Charge-Transfer Emission of Mixed Organic Cocrystal Microtubes over

Dashed circle shows single end-open tube with a hexagonal cross section. (c) TEM image of a typical phenanthrene–TCNB microtube. Inset shows the ...
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Charge-Transfer Emission of Mixed Organic Cocrystal Microtubes over the Whole Composition Range Yan-Qiu Sun, Yi-Long Lei, Xu-Hui Sun, Shuit-Tong Lee,* and Liang-Sheng Liao* Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, China S Supporting Information *

ABSTRACT: A series of crystalline mixed cocrystal microtubes comprising organic chargetransfer (CT) complexes has been prepared. The emission colors of the mixed cocrystal microtubes can be tailored from green to orange at low dopant concentrations (0 < x ⩽ 5%), while their hexagonal cross sections can transform into square ones gradually at higher concentrations (0.15 < x < 1). In addition, we can further extend the solvent-processed synthetic route to other CT pairs based on structural compatibility consideration.

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individual components. As such, they have gained increasing attention in the field of molecular crystal engineering.20,21 Typically, noncovalent interactions including halogen and hydrogen bonding, CT, and π−π stacking have been used widely to direct supramolecular cocrystal formation.22−25 Moreover, an elegant example has demonstrated that pyrene derivative (1,3,6,8-tetramethylpyrene, TMPY) can be selected as a host to form mixed molecular cocrystals based on the π−π stacking interaction between TMPY and perylene, which can be used to realize enhanced emission and lasing.26 In addition, ambipolar organic field-effect transistors (OFETs) derived from cocrystal has also been constructed by complementary hydrogen bonding interaction between dipyrrolopyridine (DP-P2P) and naphthalenediimide (NDI),27 which can not be regarded as a simple mixing of DP-P2P and NDI. The in depth understanding for cocrystal formation would also be crucial to improve the efficiency of organic solar cells (OSCs).28 To date, realization of mixed molecular cocrystals over the whole composition range still remains a difficult challenge due to undesired structural mismatch between different components, which tends to induce phase segregation and inhibit their coassembly. A luminescent CT cocrystal, composed of an electron donor and an acceptor, may be a promising candidate to access this purpose considering their inherent CT interactions and perfect structural similarity of different CT complexes. Herein, we develop a simple and available design principle for constructing a series of crystalline mixed CT microtubes of

oping provides a versatile and powerful approach to realize high-efficiency luminescence by introducing guest molecules (energy acceptor) into host material (energy donor) in organic light-emitting diode devices (OLEDs),1−4 which usually involves an efficient energy transfer process from a donor molecule to an acceptor molecule. Besides improvement in luminescence efficiency, tunable emission colors can also be achieved depending on the molar ratios of the donor and acceptor. In practice, the doping strategy can also be successfully extended to various supramolecular systems, such as organogels,5−8 DNA assemblies,9,10 inorganic/organic hybrids,11,12 and charge-transfer (CT) complex microtubes.13 Although supramolecular self-assembly methods are a versatile tool to control intermolecular interactions and energy transfer process in the solid state, successful examples have been rare because it is rather difficult to manipulate the coassembly of donor/acceptor molecules.14−16 Recently, we successfully synthesized highly efficient white-light (WL) emitting CT microtubes by incorporating a dopant of pyrene into blueemitting naphthalene-1,2,4,5-tetracyanobenzene (TCNB) matrix.13 In addition, tunable emission was also achieved in organogels through control of the efficiency of energy transfer from a blue-emitting gel of 2,3-bis(decyloxy)anthracene to specifically designed tetracene-based acceptors.17 Thus, exploring spectrum-matching and structural complementary donor/ acceptor pairs by rational design and selection of the individual components is an appropriate approach to realize tunable emission in supramolecular systems. However, a promising way to synthesize new materials is by forming cocrystals. Cocrystals are composed of two or more diverse neutral molecules18,19 and can generate novel and/or enhanced properties owing to the synergistic effects of the © XXXX American Chemical Society

