J. Phys. Chem. B 2005, 109, 6233-6238
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Chirality in Supramolecular Self-Assembled Monolayers of Achiral Molecules on Graphite: Formation of Enantiomorphous Domains from Arachidic Anhydride Feng Tao and Steven L. Bernasek* Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544 ReceiVed: October 18, 2004; In Final Form: December 22, 2004
The molecular arrangement and chirality of the self-assembled arachidic anhydride monolayer on graphite were investigated using scanning tunneling microscopy (STM). This molecule has two identical alkyl chains, linked by an anhydride group in the middle. In its extended form, one alkyl chain is shifted, with respect to the other, along the molecular backbone. Upon adsorption on graphite, this achiral anhydride spontaneously forms two types of homogeneous domains (denoted as m and m′) with mirror symmetry. The angle from the molecular chain to the row-packing direction is 98.0° ( 0.5° and 82.0° ( 0.5° for domains m and m′, respectively. Domain m is the mirror image of m′. The molecular arrangement of this self-assembled monolayer shows that domains m and m′ are two-dimensional enantiomers with opposite chiralities. This new molecular packing motif is confirmed by line-profile analyses along the molecule-chain and the row-packing directions. This finding demonstrates the spontaneous formation of highly ordered homogeneous enantiomorphous domains on graphite resulting only from weak van der Waals forces between the achiral arachidic anhydride molecules.
1. Introduction Chirality is a property of natural materials, including minerals, organic molecules, and biological structures. This chirality largely impacts the behavior of these materials. It also has a very important role in synthesized materials. For instance, stereospecificity is particularly important in the synthesis and function of enantiopure drugs.1 In fact, understanding the chiral chemistry of two-dimensional (2-D) and three-dimensional (3D) organic materials has attracted an enormous amount of attention,2-8 because of technological needs in enantioselective catalysis, enantiomeric selectivity in optical activity, chiral recognition in biomolecular systems, and the development of molecular devices and sensors based on the construction of patterned surfaces. The invention of scanning tunneling microscopy (STM) has significantly accelerated the study of chirality in two dimensions. With this unique technique, chirality can be addressed successfully at the molecular level, and even at the atomic level, for organic monolayers on solid surfaces.2-8 The determination of the absolute chirality of individual adsorbed alkenes chemisorbed on Si(100) has been demonstrated,9 as has the direct determination of the chiral center of hydrocarbons adsorbed on graphite.4 Recent STM investigations have shown that both chiral and achiral molecules can self-assemble into chiral domains. For enantiopure molecules, including tartaric acid,2 cysteine,3 smectic mesogens 4′′-[(1-methylheptyl)oxy]-3′′-nitro4′-biphenylyl-4-(5-trans-decenyloxy)benzoate,10 2,5-bis[10-(2methylbutoxy)decyloxy]-terephthalic acid,11 one derivative of formamide,12 2-eicosyloxy-5-(1-methylheptyloxy)-terephthalic acid,13 and 5-[10-(2-methylbutoxy)-decyloxy]isophthalic acid,14 their 3-D chiralities seem to be transferred into two dimensions to form chiral monolayers in an enantiospecific structure. Usually, this self-assembly of one type of enantiomer (R or S) only produces one specific arrangement. The two formed 2-D * Author to whom correspondence should be addressed. E-mail:
[email protected].
