Monolayers of Diphenyldiacetylene Derivatives: Tuning Molecular Tilt

J. Phys. Chem. B 2005, 109, 19161-19168

19161

Monolayers of Diphenyldiacetylene Derivatives: Tuning Molecular Tilt Angles and Photopolymerization Efficiency via Electrodeposited Ag Interlayer on Au Yang-Hsiang Chan and Jiann-T’suen Lin* Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 115

I-Wen Peter Chen and Chun-hsien Chen* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu, Taiwan 30013 ReceiVed: June 2, 2005; In Final Form: August 5, 2005

An electrodeposited Ag adlayer (upd, underpotential deposition) is utilized to improve monolayer photopolymerization of diphenyldiacetylene derivatives (DPDAs) that would otherwise exhibit no polymerization in solid state. Topochemical reaction of diacetylene derivatives via solid-state 1,4-addition yields polydiacetylenes which are of great importance due to properties associated with their ene-yne conjugated backbones. The polymerization efficiency heavily depends on the molecular arrangement in the crystals. For example, crystals of most DPDA derivatives show no activity for topochemical reaction because the bulky phenyl end groups space out the triple bonds and thus DPDAs require relatively large translation and rotation angles for polymerization. In principle, topochemical reaction is viable if molecules are in optimal arrangement. The upd interlayer can be applied to tune the adsorbate-substrate interactions, intermolecular spacing, and the molecular tilt angle by controlling the coverage of the Ag adlayer. It is thus possible to manipulate the molecular arrangement of DPDAs for the subsequent polymerization. Successful photopolymerization of DPDA monolayers is realized from the decrease in νCtC intensity by infrared reflectance absorbance spectroscopy, growth of ene-yne π-π* transition by UV-vis measurements, and enhanced electrochemical stability by the cathodic desorption protocol. The optimal efficiency of polymerization takes place on upd-modified substrates that can generate ∼45° tilt angle for DPDA derivatives.

Introduction Since the first report of diacetylene polymerization in solid state by Wegner,1 polydiacetylene (PDA) derivatives have been extensively examined in polycrystalline and single crystalline forms,2,3 liquid crystals,4-6 gel state,7-9 solution phase,10 and monolayers.11-28 The enormous interest in PDA derivatives arises from the highly conjugated ene-yne backbone and tailored side chains (Scheme 1) that lead to intriguing optical and electronic properties,29 promising in many potential applications such as nonlinear optical materials,30-33 ultrathin photoresists,19 semiconductors and photoconductors,34,35 biomimetic liposome for drug delivery36,37 or vesicles for biosensing,38 and chemical sensors.39-42 Among the applications, monoor multilayers of PDA on solid support have been applied in molecular assemblies,11-28 immobilization of biomolecules,40,41 and sensing.41,42 Langmuir-Blodgett (LB) techniques and selfassembled monolayers (SAMs)43,44 are common approaches to prepare densely packed diacetylene thin films for the subsequent polymerization.11-28 The general guideline for thin-film polymerization is adapted from solid state chemistry. The neighboring diacetylenes are polymerized via a 1,4-addition mechanism45,46 where, as illustrated in Scheme 1, the stacking interval (d), the tilt angle (θ) between axes of the DA moiety and stacking direction, and the distance between C1 and C4 carbons of adjacent acetylenes are 4.7-5.2 Å, 40°-60°, and not longer * To whom correspondence should be addressed. J.-T.L.: phone, +886 2 2789 8522; fax, +886 2 2783 1237; e-mail, [email protected]. C.-h.C.: phone, +886 3 573 7009; fax, +886 3 571 1082; e-mail, [email protected].

SCHEME 1: Schematic Diagram for Topochemical Polymerization of Diacetylenes

than 4 Å, respectively.47,48 For example, upon depositing LB films of 10,12-pentacosadiynoic acid onto substrate, the nominal film pressure is regulated at ∼20-25 mN/m to achieve the spatial constrains for polymerization.18,49-52 By taking advantage of the weak molecule-substrate interactions of LB films, neighboring diacetylenes can reposition themselves to maximize the polymerization efficiency.23,53 However, overexposure to UV radiation reduces the coplanar conjugated length due to the structural reorientation of the alkyl side chain groups.54,55 This problem is less severe for polymerizing SAMs of diacetylene derivatives. Such structural integrity of PDA SAMs has been ascribed to the strong sulfur-gold chemisorption which limits the molecular mobility, preserves the polymeric conformation, and thus retains the conjugation length.20,56 Therefore, further derivatization of the polymerized SAMs can be meaningful. IR and Raman measurements by Batchelder et al. indicated that

