Anion-induced ionic liquid crystals of diphenylviologens

Article abstract of DOI:10.1039/c8tc03090a

Two series of ionic liquid crystals derived from diphenylviologens, 1 and 2, were prepared and their photophysical and electrochemical properties were also investigated. The crystallographic data of two single crystals, 1-RSO₃ (n = 10) and 2-BF

Full text of DOI:10.1039/c8tc03090a

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ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

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DOI: 10.1039/C8TC03090A
Anion-induced ionic liquid crystals of diphenylviologens†
RenTzong Wanga, GeneHsiang Leeb and Chung K. Laia*
Two series of ionic liquid crystals derived from diphenylviologens 1-2 were prepared and their
photophysical and electrochemical properties were also investigated. Crystallographic data of two
single crystals 1-RSO3 (n = 10) and 2-BF4 (n = 12) showed that a mono or bilayerlamellar
structure was induced by intermolecular Hbonds. All mesophases were identified as smectic A
phases by POM textures and powder XRD data. A linear correlation plot (R2 = 0.8885) of the
dspacings with the calculated effective radius (Reff) in compounds 2-X was obtained.
Aggregationinduced emission (AIE) ranged from ca. 463529 nm observed in compounds 2-X (e.g.,
X = BF4, PF6, RSO3, OTf and NTf2) was also anions dependent. Also these compounds 2-X exhibited
thermal switch with On/Offemission mode between Cr and SmA phases. On cyclic voltammograms,
compound 1-RSO3 (n = 12) exhibited two distinct reversible redox couples, in contrast the
compound 2-RSO3 (n = 12) formed spikelike voltammetric peak potentials without redox couples.
aDepartment of Chemistry, National Central University, ChungL 32001i, Taiwan, ROC. Email:
cklai@cc.ncu.edu.tw. Fax: +886n03 4277972; Tel: + 886 03 4259207
bInstrumentation Center, National Taiwan University, Taipei 10660, Taiwan, ROC
†Electronic supplementary information (ESI) available: the 1H, 13C and 19F NMR spectra of
compounds 1-2, the DSC and TGA thermographs of compounds 2-NTf2, 2-BF4 and 2-SCN, the XRD
diffraction plots of compounds 2-BF4 and 2-SCN, the UV-Vis and PLspectra of compounds 2-NTf2,
and he cyclic voltammograms of compounds 1-RSO3 and 2-RSO3
1

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I. Introduction
Ionic liquid crystals (ILCs) which can effectively merge the characteristic of liquid crystals (LCs)
with those of ionic liquids (ILs) have entirely distinct properties from those of conventional organic
molecular compounds. Although Knight and Shaw had reported the first pyridiniumbased ILCs1
since 1938, this field of interest has been revived again during the last two decades. A few extensive
reviews2-4 for these ILCs were reported. Original researches described about pyridiniumbased ILCs
were mostly focused on the halide salts.1, 5-11 The mesophases observed were mostly smectic A
phases, in which an ionic bilayer structure sandwiched by the anions between the pyridinium ring
was proposed.5, 8, 11,12 Other counter ions were further reported.12-18 The study of the influence of the
anion has been developed in other series of ionic liquid crystals.19-22 For association to ionic bilayer
mesogens of pyridinium salts, a few series of N,N’-disubstituted-4,4’-bipyridinium salts (viologens)
were investigated.23-41 The ionic interactions inherent, same as pyridiniumbased ILCs tended to
stabilize lamellar mesophases, e.g., SmA,23-27, 30, 34-36 SmB,31 SmC41 and SmX.35, 39, 41 However, they
also displayed other mesophases such as cubics,40 columnar phases29, 33, 37 or lamellar columns.36,40
Among the viologenbased mesogens reported (Fig. 1), most of them were those structures
containing Nalky chains, but not Naryl substituted on the pyridine. This structural limitation might
be due their complicated synthesis. The importance of about Hbonds in ionic liquids42 has been
reviewed by P. A. Hunt. The ionic mesogens induced through Hbond interactions in
pyridiniumbased ILCs has also been described.12
Moreover, the merit in thermochromic,43 photochromic44 or/and electrochromic45 properties of the
viologens has recently attracting significant attention. These technologies have been potential
candidates for applications in various fields, such as singlemolecule transistors,46 LCDs,47 smart
windows or antiglare car mirrors,48-49 OLEDs,50 charge storage51 and sensors.52 In the field of
luminescent liquid crystals, the aggregationcaused quenching (ACQ) effect is common to most
luminophores, and the  stacking interactions existent between well packing molecules were
attributed or/and prompted to the formation of excimers, leading to the observed ACQ effect.
2

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Another photophysical phenomenon in sharp contrast to the ACQ effect is aggregationinduced
emission (AIE). An AIE process associated with possible mechanisms, such as restriction of
intramolecular motion (RIM), Jaggregate formation (JAF), twisted intramolecular charge transfer
(TICT) and excitedstate intramolecular proton transfer (ESIPT) has been summarized by B. Z.
Tang.53 Based on the RIM process, viologenbased ILCs which were not luminescent or weak in
solution but exhibited strong fluorescence in aggregation or solid state might provide the possibility
to fabricate novel AIEactive ILCs by introducing peripheral counter ion or flexible alkyl chains to
the viologen cores.
Fig. 1 Known examples of viologenbased mesogens with Nalkyl chain substitution.
In this work, instead of using a soft long Nalkyl chain, ionic mesogens with a rigidcore of
Nphenylpyridinium chlorides prepared via Zincke reactions54 were reported. Among them, two
types of ionic structures were constructed. One series of diphenylviologen 1-RSO3 has alkyl chains
incorporated as counter anion while the other series are di(alkoxy)phenylviologens 2-X with a
variety of counter anions incorporated; X = bis(trifluoromethane)sulfonylimide (NTf2 ),

)
alkylsulfonate (RSO3 , triflate (OTf), tetrafluoroborate (BF4 ), hexafluorophosphate (PF6 ) and
thiocyanate (SCN). Fig. 2 shows the molecular structures of ionic mesogens with rigidcore of
diphenylviologens 1 and di(4alkoxyphenyl)viologens 2. In order to understand the steric hindrance,
correlation of the alkyl chain length and/or the counter anion on the formation of the mesophases in



such types of ionic bipyridiniumbased ILCs, a systematic study on a total number of 13 ionic
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compounds was prepared and investigated. The mesomorphic properties of these ionic compounds
were studied by use of POM, DSC, TGA and XRD.
