Impact of photosensitizer orientation on the distance dependent photocatalytic activity in zinc phthalocyanine-nanoporous gold hybrid systems

Article abstract of DOI:10.1039/d0ra03891a

Nanoporous gold powder was functionalized in a two-step approach by an azide terminated alkanethiol self-assembled monolayer (SAM) and a zinc(ii) phthalocyanine (ZnPc) derivative by copper catalyzed azide-alkyne cycloaddition (CuAAC). A series of different hybrid systems with systematic variation of the alkyl chain length on both positions, the alkanethiol SAM and the peripheral substituents of the ZnPc derivative, was prepared and studied in the photooxidation of diphenylisobenzofuran (DPBF). An enhancement by nearly one order of magnitude was observed for the photosensitized singlet oxygen (¹O₂) generation of the hybrid systems compared to the same amount of ZnPc in solution caused by the interaction of the npAu surface plasmon resonance and the excited state of the immobilized sensitizer. This interaction was shown to be distance dependent, with decreasing activity for short SAMs with alkyl chain lengths < 6 methylene groups caused by quenching of the excited state via electron transfer as well as decreasing activity for SAMs with n > 8 methylene groups due to decreasing energy transfer for long distances. An unexpected distance dependent behaviour was observed for the variation of the peripheral alkyl chain on the photosensitizer revealing a planar orientation of the immobilized photosensitizer on the nanoporous gold surface by a penta-coordinated central zinc ion through interaction with free azide groups from the self-assembled monolayer.

Full text of DOI:10.1039/d0ra03891a

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Impact of photosensitizer orientation on the
distance dependent photocatalytic activity in zinc
Cite this: RSC Adv., 2020, 10, 23203
phthalocyanine–nanoporous gold hybrid systems†
ab
David Steinebrunner,
Gunter Schnurpfeil,c Mathis Kohrode,a Alexander Epp,a
¨
¨
Khaetthariya Klangnog,c Jorge Adrian Tapia Burgos,
Dieter Wohrle and Arne Wittstock
Andre Wichmann,a
ab
c
ab
*
*
¨
Nanoporous gold powder was functionalized in a two-step approach by an azide terminated alkanethiol
self-assembled monolayer (SAM) and a zinc(II) phthalocyanine (ZnPc) derivative by copper catalyzed
azide-alkyne cycloaddition (CuAAC). A series of different hybrid systems with systematic variation of the
alkyl chain length on both positions, the alkanethiol SAM and the peripheral substituents of the ZnPc
derivative, was prepared and studied in the photooxidation of diphenylisobenzofuran (DPBF). An
enhancement by nearly one order of magnitude was observed for the photosensitized singlet oxygen
(1O2) generation of the hybrid systems compared to the same amount of ZnPc in solution caused by the
interaction of the npAu surface plasmon resonance and the excited state of the immobilized sensitizer.
This interaction was shown to be distance dependent, with decreasing activity for short SAMs with alkyl
chain lengths < 6 methylene groups caused by quenching of the excited state via electron transfer as
well as decreasing activity for SAMs with n > 8 methylene groups due to decreasing energy transfer for
long distances. An unexpected distance dependent behaviour was observed for the variation of the
Received 30th April 2020
Accepted 11th June 2020
peripheral alkyl chain on the photosensitizer revealing
a planar orientation of the immobilized
DOI: 10.1039/d0ra03891a
photosensitizer on the nanoporous gold surface by a penta-coordinated central zinc ion through
interaction with free azide groups from the self-assembled monolayer.
