Diverse Redox-Active Molecules Bearing O-, S-, or Se-Terminated Tethers for Attachment to Silicon in Studies of Molecular Information Storage

Article abstract of DOI:10.1021/jo034944t

A molecular approach to information storage employs redox-active molecules tethered to an electroactive surface. Attachment of the molecules to electroactive surfaces requires control over the nature of the tether (linker and surface attachment group). We have synthesized a collection of redox-active molecules bearing different linkers and surface anchor groups in free or protected form (hydroxy, mercapto, S-acetylthio, and Se-acetylseleno) for attachment to surfaces such as silicon, germanium, and gold. The molecules exhibit a number of cationic oxidation states, including one (ferrocene), two [zinc(II)porphyrin], three [cobalt(II)porphyrin], or four (lanthanide triple-decker sandwich compound). Electrochemical studies of monolayers of a variety of the redox-active molecules attached to Si(100) electrodes indicate that molecules exhibit a regular mode of attachment (via a Si-X bond, X = O, S, or Se), relatively homogeneous surface organization, and robust reversible electrochemical behavior. The acetyl protecting group undergoes cleavage during the surface deposition process, enabling attachment to silicon via thio or seleno groups without handling free thiols or selenols.

Full text of DOI:10.1021/jo034944t

VOLUME 69, NUMBER 5
MARCH 5, 2004
© Copyright 2004 by the American Chemical Society
Diver se Red ox-Active Molecu les Bea r in g O-, S-, or Se-Ter m in a ted
Teth er s for Atta ch m en t to Silicon in Stu d ies of Molecu la r
In for m a tion Stor a ge
Arumugham Balakumar,† Andrey B. Lysenko,† Carole Carcel,† Vladimir L. Malinovskii,†
Daniel T. Gryko,† Karl-Heinz Schweikart,† Robert S. Loewe,† Amir A. Yasseri,‡ Zhiming Liu,‡
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 uly 1, 2003
A molecular approach to information storage employs redox-active molecules tethered to an
electroactive surface. Attachment of the molecules to electroactive surfaces requires control over
the nature of the tether (linker and surface attachment group). We have synthesized a collection
of redox-active molecules bearing different linkers and surface anchor groups in free or protected
form (hydroxy, mercapto, S-acetylthio, and Se-acetylseleno) for attachment to surfaces such as
silicon, germanium, and gold. The molecules exhibit a number of cationic oxidation states, including
one (ferrocene), two [zinc(II)porphyrin], three [cobalt(II)porphyrin], or four (lanthanide triple-decker
sandwich compound). Electrochemical studies of monolayers of a variety of the redox-active
molecules attached to Si(100) electrodes indicate that molecules exhibit a regular mode of attachment
(via a Si-X bond, X ) O, S, or Se), relatively homogeneous surface organization, and robust reversible
electrochemical behavior. The acetyl protecting group undergoes cleavage during the surface
deposition process, enabling attachment to silicon via thio or seleno groups without handling free
thiols or selenols.
In tr od u ction
approaches similar to those employed for derivatizing
chromatographic media; however, the use of siloxane
chemistry often affords polymeric multilayers that tend
to be insulative.3 Other oxides such as ZrO2 have been
derivatized with molecules bearing phosphonic acid teth-
ers.4
We have been engaged in a program aimed at con-
structing devices that use the properties of molecules to
store information. Our approach employs a collection of
redox-active molecules in a self-assembled monolayer
(SAM) attached to an electroactive surface.5 The generic
The attachment of electroactive molecules to diverse
surfaces including metals (e.g., Au), semiconductors (e.g.,
Si, SnO2, TiO2), and insulators (e.g., SiO2) is essential
for studies in the field of molecular electronics. Achieving
successful device properties requires strategies that
afford (1) uniform surface coverage, (2) a regular mode
of attachment, and (3) homogeneous organization of the
electroactive molecules. A large number of electroactive
molecules bearing thiol tethers have been prepared and
examined in self-assembled monolayers (SAMs) on Au
given the facile formation of Au-S bonds.1 Much less work
has been done to attach electroactive molecules to silicon,
though a number of approaches have been described for
derivatizing Si surfaces with small molecules.2 Attach-
ment of molecules to oxide surfaces has been investigated
for purposes that are quite distinct from molecular
electronics. For example, SiO2 has been derivatized using
(2) (a) Hamers, R. J .; Coulter, S. K.; Ellison, M. D.; Hovis, J . S.;
Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J . N., J r.
Acc. Chem. Res. 2000, 33, 617-624. (b) Buriak, J . M. Chem. Commun.
1999, 1051-1060. (c) Cleland, G.; Horrocks, B. R.; Houlton, A. J . Chem.
Soc., Faraday Trans. 1995, 91, 4001-4003. (d) Bansal, A.; Lewis, N.
