SCIENCE CHINA Chemistry

© Science China Press and Springer-Verlag Berlin Heidelberg 2012 chem.scichina.com www.springerlink.com

*Corresponding authors (email: wangd@iccas.ac.cn; wanlijun@iccas.ac.cn )

• ARTICLES • January 2013 Vol.56 No.1: 124–130

doi: 10.1007/s11426-012-4666-y

Hybrid molecular nanostructures with donor-acceptor chains

YANG Liu1,2, GUAN CuiZhong1,2, YUE Wan1,2, WU JingYi1, YAN HuiJuan1, ZHANG Xu1, WANG ZhaoHui1, ZHAN XiaoWei1, LI YuLiang1, WANG Dong1* & WAN LiJun1*

1Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China 2Graduate University of CAS, Beijing 100049, China

Received March 12, 2012; accepted April 6, 2012; published online July 13, 2012

We have fabricated hybrid molecular chain structures formed by electron acceptor compound 1 and electron donor molecules 2 and 3 at the liquid/solid interface of graphite surface. The structural details of the mono-component and the binary assemblies are revealed by high resolution scanning tunneling microscopy (STM). Compound 1 can form two well-ordered lamellar pat-terns at different concentrations. In the co-adsorption structures, compounds 2 and 3 can insert into the space between molecu-lar chains of compound 1 and form large area well-ordered nanoscale phase separated lamellar structures. The unit cell param-eters for the coassemblies can be “flexibly” adjusted to make the electron donors and acceptors perfectly match along the mo-lecular chains. Scanning tunneling spectroscopy (STS) results indicate that the electronic properties of individual molecular donors and acceptors are preserved in the binary self-assembly. These results provide molecular insight into the nanoscale phase separation of organic electron acceptors and donors on surfaces and are helpful for the fabrication of surface supramo-lecular structures and molecular devices.

phase separation, donor and acceptor, self-assembly, hybrid molecular nanostructure, liquid/solid interface

1 Introduction

Organic molecules with large π conjugated electronic paths have attracted growing interests, mainly driven by the pur-suit for flexible, high performance organic electronic devic-es [1–6]. By a careful molecule design and attaching proper functional groups, the electronic properties of conjugated organic molecules can be rationally tuned to meet different application requirements [7–10]. Aside from the molecule orbital level modulation, the assembly or hierarchical struc-tures of functional molecules in the electronic devices have profound effects on the performances of devices [11–14]. For example, the spontaneous phase separation of electron donor and acceptor at nanoscale is critical for the organic photovoltaic devices [15]. Understanding the self-assembly process of functional organic molecules at molecular and

nanoscale level could supply deep insight into the molecular structure/assembly/property relationship of organic devices. Recently, great attention has been paid to investigate the surface self-assembly process of organic functional mole-cules with large π conjugation, such as oligo-thiophenes [16, 17], fullerene derivatives [18, 19], porphyrin and phthalocyanine derivatives [20–24]. In particular, the inves-tigations of the multi-component self-assembly process provide interesting model systems to understand the micro- and nano- phase separation in organic electronic devices and inspire new application perspective in single molecular functional devices. Chen et al. found that the supramolecu-lar packing structure of CuPc/F16CuPc networks could be effectively tuned by the relative molecular mixing ratio. The intermolecular CH···F hydrogen bonds between CuPc and F16CuPc play an important role in determining the supramolecular arrangement [25]. Wang et al. reported a series of two components hierarchical assembly structures by tuning the molar ratio of 1,4-bis-pyri-dylethynyl based

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acceptor and fused thiophene based donor at the solid/ liquid interface [26]. Itaya et al. successfully fabricated a well-ordered three components structure by trapping C60 in the open spaces formed by bimolecular adlayer of ZnPc and ZnOEP on Au(111) surface in solution phase [27].

