Dithienylcyclopentene-functionalised subphthalocyaninatoboron complexes: Photochromism, luminescence modulation and formation of self-assembled monolayers on gold

Article abstract of DOI:10.1039/c1dt11644d

Subphthalocyaninatoboron (SubPc) complexes bearing six peripheral n-dodecylthio substituents and an apical photochromic dithienylperfluorocyclopentene unit were prepared. The photoinduced isomerisation of the apical substituent from the open to the ring-c

Full text of DOI:10.1039/c1dt11644d

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PAPER
Dithienylcyclopentene-functionalised subphthalocyaninatoboron complexes:
Photochromism, luminescence modulation and formation of self-assembled
monolayers on gold
Tobias Weidner,a Joe E. Baio,a Johannes Seibel†b,c and Ulrich Siemeling*b,c
Received 31st August 2011, Accepted 25th October 2011
DOI: 10.1039/c1dt11644d
Subphthalocyaninatoboron (SubPc) complexes bearing six peripheral n-dodecylthio substituents and
an apical photochromic dithienylperfluorocyclopentene unit were prepared. The photoinduced
isomerisation of the apical substituent from the open to the ring-closed form significantly influences the
photoluminescence of the covalently attached SubPc unit, which is more efficiently quenched by the
ring-closed form. Films on gold were fabricated from these multifunctional conjugates and
characterised by near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron
spectroscopy (XPS). The results are in accord with the formation of self-assembled monolayers based
on dome-shaped SubPc-based anchor groups. Their chemisorption is primarily due to the peripheral
n-dodecylthio substituents, giving rise to covalently attached thiolate as well as coordinatively bound
thioether units, whose alkyl chains are in an almost parallel orientation to the surface.
to exhibit photoswitchable properties are based on adsorbate
Introduction
molecules which contain a photochromic tailgroup connected
Surfaces that respond to external stimuli in a reversible and
to the headgroup, which is responsible for attachment to the
well-defined manner are of great current interest for science and
technology.1 The immobilisation of photoswitchable molecules
on surfaces is an especially active area of research in this
context.1d,f,i,l,q,r,s Photochromic units which have been utilised for
substrate. Essential requirements for optimum performance of
such a ‘smart’ SAM are (i) sufficient void space around the
terminal functional units in the SAM to allow their unhindered
photoisomerisation and (ii) an adsorption geometry that avoids
photoswitching on surfaces include azobenzene,2 spiropyran,3
“diving” of the terminal groups towards the surface. The tradi-
and diarylethene4 derivatives. Among the latter, dithienylcy-
tional strategy for achieving this is based on the incorporation
clopentenes have turned out to be particularly useful for pho-
of the functional adsorbate molecules into a matrix of similar,
but shorter, non-functional adsorbate species.7 A major problem
of this two-component “daisies in the lawn” approach is that
phase separation of the components can occur on the surface.8
Alternative one-component strategies have been suggested, which
involve bulky spacer groups9 or the use of large-footprint head-
groups for lateral separation.10 We and others have been addressing
the latter possibility, predominantly utilising tripodal binding
units for adsorption on gold.11 As the most recent development,
disk-like headgroups with a central binding site for functional
units have been investigated in this context. Herges, Magnussen
and coworkers have introduced the triazaangulenium ring system
for this purpose,12 while we have addressed phthalocyaninato
complexes bearing eight peripheral long-chain alkylthio groups
for multipoint attachment on gold.13 We have now extended
this approach from the flat, disk-like metal phthalocyaninato to
the concave, dome-shaped boron subphthalocyaninato platform.
Subphthalocyaninatoboron (SubPc) complexes are composed of
three N-fused diiminoisoindole units arranged around a central
four-coordinate boron atom, whose apical ligand is pointing away
from the molecular dome.14 The results of an STM investigation
toswitching, which is due to the fact that they can exhibit
high fatigue resistance together with a rapid response behaviour,
excellent thermal stability and thermal irreversibility of the
photoisomerisation.5 The chemisorption of functional molecules
from solution is an elegant and very convenient way of surface
modification, which can lead to the formation of well-ordered,
stable self-assembled monolayers (SAMs) on the substrate. The
archetypal and most commonly used adsorbate system for SAM
fabrication is that of thiols (or closely related reactive sulfur-
containing compounds like disulfides or thioacetates) on gold,
giving rise to thiolate-type SAMs.6 SAMs which are designed
aNational ESCA and Surface Analysis Center for Biomedical Problems
(NESAC/BIO), Departments of Bioengineering and Chemical Engineering,
University of Washington, Seattle, Washington, 98195, USA
bInstitute of Chemistry, University of Kassel, Heinrich-Plett-Str. 40, 34132,
Kassel, Germany. E-mail: siemeling@uni-kassel.de
cCenter for Interdisciplinary Nanostructure Science and Technology, Univer-
sity of Kassel, Heinrich-Plett-Str. 40, 34132, Kassel, Germany
† Current address: Laboratory for Nanoscale Materials Science, Empa-
Swiss Federal Laboratories for Materials Science
Uberlandstr. 129, 8600 Du¨bendorf, Switzerland
& Technology,
¨
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Scheme 1 Synthesis of compounds 3a and 3b.
