Synthesis and Photophysical Properties of Porphyrin-Functionalized Molecular Clips

Article abstract of DOI:10.1021/jo990348g

Different clip-shaped receptor molecules functionalized at one side-wall with a porphyrin unit have been synthesized by a condensation involving a diamino glucoluril derivative and a porphyrin dione. A similar condensation reaction with a porphyrin tetrao

Full text of DOI:10.1021/jo990348g

J . Org. Chem. 1999, 64, 6653-6663
6653
Syn th esis a n d P h otop h ysica l P r op er ties of
P or p h yr in -F u n ction a lized Molecu la r Clip s
J oost N. H. Reek,†,§ Alan E. Rowan,† Maxwell J . Crossley,‡ and Roeland J . M. Nolte*,†
Department of Organic Chemistry, NSR Center, University of Nijmegen, Toernooiveld 1, 6525 ED,
Nijmegen, The Netherlands, and School of Chemistry, The University of Sydney, NSW 2006, Australia
Received February 25, 1999
Different clip-shaped receptor molecules functionalized at one side-wall with a porphyrin unit have
been synthesized by a condensation involving a diamino glucoluril derivative and a porphyrin dione.
A similar condensation reaction with a porphyrin tetraone resulted in porphyrin molecule with
two receptor sites. The binding and photophysical properties of some of these porphyrin-receptor
molecules are described.
In tr od u ction
In the past we have been studying clip-shaped receptor
molecules based on diphenylglycoluril, which are able to
bind dihydroxybenzene guest molecules by means of
hydrogen bonding and π-π interactions.4 In this paper
we describe the synthesis and some properties of novel
porphyrin molecules (1-3 see Chart 1) containing a cleft
based on diphenylglycoluril. These porphyrin receptor
molecules are potentially interesting as supramolecular
catalysts.5 Furthermore, the combination of a binding site
for aromatic substrate molecules and the presence of a
photoactive component (1-3) results in fascinating sys-
tems with respect to electron and energy transfer studies.
Molecule 2 is of special interest since it contains both a
donor (porphyrin) and an acceptor site (quinone) and is
able to bind aromatic guest molecules.6 Using this
molecule we have been able to study the role in electron-
transfer processes, of aromatic molecules that are situ-
ated between the donor and the acceptor.7 In the photo-
synthetic reaction centers aromatic side chains of amino
acids are thought to enhance the electron-transfer rate,8
whereas their role in the lifetime of the charge-separated
state is not known.
In light of their special photophysical and catalytic
properties, porphyrins have proven to be very interesting
building blocks for the construction of functional supra-
molecular assemblies. Multiporphyin systems have been
synthesized and studied in order to obtain insight into
energy and electron-transfer processes,1 which are very
important with respect to the very efficient solar energy
conversion processes observed for photosynthetic bacte-
ria.2 Understanding of the underlying principles may lead
to very efficient artificial solar energy conversion devices.
From a catalytic point of view porphyrins are interesting
since the different metal complexes are able to mediate
different types of reactions such as epoxidation, cyclo-
propanation, Diels-Alder cycloadditions, cleavage of
amides, and alkane hydroxylation.3
† University of Nijmegen.
‡ University of Sydney.
§ Current address: Institute for Molecular Chemistry, Homogeneous
Catalysis, Faculty of Chemistry, University of Amsterdam, Nieuwe
Achtergracht 166, 1018 WV, Amsterdam, The Netherlands.
(1) (a) Sessler, J . L.; Wang, B.; Harriman, A. J . Am. Chem. Soc.
1995, 117, 704. (b) Harriman, A.; Magda, D. J .; Sessler, J . L. J . Phys.
Chem. 1991, 95, 1530. (c) Collin, J .-P.; Harriman, A.; Heitz, V.; Odobel,
F.; Sauvage, J .-P. J . Am. Chem. Soc. 1994, 116, 5679. (d) Tecilla, P.;
Dixon, R. P.; Slobodkin, G.; Alavi, D. S.; Waldeck, D. H. J . Am. Chem.
Soc. 1990, 112, 9408. (e) Harriman, A.; Magda, D. J .; Sessler, J . L. J .
Chem. Soc., Chem. Commun. 1991, 345. (f) Harriman, A.; Kubo, Y.;
Sessler, J . L. J . Am. Chem. Soc. 1992, 114, 388. (g) Turro, C.; Chang,
C. K.; Leroi, G. E.; Cukier, R. I.; Nocera, D. G. J . Am. Chem. Soc. 1992,
114, 4013. (h) Aoyama, Y.; Asakawa, M.; Matsui, Y.; Ogoshi, H. J . Am.
Chem. Soc. 1991, 113, 6233. (i) Hayashi, T.; Miyahara, T.; Hashizume,
N.; Ogoshi, H. J . J . Am. Chem. Soc. 1993, 115, 2049. (j) D′Souza, F. J .
Am. Chem. Soc. 1996, 118, 923. (k) Harriman, A.; Odobel, F.; Sauvage,
J .-P. J . Am. Chem. Soc. 1995, 117, 9461. (l) Vicente, M. G. H.; Cancilla,
M. T.; Lebrilla, C. B.; Smith, K. M. Chem. Commun. 1998, 2355. (m)
Nakano, A.; Osuka, A.; Yamazaki, I.; Yamazaki, T.; Nishimura, Y.
Angew. Chem. 1998, 110, 3172. For more general reviews see: (n)
Wasielewski, M. R. Chem. Rev. 1992, 92, 435. (o) Kurreck, H. Huber,
M. Angew. Chem. 1995, 107, 929.
Resu lts a n d Discu ssion
Syn th esis. Compounds 1a , 1e, and 3a were synthe-
sized by a condensation reaction of diamino clip 4b9 with
(4) (a) Sijbesma, R. P.; Kentgens, A. P. M.; Lutz, E. T. G.; van der
Maas, J . H.; Nolte, R. J . M. J . Am. Chem. Soc. 1993, 115, 8999. (b)
Reek, J . N. H.; Priem, A. H.; Engelkamp, H.; Rowan, A. E.; Elemans,
J . A. A. W.; Nolte, R. J . M. J . Am. Chem. Soc. 1997, 119, 9956.
(5) For a good example of porphyrin based supramolecular catalyst
see: (a) Anderson, H. L.; Bashall, A.; Henrick, K.; McPartlin, M.;
Sanders, J . K. M., Angew. Chem. 1994, 106, 445. (b) Walter, C. J .;
Anderson, H. L.; Sanders, J . K. M., J . Chem. Soc., Chem. Commun.
1993, 458; For a more general review see: (c) Feiters, M. C. Supramo-
lecular Catalysis. In Comprehensive Supramolecular Chemistry; Per-
gamon: Oxford, UK, 1996; (d) Sanders, J . K. M. Chem. Eur. J . 1998,
4, 1378.
(6) There is a current debate about the role of intervening aromatic
residues in electron-transfer processes; see, for instance, (a) Wasielews-
ki, M. R.; Niemczyk, M. P.; J ohnson, D. G.; Svec, W. A.; Minsek, D. W.
