Synthesis of meso-β doubly linked porphyrin tapes

Article abstract of DOI:10.1039/b302032k

The oxidation of 5,15-bis(3,5-di-tert-butylphenyl) Ni(II)-porphyrin 1b with Sc(OTf)₃ and DDQ led to production of meso-β doubly linked Ni(II)-porphyrin tapes that have large π-electronic communications over the arrays.

Full text of DOI:10.1039/b302032k

Synthesis of meso-b doubly linked porphyrin tapes†
Akihiko Tsuda, Yasuyuki Nakamura and Atsuhiro Osuka*
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
E-mail: osuka@kuchem.kyoto-u.ac.jp; Fax: (+81)75-753-3970; Tel: (+81)75-753-4008
Received (in Cambridge, UK) 20th February 2003, Accepted 18th March 2003
First published as an Advance Article on the web 7th April 2003
The oxidation of 5,15-bis(3,5-di-tert-butylphenyl) Ni(II)-
porphyrin 1b with Sc(OTf)3 and DDQ led to production of
meso-b doubly linked Ni(II)-porphyrin tapes that have large
p-electronic communications over the arrays.
were determined by MALDI-TOF mass spectroscopy; 3b (m/z
= 2216, Calcd for C144H148N12Ni3 2219), 3c (m/z = 2952,
Calcd for C192H196N16Ni4 2957), and 3d (m/z = 3694, Calcd
for C240H244N20Ni5 3695). The structures of 3a–d have been
characterized by their 1H-NMR spectra. Typically, the 1H-NMR
spectrum of the trimer 3b shows mutually coupled doublets for
eight b-protons (d = 8.44, 8.49, 8.52, 8.71, 8.73, 8.74, 9.08, and
9.38 ppm), singlets for two b-protons (8.63 and 8.98 ppm) and
meso-proton (9.24 ppm), and characteristic three sets of signals
for tert-butyl, and para- and ortho-phenyl protons at the
peripheral 3,5-di-tert-butylphenyl groups (1.52, 1.53, and 1.56
ppm for tert-butyl, 7.74, 7.76, and 7.76 ppm for para-phenyl,
and 7.82, 7.92, and 7.92 ppm for ortho-phenyl) (Supporting
Information 1†). This spectrum indicating the presence of three
identical 3,5-di-tert-butylphenyl groups in 3b is only consistent
with anti-arrangement, because the corresponding syn-isomer
should exhibit four sets of signals for the 3,5-di-tert-butylphe-
nyl groups in a ratio of 2+2+1+1. Similarly, the anti-
arrangements have been assigned for the tetramer 3c and
pentamer 3d on the basis of fully assignable peaks for the
protons of the fused porphyrin moieties and the presence of four
and five identical 3,5-di-tert-butylphenyl groups, respectively.
The absorption spectra of the porphyrin arrays 3a–d taken in
CHCl3 are shown in Fig. 1. The spectra of 3a–d exhibit roughly
distinct three regions as such in the meso-meso linked porphyrin
arrays13 and porphyrin tapes12 (bands I, II, and III as designated
Discrete p-conjugated porphyrin arrays with extensive elec-
tronic delocalization are of interest as conducting organic
materials, near-infrared dyes, nonlinear optical (NLO) materi-
als, molecular wires, molecular devices, and so forth.1–3 In these
studies, one of the central issues is to increase the electronic
interaction between the constituent porphyrins and hence
decrease the HOMO-LUMO gap. Along this line, a variety of
new conjugated porphyrin arrays have been actively ex-
ploited.4–12 A straightforward strategy for maximizing p-
overlap may be to hold the p-systems coplanar within a tape-
like framework by fusing the units edge-to-edge, hence forcing
a flat p-network.7–12 Another feature desirable for molecular
wire may be sufficient molecular length for separating two
termini and a synthetic handle for attachment to electrodes or
other functional groups.2
Recently, we have found that oxidation of metalloporphyrins
bearing sterically uncongested meso-positions provides effec-
tive synthetic routes towards directly linked and fused por-
phyrin oligomers. Ag( )-salt promoted coupling reaction of
I
Zn(II)-porphyrins afforded meso-meso linked porphyrin ar-
rays,13 and subsequent oxidation with a combination of
Sc(OTf)3
and
2,3-dichloro-5,6-dicyano-p-benzoquinone
(DDQ) led to the fully conjugated porphyrin tapes.12 The
former arrays adopt a nearly orthogonal conformation that tends
to minimize the electronic interaction between the neighboring
porphyrins, whereas the latter adopt a tape-shaped coplanar
structure that leads to extremely red-shifted absorption bands
owing to extensive p-conjugation. On the other hand, meso-b
doubly-linked diporphyrin was prepared in 53% yield from
oxidation of 5,10,15-tris(3,5-di-tert-butylphenyl) Ni(II)-por-
phyrin (1a) with (p-BrC6H4)3NSbCl6,10 but its extension to
higher arrays using 5,15-bis(3,5-di-tert-butylphenyl) Ni(II)-
porphyrin (1b) as a starting substrate was difficult due mainly to
the extensive peripheral halogenation. Here we report a simple
synthesis of oligomeric meso-b doubly-linked Ni(II)-porphyrin
arrays by the oxidation with DDQ and Sc(OTf)3.
Initially, the oxidation of 1b was attempted with 5 equiv
amounts of DDQ and Sc(OTf)3 in refluxing toluene, which gave
complicated polymeric products with poor solubility. We thus
examined the reaction conditions by changing the amounts of
the oxidants and the temperature, and found that the oxidation of
1b with 2 equiv of DDQ and Sc(OTf)3 in toluene at 50 °C for 12
h gave short and discrete oligomers. The reaction mixture was
passed through a short alumina column eluting with THF, and
the products were separated by size exclusion chromatography
(SEC) and further by silica gel column chromatography, giving
meso-b singly-linked diporphyrin 214 in 27% yield and meso-b
doubly-linked oligoporphyrins (2 ~ 5-mer) in the following
yields: 3a (2-mer, 8%), 3b (3-mer, 6%), 3c (4-mer, 5%), and 3d
(5-mer, 4%) (Scheme 1). Molecular weights of these arrays
† Electronic supplementary information (ESI) available: 1H NMR spectra.
See http://www.rsc.org/suppdata/cc/b3/b302032k/
Scheme 1
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CHEM. COMMUN., 2003, 1096–1097
This journal is © The Royal Society of Chemistry 2003

