Porphyrins Bearing Mono or Tripodal Benzylphosphonic Acid Tethers for Attachment to Oxide Surfaces

Article abstract of DOI:10.1021/jo034946d

The ability to attach redox-active molecules to oxide surfaces in controlled architectures (distance, orientation, packing density) is essential for the design of a variety of molecular-based information storage devices. We describe the synthesis of a series of redox-active molecules wherein each molecule bears a benzylphosphonic acid tether. The redox-active molecules include zinc porphyrins, a cobalt porphyrin, and a ferrocene-zinc porphyrin. An analogous tripodal tether has been prepared that is based on a tris [4-(dihydroxyphosphorylmethyl)phenyl]-derivatized methane. A zinc porphyrin is linked to the methane vertex by a 1,4-phenylene unit. The tripodal systems are designed to improve monolayer stability and ensure vertical orientation of the redox-active porphyrin on the electroactive surface. For comparison purposes, a zinc porphyrin bearing a hexylphosphonic acid tether also has been prepared. The synthetic approaches for introduction of the phosphonic acid group include derivatization of a bromoalkyl porphyrin or use of a dimethyl or diethyl phosphonate substituted precursor in a porphyrin-forming reaction. The latter approach makes use of dipyrromethane building blocks bearing mono or tripodal dialkyl phosphonate groups. The zinc porphyrintripodal compound bearing benzylphosphonic acid legs tethered to a SiO₂ surface (grown on doped Si) was electrically well-behaved and exhibited characteristic porphyrin oxidation/reduction waves. Collectively, a variety of porphyrinic molecules can now be prepared with tethers of different length, composition, and structure (mono or tripodal) for studies of molecular-based information storage on oxide surfaces.

Full text of DOI:10.1021/jo034946d

P or p h yr in s Bea r in g Mon o or Tr ip od a l Ben zylp h osp h on ic Acid
Teth er s for Atta ch m en t to Oxid e Su r fa ces
Robert S. Loewe,† Arounaguiry Ambroise,† Kannan Muthukumaran,† Kisari Padmaja,†
Andrey B. Lysenko,† Guru Mathur,‡ Qiliang Li,‡ David F. Bocian,*,§ Veena Misra,*,‡ and
J onathan S. Lindsey*,†
Departments of Chemistry and Electrical and Computer Engineering, North Carolina State University,
Raleigh, North Carolina 27695-8204, and Department of Chemistry, University of California,
Riverside, California 92521-0403
jlindsey@ncsu.edu; vmisra@eos.ncsu.edu; david.bocian@ucr.edu
Received J uly 1, 2003
The ability to attach redox-active molecules to oxide surfaces in controlled architectures (distance,
orientation, packing density) is essential for the design of a variety of molecular-based information
storage devices. We describe the synthesis of a series of redox-active molecules wherein each
molecule bears a benzylphosphonic acid tether. The redox-active molecules include zinc porphyrins,
a cobalt porphyrin, and a ferrocene-zinc porphyrin. An analogous tripodal tether has been prepared
that is based on a tris[4-(dihydroxyphosphorylmethyl)phenyl]-derivatized methane. A zinc porphyrin
is linked to the methane vertex by a 1,4-phenylene unit. The tripodal systems are designed to
improve monolayer stability and ensure vertical orientation of the redox-active porphyrin on the
electroactive surface. For comparison purposes, a zinc porphyrin bearing a hexylphosphonic acid
tether also has been prepared. The synthetic approaches for introduction of the phosphonic acid
group include derivatization of a bromoalkyl porphyrin or use of a dimethyl or diethyl phosphonate
substituted precursor in a porphyrin-forming reaction. The latter approach makes use of dipyr-
romethane building blocks bearing mono or tripodal dialkyl phosphonate groups. The zinc porphyrin-
tripodal compound bearing benzylphosphonic acid legs tethered to a SiO2 surface (grown on doped
Si) was electrically well-behaved and exhibited characteristic porphyrin oxidation/reduction waves.
Collectively, a variety of porphyrinic molecules can now be prepared with tethers of different length,
composition, and structure (mono or tripodal) for studies of molecular-based information storage
on oxide surfaces.
