Homologous, long-chain alkyl dendrons form homologous thin films on silver oxide surfaces

Article abstract of DOI:10.1039/b506710c

As suggested by X-ray crystal structures, homologous, long-chain alkyl dendrons with three carboxyl groups form thin films on silver oxide surfaces, which give reflection-absorption infrared spectra that show a linear increase in intensities of methylene C-H stretching absorptions. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506710c

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Homologous, long-chain alkyl dendrons form homologous thin films on
silver oxide surfaces{
Andr e´ A. Williams, B. Scott Day, Brett L. Kite, Melinda K. McPherson, Carla Slebodnick, John R. Morris{*
and Richard D. Gandour{*
Received (in Columbia, MO, USA) 13th May 2005, Accepted 15th August 2005
First published as an Advance Article on the web 20th September 2005
DOI: 10.1039/b506710c
As suggested by X-ray crystal structures, homologous, long-
chain alkyl dendrons with three carboxyl groups form thin
films on silver oxide surfaces, which give reflection–absorption
infrared spectra that show a linear increase in intensities of
methylene C–H stretching absorptions.
even-numbered homologous series, 3CAm#, of amphiphiles
(Scheme 1). With long reaction times, the condensation of
12
Behera’s amine with either an acid chloride or a carboxylic acid,
which was activated by DCC, afforded the triesters, 3EAm#, in
good to moderate yields of purified products. In general, the acid
chloride reaction gave higher yields than the DCC method.
Formolysis of the triesters produced the triacids, 3CAm#, in high
yields of recrystallized products. All compounds were fully
characterized.{
In the process of synthesizing these amphiphiles, 3EAm15
formed crystals suitable for X-ray analysis.{§ Examination of the
packing diagram (Fig. 1a) suggested that parallel alkyl chains
packed closely enough to resemble a SAM. In addition, we
Thin films and self-assembled monolayers (SAMs) have potential
1
applications in materials science and biological sensors. Interest in
thin films resides in their potential as inexpensive, versatile surface
coatings for applications including the control of wetting and
adhesion, chemical and microbial resistance, biocompatibility, and
molecular recognition for sensors, especially those related to
2
3,4
biology. From our own perspective, SAMs serve as well-
ordered models for investigating gas–surface collisional dynamics.
Most SAMs involve the attachment of thiol compounds to a gold
surface to form an ordered structure. Other approaches include
attaching terminal alkenes to silica and attaching chlorosilanes,
phosphonic acids, phosphonate esters, carboxylate esters, car-
boxylic acids and alcohols to non-metals and metals that have a
native oxide layer.
obtained
a lower quality crystal structure of 3CAm13,
which revealed strong hydrogen bonding between neighbouring
molecules.
Fig. 1a illustrates for 3EAm15 the crystalline nature of the
closely packed alkyl chains. This close packing is critical for self-
assembly. Anti-parallel chains pack closer than parallel chains (see
Fig. 1 legend). The head group is quite large (cross-sectional area,
2
Dendrons and dendrimers have been used to attach molecules
to surfaces. The strategy employs the dendron as a tripod or multi-
5
pod, which is attached to a large, bulky molecule. Rigid tripodal
a 5 243 s ) compared to the length (l
the chain (v 5 430.9 s ); the packing parameter P (P 5 v/al ) is
c
c
5 20.5 s) and volume of
3
13
0.089, which predicts a highly curved packing arrangement. This
structure illustrates how large head groups pack to accommodate
alkyl chain packing. As illustrated in Fig. 1a, the head groups on
adjacent molecules in one parallel sheet nestle together. Within a
ligands have been used to attach dyes to semiconductor
6
nanoparticles. Newkome-type dendrons (first, second and third
generation) that terminate with carboxylates have been used to
attach a ferrocene in close proximity to a gold electrode that is
7
coated with ammonio-terminated thiols.
