Synthesis of fluorescent carboxylic acid ligands for construction of monolayers on nanostructures

Article abstract of DOI:10.2478/s11532-012-0078-2

Two new long-chain carboxylic acids (1, 2) bearing strong fluorescent group pyrene as ligands for Self-Assembled Monolayers (SAMs) have been synthesized. The multistep targeted synthesis is accomplished by use of Pyren-1-yl methylamine hydrochloride and employing simplified synthetic protocols. Compound 2 contains a chiral center purposely introduced along the atom chain in order to make it suitable for chiro-optical studies of the resulting SAMs.

Full text of DOI:10.2478/s11532-012-0078-2

Cent. Eur. J. Chem. • 10(5) • 2012 • 1640-1646
DOI: 10.2478/s11532-012-0078-2
Central European Journal of Chemistry
Synthesis of fluorescent carboxylic acid ligands
for construction of monolayers on nanostructures
Research Article
Sushilkumar A. Jadhav1,2*
1Department of Material Science and Chemical Engineering,
Polytechnic University of Turin, 10129 Turin, Italy
2Italian Interuniversity Consortium for Materials Science and Technology
(INSTM), 50121 Firenze, Italy
Received 14 March 2012; Accepted 30 May 2012
Two new long-chain carboxylic acids (1, 2) bearing strong fluorescent group pyrene as ligands for Self-Assembled Monolayers
(SAMs) have been synthesized. The multistep targeted synthesis is accomplished by use of Pyren-1-yl methylamine hydrochloride
and employing simplified synthetic protocols. Compound 2 contains a chiral center purposely introduced along the atom chain in order
to make it suitable for chiro-optical studies of the resulting SAMs.
Abstract:
Keywords: Carboxylic acids • Fluorescent groups • Pyrene • Self-assembled monolayers (SAMs)
© Versita Sp. z o.o.
1. Introduction
of the resulting monolayers to the properties of available
building-blocks. In this context the design and synthesis
of new long-chain carboxylic acids as building-blocks
for SAMs have been boosted. Simple, fast, and
efficient synthesis methods for generation of libraries of
organic compounds to be used for formation of desired
monolayers, and their structure property relationship are
welcomed by material chemists. In the present paper we
report synthesis of two new fluorescent carboxylic acids
as building-blocks for self-assembled monolayers on
metallic nanostructures (such as nanoparticles, quantum
dots, and nanocages). Putting strong fluorescent groups
such as the polyaromatic pyrene group into the ligands
for monolayers greatly extends their applications in the
constructions of sensors for specific biomolecules, ions,
or drug molecules. Many features make fluorescence
one of the most powerful transduction mechanisms[9,10]
to report the chemical recognition event. A number of
fluorescence microscopy and spectroscopy techniques
based on the life-time, anisotropy, or intensity of the
emission of fluorescent probes have been developed
over the years where quenching or enhancing of the
Self-Assembled Monolayers (SAMs) constructed by the
chemisorptions of carboxylic acids on metallic or non
metallic nanostructures of various shapes are reported
[1]. Carboxylic acids are useful ligands (building-blocks)
for the construction of monolayers on two or three
dimensional metal and metal oxide surfaces especially
other than gold [2-4]. Carboxylic acids are found to give
stable, robust monolayers onto the surface of silver,
titanium oxide, silica surface, aluminium oxide and
several other substrates where the carboxylic acid group
gets chemisorbed on to the surface of the substrate and
anchors the molecules giving stable monolayers [5-8]. In
our previous studies we reported the formation of stable
self-assembled monolayers of carboxylic acids and
their subsequent photopolymerization on different metal
nanostructures from commercial long-chain carboxylic
acids containing diacetylenic units [3,4].
As most of the work in this regards is done on
construction of monolayers by use of commercially
available carboxylic acids which restricts the applications
* E-mail: sushil.unige@gmail.com
1640

S. A. Jadhav
2. Experimental procedure
Petroleum ether and light petroleum refer to the
fractions boiling in the range of 40-60°C and 80-110°C,
respectively. Solvents used as eluents were distilled prior
to use. Toluene, ethanol, dichloromethane, THF, TFA,
Sebacic acid, Pyren-1-yl methylamine hydrochloride,
Boc-L-Valine, 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetrame-
thyluronium hexafluorophosphate (HBTU) and benzyl
bromide were extra pure commercial products and were
used as received. Anhydrous methylene chloride was
1
syringed under argon. The H NMR and 13C Spectra
were recorded at 300 MHz.
