Molecular-Oriented Self-Assembly of Small Organic Molecules into

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Molecular-Oriented Self-Assembly of Small Organic Molecules into Uniform Microspheres Jie Ma,† Zhi-Zhou Li,† Xue-Dong Wang,*,† and Liang-Sheng Liao*,†,‡ †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, P. R. China ‡ Institute of Organic Optoelectronics, Jiangsu Industrial Technology Research Institute (JITRI), Wujiang, Suzhou, Jiangsu 215211, P. R. China S Supporting Information *

ABSTRACT: Self-assembly of small organic π-conjugated molecules into micro- and nanostructures has drawn much attention because of their wide applications in miniature optoelectronic devices. Many efforts have been focused on the controlled fabrication of organic micro/nanostructures, which are determined by various factors including kinetic and thermodynamic process but still remain poorly understood. In our research, two πconjugated molecules of (E)-3-(4-(diphenylamino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2-en-1-one (DFHP) and (E)-3-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2-en-1-one (DFPHP) with a small difference of substituent groups were designed and synthesized. By the solution-exchange method, DFHP are selfassembled into amorphous microspheres while DFPHP tend to grow into crystalline rhombic microcrystals. The head of DFHP organic molecules can form hydrogen bonds with ethanol molecules and the tail of DFHP dissolves in the dichloromethane (DCM) phase, which exhibits amphiphilic nature, contributing to the self-assembly of microspheres. In a contrast, both the head and the tail of DFPHP can have hydrogen bonds with the ethanol molecules, which leads to the ordered DFPHP molecular packing and then the formation of rhombic microcrystals. Furthermore, based on simulated growth/equilibrium morphology, DFHP form the amorphous microspheres other than the crystalline microcrystals due to the lower attachment energy (Etotal = −43.5 kcal/mol) as compared with that (Etotal = −53.1 kcal/mol) of DFPHP. Our demonstration can indeed builds the structure−morphology relationship for rational fabrication of organic micro/nanostructures, which would contribute to integrated optoelectronics.

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nanostructures is not fully clear and far behind their inorganic counterparts.25,26 Self-assembled organic micro/nanostructures have plentiful regular morphologies such as one-dimensional (1D) wires/ rods,27−29 2D plates,30 and 0D spheres.31,32 Among them, micro/nanoscale spheres potentially act as the optical component for organic optoelectronic devices.33 For example, the monolayer of polystyrene spheres can enhance light extraction in organic light-emitting diodes (OLED).34 In recent years, Prof. H. Sun reported that organic dye-doped polymer microspheres can provide the whispering gallery mode cavity for the organic solid-state lasers.35 However, most of the previous reports demonstrate the microspheres based on polymers; there is very little research about small molecules. As compared with organic polymer molecules, small organic molecules inherently have a series of advantages such as varying molecular structure, molecular weight, and simple synthetic method.36 However, the growth mechanism of microspheres based on small organic molecules, especially the relationship

ontrolled synthesis of inorganic semiconductor and/or metal micro/nanostructures with well-defined sizes, shapes, and compositions has provided a powerful tool for tailoring their properties and paved the way for far-reaching applications ranging from optoelectronics,1,2 to catalysis,3,4 to plasmonics,5,6 and to medical diagnostics.7,8 Organic micro/ nanostructures are fundamentally different from inorganic ones, because of weak van der Waals intermolecular interactions.9,10 The growth morphology of organic micro/nanostructures coupled with its size is of great importance for industrial separation, purification, and storage processes.11,12 Moreover, they play a crucial role in developing novel organic semiconductor technologies, such as single-crystal microlasers,13−15 transistor arrays,16−18 sensors,19−21 and photoconductors.22,23 For instance, organic single-crystalline hexagonal-microdisks (HMDs) of p-distyrylbenzene (DSB) have been demonstrated as the whispering-gallery-mode (WGM) microresontors with a cavity quality-factor (Q) of ∼210 for the laser action. However, although tailor-made molecules can generally be obtained by organic synthesis, the molecules can also grow up into different shapes according to different self-assembly processes.24 Our understanding on the growth mechanism of organic micro/ © 2017 American Chemical Society