Fig. 2 The molecular structures of diphenylviologens 1-RSO3 and di(4alkoxyphenyl)viologens
2-X.
The crystallographic data obtained from two representative crystals 1-RSO3 (n = 10) and 2-BF4 (n =
12) established the crystal structures and molecular structures in this type of ILCs. Powder XRD data
also supported the proposed model of the anions sandwiched between the bipyridinium rings.5, 8, 11, 18
XRD data indicated that a monolayer and a bilayer lamellar structure in the crystal lattices was
formed for di(4-alkoxyphenyl)viologens 2 and diphenylviologens 1, respectively. There continues to
be a growing interest in recent developments44, 55-59 for AIEactive viologen derivatives. Both
photophysical and electrochemical properties of the rigidcore of viologens 1-2 were also studied.
II. Results and discussion
Synthesis and characterization
The synthetic pathways to compounds 1-2 are listed in Scheme 1. The precursors of diphenyl
viologen dichloride 1-Cl and di(4-alkoxyphenyl)viologen dichlorides 2-Cl (R = CnH2n+1, n = 10, 12,
14) via Zincke reactions followed by the methods in the literature60-61 were obtained. These
viologen‒based precursors were identified by 1HNMR and 19FNMR spectroscopy. For example, 
7.80–7.82 (m, 6H) and 8.01 (dd, 4H, J = 7.5, 1.5 Hz) were assigned to the Ph‒H and  9.15 (d, 4H, J
= 6.6 Hz), 9.75 (d, 4H, J = 6.6 Hz) were assigned to the Py‒H for diphenyl viologen dichloride 1-Cl.
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Similarly, two characteristic peaks at  7.30 (d, 4H, J = 9.0 Hz) and 7.86 (d, 4H, J = 8.7 Hz) were
assigned to the Ph‒H and the other peaks at  8.88 (br, 4H) and 9.49 (br, 4H) were assigned to the
Py‒H for di(4alkoxyphenyl)viologen dichloride 2-Cl) Then methanol used as a phasetransfer
medium was added in the metathesis of counter anions. The soft counter anion salt solution, for
example sodium alkylsulfonate (RSO3Na), sodium tetrafluoroborate (NaBF4), potassium
hexafluorophosphate (KPF6), lithium triflate (LiOTf), lithium bis(trifluoromethane)sulfonimide
(LiNTf2) or potassium thiocyanate (KSCN) dissolved in water was added to their precursors 1-Cl or
2-Cl dissolved in methanol/dichloromethane (CH3OH/CH2Cl2 = 1/5). The reaction was continuously
stirred at 35–40 C for 3 h. This ionexchange reaction was monitored using an alumina TLC plate
with an eluent of CH3OH/CH2Cl2 (5/95) in which the retention factor (Rf ~ 0.6) of the product was
higher than the Rf ~ 0.1 of the precursor. The organic layer was passed through a short alumina
(Al2O3) flash column. The products were obtained after recrystallization from isopropanol/ethyl
acetate mixture. The 13CNMR spectra of compounds 2-X were not obtained due to their poor
solubility in common solvents. The 1H or 19Fnuclear magnetic resonance spectroscopy (NMR)
was used to identify their structures, for example, on 1HNMR spectra, all the PyHpeaks for
bipyridinium based derivatives 1-X and 2-X were easily differentiated with their precursors 1-Cl
and 2-Cl. The individual peak integration of bipyridinium and alkylsulfonate for either 1-RSO3 or
2-RSO3 was equivalently obtained, confirming the stoichiometric coefficient for each moiety of the
ion pair. On 19FNMR spectra, for example, the characteristic peaks appeared at  = 153.57 and
153.62 (s, BF4 ), 74.4 (d, J = 707 Hz, PF6 ), 79.6 (s, OTf) and 80.2 (s, NTf2 ) identified and
confirmed all of the fluorinecontaining compounds. Positive (or negative) ion moieties were
recorded using the electrospray ionizationhigh resolution mass spectrometry (ESIHRMS) to
confirm the target structures.



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Single crystal and molecular structures of N,N’-diphenyl-4,4’-bipyridinium di(decylsulfonate)
1-RSO3 (n = 10) and N,N’-bis(4-dodecoxyphenyl)-4,4’-bipyridinium di(tetrafluoroborate) 2-BF4
(n = 12).
In order to understand the correlation between the ionic molecular structures and mesomorphic
behavior of these two types of ionic mesogens, two single crystals of the mesogenic compounds
1-RSO3 (n = 10) and 2-BF4 (n = 12) suitable for crystallographic analysis were obtained by slow
vaporization from mixed solvent of isopropanol/ethyl acetate at room temperature and their
structures resolved. Fig. 3 shows two molecular structures with the atomic numbering schemes.
Table 1 lists their crystallographic and structural refinement data for the two crystals. They all
crystallize in a triclinic space group P-1 with a Z = 1 and Z = 2, respectively.
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Table 1 Crystallographic and refinement details for compounds 1-RSO3 and 2-BF4.