rsc.li/rsc-advances
therapy (PDT).7–12 All of these processes are based on a photo-
physical interaction between the nanostructured metal support
Introduction
and the attached functionality, mostly in the form of energy
transfer from the plasmon resonance to the excited state of the
attached organic or organometallic moiety.7,13,14
The eld of hybrid materials consisting of a nanostructured
metal and various organic or organometallic functionalities has
recently gained growing attention due to their possible appli-
cations in different elds ranging from catalysis and sensor
applications to new medical treatments.1–4 Especially regarding
their photophysical properties those hybrid systems are of
emerging interest as many of the used nanostructured metal
skeletons show special optical properties due to their surface
plasmon resonance.4–6 Therefore, several hybrid systems were
designed and recently reported as alternatives for classical
organometallic systems for example in photon upconverting
systems or as completely new approaches like plasmonic
sensing or medical applications like photodynamic cancer
The probably best understood type of hybrid system in this
context is the one usually used for PDT consisting of either
a metal phthalocyanine or porphyrin derivative attached to gold
nanoparticles (AuNPs). Since their rst report by Russell and
coworkers in 2002, several studies on this type of hybrid mate-
rials were performed showing that such systems are superb 1O2
sensitizers with increased photocatalytic activities compared to
the analogous complexes used in solution.15,16 The underlying
photophysical interactions were investigated in detail in the
group of Lemmetyinen, showing that various energy and elec-
tron transfer processes occur in such systems, which are also
1
responsible for the observed increase in O2 formation.13,17
aInstitute of Applied and Physical Chemistry and Center for Environmental Research
and Sustainable Technology, University Bremen, Leobener Str. UFT, 28359 Bremen,
Germany. E-mail: awittstock@uni-bremen.de
1
In the context of photocatalytical O2 based oxidation reac-
tions, that are also of interest in industrial processes, colloidal
nanoparticle systems are only of minor interest as truly
heterogeneous systems are usually better in terms of lifetime
and recyclability. Here, nanoporous gold (npAu, Fig. 1b) could
be a valuable alternative, as it shows comparable photophysical
properties of AuNPs while fullling the criteria of a truly
bMAPEX Center for Materials and Processes, University Bremen, Bibliothekstr. 1,
28359 Bremen, Germany
cOrganic and Macromolecular Chemistry, University Bremen, Leobener Str. NW2,
28359 Bremen, Germany. E-mail: woehrle@uni-bremen.de
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03891a
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performed at the SAM alkanethiol as well as on the peripheral
side chains of the ZnPc derivative (Fig. 2). The length of the alkyl
chains, but also the orientation of the macrocycle on the surface
contributes to the overall distance between the ZnPc and the
npAu surface. Therefore, a careful investigation of both distance
determining parameters in nanoscaled systems is essential as it
can help to improve the design and understanding of novel
hybrid systems in this recently strong and fast growing research
eld.
Results and discussion
Synthesis and characterization
The npAu powder support was prepared employing a free
corrosion method of the starting alloy disk (Ag/Au, 70 : 30 at%)
in concentrated HNO3 and powdering of the obtained npAu
disk as described previously.26 The resulting powder showed
a mean particle size of around 28 mm and pore sizes in the range
of 40 nm.26 The residual silver concentration was below 1 at%,
which was conrmed by EDX spectroscopy and shows good
agreement to previously reported results.28–32
The organic functionalization of the npAu powder was ach-
ieved following a two-step approach established in our group
(Fig. 2).25–27 In a rst step, the surface of the npAu powder was
functionalized with an azide-terminating SAM using acetyl-
protected azidoalkylthioates. For the systematic variation of
the distance between the immobilized photosensitizer and the
npAu surface, SAMs with alkyl chain lengths ranging from nSAM
¼ 3 to 11 CH2 groups between the sulphur anchor and the free
azide group were employed. In the following step, the respective
ZnPc derivative with either a propynoxy- or hexynoxy-group with
nZnPc ¼ 1 or 4 CH2 groups as peripheral substituent was bound
to the free azide groups of the SAM via copper catalyzed azide-
alkyne cycloaddition (CuAAC). By this method a complete set
of hybrid systems was obtained with chain lengths ranging from
nSAM ¼ 3 to 11 CH2 groups in the basic SAM for both ZnPc
derivatives (Table 1).
Each of the obtained hybrid systems was analyzed in detail
regarding its chemical composition and porous structure of the
support. It was shown by SEM that the porous structure of the
npAu powder stayed unchanged at 40 nm during the function-
alization. The energy dispersive X-ray spectroscopy (EDX)
measurements of all samples showed a homogeneous zinc
distribution, which was conrmed by EDX mapping and line
scan experiments. This conrms previous results of selected
systems as described previously by us.26,27
Fig.
