5696
Langmuir 2001, 17, 5696-5702
The Reaction between
Tetrakis(diethylamino)tin and Indium Tin
Oxide
Eric L. Bruner, Amelia R. Span, Steven L. Bernasek,* and
Jeffrey Schwartz*
Department of Chemistry, Princeton University, Princeton,
New Jersey 08544-1009
Received April 2, 2001. In Final Form: June 12, 2001
Indium tin oxide (ITO) is a common anodic material for
modern optoelectronics,1-4 and it has been proposed5 that
organizing a dipole at the ITO surface can change the
work function of that material, directly affecting its hole
injection propensity in a device. Surface adsorption of
organics can create this surface dipole; systematic variation of these organics could provide a means to control
device characteristics. For example, changes in the work
function of gold6 or copper7 electrodes can be effected by
self-assembly of variously substituted organic thiols,6 but
thiols do not bind strongly to ITO.8 Our approach to surface
modification of ITO with organics involves using “linker”
complexes which react with surface hydroxyl groups of
the mixed oxide 9,10 and which maintain substitutionally
reactive functionality following surface chemisorption.
Ligand exchange, or metathesis, by proton transfer in
surface complexes proceeds rapidly and to completion,
when the attacking reagent is of greater acidity than the
“leaving group” and when kinetic barriers to proton
transfer are low.11 Accordingly, simple Zr12 or Sn10
alkoxides are viable precursors of stable interfaces
between ITO and carboxylic acids12 or phenols through
sequences of surface deposition and ligand metathesis,10,13
carboxylic acids (pKa ≈ 5)14 or phenols (pKa ≈ 10)14 are
more acidic than the tert-butyl alcohol “leaving group”
(pKa ≈ 18),14 and kinetic barriers to proton transfer
between the oxygens of reagent and ligand are low.11 By
these precedents, metathesis should be possible with
arylthiols (pKa ≈ 6),15 but surface modification with weakly
acidic carbon acids may not succeed. For example, the
* To whom correspondence should be addressed.
(1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(2) Campbell, I. H.; Davids, P. S.; Smith, D. L.; Barashkov, N. N.;
Ferraris, J. P. Appl. Phys. Lett. 1998, 72, 1863.
(3) Antoniadis, H.; Miller, J. N.; Roitman, D. B.; Campbell, I. H.
IEEE Trans. Electron Devices 1997, 44, 1289.
(4) Burrows, P. E.; Bulović, V.; Shen, Z.; Forrest, S. R.; Thompson,
M. E. IEEE Trans. Electron Devices 1998, 44, 1188.
(5) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. (Weinheim,
Ger.) 1999, 11, 605.
(6) Zehner, R. W.; Parsons, B. F.; Hsung, R. P.; Sita, L. R. Langmuir
1999, 15, 1121.
(7) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.;
Barashkov, N. N.; Ferraris, J. P. Appl. Phys. Lett. 1997, 71, 3528.
(8) Yan, C.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Langmuir
2000, 16, 6208.
(9) Purvis, K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. J. Am. Chem.
Soc. 2000, 122, 1808.
(10) Span, A. R.; Bruner, E. L.; Bernasek, S. L.; J., S. Langmuir
2001, 17, 948.
(11) Miller, J. B.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc.
1993, 115, 8239.
(12) VanderKam, S. K.; Gawalt, E. S.; Schwartz, J.; Bocarsly, A. B.
Langmuir 1999, 15, 6598.
(13) VanderKam, S. K.; Lu, G.; Bernasek, S. L.; Schwartz, J. J. Am.
Chem. Soc. 1997, 119, 11639.
(14) Carey, F. A. Organic Chemistry, 4th ed.; McGraw-Hill Higher
Education: Boston, 2000.
