Solvent-free luminescent organic liquids

Article abstract of DOI:10.1002/anie.201108853

Illuminating! Isolation of a π-core by covalently attached flexible hydrocarbon chains has been employed to synthesize blue-emitting oligo(p-phenylenevinylene) (OPV) liquids with tunable viscosity and optical properties. A solvent-free, stable, white-light emitting ink/paint, which can be applied onto various surfaces and even onto LEDs, was made by blending of liquid OPVs with emissive solid dopants. Copyright

Full text of DOI:10.1002/anie.201108853

Angewandte
Chemie
DOI: 10.1002/anie.201108853
Luminescent Liquids
Solvent-Free Luminescent Organic Liquids**
Sukumaran Santhosh Babu, Junko Aimi, Hiroaki Ozawa, Naoto Shirahata, Akinori Saeki,
Shu Seki, Ayyappanpillai Ajayaghosh, Helmuth Mçhwald, and Takashi Nakanishi*
[
1]
A prospective research scenario of organic electronics
utilizes noncovalent interactions to assemble optoelectroni-
benefits, for example, nonvolatility, processing under solvent-
free conditions, tunable optoelectronic functions, a high
density of electronically active p-conjugated moieties, or the
ability to act as solvent/matrix for other organic or inorganic
components. Furthermore, the liquid materials are tractable,
and can be easily treated and used on a bulk scale.
[2]
cally active molecules to attain improved performance in
[
3]
[4]
devices, such as field-effect transistors and solar cells,
because it may enable simple and cheap manufacture as
well as easy defect annealing. In order to compete with
inorganic materials, utmost care is needed for organic
substances, from molecular design to self-organization, fab-
rication into devices in a predictable way, and finally to end-
use applications with improved performance and longevity.
Therefore, alternative and qualitatively different approaches
should be considered in this direction.
The linear p-conjugated molecule oligo(p-phenylene-
vinylene) (OPV) was chosen as the functional core moiety.
OPV has been widely studied in organic optoelectronics
because it has excellent stability and emission characteristics,
[
4,5]
[
2,14]
as well as self-assembly properties.
Herein, we report the
synthesis of a series of room-temperature liquid OPVs (1–4,
Figure 1a), as well as their use as a solvent/matrix and a blue-
emitting component for the preparation of liquid inks that
emit white light. This study paves the way to light-emitting
liquids which can be painted onto various surfaces that have
different geometries.
Softening of the optoelectronic functional materials is one
such example towards printable organic electronics. The
formulation of solvent-free organic materials, such as ionic
[
6]
[7]
liquids, ionic liquids that contain nanoparticles, or organic
[8]
chromophores, is an emerging and challenging area which
aims to find replacements for self-assembled organic semi-
conductors. Recently, uncharged room-temperature organic
liquids have been introduced as new functional liquids by
isolating the p-core through the use of low-viscosity organic
chains. Room-temperature, solvent-free organic liquids, such
Room temperature, solvent-free, liquid OPVs were syn-
thesized by substituting two different OPV cores with low-
viscosity hydrocarbons, such as branched aliphatic chains
(Figure 1a). For example, the complex viscosities (h*) of the
branched alkyl bromide uncoupled (2b) and coupled to
benzaldehyde (2a) (Scheme S1 in the Supporting Informa-
tion) are 1.03 and 0.01 Pas, respectively, at an angular
[
9]
[
10]
[11]
[12]
as phthalocyanines,
porphyrins,
carbazoles
and ful-
[
13]
lerenes
have been reported as proof of this concept.
ꢀ
1
However, it is still premature to deliver excellent end-use
performance and meaningful applications. The use of organic
liquids at room temperature is expected to provide several
frequency of w = 10 rads (Figure S1 in the Supporting
Information). The molecular design strategy, which includes
the position of the alkyl chain substituent and the extent of
chain branching (Figure 1a) are extremely important to tune
the physical features of the liquid. The targeted branched
chain, coupled OPV derivatives 1–4 were obtained as pale
yellow viscous fluids at room temperature, whereas the
reference molecules 5 and 6 were solids. All of the OPV
[*] Dr. S. Santhosh Babu, Dr. J. Aimi, Dr. H. Ozawa, Dr. N. Shirahata,
Dr. T. Nakanishi
National Institute for Materials Science, Tsukuba (Japan)
E-mail: nakanishi.takashi@nims.go.jp
Dr. A. Saeki, Prof. S. Seki
Department of Applied Chemistry, Graduate School of Engineering
Osaka University (Japan)
1
derivatives were unambiguously identified by H NMR
spectroscopy and MALDI-TOF mass spectrometry. The
1
H NMR spectrum (Figure S2 in the Supporting Information)
Prof. A. Ajayaghosh
Photosciences and Photonics group, National Institute for Inter-
disciplinary Science and Technology (NIIST), CSIR (India)
and thermogravimetric analysis (TGA, Figure S3 in the
Supporting Information) of the fluids indicates the absence
of residual solvent in the bulk materials, hence the fluid-like
behavior is an inherent property of the single bulk compo-
nent.
Prof. Dr. H. Mçhwald
Department of Interfaces, Max Planck Institute of Colloids and
Interfaces (Germany)
[
**] We thank Dr. T. Sato, Z. Rao, Dr. S. Samitsu, and Dr. Z. Schnepp at
NIMS for help with determining the absolute quantum yield,
rheology, DSC, and XRD measurements. We also acknowledge Dr.
M. J. Hollamby, Dr. K. Sugiyasu, and Dr. M. Takeuchi at NIMS for
meaningful discussions. This study was supported partially by
KAKENHI (23685033) from the MEXT (Japan) and the Shorai
Foundation for Science and Technology. Support from DAE (Out-
standing Researcher Award to A.A.) and CSIR, Government of India
is acknowledged.
The fluid was evaluated by differential scanning calorim-
etry (DSC) and rheology analyses. As shown in Figure 1b, the
cooling traces of the DSC thermogram indicate that 1–4 have
relatively low glass transition temperatures (T ) between ꢀ43
g
to ꢀ558C (Table S1 in the Supporting Information), which
shows that over a wide range of temperature, until decom-
position at around 3008C (estimated by TGA), these com-
pounds exist as solvent-free fluids. The molar heat capacities
(
Cmol) of 1–4 at the glass transition temperature are between
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108853.
ꢀ
1
ꢀ1
639.4 (1) and 679.5 Jmol
K
(4, Table S1 in the Supporting
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Figure 2. Comparison of the normalized a) absorption and b) emission
spectra of solvent-free 2 (red) and 4 (blue) in bulk-liquid state (dashed
line) supported on quartz plate and dichloromethane solution (solid
ꢀ5
line). c=5ꢀ10 m, l=1 mm, l =360 nm for 2 and l =385 nm for
ex
ex
4. Insets show images of 2 under a) visible and b) UV light (365 nm).
chains contribute to the assembly and solid characteristics. In
such cases, the melting point stays at high temperatures, 171
and 648C for 5 and 6, respectively. Furthermore, X-ray
diffraction (XRD) analyses of the liquid samples showed only
two halos that correspond to the average distance between
the disordered OPV core (13.6–18.2 ꢀ) and the molten
aliphatic chains (4.6–4.9 ꢀ, Figure S5a in the Supporting
Information). The XRD results point to no regular molecular
ordering in the densely dispersed OPV cores in liquids 1–4.
The isolation of the OPV core by branched hydrocarbon
chains thus enables the OPVs to be liquid at room temper-
Figure 1. a) Chemical structures of OPVs 1–6. b) The cooling trace of
DSC thermograms of 1 (red), 2 (green), 3 (dark blue), and 4 (light
blue) showing the glass transition temperatures (T ). c) Variation of
g
1
ature, and basic H NMR, TGA, DSC, XRD, and rheology
storage modulus (G’, circles) and loss modulus (G’’, squares) versus
angular frequency on double-logarithmic scale and d) complex viscos-
ity (h*) versus angular frequency on double-logarithmic scale for
experiments lead to the conclusion that fluidity is an inherent
characteristic of these compounds (Table S1 in the Supporting
Information).
Comparison of the absorption and emission spectra of 1–4
in the solvent-free liquid state and in homogeneous solution
1
(red), 2 (green), 3 (dark blue), and 4 (light blue); shear strain
(g)=0.1.
(Figure 2, see also Figures S6 and S7 in the Supporting
Information). As the viscosity decreases, the molar heat
capacity increases, and this can be correlated with the fluidity
of 1–4. The loss modulus G’’ is higher than the storage
modulus G’ over the measured angular frequency range of
Information) indicates that in the solvent-free liquid state, p–
p interactions between the cores are efficiently inhibited by
the introduction of branched alkyl chains. The slightly
broader absorption and emission spectra in the solvent-free
state relative to monomers in dichloromethane solution
illustrate that the OPV core is in an almost monomerically
dispersed and highly dense state, but probably with small
clustering. The optical inhomogeneity of the solvent-free
liquid leads to light scattering, which appears as a tail that
ꢀ
1
w = 0.1–100 rads and shear strain of g = 0.1. This result
clearly indicates that the target molecules 1–4 behave as
Newtonian liquids (Figure 1c). The value of h* was found to
ꢀ1
vary significantly from 34.6 (1) to 0.64 Pas (4) at w = 10 rads
Figure 1d, see also Table S1 in the Supporting Information).
(
[
15a]
This result shows that a (2,4,6)-substitution pattern on both
sides of the phenyl moieties and swallow-tailed aliphatic
chains are more effective than a (3,5)-substitution pattern and
hyper-branched chains to attain low-viscosity OPV liquids. In
other words, together with the spectroscopic results discussed
below (Figure 2, see also Figure S6 in the Supporting Infor-
mation), this molecular design can efficiently disturb inter-
molecular p–p interactions of the OPV core by steric
hindrance via soft tail attachment (Figure S4 in the Support-
ing Information) and as a consequence, low-viscosity organic
liquids are produced. Comparing the physical properties of
the liquids 1–4 with the two solid reference compounds 5 and
extends to longer wavelengths (Figure 2a).
In Figure 2b,
the spectral peak that corresponds to the 0–0 vibronic band
(lmax = 400 nm (2), 430 nm (4)) of the S !S transition of the
1
0
fluorescence spectra in solution and solvent-free states are
almost identical, whereas the 1–0 band (lmax = 418 nm (2),
453 nm (4)) has a lower intensity in the solvent-free liquids,
which indicates a difference in the stabilization of the excited
[15b]
states in the solvent-free versus solution states.
The insets
of Figure 2a and b show images of 2 at room temperature
under visible and UV light (l = 365 nm), respectively, which
ex
shows the viscous and blue emissive characteristics in the
liquid state. All of the liquids 1–4 emit blue light in the
solvent-free bulk state with an absolute fluorescence quantum
yield of 45–48% (Table S2 in the Supporting Information). To
have an insight into our design strategy, the refractive index
(RI) was also evaluated by using 589.3 nm light. The RI values
ranged from 1.537 (1) to 1.520 (4, Table S1 in the Supporting
Information). The observed RI values are close to those of
6
(Figure 1a, see also Table S2 in the Supporting Informa-
tion), in which the OPV core has methoxy and dodecyloxy
substituents, respectively, stresses the importance of the
branched aliphatic side chains for tuning the intermolecular
interactions. In compound 6, p–p interactions between the
aryl groups and van der Waals forces between the linear alkyl
3
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amorphous solids of distyrylbenzene derivatives with bulky
simply blending with a spatula. A larger amount of the solutes
[16]
substituents. Interestingly, even if the trend in RI has only
a small variation, it follows the trend of decreasing viscosity
from 1 to 4, and clearly shows the importance of the extent of
OPV core density and isolation in tuning the viscosity and RI
values. To understand how the core isolation affects the
optoelectronic properties, transient photoconductivity experi-
ments were carried out (Figure S8 in the Supporting Infor-
mation).
could be dissolved into the OPV matrix by heating or
dissolving in a suitable solvent and then evaporating the
solvent. This interesting finding motivated us to tune the
emission properties of liquid OPVs to achieve white-emitting
liquids. As indicated in Figure 3a, pastes that emit white light
were prepared by blending liquid OPVs with Alq3 and
rubrene in a watch glass with a spatula. The supporting video
file (see the Supporting Information) also clearly demon-
strates the method adopted for the preparation of the pastes
within a minute. We used 2 and 4 for preparing white-emitting
composites, which are referred to as composite I and II,
respectively. Blending 2 with Alq3 and rubrene in the molar
ratio of 1:1.65:0.23 and annealing the sample at 408C for
2 min led to comparatively good white emission with a Com-
mission Internationale de lꢁꢂclairage (CIE) 1931 chromaticity
Organic materials that emit white light are particularly
important because durable materials with improved color
purity and stability are in high demand for the development of
large-area devices through low-cost manufacturing technol-
[
17]
[18]
ogy. Except for a few examples, in most cases of solution-
[
19]
[20]
or gel-based
white-light emission, color stability and
purity cannot be retained in solvent-free conditions, such as
coated thin films or xerogels, whereas in the case of solid
samples, processing becomes a tedious task. This situation
demands suitable functional materials that have better
emissive features in solvent-free conditions as well as facile
and effortless processing characteristics. Liquid OPVs 1–4 can
be used as solvents or matrices for other organic or inorganic
components to obtain useful hybrid materials. For instance,
[
21a]
diagram
coordinate value of (0.33, 0.34) in the solvent-free
state. A reasonably large spectral width that ranges from 400
to 700 nm, which matches the standard spectral width of white
[21]
emission, was observed (Figure 3b). A possible advantage
of our liquid composites is the reduced interface resistance
between the components in the solvent-free liquid matrix,
which presumably leads to partial energy transfer between
the matrix host and the guest molecules to give pure white
3
0 mg of green-emitting tris(8-hydroxyquinolinato)alumi-
[21b]
nium (Alq3) and 2 mg of orange-emitting 5,6,11,12-tetraphe-
emission.
The photoexcited carrier decay of OPV samples
nylnaphthacene (rubrene) were dissolved in 100 mg of 2 by
is very fast and the major component (ca. 80%) has a lifetime
Figure 3. a) Illustration of the preparation of solvent-free white-emitting liquid composite. b) Emission spectrum of composite I (l =360 nm)
ex
supported on quartz plate; inset shows the image of the white emission from composite I. c) Fluorescence matrix scan of composite I.
Angew. Chem. Int. Ed. 2012, 51, 3391 –3395
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of 600 ps. The lifetime of the composite I was much shorter
than that of composite 2 (Figure S9 in the Supporting
Information). This result supports the possibility of energy
transfer between OPV component and the dopants. The
molar ratio between the three components was found to be
significant in obtaining pure white emission (Figure S10 in the
Supporting Information). Furthermore, annealing of the
sample changes the interface characteristics towards
improved brightness of the white emission. Fluorescence
matrix scan measurements of composite I, which has a CIE
liquid-host matrix and three emissive components in the
present system to obtain a feasible white-emitting liquid
composite with color purity.
To efficiently address the advantage of the current system,
as shown in Figure 4, we carried out experiments to tune the
emission from blue-white (i) to pure white (ii) and further to
red-white (iii). However, for practical demonstration of the
liquid composite, it is of particular importance to have
tunable white emission, such as blue-white (cool), pure white,
and yellow-white (warm) colors, which can find applications
[
23]
coordinate value of (0.33, 0.35, l = 360 nm), exhibited broad
according to requirements. This was obtained from com-
posite I when the molar ratio of the components (2/Alq3/
rubrene) was slightly varied (Figure 4a–c). As a realistic
demonstration, the lower viscosity value of the white-emitting
liquid composite II enabled it to be used as ink for a roller-ball
pen to write directly on various surfaces, which include paper
and solid surfaces. The writing is then readable under
a 365 nm UV lamp. Figure 4d shows an image of text written
on paper with white-emitting ink. As another application, the
composite material was used as a paint to coat large areas of
various surfaces. As the viscosity of composite II is 3.2 Pas,
a paintbrush was suitable for coating the surfaces. Figure 4e
ex
emission from 400–700 nm (Figure 3c), and the emission can
be tuned by using different excitation wavelengths (Fig-
ure S11 in the Supporting Information). An important feature
of composites I and II is that white emission can be obtained
across a wide range of excitation wavelengths from 350–
4
30 nm (Figure 3c, see also Figure S12 in the Supporting
Information). The achievement of white emission by low-
energy excitation above 400 nm is energetically favored and
can reduce the thermal energy loss. It will save electric power
consumption and prolong device lifetime. Composites I and II
have an absolute quantum yield of 35% and 37%, respec-
tively. These values indicate that, unlike other materials that
2
shows an image of white emission from a 5 ꢃ 5 cm area of the
[19,20]
emit white light,
the quantum yield is not reduced to
composite that was painted onto a phenol-formaldehyde-type
black surface by using a brush. The applicability of the
composites as a tractable material has been realized, as white-
light emission was obtained by coating composite II onto
a commercially available UV-LED (375 nm). As shown in
a large extent relative to the parent liquid samples. Compo-
sites I and II maintain their strongly emissive features, which
is a good indication of the practical demonstration of
composites I and II as white light emitting liquid materials.
DSC and rheology analyses of the white-light-emitting
pastes that were prepared by blending the components
(
Figure S13 in the Supporting Information) clearly indicates
that the fluidity is retained. Composites I and II have glass
transition temperatures of ꢀ45.1 and ꢀ45.98C, respectively.
Hence, there was no considerable change in T upon mixing
g
the liquid OPVs with the solid dopants. Even in the presence
of Alq3 and rubrene, the viscosity remains 3.2 Pas in the case
of composite II, and the material retains the Newtonian
fluidity (Figure S13b,c in the Supporting Information). In
order to compare with the solvent-free white emission, we
carried out fluorescence studies of the OPVs in a solution of
dichloromethane. These experiments revealed that a signifi-
cantly different molar ratio is needed to achieve white
emission in homogenous solution (Figure S14 in the Support-
ing Information). The different molar ratios that are required
to produce white light emission in the homogenous solution
versus the solvent-free state could be attributed to the
[
16]
difference in the (average) donor–acceptor distance and
the extent of direct excitation of the components. It should be
noted that drop-casted films of the white-emitting dichloro-
methane solutions did not exhibit white emission, because
a different molar ratio is needed in the solvent-free state
(
Figure S15i in the Supporting Information). This is probably
a serious issue when samples are integrated into a device
surface from volatile solutions with simultaneous solvent
Figure 4. a) Normalized emission spectra of composite I with molar
ratios of 2/Alq3/rubrene of 1.0:1.50:0.05 (blue), 1.0:1.65: 0.23 (green),
and 1.0:1.65:0.30 (red) supported on a quartz plate (lex =360 nm)
with corresponding b) CIE coordinate values and c) images. Images of
d) text written on paper with composite II in a rollerball pen,
[
22]
evaporation to get white-emitting thin films.
experiments with a solid composite of 6, Alq3, and rubrene
Figure S15ii in the Supporting Information) as well as
Control
(
2
e) 5ꢀ5 cm area coated with composite II using a brush and exposed
a composite of liquid 2 and Alq3 (Figure S15iii in the
under UV light (365 nm), and f) commercially available UV-LED
Supporting Information) revealed the advantage of the
(375 nm) before (left) and after (right) coating with composite II.
3
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Figure 4 f, bright white emission was observed when a thin
layer of composite II was applied onto the UV-LED. This
experiment shows the feasibility of the composites as a cost
effective white-emitting material. Hence, as well as being
a white-emitting liquid, our composite liquids have solved
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[
[
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a longstanding problem with solution-
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[
[
[
white-emissive materials, that is, self-quenching in the dried,
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In conclusion, blue-emitting, environmental friendly, low-
viscosity, room-temperature liquids that contain an OPV core
have been synthesized and fully characterized as solvent-free
functional organic liquids. This study demonstrates the ability
of a highly dense but monomerically dispersed p-core system
as an emissive liquid matrix to accommodate various guest
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produce nonsticky films for white-emitting displays is under
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