An unusual self-assembly of a deuteroporphyrin 4-aminothiophenol derivative on Au(111) surfaces

Article abstract of DOI:10.1039/c3nj00118k

An unexpected ruthenium(ii)-deuteroporphyrin 4-aminothiophenol (4-ATP) thiolester derivative (Ru-4ATPP) was obtained by the condensation of 4-ATP with carbonyl[13,17-bis(propanoic acid)-2,7,12,18-tetramethylporphyrinato] ruthenium(ii). Ru-4ATPP was self-assembled on Au(111) surfaces through the terminal amino groups, as shown by cyclic voltammetry (CV) and confirmed by X-ray photoelectron spectroscopy (XPS). A slow and incomplete isomerisation process of the self-assembled 4-RuATPP thiolester to its amido analogue was observed, due to the catalytic effect of the gold surface. Consequently, the formed amido isomer of Ru-4ATPP became self-assembled on Au(111) through the resulting terminal thiolate groups, a condition characterized by CV reductive desorption (peak at ca. -0.7 eV) and XPS (S2p peaks at 162.1 and 163.3 eV).

Full text of DOI:10.1039/c3nj00118k

NJC
View Article Online
View Journal
PAPER
An unusual self-assembly of a deuteroporphyrin
4-aminothiophenol derivative on Au(111) surfaces†
Cite this: DOI: 10.1039/c3nj00118k
Rudy Martin,a Roberto Cao,*a Franz-Peter Montforts*b and
Paul-Ludwig M. Noeskec
An unexpected ruthenium(II)-deuteroporphyrin 4-aminothiophenol (4-ATP) thiolester derivative (Ru-4ATPP)
was obtained by the condensation of 4-ATP with carbonyl[13,17-bis(propanoic acid)-2,7,12,18-
tetramethylporphyrinato]ruthenium(II). Ru-4ATPP was self-assembled on Au(111) surfaces through the
terminal amino groups, as shown by cyclic voltammetry (CV) and confirmed by X-ray photoelectron
spectroscopy (XPS). A slow and incomplete isomerisation process of the self-assembled 4-RuATPP
thiolester to its amido analogue was observed, due to the catalytic effect of the gold surface. Consequently,
the formed amido isomer of Ru-4ATPP became self-assembled on Au(111) through the resulting terminal
thiolate groups, a condition characterized by CV reductive desorption (peak at ca. À0.7 eV) and XPS
(S2p peaks at 162.1 and 163.3 eV).
Received (in Porto Alegre, Brazil)
30th January 2013,
Accepted 24th March 2013
DOI: 10.1039/c3nj00118k
www.rsc.org/njc
presence of aromatic compounds as spacer arms between the
latter and the surface is recommended in order to guarantee an
1. Introduction
Recently we reported a gold electrode modified with a
ruthenium(II)-deuteroporphyrin alkyldisulfide derivative
efficient electron transfer (ET) process. In this sense, 4-amino-
thiophenol (4-ATP) constitutes an adequate alternative. 4-ATP
has been used for the self-assembly on metal nanoparticles3–7
and electrodes.8–11 The rigidity conferred to the monolayer, the
delocalized p-electrons of the aromatic ring, as well as the
presence of an amino group as a scaffold for further modifica-
tions are some of the advantages in the use of such a compound
in the formation of SAMs on gold surfaces.12 SAMs are known to
modify the properties of metal surfaces, a process that could
catalytically favor different types of reactions. Nevertheless, such
studies have received very little attention.
SAMs of 4-ATP on gold surfaces have been characterized by
CV,4,6,8,11–14 XPS,3,4,6,8,13,15,16 and scanning tunnelling micro-
scopy (STM),11,13,17 among other techniques. It is well known
that in alkali medium, voltammograms of thiolated derivative
SAMs can offer a sharp reduction peak as a result of the
disruption of Au–SR bonds.18–23 For SAMs of aromatic thiols
on gold, a wide range of values has been reported (À0.6 to À1.5 V)
especially for thiophenol and its derivatives.11,13,24–27 In the
particular case of SAMs of 4-ATP and also its 3- and 2-ATP
isomers the reductive desorption at low pH gave peaks between
([Ru(Pds)(CO)]). The modified electrode was successfully tested
for the electrochemical detection of nitric oxide in aqueous
solution.1 The formed self-assembled monolayer (SAM) of
[Ru(Pds)(CO)] was characterized by cyclic voltammetry (CV) and
X-ray photoelectron spectroscopy (XPS). The recorded CV reductive
desorption peak at À0.92 V indicated the association of the SAM
through the thiolate groups after the rupture of the S–S bond.
