Multiply-fused porphyrins - Effects of extended π-conjugation on the optical and electrochemical properties

Article abstract of DOI:10.1039/c3cc42831a

A novel quadruply-fused porphyrin has been synthesized with a facilely prepared precursor in a high yield. A detailed comparison of the physical properties of a series of fused porphyrins revealed remarkable effects of the ring fusion on lowering LUMO levels rather than HOMO levels.

Full text of DOI:10.1039/c3cc42831a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Multiply-fused porphyrins—effects of extended
p-conjugation on the optical and electrochemical
properties†
Tomoya Ishizuka,*a Yuta Saegusa,a Yoshihito Shiota,b Kazuhisa Ohtake,a
Kazunari Yoshizawab and Takahiko Kojima*a
Cite this: Chem. Commun., 2013,
49, 5939
Received 17th April 2013,
Accepted 13th May 2013
DOI: 10.1039/c3cc42831a
www.rsc.org/chemcomm
A novel quadruply-fused porphyrin has been synthesized with a facilely meso-b-arene-fused derivatives10–14 exhibit larger red-shifts of the
prepared precursor in a high yield. A detailed comparison of the physical optical absorption bands and smaller HOMO–LUMO gaps, com-
properties of a series of fused porphyrins revealed remarkable effects of pared to b–b-arene fused derivatives.9 However, the ring-fusion
the ring fusion on lowering LUMO levels rather than HOMO levels.
reactions reported so far are highly limited and the synthetic
procedures for the precursors are time-consuming.
p-Conjugated organic molecules exhibiting long-wavelength
absorption and a high tendency to form intermolecular p–p
stacking structures have attracted considerable attention due to
their potential for application in organic semiconductors and
Herein, we report a facile and efficient procedure for the
preparation of a quadruply ring-fused porphyrin (4 in Fig. 1), in
which the four meso-phenyl groups are covalently bonded to the
b-carbons of the pyrrole rings at the ortho-positions. The precursor
of the fused porphyrin, zinc(II) 2,3,12,13-tetrabromo-tetraphenyl-
porphyrinate (ZnTPPBr4), was easily prepared with a two-step
procedure from TPP15 and thus can be easily modified by
changing tetraarylporphyrin precursors. In addition, the ring-
fused porphyrins are highly planar with extended p-conjugation,
showing strong p–p stacking tendency and physical properties
based on the narrowed HOMO–LUMO gaps.
The ring-fusion reactions of ZnTPPBr4 were conducted by
direct C–H activation in the presence of a Pd catalyst.16 The key
factor of the reaction is the selection of the Pd catalyst. The usual
molecular Pd catalysts, such as Pd(PPh3)4, did not afford the fully
fused product 4, but the partially fused products, 1 and 2 (Table S1
in the ESI†).14c In sharp contrast, when Pd-nanoclusters derived
from [Pd(Z3-C3H5)Cl]2 were employed as the catalyst (Scheme 1),17
the quadruply fused 4 was obtained in 79% yield accompanying
the triply fused 3 as a minor product in 11% yield. The isolation of
4 was very easily performed by virtue of its low solubility in
organic solvents. The reaction mixture was filtered to remove
the Pd catalyst and other insoluble materials, and then the solvent
of the filtrate was evaporated under vacuum. The residual solid
was dissolved in THF and the remaining violet solid was filtered
organic photovoltaic cells.1–3 Tetraphenylporphyrin (TPP),4
a
representative of synthetic porphyrins,5 and its derivatives are
promising candidates as functional dyes for the purpose men-
tioned above, because of the highly-extended p-conjugation,
facileness of the synthesis and introduction of functional
groups, and the robustness among the organic dyes.4 TPP,
however, cannot form strong intermolecular p–p stacking with
the porphyrin core due to the steric repulsion of the meso-phenyl
groups,6 and thus it cannot be directly used to construct p–p
stacked one-dimensional molecular wires, which are very impor-
tant to construct molecule-based optoelectronic materials.7
On the other hand, modification of porphyrins by introduc-
tion of fused rings on the periphery of the porphyrin aromatic
circuit has been intensively studied recently,8–14 because of the
merits of the unique physical properties derived from the
narrowed HOMO–LUMO gaps. The ring-fusion strategy has been
successfully applied to obtain a chromophore showing an absorp-
tion band over 1400 nm based on one porphyrin unit,12 and by
increasing the number of porphyrin units fused, the longest
absorption band can reach the infrared region.13 In particular,
a Department of Chemistry, Graduate School of Pure and Applied Sciences,
University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Japan.
