Physisorption Gives Narrower Orientational Distribution than

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Physisorption Gives Narrower Orientational Distribution than Chemisorption on a Glass Surface: A Polarization-Sensitive Linear and Nonlinear Optical Study Shoichi Yamaguchi,† Haruko Hosoi,‡,† Megumi Yamashita,‡ Pratik Sen,†,§ and Tahei Tahara*,† †

Molecular Spectroscopy Laboratory, Advanced Science Institute (ASI), RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan, and Department of Biomolecular Science, Faculty of Sciences, Toho University, 2-2-1 Miyama, Funabashi 274-8510, Japan

‡

ABSTRACT Linear and nonlinear polarization-sensitive spectroscopic techniques were employed to determine the orientational distribution of a probe molecule physisorbed and chemisorbed on a fused-silica glass surface. The chemisorption was through a well-known specific reaction between the maleimide group of the probe molecule and the thiol group of the surface. In the physisorption, the molecule was bound to the surface by the weaker van der Waals forces, dipole-dipole interaction, or both. Despite the absence of the strong interaction between the molecule and the surface in the physisorption, it was found that the physisorption gives narrower orientational distribution than the chemisorption. The strong covalent bond in the chemisorption is not effective to increase the orientational order. It is highly likely that the present conclusion is applicable to a variety of probe molecules having a functional group for a specific reaction of chemisorption. SECTION

Surfaces, Interfaces, Catalysis

A

dsorption is an essential factor of surface reactions such as corrosion and heterogeneous catalysis.1,2 Adsorbed molecules on a surface can have specific orientation, owing to anisotropic forces between the adsorbates and surface. Depending on the nature of the forces, there are two types of adsorption. In chemisorption, the adsorbates are strongly bound to the surface by chemical bonds. In physisorption, the adsorbates are held by the weaker van der Waals forces, dipole-dipole interaction, or both. Then, it is natural to expect that the chemisorbed molecules will show much narrower orientational distribution than the physisorbed ones. However, this expectation has never been systematically examined because of technical difficulty in determining the orientational distribution precisely.3-11 Although the orientational distribution has been successfully determined in several reports,6-11 it is still difficult to elucidate how the orientational distribution is influenced by adsorbatesurface interactions with the surface density of adsorbates kept low enough to avoid adsorbate-adsorbate interactions. Here we show an application of linear and nonlinear optical techniques to the measurement of the orientational distribution of a probe molecule adsorbed on a glass surface with such a low density. First of all, we determined the “up” versus “down” alignment of the probe molecule by a heterodynedetection technique, which has never been addressed by the previous studies. Then, we measured the polarization dependence of linear reflection (LR) and second harmonic generation (SHG) to obtain linear and nonlinear optical

r 2010 American Chemical Society

susceptibilities with the highest data quality ever achieved. We formulated parallel framework to calculate molecular orientational parameters from the linear and nonlinear susceptibilities and determined the ensemble average and standard deviation of the orientational distribution. We unequivocally demonstrate that for this probe molecule the physisorption gives narrower orientational distribution than the chemisorption, which is contrary to the intuitive expectation. Figure 1 shows the probe molecule, PyMPO maleimide12 physisorbed and chemisorbed on the glass surface. The physisorption was simply realized by casting an ethanol solution of PyMPO maleimide on an unmodified fused-silica glass surface and drying it for 10 s. For chemisorption, the probe molecule was adsorbed on a thiol-modified surface of the same fused-silica glass through the specific chemical reaction between the maleimide group and the thiol group, which is one of the most widely utilized chemical reactions for probe molecules in bioscience. The surface density of PyMPO was controlled at 0.3 to 0.4 molecules nm-2 in each adsorption (Supporting Information), which is low enough to avoid adsorbate-adsorbate interactions. The orientation of the probe molecule is parametrized by the tilt angle θ of the S1 r S0 transition dipole moment of Received Date: August 13, 2010 Accepted Date: August 19, 2010 Published on Web Date: August 26, 2010

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DOI: 10.1021/jz101142m |J. Phys. Chem. Lett. 2010, 1, 2662–2665

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Figure 1. Schematics of PyMPO physisorbed (blue) and chemisorbed (red) on the glass surfaces. θ represents the tilt angle between the transition dipole of PyMPO (molecular z axis) and the surface normal (laboratory Z axis). See the Supporting Information for the details.

