Poly-ortho-functionalizable tetraarylporphycene platform-synthesis of octacationic derivatives towards the layer-by-layer design of versatile graphene oxide photoelectrodes

Article abstract of DOI:10.1002/adma.201203459

The most highly charged water-soluble poly-ortho-substituted tetraarylporphycene (Pc⁸⁺) has been synthetized. The latter, which interacts in aqueous solutions with graphene oxide (GO) forming a photoactive ensemble Pc⁸⁺/GO, is immobilized by means of the Layer-by-Layer (LbL) technique onto ITO electrodes to afford novel solar energy conversion devices. Copyright

Full text of DOI:10.1002/adma.201203459

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Poly-Ortho-Functionalizable Tetraarylporphycene Platform–
Synthesis of Octacationic Derivatives Towards the Layer-by-
Layer Design of Versatile Graphene Oxide Photoelectrodes
Wolfgang Brenner, Jenny Malig, Rubén D. Costa, Dirk M. Guldi,* and Norbert Jux*
Dedicated to the memory of Professor Christian Claessens
Porphycene, the first isolated and most stable constitutional
isomer of porphyrin, was synthesized for the first time in 1986
by Vogel and co-workers.[1] To this end, 2,7,12,17-tetra-n-propyl-
porphycene is by far the most commonly studied porphycene
derivative owing to its ease of synthesis and its remarkable
solubility.[2] In the early years, porphycenes were investigated
as therapeutic agents in the field of photodynamic therapy.[3]
Given its structural similarity with porphyrins it is, however,
surprising that porphycenes were hardly ever explored in areas
such as catalysis,[4] protein mimicry,[5] and material science.[6]
As far back as 1966, the concept of layer-by-layer (LbL) assem-
blies attracted considerable attention.[7] The LbL concept gained
new momentum in the 1990s. Here, multicomposite films were
realized by the consecutive adsorption of polyanions and poly-
cations[8] offering a myriad of unique incentives by employing
oppositely charged building blocks–molecules, particulates,
and polymers. Charged polymers as LbL building blocks bring
along a number of disadvantages. In particular, polymers are
poly-disperse and usually do not adopt a defined and controlled
shape and/or orientation within the deposited layers. Taken the
aforementioned into concert, precise fine-tuning of the prop-
erties of polymeric thin films emerges as a real challenge. For
example, Park et al. observed layers with varying charge den-
sity.[9] Molecular rather than polymeric oligoelectrolytes assist
in circumventing such drawbacks, as their structures and their
properties are readily modified and adjusted. Such a strategy
provided novel, hierarchically ordered materials with features
applicable for electronic[10] and mechanical uses.[8,11]
energy conversion,[13] electron donor-acceptor nanoensem-
bles,[14] and functionalization of nanoparticles.[15] Recently, we
have realized the implementation of these 8-fold negatively-
charged porphyrins by means of LbL assembly to pyrene+
functionalized graphene.[16] Importantly, the electron transfer
dynamics were strongly impacted by the presence of electron
accepting graphene. Encouraged by these results we adapted
the aforementioned concept to porphycenes, which act as elec-
tron acceptors rather than as an electron donor.
In contrast to porphyrins, porphycenes display – besides the
typical Soret band – absorption peaks with high extinction coef-
ficients in the red region of the solar spectrum. This renders
them very interesting candidates for the fabrication of solar
cells. Surprisingly, up to now there has been only one publi-
cation dealing with the possible incorporation of porphycenes
into solar cells.[17]
Within the scope of the current study, we focused on an 8-fold
positively-charged, water-soluble porphycene (Pc8+) (Scheme 1) .
Pc8+ is, to the best of our knowledge, the most highly charged
porphycene synthesized to date. To demonstrate the applicability
of this electron acceptor in solar energy conversion devices,
we tested negatively charged graphene oxide (GO), which has
recently been used as a hole transport layer in polymer solar
cells.[18] Control over successfully assembling Pc8+ and GO to
yield novel photoactive electrodes was possible through electro-
static interactions in solution and on electrode surfaces.