Received: June 29, 2017 Revised: July 31, 2017 Published: August 23, 2017 4527

DOI: 10.1021/acs.cgd.7b00910 Cryst. Growth Des. 2017, 17, 4527−4532

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between desirable morphologies and the molecular structure, remains a big challenge.37 Herein, two π-conjugated molecules of (E)-3-(4(diphenylamino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2en-1-one (DFHP) and (E)-3-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2-en-1-one (DFPHP) with the small, different substituent groups on the part of triphenylamine were designed and synthesized, as model compounds, to explore the growth mechanism of their selfassembled organic microstructures, especially the organic microspheres. Through the facile solution-exchange method, we can obtain DFHP amorphous microspheres and crystalline DFPHP rhombic microcrystals. According to the interaction between these two organic molecules and the solvent molecules, one tail of the DFHP organic molecule has a hydrogen bond with an ethanol molecule and another side of DFHP dissolved in the DCM phase, which exhibits amphiphilic nature contributing to the formation of microspheres. In contrast, both sides of DFPHP can form hydrogen bonds with the ethanol molecules, which lead to the ordered DFPHP molecular packing with formation of regular crystalline rhombic microcrystals. Furthermore, based on growth morphology of these organic molecules, DFHP molecules form microspheres other than the crystalline microspheres due to the lower attachment energy (Etotal = −43.5 kcal/mol) as compared with that (Etotal = −53.1 kcal/mol) of DFPHP growth morphology. Our demonstration reveals the growth mechanism for these self-assembled solid-state microspheres based on small organic molecules, which would contribute to organic optoelectronics. As shown in Scheme 1, two organic π-conjugated small molecules of (E)-3-(4-(dipheylamino)-1-(4-fluoro-2Scheme 1. Synthetic Route of DFHP (X = H) and DFPHP (X = OMe) Organic Molecules

Figure 1. (a,b) Bright-field optical micrograph and the corresponding fluorescence microscopy image of the self-assembled DFHP microspheres. The scale bars are both 20 μm. (c) SEM image of these asprepared DFHP microspheres with a scale bar of 5 μm. Inset: the magnified SEM image of one typical microsphere with the diameter of 1.3 μm (d) Histogram of diameter distribution of organic microspheres. (e,f) Bright-field optical micrograph and the corresponding fluorescence microscopy image of these self-assembled DFPHP microcrystals. The scale bars are both 100 μm.

hydroxyphenyl)prop-2-en-1-one (DFHP) and (E)-3-(4-(bis(4methoxyphenyl)amino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2-en-1-one (DFPHP) were designed and synthesized through the classical Claisen-Schmidt condensation reaction (Figures S1 and S2, Supporting Information). The facile solution-exchange method is utilized to fabricate DFHP and DFPHP nano/microstructures.38 In the typical fabrication process, DFHP (4.1 mg) is dissolved in the solvent of CH2Cl2 (1.0 mL). To this solution (∼10 mmol/L), 10 mL of ethanol was added under vigorous stirring. After 10 min standing, this mixture is dropped onto a clean glass substrate (Brand: Electron Microscopy Sciences). After the solvents evaporated, DFHP molecules turn into dense and homogeneous hemispherical solid microstructures (Figure 1a). The dense microspheres emit orange light stimulated by UV light (300−380 nm) from the fluorescence microscopy image (Figure 1b). Furthermore, Figure 1c presents the SEM image of these asprepared DFHP microstructures, which are presented as a hemispherical morphology, whose diameter (D) ranges from

