Nanosized π-conjugated molecules based on truxene and porphyrin: Synthesis and high fluorescence quantum yields

Article abstract of DOI:10.1021/ol051221i

(Chemical Equation Presented) A series of novel nanosized π-conjugated molecules based on both truxene and porphyrin moieties with high fluorescence quantum yields have been prepared via the Suzuki cross-coupling and the Lindsey reactions. The investigati

Full text of DOI:10.1021/ol051221i

ORGANIC
LETTERS
2005
Vol. 7, No. 19
4071-4074
Nanosized
π-Conjugated Molecules
Based on Truxene and Porphyrin:
Synthesis and High Fluorescence
Quantum Yields
Xiao-Fei Duan, Jin-Liang Wang, and Jian Pei*
The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry
of Education, College of Chemistry and Molecular Engineering, Peking UniVersity,
Beijing 100871, China
jianpei@pku.edu.cn
Received May 24, 2005
ABSTRACT
A series of novel nanosized
π-conjugated molecules based on both truxene and porphyrin moieties with high fluorescence quantum yields
have been prepared via the Suzuki cross-coupling and the Lindsey reactions. The investigation of optical properties demonstrates that various
aryl groups as the antenna efficiently enhance the intramolecular and intermolecular energy transfer. These nanosized macromolecules emitting
intense red light might be good candidates for photonic and electronic devices.
Recently, the design and synthesis of nanosized π-conjugated
macromolecules has rapidly become a vigorous research topic
as a result of their momentous applications, such as active
components in advanced materials.1 π-Conjugated dendrim-
ers have received particular attention in this field because
of their regularly branched three-dimensional architectures,
nanosized rigid structures, and monodisperse nature, as well
as the ease of introduction of functional groups.2 Such
features of π-conjugated dendrimers are therefore appropriate
for various applications in material science, including organic
light-emitting diodes (OLED),3 nonlinear optics,4 and nano-
fibers by self-assembly.5
synthetic approach.6 Importantly, their biological activity as
antennas and the efficient light-harvesting abilities in photo-
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Porphyrins as building blocks provide an extensively
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10.1021/ol051221i CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/13/2005

synthetic bacteria as well as the use in molecular conductive
wires and optoelectronic devices have received extensive
attention.7 Although considerable efforts have been made for
exploration of multiporphyrin arrays with flexible8 and rigid
dendrons,9 to develop π-conjugated porphyrin-containing
dendrimers as the core and/or as the bridge for the nano-
materials has still been of recent concern.
Scheme 1a
In this contribution, we present a series of π-conjugated
nanosized macromolecules containing two chromophores
(porphyrin and truxene moieties). For example, the diameter
of 3 is ca. 5.2 nm (see SI S19-S21), which represents one
of the largest known conjugated dendrimers based on
prophyrin.9 We also develop a highly convenient diversity
approach to such porphyrin-containing π-conjugated den-
drimers through the Suzuki coupling and the Lindsey
reactions. We achieve highly intense red light emission from
1-6, because truxene moieties with long alkyl chains
radically enhance the solubility and suppress the aggregation
in solid states and obviously reduce the self-quench of
porphyrin’s fluorescence.
The readily available starting material 14 was reported in
our previous contributions.10 Scheme 1 shows the approach
to 7-12 as the precursors of 1-6. The Suzuki cross-coupling
reaction of tribromotruxene with 4-formylbenzeneboronic
acid catalyzed by Pd(PPh3)4 in a mixture of THF and aqueous
sodium carbonate solutions provided 7 in moderate yield
(70%). Finally, we obtained 8, 9, 10, and 12 from 7 and 11
from 13 in good yields (70-90%) by using various aryl-
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Karotki, A.; Kruk, M.; Dzenis, Y.; Rebane, A.; Suo, Z.; Spangler, C. W. J.
Phys. Chem. B 2004, 108, 4221.
a Reagents and conditions: (a) 4-formylbenzeneboronic acid,
Pd(PPh3)4, THF, reflux; (b) arylboronic acid, Pd(PPh3)4, THF,
reflux; (c) NBS, CH2Cl2/acetic acid, 0 °C; (d) 2-thiophenylboronic
acid, Pd(PPh3)4, THF, reflux.
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boronic acids. Bromination of 10 with NBS in CH2Cl2 and
acetic acid (v/v ) 1:1) afforded 13 (90%).
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Yamazaki, T.; Yamazaki, I. J. Am. Chem. Soc. 2003, 125, 10625. (c)
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Fre´chet, J. M. J. Chem. Commun. 2000, 18, 1701. (e) Uyemura, M.; Aida,
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T.; Matsuda, H.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 6350. (j) Jang,
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Rajesh, C. S.; Modarelli, D. A. J. Org. Chem. 2003, 68, 247. (c) Pillow, J.
N. G.; Halim, M.; Lupton, J. M.; Burn, P. L.; Samuel, I. D. W.
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As outlined in Scheme 2, the products through the Adler
reaction using propionic acid as the catalyst and solvent were
too difficult to be purified.11 Trifluoroacetic acid was not
powerful enough to construct such large molecules.12 For-
tunately, the condensation of 7-12 with distilled pyrrole
using BF3‚Et2O and ethanol as catalyst and CH2Cl2 as solvent
in an aerobic dark environment, following by DDQ oxidation
and Et3N neutralization, afforded porphyrins 1-6 at about
20% yields.13 We also employed the Suzuki reaction of 1
with various arylboronic acid to produce 2-6; however, the
yields were quite low and it was difficult to purify the desired
products.14 Therefore, such fairly convenient and highly
diversified convergent strategy provides us the possibility
to develop nanosized π-conjugated porphyrins with various
aromatic substituents at outermost.
(11) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
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Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827.
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Org. Lett., Vol. 7, No. 19, 2005

