Comparison of the photocatalytic activity of novel hybrid photocatalysts based on phthalocyanines, subphthalocyanines and porphyrins immobilized onto nanoporous gold

Article abstract of DOI:10.1039/d1ra01331a

A series of different singlet oxygen photosensitizers was immobilized onto nanoporous gold powder with a mean pore size of 40 nmviacopper catalyzed azide-alkyne cycloaddition. The attachment of phthalocyanine and porphyrin derivatives was performed on the peripheral substituent of the macrocycle, whereas the subphthalocyanine derivatives were attachedviathe axial substituent with respect to the macrocyclic ring system. All obtained hybrid systems were studied in the photooxidation of 2,5-diphenylfuran as a chemical singlet oxygen quencher and showed increased photocatalytic activity compared to the same amount of the corresponding photosensitizer in solution due to photoinduced interactions of the plasmon resonance of the nanostructured gold support and the attached photosensitizer. The understanding of the different photophysical interactions depending on the coordination mode of the macrocycle as well as the position of the absorbance in the electromagnetic spectrum is an important point in the development towards highly active hybrid photocatalysts covering a broad absorption range within the spectrum of visible light.

Full text of DOI:10.1039/d1ra01331a

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Comparison of the photocatalytic activity of novel
hybrid photocatalysts based on phthalocyanines,
subphthalocyanines and porphyrins immobilized
onto nanoporous gold†
Cite this: RSC Adv., 2021, 11, 11364
ab
c
abd
David Steinebrunner,
Burgos,
Gunter Schnurpfeil, Jan Thayssen,
Jorge Adrian Tapia
¨
ab
a
c
ab
Andre Wichmann, Dieter Wohrle* and Arne Wittstock
*
¨
A series of different singlet oxygen photosensitizers was immobilized onto nanoporous gold powder with
a mean pore size of 40 nm via copper catalyzed azide–alkyne cycloaddition. The attachment of
phthalocyanine and porphyrin derivatives was performed on the peripheral substituent of the
macrocycle, whereas the subphthalocyanine derivatives were attached via the axial substituent with
respect to the macrocyclic ring system. All obtained hybrid systems were studied in the photooxidation
of 2,5-diphenylfuran as a chemical singlet oxygen quencher and showed increased photocatalytic
activity compared to the same amount of the corresponding photosensitizer in solution due to
photoinduced interactions of the plasmon resonance of the nanostructured gold support and the
attached photosensitizer. The understanding of the different photophysical interactions depending on
the coordination mode of the macrocycle as well as the position of the absorbance in the
electromagnetic spectrum is an important point in the development towards highly active hybrid
photocatalysts covering a broad absorption range within the spectrum of visible light.
Received 18th February 2021
Accepted 11th March 2021
DOI: 10.1039/d1ra01331a
rsc.li/rsc-advances
general, this class of metal complex combines very high
extinction coefficients, good triplet state quantum yields and
Introduction
1
Singlet oxygen ( O
2
) as one sort of reactive oxygen species (ROS) lifetimes with reasonable chemical stability, which is oen
5–7
is of special interest, for example as a reactant in many modern a problem in the case of the all organic photosensitizers.
organic transformations or as active part in photodynamic Since such complexes also suffer from singlet oxygen induced
1–3
therapy (PDT). The formation of this ROS is usually achieved self-decomposition, the lifetime of the photosensitizers can be
either by direct chemical release from ozonides or hydrogen drastically improved by immobilization on a support, as it was
peroxide and sodium hypochlorite or by photochemical sensi- shown in the case of zinc(II) phthalocyanines when immobilized
8
tization employing various photosensitizers that transfer energy onto an inert silica support.
3
from an excited triplet state to the ground state of oxygen ( O ,
A promising new approach towards highly active singlet
S ) under formation of reactive singlet oxygen ( O , D
g 2 g
). Apart oxygen sensitizers is the combination of a classical porphyr-
2
3
1
1
4
from all organic photosensitizers like rose bengal or methylene inoid sensitizer with an optically active support, where the
blue, porphyrinoid metal complexes like zinc(II) phthalocya- photocatalytic activity can be further improved by interactions
9
,10
nine- or zinc(II) porphyrin derivatives are among the most used between the active support and the immobilized sensitizer.
5,6
and studied photocatalysts for singlet oxygen sensitization. In Nanostructured gold systems, as for example gold nanoparticles
AuNPs) or nanoporous gold (npAu), both show strong absorp-
(
tion of visible light due to their surface plasmon resonance and
a
Institute of Applied and Physical Chemistry, Center for Environmental Research and are therefore ideal candidates in the context of such novel
Sustainable Technology, University Bremen, Leobener Str. UFT, 28359 Bremen,
9–13
hybrid photocatalysts.
