Meso-meso-linked subporphyrin dimer

Article abstract of DOI:10.1002/chem.201303767

Directly meso meso linked subporphyrin dimer 5 and 7 were synthesized by Ni-mediated reductive coupling of the corresponding meso-bromosubporphyrins. Dimer 5 exhibited a largely perturbed absorption spectrum, a redshifted and enhanced fluorescence spectru

Full text of DOI:10.1002/chem.201303767

COMMUNICATION
DOI: 10.1002/chem.201303767
meso–meso-Linked Subporphyrin Dimer
Masaaki Kitano,[a] Jooyoung Sung,[b] Kyu Hyung Park,[b] Hideki Yorimitsu,[a]
Dongho Kim,*[b] and Atsuhiro Osuka*[a]
The subporphyrin, a genuine ring-contracted porphyrin
consisting of three pyrrole units and three methine carbons,
has emerged as a new functional pigment in view of its dis-
tinct 14p-electronic aromatic system, and tunable absorp-
tion and emission properties.[1,2] In particular, meso-aryl sub-
stituents provide significant influences on the electronic
nature of subporphyrins by virtue of their almost free rota-
tion at room temperature, as was reported for meso-(oligo-
1,4-phenyleneethynylene)- and meso-(4-aminophenyl)-sub-
stituted subporphyrins.[3] Herein, we report the synthesis of
a meso–meso linked subporphyrin dimer 5, which is an im-
portant molecule for comparison with the numerous exam-
ples of meso–meso linked ZnII–porphyrin arrays already de-
scribed in the literature.[4,5] Directly meso–meso linked ZnII–
diporphyrin 2 was prepared by AgI-promoted oxidative di-
merization of ZnII porphyrin 1. In diporphyrin 2, the two
porphyrins are rigidly held at nearly perpendicular orienta-
tion with hindered rotation at the meso–meso linkage, which
leads to minimum electronic interaction between the two
porphyrins despite the direct connection.[6] The absorption
spectrum of 2 shows a split Soret band, which can be inter-
preted in terms of simple exciton coupling of the two ZnII–
porphyrins without significant conjugative interaction. In
line with this feature, state-to-state excitation energy hop-
ping is common for meso–meso linked ZnII–porphyrin
arrays.[6,7]
3 took place. These negative results led us to explore reduc-
tive coupling[10] of 4. A solution of an equivalent amount of
[NiACTHNUTRGNE(NUG cod)2] and 4 in DMF was heated at 808C for 2 h under
argon atmosphere.[11] After usual work-up, meso–meso
linked subporphyrin dimer 5 was obtained in 31% yield
along with the recovery of 3 (27%). An excess amount of
1,5-cyclooctadiene was added with the intention of securing
catalyst duration, which led to a remarkable improvement
in yield of 5 (72%) with suppression of debromination
(Scheme 1).
Scheme 1. Porphyrins 1, 2 and subporphyrins 3–6.
Recently, meso-free and meso-bromosubporphyrin 3 and
4 have been explored and used for the synthesis of various
meso-A2B-type-substituted subporphyrins.[8] By following
the synthesis of 2, we initially attempted oxidative coupling
of 3. Various oxidizing agents, such as AgPF6,[4] AgPF6, and
I2,[9a,b] and [bis(trifluoroacetoxy)iodo]benzene[9c] were trialed
in attempted oxidative dimerization reactions of 3, but the
formation of 5 was not observed, and only decomposition of
The high-resolution electrospray ionization time-of-flight
(HR ESI-TOF) mass spectrum of 5 revealed an intense
borenium cation peak at m/z 817.3095 (calcd for
1
C55H35B2N6O1 817.3070 [5ÀOMe]+). The H NMR spectrum
of 5 shows a broad spectrum at room temperature, which
has been ascribed to slow rotation of the meso–meso linkage
comparable to NMR time scale (Figure 1). Consistent with
this interpretation, 5 shows a clear 1H NMR spectrum at
À408C that consists of five sets of signals due to the b-pyr-
rolic protons and two sets of signals due to the ortho, meta,
and para protons of the meso-phenyl groups, differentiating
the concave and convex sides of the bowl-shaped subpor-
phyrin macrocycle (Figure 1). Characteristically, a signal due
to H6 at the concave face is upfield shifted, and H1 and H2
at the convex face are downfield shifted. At 1208C, the
1H NMR spectrum of 5 became simpler due to smooth rota-
tion of the meso–meso linkage. These rotational features of
5 are distinctly different from those of 2.
