Molecular Motion of Electrically Stimulated Self-Assembled

of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8562, .... Hao-Li Zhang , Stephen D. Evans , Kevin Critchley , Hitoshi...
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J. Phys. Chem. B 2002, 106, 7142-7145

Molecular Motion of Electrically Stimulated Self-Assembled Monolayers on Gold Surface Hitoshi Fukushima*,†,‡ and Takashi Tamaki§ Joint Research Center for Harmonized Molecular Materials (JRCHMM)-Japan Chemical InnoVation Institute (JCII), c/o AdVanced Industry Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Institute for Materials and Chemical Process (IMCP), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8562, Japan ReceiVed: October 30, 2001; In Final Form: March 15, 2002

The molecular motion of self-assembled monolayers (SAMs) on gold was examined using surface plasmon resonance (SPR) under an electric field. Electrically stimulated SAMs are generated from asymmetric disulfides containing a cyano-fluoro-terphenyl moiety that has a large anisotropic factor of the dipole moment. SPR was combined with a liquid crystal (LC) cell, which only detects a strong influence of the electric field on SAMs. A slight change in the SPR angle was observed upon the application of an electric field in the cell. The response time to saturation of the change of the SPR resonance angle is approximately 150 ms, which is not as fast as those of conventional liquid crystal materials. Furthermore, the relaxation time is much slower than in the case of a conventional liquid crystal cell. This is the first demonstration of an electrically induced motional behavior of a SAM that is chemically attached to a gold surface.

Introduction Self-assembled monolayers (SAMs) are ideal ultrathin films to amplify molecular scale functions to the macroscopic level, such as surface properties.1 In particular, the accurate motion control of picodroplets on functionalized surfaces is one of the attractive objectives in order to develop microdevices using picoliquid chemistry.2 Studies of SAMs3,4 have already shown that the high-sensitivity SAM surfaces in contact with liquid crystal enabled the orientation of bulk liquid crystals to be controlled efficiently. SAMs generated from asymmetric disulfides or triethoxysilane derivatives containing an azobenzene moiety have induced simultaneous change of surface properties, such as the refractive index and contact angle, upon irradiation with UV light and visible light, respectively.5 Moreover, the same azobenzene system consisting of a calix[4]resorcinarene derivative monolayer on a silica plate could induce the stepwise motion of liquids along a surface exposed to asymmetrical photoirradiation.6 Electrically induced dynamics of liquid crystal bulk adjacent to a solid surface has also been investigated by various methods such as time-resolved infrared spectroscopy and polarization modulated spectroscopy.7 Recently, conductance switching in single molecules of SAMs on gold was detected in a nanoscale area using a STM tip. This switching was due to conformational changes in the molecules on the metal surface.8 However, no study has been reported to date on electrically stimulating the orientational changes driven by the molecular motion of SAMs chemisorbed onto metal surfaces. The main investigation should address the following issues: (1) how the molecular oscillation of monolayer attached to a metal surface is generated by the electric field, and (2) how the nanoscale motion of SAMs induced by an electric field is * Corresponding Author. E-mail: [email protected]/ [email protected]. † Original Address: TPRC, SEIKO EPSON Corporation, Fujimi 281, Suwa-gun, Nagano 392-8502, Japan. ‡ JRCHMM-JCII. § IMCP-AIST.

propagated and amplified over bulk materials such as liquid crystals. To achieve these objectives, the SAM generated from newly designed and synthesized disulfide was introduced to a metal surface combined with the novel surface plasmon resonance (SPR) technique. Using the above novel SAM system, we present here the first evidence of electrically stimulated molecular motion of SAMs generated from asymmetrical disulfide LC1 that has a large anisotropic factor of the dipole moment. Results and Discussion To induce the maximum response of molecular dynamics in the SAM, a cyano-fluoro-terphenyl moiety, which has a large anisotropic factor of the dipole moment,9 was introduced into an asymmetric disulfide skeleton. The general strategy for the design of an appropriate adsorbent molecule is based on two requirements: (1) a high degree of anisotropic factor of the dipole moment and (2) spatial free volume between adjacent molecules on the substrate surface. To fulfill these requirements, the asymmetrical disulfide LC1 shown in Scheme 1 was synthesized. The SAM generated from LC1 was characterized by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) prior to characterization of the dynamic change. The νsCN band at 2233 cm-1 was sharp, which indicates that the CN bond attached to the benzene ring is directed toward the surface normal. The substrate with the SAM generated from LC1 is employed as an electrode for the liquid crystal cell, as shown in Figure 1. To study the electrically stimulated motion of the LC1 SAM, a specific liquid crystal cell including this SAM was designed so as to monitor the substantial dynamics of the tailored SAM under the application of an electric field. LC1 was dissolved in dichloromethane (0.2 mM), and 500 Å of gold film deposited onto a substrate, which has the same refractive index as the prism in the apparatus (n ) 1.73), was immersed in the solution for 6 h. The gold substrate chemisorbed by LC1 was rinsed