DOI: 10.1039/C8TC03090A
1-RSO3 (n = 10)
2-BF4 (n = 12)
Empirical formula
Formula weight (g mol-1)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C42 H68 N2 O10 S2
825.10
200(2)
1.54178
Triclinic
C46H67B2F8N2O2.50
861.63
200(2)
1.54178
Triclinic
P-1
P-1
a (Å)
b (Å)
c (Å)
 (°)
6.2986(10)
9.3042(2)
18.9025(4)
83.2337(7)
85.7251(71)
86.4000(8)
1095.34(4)
1
7.7273(2)
10.0812(3)
30.1503(9)
98.0699(10)
93.6023(11)
93.0372(11)
2316.32(11)
2
 (°)
 (°)
Volume (Å3)
Z
Density (calculated) (Mg/m3)
Crystal size (mm3)
θ range for data collection (o)
Data/restraints/parameters
Final R1, wR2
1.251
1.235
0.253x0.115x0.085
2.359 to 69.996
4147/6/291
R1 = 0.0395,
wR2 = 0.1063
0.353x0.223x0.026
2.968 to 69.985
8748/16/572
R1 = 0.0692,
wR2 = 0.1827
The overall molecular shapes of two crystals were considered as linear shapes, and the molecular
lengths were ca. 36.62 Å (atom C21‒C21 in 1-RSO3) and 47.64 Å (atom C23‒C23 in 2-BF4). Both
bipyridinerings in crystals 1-RSO3 and 2-BF4 were nearly coplanar, leading to a small dihedral
angle of 1.1 and 0.4. The small dihedral angles obtained were quite different from twisted
bipyridine rings of other dialkylviologens.38 A relatively large dihedral angle of ca. 32.5‒47.0
between pyridine and phenyl ring in these diphenylbipyrindiniums was observed. Two counter ions
(decyl)SO3¯ and BF4¯ in both crystals were orderly intercalated between the bipyridinium layers.
Interestingly, two hydrated water molecules were cooperated in crystal lattice 1-RSO3.
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Fig. 3 Two ORTEP plots of 1-RSO3 (n = 10, top) and 2-BF4 (n = 12, bottom) with the atomic
numbering schemes, thermal ellipsoids are shown at the 50% level. Hydrogens were omitted
for clarity.
In the crystal lattices, intermolecular Hbonds and Coulombic forces between cations and anions
were found to be more dominant. Indeed, Hbonds with a weak to medium strength were observed,
for example, Hbonds (CH…O) of bipyridiniumH and sulfonateO anion or hydrated water in
crystal 1-RSO3 (n = 10), Hbonds CH…F of bipyridiniumH and borateF in crystal 2-BF4 (n = 12)
were all observed. These Hbond lengths ranged approximately from d = 1.947  2.507 Å (in
1-RSO3), from d = 2.286  2.591 Å (in 2-BF4, Fig. 4). The formation of Hbonds was responsible
for a better or preferred packing both in the solid or/and the liquid crystal states. Some Hbonds
observed between the cations and anions in these two crystals are listed in Table 2.
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Fig. 4 Hbonds (red dash line) connected around the bipyridinium with counter anions observed in
crystals 1-RSO3 (n = 10, left plot) and 2-BF4 (n = 12, right plot).
Table 2 Intermolecular Hbonds observed in crystal 1-RSO3 and 2-BF4.
Compd.
No. of H-bonds
Bond distance/Å
Bond angle/o
1-RSO3
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
O3H4A = 1.947
H5BO4 = 1.987
H4O5 = 2.027
O1H4B = 2.137
H1O2 = 2.263
H7O5 = 2.310
H2O5 = 2.400
H8O2 = 2.450
H11O1 = 2.507
H1F8 = 2.286
H11F1 = 2.487
H7F2 = 2.509
H5F7 = 2.522
H1F6 = 2.584
H5F5 = 2.591
 S1O3−H4A = 122.8
 O5H5B−O4 = 170.4
 C4H4−O5 = 163.8
 S1O1−H4B = 103.8
 C1H1−O2 = 162.9
 C7H7−O5 = 138.6
 C2H2−O5 = 171.0
 C8H8−O2 = 151.3
 C11H11−O1 = 171.0
 C1H1−F8 = 169.4
 C11H11−F1 = 152.4
 C7-H7−F2 = 120.8
 C5-H5−F7 = 126.9
 C1H1−F6 = 124.7
 C5H5−F5 = 144.2
2-BF4
On the other hand, Coulombic forces and intermolecular  interactions in these ILCs were also
critical in these lattices, as shown in Fig. 5. Considering the formal charge between the more positive
+
N atoms and nearby standing negative F (or O) atoms, the crystal packing model was mainly
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driven by the Coulombic interactions. Crystallographic data indicated that the distancDeOoI:f10t.h10e39/C8TC03090A
Coulombic force when packed with sulfonate anion was approximately d = 3.9914.042 Å in
1-RSO3, which was weaker than those in 2-BF4, d = 2.8822.922 Å. Furthermore, weak
intermolecular  interactions between the neighboring diphenylbipyridiniums were also observed
in crystals 1-RSO3. A  distance ranged at d = 3.6803.756 Å was obtained in 1-RSO3. However,
a distance of d = 7.727 Å showed no intermolecular  interactions in 2-BF4 due to the intercalated
counter anions. The Coulombic forces (d = 3.9914.042 Å) were much weaker than the Hbonds (d
= 1.9472.507 Å) in 1-RSO3, whereas the Coulombic forces (d = 2.8822.922 Å) were just slightly
weaker than the Hbonds (d = 2.2862.591 Å) in 2-BF4. In contrast, the intermolecular 
interactions (d = 3.6803.756 Å) were slightly stronger than the Coulombic forces (d = 3.9914.042
Å) in 1-RSO3.
Fig. 5 Coulombic forces (red dash line) and intermolecular interactions (blue dash line) observed in
1-RSO3 (n = 10, left) and 2-BF4 (n = 12, right) in the crystal lattices.
All the terminal alkyl chains were interdigitated between layers by a style of layerbylayer and
headtotail arrangements (see Fig. 6). The lamellar bilayer structures in compounds 1-RSO3 were
formed in a headtohead arrangement with sulfonatehead groups in apposition. Bipyridiniums
were aligned in a linebyline style; the counter alkylsulfonate anions were similarly intercalated or
linked (Fig. 6). However, monolayered lamellar structures in compounds 2-BF4 were formed by an
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endtoend style in which the counter BF4¯ was in fact sandwiched between the cationic
bipyridinium rings (Fig. 6). The bipyridiniums in compound 1-RSO3 inclined at the lamellar level,
but the di(alkoxyphenyl)bipydiniums in compounds 2 stood at the upright direction.
Fig. 6 Crystal packing viewed down aaxis. Interdigitated bilayers in diphenylviologen 1-RSO3 (n
= 10, left plot) and interdigitated monolayers in di(dodecoxyphenyl)viologen 2-BF4 (n = 12, right
plot).
