A porphyrin fused to four anthracenes

Article abstract of DOI:10.1021/ja109671f

We have synthesized a fused tetraanthracenylporphyrin by oxidation of a meso-anthracenyl nickel(II) porphyrin with FeCl₃. This compound exhibits an intense red-shifted absorption spectrum (λ_max = 1417 nm; ε = 1.2×10⁵ M

Full text of DOI:10.1021/ja109671f

Published on Web 12/09/2010
A Porphyrin Fused to Four Anthracenes
Nicola K. S. Davis, Amber L. Thompson, and Harry L. Anderson*
Chemistry Research Laboratory, UniVersity of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.
Received October 27, 2010; E-mail: harry.anderson@chem.ox.ac.uk
Scheme 1. Synthesis of Fused Tetraanthracenylporphyrin Ni-1
Abstract: We have synthesized a fused tetraanthracenylporphy-
rin by oxidation of a meso-anthracenyl nickel(II) porphyrin with
FeCl3. This compound exhibits an intense red-shifted absorption
spectrum (λmax ) 1417 nm; ε ) 1.2 × 105 M-1 cm-1) and a small
electrochemical HOMO-LUMO gap (0.61 eV). The crystal
structure shows that it forms π-stacked dimers with a short Ni · · ·Ni
distance (3.32 Å).
The synthesis of large, flat π systems, or “molecular graphenes”,
is a growing field of research aimed at creating materials for use
as organic semiconductors and nonlinear optical dyes.1 Recently,
work in this area has focused on π-expanded porphyrins, in which
aromatic units are fused to the porphyrin ring.2-10
For over 35 years, chemists have been intrigued by the possibility
of synthesizing fused tetraanthracenylporphyrins.11 The high sym-
metry and large π system of such a molecule suggest that it should
have unusual optoelectronic properties. Here we report the synthesis
of a fully fused tetraanthracenylporphyrin, Ni-1. The near-IR (NIR)
absorption of this porphyrin is remarkably red-shifted and intense
(λmax ) 1417 nm; ε ) 1.2 × 105 M-1 cm-1). The crystal structure
of Ni-1 shows that it forms tight π-stacked dimers and suggests
that similar molecules with less bulky side chains may form discotic
columnar liquid crystals with useful charge-transport behavior.12
formed when Zn-5 was treated with scandium(III) triflate and 2,3-
dichloro-5,6-dicyanobenzoquinone (DDQ). However, we were not
able to isolate Zn-1 from the complex mixture of products, so we
explored other oxidants. Iron(III) chloride has been widely used to
fuse aromatic moieties to the porphyrin periphery.1,3,5b,6,7b This
reagent demetalates zinc porphyrins, so we tested its reaction with
the nickel(II) complex Ni-5. We were delighted to find that
treatment of Ni-5 with excess FeCl3 in dichloromethane results in
eightfold oxidative ring closure to give the fully fused product, Ni-
1, in 49% yield.
Crystals of Ni-1 were grown by diffusing ethanol vapor into a
solution of the porphyrin in benzene and then analyzed by X-ray
diffraction.14 Molecules of Ni-1 form π-stacked dimers in the crystal
(Figure 1), with two crystallographically independent porphyrin
molecules. The planes of the two molecules are almost parallel
(angle between the mean planes of the 24-atom porphyrin cores:
1.4°), and the two porphyrins in the dimer are twisted by ∼20°
with respect to each other. The mean distance of the core of one
porphyrin to the plane of the other is 3.41 Å, with a Ni · · ·Ni
distance of 3.316(2) Å. Both porphyrins adopt ruffled conforma-
tions; the mean deviation from planarity for the 24 atoms of each
porphyrin core (0.20 Å) is similar to that in a typical meso-
tetrasubstituted nickel(II) porphyrin and less than in Osuka’s
We previously found that expanded porphyrins can be difficult
to purify or characterize because of aggregation and that this
problem can be alleviated by attaching bulky aryl ether sub-
stituents.7b Hence, we adopted the same strategy here. Bromoan-
thracene 2 was synthesized in 79% yield by reacting anthrone 3
with phosphorus tribromide (Scheme 1). Lithium-halogen ex-
change followed by addition of pyrrole-2-carboxaldehyde gave
alcohol 4 (not isolated), which was tetramerized with propionic
acid to give H2-5 in 10% yield.13
quadruply azulene-fused porphyrin (0.40 and 0.46 Å).5b The H
1
NMR spectrum of Ni-1 in C6D6 exhibits just six resonances at
normal chemical shifts (see the Supporting Information), showing
that the dimer aggregate dissociates in solution.
Initial experiments (using MALDI mass spectrometry and NMR
analysis) indicated that the fully fused zinc derivative Zn-1 was
Conversion of Ni-5 to Ni-1 results in a dramatic change in the
UV-vis-NIR absorption spectrum (Figure 2). The absorption
9
30 J. AM. CHEM. SOC. 2011, 133, 30–31
10.1021/ja109671f  2011 American Chemical Society

