1,10-phenanthrolinium ionic liquid crystals

Article abstract of DOI:10.1021/la1047276

The 1,10-phenanthrolinium cation is introduced as a new building block for the design of ionic liquid crystals. 1,10-Phenanthroline, 5-methyl-1,10- phenanthroline, 5-chloro-1,10-phenanthroline, and 4,7-diphenyl-1,10- phenanthroline were quaternized by rea

Full text of DOI:10.1021/la1047276

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© 2011 American Chemical Society
1,10-Phenanthrolinium Ionic Liquid Crystals
Thomas Cardinaels,† Kathleen Lava,‡ Karel Goossens,‡ Svetlana V. Eliseeva,‡ and
Koen Binnemans*,‡
†SCK CEN, Institute for Nuclear Materials Sciences, Boeretang 200, B-2400 Mol, Belgium, and
3
‡Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O. Box 2404,
B-3001 Heverlee, Belgium
Received November 27, 2010. Revised Manuscript Received December 21, 2010
The 1,10-phenanthrolinium cation is introduced as a new building block for the design of ionic liquid crystals. 1,10-
Phenanthroline, 5-methyl-1,10-phenanthroline, 5-chloro-1,10-phenanthroline, and 4,7-diphenyl-1,10-phenanthroline
were quaternized by reaction with 1,3-dibromopropane or 1,2-dibromoethane. The resulting cations were combined
with dodecyl sulfate or dioctyl sulfosuccinate anions. The influence of both the cation and anion type on the thermal
behavior was investigated. Several of the complexes exhibit mesomorphic behavior, with smectic E phases for the
dodecyl sulfate salts and smectic A phases for the dioctyl sulfosuccinate salts. Structural models for the packing of the
1,10-phenanthrolinium and anionic moieties in the liquid-crystalline phases are presented. The ionic compounds show
fluorescence in the solid state and in solution.
Introduction
positions by a 2-substituted imidazole ring led to imidazo-
[4,5-f]-1,10-phenanthrolines, which are very versatile ligands for
the design of liquid-crystalline metal complexes (metallomeso-
gens).9 Related mesogenic ligands are the pyrazino[2,3-f]-1,10-
phenanthrolines.10
In this article, we present a novel approach to liquid crystals
based on 1,10-phenanthroline. 1,10-Phenanthroline and some of
its derivatives were quaternized by the reaction of their two
nitrogen atoms with an R,ω-dibromoalkane. Exchanging the
bromide ions of the resulting 1,10-phenanthrolinium bromides
for dodecyl sulfate (DOS) or dioctyl sulfosuccinate (DOSS)
anions resulted in a new class of ionic liquid crystals.11,12
1,10-Phenanthroline and its derivatives are well-known, versa-
tile bidentate ligands that can form stable complexes with many
metal ions in different oxidation states.1-3 It is also possible to
functionalize the 1,10-phenanthroline moiety in many ways to
synthesize different types of liquid crystals. The aromatic core can
be extended to form calamitic (rodlike) liquid crystals, but it can
also be used to design discotic (disklike), bent-core or polycatenar
liquid crystals. Polycatenar (literally “many-tailed”) liquid crys-
tals consist of an extended, linear, usually aromatic core that is
decorated with several long alkoxy chains at both ends.4 The first
3,8-disubstituted 1,10-phenanthroline-containing calamitic liquid
crystals were described by Bousquet and Bruce and showed
smectic and nematic phases.5 Related tetracatenar ligands and
their corresponding rhenium(I) complexes exhibit a hexagonal
columnar mesophase (Colh).6 In this case, ester linkages and triple
bonds were used to connect the aromatic units in order to develop
a long aromatic core to counterbalance the bulky metallic frag-
ment. A rich mesomorphism, including smectic C, cubic, hex-
agonal, and rectangular columnar phases, depending on the
alkoxy chain length, is exhibited by a rigid, extended, tetracatenar
3,8-disubstituted 1,10-phenanthroline with acetylenic linkers.7
Other mesomorphic systems include tetrahedral copper(I) com-
plexes derived from a nonsymmetrical phenanthroline, which
were found to show an oblique columnar mesophase (Colo).8
The functionalization of 1,10-phenanthroline in the 5 and 6
Experimental Section
Defect textures of the mesophases were observed with an
Olympus BX60 polarizing optical microscope equipped with a
Linkam THMS600 hot stage and a Linkam TMS93 program-
mable temperature controller. DSC traces were recorded with a
Mettler-Toledo DSC822e module (heating/cooling rate of 10 °C
min-1, helium atmosphere). Powder X-ray diffractograms were
recorded with a Bruker AXS D8 Discover diffractometer
mounted with a copper X-ray ceramic tube, working at 1.6 kW.