(15) Delby, K. N.; Jencks, W. P. J. Chem. Soc., Perkin Trans. 2 1997,
8, 1555.
preparation of a surface tin acetylide by metathesis
between an acetylene (pKa ≈ 25)16 and a surface tin
alkoxide would not be anticipated. However, the preparation of tin acetylides in solution from tin amides17-21 (for
the amine “leaving group,” pKa ≈ 36),22 is well-known,
and the reaction between tetrakis(diethylamino)tin (1)
and silica, followed by treatment with an acetylene at
elevated temperature, has been reported23,24 to yield
surface bound tin monoacetylides. Unfortunately, although qualitative X-ray photoelectron spectroscopic
(XPS) data were obtained, which do show the presence of
C and Sn on the silica surface, no direct evidence was
offered for the chemical reaction between 1 and silica, nor
was the stoichiometry of any resulting surface bound
materials reported. It was therefore important, as a
starting point, to elucidate details of the chemistry of 1
with an hydroxylated surface, since metathesis would give
surface bound products of comparable stoichiometry. We
report herein details of the reaction between 1 and
hydroxylated ITO, both under “normal” laboratory conditions and in ultrahigh vacuum (UHV). We find that surface
bound tin amides are indeed formed and that the
composition of these amides evolves with increasing
temperature of the surface. We have also found that these
surface tin amides can react with acetylenes to give the
corresponding surface bound acetylide derivatives, but
slowly.
Experimental Section
General. Our UHV chamber25 is equipped with a Mattson
Research Series Fourier transform infrared spectrometer (FTIR),
a quadrupole mass spectrometer (QMS), and an XPS spectrometer. ITO on glass (Colorado Concept Coatings, 15 Ω/0, 1500 Å)
was cleaned by sonication in detergent and then by rinsing
successively with water, hot trichloroethylene, acetone, and
methanol. Slides thus treated were stored under vacuum and
then bombarded with Ar+ ions after transfer into UHV to remove
residual organics. XPS analysis of the cleaned ITO showed the
Sn/In ratio to be ca. 6%; the In signal serves as an internal
standard for determination of surface tin complex stoichiometries,
as described previously.10 Deconvolution of XP spectra was done
using “XPSPEAK” freeware, available at http://www.phy.cuhk.edu.hk/∼surface/. Hydroxylation of the ITO was accomplished
in UHV through five cycles of water dose (180 K) desorption (290
K);9 XPS showed approximately 8% of the surface oxygen to be
OH (O[1s] binding energy [BE] ) 533.1 eV). 1 was used as received
(Aldrich) and was stored under nitrogen. Substituted phenylacetylenes (p-X-C6H4CtC-H; X ) H (Acros), F and OCH3
(Aldrich), and NO2 (Toronto Research)) were either used as
received or distilled before use. Surface tin phenylacetylides were
characterized by diffuse reflectance infrared spectroscopy (DRIFT)
using a Nicolet 730 FTIR spectrometer equipped with a SpectraTech DRIFT accessory. The IR spectrum of 1 was acquired in
(16) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
Chemistry, 3rd ed.; Harper & Row: New York, 1987.
(17) Davies, A. G. Organotin Chemistry; VCH: New York, 1997;
Chapter 9.
(18) Jousseaume, B. J. Chem. Soc., Chem. Commun. 1984, 1452.
(19) Williamson, B. L.; Stang, P. J. Synlett 1992, 199.
(20) Jones, K.; Lappert, M. F. J. Organomet. Chem. 1965, 3, 296.
(21) Cauletti, C.; Furlani, C. Gazz. Chim. Ital. 1988, 118, 1.
(22) House, H. O. Modern Synthetic Reactions, 2nd ed.; W. A.
Benjamin, Inc.: Menlo Park, CA, 1972.
(23) Yam, C. M.; Kakkar, A. K. J. Chem. Soc., Chem. Commun. 1995,
8, 907.
(24) Yam, C. M.; Dickie, A.; Malkhasian, A.; Kakkar, A. K.; Whitehead,
M. A. Can. J. Chem. 1998, 76, 1766.
(25) Miller, J. B.; Bernasek, S. L.; Schwartz, J. Langmuir 1994, 10,
2629.
10.1021/la010495l CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/04/2001
Notes
Langmuir, Vol. 17, No. 18, 2001 5697
Table 1. X-ray Photoelectron Spectral Analysis of the Reaction between ITO and 1
XPS
temp
(K)
complex obsd.
185
multilayer of Sn(NEt2)4, 1
293
mixture of 2 and 3 (1:4)
[ITO]-[O]1-Sn(NEt2)3, 2
[ITO]-[O]2-Sn(NEt2)2, 3
a
380
mixture of 3 and 4 (1:1.5)
[ITO]-[O]3-Sn(NEt2)1, 4
450
mixture of 3 and 4 (1:9)
atomic
species
binding
energy (eV)a
Sn(3d5/2)
C(1s)
N(1s)
Sn(3d5/2)
C(1s)
N(1s)
C(1s)
N(1s)
Sn(3d5/2)
C(1s)
N(1s)
Sn(3d5/2)
487.5
285.3, 286.0
399.1
487.5
286.7, 287.6
400.6
285.2, 286.3
399.3
486.9
284.6, 285.7
399.1
486.6
atomic ratiosb
C/ Sn ) 16
C/ N ) 4
C/ Sn ) 9.3, C/ N ) 4.2, N/ Sn ) 2.1
C/ Sn ) 5.9, C/ N ) 4.0, N/ Sn ) 1.4
C/ Sn ) 4.6, C/ N ) 4.0, N/ Sn ) 1.1
b
Calibrated against In(3d5/2). Experimental sensitivity factors determined from atomic ratios in the defined species Sn(NEt2)4 (1).
a nitrogen-filled glovebox using a Midac Illuminator FTIR
spectrometer equipped with a Surface Optics specular reflectance
accessory.
1-Ethynyl-4-(trifluoromethyl)benzene.26 A solution of
4-iodo-(trifluoromethyl)benzene (3.0 mmol, 0.816 g), palladium
dichloride (0.06 mmol, 0.0106 g), triphenylphosphine (0.42 mmol,
0.110 g), and copper(I) iodide (0.15 mmol, 0.029 g) in diisopropylamine (5 mL) was stirred under nitrogen at 0 °C. A second
solution of trimethylsilylacetylene (3.6 mmol, 0.354 g) in diisopropylamine (2 mL) was added by syringe. The reaction was
warmed to room temperature and stirred vigorously. Additional
diisopropylamine (4 mL) was added after 1 h. Gas chromatography and mass spectrometry (GC-MS) analysis showed the
coupling reaction to be complete after a total of 2.5 h. The reaction
mixture was concentrated by rotary evaporation. The residue
was suspended in hexanes (15 mL), filtered through Celite, and
concentrated again by rotary evaporation. The resulting brown
liquid was distilled to give 0.479 g (2 mmol, 66%) of 1-(trimethylsilyl)ethynyl-4-(trifluoromethyl)benzene as a colorless
oil (1H NMR [300 MHz, CDCl3]: δ 7.50 [s, 4 H], 0.25 [s, 9 H]),
which was then dissolved in methanol (2.5 mL) containing
pulverized KOH (0.4 mmol, 0.026 g). GC-MS analysis indicated
removal of the silyl protecting group within 30 min. The reaction
mixture was diluted with water (4.8 mL), extracted with ether,
and dried (MgSO4). The ether was removed by distillation under
nitrogen to give 0.236 g of the product (1.4 mmol, 70%) as a clear
liquid. 1H NMR (300 MHz, CDCl3): δ 7.58 (s, 4 H), 3.18 (s, 1 H).
IR (neat): 3306, 1624, 1612, 1408, 1326, 1171, 1129, 1067, 1017,
843 cm-1.
[ITO]-[O]1-Sn(NEt2)3/[ITO]-[O]2-Sn(NEt2)2 (2/3). A horizontal tube reaction chamber equipped with two stopcockregulated reservoirs (one for 1, and the other for the phenylacetylene) was fitted with samples of conventionally cleaned
ITO/glass. The chamber was evacuated to 10-4 Torr at room
temperature. The ITO/glass samples were exposed to vapor of
1 for three cycles each of 15 min with the chamber open to the
vacuum, followed by 30 min with the chamber isolated from the
vacuum source. The ITO substrates were cooled by external dry
ice during the second and third cycles, and the reservoir of 1 was
gently heated to 55 °C during the third cycle, after which time
a film of 1 was visible on the glass slides. The reservoir of 1 was
then closed, and excess 1 was removed from the ITO by evacuation
at room temperature for at least 1 h, until no 1 was visible.
[ITO]-[O]1-Sn(C≡C-C6H4)3/[ITO]-[O]2-Sn(C≡C-C6H4)2
(5a/6a). Phenylacetylene was vapor deposited onto 2/3 at reduced
pressure. The reservoir containing phenylacetylene was opened
to the reaction chamber, which was still under active vacuum
following removal of excess 1. The stopcock between the reaction
chamber and vacuum source was then immediately closed.
Substrate slides were gently cooled by placing dry ice below the
chamber until a film of phenylacetylene was visible on the slides.
The dry ice was then removed, and the reservoir was isolated
from the reaction chamber. Samples of 2/3 were exposed to
phenylacetylene in this way for 4 h before excess phenylacetylene
was removed by evacuation at room temperature.
(26) Crisp, G. T.; Flynn, B. L. J. Org. Chem. 1993, 58, 6614.
Figure 1. Infrared spectra of 1 on ITO: (a) multilayer of 1
prepared in UHV; (b) film of 1 evaporated on ITO at -78 °C
at 10-4 Torr.
[ITO]-[O] 1 -Sn(C≡C-C 6 H 4 -F) 3 /[ITO]-[O] 2 -Sn(C≡CC6H4-F)2 (5b/6b). Samples of 2/3 were exposed to 1-ethynyl-4fluorobenzene as described for phenylacetylene, but the exposure
time totaled only ca. 1 h. Excess 1-ethynyl-4-fluorobenzene was
removed by evacuation at room temperature.
[ITO]-[O]1-Sn(C≡C-C6H4-OCH3)3/[ITO]-[O]2-Sn(C≡CC6H4-OCH3)2 (5c/6c). Samples of 2/3 were exposed to 1-ethynyl4-methoxybenzene as described for phenylacetylene, but the
exposure time was 3.5 h. Dry ice was placed beneath the slides
only during the final hour of exposure. Excess 1-ethynyl-4methoxybenzene was removed by evacuation at room temperature.
5698
Langmuir, Vol. 17, No. 18, 2001
Notes
Scheme 1. Synthesis of ITO Surface Bound Tin Acetylides
Results and Discussion
tion at low temperature and to elucidate the thermal
evolution9 of such surface tin amide species. Deposition
of 1 was accomplished from the vapor phase onto the
hydroxylated ITO surface at 185 K. XPS analysis showed
complete attenuation of the In(3d5/2) and O(1s) core level
signals, indicative of formation of a multilayer of 1 (see
Table 1). Under these conditions, the Sn(3d5/2) signal
measured is due entirely to 1, and C(1s)/Sn(3d5/2) and
N(1s)/Sn(3d5/2) sensitivity factor ratios can be determined
experimentally according to the stoichiometry of 1. The
FT-RAIRS spectrum of multilayer 1 (recorded at 150 K)
showed prominent peaks for νC-H at 2952 and 2927 cm-1
and two weaker, broader peaks centered at 2867 and 2819
cm-1 (Figure 1a); this pattern is similar to that recorded
for a film of 1 evaporated onto conventionally prepared
ITO at -78 °C and 10-4 Torr (2962, 2925, 2861, and 2831
[br] cm-1; Figure 1b). Thermal desorption of excess 1 was
accomplished at 293 K over 15 min, and the composition
of the chemisorbed tin amide was determined by XPS to
be a mixture of tris-amide [ITO]-[O]1-Sn(NEt2)3, 2, and
bis-amide [ITO]-[O]2-Sn(NEt2)2, 3 (C/Sn ) 9.3, N/Sn )
2.1, 2:3 ) 1:4, Table 1). Whereas O(1s) signals for lattice
oxygens of ITO were again apparent, the signal for surface
OH was substantially lost, showing that this functionality
was consumed on formation of 2/3. The FT-RAIRS
spectrum of 2/3 (recorded at 150 K) showed a prominent
peak for νC-H at 2970 cm-1 and two weaker, broader peaks
centered at 2915 and 2876 cm-1 (Figure 2). Heating the
substrate to 380 K for 10 min caused further surface OH
group reaction, to yield a 1.5:1 mixture of 3 and monoamide
species [ITO]-[O]3-Sn(NEt2)1, 4 (C/Sn ) 5.9, N/Sn ) 1.4,
Table 1). Heating the initial mixture of 2 and 3 to 293 K
for 10 h or to 450 K for 10 min gave 3 and 4 (C/Sn ) 4.6,
N/Sn ) 1.1, 3:4 ) 1:9, Table 1). This stepwise reactivity
of surface tin amides with OH groups on ITO (Scheme 1)
closely parallels that of their surface Sn alkoxide analogues
on this surface10 and on others of low hydroxyl group
content.27 The C(1s) and N(1s) binding energies of
component complexes were determined by deconvolution
The reaction between ITO and 1 was studied in UHV
to determine the stoichiometry of surface complex forma-
(27) Lu, G.; Purvis, K.; Schwartz, J.; Bernasek, S. L. Langmuir 1997,
13, 5791.
Figure 2. Infrared spectra of 2/3 on ITO prepared on ITO in
UHV.
[ITO]-[O] 1-Sn(C≡C-C6H4-NO 2)3/[ITO]-[O] 2-Sn(C≡CC6H4-NO2)2 (5d/6d). Because of its relatively low volatility,
treatment of 2/3 with 1-ethynyl-4-nitrobenzene was accomplished
through four cycles of exposure as described for deposition of 1
onto ITO. The first cycle was performed at room temperature,
and the second and subsequent cycles involved cooling the
substrate with external dry ice. The 1-ethynyl-4-nitrobenzene
reservoir was slowly warmed to 70 °C during the third and fourth
cycles. Excess 1-ethynyl-4-nitrobenzene was removed by evacuation for 15 h at room temperature, followed by heating to 70 °C
with continued evacuation.
[ITO]-[O] 1 -Sn(C≡C-C 6 H 4 -CF 3 ) 3 /[ITO]-[O] 2 -Sn(C≡CC6H4-CF3)2 (5e/6e). Samples of 2/3 were exposed to 1-ethynyl4-(trifluoromethyl)benzene as described for phenylacetylene;
exposure time was 4 h, but substrate cooling with dry ice was
not required. Excess 1-ethynyl-4-(trifluoromethyl)benzene was
removed by evacuation at room temperature.
Notes
Langmuir, Vol. 17, No. 18, 2001 5699
Figure 3. C(1s) XP spectra for surface tin amide species: (a) multilayer of 1; (b) substrate heated to 293 K, showing a mixture
of 2 (higher binding energy peaks) and 3 (1:4); (c) substrate heated to 380 K, showing a mixture of 3 (higher binding energy peaks)
and 4 (1:1.5); (d) substrate heated to 450 K, showing a mixture of 3 and 4 (1:9).
of the corresponding XP spectra of mixtures (Figures 3
and 4). Deconvolution of C(1s) binding energy data9,10 for
1 was done with the assumption that the intensities for
the two types of carbon signals in the ethyl group, N-CH2CH3 and N-CH2CH3, were equal; binding energy differences obtained for these carbons were the same (0.7 eV)
as those reported for ethylamine.28 Carbon(1s) binding
energies for 3 and 4 were similarly obtained from
deconvolution of the XP spectra measured for material
heated to both 380 and 450 K, taking into account relative
amounts of 3 and 4 present at these temperatures (Table
1) as determined from integration of total C(1s), N(1s),
(28) Gelius, U.; Heden, P. F.; Hedman, J.; Lindberg, B. J.; Manne,
R.; Nordberg, R.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 2, 70.
and Sn(3d5/2) signals. Binding energies for 2 were obtained
analogously from data for material warmed to 293 K.
Nitrogen(1s) binding energies for individual amide complex components were also determined based on deconvolution of spectra weighted according to relative amounts
of amide complexes present (Figure 4), though error in
assignment for N(1s) binding energies is likely to be higher
than that for C(1s) (for C[1s] error is (0.2 eV), given the
4-fold higher signal-to-noise ratios measured for the latter
element. Fitting N(1s) peaks of 2 and 3 using binding
energies of 1 and 4 served as an independent validation
of peak position assignments made based on integration
ratios; both deconvolution procedures gave similar peak
position values, within experimental error.
5700
Langmuir, Vol. 17, No. 18, 2001
Notes
Figure 4. N(1s) XP spectra for surface tin amide species: (a) substrate heated to 293 K, showing a mixture of 2 (higher binding
energy peaks) and 3 (1:4); (b) substrate heated to 380 K, showing a mixture of 3 (higher binding energy peaks) and 4 (1:1.5); (c)
substrate heated to 450 K, showing a mixture of 3 and 4 (1:9).
In a parallel series of experiments, samples of ITO/
glass were cleaned and dried under conventional conditions and were placed in a simple reactor connected to
reservoirs of 1 and a particular phenylacetylene.10 This
tube could be evacuated and either heated with external
heating tape or cooled by externally applied dry ice. A
multilayer of 1 was deposited on the ITO substrate at
-78° C by evaporation from a reservoir held at 55 °C at
10-4 Torr (IR: νC-H ) 2962, 2925, and 2831 cm-1; Figure
3a). Excess 1 was removed in vacuo at room temperature
to give 2/3, by analogy with material produced by similar
thermal processing in UHV.
Acetylides are interesting targets for possible surface
dipole introduction5 in the context of device behavior
modification through work function change. Because the
Sn-acetylide bond is linear,17,29 binding a substituted
acetylide moiety to ITO might optimize the effective dipole
moment normal to the surface5,6,30,31 imparted by surface
molecular coordination. (In contrast, because the SnS-C bond is bent,17,32,33 conformational variation in surface
tin thiolate geometries might give rise to net surface dipole
normal components that are far smaller than those
expected based on the dipole moment of the free ligands.)
Samples of 2/3 were exposed to vapor of each of a series
(29) Khakin, L. S.; Grikina, O. E.; Sipachev, V. A.; Granovsky, A. A.;
Nikitin, V. S. Russ. Chem. Bull. 2000, 49, 620.
(30) Nüesch, F.; Rotzinger, F.; Si-Ahmed, L.; Zuppiroli, L. Chem.
Phys. Lett. 1998, 288, 861.
(31) Zuppiroli, L.; Si-Ahmed, L.; Kamaras, K.; Nüesch, F.; Bussac,
M. N.; Ades, D.; Siove, A. Eur. Phys. J. B 1999, 11, 505.
(32) Bravo, J.; Cordero, M. B.; Casas, J. S.; Sánchez, A.; Sordo, J.;
Castellano, E. E.; Zukerman-Schpector, J. J. Organomet. Chem. 1994,
482, 147.
(33) Labib, L.; Khalil, T. E.; Iskander, M. F. Polyhedron 1996, 15,
349.
Notes
Langmuir, Vol. 17, No. 18, 2001 5701
Scheme 2. Ligand Metathesis Illustrated for 3 via Putative Tin-Acetylene Complex 8
Table 2. X-ray Photoelectron Spectral Analysis of the
Reaction between 3/4 and p-NO2(C6H4)CtCH
XPS
temp
(K)
complex obsd.
185 multilayer
p-NO2(C6H4)C≡CH
273 [ITO]-[O]2-Sn(C≡C[C6H4]NO2)2,
[ITO]-[O]3-Sn(C≡C[C6H4]NO2)1
6d/7d
atomic
species
binding
energy (eV)a
C(1s)
N(1s)c
O(1s)c
Sn(3d5/2)
C(1s)
N(1s)c
O(1s)c
286.3, 285.3
407.9
535.3
X
285.2
407.3
534.1
atomic
ratiosb
O/N ) 2
Oc/Nc ) 2
Nc/Sn ) X
Nd/Sn ) X
Oc,e/Sn ) 1.1
a Calibrated against In(3d ). b Experimental sensitivity factors
5/2
determined from atomic ratios of defined species p-NO2(C6H4)CtCH.
c N(1s) and O(1s) for the NO group. d For 3/4. e For complete
2
conversion of 3 and 4 (1:9) to 6d/7d (1:9), expected O/Sn ) 2.2:1.
Figure 5. DRIFT analysis of the reaction between phenylacetylene and 2/3 (lower trace) to give 5a/6a, and a control
(upper trace).
of substituted phenylacetylenes (p-X-C6H4CtC-H; X )
H, F, CF3,26 OCH3, and NO2), with substrate cooling to
form multilayers. The multilayers were then desorbed in
vacuo, at room temperature, to yield the surface acetylides
(for 1-ethynyl-4-nitrobenzene, the reservoir was warmed
for initial deposition, as was the substrate for multilayer
desorption). Conversion of 2/3 to the corresponding
acetylides (5/6) was slow at 10-4 Torr, perhaps because of
Figure 6. XP spectra following partial conversion of 3/4 to
6d/7d by reaction with 1-ethynyl-4-nitrobenzene: (a) N(1s);
(b) O(1s).
a relatively high kinetic barrier to proton transfer11 from
the carbon acid to the amide ligand in a weakly bound
tin-acetylene complex intermediate (8, Scheme 2);34,35
exchange was, however, complete at this reaction pressure. Product tin acetylides were identified by DRIFT,
5702
Langmuir, Vol. 17, No. 18, 2001
noting the introduction of peaks characteristic of the
various acetylenes and the disappearance of νC-H signals
both for 2/3 and for the terminal C-H bond of the
free acetylenes (Figure 5). In control experiments, it was
found that no surface acetylenic materials remained on
the ITO surface in the absence of 2/3, following simple
evacuation.
Attempts were made to measure the stoichiometry of
tin acetylide formation by metathesis in UHV, but they
were hampered by slow ligand exchange rates, which are
likely to be exacerbated by low residence times, expected
for weak complexation of the acetylene to tin, and at
operating background pressures e10-7 Torr. Nonetheless,
partial metathesis could be observed for reasonable
exposure times and pressures. For example, a sample of
3/4 (1:9) prepared from 1 by heating to 450 K was cooled
to 190 K and exposed to 1-ethynyl-4-nitrobenzene for 20
min at 2 × 10-7 Torr, which gave a multilayer of the
acetylene on the substrate (Table 2). Thermal desorption
of excess acetylene was accomplished at 293 K over a period
of 10 min. Following two further cycles of dose/desorption,
XPS analysis (Figure 6) showed that metathesis to the
corresponding surface tin acetylides, 6d/7d, was about
one-third to one-half complete, based on comparative nitro
group N(1s) and O(1s) data (Table 2); we have noted similar
requirements for multiple dose/desorption cycles to ensure
(34) For examples of isolable 5-coordinate tin complexes, see: Van
Koten, G.; Noltes, J. G. In Adv. Chem. Ser. 1976, No. 157, 275.
(35) For an example of an isolable 6-coordinate tin alkoxide complex,
see: Reuter, H.; Kremser, M. Z. Anorg. Allg. Chem. 1991, 598/599,
259.
Notes
complete metathesis even using kinetically more reactive
systems, such as carboxylic acids or phenols.13,36
Conclusions
Synthesizing a surface tin amide complex on ITO
enables strong bonding of substituted arylacetylenes to
the ITO as surface tin acetylides. Since previous work has
shown that acetylenic group substitution in a thiol can
lead to large changes in the work function of gold,6 it is
now important to determine how acetylide coordination
can affect the work function of an ITO electrode to which
it is indirectly bonded. Accordingly, ultraviolet photoelectron spectroscopic measurements are now in progress,
and organic light-emitting devices are being constructed
in UHV so that device-characteristic determinations can
be made using these materials.
Acknowledgment. The authors thank the National
Science Foundation and the New Jersey Center for
Optoelectronic Materials for their support of this research.
Supporting Information Available: The N(1s) XPS
spectrum of the multilayer of tetrakis(diethylamino)tin (1); the
Sn(3d5/2) XPS spectra of [ITO]-[O]1-Sn(NEt2)3, 2, [ITO]-[O]2Sn(NEt2)2, 3, and [ITO]-[O]3-Sn(NEt2)1, 4; and the IR spectra
(3800-800 cm-1) of multilayer 1, 5b/6b, 5c/6c, 5d/6d, and 5e/
6e. These materials are available free of charge via the Internet
at http://pubs.acs.org.
LA010495L
(36) Purvis, K. L.; Lu, G.; Schwartz, J.; Bernasek, S. L. Langmuir
1998, 14, 3720.