Meso-trialkyl-substituted subporphyrins

Article abstract of DOI:10.1002/anie.200906005

"Chemical Equation Presented" The desulfurization of meso-trithienylsubporphyrins leads to meso-trialkylsubporphyrins (see scheme). Oxidation at the benzylic position of the meso-alkyl substituent of 1 has been used to prepare a valeryl-substituted subpor

Full text of DOI:10.1002/anie.200906005

Angewandte
Chemie
DOI: 10.1002/anie.200906005
Porphyrinoids
Meso-Trialkyl-Substituted Subporphyrins**
Shin-ya Hayashi, Yasuhide Inokuma, Shanmugam Easwaramoorthi, Kil Suk Kim,
Dongho Kim,* and Atsuhiro Osuka*
In recent years, subporphyrins, which are ring-contracted
porphyrins, have emerged as promising functional molecules
because of their attractive features, including bowl-shaped
structures, 14p-electronic aromatic systems, porphyrinlike
spectral characteristics, intense fluorescence, and supramolec-
ular chemistry based on axial chelation of the boron atom.[1–3]
Notably, the free rotation of meso-aryl substituents of
subporphyrins leads to the large electronic effects that give
rise to highly perturbed optical properties, as demonstrated
by oligo-1,4-phenyleneethynylene and 4-aminophenyl substi-
tuted subporphyrins.[4] Despite this progress, the chemistry of
subporphyrins still remains in its infancy and an effective
synthetic entry into novel subporphyrins is highly desirable.
Considering the important roles that meso-alkyl-substituted
porphyrins have played in the development of porphyrin
chemistry since the first porphyrin synthesis in 1935,[5] it is
desirable to study the chemistry of meso-trialkyl-substituted
subporphyrins, but the synthesis of this class of molecules has
not been reported to date. Herein, we report the first
synthesis of meso-trialkyl-substituted subporphyrins.
route that involved the synthesis of meso-thienyl subporphyr-
ins, and subsequent reductive desulfurization with Raney
nickel. Thus, subporphyrins 1a and 1b were prepared by using
our protocol in 1.7 and 3.7% yield, respectively (Scheme 1).
Scheme 1. Synthesis of meso-trialkylsubporphyrins 2a–c.
We initially applied our synthetic protocol for meso-
triaryl-substituted subporphyrin[1c] to the synthesis of meso-
trialkyl-substituted subporphyrins. Pyridine-tri-N-pyrrolyl-
borane was condensed with 1-pentanal under various reaction
conditions by changing reaction parameters such as solvent,
temperature, molar ratio, and any possible additives. Unfortu-
nately, the desired meso-tributyl subporphyrin was not
detected. These negative results led us to explore an indirect
The higher yield of 1b may be ascribed to the absence of the
free a-thienyl position, since free a-thiophenes are prone to
oxidative degradation. It is noteworthy that these compounds
are the first examples of subporphyrins that bear five-
membered aromatic heterocycles. Single-crystal X-ray dif-
fraction analysis revealed the bowl-shaped structure of 1b
(Figure 1), in which the dihedral angles[6] of the meso-thienyl
[*] S. Hayashi, Dr. Y. Inokuma, Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science, Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
Dr. S. Easwaramoorthi, K. S. Kim, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-electronic Systems
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Figure 1. X-ray crystal structure of subporphyrin 1b (thermal ellipsoids
are set at the 50% probability level).
[**] This work was supported by Grants-in-Aid (no. 19205006 (A), and
20108001 “pi-Space”) from MEXT (Japan). Y.I. thanks the JSPS for a
Research Fellowship for Young Scientist. The work at Yonsei
University was supported by the Star Faculty and World Class
University (2008-1955) We acknowledge programs from MEST. an
AFSOR/AOARD Grant (FA4869-08-1-4097), and a grant from the
fundamental R&D Program for Core Technology of Materials
funded by the Ministry of Knowledge Economy, Republic of Korea.
