Efficient Luminescent Microtubes of Charge-Transfer Organic

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Efficient Luminescent Microtubes of Charge-Transfer Organic Cocrystals involving TCNB, Carbazole Derivatives and Pyrene Derivatives Hao Sun, jing peng, kun zhao, Rabia Usman, Arshad Khan, and Mingliang Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01302 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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Crystal Growth & Design

Efficient Luminescent Microtubes of Charge-Transfer Organic Co-crystals involving TCNB, Carbazole Derivatives and Pyrene Derivatives Hao Sun, † Jing Peng, † Kun Zhao, † Rabia Usman, †Arshad Khan, ‡ and Mingliang Wang † * † School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China ‡ State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, P. R. China

*Corresponding

author.

Tel.:

+862585092237.

E-mail

[email protected].

ACS Paragon Plus Environment

address:

(M.W.)


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ABSTRACT: A series of binary CT co-crystals and microtubes involving 9-formylcarbazole, 9-acetylcarbazole, 1-formylpyrene and 1-acetylpyrene as CT donors and 1,2,4,5-tetracyanobenzene (TCNB) as CT acceptor were obtained via molecular self-assembly. Crystal structural analysis revealed that these organic co-crystals were constructed with D··A··D··A··D (Ia and IIb) and DAD···DAD (Ib and IIa) mode through π···π, hydrogen bond, C−H···π and C−N···π interactions. Infrared spectroscopy, thermal property profiles and solid-state absorption spectroscopy were recorded to understand physicochemical properties of all CT materials. The morphology and structure of microtubes were detected by SEM and PXRD. Investigation of luminescent properties shows that stepwise increase of luminescent efficiency from pristine donors to microtubes can be realized. These results provide an effective way for exploring new luminescent materials.

 INTRODUCTION In the last decades, research on organic luminescent materials has gained extensive attentions in consideration of their remarkable properties and plenty luminant organic molecules have played predominant roles by prepared as sensors,1-5 whiteners,6-8 colorants9-11 and active emitters in light-emitting diodes12-17. Supramolecular organic co-crystal chemistry which focuses on the study of various noncovalent interaction, such as hydrogen bonds, C-H•••π, π•••π and charge-transfer (CT) interactions induced by molecular recognition has also won numerous concern in recent years because of their remarkable structural topologies and potential applications.18 Co-crystal strategy can realize the introduction of diverse neutral molecules19,20 and is likely to obtain superior properties because of synergistic effects of the individual components21-23. Therefore, preparaing organic co-crystals can be an effective way to acquire novel luminescent materials. Moreover, various intermolecular interactions in co-crystal materials can produce modulation of emission colors and improvement in luminescence efficiency. 24 To date, the ordered self-assembly of diverse chromophores offer a unique alternative approach to develop molecular co-crystals. On the other hand, CT interaction, among other noncovalent interactions, demonstrates its superiority in


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achieving organic co-crystals with outstanding fluorescent properties. amounts

of

luminescent

CT

co-crystals

were

prepared

25,26

Large

involving

1,2,4,5-tetracyanobenzene (TCNB) as a typical CT electron acceptor and their emission can extensively cover visible region from blue through green to red.

27,28

Furthermore, a series of organic CT materials possessing distinctive morphology of microtubes which can be served as active optical waveguides has been prepared by reprecipitation

method.29,30

Comparing

with

aggregation-induced

quenching,

aggregation-induced emission enhancement (AIEE) effect may attribute to the luminescent behavior of the CT materials which exhibit high emission the solid state but

