Triphenylene Substituted Pyrene Derivative: Synthesis and Single

Article pubs.acs.org/JPCC

Triphenylene Substituted Pyrene Derivative: Synthesis and Single Molecule Investigation Xue-mei Zhang,†,‡ Hai-feng Wang,†,§ Shuai Wang,‡ Yong-tao Shen,‡ Yan-lian Yang,‡ Ke Deng,*,‡ Ke-qing Zhao,*,§ Qing-dao Zeng,*,‡ and Chen Wang*,‡ ‡

National Center for Nanoscience and Technology (NCNST), Beijing 100190, P. R. China College of Chemistry and Materials Science, Sichuan Normal University, Chengdu Sichuan 610066, P. R. China

§

S Supporting Information *

ABSTRACT: A novel donor−acceptor material based on pyrene derivative with two substituted triphenylenes (Py-TP2) is synthesized via the Sonogashira coupling reaction. The structure and physical chemistry properties of the target molecule have been discussed, ranging from the traditional 1H NMR and high-resolution mass spectroscopy (HRMS), over UV and PL spectra, and to the surface science research. The results revealed that the Py-TP2 molecule shows a narrowed energy gap between LUMO− HOMO and a bathochromic shift of 27 nm in the solid state as compared to that in solution, which is important for its practical applications in optoeletronic devices. Moreover, combined with DFT calculations, our STM results clearly show that the PyTP2 molecule assembled into a stable long-ranged zigzag structure on HOPG surface. The interesting results in this contribution will boost the physical chemistry study of other functional materials under such methods.

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electronics.18−20 However, in light of its characteristic chemical structure of aromatics and 3-fold rotation axes, triphenylene (TP) has been traditionally chosen as a building block for fabrication of photoelectric materials, such as liquid crystals (LCs). More interestingly, triphenylene DLCs as an organic semiconductor shows fast charge carrier mobility in the range of 10−1 to 10−3 cm2 V−1 s−1. It has shown good performance in electronic devices such as thin film transistors, organic light emitting diodes, and organic solar cells.21−26 Besides, when attached with aliphatic chains, triphenylene derivatives display the good self-assembling nature of dynamic molecular systems as well as the high solubility into common solvents, and these amazing properties can be tuned by modification of the molecular structures.27−29 However, to the best of our knowledge, reports on functional material derived from pyrene and triphenylene are exceedingly rare, in spite of these potentially attractive features. With this aim in mind, pyrene and triphenylene are both chosen for constructing a novel molecule material. Through Sonogashira coupling reaction, a novel pyrene derivative with two substituted triphenylenes (Py-TP2, Scheme 1) is synthesized. On a solid surface, the assembled structure and electrical properties of Py-TP2 have been investigated at the single molecule level.

INTRODUCTION With respect to optoelectronic devices, not only the properties of individual molecules but also the conformation, orientation, and packing structures of these molecules on the solid substrate play an essential role in determining the performance of these devices.1 Therefore, controlling of the molecular arrangement and understanding the properties of single molecules at the interface have important consequences for fabrication of devices.2,3 In recent years, because of its capability to resolve spatial information at the atomic scale, scanning tunneling microscopy (STM) has been proven as an ideal technique for investigating the assemblies adsorbed onto a highly oriented pyrolytic graphite (HOPG) surface or metal surface, such as Au(111) or Cu(110), and so forth.4−9 Also, scanning tunneling spectroscopy (STS) could be utilized to probe qualitatively the positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of single molecules or even different parts of one single molecule.10−14 Herein, the STM/STS technique is utilized to probe the structural properties of a triphenylene-substituted pyrene derivative on a solid surface. Pyrene (Py) has been one of the most studied candidates in fundamental and applied photochemical research since its discovery in 1837.15 Pyrene derivatives show the unique optical property of high fluorescent quantum yields in solution, solid state, and liquid crystalline phase.16,17 Recently, thanks to its excimer fluorescence properties, Py and its derivatives have been widely employed as fluorescence probes for sensing environmental parameters such as temperature, pressure, or pH, and also as organic semiconductors for application in materials science and organic © 2012 American Chemical Society

