Ligand-Assisted Back Energy Transfer in Luminescent TbIII Complexes for Thermosensing Properties

Article abstract of DOI:10.1002/chem.201804392

A luminescent Tb^III complex with a hexafluoroacetylacetone (hfa) ligand shows a characteristic back energy transfer (BEnT), which leads to high temperature sensitivity and potential application as a thermosensitive paint. Ligand-assisted BEnT was observed when a phosphine oxide ligand was introduced into Tb(hfa)₃ complex, which was shown to affect the activation energy (ΔE_a) and frequency factor (A) in the BEnT process between Tb^III ion and hfa ligands. According to temperature-dependent emission-lifetime measurements of mononuclear Tb(hfa)₃ complexes with monodentate phosphine oxides and polynuclear Tb(hfa)₃ complexes with bidentate phosphine oxides, the ΔE_a and A values of polynuclear Tb^III complexes were smaller than those of mononuclear Tb^III complexes. Phosphorescence spectra and lifetimes of each Gd(hfa)₃ complex revealed that excited states of hfa ligands in Tb^IIIcomplexes differed from those of the polynuclear Tb^III complexes and mononuclear Tb^III complexes. The differences in the ΔE_a and A values between polynuclear and mononuclear Tb^III complexes were caused by the formation of different excited states, such as delocalization of the excited state in the polynuclear Tb^III complexes and localization of excited states in the mononuclear Tb^III complexes. In particular, small ΔE_a and A values of polynuclear Tb^III complexes provided high, effective activation of the BEnT at low temperature, which resulted in high-sensitive temperature-dependent phosphor materials over a wide temperature range.

Full text of DOI:10.1002/chem.201804392

A Journal of
Accepted Article
Title: Ligand-assisted Back Energy Transfer in Luminescent Tb(III)
Complexes for Thermo-sensing Properties
Authors: Masanori Yamamoto, Yuichi Kitagawa, Takayuki Nakanishi,
Koji Fushimi, and Yasuchika Hasegawa
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To be cited as: Chem. Eur. J. 10.1002/chem.201804392
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Ligand-assisted Back Energy Transfer in Luminescent Tb(III)
Complexes for Thermo-sensing Properties
Masanori Yamamoto[a], Yuichi Kitagawa[b], Takayuki Nakanishi[c], Koji Fushimi[b], Yasuchika
Hasegawa*[b]
Abstract: Luminescent TbIII complex with hexafluoroacetylacetone
(hfa) ligand shows a characteristic back energy transfer (BEnT),
which leads to high temperature sensitivity for thermo-sensitive paint.
Ligand-assisted BEnT by introducing of phosphine oxide into
Tb(hfa)3 complex defines as an effect of controlling activation energy
(ΔEa) and frequency factor (A) in BEnT process between TbIII ion
and hfa ligands. According to temperature-dependent emission
lifetime measurements of mononuclear Tb(hfa)3 complexes with
monodentate phosphine oxides and polynuclear Tb(hfa)3 complexes
with bidentate phosphine oxides, the ΔEa and A of polynuclear TbIII
complexes were smaller than those of mononuclear TbIII complexes.
Phosphorescence spectra and lifetimes of each Gd(hfa)3 complex
revealed that excited states of hfa ligands in TbIIIcomplex differs from
polynuclear TbIII complexes and mononuclear TbIII complexes. The
difference of ΔEa and
A
value between polynuclear and
mononuclear TbIII complexes are caused by the formation of
different excited states,such as delocalization of the excited state in
the polynuclear TbIII complexes and localization of excited states in
the mononuclear TbIII complexes. In particular, the small ΔEa and
small A values of polynuclear TbIII complexes provide high effective
activation of the BEnT under low temperature, resulting in high-
sensitive temperature-dependent phosphor materials over wide
range temperature.
Figure 1. (a) Conceptual image of this study. (b) Chemical
structure of Tb(hfa)3(phosphine oxide) complexes.
times (emission lifetime > 1 µs) based on the 4f-4f transitions.[13-
16]
In 2003, Amao and co-workers reported luminescent
thermosensitive paint using the vibrational relaxation process of
a red luminescent EuIII complex.[14] In 2004, we described the
temperature-sensing ability of a green luminescent TbIII complex
based on the back energy transfer (BEnT).[16] The energy
transfer process from TbIII to EuIII in lanthanide coordination
polymer has also been used for a ratiometric luminescent
thermosensor.[17] The thermosensitive paints containing
luminescent lanthanide complexes have also been widely
investigated. [18-22]
Introduction
Thermosensitive paints containing luminescent materials enable
temperature information about the substance surface to be
obtained with high spatial resolution and a fast response.[1]
Luminescent thermosensors are expected to be useful for
capturing surface temperature dynamics of vehicles, aircrafts
and chemical reactors.[2,3] The sensing mechanism is based on
the temperature-depended luminescence intensities and
lifetimes of organic compounds[4-6], metal complexes[7-9] and
Luminescent TbIII complexes with BEnT are the most
useful in molecular design unit for high-speed thermosensor.
Here, we focus on the BEnT from excited state of TbIII (5D4) to
excited triplet state of the aromatic ligand (T1) for thermosensing.
The small energy gap between the 5D4 and T1 states (<1850 cm-
1) leads to effective BEnT in TbIII complexes.[23] To control of the
energy gap, pyridyl, diketonate and carbonyl ligands with
aromatic derivatives in TbIII complexes have been
nanoparticles[10-12]
containing lanthanide complexes have attracted attention for
precise temperature imaging with narrow emission bands (full-
width at half maximum (FWHM) < 10 nm) and long emission life-
. In particular, luminescent thermosensors
investigated.[24-28]
Among
these
ligands,
the
hexafluoroacetylacetonate (hfa) ligand is one of the most
promising candidates for development of thermosensitive
luminescent TbIII complexes (energy gap of hfa = 1700 cm-1).