no emission in dilute solution.31-33 Thus, using such supramolecular strategy

may provide a novel and useful path to obtain promising luminescent materials with high efficiency. Carbazole derivatives and pyrene derivatives exhibit excellent optical and electronic properties because of the basic chromophores, and the introduction of formyl and acetyl-substituents contributes to the formation of CT co-crystals and the modulation of fluorescence emission.34-38 Herein, we offered successful illustration of obtaining high solid-state luminescence efficiency co-crystals through modulation of intermolecular packing modes involving 9-formylcarbazole, 9-acetylcarbazole, 1-formylpyrene and 1-acetylpyrene as CT donors (D) and 1,2,4,5-tetracyanobenzene (TCNB) as CT acceptor (A) via a classical solution co-crystallization process (scheme 1). Additionally, luminescent microtubes of the four co-crystals were prepared by reprecipitation method and the solid-state fluorescence efficiency show an obvious increase from donors through co-crystals to microtubes (co-crystal IIa for exception). The AIEE effect and synergistic effect contribute to the origin of such tendency which may has certain significance to construct miniaturized luminescent devices from CT complex materials.

 EXPERIMENTAL SECTION Material Synthesis. 9-formylcarbozole (CAS: 39027-95-7), 9-acetylcarbazole (CAS: 574-39-0), 1-formylpyrene (CAS: 3029-19-4), 1-acetylpyrene (CAS: 3264-21-9),



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1,2,4,5-Tetracyanobenzene (CAS: 712-74-3) and solvents were purchased from Alfa and used directly without further purification. CT complexes Preparation. Co-crystal Ia. equimolar 9-formylcarbazole and TCNB were dissolved in 15mL mixed solvent of acetonitrile/dichloromethane (2:1) in a conical flask. Crystallization of slow evaporating process at room temperature lasts for 7 days and then yellow schistose co-crystals yielded. Co-crystal Ib. equimolar 9-acetylcarbazole and TCNB were dissolved in 15mL mixed solvent of acetonitrile/dichloromethane (2:1) in a conical flask. Crystallization of slow evaporating process at room temperature lasts for 7 days and then yellow acicular co-crystals yielded. Co-crystal IIa. equimolar 1-formylpyrene and TCNB were dissolved in 15mL mixed solvent of acetonitrile/dichloromethane (2:1) in a conical flask. Crystallization of slow evaporating process at room temperature lasts for 7 days and then orange rod-like co-crystals yielded. Co-crystal IIb. equimolar 1-acetylpyrene and TCNB were dissolved in 15mL mixed solvent of acetonitrile/dichloromethane (2:1) in a conical flask. Crystallization of slow evaporating process at room temperature lasts for 7 days and then orange acicular co-crystals yielded. Microtubes of co-crystal Ia/Ib were synthesized by CT-induced self-assembly and solvent etching effect. In a typical synthesis, 10 mM co-crystals were adequately dissolved in 10 mL acetonitrile. The plained mother liquor (2.5 mL) was injected into 10 mL mixed solvent of 1:4 (v/v) ethanol/water a few minutes later. Flocculent suspension appeared within several seconds. IIa-complex and IIb-complex were also prepared by similar experiment procedure, except the mixed solvent of ethanol/water was set to 2:3. A small amount of colloidal samples were placed on a clean quartz substrate and dried under vacuum for SEM picture. The remanent samples were collected by centrifuging at water,

then dried

6000

r/min and repeatedly washed with

at 100 °C for other property tests.


ultrapure

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Measurement Studies.39,40 Powder X-ray diffraction (PXRD) patterns for co-crystals and microtubes were detected by an 18 kW advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). Single X-ray diffraction data for co-crystals were collected on Nonius CAD4 diffractometer with Mo Kα radiation (λ = 0.71073 Å). The structures were analyzed by direct methods using the SHELXL-2014/7 program and refined anisotropically using a full-matrix least-squares procedure. All non-hydrogen atoms were refined anisotropically and were inserted at the calculated positions and fixed at their positions. Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA) profiles were depicted using a Mettler-Toledo thermogravimetric analyzer from 50 to 350 °C at 10 °C/min under a dry nitrogen purge (20 mL/min). The morphologies and sizes of the samples were detected by using field-emission scanning electron microscopy (Navo Nano SEM450) at acceleration voltages of 30 kV Spectroscopic Measurements.21-23,39,40 UV−vis absorption spectra were recorded by using Shimadzu UV-2600 spectrometer. Infrared spectra were detected by using Bruker Tensor 27 FT-IR spectrometer with KBr pellets. Fluorescence spectra were recorded by using Horiba FluoroMax 4 spectrofluorometer. Solid-state fluorescent quantum yields were detected by using Quanta-υ accessory with excitation source of 320 nm. Fluorescence lifetime measurements for all materials were recorded by the time-correlated single-photon counting technique (TCSPC) using a TemPro Fluorescence Lifetime System (Horiba Jobin Yvon) equipped with a NanoLed excitation wavelength at 340 nm.

