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Functional Inorganic Materials and Devices

Facile and Equipment-Free Data Encryption and Decryption by Self-Encrypting Pt(II) Complex Jiajia Kang, Jun Ni, Mengmeng Su, Yanqin Li, Jian-Jun Zhang, Huajun Zhou, and Zhong-Ning Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21221 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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ACS Applied Materials & Interfaces

Facile and Equipment-Free Data Encryption and Decryption by Self-Encrypting Pt(II) Complex Jiajia Kang,† Jun Ni,*,†,‡ Mengmeng Su,† Yanqin Li,† Jianjun Zhang,† Huajun Zhou,*,§ and Zhong-Ning Chen*,‡ †College

of Chemistry, Dalian University of Technology, Linggong Road No. 2, Dalian 116024,

P. R. China ‡State

Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, Fuzhou 350002, China §High-Density

Electronics Center, University of Arkansas, Fayetteville, Arkansas 72701, United

States KEYWORDS: Pt(II) complex, self-encryption, stacking mode-intervened luminescence switching, data encryption-decryption, equipment-free

ACS Paragon Plus Environment

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ABSTRACT: Luminescence switching materials (LSMs) are vital to various data securityrelated techniques including data encryption-decryption. Here we reported a family of pseudopolymorphs

based

on

a

diimine-platinum(II)

complex,

Pt(Me3SiC≡CbpyC≡CSiMe3)(C≡CC6H4Br-3)2 (1) and systematically studied the influence of stacking modes on luminescence switching behaviors. Upon exposure to heat or THF vapor, these pseudopolymorphs exhibit unusual stacking mode-intervened luminescence switching (SMILS) property that non-columnar and quasi-columnar pseudopolymorphs undergo singleand multi-step conversion processes respectively to same non-columnar products. Systematic studies revealed that the unique SMILS behavior is caused by the existence of stable intermediate products as well as different conversion processes of pseudopolymorphs with distinct stacking modes. Such new property leads to the self-encryption function of 1 which is very important for improving the existing data encryption-decryption technique. On this basis, we developed a facile, reusable, equipment-free technique with 1 as the only starting material and realized data encryption-decryption successfully.

INTRODUCTION Data security has played an increasingly important role in every corner of our society including people’s daily lives, economy, and military fields. For protecting data security, a few kinds of techniques including security printing, anti-counterfeiting, and data encryption-decryption have been developed.1-16 Among them, the data encryption-decryption technique is considered one of the securest methods because the data can be well hidden in the background at the encryption state and then displayed on the background under specific conditions. To realize the data encryption and decryption, both data and background must have the same initial color and luminescence, meanwhile, they must exhibit some differences under specific conditions.


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Luminescence switching materials (LSMs) that exhibit color and luminescence changes in response to external stimuli have been used as functional material (data-carrying material) in data encryption-decryption devices.1-6 In addition, some luminescent materials with long phosphorescence lifetime were also used for achieving the data encryption-decryption.7,8 For example, a novel device has been reported using [ppy2IrNH]+(PF6)− and BODIPY dye as functional and background materials which exhibited excellent data encryption and decryption performance.8 However, such techniques have several inherent limitations which severely restricts its broad deployment. First, the choice of the background material is very limited due to the required features mentioned above. Second, the two materials significantly differ in compositions and thus their initial colors and luminescent properties are not close enough to achieve full data encryption. Third, the device can neither be regenerated nor be reused because data erasure can only be achieved by destroying the device, leading to high cost. How to improve the conventional data encryption-decryption technique faces the big challenges. Recently, Huang and co-workers successfully realized the data encryption-decryption by a unique luminescent Ir(III) complex without adding other background material.9 Under electric stimulus, this complex exhibited no changes in color and luminescence but its emission lifetime changed significantly by comparison to the initial sample. This property makes the complex can be used as both functional and background material and realize self-encryption by a Pt electrode “pen”. Moreover, due to the different emission lifetime, the information can be decrypted and recognized by photoluminescence lifetime imaging microscopy (PLIM). Importantly, as only one starting material was used, the device exhibited good regeneration and reusability. This new strategy points the way for the development of encryption-decryption technology. Inspired by it, we believe that if LSMs also have the self-encryption function, the data reading can be