C O M M U N I C A T I O N S
-0.44 V and a first reduction at -1.05 V (E1ox - Er1ed ) 0.61 eV;
all potentials relative to internal ferrocene, Fc/Fc+), whereas Ni-5
has the typical electrochemistry of a porphyrin monomer (Eo1x
0.61 V; E1red ) -1.81 eV; Eo1x - Er1ed ) 2.42 eV).
)
In conclusion, we have synthesized a fused tetraanthracenylpor-
phyrin, Ni-1, by the oxidation of a meso-anthracenyl porphyrin,
Ni-5, through a reaction first proposed by Yen in 1975.11 The key
to this successful synthesis was the use of bulky aryloxy substituents
to facilitate oxidative ring closure and to hinder formation of
extended aggregates, although these substituents do not prevent the
formation of dimeric aggregates in the solid state. This porphyrin
has an exceptionally small HOMO-LUMO gap (λmax ) 1417 nm;
E1ox - E1red ) 0.61 eV). These results suggest that fused tetraan-
thracenylporphyrins may by useful materials for light harvesting
and charge transport in photovoltaic devices.
Acknowledgment. We thank the EPSRC and DSTL for support,
the EPSRC Mass Spectrometry Service (Swansea) for mass spectra,
the Diamond Light Source for beam time on I19 (MT1858), and
Kirsten E. Christensen and David R. Allan for assistance.
Supporting Information Available: Procedures for the synthesis
of Ni-1, spectroscopic data, and crystallographic data (CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) Zhi, L.; Mu¨llen, K. J. Mater. Chem. 2008, 18, 1472–1484.
(2) Lash, T. D.; Werner, T. M.; Thompson, M. L.; Manley, J. M. J. Org. Chem.
2001, 66, 3152–3159.
(3) Gill, H. S.; Harmjanz, M.; Santamar´ıa, J.; Finger, I.; Scott, M. J. Angew.
Chem., Int. Ed. 2004, 43, 485–490.
(4) Yamane, O.; Sugiura, K.; Miyasaka, H.; Nakamura, K.; Fujimoto, T.;
Nakamura, K.; Kaneda, T.; Sakata, Y.; Yamashita, M. Chem. Lett. 2004,
33, 40–41.
Figure 1. Two orthogonal views of the dimeric arrangement of Ni-1 in
(5) (a) Nakamura, N.; Aratani, N.; Shinokubo, H.; Takagi, A.; Kawai, T.;
Matsumoto, T.; Yoon, Z. S.; Kim, D. Y.; Ahn, T. K.; Kim, D.; Muranaka,
A.; Kobayashi, N.; Osuka, A. J. Am. Chem. Soc. 2006, 128, 4119–4127.
(b) Kurotobi, K.; Kim, K. S.; Noh, S. B.; Kim, D.; Osuka, A. Angew. Chem.,
Int. Ed. 2006, 45, 3944–3947.
the crystal (hydrogens have been omitted for clarity).
(6) Hayashi, S.; Tanaka, M.; Hayashi, H.; Eu, S.; Umeyama, T.; Matano, Y.;
Araki, Y.; Imahori, H. J. Phys. Chem. C 2008, 112, 15576–15585.
(7) (a) Davis, N. K. S.; Pawlicki, M.; Anderson, H. L. Org. Lett. 2008, 10,
3945–3947. (b) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. Org.
Lett. 2010, 12, 2124–2127.
(8) Sooambar, C.; Troiani, V.; Bruno, C.; Marcaccio, M.; Paolucci, F.; Listorti,
A.; Belbakra, A.; Armaroli, N.; Magistrato, A.; De Zorzi, R.; Geremia, S.;
