5,10,15,20-Tetrakis[4′-(terpyridinyl)phenyl]porphyrin and its Ruii complexes: Synthesis, photovoltaic properties, and self-assembled morphology

Article abstract of DOI:10.1039/b707852h

A novel tetrakis(terpyridinyl)porphyrin derivative and its Ru^II complexes were efficiently synthesized using microwave enhanced synthesis and shown to possess photovoltaic properties. Transmission electron microscopy and selected area electron

Full text of DOI:10.1039/b707852h

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5,10,15,20-Tetrakis[49-(terpyridinyl)phenyl]porphyrin and its RuII
complexes: Synthesis, photovoltaic properties, and self-assembled
morphology{
Tae Joon Cho,a Carol D. Shreiner,a Seok-Ho Hwang,a Charles N. Moorefield,b Brandy Courneya,a
Luis A. God´ınez,c Juan Manr´ıquez,c Kwang-Un Jeong,a Stephen Z. D. Chenga and George R. Newkome*ab
Received (in Austin, TX, USA) 23rd May 2007, Accepted 3rd August 2007
First published as an Advance Article on the web 20th August 2007
DOI: 10.1039/b707852h
A novel tetrakis(terpyridinyl)porphyrin derivative and its RuII
porphyrin derivative including the Soret band at 423 nm and 4
Q-bands at 519, 554, 593, and 652 nm.
complexes were efficiently synthesized using microwave
enhanced synthesis and shown to possess photovoltaic proper-
ties. Transmission electron microscopy and selected area
electron diffraction were used to investigate its nanowire
self-assembly.
The tetrakisRuII complex 3 (Scheme 1) was synthesized by
combining porphyrin 2 with 4 equiv. of the paramagnetic
[49-tolylterpyridine]RuIII adduct6 5 in the presence of a catalytic
amount of N-ethylmorpholine in ethylene glycol; the suspension
was subjected to microwave irradiation (450 W, 20 min.). The
crude product was purified using column chromatography (silica,
H2O : sat. aq. KNO3 : MeCN 5 1 : 1 : 10) and subsequently
treated with methanolic NH4PF6 to convert the counterions to
Porphyrin-based polypyridinyl systems have been widely studied
due to their attractive photophysical properties.1 Elegant work by
Sauvage and coworkers2 has resulted in the synthesis of diad and
triad terpyridinyl porphyrin metal complexes. More recently,
Elliott et al.,3 reported the synthesis of a tetrakisbipyridinylpor-
phyrin and its Ru complex for use as a catalyst in electrochemical
olefin epoxidation. These works provided the inspiration to design
and synthesize a four-directional tetrakis(terpyridinyl)porphyrin,
which has the potential to be a suitable core for construction of
porphyrin containing metallodendritic motifs possessing unique
photovoltaic properties. Herein, we report the efficient, micro-
wave-assisted synthesis of a tetrakis(terpyridinyl)porphyrin and its
heteroleptic RuII complexes and their photovoltaic properties.
49-(p-Formylphenyl)terpyridine (1) was prepared according to
the literature procedure.2a The Adler method4 of refluxing
aldehyde 1 with 1 equiv. of pyrrole in propionic acid was
employed to synthesize the tetrakis(terpyridinyl)porphyrin (2,
Scheme 1), which was isolated in 3% yield after purification using
column chromatography (Al2O3, CHCl3). The structure of 2 was
confirmed (1H NMR) by singlets observed at 22.64 ppm assigned
to the porphyrin internal amine protons, 9.00 ppm for the outer
pyrrole protons, and 9.10 ppm representing the 39,59-tpyHs
(terpyridine 5 tpy, these signals integrated as expected in a 4 : 4
: 1 ratio), along with a mass peak (ESI-MS) at m/z 1539.2 [M +
H]+. Alternatively, the microwave enhanced synthesis5 (400 W,
10 min.) of porphyrin 2 gave rise to an improved, albeit still low,
overall yield (12%) and decreased reaction time. The UV/Vis
spectrum of 2 showed typical absorption peaks expected for a
PF6 to aid in solubility. Structural support (1H NMR) for
2
complex 3 included resonances attributed to the 39,59-tpyHs of
inner terpyridine [9.49 ppm (8H)] and outer terpyridine [9.08 ppm
(8H)] moieties. Resonances assigned to the pyrrole and amines of
the porphyrin ring were observed as expected at 9.24 (8H) and
22.54 ppm (2H), respectively. The methyl group on the outer
terpyridine appeared as a singlet [2.59 ppm (12H)] while the ESI-
mass spectrum for 3 displayed molecular ion peaks at m/z 5 954.5
[M 2 4PF6]+, z 5 4, (calcd. 954.4), 734.6 [M 2 5PF6]+, z 5 5,
(calcd. 734.5), 588.1 [M 2 6PF6]+, z 5 6, (calcd. 587.9), 483.2 [M 2
7PF6]+, z 5 7, (calcd. 483.2), 404.5 [M 2 8PF6]+, z 5 8, (calcd.