K.S.K. thanks the MEST for a fellowship of the BK21 program.
substituents are all small (31.7, 35.6, and 48.28), thus allowing
the strong electronic conjugation of these substituents with
the subporphyrin core. These dihedral angles are slightly
smaller than those of meso-triphenyl subporphyrin 3 (38.3,
45.7, and 48.18), which reflect the compact size of the meso-
thienyl substituents.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906005.
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The UV/Vis absorption spectra of
1a and 1b exhibit broadened Soret-
like bands at 394 and 400 nm, respec-
tively, which are red-shifted by 21 and
27 nm from that of 3 at 373 nm
(Figure 2 and the Supporting Infor-
mation). The Q-like band of 1b was
observed as a considerably broadened
band at 534 nm. The fluorescence of
(kr = 1/t0; t0 = tF/FF) were 5.4 ꢀ 107 sÀ1 and 8.0 ꢀ 107 sÀ1 for 3
and 1b, respectively. The natural radiative rate of a molecule
has often been related to the radiative size of a chromo-
phore.[4a] Thus, the larger radiative rate of 1b than 3 is
attributed to the relatively facile rotation of the thienyl
groups around the subporphyrin core. This rotation facilitates
larger electronic interactions between the meso substituents
and the core.
We then examined the reductive desulfurization of meso-
thienyl-substituted subporphyrins. A solution of 1b in toluene
was treated with W-7 Raney Nickel[7] at 408C for 30 min, and
TLC analysis confirmed the consumption of 1b. Comparison
1
of the H and 11B NMR spectra of the reaction mixture with
those of meso-triaryl-substituted counterparts indicated the
formation of meso-tripentylsubporphyrin 1b, along with
several over-reduced products (see the Supporting Informa-
tion).[1c,8] Since these reduction products were difficult to
separate, the reaction mixture was oxidized with o-chloranil
to give a simpler mixture, from which 2b was isolated in 50%
yield. In the same manner, meso-tributylsubporphyrins 2a
was obtained in 48% yield.
By following essentially the same procedures, meso-tris(2-
benzo[b]thienyl)-subporphyrin 1c was prepared in 0.9% yield
from the reaction of 2-formyl-benzothiophene with pyridine-
N-tripyrrolylborane. Compound 1c was converted to meso-
tris(2-phenylethyl)subporphyrin 2c in 43% yield.
Figure 2. UV/Vis absorption (solid lines) and fluorescence (dashed
lines) spectra of subporphyrins 1b, 2b, 3, and 4 in CH2Cl2.
The high-resolution ESI mass spectrum of 2b shows an
intense borenium cation peak at m/z 452.3232 (calcd for
1b was observed at 632 nm with a fluorescence quantum yield
FF = 0.45. To the best of our knowledge, the FF value of 1b is
the highest reported to date for simple porphyrinoid systems
without charge-transfer interactions. These spectral data
indicate large electronic influences of the meso-thienyl
substituents on the subporphyrin core, and the large Stokes
shift (Table S1 in the Supporting Information) suggests a large
structural change in the excited state. The fluorescence
lifetime of 1b measured by the time-correlated single
photon counting method (TCSPC) was 5.6 ns, which is
approximately twice that of 3 (2.4 ns; Figure 3 and the
Supporting Information). Furthermore, the natural radiative
rate constants calculated from FF and tF using the relationship
1
C30H39N3B1 = 452.3237 [2b-OMe]+). The H NMR spectrum
of 2b shows a singlet at d = 8.06 ppm for the six b-pyrrolic
protons, a set of five signals at d = 3.92, 2.30, 1.72, 1.53, and
0.97 ppm for the protons in meso-pentyl substituents, and a
singlet at d = 0.75 ppm for the axial B methoxy protons. The
chemical shift of the b-pyrrolic proton signal of 2b is shifted
upfield from that of 3 (d = 8.12 ppm)[1c] because of the
absence of the local anisotropic ring current effect of the
meso-phenyl substituent. The signal for the boron atom in the
11B NMR spectra was observed at d = À15.0 ppm, which is
similar to that of 3 at d = À15.3 ppm.