2.1. Experimental procedure for synthesis of
compound 1 from Scheme 1
Synthesis of diethyl sebacate (4) (Scheme 1,
step i).
Figure 1. Fluorescent carboxylic acid monolayer anylate
interactions.
Sebacic acid (26 g, 129 mmol) was taken in a
250 mL single-neck flask, to which ethanol (63.5 mL)
and concentrated sulphuric acid (5 mL) were added. The
solution was then refluxed overnight. After the solvent
was removed by vacuum evaporation, the residue was
treated with water (200 mL) and extracted with ether
three times. The ether layer was washed with 5%
sodium bicarbonate and then several times with water,
and dried over sodium sulphate. Evaporation of the
solvent gave a dense liquid which was vacuum distilled.
The product distillates out at 156-158°C at 6 mmHg as a
transparent liquid: 24.8 g (75%). 1H NMR (CDCl3, ppm)
δ = 1.22-1.29 (14H, m), 1.58-1.62 (4H, m), 2.27 (4H, t,
J = 7.5), 4.11 (4H, q, J = 6).13C NMR (CDCl3, ppm) δ =
14.33, 25.02, 29.16, 34.40, 60.24, 173.92. Data was in
accordance with reported analysis [16].
Figure 2. Newly synthesized long-chain fluorescent carboxylic
acids.
Synthesis of sebacic acid monoethyl ester (5)
(Scheme 1, step ii).
fluorescence upon interactions with the analytes can
be monitored (Fig. 1). These are enormously sensitive
techniques that allow even the detection of single
molecules. Fluorescence does not consume analytes
and no reference is required. Light can travel without
physical wave-guide, facilitating enormously the
technical requirements for constructions of a sensor
[11]. Pyrene is preferred fluroscent precursor for putting
in monolayer building-block for many reasons as it is
one of the most prominent standard chromophores
for probing fluorescence in relation to excimer
formation. Although attached to a conducting surface,
these monolayers bearing pyrene display significant
fluorescence from a pyrene excited state. The SAMs
obtained from pyrene derivatives are so stable
that extensive washing did not result in any loss of
fluorescence intensity [12-15].
In
a
two-neck round-bottom flask the side
arm of which is corked, a mixture of sebacic acid
(32.32 g, 160 mmol), diethyl sebacate (24 g, 93 mmol),
and concentrated hydrochloric acid (4 mL, d 1.19) was
added. A reflux condenser was connected to the top
of the distilling flask. The flask was heated in an oil
bath at 160–170°C until the mixture was completely
homogeneous. The temperature of the bath was then
lowered to 120–130°C, and 9.6 mL (165 mmol) of
95% ethyl alcohol was added to the solution through
the condenser. The mixture was allowed to reflux for
two hours. At the end of this period an additional 2 mL
portion of ethyl alcohol was poured into the solution
and refluxing was continued for two more hours. The oil
bath was allowed to cool to about 75°C and the reaction
mixture was subjected to distillation under reduced
1641

Synthesis of fluorescent carboxylic acid ligands
for construction of monolayers on nanostructures
pressure, using a water pump. The temperature of the (2H, t, J = 6), 2.28 (2H, t, J = 6), 5.16 (2H, d, J = 6), 5.76
bath was increased slowly and distillation was continued (1H, b), 7.95-8.28 (9H, m). 13C NMR (CDCl3, ppm) 24.73,
until the bath temperature reached about 125°C. 25.09, 28.50, 28.94, 29.50, 34.4, 36.71, 42.0, 123.78,
The bath was again cooled to 75–80°C and the 124.98, 125.41, 126.34, 126.90, 131.01, 133.24, 139.0.
distillation was continued at lower pressure, using 173.1, 179.05. Anal. found C, 78.0 H, 7.14 N, 3.37. Anal.
an oil pump. The first fractions consisted of a little calculated for C27H29NO3: C, 78.04 H, 7.03 N, 3.37. Anal.
alcohol, water, and n-butyl ether. The next fraction found C, 78.0 H, 7.14 N, 3.37.
was ethyl sebacate, bp 156–158°C at 6 mm of Hg.