RESULT AND DISCUSSION Co-crystal structures. Single-crystal X-ray diffraction (SXRD) analysis was performed for the co-crystals to determine the supramolecular interactions and self-assembly fashion in the steric configuration. Co-crystal Ia and IIb includes one donor molecule and one acceptor molecule, while Ib and IIa crystallize in a 2:1 stoichiometric ratio of donor and acceptor in an asymmetric unit. All the



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crystallographic data are presented in Table 1. All interaction parameters are listed in Table 2, 3 and 4 respectively. Co-crystal Ia crystallized in the monoclinic crystal system with the space group of P21/n and comprises of one 9-formylcarbazole molecule and one TCNB molecule in the asymmetric unit. 9-formylcarbazole molecules and TCNB molecules aligned alternatively forming a typical mixed stack (D···A···D···A···) arrangement through C-N···π interaction, π···π interaction and charge transfer interaction. The columnar structure was regarded as main configuration to maintain the co-crystal structure. The parallel stacked columns associate together via C15-H15A···N5 and C18-H18···O interactions, while columns with different orientations were jointed through C9-H9A···N3 and C11-H11A···N3 interactions. Co-crystal Ib crystallized in the monoclinic crystal system with the space group of P21/n and includes one 9-acetylcarbazole molecule and half TCNB molecule in the asymmetric unit. Each TCNB molecules is sandwiched in a face-to-face manner (D···A···D) between two 9-acetylcarbazole molecules to constitute the same structural units via π···π interaction and charge transfer interaction. C-H···O hydrogen bonds principally linked the parallel structural units, and C-H···π interactions between adjacent 9-acetylcarbazole result in the structural units towards different directions. Co-crystal IIa crystallized in the monoclinic crystal system with the space group of P21/n and consists of one 1-formylpyrene molecule and half TCNB molecule in the asymmetric unit. Bearing a resemblance to co-crystal Ib, co-crystal IIa forms sandwich-like units via π···π interactions between 1-formylpyrene molecules and TCNB molecules. However, neighboring pyrene molecules point to the same orientation duo to the obvious π···π interaction along the b axis other than donors in co-crystal Ib. The adjacent columns formed by structural units are jointed by C-H···O hydrogen bonds. In spite of the same stoichiometric ratio of co-crystal Ia, co-crystal IIb crystallized in the orthorhombic crystal system with the space group of Pna21. The similar columnar framework with typical mixed stack (D···A···D···A···) arrangement is


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established by 1-acetylpyrene molecules and TCNB molecules through C-N···π interaction, π···π interaction and charge transfer interaction. Moreover, TCNB molecules act as a bridge to connect neighboring molecules via C−H··· N hydrogen bonds which contributes to extend further the network. FT-IR spectroscopy. Vibration spectroscopy was performed to expound the non-covalent interactions and CT interactions existing in the co-crystals. As shown in Figure 5, IR spectra show high purity of co-crystals and truly a composite of monomers. Compared to the parent components, frequency shifts of main bands in the co-crystals, such as C≡N stretching peak of free TCNB, appear to be similar in all four co-crystals with a small red shift of 1−2 cm−1. It is obvious that the C−H stretching bands (3049 and 3113 cm−1) of free TCNB tend to decrease in frequency shift upon co-crystals formation with donor molecules (Ia: 3043 and 3113 cm−1, Ib: 3035 and 3112 cm−1, IIa: 3039 and 3109 cm−1, IIb: 3040 and 3103 cm−1) which may be ascribed to the different π···π interactions in these crystals 41. These changes could be the proof for the existence of D–A interactions and first step for the further exploration of CT interactions. Thermal behavior. In order to appraise the thermal properties of the crystals, thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) were undertaken. As shown in Figure 6, the smooth baseline with appearance of one single sharp melting peak indicates that no transformation of crystal form or solvent removal occurs. DSC profile depicts that single phase transition endothermic peaks of the co-crystals appear between the endothermic peaks of the donors and TCNB, which reflects significantly new stacking patterns and intermolecular interactions after



