Supramolecular Design of Highly Efficient Two-Component Molecular

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Supramolecular Design of Highly Efficient Two-Component Molecular Hybrids Towards Structure and Emission Properties Tailoring Arshad Khan, Rabia Usman, Mir Sayed, Rongrong Li, Hui Chen, and Nongyue He Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01922 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Crystal Growth & Design

Supramolecular Design of Highly Efficient Two-Component Molecular Hybrids Towards Structure and Emission Properties Tailoring

Arshad Khan ab* †, Rabia Usman a †, Sayed Mir Sayed a, Rongrong Lid, Hui Chenc, Nongyue He ac*

a

State Key Laboratory of Bioelectronics, National Demonstration Center for Experimental

Biomedical Engineering Education, Southeast University, Nanjing 210096, P. R. China b

School of Material Science and Engineering, Hunan University Lushan S Rd, Yuelu Qu,

Changsha, Hunan, China, 410006 c

Economical Forest Cultivation and Utilization of 2011 Collaborative Innovation Center in

Hunan Province, Hunan Key Laboratory of Biomedical Nanomaterials and Devices, Hunan University of Technology, Zhuzhou 412007, China d

School of Pharmaceutical Chemical and Materials Engineering, Taizhou University, Taizhou,

Zhejiang, 318000, P. R. China.

† These authors contributed equally to this work

*Corresponding authors. Tel.: +862583790885. E-mail address: [email protected], (N.H.) [email protected]

ACS Paragon Plus Environment

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Abstract: Exploiting molecular systems to attain tunable emission characteristics out of the organic fluorophore is of great pertinence for the construction of solid-state luminescent materials owing to their fascinating applications in optoelectronics. Herein, we have designed three Charge transfer (CT) molecular assemblies utilizing carbazole luminophores (9-benzoyl carbazole (BC), 9-(para Tolyl carbazole (TC) and N-(4-formyl phenyl) carbazole (FC)) as donor compounds and 1,2,4,5tetracyanobenzene (TCNB) as an acceptor building block to tailor the structure and emission properties. Unlike, TC and FC, BC has a carbonyl spacer between carbazole and phenyl group, which allow more short contacts and therefore, effect the binary crystalline self-assembly. Detail structural and spectroscopic studies revealed the formation of alternate sandwich motifs between the donors and acceptor in a cofacial fashion, which resulted in tunable molecular structure and photophysical properties. Cocrystals XII and XIII present identical emissions due to the similar molecular packing modes (DAD···DAD) and stoichiometric ratio (2:1) whereas, cocrystal XI exhibits varied molecular packing features (DADA) and molar ratio (3:2) with yellow-green fluorescence. The present study demonstrates the significance of molecular design as an effective route that could fine-tune the molecular packing, stoichiometry, and luminescence characteristics, thereby; bring out efficient optical properties out of the single component via cocrystal strategy for construction of novel solid state luminescent materials.


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Introduction Interest in organic solid-state fluorescent materials has evoked tremendously in recent years owing to their tailorable structure and properties based on the molecular level tuning with emerging applications in light emitting diodes 1, 2, photovoltaic devices3, sensors4, lasers5, 6 etc. However, to obtain such materials can be very challenging in the wake of their non- fluorescent behavior upon formation of molecular aggregates in the solid state due to aggregation-caused quenching (ACQ)7. It is suggested that supramolecular interactions particularly, hydrogen-bonding and π···π interactions have played a crucial part in regulating the optical properties 8. Hence, to tune the emission characteristics of a given organic chromophore, sophisticated chemical alteration offers a facile scheme to control different interactions and molecular stacking in the solid state to accomplish new functional molecular materials 9-11. Organic cocrystals, are crystalline single-phase materials formed of electron-rich donors and electron-poor acceptors in a definite stoichiometric ratio, offer an efficient route to modulate the physiochemical properties based on the various supramolecular interactions 8, 12-17. Much attention has been devoted to the design of organic cocrystals in recent times by virtue of their intriguing properties particularly for organic optoelectronic applications