1 Structure and corresponding optical spectra for (a) the
photosensitizer zinc(II) phthalocyanine with either peripheral propy-
noxy- (1, cyan) or hexynoxy-substituents (2, black) and (b) nanoporous
gold with pore sizes of 40 nm.
heterogeneous, monolithic catalyst.3,18–20 This monolithic
material consists of ligaments and pores on the order of 40 nm
throughout macroscopic dimensions of up to centimeters
(Fig. 1b, inset). In addition, npAu was already shown to be an
efficient catalyst for oxidation reactions in both, gas and liquid
phase systems.21–24 On the other hand, the same principles for
functionalization of the npAu surface with organic compounds
such as macrocyclic metal complexes can be applied as in the
case of AuNPs.
We recently reported a comparable system where a ZnPc
derivative was immobilized on different shaped npAu supports,
as examples for a truly heterogeneous monolithic system,
a heterogeneous powder catalyst as well as a photocatalytic
coating.25–27 These new hybrid photocatalysts showed high
overall 1O2 sensitization activities, where the activity was an
order of magnitude above the activity of the same amount of the
photosensitizer in solution.26
The amount of immobilized sensitizer in every hybrid system
was determined by quantication of the amount of Zn in the
system by inductively coupled plasma mass spectrometry (ICP-
MS) aer dissolution of the corresponding sample in ultrapure
aqua regia. The obtained values as well as the light exposed
fractions of the immobilized sensitizer are summarized in
Table 1. The amount of light exposed sensitizer was around 1.5
ꢀ 10ꢁ10 mol for every hybrid system which was determined
from the immobilized sensitizer amount and a penetration
depth of 300 nm for visible light into the nanoporous
structure.26
By selective irradiation of only the plasmon resonance of
npAu (Fig. 1b) it was shown that a contribution by energy
transfer to the attached ZnPc (Fig. 1a) is responsible for the
enhanced activity.26 The investigation of the distance depen-
dency of the energy transfer between both parts of the hybrid
system is therefore of immense interest to optimize the system
1
regarding its O2 sensitization activity. The system itself offers
two different positions to achieve hybrid materials with several
distances between the npAu surface and the attached photo-
sensitizer. The variation of the alkyl chain length can be
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Fig. 2 Schematic representation of the two step npAu powder functionalization: (a) formation of the azide terminated SAM using azidothioates
with various alkyl chain lengths, and (b) copper catalyzed azide-alkyne cycloaddition (“click chemistry”) to bind the peripherally alkyne-
substituted ZnPc derivatives 1 and 2. (npAu modified from [ref. 20]).
simultaneously.26,27 The decrease of DPBF was followed by UV-
Vis spectroscopy at l ¼ 415 nm and the photooxidations were
Table 1 Overview over the series of hybrid systems with systematic
variation of the ZnPc–npAu distance and the corresponding immo-
bilization fractions. Hybrid – abbreviation used in the text for the
quantied by their corresponding turnover numbers (TON) and
corresponding hybrid system, nSAM – total number of CH2 groups in
turnover frequencies (TOF, Fig. S1–S18†).
the alkylthiol SAM, nZnPc – number of CH2 groups in the peripheral
substituent of the ZnPc photosensitizer, Zn [mg gCatalystꢁ1] – immobi-
lized Zn content given as mg per g hybrid catalyst as obtained by ICP-
MS measurements and ZnPcIrr. [mol] – determined photosensitizer
amount immobilized on npAu and irradiated during the photocatalytic
studies
For the ZnPc derivative 1 with peripheral propynoxy-
substituents the TOF values showed a clear trend with
maximum activity for the hybrid H1-7 build of a SAM with seven
methylene groups. In contrast, for longer and shorter distances
the photocatalytic singlet oxygen sensitization activity decreases
rapidly (Table 2, Fig. 3 and 4). Similar trends were observed with
decreasing photocatalytic activities by irradiation of mainly the
ZnPc using a 700 nm bandpass lter. A further decrease was
observed for irradiation of only the plasmon resonance of npAu
employing a 550 nm bandpass lter. The highest activities
using the 550 nm cut-on lter clearly shows the synergistic
effect of the npAu surface plasmons for the generation of singlet
oxygen (Table 2).
