Molecularly ordered decanethiolate self-assembled monolayers on Au(111) from in situ cleaved decanethioacetate: An NMR and STM study of the efficacy of reagents for thioacetate cleavage

Article abstract of DOI:10.1021/la100103k

The cleavage of decanethioacetate (C10SAc) has been studied by ¹H nuclear magnetic resonance (NMR) spectroscopy and scanning tunneling microscopy (STM) imaging of in situ prepared decanethiolate self-assembled monolayers (SAMs) on Au(111). Solutions of C10SAc (46 mM) and previously reported cleavage reagents (typically 58 mM) in CD₃OD were monitored at 20 °C by NMR spectroscopy. Cleavage by ammonium hydroxide, propylamine, or hydrochloric acid was not complete within 48 h; cleavage by potassium carbonate was complete within 24 h and that by potassium hydroxide or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) within 2 h. Similar cleavage rates were observed for phenylthioacetate. The degree of molecular ordering determined by STM imaging increased with increasing extent of in situ cleavage by these same reagents (2.5 mM C10SAc and 2.5 mM reagent in ethanol for 1 h, then 16 h immersion of Au/mica). Less effective cleavage reagents did not cleave the C10SAc sufficiently to decanethiol (C10SH) and gave mostly disordered SAMs. In contrast, KOH or DBU completely cleaved the C10SAc to C10SH and led to well-ordered SAMs composed of (3 × 3)R30° domains that are indistinguishable from SAMs grown from C10SH. Monolayer formation from thioacetates in the absence of cleavage agents is likely due to thiol or disulfide impurity in the thioacetates. Eliminating disulfide by using Bu ₃P as a sacrificial reductant also helped to produce good molecular order in the SAM. The methods presented here allow routine growth of molecularly ordered alkanethiolate SAMs from thioacetates using reagents of ordinary purity under ambient, benchtop conditions.

Full text of DOI:10.1021/la100103k

pubs.acs.org/Langmuir
©
2010 American Chemical Society
Molecularly Ordered Decanethiolate Self-Assembled Monolayers on
Au(111) from in Situ Cleaved Decanethioacetate: An NMR and STM Study
of the Efficacy of Reagents for Thioacetate Cleavage
†
‡
‡
,‡
Anuradha Singh, Daminda H. Dahanayaka, Abhijit Biswas, Lloyd A. Bumm,* and
,
†
Ronald L. Halterman*
†
‡
Department of Chemistry and Biochemistry, 620 Parrington Oval, and Homer L. Dodge Department of
Physics and Astronomy, 440 W Brooks St., University of Oklahoma, Norman, Oklahoma 73019
Received January 8, 2010. Revised Manuscript Received May 30, 2010
1
The cleavage of decanethioacetate (C10SAc) has been studied by H nuclear magnetic resonance (NMR) spectroscopy
and scanning tunneling microscopy (STM) imaging of in situ prepared decanethiolate self-assembled monolayers (SAMs) on
Au(111). Solutions of C10SAc (46 mM) and previously reported cleavage reagents (typically 58 mM) in CD OD were
3
monitored at 20 ꢀC by NMR spectroscopy. Cleavage by ammonium hydroxide, propylamine, or hydrochloric acid was not
complete within 48 h; cleavage by potassium carbonate was complete within 24 h and that by potassium hydroxide or
1
,8-diazabicyclo[5.4.0]undec-7-ene (DBU) within 2 h. Similar cleavage rates were observed for phenylthioacetate. The degree
of molecular ordering determined by STM imaging increased with increasing extent of in situ cleavage by these same reagents
2.5 mM C10SAc and 2.5 mM reagent in ethanol for 1 h, then 16 h immersion of Au/mica). Less effective cleavage reagents
did not cleave the C10SAc sufficiently to decanethiol (C10SH) and gave mostly disordered SAMs. In contrast, KOH or DBU
(
√
√
completely cleaved the C10SAc to C10SH and led to well-ordered SAMs composed of ( 3 ꢀ 3)R30ꢀ domains that are
indistinguishable from SAMs grown from C10SH. Monolayer formation from thioacetates in the absence of cleavage agents
3
is likely due to thiol or disulfide impurity in the thioacetates. Eliminating disulfide by using Bu P as a sacrificial reductant also
helped to produce good molecular order in the SAM. The methods presented here allow routine growth of molecularly
ordered alkanethiolate SAMs from thioacetates using reagents of ordinary purity under ambient, benchtop conditions.
Introduction
disulfides typically have lower solubility and vapor pressure,
properties that can affect the nature of SAM formation. The pre-
sence of disulfides in a nonrigorously purified thiol sample can
Self-assembled monolayers (SAMs) have become an important
practical tool in surface science because of their ease of prepara-
8
1
lead to SAMs with a higher defect density and a reduced degree
tion and benchtop-friendly stability. Their surface chemical and
9
of molecular order. In the course of this study, we have found
physical properties can be tailored by changing the exposed
chemical functional groups, which can also be spatially patterned.
