Materials Letters 130 (2014) 91–93

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Materials Lettersjournal homepage: www.elsevier.com/locate/matlet

Thermo-sensitive luminescent materials composed of Tb(III)and Eu(III) complexes
Yuichi Hirai, Takayuki Nakanishi, Kohei Miyata, Koji Fushimi, Yasuchika Hasegawa n
Faculty of Engineering, Hokkaido University, North 13 West 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan

art ic l e i nf oa b s t r a c t

Article history:
Received 10 April 2014
Accepted 12 May 2014
Available online 20 May 2014

Thermo-stable lanthanide coordination polymers [Tb,Eu(hfa)3(dpbp)]n composed of Tb(III) ion, Eu(III)ion, bidentate phosphine oxide (dpbp: 4,4’-bis(diphenylphosphoryl)biphenyl), and low-vibrationalfrequency hexaﬂuoroacetylacetonate (hfa) ligand are reported. The lanthanide coordination polymers[Tb,Eu(hfa)3(dpbp)]n (Tb/Eu ¼1, 10, 250, 500, and 750) are characterized by XRD, emission spectra andemission lifetime measurements. The energy transfer efﬁciencies among lanthanide ions are estimatedusing the emission lifetimes of solid-state coordination polymers. Characteristic emission propertiesof lanthanide coordination polymers composed of Tb(III) ions and a small amount of Eu(III) ions
(Tb/Eu ¼750) have been observed for the ﬁrst time. The energy transfer between lanthanide ions incoordination polymers is described in terms of solid-state materials.
& 2014 Elsevier B.V. All rights reserved.

Keywords:
Coordination polymer
Complex
Luminescence
Lanthanide

1. Introduction
Lanthanide complexes with characteristic narrow emission
bands and long emission lifetimes have been regarded as attractive luminescent materials for use in light-emitting diodes (LEDs),
lasers, and optical ﬁbers [1–3]. Various types of luminescentlanthanide complexes have been reported [3,4]. In particular,
red-luminescent Eu(III) complexes and green-luminescent Tb(III)complexes with characteristic organic ligands are expected to be
useful in application such as chemical and biological sensors [5,6].
These sensors composed of lanthanide complexes provide effective information for detection of proton, metal ions and biologicalcompounds in liquid media [7–9].
Luminescent physical sensors for measurements of temperatureand pressure on the material surface are also expected to be useful inapplications in the ﬁelds of ﬂuid dynamics and aeronautical engineering [10]. Luminescent physical sensors based on lanthanide complexes
have been investigated, recently [11,12]. In 2003, Amao andco-workers reported the ﬁrst temperature sensors based on luminescent Eu(III) complexes [11]. We have also reported lanthanide coordination polymers composed of Tb(III) and Eu(III) complexes, [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu¼ 99), for sensing temperature between 200 and
500 K [12]. The remarkable thermo-sensitive properties of [Tb,Eu(hfa)3(dpbp)]n are caused by the energy transfer between Tb(III) andEu(III) ions in coordination polymer matrix. At the present stage,
photophysical studies on the energy transfer from Tb(III) ions to Eu(III)nCorresponding author. Tel./fax: þ 81 11 706 7114.
E-mail address: hasegaway@eng.hokudai.ac.jp (Y. Hasegawa).

http://dx.doi.org/10.1016/j.matlet.2014.05.074
0167-577X/& 2014 Elsevier B.V. All rights reserved.

ions have been well-known using mono- and di-nuclear lanthanide
complexes in homogeneous organic and aqueous media [13,14]. We
consider that the energy transfer mechanisms of [Tb,Eu(hfa)3(dpbp)]nmight be different from those in previous reported homogeneous
systems in solution. Kinetic analysis of the photophysical properties insolid-state coordination polymer is directly linked with not only novel
aspects in the ﬁelds of solid-state photochemistry but also development of effective thermo-sensors for measurement on the materialsurface.
Here, we prepared [Tb,Eu(hfa)3(dpbp)]n with various Tb/Eu ratios(1, 10, 250, 500, and 750) for analysis of thermosensitive performanceand their energy transfer efﬁciency. The temperature-dependence ofthe energy transfer efﬁciency between Tb(III) ions and Eu(III) ions isreported for the ﬁrst time. Prepared [Tb,Eu(hfa)3(dpbp)]n was characterized by IR and XRD measurements. The energy transfer efﬁciencies between 100 and 350 K were estimated using an opticalcryostat for precise photophysical measurements. We found characteristic photophysical properties of lanthanide coordination polymers composed of Tb(III) ions and a small amount of Eu(III) ions (Ln(III) ratio: Tb/Eu¼750). In this study, characteristic energy transfer
efﬁciency between lanthanide ions in solid-state coordination polymers is described herein for development of thermo-sensitive
luminescent materials.

