Synthesis of cyclic hexameric porphyrin arrays. Anchors for surface immobilization and columnar self-assembly

Article abstract of DOI:10.1021/jo034861c

To investigate new architectures for the self-assembly of multiporphyrin arrays, a one-flask synthesis of a shape-persistent cyclic hexameric array of porphyrins was exploited to prepare six derivatives bearing diverse pendant groups. The new arrays conta

Full text of DOI:10.1021/jo034861c

Syn th esis of Cyclic Hexa m er ic P or p h yr in Ar r a ys. An ch or s for
Su r fa ce Im m obiliza tion a n d Colu m n a r Self-Assem bly
Kin-ya Tomizaki,† Lianhe Yu,† Lingyun Wei,‡ David F. Bocian,*,‡ and J onathan S. Lindsey*,†
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, and
Department of Chemistry, University of California, Riverside, California 92521-0403
jlindsey@ncsu.edu; david.bocian@ucr.edu
Received J une 19, 2003
To investigate new architectures for the self-assembly of multiporphyrin arrays, a one-flask synthesis
of a shape-persistent cyclic hexameric array of porphyrins was exploited to prepare six derivatives
bearing diverse pendant groups. The new arrays contain 6-12 carboxylic acid groups, 12 amidino
groups, 6 thiol groups, or 6 thiol groups and 6 carboxylic acid groups in protected form (S-acetylthio,
TMS-ethyl, TMS-ethoxycarbonyl). The arrays contain alternating Zn and free base (Fb) porphyrins
or all Zn porphyrins. The one-flask synthesis entails a template-directed, Pd-mediated coupling of
a p/p′-substituted diethynyl Zn porphyrin and a m/m′-substituted diiodo Fb porphyrin. The porphyrin
building blocks (trans-A2B2, trans-AB2C) contain the protected pendant groups at nonlinking meso
positions. A self-assembled monolayer (SAM) of a Zn3Fb3 cyclic hexamer containing one thiol group
on each porphyrin was prepared on a gold electrode and the surface-immobilized architecture was
examined electrochemically. Together, the work reported herein provides cyclic hexameric porphyrin
arrays for studies of self-assembly in solution or on surfaces.
In tr od u ction
porphyrins joined via diphenylethyne linkers (Scheme
1).10 This cyclic array is shape persistent with a cavity
diameter of 30-35 Å. The cyclic hexamer was prepared
by a one-flask reaction of a diethynyl zinc porphyrin (Zn -
2) and a diiodo free base porphyrin (3) in the presence of
a tripyridyl template (4). The cyclic hexamer also has
been prepared in a stepwise manner where the template
is used in the final reaction of components (pentamer +
monomer, or trimer + trimer) to create the hexamer.11
The template is essential for the one-flask synthesis and
can be used to augment the stepwise synthesis. Regard-
less of approach, the template is displaced upon chro-
matographic workup of the reaction mixture. The cyclic
hexamer (bearing mesityl groups at all nonlinking por-
phyrin meso-positions) is termed Zn 3F b3-1a (previously
termed cyclo-Zn3Fb3U-p/m). The cyclic hexamer contain-
ing all Zn porphyrins (Zn 6-1a ) is readily formed by
metalation. Gossauer has prepared an even larger cyclic
hexameric array incorporating arylethyne linkers.12
The primary motivation for preparing the cyclic hex-
amers was to study light-harvesting phenomena. The
A wide variety of multiporphyrin arrays have been
prepared for studies related to biological and materials
chemistry.1-6 Cyclic architectures of porphyrins in de-
fined 3-dimensional structures have been employed in
diverse studies7 and are members of a broader class of
shape-persistent nanoscale molecular architectures that
have elicited wide interest.8 Anderson and Sanders
pioneered the synthesis of cyclic arrays (primarily trimers
and tetramers) of porphyrins joined by diphenylethyne
or diphenylbutadiyne linkers and investigated their
host-guest properties.7,9 We extended this line of work
to the synthesis of a cyclic hexameric porphyrin array
comprised of alternating Zn porphyrins and free base (Fb)
† North Carolina State University.
‡ University of California.
* Corresponding author.
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10.1021/jo034861c CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/17/2003
J . Org. Chem. 2003, 68, 8199-8207
8199

Tomizaki et al.