[a]
M. Yamamoto
Graduate School of Chemical Sciences and Engineering, Hokkaido
University, North 13 West 8, Kita-ku, Sapporo, Hokkaido, 060-8628
(Japan).
Dr. Y. Kitagawa, Dr. K. Fushimi, Prof. Dr. Y. Hasegawa
Faculty of Engineering, Hokkaido University, North 13 West 8, Kita-
ku, Sapporo, Hokkaido, 060-8628 (Japan).
[24,25]
The hfa ligand also forms hydrogen bond networks in solid
[b]
[c]
TbIII complexes, which promotes the thermal stability in the high-
temperature range.[29,30] The luminescent units composed of
Tb(hfa)3 complexes are suitable for thermosensitive paints based
on the BEnT.
Dr. T. Nakanishi
Faculty of Industrial Science and Technology, Tokyo University of
Science, 6-3-1, Shinjuku, Katsushika-ku, Tokyo, 125-8585 (Japan).
In general, the BEnT is depended on the activation energy
(ΔEa) and frequency factor (A) in the Arrhenius analysis.[27, 31]
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Controlling ΔEa and A in Tb(hfa)3 complexes improves the
thermosensing ability for adjustable sensing range and
temperature. In this study, activation energy control in BEnT of
Tb(hfa)3 complex is reported using activation energy controllers
(AECs). The conceptual image of AECs is shown in Figure 1a.
Here, we select the aromatic phosphine oxide ligand as an AEC
molecule. The ΔEa and A values of BEnT can be tuned by
introducing the phosphine oxide ligand with a specific interaction
with hfa ligand. The phosphine oxide ligand strongly coordinates
to the lanthanide ion by P=O group.[32] The P=O group is also
useful for enhancing the luminescence of lanthanide complexes
based on suppression of nonradiative process via vibrational
relaxation.[32] Luminescent TbIII complexes with hfa and
phosphine oxide ligands, mononuclear [Tb(hfa)3(AEC)] and
polynuclear
[Tb(hfa)3(biphepo)],
[Tb(hfa)3(dpbp)]n
[Tb(hfa)3(AEC)]n
:
[Tb(hfa)3(tppo)2],
[Tb(hfa)3(dpb)]n,
[Tb(hfa)3(dppy)2],
and
[Tb(hfa)3ZnCl2(dppy)2]n
(tppo:
trisphenylphosphine
diylbis(diphenylphosphine
pyridyldiphenylphosphine
oxide,
biphepo:
oxide),
oxide,
[1,1’-biphenyl]-2,2’-
dppy:
dpb:
4-
1,4-
phenylenebis(diphenylphosphine oxide), dpbp: [1,1’-biphenyl]-
4,4’-diylbis(diphenylphosphine oxide)), were prepared for
estimation of tunable ΔEa and A values in their BEnT (Figure 1b).
The emission properties of [Tb(hfa)3(AEC)] were evaluated using
the emission spectra, emission lifetimes and emission quantum
yields. To estimate the ΔEa and A values in BEnT, Arrhenius
analysis was performed using temperature-dependent emission
lifetimes measurements. The triplet state lifetimes of hfa ligands
were estimated using polynuclear and mononuclear GdIII
complexes. Introduction of phosphine oxide ligands to Tb(hfa)3
complexes leads to control of the BEnT process, which is called
ligand-assisted BEnT. This is the first report of control of
thermosensing properties of the Tb(hfa)3 complex by ligand-
assisted BEnT.
Figure 2. Emission spectra of (a) mononuclear [Tb(hfa)3(AEC)]
and (b) polynuclear [Tb(hfa)3(AEC)]n in the solid state at room
temperature. Excited at 350 nm.
Information and Figure S8). In polynuclear [Tb(hfa)3(AEC)]n, the
FRET-like energy transfer between TbIII ions should not be
inefficient.
Single
crystals
of
[Tb(hfa)3(biphepo)],
[Tb(hfa)3(dppy)2] and [Tb(hfa)3ZnCl2(dppy)2]n were not obtained,
and these structure could not be determined.
The emission spectra of the mononuclear [Tb(hfa)3(AEC)]
and polynuclear [Tb(hfa)3(AEC)]n complexes in solid state at RT
are shown in Figure 2. Emission bands are observed at 487, 548,
582, and 623 nm, which are attributed to 4f−4f transitions of the
TbIII complexes (5D4→7FJ, where J = 6, 5, 4, and 3, respectively).
The Stark splitting shapes at 548nm (5D4→7F5) for mononuclear
[Tb(hfa)3(AEC)] are similar to those for polynuclear
[Tb(hfa)3(AEC)]n. In particular, the stark splitting shapes of
[Tb(hfa)3(tppo)2] and [Tb(hfa)3(dppy)2] are much similar to those
of [Tb(hfa)3(dpb)]n and [Tb(hfa)3(dpbp)]n (see Figure S9a).
Generally, the Stark splitting shape of emission spectrum of a
lanthanide complex is dependent on the coordination geometry.
From Figure S9b, the coordinate structure of [Tb(hfa)3(dppy)2]
should also be eight-coordinate square antiprism structure, such
as [Tb(hfa)3(dpbp)]n. We found that the emission spectral shape
of [Tb(hfa)3(biphepo)] with eight-coordinate square antiprism
structure is similar to that of [Tb(hfa)3ZnCl2(dppy)2]n.
