Adsorption characteristics of tripodal thiol-functionalized porphyrins on gold

Article abstract of DOI:10.1021/jp0537005

X-ray photoelectron and Fourier transform infrared spectroscopy studies are reported for self-assembled monolayers (SAMs) of two tripodal thiol-functionalized metalloporphyrins (Zn and Cu) and three benchmark tripods on gold substrates. The tripodal unit common to all five molecules is 1-(phenyl)-1,1,1 -tris(4-mercaptomethylphenyl)methane (Tpd). Both porphyrins contain S-acetyl-protected thiols and are linked to the 4-position of the phenyl ring of Tpd via a phenylethyne group. The benchmark molecules include (1) two tripods containing a bromine atom at the 4-position of the apical phenyl ring, one a free thiol and the other its S-acetyl-protected analogue, and (2) a S-acetyl-protected tripod containing a phenylethyne unit at the 4-position of the apical phenyl group. Together, the spectroscopic studies reveal that none of the five tripodal molecules bond to the gold surface via all three sulfur atoms. Instead, the average number of bound thiols ranges from 1.5 to 2, with the porphyrinic molecules generally falling at the middle to upper end of the range and the smallest benchmark tripods falling at the lower end. Similar surface binding is found for the S-acetyl-protected and free benchmark tripods, indicating that the presence of the protecting group does not influence binding. Furthermore, the surface binding characteristics of the SAMs are not sensitive to deposition conditions such as solvent type, deposition time, or temperature of the solution.

Full text of DOI:10.1021/jp0537005

J. Phys. Chem. B 2005, 109, 23963-23971
23963
Adsorption Characteristics of Tripodal Thiol-Functionalized Porphyrins on Gold
Lingyun Wei,† Hugo Tiznado,† Guangming Liu,† Kisari Padmaja,‡ Jonathan S. Lindsey,*,‡
Francisco Zaera,*,† and David F. Bocian*,†
Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and Department of
Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204
ReceiVed: July 6, 2005; In Final Form: October 13, 2005
X-ray photoelectron and Fourier transform infrared spectroscopy studies are reported for self-assembled
monolayers (SAMs) of two tripodal thiol-functionalized metalloporphyrins (Zn and Cu) and three benchmark
tripods on gold substrates. The tripodal unit common to all five molecules is 1-(phenyl)-1,1,1-tris(4-
mercaptomethylphenyl)methane (Tpd). Both porphyrins contain S-acetyl-protected thiols and are linked to
the 4-position of the phenyl ring of Tpd via a phenylethyne group. The benchmark molecules include (1)
two tripods containing a bromine atom at the 4-position of the apical phenyl ring, one a free thiol and the
other its S-acetyl-protected analogue, and (2) a S-acetyl-protected tripod containing a phenylethyne unit at
the 4-position of the apical phenyl group. Together, the spectroscopic studies reveal that none of the five
tripodal molecules bond to the gold surface via all three sulfur atoms. Instead, the average number of bound
thiols ranges from 1.5 to 2, with the porphyrinic molecules generally falling at the middle to upper end of the
range and the smallest benchmark tripods falling at the lower end. Similar surface binding is found for the
S-acetyl-protected and free benchmark tripods, indicating that the presence of the protecting group does not
influence binding. Furthermore, the surface binding characteristics of the SAMs are not sensitive to deposition
conditions such as solvent type, deposition time, or temperature of the solution.
I. Introduction
CHART 1
Owing to their potential applications in nanopatterning,
chemical sensors, and molecular electronics, self-assembled
monolayers (SAMs) of thiol-derivatized molecules on gold
substrates have been widely studied.1-3 Typically, these mol-
ecules are functionalized with a single thiol for attachment to
the gold substrate,4 because the small size of a single sulfur-
atom anchor may afford superior packing in the SAM. On the
other hand, the integrity of the surface attachment with a single-
atom anchor is pinned solely on that Au-S bond. One strategy
for achieving enhanced robustness of surface attachment and
better control of molecular orientation is to use multiple
anchoring atoms. Toward this end, a variety of molecules have
been designed with multiple thiol tethers, including tripodal units
having each leg of the tripod bearing a sulfur anchoring atom.
A comprehensive list of references to the tripodal literature is
provided in the Supporting Information.
Our group has been engaged in a program aimed at construct-
ing devices that use the redox states of porphyrinic molecules
as the active medium for information storage.5-17 In this design,
a molecular monolayer is attached to an electroactive surface
such as gold or silicon. In our past studies of porphyrinic
molecules on gold, a number of thiol anchors have been used
for surface attachment. In a recent study, we examined SAMs
of molecules functionalized with a tetraarylmethane-based
tripodal thiol anchoring group (Chart 1),9 similar to that
employed by others to prepare molecular SAMs for a variety
of proposed applications, including organic light-emitting diodes,
sensors, and molecular electronic devices.18-20 An unexpected
observation from our studies was that the surface coverage of
the molecules with the tripodal thiol anchoring group was
relatively low (∼3 × 10-11 mol‚cm-2),9 ∼3-fold lower than
that achieved with analogous molecules containing a monopodal
thiol anchor (∼8 × 10-11 mol‚cm-2).13,14 We speculated that
the low coverage obtained for our molecular tripods might be
due to the relatively large molecular footprint of the tetraaryl-
methane base unit. This result prompted us to investigate the
surface-binding characteristics of the molecular tripods in more
detail.
