Amorphous formability and temperature-sensitive luminescence of lanthanide coordination glasses linked by thienyl, naphthyl, and phenyl bridges with ethynyl groups

Article abstract of DOI:10.1246/bcsj.20160379

The glass-transition properties and temperature-sensitive luminescence of lanthanide (Ln(III)) coordination compounds are reported. The glass formability was systematically provided by incorporation of bent-angled phosphine oxide (2,5-bis-(diphenylphosphorylethynyl)thiophene: dpet, 2,7-bis(diphenylphosphorylethynyl)naphthalene: dpen, 1,3-bis(diphenylphosphorylethynyl)benzene: m-dpeb) ligands with thienyl, naphthyl, phenyl cores, and ethynyl groups. The glass-transition points were clearly identified for all Ln(III) coordination compounds (T_g = 65-87°C). The Tb(III)/Eu(III) mixed coordination glass [Tb,Eu(hfa)₃(m-dpeb)]₃ (hfa: hexafluoroacetylacetonate) also showed green, yellow, orange, and red photoluminescence depending on temperature.

Full text of DOI:10.1246/bcsj.20160379

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Amorphous Formability and Temperature-Sensitive Luminescence of Lanthanide
Coordination Glasses Linked by Thienyl, Naphthyl, and Phenyl Bridges with
Ethynyl Groups
Yuichi Hirai, Pedro Paulo Ferreira da Rosa, Takayuki Nakanishi, Yuichi Kitagawa,
Koji Fushimi, and Yasuchika Hasegawa*
Advance Publication on the web December 27, 2016
doi:10.1246/bcsj.20160379
© 2016 The Chemical Society of Japan

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Amorphous formability and temperature-sensitive luminescence of lanthanide
coordination glasses linked by thienyl, naphthyl, and phenyl bridges with ethynyl
groups
Yuichi Hirai,1 Pedro Paulo Ferreira da Rosa,2 Takayuki Nakanishi,2 Yuichi Kitagawa,2 Koji Fushimi,2 and Yasuchika
Hasegawa*2
1Graduate School of Chemical Sciences and Engineering, Hokkaido University, Northꢀ13 Westꢀ8, Kitaꢀku, Sapporo, Hokkaido 060ꢀ8628
2Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Northꢀ13 Westꢀ8, Kitaꢀku, Sapporo, Hokkaido 060ꢀ8628
Eꢀmail: <hasegaway@eng.hokudai.ac.jp>
Abstract
The glass transition properties and temperatureꢀsensitive
luminescence of lanthanide (Ln(III)) coordination compounds
are reported. The glass formability was systematically provided
by incorporation of bentꢀangled phosphine oxide
Here, we focus on systematic construction of Ln(III)
coordination compounds with glass forming ability by simply
altering the aryl cores of organic bridging ligands to broaden
the range of molecular designs. Ln(III)ꢀmixed coordination
glasses
may
also
provide
transparent
and
(2,5ꢀbis(diphenylphosphorylethynyl)thiophene:
2,7ꢀbis(diphenylphosphorylethynyl)naphthalene:
dpet,
dpen,
temperatureꢀdependent luminescent materials. We have
previously reported that the Tb(III)/Eu(III) mixed rigid
coordination polymers “chameleon luminophores” show
brilliant green, yellow, orange, and red photoluminescence
depending on temperature.13 The emission colors and
temperatureꢀresponsive regions were also tuned by
Tb(III)/Eu(III) mixture ratios.14 These compounds are expected
as temperature/pressure sensitive paints (TSPs/PSPs) in the
field of fluid dynamics and aeronautical engineering to
visualize the fluid phenomena such as aerodynamic
heating.15ꢀ17 The Ln(III)ꢀmixed coordination glasses would
achieve dramatically improved coating property on materials
surface for highꢀconcentration TSPs/PSPs without polymer
matrices.
In this study, the construction of Ln(III) coordination
glasses and the control of physical properties are reported. We
prepared three bidentate phosphine oxide ligands with thienyl,
naphthyl, and phenyl cores (Figure 1b: dpet, dpen, mꢀdpeb).
Thermal properties of the corresponding Ln(III) coordination
compounds were characterized by DSC and TGꢀDTA
measurements. The emission properties were evaluated based
on the photophysical parameters such as emission quantum
yields and lifetimes. The glass transition properties and
temperatureꢀsensitive luminescence of Ln(III) coordination
glasses are demonstrated.
