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Pressure‐Tuned Structure and Property of Optically Active Nanocrystals

2016; Volume: 28; Issue: 10 Linguagem: Inglês

10.1002/adma.201504819

1521-4095

Feng Bai, Binsong Li, Kaifu Bian, Raid Haddad, Huimeng Wu, Zhongwu Wang, Hongyou Fan,

Mass Spectrometry Techniques and Applications

Investigations through high-pressure X-ray scattering and spectroscopy in combination with theoretical computations shows that high-pressure compression can systematically tune the optical properties and mechanical stability of the molecular nanocrystals. Nanocrystals with unique morphology are relevant to a wide variety of optical and electronic applications such as photo­catalysis, sensing, etc.1-4 Precise control of structural para­meters through nanoscale engineering to improve optical and electronic properties of functional nanocrystals continuously remains an outstanding challenge. Previous work has been conducted largely at ambient pressure and relies on specific chemical or physical interactions such as van der Waals interactions, dipole–dipole interactions, chemical reactions, ligand–receptor interactions, etc. For example, molecular nanocrystals are formed through self-assembly of a single molecular precursor as a building block via covalent or noncovalent interactions including π–π stacking, hydrophobic–hydrophobic interaction, ligand coordination, etc.1, 2, 4 The properties of the molecular nanocrystals result as a synergy not only from their individual building blocks but also from collective effects due to molecular coupling within the self-assembled nanocrystals. To manipulate the former, one can modify the molecular building blocks or compositions so as to tune the property of individual nanocrystals. For the latter, the building blocks must be assembled into ordered nanocrystals with controlled spacing such that coupling between nearby molecular building blocks leads to new physics and collective properties throughout the self-assembled network. Ability to control separation distance or binding between building blocks is therefore critical. Chemical and synthetic routes have traditionally been the dominant method in order to tune structure, functionality, and properties of these nanocrystals.4-7 Recently, there are significant interests in using external forces as an effective means to control nanomaterial phases and structures for designing and engineering nanomaterials.8 Quan et al. reported reversal of Hall–Petch Effect in structural stability of nanocrystals and associated variation of phase transformation depending on nanocrystal sizes.9 Based on these works, they further demonstrated pressure-induced switching between amorphization and crystallization in nanoparticles for harvesting of metastable nanocrystal phases.10 Sun and co-workers showed stress-induced reduction of crystalline stacking faults under pressure, leading to an increase of symmetry of diffraction peaks and mechanical performance.11 Yan et al. reported pressure-based processing of structural transformation of hydrogen bonded nanomaterials.12, 13 Despite these progresses in structural manipulation through external pressure, ability to tune property of nanocrystals and its correlation to the corresponding structure is still limited and desired. Herein, we showed a reversible tuning of optical property and structure of molecular nanocrystals under pressure. The molecular nanocrystals were synthesized using an optically active chromophore, zinc tetra-pyridyl porphyrin (ZnTPP) as a building block. The nanocrystals were loaded into a diamond anvil cell at ambient pressure, then subjected to pressures of 0–15 GPa to induce mechanical compression of unit cell lattices, and ultimately induced changes to the optical properties. Through in situ high-pressure wide-angle X-ray scattering (HP-WAXS), it was observed that up to an external pressure of 7 GPa, the unit cell lattice dimension of the nanocrystals could be systematically and reversibly manipulated and controlled. This allowed fine-tuning of the crystal structure of the nanocrystals through changing both the bond angle and length. More importantly, UV–vis absorption spectroscopy studies and fluorescent imaging showed that stress could essentially tune the optical property of the ZnTPP nanocrystals. We observed that the overall crystal framework turned into amorphous when external pressures were greater than 9 GPa. Investigation through high-pressure, spectroscopy, HP-WAXS in combination with theoretical computations shows that pressure can effectively tune optical property and manipulate the mechanical stability of the