"Blackening" porphyrins by conjugation with quinones

Article abstract of DOI:10.1002/anie.200804143

From red to black porphyrins: "Black" porphyrins absorb visible light effectively at all wavelengths and are available by conjugation of four benzoquinone units to a porphyrin core. These robust molecular components are likely to be useful in optoelectron

Full text of DOI:10.1002/anie.200804143

Angewandte
Chemie
DOI: 10.1002/anie.200804143
Black Porphyrins
“Blackening” Porphyrins by Conjugation with Quinones**
Srinivas Banala, Thomas Rꢀhl, Klaus Wurst, and Bernhard Krꢁutler*
Dedicated to Professor Heinz Falk on the occasion of his 70th birthday
Heme and chlorophyll are fascinating porphyrinoid “pig-
ments of life” in plants and in blood.[1,2] Their respective
characteristic bright green and red colors are a consequence
of the efficient absorption of narrow sections of visible
light.[3,4] Heme and chlorophyll were targets of total synthe-
ses[5–7] and have inspired the synthesis of artificial analogues
that serve in the exploration of their properties as pigments.[8]
Non-natural porphyrins are currently of interest as cata-
lysts,[2,9] as redox systems,[10] as robust molecular components
in optical[11] and other nanoscopic devices,[12–14] and in medical
applications.[15]
benzene at 1408C and in the presence of a 24-fold excess of
benzoquinone (Scheme 1). The crude reaction mixture was
oxidized with dicyanodichlorobenzoquinone (DDQ) and
then treated with zinc(II) acetate to give the zinc tetraquino-
noporphyrin 2-Zn in 86% yield.
Many of the applications of porphyrins rely on their
characteristic bright colors and the associated properties.[16,17]
Herein we describe “black” porphyrins, obtained by specific
modification of the porphyrin chromophore with benzoqui-
nones. The absorption spectra of these tetrapyrroles feature
broad and intense electronic transitions covering all wave-
lengths of visible light. Pigments that absorb a large fraction
of sunlight are in great demand.[18] Indeed, “black” leaves
have been proposed to increase plantsꢀ capacity in collecting
solar energy by biological photosynthesis.[19]
The symmetrical b,b’-tetrasulfolenoporphyrin (1-2H) was
prepared earlier as a versatile reactive building block for
further covalent attachment of functional units.[20] Its syn-
thesis was first developed in conjunction with the attachment
of C60-fullerene units by [4+2] cycloaddition reactions.[21]
Following a related strategy, but using a new “add & lock”
methodology, covalent conjugates with quinones could be
obtained systematically. Making use of the dienophilic and
oxidative properties of quinones, we have achieved an
effectively irreversible attachment of up to four electronically
conjugated benzoquinone units, giving porphyrins with sys-
tematically extended p systems.
The “black” tetraquinonoporphyrin 2-Zn was prepared by
exhaustive thermolysis of a solution of 1-2H in 1,2-dichloro-
Scheme 1. Synthesis of the zinc tetraquinonoporphyrin 2-Zn. Ar=3,5-
di-tert-butylphenyl.
[*] Dr. S. Banala, Dr. T. Rꢀhl, Prof. Dr. B. Krꢁutler
Institute of Organic Chemistry, University of Innsbruck
6020 Innsbruck (Austria)
Thermolysis of a solution of the porphyrin 1-2H and a 19-
fold excess of benzoquinone in 1,2-dichlorobenzene for only
45 minutes at 1408C gave (at about 66% conversion) a
mixture of adducts, which was oxidized with DDQ. Subse-
quent treatment with zinc acetate furnished, after purification
by chromatography, the five benzoquinone–porphyrin con-
jugates 2-Zn to 6-Zn. The products were isolated as dark
microcrystalline solids, and they were identified as the adduct
3-Zn (32.4% yield), diagonal bis-adduct 4-Zn (3% yield),
lateral bis-adduct 5-Zn (7.9% yield), tris-adduct 6-Zn (6.4%
yield), and tetra-adduct 2-Zn (0.7% yield, see Scheme 2).
