Full-color tunable photoluminescent ionic liquid crystals based on tripodal pyridinium, pyrimidinium, and quinolinium salts

Article abstract of DOI:10.1021/ja3001979

Color-tunable luminescent ionic liquid crystals have been designed as a new series of luminescent materials. To achieve tuning of emission colors, intramolecular charge transfer (ICT) character has been incorporated into tripodal molecules. A series of th

Full text of DOI:10.1021/ja3001979

Article
pubs.acs.org/JACS
Full-Color Tunable Photoluminescent Ionic Liquid Crystals Based on
Tripodal Pyridinium, Pyrimidinium, and Quinolinium Salts
Kana Tanabe,† Yuko Suzui,‡ Miki Hasegawa,‡ and Takashi Kato*,†
†Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo
113-8656, Japan
‡Department of Chemistry and Biological Science, College of Science and Engineering, Aoyama-Gakuin Univerisity, 5-10-1
Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan
S
* Supporting Information
ABSTRACT: Color-tunable luminescent ionic liquid crystals
have been designed as a new series of luminescent materials.
To achieve tuning of emission colors, intramolecular charge
transfer (ICT) character has been incorporated into tripodal
molecules. A series of the compounds has three chromophores
in each molecule, incorporated with both electron-donating
moieties such as alkylaminobenzene and alkoxybenzene, and
electron-accepting moieties such as pyridinium, pyrimidinium,
and quinolinium parts. These C3-symmetrical molecules self-
assemble into liquid-crystalline (LC) columnar (Col)
structures over wide temperature ranges through nanosegregation between ionic moieties and nonionic aliphatic chains.
Photoluminescent (PL) emissions of these tripodal molecules are observed in the visible region both in the self-assembled
condensed states and in solutions. For example, a pyrimidinium salt with didodecylaminobenzene moieties exhibits yellowish
orange emission (λem = 586 nm in a thin film). Multicolor PL emissions are successfully achieved by simple tuning of changing
electron-donating and electron-accepting moieties of the compounds, covering the visible region from blue-green to red. It has
been revealed that ICT processes in the excited states and weak intermolecular interactions play important roles in the
determination of the PL properties of the materials, by measurements of UV−vis absorption and emission spectra, fluorescence
lifetimes, and PL quantum yields.
chains (Figure 1). To date, color-tunable emissions have not
been obtained for ionic liquid crystals in bulk states.
INTRODUCTION
■
Photoluminescence of organic compounds is a conventional,
but still attractive phenomenon from the viewpoints of both
fundamental science and practical applications.1 Among a large
amount of organic fluorophores including neutral and charged
molecules, luminescent ionic molecules are fascinating
candidates for a wide variety of applications such as sensors2
and optoelectronic devices3 because they have unique features
such as high solubility in polar solvents, strong interfacial
dipoles from the ionic moieties, possibility of ionic migration
under an applied electric field, and strong molecular
aggregation behavior due to the electrostatic interactions.
These ionic luminescent molecules have organic aromatic
chromophores. These materials are desired to exhibit
luminescent properties in both solutions and solid states for
various applications. Studies on the photoluminescent (PL)
properties in solutions are essential for understanding the
photophysics of the isolated chromophores, while those in the
condensed states are necessary as well for various optoelec-
tronic applications such as organic fluorescent sensing film,4
organic light-emitting diodes, and organic solid-state lasers.5
Herein, we report on new PL ionic liquid crystals exhibiting
columnar liquid-crystalline (LC) states. The basic molecular
design is push−pull type C3-symmetrical ions bearing long alkyl
However, emissions from organic chromophores are usually
suppressed in their condensed states even if they are highly
emissive in solutions. This general phenomenon occurs due to
the concentration quenching caused by intermolecular
interactions such as energy transfer and excimer formation. In
some cases, opposite phenomena are observed in which
chromophores are more emissive in the aggregated states
than in the solutions. This behavior is called as aggregation-
induced emission (AIE) or aggregation-induced emission
enhancement (AIEE), which is often explained by the
restriction of the molecular motion in the aggregated states.6
“Aggregated states” represent various bulk states with no
solvent such as powder, thin films, single crystals, and liquid
crystals as well as aggregation in solutions. Among them, liquid
crystals are quite attractive materials because they have both
liquid-like fluidity and crystal-like order and have a wide variety
of applications such as electric and optical functional devices.7,8
Ionic liquid crystals have attracted much attention9 because
of their potential to act as organized reaction media, anisotropic
Received: January 8, 2012
Published: February 28, 2012
© 2012 American Chemical Society
5652
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
Figure 1. Schematic illustrations of molecular design.
Figure 2. Molecular structures of tripodal pyridinium salts Py1−3, pyrimidinium salts Pm1−3, and quinolinium salts Q1−3.
ion-conductive materials,10 and redox-active materials.11
Despite their potential applications, there are only a few PL
ionic liquid crystals reported such as pyrilium,12 oxadiazolylpyr-
idinium,13 benzobis(imidazolium),14 and pyrazolium salts.15
Recently, Ziessel and co-workers have prepared highly
luminescent ionic liquid crystals based on BODIPY and
anthracene cores.16 The limited number of luminescent ionic
liquid crystals is partially due to the smallness of the π-
conjugation of ionic LC molecules. Moreover, their small π-
conjugation restricts the variety of the emission color. Hence,
sufficient insight has not yet been obtained either on the
fundamental properties or on the applications. The develop-
ment of the methodology for the design on the LC molecules
based on simpler luminescent ionic cores would enable us to
gain the insight into the photophysical properties of ionic
compounds not only as isolated molecules but also as self-
assembled LC materials, leading to the development of a new
series of color-tunable luminescent materials.
intramolecular charge transfer (ICT) reactions. It is well-
known that aminobenzonitrile derivatives in polar solvents
show dual fluorescence, which originates from locally excited
and twisted ICT (TICT) states.17 Not only such neutral D−A
molecules, but also an ionic D−A system can exhibit such
fluorescence from an excited ICT state. For example, Fromherz
et al. have reported that the emission from excited TICT state
of aminophenyl pyridinium salts is observed in polar
solutions.18 To date, however, most of the study on the
emission of such ionic D−A compounds has been conducted in
solutions, not in the self-assembled condensed states. Here, we
report on the self-assembled structures and the photophysical
properties of tripodal ionic LC compounds.