SAMs can be used as a 2D matrix for supporting and isolat-
that the resulting SAMs formed in the presence of disulfides were
often much more disorderedthandesired for molecularly resolved
STM imaging. Our desired outcome was to develop methodology
that could achieve high-quality, well-ordered, high-density, crys-
talline SAMs from organothiols under ambient benchtop condi-
2
ing molecules, such as candidate molecules for electronics. The
most widely studied system on metallic surfaces is alkanethiol/
Au(111), where the monolayer is anchored to the surface by
Au-S chemisorption and stabilized by the intermolecular inter-
actions of the rest of the molecule. The chemisorption of orga-
nothiols from solution or vapor onto gold is often a successful
method to form molecularly ordered organothiolate monolayers.
SAMs analogous to those of alkanethiols have also been grown
√
√
tions, namely, the ( 3 ꢀ 3)R30ꢀ structure readily prepared from
pure decanethiol.
Deprotection of Thiol Derivatives. An approach to obtain
thiols for SAM applications that minimizes undesirable disulfide
formation is the use of a suitably protected thiol that can be
deprotected in situ just before or during SAM formation.
1,3-5
from other derivatized molecules containing a thiol headgroup.
1
0,11
12
Thiocyanates,
S-tritylalkanethiols, and especially acetyl-
As described in the literature, we have also found that, under
ambient, benchtop conditions organothiol samples contained and
were prone to form disulfides. Compared to the parent thiol,
3
,13-16
protected thiols (thioacetates)
have been developed as thiol
(
8) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem.
Soc. 1987, 109, 3559.
*
(
E-mail: bumm@nhn.ou.edu; rhalterman@ou.edu.
1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M.
Chem. Rev. 2005, 105, 1103.
2) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.;
Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705.
3) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.;
Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. J. Am. Chem. Soc. 1995,
17, 9529.
(9) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K. E.;
Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 2003, 125, 2597.
(10) Dreesen, L.; Volcke, C.; Sartenaer, Y.; Peremans, A.; Thiry, P. A.;
Humbert, C.; Grugier, J.; Marchand-Brynaert, J. Surf. Sci. 2006, 600, 4052.
(11) Ciszek, J. W.; Stewart, M. P.; Tour, J. M. J. Am. Chem. Soc. 2004, 126,
13172.
(12) Inman, C. E.; Reed, S. M.; Hutchison, J. E. Langmuir 2004, 20, 9144.
(13) Holmes, B. T.; Snow, A. W. Tetrahedron 2005, 61, 12339.
(14) Lau, K. H. A.; Huang, C.; Yakovlev, N.; Chen, Z. K.; O’Shea, S. J.
Langmuir 2006, 22, 2968.
(15) Stapleton, J. J.; Harder, P.; Daniel, T. A.; Reinard, M. D.; Yao, Y.; Price,
D. W.; Tour, J. M.; Allara, D. L. Langmuir 2003, 19, 8245.
(16) Cai, L.; Yao, Y.; Yang, J.; Price, D. W.; Tour, J. M. Chem. Mater. 2002, 14,
2905.
(
(
1
(4) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164.
(5) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665.
(6) Collman, J. P.; Devaraj, N. K.; Eberspacher, T. P. A.; Chidsey, C. E. D.
Langmuir 2006, 22, 2457.
7) Niklewski, A.; Azzam, W.; Strunskus, T.; Fischer, R. A.; W o€ ll, C. Langmuir
004, 20, 8620.
(
2
Langmuir 2010, 26(16), 13221–13226
Published on Web 07/21/2010
DOI: 10.1021/la100103k 13221

Article
Singh et al.
precursors to circumvent disulfide formation while allowing for
facile deprotection and monolayer formation. The approach has
proved particularly effective for self-assembly of dithiols, where
disulfide formation would lead to multilayer formation and,
worse, polymerization. Favoring the application of the thio-
acetates are their convenient preparation from alkyl halides, their
effective prevention of oxidative formation of disulfides upon
storage, and the ready ability to cleave the thioester to release
the thiol.
Deprotection of Thioacetates. Since thioacetates have seen
the greatest scope of applications, we have focused our efforts on
the use of thioacetate protection and cleavage to address our
desire for a simple, reliable method that can be used to produce
well-ordered SAMs under ambient conditions with routine bench-
top procedures that do not require heroically pure materials.