Scheme 2a
a Reagents and conditions: (a) (i) BF3‚Et2O, Ethanol, CH2Cl2,
(ii) DDQ, (iii) NEt3; (b) propionic acid, reflux; (c) (i) trifluoroacetic
acid, (ii) DDQ, (iii) NEt3.
Figure 1. Normalized absorbance (top) and fluorescence (bottom)
spectra of 1-6 in solution states at room temperature. Emission
spectra were obtained upon excitation at the absorption maxima.
Porphyrins 1-6 were readily soluble in common organic
solvents such as toluene, CH2Cl2, chloroform, and THF. The
structure and purity of all new compounds was characterized
All PL spectra in dilute THF solutions exhibited charac-
teristics of the porphyrin chromophore (at about 650 nm)
and truxene derivatives (at about 300-400 nm) as well as
another peak at 400-500 nm, respectively. The fluorescence
quantum yields (ΦF) of 1-6 were from 0.19 to 0.30, which
was much higher than lots of porphyrin derivatives; for
example, the quantum yield of TPP was about 0.11.15
Compound 6 exhibited strong red light emission with high
fluorescence quantum yield (ΦF ) 0.30), indicating that it
could be a good candidate for red-emitting materials.
To understand the energy transfer we first carried out the
PL measurement at the absorption maximum of the dendron
and the core. The photoluminescent excitation (PLE) spectra
of 1-6 were also recorded under excitation at 650 nm across
the absorption spectrum (see SI S15-S17). Both PL and PLE
results demonstrated that highly efficient energy transfer from
the truxene moieties to the porphyrin core was achieved
owing to the antenna effect of eight aryl branches at the outer
space.
1
clearly with H and 13C NMR spectra and matrix-assisted
laser desorption ionization time-of-flight (MALDI-TOF)
1
mass spectroscopy (see SI S22-S60). H NMR spectra
showed expected signals for the inner proton of porphyrin
moiety at around -2.50 ppm together with the aromatic
protons (at about 9.10 ppm) corresponding to porphyrin
moieties.
The absorption and photoluminescent (PL) spectra of
porphyrin derivatives 1-6 were recorded both in dilute THF
solutions (10-6 M) and in thin films as shown in Figures 1
and 2. Table S1 summarizes their physical properties. In
Figure 1, 1-6 show the main features of absorption spectra
including four Q-bands in the region of 500-700 nm (which
were consistent with a free-base porphyrin), the Soret band
at 425 nm, and the truxene moiety at 315-380 nm,
respectively. We also observed that to introduce the different
aryl groups resulted in a red-shift of the absorption peaks of
truxene moieties (328 nm for 2, 334 nm for 3, 340 nm for
4, 375 nm for 5 and 341 nm for 6, respectively) due to the
increase of the effective conjugation length through π-π*
delocalization. In comparison with TPP (tetraphenyl-
porphyrin),9h the Soret band of 2-6 exhibited slight red-
shift (about 9 nm).
Solid films of 1-6 on quartz plates were spin-coated with
10 mg/mL THF solutions at 1000 rpm and recorded in Figure
2. We observed that absorption behaviors of 1-6 in films
were almost identical with those in solutions, which implied
(15) Seybold, P. G.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1.
Org. Lett., Vol. 7, No. 19, 2005
4073

reduced the π-π aggregation of the porphyrin core. Interest-
ingly, we observed that emission peaks in the blue region
were obviously decreased from 1 to 6, which were identical
with the red-shift of the absorption behaviors from 1-6. We
also found more efficient energy transfer in films than in
solutions, because it was enhanced in solid films and the
torsion angle between porphyrin and truxene moieties was
smaller in films than in solutions.16 With alteration of the
outermost aryl units, irradiation of the truxene segments
resulted in the intensity of emission bands between 350 and
500 nm weakening and the one of the porphyrin at around
650 nm increasing. This indicated that the efficiency of
energy transfer from truxene arms to the porphyrin core
enhanced with the increase of the effective conjugation length
from benzene to the fluorene units. The remaining emission
in blue region accounted for the competition between the
direct emission from truxene moieties and energy transfer
from truxene to porphyrin core.
In summary, we have developed a convenient and highly
diversified route to construct novel, monodispersed, nano-
sized π-conjugated dendrimers based on truxene and por-
phyrin segments. These nanosized molecules show intense
red light emitting properties with high fluorescence quantum
yields both in solutions and in films. We also demonstrate
that excitation of the truxene moieties leads to energy transfer
to the luminescent core, and such energy transfer relies on
the aromatic units at the outermost. Evaluating new materials
based on this molecular backbone and applications in
optoelectronic devices are still in investigation in our
laboratory.
Figure 2. Normalized absorbance (top) and fluorescence (bottom)
spectra of 1-6 in film states at room temperature. Emission spectra
were obtained upon excitation at the absorption maxima.
Acknowledgment. This work was supported by the
Major State Basic Research Development Program (no.
2002CB613402) from the Minister of Science and Technol-
ogy, China and National Natural Science Foundation of
China (NSFC 20425207, 50473016, and 90201021).
that intermolecular π-π aggregation was suppressed in films
owing to the long alkyl chains. However, the Soret band of
the porphyrin core red-shifted about 8 nm (433 nm) relative
to those in solutions (425 nm).
In thin films, 1-6 exhibited intense light emission at about
650 nm, which blue-shifted about 10 nm compared with TPP
as a result of the large torsion angle between the porphyrin
core and the truxene moieties. All results indicated that the
large steric effect of the truxene skeleton significantly
Supporting Information Available: Experimental pro-
cedures, additional data and details. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL051221I
(16) Jiang, B.; Yang, S.-W.; Jones, W. E., Jr. Chem. Mater. 1997, 9,
2031.
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