Monolayers of diphenyldiacetylene derivatives: Tuning molecular tilt angles and photopolymerization efficiency via electrodeposited Ag interlayer on Au

Article abstract of DOI:10.1021/jp0529366

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 ??_Ca?? intensity by infrared reflectance absorbance spectroscopy, growth of ene-yne ??-??* 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 a??45?° tilt angle for DPDA derivatives. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0529366

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 photopo-
lymerization 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
SCHEME 1: Schematic Diagram for Topochemical
Polymerization of Diacetylenes
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 ap-
plications such as nonlinear optical materials,30-33 ultrathin
photoresists,19 semiconductors and photoconductors,34,35 bio-
mimetic liposome for drug delivery36,37 or vesicles for biosens-
ing,38 and chemical sensors.39-42 Among the applications, mono-
or 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 self-
assembled 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 po-
lymerization is adapted from solid state chemistry. The neigh-
boring diacetylenes are polymerized via a 1,4-addition mech-
anism45,46 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
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
* To whom correspondence should be addressed. J.-T.L.: phone, +886
2 2789 8522; fax, +886 2 2783 1237; e-mail, jtlin@chem.sinica.edu.tw.
C.-h.C.: phone, +886 3 573 7009; fax, +886 3 571 1082; e-mail,
chhchen@mx.nthu.edu.tw.
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
Chan et al.
CHART 1
the suppression of oxide formation.61-66 The modified surface
promotes the headgroup-substrate anchoring61-64,67 and in-
creases 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
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-(tri-
alkylsily)ethyl protecting group is stable and can be readily
deprotected by introducing TBAF (tetrabutylammonium fluo-
ride) 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 po-
tentiostated at a constant potential (vide infra).
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.
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 cross-
couples 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 potential-
assisted assembly.75-78 During monolayer assembly, the sub-
strate is potentiostated at a constant potential of -300 mV (vs
+/0
EAg ). Compared to IR spectra measured from the conventional
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 electro-
chemical 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 upd-
modified gold substrates demonstrated a better SAM reproduc-
ibility61 than those on the corresponding bulk materials due to
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)
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19163
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
+/0
headgroup. In the following, 250, 100, or 46 mV (vs EAg
)
corresponding isotropic ones (bottom row) in γ
mode for
in Figure
Si(CH3)3
Si(CH3)3
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
IV and in a2 for I-IV. The disappearance of γ
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
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. ø 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
ν
C-O), and trimethylsilane (γSi(CH ) ). The peak positions and
3 3
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
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
transition dipole directionb,c
∼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
phenyl ring: a1, | C2V axis
phenyl ring: a1, | C2V axis
ν
19a(CdC), in-plane stretching
ν(C-O), stretching
ν
18b(CH), in-plane bending
18a(CH), in-plane bending
phenyl ring: b2,
C2V axis
ν
phenyl ring: a1, | C2V axis
γ(Si(CH3)3), skeletal vibration
ν17b(CH), out-of-plane bending
phenyl ring: a2,
C2V axis
a The peak intensity is negligible. b Numbers, modes defined in ref 80. c 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 Ib /Ia and Molecular Tilt Anglesa of I-IV SAMs Prior to Photopolymerization
2
1
Au/Ag(upd, 46 mV)
Au/Ag(upd, 100 mV)
Ib /Ia θ (deg)
Au/Ag(upd, 250 mV)
Ib /Ia θ (deg)
bulk
compds
Ib /Ia
Ib /Ia
θ (deg)
2
1
2
1
2
1
2
1
I
II
III
IV
3.65
2.29
1.06
2.63
2.30( 0.17
1.45( 0.47
0.87( 0.04
2.15( 0.25
32 ( 2
32 ( 8
39 ( 1
39 (3
2.55 ( 0.64
1.49 ( 0.26
1.07 ( 0.14
2.35 ( 0.21
35 ( 7
33 ( 5
45 ( 4
42 ( 3
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 Ib /Ia . See Supporting Information for details. This table is calculated with a1 at
2
1
∼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 Ib /Ia . Because of the missing
2
1
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 in-
plane 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 Ib /Ia yields θ values, which
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.
2
1
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,
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 deriva-
tives 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 experi-
ments were conducted in nonaqueous solution to attain suf-
ficiently negative potential limits.86 Figure 4 records the

Diphenyldiacetylene Derivative SAMs on Au/Ag(upd)
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 electro-
chemically 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.
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 polymer-
ized 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
E
Ag/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.
Conclusion
IRAS spectra show that potential-assisted assembly is effec-
tive 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 de-
sorption of the UV-illuminated SAMs takes place at a more
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
+/0
mV versus EAg . 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.
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).

19166 J. Phys. Chem. B, Vol. 109, No. 41, 2005
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)eth-
yl)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, CDCl3): 0.04 (s, 9H), 0.90-0.95 (m, 2H), 2.94-
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, CDCl3): 2.47 (s, 3H), 7.16 (d, J ) 8.7 Hz, 2H),
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.
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 polym-
erization efficiency, the decrease in peak intensity of νC≡C is
monitored against UV-illumination time. Over the 2-h illuminat-
ing 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.
Methyl 4-(4-(4-(2-(Trimethylsilyl)ethylthio)phenyl)buta-
1,3-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)trimethyl-
silane, 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.
Synthesis of Compounds I-IV. 1,4-Bis(4-(2-(trimethylsi-
lyl)ethylthio)phenyl)buta-1,3-diyne, III. To a round-bottomed
single-necked flask were added 2.45 g (7.99 mmol) of 1-(2-
trimethylsilanyl-ethylsulfanyl)-4-trimethylsilanylethynyl-ben-
zene (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
MALDI MS: m/z 392 (M+). H 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 C23H24O2-
SSi: C, 70.37; H, 6.16. Found: C, 69.90; H, 5.82.
Preparation of upd-Modified Gold Substrate. The sub-
strates 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
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 CH2-
Cl2 and extracted with brine. The organic layer was separated

Diphenyldiacetylene Derivative SAMs on Au/Ag(upd)
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19167
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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
+/0
potential of the gold substrates was 650 mV (vs EAg ). The
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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
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Acknowledgment. The authors thank the National Science
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Chemistry (NTHU) for generous financial and research support.
Supporting Information Available: Details of synthetic
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θ. This material is available free of charge via the Internet at
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