10.1021/jp0529366 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/22/2005

19162 J. Phys. Chem. B, Vol. 109, No. 41, 2005 CHART 1

Chan et al. the suppression of oxide formation.61-66 The modified surface promotes the headgroup-substrate anchoring61-64,67 and increases the thermal61,66,68 and electrochemical65,69,70 stability of SAMs. The molecular tilt angle and packing density of n-alkanethiol SAMs can be fine-tuned by adjusting the coverage of the upd adlayer,59 which will be shown here opening a novel avenue to activate polymerization of DPDA SAMs. Herein, we synthesize a series of DPDAs displayed in Chart 1, potential candidates for third-order nonlinear optics33,71 and for high spin polymers.72,73 The Au/Ag(upd) system is prepared as the substrate to explore how the upd coverage affects the stacking interval (d), intermolecular angle (θ), and polymerization efficiency of the DPDA SAMs. Results and Discussion

the polymerized methyl-terminated diacetylenes exhibited a high degree of local order and were surrounded by amorphously packed domains.56 The films were stable against washing with chloroform. Similar results were observed on hydroxyl- and carboxylate-terminated diacetylene thiol SAMs by groups from Crooks19-22 and Evans23-26,29 who confirmed the feasibility of polymerizing DA-containing SAMs and demonstrated the film stability under conditions of organic solvents, electrochemical environments, and elevated temperature.22 To cope with the aforementioned spatial requirement for polymerizing monomeric SAMs, the molecules are derivatized with polymethylene spacers between the DA moiety and the anchoring headgroup. The spacer offers freedom to adjust their relative positions and to improve polymerization efficiency. The lattice strain generated by the ene-yne hybridization can be released by the longer spacer more easily than by the shorter ones.24 Apparently, it is a challenge to assemble PDA SAMs with the spacer as short and rigid as one aryl ring (Chart 1), and thus PDA SAMs of such fully conjugated molecules have never been reported. For topochemical reaction, formation of PDA exhibits minimum movement of diacetylene moiety and thus the packing arrangement of monomers in the crystalline lattice plays a decisive role. For example, most diphenyldiacetylene derivatives (DPDAs) are found not polymerizable in solid state.47,48,57 This is attributed to the attraction between neighboring phenyl rings and their bulky size58 whose steric effect on monomer stacking requires relatively large translation and ring rotation angles to proceed topochemical 1,4-addition.33,47 With dinitro, diamino, or trifloromethyl substituents at the meta or ortho position, the lattice stacks differently from that of DPDA. Some crystalline phases of the DPDA derivatives become reactive.33,57 For DPDA monolayers assembled on substrate, the intermolecular π-π stacking is not the only factor to decide the packing arrangement of monomers. The sulfur-substrate chemisorption, if adjustable, might be used to tune the monomer packing structure, similar to that of the sidearms derivatized on aryl rings. We recently demonstrated that the packing structure and the molecular tilt angle of n-alkanethiol SAMs can be influenced by a pre-electrodeposited foreign metal on the substrate,59 namely, underpotential deposition (upd). Upd is an electrochemical process that a submonolayer or full monolayer of foreign metal atoms can be electrochemically deposited at a potential positive of the reversible Nernst potential of its bulk form because of stronger adatom-substrate interactions than those between adatoms.60 Studies of monolayers on updmodified gold substrates demonstrated a better SAM reproducibility61 than those on the corresponding bulk materials due to

