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Multicolor Ultralong Organic Phosphorescence through Alkyl Engineering for 4D Coding Application Xuan Wang, Huili Ma, Mingxing Gu, Changqing Lin, Nan Gan, Zongliang Xie, He Wang, Lifang Bian, Lishun Fu, Suzhi Cai, Zhenguo Chi, Wei Yao, Zhongfu An, Huifang Shi, and Wei Huang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Chemistry of Materials

Multicolor Ultralong Organic Phosphorescence through Alkyl Engineering for 4D Coding Application Xuan Wang,†,# Huili Ma,†,# Mingxing Gu,† Changqing Lin,† Nan Gan,† Zongliang Xie,‡ He Wang,† Lifang Bian,† Lishun Fu,† Suzhi Cai,† Zhenguo Chi,‡ Wei Yao,† Zhongfu An*†, Huifang Shi*† and Wei Huang*†,§ †Key

Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China ‡PCFM Lab, GDHPPC Lab, KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Material and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, China. Institute of Flexible Electronics (IFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi’an 710072, China. §

ABSTRACT: Luminogens with colorful ultralong organic phosphorescence (UOP) are in high demand for various potential applications in optoelectronics. Herein we reported a concise approach to tuning ultralong organic phosphorescence based on same chromophores of carbazole and phthalimide units through alkyl engineering. With flexible alkyl increasing, the UOP emission colors can be controllably tuned from green to orange along with lifetime variation. Furthermore, these phosphors were endowed with unexpected visible-light excitation, mechanochromism and mechanoluminescence (ML) properties simultaneously. Additionally, the colorful UOP with diverse emission lifetime was firstly applied to 4D code for information encryption. These findings will open a door to explore multifunctional organic phosphorescence materials and expand their potential applications.

Information security has drawn considerable attentions as the exponential growth of information communications in the era of big data, which significantly promotes the development of technologies for information coding.1,2 To date, coding techniques can be divided into 1D coding based on lines and spaces with different widths, 2D coding with matrix and stacking as well as 3D coding with blue, green, red and black colorful modules (Figure 1a), which came up short for coding, encryption and storage density of information in practical applications owing to the limited storage capacity and low security.3-5 More dimensional coding techniques are urgently needed for data anti-counterfeiting. Recently, ultralong organic phosphorescence (UOP), that is, organic persistent luminescence, demonstrates great potential for coding techniques owing to its long-lived emission lifetime, large Stokes shift and rich excited state characteristics.6-9 With great efforts on enhancing intersystem crossing (ISC) between singlet and triplet excited states by introduction of heavy halogen atoms10 and aromatic carbonyl groups11 and suppressing nonradiation transient through construction of rigid environment12 with some feasible strategies, such as crystalline inducement,1316 H-aggregation,17-19 host-guest doping,20-22 polymerization,2326 etc., series of UOP materials with ultralong lifetimes,27-29 high quantum yields30 or colorful emissions31 have been developed for optoelectronic applications ranging from display and chemosensor to bioimaging and anti-counterfeiting.32-38 Notably, UOP emission color can be tuned with distinct molecular architectures39,40 or different crystalline arrangements.41 Despite great success among these UOP

materials, it remains a formidable challenge to tune emission color of ultralong organic phosphorescence within the same chromophore for novel applications.

Figure 1. The development of coding technology and design of target molecules. (a) Progress of coding technology. (b) The design concept for target UOP molecules with multi-functions through alkyl engineering. Note that the color of the leaves represent the UOP emission of the target phosphors, respectively.

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Figure 2. Photophysical properties of aromatic imide derivatives in the solid state under ambient conditions. (a) Steady-state photoluminescence (black line) and phosphorescence (orange line) spectra of CzEL, CzPL, CzBL and CzFL. Insets show photographs of CzEL, CzPL, CzBL and CzFL under 365 nm UV light on and off. (b) Excitation spectra of CzEL (black line), CzPL (orange line), CzBL (blue line) and CzFL (green line) by monitoring emission peaks at 501, 553, 520 and 561 nm, respectively. The gray curve is a photoluminescence spectrum of an iPhone 6 LED. Insets: Photographs of CzEL, CzPL, CzBL and CzFL after removal of excitation by an iPhone LED. (c) Lifetime decay profiles of the ultralong phosphorescence bands of CzEL, CzPL, CzBL and CzFL phosphors. (d) Excitationphosphorescence mapping of the phosphors.

