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Assembly and Crystal Structure of a Photoactive Array of Five Porphyrins.

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internal standard. CV: scan rate, 2OmVs-'. DPV: pulse amplitude, 50 mV; pulse
width, 50 ms; pulse period, 200 ms; scan rate, 20 mVs-'.
Received: November 21. 1994 [Z7487IE]
German version: Angew. Chem. 199.5, 107, 1228
Keywords: ab initio calculations . electrochemistry . lanthanide
compounds
[l] For a recent review on metallofullerenes. see: D . S. Bethune. R. D. Johnson,
J. R. Salem, M. S. de Vries, C. S. Yannoni, Nature 1993, 366, 123.
[2] K. Kikuchi, S. Suzuki. Y Nakao, N. Nakahara, T. Wakabayashi. H. Shiromark!. K. %to. I. Ikemoto, Y. Achiba, Chem. P h w Lett. 1993, 216, 67.
[3] K . Yamamoto, H. Funasaka. T. Takahashi. T. Akasaka. J. Phys. Chem. 1994,
98. 2008.
[4] a) H. Shinohara, H. Yamaguchi, Y Hayashi. H . Sato, M. Ohkochi. Y Ando,
Y Saito, 1 Phys. Chem. 1993, 97,4259; b) H . Shinohara, N. Hayashi, H. Sato,
Y Saito. X.-D. Wang, T. Hashizume, T. Sakurai, ibid. 1993, 97, 13438.
151 R. Beyers. C.-H. Kiang, R. D. Johnson, J. R. Salem, M. S . de Vries, C. S.
Yannoni, I).S. Bethune. H . C. Dorn. P. Burbank, K . Harich. S. Stevenson,
Nature 1994, 370, 196.
[6] Suzuki, T.; Maruyama, Y; Kato, T.; Kikuchi. K.; Achiba. Y. J. A m . Chem. Soc.
1993, 115, 11006.
[7] K. Kikuchi, Y. Nakao. S . Suzuki. Y. Achiba, T. Suzuki. Y. Maruyama. J. A m .
ChiJm.Soc. 1994, 116, 9367.
[8] K. Yamamoto, H. Funasaka. T. Takahashi. T. Akasaka, T. Suzuki, Y. Maruyama, J. Phys. Chem. 1994. 98, 12833.
[9] S. Hino, H . Takahashi. K. Iwasaki, K. Matsumoto, T. Miyazaki, S. Hasegawa,
K. Kikuchi, Y Achiba, Phys. Rev. Leu. 1993, 71, 4261.
[lo] D. M. Poirier, M. Knupfer. J. H . Weaver. W. Andreoni, K . Laasonen, M.
Parrinello. D. S. Bethune, K . Kikuchi, Y. Achiba, Phys. Rev. B 1994.49, 17403.
[11] K . Laasonen, W. Andreoni, M. Parrinello, Science 1992, 258, 1916.
[12] a) S. Nagase. K . Kobayashi, T. Kato. Y. Achiba, Chem. Phys Lett. 1993, 201.
475; b) S. NagaSe. K. Kobayashi. ibid. 1993, 214. 57; c) S. Nagase, K.
Kobayashi, ibid. 1994. 228, 106; d ) S. Nagase, K. Kobayashi, ibid. 1994, 231,
319; e) S. NagaSe. K. Kobayashi, J. Chem. Soc.. Chern. Commun. 1994, 1837.
[13] M. M. Alvarez. E. G . Gillan, K. Holczer, R. B. Kaner, K. S. Min, R. L.
Whetten. J. Phys. Chem. 1991, 95, 10561.
[14] For experimental details, see ref. [2]. Retention times for La,(aC,,: 490 min
(polystyrene column, JAIGEL Co., CS, eluant) and 9.4 min (Buckyclucher I
column, Regis Co., toluene eluant).
[15] On the basis of the mass spectrometric studies. Whetten et al. suggested that:
1) La,@ C,, has an ionization potential higher than La(u;C,,, and 2) La,@ C,,
has an electron affinity higher than known fullerenes. These observations are
consistent with our electrochemical results. C. Yeretzian. K. Hansen, M. M.