Whereas colloidal AuNP based hybrid
Germany. E-mail: awittstock@uni-bremen.de
b
materials behave more like a pseudo homogeneous system,
monolithic npAu systems represent a truly heterogeneous
material as hybrid photosensitizer. Therefore, AuNP based
systems are promising candidates in PDT applications due to
MAPEX Center for Materials and Processes, University Bremen, Bibliothekstr. 1,
2
8359 Bremen, Germany
c
Organic and Macromolecular Chemistry, University Bremen, Leobener Str. NW2,
8359 Bremen, Germany. E-mail: woehrle@uni-bremen.de
2
d
Institute for Organic and Analytical Chemistry, University Bremen, Leobener Str. UFT, their homogeneous behavior and the biocompatibility of
14–16
2
†
1
8359 Bremen, Germany
AuNPs.
npAu on the other hand is a perfect example for
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra01331a
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Scheme 1 Molecular structures of the different substituted photo-
sensitizers; zinc(II) phthalocyanines (ZnPcs, 1-2), the tetraphenylpor-
phyrins (TPPs, 3–6) and the boron(III) subphthalocyanines (BsubPcs, 7–
9).
systems. The effect of the position of the sensitizer absorption
and the resulting photophysical interactions will be discussed
in detail. Besides that, the inuence of the alkyl chain length on
the photosensitizer and the resulting overall orientation
resulting from axial and peripheral sensitizer attachment are of
major interest in this study.
Fig. 1 (a) SEM micrograph of the npAu structure with a magnification Results and discussion
of 100k; (b) corresponding transmission spectrum of a npAu foil in
a DMF environment with same pore sizes showing the longitudinal
Synthesis and characterization
(
490 nm) and transverse (580 nm) absorption of the plasmon
Nanoporous gold powder with a mean pore size of 40 nm was
prepared employing a free corrosion method selectively leach-
resonance.
9,10
ing Ag out of an Ag/Au alloy. Organic functionalization of the
obtained npAu support was achieved following a two-step
a heterogeneous catalyst that can be easily employed in both,
batch and ow reactor systems.
The nanoporous structure of npAu can be easily obtained by
corrosion of a suitable bimetallic alloy such as Ag/Au, where the
macroscopic shape of the material is already determined by the
shape of the starting alloy.
a complex process consisting of leaching of the less noble ad-
metal followed by reorganization of the gold atoms in the
evolving ligaments.
parameter for the average size of the obtained pores and liga-
9,10
approach as reported previously by us (Scheme 2). In a rst
17–20
step, a SAM employing 6-azidohexyl thioacetate as linker unit is
prepared on the npAu surface resulting in an azide function-
alized surface. In the second step, the different photosensitizers
were attached to the azide-terminated SAM via CuAAC to give
the desired hybrid systems.
All of the obtained hybrid systems were analyzed regarding
the pore sizes of the support and the distribution of the
immobilized photosensitizer by scanning electron microscopy
17–20
The evolution of nanoporosity is
17–21
The dealloying time thereby is a crucial
(SEM) and energy dispersive X-ray (EDX) spectroscopy. The pore
ments in the material, which also have a direct inuence on the
sizes were found to stay constant at 40 nm unaffected by the
functionalization. The employed photosensitizers all showed
a homogeneous distribution over the npAu support, which was
22–24
position of the surface plasmon resonance.
used a npAu powder with average pore size of around 40 nm
Fig. 1a) showing a typical optical behavior with the longitudinal
In this study we
(
10
shown by EDX mapping and line scan experiments. For the
part of the plasmon resonance at 490 nm and the transverse
part at around 580 nm (Fig. 1b).
quantication of the amount of immobilized photosensitizer,
On this active support, different photosensitizers from the
classes of phthalocyanines, porphyrins and sub-phthalocyanines
(Scheme 1) were immobilized following a two-step approach of
self-assembled monolayer (SAM) formation and subsequent
attachment via copper catalyzed azide–alkyne cycloaddition
(CuAAC). The different classes of photosensitizers were selected
to obtain various hybrid systems covering the whole range of
visible light absorption as a complete set of panchromatic singlet
oxygen sensitizing systems.
All of the obtained hybrid systems were studied in the
photooxidation of 2,5-diphenylfuran (DPF) as singlet oxygen
Scheme 2 Schematic representation of the npAu hybrid preparation;
formation of an azide terminated SAM using 6-azidohexyl thioacetate
followed by attachment of the different photosensitizers via CuAAC
quencher, allowing
a comparison between the different
(“click reaction”).