In this article, we report the nanoscale phase separation of electron donor and acceptor molecules into hybrid mo-lecular chain-like nanostructures on highly oriented pyro-lytic graphite (HOPG) surface. The chemical structures of three molecules in this study are shown in Scheme 1. Com-pound 1 is featured as two naphthalene diimide cores con-nected via a bis-ethynyl bridge. Naphthalene diimide is an intensively studied molecular acceptor building block in organic electronics. Compound 2 has a fused-nine-ring thienoacene, and compound 3 has a porphyrin core. Both of them are molecular donor materials. Molecular scale resolu-tion scanning tunneling microscopy (STM) is employed to reveal the structure and electronic properties of hybrid self-assembly on HOPG surface. We found that acceptor compound 1 form a linear chain-like structure, which can define the adsorption and assembly of donor compounds 2 and 3 to highly dispersed chain-like nanostructures on sur-face. These results provide molecular insight into the na-noscale phase separation of organic acceptors and donors on surfaces and are helpful for the fabrication of surface su-pramolecular structures and molecular devices.

2 Experimental

2.1 Materials

The synthesis of compounds 2 and 3 has been described in literature [28, 29]. According to literature, the synthesis route of compound 1 is shown in Scheme 2 [30]. Com-pounds 1 and 2 were dissolved in 1-phenyloctane from TCI. Compound 3 was dissolved in HPLC grade toluene. All the solvents were used without further purification. The molec-

Scheme 1 Chemical structures of compound 1, 2, and 3.

Scheme 2 Synthetic route to compound 1 [30].

ular assemblies were prepared by depositing a droplet (~2 L) of pure phase solution or a mixture with specific mo-lecular ratio on a freshly cleaved atomically flat HOPG sur-face (quality ZYB, Veeco Inc., Santa Barbara, CA).

2.2 STM and STS measurements

STM measurements were performed on a Nanoscope IIIa SPM (Veeco Inc., Santa Barbara, CA) at the liquid/solid interface at room temperature and under ambient condition. The tips were mechanically cut Pt/Ir wires (90/10). All STM images present in the paper were recorded in the constant current mode. The specific tunneling conditions of each figure are given in the corresponding figure captions. STS measurements were carried out by applying a modulation to the bias voltage. A lock-in amplifier was used to collect dI/dV-V signals. The parameters in the amplifier are: A = 50 mV, f = 5 K, time constant = 1 ms and sensitivity = 100 mV. The feedback of STM control was turned off during STS measurements. The drift-free high resolution STM images were collected before STM measurement to ensure that the STS spectra were collected on the desired molecules.

3 Results and discussion

3.1 Ordered adlayer of 1

Compound 1 can form two ordered self-assembled struc-tures at liquid/HOPG interface depending on the molecule concentration. Figure 1 shows STM image of the adlayer prepared by dropping 2 L of 1-phenyloctane containing 2.5×105 M compound 1 onto the HOPG surface. A highly ordered lamellar structure is revealed in Figure 1(a). The adlayer clearly consists of bright dots arranging into a dou-ble line lamellar structure. Each bright dot can be attributed to a naphthalene diimide core. The inter-distance of two adjacent dots in the white ring is 1.8±0.1 nm, which is con-sistent with the dimension of compound 1 backbone. The main axis of the molecule 1 shows a ~ 8° rotation relative to the lamellar vector a. The dark area between neighboring lamella is occupied by the alkyl chains attached to the aro-matic backbone, which are not clearly revealed in the STM image. A unit cell is outlined in Figure 1(a) with the param-eters a = 2.1±0.1 nm, b = 3.2±0.1 nm, = 80°±2°, which corresponds to a surface coverage of 0.3 molecule/nm2. A

126 Yang L, et al. Sci China Chem January (2013) Vol.56 No.1

Figure 1 STM images of 1 adlayer on HOPG surface obtained at low concentration. (a) High-resolution STM image. Tunneling conditions: Ebias = 575 mV, Itip = 210 pA. (b) Proposed structure model.

structure model is proposed in Figure 1(b). The neighbor molecules along the molecular chain may interact with each other via dipole-dipole interaction between carbonyl groups. On the other hand, we found that the unit cell defined space does not allow full interdigitation of alkyl chains of neigh-boring molecules. The van der Waals interaction between alkyl chains is not so significant due to the relatively short alkyl substitution length [31]. Therefore, the attached alkyl chains are not shown for clarity.