1.2 to 1.5 μm. The magnified SEM image of an individual hemisphere is shown in the top right inset of Figure 3a. From this image, we can find that these self-assembled microstructures possess a hemispherical shape. In addition, Figure 1d shows the size distribution of DFHP microstructures in a certain range for SEM images, and the size of organic microspheres is in accord with the normal distribution. The percentage of spheres with D of 1.35 μm is about 40%, which indicates that the spherical microstructure fabricated by DFHP molecule possesses good homogeneous properties. Besides, as shown in Figure S5a, these DFHP microstructures have spherical morphology. Furthermore, the AFM image (Figure S5b) shows that our obtained DFHP microstructures are microscale hemispheres. As compared with these as-prepared hemispherical microstructures of DFHP fabricated by this facile solution-exchange method, DFPHP molecules spontaneously assembled into rhombic crystal-like microstructures rather than microspheres under the same preparation condition (Figure 1e). In addition, the rhombic organic microcrystals have an edge length ranging from 5.0 to 20.0 μm. In addition, Figure 1f shows the microscopy fluorescence image of these as-prepared DFPHP microcrystals, which exhibited intense red emission with the optical wave guiding characteristic under the excitation of UV band (330−380 nm) from a mercury lamp. In a short conclusion, organic hemispheres and rhombic microcrystals are obtained from DFHP and DFPHP molecules, respectively. 4528

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Figure 2. (a) Simulated growth morphology of DFHP molecules based on the attachment energies using Materials Studio package. Inset: photograph of the millimeter-scale DFHP single crystals by the recrystallization. (b) Simulated growth morphology of DFPHP molecules based on the attachment energies using the Materials Studio package. Inset: photograph of the centimeter-scale DFPHP single crystals by recrystallization.

In order to investigate the growth mechanism of these selfassembled organic microstructures, we simulated the growth morphology of DFHP and DFPHP crystals based on the attachment energies using the Materials Studio package. From the calculated results, the growth morphology of DFHP molecules is predicted as the cubic plate-like structure (Figure 2a), which is totally different from our as-prepared organic microspheres. For comparison, the simulated DFPHP growth morphology is predicted as rhombic plate-like morphology (Figure 2b), which is consistent with our as-prepared rhombic organic microcrystals. Especially, by the recrystallization, we can prepare centimeter-scale DFPHP single crystals (inset of Figure 2b), which has the same morphology as the simulated growth morphology. In sharp contrast, we can only obtain millimeterscale DFHP crystals (inset of Figure 2a), which can indicate that DFPHP molecules have better crystallinity than that of DFHP molecules. Based on the single-crystal X-ray analysis, these as-prepared DFHP bulk single crystals belong to the monoclinic system, space group P1̅ with cell parameters of a = 8.090(18) Å, b = 9.486(2) Å, c = 13.310(3) Å, α = 92.34(10), β = 96.68 (9), γ = 94.62(10) (Table S1). As compared with DFHP crystals, DFPHP has the same crystal system and space group with cell parameters of a = 10.479(7) Å, b = 11.511(5) Å, c = 12.942(5) Å, α = 96.10(3), β = 108.46(3), γ = 116.47(3) (Table S2). Further insights into the different growth mechanism, we calculate the attachment energy Eattach {hkl}s and surface energy γ{hkl} of various crystal faces (hkl) of DFHP and DFPHP growth morphology and equilibrium morphology based on the unit cell of these two organic crystals (Figures S4 and S5).39 As seen from the calculated results (Table 1), for DFHP, Eattach {001}s > attach attach attach Eattach {100}s > E{011}s > E{101̅}s > E{010}s. Meanwhile, the values of surface energy of various crystal faces (hkl) arrange in the reverse order, γ{010}s > γ{101̅}s > γ{011}s > γ{100}s > γ{001}s. Correspondingly, the (001) and (100) crystal planes have the largest exposed facet area percentage of ∼60%. In any event, both growth and equilibrium morphology (Figure S6) are mainly bounded by the lowest surface energy (001) crystal faces with γ{001}s = 16.6 kcal/mol (Table 1). In comparison, for DFPHP, the calculated attachment Eattach {hkl}s and surface energy γ{hkl} of various crystal faces (hkl) follows the order (Table 2):