S. J . Phys. Chem. B 1998, 102, 4058-4060. (e) Linford, M. R.; Fenter,
P.; Eisenberger, P. M.; Chidsey, C. E. D. J . Am. Chem. Soc. 1995, 117,
3145-3155. (f) Allongue, P.; de Villeneuve, C. H.; Pinson, J .; Ozanam,
F.; Chazalviel, J . N.; Wallart, X. Electrochim. Acta 1998, 43, 2791-
2798.
(3) Haller, I. J . Am. Chem. Soc. 1978, 100, 8050-8055.
(4) (a) Katz, H. E. Chem. Mater. 1994, 6, 2227-2232. (b) Gardner,
T. J .; Frisbie, C. D.; Wrighton, M. S. J . Am. Chem. Soc. 1995, 117,
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† North Carolina State University.
‡ University of California.
(1) (a) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (b) Gooding, J .
J .; Mearns, F.; Yang, W.; Liu, J . Electroanalysis 2003, 15, 81-96.
10.1021/jo034944t CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/11/2003
J . Org. Chem. 2004, 69, 1435-1443
1435

Balakumar et al.
design of the molecules includes a redox-active unit, a
linker, and a surface attachment group. Information is
stored in the distinct redox states of the molecules. As
such, the charge-storage molecules constitute molecular
capacitors. The surface attachment group serves a me-
chanical role in tethering the molecule to the surface and
an electronic role in providing communication between
the surface and the redox-active unit. The duration of
charge storage (i.e., charge-retention time) depends on
the nature of the redox-active unit and the length and
composition of the linker.
We have prepared a large number (100+) of redox-
active molecules that bear thiol-derivatized tethers. The
redox-active molecules include ferrocenes and a wide
variety of porphyrinic macrocycles in diverse molecular
architectures; the architectures include porphyrin mono-
mers,6,7 ferrocene-porphyrins,8 multiporphyrin arrays,9,10
triple-decker sandwich compounds composed of lan-
thanide metals coordinated by porphyrin and phthalo-
cyanine ligands,11,12 and dyads13 of such triple deckers.
The thiol-derivatized molecules have been examined in
SAMs on Au. The major objectives of the studies on Au
were to carry out fundamental studies of the effects of
molecular architecture and linker composition/length on
charge-retention properties and rates of electron trans-
fer.14-16 For practical applications, we recently examined
a much more limited selection of compounds on Si(100).17,18
The molecules examined were 4-(hydroxymethyl)phen-
ylferrocene (1-OH) and 5-[4-(hydroxymethyl)phenyl]-10,-
15,20-trimesitylporphinatozinc(II) (2-OH). For surface
dilution effects, the inert adsorbate biphenylmethanol (3-
OH) was employed. In each case, the benzyl alcohol
tether afforded an alkyl siloxane connection upon attach-
ment to the silicon surface. We also have prepared a
ferrocenylmethylphosphonic acid, which was attached to
a thin layer of SiO2 on a silicon substrate.19
In this paper, we describe the synthesis of a much more
diverse collection of candidate charge-storage molecules
that incorporate a terminal functional group suitable for
attachment to silicon. We also report the electrochemical
properties of a representative selection of the molecules
attached to silicon. One set of ferrocenes, porphyrins, and
biphenylmethyl derivatives has alcohol, thiol, or selenol
termini (in free or protected form) for comparative studies
of the effect of the attachment atom on electron-transfer
rates. The motivation for this study originated in part
from a report by Ratner, who predicted the conductivity
of a tether depends on the surface attachment atom,
increasing along the series O, S, and Se.20 A group of
porphyrin-alcohol/thiols was designed to have minimal
facial encumbrance, thereby offering no steric barrier to
lateral electron-transfer interactions in SAMs. Two triple
deckers have been prepared that incorporate alcohol
tethers of different length. The synthetic work described
herein spanned a period of years, during which the
synthetic methods for preparing porphyrins have evolved.
Accordingly, both traditional statistical methods and
more recent rational methods have been employed for the
synthesis of porphyrins.
This paper is the first in a series. The second paper in
this series describes methodology for preparing diverse
porphyrinic molecules bearing phenylphosphonic acid
tethers for attachment to oxides (e.g., SiO2, TiO2).21 The
third paper in the series describes methodology for
preparing porphyrins bearing benzylphosphonic acid
tethers or tripodal tethers composed of benzylphosphonic
acid legs.22 The tripodal tethers are employed to enforce
a vertical orientation of porphyrins at a defined distance
from the surface. The fourth paper in the series describes
tripodal tethers bearing a protected benzylthiol group on
each leg of the tripod and a redox-active molecule at the
vertex of the tripod.23 The redox-active molecules include
a ferrocene, a porphyrin, a phthalocyanine, a ferrocene-
porphyrin, and two examples of triple decker sandwich
compounds. Taken together, this work provides the
foundation for the synthesis of diverse redox-active
molecules suitable for attachment to a range of surfaces
for studies of information storage.
