Dithia-crown-annelated tetrathiafulvalene disulfides: Synthesis, electrochemistry, self-assembled films, and metal ion recognition

Article abstract of DOI:10.1021/jo000115l

The synthesis and electrochemistry of a series of tetrathiafulvalene (TTF) and dithia-crown-TTF derivatives attached with one or two disulfide group(s) 7a-f are reported. The self-assembled monolayers (SAMs) of these TTF disulfides on gold were prepared and characterized by reflection - absorption infrared spectroscopy. The SAMs are extremely stable under a wide variety of conditions and over extended periods of time and show remarkable electrochemical stability upon repeated potential scans. SAMs of the crown-TTF disulfides 7c,d,f can recognize alkali metal ions, and the process can be monitored following the electrochemical potential shift of the surface-confined TTF group.

Full text of DOI:10.1021/jo000115l

3
292
J . Org. Chem. 2000, 65, 3292-3298
Dith ia -Cr ow n -An n ela ted Tetr a th ia fu lva len e Disu lfid es: Syn th esis,
Electr och em istr y, Self-Assem bled F ilm s, a n d Meta l Ion
Recogn ition
Sheng-Gao Liu, Haiying Liu, Krisanu Bandyopadhyay, Zhiqiang Gao, and Luis Echegoyen*
Department of Chemistry, University of Miami, Coral Gables, Florida 33124
Received J anuary 27, 2000
The synthesis and electrochemistry of a series of tetrathiafulvalene (TTF) and dithia-crown-TTF
derivatives attached with one or two disulfide group(s) 7a -f are reported. The self-assembled
monolayers (SAMs) of these TTF disulfides on gold were prepared and characterized by reflection-
absorption infrared spectroscopy. The SAMs are extremely stable under a wide variety of conditions
and over extended periods of time and show remarkable electrochemical stability upon repeated
potential scans. SAMs of the crown-TTF disulfides 7c,d ,f can recognize alkali metal ions, and the
process can be monitored following the electrochemical potential shift of the surface-confined TTF
group.
In tr od u ction
Functionalized self-assembled monolayers (SAMs) have
SAMs on gold electrodes of n-mercaptoalkyl tetrathiaful-
valenecarboxylate terminated in a thiol group, even
10
though the SAMs were relatively unstable. (iii) Re-
received considerable attention because of their ability
to easily modify the chemical and physical properties of
the substrate surfaces to which they are bound and to
5a
5b
cently, Reinhoudt et al. and Moore et al. reported the
incorporation of crown-ether groups into SAMs and their
responses as potential metal ion sensors. In one case,
they prepared simple 12-crown-4 and 15-crown-5 deriva-
tives with appended single chains terminated in a thiol
group, and impedance spectroscopy was used to monitor
1
-3
provide a variety of chemically tailored surfaces. SAMs
also offer an easy route to prepare active and fast-
responding sensors if appropriate receptor groups are
linked to a transduction element (such as a redox center
or an optically absorbing group).4 Molecular recognition
events on the surfaces of these well-organized assemblies
can thus be used to monitor and sense specific analytes
in solution. Illustrative examples of receptor molecules
that have been incorporated in SAMs include calix[4]-
5a
the ion binding events on the surface. The other
-6
reported dithia-crown-annelated TTF derivatives that
also contained single alkyl chains terminated in a thiol
5b
group. This work exploited the direct electrochemical
response of the crown-TTF to measure the effect of ion
complexation, similar to the work in homogeneous solu-
1
,4
3
resorcinarenes,
2,2′-thiobisethylacetoacetate, crown
9
tion reported by Becher et al. However, these SAMs were
5
6-8
ethers, and cyclodextrins. In the present work, we are
particularly interested in the use of immobilized crown
ether groups as potential ion sensors in SAMs, employing
the redox active tetrathiafulvalene (TTF) as the trans-
duction group.5b The overall concept is based on the
following observations: (i) Becher et al. reported that
appropriate dithia-crown-TTFs can recognize alkali metal
ions in homogeneous solution due to an inductive effect
apparently not very stable under several conditions, and
the electrochemical responses observed were rather weak
5b
and ill-resolved. Very recently, we reported very stable
SAMs of bis-thioctic ester derivative of TTFs which
11
exhibit well-resolved, surface-confined electrochemistry.
Here we report the synthesis, electrochemistry, SAM
formation and characterization, and alkali metal cation
recognition properties of the SAMs of a series of TTF and
dithia-crown annelated TTF derivatives containing one
or two surface active disulfide group(s) to anchor the
systems to the metal surfaces.
9
on the polarizable TTF system. (ii) Ward et al. described
*
To whom correspondence should be addressed. E-mail: eche-
goyen@miami.edu. Fax: 305 284 4571.
1) Velzen, E. U. T.; Engbersen, J . F. J .; Lange, P. J .; Mahy, J . W.
C.; Reinhoudt, D. N. J . Am. Chem. Soc. 1995, 117, 6853.
2) Offord, D. D.; Sachs, S. B.; Ennis, M. S.; Eberspacher, T. A.;
Griffin, J . H.; Chidsey, C. E. D.; Collman, J . P. J . Am. Chem. Soc. 1998,
20, 4478.
3) Weizman, Y. G. H.; Shanzer, J . L. A.; Rubinstein, I. Chem. Eur.
J .1996, 2, 759.
4) Schonherr, H.; Vancso, G. J .; Huisman, B. H.; van Veggel, F. C.
J . M.; Reinhoudt, D. N. Langmuir 1997, 13, 1567.
5) (a) Flink, S.; Boukamp, B. A.; Berg, A. v. d.; van Veggel, F. C. J .
(
Exp er im en ta l Section
(
Electr och em istr y. All electrochemical experiments were
performed on a BAS 100W electrochemical analyzer (Bioana-
lytical Systems, West Lafayette, IN) at room temperature with
a three-electrode configuration containing 0.1 M tetrabutyl-
1
(
(
6
ammonium hexafluorophosphate (TBAPF ) as the supporting
electrolyte, which was recrystallized twice from ethanol and
dried under vacuum. A glassy carbon (GC, L 3 mm) or the
SAM-modifided spherical gold electrodes were used as the
working electrodes, and the counter electrode was a platinum
(
M.; Reinhoudt, D. N. J . Am. Chem. Soc. 1998, 120, 4652. (b) Moore,
A. J .; Goldenberg, L.; Bryce, M. R.; Petty, M. C.; Monkman, A. P.;
Marenco, C.; Yarwood, J .; J oyce, M. J .; Port, S. N. Adv. Mater. 1998,
1
0, 395.