It is well known that alkyl engineering plays a critical role in rational modification of photophysical properties including absorbance, photoluminescence, charge transfer, etc., among organic optoelectronic functional materials with donor-acceptor architectures for potential applications via tuning molecular packing in solid state.42-44 Considering that molecular packing in crystalline state shows significant influence on UOP under ambient conditions, we propose that introduction of alkyl units may endow organic phosphorescence materials with unexpected functions in crystal. Thus, we synthesized a series of organic phosphors composed of electron-donating carbazole and electron-withdrawing phthalimide units as well as flexible alkyl chains. Carbazole is a typical building block for UOP45,46 and phthalimide demonstrates great potential for mechanoluminescence (ML),47,48 enabling UOP to be multifunctional (Figure 1b). With flexible alkyl chain variation, colorful ultralong phosphorescence from green to orange was obtained after cease of UV excitation, which can also be observed by visible-light irradiation. Simultaneously, both ML and mechanochromism were observed. Additionally, the colorful UOP with different emission lifetime feature were successfully applied to 3D/4D coding initially.

In this study, a series of aromatic imide derivatives, namely, 2-(2-(9H-carbazol-9-yl) ethyl) isoindoline-1,3-dione (CzEL), 2-(3-(9H-carbazol-9-yl) propyl) isoindoline-1,3-dione (CzPL), 2-(4-(9H-carbazol-9-yl) butyl) isoindoline-1,3-dione (CzBL) and 2-(5-(9H-carbazol-9-yl) pentyl) isoindoline-1,3-dione (CzFL) were readily synthesized via two-step reactions with yields of over 42% (Scheme S1). The chemical structures and purity of these target compounds were fully characterized by NMR spectroscopy, matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOFMS), elemental analysis, high-performance liquid chromatogram (HPLC) (Figure S1-S17) and X-ray single crystal diffraction. There is no impurity in the target compounds. Notably, both CzEL and CzBL in crystalline state show Z-like molecular configurations, distinctly different from CzPL and CzFL with L-shaped structures (Figure S18), indicating there exists odd-even effect of alkyl linkers on molecular configurations. The photophysical properties of these molecules were studied in detail through absorption, steady-state photoluminescence (PL) and excitation spectra in both solution and solid state. The absorption spectra of these molecules in

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Chemistry of Materials dilute dichloromethane (50 μM) were nearly consistent with peaks at 263,

Figure 3. Mechanoluminescence and mechanochromism properties of aromatic imide derivatives in solid state under ambient conditions. (a) PL spectra of CzFL before and after grinding. (b) CIE coordinate for CzFL before and after grinding. (c) PXRD of CzFL solid before and after grinding. (d) Mechanoluminescent spectrum of CzBL. Inset: ML image of CzBL as-prepared sample upon grinding with a spatula in dark room. (e) Dipole moments of CzEL, CzPL, CzBL and CzFL molecules.

295, 331 and 345 nm. Their steady-state PL spectra of these phosphors from CzEL to CzFL showed emission peaks at 357, 362, 360 and 370 nm in solution, respectively, which might be attributed to locally excited states (Figure S19). For CzEL and CzPL, there exist another emission bands at around 564 and 550 nm assigned to intramolecular charge transfer (ICT) states, respectively.49 Notably, the phosphorescence spectra of CzEL, CzPL, CzBL and CzFL in dilute dimethyl tetrahydrofuran (50 μM) and 5% wt doped in PMMA at 77 K were completely identical (Figure S21 and S22), indicating alkyl groups have less influence on their triplet excited states in single molecule level. In solid state, with alkyl length increasing from CzEL to CzFL, they exhibited sky blue, deep blue, green and blue emission under 330 nm excitation, respectively (Figure 2a). Surprisingly, these compounds showed colorful persistent luminescence ranging from green to orange with alkyl increasing under ambient conditions, after the UV-light excitation source was removed (SV1-SV4). Notably, phosphorescence is sensitive to oxygen.33 With atmosphere change from vacuum to oxygen, it was found that the intensity of persistent luminescence deceased for these phosphors (Figure S25), indicating the phosphorescence nature of the persistent luminescence. To the best of our knowledge, this is the first molecular system with multicolor UOP via alkyl engineering. From Figure 2b, we noticed that the excitation spectra showed a blue shift as alkyl chain increased. Moreover, CzEL, CzPL and CzBL phosphors can be excited by visible light, such as incandescent light, or even mobile phone

