Molecular design of photoswitchable surfaces with controllable wettability

Article abstract of DOI:10.1039/c0jm03528a

In this work, we developed a photocontrollable substrate which was prepared using an azobenzene-containing self-assembled monolayer (SAM) on the silicon surface via the chemisorption of 3-glycidoxypropyltrimethoxysilane (GPTS) and 4-(4′-aminophenylazo) benzoic acid (APABA). The prepared surfaces were chemically characterized by X-ray photoelectron spectroscopy (XPS). The reversible photoswitching performance of APABA molecules were investigated by UV spectroscopy in dimethylsulfoxide (DMSO) solution. To understand and control this reversible photoswitchable mechanism and wettability properties, contact angle measurements were performed by using a variety of liquids after UV and visible light irradiation. These contact angle results are used to approximate the components of the APABA-modified surface energy under UV and visible light using the Lifshitz-van der Waals/acid-base approach. The total surface energy (γ_s) after visible light irradiation (for trans formation) was calculated to be 37.28 mJ m^-2, whereas the value after UV light exposure (for cis formation) was also calculated to be 36.95 mJ m^-2. All the results demonstrate the great potential to control molecular events within and on the surfaces of molecular constructs using light. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm03528a

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PAPER
Molecular design of photoswitchable surfaces with controllable wettability†
a
Gokc¸en Birlik Demirel, Nursel Dilsiz,b Mehmet C¸ akmakc and Tuncer C¸ aykaraa
*
€
Received 18th October 2010, Accepted 22nd November 2010
DOI: 10.1039/c0jm03528a
In this work, we developed a photocontrollable substrate which was prepared using an azobenzene-
containing self-assembled monolayer (SAM) on the silicon surface via the chemisorption of 3-
glycidoxypropyltrimethoxysilane (GPTS) and 4-(40-aminophenylazo) benzoic acid (APABA). The
prepared surfaces were chemically characterized by X-ray photoelectron spectroscopy (XPS). The
reversible photoswitching performance of APABA molecules were investigated by UV spectroscopy in
dimethylsulfoxide (DMSO) solution. To understand and control this reversible photoswitchable
mechanism and wettability properties, contact angle measurements were performed by using a variety
of liquids after UV and visible light irradiation. These contact angle results are used to approximate the
components of the APABA-modified surface energy under UV and visible light using the Lifshitz–van
der Waals/acid–base approach. The total surface energy (gs) after visible light irradiation (for trans
formation) was calculated to be 37.28 mJ mꢀ2, whereas the value after UV light exposure (for cis
formation) was also calculated to be 36.95 mJ mꢀ2. All the results demonstrate the great potential to
control molecular events within and on the surfaces of molecular constructs using light.
photoswitchable SAMs on silicon surfaces have already been
Introduction
described.8–15 Sekkat et al. have reported a photoswitchable
surface which consists of the chemisorption of triethoxysilane
containing an azobenzene moiety and they observed that this
preparation process did not allow formation of a dense
azobenzene monolayer, probably because of the difficulties with
purification during the synthesis of the azo-silane.10 On the other
hand, Siewierski and co-workers used a variety of preparation
methods that give rise to a higher density of monolayer con-
taining azobenzene moieties.9 In their procedure, azobenzene
moieties are grafted onto a densely functionalized SAM.9 Cook
et al. reported a photoswtiching of ‘on’ and ‘off’ coordination
sites in a self-assembled monolayer containing an azobenzene
chromophore. They used this photoswtichable system to control
of the releasing of metallated macrocycles.11 In another study,
Sortino et al. prepared a cationic azobenzene SAM on a platinum
electrode to control the immobilization and to release of mole-
cules rich in negative charges.12
Stimuli-sensitive materials are able to change certain properties
in response to specific external stimuli such as heat or light. Light
can be used as a precise stimulus by selecting suitable wave-
lengths, polarization directions, and intensity, allowing non-
contact control. Due to the substantial changes observed in
material properties during light irradiation, these materials have
been investigated as active components for a variety of applica-
tions from lithography1 to non-linear optical devices,2 to all-
optical switches,2 and even data storage.2 Azobenzene derivatives
are typical compounds adapted to prepare such a photo-
switchable surface. The physical and chemical properties of the
two isomers are different3 and the transition between the trans
and cis isomers causes a change in dipole moment resulting in
a change of surface wettability.4–6 The incorporation of this
photoactive unit and related ‘‘azo’’ chromophores into self-
assembled monolayers (SAMs) have been well studied.7–10 SAMs
are among the most attractive option for controlling molecular
structure on a surface.7 Many different kinds of preparation of
Herein, we have developed a photoswitchable surface with
a dense monolayer containing the 4-(40-aminophenylazo) ben-
zoic acid chromophore on an epoxy-terminated silicon surface.