6) Henke, C.; Steinem, C.; J anshoff, A.;. Steffan, G.; Luftmann, H.;
Sieber M.; Galla, H. Anal. Chem. 1996, 68, 3158.
7) Weisser, M.; Nelles, G.; Wohlfart, P.; Wenz, G.; Mittler-Nher, S.
J . Phys. Chem. 1996, 100, 17893.
8) Rojas, M. T.; Koniger, R.; Stoddart, J . F.; Kaifer, A. E. J . Am.
Chem. Soc. 1995, 117, 325.
(
(9) Hansen, T. K.; J orgensen, T.; Stein, P. C.; Becher, J . J . Org.
Chem. 1992, 57, 6403.
(10) Yip, C.-M.; Ward, M. D. Langmuir 1994, 10, 549.
(11) Liu, H.-Y.; Liu, S.-G.; Echegoyen, L. Chem. Commun. 1999,
1493.
(
(
1
0.1021/jo000115l CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/14/2000

Dithia-Crown-Annelated Tetrathiafulvalene Disulfides
J . Org. Chem., Vol. 65, No. 11, 2000 3293
wire (L 1 mm). An aqueous Ag/AgCl or nonaqueous Ag/Ag+
electrode served as reference. Both the counter and the
reference electrodes were directly immersed in the electrolyte
solution. The scan rate was generally 100 mV s-1 unless
otherwise specified.
Gen er a l Syn th etic P r oced u r e of 2b-d .12 To a -10 °C
mixture of ethylene glycol (tri-, tetra-, or penta-) (0.1 mol), dry
THF (70 mL), and pyridine (3 mL, 37 mmol) was added
103.56, 115.03, 127.24, 131.48. FAB -MS: m/z 530 (M , 100).
HRMS: found 530.0049, calcd for C18 530.0054.
+
+
26 6 6
H O S
Syn th esis of 6e,f. Monoalcohols 6e-f were prepared by
controlled reduction of one of the two ester groups of the
corresponding diester-TTF derivatives using NaBH -LiCl in
a mixed solvent of dry THF and absolute ethanol (4:1 v/v),
similar to the preparation of 6c as described above, or
separated as a minor byproduct during the preparation of 6a
and 6d , respectively. It should be pointed out that only 6f was
separated as a byproduct in the preparation of 6d . 6e: pale
4
14
3
dropwise (ca. 30 min) 7.5 mL of PBr (80 mmol). After the
addition, the mixture was stirred for an additional 5 h at 0 °C
and then left unstirred overnight. THF was removed, the
-
1
yellow solid. FT-IR (KBr, cm ): 3249 (m, -OH), 1709 (s, Cd
1
mixture was extracted with ether and dried over MgSO
4
, and
O). UV-vis (CH
(DMSO-d , ppm): 3.39 (s, 4H), 3.72 (s, 3H), 4.66 (d, 2H, J )
5.90 Hz), 6.21 (t, 1H, J ) 5.93 Hz). C NMR (DMSO-d
ppm): 29.51, 52.79, 59.91, 104.64, 112.85, 112.95, 113.20,
2 2
Cl , λmax, nm): 310, 331, 429. H NMR
the solvent was removed to give a yellow oil, which was
distilled under high vacuum to yield a colorless oil fraction at
ca. 220 °C. The yield was around 60%. The compounds were
6
1
3
6
,
+
1
+
+
characterized by FAB -MS and H NMR, which agreed well
115.07, 158.52, 159.38. MS (FAB ): 382 (M , 100). 6f: pale
1
2
-1
with the published data and the proposed structure.
Gen er a l Syn th etic P r oced u r e of 3b-d . To a 1000 mL
round-bottom flask were added 2b-d (10.0 mmol), the Zn salt
yellow solid. FT-IR (KBr, cm ): 3250 (m, -OH), 1050 (m, CO).
1
H NMR (CDCl , ppm): 3.04 (t, J ) 6.3 Hz, 4H), 3.67-3.72
3
+
(m, 20H), 4.40 (d, J ) 0.9 Hz, 2H), 6.24 (s, 1H). FAB -MS:
+
1
(5.0 mmol), and acetone (500 mL). The mixture was bubbled
28 6 6
m/z 544 (M , 100). HRMS: found 544.0208, calcd for C19H O S
with nitrogen for about 10 min and then heated to reflux under
nitrogen for 2 days. After removal of the solvent, the mixture
544.0210.
Gen er a l P r oced u r e for th e P r ep a r a tion of TTF Bis-
a n d Mon od isu lfid es 7a -f. To a 5 mL of CH Cl were added
was extracted with CH
was then washed with water (3 × 50 mL), separated, and dried
over MgSO and the solvent removed to give a red oil, which
was subjected to column chromatography on silica gel (230-
00 mesh), eluting with a CH Cl /acetone mixture. The yield
2
Cl
2
(3 × 100 mL). The organic solution
2
2
the alcohol-functionalized TTF (0.1-0.2 mmol) and thioctic
acid (2.5 molar equiv for dialcohols 6a -d or 1.2 molar equiv
for monoalcohols 6e-f). The mixture was stirred for 15 min
4
4
2
2
2
at 0 °C (ice/water bath) under N . Then 1,3-dicyclohexylcar-
was about 60-75%. Characterization of compounds 3b-d
bodiimide (DCC) (3 molar equiv for dialcohols 6a -d or 1.5
molar equiv for monoalcohols 6e-f) and 4-(dimethylamino)-
pyridine (DMAP) or 4-pyrrolidinopyridine (0.6 molar equiv for
dialcohols 6a -d or 0.3 molar equiv for monoalcohols 6e-f) in
agreed well with that reported by Becher et al., who synthe-
9
sized the compounds using the dithiolate salts rather than 1.