[a] M. Kitano, Prof. Dr. H. Yorimitsu, 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
[b] J. Sung, K. H. Park, Prof. Dr. D. Kim
Department of Chemistry and Spectroscopy of
p-Functional Electronic System, Yonsei University
Seoul 120-749, (Korea)
E-mail: dongho@yonsei-u.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303767.
Chem. Eur. J. 2013, 19, 16523 – 16527
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
16523

analysis revealed a directly meso–meso linked dimer struc-
ture, in which the mean bowl depth is 1.33 ꢁ and the center-
À
to-center distance (B B distance) is 7.21 ꢁ and the C-
À
G
N
angle between the two subporphyrin cores is 55.58
(Figure 3).[13] The C
(meso) C
E
À
is similar to that (1.47 ꢁ) of meso–meso linked ZnII–dipor-
phyrin 2, but the dihedral angle of 5-OBn is considerably
smaller than that (86.18) of 2. The latter structural feature
may lead to stronger electronic interaction between the two
subporphyrin units.
Figure 1. 1H NMR spectra of 5 at various temperatures.
To evaluate the rotation barrier of the meso–meso link-
age, meso–meso linked subporphyrin dimer 7 was prepared
by the same method, which displays a singlet signal due to
1
the tolyl protons. Similar to 5, the H NMR spectrum of 7
exhibits temperature-dependent spectral changes. By simu-
lating the observed spectral changes of signals due to the
tolyl proton of 7 (Figure 2),[12] the values for DH°, DS°, and
°
DG298 of the rotation barrier have been determined to be
14.7 kcalmolÀ1, 1.28 calmolÀ1 KÀ1, and 14.3 kcalmolÀ1, re-
spectively.
Subporphyrin dimer 5 was converted to its B,B’-dibenzyl-
ACHTUNGTRENNUNGoxy congener 5-OBn by heating in the presence of benzyl al-
cohol. Slow recrystallization from a mixture of CHCl3 and
toluene gave nice crystals of 5-OBn. Single-crystal XRD
Figure 3. X-ray crystal structure of 5-OBn: a) perspective and b) side
views. Thermal ellipsoids are represent 50% probability. Solvent mole-
cules are omitted for clarity.
The UV/Vis absorption spectra of 5 and its reference mo-
nomer 6 in toluene are shown in Figure 4. Although the sub-
porphyrin monomer 6 shows a Soret-like band at 375 nm
and Q-like bands at 462 and 486 nm, that of meso–meso
linked dimer 5 is significantly altered, featuring a Soret-like
band at 403 nm with a broad shoulder around 388 nm and
broadened Q-like bands with peaks at 462 and 516 nm and
a tail extending over 550 nm. These absorption features sug-
gest stronger electronic interactions in 5 than those in 2.
Figure 2. Temperature-dependent 1H NMR spectral changes of the
methyl group of 7 (left) and a plot of ln(k/T) versus 1/T (right).
16524
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16523 – 16527

meso–meso Linked Subporphyrin Dimer
COMMUNICATION
Figure 4. UV/Vis absorption (solid lines) and fluorescence (dashed lines)
spectra of 5 and its reference monomer 6 in toluene.
The absorption spectrum of 2 displays a split Soret band at
416 and 451 nm and slightly redshifted Q bands, which has
been interpreted in terms of exciton coupling with negligible
conjugative interaction.[5,6] Therefore, conjugative interac-
tions of the two subporphyrin units are suggested for 5 in
addition to excitonic interaction. Fluorescence spectrum of 5
in toluene is largely redshifted to 593 nm and is substantially
enhanced (FF =0.40) with a large Stokes shift (2520 cmÀ1),
compared to that of 6 (at 517 nm, FF =0.16, and 1230 cmÀ1).
