Temperature dependent NIR emitting lanthanide-PMO/silica hybrid materials

Article abstract of DOI:10.1039/c7dt01620d

Two materials-A mesoporous silica (MS) and a periodic mesoporous organosilica (PMO) functionalized with dipyridyl-pyridazine (dppz) units were grafted with near-infrared (NIR) emitting lanthanide (Nd³⁺, Er³⁺, Yb³⁺) complexes in an attempt to obtain hybrid NIR emitting materials. The parent materials: dppz-vSilica and dppz-ePMO were prepared by a hetero Diels-Alder reaction between 3,6-di(2-pyridyl)-1,2,4,5-Tetrazine (dptz) and the double bonds of either ethenylene-bridged PMO (ePMO) or vinyl-silica (vSilica) and subsequent oxidation. The dppz-vSilica is reported here for the first time. The prepared lanthanide-PMO/silica hybrid materials were studied in depth for their luminescence properties at room temperature and chosen Nd³⁺ and Yb³⁺ samples also at low temperature (as low as 10 K). We show that both the dppz-vSilica and dppz-ePMO materials can be used as "platforms" for obtaining porous materials showing NIR luminescence. To obtain NIR emission these materials can be excited either in the UV or Vis region (into the π → π? transitions of the ligands or directly into the f-f transitions of the Ln³⁺ ions). More interestingly, when functionalized with Nd³⁺ or Yb³⁺ β-diketonate complexes these materials showed interesting luminescence properties over a wide temperature range (10-360 K). The Yb³⁺ materials were investigated for their potential use as ratiometric temperature sensors.

Full text of DOI:10.1039/c7dt01620d

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Temperature dependent NIR emitting lanthanide-
PMO/silica hybrid materials†
Cite this: DOI: 10.1039/c7dt01620d
Anna M. Kaczmarek, *‡a Dolores Esquivel, ‡b Judith Ouwehand,c
a
Pascal Van Der Voort, c Francisco J. Romero-Salguero b and Rik Van Deun
*
Two materials – a mesoporous silica (MS) and a periodic mesoporous organosilica (PMO) functionalized
with dipyridyl-pyridazine (dppz) units were grafted with near-infrared (NIR) emitting lanthanide (Nd3+
,
Er3+, Yb3+) complexes in an attempt to obtain hybrid NIR emitting materials. The parent materials: dppz-
vSilica and dppz-ePMO were prepared by a hetero Diels–Alder reaction between 3,6-di(2-pyridyl)-
1,2,4,5-tetrazine (dptz) and the double bonds of either ethenylene-bridged PMO (ePMO) or vinyl-silica
(vSilica) and subsequent oxidation. The dppz-vSilica is reported here for the first time. The prepared
lanthanide-PMO/silica hybrid materials were studied in depth for their luminescence properties at room
temperature and chosen Nd3+ and Yb3+ samples also at low temperature (as low as 10 K). We show that
both the dppz-vSilica and dppz-ePMO materials can be used as “platforms” for obtaining porous materials
showing NIR luminescence. To obtain NIR emission these materials can be excited either in the UV or Vis
region (into the π → π* transitions of the ligands or directly into the f–f transitions of the Ln3+ ions). More
interestingly, when functionalized with Nd3+ or Yb3+ β-diketonate complexes these materials showed
interesting luminescence properties over a wide temperature range (10–360 K). The Yb3+ materials were
investigated for their potential use as ratiometric temperature sensors.
Received 3rd May 2017,
Accepted 5th June 2017
DOI: 10.1039/c7dt01620d
rsc.li/dalton
lead to very efficient luminescent materials additionally
showing high processability. Among them organic functiona-
1. Introduction
The remarkable luminescence properties of lanthanide ions lized mesoporous silica (MS) and periodic mesoporous organo-
(Ln3+) are largely due to the well-shielded nature of the f–f silica (PMO) materials have attracted particular attention as
transitions.1–3 Ln3+ ions show characteristic narrow line like hybrid hosts for lanthanide complexes and very recently
emission peaks ranging from the ultraviolet-visible (UV-Vis) to lanthanide polyoxometalates (POMs) for the very first time.5–10
near-infrared (NIR) region, as well as exhibit relatively long Well-ordered nanoporous structures, large surface areas and
luminescence lifetimes. Yet, because the f–f transitions of possibility of incorporating functional organic groups make
lanthanides are parity forbidden, their absorption coefficients both organic–inorganic hybrid materials promising supports
are very low and as a result of this they exhibit low emission for binding Ln3+ compounds.
intensities. In order to overcome this problem various in-
Mesoporous silicas (MS) such as MCM-41 and SBA-15 have
organic materials and hybrid organic–inorganic materials, been functionalized with chelating organic ligands, including
which allow exciting the Ln3+ ions through charge-transfer modified β-diketonates, pyridyl groups, 1,10-phenathroline
bands or organic ligands, have been designed.4 This has led and calix[4]arene derivatives.11–17 Other suitable sensitizer
scientists to explore various Ln3+ hybrid systems, which could ligands as bipyridine, acridone, mercaptobenzoic acid or
2-phenoyltrifluoroacetonate have been successfully incorpor-
ated as bridging organic compounds into PMO
aL3 Luminescent Lanthanide Lab, Department of Inorganic and Physical Chemistry,
framework.18–21 Recently, we have reported the incorporation
Ghent University, Krijsglaan 281-S3, B-9000 Ghent, Belgium.
of dipyridyl-dihydropyridazine moieties on the surface through
E-mail: anna.kaczmarek@ugent.be, rik.vandeun@ugent.be
bDepartamento de Química Orgánica, Instituto Universitario de Investigación en
Química Fina y Nanoquímica IUIQFN, Facultad de Ciencias, Universidad de
Córdoba, Campus de Rabanales, Edificio Marie Curie, E-14071 Córdoba, Spain
cCOMOC, Department of Inorganic and Physical Chemistry, Ghent University,
Krijsglaan 281-S3, B-9000 Ghent, Belgium
hetero Diels–Alder reaction between the double bonds on an
ethenylene-bridged PMO and a tetrazine derivative.5 Tetrazine-
derived ligands can be employed as sensitizers for Ln3+
ions and due to their low triplet level they can be well matched
with the accepting levels of NIR emitting lanthanides such as
Nd3+, Er3+, Yb3+. For example the 3,6-bis(2-pyridyl)tetrazine
ligand together with thenoyltrifluoroacetone have been used
†Electronic supplementary information (ESI) available: N2 sorption isotherms,
additional luminescence data. See DOI: 10.1039/c7dt01620d
‡These authors contributed equally to the work.