404.7).
Zinc metallation7 of the porphyrin complex 3 (Cl2 counterions)
was achieved by refluxing with 1 equiv. of Zn(OAc)2 in MeOH for
2
3 h to give complex 4 (72%). The 1H NMR of 4 (PF6
counterions) exhibited a similar resonance pattern to that of 3 with
the expected disappearance of the internal amine proton peak.
Molecular ion peaks (ESI-MS) were observed at m/z 5 970.6
[M 2 4PF6]+, z 5 4, (calcd. 970.4), 747.5 [M 2 5PF6]+, z 5 5,
(calcd. 747.3), 598.6 [M 2 6PF6]+, z 5 6, (calcd. 598.6), 492.3 [M 2
7PF6]+, z 5 7, (calcd. 492.3), 412.8 [M 2 8PF6]+, z 5 8, (calcd.
412.7).
A peripherally modified complex employing 49-(p-decyloxyphe-
nyl)terpyridine to enhance solubility was prepared by a modified
procedure of Constable et al.8 This terpyridine was then treated
with 1 equiv. of RuCl3 in refluxing EtOH to give (90%) the
paramagnetic [49-(p-decyloxyphenyl)terpyridine]RuIII adduct 6.
Tailored tetrakisRuII complexes 7 and 8 (Scheme 1) were
synthesized using the same procedures for 3 and 4, respectively
(7: 36%, 8: 81% yield). Structural support (1H NMR) for complex
7 included resonances for the 39,59-tpyHs of inner terpyridine
[9.39 ppm (8H)] and outer terpyridine [9.02 ppm (8H)] moieties.
Resonances attributed to the outer pyrrole and the inner amine
protons were observed at 9.23 (8H) and 22.53 ppm (2H),
respectively. The alkyl methyl and –OCH2–R groups on the
aDepartment of Polymer Science, The University of Akron, Akron,
OH 44325, USA. E-mail: newkome@uakron.edu
http://www.dendrimers.com;
bMaurice Morton Institute of Polymer Science, The University of Akron,
Akron, OH 44325, USA
cElectrochemistry Department, Centro de Investigacio´n y Desarrollo
Tecnolo´gico en Electroqu´ımica, Quere´taro, 76700, Me´xico
{ Electronic supplementary information (ESI) available: supportive
analytical data and photovoltaic properties. See DOI: 10.1039/b707852h
4456 | Chem. Commun., 2007, 4456–4458
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Scheme 1 Synthesis of the porphyrin–Ru-complex: a: propionic acid, 400 W, 10 min. microwave reactor, b: ethylene glycol, 450 W microwave reactor, 20
min., c: Zn(OAc), MeOH, reflux, 3 h, followed by NH4PF6.
decyloxy chain appeared as two triplets [0.92 ppm (12H)] and
[4.19 ppm (8H)], respectively. The ESI-mass spectrum for 7
displayed molecular ion peaks at m/z 5 1096.0 [M 2 4PF6]+, z 5 4,
(calcd. 1096.7), 848.6 [M 2 5PF6]+, z 5 5, (calcd. 848.6), 682.8
[M 2 6PF6]+, z 5 6, (calcd. 682.8), 564.8 [M 2 7PF6]+, z 5 7,
is associated to the excitation of the ZnII–porphyrin and Stpy–
RuII–tpyT complexes around 425 nm and 495 nm, respectively.10
The photoelectrochemical response (Fig. 1) for the ZnII–porphyrin
having the long hydrocarbon chain (8) is larger than that
possessing only the methyl group (4) by a factor of y3 (at
425 nm). In the following stage, we obtained j-E plots (ESI; S3{) in
order to calculate their photovoltaic performance. Thus, Table 1
shows that the global photoconversion efficiency (g) for TiO2/8
system is almost 10 times larger than for TiO2/4. The photovoltaic
results (Table 1) suggest that the peripheral aliphatic groups
promote a decrease in recombination of the photogenerated
electrons.11
1
(calcd. 564.6), 476.0 [M 2 8PF6]+, z 5 8, (calcd. 475.9). The H
NMR of the Zn-metallated porphyrin complex 8 showed a similar
absorption pattern to that of 7 except for the disappearance of
the internal amine proton peak. Molecular ion peaks were
observed at m/z 5 1111.6 [M 2 4PF6]+, z 5 4, (calcd. 1112.2),
861.1 [M 2 5PF6]+, z 5 5, (calcd. 860.8), 693.3 [M 2 6PF6]+, z 5 6,
(calcd. 693.2), 573.7 [M 2 7PF6]+, z 5 7, (calcd. 573.4), 483.6 [M 2
8PF6]+, z 5 8, (calcd. 483.6).
Estimation of electron lifetimes (tn) for DSSC constructed with
dyes 8 and 4 was calculated from electrochemical impedance
spectroscopy experiments under polychromatic illumination
(2.2 mW cm22) according to the methodology previously reported
by Bisquert et al.12a and Gra¨tzel et al.12b Experimental spectra were
fitted to the corresponding equivalent circuit12a (Table 1 and ESI;
S7{) revealing that tn for TiO2/8 and TiO2/4 systems were 5.14 ms
and 0.13 ms, respectively. This suggests that electron recombina-
tion for TiO2/8 system is almost 40 times slower than for TiO2/4.