structures from pure R and pure S enantiomers, respectively, are enantiomorphous. Rarely, a single enantiomer might form quasi-enantiomorphous structures.15 These are 2-D structures with mirror symmetry in the same domain. However, such structures are not really enantiomorphous on a molecular scale, because all the molecules come from one type of enantiomer. The molecular arrangement and formed unit cell might seem to be enantiomorphous. For the adsorption of racemate,2,3,12 the R and S enantiomers may spontaneously form two specific structures with opposite chiralities upon adsorption. The two types of structures are enantiomorphous. This process is also called spontaneous phase separation. However, for achiral molecules, their possible chiral behaviors are different from molecules with chirality in three dimensions. Because of structural considerations, some substrates may induce chirality for these achiral molecules upon adsorption, because of the reduction of symmetry at the surface.7 In another case, a mismatch between an achiral adsorbate and an achiral substrate may result in distortion upon adsorption, which leads to chirality in the adsorbed monolayer.8 Beyond the induction of chirality for individual molecules, the self-assembly of these achiral molecules on graphite may produce both left-hand and righthand monolayer structures. These structures are related to each other by a reflection in a mirror plane and cannot be superimposed onto each other by any 2-D rotational or translational symmetry operation, thereby producing opposite 2-D chiralities. With respect to the mirror symmetry of the two types of domains formed in this case, they are similar to the formation of enantiomorphous structures from a racemic mixture. The achiral molecules in three dimensions, which may be induced to present chiralities upon adsorption, mainly include carboxylic acids, anhydrides, esters, and amides, because of the asymmetry of the carboxyl group in these molecules. Recent investigations16-18 of the assembly of myristic, palmitic, stearic, arachidic, behenic, heptadecanoic, and nonadecanoic acids demonstrated an odd-even effect in their molecular arrangements and corresponding chiral structures. Fatty acid molecules
10.1021/jp0452397 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/24/2005
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Figure 1. Possible molecular arrangement of a carboxylic acid molecule with an even number of C atoms (such as CH3(CH2)10COOH) and a carboxylic acid molecule with an odd number of C atoms (such as CH3(CH2)9COOH). Panels a, b, and c show the even-numbered C-atom series, whereas panels d, e, and f represent the odd-numbered C-atom series. The red dashed line is the mirror plane between two two-dimensional (2-D) enantiopure images.
with an even number of C atoms spontaneously separate into two types of domains with opposite chiralities upon selfassembly on graphite. In contrast, acids with an odd number of C atoms form structures of racemic mixtures. This type of oddeven effect has not been thoroughly clarified. The mechanistic understanding of the molecular arrangement and corresponding chiral chemistry for the 2-D domains formed from other achiral molecules adsorbed on graphite is still in its starting stage. The objectives of this study are (i) to further examine the 2-D chirality of adsorbed achiral molecules and (ii) to investigate the stereochemical effects of molecular structure and functionality on chiral chemistry in the self-assembled monolayers of these molecules on graphite. 2. Experimental Section All of the studies described here were performed with a laboratory-built STM system that was operating at the liquid/ solid interface under ambient conditions. The single-tube scanner and tip were horizontally mounted. The tip was fabricated by mechanically cutting 0.25 mm platinum/iridium wire (Pt/Ir ) 90/10) from Goodfellow. Highly oriented pyrolytic graphite (HOPG) of ZYA grade was obtained from Advanced Ceramics Corporation. Arachidic anhydride was purchased from Aldrich Chemical Company with a purity of 98% and used without further purification. Arachidic anhydride was dissolved into 1-phenyloctane (99%, Aldrich) to produce an almost-saturated solution. The graphite was freshly cleaved before imaging. Samples were prepared by gently depositing one drop of this prepared solution onto graphite. Samples were positively biased. All images were obtained in constant current mode and calibrated with the bare HOPG hexagonal lattice. Experiments were repeated with different tips and samples, to check for the reproducibility and to ensure that the images were not influenced by tip and sample artifacts. 3. Results Saturated fatty acid molecules typically self-assemble on graphite in two classes of distinct structures: enantiomorphous
