Systematic Synthesis of Porphyrin Dimers Linked by Conjugated Oligoacetylene Bridges

Article abstract of DOI:10.1246/cl.2003.694

A series of α,ω-bis(meso-porphyrinyl)oligoacetylenes were systematically prepared. The exciton interaction between the porphyrins gradually decreased with increasing number of acetylenes, whereas the interaction between the porphyrin and the oligoacetylene unit appeared.

Full text of DOI:10.1246/cl.2003.694

694
Chemistry Letters Vol.32, No.8 (2003)
Systematic Synthesis of Porphyrin Dimers Linked by Conjugated Oligoacetylene Bridges
Kazuya Nakamura,y Tatsuhiko Fujimoto,y Satoshi Takara,y Ken-ichi Sugiura,Ãy;yy Hitoshi Miyasaka,yy Tomohiko Ishii,yy
Masahiro Yamashita,yy and Yoshiteru SakataÃy
yThe Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
yyDepartment of Chemistry, Graduate School of Science, CREST, JST, Tokyo Metropolitan University,
1-1 Minami-Ohsawa, Hachi-Oji, Tokyo 192-0397
(Received May 12, 2003; CL-030406)
A series of a,w-bis(meso-porphyrinyl)oligoacetylenes were
Ar
systematically prepared. The exciton interaction between the
porphyrins gradually decreased with increasing number of ace-
tylenes, whereas the interaction between the porphyrin and the
oligoacetylene unit appeared.
O
O
N
N
N
N
NiII
Ar =
Classic a,w-bis(aryl)oligoacetylenes (BAOAs) have been
the focus as important component molecules for interdisciplina-
ry advanced material chemistry.1,2 Contemporary studies of this
class of materials were started after the pioneering work of Na-
kagawa and co-workers from 1964 to 1973.3 They comprehen-
sively investigated a variety of BAOAs capped with the follow-
ing aromatic hydrocarbons, e.g., phenyl, naphthyl, anthryl,
phenanthryl, chrysenyl, fluorenyl, biphenyl-4-yl, and pyrenyl.
Accompanying the development of such materials in which
the terminal groups are replaced by functional p-systems, re-
search interest has shifted from the established research objec-
tive of elucidating the electronic interaction between the termi-
nal groups through the oligoacetylene unit (OAU),3 via their
excited state chemistries involving intramolecular photoinduced
electron and/or energy transfer between the terminal groups
through the OAU,1 to the single-molecule-based nano science
of transporting electrons through the OAU.2 The BAOAs cap-
ped with metalloporphyrins at the meso-position with a large
atomic coefficient, which were prepared for the first time by Ar-
nold and co-workers in 1978,4b should be interesting,4,5 because
the molecules are sufficiently large to enable detection by scan-
ning tunneling microscopy as individual molecules,6 in addition
to the well-known multiple functions associated with their ex-
panded p-electronic systems.7 In this work, we systematically
prepared and spectroscopically characterized a,w-bis(porphyri-
nyl)oligoacetylenes, 11-14.
The synthetic route for 11-14 is shown in Scheme 1. Retro
synthetic consideration made us adopt the oxidative coupling
reaction of oligoethynylporphyrins (4, 6, 8, and 10, vide infra)
as the final step, thereby producing the desired compounds
11-14. An important key intermediate, 4, having the substituents
to increase solubility,8 was prepared according to a general
method.4a Extension of the OAU was performed by the mixed
coupling reaction of 5, derived from 4, and 30 equiv. of trimeth-
ylsilylacetylene. Although the desired diyne derivative 6 was
the main product of this reaction, unseparable by-product con-
taminated it. Continuous introduction of dry air to the reaction
mixture solved this problem to give 6 along with trace of 11 and
the expected 1,4-bis(trimethylsiliy)-1,3-butadyne. This reaction
condition is applicable to the further extension of the OAU, af-
fording 8 from 7 (70% yield) and providing 10 from 9 (50%).