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for obtaining NIR emitting complexes with Nd3+, Er3+, and was suspended in 6 mL of acetic acid and stirred on a
Yb3+.22
magnetic stirrer. 174 mg (2.52 mmol) of sodium nitrite was
A precise measurement of temperature can be crucial in added in small portions to the solution and an immediate
various fields of application, such as cryogenics, safety sector, color change from orange to dark red was observed. After the
biology, medicine, engineering or chemical reaction monitor- reaction was completed the solution was slowly poured into a
ing, to give a few examples.23–25 The traditional, so-called beaker filled with ice. At this point the color changed from red
contact thermometers, are not suitable for temperature to dark purple-red. The product was left to crystallize for
measurements of fast-moving objects or at the submicron several minutes. It was next isolated by filtration and the
scale (such as in microfluids, microcircuits or in cells). Due to residue was washed with water (20 mL) and dried in vacuo to
the constraints of conventional thermometers many new yield 3,6-bis(2-pyridyl)tetrazine as a purple solid.
thermometers based on various optical methods are being
The compound was characterized by elemental analysis
developed.26 Among them much attention is being given to (calculated: 61.07% C, 3.41% H, 35.58% N; found: 60.43% C,
luminescence based thermometry and various materials such 3.22% H, 35.88% N).
as quantum dots, lanthanide doped inorganic phosphors,
Synthesis of dipyridyl-pyridazine functionalized ethenylene-
lanthanide doped semiconductors, lanthanide MOFs or fluo- bridged PMO (dppz-ePMO), see Scheme 1. The surface Diels–
rescent polymers have shown promising results, to give a few Alder reaction of an ethenylene-bridged PMO (ePMO) with
examples.27–29 There are various ways of assessing the temp- 3,6-di(2-pyridyl)-1,2,4,5-tetrazine as diene was carried out
erature or temperature change of a material by employing following a method already reported by Esquivel et al. for the
luminescence features. Most often the steady state intensity of formation of surface Diels–Alder adducts on periodic meso-
one (or two) transition(s), or the luminescence lifetime is porous organosilicas.5 In a typical procedure 0.3 g of 100%
used.30 As Ln3+ emission in the NIR region is in general much trans ethenylene-bridged PMO (ePMO),32 previously heated
weaker then in the Vis region, NIR emitting thermometers are under vacuum at 120 °C overnight, was added to a mixture of
substantially less frequently reported.
0.25 g tetrazine derivative dissolved in 20 ml of dodecane. The
In this work we report the preparation of a mesoporous mixture was aged in an autoclave at 200 °C for 11 days. After
silica and a periodic mesoporous organosilica material, which cooling down the solid was filtered and washed several times
are both functionalized with dipyridyl-pyridazine (dppz) units with chloroform. Additionally, in order to remove possible
by a hetero Diels–Alder reaction between the substituted tetra- diene physically adsorbed into the pores, the material was
zine (3,6-di(2-pyridyl)-1,2,4,5-tetrazine) and the double bonds refluxed in chloroform for 8 h (this procedure was repeated
of either vinyl-silica or ethenylene-bridged PMO. We have pre- two times). The solid product was collected on a filter, washed
viously reported a similar material – a dpdp-ePMO@Eu (dpdp with chloroform and acetone and dried at 120 °C under
= dipyridyl-dihydropyridazine) and studied its luminescence vacuum. Last, the product was aromatized using NaNO2 follow-
properties in the Vis region.5 Here we prepare a modified ing procedures previously reported.33 The procedure was
dppz-ePMO and novel dppz-vSilica material. These two repeated two times, and the material was washed excessively
materials are further grafted with Nd3+, Er3+, and Yb3+ and dried at 120 °C under vacuum afterwards. The nitrogen
β-diketonate complexes in an attempt to obtain hybrid NIR content on dppz-ePMO material was 1.0% (based on CHN
emitting materials. Additionally, we study the temperature results).
dependent luminescence properties of dppz-ePMO and dppz-
Synthesis of dipyridyl-pyridazine functionalized vinyl silica
vSilica materials functionalized with β-diketonate complexes of (dppz-vSilica), see Scheme 1. The vSilica material was syn-
Nd3+ and Yb3+
.
thesized according to the following procedure. 7.00 g of P123
was dissolved in 210 mL 2 M HCl and left to stir for
45 minutes at room temperature (until completely dissolved).
Next, 13.1 mL of TEOS was added and the solution was stirred
45 minutes at 40 °C. Then 3.1 mL of vinyltriethoxysilane
(VTES) was added and stirred for 20 h. A white precipitate was
formed and left to age at 100 °C for 24 h. After filtering and
washing with water and ethanol the surfactant was extracted
three times from a mixture of ethanol and 1 M HCl in appro-
2. Experimental section
All reactants and solvents were used as received without
further purification.
