Characterization of self-assembled monolayers of porphyrins bearing multiple thiol-derivatized rigid-rod tethers

Article abstract of DOI:10.1021/ja047723t

A series of multithiol-functionalized zinc porphyrins has been prepared and characterized as self-assembled monolayers (SAMs) on Au. The molecules, designated ZnPS_n (n = 1-4), contain from one to four [(S-acetylthio)methyl]phenylethynylphenyl groups appended to the meso-position of the porphyrin; the other meso-substituents are phenyl groups. For the dithiol-functionalized molecules, both the cis- and the trans-appended structures were examined. The ZnPS_n SAMs were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPS_n SAMs. (1) The ZnPS_n molecules bind to the Au surface via a single thiol regardless of the number of thiol appendages that are available per molecular unit. (2) The porphyrins in the ZnPS₃ and ZnPS₄ SAMs bind to the surface in a more upright orientation than the porphyrins in the ZnPS₁, cis-ZnPS₂, and trans-ZnPS₂ SAMs. The porphyrins in the ZnPS₃ and ZnPS₄ SAMs are also more densely packed than those in the cis-ZnPS₂ and trans-ZnPS₂ SAMs. The packing density of the ZnPS₃ and ZnPS₄ SAMs is similar to that of the ZnPS ₁ SAMs, despite the larger size of the molecules in the former SAMs. (3) The thermodynamics and kinetics of electron transfer are generally similar for all of the ZnPS_n SAMs. The general similarities in the electron-transfer characteristics for all of the SAMs are attributed to the similar binding motif.

Full text of DOI:10.1021/ja047723t

Published on Web 08/31/2004
Characterization of Self-Assembled Monolayers of Porphyrins
Bearing Multiple Thiol-Derivatized Rigid-Rod Tethers
Amir A. Yasseri,† Dennis Syomin,† Vladimir L. Malinovskii,‡ Robert S. Loewe,‡
Jonathan S. Lindsey,*,‡ Francisco Zaera,*,† and David F. Bocian*,†
Contribution from the Department of Chemistry, UniVersity of California,
RiVerside, California 92521-0403, and Department of Chemistry,
North Carolina State UniVersity, Raleigh, North Carolina 27695-8204
Received April 20, 2004; E-mail: jlindsey@ncsu.edu; francisco.zaera@ucr.edu; david.bocian@ucr.edu
Abstract: A series of multithiol-functionalized zinc porphyrins has been prepared and characterized as
self-assembled monolayers (SAMs) on Au. The molecules, designated ZnPSn (n ) 1-4), contain from
one to four [(S-acetylthio)methyl]phenylethynylphenyl groups appended to the meso-position of the porphyrin;
the other meso-substituents are phenyl groups. For the dithiol-functionalized molecules, both the cis- and
the trans-appended structures were examined. The ZnPSn SAMs were investigated using X-ray photo-
electron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The
studies reveal the following characteristics of the ZnPSn SAMs. (1) The ZnPSn molecules bind to the Au
surface via a single thiol regardless of the number of thiol appendages that are available per molecular
unit. (2) The porphyrins in the ZnPS3 and ZnPS4 SAMs bind to the surface in a more upright orientation
than the porphyrins in the ZnPS1, cis-ZnPS2, and trans-ZnPS2 SAMs. The porphyrins in the ZnPS3 and
ZnPS4 SAMs are also more densely packed than those in the cis-ZnPS2 and trans-ZnPS2 SAMs. The
packing density of the ZnPS3 and ZnPS4 SAMs is similar to that of the ZnPS1 SAMs, despite the larger
size of the molecules in the former SAMs. (3) The thermodynamics and kinetics of electron transfer are
generally similar for all of the ZnPSn SAMs. The general similarities in the electron-transfer characteristics
for all of the SAMs are attributed to the similar binding motif.
electrodes and establishing direct electrical contact to both.12-14
In this regard, dithiol-functionalized molecules have been
implemented in a variety of two-terminal devices to explore
phenomena related to molecular electronics, including conduc-
tance switching in nanopores,15-17 and resonant tunneling
behavior in break junctions.18
We have been engaged in a program aimed at constructing
devices that use the redox properties of porphyrinic molecules
to store information.14,19-22 In the course of these studies, we
I. Introduction
Self-assembled monolayers (SAMs) of thiol-derivatized
molecules on Au substrates have been widely studied in
connection with their potential applications in the general area
of molecular-based electronics.1,2 Molecules have been designed
to be tethered to the Au substrate via one or more thiols,
including tripodal units bearing three thiols.3-11 Molecules also
have been constructed with two thiols at opposite ends, thus
affording the opportunity of spanning two different metal
(12) Jagessar, R. C.; Tour, J. M. Org. Lett. 2000, 2, 111-113.
(13) Tour, J. M.; Jones, 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.
† University of California, Riverside.
‡ North Carolina State University.
(1) Kwok, K. S.; Ellenbogen, J. C. Mater. Today 2002, 5, 28-37.
(2) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378-
4400.
(3) Whitesell, J. K.; Chang, H. K. Science 1993, 261, 73-76.
(4) Fox, M. A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14, 816-
820.
(14) Schweikart, K.-H.; Malinovskii, V. L.; Yasseri, A. A.; Li, J.; Lysenko, A.
B.; Bocian, D. F.; Lindsey, J. S. Inorg. Chem. 2003, 42, 7431-7446.
(15) Chen, J.; Wang, W.; Klemic, J.; Reed, M. A.; Axelrod, B. W.; Kaschak,
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D. S.; Bennett, D. W. Ann. N.Y. Acad. Sci. 2002, 960, 69-99.
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M. Appl. Phys. Lett. 2000, 77, 1224-1226.
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Solid Films 1998, 327-329, 477-480.
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J. M.; Weiss, P. S. Science 2001, 292, 2303-2307.
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Chem. 2000, 11, 549-556.
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Otsubo, T. Chem. Commun. 2001, 1830-1831.
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H. V. Phys. ReV. Lett. 2002, 88, 176804/1-176804/4.
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Acta 2002, 85, 788-798.
(19) Balakumar, A.; Lysenko, A. B.; Carcel, C.; Malinovskii, V. L.; Gryko, D.
T.; Schweikart, K.-H.; Loewe, R. S.; Yasseri, A. A.; Liu, Z.; Bocian, D.
F.; Lindsey, J. S. J. Org. Chem. 2004, 69, 1435-1443.
(9) Zhu, L.; Tang, H.; Harima, Y.; Yamashita, K.; Aso, Y.; Otsubo, T. J. Mater.
Chem. 2002, 12, 2250-2254.
(20) 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.
(21) Gryko, D.; Li, J.; Diers, J. R.; Roth, K. M.; Bocian, D. F.; Kuhr, W. G.;
Lindsey, J. S. J. Mater. Chem. 2001, 11, 1162-1180.
(10) Hirayama, D.; Takimiya, K.; Aso, Y.; Otsubo, T.; Hasobe, T.; Yamada,
H.; Imahori, H.; Fukuzumi, S.; Sakata, Y. J. Am. Chem. Soc. 2002, 124,
532-533.
(11) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.; Lindsey, J. S.;
Bocian, D. F. J. Org. Chem. 2004, 69, 1461-1469.
