Atomistic Simulations of Self-Assembled Monolayers That Contain

Also the effect of relative positions of the azobenzene group within the chain is .... Alexey A Sokol , C.Richard A Catlow , Selma Hansal , Wolfgang E...
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Langmuir 1996, 12, 3024-3030

Atomistic Simulations of Self-Assembled Monolayers That Contain Azobenzene L. Xing and W. L. Mattice* Institute of Polymer Science, University of Akron, Akron, Ohio 44325-3909 Received October 20, 1995. In Final Form: March 25, 1996X Self-assembled monolayers containing azobenzene groups covalently incorporated into some of the chains are of particular interest because of the photoisomerizable character of the azobenzene unit. A potentially interesting system is one where the terminal groups in the chains that contain azobenzene are different from the other terminal groups. The configurational states of the azobenzene are then expected to control the relative protrusion of the terminal groups and alter the mole fraction of these groups present on the free surface, which directly influences the properties of the monolayer. Molecular dynamics simulations have been performed for mixed systems of a hydrocarbon chain containing azobenzene surrounded by pure hydrocarbon chains on a planar silica substrate. With a surface area assigned roughly three times larger than the usual area for a pure hydrocarbon chain, the azobenzene-containing chain exhibits an observable chain length difference between the trans and the cis configurations. Also the effect of relative positions of the azobenzene group within the chain is studied, and it is found that head and tail connections introduce larger variations in chain length than center connection during the trans h cis isomerization. Both effects lead to the alteration of the relative protrusion of functional groups at the chain end, thus the characteristics of the monolayer are altered and controlled at the molecular scale.

Introduction Self-assembled monolayers (SAMs) are a class of molecular assemblies that are prepared by spontaneous adsorption of molecules from solution onto a solid substrate.1 A self-assembling surfactant molecule participates in three types of interactions. The first and the most important one is the chemisorption on the substrate, which distinguishes the SAMs from the LangmuirBlodgett (LB) films. The interchain van der Waals interaction has to be strong enough for the molecules to self-assemble. The third interaction comes from the terminal functionalities. Due to the possibility of chemical reaction with the substrate, the self-assembly compounds have a distinct advantage in both thermal2 and chemical3 stability over the classical LB technique. Both the synthesis and the characterization of SAMs have been mostly attempted on a gold surface, probably because gold can be treated under ambient conditions since it does not have a stable oxide. Among the various functional groups available, such as the thiols, sulfides, and disulfides, the thiol group forms the strongest interaction with the gold surface. Another popular system is alkylsiloxane molecules on a silica surface. Because of the polymeric nature of the siloxane bonds, the SAMs made of alkylsiloxane on silica are not as ordered as those of alkanethiols on gold. But also because of the polymeric bonding, the former are mechanically more stable than the latter.4 Recent work on self-assembled systems provided not only a more detailed probe of the structure of the typical SAMs, e.g., alkanethiol molecules on gold,5-11 X

Abstract published in Advance ACS Abstracts, May 15, 1996.

(1) Ulman, A. An Introduction to Ultra-thin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press, Inc.: San Diego, CA, 1991. (2) Cohen, S. R.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Wiley and Sons: New York, 1980. (3) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (4) Xiao, X. D.; Liu, G. Y.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600. (5) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (6) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (7) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853.

