Bis-anthracene fused porphyrins: Synthesis, crystal structure, and near-IR absorption

Article abstract of DOI:10.1021/ol100619p

Figure presented Synthesis of fused bis-anthracene porphyrin monomers and dimers has been achieved by oxidative ring closure using FeCl₃ and Sc(OTf)₃/DDQ, respectively. The fused compounds display red-shifted absorption spectra with maxima in the near-IR at 973 and 1495 nm, respectively, and small electrochemical HOMO-LUMO gaps. The crystal structure of the fully fused bis-anthracene porphyrin shows that it has a regular planar π-system.

Full text of DOI:10.1021/ol100619p

ORGANIC
LETTERS
2010
Vol. 12, No. 9
2124-2127
Bis-Anthracene Fused Porphyrins:
Synthesis, Crystal Structure, and
Near-IR Absorption
Nicola K. S. Davis, Amber L. Thompson, and Harry L. Anderson*
Oxford UniVersity, Department of Chemistry, Chemistry Research Laboratory 12
Mansfield Road, Oxford OX1 3TA, U.K.
harry.anderson@chem.ox.ac.uk
Received March 16, 2010
ABSTRACT
Synthesis of fused bis-anthracene porphyrin monomers and dimers has been achieved by oxidative ring closure using FeCl3 and Sc(OTf)3/
DDQ, respectively. The fused compounds display red-shifted absorption spectra with maxima in the near-IR at 973 and 1495 nm, respectively,
and small electrochemical HOMO-LUMO gaps. The crystal structure of the fully fused bis-anthracene porphyrin shows that it has a regular
planar π-system.
The fusion of aromatic rings to porphyrin cores has received
much attention recently.1-12 The unusual optical and elec-
tronic properties of these expanded π-systems make them
interesting for a variety of applications including photody-
namic therapy,13 two-photon absorption,14 nonlinear op-
tics,8,15-17 organic semiconductors18 and photovoltaics.6,19,20
(1) Yamane, O.; Sugiura, K.; Miyasaka, H.; Nakamura, K.; Fujimoto,
(11) Lash, T. D.; Werner, T. M.; Thompson, M. L.; Manley, J. M. J.
Org. Chem. 2001, 66, 3152–3159.
T.; Nakamura, K.; Kaneda, T.; Sakata, Y.; Yamashita, M. Chem. Lett. 2004,
33, 40–41
.
(12) Richeter, S.; Jeandon, C.; Kyritsakas, N.; Ruppert, R.; Callot, H. J.
J. Org. Chem. 2003, 68, 9200–9208.
(2) Tokuji, S.; Takahashi, Y.; Shinmori, H.; Shinokubo, H.; Osuka, A.
Chem. Commun. 2009, 1028–1030
(3) Kurotobi, K.; Kim, K. S.; Noh, S. B.; Kim, D.; Osuka, A. Angew.
Chem., Int. Ed. 2006, 45, 3944–3947
.
(13) Bonnett, R. Chem. Soc. ReV. 1995, 24, 19–33.
(14) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L.
Angew. Chem., Int. Ed. 2009, 48, 3244–3266.
.
(4) Davis, N. K. S.; Pawlicki, M.; Anderson, H. L. Org. Lett. 2008, 10,
(15) Calvete, M.; Yang, G. Y.; Hanack, M. Synth. Met. 2004, 141, 231–
3945–3947
.
243.
(5) Jimenez, A. J.; Jeandon, C.; Gisselbrecht, J.-P.; Ruppert, R. Eur. J.
(16) Fei, H. S.; Han, L.; Ai, X. C.; Yin, R.; Shen, J. C. Chin. Sci. Bull.
Org. Chem. 2009, 5725–5730
(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) Gill, H. S.; Harmjanz, M.; Santamar´ıa, J.; Finger, I.; Scott, M. J.
Angew. Chem., Int. Ed. 2004, 43, 485–490
.
1992, 37, 298–301.
(17) Senge, M. O.; Fazekas, M.; Notaras, E. G. A.; Blau, W. J.;
.
Zawadzka, M.; Locos, O. B.; Mhuircheartaigh, E. M. N. AdV. Mater. 2007,
19, 2737–2774
.
.
(18) Sayyad, M. H.; Saleem, M.; Karimov, K. S.; Yaseen, M.; Ali, M.;
Cheong, K. Y.; Noor, A. F. M. Appl. Phys. A: Mater. Sci. Process. 2009,
98, 103–109.
(8) Aratani, N.; Kim, D.; Osuka, A. Chem. Asian J. 2009, 4, 1172–
1182
.
(9) Cammidge, A. N.; Scaife, P. J.; Berber, G.; Hughes, D. L. Org. Lett.
(19) Tanaka, M.; Hayashi, S.; Eu, S.; Umeyama, T.; Matano, Y.; Imahori,
2005, 7, 3413–3416
(10) Sooambar, C.; Troiani, V.; Bruno, C.; Marcaccio, M.; Paolucci,
F.; Listorti, A.; Belbakra, A.; Armaroli, N.; Magistrato, A.; De Zorzi, R.;
.
H. Chem. Commun. 2007, 2069–2071.
(20) Mai, C.-L.; Huang, W.-K.; Lu, H.-P.; Lee, C.-W.; Chiu, C.-L.;
Liang, Y.-R.; Diau, E. W.-G.; Yeh, C.-Y. Chem. Commun. 2010, 46, 809–
811.
Geremia, S.; Bonifazi, D. Org. Biomol. Chem. 2009, 7, 2402–2413
.
10.1021/ol100619p  2010 American Chemical Society
Published on Web 04/06/2010