2.1. Synthesis
Synthesis of 3,6-di(2-pyridyl)-1,2,4,5-tetrazine. The tetrazine priate amounts. The product was dried under vacuum at
derivative was synthesized according to the following pro- 120 °C overnight. The dppz-vSilica material was obtained in a
cedure, which is a modified synthesis based on that previously similar approach as that described for the preparation of dppz-
reported in literature.31 10.0 g (96.0 mmol) 2-cyanopyridine ePMO, yet in milder conditions. To obtain the desired product
and 15.1 g (300 mmol) hydrazine hydrate were stirred for 16 h a flask reaction (under nitrogen) was carried out (set at 120 °C
at 90 °C in an inert atmosphere. The mixture was then cooled for 7 days). The nitrogen content on dppz-vSilica material was
to room temperature and filtered and washed with water and 0.65% (based on CHN results).
then ethanol and dried under vacuum. Orange dihydrotetra-
Synthesis of dppz-ePMO material grafted with LnCl3 (dppz-
zine was obtained. 200 mg (0.828 mmol) of dihydrotetrazine ePMO@LnCl3). For the preparation of the dppz-ePMO@LnCl3
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Scheme 1 Synthesis of dppz-ePMO/vSilica@LnL3 materials.
materials, a certain amount of LnCl3·6H2O (Ln = Nd3+, Er3+,
Synthesis of dppz-ePMO material grafted with Ln(tta)3/Ln
Yb3+) was dissolved in 20 mL methanol and then dppz-ePMO (bta)3 complexes (dppz-ePMO@Ln(tta)3 and dppz-ePMO@Ln
was added to this solution. The molar ratio of LnCl3 to dppz- (bta)3), see Scheme 1. A similar procedure was used as for the
ePMO was 4 : 1. The mixture was stirred for 10 minutes at synthesis of dppz-ePMO@LnCl3 except that Ln(tta)3or Ln(bta)3
room temperature. Next it was sealed in glass tubes and placed complexes were used. The molar ratio of Ln3+ complexes to
on a heating block set at 120 °C for 24 h. After cooling to room dppz-ePMO was maintained at 4 : 1.
temperature the product was filtered and washed several times
with methanol (in order to remove any physisorbed lanthanide (bta)3 complexes (dppz-vSilica@Ln(tta)3 and dppz-vSilica@Ln
salt). The product was dried in a vacuum oven at 50 °C. (bta)3), see Scheme 1. A similar procedure was used as for the
Synthesis of dppz-vSilica material grafted with Ln(tta)3/Ln
Synthesis of dppz-vSilica material grafted with LnCl3 (dppz- synthesis of dppz-vSilica@LnCl3 except that Ln(tta)3or Ln(bta)3
vSilica@LnCl3). For the preparation of the dppz-vSilica@LnCl3 complexes were used. The molar ratio of Ln3+ complexes to
materials, a certain amount of LnCl3·6H2O (Ln = Nd3+, Er3+, dppz-vSilica was maintained at 4 : 1.
Yb3+) was dissolved in 20 mL methanol and then dppz-vSilica
2.2. Characterization
was added to this solution. The molar ratio of LnCl3 to dppz-
ePMO was 4 : 1. The mixture was left to stir for 3 h on a mag- XRD patterns were recorded by a Bruker D8 DISCOVER A25
netic stirrer with heating at 50 °C. The dppz-vSilica@LnCl3 diffractometer or Thermo Scientific ARL X’TRA diffractometer
materials were prepared at lower temperatures and the reac- equipped with a Cu Kα (λ = 1.5405 Å) source, a goniometer and
tions were carried out for a shorter time compared to the a Peltier cooled Si (Li) solid state detector.
dppz-ePMO@LnCl3 materials, due to the lower thermal stabi-
Nitrogen adsorption–desorption isotherms were measured
lity of MS materials. After cooling to room temperature the by using a Micromeritics TriStar 3000 analyzer at −196 °C. The
product was filtered and washed several times with methanol. samples were vacuum dried for 24 h at 120 °C before the
The product was dried in a vacuum oven at 50 °C.
measurements. Surface areas were calculated using the
Synthesis of Ln(tta)3·2H2O and Ln(bta)3·2H2O complexes Brunauer–Emmett–Teller (BET) method. Pore size distri-
(Ln = Nd3+, Er3+, Yb3+). The complexes were prepared accord- butions were obtained by analysis of the desorption branch of
ing to previously reported procedures.34,35 The complexes were the isotherms using the Barrett–Joyner–Halenda (BJH)
characterized by elemental analysis.
method.
The 13C MAS NMR spectra were recorded at 100.61 MHz on
Nd(tta)3·2H2O (NdC24H16F9O8S3) calculated: 34.16% C;
1.91% H; found: 35.03% C; 2.15% H. Er(tta)3·2H2O a Bruker AVANCE-400 WB dual channel NMR spectrometer at
(ErC24H16F9O8S3) calculated: 33.25% C; 1.86% H; found: room temperature. An overall of 1000 free induction decays
34.10%% C; 1.92% H.
were accumulated. The excitation pulse and recycle time were
Yb(tta)3·2H2O (YbC24H16F9O8S3) calculated: 33.03% C; 6 ms and 2 s. Chemical shifts were measured relative to tetra-
1.85% H; found: 33.87% C; 2.08% H. Nd(bta)3·2H2O methylsilane standard. All samples were dried at 120 °C for
(NdC30H22O8F9) calculated: 43.64% C; 2.69% H; found: 24 h prior to analysis.
43.89% C; 2.85% H.
Elemental analysis (CHNS) was performed on a Thermo
Er(bta)3·2H2O (ErC30H22O8F9) calculated: 42.45% C; 2.61% Flash 2000 elemental analyzer by using V2O5 as catalyst.
H; found: 43.06% C; 2.80% H. Yb(bta)3·2H2O (YbC30H22O8F9)
calculated: 42.17% C; 2.59% H; found: 42.51% C; 2.88% H.
The luminescence of solid samples was studied. Solid pow-
dered samples were put between quartz plates (Starna cuvettes
for powdered samples, type 20/C/Q/0.2).
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Luminescence measurements were performed on an
Edinburgh Instruments FLSP920 UV-vis-NIR spectrometer
setup using a 450 W xenon lamp as the steady state excitation
source and a Hamamatsu R5509-72 photomultiplier operating
at −80 °C. The time-resolved measurements were performed
using a Continuum Surelite I laser (450 mJ @1064 nm), operat-
ing at a repetition rate of 10 Hz and using the third harmonic
(355 nm) as the excitation source, and the photomultiplier
detector mentioned above. The temperature dependent
measurements were measured using an ARS closed cycle cryo-
stat at a temperature range between 10 K–360 K (with a temp-
erature step of 60 K).
