A versatile synthetic strategy for nanoporous gold-organic hybrid materials for electrochemistry and photocatalysis

Article abstract of DOI:10.1016/j.tet.2014.03.091

Nanoporous gold (npAu) was employed as high surface area substrate for immobilization of redox- and photooxidative-active organic molecules. A two-step synthetic routine is demonstrated as a versatile and robust method for immobilization of various molecu

Full text of DOI:10.1016/j.tet.2014.03.091

Tetrahedron xxx (2014) 1e7
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A versatile synthetic strategy for nanoporous goldeorganic hybrid
materials for electrochemistry and photocatalysis
a
b
a
a
€
€
Andre Wichmann , Gunter Schnurpfeil , Jana Backenkohler , Lena Kolke ,
c
b
a
a,
*
€
€
Vladimir A. Azov , Dieter Wohrle , Marcus Baumer , Arne Wittstock
a University of Bremen, Center for Environmental Research and Sustainable Technology and Institute of Applied and Physical Chemistry, Leobener
Strasse UFT, 28359 Bremen, Germany
b University of Bremen, Institute for Organic and Macromolecular Chemistry, Leobener Strasse NW2, 28359 Bremen, Germany
c University of Bremen, Institute of Organic Chemistry, Leobener Strasse NW2, 28359 Bremen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanoporous gold (npAu) was employed as high surface area substrate for immobilization of redox- and
photooxidative-active organic molecules. A two-step synthetic routine is demonstrated as a versatile and
robust method for immobilization of various molecules. First, self-assembled monolayers (SAMs) of
thiols containing an azide moiety were prepared on npAu substrates. Then, alkyne-modified ferrocene,
tetrathiafulvalene, and zinc(II)phthalocyanine derivatives were covalently bound via the click reaction to
this linker. Following the provided synthetic procedures high performance composite materials are
generated for electrochemistry and photochemistry. The robust bonding between the organic functional
group and the gold support provides stability even under strongly oxidizing conditions (applied potential
or singlet oxygen).
Received 30 January 2014
Received in revised form 24 March 2014
Accepted 26 March 2014
Available online xxx
Keywords:
Nanoporous gold
Self-assembled monolayers
Metaleorganic-hybrid electrodes
Photochemistry
Ó 2014 Elsevier Ltd. All rights reserved.
Click-chemistry
1. Introduction
extremely homogeneous porous structure is formed.10 About 70
percent of the material is void volume making the material pene-
In nanotechnology the modification of nanomaterials with
functional groups is of increasing interest. Such hybrid-materials
containing an organic functionality on the one hand and an in-
organic substrate on the other hand are important in different
fields of physics, chemistry, and electronics. Recently, porous
goldeorganic molecules hybrid-materials were employed for pos-
sible applications as biosensors1e4 (e.g., glucose or ATP detection)
and also as actuator5 and high-strength composite materials.6 Key
to these various applications is the high electrical conductivity of
the material, its porosity increasing the surface area and thus
sensitivity, as well as the unique optical and electronic properties
generated by the nanostructure.5e7
A substrate material, which became very popular in this context
is nanoporous gold (npAu).4e9 This monolithic gold material ex-
hibits a continuous 3D porous structure of nanosized gold liga-
ments and pores on the order of a few tens of nanometers. Based on
a self-organization process of gold atoms during the preparation of
the material by corrosion of an Au alloy (AueAg or AueCu) an
trable for gases and liquids; hence, the entire surface of this gold
sponge (w10 m2/g) is exposed to the surrounding media. Since
about ten years several publications reported on the high catalytic
potential of the material for various reactions in gas and liquid
phase.11e13 Accordingly, further engineering of surface reactivity by
organic functional groups is an obvious approach for tailoring the
surface chemistry and thus reactivity of the material for applica-
tions in catalysis, electrochemistry and sensors.
The most stable form for immobilization of an organic entity on
a gold surface is the formation of a chemical bond between both
partners. This concept is well established in the context of self-
assembled-monolayers (SAMs) on gold substrates.14e16 Thiol or
disulfide groups of the organic partner react with the gold surface
forming a chemical bond. Common methods to functionalize
nanoporous gold surfaces, hence, use molecules containing such
groups (e.g., functionalized cysteine/lactase), attaching them di-
rectly to the surface.5,8,17 Alternative approaches use physical en-
trapment of glucose-oxidase in a sulfur containing polymer1
(poly(3,4-ethylenedioxythiophene)) or binding of glucose-oxidase
onto an already existing monolayer, which provides chemical
reactivity in the form of acid-groups.3 Yet, a general synthetic route
* Corresponding author. Tel.: þ49 421 21863400; fax: þ49 421 21863188; e-mail
for immobilization providing
a maximum of stability and
address: awittstock@uni-bremen.de (A. Wittstock).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.03.091
Please cite this article in press as: Wichmann, A.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.03.091

2
A. Wichmann et al. / Tetrahedron xxx (2014) 1e7
reproducibility is missing. The motivation of our work, hence, is to
establish and describe a general and versatile two-step synthetic
approach: (A) the formation of a suitable SAM on the npAu surface
and (B) binding the catalytic active entity to a specific linker mol-
ecule of the previously formed SAM. As compared to the direct
synthesis of such functional thiols the proposed two-step approach
bears several advantages (see also Ref. 16 for more details), (1) it
generalizes the synthetic procedure (2) it enables the bonding of
functionalities/catalysts, which are not available containing thiol
moieties, making it a very versatile approach, (3) it takes advantage
of the order of the preexisting SAMdsmaller molecules show faster
adsorption kinetics and, thus, less defects in the SAM (4) the syn-
thesis of functionalized thiols is typically laborious (in case of sulfur
containing molecules, peptides or proteins even a considerable
challenge). Thiols also exhibit a distinct propensity to oxidation
limiting their storage capability, which renders the synthesis of
larger quantities of expensive thiols uneconomic. The described
two-step procedure might, hence, be even the only suitable way of
immobilization for many organic functionalities.
