Ionic liquid crystals derived from 4-hydroxypyridine

Article abstract of DOI:10.1039/c0sm01376e

Monoalkylation of 4-hydroxypyridine gave either neutral N-alkyl-4-pyridones or 4-alkoxypyridines depending on the site (N or O) of alkylation. Further alkylation of these two neutral compounds yielded ionic liquid crystals (ILCs) of N-alkyl-4-alkoxypyridinium bromide, having two long alkyl chains. Alternatively, simple protonation of the two neutral compounds with HCl also induced the formation of ILCs ofN-alkyl-4-hydroxypyridiniumchloride or 4-alkoxypyridiniumhydrochloride.Crystal structures of two different types of ionic liquid crystals and one neutral compound were determined via single crystal X-ray diffraction.Mesophase properties were examined by the technique of differential scanning calorimetry, polarized optical microscopy, and powder X-ray diffractometry. Anions influence the structure and stability of themesophase in the dialkylated compounds. The self-assembly behavior of some typical compounds in solution was studied using the technique of NMR diffusion order spectroscopy. Finally, stable Au nanoparticles stabilized by a pyridinium salt were prepared and characterized. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0sm01376e

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PAPER
Ionic liquid crystals derived from 4-hydroxypyridine†
*
Jung-Tang Lu, Ching-Kuan Lee and Ivan J. B. Lin
Received 25th November 2010, Accepted 5th January 2011
DOI: 10.1039/c0sm01376e
Monoalkylation of 4-hydroxypyridine gave either neutral N-alkyl-4-pyridones or 4-alkoxypyridines
depending on the site (N or O) of alkylation. Further alkylation of these two neutral compounds yielded
ionic liquid crystals (ILCs) of N-alkyl-4-alkoxypyridinium bromide, having two long alkyl chains.
Alternatively, simple protonation of the two neutral compounds with HCl also induced the formation of
ILCsof N-alkyl-4-hydroxypyridinium chloride or 4-alkoxypyridinium hydrochloride. Crystal structures of
two different types of ionic liquid crystals and one neutral compound were determined via single crystal
X-ray diffraction. Mesophase properties were examined by the technique of differential scanning
calorimetry, polarized optical microscopy, and powder X-ray diffractometry. Anions influence the
structure and stability of the mesophase in the dialkylated compounds. The self-assembly behavior of some
typical compounds in solution was studied using the technique of NMR diffusion order spectroscopy.
Finally, stable Au nanoparticles stabilized by a pyridinium salt were prepared and characterized.
O-substituted pyridines (Scheme 1). These N- or O-substituted
Introduction
products could further react with alkyl halide to produce
N-alkyl-4-alkoxypyridinium salts, or react with hydrochloric
acid to give N-alkyl-4-hydroxypyridinium and N-H-4-alkoxy-
pyridinium salts. We study their assembly behaviors in solution,
solid, and the mesophase. A pyridinium salt was also used as
a protecting ligand, to fabricate the gold nanoparticles.
Ionic liquid crystals (ILCs)1 are amphiphiles and are special type of
ionic liquids (ILs).2 ILs are a new generation of green solvent,
having low vapor pressure, high polarity and tunable compatibility
towards various solvents. ILCs, having either positively or nega-
tively charged ionic head groups tend to assemble so as to provide
partially ordered environments through Coulombic, hydrophobic,
dipole–dipoleandhydrogenbondinginteractions, andusuallyform
a lamellar mesophase. Recent developments show that cationic
amphiphiles have many interesting properties and have been used
as templating and protecting agents for nanoparticle formation,3
gelators,4 ionic conductors, 5 surfactants,6 ionic liquids,7 and anti-
bacterial agents.8 The most widely studied systems of ILCs include
ammonium,9 phosphonium,10 imidazolium,11 pyrrolidinium12 and
pyridinium salts.11a–b,13 Our group have been working on imidazo-
lium based ILCs,14 and now wish to extend our study to ILCs of
pyridinium salts. Most of the known pyridinium based ILCs are
salts with only one alkyl chain. Liquid crystals of pyridinium salts
with two long alkyl chains are rare; for example, protonation of
pyridine with two alkyl chains at the 2 and 6 positions produces
pyridinium LCs;13f another example has an N-alkyl chain and an
oxadiazole group at the 4-position of the pyridine ring.13g
Results and discussion
Synthesis
N-Alkyl-4-pyridones. This series of compounds (CnPyO, Cn
stands for CnH2n + 1, n ¼ 12, 14, 16 and 18), a type of zwitterion,
were produced through a slight modification of a known reac-
tion.15 Addition of a phase transfer agent, tetrabutylammonium
In this work we chose 4-hydroxypyridine as our starting
material to prepare pyridinium salts. Monoalkylation of
4-hydroxypyridine could produce N-substituted pyridones or
Department of Chemistry, National Dong-Hwa University, Shoufeng,
Hualien, 974, Taiwan. E-mail: ijblin@mail.ndhu.edu.tw; Fax: +886-3-
863-3570; Tel: +886-3-863-3599
† Electronic supplementary information (ESI) available: The crystal
data, NMR and IR spectra are provided in the electronic supporting
information. CCDC reference numbers [802278–802280]. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0sm01376e
Scheme 1 (i) CnH2n+1Br, 2N NaOH, TBAB, THF, reflux, 24 h; (ii) HCl
(conc.), CH3OH, RT, 2 h; (iii) CnH2n+1Br, 85 ꢀC, 24 h; (iv) after (iii),
NH4X (X ¼ BF4, PF6), ethanol, RT, 2 h.
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bromide (TBAB), improves the yields substantially (Scheme 1).
In our hands, an increase in the yields from 30–40% to 70–80%
was observed. Although isomerization of an O-arylated product
to an N-arylated product had been claimed to be a part of the
reasons for the preferential formation of an N-substituted pro-
duct,15b such isomerization did not occur in our case, as moni-
1
tored by H-NMR spectroscopy.
4-Alkoxypyridines. Several approaches are known for the
preparation of the title compounds. Attempts to produce
4-alkoxypyridine (PyOCn, n ¼ 12, 14, 16 and 18) via direct
alkylation of 4-hydroxypyridine results in low product yields.
Two other synthesis methods are known for 4-alkoxylated
products.16,17 With slight modification, the method described in
ref. 18 was used. In this work, long chain alcohols were heated in
DMSO with NaOH at 100 ꢀC for 1 h, then 4-chloropyridine was
added and reacted for 24 h to produce 4-alkoxylated products
(Scheme 2).
Pyridinium salts. The preparation of N- and 4-disubstituted
pyridinium salts are schematically represented in Scheme 1.
N-Alkyl-4-alkoxypyridinium
bromides
(denoted
as
[CnPyOCn][Br], n ¼ 10, 12, 14, 16 and 18) were synthesized by the
reaction of either N-alkyl-4-pyridones or 4-alkoxypyridines with
alkyl bromide at 80 ꢀC. Metathesis of the bromide salts by
ammonium hexafluorophosphate (NH4PF6) and ammonium
tetrafluoroborate (NH4BF4) produced N-alkyl-4-alkoxypyr-
idinium hexafluorophosphate ([CnPyOCn][PF6], n ¼ 10, 12, 14,
16 and 18) and N-alkyl-4-alkoxypyridinium tetrafluoroborate
([CnPyOCn][BF4], n ¼ 10, 12, 14, 16 and 18), respectively.
N-Alkyl-4-hydroxypyridinium chlorides ([CnPyOH][Cl], n ¼ 12,
14, 16 and 18) and 4-alkoxypyridinium hydrochlorides
([HPyOCn][Cl], n ¼ 12, 14, 16 and 18) were obtained through
direct protonation of N-alkyl-4-pyridones and 4-alkoxypyridines
with concentrated HCl.