flashlight, whose multicolor phosphorescence can still last for several seconds (SV5-SV7). For CzFL crystal, UOP can hardly be observed after the LED source was removed, which was ascribed to inefficient irradiation due to a small overlap between the emission spectrum of the excitation source and the absorption spectrum of the phosphor (Figure 2b). The main emission bands of CzEL, CzPL, CzBL and CzFL exhibited ultralong lifetimes with 762, 605, 222 and 603 ms, respectively, in the crystalline state under ambient conditions (Figure 2c). In solution, there existed low PL quantum efficiency, which was as low as 2.0%. However, the quantum yields of these molecules showed great improvement in solid state, indicating crystallization-induced emission enhancement behavior. Remarkably, the quantum yields of CzEL in the crystal reached up to 61.6% (Figure S26). To further understand the ultralong phosphorescence of all compounds in solid state, 3D-scanning of excitationphosphorescence emission was performed. As shown in Figure 2d, the optimal excited wavelengths were in or close to visible light area. After excitation by simulated sunlight for 5 mins, the absolute illuminance of CzEL crystal remained at approximately 306 mcd m-2 after a delay time of 1 s and lasted up to 138 s within 0.32 mcd m-2 as recognized by naked eye. Other two phosphors, CzPL and CzBL, could last up to 74 and 123 s, respectively (Figure S27). Occasionally, it was found that these compounds also showed mechanochromism under external stimuli. As depicted

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in Figure 3a and 3b, CzFL showed an obvious red shift (26 nm) with a remarkable emission color change from as-prepared (450

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nm) to the grinding sample (476 nm). And CzEL, CzPL and

Figure 4. Investigation on mechanism of multicolor ultralong organic phosphorescence in crystal state. (a) Phosphorescence spectra and photographs of CzEL, CzPL, CzBL and CzFL crystals under 365 nm off with the time delay at 77 K. (b) Lifetime decay profiles of the ultralong phosphorescence bands of 500 and 554 nm for CzBL at 77 K. (c) Phosphorescence spectra of CzEL, CzPL, CzBL and CzFL and schematic representation of the arrangement of two chromophores in amorphous state. Note that the alkyls were left out in amorphous model. (d), (e) Intermolecular interactions in CzEL and CzFL crystals.

CzBL materials showed red-shift of 5, 19 and 11 nm before and grinding was ascribed to variation of crystalline degree, which after grinding, respectively (Figure S28-30). Notably, UOP was confirmed by powder X-ray diffraction in Figure 3c and emission can still be observed after the phosphors were ground S28-S30. Combined with single crystal analysis, it is found that (Figure S31). However, after doping in PMMA with different the phosphors with a larger red-shifted degree showed more concentration (10, 50 and 80%), no UOP was observed (Figure twisted structures with alkyl chain changed. Thus we reasoned S32), indicating the UOP emission was from the aggregated that the flexible alkyl chain played a critical role in tuning the states. The PL emission color change of these luminogens after spectral change under external force stimulation. Table 1. Single crystal parameters of CzEL, CzPL, CzBL and CzFL. Compound

Space group

Space point group

Symmetry

ML

Dipole moment

CzEL

P212121

222

Chiral

Active

4.5195

CzPL

P-1

-1

Centrosymmetric

Inactive

4.0473 3.4523

CzBL

Pca21

mm2

Noncentrosymmetric

Active

4.3351

CzFL

Pbca

mmm

Centrosymmetric

Inactive

3.5828

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Figure 5. Applications of multi-functionalized ultralong phosphors. (a) A dual-encryption model of anti-counterfeiting and the formation of 3D/4D code of data encryption. (b) Photographs of the pattern of “peacock” before and after 365 nm lamp and LED lamp off. (c) Two type codes called 3D and 4D codes for data encryption.