Fig. 2. Structures of 11-azidoundecane-1-thiol 1 and the alkyne-substituted ferrocene,
tetrathiafulvalene and phthalocyanine derivates 2e4.
However, such an approach strongly depends on the step (B),
the reaction between the catalytically active species and the pre-
existing SAM. This reaction has to fulfill the following conditions:
mild reaction conditions avoiding thermal coarsening of the gold
substrate, high reaction rates, and formation of a stable bond in
various reaction media, also under applied electrochemical po-
tentials. We, thus, opted for the copper(I) catalyzed reaction be-
tween an alkyne compound and an azide (linker in SAM) known as
‘click-reaction’ named by Sharpless in 2001.18 In order to demon-
strate the versatility of our synthetic approach we synthesized
exemplary organic molecules with the according moieties and
tested the derived npAu/organic composites for applications in
electrochemical environments and photo-catalysis. For the first
application two different compounds were synthesized and
immobilized onto npAu: redox-active ferrocene19,20 and tetrathia-
fulvalene.21 For the second application as a photocatalyst a zinc(II)
phthalocyanine was synthesized and immobilized. This class of
molecules (metallophthalocyanines) is an important (photo-) cat-
alysts in industry for the photooxidation and oxidation of different
substrates by the reaction with molecular oxygen.22
immobilized on the surface of npAu the following reaction steps
were carried out:
1. Preparation of npAu,
2. Synthesis of the linker 11-azido-1-undecanethiol 1,
3. Synthesis of the ferrocene, tetrathiafulvalene, and phthalocy-
anine derivates 2e4 containing alkyne groups,
4. Synthesis of tris(benzyltriazolylmethyl)amine (TBTA)
copper ligand for the click-reaction,
5 as
5. Binding of linker 1 (and 1-octanethiol as a spacer) on npAu,
6. Binding of the alkyne-substituted compounds 2e4 via the click
reaction at the modified npAu surface.
The preparation of the npAu substrate (step 1) was carried out
following procedures described previously in Refs. 12,23. For the
functionalization of 2e4, disks of a Au30eAg70 alloy of about 6 mm
diameter and 100 or 200 mm thickness were submersed in concen-
trated nitric acid for 24 h and 48 h, respectively, and subsequently
dried in air. Scanning-electron micrographs of cross sections of these
disks show the homogeneous porosity of the material (Fig.1). In the
following (step 2), the compound 11-azido-1-undecanethiol 1 was
synthesized (Fig. S1, see Supplementary data for synthetic details
and numbering). Starting with the commercially available 11-
bromo-1-undecanol 6, the intermediates 11-azido-1-undecanol 7,
and 11-azido-1-undecanol,1-methanesulfonate 8 were synthesized
as described in literature.24 In an alternative pathway an in-
termediate component 11-azido-1-undecanol,1-thioacetate 9 was
firstly obtained by nucleophilic substitution of 8 with potassium
thioacetate. In a following step the component 1 could be obtained
by using methanol/acetyl chloride instead of introducing gaseous
HCl.24
2. Results and discussion
2.1. Synthesis
For the preparation of hybrid-materials consisting of ferrocene,
tetrathiafulvalene, and phthalocyanine derivates (Fig. 2)
In step 3, the alkyne-substituted compounds 2e4 were syn-
thesized. 1-Ferrocenyl-2-propyn-1-one 2 was obtained as de-
scribed by Barriga et al.25 starting from ferrocenecarbaldehyde 10
(Fig. S2). 4-carboxytetrathiafulvalene, propargylester 3 was syn-
thesized starting with the basic compound tetrathiafulvalene 12 up
to the intermediate tetrathiafulvalene-4-carboxylic acid 15 by fol-
lowing the pathway of Garin et al.26 The alkyne terminated TTF
product was prepared in a two-step sequence from the previously
reported 4-carboxytetrathiafulvalene.26 First, the acyl chloride was
synthesized by treating the carboxylic acid with oxalyl chloride and
N,N-dimethylformamide. Afterward the esterification was achieved
by addition of propargyl alcohol and N,N-diisopropylethylamide
(Fig. S3) to obtain 3.
Fig. 1. Scanning-electron micrograph of a cross-section of a npAu disk.
Please cite this article in press as: Wichmann, A.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.03.091

A. Wichmann et al. / Tetrahedron xxx (2014) 1e7
3
Fig. 3. Schematic illustration of the attachment of the alkyne-substituted compounds 2e4 to npAu; (a) immobilization of the linker 1 and a spacer molecules (1:3) (SAM formation),
(b) functionalization with active species 2e4 using the click-reaction, (c) nanoporous gold system with a specific functionality.
Next (part of step 3), 2,9,16,23-tetrakis(4-hex-5-yn-oxy)phtha-
locyanine-zinc(II) 4 was synthesized after modification of a pro-
cedure reported by Quinton et al.27 and Meder et al.28 (Fig. S4).