Subporphyrins 2a–c are quite soluble in organic solvents
including methanol and hexane, thus making their crystal-
lization rather difficult. Therefore, in order to obtain single
crystals suitable for X-ray diffraction, we examined many
groups for the axial ligand of 2b. Fortunately, we found that
suitable crystals could be obtained by slow recrystallization of
an ether solution of 3,5-dinitrobenzyl-subporphyrin DN-2b
that was prepared by heating a toluene solution of 2b at reflux
in the presence of 3,5-dinitrobenzylalcohol. The crystal
structure of DN-2b displays a bowl-shaped C3-symmetric
structure with a bowl depth of 1.37 ꢁ, which is the distance
from the boron atom to the mean plane of the six peripheral
À
b-carbon atoms (Figure 4). The bond distances of three B N
À
and B O bonds are 1.48, 1.49, 1.49, and 1.46 ꢁ, respectively.
The UV/Vis absorption spectrum of 2b in CH2Cl2 exhibits
Soret-like bands at 360 nm with a shoulder at 344 nm, and Q-
like bands at 450 and 472 nm, which are both blue-shifted and
more intense than those of 3 (Figure 2). Importantly, the
comparison of the optical data of 2b and 3 reveals a
substantial electronic conjugation of the meso-phenyl
Figure 3. Fluorescence decay profiles of subporphyrins 1b, 2b, 3, and
4 in CH2Cl2 obtained by the TCSPC method (lex =420 nm; the decays
are collected at the respective fluorescence maxima).
322
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Figure 4. Crystal structure of subporphyrin DN-2b (thermal ellipsoids
are set at the 50% probability level).
groups to the subporphyrin core in 3; in contrast, this
conjugation is only marginal for porphyrins. It is noteworthy
that the absorption spectrum of 2b indicates clear vibronic
structures, which are reminiscent of the absorption features of
typical porphyrins, for the Soret-like band as well as the Q-
like bands. The steady-state fluorescence spectrum of 2b also
exhibits a vibronic structure with peaks at 476 and 507 nm,
which are a mirror image of the Q-like band spectrum. The
small Stokes shift value (489 cmÀ1) reflects the rigid structure
of subporphyrin core in 2b, because the radiative part of the
chromophore is limited to the core alone. The fluorescence
quantum yield of 2b was also determined to be small (0.07).
The fluorescence lifetime of 2b was measured to be 3.2 ns,
and the natural radiative rate constant was calculated to be
2.2 ꢀ 107 sÀ1. The larger radiative rates of 1b and 3 with
respect to that of 2b (Table S1 in the Supporting Information)
confirm that the intrinsic photophysical properties of sub-
porphyrins are remarkably influenced by conjugated inter-
actions with meso-aryl substituents. In addition, the larger
Stokes shifts (Table S1 in the Supporting Information) for the
meso-aryl subporphyrins are due mainly to the different
dihedral angles between the core and the aryl groups in the
ground and excited states. Thus, the enhanced fluorescence
properties of 3 (FF = 0.13) and meso-trithienyl subporphyrins
(1a, FF = 0.35; 1b, FF = 0.45; 1c, FF = 0.47) are considered to
arise from the effective electronic interactions with the meso-
aryl substituents.
Figure 5. Cyclic voltammograms of subporphyrins 1b, 2b, and 3.
Fc/Fc+ =ferrocene/ferrocenium.
ment of 2b with ten equivalents of 2,3-dichloro-5,6-dicyano-
benzoquinone (DDQ) in the presence of water in THF at
608C[9] afforded meso-valeryl subporphyrin 4 in 24% yield
(Scheme 2). The high-resolution ESI mass spectrum of 4
Scheme 2. Synthesis of meso-valeryl subporphyrin 4.