1,3ꢀbis(diphenylphosphorylethynyl)benzene: mꢀdpeb) ligands
with thienyl, naphthyl, phenyl cores and ethynyl groups. The
glass transition points were clearly identified for all Ln(III)
coordination compounds (Tg = 65ꢀ87˚C). The Tb(III)/Eu(III)
mixed coordination glass [Tb,Eu(hfa)3(mꢀdpeb)]3 (hfa:
hexafluoroacetylacetonate) also showed green, yellow, orange,
and red photoluminescence depending on temperature.
1. Introduction
Amorphous molecular materials or molecular glasses
based on organic compounds are promising candidates for
optical devices such as displays, lightings and LEDs because of
their isotropy, transparency, and high processability.1ꢀ3 In
particular, πꢀconjugated molecular glasses have attracted
attention for their tunable photophysical properties. Nakano
and Shirota have reported that a series of C3 starburst organic
compounds exhibits glass transition and electrical
conductivity.3,4 Bhowmik and coꢀworkers demonstrated the
fluorescence and amorphous properties of molecular materials
based on quinoline amines.5 The molecular glasses can be
expected as potential electroꢀ and photoꢀactive materials.
Luminescent
lanthanide
(Ln(III))
coordination
compounds are quite attractive for optical devices and
bioꢀprobes due to their pure color emission (fwhm < 20 nm)
and long emission lifetimes (τ ≈ ms).6ꢀ9 They are usually
dissolved in polymer matrices such as polymethyl methacrylate
for transparent luminescent materials to avoid light scattering
from crystal grain boundary of the compounds.10,11 We recently
succeeded in synthesizing luminescent and amorphous Ln(III)
complexes composed of Eu(III) ions, lightꢀharvesting
hexafluoroacetylacetonate (hfa) ligands, and 120˚ꢀangled
bridging ligands with ethynyl groups (Figure 1, mꢀdpeb).12
Quantum calculations and mass spectrometry revealed the
formation of trimer structure with pseudoꢀC3 axis (Figure 1,
[Eu(hfa)3(mꢀdpeb)]3). The glass formability should be caused
by the thermodynamically nonꢀequilibrium states that arise
from multiple quasiꢀstable states, resulting in suppressing easy
crystal packing to form amorphous solid without polymer
matrices.
Figure 1. (a) The molecular design of Ln(III)
coordination glasses and (b) chemical structures of organic
ligands.

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2. Experimental
2.1 General methods
chromatography on SiO2 using hexane and ethyl acetate as
mixed eluent (hexane : ethyl acetate = 5 : 1) to afford
2,7ꢀbis(trimethylsilylethynyl)naphthalene (5.1 g, 16 mmol).
2,7ꢀbis(trimethylsilylethynyl)naphthalene (5.1 g, 16 mmol) was
dissolved in THF (100 mL) and 1 M KOH aqueous solution (45
mL, 45 mmol) was added. The mixture was stirred overnight.
The product was extracted with hexane and concentrated to
afford 2,7ꢀdiethynylnaphthalene (2.5 g, 14 mmol). The
2,7ꢀdiethynylnaphthalene (2.5 g, 14 mmol) was dissolved in
dry Et2O (80 mL) under an Ar atmosphere. A solution of 1.6 M
nꢀBuLi (20 mL, 31 mmol) was added dropwise at −80˚C. The
mixture was allowed to stir for 3 h at −10˚C, after which a
PPh2Cl (5.6 mL, 31 mmol) was added dropwise at −80˚C. The
mixture was gradually brought to room temperature and stirred
overnight. The product was extracted with dichloromethane,
concentrated, and reꢀdissolved in dichloromethane (60 mL),
followed by addition of a 30% H2O2 aqueous solution (10 mL).
The reaction mixture was stirred for 3 h at 0˚C. The product
was extracted with dichloromethane and recrystallized. The
obtained crude powder was washed with ethyl acetate.
All chemicals are reagent grade and used without further
1
purification. H NMR (400 MHz) spectra were recorded on a
JEOL ECS400. Chemical shifts were reported in δ ppm,
referenced to an internal tetramethylsilane standard for 1H
NMR spectroscopy. Infrared spectra were recorded on a
JASCO FTIRꢀ420 spectrometer using KBr pellets. Elemental
analyses were performed by an Exeter Analytical CE440. Mass
spectroscopy was performed by a Thermo Scientific Exactive
instrument. Differential thermal analysis was performed on a
Shimadzu DSCꢀ60 Plus under a nitrogen atmosphere at a
1
heating/cooling rate of 5˚C min− . Thermogravimetric analyses
were conducted on a Shimadzu DTGꢀ60 under a nitrogen
1
atmosphere at a heating/cooling rate of 5˚C min− .