Results and Discussion
Photophysical properties at room temperature
The mononuclear [Tb(hfa)3(AEC)] and polynuclear
[Tb(hfa)3(AEC)]n
[Tb(hfa)3(dppy)2],
([Tb(hfa)3(tppo)2],
[Tb(hfa)3(dpb)]n,
[Tb(hfa)3(biphepo)],
[Tb(hfa)3(dpbp)]n
and
[Tb(hfa)3ZnCl2(dppy)2]n)
were
synthesized
by
reacting
[Tb(hfa)3(H2O)2] precursor with phosphine oxide ligands in
methanol under reflux. Single crystals of mononuclear
[Tb(hfa)3(AEC)] and polynuclear [Tb(hfa)3(AEC)]n were prepared
by recrystallization from methanol solutions at room temperature
(RT). The structural images and crystal data of [Tb(hfa)3(tppo)2],
[Tb(hfa)3(dpb)]n and [Tb(hfa)3(dpbp)]n are shown in Figure S1
and Table S1, respectively. From the single-crystal X-ray
structural analysis results, the coordination sites of the TbIII ion
are comprised of three hfa and two phosphine oxide ligands
(AEC ligands). Their coordination geometries based on the
shape measure estimations are categorized as eight-coordinate
square antiprism structures (see Figure S2 and Table S2-S4).
The critical distance for dipole-dipole energy transfer between
TbIII ions was estimated to be approximately 5 Å (see supporting
The emission decay of mononuclear [Tb(hfa)3(AEC)] and
polynuclear [Tb(hfa)3(AEC)]n complexes in solid state at 300 K
are shown in Figure S3. The emission lifetimes of the
[Tb(hfa)3(AEC)] complexes (τobs) were determined from the slope
of the logarithmic decay profiles. The emission decay of the
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mononuclear [Tb(hfa)3(AEC)] and polynuclear [Tb(hfa)3(AEC)]n
complexes showed microsecond-scale emission lifetime by
estimating single-exponential fitting. The overall emission
quantum yields (Φtot) by excitation of the hfa ligands in solid
state at RT were measured using the integration sphere. The
photophysical parameters (Φtot and τobs, RT) are summarized in
Table 1. The Φtot values of [Tb(hfa)3(tppo)2], [Tb(hfa)3(biphepo)]
and [Tb(hfa)3(dppy)2] are 9.2%, 11%, and 10%, respectively. In
contrast, the Φtot values of [Tb(hfa)3(dpbp)]n (17%) and
[Tb(hfa)3ZnCl2(dppy)2]n (35%) are significantly larger than those
of mononuclear [Tb(hfa)3(AEC)] complexes, although the Φtot
value of [Tb(hfa)3(dpb)]n (10%) is similar to those of
mononuclear [Tb(hfa)3(AEC)] complexes. These results indicate
that the photosensitized emission quantum yields of polynuclear
TbIII complexes are dependent on the phosphine oxide linker
moiety. We consider that introduction of phosphine oxide ligand
affects the photosensitized energy transfer process between the
hfa ligands and the TbIII ion.
Temperature-dependent emission properties
To estimate the effect of BEnT in the mononuclear
[Tb(hfa)3(AEC)] and polynuclear [Tb(hfa)3(AEC)]n complexes,
the temperature-dependent emission lifetimes in solid state were
measured in the range 90–400 K. The logarithmic lifetime
profiles of
a
typical TbIII complex with the tppo ligand,
[Tb(hfa)3(tppo)2], are shown in Figure 3a. From Figure 3a,
emission decay profiles from 100 K to 200 K were shown single-
exponential decays, but emission decay profiles from 250 K to
400
K were shown multi-exponential decays. In higher
temperature range, we consider that at least two types of
emission species are coexisted in solid states. The emission
decay of [Tb(hfa)3(tppo)2] analyzed single-exponential decays at
each temperature by treating emission lifetime as an average
value. The emission lifetimes of the mononuclear
[Tb(hfa)3(tppo)2] complexes at 100 K, 200 K, 300 K, and 400K
are 806 µs, 709 µs, 86 µs, and 5.6 µs, respectively. The longer
emission lifetimes at 100-200 K are because of suppression of
the BEnT process from emitting level of the TbIII ion (5D4) to the
triplet state of the hfa ligands (T1). The temperature-dependent
emission lifetimes are observed for all of the mononuclear
[Tb(hfa)3(AEC)] and polynuclear [Tb(hfa)3(AEC)]n complexes.
（see Figure S4 and S5）.
Figure 3. (a) Temperature-dependent emission lifetime decays
of [Tb(hfa)3(tppo)2] in the solid state among 100-400 K. Excited
at 355 nm (third harmonics of Q-switched Nd:YAG laser. fwhm
=5 ns, λ = 1064 nm, black line: 100 K, purple line: 150 K, blue
line: 200 K, green line: 250 K, yellow line: 300K, orange line: 350
K, red line 400 K). (b) Arrhenius plots of each each Tb(hfa)3
complex in the solid state.
Table 1. Photophysical properties and Arrhenius parameters of [Tb(hfa)(AEC)]
Φtot
τobs, RT
ΔEa
A
Conformation
%
µs
cm-1
s-1
To analyze the BEnT mechanism, we estimated the BEnT
rate constants (kBEnT) by kinetic analysis. The temperature
dependence of kBEnT is expected to follow an Arrhenius-type
equation with an activation energy ΔEa. [17] kBEnT is given by
[Tb(hfa)3(tp
po)2]
[Tb(hfa)3(bi
phepo)]
[Tb(hfa)3(dp
py)2]
Mononuclear
Mononuclear
Mononuclear
Polymer
9.2±1
11±1
10±1
9.7±1
17±2
35±2
209±3
267±12
482±9
73±5
3200
4000
3600
1700
1900
2500
1.98×1010
3.56×1011
1.06×1010
4.06×107
3.68×107
7.92×107
1
1
∆ꢀ!
[Tb(hfa)3(dp
b)]n
[Tb(hfa)3(dp
bp)]n
[Tb(hfa)3Zn
Cl2(dppy)2]n
ln
−
= ln ꢀ!"#$ = ln ꢀ −
∙ ꢀ!!
1 ,
ꢀ!"# ꢀ!"#
ꢀ
Polymer
215±4
610±7
where τobs, τ90K, A, ΔEa, R and T are the observed emission
lifetime, standard emission lifetime at 90 K, frequency factor,
activation energy, gas constant and temperature, respectively.