Zn(II) was selected instead of Mn(II) the as central metal ion be-
cause zinc(II)phthalocyanine exhibits under irradiation a high sin-
glet oxygen quantum yield.29 Tris(benzyltriazolylmethyl)amine
(TBTA, step 4) 5 was formed in a click reaction from tripropargyl-
amine 19 and benzyl azide 20 in a procedure described by Chan
et al. (Fig. S5).30
2.2. Redox active properties of the ferrocene and tetrathia-
fulvalene derivates 2,3 on the surface of npAu
The alkyne terminated ferrocene derivates 2 were bonded to the
npAu surface according to above described procedure. The resulting
ferroceneenpAu-system was characterized using cyclic voltam-
metry (Fig. 5a). The current density (related to the geometric sur-
face of the catalysts) of the redox active molecule attached to the
npAu electrode was nearly two orders of magnitude higher com-
pared to the reference system on a planar gold surface. This en-
couraged us to extend this procedure to the synthesis of the
tetrathiafulvalene derivate 3. The possibility of a stepwise oxida-
tion/reduction via the two oxidation states (TTFþ and TTF2þ) makes
After synthesis of these various compounds the two-step im-
mobilization (steps 5 and 6, cf. Fig. 3) was carried out as follows. The
first step (5) is the formation of a self-assembled monolayer (SAM)
on the nanostructured gold material containing two different types
of molecules, (1) a linker molecule 1 with an azide terminated
group and (2) a shorter ‘spacer’ 1-octanethiol without any func-
tional group at the other end of the C-chain. The latter alkanethiol,
acting as diluent spacer, was added in a 3:1 ratio with respect to the
linker molecule in order to avoid mutual steric hindrances of the
space-filling active molecules 2e4 on the surface of npAu.
The progress of self-assembly of 1 together with the spacer 1-
octanethiol was monitored by cyclic voltammetry (CV, cf. Fig. 4).
The electrolyte contains K3Fe(CN)6, which serves as a probe re-
agent. The reversible redox signals of the K3Fe(CN)6 appear at
360 mV (oxidation: Fe2þ/Fe3þ
) and 80 mV (reduction:
Fe3þ/Fe2þ), respectively, indicating the availability of free ad-
sorption sites on the gold surface before the immobilization.
Depending on the degree of formation of the monolayer the redox
signals are decreasing. After three days no noticeable signals are
visible any more, indicating that a monolayer is formed and the
adsorption sites are blocked by the thiols 1-octanethiol and 1.
In a subsequent copper(I) catalyzed reaction (step 6), active
species (catalyst) containing the alkyne moiety 2e4 (cf. Fig. 2) were
covalently bound to the azide terminated SAM (at room tempera-
ture, i.e., w20 ꢀC, total reaction time 24 h). In all ‘click-reactions’
tris(benzyltriazolylmethyl)amine (TBTA) 5 was added. Chidsey et al.
showed that this compound acts as a co-catalyst to the copper
during reaction (coordinating the Cu intermediates) and increases
the reaction rate by a factor of up to 20.31
Fig. 4. Cyclic voltammograms (CV) of npAu after different periods of time during
formation of a SAM (linker 1 and spacer octane-1-thiol, step 5). The CV was recorded in
0.1 M KCl (ACROS, p.A.) with 5 mM K3Fe(CN)6 (Merck, p.A.) versus Ag/AgCl and a scan
rate of 10 mV/s. The characteristic oxidation (360 mV) and reduction (80 mV) signals of
ferrocyanide decreases as a result of the formation of the SAM and the concomitant
decrease of free adsorption sites.
Please cite this article in press as: Wichmann, A.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.03.091

4
A. Wichmann et al. / Tetrahedron xxx (2014) 1e7
the two peroxides as ene-products (Scheme 2). The peroxides can
be converted to rose oxide.22 The photooxidation of (S)-(ꢁ)-citro-
nellol finally to rose oxide is an industrial process in annual multi-
ton quantities employing Rose Bengal as photosensitizer.
Scheme 2. Photocatalytic oxidation of (S)-(ꢁ)-citronellol catalyzed by ZnPc.
The photo-oxidative activities of ZnPc 4 on npAu were de-
termined by measuring the oxygen consumption over time for the
oxidation of citronellol (molar ratio citronellol to ZnPc¼104:1) in
a photoreactor under irradiation with visible light.22 The oxygen
consumption in the dark is negligible. Under irradiation employing
the pure nanoporous gold no oxygen was consumed. By function-
alization of the nanoporous gold with the phthalocyanine 4 a re-
producible conversion of w37% citronellol (c0¼0.5 mmol; 25 mg
npAu, 6 mm in diameter) over 24 h was measured (Fig. 6). The
conversion of citronellol to the peroxides was confirmed by using
gas chromatography and mass spectrometry (GCeMS). For com-
parison, a flat gold surface (1.8ꢂ1.8 cm) was modified with the ZnPc
as well. No oxygen consumption at all was detected indicating that
the amount of the dye on the planar gold surface is too low, and
therefore a dye-functionalization on planar surfaces is not suitable
for this application. As a benchmark of this system we quantified
the amount of Zn (which means ZnPc 4) immobilized on the gold
surface. The amount of w5ꢂ10ꢁ8 mol ZnPc on 25 mg npAu was
confirmed by inductively coupled plasma optical emission spec-
trometry (ICP-OES). In a homogeneous photooxidation (in the ab-
sence of npAu) no oxygen consumption was detected using the
same amount of ZnPc in solution (5ꢂ10ꢁ8 mol in 50 mL) indicating
a positive influence of the nanoporous gold. It can be speculated
that this enhanced activity is related to the unique electronic and
optical properties of the npAu, e.g., a plasmonic effect.32,33 For
a more detailed insight further investigations are required, which
are in progress. Also, we detect that only a fraction of the ZnPc 4
provided in solution (corresponding to 1/3 of a monolayer) was
actually immobilized onto the substrate (w0.01 of a monolayer
determined by ICP-MS). This deviation might be explained by the
Fig. 5. Cyclic voltammograms of the electro-active species 2,3 on nanoporous gold
substrate. (a) CV of ferrocene derivate 2: the current density of the npAu electrode is
higher than for the planar Au electrode by a factor of w60. The CV was recorded in
0.1 M HClO4 (Sigma Aldrich, ACS reagent 70%) versus a Ag/AgCl electrode with a scan
rate of 10 mV/s. (b) CV of TTF bonded onto an npAu electrode and the flat gold sub-
strate as reference. The current density of the npAu functionalized TTF-system is
higher by a factor of 30 compared to the planar reference system. The CV was recorded
in 0.1 M KCl (ACROS, p.A.) versus Ag/AgCl with a scan rate of 50 mV/s.