The luminescence decay curves of the samples were
measured when excited into the maximum of the ligand band
and monitored at the appropriate wavelength. The decay
curves could be well fitted using
function.
a double-exponential
Fig. 2 Powder XRD patterns of vSilica, dppz-vSilica, and dppz-
vSilica@Nd(tta)3.
3. Results and discussion
second-order reflection peaks (110) and (200) are observed.
This is typical for materials with a hexagonal arrangement of
uniform pores. The X-ray diffraction patterns of the dppz-
ePMO and dppz-vSilica are almost unchanged when compared
to the parent materials (ePMO and vSilica, respectively). This
indicates that the ordered hexagonal mesostructure (P6mm) is
retained after the hetero Diels–Alder reactions.
The Ln3+-dppz-ePMO/vSilica hybrid materials (Ln = Nd3+, Er3+,
Yb3+) were obtained in two steps (Scheme 1). First, the success-
ful formation of dppz-ePMO/vSilica through Diels–Alder cyclo-
addition between the double bonds of the ePMO/vSilica and
3,6-di(2-pyridyl)tetrazine was carried out (step a in Scheme 1).
Next, these materials (dppz-ePMO/vSilica) were further functio-
nalized with Ln3+ β-diketonate complexes giving rise to NIR
emitting hybrid porous materials (step b in Scheme 1).
The N2 adsorption–desorption isotherms of ePMO, dppz-
ePMO, vSilica and dppz-vSilica are shown in Fig. S1.† Both
parent materials (ePMO and vSilica) exhibit type IV isotherms
with H1-type hysteresis loops at relative pressures in the range
0.5–0.8, characteristic of mesoporous materials with uniform
mesopores. The BET surface areas, pore volumes and pore dia-
meters are 722 m2 g−1, 0.78 cm3 g−1 and 5.8 nm, for ePMO
and 805 m2 g−1, 0.92 cm3 g−1 and 5.4 nm for vSilica, respect-
ively. After functionalization, dppz-ePMO and dppz-vSilica
showed similar nitrogen adsorption–desorption isotherms,
which confirmed their mesoporous structure. A pronounced
decrease of the surface area, pore volume and pore size was
observed after the formation of surface Diels–Alder adducts.
These results confirm the phenomenon of surface decoration
into the pores of ePMO and vSilica with Diels–Alder adducts,
previously reported in this type of reaction with other dienes.36
The solid-state 13C CP/MAS NMR spectrum of the vSilica
material showed two intense signals at 138 and 130 ppm
corresponding to the sp2 carbons of the vinyl groups grafted
on the silica surface (Fig. 3). Additional signals of residual sur-
factant (76 and 71 ppm) and non-hydrolyzed ethoxy groups
(58 and 18 ppm) were present. After the Diels–Alder reaction
with the tetrazine derivative, dppz-vSilica showed new bands
in the aromatic region (160–120 ppm) confirming the success-
ful formation of the surface Diels–Alder adduct. Additionally,
signals at 6 and 19 ppm were assigned to Si–CH2–CH3 func-
tionalities formed under the reaction conditions.37 The 13C
CP/MAS NMR spectrum of the dppz-ePMO material corrobo-
rated the results obtained in our previous work.5
3.1. Dipyridyl-pyridazine functionalized parent materials
(dppz-ePMO and dppz-vSilica)
To characterize the materials, powder X-ray diffraction analysis
was performed on ePMO, dppz-ePMO, vSilica and dppz-vSilica
as shown in Fig. 1 and 2. For both the dppz-ePMO and dppz-
vSilica a strong reflection (100) at low 2θ as well as two small
Fig. 1 Powder XRD patterns of ePMO, dppz-ePMO, and dppz-
ePMO@Nd(tta)3.
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thesized for the first time in this work) functionalized with
β-diketonate complexes of Nd3+, Er3+ and Yb3+ and investigated
their NIR luminescence properties in detail. Two β-diketonate
ligands were chosen for the study – 2-thenoyltrifluoroacetone
(tta; triplet level: 20 400 cm−1) and benzoyltrifluoroacetone
(bta; 21 600 cm−1). These β-diketonate ligands have a double
function in these materials. First of all, they shield the first
coordination sphere of the Ln3+ ion from H2O molecules and
quenching groups and second they provide an additional
ligand through which they can be excited. Ligands with C–F
groups instead of C–H groups have been chosen to prevent the
quenching of the NIR emission. The combined excitation–
emission spectra of all these complexes have been presented
in the ESI (Fig. S2–S4†). The importance of NIR Ln3+ and their
use for silica-based materials has previously been extensively
visualized by numerous publications.38–42 These kind of
materials can find applications in bio-imaging, bio-sensing, in
laser systems, fiber-optic communications, thermometers, etc.
Fig. 4 presents the combined excitation–emission spectra of
the Nd3+ β-diketonate functionalized dppz-ePMO and dppz-
vSilica compounds. For the sake of comparison the dppz-
ePMO and dppz-vSilica materials were also functionalized with
the NdCl3 salt and the luminescence spectra of these materials
have been presented in the ESI (Fig. S5 and S6†). It can clearly
be seen that the dppz-ePMO@NdL3 materials differ from the
dppz-ePMO@NdCl3 and dppz-vSilica@NdCl3 materials. In the
excitation spectra of the dppz-ePMO@NdL3 and dppz-
Fig. 3 13C CP/MAS NMR of vSilica and dppz-vSilica (*, ethanol
residues).