Received: July 24, 2014 Revised: December 26, 2014

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DOI: 10.1021/cm5027249 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials (anthracene)x(phenanthrene)1−x(TCNB) (0 ⩽ x ⩽ 1). Tunable emission from green to orange has been realized by altering the dopant concentration of anthracene in phenanthrene−TCNB host (0 < x ⩽ 5%), suggesting efficient energy transfer processes from phenanthrene−TCNB to anthracene−TCNB can occur owing to diverse CT features and the perfect structural match between them. The formation of the threecomponent mixed molecular cocrystals was also achieved even at higher concentration of anthracene (0.15 ⩽ x ⩽ 1). Hence, the (anthracene)x(phenanthrene)1−x(TCNB) microtubes can be regarded as a homogeneous solid solution composed of phenanthrene−TCNB and anthracene−TCNB based on spectral and structural analysis results. It is worth noting that hexagonal phenanthrene−TCNB microtubes (x = 0) would transform into square anthracene−TCNB microtubes (x = 1) gradually, which can be well elucidated according to their crystal structural simulation results. The design principle for mixed CT complex cocrystals can also be extended to other host/guest pairs. Moreover, the growth mechanism of (anthracene)x(phenanthrene)1−x(TCNB) microtubes has also been proposed, which provides a promising model for exploration of such complicated supramolecular system. As reported previously,13 TCNB-based CT complexes, such as naphthalene−TCNB, can form highly luminescent singlecrystalline microtubes. Here, we selected anthracene and phenanthrene as electron donors and TCNB as an electron acceptor (Scheme 1) to synthesize mixed CT complex

Figure 1. SEM images of phenanthrene−TCNB microtubes at (a) low and (b) high magnification. Dashed circle shows single end-open tube with a hexagonal cross section. (c) TEM image of a typical phenanthrene−TCNB microtube. Inset shows the corresponding SAED pattern. (d) Molecular packing of phenanthrene−TCNB along the C axis, i.e., the [001] direction.

Scheme 1. Molecular Structures of (1) Anthracene, (2) Phenanthrene, (3) 1,2,4,5-Tetracyanobenzene (TCNB), (4) Fluorene, and (5) Carbazole

cocrystals due to the well-matched structural relationship between phenanthrene−TCNB and anthracene−TCNB. Scanning electron microscopy (SEM) images at different magnification ratios exhibit that phenanthrene−TCNB microtubes were also synthesized successfully by this solution-processed strategy, as shown in Figure 1a,b. It reveals clearly that 1D hollow tubular structure with a hexagonal cross section was obtained, as confirmed by the single end-open tube (dashed circle). Each individual tube has a length of ∼40 μm and a diameter of ∼3 μm. Transmission electron microscopy (TEM) image (Figure 1c) further demonstrates that a single microtube has an inner diameter of ∼900 nm and wall thickness of ∼650 nm. In addition, the corresponding selected area electron diffraction (SAED) pattern (Figure 1c inset) combined with the powder X-ray diffraction (XRD) pattern (Figure 2d) shows that phenanthrene−TCNB microtubes have a single-crystalline structure growing along the [001] direction. Meanwhile, the molecular packing configuration in the bulk crystal of phenanthrene−TCNB29 (Figure 1d) represents that the twocomponent molecules are nearly planar and stacked alternately along the C axis, suggesting an intrinsic CT interaction between phenanthrene and TCNB. Furthermore, a 2:3 (v/v) ethanol/ water mixture as a poor solvent was required to induce the formation of tubular structures. Instead of the well-defined

Figure 2. (a) Steady-state emission spectra of doped phenanthrene− TCNB microtube films spin-coated on a quartz substrate at a doping concentration of 0 (black), 0.05% (green), 0.5% (yellow), and 5% (blue) upon excitation with 365 nm. Insets show the corresponding photographs of doped phenanthrene−TCNB microtubes under daylight (upper row) and a UV lamp (365 nm, bottom row). (b) Solid-state absorption spectra of doped phenanthrene−TCNB microtubes at different doping ratios. (c) Excitation and emission spectra of phenanthrene−TCNB (green) and anthracene−TCNB (orange) microtubes. The excitation wavelength is 365 nm. (d) XRD patterns of doped phenanthrene−TCNB microtubes at different doping ratios.

microtubes, solid micro- and nanorods were obtained due to CT-induced interaction if only water was used as a poor solvent (Figure S1, Supporting Information). Hence, it can be concluded that ethanol-etching process after CT-induced selfassembly should be responsible for the formation of phenanthrene−TCNB microtubes, similar to that of naphthalene−TCNB microtubes.13 Subsequently, we tried to synthesize doped phenanthrene−TCNB microtubes by introducing anthracene as a dopant by a similar synthesis process. Figure S2, Supporting Information, clearly demonstrates that the hexagonal tubular structures of doped phenanthrene− TCNB complex remain unchanged at 0.5% anthracene as B