In tr od u ction
period during which the oxidized molecules remained
charged (i.e., the charge-retention time) depends quite
sensitively on the length of the tether (linker and surface
attachment group). For example, as the number of
methylene groups in the tether phenyl-(CH2)n-S- in-
creased along the series 0, 1, 2, and 3, the charge-
retention time increased from 116, 167, 656, to 885 s.2
The rate of electron-transfer (reading process) also slowed
with increase of linker length.3 Moreover, the quality
(uniformity, integrity) of the self-assembled monolayers
(SAMs) increased in going from the phenylthio tether
(n ) 0) to the phenylalkylthio tethers (n ) 1-3). These
results prompted us to undertake the synthesis of por-
phyrins bearing tethers longer than the phenylphospho-
nic acid tethers described in the previous paper.
In the preceding paper we described the synthesis of
a variety of porphyrinic species bearing phenylphospho-
nic acid tethers for attachment to oxide surfaces.1 Our
chief application of such molecules is for studies of
molecular-based information storage.2 Upon attachment
to an electroactive oxide surface, the porphyrinic species
can be charged at a given electrochemical potential. The
writing process for information storage entails the bulk
oxidation of the molecules at the surface, and the reading
process entails the bulk reduction to the neutral state.
One of the considerable attractions of molecular infor-
mation storage is the ability to tune the properties of the
charge-storage molecules through molecular design. In
studies of thiol-derivatized porphyrins, we found that the
The tethers of interest include benzylphosphonic acid,
hexylphosphonic acid, and tripodal phosphonic acids. The
benzyl and hexyl linkers are longer than a phenyl unit,
and the tripodal tether is expected to anchor the redox-
active molecule in a 3-point contact and thereby enforce
a vertical orientation of the charge-storage molecule. The
† Department of Chemistry, North Carolina State University.
‡ Department of Electrical and Computer Engineering, North
Carolina State University.
§ University of California.
(1) Muthukumaran, K.; Loewe, R. S.; Ambroise, A.; Tamaru, S.-I.;
Li, Q.; Mathur, G.; Bocian, D. F.; Misra, V.; Lindsey, J . S. J . Org. Chem.
2004, 69, 1444-1452.
(2) Roth, K. M.; Dontha, N.; Dabke, R. B.; Gryko, D. T.; Clausen,
C.; Lindsey, J . S.; Bocian, D. F.; Kuhr, W. G. J . Vac. Sci. Technol., B
2000, 18, 2359-2364.
(3) Roth, K. M.; Gryko, D. T.; Clausen, C.; Li, J .; Lindsey, J . S.; Kuhr,
W. G.; Bocian, D. F. J . Phys. Chem. B 2002, 106, 8639-8648.
10.1021/jo034946d CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/11/2003
J . Org. Chem. 2004, 69, 1453-1460
1453

Loewe et al.
key design issues for tripods are (1) the nature of the
atom or molecular unit to which the three legs of the
tripod are attached, (2) the composition and length of the
tripod legs, and (3) the nature of the three terminal
groups for surface attachment. Diverse tripodal tethers
have been prepared for attaching molecules to surfaces.
Tripods containing a C atom,4-16 a Si atom,17,18 or an
adamantane13,19-22 unit at the central core of the tripod
have been prepared. The tripod legs include methyl,9,21
ethyl,8 propyl,4,6,7,9,10 alkyl ether,5 phenyl,13,19,20 ben-
zyl,11,12,14,16 biphenyl,15 diphenylethyne,17,22 oligophenyl,18
and oligoethynylphenyl22 structures. The terminal groups
include thiol,4,6,7,9,11,12,14,16,21 S-acetylthio,10,12,14,17,22 thio-
cyanate,9 alcohol,9,21 ester,5,13,19-21 carboxylic acid,5,8,13,20
allyl,18 diethyl phosphonate,15 or phosphonic acid15 groups.
