Morphology-Retained Photoconversion Reaction of Anthracene

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Morphology-Retained Photoconversion Reaction of Anthracene Single Crystal: A New Approach for Organic Heterostructures Minkyung Lee,†,‡ Chibeom Park,†,§ Jungah Kim,‡ Jinho Lee,‡ Soyoung Kim,‡ Jin Young Koo,*,§ and Hee Cheul Choi*,‡,§ ‡

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Department of Chemistry, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Namgu, Pohang 790-784, Korea § Center for Artificial Low-Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-Ro, Pohang 790-784, Korea S Supporting Information *

ABSTRACT: Morphology-retained solid-state photoconversion of anthracene (AN) to 9,10-anthraquinone (PC-ANQ) and dipara-anthracene (PC-DPA) was accomplished by irradiating mercury lamp light to plate-shaped AN single crystal in oxygen and argon atmosphere, respectively. The photoconverted crystals retained the original plate shape morphology of the starting AN crystal, whereas the emission profile and crystal structure were significantly changed. The electrical conductivity of PC-ANQ crystal is 5 orders of magnitude greater than that of the starting AN crystal, whereas the PC-DPA crystal exhibits a decreased conductivity. The AN/PC-ANQ/PC-DPA heterostructures with smooth interface were successfully obtained by inducing the photoconversion only at the desired area. KEYWORDS: organic heterostructure, area-selective photoreaction, solid-state photoconversion, morphology-retained photoreaction, molecular rearrangement

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oth optical1 and electrical2,3 properties of organic crystals primarily rely on the arrangements of unit molecules in the crystal, which are generally distinguished by the crystal morphology. Because such properties are directly related to the orbital overlap between neighboring molecules, the properties of organic single crystals are frequently influenced by the measurement direction of the target crystal, e.g., polarization of illumination4 or location of electrodes for electrical property measurements.3 In general, however, it is very difficult to achieve the desired arrangement of molecules in a crystal because the crystal planes are quickly determined during the early stage of crystallization process5 and rarely modifiable throughout the crystallization process. This implies that accessible facets having specific molecular arrangements are limited by the crystal morphology. Therefore, it is important to develop methodologies to obtain a specific morphology of crystals for specific target molecules for a better understanding of fundamental sciences as well as an opportunity to maximize the property of interest for practical applications. Although numerous methods3,6,7 have been developed to control the morphology of organic crystals, most of them still depend primarily on the crystallization process. One potential approach is to convert the chemical moiety having a specific crystal morphology into another one, eventually into the desired chemical moiety while keeping the original morphology intact. By doing this, the new chemical moiety would have © XXXX American Chemical Society

a chance for possessing a specific morphology that would not have been available by conventional crystallization process. A photoconversion reaction is a good candidate for this purpose. Solution phase photoreaction has been widely used for various organic syntheses, and solid-state photoreaction has been also studied8−18 over the past 50 years. One important advantage of solid-state photoreaction is that the overall morphology of the crystal is mostly maintained during and after the reaction, although it can cause minor changes such as bending9,10,16 or curling of structure.19 Moreover, the photoreaction can be applied only at desired area by controlling the region where the light is irradiated, which enables the synthesis of more complicated functional systems including heterostructures. Herein, we report a photochemical approach to obtain ANQ and DPA platy crystals from AN platy single crystals via solid-state photoconversion reaction. It is noteworthy that the plate shapes of ANQ and DPA crystals have not been obtained by the vapor phase crystallization method. This result proves that when a specific crystal morphology is not available for a certain molecule (ANQ, DPA in this case), the morphology can be achieved by crystallization of another molecule (AN in this case) followed by photoreaction to Received: August 5, 2018 Accepted: September 25, 2018 Published: September 25, 2018 A

DOI: 10.1021/acsami.8b13369 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

reactions were performed in a closed chamber with a continuous flow of oxygen for the formation of ANQ, and argon for the formation of DPA. (Scheme S1) We first investigated the photoluminescence (PL) change of AN plate upon the photoconversion reaction. The PL microscope images show that the starting AN platy crystal exhibits a uniform blue PL. However, when the photoconversion reaction was conducted in an oxygen atmosphere, the red PL emerged from the edge of the crystal after 3 h irradiation and covered entire platy crystal after 12 h irradiation. Upon prolonged light irradiation, yellow PL appeared from the edge and spread again toward the center region. (Figure 2a, top row) When the same photoconversion