SCHEME 1. Cyclic Hexa m er w ith th e Bou n d Tr ip yr id yl Tem p la tea
a
Three nonlinking meso-aryl groups have been omitted for clarity.
tripyridyl template or a dipyridyl porphyrin can be bound
as a guest in the cyclic hexameric host in a nearly
quantitative manner at submicromolar concentrations
(Kassoc > 3 × 108 M-1).13 Photophysical studies of Zn 3F b3-
1a characterized the energy-transfer dynamics from a Zn
porphyrin to adjacent Fb porphyrins in the backbone of
the wheel,10 and from the porphyrins in the wheel to a
dipyridyl-substituted Fb porphyrin bound in the cyclic
hexamer in a wheel-and-spoke architecture.13 The facile
synthesis, shape-persistent architecture, avid host-guest
binding phenomena, and efficient energy-transfer proper-
ties prompted consideration of applications for analogous
cyclic hexamers in supramolecular and materials chem-
istry.
bearing free thiols15-17 or S-acetylthio esters16-21 have
been prepared, as have porphyrin disulfides.21,22 The
S-acetylthio-derivatized compounds can be used in SAM
formation as the ester undergoes cleavage upon exposure
to the gold substrate, thereby obviating handling of free
thiols.17,23 To our knowledge, the largest multiporphyrin
arrays that have been employed in the formation of SAMs
contain three porphyrins anchored to the surface via two
thio tethers.20
In this paper, we describe the design and synthesis of
a family of cyclic hexamers bearing functional groups
arrayed along the wheel. The functional groups (carboxyl-
ic acid, amidine, thiol) in protected form are incorporated
in trans-AB2C- and trans-A2B2-porphyrin building blocks.
We felt that with appropriate derivatization, a cyclic
hexamer would afford a corral-like architecture in a self-
assembled monolayer (SAM) and that such a structure
could be used as a baseplate or anchor for columnar self-
assembly of cyclic hexamers yielding tubular structures.
Encouragement for the latter notion was provided by
preliminary X-ray scattering studies of Zn 3F b3-1a at high
concentration in toluene, showing that at saturation the
hexamer organizes into crystal-like arrays consistent
with columnar stacking (Dr. David Tiede, unpublished
data). Columnar or tubular structures constitute a
characteristic motif in both biological and artificial
systems.14 The formation of SAMs occurs readily upon
exposure of thiol-derivatized porphyrins to a gold sub-
strate. A variety of porphyrin-containing compounds
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8200 J . Org. Chem., Vol. 68, No. 21, 2003

Synthesis of Cyclic Hexameric Porphyrin Arrays
CHART 1
The porphyrins are used as substrates in the template-
directed one-flask synthesis of cyclic hexamers. A cyclic
hexamer bearing six thiol groups has been employed to
form a SAM that has been characterized electrochemi-
cally. The examination of columnar architectures is
beyond the scope of this paper and will be described
elsewhere.
in Chart 1. Each porphyrin possesses two nonlinking
meso substituents. Because the cyclic hexamers are
prepared by reaction of an R1/R2-diethynyl porphyrin and
an R3/R4-diiodo-porphyrin, up to four distinct substituents
can be introduced on the nonlinking meso-positions. In
cyclic hexamers of type I, all nonlinking meso-substitu-
ents are identical (R1-4 ) R). In type II, the two substit-
uents on the diethynyl porphyrin are identical with one
another (R1, R2 ) R) and the two substituents on the
diiodo porphyrin are identical with one another (R3, R4
) R′). In type III, the two porphyrins each bear the same
two distinct substituents (R1, R3 ) R; R2, R4 ) R′). Note
that the porphyrins can rotate about the diphenylethyne
linker, enabling interchange of the groups that project
Resu lts a n d Discu ssion
(1) Molecu la r Design . Cyclic hexamers with three
distinct patterns of substituents were designed as shown
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Tomizaki et al.
on each side of the array. Such rotation is inconsequential
for types I and II, where a given porphyrin has two
identical nonlinking substituents. In type III, each por-
phyrin has two distinct nonlinking substituents. Thus,
the architecture displayed in Chart 1 for type III cyclic
hexamers is only one possible accessible conformation.
dipyrromethanes 5b,25 5c,26,27 5d ,27 5e,26 and 5f25 were
prepared in the same manner.