Herein, we examine the surface-bonding and structural
characteristics of SAMs of two tripodal thiol-functionalized
metalloporphyrins (Zn and Cu) and three benchmark tripods
(Chart 2). The tripodal unit common to all five molecules is
* Authors to whom correspondence should be addressed (e-mail
jlindsey@ncsu.edu, Francisco.Zaera@ucr.edu, or David.Bocian@ucr.edu).
† University of California.
‡ North Carolina State University.
10.1021/jp0537005 CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/24/2005

23964 J. Phys. Chem. B, Vol. 109, No. 50, 2005
Wei et al.
CHART 2
1-(phenyl)-1,1,1-tris(4-mercaptomethylphenyl)methane (Tpd).
The two metalloporphyrins (ZnP-Tpd and CuP-Tpd) bear
mesityl groups at the three nonlinking meso positions, are linked
to the 4-position of the phenyl ring of Tpd via a phenylethyne
group attached to the fourth meso position, and contain S-acetyl-
protected thiols. The benchmark molecules include (1) two
tripods each containing a bromine atom at the 4-position of the
apical phenyl ring, one a free thiol (Tpd-1′) and the other its
S-acetyl-protected analogue (Tpd-1), and (2) a S-acetyl-
protected tripod containing a phenylethyne unit at the 4-position
of the apical phenyl ring (Tpd-2). The surface-bonding and
structural characteristics of the five tripods were examined using
X-ray photoelectron (XPS) and Fourier transform infrared
(FTIR) spectroscopy. The salient observation that emerges from
these studies is that none of the five molecular tripods bind to
the gold surface via all three sulfur atoms.
silica (particle size ) 32-63 µm) for sample purification.
Absorption spectra were collected in toluene for sample
characterization. 1H NMR (300 MHz) and 13C NMR (75 MHz)
spectra were obtained in CDCl3 unless otherwise noted. Ad-
ditional characterization was carried out by laser desorption mass
spectrometry (LD-MS) in the absence of a matrix and/or by
fast atom bombardment mass spectrometry (FAB-MS).
2. a. ZnP-Tpd was prepared as described in the literature.9
2. b. 5,10,15-Trimesityl-20-[4-(3-methyl-3-hydroxybut-1-yn-
1-yl)phenyl]porphinatocopper(II) (CuP-3). A sample of 5,10,15-
trimesityl-20-[4-(3-methyl-3-hydroxybut-1-yn-1-yl)phenyl]por-
phyrin (P-3)21 was treated with copper acetate monohydrate to
give the copper chelate CuP-3 (Scheme 1). Following a general
procedure,22 a solution of P-3 (206 mg, 0.250 mmol) in
CH2Cl2/MeOH (1:1) (5 mL) was treated with Cu(OAc)2‚H2O
(1.25 g, 6.25 mmol) at room temperature. The progress of the
reaction was monitored by TLC. The reaction was complete in
1.5 h. Chromatographic purification (silica, CH2Cl2) afforded
a reddish purple solid (217 mg, 98%): LD-MS obsd 883.4;
FAB-MS obsd 883.3461, calcd 883.3537 (C58H52CuN4O); λabs
418, 541 nm.
II. Experimental Section
A. Synthesis. 1. General Procedures. Dry toluene and dry
triethylamine (TEA) were prepared by distillation over CaH2.
The CHCl3 was of reagent grade and contained 0.9% v/v
ethanol. All other chemicals were of reagent grade and were
used as received. All of the palladium-catalyzed couplings were
carried out under argon using standard Schlenk techniques.
Adsorption column chromatography was performed using flash
2. c. 5-(4-Ethynylphenyl)-10,15,20-trimesitylporphinatocop-
per(II) (CuP-4). Reaction of CuP-3 with sodium hydroxide in
refluxing toluene provided CuP-4 in quantitative yield (Scheme
1). Following a general procedure,15 a solution of porphyrin

Tripodal Thiol-Functionalized Porphyrins
J. Phys. Chem. B, Vol. 109, No. 50, 2005 23965
SCHEME 2
SCHEME 1
mg, 20. µmol), and P(o-tol)3 (48 mg, 160 µmol) were added.