Arrhenius plots of BEnT for the mononuclear [Tb(hfa)3(AEC)]
and polynuclear [Tb(hfa)3(AEC)]n complexes are shown in
Figure 3b, and the ΔEa and A values are given in Table 1. The
BEnT process diagram is shown in Figure S6. From Figure 3b,
Polymer
the Arrhenius plot of mononuclear [Tb(hfa)3(AEC)] and
polynuclear [Tb(hfa)3(AEC)]n could be analyzed by linear
approximation. These results indicate that the BEnT process is
only composed of between TbIII ions and hfa ligands. The ΔEa
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values
of
polynuclear
[Tb(hfa)3(AEC)]n
complexes
([Tb(hfa)3(dpb)]n, [Tb(hfa)3(dpbp)]n and [Tb(hfa)3ZnCl2(dppy)]n)
are smaller than those of mononuclear [Tb(hfa)3(AEC)]
complexes
([Tb(hfa)3(tppo)2],
[Tb(hfa)3(biphepo)]
and
[Tb(hfa)3(dppy)2]). The A values of polynuclear [Tb(hfa)3(AEC)]n
are also smaller than those of mononuclear [Tb(hfa)3(AEC)].
According to the polynuclear TbIII complexes, the ΔEa
value of [Tb(hfa)3(dpb)]n (ΔEa = 1700 cm-1) and [Tb(hfa)3(dpbp)]n
(ΔEa
=
1900 cm-1) are smaller than those of
[Tb(hfa)3ZnCl2(dppy)]n (ΔEa = 2500 cm-1). The A value of
[Tb(hfa)3(dpb)]n (A = 4.06×107 s-1) and [Tb(hfa)3(dpbp)]n (A
=73.68×107
s-1)
are
also
smaller
than
those
of
[Tb(hfa)3ZnCl2(dppy)]n (A = 7.92×107 s-1).The smaller ΔEa and A
values of polynuclear [Tb(hfa)3(AEC)]n complex promote
effective activation of BEnT under lower temperature, resulting
in high-sensitive temperature-dependent phosphor materials
over a wide range temperature. The phosphine oxide ligand
(activation energy controller: AEC) should dominate not only the
TbIII molecular structures (mononuclear or polymer) but also the
BEnT process (ΔEa and A values).
Phosphorescence
[Gd(hfa)3(AEC)] and polynuclear [Gd(hfa)3(AEC)]n
The calculated ΔEa and values of mononuclear
properties
of
mononuclear
A
[Tb(hfa)3(AEC)] and polynuclear [Tb(hfa)3(AEC)]n complexes are
directly linked to the triplet levels of hfa ligands in the Tb(hfa)3
complexes. In this study, to estimate the triplet levels of the hfa
ligands, mononuclear [Gd(hfa)3(AEC)] and polynuclear
Figure 4. (a) Phosphorescence spectra of mononuclear
[Gd(hfa)3(AEC)] and polynuclear [Gd(hfa)3(AEC)]n in the solid
state at 100 K. Excited at 350 nm (Time delay = 0 ms). (b)
Chemical structure of Gd(hfa)3(phosphine oxide) complexes.
[Gd(hfa)3(AEC)]n
[Gd(hfa)3(biphepo)],
complexes
[Gd(hfa)3(dppy)2],
([Gd(hfa)3(tppo)2],
[Gd(hfa)3(dpb)]n,
[Gd(hfa)3(dpbp)]n and [Gd(hfa)3ZnCl2(dppy)]n) were prepared by
the same synthetic method as that used to prepare the
mononuclear [Tb(hfa)3(AEC)] and polynuclear [Tb(hfa)3(AEC)]n
complexes. Generally, the triplet state properties of
photosensitized ligands in lanthanide complexes can be
evaluated by phosphorescence measurements of the GdIII
complexes, because the excited energy level of the GdIII ion is
higher than the T1 level of photosensitized hfa ligand. We
confirmed that phosphine oxide ligands were not excited at 350
or 355 nm based on diffuse reflection spectra of GdIII complexes
and phosphine oxide ligand (see Figure S11). The
phosphorescence spectra of the mononuclear [Gd(hfa)3(AEC)]
and polynuclear [Gd(hfa)3(AEC)]n complexes in solid states at
100 K are shown in Figure 4. The phosphorescence spectrum
shapes of the polynuclear [Gd(hfa)3(dpb)]n and [Gd(hfa)3(dpbp)]n
complexes are broader than those of the mononuclear
Table 2. Triplet energies and phosphorescence lifetimes of hfa ligands
ΔE(T1-5D4 )
E(T1)
τobs,1
τobs,2
Conformation
Mononuclear
Mononuclear
cm-1
cm-1
ms
ms
2.00
(70%)
9.12
(30%)
[Gd(hfa)3
(tppo)2]
21700
21800
1300
[Gd(hfa)3
(biphepo)
]
2.94
(68%)
9.41
(32%)
1400
[Gd(hfa)3
(dppy)2]
1.96
(71%)
8.25
(29%)
Mononuclear
Polymer
21600
22400
22300
21700
1200
2000
1900
1300
3.31
(79%)
9.54
(21%)
[Gd(hfa)3
(dpb)]n
[Gd(hfa)3(AEC)]
and
polynuclear
[Gd(hfa)3ZnCl2(dppy)2]n
complexes. These results indicate that the vibrational structure
of the excited hfa ligand in the polynuclear [Gd(hfa)3(dpb)]n and
[Gd(hfa)3(dpbp)]n complexes are considerably different from
those of the mononuclear [Gd(hfa)3(AEC)] and polynuclear
[Gd(hfa)3ZnCl2(dppy)2]n
phosphorescence bands
3.30
(80%)
10.2
(20%)
[Gd(hfa)3
(dpbp)]n
Polymer
[Gd(hfa)3
ZnCl2(dp
py)2]n
2.02
(65%)
7.75
(35%)
Polymer
complexes.