Fig. 1 (a) Perspective view and atomic labeling for the compound
[C16PyO]$H2O; thermal ellipsoids are set at 50%. Hydrogens are omitted
ꢀ
ꢀ
for clarity. Selected bond lengths [A] and angles [ ]: O(1)–C(3) 1.270(3);
N(1)–C(1) 1.335(3); N(1)–C(5) 1.339(3); N(1)–C(6) 1.479(3); C(1)–C(2)
1.352(3); O(4)–C(5) 1.352(3); C(1)–N(1)–C(5) 118.3(2); C(5)–N(1)–C(6)
120.2(2); O(1)–C(3)–C(2) 123.2(3); N(1)–C(6)–C(7) 115.3(2). (b) Crystal
packing of [C16PyO]$H2O viewed down the b-axis. (c) H-bonding inter-
00
0
ꢀ
actions between H2O and pyridones (A): H (5A)/O(2) 2.615; H (6B)/
O(2) 2.676; H(2B)/O(1) 1.917; H00(5A)/O(1) 2.575. Atoms with prime
(0) label are at (x, 1/2 ꢁ y, ꢁ1/2 + z) and the double prime (00) label at (x,
1.5 ꢁ y, ꢁ1/2 + z).
Crystal structure
zwitterion (Scheme 3). The alkyl chain stretches outward at
a tilting angle of ca. 30ꢀ from the plane extended from the
pyridine ring. The molecules are packed in a non-interdigitated
bilayer fashion and the molecular rods tilt ca. 68ꢀ from the
normal of the layer plane (a–c plane) (Fig. 1(b)). Extended
hydrogen bonding interactions are observed among pyridones
[C16PyO]$H2O. Crystals of this compound suitable for single
crystal X-ray diffraction were obtained from the CH2Cl2/hexane
solvent system. The source of the hydrate is from the trace of
water in the non-treated solvent. The crystal data are given in
ESI (Table S1-S2),† and an ORTEP drawing in Fig. 1(a). The
00
0
ꢀ
ꢀ
and the hydrate water: H (5A).O(1) (2.575 A), H (6B).O(2)
C(3)–O(1) bond length of 1.270(3) A is comparable to the known
pyridone CO bond length (1.274(6) A).18 For reference, the C]O
ꢀ
double bond is 1.20 A, and C–O single bond is about 1.33 A.
00
ꢀ
ꢀ
ꢀ
ꢀ
(2.676 A), H (5A).O(2) (2.615 A) and H(2B).O(1) (1.917 A)
(Fig. 1(c)).
19
ꢀ
ꢀ
The N(1)–C(1) (1.335(3) A) and N(1)–C(5) (1.339(3) A) bonds
ꢀ
ꢀ
are ca. 0.02 A longer than the corresponding N–C bonds in 4-
[C16PyOC16][Br]$H2O. Crystals of this compound were
obtained from the CH2Cl2/ether solvent system. The crystal data
is shown in ESI (Table S3-S4),† and the rod-like structure is
drawn in Fig. 2(a). The bond lengths and angles of this
compound are normal. The N-alkyl and -alkoxy chains stretch in
opposite directions and tilt ca. 39ꢀ and 48ꢀ from the extended
ꢀ
hydroxypyridine (1.319 A); while the C(1)–C(2) (1.352(3) A) and
ꢀ
ꢀ
C(4)–C(5) (1.352(3) A) bonds are ca. 0.03 A shorter than those of
ꢀ
the corresponding C–C bonds found in 4-hydroxypyridine (1.384
ꢀ
A). Consequently, the nature of this compound is a resonance
hybrid of the dearomatized N-alkyl-4-pyridone and the
Scheme 2 (i) Excess of NaOH, DMSO, 100 ꢀC, 1 h; (ii) 4-chloropyr-
idine, 100 ꢀC, 24 h.
Scheme 3
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pyridinium ring plane, respectively. This compound has
ꢀ
a molecular length of ca. 42.7 A, and packs in a non-interdigi-
ESI (Table S5-S6),† and the molecular structure is shown in
Fig. 3(a). The alkyl chain extends at a tilting angle of ca. 32ꢀ
from the pyridinium ring plane. This compound has normal
bond angles and lengths compared with those found in the
pyridinium salts.13d–e The molecular cation has a structure
similar to that of [C16PyOC16][Br]$H2O. The packing diagram
shows that these molecules are stacked in an interdigitated
bilayer fashion (Fig. 3(b)) with a repeating layer distance of ca.
tated monolayer fashion (Fig. 2(b)). Hydrogen bonding inter-
actions are formed among bromide, H2O and pyridinium
ꢀ
ꢀ
cations: H(1A).Br(1) (2.743 A), H(2A).Br(1) (2.521 A),
H00(2B).Br(1) (2.598 A), H(6A).Br00(1) (2.865 A),
ꢀ
ꢀ
H00(5A).Br00(1) (2.809 A), H(2C).O(2) (2.452 A) and
ꢀ
ꢀ
00
ꢀ
H (4A).O (2) (2.344 A) (Fig. 2(c)). One can consider two pyr-
idinium salts as a pair, in which the two pyridinium cores are
associated in a head to tail fashion via hydrogen bondings as
described above, and the p–p interactions with a distance of 3.4
ꢀ
29.5 A. Hydrogen bonding interactions are formed among
hydrated water, chloride and pyridinium moieties: H(3).Cl(1)
0
0
ꢀ
ꢀ
ꢀ
(2.653 A), H (4).Cl(1) (2.874 A), H (1A).Cl(1) (2.155 A),
ꢀ
H00(1).O (1) (2.595 A),
H
(2).O000(2) (2.539 A),
ꢀ
A (Fig. 2(d)). Tilting of the molecular rod ca. 45 from the
ꢀ
ꢀ
000
ꢀ
normal of the layer plane gives a layer distance of 33.4 A. In
ꢀ
H*(20B).O (2) (2.590 A) (Fig. 3(c)). Hydrogen bonding
interactions between the pyridinium cores are the cause of their
LC phase formation.
general, an alkyl chain favors a staggered conformation and
adopts a zigzag fashion, as is observed between C(22) and C(37).
The ‘‘gauche conformation’’ observed at the alkyl chain next to
the oxygen atom is probably due to the favorable intramolecular
ꢀ
hydrogen bonding between H(8B) and O(1) (2.636 A), and
maximizing the hydrophobic interactions in the crystal packing.
[C16PyOH][Cl]$H2O. Crystals were obtained from the
CH2Cl2/hexane solvent system. The crystal data is given in the
Fig.
2
(a) Perspective view and atom labeling of compound
Fig. 3 (a) Perspective view and atomic labeling of compound
[C16PyOC16][Br]$H2O; H atoms omitted for clarity. Selected bond
[C16PyOH][Cl]$H2O; H atoms are omitted for clarity; theꢀrmal ellipsoids
ꢀ
ꢀ
ꢀ
are set at 50%. Selected bond lengths [A] and angles [ ]: O(1)–C(21)
lengths [A] and angles [ ]: O(1)–C(3) 1.335(2); O(1)–C(6) 1.467(2); N(1)–
C(22) 1.491(2); C(1)–C(2) 1.376(3); C(4)–C(5) 1.366(3); C(3)–O(1)–C(6)
118.4(2); C(1)–N(1)–C(5) 119.7(2); O(1)–C(6)–C(7) 107.3(2); N(1)–
C(22)–C(23) 109.2(2). (b) Crystal packing of [C16PyOC16][Br]$H2O
viewed down the b-axis. (c) H-bonding interactions between the H2O,
1.328(2); N(1)–C(5) 1.482(2); C(1)–C(2) 1.366(3); C(3)–C(4) 1.360(3);
O(1)–C(21)–C(4) 123.1(2); C(3)–N(1)–C(2) 119.4(2); N(1)–C(5)–C(6)
113.5(2). (b) Crystal packing of [C16PyOH][Cl]$H2O viewed down the b-
ꢀ
axis. (c) H-bonding interactions between H2O, chloride and cations (A):
H(3).Cl(1) 2.653; H0(4).Cl(1) 2.874; H0(1A).Cl(1) 2.155; H00(1).O
(1) 2.595; H (2).O000(2) 2.539; H*(20B).O000(2) 2.590. The additional
symbols of (0), (00), (000) and (*) indicate that these atoms are at (1 ꢁ x, 2 ꢁ
y, 1 ꢁ z), (3 ꢁ x, 3 ꢁ y, 1 ꢁ z), (1 + x, 1 + y, z) and (1 ꢁ x, 2 ꢁ y, ꢁz),
respectively.