Mesomorphic properties and thermal stability
The thermal behavior of compounds 1-RSO3 and 2-X (X = RSO3, NTf2, OTf, BF4, PF6 and SCN)
was investigated using polarized optical microscopy (POM) and differential scanning calorimetry
(DSC). The transition temperatures and enthalpies of the compounds 2 are listed in Table 3. All
these viologens 1-2 exhibited mesogenic behavior, which was expected for rodlike molecules. On
cooling from the isotropic liquid phase, smectic A phase with a focalconic texture or homeotropic
texture was common observed (Fig. 7). These SmA phases were also confirmed as bilayer structures
for 1-RSO3 or monolayer structures for 2-X by using powder Xray diffraction experiments (see
discussion later). For the series of viologen 1-RSO3, the melting temperature remained at Tmp
=
88.7‒113.1 C, whereas the clearing temperature (Tcl) increased with carbon length, i.e., Tcl = 217 (n
= 10) < 256 (n = 12) < 275 C (n = 14). The temperature range of SmA phases increased with carbon
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length, TSmA = 118.5 (n = 10) < 196.3 (n = 12) < 223.0 C (n = 14) on cooling.
For the series of viologens 2-X, all compounds exhibited SmA phases. However, the clearing
temperature was quite sensitive to the anions incorporated; Tcl = 337.0 (2-OTf) > 336.0 (2-BF4) >
312.0 (2-PF6) > 304.8 (2-NTf2) > 274.0 (2-SCN) > 220.2 C (2-RSO3) for the n = 12 derivative.
This decreasing in clearing temperature was not quite correlated with effective radius of the anions
(discussed later). Not surprisingly, compound 2-RSO3 substituted with both alkyl-chains at cation
and anion moieties exhibited higher melting temperatures by Tmp = 31.7 (n = 10) < 43.8 (n = 12) <
50.0 C (n = 14) and lower clearing temperatures Tcl = 0.3 (n = 10) < 35.8 (n = 12) < 71.3 C (n =
14) than those of compounds 1-RSO3. This change in both clearing and melting temperature led to a
smaller temperature range of SmA phase, i.e. TSmA = 92.4 (n = 10) ‒ 86.0 C (n = 14) which might
be attributed to the much more hydrophobic interaction than Coulumbic and Hbond interactions.
Similarly, for the series of viologens 2-NTf2, the melting temperature remained at 103.8‒104.5 C,
but the clearing temperature (Tcl) increased with carbon length, i.e., Tcl = 254.3 (n = 10) < 304.8 (n =
12) < 328.0 C (n = 14). The temperature range of SmA phase increased with carbon length, i.e.
TSmA = 156.3 (n =10) < 203.3 (n = 12) < 228.8 C (n = 14) on cooling. The dependence of the
mesogenic behavior was often observed in melting and clearing temperature increase with elongation
of alkyl chain.7
Fig. 7 Optical textures observed: (a) SmA phase by 1-RSO3 (n = 10) cooling at 210 C; (b) SmA
phase by 2-NTf2 (n = 10) at 247 C; (c) SmA phase by 2-RSO3 (n = 10) at 205 C.
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In order to understand the effect of counter anions incorporated in such type of mesogenic
diviologen 2-X, six compounds with different anions (X = BF4, PF6, OTf, NTf2, RSO3, SCN) were
prepared and their mesomorphic behavior was compared. Surprisingly, all compounds 2-X were
mesogenic regardless of the type of counter anions, as shown in Fig. 8. These bipyridiniumbased
mesogens showed a higher Tcl and wider TSmA than those of pyridiniumbased mesogens in the
literatures.12 Four ionic salts with small anionic moiety (X = BF4, PF6, OTf, SCN) resulted in
decomposition before clearing point. An extra more ordered SmX phase for compounds 2-BF4 or
2-SCN were observed which was also confirmed by powder XRD (see discussion later).
Interestingly, a new volumebased approach predicting thermophysical behavior of ionic liquids
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and ionic liquid crystals was reported by Nelyubina.62
DOI: 10.1039/C8TC03090A
Fig. 8 Bar graphs showing the thermal mesomorphism of compounds 2-X (for all n = 12).
The thermal stability of compounds 2-X (n = 12) was also performed by thermogravimetric analyses
(TGA) under nitrogen atmosphere, shown in Fig. 9. Different counter anion showed different
thermal stabilities. All six compounds showed good thermal stability at a temperature below ca. 270
C. The decomposition temperatures were obtained for a 5% weight loss. The relative stability of
these compounds 2-X followed by a Tdec = 344.0 (2-NTf2) > 337.2 (2-OTf) > 336.5 (2-BF4) > 312.5
(2-PF6) > 299.5 (2-RSO3) > 274.5 C (2-SCN).
Fig. 9 The TGA thermograms of compounds 2-X.
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DOI: 10.1039/C8TC03090A
Powder Xray diffractions
Variable temperature powder Xray diffraction experiments were performed to confirm the structure
of the SmA phases for compounds 1-2. All the samples (except 2-BF4, 2-PF6, 2-SCN) were first
heated above their Tcl, then cooled down to a mesophase temperature and the diffraction data were
collected. The Xray diffraction data are summarized in Table 4. All compounds 1-2 were
rodshaped molecules, and they exhibited SmA phases. In most cases, all XRD patterns showed a
typical lamellar diffraction pattern of layered structures in which one strong peak and one very weak
peak at small angle region and also a broad halo peak at large angle were observed. These two
diffraction peaks at lower angle were assigned as index 001 and 002, which were typically
characteristics of layer structures observed for a SmA phase. For example, for compounds 1-RSO3 (n
= 12) at 180.0 C a diffraction pattern with one strong peak at 27.1 Å and one very weak peak at 13.6
Å at small angle region was observed (Fig. 10). The lamellar thickness of 27.1 Å for compound
1-RSO3 (n = 12) with a bilayer structure was slightly shorter than that of compound 2-RSO3 at 30.8
Å with a monolayer structure. Other compounds 2-X exhibited similar diffraction patterns of SmA
phase as compound 1-RSO3. However, the layer distance or thickness for the compounds 2-X in
SmA phase was sensitive to the compounds; d = 38.9 (2-SCN) > 36.2 (2-BF4) > 35.6 (2-PF6) > 34.8
(2-OTf) > 30.8 (2-RSO3) > 30.5 Å (2-NTf2). Additional two peaks at 13.0 and 9.8 Å for compound
2-SCN assigned as indices 003, 004, and one peak at 11.7 Å for compound 2-OTf assigned as 003
were also observed. Furthermore, two compounds 2-BF4 and 2-SCN also showed a SmX phase at
lower temperature at 140 and 150 C, respectively.