S0743-7463(95)00915-2 CCC: $12.00

and alkylsiloxane on silica,4,12-14 but also diverse combinations of organic molecules and substrates have been employed for self-assembly processes.15-20 Although a vast variety of experimental techniques have been used to characterize the properties of those systems, e.g., contact angle, ellipsometry, transmission electron microscopy, infrared spectroscopy, X-ray, scanning tunneling microscopy, and atomic force microscopy, the structure at the molecular level remains unclear. Computer simulations were carried out by Klein et al. for alkanethiols adsorbed onto a gold substrate.21-25 Two models are suggested due to the inadequate experimental indentification of the direction of the sulfur-methylene bond on the surface.21 In one model the S-C bond is nearly parallel to the substrate, while in the other it rotates freely. Both systems consist of ordered, oriented chains uniformly tilted with respect to the surface normal and gave film thickness consistent with experiments. Simulations at different temperatures reveal distinct phases with different kinds of disorder, and a one-dimensional “phase (8) Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116. (9) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825. (10) Robison, G. N.; Freedman, A.; Graham, R. L. Langmuir 1995, 11, 2600. (11) Dhirani, A.; Hines, M. A.; Fisher, A. J.; Ismail, O.; GuyotSionnest, P. Langmuir 1995, 11, 2609. (12) Jin, Z. H.; Vezenov, D. V.; Lee, Y. W.; Zull, J. E.; Sukenik, C. N.; Savinell, R. F. Langmuir 1994, 10, 2662. (13) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. Langmuir 1995, 11, 1190. (14) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215. (15) Israelachvili, J. Langmuir 1994, 10, 3774. (16) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (17) Flokers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (18) Peanasky, J.; Schneider, H. M.; Granick, S.; Kessel, C. R. Langmuir 1995, 11, 953. (19) Gu, Y.; Lin, Z.; Butera, R. A.; Smentkowski, V. S.; Waldeck, D. H. Langmuir 1995, 11, 1849. (20) Frisbie, C. D.; Wollman, E. W.; Wrighton, M. S. Langmuir 1995, 11, 2563. (21) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994. (22) Hautman, J.; Klein, M. L. J. Chem. Phys. 1990, 93, 7483. (23) Hautman, J.; Bareman, J. P.; Mar, W.; Klein, M. L. J. Chem. Soc., Faraday Trans. 1991, 87, 2031. (24) Hautman, J.; Klein, M. L. Phys. Rev. Lett. 1991, 67, 1763. (25) Mar, W.; Klein, M. L. Langmuir 1994, 10, 188.

© 1996 American Chemical Society

Simulations of Azobenzene SAMs

diagram” was obtained.22 Also molecular dynamics calculations were used to study the dense monolayers terminated by different functional groups.23 Monolayers formed from molecules with polar terminal groups adopted more rigid structures. All of the previous work was based on a united-atom method. Then an all-atoms model, which provided a more detailed description, was applied and compared to the united-atom model.25 A Monte Carlo study was also performed for the alkanethiols adsorbed on a gold surface.26-29 In sufficiently large systems, welldifferentiated domains appeared which were absent in small systems.29 Using molecular dynamics simulations the phase behavior of a model Langmuir monolayer had been investigated in the regime near close packing as well.30 The earliest computer simulation on alkylsiloxane molecules on an ordered substrate was done by Zhan et al.31 The system consisted of 36 alkylsiloxane molecules terminated by hydroxyl groups in a simulation cell with periodic boundary conditions in the plane of the surface. Different lattices with the same surface density of hydrocarbon chains were studied using molecular dynamics at room temperature. The tilt angle of the chains, and thus the thickness of the surface, is determined not only by the surface density but also by the different grafting patterns. The average thickness of the monolayers for the square lattice is smaller than that of the triangle lattice, because the chains in the triangle lattice exhibit a smaller tilt angle. The fraction of the gauche defects is fairly low, and most of them occurred near the tail of the chains. The pattern of the free surface formed by the tail atoms is found to be disordered. Most of the previous work was performed on systems containing one kind of chain, although different functional groups might be attached to the chain ends. In addition to the pure hydrocarbon layers, the mixed systems of both hydrocarbon and chains containing azobenzene groups are of interest because of the photoisomerizable properties of the azobenzene unit. Such a photoresponsive unit has been widely applied in photochemical studies, e.g., photoregulated conformation of azobenzene-containing polypeptides,32 liquid crystal alignment by azobenzene monolayers,33 reversible optical storage materials from azo polymers,34 and illumination-induced modification of LB films.35 In this paper, we are interested in the chain length changes caused by the trans h cis isomerization of the azobenzene unit. Figure 1 shows the basic idea of designing such a “smart” surface. Different functional groups are bonded to the ends of chains with or without azobenzene incorporation. We conjecture that for the trans configuration of the azobenzene group, the end groups attached to the azobenzene-containing chain stick out of the surface formed by the surrounding hydrocarbon chains, while with the azobenzene group in the cis configuration, they will be buried inside the monolayer. And the isomerizations can be reversibly introduced by irradiation at appropriate wavelength. The change of the chain length (26) Seipmann, J. I.; McDonald, I. R. Mol. Phys. 1992, 75, 255. (27) Seipmann, J. I.; McDonald, I. R. Mol. Phys. 1993, 79, 457. (28) Seipman, J. I.; McDonald, I. R. Phys. Rev. Lett. 1993, 70, 453. (29) Seipmann, J. I.; McDonald, I. R. Langmuir 1993, 9, 2351. (30) Karaborni, S. Langmuir 1993, 9, 1334. (31) Zhan, Y. J.; Xing, L.; Mattice, W. L. Langmuir 1995, 11, 2103. (32) Diardelli, F.; Pieroni, O.; Fissi, A.; Houben, J. L. Biopolymers 1984, 23, 1423. (33) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992, 8, 2601. (34) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2268. (35) Seki, T.; Ichimura, K.; Fukuda, R.; Tanigaki, T.; Tamaki, T. Macromolecules 1996, 29, 892.