Figure 2(a) is a large-scale STM image of the adlayer prepared by dropping 2 L of 1-phenyloctane containing 1×104 M compound 1 onto the HOPG surface. Well- defined ordered network patterns are observed over 100 × 100 nm2 with almost no point defect. Detailed information about the arrangement of molecules in the adlayer is dis-closed from the high resolution STM image and shown in Figure 2(b). The ordered assembly can be resolved as bright dots arranging along two directions “a” and “b”. The mole-cules form double zigzag lines along “a” direction and dou-ble parallel lines along “b” direction. Occasionally, line defects along “a” direction with one line or three lines or even multiple molecular lines are observed in STM images, as shown in Figure 2(c), indicating that the main axis of molecules arranges along “a” direction. On the basis of the above analysis, a structure model is proposed in Figure 2(d). The unit cell parameters are measured to be a = 3.2±0.1 nm, b = 6.0±0.1 nm, = 85°±2°. The position of alkyl chain is not clearly revealed in STM image, so the alkyl chains are represented by methyl group in the structure model for clar-ity. Compared with the lamellar structure obtained at low concentration, molecules 1 along “a” direction are stacked to further maximize the interaction between carbonyl groups. On the other hand, the parallel-arranged dimer of molecule 1 is “inserted” in between adjacent molecular la-mella, which highlights the relative weak intermolecular interaction between adjacent lamella.

3.2 Adlayer of compound 2

Figure 3(a) is a typical large scale STM image acquired on the adlayer of molecule 2 on HOPG surface. The molecules

Figure 2 (a) Large-scale STM image of 1 at high concentration. Tunnel-ing conditions: Ebias = 646 mV, Itip = 401 pA. (b) High-resolution STM image. Tunneling conditions: Ebias = 672 mV, Itip = 542 pA. (c) Varieties of hybrid molecular chains formed by compound 1. Tunneling conditions: Ebias = 732 mV, Itip = 354 pA. (d) Proposed structural model for structure in (a) and (b).

Figure 3 (a) Typical large-scale STM image of compound 2. Tunneling conditions: Ebias = 473 mV, Itip = 488 pA. (b) High-resolution STM image. Tunneling conditions: Ebias = 284 mV, Itip = 319 pA. (c) Proposed structure model in (a) and (b).

adsorb on HOPG surface and self-organize into well- ordered adlayer extended over 100 nm. Shown in high- resolution STM image (Figure 3(b)), the molecular adlayer are composed of close packed array of molecular clusters. Each cluster can be resolved as two elliptic spots slightly shifted along their short axes. By comparing the STM fea-ture and molecular structure of compound 2, each elliptic spot is ascribed to the backbone of a single molecule. A unit cell for the adlayer is outlined with the lattice parameters of a = 2.8±0.1 nm, b = 3.2±0.1 nm, = 49°±1°. Figure 3(c) is a proposed structural model for the adlayer. The molecules arrange in a back-to-back configuration to form a molecular dimer, which serves as the basic structure unit and form close-packing array on the surface. The weak intermolecular S-S interaction may be responsible for the formation of mo-lecular dimmer [32, 33]. The alkyl chains may be adsorbed randomly in the space between molecular clusters and are not revealed in the STM images.

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We found that compound 3 cannot form ordered adlayer on HOPG, which may be ascribed to the high mobility and weak intermolecular interaction of porphyrin on HOPG [22].

3.3 Co-adsorption structures of compounds 1 and 2

We further investigated the coassembly of compound 1 and compound 2 to understand the nanoscale phase separation of electron-accepter and electron-donor materials. Figure 4(a) is a large scale STM image of the co-adsorption struc-ture formed from a mixed solution of compound 1 and 2 with 2.5×105 M and 5×105 M, respectively. The molar ratio of 1 and 2 is 1:2. A highly ordered lamellar self- assembly different from the monocomponent assembly structures is obtained. Although several defects (indicated by arrows in Figure 4(a)) can be found in the adlayer, the long-range ordered structure can extend over 150 nm. From the STM feature in Figure 4(b) and molecular structures, we can easily ascribe row A to compound 1 and row B to compound 2. The point defect outlined with white circle further proves that compound 1 shows as two spots with a dark gap in STM image. Different from the double-line feature in assembly structure of compound 1 itself, the molecules form single-line molecular rows in the co- adsorption assembly. In contrast, the similar dimer building units of compound 2 molecules form molecular rows and are arranged alternately with row of compound 1 in the co-adsorption adlayer. The unit cell parameters are meas-ured to be a = 2.7±0.1 nm, b = 3.5±0.1 nm, = 62°±2°. The unit cell vector along “a” direction is almost the same as that of compound 2. At the same time, molecules 1 in row A adjust the packing geometry slightly so that the aromatic backbone of compound 1 is almost parallel to the unit cell direction “a”. As a result, the periodicity of molecular rows of compound 1 and compound 2 perfectly matches in “a” direction to form highly ordered nanoscale phase separated molecular chains. On the basis of the above analysis, a structure model for the binary structure is proposed in Fig-ure 4(c). We note that the adlayer structures show intriguing dependence on the molar ratio of two components in the solution phase. When compound 1 is excessive in solution phase, the coexistence of pure phase of compound 1 and the co-adsorption structures is observed. On the other hand, if