2.2 Syntheses
2.2.1
Preparation
of
2,5ꢀbis(diphenylphosphorylethynyl)thiophene (dpet)
Trimethylsilylacetylene (9.5 mL, 69 mmol) was added in one
portion to a solution of 2,5ꢀdibromothiophene (3.0 mL, 27
mmol), PdCl2(PPh3)2 (0.98 g), CuI (0.34 g), PPh3 (0.94 g) in
diisopropylamine/dry THF (80 mL/80 mL) under Ar
atmosphere and stirred at room temperature for 20 min. The
mixture was stirred at 50˚C for 9 h, and then allowed to cool to
room temperature. The ammonium salt was removed by
filtration and the mixture was extracted with hexane and
concentrated. The obtained crude oil was purified by column
chromatography on SiO2 using hexane as an eluent to afford
2,5ꢀbis(trimethylsilylethynyl)thiophene (6.5 g, 24 mmol). The
resulting 2,5ꢀbis(trimethylsilylethynyl)thiophene (6.5 g, 24
mmol) was dissolved in methanol (100 mL) and 1 M KOH
aqueous solution (60 mL, 60 mmol) was added. The mixture
was stirred for 3 h. The product was extracted with hexane,
concentrated, and purified by column chromatography on SiO2
using hexane as an eluent to afford 2,5ꢀdiethynylthiopehene
(2.1 g, 16 mmol). The 2,5ꢀdiethynylthiophene (2.1 g, 16 mmol)
was dissolved in dry Et2O (80 mL). A solution of 1.6 M nꢀBuLi
(23 mL, 35 mmol) was added dropwise to the solution at −
80˚C. The mixture was allowed to stir for 3 h at −10˚C, after
which a PPh2Cl (6.2 mL, 34 mmol) was added dropwise at −
80˚C. The mixture was gradually brought to room temperature,
and stirred overnight. The product was extracted with
dpen: Yield: 3.6 g (34%). 1H NMR (400 MHz, CDCl3, 25 ˚C) δ
8.12 (s, 2H, ꢀCH), δ 7.89ꢀ7.95 (m, 8H, ꢀCH), δ 7.82ꢀ7.84 (d, 2H,
ꢀCH), δ 7.64ꢀ7.66 (dd, 2H, ꢀCH), δ 7.48ꢀ7.59 (m, 12H,
ꢀCH) ppm. ESIꢀMass (m/z): calcd for C38H27O2P2 [M+H]+,
577.15; found, 577.15. Anal. Calcd for C38H26O2P2: C, 79.16;
H, 4.55%. Found: C, 79.05; H, 4.70%.
2.2.3 Preparation of Eu(III) coordination glasses Eu(hfa)3(X)3
(X = dpet, dpen).
The phosphine oxide ligand (0.8 mmoL) was dissolved in
methanol (40 mL), and Eu(hfa)3(H2O)2 / methanol (0.8 mmoL /
40 mL) solution was added dropwise. The solution was stirred
for 3 h at 50˚C. The reaction mixture was concentrated,
reꢀdissolved in methanol (5 mL), and then hexane (15 mL) was
added. The organic solvents were removed by decompression
to form amorphous solids.
[Eu(hfa)3(dpet)]3: Yield 0.62 g (20%). IR (KBr) 1656 (st, C=O),
1
1144 (st, P=O), 1100ꢀ1253 (st, CꢀOꢀC and st, CꢀF) cm−
.
FABꢀMass (m/z): [Mꢀhfa]+ calcd for C136H74Eu3F48O22P6S3,
3710.9; found, 3709.9. Anal. Calcd for C141H75Eu3F54O24P6S3:
C, 43.24; H, 1.93%. Found: C, 44.02; H, 2.24%.