this molecule interesting for redox switches.21 Also in this case, the
combination of the npAu electrode and tetrathiafulvalene derivate
3 is more sensitive (by a factor of w30) as compared to the refer-
ence TTF-system on a planar gold surface (Fig. 5b). The electro-
chemical characterization of 2 and 3 dissolved in CH2Cl2 (not bound
to the npAu electrode) is shown in Fig. S6 of the Supplementary
data.
2.3. Photocatalytic properties of the phthalocyanine derivate
4 on the surface of the npAu
As mentioned before, zinc(II)phthalocyanine (ZnPc) is an active
photocatalyst sensitizer in photooxidations. By excitation of the
sensitizer in presence of oxygen O2(3Sgꢁ), singlet oxygen (O2(1Dg)) is
obtained followed by energy transfer to triplet oxygen, which is
responsible for the photooxidation (Scheme 1). One example is the
well-known photooxidation of (S)-(ꢁ)-citronellol, which results in
Fig. 6. Photooxidation of citronellol (0.5 mmol in 50 mL ethanol) in the presence of
a npAu disk (m¼25 mg; thickness of 100
m
m; irradiated area of 19 mm2) functionalized
with the ZnPc derivate 4. The squares indicate pressure drops measured by the in-
strument. The dotted line represents the deduced conversion of citronellol as a guide
for the eye.
Scheme 1. Generation of singlet oxygen (O2(1Dg)) based on a photosensitizer (Sens)
system.
Please cite this article in press as: Wichmann, A.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.03.091

A. Wichmann et al. / Tetrahedron xxx (2014) 1e7
5
multifunctionality of the ZnPc molecule potentially binding to
more than one azide group on the npAu surface. In this context, for
future investigations Zn(II)phthalocyanines with only one acety-
lene functionality should be synthesized.
(ppm) downfield from tetramethylsilane using the residual CHCl3
(7.26 ppm for 1H, 77.0 ppm for 13C) signals as a reference. Low
resolution mass spectra were measured on Finnigan MAT 8200 or
Finnigan MAT 95 spectrometers for electron ionization (EI, 70 eV),
and on a Bruker Esquire-LC spectrometer for electrospray ionization
(ESI). High resolution MS-spectra (HRMS) were measured on
a Finnigan MAT 95 spectrometer (EI, 70 eV). Measured isotopic
patterns of the Mþ ions of all reported compounds were fully
consistent with the calculated ones. Melting points were de-
termined using capillary melting point apparatus and are un-
corrected. Rf values were determined using 0.2 mm silica gel F-254
TLC cards. Flash chromatography (FC) was carried out using
3. Conclusions
The aim of this study was to demonstrate a versatile and re-
producible synthetic approach for a stable immobilization of dif-
ferent organic molecules onto a highly porous gold substrate,
namely the nanoporous gold. The open porosity of the monolithic
nanostructure in conjunction with its high electrical conductivity
makes it an ideal candidate for applications in catalysis and elec-
trochemistry. Electrochemically and photochemically active mole-
cules were used to demonstrate the potential of such systems. In
a first step, a suitable SAM with reactive azide units was bound onto
the npAu substrate serving as a stable platform for further func-
tionalization with the reactive molecules (catalyst). We presented
an alternative and more easy binding of an azide-linker as well as
the successful immobilization of three compounds containing
alkyne functionalities for potential sensor and photocatalytic ap-
plications. The redox active species (ferrocene- and tetrathiafulva-
lene derivates) on npAu and the ZnPcenpAu system for
photooxidation of citronellol showed very appreciable activity. In
both cases the activity of the nanoporous gold derived system was
superior to reference systems on planar gold substrates, owing to its
nanoporosity and, thus, increased surface area. In the case of the
electrochemical active system, the sensitivity could be enhanced by
nearly two orders of magnitude. The immobilized photocatalyst on
the planar gold surface showed no detectable reactivity, while the
ZnPcenpAu-system reaches almost 40% conversion of 0.5 mmol
citronellol even though the size of the sample and irradiated geo-
metric area, respectively, was w20 times smaller than that of the
planar substrate. The finding that the ZnPcenpAu showed even
higher conversion than the same amount of ZnPc dissolved in the
reaction medium indicates an enhancement from the nanoporous
gold substrate possibly by surface plasmons. The demonstrated
synthetic routine provides a versatile tool for generating high per-
formance organic/npAu composite materials with a robust chem-
ical bond between the organic functionality and the Au substrate.
230e440 mesh (particle size 36e70 mm) silica gel.
4.2. Preparation of the planar and nanoporous gold
substrates
In all photochemical experiments nanoporous gold disks with
a diameter of 6 mm and a thickness of 100 mm were used, prepared
by etching AgeAu alloys (70 at% Ag, 30 at% Au) with 50 mL con-
centrated nitric acid (24 h, HNO3, 65 wt %, OmniTrace EMD
Chemicals). For functionalization of npAu with redox-active mole-
cules 2,3 disks with 200 mm thickness were generated by corrosion
in 50 mL concentrated nitric acid over 48 h. The master alloys were
prepared in a procedure described before by us.11 After washing
with three times with 30 mL deionized water, the nanoporous gold
samples were dried 24 h at ambient conditions. The residual silver
content of the obtained material was below 1 at% as confirmed by
AAS.23 The npAu show reproducible pore sizes of w40 nm (cf.