1
3
Syn th esis of 5c. To a 50 mL round-bottom flask were
added 3c (1.87 g, 4.66 mmol), 4 (1.10 g, 4.66 mmol), and 10
mL of freshly distilled triethyl phosphite. The mixture was
stirred and heated to 110 °C (oil temperature) under Ar for a
period of 2 h. After the excess triethyl phosphite was evapo-
rated under high vacuum, the residue was then subjected to
column chromatography on silica gel (70-230 mesh) eluting
2 2
5 mL of cold CH Cl were added, and the mixture was stirred
for another 15 min at 0 °C. The cooling bath was then removed
and the solution allowed to warm to room temperature. After
being stirred for 48 h under N , the reaction mixture was
2
filtered through a fine glass frit to yield a clear filtrate and
the insoluble urea byproduct as a fine white-gray powder. The
clear filtrate was washed with water (3 × 50 mL), 5% acetic
acid aqueous solution (3 × 20 mL), and finally again with
with 2:1 CH
the eluent was changed to 10:1 CH
fraction was the target compound and gave a 63% yield as a
2
Cl
2
/hexane. After the first fraction was eluted,
2
2
Cl /acetone. The last
water (3 × 30 mL). The organic layer was dried over MgSO
4
,
purple solid. FT-IR (KBr, cm ): 1722 (vs, CdO). 1H NMR
-1
filtered, and evaporated, and the residue was subjected to
column chromatography.
(
CDCl , ppm): 3.01 (t, J ) 6.3 Hz, 4H), 3.64-3.66 (m, 12H),
3
1
3
3
5
1
5
.70-3.75 (m, 4H), 3.83 (s, 6H). C NMR (CDCl
3
, ppm): 35.92,
7a . Silica gel (230-400 mesh) eluting with CH
2 2
Cl afforded
the compound as a red oil. Yield: 84%. 1H NMR (CDCl
,
3.57, 69.84, 70.66, 70.96, 70.98, 108.93, 112.86, 128.39,
3
+
+
32.10, 160.07. FAB -MS: m/z 586 (M , 100). HRMS: found
85.9954, calcd for C20 585.9952.
Syn th esis of 6c. To a 100 mL three-necked round-bottom
ppm): 4.90 (4H, s), 3.56-3.54 (2H, m), 3.29 (4H, s), 3.23-3.15
H
26
O
8
S
6
(4H, m), 2.51-2.41 (2H, m), 2.38-2.33 (4H, m), 1.93-1.90 (2H,
1
3
13
3
m), 1.76-1.71 (8H, m), 1.58-1.55 (4H, m). C NMR (CDCl ,
flask were added 5c (0.8 g, 1.36 mmol), 30 mL of dry THF,
and 7.5 mL of methanol. The mixture was stirred and cooled
ppm): 173.10, 140.17, 129.95, 114.39, 114.36, 106.91, 60.31,
58.30, 56.29, 55.17, 37.00, 35.28, 33.58, 30.60, 25.81, 25.07,
+
to 0 °C. Then, NaBH
2.03 mmol) were added under nitrogen. After being stirred
4
(0.415 g, 10.97 mmol) and LiCl (0.51 g,
24.18, 23.35. UV-vis (CHCl
3
, λmax, nm): 335, 309. FAB -
+
1
MS: m/z 731 (M , 100). HRMS: found 730.9735, calcd for
10 730.9742.
7b. Silica gel (230-400 mesh) eluting with hexane/ethyl
acetate (1:1) afforded the compound as a red-orange solid.
at 0 °C for about 4 h, the cooling bath was removed and stirring
was continued for an additional 1 h at room temperature. Then
the reaction mixture was cooled again, and 25 mL of saturated
26 34 4
C H O S
1
NH
4
Cl aqueous solution was added dropwise at 0 °C. After
3
Yield: 87%. H NMR (CDCl , ppm): 4.91 (4H, s), 3.91 (4H, t,
the addition, the mixture was extracted with AcOEt (3 × 80
J ) 5.34 Hz), 3.74-3.67 (12H, m), 3.58 (4H, t, J ) 5.40 Hz),
3.19-3.14 (4H, m), 2.99 (2H, t, J ) 5.90 Hz), 2.45-2.37 (4H,
m), 1.94-1.85 (2H, m), 1.79-1.63 (8H, m), 1.49-1.46 (4H, m).
C NMR (CDCl
128.82, 111.84, 108.38, 71.98, 71.51, 70.38, 60.38, 58.37, 56.36,
40.62, 38.95, 37.05, 36.19, 34.17, 26.72, 25.07. UV-vis (CHCl ,
3
mL), and the combined organic layers were washed with brine
(
2 × 40 mL), separated, and dried over Na
2 4
SO . After removal
1
3
of the solvent, most of the dialcohol TTF 6c was collected as a
3
, ppm): 173.70, 173.17, 154.47, 142.04, 129.92,
pale yellow solid and purified by washing the mixture with
CH
2
Cl
2
. The filtrate was subjected to flash chromatography
Cl
acetone. The total yield was 85%. FT-IR (KBr, cm ): 3340
+
+
on silica gel (70-230 mesh) eluting with 2:1 (v/v) CH
2
2
/
λ
max, nm): 327.5, 304.0. FAB -MS: m/z 862 (M , 60). HRMS:
found 862.0453, calcd for C32 10 862.0451.
c. Silica gel (230-400 mesh) eluting with 20:1 CH
acetone afforded the compound as a red-orange solid. Yield:
-
1
46 7
H O S
1
(
br, s, -OH), 1090 (s, CO). H NMR (DMSO-d
6
, ppm): 3.02 (t,
7
2 2
Cl /
J ) 6.3 Hz, 4H), 3.51-3.55 (m, 12H), 3.62 (t, J ) 6.2 Hz, 4H),
1
3
4
(
.24 (d, J ) 5.7 Hz, 4H), 5.51 (t, J ) 5.7 Hz, 2H). C NMR
1
8
(
2
0%. H NMR (CDCl
m, 8H), 1.90-1.95 (m, 2H), 2.33-2.38 (m, 4H), 2.44-2.53 (m,
H), 3.02 (t, J ) 6.3 Hz, 4H), 3.13-3.18 (m, 4H), 3.51-3.60
m, 2H), 3.66-3.68 (m, 12H), 3.70-3.76 (m, 4H), 4.89 (s, 4H).
3
, ppm): 1.46-1.48 (m, 4H), 1.65-1.72
DMSO-d , ppm): 34.93, 56.58, 68.94, 69.56, 70.01, 70.03,
6
(
(12) (a) Delaney, P. A.; J ohnstone, R. A. W.; Entwistle, I. D. J . Chem.