In this context, cationic organic compounds Py1−3, Pm1−3,
and Q1−3 were designed (Figure 2). In our molecular design,
each of three kinds of electron donating dodecyloxy- or
dodecylaminophenyl groups is attached to each of three kinds
of electron-accepting cationic parts. As electron-donating parts,
3,4-didodecyloxybenzene (for compounds Py1, Pm1, and Q1),
3,4,5-tridodecyloxybenzene (for compounds Py2, Pm2, and
Q2), or 4-(N,N-didodecylamino)benzene moieties (for com-
pounds Py3, Pm3, and Q3) are introduced at three terminals of
the tripodal molecules. For electron-accepting parts, pyridinium
To obtain multicolor fluorescence from small π-conjugated
moieties, it is a useful approach to attach both electron-
donating and -accepting parts to the ends of a chromophore.
Such donor (D)−acceptor (A) type chromophores can exhibit
photoinduced fluorescence emission via the excited-state
5653
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
Scheme 1. Synthetic Routes of Tripodal Pyridinium, Pyrimidinium, and Quinolinium Salts
salts (for compounds Py1−3), pyrimidinium salts (for
compounds Pm1−3), or quinolinium salts (for compounds
Q1−3) are incorporated with these compounds. Terminal long
alkyl chains are expected to enhance the nanosegregation
between ionic and nonionic parts, which lead to the formation
of self-assembled LC structures. As an entire molecular shape,
tripodal shape is employed for all of the compounds, which
would be of advantage to chemoselective molecular and anion
recognition,19 leading to the possibility of applications for
sensory materials. In our preliminary communication, thermal
properties and photophysical properties was reported only for
pyridinium salt Py2.20 C3-symmetrical molecular shape21 would
facilitate the formation of self-assembled columnar structures
that can be easily aligned, leading to one-dimensional carrier
transport10 and polarized photoluminescence.22
were characterized by 1H and 13C NMR spectroscopy,
elemental analysis, and MALDI-TOF mass spectroscopy.
Liquid-Crystalline Properties. All of the compounds
exhibit LC columnar phases (Figure 3, Table 1). The self-
assembled structures of these tripodal compounds were studied
by differential scanning calorimetry (DSC) and X-ray
diffraction (XRD) measurements. Compound Py1 shows a
crystal (Cryst)−rectangular columnar (Colr) phase transition at
RESULTS AND DISCUSSION
■
Syntheses of the Tripodal Salts. All of the tripodal
compounds were prepared by quaternization reactions of the
corresponding phenylpyridine, phenylpyrimidine, or phenyl-
quinoline derivatives with 1,3,5-tribromomethylbenzene fol-
lowed by the counteranion exchange from bromide to
hexafuluorophosphate in moderate or high yields (Scheme 1,
see Supporting Information in details). All of the compounds
Figure 3. Polarized optical micrograph of the tripodal pyridinium salt
Py1 at 235 °C in the Col phase. Arrows indicate the directions of
polarizer and analyzer axes.
5654
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
Table 1. Thermal Properties of the Tripodal Salts
a
compound
phase transition behavior
lattice parameters
Py1
Cryst 48 (14) Colr 120 (19) Col
a = 74.0 Å, b = 34.5 Å
b
c
240 Iso
(Colr)
Py2
Cryst1 13 (6) Cryst2 24 (7) Colr
a = 77.4 Å, b = 33.4 Å
b
d
245 Iso
(Colr)
Py3
Cryst −30 (0.4) Colr 105 (4) Cub a = 73.1 Å, b = 35.7 Å
b
e
180 Iso
(Colr)
b
Pm1
Pm2
Cryst 86 (19) Colr 210 Iso
a = 76.4 Å, b = 42.3 Å
f
(Colr)
b
Cryst −32 (25) Colr 240 Iso
a = 73.4 Å, b = 29.1 Å
d
(Colr)
d
Pm3
Q1
Glassy −10 Colh 176 (0.8) Iso
a = 40.9 Å (Colh)
Cryst 56 (11) Colr 191 (2) Iso
a = 81.0 Å, b = 41.7 Å
g
(Colr)
h
Q2
Q3
Cryst −37 (26) Colh 223 (3) Iso
a = 42.9 Å (Colh)
b
i
Glassy 79 (0.3) Colh 210 Iso
a = 42.1 Å (Colh)
a
Transition temperatures and enthalpies (kJ mol−1, in parentheses)
were determined by DSC measurements on the second heating at 10
°C min−1. Cryst, crystalline; Glassy, glassy; Colh, hexagonal columnar;
Colr, rectangular columnar; Col, unidentified columnar; Cub,
b
unidentified cubic; Iso, isotropic; dec., decomposition. The transition
temperatures were determined by polarizing optical microscope.
c
d
e
Measured at 80 °C. Measured at 140 °C. Measured at 50 °C.
f
g
h
Measured at 180 °C. Measured at 160 °C. Measured at 90 °C.
Measured at 100 °C.
i
48 °C. In the formation of LC columnar structures, segregation
between ionic pyridinium units and nonionic aliphatic parts
could be an important driving force in a way similar to
previously reported ionic liquid crystals (Figure 4).9−11 When
the number of the alkoxy chains is increased from six to nine,
the melting temperatures are decreased. For example, the
melting temperature of compound Py2 with nine terminal
dodecyloxy chains (Tm = 24 °C) is lower than that of
compound Py1 with six terminal dodecyloxy chains (Tm = 48
°C). As the number of alkoxy chains increases, central ionic
cores become more surrounded by the flexible aliphatic
moieties, which would lead to the stabilization of the LC
columnar structures. As the temperature rises above 240 °C, all
of the compounds gradually start to exhibit phase transitions to
isotropic liquids or to decompose before reaching the
isotropization temperatures with disappearance of the optical
textures, which does not recover after the thermal decom-
position.
