Dibenzothiophene Derivatives: From Herringbone to Lamellar Packing

Aug 9, 2010 - William A. Ogden , Soumen Ghosh , Matthew J. Bruzek , Kathryn A. McGarry , Luke Balhorn , Victor Young , Jr. , Lafe J. Purvis , Sarah E...
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DOI: 10.1021/cg100863q

Dibenzothiophene Derivatives: From Herringbone to Lamellar Packing Motif

2010, Vol. 10 4155–4160

Chengliang Wang,†,§ Huanli Dong,† Hongxiang Li,‡ Huaping Zhao,† Qing Meng,† and Wenping Hu*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, CAS, Beijing 100190, China, ‡Laboratory of Material Science, Shanghai Institute of Organic Chemistry, CAS, Shanghai 200032, China, and §Graduate University of Chinese Academy of Sciences, Beijing 100039, China Received June 29, 2010; Revised Manuscript Received July 25, 2010

ABSTRACT: It is generally believed that π-π stacking would be much more efficient than herringbone stacking for the transporting of charge carriers. The electron-withdrawing group sulphone unit was introduced into dibenzothiophene (DBT) derivatives, and lamellar structures were observed in the single crystals of the products along with strong, long-range π-π intermolecular interactions. As a contrast, the reduced materials adopted herringbone packing. We contributed this change of packing motif to the polarity of the sulphone unit. These results are meaningful to the molecular design to obtain π-π stacking.

Crystal engineering is to design and synthesize functional molecular solid-state structures, or arrangements, through tuning the intermolecular interactions.1 The arrangement, namely the packing motif, plays a very important role in the organic electronic device performance.2-4 For instance, the efficient charge transport of the organic field-effect transistors (OFETs) was strongly dependent on the molecular packing structures. Research has shown that there are four main possible packing motifs in organic solid states, named (1) typical herringbone packing without π-π overlap between neighbor molecules (e.g., pentacene5); (2) nonclassical herringbone packing with π-π overlap between neighbor molecules, which is also called slipped π-stacking in some citations6 (e.g., rubrene7); (3) lamellar packing, one-dimensional (1-D) π-stacking (we take 1,2,3,4-tetrafluoroanthracene8 as an example to demonstrate this packing motif); and (4) lamellar packing, two-dimensional (2-D) π-stacking (e.g., TIPS-PEN9). These kinds of packing motifs and the illustrations were presented in Figure 1. Although pentacene, which is a benchmark of the organic semiconductors and adopts herringbone stacking in crystals with C-H 3 3 3 π intermolecular interactions instead of π-π interactions, showed high FET performance with mobility up to 5 cm2/(V s) obtained from its polycrystalline thin films,10 it is generally believed that the π-π stacking would be the most efficient way to transport charge carriers. And almost all of the highest mobilities of FETs reported so far were using semiconductors which adopted the π-π stacking motif as active layers, such as rubrene (the highest performance for p-type single crystal transistors), TiOPc (the highest performance for p-type thin film transistors), compound 1 (the highest performance for n-type single crystal transistors), compound 2 (the highest performance for n-type thin film transistors), and compound 3 (the highest performance for n-type thin film transistors based on oligomers). Therein, rubrene adopts slipped π-π stacking and the other four compounds show 2-D π-π stacking. These results give a very clear conclusion of the importance of π-π stacking *To whom correspondence should be addressed. E-mail: [email protected]. Fax: 86 10 62527295. Telephone: 86 10 82615030.