ꢁ1
Hybrid
nSAM
nZnPc
Zn [mg gCatalyst
]
ZnPcIrr. [mol]
H1-3
H2-3
H1-4
H2-4
H1-5
H2-5
H1-6
H2-6
H1-7
H2-7
H1-8
H2-8
H1-9
H2-9
H1-10
H2-10
H1-11
H2-11
3
3
4
4
5
5
6
6
7
7
8
8
9
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
154.9
151.7
235.3
191.1
161.2
165.9
165.8
189.6
170.1
150.0
147.7
133.4
155.9
187.6
151.6
176.1
157.9
183.9
1.4 ꢀ 10ꢁ10
1.4 ꢀ 10ꢁ10
2.1 ꢀ 10ꢁ10
1.8 ꢀ 10ꢁ10
1.5 ꢀ 10ꢁ10
1.5 ꢀ 10ꢁ10
1.5 ꢀ 10ꢁ10
1.7 ꢀ 10ꢁ10
1.6 ꢀ 10ꢁ10
1.4 ꢀ 10ꢁ10
1.3 ꢀ 10ꢁ10
1.2 ꢀ 10ꢁ10
1.4 ꢀ 10ꢁ10
1.7 ꢀ 10ꢁ10
1.4 ꢀ 10ꢁ10
1.6 ꢀ 10ꢁ10
1.5 ꢀ 10ꢁ10
1.7 ꢀ 10ꢁ10
For the photophysical interactions, two different pathways
are possible in such hybrid materials, which were shown spec-
troscopically in detail on AuNP systems. Lemmetyinen and
9
10
10
11
11
Distance dependent photocatalytic activity
For the determination of the photocatalytic activity of each
ZnPc–npAu hybrid system, the photocatalysts were studied in
the photooxidation reaction of 1,3-diphenylisobenzofuran
(DPBF). DPBF is the most used chemical quencher for singlet
oxygen and reacts selectively and almost quantitatively via an
endoperoxide to 1,2-dibenzoylbenzene (Fig. 3, inset). The
photooxidation reactions were conducted in a self-built photo-
catalytic setup described previously under irradiation with
a 550 nm cut-on lter, 700 nm bandpass or 550 nm bandpass
lter to achieve selective irradiation of the immobilized ZnPc,
the npAu plasmon resonance or both active sites
Fig. 3 DPBF conversion curves exemplarily showing the activity for
a short SAM (H1-3, gray) with strong quenching by electron transfer
from ZnPc to npAu, a medium SAM (H1-7, blue) with highest energy
transfer from npAu to ZnPc and a long SAM (H1-11, black) with
decreasing energy transfer from npAu to ZnPc. Inset: reaction scheme
for the photooxidation of DBPF used for the determination of 1O2
production as selective chemical quencher.
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Table 2 Summarized photocatalytic activities determined for the two
series of hybrid systems with systematic variation of the ZnPc–npAu
distance under irradiation with a 550 nm cut-on (TOFco), 700 nm
bandpass (TOF700) and 550 nm bandpass (TOF550) filter. dSAM [nm] –
calculated thickness of the SAM showing the distance between the
npAu surface and the terminal N atom of the azide groups of the SAM
with either a short (s) or long (l) axial coordination distance to the
central Zn ion of the immobilized ZnPc derivative
predominant. This process acts as quenching of the excited
state of the ZnPc photosensitizer with negative inuence on the
singlet oxygen formation. For the hybrid systems H1-7 to H1-11
the distance between donor and acceptor is beyond the electron
transfer regime, and therefore only energy transfer takes place
with decreasing activity towards larger distances.