Some of the tripods bear redox-active groups including
ferrocene,8,9 viologen,15 fullerene,5,12,14 ruthenium-tris-
(bpy),13,19 or oligothiophene12,14,16 units. Dendrimeric tri-
pods bearing more than three sites of attachment also
have been prepared.4,5,8,18,23
A tripod built around a tetraarylmethane structure
containing three terminal phosphonic acid groups ap-
peared most attractive for our purposes owing to the
rigid, compact, and tetrahedral architecture. The tripods
of this type that have been prepared incorporate meth-
ylthiol11,12,14,16 or ester13,19 termini attached to phenyl legs
or dialkyl phosphonate termini attached to biphenyl
legs.15 The synthesis of the thiol-terminated tetraaryl-
methane tripod proceeded through the valuable inter-
mediate 1,1,1-tris(4-bromomethylphenyl)(4-bromophenyl)-
methane.14 We felt that the route for preparing this
intermediate could be adapted to incorporate porphyrins
and phosphonic acid groups.
In this paper we describe the synthesis of a selection
of porphyrins bearing benzylphosphonic acid, hexylphos-
phonic acid, and tripodal phosphonic acid groups. We
then describe the electrochemical characteristics of a
porphyrin-tripodal phosphonic acid compound tethered
to a SiO2 dielectric layer on a Si platform. Taken together,
this work provides the basis for the design and synthesis
of porphyrin-linker-phosphonic acid constructs for stud-
ies of molecular information storage.
Resu lts a n d Discu ssion
1. Syn th esis. Zin c P or p h yr in s Bea r in g Sin gle
Tet h er s. (a ) Ben zylp h osp h on ic Acid Tet h er s. The
preparation of a porphyrin bearing one phosphonic acid
group requires the availability of a suitable phosphonate
aldehyde. Compound 3 was prepared in three steps
from commercially available R-bromo-p-toluic acid fol-
lowing a literature procedure without characterization
data (Scheme 1).24 An Arbuzov reaction of R-bromo-p-
toluic acid and triethyl phosphite afforded compound 1
in 77% yield. Reduction of 1 with borane-THF furnished
benzyl alcohol 2, which upon oxidation with PCC gave
aldehyde 3 in 77% yield (two steps). A mixed-aldehyde
condensation25 of 3, mesitaldehyde, and pyrrole at high
concentration26 using BF3‚O(Et)2/ethanol cocatalysis
(achieved by reaction in CHCl3 containing 0.75% etha-
nol)27 gave a mixture of porphyrins, from which the
desired A3B-porphyrin 4 was obtained in 9.4% yield. The
mixed-aldehyde condensation procedure is a statistical
process and was employed because rational routes are
not yet available for the synthesis of A3B-porphyrins
where A ) mesityl. Metalation of 4 with Zn(OAc)2‚2H2O
afforded Zn 4 in 94% yield. Treatment of Zn 4 to the same
conditions employed in the previous paper1 to cleave di-
tert-butyl groups [TMS-Br (15 equiv) and TEA (20 equiv)
in refluxing CHCl3] caused cleavage of the ethyl protect-
ing groups to afford porphyrin-benzylphosphonic acid
Zn 5 in 78% yield.
An alternate route to porphyrin 4 is shown in Scheme
2. A mixed-aldehyde condensation of R-bromo-p-tolual-
dehyde (6),28 mesitaldehyde, and pyrrole afforded the
desired A3B-porphyrin 7 bearing one bromomethylphenyl
group in 16% yield. This valuable porphyrin building
block, as with other bromomethylporphyrins,29 can be
functionalized with a wide variety of nucleophiles. For
example, treatment of 7 with triethyl phosphite in an
Arbuzov reaction or sodium diethyl phosphite in THF
gave porphyrin 4 in 80% or 73% yield, respectively. All
three routes afford porphyrin 4 in a straightforward
manner and differ mainly in the order of introduction of
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F. P. Tetrahedron 1999, 55, 7829-7834. (d) Buchler, J . W.; Simon, J .
R. Eur. J . Inorg. Chem. 2000, 2615-2621. (e) Salom-Roig, X. J .;
Chambron, J .-C.; Goze, C.; Heitz, V.; Sauvage, J .-P. Eur. J . Org. Chem.
2002, 3276-3280.
(21) Kittredge, K. W.; Minton, M. A.; Fox, M. A.; Whitesell, J . K.
Helv. Chim. Acta 2002, 85, 788-798.