Tetrahedron 1989, 45, 4785. (b) Kumar, K.; Lin, Z.; Waldeck, D. H.;
Zimmt, M. B. J . Am. Chem. Soc. 1996, 118, 243. (c) Lawson, J . M.;
Paddon-Row: M. N.; Schuddelbom, W.; Warman, J . M.; Clayton, A.
H.; Ghiggino, K. P. J . Phys. Chem. 1993, 97, 13099.
(2) (a) For a review, see Fox, M. A. Photoinduced electron transfer;
Chanon, M. A., Ed.; Elsevier: New York, 1988. (b) Barber, J . Nature
1988, 333, 114. (c) Feher, G.; Allen, J . P.; Okamura, M. Y.; Rees, D. C.
Nature 1989, 339, 111. (d) Fleming, G. R.; Martin, J . L.; Breton, J .
Nature 1988, 333, 190. (e) Barber, J .; Andersson, B. Nature 1994, 370,
31. (f) Allen, J . P.; Feher, G.; Yeates, T. O.; Komiya, H.; Rees, D. C.
Proc. Nat. Acad. Sci. U.S.A. 1987, 84, 5730.
(3) (a) Groves, J . T.; Quinn, R. J . Am. Chem. Soc. 1985, 107, 5790.
(b) Callot, H. J .; Metz, F.; Piechoki, C. Tetrahedron 1982, 2365. (c)
Smegal, J . A.; Hill., C. L. J . Am. Chem. Soc. 1983, 105, 3515. (d)
Collman, J . P.; Tanaka, H.; Hembre, R. T.; Brauman, J . I. J . Am. Chem.
Soc. 1990, 112, 3689. (e) Bartley, D. W.; Kodadek, T. Tetrahedron Lett.
1990, 31, 6303. (f) Aoyama, T.; Motomura, T.; Ogoshi, H. Angew.
Chem., Int. Ed. Engl. 1989, 28, 921.
(7) Part of this work has been published as a preliminary com-
munication: Reek, J . N. H.; Rowan, A. E.; de Gelder, R.; Beurskens,
P. T.; Crossley, M. J .; De Feyter, S.; de Schryver F.; Nolte, R. J . M.
Angew. Chem., Int. Ed. Engl. 1997, 36, 361.
(8) Several authors in Reaction Centers of Photosynthetic Bacteria;
Michel-Beyerle, M.-E., Ed.; Springer-Verlag: Berlin, 1992.
10.1021/jo990348g CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/12/1999

6654 J . Org. Chem., Vol. 64, No. 18, 1999
Reek et al.
Ch a r t 1
Sch em e 1a
isomer, which could not be separated. After metalation
to the zinc porphyrin, however, the two isomers were
separated by column chromatography.
The synthesis of 2 started from the dinitro half-clip
6.9 The second side-wall was attached by treating 6 with
hydroquinone in the presence of p-toluenesulfonic acid
using 1,2-dichloroethane as the solvent. This hydro-
quinone ring was subsequently oxidized by bubbling air
through a solution of the compound in DMSO/pyridine,
which contained Cu2Cl2. After the reduction of the nitro
groups with Pd/carbon and triethylammonium formate
in THF/MeOH, the condensation reaction with porphyrin-
2,3-dione 5 was carried out. The yield of 2a was 20% with
respect to 7a .
During the sequence of reactions, a new compound (9)
was obtained as a side product. It had a quinone-like side-
wall, and double bonds between the side-wall and the
methylene bridges. This same side reaction was observed
during the oxidation of the 1,4-dimethoxybenzene/hy-
droquinone clip 10, yielding 12 as a side product (Scheme
2). The two AX patterns normally observed in the 1H
NMR spectrum for the CH2N protons of a clip molecule
with two different side-walls9 were altered and only one
AX pattern and one singlet at low field (8.1 ppm) were
visible. The latter signal had the intensity of two protons
and is assigned to the doubly bonded bridge between the
diphenylglycoluril unit and the side-wall.
a
(i) Reflux over mol sieves in CH2Cl2 under N2; (ii) hydro-
quinone, p-toluenesulfonic acid, 1,2-dichloroethane, reflux over mol
sieves; (iii) Cu2Cl2, pyridine, DMSO, oxygen; (iv) Pd/C, TEAF, THF/
MeOH (1:1, v/v).
porphyrin-2,3-dione 510 and porphyrin 2,3,12,13-tetraone
8,11 respectively (Scheme 1). In the latter reaction two
isomers were formed, viz., the C-shaped and the S-shaped
The zinc derivatives of the porphyrins were obtained
by treating the free base porphyrins with an excess of
Zn(OAc)2 in DMF/toluene (90% yield). The copper deriva-
tive 1c was synthesized by refluxing 1a in DMF/toluene
with an excess of Cu(OAc)2. The gold porphyrin 1d was
obtained by refluxing 1a for 2 days in acetic acid in the
presence of KAuCl4 and NaOAc.1c
(9) Reek, J . N. H.; Elemans, J . A. A. W.; Nolte, R. J . M. J . Org. Chem.
1997, 62, 2234.
(10) Crossley, M. J .; King, L. G. J . Chem. Soc., Chem. Commun.
1984, 920.
(11) Crossley, M. J .; Govenlock, L. J .; Prashar, J . K. J . Chem. Soc.,
Chem. Commun. 1995, 2379.
Reference compounds 13a and 13b were prepared in
order to study the influence of the dimethoxyquinoxaline

Porphyrin-Functionalized Molecular Clips
J . Org. Chem., Vol. 64, No. 18, 1999 6655
Sch em e 2a
a
(i) Cu2Cl2, pyridine, DMSO, oxygen.
unit attached to the porphyrin upon the electronic
properties of the latter ring. These derivatives were
synthesized by a condensation reaction of 1,4-dimethoxy-
2,3-diaminobenzene with 5, yielding 13a in 80% yield.
The zinc porphyrin 13b was synthesized from 13a in the
same way as 1b from 1a .
solid state.12 The large porphyrin wall wraps around the
back of the diphenylglycoluril part of its partner, and
interacts with a phenyl group on the convex side.
Although no crystals suitable for X-ray analysis could be
grown from 2, we expect that a single molecule of 2a will
have approximately the same structure as a single
molecule of 1a , since the X-ray structures of 4a and 11
were found to be very similar.4a,12 The edge-to-edge
distance between the electron donor (free base and zinc
porphyrin) and the electron acceptor (benzoquinone) in
2, therefore, will be approximately 6.5 Å, with a center-
to-center distance of 9 Å.
Molecules 9 and 12 have a geometry that is different
from those of 1a and 2. The conformations of the free
base analogues of 9 and 2a were calculated with the help
of molecular mechanics using the CHARMm force field
in the QUANTA modeling package.13 The structure of 2a
(Figure 2) was found to be very similar to the X-ray
structure of 1a , the main difference being the bent form
of the porphyrin in the latter molecule. This bending is
the result of the interactions between the two clip
molecules in the dimeric structure of 1a . According to
the calculations the doubly bonded connection in 9 results
in a side-wall that is perpendicular to the other aromatic
side-wall, hence the molecule does not possess a cleft
(Figure 2). The center-to-center distance between the
porphyrin unit and the quinone-like moiety in 9 is 15 Å,
and the planes make an angle of 101°.
