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In-situ construction of 1D component-interchange organic core/ shell microrods for multi-color continuous-variable optical waveguide Ming-Peng Zhuo, Xi Yu Fei, Yi Chen Tao, Jian Fan, Xue-Dong Wang, Wanfeng Xie, and Liang-Sheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22317 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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ACS Applied Materials & Interfaces

In-Situ Construction of 1D Component-Interchange Organic Core/Shell Microrods for Multi-Color Continuous-Variable Optical Waveguide Ming-Peng Zhuoa, Xi-Yu Feia, Yi-Chen Taoa, Jian Fana,b, Xue-Dong Wanga*, Wan-Feng Xie c* and Liang-Sheng Liaoa,b*

aInstitute

of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China bInstitute

of Organic Optoelectronics, Jiangsu Industrial Technology Research Institute

(JITRI), Wujiang, Suzhou, Jiangsu 215211, P. R. China cSchool

of Electronics & Information Engineering, Qingdao University, Qingdao 266071,

China Keywords: Component-interchange, Core/shell, Organic Photonics, Energy-transfer (ET), Optical waveguide

ACS Paragon Plus Environment

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Abstract

The core/shell micro/nanostructures with versatility, tunability, stability, dispersibility, and biocompatibility are widely applied in optics, biomedicine, catalysis, and energy. Organic micro/nanocrystals have significant applications in miniaturized optoelectronics due to their controllable self-assembly behavior, tunable optical properties, and tailor-made molecular structure. Nevertheless, the advanced organic core/shell micro/nanostructures, which incorporate the multi-functionality, the flexibility and the higher compatibility, are rarely demonstrated due to the dynamic nature of molecular self-assembly and the complex epitaxial relationship of material combination.

Herein,

we

demonstrate

the

one-dimensional

(1D)

organic

core/shell

micro/nanostructures with component-interchange, which originates from the 4,4'-((1E,1'E)-(2,5dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dipyridine (DPEpe) single-crystal microrods or the DPEpe-HCl single-crystal microrods after a reversible protonation or deprotonation process. Notably, the DPEpe/DPEpe-HCl core/shell microrods display vivid visualizations of tunable emission-color via an efficient energy-transfer (ET) process during the stepwise formation of shell layer. More significantly, these DPEpe/DPEpe-HCl and DPEpe-HCl/DPEpe core/shell microrods cooperatively demonstrate the multi-color optical waveguide properties continuously adjusted from green (CIE (0.326, 0.570)), to yellow (CIE (0.516, 0.465)), and to red (CIE (0.614, 0.374)). Our investigation provides a new strategy to fabricate the organic core/shell micro/nanostructures, which can eventually contribute to the advanced organic optoelectronics at micro/nanoscale.


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Introduction

The core/shell micro/nanostructures have attracted considerable attention in both fundamental study and potential applications due to versatility,1,2 tunability,3 stability,4 dispersibility,5 and biocompatibility,6 which were extensively applied in catalysis,7 energy storage8,9 biomedicine,10 and optics.11-13 To date, many efforts have been directed towards core/shell inorganic nanomaterials to realize the superior and multifunctional chemistry and physics properties. For instance, Prof. Peng et, al. demonstrated that the enhancement of the photoluminescence quantum yields of CdS/CdSe core-shell quantum dots from several percent to above 50% through protecting the core with a shell layer to passivate the surface trap states.14 Besides, Almutairi and coworkers have designed lanthanide-based core/shell/shell nanoparticles as the triple-modal contrast agents to achieve concurrently enhanced performance in photoluminescence, resonance imaging, and computed tomography.2 It is commonly known that the organic micro/nanostructures have the characteristic advantages of the controllable selfassembly behavior,15-17 the tunable optical properties,18,19 and the tailor-made molecular


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structure,20,21 enabling them great significant potential in the flexible optoelectronics. Moreover, the core/shell structures have combined advantages of the shell and the core, whose multifunctional optoelectrical properties make them as promising candidates for the optoelectronic and the thermoelectric devices.22,23 Nevertheless, the controllable synthesis of the organic core/shell micro/nanostructures remains be unexplored.