2.2. Experimental procedure for synthesis of
Ethyl hydrogen sebacate was collected at 183–187°C
at 6 mm of Hg. The distillation gives product as white
compound 2 from Scheme 2
Tert-butyl 3-methyl-1-oxo-1-(pyren-1-ylmethyl-
amino) butan-2-yl carbamate (9) (Scheme 2, step i)
[19,20].
Boc-L-valine (Aldrich 99.9%, 1.217 g, 5.6 mmol) was
taken into a 250 mL single-neck flask and 25 mL of dry
DCM was added. The resulting solution was cooled to
0°C in an ice bath and 2-(1H-benzotriazol-1-yl)-1,1,3,3-
1
solid 19.1g (52%), mp = 35.6-35.9°C. H NMR (CDCl3,
ppm) δ = 1.22-1.30 (11H, m), 1.58-1.62 (4H, m), 2.25-
2.36 (4H, m), 4.11 (2H, q, J = 6).13C NMR (CDCl3,
ppm) δ = 14.46, 24.84, 25.13, 29.18, 29.25, 34.25,
34.56, 60.44, 174.20, 180.17. Data was consistent with
reported analysis [17].
9-[(Pyren-1-ylmethyl)carbamoyl]nonanoic acid
ethyl ester (7) (Scheme 1, step iii).
tetramethyluronium
hexafluorophosphate
(HBTU)
Sebacic acid monoethyl ester (5) (2 g, 8.7 mmol)
and dicyclohexylcarbodiimide (1.98 g, 9.6 mmol) were
taken into a two-neck flask; approximately 15 mL
of dry DCM were added and the solution was kept
under stirring at 0°C for 1 h. Pyren-1-yl methylamine
hydrochloride (6) (2.36 g) was taken into a second flask,
adding 75 mL of dry DCM and then triethylamine until
the solution became clear. The latter solution was then
added to the former one and the mixure was kept under
stirring overnight at room temperature. At the end of the
reaction the solution was filtered to remove urea and the
solvent was taken off. The crude residue was purified
by column chromatography using 1:1 petroleum ether/
ethyl acetate as eluent, yielding a white solid (433 mg,
(2.37 g, 6.16 mmol), keeping the solution under stirring at
0°Cfor30minutes.Aseparatesolutionof(pyren-1ylmethyl)
amine hydrochloride (1.5 g, 5.6 mmol) in dry DCM
(80 mL) was prepared by addition of triethylamine until
the solution became clear. This solution was then added
to the previous cooled solution containing valine and
HBTU. The reaction mixture was kept under stirring
overnight at room temperature. After completion of the
reaction the reaction mixture was filtered, the solvent
was removed to get a pinkish white residue which was
then dissolved into a sufficient amount of diethyl ether:
some white solid (TEA.HCl) remained undissolved,
which was filtered off. The ether layer was washed three
times with water, dried on anhydrous sodium sulfate.
Evaporation of ether gives the crude product which
was recrystallized from ethanol to give yellowish white
crystals (900 mg, 38%). 1H NMR (CDCl3, ppm): δ = 0.89
(3H, d, J = 3), 0.91 (3H, d, J = 3), 1.33 (9H, s), 2.2 (1H,
m), 3.92 (1H, dd, J1 = 3, J2 = 6 ), 5.01 (1H, b), 5.18 (2H,
t, J = 4.5), 6.29 (1H, b), 7.95-8.26 (9H, m). 13C NMR
(CDCl3, ppm): δ = 17.98, 19.62, 28.43, 30.87, 42.06,
60.48, 123.01, 124.91, 124.98, 125.24, 125.57, 125.61,
126.30, 127.33, 127.54, 127.78, 128.44, 129.19, 130.97,
131.01, 131.45, 131.48, 171.45.