convert it to the desired molecule. As a proof of concept, we investigated the solid-state photoconversion reaction of AN to ANQ and AN to DPA. The plate shape morphology of the starting AN crystal was retained without any observable damage or changes during the light irradiation, and the successful photoconversion reaction was confirmed by 1H NMR, gas chromatography−mass spectrometry (GC-MS), and crystal structure analysis. Interestingly, the resulting PC-ANQ platy crystal exhibited dramatically increased electrical conductivity, which is attributed to the enhanced π−π orbital overlap. The AN/ANQ and AN/DPA heterostructure crystals were also demonstrated by the area-selective photoconversion reaction on AN platy crystal. AN molecules preferentially crystallize to a plate shape by the physical vapor transport (PVT) method (Figure S1) and AN dissolved in organic solvents readily undergoes structural transformations to ANQ or DPA upon mercury lamp light irradiation.20 It should be noted that such reactions are also possible in solid-state AN.21,22 Meanwhile, it is difficult to directly crystallize ANQ or DPA into a plate shape crystal because ANQ prefers to grow into a wire form and DPA decomposes into AN during the PVT process. Taking all of these phenomena into account, we attempted a photoconversion reaction of a single crystal AN plate to obtain a plate shape ANQ and DPA crystals. Figure 1 schematically

Figure 2. PL change upon photoconversion reaction of AN platy crystals. (a) PL microscope images of individual crystal depending on reaction time (scale bar: 50 μm). (b) PL spectra obtained from multiple crystals before and after the photoconversion reaction.

reaction was conducted in an argon atmosphere, a completely different PL change behavior was observed. Instead of the emergence of red or yellow PL from the edge, the blue PL of starting AN crystal became dim at a few spots inside the crystal and spread to entire crystal as the light irradiation was continued. (Figure 2a, bottom row) Such a difference in PL change implies that oxygen is involved in the photoconversion reaction in the former case and its diffusion inside the crystal limits the reaction rate, whereas the reaction in the latter case occurs without the gas molecules involved. The PL changes were also monitored in the PL spectra recorded from the photoconversion reaction of multiple crystals (Figure 2b). In oxygen atmosphere, two distinctive peaks of starting AN crystals shown at 420 and 440 nm completely disappeared after 12 h reaction, and one broad peak newly appeared at 570 nm instead. The new peak gradually shifted toward shorter wavelength, and the final spectrum became similar to that of ANQ wire grown by PVT method. (Figure S2a) However, when the reaction was undergone in argon atmosphere, two PL peaks of original AN crystals became one broad PL peak with less intensity, which is similar to that of DPA. (Figure S2b) The similar PL characteristic after the photoconversion

Figure 1. Schematic view of photoconversion reaction to form plate shape anthraquinone (ANQ), dipara-anthracene (DPA) (top) AN/ ANQ, and AN/DPA heterostructures (bottom) from a plate-shaped AN single crystal.

presents our solid-state photoconversion reaction of AN crystal into ANQ (photo-oxidation) and DPA (photodimerization). The AN platy crystals were first grown on a Si substrate by PVT method, then light (mercury lamp) was irradiated directly (123.5 mW/cm2) or through an optical microscope objective lens with a band-pass filter (λex = 330−380 nm, 6.63 mW/cm2) to convert them into ANQ or DPA crystals. Especially for the area-selective photoconversion reaction, the light was irradiated through a chrome photomask. All of the photoconversion B

DOI: 10.1021/acsami.8b13369 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a) Average conductivity changes upon photoconversion reaction of AN platy crystal. The error bars represent the standard deviation. (b) Conductivity with respect to corresponding PL peak positions shown in PC-ANQ crystal devices. The gray arrow represents the progression of photoreaction and the horizontal dashed line indicates the average conductivity of starting AN crystal devices. (c) XRD spectra of photoconverted crystals and AN crystal. (d, e) Proposed conduction pathway (blue arrows) of AN and PC-ANQ crystal with molecular arrangements perpendicular to (001) plane of respective crystal structure.

peak at 578 nm. (Figure S7) Hence, the red PL shown in the photoconversion reaction is attributed to the modulation of ANQ PL by residual AN in PC-ANQ crystal. The electrical property of the organic crystal is largely dependent on the molecular orientation in the crystal.3,25,26 The morphology conservation during the solid-state photoconversion reaction implies that the translational movement of each molecule occurs locally, which is very small or negligible. However, the intermolecular interactions between converted molecules trigger molecular rearrangement, and this gives a chance to build new charge transport pathways. Indeed, we found a significant increase in the electrical conductivity of PCANQ crystal, compared to that of starting AN crystal which is known to have low electrical conductivity. (Figure 3a). An average electrical conductivity of 6.67 × 10−10 S/cm was obtained from multiple AN crystal devices (Table S1), among which some devices were converted to PC-DPA and the others were converted to PC-ANQ while the crystals were continuously monitored by PL microscope. PC-DPA showed an average conductivity of 2.28 × 10−10 S/cm, which is slightly