Organic low dimensional micro/nanomaterials with unique molecular packing mode, minimized defects, and low-temperature processing, exhibit great optoelectronic applications in organic light-emitting diodes (OLEDs),24,25 organic field-effect transistors (OFETs),26,27

organic

solid-state

lasers

(OSSLs),18,19,22

organic

active

optical

waveguides,28,29 and organic light-emitting transistors (OLETs).30,31 Most recently, we achieved a wavelength-tunable near-infrared OSSL based on the self-assembled organic single-crystalline nanowires through a facile room-temperature solution-processing approach.20 Compared with single-component or uniformly doped organic low dimensional micro/nanocrystals, the core/shell structures have unique merits, such as combination of multi-functionality for microelectronic chips as well as stronger flexibility


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and good compatibility for diverse application requirements.32 Zhao et, al. developed organic core/shell nanowires with waveguiding core and chemiluminogenic shell for rapid and selective optical sense of traces H2O2 gas.33 Although the core/shell structures have made great achievements, the dynamic nature of molecular self-assembly as well as complex epitaxial relationships of material combinations making it challenging to construct the organic core/shell micro/nanostructures.34-36

Herein, we demonstrate the one-dimensional (1D) organic core/shell microrods with flexible component-interchange (Figure 1g) via a reversible protonation or deprotonation process originating from the self-assembled green-emissive single-crystal microrods of 4,4'-((1E,1'E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dipyridine (DPEpe) or the red-emissive single-crystal DPEpe-HCl microrods, which can simultaneously act as the core layer or the shell layer. Interestingly, there is an obvious emission-color modulation from green (CIE (0.334, 0.565)) to red (CIE (0.524, 0.456)) in the DPEpe/DPEpe-HCl core/shell microcrystals during the stepwise formation of the shell layer via an efficient energy-transfer (ET) process. What’s more, the consecutive tunable-


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color optical waveguide from green (CIE (0.326, 0.570)) to yellow (CIE (0.516, 0.465)), and to red (CIE (0.614, 0.374)) has been achieved through the cooperative function of the DPEpe/DPEpe-HCl and DPEpe-HCl/DPEpe core/shell microstructures, which demonstrates the multifunctionality of organic photonics in the integrated optoelectronic devices.

Results and Discussion

As shown in Figure 1a, the DPEpe and DPEpe-HCl powders under the UV lamp display intense green and bright red photoluminescence, respectively. The emission peaks for DPEpe and DPEpe-HCl powders (Figure 1b) occur at 525 and 610 nm, respectively, confirming the formation of DPEpe-HCl.37 Furthermore, the protonation of the nitrogen on pyridine unit is evidenced by the obvious chemical downfield shift of the Ha and Hb close to the pyridine moieties (inset of Figure 1b).38,39 The protonation results into an increased electron-withdrawing ability, which leads to an intermolecular charge transfer and a red shift in the photoluminscence.40 It is highly consistent with the variation in the electronic


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structures from DPEpe to DPEpe-HCl supported by theoretical result (Figure S1) and experimental cyclic voltammogram data (Figure S2).

It should be noted that both the organic microcrystals of DPEpe and DPEpe-HCl were prepared via a facile solution-evaporation method at room temperature (detail experiment in Supporting Information). The fluorescence microscopy (FM) images of the 1D selfassembled micro-rods of DPEpe (green emission) and DPEpe-HCl (red emission) are depicted in Figure 1c and 1d, respectively, which display the typical optical waveguide characteristic with bright emission from both tips and weaker emission from the bodies.28 The SAED patterns (Figure 1e and 1f) of the as-prepared DPEpe and DPEpe-HCl microrods clearly indicate a strong crystallinity, which is accorded with the corresponding XRD patterns with intense diffraction peak (Figure S3). The TEM images of the as-prepared microrods (insets of Figure 1e and 1f) show regular morphology and smooth surface, which have the potential to act as the building blocks for optoelectronic devices. Besides, the as-prepared DPEpe organic microcrystals identify with the simulated growth morphology of DPEpe with rod-like morphology (Figure S4). The measured d-spacing