Scheme 1.
Synthesis of Anthracene-Fused Porphyrinsa
a
a denotes R ) n-C8H17, b denotes R ) 2,4,6-trimethylphenyl.
Anthracene matches well to the geometry of the porphyrin
ring periphery, indicating that fusion may occur without loss
of planarity to generate highly delocalized π-systems with
strong near-IR absorption. Previously, we reported the
synthesis of a pair of fused monoanthracene porphyrins and
demonstrated that electron-donating alkoxy substituents are
required on the anthracene in order to achieve oxidative
fusion to the porphyrin.4 Here we report the synthesis of a
fused bis-anthracene porphyrin monomer and dimer, together
with an investigation of their optical and electrochemical
properties. We demonstrate that bulky aryl ether substituents
can be used to limit the aggregation of these large flat
π-systems.
Anthracene porphyrins Ni-1b and Zn-2b were synthesized
as shown in Scheme 1. Initially, synthesis was carried out
with n-octyl ether substituents to promote solubility. 10-
Bromo-1,8-bis(octyloxy)anthracene 4a was subjected to
Suzuki coupling with diboronic ester Zn-3 to afford bis- and
monoanthracene porphyrins Zn-5a and Zn-6a. A variety of
conditions were explored for fusion of the anthracene units
to the ring periphery. When Zn-5a was treated with
Sc(OTf)3/DDQ the only product isolated was the partially
fused porphyrin Zn-7a; treatment of Zn-7a with further
Sc(OTf)3/DDQ resulted in no reaction.
Purification and characterization of Ni-1a proved to be
challenging due to strong aggregation. A clean MALDI-TOF
1
mass spectrum was obtained; however, the H NMR spec-
trum was broad, even at 140 °C in C2D2Cl4.
Osuka and Tsuda have reported the synthesis of porphyrin
tapes consisting of porphyrin units fused across the meso, ꢀ
and ꢀ′ positions with Sc(OTf)3/DDQ.22 Reaction of Zn-6a
under these conditions yielded the fully fused porphyrin
dimer Zn-2a in a one-pot procedure, as confirmed by the
MALDI-TOF spectrum. The product displayed absorption
in the near-IR, consistent with a highly conjugated porphyrin
system; however, purification of Zn-2a by chromatography
proved impossible due to its low solubility. Strong aggrega-
1
tion led to a featureless H NMR spectrum.
In order to circumvent recurring problems of insolubility
and aggregation, the anthracene moiety was redesigned to
bear bulky 2,4,6-trimethylaryl ether substituents. The use of
bulky aryl ether substituents to limit aggregation has been
successfully employed in phthalocyanines, which are notori-
ous for their insolubility and aggregation.23
Synthesis of anthracene triflate 4b was achieved as shown
in Scheme 2. Nucleophilic aromatic substitution of 1,8-
dichloroanthraquinone by 2,4,6-trimethylphenol gave an-
thraquinone 8.24 Reduction with NaBH4 followed by acid
work up gave anthracenone 9, which was converted into
anthracene triflate 4b by treatment with lithium hexameth-
yldisilazane and triflic anhydride at -78 °C.25,26
Iron(III) chloride has been widely used for oxidative ring
closures.3,19,21 In order to apply this methodology, the
coordinated zinc metal was replaced with nickel(II) to prevent
demetalation. When Ni-5a was treated with 10 equiv of FeCl3
in dichloromethane, the product was found to be partially
fused porphyrin Ni-7a. Subjection of Ni-7a to a further 10
equiv of FeCl3 led to formation of fully fused bis-anthracene
porphyrin Ni-1a.
(22) Tsuda, A.; Osuka, A. Science 2001, 293, 79–82.
(23) McKeown, N. B.; Makhseed, S.; Msayib, K. J.; Ooi, L. L.;
Helliwell, M.; Warren, J. E. Angew. Chem., Int. Ed. 2005, 44, 7546–7549.
(24) Brewis, M.; Clarkson, G. J.; Humberstone, P.; Makhseed, S.;
McKeown, N. B. Chem.sEur. J. 1998, 4, 1633–1640.
(25) Prinz, H.; Wiegrebe, W.; Mu¨ller, K. J. Org. Chem. 1996, 61, 2853–
(21) Wu, J.; Gherghel, L.; Watson, M. D.; Li, J.; Wang, Z.; Simpson,
2856.
C. D.; Kolb, U.; Mu¨llen, K. Macromolecules 2003, 36, 7082–7089
.
(26) Toyota, S.; Makino, T. Tetrahedron Lett. 2003, 44, 7775–7778
.
Org. Lett., Vol. 12, No. 9, 2010
2125