Alvarez. K. S. Min. E. G . Gillan. K. Holczer. R. B. Kaner, R. L. Whetten,
Chem. Phys. Let,. 1992, 196. 337.
[16] D . Duhois, K. M. Kadish, S. Flanagan, L. J. Wilson. J. Am. Chem. Soc. 1991,
113,1773.
[17] Q. Li. F. Wudl, C. Thilgen, R. L. Whetten. F. Diederich, F. J. Am. Chem. Sac.
1992, 114, 3994.
[18] Computational details: Calculations were carried out using Hartree-Fock
(HF) molecular orbital method and the Gaussian 92/DFT program. The effective core potential and basis set developed by Hay and Walt were used on La
but the outermost core electrons in the 5s25p6 configuration were explicitly
treated as valence electrons [lsa]. The basis set of La was (5s5p3d)/[4s4p3d] in
standard notation. The split-valence 3-21G hasis set was used for C [18 b].
These give 782 contracted Gaussian functions for La,(dC,,. The geometry
optimized with the AM1 method was used for the C,, cage [lSc]. a) P. J. Hay,
W. R. Wadt. J . Chem. Phys. 198.5, 82. 299; b) J. S. BinkIey, J. A. Pople, W J.
Hehre, J . Am. Chem. Sac. 1980, 102, 939; c) M. J. S. Dewar, E. G . Zoebisch,
E. F. Healy. J. J. P. Stewart, ibid. 1985, 107, 3902.
[19] The a b initio calculations by Nakao et al. suggest that /h-C8, has the largest
bond energy among the seven C,, isomers. K. Nakao. N. Kurita. M. Fujita,
Phys. Rev. B 1994, 49. 11415.
[20] a) P. W. Fowler, Chem. P h w Leu. 1986, 131, 444. b) P. W. Fowler, D . E.
Manolopoulos. Nature 1992, 355. 428.
[21] Because we keep the Ih symmetry of the C,, cage during our calculations. this
is not due to the Jahn-Teller distortion.
I221 I n this case. the us orbital is raised from -4.4 to -0.1 eV.
Assembly and Crystal Structure of a Photoactive
Array of Five Porphyrins""
Sally Anderson,* Harry L. Anderson, Alan Bashall,
Mary McPartlin,* and Jeremy K. M. Sanders*
The investigation of conformationally homogeneous porphyrin assemblies is essential for the better understanding of
energy and electron transfer in natural photosynthetic reaction
centers.['] This area of research has been stimulated by the characterization of natural photosynthetic reaction centers by
Deisenhofer, Huber, and Michel,['] and by the theoretical work
of Marcus.131 Structures containing many porphyrin chromophores are of particular interest as models for light-harvesting antennae; only a few antennae models containing more than
three porphyrin units have been studied,[,] and all of these structures are rather flexible, which hampers understanding of their
photophysical properties. The structure of the five-porphyrin
complex presented in this communication is exceptionally well
defined; its conformation in solution, deduced by NMR, is essentially identical to that in the solid state, as determined by
single-crystal X-ray diffraction. It should be straightforward to
vary the structure of this complex systematically, because the
central porphyrin is inserted in the last step with strong but
reversible noncovalent binding interactions.
Corey-Pauling-Koltun (CPK) models of the cyclic zinc porphyrin tetramerI5I Zn,-1 indicate that it is rather flexible, and
NMR spectroscopy confirms that the molecule exists in several conformations, which interchange slowly on the chemical shift time scale.r5".6l
One possible conformer is shown schematically in Figure 1.