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Scheme 3 Reaction scheme for the photooxidation of the chemical
singlet oxygen quencher 2,5-diphenylfuran (DPF).
the hybrid systems were dissolved in ultra-pure aqua regia and
subsequently analyzed by inductively coupled plasma mass
spectrometry (ICP-MS), determining the fraction of the central Fig. 2 UV-Vis spectra for the photosensitizer zinc(II) phthalocyanine
with either peripheral propynoxy- (1, cyan) or hexynoxy-substituents
(2, black) recorded in DMF.
metal compared to the support quantity (Table S1†). In the case
ꢀ
1
of immobilized ZnPcs, quantities of around 150 mg g hybrid
9,10
catalyst were found as determined already in earlier studies.
For the TPP derivatives slightly lower quantities of around 110
ꢀ
1
absorption of the Q-band around 680 nm with high molar
mg g
catalyst were determined whereas for the BsubPcs
ꢀ
1
ꢀ1
ꢀ
1
extinction coefficients of 140 000 L mol cm and a weaker
absorption at around 360 nm of the Soret-band (Fig. 2).
quantities of around 500 mg g were found due to the higher or
lower sterical demand of the macromolecules.
Under panchromatic irradiation between 400 and 800 nm,
both ZnPc derivatives show singlet oxygen sensitization respon-
sible for the DPF oxidation. The activity of both derivatives in
solution revealed similar TON indicating the same photocatalytic
activities, corresponding to a total DPF conversion of around 10%
aer 30 min irradiation time. In contrast, aer immobilization
onto the npAu support, both ZnPc hybrid systems showed an
increase in activity, even stronger for the hybrid consisting of the
propynoxy-substituted ZnPc derivative (Fig. 3). The origin of the
increased photocatalytic activity of the ZnPc photosensitizer
when immobilized onto the npAu support was shown in earlier
studies to be a consequence of the heavy-atom effect as well as
energy transfer of the nanostructure caused plasmon resonance
The photocatalytic singlet oxygen sensitization activity of the
heterogeneous hybrid photocatalysts was determined by irra-
diation of the catalysts with a 300 W Xe arc lamp and a light
ꢀ2
intensity of 180 mW cm within a self-built setup described
1
0,25
1
elsewhere.
For the quantication of O
2
formed during the
quencher, allowing
1
catalyst run, DPF was used as chemical O
2
to follow the reaction progress via UV-Vis spectroscopy by the
decrease of the DPF absorption band employing Lambert Beer's
rule. DPF is structurally related to the standard chemical
quencher 1,3-diphenylisobenzofuran (DPBF), reacting selec-
1
tively with O via an endo-peroxide to a diketo species (Scheme
2
3). This quencher was used instead of DPBF as the absorption is
signicantly blue-shied and therefore irradiation without self-
decomposition is also possible below 500 nm, where the main
absorption band of the porphyrin derivatives is located. For
every DPF photooxidation, the corresponding turnover
numbers (TON) and turnover frequencies (TOF) were calculated
for quantication of the activity based on the fraction of irra-
diated photosensitizer as described in detail within the ESI.†
Noteworthy, the bare npAu without attached sensitizer showed
no catalytic activity, i.e., generation of singlet oxygen under all
studied conditions.
10,12,13
to the attached photosensitizer.
The orientation of the immobilized ZnPc derivatives on the
npAu surface was shown to adopt a planar geometry due to the
multidentate properties of the peripheral substituents enabling
25
up to four binding sites to the azide groups of the SAM. Due to
the planar orientation, it was also shown that the central zinc
Phthalocyanines on npAu
The photocatalytic activity of the zinc(II) phthalocyanine (ZnPc)
derivatives immobilized on various npAu supports had been
investigated already in detail in the photooxidation of
10,23,25
DPBF.
However, this class of hybrid material was also
analyzed in the photooxidation of DPF, acting as reference
system in comparison to the novel hybrid materials employing
other macrocyclic metal complexes. For this purpose, two zin-
c(II) phthalocyanines containing either propynoxy- (1) or hex-
ynoxy- (2) peripheral substituents were studied in the
photooxidation reaction of DPF before and aer immobilized
on a npAu powder support. The ZnPc derivatives exhibit the
Fig. 3 Photocatalytic DPF conversion given as TON vs. reaction time
employing either the ZnPc with peripheral propynoxy- (1, H1, cyan) or
hexynoxy-substituents (2, H2, black) in solution (dashed line) and when
typical phthalocyanine absorption behavior with the maximum immobilized on the npAu support (bold line).
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The UV-Vis spectra of those TPP derivatives all exhibit the
typical porphyrinoid absorption characteristics, with the main
absorption of the Soret-band around 420 nm and a weaker and
splitted Q-band between 500–650 nm. As for the ZnPc deriva-
tives, the ZnTPP sensitizers exhibit identical UV-Vis spectra,
with a maximum molar extinction coefficient of 230 000 L
ꢀ
1
ꢀ1
mol cm , signicantly higher than for the ZnPc sensitizers.
