Effective π-extension of carbazole-based thiaporphyrins by peripheral phenylethynyl substituents

Article abstract of DOI:10.1021/ol401403n

Several tetrakis(phenylethynyl)- and (phenylethynylphenylethynyl)- substituted carbazole-based thiaporphyrins were synthesized. These π-extended porphyrins display remarkably intensified and red-shifted absorption bands in the NIR region up to 1126 nm due

Full text of DOI:10.1021/ol401403n

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Effective π‑Extension of Carbazole-Based
Thiaporphyrins by Peripheral
Phenylethynyl Substituents
Chihiro Maeda,*,† Mikako Masuda, and Naoki Yoshioka*
Department of Applied Chemistry, Faculty of Science and Technology, Keio University,
Kohoku-ku, Yokohama 223-8522, Japan
cmaeda@okayama-u.ac.jp; yoshioka@applc.keio.ac.jp
Received May 17, 2013
ABSTRACT
Several tetrakis(phenylethynyl)- and (phenylethynylphenylethynyl)-substituted carbazole-based thiaporphyrins were synthesized. These
π-extended porphyrins display remarkably intensified and red-shifted absorption bands in the NIR region up to 1126 nm due to perturbation
by the phenylethynyl substituents.
Fused porphyrins have received considerable attention
due to their unusual optical and electronic properties.1,2 Of
particular interest is the extended π-conjugation of these
macrocycles, as compared to regular porphyrins, which
may lead to the application of such compounds in photo-
dynamic therapy,3 nonlinear optical devices,4 and photo-
voltaics.5 These fused porphyrins are typically synthesized
via intramolecular oxidative coupling reactions.
Carbazole derivatives have often been studied as novel
materials since they are highly emissive, electron conduct-
ing, easily modified, and chemically stable.6 Since carba-
zole is a benzene-fused pyrrole, its incorporation into fused
porphyrins presents interesting possibilities.7ꢀ9 Recently,
we reported a multiple annulation strategy which allows
the synthesis of novel porphyrinoids from 1,3-butadiyne-
bridged cyclic carbazole oligomers.10 Among these, the
† Present address: Division of Chemistry and Biochemistry, Graduate
School of Natural Science and Technology, Okayama University, Tsushima,
Okayama 700-8530, Japan.
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42, 1921. (c) Mori, H.; Tanaka, T.; Osuka, A. J. Mater. Chem. C 2013, 1,
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r
10.1021/ol401403n
XXXX American Chemical Society