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Figure 1. Schematic of the effects of trans h cis isomerization on the relative protrusion of the functional group bonded to the end of the chain that contain azobenzene: (left) azobenzene group in the trans configuration; (right) azobenzene group in the cis configuration.

will lead to the alteration in the composition of functional groups at the free surface, and thus the characteristics of the surface can be varied and controlled by the photoirradiation. Of particular interest will be the changes in the affinity of the model for protein adsorption. Whitesides found that protein adsorption increased as the mole fraction of methyl-terminated chains increased in a mixed monolayer derived from hydroxyl- and methyl-terminated amphiphiles.36 From the model proposed in Figure 1, we predict that the amount of adsorbed protein will depend on the configuration of the azobenzene. The objectives of the present article are to provide information pertinent to the answers to the following questions: First, how do the chains behave differently when the incorporated azobenzene groups in the trans form are converted to the cis form? Secondly, in the mixed SAMs, will the trans h cis transition of azobenzene result in readily observable differences in the relative protrusions of the functional groups connected to the chain end? Thirdly, under what circumstances will it be the most effective to convert the end groups from “protruding” to “hidden” upon the photoinduced trans to cis isomerization of NdN? And finally, what are the details of the rotational isomeric states? The Model Atomistic inclusion of the substrate not only will increase the size of the system dramatically but also may be ambiguous because the amorphous nature of silica layer formed upon the exposure of the silicon wafer to the atmosphere smears out the exact construction of the surface. The compromise we have taken is to represent explicitly the alkylsiloxanes derived from the chemisorption of alkyltrichlorosilanes on silica but exclude the substrate entirely. The simulation cell consists of a two-dimensional 8 × 8 square lattice with the dimensions of 36.64 × 36.64 Å in the x-y plane, corresponding to the surface density of 21 Å2/chain.37 By applying the periodic boundary conditions in both x and y directions, an infinite monolayer is built up. Figure 2 depicts the initial configuration of the periodic cell from the top of the surface and along the surface normal. A single methyl-terminated chain (36) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (37) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852.