When
meso-tetra(4pyridy1)porphyrin H,Py,P is added to a solution of cyclic tetramer
Zn,-1, a strong 1 : I
4
complex forms, which
is in slow exchange
with excess of either
component on the
Zn4-l
'H NMR
chemical
shift time scale and runs on a silica column as a single band. The
H,-Py,P ligand locks the tetramer in a single symmetrical conformation: the signals of the ester methyl (Me,) and ring methyl
(Me,) groups each give rise to two singlets of equal intensity
separated by Ah = 0.28 and 0.05, respectively, which rules out
the D,, conformation shown schematically in Figure 1. Similar
splittings are also observed in the 13CNMR spectrum. Molecular mechanics calculations and CPK models indicate that the
[*I
['I
[**I
1096
Q VCH Verlugsgesellschuft mbH, 0-69451 Weinheim, 1995
Dr. S. Anderson.''] Dr. H. L. Anderson."' Dr. J. K. M. Sanders
Cambridge Centre for Molecular Recognition
University Chemical Laboratory
Lensfield Road, GB-Cambridge CB2 1EW (UK)
Telefax: Int. code + (1223)336913
Prof. M. McPartlin, A. Bashall
School of Chemistry, University of North London
Holloway Road, GB-London N7 8DB (UK)
Telefax: Int. code + (171)753-5402
Current address: Dyson Perrins Laboratory
South Parks Road. GB-Oxford OX1 3QY (UK)
Telefax: lnt. code + (18651275674
This work was supported by the Science and Engineering Research Council
(UK), RhGne Poulenc Rorer, Magdalene College Cambridge, and Trinity College Cambridge. We are grateful to Prof. F. Diederich (ETH Zurich, Switzerland) for generously providing facilities for fluorescence measurements.
i10.00+ .2510
0570-0833~9SjlOlO-I096!
Angew. Chem. Int. Ed. Engl. 1995, 34, No. 10
COMMUNlCATlONS
Fig. 1 One possible conformer of cyclic tetramer Zn,-1. and the complex formed
with H,-Py,P.
complex H,-Py,P.Zn,-l prefers to adopt a tub-shaped geometry reminiscent of cyclooctatetraene (Fig. 2). This D,,symmetry
is consistent with the 'H and 13C NMR spectra.['] Split signals
result from the two pyrrole environments shown in Figure 2.
Fig. 3. Crystal structure of the tetrameric zinc porphyrin complex H,-Py,P,Zn,-l.
a) Side view. b) A view perpendicular to the central porphyrin plane showing the
crystallographic S, symmetry and the orthogonal orientation of the four peripheral
zinc porphyrins of the Zn,-1 unit.
I
I
I
I
I
1
I
10.0
9.0
8.0
7.0
6.0
5.0
4.0
*
I
I
3.0
2.0
7
-5.0
6
Fig. 2. Idealized structure and 'H NMR spectrum of the H,-Py,P.Zn,-l complex.
Most functional groups have been omitted for clarity. Axial pyrrole units are shaded
black and equatorial units unshaded.
The H, and H, pyridine protons of H,-Py,P appear in the
complex as sharp doublets at 6 = 2.26 and 5.77, respectively.
The H, protons are shifted upfield by A6 = 6.81 and the H,
protons by A6 = 2.41 on complexation to the cyclic tetramer.
The magnitude of these upfield shifts, when compared with
those in other pyridine-zinc porphyrin complexes,['' shows that
all four porphyrins of Zn,-1 must be bound simultaneously. The
N-H signal of the ligand is shifted upfield by A6 = 2.9 to
6 = - 4.8, as would be expected from the additive effect of four
porphyrin ring currents perpendicular to the ligand plane and in
close proximity.
The symmetry of the tetrazinc complex H,-Py,P.Zn,-l in the
solid state is consistent with that observed in solution. X-ray
structure analysis[g1of a well-formed, but very poorly diffracting crystal established the structure shown in Figure 3 with exact crystallographic S,, and idealized D,, symmetry. The four
zinc porphyrin rings are orthogonal to the central H,-Py4P porA n x i w Cliem. Inl. Ed. Engl. 1995, 34, N o . 10
0 VCH
phyrin unit (dihedral angle between N, planes 91.2") and highly
bowed. The bonds from the central porphyrin pyridyl nitrogen
atoms to the surrounding zinc atoms, responsible for the conformational homogeneity of the tetramer, are equal by symmetry,
with a Zn-N length of 2.20(2) A. The N, donor set is planar to
within 0.06 A; the zinc atom lies 0.29 8, out of the plane towards
the pyridyl group.
In the solid state the tetramer molecules form infinite stacks,
parallel to the c axis, and each alternate molecule is rotated
relative to its neighbors along this axis as shown in Figure 4a.