ꢀ
anion and cation (A): H(1A).Br(1) 2.743; H(2A).Br(1) 2.521;
H00(2B).Br(1) 2.598; H(6A).Br00(1) 2.865; H0(5A).Br00(1) 2.809;
H(2C).O(2) 2.452; H0(4A).O00(2) 2.344. The additional symbols of (0)
and (00) indicate that these atoms are at (1 ꢁ x, ꢁ1 ꢁ y, 1 ꢁ z) and (1 ꢁ x,
ꢀ
ꢁy, 1 ꢁ z). (d) The p–p interaction of 3.4 A between pyridinium rings.
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Liquid crystalline properties
83.7 ꢀC, and the clearing process occurs at 129.0 ꢀC. Compounds
of n ¼ 16 and 14 have a similar phase transition behavior to that
of n ¼ 18, but for n ¼ 12, the mesophase is observed only in the
cooling process. For n ¼ 10, only a simple melting process occurs
Liquid crystalline behaviors of the pyridinium salts were studied
by polarized optical microscopy (POM), differential scanning
calorimetry (DSC) and powder X-ray diffraction (PXRD). In all
cases, SmA mesophases are detected. To avoid the influence of
H2O on the thermal behavior in the process of DSC measure-
ments, the hydrated samples were heated at 100 ꢀC for 1 h on the
sample holder, in which the cover of the cell lid was punched with
a hole to allow the evaporation of water. The phase transitions
and thermodynamic data of these ionic liquid crystals are shown
in Table 1. The layer distances of these compounds at different
phases and temperatures are given in Table 2. The neutral
compounds, [CnPyO] and [PyOCn] are non-mesomorphic.
The POM image of [CnPyOCn][Br] shows that upon melting,
the fluid exhibits focal-conic texture and homeotropic domain,
the latter is caused by the alignment of the molecular vector
orthogonal to the substrate but parallel to the light source,
a typical behavior for the SmA mesophase. The DSC thermo-
gram for the compound with n ¼ 18 shows that the phase tran-
sition from the crystal to the mesophase appears at 86.9 ꢀC with
an enthalpy of 73.7 kJ molꢁ1, and the transition to an isotropic
liquid appears at 132.9 ꢀC with an enthalpy of 1.7 kJ molꢁ1. The
small enthalpy value upon clearing is consistent with the exis-
tence of the mesophase. The compounds of n ¼ 16, 14, 12 and 10
have melting temperatures at 78.5, 76.9, 69.6 and 62.8 ꢀC and
clearing temperatures at 133.3, 135.5, 120.2 and 69.3 ꢀC,
respectively. The chain length has a minimum influence on the
clearing process for n ¼ 18, 16 and 14, suggesting Coulombic,
hydrogen bonding and dipole, but not hydrophobic, interactions
are dominating in this process. These salts have a mesophase
temperature range of DT of about 55 ꢀC, except for n ¼ 10, which
has a DT value of 6.5 ꢀC. It appears that the hydrophobic
interactions arising from the ten-carbon chain length is barely
large enough to sustain the formation of the mesophase for the
compound where n ¼ 10.
ꢀ
at 37.9 C and due to severe super-cooling, the freezing process
does not occur above 0 ꢀC; it thus behaves as a room temperature
ionic liquid. The chain length influences both the melting and
clearing processes; this is especially true when comparing to the
clearing temperature of the bromide salts of n ¼ 18, 16 and 14.
All the above results suggest a weaker cation–anion interaction
ꢁ
ꢁ
for the PF6 than for the Brꢁ. The PF6 anion also causes
a smaller mesophase range in this series. Interestingly, the dif-
fractogram shows that in the solid state the layer distance of
ꢁ
ꢀ
ꢀ
23.9 A for the PF6 salt of n ¼ 16 is 9.3 A shorter than the
corresponding bromide salt. It is likely that the larger anion
allows the carbon chains to interdigitate to have more efficient
packing. Upon heating to 90 ꢀC, above the melting point, the
ꢀ
ꢀ
layer distance increases from 23.9 A in the solid state to 29.0 A in
ꢀ
the mesophase; the latter is ca. 5 A shorter than the non-inter-
digitated Brꢁ analogue suggesting a partially interdigitated alkyl
chain in the mesophase. Upon increasing the temperature further
to 100 and 110 ꢀC in the mesophase, there is a slight decrease in
ꢀ
the layer distance (28.8 and 28.6 A respectively), again consistent
with a SmA mesophase. A similar change of layer distance from
the solid to the mesophase was observed for the compounds of
n ¼ 18 and 14.
ꢁ
For the BF4 series, similar textures are observed in the mes-
ꢁ
ophase as for the Brꢁ and PF6 series. DSC thermograms show
that for the compounds of n ¼ 18, 16, 14 and 12, the melting
ꢀ
processes occur at 69.8, 63.0, 51.2 and 39.4 C and the clearing
ꢀ
processes appear at 127.9, 113.5, 98.2 and 77.3 C respectively.
For n ¼ 10, only a simple melting process is observed at 46.8 ꢀC.
ꢁ
At room temperature, these BF4 compounds have slightly
ꢀ
longer layer spacings (ca. 2–3 A) than the PF6ꢁ compounds, but
ꢁ
ꢀ
are substantially shorter (ca. 6–8 A) than the corresponding Br
series. An interdigitated monolayer packing with a smaller tilting
angle from the normal plane than the PF6ꢁ salts is proposed for
the solid structure. In the mesophase, a lamellar structure with
layer spacings very similar to the PF6ꢁ salts but much shorter (up
PXRD was utilized to characterize the mesophase structures.
Since single crystal structural data of [C16PyOC16][Br] is avail-
able, results of this compound will be discussed first. The solid
sample shows a set of three equally spaced peaks at small angle
regions corresponding to (001), (002) and (003) reflections, sug-
gesting a well-defined lamellar structure with a layer distance of
ꢁ
ꢀ
to ca. 5 A) than the Br analogues are also observed, thus
a partially interdigitated chain is proposed.
Among the [CnPyOH][Cl] compounds (n ¼ 18, 16, 14 and 12),
only n ¼ 18 and 16 exhibit liquid crystalline properties with the
SmA mesophase as evidenced by the textures observed under
POM and results of DSC and PXRD studies. The melting
temperatures for compounds of n ¼ 18 and 16 are at 87.0 and
84.5 ꢀC, respectively, and clearing temperatures are 157.1 and
114.9 ꢀC, respectively. As revealed by the single crystal structure
for the compound of n ¼ 16, an interdigitated bilayer stacking
with molecular rods tilting 15ꢀ from the layer plane gives a layer
ꢀ
33.2 A, consistent with that from single crystal results (Fig. 4). If
the temperature is raised to a mesophase at 90 ꢀC, the layer
ꢀ
spacing increases slightly to 34.4 A and a faint halo at medium
ꢀ
angle is observed. Upon further heating to 100 and 110 C, the
ꢀ
layer distance decreases gradually to 33.8 and 32.9 A. The slight
increase of layer distance from the crystal to the mesophase is
presumably caused by the alignment of the molecular rod
perpendicular to the layer plane, which tends to elongate the
layer spacing, and at the same time thermal motion of the chains
at the mesophase tends to shorten the layer spacing (Scheme 4).