Since the pioneering work by Vogel et al.[1] different routes
for the synthesis of porphycenes have been explored, focusing
on experimental facilitation,[19] avoidance of unstable interme-
diates,[20] exploration of an easily modifiable bipyrrole,[21] and
reduction of synthetic steps.[22] The formation of appropriately
substituted pyrroles is more challenging and requires the cor-
responding aldehydes as building blocks and starts with the for-
mation of the respective nitroolefins followed by a Barton–Zard
reaction, delivering the desired pyrroles in yields of up to 90%.[23]
Due to steric demands imposed by aldehyde 1 (Scheme 1),
the corresponding nitro-olefin could not be prepared at all.
In a similar approach, the ethyl cinnamate derived from 1 for
the Sanchez-García’s route was assumed not to be accessible
either.[22] Finally, the desired pyrrole was synthesized under
more drastic conditions. In a slight modification of the pyrrole
formation described by Matsumoto et al.,[24] DBU (1,8-diazabi-
cyclo[5.4.0]undec-7-ene) was added to a mixture of aldehyde 1
(Scheme 1) and ethyl isocyanoacetate.[12] The latter led to a 20%
yield for a 24 hours reaction. In the context of iodination of the
free α-position of the pyrrole, we modified a procedure first
In this context, we have recently reported the design, syn-
thesis, and characterization of an 8-fold positively-charged por-
phyrin,[12] followed later by an 8-fold negatively-charged por-
phyrin.[13] By employing di-ortho-functionalized aryl rings as
substituents in the meso-positions, aggregation was prevented
and the charged substituents are well aligned. These poly-ortho-
substituted porphyrins have played an important role in solar
W. Brenner, J. Malig, Dr. R. D. Costa,
Prof. D. M. Guldi, Dr. N. Jux
Department of Chemistry & Pharmacy
and Interdisciplinary Center for Molecular Materials
Friedrich-Alexander-Universitat
Erlangen-Nurnberg, 91054 Erlangen, Germany
E-mail: dirk.guldi@chemie.uni-erlangen.de;
norbert.jux@chemie.uni-erlangen.de
DOI: 10.1002/adma.201203459
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Scheme 1. Synthesis of porphycene Pc8+. In the boxes photophysically investigated porphycenes are shown.
described by Dolphin et al.[25] In particular, by dissolving pyr-
role in acetic acid and treating it with an excess of iodine mono-
chloride afforded α-iodinated pyrrole 3 in quantitative yields.[26]
Following iodination, Ullmann coupling was conducted before
bipyrrole 4 was saponified and decarboxylated. The unsub-
stituted bipyrrole 5 was then subjected to Vilsmeier-Haack
formylation.
bearing eight bromomethyl substituents. The need for such
a long reaction time relates to the slow replacement of chlo-
romethyl with bromomethyl groups. With the poly-ortho-bro-
minated porphycene 8 in hands, the 8-fold positively-charged
tetraarylporphycene Pc8+ was obtained by heating with tert-
butylpyridine in toluene at reflux temperature.
The electronic properties of porphycene PcOMe and the pos-
itively charged porphycene Pc8+ in the ground state resemble
those known for tetrakis-(tert-butylphenyl) porphycene (Pc).[26] As
shown in Figure 1, the Soret- and Q-bands are 4 nm (284 cm−1 )
and 14 nm (328 cm−1 ) hypsochromically shifted in PcOMe
with respect to Pc. In agreement with solvatochromic effects,
the absorption bands of Pc8+ are 5 nm (354 cm−1 ) and 7 nm
(166 cm−1 ) red-shifted with respect to PcOMe.
Mass spectra analysis confirmed that during the formyla-
tion reaction with DMF/POCl3 substitution of some methoxy
groups by chloride atoms took place. Owing to the stability of
halogenated compounds under McMurry conditions,[27] a low-
valent titanium-mediated coupling procedure was employed.
For analysis, PcOMe was separated by column chromatography.