(22) Li, Q.; Rukavishnikov, A. V.; Petukhov, P. A.; Zaikova, T. O.;
J in, C.; Keana, J . F. W. J . Org. Chem. 2003, 68, 4862-4869.
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1454 J . Org. Chem., Vol. 69, No. 5, 2004

Porphyrins with Tripodal Benzylphosphonic Acid Tethers
SCHEME 1
SCHEME 2
The synthesis of a porphyrin-phosphonic acid bearing
p-tolyl groups at all nonlinking meso positions is shown
in Scheme 3. The synthesis relies on the rational con-
densation of a dipyrromethane and a dipyrromethane-
dicarbinol.30 Reaction of 3 with excess pyrrole under TFA
catalysis afforded dipyrromethane 9 in 46% yield. The
condensation of 9 and 10-d iol31 in CH2Cl2 using InCl3
as catalyst32 followed by oxidation with DDQ afforded the
free base porphyrin. The reaction of crude free base
porphyrin with Zn(OAc)2‚2H2O gave the zinc porphyrin
Zn 11. However, the insolubility of Zn 11 in typical
solvents (CHCl3, THF, toluene, and mixtures thereof)
prevented analysis. A suspension of Zn 11 in CH2Cl2 was
treated with TFA, affording the free base porphyrin 11
in 12% overall yield. Free base porphyrin 11 showed good
solubility and was readily characterized.
the diethoxyphosphoryl group. Porphyrin 7 could also be
treated with trimethyl phosphite in an Arbuzov reaction
affording porphyrin 8 in 79% yield. Zinc insertion af-
forded porphyrin Zn 8 in 98% yield. The methyl groups
were cleaved under the same conditions employed for
Zn 4, affording porphyrin Zn 5 in 77% yield. On the basis
of this single comparison, the methyl and ethyl protect-
ing groups seem comparable in affording the correspond-
ing porphyrin-phosphonic acid.
(30) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J .; Lindsey, J . S. J .
Org. Chem. 2000, 65, 7323-7344.
(31) Gryko, D.; Lindsey, J . S. J . Org. Chem. 2000, 65, 2249-2252.
(32) Geier, G. R., III; Callinan, J . B.; Rao, P. D.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 2001, 5, 810-823.
J . Org. Chem, Vol. 69, No. 5, 2004 1455

Loewe et al.
SCHEME 3
SCHEME 4
(b) Hexylp h osp h on ic Acid Teth er . To explore the
effect of tether length on the electron-transfer properties
of porphyrin SAMs, we prepared a porphyrin that bears
a hexylphosphonic acid tether (Scheme 4). Condensation
of dipyrromethane 1233 and 10-d iol using InCl3 followed
by oxidation with DDQ afforded porphyrin 13 in 24%
yield. Metalation furnished Zn 13 in 85% yield. An
Arbuzov reaction of Zn 13 and triethyl phosphite afforded
porphyrin Zn 14 in quantitative yield. Treatment with
TMS-Br/TEA in refluxing CHCl3 gave porphyrin-hexyl-
phosphonic acid Zn 15 in 88% yield.
P or p h yr in Ar ch itectu r es for In cr ea sed Mem or y
Den sity. Molecules with an increased number of cationic
oxidation states can afford increased memory density. We
have employed this approach in the construction of
S-acetylthio-derivatized ferrocene-porphyrins34 as well as
other multiredox arrays.35 A ferrocene-porphyrin can
store two bits of information34 provided the porphyrin and
ferrocene are weakly coupled electronically: the porphy-
rin macrocycle can be cycled between three states (neu-
tral, monocation, dication) while the ferrocene provides
access to a fourth state (ferrocene monocation).
The synthesis of a ferrocene-porphyrin bearing a
dimethyl phosphonate group is outlined in Scheme 5.
Condensation of 4-ferrocenylbenzaldehyde (16),34,36 R-bro-
mo-p-tolualdehyde (6), and 5-mesityldipyrromethane (17)37
afforded a mixture of three porphyrins, which was treated
with trimethyl phosphite in an Arbuzov reaction. Chro-
matography afforded the dimethyl ferrocene-porphyrin-
phosphonate 18 in 20% yield. Metalation of 18 with
Zn(OAc)2‚2H2O gave the zinc chelate (∼40%), but this
(33) Gryko, D.; Li, J .; Diers, J . R.; Roth, K. M.; Bocian, D. F.; Kuhr,
W. G.; Lindsey, J . S. J . Mater. Chem. 2001, 11, 1162-1180.
(34) Gryko, D. T.; Zhao, F.; Yasseri, A. A.; Roth, K. M.; Bocian, D.
