Synthesis of porphyrins bearing hydrocarbon tethers and facile covalent attachment to Si(100)

Article abstract of DOI:10.1021/jo049439q

The use of redox-active molecules as the active storage elements in memory chips requires the ability to attach the molecules to an electroactive surface in a reliable and robust manner. To explore the use of porphyrins tethered to silicon via carbosilane

Full text of DOI:10.1021/jo049439q

Syn th esis of P or p h yr in s Bea r in g Hyd r oca r bon Teth er s a n d F a cile
Cova len t Atta ch m en t to Si(100)
Zhiming Liu,† Amir A. Yasseri,† Robert S. Loewe,‡ Andrey B. Lysenko,‡
Vladimir L. Malinovskii,‡ Qian Zhao,§ Shyam Surthi,§ Qiliang Li,§ Veena Misra,*,§
J onathan S. Lindsey,*,‡ and David F. Bocian*,†
Department of Chemistry, University of California, Riverside, California 92521-0403,
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204,
and Department of Electrical and Computer Engineering, North Carolina State University,
Raleigh, North Carolina 27695-8204
david.bocian@ucr.edu; jlindsey@ncsu.edu; vmisra@eos.ncsu.edu
Received April 5, 2004
The use of redox-active molecules as the active storage elements in memory chips requires the
ability to attach the molecules to an electroactive surface in a reliable and robust manner. To explore
the use of porphyrins tethered to silicon via carbosilane linkages, 17 porphyrins have been
synthesized. Fourteen porphyrins bear a tether at a single meso site, and three porphyrins bear
functional groups at two â sites for possible two-point attachment. Two high-temperature processing
methods (400 °C under inert atmosphere) have been developed for rapid (minutes), facile covalent
attachment to Si platforms. The high-temperature processing conditions afford attachment either
by direct deposition of a dilute solution (1 µM-1 mM) of the porphyrin sample onto the Si substrate
or sublimation of a neat sample onto the Si substrate. The availability of this diverse collection of
porphyrins enables an in-depth examination of the effects of the tether (length, composition, terminal
functional group, number of tethers) and steric bulk of nonlinking substituents on the information-
storage properties of the porphyrin monolayers obtained upon attachment to silicon. Attachment
proceeds readily with a wide variety of hydrocarbon tethers, including 2-(trimethylsilyl)ethynyl,
vinyl, allyl, or 3-butenyl directly appended to the porphyrin and iodo, bromomethyl, 2-(trimethyl-
silyl)ethynyl, ethynyl, vinyl, or allyl appended to the 4-position of a meso-phenyl ring. No attachment
occurs with substituents such as phenyl, p-tolyl, mesityl, or ethyl. Collectively, the studies show
that the high-temperature attachment procedure (1) has broad scope encompassing diverse
functional groups, (2) tolerates a variety of arene substituents, and (3) does not afford indiscriminate
attachment. The high-temperature processing conditions are ideally suited for use in fabrication
of hybrid molecular/semiconductor circuitry.
In tr od u ction
the active storage medium, and information is stored in
the discrete redox states of the molecules. The focus of
our work has been developing a hybrid architecture,
where the molecular material is attached to a semicon-
ductor platform. The implementation of hybrid molecular/
semiconductor architectures as a transition technology
leverages the vast infrastructure of the semiconductor
industry with the advantages afforded by molecule-based
active media.
The field of molecular electronics has been driven in
part by the prospect that devices that rely on the bulk
properties of semiconductors will fail to retain the
required characteristics to function when feature sizes
reach nanoscale dimensions. As a consequence, there has
been much interest in developing molecule-based elec-
tronic materials for use in both memory architectures and
circuit elements.1 Toward this goal, we have been en-
gaged in a program aimed at constructing devices that
use the properties of molecules to store information.2-6
In our approach, a collection of redox-active porphyrinic
molecules attached to an electroactive surface serves as
The success of such a hybrid architecture requires (1)
a straightforward means of attaching porphyrins to an
(3) 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.
(4) Liu, Z.; Yasseri, A. A.; Lindsey, J . S.; Bocian, D. F. Science 2003,
302, 1543-1545.
(5) 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.
(6) 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.
† University of California.
‡ Department of Chemistry, North Carolina State University.
§ Department of Electrical and Computer Engineering, North
Carolina State University.
(1) For recent reviews, see: (a) Kwok, K. S.; Ellenbogen, J . C. Mater.
Today 2002, 5, 28-37. (b) Carroll, R. L.; Gorman, C. B. Angew. Chem.
Int. Ed. 2002, 41, 4378-4400.
(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.
10.1021/jo049439q CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/17/2004
5568
J . Org. Chem. 2004, 69, 5568-5577

Silicon-Tethered Porphyrins
electroactive surface, particularly large-wafer silicon, and
(2) a robust linkage that can withstand large numbers
of redox cycles. A number of methods have been devel-
oped for covalent attachment of organic molecules to
silicon surfaces.7 For example, the reaction of Si (hydrogen-
passivated or chlorine-modified) with an alcohol affords
the self-assembled film containing RO-Si linkages.
However, the reaction requires use of neat liquids or a
very high concentration of the molecules to be attached.8-11
Porphyrins generally have low solubility in organic
solutions, with concentrations of ∼50 mM being a typical
upper limit. The method we previously developed for
attaching porphyrins to Si platforms (either hydrogen-
passivated or iodine-modified) involved depositing a drop
of solution containing the porphyrin compound in a high-
boiling solvent (e.g., benzonitrile, bp 191 °C) onto a
photolithographically patterned micrometer-size Si elec-
trode, followed by heating at ∼170 °C for several hours,
during which time additional solvent was added to the
sample area.6 This method afforded attachment of por-
phyrins6 (and ferrocenes3,6) to Si(100) via tethers that are
terminated with OH, SAc, and SeAc groups, yielding
RO-Si, RS-Si, and RSe-Si linkages (the acetyl protect-
ing group is cleaved upon attachment) where R repre-
sents the tether and accompanying redox-active unit.12
This procedure produced high quality monolayers useful
for academic studies but was unsuited for reproducible
fabrication of memory chips on large Si wafers. In
addition, in the past few years it has become apparent
that more stable monolayers are generally obtained with
carbosilane linkages (RC-Si) than alkoxysilane linkages
(RO-Si). Achieving a stable linkage of the redox-active
unit to the Si surface is essential because as many as
1015 cycles may be encountered over an operational
lifetime in a memory chip.13
employed for attachment to Si via thermal,15,20,21 free
radical,15 photochemical (UV),22-24 and Lewis acid medi-
ated reactions.25,26 Alkynes have been less studied but
generally appear to react via the same methods as for
alkenes, including thermal,27 free radical,15 photochemi-
cal,28 Lewis acid mediated,26,28 and electrografting pro-
cesses.28 Of these, the thermal methods (typically ∼100
°C) with alkenes or alkynes appeared to be most ap-
plicable for attaching porphyrinic compounds to large-
scale Si wafers. However, the requirement for use of very
high concentrations of reactants appeared to exclude such
an application.
The success of our thermal attachment method with
alcohol tethers (both for porphyrins and other types of
molecules)6 combined with the fact that porphyrins are
known to be stable at very high temperatures (400 °C
under inert atmosphere conditions)4 where other types
of organic molecules decompose prompted us to explore
very-high-temperature processing strategies. We devel-
oped two high-temperature processing conditions that
enable attachment to Si(100) of porphyrins containing a
wide variety of functional groups. The conditions entail
direct deposition of the sample onto the Si substrate or
sublimation onto the Si substrate. The porphyrins ex-
amined initially were those that bear functional groups
known to attach to silicon, such as the benzyl alcohol
porphyrin Zn 1.6 We subsequently found that a number
of hydrocarbon tethers also afford attachment. The latter
finding prompted the synthesis of a systematic set of
porphyrins bearing a wide variety of hydrocarbon tethers.
A number of methods have been developed for deriva-
tizing silicon surfaces via carbosilane linkages.7 The
methods include pyrolysis of diacyl peroxides,14,15 reaction
of Grignard reagents (with halogenated silicon sur-
faces),16 and electrografting of aryldiazonium salts,17 alkyl
halides,18 or Grignard reagents.19 Alkenes have been
In this paper, we first describe the synthesis of the
porphyrins bearing hydrocarbon tethers. We then de-
scribe the two high-temperature processing methods for
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J . Org. Chem, Vol. 69, No. 17, 2004 5569

Liu et al.
CHART 1
CHART 2
attachment to silicon substrates. Taken together, this
work greatly expands the scope of porphyrins that can
be attached to silicon and enables attachment under
conditions suitable for reproducible fabrication.
Zn 4,31 and Zn 532 have been prepared previously. A report
on the charge-storage properties of the various porphyrin
molecules is beyond the scope of this paper and will be
described elsewhere.
Resu lts a n d Discu ssion
2. Syn th esis of P or p h yr in s Bea r in g Meso-Lin k ed
Teth er s. A. Sta tistica l Ap p r oa ch . Two A3B-porphyrins
were prepared [A ) mesityl; B ) 4-vinylphenyl (Zn 6) or
4-allylphenyl (Zn 7)] for attachment to Si(100). Porphy-
rins bearing three meso-mesityl groups are not available
via rational synthesis but can be prepared via statistical
mixed-aldehyde condensations. The synthesis of Zn 6 is
shown in Scheme 1. 4-Iodobenzaldehyde was protected
as the dimethyl acetal (20) in 96% yield. Kumada cross-
coupling33 of 20 and vinylmagnesium bromide afforded
acetal 21 in 64% yield. Removal of the acetal protecting
group of 21 was not attempted given the high reactivity
of the styryl moiety under either acidic or basic condi-
tions. A mixed-aldehyde condensation34 was carried out
at elevated concentration35 using BF3‚O(Et)2-EtOH co-
catalysis36 with acetal 21, mesitaldehyde, and pyrrole.
Oxidation with DDQ afforded a mixture of porphyrins.
The porphyrin mixture was treated with zinc acetate to
give the corresponding zinc porphyrins. Porphyrins that
have substituents of similar polarity but different degrees
of facial encumbrance are more readily separated as the
zinc chelates than as the free base forms.31 Chromatog-
raphy afforded Zn 6 in 14% yield.
1. Molecu lar Design . The porphyrins examined herein
are designed to probe (1) the effects of the tether (length,
composition, terminal functional group) on the ease of
attachment and quality of the resulting monolayers on
silicon, (2) the ability to attach to silicon via two linkages,
and (3) the effects of the size and pattern of nonlinking
substituents on the charge-storage properties of the
resulting monolayers on silicon. The motivation for these
studies stems from the fact that the electron-transfer
rates vary depending on the nature of the tether2,5 and
on the surface density6 of the attached redox-active
molecules. Most of the porphyrins are zinc chelates, bear
a tether at one meso site, and incorporate inert groups
at the three nonlinking meso sites. Within this class, one
set of molecules varies the nature of the surface attach-
ment group (iodo, bromomethyl, ethyne, vinyl, allyl;
Zn 2-Zn 7; Chart 1) with mesityl groups at the three
nonlinking meso sites. A second set of molecules varies
the steric bulk of the nonlinking meso substituents (p-
tolyl vs mesityl) and the length of the tether (4-vinylphe-
nyl vs 4-allylphenyl) [Zn 6 and Zn 7, Chart 1; Zn 10 and
Zn 11, Chart 2]. A third set incorporates alkene-termi-
nated tethers of different length (vinyl, allyl, 3-butenyl,
4-vinylphenyl, 4-allylphenyl) with nonlinking p-tolyl
groups [Zn 10-Zn 14, Chart 2]. A fourth set of molecules
employs a fixed tether (4-allylphenyl) and varies the size
of the two flanking meso substituents (p-tolyl, methyl)
and the distal meso substituent (p-tolyl, mesityl, 2,4,6-
triethylphenyl) [Zn 14-Zn 19, Chart 3]. Two additional
zinc porphyrins bear two halo or two vinyl groups at
porphyrin â positions. The final porphyrin is a free base,
core-modified monothia-porphyrin with two bromo groups
at the â-thiophene positions. Porphyrins Zn 2,29 Zn 3,30
Allyl-porphyrin Zn 7 was prepared as shown in Scheme
2. Acetal 2237 was treated with a biphasic solution38 of
(31) Wagner, R. W.; J ohnson, T. E.; Lindsey, J . S. J . Am. Chem.
Soc. 1996, 118, 11166-11180.
(32) Wagner, R. W.; J ohnson, T. E.; Li, F.; Lindsey, J . S. J . Org.
Chem. 1995, 60, 5266-5273.
(33) Tamao, K.; Sumitani, K.; Kumada, M. J . Am. Chem. Soc. 1972,
94, 4374-4376.
(34) Lindsey, J . S.; Prathapan, S.; J ohnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
(35) Wagner, R. W.; Li, F.; Du, H.; Lindsey, J . S. Org. Process Res.
Dev. 1999, 3, 28-37.
(36) Lindsey, J . S.; Wagner, R. W. J . Org. Chem. 1989, 54, 828-
(29) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J . S. Chem.
Mater. 1999, 11, 2974-2983.
(30) 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.
836.
(37) 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.
(38) Lindsey, J . S.; Brown, P. A.; Siesel, D. A. Tetrahedron 1989,
45, 4845-4866.
5570 J . Org. Chem., Vol. 69, No. 17, 2004

Silicon-Tethered Porphyrins
CHART 3
SCHEME 1
SCHEME 2
aqueous TFA and CH2Cl2 to afford 4-allylbenzaldehyde
(23) in 81% yield. A mixed-aldehyde condensation34 of 23
with mesitaldehyde and pyrrole was carried out using
BF3‚O(Et)2-EtOH cocatalysis36 followed by DDQ oxida-
tion. Zinc insertion and chromatographic workup afforded
Zn 7 in 12% yield.
B. Ra tion a l Ap p r oa ch . To achieve a scalable syn-
thesis, we have investigated meso substituents that are
compatible with a rational synthesis of porphyrins. The
rational synthesis relies on the condensation of a dipyr-
romethane and a dipyrromethane-dicarbinol.39 The syn-
thesis of the dipyrromethanes and 1,9-diacyldipyr-
romethanes is described below.
and workup via chromatography and Kugelrohr distil-
lation,41 but recently we found that milder acids could
be employed in conjunction with a more simple purifica-
tion procedure via direct crystallization.42 These proce-
dures were employed to prepare dipyrromethanes 24-
31 (Scheme 3). In method A,42 an aldehyde (21, 23,
mesitaldehyde, or 4-pentenal) was condensed with pyr-
role (100 equiv) under InCl3 (0.1 equiv) catalysis at room
temperature for 1.5 h, followed by quenching of the
reaction with powdered NaOH, filtration to remove
neutralized catalyst, removal of pyrrole, and recrystal-
lization (or column chromatography). In this manner, the
new dipyrromethanes 25, 27, 28 and the known dipyr-
romethane 3042 were prepared in good yields. Application
of method A to 3-butenal diethyl acetal was unsuccessful.
Therefore, the latter was condensed with excess pyrrole
(40 equiv) under TFA (0.1 equiv) catalysis at room
temperature for 10 min (method B).41 Analysis of the
crude reaction mixture by GC showed a much higher
percentage of N-confused dipyrromethane (∼25%) than
is typically observed (∼5%) under these conditions.
Syn th esis of Dip yr r om eth a n es. The synthesis of
dipyrromethanes can be achieved via the one-flask reac-
tion of an aldehyde with excess pyrrole.40-42 The synthetic
method has generally employed TFA as the acid catalyst
(39) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J .; Lindsey, J . S. J .
Org. Chem. 2000, 65, 7323-7344.
(40) Lee, C.-H.; Lindsey, J . S. Tetrahedron 1994, 50, 11427-11440.
(41) 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.
(42) Laha, J . K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.;
Lindsey, J . S. Org. Process Res. Dev. 2003, 7, 799-812.
J . Org. Chem, Vol. 69, No. 17, 2004 5571

Liu et al.
SCHEME 3a
SCHEME 4
In this manner, each dipyrromethane (28-31) was
separately treated with EtMgBr in toluene followed by
reaction with the appropriate acid halide (p-toluoyl
chloride or acetyl bromide). Subsequent reaction with
dibutyltin dichloride gave the corresponding tin complex,
which was readily isolated by passage through a silica
pad followed by precipitation from MeOH. The diacyl-
dipyrromethane-tin complexes 32-Sn Bu 2-35-Sn Bu 2
were isolated in yields of 39-57%.
Syn th esis of P or p h yr in s. The meso-substituted por-
phyrins Zn 9 and Zn 11-Zn 19 were prepared by reaction
of a dipyrromethane and a dipyrromethane-dicarbinol
(Scheme 5). The dipyrromethane-dicarbinols were pre-
pared by reduction with NaBH4 of the corresponding 1,9-
diacyldipyrromethane-tin complex45 or of the unsubsti-
tuted 1,9-diacyldipyrromethane.39 The complete reduction
of the tin(IV) complexes requires a slightly longer reac-
tion time (∼2 h) versus that of the uncomplexed diacyl-
dipyrromethane (40 min). The dipyrromethane + dipyr-
romethane-dicarbinol condensation was performed at
room temperature with catalysis by TFA (30 mM) in CH3-
CN39 or Yb(OTf)3 (3.2 mM) in CH2Cl2.46 Subsequent
oxidation with DDQ and zinc metalation afforded the
target porphyrin.
The use of mild Lewis acids in CH2Cl2 generally affords
slightly higher yields and a more facile workup than use
of TFA in CH3CN. However, attempts to prepare Zn 9
(entry 1) using either InCl3 or Yb(OTf)3 gave lower yields
(3-11%) than with TFA (13%). The yields of porphyrins
Zn 16 and Zn 19 were somewhat low (∼10%), which is
attributed to the bulky 2,4,6-triethylphenyl moiety. Also,
porphyrins bearing meso-methyl groups (entries 8-10)
gave lower yields than the analogous porphyrins bearing
meso-p-tolyl groups (entries 5-7). In each case, the
porphyrin-forming reaction was rapid (<30 min). Analy-
a
Method A ) InCl3 (0.1 equiv), pyrrole (100 equiv) rt, 1.5 h;
method B ) TFA (0.1 equiv), pyrrole (40 equiv) rt, 10 min; method
C ) MgBr2 (0.5 equiv), pyrrole (100 equiv) rt, 1 h.
Nevertheless, the two regioisomers were readily sepa-
rated by column chromatography, affording 24 as a
viscous oil in 36% yield. The known dipyrromethanes 2643
and 2941 were also prepared following method B. The
condensation of 2,4,6-triethylbenzaldehyde44 with excess
pyrrole (100 equiv) under MgBr2 (0.5 equiv) catalysis at
room temperature for 1 h (method C)42 afforded 31 as a
viscous oil in 57% yield after column chromatography.
All attempts to prepare 5-vinyldipyrromethane (to serve
as a precursor to porphyrin Zn 10) from acrolein or
acrolein diethyl acetal via method A or B were unsuc-
cessful.
Syn th esis of 1,9-Dia cyld ip yr r om eth a n es. The syn-
thesis of several 1,9-diacyldipyrromethanes is shown in
Scheme 4. Treatment of a dipyrromethane with EtMgBr
in toluene followed by reaction with an acid chloride
typically affords a mixture of the 1-acyldipyrromethane
and 1,9-diacyldipyrromethane.39 Acyldipyrromethanes
typically afford amorphous powders upon attempted
crystallization and streak extensively on column chro-
matography. To facilitate isolation of the 1,9-diacyldipyr-
romethane, the crude acylation mixture is treated with
dibutyltin dichloride. The diacyldipyrromethane-tin com-
plex, which forms selectively, typically is nonpolar and
crystallizes readily.45
(43) Wilson, G. S.; Anderson, H. L. Synlett 1996, 1039-1040.
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(46) Geier, G. R., III.; Callinan, J . B.; Rao, P. D.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 2001, 5, 810-823.
5572 J . Org. Chem., Vol. 69, No. 17, 2004

Silicon-Tethered Porphyrins
SCHEME 5a
a
Method A ) TFA, MeCN, rt; method B ) Yb(OTf)3, CH2Cl2, rt.
sis by laser desorption mass spectrometry (LDMS)47 of
the crude reaction mixtures after bulk oxidation showed
no evidence of the presence of any other porphyrin
species.
One porphyrin that was not available via this route
was the vinyl-porphyrin Zn 10, owing to the lack of access
to 5-vinyldipyrromethane (vide supra). The synthesis of
Zn 10 was achieved following the route outlined in
Scheme 6. Diacyldipyrromethane 3248 was reduced with
NaBH4 and the resulting 32-d iol was condensed with
dipyrromethane (36)41,49 in CH2Cl2 containing Yb(OTf)3
at room temperature. Subsequent oxidation with DDQ
afforded free base porphyrin 37 in 33% yield. Porphyrin
37 was iodinated at the meso position using I2 and (CF3-
CO2)2IC6H5 in CHCl3/pyridine50 to furnish free base
porphyrin 8 in 82% yield. Zinc insertion afforded Zn 8 in
75% yield. Porphyrin Zn 8 was then subjected to a Stille
cross-coupling reaction51 with (vinyl)tributyltin and Pd-
(PPh3)4 to afford vinylporphyrin Zn 10 in 77% yield.
3. Syn th esis of P or p h yr in s Bea r in g â-lin k ed Teth -
er s. Porphyrin Zn 8 was of great interest for surface
attachment because the meso-iodo substituent would
potentially afford a C-Si bond from the porphyrin meso
carbon, thereby affording a porphyrin monolayer with
direct attachment to the surface. However, Zn 8 displayed
poor surface attachment and redox behavior, presumably
due to steric hindrance of the flanking hydrogen atoms
(47) (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.
(48) Gryko, D.; Lindsey, J . S. J . Org. Chem. 2000, 65, 2249-2252.
(49) Wang, Q. M.; Bruce, D. W. Synlett 1995, 1267-1268.
(50) (a) Shultz, D. A.; Gwaltney, K. P.; Lee, H. J . Org. Chem. 1998,
63, 769-774. (b) Shanmugathasan, S.; J ohnson, C. K.; Edwards, C.;
Matthews, E. K.; Dolphin, D.; Boyle, R. W. J . Porphyrins Phthalocya-
nines 2000, 4, 228-232.
(51) Stille, J . K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508-524.
J . Org. Chem, Vol. 69, No. 17, 2004 5573

Liu et al.
SCHEME 6
SCHEME 7
at the 3- and 7-positions of the porphyrin. A next-best
design entailed placement of halo substituents at the
flanking â-positions of the porphyrin, which again should
afford a monolayer with direct C-Si bonds from the
porphyrin to the surface.
the free base porphyrin. Zinc insertion afforded porphyrin
Zn 40 in 38% yield (using InCl3) or 46% yield [using Yb-
(OTf)3]. We also attempted to brominate porphyrin Zn 40
with NBS to give a 3,5,7-tribrominated porphyrin, which
could potentially bind to a Si(100) surface at three sites
of the porphyrin backbone (not shown). However, 1H
NMR analysis of the resulting product showed a mixture
of several polybrominated porphyrin species that proved
inseparable (TLC showed one spot under a variety of
conditions). The reaction of Zn 40 with tributyl(vinyl)tin
and Pd(PPh3)4 in THF afford the corresponding 3,7-
divinylporphyrin (Zn 41) in 86% yield. This porphyrin
bears two sites for attachment to Si(100) and may provide
increased stability as a result of the double tether.
We also prepared a porphyrin bearing bromine atoms
attached to two adjacent â-positions. For this design, we
settled on a core-modified mono-thia (N3S) porphyrin
because of the greater ease of synthesis as compared to
the analogous N4-porphyrin. The synthesis of the N3S-
porphyrin followed a “3 + 1” approach.54 2,5-Dimethyl-
thiophene was exhaustively brominated with Br2 (7
equiv) in CH2Cl2 to afford the tetrabromothiophene
The synthesis of a porphyrin (Zn 40) bearing two
â-bromo substituents is shown in Scheme 7. The three
nonlinking meso substituents include two 3,5-di-tert-
butyl phenyl groups and one mesityl group to aid in the
solubility of the dibromo-porphyrin. Treatment of dia-
cyldipyrromethane 3852 with 2 equiv of Br2 in CHCl3 at
0 °C for 10 min afforded 39 in 54% yield. The acyl
substituents at the 1- and 9-positions direct the electro-
philic substitution to the 3- and 7-positions of the
dipyrromethane backbone.53 Reduction of 39 with NaBH4
afforded the corresponding dipyrromethane-dicarbinol,
which was immediately condensed with dipyrromethane
30 in CH2Cl2 containing either InCl3 or Yb(OTf)3 to yield
(52) 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.
(53) Lee, C.-H.; Li, F.; Iwamoto, K.; Dadok, J .; Bothner-By, A. A.;
Lindsey, J . S. Tetrahedron 1995, 51, 11645-11672.
5574 J . Org. Chem., Vol. 69, No. 17, 2004

Silicon-Tethered Porphyrins
SCHEME 8
deposition approach. In this procedure, the porphyrin was
first dissolved in an organic solvent and a small drop (1
µL) of the resulting dilute solution was placed onto a
micrometer-size Si microelectrode that was contained in
a sealed vial maintained under inert atmosphere (see
Experimental Section for additional details). The vial was
then placed on a hot plate preheated to a particular
temperature, and the system was “baked” for a specified
time. The Si platform was then cooled, washed to remove
nonattached porphyrin, and interrogated voltammetri-
cally to investigate the quality of the monolayer and
determine the surface coverage (by integration of the
peaks in the voltammogram).
The “best” attachment conditions for direct deposition
were determined via a systematic study using porphyrins
Zn 1 and Zn 7 that probed the effects of varying the
baking temperature, the baking time, the concentration
of the porphyrins in the deposition solution, and the
nature of the deposition solvent. The first three of these
variables are not independent; however, the studies
revealed the following general trends: (1) As the baking
temperature is increased, the surface coverage monotoni-
cally increases. For example, increasing the baking
temperature from 100 to 400 °C (baking time 30 min;
deposition solution porphyrin concentration 1 mM) in-
creased the surface concentration from 1 × 10-11 mol
cm-2 to ∼8 × 10-11 mol cm-2 (the saturating coverage
for the porphyrin is ∼10-10 mol cm-2). At temperatures
above 400 °C, no further attachment is achieved and the
system degrades. (2) As the baking temperature is
increased, the time required to achieve the highest
surface coverage monotonically decreases. For example,
a baking time of 1 h was required to achieve maximum
coverage at 200 °C. This time was reduced to 2 min when
the baking temperature was elevated to 400 °C. (3) As
the concentration of the porphyrin in the deposition
solution was increased from 1 to 100 µM, the surface
coverage for a given baking time and temperature
systematically increased. Increasing the porphyrin con-
centrations above 100 µM had little effect on the cover-
age. (4) Both high-boiling (benzonitrile, bp ) 191 °C) and
low-boiling (THF, bp ) 66 °C) solvents yielded essentially
identical results for a particular set of deposition and
baking conditions. The best conditions were applied to
several porphyrins. Figure 1 (top panels) shows repre-
sentative cyclic voltammograms of Zn 1 and Zn 7 obtained
by attaching the porphyrins using a 100 µM deposition
solution followed by baking at 400 °C for 2 min.
Su blim a tion . The next high-temperature attachment
procedure involved an indirect deposition approach. In
this procedure, a small quantity of the porphyrin (<1 mg)
was placed in the bottom of a cylindrical glass container
whose diameter permitted insertion into the heating vial.
The top of the container was flat to allow the Si platform
to be placed on top with the micrometer-size electrode
facing downward (∼3 mm above the solid sample). The
vial was sealed, purged with Ar, and placed on a hot plate
preheated to a particular temperature, and the porphyrin
was sublimed for a specified time. The Si platform was
then cooled, washed to remove nonattached porphyrin,
and interrogated voltammetrically to investigate the
quality of the monolayer and determine the surface
coverage. Representative cyclic voltammograms of Zn 1
and Zn 7 are shown in Figure 1 (bottom panels). Both of
derivative 42 in 88% yield after recrystallization (Scheme
8). (Attempts to prepare an analogous tetrabrominated
pyrrole following the same route were unsuccessful.)
Treatment of 42 with excess pyrrole (100 equiv) afforded
the NSN-tripyrrane 43 in 55% yield. The reaction was
fast (5 min), and no catalyst was needed. The reaction
presumably is autocatalytic, where the liberated HBr
catalyzes the condensation. Analysis of the crude reaction
mixture by GC-MS showed three major peaks (2.2:1.0:
0.7 ratio) each with m/z ) 400, which were assigned as
tripyrrane 43, N-confused tripyrrane, and doubly N-
confused tripyrrane. The desired tripyrrane 43 was
isolated (93% purity by GC) after column chromatogra-
phy and recrystallization. Reduction of 2,5-diacylpyrrole
4455 with NaBH4 and condensation of the resulting
pyrrole-dicarbinol (44-d iol) with 43 under InCl3 cataly-
sis46 afforded the desired N3S-porphyrin 45 in 24% yield
(Scheme 8). Analysis of the crude reaction mixture by
LDMS showed no evidence of any other porphyrins.
4. Atta ch m en t Meth od s. Ba k in g. The initial high-
temperature attachment procedure involved a direct
(54) Latos-Grazynski, L. In The Porphyrin Handbook; Kadish, K.
M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA,
2000; Vol. 2, pp 361-416.
(55) Barbero, M.; Cadamuro, S.; Degani, I.; Dughera, S.; Fochi, R.;
Gatti, A.; Prandi, C. Gazz. Chim. Ital. 1990, 120, 619-627.
J . Org. Chem, Vol. 69, No. 17, 2004 5575

Liu et al.
F IGURE 1. Fast-scan cyclic voltammograms (100 V s-1) of monolayers of Zn 1 and Zn 7 on p-type Si(100) microelectrodes. The
solvent/electrolyte overlayer is composed of propylene carbonate containing 1.0 M Bu4NPF6.
these molecules were attached by subliming at 400 °C
for 20 min. The “best” attachment conditions for indirect
deposition were determined via a systematic study that
probed the effects of varying the sublimation temperature
and baking time. At temperatures below 300 °C, rela-
tively little attachment was achieved via the sublimation
method. At 400 °C, the surface coverage monotonically
increased as the sublimation time was increased. No
further coverage was observed for times longer than 20
min. It is noteworthy that although sublimation of
porphyrinic compounds is a well-known process for
purification and for generation of thin films,56 the sub-
limation process developed herein employs porphyrins
bearing a reactive tether and enables fabrication of
surface-attached monolayers of the porphyrins.
Scop e of Ap p lica tion . With the baking and sublima-
tion methods in hand for attachment of porphyrins to Si,
porphyrins Zn 1-Zn 19, Zn 40, Zn 41, and 45 were exam-
ined for attachment using the same deposition conditions
(baking temperature 400 °C; baking time 2 min; deposi-
tion solution porphyrin concentration 1 mM). The func-
tional groups that afforded attachment include 2-(tri-
methylsilyl)ethynyl, vinyl, allyl, and 3-butenyl directly
appended to the porphyrin; and iodo, bromomethyl,
2-(trimethylsilyl)ethynyl, ethynyl, vinyl, and allyl ap-
pended to the 4-position of a meso-phenyl ring. The
surface coverage varied somewhat as a function of
porphyrin and/or linker type; however, the surface cover-
ages were typically in the range 4 × 10-11 to 8 × 10-11
mol cm-2. Attachment was not achieved for the sterically
congested porphyrins meso-iodo, Zn 8; â-dibromo, Zn 40;
and vicinal â-dibromo, 45. In general, the surface cover-
ages and characteristic features of the voltammograms
obtained via the sublimation method (sublimation tem-
perature 400 °C; sublimation time 20 min) are quite
similar to those obtained via the baking method, indicat-
ing covalent attachment and robust electrochemical
behavior.6 The relatively narrow voltammetric waves and
the absence of visible surface oxidation at high potentials
suggest that the porphyrins are packed relatively uni-
formly and fully cover the surface. As controls, the zinc
chelates of various porphyrins that lack functional groups
were examined, including 2,3,7,8,12,13,17,18-octaeth-
ylporphyrin, meso-tetraphenylporphyrin, meso-tetra-p-
tolylporphyrin, and meso-tetramesitylporphyrin. No at-
tachment was observed for any of these porphyrins, as
was evident from the observation that the baked film was
completely removed by washing (in addition, no voltam-
metric peaks were observed). Collectively, these results
indicate that the high-temperature attachment procedure
(1) has broad scope encompassing diverse functional
groups, (2) tolerates a variety of arene substituents, and
(3) does not afford indiscriminate attachment. Finally,
the Zn 7 monolayer was used in an initial series of tests
to evaluate the robustness to electrochemical cycling of
the carbosilane tethered porphyrins. This test was con-
(56) (a) Perlovich, G. L.; Golubchikov, O. A.; Klueva, M. E. J .
Porphyrins Phthalocyanines 2000, 4, 699-706. (b) Semyannikov, P.
P.; Basova, T. V.; Grankin, V. M.; Igumenov, I. K. J . Porphyrins
Phthalocyanines 2000, 4, 271-277. (c) Torres, L. A.; Campos, M.;
Enriquez, E.; Patin˜o, R. J . Chem. Thermodyn. 2002, 34, 293-302.
5576 J . Org. Chem., Vol. 69, No. 17, 2004

Silicon-Tethered Porphyrins
ducted as described previously4 and showed that the
voltammetric characteristics of the monolayer were
unchanged after ∼1010 redox cycles.
benzonitrile, THF, and CH2Cl2. The choice of solvent was
primarily dictated by solubility of the porphyrin rather than
any specific characteristics of the solvent. However, monolay-
ers prepared using benzonitrile or THF exhibited superior
voltammetric characteristics relative to those prepared using
CH2Cl2, likely as a result of the fact that the halogenated
solvent can react with the surface at high temperature.
Prior to introduction of the porphyrin and baking, the Si
microelectrode was placed in a vial. The vial was sealed with
a Teflon cap and purged with Ar for 15 min. A syringe
containing the porphyrin solution was inserted through the
Teflon cap, and a drop of the solution was placed onto the
microelectrode. The solvent was then allowed to dry under the
continued Ar purge. The purge was stopped, the vial was
transferred to the hot plate at the preset temperature, and
the electrode was baked for a specified time. The temperatures
investigated ranged from 200 to 450 °C; the times ranged from
2 to 30 min. The vial was then removed from the hot plate,
and the Ar purge was reinitiated. After the vial had reached
room temperature, solvent was syringed into the vial to wash
the electrode and remove nonattached porphyrin. In some
cases, the microelectrode was removed from the vial and
washed in air. No difference was observed in the voltammetric
characteristics of the monolayers for electrodes washed under
inert versus ambient conditions.
For the sublimation procedure, the vessel containing the
solid porphyrin was placed into the vial, the microelectrode
was placed on top of the vessel, and the vial was sealed. The
vial was then purged gently (to prevent displacing the micro-
electrode from the vessel) for 15 min. The purge was stopped,
and the vial was transferred to the hot plate at the preset
temperature for a specific time. Times in the range 2-20 min
were investigated. The vial was then removed from the hot
plate and allowed to cool to room temperature. The microelec-
trode was then removed and washed to remove nonattached
porphyrin.
Con clu sion s
We have prepared a set of porphyrins bearing carbon
tethers for attachment to Si(100). Collectively, the studies
reported herein indicate that porphyrins bearing a
variety of functional groups can be covalently attached
to Si via high-temperature processing. The baking and
sublimation methods are complementary and together
afford a nearly universal strategy for attaching porphy-
rins. The baking method employs the porphyrin in a
dilute solution (1 µM-1 mM), whereas the sublimation
method employs the porphyrin as a neat solid. We note
that the success of both approaches apparently does not
depend on melting the porphyrins. Indeed, the melting
points of the porphyrins ranged from 230 °C (Zn 19) to
435 °C (Zn 14), yet good quality monolayers were obtained
regardless of melting point value. The baking approach
is essentially “dry” inasmuch as only a small amount of
solvent is used in the attachment process; the sublima-
tion approach is totally “dry” in that no solvents are
required in the process. This latter process is particularly
appealing from a semiconductor processing perspective,
wherein uniform attachment of molecules to very large
(30 cm) Si wafers might be anticipated in the manufac-
ture of future generation hybrid molecular/semiconductor
devices.
Exp er im en ta l Section
Electr och em ica l Stu d ies a n d Atta ch m en t P r oced u r e.
The porphyrins were attached to Si microelectrodes (100 µm
× 100 µm) that were prepared photolithographically from
device-grade wafers (B-doped Si(100); F ) 0.005-0.1 Ω cm).
The procedure for preparing these microelectrodes has been
described in detail.6 The electrochemical procedures, tech-
niques, and instrumentation were also the same as described
previously.6 The surface coverage of the molecules was deter-
mined by integrating the peaks in the voltammogram. The
temperature of the Si platform was measured by attaching a
thermocouple directly to the platform.
The basic procedures for attachment via the baking and
sublimation methods are as described in Results and Discus-
sion. Additional details of these procedures are described
below.
For the baking procedure, porphyrin concentrations in the
range 1 µM to 3 mM were investigated. The solvents included
Ack n ow led gm en t. This work was supported by the
Center for Nanoscale Innovation in Defense and DARPA/
DMEA (under award number DMEA90-02-2-216) 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.
Su p p or tin g In for m a tion Ava ila ble: Complete Experi-
mental Section describing the synthesis of all new compounds,
summary of melting point data for the porphyrins, and
characterization data for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O049439Q
J . Org. Chem, Vol. 69, No. 17, 2004 5577