Figure 4 (a) Large-scale and (b) high resolution STM image of co-adsorption adlayer of compound 1 and 2 (1:2). Tunneling conditions: (a) Ebias = 700 mV, Itip = 300 pA; (b) Ebias = 679 mV, Itip = 300 pA; (c) Pro-posed structure model.

compound 2 is excessive in the mixed solution, similar bi-nary structure is obtained but with lots of defects with missing compound 1.

Interestingly, we found that the STM contrast of com-pound 1 and 2 in the co-adsorption adlayer strongly depend on the tunneling bias. To further investigate the electronic properties of individual component in the nano-phase sepa-rated co-assembly, scanning tunneling spectroscopy is car-ried out [34]. Figure 5 shows the STS results of compound 1 and 2 in single and binary structures. dI/dV-V curves are measured by AC modulated bias using lock-in technique. Figure 5(a) and (b) show the typical dI/dV-V curves of compound 1 and 2 in single-component molecular adlayer. HOMO and LUMO energy for compound 1 and 2 can be confirmed by the method as shown in the curves. Over 100 STS curves are collected and statistically analyzed for each molecule. Histogram for 1 and 2 in pure phase and binary adlayer is shown in Figure 5(c), (d), (e) and (f), respectively. From the histogram, average HOMO and LUMO energy can be got by Gaussian fitting. Table 1 shows the statistical distributions of HOMO and LUMO energy levels from a large number of STS spectra for 1 and 2 in mono-compo- nent molecular adlayer and in hybrid molecular nanostruc-ture. The HOMO and LUMO levels for compound 1 in mono-component molecular adlayer from STS results are 0.85±0.14 and 0.72±0.06 eV, respectively, with a gap of 1.57±0.20 eV, and those in binary structure are 0.76±0.17 and 0.71±0.19 eV, with a gap of 1.47±0.36 eV. Meanwhile, the orbital energy for compound 2 in mono-component adlayer are 0.92±0.11 (HOMO) and 0.99±0.14 eV (LUMO), with a gap of 1.91±0.25 eV, and those in binary structure are 0.97±0.18 and 0.92±0.16 eV, with a gap of 1.89±0.34 eV. It can be inferred that the energy levels for 1 and 2 in binary structure are almost the same as those in mono-component adlayer considering the statistic error. In addition, the energy gaps from STS results are well con-sistent with theoretical calculation results, indicating that the energy levels of 1 and 2 are insensitive to change in the hybrid supramolecular architecture.

3.4 Co-adsorption structures of compounds 1 and 3

The successful observation of co-assembly of molecular

Table 1 HOMO and LUMO energy level of compounds 1 and 2 on HOPG by STS and theoretical calculation

STS results (eV) Theoretical result (eV)

HOMO LUMO HOMO-

LUMO gap HOMO-LUMO gap

1 a) 1 b) 2 a) 2 b)

0.85±0.14 0.76±0.17 0.92±0.11 0.97±0.18

0.72±0.06 0.71±0.19 0.99±0.14 0.92±0.16

1.57±0.20 1.47±0.36 1.91±0.25 1.89±0.34

1.43

1.94

a) Results from mono-component molecular adlayer. b) Results from bi-component molecular adlayer.