These results are consistent with an SmA mesophase. Similar
arguments apply to the other members in the bromide series.
The PF6ꢁ series compound of n ¼ 10 is non-mesomorphic. For
the other members, fan and homeotropic textures are observed
under POM, again suggesting a SmA mesophase (Fig. 5 for n ¼
16). For the compound of n ¼ 18, the DSC thermogram shows
that the melting process from crystal to mesophase occurs at
distance of 29.5 A. Upon heating to 90 ꢀC in the mesophase,
ꢀ
ꢀ
a slight elongation of layer distance to 31.7 A is observed. This
result suggests a partially interdigitated lamellar mesophase
(Scheme 5).
All the [HPyOCn][Cl] compounds (n ¼ 18, 16, 14 and 12)
exhibit liquid crystalline properties similar to the SmA meso-
phase. DSC thermograms reveal that the melting temperatures
are at 124.0, 124.2, 122.4 and 121.8 ꢀC for compounds of n ¼ 18
to 12, respectively. The corresponding clearing temperatures are
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Table 1 Phase transition temperatures (ꢀC) and corresponding enthalpies (kJ molꢁ1), in parentheses, for the pyridinium salts (Cr ¼ crystal; Cr* ¼ other
crystal; SmA ¼ smectic A; I ¼ isotropic and n. d. means the phase temperature transition by polarized optical microscope).
n ¼ 18
16
14
[CnPyOCn][Br]
12
10
n ¼ 18
16
[CnPyOCn][PF6]
14
12
10
n ¼ 18
16
[CnPyOCn][BF4]
14
12
10
n ¼ 18
16
[CnPyOH][Cl]
[HPyOCn][Cl]
14
12
n ¼ 18
16
14
12
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Table 2 The d-spacings of pyridinium salts from powder X-ray
diffraction. (RT ¼ room temperature)
ꢀ
ꢀ
d-Spacing/A (T/ C)
Compounds
n
Solid state
Meosphase
[CnPyOCn][Br]
18
16
14
12
10
18
16
14
18
16
14
12
18
16
18
16
14
12
37.1 (RT)
33.2 (RT)
29.7 (RT)
26.9 (RT)
23.3 (RT)
26.2 (RT)
23.9 (RT)
21.3 (RT)
29.2 (RT)
25.6 (RT)
23.1 (RT)
20.5 (RT)
31.8 (RT)
29.5 (RT)
30.9 (RT)
28.3 (RT)
25.7 (RT)
23.5 (RT)
37.1 (100)
33.8 (100)
29.9 (100)
27.1 (100)
22.8 (100)
31.2 (100)
28.8 (100)
25.8 (85)
32.2 (90)
29.7 (90)
26.9 (90)
24.3 (70)
34.1 (90)
31.7 (90)
37.4 (130)
35.1 (130)
33.0 (130)
32.1 (130)
[CnPyOCn][PF6]
[CnPyOCn][BF4]
Fig. 5 The texture of [C16PyOC16][PF6] at the mesophase (99 ꢀC) upon
heating.
[CnPyOH][Cl]
[HPyOCn][Cl]
Scheme 5 Schematic drawing of transitions of [C16PyOH][Cl]; (a) at the
crystal phase, (b) at the mesophase (100 ꢀC).
Fig. 4 The PXRD spectrum of [C16PyOC16][Br].
packing, similar to that of [C16PyOH][Cl]. Upon heated to
ꢀ
130 C in the mesophase, an elongation of the layer distance to
ꢀ
35.1 A is observed. For all the compounds of [HPyOCn][Cl], an
ꢀ
increment of layer spacing of ca. 7 A from the solid state to the
mesophase is always found. This result suggests a structure of
a non-interdigitated lamellar mesophase.
Thermal behaviors of these compounds are compared. Anions
play an important role in the structure and mesophase behavior
ꢁ
ꢁ
for the dialkylated pyridinium compounds. The PF6 and BF4
ions are unable to sustain ordered ionic motion for the chain
length of n ¼ 10; only at a chain length of n S 12 do hydrophobic
and ionic interactions come into play for the formation of the
liquid crystals. This anion effect has been reported in other
pyridinium salts.13i The ionic size also affects the crystal packing
and mesophase structure. Thus for dialkyl substituted bromide
compounds, a non-interdigitated monolayer structure is adopted
Scheme 4 Schematic drawing of [C16PyOC16][Br]; (a) at the crystal
phase, (b) at the mesophase (100 ꢀC).
ꢁ
in the crystal state and mesophase, whereas for the PF6 and
ꢁ
142.6, 142.0, 139.3 and 132.5 ꢀC accompanied by decompositions
near the clearing points; nevertheless, the enthalpy changes are
still observable. The melting and isotropic points are very
insensitive to the chain lengths, suggesting that the hydrophobic
interactions are the least significant factor in the thermal
behavior of the [HPyOCn][Cl] series. The PXRD experiments
BF4 salts, interdigitated structures are found in the crystalline
state and partially interdigitated structures are adopted in the
mesophase. Presumably, the larger anion allows the chain
interdigitation to have efficient chain–chain hydrophobic inter-
actions. Interestingly, while the two mono-alkylated CnPyO and
PyOCn are non-mesomorphic, their protonated products do
show liquid crystalline properties, an indication of the role of
ionic interaction in the formation of the mesophase. Of these two
ꢀ
show that the compound of n ¼ 16 has a layer distance of 28.3 A
at room temperature, consistent with an interdigitated bilayer
3496 | Soft Matter, 2011, 7, 3491–3501
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series of products, [CnPyOH][Cl] exhibits a wider mesophase
range than the [HPyOCn][Cl] series, presumably the former have
stronger cation–anion interactions. Although the latter series of
compounds display a narrower mesophase range, all of them are
mesomorphic. This is not the case for the former, only the
compounds of n ¼ 16 and 18 are mesomorphic. These results
suggest that the flexible chain has to be long enough to disrupt
the strong ionic interactions to initial and maintain the meso-
phase. Similar N-protonated compounds [HPyC12][X] (X ¼ Cl,
Br and I) are known,13a however, they are non-mesomorphic.
Here, the oxygen atom in the alkoxyl chain of [HPyOC12][Cl]
plays an important role in the formation of the mesophase.
Presumably, the oxygen atom provides richer hydrogen bonding
interactions to promote the formation of the mesophase.