1
52.6%), mp 120.5-120.9°C. H NMR (CDCl3, ppm) δ =
1.21-1.26 (15H, m), 2.18-2.25 (4H, m), 4.10 (2H, q, J =
6), 5.16 (2H, d, J = 6), 5.16 (2H, d, J = 3), 5.75 (1H, b),
7.95-8.28 (9H, m). 13C NMR (CDCl3, ppm) δ = 14.48,
25.11, 29.22, 29.27, 29.32, 29.41, 34.53, 37.02, 42.25,
60.38, 123.14, 124.93, 124.98, 125.28, 125.59, 125.64,
126.35, 127.54, 127.81, 128.47, 129.29, 130.98, 131.47,
131.49, 172.86, 174.08.
9-[(Pyren-1-ylmethyl)carbamoyl]nonanoic acid
(1) (Scheme 1, step iv) [18].
The substrate (600 mg, 1.35 mmol) was taken into
a single-neck flask and 75 mL of approximately 0.5 N
KOH/EtOH was added, the solution was kept under
magnetic stirring at 45°C for 1.5 h. The reaction mixture
was then diluted with water and acidified with 1N HCl till
pH 2. The product separated out as a white solid which
was filtered and dried. The product was recrystallized
from THF/water (80:20) to obtain a white crystalline solid
2-Amino-3-methyl-N-(pyren-1-ylmethyl)
butanamide (10) (Scheme 2, step ii) [21].
Compound 9 (300 mg, 0.7 mmol) was added to
trifluroacetic acid (2.4 mL) kept at 0°C, and the resulting
thick dark brown solution was kept under stirring for
approximately 2 hours. The progress of reaction was
monitored by TLC. After completion of the reaction 5%
sodium bicarbonate solution was added till the solution
became alkaline (pH 8) before extraction (three times)
with ethyl acetate. The organic layer was washed with
1
(390 mg, 69%), mp 163.4-163.7 °C. H NMR (CDCl3,
ppm) δ = 1.22-1.28 (8H, m), 1.53-1.63 (4H, m), 2.23
1642

S. A. Jadhav
water, dried over sodium sulfate and evaporated to get
10-[3-Methyl-1-oxo-1-(pyren-1-ylmethylamino)
the crude product as a white residue which was purified butan-2-ylamino]-10-oxodecanoicacid(2)(Scheme2,
on silica gel column (1:1 hexane/ethyl acetate as eluent step v).
(145 mg, 63%). 1H NMR (CDCl3, ppm) 0.89 (3H, d, J =
In a single-neck flask equipped with a vacuum
6), 0.99 (3H, d, J = 6), 2.04 (1H, m), 3.33 (1H, b), 5.12 system, compound 12 (109 mg, 0.18 mmol) was
(1H, dd, J1 = 6, J2 = 12 ), 5.23 (1H, dd, J1 = 6, J2 = 6), dissolved in approximately 100 mL of 1:1 THF/ethanol,
7.72 (1H, b), 7.97-8.31 (9H, m). 13C NMR (CDCl3, ppm): followed by 101 mg of palladium on carbon (10%). The
δ = 18.5, 31.7, 41.8, 59.9, 123.0, 123.7, 124. 9, 125.2, vacuum hydrogen cycles were repeated at least two
125.6, 126.3, 126.6 133.3, 139.0, 171.33.
Dodecanedioic acid monobenzyl ester (11) hydrogen atmosphere for 25 minutes, the reaction being
(Scheme 2, step iii). monitored by TLC. After disappearance of substrate,
times. The reaction mixture was kept stirring under
Sebacic acid (4.04 g, 20 mmol) reacted with the reaction stopped. The mixture was filtered twice to
potassium hydroxide (20 mmol) in water (10 mL), and remove the catalyst and concentrated to yield the crude
was stirred for 1 h at room temperature. Enough toluene product which was then recrystallized from ethanol to
was added and the azeotrope was evaporated to get an off-white solid (46 mg, 50%), mp 189.9-191°C.
dryness to yield 4.8 g (20 mmol) of potassium sebacate. 1H NMR (DMSO-D6, ppm) 0.80 (6H, d, J = 6), 1.09-1.15
Toluene (60 mL) was added, followed by 0.6 g (2 mmol) (8H, m), 1.94 (1H, m), 2.46-2.48 (4H, m), 4.12 (1H, dd,
of tetrabutylammonium bromide (TBAB) and 4.6 mL J1 = 3, J2 = 6), 4.98 (2H, d, J = 6), 7.85 (1H, d, J =
(20 mmol) of benzyl bromide. The reaction mixture was 6 ), 8.00-8.28 (9H, m). 8.63 (1H, dd, J1 = 6, J2 = 6).