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significantly simplified because color and luminescence changes can be readily recognized by the naked eye. That is to say, data encryption-decryption can be realized without using any equipment, thus further improving the data encryption-decryption technique. To achieve the selfencryption, LSMs must satisfy the two conditions at the same time. They have two isomers with the same color and luminescence (including emission lifetime and quantum yield) which can be easily converted to each other and exhibit different color and/or luminescent changes to a same stimulus. However, existing LSMs are not meet the requirements because that for conventional luminescence switching property, all different isomers or pseudopolymorphs (i and ii) will be indiscriminately transformed to the same product (iii) under the same stimulus (Figure 1a).17-19 Luminescent square-planar platinum(II) complexes have been well studied due to their rich excited state properties as well as the unique self-assembly and aggregation abilities in both liquid and solid states.20-25 Furthermore, the spectroscopic properties of them are usually sensitive to the changes of intermolecular interactions such as Pt–Pt contact, hydrogen bonding or aromatic π–π stacking caused by external stimuli, leading to their versatile luminescence switching behaviors and wide range of applications.17,18,23-28 Previous studies revealed that diimine Pt(II) bis-acetylide complexes can form different pseudopolymorphs with distinct stacking modes through various intermolecular interactions, and some of them have the same color and luminescence properties.28-31 Recently, a few Pt(II) complexes were reported to exhibit unusual irreversible luminescence switching, two-step luminescence switching and shape memory function properties, which demonstrated that the stacking modes may affect luminescent switching properties to some extent.32-35 Although the roles of stacking mode played in the luminescent switching process are still unclear, all above findings suggested that diimine Pt(II)


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bis-acetylide complex may be the potential luminescence switching material with self-encryption function.

Figure 1. (a) Conventional luminescence switching in which isomers or pseudopolymorphs (i and ii) with different stacking modes are converted to the same product (iii) under a same stimulus. (b) Stacking mode-intervened luminescence switching (SMILS) in which pseudopolymorphs (iv) and (v) with different stacking modes undergo two- and one-step respectively to afford the same final product (vii) under a same stimulus. (c) Schematic diagram of the equipment-free encryption-decryption technique enabled by SMILS. Herein we reported a family of pseudopolymorphs based on a diimine-platinum(II) complex, Pt(Me3SiC≡CbpyC≡CSiMe3)(C≡CC6H4Br-3)2 (1), bearing 5,5’-Bis(trimethylsilyethynyl)-2,2’bipyridine and 3-bromophenylacetylide ligands. The choice of two ligands were conductive to the formation of planar Pt(II) complex so that more enriching excited states can be achieved through the formation of intermolecular Pt-Pt interactions. Furthermore, non-axisymmetric 3bromophenylacetylene ligand not only has the strong ability to form intermolecular interactions


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with different solvated molecules but also can lead to the various stacking structures of the complex, providing the opportunity for creating unusual luminescence switching behaviour. These pseudopolymorphs stacked in the quasi-columnar or non-columnar structures and exhibited unusual stacking mode-intervened luminescence switching (SMILS) properties to the same special stimuli. Upon heating or exposure to THF or toluene vapor, quasi-columnar stacking structures (iv) tend to be converted to non-columnar product (vii) in multi-step processes via quasi-columnar intermediates (vi) while non-columnar stacking structures (v) will be converted to the same non-columnar product (vii) in a single-step process (Figure 1b). Systematic studies revealed that the unique SMILS behavior is caused by the existence of stable intermediate products as well as different conversion processes of pseudopolymorphs with distinct stacking modes. To the best of our knowledge, it is the first systematic study of the correlation between the stacking mode and luminescence switching behavior. Moreover, the SMILS property in response to heat and THF vapor make 1 has the self-encryption function. On this basis, a simple SMILS-enabled technique using 1 as only starting material was designed and successfully realized the data encryption-decryption without needing any equipment (Figure 1c). This new technique is low-cost, reusable, facile operation and therefore holds potentials for broader deployment. RESULTS AND DISCUSSION Preparation and Structures. Complex 1 was synthesized in a high yield from the reaction between Pt(Me3SiC≡CbpyC≡CSiMe3)Cl2 and 3-bromophenylacetylene. 1 has good solubility and stability in common organic solvents and thus can be easily processed in solutions. Eight pseudopolymorphs of 1 including five yellow crystals, namely 1·½(CH2Cl2), 1·½(CHCl3), 1·Acetone, 1·¼(Toluene), and 1·Toluene, two orange crystals, namely 1·½(ClCH2CH2Cl) and