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Xing and Mattice

fully atomistic model. Partial charges were included in the calculations, and the Discover CVFF force field38 was utilized in minimization and dynamics simulations. The cutoff distance for the nonbond interactions is 10 Å, and the time step for the integration of the equation of motion is 1 fs. Temperature was increased to 300 K and the system was equilibrated for 50 ps, then the dynamic trajectories were obtained for 200 ps, from which the characteristics of the molecules are analyzed. Hydrogen atoms were included explicitly in the computations of the trajectories, but they are ignored in the subsequent analysis of the trajectories. Results and Discussions

Figure 2. Initial configuration of the simulation cell viewed from the top of the surface and along the surface normal. The length of the sides of the square box is 36.64 Å.

which contains the azobenzene group ((HO)3Si(CH2)a-pC6H4NdN-p-C6H4(CH2)bCH3 (Az)) is surrounded by 63 hydroxyl-terminated hydrocarbon chains ((HO)3Si(CH2)16OH (Hc)). To avoid the diffusion apart of the molecules, the head atom (Si) of each chain is fixed in the x-y plane during the simulation. In Figure 2 the two Hc chains above and below the Az chain are pictured lighter than the others to facilitate the description of the three kinds of lattices studied, namely lattices a, b, and c. Lattice a is a fully occupied one, with one chain sitting on each site. In lattice b, one of the light Hc chains is removed and the corresponding site is left vacant, to offer more free space for the motion of the Az chain. In lattice c, both of the light Hc chains are removed, and an even larger surface area is available to the Az chain. Figure 3 is the side view of the packing of the chains in lattice a. The molecule containing the azobenzene group is shown in the spacefilled model, while the stick model represents the Hc chains. In composing lattices b and c, one or two Hc’s are taken out without leaving the silicon atoms in the lattice. As already pointed out, the silicon atoms are the bonding agents to the substrate but not the substrate itself. The absence of chemisorption on a proper site leads to the absence of the whole Hc molecule, leaving a “hole” on the lattice. The number of methylene groups in Az is chosen so that its total chain length closely matches that of Hc when both chains are extended. Therefore the trans h cis isomerizations translate the end groups of Az reversibly from “protruding” to “hidden”. By trying different values, it is found that altogether nine methylene groups is the appropriate choice. Keeping the total number of methylene groups constant, we have various combinations of subscripts a and b, which designate the relative position of the azobenzene group within the chain. Different connections with the azobenzene close to the substrate, near the middle of the chain, and near the end of the chain are denoted by head (a ) 2, b ) 7), center (a ) 4, b ) 5), and tail (a ) 7, b ) 2) connections, respectively. Biosym’s InsightII version 2.3.5 and Discover version 2.96 were employed in the computer simulation with a

Single Chain Behavior. We start with the study of a single Az chain in the fully extended state with the azobenzene group in the trans configuration and in a local energy minimum with the azobenzene group in the cis configuration, both in vacuum. The results are listed in Table 1. The molecule with the azobenzene in the trans configuration is the lower energy state. The end-to-end distance of the trans chain is almost three times as large as that in the cis chain in their optimized conformations. The differences in the components of the radius of gyration along the three principal axes also indicate that the trans Az chain is in a much more stretched shape, with the direction of the end-to-end vector nearly coincident with the major principal axis. The cis Az chain is less asymmetric, and the end-to-end vector is less strongly correlated with the major principal axis. Lattice Effects. The Az chain was put into a “forest” of Hc molecules, forming lattices a, b, and c, respectively. The chains tilt along a certain direction in all the different packings. Figure 4 is the time-averaged density profile for the Az chain in the different lattices along the surface normal, including all the non-hydrogen atoms, with the z coordinates of Si’s shifted to zero. The solid curves are for the Az chain in the cis configuration, while the dotted ones are for the trans configuration. Similar to what has been observed before,31 the peaks are relatively sharp near the surface and become more diffuse toward the chain end, which indicates that the atoms close to the free surface undergo more extensive random motions than those near the substrate. The relative protrusion of the chain along the surface normal can be deduced from the region where the density profile falls to zero. In lattice a, which is the most dense packing, the density profiles for both trans and cis tail off at almost the same distance, between 22 and 23 Å. In lattice b, where the Az chain has access to a larger surface area, the trans Az extends about 1 Å further than the cis Az. In the most sparsely packed lattice, namely, lattice c, a readily observable chain length difference of about 3 Å between trans and cis is obtained. Also observed is a change in the roughness of the density profile, arising from the increase in the random motion of the Az chain, due to its less congested environment. From lattice a through b to c, the peaks representing the preferred positions of the carbon atoms become less distinct, and it becomes more and more probable that the carbon atoms move around a broader range of length scale instead of being positioned within a small range. To describe the orientation of a chain, three solid angles are defined,31 namely, the tilt angle, which is the angle between the chain axis and the surface normal, the procession angle, which is the rotational angle of the chain (38) Force field parameters are available as supporting information in ref 31.

