Photoluminescent Anisotropy Amplification in Polymorphic Organic

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Photoluminescent Anisotropy Amplification in Polymorphic Organic Nanocrystals by Light-Harvesting Energy Transfer Meng-Jia Sun, Yingying Liu, zeng wei, Yong Sheng Zhao, Yu-Wu Zhong, and Jiannian Yao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02055 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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Photoluminescent Anisotropy Amplification in Polymorphic Organic Nanocrystals by Light-Harvesting Energy Transfer Meng-Jia Sun,†,§,‡ Yingying Liu,†,§,‡ Wei Zeng,†,§ Yong Sheng Zhao,*,†,§ Yu-Wu Zhong,*,†,§ and Jiannian Yao*,†,§ †Key

Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China §University

of Chinese Academy of Sciences, Beijing 100049, China

Supporting Information Placeholder ABSTRACT: Polymorphism and anisotropy are fundamental phenomena of crystalline materials. However, the structuredependent photoluminescent (PL) anisotropy in polymorphic organic crystals has remained unexplored. Herein, two polymorphic nanocrystals, green-emitting nanorods (PtD-g) and yellow-emitting nanoplates (PtD-y), were obtained from a platinum(II)- -diketonate complex. The PtD-y crystals display remarkable PL anisotropy with an anisotropy ratio of up to 0.87 whereas the PtD-g crystals are nearly unpolarized. The polarization properties are rationalized on the different molecular packing of these crystals. By light-harvesting energy transfer, the PtD-y crystals are successfully used to amplify the emission polarization of a red-emitting platinum acceptor (PtA) doped into the donor crystalline matrix, which is otherwise weakly polarized as pure crystals.

Organic and organometallic crystals are commonly formed as cooperative results of various noncovalent interactions, including hydrogen/halogen bonding, stacking, metallophilic interactions and so on.1-5 These interactions are easily influenced by external conditions such as light, solvents, and temperature, providing polymorphyic crystal structures from a particular chemical substance.6 By taking advantage of crystalline polymorphism, many types of organic and organometallic aggregates with enormous structural diversity and photoluminescent (PL) properties have been reported.7-11 However, there are few examples on the use of molecular doping techniques12-14 in polymorphic materials for exploring the structure-property relationship between different polymorphic states. The long-range anisotropic packing of chromophores in ordered assemblies often leads to distinct PL anisotropy, which could be used to reveal the molecular orientation in nanocrystals and induce the PL anisotropy of guest molecules by highly-efficient energy transfer (EnT) processes.15-18 Because of their intrinsic ordered molecular packing and efficient exciton migration, EnT processes are generally accelerated in organic crystals.19-21 Although PL color tuning and emission amplification via EnT have been

demonstrated in various molecularly doped crystals,13a,22a,23-26 EnT coupled with efficient PL anisotropy transfer in wellordered polymorphic nanostructures has not been reported yet. Such study is of significance in probing the donoracceptor orientation relationship in doped crystals and manipulating polarized PL signals in different polymorphic environment. To date, a vast number of photofunctional metal complexes have been reported; however, none of them have been illustrated to show PL anisotropy transfer in binary nanostructures. Square planar Pt complexes are known to form light-emitting polymorphic nanostructures by Pt-Pt and/or - stacking.4,27-31 We selected two platinum(II)- diketonate complexes as the EnT donor (PtD) and acceptor (PtA), respectively (Figure 1a). The similar structures and matched energy levels between PtD and PtA are beneficial for studying the EnT process in crystalline states. Two polymorphic crystals, PtD-g and PtD-y, were obtained from PtD, displaying different polarization-sensitive PL properties. The superior PL anisotropy of PtD-y can be efficiently transferred to acceptor with remarkable amplification along with the light-harvesting EnT process in doped crystals. In CH2Cl2, PtD and PtA display typical absorptions of platinum(II) complexes with multiple singlet and triplet charge-transfer transitions from 350 nm to visible regions, including metal-to-ligand (MLCT) and ligand-to-ligand charge-transfer (Figure 1e).32,33 PtD displays broad PL spectrum with an emission peak at 530 nm and PtA emits at 603 nm with distinct vibronic structures. PtD-g and PtD-y were obtained by a quick reprecipitation method with an emission quantum yield of 22.5% and 39.8%, respectively (Table S1). PtD-g was prepared by injecting a low concentration of PtD (2 mM) in CH2Cl2 to a large amount of propanol as poor solvent. PtD-y was formed using the same procedure with a relatively higher concentration (10 mM). In the middle-concentration condition, a mixture of PtD-g and PtD-y were obtained (Figure S8). The emission of PtD-g shows characteristic vibronic structures of ligand-centered 3F F (3IL) transition; while that of PtD-y is structureless. These emissions are attributed to an admixture of 3MLCT and 3IL transitions,32 which is also supported by TDDFT calculations (Figure S9).

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in which the highly anisotropic donor surroundings absorbed photons and funneled the excitation energy to the molecularly distributed acceptor in a polarized form. The same polarized orientation between the donor and acceptor indicates that the of acceptor is also aligned parallel to that of donor on the (001) plane. In contrast, the acceptor doped in the unpolarized PtD-g crystal displays weak PL anisotropy ( = 0.22 ± 0.02) without obvious amplification effect (Figures 4f and S26). This suggests that the acceptor molecules adjust their orientations to the donor matrix accompanied by the transfer of PL anisotropy. In conclusion, we have successfully prepared two polymorphic crystalline nanostructures, PtD-g and PtD-y, from a platinum(II)- -diketonate complex. The excellent light-harvesting and EnT properties in these materials are realized by doping a low-energetic red-emitting platinum acceptor. Moreover, remarkable PL anisotropy coupled with EnT process was observed in the pure and doped PtD-y crystal. The highest PL anisotropy ratio reaches to 0.87 and 0.82 for the pure PtD-y donor and doped acceptor emission, respectively, indicating the excited anisotropic energy of the donor could be effectively transferred to the acceptor with a significant amplification effect. This work demonstrates that packing mode has a significant impact on the excited state and PL anisotropy properties of crystalline materials, suggesting the great potential of polymorphic nanostructures for applications in multifunctional nanophotonic devices.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS publications website at DOI: 10.1021/jacs.xxxxxxx. Additional experimental details and data (PDF) Crystal data of PtD-g, PtD-y and PtA in the cif format

AUTHOR INFORMATION Corresponding Authors *[email protected] (Y.-W.Z.);

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*[email protected] (Y.S.Z.); *[email protected] (J.Y.)

ORCID Yu-Wu Zhong: 0000-0003-0712-0374 Yong Sheng Zhao: 0000-0002-4329-0103

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (grants 91622120, 21872154, 21601194, and 21472196) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant XDB12010400) for funding support.

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