E-mail: ishizuka@chem.tsukuba.ac.jp, kojima@chem.tsukuba.ac.jp;
Fax: +81-29-853-6503
b Institute for Materials Chemistry and Engineering, Kyushu University, Motooka,
Nishi-Ku, Fukuoka 819-0395, Japan
† Electronic supplementary information (ESI) available: Complete experimental
details of syntheses, compound characterization, summary of optical absorption and
redox potentials, details of crystal structural analyses, DFT-optimized structures
and cyclic and differential-pulse voltammograms. CCDC 929175–929178. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc42831a
Fig. 1 Structures of fused porphyrins 1–4.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5939--5941 5939

View Article Online
Communication
ChemComm
Scheme 1 Synthetic route to quadruply-fused porphyrin 4.
to give a red-brown filtrate. The solid obtained mainly included 4
and the recrystallization from THF–EtOH in the presence of
1 drop of pyridine to increase the solubility gave 4 with a pyridine
molecule as an axial ligand (4-py) in the pure form. In addition,
the major component of the red-brown filtrate in THF was the
triply fused 3 and further purification using preparative-scale thin-
layer chromatography on silica gel followed by recrystallization
from THF–hexane gave violet crystals of 3. The singly-fused 1 and
doubly-fused 218 were also obtained from the reactions under
different conditions using Pd-nanoclusters derived from Pd(OAc)2
as catalysts (entry 2 in Table S1 in the ESI†).17,19
Fig. 2 Crystal structures of 1-THF (a), 2 (b), 3 (c), and 4-py (d). The thermal
ellipsoids are drawn with 50% probability.
Characterization of the ring-fused porphyrins was performed phenyl group in the ring-fused moieties (see Fig. 1). The C–C bond
using 1H NMR spectroscopy, MALDI-TOF-MS spectrometry, lengths of non-fused phenyl groups and the C–C bond lengths except
elemental analysis19 and X-ray crystallography (vide infra). The the Cipso–Co(fus) bond in the fused phenyl rings were almost equal to
1H NMR spectrum of 4 in DMSO-d6 displayed a simple signal ca. 1.39 Å, whereas the Cipso–Co(fus) bond lengths were ca. 1.44 Å in
1
pattern reflecting the symmetric structure, and eight H NMR common to all the crystal structures of the fused porphyrins. The
signals including three signals due to a free pyridine molecule20 elongation of the Cipso–Co(fus) bond by ca. 0.05 Å indicates that a
were observed (Fig. S2 in the ESI†). Each signal of 4 was assigned single bond is localized between Cipso and Co(fus), and thus the
1
on the basis of the H–1H COSY spectrum (Fig. S3 in the ESI†). p-conjugation circuit always avoids the Cipso–Co(fus) bond and the
The 1H NMR spectrum of 3 showed a relatively complicated isolated aromaticity of the meso-phenyl groups is almost lost. There-
pattern due to the low-symmetric structure (Fig. S2b in the ESI†). fore, the larger aromatic circuits soaking into the fused phenyl rings
Recrystallization of 1–4 gave single crystals appropriate for X-ray in the fused porphyrins are recognized in the crystal structures.
diffraction analysis.21 The ORTEP drawings of 1-THF, 2, 3, and 4-py
The fused porphyrins obviously showed intermolecular p–p
are shown in Fig. 2. Compounds 1 and 4 took a THF and a pyridine stacking in the crystal packing and the p–p stacking distances
(py) molecule, respectively, from the recrystallization solvents as axial gradually shortened as the number of the fused rings increased.