Newkome-type dendrons are also used to impart water
8
solubility on very hydrophobic aromatic groups. Amphiphiles
that contain second generation Newkome-type dendrons, and
11
and calixarene, have also
9,10
lipophilic tails attached to fullerene
11
been made; the latter forming a persistent micelle. While the
water-solubilizing properties of Newkome-type dendrons initiated
this project, the discovery of the facile formation of a thin film on a
silver oxide surface forms the basis of this report, which begins
with their synthesis and single crystal X-ray analyses.
Interest in systematically exploring the aqueous properties of
amphiphiles with very long alkyl chains led us to synthesize an
Department of Chemistry MC 0212, Virginia Tech, Blacksburg,
VA 24061, USA. E-mail: jrmorris@vt.edu; gandour@vt.edu;
Fax: (+1)540 231 3255; Tel: 01 540 231 3731 or 01 540 231 2472
{
Electronic supplementary information (ESI) available: Synthesis and
Scheme 1 Reagents and conditions (% isolated yield). a: X 5 Cl, TEA,
dry PhH, 48 h, rt (77–84%); b: X 5 OH, DCC, HOBt, dry THF, 7 d, rt
characterisation data for all the compounds prepared. See http://
dx.doi.org/10.1039/b506710c
{ Please contact John Morris about the surface studies or Richard
Gandour about the synthetic aspects of this work.
(61–82%); c: 99% HCOOH, 18 h, rt (65–86%). Nomenclature: 3E (three
t
2 2 2 2
–CH CH COO Bu), 3C (three –CH CH COOH), Am (amido, –CONH–).
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Fig. 1 (a) X-Ray crystal structure of 3EAm15. This view illustrates that anti-parallel chains pack more closely than parallel chains. For anti-parallel
chains, every other carbon lies close to a corresponding carbon in the neighbouring chain; C…C distances range from 4.00–4.12 s. For parallel chains, all
…
the carbons lie approximately the same distance apart; C C distances range from 4.96–5.14 s. An intermolecular hydrogen bond between the amide N–H
…
blue sphere) on one molecule and an ester carbonyl oxygen (red sphere) on a parallel neighbouring molecule is indicated by a dashed line. The N O
(
distance is 2.970(3) s and the N–H…O angle is estimated to be 120.3u. (b) X-Ray crystal structure of 3CAm13. The chain packing illustrates that anti-
parallel chains pack significantly closer than parallel chains. For anti-parallel chains, every other carbon lies close to a corresponding carbon in the
neighbouring chain, dashed green lines indicate the C…C distance (3.88–4.12 s). Six strong intermolecular hydrogen bonds form with four neighbouring
molecules. There are four acid–acid (O…O distances, 2.636(4)–2.656(4) s) and two acid–amide (O…O distances, 2.676(4) and 2.660(4) s) hydrogen bonds
(not shown). In the latter, the acid O–H is the donor and the amide carbonyl oxygen is the acceptor.
parallel sheet, a hydrogen bond connects two head groups, with
the amide hydrogen as the donor and an ester-carbonyl oxygen as
the acceptor.
Fig. 2a shows reflection absorption infrared spectra (RAIRS),
recorded for a series of adsorbed acids (3CAm#, where # 5 13, 15,
17, 19 and 21). The spectra show narrow, intense peaks for the
21
Fig. 1b illustrates the alkyl chain packing and hydrogen bonding
for 3CAm13. Anti-parallel chains pack significantly closer than
parallel chains. All three carboxyls of the head group are involved
in hydrogen bonding. Two carboxyls on each head form acid
dimers with a molecule in an adjacent unit cell. The third carboxyl
forms a hydrogen bond with a neighbouring molecule; the
carboxyl hydrogen as the donor and the amide oxygen as the
acceptor.
CH symmetric and asymmetric stretches located at 2856 cm
2
21
19
and 2928 cm , which resemble those for alkanes in liquids. The
intensities of the methylene peaks are consistent with those
expected for a uniform layer, or layers, of alkane chains on the
19,20
surface.
However, these thin films lack the highly ordered
structure often observed for long chain fatty acids on silver.
Ordered SAMs are evident from a red-shifted methylene
1
9
21
asymmetric stretch below about 2920 cm and clearly resolved
21
21
The close packing of alkyl chains in the above crystal structures
suggests a possible arrangement for thin film formation on a
surface. Given the tripodal arrangement of carboxyls in 3CAm#,
all three carboxyls could not attach to the surface; the footprint
being too large for close packing of parallel alkyl chains. As
suggested by Fig. 1b, a carboxyl group could assist in the self-
assembly through lateral hydrogen bonding with a neighbouring
molecule.
methyl stretches at 2965 cm and 2885 cm . The lack of order in
these systems is attributed to the large footprint of the bulky head
groups and competition between surface adsorption and chain–
chain hydrogen bonding interactions of neighbouring carboxylic
acids. Despite the disordered nature of the system however, the
molecules do resist extensive rinsing with ethanol and appear to
form a stable thin film. Fig. 2b demonstrates that the methylene
stretching intensity is directly proportional to the number of
methylenes in the triacid used to make the film. The linear
dependence of the methylene peak intensity on chain length
demonstrates that the overall coverage and density of acids
adsorbed on each surface is constant, a characteristic of uniform
molecular adsorption on a surface. This uniform absorption could
be either a monolayer, bilayer or multilayer thin film. Future
studies will more fully characterize these thin films and find ways
to drive an ordered assembly of tripodal molecules.
Long-chain fatty acids often form well-ordered, densely-packed
14–17
SAMs on the native oxide surface of silver.
The packing
structures of 3EAm15 and 3CAm13 in the bulk crystal suggests
that monolayers or bilayers may also form on silver oxide. We
have explored this possibility by immersing a freshly prepared
1
8
silver oxide sample (evaporated silver on annealed mica ) in a
mM ethanolic solution of a particular triacid for 18 h. Upon
1
removal of the silver sample from solution, the surface was rinsed
extensively in absolute ethanol and dried completely in a high
pressure stream of ultra-pure nitrogen.
Given the relative ease of synthesis of these dendrons, one can
readily imagine several strategies for expanding this project. One
5
054 | Chem. Commun., 2005, 5053–5055
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at Virginia Tech; for X-ray analysis by NSF (grant CHE-0131128),
who funded the purchase of the Oxford Diffraction Xcalibur2
single crystal diffractometer; and for surface studies by ARO
(grant W911NF-04-01-0195). We thank Professor George R.
Newkome for samples of Behera’s amine.
Notes and references
§
Crystal data for 3EAm15: Empirical formula 5 C38
weight 5 653.96, crystal system 5 triclinic, space group 5 P1, a =
.6653(9), b = 9.9138(10), c = 35.868(3) s, a = 82.928(8), b = 89.815(10), c =
7
H71NO , formula
¯
5
8
3
¯
1.801(10) oˆ , unit cell volume 5 1978.6(4) s , Z 5 2, absorption
21
coefficient 5 0.074 mm , reflections collected 5 13578, independent
reflections 5 7015 (Rint = 0.0706), final R indices [I>2 u¨ (I)] R1 = 0.0658
wR2 = 0.1215, CCDC 271916. Crystal data for 3CAm13: Empirical
7
formula 5 C24H43NO , formula weight 5 457.59, crystal system 5 triclinic,
¯
space group 5 P1, a = 5.4497(8), b = 15.755(2), c = 29.520(4) s, a =
3
¯
9
3.557(11), b = 91.632(11), c = 91.894(12) oˆ , unit cell volume 5 2527.2(6) s ,
21
Z 5 4, absorption coefficient 5 0.087 mm , reflections collected 5 18103,
independent reflections 5 8931 (Rint not calculated due to twin refinement),
final R indices [I>2 u¨ (I)] R1 = 0.1253 wR2 = 0.3922, CCDC 271917. See
http://dx.doi.org/10.1039/b506710c for crystallographic data in CIF or
other electronic format.
1
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21
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1
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We gratefully acknowledge financial support for synthesis by
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2
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