vSilica@NdL3 materials
a
broad band in the range
250–500 nm is present, attributed to the π → π* transitions of
the ligands. The maximum of this broad band varies a bit for
each of the samples and seems to be partially dependent of
the employed β-diketonate ligand. In the materials where
3.2. Structural characterization of Ln3+-dppz-ePMO/vSilica
hybrid materials (Ln = Nd3+, Er3+, Yb3+)
After binding of Ln3+ to dppz-ePMO and dppz-vSilica, all
resulting materials remained intact the ordered hexagonal
mesostructure. The powder XRD patterns of dppz-ePMO and
dppz-vSilica bonded to Nd(tta)3 are shown in Fig. 1 and 2,
respectively. Likewise, the surface properties are maintained
after grafting with lanthanide complexes.5
3.3. Luminescence properties of Ln3+ functionalized dppz-
ePMO/vSilica hybrid materials (Ln = Nd3+, Er3+, Yb3+)
In our previous work we have studied the luminescence pro-
perties of ePMO functionalized with dipyridyl-dihydropyrida-
zine and further with Eu3+ ions (in the form of EuCl3 or
Eu(tta)3 complex). We observed that both in the case of the
EuCl3 and Eu(tta)3 functionalized materials energy is not
completely transferred from the dipyridyl-dihydropyridazine
(dpdp) ligand to the Eu3+ ions. This suggests that the triplet
level of the ligand is located only slightly higher than the
accepting level of Eu3+ (18 674 cm−1) and that it could be a
ligand very well suited for the design of NIR emitting Nd3+
Er3+ and Yb3+ materials.
,
The dpdp (previously employed for the preparation of our
Eu@PMO materials) and dppz ligands are very similar and we
do not expect many differences in their energy harvesting
ability or triplet level.5 Here, we have prepared dppz-ePMO and
Fig. 4 Combined excitation–emission luminescence spectra of (a)
dppz-ePMO@Nd(tta)3, (b) dppz-ePMO@Nd(bta)3, (c) dppz-vSilica@Nd
(tta)3, and (d) dppz-vSilica@Nd(bta)3. All of the emission spectra where
recorded when excited into the maximum of the broad bands and
dppz-vSilica materials (the dppz-vSilica material is new, syn- observed at the 4F3/2
→
4I11/2 transition peaks.
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Nd(tta)3 complexes were employed for the functionalization the
maximum is around 350.0 nm. When the Nd(bta)3 complexes
were used instead the maximum is shifted slightly towards
lower wavelengths – 327.0 nm for dppz-ePMO@Nd(bta)3 and
336.0 nm for dppz-vSilica@Nd(bta)3. Aside from the broad
band, several sharp peaks (some embedded on the broad
band), which could be assigned to f–f transitions of Nd3+ can
be observed in the excitation spectra of these samples. In the
case of all four materials the emission spectra consist of the
4
three characteristic Nd3+ emission peaks: F3/2
→
4I9/2
4
4
4
4
(895.0–890.0 nm), F3/2 → I11/2 (1065.0 nm), and F3/2 → I13/2
(1335.0 nm). A detailed assignment of all excitation and emis-
sion peaks in these spectra can be found in Table S1.† These
materials show strongest emission when they are excited into
the maximum of the broad band, but as has been shown in
Fig. 5 for the dppz-vSilica@Nd(tta)3 materials, good lumine-
scence properties can also be obtained when exciting in the
visible region, into some of the f–f transitions of Nd3+ (e.g. at
462.0 nm or 739.0 nm).
Fig. 6 presents the combined excitation–emission spectra of
the Yb3+ β-diketonate functionalized dppz-ePMO and dppz-
vSilica compounds. A strong broad emission peak with a
maximum around 985.0 nm was observed for all studied
materials – dppz-ePMO@Yb(tta)3, dppz-ePMO@Yb(bta)3, dppz-
vSilica@Yb(tta)3, and dppz-vSilica@Yb(bta)3 when exciting
into the maximum of the broad excitation band centered
around 350.0 nm (slightly varied depending on sample). In
Fig. S7† the combined excitation–emission spectrum of dppz-
vSilica@YbCl3 has been presented for comparison.
Fig. 6 Combined excitation–emission luminescence spectra of (a)
dppz-ePMO@Yb(tta)3, (b) dppz-ePMO@Yb(bta)3, (c) dppz-vSilica@Yb
(tta)3, and (d) dppz-vSilica@Yb(bta)3. All of the emission spectra where
recorded when excited into the maximum of the broad bands and
observed at the 2F5/2
→
2F7/2 transition peaks.
When exciting the Er3+ materials at 350.0 nm one strong,
broad peak located at around 1530.0–1535.0 nm, which can be
assigned to the 4I13/2 → 4I15/2 “telecom” transition, was observed
for the dppz-ePMO@Er(tta)3, dppz-vSilica@Er(bta)3, and
dppz-vSilica@Er(tta)3 materials (Fig. 7). The recorded emission
spectra were very weak in all three cases. No signal was detected
for the dppz-ePMO@Er(bta)3 material. This is due to the strong
quenching of the material, the strong quenching can also be
observed for the Nd3+ and Yb3+ materials.
Fig. 7 Emission spectra of Er3+ containing materials (from top to
bottom): dppz-ePMO@Er(tta)3, dppz-vSilica@Er(bta)3, and dppz-
vSilica@Er(tta)3. Only very weak emission is observed for these materials.
No NIR emission was detected for the dppz-ePMO@Er(bta)3 material.
In order to compare the luminescence properties of the
dppz-ePMO@Ln3+ and dppz-vSilica@Ln3+ materials we have
carried out decay time measurements for all of the Nd3+ and
Yb3+ samples. The Er3+ samples were excluded from further
studies due to their low luminescence intensity. For all
samples the decay times were measured when exciting into the
maximum of the broad ligand band and observing at the most
intensive emission peak. All decay profiles have been pre-
sented in the ESI (Fig. S8–S15†) and all of the decay times have
been overviewed in Table 1. The decay curves of all samples
could be well fitted only when using a double-exponential
function. This suggests that there is more than one coordi-
nation environment around the Ln3+ ions in these hybrid
mesoporous materials. When comparing the decay times of
Fig. 5 Compiled emission spectra of dppz-vSilica@Nd(tta)3 excited at
different wavelengths.