Str u ctu r es. Purple crystals of 1a suitable for a X-ray
structure analysis were grown by slow diffusion of diethyl
ether into a chloroform solution of this compound.9 The
crystal structure of 1a is monoclinic and the unit cell
contains 4 molecules of 1a , which are packed in dimeric
pairs perpendicular to each other (Figure 1). The di-
phenylglycoluril unit in the X-ray structure of 1a is quite
similar to that in the X-ray structure of 4a .4a The twist
in the diphenylglycoluril framework of molecule 1a is
very small which is clear from the small dihedral angle
C21-C9-C9′-C21′ (5.5° as compared to 22° in clip 4a ).
The methoxy groups of the 1,4-dimethoxybenzene side-
wall point upward in the same way as in the crystal
structure of 4a , whereas the methoxy groups of the
porphyrin wall point outward. The latter groups are not
able to point upward due to steric interactions with the
porphyrin unit.
The two dimethoxybenzene groups attached to the
diphenylglycoluril unit of 1a define a tapering cleft, with
a center-to-center distance of 6.28 Å. The carbonyl groups
make the same angle (37°) as in 4a (39°), and the
distances between the oxygen atoms of the carbonyl
groups are almost equal (5.58 Å, as compared to 5.52 Å
in 4a ). The porphyrin wall of 1a is nonplanar and bends
toward the cavity with an out of plane angle of 15°,
resulting in the porphyrin unit and the opposite dimeth-
oxybenzene wall being parallel. This bending can be
attributed to favorable stacking interactions between the
two molecules in the dimeric pair (Figure 1). The
dimethoxybenzene wall of one molecule occupies the cleft
of its dimeric partner, forming a structure which is also
found in other clip molecules in both solution and the
As mentioned above the S-shaped and the C-shaped
isomers of clip molecule 3b were separated by column
chromatography. Unfortunately we were unable to grow
crystals suitable for X-ray diffraction of either the C- or
the S-shaped species. All spectroscopic data for the two
(12) Reek, J . N. H.; de Gelder, R.; Elemans, J . A. A. W.; Rowan, A.
E.; Beurskens, P. T.; Nolte, R. J . M. Submitted for publication.
(13) The structures were generated by changing the X-ray structure
of 1a in the molecular editor of the Quanta packages. Minimizations
were carried out using the CHARMm Force Field (NABR; CHARMm
Force Field).

6656 J . Org. Chem., Vol. 64, No. 18, 1999
Reek et al.
F igu r e 1. X-ray structure of 1a . A side-view of the dimeric complex (left) and the front-view of the monomer are shown (right).
Hydrogen atoms have been omitted for clarity.
structures were very similar. To distinguish between the
two species, host-guest binding experiments were car-
1
ried out and followed by H NMR spectroscopy.
Solid-liquid extraction experiments were performed
using a dimeric resorcinol guest molecule (14),14 that was
insoluble in chloroform. Only one of the isomers was able
to solubilize the substrate by forming a strong complex.
Large upfield shifts in the 1H NMR spectrum were
observed for both the guest molecule and this host
molecule, indicating that the former molecule was bound
in the cleft of the latter. The other isomer was unable to
solubilize 14, and consequently no complex formation was
1
observed by H NMR. A similar solid-liquid extraction
experiment using 1f as the receptor showed that this clip
is also unable to solubilize 14. Molecular modeling
calculations revealed that 14 fits nicely into the C-shaped
structure of 3b, forming hydrogen bonds with the two
diphenylglycoluril parts of the molecule (Figure 3). From
this calculation and the fact that clip molecule 1f is
unable to bind guest molecule 14, we propose that the
C-shaped structure of 3b is the isomer that binds 14. An
experiment to confirm the shapes of the two struc-
tures was carried out using the bulky trans-pyridyl-
porphyrin 15. This porphyrin molecule was added step-
wise to separate solutions of the S-shaped and the
C-shaped isomers of 3b, and the results were monitored
by both 1H NMR and UV-vis spectroscopy. Upon the
addition of 15 in both cases the Q-bands in the UV-vis
spectra of the zinc-porphyrin 3b shifted to the red,
indicating that the pyridine ligands were complexed to
the zinc centers.15
porphyrin shifted to lower field. When 15 was complexed
to the S-shaped isomer, a shift of the protons of the
p-dimethoxybenzene side-walls of the latter molecule was
observed, indicating that these protons were in the
shielding zone of the complexed porphyrin ligand. In the
complex with the C-shaped isomer, no such shifts of the
side-wall protons of 3b were observed. In this case the
complexation must have taken place on the outside of
the molecule (Figure 4).
Bin d in g P r op er ties. We have shown previously that
molecular clips of type 4a bind dihydroxybenzene guests
by hydrogen bonding with its carbonyl functions and π-π
stacking interactions with its aromatic walls (Ka resor-
cinol ) 2600 M-1).4 1H NMR titration experiments with
1a and the guest 5-pentylresorcinol (olivetol) in CDCl3
revealed that the latter molecule is bound weakly in the
cleft of 1a (Ka ) 25 M-1). The more electron poor guest,
hexyl 3,5-dihydroxybenzoate, formed a stronger complex
with host molecule 1a (Ka ) 120 M-1). From the calcu-
This complexation was confirmed by 1H NMR spec-
troscopy, which showed that the signals of the zinc-
(14) Nunen, van H. L. M., Ph.D. Thesis, 1995, University of
Nijmegen, The Netherlands.
(15) (a) Dolphin, D., Ed. The Porphyrins; Academic: New York, 1978.
(b) Fleischer, E. B.; Shachter, A. M. Inorg. Chem. 1991, 30, 3763.

Porphyrin-Functionalized Molecular Clips
J . Org. Chem., Vol. 64, No. 18, 1999 6657
F igu r e 2. Calculated structures (CHARMm force field) of 2 and 9. tert-Butyl groups have been omitted for clarity.
F igu r e 3. Computer-modeled structure of the complex between the C-shaped structure of 3b and the guest 14.
F igu r e 4. Complexation of trans-bipyridyl-porphyrin 15 to the S-shaped isomer (left) and the C-shaped isomer (right) of 3b.
lated complex-induced shift values it followed that the
guest was bound in an slightly off-center position between
the xylylene walls of 1a , as was found for complexes with
other clip molecules.4 The complexation of dihydroxy-
benzenes in zinc porphyrin clip 1f was somewhat stronger
(hexyl 3,5-dihydroxybenzoate: Ka ) 540 M-1). This

6658 J . Org. Chem., Vol. 64, No. 18, 1999
Reek et al.
feature was also observed for other metal-functionalized
clip molecules.9 It is known that the incorporation of
metals into porphyrins increases the π-π stacking
interactions of the latter molecules.15a Binding studies
in CCl4 using an external solvent as the lock signal and
the reference revealed that the association constant
between hexyl 3,5-dihydroxybenzoate and 1f is much
stronger (Ka ) 2 × 103 M-1) in this solvent. The binding
affinities of 2 are expected to be lower than those of 1:
on the basis of measurements carried out with 4a and
12 we estimate that the difference is about 3 kJ /mol.4,9
The geometry of the complex with 2 will be similar to
that of the complex with 1f, i.e., the guest will be located
between the zinc porphyrin and the quinone function.