The introduction of acyl groups at the 1- and 9-posi-
tions of a dipyrromethane can be achieved in two ways:
the direct diacylation of the dipyrromethane with identi-
cal substituents at the 1- and 9-positions in a one-flask
process, or the stepwise acylation with two different
substituents at the 1- and 9-positions (Scheme 2).26 The
direct diacylation method often affords a mixture of the
diacyldipyrromethane and the monoacyldipyrromethane,
which can be difficult to separate. The stepwise acylation
method sometimes affords cleaner reactions than the
The substituents of interest include groups that enable
surface immobilization (thiol, carboxy, amidine) and
possible columnar self-assembly (carboxy, amidine). The
previous cyclic hexamer (Zn 3F b3-1a ) employed mesityl
groups at all nonlinking meso-positions. To determine the
effects of lesser steric bulk, an aryl group lacking 2,6-
dimethyl groups was employed (Zn 3F b3-1b). The designs
of Zn 3F b3-1d and Zn 3F b3-1e (incorporating meso-H or
tert-butylphenyl groups) also enable comparison of the
effects of steric hindrance toward binding. The remaining
cyclic hexamers contain one set of substituents for surface
immobilization or binding (Zn 3F b3-1d -g) or two sets of
substituents (Zn 3F b3-1c,h ).
SCHEME 2
(2) Syn th esis of P or p h yr in Bu ild in g Block s. The
cyclic hexamers of type I and II require trans-A2B2-
porphyrins while type III requires trans-AB2C-porphy-
rins. The rational synthesis of trans-A2B2-porphyrins is
achieved by the reaction of a dipyrromethane + aldehyde
or the self-condensation of a dipyrromethane-mono-
carbinol, while that of trans-AB2C-porphyrins proceeds
by the reaction of a dipyrromethane + dipyrromethane-
dicarbinol. The protected forms of the carboxy, amidine,
and thiol functional groups include the 2-(trimethylsilyl)-
ethyl ester, the bis[2-(trimethylsilyl)ethyl carbamate],
and the S-acetylthio ester, respectively. Several mono-
meric porphyrins bearing amidino groups have been
prepared, in which case protecting groups were not
employed because further synthetic manipulations were
not performed.24
Dip yr r om eth a n e P r ecu r sor s. The reaction of 4-(S-
acetylthiomethyl)benzaldehyde19 with excess pyrrole un-
der TFA catalysis in a one-flask process25 afforded
dipyrromethane 5a in 45% yield (eq 1). The known
8202 J . Org. Chem., Vol. 68, No. 21, 2003

Synthesis of Cyclic Hexameric Porphyrin Arrays
SCHEME 3
direct acylation method. While all diacyldipyrromethanes
required herein incorporated the same acyl substituents
at the 1- and 9-positions, we employed both the direct
diacylation approach (method 1) and the stepwise acyl-
ation approach (method 2), the latter employing the same
acyl component in both steps to obtain the requisite
diacyldipyrromethanes. Thus, treatment of a dipyrro-
methane (5) with 5 equiv of EtMgBr in toluene followed
by 2.5 equiv of the corresponding acid chloride produced
a mixture of monoacyl and diacyldipyrromethanes, which
was separated by column chromatography. In this man-
ner (method 1), diacyldipyrromethanes 6a , 6d , 6e, and
6f26 were obtained in modest yield.
Method 2 was applied in selected cases, requiring S-2-
pyridyl benzothioates. S-2-Pyridyl 3-iodobenzothioate
(7a ) was made previously;27 7b was prepared in 83% yield
by reaction of 4-[2-(triisopropylsilyl)ethynyl]benzoyl chlo-
ride26 and 2-mercaptopyridine. Thus, treatment of a
dipyrromethane (5) with EtMgBr in THF followed by a
pyridyl benzothioate (7a , 7b) afforded the monoacyl-
dipyrromethane (8a , 8b) in good yield (82%, 71%, respec-
tively). Monoacyldipyrromethanes 8c and 8d were pre-
pared according to the literature.27 The second acylation
of 8a -d with the requisite acid chloride afforded 6a -d
in 57-65% yield. The TIPS groups in diacyldipyr-
romethanes 6b and 6f were removed with TBAF in THF,
affording 6g and 6h in 62% and 71% yield, respectively
(eq 2).
tion of a diacyldipyrromethane (6) with NaBH4 in THF/
methanol afforded the corresponding dipyrromethane-
dicarbinol (6-OH). The latter was directly condensed with
a dipyrromethane (5) in acetonitrile in the presence of
TFA, followed by oxidation with DDQ. The Fb porphyrins
9-14 were obtained in 19-28% yield. Treatment of the
tr a n s-AB2C-p or p h yr in s. The synthesis of the desired
trans-AB2C-porphyrins is shown in Scheme 3. The reduc-
(24) (a) Kirby, J . P.; van Dantzig, N. A.; Chang, C. K.; Nocera, D.