Simulations of Azobenzene SAMs

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Figure 3. Simulation cell for lattice a viewed from the side of the box. The Az chain is shown in the space-filled model, with the azobenzene group in the trans (top) and the cis (bottom) configurations, respectively. Table 1. Single Az Chain Behavior

trans cis

principal moments of the radius of gyration (Å)

projection of the principal axis on the end-to-end vector

potential energy (kJ/mol)

end-to-end distance (Å)

(S21)1/2

(S22)1/2

(S23)1/2

1

2

3

171.4 200.8

21.9 7.40

6.56 3.50

1.19 2.27

0.511 0.992

0.999 0.831

0.0361 0.191

0.0411 0.522

axis along the surface normal, and the rotational angle of the chain about its molecular axis. Figure 5 shows the tilt angles of the Az molecule along the chain, starting from the carbon atom bonded to the silicon surface and ending with the carbon atom in the terminal methyl group. The heavy atoms are numbered continuously along the chain, except for the four carbon atoms in each C6H4 unit that are bonded to hydrogen atoms, which are not included. The locations of the C6H4 rings are denoted by the absence of points at positions 6 and 11 in Figure 5. The C-NdN-C unit is atoms 7-10, and atoms 5 and 12 are the aromatic carbon atoms bonded

to the methylene groups. Again cis is plotted with a solid line, while trans is plotted with a dotted line. In lattice c, the tilt angle keeps decreasing along the chain, and the decrease for cis is greater than that for trans. The continuous decrease in tilt angle is not seen in lattice a, nor for the cis chain in lattice b. Detailed analysis shows that although the trans azobenzene unit itself is slightly longer than the cis azobenzene unit, the difference is not sufficient to account for the complete contribution to the total chain length difference between the trans Az and the cis Az in lattice c. If we relate the variance of the tilt angle to the degree of the deviation of the chain from a

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Figure 4. Density profiles of the Az chain along the surface normal in lattice a (top), b (middle), and c (bottom). The solid lines are for the azobenzene groups in the cis configurations, while dotted lines are for the trans configurations.

straight line, another reason for the smaller chain length in cis than in trans can be deduced, which is the bending of the cis Az chain. It is noticeable that in lattices b and c the density have some finite values at z equal to zero, and the tilt angles of the first one or two atoms are greater than 90°, indicating the extension of atoms into the negative z region. The extension goes at most to ∼-1 Å, which does not really imply penetration into the substrate, since there are two Si-O bonds from the x-y plane to the substrate and the surface roughness of the substrate is generally about a few angstroms. Since the motivation for this study is to understand the behavior of the tails, we will not report detailed descriptions of the system close to the substrate. The average values of the chain length and their standard deviations over the simulation time period are summarized in Table 2. The differences are the subtractions of chain length in cis from that in trans, with the uncertainties estimated according to the theory of error propagation. The numbers indicate that the chains in lattice a are too densely packed to let the Az chain adopt its preferred shorter (but thicker) conformation in the cis state. In lattice b, the average distance of the terminal groups to the substrate gives ∼1 Å change from trans to cis, which might not be sufficient to cause any distinction in surface properties. However, a change of ∼3 Å from trans to cis is obtained in lattice c, which is significant compared to the total thickness of the film, specifically about 20 Å, and is larger than the fluctuations in the chain lengths. Since we have not observed the phenomena of interest in the first two lattices, a and b, the remaining studies are conducted on lattice c. Connection Effect. Next, the effect of different positions of the azobenzene group within the Az chain on the chain length are studied. Three kinds of connections