The series of the ZnPc derivative 2 with peripheral hexynoxy-
substituents surprisingly showed a similar trend as the series of
the ZnPc derivative 1 with the maximum activity for the hybrid
H2-7. The activities for all other hybrids of this series showed
the same electron and energy transfer behaviour for longer and
shorter chain lengths of the SAM as already discussed for the
rst series. This observation is quite unexpected as, for an
example, H1-6 and H2-3 should have the same distance between
the immobilized sensitizer and the npAu surface due to the
same number of CH2 groups if the linking unit is fully linearly
extended. Therefore H1-6 and H2-3 should show a comparable
photocatalytic activity. But in fact, the observed activities exhibit
a completely contrary result as H2-3 shows the lowest activity of
all hybrid systems whereas H1-6 is one of the most active
hybrids.
The same distance dependent behaviour for both ZnPc series
shows unambiguously that the overall distance between the two
active centers is mainly determined by the chain length of the
employed azidoalkylthioate used for the SAM formation. This
fact is indeed contrary to a fully linearly extended orientation of
the immobilized sensitizer and therefore a more detailed
discussion about the orientation of the ZnPc macrocycle on the
npAu surface is essential to understand the observed photo-
catalytic activities.
Hybrid TOFco [minꢁ1
]
TOF700 [minꢁ1
]
TOF550 [minꢁ1
]
dSAM [nm]
H1-3
H2-3
H1-4
H2-4
H1-5
H2-5
H1-6
H2-6
H1-7
H2-7
H1-8
H2-8
H1-9
H2-9
988.9
719.6
639.7
428.3
1729.1
975.8
99.2
60.6
0.96 (s)
0.96 (l)
1.07 (s)
1.07 (l)
1.21 (s)
1.21 (l)
1.32 (s)
1.32 (l)
1.46 (s)
1.46 (l)
1.58 (s)
1.58 (l)
1.71 (s)
1.71 (l)
1.83 (s)
1.83 (l)
1.96 (s)
1.96 (l)
2347.8
1905.7
2596.6
1984.8
2961.9
2400.9
3670.8
3125.1
2934.9
2329.8
2634.1
1807.9
186.8
127.8
243.9
136.9
294.4
171.2
304.3
260.7
254.1
187.2
238.3
149.4
171.2
119.7
119.5
53.5
1979.4
1070.4
2164.9
1308.8
2481.8
1826.1
1872.9
1503.2
1624.7
1316.7
1601.6
1026.8
949.1
H1-10 2452.6
H2-10 1497.8
H1-11 1619.4
H2-11
835.9
552.3
Orientation of the photosensitizer on the surface
The distance between the immobilized photosensitizer and the
npAu support is determined by the chain length of the
employed SAM, but at a certain degree also by the orientation of
the photosensitizer on the surface. Taking two possible orien-
tations into account, namely a fully extended perpendicular
orientation in respect to the support, as well as a planar, parallel
Fig. 4 Photocatalytic 1O2 sensitization activities for the hybrids with
a systematic variation of the alkyl chain lengths plotted as function of orientation, the distance between the central Zn ion and the
the SAM chain length. Series H1-3–H1-11 with the peripheral propy-
npAu surface shows a signicant deviation. Whereas, for
noxy-functionalized ZnPc derivative 1 is the cyan trace and series H2-
example, system H2-6 in planar conguration shows a separa-
3–H2-11 with peripheral hexynoxy-functionalities (ZnPc 2) the black
tion distance of around 1.3 nm, the distance between the Zn ion
trace.
and the npAu surface enlarges to nearly 1.9 nm for a perpen-
dicular orientation.