Figure 5. X-ray diffraction patterns of the tripodal salts (a) Py2, (b)
Pm2, and (c) Q2 at 140 °C in the Col phases.
shows the XRD patterns of compounds Py2, Pm2, and Q2
measured at 140 °C. The XRD pattern of pyridinium salt Py2
shows two intense peaks at 38.7 and 30.7 Å and two weak peaks
at 17.1 and 10.9 Å, which are assigned to the (20), (11), (02),
and (13) reflections of a rectangular lattice with the lattice
parameters of a = 77.4 Å and b = 33.4 Å (Figure 5a).
Pyrimidinium salt Pm2 exhibits analogous XRD patterns
(Figure 5b), suggesting that Pm2 self-assembles into Colr
structures in a manner similar to Py2. Meanwhile, the XRD
pattern of quinolinium salt Q2 shows only one intense peak at
The preferable self-assembled structures are changed from
Colr to hexagonal columnar (Colh) structures via a structural
change of ionic cores such as expansion of the hetero rings (i.e.,
quinolinium salts) and the introduction of nitrogen atoms (i.e.,
pyrimidinium salts). All three pyridinium salts Py1−3 form
Colr phases, while pyrimidinium salts Pm3 and quinolinium
salts Q2 and Q3 form Colh structures. For example, Figure 5
Figure 4. A schematic representation of the self-assembled columnar structure of tripodal salts. PF6 anions are omitted for clarity.
5655
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
36.5 Å and a weak peak at 17.6 Å. In conjunction with a fan
texture in POM image of Q2, characteristic of a columnar
phase, these peaks can be assigned to the (10) and (20)
reflections of a hexagonal columnar structure (Figure 5c,
Supporting Information). This behavior can be explained by
the flexibility of the three arms of tripodal molecules. As the
ionic cores become larger, intramolecular rotation of the arms
may become restricted due to steric repulsions between ionic
cores. In a similar way, the introduction of nitrogen atoms can
lead the restriction of the intramolecuar rotation due to the
electrostatic repulsion between the lone pair of nitrogen atom
and the closest π-electron systems. Only a little rotational
flexibility of tripodal molecules can lead to comparatively
circular molecular shape. Such molecules would stack along
their perpendicular axises, leading to columnar structures that
self-assemble into hexagonal order.
thin films are much lower than those in the chloroform
solutions. For example, Φf of Py1 in the condensed state is
0.116, while that in dichloromethane solution is 0.414. It is a
common phenomenon that the formation of delocalized
excitons in the condensed states quenches the emission of
organic lumminophores.1 In contrast, the bulk states of
pyrimidinium Pm1−3 and quinolinium salt Q1−3 are more
emissive than the solution states except for Pm2. In particular,
Φf values of quinolinium salts Q1−3 are about 10−20 times
larger than those in solutions. For example, Q2 is emissive in
the LC state with the Φf of 0.088, while it is nearly nonemissive
in the solution (Φf = 0.004). The enhanced emission is ascribed
to the aggregation-induced emission enhancement (AIEE).6
Although the origin of this AIEE phenomenon is not fully
understood, aggregation-induced restriction of the molecular
motion or planarization of the distorted chromophore units
may occur as Tang et al. have explained on the AIE or AIEE
behavior of pentaphenylsilole and biphenyl derivatives.6 In the
present study, the rotation between cationic heteroring and
electron-donating phenyl ring could be restricted or slow down
when the molecules assemble with each other. In the case of
quinolinium salts Q1−3, due to the steric hindrance of three
quinoline rings of the tripodal molecules, chromophores could
not assemble into close-packing structures even in the
condensed states, whereas pyridinium salts Py1−3 and
pyrimidinium salts Pm1−3 have relatively large spaces at the
center of the molecules. As a result, photoluminescence from
only Q1−3 is significantly enhanced in the condensed states. In
addition, these PL wavelengths are not definitely changed by
the thermal phase transitions between Cryst and LC phases.
For example, compound Py1 shows bluish green photo-
luminescence with a peak at 515 nm in the Cryst phase at room
temperature, and at 512 nm in the Col phase at 100 °C (Figure
7). Only a little change of the peak positions is observed upon
the Cryst−Col phase transition. This behavior indicates that
interactions between the cationic chromophores of the tripodal
salts are not affected by the self-assembled structures.
Photophysical Properties in the Condensed States. As
shown in Figure 6, all of the compounds exhibit PL emissions
Figure 6. Photographs (left) and selected photoluminescence spectra
(right) of the tripodal compounds (a) Py1, (b) Py2, (c) Py3, (d)
Pm1, (e) Pm2, (f) Pm3, (g) Q1, (h) Q2, and (i) Q3 in the condensed
states. These photographs were taken under UV light (365 nm)
irradiation in the bulk states at room temperature.
in their LC or solid states. It is noteworthy that the emission
colors in this series cover the visible region from blue to red
even in their condensed states. This is the first example of the
series of PL ionic liquid crystals that show multicolored
luminescence. Details in photophysical properties of the
compounds are shown in Table 2.