˚
The emitted Cu KR radiation (λ = 1.5418 A) was focused on the
€
sample by a Gobel mirror. All of the samples (without a thermal
history) were prepared by spreading the compounds on a thin,
clean silicon wafer. Diffractograms were recorded using Bragg-
Brentano reflection geometry (θ/2θ setup) at an angular
resolution (in 2θ) of 0.03° per step. The deviation between the
temperature on the surface of the sample holder and the set
temperature was about 3%. The scattering signal was recorded
with a 1D detector (LynxEye). Absorption spectra were measured
on a Varian Cary 5000 spectrophotometer on freshly prepared
methanol solutions of the 1,10-phenanthrolinium salts in quartz
*Corresponding author. E-mail: koen.binnemans@chem.kuleuven.be.
Fax: þ32 16 327992.
(1) Sammes, P. G.; Yahioglu, G. Chem. Soc. Rev. 1994, 23, 327–334.
(2) Accorsi, G.; Listorti, A.; Yoosaf, K.; Armaroli, N. Chem. Soc. Rev. 2009, 38,
1690–1700.
(3) Bencini, A.; Lippolis, V. Coord. Chem. Rev. 2010, 254, 2096–2180.
^
(4) Nguyen, H. T.; Destrade, C.; Malthete, J. Adv. Mater. 1997, 9, 375–388.
(5) Bousquet, S. J. P.; Bruce, D. W. J. Mater. Chem. 2001, 11, 1769–1771.
(6) Date, R. W.; Iglesias, E. F.; Rowe, K. E.; Elliott, J. M.; Bruce, D. W. Dalton
Trans. 2003, 1914–1931.
(9) Cardinaels, T.; Ramaekers, J.; Nockemann, P.; Driesen, K.; Van Hecke, K.;
Van Meervelt, L.; Lei, S. B.; De Feyter, S.; Guillon, D.; Donnio, B.; Binnemans, K.
Chem. Mater. 2008, 20, 1278–1291.
(7) Cardinaels, T.; Ramaekers, J.; Nockemann, P.; Driesen, K.; Van Hecke, K.;
Van Meervelt, L.; Wang, G.; De Feyter, S.; Iglesias, E. F.; Guillon, D.; Donnio, B.;
Binnemans, K.; Bruce, D. W. Soft Matter 2008, 4, 2172–2185.
(8) Ziessel, R.; Pickaert, G.; Camerel, F.; Donnio, B.; Guillon, D.; Cesario, M.;
Prange, T. J. Am. Chem. Soc. 2004, 126, 12403–12413.
(10) Cardinaels, T.; Ramaekers, J.; Driesen, K.; Nockemann, P.; Van Hecke, K.;
Van Meervelt, L.; Goderis, B.; Binnemans, K. Inorg. Chem. 2009, 48, 2490–2499.
(11) Guan, Y.; Zakrevskyy, Y.; Stumpe, J.; Antonietti, M.; Faul, C. F. J. Chem.
Commun. 2003, 894–895.
(12) Binnemans, K. Chem. Rev. 2005, 105, 4148–4204.
2036 DOI: 10.1021/la1047276
Published on Web 01/20/2011
Langmuir 2011, 27(5), 2036–2043

Cardinaels et al.
Article
Chart 1. Overview of the 1,10-Phenanthrolinium Salts
Table 1. Transition Temperatures and Thermodynamic Data for the
1,10-Phenanthrolinium Compounds
Suprasil cells (optical path length, 1 cm). Luminescence spectra
were recorded on an Edinburgh Instruments FS900 or FS920
steady-state spectrofluorimeter for solid-state samples or solu-
tions in methanol, respectively. These instruments are equipped
with a 450 W xenon arc lamp, a microsecond flash lamp (pulse
length, 2 μs), and a red-sensitive (300-850 nm, Hamamatsu
R928P on Edinburgh Instruments FS900) or extended red-sensi-
tive (185-1010 nm, Hamamatsu R2658P on Edinburgh Instru-
ments FS920) photomultiplier. The excitation wavelength was set
at 308 nm for compounds 1b, 1c, 2b, 2c, 5b, and 5c and at 320 nm
for compounds 4b and 4c. Quantum yields were determined by a
comparative method using solutions of rhodamine 101 (Aldrich)
in ethanol (Q = 100%) and quinine sulfate (Fluka) in 1 N H2SO4
(Q = 54.6%) as standards with an estimated error of 20%.13 The
synthesis and characterization of the ionic liquid crystals are
described in the Supporting Information.