Self-assembled monolayers have been widely employed to
bestow new functionalities of metal surfaces.2 When metal
electrodes are modified with SAMs containing metal ions the
a
´
Laboratorio de Bioinorganica, Universidad de La Habana, Zapata y G, Vedado,
La Habana, 10400 Cuba. E-mail: caov@fq.uh.cu; Fax: +53-7333502;
Tel: +53-78792145
b Institute of Organic Chemistry, University of Bremen, Leobener Strasse NW 2,
28359, Bremen, Germany. E-mail: mont@uni-bremen.de; Fax: +421-218-63120;
Tel: +421-218-63120
c Fraunhofer-Institute for Manufacturing Technology and Applied Materials Research
(IFAM), Adhesive Bonding Technology and Surfaces, Wiener Str. 12, 28359,
Bremen, Germany
† Electronic supplementary information (ESI) available: Fig. S1: cyclic voltammo- 1.51 and À1.45 V.13 In contrast, the reductive desorption
grams of Ru-4ATPP SAMs in the anodic region. Fig. S2: reductive desorption CV in
potential of 4-ATP SAMs on gold electrodes employing an alkali
solution lied at around À0.73 V.11
the cathodic region of a stored solution of Ru(4-ATPP) on days 2–8 after being
prepared. Fig. S3: HMBC spectra in the carbonyl region of Ru-4ATPP in the
Here we report the unusual synthesis and self-assembly of a
presence and absence (days 0 and 10) of AuCl3. Fig. S4: HMBC spectra in the
ruthenium(II)-deuteroporphyrin derivative containing 4-ATP as a
spacer arm (Ru-4ATPP, Fig. 1), including an isomerization process.
carbonyl region of Ru-4ATPP ten days after being prepared. See DOI: 10.1039/
c3nj00118k
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem.

View Article Online
Paper
NJC
used. The Au(111) slides were clamped against an O-ring, which
defined the geometric area of the working electrode as 0.58 cm2.
The electrolyte solutions were degassed (30 min) with Ar prior
to each experiment.
2.4. Spectroscopy
Electronic spectra were recorded using a Varian Cary spectro-
photometer interfaced with a microcomputer for data acquisi-
tion. IR spectra were recorded using a Perkin-Elmer Paragon
500 FT-IR spectrometer with samples prepared as KBr tablets.
1H- and 13C-NMR spectra were obtained using Bruker DPX-200
and AVANCE DRX-600 MHz spectrometers at 300 K; all
chemical shifts were referenced to TMS lock signal. Ru-4ATPP
(30 mg) was dissolved in CD2Cl2 (0.5 mL) + d5-pyridine (5 mL).
MS were obtained using a Finnigan MAT 8200 spectrometer [EI
(70 eV) and ESI†]. The X-ray photoelectron spectra (XPS) were
obtained on Surface Science Instruments X- and M-probe
spectrometers using monochromatic Al-Ka X-ray sources (hn =
1486.6 eV). The BE scales for the monolayers on gold were
referenced by setting the Au4f7/2 BE to 84.0 eV. The XPS signals
were deconvoluted employing the Voigt function29 and con-
straining the customary 1.2 eV peak separations and also the
2 : 1 area ratio of the S2p3/2 and S2p1/2 components of the sulfur
doublet. The binding energies, areas under the curves, and full
widths at half-maximum (fwhm) were unconstrained in all the
deconvolution procedures.
Fig. 1 Schematic representation of Ru-4ATPP.
2. Experimental part
2.1. Chemicals
4-Aminothiophenol, 4-ATP (97%), was obtained from Aldrich
and used without further purification. Aniline (Ani) (99%) and
thiophenol (TP) (95%) were obtained from Acros and Aldrich,
respectively, and distilled prior to use. The preparation of the
disulphide derivative of 4-ATP (4-ATPds) was performed by a
mild oxidation of 4-ATP as described elsewhere.28 Tetrabutyl-
ammonium hexafluorophosphate, TBAHFP (electrochemical
grade), and isobutylchloroformate (ClCO2iBu) were purchased
from Fluka and used without further purification. CH2Cl2
(analytical grade) was obtained from Merck, and dried with
phosphorous pentoxide, distilled and degassed employing the
freeze–pump–thaw cycling. Other solvents were dried using
standard methods. Doubly distilled water was always used.