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values of 7.1, 4.3 Å with an angle of 48.2o (Figure 1e) is different from that of 2.9, 4.0 Å with an angle of 67.6o (Figure 1f), which indicates the formation of DPEpe-HCl organic microcrystals. Furthermore, under the HCl gas environment, the surface molecules on the DPEpe organic microrod will convert into DPEpe-HCl, forming the shell layer in situ. The shell layer will act as a protecting layer that envelops the core part molecules and forms a DPEpe/DPEpe-HCl core/shell microrod structure (Figure 1g). Similarly, fumed by NH3 gas, DPEpe-HCl organic microrod will convert into DPEpe-HCl/DPEpe core/shell organic microrod, which is attributed to the reversion of the protonated nitrogen on pyridine.39,41 Thus, these emissive DPEpe and DPEpe-HCl organic single-crystalline microrods have been successfully prepared via a facile solution method, which are the potential building blocks to construct core/shell organic micro/nanostructures (Figure 1g).

The formation process of the DPEpe/DPEpe-HCl core/shell organic microrods was accurately quantified via fluorescence microscopy techniques. Figure 2a displays an obvious variation in emission color of organic microcrystals from intense green emission to bright green-yellow emission after being fuming HCl gas for 15 min, and then to yellow


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emission after another 15 min. After further fuming, the fluorescence of the organic microrods will change into orange-red (60 min) and bright red (80 min). The consistent XRD spectra under various fuming time (Figure S5) further illustrate that the protonation process only occurs on the surface of DPEpe microcrystals. As shown in Figure 2b, the intensity at green region decreases with the increasing fuming time, and the intensity at the red region synchronously increases due to the emission of DPEpe-HCl molecules, which causes a tunable emission color from green (CIE (0.334, 0.565)) to red (CIE (0.524, 0.456)) (Figure 2c). From the Figure 2d, the ratio of Cl element sharply increased from 1.2 % to 10.5 % after the initial HCl gas fuming (60 min), which indicates the formation of DPEpe-HCl. After that, the ratio of Cl element increases from 10.5 % to 12.2 % after prolonging the fuming time to 180 min. The element ratios of N and O elements exhibit a diametrically opposed tendency compared with that of Cl element, as well as maintaining a high ratio more than 43% during the entire process. The superficial DPEpe molecules of the organic microcrystals in situ convert into the dense shell layer DPEpe-HCl, which can prevent the further protonation of the DPEpe molecules at the core part. Hence, the


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DPEpe/DPEpe-HCl core/shell organic microcrystals with tunable emission-color from green to yellow, and to red, can be achieved by adjusting the fuming time under HCl gas.

To make further insight into the tunable emission color mechanism of DPEpe organic microcrystal after being fumed by HCl gas, the time-resolved PL decay transient at different emission peaks and spectrally-resolved lifetime at different location in the organic microcrystal have been performed. The obvious spectral overlap between the DPEpe emission and DPEpe-HCl absorption spectra (Figure S6a) is the necessary condition for the presence of an ET process from excited DPEpe molecules to DPEpeHCl molecules.42-44 The emission of DPEpe organic microcrystals after being fumed by HCl gas for different time decays mono-exponentially at emission peak at both 520 nm and 630 nm (Figure 3a and 3b). As shown in Figure S6b, except absorbing energy and radiative transition process, the DPEpe-HCl molecules emit red light have an extra energy transfer process (~5 ns) that the excitons transfer from the S1 level of DPEpe to S1 level of DPEpe-HCl, which maybe leads to an increased lifetime for DPEpe-HCl.45 As shown in Figure 3c, the lifetime at the emission peak at 520 nm for the DPEpe organic