Although all these salts show concentration dependent D and RH
values, this behavior is more pronounced for the dialkylated salt
than for the monoalkylated compounds, and suggests that the
degree of aggregation is greater for the dialkylated salt than the
monoalkylated salts. This could possibly be due to the additional
hydrophobic interactions provided by the second alkyl chain in
the dialkylated salt. A similar concentration-dependent behavior
has been found for 1-(4-dodecyloxybenzyl)-3-methyl-1H-imida-
zolium bromide11e and 3,30-hexadecyl-1,10-(1,3-mesitylene)-
bis(methylene)imidazolium iodide.21
Stabilization of gold nanoparticles
Studies of gold nanoparticles (AuNPs) have been the subject of
great research interest, due to their unique size- and shape-
dependent properties, and have potential applications in catal-
ysis, biology, and nanotechnology.22 To prepare stable colloidal
AuNPs in solution, stabilizers are usually required. Other than
the commonly used thiols,23 phosphines,24 imidazolium salts,3e, 3g, 25
and the N-monoalkylated pyridinium salt, 4-(dimethylamino)-
pyridine have also been used to stabilize AuNPs.26 In this work,
the dialkylated salt [C16PyOC16][BF4] was examined for its
ability to stabilize AuNPs. HAuCl4 mixed with pyridinium salts
in CH2Cl2 was stirred until the solution becomes clear. Then an
aqueous NaBH4 solution was added to reduce the gold complex,
after which the CH2Cl2 layer turned ruby red indicating the
formation of AuNPs. This colloidal gold solution is stable over
months in air. This is in contrast to the previous observation that
AuNPs stabilized by octadecylamine and triphenylphosphine
could be stabilized for only a few days in chlorinated solvents.27
This colloidal gold shows a strong surface plasmon band28 at 524
nm in the UV-Vis spectrum (Fig. 6(a)). Under a transmission
electron microscope (TEM), samples prepared by dipping the
colloidal solution onto a copper grid reveals spherical AuNPs of
ca. 8 nm assembled in a well ordered 2D array (Fig. 6(b)). The
nature of [C16PyOC16][BF4] on the surface of AuNPs was also
studied by 1H-NMR spectroscopy. Signals between 0.5 and
2.0 ppm, assignable to alkyl chains are broad, and signals for the
pyridinium ring essentially disappear (Fig. S2†), indicating that
the pyridinium ring head is close to the surface of AuNP and the
alkyl chains are stretched away from the AuNP (Fig. 6(c)). This
argument is supported by the studies of FT-IR where the signals
due to the pyridinium ring of AuNPs are barely observed, in
contrast to those sharp bands observed for the free stabilizer
(Fig. S3†). Moreover, the TEM image also shows that the
Aggregation in solution
The self-assembly properties of molecules in solution were
studied employing the technique of diffusion-ordered spectros-
copy (DOSY) NMR at various concentrations. The diffusion of
chemical molecules in a solvent can be measured using the
pulsed-field gradients. Depending on the size, shape, and
concentration of the compounds in solution, the diffusion
constants (D) obtained by the Stokes–Einstein equation can be
applied to get the relative average hydrodynamic radius (RH).20
RH is the radius of a hypothetical sphere that diffuses with the
same speed as the particle under examination. In this text, two
neutral compounds PyOC16 and C16PyO, and three ionic
compounds [HPyOC16][Cl], [C16PyOH][Cl] and [C16PyOC16][Br]
were studied to measure the D and RH values (Table 3). For the
neutral compounds, PyOC16 and C16PyO, the D values change
only slightly upon varying its concentration from 12.5 to 100
mM. When HCl was added, these two pyridinium salts exhibit
the concentration-dependent self-assembly phenomena in solu-
tion. For [HPyOC16][Cl], the diffusion coefficient of 8.4 ꢂ 10ꢁ10
m2 sꢁ1 (RH ¼ 4.7 A) at the concentration of 12.5 mM increases
ꢀ
gradually to 6.4 ꢂ 10ꢁ10 m2 sꢁ1 (RH ¼ 6.2 A) at 100 mM. For the
ꢀ
other protonated compound [C16PyOH][Cl], the diffusion coef-
ficient increases from 7.0 ꢂ 10ꢁ10 m2 sꢁ1 (RH ¼ 5.7 A) at 12.5 mM
ꢀ
to 5.2 ꢂ 10ꢁ10 m2 sꢁ1 (RH ¼ 7.6 A) at 50 mM. The diffusion rate
ꢀ
of [C16PyOH][Cl] at 100 mM is not measured, due to the solu-
bility constrain. For the dialkylated ionic compound
1
[C16PyOC16][Br], the H chemical shift changes and diffusion
coefficients are also concentration dependent (Fig. S1†). The RH
ꢀ
ꢀ
value is doubled (9.2 A) from 12.5 mM (RH ¼ 4.4 A) to 100 mM.
Table 3 Diffusion coefficient values obtained for N- or O-substituted pyridine and pyrinium salt at different concentrations in CDCl3.
Diffusion coefficienta (Hydrodynamic radius)b
Concentrations (mM)
PyOC16
C16PyO
[HPyOC16][Cl]
[C16PyOH][Cl]
[C16PyOC16][Br]
12.5
25.0
50.0
100.0
13.1(3.0)
10.2(3.9)
10.4(3.8)
10.4(3.8)
9.7(4.1)
8.8(4.5)
8.9(4.5)
8.6(4.6)
8.4(4.7)
7.9(5.0)
6.9(5.8)
6.4(6.2)
7.0(5.7)
6.6(6.0)
5.2(7.6)
9.1(4.4)
6.6(6.1)
5.8(6.9)
4.3(9.2)
a
In units of 10ꢁ10 m2 sꢁ1. Measured at 25 ꢀC in CDCl3 (viscosity ¼ 0.55 kg sꢁ1 m). Data were measured using the double-STE pulse sequence using
a diffusion time of 50 ms and bipolar gradients with duration of 1 ms. The sample was not spun. Data were acquired and processed using the
automation programs included in the TOPSPIN software package. 16 spectra were acquired using a linear gradient ramp from 2 to 95% using sine-
shaped gradients. b In units of 10ꢁ10 m. The values were obtained after applying the Stokes–Einstein equation.
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3–5 mg samples, 10 ꢀC minꢁ1 heating and cooling rates and
calibrated with indium and tin standards). Optical character-
ization of the pyridinium salts were performed by a Zeiss Axio-
plan 2 polarizing microscope equipped with a Mettler Toledo
FP82 hot stage and Mettler Toledo FP90 central processor. The
powder X-ray diffraction data were determined from the
Wiggler-A beamline of the National Synchrotron Radiation
Research Center (NSRRC). Diffraction patterns were recorded
in q/2q geometry with step scans normally 0.02 degree in 2q ¼
1–25 degree stepꢁ1 sꢁ1 and a gas flow heater was used to control
the temperature. The powder samples were charged in Linde-
mann capillary tubes (80 mm long and 0.01 mm thick) from
Charles Supper Co. with an inner diameter of 0.10 or 0.15 mm.
Single crystal X-ray diffraction data were collected on a Bruker
SMART diffractometer equipped with a CCD array detector
Fig. 6 (a) The UV-Vis spectrum of AuNPs and the absorption is
524 nm. (b) The TEM image for AuNPs stabilized by [C16PyOC16][BF4]
in CH2Cl2. (c) The diagram for pyridinium salts protect the AuNPs.
with graphite monochromatized Mo-Ka radiation (l
¼
ꢀ
0.71073 A) in 4 and u scan modes. FT-IR spectra were measured
with a Mattson Galaxy Series FT-IR 3000 spectrophotometer.
The UV–Vis spectra were obtained on a PerkinElmer Lambda
750 spectrophotometer. The images of nanoparticles were
acquired by an Analytical Transmission Electron Microscope
(JEOL JEM-3010). All diffusion NMR experiments were
acquired at 298 K on a BRUKER 600 UltraShieldꢀ equipped
with a 5 mm BBO inverseprobe. Diffusion experiments were
performed with a diffusion time of 50 ms and a longitudinal
Eddy-current delay (LED) of 5 ms. Hydrodynamic radii were
obtained through the Stokes–Einstein equation (D ¼ kT/6phRh),
D is the diffusion coefficient (m2 sꢁ1), k is the Boltzmann constant
(m2 kg sꢁ2 Kꢁ1), T is the absolute temperature (K), h is the
viscosity of solvent (kg sꢁ1 mꢁ1) and RH is the hydrodynamic
radius (m).
distance between the particles is ca. 2 nm, which is consistent
with the spacing of
[C16PyOC16][BF4].
a bilayered packing of U-shaped
Conclusion
N-Alkylpyridones CnPyO and O-alkoxypyridines PyOCn are
prepared in good yields via controlled alkylation. Both the
neutral compounds of CnPyO and PyOCn are non-mesomorphic.