The nature of the tetra peripheral substituted ZnPc deriva-
coworkers observed energy transfer from both, the excited ZnPc
tive indeed can easily adopt such a planar conguration by
to the AuNPs and vice versa.13,17 Also, electron transfer from the
undergoing multiple CuAAC reactions with up to four binding
ZnPc to the AuNPs resulting in the corresponding phthalocya-
sites. There are some studies about the orientation of phtha-
nine radical cation was observed.13 In general, energy transfer is
locyanines and closely related macrocycles like porphyrins and
a long ranging interaction with decreasing efficiency from 1 to
porphyrazines on Au surfaces reported in literature, but all of
10 nm distance, whereas electron transfer is usually limited by
them were prepared employing thiol-containing derivatives as
a distance between donor and acceptor of around 1–2 nm in
precursors for SAM formation and none of those systems was
alkanethiol-bridged systems.33,34 The decreasing singlet oxygen
prepared using the CuAAC reaction.35–40 Therefore, although for
sensitization activity for the hybrid systems H1-3 to H1-6 is
some systems a planar orientation is reported, the hybrid
therefore a result of the competition between energy and elec-
system reported here may vary in this respect due to the
tron transfer, where electron transfer for shorter distances gets
different preparation method. In addition, all of these
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experiments were performed on single crystal surfaces and
therefore those results, but also most of the analysis tech-
niques, are not directly transferable to a nanoporous surface.
The UV-Vis spectra of H1-7 and H2-7 when prepared on
a light penetrable, 100 nm thick npAu foil exhibit an interesting
feature where the absorption of the ZnPc Q-band is red-shied
compared to the absorption of the same sensitizer in solution,
both times measured in a DMF environment (Fig. 5). Such
a redshi of the Q-band of phthalocyanines and porphyrins is
known to be caused by axial coordination of a nitrogen atom
from an organic amine.41–44 The binding of an additional ligand
in axial position is a common feature of zinc phthalocyanines
and porphyrins derivatives, even forming stable crystal struc-
tures, supramolecular dimers and trimers as well as hybrid
systems directly immobilized on amine-terminated SAMs.45–53
In addition, the penta-coordinating ability of the Zn central ion
of zinc porphyrins is used frequently as template in many
supramolecular synthesis approaches.54–57
The observed redshi aer immobilization of the ZnPc onto
the npAu surface suggests a similar penta-coordinated central
ion, which is possible by interactions with free electron pairs of
the azide groups from the SAM, representing the main pathway
for photoinduced interactions in the hybrid systems (Fig. 6).
Such an expected interaction might be even forced when the
tetrasubstituted photosensitizer undergoes two or more
triazole-based connections to the SAM, bringing the Zn central
ion in a template-like direction to the free azide groups of the
SAM. Such a coordination mode would result in a distance
between the immobilized ZnPc and the npAu surface, which is
determined by the chain length of the SAM as well as the axial
coordination strength and can explain the observed distance
dependent photocatalytic behaviour of the hybrid systems. The
slightly higher activity that was found for the series of the ZnPc
derivative bearing propynoxy-substituents can be explained by
a stronger interaction of the Zn ion with the azide groups
compared to the hexynoxy-substituted ZnPc derivative due to
the shorter peripheral substituent as indicated by the different
shis of the Q-band aer immobilization (Fig. 5).
Fig.
6
Simplified schematic illustration of the axial coordination
between an immobilized ZnPc molecule on the npAu surface repre-
senting the dominant pathway for photoinduced interactions in the
ZnPc–npAu hybrid systems. Peripheral substituents of the ZnPc
forming covalent connections to the npAu surface through CuAAC
were omitted for clarity.
Another interesting point is the position of the maximum
photocatalytic activity for the hybrid systems with nSAM ¼ 7
methylene groups in the SAM. By comparison of the photo-
catalytic activities and the calculated distances between the Zn
center and the npAu surface (Table 2), the maximum was found
for a distance around 1.5 nm. This nding is also important to
that extend that a distance slightly above 1 nm is oen
described as the border until electron transfer is possible
through alkanethiol-based bridges and only energy transfer is
observed for further increasing distances.33,34 In hybrid systems
of pyrene-functionalized AuNPs with different alkylthiol linkers,
photoinduced electron transfer to the AuNP was observed in 5
methylene containing bridges but not for similar systems con-
taining bridges of 8 methylene groups.33 Therefore, the
observed maximum activity around 1.5 nm also supports our
suggested planar conguration and our hypothesis of an
interaction with the free azide groups.