(arrangements) on the properties. And their chemical structures, packing motifs (arrangements), and properties are described in Table 1. Lots of research has been accomplished (mainly by Anthony et al.) to prevent the C-H 3 3 3 π intermolecular interactions in order to obtain π-π stacking (or lamellar structure) in the crystals through substituting at the peri-positions of acenes.4,15,16 Rubrene and TIPS-PEN were the most representative molecules, which adopted slipped π-π stacking and lamellar structure, respectively, due to this strategy. Besides this strategy, introducing hydrogen bonds,17,18 halogenhalogen19 or chalcogen-chalcogen20,21 interactions, or increasing the C/H ratio21 of the molecules was also used to obtain lamellar structures. Moreover, introducing polarity, usually as electron-withdrawing groups, into the molecular structure also could result in lamellar structure. The design conception is quite concise and explicit. As we all know, p-type organic semiconductors usually possess a large π-conjugated system, and this electronrich system would lead to a herringbone structure for the electrostatic repulsion. If polarity is introduced into the chemical structure, the dipole-dipole attraction would counterbalance the electrostatic repulsion and this balance might result in lamellar structure. On the other hand, polar groups might also introduce hydrogen bonds and coordinate bonds into molecular arrangements,1 which might dramatically affect the packing motif along with the charge transport properties. However, there are rare citations in the literature16,22 that discuss this kind of strategy to obtain π-π stacking in crystal structures. Using linear acenes end-functionalized with a 1,4quinone moiety, Nuckolls’s group23 obtained a lamellar structure with π-π interactions. They ascribed the π-stacking to the dipolar structure with one end having electron-rich groups and the other end electron-withdrawing groups. Similar results (Figures 1C and 2A)8,24 were also obtained (in work directed by Swager and Gavezzotti et al.) from the acenes end-functionalized with a tetrafluoro-moiety. π-π stacking was also observed in the crystals of some derivatives end-capped with electron-withdrawing groups at both ends (e.g., trifluoromethylphenyl14 derivatives). In our previous

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Figure 1. Possible packing motifs in crystals. (A) herringbone packing (example, pentacene, RefCode PENCEN from CSD); (B) herringbone packing with π-π overlap between neighbor molecules, slipped π-stacking (example, rubrene, RefCode QQQCIG01 from CSD); (C) lamellar motif, 1-D π-stacking (example, 1,2,3,4-tetrafluoroanthracene, RefCode MIKGOD from CSD); (D) lamellar motif, 2-D π-stacking (example, TIPS-PEN, RefCode VOQBIM from CSD). Protons are omitted for clarity.

Table 1. Chemical Structures, Packing Motifs, and Mobilities of Representative Molecules Which Showed the Highest Mobilities in the Related Area

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Scheme 2. Synthesis Routes of DBT Derivatives

Figure 2. Lamellar motif obtained by introducing polarity into the chemical structure to balance the electrostatic repulsion: (A) 1,2,3,4tetrafluoroanthracene (RefCode MIKGOD from CSD), (B) 3 (the crystallographic data of 3 can be downloaded free from the Internet as Supporting Information for ref 14 at http://pubs.acs.org). Protons are omitted for clarity.

Scheme 1. DBT and Its Derivatives: From Herringbone to Lamellar Packing Motif

work, we reported some dibenzothiophene (DBT) derivatives25 and their applications in OFETs. In the following studies, we found that, by introducing sulphone groups, the packing motifs of two derivatives changed from herringbone into lamellar structure, as shown in Scheme 1. We attributed this change to the introduction of polar electron-withdrawing groups into the molecular structures. The relevant characteristics were also presented in this communication with the expectation that they would be helpful to molecular design through analysis of the structures and properties. The synthesis routes of the three derivatives of DBT were outlined in Scheme 2. The synthesis and the physicochemical properties of dibenzo[b,d]thiophene 5,5-dioxide (DODBT) could be easily found,26-28 and the synthesis of 3,7-bis(phenylethynyl)dibenzo[b,d]thiophene (BEDBT) has been reported in