This host-guest complex, therefore, was felt to be an
interesting model for studying the influence of an inter-
vening aromatic molecule upon the electron-transfer
process between a porphyrin donor and a quinone accep-
tor (vide infra).7
observed shifts were rather small suggesting that the
strength of the dimeric complex was weak. This is
probably due to the steric hindrance caused by the phenyl
groups on the porphyrin ring. The 1H NMR signals of
the C-shaped isomer of 3b not only shifted upon an
increase in concentration, but also broadened. This
suggests that the rate of exchange between the mono-
meric and the dimeric species is slow. The higher energy
barrier for dimerization of the C-shaped isomer of 3b is
not unexpected since in the dimer the molecules fit
tightly together, and two large surfaces have to be
solvated upon breaking the complex. In Figure 5 the
calculated structures of the dimeric species of both the
S- and C-shaped isomers of 3b are shown. The dimer of
S-shaped 3b is of interest because it has two clefts which
can further interact with other molecules of 3b to form
a long chain of organized porphyrins.17 The propensity
of our porphyrin-containing clip molecules to self-associ-
ate makes them interesting building blocks for construct-
ing organized multiporphyrin arrays in order to study
tandem electron transfer and tandem energy transfer
processes.
Electr och em istr y. The redox properties of porphyrin
clip molecules 1, 2, 9, and those of the model compounds
13 were studied by cyclic voltammetry (CV) and dif-
ferential pulse voltammetry (DPV). In Table 1 the
reduction potentials of the different porphyrins are given.
All first reductions of the porphyrin π-systems were
reversible or nearly reversible, except the reduction of
the π-system of porphyrin clip 2. This, however, is due
to the quasireversible reduction of the quinone unit of
this molecule, which leads to adsorption on the elec-
trode.18 As expected, metalation of the porphyrin resulted
in a change in the reduction potential. The second
reduction of 1b and 1c is shifted approximately 200 mV
to lower potential. The charged Au(III) porphyrin clip 1d
displayed a reduction at higher potential (-0.91 V vs Fc/
Fc+), which is similar to the reduction potential of quinone
(-0.93 V). This Au(III) porphyrin, therefore, can be used
as an electron acceptor in electron-transfer processes.1c
Important with respect to possible photoinduced elec-
tron-transfer processes are the oxidation potentials of the
electron-donating group (zinc porphyrin) and the reduc-
tion potential of the electron-accepting group (quinone)
(see Table 1). The first oxidation potentials of the zinc
porphyrins in 1b, 1f, 2b, and 9 were all reversible and
at approximately 0.3 V vs Fc/Fc+, which is similar to
potentials found for comparable systems in the litera-
ture.15 The free base and the copper porphyrins were
somewhat harder to oxidize than the zinc porphyrin
molecules (0.4 and 0.5 V, respectively). No oxidation of
the gold porphyrin 1d was observed. The reduction of the
quinone group of clip 2b occurred at -0.93 V and was
quasireversible. The latter was confirmed by scan rate
studies. It is known that quinone groups can polymerize
and interact with the electrode.18
Self-Associa tion . Some clip molecules are able to
dimerize not only in the solid state but also in solution.
Water-soluble clip molecules were shown to have a very
strong self-association which is driven by hydrophobic
effects.16 The porphyrin-functionalized molecules were
also found to form dimers in solution, with a geometry
similar to that observed in the X-ray structure of 1a (see
below). Upon diluting a concentrated solution of 1a in
1
CDCl3, the signals of the aromatic walls in the H NMR
spectrum shifted downfield, indicating that the dimeric
complex of 1a dissociated. The shifts were not large
enough for a self-association constant to be calculated.
The metal-functionalized porphyrin clip molecules showed
a stronger self-association behavior, probably due to a
decrease in electron density on the porphyrin side-wall.
As was found for the binding of dihydroxybenzene guest
molecules, a decrease in electron density and hence a
decrease in electrostatic repulsion between aromatic
rings leads to higher association constants. In the case
of molecular clips 1b, 1d , and 1f the shifts were large
enough to calculate the self-association constant: Kas
)
18 M-1 for 1b and 1f, and Kas ) 12 M-1 for 1d . (For 1c
no measurements were carried out).
The p-dimethoxybenzene side-wall and the methylene
bridges to which the porphyrin ring is attached, showed-
together with the methoxy groupssthe largest shifts
upon dilution, indicating that the dimeric complex had
a geometry similar to that in the crystal structure (Figure
1). The self-association of 1f was measured at various
temperatures. At lower temperatures the dimeric com-
plex was found to be more stable, indicating that the
process of dimerization is enthalpy driven (a similar
observation was made for clip molecules which were
monofunctionalized with a phenanthroline ring).12 At
very low temperatures larger aggregates were formed in
CCl4. For instance, the UV-vis spectra broadened upon
cooling the solution to -200 °C, to such an extent that
Q-bands could no longer be distinguished. At room
temperature the spectrum very slowly ((15 min) re-
turned to the normal spectrum. The fluorescence inten-
sity also decreased at lower temperature, supporting the
idea that larger aggregates were formed.
The presence of an excess of hexyl 3,5-dihydroxyben-
zoate guest had a small influence on the oxidation
potential of 1f and 2b. The reduction of the quinone group
of 2b, was found to be shifted 0.2 V to higher potential.
1H NMR dilution studies on the S-shaped isomer of 3b
revealed that this molecule forms dimers as well. The
(17) Upon diluting a concentrated solution of the reference com-
pound 13b, no 1H NMR shifts were observed, indicating that this
compound does not self-associate, besides the normal aggregation
behavior known for porphyrin molecules.15
(16) Reek, J . N. H.; Kros, A.; Nolte, R. J . M. Chem. Commun. 1996,
245.
(18) Lindsey, A. S. In Polymeric Quinones, The Chemistry of Quinoid
Compounds; Patai, S., Ed.; J ohn Wiley and Sons: New York, 1974.

Porphyrin-Functionalized Molecular Clips
J . Org. Chem., Vol. 64, No. 18, 1999 6659
F igu r e 5. Calculated structures of the dimers of the C-shaped (under) and S-shaped (above) isomers of 3b.
F igu r e 6. Typical absorption (A) and emission spectra (B) of zinc-porphyrins to which the conjugated quinoxaline unit is attached.
The reduction was now chemically irreversible, resulting
in a small reoxidation current. The reduction of the
quinone like side-wall in 9 was observed at -1.45 V (vs
Fc/Fc+) and found to be quasireversible as well. This group
is harder to reduce than to the quinone unit in 2b and
hence is not a good electron acceptor compared to the
quinone side-wall.