of
The
broader
and
[Gd(hfa)3(dpb)]n
[Gd(hfa)3(dpbp)]n could be caused by the change of the potential
curvature of the triplet state in the excited hfa ligands. From the
phosphorescence spectra, the estimated triplet energy levels of
the hfa ligands in the GdIII complexes E(T1) are summarized in
Table 2. We also give the energy gap ΔE(T1-5D4) between the
triplet level of the hfa ligands (T1) and emitting level of Tb
5
(energy gap of D4 = 20400 cm-1). The ΔE(T1-5D4) values of the
polynuclear
TbIII
complexes
[Tb(hfa)3(dpb)]n
and
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[Tb(hfa)3(dpbp)]n were larger than those of mononuclear
[Tb(hfa)3(AEC)] complexes, although the ΔEa values of the
BEnT process for [Tb(hfa)3(dpb)]n and [Tb(hfa)3(dpbp)]n are
smaller than those of mononuclear [Tb(hfa)3(AEC)] complexes.
From these photophysical conflicts, we propose that the BEnT
process is related to the triplet states lifetimes of hfa ligands in
our systems.
Here, the phosphorescence decay measurements were
performed to estimation of the triplet state lifetimes on the
excited hfa ligands. The logarithmic lifetime profiles of the GdIII
complexes are shown in Figure S7, and the calculated
phosphorescence lifetimes are summarized in Table 2. The
phosphorescence decay exhibited double exponential decay at
100 K. Note that the Tb(hfa)3 units is composed of three identical
hfa ligands from single-crystal X-ray structural analyses. The
double exponential decay of hfa phosphorescence is caused by
the presence of at least two types of excited hfa ligands in the
solid state. The phosphorescence lifetimes of the polynuclear
[Gd(hfa)3(dpb)]n and [Gd(hfa)3(dpbp)]n complexes are longer
than those of the mononuclear [Gd(hfa)3(AEC)] and polynuclear
[Gd(hfa)3ZnCl2(dppy)2]n complexes.
Here, we here consider that the polynuclear
[Gd(hfa)3(dpb)]n and [Gd(hfa)3(dpbp)]n complexes promote
energy transfer hopping (migration) of excited TbIII ions on the
dpb and dpbp linker ligands, [33] resulting in delocalization of the
excited triplet state of hfa ligand. This excited delocalization is
related to the broader phosphorescence spectra and long
phosphorescence lifetimes of the hfa ligands, which promotes
ligand-assisted BEnT. The excited states of mononuclear
[Tb(hfa)3(AECs)] complexes with large ΔEa and large A values
only depend on localization between TbIII and hfa ligands in the
forward and back energy transfer process, resulting in short
phosphorescence lifetime and large frequency factor A (Figure
5b). For the polynuclear [Tb(hfa)3ZnCl2(dppy)2]n complex, the
ΔEa and A values are similar to those of the polynuclear
[Tb(hfa)3(dpb)]n and [Tb(hfa)3(dpbp)]n complexes. However, the
phosphorescence spectrum shape of the polynuclear
[Gd(hfa)3ZnCl2(dppy)2]n complex is the same as that of
mononuclear [Gd(hfa)3(dppy)2] complex. From these results, we
consider that [Tb(hfa)3ZnCl2(dppy)2]n shows localized forward
and back energy transfer between the hfa and TbIII ions (short
phosphorescent lifetimes and phosphorescent spectrum shapes),
and delocalized energy hopping of excited TbIII ions via the
phosphine oxide linker ligands (show small ΔEa and small A)
(Figure 5c).
Conclusions
Introduction of phosphine oxide ligands (Activation Energy
Controllers: AECs) to Tb(hfa)3 complexes leads to control of the
BEnT process (the ligand-assisted BEnT). From Arrhenius
analysis based on the temperature-dependent emission lifetime
measurements, the ΔEa and A values are smaller for the
polynuclear [Tb(hfa)3(AEC)]n complexes than for the
mononuclear [Tb(hfa)3(AEC)] complexes. In particular,
[Tb(hfa)3ZnCl2(dppy)2]n shows a wider sensing range and larger
emission quantum yields than the previous reported chameleon
luminophore structure [Tb0.99Eu0.01(hfa)3(dpbp)]n.[17,33] The
phosphorescent lifetimes of the hfa ligands in the GdIII
coordination polymers are longer than those in mononuclear
[Gd(hfa)3(AEC)] complexes. We consider that the energy
hopping of excited TbIII ions and hfa ligands is introduced by the
phosphine oxide linker ligand in polynuclear [Gd(hfa)3(AEC)]n
complexes. The ΔEa and A values in the BEnT process are a
strongly correlated with the excitation lifetimes of the hfa ligands.
Introduction of phosphine oxide ligands to Tb(hfa)3 complexes is
expected to control the BEnT process. This is the first report of
the thermosensing properties of the Tb(hfa)3 complex by ligand-
assisted BEnT.
Experimental Section
Materials: Terbium acetate tetrahydrate (>99.9%), n-BuLi (in n-hexane,
1.6 M) and hydrogen peroxide were purchased from Kanto Chemical Co.,
Inc. Hexafluoroacetylacetone (>95.0%), triphenylphosphine oxide
(>98.0%),
2,2’-bis(diphenylphosphino)biphenyl
(>98.0%),
1,4-
dibromobenzene (>99.0%), 4,4’-dibromobiphenyl (>98.0%) and
chlorodiphenylphosphine (>97.0%) were obtained from Tokyo Chemical
Industry Co., Ltd. All other chemicals and solvents were reagent grade
and were used without further purification.