2. Experimental
Preparation of coordination polymers [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu¼1): All chemicals were of reagent grade and used as received.

92Y. Hirai et al. / Materials Letters 130 (2014) 91–93

Phosphine oxide ligand (4,4’-bis (diphenylphosphoryl) biphenyl:dpbp) was synthesized as reported [12]. Phosphane oxide ligand(dpbp, 0.44 g), Tb(hfa)3(H2O)2 (0.33 g) and Eu(hfa)3(H2O)2 (0.32 g)
were dissolved in methanol. The solution was reﬂuxed whilestirring for 2 h to give white precipitates. The precipitates wereﬁltered, washed with methanol and chloroform several times, anddried in vacuo.
To determine Eu:Tb ratios of [Tb,Eu(hfa)3(dpbp)]n, ICP-AES wasconducted (Shimadzu ICPE-9000). Concentrations were calculated
and averaged from the emission intensities of Eu and Tb(λ ¼381.917 and 350.917 nm, respectively) for each sample using
the appropriate calibration curves. These analyses revealed a ratioof Tb:Eu ¼0.498:0.502 (calcd 1:1). [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu ¼10, 250, 500, 750) were synthesized by the same method.
Tb:Eu ¼0.908: 9.23  10  2 (calcd 10:1), 0.996:3.93  10  2(calcd 250:1), 0.998:2.04  10  2 (calcd 500:1), 0.999: 1.30  10  2(calcd 750:1). IR (KBr): 3067 (st, arC–H), 2895 (st, C–H), 1650 (st,CQO), 1493 (arC–C), 1253–1142 (st, C–F), 1125 (st, PQO)
cm  1(JASCO FT/IR-420 spectrometer).
Optical measurements: Emission spectra were recorded on aJASCO F-6300-H spectrometer. Emission lifetimes of Tb(III) ions incoordination polymers were measured using the third harmonics(355 nm) of a Q-switched Nd:YAG laser (Spectra Physics, INDI-50,fwhm¼ 5 ns, l ¼1064 nm), ﬁlter transmitting wavelength shorter
than 550 nm, and photomultiplier (Hamamatsu photonics R5108,response time r 1.1 ns). Emission lifetimes and emission spectra in
the range 100–350 K were measured by a cryostat (Thermal BlockCompany, SA-SB245T) and a temperature controller (Oxford,Instruments, ITC 502S). The emission quantum yields of coordination polymers excited at 380 nm were estimated using JASCO

Energy Transfer

Green at 100 K

Yellow at 200 K

Red at 300 K

Intensity (a.u.)87654321500550
600650
Wavelength (nm)700

Fig. 1. (a) Chemical structures of energy transfer process between Tb(III) and Eu(III)ions in lanthanide coordination polymer [Tb,Eu(hfa)3(dpbp)]n. (b) Luminescence
images of [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu ¼10) in cryostat at 100, 200 and 300 K.
(c) Temperature-dependence of emission spectra of [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu¼ 10) in the solid state (1:100 K, 2:150 K, 3:200 K, 4:250 K, 5:300 K, 6:350 K,
7:400 K, 8:450 K), excited at 380 nm.

F-6300-H spectrometer attached to JASCO ILF-53 integrating
sphere unit (ϕ ¼100 nm).

