Non-Covalent Assembly of a Photoswitchable Surface

Article abstract of DOI:10.1021/ja039230e

A noncovalently bound multilayered thin film in which individual layers are linked by metal ligand interactions undergoes a photochemically initiated permanent change in surface wettability. The film consists of three separate layers: a SAM on gold of 4-[(10-mercaptodecyl)oxy]pyridine-2,6-dicarboxylic acid, a layer of Cu(II) ions that are deposited onto the SAM and bind symmetrically in the site provided by the two carboxylate groups and the pyridyl nitrogen atom, and a layer of cis-2,2′-dipyridylethylene, which caps the Cu(II) layer by complexation through both pyridyl nitrogen atoms (Film I). Photoexcitation of the film in chloroform at 300 nm leads to substantial cis-trans isomerization as indicated by conductivity, impedance, grazing incidence IR, and contact angle measurements. The latter show a decrease in contact angle (increase in wettability) of 17°, which is attributed to exposure of both the underlying Cu(II) layer and one of the pyridyl ring nitrogen atoms following isomerization to the trans isomer. Copyright

Full text of DOI:10.1021/ja039230e

Published on Web 01/08/2004
Non-Covalent Assembly of a Photoswitchable Surface
Christopher G. F. Cooper,* John C. MacDonald,* Ernesto Soto, and W. Grant McGimpsey*
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609
Received October 25, 2003; E-mail: wgm@wpi.edu
We recently reported a noncovalent strategy for the fabrication
of highly ordered thin films with the ability to generate electric
current upon irradiation. In these systems, multiple layers of organic
ligands are assembled by complexation with layers of transition-
metal ions such as Cu(II).1 Our intention in undertaking this
multilayer approach is to show that the construction of complex,
multicomponent supramolecular devices can be achieved nonco-
valently in considerably less time and with less effort than by
covalent methods. To demonstrate its versatility, we have now
applied this approach toward the construction of surfaces with
photoswitchable wettability.
Control of surface wettability is currently of interest due to its
importance in the fields of micro- and nanofluidics. Whitesides et
al. and others have devoted considerable effort to the creation of
multifunctional surfaces with variable wettability using microcontact
2
-5
printing techniques. More recently, several groups have shown
that wettability can be controlled by externally applied stimuli such
as light or an electric field. 6-9 Building on these studies and our
Figure 1. Structures of thin films I and II. I can be converted to II by
irradiation with 300-nm light.
noncovalent method of assembly, we report a preliminary study in
which the wettability of a multilayered surface capped with 2,2′-
dipyridylethylene ligands can be switched permanently by photo-
induced cis-trans isomerization.
Table 1. Contact Angle Measurements for Films I and II
film
contact angle (deg)
gold
film I
76 ( 1.0
For this work, we have assembled multilayered thin films I and
II, as shown in Figure 1. Each film consists of a 2,2′-dipyridyl-
ethylene light absorbing group coupled to a gold surface via metal-
ligand interactions. These films were fabricated by self-assembly
of 4-[(10-mercaptodecyl)oxy]pyridine-2,6-dicarboxylic acid on a
clean gold surface, followed by the deposition of Cu(II) ions (from
component 1 (pyridyl-capped decanethiol)
component 2 (Cu(II) ions)
component 3 (cis-2,2′-dipyrydylethylene)
film II
component 3 (trans-2,2′-dipyrydylethylene)
film I irradiated
76.0 ( 1.5
58.0 ( 2.0
76.5 ( 2.5
63.5 ( 0.5
57.0 ( 2.0
64.0 ( 2.0
film II irradiated
2
CuBr ) that complex with the pyridine headgroup of the previous
layer, and finally, deposition of the 2,2′-dipyridylethylene ligand
in either the cis (film I) or trans (film II) form, which serves to
cap the Cu(II) ions.
Conductivity, impedance, contact angle, and grazing incidence
IR experiments were carried out on films I and II after the addition
of each layer and confirmed the ordered deposition of each
component. Conductivity values (CV) for both I and II obtained
angle measurements shown in Table 1 and IR spectra (not shown)
also confirm that substantial changes in the surface occur following
addition of each layer. In particular, the IR measurements show
conclusively the presence of each of the added layers. Conductivity,
impedance, and IR data are available as Supporting Information.
Deposition of cis- and trans-dipyridylethylene leads to differences
in the surface wettability with the cis isomer providing a more
hydrophobic surface as determined from contact angle measure-
ments. The difference in contact angle of ∼13° between I and II
is greater than that reported for a variety of photoisomerizable thin
3 6
in an aqueous solution of K [Fe(CN) ] change as the individual
components are deposited sequentially onto the gold surface. The
3
+
2+
CV of the bare gold surface shows the Fe /Fe redox peaks for
9
6
oxidation and reduction of ferricyanide, whereas deposition of the
films that typically exhibit changes of ca. 9°. In addition, the CV
pyridine-capped decanethiol yields conductivity values (measured
in the range -0.5 to +0.6 V versus SCE) that indicate the formation
of an insulating monolayer with few defects. After the monolayer
is exposed to a solution of Cu(II) ions, the CV of the film is nearly
identical to that of bare gold with only a small decrease in peak
current. This result indicates that the Cu(II) ions promote tunneling
of electrons between the gold surface and the solution. Deposition
of the dipyridylethylene ligands again results in attenuated con-
ductivity consistent with formation of an insulating layer on the
surface.
results indicate that the cis capped surface is less conductive than
the trans capped surface. Recalling that impedance measurements
indicate well-ordered films in each case and that IR studies confirm
the presence of the dipyridylethylene in each film, the difference
in surface hydrophobicity can be attributed to differences in the
electrostatics of the two surfaces that arise from different orienta-
tions of the two isomers. Molecular modeling by our group and
previous studies by others involving nickel-dipyridylethylene
complexes10 indicate that the cis isomer forms a symmetrical
bidentate Cu(II) complex (Figure 1) that efficiently “caps” the metal
ion, producing a hydrophobic packing arrangement on the surface.
The trans isomer, however, is not able to form a stable bidentate
These results, in addition to impedance observations, are
consistent with our previous studies of multilayered films. Contact
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10.1021/ja039230e CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
cis photoisomerization equal to 0.8412 and 0.12,13 respectively.
Irradiation of II, however, does not yield any cis product as
indicated by the lack of change in the CV, IR, and contact angle
data. Failure of the trans-dipyridylethylene units to undergo
photoisomerization to the cis isomers could be caused by ordered
packing of the trans-dipyridylethylene in II that sterically inhibits
the structural reorganization necessary for trans-cis isomerization
to occur. The CV and impedance measurements of II indicate that
the dipyridylethylene headgroups in fact are well-ordered in II.
Another contributing factor could be that the solution photochem-
istry of trans-2,2′dipyridylethylene differs from that of the Cu(II)
complex on the surface.
The system described here demonstrates that noncovalent mul-
tilayer assembly provides a convenient means to fabricate highly
ordered thin films with wettability that can be switched. Although
the wettability of surfaces functionalized with dipyridylethylene is
not reversible, this work provides proof-of-concept that chro-
mophores with photoswitchable molecular structures can be attached
noncovalently to surfaces, that noncovalent multilayer films
terminated with such chromophores have well-ordered surface
structures, that the wettability of such surfaces can be controlled
photochemically, and that such multilayer thin films are stable
before and after photoinduced changes in structure at the surface.
We currently are using this methodology to incorporate photochro-
mic molecules into similar noncovalent multilayer films to provide
reversible switching of wettability. We expect that such systems
will have broad application beyond wettability by providing new
opportunities to control a variety of important surface-based
processes such as metal-centered redox chemistry and heterogeneous
catalysis.
Figure 2. Grazing angle IR spectra of films I, II, and irradiated I.
complex because of its elongated structure. Although we have not
yet determined the mode of binding, the trans isomer is likely
monodentate, as shown in Figure 1. The structure shown in Figure
1
helps explain the enhanced wettability provided by this isomer
because the Cu(II) ion in this case is not completely coordinated
and, therefore, is free to complex with water. The uncoordinated
pyridine ring also is free to bind water via hydrogen bonding at
the ring nitrogen. Impedance measurements on II in fact show
elevated capacitance values for the trans capped system, which
indicates that diffusion occurs between the solution and the layer
of metal ions. Further support for the binding arrangement described
is provided by experiments carried out with cis- and trans-stilbene,
which could not be deposited on the Cu(II) layer.
The contact angle, CV, impedance, and IR data also help to
characterize the changes that occur following photoexcitation of
the films. Exposure of I to 300 nm irradiation in chloroform11 in
the presence or absence of oxygen results in a substantial decrease
in the contact angle, from 76.5° to 57°, a change that is consistent
with cis-trans isomerization. We note that the contact angle
obtained is somewhat smaller than that of unirradiated II. The
smaller angle is likely a result of different packing arrangements
in the films. Also, the conductivity of I increases following
irradiation so as to be nearly identical to that of unirradiated II,
while impedance measurements indicate that the irradiated film
remains a well-ordered system. The clearest confirmation of
isomerization of I is provided by IR measurements. Figure 2 shows
the IR spectra of unirradiated I and II and I following irradiation.
While the IR absorption bands that are normally used to distinguish
between cis- and trans-dipyridylelthylenes lie at frequencies lower
Acknowledgment. We thank the National Science Foundation
for financial support of this work. (NSF-NIRT DMI-0210258).
Supporting Information Available: Synthesis, film assembly
methods, and characterization data (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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