our previous work.25 3,7-Bis(phenylethynyl)dibenzo[b,d]thiophene 5,5-dioxide (DOBEDBT) was synthesized through a similar route to its reduced counterparts BEDBT by using a Pd catalyzed modified Sonogashira coupling reaction. All three compounds were soluble in normal organic solvents (dichloromethane, tetrahydrofuran, toluene, chlorobenzene, etc.) and easily formed crystals, and they were all purified through recrystallization. As for BEDBT, it showed white rodlike crystals, while, as for DOBEDBT, it displayed pale yellow grains. The absorption and fluorescence emission spectra of DOBEDBT in dilute solution (CH2Cl2) at the concentration of 10-5 M were shown in Figure 4A. According to the UVvis spectra, DOBEDBT showed absorption peaks similar to those of BEDBT, but broader absorption peaks were observed with maximum absorption about 30 nm red shift for DOBEDBT. Previous reports26,29,30 have shown that the sulphone group was not a significant bathochromic group, so that it was not likely responsible for the red shift. In conclusion, the red shift might be attributed to the enhancement of the conjugated degree for DOBEDBT. The optical energy gaps determined from the initial absorption were 3.35 and 3.12 eV for BEDBT and DOBEDBT, respectively. To investigate the position change of the frontier orbital of DOBEDBT compared with BEDBT, density functional theory (DFT) calculations were performed using Gaussion 0331 at the B3LYP/6-31G(d) level. For BEDBT and DOBEDBT, the largest coefficients in the highest occupied molecular orbital (HOMO) were mainly delocalized on the hole molecules, as shown in Figure 3A and C. However, the coefficients in the lowest unoccupied molecular orbital (LUMO) were most located on the dibenzothiophene core as for DOBEDBT compared to BEDBT, suggesting the significant role of the electron-withdrawing group (sulphone unit) for DOBEDBT. The energy values of the HOMO and LUMO levels of BEDBT and DOBEDBT were calculated by using DFT (the HOMO levels were -5.41 and -5.74 eV; the LUMO levels were -1.85 and -2.31 eV, for BEDBT and DOBEDBT, respectively). The cyclic voltammogram (CV) of DOBEDBT showed irreversible oxidation peaks in CH2Cl2 solution at the concentration of 10-3 M (Figure 4B) using Bu4NPF6 (0.1 M) as supporting electrolyte. The electrochemical property of BEDBT was also described here for comparison. From Figure 4B, the onset oxidation potentials of DOBEDBT moved 0.33 V by comparison with BEDBT (the HOMO energy level of DOBEDBT was 0.33 eV lower than that of BEDBT, coincident with the result obtained by DFT calculations). The HOMO

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Figure 3. HOMO orbitals (A and C) and LUMO orbitals (B and D) of BEDBT and DOBEDBT obtained by using DFT calculations: (A and B) BEDBT; (C and D) DOBEDBT.

Figure 5. (A) Herringbone packing motif without π-π stacking of DBT (RefCode DBZTHP01 from CSD); (B) lamellar packing motif of DODBT; (C) hydrogen bonds between neighbor molecules; (D) overlap between neighbor molecules of DODBT along with π-stacking (gray, bottom layer; blue, middle layer; green, top layer); the opposite orientation in sequence could be found. Protons are omitted for clarity. Figure 4. (A) UV-vis absorption (solid line) and fluorescence emission spectra (broken line) of BEDBT and DOBEDBT; (B) cyclic voltammograms and (C) orbital levels of DBT (theory level),32 DODBT (theory level),26 BEDBT,25 and DOBEDBT.

energy level of DOBEDBT determined at the onset oxidation potentials was -6.20 eV by using ferrocene (Cp2Fe) as internal standard substance. This high ionization potential (IP, equal to HOMO) suggested the high stability of DOBEDBT and could be ascribed to the high IP of DBT and the electronwithdrawing behavior of sulphone groups. The LUMO energy level of DOBEDBT was calculated roughly from its energy gap and HOMO energy level as -3.08 eV, about 0.56 eV lower than that of BEDBT (also consistent with the DFT calculations), suggesting its strong tendency to lower the LUMO energy level. And the lower LUMO energy level might facilitate electron injecting, which was essential for n-channel FETs. All the HOMO/LUMO energy levels and gaps of BEDBT and DOBEDBT were summarized in Figure 4C as well as DBT and DODBT for comparison. The arrangements of DODBT33 and DOBEDBT34 were investigated by their X-ray diffraction (XRD) measurements.