The structures of the five adducts were established by
mass spectrometric analysis, and homo- and heteronuclear
Fax: (+43)512-507-2892
E-mail: bernhard.kraeutler@uibk.ac.at
Dr. K. Wurst
Institute of General, Inorganic and Theoretical Chemistry
University of Innsbruck (Austria)
[**] The authors thank Thomas Mꢀller, Simone Moser, and Prof. Ernst
Ellmerer for mass and NMR spectra. The work in Innsbruck was
supported by the Austrian National Science Foundation (FWF
project no. P-17437) and the Tyrolean Science Foundation (project
no. UNI 404/22).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804143.
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Figure 1. X-ray crystal structures of zinc tetraquinonoporphyrin 2-Zn
(top) and of the “lateral” zinc bisquinonoporphyrin 5-Zn (bottom).
(ORTEP plots; H atoms are omitted for clarity).
The conjugated quinone moieties have a remarkable and
systematic effect on the UV/Vis spectra, intensifying the
absorption bands at long wavelengths and shifting their
maxima by 133 nm (in CH2Cl2; from 592 nm for 1-Zn[20] to
725 nm for the tetra-adduct 2-Zn). The position of the
absorption maxima is also strongly solvent dependent: for
example, in the UV/Vis spectra of 2-Zn the maximum shifts
from 710 nm in petroleum ether, to 725 nm in CH2Cl2, to
736 nm in MeOH. At the same time, the spectral features of
typical metalloporphyrins disappear completely. Instead of
the diagnostic strong Soret band near 400 nm and weaker
Scheme 2. Synthesis of the zinc b,b’-quinonoporphyrins 2-Zn to 6-Zn.
Ar=3,5-di-tert-butylphenyl.
NMR spectroscopy. The NMR spectra are compatible with an
effectively planar structure of the extended porphyrin-naph-
thoquinone moiety, and the specific addition pattern for the
diagonal and lateral bis-adducts 4-Zn and 5-Zn could be
determined (see the Experimental Section and the Support-
ing Information).
Both the tetra-adduct 2-Zn and the lateral bis-adduct 5-
Zn gave monoclinic single crystals suitable for X-ray analysis.
The crystal structures confirm the constitutional assignment
(deduced from the NMR and mass spectra) and indicate
largely planar porphyrin structures. They also reveal that the
tetra-adduct 2-Zn contains a hexacoordinate Zn center, to
which two molecules of methanol are bound by means of long
Q bands near 550 nm (as in the spectrum of 1-Zn),[3,20]
a
multiband structure covering the whole visible range is
evident in the spectra of 6-Zn and 2-Zn (Table 1 and
Figure 2). A gradual reduction of the characteristic band
near 400 nm (Soret band of porphyrins, see Table 1) thus also
occurs upon introduction of one to four conjugated benzo-
quinone units. However, integration of the absorption spectra
of solutions in CH2Cl2 of the zinc porphyrinoids 2-Zn to 6-Zn
(from about 380 to 800 nm) indicates a similar total (normal-
À
Zn O bonds (2.308 ꢁ) resulting in an unusual pseudo-
octahedral coordination sphere at the zinc center (Figure 1).
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Table 1: UV/Vis spectral characteristics of the zinc porphyrin 1-Zn[20] and
of the zinc quinonoporphyrins 2-Zn to 6-Zn (in CH2Cl2).
occur upon introduction of (up to four) conjugated benzo-
quinone units: The solution of 6-Zn is dark brown, while that
of the tetra-adduct 2-Zn is black.
[a,b]
[a,c]
Cmpd
lmax (lge)
lmax (lge)
Absorption[d] (lge)
400 nm
700 nm
The covalent attachment and conjugation of quinone units
at the b positions of the porphyrins also has a dramatic effect
on the luminescence properties. Whereas the zinc porphyrin
1-Zn exhibits a typical bright red luminescence, the lumines-
cence of all of the quinone conjugates is hardly detectable
(see Figure S1 in the Supporting Information). Thus, these
porphyrinoid compounds represent a class of zinc complexes
that absorb light strongly throughout the whole visible range,
and (not unexpectedly[22]) they hardly emit. The porphyrin
and benzoquinone building blocks, simply linked and con-
jugated at the porphyrinoid b positions, result in remarkable,
tunable photophysical properties.