[Eu(hfa)3(dpen)]3: Yield 0.52 g (16%). IR (KBr) 1656 (st, C=O),
1
1144 (st, P=O), 1100ꢀ1256 (st, CꢀOꢀC and st, CꢀF) cm−
.
dichloromethane,
concentrated,
and
reꢀdissolved
in
FABꢀMass (m/z): [Mꢀhfa]+ calcd for C154H86Eu3F48O22P6,
3843.1; found, 3842.0. Anal. Calcd for C159H87Eu3F54O24P6: C,
47.17; H, 2.17%. Found: C, 46.85; H, 2.39%.
dichloromethane (60 mL), followed by addition of a 30% H2O2
aqueous solution (11 mL). The reaction mixture was stirred for
3 h at 0˚C. The product was extracted with dichloromethane
and purified by column chromatography on SiO2 using ethyl
acetate and hexane as mixed eluent (ethyl acetate : hexane = 2 :
1). The residue was concentrated and recrystallized in ethyl
acetate. The obtained powder was washed with ethyl acetate.
dpet: Yield: 3.6 g (26%). 1H NMR (400 MHz, CDCl3, 25 ˚C) δ
7.83ꢀ7.89 (m, 8H, ꢀCH), δ 7.49ꢀ7.62 (m, 12H, ꢀCH), δ 7.35 (s,
2H, ꢀCH) ppm. ESIꢀMass (m/z): calcd for C32H23O2P2S [M+H]+,
533.09; found, 533.09. Anal. Calcd for C32H22O2P2S: C, 72.17;
H, 4.16%. Found: C, 72.31; H, 4.37%.
2.3 Optical measurements
UVꢀVis absorption spectra were recorded on a JASCO Vꢀ670
spectrometer. Emission and excitation spectra were recorded on
a HORIBA Fluorologꢀ3 spectrofluorometer and corrected for
the response of the detector system. The temperatureꢀdependent
emission spectra were measured with a nitrogen bath cryostat
(Oxford Instruments, Optistat DN) and a temperature controller
(Oxford, Instruments, ITC 502S). Emission lifetimes (τobs) were
measured using the third harmonics (355 nm) of a Qꢀswitched
Nd:YAG laser (Spectra Physiscs, INDIꢀ50, fwhm = 5 ns, λ =
1064 nm) and a photomultiplier (Hamamatsu photonics, R5108,
response time ≤ 1.1 ns). The Nd:YAG laser response was
2.2.2
Preparation
of
2,7ꢀbis(diphenylphosphorylethynyl)naphthalene (dpen)
Trimethylsilylacetylene (5.5 mL, 40 mmol) was added in one
portion to a solution of 2,7ꢀdibromonaphthalene (5.3 g, 19
mmol), PdCl2(PPh3)2 (0.55 g), CuI (0.19 g), PPh3 (0.53 g) in
diisopropylamine/dry THF (40 mL/100 mL) under an Ar
atmosphere and stirred at room temperature for 20 min. The
mixture was stirred at 50˚C overnight, and then allowed to cool
to room temperature. The ammonium salt was removed by
filtration and the mixture was extracted with ethyl acetate and
concentrated. The obtained crude oil was purified by column
monitored with a digital oscilloscope (Sony Tektronix,
TDS3052, 500 MHz) synchronized to the singleꢀpulse
excitation. Emission lifetimes were determined from the slope
of logarithmic plots of the decay profiles. The emission
quantum yields excited at 380 nm (Φtot) were estimated using
JASCO Fꢀ6300ꢀH spectrometer attached with a JASCO ILFꢀ53
integrating sphere unit (φ = 100 mm). The wavelength
dependence of the detector response and the beam intensity of
Xe light source for each spectrum were calibrated using a

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standard light source.
heating process), the chemical structures on right shows the
bridging ligands.
3. Results and Discussion
The organic bridging ligands for formation of Eu(III)
coordination glasses were prepared as shown in Scheme S1
(see supplementary information). The ethynyl groups were
Table 1. Thermal properties of an Eu(III) coordination polymer
[Eu(hfa)3(dpt)]n and coordination glasses [Eu(hfa)3(X)]3.