However, upon further reaction with alkyl bromide produce
dialkylated pyridinium salts possessing LC properties. Proton-
ating the two neutral series of compounds also induce an SmA
LC phase; which has the potential to serve as proton transfer
materials with directional order. Dialkylated pyridinium salts
have a similar mesophase temperature range comparable to those
of other reported pyridinium salts with a single core. The effect of
an anion on the mesophase range of N-alkyl-4-alkoxypyridinium
General method for the synthesis of N-alkyl-4-pyridone
monohydrate
To a mixture of 4-hydroxypyridine (5.00 g, 52.57 mmol) and
tetrabutylammonium bromide (TBAB) (1.69 g, 5.24 mmol) in
50 mL THF was added with an aqueous NaOH solution (2N,
25 mL). This resultant solution was stirred until the muddy
solution became clear. 1-Bromooctadecane (14.01 g, 42.05
mmol) was then added to this solution with refluxing for 24 h at
75 ꢀC. After cooling, the THF solvent was removed by a rotary
evaporator, and the crude product was extracted by CH2Cl2 and
water (1 : 0.8). The organic layer was collected and the CH2Cl2
was removed by a rotary evaporator. The crude product was
treated with ether, and then filtered by gravitation the filtrate
collected, because [C18PyO]$H2O was dissolved in ether. This
step was repeated several times. The ether solvent was removed
and a white product was obtained N-octadecyl-4-pyridone with
ꢁ
ꢁ
salts is Brꢁ > BF4 > PF6ꢁ. The larger sized anions (PF6 and
BF4ꢁ) favor the assembly via interdigitated alkyl chains in the
solid, and partial interdigitation in the mesophase. While the
pyridinium salts of [HPyOC16][Cl], [C16PyOH][Cl] and
[C16PyOC16][Br] display concentration-dependent aggregation in
CDCl3, the neutral compounds of PyOC16 and C16PyO are not.
Finally, [C16PyOC16][BF4] is a good stabilizer for spherical
AuNPs (ca. 8 nm) in CH2Cl2 for months. Here, we provide an
alternative facile route for the synthesis of stable AuNPs without
adding other phase transfer agents such as tetraoctylammonium
bromide (TOAB).
Experimental section
1
a yield of about 80%. H-NMR (300 MHz, CDCl3): d ¼ 0.87
All the solvents and reagents were purchased from Aldrich,
Acros, J. T. Baker and used as received. 1H-NMR (300 &
400 MHz) spectra were recorded on an Avance DPX300
BRUKER, AvanceII 400 BRUKER and the chemical shifts (d)
were given in ppm and referenced to residual solvent signals.
Elemental analyses were performed in the Taiwan Instrument
Center. In this content, the phase transitions and thermodynamic
data were determined by differential scanning calorimetry
(Mettler Toledo DSC822 equipped with cryostatic cooling,
(t, 3J ¼ 7 Hz, 3H, CH3), 1.24–1.44 (m, 30H, CH2), 1.77 (t, 3J ¼ 7
Hz, 2H, CH2), 3.81 (t, 3J ¼ 7 Hz, 2H, CH2), 6.46 (d, 3J ¼ 7 Hz,
2H, aromatic CH), 7.35 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal.
calcd. for C23H43NO2: C 75.56; H 11.85; N 3.83. Found: C 75.43;
H 11.75; N 3.74%.
N-Hexadecyl-4-pyridone. Typical data for [C16PyO]$H2O:
1
white solid, yield 76%. H-NMR (300 MHz, CDCl3): d ¼ 0.84
(t, 3J ¼ 7 Hz, 3H, CH3), 1.24–1.29 (m, 26H, CH2), 1.76 (t, 3J ¼ 7
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3
Hz, 2H, CH2), 3.79 (t, 3J ¼ 7 Hz, 2H, CH2), 6.45 (d, J ¼ 7 Hz,
(m, 16H, CH2), 1.44–1.51 (m, 2H, CH2), 1.86–1.93 (m, 2H, CH2),
4.24 (t, 3J ¼ 6 Hz, 2H, CH2), 7.23 (d, 3J ¼ 7 Hz, 2H, aromatic),
8.57 (d, 3J ¼ 7 Hz, 2H, aromatic). Anal. calcd. for C17H32NO2Cl:
C 64.23; H 10.15; N 4.41. Found: C 64.79; H 10.16; N 4.39%.
2H, aromatic CH), 7.32 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal.
calcd. for C21H39NO2: C 74.72; H 11.65; N 4.15. Found: C 75.09;
H 11.70; N 4.13%.
N-Tetradecyl-4-pyridone. Typical data for [C14PyO]$H2O:
1
white solid, yield 75%. H-NMR (300 MHz, CDCl3): d ¼ 0.87
General method for the synthesis of N-alkyl-4-alkoxypyridinium
bromide monohydrate
(t, 3J ¼ 7 Hz, 3H, CH3), 1.25–1.30 (m, 22H, CH2), 1.76 (t, 3J ¼ 7
3
Hz, 2H, CH2), 3.75 (t, 3J ¼ 7 Hz, 2H, CH2), 6.39 (d, J ¼ 7 Hz,
2H, aromatic CH), 7.28 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal.
calcd. for C19H35NO2: C 73.74; H 11.40; N 4.53. Found: C 73.56;
H 11.50; N 4.45%.
A mixture of [C18PyO]$H2O (1.00 g, 2.74 mmol) and 1-bro-
mooctadecane (1.08 g, 3.26 mmol) was treated with stirring at
85 ꢀC for 24 h without adding any solvent. After cooling, the
solid was obtained and hexane added to remove the remaining
1-bromooctadecane. The crude product was obtained by filtra-
tion. Recrystallization from CH2Cl2 and ether gave the white
solid of N-octadecyl-4-octadecyloxypyridinium bromide with
a yield of about 83%. Typical data for [C18PyOC18][Br]$H2O:
1H-NMR (400 MHz, CDCl3): d ¼ 0.85 (t, 3J ¼ 7 Hz, 6H, CH3),
N-Dodecyl-4-pyridone. Typical data for [C12PyO]$H2O: white
solid, yield 76%. 1H-NMR (300 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7
Hz, 3H, CH3), 1.24–1.29 (m, 18H, CH2), 1.75 (t, 3J ¼ 7 Hz, 2H,
3
3
CH2), 3.75 (t, J ¼ 7 Hz, 2H, CH2), 6.38 (d, J ¼ 7 Hz, 2H,
aromatic CH), 7.29 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C17H31NO2: C 72.55; H 11.10; N 4.98. Found: C 72.91; H
11.27; N 5.00%.
3
1.21–1.44 (m, 60H, CH2), 1.80–1.94 (m, 4H, CH2), 4.27 (t, J ¼
6 Hz, 2H, CH2), 4.69 (t, 3J ¼ 7 Hz, 2H, CH2), 7.43 (d, 3J ¼ 7 Hz,
2H, aromatic CH), 9.13 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal.
calcd. for C41H80NO2Br: C 70.45; H 11.54; N 2.00. Found: C
70.38; H 11.60; N 1.98%.
General method for the synthesis of 4-alkoxypyridinium
hydrochloride
A mixture of [PyOC18] (0.20 g, 0.58 mmol) was dissolved in 25
mL of methanol and added 1 mL of concentration of hydro-
chloric acid (HCl) with stirring at room temperature for 40 h.
Then the methanol solvent was removed by a rotary evaporator.