Received: September 26, 2012 Revised: November 28, 2012 Published: November 30, 2012 307

dx.doi.org/10.1021/jp3095616 | J. Phys. Chem. C 2013, 117, 307−312

The Journal of Physical Chemistry C

Article

Scheme 1. Schematic Presentation for Synthesis Route of Py-TP2

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Ar. The mixture was stirred at 80 °C for 24h. After reaction completion, the product purified by using column chromatography and crystallization from ethanol, was collected (204 mg, yield 59%) 2. STM/STS Investigation. 2.1. Sample Preparation. PyTP2 was dissolved in toluene with the concentration less than 1.0 × 10−4 M. A droplet of solution (0.4 μL) containing PyTP2 was deposited onto the freshly cleaved highly oriented pyrolytic graphite (HOPG, grade ZYB, advanced Ceramics Inc., Cleveland, USA). After evaporation of the solvent, the sample was studied by STM. 2.2. STM/STS Measurement. STM investigation is performed with a Nanoscope IIIa scanning probe microscope system (Bruker, USA) under atmosphere conditions. All STM images provided are raw data and are acquired in constant current mode using a mechanically formed Pt/Ir (80/20) tip. The drift for all presented STM images is calibrated by using the underlying graphite lattice as a reference. Besides, all of the molecular models are built with a HyperChem software package. For STS experiments, the tunneling spectra were performed by adding a dithering modulation (peak-to-peak 20−30 mV) to the bias voltage, and the bias was scanned through the designated voltage range. Specifically, the tip was respectively located above the different parts (Py and TP) with feedback turned off. Then the modulated tunneling current from STM was fed into a lock-in amplifier, and consequently the dI/dV signal obtained from the lock-in amplifier was recorded. More than 50 curves have been collected for different groups to achieve the highest signal-to-noise ratio and to ensure the reliability and reproducibility. 3. Computational Details. The theoretical calculation was performed using DFT provided by the DMol3 code.33 We use the periodic boundary conditions (PBC) to describe the 2D periodic structure on the graphite in this work. The Perdew and Wang parametrization34 of the local exchange-correlation

EXPERIMENTAL SECTION 1. Synthesis of 1,6-di(3,6,7,10,11-pentakis(butoxy) triphenyl-ene-2-yl undec-10-ynoate)pyrene (Py-TP2). Scheme 1 summarizes the standard Sonogashira coupling reaction synthetic route to the pyrene-cored compound: 1,6di(3,6,7,10,11-pentakis (butoxy) triphenylene-2-yl undec-10ynoate)pyrene (Py-TP2), studied in this contribution. The target molecule was synthesized as described below briefly and characterized by using 400 MHz 1H NMR and high resolution mass spectroscopy (HRMS) (Supporting Information). 1.1. Synthesis of 1,6-Dibromopyrene (1).30 To the cold solution of Pyrene (3.06 g, 15 mmol) in CCl4 (60 mL), the solution of Br2 (4.8 g, 30 mmol) in CCl4 (60 mL) was added slowly with stirring. The solution was stirred in room temperature for another 12 h. After reaction completion, the product was collected by filtration and purified by crystallization from toluene. Yellow crystal product with melting point of 230 °C was obtained (1.2 g, 29%). 1.2. Synthesis of 3,6,7,10,11-Pentakis(butoxy)triphenylene-2-yl undec-10-ynoate (TP(OC4H9)5(O2CC8H16CCH), 2).31 It was synthesized by the method described above through the reaction of 3,6,7,10,11pentakis(butyloxy)triphenylene-2-ol (1 g, 1.65 mmol) with undec-10-ynoic acid (0.452 g, 2.48 mmol) in the presence of DCC (0.51 g, 2.48 mmol) and DMAP (0.1 g, 0.8 mmol) in CH2Cl2 (50 mL). White solid product was obtained (1.053 g, yield 83%). 1.3. Synthesis of 1,6-di(3,6,7,10,11-Pentakis(butoxy)triphenylene-2-yl undec-10-ynoate)pyrene (Py-TP2).32 To the mixture of 1,6-dibromopyrene (72 mg, 0.2 mmol), CuI (1.91 mg, 0.01 mmol), Pd(PPh3)2Cl2 (14.1 mg, 0.02 mmol) and PPh3 (10.49 mg, 0.04 mmol) in dry pyridine (3 mL),the solution of TP(OC4H9)5(O2CC8H16CCH) (461 mg, 0.6 mmol) in triethylamine (3.5 mL) was added underprotection of 308