Synthesis of DPDA Derivatives for Monolayer Assembly. In nondeareated solutions, the proneness of oxidative S-S coupling of thiols turns dithiols into insoluble poly(disulfides) facilely. Aromatic or conjugated R,ω-dithiols are particularly unstable. For example, black precipitates develop rapidly in the solution of R,ω-dithiols analogous to I and III. Therefore, protection of the sulfur headgroup for I-IV is necessary because of the stability concerns in preparing of SAM deposition solutions and the subsequent storage. Ideal protecting groups should be stable under harsh synthetic conditions and can be easily removed prior to monolayer assembly. Typical protecting groups for mercaptoaryls, such as methyl (I and II) or ethyl groups, are hard to be deprotected. Acetyl groups are labile in either acidic or basic solutions and thus are impractical if further synthetic procedures are required, such as cross-coupling for preparing asymmetric monothiols (e.g., II and IV). A 2-(trialkylsily)ethyl protecting group is stable and can be readily deprotected by introducing TBAF (tetrabutylammonium fluoride) in THF.74 However, upon exposure to ambient atmosphere, the solution of deprotected III precipitates promptly. To circumvent these problems, SAMs of I-IV are prepared by potential-assisted assembly75-78 in which the substrate is potentiostated at a constant potential (vide infra). The general synthetic route for I-IV is shown in Scheme 2 where common conditions for Glaser-type or Sonogashira coupling are utilized to produce diacetylenes. Homo coupling of 1 results in III which is then deprotected by TBAF and reacted with CH3I to afford methyl terminated I. Asymmetric II and IV are prepared with a copper catalyst (CuI) that crosscouples iodoacetylene (2) with terminal phenyl acetylenes. The reaction produces a mixture of DPDAs among which the desired product can be easily separated by column chromatography. Electrochemically Assisted Assembly of DPDA SAMs. Due to the aforementioned concerns associated with protection of the thiol headgroup, I-IV SAMs are prepared by potentialassisted assembly.75-78 During monolayer assembly, the substrate is potentiostated at a constant potential of -300 mV (vs EAg+/0). Compared to IR spectra measured from the conventional immersion-and-waiting procedures (see Supporting Information), this potential-assisted method significantly increases the peak intensity of the characteristic vibrational modes of I-IV monolayers (upper row of Figure 1), in a good agreement with literature reports that such an electrochemical method facilitates assembly of dialkyl or aryl sulfide compounds.76-78 It should be noted that the substrate is a gold film premodified by an Ag upd adlayer, Au/Ag(upd). When bare gold is used as the substrate for the potential-assisted assembly, the increase in monolayer peak intensity is limited. This discrepancy suggests that Au/Ag(upd) promotes anchoring of the protected sulfur

Diphenyldiacetylene Derivative SAMs on Au/Ag(upd)

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Figure 1. IR spectra of (A) I, (B) II, (C) III, and (D) IV obtained in KBr pellets (bottom row) and from monolayers (upper row, on Au/Ag(upd, 46 mV)). The vertical scale of absorbance is identical for the four monolayer spectra.

SCHEME 2

headgroup. In the following, 250, 100, or 46 mV (vs EAg+/0) shown in the parentheses of Au/Ag(upd) specify the upd potential used for substrate modification. The coverage of Ag adatoms ranges from ∼33% at Au/Ag(upd, 250 mV) to a full monolayer at Au/Ag(upd, 46 mV) where the nearest neighbor spacings span from 0.43 to 0.29 nm, respectively.60,79 Effect of Ag Coverage on SAM Structure. A simple model of DPDA derivatives tilted from the surface normal (θ) is sketched in Figure 2. For the phenyl rings, the in-plane vibrational modes a1 (∼1589, 1483 and 1013 cm-1) and b2 (∼1095 cm-1) are, respectively, parallel and perpendicular to the C2V axis of p-benzene. The out-of-plane mode, a2, is perpendicular to both a1 and b2. χ is the rotation angle of the molecule around the a1 axis. The molecular tilt angle, θ, can be derived from the intensity of these transition dipoles. Typical IR spectra of I-IV prepared in KBr pellets and SAMs are displayed in the bottom and upper rows of Figure 1, respectively. The molecules in KBr pellets are ground thoroughly and thus presumably exhibit isotropic orientation for deducing the molecular tilt angles of I-IV SAMs. Denoted in Figure 1 are the characteristic vibrational modes for acetylene (νCtC), disubstituted p-benzene moiety (a1, a2, and b2), ester (νCdO and νC-O), and trimethylsilane (γSi(CH3)3). The peak positions and vibrational modes are summarized in Table 1. The upper row in Figure 1 displays infrared reflection absorption spectroscopy (IRAS) spectra of I-IV SAMs on Au/ Ag(upd, 46 mV). The monolayer spectra differ from the

corresponding isotropic ones (bottom row) in γSi(CH3)3 mode for IV and in a2 for I-IV. The disappearance of γSi(CH3)3 in Figure 1D confirms that formation of S-substrate bond via the potential-assisted assembly is through cleavage of the alkyl sidearm (S-CH2) rather than breaking the S-aryl bond.81 The

Figure 2. Proposed binding schemes for I-IV self-assembly and corresponding vibrational modes for p-benzene moieties. a1 and b2 reside on the phenyl planes to which a2 is perpendicular.

19164 J. Phys. Chem. B, Vol. 109, No. 41, 2005

Chan et al.

TABLE 1: Peak Assignments for Diphenyldiacetylene in KBr Pellets and in Monolayers peak position (cm-1) in KBr

in monolayers

vibrational modeb

∼2210 1713 1586 1486 1276 1092 1013 ∼840 818

∼2214 1738 1589 1483 1286 1095 1013 ∼840 a

ν(CtC), stretching ν(CdO), stretching ν8a(CdC), in-plane stretching ν19a(CdC), in-plane stretching ν(C-O), stretching ν18b(CH), in-plane bending ν18a(CH), in-plane bending γ(Si(CH3)3), skeletal vibration ν17b(CH), out-of-plane bending

a

b

transition dipole directionb,c

phenyl ring: a1, | C2V axis phenyl ring: a1, | C2V axis phenyl ring: b2, ⊥ C2V axis phenyl ring: a1, | C2V axis phenyl ring: a2, ⊥ C2V axis

c

The peak intensity is negligible. Numbers, modes defined in ref 80. The directions of the transition dipoles, a1, a2, and b2, with respect to the phenyl ring are depicted in Figure 2.

TABLE 2: Intensity Ratio of Ib2/Ia1 and Molecular Tilt Anglesa of I-IV SAMs Prior to Photopolymerization compds

bulk Ib2/Ia1

I II III IV

3.65 2.29 1.06 2.63

Au/Ag(upd, 46 mV) Ib2/Ia1 θ (deg) 2.30( 0.17 1.45( 0.47 0.87( 0.04 2.15( 0.25

32 ( 2 32 ( 8 39 ( 1 39 (3

Au/Ag(upd, 100 mV) Ib2/Ia1 θ (deg) 2.55 ( 0.64 1.49 ( 0.26 1.07 ( 0.14 2.35 ( 0.21

35 ( 7 33 ( 5 45 ( 4 42 ( 3

Au/Ag(upd, 250 mV) Ib2/Ia1 θ (deg) 3.43 ( 0.40 1.90 ( 0.28 1.31 ( 0.15 2.56 ( 0.19

43 ( 3 40 ( 4 51 ( 3 44 ( 2

a θ is estimated by taking the arctangent of the normalized Ib2/Ia1. See Supporting Information for details. This table is calculated with a1 at ∼1013 cm-1. The other two a1’s give similar tilt angles.

lack of a2 indicates that the rings are parallel to the surface normal according to the surface dipole selection rule82 in which only vibration modes with transition dipole moments aligned perpendicular to the metal substrate can interact with the IR beam and appear in the IRAS spectra, whereas those aligned parallel with the substrate are missing. With the collective alignment of the p-benzene moiety, monolayers I-IV are likely highly ordered. The molecular tilt angle, θ, can be estimated by the intensity ratio of the orthogonal vectors of Ib2/Ia1. Because of the missing of a2 in the IRAS spectra, the benzene rings of the linear DPDA moiety lie on the X-Z plane (Figure 2) and so do the two inplane modes, b2 and a1. Therefore, calculation of θ becomes straightforward (see Supporting Information for details). After the peak intensity is normalized by those of isotropic KBr samples, taking the arctangent of Ib2/Ia1 yields θ values, which are summarized in Table 2. Overall speaking, a more positive upd potential and thus a lower coverage of Ag adatoms result in a larger molecular tilt angle although the difference is only by ∼10° at most. It is well documented that, at submonolayer coverage, the Ag adlayer exhibits an open lattice structure on gold.60 After the subsequent assembly of thiol monolayers, IRAS and scanning tunneling microscopy (STM) studies do not support the possibility of formation of silver islands and giving rise to uncovered gold.66,83 Note that the size of the sulfur headgroup is too large to fit in the interstices of Ag adatoms even at the lowest coverage and, hence, formation of the S-Au bond is unlikely. Taking together the S-Ag bonding scheme and the molecular tilt angle associated with Ag coverage, it is plausible that I-IV more or less adopt some preferential arrangement with respect to the Ag adatoms such that a smaller Ag coverage (i.e., a larger nearest-neighbor distance) increases the intermolecular spacing and thus the tilt angle. Effect of upd Coverage on Polymerization Efficiency. Polymerization of the monolayer is realized by the presence of the π-π* transition of polymerized diacetylene. For a fully conjugated PDA, this π-π* transition is typically located at ∼620-640 nm.20,24 The peak position shifts to ∼540 nm if the average conjugation length is reduced due to structural disorder or strain.20,24 We chose to focus this study on IV SAMs because,

Figure 3. UV-vis spectra of IV monolayers on Au/Ag(upd, 250 mV) before (bottom trace) and after subjected to UV illumination for 5 (middle) and 30 min (top) under N2(g) atmosphere.