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1·THF(O), and one red 1·THF(R), were separately crystallized out by layering petroleum ether on the corresponding solutions of 1 (Table S1). Both 1·THF(R) and 1·Toluene are intermediates during the formation of 1·THF(O) and 1·¼(Toluene), respectively. According to the X-ray crystallographic analyses, eight pseudopolymorphs containing the same Pt(II)–moieties are stacked in two modes: quasi-columnar and non-columnar (Figure 2). In 1·½(CH2Cl2) (Figure 2a), 1·½(CHCl3) (Figure 2b) and 1·Toluene (Figure 2c), adjacent Pt(II)-moieties are arranged in an antiparallel packing pattern (Figure S1) to form quasi-columnar stacking structures with the inclined angles being 74.8, 85.4, and 72.2°, respectively (Note: the inclined angle of a columnar structure is 90°). The shortest Pt···Pt distances are 4.664, 4.232, and 4.127 Å, respectively, indicating the absence of intermolecular Pt–Pt contacts in them (see Supplementary Table 2). In contrast, adjacent Pt(II)-moieties in 1·THF(R) (Figure 2d) are alternately stacked in staggered and antiparallel packing patterns (Figure S1) to also afford a quasi-columnar stacking with its inclined angle being 73.2°. Interestingly, the staggered packing pattern enables adjacent moieties to be more compact, resulting in the shortest Pt···Pt distance being 3.363 Å (Table S2), stronger intermolecular Pt–Pt interaction, and a deeper color compared to other pseudopolymorphs. In the four non-columnar stacking structures, adjacent moieties are packed in an antiparallel pattern (stacking with their inclined angles being smaller than 45° is considered non-columnar). In 1·Acetone (Figure 2e) and 1·THF(O) (Figure 2f), the shortest Pt···Pt distances are larger than 4.5 Å (Table S2), and adjacent moieties are packed into scalene cylinder-shaped structures with the inclined angles being 39.7° and 41.7°, respectively. In 1·¼(Toluene) (Figure 2g) and 1·½(ClCH2CH2Cl) (Figure 2h), the shortest Pt···Pt distances are 4.309 and 3.606 Å (Table S2), respectively and neighboring dimeric motifs are connected through C-H···π(C≡C) and C-H···Br bonds to form herringbone-shaped structure (Figure S2). Their dihedral angles between adjacent


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packing motifs being 45.9° and 64.3°, respectively, while their inclined angles can both be taken as 0°.

Figure 2. The chemical structure of complex 1 and stacking modes of eight pseudopolymorphs. (a–d) Quasi-columnar stacking mode and (e–h) Non-columnar stacking mode with (e–f) and (g– h) being the scalene cylinder and herringbone stacking, respectively. Note: atoms in solvent molecules, Pt, and other atoms are represented by the space-filling (green: Cl; gray: C; red: O; light-gray: H), standard, and wire/stick modes, respectively. The diverse stacking structures of these solvated forms are mainly attributed to the location and space volumes of different solvent molecules as well as rich intermolecular interactions. Generally, halohydrocarbon molecules are located between two phenylacetylene ligands of the Pt(II)-moieties, favoring the construction of quasi-columnar stacking structure (Figure S3). However, the relative large ClCH2CH2Cl molecule can cause a non-columnar stacking structure


8

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to achieve a close packing. In 1·Acetone, acetone molecules are held near the pyridinyl ligands of the Pt(II)-moiety via strong C-H···O hydrogen bonds and the π(C=O)···π (pyridine ring) interactions (Figure S3), resulting in its non-columnar stacking. Interestingly, two pairs of solvated forms, i.e., 1·THF(R) and 1·THF(O), 1·Toluene and 1·¼(Toluene), crystallize with the same solvent molecules in different stacking modes. In 1·THF(R) and 1·THF(O), THF molecules in both structures are perpendicular to or coplanar with the Pt(II)-moieties (Figure S3), respectively, resulting in two different stacking modes. Although the toluene molecules in 1·Toluene and 1·¼(Toluene) are both perpendicular to the Pt(II)-moieties (Figure S3), the severely disordered toluene molecules in 1·¼(Toluene) effectively separate the antiparallel dimeric motifs and make them arrange in different directions to achieve a close packing, leading to a non-columnar stacking structure. Photophysical Properties. The UV-Vis absorption spectrum of 1 in CH2Cl2 at room temperature (Figure S4) is similar to those of reported Pt(II)-complexes with the Me3SiC≡CbpyC≡CSiMe3 ligand.28 The high-energy bands at 270–370 nm are mainly attributed to the ligand-centered transition. The broad low-energy band with the maximum centered at 435 nm is ascribed to dπ(Pt)→π*(Me3SiC≡CbpyC≡CSiMe3) 1MLCT (metal to ligand chargetransfer) and π(C≡CC6H4Br-3)→π*(Me3SiC≡CbpyC≡CSiMe3) 1LLCT (ligand to ligand chargetransfer) transitions. In its degassed CH2Cl2 solution, 1 emits a bright orange luminescence peaked at 603 nm which can be attributed to the 3MLCT/3LLCT excited states (Table 1 and Figure S4). Figure 3 displays the photographic images under ambient light and UV light irradiation (λex= 365 nm), UV-Vis absorption and emission spectra of eight pseudopolymorphs in the powder state while Table 1 lists their luminescence data. Suspension solutions of 1 in CH2Cl2 were