ligands on the central ZnII ions, whereas 2 and 3 did not have any The THF-ligated 1 formed p–p stacked dimers, in which the
axial ligand. As one of the characteristics of the ring-fused porphyrins interplanar distance of the two porphyrins was 3.46 Å (Fig. S5 in
in the crystal structures, the bond lengths between the central ZnII the ESI†). On the other hand, compounds 2 and 3 having no axial
ion and the nitrogen atoms of the pyrrole rings involved in the ring- ligands formed p-stacked one-dimensional (1D) supramolecular
fused structures were much shortened than those for the ZnII and arrays in the crystals (Fig. S6 and S7 in the ESI†). Both the 1D
the nitrogen atoms of non-fused pyrrole rings (Table S2 in the ESI†): arrays with 2 and with 3 exhibit two interplanar distances of 3.43
for instance, in 4-py, the bond distances of Zn–N2 and Zn–N4 were and 3.58 Å for 2, and 3.28 and 3.36 Å for 3, respectively. With respect
found to be 1.894(2) Å and 1.900(2) Å, respectively, involving the to the p–p stacking distances of the three crystal structures, the
fused-ring nitrogens, whereas those of Zn–N1 and Zn–N3 were order is 1-THF (3.46 Å) > 2 (3.43 Å) > 3 (3.28 Å). This order indicates
2.196(5) Å and 2.126(5) Å, respectively, involving the non-fused that increasing the number of the fused rings enhances the
pyrrole nitrogens. On the other hand, the Zn–N bond lengths in planarity of the molecule and simultaneously increases the number
ZnTPP were almost identical.6 The difference in the Zn–N bond of the p-electrons involved in the p-conjugation. Due to the axial
lengths observed for the fused and non-fused pyrroles is probably pyridine ligand, 4-py showed a doming distortion of the fully fused
caused by the rigidity of the fused ring structure, which results in porphyrin ligand in the crystal structure and the porphyrin core did
shrinkage of the N2ꢀꢀꢀN4 distance and elongation of the N1ꢀꢀꢀN3 not exhibit any intermolecular p–p stacking (Fig. S8 in the ESI†).
distance. The rhombic deformation of the porphyrin core by the ring
To confirm the electronic effects of the ring-fusion on the
fusion is confirmed by the density-functional-theory (DFT)-optimized porphyrin p-conjugation, we have measured the electronic
structure of the free-base porphyrin of 4 (Fig. S4 in the ESI†), where absorption spectra and cyclic and differential-pulse voltammo-
the N1ꢀꢀꢀN3 distance is 4.59 Å and the N2ꢀꢀꢀN4 distance is 3.43 Å. grams (CV and DPV) of the fused porphyrins (Table S3 in the
Another feature found in the crystal structures of the fused porphyrins ESI†). Due to the low solubility of 3 and 4, we employed dimethyl-
was the long bond lengths between the ipso-carbon (Cipso) of the formamide (DMF) as the solvent for absorption spectroscopy and
fused phenyl groups and the ortho-carbon (Co(fus)) of the same electrochemical measurements. In the absorption spectra, both the
c
5940 Chem. Commun., 2013, 49, 5939--5941
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
by Grants-in-Aid (no. 20108010, 22750118, and 24245011) from
Japan Society for the Promotion of Science (JSPS).
Notes and references
1 (a) J. Wu, W. Pisula and K. Mu¨llen, Chem. Rev., 2007, 107, 718; (b) T. Aida,
¨
E. W. Meijer and S. I. Stupp, Science, 2012, 335, 813; (c) D. Gorl, X. Zhang
and F. Wu¨rthner, Angew. Chem., Int. Ed., 2012, 51, 6328.
¨
2 (a) L. Schmidt-Mende, A. Fechtenkotter, K. Mu¨llen, E. Moons, R. H.
Friend and J. D. MacKenzie, Science, 2001, 293, 1119; (b) X. Feng,
V. Marcon, W. Pisula, M. R. Hansen, J. Kirkpatrick, F. Grozema,
D. Andrienko, K. Kremer and K. Mu¨llen, Nat. Mater., 2009, 8, 421.
3 Y. Yamamoto, T. Fukushima, Y. Suna, N. Ishii, A. Saeki, S. Seki,
S. Tagawa, M. Taniguchi, T. Kawai and T. Aida, Science, 2006, 314, 1761.
4 (a) The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2000, vol. 1; (b) M. O. Senge,
M. Fazekas, E. G. A. Notaras, W. J. Blau, M. Zawadzka, O. B. Locos
and E. M. N. Mhuircheartaigh, Adv. Mater., 2007, 19, 2737.