Fluorescence quantum yields (Φf) for these compounds in
annealed films were determined with an absolute PL quantum
yield measurement system (see Experimental Section for
details). For pyridinium salts Py1−3, the values of Φf in the
Photophysical Properties in Solution. All of the tripodal
compounds exhibit fluorescence emission both in solution as
well as in the condensed states (Table 2). For example,
pyridinium salt Py1 shows blue-green emission in the
dichloromethane solution with the absorption maximum at
386 nm and the emission maximum at 511 nm. This emission
may originate from the ICT excited state, due to that the
a
Table 2. Photophysical Properties of the Tripodal Salts
b
in solution
in annealed films
c
d
e
compound
λabs /nm
λem /nm
Stokes shift/eV
Eg/eV
Φf
λabs/nm
λem/nm
Φf
state
Py1
Py2
Py3
Pm1
Pm2
Pm3
Q1
386
375
511
518
530
526
524
535
598
608
651
0.79
0.91
0.34
0.64
0.67
0.19
0.84
0.99
0.42
2.76
2.74
2.39
2.59
2.58
2.30
2.38
2.51
2.00
0.414
0.153
0.152
0.064
0.026
0.021
0.012
0.004
0.005
380
375
515
520
542
560
578
586
563
577
652
0.116
0.043
0.048
0.093
0.011
0.037
0.126
0.088
0.042
Cryst
Colr
463
432
Colr
414
399
Cryst
Colr
408
388
494
465
Colh
Cryst
Colh
Glassy
323, 425
321, 409
288, 533
323, 408
322, 391
286, 506
Q2
Q3
a
λabs, absorption maxima; λem, emission maxima; Eg, HOMO−LUMO gaps estimated from the onset positions of the UV−vis absortption spectra;
Φf, fluorescence quantumn yields.; Cryst, crystalline; Glassy, glassy; Colh, hexagonal columnar; Colr, rectangular columnar. Excitation wavelengths
b
are consistent with λabs of the same samples. Annealed at 180 °C for 10 min and then cooled to room temperature. Measured at room temperature.
c
d
e
Measured in 1.0 × 10−5 M dichloromethane solutions. Measured in 1.0 × 10−6 M dichloromethane solutions. Measured in chloroform solutions.
5656
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
Figure 7. Photoluminescence spaectra of Py1 in the Cryst phase at
room temperature and in the Col phase at 50, 100, 150, and 200 °C.
The excitation wavelength is 380 nm.
Figure 9. Normalized absorption (solid lines) and photoluminescence
spectra (dashed lines) of the pyridinium salts Py1 (indicated in black),
Py2 (red), and Py3 (blue) in dichloromethane solutions.
structures of the chromophores have both electron donor and
acceptor substituents. To confirm the ICT character of the
tripodal molecule, frontier molecular orbitals have been studied
by density functional theory (DFT) calculations for the model
compound. Figure 8 shows the highest occupied molecular
orbital (HOMO), second HOMO (HOMO−1), and lowest
unoccupied molecular orbital (LUMO) of the model
compound for Py1 with shorter alkyl chains. HOMO and
HOMO−1 localize the electron-donating dialkoxy benzene
units, while LUMO localizes the electron-accepting pyridinium
parts, and the three chromophores are independent from each
other. Compound Py1 with longer alkyl chains may have
analogical electronic structure, which would lead to the ICT
feature of the molecule. To study the origin of the ICT
emission in detail, substituent and solvent effects were studied
(vide infra).
Effects of Electron-Donating Parts on the Photo-
physical Properties. To examine the effects of electron-
donating moieties on the photophysical properties, the
electronic properties of electron-donating parts are directly
compared in the single core skeleton. In the single skeleton, the
electron-donating abilities are considered to be increased in the
following order: 3,4-didodecyloxybenzene (electron-donating
parts for compounds Py1, Pm1, and Q1), 3,4,5-tridodecylox-
ybenzene (for Py2, Pm2, and Q2), and 4-(N,N-
didodecylamino)benzene (for Py3, Pm3, and Q3). For
representative examples, absorption and emission spectra of
pyridinium salts Py1−3 are given in Figure 9. On the core
skeleton with pyridinium salts (compounds Py1−3), the
corresponding wavelengths of absorption maxima are 386,
375, and 463 nm in dichloromethane solutions, respectively.
The bathochlomic shift of absorption wavelength is obtained in
the order of Py2, Py1, and Py3. On the other single skeleton,
the same tendency is observed, where 3,4-didodecyloxybenzene
(for Py1, Pm1, and Q1) induces more bathochromic shift in
the absorption wavelength than expected from the number of
alkoxy chains. This tendency can be explained by the
asymmetrical structure of 3,4-didodecyloxybenzene in the
single chromophore; however, details are under investigation.
In the PL spectra of compounds Py1−3, the wavelength of
emission maxima are 511, 518, and 530 nm, respectively. In
addition, the values of the Stokes shift are decreased in the
order of Py2, Py1, and Py3.
Frontier molecular orbitals have been studied by DFT
calculations for the model monocationic compounds with
shorter alkyl chains (Figure 10). For the model compounds of
Py1 and Py2 having alkoxy chains as their electron-donating
groups, the HOMO orbitals are located on the electron-
donating groups, whereas the LUMO orbitals are localized on
the electron-accepting cationic parts. These orbitals are
considered as largely π orbitals of the electron-donating
alkoxyphenyl groups and π* orbitals of the electron-accepting
pyridinium units, respectively. For the model compound of Py3
having an alkylamino group, the HOMO is comparatively
delocalized all over the cationic chromophore, which is
explained by an involvement of the lone electron pair on the
Figure 8. Molecular structure of the model tripodal compound for pyridinium salt Py1 and representation of the frontier orbitals (HOMO,
HOMO−1, and LUMO) of the model compounds obtained from DFT calculations at the B3LYP/6-31G* level.
5657
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
Figure 12. Representation of the frontier orbitals of the model
compounds for compounds Py1, Pm1, and Q1 obtained from DFT
calculations at the B3LYP/6-31G* level. Molecular structures of the
model compounds are indicated on the top of the figure. Upper
orbitals indicate LUMOs, and the bottom orbitals indicate HOMOs
for the model compounds.