F.; Kuhr, W. G.; Lindsey, J . S. J . Org. Chem. 2000, 65, 7356-7362.
(35) Balakumar, A.; Lysenko, A. B.; Carcel, C.; Malinovskii, V. L.;
Gryko, D. T.; Schweikart, K.-H.; Loewe, R. S.; Yasseri, A. A.; Liu, Z.;
Bocian, D. F.; Lindsey, J . S. J . Org. Chem. 2004, 69, 1435-1443.
(36) (a) Coe, B. J .; J ones, C. J .; McCleverty, J . A.; Bloor, D.; Cross,
G. J . Organomet. Chem. 1994, 464, 225-232. (b) Imrie, C.; Loubser,
C.; Engelbrecht, P.; McCleland, C. W. J . Chem. Soc., Perkin Trans. 1
1999, 2513-2523.
(37) Littler, B. J .; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J . S. J . Org. Chem. 1999, 64, 1391-1396.
1456 J . Org. Chem., Vol. 69, No. 5, 2004

Porphyrins with Tripodal Benzylphosphonic Acid Tethers
SCHEME 5
SCHEME 6
of 19 with CuCN afforded 20 in 60% yield (76% based on
recovery of starting material 19). Radical bromination
of 20 using NBS (1.1 equiv per methyl group) and AIBN
in refluxing benzene furnished crude tribromo nitrile 21
in ∼90% purity. 1H NMR spectroscopy showed the
presence of unreacted p-tolyl resonances, indicating
incomplete bromination. The mono and dibromo products
were not easily removed from the reaction mixture; thus,
the crude material was carried forward. Reduction of
crude 21 with DIBALH gave aldehyde 22, which was
converted to the acetal (23) using TiCl4 in CH2Cl2/
methanol. Subsequent reaction with triethyl phosphite
at 100 °C for 6 h afforded 24 in 54% yield (from 20). The
acetal was cleaved during the acidic workup that was
employed to convert the odorous triethyl phosphite to
diethyl phosphite. Although each member of the series
of compounds 21-23 was ∼90% pure owing to the
presence of partially brominated species, 24 was obtained
in pure form. Condensation of 24 with excess pyrrole
under new reaction conditions (InCl3 as catalyst)39 af-
forded dipyrromethane 25 in 77% yield.
compound proved difficult to characterize because of its
insolubility in CHCl3, THF, and toluene. Attempted
deprotection of the zinc porphyrin-phosphonate also
afforded an intractable solid.
We have also explored the use of cobalt(II)porphyrins
to serve as molecules that can provide three cationic
oxidation states: the mono- and dication porphyrin
radicals and a metal-centered Co(II)/Co(III)38 oxidation.
The synthesis of a cobalt porphyrin-phosphonic acid is
shown in Scheme 6. Porphyrin 4 was treated with Co-
(OAc)2 to yield the cobalt porphyrin Co4 in 68% yield.
Cleavage of the ethyl protecting groups using the same
procedure described above (TMS-Br/TEA in refluxing
CHCl3) furnished the porphyrin-phosphonic acid Co5 in
92% yield.
Dipyrromethane 25 serves as a valuable synthetic
intermediate for condensation with dipyrromethane-
dicarbinols30 to afford porphyrins bearing a tripodal
phosphonate tether. Thus, condensation of 25 and dipyr-
romethane-dicarbinol 10-d iol31 with catalysis by InCl3
followed by oxidation with DDQ gave the free base
porphyrin. Metalation gave zinc porphyrin Zn 26 in 11.3%
overall yield. Deprotection using 5 equiv of TMS-Br and
6.7 equiv of TEA per phosphonate group afforded por-
phyrin Zn 27 in 82% yield (Scheme 8).