domains and structures of a racemic mixture. To further understand these observed chiral structures for achiral molecules on graphite, Figure 1 shows these two possibilities for both evennumbered and odd-numbered carbon-chain fatty acid molecules. CH3(CH2)10COOH and CH3(CH2)9COOH are used as the examples for the two types of acids. For the possible structure of a racemic mixture of the even-numbered carbon chain acid shown in Figure 1c, the H atoms of half of the COOH groups are much closer to the H atoms of their adjacent CH3 groups (labeled with green ellipses) than those in the chiral domains, marked with green circles in Figure 1a and 1b. This steric repulsion should make this racemic structure unstable. On the other hand, the van der Waals interaction between adjacent alkyl chains is maximized in the enantiomorphous structures. This interaction is weakened in the racemic structures, because of the reduced overlap of the adjacent alkyl chains along the molecular long-axis direction. Thus, structural analysis suggests that thermodynamic factors determine the preferred formation of the two types of 2-D enantiomer domains with opposite chiralities (see Figure 1a and 1b). For the odd-numbered acid, a similar difference exists between enantiomorphous domains (Figure 1d and 1e) and the racemic structure (Figure 1f). In this case, these two chiral domains have larger steric repulsion and weaker van der Waals interaction than the racemic mixture. This suggests the racemic structure is thermodynamically preferred over chiral domains for odd-numbered acids. The more energetically favorable 2-D molecular arrangement and chiral chemistry is consistent with the experimental observations16-19 that only domains of the racemic structure were observed for odd-numbered acids and only the enantiomorphous structure was observed for even-numbered acids. The analysis of STM images and the structural model based on fatty acids demonstrate that subtle structural variations of individual molecules can result in significant differences in the chiral structure formed upon adsorption. The complexity of molecular packing patterns and the chiral chemistry of achiral molecules on the surface is clearly shown. The recent investigation of monolayers of stearic acid palmityl ester shows that both 2-D enantiopure domains and domains of 2-D racemic mixtures can be spontaneously formed,20 further suggesting the diversity
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Figure 2. Two 2-D enantiomers of arachidic anhydride with opposite chiralities lying on the substrate with molecular carbon skeletons parallel to surface. The gray plane represents the substrate under the molecules, and the green dashed line shows the mirror plane between the two enantiomers. The conformations of the two 2-D enantiomers are obtained by optimization with density functional theory (DFT).
of the expression of chirality in self-assembled monolayers of achiral molecules. Therefore, further exploration of molecular arrangements and the possible chirality of self-assembled monolayers formed from achiral molecules is necessary. Anhydride molecules are a quite different category of achiral molecules. In this case, there are two interior carboxyl groups linking two alkyl chains displaced from each other along the molecular backbone. Almost all of the reported chiral domains formed from chiral or achiral molecules involve hydrogen bonding in the formation of the ordered structures. For anhydride molecules, no hydrogen bonding can contribute to the formation of the self-assembled monolayer. Thus, the anhydride molecule is an ideal achiral molecule to explore the molecular packing structure and the possible chirality of self-assembled monolayers formed strictly because of van der Waals interactions. While some categories of long-chain hydrocarbons are achiral in three dimensions, adsorption with their C-atom skeleton parallel to the substrate results in a loss of mirror-plane symmetry, producing two nonsuperimposable forms of the molecules on the surface. At the level of individual molecules (as in Figure 2), achiral arachidic anhydride molecules display 2-D chirality and should lead to equal numbers of 2-D enantiomers upon adsorption on the surface. Because of the combination of van der Waals forces between two adjacent molecules and between molecules and the substrate lattice underneath, these molecules may self-assemble into two types of ordered structures: domains of a racemic mixture or two types of enantiopure domains with opposite chiralities. Figure 3a is a typical STM image of a self-assembled monolayer composed of arachidic anhydride physisorbed on graphite. For long-chain alkanes and alcohols, the monolayer normally consists of molecules lying flat on the graphite in a close-packed arrangement and packing side by side to form lamellae. Two adjacent lamellae are separated by a narrow trough comprised of alternative dark and bright spots in the images. However, the arachidic anhydride monolayer is relatively homogeneous without obviously separated lamellae, suggesting that these molecules arrange with an interdigitating pattern. The image in Figure 3a seems to be somewhat similar to the self-assembled fatty acid monolayer. However, there are distinct differences. Figure 3a shows that the image is composed of two parts: one is the ordered bright bands packing parallel to each other, and the other is the dark-hole rows (marked with black arrows) with discontinuous dark holes lying between the two neighboring bright spots. Importantly, the angle between the dark-hole row and the molecule-chain directions is 82° ( 0.5°, which is different from the 90° angle that is observed for fatty acid structures. STM images show that the distance along the chain direction between two adjacent dark-hole rows is 25.5 ( 0.2 Å; On the basis of the molecular structure and dimension of
Figure 3. Two types of domains (m and m′) formed by self-assembly of achiral arachidic anhydride: (a) domain m, scan area is 168 × 168 Å; (b) domain m′, scan area is 135 × 135 Å. Dark arrows show the black-hole rows, and turquoise arrows are the direction of molecular chains. The pink kink demonstrates one molecule packs on two adjacent lamellae. The angles from the molecule-chain direction to the blackhole row, counterclockwise, are 82° ( 0.5° in panel a and 98° ( 0.5° in panel b.