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microcrystals displays a vastly decrease from 2.6 ns to 1.6 ns after staying in HCl gas environment for 60 min, which is contrary to that at 650 nm with an augment from 4.6 ns to 6.3 ns. The decreasing peak lifetime of the DPEpe molecules is attributed to the increasing density of low energy states for energy migration, as well as the lifetime increasing in the red emission section is owing to the increased DPEpe-HCl molecule contribution with an intrinsically slower PL decay. 46, 47 As shown in Figure 3d-1 and S6b, the excited excitons on the singlet energy level of core layer DPEpe molecules can effectively transfer to the singlet energy level of the shell layer DPEpe-HCl molecules through an ET process, leading into red emission. Due to the protection of the DPEpeHCl shell layer, the protonation of DPEpe molecules at the core layers was passivated, which results into the stabilized decay lifetime after a long fuming time (180 min) under HCl gas. As shown in the Figure 3e and S8a, the maximal PL intensity appeared at spatial location A2 (Figure 3d-2), which is 0.5 μm beneath the surface. Moreover, it should be noted that the efficiency of ET also depends upon the distance between the DPEpe and DPEpe-HCl molecules, which is typically below 6 nm.45 Therefore, it is indicated that the thickness of the shell layer is about 0.5 μm. Figure 3f and S8b clearly demonstrate an


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increase of lifetime (PL peak at 520 nm) at location with the increasing distance from 0 to 1 μm, which clarifies that the core part is the DPEpe crystal. Therefore, the DPEpe/DPEpe-HCl core/shell organic microrods with a tunable emission-color display a high-efficiency ET process from core to shell.

Figure 4a displays an obvious blue-shift with the maximum peak from 453 to 397 nm in the absorption spectra of DPEpe-HCl ethanol solutions after adding excess NH3·H2O, which is due to the deprotonation from DPEpe-HCl molecules to DPEpe molecules (inset of Figure 4a), as well as the previous investigations.39,48 Fumed by HCl gas, the DPEpe organic microrods can be converted into the DPEpe/DPEpe-HCl core/shell microrods via a protonation process. Similarly, after fumed by the NH3 gas for 240 min, the DPEpe-HCl organic microrods will convert into DPEpe-HCl/DPEpe core/shell organic microrods via a deprotonated process. As shown in Figure 4b, the PL spectra of the DPEpe-HCl/DPEpe core/shell microrods can be disassembled into two sectional corresponding to the emission of DPEpe and DPEpe-HCl molecules, which is agreed with the corresponding CIE chromaticity diagram (Figure S8). Furthermore, these as-prepared DPEpe-


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HCl/DPEpe core/shell organic microrods display intense yellow emission under excitation of UV-light (Figure 4c), which can be attributed to the combination of green emission of DPEpe and red emission of DPEpe-HCl. In contrast, only weak red emission was observed with the excitation of green-light (Figure 4d). There are two reasons for this phenomenon. One is the absence of the green fluorescence originated from DPEpe molecules. The other is the prevention of the compact shell layer forming from the DPEpe molecules at the surface, which weakens the excited energy for the core part of DPEpeHCl and leads the weak red emission. Thus, the DPEpe-HCl/DPEpe core/shell organic microrods were indeed successfully prepared after a deprotonation process on the DPEpe-HCl microrod in the NH3 vapor environment.

Inspired by the tunable emission-color and the core/shell microstructure as well as the typical optical waveguide behavior, we conduct the optical waveguide measurement based on both single-component organic microrods of DPEpe or DPEpe-HCl, and core/shell organic microrods of DPEpe/DPEpe-HCl or DPEpe-HCl/DPEpe. Figure 5a and 5c display the micro-area FM images obtained from a DPEpe microrod (length of 66.1


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μm) and DPEpe-HCl microrod (length of 50.3 μm), respectively, by accurately shifting the excitation laser spots (diameter ~2 μm) along the length of rod. Figure 5b and 5d clarify the PL signals detected from the tip of the DPEpe microrod based on the various photons propagation distance. The obvious emission shift is absent during increasing the propagation distance in both DPEpe and DPEpe-HCl organic microrod, which agrees with the CIE coordinates (Figure 5i) of (0.225, 0.518) and (0.518, 0.452), respectively. The PL intensity at the excited site along the body of the prepared 1D microcrystal (Ibody) and at the emitting tip (Itip) were recorded, and the ratio Itip/Ibody shows a single-exponential decay against propagation distance, which indicates an active nature of the optical waveguide.28 The optical-loss coefficient (R) was calculated by single-exponential fitting Itip/Ibody = Aexp(-RD), where D is the distance between the excited site and the emitting tip.49 Accordingly, RDPEpe = 0.0102 dB/μm and RDPEpe-HCl = 0.0181 dB/μm at 520 and 650 nm, respectively,