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Chemistry of Materials dopant, which can also be observed by a single broken tube (dashed line). To further determine whether anthracene has been doped into the phenanthrene−TCNB microtubes successfully, the steady-state emission spectra of doped phenanthrene−TCNB microtube films spin-coated on a quartz substrate at different doping ratios from 0 to 5% were performed by excitation with 365 nm. As shown in Figure 2a, the spectrum from undoped phenanthrene−TCNB microtube film (black curve) shows a broad structureless band around 548 nm, which is derived from a CT transition from the HOMO of phenanthrene to the LUMO of TCNB.30−32 Obviously, doped phenanthrene− TCNB microtube films only show single emission bands due to the spectrum superposition between phenanthrene−TCNB and anthracene−TCNB. The emission band (blue curve) is red-shifted to 565 nm at 5% anthracene as dopant, corresponding to a typical orange emission of anthracene− TCNB component. Similarly, the emission peaks of doped phenanthrene−TCNB microtube films are also consistent with the microarea PL spectra recorded from the corresponding single microtube of (anthracene)x(phenanthrene)1−x(TCNB) (Figure S7, Supporting Information), suggesting that anthracene molecules are well-dispersed in phenanthrene−TCNB host. In addition, the corresponding photographs of doped phenanthrene−TCNB microtubes (Figure 2a inset) excited with a UV lamp (365 nm) exhibit that tunable emission colors from green through yellow to orange can be achieved with increasing doping concentration of anthracene. Solid-state absorption spectra of pure and doped phenanthrene−TCNB microtubes at different doping ratios were also carried out, as shown in Figure 2b. Besides the absorption band at 449 nm arising from phenanthrene−TCNB component when remaining unchanged, another band at 519 nm due to the characteristic absorption of anthracene−TCNB component is significantly enhanced when the doping ratios of anthracene are increased from 0 to 5%. Meanwhile, the excitation and emission spectra of phenanthrene−TCNB and anthracene−TCNB microtubes is shown in Figure 2c. Obviously, the good overlap between the fluorescence spectrum of phenanthrene−TCNB complex and the excitation spectrum of anthracene−TCNB complex in the wavelength range of 450−550 nm, probably induce the Förster resonance energy transfer (FRET) between them. Besides, it is worth noting that the unit cells of the two crystals have similar lattice parameters (a = 9.413 Å, b = 13.104 Å, and c = 7.206 Å for phenanthrene−TCNB, and a = 9.505 Å, b = 12.748 Å, and c = 7.417 Å for anthracene−TCNB).29,33 Close structural compatibility between phenanthrene−TCNB and anthracene−TCNB makes it feasible to introduce anthracene as a dopant to phenanthrene−TCNB host. Except for some slight change of the relative intensity ratios between (110) and (020) planes, the X-ray powder diffraction (XRD) patterns of doped phenanthrene−TCNB microtubes remain almost unchanged (see Figure 2d), similar to the case of pure single-crystalline phenanthrene−TCNB microtubes. Thus, we can infer that the incorporation of anthracene did not destroy the original crystal structure of the phenanthrene−TCNB microtubes, which is also beneficial for efficient energy transfer process between phenanthrene−TCNB and anthracene−TCNB. The possible energy transfer process is shown in Figure 3a. As shown in the schematic, a suitable distance between donor and acceptor has been reached due to a trace of anthracene molecules well-dispersed in the phenanthrene−TCNB complex

Figure 3. (a) Schematic representation of the energy transfer in doped CT complex microtubes. (b) Time-resolved fluorescence decay of doped phenanthrene−TCNB microtubes excited with a 370 nm laser.

