite for further self-assembly process and thus not further discussed. Based on the most favorable IPDD dimer, we further optimized the two-layer IPDD tetramer (Figure e, other unfavorable structures in Figure S4), which is 126.3 kcal/mol lower than the separated four IPDD monomers. It suggested that the self-assembly of IPDD is a highly thermodynamically favorable process. The various weak interactions, including the perpendicular π–π interaction between the two layers (ca. 3.384 Å) and the attractive hydrogen bonding interaction (the average H-bond distance is ca. 2.358 Å), play a significant role in stabilizing these structures. According to the structural information of the above-mentioned IPDD, we predicted that IPDD molecules may form a double-stranded helical structure in which each layer consists of coupled enantiomorphism isomers arranged side by side reversely and the neighboring layers are in parallel at a stagger angle in a direction along the axis (see Figure a). In addition, the four aromatic parts as fan blades, except the IPD group, provide the possibility of further superior superstructures via an antiparallel arrangement based on π stacking interaction in different directions.

Figure 2

(a) Double-stranded helical arrangement of IPDD, as represented by space-filling models: each helical strand is shown in different colors for clarity. (b, c) Atomic force microscopy (AFM) images of IPDD in petroleum ether/ethyl acetate (6:1) at a concentration of 5 × 10–4 mol/L after self-assembly for 1 h. (i) and (ii) are the axial and horizontal height profiles of columnar helices, respectively, in (c).

To corroborate the speculation, IPDD was dissolved in a mixed solvent of petroleum ether and ethyl acetate considered as poor and good solvents, respectively, after ultrasonic treatment for 1 h. The concentration of the solution was 5 × 10–4 mol/L. By keeping the volume ratio of petroleum ether/ethyl acetate as 6:1 and allowing to stand at room temperature for 1 h, the aggregation of the IPDD molecules occurred at the first level. As shown in AFM images (Figure b,c), the columnar helices with a height of 3 ± 0.5 nm were assembled in the initial period, which approximate to the size of coupled enantiomorphism isomers of IPDD (∼2.7 nm), which was further supported by an X-ray diffraction pattern (Figure S5a). In addition, these columnar helices were partly packed in parallel and formed into bundles, which was most likely driven by intermolecular π–π stacking interactions of the four aromatic parts as fan blades outside. Particularly, the aromatic planes of IPD groups possibly made a significant contribution to the stabilization between layers via π–π stacking interactions. This assertion was further supported by the X-ray diffraction pattern and Fourier transform infrared (FT-IR) spectroscopy studies. Figure S5a shows a diffraction peak at ca. 2θ = 22.62° (interplanar spacing of 3.8 Å) for IPDD, suggesting the presence of strong π–π stacking in these assemblies.[16] The FT-IR spectrum of IPDD exhibited absorption peaks at around 1643 and 1693 cm–1 (Figure S5b), which are consistent with an antiparallel arrangement of the aromatic rings in IPDD.[17] Besides, these assembled structures of columnar helices were reinforced owing to the stereohindrance effect of the other four rigid planes outside, except the IPD groups. Considering the size of columnar helices and the single-crystal diffraction analyses of IPDD, the formation mechanism of the columnar helices was in accordance with that shown in Figure a, in which the enantiomorphism isomers of IPDD adopt a similar packing mode as single-crystal diffraction. The coupled enantiomorphism isomers adopted the side-by-side arrangement reversely between the aromatic planes of IPD groups in the horizontal direction to form a monolayer, and the monolayer of coupled enantiomorphism isomers approach each other in parallel at a stagger angle in the axis direction based on π–π stacking interactions and stereohindrance effect, finally leading to the formation of helical structures.

By increasing the assembly time to 12 h, the growth of primitive self-assembled double-stranded helices led to the formation of the next dimensional structure. The scanning electron microscopy (SEM) images (Figures a and S6a) revealed the expected fibrils with a length of few hundreds of micrometers and a width of several tens of nanometers, which was also clearly observed from the transmission electron microscopy (TEM) images (Figure S7). Besides, these fibrils were found to be of superior flexible materials owing to the four aromatic parts as fan blades on the outside of double-stranded helices, which provide enhanced selectivity of the assembly direction and stability of the assembly structure. Moreover, the antiparallel arrangement of the four aromatic parts as fan blades on the outside between fibrils also gave rise to these fibrils that easily stack into bundles (Figure S6b). As expected, with further increasing the assembly time to 24 h, the assembly dimension was ulteriorly upgraded and the asymmetry emerged. The linear fibrils intertwined with each other in a twisted mode and was clearly observed by SEM (Figure b). As shown in Figure c, coiled-coil microfibers were generated.

Figure 3

SEM images of IPDD in petroleum ether and ethyl acetate (volume ratio 6:1) for different assembly times: (a) 12 h and (b, c) 24 h.