10.1021/jp0155765 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/28/2002

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J. Phys. Chem. B, Vol. 106, No. 29, 2002 7143

SCHEME 1: Molecular Structure of LC1 and the Schematic Image of SAM Generated from LC1

Figure 1. Schematic image of SPR apparatus attached to a liquid crystal cell to monitor the dynamics of LC1 SAM induced by an electric field in real time.

well with dichloromethane and dried under nitrogen. Liquid crystal, which was inserted into the cell with a gap (6µm) between the LC1 SAM-modified gold substrate and the ITO electrode, exhibits an almost zero anisotropic factor of the dipole moment (∆ = 0). This specific liquid crystal was formed by mixing two liquid crystal materials.10 The liquid crystal cell filled with this specific liquid crystal allows us to detect only the regional phenomena of dynamic change in the LC1 SAM onto the gold surface under an electric field. Furthermore, the advantage of using the liquid crystal bulk in contact with the LC1 SAM surface is that slight changes of SAM dynamics could be amplified due to the large anisotropic factor of the refractive index. It would be possible to enhance even a molecular scale of motional changes that may occur on the LC1 SAM surface due to the electric field. The apparatus used to monitor the substantial dynamics of the SAM on gold is based on the principle of SPR,11 which allows us to characterize the molecular scale motion of monolayer thickness on the gold surface. The details of the SPR system are shown in Figure 1. This SPR system is specifically designed to detect the change of an organic thin layer having a high refractive index such as that of a liquid crystal.12 The prism has a refractive index of 1.73 and thus exhibits SPR absorbance in a wide range of thin film measurement with 670 nm incident light. Dynamic changes in the SPR angle under an electric field were observed at the rate of 1 plot/0.1 ms. An electric field was induced between the ITO electrode (anode) and the gold surface (cathode) by applying a dc voltage (10, 15, and 20 V). The results of SPR measurement are shown in Figure 2. For the cell fabricated with the LC1 SAM, a clear change of SPR angle was observed upon the application of an electric field in the cell. The response time is approximately 150 ms to saturation of the change of the SPR resonance angle, which is not as fast as those of conventional liquid crystal materials.7 Furthermore, the relaxation time is much slower than in the case of a conventional cell.7 As the electric field becomes stronger in the cell, the slight change in the SPR angle caused by the change in the dielectric constant in the LC1 SAM is amplified further (Figure 2). Furthermore, this result differs from the response in photoisomerization of silylated surfaces, which were tethered by azobenzene derivatives. In particular, the relaxation time of the photoisomerized surface is faster than that of the electrically stimulated LC1 SAM surface.13 Negligible response of the SPR angle from the LC1 SAM surface was observed when the polarity of the electric field was reversed.

Figure 2. Real time characterizations of the SPR angles generated from the LC1 SAM on gold surface: (a) when dc voltage is on; (b) when dc voltage is off. The result generated from hexadecanethiol (HDT SAM) is also shown for comparison The dielectric anisotropy of the liquid crystal used here is ∆ = 0.

We assumed that this phenomenon arose due to the unfavorable conformation of the terphenyl moiety with the alkyl chain. The vector of the dipole moment combined with the vector of the reverse electric field in the cell is in a nearly horizontal direction along the gold surface. This molecular motion may be hindered by the steric interference between terphenyl moieties in the SAM. On the other hand, the initial vector of the electric field stimulates motion in the direction for favorable conformational

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Figure 3. Real time characterization of the SPR angle generated from the TL213 liquid crystal bulk, which comes in contact with the LC1 SAM on the gold electrode. TL213 has a high degree of the dielectric anisotropy (∆ = +5.7).