nanocrystals. The synthesis of the ZnTPP nanocrystals was conducted through confined self-assembly of ZnTPP in an acid–base neutralization process (see detailed experimental procedures in the Supporting Information).1 Briefly, a 0.5 mL of 0.01 m ZnTPP acidic solution (0.2 m HCl) was added into a 9.5 mL of basic surfactant solution containing 0.02 × 10−3 m NaOH with continuous stirring at room temperature for 48 h. Surfactant cetyltrimethylammonium bromide (CTAB) with concentration of 0.01 m was used in the confined self-assembly. The final ZnTPP nanocrystals were collected through centrifuging and washed with DI water to remove free surfactants. The ZnTPP nanocrystals exhibit a well-defined external morphology and are uniform in size, with an average diameter of 100 nm and length of 2–3 μm. The uniformity of the 1D nanocrystals allows them to form highly ordered arrays, as shown in the representative scanning electron microscopy (SEM) image in Figure 1A. X-ray diffraction (XRD) patterns of the ZnTPP nanocrystals (Figure 1B) are consistent with a hexagonal space group 148 (SG148) with the unit cell dimensions a = b = 33.110 Å, c = 9.273 Å, α = β = 90°, and γ = 120°.1, 2, 14 Each Zn porphyrin molecule is bound to four neighbor porphyrins through four identical Zn–N axial ligations: two at the Zn core and two at the opposite ends of its peripheral pyridyl groups. Such self-assembly packing leads to formation of periodic micropores within the nanowire crystal framework. The ZnTPP molecule’s two remaining pyridyl groups are unligated and exposed at surfaces of the micropores. From the crystal structure (shown in Figure 1C), the periodicity (or pore diameter) was measured to be ≈ 9.6 Å and the pore center-to-center distance is ≈20 Å. Additionally, every molecule in the unit cell has two identical π–π interactions with its counterparts in adjacent unit cells in the (plus/minus) c-direction. As shown in Figure S1 (Supporting Information), the absorption spectrum of the nanocrystals differs from those of ZnTPP molecules in that the B-(Soret) and Q-bands split and redshifted. As pressure was applied, the peaks in the UV–vis absorption spectrum (Figure 2A,B) incrementally redshifted and the peaks broadened until some merged and the peak positions were difficult to discern upward of 4 GPa. These spectral changes were reversible up to the pressures tested. Figure 2C shows fluorescent photographs of the nanocrystals fluorescing (red) at various pressures when excited by green laser (532 nm). The fluorescence diminished with pressure and disappeared above 3 GPa. Fluorescence may have diminished directly as a result of the structural changes with pressure, or possibly due to reabsorption by the crystal as the absorption spectrum broadens and shifts. The fluorescence returns and brightens as pressure is gradually released (Figure 2C). In order to test the mechanical stability of the nanocrystals, XRD spectra (Figure 3A) were collected as external pressure was gradually applied until the characteristic pattern disappeared. The pattern shifts to larger diffraction angles with increasing pressure, as expected, and the peaks broadened until many merged or no longer appeared. While the patterns are difficult to discern upward of 3 GPa, they could still be indexed within the same space group up to 4.6 GPa (Figure 3B). Beyond that, the crystal structure morphs until the nanocrystals appear to have been completely crushed by 9 GPa and do not rearrange/reorganize after releasing pressure. Simulations of the nanocrystals were performed using the semiempirical PM6 method to study the geometries and energetics of the pressurized systems.15 The molecules were placed in a periodic cell of dimensions determined by indexation of the XRD patterns at a given pressure. Initial atomic positions were taken as the fractional coordinates of the X-ray crystal structure at ambient pressure, then the structure was allowed to relax except for the core Zn atom positions. Figure 3C shows how the calculated crystal binding energy relates to the shrinking unit cell dimensions with pressure. The calculations exhibited an energetic critical point for the crystal stability around 4.6 GPa, in agreement with the experimental results where the XRD pattern could only be indexed in the same space group up to this point. On examining the structural changes (Figure 3B) that occur with initial compression (reversible region), it was observed that the percent shrinkage was greater in the c-direction than in the (equivalent) a- and b-directions. In order to rationalize this disproportionality, simulations were again performed on the nanocrystals, this time artificially