matrix, which facilitates efficient energy transfer from the excited phenanthrene−TCNB to anthracene−TCNB molecules. Actually, by adjusting the dopant concentration, the tunable emission could be achieved in the three component CT complex microtubes obtained by highly efficient Förster resonance energy transfer (FRET) from the excited phenanthrene−TCNB to anthracene−TCNB molecules. Meanwhile, time-resolved fluorescence decay profiles of pure and doped phenanthrene−TCNB microtubes are presented in Figure 3b, which provide an additional evidence to support the energy transfer process in the present CT-induced supramolecular system. The two-exponential average lifetime measured for the pure phenanthrene−TCNB microtubes is calculated to be 56.4 ns, which is shortened to 23.9 ns at 5% anthracene as dopant (Table S5, Supporting Information). The lifetimes of phenanthrene−TCNB component are gradually shortened by increasing doping concentration of anthracene, further revealing the occurrence of efficient energy transfer from phenanthrene−TCNB to anthracene−TCNB in the doped CT complex microtubes. As a consequence, an emission-selfabsorption process in the present system can be excluded in light of the continuous lifetime change of phenanthrene− TCNB. The quantum yields of pure and doped phenanthrene− TCNB microtubes were also recorded in order to investigate the luminescence efficiency of the doped CT system, as shown in Figure S6, Supporting Information. Remarkably, we can find that the quantum yield is decreased from 40.61% (pure phenanthrene−TCNB) to 30.18% (5% dopant concentration). Similar to the case of naphthalene−TCNB microtubes,13 phenanthrene−TCNB microtubes also exhibit the features of an aggregation-induced emission (AIE) effect, which suggests that a solid environment is needed to reduce rapid internal conversion and recombination in solution, as reported previously.34 Thus, the occurrence of efficient energy transfer from phenanthrene−TCNB with a high luminescence efficiency to low efficiency anthracene−TCNB at low dopant concentrations can be demonstrated. Then the quantum yield slightly declines to 27.09% at 30% doping concentration and basically keeps unchanged even for pure anthracene−TCNB, which infers that the energy transfer efficiency from phenanthrene−TCNB to anthracene−TCNB is close to 100% for dopant concentration over 5%. On the basis of the above-mentioned results, it has been well demonstrated that the tubular coassembly of phenanthrene− TCNB and anthracene with low doping concentration (0 < x ⩽ 5%) can be realized successfully. Next we investigate whether higher concentration of anthracene can be effectively C

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Chemistry of Materials introduced into phenanthrene−TCNB microtubes considering the high structural similarity between phenanthrene−TCNB and anthracene−TCNB, which is important for the formation of mixed molecular cocrystals. Similar two step solvent-etching coassembly was also used to synthesize (anthracene)x(phenanthrene)1−x(TCNB) microtubes (0.15 ⩽ x < 1), except the volume ratio of ethanol and water was slightly changed. Spectral measurements (Figure 4) of (anthracene)x(phenanthrene)1−x(TCNB) microtubes (0.15 ⩽ x < 1)

Figure 5. XRD patterns of mixed CT microtubes of (anthracene)x(phenanthrene)1−x(TCNB) (0.15 ⩽ x ⩽ 1).

The fluorescence microscopy tests of mixed CT microtubes of (anthracene)x(phenanthrene)1−x(TCNB) (0 ⩽ x ⩽ 1) excited by UV light were also carried out. As shown in Figure 6, we can observe that all mixed CT microtubes show uniform

Figure 4. (a) Steady-state emission spectra of (anthracene)x(phenanthrene)1−x(TCNB) (x = 0.15, 0.3, 0.5, 0.75) and pure anthracene−TCNB microtube spin-coated on a quartz substrate upon excitation with 365 nm. Insets show the corresponding photographs of the mixed CT complex microtubes under daylight (upper row) and a UV lamp (365 nm, bottom row). (b) Solid-state absorption spectra of (anthracene)x(phenanthrene)1−x(TCNB) (x = 0.15, 0.3, 0.5, 0.75) and pure anthracene−TCNB microtube films.