3. Results and discussion
The structures of lanthanide coordination polymers wereanalyzed using powder XRD analyses. All signals are calibrated
by the signal of silicon powder at 28.41. Observed signals at 7.51,8.81, 9.41, 10.31, 20.11 and 21.51 are attributed to the geometricalstructures of [Tb,Eu(hfa)3(dpbp)]n (see Supporting information
Fig. S1). We found that the signals of [Tb,Eu(hfa)3(dpbp)]n agreewith those of previously reported [Eu(hfa)3(dpbp)]n [12]. Theseresults indicate that the geometrical structures of [Tb,Eu(hfa)3(dpbp)]n are the same as those of [Eu(hfa)3(dpbp)]n. According to our previous data using X-ray single crystal analysis of [Eu(hfa)3(dpbp)]n, distances between Eu(III) ions inter- and intrapolymer chains have been found to be 11.4 Å and 13.6 Å, respectively. The critical distance for dipole–dipole energy transfer
between lanthanide ions has been estimated to be approximately12 Å [15]. In the solid crystals of [Tb,Eu(hfa)3(dpbp)]n, positions oflanthanide ions are ﬁxed in the crystal packing structure. Theenergy transfer is generally dependent on the distance between
donor and acceptor. From the point of distance for energy transfer,
we propose that inter-polymer chains energy transfer might bedominated because of the shorter distance (11.4 Å). We consider
that distances among Ln(III) ions in the solid-state polymer matrix
lead to effective energy transfer among Ln(III) ions.
To estimate the energy transfer efﬁciency between Tb(III) ionsand Eu(III) ions in solid-state polymer matrix, we measured
temperature-dependent emission spectra of [Tb,Eu(hfa)3(dpbp)]n.
Temperature-dependent emission spectra of [Tb,Eu(hfa)3(dpbp)]n(Tb/Eu ¼10) in the solid state in the temperature range of 100–450 K are shown in Fig. 1(c). The characteristic emission bands at488 nm, 543 nm and 613 nm are attributed to the 4f–4f transitionsof Tb(III) (5D4–7F6, 5D4–7F5) and Eu(III) (5D0–7F2), respectively.
The emission quantum yield and lifetime at 300 K (excited atπ–πn transition band of hfa ligands) were 59.8% and 0.11 ms,respectively.
The emission intensities of Tb(III) ions in [Tb,Eu(hfa)3(dpbp)]ndecrease with increasing temperature. We also observed that theemission intensities of Eu(III) ions in [Tb,Eu(hfa)3(dpbp)]n increase
with rise of temperature. The ratio of the luminescent intensitiesof Tb(III) and Eu(III) ions (IEu/ITb) in [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu ¼10, 250, 500, and 750) is shown in Fig. 2(a). The luminescentintensity ratio (IEu/ITb) depends on the concentration of Tb(III) andEu(III) ions. We think that the emission spectral ratios in various
concentrations of [Tb,Eu(hfa)3(dpbp)]n are directly linked to theenergy transfer process between Tb(III) and Eu(III) ions.
The temperature-dependence of emission spectra in [Tb,Eu(hfa)3(dpbp)]n are attributed to the energy transfer between Tb(III) and Eu(III) ions. The energy transfer efﬁciency from the Tb(III)to Eu(III) ion (ηTb–Eu) is estimated by the following equation [16]:τð1Þ
nTb–Eu ¼ 1  obs

τTbin which τobs and τTb are the emission lifetimes of [Tb,Eu(hfa)3(dpbp)]n and [Tb(hfa)3(dpbp)]n, respectively. The lifetime
measurements were monitored at the 4f–4f transition bands ofTb(III) ion, 5D4–7F6 (545 nm) and 5D4–7F5 (488 nm).
The temperature dependence of the energy transfer efﬁciency
ηTb–Eu of [Tb,Eu(hfa)3(dpbp)]n are shown in Fig. 2(b). We observed
the energy transfer efﬁciencies between Tb(III) and Eu(III) ions at
100 K were equal to approximately zero. The photophysicalphenomena is similar to that in previous study on energy transfer
between Tb(III) and Eu(III) ions in solid state [17]. According to theY. Hirai et al. / Materials Letters 130 (2014) 91–93

2

photophysical properties using opto-magnetic measurements of[Tb,Ln(hfa)3(dpbp)]n (Ln¼Nd, Sm, Eu, Gd, Dy, and Yb). In the solidstate polymers, we also think that the π-orbital in ligands also
affects the thermo-sensing properties. Those studies will provide
novel aspects of the energy transfer mechanisms in solid-state
lanthanide coordination polymers.

1Acknowledgments

3Intensity ratio (IEu /ITb)

93

0
100

150200250
Temperature (K)300350This work was partly supported by Grants-in-Aid for ScientiﬁcResearch on Innovative Areas of “New Polymeric Materials Basedon Element-Blocks (No. 2401)” (2401) of Ministry of Education,Culture, Sports, Science and Technology (MEXT) of Japan.

Appendix A. Supporting information

100Tb-Eu(%)

Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.matlet.2014.05.074.

50References

0-50

100150200250Temperature (K)300350Fig. 2. (a) Temperature dependence of relative emission intensities (IEu/ITb, ▲: Tb/
Eu¼ 10, ●: Tb/Eu ¼ 250, ■: Tb/Eu ¼ 500 and ▼: Tb/Eu ¼ 750). (b) The temperaturedependence of energy transfer efﬁciencies of [Tb,Eu(hfa)3(dpbp)]n (ηTb–Eu, ♦: Tb/
Eu¼ 1, ▲: Tb/Eu ¼10, ●: Tb/Eu ¼250, ■: Tb/Eu ¼500 and ▼: Tb/Eu ¼750).