1
3
Soc., Perkin Trans. 1, 1986, 1855. (b) Dutton, P. J .; Fyles, T. M.;
Mcdermid, S. J . Can. J . Chem. 1988, 66, 1097. (c) Dann, J . R.; Chiesa,
P. P.; Gates, J . W., J r. J . Org. Chem. 1961, 26, 1991
3
C NMR (CDCl , ppm): 24.72, 25.13, 25.81, 28.85, 33.92,
34.15, 34.76, 35.89, 38.72, 40.42, 49.31, 56.45, 58.16, 69.85,
(
13) The synthesis of diester and alcohol functionalized TTFs 5b,d
and 6a ,b,d were reported in our recent paper: Liu, S.-G.; Echegoyen,
L. Eur. J . Org. Chem., in press.
(14) (a) Liu, S.-G.; Cariou, M.; Gorgues, A. Tetrahedron Lett. 1998,
39, 8663. (b) Fox, M. A.; Pan, H. L. J . Org. Chem. 1994, 59, 6519.

3
294 J . Org. Chem., Vol. 65, No. 11, 2000
Liu et al.
7
0.70, 71.02, 108.43, 111.29, 128.35, 129.78, 133.13, 172.95.
are reported as - log(R/R
0
), where R and R
tivities of the sample and reference, respectively.
0
are the reflec-
+
+
FAB -MS: m/z 906 (M , 100). HRMS: found 906.0709, calcd
for C34 10 906.0713.
d . Silica gel (230-400 mesh) eluting with hexane/ethyl
acetate (1:2) afforded the compound as an orange solid. Yield:
H
50
O
8
S
The transmission infrared spectra of the compounds 7b-d
were recorded for thin films prepared by putting a drop of the
solution of the compound on a quartz cell and then evaporating
with a flow of Ar in the same spectrometer using 500 scans
7
1
8
3
5%. H NMR (CDCl , ppm): 4.90 (s, 4H), 4.07 (4H, t, J ) 6.0
-
1
Hz), 3.73-3.67 (20H, m), 3.49-3.46 (2H, m), 3.13-3.04 (4H,
with 4 cm resolution.
m), 2.45-2.39 (2H, m), 2.37-2.34 (4H, m), 1.96-1.91 (2H, m),
1
.67-1.61 (8H, m), 1.38-1.29 (4H, m). UV-vis (CHCl , λmax,
3
Resu lts a n d Discu ssion
+
+
nm): 325.50, 304.00. FAB -MS: m/z 950 (M , 100). HRMS
+
(FAB ) m/z found 950.0977, calcd for C36
H
54
O
9
S
10 950.0975.
Syn th esis. Generally speaking, most of the reported
5
7
e. Silica gel (230-400 mesh) eluting with CH
2
2
Cl afforded
SAMs on gold are derived from alkanethiols or from
the compound as a red oil. Yield: 85%. 7e was also prepared
following the same procedure but using dry THF instead of
CH
17-19
n-alkyl disulfides.
To generate the thiol group(s), a
well-established method is to convert -Br to -SH by
1
2
Cl
2
to yield 83%. H NMR (CDCl
3
, ppm): 5.34 (2H, s), 3.81
refluxing RBr with thiourea in ethanol followed by
(
2
1
3H, s), 3.61-3.56 (1H, m), 3.30 (4H, s), 3.19-3.14 (1H, m),
hydrolysis with potassium hydroxide.5
b,10,20
However, the
.88-2.66 (1H, m); 2.42-2.38 (2H, m), 1.95-1.90 (2H, m),
.72-1.64 (4H, m), 1.53-1.48 (2H, m). 1 C NMR (CDCl
3
,
3
yield is normally somewhat low, and the SAMs of these
ppm): 172.55, 160.17, 147.14, 121.62, 114.36, 112.62, 108.53,
mono-thiols are typically electrochemically unstable upon
repeated voltammetric cycling.5
b,10
Very recently a novel
6
3
4
0.99, 56.66, 56.10, 53.05, 40.59, 38.90, 35.31, 34.96, 34.08,
0.65, 29.02, 25.87, 25.03, 24.93. UV-vis (CHCl
3
, λmax, nm):
TTF compound containing four thiol groups was reported,
which formed very robust SAMs on gold electrodes, and
these were remarkably stable after repeated voltammet-
+
+
+
32, 332, 305. FAB -MS: m/z 570 (M , 100). HRMS (FAB ):
m/z found 569.9283, calcd for C19
22 4 8
H O S 569.9284.
7
f. Silica gel (70-230 mesh) eluting with 10:1 CH
2
Cl
acetone afforded a red-orange solid. Yield: 90%. H NMR
CDCl , ppm): 6.34 (s, 1H), 4.82 (s, 2H), 3.73-3.67 (m, 20H),
.49-3.46 (m, 1H), 3.17-3.14 (m, 2H), 3.06-3.02 (m, 4H),
.45-2.39 (m, 1H), 2.37-2.34 (m, 2H), 1.96-1.91 (m, 1H),
2
/
21
ric cycling. The use of such multiple anchoring sites
1
provides very strong adherence of the compound to the
metal surfaces, especially if the sulfur atoms are present
within the same ring structure, in which case they exhibit
(
3
3
2
1
1
+
+
22
.67-1.61 (m, 4H), 1.38-1.29 (m, 2H). FAB -MS: m/z 732 (M ,
00). HRMS: found 732.0539, calcd for C27 732.0540.
Mon ola yer P r ep a r a tion . A L 250 µm gold wire (99.9999%)
a chelate effect. For these reasons, the use of thioctic
H
40
O
7
S
8
acid derivatives to anchor SAMs on metal surfaces has
received considerable recent attention.1
7-19
In addition,
was cleaned by boiling in 37% nitric acid overnight and rinsed
with copious amounts of 18 MΩ water (Barnstead). Spherical
gold electrodes were prepared by heating the wire in a natural
thioctic acid is commercially available and easy to
incorporate into a wide variety of structures via simple
condensation reactions, as reported in the present work.
This method is much easier and more effective, involving
a single reaction and affording high yields by attaching
thioctic acid to TTF derivatives incorporating hydroxy
group(s) (Scheme 1), which affords a mild and direct
introduction of the disulfide group(s) to the TTF deriva-
2
gas/O flame until a small gold sphere formed on the end of
the wire followed by cooling in the water (Barnstead). This
process leads to the formation of well-defined Au(111) facets,
1
5
as described in earlier reports. The gold wire was then sealed
into a glass capillary leaving only the gold sphere exposed.