1
The steric demand of the substituents is derived from the H
NMR spectra, especially from the signals of the phenyl-CH2
groups (Figure S1). Interestingly, the C–C rotation centered
at the phenyl/pyrrole connection is not hindered in pyrrole 2
(one singlet), but inhibited in iodopyrrole 3 and bipyrrole tet-
racarboxylic ester 4 (two doublets). In stark contrast, removing
the esters in the 3,3’,5,5’-positions releases the rotation (one
singlet), before the signals appear again as two doublets in the
resulting porphycene PcOMe.
Next, the electrochemical features were determined for Pc8+
(10−4 M) in acetonitrile and for PcOMe (10−4 M) in THF with
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as
supporting electrolyte and Fc/Fc+ as standard versus Ag/Ag+. In
terms of reduction, two quasi-reversible reduction steps evolve
at −0.65 and −0.93 V for PcOMe as well as at −0.41 and −1.14 V
for Pc8+. In terms of oxidation, the two oxidation steps at +0.86
and +1.24 V for PcOMe as well as at +0.86 and +1.27 V for Pc8+
are quasi-reversible.
Following the McMurry reaction, the mixture of different
porphycenes PcOMe/7 was treated with hydrobromic acid
in acetic acid for seven days to yield exclusively porphycene 8
Initial insights into the excited state characteristics of PcOMe
and Pc8+ stem from fluorescence spectroscopy. When compared
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Our initial investigations focused on titration assays of por-
phycene Pc8+ and graphene oxide (GO) monitoring the changes
of the absorption and emission features of Pc8+ as different con-
centrations of GO were added. In the ground state, an increase of
the absorbance was measured when GO was present (Figure S2)
suggesting the formation of Pc8+/GO electron donor-acceptor
hybrids. In the excited state, the fluorescence of Pc8+ is gradu-
ally quenched upon GO addition (Figure S2). The latter reflects
in readily formed Pc8+/GO either an effective transduction of
excited state energy from photoexcited Pc8+ to GO or an effec-
tive electron transfer from GO to photoexcited Pc8+.[29]
As a complement to the latter, the changes in fluorescence
lifetimes were monitored for different Pc8+/GO ratios (Figure 1).
In the absence of GO, the fluorescence time profile of Pc8+, as
determined, for example, at the 650 nm fluorescence maximum
by exciting at 403 nm, is dominated by long-lived component of
6.2 ns (94%) (vide infra). Upon adding GO, only a short-lived
component, which is, however, masked by the instrumental
resolution of 0.1 ns, is seen in the fluorescence decay of Pc8+.
We consider the latter as yet another attest for the association of
Pc8+ and GO owing to electrostatic interactions.
To exclude aggregation due to charge neutralization a solu-
tion of Pc8+/GO was drop casted onto a silicon wafer and tested
by means of atomic force microscopy (Figure S3). Given the
fact that single layer flakes of GO are observed indicates that
Pc8+/GO is non-aggregated.
Encouraged by these results, we turned to fabricate photo-
electrochemical cells with photoactive indium tin oxide (ITO)
electrodes modified by the sequential LbL deposition of Pc8+
and GO, that is, ITO/PDDA/PSS/(Pc8+/GO)x. As reference elec-
trodes, GO was replaced with PSS to result in ITO/PDDA/PSS/
(Pc8+/PSS)x. Here, PDDA is poly-(diallyl-dimethylammonium),
PSS is poly-(sodium-4-styrenesulfonate), and x varies from 1 to
14. To complete the devices we utilized polysulfide electrolytes
(i.e., equimolar 3 M Na2S/S/NaOH) and Cu2S-based counter
electrodes–see Supporting Information. The LbL formation was
monitored after each deposition step by absorption and fluores-
cence spectroscopy. Of particular importance is the fact that the
resulting absorption spectra of Pc8+ in both ITO/PDDA/PSS/
(Pc8+/GO)x, and ITO/PDDA/PSS/(Pc8+/PSS)x resemble those
seen in aqueous solutions and that the overall absorptions
increase linearly as a function of deposition steps (Figures 2, S4,
and S5). In complementary experiments the film thickness was
probed by ellipsometric measurements (Figure 2). To this end,
we note a linear relationship, which corroborates the absorp-
tion measurements. In ITO/PDDA/PSS/(Pc8+/GO)x a quantita-
tive fluorescence quenching of Pc8+ is discernable upon 403 nm
excitation. This is in good agreement with the homogeneous
experiments performed in water (vide supra). Finally, to confirm
the notion of a controlled two-dimensional film growth, scan-
ning electron microscopy (SEM) images were performed. The
latter shows a good coverage of the ITOs featuring the absence
of GO aggregation (Figures 2 and S6). As such, we conclude
the successful control over the homogeneous LbL deposition as
means to integrate Pc8+ and GO onto photoelectrodes.