The MgTPP exhibits a weak bathochromic shi in respect to the
2
ZnTPP derivatives, whereas the metal-free H TPP shows a hyp-
sochromic shi and an additional splitting of the Q-band,
33
a typical observation for metal-free porphyrinoids. The
molar extinction coefficients for those two derivatives were
Fig. 4 UV-Vis spectra for different tetraphenylporphyrin photosensi- found in the same range as for the ZnTPP derivatives (Fig. 4).
tizers: zinc(II) tetraphenylporphyrin with either peripheral propynoxy-
Although the ZnTPP sensitizers offer signicantly higher
(3, cyan) or hexynoxy-substituents (6, black) as well as the magne-
extinction coefficients in comparison to the ZnPc sensitizers,
the observed photocatalytic activities of the sensitizers in
solution showed the same singlet oxygen sensitization activity.
This can be explained by the position of the main absorption
band at higher energies, leading to a larger energy difference
between the excited triplet state of the sensitizer with respect to
the singlet oxygen energy level, thus making energy transfer less
efficient. Aer immobilization onto the npAu powder, the
sium(II) tetraphenylporphyrin (5, blue) and metal-free tetraphenylpor-
phyrin (4, orange) both bearing peripheral hexynoxy-substituents. All
spectra were recorded in DMF.
ion forms an additional axial coordination to free azide groups of
the SAM resulting in a penta-coordinated central ion known for
25–32
various ZnPc and ZnTPP derivatives.
Within this orientation of
the immobilized ZnPc, the axial coordination is stronger for the ZnTPPs showed a similar behavior as the ZnPcs, again with
ZnPc derivative with shorter peripheral substituents, explaining stronger enhancement for the propynoxy-substituted ZnTPP
the stronger observed increase in photocatalytic activity for the derivative, but an overall smaller increase in activity in
comparison to the ZnPc derivatives (Fig. 5). The weaker increase
in activity can be explained by the lower spectral overlap of the
ZnTPP absorption band with the plasmon resonance of the
npAu support. In addition, the main interaction is expected to
proceed via the Q-band of the TPP sensitizer, having molar
extinction coefficients in the order of one magnitude below the
Q-band absorption of the ZnPc derivatives. Nevertheless, the
higher activity of the ZnTPP based hybrid H3 in comparison to
hybrid H6 indicates a similar penta-coordinated geometry of
the central Zn ion as observed for the ZnPc photosensitizers.
The MgTPP derivative showed similar behavior as compared
to the ZnTPP derivatives, but with signicantly lower activity for
both, the sensitizer in solution as well as immobilized on the npAu
support. An interesting behavior was observed for the metal-free
hybrid system H1 in comparison to H2.
Porphyrins on npAu
For the investigation of different TPP based photosensitizers,
zinc(II) tetraphenylporphyrin (ZnTPP, 3, 6) derivatives with
peripheral propynoxy- and hexynoxy-substituents similar to the
ZnPc sensitizers were used. In addition, a magnesium(II) tetra-
phenylporphyrin (MgTPP, 5) as well as a metal-free tetraphe-
2
nylporphyrin (H TPP, 4) were chosen to determine the inuence
of the central metal cation as well as the situation, where the
penta-coordinated geometry could not be formed due to the
lack of the central metal ion.
2
H TPP derivative, where the direct interaction via axial coordina-
tion is excluded. In only this case, the observed photocatalytic
activity aer immobilization onto the npAu was smaller than
compared to the photosensitizer in solution. However, the amount
of immobilized sensitizer could only be assumed to be the same as
for the other TPP derivatives, as the lack of the central metal ion
makes the determination via ICP-MS impossible.
Subphthalocyanines on npAu
The class of boron(III) subphthalocyanines (BsubPcs) used for
immobilization was chosen because of their good singlet
34
oxygen sensitization activity and high chemical stability. The
macrocyclic ring system in this class of sensitizers is structurally
similar to the common phthalocyanine ring, comprising of only
three azomethine-bridged isoindole units around a boron
central ion (Scheme 1). In addition, this class of sensitizers
offers the possibility of a direct axial linkage to the npAu
Fig. 5 Photocatalytic DPF conversion given as TON vs. reaction time
employing either the ZnTPP with peripheral propynoxy-(3, H3, cyan),
with hexynoxy-substituents (6, H6, black), the MgTPP (5, H5, blue) or
2
the metal-free H TPP (4, H4, orange) in solution (dashed line) and
when immobilized on the npAu support (bold line).