2
The gold sphere was annealed in the natural gas/O flame
again and cooled under an Ar atmosphere before monolayer
preparation. The geometric areas of the gold electrodes were
calculated from the slopes of the linear plots of the cathodic
peak current versus the square root of the scan rate obtained
tives. Moreover, the SAMs of the TTF disulfides are much
10,5b
more stable than those previously reported.
Thus,
TTF disulfides 7a -f were prepared according to the
procedures shown in Scheme 1 starting from 4,5-ethyl-
enedithio-1,3-dithiole-2-thione (3a ), 5,8,11-trioxa-2,14,-
16,18-t et r a t h ia bicyclo[13.3.0]oct a dec-1(15)-en e-17-
thione (3b), 5,8,11,14-tetraoxa-2,17,19,21-tetrathiabicyclo-
3
+
for the diffusion-controlled reduction of Ru(NH
3
)
6
. We em-
-
6
2
ployed a diffusion coefficient of 7.5 × 10 cm /s (at 25 °C in
1
0
0
.1 M NaCl). Typical values for the geometric area of the
2
electrode varied between 0.01 and 0.02 cm . Surface coverages
of the SAMs of the TTF derivatives were calculated by
integration of the current during the first scan.
[
16.3.0]hexaicos-1(18)-ene-20-thione (3c), or 5,8,11,14,-
1
1
7-pentaoxa-2,20,22,24-tetrathiabicyclo[19.3.0]tetraicos-
Monolayers were typically prepared by immersing the
freshly prepared gold-bead electrodes into a THF solution
containing the appropriate disulfides in mM concentrations
(21)ene-23-thione (3d ), all of which were synthesized
9
according to well-established procedures with a slight
modification (Scheme 1). The modification consisted of
reacting bis(tetrabutylammonium)-bis(1,3-dithiole-2-thi-
1
1,16
for 24-48 h.
To investigate the possibility of a solvent effect
Cl were
on the formation of the monolayers, DMSO and CH
2
2
also employed. After removal from solution, the gold-bead
electrodes were rinsed with the appropriate solvent and dried
in air.
one-4,5-dithiolate)-zincate(II), the Zn salt 1, (n-Bu
4 2
N) -
Zn(dmit) , rather than the dithiolate salt reported by
2
9
Becher et al. with the corresponding 1,2-dibromoethane
(2a ) or dibromo glycols (2b-d ) in acetone at reflux for
about 2 days under nitrogen using high-dilution tech-
niques (see Experimental Section). The yields (60-75%)
for 3b -d are reasonably high compared to those of
In fr a r ed Sp ectr oscop y. For monolayer characterization,
the reflection-absorption infrared spectra (RAIRs) were re-
corded on a Perkin-Elmer 2000 FT-IR spectrophotometer,
equipped with a grazing angle (80°) infrared reflection acces-
sory and a ZnSe wiregrid polarizer from International Crystal
Laboratory. The spectra were recorded with a liquid nitrogen
cooled MCT detector and the measurement chamber was
continuously purged with nitrogen gas during the measure-
(
(
(
17) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1992, 64, 1998.
18) Wang, Y.; Kaifer, A. E. J . Phys. Chem. B 1998, 9922.
19) Blonder, R.; Willner, I.; Buckmann, A. F. J . Am. Chem. Soc.
-
1
ments. Typically 1000 scans with 4 cm resolution were per-
formed to get the average spectra. A clean and freshly prepared
gold plate was used to record the reference spectra. The RAIRs
1
998, 120, 9335.
20) (a) Creager, S. E.; Rowe, G. K. J . Electroanal. Chem. 1994, 370,
203. (b) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993,
, 786. (c) Hudhomme, P.; Blanchard, P.; Salle, M.; Moustarder, S. L.;
(
9
Riou, A.; J ubault, M.; Gorgues, A.; Duguay, G. Angew. Chem., Int. Ed.
Engl. 1997, 36, 878.
(21) Fujihara, I.; Nakai, H.; Yoshihara, M.; Maeshima, T. Chem.
Commun. 1999, 737.
(
15) Schneir, J .; Sonnenfeld, R.; Marti, Q.; Hansma, P. K.; Demuth,
J . E.; Hamers, R. J . J . Appl. Phys. 1988, 63(3), 717.
16) Arias, F.; God ´ı nez, L. A.; Kaifer, A. E.; Echegoyen, L. J . Am.
Chem. Soc. 1996, 118, 6086.
(
(22) Shon, Y.-S.; Lee, T. R. Langmuir 1999, 15, 1136.

Dithia-Crown-Annelated Tetrathiafulvalene Disulfides
J . Org. Chem., Vol. 65, No. 11, 2000 3295
Sch em e 1. Syn th etic P r oced u r es for Com p ou n d s 7a -f
Ch a r t 1
Ta ble 1. Electr och em ica l Da ta of 7a -f in THF -TBAP F 6
-
1
a t Room Tem p er a tu r e w ith a Sca n Ra te of 100 m V s
compd
E1/21 (∆Ep)/mV
E1/22 (∆Ep)/mV
7
7
7
7
7
7
a
b
c
d
e
f
655 (55)
644 (67)
685 (58)
637 (67)
726 (66)
637 (52)
866 (52)
832 (80)
822 (56)
790 (77)
949 (58)
799 (62)
TTF derivatives 6. In general, the reduction products
should be mixtures of mono- and dialcohol TTFs and the
separation could be difficult. However, only trace amount
of the monoalcohol TTF was present in the reaction
mixture for all the cases and, due to the great differential
solubility between the mono- and dialcohol products in
9
Becher’s. It should be pointed out that the large scale
synthesis of 2b-d was reported long ago. Our procedure
was slightly different, since THF was used as the solvent
and the reaction gave higher yields. Cross coupling
between the 1,3-dithiole-2-thione 3a -d and 4,5-bis-
CH
2
Cl
2
, separation was accomplished by washing the
Cl (6a -c). Alter-
crude product mixture with cold CH
2
2
natively, due to their polarity difference, analytically pure
di-alcohols were readily obtained by a flash column
chromatography on silica gel (70-230 mesh) eluting with
(
methoxycarbonyl)-1,3-dithiole-2-one (4) in neat triethyl
phosphite at 110-140 °C afforded the unsymmetrically
substituted crown-ether TTF diester 5 in reasonably high
yields (40-60%). Theoretically speaking, symmetric TTF
derivatives (Chart 1) tetramethoxycarbonyltetrathia-
fulvalene (8) and bis(ethylenedithio)tetrathiafulvalene
a CH
:2 CH
synthesized in high yield by direct coupling between the
corresponding alcohols with thioctic acid in CH Cl in the
2
Cl
2
/acetone mixture (6c: 2:1 CH
2 2
Cl /acetone; 6d :
5
2
2
Cl /acetone). Finally, compounds 7a -f were
2
2
presence of DCC and DMAP or 4-pyrrolidinopyridine. It
should be stressed that this direct coupling reaction to
prepare TTF disulfides is general, straightforward and
gives high yields (>80%).