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co-crystallization. In addition, TGA diagram illustrates decomposition of the co-crystals occurs around 275 °C and the decomposition rates for all four co-crystals exceed 88%. The results suggest the potential formation of CT co-crystals. Solid-state absorption spectra. For the sake of investigating the CT state of these co-crystals, diffuse reflectance absorption spectroscopy (DRS) was adopted. Attributed to the CT transitions, new extensive red-shifted absorption peaks occur at around 400 nm for Ia and Ib, and 500 nm for IIa and IIb in the absorption spectrum (Figure 7).42 Compared with Ia and Ib, absorption bands of IIa and IIb shift to longer wavelength which reveals that the HOMO-LUMO gap of CT interaction is smaller. Primary absorption bands of donors and TCNB are in accord with the absorption bands of corresponding co-crystals which also manifests the constitution of CT co-crystals evidently. In addition, absorption bands of Ia and IIa keep high similarity with absorption band of Ib and IIb, respectively, due to their similar structure. As we can see in Fig 7, IIa and IIb appear darker color than Ia and Ib due to the longer wavelength of CT absorption band which are the prominent characteristics of CT co-crystals. Scanning Electron Microscope (SEM) Image and Powder X-ray Diffraction Analysis (PXRD). As shown in Figure 8, Scanning electron microscopy (SEM) images were recorded at different magnification ratios which demonstrate that microtubes were synthesized successfully by the solution-processed method. It intuitively illuminates that hollow tubiform structure with a length about 40 μm for most individual tubes was obtained. Additionally, the longer microtubules of coprecipitation IIa may be caused by the more sequential π···π stacking mode along b axis.


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To further demonstrate the invariability of co-crystal structure and intermolecular interactions in the microtubes, XRD patterns of co-crystals and coprecipitation materials were measured as shown in Figure 9. Although there are non-significant differences between the diffraction intensities of co-crystals and coprecipitation materials, we can see that the positions of main peaks remain unchanged. Therefore, it can be speculated that the coprecipitation process does not induce phase segregation in the microtubes, indicating that the spatial construction of microtubes are consistent with the co-crystal structure. Additionally, ethanol-etching process should be responsible for the formation of microtubes during the CT-induced selfassembly process.29,30 Fluorescent Characteristics. All materials were subjected to fluorescence analysis to get insight into the luminescent behavior (Figure 10). Excited at 365 nm, co-crystal Ia and Ib show broad emission peaks at 500 and 510 nm, respectively. For co-crystal IIa and IIb, emission peaks appear at 598 and 561 nm when they are excited at 380 nm. Compared with the single components, the emission spectra of co-crystals exhibit obvious bathochromic shifts in the visible region, suggesting the formation of a typical CT transition state in these solids.43 Furthermore, the emission spectra of co-crystals and coprecipitation materials trend almost the same with a slight difference of 1-3 nm. To further appraise the optical−physical properties of these materials, we recorded their solid-state emission quantum yields and fluorescence lifetime as shown in Table 5. It is significant to notice that fluorescence efficiency of co-crystals depicts remarkable increase which is mainly attributed to the AIEE effect. However, the compact π···π stacking arrangement between adjacent pyrene molecules in co-crystal IIa induce concentration- or aggregation-quenching effect and, therefore, the quantum yields tend to decrease. In addition, we assume that ethanol-etching process triggers relatively loose molecular packing which results in a further rise of quantum yields and lifetime from co-crystals to coprecipitation materials. Remarkably, the increase in fluorescence lifetime from donors to co-crystals owes to the stronger stability of the CT state generated by TCNB than S0−S1 process.24



