β-to-β 2,5-Pyrrolylene-Linked Cyclic Porphyrin Oligomers

Article abstract of DOI:10.1002/chem.201601306

β-to-β 2,5-Pyrrolylene linked cyclic porphyrin oligomers have been synthesized by Suzuki–Miyaura coupling of 2,5-diborylpyrrole and 3,7-dibromoporphyrin. The cyclic porphyrin oligomers exhibit roughly coplanar structures, strong excitonic coupling, small electrochemical HOMO–LUMO gaps, and ultrafast excitation energy transfer between the neighboring porphyrins via the pyrrolylene bridge.

Full text of DOI:10.1002/chem.201601306

DOI: 10.1002/chem.201601306
Communication
&
Porphyrinoids
b-to-b 2,5-Pyrrolylene-Linked Cyclic Porphyrin Oligomers
Yutao Rao,[a] Jun Oh Kim,[b] Woojae Kim,[b] Guangming Zhong,[a] Bangshao Yin,[a]
Mingbo Zhou,[a] Hiroshi Shinokubo,[c] Naoki Aratani,[d] Takayuki Tanaka,[e] Shubin Liu,[f]
Atsuhiro Osuka,*[e] Dongho Kim,*[b] and Jianxin Song*[a]
tation energy transfer as a consequence of large electronic in-
Abstract: b-to-b 2,5-Pyrrolylene linked cyclic porphyrin
oligomers have been synthesized by Suzuki–Miyaura cou-
teraction of the constituent porphyrins. Pyrrole is a quite at-
tractive bridge in cyclic porphyrin arrays in view of its electron-
pling of 2,5-diborylpyrrole and 3,7-dibromoporphyrin. The
rich properties, significant conjugation with other segments,
cyclic porphyrin oligomers exhibit roughly coplanar struc-
a potential hydrogen-bonding donor, and possible interconver-
tures, strong excitonic coupling, small electrochemical
sion between amine-type and imine-type forms. Despite these
HOMO–LUMO gaps, and ultrafast excitation energy trans-
promises, pyrrole-bridged porphyrin nanorings are rare. To the
fer between the neighboring porphyrins via the pyrroly-
best of our knowledge, meso-to-meso pyrrole-bridged porphy-
lene bridge.
rin nanorings were synthesized as a single example by Suzuki–
Miyaura crossing coupling of 5,10-dibromoporphyrin and 2,5-
diborylpyrrole.[6] As an extension of this work, we report the
Cyclic porphyrin arrays have attracted wide attention owing to
their potential applications, such as synthetic models of artifi-
cial photosynthetic antenna and large host molecules possess-
ing convergent multidentate coordination sites.[1–3] Cyclic por-
phyrin arrays are usually linked with acetylenic, aryl, heterocy-
clic, and other bridges. As rare and extreme cases, two spacer-
less directly meso-to-meso and b-to-b linked cyclic porphyrin
arrays have been explored as photosynthetic models.[4,5] These
porphyrin nanorings display very efficient intramolecular exci-
synthesis of b-to-b 2,5-pyrrolylene-bridged cyclic porphyrin
dimer and trimer (Figure 1).
3,7-Dibromo-10,15,20-triaryl NiII porphyrin 1Ni was prepared
by bromination of 3,7-diboryl-10,15,20-triaryl NiII porphyrin
[a] Y. Rao, G. Zhong, Dr. B. Yin, Dr. M. Zhou, Prof. Dr. J. Song
Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research (Ministry of Education of China)
Key Laboratory of Application and Assembly of
Organic Functional Molecules
Hunan Normal University, Changsha 410081 (China)
Fax: (+86)731-8478-6676
E-mail: jxsong@hunnu.edu.cn
[b] J. O. Kim, W. Kim, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University
Seoul 03722 (Korea)
E-mail: dongho@yonsei.ac.kr
[c] Prof. Dr. H. Shinokubo
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University, Nagoya 464-8603 (Japan)
[d] Prof. Dr. N. Aratani
Nara Institute of Science and Technology (Japan)
[e] Dr. T. Tanaka, 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
[f] Prof. Dr. S. Liu
Division of Research Computing, Information Technology Services
University of North Carolina, Chapel Hill, North Carolina 27599 (USA)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
http://dx.doi.org/10.1002/chem.201601306.
Figure 1. Porphyrin compounds studied herein.