1-Zn
3-Zn
4-Zn
5-Zn
6-Zn
2-Zn
590 (3.92)
614 (4.09)
648 (4.37)
642 (4.44)
680 (4.42)
725 (4.90)
424 (5.85)
418 (5.18)
408 (4.84)
425 (4.95)
425 (4.53)
418 (4.66)
4.19
4.61
4.65
4.58
4.35
4.49
<2.0
2.6
3.1
2.7
4.1
4.5
[a] Wavelength in nm. [b] Band at longest wavelength. [c] Dominant band
near 400 nm. [d] Absorptions at the boundaries of visible light.
ized) absorption cross section in the visible range for all five
compounds.
Solutions of the porphyrins 1-Zn to 6-Zn (and of the
deconjugated quinonoporphyrin 7-Zn) are displayed in
Figure 3. It is apparent that remarkable changes in color
As shown, porphyrins with conjugated quinone units at
the b positions are easily obtained from the sulfolenopor-
phyrin 1-2H.[20] Here, we have studied the quinonoporphyrins
2-Zn to 6-Zn more specifically. Their absorption bands were
Figure 2. Effect of b,b’-bound quinone substituents. UV/Vis spectra of 1-Zn and of 2-Zn to 6-Zn in CH2Cl2 (absorption not normalized; visible
range (400–700 nm) highlighted).
Figure 3. Colors of solutions of zinc porphyrins 1-Zn to 7-Zn (in MeOH, ca. 50 mm, 5–10% CH2Cl2).
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> 3208C), which was characterized by mass spectrometry and by UV/
Vis and 1H NMR spectroscopy.
found to broaden and shift gradually with the number of
attached quinone units. The optical properties of 2-Zn to 6-Zn
indicate electronic features that call for theoretical studies, as
well as for better characterization in photophysical and
electrochemical experiments. Indeed, the new quinonopor-
phyrins are expected to exhibit interesting electrochemical
properties as a result of the directly conjugated porphyrin and
benzoquinone moieties. Both types of basic chromophores
are well-studied biological or artificial redox systems.[23,24] The
zinc tetraquinonoporphyrin 2-Zn is thus expected to accept a
load of at least 10 electrons per molecule. Such quinonopor-
phyrins clearly promise to expand the range of redox proper-
ties of the “simple” porphyrins with a modified periphery.[10]
Solutions of 2-Zn absorb light in all of the visible range
(e.g. in CH2Cl2: e > 8800) and do not decompose noticeably
when exposed to daylight. They show negligible luminescence
and the colors of the solutions range from gray to black,
depending on the concentration. Thus, the porphyrinoid 2-Zn
is the first example of a (mononuclear) “black” porphyrin.[25]
In this respect, on a molecular level its visual color properties
are the same as those of nanoscopic carbon materials with
extended p systems, such as graphene,[26] graphite, and nano-
tubes.[27] Similar to the porphyrin 1-2H, the porphyrins 2-Zn
to 6-Zn are also reactive building blocks, and they could be
used for further modification of the porphyrin structure or for
assembly of larger covalent porphyrin arrays.
UV/Vis (CH2Cl2, c = 9.712 ꢂ 10À6 m): lmax (lge): 725 (4.90), 668 sh
(4.29), 555 (4.95), 453.5 sh (4.52), 418 (4.66), 365 (4.53), 325 (4.39),
254 nm (4.99); lmin (lge): 690 (4.34), 634 (3.94), 482 (4.37), 387 (4.42),
341 (4.41), 306 nm (4.31). 1H NMR (300 MHz, in CDCl3): d = 1.54 (s,
72H), 6.97 (s, 8H), 7.97 (s, 8H), 8.11 (brs, 8H), 8.33 ppm (brs, 4H).
FAB MS (C108H100N4O8Zn; exact mass = 1644.683). m/z (%): 1651.8
(32), 1650.8 (48), 1649.8 (71), 1648.8 (86), 1647.8 (100) 1646.8 (95),
1645.8 (79, [M+H]+), 1644.8 (61, M+).