2.5. Preparation of carbonyl[13,17-bis(S-(4-aminophenyl)propane-
dioate-yl))-2,7,12,18-tetramethylporphyrinato]ruthenium(II)
[Ru(4-ATPP)(CO)]
Ru(DPdc)(CO)1 (100 mg, 0.156 mmol) was dissolved under
argon in dry THF (50 mL) and freshly distilled triethylamine
(1 mL) was added. The resulting dispersion was stirred under
cooling (À15 1C) and a solution of isobutylchloroformate (85 mg,
0.624 mmol, 4 eq.) in THF (2 mL) was added. The reaction
mixture was stirred for 2 h. The temperature was increased to
À10 1C and 2 mL of solution (58 mg, 0.468 mmol, 3 eq.) of
4-aminothiophenol was slowly added. The reaction mixture
(under argon) was stirred at RT for 4 h. The resulting reaction
mixture was extracted with CH2Cl2 and washed with saturated
solutions of NaHCO3 and NaCl and finally with water. The
organic phase was evaporated and the crude brown solid was
purified by column chromatography (silica, CH2Cl2 :MeOH:EtOAc,
10 : 0.4 : 0.4) to yield 99 mg (75%) of a bright orange solid. TLC
(silica gel, CH2Cl2 : MeOH, 10 : 1). Rf = 0.59. UV/Vis (CH2Cl2):
2.2. Preparation of the self-assembled monolayer
The gold slides used (200 nm of gold evaporated on glass with a
pre-layer of 2–4 nm of chromium; Gold Arrandee) were cleaned
before each experiment by immersing in piranha solution for
5 min and then washed with significant amounts of ethanol
and water (two times). After cleaning, the slides were flame-
annealed in an n-butane-gas flame to produce a flat surface
with a predominant Au(111) crystallographic orientation.
The self-assembled monolayers of Ru-4ATPP were formed on
Au slides using 1 mM solutions in CH2Cl2. The immobilization
was carried out in a sealed flask (24 hours at RT in the dark)
containing both the Ru-4ATPP solution and the gold slide.
Once the gold slide was removed from the Ru-4ATPP solution
it was rinsed with significant amounts of CH2Cl2, dried under
argon, and stored (if necessary) in sealed flasks. The samples
were stored (at RT in the dark) for CV and XPS studies.
l
max, nm. e (MÀ1 cmÀ1) = 314 nm (13 800), 394 (150 000), 516
(10 500), 546 (14 400). IR (KBr): n = 3453, 3367 cmÀ1 (m, NH2),
2920 (s, CH), 2848 (s, CH), 1933 (s, CRO), 1661 (m, CQO,
thiolester), 1591 (m, d(NH2)). 1H-NMR: (600 MHz, CD2Cl2 + 5 mL
d5-pyridine), d = 3.36 (s, 3H, CH3), 3.38 (m, 4H, CH2 a to
2.3. Electrochemistry
All glassware was cleaned using a piranha solution followed by carbonyl), 3.40 (s, 3H, CH3), 3.48 (s, 3H, CH3), 3.52 (s, 3H,
rinsing with bi-distilled water (l r 4 mS). Cyclic voltammetry CH3), 4.21 (m, 4H, CH2 b to carbonyl), 5.49 (broad, s, 4H, NH2),
was performed using a commercial EG&G PAR-384 model 6.43 (m, 4H, CHar), 7.12 (m, 4H, CHar), 8.74 (s, 1H, 8-H), 8.77
equipment. A one compartment Teflon cell, fitted with a Pt foil (s, 1H, 3-H), 9.64 (s, 1H, 10-H), 9.68 (s, 1H, 5-H), 9.71 (s, 1H,
counter electrode and a Ag/AgCl(sat) reference electrode, was 20-H), 9.77 (s, 1H, 15-H). 13C-NMR (CQO): 199.7 ppm.
c
New J. Chem.