Fig. 1). Different planar polycrystalline gold substrates were used as
reference systems for comparison with the npAueorganic hybrid
materials. In the electrochemical experiments gold foils with di-
mensions of ca. 10ꢂ13ꢂ0.5 mm3 for immobilization of ferrocene
derivate 2 and ca. 10ꢂ10ꢂ0.5 mm3 for immobilization of tetra-
thiafulvalene derivate 3 were used. Before immobilization the gold
substrates were cleaned by applying a potential of ꢁ1.8 V for
3 min in 0.5 M H2SO4. In case of immobilization of phthalocyanine
derivate 4 a SiO2-substrate (ca. 1.8ꢂ3.6 cm2) was sputtered with
gold (Cressington Sputter Coater 108auto).
4.3. Organic functionalization of the nanoporous gold
substrate
4. Experimental
4.1. General information
The nanoporous gold disk was immersed in an ethanolic solu-
tion (5 mL) containing 50 mM 11-azido-1-undecanethiol 1 and
150 mM 1-octanethiol (Sigma Aldrich, >98.5%) for 3 days at room
temperature. The samples were washed immediately three times
by dipping them in ethanol for 15 min removing the excessive
linker 1 and the 1-octanethiol.
The electro-active compounds (ferrocene and tetrathiafulvalene
derivates 2,3) were attached subsequently in a reaction using
0.25 M of Fc or TTF alkyne derivatives, 0.1 M Cu[TBTA]PF6, and
0.25 M hydroquinone (Sigma Aldrich, >99.0%) in 5 mL DMSO
(Merck, p.A.)/H2O 3:1 for 24 h. For the immobilization of the
phthalocyanine derivate 4 a mixture of 5 mL THF (Merck, p.A.)/H2O
3:1 was used as solvent. After immobilization, the samples were
washed immediately three times by dipping them for 15 min in
DMSO in the case of the Fc and TTF derivates 2,3 or in THF in the
case of the phthalocyanine derivate 4.
1-Ferrocenyl-2-propyn-1-one 2 (Fig. S2)25 and tris(benzyl-
triazolylmethyl)amine (TBTA) 5 (Fig. S5)30 were prepared as de-
scribed before. 11-Azido-1-undecanethiol 1 was synthesized as
shown in Fig. S1. The starting material 11-bromo-1-undecanol 6 is
commercially available (SigmaeAldrich). The compounds 11-azido-
1-undecanol 7 and 11-azido-1-undecanol,1-methanesulfonate 8
were synthesized as described in literature.24 The compounds 11-
azido-1-undecanol,1-thioacetate 9 and 11-azido-1-undecanethiol
1 were synthesized as described in the Experimental part. The al-
kyne-modified tetrathiafulvalene derivate 3 was synthesized fol-
lowing the general procedure reported by Garin et al.26 up to
compound tetrathiafulvalene-4-carbonyl chloride 16; the synthesis
of compound 3 is described in the experimental part (Fig. S3). The
synthesis of metallo-phthalocyanine 4 (shown in Fig. S4) was
similar to a procedure from D. Quinton et al.,27 i.e., Meder et al.,28
modifications are described in the Experimental part.
4.4. Determination of the phthalocyanine derivate 4 by in-
ductively coupled plasma optical emission spectrometry (ICP-
OES)
Reagent grade chemicals and solvents were used without fur-
ther purification unless otherwise stated. All reactions were carried
out under atmosphere of dry N2. NMR spectra were recorded using
Bruker Avance DPX-200 spectrometer and Bruker Avance NB-360
Subsamples (w10 mg of functionalized npAu) were weighed
(balance Mettler MD-205) and transferred in Teflon TFM(R) HP120
spectrometer. Chemical shifts (d) are reported in parts per million
Please cite this article in press as: Wichmann, A.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.03.091

6
A. Wichmann et al. / Tetrahedron xxx (2014) 1e7
digestion vessels. 6 mL 14.4 mol/L nitric acid and 3 mL 10.2 mol/L
hydrochloric acid were added. Closed vessels were subjected to
a microwave-assisted digestion (ca. 210 ꢀC for 40 min; MLS 1200
mega system, MWS GmbH, Leutkirch, Germany). Solutions ob-
tained were clear, they were evaporated to near dryness by mi-
crowave action (2ꢂ40 min). Residues of ca. 0.5 mL each were
determined gravimetrically, and 3 mL 0.5 mol/L nitric acid was
added. The mixtures were subjected to a 10 min dissolution step in
closed vessels under mild microwave action (250 W). Final volumes
were calculated applying a residue density of 1.05. Two procedural
blanks were run in the same sequence.
4.8. Synthesis
4.8.1. Synthesis of 11-azido-1-undecanol,1-thioacetate (9). 1.46 g
(5 mmol) 11-azido-1-undecanol,1-methanesulfonate 8 and 1.14 g
(2 mmol) potassium thioacetate were dissolved in 50 mL DMF and
stirred for 24 h at room temperature under an argon atmosphere.
Afterward 300 mL water was added. The aqueous solution was
extracted with ether for three times, and then the collected organic
layers were washed with water for three times. The organic phase
was dried by filtering over cotton wool. After removing the solvent
and drying in vacuum the residue was purified by column chro-
matography (silica gel, CH2Cl2). Yield: 1.18 g (4.35 mmol, 87%). 1H
The Zn amount was measured by using a VARIAN Vista Pro. For
the inductively coupled plasma optical emission spectrometry (ICP-
OES) a radial plasma observation was used. The samples prepared
as described above were diluted by a factor of two in 1% HNO3.