(5) 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.
(6) Gryko, D. T.; Clausen, C.; Lindsey, J . S. J . Org. Chem. 1999,
64, 8635-8647.
(7) 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.
(8) 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.
(9) Clausen, C.; Gryko, D. T.; Dabke, R. B.; Dontha, N.; Bocian, D.
F.; Kuhr, W. G.; Lindsey, J . S. J . Org. Chem. 2000, 65, 7363-7370.
(10) Clausen, C.; Gryko, D. T.; Yasseri, A. A.; Diers, J . R.; Bocian,
D. F.; Kuhr, W. G.; Lindsey, J . S. J . Org. Chem. 2000, 65, 7371-7378.
(11) 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.
(12) 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.
(13) Schweikart, K.-H.; Malinovskii, V. L.; Diers, J . R.; Yasseri, A.
A.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J . S. J . Mater. Chem. 2002,
12, 808-828.
(14) 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.
(15) Roth, K. M.; Lindsey, J . S.; Bocian, D. F.; Kuhr, W. G. Langmuir
2002, 18, 4030-4040.
(16) Roth, K. M.; Liu, Z.; Gryko, D. T.; Clausen, C.; Lindsey, J . S.;
Bocian, D. F.; Kuhr, W. G. ACS Symposium Series 2003, 844, 51-61.
(17) Li, Q.; Mathur, G.; Homsi, M.; Surthi, S.; Misra, V.; Malinovskii,
V.; Schweikart, K.-H.; Yu, L.; Lindsey, J . S.; Liu, Z.; Dabke, R. B.;
Yasseri, A.; Bocian, D. F.; Kuhr, W. G. Appl. Phys. Lett. 2002, 81,
1494-1496.
Resu lts a n d Discu ssion
A. F er r ocen es a n d P or p h yr in s Bea r in g Alcoh ol,
Th iol, or Selen oa ceta te Gr ou p s. We sought to prepare
ferrocenes, porphyrins, and inert analogues bearing
benzyl linkers with different heteroatom (O, S, or Se)
termini for attachment to silicon or gold. The structures
of the target compounds are shown in Chart 1. Several
of these compounds (1-OH,18 2-OH,18 2-SAc,7 3-SAc,24
3-SH25) are known, and 3-Cl and 3-OH are commercially
available. The general strategy for preparing the alcohols
(20) Yaliraki, S. N.; Kemp, M.; Ratner, M. A. J . Am. Chem. Soc.
1999, 121, 3428-3434.
(21) 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.
(22) 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.
(18) Roth, K. M.; Yasseri, A. A.; Liu, Z.; Dabke, R. R.; 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.
(19) Li, Q.; Surthi, S.; Mathur, G.; Gowda, S.; Sorenson, T. A.;
Tenent, R. C.; Kuhr, W. G.; Tamaru, S.-I.; Lindsey, J . S.; Liu, Z.;
Bocian, D. F.; Misra, V. Appl. Phys. Lett. 2003, 83, 198-200.
(23) Wei, L.; Padmaja, K.; Youngblood, W. J .; Lysenko, A. B.;
Lindsey, J . S.; Bocian, D. F. J . Org. Chem. 2004, 69, 1461-1469.
(24) Haslanger, M. F.; Varma, R. K.; Gordon, E. M. Ger. Offen. DE
3718450 A1, 1987.
1436 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules for Attachment to Silicon
CHART 1
SCHEME 1
is to synthesize the corresponding ester followed by
reduction to the alcohol. The general strategy for the
remaining compounds begins with a halomethyl deriva-
tive, available commercially or by direct synthesis. Treat-
ment of the halomethyl compound with KSAc or KSeAc
affords the thio or selenyl ester, which can be hydrolyzed
to give the free thiol or selenol if desired. The S-acetylthio
derivatives of diverse molecules are known to undergo
deprotection on exposure to Au, thereby enabling in situ
surface attachment via the Au-S linkage without han-
dling free thiols.6,26 We prepared several compounds
bearing free thiols for studies of surface coverage densi-
ties. The selenols are more air-sensitive than thiols and
readily form the corresponding diselenides.27 Tour has
prepared various protected selenols and studied their
deprotection chemistry.28 Given the sensitivity of free
selenols, we decided to maintain the selenyl derivatives
in the Se-acetylseleno form for attempted in situ depro-
tection or, if needed, chemical deprotection immediately
prior to use.
1-Br with KSAc in DMF29 at room temperature for 1 h
afforded the corresponding S-acetylthio-derivatized fer-
rocene (1-SAc) in 38% yield. In a similar way, KSeAc30
reacted with 1-Br generated in situ, affording 1-SeAc
in 29% yield. Treatment of 1-SAc in CH2Cl2 with metha-
nolic KOH at room temperature afforded the ferrocene-
thiol 1-SH in 61% yield.