arachidic anhydride (Figure 2), the superimposed red bar in the image (Figure 3a) represents one molecule lying flat on the surface, adopting an all-trans conformation. The distance between two dark-hole rows along the chain direction is 51.3 ( 0.3 Å, which is consistent with the calculated molecular length for the stretched-out molecules along the chain direction. Therefore, every molecule seems to adsorb in two adjacent lamellae, in an interdigitated structure.
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Figure 4. (a) Proposed molecular arrangements for the observed images in Figures 3a and 5a. The image contains three lamellae. The red right triangle is used to calculate the angle from the molecule-chain direction to the black-hole row direction counterclockwise. The ordered lamellae are labeled A, B, C, D, ... Each lamella has the width of half the entire molecular length. Because of the interdigitated arrangement of chains, each molecule bridges between two adjacent lamellae. Thus, each molecule is labeled with AB or BC, etc., depending on which two adjacent lamellae it occupies. The superscripts l and r refer to the left and right alkyl chains in a molecule. The subscripts n, n+1, n+2, ... label the packing order of the molecules in a lamella. In this way, each chain of every molecule has a specific label, simplifying the description of the pattern arrangement. (b) A calculation from this right triangle gives an angle of 81.5° from the molecule-chain direction to the black-hole row counterclockwise.
TABLE 1: Parameters Measured from Experimental Images and Calculated from the Arrangement Structures of Two Enantiopure Images
parameter
Experimental Results
Structural Models
domain m
domain domain m m′
domain m′
distance between two 51.3 ( 0.3 Å 51.5 ( 0.3 Å 51.66 Å 51.66 Å interval black holes in the molecule-chain direction distance between two 8.7 ( 0.1 Å 8.7 ( 0.1 Å 8.52 Å 8.52 Å adjacent bright spots in black-hole direction angle between the 82° ( 0.5° 98° ( 0.5° 81.5° 98.5° molecule-chain and black-hole directions
4. Discussion Based on close observation of the image features in Figure 3 and the structural parameters in Table 1, a model for this structure, as shown in Figure 4a, is proposed. In this model, every lamella (labeled as A, B, C, D, E, ...) is composed of left (l) or right (r) alkyl chains of different molecules, and each molecule contributes to two adjacent lamellae. Every alkyl chain l or r is labeled using (AB) [or (BC), (CD), (DE), ...] n, n+1, n+2, n+3,.... In this nomenclature, AB (or BC, CD, DE, ...) shows those molecules bridging two adjacent lamellae A and B (or B and C, C and D, D and E, ...). The subscripts n, n+1, n+2, n+3, ... represent the order of the alkyl chains in a lamella. For r l instance, lamella B is comprised of (BC) ln, (AB) n+1 , (BC) n+2 , r l r r l (AB) n+3, (BC) n+4, (AB) n+5, and so forth. (BC) n, (CD) n+1 , r l r l , (CD) n+3 (BC) n+2 , (BC) n+4, (CD) n+5, and so forth comprise lamella C. In a lamella, every chain such as (BC) ln and r l of lamella B shifts from its adjacent (BC) n+2 and (AB) n+1 r (AB) n+3 chain by half units. This shift, which is labeled with a violet dashed arrow in Figure 4, exactly forms an angle of 81.5° from the molecule-chain to dark-hole row directions counterclockwise, which is consistent with the measured angles from the observed images (see Table 1). This angle was obtained from the calculation described in Figure 4b. In addition, at the
dark-hole row, the distance between two adjacent anhydride groups is in good agreement with the measured average distance in the STM images (see Table 1). The structure in Figure 4a is also presented with a ball-and-cylinder model in Figure 5c. The dark-hole rows in Figures 4a and 5c are comprised of alternating anhydride groups and uncovered regions of graphite. Obviously, the length of the uncovered portion (marked with a black box) at the dark-hole row is larger than that of the anhydride group (marked with a green box). Therefore, the rectangular dark region and the narrower bright spot between two neighboring dark areas correspond to the uncovered graphite surface and the anhydride group, respectively. Generally, both topographic variation and the electronic properties of the organic monolayer will contribute to the contrast of the STM image. A simple