PyMPO (molecular z axis) with respect to the surface normal (laboratory Z axis), as shown in Figure 1. First, the range of Æθæ, that is, up versus down alignment, was determined for physisorbed and chemisorbed PyMPO by heterodyne-detected electronic sum frequency generation (HD-ESFG).13,14 The brackets represent the ensemble average for the orientational distribution. HD-ESFG provides complex χ(2) spectra of surface molecules (χ(i) is the ith-order optical susceptibility), which directly indicates the sign of Æcos θæ. The complex χ(2) spectra of the physisorption and chemisorption samples (Figure S7 of the Supporting Information) showed that Æcos θæ is positive for each adsorption. Therefore, the range of Æθæ is from 0 to 90, which corresponds to the up alignment of physisorbed and chemisorbed PyMPO. The orientational distribution can be characterized by the ensemble average and standard deviation of θ. These two quantities, Æθæ and (Æ(θ - Æθæ)2æ)1/2 ( σ), can be obtained numerically from experimental values of Æcos2 θæ and Æcos3 θæ/ Æcos θæ, if the functional form of the orientational distribution


Figure 2. (a) Polarization SHG and (b) polarization LR data of the physisorption (blue) and chemisorption (red) samples. The open circles represent the experimental data, and the solid lines stand for theoretical fits. The input wavelength was 800 nm for SHG and 400 nm for LR. See the Supporting Information for the details.

is mathematically assumed.7 The polarization dependence of SHG was employed to determine Æcos3 θæ/Æcos θæ. Figure 2a shows the polarization SHG data of the physisorption and chemisorption samples. The horizontal axis represents the linear polarization angle of the input fundamental light at 800 nm, and the vertical axis stands for the SHG intensity at 400 nm. The fitting analyses of these data yielded the ratios of the χ(2) tensor elements in resonance with the S1 r S0 (2) transition of PyMPO (Supporting Information): χ(2) ZZZ/χXZX is 0.73 and 1.1 for the physisorption and chemisorption samples, respectively. Then, Æcos3 θæ/Æcos θæ can be determined by

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using the following equation15 ð2Þ

Æcos3 θæ χ ¼ ð2Þ ZZZ ð2Þ Æcos θæ χZZZ þ 2χXZX which is on condition that the molecular hyperpolarizability of PyMPO has only one predominant tensor element along the molecular z axis. (See the Supporting Information.) For the physisorption and chemisorption samples, Æcos3 θæ/Æcos θæ was determined to be 0.27 and 0.36, respectively. To determine Æcos2 θæ, the polarization dependence of LR was employed. Figure 2b shows the polarization LR data of the physisorption and chemisorption samples. The horizontal axis represents the linear polarization angle of the input light at 400 nm, and the vertical axis stands for the difference of the reflection intensity between the surfaces with and without PyMPO. The fitting analyses of these data yielded the ratios of the χ(1) tensor elements, where χ(1) is in resonance with the (1) S1 r S0 transition of PyMPO: χ(1) ZZ /χXX is 0.49 and 0.35 for the physisorption and chemisorption samples, respectively. Then, Æcos2 θæ is obtained from the following equation16 ð1Þ

Æcos2 θæ ¼

χZZ ð1Þ χZZ

ð1Þ

þ 2χXX

For the physisorption and chemisorption samples, Æcos2 θæ was determined to be 0.20 and 0.15, respectively. Here we assume that the orientational distribution is expressed by a modified Gaussian function introduced by Simpson and Rowlen,17 which is physically the most relevant. From numerical calculations with all possible inputs for the parameters of the modified Gaussian function, we can obtain numerical relationships between Æθæ, σ, Æcos3 θæ/Æcos θæ, and Æcos2 θæ. When Æcos3 θæ/Æcos θæ is fixed at the experimental value of the physisorption sample (0.27), the values of Æθæ and σ cannot be determined uniquely, but one-to-one relationship between Æθæ and σ can be determined, as graphically shown in Figure 3a with a solid blue line. (Because HD-ESFG has shown 0 < Æθæ < 90, the vertical axis in Figure 3a does not have to be larger than 90.) This one-to-one relationship means that any pair of Æθæ and σ on the solid blue line can realize Æcos3 θæ/ Æcos θæ of 0.27. Consequently, the orientational distribution cannot be determined uniquely from the polarization SHG data alone, even with the assumption of the modified Gaussian function. However, when Æcos2 θæ is fixed at the experimental value of the physisorption sample (0.20), another one-toone relationship between Æθæ and σ can be obtained, which is also shown in Figure 3a with a dotted blue line. The crossing point (indicated by a blue open circle) of the solid and dotted blue lines gives the unique pair of Æθæ = 66 and σ = 12, which can simultaneously realize Æcos3 θæ/Æcos θæ of 0.27 and Æcos2 θæ of 0.20. This unique pair gives the orientational distribution for the physisorption sample, which is shown in Figure 3b with a blue line. The same analysis was performed for the chemisorption sample. The red lines in Figure 3a represent one-to-one relationships that satisfy the experimental values of SHG and LR of the chemisorption sample. The crossing point (indicated by a red open circle) of the solid and dotted red lines gives the unique pair of Æθæ = 83 and σ = 23.