Such large changes in the fluorescence spectrum were not
observed for 2. Fluorescence lifetime of 5 has been deter-
mined by single-photon counting to be 2.3 ns, which is slight-
ly shorter than that (2.8 ns) of 6. The fluorescence spectrum
of 5 is rather insensitive to solvent polarity and did not
show quenching even in polar solvents such as THF and
DMSO, different from recently reported boron–dipyrrome-
thene (BODIPY) dimers.[14] This has been ascribed to small
S1-excitation energy of 5 (2.09 eV) due to large electronic
interaction, which is not sufficient to induce so-called sym-
metry-breaking intramolecular charge separation.[15]
The transient absorption spectra in toluene after photoex-
citation at 540 nm were recorded (Figure 5). Characteristi-
cally, the transient stimulated emission (SE) band of 5 was
clearly observed, and its peak position was gradually red-
shifted in 30 ps time range, whereas SE band of 6 was very
weak, and the excited state absorption spectra did not show
a significant spectral shift. We can suggest that this dynamic
Stokes shift of S1 fluorescence in 5 is probably caused by
continuous structural changes of dihedral angle in the excit-
ed state. These features are distinct characteristics of 5, not
observed for 2. Because of rigid and orthogonal geometry of
2, the molecular orbital (MO) interaction between the two
porphyrin cores is weak.[16] However, the situation is differ-
ent for 5, in which the rotation around the direct meso–
meso linkage leads to efficient MO interactions between the
constituent subporphyrins. It is conceivable that the torsion-
al relaxation after photoexcitation may cause more coplanar
geometry, and the Qz (z direction along the long molecular
axis) state transition becomes stabilized and enhanced by in-
creasing electronic communication between the two subpor-
phyrins cores.[17] Therefore, the enhanced Qz state transition
Figure 5. Transient absorption spectra of a) 5 and b) 6 in toluene obtained
by photoexcitation at l=510 and 460 nm, respectively.
and large oscillator strength of 5 can explain the observed
large S1 fluorescence quantum yield and clear dynamic
Stokes shift of SE bands.
Interestingly, the anisotropy decay profile of 5 is distinctly
different from those of 2 and 6 (Figure 6).[6] The anisotropy
decay profile of 6 shows an initial anisotropy value of 0.1 re-
sulting from an ultrafast in-plane dephasing process between
Qx and Qy transitions. The anisotropy decay profile of dipor-
phyrin 2 revealed an ultrafast depolarization process, in
which anisotropy value decreases from 0.4 to 0.04 with
Figure 6. Temporal profiles of femtosecond transient absorption anisotro-
py decays of 5 (black) and 6 (gray) in toluene monitoring at l=620 and
580 nm with photoexcitation of Q bands at l=540 and 480 nm, respec-
tively.
Chem. Eur. J. 2013, 19, 16523 – 16527
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16525

D. Kim, A. Osuka et al.
Figure 7. MO diagrams of 5 calculated with Gaussian 09 package (all energy levels were calculated at the B3LYP/6-311G(d) level. Each MO was visual-
ized with cubegen program).[20]
a time constant of <200 fs.[18] Such a fast depolarization pro-
cess is attributable to a mixing of all relaxation processes,
such as in-plane dephasing of Qx and Qy in porphyrin mono-
mer, energy hopping between the two transition dipoles, and
internal conversion from Qx or Qy state of monomeric unit
to Qz state of the dimer.[6] Likewise, porphyrin arrays hold-
ing monomeric character are known to show complicated
depolarization processes due to ultrafast dephasing of the
two degenerate transition dipoles in the porphyrin mono-
mer, as well as the energy transfer between the porphyrin
monomers.[18,19] In contrast, the anisotropy decay profile of
subporphyrin dimer 5 shows a high initial anisotropy value
of 0.4 and slow single exponential decay with a time con-
stant of 300 ps. This slow decay can be assigned as a rotation-
al diffusion. Based on this high initial anisotropy value and
rather simple depolarization process of 5, we can propose
that the p conjugation extends to the whole molecule, and
hence 5 can be regarded as a one-quantum system.
spread over the two subporphyrin units, indicating the delo-
calized nature of electronic states of 5.
The electrochemical properties of 5 were examined by
cyclic voltammetry (CV) in CH2Cl2 containing 0.1m
Bu4NPF6 as a supporting electrolyte versus ferrocene/ferro-
cenium cation (see the Supporting Information). One-elec-
tron oxidation waves of 5 were observed at 0.60 and 1.03 V,
and reduction wave was observed at À1.78 V, whereas those
of 6 were observed at 0.82 and À1.84 V. These data allowed
us to determine the electrochemical HOMO–LUMO gaps
of 5 and 6 to be 2.38 and 2.66 eV, respectively. The two oxi-
dation waves of 5 have been assigned as split first oxidation
waves (one electron per subporphyrin), as was judged from
the results of other electronically interacting diporphyrins.[22]
Potential difference between the first and second oxidation
waves (DE=EOX2ÀEOX1) of 5 is 0.43 V, which is substantial-
ly larger than that (0.11 V) of 2 and is comparable with that
(0.42 V) of a fully conjugated ZnII porphyrin tape, again
supporting the large electronic interaction in 5.[22]
The molecular structures and MO diagrams of 5 were op-
timized at B3LYP/6-311G(d) level by using the Gaussian 09
package (Figure 7).[20] We have previously reported that
meso-triphenyl-substituted subporphyrin 6 possesses quite
porphyrin-like four orbitals, a1u-like HOMOÀ1, a2u-like
HOMO, and eg-like LUMO and LUMO+1.[2b,21] The
HOMO and LUMO of subporphyrin dimer 5 are both
In summary, directly meso–meso linked subporphyrin
dimer 5 and 7 were synthesized by Ni-mediated reductive
coupling of the corresponding meso-bromosubporphyrins.