Figure 1. Upper part–absorption spectra of PcOMe (light grew) in THF,
Pc8+ (black) in THF, and Pc (dark grey) in THF. Central part–3D fluores-
cence of Pc8+ in water. Lower part–time-correlated single-photon counting
of Pc8+ in water in the absence (black) and in the presence of variable
concentrations of GO (dark grey to light grey)–excitation wavelength of
403 nm and emission wavelength of 650 nm.
to Pc, the fluorescence maxima are hypsochromically shifted to
652 nm for PcOMe (in THF) and 660 nm for Pc8+ (in water),
which agrees with the ground state features (vide supra). The
fluorescence quantum yields were determined by using the
gradient method[28] as 0.9 and 0.4 for PcOMe and Pc8+, respec-
tively. Finally, the major components in the fluorescent excited
state decay are 6.5 ns (PcOMe) and 6.2 ns (Pc8+).
The modified ITO/PDDA/PSS/(Pc8+/GO)x reveal prom-
ising photoresponse profiles under AM 1.5 conditions, which
increase as a function of Pc8+/GO sandwich layers (Figure 3).
Starting at around 10 layers, we note a plateau in the performance
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Figure 3. Upper part–photoaction spectrum of a ITO/PDDA/PSS/(Pc8+
/
GO)14 photoelectrode under AM 1.5 conditions. Central part − photo-
current-voltage characteristics of ITO/PDDA/PSS/(Pc8+/GO)x photo-
electrodes with increasing number of Pc8+/GO sandwich layers–up to
14–under AM 1.5 conditions. Lower part − photocurrent-voltage charac-
teristics of ITO/PDDA/PSS/(Pc8+/PSS)x photoelectrodes with increasing
number of Pc8+/PSS sandwich layers–up to 10–under AM 1.5 conditions.
Figure 2. Upper part–plot of absorbances at 385 and 650 nm of Pc8+
GO LbL assemblies on ITO/PDDA/PSS with increasing number of Pc8+
GO sandwich layers–up to 14. The inset shows the absorption spectra.
Central part–ellipsometric thickness of Pc8+/GO LbL assemblies on Si/
PDDA/PSS as a function of Pc8+/GO sandwich layers–up to 14. Lower
part–SEM image of Pc8+/GO LbL assembly on ITO/PDDA/PSS.
/
/
Na3PO4 and sodium ascorbate purged with O2 and Ar) rather
than polysulfide (Figure S7).
reaching short-circuit current densities (Jsc) between 100
and 120 μA/cm2 and open-circuit voltages (Voc) of around
0.15 V. Under monochromatic illumination, that is, the inci-
dent photon to charge carrier efficiency (IPCE) spectrum, a
good resemblance with the absorption features prompts to the
photosensitizing features of Pc8+ (Figure 3). Next, the direction
of charge flow was probed by systematically varying the electro-
lyte composition from an electron mediator (i.e., Na3PO4 solu-
tion purged with O2 and Ar) to a hole mediator (i.e., mixture of
In the absence of GO, namely in ITO/PDDA/PSS/(Pc8+/
PSS)x devices, free charge carriers are generated through inter-
facial interactions between photoexcited Pc8+ and the redox
electrolyte. Such a mechanism is, however, only effective when
a maximum of two Pc8+/PSS sandwich layers are present.
Charge formation at the interface, charge transport through the
sandwich layers, and charge collection at the ITO electrodes is
blocked upon increasing the number of layers. As a matter of
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R. Davidi, C. Averbuj, S. Kimel, J. Photochem. Photobiol. B: Biology
1995, 30, 115–121.