(
BEDT-TTF), or 1,4,5,6-tetrahydro-2(3),6(7)-bis(4,7,10-
trioxa-1,13-dithiatridecane-1,13-diyl)-1,4,5,8-tetrathiaful-
valene (9), or 1,4,5,6-tetrahydro-2(3),6(7)-bis(4,7,10,13-
tetraoxa-1,16-dithiahexadecane-1,16-diyl)-1,4,5,8-tet-
rathiafulvalene (10), or 1,4,5,6-tetrahydro-2(3),6(7)-bis-
4,7,10,13,16-pentaoxa-1,19-dithianonadecene-1,19-diyl)-
,4,5,8-tetrathiafulvalene (11) could be formed in addition
to the unsymmetrical ones (5a -d ) during such cross-
coupling reactions. However, except for 8 which was
formed in all cases and for a small amount BEDT-TTF
formed during the preparation of 5a , we did not observe
any symmetric dithia-crown annelated TTF derivatives
9, 10, or 11, Chart 1) during the preparation of 5b-d .
This reflects the low reactivity of the 1,3-dithiole-2-thione
b-d to self-coupling.11 Reduction of the two ester groups
of 5 using NaBH and LiCl in a mixed solvent of THF
and methanol (4:1 v/v)14 gave the corresponding dialcohol
Electr och em istr y a n d Com p lexa tion Stu d ies in
Solu tion . The electrochemistry and the complexation
(
1
+
properties of compounds 7a -f with alkali metal ions (Li ,
+
+
Na , and K ) in solution were investigated by cyclic
voltammetry.
As shown in Table 1, TTF derivatives 7a -f undergo
6
two, one-electron, redox processes in THF-TBAPF on
a glassy carbon electrode, giving two pairs of well-defined
9
,23,24
redox peaks with ∆E
p
<80 mV, typical for TTFs.
The
(
oxidation potentials (except for the first oxidation of 7c),
3
(
23) (a) For a recent review, see: Bryce, M. R. J . Mater. Chem. 1995,
, 1481. (b) Devonport, W.; Bryce, M. R.; Marshallsay, G. J .; Moore, A.
J .; Goldenberg, L. M. J . Mater. Chem. 1998, 8, 1361.
4
5

3
296 J . Org. Chem., Vol. 65, No. 11, 2000
Liu et al.
Ta ble 2. Electr och em ica l Recogn ition for 7b,c,d (1.0
m M) in th e P r esen ce of Differ en t Alk a li Meta l Ion s (ca . 5
m M) in THF Solu tion
7
b
7c
7d
ions
E1/21
E1/22
E1/21
E1/22
E1/21
E1/22
LiPF6
NaPF6
KPF6
0
+10
0
0
0
0
0
+11
+14
0
+10
+11
0
+40
+15
0
+30
+20
especially for the second oxidation, show a small but
evident trend: 7a > 7b > 7c > 7d . This correlates with
the electron-donating ability of the crown-ether group.13
In addition, the current varies linearly with the square
root of the scan rate indicative of diffusion controlled
processes and the peak-to-peak separation increases
slightly with the scan rate indicating a slow electron-
transfer process.
F igu r e 1. CVs of 1.0 mM of 7d in THF solution containing
0
.1 M TBAPF
The working electrode was a platinum electrode, and Ag/AgCl
+ 0.1 M TBAPF in THF was the reference electrode.
6 6
in the absence and presence of 5.0 mM NaPF .
Complexation in homogeneous solution was also stud-
ied by cyclic voltammetry. As discussed above, 7a -f
exhibit two, one-electron, reversible oxidation waves. The
CVs were also recorded after addition of ca. 5 molar equiv
6
crown-ether annelated TTFs 9, 10, and 11 (Chart 1)
shows that for 7b the shifts are identical to those
observed for 9, while those of 7c,d , are different from
those of 10 or 11. Both Eox1 and Eox2 changed for 7c,d
upon metal ion additions, but only the first oxidation
potentials were positively shifted for 10 or 11. The
+
+
+
+
of MPF
solutions. The results are summarized in Table 2. As can
be seen the addition of 5 mM LiPF , NaPF , or KPF to
a 1.0 mM solution of 7a or 7e in THF-TBAPF system
6
(M ) Li , Na , or K ) to THF-TBAPF
6
6
6
6
6
+
+
+
had no effect on the observed potentials, indicating that
the alkali metal ions do not interact with the thioctic
potential shifts for 7c follow the order Na ∼ K > Li ,
for 7d it is Na > K > Li , but for 10, the trend is Na
+
+
+
+
+
+
+
+
+
acid-TTF moieties. Addition of 5 mM NaPF
6
to a 1.0 mM
> K > Li and for 11, the order is K > Na > Li .
THF solution of 7b results in a 10-mV positive potential
Ch a r a cter iza tion of Mon ola yer s by Reflection -
Absor p tion In fr a r ed Sp ectr oscop y (RAIR). RAIR
spectroscopy was used to provide information about
monolayer formation and also about the degree of orga-
nization. The conventional transmission spectrum of
•+
shift of the first redox peak (TTF/TTF ), while the second
+
.
2+
reduction process (TTF /TTF ) remains unchanged.
These observations are similar to those previously re-
ported for analogous compounds 9-11 (Chart 1) in
homogeneous solution and could be interpreted in terms
of complex dissociation due to a repulsive interaction
compound 7b (spectrum not shown here) shows peaks
-1
at 2925 and 2852 cm
due to the asymmetric and
in the alkyl
chains, respectively. A strong band is observed at 1731
+. 9
+
+
after initial formation of TTF . However, the Na or K
symmetric stretching modes of the -CH
2
induced potential shift for either 7c or 7d in THF solution
is between +10 and +40 mV for both the first and second
redox processes (Table 2). This is different from previous
observations with similar compounds.9
25
-1
26
cm that is due to the ester -CdO group. Peaks at
-1
1
157 and 1129 cm are assigned to the symmetric -C-
27
O-C- stretching from the anchoring chain. Moreover,
As an example, in the case of 7d , the shifts in the
-1
a peak at 1232 cm is due to the antisymmetric -C-
+
•+
presence of Na are +40 mV (TTF/TTF ) and +30 mV
O-C- band. The transition dipole of the former band
should be aligned parallel to the axis bisecting the
C-O-C angle while that of the latter should be perpen-
•
+
2+
(
TTF /TTF ), for the first and second oxidations, re-
spectively (Figure 1). K also causes a positive potential
shifts, but smaller than those caused by Na (Table 2).
