© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1989 wileyonlinelibrary.com COMMUNICATION Pressure-Tuned Structure and Property of Optically Active Nanocrystals Feng Bai,* Binsong Li, Kaifu Bian, Raid Haddad, Huimeng Wu, Zhongwu Wang, and Hongyou Fan* Prof. F. Bai Key Laboratory for Special Functional Materials of the Ministry of Education Henan University Kaifeng 475004, P. R. China E-mail: baifengsun@gmail.com Dr. B. Li, Dr. K. Bian, Dr. H. Wu, Prof. H. Fan Sandia National Laboratories Advanced Materials Laboratory 1001 University Blvd. SE, Albuquerque, NM 87106, USA E-mail: hfan@sandia.gov Dr. R. Haddad, Prof. H. Fan Department of Chemical and Biological Engineering Center for Micro-Engineered Materials University of New Mexico Albuquerque, NM 87131, USA Dr. Z. Wang Cornell High Energy Synchrotron Source Cornell University Ithaca, NY 14853, USA DOI: 10.1002/adma.201504819 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 trans- formation 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 transforma- tion of hydrogen bonded nanomaterials. [12,13] Despite these pro- gresses 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 proper- ties. 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 con- trolled. 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. Investiga- tion through high-pressure, spectroscopy, HP-WAXS in combi- nation 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 Nanocrystals with unique morphology are relevant to a wide variety of optical and electronic applications such as photo- catalysis, sensing, etc. [1–4] Precise control of structural para- meters through nanoscale engineering to improve optical and electronic properties of functional nanocrystals continu- ously remains an outstanding challenge. Previous work has been conducted largely at ambient pressure and relies on spe- cific 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 non- covalent 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 nanocrys- tals 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 Adv. Mater. 2016, 28, 1989–1993 www.advmat.de www.MaterialsViews.com