STM Investigation of the Photoisomerization and Photodimerization of

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STM Investigation of the Photoisomerization and Photodimerization of Stilbene Derivatives on HOPG Surface Ling-yan Liao,†,¶ Yi-bao Li,‡,¶ Xue-mei Zhang,† Yan-fang Geng,*,† Jun-yong Zhang,§ Jing-li Xie,*,§ Qing-dao Zeng,*,† and Chen Wang*,† †

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, National Center for Nanoscience and Technology (NCNST), 11 Zhongguancun Beiyitiao, Beijing 100190, China ‡ Key Laboratory of Organo-pharmaceutical Chemistry, Gannan Normal University, Ganzhou 341000, China § College of Biological, Chemical Science and Engineering, Jiaxing Uninversity, Jiaxing 314001, China ABSTRACT: The photochemical reactions of stilbene and its derivatives have been extensively investigated in the gas and liquid phases but less on surfaces. In this work, scanning tunneling microscopy (STM) has been employed to investigate the photoisomerization and photodimerization of two stilbene derivatives on HOPG surface. After UV-light irradiation, one stilbene derivative performs photoinduced trans to cis isomerization and a well-ordered Kagomé network with two types of cavities forms at the 1-phenyloctane/HOPG interface, which is totally different from the lamella structures (before irradiation). Another stilbene derivative can also perform trans to cis photoreaction and the assembly structures have two conformations. With the increase of the irradiation time, one of the two conformations according to the topochemical postulates can further perform [2 + 2] photodimerization reaction. The difference in alkoxy substitution is reflected in the self-assembled monolayer and consequently also in the photoreactivity.

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past few decades.6−10 It has also been of great interest to study the isomerization reactions on surfaces. Understanding and controlling conformational changes of molecules adsorbed on surfaces is one of the most actively pursued goals related to fundamental electronic device miniaturization.11 So far, many isomerization reactions have been studied for molecules adsorbed on surfaces1,12−27 by activating the reactions via photon excitation,20 by means of tunnel electrons, or with the electrostatic field under the tip of STM.15−17 Azobenzene and its derivatives have been considered as two prototypical models for trans−cis isomerization studies and heavily studied on surface as molecular switches.28 Also there are massive investigations about trans−cis photoisomerization of azobenzene on surface in our previous works.29−32 The photoisomerization reactivities of stilbene and its derivatives (Scheme 2) are another class of important reaction that involves the internal rotation around a carbon−carbon double bond and has been extensively investigated in the gas and liquid phases.33−35 However, there are fewer investigations about isomerization reactions of stilbene and its derivatives on surfaces. When stilbene-like molecules adsorb on surfaces and form monolayers, the internal rotation degrees of the molecules is reduced.36 As reported in the literature, molecular islands,14,17 isolated species,15,16 and steric hindrance37 arising from the surface atoms or the neighboring molecules are

INTRODUCTION Stilbene and its derivatives are good materials for molecular electronics due to their properties of organic conductors, photoswitches, organic displays, or biosensors.1−4 Many stilbene derivatives are natural products widely existing in plants, and several of them, such as resveratrol, 3,4′,5trihydroxy-trans-stilbene found in grapes, are reported to provide several health benefits.5 Stilbene and its derivatives have so many advantages and have been of interest for more than half a century. Here, we choose two stilbene derivatives (see Scheme 1) as photochemical materials to investigate their Scheme 1. Chemical Structures of Two Stilbene Derivatives

photoresponses on a highly oriented pyrolitic graphite (HOPG) surface through the scanning tunneling microscopy (STM) technique. STM is a powerful technique and can probe at an atomic scale. Isomerization and especially trans−cis photoisomerization reactions of aromatic molecules have been extensively studied in both solution phase and gas phase environments during the © 2014 American Chemical Society