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
NJC
Paper
MS (ESI, CH2Cl2 : MeOH) positive: m/z 875 [M + Na]+, negative:
m/z 850 [M À 2H+], m/z 886 [M + ClÀ].
3. Results and discussion
Ru(4-ATPP) was synthesized by the condensation of 4-ATP
with carbonyl[13,17-bis(propanoic acid)-2,7,12,18-tetramethyl-
porphyrinato]ruthenium(II).1 We expected a conjugation between
the carboxylic group of the porphyrin derivative and the amino
group of 4-ATP. Such a product would leave the thiol of 4-ATP
available for the self-assembly on a gold electrode.
The presence of the n(NH) at 3454 cmÀ1 and 3366 cmÀ1 and
the absence of n(SH) bands at ca. 2550 cmÀ1 in the IR spectrum
of the product indicated that the thiol group actually participated
in the condensation reaction. Additionally, a peak at 199.7 ppm
(13C-NMR) confirmed the formation of a thiolester group
instead of an amido one.30 Therefore, the unexpected product
obtained corresponded to Ru-4ATPP (Fig. 1). The achieved
thiolester conjugation is less stable than the expected amido
one but kinetically more favored, which should be the reason
for the formation of the unexpected product.
The formation of a SAM of Ru-4ATPP on gold surfaces would
impose the participation of both of its amino groups. This
mode of self-assembly (Au–N), although not usual, has been
reported before.16,31 Nevertheless, this situation required a
detailed characterization of the SAM of Ru-4TPP, especially by
CV for which no report has been found.
Fig. 2 Reductive desorption CV in the cathodic region (in KOH 0.5 M) of:
(a) Ru-4ATPP; (b) 4-ATP (c) TP (d) 4-ATPds; (e) Ani. Scan rate = 100 mV sÀ1
.
(c) 4-ATPds, a chemical analogue of 4-ATP, presented a
single peak at À0.87 V.
(d) Ani, with only N as a donor atom, presented two peaks at
À0.86 and À0.99 V.
3.1. Cyclic voltammetric determination
From these observations we assumed the following
The electrochemical behavior of ruthenium(II)-porphyrin SAMs assignations:
on gold electrodes is characterized by a reversible redox couple
(a) The peak recorded between À0.88 and À0.86 V should
within 0.2 V and 0.8 V that directly involves the metal center.32,33 correspond to the disruption of the Au–N(amino) bond.
The cyclic voltammogram (CV) of gold electrodes modified with
(b) The cathodic peaks at À0.71 and À0.69 should be
Ru-4ATPP in the anodic range presented a reversible redox pair assigned to the reductive disruption of the Au–S(thiolate) bond.
(DE = 36 mV, E1/2 = 0.54 V) which corresponds to coordinated Cathodic peaks at ca. À0.70 V have earlier received the same
ruthenium(II) to the porphyrin macrocycle as a whole (Fig. S1 assignation.11,24,25
in ESI†).
(c) The peak of Ani at À0.99 V was not assignable but could
The reductive desorption CV (0.5 M KOH) of the Ru-4ATPP correspond to a highly stable self-assembly of Ani through
SAM immediately after preparation gave a single cathodic peak Au–N(amino) bonds.
at À0.87 V (Fig. 2a), which was assigned to the disruption of the
Voltammograms of Ru-4ATPP were also recorded over time
Au–N bond in the SAM of Ru-4ATPP (Fig. 1). In order to confirm (1–20 days), as presented in Fig. 3, where significant variations
this assignation we decided to extend the CV reductive were observed. With only 24 hours after rinsing the gold
desorption studies to free 4-ATP (Fig. 2b) and its aromatic electrode modified with the Ru-4ATPP SAM (day 1) the CV
analogues: thiophenol (TP, Fig. 2c), the disulfide analogues of presented a new reductive desorption peak at À0.69 V assigned
4-ATP (4-ATPds, Fig. 2d), and aniline (Ani, Fig. 2e). It is to the Au–S(thiolate) bond that increased in intensity in the
necessary to underline that these SAMs were prepared in following days while the initial one at À0.87 V (Au–N(amino)
CH2Cl2 without the presence of any acid that could favor the bond) decreased. The peak at À0.87 V not only decreased in
protonation of the amino groups.