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macrocycles. Whereas the binding to the npAu is achieved in
peripheral position in the case of the phthalocyanine and
porphyrin photosensitizers, in the case of the BsubPcs the
binding to the npAu is achieved in axial position. Therefore, the
secondary axial coordination point in this case is blocked,
leading to the situation where not only the chain length of the
SAM determines the distance between the npAu surface and the
attached sensitizer, but the chain length in axial position also
contributes to the overall distance.
This is reected by the obtained photocatalytic activities of
the BsubPcs, where the derivative 8/H8 showed a higher
increase in activity aer immobilization compared to the
Fig.
6
UV-Vis spectra for the photosensitizer boron(III) sub- sensitizers with a shorter and longer alkyl chain. This can be
phthalocyanine with either axial propynoxy- (7, cyan), hexynoxy- (8,
black) or undecynoxy-substituents (9, magenta) recorded in DMF.
explained by the fact that this distance shows the best case for
successful energy transfer from the npAu to the photosensitizer,
whereas in the case of the undecynoxy-substituent the energy
transfer efficiency decreases due to the large distance of around
support, which also results in a non-planar, bowl-shaped
geometry of the photosensitizer.
3
nm. On the other hand, in the case of the propynoxy-
substituted BsubPc photosensitizer with a distance of around
nm, electron transfer from the photosensitizer to the npAu
34
The UV-Vis spectra of the three different BsubPc derivatives
–9 show similar absorption bands as the ZnPc derivatives,
1
7
support is dominant resulting in quenching of the excited state
of the photosensitizer. In addition, BsubPcs are known to act as
strong electron donors resulting in photoinduced charge
transfer to the npAu enabling the needed energy transfer to
molecular oxygen and thus resulting in smaller singlet sensiti-
however exhibiting a large blue-shi of both, the Soret- and the
Q-band. The strongest absorption band in this class of sensi-
tizer is again the Q-band at around 550 nm (Fig. 6). The molar
extinction coefficients are very low in comparison to the parent
ꢀ
1
ꢀ1
ZnPc photosensitizers with values around 21 000 L mol cm
.
35–37
zation activity.
On the other hand, those photosensitizers exhibit the largest
spectral overlap with the plasmon resonance of the npAu
support, making this combination a promising design for
a novel hybrid material.
Comparison of the different hybrid systems
For the nal comparison of the photocatalytic activities of the
As consequence of the small extinction coefficients, all of the different photosensitizers in solution as well as aer immobi-
BsubPc derivatives showed only small singlet oxygen sensitiza- lization onto the npAu surface, the corresponding TOF values
tion activities in solution, exhibiting TOF values of around for all systems were determined (Fig. S1–S9†). In the case of the
ꢀ
1
3
0 min . In comparison to the earlier employed ZnPc and ZnPc (1,2) and ZnTPP (3,6) photosensitizers, TOF values around
ꢀ1
ZnTPP photosensitizers, those class of sensitizers showed the 130 min were observed for both derivatives when subjected to
lowest overall activity in solution (Fig. 7). However, aer DPF oxidation providing the photosensitizer in solution (Fig. 8).
immobilization onto the npAu support, a completely different
behavior regarding the alkyl chain length of the sensitizer was all showed an increase in
Aer immobilization onto the npAu, those photosensitizers
1
O sensitization activity, in both
2
observed. The reason for this behavior is the different substi- cases with a larger increase for the propynoxy-substituted ZnPc
tution position compared to the phthalocyanine and porphyrin and ZnTPP derivatives (H1,H3). The origin of the difference in
enhancement was shown to result from stronger axial
Fig. 7 Photocatalytic DPF conversion given as TON vs. reaction time
employing either the BsubPc with peripheral propynoxy-(7, H7, cyan), Fig. 8 Comparison of the determined photocatalytic activities given
with hexynoxy- (8, H8, black) or with undecynoxy-substituents (9, H9, as TOF values for DPF oxidation for the different photosensitizers
magenta) in solution (dashed line) and when immobilized on the npAu when provided in solution (cyan) and after immobilization onto the
support (bold line).
npAu support (black).
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18
38–40
coordination of the central metal ion to free azide groups from
a
literature procedure.