Single crystals of DODBT and DOBEDBT suitable for XRD were grown from chlorobenzene and toluene solution, respectively. From Figures 5B and 6B, lamellar packing motifs were observed in the DODBT and DOBEDBT single crystals. However, as a contrast, the reduced materials DBT and BEDBT adopted herringbone stacking (as shown in Figures 5A and 6A), which indicated that the sulphone groups indeed made a great difference in the molecular arrangements. As for DODBT, the single crystals belonged to a crystal system of monoclinic space group C2/c with unit-cell parameters of a = 10.043(2), b = 13.844(3), c = 7.0241(14) A˚ and β = 91.34(3)°. Every DODBT molecule interacted with four neighbor molecules by hydrogen bonds (S-O 3 3 3 H) as shown in Figure 5C and with two molecules by π-π interactions. Molecules along π-stacking adopted the opposite orientation in sequence, which suggested that the π-π stacking was induced by the polar groups. The distance between two π-faces was about 3.428 A˚ (Figure 5B), and the π-π overlap between neighbor molecules along π-stacking was almost as large as that of a benzene ring (Figure 5D). The single crystals of DOBEDBT belonged to a crystal system of triclinic space group P1 with unit-cell parameters of a = 10.031(2), b = 10.146(2),

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Figure 6. (A) Herringbone packing motif without π-π stacking of BEDBT; (B) lamellar packing of DOBEDBT, (C) interactions between one DOBEDBT molecule and its neighbor molecules (red dashed line, hydrogen bonds; green dashed line, C-H 3 3 3 π interactions; π-π interactions are not displayed); (D) overlap between neighbor molecules of DOBEDBT along with π-stacking (gray, bottom layer; blue, middle layer; green, top layer); the opposite orientation in sequence could be found. Protons are omitted for clarity.

c = 11.316(2) A˚ and R = 100.63(3)°, β = 100.92(3)°, γ = 109.63(3)°. And a lamellar packing motif was observed in the crystals of DOBEDBT, too. But very interestingly, due to the polarity caused by the introduction of sulphone groups, molecules of DOBEDBT revealed a nonplanar structure. The dihedral angle between the plane of the twisting phenyl unit and the plane of the DBT core was about 64.85°. In addition, every DOBEDBT molecule interacted with two molecules by hydrogen bonds (S-O 3 3 3 H), and the distances between O atoms and H atoms on the DBT core were about 2.506 A˚ and those between O and H atoms on the phenyl units were about 2.606 A˚. Besides the hydrogen-bonded contacts, there were also C-H 3 3 3 π interactions between DOBEDBT and two adjacent molecules and π-π interactions with two other molecules. Similar to the case of DODBT, molecules along π-stacking also adopted the opposite orientation in sequence and there were two kinds of distances between two π-faces, about 3.370 and 3.415 A˚, respectively. These differences between the crystal structures of the sulphone-based derivatives DODBT and DOBEDBT and those of the reduced materials DBT and BEDBT suggested the important role of the sulphone in the molecular arrangements, which would be helpful to obtain π-π stacking. In conclusion, the strategies to obtain π-π stacking were briefly reviewed and two DBT derivatives were obtained with a lamellar packing motif. This might be attributed to the polar group (sulphone unit), which balanced the electrostatic repulsion. As for DOBEDBT, the largest coefficients in the LUMO were mostly located on the dibenzothiophene core compared

to those of BEDBT by using DFT calculations, and the polar group led to a much lower LUMO energy level, about 0.56 eV lower than its reduced counterpart, BEDBT, according to the CV measurements. The sulphone-based derivatives adopted lamellar structures in their crystals with molecules orientated oppositely along π-stacking in sequence, while the reduced materials showed herringbone packing motifs. This result suggested that the π-π stacking was induced by the polar groups, and it is meaningful to the molecular design of the π-stacking motif. The potential application of DOBEDBT in organic electronics is underway. Acknowledgment. The authors would like to thank Ms. Hua Geng and Prof. Zhigang Shuai for DFT calculations and further discussions. This work was supported by the National Natural Science Foundation of China (60771031, 60736004, 20571079, 20721061, and 50725311), the Ministry of Science and Technology of China (2006CB806200, 2006CB932100), and the Chinese Academy of Sciences. Supporting Information Available: Description of experimental procedures and instruments, and crystallographic information files (CIF) of DODBT and DOBEDBT. This material is available free of charge via the Internet at http://pubs.acs.org.

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