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Figure 1. (a) Photoluminescence spectrum, and (b) UV absorption of Py-TP2. (The powder film was prepared by putting solid powder of the sample between a quartz glass and a metal sheet, extruded into sheet and then tested; and the concentration for all solutions was 1 × 10−5 M.)

attributed to the pyrene and the long alkyl chains indicating less contribution to the tunneling current than TP parts. More interestingly, along the red arrow, the TP parts, belonging to different molecules, alternately assemble into lines of beads. The Py and long-alkyl chains, bridging with two lines of beads, form a typical herringbone arrangement. By carefully inspecting the high resolution STM image, we have proposed the molecular model illustrating the packing arrangements of Py-TP2 molecules in part c of Figure 2. Within the assembled molecular architecture, two Py-TP2 molecules construct a dimer through the van der Waals’ interactions between the alkyl chains in the TP groups, and each pair of dimer piles up to form an extended zigzag architecture. DFT calculations have been performed to investigate the selfassembled behavior. Results show that the Py-TP2 molecule has two different energetically equivalent geometries. For one isomer (drawn in pink), the end TP groups of Py-TP2 exhibit an anticlockwise orientation relative to the center Py of the molecule, whereas for the other (drawn in cyan), the end TP groups exhibit a clockwise orientation. In a unit cell, every two isomers form a dimer through the van der Waals’ interactions (about −47.173 kcal/mol). The calculated parameters of the unit mesh are a = 6.30 nm, b = 3.30 nm, and α = 90°, which agrees well with the experimental values (a = 6.5 ± 0.1 nm, b = 3.2 ± 0.1 nm, and α = 87 ± 2°). We also investigate the interaction between the Py-TP2 molecules and the graphite substrate. Because of the aromatic rings in the molecule, the interaction between the absorbed molecule and substrate is very strong (about −229.57 kcal/ mol), which indicates that the Py-TP2/graphite system is most energetically stable.37 3. Single Electrical Property of Py-TP2 on the HOPG Surface. To investigate the electronic structure of two separate groups of the Py-TP2 molecule in contact with the graphite surface, STS measurements have further been carried out by locating the STM tip on top of the triphenylene group and pyrene moiety, separately. The apparent gap values between HOMO and LUMO as well as the positions of HOMO and LUMO (edges) are illustrated using the dI/dV vs V curves, which is considered to reflect the density of states for the adsorbate (Figure 3). With this approach, the gap edge defined by cross-point of the tangents of the platform and uplifted part of the dI/dV spectra are considered to represent the edge of HOMO and LUMO, thus the apparent energy gap between HOMO and LUMO is estimated from the separation between

energy are applied in the local spin density approximation (LSDA) to describe exchange and correlation. We expanded the all-electron spin-unrestricted Kohn−Sham wave functions in a local atomic orbital basis. In such double-numerical basis set polarization was described. All calculations were all-electron ones, and performed with the Extra-Fine mesh. Self-consistent field procedure was done with a convergence criterion of 0.5 × 10−5 a.u. on the energy and electron density.

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RESULTS AND DISCUSSION 1. Photophysical Properties. The photoluminescence (PL) spectrum of Py-TP2 was collected both in DCM (1 × 10−5 M) and in thin films, and its UV−vis absorption was recorded in dilute solutions (1 × 10−5 M) in CH2Cl2 (DCM) and cyclohexane. Its fluorescence spectrum (part a of Figure 1) in DCM displays a well-resolved fluorescence band at ∼448 nm. While in thin film, the fluorescence spectrum shows an obvious red-shift, with the maximum absorption wavelength of about 475 nm, possibly due to the enhanced aggregation in the thin film which is important for its practical applications in optical devices. For a better insight into the photoluminescent mechanism, we measured the UV−vis absorption in two different solvents, and the results were shown in part b of Figure 1. The strongest absorption peak showed a bathochromic shift with the solvent’s polarity increasing indicating that the electronic excited state is S1 (π, π*)35 and the fluorescence emission should be the S1 (π, π*) to S0 transition. In the solid state, a larger degree of intermolecular π−π stacking and aggregation resulted in the HOMO level higher compared with that of in solution,36 therefore, the energy gap between LUMO−HOMO is narrowed and the emission peak in solid state showed a bathochromic shift of 27 nm than that in solution. 2. Self-Assembled Structure of Py-TP2 at the HOPG/1Phenyloctane Interface. When adsorbed on HOPG at the solid/liquid interface, the 2D assembly of Py-TP2 is investigated by STM under ambient conditions. As shown in part a of Figure 2, Py-TP2 molecules can form large-area and homogeneous assemblies on HOPG with a long-ranged zigzag feature. The detailed packing arrangement of Py-TP2 molecules is shown in part b of Figure 2. Because of the high electron density of three aromatic rings, the TP groups appear bright and have a triangular shape (highlighted by the blue triangles). The linear area with lower contrast (drawn in white) could be 309