As shown in Table 2, the affinity sequence for 7d is Na
> Li , for 7c, it is Na ∼ K > Li .
On the whole, it can be concluded that (i) the observed
shifts, as expected, were always anodic (more positive);
ii) the shifts, if any, of the first oxidation peak were
always larger than those for the second one; (iii) no Li
effect was observed with any of the compounds; (iv) the
shifts for compound 7d are larger than those for com-
pounds 7b or 7c; and (v) the largest shifts observed for
compound 7d were found in the presence of sodium ions,
+
+
dicularly aligned. Besides these, absorbances at 891, 783,
+
-1
7
31, and 668 cm are assigned to various -C-S stretch-
+
+
+
+
+
>
K
-1
ing modes of the TTF moiety and the peak at 1540 cm
corresponds to the CdC ring stretch of the TTF ring.
10
Since compounds 7 (b, c, and d ) are structurally similar,
only data for 7b are presented, both from the conven-
tional transmission spectrum along with those of the
SAM on a gold surface; see Table 3.
Figure 2 shows the RAIR spectra of monolayers of
three different compounds at two different frequency
ranges. Comparisons of the conventional transmission
spectrum with those of the monolayers in the higher
frequency region reveal that for all three monolayers the
(
+
+
+
+
with a general order of Na > K > Li .
Evidence that the shifts observed arise from complex-
ation comes from the fact that 7a and 7e (no crown ether
present in either) showed no cation effects and 5b and
2
peak positions for the -CH symmetric and antisymmet-
ric stretching are the same or shifted to higher wave-
number, suggesting a disordered or liquidlike packing
5
d , which do possess a crown-ether moiety, showed
1
3
almost the same behavior.
28
environment of the methylene chains in the monolayers.
Comparing the shifts observed for 7b-d with those
9
obtained by Becher et al. for the analogous symmetric
(25) Allara. D. L.; Nuzzo. R. G. Langmuir 1985, 1, 52.
(
26) Socrates, G. In Infrared Characteristic Group Frequencies; J ohn
(24) Moore A. J .; Bryce, M. R.; Batsanov, A.-S.; Green, A.; Howard,
Wiley & Sons: 1994.
J . A. K.; McKervey, M. A.; McGuigan, P.; Ledoux, I.; Orti, E.; Viruela,
R.; Viruela, P. M.; Tarbit, B. J . Mater. Chem. 1998, 8, 1173.
(27) Pertsin, A. J .; Grunze, M.; Garbuzova, I. A. J . Phys. Chem. 1998,
102, 4918.

Dithia-Crown-Annelated Tetrathiafulvalene Disulfides
J . Org. Chem., Vol. 65, No. 11, 2000 3297
F igu r e 2. RAIR spectra of the monolayers of 7b,c,d in two
different frequency ranges.
Ta ble 3. Vibr a tion a l Mod e Assign m en t (cm -1) of Ma jor
P ea k s Obser ved by Tr a n sm ission IR Sp ectr a of 7b a n d
by RAIR Sp ectr oscop y of Differ en t Mon ola yer s on a
Gold Su r fa ce
transmission
RAIR
RAIR
RAIR
IR of 7b
of 7b
of 7c
of 7d
assignment
2
2
1
1
1
1
1
925
852
731
540
232
157
129
2928
2848
1737
1571
1241
1175
1122
2925
2851
1731
2925
2851
1737
νas(-CH2)
νa(-CH2)
ν (-CdO)
ν (CdC) central
νas (-C-O-C-)
νs (-C-O-C-)
1246
1175
1122
946
859
740
1245
1178
1124
950
860
744
9
8
7
7
6
74
91
83
31
68
ν (C-S) from TTF
893
666
672
670
F igu r e 3. CVs of SAMs of 7c,f in THF solution containing
This is expected since molecules with short alkyl chains
are known to form disordered monolayers. Moreover, the
presence of crown ether terminal groups is probably also
responsible for the less ordered structure of these mono-
layers. It is apparent from the peak positions that
increasing the size of the appended crown results in more
disordered monolayers, as expected.
0
.1 M TBAPF
6
at different scan rates of 100 (the lowest
current), 200, 400, 600, and 1000 (the highest current)
-
1
mV s
.
this case is oriented almost perpendicular to the surface.
2
9
Calculations based on the -C-O-C- vibration suggest
a ∼60° inclination of the -C-O-C- portion attached to
the TTF moiety for the monolayer of 7d and ∼80° for that
of 7b, with respect to the surface. The corresponding
angle is 68° for the monolayer of 7c. These differences
in orientation may be due to the changes in the size of
the terminal crown ether group.
Comparison of the RAIR spectra in the lower frequency
-
1
range (1800-600 cm ) provides information about the
orientation of the molecules. For the monolayer of 7b, a
very weak peak was observed for the -CdO stretching
-1
at 1737 cm . The peak due to the -CdC- from the rings
and -C-O-C- stretching are also very weak. On the
other hand, the -C-S stretching at 666 cm- is very
intense (Figure 2) compared to the one observed in the
conventional transmittance spectrum. From the surface
selection rule this differential spectral intensity indicates
that the two -CdO groups and the -C-O-C- from the
anchoring chain as well as the -CdC- in the rings are
oriented almost parallel to the metal surface while the
Electr och em ica l a n d Recogn ition P r op er ties of
th e SAMs of th e Disu lfid es. Figure 3 presents two
typical surface-confined CVs of the SAMs of TTF disul-
fides 7c and 7f. The electrochemical data are collected
in Table 4. As can be seen in Figure 3 and Table 4, for
all cases studied the redox peak currents are proportional
to the scan rate and the peak shapes, peak potentials
and peak-to-peak separations are independent of the scan
1
-
1
-
C-S- part from the crown ether is more perpendicular.
rate up to 1000 mV s , indicative of well-behaved
surface-confined electrochemical processes. The anodic
and cathodic peak-to-peak separations are less than 20
mV in all cases, and the full width at half-maximum
(fwhm) is 95-110 mV in THF solution. For surface-
confined redox centers where no lateral interactions exist
In contrast, the RAIR spectra of the monolayers of 7c
and 7d show an intense peak for the -CdO stretching
and a strong absorption due to the -C-O-C- symmetric
vibrations but none is observed for the -C-S stretching.