From a comparative analysis of the cathodic peaks of 4-ATP, compared with the CV at day 0 (Fig. 3), a process that can only
TP, Ani and 4-ATPds we observed that: be attributed to an increase in the organization of the SAM,
(a) 4-ATP, with S and N as potential donor atoms, presented something normally expected to happen over time.
intensity but also was shifted to more negative values when
an intense peak at À0.71 V, a very weak one at À0.88 V (Fig. 2b).
The intensities of the peaks assigned to the Au–N(amino)
and Au–S(thiolate) bonds were compared when referred to the
An oxidative peak at À0.60 V was also recorded.
(b) TP, with only S as a possible donor atom, presented one sum of the areas of both peaks. Already on the 7th day the area
single peak at À0.69 V, also present in 4-ATP.
of the new peak at ca. À0.7 V (Au–S(thiolate) bond) constituted
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem.

View Article Online
Paper
NJC
Fig. 3 Reductive desorption CV of Ru-4ATPP recorded at different days after the
formation of the SAM.
34% of the sum of the areas of both peaks, while on the 20th
day this value increased to 65% (Fig. 3, inset).
The observed variations in intensity of the Au–N (amino)
peak and the additional presence of a peak assigned to the
Au–S(thiolate) bond could be interpreted as a slow isomeriza-
tion of self-assembled Ru-4ATPP from its thiolester form to the
corresponding amido isomer.
Fig. 4 XPS spectra of Ru-4ATPP SAM in: (a) N1s region and (b) S2p region.
The proposed isomerization of Ru-4ATPP should be
attributed to the catalytic role of the gold surface since when
its solution was prepared and stored for CV determination for
eight days no significant variation was observed. This study was
performed by only putting in contact the Ru-4ATPP solution
with the gold electrode for each CV determination (days 2, 4, 6
and 8) (Fig. S2 in ESI†). Therefore, the isomerization process
should not be attributed to the hydrolysis of the thiolester
group since it only occurs in the presence of the gold surface.
The more intense peak at 398.3 eV should correspond to the
four equivalent nitrogen atoms of the porphyrin macrocycle.1
The peak at 399.4 eV was assigned to the nitrogen atoms of the
amino groups associated to gold.16 The third and less intense
peak at 400.2 eV could be attributed to unassociated nitrogen
(amino group) atoms. The presence of the three N1s peaks
could be interpreted as due to the formation of a mixed SAM of
Ru-4ATPP associated in two different forms: (a) through the
terminal amino group of the thiolester isomer and (b) through
the thiolate group of its amido isomer.
3.2. XPS determination
In order to confirm the variations observed in the voltammo-
The analysis of the XPS spectrum in the S2p region (Fig. 2b)
grams recorded over time, a XPS study of the gold surface also offered indications of the isomerization process of
modified with a SAM of Ru-4ATPP was performed. Ru-4ATPP. Two pairs of peaks were recorded. One pair of S2p
XPS determination of SAMs of 4-ATP on gold indicated peaks was observed at 162.1 and 163.3 eV, which should be
that the formation of Au–N4,16,31 bonds or free amine34,35 is assigned to self-assembled sulfur atoms of the thiolester group.
characterized by a N1s peak at ca. 399 eV. On the other hand, The other pair of S2p peaks was recorded at 163.9 and 165.1 eV
associated thiolate groups present a pair of S2p peaks at around which should correspond to unassociated sulfur atoms.36,37 In
161–163 eV, while when free, both peaks are shifted to higher this sense it is important to point out that Ashwell et al reported
energies.36
the S2p peaks of 4-ATP at 161.9 and 163.1 eV when a
The XPS spectrum in the N1s region of a gold surface Au–S(thiolate) bond was formed, while the free thiol group
modified on the 7th day with a SAM of Ru-4ATPP was resolved (Au–N bond) was recorded at 163.6 and 164.8 eV.16
into three peaks (Fig. 4a). The assignations were made mainly
It is important to underline that the S : N atomic ratio
considering an earlier report on the angle-resolved XPS study determined by XPS (1 : 2.98), in a high concordance with the
of 4-ATP self-assembled on gold electrodes, where the peak theoretical ratio (1 : 3), indicates that no decomposition took
at 400.1 eV was assigned to the free amino group when a place during the self-assembly and isomerization process.