The stirring was continued at 65 °C. Three additional portions
of Pd2(dba)3 (18 mg, 20. µmol) and P(o-tol)3 (48 mg, 160 µmol)
were added at 5.25, 8.75, and 11.5 h. After 15 h, additional
portions of Pd2(dba)3 (36 mg, 39 µmol), and P(o-tol)3 (96 mg,
320 µmol) were added, followed by the addition of pheny-
lacetylene (100 µL, 910 µmol). The stirring was continued at
65 °C. The reaction was stopped after a total of 36 h. The
solution was concentrated. The resulting residue was chromato-
graphed [silica, hexanes/ethyl acetate (85:15)], affording a
colorless liquid (29.6 mg, 22%): 1H NMR (400 MHz) δ 2.30-
2.42 (m, 9H), 4.08 (s, 6H), 7.00-7.24 (m, 14H), 7.28-7.44
(m, 5H), 7.46-7.56 (m, 2H); 13C NMR δ 30.57, 33.11, 64.39,
89.32, 89.68, 121.05, 127.74, 128.07, 128.12, 128.26, 128.44,
128.53, 131.02, 131.11, 131.33, 131.43, 131.80, 132.16, 135.23,
135.46, 145.44, 146.97, 195.39; FAB-MS obsd 684.1848, calcd
684.1827 (C42H36O3S3).
2. e. 5-[4-[2-[4-[1,1,1-Tris[4-(S-acetylthiomethyl)phenyl]-
methyl]phenyl]ethynyl]phenyl]-10,15,20-trimesitylporphinato-
copper(II) (CuP-Tpd). A similar reaction of Tpd-120 with
CuP-4 provided CuP-Tpd (Scheme 3). A Schlenk flask
containing Tpd-1 (121 mg, 182 µmol), CuP-4 (100 mg, 121
µmol), Pd2(dba)3 (16 mg, 18 µmol), and P(o-tol)3 (44 mg, 150
µmol) was alternatively evacuated and purged with argon three
times. Toluene/TEA (5:1, 8.4 mL) was added, and the mixture
was stirred at 65 °C. The progress of the reaction was monitored
by TLC. The reaction was complete in 5.5 h. The solvent was
evaporated and the residue chromatographed (silica, hexanes/
CH2Cl2 1:1), affording a reddish purple solid (58 mg, 34%):
LD-MS obsd 1407.3; FAB-MS obsd 1407.4319, calcd 1407.4376
(C89H76CuN4O3S3); λabs 420, 540 nm.
CuP-3 (193 mg, 218 µmol) in toluene (15 mL) was treated with
powdered NaOH (348 mg, 8.70 mmol) under reflux conditions.
The reaction was monitored by TLC. The reaction was stopped
after 5 h. The reaction mixture was then cooled to room
temperature, washed with water, and extracted with CH2Cl2.
The organic layers were combined and concentrated. The
resulting residue was dissolved in CH2Cl2 and filtered through
a short pad of silica, affording a dark pink solid. The reaction
was quantitative: LD-MS obsd 825.3; FAB-MS obsd 825.3027,
calcd 825.3018 (C55H46CuN4); λabs 418, 542 nm.
2. d. 1-(4-Phenylethynylphenyl)-1,1,1-tris[4-(S-acetylthio-
methyl)phenyl]methane (Tpd-2). The key reaction for the
synthesis of the target molecules is the palladium-mediated
Sonogashira coupling of a bromoaryl and an ethynylaryl
molecule (Scheme 2), which was carried out using conditions
developed for porphyrin substrates.23,24 A Schlenk flask contain-
ing Tpd-120 (132 mg, 199 µmol), Pd2(dba)3 (18 mg, 20 µmol),
and P(o-tol)3 (48 mg, 160 µmol) was alternatively evacuated
and purged with argon three times. Toluene/TEA (5:1, 5.3 mL)
and phenylacetylene (1.0 mL of 0.136 mM solution in toluene)
were added, and the mixture was stirred at 65 °C. The progress
of the reaction was monitored by TLC. After 2 h, additional
portions of phenylacetylene (50.0 µL, 455 µmol), Pd2(dba)3 (18
2. f. 1-(4-Bromophenyl)-1,1,1-tris(4-mercaptomethylphenyl)-
methane (Tpd-1′). Reductive cleavage of the thioacetyl func-
tionality of Tpd-120 with LiAlH4 provided the free thiol tripod
Tpd-1′ in 14% yield (Scheme 4). Following a general proce-
dure,25 lithium aluminum hydride (1 M solution in THF, 4.6
mL) was added dropwise to a solution of Tpd-1 (514 mg, 0.770

23966 J. Phys. Chem. B, Vol. 109, No. 50, 2005
Wei et al.