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Fig. 10 The powder Xray diffraction plots by compound 1-RSO3 (n = 12, top) at 180 C, compound
2-RSO3 (n = 12, middle) at 210 C and compound 2-NTf2 (n = 12, bottom) at 180 C.
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Table 4 Detailed indexation by powder XRD for compound 1-2 (all n = 12)
Compds
Mesophase
temp.
dSpacing(Å)
obs. (calcd.)
Miller
indices
a
1RSO3
SmA at 180 C
27.1 (27.1)
13.6 (13.6)
5.0 (br)
001
002
halo
2SCNb
SmX at 140 C
39.9 (39.9)
19.9 (20.0)
13.3 (13.3)
9.9 (10.0)
4.7 (br)
001
002
003
004
halo
halo
001
002
003
004
halo
4.2 (br)
SmA at 160 C
38.9 (38.9)
19.4 (19.5)
13.0 (13.0)
9.8 (9.7)
4.2 (br)
b
2BF4
SmX at 150 C
SmA at 160 C
36.0 (36.0)
18.1 (18.0)
4.7 (br)
36.2 (36.2)
18.2 (18.1)
4.8 (br)
001
001
halo
001
002
halo
b
2PF6
SmA at 260 C
SmA at 210 C
35.6 (35.6)
17.9 (17.8)
4.7 (br)
001
002
halo
2OTfa
34.8 (34.8)
17.5 (17.4)
11.7 (11.6)
4.8 (br)
001
002
003
halo
a
2RSO3
SmA at 210 C
SmA at 180 C
30.8 (30.8)
15.5 (15.4)
5.0 (br)
001
002
halo
a
2NTf2
30.5 (30.5)
5.0 (br)
001
halo
a: temperature taken on the cooling process.
b: temperature taken on the heating process.
The dependence of dspacing in the layered SmA phase with the carbon lengths was also performed.
The plots of lamellar thickness with various carbon length for 1-RSO3 (R2 = 0.9954), 2-RSO3 (R2 =
0.9409) and 2-NTf2 (R2 = 0.9995) were shown in Fig. 11. For example, all XRD data showed a good
17

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correlation of the dspacing with the carbon length, for example, i.e. d = 24.7 (n = 10) < 27.3 (n = 12)
< 29.5 Å (n = 14) in compounds 1-RSO3. The other two series of compounds 2-RSO3 and 2-NTf2
showed a similar trend of dspacing with carbon length.
Fig. 11 The correlation of dspacing of Lamellar thickness with carbon length in 1-RSO3, 2-RSO3
and 2-NTf2.
Another interesting feature was observed on the effect of dspacing with counter anions in
compounds 2-X. The effective ionic radii are calculated assuming that the studied ions have a
spherical shape by the equation of V = (4/3)R3. The molecular volume (V) of various anions were
provided to calculate the effective radii (Reff) of anions: [SCN] = 1.93 Å,63 [BF4] = 2.05 Å,64 [PF6]
= 2.59 Å,65 [TfO] = 2.75 Å65 and [Tf2N] = 3.72 Å.66 The results appeared that the d‒spacing
decreased with increasing ion radius (Fig. 12, R2 = 0.8885). Those results showed negatively
correlated with the XRD data obtained; larger dspacing for smaller anions (i.e., SCN) and smaller
dspacing for large anions (i.e. TfO, Tf2N and RSO3 ). The broad halo peaks also showed an



positive correlation; i.e. d = 4.2 Å (smaller SCN) < d = 4.74.8 Å (medium anion, BF4 , PF6 , OTf)


< d = 5.0 Å (larger anion, NTf2 or RSO3 ). The degree of alkoxy interdigitation might be attributed
to the change in the lamellar distance; the more interdigitating for large counter anions, the shorter
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dspacing it exhibited.
Fig. 12 The dependence of the dspacing with the effective radius of counter anions in compounds
2-X.
Photophysical and electrochemical properties
Photophysical characteristics for all viologens 1-2 (all n = 12) are listed in Table 5. The absorption
max peak of diphenylviologen 1-RSO3 in methanol at 314 nm is attributed to ‒* transition.
Di(4-alkoxyphenyl)viologens 2-RSO3 exhibits a redshift by max= 68.2 nm due to the
incorporation of two more electrondonating alkoxy chains (Fig. 13). All max for compounds 2-X
were not sensitive to the counter anions, and they were all ranged in max = 378.6385.0 nm due to
their similar core structures. Interestingly, the absorption max peaks for these viologens 1 or 2 in
methanol at 314 nm or 379‒385 nm are shorter than those in dichloromethane at 320 nm or 396‒407
nm. These negative solvatochromism corresponded to hypsochromic shift (or blue shift) with
increasing solvent polarity. All methanol solutions of viologens 1-2 were detected without any
observable signals on photoluminescence. But the 2-RSO3 will aggregate in the aqueous mixture
with high water fractions (ƒw) needed to turn on their light emission process. As shown in Fig. 14,
weak PL signals are recorded at ƒw < 50% because the luminogens 2-RSO3 and 2-NTf2 are fully
dissolved and quenched in these solvent compositions. The PL intensity started to enhance at ƒw >
50% till it reached to the maximum intensity at ƒw ~ 70%. Normally, these viologenbased
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luminogens except 1-RSO3 and 2-SCN emitted variable lights at the maximum wavelength of
463‒529 nm from skyblue to yellow. The quantum yields were ranged in 0.014‒0.097 (Fig. 13 &
Table 5). Surprisingly, the representative solidfilm of viologen 2-NTf2 exhibited an onoff mode
for photoluminescence when excited at 522 nm (onmode) in crystal phase and quenched (offmode)
in SmA phase (Fig. 15). The flexible alkyl chains enhanced the nonradioactive relaxation while
melting at SmA phase.