reaction to ANQ and DPA signifies that the photoconverted plate shape ANQ and DPA (PC-ANQ, PC-DPA) crystals were generated from the AN crystals as a result of the solid-state photoreaction. Interestingly, the scanning electron microscope (SEM) images clearly show that the original morphology was retained well despite the large spectral changes. (Figure S3) The successful photoconversion was also confimed by 1H NMR showing only DPA and ANQ signal without AN siganl. (Figures S4 and S5) During the photoreaction of PC-ANQ, we found an intermediate region which showed red PL in response to light irradiation time. Therefore, we intentionally stopped the reaction and check the composition of the PCANQ crystal (red PL, 12 h irradiation). 1H NMR and GC-MS give two components; ANQ as a major component and AN as a minor component. (Figure S6) To confirm the effect of residual AN molecules on the PL of PC-ANQ crystal, we synthesized AN/ANQ mixed crystals by liquid−liquid interfacial precipitation (LLIP) method.23,24 Interestingly, we observed dramatic PL difference depending on the mixing ratio of AN and ANQ, and 1:9 mixed crystal exhibited red PL with a C

DOI: 10.1021/acsami.8b13369 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. Photoconverted heterostructure. PL images of (a) AN/PC-ANQ, (b) AN/PC-DPA, and (c) PC-ANQ/PC-DPA heterostructure. The PL spectra of each region are shown in Figure S14. (d) AFM height and (e) phase images of AN/PC-ANQ corresponding to the dashed-line box region in a. (f) Height (top) and phase (bottom) profiles along the dashed lines in d and e, respectively.

appeared at 11.46° 2θ, which corresponds to either (001) or (200) plane. Although it is difficult to clarify the exact nature of crystal plane due to the very small discrepancy, it is likely to be (001) plane due to the involvement of less molecular rearrangement during photoconversion reaction compared to (200) plane. In both cases, however, the ANQ molecules arrange face-to-face within the crystal planes. (Figure 3e, S10) Therefore, the significant increase of electrical conductivity of PC-ANQ is likely to be the result of the increased overlap of π−π orbitals between adjacent molecules (π−π distance: 3.527 Å). On the other hand, the decrease of electrical conductivity occurred in PC-DPA needs to be explained in a different way. Although the photodimerization process rearranges two adjacent molecules face-to-face, [2 + 2] cycloaddition reaction breaks electron conjugation within each molecule, and the resulting deformation of molecular frame reduces orbital overlap, which eventually makes electrons’ hopping difficult. The most remarkable advantage of solid-state photoreaction is that the conversion region can be readily controlled. By utilizing this area-selectivity, we successfully synthesized lateral heterostructures in a crystal, which is the first demonstration to the best of our knowledge. Selective light irradiation was realized by inserting a chrome photomask in the middle of the light path of microscope, (Scheme S1) similar to the commercial noncontact lithography technique. Figure 4a shows the PL image of the resulting AN/PC-ANQ lateral heterostructure where the blue and red PL areas were clearly defined from the light irradiated and blocked area, respectively. Negligible difference of PL spectra shown in the depth profile at AN and PC-ANQ region suggests that the heterostructure was generated throughout the almost entire crystal, not just on the surface. (Figure S11) The surface of AN/PC-ANQ heterostructure was further analyzed by atomic force microscopy (AFM). The phase image showed a clear difference between AN and PC-ANQ area. However, in the height image, small bump was formed along the boundary of AN and PC-ANQ area whereas the overall heights of both parts are same. (Figure 4d−f) Although the origin of this bump at the boundary is not clear, we believe that it is related with the oxygen diffusion since the reaction occurs much faster at