Scheme 2. Synthesis of Anthracene Triflate 4b
Suzuki coupling between 4b and Zn-3 led to the formation
of Zn-5b and Zn-6b in 15 and 44% yields, respectively.
Demetalation of Zn-5b with TFA and treatment with
Ni(acac)2 gave porphyrin Ni-5b, which underwent intramo-
lecular oxidative ring closure with FeCl3 to give Ni-7b in
72% yield. Further reaction with FeCl3 gave the fully fused
porphyrin Ni-1b. However, this compound was contaminated
with chlorinated byproduct, which was difficult to remove,
so efforts were directed toward suppressing chlorination.
AgOTf was added to the reaction mixture to scavenge
chloride ions, and this strategy reduced the degree of
chlorination. Small amounts of monochlorinated fused por-
Figure 1. Two orthogonal views of the molecular structure of Ni-
1b in the crystal (hydrogens omitted for clarity).27
1b, which shows sharp signals, in contrast to the very broad
spectrum of Ni-1a.
1
phyrin were still observed by MALDI-TOF and H NMR
The bis-anthracene-fused dimer Zn-2b was synthesized
in a one-pot procedure from Zn-6b using Sc(OTf)3/DDQ in
a 13% yield. It is soluble in many organic solvents and gives
of the crude reaction mixture, but these byproducts were
removed by silica chromatography to give Ni-1b in 15%
yield.
1
a sharp H NMR spectrum in d5-pyridine.
Crystals of Ni-1b were grown by addition of methanol to
a solution in chloroform. Although the crystals were small
and highly solvated, with 52% of the unit cell being occupied
by disordered solvent, we were able to obtain a low-
resolution crystal structure (Figure 1).27 This structure
confirms the regular planar geometry of the π-system and
shows that the aryl ether substituents lie perpendicular to
the anthracene-porphyrin core, thereby inhibiting π-stacking.
The effectiveness of these substituents in preventing ag-
Unfused porphyrins Ni-5b and Zn-6b display absorption
spectra typical of simple porphyrin monomers, with intense
Soret bands at 427 and 429 nm, respectively (Figure 2).3,6
However expansion of the π-system by fusion to an
anthracene unit greatly perturbs the electronic structure. As
a result, the absorption spectrum of Ni-7b is red-shifted,
displaying a λmax at 828 nm and a highly distorted spectral
shape, with lower peak intensities than Ni-5b. Such perturba-
tions have been reported previously in low-symmetry por-
phyrins.3,19 Fusion of the second anthracene unit to the
porphyrin core in Ni-1b restores the D2h symmetry to give
a simpler spectrum and a larger red-shift (λmax 973 nm). The
sharpness and intensity of this absorption band is unusual
(ε ) 1.4 × 105 M-1 cm-1; fwhm ) 300 cm-1); porphyrins
fused to naphthalene or azulene rings display broad peaks
with lower intensity.3,9,19 The spectrum of Zn-2b extends
well into the near-IR with a λmax at 1495 nm. Furthermore,
it displays a similar spectral shape to Ni-1b indicating that
the chromophore also possesses D2h-symmetry.
1
gregation is demonstrated by the H NMR spectrum of Ni-
(27) X-ray diffraction data for crystals of Ni-1b were collected at 150
K (Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105–107) with
synchrotron radiation using I19 (EH1) at the Diamond Light Source. Data
were collected and reduced using CrystalClear (Rigaku Inc., 2009). The
structure was solved using SIR92 (Altomare, A.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl.
Crystallogr. 1994, 27, 435) and refined using the CRYSTALS software
suite. Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin,
D. J. J. Appl. Crystallogr. 2003, 36, 1487. The structure contains large
solvent accessible voids comprising weak, diffuse electron density. The
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 (Spek, A. J. Appl. Crystallogr. 2003, 36, 7-13; van der Sluis,
P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194–201) integrated with the
CRYSTALS software. The sample consisted of very thin plates which were
highly prone to solvent loss, leading to very weak data with high agreement
factors and the requirement for copious restraints. Full refinement details
are given in the SI and the CIF file. Crystallographic data (excluding
structure factors) have been deposited with the Cambridge Crystallographic
Data Centre (CCDC 771217), and copies of these data can be obtained
free of charge via www. ccdc.cam.ac.uk/data_request/cif.
Cyclic and square wave voltammetry were carried out on
Ni-1b, Ni-5b, Ni-7b, Zn-2b, and Zn-6b to determine their
redox potentials (Table 1). Porphyrins Ni-5b and Zn-6b
red
display E1ox-E1 values typical of unfused porphyrin
monomers.28,29 The separation of E1 and E1 decreases
as the π-system is expanded by fusion to anthracene units.
ox
red
2126
Org. Lett., Vol. 12, No. 9, 2010