18-25,

although the co-crystal

“Qinhydrone” was already discovered in 1844 26, whereas the definition of “cocrystal” has been established with a much broader intention since 2018 27. Due to limited advances, many challenges are still facing by the scientific community as how to realize effective cocrystallization of the two different components. In this direction, many strategies (vapor-phase, and mechanochemical techniques) have been reported but solution based procedure is the most preferable to generate good quality crystals which is essential for further structural characterization. More interestingly, the facile synthesis via solution-based approach makes the cocrystal approach more promising for exploration of novel physical and chemical phenomena compared to the single component system, where tedious multi-step reactions are involved. Carbazole derivatives are well-known dyes and have been used as host materials for their excellent optical and electronic properties, and the introduction of carbonyl and methyl as substituents contribute to the formation of hydrogen bonds and C-H···π interactions, which could create more packing modes and form co-crystals easier, while the carbazole core will facilitate in planer configuration (π···π) which leads to the modulation of fluorescence emission 28-34. We believe that using carbonyl group as a spacer and as a side chain may achieve different stacking configuration



that will result in tailorable structure and emission. However, examples of two-component luminescent CT cocrystals of carbazole-based compounds are rarely explored to date.13, 14, 35. In this work, we have designed two-component molecular aggregate utilizing the carbazole derivatives (9-benzoyl carbazole (BC), 9-(para-Tolyl carbazole (TC) and N-(4-formyl phenyl) carbazole (FC) as donors and TCNB as acceptor (scheme 1) via cocrystallization procedure. The choice of the donors By introducing the carbonyl spacer in BC, the color, stoichiometry, and molecular packing of cocrystal XI have been altered compared to the XII and XIII. The asobtained luminescent CT materials demonstrate efficient tunable packing modes and emissions compared to their individual molecular precursors because of the mixed sandwich structure. Hence, the present work presents a facile approach towards understanding the role of supramolecular interactions for the rational design of molecular systems that possess desire tunable molecular structure and photophysical characteristics.

EXPERIMENTAL SECTION The three donor compounds and 1,2,4,5-Tetracyanobenzene were acquired from a commercial supplier. Analytical grade solvents were used throughout the experiment. Preparation of the CT Co-Crystal Synthesis of XI. The molecular cocrystal XI was obtained by dissolving BC and TCNB in 1:1 ratio in 15 ml dichloromethane/acetonitrile mixed solvent in a 50 ml flask. The flask was kept at room temperature for several days to obtain block green crystals in a good quantity. Synthesis of XII. The molecular cocrystal XII was obtained by dissolving TC and TCNB in 1:1 ratio in 15 ml dichloromethane/acetonitrile mixed solvent in a 50 ml flask. The flask was kept at room temperature for several days to yield block red crystals in good quantity. Synthesis of XIII. The molecular cocrystal XIII was obtained by dissolving FC and TCNB in 1:1 ratio in 15 ml dichloromethane/acetonitrile mixed solvent in a 50 ml flask. The flask was kept at room temperature for several days to harvest block red crystals in good quantity. Measurement Studies


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The PXRD patterns for the crystals were recorded using an 18 KW advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). Single X-ray diffraction data for crystals XI, XII and XIII were collected on Nonius CAD4 diffractometer with Mo Kα radiation (λ= 0.71073 Å). The structures were solved by direct methods using the SHELXL-2014/7 program and refined anisotropically using full-matrix least-squares procedure36. All non-hydrogen atoms were refined anisotropically and were inserted at their calculated positions and fixed at their positions. Spectroscopic Measurements. Infrared spectra were recorded on a Bruker Tensor 27 FT-IR spectrometer with KBr pellets. UVVis absorption spectra were collected on a Shimadzu UV-3600 spectrometer. Fluorescence spectra were obtained on a Horiba FluoroMax 4 spectrofluorometer. Fluorescence microscopy images. Fluorescence microscopy images were collected on Olympus BX51 imaging system excited at 365 nm. Solid fluorescent quantum yields were performed using Quanta-φ accessory with excitation wavelength at 320 nm. Fluorescence lifetime measurements for the crystals were undertaken by the time-correlated single-photon counting technique (TCSPC) using a TemPro Fluorescence Lifetime System (Horiba Jobin Yvon) equipped with a NanoLed excitation source of 340 nm. Solid-state structures The Single-crystals of the three cocrystals were subjected to single crystal X-ray diffraction (SXRD) to determine the various non-covalent routs and molecular packing in the crystalline state. The detail of the structural refinement, hydrogen bonds, and various π···π aromatic interactions are summarized in Tables 1, 2, and 3. Cocrystal XI: Cocrystal XI belongs to triclinic crystal system with two molecules of BC and three molecules of TCNB in the asymmetric unit. The crystal structure of XI represents typically mixed stack (DADA) alternate layer assembly with a non-uniform centroid distance (Table 3) between the D-A pair through collaborative CT interaction and C-H···N hydrogen bonds (figure