as a result of the relatively reactive ester end group, IV can be further derivatized more facilely than its analogues of I-III. Exemplified in Figure 3 are transmission UV-vis spectra of IV SAMs on Au/Ag(upd, 250 mV) prior to and after illuminated with UV light (254 nm) for 5 and 30 min. The growth of the broad band demonstrates that polymerization of DPDA derivatives indeed takes place on Au/Ag(upd, 250 mV). The peak maximum at 590 nm shows a rather short ene-yne moiety, suggesting that the polymerization takes place only locally. Nonetheless, this example shows that Au/Ag(upd) can be utilized to improve monolayer polymerization of DPDAs that would otherwise exhibit no polymerization in solid state. More evidence for the success in developing PDA monolayers is revealed by the voltammograms of electrochemically reductive desorption. A more negative peak potential indicates a more stable organosulfur monolayer.75,84 It has been shown that SAMs stabilized by the upd interlayer require a desorption potential more negative than the corresponding monolayers on bare gold.60,69,70,85 To avoid interfering our observation of the reductive desorption process by hydrogen evolution, the experiments were conducted in nonaqueous solution to attain sufficiently negative potential limits.86 Figure 4 records the

Diphenyldiacetylene Derivative SAMs on Au/Ag(upd)

Figure 4. Reductive desorption of IV SAMs on Au/Ag(upd, 250 mV) before (dash line) and after (solid line) being subjected to UV illumination for 30 min under N2(g) atmosphere. Conditions: 0.1 M TBAP in acetonitrile; deareated with N2(g); scan rate, 0.1 V/s; reference, a quasi-reference electrode of a chloride-coated silver wire.

desorption waves for monomeric (dashed curve) and polymerized IV (solid curve) on Au/Ag(upd, 250 mV). Prior to electrochemical measurements, SAMs for the solid curve are polymerized by 30 min of UV illumination. The solid curve exhibits a well-defined desorption peak centered at ∼1.9 V vs EAg/AgCl, ca. 0.8 V negative from that of IV monomers (dashed line). Such enhanced stability for the polymerized monolayer is ascribed to the higher molecular weight and multidentate anchoring on the substrate. However, the reductive current of the polymerized film starts from -1.0 V, indicative of presence of unpolymerized DPDA monomers. Figures 5 and 6 demonstrate the effect of upd coverage on photopolymerization of the DPDA SAMs. The Au/Ag(upd) substrates are premodified at upd potentials of 46, 100, and 250 mV versus EAg+/0. Typical IRAS spectra of IV SAMs obtained before and after UV illumination are displayed in the lower and upper rows of Figure 5, respectively. Prior to UV illumination, the intensities of νCdO at 1738 cm-1, νC-O at 1286 cm-1, and νC≡C stretching for IV on Au/Ag(upd, 46 mV) are stronger than those prepared on the other two upd potentials. The difference in νC≡C peak intensity is consistent with the fact that a smaller molecular tilt angle (Table 2) leads to a longer projection length of the νC≡C transition dipole along the surface normal, a larger molecular packing density, and thus stronger peak intensity.

J. Phys. Chem. B, Vol. 109, No. 41, 2005 19165 After UV illumination for 30 min, the intensity of the νC≡C peak diminishes pronouncedly while the change for other peaks is insignificant. Therefore, the difference in the evolution of peak intensities is arising from photopolymerization, rather than loss of DPDA monomers. Noticeably, the intensity decrease in the νC≡C peak is dependent on the upd potential. Taking together the growth of π-π* transition in UV-vis spectra (Figure 3) and the enhanced monolayer stability examined by electrochemically reductive desorption (Figure 4), the decrease in the intensity of νC≡C indicates the degree of polymerization. Accordingly, Figure 5 suggests that the extent of monolayer polymerization can be tuned by the coverage of an Ag interlayer. Figure 6 is plotted by the fraction change of νC≡C peak intensity against UV illumination time because it is better defined, easier to measure, and more quantitative than the broad band of the PDA π-π* transition in UV-vis spectra and the electrochemical desorption methods.22 Over the 2-h period, monolayers exhibiting the largest drop in νC≡C intensity appear to be III on Ag/Ag(upd, 100 mV) and IV on Ag/Ag(upd, 250 mV) whose molecular tilt angles on average are ∼45°, similar to the optimal angle for polymerization in solid state. For films with θ of ∼5° larger or smaller than 45°, their polymerization turns out to be less completed. Further off from the optimal angle, such as II on Au/Ag(upd, 46 mV), the fractional decrease in νC≡C intensity is even less apparent. The plots for compounds I and II are not provided because their weak transition dipoles make it difficult to track the change in νC≡C peak intensity with accuracy. Conclusion IRAS spectra show that potential-assisted assembly is effective in anchoring SAMs of the protected DPDA derivatives, I-IV, on Au/Ag(upd). The same protocol, however, results in ill-defined SAMs on bare gold, suggesting that Au/Ag(upd) exhibits a stronger tendency in sulfur-substrate bond formation than that on bare gold. The phenyl rings of the DPDA SAMs are parallel to the surface normal. The molecular tilt angles appear dependent on the upd potential. A more positive upd potential gives a lower coverage of Ag adatoms and a larger tilt angle. UV-vis spectra reveal the presence of ene-yne π-π* transition. The maximum absorbance for polymerized IV SAMs resides at 590 nm, suggesting that the ene-yne moiety is not fully conjugated. Linear scan voltammograms show that desorption of the UV-illuminated SAMs takes place at a more

Figure 5. IRAS spectra of IV monolayers on (A) Au/Ag(upd, 46 mV), (B) Au/Ag(upd, 100 mV), and (C) Au/Ag(upd, 250 mV) prior to (bottom row) and after exposed to UV illumination for 30 min (upper row).

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Figure 6. Fractional change of the ν(CtC) intensity as a function of UV illumination time under N2(g) for compounds III (left panel) and IV on Au/Ag(upd). The squares, diamonds, and circles denote Au/Ag(upd, 46 mV), Au/Ag(upd, 100 mV), and Au/Ag(upd, 250 mV), respectively.

negative potential than those prior to polymerization. The enhanced stability is attributed to the formation of multiple anchoring legs via photopolymerization. To study the polymerization efficiency, the decrease in peak intensity of νC≡C is monitored against UV-illumination time. Over the 2-h illuminating period, the maximum decrease in the intensity of νC≡C peak is found for films with ∼45° tilt angles. With the model system of DPDA SAMs, this study demonstrates that the upd-modified substrate can be utilized to manipulate the SAM structure and the monolayer reactivity. Experimental Section Materials. All chemicals were used as received unless otherwise noted. We report here the design and synthesis a series of mono- and dithiol-terminated DPDA derivatives where the sulfur headgroups are protected by methyl or (trimethylsilyl)ethyl (Scheme 2). The synthetic procedures for compounds I-IV were described as follows. Synthesis of Compounds I-IV. 1,4-Bis(4-(2-(trimethylsilyl)ethylthio)phenyl)buta-1,3-diyne, III. To a round-bottomed single-necked flask were added 2.45 g (7.99 mmol) of 1-(2trimethylsilanyl-ethylsulfanyl)-4-trimethylsilanylethynyl-benzene (see Supporting Information for synthetic details), 56 mg (0.08 mmol) of Pd(PPh3)2Cl2, 76 mg (0.4 mmol) of CuI, 105 mg (0.4 mmol) of PPh3, and 1.65 g (12 mmol) of potassium carbonate in 100 mL of CH3OH/THF (1:1, v/v). The mixture was stirred for 4 h at room temperature, and then the solvent was removed using a rotary evaporator. The residue was poured into CH2Cl2 and extracted with brine. The organic layer was separated and dried over MgSO4, and the solvent was removed under reduced pressure. The crude product was purified by column chromatography on aluminum oxide, eluting with hexane to yield 2.61 g (70%) of III. MALDI MS: m/z 466 (M+). 1H NMR (δ, CDCl3): 0.04 (s, 18H), 0.90-0.94 (m, 4H), 2.93-2.98 (m, 4H), 7.18 (d, J ) 8.6 Hz, 4H), 7.39 (d, J ) 8.6 Hz, 4H). Anal. Calcd for C26H34S2Si2: C, 66.89; H, 7.34. Found: C, 66.53; H, 7.28. 1,4-Bis(4-(methylthio)phenyl)buta-1,3-diyne, I. To 0.5 g (1.12 mmol) of III was added 5 mL (5.0 mmol) of THF solution containing TBAF under nitrogen, and then the solution was stirred for 40 min. Iodomethane (3 mL, 48.19 mmol) was added, and the mixture was stirred for 12 h. The solvent was removed using a rotary evaporator, and the residue was dissolved in CH2Cl2 and extracted with brine. The organic layer was separated