Dimer 5 exhibited a largely perturbed absorption spectrum,
a redshifted and enhanced fluorescence spectrum, higher ox-
idation potentials with a large potential difference between
16526
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 16523 – 16527

meso–meso Linked Subporphyrin Dimer
COMMUNICATION
Org. Lett. 2012, 14, 2694; c) M. Kitano, S. Hayashi, T. Tanaka, N.
Aratani, A. Osuka, Chem. Eur. J. 2012, 18, 8929.
[9] a) A. Nakano, H. Shimidzu, A. Osuka, Tetrahedron Lett. 1998, 39,
9489; b) X. Shi, L. S. Liebeskind, J. Org. Chem. 2000, 65, 1665; c) L.-
M. Jin, L. Chen, J.-J. Yin, C.-C. Guo, Q.-Y. Chen, Eur. J. Org. Chem.
2005, 3994.
[10] X.-Q. Lu, Y. Guo, Q.-Y. Chen, Synlett 2011, 77.
[11] M. F. Semmelhack, P. M. Helquist, L. D. Jones, J. Am. Chem. Soc.
1971, 93, 5908.
the first and second oxidation waves, and a long-lived large
anisotropy of S1 state. These properties have been interpret-
ed in terms of the effective conjugative interactions of the
two subporphyrin units due to the small rotational barrier of
the meso–meso bond. Exploration of more elaborated sys-
tems, such as meso–meso linked subporphyrin oligomers and
porphyrin–subporphyrin hybrids, are actively pursued in our
laboratories.
[12] F. P. Gasparro, N. H. Kolodny, J. Chem. Educ. 1977, 54, 258.
[13] Crystal data for 5-OBn: C68H46B2N6O2·2C7H8; Mw =1185.00; triclin-
¯
ic; space group P1 (No. 2); a=12.1018(3), b=15.8455(3), c=
17.9185(4) ꢁ; a=103.4753(13), b=104.6738(13), g=103.5719(13)8;
V=3070.89(12) ꢁ3; T=93 K; 1calcd =1.282 gcm–3; Z=2; 28270 re-
flections measured; 9528 unique (Rint =0.0786); R1 =0.0992 [I>
2.0s(I)]; wR2 =0.3195 (all data); GOF=1.075. CCDC-931802 con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] M. T. Whited, N. M. Patel, S. T. Roberts, K. Allen, P. J. Djurovich,
S. E. Bradforth, M. E. Thompson, Chem. Commun. 2012, 48, 284.
[15] a) W. Rettig, Angew. Chem. 1986, 98, 969; Angew. Chem. Int. Ed.
Engl. 1986, 25, 971; b) F. Schneider, E. Lippert, Ber. Bunsen-Ges.
1968, 72, 1155; c) N. Nakashima, M. Murakawa, N. Mataga, Bull.
Chem. Soc. Jpn. 1976, 49, 854.
Acknowledgements
The work at Kyoto was supported by Grant-in-Aid (No. 25220802(S)) for
Scientific Research from MEXT of Japan. M.K. acknowledges a JSPS
Fellowship for Young Scientists. The work at Yonsei was supported by
the Global Frontier R&D Program of the Center for Multiscale Energy
System (2012-8-2081), Mid-career Researcher Program (2010-0029668) of
National Research Foundation (NRF) grant funded by MEST of Korea,
and AFSOR/AOARD grant (no. FA2386-09-4092).
Keywords: anisotropy
·
conjugative
interaction
·
porphyrinoids · reductive coupling · subporphyrins
[16] T. Miyahara, H. Nakatsuji, J. Hasegawa, A. Osuka, N. Aratani, A.
Tsuda, J. Chem. Phys. 2002, 117, 11196.
[17] a) S. Cho, M.-C. Yoon, K. S. Kim, P. Kim, D. Kim, Phys. Chem.
Chem. Phys. 2011, 13, 16175; b) G. Duvanel, J. Crilj, A. Schuwey, A.
Gossauer, E. Vauthey, Photochem. Photobiol. Sci. 2007, 6, 956.
[18] a) K. Wynne, R. M. Hochstrasser, Chem. Phys. 1993, 171, 179; b) R.