Chem. Eur. J. 2016, 22, 8801 – 8804
8801
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
with CuBr2 in THF.[5] Initially, Suzuki–Miyaura cross-coupling of
1Ni and 2,5-diborylpyrrole[7] was conducted under the same
conditions used for meso-to-meso pyrrole-bridged porphyrin
arrays (Pd2(dba)3, PPh3, CsF, Cs2CO3, DMF, toluene, reflux,
24 h).[6] Mass analysis of the reaction mixture indicated forma-
tion of only linear oligomers and other unknown byproducts.
Then we changed PPh3 by P(tBu)3. To our delight, this reaction
proceeded to give cyclic porphyrin dimer 2Ni and trimer 3Ni
in 30% and 15% yields, respectively, after separation over
a silica gel column. High-resolution electrospray-ionization
time-of-flight (HR-ESI-TOF) mass measurement detected the
parent ion peak of 2Ni at m/z=1987.0353 (calcd for
C132H146N10Ni2 =1987.0439 ([M]+)), and that of 3Ni at m/z=
2981.5607 (calcd for C198H219N15Ni3 =2981.5731 ([M]+)). The
1H NMR spectra of 2Ni and 3Ni are both quite simple, exhibit-
ing a set of signals for the porphyrins and bridges, indicating
the molecular symmetry. The protons of the porphyrins were
observed similarly for 2Ni and 3Ni, but the signals for the pyr-
role bridges are remarkably different. The pyrrolic NH and b-
protons are observed at d=10.36 and 7.23 ppm for 2Ni but
the corresponding protons are observed at d=9.59 and
7.96 ppm for 3Ni. These chemical shifts differences may reflect
different conformations of the pyrrole bridges, as shown in
Figure 2.
Our attempts to demetalate 2Ni and 3Ni under acidic condi-
tions failed. Dibromo free-base porphyrin 1H was prepared by
demetalation of dibromo CuII porphyrin 1Cu and was used for
the Suzuki–Miyaura coupling reaction with 2,5-diborylpyrrole
to yield 2H and 3H in 20% and 9% yields, respectively. 2Zn
and 3Zn were prepared almost quantitatively by ZnII ion inser-
tion of 2H and 3H. Singly 2,5-pyrrolylene-bridged dimers 4m
(M=Zn, Ni, 2H), 3,7-di(pyrro-2-yl)porphyrin monomers 5m
(M=Zn, Ni, 2H), and 2-(pyrro-2-yl)porphyrin monomers 6m
(M=Zn, Ni, 2H) were also synthesized by the same procedure
as that of 2M (M=Zn, Ni, 2H) for comparison of their spectro-
scopic properties.
Figure 3 shows the normalized UV/Vis absorption and fluo-
rescence spectra of a series of ZnII porphyrins measured in
Figure 2. X-ray crystal structure of 2Ni and 3Ni. a) Top view and b) side
view of 2Ni and c) top view and d) side view of 3Ni. Ellipsoids are set at
50% probability; substituents on phenyl groups and solvent molecules are
omitted for clarity.
Figure 3. Normalized UV/Vis absorption (c) and fluorescence (a) spec-
tra of a) 2Zn, b) 3Zn, c) 4Zn, d) 5Zn, and e) 6Zn in CH2Cl2.
CH2Cl2. Mono-pyrro-2-yl-substituted 6Zn shows a Soret band
at 420 nm and Q-band at 549 nm, both of which are slightly
red-shifted compared to those of 5,10,15-triaryl ZnII porphyrin
(Soret band at 416 nm and Q-bands at 543 and 579 nm).[9] The
0–0 band in the fluorescence spectrum of 6Zn was observed
at 605 nm with FF =0.027, which is red-shifted and also very
slightly enhanced as compared with that of the reference por-
phyrin (lfl =587 nm and FF =0.023). On the other hand, di(pyr-
ro-2-yl)-substituted 5Zn exhibits a more red-shifted and broad-
er absorption spectrum with bands at 429 and 557 nm and
fluorescence with lfl =618 nm and FF =0.011, which implies
that two pyrrole units attached to adjacent b positions signifi-
cantly affect the electronic structure of the porphyrin core.
Definitive structural confirmations of 2Ni and 3Ni have
been accomplished by X-ray diffraction analysis, which re-
vealed diporphyrin and triporphyrin structures bridged by 2,5-
pyrrolylene linkages (Figure 2).[8] Dimer 2Ni displays a rather
bent gable structure with a diporphyrin dihedral angle of
111.68, in which the pyrrole bridges are pointing inward. Trimer
3Ni shows a more coplanar triangular molecular shape with
the pyrrole bridges pointing outward. The structure of 3Ni is
distinctly more coplanar. The center-to-center distances are
10.72 Š for 2Ni and 13.11 Š for 3Ni, respectively.
In the next step, free-base porphyrin oligomers, 2H and 3H,
and ZnII porphyrin oligomers, 2Zn and 3Zn, were prepared.