Chan et al. and dried over MgSO4, and the solvent was removed under reduced pressure. The crude product was purified by column chromatography on aluminum oxide, eluting with hexane to yield 0.28 g (89%) of I. MALDI MS: m/z 294 (M+). 1H NMR (δ, CDCl3): 2.47 (s, 6H), 7.16 (d, J ) 8.4 Hz, 4H), 7.40 (d, J ) 8.4 Hz, 4H). Anal. Calcd for C18H14S2: C, 73.43; H, 4.79. Found: C, 72.94; H, 4.73. Trimethyl(2-(4-(4-phenylbuta-1,3-diynyl)phenylthio)ethyl)silane, 3. Compound 3 was prepared following the general procedure for the Sonogashira coupling reaction87,88 using 0.56 g (5.46 mmol) of ethynylbenzene, 1.3 g (3.61 mmol) of (2-(4(2-iodoethynyl)phenylthio)ethyl)trimethylsilane (see Supporting Information), 137 mg (0.20 mmol) of PdCl2(PPh3)2, 58 mg (0.31 mmol) of CuI, and 150 mg (0.57 mmol) of PPh3 in 50 mL of freshly distilled THF and 5 mL of diisopropylamine. The reaction was stirred at 80 °C for 24 h and worked up. The crude product was purified by an aluminum oxide column, eluting with CH2Cl2/hexane (10-20:100, v/v). The desired fractions were collected and concentrated to afford 398 mg (33%) of 3. 1H NMR (δ, CDCl ): 0.04 (s, 9H), 0.90-0.95 (m, 2H), 2.943 2.98 (m, 2H), 7.19 (d, J ) 8.6 Hz, 2H), 7.29-7.35 (m, 3H), 7.40 (d, J ) 8.6 Hz, 2H), 7.49-7.52 (m, 2H). Methyl(4-(4-phenylbuta-1,3-diynyl)phenyl)sulfane, II. To 0.3 g (0.90 mmol) of 3 was added 3 mL (3.0 mmol) of THF solution containing TBAF under nitrogen, and then the mixture was stirred for 40 min. Iodomethane (3 mL, 48.19 mmol) was added, and the mixture was stirred for 18 h. The solvent was removed using a rotary evaporator, and the residue was dissolved in CH2Cl2 and extracted with brine. The organic layer was separated and dried over MgSO4, and the solvent was removed under reduced pressure. The crude product was purified by column chromatography on aluminum oxide, eluting with hexane to yield 0.18 g (82%) of II. MALDI MS: m/z 248 (M+). 1H NMR (δ, CDCl ): 2.47 (s, 3H), 7.16 (d, J ) 8.7 Hz, 2H), 3 7.28-7.35 (m, 3H), 7.41 (d, J ) 8.7 Hz, 2H), 7.49-7.51 (m, 2H). Anal. Calcd for C17H12S: C, 82.22; H, 4.87. Found: C, 82.48; H, 4.83. Methyl 4-(4-(4-(2-(Trimethylsilyl)ethylthio)phenyl)buta1,3-diynyl)benzoate, IV. Compound IV was prepared following the general procedure for the Sonogashira coupling reaction using 0.64 g (4.0 mmol) of methyl 4-ethynylbenzoate, 1.1 g (3.1 mmol) of (2-(4-(2-iodoethynyl)phenylthio)ethyl)trimethylsilane, 110 mg (0.16 mmol) of Pd Cl2(PPh3)2, 58 mg (0.31 mmol) of CuI, and 97 mg (0.37 mmol) of PPh3 in 50 mL of freshly distilled THF and 5 mL of triethylamine. The reaction was allowed to proceed at 60 °C for 24 h and then worked up. The crude product was purified by an aluminum oxide column, eluting with CH2Cl2/hexane (1:1 by vol.). The desired fractions were collected and concentrated to afford 0.86 g (72%) of IV. MALDI MS: m/z 392 (M+). 1H NMR (δ, CDCl3): 0.02 (s, 9H), 0.90-0.95 (m, 2H), 2.94-2.98 (m, 2H), 3.90 (s, 3H), 7.19 (d, J ) 8.7 Hz, 2H), 7.41 (d, J ) 8.7 Hz, 2H), 7.55 (d, J ) 8.1 Hz, 2H), 7.98 (d, J ) 8.0 Hz, 2H). Anal. Calcd for C23H24O2SSi: C, 70.37; H, 6.16. Found: C, 69.90; H, 5.82. Preparation of upd-Modified Gold Substrate. The substrates were 200 nm of gold film with 5 nm of Cr underlayer prepared by thermal evaporation onto glass slides. Prior to thermal evaporation, the glass slides were cleaned with piranha solution, a mixture of 1:3 (v/v) 30% H2O2 and concentrated H2SO4. The upd solution was prepared from reagent grade chemicals and Millipore-Q purified water (18 MΩ cm-1). The solution of underpotential deposition process for Ag is 0.6 mM Ag2SO4 (Fisher Scientific) dissolved in 0.1 M H2SO4. A silver wire was used as the reference electrode and a Pt wire as the