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Chem. 2000, 65, 1084-1092.
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Tomizaki et al.
CHART 2
SCHEME 5
SCHEME 4
in 42% yield. Deprotection of the TMS group with TBAF
on silica gel afforded 19 in 92% yield. Treatment of 19
with methanolic zinc acetate afforded the zinc chelate Zn -
19. Alternatively, porphyrin Zn -19 was prepared by
metalation of 18, yielding Zn -18, followed by deprotection
of the TMS group affording Zn -19. While the yield of each
of these steps is >90%, the limited solubility of 19 in CH2-
Cl2/CHCl3 makes the latter route (via Zn -18) preferred
for the preparation of Zn -19.
Fb porphyrins with methanolic zinc acetate afforded the
corresponding zinc chelates in >90% yield.
tr a n s-A2B2-P or p h yr in s. Porphyrins 15-17 (Chart 2)
were prepared by the self-condensation of a dipyr-
romethane-monocarbinol.27 The synthesis of a trans-A2B2-
porphyrin lacking nonlinking meso-substituents is shown
in Scheme 4. A wide variety of related diarylporphyrins
have been prepared by Boyle and co-workers.28 The
reaction of dipyrromethane 5f and 4-[2-(trimethylsilyl)-
ethynyl]benzaldehyde under minimal scrambling condi-
tions29 afforded the TMS-protected ethynyl porphyrin 18
A porphyrin bearing two amidino groups was prepared
as shown in Scheme 5. Condensation of dipyrromethane
5e with the appropriate aldehyde in CH2Cl2 under TFA
catalysis afforded 20 or 21 in 17% or 8.8% yield, respec-
tively. The solubility of 21 was extremely poor in CHCl3,
CH2Cl2, or toluene. The zinc chelates Zn -20 and Zn -21
were treated with Weinreb’s amide transfer reag-
ent [AlCl(CH3)(NH2), freshly prepared from equimolar
amounts of ammonium chloride and Al(CH3)3 in tolu-
ene]30 at 80 °C for 3-4 days, affording Zn -22 and Zn -23
(28) (a) Elgie, K. J .; Scobie, M.; Boyle, R. W. Tetrahedron Lett. 2000,
41, 2753-2757. (b) Sutton, J . M.; Clarke, O. J .; Fernandez, N.; Boyle,
R. W. Bioconjugate Chem. 2002, 13, 249-263.
(29) Littler, B. J .; Ciringh, Y.; Lindsey, J . S. J . Org. Chem. 1999,
64, 2864-2872.
8204 J . Org. Chem., Vol. 68, No. 21, 2003

Synthesis of Cyclic Hexameric Porphyrin Arrays
SCHEME 6
F IGURE 1. Analytical SEC traces in the synthesis of Zn 3F b3-
1c. (A) The crude reaction mixture obtained with AsPh3/Pd2-
(dba)3. Peaks from tR ) 6-9 min are attributed to high
molecular weight material (HMWM). (B) Purified cyclic hex-
amer Zn 3F b3-1c.
with a molar ratio of Zn -2:3:4:Pd2(dba)3:AsPh3 ) 1.0/1.0/
1.0/0.30/2.4 in toluene/TEA (5:1) at 35 °C (Scheme 1).10
We investigated two changes to the reaction conditions:
(1) We changed the ratio of Zn -2:3:4 to 1.0/1.0/0.33. (2)
We examined the use of P(o-tol)3 in place of AsPh3, which
afforded diminished side reactions in the synthesis of
diarylethyne-linked multiporphyrin arrays, thereby fa-
cilitating separation and purification.35 In the one case
examined (Zn 3F b3-1e), the isolated yield was 5.1%
(AsPh3) or 1.8% (P(o-tol)3). Accordingly, each Pd-mediated
coupling was performed with use of the conditions
previously employed for the synthesis of Zn 3F b3-1a but
with a 1.0/1.0/0.33 ratio of Zn -2:3:4.