6-Azidohexyl-1-thioacetate
as
the SAM for the derivatives bearing shorter peripheral substit- building block for SAM formation, tris(benzyl-triazolylmethyl)
25
1
41,42
uents. In addition, a larger enhancement of O sensitization amine (TBTA)
as co-catalyst for the CuAAC reaction and
2
was observed for the ZnPc derivatives compared to the ZnTPP the
photosensitizers due to a larger spectral overlap integral phthalocyanine zinc(II) (ZnPc-3, 1), 2,9,16,23-tetrakis(5-hexyn-
between the sensitizer absorption and the plasmon resonance 1-yloxy)phthalocyanine zinc(II) (ZnPc-6, 2), and 5,10,15,20-tet-
photosensitizers 2,9,16,23-tetrakis(2-propyn-1-yloxy)
43
9
of the npAu support.
rakis(4-(prop-2-yn-1-yloxy)phenyl)porphyrin zinc(II) (ZnTPP-3,
44
The importance of the axial coordination of the central metal 3) were all synthesized as described elsewhere. The following
ion and its inuence onto the overall activity of the hybrid chemicals and solvents were bought as indicated and were all
material was shown exemplarily with the ZnTPP derivative (6/ used as received without further purication: Cu(MeCN) PF6
4
H6), where for comparison also the corresponding MgTPP (5/ (97%, Aldrich), hydroquinone (Merck), 2,5-diphenylfuran (DPF,
H5) and H TPP (4/H4) derivatives were investigated. In the case >98.0%, TCI), EtOH (abs., reagent grade, VWR), THF (reagent
2
of the MgTPP derivative, a weaker increase in activity was found grade, $99.0%, VWR), DMF (analytical reagent grade, $99.5%,
aer immobilization onto the npAu in comparison to the parent VWR), HNO
ZnTPP derivative, due to a weaker axial coordination of the (NORMATOM, ultra-pure for trace analysis, 67%, VWR) and HCl
magnesium central ion. The H TPP derivative, where no axial (NORMATOM, ultra-pure for trace analysis, 34%, VWR).
3 3
(analytical reagent grade, 65%, VWR), HNO
2
coordination can be formed, was found to be the only derivative
where no increase in activity aer immobilization was observed porphyrin (H TPP-6, 4). Adapting a literature procedure,
Synthesis of 5,10,15,20-tetrakis([hex-5-yn-1-yloxy]phenyl)
45
2
as result of the lack of the axial coordination.
In contrast to the ZnPc and ZnTPP photosensitizers, the TOF mmol) was dissolved in dry acetonitrile (50 mL) under nitrogen
values for DPF oxidation of the BsubPc derivatives in solution atmosphere. Subsequently, dry K CO (205 mg, 1.48 mmol,
5,10,15,20-tetra-(4-hydroxyphenyl) porphyrin (100 mg, 0.148
2
3
ꢀ
1
ꢁ
were found to be only around 30 min as result of the molar dried at 180 C under vacuum for 24 h) was added and the
extinction coefficients that are one order of magnitude smaller mixture was stirred at room temperature for one hour before 6-
than for the other two classes of photosensitizers. Aer immo- bromohex-1-yne (113 mg, 0.7 mmol) was added and the mixture
bilization onto the npAu support, the BsubPc derivatives was stirred under reux for further 24 h. Aer the mixture was
showed different properties compared to the ZnPc and ZnTPP poured onto cold HCl (1 N), the blue-purple solid was isolated
derivatives, with only an increase in activity for the hybrid H8. by ltration and redissolved in CH Cl aer drying. Then the
2
2
organic layer was washed with aqueous Na CO (250 mL, 5%)
2
3
and H
over Na
In this study, different singlet oxygen photosensitizers were sure to yield the desired compound as blue-purple crystals (87%
2
O for three times. The organic layer was isolated, dried
Conclusions
2
SO and the solvent was removed under reduced pres-
4
1
immobilized successfully onto a npAu support and studied in yield). H NMR (600 MHz, CDCl
3
, ppm): d ¼ 8.89 (s, 8H), 8.13 (d,
the photooxidation of DPF. Besides the previously studied 8H), 7.30 (d, 8H), 3.95 (t, 8H), 2.3–2.2 (m, 8H), 1.92–1.88 (m, 4H),
hybrid systems bearing ZnPc derivatives as immobilized chro- 1.77–1.73 (m, 8H), 1.56–1.48 (m, 8H), ꢀ2.68 (s, 2H). HR-MS: m/z
+
+
mophores, hybrid systems comprising of different TPP and ¼ 999.48411 [M + H] , calc. for C68
H
N
4
O
m/z ¼ 999.48438.
63
4
BsubPc derivatives were studied and compared to the parent UV-Vis (CH Cl ): l
max
[nm] ¼ 421, 556, 594, 648.
2
2
hybrid system. The original ZnPc based hybrid systems showed
the highest overall photocatalytic activity as well as the highest
increase in activity aer immobilization onto the npAu support.