1a). TCNB molecule present between the two π stack columns stabilizing the neighboring TCNB molecules in π stack motif via C-H···N together with C4H4···π (2.90 Å) interactions along b-axis (Figure S1 of supporting information). In the layer structure, the donor and acceptor units are linked via CH···N and CH···O hydrogen bonds, constituting 2D sheet along a-axis as depicted in figure (Figure 1b). Cocrystal XII: Cocrystal XII crystallized in triclinic crystal system with two molecules of donors and one molecule of acceptor present in the asymmetric unit. In cocrystal XII the donor and acceptor aligned alternatively in a cofacial manner via CT interactions and CH17A···π (2.67Å to form mixed stack (DAD···DAD) 1D layer structure (figure 2a). TCNB molecule act as a bridge to connect donor molecules through a hydrogen bond (C-H···N3) (table 3) and weak C-H15···π (3.22Å) to form an extended 2D sheet along the c-axis (figure 2b). Cocrystal XIII: Complex XIII comprises two molecules of donors and one molecule of the acceptor in the asymmetric unit and crystallized in triclinic space group Pī. Interestingly, the molecular packing in XIII resembles the complex XII, with acceptor molecule become sandwiched between the two carbazole units forming an alternate mixed stack structure supported by CT π···π and C17H7···π (2.817 Å) interactions (figure 3a). The TCNB molecules hold the donor molecules by (C-H···N) hydrogen bonds forming a 2D network along c-axis as illustrated in figure 3b. Powder X-ray diffraction (XRD) Powder diffraction curves of the as-prepared cocrystals are different compared to the pristine samples, revealing good ordered CT self-assemblies have been generated because of D-A interactions (Figure S2, S3, S4 supporting information). Interestingly, the simulated PXRD profile


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(figure 4) of the three complexes are well coincided with the experimental PXRD curves of the ground crystals, signifying the same molecular arrangement. FT-IR Spectroscopy FT-IR spectroscopy is recorded to revel the nature of D-A interactions in the CT composites by observing the frequency changes in the main modes (C_H and C=C stretching bands) of the cocrystals against the unbound TCNB crystals. The three Complexes (XI: 3105 and 3034 cm-1; XII: 3105 and 3042 cm-1; and XIII: 3106, 3044) displayed a downshift in the C_H vibration band compared to the free TCNB (3113 and 3047 cm-1), while 1484 cm-1 (C=C stretching bands) of TCNB shifted to 1488 cm-1 for XI, 1489 cm-1 for XII, and 1489 cm-1 for XIII, indicating the increasing electron density of benzene ring upon cocrystals formation 37 (figure 5). It is believed, that the different hydrogen bonds and CT interactions between the donor and acceptor result in the decrease in C_H bands frequency shift and increase in (C=C stretching bands) upon generation of CT supramolecular self-assembly, which is also reported in the previous work 8, 38. Optical Characteristics Solid-state absorption studies Solid-state absorption has been widely used to corroborate the formation of CT state in the CT complexes

39-41.

UV-vis was performed for the as-prepared molecular aggregates and compared

with the individual donors (Figure S5). Remarkably, the three CT complexes reveal broad red shift absorption bands in the visible region 423-600 nm for XII and XIII and absorption band at 437 nm for XI: with respect to the single components systems, which stated obvious charge transfer between the D-A pair 42, 43. The formation of CT complexes is moreover reflected in the cocrystals colors (figure 6) under the fluorescence microscope using UV light showing tunable luminescence



from green to red relative to blue emitting monomers crystals, which can be ascribed to the successful accomplishment of binary CT molecular self-assembly process 44, 45. Luminescent properties of the CT complexes Solid-state fluorescence investigation was undertaken to explore the relationship of D-A arrangement on the luminescence behavior of the crystals. As depicted in figure 7, the three cocrystals (XI-515; XII: 601; XIII: 604) exhibit new red shift emissions against the monomeric compounds. Due to the mixed stack π···π modes, stable CT state has been attained between the donor and acceptor species which led to the bathochromically shifted emissions in these cocrystals 10, 15, 46, 47.