Apparatus: 1H NMR spectra were recorded using a JEOL JNM-ECS400
(400 MHz). 1H NMR chemical shifts were determined by
tetramethylsilane (TMS) as an internal standard. Infrared spectra were
Figure 5. (a) Excited energy delocalization image of polynuclear
[Tb(hfa)3(dpb)]n and [Tb(hfa)3(dpbp)]n. (b) Excited energy
localization image of mononuclear [Tb(hfa)3(AEC)]. (c) Excited
recorded using
a JASCO FT/IR-4600. Elemental analyses were
performed using an Exeter Analytical, Inc. CE-440 Elemental Analyzer.
energy
delocalization
image
of
polynuclear
[Tb(hfa)3ZnCl2(dppy)2].
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10.1002/chem.201804392
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Preparation of [1,1’-biphenyl]-2,2’-diylbis(diphenylphosphine oxide)
CDCl3 / TMS): δ7.67-7.80 (m, 16H, p-C6H5, C6H4), 7.45-7.60 (m, 12H, p-
(biphepo) C36H28O2P2: Biphepo was obtained by the same method as
C6H5, C6H4) ppm. IR (ART): 1120 (st, P=O) cm–1
.
previously
reported
by
Nakamura
et
al.[34]
2,2’-
Bis(diphenylphosphino)biphenyl (500 mg, 1.60 mmol) added into 20 ml of
dichloromethane, and cooled under 0˚C and then 30% H2O2 aqueous
solution (30%) was add to the solution. The reaction mixture was stirred
Preparation of [ZnCl2(dppy)2] C34H28Cl2N2O2P2Zn. [ZnCl2(dppy)2] was
prepared according to literature references.[35] 1H NMR (400MHz,
CDCl3/TMS): δ8.84-8.89 (t, 4H, py), 7.71-7.77 (d, 4H, py), 7.62-7.70 (m,
12H, Ar), 7.50-7.57 (m, 8H, Ar) ppm. ESI-MS calcd. for [ZnCl(dppy)2]+:
692.03 Found, 692.03. Elemental analysis calcd (%) for
C34H28Cl2N2O2P2Zn: C 58.77, H 4.06, N 4.03; found: C 59.27, H 4.31, N
3.73.
for
3
h. After the reaction, the product was extracted with
dichloromethane (30 ml × 3), and then washed with brine for three times.
The dichloromethane solution was dried using anhydrous MgSO4 and
concentrated to dryness. The biphepo was recovered as a white solid.
Yield: 336 mg (38%). 1H NMR (400 MHz, CDCl3 / TMS): δ7.73-7.69(t, 4H),
7.66-7.63(t, 4H), 7.53-7.50(t, 2H), 7.47-7.43(t, 4H), 7.36-7.33(t, 2H),
7.31-7.28(t, 4H), 7.20-7.15(m, 6H), 7.00(t, 2H)ppm. IR (ART): 1210 (st,
Preparation of [Tb(hfa)3(H2O)2] C15H7F18O8Tb: [Tb(hfa)3(H2O)2] was
prepared according to literature reference.[16] Terbium acetate
tetrahydrate (2.4 g, 5.9 mmol) was dissolved in distilled water (30 mL).
Hexafluoroacetylacetone (4.0 g, 19 mmol) was added dropwise to the
solution and let stirred for 3 h at room temperature to form pale yellow
precipitates. The reaction mixture was filtered and washed with distilled
water. The resulting powder was used without further purification for the
next step. Yield 4.1 g (85%). IR (ART): 1647 (st, C=O), 1251, 1204,1135
(st, C-F) cm-1. Elemental analysis calcd (%) for C15H7F18O8Tb: C 22.08, H
0.86; found: C 21.68, H 1.14.
P=O) cm–1
.
Preparation of 4-pyridyl diphenyl phosphine oxide (dppy)
C17H14NOP: Dppy was prepared according to literature references.[35] 1H
NMR (400MHz, CDCl3/TMS): δ8.74-8.79 (t, 2H, py), 7.48-7.71 (m, 12H,
Ar) ppm. ESI-MS calcd. for [M+H]+: 280.09 Found, 280.09. Elemental
analysis calcd (%) for C17H14NOP: C 73.11, H 5.05, N 5.02; found: C
73.03, H, 5.07; N, 4.95.
Preparation of 1,4-phenylenebis(diphenylphosphine oxide) (dpb)
Preparation of [Tb(hfa)3(tppo)2] C51H33F18O8P2Tb: Methanol (50mL)
containing [Tb(hfa)3(H2O)2] (294 mg, 0.36 mmol) and triphenylphosphine
oxide: tppo (200 mg, 0.72 mmol) was fluxed under stirring for 3 h. The
reaction mixture was concentrated with evaporator. Recrystallization from
methanol gave colorless crude crystals. Yield: 150 mg (31%). IR (ART):
1653 (st, C=O), 1250,1140 (st, C-F), 1161, 1122 (st, P=O) cm-1.
Elemental analysis calcd (%) for C51H33F18O8P2Tb: C 45.83, H 2.49;
found: C 45.19, H 2.49.