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deposited to the quartz slides and dried by nitrogen flow, resulting in evenly distributed powders on the slides. Then these quartz slides were exposed to corresponding VOC vapors for 15 min (for 1·THF(R) and 1·Toluene, the exposure time was ca. 1 min and 2.5 min, respectively). The five yellow forms have very similar UV-Vis absorption with the low-energy absorption in the range at 455–554 nm arising from both 1LLCT and 1MLCT transitions. The low-energy absorption of orange 1·½(ClCH2CH2Cl) and 1·THF(O) exhibits slight red-shifts. In comparison, that of red 1·THF(R) presents a much more pronounced red-shift in the range at 540–630 nm while extending to 720 nm mainly due to its short Pt···Pt distances and significantly boosted PtPt interactions.

Figure 3. (a) Photographic images under ambient light and UV light irradiation (λex = 365 nm), (b) UV-Vis absorption and (c) emission spectra of eight pseudopolymorphs of 1 in the powder state.


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Upon irradiation at 360 < λex < 500 nm, the five yellow forms emit very similar and bright yellow luminescence with vibrionic-structured emissions peaked at 558–566 and 603–610 nm due to dπ(Pt) → π*(Me3SiC≡CbpyC≡CSiMe3) 3MLCT and π(C≡CC6H4Br-3) → π*(Me3SiC≡CbpyC≡CSiMe3) 3LLCT triplet states. 1·½(ClCH2CH2Cl) and 1·THF(O) have similar emission origins as those of the yellow forms and display orange luminescence with emissions peaked at 610 nm for 1·½(ClCH2CH2Cl) and 617 nm for 1·THF(O), respectively. In contrast, 1·THF(R) emits a red luminescence at 718 nm with lifetime and quantum yield being 0.189 µs and 0.9%. The dramatic red-shift of 1·THF(R) in luminescence compared to other solvated forms is due to its short Pt···Pt distance, significantly enhanced intermolecular Pt–Pt contact, and the resultant change of the lowest-excited states from 3MLCT/3LLCT states to 3MMLCT (metal-metal to ligand charge-transfer)/3LLCT states.28 Table 1. Luminescence data for 1 in CH2Cl2 solution and eight pseudopolymorphs in solid state. sample

medium

em (nm)

em (s)

em (%)

1

CH2Cl2 solution

603

0.279

5.06a

1·½(CH2Cl2)

solid state

561, 607, 660 sh

0.906

12.06

1·½(CHCl3)

solid state

561, 607, 660 sh

0.922

12.03

1·Acetone

solid state

566, 610, 662 sh

0.886

11.59

1·THF(O)

solid state

578, 617, 671 sh

0.613

6.26

1·THF(R)

solid state

718

0.189

0.90

1·Toluene

solid state

562, 606, 654 sh

1.309

11.70

1·¼(Toluene)

solid state

558, 603, 643 sh

1.431

10.62

1·½(ClCH2CH2Cl)

solid state

610

0.429

4.07

a The

quantum yield was estimated relative to [Ru(bpy)3](PF6)2 in CH3CN as the standard (Φem =

6.2%).