The oxidation and reduction potentials of subporphyrins
were measured by cyclic voltammetry. In CH2Cl2 solution, 3
exhibited the first oxidation and reduction potentials at 0.82
and À1.84 V, respectively (Figure 5), while 1b exhibited two
reversible oxidation potentials at 0.45 and 0.84 V, and a
reduction potential at À1.71 V. Accordingly, the electro-
chemical HOMO–LUMO (HOMO = highest occupied
molecular orbital, LUMO = lowest unoccupied molecular
orbital) gap of 1b was only 2.16 eV, which was considerably
lower than that of 3 (2.67 eV). This value is approximately
consistent with MO calculations at the B3LYP/6-31G* level
(Figure S9 in the Supporting Information). On the other
hand, the subporphyrin 2b exhibited the reversible oxidation
process at 0.77 V, but the reduction was outside the potential
window. Thus, the measurements were carried out in aceto-
nitrile, in which oxidation and reduction potentials of 0.78 and
À1.82 V (3) and 0.77 and À2.16 V (2b) were recorded
(Figures S7 and S8 in the Supporting Information).
revealed an intense borenium cation peak at m/z 466.3024
(calcd for C30H37N3O1B1 = 466.3029 [4-OMe]+). The 1H NMR
spectrum reflects the Cs symmetry of 4 and shows the three b-
proton signals as a singlet at 8.45 ppm and as a pair of
doublets at d = 8.11 and 8.08 ppm, and a set of signals for the
valeryl group at d = 3.84, 2.08, 1.64, and 1.08 ppm. The
11B NMR spectrum of 4 exhibits a peak at d = À14.9 ppm. The
absorption spectrum of 4 shows a Soret-like band at 371 nm
and Q-like bands at 461 and 490 nm. Both the absorption
bands are red-shifted and are less intense than those of 2b.
The fluorescence spectrum of 4 exhibits a broad band at
519 nm (FF = 0.04) with tF = 1.1 ns. The small fluorescence
quantum yield may originate from charge-transfer-type
interactions because of the electron-withdrawing nature of
the carbonyl group, or mixing of its np* character. The
oxidation and reduction potentials of 4 are positively shifted
to 1.11 and À1.54 V, respectively (Table S2 in the Supporting
Information). Hence, it may be concluded that the simple
Finally, we examined the chemical transformations at the
benzylic positions of the meso-alkyl-substituents in 2b. Treat-
Angew. Chem. Int. Ed. 2010, 49, 321 –324
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Communications
45, 961 – 964; c) E. Tsurumaki, S. Saito, K. S. Kim, J. M. Lim, Y.
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439.
modification at the benzylic position led to large changes in
the optical properties of subporphyrin.
In summary, the first synthetic route to meso-trialkylsub-
porphyrins proceeds by desulfurization of meso-trithienyl-
subporphyrins, which are attractive molecules that exhibit
strongly perturbed absorption and fluorescence spectra
induced by large electronic interactions with meso-thienyl
substituents. Meso-alkylsubporphyrins are very important in
understanding the electronic systems of subporphyrins as they
show the optical and electrochemical properties of the
unmodified subporphyrin macrocycle. Furthermore, oxida-
tion at the benzylic position of the meso-alkyl substituent has
been used to prepare a valeryl-substituted subporphyrin that
exhibits highly perturbed optical and electrochemical proper-
ties. This work provides a firm basis for the exploration of
functional subporphyrins, which is currently under investiga-
tion.
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Received: October 26, 2009
Published online: December 8, 2009
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in CDCl3 indicated three peaks at d = À15.0, À12.1, and
À7.9 ppm, which were assigned for 2b, meso-tripentylsubchlorin,
and meso-tripentylbacteriosubchlorin, respectively. The reduc-
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catalyst and reaction time (see the Supporting Information).
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Keywords: desulfurization · fluorescence · macrocycles ·
porphyrinoids
.
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