Peripherally ethynylated carbazole-based core-modified porphyrins

Article abstract of DOI:10.1039/c2ob25645b

Peripherally ethynylated carbazole-based core-modified porphyrins were synthesized by sequential metal-catalyzed coupling and annulation reactions. Experimental results and DFT calculations both confirm that the π-conjugated networks of the resulting porp

Full text of DOI:10.1039/c2ob25645b

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Volume 10 | Number 27 | 21 July 2012 | Pages 5153–5316
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Chihiro Maeda and Naoki Yoshioka
Peripherally ethynylated carbazole-based core-modified porphyrins

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COMMUNICATION
Peripherally ethynylated carbazole-based core-modified porphyrins†
Chihiro Maeda* and Naoki Yoshioka
Received 30th March 2012, Accepted 10th May 2012
DOI: 10.1039/c2ob25645b
Peripherally ethynylated carbazole-based core-modified por-
phyrins were synthesized by sequential metal-catalyzed coup-
ling and annulation reactions. Experimental results and DFT
calculations both confirm that the π-conjugated networks of
the resulting porphyrins effectively delocalize over the entire
macrocycle, including the ethynyl substituent groups.
Significant effort has been devoted to the synthesis of porphyrins
and porphyrinoids. Both are of exceptional importance in
various biological processes, such as photosynthesis, and have
numerous applications in catalysis, anion sensing and optical
Scheme 1 Synthesis of 1a and 1b.
devices.1 One of the most intriguing aspects of porphyrin chem-
istry is the potential for expansion of the π-conjugated system
within the molecule, since such extended conjugation allows
tailoring of the various useful properties of these macrocycles.
a result of pursuing this strategy, we report the peripheral ethyny-
lation of the core-modified porphyrin 1b.
Initial work focused on the peripheral functionalization of 1a.
Reaction of 1a with Br2 resulted in a mixture of several bromi-
nated products which could not be readily separated from one
another. However, treatment of 1a with 5 equiv. of NBS gave the
tetrabrominated compound 2 in 88% yield. The Stille coupling
reaction of 2 with tributyl(trimethylsilylethynyl)tin provided the
To date, this expanded conjugation has been achieved by the
introduction of acetylene moieties,2 the fusion of aromatic
rings,3 and ring expansion.4
Carbazole-based materials have also been extensively studied
due to their unique properties. They can be highly emissive and
electron conducting and are also both chemically stable and
capable of polymerization and metal catalysed cross coupling.5
tetrakis(trimethylsilylethynyl)-substituted product 3a. Slow dif-
fusion of methanol vapor into a dichloromethane solution of 3a
produced crystals with quality suitable for X-ray diffraction
Since a carbazole is simply a benzene-fused pyrrole, the incor-
poration of carbazole units into fused porphyrins presents inter-
analysis. The crystal structure data resulting from this analysis
esting possibilities. However, despite the potential usefulness of
provides unambiguous elucidation of the structure of 3a, con-
firming the presence of four trimethylsilylethynyl substituents at
the thiophene moieties (Fig. 1).‡ Product 3a was next oxidized
to 3b using MnO2. Subsequently, a series of synthetic procedures
such carbazole-based porphyrinoids, only a few examples have
been reported to date.6,7 Recently, we reported a multiple annula-
tion strategy which allows the synthesis of novel porphyrinoids
from 1,3-butadiyne-bridged cyclic carbazole oligomers
(Scheme 1).8 Among these, the tetrabenzo-fused core-modified
was performed in order to attach ethynyl groups to the carbazole
moieties. The intermediate 3,6-dibromo-1,8-diiodocarbazole (4)
porphyrin 1b exhibits distinct aromaticity as well as near infrared
was first prepared from 3,6-dibromocarbazole by reaction with
(NIR) absorption up to 1050 nm. It is evident that the π-conju-
iodine monochloride. The Sonogashira coupling reaction of 4
gated circuit of 1b can be considered as an expanded 34 π elec-
with trimethylsilylacetylene provided 3,6-dibromo-1,8-bis(tri-
tron system, the presence of which is likely responsible for the
methylsilylethynyl)carbazole (5). The bromo substituents of 5
significant red-shifted absorption of this molecule. The addition
were converted into triisopropylsilylethynyl groups through a
of various substituent groups to this macrocycle is seen as a
Stille coupling reaction. The trimethylsilyl protecting groups of
means to further modify its optical and electronic properties. As
6 were then selectively removed by reaction with 2.5 equiv. of
tetrabutylammonium fluoride to give 7. The Glaser coupling
reaction of 7 gave the cyclic carbazole dimer 8 in 27% yield.
Finally, the double annulation reaction of 8 with Na2S afforded
9a.9 The high-resolution electrospray-ionization (HR-ESI) mass
spectrum of 9a exhibited the parent ion peak at m/z = 1214.6260
(determined from C76H97N2S2Si4 [M–H]− = 1214.6207). Similar
to 3a, 9a was oxidized to 9b using MnO2 (Scheme 2) .
Department of Applied Chemistry, Faculty of Science and Technology,
Keio University, Kohoku-ku, Yokohama 223-8522, Japan. E-mail: cmaeda@
applc.keio.ac.jp; Fax: +81 45-566-1551; Tel: +81 45-566-1884
†Electronic supplementary information (ESI) available: Experimental
details and compound data. CCDC 871499. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2ob25645b
5182 | Org. Biomol. Chem., 2012, 10, 5182–5185
This journal is © The Royal Society of Chemistry 2012