128 Yang L, et al. Sci China Chem January (2013) Vol.56 No.1

Figure 5 STS measurements on compound 1 and 2. Histogram of the gap edge of compound 1 in single molecular adlayer (c) and binary structure (e); compound 2 in single molecular adlayer (d) and binary structure(f). (a) and (b) is typical dI/dV-V curve of 1 and 2 in single molecular adlayer.

acceptor 1 with compound 2 encourages us to investigate the coassembly of compound 1 and the electron donor compound 3. Dropping 1.5 L mixture of 1 dissolving in 1-phenyloctane and 3 dissolving in toluene on HOPG sur-face, we obtained a self-assembly film on the surface. The molar ratio of 1 and 3 is 1:1. Figure 6(a) shows the large scale STM image of the co-adsorption structure. Interest-ingly, a similar well-ordered lamellar co-adsorption struc-ture is formed. Although the point defects density is rela-tively high in the adlayer, as indicated by the white arrows, they do not induce any dislocation in the adlayer. From the

high-resolution STM image in Figure 6(b), the characteristic square feature of molecule 3 is well revealed in the STM image. We can easily ascribe molecular row A to compound 1 and molecular row B to compound 3. According to the length of the backbone of compound 1, one molecule can be determined by two dots along the row A marked by a white circle (Figure 6(b)) , and it can serve as a marker for deter-mining the arrangement of compound 1 in molecular rows A and their relative position to molecular row B. The rela-tive position of two components in the coassembly adlayer is very similar to that of compound 1 and compound 2. A

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Figure 6 Co-adsorption structure formed by compound 1 and 3 (1:1). (a) Large-scale STM image and (b) high resolution STM image. Tunneling conditions: (a) and (b) Ebias = 455 mV, Itip = 300 pA. (c) Proposed structure model for coassembly structure.

unit cell is outlined with the measured parameters of a = 2.1±0.1 nm, b = 2.8±0.1 nm, = 66°±2°. On the basis of the periodicity and molecular arrangement, a structure model is proposed in Figure 6(c). Intriguingly, the periodicity along “a” direction is almost the same as that of pure phase of compound 1. In this co-assembly, the adlayer structure is more determined by the native assembly behavior of com-pound 1, due to the relatively small size of compound 3.

As revealed by high resolution STM images, the co- adsorption of compound 1 with either compound 2 or com-pound 3 results in the formation of alternately arranged mo-lecular chains of donor/acceptor binary structures. The co-assembly structures are resembled to the lamellar struc-ture of compound 1 with electron-donor accommodated between neighboring lamella. However, the subtle differ-ence of adlayer structure is observed in these two binary co-assemblies. For instance, the unit cell vector along mo-lecular chains for the co-assemblies can be “flexibly” changed to match the electron - donors with different sizes. As shown in Figure 4, the unit cell vector “a” in co-assembly of 1 and 2 is close to that of mono-component adlayer of molecule 2. In contrast, the unit cell vector “a” of co-assembly of 1 and 3 (Figure 5) coincides with that of compound 1 (Figure 1) due to the smaller dimension of compound 3. As a result, the electron donors and acceptors are perfectly matched along the molecular chains. Among many possibilities, we tentatively propose the adsorption induced substrate/adsorbate dipole as a major driving force for the formation of binary assemblies. Due to the distinctly different electronic properties, the adsorption of electron donors and acceptors can produce the opposite dipole mo-ments normal to the surface at the periphery of each mole-cule. The weak dipole interactions may favor the formation of alternately arranged molecular chains on surfaces.

4 Conclusions

In summary, we have fabricated hybrid molecular chain structures formed by electron acceptor compound 1 and electron donor compounds 2 and 3 at the liquid/solid inter-face of HOPG surface. The details of the individual adlayer and the binary co-assemblies are revealed by STM. Com-

pound 1 can form two well-ordered patterns at different concentration. A double line lamellar structure is obtained at low concentration, whereas a network structure with in-serted molecular dimer is obtained at high concentration. Compound 2 can form large well-ordered structures with molecular dimers, owing to the S-S interaction. In the co-adsorption structures, compounds 2 and 3 can insert into the space between molecular chains of compound 1 and form large area well-ordered structures. The unit cell pa-rameters for the coassemblies can be “flexibly” adjusted to make the electron donors and acceptors perfectly match along the molecular chains. STS results indicate the elec-tronic properties of individual molecular donors and accep-tors are preserved in the binary self-assembly. These results provide molecular insight into the nanoscale phase separa-tion of organic acceptors and donors on surfaces and are helpful for the fabrication of surface supramolecular struc-tures and molecular devices.

The authors thank the financial supports from National Basic Research Program of China (2011CB808700 and 2011CB932300), National Natural Science Foundation of China (21121063, 91023013), and the Chinese Academy of Sciences.

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