Received: June 4, 2014 Revised: July 5, 2014 Published: July 7, 2014 15963

dx.doi.org/10.1021/jp505511e | J. Phys. Chem. C 2014, 118, 15963−15969

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molecular assemblies which are formed through inclusion,46−48 hydrogen-bonded,49,50 or coordination.51−60 Here, we observe that assembly of molecule 2 on HOPG surface can also perform photochemical [2 + 2] reaction. In our previous research,61 we successfully designed and synthesized different structural stilbene derivatives and got different 2D monolayer assembled structures. Furthermore, we studied the photochemical responses of two of them through scanning tunneling microscopy. To the best of our knowledge, there are fewer STM studies about the photochemical reaction of stilbene and its derivatives on surface.14,62,63 In this present work, we successfully observe that the monolayer structures of molecules 1 and 2 (see Scheme 1) can perform photoisomerization reactivities after UV irradiation. On the other hand, with the increase of the irradiation time the photodimerization [2 + 2] reaction occurs in the assembled structure of molecule 2. These results can offer experimental and theoretical guidance for the fabrication of molecular optoelectronic devices.

Scheme 2. A Schematic Representation of the Reversible Cis/Trans Photoisomerization and Photodimerization of Stilbene Unit

expected to have an obvious influence on the isomerization reactivities on surfaces, and new molecular conformations that are unstable in the gas or liquid phases may be stabilized through weak surface interactions. In this paper, we choose two stilbene derivatives with different alkoxy chain lengths (see Scheme 1). The two molecules have different interactions with the substrate or with the neighboring molecules due to the difference in alkoxy chain length. Another photochemical pathway of stilbene and its derivatives is the dimerization (Scheme 2), which leads to cyclobutanes and was first discovered by Ciamician and Silberat at the beginning of the 20th century.38 Photochemical [2 + 2] reactions, especially the photodimerization of cinnamic acid derivatives, have been studied extensively in three dimensions (3D). Through the pioneering work of Schmidt and coworkers, the prerequisites for [2 + 2] photodimerization reactivities in the crystals are now well founded.39,40 With some exceptions,41−44 this cycloaddition reaction requires that the double bonds of neighboring molecules arrange in a parallel manner and make contact at a distance of 4.2 Å or less.45 Some reports have demonstrated that in the solid state the packing of molecules “strictly” controls [2 + 2] photodimerization.42,43 Some of compounds can dimerize in the crystals upon UV irradiation, while in solution the photodimerization cannot occur. But it can be performed in solution by utilizing

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EXPERIMENTAL SECTION All solvents for STM experiments were purchased from Acros Co. and used without further purification. All of the studied samples were dissolved in 1-phenyloctane, and the concentration of all the studied solutions for STM investigation was less than 1.0 × 10−4 mol L−1. A small droplet of solution containing molecule 1 or 2 was deposited onto freshly cleaved highly oriented pyrolytic graphite (HOPG, grade ZYB, NTMDT, Russia). A few minutes later, the sample was characterized by STM. Then, the studied sample was irradiated with UV light at 365 nm for different time. After that, the STM investigation was performed. The 365 nm UV light for irradiation experiments was obtained from a xenon lamp (PLS-SXE 300, 50 W) with a 365 nm glass filter (Figure 1), and all these light systems were bought from Peking Perfect Co. (CHN). During the irradiation process, the temperature of the sample was kept between 20 and 25 °C, the lamp was at a distance of 25 cm from the sample, and the area of light emitted by the lamp was much larger than the size of the sample. All of

Figure 1. Spectra of xenon lamp with filter λmax = 365 nm. 15964

dx.doi.org/10.1021/jp505511e | J. Phys. Chem. C 2014, 118, 15963−15969

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the irradiation experiments as well as STM investigation were operated in the dark. The STM investigation was performed with a Nanoscope IIIa scanning probe microscope system (Bruker, USA) under atmospheric conditions. All STM images presented were acquired in constant current mode using a mechanically formed Pt/Ir (80/20) tip. All images presented here were flattened to correct the tilting effect of the substrate, and the thermal drift was corrected using the underlying graphite lattice as a reference. Molecular models were constructed using a HyperChem software package.