stirred at reflux for 5 hours. Cooling of the mixture to 13C NMR (DMSO-D6, ppm) 18.97, 19.97, 25.17, 29.21,
room temperature and evaporation of solvent gave a 34.41, 35.91, 38.12, 39.11, 39.36, 58.56, 124.01, 125.3,
white residue which was purified on silica gel column 125.82, 125.9, 125.91, 127.44, 127.69, 128.08, 128.75,
(2:1 hexane/ethyl acetate as eluent). Yield 7.5 g (65%), 130.99, 131.46, 133.53, 171.80, 172.94, 175.20. Anal.
1
mp 39.9-40.3 °C. H NMR (CDCl3, ppm) 1.3 (8H, m), calced. for C32H38N2O4: C, 74.68 H, 7.44 N, 5.44. Anal.
1.65 (4H, m), 2.35 (4H, t, J = 7.2), 5.12 (2H, s), 7.29 (5H, found C, 74.5 H, 7.50 N, 5.39.
m). 13C NMR (CDCl3, ppm) 24.63, 24.90, 28.94, 29.00,
33.94, 34.31, 66.05, 128.11, 128.49, 136.29, 173.55,
179.46.
3. Results and discussion
Benzyl 10-[3-methyl-1-oxo-1-(pyren-1-ylmethyl-
3.1.
Synthesisof10-Oxo-10-(pyren-1-ylmethyl-
amino) decanoic acid (1, Py-C)
amino)butan-2-ylamino]-10-oxodecanoate
(Scheme 2, step iv).
(12)
The synthesis has been performed (Scheme 1) starting
from commercial sebacic acid (3) through the synthesis
of ethyl sebacate (4) followed by its reaction with ethanol
and with excess sebacic acid to get ethyl hydrogen
sebacate 5. This monoester of the sebacic acid was
then condensed with pyrene methyl amine hydrochloride
(6) by using dicylcohexyl carbodiimide (DCC)
as the condensing reagent in dry dichloromethane
solvent and triethyl amine. Here triethylamine (TEA)
serves not only as a mild base to catalyse the reaction
but also help to neutralize the hydrochloride salt
of the amine used. The condensation thus yields
the intermediate ester (7) which upon hydrolysis
with 0.5N KOH in ethanol gives the required acid (1)
in good yields. In literature the condensation of a long-
chaincarboxylicacidwithaminehydrochloridesaltshave
been reported by means of an approach which involves
first the conversion of the acid into an acid chloride
by using thionyl chloride the same report also uses
In to a single neck flask compound 11 (132 mg,
0.45mmol)wasdissolvedin5mLofdrydichloromethane,
and HBTU (258 mg, 0.68 mmol) was added. The
solution was cooled to 0°C and kept under stirring for
30 minutes. In to another flask compound 10 (150 mg,
0.45mmol) wasdissolvedin8mLofdrydichloromethane
and triethylamine (69 mg, 0.68 mmol) this solution was
added to the initial solution and kept under stirring
overnight at room temperature. After completion of the
reaction the reaction mixture was filtered the solvent was
removed to get the crude residue which was purified
by silica gel coloumn (Hexane/ Ethyl acetate 2:1).
Yield 179 mg (66%). mp 183.1-183.4°C. 1H NMR (CDCl3,
ppm) 0.89 (3H, d, J = 3), 0.92 (3H, d, J = 3), 1.17-1.2
(8H, m), 1.46-1.5 (4H, m), 2.1-2.22 (3H, m), 2.32 (2H, t,
J = 9), 4.24 (1H, dd, J1 = 9, J2 = 6), 5.03 (1H, dd, J1 = 6,
J2 = 6), 5.21 (1H, dd, J1 = 6, J2 = 6), 6.08 (1H, d, J = 9),
6.45 (1H, b), 7.33-7.36 (5H, m), 7.92-8.21 (9H, m). 13C
NMR (CDCl3, ppm) 18.53, 19.55, 25.08, 29.22, 34.50,
42.06, 58.73, 66.30, 122.96, 124.98, 125.54, 125.60,
126.27, 127.35, 127.52, 127.75, 128.39, 136.33, 171.20,
173.45, 173.87.
a
potentially dangerous substance BOP [15].