Herein, we highlight pertinent literature examples of in situ
deprotection of thioacetates for self-assembly of thiolate mono-
layers, provide new NMR spectroscopy data for the extent of
cleavage of decanethioacetate by various reagents in deuterated
methanol or tetrahydrofuran, and show by STM imaging that
conditions leading to faster, more complete thioacetate deprotec-
tion yield larger fractions of crystalline, molecularly ordered
domains in the resultant decanethiolate monolayers formed by
in situ deprotection of decanethioacetate.
absence of the carbonyl stretch from the thioacetate implied that
deprotection of the molecules incorporated into the SAM was
complete. Several other studies on SAM formation cited this initial
work as support for in situ cleavage of other arylthioacetates and
alkanethioacetates in various solvents through the addition of a
wider range of exogenous bases. These conditions include the use of
NH OH in EtOH, THF, or acetone-MeOH; Cs CO in acetone/
4
2
3
16
MeOH or conc. H SO in CH Cl /MeOH; triethylamine or
2
4
2
2
17
15
NH OH in DMF; NH OH or conc. H SO in EtOH; NaOH
in EtOH; or EtOH/H O. Concentrations of the thioacetate
4
4
2
4
18
7
2
compound ranged 0.1-1 mM and concentration of the cleaving
agent 0.1 mM to 0.2 M. The cleaving agent to thioacetate mole
ratio ranged from nearly unity to over a thousandfold excess.
Direct Use of Thioacetates. It has also been speculated that
no cleavage agent is required to form the thiolate SAM, because
it seemed that direct deprotection of the thiol could occur at the
3
,14,19-22
gold surface
SPR, XPS, and TOF-SIMs measurements
for thioacetates of alkyl chains and oligo(phenylene ethylnylene)-
based R,ω-dithioacetates in EtOH, THF, or 1:1 dichloromethane/
EtOH showed that full monolayers can be formed directly from
1
4
thioacetates. Utilizing surface characterization by XPS and
IRRAS, Lee et al. found that SAMs formed from alkanethio-
acetate are more disordered and less densely packed than the
2
1
similar alkanethiol-derived SAMs. In followup work, W o€ ll et al.
showed that an ethanolic solution of decanethioacetate carefully
purified to be free from thiol formed a molecularly ordered flat-
lying decanethiolate monolayer on gold by STM. Only small
regions of high-density upright-standing molecules were observed
at defect sites and at the boundaries between larger phases of flat-
An initial report on the in situ cleavage of organothioacetates
3
for chemisorption onto gold used aqueous NH OH in THF. The
4
characterization of the resulting self-assembled monolayers in this
and subsequent reports has relied chiefly on thickness determina-
tions by ellipsometry, X-ray photoelectron spectroscopic (XPS),
infrared reflection absorption spectroscopy (IRRAS), near-edge
X-ray absorption fine structure (NEXAFS), electrochemistry,
contact angle, and/or surface plasmon resonance (SPR) measure-
2
3
lying molecules. Thus, if direct cleavageoccurs, it is not expected
to occur at a rate sufficient to allow formation of large regions
of dense upright phases. Additionally, XPS and IRRAS studies
show that the molecule-gold interface in thioacetate-derived
SAMs is predominantly thiolate; thioacetate, if present, is only
1
,3
ments. Although these techniques measure bulk properties,
such as average effective thickness and molecular tilt (IRRAS and
NEXAFS), molecular ordering is not measured but, in the case of
IRRAS, can be inferred from comparison to appropriate model
systems. An accurate assessment of the molecular order and
crystallinity of the SAM requires a direct imaging technique such
as STM or a surface diffraction technique such as grazing incident
X-ray diffraction (GIXRD) or low-energy atom diffraction
1
4,21
present as a minority.
In summary, SAMs derived from
thioacetates, of ordinary purity with or without cleavage by
exogenous base, have been shown to produce thiolate SAMs with
monolayer coverage, but with a large degree of uncertainty
regarding long-range crystalline molecular ordering.
(
LEAD). One study used STM imaging to characterize SAMs
Results and Discussion
produced by an in situ deprotection of thioacetates, but molecular
resolution was not obtained. To our knowledge, no systematic
assessment of the molecular order of SAMs formed by cleavage of
thioacetates has been undertaken.
Tour et al. reported that, when acetyl-protected arylthiols
Given this diverse background for in situ cleavage under vari-
ous conditions, we initiated an STM study to compare the struc-
ture and order of SAMs grown from decanethioacetates with and
without the presence of a thioacetyl cleaving agent. As discussed
below, we did observe striking differences in the quality of high-
density, upright-standing SAMs produced in the presence of vari-
ous cleaving agents. Although the exact percentages of mole-
cularly ordered SAM regions were not quantified, the SAMs
formed in the presence of different thioacetate cleaving agents
ranged from almost completely disordered regions to almost
7
(
0.1 to 41 mM) were initially reacted with NH OH in THF before
4
exposing to gold surfaces, the resulting layer thickness was closer
to that expected for a full monolayer of standing up molecules,
than when no exogenous base was added (ellipsometry and
3
XPS). This result was attributed to a higher concentration of
organothiolate being present due to base-catalyzed hydrolysis.
Thiolate adsorption to the surface without exogenous base was
suggested to possibly occur through initial adsorption of the
thioacetates to the gold surface or by limited thioacetate hydro-
lysis by trace wateror enols of the thioesters. NMR measurements
were reported to indicate that hydrolysis of the thioacetate in
(
17) Shaporenko, A.; Elbing, M.; Blaszczyk, A.; von Hanisch, C.; Mayor, M.;
Zharnikov, M. J. Phys. Chem. B 2006, 110, 4307.