performed by Yanagi et al. suggest that chloridosubphthalocyani-
natoboron derivatives bearing six peripheral long-chain alkylthio
substituents are suitable for SAM formation on gold.15 We note
that very recently the anchoring of SubPc complexes on gold was
achieved through dangling a-lipoic acid-based disulfide units,16
which are known to undergo an oxidative addition reaction with
the gold substrate, leading to covalent gold-thiolate bonds.17
Thioethers are assumed to be more mobile than thiolates on a gold
surface, which may be important for lateral diffusion during self-
assembly, to avoid the formation of incomplete monolayers. The
present study, which focuses on SubPc-based adsorbate species
with six peripheral SR units, is in continuation of our previous
work, where we have investigated the use of two,18 three,11b,c,f four,18
and eight thioether units13 for multipoint attachment on gold.
of BCl3 in xylene, following a protocol developed by Torres
and coworkers.21 The dithienylcyclopentene derivatives 2aH2
22
and 2bH2 were prepared from 1,2-bis(2-chloro-5-methylthien-3-
yl)hexafluorocyclopentene and the respective bromophenol utilis-
ing standard Suzuki cross-coupling methodology.23
In the final step of the synthesis, the Cl ligand is substituted by
a phenolato ligand of the type 2H. This was easily achieved by the
reaction of 1 with one equivalent of 2aH2 and 2bH2, respectively,
in toluene solution. The products were obtained as dark violet
solids in ca. 60% yield after column chromatography.
The chemical composition of 3a and 3b was established by
high-resolution mass spectrometry (HRMS), and their purity was
verified by elemental analysis. They were thoroughly characterised
by IR and multinuclear NMR spectroscopy, which revealed no
unexpected features. The characteristic n(B–O) vibrational band is
observed at ca. 1050 cm-1 for both compounds, which is very close
to the value of 1052 cm-1 reported for the parent PhO-substituted
SubPc.24 The respective hexafluorocyclopentene ring of 3a and 3b
bears two different thienyl substituents and therefore gives rise to
three signals in the 19F NMR spectrum. The signal due to central
CF2 unit (d ca. -132 ppm) is clearly separated from the signals of
the flanking CF2 groups (d ca. -110 ppm), which exhibit only a
very small chemical shift difference (£ 1 ppm).
Results and discussion
Synthetic work
The synthesis of the target compounds 3a and 3b is outlined
in Scheme 1. The n-dodecylthio-substituted chloridosubphthalo-
cyaninatoboron complex 119 was prepared by cyclotrimerisa-
tion of 4,5-bis(n-dodecylthio)phthalodinitrile20 in the presence
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Table 1 UV-vis spectroscopic data at room temperature (CH2Cl2 solu-
tion, l > 290 nm)
Compounds 3a and 3b contain two covalently attached photo-
physically active units, viz. the photochromic dithienylcyclopen-
tene moiety of the type 2H and the SubPc-based headgroup,
which is a strong chromophore and fluorophore.25 We note that the
groups of Branda and Irie have demonstrated independently that
the photoinduced isomerisation of dithienylethene substituents
significantly influences the luminescence behaviour of covalently
attached porphyrins, a switch-induced change known as lumi-
nescence modulation.5a,26 A range of other fluorophores based,
for example, on BODIPY27 or rhodamine dyes28 have also been
utilised in conjunction with dithienylethene-type switches for
luminescence modulation.29
l
max/nm, log(e/M-1 cm-1)
1
307, 4.75; 383, 4.36; 412, 4.37; 602, 3.89
297, 4.45
2aH2
2a¢H2
2bH2
2b¢H2
3a
276, 4.24; 310, 4.30; 365,a 4.03; 588, 4.14
294, 4.55
337, 4.37; 381, 4.26; 581, 2.28
294, 4.95; 389, 4.40; 414, 4.41; 601, 4.92
295, 4.99; 383, 4.45; 412, 4.46; 601, 4.96
3b
a Shoulder.
The UV-vis spectra of 3a and 3b are shown in Fig. 2, to-
gether with the spectra of the chloridosubphthalocyaninatoboron
complex 1 and the dithienylcyclopentene derivatives 2aH2 and
2bH2, which served as starting materials. Pertinent absorption
data for these compounds as well as the ring-closed isomers 2a¢H2
and 2b¢H2 are collected in Table 1. It is well known that the
influence of the apical ligand on the spectroscopic and redox
orbitals of the basal SubPc unit is very small.30 To a good
approximation, therefore, the absorption of the compounds of
type 3 can be viewed as a superposition of the absorption of the
apical dithienylcyclopentene-based ligand and the basal SubPc
unit.