A set of survey reactions (10-mL scale, Schlenk line)
was performed for the synthesis of each of the cyclic
hexamers. The reactions make use of a refined synthesis
of template 4.11 Each reaction was completed within 1-3
h as indicated by analytical size exclusion chromatogra-
phy (SEC). The SEC traces of each crude mixture showed
high molecular weight material (HMWM) and the desired
cyclic hexameric array (Figure 1). The molecule ion peak
of most porphyrin monomers and multiporphyrin arrays
was readily detected by laser desorption mass spectrom-
etry (LDMS) without an added matrix,36 as we have
observed previously in the synthesis of Zn 3F b3-1a .10
However, LDMS spectra of the cyclic hexamers bearing
ester groups could not be obtained. Among several LDMS
matrices examined, only 1,4-bis(5-phenyloxazol-2-yl)-
benzene (POPOP) and dithranol clearly gave the molecule
ion peak for most of the cyclic hexamers. In general the
observed mass was within (0.2% of the expected mass.
The preparative syntheses of cyclic hexamers were
performed in the same manner and gave product distri-
butions that were essentially identical with those ob-
in 79% and 54% yield, respectively. Metalation of 20 and
21 with zinc was essential to avoid production of the
aluminum chelates Al-20 and Al-21 upon reaction with
AlCl(CH3)(NH2).
To improve the solubility of the amidino porphyrins,
we considered the established acetyl,31 ethoxycarbonyl,32
and benzyloxycarbonyl33 protecting groups for the ami-
dino functionality, but our requirements for deprotection
under gentle conditions at the cyclic hexamer stage led
to use of the 2-(trimethylsilyl)ethoxycarbonyl group. The
TMS-ethyne groups in Zn -22 were deprotected with
TBAF in methanol. The resulting ethynyl porphyrin was
treated with 2-(trimethylsilyl)ethyl p-nitrophenyl carbon-
ate (24)34 in the presence of NaOH in THF/H2O (1:1),
affording the putative Namino-protected porphyrin. Limited
solubility of this species caused difficulties in chromato-
graphic purification. Accordingly, treatment with 24 in
the presence of NaH in THF afforded Zn -25 in 25%
overall yield. Similarly, Zn -23 was demetalated with TFA
and the amidino moieties were protected with 2-(tri-
methylsilyl)ethoxycarbonyl groups, affording 26 in 38%
yield (Scheme 6).
(3) Syn th esis of Cyclic Hexa m er s. The conditions
for the one-flask reaction of diethynyl porphyrin Zn -2 and
diiodo porphyrin 3 with template 4 to give the cyclic
hexamer Zn 3F b3-1a employed each porphyrin at 2.5 mM
(30) (a) Basha, A.; Lipton, M.; Weinreb, S. M. Tetrahedron Lett.
1977, 4171-4174. (b) Levin, J . I.; Turos, E.; Weinreb, S. M. Synth.
Commun. 1982, 12, 989-993. (c) Garigipati, R. S. Tetrahedron Lett.
1990, 31, 1969-1972.
(31) Luckenbach, G. Chem. Ber. 1884, 17, 1421-1428.
(32) (a) Olin, J . F.; Dains, F. B. J . Am. Chem. Soc. 1930, 52, 3322-
3327. (b) Massa, S.; Di Santo, R.; Artico, M. J . Heterocycl. Chem. 1990,
27, 1131-1133.
(33) Su, T.; Naughton, M. A. H.; Smyth, M. S.; Rose, J . W.; Arfsten,
A. E.; McCowan, J . R.; J akubowski, J . A.; Wyss, V. L.; Ruterbories, K.
J .; Sall, D. J .; Scarborough, R. M. J . Med. Chem. 1997, 40, 4308-4318.
(34) Rosowsky, A.; Wright, J . E. J . Org. Chem. 1983, 48, 1539-1541.
(35) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J . S. Chem.
Mater. 1999, 11, 2974-2983.
(36) Srinivasan, N.; Haney, C. A.; Lindsey, J . S.; Zhang, W.; Chait,
B. T. J . Porphyrins Phthalocyanines 1999, 3, 282-291.
J . Org. Chem, Vol. 68, No. 21, 2003 8205

Tomizaki et al.
