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ARTICLE
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A water-soluble Tb complex as a temperature-sensitive
luminescent probe
Jorge H.S.K. Monteiro, Fernando A. Sigoli, and Ana de Bettencourt-Dias
Abstract: The water soluble [Tb(dipicCbz)3]3− (dipicCbz = 4-(9H-carbazol-9-yl-)pyridine-2,6-dicarboxylato) complex was isolated
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and evaluated as a temperature sensor in water. The 1:3 (Tb :dipicCbz ) stoichiometry in solution was confirmed by lumines-
cence titration and high-resolution mass spectrometry. The quantum yield of sensitized emission is 3.8% ± 0.4% at 25.0 ± 0.1 °C,
and the emission intensity depends on the temperature in the range of 5–70 °C with a relative thermal sensitivity of 3.4% °C−1 at
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5 °C and temperature resolution < 0.01 °C in the range of 30–40 °C. The reversibility of this behavior was demonstrated for three
heating–cooling cycles. Calculations of the energy gap between donor and acceptor show that the temperature dependence of
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the emission intensity is due to back-energy transfer from the Tb D excited state to the triplet and twisted intramolecular
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charge transfer (TICT) states of the dipicCbz. The assignment of one of the energy levels as a TICT state was confirmed by the
temperature-dependent behavior of the phosphorescence band.
Key words: lanthanides, terbium, temperature probe, luminescence.
Résumé : Nous avons isolé le complexe hydrosoluble [Tb(dipicCbz)3]3− (dipicCbz = 4-(9H-carbazol-9-yl)pyridine-2,6-dicarboxylato)
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et nous l’avons évalué en tant que capteur de température dans l’eau. La stœchiométrie de 1:3 (Tb :dipicCbz ) a été confirmée
en solution par titrage de luminescence et par spectroscopie de masse a` haute résolution. Le rendement quantique de l’émission
sensibilisée est de 3,8 ± 0,4 % a` 25,0 ± 0,1 °C, et l’intensité de l’émission dépend de la température dans l’intervalle de 5 a` 70 °C,
avec une sensibilité thermique relative de 3,4 % °C−1 a` 35 °C et une résolution de température < 0,01 °C dans l’intervalle de 30 a`
40 °C. Nous avons démontré le caractère réversible de ce comportement sur trois cycles de chauffage-refroidissement. Les calculs
de la bande d’énergie interdite entre le donneur et l’accepteur montrent que la relation de dépendance entre l’intensité de
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l’émission et la température est attribuable au transfert d’énergie réversible de l’état excité D du Tb a` l’état triplet et a` l’état de
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transfert de charge intramoléculaire déformé dit « TICT » (twisted intramolecular charge transfer) du ligand dipicCbz.
L’attribution de l’un des niveaux d’énergie a` un état TICT a été confirmée par le fait que la bande de phosphorescence varie en
fonction de la température. [Traduit par la Rédaction]
Mots-clés : lanthanides, terbium, sonde de température, luminescence.
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Introduction
time response, and precision at the molecular level. Organic dyes
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have been extensively used for this purpose; yet, they have
issues associated with photobleaching, short emission lifetimes,
small Stokes shifts, and in some cases, change in the energy of
the emission band as a function of the pH or solvent system.
Lanthanide (Ln ) ions, with their narrow emission bands, long
emission lifetimes, and energy of the emission band largely inde-
pendent of the local environment, are highly attractive for lumi-
However, the emission is based on parity
forbidden 4f–4f transitions and thus direct excitation of the Ln is
an inefficient process. To circumvent this limitation, Ln com-
plexes are used, in which organic chromophores bound to the
In these systems, the ligand ab-
sorbs energy and then transfers it to the metal ion, which then
Temperature is an important thermodynamic and kinetic vari-
able in reactions and biochemical processes. In living systems,
abnormalities in the temperature are indicative of diseases. For
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example, cancer cells have a higher temperature due to high me-
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tabolism.2,3 Therefore, in situ measurement of cell temperature is
important, and compounds that are able to sense temperature at
the cellular level, often referred to as nano-thermometers, can be
used to probe the local temperature in cells and thus can be used
to diagnose diseased tissue.
Temperature measurements can be achieved by electrical, me-
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nescence sensing.
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chanical, and optical methods. Electrical measurements are
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based on changes in the resistance, voltage, conductivity, or elec-
trical capacity of the probe, whereas mechanical measurements
are based on the deflection of a cantilever composed by two dif-
Ln are used as sensitizers.
emits light. The process is commonly referred to as the antenna
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ferent materials. The electrical and mechanical measurements
effect.
This leads to desirable large Stokes shifts of sensitized
are very sensitive but can be only used to obtain thermal images
of surfaces and do not offer spatial resolution. Luminescence-
based measurements are advantageous due to noncontact, real-
emission.
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Due to the above-mentioned characteristics, development of
nano-thermometers based on luminescent Ln compounds is
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Received 14 July 2017. Accepted 18 September 2017.
J.H.S.K. Monteiro. Institute of Chemistry, University of Campinas, Campinas, São Paulo, Brazil; Department of Chemistry, University of Nevada, Reno,
NV 89557, USA.
F.A. Sigoli. Institute of Chemistry, University of Campinas, Campinas, São Paulo, Brazil.
This paper is part of a Special Issue entitled “2016 Gordon Research Conference on Electron Donor Acceptor Interactions (GRC–EDAI)”.
Can. J. Chem. 00: 1–6 (0000) dx.doi.org/10.1139/cjc-2017-0436
Published at www.nrcresearchpress.com/cjc on 14 November 2017.