UV/Vis spectra (in CH2Cl2): 3-Zn (c = 1.18 ꢂ 10À5 m): lmax (lge):
614 (4.09), 579 (4.42), 491 sh (4.79), 458 (4.96), 418 (5.18), 334 nm-
(4.37). 4-Zn (c = 4.02 ꢂ 10À5 m): lmax (lge): 648 (4.37), 604 (4.16), 505
(4.88), 408 nm (4.84). 5-Zn (c = 4.01 ꢂ 10À5 m): lmax (lge):642 (4.44),
604 (4.38), 504 (4.82), 483sh (4.80), 425 (4.95), 335 nm (4.43). 6-Zn
(c = 3.42 ꢂ 10À5 m): lmax (lge): 680 (4.42), 639 (4.11), 522 (4.66), 425
(4.53), 360 (4.24), 273 nm (4.79). 7-Zn (c = 2.94 ꢂ 10À5 m): lmax
(lge):589sh (3.44), 551 (4.43), 423 (5.69), 402 sh (4.75), 327 nm (4.34).
X-ray structure analyses: Data collection on a Nonius Kappa
CCD, equipped with graphite-monochromated MoKa radiation (l =
0.71073 ꢁ). Data for 2-Zn: C110H108N4O10Zn·2EtOAc, Mr = 1887.58,
monoclinic, P21/c, a = 17.6879(4), b = 18.9404(4), c = 17.4055(3) ꢁ,
b = 115.915(2)8, V= 5244.76(19) ꢁ3, Z = 2, 1calcd = 1.195 gcmÀ3, T=
233 K, R1 = 0.0622, wR2 = 0.1753 for 7328 reflections with I > 2s(I).
Data for 5-Zn: C96H102N4O9S2Zn·5CH2Cl2, Mr = 2009.94, monoclinic,
P21/c, a = 27.9317(7), b = 21.413(5), c = 17.5983(3) ꢁ, b = 102.422(2)8,
V= 10279(3) ꢁ3, Z = 4, 1calcd = 1.299 gcmÀ3, T= 233 K, R1 = 0.0846,
wR2 = 0.2194 for 10100 reflections with I > 2s(I). CCDC 699276 and
699277 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
“Black” porphyrins are presented here as new low-
molecular-weight compounds of possible use in optoelec-
tronic and redox systems. Their atypical optical and electronic
properties are likely to be systematically tunable. These
robust and structurally versatile b,b’-quinonoporphyrins
appear to be of interest in solar-energy-converting devi-
ces,[18,28] and in other applications, such as photoprotection[29]
and information storage.[11]
Received: August 22, 2008
Published online: December 9, 2008
Keywords: cycloaddition · dyes/pigments · porphyrinoids ·
.
quinones · zinc
Experimental Section
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Synthesis of 2-Zn:
A deoxygenated solution of 1-2H (52 mg,
36.52 mmol) and p-benzoquinone (160 mg,1.48 mmol, 40 equiv) in
16 mL of 1,2-dichlorobenzene (DCB) was heated and stirred for 15 h
at 1408C. The solvent was removed from the mixture at 558C under
high vacuum (ca. 0.03 mbar), and the residue was redissolved in
20 mL of CH2Cl2 and 1 mL of NEt3. Then DDQ (200 mg, 0.88 mmol,
24 equiv) was added, and the mixture was heated to reflux for 6 h
under argon. The cold reaction mixture was washed with aq. NaHCO3
(3 ꢂ 25 mL), and the porphyrins were extracted with CH2Cl2 (4 ꢂ
50 mL). The organic extracts were filtered through a column of
silica gel 60, which was washed with CH2Cl2/MeOH (99:1). The
combined eluates were concentrated to dryness under reduced
pressure and the dark residue was dissolved (in 25 mL of CH2Cl2
and 2.5 mL of MeOH). After addition of Zn(OAc)2·2H2O (100 mg,
456 mmol) the suspension was heated to reflux for 1.5 h. The cold
mixture was washed with saturated aq. NaHCO3 (3 ꢂ 40 mL), and
porphyrins were extracted with CH2Cl2 (200 mL). The organic extract
was filtered through a silica gel 60 column with CH2Cl2/MeOH (99:1),
and the dark eluate was concentrated to dryness under reduced
pressure. The residue was dissolved in CH2Cl2 (2 mL), ethyl acetate
(10 mL) and 10 mL of MeOH were added. The flask was left open in
the dark for slow evaporation. Within 2 days black crystals separated
out, which after drying gave 46.6 mg of 2-Zn. A second crop of
crystalline 2-Zn (5.2 mg) of was obtained in a similar fashion. Thus a
total of 51.8 mg (86.1%) of black crystalline 2-Zn was obtained (m.p.
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