Large channels parallel to the a and b axes (shown in cross
section in Fig. 4a) lie between adjacent molecules. The stacks do
not adopt a close-packed arrangement, but pack to leave large,
square channels running the length of the crystal (Fig. 4b) in the
c direction. Apparently, rc-rc interactions between porphyrins
from adjacent stacks determine this arrangement; the minimum
interporphyrin distance is 3.66A and the offset between the
porphyrin centers 4.49 A. Extended regions of electron density
are consistent with occupation of the three sets of mutually
perpendicular channels by randomly oriented solvent molecules. Similar large channels running the length of the crystal,
and accommodating a large number of solvent molecules, were
also found in a related trinuclear zinc porphyrin,[''l and both
structures are reminiscent of the "porphyrin sponges" reported
for tetraarylporphyrins." 'I
Verlugsgesell.~chafimbH, 0-69451 Weinheim,1995
0570-0833/9S/l010-1097 3 10.00 f .25:0
1097
COMMUNICATIONS
Fig. 4. a) The stacks of H,-Py,P.Zn,-l molecules parallel to the c axis. showing a cross section of the channels running parallel to the h axis (identical channels
parallel t o the a axis are generated by symmetry). b) A view down the stacks of H,-Py,P.Zn,-I showing the square cross section of the large channels that run parallel to
the c axis
The association constant between Zn,-1 and H,-Py,P was
found to be 2 1 1 x 1 0 1 o ~ -at
l 30°C in dichloromethane by
using UV/visible spectroscopy (Fig. 5) .[12] Although this is a
very strong binding constant, it is much lower than would be
expected by extrapolation from the binding constants of the
cyclic dimer for 4,4'-bipyridy1(6 x 10' M - ') and the cyclic trimer
for tris(4-pyridy1)triazine (1 x 1 0' M - ').['I
Presumably the en-
'""1
II
!
X
["/.I
-
7
0.0
1 .o
Equiv. H,-Py,P
2.0
3.0
Fig. 5. Binding curve for cyclic tetramer binding to H,-Py,P. Experimental points
.
curve shows strong-binding
on the calculated curve for K = 2 x 1 0 ' o ~ - ' Dashed
limit. X = fraction of bound H,-Py,P.
1098
G VCH Verlagsgrsellschufi mhH, 0 - 6 9 4 5 f Wemherm. 1995
tropy and enthalpy associated with locking the conformation of
the cyclic tetramer into a tub geometry are unfavorable.
The absorption spectrum of the H,-Py,P.Zn,-l complex is
essentially the sum of the absorption spectra of the two component parts. However, the fluorescence emission spectrum of the
complex is around 1000 times less intense than that of either
component; even the residual fluorescence of the 1 :1 mixture
can mainly be accounted for by traces of the dissociated components (Fig. 6). Similar fluorescence quenching is observed in the
Zn-Py,P. Zn,-1 complex, where the central tetrapyridylporphyrin has been metalated with zinc. This behavior is much
more dramatic than that observed in Lindsey's five-porphyrin
array, where the fluorescence of only the peripheral zinc porphyrins was quenched.[4d]A possible explanation for the fluorescence quenching in our system is that photoinduced electron
transfer occurs, as in natural photosynthetic reaction centers,
followed by nonradiative decay of the charge-separated state.
This is one of the most conformationally homogeneous and
well-characterized mimics of the photosynthetic reaction center,
and it is also one of the most versatile. Reversible interactions
leading to noncovalent bonds are often thought of as more
flexible than covalent bonds, but this system demonstrates that
many cooperative weak interactions can yield a well-defined
supramolecular object. It should be possible to insert a range of
tetrapyridylporphyrin ligands into the cyclic tetramer to give a
variety of models for photosynthetic reaction centers with iden-
0S70-083319511010-10988 fO.00-t ,2510
Angrw Chem. I n f . Ed. Engl. 1995. 34, N o . 10
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1
[4] a) J. Davila, A. Harriman, L. R. Milgrom. Chem. Phj.5. Lrrr. 1987, 136, 427430; b) 0. Wennerstrom. H. Ericsson. I. Raston. S. Svensson, W. Pimlott,
Tetrahedron Lett. 1989.30,1129-1132;c)T. Nagatd.A Osuka. K. Maruyama,
J . A m . Chem. SO<.1990,112, 3054-3059; d ) S. Prdthapan. T. E. Johnson, J. S.