Stea d y-Sta te Absor p tion Sp ectr oscop y. The con-
jugated quinoxaline group attached to the porphyrin
units induces a symmetry element which is different from
normal tetraphenyl porphyrin (TPP), resulting in some-
what deviating absorption spectra. In the case of the free
base porphyrins 1a , 1e, 2a , and 13a a large Soret band
at approximately 440 nm was observed, as well as two
Q-bands at 530 and 600 nm, and two very small Q-bands
at 568 and 650 nm. The zinc-porphyrin molecules 1b, 1f,
2b, 9, and 13b showed a broad solvent dependent Soret
band with one or two maxima between 420 and the 450
nm, two larger Q-bands at 570 and 612 nm, and a small
Q-band at 530 nm (see Figure 6A). The porphyrin unit
in 3 has a different symmetry and hence a different
absorption spectrum. For the free base porphyrin 3a , the
Soret band was observed at 463 nm, and the Q-bands
(in decreasing intensities) at 540, 619 and 673 nm. The
zinc-porphyrin in 3b gave absorption bands at 475 nm
(Soret band) and at 661 nm and in addition to this some
small bands at 560, 586, 610, and 630 nm (Q-bands),
similar to the pattern seen for chlorins.15
Stea d y-Sta te F lu or escen ce Sp ectr oscop y. A typi-
cal emission spectrum of a zinc porphyrin with an
attached conjugated quinoxaline unit, is shown in Figure
6B. Two maxima are observed at 620 and 680 nm.
Comparison of the fluorescence quantum yields of the
different zinc-porphyrin molecules can provide informa-
tion about additional processes involved in the decay of
the excited state, i.e., photoinduced electron transfer.
In CCl4 the quantum yields of 1b, 1f, 2b, and 13b were
comparable to those observed for Zn(TTP) (Table 2). In
the more polar solvents CH2Cl2 and CHCl3, the fluores-
cence quantum yield of 2b was approximately 10 times

6660 J . Org. Chem., Vol. 64, No. 18, 1999
Reek et al.
Ta ble 1. Ha lfw a ve P oten tia ls (fr om CV a n d DP V) for th e
Red u ction a n d Oxid a tion of Differ en t P or p h yr in Clip
Molecu lesa
this state has a larger dipole moment than the ground
state.
P h ot oin d u ced Ch a r ge Sep a r a t ion . Within the
framework of the Marcus theory,19 the energy barrier ∆Gq
for electron transfer can be expressed by eq 1:
reduction potentials
oxidation potentials
porphyrin
E1/2 (V)
∆Ep (mV)
E1/2 (V)
∆Ep (mV)
1a
-1.61
-1.79
-1.69
-2.02
-1.62
-2.00
-0.91
-1.44
-1.67
-1.86
-1.78
-2.04
60
63
42
69
52
54
66
57
70
80
70
90
0.40
62
∆Gq ) (∆G° + λ)2/4λ
(1)
1b
1c
0.30
0.52
70
70
where -∆G° is the driving force and λ is the overall
reorganization energy. The driving force for photoinduced
electron transfer ∆G° can be calculated using eq 2:
1d
1e
-
0.42
-
56
∆G° ) e[EOX - ERED] - ES - (e2/4πꢀSꢀ0RCC) )
e[EOX - ERED] - ES - (14.4/ꢀSRCC) (2)
1f
0.30
0.56
0.29
0.58
0.30
0.56
0.28
0.32
0.57
76
80
80
125
68
98
69
73
107
1f + guest
where EOX refers to the redox potential for one-electron
oxidation of the Zn porphyrin (0.30 V vs Fc/Fc for 2b),
2b
-0.93b
-1.77
-2.06
-0.78b
-1.45b
-1.67
-1.86
-1.75
-2.04
74
70
-
55
70
80
65
70
+
2b + guest
9
13a
13b
ERED refers to the redox potential for one-electron reduc-
+
tion of the quinone (-0.93 V vs Fc/Fc for 2b), ES is the
singlet excitation energy of the porphyrin unit (2.18 eV
for 2b), and RCC is the center-to-center distance between
the porphyrin and the quinone. When the distance is
large and/or the solvent is polar, the last term of eq 2
equals zero. In the case of 2b the distance is ap-
proximately 9 Å, indicating that the last term of eq 2 is
significant. From eq 2 we can calculate that for 2b the
driving force (-∆G°) in dichloromethane is 1.13 eV. The
reorganization energy λ for similar systems has been
estimated to be 0.9-1 eV in polar solvents.20 Assuming
that the reorganization energy is comparable in our
system we can calculate that the energy barrier for
electron transfer in 2b in dichloromethane is very low.
To calculate the energy barrier in other solvents the
electrochemical data of eq 2 should be corrected for the
solvation energy of the ions in these solvents, since the
electrochemistry was only performed in dichloromethane.
This correction can be made using the Born equation (eq
3),
a
Measured in CH2Cl2 with 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAH) as supporting electrolyte. A Pt working
electrode, a Pt auxiliary electrode, and a Ag/AgI (0.1 M-1 TBAH,
+
0.02 TBAI) electrode were used. Potentials are reported vs Fc/Fc
in CH2Cl2. bThese values are for the quinone side-wall.
Ta ble 2. F lu or escen ce Qu a n tu m Yield s of Differ en t
P or p h yr in s Mea su r ed in Va r iou s Ch lor in a ted Solven tsa
porphyrin
CH2Cl2
CHCl3
CCl4
1b
1f
2b
13b
Zn(TPP)
0.016
0.016
0.002
0.024
0.025
0.016
0.016
0.002
-
0.022
0.020
0.020
0.024
0.025
-
a
Excitation wavelength 572 nm.
Ta ble 3. F lu or escen ce Qu a n tu m Yield s of P or p h yr in s 1f
a n d 2b Mea su r ed in Va r iou s Non ch lor in a ted Solven tsa
∆G°solv ) -(e2/2){(1/rd+) + (1/ra-)}(1 - 1/ꢀ) (3)
1f
λem (max)
2b
λem (max)
solvent
ꢀ
Φf
Φf
where rd+ and ra- are effective radii of the cation and the
anion radical. Assuming radical distances of 5 Å, one can
calculate that there is still a positive driving force (0.70
for CCl4 and 0.59 for hexane) in apolar solvents. Fur-
thermore, the corrected ∆G values in the different
solvents are all within the Marcus normal region. It is
clear from the fluorescence data, however, that there is
a significant barrier to the electron-transfer process in
the apolar solvents CCl4 and hexane.
c-hexane
hexane
diethyl ether
THF
dibutyl ether
acetonitrile
2.03 0.020
2.05 0.018
4.20 0.012
7.58 0.018
3.10 0.016
37.50 0.0052
615 nm
615 nm
624 nm
625 nm
626 nm
635 nm
0.018
0.013
0.004
0.004
0.004
0.001
616 nm
616 nm
624 nm
625 nm
625 nm
633 nm
a
Excitation wavelength 572 nm.
lower than that of the other molecules, e.g., reference
compound 13b. This suggests that in these more polar
solvents electron transfer can occur from the excited state
of the zinc porphyrin to the quinone side-wall.
In flu en ce of Gu est Molecu les on th e Electr on
Tr a n sfer . We showed that dihydroxybenzene guest
molecules can be bound in the cleft of molecules 1 and 2.