were first performed to investigate the formation of mixed CT cocrystals containing various amounts of anthracene. One can observe that all mixed CT microtube films (Figure 4a inset) exhibit intense orange emission upon excitation with 365 nm. Figure 4a shows that the emission peaks around 568 nm derived from anthracene−TCNB component remain almost unchanged even when the molar ratio of anthracene is set to 0.75, except for a slight red-shift. As shown in Figure 4b, the absorption spectra of mixed CT complex microtube films exhibit two broad structureless CT bands originating from phenthracene−TCNB and anthracene−TCNB, and characteristic absorption bands of phenanthrene−TCNB component disappear with increasing molar ratios of anthracene. Meanwhile, the typical absorption bands of anthracene−TCNB are red-shifted gradually, dissimilar to the case in doped phenanthrene−TCNB microtubes (0 < x ⩽ 5%). These spectra analysis results reveal that luminescent mixed CT cocrystals containing higher molar ratios of anthracene could be achieved (0.15 ⩽ x < 1). . To further demonstrate whether the anthracene/phenanthrene ratios can be varied continuously in mixed CT microtubes, XRD patterns of (anthracene)x(phenanthrene)1−x(TCNB) (0.15 ⩽ x ⩽ 1) were also measured, as shown in Figure 5. Although the positions and intensities of the diffraction peaks are dependent on the composition, we can see that the positions of (110) and (020) planes remain unchanged and that the relative intensity ratios between them increase gradually in the (anthracene)x(phenanthrene)1−x(TCNB) microtubes, when the molar ratios of anthracene increase from 0.15 to 0.75. Hence, it can be inferred that the introduction of higher molar ratios of anthracene does not induce phase segregation in the mixed CT microtubes, suggesting that each sample of mixed complex is composed of almost uniform composition and crystal structure.

Figure 6. Fluorescence microscopy images of mixed CT microtubes of (anthracene)x(phenanthrene)1−x(TCNB) (0 ⩽ x ⩽ 1) at x = (a) 0, (b) 0.05%, (c) 0.5%, (d) 5%, (e) 0.15, (f) 0.3, (g) 0.5, (h) 0.75, and (i) 1, by excitation with unfocused UV light (330−380 nm).

emission regardless of doping ratios of anthracene, revealing anthracene molecules are uniformly distributed in individual tubes. Meanwhile, the emission colors of mixed CT microtubes are consistent with those of their corresponding microtube films when excited with UV light. Besides, whole and microarea P L s p ec t r a o f si n g le m ic r o t u b e s o f ( a n t h r a c e ne)x(phenanthrene)1−x(TCNB) (0 ⩽ x ⩽ 1) were shown in Figure S7, Supporting Information. We can observe that the two spectra are almost the same, which reveals that each microtube emits uniform light. All of the measurement results mentioned above confirm that anthracene−TCNB and phenanthrene−TCNB form a homogeneous solid solution, similar to the case of the doped phenanthrene−TCNB microtubes (0 < x ⩽ 5%). The photostability of the mixed CT microtubes (Figure S8, Supporting Information) has also been studied by analyzing the fluorescence intensity decays of three microtube films irradiated with and without a 250 W UV lamp (365 nm). It is obvious that phenthracene−TCNB is photosensitive, and the relative fluorescence intensity ratio of the phenthracene−TCNB microtube film is 26.8% compared with its initial state after 24 h. Otherwise, the relative fluorescence intensity of anthracene−TCNB microtube film only slightly decreases, suggesting that its photostability is much better than that of phenthracene−TCNB. In addition, the photostability of (anthracene)0.5(phenanthrene)0.5(TCNB) is close to that of anthracene−TCNB. On the basis of the above results, we can D

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Chemistry of Materials infer that the doping process is beneficial to increase the photostability of the mixed CT cocrystals. Indeed, higher molar ratios of anthracene are also used to induce morphology transformation of mixed CT microtubes of (anthracene)x(phenanthrene)1−x(TCNB), which have been clearly demonstrated by the SEM and TEM results shown in Figure 7. Specifically, one can observe from Figure 7a−c that

Figure 8. Fluorescence microscopy images of mixed CT microtubes of (carbazole)x(fluorene)1−x(TCNB) (0 ⩽ x ⩽ 1) at x = (a) 0, (b) 0.05%, (c) 0.5%, (d) 5%, (e) 0.25, (f) 0.5 and (g) 1, excited with UV light. The scale bars are 10 μm.

quartz substrate were also examined, as shown in Figure 9a. The emission band at 512 nm originating from fluorene−

Figure 7. SEM images of mixed CT complex microtubes of (anthracene)x(phenanthrene)1−x(TCNB) (x = 0.15) at (a) low and (b) high magnification. (c) TEM image of a typical (anthracene)0.15(phenanthrene)0.85(TCNB) microtube. Dashed circle shows single end-open tube with a hexagonal cross section. SEM images of mixed CT complex microtubes of (anthracene)0.5(phenanthrene)0.5(TCNB) at (d) low and (e) high magnification. (f) TEM image of a typical (anthracene)0.5(phenanthrene)0.5(TCNB) microtube. SEM images of anthracene−TCNB microtubes at (g) low and (h) high magnification. (i) TEM image of a typical anthracene−TCNB microtube. Inset shows the corresponding SAED pattern.

