Layer-by-layer assembly of Zn(II) and Ni(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin multilayers on Au using copper catalyzed azide-alkyne cycloaddition{

Alexandra Krawicz,a Joseph Palazzo,b Gwo-Ching Wangb and Peter H. Dinolfo*a

Received 9th March 2012, Accepted 7th June 2012

DOI: 10.1039/c2ra20440a

We have developed a versatile layer-by-layer (LbL) fabrication method to assemble porphyrin based

multilayer thin-films on electron-beam evaporated Au surfaces utilizing copper(I) catalyzed azide-

alkyne cycloaddition (CuAAC) as both a means of anchoring the films to the Au surface and coupling

the individual molecular layers together. The molecular based multilayer films are comprised of Zn(II)

and Ni(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin and a bis-azido linker layer. Herein, we

describe the fabrication and characterization of multilayer films on Au surfaces modified with an

azido-terminated alkanethiol self assembled monolayer. The absorbance growth trends, as followed

by UV-vis absorption, show a consistent linear increase that extends over tens of bilayers. Multilayer

film thicknesses were obtained from spectroscopic ellipsometry, using a Cauchy model applied over

the transparent range, and resulted in a consistent linear growth trend. Optical constants, index of

refraction and extinction coefficients, were then determined using an oscillator model over the entire

visible region. The resulting extinction coefficients were consistent with those typical of Zn(II) and

Ni(II) porphyrin absorption spectra. The topology of the films and surface roughness was analyzed by

tapping mode atomic force microscopy (TM-AFM) and confirmed the continuous nature of the films.

X-Ray photoelectron spectroscopy (XPS) was consistent with the expected elemental composition of

the porphyrin based films assembled on Au surfaces. Additionally, XPS was used to examine the

utility of ethylenediaminetetraacetic acid disodium salt (Na2EDTA) as a Cu chelator to remove

adventitious catalyst following multilayer fabrication.

Introduction

The modification of electrode surfaces using molecularly ordered

thin films, with tunable electrochemical and photophysical

properties, has widespread applications in the field of molecular

electronics, and photovoltaics, among others.1–4 One of the most

common methods of adding functionality to electrode surfaces is

through the use of self assembled monolayers (SAMs).5 Another

method is the layer-by-layer (LbL) fabrication technique, which

can generate ordered thin films, composed of multiple building

blocks, efficiently and inexpensively.6 This methodology has the

potential to control the orientation and ordering of the films’

components at the molecular level through simple solution

deposition techniques. The LbL approach is a convenient and

precise technique, which allows for the facile engineering of

electronic, photophysical, and chemical properties into the

nanostructured films.7,8

LbL thin film formation can be accomplished by a series of

sequential self-limiting coupling reactions that each deposit a single

layer of material on the surface at a time. A wide variety of multilayer

thin films have been assembled on different substrate surfaces via this

technique using polymer, inorganic and molecular building blocks.

There exist a variety of interlayer coupling reactions which have been

explored as coupling techniques for the LbL assembly method.

Examples include electrostatically assembled polyelectrolytes,8

alpha-zirconium phosphate coupled dyes,9–16 Langmuir–Blodgett

films, palladium-pyridyl coordination,17–21 polymeric layers made

through siloxane polymerization,22–28 or various other organic

reactions.29–34

We have recently developed a molecular LbL thin film

fabrication methodology utilizing copper(I)-catalyzed azide-

alkyne cycloaddition (CuAAC) reactions as both a means to

link the layers together and attach them to oxide surfaces.35–37

This technique was used to assemble nanoscale multilayer films

of tetraphenyl-Zn(II)-porphyrins and perylenediimide building

blocks on several substrates, including SiO2, indium tin oxide

(ITO), quartz and glass. The resulting films showed reproducible

and linear growth trends for absorbance and thickness over tens

aDepartment of Chemistry and Chemical Biology, Rensselaer PolytechnicInstitute, 110 8th Street, Troy, NY, 12180, USA. E-mail: dinolp@rpi.edubDepartment of Physics, Applied Physics and Astronomy, RensselaerPolytechnic Institute, 110 8th Street, Troy, NY, 12180, USA{ Electronic Supplementary Information (ESI) available: experimentalprocedures, visible transmission and specular reflection, TM-AFMimages, and survey and high-resolution XPS spectra. See DOI:10.1039/c2ra20440a

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of layers.35–37 Discovered in 2001 by Sharpless and coworkers38

and Meldal and coworkers,39 CuAAC has been popularized as a

rapid, facile, and robust cycloaddition reaction utilizing inex-

pensive Cu(I) catalyst. This simple reaction is tolerant of a

variety of conditions and other functional groups, and has been

used extensively as a surface modification technique on multiple

surfaces.40–45 While a few other groups have used this reaction to

fabricate triazole-linked polymer based multilayers,46–49 to our

knowledge we were the first to report the use of CuAAC to build

molecular multilayer films.35–37

In this report, we describe the extension of this LbL multilayer

fabrication method to include tetraphenylporphyrin building

blocks assembled on electron-beam (e-beam) evaporated Au

surfaces that will allow for the examination of charge transport

properties via electrochemical methods and scanning probe

microscopies.50 E-beam evaporated gold provides an ideal

surface for the growth of nanostructured thin films due to its

relatively smooth surface and ability to form high-quality mixed

azido-alkane SAMs required for CuAAC based multilayer

growth.40,41,51,52 Multilayer formation on Au electrode surfaces

opens up the possibility of additional characterization methods

for nanoscale thin films and provides a reproducible platform to

analyze electrochemical electron transfer rates.51 Since our initial

report of molecular LbL thin film fabrication on oxide surfaces,

others have used CuAAC for LbL generation of phenyl-triazole

based molecular wires on Au(111) electrodes.53

We have been particularly interested in the use of porphyrin

building blocks in the assembly of these films. The highly tunable

electrical and optical properties of porphyrins have led to their

use in a wide range of materials chemistry applications.54

Multilayer thin films assembled using porphyrin based molecular

building blocks could lead to a wide range of applications

including artificial photosynthetic processes,3,55,56 semiconduc-

tor sensitization,57–60 and catalysts,42,61–63 among others.64,65

Previously, porphyrin based molecular multilayer films have

been assembled in a LbL fashion using a variety of covalent and

non-covalent coupling methods including, transition metal

coordination,14–16,66–68 electrostatic interactions between electro-

lytes,69–71 and purely organic linkages.32–34

Herein, we describe the synthesis and characterization of

multilayer films of Zn(II) and Ni(II) 5,10,15,20-tetra(4-ethynyl-

phenyl)porphyrin (1 and 2 respectively) assembled by CuAAC

on e-beam evaporated Au surfaces. Multilayer growth was

monitored by UV-visible spectroscopy, observing an increase in

absorbance at the porphyrin Soret and Q-bands with each

additional porphyrin layer. Spectroscopic ellipsometry was used

to determine the film thickness and optical constants, which are

important to predict or understand the nonlinear optical

properties of these multilayers. The surface morphology was

explored by AFM, commonly employed in the analysis of films

composed of porphyrins and phthalocyanines,72 to obtain a

representative image of the topology and to inspect the integrity

and roughness of the film. X-Ray photoelectron spectroscopy

(XPS) was used to analyze the chemical composition, along with

determining the left over copper catalyst in the film structures.

Porous materials created via CuAAC often contain excess copper

ions, but several extraction methods have been employed to

remove the adventitious catalyst.73,74 XPS was used herein to

determine the amount of copper remaining in the Au supported

porphyrin multilayer films before and after treatment with

ethylenediaminetetraacetic acid disodium salt (Na2EDTA).

Results and discussion

Multilayer growth

Fig. 1 outlines our methodology for assembling molecular

multilayers using CuAAC reactivity. The process relies on two

sequential self-limiting CuAAC reactions of a multi-ethynyl

functionalized tetraphenylporphyrin (1 or 2) and a multi-azido

linker (3 or 4). The fabrication process begins with a mixed

azido-alkane SAM formed on an e-beam evaporated Au surface

to provide the initial attachment point. The azide-terminated

SAM is then reacted with the ethynyl functionalized porphyrin (1

or 2) under CuAAC conditions (step 1, Fig. 1). This step results

in a densely packed monolayer of porphyrin attached to the

SAM through 1,4-subsituted 1,2,3-triazole linkages and a surface

that is now terminated in acetylene groups. After a series of

solvent washes to remove unreacted starting material and

catalyst, another CuAAC reaction is performed on the surface

with a multi-azido linker creating an azide terminated surface

(step 2 in Fig. 1). The combination of steps 1 and 2 result in one

molecular bilayer added to the surface. The two self-limiting

reactions are then repeated sequentially to yield additional

molecular bilayers which are covalently attached to the surface

via 1,4-subsituted 1,2,3-triazoles.

Fig. 2 shows the visible absorption spectra taken throughout

the fabrication of a multilayer of 1 and 4 on a glass supported

20 nm thick, optically semi-transparent Au surface. With each

consecutive CuAAC reaction of 1 with the azide terminated

surface there was a consistent increase in absorbance for the

Soret peak at y440 nm. The refractive index of Au changes

dramatically over the visible region; n is y1.45 from 400–450

nm, but drops sharply to y0.13 in the range of 600–900 nm.75

This drastic difference in refractive index creates artifacts in the

absorbance and reflectance spectra of multilayers grown on Au.

In the region between 600–900 nm, the Fresnel equations predict

that a large percentage of incident light is reflected back from the

surface due to the significant difference in the refractive index

between water (n = 1.33), in which the sample is placed, and Au

(y0.13). As the multilayer is assembled on the surface, the

refractive index rises to that of the film (y1.5, vide infra)

resulting in less reflected light (greater transmission) and the

appearance of a negative absorption region. Within that

increased transmission range, the appearance of two Q-bands

associated with 1 can clearly be seen at 560 and 600 nm. Similar

absorptivity changes were observed when linker 3 was incorpo-

rated into the films (Figure S1{).

Multilayer fabrication was also followed using near-normal

specular reflection off of thicker Au surfaces (100 nm). Fig. 3

shows the Fresnel reflectivity for the visible region throughout

multilayer growth from 1 through 16 bilayers of 1 and 4. The

spectra consistently decrease in reflectivity as each bilayer is

added to the Au surface. The spectra show similar porphyrin

features as found in the transmission mode measurements (see

Fig. 2 above) with decreases in reflectivity at the Soret and

Q-band region. The bands are shifted towards longer wave-

lengths by approximately 10–20 nm due to complications from

the Kramers–Kronig effect. The complex index of refraction (n)

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is composed of the real refractive index (n) and the imaginary

part (ik) according to the Kramers–Kronig relationship (eqn (1)),

where k is the extinction coefficient of the film.76

n(l) = n(l) + ik(l) (1)

Due to the high optical density of the porphyrin based

multilayers, the reflectivity spectra of these samples show a first-

derivative like line shape, with increased reflectance on the high

energy side of the absorption features, as predicted by the

Kramers–Kronig effect.77 Similar trends in specular reflectivity

changes were observed for multilayers formed with 2 and 4

(Figure S2{).

Spectroscopic ellipsometry

Ellipsometry is a non-destructive surface analysis technique that

is capable of determining the thickness and optical properties of

nanoscale thin films. Spectroscopic ellipsometry measures the

change in phase (D) and amplitude (Y) of elliptically polarized

light reflected off of a substrate surface as a function of

wavelength (l). The data for D(l) and Y(l) are then fit to a

Fig. 1 Schematic representation of molecular LbL multilayer growth using CuAAC reactivity on Au surfaces.

Fig. 2 Top: UV-Vis spectra of multilayers of 1 and 4 assembled on an

azide-terminated SAM on optically transparent e-beam evaporated Au

(20 nm thickness). The absorption spectra for porphyrin layers are

shown. Bottom: Absorbance vs. the number of bilayers at the Soret and

Q-bands illustrate the linear dependence for multilayer growth.

Fig. 3 Specular reflectivity scans of 1 through 16 bilayers of 1 and 4 on

100 nm thick Au. Only porphyrin layer spectra are shown for clarity.

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model describing the complex index of refraction (n) and

thickness of the material as described above in eqn (1) for the

Kramers–Kronig relationship.76 The porphyrin based films

described in this study have strong, localized absorptions in

the visible region due to the Soret and Q-band transitions, thus

somewhat complicating the analysis of the ellipsometry data.

Spectroscopic ellipsometry was employed throughout the

growth of multilayers on Au substrates to provide information

on both the thickness and optical constants of the thin films.

Fig. 4 shows the measured D(l) data in the range of 405–742 nm,

collected at a 65u angle of incidence, for multilayers of 1 and 4.

The D(l) parameters are particularly sensitive to thickness

changes of the material.78–80 As additional bilayer reactions are

performed on the Au surface, there is a consistent decrease in

D(l), especially in the region around 460 nm where the Soret

band of 1 is located. This is consistent with the increasing

multilayer thickness from 4 to 16 bilayers. Similar trends in D(l)

were observed during multilayer growth using the other

molecular components outlined in Fig. 1.

To calculate the thickness of the multilayer thin films, we

employed the Cauchy dispersion model (eqn (2)), over the non-

absorbing region (674–741 nm) of the porphyrin, to describe the

refractive index (n) as a function of the wavelength (l).80

n(l)~AzB

l2z

C

l4(2)

This methodology allows for the straightforward determina-

tion of film thickness, without additional fitting parameters

required to describe k(l) for the material. Ellipsometry has been

used to find the film thickness of other absorbing materials such

as porphyrins,32,81 phthalocyanines,55,72,82,83 LbL assembled

polypyridyl–PdCl2 films,21 and even CuAAC coupled dendri-

meric films49 using similar methods.

Fig. 5 shows the ellipsometrically determined film thickness

versus the number of bilayers added to the Au surface of 1 and 2,

with linker 4. The results show the expected linear increase in

film thickness versus the number of bilayers, with growth rates of

1.5 and 1.8 nm per bilayer for films comprised of 1 and 4, and 2

and 4 respectively. The film growth rates of multilayers on Au

are slightly lower than those of similar multilayers on Si(100)

measured by both ellipsometry and X-ray reflectivity.37 There

are several possible explanations for the differences in growth

rates of these films such as the higher surface roughness of the

e-beam evaporated Au as compared to the native oxide of

Si(100) substrates. Au substrates generated by e-beam evapora-

tion predominantly form a Au(111) surface, with varying grain

sizes of 45–60 nm, depending on evaporation conditions and

rates.5,84 Surfaces roughnesses of e-beam evaporated Au also

vary somewhat, with typical values of around 2–4.5 nm

(rms).85,86 The native oxide or Si(100) substrates on the other

hand, show rms roughness values of less than 1 nm, depending

on the cleaning methods used.87,88 In addition to differences in

surface roughness, the alkane-thiol based SAMs on Au(111)

have a higher packing density than alkyl-siloxane SAMs on

silicon.5 A dense monolayer of azido-alkane thiol SAM moieties

may promote multiple click reactions of one porphyrin with the

substrate, in turn changing the angle of growth and reducing the

number of available acetylene groups for the next surface

reaction. Thus, given the dendritic nature of the molecule

building blocks, the multilayer growth scheme outlined in Fig. 1

represents a simplified view of the bonding configurations within

the film. We previously compared the X-ray reflectivity

determined film thicknesses of similar porphyrin multilayer

films assembled on Si(100) : SiO2 substrates with the intermo-

lecular distances obtained from DFT molecular modelling to

predict a range of potential growth angles. A molecular growth

angle of 45u was calculated for a purely trans-bonding mode

(using the 5- and 15-meso-positions on the porphyrin), whereas a

70u angle was calculated for a cis-bonding mode (5- and 10-meso-

positions on the porphyrin).37 Polarized GATR (grazing angle

attenuated total reflectance) IR spectra of porphyrin multilayers

assembled on ITO, a rougher surface than either e-beam

evaporated Au or Si(100) : SiO2, yielded a porphyrin orientation

of y45u, suggesting the trans-bonding mode dominates the

growth patterns.36 Nevertheless, the growth patterns of these

multilayer films are likely influenced by both the substrate

roughness and packing density of azides for the initial surface

attachment layer. In this work, the surface density of the azides

from the alkanethiol SAMs on e-beam evaporated Au is likely

higher than that on oxide surfaces examined previously.

Additionally, with the multi-functional porphyrin building

Fig. 4 Ellipsometry D trend for increasing number of bilayers of 1 and 4

assembled on Au and acquired at a 65u angle of incidence.

Fig. 5 Plot of film thickness vs. number of bilayers for multilayer films

of 1 and 4, and 2 and 4. The error bars were calculated based on standard

deviation and are on the scale of the data point size, and therefore not

shown.

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blocks employed, it is possible that cross-linking can occur

within the multilayer structure, leading to a combination of

binding motifs present within the film.

Optical constants have also been obtained ellipsometrically for

absorbing films, containing porphyrins and phthalocyanines, by

modeling the dielectric functions of the material with an

oscillator model covering the full spectral range.72,82,89 These

methods typically use a fixed film thickness obtained from

Cauchy modeling in the transparent region, or other methods.89

We performed a full spectroscopic ellipsometry analysis of the

thicker multilayer structures. The upper plots in Fig. 6 show

representative ellipsometry data (D(l) and Y(l)) and fits for the

range of 410.0 to 741.5 nm (44 data points) for multilayers of 1

and 4, and 2 and 4 on Au(111) surfaces. The resulting model for

D(l) and Y(l), shown as solid lines, is in excellent agreement

with the measured data.

The bottom plots show the resulting n(l) and k(l) profiles for

the multilayer films as derived from the modeled dielectric

constants. As can be observed in the lower plots of Fig. 6, the

k(l) profiles closely match those of the absorption profile obtained

from UV-vis spectroscopy for the porphyrin multilayers

assembled on transparent Au (see Fig. 2). The multilayers

containing 1 (left plots) show a Soret absorbance feature at

435 nm and Q-bands at 560 and 610 nm. These are red-shifted

relative to the solution spectra of 1 due to aggregation effects within

the film.35–37 The k(l) profile for multilayers containing 2 display

Soret and Q-band features at 410 and 530 nm, which are closer to

those of the solution spectra of 2. Additionally, the n(l) spectrum of

both porphyrin based multilayers matches that of the specular

reflectivity assembled on opaque Au(111) substrates (see Fig. 3 and

S2{). The refractive index (n) for the porphyrin multilayer films is in

the range of 1.4–1.5 for films of 1 and 4, and 1.45–1.55 for films of 2

and 4 in the non-resonant region (674–741.5 nm).

Although optical constants for phthalocyanine are more

commonly reported in the literature than for porphyrin films,

the n(l) spectrum of vacuum evaporated thin films of free base

porphyrin was reported to be between 1.2–1.4 (in the range of 4–

12 eV) with prominent features between 0.5–4eV.90 Additionally,

the k(l) profile had prominent features that reflected the UV-vis

absorption profile.90 The literature value of the extinction

coefficient for phthalocyanine films from spectroscopic ellipso-

metry also shows agreement with the absorption spectra and the

refractive index range is comparable to our porphyrin multi-

layers.91 Others have reported refractive index values for

phthalocyanine containing films in the range of 1.6–1.8 in the

nonresonant region of the visible spectrum.72

Fig. 6 Spectroscopic ellipsometry data (410.0 to 741.5 nm, 44 data points) and fitting results for multilayers of 1 and 4 (left panels, a–c) and 2 and 4

(right panels, d–f). The top and middle panels show the D and Y data (open symbols) and fits (solid lines) collected at an incident angle of 55, 60, 65, 70,

and 75u. The bottom panel shows the resulting optical constants for the multilayer films as determined by the oscillator model. The index of refraction

(n) is shown as a dashed red line and the extinction coefficient (k) as a solid blue line.

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Tapping mode atomic force microscopy

Topography and surface roughness was analyzed with tapping

mode atomic force microscopy (TM-AFM) to confirm the

continuity and integrity of the film on the Au surfaces. Fig. 7

shows representative 1 mm 6 1 mm topography and amplitude

TM-AFM images for multilayers comprised of 2 and 4. The

amplitude images remove long-range variations and offsets, and

therefore are sometimes better for visualization of the surface

features.92 The values of root mean squared roughness (rms) for

the images in Fig. 7 are 2.1, 2.6 and 3.7 nm for 1, 10 and 20 bilayers

of 2 and 4, respectively. The TM-AFM images of the multilayers

of 1 and 4 are shown in Fig. S3{ and yielded rms values of 2.5, 2.1

and 3.8 for 4, 10 and 16 bilayers, respectively. Larger area scans of

10 mm 6 10 mm show continuous multilayer films and comparable

roughness values. There was no apparent surface damage after

repeated TM-AFM scans of the same area. The rms tends to

increase slightly as more layers are deposited, and is similar to

observations made for similar multilayers on silicon.37 The

roughness of the multilayer films is similar to that of the

underlying Au surface,85,86 suggesting that the film morphology

is templated somewhat by the underlying substrate.55

X-Ray photoelectron spectroscopy and copper removal methods

We employed X-ray photoelectron spectroscopy (XPS) to

confirm the surface attachment of the molecular components

and to estimate the amount of copper catalyst remaining in the

multilayer film. XPS is a common surface characterization

technique and has been used to analyze porphyrin films on

gold93 and other surfaces.32 Copper contamination was reported

by others who used CuAAC reactivity in the assembly of

LbL films and the functionalization of acetylene-terminated

monolayers.94 In some cases, the remaining copper catalyst was

removed by EDTA37 or other chelating agents.73

A representative XPS spectrum of a multilayer film consisting

of 2 bilayers of 1 and 4 before and after treatment with 0.01 M

Na2EDTA is shown in Fig. 8. (The XPS spectra for multilayers

of 1 and 3 are shown in Fig. S4.{) The spectrum shows all of the

expected atomic peaks for the empirical formula for the 1 and 4

multilayer structure. High resolution spectra of the N 1s peak

(Fig. S5{) display a broadened spectrum with binding energies

around 398–401 eV due to multiple types of nitrogen atoms

in the film, including the porphyrin ring, 1,2,3-triazole and

unreacted azide groups. Additionally, high-resolution spectra for

the Zn 2p1/2 and 2p3/2 (Fig. S6{) show peaks at binding energies

of 1044.6 ¡ 0.2 and 1021.7 ¡ 0.2 eV, respectively, in close

agreement with values previously reported for other Zn

porphyrins.4,32,95–99

Table 1 contains atomic composition data taken from XPS

spectra for 2 bilayers of 1 and 3 and 1 and 4, before and after

treatment with Na2EDTA(aq). Table 1 also includes a compar-

ison of the atomic ratios of Cu and N to that of Zn. It is clear

from the XPS elemental spectra that some copper catalyst

remains in the film following multilayer fabrication. In an

attempt to remove the left over Cu, we exposed each film to a

0.01 M Na2EDTA solution (pH = 4.8) following each CuAAC

reaction. As can be seen in Table 1, the percentage of Zn did not

change significantly after Na2EDTA(aq) treatment and stayed

within 0.9 to 1.2%. Meanwhile, the amount of Cu detected was

between 1–2.8 times higher with respect to Zn before

Na2EDTA(aq) treatment, but dropped to a ratio of 1–1.3

afterwards.

While Na2EDTA was not effective in removing all of the

CuAAC catalyst from the multilayer films, XPS was able to

Fig. 7 TM-AFM images of 1 (left), 10 (middle) and 20 (right) bilayers of multilayers based on 2 and 4. The upper images are topography AFM height

profiles and the lower are amplitude scans, a pseudoderivative of the topography data to emphasize the structural characteristics. All TM-AFM images

areas are 1 mm 6 1 mm.

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provide information about the nature of the remaining Cu

species. Fig. 9 shows the high-resolution spectra of the Cu 2p3/2

peaks for multilayers of 1 and 3 and 1 and 4 before and after

treatment with Na2EDTA(aq). The high-resolution XPS spectra

show a broad set of Cu 2p1/2 and 2p3/2 peaks, consistent with

multiple Cu species. Deconvolution of the spectra, shown in

Fig. S7{, reveal the presence of both Cu(I) and Cu(II) species at

binding energies of 932.4 ¡ 0.2 and 934.4 ¡ 0.2 eV,

respectively.100–102 Table 2 contains a comparison of the

Cu(II)/Cu(I) ratios, from deconvolution of the Cu 2p3/2 peaks,

before and after Na2EDTA(aq) treatment. Following treatment

with Na2EDTA, the XPS data shows a marked decrease in the

Cu(II) 2p3/2 peaks relative to Cu(I) (see the ESI{), and the

disappearance of the Cu(II) satellite peaks at approximate

binding energies of 944 and 962 eV. This data suggests that

Na2EDTA(aq) is more effective at removing Cu(II) species than

Cu(I) from the multilayer films.

We previously found that untreated multilayer films of 1 and 3

assembled on the native oxide of Si(100) substrates had a

Cu : Zn ratio of 1.36 : 1, whereas films of 1 and 4 had a

significantly lower ratio, 0.16 : 1.37 We proposed that the higher

amounts of Cu in the multilayers with linker 4 may be a result of

the two sulfonates compared to 3 which does not contain any

anionic groups. The Na+ peak in the high-resolution XPS spectra

was relatively small for the expected atomic composition of the

multilayers grown with 4, suggesting that Cu ions exchanged

with the sulfonate cations of 4. The comparable amount of Cu

present in the multilayers containing linker 3 suggests that there

Fig. 8 XPS spectra of multilayers of 1 and 4 before (green dashed line) and after (red solid line) treatment with Na2EDTA(aq).

Table 1 Percentage atomic composition data derived from XPS data of multilayer films

Atomic peak 1 and 3 1 and 3 Na2EDTA 1 and 4 1 and 4 Na2EDTA Monolayer of 5 Monolayer of 5 Na2EDTA

C 1s 69.3 68.9 66.3 63.2 63.8 64.0N 1s 10.2 9.8 9.5 11.5 9.5 5.8O 1s 8.2 7.5 7.1 6.5 0.6 3.7Au 4f7/2 8.8 11.1 12.3 16.1 22.8 23.5Cu 2p3/2 2 1.3 2.5 1.6 1.3 0.1Zn 2p3/2 1.2 1.1 0.9 1.2 1.8 1.4S 2p 0.3 0.3 1.4 ,0.1 1.3 1.5

Ratios of atomiccomposition to Zn

Cu 2p3/2 1.7 1.2 2.8 1.3 0.7 0.1Zn 2p3/2 1.0 1.0 1.0 1.0 1.0 1.0N 1s 8.5 8.9 10.6 9.6 5.4 4.2

Fig. 9 High resolution spectra of the Cu 2p3/2 peak for multilayer films of 2 bilayers of 1 and 3 (left) and 1 and 4 (right) before (dashed red line) and

after (solid green line) treatment with Na2EDTA(aq).

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is another functional group, such as unreacted alkynes in 1, that

is capable of sequestering Cu. There is a wide range of

structurally characterized Cu(I)–alkyne complexes known in

the literature.103

We previously reported the structural characterization of

multilayer films of 1 and 3 and 1 and 4 on ITO electrode surfaces

using grazing-angle attenuated total reflectance FTIR spectro-

scopy.36 These results, combined with previous thickness

determinations,37 established that the porphyrin building blocks

are bonded in a trans fashion within the multilayer films, using

only two of the available phenyl-ethynyl groups. Thus, the

remaining ethynyl groups from 1 could potentially form a Cu(I)–

acylide complex, thus sequestering Cu in the film. This is

consistent with the observed Cu(I) peaks in the XPS spectra of

the multilayers following Na2EDTA(aq) treatment.

To further explore the possibility of Cu(I)–acetylide formation

within the multilayer structures comprised of 1, we examined the

retention of Cu by a monolayer film of a monoethynyl

functionalized porphyrin, Zn(II) 5-(4-ethynylphenyl)-10,15,20-

tri-phenyl porphyrin (5). The CuAAC reaction of the single

phenyl-ethynyl group with the azido surface would not leave an

unreacted alkyne for Cu(I) binding. Comparison of monolayer

films of 5 before and after Na2EDTA(aq) showed that the vast

majority of Cu(I) and Cu(II) were removed, supporting the

assignment of Cu(I)–acetylide binding as the mode of Cu

retention in the films (see Fig. S8{). Efforts are currently

underway to synthesize trans-di-ethynyl molecules for LbL

formation that would limit the number of unreacted functional

groups within the films that could sequester excess Cu.

Conclusions

The UV-vis spectroscopy and spectroscopic ellipsometry results

show that the Au(111) surfaces were effectively functionalized

with an azide terminated SAM creating a functional platform

onto which porphyrins were clicked. Furthermore these azide

terminated surfaces enabled the growth of the mixed porphyrin

based multilayers as evidenced by the absorbance and film

thickness trends. Deviations from linear trends in film thickness

may stem from the changing surface properties of the surface onto

which the layers are deposited. This technique of thin-film

multilayer deposition can control the thickness, bonding archi-

tecture, and thus the overall structure and properties of the

macroscopic film. The methodology also enables the introduction

of different functionalities into the molecular multilayer films.

Acknowledgements

This material is based upon work supported by the National

Science Foundation under Grant No. 0333314 and Rensselaer

Polytechnic Institute.

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