Diphenyldiacetylene Derivative SAMs on Au/Ag(upd) counter electrode. The electrodes were flame-annealed before experiments. The upd process was carried out with a CHI 604 potentiostat (CH Instruments Inc., Austin, TX). The initial potential of the gold substrates was 650 mV (vs EAg+/0). The scan rate was 20 mV/s. The final potentials were 250, 100, and 46 mV where the electrode was removed from the solution under potential control. The Au/Ag(upd) substrates were rinsed several times with pure ethanol and then blown dry with a stream of nitrogen. The deposition and stripping peaks were more distinct than those of upd on polycrystalline gold but not as well defined as peaks on Au(111)60 indicating that the substrates were preferentially (111) oriented but polycrystalline in nature. Potential-Assisted SAMs. The substrates were prepared as mentioned previously. Around 0.5 mM DPDAs were prepared in a mixture of EtOH/MeOH/acetone (15:1:2, v/v/v) containing a supporting electrolyte of 0.5 mM TBAP (tetrabutylammonium perchlorate) in 50-mL vials. The monolayers were deposited by applying a constant potential of -300 mV (vs EAg+/0) for at least 4 h in the adsorbate solutions.75-78 After deposition, the substrates were removed from the solutions and were then rinsed with ethanol and dried in flowing N2 gas. Further polymerization was carried out by exposure to a 254-nm UV lamp (model UVG-11, Ultra-Violet Products Inc.) with a sample-source distance of approximately 2 cm in a nitrogen-purging dark chamber. After polymerization, the substrates were rinsed with ethanol and blown dry with a stream of nitrogen. Characterization of SAMs. Reflection-absorption spectroscopy was measured using an FT-IR spectrometer (PerkinElmer System 2000) equipped with a liquid nitrogen-cooled MCT detector. A single reflection mode with the p-polarized light at 85° grazing angle of the incident light was used. The light path, MCT detector, and sample chamber were purged with dry nitrogen during measurement. An argon-cleaned or flamecleaned gold substrate was used as the reference. The IR spectra were collected with a total of 1024 scans of both the sample and the reference at 4 cm-1 resolution. For UV-vis measurements, the transparent substrates were prepared by thermal evaporation of 15 nm of Au onto quartz substrates pretreated with (3-mercaptopropyl)trimethoxysilane (MPS).89 Acknowledgment. The authors thank the National Science Council (R.O.C.), Academia Sinica, and the Department of Chemistry (NTHU) for generous financial and research support. Supporting Information Available: Details of synthetic procedures for starting materials, IRAS spectra of DPDA SAMs on bare gold, and equations to estimate the molecular tilt angle, θ. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wegner, G. Z. Naturforsch. 1969, 24b, 824-832. (2) Ouyang, X.; Fowler, F. W.; Lauher, J. W. J. Am. Chem. Soc. 2003, 125, 12400-12401. (3) Crihfield, A.; Hartwell, J.; Phelps, D.; Walsh, R. B.; Harris, J. L.; Payne, J. F.; Pennington, W. T.; Hanks, T. W. Cryst. Growth Des. 2003, 3, 313-320 and references therein. (4) Chang, J. Y.; Baik, J. H.; Lee, C. B.; Han, M. J.; Hong, S.-K. J. Am. Chem. Soc. 1997, 119, 3197-3198. (5) Hammond, P. T.; Rubner, M. F. Macromolecules 1997, 30, 57735782. (6) Okuno, T.; Izuoka, A.; Ito, T.; Kubo, S.; Sugawara, T.; Sato, N.; Sugawara, Y. J. Chem. Soc., Perkin Trans. 2 1998, 889-895. (7) Tamaoki, N.; Shimada, S.; Okada, Y.; Belaissaoui, A.; Kruk, G.; Yase, K.; Matsuda, H. Langmuir 2000, 16, 7545-7547. (8) George, M.; Weiss, R. G. Chem. Mater. 2003, 15, 2879-2888. (9) Aoki, K.; Kudo, M.; Tamaoki, N. Org. Lett. 2004, 6, 4009-4012.

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