18) Cheng, L.; Yang, J.; Yao, Y.; Price, D. W.; Dirk, S. M.; Tour, J. M.
Langmuir 2004, 20, 1335.
19) Cai, L. T.; Skulason, H.; Kushmerick, J. G.; Pollack, S. K.; Naciri, J.;
(
(
Shashidhar, R.; Allara, D. L.; Mallouk, T. E.; Mayer, T. S. J. Phys. Chem. B 2004,
108, 2827.
THF-d was complete within 10 min upon exposure to aqueous
8
NH OH and that other bases such as N,N-dipropylamine or 4-N,
N-dimethylaminopyridine (DMAP) were less effective. In that
same study, infrared measurements of one of the phenylene
ethynylene molecules showed that these rigid-rod molecules were
standing up, oriented within 20ꢀ of the surface normal, and had
infrared intensities consistent with a full monolayer. Furthermore,
(20) Nakashima, H.; Furukawa, K.; Ajito, K.; Kashimura, Y.; Torimitsu, K.
Langmuir 2005, 21, 511.
4
3
(
21) B ꢀe thencourt, M. I.; Srisombat, L.-o.; Chinwangso, P.; Lee, T. R. Langmuir
2009, 25, 1265.
22) Kang, Y.; Won, D.-J.; Kim, S. R.; Seo, K.; Choi, H.-S.; Lee, G.; Noh, Z.;
Lee, T. S.; Lee, C. Mater. Sci. Eng. C 2004, 24, 43.
23) Badin, M. G.; Bashir, A.; Krakert, S.; Strunskus, T.; Terfort, A.; W o€ ll, C.
Angew. Chem., Int. Ed. 2007, 46, 3762.
(
(
1
3222 DOI: 10.1021/la100103k
Langmuir 2010, 26(16), 13221–13226

Singh et al.
Article
Figure 2. STM image of SAM on Au(111) grown for 16 h from
Figure 1. STM image of SAM on Au(111) grown for 16 h from
C10SAc. The image is 100 nm ꢀ 100 nm. Tunneling conditions are
C10SAc, precleaved for 1 h in NH
4
OH. The image is 100 nm ꢀ
00 nm. Tunneling conditions are Vsample = -1 V and itunnel
pA. See text for details.
1
1
=
V
sample = -1 V and itunnel = 1 pA. See text for details.
completely ordered regions. This qualitative information is suffi-
cient in setting procedures that favor the formation of well-
ordered decanthiolate SAMs on Au(111).
monolayer is kinetically stable as would be anticipated for high-
coverage, standing-up monolayers. Repeating the SAM growth at
higher temperature (60 ꢀC) with a fresh substrate likewise did not
improve the order.
The acetyl-protected decanethiol was purified by simple bulb-
to-bulb vacuum distillation. The purified sample used in these
studies contained 0.04% decanethiol and 0.01% decanedisulfide
as determined by GC-MS integration using total ion current. The
solutions were prepared in absolute ethanol and were 2.5 mM in
the thioacetate. If a thioacetyl cleaving agent was used, its con-
centration was also 2.5 mM, and a solution of the mixture was
allowed to stand for 1 h prior to use allowing time for cleavage to
occur. All solution work was carried out under benchtop (ambient)
Cleavage of C10SAc by NH OH. Following the literature
4
precedent, we next added NH OH as the thioacetyl cleaving agent
4
in an attempt to form decanethiol in situ, allowing 1 h for the
cleavage reaction to proceed before the Au substrate was
1
4-16
introduced.
The STM images show a SAM with generally
poor, but improved order (Figure 2). The structure is manifested
by two distinct types of regions with different heights. In the lower
regions, generally no molecularly resolved imaging could be
conditions, with noattempts to exclude oxygen. Freshly H flame-
2
annealed (cleaned) Au(111)/mica substrates were immersed in
this solution for 16 h. They were then removed, rinsed in absolute
√
√
observed. In the higher regions, a ( 3 ꢀ 3)R30ꢀ molecular
lattice was sometimes observed. The statistical percentage of
molecularly ordered domains was not determined.
EtOH, and blown dry with dry N . All sample preparation was
2
performed at room temperature under ambient atmospheric
conditions. Constant current STM imaging was performed in a
While these highly disordered SAMs are disappointing, our
results are nevertheless consistent with the results of Tour and
others who found near-monolayer coverages of high-density,
standing-up molecules of undetermined molecular ordering.
However, our results stand in contrast to the rigorously purified
thioacetate in the work of W o€ ll et al., where highly ordered SAMs
of the lying-down phase of decanethiolate are predominantly
dry N purged atmosphere, under conditions known to routinely
2
produce excellent molecularly resolved images of decanethiol/
2
4
Au(111) SAMs.