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RESULTS AND DISCUSSION 1. Photoisomerization in Molecule 1 Assemblies. As a typical photosensitive compound, molecule 1 will isomerize from the trans to cis form after irradiation with UV light. First, the solution-phase photoisomerization behavior of 1 was studied in chloroform by UV−vis spectroscopy. Figure 2

Figure 3. (a) An STM image of the molecule 1 monolayer, Iset = 289.9 pA, Vbias = 570.7 mV, scale bar = 20 nm. (b) An STM image of the molecule 1 network after 10 min of UV light irradiation, Iset = 262.5 pA, Vbias = −600.3 mV, scale bar = 20 nm. (c) A high-resolution STM image of the outlined area (the red frame), Iset = 262.5 pA, Vbias = −600.3 mV, scale bar = 5 nm. (d) A suggested model corresponding to part c.

six B-type cavities. The inner widths for cavities A and B are 19.1 and 7.9 Å, respectively. Figure 3c is the high-resolution image of the outlined area (the red frame) in Figure 3b, and importantly, it reveals the adsorption conformation of molecule 1 after UV irradiation at 365 nm at the 1-phenyloctane/HOPG interface. As revealed in this image (Figure 3c), the molecular conformation is quite different from that in Figure 3a. The stilbene aromatic core of each molecule now appears as a “V” shape due to the transformation of the aromatic core from its trans to the cis isomer.17,20,29−32,65 The bright rod L1 is measured to be 1.1 ± 0.1 nm in Figure 3c, which is shorter than 1.3 ± 0.1 nm (the length of the bright rod before UV irradiation).61 The result shows that there is a decrease in the length of one stilbene core after isomerization from trans to cis, which is similar to the isomerization of azobenzene derivatives on surfaces.17,29−32 In every six molecules which construct Atype cavity, the stilbene cores of three molecules show a clockwise direction (indicated by the yellow arrow in Figure 3c), whereas the other three show a counterclockwise direction (indicated by the black arrow in Figure 3c), thus leading to a chiral conformation. To better illustrate the chiral conformation, here we define the clockwise direction as an Rconfiguration and the counterclockwise direction as an Sconfiguration (as shown in Figure 3c). The well-ordered network can be formed after UV illumination, which is attributed to the interdigitation of alkoxy chains. Interestingly, the interdigitation of alkoxy chains is insufficient. An A-type cavity is constructed by three pairs of interdigitated alkoxy chains and three single alkoxy chains, which displays an amazing odd−even alternating organization. The distances (L2 and L3) between two bright rods are both determined as 0.9 ±

Figure 2. UV−vis spectra of molecule 1 (c = 6 × 10−6 M) in chloroform: before (trans-1, black) and after UV light irradiation at 365 nm for 10 min (cis-1, red).

shows the absorption spectra of 1 before (trans-1, black) or after (cis-1, red) UV light irradiation. The absorption maxima are located at 323 nm for the trans-1 and 273 nm for the cis-1, which agrees with the report.64 The results confirm the photochemical reaction and the existence of the cis isomer. Then we investigated the photoisomerization reaction of 1 on surface. As shown in Figure 3a, 1 formed a large area of wellordered lamellae at the 1-phenyloctane/HOPG interface.61 After the self-assembled structure of 1 was observed, the studied sample was irradiated at 365 nm for 10 min and the photoisomerization behavior of 1 was then studied with STM. Images observed after illumination for more than 20 min showed the same packing pattern like that in Figure 3b. As displayed in the image of Figure 3b, the STM result reveals that a new packing pattern has been formed after UVlight irradiation. The new pattern is totally different from the lamellae structure in Figure 3a. A well-ordered Kagomé network with two types of cavities (marked A and B in Figure 3c) is formed, which is totally different from that in Figure 3a. The comparatively larger A-type cavity with 6-fold symmetry is composed of six molecules, while the triangular B-type cavity is formed by three molecules. Each A-type cavity is surrounded by 15965