Chem. Eur. J. 2016, 22, 8801 – 8804
www.chemeurj.org
8802
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
b-to-b Pyrrolylene-bridged linear porphyrin dimer, 4Zn, ex-
hibits considerably enhanced 0–0 band intensity and relatively
high fluorescence quantum yield compared to those of por-
phyrin monomers (Table 1). Dimer 4Zn shows a large radiative
rate constant, which is consistent with TD-DFT calculation that
shows relatively high oscillator strength in the S1 transition.
Furthermore, the previous study on b-to-b directly linked por-
phyrin dimers, which reports its large fluorescence quantum
yield and radiative rate constant owing to enhanced through-
space MO interactions,[10] is in good agreement with our obser-
vations on 4Zn.
monomeric moiety, obviously was positive, reflecting the ar-
rangement of transition dipole moment within the triangular
three porphyrin units.[5b] The time-resolved fluorescence aniso-
tropy measurements after photoexcitation at 420 nm clearly
exhibit a positive initial anisotropy value in line with the excita-
tion anisotropy result.
To get more detailed information on the EET processes oc-
curring in 3Zn, we have performed femtosecond transient ab-
sorption anisotropy (TAA) measurements. Pump wavelength
was tuned to the lowest state of Q-band (lpump =610 nm),
which enabled us to exclude complicated processes arising
from the upper excited states. Furthermore, multiple exciton
generation was suppressed by using low pump-power condi-
tions. As shown in Figure 4, fast depolarization process with
Table 1. Fluorescence quantum yields, fluorescence lifetimes, and radia-
tive- and non-radiative rate constants of 2Zn–6Zn.
[a]
[b]
[c]
Compound
Ffl
tfl
kr[c]
[107 sÀ1
knr
[ns]
]
[108 sÀ1
]
2Zn
3Zn
4Zn
5Zn
6Zn
0.002
0.010
0.065
0.011
0.027
1.4
1.5
1.7
2.4
2.4
0.14
0.63
3.8
0.45
1.1
7.1
6.6
5.5
4.1
4.0
[a] Absolute quantum yields for 2Zn-6Zn are measured. [b] Fluorescence
lifetimes are measured by time-correlated single photon counting tech-
nique in toluene solution. [c] Radiative and non-radiative rate constants
are calculated on the basis of following equations: Ffl =kr/(kr +knr) and
tfl =1/(kr +knr).
Cyclic porphyrin oligomers 2Zn and 3Zn show distinctive
spectral features in their absorption and fluorescence spectra.
Cyclic dimer 2Zn shows smeared long-tailing Q-band and
a fluorescence peak around 755 nm with a large Stokes shift.
Distinct excitonic coupling is evinced by a split Soret band
(lmax =419 and 496 nm). Based on the TD-DFT calculations with
the optimized geometry of 2Zn, we found that the lowest ex-
cited state of 2Zn is optically forbidden (Supporting Informa-
tion, Figure S41), which accounted for its small fluorescence
quantum yield (Table 1) and largely red-shifted fluorescence
Figure 4. Transient absorption anisotropy (TAA) decay profiles of 3Zn in tol-
uene, where the pump and probe wavelengths are 610 nm and 520 nm, re-
spectively.
a time constant of 230 fs was observed. This decay has been
ascribed to energy transfer between the adjacent porphyrins.
Based on the simple polygon model proposed by Fleming
et al.,[11] we could evaluate the excitation energy transfer time
of cyclic 3Zn as 690 fs. Importantly, this time constant of 3Zn
is approximately 200 times faster than the calculated rate of
Fçrster-type resonance energy transfer (FRET; details are given
in the Supporting Information). Since the FRET mechanism
only depends on through-space energy transfer, the discrepan-
cy between the experimentally observed and calculated EET
rates indicates the importance of through-bond interaction via
the pyrrole linkage. Furthermore, EET processes occurring in
femtosecond time scale of 3Zn indicates the strong electronic
interactions between coplanar porphyrin moieties in the excit-
ed state. Interestingly, the EET time constant of 3Zn is slightly
slower than that of direct b-to-b linked cyclic porphyrin trimer
(360 fs) but faster than that of 1,3-butadiyne bridged cyclic
porphyrin trimer (1.4 ps).[5] This comparison suggested that
a 2,5-pyrrolylene bridge is effective in mediating the through-
bond electronic interaction.
spectrum. Cyclic trimer 3Zn also exhibits a split Soret (lmax
=
416 and 510 nm) and Q-bands (lmax =557 and 605 nm) with
a relatively high extinction coefficient and enhanced Q(0,0)
transition. These absorption data strongly suggest the large
electronic interactions in 2Zn and 3Zn.