Figure 10. Representation of the frontier orbitals of the model
compounds for pyridinium salts Py1−3 obtained from DFT
calculations at the B3LYP/6-31G* level. Molecular structures of the
model compounds are indicated on the top of the figure. Upper
orbitals indicate LUMOs, and the bottom orbitals indicate HOMOs
for the model compounds.
For all of the compounds, the HOMOs are almost totally
localized on the electron-donating dialkoxybenzene moieties,
while the LUMOs are mainly localized on the electron-
accepting cationic parts, in a manner similar to the discussion
on the effect of electron-donating nature.
nitrogen atom of the dialkylamino group in the conjugation of
the phenylpyridine backbone. As a result of this elongation of
conjugation, increase in the absorption and emission maxima
might be obtained for the alkylaminobenzene derivative Py3 as
compared to the corresponding alkoxybenzene derivatives Py1
and Py2.
Effects of Electron-Accepting Parts on the Photo-
physical Properties. The direct comparison of the electronic
properties of electron-accepting moieties is conducted as well
as electron-donating parts. Figure 11 shows the absorption and
Solvent Effects on the Photophysical Properties. To
study the CT character of the tripodal salts, solvent effects on
the PL properties were studied. For example, the results of
pyridinium salts Py1−3 are shown below. Photoluminescence
spectra were measured in different solvents with various
polarities (see the Supporting Information). All of the
compounds show obvious positive solvatochromism in the PL
spectra except for cyclohexane solutions, where the molecules
are considered to aggregate with each other because the solvent
polarity is too low to disperse such ionic molecules. This
behavior is illustrated more clearly in Figure 13 where the
emission maxima (νmax, in wavenumber) are plotted against the
solvent polarity parameter ET(30).23 For all of the compounds,
νmax values decrease with increasing solvent polarity, obeying
close-to-linear relationship for ET(30) values. Moreover,
relatively large Stokes shifts (0.4−2.2 eV) and a lack of fine
structure emission of the tripodal compounds also indicate the
Figure 11. Normalized absorption (solid lines) and photolumines-
cence spectra (dashed lines) of the pyridinium salts Py1 (indicated in
black), Pm1 (green), and Q1 (orange) in dichloromethane solutions.
emission spectra of compounds Py1, Pm1, and Q1 having 3,5-
didodecylaminobenzene as the single electron donating part.
Both for pyrimidinium salt Pm1 and quinolinium salt Q1, the
absorption maxima are red-shifted as compared to those of
corresponding pyridinium salt Py1 by 28 and 39 nm,
respectively (λabs = 386 nm for Py1; λabs = 414 nm for Pm1;
λabs = 425 nm for Q1). The same tendency is observed in the
emission maxima (λem = 511 nm for Py1; λem = 526 nm for
Pm1; λabs = 598 nm for Q1). This is attributed to more
electron-deficient property of pyrimidine ring and π-expanded
nature of quinoline ring as compared to pyridine ring, either of
which results in low-lying LUMO level, leading to red-shifted
absorption and emission. Figure 12 depicts frontier orbitals of
the model monocationic compounds for Py1, Pm1, and Q1.
Figure 13. The dependence of the emission maxima for compounds
Py1 (indicated in black), Py2 (red), and Py3 (blue) on the solvent
polarity parameter ET(30). The data recorded in cyclohexane are
omitted for linear regression analyses due to low solubility of the
compounds.
5658
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
Table 3. Fluorescence Lifetimes of the Pyridinium Salts Py1−3 in Thin Films
a
b
as-spin-coated film
annealed film
compound
λem/nm
τ1/ns
τ2/ns
A1
A2
χ
τ1/ns
τ2/ns
A1
A2
χ
Py1
385−470
450−588
550−650
455−593
477−615
4.3
4.0
6.9
6.5
2.7
15.4
11.9
19.3
16.1
11.9
0.94
0.83
0.88
0.95
0.95
0.06
0.17
0.12
0.05
0.05
1.005
1.121
1.166
1.022
1.144
<2
4.1
5.9
2.9
3.7
6.9
12.5
23.3
8.7
0.97
0.95
0.95
0.91
0.98
0.03
0.05
0.05
0.09
0.02
1.018
1.051
0.991
0.999
1.040
Py2
Py3
17.3
a
b
Spin-coated from chloroform solutions. Annealed at 180 °C for 10 min and then cooled to room temperature.
strong ICT character of the chromophores. This indicates
efficient charge shifts in the excited states of these compounds,
which are compatible with the frontier orbital pictures of the
model monocationic compounds obtained through DFT
calculations as mentioned above.
Fluorescence Lifetime Measurements. To study the
origin of the PL emissions, the fluorescence lifetimes for the
tripodal salts were measured both in solutions and in the
condensed states. In chloroform solutions, it was revealed that
these compounds have two PL components with lifetimes of
nanosecond order. For example, compound Py1 has two PL
components with lifetimes of ca. 1 and 5 ns. Fluorescence
lifetime measurements in the condensed states were also
conducted for both as-spin-coated films and annealed spin-
coated films.
For the as-spin-coated films in which tripodal molecules are
rather randomly assembled with each other with electrostatic
interactions and van der Waals interactions, two emissive
components have been observed with mostly slower lifetimes
than those in solutions. As shown in Table 3, all of the
molecules decay through two relaxation pathways. For example,
the lifetimes of compound Py1 in an as-spincoated film are
revealed to be 4 ns (83%) and 12 ns (17%) at emission
wavelength in the range of 450−588 nm. Two emissive
components are observed even in the films, both of which can
be attributed to monomeric components from intramolecular
CT states. The shorter ones can be assigned to be the emissive
components with little perturbation and the longer ones to be
those without obvious intermolecular interactions (see also the
Supporting Information). Their vibrational deactivation path-
ways are preferably suppressed in the as-spin-coated films
because of the restriction of the intramolecular vibrational and
rotational motions.