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 CONCLUSION In summary, four binary CT co-crystas were successfully prepared by solvent evaporation method and four luminescent microtubes with same components were obtain by coprecipitation process. Structure and PXRD analysis reveals that mixed stacking of D··A··D··A··D and DAD···DAD arrangement exist in co-crystals and the co-crystals and microtubes possess similar intermolecular interactions which construct the steric configuration. Investigations of optical-properties indicate that tunable emission and enhancement of luminescent efficiency can be realized via such co-crystal strategy, which may provide a novel way for the next-generation organic luminescent materials. Furthermore, the preparation of microtubes may have potential applications for the design of luminescent devices.

 ASSOCIATED CONTENT Supporting Information CIF files of co-crystal Ia, Ib, IIa, and IIb were provided in supporting information on the ACS Publications website.

 AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. ORCID Wang Mingliang: 0000-0002-3934-6100 Notes The authors declare no competing financial interest.

 ACKNOWLEDGEMENTS This project is supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (1107047002).

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Scheme 1. Chemical Structures of Acceptor and Donor Molecules. Figure 1. Structure of co-crystal Ia (a) Ellipsoid plots of co-crystal Ia (b)Stacking mode and intermolecular interactions of co-crystal Ia. Figure 2. Structure of co-crystal Ib (a) Ellipsoid plots of co-crystal Ib (b)Stacking mode and intermolecular interactions of co-crystal Ib. Figure 3. Structure of co-crystal IIa (a) Ellipsoid plots of co-crystal IIa (b)Stacking mode and intermolecular interactions of co-crystal IIa. Figure 4. Structure of co-crystal IIb (a) Ellipsoid plots of co-crystal IIb (b)Stacking mode and intermolecular interactions of co-crystal IIb. Figure 5. FT-IR spectra of the co-crystals. Figure 6. DSC and TGA profiles of crystals. Figure 7. (a) Solid state absorption spectra of CT co-crystals. (b) Photograph of co-crystals under visible light. Figure 8. SEM images of the microtubes. Figure 9. XRD patterns of the co-crystals (red) and coprecipitation materials (black). Figure 10. Solid-state fluorescence spectra of donors, co-crystals and coprecipitation materials (λex=380 nm for IIa and IIb, λex=365 nm for other samples). Table 1. Crystal Data and Structure Refinement. Table 2. Intermolecular Hydrogen Bonds Parameters in Co-crystals. Table 3. Face-to-Face π-Stacking Interactions in Co-crystals. Table 4. C−H···π and C−N···π Interactions in Co-crystals. Table 5. Solid-State Photophysical Properties for donors, co-crystals and coprecipitation materials.



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Scheme 1.

’ Figure 1.

Figure 2.

Figure 3.


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Figure 4.

Figure 5.

Figure 6.



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Figure 7.

Figure 8.


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Figure 9.

Figure 10. Table 1. crystal

Ia

Ib

IIa

IIb

Formula

C23H11N5O

C19H12N3O

C22H11N2O

C28H14N4O

Formula weight

373.37

298.32

319.33

422.43

Temperature/K

293

293

293

293

Crystal size/mm3

0.30 × 0.20 × 0.10

0.20 × 0.20 × 0.10

0.20 × 0.20 × 0.10

0.20 × 0.20 × 0.10

Morphology

schistose

acicular

rod-like

acicular

Crystal system

Monoclinic

Monoclinic

Monoclinic

Orthorhombic

Space group

P21/n

P21/n

P21/n

Pna21

a/ Å

7.5190 (15)

10.619 (2)

8.5800 (17)

7.1500 (14)

b/ Å

7.1310 (14)

6.0200 (12)

11.091 (2)