Table 1. Redox Potentials of Compounds Ni-1b, Ni-7b, Ni-5b,
Zn-2b, and Zn-6ba
ox
red
ox
red
compd
E1 (V)
E1 (V)
E1 - E1
Ni-1b
Ni-7b
Ni-5b
Zn-2b
Zn-6b
0.07
0.08
0.63
-0.24
0.44
-1.20
-1.33
-1.47
-1.01
-1.56
1.27
1.41
2.10
0.77
2.00
a In THF with 0.1 M Bu4NPF6 vs internal ferrocene/ferrocinium.
employ FeCl3 to synthesize the fully fused bis-anthracene
porphyrin monomer. Use of bulky aryl ether groups on the
anthracenes is an effective strategy for preventing aggrega-
tion. The crystal structure of the fully fused porphyrin Ni-
1b demonstrates the planarity of the central π-system and
shows that the aryl ethers are perpendicular to this core.
Anthracene-fused porphyrins display extensive absorption in
the near-IR and small HOMO-LUMO gaps indicating
potential application in the fields of nonlinear optics8,15-17
and solar cells.6,19,20 The optimal HOMO-LUMO gap for
a dye-sensitized solar cell is about 1.3 eV, which matches
the 1.28 eV gap of Ni-1b.31
Acknowledgment. We thank the EPSRC and DSTL for
support and the EPSRC Mass Spectrometry Service (Swansea)
for mass spectra. We also thank Diamond Light Source for
an award of beamtime on I19 and all the instrument scientists
on the beamline for help and assistance.
Figure 2. UV-vis-NIR absorption spectra in CHCl3/1% pyridine
for (a) Ni-5b (gray), Ni-7b, ε × 3 (black dash), Ni-1b (black), (b)
Zn-6b (gray) and Zn-2b (black).
Supporting Information Available: Experimental details
and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
These electrochemical gaps match well with the optical gaps
of 1.50, 1.28, and 0.83 eV measured for the fused porphyrins
Ni-7b, Ni-1b, and Zn-2b, respectively, at their near-IR
maxima (in CHCl3 with 1% pyridine). Furthermore, dimer
Zn-2b displays optical and electrochemical properties similar
those reported for triply fused porphyrin dimers and trim-
ers.28,30
In conclusion, fully fused bis-anthracene porphyrin mono-
mers and dimers have been synthesized for the first time.
Oxidation with Sc(OTf)3/DDQ was used to form the an-
thracene end-capped fused dimer, while it was necessary to
OL100619P
(28) Fendt, L. A.; Fang, H.; Plonska-Brzezinska, M. E.; Zhang, S.;
Cheng, F.; Braun, C.; Echegoyen, L.; Diederich, F. Eur. J. Org. Chem.
2007, 4659–4673.
(29) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem.
1984, 23, 817–824.
(30) Cheng, F. Y.; Zhang, S.; Adronov, A.; Echegoyen, L.; Diederich,
F. Chem.sEur. J. 2006, 12, 6062–6070.
(31) Snaith, H. J. AdV. Funct. Mater. 2010, 20, 13–19.
Org. Lett., Vol. 12, No. 9, 2010
2127