TABLE 1. Syn th esis of Cyclic Hexa m er s
precursors
type
yield, %
cyclic hexamer
Zn -2 + 3
I
I
I
II
II
III
III
III
5.3
4.1
0.7
5.9
5.1
8.7
9.6
3.6
Zn 3F b3-1a
Zn 3F b3-1b
Zn 3F b3-1ca
Zn 3F b3-1d
Zn 3F b3-1e
Zn 3F b3-1f
Zn 3F b3-1g
Zn 3F b3-1h
Zn -16 + 17
Zn -25 + 26
Zn -19 + 17
Zn -15 + 17
Zn -9 + 12
Zn -10 + 13
Zn -11 + 14
a
Characterization data were insufficient to establish identity.
tained in survey reactions. Chromatography on silica
removed non-porphyrin species and some of the HMWM
species, while preparative SEC (repetitive) removed the
HMWM species and gave the pure cyclic hexamer. In this
manner, the cyclic hexamers Zn 3F b3-1b,d -g were ob-
tained in yields ranging from 3.6% to 9.6% (Table 1). The
product of the reaction leading to cyclic hexamer Zn 3F b3-
1c was obtained in 0.7% yield. Several all-Zn hexamers
[Zn 6-1b,d ,f,g] were prepared by treatment of the corre-
sponding arrays with zinc acetate.
F IGURE 2. Voltammetry of Zn 3F b3-1g in a SAM on Au. The
solvent was CH2Cl2 containing 1.0 M Bu4NPF6; the scan rate
was 100 V s-1
.
potentials are similar to those of monomeric Zn and Fb
porphyrins, indicative of the weak interactions between
the constituents of the hexamer.10
The electrochemical behavior of Zn 3F b3-1g self-as-
sembled on gold was also investigated by using methods
employed previously (see Supporting Information). The
fast-scan voltammogram (100 V/s) of the Zn 3F b3-1g SAM
is shown in Figure 2. The general appearance of the
voltammogram of the SAM is similar to that observed in
solution. Two distinct waves are observed at ∼0.68 and
∼1.27 V. These waves straddle a broader wave centered
at ∼1.0 V. The waves contributing to the broad wave are
modestly resolved and occur at ∼0.98 and ∼1.03 V. The
waves at ∼0.68 and 1.27 V are attributed to the first
oxidation of the three Zn porphyrins and the second
oxidation of the three Fb porphyrins. The wave at 1.03
V is attributed to the second oxidation of the three Zn
porphyrins, whereas the wave at 0.98 V is attributed to
the first oxidation of the three Fb porphyrins. The
assignment of these waves to the Zn and Fb porphyrins,
respectively, is consistent with the trends observed for
monomeric Zn and Fb tetraarylporphyrins in solution
(the second oxidation of a Zn porphyrin is at a higher
potential than the first oxidation of the analogous Fb
porphyrin37). In addition, our previous studies of mono-
meric Zn porphyrin SAMs (in which the porphyrin is
similar to that of the constituent porphyrin in the
hexamer) have shown that the first and second waves
occur near ∼0.70 and ∼1.05 V.19
The cyclic hexameric porphyrin arrays Zn 3F b3-1b,d -g
were characterized by TLC analysis, analytical SEC,
1
MALDI-MS, UV-vis absorption spectroscopy, H NMR
spectroscopy, and fluorescence spectroscopy. The cyclic
hexamers were sufficiently soluble in THF-d8 [Zn 3F b3-
1d and Zn 6-1d ,g] or CDCl3 [Zn 3F b3-1b,e-h and Zn 6-
1b,f] to obtain well-resolved 1H NMR spectra. It was
1
difficult to obtain a clear H NMR spectrum of putative
hexamer Zn 3F b3-1c due to the small amount obtained
(∼1.0 mg). While analytical SEC of the purified product
showed a single sharp peak at tR ) 9.29 min, no firm
evidence is in hand to confirm the expected structure.
It is noteworthy that no significant change in yield
(5.9% versus 5.3%) was observed for the formation of the
cyclic hexamer in the synthesis of the less sterically
hindered cyclic hexamer Zn 3F b3-1d versus Zn 3F b3-1a .
The former bears a meso-H while the latter bears a meso-
mesityl group at the nonlinking position of the Zn
porphyrin. The template-directed formation of the cyclic
hexamer entails binding of tripyridyl template 4 at the
apical position of the Zn porphyrin. Thus, an enhanced
template effect was not observed upon removal of the
meso-mesityl groups.