Bonifazi, D. Org. Biomol. Chem. 2009, 7, 2402–2413.
(9) Jiao, C. J.; Huang, K. W.; Guan, Z. P.; Xu, Q. H.; Wu, J. S. Org. Lett.
2010, 12, 4046–4049.
(10) Diev, V. V.; Hanson, K.; Zimmerman, J. D.; Forrest, S. R.; Thompson,
M. E. Angew. Chem., Int. Ed. 2010, 49, 5523–5526.
(11) Yen, T. F. The Role of Trace Metals in Petroleum; Ann Arbor Science
Publishers: Ann Arbor, MI, 1975.
(12) (a) Pisula, W.; Feng, X.; Mu¨llen, K. AdV. Mater. 2010, 22, 3634–3649.
(b) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Ha¨gele, C.; Scalia,
G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schrievogel, A.; Tonsoni, M. Angew.
Chem., Int. Ed. 2007, 46, 4832–4887.
(13) Volz, H.; Scha¨ffer, H. Chem.-Ztg. 1985, 109, 308–309.
(14) Data for Ni-1 were collected at low temperature (see: Cosier, J.; Glazer,
A. M. J. Appl. Crystallogr. 1986, 19, 105–107) with synchrotron radiation
using I19 (EH1) at the Diamond Light Source. The data were collected
and reduced using CrystalClear (Rigaku Inc., 2009). The structure was
solved using SuperFlip (see: Palatinus, L.; Chapuis, G. J. Appl. Crystallogr.
2007, 40, 786-790)and refined using full-matrix least-squares within
CRYSTALS (see: Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout,
K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487). In addition to
ordered solvent, the structure contains large solvent-accessible voids
comprising diffuse electron density. Discrete Fourier transforms of the void
regions were treated as contributions to the A and B parts of the calculated
structure factors using PLATON/SQUEEZE (see: Spek, A. L. J. Appl.
Crystallogr. 2003, 36, 7–13). Full refinement details are given in the CIF
file in the Supporting Information. The crystallographic data (excluding structure
factors) have been deposited with the Cambridge Crystallographic Data Centre
(CCDC 802858) and can be obtained via www. ccdc.cam.ac.uk/data_request/cif.
Figure 2. UV-vis-NIR spectra of unfused tetraanthracenylporphyrin Ni-5
(gray) and fused tetraanthracenylporphyrin Ni-1 (black) in toluene.
spectrum of Ni-1 shows a maximum at 1417 nm (optical
HOMO-LUMO gap: 0.87 eV). This peak is extremely sharp (ε )
1.2 × 105 M-1 cm-1; fwhm ) 284 cm-1), reflecting the high
symmetry and rigid geometry of the chromophore. The Q band of
Ni-1 occurs at a longer wavelength than those reported for all other
porphyrin monomers,5b and it is more red-shifted than those of most
conjugated porphyrin oligomers. As expected, Ni-1 has a small
electrochemical HOMO-LUMO gap and is easily oxidized. Square-
wave and cyclic voltammetry were carried out on Ni-5 and Ni-1
in THF with 0.1 M NBu4PF6. Ni-1 shows a first oxidation wave at
JA109671F
9
J. AM. CHEM. SOC. VOL. 133, NO. 1, 2011 31