measurements at 300 K, the energy transfer efﬁciencies ηTb–Eu arefound to be 99%, 65%, 7.4% and 8.2% for Tb/Eu ¼1, 10, 250 and 500,respectively. These results indicate that the energy transfer efﬁciency ηTb–Eu depends on the concentration of the energy transfer
acceptor, Eu(III) ions. We also found that the energy transfer
efﬁciency ηTb–Eu of [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu ¼750) showednegative value (  33%), which indicates that emission lifetime ofTb(III) ions in [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu ¼750) is larger than thatof [Tb(hfa)3(dpbp)]n (see Supporting information Table S1). Theenergy transfer mechanisms in solid-state of these polymers
might be different from general Förster mechanism in liquidmedia.
4. Conclusion
We synthesized lanthanide coordination polymers with various
ionic ratios [Tb,Eu(hfa)3(dpbp)]n (Tb/Eu ¼ 1, 10, 250, 500, and 750)
and measured the temperature-dependent emission spectra andlifetimes. We found that the thermo-sensitive performance andenergy transfer efﬁciency from Tb(III) ions to Eu(III) ions aredependent on Tb/Eu ratios in lanthanide coordination polymers.
In particular, characteristic energy transfer properties of lanthanide coordination polymers composed of Tb(III) ions and a small
amount of Eu(III) ions (Tb/Eu ¼750) have been observed forthe ﬁrst time. We are now trying to analyze the unusual[1] Koen. B. Lanthanide-based luminescent hybrid materials. Chem Rev
2009;109:4283–374.
[2] Ken K, Yasuhiro K, Yoshi O. Plastic optical ﬁbers and ampliﬁers containing
lanthanide complexes. Chem Rev 2002;102:2347–56.
[3] Sebastiaan A, Jos JMH, van den E, Ilpo M, Maxime AS, Wen TF, et al.
Substituted phenanthrolines as antennae in luminescent EuIII complexes. Eur
J Inorg Chem 2013;36:6137–46.
[4] Silvanose B, Gopakumar N, Jean CGB, Scopelliti R, Kim HK, Reddy MLP. Brilliant
photoluminescence and triboluminescence from ternary complexes of DyIIIand TbIII with 3-phenyl-4-propanoyl-5-isoxazolonate and a bidentate phosphine oxide coligand. Inorg Chem 2013;52:8750–8.
[5] Matthew ST, Marlin H, Dalibor S. Cooktails of Tb3 þ and Eu3 þ complexes: ageneral platform for the design of ratiometric optical probes. J Am Chem Soc
2007;129:7570–7.
[6] Daniel G, Stefan S, Hans GL, Niko H. Six-color time-resolved förster resonanceenergy transfer for ultrasensitive multiplexed biosensing. J Am Chem Soc
2013;135:1102–9.
[7] Thorﬁnnur G, Joseph PL, Katell S, Anrew JH. pH responsive Eu(III)-phenanthroline supramolecular conjugate: novel “Off–On–Off” luminescent signaling in
the physiological pH range. J Am Chem Soc 2003;125:12062–3.
[8] Song D, En M, Zhong MS, Hongjie Z. A layer-structured Eu-MOF as a highly
selective ﬂuorescent probe for Fe3 þ detection through a cation-exchange
approach. J Mater Chem 2012;22:16920–6.
[9] Jean CGB. Lanthanide luminescence for biomedical analyses and imaging.
Chem Rev 2010;110:2729–55.
[10] Michael S. The art of ﬂuorescence imaging with chemical sensors. Angew
Chem Int Ed 2012;51:3532–54.
[11] Masaya M, Shinji K, Tokuji M, Yutaka A. Characterization of an ultrathin
polymer optode and its application to temperature sensors based on luminescent europium complexes. J Mater Chem 2003;13:2875–9.
[12] Kohei M, Yuji K, Takayuki N, Atsushi. K, Masako K, Koji F, et al. Chameleon
luminophore for sensing temperatures: control of metal-to-metal and energy
back transfer in lanthanide coordination polymers. Angew Chem Int Ed
2013;52:6413–6.
[13] Jean FL, Laure G, César B, Tomasz AW, Prasun M, David HW, et al. Optimizing
sensitization process in dinuclear luminescent lanthanide oligomers: selectionof rigid aromatic spacers. J Am Chem Soc 2011;133:16219–34.
[14] Alexander MF, Pascal HF, Peter H, Alan MK, David P. Experimental measurement
and theoretical assessment of fast lanthanide electronic relaxation in solution
with four series of isostructural complexes. J Phys Chem A 2013;117:905–17.
[15] Yasuhika H, Kei M, Yuji W, Jeong HK, Nobuaki N, Tatsuhiko Y, et al.
Characteristic emission of β-diketonato Nd(III) complexes dressed with perﬂuoroalkyl groups in DMSO-d6. Chem Phys Lett 1996;260:173–7.
[16] Claude P, Jean CGB, Gérald B, Gérald H, Alan FW. Self-assembly and photophysical properties of lanthanide dinuclear triplet-helical complexes. J Am
Chem Soc 1993;115:8197–206.
[17] Carlos DSB, Patricia PL, JOS Nuno, Angel M, Victor SA, Fernando P, et al. A
luminescent molecular thermometer for long-term absolute temperature
measurements at the nanoscale. Adv Mater 2010;22:4499–504.