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Figure 2. (a) Large scale and (b) high resolution STM image of the Py-TP2 assemblies at 1-phenyloctane/HOPG interface. Iset = 95.78 pA, Vbias = 1.129 V. A colored schematic model was superimposed on b to present a Py-TP2 molecule. (c) A suggested molecular model for the observed area in b. To visually illustrate the formed zigzag architecture, neighboring molecules are drawn in two kinds of colors.

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CONCLUSIONS A newly pyrene derivative with two substituted triphenylene groups was synthesized through the Sonogashira coupling and characterized by 1H NMR and HRMS (ESI). Its photophysical properties were recorded by the UV absorption and photoluminescence (PL) spectra, which shows an obvious red shift for the emission peak in solid state as compared to that in solution. Moreover, to better understand its conformation behavior on solid surface, the target molecule was investigated by the scanning tunneling microscopy and spectroscopy (STM/STS) at the single molecule level. On the HOPG surface, our high resolution STM results reveal that the Py-TP2 molecule can fabricate a long-ranged zigzag structure in which the triphenylene parts alternately assemble into lines of beads, whereas the bridged pyrene and long-alkyl chains form a typical

these two edges. Determined from the dI/dV vs V spectra, the apparent energy gaps of two parts of Py-TP2 are estimated to be 2.20 ± 0.10 eV and 2.63 ± 0.10 eV for Py and TP, respectively. We have also performed the DFT calculations to investigate the electronic properties of two parts of Py-TP2. The calculated energy gaps of pyrene and triphenylene groups are 2.13 and 2.77 eV respectively, which agrees well with the experimental values (Table 1). Relative to the Fermi level of the substrate (0 bias location in dI/dV vs V spectrum), the energy gap centers for Py and TP are estimated to be shifted negatively for 0.10 and 0.13 eV, respectively. This demonstrates that both pyrene and triphenylene can be used as one of n-type materials in air, thus Py-TP2 molecule should also have some potential application as n-type semiconductor.38 310



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Physics, Chinese Academy of Sciences (2010KL0010) are also gratefully acknowledged.

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Figure 3. Typical dI/dV curves of the pyrene and triphenylene for the Py-TP2 molecule.

Table 1. Electronic Properties Measured by STS and Theoretical Calculation of Investigated Py-TP2 Molecule pyrene

triphenylene

1.01 −1.19 2.20 −0.10 −2.95 −5.08 2.13

1.18 −1.45 2.63 −0.13 −1.92 −4.69 2.77

inflection I inflection II gap (STS)/eV Ef (offset)/eV LUMO/eV HOMO/eV gap (therotical)/eV

herringbone arrangement. In parallel, dI/dV vs V spectrum has showed that the energy gap centers for these two parts are estimated to be shifted negatively, relative to the Fermi level of the substrate. The interesting results of such donor−acceptor system remind us exploring its potential application in preparation of important films and optoelectronic devices in the near future.

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ASSOCIATED CONTENT

S Supporting Information *

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H NMR and HRMS (ESI) spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Q.Z.) E-mail: [email protected], fax: 86-10-62656765; (C.W.) E-mail: [email protected], fax: 86-10-62656765; (K.D.) [email protected], fax: 86-10-62656765. (K.-q.Z.) Email: [email protected], fax: 86-28-84764743. Author Contributions †

These authors contribute to this work equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Nos. 2011CB932303, 2013CB934200). The National Natural Science Foundation of China (Nos. 21073048, 51173031, 91127043, 50973076, 20933008), and Key Laboratory of Optoelectronic Materials Chemistry and 311



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Triphenylene Substituted Pyrene Derivative: Synthesis and Single

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