compound transitiona T (°C)b ΔH (kJ mol-1
)
ΔS (J K-1 mol-1
)
1b
Cr1 f Cr2
Cr2 f Cr3
Cr3 f SmE 143
88
133
44.5
17.8
17.2
123
44
41
SmE f dec >200
2b
3b
Cr1 f Cr2 87
Cr2 f SmE 149
SmE f dec >200
34.6
11.5
96
27
Cr1 f Cr2
Cr2 f Cr3
Cr3 f Cr4
40
62
100
1.5
74.4
0.9
3.2
5
222
2
Cr4 f SmE 185
SmE f dec >200
7
4b
5b
Cr1 f Cr2
Cr2 f I
34
1.8
14.7
0.9
6
32
3
190c
41
Cr1 f Cr2
Cr2 f Cr3
Cr3 f dec
Results and Discussion
93
>200
61.9
169
1,10-Phenanthroline and its derivatives 5-methyl-1,10-phenan-
throline, 5-chloro-1,10-phenanthroline, and 4,7-diphenyl-1,10-
phenanthroline (bathophenanthroline) have been used as starting
materials. An overview of the 1,10-phenanthrolinium salts is
given in Chart 1. Bromide salts 1a-5a were prepared via double
quaternization (Menschutkin reaction) of the respective 1,10-
phenanthroline compound with 1,3-dibromopropane (1a-4a) or
1,2-dibromoethane (5a) in nitrobenzene.14 DOS salts 1b-5b were
synthesized via a metathesis reaction between bromide salts
1a-5a and silver dodecyl sulfate in ethanol. DOSS salts 1c-5c
were prepared via a metathesis reaction between bromide salts
1a-5a and sodium dioctyl sulfosuccinate in water. Yields were in
the range of 65-94% for all of the 1,10-phenanthrolinium salts.
1c
2c
3c
Cr f SmA 81c
28.6
0.1
15.2
2.4
8.7
2.3
0.1
6.8
2.1
81
0.2
45
5
26
5
0.2
17
5
SmA f I
171
Cr f SmA 63c
SmA f I
157c
Cr f SmA 65c
SmA f I
g f I
Cr f SmA 116
SmA f I 182
146
66
4c
5c
a Abbreviations: Cr, Cr1, Cr2, Cr3, and Cr4, crystalline phases; g,
glass; SmE, smectic E phase; SmA, smectic A phase; I, isotropic liquid;
dec, decomposition. b Onset temperatures obtained by DSC at heating/
cooling rates of 10 °C min-1 (helium atmosphere). c Peak temperature.
None of the bromide salts (1a-5a) were liquid-crystalline,
which is not unexpected because these compounds do not
contain any aliphatic chains. Except for the compounds contain-
ing the 4,7-diphenyl-1,10-phenanthrolinium cation (4b and 4c)
and DOS salt 5b, the DOS (1b-3b) and DOSS (1c-3c and 5c)
salts were liquid-crystalline. Considering the data in Table 1
(phase behavior of 1b, 5b, 1c, and 5c), one can expect that
compound 5b is intrinsically able to form a mesophase but its
melting point is so high that the material thermally decomposes
before it can melt into the liquid-crystalline phase. Upon
heating, the DOS salts (1b-3b) showed a smectic E phase,
whereas the DOSS salts (1c-3c and 5c) showed a smectic A
phase. The identification of the mesophases was confirmed by
PXRD (see below).
The purity of all of the compounds was verified via 1H and 13
NMR spectroscopy and CHN elemental analysis.
C
The thermal properties of all of the 1,10-phenanthrolinium
compounds were examined by polarizing optical microscopy
(POM) and differential scanning calorimetry (DSC) and for some
compounds by powder X-ray diffraction (PXRD) at elevated
temperatures. The transition temperatures and thermodynamic
data for the dodecyl sulfate and the dioctyl sulfosuccinate salts are
summarized in Table 1.
With the exception of the DOSS salts (1c-5c), two or more
crystalline phases were observed for the 1,10-phenanthrolinium
salts, which is not unusual for ionic compounds. The absence of
crystalline polymorphism for the DOSS salts (1c-5c) can be
attributed to the large, bulky anions, which prevent good pack-
ing within the crystalline solid phase. It should be mentioned
that all of the DOSS salts (1c-5c) were obtained as waxy solids.
The smectic A phases of the DOSS salts (1c-3c and 5c) were
identified by polarizing optical microscopy via the formation of
^
batonnets on cooling from the isotropic liquid (Figure 1). On
^
further cooling, these batonnets coalesced into fans so that a fan
(13) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107–1114.
(14) Yamada, M.; Nakamura, Y.; Kuroda, S.; Shimao, I. Bull. Chem. Soc. Jpn.
1990, 63, 2710–2712.
texture with homeotropic domains was formed (Figure 1). The
smectic E phases of the DOS salts showed an oily streak texture
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Cardinaels et al.
Figure 2. (a) Thermal behavior of 1,10-phenanthrolinium do-
decyl sulfate (DOS) salts 1b-5b. Abbreviations: Cr1, Cr2, Cr3,
and Cr4, crystalline phase; SmE, smectic E phase. (b) Thermal
behavior of 1,10-phenanthrolinium dioctyl sulfosuccinate
(DOSS) salts 1c-5c. Abbreviations: Cr, crystalline phase;
SmA, smectic A phase.
of compound 2c (second heating/cooling cycle) are given in
Figure 3. The mesophase temperature range was about 95 °C
for DOSS salts 1c-3c, but the mesophase temperature range for
DOSSsalt 5c was smaller (66 °C). Mesomorphic DOS salts 1b-3b
decomposed in the liquid-crystalline phase before the clearing
point was reached. DOSS salts 1c-5c had lower melting points
than DOS analogues 1b-5b.