Xing and Mattice

Figure 5. Tilt angle of the Az molecule along the chain in lattice a (top), b (middle), and c (bottom), with cis in solid lines and trans in dotted lines. Error bars are standard deviations. Table 2. Lattice Effects on Az Chain Length (Å), As Judged by the z Coordinate of the Carbon Atom in the Terminal Methyl Group trans cis difference

a

b

c

21.9 ( 0.4 21.5 ( 0.4 0.4 ( 0.6

19.2 ( 0.7 18.3 ( 0.5 0.9 ( 0.9

20.2 ( 0.5 17.3 ( 0.6 2.9 ( 0.8

are used, namely, head, center, and tail connections. The density profiles for the Az chain in the different connections are shown in Figure 6. Again the peaks become less distinct approaching the free surface, indicating the increased motion by the end of the molecules. All of the three connections produce a large difference in chain length between trans and cis. The difference cannot be attributed completely to the difference in length of the azobenzene unit itself. If we exam the tilt angles of the Az molecules plotted in Figure 7, the larger variances in the cis chain than those of the trans chain for all the different connections imply the bending of the chains is stronger in the cis configuration. To gain an insight into the location of the terminus of the Az chain with respect to the termini of the surrounding hydrocarbon chains, the density profiles for the two types of end groups are plotted separately. The solid lines in Figure 8 represent the density profile of the carbon atoms in the methyl groups with azobenzene in the cis form, while the dotted lines are for the oxygen atoms in the terminal hydroxyl groups of the overall chains that do not contain the azobenzene. The methyl groups are buried in the surface formed by the hydroxyl groups in all the three different connections, with the azobenzene unit in the cis configuration. Correspondingly, the density profiles for methyl groups with azobenzene in the trans form are diagrammed in the solid lines in Figure 9, with those of the hydroxyl groups of the matrix depicted in dotted lines. In both head and tail connections, the methyl groups stick out of the surface of OH’s, while in the center

Simulations of Azobenzene SAMs

Figure 6. Density profiles of the Az chain along the surface normal in head (top), center (middle), and tail (bottom) connections, with cis in solid lines and trans in dotted lines.

Figure 7. Tilt angle of the Az chain at different atoms in the head (top), center (middle), and tail (bottom) connections, with cis in solid lines and trans in dotted lines. Error bars are standard deviations.

connection, the two kinds of functional groups are positioned at approximately the same height. The profiles represented by the surface hydroxyl groups are affected very little by the change in conformation of the azobenzene

Langmuir, Vol. 12, No. 12, 1996 3029

Figure 8. Density profiles of the end groups along the surface normal. The solid lines are for the methyl groups bonded to the Az chain ends with azobenzene in the cis configuration, and the dotted lines are for the hydroxyl groups bonded to the hydrocarbon chain ends.

units, as can be seen by comparing the dotted lines in equivalent panels of Figures 8 and 9. The average Az chain lengths with standard deviations for different connections are summarized in Table 3, with the corresponding values for the Hc molecules shown in the parentheses. The differences between trans and cis are around 4 Å in both head and tail connections of the azobenzene unit, and are about 1 Å smaller for the center connection, due to the slight shrinkage of the chain in that particular connection with azobenzene in the trans configuration. The sizes of the Az chain with different connections in lattice c are summarized in Table 4. The three principal moments of the radius-of-gyration show that the Az chain in the trans configuration is always more extended in a particular direction than when in the cis configuration. That particular direction is quite close to the direction of the end-to-end vector, which can be deduced from the projections of the principal axis on the end-to-end vector. The product of the two smaller principal moments of the radius-of-gyration gives a quantitative measure of “cross section”, which is 2-3 times larger in the cis chain than those in the trans chain. The radiation-induced trans h cis transition will reduce the chain extension, (S21)1/2, and increase its cross section, (S22S23)1/2. Compared with the values presented in Table 1, the Az chain is more stretched along the direction of the end-to-end vector in lattice c, for both the trans and the cis configurations, than is the single chain in the vacuum. Although the change in the largest principal moment of the radius-of-gyration is not so significant for the Az chain in the trans form, the cis Az in lattice c is extremely extended with regard to the single chain, due to its interactions with the embracing Hc chains in the lattice.