change of the terphenyl moiety, which allows the terphenyl moiety to gain more free space above the dodecyl chain of the adjacent disulfide in the SAM. However, the details of this mechanism are yet to be investigated. The liquid crystal cell with hexadecanethiol on gold exhibited no change of the SPR angle under the same experimental conditions, indicating that only the LC1 SAM responded to the electric field, leading to the motional change of the SAM. To clarify the assumption of the independent motion of liquid crystal on the LC1 SAM surface, another cell having a conventional liquid crystal (TL213)10 with the LC1 SAM on a gold electrode was prepared. The result of SPR measurement is shown in Figure 3. The response time and relaxation time of this cell under an electric field are much faster than those of the LC1 cell having liquid crystal with a negligible anisotropic factor of the dipole moment (∆ = 0). The major driving force of this motion originated from the liquid crystal at the interfacial layer on the LC1 SAM. Moreover, the degree of change of the SPR resonance angle is much greater compared to the case shown in Figure 2. This implies that independent motion of the liquid crystal inserted into the LC1 cell should cause the large change of the SPR resonance angle as well as the faster response to the electric field. From these results of control experiments, the slight change of the SPR resonance angle in Figure 2 is attributed to the independent dynamics of the LC1 SAM on the gold surface. Interestingly, the change of the SPR angle initiated by LC1 SAM dynamics induced the reduction of the refractive index at the interfacial layers on gold whereas the dynamics of liquid crystal itself, as shown in Figure 3, was opposite. Following the above assumption, this reduction may be caused by the conformational change of the terphenyl moiety, which induces the decrease of the molecular density of the SAM. This conformation is not thermodynamically stable; this is proved by the fact that the SPR angle of the LC1 SAM on gold returns to the initial value of the SPR angle when dc voltage is cut off. The strength of the electric field seems to play a crucial role in overcoming the energy barrier of conformational change of the terphenyl moiety with an alkyl spacer. We observed no increase of the SPR angle with electric field over 20 V. The substantial change of the SPR angle (0.030; 20 V) corresponds

Letters to 4.2 × 10-4 of the refractive index according to simulative calculation of the SPR curve if the thickness of LC1 SAM is assumed to be constant. This result also indicates that this surface dynamics occurred in the range of 100 Å thickness on the LC1 SAM, if the refractive index is assumed to be constant. From SPR calculation14 of LC1 SAM on gold, the thickness of the LC1 SAM is found to be approximately 25 Å. If we assume that this result is dictated by thickness under a constant refractive index, the maximum change of thickness is deduced to be approximately 5-6 Å by SPR calculation;14 this assumption is not reasonable for explaining the change of 100 Å thickness. On the other hand, it is possible to assume that this result is influenced by the slight change of the refractive index, which is correlated to the molecular orientation of the SAM interface, because both the LC1 SAM and liquid crystal have a large anisotropic refractive index. Even a small change in the tilt angle of the terphenyl moiety of LC1 attached to the gold surface may cause a large change of the refractive index in the SAM on gold. Another question is the limited level of amplification of the LC1 SAM by the electric field. The motion of the LC1 SAM due to the electric field seems insufficient to induce the long-range anchoring effect from the LC1 SAM surface to liquid crystal. The mechanism of this limited range of molecular motion in the LC1 SAM is still not fully understood. However, the reorientation of photoisomerized azobenzene surfaces also requires a greater amount of energy to accelerate the photoalignment of liquid crystal compared to the energy required to activate the azobenzene monolayer.15 Thus, we assume that the energy supplied by the electric field is insufficient to induce the full reorientation of liquid crystal, which is in contact with the electrically sensitive LC1 SAM. Further study to clarify the mechanism including direct observation of the LC1 SAM surface using STM as well as additional experiments on the SPR system is under way. Regarding the origin of this change of the refractive index, the influence of natural fluctuation, which is caused by liquid crystal molecules on the SAM surface, is highly unlikely due to the result of control experiment using a HDT SAM, as mentioned. Furthermore, another control experiment using a bare gold surface as the electrode in the liquid crystal cell also showed no change of the SPR angle with 20 V of electric field. The dynamic behavior of the bulk liquid crystal phase adjacent to either gold or the HDT SAM strongly depends on the strength of the dielectric anisotropy of the bulk liquid crystal itself. If there is no dielectric anisotropy of the bulk liquid crystal, no SPR response is observed at the interface between the HDT surface and the bulk liquid crystal. SPR measurement plays a crucial role only in the detection of the change of the refractive index adjacent to the SAM surface, and the sensitivity of the SPR response decays exponentially over 1000 Å from the gold surface. In particular, compared to the result for the TL213 liquid crystal, the cell fabricated by the HDT SAM and liquid crystal that has no dielectric anisotropy exhibited no SPR response under an electric field. This proved that this cell completely inhibits the field response of the liquid crystal phase adjacent to the HDT SAM. Thus, these results strongly support that the SPR change is the result of only motional behavior caused by the electrically induced LC1 SAM. Conclusions Dynamic change of electrically induced self-assembled monolayers on gold was observed in real time using the SPR technique. The property of molecular motion in SAMs exhibited a novel response to an applied electric field compared to