Au–S(thiolate) bond was formed, while the peak at 399.1 eV
The percentage of the peaks corresponding to each isomer
was assigned to the associated amino group (Au–N(amino) was determined with respect to the sum of the areas of them for
bond).16
each element. In the case of the N1s peaks at 399.4 and 400.2 eV,
c
New J. Chem.
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
NJC
Paper
the sum of both areas was considered as 100% of the nitrogen Moreover, the desorbed Ru-4ATPP could present strong intra-
contained in the 4-ATP residue. 41% corresponded to the peak molecular interactions (mainly p–p stacking) with the remain-
at 400.2 eV (Au–S(thiolate) bond), while the rest (59%) was ing self-assembled molecules present in the formed SAM.
covered by the N1s peak at 399.4 eV (Au–N(amino) bond).
A direct interaction of the sulfur atoms of the thiolester
A similar result was obtained when comparing the intensities group with gold, a process that weakens the S–C(QO) bond,
of the S2p peaks. The intensity of the S2p3/2 peak, corres- should have played an important role in the isomerization
ponding to the Au–S bond (at 162.1 eV), represented ca. 39% process. Considering this possibility, an additional experiment
of the total area while the peak at 163.9 eV (Au–N bond) covered was carried out. Ru-4ATPP was set to interact with AuCl3 and
61%. These results are in agreement with those obtained by the resulting product was characterized by a 13C–1H Hetero-
reductive desorption CV for the same period of time, where the nuclear Multiple Bond Correlation (HMBC) experiment (Fig. S3
area of the peak corresponding to the Au–S(thiolate) bond in ESI†). The HMBC spectrum of Ru-4ATPP (in DMSO-d6)
constituted 34% of the sum of the areas of both peaks.
presented a signal at 199.7 ppm corresponding to the carbon
The formation of a mixed SAM could also be interpreted by of the thiolester group. This carbonyl group showed cross peaks
the presence of Ru-4ATPP only in its thiolester form but asso- with the aliphatic protons at 3.38 and 4.21 ppm, which are two
ciated both through the terminal amino group and through the and three bonds away, respectively (see S3 in ESI†). No other
thiolester group in a bent position of the spacer arm.
carbonyl signal was recorded. When the same experiment was
The catalytic participation of a capped Au(111) surface is an repeated for ten days after preparing the Ru-4ATPP solution no
unusual result but has not been excluded before. In this sense, variation was observed (Fig. S4 in ESI†). The corresponding
it is interesting to mention an earlier report on the role of a HMBC experiment for an equimolar mixture of Ru-4ATPP and
SAM of 4-ATP on gold in the electrochemical polymerization of AuCl3 (in DMSO-d6) gave a signal at 174.5 ppm (no signal
aniline.8
at 199.7 ppm) that showed cross peaks with the protons
Additionally, we decided to perform ab initio calculations of mentioned before (Fig. S3 in ESI†). This result suggests the
the simplest system: 4-ATP. The delocalized p system of the complete isomerization of the thiolester isomer into the amido
aromatic ring is extended over the amino group but does not one, a process that should have been favored by the high
include the thiol. This favors a high negative density on the mobility and accessibility between both reagents in solution.
nitrogen atom, an important factor when considering a nucleo- The isomerization process should have been induced by the
philic attack on the carbonyl group of the thiolester isomer as strong interaction between two ‘‘soft’’ atoms: Au(III) and S.
represented in Fig. 5.
The determination by HMBC of a unique species, the amido
The presence of terraces and adatoms on the Au(111) surface isomer of Ru-4ATPP, indicates that the only interpretation
should have favored the necessary vicinity between the carbonyl possible for the formation of a mixed SAM on gold is the
and the amino groups of self-assembled Ru-4ATPP in order to isomerization process. The differences between Au(III) and
carry out the nucleophilic attack. Although a sulfur atom in a metallic gold, being both soft, permit us to make such compar-
thiolester moiety cannot provoke a high partial positive charge ison. We took into consideration that when a sulfur donor atom
over the carbon (CQO) atom the relatively weak S–C(QO) bond interacts with gold, at any oxidation state, a Au(I)–S bond is
would favor the proposed nucleophilic attack by the electro- expected to be formed. The complete isomerization in the
nically rich amino groups.
presence of AuCl3 should be attributed to the absence of the
It can be assumed that the desorbed Ru-4ATPP, in equili- steric impediments that are present when the process occurs on
brium with the associated form, does not completely separate a gold surface.
from the gold surface, achieving a position in which the free
The unavoidable irregularities of the gold surface due to the
amino group could interact with the thiolester carbonyl group presence of terraces and adatoms should have also favored the
and provoke its substitution (Fig. 5). This criterion is based interactions with the sulfur atoms weakening the thiolester
on the characteristics of the CV of Ru-4ATPP (Fig. 2) where bond.
the oxidative reabsorption peaks are of very low intensity.