Figure 1. Mesophase defect textures observed by polarizing opti-
cal microscopy (POM). (Top) Oily streak texture of the smectic E
phase of 1b at 150 °C (200ꢀ magnification). (Middle) Appearance
^
of batonnets during the formation of the smectic A phase of 2c at
To obtain more information about the molecular arrangement
of the 1,10-phenanthrolinium compounds in the smectic phases,
the liquid-crystalline compounds were further investigated by
PXRD. Unfortunately, no well-resolved peaks could be observed
for DOS salt 3b because of partial thermal decomposition above
the rather high melting point (185 °C). Because of POM textures
similar to those of 1b and 2b, the mesophase of 3b was assumed to
be the same as that of compounds 1b and 2b (see below). Table 2
gives an overview of the Bragg reflections collected from the
X-ray diffractograms of DOS salts 1b and 2b and DOSS salts 1c,
2c, 3c, and 5c. The diffractograms of DOS salts 1b and 2b showed
several strong equidistant reflections in the small-angle region.
These reflections are related to the thickness d of the smectic
layers. In the wide-angle region, two sharp peaks were visible.
These reflections indicate a 2D ordering of the molecules within
the smectic layers and were indexed as the (110) and (200)
155 °C on cooling from the isotropic liquid phase (100ꢀ
magnification). (Bottom) Fan texture with homeotropic domains
of the smectic A phase of 2c at 150 °C (100ꢀ magnification).
with pseudohomeotropic domains (Figure 1), a texture that is
usually not observed for smectic E phases.15,16
In Figure 2, an overview of the thermal behavior of the DOS
(1b-5b) and DOSS (1c-5c) salts is given. For the DOS salts
(1b-5b), the lowest melting point was observed for the unsub-
stituted 1,10-phenanthrolinium cation (1b). However, for the
DOSS salts (1c-5c), the lowest melting point was observed for
the 5-methyl-1,10-phenanthrolinium cation (2c). The DSC traces
(15) Gray, G. W.; Goodby, J. W. Smectic Liquid Crystals: Textures and
Structures; Leonard Hill: Glasgow, 1984.
(16) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim, Germany,
2003.
2038 DOI: 10.1021/la1047276
Langmuir 2011, 27(5), 2036–2043

Cardinaels et al.
Article
reflections of a 2D centered rectangular lattice. No other (hkl)
signals (mixing of (00l) and (hk0), with h and/or k ¼ 0, l ¼ 0),
which could have pointed to substantial interlayer coupling, were
From the diffractograms obtained for the smectic E phases
shown by DOS salts 1b and 2b, parameters AM, a, and b could be
calculated (Table 2). AM corresponds to the area occupied by the
ionic clusters (a head-to-head arrangement of two 1,10-phenan-
throlinium dications and four DOS anions) within the smectic
layers. It was estimated using the relation AM = 2VM/d, where
VM is the molecular volume (estimated as VM = (M/0.6022)f, M
is the molecular mass (g mol-1), and f is a temperature correction
factor (f = 0.9813 þ 7.474 ꢀ 10-4T, with T in °C)), and d is the
smectic layer thickness.17 At 161 °C, this calculation yields a value
˚
found. Only a diffuse signal centered at about 4.4 A was
additionally observed in the wide-angle region. This signal is
related to the lateral short-range order of the molten aliphatic
chains. The diffractograms of DOSS salts 1c, 2c, 3c, and 5c
showed up to three sharp, equidistant reflections in the small-
angle region, corresponding to the layer thickness d of the smectic
A layers. In the wide-angle region, a diffuse signal was observed at
2
˚
˚
about 4.5 A corresponding to the lateral short-range order of the
of AM ≈ 90 A for both 1b and 2b. Parameters a and b are the
molten aliphatic chains.
dimensions of the 2D rectangular lattice unit cell in the smectic E
phase (for a 2D rectangular lattice, 1/dhk2 = h2/a2 þ k2/b2). For 1b
and 2b, similar values were found for a and b: about 7.7 and about
˚
5.4 A, respectively. Consequently, the area of the rectangular
2
˚
lattice unit cell equals about 42 A . Considering these values, it
can be concluded that the four alkyl chains of an ionic cluster are
almost fully interdigitated (the cross-sectional area of one fully
2
18
˚
stretched aliphatic chain equals about 23.4 A at 161 °C). In
Figure 4, a model of the molecular arrangement in the smectic E
phases is proposed. In the model, the smectic layers are composed
of ionic sublayers on one hand and aliphatic sublayers on the
other hand. This is due to electrostatic interactions between the
positively and negatively charged ionic headgroups and van der
Waals interactions between the relatively long aliphatic chains,
which both lead to microphase separation. Each ionic sublayer is
in fact a bilayer of 1,10-phenanthrolinium dications. Such a
bilayer arrangement has been observed for other ionic liquid
crystals.19,20 The planes of the dications are oriented perpendi-
cular to the smectic layers, thus exposing their positive charges
most effectively to the sulfate anions. Inside each ionic bilayer,
the 1,10-phenanthrolinium moieties are arranged in such a way
that a herringbone molecular packing is achieved, which is typical
Figure 3. DSC trace of 2c: second heating (upper curve) and
cooling (lower curve) runs at a heating/cooling rate of 10 °C min-1
in a He atmosphere. Abbreviations: Cr, crystalline phase; SmA,
smectic A phase; I, isotropic liquid.