The A3B-type porphyrin 2-Br , which bears three mesi-
tyl groups, is not available via rational synthetic meth-
ods. Synthesis of 2-Br was realized by a mixed-aldehyde
condensation31 at elevated concentration32 of pyrrole,
mesitaldehyde, and R-bromo-p-tolualdehyde7,33 with BF3‚
O(Et)2-ethanol cocatalysis34 (achieved by reaction in
CHCl3 containing ethanol as stabilizer). The resulting
mixture of porphyrins was treated with Zn(OAc)2‚2H2O
to give the zinc porphyrins. A mixture of porphyrins
where the porphyrins possess different degrees of facial
encumbrance is more readily separated upon forming the
zinc chelates because the facial encumbrance modulates
the affinity of the apical zinc site for adsorption media.31
We have previously prepared a hydroxymethyl-substi-
tuted ferrocene (1-OH) by reduction of the corresponding
ester.18 Treatment of 1-OH with PPh3 and CBr4 in dry
diethyl ether at room temperature afforded 1-Br in 46%
yield (Scheme 1). This compound was unstable on silica
and was used immediately in the next step. Reaction of
(25) Brown, T. J .; Chapman, R. F.; Cook, D. C.; Hart, T. W.; McLay,
I. M.; J ordan, R.; Mason, J . S.; Palfreyman, M. N.; Walsh, R. J . A.;
Withnall, M. T.; Aloup, J .-C.; Cavero, I.; Farge, D.; J ames, C.; Mondot,
S. J . Med. Chem. 1992, 35, 3613-3624.
(26) 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.
(29) Zheng, T.-C.; Burkart, M.; Richardson, D. E. Tetrahedron Lett.
1999, 40, 603-606.
(30) (a) Koketsu, M.; Nada, F.; Hiramatsu, S.; Ishihara, H. J . Chem.
Soc., Perkin Trans. 1 2002, 737-740. (b) Kageyama, H.; Takagi, K.;
Murai, T.; Kato, S. Z. Naturforsch. 1989, 44b, 1519-1523.
(31) Lindsey, J . S.; Prathapan, S.; J ohnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
(27) (a) Klayman, D. L. In Organic Selenium Compounds: Their
Chemistry and Biology; Klayman, D. L., Gu¨nther, W. H. H., Eds.; J ohn
Wiley & Sons: New York, 1973; pp 67-171. (b) Sonoda, N.; Ogawa,
A. In The Chemistry of Organic Selenium and Tellurium Compounds;
Patai, S., Rappoport, Z., Eds.; J ohn Wiley & Sons: New York, 1986;
Vol. 1, pp 619-665.
(32) Wagner, R. W.; Li, F.; Du, H.; Lindsey, J . S. Org. Process Res.
Dev. 1999, 3, 28-37.
(33) Wen, L.; Li, M.; Schlenoff, J . B. J . Am. Chem. Soc. 1997, 119,
7726-7733.
(28) Reinerth, W. A.; Tour, J . M. J . Org. Chem. 1998, 63, 2397-
2400.
(34) Lindsey, J . S.; Wagner, R. W. J . Org. Chem. 1989, 54, 828-
836.
J . Org. Chem, Vol. 69, No. 5, 2004 1437

Balakumar et al.
SCHEME 2
CHART 2
ing porphyrins. Porphyrins bearing pentyl groups at the
meso positions have very little steric hindrance, yet such
porphyrins exhibit high solubility in organic solvents.35
Four target molecules that incorporate pentyl groups at
the three nonlinking meso positions are shown in Chart
2. The linker is either a benzyl unit or a pentyl unit, while
the surface attachment groups include an alcohol (for
attachment to silicon) or a protected thiol (for attachment
to gold or silicon).
SCHEME 3
The synthesis of A3B-porphyrins 4a -c was achieved
by mixed-aldehyde condensation of pyrrole, hexanal, and
a given aldehyde followed by oxidation with DDQ and
chromatographic purification. The aldehydes employed
to introduce the tether were 5,36 6 (prepared from
6-bromohexanal37 and used immediately owing to insta-
bility), and methyl 4-formylbenzoate (7). The polarity
difference between the pentyl and ester/S-acetylthioester
groups enabled facile separation, affording the free base
porphyrins in ∼9%, 7%, and 16% yields, respectively.
Subsequent metalation gave the zinc chelates 8, 4b, and
9 in 64%, 63%, and 73% yields, respectively. Finally,
reduction of porphyrins 8 and 9 using excess LiAlH4
afforded the target porphyrin alcohols 4a and 4c in 87%
and 92% yields, respectively (Scheme 4).
