A Spin-Active, Electrochromic, Solvent-Free Molecular Liquid Based on Double-Decker Lutetium Phthalocyanine Bearing Long Branched Alkyl Chains

Article abstract of DOI:10.1002/asia.201800175

Synthesis and characterization of a novel, multifunctional, solvent-free room-temperature liquid based on alkylated double-decker lutetium(III) phthalocyanine (Pc₂Lu) are described. Lowering of the melting point and viscosity of intrinsically solid Pc₂Lu compounds has been achieved through the attachment of flexible, bulky, and long branched-alkyl chains, that is, thio-2-octyldodecyl, to the periphery of the Pc₂Lu unit. The embedded Pc₂Lu unit maintains its inherent molecular functions, such as spin-active nature and electrochromic behavior in the liquid state. Comparison of the properties with a solid-like Pc₂Lu derivative, functionalized with shorter alkyl chains, that is, thio-2-ethylhexyl, underlines the importance of the hampering effect on the π–π interactions of neighboring Pc₂Lu molecules by bulkier and longer branched-alkyl chains. This study could possibly pave the way for novel multifunctional liquids whose spin-activities are associated with their rheological or optoelectronic properties.

Full text of DOI:10.1002/asia.201800175

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: A Spin-Active, Electrochromic, Solvent-Free Molecular Liquid
Based on Double-Decker Lutetium Phthalocyanine Bearing Long
Branched Alkyl Chains
Authors: Agnieszka Zielinska, Atsuro Takai, Hiroya Sakurai, Akinori
Saeki, Marcin Leonowicz, and Takashi Nakanishi
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A Spin-Active, Electrochromic, Solvent-Free Molecular Liquid Based on Double-Decker
Lutetium Phthalocyanine Bearing Long Branched Alkyl Chains
Agnieszka Zielinska,[a,b] Atsuro Takai,[a] Hiroya Sakurai,[a] Akinori Saeki,[c] Marcin Leonowicz,[b] and
Takashi Nakanishi*[a]
Abstract: Synthesis and characterization of a novel, multifunctional,
solvent-free room-temperature liquid based on alkylated double-
decker lutetium (III) phthalocyanine (Pc2Lu) are described. Lowering
of the melting point and viscosity of intrinsically solid Pc2Lu
compounds has been achieved through the attachment of flexible,
bulky, and long branched-alkyl chains, i.e. 2-octyldodecyl, to the
periphery of the Pc2Lu unit. The embedded Pc2Lu unit maintains its
inherent molecular functions, such as spin-active nature and
electrochromic behavior in the liquid state. Comparison of the
properties with a solid-like Pc2Lu derivative, functionalized with
shorter alkyl chains, i.e. 2-ethylhexyl, underlines the importance of the
hampering effect on the – interactions of neighboring Pc2Lu
molecules by bulkier and longer branched-alkyl chains. This study
could possibly pave the way for novel multifunctional liquids whose
spin-activities are associated with their rheological or optoelectronic
properties.
transferred from the inherent molecular functions of the -
conjugated core.
Numerous studies of alkylated -molecules have proven that
the substitution pattern, length, and level of branching of alkyl
chains are very important factors for determining the bulk
properties.[13,14] The bulky alkyl chains can protect the functional
core moiety from photodegradation.[3b] Further adjustment of the
optoelectronic properties of alkylated -FMLs by doping of -
conjugated molecular additives has also been achieved.[3–5,7,15,16]
The lack of necessity of polymer matrix or solvent can allow higher
concentration/density of functional -units in the neat liquid, and
may also help with the fabrication of thin films or highly defined
elements in confined spaces. Therefore, we believe the core
isolated and protected alkylated -FMLs can lead to the creation
of various new soft materials with predictable optoelectronic
functions. The functions we focus on in this study are spin
behavior, magnetic property, and electrochromism of a neutral
radical at the highly dense solvent-free liquid states.