This observation strongly suggests that -CdO group in
(
28) Porter, M. D.; Thomas, B. B.; Allara, D. L.; Chidsey, C. E. D. J .
(29) Han, S. W.; Ha, T. H.; Kim, C. H.; Kim, K. Langmuir 1998, 14,
6113.
Am. Chem. Soc. 1997, 109, 3559.

3
298 J . Org. Chem., Vol. 65, No. 11, 2000
Liu et al.
Ta ble 4. Electr och em ica l Da ta of SAMs of th e TTF
Ta ble 7. Electr och em ica l Sh ift for th e SAMs of 7b-d in
Disu lfid es in THF -TBAP F 6 System a t Room
th e P r esen ce of Differ en t Alk a li Meta l Ion s (ca . 5 m M)
-
1
Tem p er a tu r e w ith a Sca n Ra te of 100 m V s
SAMs of 7b
SAMs of 7c
SAMs of 7d
metal
ions
SAMs of the
disulfides
E1/21
E1/22
E1/21
E1/22
E1/21
E1/22
E1/21 (∆Ep)/mV
E1/22 (∆Ep)/mV
coveragea
LiPF6
NaPF6
KPF6
0
0
0
0
0
+40
+45
0
+35
+38
0
+60
+20
0
+55
+30
7
7
7
7
7
7
a
b
c
d
e
f
654 (25)
641 (12)
661 (9)
623 (20)
719 (9)
597 (7)
904 (15)
846 (5)
841 (7)
798 (15)
996 (13)
817 (7)
1.58
1.16
1.79
0.86
2.21
1.07
+10
0
dependent (Table 5), similar to what is observed in
solution, typical of TTF electrochemistry.33 In aqueous
solution, it is impossible to detect a CV response for any
of the SAMs studied.
a
In units of 10-10 mol/cm-2
.
Ta ble 5. Solven t Effect on th e Electr och em istr y of th e
SAMs of 7b a t Room Tem p er a tu r e w ith a Sca n Ra te of
For comparison, solution electrochemical data of 7b are
summarized in Table 6 in different solvents.
-
1
1
00 m V s
The alkali metal cation recognition abilities of the
SAMs of 7a -f were checked by CV in the presence of
different alkali metal ions and the data are summarized
in Table 7. The results of 7f are almost identical to those
of 7d , which contain the same dithia-crown ether. As
reported in homogeneous solution, a significant anodic
shift was observed for both the first and second redox
processes of the SAMs of 7b,c,d ,f in THF solution when
solvent
E1/21 (∆Ep)/mV
E1/22 (∆Ep)/mV
THF
CH2Cl2
ODCB
CHCl3
641 (12)
502 (22)
557 (42)
543 (19)
846 (5)
769 (9)
828 (46)
799 (15)
a
a
o-Dichlorobenzene.
Ta ble 6. Solven t Effect on th e Electr och em istr y of 7b in
Solu tion a t Room Tem p er a tu r e w ith a Sca n Ra te of 100
+
Na was added. However, there is no potential shift
-
1
observed for the SAMs of 7a or 7e, in which no crown
m V s
ether group is present. These results indicate that the
solvent
E1/21 (∆Ep)/mV
E1/22 (∆Ep)/mV
+
potential shifts result from the interaction of M with
THF
CH2Cl2
ODCB
644 (67)
550 (75)
524 (62)
832 (80)
875 (75)
839 (72)
the dithia-crown ether of 7b,c,d ,f. The affinity sequence
+
+
+
for the SAMs of 7b,d ,f was Na > K > Li . However,
+
+
+
for 7c, the order is Na ∼ K > Li , similar to the solution
behavior.
between them and in rapid equilibrium with the elec-
trode, identical surface waves with a zero peak-to-peak
separation and a peak width of 90.6/n mV are observed
Con clu sion s
(
90.6 mV fwhm for a one-electron transfer).30-32 The fact
A general, effective and straightforward synthetic
procedure has been developed to prepare dithia-crown
annelated TTF disulfides with high yields. The self-
assembled monolayers (SAMs) of these TTF disulfides
on gold spheres have been prepared and characterized
by reflection-absorption infrared spectroscopy, which
show different orientations due to changes in the termi-
nal crown size. These electrochemically active SAMs
showed well-defined surface confined redox waves char-
acteristic of the TTF moiety, extreme stablity under a
wide variety of conditions and over extended periods of
time and especially, remarkable electrochemical stability
upon repeated potential scans. SAMs of the crown-TTF
disulfides 7c,d ,f can recognize alkali metal ions and the
process can be easily monitored following the potential
shift of the surface-confined TTF group. These recognition
properties and almost indefinite stability show promise
as potential thin-film sensors for electrochemically inac-
tive metal ions.
that the peak-to-peak separations are nonzero but inde-
pendent of scan rate indicates that slow charge transfer
kinetics are not the cause of the observed results. Similar
behavior has been previously observed for surface-
confined redox centers, and explained on the basis of
nonequilibrium states due to slow rate processes, such
as electron transfer from the TTF redox center to the gold
electrode via tunneling.
The formal potentials of the SAMs of 7d are slightly
shifted cathodically compared with those of the SAMs of
b,c. The formal potentials of the SAMs of 7a are higher
than those of 7b. The formal potentials of SAMs of 7e
are 60 mV higher than those of the SAMs of 7a , surely
7
the result of the -CO
2 3
CH group in 7e. The SAMs of
a -f are very stable and their electrochemical responses
7
remain essentially unchanged, especially for those of 7a -
d , after more than 100 scans because the four sulfurs
can bind strongly to the gold surface. By far these are
the most robust monolayers that we have ever worked
with.
Ack n ow led gm en t. This work was supported by the
National Science Foundation, grant CHE-9816503.
It should be pointed that the electrochemical behavior
of the SAMs of the TTF disulfides is highly solvent
J O000115L
(
(
(
30) Chidsey, C. E. D. Science 1991, 251, 919
31) Laviron, E. J . Electroanal. Chem. 1974, 52, 395.
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