Lindsey, ihid. 1993. /IS.7519-7520; e) R . W. Wagner. J. S. Lindsey, ibid. 1994,
116, 9759-9760.
[S] The cyclic porphyrin tetramer Zn,-1 was synthesked with meso-tetra(4pyridy1)porphyrin (HI-Py,P) as a template. as described previously: a) S. An.
104, 921 -924;
derson, H. L. Anderson, J. K. M. Sanders, Angew. C h ~ m1992,
Angeit. Chem. fn!. Ed. Engl. 1992.31.907-910; b) S . Anderson, H. L. Anderson, J. K . M. Sanders, A t < . Chem. Res. 1993,26,469-475; c) H. L. Anderson,
R. P. Bonar-Law. L. G. Mackay, S. Nicholson, J. K. M. Sanders, Proc. NATO
Science Forum on Suprumol. Chem., Kluwer. 1992, 359 -374; d) S. Anderson,
H. L. Anderson, I. K. M. Sanders. unpublished.
[6] The conformers of the cyclic tetramer interconvert rlowly relative to the
' H NMR chemical shift time scale in CID,CI, up to 100 'C.
[7] NMR spectra consistent with D,, symmetry are observed, because tautomerism of the HI-Py,P N-H protons is fast on the NMR time scale.
[8] a ) H. L. Anderson, J. K. M. Sanders. J . Chem. Sot. <'hem. Comniun. 1989,
1714-1715: b) H. L. Anderson, J. K. M. Sanders, Angeu. Chem. 1990, 102,
1478-1480: Angeiv. Chem. int. Ed. Engi. 1990, 29. 1400-1403.
CZ,,,H,,,CI,N,,O,,
Zn,.
[9] Crystal data for H2-Py,P.Zn,-I.5C,H,,.3CHCI,,
M , = 5287.26. tetragonal. space group P4/nnc (No. 126). u = 35.860(8),c =
29.244(6)& 2 = 4 . V=37606.02.&'. pcA,c,=0.934gcm~',F(OOO)=11072,
{t(MoKJ = 0.37 mm-I. Crystals of H,-Py,P Zn,-1 were very fragile and
rapidly lost solvent in air. so data were collected for a crystal sealed with
mother liquor i n a Lindemann tube. 16183 (7562 unique) data were collected
in the 0 range of 3-19" using a Phillips PW1100 diffractometer with 01-20
scans. The structure was solved with difficulty by direct methods [SHELXS861; the center of the molecule lies on a site of S, symmetry, and the main
skeleton of the structure was only slowly revealed during many subsequent
difference-Fourier syntheses. Only the first two carbon atoms of each methyl
propanoate side chain (that is. the two carbon atoms closest to the zinc porphyrins) were located. and there was an unresolved region of extended electron
density near to the terminal atom of each two atom chain. The presence of
approximately three chloroform and five cyclohexane inolecules per tetramer
was indicated by analysis of the 'H NMR spectrum of crystals of H,Py,P.Zn,-l in CD2C12(that is, 103.5 e per asymmetric unit). No recognizable
solvent molecules could be identified, but many regions of extended electron
density were located in the channels running parallel to the three cell axes. The
highest residual maxima were included in the refinement
their population parameters adjusted to correspond to the expected total-solvent and side-chain electron density. The structure was refined on F2
[SHELXL-931 using all 7562 independent refelections. Due to the paucity of
data, geometrical restraints were applied to chemically equivalent bonds and to
make the phenyl and pyridyl groups planar. wR, = [ ~ W ( F : - F ~ ) / ~ F ~=] ' " ' ~
0.471, R, = ~ l l F o ~ - ~ F=~0.167
~/~
[forF 1202refectionswith
o
~~>220(F:)],
goodness of fit on F2 = 0.893 on 532 parameters. Further details of the crystal
structure investigation are available on request from the Director of the Cambridge Crystallographic Data Centre, 12 Union Road. GB-Cambridge CB2
1 EZ (UK) on quoting the full journal citation.
(101 H. L. Anderson, A. Bashall, K. Henrick, M. McPartlin. J. K. M. Sanders,
Angen,. Chem. 1994, 106. 445-447; Angew. Chem In/. Ed. EngI. 1994. 33,
429 -431.