STM Studies of Direct Adsorption of C10SAc. SAMs
prepared from C10SAc without a cleaving agent appeared to
consist of high-density upright chains that exhibited almost no
discernible molecular ordering by STM imaging. The Au substrate
step edges and vacancy islands were observed (characteristic of
alkanethiol SAMs). Very rarely observed were small islands of the
2
3
observed. Our C10SAc samples, purified by bulb-to-bulb vacuum
distillation and stored in glass vials, contained traces of decan-
ethiol (0.04%) and decanedisulfide (0.01%). The trace thiol could
be present through co-distillation or through trace hydrolysis on
the glassware, and although it was below the detection limit of
normal NMR measurements, its presence could be determined
by GC-MS measurements. It seems reasonable that trace thiol,
or perhaps even trace disulfide, could adsorb in preference to
thioacetate. As discussed below, the extent of thioacetate cleavage
√
√
expected ( 3 ꢀ 3)R30ꢀ crystalline-molecular order, which were
higher than the surrounding monolayer. (Figure 1 and Supporting
Information Figure S12). Deposition of decanethiol under iden-
tical conditions leads to well-ordered monolayers composed of
√
√
(
3 ꢀ 3)R30ꢀ domains. Attempts to backfill the decanethio-
acetate-produced high-density but molecularly disordered SAM
with decanethiol by re-exposure of the SAM to decanethiol (vapor
or in ethanol at 60 ꢀC) did not increase the molecular ordering of
the SAM; this result indicates that the originally formed disordered
by NH OH under our monolayer-forming conditions is perhaps
4
too limited to provide a sufficiently high concentration of decan-
ethiolate needed for the formation of a uniformly well-ordered
monolayer. Therefore, we propose that either trace thiol or
disulfide already present inour thioacetate samples is the principle
component involved in formation of the monolayers. At the low
(
24) Bumm, L. A.; Arnold, J. J.; Charles, L. F.; Dunbar, T. D.; Allara, D. L.;
Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017.
Langmuir 2010, 26(16), 13221–13226
DOI: 10.1021/la100103k 13223

Article
Singh et al.
Figure 3. C10SAc (46 mM in CD OD) cleavage by NMR moni-
3
Figure 4. C10SAc (46 mM in CD
toring. A: K CO (46mM).B:NH
D: KOH (58 mM).
3
OD) cleavage by NMR moni-
4
OH(575mM).C:DBU(58mM).
4
toring. A: no additive. B: NH OH (115 mM). C: HCl (250 mM). D:
propylamine (70 mM).
2
3
conditions that led to faster and more complete cleavage of the
thioacetate to form decanethiolate should be better for forming
the well-ordered monolayers typically seen when decanethiol is
used directly.
concentrations of decanethiol likely present in our samples,
initially physisorbed thioacetate could inhibit formation of well-
ordered crystalline regions, in a manner analogous to that
reported for disulfides.
The cleavage of thioacetates under basic conditions has been
studied mechanistically and was found to primarily occur by a
slow addition of hydroxide (formed by the reaction of exogenous
bases such as amines to water) to the thioacetate to form a
tetrahedral intermediate that rapidly breaks down to form the
NMR Studies of C10SAc Cleavage. In order to gain insight
into the extent of thioacetate cleavage, we examined NH OH for
4
the cleavage of the thioacetate of decanethiol. In contrast to the
report in the literature that NH OH completely cleaved arylthio-
4
2
5,26
acetates within 10 min in THF-d , we noted a slow cleavage of
cleavage products.
Similarly, protonation would activate the
8
C10SAc (58 mM) and NH OH (115 mM) in CD OD at room
thioacetates toward addition of water or alcohols to form a
tetrahedral intermediate that could decompose to liberate the
thiol. We examined the use of aqueous concentrated HCl for acid-
4
3
temperature under air (58% cleavage after 48 h, Figure 3). During
the time course of this reaction, the disulfide concentration also
steadily increased to 31% after 48 h (Supporting Information
catalyzed cleavage in CD OD and n-propylamineas an additional
3
1
Figure S4). The reaction course was monitored by H NMR
amine base that could potentially react through acyl transfer
to the amine as well as undergo the base-catalyzed cleavage.
Although these reaction conditions led to some cleavage of the
thioacetate, they also did not reach completion within 48 h even at
these relatively high concentrations needed for the NMR mea-
surements (Figure 3). In each case, the amount of disulfide
steadily increased;to 11% after 48 h with HCl and 48% with
propylamine (Supporting Information Figures S5 and S6). Since
the weak base sodium citrate is often used in the production of
spectroscopy by integrating the methylene hydrogen atoms
adjacent to the sulfur atom. These hydrogen atoms give char-
acteristic and different signals in CD OD for the thioacetate
3
(
AcSCH -, 2.85 ppm), the thiol (HSCH -, 2.50 ppm) and the
2
2
disulfide (-CH S-SCH -, 2.68 ppm) groups (see Supporting
2
2
Information Figures S1-S3). The disappearance of the acetyl
methyl group (2.30 ppm) can also be monitored for the extent of
thioacetate cleavage. The detailed hydrolysis or methanolysis fate
2
7
of the acetate group itself was not determined. When the NH OH
gold nanoparticles, we also examined its efficacy to cleave thio-
acetates. When sodium citrate (58 mM) and C10SAc (46 mM in
4
concentration was increased to 575 mM, the cleavage was com-
plete within 6 h (Figure 4). The extent of cleavage in the 2.5 mM
solutions used to grow the SAMs will be considerably lower than
observed in our NMR studies, where higher concentrations were
used to improve NMR signal levels.