dx.doi.org/10.1021/jp505511e | J. Phys. Chem. C 2014, 118, 15963−15969

The Journal of Physical Chemistry C

Article

As shown in Figure 4b, upon irradiation by UV light, domains of reagent and product of this photoinduced reaction have been simultaneously imaged with submolecular resolution and identified at the graphite/liquid interface. Domain A in Figure 4b is the same as that in Figure 4a, which represents the unchangeable parts. Though domain B displays the lamella structure similar to that in domain A, the bright spots become brighter, which is ascribed to the adsorption of the isomerized molecule 2 (cis-2).17,31,32 On the other hand, as shown in the high-resolution STM image (Figure 4c), the size of every bright spot is determined to be L5 = 1.0 ± 0.1 nm, W5 = 1.3 ± 0.1 nm, or L5′ = 1.0 ± 0.1 nm, W5′ = 1.1 ± 0.1 nm, which is about twice the width of a benzene ring, indicating that each spot is composed of two cis-2 molecules. The two neighboring bright spots form a dimer marked by the white rectangle and then make up each lamella. On careful inspection of the highresolution image (Figure 4c), in each dimer the two bright spots show a quite different conformation from each other. If we divide a cis-2 molecule into two parts, the double bond as a head and the two benzene rings as a tail, the two aromatic cores appear in a “head-to-head” manner in the bright spot as indicated by the white arrow in Figure 4c, whereas in another bright spot (indicated by the yellow arrow) the conformation of the two aromatic cores adopts a “head-to-tail” fashion. Here we define the head-to-head manner as the conformation 1 and the head-to-tail fashion as the conformation 2. In each lamella the molecules of the row along the blue arrow show conformation 1 and the molecules of the row along the red arrow display conformation 2. On the basis of these analyses, a suggested molecular model is proposed in Figure 4d. Figure 4c shows a unit cell with parameters a = 2.2 ± 0.1 nm, b = 3.6 ± 0.1 nm, and α = 114 ± 2°. Finally, we investigated the solution-phase photoisomerization behavior of 2 in chloroform by UV−vis spectroscopy. Figure 6 shows the absorption spectra of 2 before (trans-2, black) or after (cis-2, red) UV light irradiation. The absorption maxima are located at 323 nm for trans-2, 274 nm for cis-2, which agrees with the report.64 Similar to that of 1, the results also confirm the photochemical reaction and the existence of the cis isomer. Compared with the free movement in solution phase, the molecules adsorbed on a solid surface are usually limited within the two dimensions. Molecular islands,14,17 or isolated species,15,16 steric hindrance37 involving the surface atoms or the neighboring molecules are expected to have a strong influence on the isomerization reactions. Here, for compound 2, prolonging the alkoxy chain of compound 1, the molecule−molecule and molecule−substrate interactions are changed, which affects its self-assembly behavior and also the photochemical reactions. As a result, we got completely different assembly patterns after irradiation. 3. Photoinduced [2 + 2] Reaction of Molecule 2. Stilbene and its derivatives are known to undergo two different reactions upon photochemical excitation. The first is an E/Z isomerization and the second is a [2 + 2]-cycloaddition of two stilbene cores. The [2 + 2]-cycloaddition can lead to the formation of cyclobutanes and during cyclobutane formation the carbon−carbon double bond in stilbene core is replaced by a carbon−carbon single bond, which causes the loss of conjugation.70 Here after a physisorbed monolayer of molecule 2 was formed at the graphite/1-phenyloctane interface, UV light was introduced to irradiate the monolayer at 365 nm for 20 min; as a result, the photoinduced [2 + 2] reaction occurred and the result was characterized by STM. Images observed after

0.1 nm in Figure 3c, which agrees with the lengths of single and interdigitated alkoxy chains. The host properties of 2D nanoporous networks for guest molecules and the dynamic behavior of guests inside the pores were studied extensively.66−69 Usually, a rigid network system leads to size selectivity; guests smaller than the cavity can be accommodated inside, whereas the larger ones are excluded. Here, molecule 1 can form nanoporous networks after UV light irradiation and each pore can accommodate one 1-phenyloctane molecule (indicated by the white arrow in Figure 3c). The length L4 is determined as 1.3 ± 0.1 nm in Figure 3c, which agrees with the length of a 1-phenyloctane molecule. Based on the above STM result’s analyses, a suggested molecular model is proposed in Figure 3d. Figure 3c shows a unit cell with parameters a = 3.9 ± 0.1 nm, b = 4.0 ± 0.1 nm, and α = 121 ± 2°. 2. Photoisomerization in Molecule 2 Assemblies. Molecules 2 and 1 have similar chemical structures except for the length of the substituted alkoxy chains. Different alkoxy chain length causes dramatical change in their self-assembly behavior and also consequently leads to the different photochemical results on the HOPG surface. As displayed in Figure 4a, at an appropriate concentration (