Photophysical characterisation
The photoisomerisation of compounds 2aH2 and 2bH2 was
investigated in dichloromethane solution (Fig. 1). Irradiation with
UV light (l = 313 nm) caused the expected spectral changes due to
an increasing conversion of the colourless open to the blue ring-
closed form (2H2 → 2¢H2). The observation of an isosbestic point
indicates a simple equilibrium between these two species in each
case. The ring-closed isomer 2¢H2 exhibits an absorption at lmax
ª
585 nm, which is responsible for the blue colour of the solution
(Table 1). A second absorption of this isomer is located in the
region between ca. 350 and 400 nm. These results are in line with
those reported by Lehn and coworkers for 2bH2 in acetonitrile
solution.22b,c
Fig. 2 Absorption spectra of compounds 1–3 in dichloromethane
solution.
The effect of irradiation with UV light (l = 313 nm) on the
absorption is exemplarily shown for 3b in Fig. 3. As was observed
before in the analogous experiment with 2aH2 and 2bH2 (vide
supra), isosbestic points indicate a simple equilibrium between
two species in each case. Formation of the ring-closed form 3¢
causes pronounced absorption increases in the regions of ca.
340 nm and 490 nm, where the SubPc unit exhibits absorption
minima. In addition, a slight absorption increase occurs close to
the absorption maximum of the SubPc unit at ca. 600 nm, where
the ring-closed form exhibits a strong maximum, too. Prolonged
irradiation (t > ca. 20 min) turned out to lead to bleaching of
the absorption at ca. 600 nm and strong deviation of the spectra
from isosbestic behaviour, which suggests a decomposition of the
SubPc unit. The tendency of SubPc derivatives to be photolabile
in solution has been described before.31
Fig. 1 Effect of UV irradiation (l = 313 nm) on the absorption spectrum
of 2aH2 (top) and 2bH2 (bottom) in dichloromethane solution.
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1
Table 2 Half-wave potentials E vs. ferrocenium/ferrocene and peak-to-
2
peak separations DEp in V for the first oxidation and reduction process of
3a and 3b (CH2Cl2, 0.1 M [NBu4][PF6] supporting electrolyte, v = 0.1 V
s-1)
0/-, DEp
E
+/0, DEp
1
2
1
2
E
3a
3b
-1.61, 0.11
-1.57, 0.07
0.51, 0.07
0.51, 0.07
Electrochemical characterisation
SubPc derivatives are redox-active compounds, which undergo a
one-electron oxidation at a potential of ca. 0.5 V and a one-electron
reduction at a potential of ca. -1.5 V vs. the ferrocenium/ferrocene
couple.14b While the reduction process is usually reversible, the
oxidation may be accompanied by gradual decomposition, leading
to deviations from electrochemical reversibility. Compounds 3a
and 3b were investigated by cyclic voltammetry (Table 2). Po-
tential windows were chosen such that only the first oxidation
or reduction process took place, because otherwise irreversible
behaviour was observed. Both compounds undergo the expected
one-electron oxidation and reduction under these conditions at
Fig. 3 Effect of UV irradiation (l = 313 nm) on the absorption spectrum
of 3b in dichloromethane solution.
An important photophysical feature of SubPc derivatives is
their strong luminescence, which usually exhibits a rather small
Stokes shift.32 Compounds 3a and 3b exhibit essentially identical
photophysical behaviour. Their respective luminescence maximum
is located at a wavelength of ca. 620 nm. Fig. 4 shows the
absorption and emission spectra of compound 3b together with the
excitation spectrum recorded at the emission maximum. The emis-
sion intensity is largest at the excitation wavelengths corresponding
to the two strongest absorption maxima of almost equal intensity
located at l = 294 nm (log e = 4.95) and l = 601 nm (log e = 4.92).
The emission intensity decreases considerably upon irradiation
with UV light (l = 313 nm), which induces photoisomerisation of
the dithienylcyclopentene moiety from the open to the ring-closed
form (3b → 3¢b). This switch-induced change (i.e., modulation) of
luminescence can be explained by the fact that the ring-closed form
is able to effect efficient emission quenching of covalently attached
luminophores by resonant energy transfer.5a,26,29 We have observed
an intensity decrease of almost 50% at the emission maximum
during 10 min of irradiation (l = 313 nm). Although our UV-
vis spectroscopic results indicate that photodegradation occurs
only to a minor extent during this fairly short time span (vide
supra), a more detailed analysis of the luminescence modulation
is not meaningful due to uncertainties concerning the nature and
photophysical properties of the degradation products.