P or p h yr in s Bea r in g Tr ip od a l P h osp h on ic Acid
Teth er s. Our design for porphyrins bearing tripodal
phosphonic acid tethers incorporates a p-phenylene group
between the porphyrin and the methane-carbon vertex
of the tripod. The three legs of the tripod are provided
by benzylphosphonic acid groups. The synthesis we
developed proceeds via a dipyrromethane bearing the
tripod with protected phosphonic acid groups (Scheme
7).
The synthesis begins with 1-(4-bromophenyl)-1,1,1-tri-
p-tolylmethane (19).14 Rosenmund-von Braun reaction
Similarly, a porphyrin was prepared that bears a free
ethynyl group, which could be used for Sonogashira
(38) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
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Lindsey, J . S. Org. Process Res. Dev. 2003, 7, 799-812.
J . Org. Chem, Vol. 69, No. 5, 2004 1457

Loewe et al.
SCHEME 7
SCHEME 8
also characterized by 31P NMR spectroscopy using H3-
PO4 as an external standard. In a few cases, solubility
limited purification and analysis. Tri-p-tolylporphyrin
Zn 11 was sparingly soluble (CHCl3, THF, or toluene),
and trimesitylporphyrin Zn 4 displayed good solubility in
these solvents. The greater bulk of the mesityl versus
p-tolyl group suppresses cofacial aggregation between
porphyrins. The free base analogues of both porphyrins
display good solubility. Porphyrin Zn 18 was of such poor
solubility that no characterization data could be obtained,
whereas the free base analogue, 18, displayed good
solubility. The limited solubility of Zn 11 and Zn 18 but
not their free base analogues is attributed to coordination
of the dialkyl phosphonate of one porphyrin to the apical
site of the zinc porphyrin of another porphyrin. Each
porphyrin bearing a tripodal phosphonate tether dis-
played good solubility in common organic solvents. Por-
phyrin Zn 27, which bears three phosphonic acid groups,
was quite soluble in water as well as organic solvents.
oligomerization with porphyrin monomers.The ethynyl
unit was incorporated via 29-d iol, which was obtained
by treatment of ethynyl diacyldipyrromethane 2840 with
TBAF to give 29 followed by reduction with NaBH4. The
condensation of 25 and 29-d iol in CH2Cl2 with Yb(OTf)3
as catalyst32 followed by oxidation with DDQ gave the
free base porphyrin. Metalation afforded zinc porphyrin
Zn 30 in 24% yield (Scheme 9).
Ch em ica l Ch a r a cter iza tion a n d Solu bility P r op -
er ties. All porphyrins were characterized by absorption
spectroscopy, 1H NMR spectroscopy, LDMS,41 and FABMS.
The phosphonate-containing compounds generally were
(41) (a) Fenyo, D.; Chait, B. T.; J ohnson, T. E.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 1997, 1, 93-99. (b) Srinivasan, N.; Haney,
C. A.; Lindsey, J . S.; Zhang, W.; Chait, B. T. J . Porphyrins Phthalo-
cyanines 1999, 3, 283-291.
(40) Cho, W.-S.; Kim, H.-J .; Littler, B. J .; Miller, M. A.; Lee, C.-H.;
Lindsey, J . S. J . Org. Chem. 1999, 64, 7890-7901.
1458 J . Org. Chem., Vol. 69, No. 5, 2004

Porphyrins with Tripodal Benzylphosphonic Acid Tethers
SCHEME 9
F IGUR E 1. Cyclic voltammetry of Zn 27 on SiO2 (Tox
1.3 nm) with scan rates of 20 (lower amplitude) and 100
(higher amplitude) V s-1
)
.
of the monolayers. However, at higher scan rates, the
reduction currents are lower than the oxidation currents.
The diminished reduction current is attributed to limited
minority carriers available in the p-type semiconductor
at high scan rates. The integrated current (∼2.25 ×
1013 electrons/cm2) corresponds to a molecular coverage
on the SiO2 surface of 3.7 × 10-11 mol/cm2. These vol-
tammetric characteristics indicate that the porphyrin
bearing a tripodal phosphonic acid tether forms a robust,
electrically well-behaved monolayer on the SiO2 surface.