2
General procedure for the metalation of H TPP-6
46
In addition, most of the photosensitizers used in this study Aer a modied literature procedure, a solution of Mg(OAc)
showed increased photocatalytic activity aer immobilization, as or Zn(OAc) (0.655 mmol) in MeOH was added to a solution of
result of synergistic energy transfer from the npAu plasmon TPP (4, 0.488 mmol) in CHCl (30 mL) and the resulting
2
2
H
2
3
resonance to the attached macromolecules, with different mixture was stirred at room temperature for 1 h. Aer TLC
enhancement factors depending strongly on the position of the conrmed completion of the metalation reaction, the mixture
sensitizer absorption and the spectral overlap with the plasmon was poured onto H O (100 mL) followed by extraction with
2
resonance. Therefore, the appropriate selection of the sensitizer CH Cl . The organic layer was separated, washed with H O and
2
2
2
can be based on the process conditions, offering a complete set of brine and was dried over anhydrous Na SO . Aer ltration, the
2
4
heterogeneous photocatalysts with absorption variability and wide solvent was removed under reduced pressure to get the desired
applicability. In addition, the presented set of hybrid materials can compound as a purple powder.
be employed in further studies as building blocks towards highly
active, panchromatic heterogeneous photocatalysts.
5,10,15,20-Tetrakis(4-(hex-5-yn-1-yloxy)phenyl)porphyrin mag-
1
nesium(II) (MgTPP-6, 5). Yield: 89%. H NMR (600 MHz, CDCl
ppm): d ¼ 8.85 (s, 8H), 8.20 (m, 8H), 7.72 (m, 8H), 3.98 (t, 8H), 2.1–
.2 (m, 8H), 1.9–1.88 (m, 4H), 1.74–1.68 (m, 8H), 1.61–1.48 (m, 8H).
3
,
2
Experimental
Materials
+
+
HR-MS: m/z ¼ 1051.50052 [M + H] , calc. for C H MgN O m/z ¼
7
0
67
4 4
1
051.50073. UV-Vis (CH Cl ): l
[nm] ¼ 428, 566, 608.
2
2
max
Ag/Au alloy disks (70 : 30 at%, 5 mm diameter, 200 mm thick-
ness) for npAu preparation were obtained according to zinc(II) (ZnTPP-6, 6). Yield: 92%. H NMR (600 MHz, CDCl ,
3
5,10,15,20-Tetrakis(4-(hex-5-yn-1-yloxy)phenyl)porphyrin
1
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ppm): d ¼ 8.88 (s, 8H), 8.21 (d, 8H), 7.77 (m, 12H), 3.98–3.94 (m, Bruker AVANCE NEO spectrometer and HR-MS (ESI positive,
8
1
H), 2.22–2.18 (m, 8H), 1.92–1.88 (m, 4H), 1.74–1.70 (m, 8H), direct injection) recorded with a Bruker Impact II spectrometer.
+
.57–1.52 (m, 8H). HR-MS: m/z ¼ 1061.39767 [M + H] , calc. for All UV-Vis spectra were recorded using a UV-1600PC spec-
+
C H N O Zn m/z ¼ 1061.39788. UV-Vis (CH Cl ): l
[nm] ¼ trometer from VWR with a resolution of 1 nm. Surface analysis
of the npAu supports and the nal hybrid materials was per-
formed on a scanning electron microscope (SEM, Supra 40,
Zeiss) operated at 15.0 kV acceleration voltage, 300 pA probe
current and 10 mm working distance and equipped with
6
8
61
4
4
2
2
max
4
25, 558, 599.
General procedure for BsubPc preparation
Based on general literature procedure,
phthalocyanine chloride (100 mg, 0.231 mmol) and AgOTf
75 mg, 0.292 mmol) were dissolved in dry toluene (5 mL) and the
4
7
a
boron sub-
a Bruker XFalsh 6/30 EDX detector. For the quantication of
immobilized photosensitizer, the hybrid material (10 mg) was
dissolved in ultrapure aqua regia (2 mL) prior to determination
by ICP-MS using an iCAP-Q spectrometer form Thermo Fisher
Scientic.
(
mixture was stirred for 2 h under an argon atmosphere. Aer TLC
showed full consumption of BSubPcCl, DIPEA (50 mL, 0.29 mmol)
was added followed by addition of the appropriate u-alkyn-1-ol
(1.39 mmol) at which point a color change from purple to red
was observed. Aer continued stirring for 20 h, the reaction DPF photooxidation over npAu hybrids
2
mixture was passed through a short plug of SiO and eluted with
EtOAc (100 mL). The ltrate was concentrated under reduced
pressure, and the solid residue was dissolved in a small volume of
2 2 2
CH Cl before subjected to ash column chromatography (SiO ,
gradient elution from 5–10% EtOAc/toluene) to afford the desired
products with yields between 40–45% as pink powders.