Kumble, S. Palese, V. S.-Y. Lin, M. J. Therien, R. M. Hochstrasser, J.
Am. Chem. Soc. 1998, 120, 11489; c) H. S. Cho, N. W. Song, Y. H.
Kim, S. C. Jeoung, S. Hahn, D. Kim, S. K. Kim, N. Yoshida, A.
Osuka, J. Phys. Chem. A 2000, 104, 3287.
[19] a) M.-C. Yoon, S. Cho, P. Kim, T. Hori, N. Aratani, A. Osuka, D.
Kim, J. Phys. Chem. B 2009, 113, 15074; b) I.-W. Hwang, D. M. Ko,
T. K. Ahn, Z. S. Yoon, D. Kim, X. Peng, N. Aratani, A. Osuka, J.
Phys. Chem. B 2005, 109, 8643.
[1] a) Y. Inokuma, A. Osuka, Dalton Trans. 2008, 2517; b) T. Torres,
Angew. Chem. 2006, 118, 2900; Angew. Chem. Int. Ed. 2006, 45,
2834; c) A. Osuka, E. Tsurumaki, T. Tanaka, Bull. Chem. Soc. Jpn.
2011, 84, 679.
[2] a) Y. Inokuma, J. H. Kwon, T. K. Ahn, M.-C. Yoo, D. Kim, A.
Osuka, Angew. Chem. 2006, 118, 975; Angew. Chem. Int. Ed. 2006,
45, 961; b) Y. Inokuma, Z. S. Yoon, D. Kim, A. Osuka, J. Am.
Chem. Soc. 2007, 129, 4747; c) N. Kobayashi, Y. Takeuchi, A. Matsu-
da, Angew. Chem. 2007, 119, 772; Angew. Chem. Int. Ed. 2007, 46,
758; d) Y. Takeuchi, A. Matsuda, N. Kobayashi, J. Am. Chem. Soc.
2007, 129, 8271.
[20] Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A.
Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ꢂ.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc., Wallingford CT, 2009.
[3] a) Y. Inokuma, S. Easwaramoorthi, S. Y. Jang, K. S. Kim, D. Kim, A.
Osuka, Angew. Chem. 2008, 120, 4918; Angew. Chem. Int. Ed. 2008,
47, 4840; b) Y. Inokuma, S. Easwaramoorthi, Z. S. Yoon, D. Kim, A.
Osuka, J. Am. Chem. Soc. 2008, 130, 12234.
[4] a) A. Osuka, H. Shimidzu, Angew. Chem. 1997, 109, 93; Angew.
Chem. Int. Ed. Engl. 1997, 36, 135; b) A. Nakano, A. Osuka, I. Ya-
mazaki, T. Yamazaki, Y. Nishimura, Angew. Chem. 1998, 110, 3172;
Angew. Chem. Int. Ed. 1998, 37, 3023; c) N. Aratani, A. Osuka,
Y. H. Kim, D. H. Jeong, D. Kim, Angew. Chem. 2000, 112, 1517;
Angew. Chem. Int. Ed. 2000, 39, 1458.
[5] a) N. Aratani, A. Osuka, Bull. Chem. Soc. Jpn. 2001, 74, 1361; b) D.
Kim, A. Osuka, Acc. Chem. Res. 2004, 37, 735; c) N. Aratani, D.
Kim, A. Osuka, Acc. Chem. Res. 2009, 42, 1922.
[6] a) Y. H. Kim, D. H. Jeong, D. Kim, S. C. Jeoung, H. S. Cho, S. K.
Kim, N. Aratani, A. Osuka, J. Am. Chem. Soc. 2001, 123, 76; b) D.
Kim, A. Osuka, J. Phys. Chem. A 2003, 107, 8791.
[21] M. Gouterman, J. Mol. Spectrosc. 1961, 6, 138.
[22] A. Tsuda, H. Furuta, A. Osuka, J. Am. Chem. Soc. 2001, 123, 10304.
[7] N. Aratani, H. S. Cho, T. K. Ahn, S. Cho, D. Kim, H. Sumi, A.
Osuka, J. Am. Chem. Soc. 2003, 125, 9668.
[8] a) M. Kitano, S. Hayashi, T. Tanaka, H. Yorimitsu, N. Aratani, A.
Osuka, Angew. Chem. 2012, 124, 5691; Angew. Chem. Int. Ed. 2012,
51, 5593; b) T. Tanaka, M. Kitano, S. Hayashi, N. Aratani, A. Osuka,
Received: September 26, 2013
Published online: November 4, 2013
Chem. Eur. J. 2013, 19, 16523 – 16527
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16527