C30H24O2P2:
1,4-phenylenebis(diphenylphosphine
oxide)
was
synthesized according to the published procedure.[36] A solution of n-BuLi
(12.5 mL, 1.6 M, 20 mmol), was added dropwise to a solution of 1,4-
dibromobenzene (2.36 g, 10 mmol) in dry THF (30 mL) at –80˚C. The
addition was completed in ca. 15 min during which time a yellow
precipitate was formed. The mixture was allowed to stir for 3 h at –10˚C,
after which a chlorodiphenylphosphine (2.7 mL, 20 mmol) was added
dropwise at –80˚C. The mixture was gradually brought to room
temperature, and stirred for 14 h. The product was extracted with
dichloromethane, the extracts washed with brine for three times and
dried over anhydrous MgSO4. The obtained white solid and
dichloromethane (ca. 40 mL) were placed in a flask. The solution was
cooled to 0˚C and then 30% H2O2 aqueous solution (5 mL) was added to
it. The reaction mixture was stirred for 2 h. The product was extracted
with dichloromethane, the extracts washed with brine for three times and
dried over anhydrous MgSO4. The solvent was evaporated, and resulting
residue was washed with ethyl acetate for several times, and filtered in
vacuum to afford a white powder. Yield: 2.5 g (52%). 1H NMR (400 MHz,
CDCl3 / TMS): δ7.48-7.78 (m, 24H, p-C6H5, C6H4) ppm. IR (ART): 1121
Preparation of [Tb(hfa)3(biphepo)] C51H31F18O8P2Tb: Methanol (100
mL) containing [Tb(hfa)3(H2O)2] (294 mg, 0.36 mmol) and biphepo (200
mg, 0.36 mmol) was refluxed under stirring for 3 h. The reaction mixture
was concentrated with evaporator. Recrystallization from methanol gave
white crystal. Yield: 176 mg (36%). IR (ART): 1654 (st, C=O), 1253, 1140
(st, C-F), 1180, 1124 (st, P=O) cm-1. Elemental analysis calcd (%) for
C51H31F18O8P2Tb: C 45.90, H 2.34; found: C 45.53, H 2.43.
Preparation of [Tb(hfa)3(dppy)2] C49H31F18N2O8P2Tb: Methanol (50mL)
containing [Tb(hfa)3(H2O)2] (294 mg, 0.36 mmol) and dppy (200 mg, 0.72
mmol) was fluxed under stirring for 3 h. The reaction mixture was
concentrated with evaporator. Recrystallization from methanol gave
colorless crude crystals. Yield: 110 mg (23%). IR (ART): 1652 (st, C=O),
1251, 1140 (st, C-F), 1170-1108 (st, P=O) cm-1. Elemental analysis calcd
(%) for C49H31F18N2O8P2Tb: C 43.97, H 2.33, N 2.09; found: C 44.19, H
2.45, N 1.87.
(st, P=O) cm–1
.
Preparation of [1,1’-biphenyl]-4,4’-diylbis(diphenylphosphine oxide)
(dpbp) C36H28O2P2: 4,4’-bis(diphenylphosphoryl)biphenyl was
synthesized according to the published procedure. [36] A solution of n-
BuLi (9.3 mL, 1.6 M hexane, 15 mmol), was added dropwise to a solution
of 4,4’-dibromobiphenyl (1.9 g, 6.0 mmol) in dry THF (30 mL) at –80˚C.
The addition was completed in ca. 15 min during which time a yellow
precipitate was formed. The mixture was allowed to stir for 3 h at –10˚C,
after which a chlorodiphenylphosphine (2.7 mL, 15 mmol) was added
dropwise at –80˚C. The mixture was gradually brought to room
temperature, and stirred for 14 h. The product was extracted with
dichloromethane, the extracts washed with brine for three times and
dried over anhydrous MgSO4. The obtained white solid and
dichloromethane (ca. 40 mL) were placed in a flask. The solution was
cooled to 0˚C and then 30% H2O2 aqueous solution (5 mL) was added to
it. The reaction mixture was stirred for 2 h. The product was extracted
with dichloromethane, the extracts washed with brine for three times and
dried over anhydrous MgSO4. The solvent was evaporated, and resulting
residue was washed with ethyl acetate for several times, and filtered in
vacuum to afford a white powder. Yield: 1.1 g (33%). 1H NMR (400 MHz,
Preparation of [Tb(hfa)3(dpb)]n C45H27F18O8P2Tb: [Tb(hfa)3(H2O)2] (343
mg, 0.42 mmol) and dpb (200 mg, 0.42 mmol) were dissolved in
methanol (15 mL). The solution was refluxed while stirring for 3 h to give
a white precipitate. The precipitate was filtered, washed with methanol for
several times, and dried in vacuum. Yield: 213 mg (40%). IR (ART): 1652
(st, C=O), 1252, 1138 (st, C-F), 1186-1106 (st, P=O) cm-1. Elemental
analysis calcd (%) for C45H27F18O8P2Tb: C 42.95, H 2.16; found: C 43.28,
H 2.04.
Preparation of [Tb(hfa)3(dpbp)]n C51H31F18O8P2Tb: [Tb(hfa)3(H2O)2]
(294 mg, 0.36 mmol) and dpbp (200 mg, 0.36 mmol) were dissolved in
methanol (15 mL). The solution was refluxed while stirring for 3 h to give
a white precipitate. The precipitate was filtered, washed with methanol for
several times, and dried in vacuum. Yield: 184 mg (38%). IR (ART): 1652
(st, C=O), 1250, 1139 (st, C-F), 1182, 1123 (st, P=O) cm-1. Elemental
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10.1002/chem.201804392
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analysis calcd (%) for C51H31F18O8P2Tb: C 45.90, H 2.34; found: C 45.65,
H 2.19.
The error of the phosphorescence spectra at 430 nm (phosphorescence
edge of GdIII complexes) was estimated to be 5 cm-1. Emission lifetimes
(τobs) of coordination polymers were measured using the third harmonics
(355 nm) of a Q-switched Nd:YAG laser (Spectra Physics, INDI-50,
FWHM = 5 ns, λ = 1064 nm) and a photomultiplier (Hamamatsu
photonics, R5108, response time ≤ 1.1 ns). The Nd:YAG laser response
was monitored with a digital oscilloscope (Sony Tektronix, TDS3052, 500
MHz) synchronized to the single-pulse excitation. Emission lifetimes were
determined from the slope of logarithmic plots of the decay profiles.
Emission lifetimes in the range 100–400 K were measured using a
cryostat (Thermal Block Company, SA-SB245T) and a temperature
controller (Oxford, Instruments, ITC 502S). We carried out all
photophysical measurements of TbIII and GdIII complexes with cut filter
under 385 nm (LU0385 longpass filter, Asahi Spectra Co., Ltd.).The
emission quantum yields excited at 350 nm (Φtot) were estimated using a
JASCO F-6300-H spectrometer attached with JASCO ILF-533 integrating
sphere unit (φ = 100 nm). The wavelength dependence of the detector
response and the beam intensity of the Xe light source for each spectrum
were calibrated using a standard light source.
