Synthesis and characterization of luminescent tricationic salts of mesitylene and stilbazolium moieties

Article abstract of DOI:10.1016/j.molstruc.2012.03.024

A series of luminescent tricationic salts having mesitylene and stilbazolium moieties with various organic counterions were synthesized by using quaternization and metathesis reactions. Their chemical structures were characterized by ¹H, ¹⁹F and ¹³C NMR spectroscopy, and elemental analysis. Their crystalline and amorphous properties were examined by differential scanning calorimetry, polarized optical microscopy, and variable-temperature X-ray diffraction studies. Their thermal stabilities were determined by thermogravimetric analyses, which were in the broad temperature ranges of 239-317°C in nitrogen. Their optical properties were studied by UV-Vis and photoluminescent spectroscopy. The light-emission properties were dependent on the polarity of organic solvents (positive solvatochromism) but independent on the chemical structures of organic counterions. They emitted blue light in various organic solvents in a broad range of wavelengths of 470-497 nm, but emitted green light in a narrow range of wavelengths of 506-515 nm in their solid states.

Full text of DOI:10.1016/j.molstruc.2012.03.024

Journal of Molecular Structure 1019 (2012) 174–182
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and characterization of luminescent tricationic salts of mesitylene
and stilbazolium moieties q
Tae Soo Jo a, William L. McCurdy a, Ontida Tanthmanatham a, Tae K. Kim a, Haesook Han a,
Pradip K. Bhowmik a, , Benoît Heinrich b, Bertrand Donnio b
⇑
a Department of Chemistry, University of Nevada at Las Vegas, 4505 Maryland Parkway, Box 454003, Las Vegas, NV 89154, USA
b Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504, CNRS-Université de Strasbourg, BP 43, 23 rue de Loess, F-67034 Strasbourg Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of luminescent tricationic salts having mesitylene and stilbazolium moieties with various organic
counterions were synthesized by using quaternization and metathesis reactions. Their chemical struc-
tures were characterized by 1H, 19F and 13C NMR spectroscopy, and elemental analysis. Their crystalline
and amorphous properties were examined by differential scanning calorimetry, polarized optical micros-
copy, and variable-temperature X-ray diffraction studies. Their thermal stabilities were determined by
thermogravimetric analyses, which were in the broad temperature ranges of 239–317 °C in nitrogen.
Their optical properties were studied by UV–Vis and photoluminescent spectroscopy. The light-emission
properties were dependent on the polarity of organic solvents (positive solvatochromism) but indepen-
dent on the chemical structures of organic counterions. They emitted blue light in various organic sol-
vents in a broad range of wavelengths of 470–497 nm, but emitted green light in a narrow range of
wavelengths of 506–515 nm in their solid states.
Received 5 January 2012
Received in revised form 2 March 2012
Accepted 9 March 2012
Available online 23 March 2012
Keywords:
Tricationic ionic liquid
Photoluminescence
UV–Vis spectroscopy
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
of ILs, when properly designed. For example, ammonium-based
dicationic phosphate salt liquids [19–21], imidazolium-based
Ionic liquids (ILs) are molten salts at room temperature and
composed of cations and anions in their chemical structures. These
materials have drawn much attention over the past years due to
their potential applications as substitutes for volatile organic sol-
vents in organic synthesis [1], solvents for high-temperature reac-
tions [2], electrochemical applications in photovoltaic cells and
fuel cells [3–6], high-temperature lubricants [7], liquid–liquid
extractions [8–10], and mass spectrometric applications [11–13].
The properties of ILs can be easily tuned by change of the chemical
structures or combination of various cations and anions. Design
and synthesis of new salts leading to ILs are an important class
of materials that have potential applications in chemistry, physics,
and material science and technology [14–17].
In general, multi-charged ILs have been shown to have a greater
range of physical properties than traditional monocationic types
such as higher density, glass transition, melting point, surface ten-
sion, and shear viscosity [18]. Since the number of possible combi-
nations of cationic and anionic moieties in multi-charged ILs are
greater than those of monocationic such liquids, multi-charged
ILs can be considered as an extension to the structural variations
dicationic ILs [22], pyridinium based dicationic ILs [23], and
pyrrolidium cation based ILs [24] were synthesized to explore their
physical properties. Note here that they displayed better physical
properties than those of monocationic ILs. Even though materials
chemist faces a challenge to synthesize multi-charged ILs because
of strong, long-range intermolecular interactions, however, the
incorporation of the bulky organic anions such as bis(trifluoro-
methylsulfonyl)imide (ꢀN(Tf)2) not only decreases the melting
point but also increases the thermal stability of ILs.
Armstrong and co-workers reported the effect of chemical struc-
tural modifications on the physicochemical properties of symmetri-
cal tricationic ILs containing ꢀN(Tf)2, hexafluorophosphate (ꢀPF6),
tetrafluoroborate (ꢀBF4), and trifluoromethanesulfonate (TfOꢀ).
The densities of ILs with similar cationic moieties are affected by
the counterions in the order ꢀN(Tf)2 > ꢀPF6 > TfOꢀ
>
ꢀBF4; and the
thermal decomposition temperatures of tricationic ILs are higher
(often >400 °C) than those of monocationic ionic liquids with a
few exceptions (e.g. 1-ethyl-3-methyl-imidazolium salt with trifli-
mide counterion has a thermal stability of 400 °C). The melting
points of tricationic ILs are affected by the flexibility of the central
core systems; for example, ILs with more flexible core structure ex-
hibit low melting points when compared with those of having rigid
core structure. The solubility of these salts in water and heptane is
very similar to that of monocationic and dicationic ILs and depends
largely on the choice of anions [25,26]. Furthermore, the mesitylene
q
This article is dedicated to Professor Donald W. Boerth with best wishes on the
occasion of his 65th birthday.
⇑
Corresponding author. Tel.: +1 702 895 0885; fax: +1 702 895 4072.
E-mail address: pradip.bhowmik@unlv.edu (P.K. Bhowmik).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.03.024

T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
175
core with bis and trisimidazolium salts exhibit liquid-crystalline
(LC) properties with various organic anions (ꢀI, ꢀBF4, and ꢀN(Tf)2)
[27]. Additionally, organic salts having fluorescent properties are
receiving considerable attention due to their unique attributes.
They not only exhibit tunable electronic and physical characteris-
tics, including solution-based and solid-state fluorescence, but also
offer other advantages ascribed specifically to their charged nature.
For example, benzobis(imidazolium) salts are an excellent class of
such organic salts [28]. Multicharged salts also possess greater
promise as therapeutics (e.g., inhibition of protein kinase C) in sev-
eral animal models than the corresponding monocationic salts
[29,30].
optical microscopy (POM, Nikon, Model Labophot 2) equipped with
a pair of crossed polarizer. Thermogravimetric analyses (TGA) of
these salts were performed using a TGA Q50 instrument at a heating
rate of 10 °C/min in nitrogen. The VTXRD measurements of two such
salts 5 (from 20 to 200 °C) and 6 (from 20 to 80 °C) were carried out
on both samples for both heating and cooling cycles, with tempera-
ture steps of 20 and 10 °C, respectively. Samples were exposed to X-
ray beam for about one hour at each temperature. The XRD patterns
were obtained using the following setup. A linear monochromatic
CuKa1 beam (k = 1.5405 Å) was obtained using a sealed-tube gener-
ator (900 W) equipped with a bent quartz monochromator. The
crude powder was filled in Lindemann capillaries of 1 mm diameter
Stilbazoles and stilbazolium salts are a class of organic crystalline
materials which exhibit many interesting physical, optical, and opto-
electronic properties, which have been studied extensively [31–35].
For example, 4-N,N-dimethylamino-40-N0-methyl-stilbazolium tosyl-
ate, 4-N,N-dimethylamino-40-N0-methyl-stilbazolium p-hydroxy-
benzenesulfonate, 4-N,N-dimethylamino-40-N0-met hyl-stilbazolium
p-aminobenzenesulfonate, and 4-N,N-dimethylamino-40-N0-methyl-
stilbazolium p-dimethylaminobenzenesulfonate are used in a variety
of nonlinear optical devices [36]. Due to their heterocyclic structures,
they have potential to be used as cationic species via quaternization
reaction. In this study, we describe the synthesis of new luminescent
tricationic salts from mesitylene core containing trans-4-octyloxy-40-
stilbazolium moieties with various organic counterions as shown in
Scheme 1. Their chemical structures, thermal, morphological (crys-
talline or amorphous), and optical properties were characterized by
using various experimental techniques.
and 10 lm wall thickness. The diffraction pattern was recorded with
a curved INEL CPS120 counter gas-filled detector linked to a data
acquisition computer; periodicities up to 60 Å can be measured,
and the sample temperature controlled to within 0.05 °C from 20
to 200 °C. In each case, exposure times were varied from 1 to 24 h.
The UV–Vis absorption spectra in organic solvents were recorded
with a Varian Cary 3 Bio UV–Vis spectrophotometer at rt. Photolu-
minescence spectra in solution state were recorded with a Perkin–
Elmer LS 55 luminescence spectrometer with a xenon lamp light
source.
2.2. Synthesis of salt 3
1,3,5-tris(bromomethyl)mesitylene, 1, (0.300 g, 0.752 mmol)
was reacted with excess of trans-4-octyloxy-40-stilbazole, 2,
(0.745 g, 2.41 mmol) in acetonitrile (15 mL) and heated to reflux
for 48 h to give 3. The solvent was removed by a rotary evaporator
to yield the crude salt, which was then recrystallized from ethanol.
The yellow crystals were dried in vacuo at 80 °C for 48 h.