A rational synthetic route was employed for the
synthesis of porphyrin 4d . Dipyrromethane 1038 and
dipyrromethane-dicarbinol 11-d iol (derived by reduction
of 1139) were condensed under new acid catalysis condi-
tions [Yb(OTf)3 in CH2Cl2 at room temperature]40 followed
by oxidation with DDQ, affording porphyrin 12 in 8.8%
yield. TLC and laser-desorption mass spectrometry in the
The required porphyrin 2-Br was separated by chroma-
tography (silica, hexanes/CH2Cl2) in 16% yield (Scheme
2). Compound 2-Br was treated with KSeAc to give
2-SeAc in 63% yield. Treatment of the known porphyrin
2-SAc with methanolic KOH in CH2Cl2 gave the porphy-
rin-thiol 2-SH in 78% yield.
To obtain the biphenylmethyl derivatives, 4-phenyl-
benzyl chloride was treated with KSAc in DMF to obtain
3-SAc in 92% yield, which upon hydrolysis with KOH in
methanol gave 3-SH in 74% yield. Compound 3-SAc has
been described in a patent,24 and the hydrolysis product
3-SH has been prepared as a reaction intermediate.25
Similar reaction with KSeAc in DMF gave 3-SeAc in 56%
yield (Scheme 3).
(35) Lindsey, J . S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.;
Marguerettaz, A. M. J . Org. Chem. 1987, 52, 827-836.
(36) Mu¨ller, K.; Prinz, H. J . Med. Chem. 1997, 40, 2780-2787.
(37) Trost, M.; Warner, R. W. J . Am. Chem. Soc. 1982, 22, 6112-
6114.
B. Ster ica lly Un en cu m ber ed P or p h yr in s Bea r in g
Alcoh ol or S-Acetylth io Gr ou p s. Our prior studies of
information storage have employed porphyrins bearing
mesityl groups at the nonlinking meso positions. Mesityl
groups suppress cofacial interactions between neighbor-
(38) Tomizaki, K.-Y.; Yu, L.; Wei, L.; Bocian, D. F.; Lindsey, J . S. J .
Org. Chem. 2003, 68, 8199-8207.
(39) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J .; Lindsey, J . S. J .
Org. Chem. 2000, 65, 7323-7344.
(40) Geier, G. R., III.; Callinan, J . B.; Rao, P. D.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 2001, 5, 810-823.
1438 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules for Attachment to Silicon
SCHEME 4
SCHEME 5
SCHEME 6
absence of a matrix (LDMS)41 showed the presence of
trace levels of meso-tetrapentylporphyrin, indicating the
occurrence of a low level of acidolysis leading to the
formation of undesired porphyrins (i.e., scrambling). A
low level of scrambling is characteristic of pentyl-
substituted dipyrromethane-dicarbinols such as 11-
d iol.40 The target free base porphyrin 12 was readily
purified. Metalation afforded the zinc chelate 4d in 66%
yield (Scheme 5).
C. P or p h yr in s Bea r in g Diver se Lin k er s a n d Su r -
fa ce Atta ch m en t Gr ou p s. An A3B-porphyrin bearing
a long alkyl tether was prepared to examine the effects
of a long, nonaryl spacer on the charge-retention proper-
ties. The long-chain aldehyde was prepared using the
strategy reported for 7-(S-acetylthio)heptanal7 (Scheme
6). Oxidation of 16-bromo-hexadecanol (13) with PCC on
Celite42 gave 16-bromohexadecanal (14, 55% yield), which
upon substitution with KSAc gave the desired S-acetylthio-
protected aldehyde 15 in 53% yield. A mixed-aldehyde
condensation of pyrrole, mesitaldehyde, and 15 at el-
evated concentration32 with BF3‚O(Et)2-ethanol coca-
(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.
(42) Kurth, M. J .; O’Brien, M. J .; Hope, H.; Yanuck, M. J . Org. Chem.
1985, 50, 2626-2632.
talysis34 followed by oxidation with DDQ afforded a
readily separable mixture of porphyrins. The free base
porphyrin was isolated (13% yield) and metalated to give
zinc porphyrin 16 in 11% overall yield.
J . Org. Chem, Vol. 69, No. 5, 2004 1439

Balakumar et al.
SCHEME 7
SCHEME 8
We also prepared an A3B-type porphyrin bearing a
phenol group to investigate the effects of attachment to
a silicon surface without the intervening methylene group
as in 2-OH. Zinc porphyrin 17 was prepared in 50% yield
by metalation of 5-(4-hydroxyphenyl)-10,15,20-trimesi-
tylporphyrin.43
dication porphyrin radicals and a metal-centered Co(II)/
Co(III) oxidation. Treatment of porphyrin 2122 with Co-
(OAc)2 afforded cobalt porphyrin 22 in 56% yield. Treat-
ment of the latter with KSAc in THF furnished the
S-acetylthio-derivatized cobalt porphyrin 23 in 70% yield
(Scheme 8).
