Phosphorus(V) porphyrins with axial carbazole-based dendritic substituents

Article abstract of DOI:10.1021/ol062979k

(Chemical Equation Presented) Three phosphorus(V) porphyrins with axial carbazole-based dendritic substituents (D-A-D) have been designed and synthesized, which are nonfluorescent due to their effective electron transfer from the carbazole dendron to the

Full text of DOI:10.1021/ol062979k

ORGANIC
LETTERS
2007
Vol. 9, No. 5
797-800
Phosphorus(V) Porphyrins with Axial
Carbazole-Based Dendritic Substituents
Tinghua Xu, Ran Lu,* Xingliang Liu, Xiangqian Zheng, Xianping Qiu, and
Yingying Zhao
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education,
College of Chemistry, Jilin UniVersity, Changchun 130012, PRC
luran@mail.jlu.edu.cn
Received December 9, 2006
ABSTRACT
Three phosphorus(V) porphyrins with axial carbazole-based dendritic substituents (D-A-D) have been designed and synthesized, which are
nonfluorescent due to their effective electron transfer from the carbazole dendron to the excited porphyrin within the dendritic matrix. The
incident photon to current conversion efficiencies (IPCE) spectra demonstrate that the molecular structure of the dendrimers can significantly
affect the photovoltaic response to the visible light.
Because of their outstanding photophysical and redox
properties, porphyrins have been widely used as building
blocks in the construction of π-conjugated systems for their
potential applications in molecular electronics, photochemical
energy conversion and storage, switches, etc.1 Recently,
various rigid and flexible dendritic structures have been
designed to connect with porphyrin cores as artificial
molecular systems to investigate their energy and/or electron
transfer. Most of these dendritic porphyrins reported so far
are modified by the functional substitutes in the peripheral
meso- or pyrrole-â position via covalent linkages, which may
require extensive synthetic efforts.2-4 However, the “axial-
bonding” strategy appears to be an attractive approach for
the facile preparation of porphyrin arrays with well-tunable
photo- and electrochemical properties through changing the
π-orbital interactions.5,6 [P(TPP)Cl2]Cl is a well-known axial-
bonding building block, and the P-Cl bonds are reactive
toward phenols and alcohols.7 On the other hand, carbazole
is also an appealing molecule because of its intense
luminescence, electron efficiency, and potential for dendritic
construction.8,9 Our group has previously reported photo-
induced intramolecular energy transfer in the porphyrins with
four monodisperse dendritic carbazole arms.10 Therefore, the
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Ceroni, P.; Bergamini, G.; Marchioni, F.; Balzani, V. Prog. Polym. Sci.
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10.1021/ol062979k CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/01/2007

synthesis of D-A-D-type molecules based on carbazole
dendron-derived phosphorus(V) porphyrins, which are strong
electron acceptors, may lead to novel materials with fascinat-
ing photo- and electrophysical properties. To the best of our
knowledge, there is no example of an axially linked
phosphorus(V) porphyrin with a rigid dendron. Herein, we
present the preparation and characterization of novel phos-
phorus(V) porphyrins with axial monodisperse dendritic
carbazoles 1-3. Notably, a photovoltaic response to the
visible light is observed in the incident photon to current
conversion efficiencies (IPCE) spectra.
Scheme 1. Syntheses of Cz-G1-OMe and Cz-G2-OMe
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The synthetic routes for the carbazole dendrons 12-14
are shown in Schemes 1, 2, and 3, respectively. To improve
Scheme 2. Synthesis of Cz-G3-OMe
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Scheme 3. Syntheses of Cz-Gn-OH
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prepared in a yield of 54% by Friedel-Crafts alkylation of
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Org. Lett., Vol. 9, No. 5, 2007