30.709 (6)

c/ Å

34.566 (7)

24.732 (5)

16.435 (3)

9.4910 (19)

α/°

90

90

90

90

β/°

95.93 (3)

102.33 (3)

97.56 (3)

90

γ/°

90

90

90

90

V/ Å 3

1843.4 (6)

1544.6 (6)

1550.4 (5)

2083.9 (7)

Z

4

4

4

4

ρ(calcd)/Mgm-3

1.345

1.283

1.386

1.346

θ range for data

1.185-25.368

1.686-25.367

2.221-25.372

1.326-25.370

collection/°F(000)

768

620

660

872

ref collected/unique

4600/4600

3007/3007

2841/2841

2050/2050

R1,wR2(I>2σ(I))

0.0593,0.1293

0.0736,0.1563

0.0679,0.1261

0.0538,0.1160

R1,wR2(all data)

0.1244,0.1545

0.1588,0.1852

0.1578,0.1529

0.0964,0.1389

Goodness-of-fit, S

0.987

1.029

0.971

1.094

CCDC

1574408

1574409

1574410

1574411



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Table 2. crystal

interaction

D-H(Å)

H···A(Å)

D···A(Å)

angle(°)

Ia

C15-H15A···N5 C18-H18···O C9-H9A···N3 C11-H11A···N3 C17-H17A···O C20-H20A···O C13-H13A···N2 C23-H23A···N4

0.93 0.93 0.93 0.93 0.93 0.93 0.93 0.93

2.62 2.25 2.70 2.69 2.17 2.35 2.65 2.69

3.550 3.176 3.399 3.471 3.100 3.202 3.327 3.522

175 172 133 143 176 152 130 149

Ib IIa IIb

Table 3.

crystal interaction dπ-π,dc-c(Å)a angle(°)b TCNB···carbazole 3.431,3.761 4.34 Ia TCNB···carbazole 3.443,4.179 4.16 Ib TCNB···pyrene 3.434,3.658 1.98 IIa pyrene···pyrene 3.450,3.795 2.20 TCNB···pyrene 3.444,3.664 1.42 IIb a

The interplanar separation (dπ−π) and the closest centroid distance (dc−c). bThe angles were measured between the mean planes of adjacent rings respectively. Table 4.

crystal interaction C20-N2···carbazole Ia C21-N3···carbazole C21-N3···carbazole C4-H4···carbazole Ib C5-H5···carbazole IIa C26-N2···pyrene IIb C28-N3···pyrene

distance(Å)a 3.650 3.305 3.808 2.992 2.777

angle(°)b 70.7 84.2 82.5 137 148

3.623 3.501

73.1 83.0

a

The distances were measured from the hydrogen atom or nitrogen atom to the center of the aromatic ring. bThe angles were measured between C−H−C or C−N−C. Table 5.

ΦF (%)/τF (ns) Ia Ib IIa IIb

donor 0.46/4.92 4.79/3.12 4.68/2.02 0.95/2.19

co-crystal 7.18/22.59 12.57/12.69 4.18/5.25 11.99/26.43


coprecipitation 15.79/26.94 15.17/15.70 8.64/24.72 15.74/21.22

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Table of Contents Graphic: For Table of Contents Use Only

Efficient Luminescent Microtubes of Charge-Transfer Organic Co-crystals involving TCNB, Carbazole Derivatives and Pyrene Derivatives Hao Sun, † Jing Peng, † Kun Zhao,† Rabia Usman, †Arshad Khan, ‡ and Mingliang Wang † * † School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China ‡ State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, P. R. China

Four CT co-crystals and microtubes involving TCNB, carbazole derivatives and pyrene derivatives were obtained via molecular self-assembly. Structure and PXRD analysis reveals that the co-crystals and microtubes possess similar intermolecular interactions. Investigations of optical-properties indicate that tunable emission and stepwise increase of luminescent efficiency are realized via such strategy.


Efficient Luminescent Microtubes of Charge-Transfer Organic

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