In previous studies, the coverage of the npAu surface was
found to be around 1% of a monolayer, calculated on the
assumption that a complete monolayer is based on a 1 : 1 ratio
of immobilized photosensitizer molecules to available Au
surface atoms. Considering the planar orientation of the
photosensitizer on the surface as well as the van der Waals
radius of gold, it is obvious that this assumption cannot
represent the steric demand of a ZnPc molecule. As a single
ZnPc core without any attached peripheral substituents already
covers an area of at least 25 gold atoms when adopting a planar
orientation on the Au surface, a value of 1% of the previous
Fig. 5 UV-Vis spectra showing the absorption of the Q-band of the
photosensitizer ZnPc with either peripheral propynoxy- (1, H1-7, cyan)
or hexynoxy-substituents (2, H2-7, black) in solution (bold line) and
when immobilized on a npAu foil (dashed line).
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assumption would correspond to a value of at least 25% of grade, $99.5%), HNO3 (analytical reagent grade, 65% and
a ZnPc monolayer on the Au surface. NORMATOM, ultra-pure for trace analysis, 67%) and HCl
In general, it should be mentioned that the “click-method- (NORMATOM, ultra-pure for trace analysis, 34%) were bought
ology”, namely the CuAAC reaction, is a convenient and versa- at VWR and were used as received without further purication.
tile method oen used for the functionalization of
nanostructured surfaces. The attachment of photosensitizers,
especially of phthalocyanines or porphyrins derivatives,
employing this method is a quite common synthesis strategy for
Synthesis of the ZnPc derivatives
2,9,16,23-Tetrakis(2-propyn-1-yloxy)phthalocyanine zinc(II) (1)66
the preparation of various hybrid materials.58–62 Although used
frequently, the characterization of as prepared hybrid materials
is oen challenging, which might be the reason that little is
known about the orientation of the macromolecules in such
hybrid systems. However, especially when investigating photo-
induced interactions, the orientation can have a crucial impact
on the overall donor–acceptor distance and therefore possible
interactions between immobilized sensitizers and free func-
tional groups of the SAM need to be taken into consideration
and discussed properly in the context of new hybrid materials.
and 2,9,16,23-tetrakis(5-hexyn-1-yloxy)phthalocyanine zinc(II)
(2)25 were synthesized by cyclotetramerization of 4-(2-
propynyloxy)-phthalonitrile or 7-(2-hexynyloxy)-phthalonitrile
respectively with Zn(CH3COO)2$2H2O as reported previously
(Scheme S1†).
Synthesis of azidoalkylthioacetates
3-Azidopropyl-1-thioacetate (3),67–69 4-azidobutyl-1-thioacetate
(4),69,70 5-azidopentyl-1-thioacetate (5),71–73 6-azidohexyl-1-
thioacetate (6),73–75 7-azidoheptyl-1-thioacetate (7),71,73,76 8-
azidooctyl-1-thioacetate (8),77–79 9-azidononyl-1-thioacetate
(9),79–81
10-azidodecyl-1-thioacetate
(10)81–83
and
11-
Conclusions
azidoundecyl-1-thioacetate (11)25 were all synthesized starting
from the corresponding n-bromo-1-alkanol via n-azidoalkan-1-
ol and n-azidoalkyl methanesulfonate according to literature
procedures (Scheme S2†).
In this study the distance dependent interactions in hybrid
systems based on zinc(II) phthalocyanine derivatives immobi-
lized on a npAu support were studied. For this purpose,
a systematic variation of the distance between the two chro-
mophores was performed by variation of the alkyl chain length
within the SAM as well as on the peripheral substituents on the
ZnPc derivative. The whole series of hybrid systems was tested
in the photooxidation of DPBF as standard reaction for singlet
oxygen sensitizing systems.
The distance dependent photocatalytic activities, the low
concentration of the immobilized photosensitizer as well as the
redshi of the Q-band all supports the suggested planar
orientation of the phthalocyanine ring with respect to the npAu
surface, also providing evidence for a penta-coordinated Zn
central ion interacting with the terminal azide groups of the
SAM. The orientation of the sensitizer as consequence of the
chosen preparation strategy using the established CuAAC
reaction is an important step towards a better understanding of
photoinduced interactions in hybrid systems as well as
a template tool for the design and synthesis of novel hybrid-
based materials.