Table 5 Summary of photophysical data for compounds 1-2 (all n = 12).
Compds
Absmaxa/nm
Absmaxb/nm
AIE maxc,d/(Φ/nm)
1-RSO3
2-RSO3
2-BF4
314.0
382.2
384.4
380.4
382.8
385.0
378.6
320.0
395.5
399.0
402.0
400.5
406.5
400.5
n.d.
529.0 (0.078)
523.5 (0.014)
463.0 (0.078)
478.7 (0.068)
508.1 (0.097)
n.d.
2-PF6
2-OTf
2-NTf2
2-SCN
a: The solution (10 ppm) in methanol was measured at room temperature.
b: The solution (10 ppm) in CH2Cl2 was measured at room temperature.
c: Quantum yield was calculated with anthracene in ethanol (Φ =0.27) as the standard.
d: The suspended mixture (20 M in 70%ƒw water/methanol) was measured at room
temperature.
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Fig. 13 (a) UV absorption of compounds 1-2 in MeOH (solid lines) & CH2Cl2 (dash lines); (b)
fluorescence spectra and photographic aggregation images for compounds 1-2 in 70% ƒw
water/methanol mixtures (The insert graphs were taken under UV illumination).
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Fig.14 (a) Photoluminescence spectra of compound 2-RSO3 (n = 12; left plot) in methanol and
water/methanol mixtures (concentration  20 M); (b) Plots of the emission intensities v.s. water
fractions (ƒw) for 2-RSO3 (n = 12, right plot) & 2-NTf2 (n = 12, right plot) in methanol and
water/methanol mixtures (concentration  20 M). PL Spectra were excited at 525 and 505 nm for
2-RSO3 and 2-NTf2.
Fig. 15 Photoluminescence spectra (redline in Cr phase and blueline in SmA phase) and the two
photographs of 2-NTf2. The test chip was measured by Konica Minolta CS1000S spectro
radiometer under 365 nm UV light irradiation and the inserted graphs were taken under UV
illumination.
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The electrochemical redox reactions of viologens (Scheme 2) were often followed byDaOIt:w10.o1039/C8TC03090A
sequential oneelectrontransfer reactions; from divalent cation (Vio2+) to form firstly a radical
cation (Vio˙+) and secondly a neutral bipyridinylidene (Vio0). The E1 and E2 values were obtained
from the potential of the first and second redox process.
Scheme 2 Electrochemical redox reactions in viologens.
For the voltammetric measurements, use of the half wave potential E = (Ep(anodic) + Ep(cathodic))/2 can
be compared to the value EFc of the ferrocene/ferrocenium as the internal standard to determine the
relative potential.67-68 Fig. 16a presents typical cyclic voltammograms with reversible couples for the
first redox process (Vio2+/Vio˙+) of viologens 1-RSO3 and 2-RSO3 at varying sweep rates from 0.025
to 1 V/s. Peaks separation increased with increasing scan rate. Good linear tendencies (R2 = 0.9953,
0.9937) of anodic peak current on square root of sweep rate for their reductions (Vio2+/ Vio˙+)
observed at Fig. 16b indeed well met the Rendles–Sevcik equation; Ip = 0.4463
F3/2R-1/2n3/2c0AD1/21/2, where is the sweep rate and v is the sweep rate.
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Fig. 16 Cyclic voltammetry for compounds1-RSO3 and 2-RSO3 at varying sweep rates (top plots);
dependence of anodic peak current on square root of sweep rate for their reductions (Vio2+/ Vio˙+)
(bottom plot).
Moreover, the viologen 1-RSO3 (n = 12) exhibits two distinct, reversible redox couples on cyclic
voltammogram, and its half wave potentials E1 = 0.718 and E2 = 0.970 V (vs EFc) measured in
DMSO (Fig. 17) were similar to those performed in DMF.69 The viologen 2-RSO3 containing long
chains exhibited E1 = 0.727V (vs EFc) and spikelike voltammetric peak potentials (EPc-1 = 0.055
and EPa-2 = 0.315 V; vs Ag/AgCl) without redox couples on its cyclic voltammogram. This
behavior has been concluded70-72 that the spike response originates from potentialdriven phase
transition associated with reduction of the long chain containing viologen. It leaded to form a 2D
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condensed film (Fig. 18) on the graphite cathode surface, was different to that nonalDkOyI:l1c0.h10a3i9n/C8TC03090A
containing viologen formed gaslike adsorption layer. The results of bulk materials above may
develop new prospects for redoxactive (or electrochromic) liquid crystals.
Fig. 17 Voltammograms of viologens: (a) two reversible redox couples of 1-RSO3 (left)
and (b) spikelike peaks of 2-RSO3 (right).
Fig. 18 Different behaviors of viologens 1(left) and 2 (right) exhibited on an electrode surface.
III. Experimental
General materials and methods
All chemicals and solvents were reagent grade from Alfa Aesar, Sigma Aldrich or Acros chemical Co.
and used as received.
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NMR analysis: 1HNMR (300 MHz), 13CNMR (75 MHz) and 19FNMR (282 MHzD)OsIp: 1e0c.1t0r3a9/wC8eTCre03090A
recorded on Bruker DRS300.
Elemental analyses: Elemental analyses were made on Elementar vario EL III.
ESIMS analysis: Mass spectra of positive (or negative) ion moiety were recorded on JEOL
JMS700 by electrospray ionization method.
DSC analysis: Differential scanning calorimetry (DSC) measurements were performed on samples of
about 3mg. Phase transitions were measured on Mettler DSC822 and calibrated with a pure indium
sample. All phase transitions were determined by a scan rate of 10.0 C/min. The heating and cooling
rates were 10°C/min.
TGA analysis: All measurements were performed on samples of about 4.0 mg determined on Mettler
SDTA 851 by a scan rate of 20.0 C/min.