2. Tr ip le-Deck er Sa n d w ich Com p ou n d s Bea r in g
Alcoh ol Gr ou p s. Triple-decker sandwich coordination
compounds (composed of two lanthanides and three
porphyrinic ligands)46 provide a straightforward means
for storage of multiple bits of information.11-13 To attach
triple deckers to silicon, we sought to prepare triple
deckers bearing alcohol tethers of different length. The
synthesis of triple deckers of composition (Pc)Eu(Pc)Eu-
(Por) (where Pc and Por indicate phthalocyaninato and
porphyrinato ligands, respectively) begins with the reac-
tion of a porphyrin and Eu(acac)3‚nH2O in refluxing 1,2,4-
trichlorobenzene (1,2,4-TCB).47,48 The resulting europium-
porphyrin half-sandwich complex49 is then treated with
a (Pc)Eu(Pc) double decker under continued reflux. This
method is superior to the statistical route wherein the
porphyrin, phthalocyanine, and europium salt are reacted
simultaneously, affording a mixture of several types of
triple deckers.
To investigate the effects of lateral interactions among
porphyrins in a SAM, we prepared a porphyrin bearing
two carboxy groups in the flanking “wing” positions. The
mixed condensation of 4-tert-butylbenzaldehyde, alde-
hyde 18,6 and dipyrromethane 1944 was carried out using
the minimal-scrambling conditions45 that were identified
from studies of dipyrromethane + aldehyde reactions for
use with unhindered aldehydes. The free base porphyrin
bearing two protected carboxy groups in a trans position
and one S-acetylthio tether was obtained in 5.1% yield
(Scheme 7). Subsequent metalation gave the target zinc
porphyrin 20 in 93% yield.
The first triple-decker alcohol was prepared by reaction
of porphyrin ester 2450 and Eu(acac)3‚nH2O in refluxing
D. Molecu les w ith In cr ea sed Nu m ber of Oxid a -
tion Sta tes. 1. Coba lt P or p h yr in s. We have explored
the use of cobalt(II) porphyrins to serve as molecules that
provide three cationic oxidation states: the mono- and
(46) (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.
(47) Chabach, D.; De Cian, A.; Fischer, J .; Weiss, R.; El Malouli
Bibout, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 898-899.
(48) Gross, T.; Chevalier, F.; Lindsey, J . S. Inorg. Chem. 2001, 40,
4762-4774.
(49) (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.
(43) (a) Zhang, X.-X.; Wayland B. B. J . Am. Chem. Soc. 1994, 116,
7897-7898. (b) Zhang, X.-X.; Wayland B. B. Inorg. Chem. 2000, 39,
5318-5325.
(44) Rao, P. D.; Littler, B. J .; Geier, G. R., III; Lindsey, J . S. J . Org.
Chem. 2000, 65, 1084-1092.
(45) Littler, B. J .; Ciringh, Y.; Lindsey, J . S. J . Org. Chem. 1999,
64, 2864-2872.
1440 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules for Attachment to Silicon
SCHEME 9
DDQ. The desired porphyrin 29 was readily separated
owing to the polarity of the hydroxyl-substituted protect-
ing group. Treatment of 29 with Eu(acac)3‚nH2O in
refluxing 1,2,4-TCB followed by double decker (t-Bu4-
Pc)2Eu (25) under continued reflux afforded the depro-
tected ethynyl-triple decker 30 in 65% yield. The latter
was coupled with 4-iodobenzyl alcohol under standard
Sonogashira Pd-coupling conditions53 to give the desired
triple-decker alcohol (31) in 58% yield following workup
using preparative SEC (Scheme 10).
E. Electr och em ica l Stu d ies of Mon ola yer s on Si-
(100). The redox-active molecules bearing an alcohol,
S-acetylthio, or Se-acetylseleno group were attached to
Si surfaces for electrochemical measurements. The mol-
ecules bearing an alcohol were attached to iodine-
modified surfaces, whereas the S-acetylthio- or Se-
acetylseleno-derivatized compounds were attached to
hydrogen-passivated surfaces.18 With the latter com-
pounds, cleavage of the acetyl protecting group occurred
during the deposition process, thereby enabling attach-
ment without handling the free thiols or selenols. The
details of the procedure for attaching the molecules to
Si surfaces can be found in ref 18. The basic procedure
involves placing a drop of solution containing the mol-
ecules on the surface and heating to dryness, followed
by continued heating of the dry deposit.
The electrochemical behavior was investigated for a
variety of the complexes tethered to Si(100) via an O, S,
or Se atom. Below, we describe the general characteristics
of the electrochemical behavior of the monolayers by
using several representative examples rather than pre-
senting a comprehensive analysis of all of the complexes
studied. These examples illustrate the salient features
of the electrochemical behavior of the larger body of
complexes that have been examined.