which

are

comparable

with

that

of

these

previously-reported

inorganic/organic optical waveguides.50-53 The high crystallinity and the smooth surface play the crucial role for the low optical-loss in both DPEpe and DPEpe-HCl organic microcrystal, which can effectively reduce optical loss caused by scattering. All of the


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above results elucidate that both the DPEpe and DPEpe-HCl organic micro-rods demonstrate excellent optical waveguide properties.

In contrast, the core/shell organic microcrystals demonstrate photons confinement effect and unique tunable emission color.33 The DPEpe/DPEpe-HCl (length of 45.6 μm, Figure 5e) and DPEpe-HCl/DPEpe (length of 64.3 μm, Figure 5g) core/shell organic microrods were selected to investigate the optical waveguide behaviors. The spatially resolved PL spectra of both DPEpe/DPEpe-HCl (Figure 5f) and DPEpe-HCl/DPEpe (Figure 5h) core/shell organic microrods demonstrate dramatic emission shift depended on photons propagation distance. The CIE chromaticity diagram (Figure 5i) displays an obvious red-shift from green (CIE (0.326, 0.570)) to yellow (CIE (0.502, 0.452)) at the tip of DPEpe/DPEpe-HCl core/shell organic microcrystal, as well as diametrical blue-shift from red (CIE (0.614, 0.374)) to yellow (CIE (0.512 0.477)) in the DPEpe-HCl/DPEpe core/shell organic microcrystal. Based on the aforementioned formula, DPEpe/DPEpeHCl and DPEpe-HCl/DPEpe core/shell organic microcrystal display the optical-loss coefficient of RDPEpe/DPEpe-HCl = 0.0338 dB/μm and RDPEpe-HCl/DPEpe = 0.0545 dB/μm,


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respectively. Therefore, the core/shell organic microcrystals demonstrate a multi-color continuous-variable optical waveguide from green to yellow, and to red by controlling the photons propagation distance.

Conclusion

In summary, the organic core/shell micro/nanocrystals are fabricated based on the selfassembled single-crystal micro-rods of the π-conjugated organic molecules of DPEpe or DPEpe-HCl, whose components are controllably inter-changeable via a reversible protonation/deprotonation process. During the stepwise formation of shell layer on the DPEpe single-crystal microrods, the organic micro/nanocrystals display a novel tunable color emission through an efficient ET process. Remarkably, the organic core/shell microcrystals of both DPEpe/DPEpe-HCl and DPEpe-HCl/DPEpe cooperatively demonstrate multi-color optical waveguide, which can be continuously adjusted from green (CIE (0.326, 0.570)), to yellow (CIE (0.516, 0.465)), and to red (CIE (0.6135, 0.3740)). Our organic core/shell micro/nanocrystals with component-interchange exhibit


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the outstanding tunable optical properties, which are potential building blocks for the advanced optics chip at micro/nanoscale.

Associated Content

Supporting Information

Supporting Information is available free of charge on the ACS Publications website.

Corresponding Author

* Email: [email protected] (X. -D. Wang), [email protected] (W. -F. Xie), [email protected] (L. -S. Liao) Notes The authors declare no competing financial interest.

Acknowledgement

The authors acknowledge financial support from the National Natural Science Foundation of China (No. 21703148), and the Natural Science Foundation of Jiangsu Province (BK20170330), and this project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology (CIC-Nano), by the Priority Academic Program Development of Jiangsu Higher


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Education Institutions (PAPD), and by the “111” Project of The State Administration of Foreign Experts Affairs of China.