By sequentially increasing the assembly time, the intermediate state was clearly observed at 36 h (Figure a), and a larger assembled structure formed at 48 h (Figure c), which was also observed from the TEM images (Figure S8). As shown in Figure b, multistranded coiled-coil microfibers stacked in parallel and finally formed prismatic structures also owing to the mutidirections of four aromatic parts as fan blades on the outside of coiled-coil microfibers. Interestingly, these types of prismatic structures were found to be porous in the axis direction, similar to the charcoal structure (Figure d), which can be used as adsorbing materials.

Figure 4

SEM images of IPDD in petroleum ether and ethyl acetate (volume ratio 6:1) for different assembly times: (a, b) 36 h and (c, d) 48 h.

However, the adjustment of the volume ratio of petroleum ether and ethyl acetate to 4:1 gave rise to a fire-new aggregation architecture. According to the SEM images (Figures a–d and S9), the delicate spherical shell structures appeared after similar treatment for 36 h. Fortunately, the forming process was monitored by centrifugation (Figure a–d).

Figure 5

SEM images of IPDD in petroleum ether and ethyl acetate (volume ratio 4:1) for different assembly times: (a–d) 36 h and (e, f) 48 h.

First, the fibers were stacked with each other in a way of that they surrounded one center (Figure a). Then, the spherical clew centers gradually grew larger and took a shape of hemispherical shell structures (Figure b). Finally, the delicate spherical shell structures were closed completely via further intertwining of fibers (Figures c and S9a), which was also observed from TEM images (Figure S10). Remarkably, besides the disordered wrap mode, the orbit mode with consistent orientation also emerged due to the parallel arrangement of fibers as mentioned above (Figure S9c). Moreover, IPDD dissolved in petroleum ether and ethyl acetate (4:1), and similarly treated for 48 h, could form porous prismatic structures by the mode of fusion as well (Figure f). As expected, the fusion process was also clearly observed from SEM images (Figures e and S11) and the traces of cylindrical pits left over on the surface of prismatic structures (Figure e–f) undoubtedly confirmed this process.

To further understand the conversion between these superstructures, concentration-dependent fluorescence titration experiment was carried out in the same solvent. As shown in Figure S12, when the concentration of IPDD was increased from 0.016 to 0.12 mg/mL, the bathochromic shift of the maximum fluorescence emission wavelength up to 40 nm implied the clear transition between assembled structures involving intermolecular π–π interactions, and the fluorescence quenching; finally, this indicates the formation of larger structures in accordance with the prismatic structures. Furthermore, we carried out dynamic light scattering (DLS) experiments to underpin the conversion between the multidimensional structures. When the concentration of IPDD was increased from 0.018 to 0.4 mg/mL, the sizes of assembled structures slowly enlarged in sequence, and finally remained constant above a concentration of about 0.3 mg/mL (Figure S13). According to these results, the conversion between these superstructures was further confirmed. Besides, the sizes of initial and final assembled structures corresponded to those shown in SEM images.

Based on the porous prismatic structures, the adsorption property of IPDD was investigated subsequently. Cyclic voltammetry experiment of IPDD (Figure S14) revealed the only negative half-wave potential in a voltage range of 1.5–1.5 V, which potentially make a selective response to reducing gases.[18] As expected, IPDD was sensitive to NH3 (Figure S15). Furthermore, it also showed great potential as gas sensing and adsorbing materials for organic amines.[19]

Conclusions

We represented the initial self-assembly columnar helices from a turbine-like branched aromatic molecule, which agree well with the quantum chemical calculations. Based on the columnar helices, the evolution processes of hierarchical architectures, such as fibrils, coiled-coil microfibers, porous prismatic structures, and spherical shells, were successfully monitored by SEM via adjusting the assemble times and the ratio of poor and good solvents. Strikingly, we, fortunately, captured the intermediate state of transition between different hierarchical architectures. In addition, the results of the electronic properties of the IPDD by cyclic voltammetry implied that the IPDD was selectively sensitive to reducing gases. These results shed new light on the understanding of the mechanism of hierarchical self-assembly based on helices. It is anticipated that our observation will provide useful information to seek for the factors that essentially control the assemble process and further prepare nano- and microscale functional materials.

Experimental Section

Instruments and Materials

All commercial chemicals and solvents were purchased from commercial suppliers and were used without further purification unless otherwise specified. Reactions were carried out under an inert atmosphere of high-purity nitrogen. Solvents used for synthesis and spectroscopic experiments were dried and freshly distilled prior to use. 1H NMR and 13C NMR experiments were recorded at 400 MHz on a Bruker AVANCE III HD 400 MHz spectrometer at ambient temperature in deuterated chloroform using tetramethylsilane (TMS) as an internal standard. The electron spray ionization mass spectra (ESI-MS) were recorded using a Bruker Agilent 1290-micro TOF Q II spectrometer.