2. Experimental
2.1. Instrumentation
Data for 3: FTIR (KBr, m
max cmꢀ1): 3109, 3032, 2924, 2855, 1636,
1620, 1512, 1458, 1327, 1258, 1204, 1173, 1150, 1042, 972, 887,
841, 741; 1H NMR (400 MHz, CDCl3, ppm) d = 0.83 (t, 9H, ACH2CH3),
1.28–1.45 (m, 30H, ACHaliphatic), 1.76 (q, 6H, AOCH2CH2CH2A), 1.89
(s, 9H, CHaliphatic), 2.38 (s, 9H, CaromCH3), 3.94 (t, 6H, AOCH2CH2A),
6.42 (s, 6H, ACH2NA), 6.88 (d, 6H, Harom), 6.94 (d, 3H, ACH@CHA),
7.52 (d, 6H, Harom), 7.62 (d, 3H, ACH@CHA), 8.03 (d, 6H, Harom),
9.38 (d, 6H, Harom); 13C NMR (400 MHz, CDCl3, ppm) d = 14.1, 18.3,
22.6, 26.0, 29.2, 29.4, 31.8, 59.1, 68.3, 115.1, 120.0, 124.2, 127.5,
128.7, 130.3, 141.4, 143.6, 144.0, 153.6, 161.4; Elemental Anal.
Calcd. for C75H96N3O3Br3 (1327.50): C, 67.87; H, 7.29; N, 3.17.
Found: C, 65.88; H, 7.50; N, 3.07.
The FTIR spectra of ionic compounds were recorded with a Shi-
madzu FTIR spectrometer using their neat films on KBr pellets at
rt. The 1H, 19F and 13C NMR spectra of ionic compounds 3–7 in CDCl3
were recorded by using VNMR 400 spectrometer operating at 400,
376, and 100 MHz, respectively, at rt. Their differential scanning cal-
orimetry (DSC) measurements were conducted using a TA module DSC
Q200 series in nitrogen at heating and cooling rates of 10 °C/min. The
temperature axis of the DSC thermograms was calibrated before using
the reference standard of high purity indium and tin. The polymor-
phism of some of these salts were also examined using a polarized
Scheme 1. Synthesis of tricationic salts of mesitylene and stilbazolium moieties.

176
T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
2.3. General procedure for the synthesis of salts 4–7
29.2, 29.3, 29.4, 31.8, 40.2, 57.8, 68.0, 111.4, 114.8, 119.8, 122.1,
124.1, 125.3, 127.0, 127.2, 128.4, 130.1, 141.2, 143.2, 143.4,
143.6, 152.5, 153.3, 153.4, 161.1; Elemental Anal. Calcd. for
Salts 4–7 were prepared by a metathesis reaction of 3 (1.00 g,
0.753 mmol) with excess (2.41 mmol, 3.2 equiv) of silver tosylate
or lithium triflimide, dioctyl sulfosuccinate sodium salt or methyl
orange in methanol (50 mL) on heating to reflux for 48 h. At the
end of the metathesis reaction, methanol was removed by a rotary
evaporator, and water (50 mL) was poured into the flask to remove
bromide and any excess salts. This step was repeated once or twice
more until all the bromide counterions were completely ex-
changed to desired counterions, which was confirmed by monitor-
ing the 1H NMR spectrum. Finally, they were dried in vacuo at 80 °C
for 72 h.
C117H138N12S3O12 (2000.62): C, 70.24; H, 6.95; N, 8.40; S, 4.81.
Found: C, 69.96; H, 7.09; N, 8.24; S, 4.53.
3. Results and discussion
3.1. Synthesis and chemical characterization
The synthesis of salts 3–7, depicted in Scheme 1, involved the
reaction of 1,3,5-tris(bromomethyl)mesitylene, 1, as an aromatic
core with excess of trans-4-octyloxy-40-stillbazole, 2, in CH3CN
on heating (Menschutkin reaction), and then followed by a metath-
esis reaction of 3 with the corresponding organic counterions with
their respective salts. Their chemical structures were confirmed by
using FTIR (Fig. S1), 1H, 19F and 13C NMR spectroscopy, and elemen-
tal analysis. The characteristic absorption bands of sulfonate and
triflimide groups of both symmetric and asymmetric stretching
frequencies in these salts were in the ranges of 1026–1057 and
1134–1157 cmꢀ1, respectively. In general, these symmetric bands
occur at lower frequencies with organic cations than those with
inorganic cations such as alkali metal ions [37,38].
Data for 4: FTIR (KBr, m
max cmꢀ1): 3117, 3048, 2924, 2855, 1636,
1620, 1597, 1512, 1466, 1258, 1203, 1180, 1157, 1119, 1034, 1011,
972, 841, 818, 679; 1H NMR (400 MHz, CDCl3, ppm) d = 0.89 (t, 9H,
ACH2CH3), 1.29–1.47 (m, 30H, ACHaliphatic), 1.76 (q, 6H,
AOCH2CH2CH2A), 1.98 (s, 9H, CHaliphatic), 2.17 (s, 9H, CaromCH3),
2.32 (s, 9H, CaromCH3), 3.98 (t, 6H, AOCH2CH2A), 6.13 (s, 6H,
ACH2NA), 6.71 (d, 3H, ACH@CHA), 6.88 (d, 6H, Harom), 7.01 (d,
6H, Harom), 7.29 (d, 3H, ACH@CHA), 7.41 (d, 6H, Harom), 7.66 (d,
6H, Harom), 7.74 (d, 6H, Harom), 9.01 (d, 6H, Harom); 13C NMR
(100 MHz, CDCl3, ppm) d = 14.1, 17.3, 21.2, 22.7, 26.0, 29.2, 29.4,
31.8, 57.8, 68.3, 115.1, 120.1, 124.2, 127.5, 128.8, 130.1, 139.2,
141.4, 143.5, 143.9, 144.0, 153.4, 161.4; Elemental Anal. Calcd.