(22) 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.
9
11944
J. AM. CHEM. SOC. 2004, 126, 11944-11953
10.1021/ja047723t CCC: $27.50 © 2004 American Chemical Society

Characterization of SAMs of Porphyrins
A R T I C L E S
Chart 1
have designed and synthesized a wide variety of porphyrinic
architectures and characterized their self-assembly behavior both
on metal and on semiconductor surfaces. We have also
extensively probed the kinetics of interfacial charge transfer in
the porphyrinic SAMs.23-26 To date, our studies of porphyrin-
based information storage media have focused on architectures
that are tethered to one electroactive surface. The electrical
contact to the second electrode is noncovalent in nature and is
typically established through an intervening conductive medium.
To expand on our capabilities for the design of electronic
devices at the molecular level, we have embarked on a program
to explore porphyrinic architectures in which the molecules
contain multiple functional groups for surface attachment. A
number of previous studies have been reported of multi-thiol-
functionalized porphyrin SAMs,11,27-38 prepared for a variety
of applications including nanopatterning for optical chemical
sensing, molecular biorecognition,28-35 and electrocatalytic
oxygen reduction.36-38 However, these previous studies have
typically used a single type of multithiol-functionalized por-
phyrin and have not explored how the number and location of
the thiol groups might affect the mode of surface binding and
geometry, or how these factors might influence the desired
functional characteristics.
In this paper, we report the preparation and characterization
of a series of multithiol-functionalized zinc porphyrin SAMs
on Au. These molecules, designated ZnPSn (n ) 1-4), contain
from one to four [(S-acetylthio)methyl]phenylethynylphenyl
groups appended to the meso-position of the porphyrin, the other
meso-substituents being phenyl groups (Chart 1). For the dithiol-
functionalized molecules, both the cis- and the trans-appended
structures were examined. The S-acetyl protecting group
undergoes facile cleavage upon exposure to the Au surface,
leaving an unprotected sulfur atom that binds directly to the
Au surface.13 For convenience, we will refer to the protected
and unprotected appendages as S-AcSCH2pep and SCH2pep,
respectively. The S-AcSCH2pep functionalization was chosen
because this group is relatively rigid, which is expected to
disfavor surface orientations where the porphyrin rings lay flat
or close to flat on the surface. Indeed, previous studies of
molecular SAMs based on similar rodlike designs have shown
that these molecules form highly ordered densely packed
(23) Roth, K. M.; Liu, Z.; Gryko, D. T.; Clausen, C.; Lindsey, J. S.; Bocian, D.
F.; Kuhr, W. G. ACS Symp. Ser. 2003, 844, 51-61.
(24) Roth, K. M.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. Langmuir 2002,
18, 4030-4040.
(25) 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.
(26) 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.
(27) Tomizaki, K.-y.; Yu, L.; Wei, L.; Bocian, D. F.; Lindsey, J. S. J. Org.
Chem. 2003, 68, 8199-8207.
(28) Boeckl, M. S.; Bramblett, A. L.; Hauch, K. D.; Sasaki, T.; Ratner, B. D.;
Rogers, J. W., Jr. Langmuir 2000, 16, 5644-5653.
monolayers,39,40 wherein the molecules align themselves in
upright orientations on the surface.40 A similar surface orienta-
tion for the multithiol-functionalized porphyrins (e.g., trans-
ZnPS2) could, in principle, allow the molecule to span two metal
electrodes and make direct electrical contact to both. Porphyrins
with cis-thiol functionalization (i.e., cis-ZnPS2, ZnPS3, and
ZnPS4) were included in the study to explore whether this would
afford binding to a given surface via two adjacent thiol groups.
Attachment via two thiols could afford enhanced stability and
(29) Shimazu, K.; Takechi, M.; Fujii, H.; Suzuki, M.; Saiki, H.; Yoshimura,
T.; Uosaki, K. Thin Solid Films 1996, 273, 250-253.
(30) Ishida, A.; Sakata, Y.; Majima, T. Chem. Commun. 1998, 57-58.
(31) Ishida, A.; Sakata, Y.; Majima, T. Chem. Lett. 1998, 267-268.
(32) Akiyama, T.; Imahori, H.; Sakata, Y. Chem. Lett. 1994, 1447-1450.
(33) Cordas, C. M.; Viana, A. S.; Leupold, S.; Montforts, F.-P.; Abrantes, L.
M. Electrochem. Commun. 2003, 5, 36-41.
(34) Kondo, T.; Ito, T.; Nomura, S.-I.; Uosaki, K. Thin Solid Films 1996, 284-
285, 652-655.
(35) Zhang, Z.; Hu, R.; Liu, Z. Langmuir 2000, 16, 1158-1162.
(36) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9,
3277-3283.
(39) Walzer, K.; Marx, E.; Greenham, N. C.; Less, R. J.; Raithby, P. R.; Stokbro,
K. J. Am. Chem. Soc. 2004, 126, 1229-1234.
(40) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L.
R. J. Am. Chem. Soc. 1996, 118, 3319-3320.
(37) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9,
2772-2774.
(38) Van Galen, D. A.; Majda, M. Anal. Chem. 1988, 60, 1549-1553.
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11945

A R T I C L E S
Yasseri et al.
series of experiments wherein the concentration of the porphyrin in
the deposition solution and the deposition time were systematically
varied. These experiments revealed that the surface coverage could be
varied in a controlled fashion from the low 10-12 to mid 10-11 mol
cm-2 range by varying the porphyrin concentration from ∼3 µM to
∼3 mM and the deposition time from 1 to 15 min. Saturation coverages,
which fall in the mid 10-11 mol cm-2 range for all of the porphyrins,
could be achieved by deposition from 3 mM solutions for 15 min.
Saturation-coverage SAMs for the electrochemical studies were
prepared by immersing the Au microelectrodes in a ∼3 mM porphyrin
solution for ∼15 min, followed by repeated (five times) sonication and
rinsing with CH2Cl2. Saturation-coverage SAMs for the XPS and FTIR
studies were prepared by depositing successive 50 µL aliquots of a 3
mM porphyrin solution onto the Au substrate, which was presealed in
a vial purged with Ar. After ∼15 min, the vial was opened and the
substrate was repeatedly sonicated and rinsed (five times) with CH2-
Cl2, sealed once again in the vial, and dried with a stream of Ar. The
SAMs were kept under Ar with minimum exposure to light prior to
the XPS and FTIR measurements. The SAMs prepared at less than
saturation coverage for the electrochemical and FTIR experiments were
obtained by procedures similar to those described above, except for
the use of different concentrations of the porphyrin solution and
deposition times, chosen to achieve the desired surface coverage.
D. Electrochemical Measurements. The electrochemical measure-
ments on the porphyrins in solution and SAMs were made using a Pt
wire or Au micro working electrode, respectively, a Pt counter electrode,
and a Ag reference electrode. The solvent was CH2Cl2:EtOH (85:15);
the supporting electrolyte was 1.0 M Bu4NPF6 (Aldrich), recrystallized
three times from methanol (Fisher) and dried overnight at room
temperature under vacuum. The RC time constant for the microelec-
trode/electrochemical cell was measured to be ∼5 µs, which is
sufficiently short to preclude interference with the measurement of the
electron-transfer rates.
might also alter the electron-transfer properties from those of a
single-thiol attached species.14,25
The series of ZnPSn SAMs on Au mentioned above was
investigated using a variety of techniques, including X-ray
photoelectron spectroscopy (XPS), Fourier transform infrared
(FTIR) spectroscopy, and various electrochemical methods
(cyclic voltammetry, swept-waveform AC voltammetry (SWAV),
and open circuit potential amperometry (OCPA)). The XPS
studies were used to elucidate the details of the surface bonding.
The voltammetric and FTIR studies were used to provide
information on packing density and surface orientation, respec-
tively. The SWAV and OCPA measurements were used to
obtain the rates of charge transfer in the presence of applied
potential and the rate of charge dissipation in the absence of
applied potential. Collectively, the studies reported herein
provide a detailed picture of the structural and functional
characteristics of the porphyrin SAMs and demonstrate how
these characteristics vary with the number of S-AcSCH2pep
appendages.
II. Experimental Section
A. Synthesis. The synthesis of the porphyrins is reported in the
Supporting Information.
B. Au Substrate Fabrication. 1. Microelectrodes for Electro-
chemical Studies. Both Au ball and band microelectrodes were used
for the electrochemical studies. These electrodes were prepared
according to previously described methods.24,41 Microelectrodes are
required for the measurement of the electron-transfer rates to ensure
that the RC time constant of the electrochemical cell does not limit the
measurement. The Au ball microelectrodes have the advantage of ease
of use. However, the surface morphology and geometry of these
electrodes are different from those of the Au band microelectrodes,
which are more similar to the larger area Au films required for the
spectroscopic studies. Initial studies aimed at comparing the properties
(redox potentials, surface coverages) of the porphyrin SAMs on the
Au ball with those of the band microelectrodes indicated little difference
between the two. Consequently, further electrochemical studies used
the Au ball microelectrodes.
2. Evaporated Films for Spectroscopic Studies. Au films were
used as substrates for the XPS and FTIR studies. The films were
prepared by e-beam vapor deposition of 10 nm of Cr (99.999%)
followed by 200 nm of Au (99.99%) onto the surface of a precleaned
oxidized B-doped Si (100) wafer (Silicon Valley Microelectronics).
The Cr and Au films were deposited at 0.5 and 1.5 Å s-1, respectively.
The base pressure of the chamber during the evaporation period was
maintained at <2.0 × 10-6 Torr. Upon completion of the film
deposition, the wafer was removed from vacuum and diced into ∼1
cm2 pieces. Each Au piece was immediately inserted into a VOC-type
glass vial fitted with a Teflon-silicon rubber septum (ambient exposure
of ∼5-10 min). The vial was purged with Ar (99.995%) to maintain
the sample under an inert environment. XPS survey spectra revealed
no noticeable differences in the level of carbon contamination between
Au samples used immediately and those stored under Ar for a few
days. However, to maintain self-consistency in all surface characteriza-
tion and electrochemical experiments, fresh Au substrates were used
immediately after preparation.
1. Voltammetric Characterization. Cyclic voltammograms were
recorded in a 0-1.2 V potential window with a CH Instruments
Electrochemical Analyzer (model 600A). For the SAMs, the charge
obtained by integrating the first anodic voltammetric peak was used to
determine the surface coverage. The area of the working electrode was
determined after the electrochemical measurement on each SAM. The
SAM was first stripped from the electrode by immersing in Nanostrip
solution (J. T. Baker) for 5 min followed by rinsing of the electrode.
The cleaned electrode was then placed in a 1 mM ferrocene solution
(CH2Cl2:EtOH (85:15) containing 1.0 M Bu4NPF6), and the voltam-
metric signal observed at 1 V s-1 was used to obtain the electrode area
via the Randles-Sevcik equation.42 The half-wave potential (E1/2
obtained for ferrocene in this electrochemical configuration is E1/2
)
≈
0.20 V, which serves as a reference point for the potentials of the SAMs.
2. Determination of Electron-Transfer Rates. The standard
electron-transfer rate constants (k0) of the SAMs were obtained using
the swept waveform AC voltammetry (SWAV) method. This method
has been described in detail previously, and its utility has been
demonstrated for obtaining kinetic data for porphyrin SAMs.25 The
current observed in the SWAV measurement was collected and
amplified by a homemade broad bandwidth amplifier (3db point 450
kHz).
3. Determination of Charge-Retention Times. The charge-retention
characteristics of the SAMs were determined using open circuit potential
amperometry (OCPA) as described in detail in past publications.24,26,43
E. XPS Measurements. The XPS data for the porphyrin SAMs were
collected using a Leybold EA11-MCD system equipped with a Mg
KR X-ray (1253.6 eV) source, a hemispherical analyzer, and a 18-
C. SAM Preparation. The SAMs were prepared from porphyrins
in CH2Cl2:EtOH (85:15) solutions. The mixed solvent was used because
of the low solubility of the porphyrins containing multiple S-AcSCH2pep
appendages. The surface coverage and the conditions required for
achieving saturation coverage were determined electrochemically in a
(42) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and
Applications; Wiley: New York, 2001.
(41) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.;
Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F. J. Am. Chem.
Soc. 1999, 121, 1059-1064.
(43) 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.
9
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Characterization of SAMs of Porphyrins
A R T I C L E S
multichannel detector. The main XPS chamber was maintained at a
base pressure of <3 × 10-8 Torr. The samples were introduced by
using a fast-transfer mechanism comprised of a long rod and an
intermediate pumping stage. The total time required for the introduction
of the sample was approximately 5 min. Spectra were obtained from
each sample using identical data collection parameters. The survey
(scans of 1000 eV or more) and high-resolution spectra were obtained
by averaging 10 or 50 scans, with an average dwell time of 100 or 250
ms per point and scan, respectively. Survey spectra were obtained with
a band-pass energy of 100.8 eV, which corresponds to a spectral
resolution, ∆E1/2 ≈ 1.5 eV. High-resolution spectra were obtained for
the S 2p, Zn 2p, N 1s, C 1s, and O 1s lines for all of the SAMs with
a band-pass energy of 31.5 eV (∆E1/2 ≈ 0.8 eV). To compensate for
energy shifts due to possible surface charging effects, all of the XPS
peak positions were referenced to the adventitious C 1s peak at 284.6
eV.
The spectra from the high-resolution scans were fitted to Gaussian
peaks after background subtraction (using Analytics software) for
semiquantitative analysis. In the case of the S signal, 2p doublets are
observed for both surface-bound and unbound species, which are not
fully resolved. Consequently, the S 2p lines were fitted by setting the
energy difference between the 2 p3/2 and 2 p1/2 peaks to 1.2 eV and
their area ratio to 2:1, and by maintaining a constant peak width at
half-maximum. The relative intensities and binding energies of each
pair of doublets were then allowed to vary in the fitting process. The
binding energies for the surface-bound S atoms for the different ZnPSn
SAMs were found to be identical to within experimental error; only
the relative intensities of the signals varied. The same was also the
case for the unbound S atoms of the different ZnPSn SAMs. In the
case of the N 1s and Zn 2p3/2 bands, the fits were constrained such that
the same parameters were used for the different porphyrin SAMs.