Table 2. Bragg Reflections Collected from the X-ray Diffractograms of the Enantiotropic Mesophases
a
a
Ib
hklc
parameters of the liquid-crystalline phase at temperature Td,e,f
˚
dmeas/A
˚
dcalc/A
compound
1b
30.64
15.40
10.41
7.73
4.42
4.40
VS
M
S
M
M
br
M
VS
M
S
001
002
003
004
110
hCH
200
001
002
003
004
006
hCH
110
200
001
002
003
hCH
001
hCH
30.89
15.45
10.30
7.72
SmE: T = 161 °C
VM = 1378 A
3
˚
2
˚
AM = 89.2 A
˚
a = 7.74 A
˚
b = 5.38 A
4.42
3.86
3.87
30.67
15.34
10.22
7.67
2b
30.64
15.24
10.33
7.65
5.08
4.48
4.37
3.85
23.09
11.55
7.69
SmE: T = 161 °C
VM = 1403 A
3
˚
2
˚
AM = 91.5 A
˚
S
a = 7.70 A
˚
b = 5.31 A
W
br
M
W
VS
W
VW
br
VS
br
5.11
4.37
3.85
23.09
11.54
7.70
1c
SmA: T = 100 °C
VM = 1868 A
3
˚
2
˚
AM = 161.8 A
4.45
22.75
4.46
2c
3c
5c
22.75
22.92
25.04
SmA: T = 120 °C
VM = 1921 A
3
˚
2
˚
AM = 168.9 A
22.92
4.46
VS
br
001
hCH
SmA: T = 120 °C
VM = 1957 A
3
˚
2
˚
AM = 170.8 A
25.04
4.55
VS
br
001
hCH
SmA: T = 120 °C
VM = 1871 A
3
˚
2
˚
AM = 149.4 A
a dmeas and dcalc are the measured and calculated diffraction spacings, respectively. b I is the intensity of the reflections: VS, very strong; S, strong; M,
medium; W, weak; VW, very weak; br, broad reflection. c hkl are the Miller indices of the reflections, and hCH is the periodicity corresponding to the
liquid-like order of the molten aliphatic chains. d T is the temperature at which the X-ray diffractogram was recorded. e VM is the molecular volume, and
AM the molecular area. a and b are the lattice parameters of the smectic E phase. f Abbreviations: SmE, smectic E phase; SmA, smectic A phase.
Langmuir 2011, 27(5), 2036–2043
DOI: 10.1021/la1047276 2039

Article
Cardinaels et al.
Figure 4. Structural model (idealized) for the smectic E phase exhibited by compound 1b. (a) Side view. The smectic layer thickness is
2
˚
˚
indicatedbyd(at161°C,d= 30.89Aand AM ≈89A (Table 2)). Inreality, thephenanthrolinemoietiesaretiltedoutoftheplane of thepaper.
(b) Top view of the 2D rectangular lattice. The lower half of an ionic bilayer is depicted in pale gray, and the upper half of this bilayer is
depictedindarkgray. Thealiphatic chains(which pointtowardandawayfromthereader) areomittedforclarity. Thedimensionsofaunitcell
˚
˚
of the 2D rectangular lattice are indicated by a and b (at 161 °C, a = 7.74 A and b = 5.38 A (Table 2)).
for E phases.15,21 From Figure 4b, it becomes clear that the
polyaromatic moieties can nicely fit on top of each other.
(Pay attention to the topography of the 1,10-phenanthrolinium
parts.) The 2D rectangular lattice unit cells are also filled with the
sulfate headgroups. The slightly rotated orientation of the 1,10-
phenanthrolinium planes with respect to the long side of the
rectangular lattice unit cells allows for an efficient electrostatic
interaction between one 1,10-phenanthrolinium dication and two
sulfate headgroups. The alkyl chains attached to the sulfate
anions point away from the ionic sublayer, and alkyl chains
attached to sulfate groups belonging to different ionic sublayers
are nearly fully interdigitated to form the aliphatic sublayers. The
E-phase structure proposed in Figure 4 differs somewhat from
previously reported E phases shown by ionic liquid crystals19,22,23
because of the fact that the phenanthrolinium moieties are
dipositive rather than having a single charge (like pyridinium or
pyrrolidinium cations). A consideration of the dimensions of the
rectangular lattice unit cell and a glance at Figure 4b (which is
drawn to scale) show that the 1,10-phenanthrolinium cations are
not free to rotate around the axis normal to the smectic layer
planes.