To investigate the electronic interaction between porphyrin
units of 3Zn, we first conducted fluorescence excitation aniso-
tropy measurements, which enabled us to compare the rela-
tive orientation of the transition dipole moments between ab-
sorbing and emitting states. As shown in Figure S21, 3Zn
shows positive constant anisotropy values throughout the
entire absorption spectrum. Given that both the rotational dif-
fusion and excitation energy transfer (EET) processes are in-
cluded at room temperature, this result indicates that the dif-
ference in angle between absorbing and emitting transition
dipole moments is smaller than 54.78 where the fundamental
anisotropy value is zero.[11] In particular, and in contrast with
the linear porphyrin oligomers, we observed that the anisotro-
py value at 420 nm, which corresponds to the Soret band of
Chem. Eur. J. 2016, 22, 8801 – 8804
www.chemeurj.org
8803
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
were performed using the supercomputing resource of the
Korea Institute of Science and Technology Information (KISTI).
Table 2. Half-wave potentials of b-to-b pyrrolylene linked porphyrin ar-
rays.[a]
compound
Eox1 [V][b]
Ered1 [V][b]
DE
Keywords: excitation energy transfer · exciton coupling ·
porphyrinoids · porphyrins · nickel
2Ni
3Ni
4Ni
5Ni
6Ni
0.18
0.26
0.27
0.32
0.36
À1.91
À1.48
À1.83
À1.82
À1.81
2.09
1.74
2.10
2.14
2.17
[1] a) S. Anderson, H. L. Anderson, J. K. M. Sanders, Acc. Chem. Res. 1993,
26, 469; b) D. Holten, D. F. Bocian, J. S. Lindsey, Acc. Chem. Res. 2002, 35,
57; c) Y. Nakamura, N. Aratani, A. Osuka, Chem. Soc. Rev. 2007, 36, 831;
d) N. Aratani, D. Kim, A. Osuka, Acc. Chem. Res. 2009, 42, 1922.
[2] a) K. Sugiura, Y. Fujimoto, Y. Sakata, Chem. Commun. 2000, 1105; b) A.
Kato, K. Sugiura, H. Miyasaka, H. Tanaka, T. Kawai, M. Sugimoto, M. Ya-
mashita, Chem. Lett. 2004, 33, 578; c) O. Shoji, H. Tanaka, T. Kawai, Y.
Kobuke, J. Am. Chem. Soc. 2005, 127, 8598; d) J. E. Raymond, A. Bhaskar,
T. D. Goodson III, N. Makiuchi, K. Ogawa, Y. Kobuke, J. Am. Chem. Soc.
2008, 130, 17212.
[3] a) M. Hoffmann, C. J. Wilson, B. Odell, H. L. Anderson, Angew. Chem. Int.
Ed. 2007, 46, 3122; Angew. Chem. 2007, 119, 3183; b) M. Hoffmann, J.
Kärnbratt, M.-H. Chang, L. M. Herz, B. Albinsson, H. L. Anderson, Angew.
Chem. Int. Ed. 2008, 47, 4993; Angew. Chem. 2008, 120, 5071; c) J. K.
Sprafke, D. V. Kondratuk, M. Wykes, A. L. Thompson, M. Hoffmann, R.
Drevinskas, W. H. Chen, C. K. Yong, J. Kärnbratt, J. E. Bullock, M. Malfois,
M. R. Wasielewski, B. Albinsson, L. M. Herz, D. Zigmantas, D. Beljonne,
H. L. Anderson, J. Am. Chem. Soc. 2011, 133, 17262; d) M. C. O’Sullivan,
J. K. Sprafke, D. V. Kondratuk, C. Rinfray, T. D. W. Claridge, A. Saywell,
M. O. Blunt, J. N. O’Shea, P. H. Beton, M. Malfois, H. L. Anderson, Nature
2011, 469, 72.
[4] Y. Nakamura, I.-W. Hwang, N. Aratani, T. K. Ahn, D. M. Ko, A. Takagi, T.
Kawai, T. Matsumoto, D. Kim, A. Osuka, J. Am. Chem. Soc. 2005, 127,
236.
[5] a) H. Cai, K. Fujimoto, J. M. Lim, C. Wang, W. Huang, Y. Tao, S. Zhang, H.
Shi, B. Yin, B. Chen, M. Ma, J. Song, D. Kim, A. Osuka, Angew. Chem. Int.
Ed. 2014, 53, 11088; Angew. Chem. 2014, 126, 11268; b) S. Lee, H.