In view of the structural discussions, we reasoned that the cocrystals XII and XIII are isostructural and have same stoichiometric ratio, thus presenting almost similar absorption and emission peaks with the appearance of red color. The carbonyl spacer in BC donor of XI effect the molecular selfassembly behavior thereby resulted in multiple hydrogen bonds (C-H···N) (Table 2) relative to the carbonyl in XII and methyl in XIII. These moderately robust hydrogen bonds (C-H···N) stabilize the ground state n (HOMO) to a greater degree compared to excited state π* (LUMO), resulting in high HOMO-LUMO gap between the TCNB and carbazole in XI against the XII/XIII cocrystals respectively, which in turn lead to color variation in these CT cocrystals XI and XII/XIII 13, 14, 48. Furthermore, solid-state fluorescence lifetime and quantum yield studies were carried out to better understand the optical properties. All the three CT complexes present increase fluorescence lifetime (Figure S6) and quantum yield values (Table 4) as a result of the CT transition state generated between the donor and acceptor 49. Considering these observations we interpreted that a stable mixed two-component CT sandwich self-assembly has been achieved that favor the formation of excimers, which in turn leads to improved photophysical properties 36, 50, 51.


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Conclusion In summary, we have successfully fabricated three novel supramolecular CT assemblies to achieve unusual stoichiometry and tunable luminescence properties via rational molecular design. Due to the identical stoichiometry (2:1) and molecular packing (DAD···DAD), cocrystals XII and XIII (red color) present similar emissions behavior relative to cocrystal XI (green color) which has different molecular packing modes (DADA) and stoichiometry (3:2), because of the carbonyl spacer. The photophysical properties of these cocrystals were dominated by the mixed sandwich motifs, as validated by the extensive structural and spectroscopic investigations. Finally, we anticipate our supramolecular design strategy leads to variation in sandwich motifs, D-A interactions and, stoichiometry, is a powerful procedure in tuning the structural and luminescent properties and can be utilized to fabricate novel optical materials via facile cocrystal strategy. Acknowledgments This research was financially supported by the National Key Research and Development Program of China (2017YFA0205301), NSFC (61701176), Hunan Key Research Project (2017SK2174), and the Economical Forest Cultivation and Utilization of 2011 Collaborative Innovation Center in Hunan Province [(2013) 448].

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(45) Li, S.; Yan, D. Two-Component Aggregation-Induced Emission Materials: Tunable One/Two-Photon Luminescence and Stimuli-Responsive Switches by Co-Crystal Formation. Adv. Opt. Mater. 2018, 6, 1800445-1800454 (46) Zhu, W.; Zhu, L.; Sun, L.; Zhen, Y.; Dong, H.; Wei, Z.; Hu, W. Uncovering the Intramolecular Emission and Tuning the Nonlinear Optical Properties of Organic Materials by Cocrystallization. Angew. Chem., Int. Ed. 2016, 55, 14023-14027. (47) Park, S. K.; Varghese, S.; Kim, J. H.; Yoon, S.-J.; Kwon, O. K.; An, B.-K.; Gierschner, J.; Park, S. Y. Tailor-Made Highly Luminescent and Ambipolar Transporting Organic Mixed Stacked Charge-Transfer Crystals: An Isometric Donor–Acceptor Approach. J. Am. Chem. Soc. 2013, 135, 4757-4764. (48) Bhowal, R.; Biswas, S.; Thumbarathil, A.; Koner, A. L.; Chopra, D. Exploring the Relationship between Intermolecular Interactions and Solid-State Photophysical Properties of Organic Co-Crystals. The J. Phys. Chem. C. 2018. (49) Liu, Y.; Zeng, Q.; Zou, B.; Liu, Y.; Xu, B.; Tian, W. Piezochromic Luminescence of Donor–Acceptor Cocrystals: Distinct Responses to Anisotropic Grinding and Isotropic Compression. Angew. Chem., Int.Ed. 2018, 57, 15670-15674. (50) Kunzelman, J.; Kinami, M.; Crenshaw, B. R.; Protasiewicz, J. D.; Weder, C. Oligo(pphenylene vinylene)s as a “New” Class of Piezochromic Fluorophores. Adv. Mater. 2008, 20, 119122. (51) Yan, D.; Delori, A.; Lloyd, G. O.; Friščić, T.; Day, G. M.; Jones, W.; Lu, J.; Wei, M.; Evans, D. G.; Duan, X. A Cocrystal Strategy to Tune the Luminescent Properties of Stilbene-Type Organic Solid-State Materials. Angew. Chem., Int.Ed.. 2011, 50, 12483-12486