However in the present approach we have used
mild reaction conditions by choosing the appropriate
1643

Synthesis of fluorescent carboxylic acid ligands
for construction of monolayers on nanostructures
HOOC
CH2 COOH
8
3
i
75%
C2H5OOC
CH2 COOC2H5
8
4
52%
ii
C2H5OOC
CH2 COOH
8
5
NH2.HCl
iii
53%
6
O
C
COOC2H5
N
H
8
7
iv 69%
O
C
COOH
N
H
8
1
Synthetic protocol for Py-C.
ii =Sebacic acid, EtOH, HCl; iii = DCC, DCM, TEA;
iv = 0.5N KOH in Ethanol.
i = EtOH, H2SO4;
Scheme 1.
condensing reagent (DCC), solvent and base (TEA)
simplifying the procedure and obtaining overall similar
yields.
Scheme 2.
Synthetic protocol for PyC-Chiral. i = HBTU, TEA;
DCM; ii = TFA; iii = Benzyl bromide, DCC; iv =
HBTU, TEA, DCM; v = H2, Pd on C.
3.2.
Synthesis of 10-[3-methyl-1-oxo-1-(pyren-
1-ylmethylamino) butan-2-ylamino]-10-
oxodecanoic acid (2, PyC-Chiral)
of the monolayers fabricated from the chiral molecule.
The synthesis starts from two commercially available
The synthesis of 10-[3-methyl-1-oxo-1-(pyren-1- reagents; pyrene methyl amine hydrochloride
ylmethylamino)2-butylamino]-10-oxodecanoic
acid (6) and Boc-L-Valine (8). In the first step the
(PyC-Chiral) (2) bearing a pyrene end-group and condensation between the acid and the amine was
provided with a chiral center along the atom chain achieved by using 2-(1H-benzotriazol-1-yl)-1,1,3,3-
has been performed. The chiral center is purposely tetramethyluronium hexafluorophosphate (HBTU) and
introduced in order to carry out chiro-optical studies TEA in dry dichloromethane to give the intermediate
1644

S. A. Jadhav
Figure 3. Normalized absorption and emission spectrum of PyC (solid line) and PyC-chiral (dotted line).
9. In addition to its high reactivity, HBTU has also
been shown to fulfill the very important requirement
4. Conclusion
of avoiding racemization at adjacent sites [19]. The
Boc-protecting group was then removed by using
trifluoroacetic acid to generate the free amine (10).
The latter was then reacted with the monobenzyl
ester of sebacic acid (11) by again using the HBTU,
TEA, DCM protocol to get the condensation
product (12). In the final step the deprotection
of the benzyl ester was performed by using
hydrogen gas and palladium on carbon (10%)
as the catalyst in THF/ethanol (50:50) to get the final
acid (2), which was then recrystallized from boiling
ethanol.
Insummarynewcarboxylicacidswithstronglyfluorescent
group pyrene have been synthesized. The selection of
end-groups has been done with the consideration of
the structure property relationship, and the requirement
of the specific applications of the resulting monolayers
constructed from these compounds. The synthetic
strategiesmentionedherecanbeappliedtothesynthesis
of other similar derivatives of varying chain lengths to
generate libraries of building blocks for self-assembled
monolayers. The experiments of constructions of
monolayers from the compounds mentioned above are
underway and the results will be reported separately.
Fig. 3 shows the absorption and emission spectra
of the compounds PyC and PyC-Chiral. The
measurements were carried out in toluene and
Acknowledgment
the excitation wavelength used was 334 nm. The Research funding from University of Genoa, Italy and
maximum absorption wavelength of the conjugated Italian Interuniversity Consortium for Materials Science
polyaromatic pyrene nucleus at 320 and 335 nm can and Technology (INSTM), Florence, Italy are gratefully
be seen.
acknowledged.
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Synthesis of fluorescent carboxylic acid ligands
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