A structural model for the SmA phases of DOSS salts 1c, 2c,
and 3c is proposed in Figure 5. The molecules segregate into ionic
and aliphatic sublayers but without additional ordering or corre-
lations within the ionic bilayers. In contrast to the dodecyl sulfate
anions, the dioctyl sulfosuccinate anions each contain two
branched alkyl chains. This explains the much higher AM values
(17) Goossens, K.; Lava, K.; Nockemann, P.; Van Hecke, K.; Van Meervelt, L.;
Driesen, K.; Gorller-Walrand, C.; Binnemans, K.; Cardinaels, T. Chem.;Eur. J.
2009, 15, 656–674.
(18) Marcos, M.; Gimenez, R.; Serrano, J. L.; Donnio, B.; Heinrich, B.; Guillon,
D. Chem.;Eur. J. 2001, 7, 1006–1013.
(19) Navarro-Rodriguez, D.; Frere, Y.; Gramain, P.; Guillon, D.; Skoulios, A.
Liq. Cryst. 1991, 9, 321–335.
(20) Goossens, K.; Nockemann, P.; Driesen, K.; Goderis, B.; Goerller-Walrand,
C.; Van Hecke, K.; Van Meervelt, L.; Pouzet, E.; Binnemans, K.; Cardinaels, T.
Chem. Mater. 2008, 20, 157–168.
(21) Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H.-W.; Vill, V. Handbook of
Liquid Crystals; Wiley-VCH: Weinheim, Germany, 1998.
2
˚
that were found for the DOSS salts (AM ≈ 162-171 A ) in
(22) Goossens, K.; Lava, K.; Nockemann, P.; Van Hecke, K.; Van Meervelt, L.;
Pattison, P.; Binnemans, K.; Cardinaels, T. Langmuir 2009, 25, 5881–5897.
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2040 DOI: 10.1021/la1047276
Langmuir 2011, 27(5), 2036–2043

Cardinaels et al.
Article
Table 3. Main Absorption and Emission Bands, Stokes Shifts and
Quantum Yields of 1,10-Phenanthrolinium DOS and DOSS Salts in
Methanol Solution (c = 10-5 M)a
Stokes
Q/% shift/cm-1 b
compound
λ
abs/nm ε/M-1 cm-1
λem/nm
1b
216
28 690
31 960
1662
29 660
31 570
1536
22 390
24 080
1758
305
447
219
309
453
227 sh
321
460
546
0.21
0.14
0.08
4056
3893
3623
2b
4b
550
552 sh,
646, 702
5b
1c
2c
4c
5c
217
306
452
216
305
448
219
309
446
227 sh
322
460
217
306
452
28 970
30 900
1320
33 540
34 601
1899
38 240
37 270
2218
27 170
30 090
2183
540, 642
544
0.02
0.20
0.11
0.35
0.02
3605
3939
4173
3754
3327
548
556
35 660
36 720
1613
532, 642
a The excitation wavelength was set at 308 nm for compounds 1b, 1c,
2b, 2c, 5b, and 5c and at 320 nm for compounds 4b and 4c. b Calculated to
be 107 ꢀ {(1/λabs) - (1/λem)}. Abbreviation: sh, shoulder.
Figure 5. Structural model (idealized) for the smectic A phase
exhibited by compound 1c. The smectic layer thickness is indicated
by d (at 100 °C, d = 23.09 A and AM ≈ 162 A (Table 2)).
and 264 nm, quaternization of the nitrogen atoms induces a
significant bathochromic shift and an additional absorption band
appears in the visible region between 440 and 460 nm.2,27,28 For
the 1,10-phenanthrolinium salts studied, neither the nature of the
anion nor the length of the bridging alkyl chain between the
nitrogen atoms has a pronounced influence on the position of the
absorption bands. Substitution of the 1,10-phenanthroline group
at the 5-position by an electron-donating methyl group, when
going from compound 1b to 2b and from 1c to 2c, resulted in the
expected small bathochromic shifts of the absorption bands. A
stronger effect is observed upon phenyl substitution, as derived
from the shifts of 11 and 17 nm for pairs of compounds 1b-4b
and 1c-4c. For all the 1,10-phenanthrolinium salts dissolved in
methanol, excitation at 308-320 nm resulted in broad-band
green fluorescence whereas excitation in the absorption band at
about 450 nm did not give any detectable fluorescence signal.
Again, a significant red shift of the emission bands can be
mentioned in comparison with those of 1,10-phenanthroline,
which displays broad-band fluorescence centered at 355 nm.