carbazole, adapting a published procedure.11 The Ullmann
condensation reaction between compound 4 and 1-iodo-4-
methoxybenzene, which was catalyzed by Cu2O in N,N-
dimethylacetamide (DMAc) at 160 °C for 24 h, gave the
first generation of the carbazole dendron, 5 (Cz-G1-OMe),
in a yield of 83%.3i,4ï,8b,c,10 The second generation of the
carbazole dendron, 8 (Cz-G2-OMe), was achieved from
compound 7 and 1-iodo-4-methoxybenzene under similar
conditions with a yield of 76%. Compound 7 was synthesized
from compounds 4 and 6 via Ullmann coupling reaction,
followed by the cleavage of the N-Ts bond under a basic
environment (70%, two steps).3i,4o,8b,c,10
Scheme 4. Syntheses of [P(TPP)(O-Gn-Cz)2]Cl (1-3) and
Compound 16
Significantly, a high yield was achieved in the preparation
of 5 and 8 based on the modified Ullmann condensation
reaction, and we were encouraged to synthesize the third
generation of the carbazole dendron, 11 (Cz-G3-OMe).8c,10
The synthetic route of compound 11 is shown in Scheme 2.
The iodination of compound 9, which was obtained from
the Ullmann condensation reaction of carbazole with 1-iodo-
4-methoxybenzene, was pursued to give compound 10. Then,
the Ullmann condensation reaction between compounds 10
and 7 was carried out at 200 °C for 60 h to yield the
carbazole dendron 11, which was recrystallized from EtOH/
THF (1:2, v/v) over four times with a yield of 64% after
flash chromatography using THF as eluent. Generally, the
preparation of dendritic oligocarbazoles of a higher genera-
tion was more difficult and the yield was not high (12-
37%), which was probably due to their lower reactivity and
easier oxidation.8b,c The methyls in O-CH3 of compound 5,
8 and 11 were easily removed by BBr3 to give the
corresponding phenols 12, 13, and 14 with yields of 80∼90%
(Scheme 3).12
increased with the generation. In addition, the reference
compound 16 was prepared by adding excess water to the
pyridine solution containing compound 15 under reflux for
12 h (Scheme 4).7a The intermediates and target molecules
were purified with column chromatography on silica gel and
characterized by FT-IR, 1H NMR, and MALDI-TOF mass
spectrometry (see Supporting Information).
The UV-vis spectra of 1-3 in CHCl3 and the film were
including two Q-bands in the range of 500-700 nm
(consistent with a reported phosphorus(V) porphyrin), to-
gether with the Soret band at ca. 440 nm, and others in the
UV region (250-350 nm) due to the carbazole units (Figure
1). The absorbance of the carbazole units was clearly
The phosphorus(V) porphyrins with axial dendritic car-
bazoles were synthesized from [P(TPP)Cl2]Cl 15 and the
phenols (12-14). As shown in Scheme 4, the dry pyridine
solution with H2TPP (5,10,15,20-tetraphenylporphyrin)13 was
refluxed for 24 h in the presence of excess POCl3 with a
good yield (74%) of compound 15.7a It was reactive toward
water and a variety of phenols in the presence of pyridine,
although air stable in both solid state and solution. Then,
the obtained phenols 12-14 were converted into the corre-
sponding axial dendritic porphyrins [P(TPP)(O-Gn-Cz)2]Cl
(1-3) in dry pyridine containing compound 15 under reflux
for 5-8 h with a yield of 79, 71, and 63%, respectively.7 It
was found that the degree of demetalation would be
promoted, as the condensation time between 15 and phenols
increased, probably as a result of HCl generated during the
reaction. The obtained axial dendritic porphyrins [P(TPP)-
(O-Gn-Cz)2]Cl (1-3) were soluble in chloroform, THF,
toluene, and CH2Cl2, and the solubility of these dendrimers
Figure 1. UV-vis absorption spectra (normalized at the Soret
bands) of 1-3 and 16 in CHCl3.
proportional to their increased number in each generation,
which indirectly reflected the flawless or perfect structures
of the as-synthesized dendrimers. Notably, there was no
distinct spectral shift or spectral broadening of the absorption
bands of the carbazole units and porphyrin core with
increasing generation.8d It was suggested that the electronic
structures of the axial porphyrins did not change when the
generation increased. As shown in Figure S1 (Supporting
Information), the Soret bands for the porphyrin cores of
compounds 1-3 in films were red-shifted compared with
those in solution, which was due to the aggregation effect.
(10) (a) Xu, T.; Lu, R.; Jin, M.; Qiu, X.; Xue, P.; Bao, C.; Zhao, Y.
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X.; Xue, P.; Tan, C.; Bao, C.; Zhao, Y. Eur. J. Org. Chem. 2006, 4014-