The addition of hexyl 3,5-dihydroxybenzoate or olivetol
to a solution of 2b in CCl4 resulted in a decrease in
fluorescence intensity (Figure 7). This decrease in fluo-
rescence intensity was not observed for either Zn(TPP),
1f, or 13b (Table 4), and hence is likely to be the result
of a photoinduced electron-transfer process.21 The addi-
tion of hexyl 3,5-dihydroxybenzoate to a CCl4 solution of
The quantum yields of 1f and 2b were also measured
in a series of non-chlorinated solvents (Table 3). Again
the quantum yields were very dependent upon the
dielectric constant of the solvent, suggesting that in the
more polar solvents the fluorescence of 2b is quenched
by a photoinduced electron-transfer process. For both
molecules, the maxima of the emission wavelengths
changed to longer wavelength as the solvent polarity
increased. The excited states of these molecules are
stabilized by more polar solvents, which suggests that
(19) Marcus, R. A. J . Chem. Phys. 1965, 43, 679.
(20) Antolovich, M.; Keyte, P. T.; Oliver, A. M.; Paddon-Row: M.
N.; Kroon, J .; Verhoeven, J . W.; J onker, S. A.; Warman, J . M. J . Chem.
Phys. 1991, 95, 1933.

Porphyrin-Functionalized Molecular Clips
J . Org. Chem., Vol. 64, No. 18, 1999 6661
the local polarity cannot, however, account fully for the
electron-transfer enhancement, and we feel that an
exchange mechanism also plays a role. More-detailed
photophysical studies on this host-guest system and the
design of new host-guest systems displaying photo-
induced electron transfer which is independent of the
solvent polarity are planned in the future in order to
obtain a more-detailed insight into the mechanism.
Con clu sion s
A series of novel porphyrin containing clip molecules
has been synthesized and characterized. These molecules
contain a binding site for dihydroxybenzene guest mol-
ecules that are bound nearby a porphyrin ring. Besides
the binding of guests, these molecules are able to form
dimers in solution and in the solid state. Although the
self-association constants of these molecules are low, the
observed type of dimeric complex is of interest for the
study of tandem electron and energy transfer processes.
Clip molecule 2 is very useful for studying electron
transfer reactions, since in this molecule the binding site
for dihydroxybenzene guests is located exactly between
a donor and an acceptor group. The photoinduced electron
transfer from the zinc porphyrin in 2 to the quinone site
appears to be dependent on the solvent polarity. In CCl4
and hexane no photoinduced electron transfer is ob-
served, but upon binding a dihydroxybenzene guest
molecule between the donor and the acceptor the fluo-
rescence of the porphyrin was quenched.
F igu r e 7. Fluorescence intensity of 2b as a function of the
guest concentration measured in CCl4. Excitation wavelength
570 nm.
Ta ble 4. F lu or escen ce Qu a n tu m Yield s of Differ en t
P or p h yr in s Mea su r ed in CCl4 in th e P r esen ce of a Gu est
Molecu lea
porphyrin
CCl4
CCl4 + guestb
1f
2b
13b
Zn(TPP)
0.020
0.020
0.024
0.025
red-shiftc
0.004
0.024
0.025
a
Excitation wavelength 572 nm. b Addition of an excess of hexyl
3,5-dihydroxybenzoate. c An additional emission band appeared at
680 nm.
1f resulted in the appearance of an extra emission band,
indicating that the emission process was altered by the
binding of the guest molecule. This emission is ascribed
to an exciplex (excited complex) between the bound guest
molecule and the porphyrin in 1f.
Exp er im en ta l Section
Gen er a l. Triethylammonium formate was distilled prior to
use. THF and diethyl ether were distilled under nitrogen from
sodium benzophenone ketyl. Dichloromethane and chloroform
were distilled from CaCl2. All other solvents and chemicals
were commercial materials and used as received. For column
chromatography Merck Silica Gel (60H) was used, and for thin-
layer chromatography Merck Silica Gel F254 plates were used.
P h ysica l Mea su r em en ts. Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) measurements were
performed using a conventional three-electrode cell, with Pt
working and auxiliary electrodes, and 0.1 M TBAH electrolyte
was used. A Ag/AgI reference electrode was employed.
Com p ou n d s. The syntheses of compounds 1a , 1e, 4, 6,
porphyrin-2,3-dione 5 and porphyrin-2,3,12,13-tetraone 8 have
been described elsewhere.9-11 All reactions with porphyrins
were carried out under nitrogen atmosphere and with the
reactions flasks covered with aluminum foil. The pyridine-
containing porphyrins used to determine the structure of 3b
were synthesized as described in the literature.15b
Zn -P or p h yr in Clip 1b. A mixture of 47 mg (0.037 mmol)
of 1a and an excess of Zn(OAc)2 (50 mg) in 2 mL of toluene
was refluxed for 16 h. After cooling, 15 mL of toluene was
added and the solution was washed with water (3×), with 5%
Na2CO3 solution (2×) and again with water. The organic
solution was evaporated to dryness and the green solid was
purified by column chromatography (CH2Cl2/MeOH, 98:2 v/v),
yielding 40 mg of 1b (80%); mp: >310 °C; 1H NMR (CDCl3) δ
8.89, 8.54 (ABq, 4H, J ) 5 Hz), 8.75 (s, 2H), 8.20-8.0 (m, 8H),
7.81-7.72 (m, 12H), 7.15 (s, 5 H), 7.14 (s, 5H), 6.62 (s, 2H),
5.92 and 3.98 (ABq, 4H, J ) 15.8 Hz), 5.56 and 3.82 (ABq,
4H, J ) 15.8 Hz) 3.86 (s, 6H) 3.72 (s, 6H); UV-vis (CHCl3)
λ/nm: 408, 447, 530, 570, 612; FAB-MS (m-nitrobenzyl alcohol)
m/z 1321 (M + H)+.
The significant enhancement of the electron transfer
upon complexation of a guest molecule is of great interest
since in the photosynthetic reaction center intervening
aromatic residues are thought to play a role in the
electron-transfer process.8 Studies on a variety of co-
valently linked donor acceptor systems, which were
shown to have through bond electron transfer, have
supported this suggestion of aromatic mediation.19 It was
suggested that a super exchange mechanism was respon-
sible for the enhancement in electron transfer. Recently,
a super exchange pathway was also suggested for the
solvent-mediated electron transfer in a donor-acceptor
system.6 This solvent-mediated transfer can be seen as
a noncovalent way of coupling the two components. In
our case the aromatic guest is selectively bound between
the donor and the acceptor. It is, of course, of interest to
know the precise mechanism of the electron transfer
process when the guest molecule is bound between the
porphyrin and the quinone. The electron transfer occurs
either via an exchange mechanism or a change in the
local polarity due to the bound guest molecule, or a
combination of both effects. From our CV measurements,
it is clear that the guest molecule changes the redox
potentials of both the donor and the acceptor species, and
hence the driving force is also modified. The changes in
(21) SPC time-resolved fluorescence measurements performed on
the complex of 2b with dihydroxybenzene showed no additional decay,
suggesting that the electron transfer between the porphyrin and the
quinone became faster than the detection limit. This indicates that
the photoinduced electron transfer is faster when an aromatic guest
is bound between the donor and the acceptor.
Cu -P or p h yr in Clip 1c. A mixture of 40 mg (0.032 mmol)
of 1a and an excess of Cu(OAc)2 (50 mg) in 2 mL of toluene
and 1 mL of DMF was refluxed for 16 h. After cooling, 25 mL
of toluene was added and the solution was washed with water
(3×), with 5% Na2CO3 solution (2×), and again with water.