approach to roughly estimate the electronic tip-sample coupling using the ionization potential (IP) was proposed:20 a functionality with lower IP than that of the alkyl chain will appear brighter if the coupling is dominated by the highest occupied molecular orbital (HOMO), whereas a higher-IP functional group should appear darker. However, this does not hold for some functional groups, and especially when the topography dominates. Other theoretical models have explained the limitation in predicting the image contrast.21 Based on perturbation theory, a theoretical calculation rationalized that highly diffuse orbitals of the adsorbate, despite being much farther in energy from the Fermi level of the graphite than the occupied state, also possibly have an important role in determining contrast.21 Although the details of electron tunneling in a system such as this are not understood well enough to determine precisely how much of the contrast is contributed from the molecule, the higher position of the interior anhydride group may make its image brighter than the uncovered substrate. This assignment is confirmed by the following line-profile studies. Except for the arrangement of the single-molecule image, Figure 3b has all the same features as Figure 3a. In Figure 3a, molecular images align on graphite from top-left to bottomright. However, they are arranged from top-right to bottom-left in Figure 3b, forming an angle of 98° ( 0.5° from the molecule-
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Figure 5. Panels a and b represent two domains with opposite chiralities, as in Figure 3a and 3b, but with smaller scan areas (scan area for panel a is 72 Å × 72 Å, and scan area for panel b is 93 Å × 93 Å) and higher resolution. The turquoise dashed line between the images in panels a and b is the mirror plane between the two enantiopure domains. Panels c and d are the corresponding molecular arrangements of the images in panels a and b, respectively. The larger molecular structures at the bottom of the figure are the two 2-D enantiomers with opposite chiralities used for packing in the images in panels a and b, respectively.
chain direction to the dark-hole row counterclockwise. This angle is the supplementary angle of 82° ( 0.5° observed in Figure 3a. This strongly suggests that Figure 3b is the mirror image of Figure 3a. A similar packing pattern (Figure 5d) can be proposed for the domain of Figure 3b. In this pattern, the angle from the molecule-chain direction to the dark-hole row counterclockwise is calculated to be 98.5°, based on the arrangement of the molecules in the model structure. Panels a and b in Figure 5 are high-resolution images corresponding to the two images in Figure 3a and 3b, respectively. Models of their molecular arrangements are displayed in Figure 5c and 5d. In Figure 5c, all the molecules have the same chirality, although each molecule has its orientation, with respect to its adjacent molecule, obtained by rotating 180° around the normal to the graphite surface. Similarly, each molecule in Figure 5d has the same chirality. Notably, the chiralities in the two domains are opposite. Therefore, they are enantiomorphous. This enantiomorphous relationship demonstrates that achiral arachidic anhydride forms two types of 2-D enantiomer domains through an interdigitating molecular packing arrangement. To some extent, they are two types of 2-D crystals with opposite chiralities. Their unit cells are comprised of two adjacent molecules (highlighted with pink boxes in Figure 5c and 5d). This proposed molecular arrangement is further supported by line profiles along the dark-hole row and molecule-chain directions. Figure 6a is the line profile along the dark-hole row (red dashed line in Figure 5a). The eight peaks correspond to
Figure 6. (a) Line profile along the black-hole row (the red dashed arrow in Figure 5a) and (b) line profile along the molecule-chain direction (the red-black dashed arrow in Figure 3b). The arrow in each panel shows the width of the valley at half the peak height. The turquoise and red bars correspond to the regions covered with one alkyl chain, marked with the black dashed line in Figure 3b, and the uncovered areas between the two adjacent chains, respectively. The lengths of these bars show the lengths of their corresponding sections of the image.