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Table 1 Luminescence decay times of mesoporous materials
Sample
τ1 (ns)/%
τ2 (ns)/%
τav (ns)
dppz-ePMO@Nd(tta)3
dppz-ePMO@Nd(bta)3
dppz-vSilica@Nd(tta)3
dppz-vSilica@Nd(bta)3
dppz-ePMO@Yb(tta)3
dppz-ePMO@Yb(bta)3
dppz-vSilica@Yb(tta)3
dppz-vSilica@Yb(bta)3
85/97
49/99.998
80/98
47/99.999
848/91
753/90
807/90
746/90
680/3
614/0.002
681/2
596/0.001
4776/9
4201/10
4491/10
4135/10
191
49
168
47
2295
2070
2195
2039
these samples it is quite clear that the dppz-ePMO@Ln3+ and
dppz-vSilica@Ln3+ materials show very similar performance.
The average decay times were calculated using eqn (S1).† For
example, for dppz-ePMO@Yb(tta)3 the average decay time was
recorded to be 2.2 µs, whereas for the equivalent dppz-
vSilica@Yb(tta)3 material it was 2.3 µs, which can be con-
sidered identical luminescence lifetimes within error margin.
For the dppz-ePMO@Nd(tta)3 and dppz-vSilica@Nd(tta)3
materials we have additionally carried out temperature
dependent luminescence studies in the 10–360 K range (Fig. 8
and 9). For both materials strong emission intensity is
observed at low temperature and gradually decreases with
temperature increase. Yet, at some temperature point the emis-
sion intensity once again starts increasing (around 260–310 K).
These changes can be attributed to the population redistribu-
tions induced by temperature.43 A higher temperature induces
a higher phonon density, increases multiphonon relaxations,
and additionally the energy transfer processes from the
ligands to the Ln3+ ions are strengthened.44 We believe the
temperature dependent luminescence trend where the emis-
sion of the sample increases at higher temperature can orig-
inate in a large part from the Ln3+ β-diketonate component. As
we have presented in the ESI (Fig. S16†) for the Ln(tta)3·2H2O
we observed a significant raise in the emission intensity with
temperature increase. The temperature dependent lumine-
scence has also been measured for the dppz-ePMO@Yb(tta)3
and dppz-vSilica@Yb(tta)3 samples and similar trends to those
Fig. 9 Temperature dependent luminescence of the dppz-vSilica@Nd
(tta)3 material.
observed for the Nd3+ samples were also observed here (Fig. 10
and 11). In the case of the Yb3+ samples not only does the
intensity of the sample change but also around 260 K the
shape of the peak changes compared to lower temperatures.
Fig. 10 Temperature dependent luminescence of the dppz-ePMO@Yb
(tta)3 material.
Fig. 8 Temperature dependent luminescence of the dppz-ePMO@Nd
Fig. 11 Temperature dependent luminescence of the dppz-vSilica@Yb
(tta)3 material.
(tta)3 material.
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4
4
The interesting temperature dependent luminescence pro- Stark components of the F3/2 multiplet as well as the F5/2
→
perties of this kind of materials could give them future appli- 4I9/2 transition is not observed. They are usually observed only
cations in the field of luminescence sensors or thermometry.
in inorganic phosphors doped with Nd3+ ions.28 When analyz-
As mentioned in the introduction, there are several ing the two Yb3+ doped materials we observed that when calcu-
different ways of accessing the temperature change of a lating the maximum intensity of the 986 nm/1015 nm peaks
material by employing luminescence features. Here, we (for dppz-ePMO@Yb(tta)3) or 990 nm/1025 nm peaks (for
focused on the change in the emission intensity with tempera- dppz-vSilica@Yb(tta)3) in a certain temperature range these
ture change. Several Nd3+ doped materials have already been materials could be considered as ratiometric temperature
studied for their potential as luminescence thermometers, sensors. Both materials showed a monotonic temperature be-
often as ratiometric thermometers.45–47 In most cases, the havior in the range of 110–310 K. For the dppz-ePMO@Yb(tta)3
4
4
intensity ratio between the F3/2(1)
→ →
4I9/2 and F3/2(2) 4I9/2 material the I986/I1015 ratio (based on the maxima of the peaks)
4
4
transitions (where F3/2(1) and F3/2(2) are two Stark com- was calculated for measurements carried out at a temperature
ponents of the 4F3/2 multiplet) is used to assess the ratiometric range of 110–310 K and has been plotted in Fig. 12a. The data
4
temperature parameter. It is also possible to use the F5/2
→
points could be well fitted with eqn (1) (R2 = 0.999). The absol-
4I9/2 to F3/2 → I9/2 intensity ratio as the ratiometric tempera- ute Sa and relative Sr sensitivities were calculated based on eqn
ture parameter.45 In the case of our dppz-ePMO@NdL3 and (2) and (3), respectively. The data have been plotted in Fig. 12b
dppz-vSilica@NdL3 materials this is not possible as the two and c. The maximum values of Sa and Sr are 0.0017 K−1
(at 110 K) and 0.17% K−1 (at 110 K), respectively.
4
4
Fig. 12 (a) Plot presenting the calibration curve for dppz-ePMO@Yb
(tta)3 material. The points depict the experimental Δ parameter and the
solid line is the best fit of the experimental points using eqn (1); (b) plot
showing the absolute sensitivity Sa values at different temperatures
(110–310 K) for dppz-ePMO@Yb(tta)3, and (c) relative sensitivity Sr
values at different temperatures (110–310 K) also for dppz-ePMO@Yb
(tta)3. The solid line is a guide for the eyes.
Fig. 13 (a) Plot presenting the calibration curve for dppz-vSilica@Yb
(tta)3 material. The points depict the experimental Δ parameter and the
solid line is the best fit of the experimental points using eqn (1); (b) plot
showing the absolute sensitivity Sa values at different temperatures
(110–310 K) for dppz-vSilica@Yb(tta)3, and (c) relative sensitivity Sr
values at different temperatures (110–310 K) also for dppz-vSilica@Yb
(tta)3. The solid line is a guide for the eyes.