SCHEME 3
SCHEME 4
dried (Na2SO4) and concentrated. TLC analysis [silica, hexanes/
CH2Cl2 (1:1)] showed the presence of two components (∼1:1
ratio) with similar retention factors. The resulting residue was
chromatographed [silica, hexanes/CH2Cl2 (1:1)] to provide a
white solid (58 mg, 14%): 1H NMR δ 1.77 (t, J ) 7.8 Hz,
3H), 3.71 (d, J ) 7.8 Hz, 6H), 7.04-7.24 (m, 14H), 7.33-
7.40 (m, 2H); 13C NMR δ 28.7, 64.1, 120.3, 127.6, 130.9, 131.3,
132.9, 139.1, 145.2, 145.9. A satisfactory mass spectrum was
1
not obtained; however, the H NMR spectrum clearly showed
the loss of the acetyl groups (absence of the singlet at δ 2.34)
from Tpd-1, as well as the appearance of the triplet owing to
the SH units (δ 1.77) and the doublet owing to the -CH2-
moieties (δ 3.71) of the product Tpd-1′.
B. Structural Characterization. 1. Chemicals and Materials.
The solvents used for the preparation of the SAMs were
anhydrous CH2Cl2 (Aldrich, 99.8%), absolute EtOH (Gold
Shield), benzonitrile (Aldrich, 99%), THF (Aldrich, 99%), and
toluene (Aldrich, 99%); all were used as received. Silicon wafers
(Silicon Valley Microelectronics) were purchased as nonoxi-
dized B-doped Si(100) (F ) 13-30 Ω‚cm). Ar and N2
(99.995%) were passed through Drierite (Fisher) and Oxyclear
(Supelco) gas purifiers prior to use.
2. Gold Substrate Preparation. The gold films were prepared
by e-beam vapor deposition of 20 nm of Cr (99.999%) followed
by 200 nm of Au (99.99%) on the surface of precleaned B-doped
Si(100) wafers. The chromium and gold films were deposited
under vacuum (P < 2.0 × 10-6 Torr) at 0.5 and 1.5 Å‚s-1
,
respectively. Upon completion of the gold deposition, the wafer
was diced into ∼1 cm2 pieces, and each piece was immediately
inserted into a VOC-type glass vial fitted with a Teflon-silicon
rubber septum. The vials were purged with Ar (99.995%) to
maintain the samples under an inert environment.
mmol) in dry THF (25 mL) cooled to 0 °C under argon. The
reaction mixture was stirred at 0 °C for ∼5 h. Excess LiAlH4
was quenched by dropwise addition of ethyl acetate (∼10 mL),
during which a greenish yellow solid separated from the
solution. The solvent was removed and the residue thus obtained
dissolved in CH2Cl2 and washed with 1 N HCl followed by
water. The organic layer was separated, and the aqueous layer
was extracted with CH2Cl2. The combined organic layer was
3. SAM Preparation. The SAMs of the porphyrin and
benchmark tripods were prepared from CH2Cl2/absolute EtOH
(85:15), THF, and toluene solutions at room temperature. For

Tripodal Thiol-Functionalized Porphyrins
J. Phys. Chem. B, Vol. 109, No. 50, 2005 23967
SAMs prepared at elevated temperature, benzonitrile was used
as the solvent. Saturation coverage SAMs for the XPS and FTIR
studies were prepared by depositing successive 50 µL aliquots
of a 3 mM solution of the porphyrin or benchmark tripod onto
the gold substrate. Previous studies of the porphyrin tripods have
shown that saturation coverage is achieved with deposition times
of e15 min,9 and FTIR studies revealed that this is also the
case for the benchmark tripods. Assembly times of 24 h did
not increase the coverage for any of the molecules (as also
revealed by FTIR studies). After molecule deposition, the
substrate was repeatedly sonicated and rinsed (five times) with
CH2Cl2 and dried with a stream of Ar.
4. X-ray Photoelectron Spectroscopy (XPS) Measurements.
The XPS data were acquired with a Leybold EA11-MCD
spectrometer equipped with a Mg KR X-ray (1253.6 eV) source,
a 100 mm hemispherical analyzer, and an 18-channel detector.
The main XPS chamber was maintained at a steady base
pressure of <2 × 10-9 Torr during analysis. The samples were
introduced by using a fast-transfer mechanism composed of a
long rod and an intermediate pumping stage. The total time
required for the introduction of the sample was ∼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 for all of the
SAMs using a band-pass energy of 100.8 eV, which corresponds
to a spectral resolution of approximately ∆E1/2 ∼ 1.5 eV.
Additional high-resolution spectra were obtained for the S 2p,
Zn 2p, Cu 2p, C 1s, and N 1s peaks using 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.
Figure 1. Survey XPS for the saturation coverage ZnP-Tpd (top)
and CuP-Tpd (bottom) SAMs on gold. (Insets) High-resolution scans
of the Zn 2p3/2 (top) and Cu 2p3/2 (bottom) spectral regions: raw data
(dots) and fits (solid).