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(12) Hohle, C.; Hofmann, U.; Schloter, S.; Thelakkat, M.; Strohriegl,
P.; Haarer, D.; Zilker, S. J. J. Mater. Chem. 1999, 9, 2205-2210.
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Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476-476. (b) Lindsey,
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The degree of red shifts depended on the generation number
of the dendrimers: the absorptions of 1, 2, and 3 in films
were red-shifted by 13, 8, and 5 nm, respectively; in other
words, the higher the generation was, the smaller the red
shift moved in the absorption (Table 1). It was deduced that
To examine the potential of the (D-A-D) triads 1-3 as
photovoltaic materials, their sandwich devices with a con-
figuration structure of Al/organic film/ITO glass were
fabricated. As shown in Figure S4 (Supporting Information),
the wavelength-dependent IPCE spectra of the devices based
on compounds 1-3 were in rough accordance with the
electronic absorption spectra of the porphyrin chromophores.
Compared to the [P(TPP)(O-G1-Cz)2]Cl (1) device, the
[P(TPP)(O-G2-Cz)2]Cl (2) device produced a more effective
photovoltaic response to the visible light owing to the lower
oxidation potential of the larger carbazole dendron,8c which
could lower the energy level of the charge separated state,
and the subsequent long-distance charge separation also
contributed to the generation of the photocurrent.14b Accord-
ing to this, the [P(TPP)(O-G3-Cz)2]Cl (3) device should
produce the most effective photovoltaic response because
compound 3 had the lowest oxidation potential and the
longest distance of charge separation. However, a poorer
photovoltaic performance than the [P(TPP)(O-G2-Cz)2]Cl (2)
device was observed, and it could be explained as followed:
because compound 3 had the largest site-isolation effect, the
resulting anion radical of porphyrin cores might be sur-
rounded by the carbazole dendron, which forbade the electron
from flowing.
In conclusion, three axial phosphorus(V) porphyrins with
dendritic carbazoles [P(TPP)(O-Gn-Cz)2]Cl 1-3 have been
designed and synthesized. Their nonfluorescence caused by
the photoinduced electron transfer may lead to potential
applications as photovoltaic materials in solar cells. The IPCE
spectra of these dendrimers demonstrate that the structure
of the dendrimers could significantly affect their photovoltaic
response to the visible light. Further investigation on the
detailed transient absorption spectra and fluorescence decay
profiles of compounds 1-3 to study the electron-transfer
process is underway.
Table 1. Comparison of the UV-vis Spectroscopic Data of
1-3
1
2
3
λmaxa [nm]
λmaxb [nm]
∆λmaxc [nm]
436
449
13
436
444
8
436
441
5
b
c
a In CHCl3. In spin-coated film. λmax(film) - λmax(solution).
the dendrons in the higher-generation dendrimers had a larger
site-isolation effect on the porphyrin core.6a Compared to
our previous results, the axial dendritic substituents sup-
pressed cofacial intermolecular excitonic interactions more
efficiently than the meso-position dendritic substituents.10b
The site isolation could also be confirmed by the polarity of
these dendrimers, which decreased with the generation,
because the Rf values were enlarged when the generation
increased (Figure S2, Supporting Information).
The fluorescence spectrum of compound 16 in CHCl3
showed a strong emission at 613 and 665 nm excited at 436
nm, which were typical values for phosphorus(V) porphy-
rins.7 When carbazole dendrons were introduced into the
axial direction of phosphorus(V) porphyrin, however, the
emission was strongly quenched (>99%) (Figure S3, Sup-
porting Information). It was suggested that the attachment
of carbazole dendrons to the axial direction of porphyrins
imparted a fast deactivation of the porphyrin singlet excited
state through electron transfer.7g,j Because porphyrins pos-
sessed broad absorption in the visible region, these phos-
phorus(V) porphyrins with axial carbazole-based dendritic
substituents [P(TPP)(O-Gn-Cz)2]Cl (1-3) might be poten-
tially applied in the solar cell.7j,14
Acknowledgment. This work is financially supported by
the National Natural Science Foundation of China (NNSFC,
No. 20574027) and the Program for New Century Excellent
Talents in University (NCET).
Supporting Information Available: Experimental pro-
cedures and characterizations of the key compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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