Table 1. Attachment and Surface Free Energies of Various Crystal Facets (hkl) of DFHP Calculated Using the Material Studio Package Miller index {hkl}

dhkl (Å)

Eattach {hkl} (kcal mol−1)

%total facet area

γ{hkl} (kcal mol−1)

{001}s {100}s {011}s {101̅}s {010}s {111̅ }̅ s {11̅0}s

13.2 8.0 7.5 7.2 9.4 5.9 6.4

−33.16 −45.05 −46.77 −50.88 −54.02 −64.29 −67.85

35.7 24.3 13.6 7.3 16.3 1.2 1.6

16.6 22.6 23.6 25.7 27.3 33.1 34.9

Table 2. Attachment and Surface Free Energies of Various Crystal Facets (hkl) of DFPHP Calculated by Using the Material Studio Package Miller index {hkl}

dhkl (Å)

Eattach {hkl} (kcal mol−1)

%total facet area

γ{hkl} (kcal mol−1)

{11̅0}s {11̅1̅}s {001}s {011̅}s {101̅}s {010}s {100}s

9.2 7.9 11.7 8.9 8.9 9.9 8.6

−43.16 −51.29 −52.12 −53.46 −62.14 −73.65 −86.67

25.6 12.4 22.8 17.6 11.7 7.9 1.5

18.1 22.8 22.7 23.7 29.7 34.2 48.5

attach attach attach attach Eattach {11̅0}s > E{11̅1}̅ s > E{001}s > E{011̅}s > E{101̅}s, and γ{101̅}s > γ{011̅}s > γ{001}s > γ{11̅1̅}s > γ{11̅0}s. The biggest exposed crystal plane is (110̅ ) with γ = 18.1 kcal/mol, which is larger than that (16.6 kcal/mol) of DFHP. For the growth of bulk single crystals, a low surface energy (16.6 kcal/mol) of DFHP represents a metastable but kinetically favored high-energy state while in a slow growth process, such as recrystallization in a sealing bottle. However, in the solution-exchange process, these DFHP molecules are inclined to form spherical morphology, especially homogeneous hemispherical solid microstructures, while exposed to atmosphere with a relatively quick growth process. In sharp contrast, DFPHP organic molecules prefer to form rhombic plate-like morphology with ordered molecular packing according to a

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relatively high surface energy of 18.1 kcal/mol.39 This is further verified by the total attachment energy (−43.5 kcal/mol) of DFHP being much larger than that (−53.1 kcal/mol) of DFPHP. Therefore, DFHP molecules tend to form these stable solid-state microspheres rather than the regular crystalline organic microcrystals. To further investigate the growth mechanism of organic DFHP microspheres, in situ bright-field optical measurements are utilized to observe the growing process of these organic microstructures. For DFHP molecule, only 3 microspheres appear at 10 s with diameters above 1 μm (Figure 3a and b). As