Scheme, Figures and Tables captions Scheme 1: Chemical structure of donors and TCNB Figure 1 a) Mixed sandwich motif in cocrystal XI b) 2D hydrogen bonding sheet between BC and TCNB Figure 2 Mixed sandwich motif in cocrystal XII b) 2D hydrogen bonding sheet between TC and TCNB Figure 3 Mixed sandwich motif in cocrystal XIII b) 2D hydrogen bonding sheet between FC and TCNB Figure 4 Experimental (black) and simulated (red) PXRD of the carbazole based precursors and cocrystals Figure 5 a) IR profiles of the CT products and TCNB a) C−H str and (c) C=C str. Figure 6 Single crystals images under fluorescence microscope at 365 nm Figure 7 Emission spectra of the cocrystals and starting materials Table 1 Crystal structure information of carbazole based CT cocrystals Table 2 Intermolecular Hydrogen Bonds in CT products Table 3 Face-to-Face π-stacking Interactions in carbazole based Cocrystals Table 4 Solid-state PLQY and fluorescence lifetime values of CT products and pristine monomers


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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 Table 1 Crystal

XI

XII

XIII

Formula

C34H16lN7O

C24H16N3

C24H14N3O

Temperature/K

293

293

293

Crystal size/mm3

0.10×0.20×0.30

0.10×0.20×30

0.10×0.20×30

Morphology

block

block

block

Crystal system

Triclinic

Triclinic

Triclinic

Space group

Pī

Pī

Pī

a/ Å

7.5640(15)

8.5100(17)

8.3670(17)

b/ Å

9.4340(19)

8.6260(17)

8.4190(17)

c/ Å

19.516(4)

13.429(3)

14.007(3)

α/deg

94.01(3)

87.30(3)

86.46(3)

β/deg

97.03(3)

79.35(3)

77.34(3)

γ/deg

98.66(3)

69.70(3)

72.03(3)

V/ Å3

1360.9(5)

908.5(4)

915.7(4)

Z

2

2

2



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ρ (calcd)/Mg m-3

1.314

1.266

1.307

θ Range for data collection/°

1.05-25.37

1.54-25.74

1.49-25.36

F(000)

554

362

374

Ref collected/unique

5401/4993

3618/3377

3608/3357

R1, wR2(I >2σ(I))

0.0650, 0.1716

0.0671, 0.1501

0.0782, 0.1746

R1, w R2 (all data)

0.1231, 0.2068

0.1539, 0.1981

0.1157, 0.2001

Goodness-of-fit,S

1.001

1.019

1.044

CCDC

1887348

1887347

1887349

Table 2 D-H (Å)

H···A (Å)

D···A (Å)

∠D-H···A (°)

C5-H5A···N6

0.930

2.401

3.301

162

C22-H22A···O

0.930

2.401

3.272

156

C25-H25A···N7

0.930

2.399

3.288

160

C32-H32A···N5

0.930

2.662

3.518

153

C19-H19A···N7

0.930

2.676

3.600

172

C9-H9A···N4

0.930

2.676

3.553

158

C8-H8···N3

0.931

2.701

3.613

167

C9-H9A···N2

0.929

2.735

3.544

146

C10-H10A···N3

0.930

2.693

3.562

149

C10-H10A···N3

0.930

2.726

3.512

143

C8-H8A···N3

0.930

2.736

3.638

164

C9-H9A…N2

0.929

2.784

3.569

143

Crystal XI

XII

XIII

Table 3


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Crystal

Interaction

dπ-π, dc-c(Å)[a]

Angle (°) [b]

XI

TCNB ··· carbazole

3.330, 3.408

0.55

TCNB ···Benzene

3.263, 4.401

0.00

XII


3.242, 3.469

8.08

XIII


3.467, 3.664

10.10

Table 4 Crystals

BC

TC

FC

XI

XII

XIII

PLQY ΦF (%)

0.48

4.79

4.85

9.6

18.5

21.5

Fluorescence lifetime τF (ns)

1.19

1.05

2.20

19.09

17.20

15.46



Supramolecular Design of Highly Efficient Two-Component Molecular Hybrids Towards Structure and Emission Properties Tailoring Arshad Khan a*, Rabia Usman a, Nongyue He ab* Hui Chen b, Rongrong Li c, Sayed Mir Sayed a, Table of Contents Three novel charge transfer molecular self-assemblies based on carbazole compounds have been designed which show tunable structure and luminescence features.


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Supramolecular Design of Highly Efficient Two-Component Molecular

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