F. FTIR Spectroscopy. The FTIR spectra of the porphyrins in both
solids and SAMs were collected at room temperature using a Bruker
Tensor 27 FTIR spectrometer. In all cases, the spectral resolution was
4 cm-1. The spectra of the solid porphyrin samples were obtained in
KBr pellets (∼ 1-2 wt % of porphyrin). These spectra were collected
in transmission mode using a room-temperature DTGS detector and
were averaged over 32 scans. The spectra of the SAMs were acquired
using a Harrick Scientific horizontal single-reflection Germanium ATR
accessory GATR (65° incidence angle). In the reflection accessory,
the SAM on the Au substrate is placed against the Ge crystal that serves
as the optical element of the GATR. A constant torque setting was
used in all measurements. During the spectra acquisition period, the
GATR accessory was purged with dry N2. Before every experiment,
the Ge crystal was cleaned with neat 2-butanone. The spectra of the
SAMs were acquired using a liquid-nitrogen cooled medium-bandwidth
MCT detector (600-4000 cm-1) and were averaged over 256 scans.
The peak-to-peak noise in the SAM spectra was e1.5 × 10-4
absorbance units.
Figure 1. Representative fast-scan voltammograms (100 V s-1) of
saturation-coverage ZnPSn (n ) 1-4) SAMs.
Table 1. Redox Potentials for Oxidation of the ZnPSn Porphyrins
in Solution and Saturation-Coverage SAMs and Surface
Concentrations for the SAMsa
/+1
E0
1/+2
E+
soln
(V)
SAM
(V)
soln
(V)
SAM
Γb
areac
Å2
2
SAM
(V)
(10-11 mol cm-
)
ZnPS1
0.47
0.48
0.48
0.49
0.49
0.54
0.52
0.53
0.62
0.62
0.69
0.74
0.73
0.70
0.71
0.82
0.84
0.89
0.97
0.96
7.6
3.0
4.7
6.7
6.5
220
550
350
250
260
cis-ZnPS2
trans-ZnPS2
ZnPS3
ZnPS4
a All redox potentials were obtained in CH2Cl2:EtOH (85:15) containing
+
1.0 M n-Bu4NPF6 and were referenced vs Ag/Ag+ (FcCp2/FcCp2 ≈ 0.19
V). The scan rates for the solution and SAM studies were 0.1 and 100 V
s-1, respectively. b Calculated from the integrated voltammetric peaks.
c Estimated from the experimental values of Γ.
to within experimental error, the potentials for all of the ZnPSn
SAMs are higher than those for the porphyrins in solution. Also,
the potentials for the ZnPS3 and ZnPS4 SAMs (E0/+1 ≈ 0.62
V) are more positive than those for the ZnPS1, cis-ZnPS2, and
trans-ZnPS2 SAMs (E0/+1 ≈ 0.53 V). In this regard, previous
studies of porphyrin SAMs have shown that the redox potentials
are influenced by both packing density and screening from
solvent and counterions.25 More densely packed or heavily
screened SAMs typically exhibit higher redox potentials. The
fact that the ZnPS3 (Γ ≈ 6.7 × 10-11 mol cm-2) and ZnPS4
(Γ ≈ 6.5 × 10-11 mol cm-2) SAMs exhibit higher redox
potentials than the cis-ZnPS2 (Γ ≈ 3.0 × 10-11 mol cm-2) or
trans-ZnPS2 (Γ ≈ 4.7 × 10-11 mol cm-2) SAMs is qualitatively
consistent with the result that the former porphyrins exhibit
higher saturation coverages than the latter. The ZnPS1 SAM,
on the other hand, does not follow the trend; this SAM exhibits
a relatively low redox potential (E0/+1 ≈ 0.53 V) despite the
fact that the saturation surface coverage is high (Γ ≈ 7.6 ×
10-11 mol cm-2). Plausibly, the phenyl substituents on ZnPS1,
which are much smaller than the S-AcSCH2pep appendages
on the other porphyrins, may be less effective at screening the
III. Results
A. Electrochemical Studies of the ZnPSn SAMs. 1. Vol-
tammetric Characteristics. Representative fast scan (100 V
s-1) cyclic voltammograms of the saturation-coverage ZnPSn
SAMs are presented in Figure 1. All of the SAMs exhibit robust
and reversible electrochemical behavior over multiple cycles
under ambient conditions. At oxidizing potentials, each SAM
exhibits two resolved voltammetric waves indicative of the
mono- and dication porphyrin radicals. The half-wave potentials
for the different ZnPSn SAMs and the surface concentrations
(Γ) at saturation coverage are summarized in Table 1. For
comparison, redox potentials for the ZnPSn porphyrins in
solution are included in Table 1.
Inspection of the redox data in Table 1 shows that while the
solution potentials for all of the ZnPSn porphyrins are the same
9
J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11947

A R T I C L E S
Yasseri et al.
Figure 3. Plot of k0 versus surface concentration for the E0/+1 state of the
ZnPSn (n ) 1-4) SAMs.
observed for ZnPS1. Finally, we note that for all of the ZnPSn
SAMs, the peak-to-peak splitting between the anodic and
cathodic waves is ∼0.03 V and does not change as a function
of surface coverage.
2. Electron-Transfer and Charge-Retention Characteris-
tics. Both the standard electron-transfer rate constants, k0, and
the charge-dissipation rates (in the absence of applied potential),
reported as charge-retention half-life times, t1/2, were measured
simultaneously for each of the ZnPSn SAMs. These studies were
undertaken because our previous work on porphyrin SAMs have
shown that the k0 and t1/2 values depend on surface coverage.25,43
The plot of k0 for the first oxidation processes (E0/+1) as a
function of surface concentration shown for the ZnPSn SAMs
in Figure 3 indicates that, indeed, higher surface coverage results
in slower rates of electron transfer; inspection of the charge-
dissipation data indicates the same trend (data not shown). At
low surface concentration, the k0 values for all of the ZnPSn
SAMs are similar to one another, ∼105 s-1. As the surface
concentration increases, the k0 values decrease and appear to
level off for all of the SAMs at k0 ≈ 2 × 104 s-1 at a surface
concentration of Γ ≈ 2 × 10-11 mol cm-2, well below the
saturation coverage of any of the SAMs. It should be noted
that the rates cannot be measured at very high surface
concentrations because of limitations in the experimental
apparatus. In terms of the charge-retention half-lives, at low
surface concentrations (Γ ≈ 2 × 10-12 mol cm-2), the t1/2 values
are <20 s, but as the surface concentration increases, they
increase, until leveling off in the t1/2 ≈ 30-50 s range at a
Figure 2. Redox potentials E0/+1 (top panel) and full-width at half-maxima
∆Ep,1/2 (anodic peak) (bottom panel) of the ZnPSn (n ) 1-4) SAMs at
various surface concentrations.
porphyrin from the solvent and counterions, thus leading to a
lower redox potential.25
To evaluate more fully the effects of surface concentration
on the electrochemical characteristics of the SAMs, voltam-
metric measurements for all of the ZnPSn SAMs were per-
formed as a function of surface coverage. The potential for each
first oxidation (E0/+1) and full-width at half-maximum for the
anodic peak of this oxidation (∆Ep,1/2) are plotted in Figure 2,
top and bottom panels, respectively. Inspection of these data
reveals the following: (1) At low surface concentrations (Γ ≈
2 × 10-12 mol cm-2), the E0/+1 values for all of the ZnPSn
SAMs are similar to one another and approach the solution
value. (2) As the surface concentration increases, the E0/+1 values
increase. (3) While the E0/+1 values for ZnPS3 and ZnPS4
continue to increase as the coverage approaches the saturation
value, the E0/+1 values for cis-ZnPS2 and trans-ZnPS2 appear
to level off before saturation coverage is reached. Leveling off
is clearly observed for ZnPS1. The trends observed for the
∆Ep,1/2 values are generally similar to those observed for the
E0/+1 values: (1) At low surface concentrations, the ∆Ep,1/2
values for all of the ZnPSn SAMs are similar to one another
and fall in the 90-100 mV range, near the thermodynamic
minimum for a redox homogeneous monolayer. (2) As the
surface concentration increases, the ∆Ep,1/2 values increase,
indicative of increasing (albeit relatively small) redox hetero-
geneity. (3) The ∆Ep,1/2 values for ZnPS3 and ZnPS4 continue
to increase as the coverage approaches the saturation value, but
the ∆Ep,1/2 values for cis-ZnPS2 and trans-ZnPS2 appear to level
off before saturation coverage is reached. Leveling off is clearly
surface concentration of Γ ≈ 2 × 10-11 mol cm-2
.
B. XPS Studies of the ZnPSn SAMs. The general surface
characteristics of a collection of e-beam evaporated Au films
deposited on Si(100) were examined by XPS prior to monolayer
deposition to survey the quality of the initial substrates.
Measurements on the clean surfaces revealed intense Au signals
near 84, 88, 335, and 353 eV, corresponding to the 4f7/2, 4f5/2
,
4d5/2, and 4d3/2 states, respectively. Weak signals were also seen
from carbon and oxygen. These signals are an indication of the
minor contamination of the Au surface commonly resulting from
adsorption of adventitious hydrocarbons and/or water present
in the ambient environment, and they point to the unreliable
nature of the C 1s signal for quantitative determinations of
surface concentrations.
The XPS signatures of the saturation-coverage ZnPSn SAMs
were examined to characterize the binding motif of the mono-
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11948 J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004

Characterization of SAMs of Porphyrins
A R T I C L E S
Figure 4. Survey XPS of the saturation-coverage ZnPS1 SAM on Au.
Inset A: High-resolution scan of the Zn 2p3/2 spectral region, showing both
the raw data (dots) and a fit (solid). Inset B: High-resolution scan of the S
2p3/2 and S 2p1/2 spectral region, raw data (dots) and fit (solid).
Table 2. Sulfur Atom Binding Energies and Atomic Ratios for
Signature Elements in the ZnPSn SAMs
Figure 5. High-resolution XPS of the saturation-coverage ZnPSn (n )
2-4) SAMs in the S 2p spectral region, raw data (dots) and fit (solid).
Each spectrum was fitted to two sets of S 2p3/2 and S 2p1/2 peaks. The low
and higher energy sets are representative of bound and unbound S,
respectively (see text).
sulfur binding energy (eV)a
estimated atomic ratio
bound
S 2p3
unbound
S 2p3
S 2p:Zn 2p3
N:Zn
/
2
b
b
SAM
ST
SB
N 1s:Zn 2p3/2
/
2
/2
ZnPS1
162.0
162.0
162.2
161.9
162.0
0.9
2.1
1.9
3.0
3.7
0.9
1
0.9
0.9
0.9
3.9
3.9
4.1
3.8
3.7
cis-ZnPS2
trans-ZnPS2
ZnPS3
163.5
163.7
163.7
163.7
centered at binding energies of 162.0 ( 0.1 and 163.2 ( 0.1
eV, respectively. In contrast, signals from two types of sulfurs
were observed for the SAMs of all of the ZnPSn molecules
containing more than one thiol (Figure 5). A list of the S 2p3/2
binding energies obtained after fitting the XPS spectrum from
each of these SAMs is given in Table 2. For each of the SAMs
exhibiting signals from two types of S atoms, the same two
binding energy values were obtained within experimental error,
162.0 ( 0.1 and 163.7 ( 0.1 eV. This result strongly suggests
that the same two types of S moieties are present in all of the
SAMs. The binding energy of 162.0 eV matches that of ZnPS1
and is in generally good agreement with that observed for neat
alkanethiols46-49 and thiol-functionalized porphyrin monolayers
chemisorbed onto Au substrates.35-37 On the other hand, the
binding energy of the second type of S is upshifted by ∼1.5
eV from that typical of an S-Au species. Previous studies of
thiol SAMs have attributed S 2p3/2 signals at these binding
energies to the presence of unbound thiols.35-37 Accordingly,
all of the ZnPSn (n ) 2-4) SAMs exhibit both bound and
unbound thiols.
To quantify the amount of bound versus unbound thiol in
the ZnPSn (n ) 2-4) SAMs, the XPS signal intensities for the
two types of S atoms were scaled against those for the Zn atom.
Those data are summarized in Table 2. The table lists the S:Zn
ratios obtained experimentally for both the total S content, ST,
and the bound S content, SB. The ST ratio reflects the stoichi-
ometry expected from the molecular structure, whereas the SB
ratio reflects the fraction of S atoms bound to the Au surface.
The ST ratios obtained experimentally are in good agreement
ZnPS4
a (0.1 eV. b ST and SB refer to the estimated total (bound + unbound)
and bound sulfur, respectively.
layer to the Au surface; acquisition of XPS data for SAMs below
saturation coverages was not possible because of the low signal
levels obtained for the key signature elements such as S. A
representative survey spectrum of the ZnPS1 SAM is shown in
Figure 4. The spectra of all of the ZnPSn SAMs exhibit multiple
peaks consistent with the molecular composition of the por-
phyrins bearing thiol groups. In particular, in addition to signals
from the bulk Au, the spectra show discrete signals for Zn, S,
and N from the porphyrin. The high-resolution Zn 2p3/2 (Figure
4, inset A) and N 1s (not shown) signals are observed at binding
energies of 1020.9 ( 0.1 and 398.2 ( 0.1 eV, respectively, in
qualitative agreement with previously reported values for zinc
porphyrins.35,43,44 Quantitative analysis of the spectra was also
carried out by scaling the spectral intensity of each element of
interest (S, Zn, N) using known atomic cross-section sensitivity
factors.45 The last column of Table 2 summarizes the experi-
mentally determined values for the N:Zn atomic ratios, which
correspond well to the 4:1 (N:Zn) atomic ratio expected for the
porphyrin within the 10-15% uncertainty in the spectral fits
of the raw data. We note that signal attenuation due to the SAMs
is negligible in these systems.
The chemical environment of the S atoms was examined in
detail. Inset B of Figure 4 shows a representative high-resolution
S 2p spectrum for the ZnPS1 SAM. The data could be fitted to
a single set of peaks with S 2p3/2 and S 2p1/2 components
(46) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A.
N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167.
(47) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323-
(44) Polzonetti, G.; Ferri, A.; Russo, M. V.; Iucci, G.; Licoccia, S.; Paolesse,
R. J. Vac. Sci. Technol., A 1999, 17, 832-839.
(45) Handbook of X-ray Photoelectron Spectroscopy; Wagner, C. D., Riggs,
W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; Perkin-Elmer
Corp.: Eden Prairie, 1978.
329.
(48) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987,
109, 733-740.
(49) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-
727.
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J. AM. CHEM. SOC. VOL. 126, NO. 38, 2004 11949

A R T I C L E S
Yasseri et al.
viewing of its weak bands. The key features observed in the IR
spectra of the SAMs are summarized below.
(1) Comparison of the SAM and solid spectra of ZnPS1,
trans-ZnPS2, ZnPS3, and ZnPS4 reveals that the frequencies
of the IR bands of each pair are similar. This result indicates
that the structure of the porphyrin macrocycle and substituent
groups is retained upon SAM formation. Note, however, that
the frequency of the ν(CdO) mode is slightly higher in the
SAMs (1698 cm-1) than in the solid (1690 cm-1). In the case
of cis-ZnPS2, all of the bands are quite weak, partially due to
the low saturation coverage (vide supra), and the similarities
between the solid and SAM spectra are more difficult to assess.
Nevertheless, the bands due to the in-plane pyrrole breathing
(998 cm-1) and out-of-plane â-hydrogen deformation (797
cm-1) modes are still apparent in the spectrum of the cis-ZnPS2
SAM, at the same values as for the solid, suggesting that the
structure of this molecule is also unaltered upon SAM formation.
(2) For the ZnPS1 SAM, there is no evidence of a ν(CdO)
vibration at 1698 cm-1 due to the acetyl protecting group,
consistent with cleavage of this group upon SAM formation.
On the other hand, each of the four porphyrins that contain
multiple S-AcSCH2pep appendages exhibits some intensity for
the ν(CdO) vibration. This observation indicates that the
upshifted signal of the unbound S atom seen in XPS is due to
an uncleaved, protected thiol, not to some other type of thiol
that has been altered by SAM formation or any interaction with
the surface. In the case of the cis-ZnPS2 SAM, the intensity of
the ν(CdO) band relative to the porphyrin bands is lower than
is observed for the trans-ZnPS2 SAM, whereas the intensities
of the XPS signals for the bound and unbound thiols are similar
for the two types of SAMs. As will be discussed below, this is
likely due to a difference in the orientation of the S-acetyl group
with respect to the Au surface.
Figure 6. FTIR spectra of the ZnPSn (n ) 1-4) solids (left panel) and in
saturation-coverage SAMs (right panel).
with those expected on the basis of the structures of the
molecules, from one to four according to the value of n in ZnPSn
(n ) 1-4). On the other hand, the SB ratio was measured to be
approximately unity for all of the multithiol porphyrin SAMs.
This implies that each of the porphyrins studied at its respective
saturation surface concentration is anchored through a single
Au-S bond.
C. FTIR Studies of the ZnPSn SAMs. The mid-frequency
(680-1800 cm-1) IR spectra of solid samples of the ZnPSn
molecules are shown in Figure 6 (left panel). The spectra exhibit
numerous bands in this frequency region. Previous studies of
structurally related porphyrins have shown that the majority of
these bands are due to in-plane modes of the porphyrin ring or
the aryl substituents, one of the most prominent features being
(3) There are certain differences in the absolute IR intensities
of the SAMs among the different ZnPSn molecules. For
instance, the cis-ZnPS2 SAM clearly exhibits lower absolute
IR intensities for all vibrational bands than any of the other
four SAMs. This result is qualitatively consistent with the
electrochemical studies, which indicate that the saturation
surface concentration for the cis-ZnPS2 SAM is significantly
lower than for the other ZnPSn SAMs (see Table 1). We also
note that the IR intensities for the ZnPS1 and trans-ZnPS2
SAMs appear to be somewhat lower than those of the ZnPS3
and ZnPS4 SAMs. However, these differences cannot be related
in a straightforward manner to differences in saturation coverage
exclusively because orientation effects also strongly influence
the appearance of the IR spectra. This point will be discussed
further below.
the porphyrin pyrrole ring breathing mode near 998 cm-1 50
. The
only prominent feature that is due to an out-of-plane mode of
the porphyrin is the band at 797 cm-1, which is due to a
â-pyrrole hydrogen deformation.51 For the ZnPSn molecules,
another important band relevant to this work is that at 1690
cm-1, which is assigned to the carbonyl stretching vibration(s)
ν(CdO) of the S-acetyl protecting group(s).13 The intensity of
this latter band generally increases (relative to the porphyrin
bands) as the number of S-AcSCH2pep appendages increases.
In the high-frequency (2600-3400 cm-1) spectral region (not
shown), only weak bands are observed, all attributable to C-H
stretches.
(4) There are differences in the relative IR intensities of the
in-plane (e.g., 998 cm-1) versus out-of-plane (797 cm-1
)
The mid-frequency (680-1800 cm-1) IR spectra of the
saturation-coverage ZnPSn SAMs are also shown in Figure 6
(right panel). The spectra of all of the ZnPSn SAMs are plotted
on the same absolute intensity scale to compare the relative
intensities of the bands, although the spectrum for the cis-ZnPS2
SAM is shown on an expanded intensity scale to facilitate the
porphyrin modes among different members of the ZnPSn series.
In particular, the IR intensities for the in-plane modes relative
to the out-of-plane mode of ZnPS1 and cis-ZnPS2 SAMs are
smaller than those of the ZnPS3 and ZnPS4 SAMs, with trans-
ZnPS2 SAM being somewhere in between. This effect, in
conjunction with the fact that in-plane modes dominate the IR
spectra, is largely responsible for the large apparent differences
in the absolute IR intensities of ZnPS1 and trans-ZnPS2 versus
ZnPS3 and ZnPS4. The relative intensities of the in-plane and
out-of-plane modes of the porphyrins in the SAM are influenced
(50) Li, X. Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Su, Y. O.; Spiro, T. G. J.
Phys. Chem. 1990, 94, 31-47.
(51) Li, X. Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Spiro, T. G. J. Am. Chem.
Soc. 1989, 111, 7012-7023.
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Characterization of SAMs of Porphyrins
A R T I C L E S
by the orientation of the molecule with respect to the plane of
the surface because only the components of the transition
moments of the vibrations of molecules adsorbed on metals
perpendicular to the plane of the surface contribute to the IR
intensities.52 The IR intensity increases monotonically as the
transition moment rotates out-of-plane and disappears when the
transition moment is parallel to the surface.53
The relative intensities of the in-plane and out-of-plane
porphyrin modes can be used to determine the average tilt angle
(R) of the porphyrin ring with respect to the surface normal.
The evaluation of the average tilt angle is predicated on certain
assumptions. (1) The porphyrin ring is planar, and thus the 998
and 797 cm-1 modes are pure in-plane and out-of-plane
vibrations, respectively. This assumption is reasonable consider-
ing that Zn tetraphenylporphyrin is planar.54 (2) The intrinsic
transition-dipole moments of the porphyrin vibrations in the
randomly oriented solid phase and in the surface-bound mono-
layer are the same. (3) The orientation of the porphyrin ring
can be described by one single angle, the average tilt from the
surface normal. Two more angles are in principle needed to
fully determine adsorption geometries, the polar angle around
the surface normal (assumed here to be random), and the rotation
around the main molecular axis (fixed at 0° with respect to the
surface plane). Preliminary work in our lab suggests that this
angle may vary from 0°, but that this variation may not affect
much the estimate of the azimuthal angle reported below. Under
these assumptions, the average azimuthal tilt angle for the
porphyrins in the SAMs can be determined from the ratios of
the relative intensities of the in-plane and out-of-plane porphyrin
modes in the SAMs versus the solids using standard meth-
ods.52,53 The angles determined from the IR data for the
saturation-coverage ZnPSn SAMs are as follows: ZnPS1, R ≈
58°; cis-ZnPS2, R ≈ 58°; trans-ZnPS2, R ≈ 50°; ZnPS3, R ≈
47°; ZnPS4, R ≈ 44°.
Figure 7. FTIR spectra of the ZnPS1 (left panel) and ZnPS4 (right panel)
SAMs at various surface concentrations. The surface concentrations and
the average tilt angles are indicated above each spectrum.
could be obtained. (2) These two porphyrins are the most
different in their nonbinding substituent groups. The IR spectra
of ZnPS1 and ZnPS4 SAMs as a function of surface concentra-
tion are shown in Figure 7, left and right panels, respectively.
The surface concentrations range from the low 10-11 mol cm-2
range (the lowest concentrations detectable by IR) to saturation.
The surface concentrations and average tilt angles are noted on
the individual spectra.
Inspection of the data shown in Figure 7 reveals the following
trends. (1) For both the ZnPS1 and ZnPS4 SAMs, the tilt angle
increases as the surface concentration decreases from the
saturation value. (2) The change in average tilt angle for the
ZnPS1 SAM is much smaller (∼7°) than that for the ZnPS4
SAM (∼21°). (3) The average tilt angles for both types of SAMs
appear to converge to a similar value of R ≈ 65° at surface
concentrations near Γ ≈ 4 × 10-11 mol cm-2, which is only
modestly below the saturation value (Γ ≈ 7 × 10-11 mol cm-2).
(4) For ZnPS4, no abrupt intensity change occurs for the
ν(CdO) band as the surface concentration decreases. If such
an intensity change were observed, it could imply binding via
a second thiol as the porphyrins become sparse on the surface.
Finally, we note that the trend in average tilt angles as a function
of surface concentration observed for the ZnPS1 and ZnPS4
SAMs qualitatively parallels the trend observed in the redox
thermodynamic parameters. In particular, for the ZnPS1 SAM,
both E0/+1 and ∆Ep,1/2 exhibit only modest changes in value
The average tilt angle for the porphyrins in the ZnPSn SAMs,
together with the number and position of the S-AcSCH2pep
appendages, determine the molecular footprint on the surface.
Accordingly, the more upright orientation for the molecules in
the ZnPS3 and ZnPS4 SAMs is likely a contributing factor to
the higher saturation surface concentrations observed for these
SAMs versus the more tilted cis-ZnPS2 and trans-ZnPS2 SAMs.
The more tilted orientation of cis-ZnPS2 versus trans-ZnPS2
may also result in the CdO transition dipole of the S-acetyl
group of the former molecule being more parallel to the surface
plane than that of the latter, leading to its attenuated IR intensity
(relative to the porphyrin modes). On the other hand, the ZnPS1
SAMs exhibit relatively high saturation surface concentrations
despite a more tilted orientation. This result is likely due to the
fact that the phenyl substituents are small relative to the
S-AcSCH2pep appendages of the other molecules.
To evaluate more fully the effects of surface concentration
on porphyrin orientation, FTIR spectra were obtained for the
ZnPS1 and ZnPS4 SAMs as a function of surface coverage.
These two SAMs were chosen for this study for two reasons.
(1) Both porphyrins can attain high surface coverage and
therefore provide a good dynamic range over which the IR data
over the concentration range Γ ≈ 1-7 × 10-11 mol cm-2
,
whereas for the ZnPS4 SAM, the changes in these redox
parameters are much larger (see Figure 2).
IV. Discussion
The studies reported herein on the series of ZnPSn SAMs
permit a systematic exploration of the factors that influence (1)
the binding mode of the SAMs to the Au surface, (2) the
orientation of the porphyrin rings with respect to the surface
(52) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-315.
(53) Yates, J. T., Jr.; Madey, T. E.; Eds. Vibrational Spectroscopy of Molecules
on Surfaces; Plenum Press: New York, 1987.
(54) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978, 17, 706-
710.
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A R T I C L E S
Yasseri et al.
plane, and (3) the packing density of the adsorbed layer. These
structural properties of the SAMs underpin the thermodynamics
and kinetics of electron transfer. In the sections below, we
address each of these issues.
tilted (R ≈ 58°) with respect to the surface normal than those
in the ZnPS3 and ZnPS4 SAMs (R ≈ 44°). The tilt angle of
the porphyrins in the trans-ZnPS2 SAM (R ≈ 50°) is intermedi-
ate between these two values. The less tilted orientation for the
porphyrins in the ZnPS3 and ZnPS4 allows these molecules to
pack more tightly than the porphyrins in the cis-ZnPS2 or trans-
ZnPS2 SAMs. Indeed, the molecular areas for the porphyrins
in the ZnPS3 and ZnPS4 SAMs are comparable to those of the
ZnPS1 SAM (see Table 2), despite the fact that the multiple
S-AcSCH2pep appendages of the former molecules are much
larger than the phenyl groups of the latter.
The first general theme that emerges from our studies is that,
regardless of the number of S-AcSCH2pep appendages available
for binding to the Au surface, binding only occurs through a
single linkage (at saturation coverage). Moreover, in the SAMs
of the multithiol-functionalized porphyrins, the unbound thiols
retain their acetyl protecting groups. This latter observation
implies that the unbound thiols either never make contact with
the Au surface or that the contact time is too short to cause
cleavage of the protecting group. This explanation seems
unlikely, however, given that attachment via multiple thiols is
not seen on the surface even at low coverages (Figure 7).
The observation that the more bulky porphyrins in the ZnPS3
and ZnPS4 SAMs can pack more tightly than the other members
of the set seems counterintuitive. We do not have a complete
explanation for this observation and can only speculate why
this trend is observed. Interactions between the S-AcSCH2pep
appendages on neighboring molecules must play a large role in
determining the packing. In particular, for the ZnPS3 and ZnPS4
SAMs, there are two S-AcSCH2pep appendages adjacent to the
SCH2pep linker to the surface. [In the case of ZnPS3, we
speculate that this is the preferred binding geometry, as opposed
to asymmetrical binding resulting in one adjacent and one
opposite S-AcSCH2pep appendage.] Possibly, the terminal
ethynylphenyl portion of these adjacent S-AcSCH2pep append-
ages can π stack with the porphyrin macrocycle of a neighboring
molecule. This type of packing would seem to be the only way
that the porphyrins in the ZnPS3 and ZnPS4 SAMs could
achieve packing densities and effective molecular areas ap-
proaching those exhibited by the porphyrins in the ZnPS1 SAM.
On the other hand, this type of π-π interaction is not possible
for the S-AcSCH2pep appendage opposite to the linker to the
surface. This might explain why the porphyrins in the trans-
ZnPS2 SAM are more tilted and less tightly packed than those
in the ZnPS3 and ZnPS4 SAMs. It should be noted, however,
that if π-π stacking does drive the packing of the latter SAMs,
one might speculate that the cis-ZnPS2 SAM would also be
tightly packed, rather than loosely as is observed. The poor
packing of the cis-ZnPS2 SAM might be explained by the
asymmetric substituent pattern (S-AcSCH2pep and phenyl) of
the groups that are adjacent to the linker to the surface.
Achieving long range π-π stacking between the single
S-AcSCH2pep appendage and a neighboring porphyrin would
require an arrangement of porphyrins on the surface that is
entropically unfavorable.
The fact that a molecule such as trans-ZnPS2 does not bind
via both thiols is not surprising given that attachment in this
fashion would require that the porphyrin lie completely flat on
the surface. This geometry is precluded because the aryl
substituents on the meso-positions of the porphyrin are not
coplanar with the porphyrin macrocycle (due to steric con-
straints),54 thereby restricting access of the second thiol to the
surface. Indeed, the S-AcSCH2pep appendage was specifically
chosen to disfavor surface geometries in which the porphyrin
ring lies (nearly) flat on the surface. Perhaps more surprising is
the observation that none of the ZnPSn molecules that contain
cis-thiol-functionalization bind via a pair of adjacent thiols,
because in those cases, the S-AcSCH2pep appendage does not
impose any steric constraints to preclude binding via two
adjacent thiols. In addition, the presence of the methylene group
in the S-AcSCH2pep appendage places the thiol moiety at the
end of a conformationally flexible tether. One explanation for
the failure of the second thiol to bind to the surface is that the
free energy associated with packing of the disklike porphyrin
molecules outweighs any additional thermodynamic stabilization
afforded by the formation of a second Au-S bond. In this
regard, changes in adsorption geometry from lying flat to tilted
upon increasing packing are common with aromatic molecules
on metal surfaces.55,56 Alternatively, the kinetics of the deposi-
tion process may also come into play in these systems. In
particular, multiple porphyrins may arrive simultaneously in the
same vicinity and begin to pack as they bind, blocking adjacent
sites and therefore precluding attachment of the second cis-
thiol. Note, however, that if this latter effect does indeed
influence the binding mode, then it must come into play even
when the SAMs are at less than saturation coverage. We further
note that binding by two adjacent thiols cannot be completely
ruled out at very low surface coverage (10-12 mol cm-2 range),
because the spectroscopic tools available do not have sufficient
sensitivity to interrogate the SAMs at these surface concentra-
tions.
The third conclusion arising from the studies reported herein
is that the packing density influences both the thermodynamics
and the kinetics of electron transfer. All of the ZnPSn SAMs
follow the same general trend as the surface concentration
increases. As the monolayers become more densely packed, the
redox process becomes less favorable, more heterogeneous, and
slower (see Figures 2 and 3). The largest effects are observed
at the lowest surface concentrations (Γ < 1 × 10-11 mol cm-2).
Interestingly, changes in the average tilt angle of the porphyrin
as a function of surface concentration appear to be correlated
with changes in the redox thermodynamics, but not the kinetics
(cf. Figure 7 with Figures 2 and 3). We are uncertain as to why
the thermodynamics and kinetics are affected differently.
Regardless, the influence of surface concentration on the redox
thermodynamics and kinetics is likely due to space-charge
effects.25 The space charge on the SAM increases as the surface
The second observation derived from this work is that the
orientation and packing density of the ZnPSn molecules on the
surface are strongly influenced by the nature (S-AcSCH2pep
versus phenyl groups), position (cis versus trans), and number
of the substituent groups. In particular, at saturation coverage,
the porphyrins in the ZnPS1 and cis-ZnPS2 SAMs are more
(55) Zaera, F. Prog. Surf. Sci. 2001, 69, 1-98.
(56) Netzer, F. P.; Ramsey, M. G. Crit. ReV. Solid State Mater. Sci. 1992, 17,
397-475.
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Characterization of SAMs of Porphyrins
A R T I C L E S
concentration increases. Space-charge effects are exacerbated
in very densely packed monolayers because the solvent/
counterions have more limited access to the redox center.
The observation that the electron-transfer kinetics exhibit only
small changes in surface-concentration regimes where the
average tilt angle (and the distance) of the porphyrin with respect
to the surface is changing the most (i.e., Γ > ∼4 × 10-11 mol
cm-2) has implications for the mechanism of electron transfer.
In particular, this observation argues against any significant
though-space contribution to the electron-transfer process, at
least in that surface concentration range, because changes in
the distance of the redox center to the surface would be expected
to strongly affect the rates if this mechanism were operative.
Instead, the results are more consistent with the notion that the
dominant mechanism for electron transfer from the porphyrin
to the surface is a through-bond nonadiabatic process that
involves the SCH2pep linker as a superexchange mediator, as
suggested by previous studies.23-25 In this regard, the fact that
the porphyrins in all of the ZnPSn SAMs in the high coverage
regime are linked to the surface via a single thiol would result
in a similar linker-mediated electronic coupling between the
molecule and the surface.
and packing density in the SAMs are determined by a complex
interplay of intra- and intermolecular forces. Our seemingly
reasonable preconceived notion concerning how the position
of potential surface binding groups (cis-thiol designs) would
affect binding did not prove correct. Similarly, the number and
placement of the S-AcSCH2pep appendages have a much larger
effect on the orientation and packing density of the molecules
than might have been expected. Indeed, the presence of multiple,
accessible surface attachment groups does not imply binding
of multiple groups upon SAM formation. Collectively, these
observations point to the difficulty of predicting the structure
of SAMs on the basis of the structure of the component
molecules alone. In this respect, predictions of the structure of
the porphyrin SAMs appear to have many of the same challenges
associated with crystal engineering, where the overall architec-
ture is dictated by the sum of a large number of subtle molecular
interactions.
Acknowledgment. This work was supported by the Center
for Nanoscience Innovation for Defense and DARPA/DMEA
(under award number DMEA90-02-2-216) and Zettacore. We
thank J. Diers for obtaining the solution electrochemical data.
Mass spectra were obtained at the Mass Spectrometry Labora-
tory for Biotechnology at North Carolina State University.
Partial funding for the facility was obtained from the North
Carolina Biotechnology Center and the NSF.
V. Outlook
The studies reported herein to elucidate the structural features
of the ZnPSn SAMs have implications that extend beyond our
immediate investigation of porphyrins for information-storage
applications. In particular, the studies show that the surface
orientation and packing density of the ZnPSn SAMs can be
controlled by the number and position of the S-AcSCH2pep
appendages. However, the surface binding mode, orientation,
Supporting Information Available: Complete synthetic pro-
cedures and spectral data for each new compound. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA047723T
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