The photoswitchable surfaces were prepared with two steps
produced as shown in Scheme 1: the first step consists of
preparing the SAMs using epoxysilane molecules, and, in the
second step, the azobenzene chromophore are grafted onto the
epoxy-modified layer. We have investigated the reversible pho-
toswitching performances of these prepared surfaces via the
evanescence field using light of appropriate wavelengths. In the
trans form, or ‘‘on’’ position, the SAMs present a coordinating
surface that can be switched ‘‘off’’ in the cis form.
aDepartment of Chemistry, Faculty of Arts and Science, Gazi University,
06500 Bexsevler, Ankara, Turkey. E-mail: gbirlik@gazi.edu.tr; Fax:
+00903122122279; Tel: +00903122021497
bDepartment of Chemical Engineering, Faculty of Engineering and
Architecture Arts and Science, Gazi University, 06500 Maltepe, Ankara,
Turkey
cDepartment of Physics, Faculty of Arts and Science, Gazi University,
06500 Bexsevler, Ankara, Turkey
† Electronic supplementary information (ESI) available: Additional
NMR spectra. See DOI: 10.1039/c0jm03528a
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different concentrations (from 5 mM to 40 mM) for different
time periods (from 1 h to 24 h) at room temperature. The
substrates were then removed from solution, washed sequentially
with DMSO, and dichloromethane, and dried with nitrogen gas.
Characterization
The 1H NMR spectra were recorded at 270 MHz using a Bruker
400MHz AV spectrometer. XPS data sets were collected with
a SPECS spectrometer by using 300 W Mg-Ka radiation. Static
water-contact angles of the sample surfaces were measured at
ꢂ
22 C in ambient air by using an automatic contact angle goni-
€
ometer equipped with a flash camera (Model DSA 100, Kruss,
Scheme 1 Preparation of photoswitchable substrates.
Germany) applying a sessile drop method. The vertical structures
of the samples, such as the ‘‘optical’’ thickness of layers, were
measured with an auto-nulling imaging ellipsometer (Nanofilm
EP3, Germany). All thickness measurements were done at
a wavelength of 532 nm with an angle of incidence of 72ꢂ. The
UV light source was a model BlueWave 200 UV Curing Spot
lamp (DYMAX, Germany) with 366 and 450 nm wavelengths.
We used the 366 nm light by utilizing band pass filter for the UV
irradiations with a flux of 2 mW cmꢀ2 on the sample surface. The
visible light source was coupled with a 410 nm-long wave length
pass filter, resulting in a flux of 10 mW cmꢀ2 on the sample
surface. UV-vis spectra were obtained by using a Shimadzu (UV-
2401PC) UV-vis recording spectrophotometer.
Materials and methods
Materials
The substrates used in this study were Si(100) wafers (n-type,
obtained from Shin-etsu, Handoutai, Japan), and cut into 5 mm
ꢁ 5 mm pieces for further modification, 4-nitrobenzoic acid, p-
phenylenediamine, 3-glycidoxypropyltrimethoxysilane (GPTS),
dimethylsulfoxide (DMSO) (99.8%), dichloromethane (99.8%),
toluen (99.8%) and sodium chlorine were purchased from
Aldrich (USA). Deionized water (18 MU cmꢀ1) was obtained
from a Millipore system.
Synthesis
Result and discussion
The photoswitchable compounds 4-(40-aminophenylazo) benzoic
acid (APABA) was synthesized according to previously reported
methods.16 1H-NMR spectra of APABA compound for trans-
and cis-positions are given in the ESI.†
Preparation of epoxysilane films
Epoxysilanes are classical compounds which have the ability to
bind covalently to other molecules that have –NH2, –COOH and
–SH groups17–20 and even to biological molecules.20–22 However
their ability to be used in several applications, and the formation
of disordered and chemically heterogeneous molecular layers are
common problems for functional silanes due to the complicated
hydrolysis/interaction/adsorption/reactivity competition of the
head-end reactive groups with the hydroxyl-terminated silicon
oxide surfaces.23 It is known that the dimers can propagate to
form an oligomerized network, and can subsequently form
a multilayer structure in the presence of silanol groups from
either free or immobilized epoxysilanes.24–26 Free epoxysilanes
may also carry both silanols and/or methoxy groups, thus, GPTS
polymerizes easily in the presence of water and can be formed
into a multilayer structure.24 The interactions between GPTS and
silica particles in aqueous solutions have been shown by Francis
et al. to involve competing hydrolysis, condensation polymeri-
zation, and adsorption of GPTS.26 They reported that the
competition between adsorption on the silica surface and
condensation polymerization of GPTS in the bulk liquid are used
to identify conditions favorable to surface modification.26 In our
case, to obtain smooth and ordering of chemisorbed layers of
silanes on hydroxyl-terminated silicon oxide surfaces, we
prepared various epoxysilane films from toluene solutions with
0.5% (v/v) to 2.0% (v/v) of the GPTS for different time periods (5
h–24 h).
Preparation of epoxy-ended surfaces
Si(100) samples were cleaned carefully by applying the following
steps: (i) they were rinsed several times with deionized water and
ethanol; (ii) then they were treated with a mixture of 70% H2SO4/
30% H2O2 at 80 ꢂC for 5 min; (iii) they were washed with ethanol
and dried under a nitrogen stream; and (iv) finally, they were
exposed in a UV/ozone chamber (Irvine, CA: Model 42, Jelight
Company Inc., USA) for 15 min prior to modification for
preparation of hydroxylated silicon surfaces. Note that the water
contact angles of these cleaned substrate surfaces were ca. 3ꢂ,
which was obtained with an automatic contact angle goniometer
(Model DSA 100, Kruss, Germany). The silicon wafers were
immediately used in order to prevent surface contamination.
The hydroxylated surfaces were immersed into different
concentrations of 3-glycidoxypropyltrimethoxysilane (GPTS)
solution in which the concentration changed from 0.5% to 2.0%
(v/v) in dry toluene at room temperature for various dipping
times (5, 10, 18, 24 h). Afterwards, the substrates were rinsed with
pure toluene and dichloromethane to remove the physically
absorbed silanes from the surface. The epoxy-terminated
surfaces were then dried under a stream of nitrogen.
Preparation of photoswitchable surfaces
Fig. 1 shows the relative ellipsometric thicknesses and water
contact angles for different GPTS concentrations and dipping
times. The ellipsometric measurements were carried out in
The optimized epoxy-terminated substrate was dipped into the 4-
(40-aminophenylazo) benzoic acid (APABA) solutions of
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ambient air. All the ellipsometric thicknesses were reported as
relative values for clarity. In the normalization the initial thick-
ness of the surfaces (1.20 nm), which is obtained from each
surface before the measurements, is subtracted from the obtained
thicknesses. Generally the relative thickness and contact angle
are higher for longer deposition times (24 h). Between 5 h and
18 h dipping time, the contact angles increase gradually from 51ꢂ
to 54ꢂ. After 18 h of dipping time, the contact angle was 61ꢂ (for
the 1% GPTS concentration). The contact angle was highest for
films dipped in the 1% GPTS solution for 24 h. Therefore,
contact angle measurements alone might indicate that the film
formation was more complete if a solution with higher dipping
time was used. However, the relative thickness of the GPTS
layer, assuming a bulk refractive index, for 18 h dipping time
from the 1% solution reached 1.35 nm but for the highest dipping
time (24h) the film thickness reaches 2.10 nm.
Preparation of photoswitchable surface
APABA grafting. The photoswitchable substrate was prepared
through a two step procedure. Briefly, in the first step, a self-
assembled epoxy-terminated layer was obtained and then in the
following step, photoswitchable APABA molecules were
attached onto the epoxy-terminated surface as seen in Scheme 1.
Self-assembled layers are one of the efficient methods to control
molecular structure on the surface.27 The photoswitchable
substrates were prepared by immersing epoxy-terminated
substrates for 1 to 24 h into a 5–40 mM solution of APABA in
DMSO.
Fig. 2A shows the relative ellipsometric thicknesses and water
contact angles for the photoswitchable substrates for a variety of
dipping times. We used the relative ellipsometric thickness for
each substrate. First we fixed the concentration of APABA solu-
tion at 20 mM, afterwards, the epoxy-terminated surfaces were
dipped into 20 mM APABA solution with different dipping times.
The results show that the contact angles of photoswitchable
substrates decreased from 55.2ꢂ to 50.9ꢂ with dipping time
increasing from 1 h to 24 h. The photoswitchable surfaces showed
more hydrophilic character than the epoxy-terminated surfaces as
expected. The APABA molecule possesses two hydrophilic
terminated functional groups. Therefore, the APABA molecule
can be attached with the carboxyl group up or down or a mixture
of both onto the epoxy-terminated surface and as a result the film
exhibits greater hydrophilicity. On the other hand, while the
hydrophilic character of the substrate increases with the grafting
of APABA molecules onto the epoxy-terminated surface, the
On the other hand, the effect of GPTS concentration on the
film layers was investigated between 0.5% (v/v) and 2% (v/v)
concentration of GPTS solution. The contact angles changed
from 50.2ꢂ to 59.4ꢂ (for the 18 h dipping time). The film thickness
of the GPTS layers also changed from 0.95 to 2.2 nm. Above 1%
(v/v) concentration of GPTS, the film thickness increased
rapidly. The theoretical thickness of a GPTS film from the
Si(100) surface to top of a molecule was calculated as 1.3 nm
using DFT calculations. In conclusion, it can be clearly said that
1% (v/v) concentration of GPTS and 18 h dipping time are
optimum for fabrication of the epoxy-terminated surfaces.
Fig. 1 (A) Effects of dipping time on epoxy-terminated layers relative
thickness and contact angle. (GPTS concentration: 1% (v/v)). (B) Effects
of GPTS concentration on epoxy-terminated layers relative thickness and
contact angle. (Dipping time: 18 h).
Fig. 2 (A) Effects of dipping time on photoswitchable substrates relative
thickness and contact angle (APABA concentration: 20 mM). (B) Effects
of APABA concentration on photoswitchable substrates relative thick-
ness and contact angle (dipping time: 24 h).
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relative thickness of films increased from 1.35 nm to 3.75 nm by
changing dipping time. It can be said that APABA molecules may
have been densely attached on the surface after 24 h.
Furthermore, the effect of APABA concentration on their
surface formation was investigated. The relative ellipsometric
thicknesses and water contact angle values of the prepared
surfaces are given in Fig. 2B. Between 5 mM and 40 mM
concentration of APABA solution, the water contact angles
changed from 55.1 to 60.7ꢂ (dipping time was kept constant at
24 h). On the other hand, the relative film thicknesses of the
APABA layer also changed from 2.9 to 4.45 nm. The photo-
switchable substrate showed the lowest contact angle value at 20
mM APABA solution for a 24 h dipping time. In solutions above
a concentration of 20 mM APABA, the contact angle values and
the film thicknesses increased and the substrates showed greater
hydrophobic character due to the unwanted binding or phys-
isorption of APABA molecules on to the surface. Based on these
results, the optimum preparation conditions of the azo layer were
determined to be a concentration of 20 mM APABA and 24 h
dipping time.
Characterization of prepared substrates
By using X-ray photoelectron spectroscopy (XPS), we can
monitor the changes in surface composition after each reaction
step in the creation of the adlayer on the silicon surface. The XPS
spectra of the surface-modified interfaces, in which the chemical
composition progresses from being terminated by hydroxyl
groups to epoxy groups, and to the photoswitchable molecule
during the surface reaction process is shown in Fig. 3. The figure
presents the overall XPS spectra for the clean Si/SiO2 substrates
used in this study, where the main peaks due to Si (2p and 2s), C
1s and O 1s are observed. At low resolution, the XPS spectrum of
a GPTS modified Si/SiO2 substrate shows no significant differ-
ence from that shown in Fig. 3. However, the peak for the C 1s
binding energy in the 280–290 eV region shows an important
change following the modification of the oxide layer with GPTS.
For the unmodified oxide the carbon peak is at 290 eV as
expected for carbon, possibly due to the presence of organic
contaminations. In the spectrum of the APABA modified pho-
toswitchable substrate, the C 1s signal increases and the N 1s
peak at 400.8 eV appears indicating the successful binding of the
APABA molecules. This provides good qualitative evidence that
the substrates were successfully modified with GPTS and
APABA, respectively.
Fig. 3 X-Ray photoelectron spectra of the Si substrates that have been
chemically modified. From bottom to top: the activated silicon substrate
with hydroxyl surface, the Si (100) substrate with an epoxide terminated
adlayer, and the substrate with APABA moieties attached.
The APABA molecule possesses two different substituents at
the para positions. To obtain the photoisomerization behavior of
the APABA molecule, a spectroscopic study was performed.
Fig. 4 shows the UV-vis absorption spectra before and after UV
irradiation of APABA molecules in DMSO solution. For the
original state, the UV-vis spectrum displays a high-intensity p /
p* band at 275 nm and a low-intensity n / p* band at 420 nm.
Upon UV irradiation, a decrease in the absorbance at 275 nm and
an increase in the absorbance at 420 nm were observable, indi-
cating a photochemical conversion from the trans to cis isomer
configuration of the APABA molecules. Irradiation of the solu-
tion for 60 min by 366 nm radiation leads to an important increase
in the band intensity at 275 nm and a decrease in the intensity of
the band at 420 nm. Afterwards the cis-APABA solution was kept
in the dark for 24 h to observe the stability of APABA molecules in
the cis state. As expected the azo band intensity merely decreases
after being kept in dark. Such a result demonstrates that the
thermal reverse reaction is quite slow. However the azo band
intensity was almost recovered after visible light irradiation for 90
min, indicating that the cis-to-trans isomerization of the APABA
had occurred. Such full reversibility can be said to be attributed to
the cis-trans photoisomerization of the APABA molecules.31
The p / p*transition of APABA allowed us to determine the
kinetics of the trans to cis photoisomerization in DMSO
Photoswitchable surface
Azobenzene and its derivatives are known to exhibit large
changes in both geometry and dipole moment as a result of UV/
visible irradiation because of reversible photoisomerization
between the Z and E configurations.28,29 The –N]N– bond
dominates the isomerization behavior of these molecules, and the
effective bond order of this moiety is influenced by the presence
of electron-donating or electron-withdrawing substituents on the
phenyl rings.30 Blevins et al. reported that, the presence of
substituents on the azobenzene phenyl rings influences the
spectroscopic and isomerization behavior of these compounds.30
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Nakagawa et al. showed that the critical area required for the
isomerization of alkyl-azobenzenes incorporated in the SAMs
2
34
ꢀ
was ca. 45 A /molecule. Therefore, a relatively low surface
coverage is necessary for a photoresponsive SAM.35 The surface
coverage G was calculated using the relation, G ¼ h ꢁ r, where h
is the elipsometric thickness of azo layer, r is the density of azo
2
ꢀ
solution. The obtained G of ca. 30 A /molecule means that the
molecules in the prepared substrate have enough space for
isomerization to occur and may not have had much intermo-
lecular interaction from the APABA molecules.
To observe the changes of the surface wettability of substrates
after UV and visible light irradiation, the contact angle
measurements were employed as a function of irradiation time.
Fig. 6 shows the evolution of the water contact angle as a func-
tion of visible and UV light irradiation time. After irradiation
with UV light (366 nm) for 180 min, the photochemical change
from trans to cis caused a change in wettability from ca. 50ꢂ to
89.9ꢂ. The water contact angle value rapidly increased with the
366 nm UV light irradiation time until a plateau value (89.9ꢂ) was
reached due to the trans-cis transition after about 180 min. Then,
these cis-surfaces were irradiated with visible light (450 nm) with
increasing irradiation time. As a result, the water contact angle
value decreased from 89.9ꢂ to 50.9ꢂ after a 450 nm visible light
irradiation time of about 240 min. Contact angle results showed
that, the trans surface showed greater hydrophilicity as expected.
APABA molecules expose their acidic head groups toward to
surface upon visible light irradiation, as a result the photo-
switchable substrate gets more hydrophilic.
Fig. 4 UV-Vis absorption study of APABA solution in DMSO as
a function of various experimental treatments.
solution. The absorbance value at 420 nm exponentially
decreased with UV irradiation time during the trans-cis transi-
tion as shown in Fig. 5B. We used the values of Attrans/cis/A0 to
characterize the kinetics of the trans to cis photoisomerization
(exposure time ¼ 0 with absorbance of A0 and exposure time of
trans–cis isomerization ¼ t with absorbance of At). The linear
plots of ln[(Attrans/cis/A0] versus time (Fig. 5A) was shown that the
trans-to-cis photoisomerization obeyed first-order kinetics.32
From the slopes, the rate constant for the trans-cis photo-
Furthermore, to provide identification of dominant function-
ality present at the surface, the contact angle measurements were
performed as a function of droplet pH as seen in Fig. 7. Data
showed that, the contact angle on the trans-APABA surface was
higher at high pH, decreased linearly and obtained a lower angle
at low pH as expected. This is because the hydrophilic acid
groups of APABA molecules are directed towards to the surface
after visible light irradiation. Therefore, after visible light irra-
diation the prepared photoswitchable surfaces display hydro-
philic character because of the presence of the acid groups of the
APABA molecule on the surface as mentioned above. On the
other hand, after UV light irradiation the APABA molecules
indicate trans-to-cis isomerization and a more hydrophobic cis-
surface was obtained. Thus, the cis surface showed lower contact
angle values at high pH.36,37 In conclusion, the observed
isomerization (kJ) is calculated to be 2.99 ꢁ 10ꢀ3 sꢀ1
.
The free volume of the azobenzene functionalized layer has
significant influence on the photoresponse of azobenzene moie-
ties. This means that a critical free volume is necessary for the
isomerization of azobenzenes in the self-assembled layers.33
Fig. 5 (A) The fitting curve of absorbance to the first order kinetics for
the trans-cis photoisomerization. (B) Dependence of absorbance at
366 nm for APABA in DMSO on the UV light exposure time during the
trans to cis photoisomerization.
Fig. 6 The evolution of the water contact angle as a function of the
visible and UV light irradiation time.
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components can be obtained. Usually, diiodomethane, water,
and formamide are used as the probe liquids. For these calcu-
lations, the advancing contact angles are applied.45,46 For the
surface energy calculations, the literature data on surface tension
and its components for the probe liquids were taken,47 and they
are listed in Table 1.
To investigate the dependence of liquid–solid interactions on
liquid properties, we measured the advancing (qadv) and receding
(qrec) contact angles for above mentioned liquids on the APABA-
modified surface by following both visible (450 nm) and UV(366
nm) irradiation. We prepared two samples for each liquid and
measured three different contact areas for each sample. Contact-
angle results for each liquid after visible/UV irradiation of the
surface are summarized in Table 2. The calculated values of the
surface free energy and its components for photoswitchable
substrates after visible and UV irradiation are shown in Fig. 8.
The components of the energy were calculated from the LWAB
approach (eqn 2), and then total value of the energy was calcu-
lated from eqn 1. The total surface energy (gs) after visible light
irradiation (trans surface) is 37.28 mJ mꢀ2, whereas the value
after UV light exposure (cis surface) is 36.95 mJ mꢀ2. We noticed
that after visible irradiation the photoswitchable surface trans-
formed into a hydrophilic surface with increased surface free
energy due to the trans form of the surface.48 Therefore the
reactive binding sites for covalent attachment from the carboxyl-
side of the molecule is possible.
Fig. 7 Variation of advancing contact angles on APABA-modified
surfaces as a function of pH values of 0.1 M B–R (Britton–Robinson)
buffer solutions.
difference in the wetting behavior is attributed to the presence of
two different isomers on the surface.38–40
Surface energy of photoswitchable substrate
The energetic properties of photoswitchable surfaces can be
characterized by the determination of the surface free energy,
which plays a very important role in various physicochemical
processes occurring at interfaces. The surface energy can be
determined by the type and magnitude of intermolecular inter-
actions.41 There is no direct reliable method for measuring the
surface energy. Indirect methods have been used for calculations.
Its determination is based on the contact angle measurements of
suitably chosen probe liquids. Although there are several
methods to estimate surface energies, the Lifshitz–van der Waals/
acid–base (LWAB) approach suggested by van Oss and co-
workers is straightforward and has been proven to be of fairly
general applicability.42,43 According to the van Oss theory, the
surface free energy gs can be expressed by eqn (1):
The electron-donor and electron-acceptor parameters of the
Lewis acid–base component very often give scattered values
for the investigated solid, which depends on the three probe
liquids used for the determination.44 By using the LWAB
approach it is possible to calculate changes not only in the
Lifshitz–van der Waals interactions gLW (London dispersion)
but also in the electron-donor gꢀ and electron-acceptor g+
interactions for photoswitchable surfaces after visible and UV
light irradiation.
The electron-donor gꢀ and electron-acceptor g+ interactions
after visible light and UV light irradiation for photoswitchable
substrates are shown in Fig. 8. For the cis substrates which are
gs ¼ gLs W + gAs B
(Eq.1)
(Eq.2)
obtained after UV exposure which showed relatively high gLW
,
pffiffiffiffiffiffiffiffiffiffiffi
low gꢀ, and no g+ interactions (Fig. 8). After visible light
irradiation, trans substrates showed higher electron-donor
interactions gꢀ than electron-acceptor g+ interactions. It can be
said that the trans-ABAPA molecules directed toward the
photoswitchable surface after visible irradiation and exposed
their polar heads outward, which increases the electron-donor
gꢀ forces. In addition to that, trans substrates also showed very
low electron-acceptor g+ interactions because the photo-
swtichable molecule can be switched to the trans form
by irradiating with light at 450 nm only with yields of about
70–90%.31
gAs B ¼ 2 gꢀs gþs
where gLs W is the nonpolar Lifshitz–van der Waals contribution,
and gAs B is the polar electron-donor/electron-acceptor (Lewis
acid–base) component of the energy. Eqn (2) states that gAs B can
be represented in terms of electron-accepting (g+s ) and electron-
donating (gꢀs ) contributions. By using this approach, the work of
adhesion WA for a liquid/solid system can be expressed as where
the subscripts S and L denote solid and liquid, respectively, and q
is the contact angle, gL is the surface tension of liquids.
ꢀ
ꢁ
(Eq.3)
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffi
WA ¼ gLð1 þ cos qÞ ¼ 2
gLs WgLLW
þ
gꢀs gþL þ gþs gꢀL
Table 1 Probe liquid free energy components (mJ mꢀ2) using calcula-
tions
Eqn (3) is often referred as the van Oss–Chaudhury–Good
equation, which provides a simple way to characterize a solid
surface by using contact angle measurements with three probe
liquids, two of which have to be polar and one of which is apo-
lar.44 Then three equations of the same type as eqn (2) can be
solved simultaneously, and thus the solid surface free energy
Liquids
gL (mN mꢀ1
)
gLLW
gLꢀ
gL+
gALB
Water
Formamide
Diiodomethane
72.8
58.0
50.8
21.8
39.0
50.8
25.5
39.6
0
25.5
2.28
0
51.0
19.0
0
3194 | J. Mater. Chem., 2011, 21, 3189–3196
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Table 2 Contact angles on the APABA-modified surfaces for various
liquids
Acknowledgements
This work was supported by The Scientific and Technology
Research Council of Turkey (Project Number: 108M018).
Contact
angle (ꢂ):
trans
Contact
angle (ꢂ):
cis
Contact
angle
change
Hysteresis
Dq
s
qadv
qrec
qadv
qrec
Liquid
Dq (ꢂ)
h
trans (ꢂ)
References
Water
Formamide
Diiodomethane 21.1
38.4
10.3
21.5
20.3
32.3
50.5 29.0 88.9 39.4
51.9 31.6 62.2 21.5
68.5 36.2 47.4 13.5
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