for C96H117N3S3O12 (1599.78): C, 72.01; H, 7.37; N, 2.62; S, 6.01.
Found: C, 69.92; H, 7.48; N, 2.69; S, 5.45.
The compound 1 was prepared from the reaction of mesitylene,
paraformaldehyde, and 33 wt% HBr in acetic acid on heating to re-
flux for 12 h. The trans-4-octyloxy-40-stilbazole, 2, was synthesized
according to the reported procedure [39]. The 1H NMR spectrum of
salt 3 showed unique resonances at d = 9.38, 8.03, 7.52 and
6.88 ppm for the protons of aromatic moieties and a set of reso-
nances at d = 7.62 and 6.94 ppm for the alkene protons of stilbazo-
lium moiety (Fig. S2). To exchange the counterion from bromide to
tosylate, salt 3 was reacted with excess of silver tosylate in meth-
anol by using a metathesis reaction. The salt 4 displayed additional
resonances at d = 7.41, 7.01 and 2.17 ppm from tosylate in its 1H
NMR spectrum (Fig. S4). The proton integral ratio of stilbazolium
salts and tosylate counterions matched to each other indicating
complete exchange of counterions. Furthermore, salt 5 (Fig. S6)
was prepared using the metathesis reaction of 3 with lithium trifli-
mide in methanol. A fluorine peak of triflimide at ꢀ78.7 ppm
appeared in 19F NMR spectrum (not shown) suggesting the pres-
ence of triflimide and the completion of exchange was further con-
firmed by elemental analysis. To have bulkier counterion, dioctyl
sulfosuccinate counterion was introduced in the stilbazolium moi-
eties by the identical metathesis reaction in a common organic sol-
vent to prepare the salt 6. The appearance of additional unique
peaks of dioctyl sulfosuccinate was confirmed by the analyses of
1H and 13C NMR spectra (Figs. S8–S9) that was suggestive of the
successful exchange of counterions. Additionally, methyl orange
counterions were incorporated into tricationic salt 3 to produce
the salt 7 by using methanol as a common organic solvent. The
introduction of methyl orange was revealed by appearance of the
unique six methyl protons signals at 2.98 ppm in its 1H NMR spec-
trum (Fig. S10). Their integral ratio was consistent with stilbazoli-
um moieties resulting in the completion of exchange. The presence
of AN(CH3)2 at 40.2 ppm in 13C NMR spectroscopy further sup-
ported the exchange of counterions (Fig. S11). The chemical struc-
tures of the salts 3–7 were confirmed by the analyses of various
proton chemical shifts in their 1H NMR spectra as listed in Table
1, e.g., the a proton signal of 3 at 6.42 ppm was shifted upfield to
6.13 ppm for 4. Similarly, this proton signal for 5–7 also appeared
further upfield indicating that this proton signal is sensitive to the
nature of counterion associated with each of the salts. Like the pro-
ton signal a, other proton signals b–c are also sensitive to the nat-
ure of counterion present in each of these salts, since they are also
shifted upfield with respect to a. In contrast, the d signals are at
Data for 5: FTIR (KBr, m
max cmꢀ1): 3125, 3071, 2924, 2855, 1620,
1597, 1512, 1466, 1350, 1319, 1258, 1196, 1134, 1057, 972, 841,
787, 756, 741; 1H NMR (400 MHz, CDCl3, ppm) d = 0.88 (t, 9H,
ACH2CH3), 1.29–1.46 (m, 30H, ACHaliphatic), 1.78 (q, 6H,
AOCH2CH2CH2A), 2.29 (s, 9H, CaromCH3), 3.98 (t, 6H, AOCH2CH2A),
5.84 (s, 6H, ACH2NA), 6.90 (d, 6H, Harom), 6.94 (d, 3H, ACH@CHA),
7.54 (d, 6H, Harom), 7.65 (d, 3H, ACH@CHA), 7.91 (d, 6H, Harom),
8.39 (d, 6H, Harom); 19F NMR (376 MHz, CDCl3, ppm) d = ꢀ78.7;
13C NMR (100 MHz, CDCl3, ppm) d = 14.1, 17.6, 22.6, 26.2, 29.2,
29.3, 29.4, 31.8, 57.9, 68.4, 115.1, 115.3, 118.3, 119.5, 121.5,
124.4, 124.7, 127.2, 128.8, 130.4, 142.6, 142.8, 143.9, 154.8,
161.9; Elemental Anal. Calcd. for C78H90N6S6F18O15 (1885.94): C,
49.67; H, 4.81; N, 4.46; S, 10.20. Found: C, 50.57; H, 5.00; N,
4.43; S, 9.81.