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ꢀ
ꢁ
ΔE
kBT
again with further temperature increases. Both the dppz-
ePMO@LnL3 and dppz-vSilica@LnL3 materials behaved in a
similar manner with temperature change. The Yb3+ materials
were evaluated for their potential use as ratiometric tempera-
ture sensors. They showed monotonic behavior in the tempera-
ture range 110–310 K.
Δ ꢀ C exp ꢁ
ð1Þ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
@Δ
@T
Sa ¼
ð2Þ
ð3Þ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
1 @Δ
Δ @T
Sr ¼ 100% ꢂ
For the dppz-vSilica@Yb(tta)3 material the I990/I1025 ratio
(based on the maxima of the peaks) was calculated for
measurements carried out at a temperature range of 110–310 K
and has been plotted in Fig. 13a. The data points could be well
fitted with eqn (1) (R2 = 0.997). The absolute Sa and relative Sr
sensitivities were calculated based on eqn (2) and (3), respect-
ively. The data have been plotted in Fig. 13b and c. The
maximum values of Sa and Sr are 0.0014 K−1 (at 110 K) and
0.13% K−1 (at 110 K), respectively. These values are slightly
lower than those of the dppz-ePMO@Yb(tta)3 material, but are
in a similar range, thus showing comparable temperature
dependent output of the two materials.
Acknowledgements
A. M. K. acknowledges Ghent University’s Special Research
Fund (BOF) for a Postdoctoral Mandate (project BOF15/PDO/
091). D. E. and F. J. R.-S. acknowledge funding of this research
by the Spanish Ministry of Economy and Competitiveness
(Project MAT2013-44463-R), Andalusian Regional Government
(FQM-346 group), Andalucía Talent Hub and Feder Funds.
References
1 G. Liu and X. Chen, Handbook on the Physics and Chemistry
of Rare Earths, ed. K. A. Gschneidner Jr., J.-C. G. Bünzli and
V. K. Pecharsky, Elsevier Science, Amsterdam, 2007, ch.
233, vol. 37, pp. 99–169.
4. Conclusions
To conclude, we have reported the preparation of a meso-
porous silica and a periodic mesoporous organosilica material,
which were both functionalized with dipyridyl-pyridazine
(dppz) units and further grafted with Nd3+, Er3+, and Yb3+
β-diketonate (2-thenoyltrifluoroacetone or benzoyltrifluoro-
acetone) complexes in an attempt to obtain hybrid NIR emitting
materials. We have previously reported the preparation of a
similarly functionalized ePMO material (functionalized with
the dipyridyl-dihydropyridazine ligand) and its grafting with
Eu3+ ions. Here, we have synthesized the dppz-ePMO and
dppz-vSilica materials, which we both further functionalized
with lanthanide complexes. We show that both the dppz-ePMO
and dppz-vSilica are very good “platform” materials for the
further functionalization with NIR emitting lanthanide ions
for luminescence applications. The dppz-vSilica material was
faster in preparation, and with very similar luminescence pro-
perties to the equivalent porous dppz-ePMO@Ln3+. The advan-
tage of the dppz-ePMO@Ln3 materials is its known higher
stability originating from the ePMO. Detailed luminescence
2 J. C. G. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34,
1048–1077.
3 S. V. Eliseeva and J.-C. G. Bünzli, Chem. Soc. Rev., 2010, 39,
189–227.
4 K. Binnemans, Chem. Rev., 2009, 109, 4283–4374.
5 D. Esquivel, A. M. Kaczmarek, C. Jimenez-Sanchidrian,
R. Van Deun, F. J. Romero-Salguero and P. Van Der Voort,
J. Mater. Chem. C, 2015, 3, 2909–2917.
6 X. Guo, H. Guo, L. Fu, H. Zhang, R. Deng, L. Sun, J. Feng
and S. Dang, Microporous Mesoporous Mater., 2009, 119,
252–258.
7 L. Sun, W. Mai, S. Dang, Y. Qiu, W. Deng, L. Shi, W. Yan
and H. Zhang, J. Mater. Chem., 2012, 22, 5121–5127.
8 L. N. Sun, H. J. Zhang, C. Y. Peng, J. B. Yu, Q. G. Meng,
L. S. Fu, F. Y. Liu and X. M. Guo, J. Phys. Chem. B, 2006,
110, 7249–7258.
9 L. N. Sun, H. J. Zhang, J. B. Yu, S. Y. Yu, C. Y. Peng,
S. Dang, X. M. Guo and J. Feng, Langmuir, 2008, 24, 5500–
5507.
studies, including temperature dependent investigations were 10 C. M. Granadeiro, S. O. Ribeiro, A. M. Kaczmarek,
carried out. These materials could be excited either through
the broad ligand band in the UV-Vis region or through the
sharp f-f transitions present in a wide range of the excitation
L. Cunha-Silva, P. L. Almeida, S. Gago, R. Van Deun, B. de
Castro and S. S. Balula, Microporous Mesoporous Mater.,
2016, 234, 248–256.
spectra. The dppz-ePMO@Ln3+ and dppz-vSilica@Ln3+ 11 B. Yan and B. Zhou, J. Photochem. Photobiol., A, 2008, 195,
materials did not show any significant differences in their
emission intensities, decay times or temperature dependent 12 Y. Li, B. Yan and H. Yang, J. Phys. Chem. C, 2008, 112,
luminescence behavior. 3959–3968.
Temperature dependent luminescence studies of chosen 13 D. B. Ambili Rai, S. Biju and M. L. P. Reddy, J. Mater.