Considering the instabilities of unprotected acetylenes such as
Ar
15 (from 1)
ii)
1: M = 2H, R = H
2: M = 2H, R = Br
3: M = NiII, R = Br
4: M = NiII, R = -CC-TMS
5: M = NiII, R = -CC-H
6: M = NiII, R = -(CC-)2-TMS
7: M = NiII, R = -(CC-)2-H
8: M = NiII, R = -(CC-)3-TMS
9: M = NiII, R = -(CC-)3-H
10: M = NiII, R = -(CC-)4-TMS
i)
Ar
ii)
iii)
iv)
v)
iv)
v)
iv)
v)
N
N
N
N
M
R
Ar
Ar
vi)
Ar
N
N
N
N
N
N
N
N
NiII
NiII
Ar
n
Ar
11: n = 1 (from 4), 12: n = 2 (from 6)
13: n = 3 (from 8), 14: n = 4 (from 10)
Scheme 1. Reagent and conditions: (i) 1.0 equiv. NBS, dry CHCl3,
0 ꢀC; (ii) excess Ni(CH3CO2)2Á4H2O, (CH2Cl)2, reflux; (iii) 3 equiv.
TMS-CꢁC-H, CuI, Pd0(PPh3)4, dry NHEt2; (iv) nBu4NF, CH2Cl2; (v )
30 equiv. TMS-CꢁC-H, CuCl, dry air, dry TMEDA-CH2Cl2; (vi)
Cu(CH3CO2)2ÁH2O, dry pyridine, 60 ꢀC.
7 and 9, the homo-coupling reaction was performed using sil-
yl-protected porphyrins,4c 4, 6, 8, and 10, affording 11-14, re-
spectively, as deeply colored stable solids under ambient condi-
tions.
Mass spectroscopic studies clearly support the structure of
9
1
11-14. The H-NMR chemical shifts of each b-proton of the
dimers are similar among 11-14, and also similar to those of
the corresponding protons of monomers, 4, 6, 8, and 10. This
phenomenon indicates that the magnetic deshielding effect of
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.8 (2003)
695
one porphyrin on the b-proton of the other porphyrin is negligi-
ble when the two porphyrins are separated by more than one bu-
ꢀ
tadiyne unit, ca. 6.8 A. Nakagawa and co-workers pointed out
18000 cmÀ1 (ca. 560 nm), and Soret-band-like Band-III at
22000 cmÀ1 (ca. 450 nm). Band-III in the absorption spectrum
of butadiyne-inserted 11 was excitonically split into two bands
with 1380 cmÀ1 width (Á), as was observed for a structurally
similar dimer.4 The excitonic coupling of 12 and 13 resembles
that the intensity of the IR-forbidden CꢁC stretching mode in-
creased with an increase in the number of acetylenes in their
BAOAs.3 A similar tendency was also observed in our system.
The highest wave number of the vibration was observed in 12
(2186 cmÀ1), and this wave number became lower with an in-
crease in the number of acetylenes, e.g., 2093 cmÀ1 for 14.
The electronic absorption spectra of the monomers having
OAUs (4, 6, 8, and 10) and the dimers (11-14) in CHCl3 are
shown in Figures 1a and 1b, respectively, along with that of ref-
erence compound 15. First of all, it should be emphasized that
vibration mode, which are typically observed in the absorption
spectra of Nakagawa et al.’s BAOAs,3 do not exist in the spec-
tra of our monomers and dimers. The shapes of the absorption
spectra of the monomers resemble that of 15; however, the ex-
tension of the OAU causes a red shift of the Soret bands.4c For 8
and 10, the Soret bands show weak splitting due to the excitonic
coupling between the porphyrin and the OAU. In contrast, the
absorption spectra of the dimers are markedly different from
those of the monomers. In the dimers, the absorption spectra
consist of three bands, Q(0,0)-band-like Band-I around
17000 cmÀ1 (ca. 590 nm), Q(1,0)-band-like Band-II at
that of 11, however, the width decreases to 1000 and 870 cmÀ1
,
respectively. These Á values decrease linearly with increasing
length of the acetylene linkage. The split disappeared in the ab-
sorption spectrum of 14. The full-width at half maximum of the
broadened absorption spectrum of 14 is 3500 cmÀ1
.
With the absorption character of the monomers in mind, we
make the following conclusions for the absorption spectra of the
dimers. For short OAU-bridged dimers such as 12 and 13, the
main exciton interactions occur between the porphyrins.4 For
the longest OAU-bridged dimer in our series (14), the interac-
tion between the excitons associated with the porphyrins is neg-
ligible. The absorption of the hexadecaoctayne part may be suf-
ficiently stabilized to interact with the porphyrin’s excitons.
This interaction broadens the Soret band of 14. The split Soret
bands of 8 and 10 support this explanation. The intermediate-
length OAU-bridged dimer 13 exhibits both interactions, the
coupling between the two porphyrins and the coupling between
the porphyrin and the dodecahexayne part.