Data for 6: FTIR (KBr, m
max cmꢀ1): 3117, 3048, 2932, 2855, 1736,
1620, 1597, 1512, 1466, 1381, 1242, 1204, 1173, 1150, 1034, 887,
841; 1H NMR (400 MHz, CDCl3, ppm) d = 0.77–0.88 (m, 45H,
ACH2CH3), 1.21–1.47 (m, 78H, ACHaliphatic), 1.73 (q, 6H, AOCH2
CH2CH2A), 2.29 (s, 9H, CaromCH3), 2.79–2.96 (m, 6H, CHaliphatic),
3.66 (dd, 3H, CHaliphatic), 3.82–3.91 (m, 12H, CHaliphatic), 4.03(s,
6H, AOCH2CH2A), 5.97 (s, 6H, ACH2NA), 7.02 (d, 6H, Harom), 7.39
(d, 3H, ACH@CHA), 7.70 (d, 3H, ACH@CHA), 7.97 (d, 6H, Harom),
8.17 (d, 6H, Harom), 8.70 (d, 6H, Harom); 13C NMR (100 MHz, CDCl3,
ppm) d = 10.8–11.0, 14.0, 14.1, 17.4, 22.6, 22.9, 23.3, 23.4, 23.6,
26.0, 28.8(d), 28.9(d), 29.1, 29.2, 29.3, 30.0, 30.2, 30.3, 31.8, 34.3,
38.4, 38.5, 38.6, 38.7, 57.7, 62.2, 67.2, 67.7, 67.8, 68.3, 115.0,
119.9, 124.4, 127.4, 128.7, 130.2, 142.0, 143.7, 143.9, 153.9,
161.5, 169.2, 171.7; Elemental Anal. Calcd. for C135H207N3S3O24
(2352.29): C, 69.93; H, 8.87; N, 1.79; S, 4.09. Found: C, 67.91; H,
8.95; N, 1.69; S, 3.93.
Data for 7: FTIR (KBr, m
max cmꢀ1): 3048, 2924, 2855, 1605, 1520,
1466, 1420, 1366, 1312, 1258, 1196, 1180, 1150, 1111, 1026, 1011,
964, 941, 841, 748, 694; 1H NMR (400 MHz, CDCl3, ppm) 0.88 (t,
9H, ACH2CH3), 1.28–1.41 (m, 30H, ACHaliphatic), 1.68 (q, 6H,
AOCH2CH2CH2A), 2.14 (s, 9H, CaromCH3), 2.98 (s, 18H, AN(CH3)2),
3.77 (t, 6H, AOCH2CH2A), 5.87 (s, 6H, ACH2NA), 6.55–6.63 (m,
15H, ACH@CHA and Harom), 7.19–7.27 (m, 9H, ACH@CHA and
Harom), 7.70–7.75 (m, 18H, Harom), 7.94 (d, 6H, Harom), 8.80 (d, 6H,
Harom); 13C NMR (100 MHz, CDCl3, ppm) d = 14.1, 17.3, 22.7, 26.0,

T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
177
Table 1
Chemicals shifts in the aromatic regions of 3–7 from their 1H NMR spectra recorded in CDCl3 at room temperature.
Entry
ArACH2AN+ (ppm) (a)
Aromatic ortho to N+ (ppm) (b)
Aromatic meta to N+ (ppm) (c)
ACH@CHA (ppm) (d)
3
4
5
6
7
6.42
6.13
5.84
5.97
5.87
9.38
9.01
8.39
8.70
8.80
8.03
7.74
7.91
8.17
7.94
7.62 and 6.94
7.29 and 6.71
7.65 and 6.94
7.70 and 7.39
*
–
*
Could not be determined due to the overlapped peaks from methyl orange counterion.
variance with respect to a, since there is no apparent trend in their
chemical shifts (Table 1 and Fig. 1). These results are in excellent
agreement with those observed in imidazolium salts either in ionic
liquids or in polymerized ionic liquids with various organic and
inorganic counterions [40–42]. Additionally, the 1H NMR spectra
shown in Fig. 1 also indicate that the a proton signal (6.42 ppm)
associated with 3 is not present in exchanged salts 4–7, suggesting
that bromide ion was exchanged with a new organic anion in each
of the synthesized salts. Elemental analyses further confirmed the
complete exchange of counterions in these salts.
transitions, which were verified by polarizing optical microscopy
(POM) studies (Fig. 2a). Like the salt 3, 4 containing tosylate coun-
terion also exhibited several endotherms prior to Tm at 265 °C and
then followed decomposition at higher temperature. All of the low-
temperature endotherms are related to polymorphism which was
also verified on observation of highly birefringent crystal phases
(Fig. 2b) by using POM studies (Fig. 2b). There were sharp textural
defects and absence of homogeneity in the domains suggesting
their crystal phases. The polymorphism phenomenon is quite com-
mon in many ionic liquids and liquid-crystalline ionic compounds
[43–46]. Note here that on transforming into liquid phase in the
first heating cycles, their crystallization processes from liquid
phase were slow as indicated in the absence of exotherms in the
cooling cycles (10 °C/min). On melting, both the salts had the ten-
dency to form glassy phase. These results suggested that exchang-
ing the bromide counterions to tosylate counterions did not
significantly depress the Tm of tricationic salt 4 when compared
with that 3, which are also at variance with the results of numer-
ous monocationic salts [1,47].