Impressively, ML was also observed in both CzEL and CzBL crystals. CzBL crystal showed an obvious ML with a major band at around 550 nm (Figure 3d), which was consistent with its phosphorescence spectrum at 298 K. From crystal data, it is easily found that the unit cells of CzEL and CzBL are chiral and non-centrosymmetric space groups of P212121 and Pca21, respectively, distinctly different from CzPL and CzFL with centrosymmetric space groups of P-1 and Pbca (Table 1). Notably, chiral and non-centrosymmetric space groups endowed the luminescent materials with the piezoelectric effect, which was beneficial to ML generation.50,51 Additionally, as shown in Figure 3e, the dipole moments of CzEL and CzBL compounds were larger than those of CzPL and CzFL molecules in crystal. Therefore, we reasoned that ML in CzEL and CzBL crystals were attributed to the influence of piezoelectric feature and large dipole moment.52 To gain deep insight into the mechanism of multicolour UOP from these phosphors, we firstly conducted a further set of experiments on phosphorescence at 77 K. These organic phosphors have obvious changes in UOP colors with the

passage of time after cease of the irradiation (Figure 4a), especially for CzPL and CzFL crystals. Such phenomenon can be explained by the different lifetimes of the broad phosphorescence emission bands varying from 470 to 550 nm (Figure 4b and S34). These results indicated that the colorful UOP in crystal may be ascribed to the multiple emission states. Furthermore, the phosphorescent intensity at short wavelenth region is largely increased, compared with that at long wavelenth region at 77 K. Hence, it can be reasonably speculated that the multiple emission states may be triggered by different luminescent centers, rather than from the higher excited states in single molecule, which generally disappeared at low temperature according to the anti-kasha’s rule.53 Subsequently, the emission band at around 470 nm in crystal is assigned to the single molecule phosphoresce, because it is exactly similar to that in solution at 77 K (Figure S21). As seen from Figure 4c, the profiles of phosphorescence spectra of these phosphors are exactly alike in amorphous state at 77 K, along with a maximum wavelength at 500 nm, indicating the welldistributed chromophores for phosphorescence generation. This

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is because the phosphors in the amorphous state have a similar intermolecular interactions between chromophores of carbazole and phthalimide, which is favorable for the formation of the well-distributed chromophores. Consequently, the band at around 500 nm in CzEL is amorphous-like phosphorescence, owing to the ordered packing between carbazole and phthalimide units. Whereas for the bands at around 520 and 550 nm, they may be triggered by the stabilization of the phthalimide and carbazole aggregation units in crystal (see Figure 4d,e), respectively. Under ambient condition, the phosphorescence profile of these phosphors in crystal were dominated by the bands at around 520, 550 or 600 nm due to the different molecular packing and intermolecular interactions, along with different lifetimes (Figure S23 and Table S2). Taken these results together, we reasoned that there might be two luminescent centres around carbazole and phthalimide luminogens for colourful UOP in crystal state. From the single crystal analysis, it was found that both carbazole and phthalimide chromophores were situated into interactions (Figure S35-S38). For CzEL, it was found that the molecular environments of carbazole and phthalimide units were similar, in which molecules were rigidly restricted by CH∙∙∙π (2.792 - 2.887 Å), C-H∙∙∙O (2.560 - 2.625 Å) and π∙∙∙π interactions (3.253 - 3.364 Å) (Figure 4d). It provides a solid evidence for the result that the profile of phosphorescence spectrum of CzEL crystal at room temperature identified with that in amorphous state at 77 K. Therefore, we inferred that the UOP emission band at 500 nm was ascribed to the molecular chromophore like that in amorphous state at 77 K (Figure 4c). For CzPL, there also existed the similar π-π stacking between carbazole and phthalimide units but weak interactions (Figure S39a), thus leading to lower intensity of emission band at around 500 nm due to intense non-radiative transitions compared with CzEL. For CzBL, there existed strong π∙∙∙π interactions (3.271 and 3.316 Å) between phthalimide and phthalimide, thus caused strong emission band at around 520 nm (Figure S39b). There are C-H∙∙∙π (2.766 and 2.873 Å) interactions around carbazole unis which induced the similar intensity peak at 550 nm. With flexible alkyl chain increase from CzEL to CzFL, chromophores of carbazole and phthalimide units exhibited well-distributed arrangements (Figure 4d, e, Figure S39 and S40), especially for CzFL with the longest alkyl chain. It was found the carbazole units in CzFL single crystal showed similar intermolecular stacking as those in carbazole single crystal (Figure 4e), which was confined by multiple C-H∙∙∙π (2.748 - 2.896 Å) and C-H∙∙∙N (2.739 Å) interactions. For phthalimide units in crystal, there only existed weak π-π stacking and C-H∙∙∙O (2.625 Å) interactions. Combined with the similar phosphorescence spectrum at 550 nm to carbazole single crystal under ambient conditions,45 we reasoned that the UOP emission of CzFL at 550 nm stemmed from the aggregation state of carbazole units. Subsequently, the UOP emission band at 520 nm was ascribed to the aggregation of phthalimide units with π-π stacking. This point was also confirmed by calculated energy levels of the lowest excited triplet states for carbazole and phthalimide chromophores in crystalline environment (Table S4). However, owing to the weak intermolecular interactions between phthalimide units, the emission lifetime of the emission band at around 520 nm was much shorter than that of the carbazole aggregation due to the fast non-radiative transitions through molecular motions,