2. E lect r och em ica l St u d ies of Mon ola yer s on
SiO2. The electrochemical behavior was investigated for
the porphyrin tripod Zn 27 tethered to a thin layer of SiO2
grown on (100) p-type Si substrates (doping density 1 ×
1018 cm-3). The surface-attachment procedure (described
in the Supporting Information) entailed the deposition
of a solution of Zn 27 in DMF on the SiO2 substrate
followed by (1) brief heating to 170 °C and (2) washing
with organic solvents. A solution of propylene carbonate
containing 1.0 M Bu4NPF6 was placed on top of the
monolayer of molecules. A silver electrode was immersed
in electrolyte to provide the reference electrode.
The data shown in Figure 1 indicates that porphyrin
tripod Zn 27 has a larger dependence on scan rates, as
demonstrated by the peak splitting, compared to the
analogous porphyrin Zn 31 bearing a p-phenylphosphonic
acid tether. This larger scan-rate dependence is at-
tributed to slower redox kinetics, which stems from the
longer length of the tripodal tether. The linker joining
the porphyrin to a phosphonic acid surface attachment
group in the tripod encompasses a p-phenylene, saturated
carbon atom, and a p-benzyl motif. At present, we are
investigating the electron-transfer and charge-retention
characteristics of various redox-active molecules tethered
to SiO2 as a function of SiO2 thickness. Both the oxide
thickness and the tether length are expected to affect the
redox kinetics. Understanding such effects is essential
for the rational design of molecular-based information
storage devices.
Representative fast-scan cyclic voltammograms of mono-
layers of Zn 27 on the SiO2 layer (thickness, Tox
)
1.3 nm) are shown in Figure 1 as a function of two scan
rates (20 and 100 V s-1). Two distinct anodic and cathodic
current peaks are observed at both scan rates, which
correspond to the formation/neutralization of the mono-
and dication radicals of the porphyrin. The oxidation of
Zn 27 appears to occur at slightly lower voltages as
compared to an analogous porphyrin (Zn 31)1 bearing a
p-phenylphosphonic acid tether. At lower scan rates, the
integrated oxidation and reduction current in each of the
waves is approximately the same, indicating that the
same amount of charge is being transferred in and out
Con clu sion s
A set of porphyrins bearing dialkyl phosphonate groups
or phosphonic acids has been prepared for attachment
to metal oxide surfaces. The phosphonate groups are
tethered to the porphyrins via benzyl or hexyl linkers,
thereby constituting analogues of the porphyrins de-
J . Org. Chem, Vol. 69, No. 5, 2004 1459

Loewe et al.
scribed in the previous paper where phenylphosphonic
acid tethers were employed. The tripodal tether is built
around a compact, robust tetraarylmethane component.
The phosphonic acid group can be introduced to the
porphyrin by Arbuzov reaction of a bromoalkyl porphyrin
or via an appropriately derivatized precursor in a por-
phyrin-forming reaction. Tripodal porphyrin-phosphonic
acid Zn 27 formed a robust, electrochemically active SAM
on SiO2. The ability to readily attach redox-active mol-
ecules to metal oxide surfaces and exercise some control
over distance and orientation via the tether architecture
(including single versus tripodal surface attachment
groups) opens a number of opportunities for studies of
molecular-based information storage. We have extended
our work with tripodal tethers by attaching diverse
redox-active molecules to a thiol-derivatized tripod for
comparative studies of electron-transfer properties, as
described in the following paper.42
Ack n ow led gm en t. This work was supported by the
DARPA Moletronics Program (MDA972-01-C-0072) and
by ZettaCore, Inc. 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 Biotech-
nology Center and the National Science Foundation. We
thank Mr. Austin Kizzie for technical assistance.
Su p p or tin g In for m a tion Ava ila ble: Complete experi-
mental procedures and relevant spectral data (1H, 13C, and
31P NMR spectra and LD-MS spectra) for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
J O034946D
(42) Wei, L.; Padmaja, K.; Youngblood, W. J .; Lysenko, A. B.;
Lindsey, J . S.; Bocian, D. F. J . Org. Chem. 2004, 69, 1461-1469.
1460 J . Org. Chem., Vol. 69, No. 5, 2004