Fluorescence Spectra

Fluorescence spectra were recorded at room temperature on an Edinburgh FLS980 spectrometer. Solvents of the spectroscopic grade were employed. The baseline was corrected by subtracting the measurement of the cuvette filled with a pure solvent used for the measurement.

Single-Crystal X-ray Analysis

The single-crystal analysis was performed on a Bruker D8 Advance diffractometer with a Cu Kα (λ = 1.5406 nm, graphite monochromatized) at a temperature of 150 K. The structures were solved by direct methods (Shelxs) and refined with anisotropic temperature factors for all nonhydrogen atoms. The hydrogen atoms were refined with fixed isotropic temperature factors in the riding mode.

SEM Measurements

The SEM micrograph was recorded on a JSM-7610 F instrument with an accelerating voltage of 10 kV. Samples for SEM measurement were prepared by depositing a droplet (ca. 5 μL) of the IPDD solution (in petroleum ether/ethyl acetate (HPLC grade), ca. 5 × 10–4 mol/L) on a silicon wafer surface with hydroxylation treatment. The sample was dried under air flow at room temperature.

AFM Measurements

The AFM experiment was performed on an NT-MDT Solver P47H-PRO instrument in the tapping mode. The sample was prepared by depositing a droplet (ca. 5 μL) of the IPDD solution (in petroleum ether/ethyl acetate (HPLC grade), ca. 5 × 10–4 mol/L) on a silicon wafer surface with hydroxylation treatment followed by drying in air.

Cyclic Voltammetry Measurement

Cyclic voltammetry was conducted on IM6ex under N2 in dry 0.1 M TBAPF6 in dichloromethane solutions and was referenced to Ag/AgCl.

IPDD Gas Sensors Measurements

IPDD gas sensors were fabricated according to a published procedure with some modifications.[19a] First, glass substrates (1 × 1 cm2) were cleaned in a mixture of H2SO4 and H2O2 (7:3, v/v), rinsed with deionized water, and dried by Ar stream. Then, the ethyl acetate solutions (15 μL; 5 mg IPDD was dissolved in 2 mL of ethyl acetate) of IPDD were spin-coated on the substrates in air at room temperature. Finally, drain and source electrodes were manufactured by vapor depositing Au (2 × 10–6 Torr, 0.5 Å/s, ∼50 nm thick) onto a semiconductor film (ca. 80 nm) through a shadow mask to obtain devices with a channel length of 200 μm and a width of 2000 μm.

Computational Details

Geometry optimizations without symmetry restrictions were carried out with the Gaussian 09[20] optimizer in association with Turbomole6.4[21] energies, which is practicable to describe larger systems. The calculations were first carried out for all molecules using the BP86 functional[22] with def2-SVP[23] basis sets in conjunction with the resolution-of-identity (RI)[24] (termed as BP86/def2-SVP). The energetics are further improved at the same level but by the addition of the D3 dispersion correction by Grimme[25] (termed as RI-BP86+D3/def2-SVP). The nature of the stationary points and the thermodynamic corrections were obtained at this level of theory.

Synthesis of 1H-Indazolo[1,2-b]phthalazinetrione Derivatives (IPDD)[15]

The reactions were carried out with phthalhydrazide (0.1 mmol), diketenes (0.20 mmol), and trifluoroacetic acid (0.020 mmol) in acetonitrile (1.0 mL) at 40 °C for 72 h. Then, the reaction solution was concentrated in vacuo and the residue was purified by flash chromatography on silica gel (petroleum ether/EtOAc) to afford the desired corresponding product in 21% yield. 1H NMR (400 MHz, CDCl3) δ 8.40–8.30 (m, 1H), 8.27–8.17 (m, 1H), 7.83–7.71 (m, 2H), 7.14 (d, J = 7.9 Hz, 2H), 7.09 (d, J = 7.9 Hz, 2H), 7.07–6.98 (m, 8H), 6.92 (d, J = 7.8 Hz, 4H), 6.81 (d, J = 16.0 Hz, 1H), 5.91 (d, J = 16.0 Hz, 1H), 5.79 (s, 1H), 3.90–3.78 (m, 1H), 3.63 (d, J = 9.9 Hz, 1H), 3.54 (t, J = 10.3 Hz, 1H), 3.44–3.25 (m, 2H), 2.32 (s, 3H), 2.30 (s, 3H), 2.27 (s, 3H), 2.18 (s, 3H).13C NMR (100 MHz, CDCl3) δ 201.7, 154.7, 154.3, 142.2, 140.7, 138.5, 138.2, 137.2, 136.7, 135.9, 133.3, 133.2, 133.0, 132.7, 131.7, 130.2, 129.6, 129.5, 129.4, 129.3, 128.6, 128.4, 128.2, 127.7, 127.5, 127.3, 126.9, 126.3, 122.8, 67.5, 58.6, 44.1, 43.9, 32.6, 21.5, 21.3, 21.1, 21.0.