tetrabenzo-fused core-modified porphyrin 1 exhibits dis-
tinct aromaticity and near-infrared (NIR) absorption due
to extended π-conjugation over the entire macrocycle.
Following this work, we investigated further π-extension
of carbazole-based porphyrins. Here we report the synthe-
sis of carbazole-based porphyrins with phenylethynyl or
phenylethynylphenylethynyl substituents (Figure 1).11
reaction with tributyltin chloride, afforded (2,6-dioctyl-
oxyphenylethynyl)tributyltin (7), which was then coupled
with1-bromo-4-trimethylsilylethynylbenzene toprovide 8.
Trimethylsilyl deprotection of 8, followed by the stan-
nylation of 9, generated [4-(2,6-dioctyloxyphenylethynyl)-
phenylethynyl]tributyltin (10).
Compounds 2a and 2b, both of which have four phenyl-
ethynyl substituents at the carbazole moieties, were sub-
sequently synthesized as follows (Scheme 1). The Stille
coupling reaction of 3,6-dibromo-1,8-bis(trimethylsilyl-
ethynyl)carbazole (11) with 7 provided 12a. The trimethyl-
silyl protection was removed by tetrabutylammonium
fluoride, and the Glaser coupling reaction of 13a gave
the cyclic carbazole dimer 14a. The annulation reaction of
14a with Na2S provided the isophlorine 15a.12 Finally, 15a
was oxidized with MnO2 to the thiaporphyrin 2a. The
phenylethynyphenylethynyl substituted thiaporphyrin 2b
was prepared from 11 in a similar manner by using 10
instead of 7. The carbazole-based thiaporphyrins bear-
ing phenylethynyl substituents at the thiophene moieties
(3a and 3b) were synthesized as follows. The Stille coupling
reaction of the tetrabrominated isophlorine 16 with either
7 or 10 provided 17a or 17b, respectively. Although 17a
and 17b were not oxidized by MnO2, their reaction with
1
PbO2 afforded 3a and 3b, respectively. The H NMR
spectra of the isophlorines15a, 15b, 17a, and 17b all exhibit
NH peaks at δ = 10 ppm, while the spectra of the oxidized
thiaporphyrin products contain no NH signals and exhibit
downfield shiftsofthe peripheral proton signals, indicating
a ring current effect.
Figure 1. Structures of the carbazole-based porphyrins.
We initially attempted to synthesize 3c but found it was
not possible to isolate the compound, due to its very low
solubility. In order to overcome this problem, octyloxy
groups were introduced at the terminal phenyl rings as
solubilizing groups to prevent aggregation of the products.
The phenylethynyltin reagents 7 and 10 were prepared
for this purpose, as shown in Scheme S1 in the Support-
ing Information. Alkylation of 2-iodoresorcinol with
1-bromooctane provided4, and the Stille coupling reaction
of 4 with tributyl(trimethylsilylethynyl)tin and subsequent
silyl deprotection of 5 gave 6. Lithiation of 6, followed by
The UV/vis/NIR absorption spectra of 2a, 2b, 3a, and 3b
all exhibitstrong Q-likebandsinthe NIR region, whichare
red-shifted in comparison to the same bands of 1 (Figure 2).
Interestingly, the central band in the 2a spectrum (at
1030 nm) is more intense than the equivalent band of 1
(934 nm), while the most pronounced absorption band of
3a (1111 nm) is more intense than the strongest band of 1
(1049 nm), suggesting that the phenylethynyl substituents
result in critical perturbation. Importantly, the spectra of
2a and 2b are very similar, as are those of 3a and 3b, which
indicates that the terminal phenylethynyl groups present in
2b and 3b make only minor contributions to the extent of
π-conjugation over the macrocycle. In addition, upon
excitation at 980 nm, 2a, 2b, and 3a exhibited weak
fluorescence with vibration bands at 1140 and 1170 nm
(Figure S22 in Supporting Information).13
(7) (a) Piatek, P.; Lynch, V. M.; Sessler, J. L. J. Am. Chem. Soc. 2004,
126, 16073. (b) Arnold, L.; Norouzi-Arasi, H.; Wagner, M.; Enkelmann,
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V.; Mullen, K. Chem. Commun. 2011, 970. (c) Arnold, L.; Baumgarten,
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M.; Mullen, K. Chem. Commun. 2012, 9640.
(8) Cabazole-containing porphyrins were formed as a consequence of
Bergman cyclization of β,β0-diethynylated porphyrins: (a) Aihara, H.;
Jaquinod, L.; Nurco, D. J.; Smith, K. M. Angew. Chem., Int. Ed. 2001,
40, 3439. (b) Nath, M.; Huffman, J. C.; Zaleski, J. M. J. Am. Chem. Soc.
2003, 125, 11484. (c) Nath, M.; Pink, M.; Zaleski, J. M. J. Am. Chem.
Soc. 2005, 127, 478.
(9) Indole-tetramers and related fused-porphyrinoids were reported by
Shinokubo and co-workers: (a) Nakamura, S.; Hiroto, S.; Shinokubo, H.
Chem. Sci. 2012, 2, 524. (b) Nakamura, S.; Kondo, T.; Hiroto, S.;
Shinokubo, H. Asian J. Org. Chem. 2013, 2, 4.
(10) (a) Maeda, C.; Yoneda, T.; Aratani, N.; Yoon, M.-C.; Lim,
J. M.; Kim, D.; Yoshioka, N.; Osuka, A. Angew. Chem., Int. Ed. 2011,
50, 5691. (b) Maeda, C.; Yoshioka, N. Org. Lett. 2012, 14, 2122. (c)
Maeda, C.; Yoshioka, N. Org. Biomol. Chem. 2012, 10, 5153. (d)
Masuda, M.; Maeda, C. Chem.;Eur. J. 2013, 19, 2971. (e) Masuda,
M.; Maeda, C.; Yoshioka, N. Org. Lett. 2013, 15, 578.
(11) For the ethynylated porphyrins, see: (a) Lin, V. S.-Y.; DiMagno,
S. G.; Therine, M. J. Science 1994, 264, 1105. (b) Screen, T. E. O.;
Lawton, K. B.; Wilson, G. S.; Dolney, N.; Ispasoiu, R.; Goodson, T.,
III; Martin, S. J.; Bradley, D. D. C.; Anderson, H. L. J. Mater. Chem.
2001, 11, 312. (c) Chandra, T.; Kraft, B. J.; Huffman, J. C.; Zaleski, J. M.
Inorg. Chem. 2003, 42, 5158. (d) Duncan, T. V.; Susumu, K.; Sinks, L. E.;
Therien, M. J. J. Am. Chem. Soc. 2006, 128, 9000.
The redoxpotentials ofthese compounds weremeasured
by cyclic voltammetry; the resulting electrochemical data
are summarized in Table 1. The electrochemical HOMO
ꢀLUMO gaps of the substituted porphyrins are all smaller
than that of 1, which is consistent with the optical
(12) For the transformation of 1,3-butadiyne to the thiophene, see:
€
(a) Lagan, J.; Arora, S. K. J. Org. Chem. 1983, 43, 4317. (b) Kromer,
J.; Rios-Carreras, I.; Fuhrmann, G.; Musch, C.; Wunderlin, M.;
€
Debaerdemaeker, T.; Mena-Osteritz, E.; Bauerle, P. Angew. Chem.,
Int. Ed. 2000, 39, 3481. (c) Sumi, N.; Nakanishi, H.; Ueno, S.;
Takimiya, K.; Aso, Y.; Otsubo, T. Bull. Chem. Soc. Jpn. 2001, 74,
979. (d) O’Connor, M. J.; Haley, M. M. Org. Lett. 2008, 10, 3973.
(13) The fluorescence was too weak to detect for 1 and 3b. The
fluorescence was not observed when excited at 400 nm.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Electrochemical Resultsa
Scheme 1. Synthesis of Carbazole-Based Porphyrins 2a, 2b, 3a,
and 3b
HOMOꢀLUMO gap
Ered1
Eox1
ΔECV
ΔEopt
(V)
(V)
(eV)
(eV)
1
ꢀ0.552
ꢀ0.418
ꢀ0.361
ꢀ0.584
ꢀ0.388
0.424
0.356
0.376
0.364
0.417
0.976
0.774
0.737
0.948
0.805
1.18
1.10
1.10
1.11
1.12
2a
2b
3a
3b
a Solvent: CH2Cl2. Supporting electrolyte: Bu4NPF6 (0.10 M).
Counter electrode: Pt. Reference electrode: Ag/Agþ. Scan rate: 0.05 V/
s. Accurate potentials were determined by the differential pulse voltam-
metry method (Supporting Information).
Figure 2. UV/vis/NIR absorption spectra in CH2Cl2 (black line,
1; red line, 2a; blue line, 2b; pink line, 3a; green line, 3b).
Inset shows the NIR region enlarged. The spectrum of 3b is
normalized.
spectroscopy results.14 DFT calculations were performed
in order to further elucidate the structures and electronic
Figure 3. MO diagram and HOMOs of (a) 1, (b) 2a, (c) 2b, (d) 3a,
and (e) 3b calculated at the B3LYP/6-31G* level.
(14) There are differences between 3a and 3b in the ΔECV and
between 2a and 3a in the reduction potentials although they show
similar optical properties. This is probably because some favorable
conformations exist in the oxidation state or in the reduction state and
such species affect the electrochemical behaviors while the absorption
spectra reflect the properties of the average structure.
properties of these compounds. In these studies, the octyl-
oxy groups were replaced with methoxy groups in order to
reduce the required calculations (Figure 3).15 The calcu-
lated HOMOꢀLUMO gaps of the ethynylated porphyrins
Org. Lett., Vol. XX, No. XX, XXXX
C