Letters conventional liquid crystal cells. This study proved that SAMs generated from molecules with strong dielectric anisotropy could perform dynamic change of the monolayer conformation upon application of an electric field. Further studies to obtain the larger amplification of SAM dynamics and to characterize the mechanism of the molecular motion are currently under way. Acknowledgment. We thank Mr. T. Tanaka and Mr. H. Tajima (Nippon Laser & Electronics Lab. Japan) for technical support for SPR instrument. We also thank Mr. S. Yamada (Seiko Epson Corporation), and Drs K. Tamada (AIST) and E. Adachi (L’Oreal, Tsukuba Centre) for helpful suggestions and discussions. The work was supported by NEDO for the Harmonized Molecular Materials theme funded through the project on Technology for Novel High-Functional Materials (AIST). Supporting Information Available: (1) Synthetic procedure of LC1, (2) chart of PM-IRRAS of LC1 SAM on gold, (3) simulative data of SPR curve for calculation (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (b) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, E. O.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417. (2) (a) Kawase, T.; Sirringhaus, H.; Friend, R. H.; Shimoda, T. SID 01 DIGEST 2001, 40. (b) Morii, K.; Seki, S.; Miyashita, S.; Towns, C. R.; Burroughes, J. H.; Friends, R. H.; Shimoda, T. Proceedings of the 10th International Workshop on Inorganic and Organic Electroluminescence; Japan Society for the Promotion of Science, 2000, 384. (c) Kopp, U. M.; de Mello, J. A.; Manz, A. Science 1998, 280, 1046. (d) Zhang, C.-X.; Manz, A. Anal. Chem. 2001, 73, 2656. (e) Mitchell, M. C.; Spikmans, V.; Manz, A.; de Mello, A. J. J. Chem. Soc., Perkin Trans. 2001, 514. (f) Knight, J.

J. Phys. Chem. B, Vol. 106, No. 29, 2002 7145 B.; Vishwanath, A.; Brody, J. P.; Austin, R. H. Phys. ReV. Lett. 1998, 80, 3863. (g) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science 1999, 283, 57. (3) Ichimura, K., Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4, 1214. (4) (a) Gupta, V. K.; Abbott, N. L. Phys. ReV. E 1996, 54, 4540. (b) Gupta, V. K.; Abbott, N. L. Science 1997, 276, 1533. (5) (a) Sekkat, Z.; Wood, J.; Geerts, Y.; Knoll, W. Langmuir 1996, 12, 2976. (b) Akiyama, H.; Tamada, K.; Nagasawa, J.; Nakanishi, F.; Tamaki, T. Trans. Mater. Res. Soc. Jpn. 2000, 25 (2), 425. (6) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624. (7) (a) Urano, I. T.; Machida, S.; Sano, K. Chem. Phys. Lett. 1995, 242, 471. (b) Tadokoro, T.; Fukazawa, T.; Toriumi, H. Jpn. J. Appl. Phys. 1997, 36, L1207. (c) Noble-Luginbuhl, R. A.; Blanchard, M. R.; Nuzzo, R. G. J. Am. Chem. Soc. 2000, 122, 3917. (8) Donhauser, Z. J.; et al. Science 2001, 292, 2303. (9) The dipole moment along the long axis of LC1 was estimated 5.3, whereas that of the short axis was 1.6 using MOPAC (PM3). (10) Both materials were purchased from Merck Japan Ltd. TL213 has a positive dielectric anisotropy (∆ = +5.7), and the refractive index was no ) 1.527, ∆n ) 0.239. The liquid crystal material (ME-1), which has the negative value of dielectric anisotropy, was specifically produced; ∆ = -4.9, no ) 1.489, ∆n ) 0.124. The mixture consists of 46 wt % TL213 and 54 wt % ME-1 so as to lead to “zero” dielectric anisotropy. The physical properties of the mixture were ∆ = 0, no ) 1.506, ∆n ) 0.177. (11) (a) Phelps, M. J.; Taylor, M. D. J. Phys. D: Appl. Phys. 1996, 29, 1080. (b) Peterlinz, A. K.; Georgiadis, R. Opt. Commun. 1996, 130, 260. (12) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913. (13) Ichimura, K.; Suzuki, Y.; Seki, T.; Kawanishi, Y.; Tamaki, T.; Aoki, K. Jpn. J. Appl. Phys. Suppl. 1989, 28-3, 289. (14) 1.540 of the refractive index for LC1 was used for the SPR measurement. The calculation of MOPAC (PM3) estimated that the length of LC1 was 31 Å. Considering the tilt angle (30°) of the alkyl chain to stabilize the lateral packing of the LC1 SAM on gold, the estimated thickness from this assumption was approximately 27 Å. The result of the SPR indicated the thickness of the LC1 SAM was approximately 25 Å. The difference may be caused by either the influence of the anisotropic factor of the refractive index or by the minor contribution of the ether bond to bend the terphenyl moiety. (15) (a) Ichimura, K.; Akiyama, H.; Kudo, K.; Ishizuki, N.; Yamamura, S. Liq. Cryst. 1996, 20, 425. (b) Akiyama, H.; Ichimura, K. J. Photopolym. Sci. Technol. 1998, 11, 181.