CD OD) were reacted, no thioacetate cleavage was observed over
3
48 h;essentially the same result as obtained with methanol-d4
alone.
Turning to stronger bases, thioacetate cleavage by K CO in
2
3
The low extent of thioacetate cleavage observed with NH OH
CD OD was studied; the NMR spectroscopic results indicated
4
3
in the methanol-d correlates with a small fraction of the thio-
acetate converted to thiol in the SAM growth solution. Although
complete cleavage within 24 h with 19% disulfide after 48 h
(Figure 4 and Supporting Information Figure S7). Presumably,
carbonate is more effective at forming a higher concentration of
4
more thiol (or disulfide) is present after treatment with NH OH,
4
-
þ
species. The two strongest bases
most of the thioacetate remains uncleaved. This supports our
the nucleophilic CD O
K
3
earlier speculation and explains why NH OH pretreatment is not
dramatically different than no cleavage agent at all in the forma-
tion of large regions of molecularly ordered SAMs.
studied, KOH and DBU, led to complete cleavage of the thioester
within minutes for KOH and within 2 h for DBU (Figure 4). In
eachcase, disulfide continued tosteadily form over the 48 h period
studied to 30% and 46%, respectively (Supporting Information
Figures S8 and S9). The rapid thioacetate cleavage by these two
reagents can be readily understood in terms of their basicity
4
In order to facilitate the formation of not only high-density and
well-defined, but also molecularly ordered decanethiolate mono-
layers on gold, we examined various exogenous bases for the in situ
cleavage of the thioacetate moiety. Given the range of reagents
and solvents reported for the thioacetate cleavage in the literature,
we initially decided to screen, by NMR spectroscopy, the ability
of several reagents to cleave C10SAc. Since our standard solvent
for SAM formation of alkanethiolates on gold is absolute
(aqueous pK of conjugate acid is about 16 in each case), which
a
-
would lead to the formation of a high concentration of CD O
(K or H-DBU ).
3
þ
þ
(
25) Bruice, T. C.; Fedor, L. R. J. Am. Chem. Soc. 1964, 86, 738.
ethanol, we used methanol-d as a suitable model solvent in these
4
(26) Gregory, M. J.; Bruice, T. C. J. Am. Chem. Soc. 1967, 89, 2121.
spectroscopic measurements. We hypothesized that reagents and
(27) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55.
1
3224 DOI: 10.1021/la100103k
Langmuir 2010, 26(16), 13221–13226

Singh et al.
Article
Figure 5. STM images SAMs on Au(111) grown for 16 h in 2.5 mM C10SAc in EtOH grown from a solution premixed for 1 h with 2.5 mM
of (A) HCl, (B) PrNH
2
, (C) K CO , (D) DBU, (E) KOH, and (F) Bu P. Each sample was grown from 2.5 mM each in ethanol (1 h premix,
2 3 3
1
6 h immersion of Au/mica). All STM images are 100 nm ꢀ 100 nm. Tunneling conditions are Vsample = -1 V and itunnel = 1 pA. See text
for details.
Although we normally do not use THF solutions for alkan-
ethiolate SAM formation on gold, we examined the extent of thio-
acetate cleavage by NMR spectroscopy in THF-d by NH OH,
crystalline order increases to an intermediate level, the structural
domain boundaries appear between neighboring crystalline do-
mains. The disordered regions appear interstitial to some ordered
regions, existing as connected areas and expanded domain
boundaries. Finally, very well-ordered SAMs have a high degree
of crystalline order and the entire surface is composed of domains
with crystalline order directly separated by domain boundaries,
but with little or no disordered regions.
8
4
Cs CO , and DBU in order to provide some additional compari-
2
3
son points to the literature. When no exogenous base was used or
when Cs CO (46 mM) or NH OH (58 mM) were reacted with
2
3
4
C10SAc (46 mM), no cleavage was observed over 48 h. The use of
more concentrated NH OH (173 mM) or DBU (58 mM) did
4
cleave the thioacetate, albeit to a lower extent than when the
We note that, when there is a low to intermediate degree of
order, the disordered regions image lower than the crystalline
regions. However, when there is a high degree of order, the sparse,
isolated disordered regions appear higher. This effect is seen
dramatically in the DBU STM image in Figure 5D. Under-
standing the origin of this effect would require better characteri-
zation of the chemical composition of these dense-disordered
regions that are higher than the ordered regions. .
reaction was studied in methanol-d (29% and 72% cleavage at
4
2
4 h, respectively).