are also smaller than that of 1. As previously reported, the
HOMO of 1 exhibits large electronic coefficients both at
the 3,6-positions of the carbazoles and at the thiophene
moieties, suggesting that efficient substituent effects occur
at these positions. The HOMOs of 2a and 3a exhibit
electronic coefficients not only at their macrocycles but
also at the phenylethynyl moieties, demonstrating effective
electronic delocalization. Conversely, the electronic coeffi-
cients associated with the terminal phenyl rings of 2b and
3b are negligible. From these results, and considering the
UV/vis/NIR absorption spectra, we may conclude that
directly substituted phenylethynyl groups perturb the
macrocycles, whereas the contributions of the terminal
phenylethynyl groups of 2b and 3b to the macrocyclic
π-conjugation are minimal.
In summary, we have synthesized several phenylethynyl
and phenylethynylphenylethynyl substituted carbazole-
based thiaporphyrins. The photophysical and electro-
chemical analysis of these products, along with DFT cal-
culations, confirmed effective π-extension resulting from
the addition of the phenylethynyl substituents. Further
exploration of novel porphyrinoids and their metal com-
plexes is currently underway in our laboratory.
Acknowledgment. This work was supported by a
Grant-in-Aid for Young Scientists (No. 23750231 (B))
from MEXT, Japan. We thank Prof. A. Osuka, Dr. T.
Tanaka, and Mr. T. Yoneda (of Kyoto University) for
fluorescence measurements and DFT calculations. C.
M. also thanks the Casio Science Promotion Founda-
tion for its support.
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari,
K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.;
Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin,
R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Supporting Information Available. Experimental pro-
cedures and compound data. This material is available
free of charge via the Internet at http://pubs.acs.org.
€
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman,
J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.02;
Gaussian, Inc.: Wallingford, CT, 2009.
The authors declare no competing financial interest.
D
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