[4] a) T. Hayashi, K. Okazaki, N. Urakawa, H. Shimakoshi, J. L. Sessler,
E. Vogel, Y. Hisaeda, Organometallics 2001, 20, 3074–3078;
b) W.-C. Lo, C.-M. Che, K.-F. Cheng, T. C. W. Mak, Chem. Commun.
1997, 1205–1206.
[5] a) T. Hayashi, H. Dejima, T. Matsuo, H. Sato, D. Murata, Y. Hisaeda,
J. Am. Chem. Soc. 2002, 124, 11226–11227; b) T. Hayashi,
D. Murata, M. Makino, H. Sugimoto, T. Matsuo, H. Sato, Y. Shiro,
Y. Hisaeda, Inorg. Chem. 2006, 45, 10530–10536; c) T. Matsuo,
H. Dejima, S. Hirota, D. Murata, H. Sato, T. Ikegami, H. Hori,
Y. Hisaeda, T. Hayashi, J. Am. Chem. Soc. 2004, 126, 16007–16017;
d) H. Nakashima, J.-y. Hasegawa, H. Nakatsuji, J. Comput. Chem.
2006, 27, 1363–1372; e) T. Matsuo, T. Tsuruta, K. Maehara, H. Sato,
Y. Hisaeda, T. Hayashi, Inorg. Chem. 2005, 44, 9391–9396.
[6] J.-M. Barbe, P. Richard, M. A. Aukauloo, C. Lecomte, P. Petit,
R. Guilard, Chem. Commun. 1994, 2757–2758.
[7] R. K. Iler, J. Colloid Interface Sci. 1966, 21, 569–594.
[8] G. Decher, Science 1997, 277, 1232–1237.
[9] S. Y. Park, M. F. Rubner, A. M. Mayes, Langmuir 2002, 18,
9600–9604.
[10] a) P. K. D. D. P. Pitigala, M. K. I. Seneviratne, V. P. S. Perera,
K. Tennakone, Langmuir 2004, 20, 5100–5103; b) S. Ponader,
K. Rosenlehner, E. Vairaktaris, C. von Wilmowsky, K. Schlegel,
F. Neukam, C. Schmidt, T. Schunk, A. Hirsch, E. Nkenke, J. Mater.
Sci.: Mater. Med. 2009, 20, 2455–2463.
[11] D. M. Kaschak, J. T. Lean, C. C. Waraksa, G. B. Saupe, H. Usami,
T. E. Mallouk, J. Am. Chem. Soc. 1999, 121, 3435–3445.
[12] N. Jux, Org. Lett. 2000, 2, 2129–2132.
fact, the latter is further corroborated by the lack of appreciable
photocurrents upon increasing the number of Pc8+/PSS sand-
wich layers (Figure 3). Therefore, the benefits that stem from
collecting and transporting electrons through the different GO
layers is demonstrated, on one hand, by a direct comparison
with the ITO/PDDA/PSS/(Pc8+/PSS)x devices and, on the other
hand, by the increase of the overall performances for the ITO/
PDDA/PSS/(Pc8+/GO)x devices upon increasing the number of
Pc8+/GO sandwich layers (Figure 3).
In summary, a sterically demanding tetraarylporphycene
PcOMe has been synthesized and characterized. The synthetic
route was modified at two critical steps, namely the formation
of the pyrrole and the subsequent iodination. The Vilsmeier-
Haack formylation led to a mixture of porphycenes (PcOMe and
7), which was converted into the all-brominated porphycene
8 (Scheme 1). 8 offers a wide variety of possibilities regarding
further functionalization. Within the scope of the current study
we concentrated on the 8-fold positively-charged, water-soluble
porphycene Pc8+. Importantly, Pc8+ displays the characteristic
traits of porphycenes such as high extinction coefficients, high
fluorescence quantum yields, and ease of reduction. In fact,
these features are true assets in the context of interacting with
GO to yield electron acceptor-donor ensembles Pc8+/GO. In the
final experiments, Pc8+ and GO were employed to construct
LbL assemblies on ITO electrodes. The performance of the cor-
responding photoelectrochemical devices highlights, for the
first time, the suitability of functionalized porphycenes as new
building blocks towards future nanohybrid materials for solar
cell applications.