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which was further confirmed by time resolved emission spectra (Figure S41). Therefore, we speculated that the different packing modes of carbazole and phthalimide chromophores in crystal lead to multiple ultralong organic phosphorescence. Given the visible light-excited multicolor UOP, a simple pattern of a peacock was fabricated, whose feathers were painted with different phosphors (Figure 5a). As shown in Figure 5b, it exhibited a colorful blue “peacock” under 365 nm UV-lamp, and a multicolor “peacock” was observed by camera after the removal of the light source. For our pattern with visible light excited by a LED light, a beautiful “flower” appeared after the removal of the excitation. Therefore, the variation of pattern information with excitation wavelength change demonstrated the potential of these materials for anti-counterfeiting and data encryption applications. Meanwhile, we designed a new type of code utilizing a dynamic color transformation over time (Figure 5a) based on these organic phosphors, which is defined as 4D coding. As shown in Figure 5c, we prepared a matrix pattern as a 3D Code 1 under excitation, which is traditional 3D code using different color modules. After the removal of excitation source, a new 3D code appeared, namely Code 2. With time delay, a series of tunable codes as information carrier were obtained at different points of time, demonstrating great potential for high density information storage and data encryption. In this case, an encrypted information of Info A that was linked to our institute website as code 3 was read out at time delay of 0.4 s. With integration of Code 1, 2, 3 and 4 in order as a new 4D code for higher level of information security was proposed to encode Info B for linking to our university website (SV8-SV11). In summary, we have developed a series of colorful ultralong organic phosphorescent materials by alkyl engineering, which were functionalized with visible-light excitation, mechanochromism and mechanoluminescence with alkyl chain increasing, UOP emission color can be tuned from green to orange after cease of visible light excitation, like an available mobile phone flashlight, under ambient conditions, which could last for 138 s totally. Moreover, bright mecholuminescence was observed in CzBL crystal. Taken the experimental results and single crystal analysis together, we reasoned that the colorful UOP emission stemmed from the different packing modes of carbazole and phthalimide chromophores in crystal. Additionally, we designed a new type of 4D coding, utilizing a dynamic color transformation over time based on these organic phosphors for the first time. This study not only provides a concise strategy to obtain novel UOP materials with versatile functions, but also extends the scope of the potential applications of UOP materials.

ASSOCIATED CONTENT Supporting Information.

Experimental details, synthetic route and characterization of target molecule, supporting photophysical spectra, theoretical data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (Z.A.).

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Chemistry of Materials *E-mail: [email protected] (H.S.). *E-mail: [email protected] (W.H.).

Author Contributions # These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (91833304, 21875104, 51673095 and 91833302), National Basic Research Program of China (973 Program, No. 2015CB932200), Natural Science Fund for Distinguished Young Scholars of Jiangsu Province (BK20180037). We are grateful to the High Performance Computing Center of Nanjing Tech University for supporting the computational resources. Xuan Wang and Huili Ma contributed equally to this work.

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