Thermooptical polarizing microscopy: Zeiss Axioplan 2 polarising microscope was used; equipped
with a Mettler FP90/FP82HT hot stage. Pictures were taken using a digital Zeiss AxioCam MRC4
camera with a resolution of 4 megapixels in combination with Zeiss AxioVision software.
PXRD: The powder diffraction data were collected from the WigglerA beam line of the National
Synchrotron Radiation Research Center (NSRRC) with a wavelength of 1.3223Å. The powder
samples were charged in Lindemann capillary tubes (80 mm long and 0.01mm thick) purchased from
Charles Supper Co. with an inner diameter of 1.0 mm,
UVVis spectrometry: Hitachi U3300 UVVis. spectrometer was used and all measurements were
performed on 10 ppm various sample solution at room temperature.
Photoluminescence spectrometry: All measurements using Horiba FluoroMax3 were performed on
various samples of 20 M solution or suspension while excited by the appointed wave length.
Cyclic voltammetry: All measurements were performed on various samples with 0.05M
tertabutylammoniumBF4 of DMSO solution. The solution filled in the cell with Ag/AgCl reference
electrode, Pt auxiliary electrode and Graphite working electrode was carried out under N2 gas
atmosphere to obtain the voltammogram.
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Synthesis of N,N’-bis(2,4-dinitrophenyl)-4,4’-bipyridinium dichloride60 3. 4,4’-Bipyridine (2.0 g,
12.8 mmol) and 2,4-dinitrochlorobenzene (8.6 g, 42.4 mmol) were dissolved in acetonitrile (25 mL).
The initially clear reaction mixture became gradually muddy and the solution was then stirred at
80 C for 24 h. Solid was collected through filtration. The product isolated as white powder was
obtained after washing with warm acetonitrile and desiccating in a vacuum oven. Yield 97%;
1H‒NMR (300 MHz, CD3OD): δ 8.43 (d, 2H, PhH, J = 8.7 Hz), 8.99 (dd, 2H, PhH, J = 2.4, 8.7
Hz), 9.11 (d, 4H, PyH, J = 6.0 Hz), δ 9.34 (d, 2H, PhH, J = 2.1 Hz), 9.67 (d, 4H, PyH, J = 5.7
Hz).
Synthesis of N,N’-diphenyl-4,4’-bipyridinium dichloride60 1-Cl. The mixture of freshly distilled
aniline (0.78 g, 8.4 mmol) and N,N’-bis(2,4-dinitrophenyl)-4,4’-bipyridinium dichloride 3 (2.25 g, 4
mmol) were slowly added N,N-dimethylacetamide (40 mL). The solution was stirred at 90–95 C for
14 h. Ethyl acetate (40 mL) was then added dropwise below 60 C. Crude product was collected. The
products isolated as white powder were obtained after recrystallization from methanol. Yield 87%;
1H‒NMR (300 MHz, DMSOd6): 7.80–7.82 (m, 6H, PhH), 8.01 (dd, 4H, PhH, J = 7.5, 1.5 Hz),
9.15 (d, 4H, PyH, J = 6.6 Hz), 9.75 (d, 4H, PyH, J = 6.6 Hz).
General synthetic procedures of N,N’-diphenyl-4,4’-bipyridinium di(dodecylsulfonate) 1-RSO3
(n = 12). To the suspension of sodium 1-dodecanesulfonate (300 mg, 1.1 mmol) in pure water (5.0
mL), 1-Cl (191 mg, 0.5 mmol) in methanol/CH2Cl2 (1/5, 30 mL) was added. This reaction was
monitored using TLC (Al2O3) with CH3OH/CH2Cl2 (5/95) as eluent and the Rf ~ 0.6 of the product
was higher than the Rf ~ 0.1 of 1-Cl. The organic layer was passed through a short Al2O3 flash
column. The crude product was obtained after solvents removed. The products 1-RSO3 isolated as
offwhite powder were obtained after recrystallization from isopropanol/ethyl acetate in an icebath.
Yield 95%; 1HNMR (300 MHz, CDCl3): 0.86 (t, 6H, CH3, J = 6.9 Hz), 1.19–1.31 (m, 36H, CH2),
1.56–1.64 (m, 4H, CH2), 2.57–2.62 (m, 4H, SO2CH2), 7.58 (m, 6H, PhH), 7.85 (m, 4H, PhH),
9.19 (d, 4H, PyH, J = 6.0 Hz), 9.36 (d, 4H, PyH, J = 6.0 Hz). 13CNMR (75 MHz, CDCl3): 14.24,
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22.82, 25.41, 29.11, 29.49, 29.58, 29.70, 29.79, 32.05, 52.21, 124.34, 128.94, 131.02, 132.09, 142.40,
145.59, 150.18. ESIHRMS (+ve): m/z calcd for C22H18N2: 155.0730 [M2+/2z]; found: 155.0726.
ESIHRMS (ve): m/z calcd for C12H25O3S: 249.1530 [Mˉ]; found: 249.1517. Anal. calcd for
C46H68N2O6S2•4H2O: C, 62.70; H, 8.69; N, 3.18; S, 7.28; found: C, 62.48; H, 8.78; N, 3.16; S, 6.99.
N,N’-Diphenyl-4,4’-bipyridinium di(decylsulfonate) 1-RSO3 (n = 10). Offwhite powder;
1HNMR (300 MHz, CDCl3):  0.85 (t, 6H, CH3, J = 6.9 Hz), 1.17–1.26 (m, 28H, CH2),
1.54–1.61 (m, 4H, CH2), 2.54–2.60 (m, 4H, SO2CH2), 7.57 (m, 6H, PhH), 7.85 (m, 4H, PhH),
9.17 (d, 4H, PyH, J = 6.3 Hz), 9.37 (d, 4H, PyH, J = 6.3 Hz). 13CNMR (75 MHz, CDCl3): 
14.24, 22.79, 25.28, 29.04, 29.45, 29.54, 29.70, 32.01, 52.02, 124.39, 128.53, 130.83, 131.80, 142.42,
145.58, 150.01. ESIHRMS (+ve): m/z calcd for C22H18N2: 155.0730 [M2+/2z]; found: 155.0726.