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Xing and Mattice Table 3. Connection Effects on Az Chain Length (Å), Comparing the z Coordinate of the Carbon Atom in the Terminal Methyl Group with the z Coordinate (in Parentheses) of the Oxygen Atom in the terminal Hydroxyl Group in Hc Chains trans cis

head

center

tail

21.3 ( 0.6 (20.1 ( 0.7) 17.3 ( 1.9 (20.3 ( 0.5)

20.2 ( 0.5 (20.4 ( 0.7) 17.3 ( 0.6 (20.5 ( 0.5)

21.6 ( 0.5 (20.2 ( 0.7) 17.1 ( 1.0 (20.3 ( 0.5)

Table 4. Size of Az Chain in Lattice c head (S21)1/2 (Å) (S22)1/2 (Å) (S23)1/2 (Å) (S22S23)1/2 (Å2)

center

tail

trans

cis

trans

cis

trans

cis

6.81 0.740 0.396 0.293

5.70 1.87 0.414 0.774

6.71 0.958 0.359 0.344

5.60 1.98 0.482 0.954

6.89 0.899 0.360 0.324

5.88 1.54 0.416 0.641

projection of principal axis on end-to-end vector 1 0.998 0.985 0.995 0.986 0.999 0.988 2 0.0550 0.166 0.0920 0.164 0.0221 0.155 3 0.0273 0.0379 0.0306 0.0412 0.0233 0.0267

Figure 9. Density profiles of the end groups along the surface normal. The solid lines are for the methyl groups bonded to the Az chain ends with azobenzene in the trans configuration, and the dotted lines are for the hydroxyl groups bonded to the hydrocarbon chain ends.

Gauche Defects. The gauche fractions of each rotatable bond of the Az chain were calculated. The results show that the total fraction of gauche conformations is higher in the cis Az than that in the trans Az, and more gauche defects are accumulated in the bonds close to the azobenzene group, independent of whether the chromophore is positioned at the head, center, or tail of the chain. Obviously, larger gauche fractions in cis Az result in a shorter end-to-end distance and, hence, less projection along the surface normal, compared to trans. Conclusions The Az chains with azobenzene in trans and in cis configurations are much more stretched in the matrix than in vacuum, due to the interactions with their surrounding Hc chains. In order to modify the surface by altering its composition of different functional groups, sufficient free space must be offered for the relaxation of the Az molecule with azobenzene in the cis configuration. When the surface area of the azobenzene-containing chain is three times that of the hydrocarbon chain, a length difference of ∼3 Å of trans from cis is obtained. Smaller surface areas for the azobenzene-containing chain do not permit the relaxation required for generation of a sig-

nificant difference in extension when the isomerization of the azobenzene occurs. This conclusion, based on simulation of SAMs, is related to the effects of azobenzene density reported in a recent experimental study of LB films.35 Various positions of the azobenzene unit connected within the chain also influence its behavior during photoisomerization. Specifically, the head and the tail connections bring about larger changes in chain length of trans from cis than that of center connection. In the former two connections, the chains in the trans form stick out of the surface, while those in cis are buried in the monolayer. Thus the modification of surface functionality at the molecular level is achieved. Details of the chain orientations are accessed through the introduction of the tilt angle, from which the reason for the difference in chain length between trans and cis, which is the bending of cis, can be deduced. If such a mixed SAM is to be synthesized, we suggest fabrication from the chain with azobenzene group in cis configuration instead of in trans. The reason is, a tightly packed SAM will probably be obtained if started from the trans form, thus the surface is not photoresponsive because the chains are too congested to relax. However, if we start from the cis chain, a looser packing near the Az chain will be formed, and cis h trans isomerizations are expected to change the protrusions of the end groups and, as a consequence, the properties of the exposing surface. Acknowledgment. This work was supported by Army Research Office. LA9509155