restrained to shrink either along the c-axis alone or in the a- and b-directions only (together to preserve symmetry). The shrinkages were set to 2% and 4% for the a- and b-dimensions, corresponding approximately to the observed shrinkages at 3.27 and 4.64 GPa, respectively. For the c-axis, shrinkages were set at 5% and 9%, corresponding to the observed shrinkages at 1.23 and 4.64 GPa. As before, initial atomic positions were taken as the fractional coordinates of the X-ray crystal structure at ambient pressure, then the structure was allowed to relax except for the core Zn atom positions. The results rationalize the shrinkage patterns obtained from the XRD indexation (Figure 3B, and Figure S2 in the Supporting Information), with larger shrinkage along c having similar effect upon crystal-binding energy as smaller shrinkage in A–B directions. The equipartition of energy between a–b plane and the c-axis implied hydrostatic pressure transmission in the crystals. Since a better understanding of the mode of crystal deformation may be useful in tweaking properties of such molecular crystal systems, we examined the physical distortion of the molecules within the crystals under pressure. Figure 4 contrasts the geometrical arrangement within crystals at 4.64 GPa (the highest pressure which could still be indexed in SG148) and at ambient pressure. It was observed that the Zn–Npyridyl distance (core to ligand) decreased less with pressure than the overall intermolecular distance (Zn–Zn) between ligated neighbors. Note that whilst the Zn–Npyridyl distance decreases from 2.34 to 2.19 Å, the intra-molecular Zn–Nporphyrin distance remained rigid at 2.07 Å). To accommodate this difference, the pyridyl group tilts and is sandwiched flatter between the two porphyrins (Figure 4), as measured by the angles Zn–Npyridyl–Cipso (pyridyl relative to molecule to which it is ligated) and Npyridyl–Cipso–Cmeso (relative to molecule to which it belongs). As noted above, every molecule in the unit cell has identical π–π interactions with two molecules in adjacent cells (plus/minus c-direction). The energetics of the π–π interactions with varying intermolecular distance were then studied using a chain of π-stacked molecules to isolate the effects of ligation from π-stacking. Figure 5A illustrates a line of π-stacked molecules along the c-direction. Rather than simulating a single π–π interactions with two molecules, a chain length of five molecules was used to reduce edge effects. The molecular geometry was optimized energetically while constraining Zn atom positions to certain Zn–Zn distances: the original c-axis length was limited to a 5% reduction to match c-axis shrinkage at 1.2 GPa, whereby the a-axis has only shrunk 1%; then to a 9% reduction to match the peak c-axis shrinkage at 3.3 and 4.6 GPa; and lastly, to a 15% reduction (a hypothetical distance to see if the c-axis shrinkage is limited by π-stacking). Note that the absence of neighboring molecules in a- and b-directions will allow geometry enhancements for optimal π-interactions. The results shown in Figure 5B suggest that π-stacking was not the limiting factor on shrinkage in the c-direction, but rather the effect of c-shrinkage on axial ligation geometry. In summary, we demonstrated an important mechanical compression method to tune and/or reversibly control both structural and optical properties in molecular nanocrystals. Investigation through HP-WAXS in combination with theoretical calculation showed that high-pressure-induced stress can effectively tune microstructure of molecular nanocrystals and manipulate their mechanical stability. More importantly, the optical properties of the molecular crystals can be tuned systematically with applied external forces. This method is simple and should be extendable to other ­self-assembled molecular and polymeric systems. The ability to exert rational control over atomic bonds and bond angles provides new opportunities for interrogation of molecular coupling and energy related applications in photocatalysis,5 pressure sensing,16 molecular nanoelectronics, and nanophotonics.17 F.B., B.L., and K.B. contributed equally to this work. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. The Cornell High Energy Synchrotron Source (CHESS) was supported by the National Science Foundation and the National Institutes of Heath/National Institute of General Medical Sciences under NSF award DMR-0936384. F.B. acknowledges the support from the National Natural Science Foundation of China (21422102, 21171049, and 50828302). Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. 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