Photoelectrochemical evaluation of the free-base porphyrin–
Stpy–RuII–tpyT complexes (3 and 7) was also conducted. First, the
surface coverage (Table 1) for each dye was investigated and
showed similar values to those of the Zn–porphyrin–Stpy–RuII–
tpyT complexes. According to the action spectra (Fig. 1) in the
presence of ZnII, complexes 4 and 8 could inject electrons on the
TiO2 semiconductor film. However, when ZnII was absent (e.g., 3
and 7), the results of photocurrent action spectra indicated that
Cyclic voltammograms (CVs) for 8, 4, 7 and 3 revealed
reversible redox peaks attributed to RuIII|RuII couple around
1.0 V vs. Fc+|Fc (SI; S1).9 Also, porphyrin-containing RuII
complexes revealed another two waves at 21.47 V and 21.60 V,
respectively, corresponding to redox processes of the terpyridine
moieties. Additional electrochemical data are discussed in the
ESI{.
Construction of dye-sensitized solar cells (DSSC) started with
nanocrystalline TiO2 electrodes that were prepared by electro-
phoretic deposition; experimental details have been reported
elsewhere.10 Action spectra obtained for ZnII–porphyrins showed
significant photocurrents at ca. 450–600 nm (Fig. 1). This behavior
Table 1 Surface coverage of the dyes and photovoltaic performance
of the DSSC
Dye aC/10211 mol cm22 Eoc/mV j/mA cm22 bff
cg (%) tn/ms
8
4
7
3
a
4.10
6.55
4.60
6.87
330
170
NA
NA
729.93
143.18
NA
0.262 2.85
0.279 0.30
5.14
0.13
NA
NA
NA
NA
NA
NA
NA
b
Dye coverage. Fill factor ff 5 Pmax/(Eoc 6 j) where Pmax denotes
Fig. 1 Normalized action spectra obtained for the dye-sensitized solar
cell (DSSC) assembled with modified TiO2 with 8 (blue line), 4 (red line), 7
(black line), and 3 (pink line).
c
maximum power cell.
(%) 5 (ff 6 Eoc 6 j) 6 100/Pinput where Pinput 5 2.2 mW cm22
Global photoconversion efficiency
.
g
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spacing was conducted using the standard evaporated thallous
chloride, which has a largest first-order spacing diffraction of
0.384 nm. SAED pattern showed that a strong diffraction
appeared on the meridian at 0.870 nm with a second order
diffraction at 0.435 nm. The d-spacing of 0.870 nm is close to the
thickness of the porphyrin complex 7 (about 0.86 nm).
Therefore, we can conclude that the normal direction of the self-
assembled porphyrin complex is parallel to the wires long direction
and nanowires generated from the single column of porphyrin
complex (disc-shape) building block. Diffused diffractions at the
meridian and quadrants (0.212 and 0.286 nm, respectively) are
likely from the alkyl chains at the end of complex 7.15
Fig. 2 The proposed charge transfer through a DSSC by using sensitized
TiO2 electrodes with Zn–porphyrin–Stpy–RuII–tpyT complexes.
Notes and references
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3 C. M. Elliott, J. R. Dunkle and S. C. Paulson, Langmuir, 2005, 21, 8605.
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Fig. 3 TEM image of the self-assembled nanowires constructed using the
discotic porphyrin complex 7. The inset image is the SAED of the bundle
of nanowires taken in the circled region of Fig. 3. Here, WD indicates the
wires long direction.
8 E. C. Constable, P. Harverson, D. R. Smith and L. Whall, Polyhedron,
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this process was interrupted.13 From this observation, we propose
an electron transfer mechanism, which assumes that electrons
introduced into the Stpy–RuII–tpyT moiety can be transfered to
the TiO2 through the Zn–porphyrins.14 This porphyrin-containing
transition metal polypyridinyl device (Fig. 2) is a potential
‘‘switch’’ for controlling charge transfer between ‘‘electron
containers’’ (Stpy–RuII–tpyT) and ‘‘electron acceptors’’ (TiO2
films).
Fig. 3 shows a TEM image of the self-assembled nanowires with
a width of ca. 7 nm, which is comparable to the diameter of
porphyrin complex 7, and 0.3–0.5 mm in length. In order to know
the detailed molecular packing inside these nanowires, selected
area electron diffraction (SAED) was conducted in the region
shown in the inset of Fig. 3. Based on the morphological
observation and its SAED, the wires long direction is in the
meridian direction of the SAED pattern. Calibration of the SAED
15 Support generously provided by NSF DMR-041780, -0705015, INT-
0405242, and AFOSR F49620-02-1-0428,02.
4458 | Chem. Commun., 2007, 4456–4458
This journal is ß The Royal Society of Chemistry 2007