[ I l l M. P. Byrn, C. J. Curtis, I. Goldberg, Y. Hsiou. S. I . Khan, P. A. Sawin. S. K.
Tendick. C. E. Strouse, J . Am. Chem. Suc. 1991. 113, 6549 -6557.
[12] Both host and guest absorb in the same region of the U V spectrum. This does
not hinder the measurement of binding constants since the extinction coefficients for the absorbing ligand are known. but rather allows an internal check
on the ligand concentration.
'
500
600
A[nrn]
-
700
800
Fig. 6. Fluorescence emission ( I ) plotted against wavelength at 1 PM in
dichloromethane for a) Zn,-l (solid line), b) H,-Py,P (dashed line). and c) H,Py,P.Zn,-l (expansion times 1000).
tical geometries but different electronic properties; this will
provide a valuable tool for analyzing the photophysics of such
systems.
Experinzental Procedure
Binding .s/iuIic.\: A solution of H,-Py,P ( 1 . 0 in
~ ~CH,CI,. also containing 6 . 0 n ~
Zn,-I) a a s titrated into a solution of Zn,-1 ( 6 . 0 n ~in CH,CI,) in a 10 mm quartz
cuvette kept at 30 C. The absorptions at 412 and 427 nm were recorded; the absorption at 412 nm was subtracted from the absorption at 421 nm to remove errors due
to the slow drift in the baseline. The results were reproduced twice. A Perkin Elmer
Lambda 2 spectrophotometer was used, which can reliably record absorption
changes of 1 x 10 -'.
Fluorcsc~~nw
fii"~/.~ilrenil'fit.s:
All spectra were recorded on a Spex I680 0.22-111 Double Spcctromctcr in emission mode exciting at 411 nm at 1 PM concentration in
argon-saturated dichloromethane.
Received: December 27, 1994 [Z7587IE]
German version: Angew. Chem. 1995. 107. 1196
Keywords: host-guest chemistry . photosynthesis . porphyrinoids
[I] a ) J. A. Cowan. J. K. M. Sanders, G. S. Beddard, R. J. Harrison, J . Chem. SOC.
Chefif.Cofmnnn 1987, 5 5 - 5 8 ; b) D. Gust. T. A. Moore, A. L. Moore, S.-J. Lee,
E. Bittersmann. D. K. Luttrull, A. A. Rehms. J. M. DeGraziano, X. C. Ma, E
Gao. R. E. Belford. T. T. Trier. Science 1990. 248. 199-201; c) M. R.
Wasieleaski, Chefif. Rev. 1992, Y2, 435-461; d ) A. Osuka, S. Nakajimd, K.
Maruyama. N. Mataga. T. Asahi. I. Yamdzaki. Y. Nishimura. T. Ohno. K.
Noraki. ./. .Am. Chrm Soc. 1993, 115, 4577-4589; e) J. L. Sessler, V. L. Capuano. A. Harriman, ihfd. 1993. 115. 4618-4628; f ) J.-C. Chambron, A. Harriman, V. Heitz. J.-P. Sauvage, h i d . 1993, 115, 6109-6114; g) J. L. Sessler, B.
Wang. A. Harriman. ihid. 1993. 115, 10418-10419; h) V. S.-Y. Lin, S. G. DiMagno, M J Therien. Sciwm 1994. 264, 1105- l l 12.
[2] a ) J. Drisenhofer, 0. Epp, K. Miki, R. Huber, H. Michel. Nature 1985, 318,
618 624: b) R. Huber. Arigcw. Chmi. 1989, 101, 849-871; A n g w . Chcm. Int.
ELI. Ei2g-I. 1989, 28. 848-869; c) J. Deisenhofer. H. Michel, {bid. 1989. I01.
872 892 and 1989, 28, 829-847.
[3] R . A Marcus. Angeuc Chem. 1993, 105. 1161-1172; Angew. Chem. h i ! . Ed.
En,?/. 1993. 32, 1111-1121.
Angel.. C h w . /nt. Ed. €ng/. 1995. 34, No. 10
6 VCH Verlugsgesellschufr mbH, 0-69451 Weinhrim, 1995
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