The fast-scan (100 V s-1) cyclic voltammograms of
monolayers of 1-SH, 2-SH, 23, and 31 are shown in
Figure 1. The voltammetric signatures of these complexes
exhibit increasing complexity, with the number of redox
waves monotonically increasing from one to four. As we
have previously discussed, access to multiple oxidation
states provides a possible basis for multibit information
storage.5,7-13
The voltammetric characteristics observed for the
monolayers of 1-SH and 2-SH on Si(100) (Figure 1A and
1B, respectively) are very similar to those we have
previously reported for monolayers of 1-OH and 2-OH.18
The ferrocene exhibits one redox wave (E1/2 ∼0.5 V), and
the porphyrin exhibits two waves (E1/2(1) ∼0.79 V; E1/2(2)
∼1.15 V). The voltammetric characteristics observed for
monolayers formed from the protected thiols 1-SAc and
2-SAc are similar to those observed for monolayers of
the unprotected thiols. These data indicate that the
S-acetylthio protecting group cleaves upon interaction
with the hydrogen-passivated silicon surface, resulting
in formation of an Si-S linkage. The cleavage of the
S-acetylthio protecting group that occurs upon interaction
of 1-SAc and 2-SAc with the hydrogen-passivated silicon
surface parallels the behavior observed upon exposure
to a gold surface.5,8,14,15 Along these lines, the voltam-
1,2,4-TCB (∼230 °C). After the half-sandwich complex
was formed, double decker (t-Bu4Pc)2Eu (25)48 was added
with continued reflux for 17 h. Chromatographic workup
including use of preparative size exclusion chromatog-
raphy (SEC)51 afforded triple-decker ester 26 in 44%
yield. Reduction of 26 with LiAlH4 in dry THF at room
temperature gave the desired triple-decker alcohol 27 in
85% yield (Scheme 9). It is noteworthy that the similar
reaction of the analogous porphyrin-alcohol (obtained by
reduction of 24) and double decker 25 did not give triple
decker 27.
The synthesis of the second triple-decker alcohol was
initated by mixed-aldehyde condensation of pyrrole,
p-tolualdehyde, and ethyne-protected aldehyde 2811 un-
der new cocatalysis conditions [BF3‚O(Et)2-NaCl in CH2-
Cl2 at room temperature]52 followed by oxidation with
(50) (a) Ono, N.; Tomita, H.; Maruyama K. J . Chem. Soc., Perkin
Trans. 1 1992, 2453-2456. (b) Tamiaki, H.; Suzuki, S.; Maruyama,
K. Bull. Chem. Soc. J pn. 1993, 66, 2633-2637. (c) Williamson, D. A.;
Bowler, B. E. Tetrahedron 1996, 52, 12357-12372.
(51) Wagner, R. W.; J ohnson, T. E.; Lindsey, J . S. J . Am. Chem.
Soc. 1996, 118, 11166-11180.
(52) Geier, G. R., III; Riggs, J . A.; Lindsey, J . S. J . Porphyrins
Phthalocyanines 2001, 5, 681-690.
(53) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 4467-4470. (b) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.;
Hagihara, N. Synthesis 1980, 627-630.
J . Org. Chem, Vol. 69, No. 5, 2004 1441

Balakumar et al.
SCHEME 10
F IGURE 1. Fast-scan (100 V s-1) cyclic voltammograms of
monolayers of (A) 1-SH, (B) 2-SH, (C) 23, and (D) 31 on p-type
Si(100) microelectrodes in propylene carbonate containing 1.0
M Bu4NPF6.
via an S- or Se-atom tether elicits robust monolayers that
are comparable in general electrochemical characteristics
to those obtained via an O-atom tether. At present, we
are investigating the detailed features of the electron-
transfer and charge-retention characteristics of the S-
and Se-attached ferrocene and porphyrin monolayers.
The fast-scan (100 V s-1) cyclic voltammogram of the
monolayer of the thiol-derivatized cobalt porphyrin 23
on Si(100) (Figure 1C) exhibits three redox waves (E1/2(1)
∼1.02 V; E1/2(2) ∼1.25 V; E1/2(3) ∼1.52 V), generally
consistent with the expectation that the metal center as
well as the porphyrin are redox active. The lowest
potential wave would be expected to be associated with
the metal center, whereas the two higher potential waves
would correspond to the first and second oxidations of
the porphyrin. The fact that the potential of the Co2+
/
Co3+ couple (nominally E1/2(1) ∼1.02 V) is quite close in
potential to the first porphyrin wave (nominally E1/2(2)
∼1.25 V) is generally consistent with a structure in which
the metal center is four-coordinate (no axial ligands). In
particular, axial ligation of cobalt porphyrins typically
shifts the Co2+/Co3+ couple to less positive potentials,
resulting in the Co2+/Co3+ potential being much less
positive than the first porphyrin oxidation potential.54 For
example, solution electrochemical studies of 23 in the
coordinating solvent THF indicate that the potential of
the Co2+/Co3+ couple is ∼0.7 V lower than that of the first
porphyrin potential. A four-coordinate cobalt ion in the
monolayer of 23 is consistent with an architecture in
which the porphyrins are well packed. In support of this
metric characteristics observed for monolayers formed
from 1-SeAc and 2-SeAc (not shown), either on silicon
or gold surfaces, are very similar to those observed for
the thiol analogues. The electrochemical results indicate
that the Se-acetylseleno protecting group undergoes
cleavage on gold and on hydrogen-passivated silicon, as
observed for the S-acetylthio protecting group. The
cleavage process enables in situ attachment without
necessitating preparation and handling of the corre-
sponding free selenols. Collectively, these studies indicate
that attachment of redox-active species to silicon surfaces
(54) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, CA, 2000; Vol. 8, pp 1-114.
1442 J . Org. Chem., Vol. 69, No. 5, 2004

Redox-Active Molecules for Attachment to Silicon
argument, voltammetric data recorded for a monolayer
of 23 using THF as the solvent yielded essentially the
same potential for the Co2+/Co3+ couple as for the
noncoordinating solvent CH2Cl2. Accordingly, the packing
in the monolayer is sufficiently tight that the metal
center is inaccessible to the THF molecules.
Con clu sion s
The synthetic pathways outlined herein provide access
to a diverse collection of redox-active compounds bearing
tethers for attachment to an electroactive surface. The
surface attachment groups, which were chosen for com-
parative studies of information storage on silicon plat-
forms, include hydroxy, mercapto, S-acetylthio, and Se-
acetylseleno groups. The acetyl-protecting unit cleaves
in situ during the process of attachment to silicon. The
redox-active molecules include ferrocene, zinc porphyrin,
cobalt porphyrin, and triple-decker lanthanide sandwich
coordination compounds. This set of compounds affords
1-4 cationic oxidation states, respectively. The porphy-
rins have been tailored in terms of facial encumbrance
and linker length. The results obtained from examining
a selection of compounds in SAMs on silicon indicate that
molecules of this design are suitable candidates for the
active elements in molecular-based information storage
devices.
The fast-scan (100 V s-1) cyclic voltammogram of the
monolayer of triple-decker alcohol 31 on Si(100) (Figure
1D) exhibits redox characteristics that are qualitatively
similar to the thiol-derivatized analogue on Au(111).11-14
However, there are certain differences in the voltamme-
try on the gold versus silicon surfaces. In particular,
monolayers on Au(111) exhibit four distinct oxidation and
reduction waves in the 0.0-1.6 V range. For the mono-
layers on Si(100), four oxidation waves are also observed
[Eox(1) ∼0.51 V; Eox(2) ∼0.82 V; Eox(3) ∼1.28 V; E1/2(4)
∼1.47 V]; however, only three reduction waves are
observed Ered (2) ∼0.70 V; Ered(3) ∼1.17 V; Ered(4) ∼1.37
V. The reduction wave that is the partner of the lowest
potential oxidation wave is completely absent. Scans to
more negative potentials (-0.5 V) and/or scans at much
slower rates (100 mV s-1) failed to detect this wave.
Regardless, the monolayer is completely reduced to the
neutral state as evidenced by the observation that
repeated scans always yield the lowest potential oxida-
tion wave. One possible explanation for the absence of
the lowest potential reduction wave is that the potential
for this process is lower in energy than the Fermi level
of the p-type Si(100), which lies at ∼0.25 eV (as deter-
mined by X-ray photoelectron spectroscopy). At potentials
greater than the Fermi energy, the Fermi level is pulled
completely into the valence band and the semiconductor
is “metal-like,” with excess carriers available for facile
redox processes with the attached molecules. On the
other hand, at potentials lower than the Fermi energy,
the Fermi level is above the valence band and the number
of carriers available for redox processes is greatly dimin-
ished. This could result in what appears to be highly
irreversible electrochemical behavior (i.e., absence of the
first reduction wave).
Ack n ow led gm en t. The cover art showing a molec-
ular-memory cube was prepared by Mr. Troy A. Barber
with use of a photograph of the Library of Congress
building (provided by Carol M. Highsmith Photography,
Inc.). This work was supported by the DARPA Mo-
letronics Program (MDA972-01-C-0072) and by Zetta-
Core, 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.
Su p p or tin g In for m a tion Ava ila ble: Complete experi-
mental procedures; 1H NMR and LDMS (or MALDI-MS)
spectra of all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
J O034944T
J . Org. Chem, Vol. 69, No. 5, 2004 1443