Nd3+ and Yb3+ materials showed that these materials exhibit a
Chem., 2009, 19, 7976–7983.
change in emission intensity with temperature change. When 14 S. M. Bruno, A. C. Coelho, R. A. S. Ferreira, L. D. Carlos,
314–322.
the temperature increases from 10 K to 260 K the intensity
gradually decreases. After 260 K the intensity starts increasing
M. Pillinger, A. A. Valente, P. Ribeiro-Claro and
I. S. Goncalves, Eur. J. Inorg. Chem., 2008, 3786–3795.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
15 V. Divya, S. Biju, R. Luxmi Varma and M. L. P. Reddy, 32 C. Vercaemst, M. Ide, B. Allaert, N. Ledoux, F. Verpoort and
J. Mater. Chem., 2010, 20, 5220–5227. P. Van Der Voort, Chem. Commun., 2007, 2261–2263.
16 H. R. Li, J. Lin, L. S. Fu, J. F. Guo, Q. G. Meng, F. Y. Liu and 33 G. P. Sagitullina, L. V. Glyzdinskaya and R. S. Sagitullin,
H. J. Zhang, Microporous Mesoporous Mater., 2002, 55, 103–107. Khim. Geterotsikl. Soedin., 2005, 858–863.
17 Y.-J. Li, B. Yan and L. Wang, Dalton Trans., 2011, 40, 34 L. R. Melby, E. Abramson, J. C. Caris and N. J. Rose, J. Am.
6722–6731. Chem. Soc., 1964, 86, 5117–5125.
18 H. Takeda, M. Ohashi, T. Tani, O. Ishitani and S. Inagaki, 35 K. Binnemans, P. Lenaerts, K. Driesen and C. Gorller-
Inorg. Chem., 2010, 49, 4554–4559. Walrand, J. Mater. Chem., 2004, 14, 191–195.
19 H. Takeda, Y. Goto, Y. Maegawa, T. Ohsuna, T. Tani, 36 D. Esquivel, E. De Canck, C. Jimenez-Sanchidrian,
K. Matsumoto, T. Shimada and S. Inagaki, Chem. Commun.,
2009, 6032–6034.
P. Van Der Voort and J. Romero-Salguero, J. Mater. Chem.,
2011, 21, 10990–10998.
20 Y. Li, B. Yan and Y. J. Li, Microporous Mesoporous Mater., 37 C. Vercaemst, J. T. A. Jones, Y. Z. Khimyak, J. C. Martins,
2010, 132, 87–93.
21 S. Biju, Y. K. Eom, J. C. G. Bunzli and H. K. Kim, J. Mater.
Chem. C, 2013, 1, 3454–3466.
22 N. M. Shavaleev, S. J. A. Pope, Z. R. Bell, S. Faulkner and
M. D. Ward, Dalton Trans., 2003, 808–814.
F. Verpoort and P. Van Der Voort, Phys. Chem. Chem. Phys.,
2008, 10, 5349–5352.
38 L. Sun, Y. Qiu, T. Liu, J. Z. Zhang, S. Dang, J. Feng,
Z. Wang, H. Zhang and L. Shi, ACS Appl. Mater. Interfaces,
2013, 5, 9585–9593.
23 D. Jaque and F. Vetrone, Nanoscale, 2012, 4, 4301–4326.
24 A. Benayas, B. del Rosal, A. Perez-Delgado, K. Santacruz-
39 Y. Liu, L. Sun, J. Liu, Y.-X. Peng, X. Ge, L. Shi and
W. Huang, Dalton Trans., 2015, 44, 237–246.
Gomez, D. Jaque, G. A. Hirata and F. Vetrone, Adv. Optical 40 Y.-J. Gu and B. Yan, J. Colloid Interface Sci., 2013, 393,
Mat., 2015, 3, 1–8. 36–43.
25 L. Li, Y. Zhu, X. Zhou, C. D. S. Brites, D. Ananias, Z. Lin, 41 P. Yang, Z. Quan, C. Li, H. Lian, S. Huang and J. Lin,
F. A. A. Paz, J. Rocha, W. Hunag and L. D. Carlos,
Adv. Funct. Mater., 2016, 26, 8677–8684.
Microporous Mesoporous Mater., 2008, 116, 524–531.
42 X. Lian and B. Yan, New J. Chem., 2013, 37, 1–3.
26 Y. Ciu, F. Zhu, B. Chen and G. Qian, Chem. Commun., 2015, 43 L. Labrador-Paez, C. Perez-Rodriguez, S. Rios, D. Alonso,
51, 7420–7431.
J. M. Caceres and I. R. Martin, Sens. Actuators, A, 2015, 233,
422–426.
44 X. Xu, Z. Wang, P. Lei, Y. Yu, S. Yao, S. Song, X. Liu, Y. Su,
L. Dong, J. Feng and H. Zhang, ACS Appl. Mater. Interfaces,
2015, 7, 20813–20819.
45 U. Rocha, C. Jacinto, W. F. Silva, I. Guedes, A. Benayas,
L. M. Maestro, M. A. Elias, E. Bovero, F. van Veggel,
J. A. G. Sole and D. Jaque, ACS Nano, 2013, 7, 1188–1199.
27 J. Rocha, C. D. S. Brites and L. D. Carlos, Chem. – Eur. J.,
2016, 22, 1–15.
28 A. Benayas, B. del Rosal, A. Perez-Delgado, K. Santacruz-
Gomez, D. Jaque, G. A. Hirato and F. Vetrone, Adv. Funct.
Mater., 2015, 3, 687–694.
29 J. Liu, R. Van Deun and A. M. Kaczmarek, J. Mater.
Chem. C, 2016, 4, 9937–9941.
30 S. Balabhadra, M. L. Dabasu, C. D. S. Brites, 46 O. A. Savchuk, J. J. Carvajal, L. De la Cruz, P. Haro
L. A. O. Nunes, O. L. Malta, J. Rocha, M. Bettinelli and
L. D. Carlos, Nanoscale, 2015, 7, 17261–17267.
Gonzalez, M. Aguilo and F. Diaz, J. Mater. Chem. C, 2016, 4,
7397–7405.
31 M. S. Robillard, H. M. Janssen, W. ten Hoeve and 47 D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek and
R. M. Versreegen, US pat, 9421274B2, 2016. M. Samoc, Nanoscale, 2012, 4, 6959–6961.
Dalton Trans.
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