Fig. 3 UV-Vis absorption spectra of fused porphyrins in DMF. 1 (black), 2 (blue),
3 (green), and 4 (red).
5 (a) P. Rothemund, J. Am. Chem. Soc., 1936, 58, 625; (b) A. D. Adler,
F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and L. Korsakoff,
J. Org. Chem., 1967, 32, 476; (c) J. S. Lindsey, I. C. Schreiman, H. C. Hsu,
P. C. Kearney and A. M. Marguerettaz, J. Org. Chem., 1987, 52, 827.
6 M. P. Byrn, C. J. Curtis, Y. Hsiou, S. I. Khan, P. A. Sawin, S. K. Tendick,
A. Terzis and C. E. Strouse, J. Am. Chem. Soc., 1993, 115, 9480.
7 (a) S. Bhosale, A. L. Sisson, P. Talukdar, A. Fu¨rstenberg, N. Banerji,
Soret- and Q-like bands gradually red-shifted upon increasing the
number of the fused rings (Fig. 3). The lowest energy of the Q-like
band was observed for 4, reaching over 1000 nm and the batho-
chromic shift was 420 nm relative to the longest Q-band of ZnTPP
in DMF. In addition, the fused porphyrins absorb almost whole
visible light and consequently, for example, the solution color of 4
in DMF is dark blue. All the fused porphyrins exhibited reversible
first-oxidation and first-reduction waves in DMF, and reversible or
pseudo-reversible second-oxidation and second-reduction waves
(Fig. S9 in the ESI†). HOMO–LUMO gaps obtained from the first-
oxidation and the first-reduction potentials become narrower with
increasing the number of fused rings. It should be noted that the
lowering of the LUMO level by ring-fusion was more remarkable
than the rise of the HOMO level: for instance, the difference of the
first reduction potentials between 4 (ꢁ0.67 V) and ZnTPP (ꢁ1.36 V)
was 0.69 V, whereas that of the first oxidation potentials of 4
(+0.65 V) and ZnTPP (+0.83 V) was 0.18 V (Table S4 in the ESI†). To
confirm the large effect of the ring-fusion on the LUMO level, we
conducted DFT calculations on 4. As a result, it was indicated that
the HOMO was delocalized mainly on the porphyrin core, whereas
the LUMO was expanded to the ring-fused moieties (Fig. S10 in the
ESI†). Therefore, the energy level of LUMO is strongly affected by
the degree of ring-fusion, compared to that of HOMO.
In summary, we have synthesized a novel quadruply-fused
porphyrin with a facilely prepared precursor in a high yield. In
addition, we have isolated and fully characterized partly-ring-fused
porphyrin derivatives. The crystal structures revealed the extension
of the p-conjugation circuits to the fused meso-phenyl groups by the
bond lengths between the ipso-carbon and the ortho-carbon bonded
to the b-pyrrole carbon. In the crystal packing, the fused porphyrins
exhibited strong intermolecular p–p stacking, reflecting the highly
planar structures. The UV-Vis spectra and the electrochemical
studies of the fused porphyrins indicate the narrowing of the
HOMO–LUMO gaps by the extension of the ring-fusion reactions.
The planar and p-extended porphyrins developed here would be
applicable to various optoelectronic materials on the basis of
the highly red-shifted absorption bands covering the entire
visible wavelength region and the strong tendency to form one-
dimensional molecular wires with intermolecular p–p stacking.