M
w of
introduced
via
SonogashiraꢀHagihara
coupling
and
bridging Tg / Decomposition
Compound
ligand /
˚C
temp.[a] / ˚C
deprotection reactions, followed by Liꢀhydrogen exchange and
oxidation reactions, resulting in bentꢀangled bridging ligands
g molꢀ1
(2,5ꢀbis(diphenylphosphorylethynyl)thiophene:
2,7ꢀbis(diphenylphosphorylethynyl)naphthalene: dpen, and
1,3ꢀbis(diphenylphosphorylethynyl)benzene: mꢀdpeb,
dpet,
[Eu(hfa)3(dpt)]n
[Eu(hfa)3(dpet)]3
[Eu(hfa)3(dpen)]3
[Eu(hfa)3(mꢀdpeb)]3
484.08
532.08
576.14
526.13
ꢀ
73
87
65[c]
322[b]
242
260
249
respectively. These ligands were reacted with Eu(III) precursor
complex, Eu(hfa)3(H2O)2. The obtained compounds were
identified by IR, mass spectrometry, and elemental analyses.
The fragments of trimer structures [Eu3(hfa)8(X)3]+ (X: dpet,
dpen, mꢀdpeb) in the mass spectra were confirmed for all
compounds. The fragments for larger m/z values also indicate
the presence of a wide range of other cyclic or linear oligomers.
Considering the results of mass spectrometry and the DFT
optimized structure as reported for [Eu(hfa)3(mꢀdpeb)]3,12 we
assume all these compounds form pseudoꢀC3 trimer structures
in amorphous solid state. The glassy states of these compounds
were also demonstrated by the halo signals of XRD patterns
(see supplementary information, Figure S1).
[a] Decomposition temperatures were estimated from
thermogravimetric curves (see supplementary information,
Figure S3). [b] Reference 18. [c] Reference 12.
Generally, coordination compounds exhibit thermal stability
compared to the organic compounds because of their linking
characters using coordination bonds. We consider that trimers
might not form longerꢀrange bonding or ordering out of each
structure. Since the present Ln(III) coordination compounds
exhibit broken C3 symmetrical structures with lack of
intermolecularꢀlinking, the stability at high temperature while
amorphous formation may be caused by stable coordination
bonds in discrete trimer complexes.
In order to determine the glass transition temperatures,
DSC measurements were performed. The cooling process
comes first, followed by heating. The glassꢀtransition
temperatures were clearly identified for all compounds, which
was characteristic of amorphous molecular materials. The
[Eu(hfa)3(dpen)]3 showed the highest Tg (Tg = 87˚C) among
these compounds. We consider this to be responsible for the
largest molecular weight and availability of aromatic surfaces
for πꢀstacking of the bridging ligand, dpen. The ethynyl groups
in bridging ligands may also play an important role in
amorphous formation by suppressing tightꢀbinding interaction
and crystallization of assembled Eu(III) coordination
compounds. The reported thiopheneꢀlinked Eu(III) coordination
compound without ethynyl groups [Eu(hfa)3(dpt)]n (dpt:
The diffuseꢀreflectance absorption spectra and emission spectra
were measured to evaluate the photophysical properties of
Eu(III) coordination glasses (Figure 3). These spectra were
normalized at maximum intensity. The absorption bands at
around 350 nm were assigned to the πꢀπ* transitions of hfa
ligands. The absorption shoulder bands of bridging ligands
were also observed at around 450 nm for [Eu(hfa)3(dpet)]3 and
[Eu(hfa)3(dpen)]3. The sharp and small absorption bands at 465
nm were due to the 7F0ꢀ5D2 transitions of Eu(III) ions. The
emission bands at 578, 591, 613, 650, and 698 nm were
5
attributed to the fꢀf transitions of D0ꢀ7FJ (J = 0, 1, 2, 3, and 4,
2,5ꢀbis(diphenylphosphoryl)thiophene)18
exhibits
no
respectively). The emission lifetimes (τobs) were measured and
analyzed as singleꢀexponential decays for each Eu(III)
coordination glass.
glassꢀtransition point (see supplementary information, Figure
S2). The glass forming ability was thus systematically
introduced by bentꢀangled bridging ligands with ethynyl groups,
that made more irregularꢀshaped molecules harder to crystallize
and more likely to form amorphous glass.
Figure 3. Diffuseꢀreflectance absorption spectra (left) and
emission spectra excited at 380 nm (right) of Eu(III)
coordination glasses (a) [Eu(hfa)3(dpet)]3, (b) [Eu(hfa)3(dpen)]3,
and (c) [Eu(hfa)3(mꢀdpeb)]3.
Figure 2. DSC thermograms of Eu(III) coordination
glasses (a) [Eu(hfa)3(dpet)]3, (b) [Eu(hfa)3(dpen)]3, and (c)
[Eu(hfa)3(mꢀdpeb)]3 (dashed line: cooling process, solid line: