A compact all-carbon tripodal tether affords high coverage of porphyrins on silicon surfaces

Article abstract of DOI:10.1021/jo0510078

Redox-active molecules designed to give high charge density on electroactive surfaces are essential for applications in molecular information storage. To achieve a small molecular footprint and thereby high surface charge density, a compound consisting of

Full text of DOI:10.1021/jo0510078

A Compact All-Carbon Tripodal Tether Affords High Coverage of
Porphyrins on Silicon Surfaces
Kisari Padmaja,† Lingyun Wei,‡ Jonathan S. Lindsey,*,† and David F. Bocian*,‡
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 May 20, 2005
Redox-active molecules designed to give high charge density on electroactive surfaces are essential
for applications in molecular information storage. To achieve a small molecular footprint and thereby
high surface charge density, a compound consisting of a triallyl tripod attached via a p-phenylene
unit to a porphyrin (1) has been synthesized. The zinc chelate of 1 (Zn-1) was attached to Si(100).
Electrochemical measurements indicate that the molecular footprint (75 Å) in the monolayer is
only ∼50% larger than the minimum achievable, indicating high surface coverage. IR spectroscopy
indicates that the bands due to the ν(CdC) (1638 cm-1) and γ(CH) (915 cm-1) vibrations present in
the solid sample (KBr pellet) are absent from the spectra of the monolayers of Zn-1, consistent
with saturation of the double bond in each of the three legs of the tripod upon the hydrosilylation
process accompanying attachment. Comparison of the relative intensities of the in-plane (998 cm-1
)
versus out-of-plane (797 cm-1) porphyrin modes indicates the average tilt angle (R) of the porphyrin
ring with respect to the surface normal is ∼46°, a value also observed for analogous porphyrins
tethered to Si(100) via monopodal carbon linkers. Accordingly, the higher packing densities afforded
by the compact tripodal linker are not due to a more upright orientation on the surface. The charge-
retention half-lives (t1/2) for the first oxidation state of the Zn-1 monolayers increase from 10 to 50
s at low surface coverage (1-5 × 10-11 mol‚cm-2) to near 200 s at saturation coverage (∼2 × 10-10
mol‚cm-2). Taken together, the high surface charge density (despite the lack of upright orientation)
of the triallyl-tripodal porphyrin makes this construct a viable candidate for molecular information
storage applications.
Introduction
the basis of stored charge.1,2 A key feature of redox-based
molecular information storage is that each memory cell
The design and synthesis of redox-active molecules
functionalized for surface attachment provides the foun-
dation for information storage devices that operate on
(1) 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.
(2) Liu, Z.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science 2003,
302, 1543-1545.
† North Carolina State University.
‡ University of California.
10.1021/jo0510078 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/27/2005
7972
J. Org. Chem. 2005, 70, 7972-7978

Compact All-Carbon Tripodal Tether
CHART 1
CHART 2
were employed with tripod 2a include a porphyrin,
phthalocyanine, ferrocene, ferrocene-porphyrin, and a
triple-decker sandwich coordination compound, whereas
a zinc porphyrin was incorporated with tripod 2b. While
tripods 2a and 2b each represent a satisfactory alterna-
tive attachment motif, the studies revealed certain
limitations to these linkers: (1) For both tripods 2a and
2b, the achievable surface coverages were consistently
lower than those for molecules containing single anchor-
atom tethers, thereby limiting the charge density of the
redox-active monolayer.9,10 (2) In the case of tripod 2a,
all three sulfur atoms do not in fact bind to the surface,
largely compromising the possible advantages of this
linker motif (unpublished results). (3) The use of alkylthio
groups for attachment to silicon is less attractive than
the use of direct alkyl attachment, given the susceptibil-
ity of the former to hydrolysis, oxidation, and perhaps
reductive displacement.
The limitations of tripods 2a and 2b prompted us to
seek an improved tripod design for linking redox-active
molecules to electroactive surfaces. Our initial focus was
new tripods for attachment to silicon substrates. In this
regard, our previous studies of porphyrin monolayers on
silicon revealed that carbon anchors are generally more
robust than anchors such as oxygen or sulfur.6,7 According-
ly, we chose to design the new tripod with alkenyl func-
tionalization to provide attachment via a carbosilane link-
age. We also sought a design wherein the tripod is more
compact than tripods 2a and 2b. We anticipated that this
might facilitate attachment to the surface via all three
legs of the tripod and potentially to higher packing
densities in the monolayers. We ultimately chose a
triallyl-substitued arene motif for the tripod. The triallyl-
arene compounds (3a-g) known to date are shown in
Chart 2.12,13 The goal was to extend this chemistry to
incorporate the triallyl tripod with a porphyrin unit.
Herein, we first describe the synthesis of the tripodal
porphyrins. We then examine the structural and electron-
transfer characteristics of one member of this set, the zinc
chelate (Zn-1), attached to Si(100). The Zn-1 monolayers
on Si(100) were examined using fast-scan cyclic voltam-
metry to quantitate the packing densities of the mono-
layers, swept waveform AC voltammetry (SWAV) to
elucidate the kinetics of electron transfer,3 open circuit
potential amperometry (OCPA) to determine the time of
charge retention after removal of the applied potential,14
must store sufficient charge for reliable readout. A
requirement for achieving high charge densities is a
relatively densely packed monolayer of redox-active
molecules. Our previous studies of porphyrinic molecules
tethered to both metal and semiconductor surfaces have
shown that the maximum achievable surface coverages
are in the mid to high 10-11 mol‚cm-2 range, which
corresponds to a molecular footprint of 200-300 Å2.1-7
This footprint is considerably larger than the 50 Å2 value
we have observed for porphyrins in Langmuir-Blodgett
films.8 If the molecular footprint of the porphyrins on the
electroactive surface could be reduced to the value
observed for the molecules in Langmuir-Blodgett films,
the charge-storage capacity of a memory cell could be
increased 4- to 6-fold.
Most of our previous studies of electroactive monolay-
ers utilized molecules that were functionalized for tether-
ing to the surface via a single anchor atom.1-7 However,
we also performed limited studies of molecules function-
alized for tethering via multiple anchor atoms, in par-
ticular tripodal linkers.9,10 The rationale for examining
the tripods was 2-fold. First, we anticipated that attach-
ment to a surface via three versus one anchor atom might
result in a more robust architecture. Second, we thought
that the more symmetrical tripodal motif might result
in higher packing densities in the monolayers. Two
different tripods were examined, each built around a
tetraarylmethane unit (Chart 1). For attachment to gold
or silicon, the feet of one tripod (2a) contained S-
acetylthiomethyl groups attached to the three aryl units.10
For attachment to a metal oxide, the feet of the other
tripod (2b) contained phosphonomethyl groups attached
to the three aryl units.9 The redox-active molecules that
(3) Roth, K. M.; Gryko, D. T.; Clausen, C.; Li, J.; Lindsey, J. S.; Kuhr,
W. G.; Bocian, D. F. J. Phys. Chem. B 2002, 106, 8639-8648.
(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) Yasseri, A. A.; Syomin, D.; Malinovskii, V. L.; Loewe, R. S.;
Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2004, 126,
11944-11953.
(6) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera,
F.; Bocian, D. F. J. Am. Chem. Soc. 2004, 126, 15603-15612.
(7) Wei, L.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.;
Bocian, D. F. J. Phys. Chem. B 2005, 109, 6323-6330.
(8) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.;
Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344-1350.
(9) 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.
(10) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.;
Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 1461-1469.
(11) Liu, Z.; Yasseri, A. A.; Loewe, R. S.; Lysenko, A. B.; Malinovskii,
V. L.; Zhao, Q.; Surthi, S.; Li, Q.; Misra, V.; Lindsey, J. S.; Bocian, D.
F. J. Org. Chem. 2004, 69, 5568-5577.
(12) Lin, S.-Y.; Hojjat, M.; Strekowski, L. Synth. Commun. 1997,
27, 1975-1980.
(13) (a) Sartor, V.; Nlate, S.; Fillaut, J.-L.; Djakovitch, L.; Moulines,
F.; Marvaud, V.; Neveu, F.; Blais, J.-C.; Le´tard, J.-F.; Astruc, D. New
J. Chem. 2000, 24, 351-370. (b) Martinez, V.; Blais, J.-C.; Astruc, D.
Org. Lett. 2002, 4, 651-653.
(14) Roth, K. M.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. Langmuir
2002, 18, 4030-4040.
J. Org. Chem, Vol. 70, No. 20, 2005 7973

Padmaja et al.
SCHEME 1
and Fourier transform infrared (FTIR) spectroscopy to
measure the relative orientations of the porphyrin rings
with respect to the surface plane.6,7
Results and Discussion
Synthesis. Our design of the porphyrin bearing a
triallyl tripod incorporates a p-phenylene unit between
the porphyrin and the central carbon of the tripod. A
p-tolyl group is present at each of the three nonlinking
meso-positions of the porphyrin. The synthesis of the
resulting A3B-porphyrin can be achieved via the conden-
sation of a dipyrromethane-dicarbinol with the dipyr-
romethane bearing the tripod.15 The synthesis of the
latter begins with commercially available 4-(trifluoro-
methyl)aniline, which upon reaction with allylmagne-
sium chloride provides the triallyl-aniline (3c)12 (Scheme
1). The reaction requires 5 equiv of allylmagnesium
chloride and involves repetition of fluoride elimination
followed by addition of the Grignard reagent to the
resulting quinone-imine. The previous synthesis at the
2-mmol scale in diethyl ether afforded 3c in 54% yield.
We carried out the reaction at the 160-mmol scale in THF
and obtained 3c in 87% yield.
The conversion of the triallyl-aniline 3c to the triallyl-
iodobenzene 4 was achieved via the Sandmeyer reaction.
Diazotization of amine 3c in the presence of hydrochloric
acid with sodium nitrite at 0-5 °C followed by treatment
with potassium iodide gave iodo compound 4 in 28% yield.
The Rosenmund-von Braun reaction of 4 with CuCN in
DMF afforded the triallyl-cyanobenzene 5 in 89% yield.
Reduction of 5 with DIBALH gave the triallyl-benzalde-
hyde 6 in 91% yield. Treatment of 6 to the standard
conditions for dipyrromethane formation (excess pyrrole
and InCl3 at room temperature)16 provided dipyrromethane
7 in 90% yield.
The condensation of the triallyl-dipyrromethane 7 with
dipyrromethane-dicarbinol 8-diol (prepared by reduction
of 8 with NaBH4)17 at room temperature in the presence
of a mild Lewis acid [Yb(OTf)3]18 followed by oxidation
with DDQ afforded free base porphyrin 1 in 29% yield.
Metalation with Zn(OAc)2‚2H2O at room temperature
afforded the desired Zn-porphyrin Zn-1 in 93% yield.
Similar metalation with Ni(OAc)2‚4H2O or Co(OAc)2
provided Ni-porphyrin Ni-1 or Co-porphyrin Co-1, re-
spectively, in high yield (Scheme 2).
Monolayer Characterization. Monolayers of Zn-1
were prepared on Si(100) substrates and examined via
electrochemical and FTIR techniques. The monolayers
were prepared on hydrogen-passivated Si(100) surfaces4
by using a high-temperature (400 °C), 2-min “baking”
procedure previously shown to give facile attachment of
alkenyl-functionalized porphyrins to Si(100) surfaces.11
The general electrochemical and vibrational character-
istics of the Zn-1 monolayers are similar to those we have
previously reported for other porphyrins tethered to Si-
(100) via monopodal carbon tethers.7 Consequently, we
will not reiterate these general features herein but rather
only describe key features that distinguish the tripodal
design from monopodal motifs.
Electrochemical Studies of Surface Coverage,
Electron-Transfer Rates, and Charge-Retention
Times. The goal of the electrochemical studies was to
determine the saturation surface coverage and elucidate
the electron-transfer and charge-retention characteristics
of the Zn-1 monolayers. These properties of the mono-
layers are described below.
A representative fast scan (100 V s-1) cyclic voltam-
mogram for the saturation-coverage Zn-1 monolayer on
a Si(100) microelectrode (100 µm × 100 µm) is shown in
Figure 1. At oxidizing potentials, the monolayer exhibits
two resolved voltammetric waves indicative of formation
of the mono- and dication porphyrin π-radicals. The redox
potentials are similar to those observed for this type of
porphyrin tethered to Si(100) with other types of link-
ers.4,6,7 The surface coverage, Γ, obtained by integrating
the voltammetric waves is ∼2.2 × 10-10 mol‚cm-2, a value
(15) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J.
Org. Chem. 2000, 65, 7323-7344.
(16) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.;
Lindsey, J. S. Org. Process Res. Dev. 2003, 7, 799-812.
(17) Tamaru, S.-I.; Yu, L.; Youngblood, W. J.; Muthukumaran, K.;
Taniguchi, M.; Lindsey, J. S. J. Org. Chem. 2004, 69, 765-777.
(18) Geier, G. R., III; Callinan, J. B.; Rao, P. D.; Lindsey, J. S. J.
Porphyrins Phthalocyanines 2001, 5, 810-823.
7974 J. Org. Chem., Vol. 70, No. 20, 2005

Compact All-Carbon Tripodal Tether
SCHEME 2
FIGURE 2. Standard electron-transfer rate constants, k0 (top
panel), and charge-retention half-lives, t1/2 (bottom panel),
versus surface concentration, Γ, for the first oxidation state
of the Zn-1 monolayers on Si(100). Note that the surface
concentration ranges shown in the panels are different.
The standard electron-transfer rate constants, k0, were
measured for the first oxidation process (E0/+1) of the Zn-1
monolayers at a variety of coverages from the low 10-12
to mid 10-11 mol‚cm-2 range. The interest in this surface-
concentration dependence of the electron-transfer rates
stems from our previous studies on porphyrin monolay-
ers, both on Si(100) and Au(111), which show that the
electron-transfer rates depend on surface coverage.3,4,6,7,10
As we have previously discussed, the electron-transfer
rates cannot be measured at very high surface concentra-
tions, i.e., those near saturation coverages, owing to
experimental limitations. Nevertheless, the available
concentration range is wide enough to compare the trends
observed for the tripodal versus monopodal porphyrins.
The electron-transfer rates of Zn-1 as a function of
surface concentration are shown in Figure 2 (top panel).
Inspection of these data reveals that the electron-transfer
rate decreases with increasing surface concentration,
similar to the behavior observed for other types of
porphyrin monolayers on both Au(111) and Si(100).3,4,6,7,10
In addition, the electron-transfer rates observed for Zn-1
at any particular surface coverage are comparable to
those observed for an analogous porphyrin with a mono-
podal carbon tether.7
that is approximately 3-fold higher than that obtained
for porphyrins with monopodal carbon (oxygen, sulfur,
or selenium) tethers. The high saturation coverage
observed for the tripodal porphyrin was highly reproduc-
ible from electrode to electrode. The surface coverage
observed for Zn-1 corresponds to a molecular footprint
of 75 Å2, a value only 50% larger than that observed for
porphyrins in Langmuir-Blodgett films.8
The charge-retention half-lives, t1/2, were also mea-
sured at selected surface coverages for the first oxidation
state of the Zn-1 monolayers, in parallel with the studies
of the electron-transfer kinetics. Again, the interest in
this surface-concentration dependence of charge-retention
stems from our previous studies on porphyrin monolay-
ers, both on Si(100) and Au(111), which show that this
FIGURE 1. Representative fast-scan (100 V s-1) voltammo-
gram of the Zn-1 monolayers on Si(100).
J. Org. Chem, Vol. 70, No. 20, 2005 7975

Padmaja et al.
plane â-pyrrole hydrogen deformation at 797 cm-1 20
.
For
the solid sample, other important features are the CdC
stretching vibration, ν(CdC), of the alkene group in the
linker at 1638 cm-1 and out-of-plane C-H deformation,
γ(CH), of this group at 915 cm-1 21
.
[Note that only weak
bands are seen in the high-frequency (2600-3400 cm-1
)
spectral region (not shown), all attributable to C-H
stretches.]
Comparison of the IR spectra of the solid and mono-
layers reveals that the bands due to the ν(CdC) (1638
cm-1) and γ(CH) (915 cm-1) vibrations are absent from
the spectra of the monolayers. The absence of these bands
is consistent with saturation of the double bond in each
of the three legs of the tripod. These data are consistent
with, although not definitive proof of, the view that each
of the three legs of the compact tripod attaches to the
Si(100) surface via a hydrosilylation reaction (as expected
when an alkene reacts with hydrogen-passivated sili-
con22).
FIGURE 3. FTIR spectra of the solid Zn-1 porphyrin in a
KBr pellet and the corresponding Zn-1 monolayers on Si(100).
process also depends on surface coverage.3,4,7,10 We were
particularly interested in the charge-retention charac-
teristics of Zn-1 at high surface coverage, beyond that
achievable for porphyrins with monopodal linkers. The
charge-retention half-lives of the Zn-1 monolayers as a
function of surface concentration are shown in Figure 2
(bottom panel). Inspection of these data reveals that the
charge-retention times for all of the monolayers increase
(charge-dissipation rates decrease) as the surface con-
centration increases, consistent with previous observa-
tions on other types of porphyrin monolayers on both
Au(111) and Si(100).3,4,6,10 The charge-retention times for
Zn-1 at surface concentrations in the low to mid 10-11
mol‚cm-2 regime are in the 10-50 s range (data not
shown in Figure 2), comparable to that observed for an
analogous porphyrin with a monopodal tether.7 As the
surface coverage is increased into the 10-10 mol‚cm-2
range, the charge-retention times increase, peaking near
200 s at saturation coverage.
The relative intensities of the in-plane (998 cm-1
)
versus out-of-plane (797 cm-1) porphyrin modes can be
used to determine the average tilt angle (R) of the
porphyrin ring with respect to the surface normal.23 The
average angle determined for the porphyrins in the Zn-1
monolayers is ∼46,° a value that is generally similar to
those we have observed for analogous porphyrins teth-
ered to Si(100) via monopodal carbon linkers.7 Accord-
ingly, the higher packing densities afforded by the
compact tripodal linker are not due to a more upright
orientation on the surface.
Conclusions
The motivation for preparing the porphyrin bearing a
compact tripodal tether was to achieve an upright
orientation and therefore a small molecular footprint on
an electroactive surface. The resulting high surface
coverage would afford a high charge density for molecular
information storage applications. The studies reported
herein demonstrate that the tripodal porphyrin does
indeed afford a reasonably small footprint (within ∼50%
of the minimum expected), but surprisingly, the mol-
ecules are not organized in an upright fashion. The ability
of the tripodal porphyrin to achieve a higher packing
density than a monopodal-functionalized porphyrin may
arise because of torsional constraints at the carbon-atom
junction of the three alkyl legs and the phenylene linker
to the porphyrin. These constraints would restrict motion
along the full length of the lever arm that links the
porphyrin to the surface. We also note that tilting is not
generally inconsistent with a small molecular footprint.
In particular, the porphyrins in the very densely packed
Langmuir-Blodgett films are significantly tilted with
FTIR Studies of Adsorption Geometry. The elec-
trochemical studies show that the tripodal linker affords
significantly higher surface coverage than either tripod
2a and 2b or any of the types of monopodal linkers we
have previously examined. The higher surface coverage
afforded by the compact tripod versus the previous more
bulky designs is easily rationalized, given the limitations
of the latter linkers. On the other hand, the higher
coverage of the compact tripod versus the monopodal
linkers is less obvious. In this regard, our previous
studies of porphyrins attached to Au(111) and Si(100) via
monopodal tethers have shown that the molecules are
significantly tilted with respect to the surface normal.5-7
Accordingly, one possible explanation for the higher
packing densities exhibited by the compact tripod is that
this design leads to a more upright orientation for the
porphyrin. To explore this issue, we probed the adsorp-
tion geometry of the porphyrin using FTIR spectroscopy.
The mid-frequency (700-1900 cm-1) IR spectrum of the
saturation-coverage Zn-1 monolayer is shown in the
bottom trace of Figure 3; the spectrum of a solid sample
of the parent porphyrin is shown in the top trace for
comparison. The IR spectra of the tripodal porphyrin in
both the monolayer and solid forms are similar to those
we have previously reported for an analogous porphyrin
with a monopodal linker.7 The key vibrational signatures
relevant to our studies are the porphyrin pyrrole in-plane
(19) Li, X. Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Su, Y. O.; Spiro,
T. G. J. Phys. Chem. 1990, 94, 31-47.
(20) Li, X. Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Spiro, T. G. J.
Am. Chem. Soc. 1989, 111, 7012-7023.
(21) Silverstein, R. M.; Bassler, G. C. Spectrophotometric Identifica-
tion of Organic Compounds; Wiley: New York, 1967.
(22) (a) Buriak, J. M. Chem. Commun. 1999, 1051-1060. (b) Buriak,
J. M. Chem. Rev. 2002, 102, 1271-1308.
(23) (a) Painter, P. C.; Coleman, M. M.; Koenig, J. L. The Theory of
Vibrational Spectroscopy and Its Application to Polymeric Materials;
Wiley: New York, 1982. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985,
1, 52-66. (c) Harrick, N. J.; Mirabella, F. M. International Reflection
Spectroscopy: Review and Supplement; Harrick Scientific Corp.: New
York, 1985. (c) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-315. (d)
Zaera, F. Int. Rev. Phys. Chem. 2002, 21, 433-471.
ring breathing mode near 998 cm-1 19
,
and the out-of-
7976 J. Org. Chem., Vol. 70, No. 20, 2005

Compact All-Carbon Tripodal Tether
respect to the surface normal.8 Finally, we note that
attachment via three legs of the compact tripod and a
tilted orientation for the porphyrin are not inconsistent.
Molecular models of the tripod on the Si(100) surface
show that tilted orientations are possible owing to the
torsional flexibility of the alkane chains in the linker.
Regardless of the origin of the increased packing density
afforded by the compact tripod, this adsorption charac-
teristic has important consequences for molecular infor-
mation storage in that higher surface coverage results
in longer charge-retention times.
Cobalt(II)-5-[4-(4-allylhepta-1,6-dien-4-yl)phenyl]-10,-
15,20-tri-p-tolylporphyrin (Co-1). A solution of 1 (13.9 mg,
18 µmol) in CHCl3 (8.5 mL) was treated with a solution of Co-
(OAc)2 (31.1 mg, 176 µmol) in MeOH (1.7 mL) at room
temperature. The reaction mixture was refluxed for 8 h with
monitoring by TLC. The reaction mixture was concentrated
and then CH2Cl2 was added. The organic solution was washed
with water, dried (Na2SO4), concentrated, and chromato-
graphed [silica, hexanes/CH2Cl2 (2:1)] to afford a red solid (14
mg, 92%): 1H NMR δ 3.80-3.98 (m, 6H), 4.08-4.26 (br, 9H),
6.00-6.20 (m, 6H), 7.10-7.23 (br, 3H), 9.65-9.95 (m, 8H),
12.6-13.6 (br, 8H), 15.60-16.60 (br, 8H); MALDI-MS obsd
848.1; FABMS obsd 847.3267, calcd 847.3211 (C57H48CoN4);
λabs 413, 530 nm.
Experimental Section
4-(4-Allylhepta-1,6-dien-4-yl)aniline (3c). Following a
literature procedure carried out at the 2-mmol scale,12 a 2 M
solution of allylmagnesium chloride in THF (0.400 L, 0.800
mol) was diluted with additional THF (1.00 L) and then treated
dropwise with a solution of 4-(trifluoromethyl)aniline (19.9 mL,
0.160 mol) in THF (210 mL) at -50 °C under argon. After
complete addition the sides of the reaction flask were rinsed
with additional THF (150 mL). The resulting mixture was
heated at reflux. The reaction was monitored by TLC for the
complete consumption of 4-(trifluoromethyl)aniline. After 3.5
h, the mixture was concentrated and then CH2Cl2 was added.
The organic solution was washed with water, dried (Na2SO4),
and filtered. The filtrate was concentrated and chromato-
graphed [silica, CH2Cl2/hexanes (2:1)], affording a pale yellow
solid (31.7 g, 87%): 1H NMR δ 2.36-2.43 (m, 6H), 3.48-3.66
(br, 2H), 4.94-5.06 (m, 6H), 5.50-5.64 (m, 3H), 6.62-6.70 (m,
2H), 7.06-7.12 (m, 2H); 13C NMR δ 42.1, 42.6, 115.0, 117.5,
127.6, 135.0, 135.8, 144.1; FABMS obsd 228.1757, calcd
228.1752 [(M + H)+, M ) C16H21N].
1-(4-Allylhepta-1,6-dien-4-yl)-4-iodobenzene (4). A solu-
tion of concentrated HCl/H2O (1:1 v/v, 44 mL) was added to a
solution of 3c (11.4 g, 50.2 mmol) in THF (80 mL). The mixture
was stirred at room temperature for 75 min and then cooled
to 0-5 °C. A chilled solution of NaNO2 (7.99 g, 116 mmol) in
water (80 mL) was added while maintaining the temperature
of the reaction mixture below 5 °C. Additional H2O (30 mL)
was added. The reaction mixture was tested for the presence
of nitrous acid with KI-starch paper. A solution of KI (14.2 g,
85.5 mmol) in H2O (16 mL) cooled to ∼5 °C was added, again
maintaining the mixture at <5 °C throughout the addition.
Additional H2O (24 mL) and THF (150 mL) were added. The
reaction mixture was then gradually allowed to warm to room
temperature. After ∼6.5 h, the reaction mixture was neutral-
ized with saturated aqueous Na2CO3 and then filtered. The
filtrate was concentrated. The resulting residue was dissolved
in CH2Cl2 and washed with water. The organic phase was
dried (Na2SO4), concentrated, and chromatographed (silica,
hexanes) to afford a colorless liquid (4.73 g, 28%): 1H NMR
(300 MHz) δ 2.38-2.50 (m, 6H), 4.94-5.12 (m, 6H), 5.44-5.64
(m, 3H), 7.00-7.12 (m, 2H), 7.58-7.70 (m, 2H); 13C NMR δ
41.8, 42.0, 43.5, 91.4, 117.8, 118.2, 129.1, 134.2, 137.3, 145.8.
Synthesis. 5-[4-(4-Allylhepta-1,6-dien-4-yl)phenyl]-10,-
15,20-tri-p-tolylporphyrin (1). Following a general proce-
dure,17,18 a solution of 817 (0.42 g, 0.60 mmol) in THF/MeOH
(10:1, 24 mL) was treated with NaBH4 (0.91 g, 24 mmol) at
room temperature. After 2 h, the reaction mixture was
quenched with saturated aqueous NH4Cl and extracted with
CH2Cl2. The organic phase was dried (Na2SO4) and concen-
trated to provide the corresponding dicarbinol 8-diol. The
resulting dicarbinol was then immediately subjected to con-
densation with dipyrromethane 7 (214 mg, 0.600 mmol) in the
presence of Yb(OTf)3 (2.98 g, 4.81 mmol) in CH2Cl2 (240 mL).
After 80 min, DDQ (409 mg, 1.80 mmol) was added and the
mixture was stirred for 1 h. Then TEA (2.40 mL) was added
and the mixture was stirred for about 30 min. The reaction
mixture was passed through a pad of silica and eluted with
CH2Cl2. The porphyrin band was collected and chromato-
graphed [silica, hexanes/ethyl acetate (9:1 f 4:1)] to provide
a pink solid (139 mg, 29%): 1H NMR δ -2.75 (s, 2H), 2.65-
2.74 (m, 15H), 5.12-5.24 (m, 6H), 5.77-5.93 (m, 3H), 7.46-
7.60 (m, 6H), 7.65 (d, J ) 8.0 Hz, 2H), 8.04-8.12 (m, 6H), 8.14
(d, J ) 8.0 Hz, 2H), 8.76-8.83 (m, 2H), 8.83-8.92 (m, 6H);
MALDI-MS obsd 791.1; FABMS obsd 791.4146, calcd 791.4114
[(M + H)+, M ) C57H50N4]; λabs 421, 516, 551, 594, 649 nm.
Zinc(II)-5-[4-(4-allylhepta-1,6-dien-4-yl)phenyl]-10,15,-
20-tri-p-tolylporphyrin (Zn-1). A solution of 1 (131.8 mg,
167 µmol) in CHCl3 (13 mL) was treated with a solution of
Zn(OAc)2‚2H2O (183 mg, 0.832 mmol) in MeOH (2.6 mL) at
room temperature. The reaction was followed by TLC. The
reaction was complete in 2 h. The reaction mixture was washed
with saturated aqueous NH4Cl and extracted with CH2Cl2. The
combined organic layer was dried (Na2SO4), concentrated, and
chromatographed [silica, hexanes/ethyl acetate (9:1)] to afford
a pink solid (132 mg, 93%): 1H NMR δ 2.66-2.78 (m, 15H),
5.12-5.28 (m, 6H), 5.80-5.96 (m, 3H), 7.50-7.60 (m, 6H), 7.67
(d, J ) 8.4 Hz, 2H), 8.04-8.14 (m, 6H), 8.16 (d, J ) 8.4 Hz,
2H), 8.88-8.94 (m, 2H), 8.94-9.02 (m, 6H); MALDI-MS obsd
852.4; FABMS obsd 852.3193, calcd 852.3170 (C57H48N4Zn);
λabs 424, 551 nm.
Nickel(II)-5-[4-(4-allylhepta-1,6-dien-4-yl)phenyl]-10,-
15,20-tri-p-tolylporphyrin (Ni-1). A solution of 1 (12.3 mg,
15 µmol) in CHCl3 (1.9 mL) was treated with a solution of Ni-
(OAc)2‚4H2O (3.8 mg, 15 µmol) in MeOH (0.5 mL) at room
temperature. The reaction was followed by TLC. The reaction
did not proceed even after 9 h. Therefore, an additional portion
of Ni(OAc)2‚4H2O (85 mg, 0.34 mmol) in MeOH (4 mL) and
CHCl3 (16 mL) was added, and the reaction mixture was
refluxed. After 11 h, some free base porphyrin remained.
Another portion of Ni(OAc)2‚4H2O (8.0 mg, 32 µmol) in MeOH
(1 mL) was added and the reaction mixture was allowed to
reflux for another 2 h. The reaction mixture was concentrated
and then CH2Cl2 was added. The organic solution was washed
with water, dried (Na2SO4), concentrated, and chromato-
graphed [silica, hexanes/CH2Cl2 (2:1)] to afford a pink solid
(12 mg, 94%): 1H NMR δ 2.60-2.71 (m, 15H), 5.08-5.22 (m,
6H), 5.72-5.90 (m, 3H), 7.42-7.52 (m, 6H), 7.60 (d, J ) 8.4
Hz, 2H), 7.84-7.92 (m, 6H), 7.95 (d, J ) 8.4 Hz, 2H), 8.66-
8.82 (m, 8H); MALDI-MS obsd 847.2; FABMS obsd 846.3284,
calcd 846.3232 (C57H48N4Ni); λabs 417, 528 nm.
4-(4-Allylhepta-1,6-dien-4-yl)benzonitrile (5). Following
a literature procedure,24 a mixture of 4 (5.07 g, 15.0 mmol),
CuCN (2.03 g, 22.6 mmol) and DMF (50 mL) was heated over
the course of 30 min until refluxing was attained. The mixture
was refluxed for 3 h. The reaction was monitored by TLC. The
mixture was poured into a flask containing crushed ice and
concentrated aqueous NH4OH (200 mL). The resulting mixture
was bubbled with oxygen for 14 h. The resulting dark blue
mixture was then filtered. The layers of the filtrate were
separated, and the aqueous layer was extracted with CH2Cl2.
The combined organic layer was dried (Na2SO4), concentrated,
and chromatographed [silica, hexanes/ethyl acetate (19:1)] to
afford a colorless liquid (3.17 g, 89%): IR (CH2Cl2) 2230 cm-1
;
1H NMR (300 MHz) δ 2.40-2.54 (m, 6H), 4.94-5.12 (m, 6H),
(24) Hanack, M.; Haisch, P.; Lehmann, H.; Subramanian, L. R.
Synthesis 1993, 387-390.
J. Org. Chem, Vol. 70, No. 20, 2005 7977

Padmaja et al.
5.40-5.62 (m, 3H), 7.36-7.48 (m, 2H), 7.56-7.68 (m, 2H); 13
NMR δ 41.6, 44.2, 109.8, 118.6, 119.1, 127.8, 132.0, 133.5,
151.7; FABMS obsd 238.1589, calcd 238.1596 [(M + H)+, M )
C17H19N].
C
revealed that the surface coverage could be varied in a
controlled fashion from the low 10-12 mol‚cm-2 to the low 10-10
mol cm-2 range (saturation coverage) by varying the porphyrin
concentration from ∼2 µM to ∼2 mM.
The monolayers for the electrochemical experiments were
prepared by dispensing a 2 µL drop of the porphyrin solution
onto the surface of the microelectrode contained in a sparged
VOC vial sealed under Ar. The monolayers prepared for the
FTIR experiments utilized much larger platforms (∼1 cm2),
and consequently required a larger drop size, ∼50 µL. After
deposition, the vial containing the Si substrate was heated on
a hotplate at 400 °C for 2 min and then removed and purged
with Ar until cooling to room temperature. Finally, the Si
substrate was rinsed and sonicated five times with anhydrous
CH2Cl2 and purged dry once again under Ar.
4-(4-Allylhepta-1,6-dien-4-yl)benzaldehyde (6). Follow-
ing a literature procedure,25 a solution of 5 (2.21 g, 9.32 mmol)
in CH2Cl2 (25 mL) was cooled to 0 °C and was treated dropwise
with a 1 M solution of DIBALH in hexanes (11.2 mL, 11.2
mmol). The mixture was allowed to slowly warm to room tem-
perature. The reaction was monitored by TLC. After 3 h, the
reaction mixture was poured into a beaker containing crushed
ice and 6 N HCl. The mixture was stirred for about 1 h. The
layers were separated and the aqueous phase was extracted
with CH2Cl2. The combined organic layer was washed with
aqueous NaHCO3 followed by water. The organic layer was
dried (Na2SO4), concentrated, and chromatographed [silica, hex-
anes/ethyl acetate (19:1)] to afford a colorless liquid (2.04 g,
91%): IR (CH2Cl2) 3078, 1705 cm-1; 1H NMR (300 MHz) δ 2.44-
2.59 (m, 6H), 4.94-5.14 (m, 6H), 5.44-5.66 (m, 3H), 7.45-
7.58 (m, 2H), 7.80-7.93 (m, 2H), 9.99 (s, 1H); 13C NMR δ 41.8,
44.3, 118.4, 127.6, 129.6, 133.8, 134.4, 153.4, 192.0; FABMS
obsd 241.1597, calcd 241.1592 [(M + H)+, M ) C17H20O].
5-[4-(4-Allylhepta-1,6-dien-4-yl)phenyl]dipyrrometh-
ane (7). Following a general procedure,16 a degassed solution
of 6 (1.54 g, 6.42 mmol) in pyrrole (45 mL, 649 mmol) at room
temperature was treated with InCl3 (136 mg, 0.615 mmol). The
mixture was stirred for 4 h, during which the mixture turned
greenish brown. The reaction was quenched by the addition
of NaOH (769 mg, 19.2 mmol, 20-40 mesh beads). The
reaction mixture was stirred for 1 h and then filtered through
a plug of cotton. The filtered material was washed with CH2-
Cl2. The filtrate was concentrated and then diluted with CH2-
Cl2. The organic solution was washed with water, then dried
(Na2SO4), concentrated, and chromatographed [silica, hexanes/
ethyl acetate (4:1)] to provide a dark brown viscous liquid (2.05
g, 90%): 1H NMR δ 2.38-2.48 (m, 6H), 4.96-5.02 (m, 4H),
5.02-5.05 (m, 2H), 5.42 (s, 1H), 5.48-5.61 (m, 3H), 5.86-5.90
(m, 2H), 6.13-6.17 (m, 2H), 6.64-6.68 (m, 2H), 7.08-7.18 (m,
2H), 7.23-7.28 (m, 2H), 7.85 (br, 2H); 13C NMR δ 42.1, 43.3,
43.7, 107.4, 108.6, 117.4, 117.8, 127.2, 128.2, 132.9, 134.7,
139.5, 144.7. FABMS obsd 356.2256, calcd 356.2252 (C25H28N2).
Monolayer Preparation and Characterization. Elec-
trode Preparation. The electrochemical measurements on
the monolayers were made using highly doped p-type Si(100)
working microelectrodes (100 µm × 100 µm) prepared via
photolithographic methods as described previously.4 A micro-
electrode was used to ensure that the RC time constant of the
electrochemical cell was sufficiently short so that the electron-
transfer kinetics of the porphyrin monolayers could be mea-
sured accurately.
A bare Ag wire was used as the counter/reference electrode.
The electrode was prepared by sonicating a section of 500 µm
diameter Ag wire in 7.0 M NH4OH, rinsing it in deionized
water and ethanol, then sonicating it in CH2Cl2 containing 1.0
M n-Bu4NPF6. The Ag wire prepared in this manner was
placed inside a 10 µL polypropylene pipet tip containing ∼5
µL of the propylene carbonate/1.0 M n-Bu4NPF6 electrolyte
solution and manipulated using a micropositioning device in
a setup previously described.4
Monolayer Preparation. All of the monolayers were
assembled on hydrogen-passivated Si(100) surfaces, prepared
as described previously,4 by using a high-temperature (400 °C),
short-time (2 min) “baking” attachment procedure previously
shown to give facile attachment of alkenyl-functionalized
porphyrins to Si(100) surfaces.11 The surface coverage and
conditions for achieving saturation coverage were determined
electrochemically in a series of experiments wherein the
concentration of the porphyrin in the deposition solution
(benzonitrile) was varied systematically. These experiments
Electrochemical Measurements. The electrochemical
measurements on Zn-1 in solution were made in a standard
three-electrode cell using Pt working and counter electrodes
and a Ag/Ag+ reference electrode. The solvent/electrolyte was
CH2Cl2 containing 0.1 M Bu4NPF6. The electrochemical mea-
surements on the Zn-1 monolayers were performed in a two-
electrode configuration using the fabricated Si(100) microelec-
trode and a Ag counter/reference electrode previously described,4
and propylene carbonate containing 1.0 M Bu4NPF6 as the
solvent/electrolyte. The RC time constant for this microelec-
trode/electrochemical cell, measured to be ∼4 µs, is sufficiently
short to preclude any significant interference with the mea-
surement of the electron-transfer rates. The cyclic voltammo-
grams were recorded using a Gamry Instruments PC4-FAS1
femtostat running PHE 200 Framework and Echem Analyst
software. The surface coverage of the porphyrins in the mono-
layer was determined by integration of the total charge in the
first anodic wave and by using the geometric dimensions of the
microelectrode. The standard electron-transfer rate constants,
k0, of the porphyrin monolayers were obtained using the same
SWAV method previously used to obtain k0 values for porphy-
rin monolayers on both Au and Si surfaces.3-6 The rates of
charge dissipation after disconnection from the applied poten-
tial, reported as charge-retention half-lives, t1/2, were obtained
using the OCPA method also used previously to obtain t1/2 val-
ues for porphyrin monolayers on both Au and Si surfaces.4,7,14
FTIR Spectroscopy. The FTIR spectra of the Zn-1 por-
phyrin in both solid and monolayer forms were collected at
room temperature with a spectral resolution of 4 cm-1. The
spectra of solid Zn-1 porphyrin were obtained in KBr pellets
(ca. 1-2 wt % porphyrin). These spectra were collected in
transmission mode using a room-temperature DTGS detector
by averaging over 32 scans.
The IR spectra of the Zn-1 monolayers were obtained using
a Harrick Scientific horizontal reflection Ge attenuated total
reflection accessory (GATR, 65° incidence angle). The Si sub-
strates were placed in contact with the flat surface of a semi-
spherical Ge crystal that serves as the optical element, and IR
spectra were collected with p polarized light using a liquid-nitro-
gen cooled medium-bandwidth (600-4000 cm-1) MCT detector
and averaging over 256 scans. The spectra of the porphyrin
monolayers were referenced against that of a hydrogen-termin-
ated Si(100) surface previously subjected to the same deposi-
tion conditions as those used to obtain the monolayer but using
only the neat deposition solvent. The Ge crystal was cleaned
with neat 2-butanone before every experiment, and the GATR
accessory was purged with dry N2 during data acquisition.
Acknowledgment. This work was supported by the
Center for Nanoscience Innovation for Defense and
DARPA/DMEA (award H94003-04-2-0404) and by Zet-
taCore, Inc.
Supporting Information Available: General experimen-
tal section and spectral data for selected compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(25) Lindsey J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
JO0510078
7978 J. Org. Chem., Vol. 70, No. 20, 2005