4. Conclusions
A thiolester derivative was obtained when 4-ATP was condensed
to carbonyl [13,17-bis(propanoic acid)-2,7,12,18-tetramethyl-
porphyrinato]ruthenium(II). The resulting Ru-4ATPP was self-
assembled on gold surfaces initially through its two free amino
groups according to CV determination. Over time, a slow
isomerization took place to give its amido isomer, a process
that has been attributed to the catalytic effect of gold. In this
isomerization the strong interaction between two soft elements
(Au and S) should have played an important role, since a
Fig. 5 Schematic representation of the possible mechanism for the isomeriza-
tion of self-assembled Ru-4ATPP: (a) the nucleophilic attack of a free amino group
and the direct weakening interaction of the sulfur atoms of the thiolester with
the gold surface are represented; (b) the formed amido isomer. Individual spacer
complete isomerization was observed when Ru-4ATPP was set
to interact with AuCl3 in solution.
arms were represented for the sake of simplicity, where R represents the
porphyrin and the second chain.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem.

View Article Online
Paper
We have observed that the self-assembly of amino groups on
gold is a stable and thermodynamically favored process, a fact
first reported by us based on CV determination.
Self-assembled monolayers can change the work function of
gold surfaces in such a way that they could catalytically favor
different types of reactions. More reports in this direction
should be expected in the near future. At present we are
studying other catalytic processes of sulfur-containing SAM
on gold surfaces, a novel topic with perspective applications.38
NJC
16 G. J. Ashwell, A. T. Williams, S. A. Barnes, S. L. Chappell,
L. J. Phillips, B. J. Robinson, B. Urasinska-Wojcik,
P. Wierzchowiec, I. R. Gentle and B. J. Wood, J. Phys. Chem.
C, 2011, 115, 4200–4208.
17 V. Arima, R. I. R. Blyth, F. Matino, L. Chiodo, F. D. Sala,
J. Thompson, T. Regier, R. D. Sole, G. Mele, G. Vasapollo,
R. Cingolani and R. Rinaldi, Small, 2008, 4, 497–506.
18 M. M. Walczak, D. D. Popenoe, R. S. Deinhammer,
B. D. Lamp, C. Chung and M. D. Porter, Langmuir, 1991,
7, 2687–2693.
19 D. F. Yang, C. P. Wilde and M. Morin, Langmuir, 1997, 13,
243–249.
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft
(DFG) for financial support of the project MO 274/24-1.
Dr Ana Lilian Montero Alejo and Joel Mieres for the theoretical
calculations and Johanes Stelten for the NMR determination
are also acknowledged.
20 D. F. Yang, C. P. Wilde and M. Morin, Langmuir, 1996, 12,
6570–6577.
¨
21 H. Hagenstrom, M. A. Schneeweiss and D. M. Kolb, Langmuir,
1999, 15, 2435–2443.
22 S. Imabayashi, M. Iida, D. Hobara, Z. Q. Feng, K. Niki and
T. Kakiuchi, J. Electroanal. Chem., 1997, 428, 33–38.
23 C. A. Widrig, C. Chung and M. D. Porter, J. Electroanal.
Chem., 1991, 310, 335–359.
Notes and references
1 R. Martin, R. Cao, A. M. Esteva and F.-P. Montforts, 24 T. Sawaguchi, F. Mizutani, S. Yoshimoto and I. Taniguchi,
J. Porphyrins Phthalocyanines, 2009, 13, 35–40. Electrochim. Acta, 2000, 45, 2861–2867.
2 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and 25 J. U. Nielsen, M. J. Esplandiu and D. M. Kolb, Langmuir,
G. M. Whitesides, Chem. Rev., 2005, 105, 1103–1169. 2001, 17, 3454–3459.
3 M. Okamura, T. Kondo and K. Uosaki, J. Phys. Chem. B, 26 Y.-T. Tao, C.-C. Wu, J.-Y. Eu, W.-L. Lin, K.-C. Wu and
2005, 109, 9897–9904. C.-h. Chen, Langmuir, 1997, 13, 4018–4023.