[13] D. M. Guldi, I. Zilbermann, G. Anderson, A. Li, D. Balbinot, N. Jux,
M. Hatzimarinaki, A. Hirsch, M. Prato, Chem. Commun. 2004,
726–727.
[14] D. M. Guldi, G. N. A. Rahman, J. Ramey, M. Marcaccio, D. Paolucci,
F. Paolucci, S. Qin, W. T. Ford, D. Balbinot, N. Jux, N. Tagmatarchis,
M. Prato, Chem. Commun. 2004, 2034–2035.
[15] J.-F. Gnichwitz, R. Marczak, F. Werner, N. Lang, N. Jux, D. M. Guldi,
W. Peukert, A. Hirsch, J. Am. Chem. Soc. 2010, 132, 17910–17920.
[16] J. Malig, C. Romero-Nieto, N. Jux, D. M. Guldi, Adv. Mater. 2012, 24,
800–805.
Experimental Section
More information on synthetic, spectroscopical, and electrochemical
characterization details as well as AFM images are included in the
Supporting Information.
[17] Y. Yamada, T. Okujima, N. Ono, D. Kuzuhara, JP2010077054.
[18] S.-S. Li, K.-H. Tu, C.-C. Lin, C.-W. Chen, M. Chhowalla, ACS Nano
2010, 4, 3169–3174.
[19] S. Nonell, N. Bou, J. Borrell, J. Teixidó, A. Villanueva, A. Juarranz,
M. Cañete, Tetrahedron Lett. 1995, 36, 3405–3408.
[20] A. Gavalda, J. I. Borrell, J. Teixido, S. Nonell, O. Arad, R. Grau,
M. Canete, A. Juarranz, A. Villanueva, J. C. Stockert, J. Porphyrins
Phthalocyanines 2001, 5, 846–852.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
[21] O. Arad, J. Morros, X. Batllori, J. Teixido, S. Nonell, J. I. Borrell, Org.
Lett. 2006, 8, 847–850.
[22] D. Sánchez-García, J. I. Borrell, S. Nonell, Org. Lett. 2009, 11,
77–79.
[23] M. Stepien, B. Donnio, Jonathan L. Sessler, Angew. Chem. Int. Ed.
2007, 46, 1431–1435.
[24] M. Suzuki, M. Miyoshi, K. Matsumoto, J. Org. Chem. 1974, 39,
1980.
[25] J. B. Paine, D. Dolphin, J. Org. Chem. 1988, 53, 2787–2795.
[26] W. Brenner, J. Malig, D. Oelsner, D. M. Guldi, N. Jux, J. Porphyrins
Phthalocyanines 2012,16, 651–662.
[27] J. E. McMurry, Chem. Rev. 1989, 89, 1513–1524.
[28] J. Y. Horiba, http://www.horiba.com/us/en/scientific/products/fluo-
rescence-spectroscopy/application-notes/quantum-yields/, 2002.
[29] In reference experiments with poly-(sodium-4-styrenesulfonate)
(PSS) and Pc8+ no fluorescence quenching was noted.
For financial support the DFG (SFB 953), ICMM, and GSMS are gratefully
acknowledged. R.D.C. acknowledges the Humboldt Foundation for
support.
Received: August 20, 2012
Revised: October 23, 2012
Published online: December 13, 2012
[1] E. Vogel, M. Köcher, H. Schmickler, J. Lex, Angew. Chem. Int. Ed.
1986, 25, 257–259.
[2] E. Vogel, M. Balci, K. Pramod, P. Koch, J. Lex, O. Ermer, Angew.
Chem. Int. Ed. 1987, 26, 928–931.
[3] a) C. Richert, J. M. Wessels, M. Mueller, M. Kisters, T. Benninghaus,
A. E. Goetz, J. Med. Chem. 1994, 37, 2797–2807; b) V. Gottfried,
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2318 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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