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Figures and Tables


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Figure 1. (a) Optical images of DPEpe and DPEpe-HCl powders under the UV lamp (λ = 375 nm). (b) The photoluminescence (PL) spectra of DPEpe (green line) and DPEpe-HCl (red line) crystals. Inset: the corresponding 1H NMR spectra of DPEpe (green line) and DPEpe-HCl (red line) in CDCl3 (C = 0.1 mM). Fluorescence microscopy (FM) image of the as-prepared (c) DPEpe organic microcrystals and (d) DPEpe-HCl organic microcrystals excited by a mercury lamp (λ = 330~350 nm). The scale bars are 20 μm. (e) SAED pattern of a typical DPEpe organic microrod. Inset: TEM image of the corresponding DPEpe organic microrod with the scale bar of 2 μm. (f) SAED pattern of a typical DPEpe-HCl organic microrod. Inset: TEM image of the corresponding DPEpe-HCl organic microrod with the scale bar of 2 μm. (g) Schematic demonstration of the DPEpe/DPEpeHCl core/shell organic microrod and DPEpe-HCl/DPEpe core/shell organic microrod.


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Figure 2. (a) FM images of DPEpe organic microcrystals after being fumed by HCl gas for 0, 15, 30, 60, and 180 min. The scale bars are 50 μm. (b) The PL spectrum and (c) the CIE chromaticity diagram of organic microcrystals corresponding to (a) excited by the UV lamp (λ = 375 nm). (d) Summary typical element (O, N and Cl) ratios (except the C and H) of the organic microcrystals corresponding to (a). The element distribution measured by energy-dispersive X-ray (EDX) mapping mode.


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Figure 3. Time-resolved PL decay transient with emission peak at (a) 520 nm and (b) 630 nm of the DPEpe organic microcrystal after being fumed by HCl gas for different time. (c) Summary lifetime for a and b. (d-1) ET energy diagram from the DPEpe molecules (core) to the DPEpe-HCl molecules (shell). (d-2) Schematic demonstration of spectrally-resolved confocal imaging for the DPEpe/DPEpe-HCl core/shell organic microrod. The PL intensity (e) and lifetime (f) corresponding to different location in the DPEpe/DPEpe-HCl core/shell organic microcrystal in d2.


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Figure 4. (a) Absorption spectra of DPEpe (green line) and DPEpe-HCl (red line) in ethanol. Inset: deprotonation mechanism of DPEpe-HCl molecules. (b) The PL spectrum of DPEpe-HCl/DPEpe core/shell organic microcrystals (blue solid line). The green and red spectra corresponding to DPEpe (green dashed line) and DPEpe-HCl (red dashed line) molecules. FM images of DPEpeHCl/DPEpe core/shell organic microcrystals with the excitation of (c) UV-light (λ = 330~380 nm) and (d) green-light (λ = 500~550 nm). The scale bars are 20 μm.


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Figure 5. (a) FM images obtained from an individual DPEpe organic microcrystal by exciting with a laser beam (λ = 375 nm) at different positions with the scale bar of 20 μm. (b) The corresponding spatially resolved PL spectrum in (a) with different separation distances. Inset: The ratios of the intensity Itip/Ibody against the distance D. Curves were fitted by an exponential decay function Itip/Ibody =Aexp(-RD). The optical waveguide characterization of (c, d) DPEpe-HCl organic microcrystal, (e, f) DPEpe/DPEpe-HCl core/shell organic microcrystal and (g, h) DPEpeHCl/DPEpe core/shell organic microcrystal, respectively. (i) The CIE chromaticity diagram corresponding to tip emission colors of the four kinds organic microcrystals as shown in (a, c, e, and g).


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Table of Content: One-dimensional (1D) organic core/shell micro/nanostructures with component-interchange originating from the DPEpe single-crystal microrods or the DPEpe-HCl single-crystal microrods were successfully achieved via a reversible protonation or deprotonation process. Notably, these 1D organic core/shell micro/nanostructures cooperatively exhibit the multi-color optical waveguide properties continuously from green, to yellow, and to red.


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In Situ Construction of One-Dimensional ... - ACS Publications

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