NMR (200 MHz, CDCl3):
d
¼3.25 (t, 3J¼6.9 Hz, 2H, CH2N3), 2.85 (t,
3J¼7.2 Hz, 2H, CH2S), 2.31 (s, 3H, CH3), 1.59e1.52 (m, 4H), 1.44e1.19
(m, 14H). 13C NMR (200 MHz, CDCl3):
d¼196.3 (SOCCH3), 51.7
(CH2N3), 30.9, 29.7, 29.6, 29.4, 29.3, 29.0, 26.9.
4.5. Cyclic voltammetry with dissolved ferrocene and tetra-
thiafulvalene derivates 2,3
4.8.2. Synthesis of 11-azido-1-undecanethiol (1). Under argon at
0 ꢀC 7 mL (95 mmol) acetyl chloride were added to 40 mL methanol.
The solution was stirred for 30 min and then 1.14 g (4.2 mmol) 11-
azido-1-undecanol,1-thioacetate 9 in 10 mL methanol was added.
Stirring was continued for 24 h. 300 mL of a saturated sodium bi-
carbonate solution was added. The mixture was extracted three
times with 100 mL of diethyl ether and then washed with a sodium
bicarbonate solution and water. The ether phase was washed with
a sodium bicarbonate solution and water. The organic layer was
separated and dried by filtration over cotton wool. The residue was
dried again in vacuum and then purified by column chromatogra-
phy (silica gel, CH2Cl2). The dark brown residue was distilled in
vacuum to give an orange-yellow liquid. Yield: 608 mg (2.65 mmol,
Cyclic voltammetry (CV) with dissolved substances was per-
formed using a computer controlled HEKA PG390 potentiostat in
a three-electrode single-compartment cell (2.5 mL) with a plati-
num disk working electrode (diameter of 1.5 mm) and a platinum
wire used as a counter electrode. A non-aqueous Ag/Agþ secondary
electrode (0.1 M TPAPþ0.01 M AgNO3 in MeCN) was used as the
reference electrode. The samples were dissolved 1ꢂ10ꢂ3 M in dry
degassed MeCN containing 0.1 M Bu4NClO4 (TBAP) as supporting
electrolyte. Ferrocene was taken as an electrochemical Ref. 34,35
ox
with the potential E ¼0.38 V versus saturated calomel electrode
1/2
(SCE) for the Fc/Fcþ couple in TBAP/CH2Cl2.36,37 A CV scan of
1ꢂ10ꢁ3 M ferrocene solution was taken after each CV measurement
for calibration purposes. Then the values of oxidation potentials
were referenced to the Fc/Fcþ couple, recalculated, and reported
versus Ag/AgCl (3 M KCl).
63%). TLC (CH2Cl2): 0.74. 1H NMR (200 MHz, CDCl3):
d
¼3.24 (t,
3J¼6.9 Hz, 2H, CH2N3), 2.56e2.45 (m, 2H, CH2SH), 1.67e1.52 (m,
4H), 1.41e1.26 (m, 14H). 13C NMR (200 MHz, CDCl3):
(CH2N3), 34.2, 29.6, 29.3, 29.2, 29.0, 28.5, 26.9, 24.8. IR (NaCl, Film):
d¼51.6
n
¼2927, 2854 (s, CH), 2095 (s, N3),1465 (m),1349 (w),1260 (m), 722
(w). MS (EI, 70 eV): m/z: 229 [M].
4.6. Cyclic voltammetry with ferrocene and tetrathiafulva-
lene derivates 2,3 immobilized on npAu
4.8.3. Synthesis of 4-carboxytetrathiafulvalene, propargylester (3). A
heavy-walled Schlenk tube equipped with a wide bore Teflon
stopcock was charged with 15 (0.1 g, 0.46 mmol) and THF (10 mL).
CV with immobilized substances was performed using a com-
puter controlled HEKA PG390 potentiostat in a three-electrode
single-compartment cell (250 mL) with a gold wire used as
a working electrode and a platinum counter electrode. An Ag/AgCl
electrode (3 M KCl) was used as the reference electrode. CV samples
were fixed with the gold wire and measured in water containing
0.1 M KCl or 0.1 M HClO4 as supporting electrolyte. The values of
oxidation potentials are reported versus Ag/AgCl.
Afterward DMF (10 mL) and oxalyl chloride (160 mL, 1.84 mmol)
were added. The reaction mixture was stirred at 70 ꢀC for 1 h and
then at room temperature overnight. After concentration in vacuo,
THF (5 mL) and propargyl alcohol (130
followed by the slow addition of N-diisopropylamine (390
m
L, 2.3 mmol) were added,
L,
m
2.3 mmol). The reaction mixture was stirred for 1 h at room tem-
perature and was concentrated in vacuo. The crude product was
purified by flash chromatography (CH2Cl2) to afford red crystals.
Yield: 0.14 g (0.36 mmol, 79%). Mp: 91 ꢀC. Rf¼0.65 (CH2Cl2). 1H NMR
(200 MHz, CDCl3):
d 7.41 (s, 1H), 6.36e6.29 (m, 2H), 4.80 (d,
4.7. Photooxidation of (S)-(L)-citronellol in organic solvents
3J¼2.44 Hz, 2H), 2.52 (t, 3J¼2.44 Hz, 1H). 13C NMR (50 MHz, CDCl3):
d
158.39, 133.51, 127.42, 119.22, 118.72, 114.26, 107.11, 7þ5.66, 53.04.