The residual HCl was removed under vacuum at 50 ꢀC and then
we could get a white product 4-octadecyloxypyridinium hydro-
N-Hexadecyl-4-hexadecyloxypyridinium bromide. Typical data
for [C16PyOC16][Br]$H2O: white solid, yield 85%. 1H-NMR
(400 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 6H, CH3), 1.22–1.46
(m, 52H, CH2), 1.84–1.96 (m, 4H, CH2), 4.28 (t, 3J ¼ 6 Hz, 2H,
3
3
CH2), 4.69 (t, J ¼ 7 Hz, 2H, CH2), 7.43 (d, J ¼ 7 Hz, 2H,
aromatic CH), 9.07 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C37H72NO2Br: C 69.13; H 11.29; N 2.18. Found: C 68.93; H
11.30; N 2.12%.
chloride with
a yield of about 98%. Typical data for
1
[HPyOC18][Cl]$0.5H2O: H-NMR (400 MHz, CDCl3): d ¼ 0.88
3
(t, J ¼ 7 Hz, 3H, CH3), 1.25–1.36 (m, 28H, CH2), 1.44–1.50
3
(m, 2H, CH2), 1.86–1.91 (m, 2H, CH2), 4.24 (t, J ¼ 6 Hz, 2H,
N-Tetradecyl-4-tetradecyloxypyridinium bromide. Typical data
for [C14PyOC14][Br]$H2O: white solid, yield 83%. 1H-NMR
(400 MHz, CDCl3): d ¼ 0.86 (t, 3J ¼ 7 Hz, 6H, CH3), 1.22–1.45
(m, 44H, CH2), 1.81–1.93 (m, 4H, CH2), 4.28 (t, 3J ¼ 6 Hz, 2H,
CH2), 7.22 (d, 3J ¼ 6 Hz, 2H, aromatic), 8.55 (d, 3J ¼ 6 Hz, 2H,
aromatic). Anal. calcd. for C23H42NOCl$0.5H2O: C 70.28; H
11.03; N 3.56. Found: C 70.16; H 10.90; N 3.39%.
3
3
CH2), 4.68 (t, J ¼ 7 Hz, 2H, CH2), 7.44 (d, J ¼ 7 Hz, 2H,
aromatic CH), 9.08 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C33H64NO2Br: C 67.55; H 10.99; N 2.39. Found: C 67.54; H
11.06; N 2.34%.
4-Hexadecyloxypyridinium hydrochloride. Typical data for
[HPyOC16][Cl]$0.5H2O: white solid, yield 91%. 1H-NMR
(400 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 3H, CH3), 1.25–1.35
(m, 24H, CH2), 1.44–1.50 (m, 2H, CH2), 1.86–1.91 (m, 2H, CH2),
4.24 (t, 3J ¼ 7 Hz, 2H, CH2), 7.22 (d, 3J ¼ 7 Hz, 2H, aromatic),
N-Dodecyl-4-dodecyloxypyridinium bromide. Typical data for
[C12PyOC12][Br]$H2O: white solid, yield 84%. 1H-NMR
(400 MHz, CDCl3): d ¼ 0.86 (t, 3J ¼ 7 Hz, 6H, CH3), 1.25–1.47
(m, 36H, CH2), 1.87 (m, 4H, CH2), 4.28 (t, 3J ¼ 6 Hz, 2H, CH2),
8.56 (d, 3J
¼
6 Hz, 2H, aromatic). Anal. calcd. for
C21H38NOCl$0.5H2O: C 69.10; H 10.77; N 3.84. Found: C 68.81;
H 10.74; N 3.80%.
3
3
4.68 (t, J ¼ 7 Hz, 2H, CH2), 7.44 (d, J ¼ 7 Hz, 2H, aromatic
3
4-Tetradecyloxypyridinium hydrochloride. Typical data for
[HPyOC14][Cl]$0.5H2O: white solid, yield 93%. 1H-NMR
(400 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 3H, CH3), 1.26–1.36
(m, 20H, CH2), 1.46–1.1.51 (m, 2H, CH2), 1.87–1.91 (m, 2H,
CH), 9.10 (d, J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd. for
C29H56NO2Br: C 65.64; H 10.64; N 2.64. Found: C 65.41; H
10.66; N 2.59%.
3
3
CH2), 4.24 (t, J ¼ 6 Hz, 2H, CH2), 7.23 (d, J ¼ 6 Hz, 2H,
N-Decyl-4-decyloxypyridinium bromide. Typical data for
[C10PyOC10][Br]$H2O: white solid, yield 80%. 1H-NMR
(400 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 6H, CH3), 1.25–1.40
(m, 28H, CH2), 1.87 (m, 4H, CH2), 4.26 (t, 3J ¼ 6 Hz, 2H, CH2),
3
aromatic), 8.57 (d, J ¼ 7 Hz, 2H, aromatic). Anal. calcd. for
C19H34NOCl$0.5H2O: C 67.73; H 10.47; N 4.16. Found: C 67.62;
H 10.54; N 4.03%.
3
3
4.67 (t, J ¼ 7 Hz, 2H, CH2), 7.45 (d, J ¼ 7 Hz, 2H, aromatic
3
4-Dodecyloxypyridinium hydrochloride. Typical data for
[HPyOC12][Cl]$H2O: white solid, yield 91%. 1H-NMR
(400 MHz, CDCl3): d ¼ 0.89 (t, 3J ¼ 7 Hz, 3H, CH3), 1.26–1.38
CH), 9.12 (d, J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd. for
C25H48NO2Br: C 63.27; H 10.19; N 2.95. Found: C 63.47; H
10.30; N 2.71%.
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General method for the synthesis of N-alkyl-4-alkoxypyridinium
hexafluorophosphate
tetrafluoroborate (NH4BF4) (0.04 g, 0.35 mmol) was dissolved in
20 mL of 83% ethanol with stirring at room temperature for 0.5
h. When the two solutions were mixed, the white precipitates
appeared at once and the solid was obtained by filtration, and
then the crude products were eluted by adding 83% ethanol and
water. Recrystallization from CH2Cl2 and hexane gave the white
A mixture of [C18PyOC18][Br]$H2O (0.20 g, 0.30 mmol) was dis-
solved in
5 mL of 95% ethanol and ammonium hexa-
fluorophosphate (NH4PF6) (0.07 g, 0.45 mmol) was also dissolved
in 5 mL of 95% ethanol with stirring at room temperature for 2 h.
Whenthetwosolutionsweremixed, thewhiteprecipitatesappeared
at once and the solid was obtained by filtration, and then the crude
products were eluted by adding ethanol and water. Recrystalliza-
tionfromCH2Cl2 andhexane gave the white solid of N-octadecyl-4-
octadecyloxypyridiniumhexafluorophosphatewithayieldofabout
solid
of
N-octadecyl-4-octadecyloxypyridinium
tetra-
fluoroborate with a yield of about 85%. Typical data for
1
[C18PyOC18][BF4]$0.5H2O: H-NMR (300 MHz, CDCl3): d ¼
0.88 (t, 3J ¼ 7 Hz, 6H, CH3), 1.26–1.45 (m, 60H, CH2), 1.86 (m,
3
3
4H, CH2), 4.27 (t, J ¼ 6 Hz, 2H, CH2), 4.42 (t, J ¼ 8 Hz, 2H,
CH2), 7.35 (d, 3J ¼ 7 Hz, 2H, aromatic CH), 8.52 (d, 3J ¼ 7 Hz,
2H, aromatic CH). Anal. calcd. for C41H78NOBF4$0.5H2O: C
70.66; H 11.43; N 2.01. Found: C 70.84; H 11.29; N 2.10%
1
91%. Typical data for [C18PyOC18][PF6]: H-NMR (300 MHz,
CDCl3): d ¼ 0.88 (t, 3J ¼ 7 Hz, 6H, CH3), 1.25–1.45 (m, 60H, CH2),
1.86 (m, 4H, CH2), 4.26 (t, 3J ¼ 6 Hz, 2H, CH2), 4.37 (t, 3J ¼ 8 Hz,
2H, CH2), 7.37 (d, 3J ¼ 7 Hz, 2H, aromatic CH), 8.41 (d, 3J ¼ 7 Hz,
2H, aromatic CH). Anal. calcd. for C41H78NOPF6: C 66.01; H
10.54; N 1.88. Found: C 65.47; H 10.49; N 1.91%.
N-Hexadecyl-4-hexadecyloxypyridinium
tetrafluoroborate.
Typical data for [C16PyOC16][BF4]: white solid, yield 77%.
1H-NMR (300 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 6H, CH3),
1.25–1.45 (m, 52H, CH2), 1.85 (m, 4H, CH2), 4.25 (t, 3J ¼ 6 Hz,
2H, CH2), 4.36 (t, 3J ¼ 8 Hz, 2H, CH2), 7.31 (d, 3J ¼ 7 Hz, 2H,
aromatic CH), 8.42 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C37H70NOBF4: C 70.43; H 11.17; N 2.22. Found: C 69.94; H
11.13; N 2.25%.
N-Hexadecyl-4-hexadecyloxypyridinium hexafluorophosphate.
Typical data for [C16PyOC16][PF6]: white solid, yield 86%.
1H-NMR (300 MHz, CDCl3): d ¼ 0.88 (t, 3J ¼ 7 Hz, 6H, CH3),
1.25–1.46 (m, 52H, CH2), 1.84 (m, 4H, CH2), 4.26 (t, 3J ¼ 6 Hz,
2H, CH2), 4.37 (t, 3J ¼ 8 Hz, 2H, CH2), 7.32 (d, 3J ¼ 7 Hz, 2H,
aromatic CH), 8.41 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C37H70NOPF6: C 64.41; H 10.23; N 2.03. Found: C 64.06; H
10.23; N 2.07%.
N-Tetradecyl-4-tetradecyloxypyridinium
tetrafluoroborate.
Typical data for [C14PyOC14][BF4]: white solid, yield 89%.
1H-NMR (300 MHz, CDCl3): d ¼ 0.88 (t, 3J ¼ 7 Hz, 6H, CH3),
1.24–1.45 (m, 44H, CH2), 1.83 (m, 4H, CH2), 4.27 (t, 3J ¼ 6 Hz,
2H, CH2), 4.43 (t, 3J ¼ 8 Hz, 2H, CH2), 7.36 (d, 3J ¼ 7 Hz, 2H,
aromatic CH), 8.53 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C33H62NOBF4: C 68.85; H 10.86; N 2.43. Found: C 68.84; H
10.96; N 2.48%.
N-Tetradecyl-4-tetradecyloxypyridinium hexafluorophosphate.
Typical data for [C14PyOC14][PF6]: white solid, yield 86%.
1H-NMR (300 MHz, CDCl3): d ¼ 0.88 (t, 3J ¼ 7 Hz, 6H, CH3),
1.24–1.45 (m, 44H, CH2), 1.84 (m, 4H, CH2), 4.26 (t, 3J ¼ 6 Hz,
2H, CH2), 4.37 (t, 3J ¼ 7 Hz, 2H, CH2), 7.32 (d, 3J ¼ 7 Hz, 2H,
aromatic CH), 8.40 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C33H62NOPF6: C 62.54; H 9.86; N 2.21. Found: C 62.01; H
9.87; N 2.23%.
N-Dodecyl-4-dodecyloxypyridinium tetrafluoroborate. Typical
data for [C12PyOC12][BF4]: white solid, yield 74%. 1H-NMR
(300 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 6H, CH3), 1.23–1.45
(m, 36H, CH2), 1.83 (m, 4H, CH2), 4.26 (t, 3J ¼ 6 Hz, 2H, CH2),
N-Dodecyl-4-dodecyloxypyridinium
hexafluorophosphate.
3
3
4.41 (t, J ¼ 7 Hz, 2H, CH2), 7.35 (d, J ¼ 7 Hz, 2H, aromatic
Typical data for [C12PyOC12][PF6]: white solid, yield 89%.
1H-NMR (300 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 6H, CH3),
1.24–1.45 (m, 36H, CH2), 1.86 (m, 4H, CH2), 4.26 (t, 3J ¼ 6 Hz,
2H, CH2), 4.37 (t, 3J ¼ 7 Hz, 2H, CH2), 7.32 (d, 3J ¼ 7 Hz, 2H,
aromatic CH), 8.41 (d, 3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd.
for C29H54NOPF6: C 60.29; H 9.42; N 2.42. Found: C 60.41; H
9.31; N 2.36%.
3
CH), 8.54 (d, J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd. for
C29H54NOBF4: C 67.04; H 10.48; N 2.70. Found: C 66.71; H
10.42; N 2.34%.
N-Decyl-4-decyloxypyridinium tetrafluoroborate. Typical data
for [C10PyOC10][BF4]: white solid, yield 62%. 1H-NMR
(300 MHz, CDCl3): d ¼ 0.86 (t, 3J ¼ 7 Hz, 6H, CH3), 1.23–1.45
(m, 28H, CH2), 1.88 (m, 4H, CH2), 4.25 (t, 3J ¼ 6 Hz, 2H, CH2),
N-Decyl-4-decyloxypyridinium hexafluorophosphate. Typical
data for [C10PyOC10][PF6]: white solid, yield 61%. 1H-NMR
(300 MHz, CDCl3): d ¼ 0.87 (t, 3J ¼ 7 Hz, 6H, CH3), 1.25–1.45 (m,
28H, CH2), 1.88 (m, 4H, CH2), 4.25 (t, 3J¼ 6 Hz, 2H, CH2), 4.36 (t,
3J ¼ 7 Hz, 2H, CH2), 7.32 (d, 3J ¼ 7 Hz, 2H, aromatic CH), 8.41 (d,
3J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd. for C25H46NOPF6: C
57.57; H 8.89; N 2.69. Found: C 57.22; H 8.91; N 2.69%.
3
3
4.36 (t, J ¼ 7 Hz, 2H, CH2), 7.31 (d, J ¼ 7 Hz, 2H, aromatic
3
CH), 8.42 (d, J ¼ 7 Hz, 2H, aromatic CH). Anal. calcd. for
C25H46NOBF4: C 64.79; H 10.00; N 3.02. Found: C 64.75; H
9.96; N 2.70%.
Formation of gold nanoparticles
The gold mixture was prepared from HAuCl4 (0.01 g, 0.03 mmol),
[C16PyOC16][BF4] (0.19 g, 0.30 mmol), and CH2Cl2 (10mL).After1
h, when the solution became clear ,an aqueous solution of NaBH4
was added (1.5 ꢂ 10ꢁ3 M, 10 mL) with violent shaking. The
colorless CH2Cl2 solution changed to ruby red and the metallic
nanoparticles were formed. The CH2Cl2 layer was selected and the
General method for the synthesis of N-alkyl-4-alkoxypyridinium
tetrafluoroborate
A mixture of [C18PyOC18][Br]$H2O (0.10 g, 0.15 mmol) was
dissolved in 130 mL of 83% ethanol and ammonium
3500 | Soft Matter, 2011, 7, 3491–3501
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and T. Cardinael, Chem. Mater., 2008, 20, 157; (h) V. Causin and
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solvent was removed, then the ethanol was added and was centri-
fuged at 9000 rpm for 10 min. Finally, the solvent was removed and
the purple gold nanoparticle were obtained.
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Acknowledgements
We thank the National Science Council of Taiwan (Grant no.
NSC 97-2113-M-259-009-MY3) for the financial support of this
work.
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