+/0
0/-
E 1
ª 0.5 V and E 1 ª -1.6 V, respectively, which compares
2
2
well to electrochemical data reported for a closely related subph-
thalocyanine bearing six peripheral n-octylthio substituents and
tBu-p-C6H4-O as the apical ligand.33 Note that oxidation processes
associated with the dithienylhexafluorocyclopentene unit occur
at potentials well above 1 V vs. ferrocenium/ferrocene,34 and are
therefore not expected tointerfere withthe SubPc-based oxidation.
SAM formation and characterisation
Thin films of 3a and 3b, respectively, were fabricated from
dichloromethane solution on solid gold substrates (see Experimen-
tal section). The films were characterised by X-ray photoelectron
spectroscopy (XPS) and near-edge X-ray absorption fine structure
(NEXAFS) spectroscopy.
XPS analysis
Average XPS determined compositions of the thin films are
reported in Table 3. The surface compositions are very similar,
showing the expected presence of carbon, nitrogen, sulfur, fluorine,
and gold (from the substrate). However, no oxygen or boron
was detected, which may be ascribed to the low percentage
of these elements in the adsorbate species. The effective film
thicknesses were determined from the IC 1s/IAu 7f intensity ratios
using previously reported attenuation lengths35 and are 17.6 and
˚
17.0 A, which is compatible with monolayers. Omitting the Au
contribution (Table 3), only two slight differences between the
theoretical and experimental compositions appear. Firstly, the
atomic percentage of S is lower than expected; secondly, there
is a slightly larger than expected N concentration. The S atoms
belong to the part of the adsorbate molecule designed for its
chemisorption on gold. Consequently, this lower than expected
sulfur percentage is compatible with an attenuation of the sulfur
signal due to inelastic scattering by the remaining parts of the
molecule, rising above the anchoring part. The slightly elevated
nitrogen concentration, too, might be explained by the SAM
architecture with considerable void space above the pyrrole units
Fig. 4 Normalised absorption, excitation and emission spectra of 3b in
dichloromethane solution.
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Table 3 Summary of XPS determined elemental compositionsa for thin films of 3a and 3b on gold
F
N
C
S
O
Au
Theor. comp.
Exp. comp.
Exp. comp. w/o Au
Exp. comp.
Exp. comp. w/o Au
4.1
4.1
84.8
5.5
1.4
0
3a
3b
2.7(0.5)
6.2(1.1)
2.0(0.6)
4.8(1.4)
2.3(1.4)
5.2(3.2)
3.0(0.6)
7.0(1.4)
36.6(1.1)
84.4(3.3)
35.4(1.1)
83.9(1.6)
1.9(0.3)
4.4(0.7)
1.8(0.2)
4.3(0.6)
n. d.b
n. d.b
n. d.b
n. d.b
56.6(1.0)
0
57.7(1.1)
0
a Values in atomic percent with experimental errors in parentheses. b Not detected.
Table 4 Angle-resolved XPS determined elemental compositionsa for thin films of 3a and 3b on gold
TOA
F
N
C
S
Au
F/N
3a
3b
Exp. comp.
Exp. comp.
0◦
2.7(0.5)
3.8
3.6
2.0(0.6)
3.8
3.7
2.3(1.4)
3.7
2.8
3.0(0.6)
1.8
1.6
36.6(1.1)
48.5
65.1
35.4(1.1)
47.8
64.5
1.9(0.3)
2.2
2.4
1.8(0.2)
3.1
56.6(1.0)
41.7
26.1
57.7(1.1)
43.5
27.8
1.2
1.0
1.3
0.7
2.1
2.3
55◦
75◦
0◦
55◦
75◦
2.4
a Values in atomic percent with experimental errors in parentheses for TOA = 0◦ (O was not detected).
at the anchor group so that the nitrogen emission would not get
attenuated as much.
Angle-resolved XPS was used to determine the concentrations
of the different species at different locations within the films. Data
acquired at photoelectron take-off angles (TOAs) 0◦, 55◦, and 75◦
are summarised in Table 4. TOA of 0◦ is normal to the surface and
will have the largest sampling depth. As expected, the Au signal
from the substrate is increasingly attenuated for both films with
increasing TOA. If the thioether groups are interacting with the
gold substrate we would expect the fluorine species to be located
at the top of the film, while the nitrogen species would be at the
opposite end, viz. down at the substrate. For the film fabricated
from 3a, the atomic ratio of F to N stays relatively constant over
this 75◦ TOA span (Table 4).
Fig. 5 High resolution N 1 s, C 1 s, and S 2p XP spectra collected from
films fabricated from 3a and 3b on gold.
In contrast, the F/N ratio for the film fabricated from 3b
increases by a factor of more than 3, indicating that 3b-based
films are more ordered than those based on 3a and the molecules
indeed adsorb preferentially with the SubPc-based anchor unit
oriented towards the surface.
High-resolution C 1 s XP spectra (Fig. 5) collected from both
films yield peak envelopes made up of four distinct peaks, one at
284.4 eV corresponding to the CH2 groups of the alkyl chains and a
small shoulder at 285.8 eV which can be attributed to either a C–N
or C–O species.36 The two peaks at higher binding energies (287.9
and 290.2 eV) correspond to CF and CF2 species, respectively.37
In both films we expect only a single nitrogen species to be present
which is nicely compatible with the single feature at 398.5 eV
observed in the N 1 s high resolution scans (Fig. 5).
The high-resolution S 2p XP spectra (Fig. 5) of both films exhibit
two S 2p doublets at ª162.0 and ª163.3 eV (S 2p3/2) related to
the thioether headgroups and the terminal thiophene moieties.
The relative intensities of these features are 1 : 5 and 1 : 1 for 3a
and 3b, respectively. No traces of oxidised sulfur species, which
would give rise to peaks at higher binding energies, are observed.
A doublet near 163 eV is generally associated with weakly bound
sulfur, unbound sulfur, or disulfide species.38,39 In the case of 3a
and 3b, this doublet can be assigned to the appended thiophene
moieties and to thioether units forming a weak coordination-type
bond to the substrate, in agreement with studies showing that
thioethers can adsorb non-destructively on gold.38,40–42 The doublet
near 162.0 eV can be assigned to thiolate species, which are due
to thioethers that undergo C–S bond cleavage upon binding to
the gold substrate.39,42 This spectral feature could, in principle,
also be due to thiophene units binding to the gold surface.18
However, this is highly unlikely in view of the steric crowding
around their sulfur atoms. For the films fabricated from 3b the
relative signal intensities of bound and unbound sulfur are 1 : 1,
which implies a coexistence of two different chemical states for
the thioether groups. However, taking the thiophene contribution
to the unbound sulfur emission into account, the predominant
binding mode of the thioether units is most likely a thiolate-type
attachment to the substrate. In contrast, the spectrum for films
fabricated from 3a exhibits only a relatively small doublet related to
the thiolate-type sulfur (17% of the total S 2p emission), suggesting
that the majority of the thioether units are either unbound or
weakly coordinated to the surface.
NEXAFS analysis. NEXAFS spectra provide information
about the electronic structure of the surface species by probing
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characteristic absorption resonances related to electronic transi-
tions from atomic core levels to unoccupied molecular orbitals.
This allows an analysis of the chemical integrity, orientation,
and alignment of surface species.43 NEXAFS spectroscopy probes
structural parameters of the film by monitoring intensity varia-
tions of spectral features with changing incidence angle of the
synchrotron light, and, thus, the angle between the electric field
vector of the incident radiation and the transition dipole moment
of the surface molecules. This effect, the so-called linear dichroism
of X-ray absorption, can be directly observed by collecting spectra
between normal and glancing X-ray incidence angles.
Carbon K-edge spectra for films fabricated from 3a and 3b,
respectively, collected at X-ray incidence angles of 70◦ and 20◦
along with the respective difference spectra are presented in Fig.
6. The spectra for both films exhibit the expected absorption edge
related to the transition of atomic C 1 s electrons into continuum
states and all the expected absorption resonances. The spectra
show a pronounced p*(C C) resonance near 285.0 eV assigned
to the aromatic ring structures. A strong Rydberg/C–H (R*)
resonance visible near 288.0 eV is mostly related to the alkyl chains.
Broad s* resonances related to C–C bonds are present at higher
photon energies (293 eV and 302 eV). Chemical impurities such as
orbital.43 The transition dipole moments of the respective R*
orbitals are parallel to the average chain orientation (assuming
all-trans conformation). This analysis yields average tilt angles
with respect to the surface normal of 61◦ 8◦ and 82◦ 8◦ for
SAMs fabricated from 3a and 3b, respectively. A higher tilt angle
corresponds to a stronger inclination of the chains. In the case of
chemisorbed 3b, therefore, the alkyl chains are in an approximately
parallel orientation to the substrate surface. Note that any disorder
in the chain conformation shifts the apparent tilt angle closer
to 55◦, which is the magic angle of NEXAFS spectroscopy. A
completely disordered chain structure will result in an apparent
angle of 55◦. The value of 61◦ determined in the case of adsorbed
3a is fairly close to that value, which suggests that the chains in this
film, instead of pointing away from the substrate surface, rather
are disordered.
Overall, the NEXAFS data indicate that both films are densely
packed and contamination-free. The data indicate a ‘flat’ ad-
sorption geometry of the thioether alkyl groups with a chain
orientation almost parallel to the gold surface. Interestingly, films
based on 3b exhibit a more ordered binding geometry, which is in
good agreement with the angle-resolved XPS data.
C
O are not detectable. The assignments of the spectral features
Conclusion
were made in accordance with Ref. 43–47.
Our results strongly support the previous notion15 that SubPc
complexes bearing six peripheral long-chain alkylthio substituents
are suitable for SAM formation on gold. We have demonstrated
their capacity to serve as anchor units even for very complex func-
tional tailgroups. In contrast to the disk-like phthalocyaninato
platform,13 the peripheral alkyl chains do not point away from the
substrate surface, which is most likely due to the dome-like shape
of the adsorbed SubPc core. The substantial differences between
SAMs fabricated from 3a and 3b, respectively, must be due to
the influence of the apical unit, which is pointing away from the
thioether-decorated anchor group in the para-substituted case 3b
and is therefore not expected to interfere with the surface binding
of the anchor group. In the meta-substituted case 3a, however,
the apical unit is able to adopt an orientation towards the anchor
unit, as is evident already from the molecular structures shown
in Scheme 1. Future work will address the photoswitchability of
the appended dithienylperfluorocyclopentene units on the gold
surface.
Fig. 6 NEXAFS carbon K-edge spectra for films fabricated from 3a and
3b on gold, acquired at 70◦, 55◦ and 20◦. The difference spectra between
the 70◦ and the 20◦ data are also shown.
The 70–20◦difference spectra show a significant linear dichroism
for the R* and s*(C–C) peaks. The negative R* difference peaks
and the inverted polarity of the s*(C–C) difference feature imply
strongly tilted alkyl chains with an orientation essentially parallel
to the surface. The dichroic ratio is substantially smaller for the
3a-based film indicating a more disordered binding geometry for
the alkyl groups compared to the 3b-based film. The absence of
any measurable angle dependence related to the aromatic rings
is expected in view of the inherently broad distribution of ring
structures throughout the molecule.11b,c
A quantitative analysis of the NEXAFS spectra was carried
out to determine the average molecular tilt angles of the thioether
chains. The orientation of the alkyl chains with respect to the
surface normal were determined using the R* transitions. The
intensities of these resonances as a function of the X-ray incidence
angle H are evaluated using published procedures for a planar
Experimental section
General considerations
All preparations involving air-sensitive compounds were carried
out under an atmosphere of dry nitrogen by using standard
Schlenk techniques or in a conventional argon-filled glove box.
Solvents and reagents were appropriately dried and purified
by conventional methods and stored under inert gas atmo-
sphere. The n-dodecylthio-substituted chloridosubphthalocyan-
inatoboron complex 119 was prepared by cyclotrimerisation of
4,5-bis(n-dodecylthio)phthalodinitrile20 in the presence of BCl3 in
xylene, following a protocol developed by Torres and coworkers.21
1,2-Bis(2-chloro-5-methylthien-3-yl)hexafluorocyclopentene was
prepared according to the published procedure.22 Elemental
analyses were carried out by the microanalytical laboratory Kolbe
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(Mu¨lheim an der Ruhr, Germany). NMR spectra were recorded
with a Varian MR-400 spectrometer operating at 400.1 MHz
for 1H. 19F NMR chemical shift values are given relative to
CCl3F as external reference. ESI mass spectra were obtained
with a quadrupole ion-trap spectrometer Finnigan LCQDECA
(ThermoQuest, San Jose´, USA). MALDI mass spectra were
obtained with a BiFlex IV spectrometer (Bruker Daltonics,
Bremen, Germany) equipped with an N2 laser (wavelength 337
nm, 3 ns pulse duration). DCTB (2-[(2E)-3-(4-tert-butylphenyl)-2-
methylprop-2-enylidene]malononitrile) was used as matrix. Mass
calibration was performed immediately prior to the measurement
with ESI Tune Mix Standard (Agilent, Waldbronn, Germany).
UV-vis spectra were recorded in dichloromethane solution with
a Lambda 900 spectrometer (Perkin Elmer, Waltham, USA). A
6 W UV lamp NU-6 KL (Benda, Wiesloch, Germany) was used
for UV irradiation. Emission and excitation spectra were recorded
in dichloromethane solution with an LS 50B Fluorescence Spec-
trometer (Perkin Elmer, Waltham, USA). Cyclic voltammetry was
performed with a computer-controlled potentiostat/galvanostat
PAR VersaStat II (Princeton Applied Research, Ametek Inc.,
Farnborough, UK), in a cyclindrical cell equipped with platinum
working and counter electrodes and a silver pseudo-reference
electrode (CH2Cl2, 0.1 M [NBu4][PF6] supporting electrolyte, N2
inert gas atmosphere, ferrocene/ferrocenium as direct reference).
cool to room temperature. Volatile components were removed in
vacuo. The crude product was purified by column chromatography
(silica gel, ethyl acetate-n-hexane 6 : 1), affording a dark violet
solid. Yield 90 mg (60%), mp 64–66 ◦C. 1H NMR (CDCl3): d 0.87
(t, J = 7.0 Hz, 18H), 1.25 (br. m, 96H), 1.54 (br. m, 12H) 1.83
(m, 12H), 2.01 (s, 3H), 2.03 (s, 3H), 3.20 (m, 12H), 5.37 (m, 1H),
5.50 (br. s, 1H), 6.73–6.82 (m, 3H), 6.87 (m, 1H), 6.92 (s, 1H),
7.07 (s, 1H), 7.10 (m, 1H), 7.29 (m, 1H), 7.46 (br. s, 1H), 8.60 (s,
6H). 13C NMR (CDCl3): d 14.1, 14.5 (br.), 22.7, 28.4, 29.1, 29.3,
29.4, 29.5, 29.6, 29.7, 29.8, 31.9, 33.6, 110.0, 112.6, 115.6, 118.5,
118.6, 118.9, 119.2, 122.6, 123.2, 127.7, 128.1, 129.5, 130.4, 133.9,
135.0, 139.9, 140.8, 141.1, 141.6, 141.8, 150.0, 150.6, 153.1, 156.7.
19F NMR (CDCl3): d -109.7 (m, 2F), -109.9 (m, 2F), -132.1 (m,
2F). HRMS/MALDI(+): m/z 2147.1341 [M]+, 2147.1383 calc.
for [C123H173BF6N6O2S8]+. Calc. for C123H173N6BF6O2S8 (2149.1):
C, 68.74; H, 8.11; N, 3.91; S, 11.94. Found: C, 67.91; H, 7.61; N,
3.67; S, 11.14%.
3b. This compound was obtained in 60% yield by a procedure
analogous to that described for 3a. Dark violet solid, mp 90–91 ◦C.
1H NMR (CDCl3): d 0.87 (t, J = 7.0 Hz, 18H), 1.25 (br. m, 96H),
1.55 (br. m, 12H) 1.85 (m, 12H), 1.90 (s, 3H), 2.12 (s, 3H), 3.23
(m, 12H), 5.47 (m, 2H), 6.86 (s, 1H), 6.92–7.02 (m, 5H), 7.23 (m,
2H), 7.59 (s, 1H), 8.60 (s, 6H). 13C NMR (CDCl3): d 14.1, 14.5
(br.), 22.7, 28.4, 29.1, 29.3, 29.4, 29.5, 29.6, 29.7, 29.8, 31.9, 33.5,
115.9, 119.2, 120.0, 121.7, 122.5, 125.5, 125.8, 126.0, 126.3, 127.1,
127.2, 128.0, 140.1, 140.3, 140.8, 141.5, 142.0, 150.6, 152.5, 156.5.
19F NMR (CDCl3): d -109.5 (m, 2F), -110.7 (m, 2F), -131.9 (m,
2F). HRMS/MALDI(+): m/z 2147.1355 [M]+, 2147.1383 calc.
for [C123H173BF6N6O2S8]+. Calc. for C123H173N6BF6O2S8 (2149.1):
C, 68.74; H, 8.11; N, 3.91; S, 11.94. Found: C, 68.88; H, 8.23; N,
3.72; S, 11.85%.
Preparative work
1,2-Bis-[5-(m-hydroxyphenyl)-2-methylthien-3-yl]hexafluoro-
cyclopentene (2aH2). nBuLi (0.94 mL of a 1.6 M solu-
tion in hexanes, 1.50 mmol) was added dropwise via sy-
ringe to a stirred solution of 1,2-bis(2-chloro-5-methylthien-3-
yl)hexafluorocyclopentene (302 mg, 0.69 mmol) in diethyl ether
(10 mL). B(OnBu)3 (430 mg, 1.87 mmol) was added after 1 h and
the mixture stirred for a further 1.5 h. THF (10 mL), Na2CO3
(3.0 mL of a 2 M aqueous solution, 6.0 mmol), [PdCl2(PPh3)2]
(21 mg, 0.03 mmol) and m-bromophenol (363 mg, 2.1 mmol) were
SAM fabrication
The gold substrates for SAM fabrication were prepared by thermal
evaporation of 100 nm gold (99.99% purity) onto polished single-
crystal silicon (111) wafers (Silicon Sense) primed with a 5
nm Ti layer for adhesion promotion. The resulting films were
polycrystalline with a grain size of 20–50 nm of predominantly
(111) crystal facets.48 The films were formed by immersion of
freshly prepared 1 ¥ 1 cm2 gold substrates in 10 mM solutions
of 3a and 3b in dichloromethane at room temperature for 10 h.
After immersion the samples were sonicated for 3 min in pure
dichloromethane. Following sonication the films were carefully
rinsed with copious amounts of dichloromethane, blown dry with
◦
added sequentially. The stirred mixture was heated to 90 C for
20 h and was subsequently allowed to cool to room temperature.
Water (20 mL) was added. The organic layer was separated off
and the aqueous phase extracted with ethyl acetate (3 ¥ 30 mL).
The combined organic layers were dried with Na2SO4. Volatile
components were removed in vacuo and the oily residue subjected
to column chromatography (silica gel, n-hexane-ethyl acetate 4 : 1).
The product was obtained as an oil, which slowly afforded a
slig◦htly blue solid upon standing. Yield 167 mg (42%), mp 82–
1
84 C (dec.). H NMR (CDCl3): d 1.96 (s, 6H, Me), 5.11 (br. s,
R
nitrogen, and kept in Parafilmꢀ-sealed plastic containers filled
2H, OH), 6.78 (m, 2H), 7.02, (m, 2H), 7.12 (m, 2H), 7.23–7.27 (m,
4H). 13C NMR (CDCl3): d 14.5, 112.5, 114.9, 118.2, 122.7, 130.3,
134.8, 141.4, 141.7, 156.0. 19F NMR (CDCl3): d -110.1 (t, J = 5.5
Hz, 4F), -131.9 (m, 2F). HRMS/ESI(+): m/z 575.05475 [M +
Na]+, 575.05446 calc. for [C27H18F6NaO2S2]+.
with nitrogen until they were characterised.
X-ray photoelectron spectroscopy
Film compositions were determined from XP spectra collected
from a Kratos AXIS Ultra DLD instrument (Kratos, Manchester,
England) equipped with a monochromatic Al-Ka X-ray source
(photon energy = 1486.6 eV). Experiments were carried out in
the electrostatic mode at a 0◦ photoelectron TOA. This TOA
was normal to the substrate. Angle-resolved measurements were
conducted at 0◦, 55◦, and 75◦ TOAs. The photoelectron energy
scale was calibrated to the Au 4f7/2 emission (84.0 eV) of the
underlying gold substrate. The XPS determined compositions
1,2-Bis-[5-(p-hydroxyphenyl)-2¢-methylthien-3-yl]hexafluoro-
cyclopentene (2bH2). This compound was obtained in 52% yield
by a procedure analogous to that described for 2aH2. Spectro-
scopic data are in accord with those reported in the literature.22
3a. Toluene (2.0 mL) was added to 2aH2 (111 mg, 0.20 mmol)
and 1 (114 mg, 0.07 mmol). The stirred mixture was heated to
130 ◦C (closed ampoule) for 16 h and subsequently allowed to
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Dalton Trans., 2012, 41, 1553–1561 | 1559
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were an average from three spots from two distinct samples.
Atomic compositions were calculated from peak areas obtained
from selected region scans (675–695 eV for F 1s; 524–544 eV
for O 1s; 390–410 eV for N 1s; 278–290 eV for C 1s; 155–
173 eV for S 2p) acquired at an analyser pass energy of 80 eV.
Molecular environments of the samples were probed by collecting
high-resolution (analyser pass energy = 20 eV) spectra from
the S 2p, N 1 s, and C 1 s regions. A linear background was
subtracted for all peak quantifications. The peak areas were
normalised by the manufacturer-supplied sensitivity factors and
surface concentrations were calculated using Casa XPS software.
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Near-edge X-ray absorption fine structure spectroscopy
NEXAFS spectra were collected at the National Synchrotron
Light Source (NSLS) U7A beamline at Brookhaven National
Laboratory, using an elliptically polarised beam with ~85% p-
polarisation. This beam line utilises a monochromator and 600 l
mm-1 grating providing a full-width at half-maximum resolution of
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spectra were brought to the standard form by linear pre-edge
background subtraction and normalising to the unity edge jump
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edge.
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Acknowledgements
This work was funded in part by NESAC-BIO (NIH grant EB-
002027). T. W. and J. E. B. thank David G. Castner for support
of this work and Daniel Fischer and Cherno Jaye (NIST) for
providing us with the experimental equipment for NEXAFS
spectroscopy and their help at the synchrotron. NEXAFS studies
were performed at the NSLS, Brookhaven National Laboratory,
which is supported by the U. S. Department of Energy, Division of
Materials Science and Division of Chemical Sciences. U. S. and J.
S. thank Andreas Winzenburg for his help with the photophysical
measurements and Jan H. Schro¨der and Ulrich Glebe for help
concerning the synthesis of compound 1.
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©