As compared to the as-spin-coated films, it was revealed that
the origin of emissions places a disproportionate emphasis on
the shorter components of the two in the annealed films. For
example, compound Py1 in an annealed film has two pathways
with lifetimes of 4 ns (95%) and 13 ns (5%) at emission
wavelength in the range of 450−588 nm, which are much more
dominated by the former component than those in the spin-
coated films mentioned above. This indicates that annealing of
the films may lead to closer packing of the self-assembled
molecules having interactions with each other, which results in
the existence of larger amounts of molecules with a fast process
of radiative decay.
pyridinium, pyrimidinium, and quinolinium salts, which self-
organize into columnar or micellar cubic LC phases via
nanosegregation between ionic and nonionic moieties. Our
finding in the present study gives us new insights into
fundamental research for the luminescent compounds on
molecular conformation, intramolecular motion, and intermo-
lecular interactions in the self-assembled condensed states.
Moreover, the results can be useful for materials design for
potential applications such as organic fluorescent sensors and
optoelectronic materials.
EXPERIMENTAL SECTION
■
General. 1H and 13C{1H} NMR spectra were recorded on a JEOL
JNM-LA400 spectrometer. Mass spectra were obtained with a
PerSeptive Biosystems Voyager-DE STR spectrometer. Elemental
analyses were carried out with a Yanaco MT-6 CHN autocorder.
Thermal properties of compounds Py1−3, Pm1−3, and Q1−3
were examined by polarizing optical micrography (POM), differential
scanning calorimetry (DSC), and X-ray diffraction (XRD) measure-
ments. DSC measurements were performed on a NETZSCH DSC204
Phoenix calorimeter at a scanning rate of 10 °C min−1. A POM
Olympus BH-51 equipped with Linkam LTS 350 hot stage was used
for visual observation. XRD patterns were obtained using a Rigaku
RINT-2500 diffractometer with a heating stage using Ni-filtered Cu
Kα radiation. UV−vis absorption and photoluminescence spectra were
measured with JASCO V-670 and a JASCO FP-777W spectrometers,
respectively. The excitation wavelength used was that of the UV−vis
absorption maximum of each sample. The fluorescence quantum yield
(Φf) was measured with an absolute PL quantum yield measurement
system (Hamamatsu C9920-02), in which excitation was provided
with a Xe lamp after passing through a monochromator.24 The
fluorescence lifetime was measured with HAMAMATSU C4334 streak
scope.
Materials and Syntheses. All reagents and solvents were
purchased from Aldrich, Tokyo Kasei, Kanto Chemical, or Wako,
and used as received. The synthetic routes used to obtain pyridinium
salts (Py1−3), pyrimidinium salts (Pm1−3), quinolinium salts (Q1−
3) are shown in Scheme 1. 4-Bromoquinoline,25 1-bromo-3,4-
didodecyloxybenzene,26 1-bromo-3,4,5-tridodecyloxybenzene,20 4-
(3,4,5-didodecyloxyphenyl)pyridine (1b),20 1,3,5-tris(4-(3,4,5-
tridodecyloxyphenyl)pyridiniummethyl)benzene bromide (4b),20 and
1,3,5-tris(4-(3,4,5-tridodecyloxyphenyl)pyridiniummethyl)benzene
hexafluorophosphate (Py2)20 were prepared according to the
literature. For other compounds, details in the syntheses and
characterization data are included in the Supporting Information.
ASSOCIATED CONTENT
■
S
* Supporting Information
CONCLUSION
■
Syntheses and characterization of compounds Py1, Py3, Pm1−
3, Q1−3, 1a, 1c, 2a−c, 3a−c, 4a, 4c, 5a−c, and 6a−c, and
additioncal photophysical properties of the compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
We have demonstrated LC materials with simple ionic cores,
which show full-color tunable emission ranging over the visible
region simply by changing electron-donating and -accepting
abilities. These LC materials are a new series of tripod-shaped
5659
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
Kumar, S. Chem. Soc. Rev. 2011, 40, 306. (r) Lemieux, R. P. Acc. Chem.
Res. 2001, 34, 845. (s) Kim, F. S.; Ren, G.; Jenekhe, S. A. Chem. Mater.
2011, 23, 682.
(9) (a) Binnemans, K. Chem. Rev. 2005, 105, 4148. (b) Bruce, D. W.
Acc. Chem. Res. 2000, 33, 831. (c) Bazuin, C. G.; Guillon, D.; Skoulios,
A.; Nicoud, J.-F. Liq. Cryst. 1986, 1, 181. (d) Ujiie, S.; Iimura, K.
Macromolecules 1992, 25, 3174. (e) Haramoto, Y.; Akiyama, Y.;
Segawa, R.; Ujiie, S.; Nanasawa, M. J. Mater. Chem. 1998, 8, 275.
(f) Katoh, M.; Uehara, S.; Kohmoto, S.; Kishikawa, K. Chem. Lett.
2006, 35, 322.
(10) (a) Yoshio, M.; Ichikawa, T.; Shimura, H.; Kagata, T.;
Hamasaki, A.; Mukai, T.; Ohno, H.; Kato, T. Bull. Chem. Soc. Jpn.
2007, 80, 1836. (b) Yoshio, M.; Mukai, T.; Ohno, H.; Kato, T. J. Am.
Chem. Soc. 2004, 126, 994.
(11) (a) Yazaki, S.; Funahashi, M.; Kagimoto, J.; Ohno, H.; Kato, T.
J. Am. Chem. Soc. 2010, 132, 7702. (b) Tanabe, K.; Kato, T. Chem.
Commun. 2009, 1864. (c) Yazaki, S.; Funahashi, M.; Kato, T. J. Am.
Chem. Soc. 2008, 130, 13206. (d) Tanabe, K.; Yasuda, T.; Yoshio, M.;
Kato, T. Org. Lett. 2007, 9, 4271.
AUTHOR INFORMATION
■
Corresponding Author
kato@chiral.t.u-tokyo.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Mr. Daisuke Kodama for the measurements of
fluorescence quantum yield and lifetime of a reference
compound. This work was partially supported by a Grant-in-
Aid for Scientific Research (A) (no. 19205017) from the Japan
Society for the Promotion of Science (JSPS) and a Grant-in-Aid
for a Scientific Research on Innovative Areas of “Fusion
Materials: Creative Development of Materials and Exploration
of Their Function through Molecular Control” (area no. 2206)
and the Global COE Program for Chemistry Innovation from
the Ministry of Education, Culture, Sports, Science, and
Technology. K.T. is grateful for financial support from the
JSPS Research Fellowship for Young Scientists.
́
(12) Markovitsi, D.; Lecuyer, I.; Clergeot, B.; Jallabert, C.; Strzelecka,
H.; Veber, M. Liq. Cryst. 1989, 6, 83.
(13) (a) Haristoy, D.; Tsiourvas, D. Chem. Mater. 2003, 15, 2079.
(b) Celso, F. L.; Pibiri, I.; Triolo, A.; Triolo, R.; Pace, A.; Buscemi, S.;
Vivona, N. J. Mater. Chem. 2007, 17, 1201.
(14) (a) Boydston, A. J.; Pecinovsky, C. S.; Chao, S. T.; Bielawski, C.
W. J. Am. Chem. Soc. 2007, 129, 14550. (b) Boydston, A. J.; Vu, P. D.;
Dykhno, O. L.; Chang, V.; Wyatt, A. R. II; Stockett, A. S.; Ritschdorff,
E. T.; Shear, J. B.; Bielawski, C. W. J. Am. Chem. Soc. 2008, 130, 3143.
REFERENCES
■
(1) Molecular Fluorescence: Principles and Applications; Valeur, B., Ed.;
Wiley-VCH: Weinheim, 2001.
(2) (a) Ho, H.-A.; Boissinot, M.; Bergeron, M. G.; Corbeil, G.; Dore,
́
K.; Boudreau, D.; Leclerc, M. Angew. Chem., Int. Ed. 2002, 41, 1548.
(b) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2003,
125, 896. (c) Wang, S.; Bazan, G. C. Adv. Mater. 2003, 15, 1425.
(d) Dwight, S. J.; Gaylord, B. S.; Hong, J. W.; Bazan, G. C. J. Am.
Chem. Soc. 2004, 126, 16850.
(3) (a) Hoven, C. V.; Garcia, A.; Bazan, G. C.; Nguyen, T.-Q. Adv.
Mater. 2008, 20, 3793. (b) Jiang, H.; Taranekar, P.; Reynolds, J. R.;
Schanze, K. S. Angew. Chem., Int. Ed. 2009, 48, 4300.
(4) (a) Thomas, S. W. III; Joly, G. D.; Swager, T. M. Chem. Rev.
2007, 107, 1339. (b) Basabe-Desmonts, L.; Reinhoudt, D. N.; Crego-
Calama, M. Chem. Soc. Rev. 2007, 36, 993. (c) Martínez-Manez, R.;
́
Sancenon, F. Chem. Rev. 2003, 103, 4419. (d) Kinami, M.; Crenshaw,
B. R.; Weder, C. Chem. Mater. 2006, 18, 946.
(5) (a) Samuel, I. D. W.; Turnbull, G. A. Chem. Rev. 2007, 107, 1272.
(b) McGehee, M. D.; Heeger, A. J. Adv. Mater. 2000, 12, 1655.
(6) (a) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009,
4332. (b) Zeng, Q.; Li, Z.; Dong, Y.; Di, C.; Qin, A.; Hong, Y.; Ji, L.;
Zhu, Z.; Jim, C. K. W.; Yu, G.; Li, Q.; Li, Z.; Liu, Y.; Qin, J.; Tang, B.
Z. Chem. Commun. 2007, 70. (c) An, B.-K.; Kwon, S.-K.; Jung, S.-D.;
Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (d) Deans, R.; Kim, J.;
Machacek, M. R.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 8565.
(e) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Soc. Rev. 2011, 40,
5361.
(15) Giner, I.; Gascon
Lafuente, C. J. Phys. Chem. C 2009, 113, 18827.
(16) (a) Olivier, J.-H.; Camerel, F.; Barbera, J.; Retailleau, P.; Ziessel,
R. Chem.-Eur. J. 2009, 15, 8163. (b) Olivier, J.-H.; Camerel, F.; Ulrich,
G.; Barbera, J.; Ziessel, R. Chem.-Eur. J. 2010, 16, 7134.
́ ́ ́
, I.; Gimenez, R.; Cea, P.; Lopez, M. C.;
́
́
(17) (a) Grabowski, Z. W.; Rotkiewicz, K.; Rettig, W. Chem. Rev.
2003, 103, 3899. (b) Bhattacharyya, K.; Chowdhury, M. Chem. Rev.
1993, 93, 507. (c) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25,
971.
(18) (a) Rocker, C.; Heilemann, A.; Fromherz, P. J. Phys. Chem.
̈
́
̃
1996, 100, 12172. (b) Ephardt, H.; Fromherz, P. J. Phys. Chem. 1993,
97, 4540. (c) Fromherz, P.; Heilemann, A. J. Phys. Chem. 1992, 96,
6864. (d) Ephardt, H.; Fromherz, P. J. Phys. Chem. 1991, 95, 6792.
(e) Ephardt, H.; Fromherz, P. J. Phys. Chem. 1989, 93, 7717.
(19) (a) Belcher, W. J.; Fabre, M.; Farhan, T.; Steed, J. W. Org.
Biomol. Chem. 2006, 4, 781. (b) Turner, D. R.; Paterson, M. J.; Steed,
J. W. J. Org. Chem. 2006, 71, 1598. (c) Wallace, K. J.; Belcher, W. J.;
Turner, D. R.; Syed, K. F.; Steed, J. W. J. Am. Chem. Soc. 2003, 125,
9699. (d) Abouderbala, L. O.; Belcher, W. J.; Boutelle, M. G.; Cragg,
P. J.; Dhaliwal, J.; Fabre, M.; Steed, J. W.; Turner, D. R.; Wallace, K. J.
Chem. Commun. 2002, 358.
(20) Tanabe, K.; Yasuda, T.; Kato, T. Chem. Lett. 2008, 37, 1208.
(21) (a) Kimura, M.; Hatano, T.; Yasuda, T.; Morita, J.; Akama, Y.;
Minoura, K.; Shimomura, T.; Kato, T. Chem. Lett. 2009, 38, 800.
(b) Isoda, K.; Yasuda, T.; Kato, T. Chem. Asian J. 2009, 4, 1619.
(c) Hatano, T.; Kato, T. Chem. Commun. 2006, 1277. (d) Stals, P. J.
M.; Smulders, M. M. J.; Martín-Rapffln, R.; Palmans, A. R. A.; Meijer,
E. W. Chem.-Eur. J. 2009, 15, 2071. (e) Lehmann, M.; Jahr, M.;
Donnio, B.; Graf, R.; Gemming, S.; Popov, I. Chem.-Eur. J. 2008, 14,
(7) Handbook of Liquid Crystals; Demus, D., Goodby, J. W., Gray, G.
W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998.
(8) (a) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed.
2006, 45, 38. (b) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605.
(c) Kato, T. Science 2002, 295, 2414. (d) Pisula, W.; Feng, X.; Mullen,
̈
K. Adv. Mater. 2010, 22, 3634. (e) Kato, T.; Yasuda, T.; Kamikawa, Y.;
Yoshio, M. Chem. Commun. 2009, 729. (f) Kato, T.; Tanabe, K. Chem.
Lett. 2009, 38, 634. (g) Donnio, B.; Bruce, D. W. Struct. Bonding
(Berlin) 1999, 95, 193. (h) Saez, I. M.; Goodby, J. W. Struct. Bonding
(Berlin) 2008, 128, 1. (i) Bushey, M. L.; Nguyen, T.-Q.; Zhang, W.;
Horoszewski, D.; Nuckolls, C. Angew. Chem., Int. Ed. 2004, 43, 5446.
(j) Palmans, A. R. A.; Meijer, E. W. Angew. Chem., Int. Ed. 2007, 46,
8948. (k) Gin, D. L.; Pecinovsky, C. S.; Bara, J. E.; Kerr, R. L. Struct.
Bonding (Berlin) 2008, 128, 181. (l) Guillon, D. Struct. Bonding
(Berlin) 1999, 95, 41. (m) van Nostrum, C. F.; Nolte, R. J. M. Chem.
Commun. 1996, 2385. (n) Rowan, S. J.; Mather, P. T. Struct. Bonding
(Berlin) 2008, 128, 119. (o) Kikuchi, H. Struct. Bonding (Berlin) 2008,
128, 99. (p) Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.;
Imam, M. R.; Percec, V. Chem. Rev. 2009, 109, 6275. (q) Bisoyi, H. K.;
́
3562. (f) Gomez-Lor, B.; Alonso, B.; Omenat, A.; Serrano, J. L. Chem.
Commun. 2006, 5012. (g) Palmans, A. R. A.; Vekemans, J. A. J. M.;
Fischer, H.; Hikmet, R. A.; Meijer, E. W. Chem.-Eur. J. 1997, 3, 300.
(h) Kobayashi, Y.; Matsunaga, Y. Bull. Chem. Soc. Jpn. 1987, 60, 3515.
(i) Lehmann, M.; Jahr, M.; Grozema, F. C.; Abellon, R. D.; Siebbeles,
L. D. A.; Muller, M. Adv. Mater. 2008, 20, 4414. (j) Hennrich, G.;
̈
Ortiz, P. D.; Cavero, E.; Hanes, R. E.; Serrano, J. L. Eur. J. Org. Chem.
2008, 4575. (k) Yasuda, T.; Shimizu, T.; Liu, F.; Ungar, G.; Kato, T. J.
Am. Chem. Soc. 2011, 133, 13437.
(22) Yasuda, T.; Ooi, H.; Morita, J.; Akama, Y.; Minoura, K.;
Funahashi, M.; Shimomura, T.; Kato, T. Adv. Mater. 2009, 19, 411.
(23) Reichardt, C. Chem. Rev. 1994, 94, 2319.
5660
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661

Journal of the American Chemical Society
Article
(24) (a) Kawamura, Y.; Sasabe, H.; Adachi, C. Jpn. J. Appl. Phys.
2004, 43, 7729. (b) Katoh, R; Suzuki, K.; Furube, A.; Kotani, M.;
Tokumaru, K. J. Phys. Chem. C 2009, 113, 2961. (c) Suzuki, K.;
Kobayashi, A.; Kaneko, S.; Takehira, K.; Yoshihara, T.; Ishida, H.;
Shiina, Y.; Oishi, S.; Tobita, S. Phys. Chem. Chem. Phys. 2009, 11, 9850.
(d) Hasegawa, M.; Kunisaki, S.; Ohtsu, H.; Werner, F. Monatsh. Chem.
2009, 140, 751.
(25) Margolis, B. J.; Long, K. A.; Laird, D. L. T.; Ruble, J. C.; Pulley,
S. R. J. Org. Chem. 2007, 72, 2232.
(26) Steinke, N.; Frey, W.; Baro, A.; Laschat, S.; Drees, C.; Nimtz,
M.; Hagele, C.; Giesselmann, F. Chem.-Eur. J. 2006, 12, 1026.
̈
5661
dx.doi.org/10.1021/ja3001979 | J. Am. Chem. Soc. 2012, 134, 5652−5661