Figure 3. (a) One-to-one relationships between Æθæ and σ for the physisorption (blue) and chemisorption (red) samples. The solid and dotted lines indicate pairs of Æθæ and σ that can reproduce the experimental values of Æcos3 θæ/Æcos θæ and Æcos2 θæ, respectively. (b) Distribution of cos θ for the physisorption (blue) and chemisorption (red) samples, given by the crossing points (open circles) in part a. See the Supporting Information for the details.

Then, the orientational distribution for the chemisorption sample is determined as shown in Figure 3b with a red line. Note that Æcos3 θæ/Æcos θæ is equal to Æcos2 θæ in the limit of an infinitesimally narrow distribution. With closer values of Æcos3 θæ/Æcos θæ and Æcos2 θæ, σ becomes smaller, which corresponds to the case of the physisorption compared with the chemisorption. It is also noted that the polarization SHG data alone do not provide correct knowledge on the orientational distribution even qualitatively. From the SHG data, Æcos3 θæ/Æcos θæ was determined to be 0.27 and 0.36 for the physisorption and chemisorption samples, respectively. On the assumption of an infinitesimally narrow distribution, Æcos3 θæ/Æcos θæ is equal to cos2Æθæ. Then, Æθæ on this assumption is 59 and 53 for the physisorption and chemisorption samples, respectively, which indicates a larger tilt angle for the physisorption. In the present analysis, however, Æθæ was determined to be 66 and 83 for the physisorption and chemisorption samples, respectively, which oppositely indicates a smaller tilt angle for the physisorption. It is definitely necessary to perform polarization LR as well as SHG measurements for correct knowledge on the orientational distribution. It is clear in Figure 3b that the orientational distribution of the physisorption sample is narrower than that of the chemisorption sample. The strong covalent bond formed between

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the maleimide and thiol groups is not effective to make the orientational distribution narrower for the chemisorption than the physisorption. Despite the absence of the strong interaction between the probe molecule and the surface in the physisorption sample, Figure 3b shows that 97% of the physisorbed molecules have θ < 90, that is, the up alignment. In the chemisorption sample, as much as 37% of the molecules have the down alignment. For the present “unexpected” result, we can think of three reasons: First, because the alkyl chains on the thiol-modified glass surface bring about less polar microenvironment than the unmodified glass surface, the thiol-modified glass surface has weaker local electric field, which can result in less effective alignment of the adsorbed molecules. Second, microscopic surface roughness in the chemisorption sample may be larger because of the presence of the silane-coupling layer, which may lead to the broader orientational distribution. Third, whereas the chemisorbed molecules are restricted in their orientation by the limitations of their covalent bonds, the physisorbed molecules can have optimized orientations to minimize their energy. Still now it is almost impossible to predict theoretically the properties of the adsorptions for a probe molecule as large as PyMPO on a solid surface as complicated as the fused-silica glass. However, it is highly likely that the present conclusion is applicable to a variety of probe molecules with a functional group for a specific reaction of chemisorption. The present experiment provides new insight into the preparation of oriented (sub)monolayer films on solid surfaces and biological interfaces.

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SUPPORTING INFORMATION AVAILABLE Detailed description of the experiments, data analyses, and theoretical models. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed.

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Present Addresses: §

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, U.P., India.

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ACKNOWLEDGMENT This work was supported by a Grant-in-Aid

for Scientific Research on Priority Area “Molecular Science for Supra Functional Systems” (no. 19056009) from Ministry of Education, Culture, Sports, Sciences and Technology (MEXT)and a Grant-in-Aid for Scientific Research (B) (no. 22350014) from Japan Society for the Promotion of Science (JSPS). We thank the RIKEN Super Combined Cluster (RSCC) for the computational resources.

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Physisorption Gives Narrower Orientational Distribution than

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