Substitution at the 5-position of 1,10-phenanthroline with a
methyl and at the 4- and 7-positions with phenyl groups led to
a bathochromic shift of the emission band, whereas shortening of
the alkyl bridge between nitrogen atoms when going from 1 to
5 resulted in the appearance of the second band at 642 nm and
a hypsochromic shift of the first band to 532-540 nm. This
latter is in line with a smaller deviation from planarity of the
1,10-phenanthroline ring system with an ethylene bridge than
with a propylene bridge and such a deviation causes a blue shift.
As in the case of the absorption spectra, the nature of the anion has
a negligible effect on the fluorescence spectra and on the quantum
yields for compounds 1, 2, and 5; however, the results for
compound 4 were unexpected. 1,10-Phenanthrolinium DOSS salt
2
˚
˚
comparison to those for the DOS salts. The larger cross-sectional
area occupied by the branched DOSS anions in comparison to the
DOS anions does not allow strict 2D intralayer ordering as in the
case of an E phase.
Many unsuccessful attempts have been made to obtain single
crystals suitable for structure determination by X-ray diffraction.
Nevertheless, the literature data on single crystals of 1,10-phe-
nanthrolinium compounds show that a propylene bridge con-
necting the two nitrogen atoms causes a much larger deformation
of the 1,10-phenanthroline ring system from a planar structure
than an ethylene bridge.24-26
Because of the ability of 1,10-phenanthroline to form com-
plexes with transition-metal ions, photophysical studies concern-
ing 1,10-phenanthroline have mainly focused on these complexes.
However, 1,10-phenanthroline itself is fluorescent. The photo-
physical properties of 1,10-phenanthroline were first studied by
Badger and Walker.27 The fluorescence spectra of the 1,10-
phenanthrolinium salts were recorded in the solid state and as a
solution in methanol (concentration, 10-5 mol L-1), all at room
temperature. The absorption and fluorescence spectra of the
methanolic solutions are given in the Supporting Information.
In Table 3, a summary of the spectroscopic properties can be
found. The absorption spectra of the 1,10-phenanthrolinium
DOS and DOSS salts show three main bands due to π f π*
and n f π* transitions. Whereas the absorption spectrum of 1,10-
phenanthroline displays two absorption bands centered at 229
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DOI: 10.1021/la1047276 2041

Article
Cardinaels et al.
are red-shifted when the aromatic system becomes less planar. As
mentioned above, a propylene bridge causes a stronger deforma-
tion of the aromatic system from planarity than an ethylene
bridge, so the compounds with the propylene bridge fluoresce at a
longer wavelength than those with an ethylene bridge. The most
intense luminescence is observed for 4,7-diphenyl-1,10-phenan-
throlinium (bathophenanthrolinium) salts 4b and 4c. These salts
show a whitish blue emission with emission maxima at 453 nm for
4b and at 492 nm for 4c. Because of the broadness of the emission
band, the fluorescence color is bluish white.
Compared to most other ionic liquid crystals, the molecular
structure of the 1,10-phenanthrolinium salts is unusual. A general
approach toward ionic liquid crystals is to extend the length of the
alkyl chains linked to the cation of an ionic liquid. For instance,
the well-studied 1-alkyl-3-methylimidazolium salts become liquid-
crystalline when the alkyl chain contains about 12 carbon
atoms, depending on the counterion.29-32 The same strategy
has been successfully used to obtain liquid-crystalline analogues
of quaternary ammonium,33 phosphonium,34,35 pyridinium,36
pyrrolidinium,17,37 piperidinium,23 and morpholinium23 ionic
liquids. Typically, these ionic liquid crystals have monocationic
cores but ionic liquid crystals with dicationic cores, e.g. piper-
azinium23 and 4,40-bipyridinium compounds,38,39 have also been
described. Provided that an anion with a long alkyl chain is used,
it is possible to obtain liquid crystallinity for a compound with a
short alkyl chain on the cation. For instance, 1,3-dimethyl-
imidazolium dodecyl sulfonate exhibits a smectic A phase.40 The
1,10-phenanthrolinium salts are conceptually related to the latter
class of ionic liquid crystals because in our compounds no long
chains are linked to the cation. It should be mentioned that the
dodecyl sulfate salts of imidazolium, pyridinium and ammonium
cations with short chains are true ionic liquids.41 One could
assume that the relatively high melting points of the 1,10-
phenanthrolinium ionic liquid crystals are due to the dicationic
character of the compounds and to the resulting high lattice
energies. However, one should realize that the dipositive charge is
delocalized so that the charge density is not very high. The
relatively elevated transition temperatures are attributed to
the strong π-π interactions between the 1,10-phenanthro-
line rings. Also, other 1,10-phenanthroline-based liquid
crystals have high transition temperatures.5-7 The thermal
behavior of 4,40-bipyridinium compounds, which are also
dicationic, shows that dipositive charges on extended cationic
cores do not necessarily lead to high melting points.38,39
Figure 6. Normalized fluorescence spectra of the dodecyl sulfate
(DOS) salts in the solid state at room temperature. The excitation
wavelengthsfor the different spectraare1b, 340 nm; 2b, 330nm; 4b,
350 nm; and 5b, 330 nm.