hydroxy group or F atom on the head side can form hydrogen bonding with ethanol molecules. In the mixed solvents, a small amount of CH2Cl2 tends to form a hemispherical drop on the top of a clean glass substrate which is covered by ethanol molecules. So, the DFHP arrangement follows this procedure: It is dissolved in the CH2Cl2 hemispherical droplet while the head side of DFHP interacts with the ethanol molecules. After the mixed solvent evaporated totally, the hemisphere based on DFHP molecules formed. For DFPHP, both head and tail parts can generate hydrogen bonding with ethanol molecules. Thus, DFPHP prefers to form microcrystal structure rather than hemispherical structure, which is consistent with the abovediscussed conclusion based on the calculated attachment energy and surface energy. Furthermore, the XRD pattern of DFHP organic microspheres shows no peaks (top panel, Figure 3j), which indicates the amorphous nature of these DFHP microstructures. In sharp contrast, the XRD pattern of these as-prepared DFPHP microstructures shows a series of characteristic peaks (bottom panel, Figure 3j), whose first peak located at 9.6 deg corresponds to the interplanar spacing of 9.2 Å, which corresponds to the crystal plane (11̅0). At the same time, the second peak located at 19.2 deg with the interplanar spacing of 4.6 Å can be ascribed to the crystal plane (22̅0). Thus, these two observed peaks both originate from the same sequence of crystal planes {110̅ }, which is also consistent with the above-discussed DFPHP growth morphology with bottom/top planes of (11̅0). Thus, these DFPHP rhombic organic microcrystals are crystalline. In summary, two π-conjugated molecules of (E)-3-(4-(bis(4methoxyphenyl)amino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2-en-1-one (DFPHP) and (E)-3-(4-(diphenylamino)phenyl)-1-(4-fluoro-2-hydroxyphenyl)prop-2-en-1-one (DFHP) with the small, different substituent groups on the triphenylamine part were designed and synthesized. By a facile bottom-up solution-exchange method, DFHP are proposed to aggregate as amorphous microspheres while DFPHP assembled into organic crystalline rhombic microcrystals. The head of DFHP has a hydrogen bond with an ethanol molecule and the tail of DFHP dissolves in the DCM phase, which exhibits amphiphilic nature contributing to the formation of microspheres. In contrast, both the head and the tail of DFPHP can form hydrogen bonds with the ethanol molecules, which lead to the ordered DFPHP molecular packing and then the formation of regular crystalline rhombic microcrystals. Moreover, according to the simulated growth/equilibrium morphology, DFHP molecules form amorphous microspheres rather than crystalline microspheres due to the lower attachment energy (Etotal = −43.5 kcal/mol) as compared with that (Etotal = −53.1 kcal/mol) of DFPHP growth morphology. Our demonstration reveals the growth mechanism of these small organic molecules microspheres, which contributes to the integrated optoelectronic devices.

Figure 3. (a−d) In situ bright-field optical micrographs of the selfassembly process of the DFHP microspheres at 0, 10, 30, and 51 s. The scale bars are all 10 μm. (e−h) In situ bright-field optical micrographs of the formation process of the DFPHP microcrystals at 0, 30, 90, and 180 s. The scale bars are all 20 μm. (i) Schematic depiction of the growth mechanism of DFHP organic microspheres and DFPHP organic microcrystals. (j) XRD patterns of DFHP organic amorphous microspheres (black line, top panel) and DFPHP organic crystalline microcrystals (red line, bottom panel).

time goes by (from 10 to 51 s), the number of microspheres with the same size increases from 3 to ∼100 (Figure 3b,c,d). Accordingly, three plate-like rhombic microcrystals of DFPHP were formed at 30 s (Figure 3e and f), and the edge length is about 0.8 μm. At 90 and 180 s (Figure 3g and h), these three DFPHP microcrystals become larger with the edge length from 1.2 to 3.0 μm; DFPHP molecules can only be attached on the small microcrystals as the solvent evaporates without forming new microcrystals. This phenomenon indicates that the DFHP molecules are not able to form microcrystals in a short time, which is consistent with the high attachment energy and the low surface energy of DFHP. As Figure 3i shows, there are two conpoments in these two compounds: the head side includes the hydroxy group and F atom, while triphenylamine or 4-methoxy-N-(4-methoxyphenyl)-N-phenylaniline is the tail side. For DFHP, only the

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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/acs.cgd.7b00910. Experimental details, 1H NMR spectra, absorption and photoluminescence spectra, photoluminescence spectra, unit cell structure, and simulated equilibrium morphology (PDF) 4530

DOI: 10.1021/acs.cgd.7b00910 Cryst. Growth Des. 2017, 17, 4527−4532

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Accession Codes

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CCDC 1538294 and 1539755 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Xue-Dong Wang: 0000-0003-0935-0835 Liang-Sheng Liao: 0000-0002-2352-9666 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Key Research and Development Program of China (Grant No. 2016YFB0400700), the Natural Science Foundation of China (Grant No. 61575136), and the Natural Science Foundation of Jiangsu Province (BK20170330). 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 Education Institutions (PAPD) and by the “111” Project of the State Administration of Foreign Experts Affairs of China.

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