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Fig. 1 X-ray crystal structure of 3a: (a) top view and (b) side view.
Hydrogen atoms except for NH protons are omitted for clarity. The
thermal ellipsoids were at the 50% probability level.
The 1H NMR spectra of 3b and 9b show considerable
downfield shifts for the peripheral protons; for 3b, Ha and Hb
appear at 10.48 and 8.65 ppm, respectively, and for 9b, Hc, Hd
and He appear at 9.08 and 8.97 ppm and 8.42 ppm, respectively,
indicating strong diatropic ring currents (ESI†). The UV-vis
absorption spectra of 3b and 9b display intensified and extre-
mely red-shifted Q-like bands at 877, 976, and 1083 nm for 3b,
and 925, 998, and 1110 nm for 9b.10 These bands display a
greater degree of red-shift as compared to those of the previously
synthesized compound 1b (Fig. 2). The absorption red-shift
observed for 9b suggests that the π-conjugation system of this
core-modified porphyrin has expanded considerably, extending
up to the benzo moieties. Additionally, the molar absorption
coefficient of Soret-like bands for these core-modified porphy-
rins is significantly lower than that for regular porphyrin.11
Scheme 2 Synthesis of tetraethynyl-substituted porphyrins 3a, 3b, 9a,
and 9b.
The oxidation and reduction potentials of 3b and 9b were
measured by cyclic voltammetry and the resulting voltammo-
grams, along with that of 1b, are shown in Fig. 3. Product 3b
exhibits oxidation waves at 0.53 and 1.01 V with reduction
waves at −0.41 and −0.73 V, while 9b shows oxidation waves at
0.62, 0.83, and 1.15 V with reduction waves at −0.32, and
−0.57 V. Both the first oxidation and reduction potentials
Fig. 2 UV/Vis/NIR absorption spectra of 1b (red), 3b (blue), and 9b
(green) in CH2Cl2.
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Fig. 4 Molecular orbital diagrams of (a) 1b, (b) 3b, and (c) 9b calcu-
lated at the B3LYP 6-31G* levels.
results demonstrate that the π-conjugated networks of these core-
modified porphyrins are extended over the entire macrocycles,
including the peripheral ethynyl groups. Further investigations
concerning the synthesis and functionalization of novel carba-
zole-containing porphyrins are currently under way in our
laboratory.
Fig. 3 Cyclic voltammograms of (a) 1b, (b) 3b, and (c) 9b (solvent:
CH2Cl2, supporting electrolyte: Bu4NPF6 (0.10 M), counter electrode:
Pt, reference electrode: Ag/Ag+, scan rate: 0.05 V s−1). Accurate poten-
tials were determined by the differential pulse voltammetry method
(ESI†).
Acknowledgements
increase in the order of 1b to 3b to 9b. The electrochemical
HOMO–LUMO gaps of 3b and 9b are evidently narrower than
that of 1b, indicating that the introduction of the ethynyl groups
reduces the HOMO–LUMO gap.
This work was supported by a Grant-in-Aid for Adaptable and
Seamless Technology transfer Program through target-driven
R&D (A-STEP) from JST, Japan (No. AS221Z00847D). We
thank Prof. A. Osuka (Kyoto-univ.) for the X-ray diffraction ana-
lyses and HR-ESI-MS measurements.
Finally, DFT calculations at the B3LYP 6-31G* level were
performed in order to obtain additional information concerning
the electronic properties of these core-modified porphyrins
(Fig. 4).12 Results of these calculations show that both HOMO
and LUMO levels are stabilized going from 1b to 3b to 9b,
which is consistent with the results of cyclic voltammetry.
Importantly, the HOMO levels of both 3b and 9b exhibit large
electronic coefficients not only on the core macrocycles, but also
on the ethynyl moieties, indicating that the electronic π electron
networks of 3b and 9b extend up to these ethynyl groups. The
nucleus-independent chemical shift (NICS) values at the centers
of these macrocycles were calculated as −11.09 for 1b, −10.03
for 3b, and −10.16 for 9b. These NICS results, along with the
1H NMR data, indicate that 3b, 9b, and 1b are all strongly
aromatic.13
Notes and references
‡Crystallographic data for 3a: C68H82N2S2Si4, M = 1103.84, monocli-
nic, space group P21/n (14), a = 19.3107(4), b = 17.0190(4), c =
23.1391(4) Å, β = 93.5198(10)°, V = 7590.3(3) Å3, Z = 4, ρcalcd
=
0.966 g cm−3, T = −180 °C, 10 918 measured reflections, 7314 unique
reflections (Rint = 0.0798), R1 = 0.0991 (I > 2σ(I)), wR2 = 0.2981 (all
data), GOF = 1.150. CCDC 871499 3a contains the supplementary crys-
tallographic data for this paper. The contribution to the scattering arising
from the presence of disordered solvents in the crystals was removed by
use of the utility SQUEEZE in the PLATON software package.14
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Conclusions
In summary, carbazole-based core-modified porphyrins bearing
ethynyl substituents on their thiophene or carbazole moieties (3b
and 9b) have been synthesized via a series of metal-catalyzed
coupling and annulation reactions. The effects of adding such
ethynyl substituents have been determined based on the results
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