In summary, the oxidation of 5,15-bis(3,5-di-tert-butylphe-
nyl) Ni(II)-porphyrin 1b with a combination of DDQ and
Sc(OTf)3 provided a series of meso-b doubly-linked porphyrin
arrays in a one-pot reaction. The porphyrin arrays exhibit
remarkably red-shifted and enhanced Q-band-like absorption
bands as well as lowered one-electron oxidation potentials upon
the increase in the number of the porphyrin, indicating
increasing electronic delocalization in the arrays. The longest
pentamer 3d has a molecular length of ca. 4.3 nm. In addition,
these arrays have free meso-positions at both ends, which can be
fabricated responding to specific purposes including molecular
wires, molecular-scale electronic devices, near-IR sensors and
dyes, and materials for nonlinear optics. Further fabrications of
these arrays are currently being investigated in our laboratory.
This work was supported by Grant-in-Aids for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan, CREST (Core Research for
Evolutional Science and Technology) of Japan Science and
Technology Corporation (JST). A. T. thanks JSPS (Japan
Society for the Promotion of Science) Research Fellowship for
Young Scientists.
Fig. 1 UV-Vis-NIR absorption spectra of meso-b doubly-linked porphyrin
arrays 3a–d taken in CHCl3 at room temperature. The absorption spectra
were normalized at 408 nm. lmax (e) of 3a: 411 (57900), 498 (33900), 531
(35400), and 741 (22800) nm. lmax (e) of 3b : 408 (67000), 567 (64600),
803 (24900), and 892 (60200) nm. lmax (e) of 3c : 408 (75000), 583 (92600),
879 (42300), and 996 (143000) nm. lmax of 3d : 408, 590, 939, and 1075
nm.
in Fig. 1). The bands I are observed at 407–410 nm, being nearly
the same positions as that of the monomer 1b, while the bands
II are split into two bands and the low-energy bands are
modestly red-shifted upon the increase of the number of the
porphyrins. More remarkably, the lowest energy bands III are
progressively red-shifted and enhanced upon an increase of the
number of the porphyrins; 741 nm for 3a, 892 nm for 3b, 996
nm for 3c, and 1075 nm for 3d, thus realizing far-infrared strong
Notes and references
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versus the reciprocal number of the porphyrins in the array (1/N)
is a straight line, hence indicating the ECL (effective conjuga-
tion length) effect (Fig. 2a), in contrast to the behavior of the
meso-meso, b-b, b-b triply linked porphyrin tapes.12,15 Its
intercept is around 1480 nm (6770 cm21) that may correspond
to a limiting value for this array. The enhancement of the bands
III can be understood in terms of increasing symmetry lowering
upon the increase in the number of the porphyrins on the basis
of Gouterman four-orbital theoretical model.16
The one-electron oxidation potential of the meso-b doubly-
linked diporphyrin 3a was determined to be 0.56 V versus
AgClO4/Ag in CHCl3 solution, which is considerably lower
than the values for the parent porphyrin monomer 1b (0.74 V)
and meso-b linked diporphyrin 2 (0.76 V) but higher than that
for completely fused diporphyrin 4 (0.46 V).10c Therefore, it
may be concluded that the energy level of HOMO orbital is
lifted upon expansion of p-system of the porphyrin. This trend
is increasingly enhanced upon an increase in the number of the
porphyrins (0.44 V for 3b, 0.38 V for 3c, and 0.35 V for 3d) as
shown in Fig. 2b, where a plot of the one-electron oxidation
potentials versus 1/N constitutes a nice straight line. The
intercept (0.21 V) of the plot may be assigned for the one-
electron oxidation potential of the “infinite” meso-b doubly-
linked porphyrin array.
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11 Analogous meso-b doubly-linked diporphyrin was reported to be also
formed in the reaction of 5,15-diaryl Ni(II)-porphyrin with TeCl4: K.
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15 A similar plot for the triply-linked arrays (ref. 12a) is not straight but
converged to 0 at infinite polymer at least up to the 12-mer.
16 M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138.
Fig. 2 (a) Plot of absorption maxima (Amax) of bands III versus the
reciprocal numbers of porphyrins (1/N) in the meso-b fused arrays. (b) Plot
of the first oxidation potentials (Eox [V] versus AgClO4/Ag taken in CHCl3)
versus 1/N. The first oxidation potentials (Eox) were obtained from cyclic
voltammetry. The indicated lines represent by equations: Amax = 13009/N
+ 6771 (cm21) with correlation coefficient, r2 = 0.998, and Eox = 0.691/N
+ 0.21 (V) with correlation coefficient, r2 = 0.999.
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