We have systematically prepared and revealed the electron-
ic absorption spectra of oligoalkynyl-bridged porphyrin dimers
along with monomers having OAUs. We are currently working
on the detailed assignment of the absorption spectra, the photo-
excited state chemistry, as well as the potential use of these
molecules in single-molecule-based nano science.
We appreciate the technical assistance provided by the Ma-
terials Analysis Centre of ISIR, Osaka University as well as
helpful discussions Dr. Motoko Asano-Someda, Dept. of Chem-
istry, Tokyo Institute of Technology, Japan.
References and Notes
1
2
R. Ziessel, Synthesis, 1999, 1839.
F. Diederich, Chem. Commun., 2001, 219; J. M. Tour, Acc. Chem. Res., 33,
791 (2000); J. M. Tour, Chem. Rev., 96, 537 (1996).
3
4
M. Nakagawa, S. Akiyama, K. Nakasuji, and K. Nishimoto, Tetrahedron,
27, 5401 (1971).
The following a,w-bis(porphyrinyl)oligoacetylenes are reported. a) acety-
lene: V. S.-Y. Lin, S. G. DiMagno, and M. J. Therien, Science, 264,
1105 (1994). b) butadiyne: D. P. Arnold, A. W. Johnson, and M.
Mahendran, J. Chem. Soc., Perkin Trans. 1, 1978, 366. c) unseparable hex-
atriyne and ocatatetrayne: D. P. Arnold and D. A. James, J. Org. Chem., 62,
3460 (1997).
5
6
Oligoacetylenes capped with meso-tetraphenylporphyrins were reported: S.
Kawabata, N. Tanabe, and A. Osuka, Chem. Lett., 1994, 1797; A. Osuka, N.
Tanabe, S. Kawabata, and I. Yamazaki, J. Org. Chem., 60, 717 (1995).
K.-i. Sugiura, H. Miyasaka, T. Ishii, and M. Yamashita, in ‘‘Chemistry of
Nanomolecular Systems: Towards the Realization of Molecular Devices,’’
ed. by T. Nakamura, T. Matsumoto, H. Tada, and K.-i. Sugiura, Spring-
er-Verlag, Berlin (2003), p 59.
7
L. R. Milgrom, in ‘‘The Colours of Life, An Introduction to the Chemistry
of Porphyrins and Related Compounds,’’ Oxford Press, Oxford (1997);
‘‘Porphyrin Handbook,’’ ed. by K. M. Kadish, K. M. Smith, and R. Guilard,
Academic Press, San Diego (2000), Vols. 1-10.
8
9
K.-i. Sugiura, H. Tanaka, T. Matsumoto, T. Kawai, and Y. Sakata, Chem.
Lett., 1999, 1193.
Selected spectroscopic data are as follows. 11: IR (KBr) 2135 (ꢀCꢁC) cmÀ1
.
12: IR (KBr) 2186 (ꢀCꢁC) cmÀ1. 13: IR (KBr) 2144 (ꢀCꢁC) cmÀ1. 14: 1H-
NMR (CDCl3) d ¼ 9:70 (s, 2H), 9.37 (d, J ¼ 4:9 Hz, 4 H), 9.00 (d,
J ¼ 4:6 Hz, 4 H), 8.92 (d, J ¼ 4:9 Hz, 4 H), 8.87 (d, J ¼ 4:6 Hz, 4 H),
7.15 (d, J ¼ 2:2 Hz, 8 H), 6.84 (t, J ¼ 2:2 Hz, 4 H), 4.11 (t, J ¼ 6:5 Hz,
16 H), 1.87 (m, 24 H), 0.98 (d, J ¼ 6:5 Hz, 48 H) ppm; MALDI-TOF
MS m/z 1915 calcd for C120H118N8Ni2O8, m/z 1918; IR (KBr) 2093
Figure 1. Absorption spectra of (a) 10 (bold solid line), 8 (bold
dotted line), 6 (thin solid line), 4 (bold dotted line), and 15 (thin sol-
id line), (b) 14 (bold solid line), 13 (bold dotted line), 12 (thin solid
line), and 11 (bold dotted line) in CHCl3.
(ꢀCꢁC) cmÀ1
.
Published on the web (Advance View) July 7, 2003; DOI 10.1246/cl.2003.694