We are grateful to Dr S. Higashibayashi (Institute for Molecular
Science, Japan) for his helpful suggestion. This work was supported
¨
E. Vauthey, G. Bollot, J. Mareda, C. Roger, F. Wu¨rthner, N. Sakai and
S. Matile, Science, 2006, 313, 84; (b) Y. Matsuo, Y. Sato, T. Niinomi,
I. Soga, H. Tanaka and E. Nakamura, J. Am. Chem. Soc., 2009,
131, 16048; (c) A. W. Hains, Z. Liang, M. A. Woodhouse and
B. A. Gregg, Chem. Rev., 2010, 110, 6689.
8 H. Mori, T. Tanaka and A. Osuka, J. Mater. Chem. C, 2013, 1, 2500.
´
´
9 (a) D. Mysliwiec, B. Donnio, P. J. Chmielewski, B. Heinrich and M. St˛epien,
J. Am. Chem. Soc., 2012, 134, 4822; (b) H. Boedigheimer, G. M. Ferrence and
T. D. Lash, J. Org. Chem., 2010, 75, 2518; (c) L. Jiang, J. T. Engle, L. Sirk,
C. S. Hartley, C. J. Ziegler and H. Wang, Org. Lett., 2011, 13, 3020.
10 (a) H. Aihara, L. Jaquinod, D. J. Nurco and K. M. Smith, Angew. Chem.,
Int. Ed., 2001, 40, 3439; (b) M. Nath, J. C. Huffman and J. M. Zaleski,
J. Am. Chem. Soc., 2003, 125, 11484; (c) L. J. K. Boerner, S. Mazumder,
M. Pink, M.-H. Baik and J. M. Zaleski, Chem.–Eur. J., 2011, 17, 14539.
11 (a) K. Kurotobi, K. S. Kim, S. B. Noh, D. Kim and A. Osuka, Angew.
Chem., Int. Ed., 2006, 45, 3944; (b) H. S. Gill, M. Harmjanz, J. Santamaria,
I. Finger and M. J. Scott, Angew. Chem., Int. Ed., 2004, 43, 485;
(c) C. Maeda, T. Yoneda, N. Aratani, M.-C. Yoon, J. M. Lim, D. Kim,
N. Yoshioka and A. Osuka, Angew. Chem., Int. Ed., 2011, 50, 5691;
(d) S. Nakamura, S. Hiroto and H. Shinokubo, Chem. Sci., 2012, 3, 524.
12 (a) N. K. S. Davis, A. L. Thompson and H. L. Anderson, J. Am. Chem.
Soc., 2011, 133, 30; (b) N. K. S. Davis, A. L. Thompson and
H. L. Anderson, Org. Lett., 2010, 12, 2124; (c) A. N. Cammidge,
P. J. Scaife, G. Berber and D. L. Hughes, Org. Lett., 2005, 7, 3413.
13 (a) A. Tsuda and A. Osuka, Science, 2001, 293, 79; (b) Y. Nakamura,
N. Aratani, H. Shinokubo, A. Takagi, T. Kawai, T. Matsumoto,
Z. S. Yoon, D. Y. Kim, T. K. Ahn, D. Kim, A. Muranaka,
N. Kobayashi and A. Osuka, J. Am. Chem. Soc., 2006, 128, 4119.
14 (a) S. Fox and R. W. Boyle, Chem. Commun., 2004, 1322; (b) D.-M. Shen,
C. Liu and Q.-Y. Chen, Chem. Commun., 2005, 4982; (c) D.-M. Shen,
C. Liu and Q.-Y. Chen, J. Org. Chem., 2006, 71, 6508; (d) S. Hayashi,
Y. Matsubara, S. Eu, H. Hayashi, T. Umeyama, Y. Matano and
H. Imahori, Chem. Lett., 2008, 846; (e) T. D. Lash, B. E. Smith,
M. J. Melquist and B. A. Godfrey, J. Org. Chem., 2011, 76, 5335.
15 P. K. Kumar, P. Bhyrappa and B. Varghese, Tetrahedron Lett., 2003,
44, 4849.
16 B. D. Steinberg, E. A. Jackson, A. S. Filatov, A. Wakamiya, M. A.
Petrukhina and L. T. Scott, J. Am. Chem. Soc., 2009, 131, 10537.
17 S. Higashibayashi and H. Sakurai, Chem. Lett., 2007, 36, 18.
18 The doubly-fused porphyrin contains three isomers, 7,22;18,40-
fused, 2, 7,22;17,34-fused, 20, as well as 7,22;8,28-fused, 200 (Fig. S1
in the ESI†). 2 was the major product and could be easily isolated by
column chromatography (see ref. 14).
19 See ESI†.
20 The pyridine molecule was originally ligated on the central ZnII of 4
during the recrystallization. Judging from the chemical shifts
observed, the pyridine molecule dissociates from the ZnII upon
dissolving in DMSO-d6.
21 The crystal structure of 2 having THF as an axial ligand has been
reported by Shen et al. (ref. 14c).
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 5939--5941 5941