Figure 9. (a) Corresponding steady-state emission spectra of (carbazole)x(fluorene)1−x(TCNB) microtube films spin-coated on a quartz substrate. The excitation wavelength is 365 nm. Insets show the photographs of (carbazole)x(fluorene)1−x(TCNB) microtube films under a UV lamp (365 nm). (b) Solid-state absorption spectra of (carbazole)x(fluorene)1−x(TCNB) microtube films. (c) Time-resolved fluorescence decay of (carbazole)x(fluorene)1−x(TCNB) microtubes excited with a 370 nm laser. (d) XRD patterns of (carbazole)x(fluorene)1−x(TCNB) microtubes.

the hexagonal cross sections of tubular (anthracene)0.15(phenanthrene)0.85(TCNB) are similar to those of pure and doped phenanthrene−TCNB microtubes (0 ⩽ x ⩽ 5%), as confirmed by the single end-open tube (dashed circle). Nevertheless, the cross sections of (anthracene)0.5(phenanthrene)0.5(TCNB) microtubes transform into square, as shown in Figure 7d−f. In addition, SEM images (Figure 5d,e) exhibit that the length-diameter ratios of (anthracene)0.5(phenanthrene)0.5(TCNB) microtubes dramatically increase, compared to those of (anthracene)x(phenanthrene)1−x(TCNB) microtubes (0 ⩽ x ⩽ 0.15). We can also see that pure anthracene−TCNB microtubes with square cross section (Figure 7g−i) were achieved when the molar ratio of anthracene is set to 1. Similar to that of phenanthrene−TCNB, the corresponding SAED pattern (Figure 7i inset) combined with the powder XRD pattern of anthracene−TCNB shows that the microtubes have also a single-crystalline structure growing along the [001] direction. To further confirm the feasibility of our design principle based on a structural compatibility consideration, we extend the two-step solvent-etching coassembly strategy to other CT systems, such as fluorene−TCNB and carbazole−TCNB. Figure 8a−g exhibit the fluorescence microscopy images of mixed CT complex microtubes of (carbazole)x(fluorene)1−x(TCNB) (0 ⩽ x ⩽ 1) excited by UV light. It can be observed clearly that the emission colors of (carbazole)x(fluorene)1−x(TCNB) microtubes from green through yellow to red can be tailored depending on doping concentration of carbazole. The corresponding steady-state emission spectra of (carbazole)x(fluorene)1−x(TCNB) microtube films spin-coated on a

TCNB is red-shifted to 592 nm arising from carbazole−TCNB. Similar to the fluorescence microscopy results, the photographs of (carbazole)x(fluorene)1−x(TCNB) microtube films (Figure 9a inset) excited with a UV lamp (365 nm) exhibit that tunable emission colors from green to red can be altered with increasing doping concentration of carbazole. Furthermore, we measured the absorption spectra of (carbazole)x(fluorene)1−x(TCNB) microtube films at different doping concentration. With the increasing doping ratios of carbazole− TCNB, a characteristic absorption peak around 495 nm due to the characteristic absorption of carbazole−TCNB is significantly enhanced, while the fluorene−TCNB characteristic absorption peak around 440 nm gradually disappears as shown in Figure 9b. Combined with the emission spectra, we confirmed the (carbazole)x(fluorene)1−x(TCNB) CT system also can be achieved over the whole composition range. Meanwhile, the fluorescence lifetimes (Figure 9c) are gradually shortened in the present (carbazole)x(fluorene)1−x(TCNB) microtube system. Thus, we inferred that energy transfer process from fluorene−TCNB to carbazole−TCNB can also occur effectively. Moreover, XRD patterns of (carbazole)x(fluorene)1−x(TCNB) microtubes shown in Figure 9d reveal that the mixed CT complexes at different doping concentration are composed of almost uniform compositions and crystal structures. E

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hexagonal cross sections. Nevertheless, the calculated surface energy of (110) plane is much larger than that of the (020) plane for the bulk crystal of anthracene−TCNB, giving rise to the occurrence of the preferred growth of (110) face. Meanwhile, the (020) face for anthracene−TCNB will disappear in order to reduce the total surface energy of the system. Thus, it can be concluded that the cross sections of phenanthrene−TCNB microtubes are hexagonal, while those of anthracene−TCNB are square. Accordingly, the doped phenanthrene−TCNB microtubes remain their original hexagonal cross sections at low dopant concentrations, and mixed CT cocrystals with higher anthracene concentrations can selfassemble to tubular structures with square cross sections.