Chung, S. Tokuji, H. Yorimitsu, A. Osuka, D. Kim, Chem. Commun. 2014,
50, 2947.
[6] J. Song, N. Aratani, H. Shinokubo, A. Osuka, Chem. Eur. J. 2010, 16,
13320.
[7] J. Takagi, K. Sato, J. F. Hartwig, T. Ishiyama, N. Miyaura, Tetrahedron Lett.
2002, 43, 5649.
[a] In dichloromethane with 0.1m Bu4PF6 vs saturated calomel electrode
(SCE). [b] Some potentials were determined by differential pulse voltam-
mogram.
Finally, the electrochemical properties of 2Ni, 3Ni, 4Ni, 5Ni,
and 6Ni were investigated by cyclic voltammetry (Table 2). The
cyclic voltammetry measurements of NiII porphyrins 2Ni–5Ni
were carried out in CH2Cl2 with 0.1m Bu4NPF6 as supporting
electrolyte (Supporting Information, Figure S27–S31). Mono-
mers 5Ni and 6Ni display electrochemical HOMO–LUMO gaps
that are typical of porphyrin monomers. In contrast, the cyclic
oligomers 2Ni and 3Ni exhibit distinctly smaller HOMO–LUMO
gaps, which is probably due to larger electronic interactions
arising from the enforced cyclic structure. These HOMO–LUMO
gaps are in good agreement with their optical HOMO–LUMO
gaps.
In summary, b-to-b 2,5-pyrrolylene linked cyclic porphyrin
dimer and trimer were synthesized by Suzuki–Miyaura cou-
pling of 2,5-diborylpyrrole and 3,7-dibromoporphyrin. The
cyclic porphyrin oligomers exhibit roughly coplanar structures,
strong excitonic coupling, and small electrochemical HOMO–
LUMO gaps. The cyclic trimer 3Zn displays the ultrafast excita-
tion energy transfer mediated by through-bond mechanism
via the pyrrole linkage. Further extension of this coupling strat-
egy to synthesize more elaborated porphyrin arrays is ongoing
in our laboratories.
[8] Crystal data for 2Ni: C172.5H146N10Ni2O9, Mw =2620.41, monoclinic, space
group P21/c, a=15.6397(17) Š, b=27.818(3) Š, c=37.340(4) Š, b=
92.791(2)8, V=16226(3) Š3, Z=4, Dcalc =1.073 gcmÀ3, T=90(2) K, 83639
measured reflections, 28474 unique reflections, R=0.0945, Rw =0.2846
(all data), GOF=1.076 (I> 2.0s(I)). Crystal data for 3Ni: C172.5H146N10Ni2,
Acknowledgements
¯
Mw =2985.01, rhombohedral, space group R3, a=29.2971(12) Š, b=
29.2971(12) Š, c=44.137(2) Š, V=32808(3) Š3, Z=6, Dcalc =0.906 gcmÀ3
,
We thank Prof. Xiaoyi Yi (Central South University, China) for
3Ni crystal measurement. The work at Hunan Normal Universi-
ty was supported by the National Natural Science Foundation
of China (Grant No. 21272065), the Scientific Research Founda-
tion for the Returned Overseas Chinese Scholars, State Educa-
tion Ministry, and Scientific Research Fund of Hunan Provincial
Education Department (Grant No. 13k027). The work at Kyoto
was supported by a Grant-in-Aid from MEXT (No. 25220802
(Scientific Research (S)). The research at Yonsei University was
supported by the Global Research Laboratory (GRL) Program
(2013K1A1A2A02050183) of the Ministry of Education, Science
and Technology (MEST) of Korea. The quantum calculations
T=90(2) K, 47021 measured reflections, 16569 unique reflections, R=
0.0883, Rw =0.2453 (all data), GOF=1.02 (I> 2.0s(I)). CCDC 976093 (2Ni)
and 976099 (3Ni) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre.
[9] K. Fujimoto, H. Yorimitsu, A. Osuka, Chem. Eur. J. 2015, 21, 11311.
[10] S. Cho, M.-C. Yoon, J. M. Lim, P. Kim, N. Aratani, Y. Nakamura, T. Ikeda, A.
Osuka, D. Kim, J. Phys. Chem. B 2009, 113, 10619.
[11] S. E. Bradforth, R. Jimenez, F. van Mourik, R. van Grondelle, G. R. Fleming,
J. Phys. Chem. 1995, 99, 16179.
Received: March 18, 2016
Revised: April 12, 2016
Published online on May 17, 2016
Chem. Eur. J. 2016, 22, 8801 – 8804
www.chemeurj.org
8804
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim