Diverse Redox-Active Molecules Bearing Identical Thiol-Terminated Tripodal Tethers for Studies of Molecular Information Storage

Article abstract of DOI:10.1021/jo0349476

To examine the effects of molecular structure on charge storage in self-assembled monolayers (SAMs), a family of redox-active molecules has been prepared wherein each molecule bears a tether composed of a tripodal linker with three protected thiol groups

Full text of DOI:10.1021/jo0349476

Diver se Red ox-Active Molecu les Bea r in g Id en tica l
Th iol-Ter m in a ted Tr ip od a l Teth er s for Stu d ies of Molecu la r
In for m a tion Stor a ge
Lingyun Wei,† Kisari Padmaja,‡ W. J ustin Youngblood,‡ Andrey B. Lysenko,‡
J onathan S. Lindsey,*,‡ and David F. Bocian*,†
Department of Chemistry, University of California, Riverside, California 92521-0403, and Department of
Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
david.bocian@ucr.edu; jlindsey@ncsu.edu
Received J uly 1, 2003
To examine the effects of molecular structure on charge storage in self-assembled monolayers
(SAMs), a family of redox-active molecules has been prepared wherein each molecule bears a tether
composed of a tripodal linker with three protected thiol groups for surface attachment. The redox-
active molecules include ferrocene, zinc porphyrin, ferrocene-zinc porphyrin, magnesium phthalo-
cyanine, and triple-decker lanthanide sandwich coordination compounds. The tripodal tether is
based on a tris[4-(S-acetylthiomethyl)phenyl]-derivatized methane. Each redox-active unit is linked
to the methane vertex by a 4,4′-diphenylethyne unit. The electrochemical characteristics of each
compound were examined in solution and in SAMs on Au. Redox-kinetic measurements were also
performed on the SAMs (with the exception of the magnesium phthalocyanine) to probe (1) the
rate of electron transfer in the presence of an applied potential and (2) the rate of charge dissipation
after the applied potential is disconnected. The electrochemical studies of the SAMs indicate that
the tripodal tether provides a more robust anchor to the Au surface than does a tether with a
single site of attachment. However, the electron-transfer and charge-dissipation characteristics of
the two tethers are generally similar. These results suggest that the tripodal tether offers superior
stability characteristics without sacrificing electrochemical performance.
In tr od u ction
rocenes and porphyrins attached via identical tethers on
Si gave comparable electron-transfer properties.4 Because
the molecules were tethered via a linker bearing only one
surface attachment group, the possibility existed that the
distinct electron-transfer characteristics stemmed from
different orientations with respect to the surface rather
than the different composition of the respective redox-
active units.
In the preceding paper, we described the synthesis of
phosphonic acid terminated tripodal tethers for attaching
redox-active groups to oxide surfaces.5 The motivation for
employing tripodal tethers was to enforce a vertical
orientation of the redox-active molecules at a fixed
distance from the surface. The tripod employed was built
around a tetraarylmethane bearing three phosphonic acid
groups. The tetrahedral projection of the aryl groups, of
which three provide surface attachment, causes the
fourth aryl group to project normal to the surface. For
studies on Au or Si, thiol tethers are ideal. A number of
tripodal tethers bearing sulfur-containing termini (SH,6-13
The rational design of molecular-based information
storage devices that rely on charge accumulation requires
an understanding of the electron-transfer dynamics of
diverse redox-active groups attached to electroactive
surfaces. In studies of self-assembled monolayers (SAMs)
of ferrocenes and porphyrins attached to Au via thiol
tethers, we found the two types of molecules to have
dramatically different electron-transfer characteristics.1,2
These differences encompassed both the electron-transfer
rates measured in the presence of applied potential and
the charge-dissipation rates in the absence of applied
potential. In particular, the electron-transfer kinetics for
the porphyrins were considerably slower (at least 10-fold)
than for the ferrocenes. Triple-decker sandwich coordina-
tion compounds composed of porphyrins, phthalocya-
nines, and lanthanide metals were found to exhibit
electron-transfer kinetics intermediate between those of
ferrocenes and porphyrins.2,3 On the other hand, fer-
† University of California.
‡ North Carolina State University.
(4) Roth, K. M.; Yasseri, A. A.; Liu, Z.; Dabke, R. B.; Malinovskii,
V.; Schweikart, K.-H.; Yu, L.; Tiznado, H.; Zaera, F.; Lindsey, J . S.;
Kuhr, W. G.; Bocian, D. F. J . Am. Chem. Soc. 2003, 125, 505-517.
(5) Loewe, R. S.; Ambroise, A.; Muthukumaran, K.; Padmaja, K.;
Lysenko, A. B.; Mathur, G.; Li, Q.; Bocian, D. F.; Misra, V.; Lindsey,
J . S. J . Org. Chem. 2004, 69, 1453-1460.