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Vapor-Induced SMILS. Upon exposure to THF or toluene vapor, non-columnar 1·Acetone and 1·½(ClCH2CH2Cl) are directly transformed to non-columnar 1·THF(O) or 1·¼(Toluene) (Figures S5 and S6), while quasi-columnar 1·½(CH2Cl2) and 1·½(CHCl3) undergo a two-step conversion process to 1·THF(O) or 1·¼(Toluene) via quasi-columnar intermediates 1·THF(R) or 1·Toluene (Figures 4 and S7-S9). This kind of vapor-induced stacking mode-intervened switching (SMILS) behavior is quite different with the conventional vapoluminescence, revealing that stacking mode is an important factor influencing the vapoluminescent property of the complex. Similar vapor-induced SMILS as such is also observed among pseudopolymorphs containing THF and toluene: non-columnar 1·THF(O) and 1·¼(Toluene) can be reversibly converted to each other in one step upon exposure to toluene or THF vapor. In contrast, upon exposure to toluene or THF vapor, quasi-columnar 1·THF(R) or 1·Toluene undergo a two-step conversion process to 1·¼(Toluene) or 1·THF(O) via 1·Toluene or 1·THF(R), respectively (Figures S5, S6 and S10). Here two issues are worthy to note. First, conventional switching is also observed in the above vapor-induced SMILS. For example, 1·Acetone and 1·½(ClCH2CH2Cl) can be converted to 1·THF(O) or 1·¼(Toluene) upon exposure to THF or toluene in one step. Second, non-columnar pseudopolymorphs cannot be converted to quasi-columnar intermediate, i.e., 1·THF(R) or 1·Toluene. On this basis, an empirical rule is developed to differentiate non-columnar from quasi-columnar stacking structures that will be discussed in the thermochromic luminescence section: if one structure cannot be converted to 1·THF(R) or 1·Toluene upon exposure to THF or toluene, it could be non-columnar. Figure 4b–d confirms the two-step conversion processes. Upon exposure to THF vapor, quasicolumnar 1·½(CHCl3) was changed to red 1·THF(R) in about 1 minute and then to orange-


12

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yellow 1·THF(O) in about another three minutes (Figure 4b). Both luminescence (Figure 4c) and PXRD analyses (Figure 4d) indicate that in the first step the starting 1·½(CHCl3) continuously diminishes to disappear while 1·THF(R) starts to appear, grow, and then dominate while in the second step 1·THF(R) continuously diminishes to disappear while 1·THF(O) continuously grows to dominate. The first step conversion is faster than the second step possibly due to both 1·½(CHCl3) and 1·THF(R) being quasi-columnar while 1·THF(O) being non-columnar. The following two observations may help to further understand the underlying mechanisms for vapor-induced SMILS. First, upon exposure to CH2Cl2, CHCl3, acetone, or ClCH2CH2Cl vapor, all other pseudopolymorphs, be quasi- or non-columnar, are directly transformed to 1·½(CH2Cl2) (Figure S11), 1·½(CHCl3) (Figure S12), 1·Acetone (Figure S13), and 1·½(ClCH2CH2Cl) (Figure S14), respectively, demonstrating the vapor-induced SMILS only occurs in the response to THF or toluene vapor. Therefore, the activation energies required for the conversions among same stacking modes and for those between different stacking modes are only slightly different from each other. This can explain why the conventional vapoluminescence is usually reversible and not affected by stacking modes. Second, upon exposure to THF, it takes one minute for quasicolumnar 1·½(CHCl3) to be converted to quasi-columnar 1·THF(R) and additional three minutes to be further converted to non-columnar 1·THF(O), suggesting 1·THF(R) is a intermediate product. In contrast, non-columnar pseudopolymorphs are converted to non-columnar 1·THF (O) directly. Therefore, we speculate vapor-induced SMILS should result from the existence of the stable intermediate products and the different conversion processes of pseudopolymorphs with distinct stacking modes.


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Figure 4. Vapor-induced SMILS. (a) The different conversion routes of non-columnar and quasi-columnar pseudopolymorphs in response to THF or toluene vapor, indicating noncolumnar and quasi-columnar forms are converted to the same product via one and two steps, respectively. (b–d) The response of 1·½(CHCl3) upon exposure to THF vapor, revealing the twostep conversion from 1·½(CHCl3) to 1·THF(O) via 1·THF(R). (b) photographic images showing the time-dependent, two-step changes of 1·½(CHCl3) powder upon exposure to THF vapor, (c) evolution of solid-state luminescence spectra and (d) PXRD diagrams of 1·½(CHCl3) in response to THF vapor, all results confirm the two-steps conversion of 1·½(CHCl3) to 1·THF(O) via 1·THF(R). Heat-Induced SMILS. Figure 5 summarizes the heat-induced SMILS and the thermal stabilities of eight pseudopolymorphs. Upon heating, the four non-columnar stacking


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pseudopolymorphs are all converted to 1-c in single-step processes while the four quasicolumnar are converted to 1-c in multi-step processes (Figure 5a). Table 2. Luminescence data for five desolvated forms obtained from heating and grinding the eight pseudopolymorphs. sample

medium

em (nm)

em (s)

em (%)

1-a

heated

957

0.012

N/A

1-b

heated

564, 610, 656 sh

0.267

2.34

1-c

heated

563, 607, 652 sh

0.219

10.25

1-d

heated

565, 611, 661 sh

1.203

11.89

1-e

ground

742

0.211

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