6662 J . Org. Chem., Vol. 64, No. 18, 1999
Reek et al.
The organic solution was evaporated to dryness, and the green
solid was purified by column chromatography (CH2Cl2/MeOH,
99:1 v/v) yielding 38 mg of 1c (90%); mp: >310 °C; UV-vis
(CH2Cl2) λ/nm: 411, 446, 532, 564, 608; FAB-MS (m-nitroben-
zyl alcohol) m/z 1318 (M + H)+.
(m-nitrobenzyl alcohol) m/z 2348 (M + H)+. HRMS Calcd for
C
148H155N16O12: 2348.201, Found: 2348.207.
Bisclip -Zn -p or p h yr in 3b: The same procedure was used
as for 1b. Purification was carried out by column chromatog-
raphy (CH2Cl2), total yield 60% after purification. The S-
shaped and C-shaped isomers could be separated by column
chromatography; Mp (both fractions): >310 °C; 1H NMR
(fraction 1, S-shape, dilute CDCl3 solution) δ 8.57 (s, 4H), 8.14
(s, 4H), 7.87, 7.84, 7.83 (3s, 8H), 7.16 (m, 20H), 6.52 (s, 4H),
5.92 and 3.94 (ABq, 8H, J ) 16 Hz), 5.50 and 3.75 (ABq, 8H,
J ) 16 Hz) 3.74, 3.66 (2s, 24H), 1.52, 1.50, 1.48 and 1.41 (4s,
72 H).; 1H NMR (fraction 2, C-shape, dilute CDCl3 solution) δ
8.56 (s, 4H), 8.14 (s, 4H), 7.86 (s, 8H), 7.16 (m, 20H), 6.52 (s,
4H), 5.91 and 3.93 (ABq, 8H, J ) 16 Hz), 5.50 and 3.78 (ABq,
8H, J ) 16 Hz), 3.74, 3.68 (2s, 24H), 1.52, 1.50, 1.48, and 1.41
(4s, 72 H) (peaks at 6.52, 5.50 and 3.68 are slightly broadened).
UV-vis (CHCl3) λ/nm: 473, 560, 586, 610, 630, 660; FAB-MS
(m-nitrobenzyl alcohol) m/z 2410 (M + H)+. HRMS Calcd for
Au -P or p h yr in Clip 1d . A mixture of 40 mg (0.032 mmol)
of 1a , 30 mg (0.080 mmol) of KAuCl4, and 10 mg (0.13 mmol)
of NaOAc in 15 mL of acetic acid was refluxed for 48 h. After
cooling, the solvent was evaporated and the solid dissolved in
CH2Cl2. The resulting solution was washed with water (3x),
with 5% Na2CO3 solution (2×) and again with water. Anion
exchange was carried out by stirring the organic phase with
saturated KPF6 solution. The organic solution was washed
with water and evaporated to dryness. The brown solid was
purified by column chromatography (CH2Cl2/MeOH, 98:2 v/v)
yielding 38 mg of 1d (90%); Mp: >310 °C; 1H NMR (CDCl3) δ
9.13 and 8.90 (ABq, 4H, J ) 5 Hz), 9.08 (s, 2H), 8.20-8.05 (m,
8H), 7.93-7.81 (m, 12H), 7.15 (s, 5 H), 7.15 (s, 5H), 6.63 (s,
2H), 5.92 and 3.96 (ABq, 4H, J ) 15.8 Hz), 5.58 and 3.81 (ABq,
4H, J ) 15.8 Hz), 3.84 (s, 6H), 3.74 (s, 6H); UV-vis (CH2Cl2)
λ/nm: 438, 549, 593; FAB-MS (m-nitrobenzyl alcohol) m/z 1451
(M-PF6)+.
Zn -P or p h yr in Clip 1f. The same procedure was used as
for 1b. Purification was carried out by column chromatography
(CH2Cl2), yield 90%; Mp: >310 °C; 1H NMR (diluted solution)
(CDCl3) δ 8.91 and 8.54 (ABq, 4H, J ) 5 Hz), 8.85 (s, 2H),
8.12, 8.07, 8.01, 7.86, 7.85, 7.77 (6s, 12H), 7.15 (m, 10 H), 6.46
(s, 2H), 5.90 and 4.01 (ABq, 4H, J ) 15.8 Hz), 5.44 and 3.69
(ABq, 4H, J ) 15.8 Hz) 3.89 (s, 6H) 3.61 (s, 6H), 1.52, 1.50,
1.48 and 1.41 (4s, 72 H); UV-vis (CHCl3) λ/nm: 412, 447, 530,
572, 615; FAB-MS (m-nitrobenzyl alcohol) m/z 1769 (M + H)+.
HRMS Calcd for C11113C1H122N10O6Zn: 1767.887; C112H123N10O6-
Zn: 1767.892; Found: 1767.882.
Qu in on e P or p h yr in Clip 2a . Compound 7b was obtained
after reduction of 220 mg (0.32 mmol) of dinitro derivative 7a
by stirring the latter in 10 mL of THF and 10 mL of MeOH in
the presence of TEAF and Pd/C.9 The Pd/C was filtered off
under nitrogen, 10 mL of CH2Cl2 and 350 mg of porphyrindione
5b were added, and the mixture was refluxed for 18 h over
molecular sieves (3 Å). After cooling, the solution was washed
with water (3×) and the organic solvents were removed in
vacuo. After column chromatography (CH2Cl2/EtOH, 99:1 v/v),
107 mg (20%) of pure 2a was obtained; mp >310 °C; 1H NMR
(diluted solution) (CDCl3) δ 8.90 and 8.56 (ABq, 4H, J ) 5 Hz),
8.73 (s, 2H), 8.20, 8.08, 8.04, 7.92, 7.88, 7.87 (6s, 12H), 7.15
(m, 10 H), 6.59 (s, 2H), 5.97 and 3.95 (ABq, 4H, J ) 15.8 Hz),
5.50 and 3.72 (ABq, 4H, J ) 15.8 Hz), 3.79 (s, 6H), 1.54, 1.52,
1.50 and 1.48 (4s, 72 H), -2.50 (s, 2H, NH); UV-vis (CHCl3)
λ/nm: 441, 531, 568, 601, 650; FAB-MS (m-nitrobenzyl alcohol)
m/z 1678 (M + 2H)+.
C
148H153N16O12Zn: 2410.115; Found: 2410.120.
Din itr o Com p ou n d 7a . A mixture of 420 mg (0.714 mmol)
of 6 and 300 mg (1.57 mmol) of p-toluenesulfonic acid in 10
mL of 1,2-dichloroethane was refluxed under nitrogen over
molecular sieves for 10 min. To this solution was added 155
mg (1.4 mmol) of 1,4-dihydroxybenzene, and refluxing was
continued for 16 h. The solvent was removed in vacuo, and
the solid was dissolved, in 10 mL of DMSO. To this solution
were added 50 mg Cu2Cl2 and 0.6 mL of pyridine and air was
bubbled through the solution for 2 h. The suspension was
poured into 50 mL of aqueous 1 N HCl and the product was
extracted with 50 mL of CHCl3. The organic layer was washed
with aqueous 1N HCl, aqueous 5% NH3 (2x) and with water,
and concentrated in vacuo. After purification, 260 mg (54%)
of pure 7a was obtained. Mp: >310 °C; 1H NMR (CDCl3) δ
6.95-7.20 (m, 10H), 6.75 (s, 2H), 5.52 and 3.85 (ABq, 4H, J )
16 Hz), 5.50 and 3.70 (ABq, 4H, J ) 16 Hz), 4.15 (s, 6H). This
compound was directly used for further synthesis.