Measurements for the photocatalytic activities were carried out
as described at 25 ꢀC in a 100 mL reaction vessel connected to
a 50 mL gas burette.22 The vessel was irradiated with a 250 W
quartz-halogen lamp of a slide projector, and the light intensity was
adjusted to w180 mW cmꢁ2. The reaction vessel was filled with
50 mL ethanol solutions containing the phthalocyanine derivate 4
immobilized on the nanoporous gold substrate (6 mm in diameter,
MS (EI): m/z 286 (100) [M]þꢃ. HRMS (EI): m/z [M] calcd for
C
10H6O2Sþ4 285.92507, found 285.92506. CV (vs Ag/AgCl, CH2Cl2):
ox1
ox2
E
1/2
¼0.42 V, E ¼0.78 V.
1/2
4.8.4. Synthesis of 4-(hex-5-yn-oxy)phthalonitrile (18). 200
mL
(1.84 mmol) 5-hexyn-1-ol and 266.9 mg (1.54 mmol) 4-
nitrophthalonitrile 17 were dissolved in 20 mL dry DMF under ar-
gon. The mixture was stirred for 15 min and then 360 mg
(2.61 mmol) ground anhydrous potassium carbonate was added.
106 mg (0.77 mmol) potassium carbonate was further added after
3 h and again after 24 h. After 26 h, water was added to the reaction
mixture. The brownish product was collected by centrifugation and
rinsed with ethanol, and then it was washed with 3ꢂ20 mL of
water, re-dissolved in toluene, and the solution dried over MgSO4.
100 mm in thickness) or for the reference measurement the com-
pound 4 dissolved in ethanol (5ꢂ10ꢁ8 mol). Then the apparatus was
flushed with pure oxygen for 10 min, and 91 mL (0.5 mmol) citro-
nellol was added to the vessel (molar ratio substrate to photosen-
sitizer was 104:1). After closing the apparatus, the reaction vessel
was irradiated under intensive magnetic stirring, and the oxygen
consumption over 24 h was recorded.
Please cite this article in press as: Wichmann, A.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.03.091

A. Wichmann et al. / Tetrahedron xxx (2014) 1e7
7
Evaporation of the solvent gave pure 4-(hex-5-yn-oxy)phthaloni-
trile as a light yellow solid. Yield: 102 mg (0.46 mmol, 30%). Mp:
97 ꢀC. 1H NMR (200 MHz, DMSO):
7.99e7.95 (AreH, m, 1H),
7.69e7.68 (AreH, m, 1H), 7.42e7.36 (AreH, m, 1H), 4.14e4.08
(CH2eOe, t, 3J¼6.9 Hz, 2H), 2.73e2.71 (eC^CH, m, 1H), 2.24e2.16
(CH2, m, 2H), 1.86e1.72 (CH2, m, 2H), 1.62e1.51 (CH2, m, 2H). IR
Supplementary data
d
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.03.091.
(KBr),
n
[cmꢁ1]¼3284 (e^CeH), 3078 (CaryleH), 2962/2945/2866
References and notes
(eCH2), 2227 (eCN), 2114 (C^C), 1603/1595 (Caryl]Caryl), 1309
(CaryleOeCalkyl). MS: (EI, 70 eV) m/z (%): 224 (5) [M], 223 (10)
[MꢁH], 144 (20) [MꢁC6H8], 127 (10) [MꢁC6H9O], 81 (90)
[MꢁC8H3N2O], 53 (60) [MꢁC10H7N2O].
1. Xiao, X. X.; Wang, M. E.; Li, H.; Si, P. C. Talanta 2013, 116, 1054e1059.
2. Kashefi-Kheyrabadi, L.; Mehrgardi, M. A. Bioelectrochemistry 2013, 94, 47e52.
3. Xiao, X.; Li, H.; Wang, M. E.; Zhang, K.; Si, P. Analyst 2014, 139, 488e494.
4. Du, X.; Liu, X.; Li, Y.; Wu, C.; Wang, X.; Xu, P. Nanoscale Res. Lett. 2013, 8, 1e6.
5. Detsi, E.; Onck, P.; De Hosson, J. T. M. ACS Nano 2013, 7, 4299e4306.
€
6. Wang, K.; Weissmuller, J. Adv. Mater. 2013, 25, 1280e1284.
4.8.5. Synthesis of 2,9,16,23-tetrakis(4-hex-5-yn-oxy)phthalocyani-
neezinc(II) (4). 2,9,16,23-Tetrakis(4-hex-5-yn-oxy)phthalocyani-
neezinc(II) was prepared by dissolving 100 mg (0.45 mmol) 4-
(hex-5-yn-oxy)phthalonitrile 18 and 50 mg (0.27 mmol) anhydrous
zinc acetate in 25 mL N,N-dimethylaminoethanol and heated at
reflux temperature (135 ꢀC) under argon for 15 h. The mixture was
then allowed to cool and methanol was added to precipitate the
product that was further collected by centrifugation. The complex
was first purified by column chromatography over silica gel using
DMF as eluent. DMF was removed under reduced pressure at 80 ꢀC.
The resulting product was then further purified by heating in
methanol for 30 min. The solid was collected by centrifugation.
€
7. Wittstock, A.; Biener, J.; Erlebacher, J.; Baumer, M.; Nanoporous Gold: From an
Ancient Technology to a Novel Material, 1st ed.; RSC, 2012, Vol. 1; p 250.
8. Huang, J. F.; Sun, I. W. Adv. Funct. Mater. 2005, 15, 989e994.
9. Okman, O.; Lee, D.; Kysar, J. W. Scr. Mater. 2010, 63, 1005e1008.
10. Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410,
450e453.
€
11. Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Baumer, M. Science 2010, 327,
319e322.
€
12. Zielasek, V.; Jurgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.;
€
Baumer, M. Angew. Chem., Int. Ed. 2006, 45, 8241e8244.