ESI-HRMS (ve): m/z calcd for C10H21O3S: 221.1217 [Mˉ]; found: 221.1208. Anal. calcd for
C42H60N2O6S2•3H2O: C, 62.50; H, 8.24; N, 3.47; S, 7.95; found: C, 62.45; H, 8.41; N, 3.38; S, 7.85.
N,N’-Diphenyl-4,4’-bipyridinium di(tetradecylsulfonate) 1-RSO3 (n = 14). Offwhite powder;
1HNMR (300 MHz, CDCl3):  0.86 (t, 6H, CH3, J = 6.9 Hz), 1.19–1.28 (m, 44H, CH2),
1.59–1.64 (m, 4H, CH2), 2.60–2.65 (m, 4H, SO2CH2), 7.60 (m, 6H, PhH), 7.84 (m, 4H, PhH),
9.24 (d, 4H, PyH, J = 5.4 Hz), 9.37 (d, 4H, PyH, J = 5.7 Hz). 13CNMR (75 MHz, CDCl3): 
14.26, 22.82, 25.34, 29.08, 29.50, 29.57, 29.72, 29.84, 32.05, 52.11, 124.35, 128.68, 130.92, 131.94,
142.39, 145.58, 150.05. ESIHRMS (+ve): m/z calcd for C22H18N2: 155.0730 [M2+/2z]; found:
155.0723. ESIHRMS (ve): m/z calcd for C14H29O3S: 277.1843 [Mˉ]; found: 277.1830. Anal. calcd
for C50H76N2O6S2•4H2O: C, 64.07; H, 9.03; N, 2.99; S, 6.84; found: C, 63.84; H, 9.18; N, 2.87; S,
6.68.
Synthesis of N,N’-bis(4-dodecoxyphenyl)-4,4’-bipyridinium dichloride 2-Cl (n = 12). A mixture
of 4-dodecoxyaniline61 (2.44 g, 8.8 mmol) and compound 3 (2.25 g, 4.0 mmol) dissolved in
N,N-dimethylacetamide (40 mL) was stirred at 90–95 C for 24 h. The product was formed. Ethyl
acetate (40 mL) was added dropwise at temperature below 60 C. The solids were collected. The
products isolated as yellow powder 2-Cl were obtained after recrystallization from methanol. Yield
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74%; 1HNMR (300 MHz, CD3OD):  0.91 (t, 6H, CH3, J = 6.6 Hz), 1.31–1.55 (m,D3O6I:H10,.103C9/HC82T)C,03090A
1.81–1.90 (m, 4H, CH2), 4.15 (t, 4H, OCH2, J = 6.5 Hz), 7.30 (d, 4H, PhH, J = 9.0 Hz), 7.86 (d,
4H, PhH, J = 8.7 Hz), 8.88 (br, 4H, PyH), 9.49 (br, 4H, PyH). 13CNMR (75 MHz, CD3OD): 
14.45, 23.74, 27.11, 30.20, 30.48, 30.73, 33.08, 69.98, 117.29, 126.88, 128.27, 136.73, 146.75,
151.09, 163.41.
General synthetic procedures of N,N’-bis(4-dodecoxyphenyl)-4,4’-bipyridinium salts 2-X (X =
BF4, PF6, OTf, NTf2, RSO3, SCN).
N,N’-Bis(4-dodecoxyphenyl)-4,4’-bipyridinium di(tetrafluoroborate) 2-BF4 (n = 12). To the
solution of 2-Cl (187 mg, 0.25 mmol) dissolved in methanol/CH2Cl2 (1/5, 30ml), sodium
tetrafluoroborate 10% aqueous solution (3.0 mL, 2.7 mmol) was added. This reaction was monitored
using TLC (Al2O3) with CH3OH/CH2Cl2 (5/95) as eluent and the Rf ~ 0.5 of the product was higher
than the Rf ~ 0.1 of 2-Cl. The organic layer was passed through a short Al2O3 flash column. The
crude product was collected after solvents removed. The products 2-BF4, isolated as white powder
were obtained after crystallization from isopropanol/ethyl acetate. Yield 95%; 1HNMR (300 MHz,
CDCl3):  0.91 (t, 6H, CH3, J = 6.5 Hz), 1.31–1.53 (m, 36H, CH2), 1.81–1.88 (m, 4H, CH2), 4.14
(t, 4H, OCH2, J = 6.5 Hz), 7.29 (d, 4H, PhH, J = 8.7 Hz), 7.83 (d, 4H, PhH, J = 6.0 Hz), 8.82 (br,
4H, PyH), 9.46 (br, 4H, PyH). 19FNMR (282 MHz, CDCl3/CD3OD, ref C6F6 = 164.9): 
153.57 and 153.62 (s, BF). ESIHRMS (+ve): m/z calcd for C46H66N2O2: 339.2557 [M2+/2z];
found: 339.2536. ESIHRMS (ve): m/z calcd. for BF4: 87.0035 [Mˉ]; found: 87.0038. Anal. calcd
for C46H66B2F8N2O2•4.5H2O: C, 59.17; H, 8.10; N, 3.00; found: C, 58.94; H, 7.46; N, 3.12.
N,N’-Bis(4-dodecoxyphenyl)-4,4’-bipyridinium di(hexafluorophosphate) (2-PF6). White powder;
1HNMR (300 MHz, DMSO-d6): 0.86 (t, 6H, CH3, J = 6.6 Hz), 1.26–1.45 (m, 36H, CH2),
1.75–1.80 (m, 4H, CH2), 4.13 (t, 4H, OCH2, J = 6.3 Hz), 7.32 (d, 4H, PhH, J = 8.7 Hz), 7.90 (d,
4H, PhH, J = 7.5 Hz), 9.01 (br, 4H, PyH), 9.62 (br, 4H, PyH). 19FNMR (282 MHz, CD3OD, ref
C6F6 = 164.9):  74.4 (d, PF, J = 707 Hz). ESIHRMS (+ve): m/z calcd for C46H66N2O2:
339.2557 [M2+/2z]; found: 339.2527. ESIHRMS (ve): m/z calcd for F6P: 144.9647 [Mˉ]; found:
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