lower than the starting AN crystal devices. However, in case of PC-ANQ, the conductivity dramatically increased to have an average value of 9.50 × 10−6 S/cm, and such an increase was more noticeable as the photoconversion reaction proceeded. (Table S2, Figure S8) Note that the PL peak position of PCANQ is a better indicator than actual irradiation time to represent the degree of conversion reaction since the reaction rate is highly dependent on the thickness and dimension of each crystal. As explained above, the PL peak position of PCANQ moves to the shorter wavelength as less AN molecules remain in the crystal. The conductivity was about 1 × 10−8 S/ cm when the PL peak was at 600 nm, and it increased up to 1 × 10−4 S/cm when PL peak shifted to 540 nm. (Figure 3b) This result indicates that the increase in conductivity mainly originates from the converted component which is ANQ. To reveal the structural contribution to the electrical conductivity, we measured X-ray diffraction (XRD) of asgrown AN and photoconverted crystals using synchrotron Xray source. The XRD pattern of as-grown AN crystals showed 4 diffraction peaks which correspond to (001), (002), (110) and (20−1) planes of the reported data.27 (Figure 3c) The limited number of diffraction peaks indicates that the AN crystals were grown to have a preferred orientation. Note that the crystal planes parallel to the substrate are only shown as diffraction peaks since we aligned the substrate surface to the X-ray beam path before measurements. Among them, the strongest (001) peak corresponds to the wide top surface of the plate shape crystal, as confirmed by single crystal X-ray experiment (Figure S9). Because the regular herringbone molecular packing (face-to-edge) occurs within this (001) plane, the charge conduction is not efficient because of the slightly mis-aligned π orbitals between adjacent AN molecules decreasing the degree of π orbital overlap (Figure 3d). PCANQ and PC-DPA also showed two or three diffraction peaks which were assigned to the reported structure.28,29 This result indicates that the crystallinity is preserved during the photoconversion process. Since the crystal morphology was retained, it is reasonable to assume that the (001) plane of AN converted into the strongest diffraction peaks shown in PCANQ and PC-DPA. In case of PC-ANQ, the main peak D

DOI: 10.1021/acsami.8b13369 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the edge of the crystal due to facile oxygen access (Figure 2a). Additional energy dispersive spectroscopy (EDS) experiment showed that the oxygen content was almost double in PCANQ area compared to the unreacted AN area, which also supports well the formation of the heterostructure. (Figure S12) The AN/PC-DPA heterostructure could be also obtained in the same way if the area-selective photoconversion reaction was performed under argon atmosphere. (Figure 4b) DPA is known to split into two AN molecule by heating or UV irradiation.30 In other words, the photodimerization reaction of AN could be reversible. However, the strong CO double bond makes it almost impossible to convert ANQ back into AN, especially in solid-state reaction. Such an irreversibility of photo-oxidation reaction enabled PC-ANQ/PC-DPA heterostructure through two-step photoconversion reaction. (Figure S13) We first made AN/PC-ANQ heterostructure with a chrome photomask (area-selective exposure) in oxygen atmosphere, and then conducted the second photoconversion reaction without chrome photomask (flood exposure) in argon atmosphere. During the second photoreaction, the preformed PC-ANQ area remained intact, as confirmed by PL image. (Figure 4c). However, when the sequence was reversed, i.e., area-selective photodimerization followed by photo-oxidation with a flood exposure, the entire area of crystal was converted into PC-ANQ. In summary, the solid-state photoconversion reaction on a plate shape AN crystal successfully generated PC-ANQ and PC-DPA crystals with the original crystal morphology retained. Time-dependent observation revealed that the photo-oxidation reaction proceeded from the edge of the AN crystal due to oxygen diffusion while photodimerization occurs globally. The PC-ANQ exhibited PL emission of intermediate states which turned out to be the effect of small amount of unreacted AN molecules. The electrical conductivity of PCANQ crystal gradually increased as the photoreaction proceeded and reached a value of five orders magnitude higher than that of the starting AN crystal due to the molecular rearrangement as a way to increase π orbital overlap. The photoconversion reaction was applied only on the selected area to achieve heterostructures of AN, PC-ANQ, and/or PC-DPA. To conclude, our solid-state photoconversion reaction enabled molecular rearrangement and organic heterostructure within an individual crystal, both of which will contribute to the development of next-generation high-performance organic heterojunction devices.

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Hee Cheul Choi: 0000-0003-1002-1262 Author Contributions †

M.L. and C.P. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS XRD measurements were performed at Pohang Accelerator Laboratory (Beamline 5D) at Pohang, Korea. We acknowledge Prof. Taiha Joo and Changmin Lee (POSTECH) for help in obtaining time-resolved photoluminescence data.

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ABBREVIATIONS AN, anthracene; PC-ANQ, photoconverted 9,10-anthraquinone; PC-DPA, photoconverted dipara-anthracene; PL, photoluminescence; SEM, scanning electron microscopy; GC-MS, gas chromatography−mass spectrometry; LLIP, liquid−liquid interfacial precipitation; XRD, X-ray diffraction; AFM, atomic force microscopy; EDS, energy-dispersive spectroscopy.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b13369. Full methods and experimental data; NMR, PL, XRD, and GC-MS spectra; SEM image, EDS profile, electrical conductivity, molecular arrangement scheme, photoreaction process (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chibeom Park: 0000-0003-0603-292X Soyoung Kim: 0000-0001-6679-7057 E

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DOI: 10.1021/acsami.8b13369 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX