A R T I C L E S
Zhong et al.
Scheme 1. Synthesis of 1-butyl-4-methylpyridinium Bromide
demonstrated by Lud et al.13 in the formation of biphenyl self-
assembled monolayers on doped ultrananocrystalline diamond
via spontaneous grafting without electrochemical induction. In
addition, the chemical reduction potential of the different
aryldiazonium salts with respect to the Fermi level of the solid
surface controls the extent of reaction.14 Thus far, most of the
diazonium salts grafted are restricted to nitrophenyl or bro-
mophenyl moieties due to the ease of acquiring these com-
mercially. In principle the concept of diazonium grafting can
be extended to a wide range of molecules, including organic
dyes or charged molecules, although this has not been explored
on the diamond platform.
Due to the rich and versatile electrochemical and photo-
chemical properties of polyoxometalates (POM), they have
attracted great attention in field of analytical chemistry, catalysis,
biology, medicine, and materials science.15 Of particular interest
is the bottom up fabrication of POM-based materials with atomic
precision through growth processes determined by self-assembly
for constructing modular molecular nanosystems. Some of the
popular methods include the use of Langmuir-Blodgett (LB)
technique16 and electrostatic layer-by-layer self-assembly (ELSA)
method17 to provide control of film thickness, structure, and
function. To create a more robust device, schemes based on
the covalent attachments of POMs through organic linkers had
been developed.18 Herein we report a strategy to graft a dye
molecule onto diamond via the facile electrochemical reduction
of in situ generated aryldiazonium salt, which allows subsequent
electrostatic assembly of phosphotungstic acid (PTA), a type
of POM, onto the positively charged terminal pyridinium end
of the coupled dye. The electrostatic interactions among the
charged groups can help to stabilize film structure and prevent
molecular aggregation, leading to the formation of well-ordered
molecular clusters.
washed with ethyl acetate and subsequently dried under vacuum
with a yield of 50%.19
4-Aminobenzaldehyde (2). Grounded sulfur flakes (5 g), sodium
sulfide nanohydrate (10 g, 41.7 mmol), and sodium hydroxide (9
g, 0.223 mol) were added to 200 mL of deionized water and then
heated for 20 min on a steam bath. Next, the mixture was added to
a hot solution of 4-nitrotoluene (16.7 g, 0.103 mol) in 95% ethanol
(100 mL), and the resulting mixture was refluxed at 80 °C for 3 h.
Deionized water (300 mL) was added to this mixture which was
then subjected to rapid vacuum distillation. The distillation was
terminated when the volume of residue decreased to about 200 mL.
Following that, the residual flask was immersed in an ice bath for
2 h for crystallization. Yellow-orange crystals of the product were
obtained via vacuum filtration and then purified by washing with
(200 mL) of cold, deionized water. The product was dried over
potassium hydroxide pellets in a vacuum desiccator for 24 h, giving
a yield of approximately 45%.20
4-(4-Aminostyryl)-1-butylpyridinium Bromide (3). 1-Butyl-
4-methylpyridinium bromide (4 g, 29.2 mmol) and 4-aminoben-
zaldehyde (2.36 g, 19.5 mmol) were added to 95% ethanol (60
mL) and refluxed at 80 °C for 24 h (see Scheme 2). The mixture
was then allowed to cool gradually before placing it in an ice bath
for crystallization to occur. Reddish-purple crystals were obtained
1
by vacuum filtration with a yield of 75%. H NMR (DMSO, 300
MHz) δ: 0.90 (3H, t, J ) 7.50 Hz), 1.26 (2H, m), 1.86 (2H, m),
4.40 (2H, t, J ) 7.20 Hz), 6.01 (2H, s), 6.62 (2H, d, J ) 8.70 Hz),
7.09 (1H, d, J ) 15.90 Hz), 7.46 (2H, d, J ) 8.70 Hz), 7.86 (1H,
d, J ) 16.2 Hz), 8.03 (2H, d, J ) 6.90 Hz), 8.75 (2H, d, J ) 6.90
Hz). 13C NMR (DMSO, 300 MHz): 13.30, 18.75, 32.41, 58.81,
113.74, 116.33, 122.23, 122.41, 130.53, 142.57, 143.39, 152.10,
153.84. ESI-MS: m/z 254.2, 197.3. FT-IR (KBr, cm-1): 3349, 3285,
3181, 3032, 2962, 2917, 1619, 1585, 1470, 1375, 1313, and 1165.
Diamond Thin Film. Polycrystalline boron-doped diamond
(BDD) films (50 µm thick) were grown on p-type Si substrates in
a commercial 2.45 GHz microwave plasma reactor (Astex) using
methanol and boron oxide mixtures. The BDD samples had a
surface resistance of 10 Ω cm and the boron doping level was
approximately 1020 cm-3. These BDD samples were used for
electrochemical experiments due to their high conductivity. Opti-
cally transparent diamond electrode was fabricated by the CVD of
a thin layer of boron-doped diamond film on quartz substrate (160
nm thick, boron concentration 7 × 1020 cm-3) according to
published procedures.1
Acid cleaning and hydrogen plasma cleaning of diamond were
used for all diamond samples. Metallic impurities were first
dissolved in hot aqua regia (HNO3/HCl ) 1:3), followed by the
removal of organic impurities from the diamond samples by hot
“piranha” solution (H2O2/H2SO4 ) 1:3) at 90 °C for 1 h. Hydrogen
termination of diamond samples was attained by microwave
hydrogen plasma treatment using 800 W microwave power and
300 sccm of hydrogen gas flow for 15 min.
II. Experimental Section
Chemical Reagents. All chemicals purchased were of the purest
grade and used as received from Sigma-Aldrich unless otherwise
stated. Phosphotungstic acid hydrate (H3[P(W3O10)4]·xH2O), also
known as tungstophosphoric acid, was used as received. All solvents
used for reaction and rinsing were of HPLC grade unless otherwise
stated. All dilution and preparation of electrolytes for electrochemi-
cal work were made with Nanopure water (18.0 MΩ·cm).
Organic Synthesis. 1-Butyl-4-methylpyridinium Bromide
(1). 1-Bromobutane (10 mL, 0.102 mol) was added to freshly
distilled 4-picoline (12.1 mL, 0.112 mol) under nitrogen; 95%
ethanol (125 mL) was added, and the mixture was refluxed for 24 h
(see Scheme 1). Light brownish-yellow crystals were precipitated
upon cooling in the refrigerator. The product was filtered and
(13) Lud, S. Q.; Steenackers, M.; Jordan, R.; Bruno, P.; Gruen, D. M.;
Feulner, P.; Garrido, J. A.; Stutzmann, M. J. Am. Chem. Soc. 2006,
128, 16884.
Surface Reaction Conditions. The grafting of the molecular
dye on H-terminated BDD via electrochemical reduction of in situ
generated aryldiazonium salt was carried out by applying five cyclic
voltammetric scans between +0.5 and -0.8 V (vs Ag/AgCl) with
a scan rate of 100 mV/s, in a solution of 1 mM of the tailor-made
molecular dye (3) and 1 mM NaNO2 in 0.5 M HBF4. The grafted
substrate was then ultrasonicated in acetone to remove any
physisorbed molecules, followed by rinsing in ethanol and water.
(14) Zhong, Y. L.; Loh, K. P.; Midya, A.; Chen, Z.-K. Chem. Mater. 2008,
20, 3137.
(15) (a) Hill, C. L. Chem. ReV. 1998, 1, 98 (Thematic Issue). (b) Pope,
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Self-Assembly to Applications; Kluwer: Dordrecht, The Netherlands,
2001.
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18294 J. AM. CHEM. SOC. VOL. 131, NO. 51, 2009