According to above spectral and structural analysis results, a possible formation mechanism for mixed CT complex microtubes of (anthracene)x(phenanthrene)1−x(TCNB) (0 ⩽ x ⩽ 1) over the whole composition range is proposed, as shown in Scheme 2. The growth processes of (anthraceScheme 2. Schematic Illustration of the Possible Formation Mechanism for Mixed CT Microtubes of (Anthracene)x(Phenanthrene)1−x(TCNB) (0 ⩽ x ⩽ 1) over the Whole Composition Range

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CONCLUSIONS In summary, a simple two step solvent etching after CTinduced coassembly method was developed to construct a series of crystalline mixed CT complex microtubes of (anthracene)x(phenanthrene)1−x(TCNB), with 0 ⩽ x ⩽ 1. The CT interactions and structural compatibility of phenanthrene−TCNB and anthracene−TCNB facilitates the formation of (anthracene)x(phenanthrene)1−x(TCNB) microtubes over the whole composition range, which provides a promising platform to investigate energy transfer and structural relationship between two different luminescent CT complexes. One interesting consequence of mixed CT complex of (anthracene)x(phenanthrene)1−x(TCNB) is that it is possible to introduce the second donor, having different electronic properties, into the crystal lattice of a molecular complex. In principle, this process can also be extended to the fabrication of more mixed molecular cocrystals comprising two and more diverse organic materials by rational design and selection of organic molecules. Specifically, realization of mixed molecular cocrystals provides a simple design principle for integration of different organic semiconductor materials with similar structural properties, which would have potential applications in optoelectronic devices, such as organic light-emitting field effect transistors (OLEFETs) and organic solar cells (OSCs).

ne)x(phenanthrene)1−x(TCNB) microtubes include low concentration doping of anthracene (0 < x ⩽ 5%) and mixed cocrystal with higher anthracene concentrations (0.15 ⩽ x < 1). On the basis of the fact that the crystalline lattice parameters of phenanthrene−TCNB and anthracene−TCNB are very similar, we can expect that the random replacement of donor components occurs without the collapse of crystalline structure. In addition, it seems reasonable that the association constant for anthracene−TCNB is higher compared to that of phenanthrene−TCNB considering their donating abilities of the two donor components. Consequently, the replacement of donor sites in the mixed CT cocrystals should also be favored by the stronger intermolecular interaction for anthracene− TCNB even at low doping concentration of anthracene (0 < x ⩽ 5%). As expected, a suitable distance of ∼2−6 nm between phenanthrene−TCNB and anthracene−TCNB can be ensured in the doped phenanthrene−TCNB microtubes, which facilitates the occurrence of highly efficient energy transfer process from phenanthrene−TCNB to anthracene−TCNB. Further, mixed CT cocrystals of (anthracene)x(phenanthrene)1−x(TCNB) can also form when anthracene with higher molar ratios (0.15 ⩽ x < 1) is introduced to phenanthrene−TCNB microtubes. Importantly, the cross sections of these mixed CT microtubes transform from hexagonal to square, which could be elucidated by structural simulation of the two pure CT complex cocrystals using the Materials Studio package (Figure S9, Supporting Information). From the simulation results, we can observe that the predicted growth pattern and thermodynamically stable morphology for phenanthrene−TCNB (Figure S9a, Supporting Information) and anthracene−TCNB (Figure S9b, Supporting Information) are both one-dimensional (1D) rod-like structures with

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ASSOCIATED CONTENT

* Supporting Information S

Experimental method, characterization, and additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (No. 61036009, 21161160446, 61177016 and 51072126), and the Key University Science Research Project of Jiangsu Province (12KJB510028). This is also a project supported by the Fund for Excellent Creative Research Teams of Jiangsu Higher Education Institutions. F

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DOI: 10.1021/cm5027249 Chem. Mater. XXXX, XXX, XXX−XXX