Photocatalytic oxidations were carried out in a self-built setup
under irradiation with a 300 W Xe-arc lamp and a light intensity of
ꢀ2
10,25
1
80 mW cm , as described elsewhere.
In addition, a saturated
3
was used as lter solution
aqueous mixture of NaNO and NaNO
2
for wavelength cutoff below 380 nm. For each photocatalytic
experiment, the reaction vessel was lled with DMF (100 mL), the
powder hybrid catalyst (25 mg) or the photocatalyst dissolved in
DMF (100 mL, 0.1 nmol, molar ratio DPF/photosensitizer
B-(prop-2-ynyl-1-oxy)-subphthalocyaninato boron(III) (BsubPc-
1
3
(
, 7). H NMR (600 MHz, CDCl , ppm): d ¼ 8.76 (dd, 6H), 7.36
3
dd, 6H), 1.55 (d, 2H), 1.31 (t, 1H). HR-MS: m/z ¼ 450.13949 [M +
30 000 : 1) was added and the setup was ushed with O2 for
+ + 11
H] , calc. for C27
H
16BN
): lmax [nm] ¼ 302, 516, 562.
B-(hex-5-ynyl-1-oxy)-subphthalocyaninato boron(III) (BsubPc-
6
O (with B) m/z ¼ 450.13965. UV-Vis
10 min to achieve gas saturation of the solvent. Prior to irradiation,
(
2 2
CH Cl
a DMF stock solution (500 mL) containing DPF (0.66 mg, 3 mmol)
was added. The decrease of DPF concentration was followed via
UV-Vis spectroscopy using Lambert Beers law at the absorption
1
6
(
, 8). H NMR (600 MHz, CDCl
3
, ppm): d ¼ 8.74 (dd, 6H), 7.37
dd, 6H), 1.61 (d, 2H), 1.32 (t, 1H), 1.22 (m, 2H), 1.12 (m, 4H).
ꢀ1
ꢀ1
maximum at 327 nm (3327 ¼ 34 000 L mol cm in DMF).
Photocatalytic activities were determined according to literature
for immobilized sensitizers by determination of the turnover
number (TON, [converted DPF] (mol)/[irradiated photosensitizer]
+
+
HR-MS: m/z ¼ 493.19407 [M + H] , calc. for C H BN O (with
3
0
22
6
1
1
B) m/z ¼ 493.19427. UV-Vis (CH Cl ): l
[nm] ¼ 302, 514,
2
2
max
562.
B-(undec-11-ynyloxy)-subphthalocyaninato boron(III) (BsubPc-
(
mol)) and turnover frequency (TOF, slope of the plot of TON versus
1
1
6
1, 9). H NMR (600 MHz, CDCl
3
, ppm): d ¼ 8.8 (dd, 6H), 7.32 (dd,
ꢀ1
48
reaction time (min )) for every hybrid system.
H), 1.59 (d, 2H), 1.34 (t, 1H), 1.17–1.05 (m, 16H) ppm. HR-MS: m/
+
+
11
z ¼ 563.27233 [M + H] , calc. for C35
H32BN
6
O (with B) m/z ¼
Conflicts of interest
5
63.27252. UV-Vis (CH Cl ): l
[nm] ¼ 300, 516, 564.
2
2
max
There are no conicts of interest to declare.
General procedure for the immobilization of the photosensitizers
npAu powder was prepared employing a free corrosion tech-
nique and functionalization with an azide-terminated SAM was
performed employing an ethanolic solution of 6-azidohexyl-1-
Acknowledgements
We gratefully acknowledge the work of Petra Witte (Department
of Historic Geology and Paleontology, University Bremen)
providing access to the electron microscope and for experi-
mental support during the measurements. In addition, we
thank Cornelia Rybarsch-Steinke (Institute of Applied and
Physical Chemistry, University Bremen) for preparation of the
ICP-MS samples. Funding of the project by Deutsche For-
schungsgemeinscha (DFG) within research grants WI 4497/3-1
and WO 237/42-1 is gratefully acknowledged.
10,25
thioacetate as described previously by us.
The immobiliza-
tion of the photosensitizers was achieved by subjecting the as
prepared azide-functionalized npAu powder to a solution of
Cu(MeCN)
mmol), hydroquinone (272.5 mg, 2.5 mmmol) and the respective
photosensitizer (1–9, 0.15 mmol) dissolved in THF/H O (5 mL,
: 1 v%). Aer a reaction time of 72 h, the reaction solution was
4 6
PF (931.8 mg, 2.5 mmmol), TBTA (1.326 mg, 2.5
2
3
discarded and the hybrid materials H1–H9 were repeatedly
washed with THF to remove any unbound and physisorbed
photosensitizer.
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