4 J. Sharma, S. Mahima, B. A. Kakade, R. Pasricha, 27 S. Kuwabata, R. Fukuzaki, M. Nishizawa, C. R. Martin and
A. B. Mandale and K. Vijayamohanan, J. Phys. Chem. B,
2004, 108, 13280–13286.
5 J. Zheng, Y. Zhou, X. Li, Y. Ji, T. Lu and R. Gu, Langmuir,
2003, 19, 632–636.
H. Yoneyama, Langmuir, 1999, 15, 6807–6812.
28 M. Kirihara, Y. Asai, S. Ogawa, T. Noguchi, A. Hatano and
Y. Hiraib, Synthesis, 2007, 3286–3289.
29 Practical Surface Analysis, ed. D. Briggs and M. P. Seah,
2nd edn, John Wiley & Sons, New York, 1996.
6 J. Sharma, N. K. Chaki, A. B. Mandale, R. Pasricha and
K. Vijayamohanan, J. Colloid Interface Sci., 2004, 272, 145–152. 30 C. M. Hall and J. Wemple, J. Org. Chem., 1977, 42,
7 A. I. Gopalan, K.-P. Lee, K. M. Manesha, P. Santhosh,
J. H. Kima and J. S. Kanga, Talanta, 2007, 71, 1774–1781.
8 J. Lukkari, K. Kleemola, M. Meretoja, T. Ollonqvist and
J. Kankare, Langmuir, 1998, 14, 1705–1715.
9 V. Arima, E. Fabiano, R. I. R. Blyth, F. D. Sala, F. Matino,
J. Thompson, G. Vasapollo, R. Cingolani and R. Rinaldi,
J. Am. Chem. Soc., 2004, 126, 16951–16958.
2118–2123.
31 A. Kumar, S. Mandal, R. Pasricha, A. B. Mandale and
M. Sastry, Langmuir, 2003, 19, 6277–6282.
32 K. M. Kadish, E. v. Caemelbecke and G. Royal, Electrochem-
istry of Metalloporphyrins in Nonaqueous Media, Elsevier
Science, New York, 2000.
33 D. A. Offord, S. B. Sachs, M. S. Ennis, T. A. Eberspacher,
J. H. Griffin, C. E. D. Chidsey and J. P. Collman, J. Am. Chem.
Soc., 1998, 120, 4478–4487.
10 W. A. Hayes and C. Shannon, Langmuir, 1996, 12, 3688–3694.
´
11 E. Valerio, L. Abrantes and A. Viana, Electroanalysis, 2008,
`
20, 2467–2474.
34 C. Lavenn, F. Albrieux, G. Bergeret, R. Chiriac, P. Delichere,
12 V. Ganesh, R. R. Pandey, B. D. Malhotra and V.
Lakshminarayanan, J. Electroanal. Chem., 2008, 619–620,
87–97.
A. Tuela and A. Demessence, Nanoscale, 2012, 4,
7334–7337.
¨
35 F. Raynal, A. Etcheberry, S. Cavaliere, V. Noel and H. Perez,
¨
13 V. Batz, M. A. Schneeweiss, D. Kramer, H. Hagenstrom, D. M.
Appl. Surf. Sci., 2006, 252, 2422–2431.
36 D. G. Castner, K. Hinds and D. W. Grainger, Langmuir, 1996,
12, 5083–5086.
Kolb and D. Mandler, J. Electroanal. Chem., 2000, 491, 55–68.
14 I. Gallardo, J. Pinson and N. VilA, J. Phys. Chem. B, 2006, 110,
˜
´
19521–19529.
37 M. I. Bethencourt, L.-o. Srisombat, P. Chinwangso and
15 M. Wirde, U. Gelius and L. Nyholm, Langmuir, 1999, 15,
T. R. Lee, Langmuir, 2009, 25, 1265–1271.
6370–6378.
38 J. Gong, Chem. Rev., 2012, 112, 2987–3054.
c
New J. Chem.
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013