3.2. Solubility of salts 3–7
The salt 3 was soluble in polar solvent methanol and even in
weakly polar solvent chloroform, but not in ether and hexane. In
contrast, other salts 4–7 were soluble in several organic solvents
ranging from low-polarity to high polarity, i.e., solvents having
low- to high-dielectric constants, but not in ether and hexane.
These solvents included tetrahydrofuran (THF), chloroform, meth-
anol, acetonitrile, and dimethyl sulfoxide (DMSO).
Fig. 3a shows the DSC thermograms of salt 5 obtained at heating
and cooling rates of 10 °C/min in nitrogen. In each of the heating cycles
there were an indistinct low-temperature (100–150 °C) endotherm
and high-temperature (ca. 200 °C) endotherm in its thermogram. Cor-
respondingly, there was a broad exotherm accompanied with a shoul-
der peak in each of the cooling cycles. The low-temperature
endotherm was related to the crystal-crystal transition (Fig. 2c) and
high-temperature one corresponded to its Tm as verified with POM
studies. These results were further verified with variable temperature
XRD studies. Fig. 4a shows the VTXRD patterns of salt 5 obtained on
heating with temperature step of 20 °C to suppress crystallization. It
melted directly into the isotropic liquid at about 200 °C (an accurate
measurement of the temperature is hampered by the large transition
range). Below 200 °C, only crystalline phases were observed, as evi-
denced by the very well structured X-ray patterns. The broad and
reproducible transition observed between 100 and 150 °C, observed
on both heating and cooling, corresponded to a crystal-to-crystal
phase transition. Small, almost undetectable structural changes of
the molecular organization were observed, as showed by the small dis-
placement to lower angle as the temperature was increased, of some of
the sharp reflections positions. On cooling, supercooling aka hysteresis
was observed, and partial crystallization was observed at about 140 °C.
The crystalline phase is likely 3D, and indexation despite the large
number of reflections into a specific space group is irrelevant (and
difficult). Fig. 3b shows the DSC thermograms of salt 6 obtained at
3.3. Thermal transitions, crystalline and amorphous properties, and
thermal stability of 3–7
To evaluate the thermal properties of the salts, differential scan-
ning calorimetry (DSC) and thermogravimetric analysis (TGA)
studies were performed the results of which are summarized in Ta-
ble 2. The salt 3 showed several endotherms prior to its melting
endotherm (Tm) at 267 °C in the first heating cycle, which was then
followed by its thermal decomposition at higher temperature. The
low-temperature endotherms were related to crystal-to-crystal
Table 2
Thermal properties of salts 3–7.
ILs
3
4
5
6
7
Tm (°C)a
Td (°C)b
267
280
265
293
204
317
70
277
230c
239
a
Crystal-to-liquid transition temperature.
Thermal decomposition was recorded at 5 wt% weight loss of a salt occurred in
b
Fig. 1. Expanded aromatic regions of 1H NMR spectra of salts (a) 3, (b) 4, (c) 5, (d) 6,
and (e) 7 (an asterisk indicates CDCl3). Xꢀ in the generalized chemical structure in
this figure represents the corresponding anion.
nitrogen.
c
Determined by using a melting point apparatus.

178
T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
Isotropic liquid
(a)
Crystalline phase
T/°C
180
160
140
120
100
80
60
40
0
5
10
15
20
25
30
2θ/°
2000
1800
1600
1400
1200
1000
800
(b)
Fig. 2. Photomicrographs of compounds (a) 3 taken at 170 °C, (b) 4 taken at 140 °C,
(c) 5 taken at 180 °C under crossed polarizers displaying crystal phases; (d) 6 taken
at 40 °C and (e) 7 taken at 150 °C under crossed polarizers exhibiting amorphous
phase (magnification 400ꢁ).
T/°C
60
50
40
30
600
400
200
0
0
5
10
15
20
25
30
2θ/°
Fig. 4. X-ray diffraction patterns for salts 5 and 6 registered on heating (with
temperature steps of 20 and 10 °C, respectively, to suppress crystallization) at
different temperatures.
heating and cooling rates of 10 °C/min in nitrogen. In the first heating
cycle it exhibited a glass transition temperature (Tg) and a broad endo-
therm (ca. 70 °C) in the first heating cycle. In the cooling cycle, it
showed only a Tg, albeit at significantly lower temperature (Fig. 3b).
In the second heating cycle, there was a Tg at slightly higher tempera-
ture than in the first heating cycle, no other transition at high temper-
ature in the thermogram. In the second cooling, it exhibited a Tg at
lower temperature and no other exotherm in the thermogram. These
results suggested that this salt was amorphous (Fig. 2d) and melted
into isotropic liquid at ca. 70 °C, which were further confirmed with
VTXRD studies. Fig. 4b shows the VTXRD patterns of salt 6 registered
on heating with temperature step of 10 °C to suppress crystallization.
This salt is an amorphous solid until ca. 70 °C, and then it transformed
into an isotropic liquid above this temperature. On cooling, it remains a
liquid. The amorphous solid consists of two broad scattering diffuse
halos at 27.5-28 Å, corresponding roughly to the molecular length in
extended conformation, and at 4.6 Å, corresponding to the molten
chains in liquid-like state.
From the results of tricationic ILs prepared from various fluorine
containing anions (ꢀN(Tf)2, ꢀPF6, ꢀBF4, and ꢀOTf), it is suggested
that fluorine containing anions significantly not only reduced the
Tm values of ILs but also increased the thermal stabilities [25,26].
The Tm value (204 °C) of 5 was significantly lower than those of 3
and 4, which is in good agreement with the results of many ionic
liquids [48]. Moreover, the salt 6 containing the flexible couterions
showed the Tg at ca. 40 °C and Tm at ca. 70 °C, which are relatively
the lowest temperatures among the salts 3–6. These results sug-
gested that it was a tricationic ionic liquid. These properties may
be related to the flexible dioctyl sulfosuccinate counterion. There
Fig. 3. DSC thermograms of salts (a) 5 and (b) 6 obtained at a heating and cooling
rate of 10 °C/min in nitrogen.

T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
179
For salt 7, there was a broad Tg in the first heating cycle, but no
other transition below the decomposition temperature in the thermo-
gram. In the second heating, it exhibited a more prominent Tg at 59 °C
only. In the cooling cycles, there was neither a Tg nor an exotherm in its
thermograms. These results suggested that it was amorphous (Fig. 2e)
and retained this property up to its decomposition temperature.
The Tm (crystalline) and fusion temperatures (amorphous salts)
for the tricationic salts with varying anions were observed as fol-
lows: dioctyl sulfosuccinate (6) < triflimide (5) < methyl orange
(7) < tosylate (4) < bromide (3). In general, the thermal properties
of the ILs are affected by several key factors such as structures
and number of cations and anions [25,26]. These thermal proper-
ties revealed that the tricationic ionic liquid 6 was prepared from
the mesitylene and stilbazolium moieties associated with the flex-
ible, bulky dioctyl sulfosuccinate anion. Note here that mesitylene
and trisimidazolium salts (N-substituted on one side with hexa-
decyl group required for the formation of LC phase) of iodide and
tetrafluoroborate formed thermotropic LC phase at 26 and 34 °C,
which persisted up to their decomposition temperatures, i.e. no
isotropic liquid transitions. In contrast, the tricationic salt of trifli-
mide ions formed a monotropic LC phase, i.e., LC phase was formed
only in the cooling cycle [27].
Fig. 5. TGA plots of compounds 3–7 obtained at a heating rate of 10 °C/min in
nitrogen.
are many reports suggesting that this counterion is capable of
imparting low-melting ionic liquids from many cationic moieties
of varied chemical structures. Furthermore, these ionic liquids also
exhibit an unusual phenomenon of liquid-liquid transitions
[49,50], but this transition was absent in the ionic liquid 6.
The decomposition temperatures (Tds) of 3–7 (temperatures at
which 5 wt% loss of salts occurred) were obtained at a heating rate
of 10 °C/min in nitrogen using TGA measurements. Their TGA plots
Fig. 6. Emission spectra of compound 4 in (a) CH3CN, (b) CH3OH, (c) THF, and (d) CHCl3.

180
T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
Fig. 7. Emission spectra of compound 5 in (a) CH3CN, (b) CH3OH, (c) THF, and (d) CHCl3.
are shown in Fig. 5. They exhibited Td values in the broad temper-
ature ranges of 239–317 °C. For salt 7, its Td was the lowest among
the salts in the series, which was presumably related to the pres-
ence of aliphatic tertiary amine group in its chemical structure.
In the case of 5, Td was the highest among the tricationic salts
examined in this study. This result may be related to the fact that
lithium triflimide (363 °C) has the highest Td among the counteri-
ons employed. Therefore, this property can significantly affect the
Td of the salt [38,51,52].
different, a change in the solvent polarity leads to differential stabil-
ization of the ground and excited states resulting in a change in the
energy gap between these electronic states. However, when com-
paring the effect of chemical structures of salts in the PL results,
there was no significant change implying that the excited states
are more affected by polarity of solvents instead of chemical
architectures.
Salt 3 showed kem = 492 and 474 nm at excitation wavelength
394 nm in CH3OH and CHCl3, respectively (not shown). However,
due to its insolubility in other organic solvents, light-emitting
properties in CH3CN, acetone, and THF could not be measured. Salt
4 displayed emission maxima at 489, 497, 482 and 471 nm with
excitation wavelengths 386, 392, 386 and 392 nm in various or-
ganic solvents (Fig. 6).
On changing the solvent from relatively low polar solvent
(CHCl3) to high polar solvent (DMSO), bathochromic shift in the
emission spectra was observed suggesting the positive solvato-
chromic effect [53,54]. After changing counterion to triflimide,
similar phenomenon was also observed for 5. It exhibited
kem = 482 nm at excitation wavelength 389 nm in THF and dis-
played kem peaks at 494 and 497 nm in MeOH and DMSO, respec-
tively (Fig. 7) indicating that the hydrophobicity of this counterion
had a minor effect in the emission spectra.
To study the effect of size of counterions, the bulky dioctyl sul-
fosuccinate ions were introduced into the salt 6. However, there
were no significant changes in its kem peaks in various organic sol-
vents (Fig. S12). For example, 3 and 6 showed kem = 492 and 494 in
methanol with excitation wavelengths at 394 and 392 nm, respec-
tively. Furthermore, 7 having methyl orange counterion exhibited
3.4. Photoluminescence properties of 3–7
Because stilbazolium moieties, which were present in all the
salts, display the luminescent property, their optical properties
were studied by UV–Vis and photoluminescent spectroscopy in
solutions as well as solid states. Each of the salts 3–7 showed an
almost identical kmax peak between 390 and 407 nm as detected
in their UV–Vis spectra recorded in DMSO, acetonitrile, methanol,
chloroform, and THF. These results could be indicative of closely
spaced p–
p⁄ transitions common to aromatic rings suggesting that
their absorption maxima were less sensitive to the polarity of these
solvents. Furthermore, the size of counterions had a minor effect
on their UV–Vis absorption spectra suggesting that there were no
significant changes occurring in their ground states.
The photoluminescence (PL) results of 3–7 indicated that a
positive solvatochromism effect was observed with the variation
of polarity of organic solvents [53,54]. In general, absorption and
emission spectra depend on the polarity of organic solvents. Since
polarities of the ground and excited state of a chromophore are

T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
181
Fig. 8. Emission spectra of compound 7 in (a) CH3CN, (b) CH3OH, (c) THF, and (d) CHCl3.
kem = 498, 496, 493, 483 and 470 nm at excitation wavelength 388,
390, 390, 386 and 390 in DMSO, CH3CN, MeOH, THF and CHCl3
(Fig. 8). Like other salts in the series, red shift was monitored by
changing the polarity of solvents. These results clearly indicated
that the polarity of solvents has a prominent effect on PL proper-
ties, but they were independent on the chemical architectures of
organic counterions.
The light-emitting properties of 3–7 in their solid states were
also studied by using solution-cast films from CHCl3 onto quartz
plates. Each of them with the exception of 7 showed a broad kem
peak in its emission spectra at 515, 529, 525 and 506 nm when
excited at 457, 400, 395, and 450 nm wavelengths of light, respec-
tively (Fig. 9). In contrast, the light-emission from the salt 7 was
too weak to measure because of the occurrence of the quenching
process. The emission spectra of thin films displayed a large red
shift [54], when compared with those of solution spectra, indicat-
ing more ordered structure formation in the stacking of aromatic
rings in the solid states. Moreover, full-widths at half-maxima
(FWHMs) of 3–6 in thin films were broader than those of solution
spectra suggesting that their light-emission spectra presumably
stemmed from a number of chromophoric species.
4. Conclusions
In summary, a tricationic salt based on stilbazolium moieties of
bromide counterions was synthesized by using quaternization
reaction and subsequently its counterions were exchanged to or-
ganic counterions including tosylate, triflimide, dioctyl sulfosucci-
nate, and methyl orange by metathesis reactions. Their chemical
structures were fully characterized by 1H, 19F, and 13C NMR spec-
troscopy, and elemental analysis. DSC and VTXRD results revealed
that the tricationic salt containing dioctyl sulfosuccinate counteri-
ons was an ionic liquid that transformed into isotropic liquid below
100 °C. The thermal stabilities of the salts were good to excellent
Fig. 9. Emission spectra of compounds 3–6 in thin films cast from CHCl3 at various
excitation wavelengths.

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T.S. Jo et al. / Journal of Molecular Structure 1019 (2012) 174–182
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P.K.B. acknowledges the University of Nevada Las Vegas (UNLV)
for New Investigation Award (NIA), Planning Initiative Award (PIA),
and Applied Research Initiative (ARI) Grants, and the donors of the
Petroleum Research Fund (PRF# 35903-B7), administered by the
American Chemical Society, and an award (CCSA# CC5589) from
Research Corporation for the support of this research. P.K.B. and
T.S.J. also acknowledge the Graduate College (UNLV) for providing
them a SPGRA grant for the years 2010–2013. The work is in part
supported by the NSF under Grant No. 0447416 (NSF EPSCoR
RING-TRUE III), NSF-Small Business Innovation Research (SBIR)
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