Figure 7. Normalized fluorescence spectra of the dioctyl sulfosuc-
cinate (DOSS) salts in the solid state at room temperature. The
excitation wavelengths for the different spectra are 1c, 340 nm; 2c,
360 nm; 4c, 360 nm; and 5c, 360 nm.
4c displays the largest quantum yield among the compounds,
0.35%; however, the quantum yield of 4b is only 0.08%, and the
two bands at 646 and 702 nm become dominating in the
fluorescence spectrum. If the quantum yields of other compounds
are considered, it can be observed that methyl substitution leads
to a lowering of the quantum yields by a factor of 1.5 to 1.8. A
decrease in the length of the alkyl bridge between the nitrogen
atoms has a detrimental effect on the fluorescence properties,
resulting in a decrease in the quantum yield by a factor of 10 when
the propylene bridge is replaced by an ethylene bridge. It should
be noticed that the quantum yields of all 1,10-phenanthrolinium
salts are quite low. It is known that the quantum yield of 1,10-
phenanthroline is rather low because of nonemissive relaxation
from the n-π* excited singlet states, which are close to the π-π*
excited singlet states.27
The fluorescence spectra of the DOS salts in the solid state are
shown in Figure 6, and those of the DOSS salts are shown in
Figure 7. The compounds fluoresce in the violet or blue region. A
visual observation of the samples makes it clear that the fluores-
cence intensity of the compounds in the solid state is appreciably
greater than in methanol solution, although the intensity is still
not very high. However, no reliable quantum yields could be
measured on the thin solid films of these compounds. The spectra
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2042 DOI: 10.1021/la1047276
Langmuir 2011, 27(5), 2036–2043

Cardinaels et al.
Article
It should be noted that no ionic liquids based on 1,10-
phenanthrolinehave beenreportedso far. 1,10-Phenanthrolinium
DOSS salts 1c-4c can be considered to be genuine ionic liquids by
the definition of ionic liquids being organic salts with a melting
point below 100 °C. Although the melting points of the DOS salts
are much higher than those of the DOSS salts, an interesting
feature of the DOS salts is the presence of the highly ordered
smectic E phase (E phase).42 This phase has also been observed in
the past for ionic liquid crystals, for instance, for pyrrolidinium,17,22
piperidinium,23 pyridinium,43 and imidazolium salts.44
most mesophases occur, no measurable fluorescence is expected
for the compounds in the mesophase.
The combination of an organic cation (with or without alkyl
chains) with a dialkyl sulfosuccinate anion is proposed as a facile
general approach toward ionic liquid crystals. We realize that the
quaternization of the nitrogen atoms in 1,10-phenanthroline, as
demonstrated in this article, is not the only method for the easy
functionalization of 1,10-phenanthroline and other nitrogen-
containing molecules. For instance, noncovalent interactions
such as H-bond formation between a nitrogen atom and the
proton of a carboxylic acid and halogen bonding are alternative
methods of functionalization.45-52 This could lead to other types
of 1,10-phenanthroline-derived liquid crystals.
Conclusions
1,10-Phenanthrolinium, 5-methyl-1,10-phenanthrolinium,
5-chloro-1,10-phenanthrolinium, and 4,7-diphenyl-1,10-phenan-
throlinium (bathophenanthrolinium) cations have been com-
bined with dodecyl sulfate (DOS) or dioctyl sulfosuccinate
(DOSS) anions to design a new class of ionic liquid crystals.
Except for the compounds containing a 4,7-diphenyl-1,10-phe-
nanthrolinium cation and DOS salt 5b, all of the DOS and DOSS
salts were found to be liquid-crystalline. The DOS salts showed a
highly ordered smectic E phase, whereas the DOSS salts showed a
disordered smectic A phase. Although the compounds exhibit a
greenish or whitish fluorescence in the solid state and dissolve in
methanol, the fluorescence intensities are weak at room tempera-
ture. The differences in the positions of the emission bands and
quantum yields can be related to the nature of the substituents
on the 1,10-phenanthroline ring and to the planarity of the
heterocyclic ring system. Given the high temperatures at which
Acknowledgment. K.G. is a postdoctoral fellow of the FWO-
Flanders, and S.V.E. is a visiting postdoctoral fellow of the FWO-
Flanders. K.L. is a research assistant of the IWT-Flanders.
Financial support by the K.U. Leuven (projects GOA 08/05
and IDO/05/005) is gratefully acknowledged. CHN elemental
analyses were performed by Dirk Henot. Powder X-ray diffracto-
grams were recorded by Dr. Jan D’Haen and Bart Ruttens
(University of Hasselt, Institute for Materials Research).
Supporting Information Available: Synthesis procedures
and characterization of all of the compounds. Absorption
and fluorescence spectra of the compounds dissolved in
methanol. X-ray diffractogram of the smectic E phase. DSC
traces. This material is available free of charge via the
Internet at http://pubs.acs.org.
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