The high-resolution spectra for the regions of interest were
fitted using Gaussian-Lorentzian lines with XPSPEAK soft-
ware. In the case of the S XPS signal, overlapping 2p spin-
split doublets are observed for two different types of sulfur
species. The spin-split doublets of each species were fitted by
setting their energy difference at 1.2 eV and their area ratio at
2p3/2:2p1/2 ) 2:1 and by also fixing the peak widths at half-
maximum at constant values. The relative intensities and binding
energies of each pair of doublets were then allowed to vary in
the fitting process, even though ultimately the binding energies
of each of the two sulfur species identified in the data turned
out to be identical for all of the porphyrin and benchmark tripod
SAMs studied here; only the relative intensities of the signals
from the two different types of sulfurs were seen to vary.
each spectrum was obtained by averaging over 256 scans. The
peak-to-peak noise in the SAM spectra was e1.5 × 10-4
absorbance unit.
III. Results
A. XPS Studies. The XPS of the saturation coverage
porphyrin and benchmark tripod SAMs were examined to
characterize the bonding motif of the molecules to the gold
surface. Representative survey spectra for the ZnP-Tpd and
CuP-Tpd SAMs are shown in Figure 1. The spectra exhibit
multiple peaks consistent with the molecular composition of
the porphyrins. In particular, in addition to signals from the bulk
gold, the spectra show discrete signals for Zn (or Cu), N, and
S. The high-resolution Zn 2p3/2 and Cu 2p3/2 spectra (Figure 1,
insets) show peaks at binding energies of 1021.0 ( 0.1 and
934.5 ( 0.1 eV, respectively, in qualitative agreement with
previously reported values for zinc7,12,26 and copper porphy-
rins,27,28 respectively. The values of the N:Zn:S and N:Cu:S
atomic ratios, obtained by scaling the peak intensities by known
atomic cross-section sensitivity factors and correcting them for
the expected photoelectron attenuation, are also consistent with
the expected 4:1:3 stoichiometry. All of this attests to the
preservation of the integrity of the molecules upon their
deposition on the surface.
5. FTIR Spectroscopy. The FTIR spectra of the porphyrin
and benchmark tripods in both solids and SAMs were collected
at room temperature using a Bruker Tensor 27 FTIR spectrom-
eter. In all cases, the spectral resolution was 4 cm-1. The spectra
of the solid porphyrin samples were obtained in transmission
mode in KBr pellets (∼1-2 wt % of porphyrin) by using a
room temperature DTGS detector and averaging over 32 scans.
The spectra of the SAMs were acquired using a Harrick
Scientific horizontal single-reflection germanium ATR accessory
GATR (65° incidence angle) and a liquid-nitrogen-cooled
medium-bandwidth MCT detector (600-4000 cm-1). The Ge
crystal that serves as the optical element of the GATR was
cleaned with neat 2-butanone before each experiment, and the
SAM/Au substrates were then pressed against it at a constant
torque, the same in all measurements. The GATR accessory
was purged with dry N2 during the data acquisition period, and
The extent of binding of the sulfur atoms to the gold surface
was examined in detail by analysis of the high-resolution S 2p

23968 J. Phys. Chem. B, Vol. 109, No. 50, 2005
Wei et al.
Figure 2. High-resolution XPS for the saturation coverage ZnP-Tpd
(top) and CuP-Tpd (bottom) SAMs in the S 2p spectral region: raw
data (dots) and fits (solid). Each spectrum was fitted to two sets of S
2p3/2 and S 2p1/2 peaks as described in the text. The lower and higher
energy sets are representative of bound (SB) and unbound (SU) sulfur
atoms, respectively.
Figure 3. High-resolution XPS for the saturation coverage Tpd-2 (top),
Tpd-1 (middle), and Tpd-1′ (bottom) SAMs in the S 2p spectral
region: raw data (dots) and fits (solid). The same analysis and
interpretation as in Figure 2 were used here.
XPS spectra for the porphyrin and benchmark tripod SAMs
shown in Figures 2 and 3. As noted in the Experimental Section,
the spectra point to the presence of two types of sulfur atoms
in the SAMs of all five tripodal molecules studied here. This is
indicated by the two binding energy values obtained upon fitting
of the data, 162.1 ( 0.1 and 163.6 ( 0.1 eV (Table 1), the
same for all molecules. On the basis of published work, the
first binding energy is known to correspond to thiol-function-
alized monolayers covalently bound to gold substrates (SB),7,29-32
whereas the second, which is upshifted by ∼1.5 eV from typical
S-Au species, can be assigned to the presence of unbound sulfur
(SU).7,33,34 Further integration of the areas under each of those
peaks was used to quantify the relative populations of the two
sulfur species (Table 1). The average number of bound thiols
was found to range from 1.5 to 2, with Tpd-1 and Tpd-1′ falling
at the lower end, Tpd-2 and CuP-Tpd somewhere in the
middle, and ZnP-Tpd at the high end. The observation that
the ratio of bound-to-unbound sulfur is similar for Tpd-1 and
Tpd-1′ indicates that the presence of the protecting group, which
cleaves upon binding to gold,15-17 does not influence binding.
[Note that the S-atom binding energies of unbound S-Ac and
S-H cannot be distinguished to within experimental error. The
retention of the protecting group for Tpd-1 is confirmed by
the IR studies (vide infra).] Finally, relative saturation coverages
were estimated for all of the SAMs by using the total area under
the S 2p XPS traces (Table 1). Clearly, the smaller Tpd-1 and
Tpd-1′ molecules reach coverages significantly higher than
those seen for the porphyrins. None of the Tpd-2, ZnP-Tpd,
or CuP-Tpd SAMs reach saturation coverages past those
obtained with the simple tripods, but, surprisingly, the zinc
porphyrin can pack ∼25% more molecules on the gold surface
than the copper analogue.
B. FTIR Studies. The mid-frequency (650-2000 cm-1) IR
spectra of the solid samples of the porphyrin and benchmark
tripods are shown in the left panel of Figure 4. The spectra are
all quite rich, as expected for the relatively large aromatic units,
and their detailed interpretation is beyond the scope of the
current investigation. Instead, we focus on features key to the
present studies, which include the following: (1) two absorptions
that distinguish the porphyrins from the benchmark tripodssa
band at 998 cm-1, which is due to an in-plane pyrrole ring-
breathing mode,35 and the peak at 797 cm-1, which is due to
an out-of-plane â-pyrrole-hydrogen deformation36 [note that
Tpd-1′ also exhibits bands in the 800 and 1000 cm-1 regions;
however, closer inspection of the spectra show that the frequen-
cies of the bands are at 804 and 1004 cm-1, which are readily
distinguishable from the signature porphyrin bands at 797 and
998 cm-1]; (2) the signature absorption of the S-acetyl protecting
groupssa band at 1693 cm-1, corresponding to the CdO
stretches.6,7 In the spectrum of Tpd-1′, the 1693 cm-1 band is
absent, but a broad weak band is observed near 2540 cm-1
,
corresponding to the S-H stretches of the free thiols.37
The corresponding mid-frequency (650-2000 cm-1) IR
spectra of the saturation coverage porphyrin and benchmark
tripod SAMs are shown in the right panel of Figure 4. The same
absolute intensity scale was used to report all of the spectra in
this panel in order to compare the relative intensities of the bands
of the different samples. Similar frequencies were measured for
most IR bands of each molecule in the SAM versus in the solid,
indicating that the structure of the molecule is not significantly
different in the two environments. A similar lack of structural
changes upon SAM formation has been observed by us for other
porphyrins bound to both gold and silicon surfaces.5-7 On the
other hand, the relative intensities of the IR bands in the SAMs

Tripodal Thiol-Functionalized Porphyrins
J. Phys. Chem. B, Vol. 109, No. 50, 2005 23969
TABLE 1: Sulfur XPS Binding Energies and Atomic Ratios and Coverages
S 2p3/2 binding energy/EV
no. of bound Sa,b
coverageb
SAM
bound (SB)
unbound (SU)
SB ) 3[A(SB)/[A(SB) + A(SU)]]
relative to Tpd-1
ZnP-Tpd
CuP-Tpd
Tpd-2
Tpd-1
Tpd-1′
162.1 ( 0.1
162.1 ( 0.1
162.0 ( 0.1
162.1 ( 0.1
162.2 ( 0.1
163.7 ( 0.1
163.5 ( 0.1
163.5 ( 0.1
163.7 ( 0.1
163.6 ( 0.1
2.0 ( 0.1
1.7 ( 0.1
1.8 ( 0.1
1.5 ( 0.1
1.6 ( 0.1
0.43 ( 0.05
0.34 ( 0.01
0.56 ( 0.05
1.00
0.96 ( 0.20
a A(SB) and A(SU) are the integrated areas of the SB and SU 2p3/2 XPS signals. b The error margins reported for the area ratios and coverages were
calculated from the analysis of multiple data sets for each type of SAM. The larger error for the coverage of Tpd-1′ arises because fewer samples
were analyzed.
Figure 4. FTIR spectra for the porphyrin and benchmark tripods, as
solids (left) and in saturation coverage SAMs (right).
Figure 5. FTIR spectra for ZnP-Tpd as a solid (bottom) and in SAMs
obtained using 3 mM (middle) and 0.1 mM (top) deposition solutions.
The latter corresponds to a SAM coverage somewhere between one-
third and half saturation.
versus solids are quite different. This is due to the fact that the
relative intensities of the bands are sensitive to the adsorption
geometry (vide infra).5-7
for ZnP-Tpd and CuP-Tpd were estimated to be ap-
proximately 41° and 43°, respectively. These values are
qualitatively similar to those we have previously found for other
types of porphyrin SAMs.5-7 In principle, an average tilt angle
could also be extracted from the IR data for the benchmark
tripods, but such analysis requires a detailed assignment of the
IR spectra, which is not presently available.
To gain additional insight into the factors that mediate the
binding of the various molecular tripods, a number of other
experiments were conducted and monitored via IR measure-
ments. In these experiments, the deposition solvent was changed
for CH2Cl2:EtOH (85:15) to THF or toluene, the deposition time
was increased from a few minutes to 24 h, the deposition
temperature was increased from room temperature to 100 °C,
and the deposition concentration was lowered from 3 to 0.1
mM. In the case of the benchmark tripods, the deposition
concentration was also increased to 25 mM. These latter (high)
concentrations cannot be achieved for the porphyrins owing to
solubility limitations. The changes in deposition solvent, time,
and temperature resulted in no measurable effect on the basic
appearance of the IR spectra. In particular, the signatures for
unbound S-acetyl-protected thiol species were always present,
with relative intensities similar to those shown for the SAMs
in Figure 4.
A key observation from the IR spectra for the SAMs of the
S-acetyl-protected molecules is that in all cases there is a clear
residual signal for the CdO stretch at 1693 cm-1 after
monolayer formation. This band is a signature for unbound
sulfurs retaining their protecting groups and, therefore, indicates
incomplete derivatization of those groups on the gold surface.
However, in the SAMs versus the solids, the intensities of the
CdO stretching vibrations are generally lower relative to those
of the other bands, in agreement with the premise of cleavage
of some but not all of the protecting groups in the SAMs. This
conclusion is in qualitative agreement with that reached from
the XPS studies. We note, however, that the IR intensities
provide only a qualitative measure of the amount of bound
versus unbound sulfur, because surface orientation effects also
influence those intensities. In fact, in the case of the Tpd-1′
SAM, no bands are evident near 2540 cm-1 that can be
attributed to S-H stretches of unbound sulfur. The absence of
these bands, despite the clear XPS evidence for unbound sulfur,
is most likely explained by such surface orientation effects: if
the transition dipole moment of the S-H stretch is nearly
parallel to the surface, the IR intensity would be null.38
The observation that the tripodal molecules do not bind via
all three sulfur atoms requires that their main axis be tilted with
respect to the surface normal. In the case of the porphyrins, the
average tilt angle of the porphyrin ring can be extracted by
analysis of the relative intensities of the in-plane pyrrole folding
versus out-of-plane â-pyrrole-hydrogen deformation modes.
The assumptions and details of this analysis can be found in
our earlier studies of porphyrin SAMs.6,7 The average tilt angles
of the porphyrin plane with respect to the surface normal (R)
In contrast to the minimal effects of solvent, temperature,
and time of deposition, lowering the deposition concentration
does alter the IR spectrum. This is illustrated in Figure 5, which
compares the IR spectra of the solid sample and the SAMs
obtained at two different deposition concentrations. Deposition
at lower concentration lowers the surface coverage to ap-
proximately one-third to half saturation, as judged by the total

23970 J. Phys. Chem. B, Vol. 109, No. 50, 2005
Wei et al.
intensity of the in- and out-of-plane porphyrin modes, and leads
to an increase of the average tilt angle of the porphyrin from
41° to 56°, as evidenced by the increased intensity of the out-
of-plane (797 cm-1) versus in-plane (998 cm-1) porphyrin
vibrations. We have previously observed similar surface cover-
age effects for other types of porphyrin SAMs on gold.7 More
importantly, a residual signal for the CdO stretch is still
apparent at 1693 cm-1 in the spectrum of the SAM at the lower
surface coverage, indicating that sulfur binding remains incom-
plete under these conditions. The intensity of the CdO stretching
vibration relative to the total intensity of the two porphyrin
modes seems to diminish at lower surface coverage, but this
does not necessarily indicate more bound sulfur in the latter
case (which would seemingly be at odds with increased tilting),
because the torsional flexibility of the acetyl groups could permit
orientations wherein the CdO bond(s) are more nearly parallel
to the plane of the surface, similar to the situation for the S-H
bonds in the Tpd-1′ SAM (vide supra). Finally, we note that
increasing the deposition concentration of the benchmark tripods
above 3 mM had no effect on the IR signature, consistent with
the view that saturation coverage is achieved with 3 mM
solutions of these tripods (vide supra).
We also note that previous studies of tripodal thiol anchors have
shown that registry effects are generally insignificant.39 In
addition, if registry were significant for complete binding of
the Tpd unit, one might expect the same number of anchoring
sulfurs for all of the five molecules studied here. The requisite
of a special ensemble of surface gold atoms to fit the adsorbing
molecule cannot account for the variations seen upon substitu-
tion at the top of the tripod unit. In addition, for each of the
molecules studied it was determined that the average number
of bound sulfurs is fractional (Table 1), indicating that binding
is not uniform throughout the SAMs. This nonuniformity might
arise from steric interactions between codepositing molecules.
A few pieces of evidence can be cited to support such a
conclusion. For one, the saturation surface coverages of the
benchmark tripods are ∼2-fold larger than those of the two
porphyrins (as determined by the XPS measurements, Table 1).
Although the former are smaller than the latter, their higher
surface coverage could result in increased intermolecular steric
interactions at the interface between the molecules and the gold
substrate. On the other hand, the steric intermolecular interac-
tions at the other end of the molecule are increased by the
addition of the phenylethyne and the more bulky porphyrinic
rings, and that may keep the tripod units farther apart at the
surface, affording them more room to accommodate the anchor-
ing gold-sulfur bonds. Regardless, the observation that incom-
plete binding of the sulfurs occurs at very low surface coverage
suggests the intermolecular steric interactions cannot be the sole
factor influencing binding. Plausibly, intramolecular steric
interactions between the relatively bulky benzyl groups of the
tripod are the primary determinant of incomplete binding.
One observation that cannot be explained by any of the above
arguments is that the number of bound sulfurs for ZnP-Tpd
appears to be somewhat greater than that for CuP-Tpd. This
behavior is observed in different deposition solvents (e.g.,
CH2Cl2:EtOH versus toluene). Any intrinsic structural differ-
ences between the zinc and copper porphyrins would be small
and cannot explain a difference in binding. At this time, we
have no explanation for the different adsorption characteristics
of ZnP-Tpd versus CuP-TPd. Also worth pointing out is
the fact that the copper porphyrin cannot reach saturation
coverages as high as those seen for ZnP-Tpd. Noting that the
porphyrins themselves are relatively rigid and cannot easily
bend, the lower coverages seen for CuP-Tpd can be qualita-
tively understood by its somewhat larger tilt on the surface
(according to the IR data), possibly a consequence of the lower
average number of sulfur atoms involved in bonding.
IV. Discussion
The studies reported herein on the porphyrin and benchmark
tripod SAMs indicate that the Tpd anchor architecture does not
lead to binding to gold surfaces via all three sulfur atoms. This
result explains our earlier observation that porphyrins containing
this linker do not afford high surface coverages.9 In particular,
the tilted orientation of the molecules, along with the relatively
large footprint of the porphyrin attached to the Tpd anchor, is
expected to lead to a relatively large overall molecular projection
on the surface. The cross-sectional area of the porphyrin tripods
can be estimated to be ∼640 Å2 on the basis of the ∼32 Å
length along the porphyrin-tripod axis and ∼20 Å width of
the porphyrin. If the molecules are tilted ∼45°, the cross-
sectional projection on the surface would be 450 Å2 (assuming
minimal torsional rotation of the porphyrin plane with respect
to the surface), which is somewhat less than the ∼550 Å2
observed for ZnP-Tpd at saturation surface coverage (∼3 ×
10-11 mol‚cm-2 ).9 However, a disordered arrangement of the
porphyrins on the surface would further increase the surface
area compared with that estimated from the cross-sectional
projection.
An interesting question is why the Tpd architecture does not
result in binding of all three sulfur atoms. The experimental
results rule out certain seemingly plausible explanations for this
binding motif.
Finally, we note that the incomplete binding of the Tpd
anchors is generally consistent with the results of studies of
other types of tripodal and multipodal thiols on gold, which
indicate that incomplete bonding is relatively common.39-44 In
some cases, the extent of bonding is dependent on the deposition
solvent, with complete binding achievable in some solvents, but
not others. Regardless, complete binding does not appear to be
achievable for the Tpd anchor. This behavior cannot be
attributed to intermolecular steric interactions as incomplete
bonding is observed even at very low surface coverage.
Accordingly, intramolecular steric interactions within the rela-
tively bulky Tpd anchor most likely influence the surface
binding. This result has implications for the general design of
molecular architectures for surface attachment and modification.
First, unfavorable interactions with the surface arising from
the presence of the S-acetyl protecting groups are excluded by
the experiments with the free thiol. In addition, the SAM appears
to have reached a stable state on the basis of the observation
that extended deposition times and elevated temperatures do
not alter the surface coverage or binding motif. These results
do not necessarily imply that the SAM has reached a global
minimum on the potential energy surface. Instead, the incom-
plete binding may reflect a local minimum with a high barrier
to the most thermodynamically favored configuration, which
arises because achieving equilibrium requires a collective
rearrangement of the entire SAM.
Second, poor registry between the sulfur atoms in the Tpd
unit and the gold atoms of the surface is excluded by molecular
modeling, which shows that the spacing between the sulfur
atoms in the Tpd unit (∼9.7 Å) combined with the flexibility
of the S-Au bonding motif permits access to three gold sites.
Acknowledgment. This work was supported by the DARPA/
DMEA (H94-003-05-2-0504; H94-003-04-2-0404; DMEA90-
02-2-216) and by ZettaCore, Inc. Mass spectra were obtained
at the Mass Spectrometry Laboratory for Biotechnology at North

Tripodal Thiol-Functionalized Porphyrins
J. Phys. Chem. B, Vol. 109, No. 50, 2005 23971
124, 532-533.
Carolina State University. Partial funding for the facility was
obtained from the North Carolina Biotechnology Center and
the NSF.
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Supporting Information Available: Absorption, LD-MS,
and 1H NMR spectra for each new compound. This material is
available free of charge via the Internet at http://pubs.acs.org.
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