Preparation of [Tb(hfa)3ZnCl2(dppy)2]n C49H31Cl2F18N2O8P2TbZn:
[Tb(hfa)3(H2O)2] (237 mg, 0.29 mmol) and [ZnCl2(dppy)2] (200 mg, 0.29
mmol) were dissolved in methanol (15 mL). The solution was refluxed
while stirring for 3 h to give a white precipitate. The precipitate was
filtered, washed with methanol for several times, and dried in vacuum.
Yield: 123 mg (29%). IR (ART): 1649 (st, C=O), 1253, 1181-1132 (st, C-
F), 1217-1106 (st, P=O) cm-1. Elemental analysis calcd (%) for
C49H31Cl2F18N2O8P2TbZn: C 39.90, H 2.12, N 1.90; found: C 39.56, H
2.35, N 1.86.
Preparation of mononuclear [Gd(hfa)3(AEC)] and polynuclear
[Gd(hfa)3(AEC)]n: Mononuclear [Gd(hfa)3(AEC)] and polynuclear
[Gd(hfa)3(AEC)]n were prepared by the same synthetic method as that
used to prepare the mononuclear [Tb(hfa)3(AEC)] and polynuclear
[Tb(hfa)3(AEC)]n complexes.
[Gd(hfa)3(tppo)2] C51H33F18O8P2Gd: IR (ART): 1652 (st, C=O),
1253,1140 (st, C-F), 1167, 1122 (st, P=O) cm-1. Elemental analysis calcd
(%) for C51H33F18O8P2Gd: C 45.89, H 2.49; found: C 45.42, H 2.89.
Computational Details: Density functional theory (DFT) geometry
optimizations of [ZnCl2(dppy)2] and Time-dependent DFT (TD-DFT) of
phosphine oxide ligands were carried out with Gaussian09 Rev D.01 by
employing the three-parameter hybrid functional of Becke based on the
correlation functional of Lee, Yang, and Parr (B3LYP).[37,38] The 6-31G*(d,
P) basis set was used for all other atoms.
[Gd(hfa)3(biphepo)] C51H31F18O8P2Gd: IR (ART): 1654 (st, C=O), 1250,
1139 (st, C-F), 1181, 1124 (st, P=O) cm-1. Elemental analysis calcd (%)
for C51H31F18O8P2Gd: C 45.90, H 2.34; found: C 46.23, H 2.45.
[Gd(hfa)3(dppy)2] C49H31F18N2O8P2Gd: IR (ART): 1652 (st, C=O), 1251,
1140 (st, C-F), 1170-1108 (st, P=O) cm-1. Elemental analysis calcd (%)
for C49H31F18N2O8P2Gd: C 44.02, H 2.34, N 2.10; found: C 44.46, H 2.47,
N 2.19.
Acknowledgements
This work was supported by JSPS KAKENHI (grant
number 18H04497). This work was also partially supported by a
Grant-in-Aid for Scientific Research on the 18H02041 from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan. This present research was supported by the
Ministry of Education, Culture, Sports, Science and Technology
through Program for Leading Graduate Schools (Hokkaido
University “Ambitious Leader’s Program”). We thank Edanz
Group (www.edanzediting.com/ac) for editing a draft of this
manuscript.
[Gd(hfa)3(dpb)]n C45H27F18O8P2Gd: IR (ART): 1653 (st, C=O), 1252,
1138 (st, C-F), 1190-1106 (st, P=O) cm-1. Elemental analysis calcd (%)
for C45H27F18O8P2Gd: C 43.00, H 2.17; found: C 43.29, H 1.92.
[Gd(hfa)3(dpbp)]n C51H31F18O8P2Gd: IR (ART): 1652 (st, C=O), 1250,
1139 (st, C-F), 1180, 1123 (st, P=O) cm-1. Elemental analysis calcd (%)
for C51H31F18O8P2Gd: C 45.95, H 2.34; found: C 45.65, H 2.19.
[Gd(hfa)3ZnCl2(dppy)2]n C49H31Cl2F18N2O8P2GdZn: IR (ART): 1650 (st,
C=O), 1181-1132 (st, C-F), 1215-1105 (st, P=O) cm-1. Elemental analysis
calcd (%) for C49H31Cl2F18N2O8P2GdZn: C 39.95, H 2.12, N 1.90; found:
C 39.75, H 2.22, N 1.95.
Keywords: Thermosensitive paint・Lanthanide complex・Back
energy transfer・Luminescence
Crystallography: Colorless single crystals of Tb(hfa)3 complexes were
mounted on the MiTeGen micromesh using paraffin oil. All
measurements were made by using a Rigaku RAXIS RAPID imaging-
plate area detector with graphite-monochromated Mo-Kα radiation. Non-
hydrogen atoms were refined anisotropically. All calculations were
performed by using the crystal-structure crystallographic software
package. CIF data was confirmed by using the checkCIF/PLATON
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Chemistry - A European Journal
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10.1002/chem.201804392
Chemistry - A European Journal
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This study presents that the effect
(Ligand-assisted BEnT) on the back
energy transfer process between Tb
Masanori Yamamoto, Yuichi Kitagawa,
Takayuki Nakanishi, Koji Fushimi,
Yasuchika Hasegawa*
ions
(hexafluoroacetylacetone) ligands for
the Tb(hfa)3 complexes with
introducing various phosphine oxide
ligands (activation energy controllers:
AECs) .
and
hfa
Page No. – Page No.
Ligand-assisted back energy transfer
in luminescent Tb(III) complexes for
thermo-sensing properties
This article is protected by copyright. All rights reserved.