Solvent-free functional molecular liquids (FMLs) are a novel
category of organic soft materials.[1,2] FMLs are created by
combining the advantages of fluids (e.g. excellent processability,
independent surface geometry, capillary action into confined
space, deformability, and amorphousness) with various functions
based on the core molecular component. In particular, FMLs
comprising optoelectronically-active -conjugated molecular units
exhibit luminescence,[3–5] photoconductivity,[5–8] and nonlinear
optics.[9] The design strategy behind those FMLs is simple in
principle: the functional core unit, such as a -conjugated
molecule, including even large macrocycles such as porphyrin
(Por)[10,11] and phthalocyanine (Pc),[9,12] is embedded by
peripherally bulky and flexible alkyl or silyl side-chain substituents.
As a result, the intrinsically solid and rigid -conjugated molecules
can be turned into a liquid at room temperature. More importantly,
this strategy can lead to isolation of the functional core unit by
steric repulsion of the alkyl chains; therefore, prediction of the bulk
optoelectronic properties as neat material can be simply
A double-decker lutetium (III) phthalocyanine (hereafter
abbreviated as Pc2Lu) was chosen as the functional core unit
because of its spin-active nature, electrochromic properties,[17,18]
and paramagnetic behavior.[19–21] Pc2Lu is a neutral radical
composed of one Lu3+ and two Pc2− macrocycles, with a hole
delocalized over the Pc rings. Unsubstituted Pc2Lu is a crystalline
solid material with limited solubility in most common organic
solvents, which makes its synthesis, processing, and application
more problematic. Pc2Lu derivatives substituted with linear alkyl
chains have a strong tendency to self-assemble into columnar
liquid crystalline structures, favorable for semiconducting
applications.[22] However, soft condensed matter with Pc2Lu units
possessing intrinsic spin-radical characteristics at room
temperature has never been achieved. Here, we develop an FML
based on the spin-radical functions of alkylated Pc2Lu.
Solvent-free, room-temperature liquid Pc2Lu was synthesized
by substituting the Pc macrocycle with bulky and flexible branched
long alkyl chains, i.e. thio-2-octyldodecyl chains, at the -position
(1a, Figure 1a). As a reference solid compound, Pc2Lu bearing
shorter thio-2-ethylhexyl chains was also synthesized (1b, Figure
1a). In both cases, racemic isomers of Guerbet alcohol[23,24]
based branched alkyl chains were chosen, due to their potential
for lowering the melting point and/or decreasing the viscosity of
liquid substances.[25] The target 1a is a brown viscous fluid, while
the reference 1b is an amorphous solid at room temperature (see
Scheme S1 for synthetic details). Both 1a and 1b were
characterized by 1H NMR (Figure S1), UV-vis absorption
spectroscopy (vide infra), and matrix-assisted laser
desorption/ionization time-of-flight mass (MALDI/TOF–MS)
analysis (Figure S2).
[a]
A. Zielinska, Dr. A. Takai, Dr. H. Sakurai, Dr. T. Nakanishi
Frontier Molecules Group, International Center for Materials
Nanoarchitectonics (WPI-MANA), National Institute for Materials
Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
E-mail: NAKANISHI.Takashi@nims.go.jp
A. Zielinska, Dr. M. Leonowicz
Warsaw University of Technology
ul. Wołoska 141, Warsaw 02-507, Poland
Dr. A. Saeki
[b]
[c]
Department of Applied Chemistry, Graduate School of Engineering,
Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Differential scanning calorimetry (DSC) was applied to
evaluate the thermal behavior of 1a and 1b (Figure 1b and Figure
S3). In the case of 1a, only one clear transition, from a glassy
Supporting information for this article is given via a link at the end of
the document.
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core–core distance between the Pc2Lu units that are separated
by disordered bulky alkyl chains. The halo signal is a much
broader feature compared with that of liquid crystalline
examples,[22,27,28] indicating inhomogeneity in the core–core
distance and conformations. The molten alkyl chains can be
observed as a broad halo in the wide-angle region (maximum at
around 13 nm−1 of q, ~4.8 Å in length). This SWAXS result well
reflects the disordered state and absence of long-range ordered
structure in 1a. In contrast, the SWAXS profile of the semi-solid
sample of 1b at 298 K shows several sharp peaks overlaid on two
broad halos, representing the coexistence of amorphous and
crystalline states (Figure S6 and Table S1).[27,28] It should be
noted that the halo in the small-angle region at 3 nm−1 of q (~20 Å
in length) corresponds to the core–core distance between the
Pc2Lu units, which is a shorter space than that seen in 1a.
The fluid character of 1a has been evaluated by means of the
rheology technique. The dependence of the storage modulus (G')
and loss modulus (G") as a function of the strain amplitude () at
296 K is presented in Figure 2a. In the strain range below 0.5%,
G' and G" showed similar values, while above 0.5%, G" exceeded
G', which indicates the liquid nature of 1a. This liquid
characteristic with a shear-thinning feature has been further
confirmed by measuring the complex viscosity (η*) varying under
different  at 296 K (Figure 2b). This behavior was also present in
the temperature dependent study. The moduli (G', G") were
obtained as a function of the angular frequency () at 296 K, and
in the temperature range of 298–328 K for  = 25% (Figures S7–
S10). All the results confirm that 1a has non-Newtonian liquid
characteristics with a shear-thinning feature. A similar non-
Newtonian liquid phenomenon was seen in linear alkyl chain
substituted liquid Por[10,11] and Pc.[12] Both alkylated Por and Pc
liquids contain relatively large -conjugated macrocycles that
might maintain subtle – interactions in their liquid state.
Figure 1. a) Chemical structures of 1a (R = 2-octyldodecyl) and 1b (R = 2-
ethylhexyl). b) The heating trace of DSC thermograms of 1a: first heating (solid
line), 1b: first heating (dotted line), and second heating (dashed line).
Both in the neat state and in solution, 1a and 1b exhibit a deep
brown color in their neutral state (Figure S11). The brown color of
the neutral Pc2Lu molecules, as opposed to the usually reported
green for the neutral Pc2Lu, is the result of the thioalkyl chains’
influence on the molecular energy levels.[18] UV-vis absorption
spectroscopy for 1a and 1b was performed in chloroform,
dichloromethane, tetrahydrofuran (THF), and n-hexane. In the
case of the THF solution of 1a and 1b, the color gradually
changed from brown to blue. The absorption spectral change
(Figure S12) corresponds to the reduction of the neutral Pc2Lu
complex. One possible explanation could be the high donor
number of THF (84 kJ mol−1);[33] similar behavior has been
reported for Pc2Tb in THF.[34] Absorption bands of the neat
samples of 1a and 1b measured with a solid powder matrix of
BaSO4 do not differ significantly, but are broader than those in
solution (Figure S11). These results indicate nonspecific, random
interactions between the core Pc2Lu in the solvent-free state,
which is consistent with the results of SWAXS and rheology. In
addition, these limited – interactions among neighboring Pc2Lu
units were confirmed by POM observation, in the form of non-
long-range ordering in 1a and 1b.
state to an isotropic state, was observed below 223 K. On the
other hand, 1b exhibited solid to isotropic liquid transition with a
melting point (Tm) at 332 K (ΔH = 27.99 kJ mol−1, ΔS = 0.48 kJ
mol−1 K−1) in the first heating trace. This transition disappeared in
the second heating trace, and instead a glassy transition
appeared below 253 K. This behavior is typical for the
supercooling phenomenon, where the kinetically trapped
supercooled liquid state does not quickly organize into a solid
state.[26] The thermodynamically stable solid state of 1b can be
obtained after thirteen additional days of storage at 278 K, which
would be the origin of the solid showing melting transition on the
first heating process. This process has been visualized by
polarized optical microscopy (POM), as shown in Figure S4.
These results indicate that the branched chains significantly lower
the transition temperatures to form a new type of fluid matter of
Pc2Lu compounds – rather thermodynamically stable room-
temperature liquid or kinetically trapped supercooled liquid, which
is totally different from the other linear alkyl- or alkyloxy-
substituted lanthanide Pc.[27–32]
To acquire further insight into the unique phase behavior of
branched alkylated Pc2Lu, small- and wide-angle X-ray scattering
(SWAXS) measurements were conducted. In the SWAXS profile
of 1a measured at 298 K, two broad halos were observed (Figure
S5). The halo in the small-angle region, appearing around the q
value of 2.7 nm−1 (~23 Å in length), corresponds to the average
Although largely overlapped -systems seen in liquid
crystalline alkylated Pc2Lu compounds are known as good n-type
semiconductors,[22] the less ordered liquid 1a and semi-solid 1b
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susceptibility as a function of the temperature was measured in
both 1a and 1b in their solvent-free state. The plots have been
fitted to match the Curie-Weiss law between 10 and 400 K for both
samples, indicating their paramagnetic behavior (Figure S14).
This result is consistent with the variable-temperature ESR
measurements in the range of 4.2–400 K (Figures S15 and S16).
The values of Curie constant C, 0.297 emu K mol−1 for 1a and
0.262 emu K mol−1 for 1b, are similar to the ones reported for
other Pc2Lu complexes.[39] The Weiss temperature, , was
estimated to be  = −0.12 K for 1a and −0.87 K for 1b. These
values are very close to zero, suggesting almost no magnetic
interaction in these compounds. It may be noticed that these
values are approximately 200 times smaller than the typical
values of  ~ 30 K observed for other Pc2Lu compounds.[39–41]
This difference may be caused by the difference in the core–core
distance between Pc2Lu units. Thus, the Weiss temperatures also
support that our Pc2Lu molecules, in particular 1a, are randomly
dispersed even in the neat state.
The electrochromic feature is an attractive property of Pc2Lu
based materials. Cyclic voltammograms (CVs) of both 1a and 1b
in dichloromethane solution are shown in Figure S17. Both 1a and
1b showed five redox states: neutral, three reduced states, and
one oxidized state. The redox potential values are almost
independent of the side chain length (Table S2).
Spectroelectrochemistry results are presented in Figures 3b, 3c,
S18, and S19. Note that each process was reversible. During
reduction, two new absorption peaks gradually arise at 745 and
570 nm for 1a (745 and 664 nm for 1b). In the oxidation process,
new peaks arise at 747 and 593 nm (749 and 595 nm for 1b).
These changes are consistent with the spectral changes for other
alkylated Pc2Lu compounds.[18] Spectroelectrochemistry of a neat
film of 1a was also performed in aqueous tetrabutylammonium
chloride (TBACl) solution. The UV-vis spectra during the first
reduction from the neutral form were monitored under applied
potential at −0.6 V vs. reference electrode Ag/AgCl/KCl (Figure
3d). The spectral change followed the same trend as that in
solution. The main peak of max = 699 nm decreases, and two new
peaks increase at 744 and 661 nm. These processes were much
faster compared to those in solution due to the limited
electrochemically active diffusion layer in the neat film. The one-
electron-reduced species of 1a, which was achieved by
immersing a thin film of neutral 1a in an aqueous Na2S2O4
reductant solution, was ESR silent (Figure 3a-I).
In summary, we investigated amorphous organic spin-active
liquid materials, which differ from the conventional organic spin-
active materials that are usually solid due to the -conjugated
parts of the molecules. The embedding of the unpaired electron
(spin radical) in the molecule by the bulky and flexible alkyl chains
causes it to be well shielded from the environment, as proven by
the results of FP-TRMC, ESR, and SQUID measurements. The
adopted molecular design can effectively weaken the
intermolecular – interactions between the Pc2Lu cores through
steric hindrance of the branched long alkyl side chains, and as a
consequence, viscous liquids at room temperature can be created.
This branched alkyl chain engineering strategy leads to isolation
Figure 2. a) Rheology results of 1a at 296 K. Storage (G’, red circles) and loss
(G” blue squares) moduli vs. strain amplitude (). Inset of a) is a photo image of
1a at 296 K under visible daylight. b) Complex viscosity (η*) vs. angular
frequency () of 1a at 296 K for different values of strain amplitude ().
exhibited relatively low photoconductivity, which was evaluated by
means of flash photolysis time-resolved microwave conductivity
(FP-TRMC) at 298 K, as shown in Figure S13.[35,36] The
photoconductivity maximum (max) of the liquid 1a was only
1.9×10−6 cm2 V−1 s−1, about one-third the max value of the solid
1b (max = 5.1×10−6 cm2 V−1 s−1). These results also indicate
that the core–core interactions of 1a are substantially, but not
completely, hampered by attached bulky and branched long alkyl
chains, which is consistent with their structural analyses.
Electron spin resonance (ESR) and superconducting
quantum interference device (SQUID) measurements were
carried out for the spin characteristics of 1a and 1b. ESR spectra
in dichloromethane (10−4 M) solution and the spectrum of the
liquid 1a at room temperature exhibited an ESR signal at g =
2.0027 ± 0.0002 and wide peak widths of about 0.9 ± 0.1 mT,
while the semi-solid 1b had a much narrower peak width of 0.1
mT at g = 2.0028 ± 0.0002 (Figure 3a). The observed difference
in the peak width between 1a and 1b in their solvent-free samples
can be understood as an effect of the Pc2Lu’s core conditions.
The bulkier and longer branched alkyl chains on 1a contribute to
sufficient weakening of the Pc2Lu-Pc2Lu interactions. On the other
hand, the shorter alkyl chains on 1b do not provide an effective
barrier between adjacent Pc2Lu units. Therefore, interactions
between the neighboring spin-active species are not limited,
which leads to narrowing of the ESR signal.[18a,37,38] The magnetic
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Figure 3. a) ESR spectra of I) a thin film of one-electron reduced species of 1a, II) a thin film of neutral 1a, III) 1a in dichloromethane (10–4 M), IV) a thin film of
neutral 1b, V) 1b in dichloromethane (10–4 M). b) UV-vis spectral changes of a thin film of 1a during the chemical reduction using Na2S2O4 (from red line to blue
lines). c) UV-vis spectral changes of 1a in dichloromethane (2 x 10–5 M) containing 0.1 M of TBAP during the electrochemical reduction at 0 V (red line) and –0.6 V
(blue lines). The inset shows a schematic illustration of the environment surrounding the Pc2Lu molecules. d) UV-vis spectral changes of a thin film of 1a pasted on
an ITO working electrode in aqueous TBACl (0.5 M) during the electrochemical reduction at 0 V (red line) and –0.6 V (blue lines). The inset shows a schematic
illustration of the environment surrounding the Pc2Lu molecules.
of the Pc2Lu core unit in the solvent-free state. Therefore, even
the neat film material can maintain the inherent molecular optical,
redox, and spin-activities. The shorter branched alkyl chains
provided less of the softening effect, but interestingly, affected
the solidification process enough to create a supercooled liquid
phase. We believe that such -FMLs with switchable spin- and
optical- activities would open up new avenues in soft, fluidic
materials that are responsive to multiple stimuli such as
magnetic field and redox.
valuable comments on this study. The authors also thank the
Soft Materials Line and the MANA TSS Group at NIMS for the
use of their facilities.
Conflict of interest
The authors declare no conflict of interest.
Keywords: lutetium double-decker phthalocyanine • functional
molecular liquids • supercooled liquid • magnetic properties •
electrochromism
Acknowledgements
This work was partly supported by the Grants-in-Aid for Scientific
Research (JSPS KAKENHI Grant Number JP25104011,
JP15H03801) from MEXT, Japan. A.Z. is grateful to WUT-NIMS
Joint Graduate School Program. The authors thank Dr. T.
Taguchi (NIMS) for the use of the rheometer, and Prof. J.
Veciana and Dr. M. Mas-Torrent (ICMAB-CSIC) for their
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10.1002/asia.201800175
Chemistry - An Asian Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
It's fluid now! We created an
amorphous and liquid lutetium
A. Zielinska, A. Takai, H. Sakurai, A.
Saeki, M. Leonowicz, T. Nakanishi*
phthalocyanine (Pc2Lu). The π-cores
of Pc2Lu are embedded in flexible,
bulky, and branched long alkyl chains,
and thus shielded well from the
environment. The inherent molecular
functions of Pc2Lu, such as spin-
active nature and electrochromic
behavior are maintained even in the
neat liquid state at room temperature.
Page No. – Page No.
A Spin-Active, Electrochromic,
Solvent-Free Molecular Liquid Based
on Double-Decker Lutetium
Phthalocyanine Bearing Long
Branched Alkyl Chains
This article is protected by copyright. All rights reserved.