€
13. Wittstock, A.; Baumer, M. Acc. Chem. Res. 2014, 47, 731e739.
14. Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164e1167.
15. Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714e10721.
16. Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev.
2005, 105, 1103e1170.
17. Qiu, H. J.; Xu, C. X.; Huang, X. R.; Ding, Y.; Qu, Y. B.; Gao, P. J. J. Phys. Chem. C
2008, 112, 14781e14785.
18. Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40,
2004e2021.
19. Ganesh, V.; Sudhir, V. S.; Kundu, T.; Chandrasekaran, S. Chem.dAsian J. 2011, 6,
2670e2694.
Yield: 31.2 mg (32 m d 9.34e9.24
mol, 7%). 1H NMR (200 MHz, DMF):
(AreH, m, 4H), 9.04e8.88 (AreH, m, 4H), 7.88e7.76 (AreH, m, 4H),
4.78e4.54 (CH2eOe, m, 8H), 2.60e2.44 (CH2, m, 8H), 2.36e2.29
(eC^CH, m, 4H), 2.26e2.16 (CH2, m, 8H), 2.08e1.86 (CH2, m, 8H).
IR: (KBr),
n
[cmꢁ1]: 3276 (e^CeH), 2935/2910/2867/2840
20. Collman, J. P.; Hosseini, A.; Eberspacher, T. A.; Chidsey, C. E. D. Langmuir 2009,
25, 6517e6521.
(CaryleH/CalkyleH), 2109 (eC^Ce), 1606 (eC]Ce arom), 1487/
1468 (CeH/CeN/CeCe), 1385/1336 (CeH/CeC/CeN), 1275 (CeH/
CeN), 1238 (CeH,CeN/CaryleCalkyl), 1120/1092 (CeH/CeC), 634 (Pc-
Ring). MS: (ESI, positive) (DMF): m/z: 961 [MþH]þ; (ESI, negative)
(DMF) m/z: 995 [MꢁCl]ꢁ, 879 [MꢁC6H9]ꢁ, 914 [MꢁC6H9þCl]2, 977
21. Segura, J. L.; Martin, N. Angew. Chem., Int. Ed. 2001, 40, 1372e1409.
€
22. Wohrle, D.; Suvorova, O.; Gerdes, R.; Bartels, O.; Lapok, L.; Baziakina, N.; Ma-
karov, S.; Slodek, A. J. Porphyrins Phthalocyanines 2004, 8, 1020e1041.
€
23. Wittstock, A.; Neumann, B.; Schaefer, A.; Dumbuya, K.; Kubel, C.; Biener, M. M.;
€
€
Zielasek, V.; Steinruck, H.-P.; Gottfried, J. M.; Biener, J.; Hamza, A.; Baumer, M. J.
Phys. Chem. C 2009, 113, 5593e5600.
[MþOH]ꢁ UVevis (DMF)
l (nm): 354, 680 (for the UVevis spectrum
please see Fig. S7 in the Supplementary data).
24. Miramon, M. L.; Mignet, N.; Herscovici, J. J. Org. Chem. 2004, 69, 6949e6952.
25. Barriga, S.; Marcos, C. F.; Riant, O.; Torroba, T. Tetrahedron 2002, 58, 9785e9792.
26. Garin, J.; Orduna, J.; Uriel, S.; Moore, A. J.; Bryce, M. R.; Wegener, S.; Yufit, D. S.;
Howard, J. A. K. Synthesis 1994, 5, 489e493.
Acknowledgements
27. Quinton, D.; Antunes, E.; Griveau, S.; Nyokong, T.; Bedioui, F. Inorg. Chem.
Commun. 2011, 14, 330e332.
28. Meder, M. B.; Haller, I.; Gade, L. H. Dalton Trans. 2005, 8, 1403e1415.
29. Shinohara, H.; Tsaryova, O.; Schnurpfeil, G.; Wohrle, D. J. Photochem. Photobiol.,
We thank the University Bremen for financial support within
the initiative ‘Func-Band’. We gratefully acknowledge the experi-
mental support (SEM) of Petra Witte (Prof. Willems, Historical
GeologydPaleontology, Geology Department of the University
€
A 2006, 184, 50e57.
30. Chan, T. R.; Hilgraf, R.; Sharpless, K. B.; Fokin, V. V. Org. Lett. 2004, 6,
2853e2855.
31. Collman, J. P.; Devaraj, N. K.; Eberspacher, T. P. A.; Chidsey, C. E. D. Langmuir
€
Bremen), (ICP-OES) of Dr. Uwe Schussler (Department of Marine
2006, 22, 2457e2464.
Chemistry, University of Bremen) and Silvana Pape (Center for
Marine Environmental Sciences, Bremen) as well as GCeMS carried
out by Dr. Thomas Dulcks and Dorit Kemken (Prof. Spiteller, De-
partment of Chemistry, University of Bremen). We also thank Jonas
Warneke (Prof. Swiderek, IAPC Universtiy of Bremen) for recording
COSY NMR spectra.
32. Lang, X. Y.; Qian, L. H.; Guan, P. F.; Zi, J.; Chen, M. W. Appl. Phys. Lett. 2011, 98.
33. Biener, J.; Nyce, G. W.; Hodge, A. M.; Biener, M. M.; Hamza, A. V.; Maier, S. A.
Adv. Mater. 2008, 20, 1211e1217.
34. Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461e466.
35. Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298, 97e102.
36. Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984, 23, 817e824.
37. Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2007, 129, 3683e3697.
€
Please cite this article in press as: Wichmann, A.; et al., Tetrahedron (2014), http://dx.doi.org/10.1016/j.tet.2014.03.091