Dim eth oxyqu in oxa lin e P or p h yr in 13a . 2,3-Diamino-1,4-
dimethoxybenzene was obtained after reduction of 40 mg
(0.175 mmol) of 2,3-dinitro-1,4-dimethoxybenzene by stirring
the latter compound in 5 mL of THF and 5 mL of MeOH in
the presence of TEAF and Pd/C.9 The Pd/C was filtered off
under nitrogen, and the solvent was evaporated in vacuo.
Subsequently 10 mL of CH2Cl2 and 80 mg of porphyrin-dione
5b were added, and the mixture was refluxed for 3 h. over
molecular sieves (3 Å). After cooling, the solution was washed
with water (3×) and the organic solvents were removed in
vacuo. After column chromatography (CHCl3/hexane, 1:1 v/v),
31 mg (35%) of pure 13a was obtained; mp >310 °C; 1H NMR
(diluted) (CDCl3) δ 8.96 and 8.93 (ABq, 4H, J ) 5 Hz), 8.77 (s,
2H), 8.04, 8.01, 7.87, 7.80 (4s, 12H), 7.01 (s, 2H), 3.82 (s, 6H),
1.54, 1.52, 1.50, and 1.48 (4s, 72 H), -2.47 (s, 2H, NH); UV-
vis (CHCl3) λ/nm: 438, 531, 568, 600, 650; FAB-MS (m-
nitrobenzyl alcohol) m/z 1226 (M + H)+.
Qu in on e P or p h yr in Clip 2b . The same procedure was
used as for 1b. Purification was carried by column chroma-
tography (CH2Cl2), yield 90%; mp: >310 °C; 1H NMR (diluted
solution) (CDCl3) δ 8.92 and 8.57 (ABq, 4 H, J ) 5 Hz), 8.86
(s, 2 H), 8.13 (s, 2 H), 8.08 (s, 2 H), 8.02 (s, 2 H), 7.92 (s, 2 H),
7.87 (s, 2 H), 7.77 (s, 2 H), 7.16 (m, 10 H), 6.57 (s, 2 H), 5.99
and 3.98 (ABq, 4 H, J ) 15.8 Hz), 5.49 and 3.72 (ABq, 4 H, J
) 15.8 Hz), 3.80 (s, 6 H), 1.52, 1.50, 1.48 and 1.41 (4s, 72 H);
UV-vis (CHCl3) λ/nm: 421, 449, 530, 571, 615; FAB-MS (m-
nitrobenzyl alcohol) m/z 1738 (M + H)+; HRMS Calcd for
Dim eth oxyqu in oxa lin e Zn -P or p h yr in 13b. The same
procedure was used as for 1b. Purification was carried out by
column chromatography (CH2Cl2), yield 90%; mp >310 °C; 1H
NMR (diluted) (CDCl3) δ 8.98 and 8.82 (ABq, 4H, J ) 5 Hz),
8.81 (s, 2H), 8.05 (d, 4H, J ) 2 Hz) 7.97 (d, 4H, J ) 2 Hz),
7.66 (t, 2H, J ) 2 Hz), 7.50 (t, 2H, J ) 2 Hz), 7.56 (s, 2H), 3.82
(s, 6H), 1.54, 1.52, 1.50 and 1.48 (4s, 72 H); UV-vis (CHCl3)
λ/nm: 412, 447, 530, 572, 615; FAB-MS (m-nitrobenzyl alcohol)
m/z 1289 (M)+. Anal. Calcd for C84H100N6O2: C, 82.31; H, 8.22;
N, 6.86. Found: C, 82.45; H, 8.29; N, 6.65.
C
110H117N10O6Zn: 1737.845; Found: 1737.840.
Bisclip -P or p h yr in 3a . In situ prepared compound 4b (0.29
mmol) was refluxed with 100 mg (0.1 mmol) of porphyrin-
tetraone 8 in CH2Cl2 for 36 h. After cooling, the mixture was
washed with water (3x), and the organic solvent was removed
in vacuo. After column chromatography (CH2Cl2), monoreacted
porphyrin and direacted porphyrin 3a were isolated. It was
not possible to separate the S-shaped isomer from the C-
shaped one (see text), leaving a mixture of the two isomers
(56% yield); mp > 310 °C;1H NMR (diluted solution) (CDCl3)
δ 8.57 and 8.54 (2s, 4H), 8.18 (m, 4H,), 7.87 (m, 8H), 7.16 (m,
20H), 6.62 and 6.58 (2s, 4H), 5.90 and 3.93 (ABq, 8H, J ) 15.8
Hz), 5.56, 5.54, and 3.90, 3.88 (2 ABq, 8H, J ) 15.8 Hz), 3.75,
3.71 (2s, 24H), 1.52, 1.50, 1.48, and 1.41 (4s, 72 H), -2.50 (s,
2H, NH); UV-vis (CHCl3) λ/nm: 463, 540, 619, 673; FAB-MS
The two side products with a double bond as connection
between the side-wall of the clip and the diphenylglycoluril
framework had the following physical properties:
1
Zn -p or p h yr in clip 9: mp: >310 °C; H NMR (CDCl3) δ
8.93 and 8.58 (ABq, 4H, J ) 5 Hz), 8.87 (s, 2H), 8.16 (s, 2H),
8.13 (s, 2H), 8.09 (s, 2H), 8.02 (s, 2H), 7.92 (s, 2H), 7.87 (s,
2H), 7.78 (s, 2H), 7.16 (m, 5H), 7.05 (m, 5H), 6.67 (s, 2H), 6.16
and 3.98 (ABq, 4 H, J ) 15.8 Hz), 4.07 (s, 6 H), 1.52, 1.50,
1.48 and 1.41 (4s, 72 H); UV-vis (CHCl3) λ/nm: 421, 449, 530,
571, 615; Field Desorption m/z 1736 (M + H)+; HRMS Calcd
for C110H115N10O6Zn: 1735.829; Found: 1735.820.

Porphyrin-Functionalized Molecular Clips
J . Org. Chem., Vol. 64, No. 18, 1999 6663
Clip 12: mp: >310 °C; 1H NMR (CDCl3) δ 8.12 (s, 2H),
7.16 (m, 5H), 6.90-7.10 (m, 5H), 6.87 (s, 2H, ArH), 6.69 (s,
2H), 5.78 and 3.84 (ABq, 4H, J ) 16 Hz), 3.89 (s, 6H). FAB-
MS (m-nitrobenzyl alcohol) m/z 587 (M + H)+.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR peak as-
signments for all compounds. This material is available free
of charge on the Internet at http://pubs.acs.org.
Ack n ow led gm en t. We thank Prof. Paddon-Row for
helpful discussions concerning the revision of this
manuscript.
J O990348G