(6) Whitesell, J . K.; Chang, H. K. Science 1993, 261, 73-76.
(7) Fox, M. A.; Whitesell, J . K.; McKerrow, A. J . Langmuir 1998,
14, 816-820.
(1) Roth, K. M.; Liu, Z.; Gryko, D. T.; Clausen, C.; Lindsey, J . S.;
Bocian, D. F.; Kuhr, W. G. Molecules as Components of Electronic
Devices; ACS Symposium Series 844; American Chemical Society:
Washington, DC, 2003; pp 51-61.
(2) 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.
(3) 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.
10.1021/jo0349476 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/11/2003
J . Org. Chem. 2004, 69, 1461-1469
1461

Wei et al.
CHART 1
SCHEME 1
SAc,13-15 and SMe16) have been described. The tripods
incorporate a carbon atom,6-10,12,13,16 a silicon atom,14
or an adamantane unit11,15 at the vertex. A tetraaryl-
methane-based tripod has been constructed using build-
ing block 1 (Chart 1), which bears three S-acetylthio-
methyl groups and one bromo group.13 Several redox-
active molecules have been attached to the vertical arm
of tripod 1, including oligothiophenes10,12 and oligothio-
phene-fullerene dyads.13
In this paper, we describe the synthesis of a family of
redox-active molecules bearing a tripodal tether with
three protected thiol groups. The synthesis makes use
of 1,1,1-tris[4-(S-acetylthiomethyl)phenyl]-1-(4-X-phen-
yl)methane, where X ) bromo (1) or iodo (2). The redox-
active molecules include a ferrocene, a porphyrin, a
phthalocyanine, a ferrocene-porphyrin, and two lan-
thanide triple decker sandwich coordination compounds
(Chart 2). We then describe a series of electrochemical
studies designed to evaluate the redox characteristics and
the electron-transfer and charge-dissipation rates of the
SAMs of the various thiol-derivatized complexes on Au.17
Taken together, this study provides the foundation for
understanding the electron-transfer properties and ki-
netics of diverse redox-active molecules in a controlled
environment on an electroactive surface.
coupling reactions with porphyrinic substrates, use of an
aryl bromide requires elevated temperature whereas the
aryl iodide reacts at room temperature.18
Treating the known compound 319 via a Sandmeyer
reaction with aqueous KI provided 4 in 65% yield upon
recrystallization. Radical bromination of 4 afforded an
inseparable mixture of poorly soluble, brominated prod-
ucts. The mixture was treated with potassium thioac-
etate, whereupon chromatographic separation proved
facile, affording the desired product 2 in 28% yield
(Scheme 1).
Resu lts a n d Discu ssion
B. Red ox-Active Un its. Several redox-active mol-
ecules (Chart 3, 5-10) were studied. Compounds 5,20 6,21
7,22 9,23 and 1024 have been prepared previously. The
synthesis of ferrocene-porphyrin 8 is shown in Scheme
2. The mixed condensation of 5-mesityldipyrromethane
(11),25 4-ferrocenylbenzaldehyde (12),26,27 and 4-(3-meth-
yl-3-hydroxybut-1-yn-1-yl)benzaldehyde (13)3 was carried
1. Syn th esis. A. Tr ip od a l Teth er s. The S-acetylthio-
derivatized tripodal molecules 113 and 2 were chosen as
the key building blocks for coupling with a variety of
redox-active molecules. The iodo-tripod 2 was developed
so that the Pd-mediated coupling with an ethyne-
substituted redox-active species could employ milder
conditions than those for bromo-tripod 1. In Pd-mediated
(18) Loewe, R. S.; Lammi, R. K.; Diers, J . R.; Kirmaier, C.; Bocian,
D. F.; Holten, D.; Lindsey, J . S. J . Mater. Chem. 2002, 12, 1530-1552.
(19) Heim, C.; Affeld, A.; Nieger, M.; Vo¨gtle, F. Helv. Chim. Acta
1999, 82, 746-759.
(20) Ambroise, A.; Wagner, R. W.; Rao, P. D.; Riggs, J . A.; Hascoat,
P.; Diers, J . R.; Seth, J .; Lammi, R. K.; Bocian, D. F.; Holten, D.;
Lindsey, J . S. Chem. Mater. 2001, 13, 1023-1034.
(21) Youngblood, W. J .; Gryko, D. T.; Lammi, R. K.; Bocian, D. F.;
Holten, D.; Lindsey, J . S. J . Org. Chem. 2002, 67, 2111-2117.
(22) Yang, S. I.; Li, J .; Cho, H. S.; Kim, D.; Bocian, D. F.; Holten,
D.; Lindsey, J . S. J . Mater. Chem. 2000, 10, 283-296.
(8) Fox, M. A.; Li, W.; Wooten, M.; McKerrow, A.; Whitesell, J . K.
Thin Solid Films 1998, 327-329, 477-480.
(9) Siiman, O.; Burshteyn, A.; Maples, J . A.; Whitesell, J . K.
Bioconjugate Chem. 2000, 11, 549-556.
(10) Zhu, L.; Tang, H.; Harima, Y.; Yamashita, K.; Hirayama, D.;
Aso, Y.; Otsubo, T. Chem. Commun. 2001, 1830-1831.
(11) Kittredge, K. W.; Minton, M. A.; Fox, M. A.; Whitesell, J . K.
Helv. Chim. Acta 2002, 85, 788-798.
(12) Zhu, L.; Tang, H.; Harima, Y.; Yamashita, K.; Aso, Y.; Otsubo,
T. J . Mater. Chem. 2002, 12, 2250-2254.
(13) Hirayama, D.; Takimiya, K.; Aso, Y.; Otsubo, T.; Hasobe, T.;
Yamada, H.; Imahori, H.; Fukuzumi, S.; Sakata, Y. J . Am. Chem. Soc.
2002, 124, 532-533.
(14) Yao, Y.; Tour, J . M. J . Org. Chem. 1999, 64, 1968-1971.
(15) 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.
(16) Hu, J .; Mattern, D. L. J . Org. Chem. 2000, 65, 2277-2281.
(17) Note that the terms “thiol tether” and “thiol-derivatized” in the
(23) 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.
(24) Li, J .; Gryko, D.; Dabke, R. B.; Diers, J . R.; Bocian, D. F.; Kuhr,
W. G.; Lindsey, J . S. J . Org. Chem. 2000, 65, 7379-7390.
(25) (a) Lee, C.-H.; Lindsey, J . S. Tetrahedron 1994, 50, 11427-
11440. (b) 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.
context of SAMs are meant to imply
a
surface-attached species
(26) 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.
containing a Au-S bond.
1462 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules with Tripodal Thiol Tethers
CHART 2
out in CH2Cl2 containing 17.8 mM TFA at room temper-
ature. Such catalysis conditions afford reaction without
acidolysis of dipyrromethane 11.28 The oxidation condi-
tions adopted here were as reported in the literature for
the synthesis of ferrocene-porphyrins.26 Because of the
presence of the easily oxidizable ferrocene unit, the acidic
reaction mixture was neutralized with DIEA prior to
oxidation (because acids are known to increase the
oxidation potential of quinones),29 and the weaker oxidant
p-chloranil was used instead of DDQ. Chromatographic
purification yielded the desired free base porphyrin 14
in 32% yield. Treatment of the latter with zinc acetate
gave zinc porphyrin 15 in 40% yield. Reaction of 15 with
sodium hydroxide in refluxing toluene proceeded smoothly
to give zinc porphyrin 8 in 76% yield.
Several distinct types of triple deckers can be prepared
depending on the nature of the three ligands (porphyrin;
phthalocyanine) and choice of lanthanide metals.30 For
the studies described herein, we prepared a triple decker
of type (Pc)Ln(Pc)Ln(Por), where H2Pc is a phthalocya-
nine, H2Por is a porphyrin; and Ln is a trivalent lan-
thanide metal ion. The (Pc)Ln(Pc)Ln(Por) triple decker
was attractive because rational synthetic methodology31,32
has recently become available for preparation of such
“type c” triple deckers, replacing statistical methods that
afford a mixture of different types of triple deckers. For
the applications described herein, we employed Eu as the
lanthanide.
Ample quantities of triple decker 9 were available from
a rational synthesis.23 The prior synthesis of ethynyl
triple decker 10 was achieved via (1) a statistical reaction
of porphyrin 16, Eu(acac)3‚nH2O, and Li2Pc that gave
trimethylsilylethynyl triple decker 17 in 9.1% yield after
extensive chromatography, followed by (2) cleavage of the
trimethylsilyl group of 17.24 We repeated the synthesis
of 10 via a new rational approach to obtain sufficient
quantities for the preparation of the tripodal-linked
target compound. The rational approach31,32 entailed
treatment of porphyrin 16 with Eu(acac)3‚nH2O in 1,2,4-
trichlorobenzene (1,2,4-TCB) at reflux (ca. 230-240 °C)
to give the porphyrin half-sandwich complex. The forma-
tion of the half-sandwich complex is readily observed by
a characteristic change of the absorption spectrum.33
After forming the half-sandwich complex of 16, Eu(Pc)2
was added. Prolonged heating of Eu(Pc)2 in 1,2,4-TCB
at 230-240 °C affords the homoleptic “all-phthalocya-
nine” triple-decker [Eu2(Pc)3], as well as the free base
phthalocyanine as a result of fragmentation of the double
(27) (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.
(28) Littler, B. J .; Ciringh, Y.; Lindsey, J . S. J . Org. Chem. 1999,
64, 2864-2872.
(29) Fukuzumi, S.; Ishikawa, K.; Hironaka, K.; Tanaka, T. J . Chem.
Soc., Perkin Trans. 2 1987, 751-760.
(30) (a) Weiss, R.; Fischer, J . In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
CA, 2003; Vol. 16, pp 171-246. (b) Ng, D. K. P.; J iang, J . Chem. Soc.
Rev. 1997, 26, 433-442.
(31) Chabach, D.; De Cian, A.; Fischer, J .; Weiss, R.; El Malouli
Bibout, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 898-899.
(32) Gross, T.; Chevalier, F.; Lindsey, J . S. Inorg. Chem. 2001, 40,
4762-4774.
(33) (a) Wong, C.-P.; Venteicher, R. F.; Horrocks, W. D., J r. J . Am.
Chem. Soc. 1974, 96, 7149-7150. (b) Wong, C.-P. Inorg. Synth. 1983,
22, 156-162.
J . Org. Chem, Vol. 69, No. 5, 2004 1463

Wei et al.
CHART 3
SCHEME 2
each phthalocyanine in the latter bears four tert-butyl
substituents. Each (t-Bu)4-phthalocyanine ligand is com-
posed of a mixture of four regioisomers owing to the
position of substitution of the four tert-butyl groups at
the perimeter of the macrocycle.34 Accordingly, triple
decker 9 is a complex mixture of (inseparable) regioiso-
mers.3,32 The absence of regioisomers in 10 greatly
1
simplifies the H NMR spectrum, facilitating character-
ization.
C. Tr ip od a l Red ox-Active Com p ou n d s. The key
reaction for the synthesis of the target molecules (Chart
2) is the Pd-mediated Sonogashira coupling of a haloaryl
tripod and an ethynylaryl redox-active molecule (Scheme
4). The coupling reactions of 1 and one of each of the
ethynyl-substituted redox-active molecules (5-6, 8-10)
were carried out using conditions18,35 that have been
developed for use with porphyrinic substrates [Pd2(dba)3
and P(o-tol)3 in toluene/TEA at 65 °C]. Such conditions
enable reaction in dilute solution (∼2.5 mM) without the
use of copper cocatalysts. The coupling reaction of 2 and
7 was carried out similarly, using Pd2(dba)3 and AsPh3
in toluene/TEA at room temperature. The reactions were
decker Eu(Pc)2. A 33% excess of porphyrin 16 over Eu-
(Pc)2 was employed in the triple-decker synthesis. The
excess 16 in the final reaction mixture was readily
recovered by chromatography. This approach afforded
triple decker 17 in 34% yield based on the porphyrin
starting material (Scheme 3). Treatment with Bu4NF at
room temperature caused cleavage of the TMS group,
affording ethynyl triple decker 10 in 89% yield. A distinct
advantage of triple decker 10 versus 9 is that the former
incorporates unsubstituted phthalocyanines, whereas
(34) (a) Sommerauer, M.; Rager, C.; Hanack, M. J . Am. Chem. Soc.
1996, 118, 10085-10093. (b) Pondaven, A.; Cozien, Y.; L’Her, M. New
J . Chem. 1992, 16, 711-718. (c) Pondaven, A.; Cozien, Y.; L’Her, M.
New J . Chem. 1991, 15, 515-516.
(35) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J . S. Chem.
Mater. 1999, 11, 2974-2983.
1464 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules with Tripodal Thiol Tethers
SCHEME 3
SCHEME 4
monitored by analytical size-exclusion chromatography
(SEC),36 laser desorption mass spectroscopy (LDMS),37
and TLC. The target compound from each reaction was
purified by column chromatography on silica and, in some
cases, preparative SEC. The isolated yields ranged from
14% to 65%. This route provided sufficient quantities for
a variety of physical studies.
2. Electr och em ical Stu dies. A. Voltam m etr ic Ch ar -
a cter istics of th e Com p lexes in Solu tion . The solu-
tion electrochemical characteristics of the various tripodal
complexes were examined prior to the studies of the
SAMs of the complexes. The solution voltammetric
characteristics of F c-Tp d (Fc0/1+ ∼0.2 V), Zn P -Tp d
(ZnP0/1+ ∼0.6 V; ZnP1+/2+ ∼0.9 V), F c-Zn P -Tp d (Fc0/1+
∼0.2 V; ZnP0/1+ ∼0.6 V; ZnP1+/2+ ∼0.9 V), Bu 8TD-Tp d
(TD0/1+ ∼0.1 V; TD1+/2+ ∼0.6 V; TD2+/3+ ∼1.0 V; TD3+/4+
∼1.3), and TD-Tp d (TD0/1+ ∼0.2 V; TD1+/2+ ∼0.6 V;
TD2+/3+ ∼1.0 V; TD3+/4+ ∼1.3 V) (not shown) are es-
sentially identical to those we have previously reported
for the complexes with the same redox centers that
contain a single S-acetylthio group rather than the
tripodal S-acetylthio tether.1-3,26,38 This is expected be-
cause the linker is relatively long and the S-acetylthio
group exerts little influence on the redox center.
The solution voltammetry of MgP c-Tp d exhibits two
oxidative waves (E1 ∼0.15 V; E2 ∼0.90 V) (not shown).
Previous studies of magnesium phthalocyanines have
typically reported the lower of these two waves, whereas
the higher wave has been reported less frequently.39 The
absorption spectrum of the solution of MgP c-Tp d ex-
hibited a doublet for the lowest-energy absorption band.
(36) Wagner, R. W.; J ohnson, T. E.; Lindsey, J . S. J . Am. Chem.
Soc. 1996, 118, 11166-11180.
(37) (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.
(38) Gryko, D. T.; Clausen, C.; Roth, K. M.; Dontha, N.; Bocian, D.
F.; Kuhr, W. G.; Lindsey, J . S. J . Org. Chem. 2000, 65, 7345-7355.
J . Org. Chem, Vol. 69, No. 5, 2004 1465

Wei et al.
Additional electrochemical studies wherein the solution
was successively diluted resulted in attenuation of the
intensity of both the lower potential voltammetric wave
and the higher-energy member of the absorption doublet.
However, the lower-potential wave still persisted, even
at the lowest concentrations (µM) where the signals were
visible. These observations are consistent with the lower-
potential voltammetric wave/higher-energy absorption
band corresponding to a π-stacked aggregate (perhaps a
face-to-face dimer). The fact that the aggregate persists
at µM concentrations indicates that the π-stacking
interactions are very strong. The pronounced tendency
of phthalocyanines to aggregate, even in dilute solution,
is well-known.40 In light of these observations, much of
the literature, which reports low potentials for the
oxidation of metal phthalocyanines, must be viewed with
caution.
F IGURE 1. Representative fast-scan (100 V s-1) cyclic vol-
tammograms of the F c-Tp d (Γ ) ∼1.6 × 10-11 mol cm-2), Zn P -
Tp d (Γ ) ∼1.1 × 10-11 mol cm-2), F c-Zn P -Tp d (Γ ) ∼0.84 ×
B. Volta m m etr ic Ch a r a cter istics of th e Com -
p lexes in SAMs. Each S-acetyl-protected tripodal com-
plex was examined for formation of a SAM on Au
microelectrodes. S-Acetylthio groups are known to un-
dergo cleavage upon exposure to gold, resulting in surface
attachment (via an S-Au bond) and formation of the
SAM without handling free thiols.41 The procedures and
instrumentation for the SAM electrochemical measure-
ments have been described.42,43
10-11 mol cm-2), and TD-Tp d (Γ ) ∼0.86 × 10-11 mol cm-2
)
SAMs. The solvent overlayer was CH2Cl2 containing 1.0 M Bu4-
NPF6.
face dimer wherein the tripod of one unit is bound to the
Au surface and the tripod of the other points into
solution.
Representative fast scan cyclic voltammograms (100
V s-1) of the F c-Tp d (Fc0/1+ ∼0.39 V), Zn P -Tp d (ZnP0/1+
∼0.71 V; ZnP1+/2+ ∼1.03 V), F c-Zn P -Tp d (Fc0/1+ ∼0.38
V; ZnP0/1+ ∼0.72 V; ZnP1+/2+ ∼1.05 V), and TD-Tp d
(TD0/1+ ∼0.33 V; TD1+/2+ ∼0.78 V; TD2+/3+ ∼1.28 V;
TD3+/4+ ∼1.47 V) assemblies are shown in Figure 1. The
general voltammetric characteristics of the assemblies
are consistent with the formation of relatively homoge-
neous SAMs wherein the complexes are tethered to Au
via the terminal sulfur atoms. The voltammetric char-
acteristics of the Bu 8TD-Tp d SAM (TD0/1+ ∼0.44 V;
TD1+/2+ ∼0.88 V; E TD2+/3+ ∼1.34 V; TD3+/4+ ∼1.58) (not
shown) are generally similar to those of the TD-Tp d
SAM; however, the waves are somewhat broader, indi-
cating a less homogeneous assembly. Interestingly, the
voltammetry of the MgP c-Tp d SAMs exhibits two oxida-
tive waves (E1 ∼0.38 V; E2 ∼1.1 V) (not shown), just as
are observed in solution. The intensity of the lower-
potential wave is attenuated (relative to the higher
potential wave) as the concentration of the complex in
the deposition solution is lowered. This observation
strongly suggests that both the monomeric and ag-
gregated MgP c-Tp d self-assemble on the Au surface.
One plausible structure for this aggregate is a face-to-
The voltammetric characteristics of the SAMs of the
different tripodal ferrocene and porphyrin complexes are
generally similar to those we have previously observed
for SAMs of complexes with the same redox center and
monothiol tethers.1-3,26,38 As is the case for the SAMs of
monothiol-tethered complexes, the formal potential of
each of the waves of the tripodal-thiol-tethered complexes
in the SAMs is more positive than that observed in
solution.17 The magnitudes of the positive shifts are
typically 0.2 V. One clear difference between the volta-
mmetric behavior of the SAMs of the tripodal-thiol-
versus monothiol-derivatized complexes is with respect
to stability under repeated voltammetric cycling. Under
ambient conditions, the tripodal complexes appear to be
considerably more stable, as is evidenced by the fact that
these assemblies could be subjected to many more (∼5-
fold) voltammetric cycles before any loss of signal was
observed. This observation suggests that all three sulfur
atoms in the tripod bind to the Au surface, yielding a
generally more robust linkage.
C. Electr on -Tr a n sfer Ch a r a cter istics of th e SAMs.
The standard electron-transfer rate constants (k0) were
measured for each redox state of the F c-Tp d , Zn P -Tp d ,
F c-Zn P -Tp d , and TD-Tp d SAMs using swept waveform
AC voltammetry (SWAV). These studies were not per-
formed on the Bu 8TD-Tp d and MgP c-Tp d SAMs. A
detailed description of the method, the data analysis
procedure, and the relation of k0 to the measured
parameters (the ratio of the peak current to the back-
ground current (Ipeak/Ibkgd) as a function of frequency) has
been reported elsewhere.2 Previous studies of other types
of porphyrin SAMs have shown that the k0 values are
particularly sensitive to the surface concentration (Γ)
when the concentration falls in the range Γ ) ∼1 × 10-12
(39) L’Her, M.; Pondaven, A. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
CA, 2003; Vol. 16, pp 117-169.
(40) Snow, A. W. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003; Vol.
17, pp 129-176.
(41) (a) Tour, J . M.; J ones, L. II; Pearson, D. L.; Lamba, J . J . S.;
Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S.
V. J . Am. Chem. Soc. 1995, 117, 9529-9534. (b) Gryko, D. T.; Clausen,
C.; Lindsey, J . S. J . Org. Chem. 1999, 64, 8635-8647.
(42) Schweikart, K.-H.; Malinovskii, V. L.; Yasseri, A. A.; Li, J .;
Lysenko, A. B.; Bocian, D. F.; Lindsey, J . S. Inorg. Chem. 2003, 42,
7431-7446.
to ∼3 × 10-11 mol cm-2 2
. In this range, the electron-
transfer rates fall by an order of magnitude as the surface
concentration increases. Consequently, the surface-
(43) Bard, A. J .; Faulkner, L. R. Electrochemical Methods: Funda-
mentals and Applications; Wiley: New York, 2001.
1466 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules with Tripodal Thiol Tethers
F IGURE 2. Ipeak /Ibkgd (dotted lines) versus frequency for the various cationic states of the F c-Tp d , Zn P -Tp d , F c-Zn P -Tp d , and
TD-Tp d SAMs. The solid lines represent fits to the data using a Randles equivalent circuit. The fitting parameters are as
follows: F c-Tp d SAM: Γ ) ∼0.63 × 10-11 mol cm-2; k0 ) ∼5.0 × 104 s-1. Zn P -Tp d SAM: Γ ) ∼1.5 × 10-11 mol cm-2; ZnP0/1+, k0
) ∼4.0 × 104 s-1; ZnP1+/2+, k0 ) ∼3.1 × 104 s-1. F c-Zn P -Tp d SAM: Γ ) ∼0.84 × 10-11 mol cm-2; Fc0/1+, k0 ) ∼4.1 × 104 s-1
;
ZnP0/1+, k0 ) ∼4.0 × 104 s-1; ZnP1+/2+, k0 ) ∼3.4 × 104 s-1. TD-Tp d SAM: Γ ) ∼0.86 × 10-11 mol cm-2; TD0/1+, k0 ) ∼6.5 × 104
s-1; TD1+/2+, k0 ) ∼6.0 × 104 s-1; TD2+/3+, k0 ) ∼4.4 × 104 s-1; TD3+/4+, k0 ) ∼3.8 × 104 s-1
.
dependence of the standard electron-transfer rates was
also investigated.
the barrier to electron transfer through the tripodal
linker is much higher than the barrier through the zinc
porphyrin. Thus, electron transfer through the tripodal
linker is rate-limiting. This view is further supported by
the observation that the electron-transfer rates for the
first oxidation of the zinc porphyrin in the Zn P -Tp d SAM
are quite similar to those of the ferrocenes in the F c-
Tp d and F c-Zn P -Tp d SAMs (cf. Figure 3, top and
middle).
(2) The electron-transfer rates for the F c-Tp d SAM are
qualitatively similar to those of the first oxidation of the
Zn P -Tp d SAM. This observation is different from that
obtained for the SAMs of the monothiol-derivatized
complexes, wherein the electron-transfer rate for the
ferrocene was much faster than for the porphyrin.2
Together, these results strongly suggest that electron-
transfer rates for SAMs of the more flexible monothiol-
derivatized complexes are influenced by direct interac-
tions of the redox unit with the surface that are precluded
in the SAMS of the more rigid (and upright) tripodal-
thiol-derivatized complexes. As we have previously dis-
cussed, the porphyrin unit in the SAMs of the monothiol-
derivatized complexes is prevented from interacting with
the surface by the bulky peripheral substituent groups
and the tendency of porphyrins to pack in an upright
fashion.4
Representative plots of Ipeak/Ibkgd for the F c-Tp d , Zn P -
Tp d , F c-Zn P -Tp d , and TD-Tp d SAMs are shown in
Figure 2. The data for Ipeak were collected at En+/(n+1)+
the data for Ibkgd were collected 0.14 V below En+/(n+1)+
The k0 values obtained from the fits of the plots of Ipeak
;
.
/
Ibkgd versus frequency at different Γ values are presented
in Figures 3 and 4. Inspection of the kinetic data reveals
certain general trends: (1) The electron-transfer rates
for the SAMs of all the tripodal-thiol-derivatized com-
plexes (with the exception of F c-Tp d , see below) are
somewhat slower (∼20%) than SAMs of complexes with
the same redox center that contain a monothiol tether.2,3
This result is consistent with the fact that the monothiol-
tethered complexes have a phenylethynylphenylmethyl
group between the redox center and the sulfur atom,
whereas the tripodal-tethered complexes have this group
and an additional benzyl group (Chart 1). (2) The
electron-transfer rates for all of the oxidation states of
all of the SAMs decrease and then level off as the surface
coverage increases. (3) The electron-transfer rates for the
oxidation states of a particular SAM monotonically
decrease with increasing oxidation state. Both of the
latter two trends are also consistent with those we have
previously reported for SAMs of the analogous monothiol-
derivatized complexes.2,3
(3) The electron-transfer rates for the first and second
oxidations of the porphyrin in the Zn P -Tp d SAM are
generally faster than those of the analogous states for
the porphyrin unit in F c-Zn P -Tp d SAM (cf. Figure 3,
middle and bottom). This behavior is consistent with the
notion that the presence of the oxidized ferrocene unit
Closer inspection of the kinetics data in Figure 3
reveals certain other trends:
(1) The electron-transfer rates for the F c-Tp d SAM are
quite similar to those of the ferrocene unit in the F c-
Zn P -Tp d SAM (Figure 3, top). This result indicates that
J . Org. Chem, Vol. 69, No. 5, 2004 1467

Wei et al.
F IGURE 4. Plots of k0 versus Γ for the four cationic states of
the TD-Tp d SAMs.
were conducted only at selected coverages in the larger
data set for which electron-transfer rates were deter-
mined. The charge-dissipation rates were measured using
open circuit potential amperometry (OCPA). A detailed
description of the method and the data analysis proce-
dure has been reported elsewhere.44 In previous studies
of charge-retention of porphyrin and other triple-decker
SAMs, we have found that charge-dissipation rates follow
approximately first-order kinetics;1-3,45 thus, we have
characterized these rates in terms of a charge-retention
half-life (t1/2).
Representative charge-retention data sets are shown
for the F c-Zn P -Tp d SAM in Figure 5. The data obtained
for the SAMs of the other complexes is similar (not
shown). The left panels in the figure show the current-
decay transients of the three different oxidized states to
the neutral state (Fc1+-ZnP0-Tpd f Fc0-ZnP0-Tpd; Fc1+
-
ZnP1+-Tpd f Fc0-ZnP0-Tpd; Fc1+-ZnP2+-Tpd f Fc0-ZnP0-
Tpd). The four traces shown for each state are the
reductive current measured from the oxidized SAMs after
selected disconnect times (10, 20, 30, and 40 s) followed
by reconnection at the open circuit potential (wherein
molecules that have remained oxidized are reduced). The
right panels show the integrated current-decay transients
for each time for the three different oxidized states. The
integrated current data were fit to a first-order rate law
(r2 > 0.98) to determine the t1/2 values. The t1/2 values at
two different surface coverages for the F c-Tp d , Zn P -
Tp d , F c-Zn P -Tp d , and TD-Tp d SAMs are listed in
Table 1.
Inspection of the data shown in Table 1 reveals that
the charge-retention characteristics of the SAMs gener-
ally parallel the electron-transfer characteristics de-
scribed above. In particular, factors that decrease the
value of k0 also decrease the rate of charge dissipation
(i.e., increase t1/2). These factors include increasing
surface coverage or the presence of charge on the
molecule (in the case of the higher oxidation states). All
of these trends are consistent with those we have
F IGURE 3. Plots of k0 versus Γ for the various cationic states
of the F c-Tp d , Zn P -Tp d , and F c-Zn P -Tp d SAMs. The top
panel compares the rates for the oxidation of the ferrocene in
F c-Tp d and F c-Zn P -Tp d ; the middle panel compares the
rates for the first oxidation of the porphyrin in Zn P -Tp d and
F c-Zn P -Tp d ; the bottom panel compares the rates for the
second oxidation of the porphyrin in Zn P -Tp d and F c-Zn P -
Tp d .
of the latter complex produces a space-charge that
provides a barrier to further oxidation. A similar effect
is observed for the TD-Tp d SAM for which the rate of
each successive oxidation decreases monotonically (Fig-
ure 4). However, for the latter SAM, the space-charge
effects arise from oxidation of a lower-potential state of
a common, delocalized redox center (the π-stack of the
complex), whereas in F c-Zn P -Tp d , the effects arise from
the oxidation of the lower-potential ferrocene unit that
is spatially separated from the zinc porphyrin unit (and
distal with respect to the surface).
D. Ch ar ge-Reten tion Ch ar acter istics of th e SAMs.
The charge-retention characteristics of the F c-Tp d , Zn P -
Tp d , F c-Zn P -Tp d , and TD-Tp d SAMs were investi-
gated in parallel with the electron-transfer kinetics.
These measurements were made on the same SAMs used
for the electron-transfer studies described above. The
effect of surface coverage on the charge-retention char-
acteristics was also investigated; however, these studies
(44) Roth, K. M.; Lindsey, J . S.; Bocian, D. F.; Kuhr, W. G. Langmuir
2002, 18, 4030-4040.
(45) 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.
1468 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules with Tripodal Thiol Tethers
TABLE 1. Com p a r ison of Electr on -Tr a n sfer Ra tes (k0)
a n d Ch a r ge-Reten tion Tim es (t1/2) for th e SAMsa
Γ
k0
molecule
(× 10-11 mol cm-2
)
(× 104 s-1
)
t1/2 (s)
F c-Tp d
2.7
0.4
2.2
8.0
22
8
Zn P -Tp d
ZnP0/1+
0.9
2.5
0.9
2.5
10
5.5
8
17
45
21
53
ZnP1+/2+
4.5
F c-Zn P -Tp d
0.45
Fc0/1+
7.2
4.1
7
4
6.2
3.5
12
21
14
25
21
30
0.93
0.45
0.93
0.45
0.93
ZnP0/1+
ZnP1+/2+
TD-Tp d
TD0/1+
0.94
2.16
0.94
2.16
0.94
2.16
0.94
2.16
6.5
2.5
6
2.2
3.4
1.8
3.8
1.5
25
49
30
53
34
55
45
94
TD1+/2+
TD2+/3+
TD3+/4+
a
Alterations in the fitting parameters for the SWAV and OCPA
data that give equally good fits change the k0 and t1/2 values by
(15%.
wich complexes is relatively straightforward. (2) The
tripodal-thiol tether provides a more robust anchor to
electroactive surfaces such as Au than does the monothiol
tether. (3) The redox-kinetic characteristics exhibited by
tripodal-thiol-attached molecules are comparable to those
of monothiol-attached species. The fact that the tripodal-
thiol tethers offer superior stability characteristics with-
out sacrificing electrochemical performance renders these
tethers attractive candidates for incorporation in molec-
ular-based information storage media.
F IGURE 5. Current decay transients (left panels) and
integrated current-decay transients (right panels) for the three
cationic states of the F c-Zn P -Tp d SAMs.
previously reported for SAMs of the mono-thio-deriva-
tized complexes.2,3 Certain detailed features of the charge-
retention characteristics also parallel the electron-
transfer behavior. For example, the t1/2 value for the
monocation of F c-Zn P -Tp d (ferrocene oxidized) is quite
similar to that of the dication (both ferrocene and zinc
porphyrin singly oxidized). This result is consistent with
the notion that electron flow through the tripodal tether
involves a barrier much higher than that for electron
transfer from the ferrocene through the zinc porphyrin.
Ack n ow led gm en t. This work was supported by the
DARPA Moletronics Programs (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 NSF. We thank Dr. J . R. Diers
for assisting with certain of the solution electrochemical
measurements.
Con clu sion s
The studies reported herein indicate that the tripodal-
thiol tether is an attractive alternative to traditional
monothiol tethers. The attractiveness of the tether
encompasses both its synthetic and physical character-
istics as follows: (1) The preparation of the tether and
its attachment to diverse types of redox-active molecules
including ferrocenes, porphyrins, and triple-decker sand-
Su p p or tin g In for m a tion Ava ila ble: Complete experi-
mental procedures and relevant spectral data (absorption,
mass, and 1H NMR) for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O0349476
J . Org. Chem, Vol. 69, No. 5, 2004 1469