(4) Electr och em ica l Stu d ies. The electrochemical
behavior of Zn 3F b3-1g was first investigated for a sample
in solution. As expected, the voltammogram of Zn 3F b3-
1g (not shown) exhibits wave characteristics of both Zn
and Fb tetraarylporphyrins. These waves are similar to
those observed for other tetraarylporphyrins.37 In par-
ticular, Zn 3F b3-1g exhibits two distinct oxidation waves
at potentials of ∼0.58 and ∼1.10 V, and a broad wave
centered near ∼0.87 V (versus Ag/Ag+; Fc/Fc+ ) 0.19 V).
The lowest potential wave corresponds to the first oxida-
tion of the three (equivalent) Zn porphyrins in the
hexamer. The broad wave corresponds to the second
oxidation of the three Zn porphyrins overlapped with the
first oxidation of three (equivalent) Fb porphyrins. The
highest potential wave corresponds to the second oxida-
tion of the three Fb porphyrins. The values of these
The fact that the oxidation potentials of the Zn and
Fb porphyrins in the Zn 3F b3-1g SAM occur at more
positive values than in solution is consistent with our
previous studies of a number of different types of por-
phyrin SAMs, including monomers and more complex
architectures.19-21 The key observation of the present
study is that the redox potentials of the three Zn
porphyrins in the Zn 3F b3-1g SAM are identical with one
another, as is also the case for the three Fb porphyrins.
The equivalent electrochemical behavior of the Zn (and
Fb) porphyrins in the hexamer is consistent with an
architecture in which all six porphyrins are in the same
proximity to the surface. Such a structure could result if
each of the six porphryins is attached to the surface via
its thiol linker. However, we have no experimental
evidence that all six of the thiols do in fact attach.
Regardless, in other architectures (for example, in which
one porphyrin is lying flat on the surface) the porphyrin
(37) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. V, pp 53-125.
8206 J . Org. Chem., Vol. 68, No. 21, 2003

Synthesis of Cyclic Hexameric Porphyrin Arrays
constituents would become electrochemically inequiva-
lent (because the various constituents would exhibit
different interactions with the surface). The relatively
simple pattern of redox waves for the Zn 3F b3-1g SAM
argues against the existence of any appreciable concen-
tration of architectures in which the porphyrins are
electrochemically inequivalent.
Finally, we note that the surface coverage of the
Zn 3F b3-1g SAM is ∼4.3 × 10-12 mol cm-2 (as determined
by integrating the first redox wave). This coverage
corresponds to a molecular area of ∼4000 Å2 per hex-
amer. This area is considerably larger than the molecular
footprint of the hexamer attached in a corral-like geom-
etry (at most 1000 Å2 per hexamer). Accordingly, the
coverage of the arrays on the surface is relatively sparse.
of gram quantities of the starting trans-A2B2- or trans-
AB2C-porphyrin building blocks.
The chemistry of cyclic hexamers has entailed multiple
types of self-assembly processes: (1) the template-
directed one-flask synthesis of the cyclic hexameric array
forming six covalent diphenylene linkers, (2) the nonco-
valent assembly of the wheel-and-spoke architecture
upon binding the tripyridyl template or the dipyridyl
porphyrin, and (3) the formation of a self-assembled
monolayer on an electroactive surface. The cyclic hex-
amers with functional groups arrayed along the wheel
perimeter now enable investigation of additional su-
pramolecular properties of these shape-persistent mac-
rocycles, including intermolecular interactions leading to
columnar structures.
Con clu sion s
Ack n ow led gm en t. This research was supported by
a grant from the NSF (CHE9988142). Mass spectra
were obtained at the Mass Spectrometry Laboratory for
Biotechnology at North Carolina State University.
Partial funding for the Facility was obtained from the
North Carolina Biotechnology Center and the NSF.
The template-directed one-flask synthesis of the cyclic
hexameric porphyrin array was applied in the prepara-
tion of six new derivatives. The one-flask synthesis is
compatible with sensitive groups, including protected
carboxyl groups, protected thiol groups, and protected
amidino groups. The cyclic hexamers have an alternating
pattern of porphyrins where the nonlinking meso-posi-
tions bear 6-12 carboxyl groups, 12 amidino groups, 6
thiol groups, or 6 thiol groups and 6 carboxyl groups. The
low yield of cyclic hexamer formation (3.6-9.6%) is offset
by the expediency of the synthesis and the availability
Su p p or tin g In for m a tion Ava ila ble: Complete experi-
mental procedures and spectral data (absorption, fluorescence,
1H NMR, LDMS) for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O034861C
J . Org. Chem, Vol. 68, No. 21, 2003 8207