Preparation of nanoporous gold
npAu powder was prepared according to a previously published
procedure by dealloying of Ag/Au disks (70 : 30 at%, 5 mm
diameter, 200 mm thickness) in concentrated HNO3 for 72 h and
cleaning with H2O. Subsequently aer dealloying the npAu
disks were powdered by fragmentation with a metal pincers.26
Obtained npAu samples exhibit a mean pore size of 40 nm as
determined by SEM measurements with a specic surface area
of around 5 m2 gꢁ1 as calculated according to a literature
procedure and also shown by BET measurements on compa-
rable npAu samples.27,84,85
npAu foils with comparable pore sizes were prepared by
oating American white gold leaf (Ag/Au 50 : 50 wt%, 1.5 ꢀ
1.5 cm, 100 nm thickness) on concentrated HNO3 for 8 h, fol-
lowed by oating on H2O according to a literature procedure.27
Hybrid preparation
Experimental
Materials
Following a general procedure,26 SAMs with different chain
lengths were prepared by covering the npAu powder with an
Alloy disks with Ag/Au ratio of 70 : 30 at% were prepared within ethanolic solution (5 mL) of azidoalkylthioacetate (3–11, 1
our group according to a previously published procedure.63 mmol) for 72 h. Aer thoroughly washing with EtOH, the as
American white gold (12 karat, Ag/Au 50 : 50 wt%) as starting prepared npAu samples were covered with a solution of the
alloy for npAu foils was bought at Noris Blattgold GmbH, ZnPc derivative (1–2, 0.15 mmol), Cu(MeCN)4PF6 (931.8 mg, 2.5
Schwabach, Germany. For the CuAAC, the catalyst Cu(MeCN)4- mmmol), TBTA (1.326 mg, 2.5 mmol) and hydroquinone (272.5
PF6 (97%) from Aldrich, hydroquinone from Merck and the co- mg, 2.5 mmmol), dissolved in a THF/H2O mixture (5 mL, 3 : 1 v%)
catalyst tris(benzyl-triazolylmethyl)amine (TBTA), which was for another 72 h. Aerwards, the as prepared hybrids were
obtained by a literature procedure,64,65 were used. The chemical repeatedly washed with THF in order to remove any physisorbed
1O2 quencher 1,3-diphenylisobenzofuran (DPBF, >95%) was material.
received from TCI. All solvents including ethanol (abs., reagent
Hybrid systems for the UV-Vis measurements on npAu foils
grade), THF (reagent grade, $99.0%), DMF (analytical reagent were prepared following the same procedure with slightly
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modied conditions as described recently for the preparation of Germany) for experimental support during the SEM and EDX
photocatalytic coatings.27
measurements and Cornelia Rybarsch-Steinke (Institute for
Applied and Physical Chemistry, University of Bremen, Bremen,
Germany) for the preparation of the ICP-MS samples. Also, we
especially thank Martin Nowak (Institute for Applied and
Physical Chemistry, University of Bremen, Bremen, Germany)
for his help during the design of the catalysis setup and the
fabrication of the reaction vessel. In addition, we thank Rebecca
Siemering, Jasmin Richter, Rike Norden and Mark Thran for
their help as part of their lab internships. Funding is gratefully
acknowledged within the grants WI 4497/3-1 and WO 237/42-1
provided by Deutsche Forschungsgemeinscha (DFG).
Characterization methods
The quantity of immobilized ZnPc on npAu was determined by
inductively coupled-plasma mass spectrometry (ICP-MS, iCAP
Q, Thermo Fisher Scientic GmbH) aer dissolving 10 mg of the
as prepared hybrid sample in ultrapure aqua regia (2 mL).
The pore size of the npAu support was determined acquiring
micrographs of the samples on a scanning electron microscope
(SEM, Supra 40, Zeiss, Germany) operated at 15.0 kV accelera-
tion voltage, 300 pA probe current and 10 mm working distance.
Characterization was performed by measuring the diameter of
at least 250 pores in the obtained SEM images using the
program ImageJ.
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