6238 J. Phys. Chem. B, Vol. 109, No. 13, 2005 eight bright spots on the red dashed line in Figure 5a. The average distances between two adjacent bright spots is 8.7 ( 0.1 Å, consistent with the average separation of two neighboring anhydride groups in the dark-hole row direction in the model, showing that the bright spots in the dark-hole row direction are the image features of the anhydride groups. In addition, the width of the valley at half-height of the peak is 4-6 Å, consistent with the calculated length of the rectangular box of uncovered graphite in Figure 4a. Therefore, the wide trough between two adjacent narrow peaks in Figure 6a corresponding to the dark hole originates from the uncovered rectangular region of the graphite substrate. Figure 6b is a line profile along the molecule-chain direction (the red-turquoise dashed line in Figure 3b). There are five steep valleys attributable to the uncovered areas between anhydride group and methyl groups. The distance between two neighboring valleys in the line profile is 25.4 ( 0.2 Å, which is in agreement with the width of a lamella, one-half the molecular length. Moreover, the width of a valley at the half depth of the valley (marked with pink arrows) in Figure 6b is ∼1/2-1/3 that in Figure 6a, in accordance with the dimension of one uncovered rectangular area in the proposed model (Figure 4a). In addition, along the molecule-chain direction (indicated by the dashed arrow in Figure 3b), there are two classes of different features, labeled with turquoise and red bars. Close observation finds that the turquoise and red bars correspond to regions covered with one chain and the uncovered area between two adjacent chains, respectively. This assignment is confirmed by the line-profile analysis. The line profile (Figure 6b) clearly shows that covered chains labeled with turquoise bars are higher than uncovered area marked with red bars. Therefore, these analyses of line profiles strongly support the molecular packing structures in the self-assembled monolayer of arachidic anhydride shown in Figures 4a, 5c, and 5d. 5. Summary The self-assembly of arachidic anhydride on highly oriented pyrolytic graphite (HOPG) has been investigated using scanning tunneling microscopy (STM). The formation of two-dimensional (2-D) enantiomorphous domains from achiral molecules without hydrogen-bonding interactions is demonstrated. The achiral arachidic anhydride carrying two 2-D stereogenic centers spontaneously forms homogeneous 2-D enantiomorphous crys-
Tao and Bernasek tals via an interdigitating molecular arrangement. This novel chiral structure is governed only by van der Waals interactions in the monolayer, in contrast to previously observed chiral structures driven by hydrogen bonding in the overlayer. Acknowledgment. This work was partially supported by the Chemistry Division of the National Science Foundation. References and Notes (1) Stefan, R. I.; van Staden, J. F.; Aboul-Enein, H. Y. Electroanalysis 1999, 11, 1233. (2) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (3) Kuhnle, A.; Lindertoh, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891. (4) Walba, D. M.; Stevens, F.; Clark, N. A.; Parks, D. C. Acc. Chem. Res. 1996, 29, 591. (5) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491. (6) Feyter, S. D.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520. (7) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (8) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2004, 126, 14233. (9) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Wolkow, R. A. Nature 1998, 392, 909. (10) Steven, F.; Dyer, D. J.; Walba, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 900. (11) De Feyter, S.; Gesquiere, A.; Grim, P. C. M.; De Schryver, F. C.; Valiyaveettil, S.; Meiners, C.; Sieffert, M.; Mullen, K. Langmuir 1999, 15, 2817. (12) De Feyter, S.; Gesquiere, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217. (13) De Feyter, S.; Gesquiere, A.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Mullen, K. Langmuir 2000, 16, 9887. (14) De Feyter, S.; Grim, P. C. M.; Rucker, M.; Vanoppen, P.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. Engl. 1998, 37, 1223. (15) Parks, D. C.; Clark, N. A.; Walba, D. M.; Beale, P. D. Phys. ReV. Lett. 1993, 70, 607. (16) Hibino, M.; Sumi, A.; Tsuchiya, H.; Hatta, I. J. Phys. Chem. B 1998, 102, 4544. (17) Fang, H.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 7311. (18) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 2000, 104, 7627. (19) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021. (20) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewia, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978. (21) Faglioni, F.; Claypool, C. L.; Lewis, N. S.; Goddard, W. A., III. J. Phys. Chem. B 1997, 101, 5996.