STM Imaging of in Situ Deprotected C10SAc. We pre-
pared companion C10SAc samples to the NMR studies for STM
imaging using different cleaving agents following the procedure
described above for the NH OH cleaved SAMs. Figure 5 shows a
4
resulting set of STM images and companion images at different
scan sizes are given the Supporting Information (Figures S13-
S17). These images were selected to be representative of the
monolayer structure observed over many samples and hundreds
of images. All samples showed Au(111) substrate steps and
vacancy islands, typical of alkanethiol SAMs. The SAM pro-
duced by HCl-treated thioacetate (Figure 5A and Supporting
Information Figure S13) exhibited the smallest fractional area of
ordered regions of this set. The HCl result is very similar to that
Cleavage of Phenylthioacetates. Because most of the litera-
ture reports are for in situ cleavage of arylthioacetates, we extended
our NMR study to the cleavage of phenylthioacetate by NH OH,
4
HCl, KOH, and DBU, but did not attempt to form SAMs from
these samples. In each case, we observed similar extents of cleav-
age as obtained with C10SAc (Supporting Information Figure S11).
Our NMR results are consistent with other cleavage measure-
2
6
ments in the literature. Thioacetate cleavage followed using UV
shows completion after 30 min (0.5 mM aryl SAc with 37.5 mM
found for NH OH, which agrees with their similarly low extent
4
1
6
of cleavage (Figure 3). As expected, n-PrNH produced slightly
NH OH in acetone/MeOH). When the levels of electrode
2
4
better order (Figure 5B). In comparison, the SAM produced
by K CO -treated thioacetate was significantly more ordered
passivation by adsorption were studied using cyclic voltammetry,
it was found that more than an hour of precleavage was required
for the adsorption rate to saturate when strong base at low
concentrations (aryl thioacetate 0.1 mM with 0.27 mM NaOH
2
3
(
Figure 5C and Supporting Information Figure S14). DBU and
KOH are highly effective, showing a high degree of order
1
7
(
Figures 5D and E, respectively).
in EtOH) was used.
The molecular order can be qualitatively assessed visually from
Effect of Added Tributylphosphine. An advantage of the in
situ cleavage methods described here is that all experiments were
run conveniently at ambient temperature with no efforts to
exclude air or to limit the interaction of the cleavage reagent with
the surface. A potential drawback of this approach is that
significant amounts of disulfide can be formed, especially under
prolonged employment of more basic conditions. In order to
combat the disulfide formation, we examined the use of the sacri-
the definition of the structural domain boundaries, which appear
between the crystalline domains of different orientation and sub-
strate registration. The most prevalent domain boundaries are the
missing zigzag row, which are observed as the straight lines
between the different domains and run in the Au Æ110æ crystallo-
graphic directions. When there are few ordered domains, there are
no structural domain boundaries; the well-ordered regions are
crystalline islands surrounded by lower disordered regions. As the
ficial reductant tributylphosphine (Bu P) to convert disulfides to
3
Langmuir 2010, 26(16), 13221–13226
DOI: 10.1021/la100103k 13225

Article
Singh et al.
2
8
thiols. When the NMR-monitored thioacetate cleavage was
performed with C10SAc (46 mM), DBU (58 mM), and Bu3P
(
order produced in the presence of a large excess of uncleaved
thioacetate is likely caused by initial physisorption of the
thioacetate onto the surface in a way that inhibits highly
ordered phases from forming.
58 mM), cleavage was again almost complete within 15 min and
was complete by 2 h. In the presence of Bu P, however, no
3
disulfide was observed even after 48 h (Supporting Information
Figure S10). The monolayer formed on gold/mica (C10SAc
Experimental Section
2
1
.5 mM, DBU 2.5 mM, Bu P 2.5 mM in ethanol, 1 h premixing,
6 h immersion) gave mostly well-ordered domains as was seen
3
Preparation of C10SAc. Following literature precedent,
potassium thioacetate (2.0 g, 17.5 mmol) was added to 1-bromo-
decane (2.0 g, 9.0 mmol) in absolute ethanol (50 mL) at room
temperature under nitrogen. After stirring for 24 h, the solvent
was removed in vacuo. The residue was partitioned between ice-
cold water (100 mL) and diethyl ether (3ꢀ 50 mL). The com-
bined organic portion was washed with water (3ꢀ50 mL), dried
with DBU alone (Supporting Information Figure S17). Thus,
neither the Bu P nor the DBU interfered with the formation of
highly ordered monolayer.
3
When C10SAc in methanol-d was reacted with just Bu P, no
4
3
cleavage was observed by NMR spectroscopy through 48 h. The
Bu P apparently did not produce enough methoxide through
deprotonation of methanol, nor was it nucleophilic enough to
directly cleave the thioacetate in methanol solution. Surprisingly,
companion STM samples using the Bu P as the cleaving agent
produced well-ordered SAMs (Figure 5F);as good as those
produced from the strong bases DBU and KOH (Figure 5D,E).
Although the size of the vacancy islands in the Bu P SAM STM
image are significantly larger than in the other images, this is not
(
MgSO ), and the solvent removed by rotary evaporation. The
4
3
crude C10SAc containingdecanethiol(0.6%) and decanedisulfide
2%) as determined by GC-MS measurements was purified through
two bulb-to-bulb distillation cycles under vacuum (0.2 mm) to give
-decanethioacetate as a pale yellow liquid (1.09 g, 56% yield). The
(
3
1
1
21
H NMR spectrum matched that reported in the literature.
GC-MS analysis of the purified decanethioacetate determined that
trace decanethiol (0.04%) and decanedisulfide (0.01%) were present
in the sample.
3
1
characteristic of the Bu P, but peculiar to this particular sample.
H NMR Spectroscopic Monitoring of Thioacetate Cleavage.
3
It is useful to speculate how additionof Bu P could lead to well-
ordered SAMs without apparent prior cleavage of the thio-
Separate solutions of C10SAc (92 mM in methanol-d
reagent to be investigated (typically 116 mM in methanol-d
mixed in equal portions to give final concentrations of 46 mM in
C10SAc and typically 58 mM in reagent just prior to measuring
4
the first NMR spectrum. These samples in methanol-d were left
in capped NMR tubes at room temperature and were periodically
monitoredbyNMRspectroscopy. The ratiosof thioacetate, thiol,
and disulfide were based on the integration of the methylene
signals at 2.85, 2.48, and 2.67 ppm for these three compounds in
CD OD, respectively.
3
Preparation of Samples for STM. Separate solutions of
2
C10SAc and the reagent to be investigated (KOH, DBU, K CO
aqueous conc. HCl, aqueous 30% NH OH, and DBU/Bu
4
were freshly prepared in absolute ethanol with initial concentra-
tions at 7.5 mM. The solutions of C10SAc and the reagent were
mixed with additional ethanol to give final concentrations of
4
) and the
) were
3
4
acetates. Bu P is a weak base but a good nucleophile. Coordina-
3
tion of the thioacetate to the gold surface may activate the
nucleophilic cleavage of the thioacetate bond through nucleophi-
lic addition of the Bu P to the carbonyl and expulsion of an
3
acetylphosphonium group. Bu P is also a reducing agent;and
3
can convert all disulfides to thiols. The trace disulfide present in
our C10SAc could lead to disordered monolayers that are a
mixture of the disulfide and thioacetate or a mixture of thiol,
disulfide, and thioacetate. Converting all the disulfide to thiol
may allow the thiol to effectively displace any initially adsorbed
thioacetate. Finally, phosphines are known to adsorb weakly on
Au surfaces and can be readily displaced bymore strongly binding
3
,
P)
3
2
9
thiols that form more ordered SAMs. Whatever the mechanism,
2.5 mM in each. The mixture was allowed to stand for 1 h prior to
we observe that equimolar concentrations of Bu P can substan-
3
use allowing time for cleavage to occur. All solution work was
carried out under benchtop (ambient) conditions, with no attempts
to exclude oxygen. H flame-annealed (cleaned) Au(111)/mica
tially improve the quality of our SAMs.
Summary. In conclusion, we can correlate the extent of
cleavage of the thioacetate of decanethiol in methanol-d4 as
determined by NMR spectroscopy with the ability of in situ
thioacetate deprotection schemes to produce larger fractions of
well-ordered crystalline decanethiolate monolayers on Au(111).
The conditions that produced most complete thioacetate cleavage
within 2 h also produced the most ordered monolayers. Addition
2
substrates were then immersed in this solution for 16 h. They were
then removed, rinsed in absolute EtOH, and blown dry with dry
N . All sample preparation was performed at room temperature
2
under ambient atmospheric conditions.
STM Measurements. Constant-current STM imaging was
performed in a dry N purged atmosphere, under conditions
known toroutinelyproduce excellent molecularly resolvedimages
2
of Bu P as a sacrificial reductant was also helpful in this regard.
3
of C10SH/Au(111) SAMs, Vsample = -1 V and itunnel = 1 pA.
Furthermore, based on our results presented here and the cited
literature, we conclude that SAMs formed using thioacetates
of ordinary purity grow from the thiol and/or disulfide
impurities even in the presence of amines and alkali metal
bases from the cleavage reagents. The poor SAM molecular
Acknowledgment. The support of NSF (DMR-0805233) and
Oklahoma State Regents for Higher Education to RLH and LAB
and NSF CAREER (CHE-0239803) to L.A.B. is acknowledged.
Supporting Information Available: Charts of cleavage pro-
gress, STM images for SAMs formed by in situ cleavage of
C10SAc. This material is available free of charge via the Internet
at http://pubs.acs.org.
(
28) Wrochem, F. v.; Scholz, F.; Schreiber, A.; Nothofer, H.-G.; Ford, W. E.;
Morf, P.; Jung, T.; Yasuda, A.; Wessels, J. M. Langmuir 2008, 24, 6910.
29) Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am. Chem.
Soc. 2000, 122, 12890.
(
1
3226 DOI: 10.1021/la100103k
Langmuir 2010, 26(16), 13221–13226