Three resorcin[4]arene-based lanthanide-coordination polymers with multifunctional photoluminescence sensing properties

Article abstract of DOI:10.1039/c8ra09777a

By utilizing a novel octacarboxylate-functionalized resorcin[4]arene as organic linkers, three lanthanide-coordination polymers, namely, [(CH₃)₂NH₂][Ln₂(HL)(H₂O)₇]·2H₂O (Ln = Tb

Full text of DOI:10.1039/c8ra09777a

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Three resorcin[4]arene-based lanthanide-
coordination polymers with multifunctional
Cite this: RSC Adv., 2019, 9, 3647
photoluminescence sensing properties†
a
a
b
a
*
*
Jia-Chen Wang, Wei Jiang and Si-Si Zhao
Hang Zhang,
By utilizing a novel octacarboxylate-functionalized resorcin[4]arene as organic linkers, three lanthanide-
coordination polymers, namely, [(CH3)2NH2][Ln2(HL)(H2O)7]$2H2O (Ln ¼ Tb (1), Eu (2) and Gd (3), H8L ¼
2,8,14,20-tetra-pentyl-4,6,10,12,16,18,22,24-octa-carboxymethoxy-resorcin[4]arene)
have
been
solvothermally synthesized and structurally characterized. Isostructural 1–3 display unique two
dimensional sandwich-based layers built with Ln3+ cations and bowl-shaped HL7ꢀ anions. Remarkably, 1
and 2 produce intensive green and red emissions respectively and long lifetimes thanks to the antenna
effect of HL7ꢀ anions. The energy level testing of 3 indicates that the newly designed ligand H8L has
a very efficient intersystem crossing process. More importantly, luminescent investigations reveal that 1
and 2 can selectively detect N,N0-dimethylformamide and Fe3+ ions with turn-on-type and turn-off-type
responses, respectively.
Received 28th November 2018
Accepted 20th January 2019
DOI: 10.1039/c8ra09777a
rsc.li/rsc-advances
compounds, ionic pollutants, pH value, gas and biomarker
molecules with a turn-off-type luminescent process.11–14 Most
Introduction
recently, some progress has been achieved in development of
turn-on-type Ln-CP luminescent sensors that were utilized to
monitor local concentration uctuations of environmentally
hazardous chemicals.15,16 However, single Ln-CP luminescent
sensors that discriminate volatile organic compounds and
metal ionic pollutants through different response types remain
exceedingly rare.17 Thus, it is an active and challenging research
eld to develop new luminescent Ln-CPs.
Currently, probing hazardous chemicals, such as volatile
organic compounds and metal ionic pollutants is of great
importance because it is highly critical in the elds of biology,
physiology, pharmacology and environmental sciences.1–3 In
light of the impacts that hazardous chemicals have on human
health and the environment, developing effective technologies
for the sensing of volatile organic compounds and metal ionic
pollutants are therefore of global importance and highly
necessary.4,5 It needs to be mentioned that uorometric tech-
niques offer relative ease of use, technical simplicity, and wide
applicability, and would likely be preferred during sensing of
volatile organic compounds and metal ionic pollutants if
available.6,7
Emerging as a class of luminescent materials, lanthanide-
coordination polymers (Ln-CPs) have afforded a great oppor-
tunity in terms of molecular sensing because of their excep-
tional uorescent features with narrow emission bands, long
emission lifetimes, and high luminescent quantum efficien-
cies.8–10 So far, uorescent Ln-CPs have been successfully
employed for luminescent sensing of harmful volatile organic
Among all lanthanide elements, Tb3+ and Eu3+ could show
intense characteristic green and red emissions, respectively,
when they are excited by energy transfer from an “antenna”
linker to Ln3+ cations under UV irradiation.18,19 Remarkably,
during the probing process the bright color changes of green or
red could even be observed by the naked eye directly.20 On the
other hand, introduction of suitable organic chromophoric
sensitizers into Ln-CPs will signicantly enhance the light
absorption ability and increase the luminescent brightness of
Ln3+ cations by “antennae effect”.21 In this regard, resorcin[4]
arenes are particularly attractive because of their bodies and
rims with various substituents, thus yielding a great diversity of
ligands.22,23 Thus far some metal–organic frameworks with
fascinating structures and properties have been synthesized by
applying the resorcin[4]arene-based ligands.24,25 Remarkably,
carboxylate-functionalized resorcin[4]arene ligands have been
proven to exhibit strong “antennae effects” in building lumi-
nescent Ln-CPs.26,27
aInstitute of Catalysis for Energy and Environment, College of Chemistry & Chemical
Engineering, Shenyang Normal University, Shenyang, 110034, P. R. China. E-mail:
zhangh@synu.edu.cn; zhaoss@synu.edu.cn
bCollege of Environmental Science and Engineering, Jilin Normal University, Siping,
136000, P. R. China
† Electronic supplementary information (ESI) available: X-ray crystallographic
data in CIF format, TGA, PXRD, luminescent spectra and tables. CCDC
1581287–1581289. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8ra09777a
Based on above consideration, we herein report three Ln-
CPs, namely, [(CH3)2NH2][Ln2(HL)(H2O)7]$2H2O [Ln ¼ Tb
(1), Eu (2), Gd (3)], assembled with a new octacarboxylate-
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functionalized resorcin[4]arene ligand, 2,8,14,20-tetra-pentyl-
4,6,10,12,16,18,22,24-octa-carboxymethoxy-resorcin[4]arene
(H8L) (Scheme 1). 1 and 2 emit unique and bright lumines-
cence in visible light region. Most importantly, they could be
applied as turn-on-type uorescent sensors for N,N0-dime-
thylformamide (DMF) and exhibit selectively uorescent turn-
off responses triggered by Fe3+ ion in comparison to other
ions.
Synthesis of [(CH3)2NH2][Tb2(HL)(H2O)7]$2H2O (1)
A mixture of H8L (27.0 mg, 0.1 mmol) and TbCl3$6H2O (18.0 mg,
0.048 mmol) was dissolved in a mixed solution of DMF (2 mL)
and water (6 mL). Then the suspension was transferred into
a Teon-lined autoclave (15 mL) and heated at 100 ꢁC for 3 days.
Colorless crystals of 1 were achieved in a 35% yield based on
Tb3+ aer cooling the reactor to room temperature. Anal. calcd.
for C66H94Tb2NO33 (Mr ¼ 1747.26): C, 45.36; H, 5.42; N, 0.80.
Found: C, 45.82; H, 5.60; N, 0.82. IR data (KBr, cmꢀ1): 3750(s),
3525(s), 3325(m), 3249(m), 2929(w), 2856(m), 1711(m), 1583(w),
1507(w), 1454(w), 1427(w), 1375(m), 1331(m), 1298(w), 1257(m),
1188(m), 1119(w), 1071(m), 939(s), 908(s), 852(s), 775(s), 725(m),
680(m), 582(s), 510(s), 413(s).
Experimental
Materials and general measurements
Commercial chemicals were used as purchased unless other-
wise mentioned. FT-IR spectra were determined on a Mattson
Alpha Centauri spectrometer. Elemental analysis data were
recorded on a Perkin-Elmer Model 240C elemental analyzer.
Powder X-ray diffraction (PXRD) patterns were determined on
a Rigaku Dmax 2000 X-ray diffractometer using CuKa radiation
(l ¼ 0.154 nm). UV-vis spectra were measured on a Cary TU-1900
doublebeam UV-vis spectrophotometer. Thermogravimetric
(TG) data were conducted on a Perkin-Elmer Model TG-7
analyzer under nitrogen gas. Luminescent spectra were per-
formed on a FLSP920 Edinburgh Fluorescence Spectrometer.
Synthesis of [(CH3)2NH2][Eu2(HL)(H2O)7]$2H2O (2) and
[(CH3)2NH2][Gd2(HL)(H2O)7]$2H2O (3)
2 and 3 were synthesized in a similar procedure as 1, in which
TbCl3$6H2O was replaced by EuCl3$6H2O (17.0 mg, 0.048
mmol) and GdCl3$6H2O (18.0 mg, 0.048 mmol), respectively.
Colorless crystals of 2 and 3 were achieved in 39% and 42%
yields, respectively, based on Ln3+ cations. Anal. calcd. for
C
66H94Eu2NO33 (Mr ¼ 1733.34): C, 45.73; H, 5.46; N, 0.80.
Found: C, 45.66; H, 5.88; N, 0.85. IR data (KBr, cmꢀ1) for 2:
3855(s), 3352(m), 2926(w), 2855(m), 1708(m), 1583(w), 1509(w),
1452(w), 1425(w), 1372(s), 1285(w), 1190(m), 1165(s), 1119(m),
1064(m), 962(s), 919(s), 848(s), 720(s), 639(s), 590(s). Anal. calcd.
for C66H94Gd2NO33 (Mr ¼ 1743.92): C, 45.45; H, 5.43; N, 0.80.
Found: C, 45.30; H, 5.71; N, 0.84. IR data (KBr, cmꢀ1) for 3:
3750(s), 3369(m), 2929(w), 2857(m), 1578(w), 1508(w), 1427(w),
1376(s), 1332 (s), 1295(w), 1189(m), 1120(m), 1071(m), 959(s),
919(s), 848(s), 726(s), 678(s), 584(s), 413(s).
Synthesis of H8L
The precursor 1,3-bis[(methoxycarbonyl)methoxy]benzene (L1)
was synthesized according to the literature procedure.28
A
mixture of L1 (2.56 g, 10.0 mmol) and hexaldehyde (1.0 g, 10
mmol) in dichloromethane (30 mL) was stirred at ice bath for
0.5 h with dropwise addition of BF3$OEt2 (6 mL). Then the
mixture was stirred at room temperature overnight. The
generated solid was collected and washed using CH2Cl2 to give
L2 in a yield of 58%. Then sodium hydroxide (1.6 g, 40 mmol),
tetrahydrofuran (150 mL), and water (150 mL) were added into
L2. The solvents were removed by water bath, and then 100 mL
water was added. The pH value of the mixture was adjusted to 1–
2 by using HCl (1.0 mol Lꢀ1), and solid H8L was achieved in
a 64% yield. Anal. calcd. for C64H80O24 (Mr ¼ 1232.50): C, 62.33;
H, 6.54; N, 0.00. Found: C, 62.42; H, 6.47; N, 0.00. IR data
(KBr, cmꢀ1) for H8L: 3404(s), 2954(m), 2929(w), 2859(m),
2657(w), 2554(w), 1737(w), 1611(s), 1586(s), 1501(w), 1432(m),
1378(m), 1292(m), 1242(s), 1187(s), 1123(s), 1098(s), 1071(s),
972(s), 907(s), 822(s), 726(s), 673(s), 516(s).
Luminescent sensing experiments
Before luminescent determination, the samples of Ln-CPs were
ground into powder. Then the suspension was prepared by
introducing the powder sample (3 mg) of Ln-CPs into each
volatile organic compounds solvent (3 mL) or stock solutions (1
ꢂ 10ꢀ2 M, 3 mL) of MClx by ultrasound treatment for 1 min.
Each suspension was transferred to a cuvette and tested by
luminescent spectrum.
Measurement for luminescent sensing of DMF vapor
A small beaker (10 mL) with the samples (10 mg) was placed
into a big sealed container (50 mL) with DMF (20 mL) for 24 h or
10 min. Subsequently, the small beaker was taken out from the
container and quickly sealed, and the emission spectra were
measured by using the solid samples. The initial emission
spectrum of the solid sample was measured in a solid sample
holder before exposure to DMF vapor.
X-ray crystallography
Crystallographic data for 1–3 were determined on an Oxford
Diffraction Gemini R Ultra diffractometer using graphite-
˚
monochromated Mo-Ka radiation (l ¼ 0.71073 A). The struc-
Scheme 1 Synthetic route of the resorcin[4]arene-based H8L ligand.
3648 | RSC Adv., 2019, 9, 3647–3652
ture were solved by direct method and rened on F2 by full-
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matrix least-squares method using the SHELXL-97 within interconnected by Tb2 cations to give a unique two dimensional
WINGX.29–31 H atoms of the organic molecules were placed sandwich-based layer (Fig. 1c).
geometrically. Further crystallographic details are listed in
Table S1.† Selected bond lengths and angles for 1–3 are listed in
Tables S2–S4.†
Luminescent properties
Solid state emission spectra of the free H8L and 1 and 2 were
determined at room temperature. For the free H8L, the emission
peak at 366 nm (lex ¼ 315 nm) corresponds to the p* / p or p*
/ n transitions,33 as shown in Fig. S2.† When excited at
295 nm, 1 emits characteristic emission peaks at 488, 543, 584,
Results and discussion
Crystal structural descriptions of [(CH3)2NH2][Tb2(HL)(H2O)7]$
2H2O (1), [(CH3)2NH2][Eu2(HL)(H2O)7]$2H2O (2) and
[(CH3)2NH2][Gd2(HL)(H2O)7]$2H2O (3)
5
7
and 623 nm, which could be ascribed to D4 / FJ (J ¼ 6–3)
transitions of the Tb3+ cations (Fig. 2a).30 The most intense
emission at 543 nm is attributed to the 5D4 / 7F5 transition of
the Tb3+ cations, resulting in an intense green emission output
in the solid state.34 Similarly, 2 exhibits characteristic transi-
tions of the Eu3+ cations upon excitation at 318 nm. As depicted
in Fig. 2b, the emission peaks at 579, 593, 614, 651, and 698 nm
1–3 are isostructural and crystallize in the same triclinic space
ꢀ
group P1. Herein, we will only describe the structure of 1 in
detail (Fig. S1†). One of the most striking features of such Ln-
CPs is the presence of numerous coordinated waters around
the Ln3+ centers. The asymmetric unit of 1 consists of two Tb3+
cations, one HL7ꢀ anion, seven coordinated water molecules,
one [H2N(CH3)2]+ cation, and two lattice water molecules
(Fig. 1a). Two Tb3+ cations exhibit different coordination
spheres. Tb1 is in a distorted square antiprism geometry, sur-
rounded by eight oxygen atoms from two different HL7ꢀ anions
and three coordinated water molecules. Tb2 is coordinated by
nine oxygen atoms from three HL7ꢀ anions and four coordi-
nated water molecules in a distorted tricapped trigonal prism
sphere. The Tb–O bond distances vary from 2.300(14) to
5
7
correspond to the D0 / FJ (J ¼ 0–4) transitions of the Eu3+
cations.35 Notably, the emission spectrum of 2 is mainly domi-
nated by the 5D0 / 7F2 transition, which is more intense than
others, leading to intense red luminescence.36 The bright colors
also demonstrate that HL7ꢀ acts as an excellent “antenna”
linker in the effectively transfer energies to Tb3+ and Eu3+
metals.37
Another critical luminescence characteristic of Ln-CPs is the
lifetime, which refers to the average time that a molecule stays
in its excited state before emitting a photon.38 Hence, the life-
32
˚
2.648(17) A, which are within the normal range. One
time values of the excited states D4 (Tb3+) and D0 (Eu3+) of 1
and 2 were monitored with 543 and 614 nm, respectively
(Fig. S3†). The observed luminescent decay proles correspond
to single exponential functions, with the lifetimes of s ¼ 0.730
ms for 1 and s ¼ 1.242 ms for 2.39 Moreover, the ligand-centered
emission is not detected, thus implying the existence of an
efficient ligand-to-metal energy-transfer process in these Ln-
CPs.40
5
5
[H2N(CH3)2]+ cation is included as a counter cation to balance
the negative charge. Seven carboxylates of each HL7ꢀ anion are
involved in coordination with ve Tb3+ cations. It is noticeable
that two HL7ꢀ anions form a sandwich-like unit via sharing two
Tb1 cations (Fig. 1b). Further, adjacent sandwich-like units are
To better understand the energy transfer process, the UV-vis
absorption spectrum of 3 was determined in methanol solution
at room temperature.41 The wavelength of the absorbance edge
is about 312 nm, indicating that HL7ꢀ has a singlet state (S1)
energy level of 32 051 cmꢀ1 (Fig. S4a†). The phosphorescent
spectrum of 3 was also recorded at 77 K to estimate the triplet
state (T1) energy level of HL7ꢀ, and obtained the T1 energy level
of 22 675 cmꢀ1 (Fig. S4b†). According to Reinhoudt's empirical
rule, the energy transfer from ligand to metal will become
effective when the energy gap between S1 and T1 of the organic
Fig. 1 (a) Coordination environments of Tb3+ cations in 1. (b) View of
the sandwich unit of 1. (c) Top view of the unique 2D sandwich-based Fig. 2 Solid state emission spectra and luminescent images of 1 (a)
network of 1.
and 2 (b).
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Scheme 2 Energy level scheme showing energy transfer processes in
1 and 2.
linker is at least 5000 cmꢀ1 41 For HL7ꢀ, the energy gap between
.
S1 and T1 is 9376 cmꢀ1, implying that the intersystem crossing
process is greatly efficient. Generally, to fulll luminescent Tb-
and Eu-CP sensors, the organic ligands should have a suitable
T1 in 22 000–27 000 cmꢀ1 to well match their energy levels (5D4,
5
20 500 cmꢀ1 for Tb3+, and D0, 17 500 cmꢀ1 for Eu3+).42 In our
Fig. 3 Emission spectra and the relative 5D4
/ /
7F5 or 5D0 7F2
case, the T1 energy level of HL7ꢀ is 22 675 cmꢀ1, indicating that
HL7ꢀ could efficiently sensitize both Eu3+ and Tb3+ emissions.42
The energy transfer process illustrated in Scheme 2 shows
ligand-to-metal energy transfers.
transition intensities of 1 (a) and 2 (b) in various volatile organic
compounds upon excitation at 295 and 318 nm, respectively (the inset
photographs show the colors for 1 and 2 dispersed in DMF (left) and
ethanol (right) under a UV light (lex ¼ 365 nm)).
the sensor is quite fast with 88.9% and 89.7% (the relative 5D4 /
7F5 or 5D0 / 7F2 transition intensities) of enhancement achieved
compared that aer 24 h, rendering 1 and 2 potential sensors for
turn-on-type detection of DMF vapor. Moreover, PXRD patterns of
1 and 2 in DMF vapor are nearly identical to the simulated one,
suggesting they are stable in the luminescent sensing process
(Fig. S6†).
Luminescent recognition of DMF
Most recently, volatile organic compounds sensing has gained
considerable interest in fundamental and practical research due
to their potential applications in optical devices, environmental
monitoring, and separation.43,44 To explore the potential appli-
cations in this eld, luminescent responses of 1 and 2 aer
incubation in various volatile organic compounds, including
chloroform, dichloromethane, ethanol, acetonitrile, tetrahydro-
furan, ether, cyclohexane, methanol and DMF, were studied in
detail. As shown in Fig. 3, the corresponding luminescence
curves still show the four characteristic emission peaks, and only
the relative 5D4 / 7F5 or 5D0 / 7F2 transition intensities of 1 and
2 were monitored under the perturbation of various volatile
organic compounds. Particularly, DMF triggers a superior lumi-
nescent turn-on effect for 1 and 2, while other volatile organic
compounds have no signicant effect on the emission. Moreover,
the inset photographs shows that only DMF can obviously
enhance the emission colors of 1 and 2, which leads to brighten
under UV light. This nding suggests that 1 and 2 are potential
sensing materials for DMF with an excellent selectivity.
The emission intensity of the Ln3+ is well-known to rely on the
energy transfer efficiency from the ligand to the Ln3+ center.45
Meanwhile, the interactions with the O–H bond of coordinated
water may lead to quenching of Ln3+ luminescence.45 DMF has
a more strong coordination ability compared to other volatile
organic compound. When coordinated waters are partially
changed by DMF, the O–H bonds surrounding the Ln3+ centers
may leave fewer and thus enhance Ln3+ emission. Hence, we infer
that the marked enhancement of their emission intensities in
response to DMF is likely caused by the displacement of coordi-
nated water molecules by DMF molecules.
Selective sensing of Fe3+ ion
Metal ion sensing and detection play a signicant role in envi-
The obvious emission intensity enhancement of 1 and 2 by
displacement in DMF encouraged us to further explore the
potential applications of them for probing DMF vapor. In order to
test the performance of 1 and 2 as DMF vapor sensor, an in situ
solid-state luminescent sensor setup was designed and used
(Fig. S5†). The luminescence response of 1 and 2 aer incubation
for 24 h under DMF vapor were measured and the results are
plotted in Fig. 4. The observation indicates 1 and 2 still exhibit
a clear enhancement effect. Besides good sensitivity, fast response
is also an important criterion for a good sensor. Hence, the solid
state emission spectra of 1 and 2 were further determined aer
ronment and life science.46–48 In this study, we also investigated
Fig. 4 Luminescent intensity for 1 and 2 upon exposure to DMF vapor
exposed in DMF vapor for 10 minutes. Clearly, the response rate of at different time.
3650 | RSC Adv., 2019, 9, 3647–3652
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luminescent 1 and 2 to sense metal ionic pollutants. The crys-
talline samples were simply soaked in aqueous solutions of
MClx (M ¼ Na+, K+, Ca2+, Cd2+, Zn2+, Mn2+, Ni2+, Mg2+, Co2+, Al3+,
Cu2+, Cr3+ Fe2+ and Fe3+, 1 ꢂ 10ꢀ2 M) for luminescence studies.
As depicted in Fig. 5, the corresponding luminescence curves
still show the four characteristic emission peaks, and only the
relative 5D4 / 7F5 or 5D0 / 7F2 transition intensities for 1 and 2
were monitored under the perturbation of various cations.
Interestingly, Fe3+ exhibits a pronounced quenching effect on
the luminescence of 1 and 2, while other metal ions have no
signicant effect with the exception of Fe2+ or Cr3+, which can
weaken the luminescence to some extent. Moreover, the inset
photographs show that only Fe3+ can completely quench the
emission colors of 1 and 2, which leads to the dark under UV
light. The above phenomenon indicate that 1 and 2 may be
considered as a promising luminescent probe for Fe3+ ions.
To further prove the luminescent quenching by Fe3+ ion,
concentration-dependent luminescent studies for 1 and 2 were
carried out in the presence of Fe3+ ion. As demonstrated in
Fig. 6, the emission intensity for the suspension of 1 and 2
sharply declines with the increase of Fe3+ concentration from
0 to 5000 mM. At the Fe3+ content of 5 ꢂ 10ꢀ3 M, the uores-
cence intensity of 1 and 2 completely disappeared. Quantita-
tively, the Stern–Volmer equation I0/I ¼ 1 + Ksv[M] can be used to
describe this quenching effect, where the values I0 and I
represent the luminescent intensities of 1 without and with
Fig. 6 Emission spectra and linear relationship for 1 in aqueous
solution of different concentrations of Fe3+
.
Volmer model. The suspension of 2 also exhibits the similar
selective sensing of Fe3+ ion. The calculated Ksv value is 3.93 ꢂ
104 based on the Stern–Volmer equation, demonstrating
a quenching effect on the uorescence of 2. Besides, a good
linear relationship is achieved with the coefficient of 0.96502
(Fig. S7†).
To understand the mechanism of the uorescence quench-
ing effect of 1 and 2 toward Fe3+ ions the UV/vis absorption data
of 1 and 2 and varied metal ions were carried out. The UV/vis
absorption spectra show that the wide absorption band from
260 to 400 nm of Fe3+ covers the range of absorption bands of 1
and 2, and is much more stronger than those of other metal
ions (Fig. S8†). This means that the UV/vis absorption of Fe3+
upon excitation may prevent the absorption of 1 and 2, and
result in the decrease or quenching of the luminescence.49,50
addition of Fe3+, respectively, [M] is the concentration of Fe3+
,
and Ksv is the quenching constant. Based on the quenching
experimental data, the Ksv value is calculated to be 1.545 ꢂ 104,
suggesting a strong quenching effect on the luminescence of 1.
The calculated linear correlation coefficient (R) in the Ksv curve
of 1 with addition of Fe3+ is 0.99685, demonstrating that the
quenching effect of Fe3+ on the luminescence of 1 ts the Stern–
Conclusions
In summary, we reported three Ln-CPs assembled with a new
octacarboxylate-functionalized resorcin[4]arene ligand and Ln3+
cations. 1 and 2 emit intense characteristic red and green emis-
sion colors. The photophysical properties demonstrated that H8L
is well suited for the sensitization of Tb3+ and Eu3+ emissions
thanks to the favorable energy level of its triplet state. 1 and 2
could be employed as potential turn-on-type uorescent sensors
for DMF and turn-off-type luminescent probe for Fe3+ ion.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We appreciate support from Prof. Jian-Fang Ma (the National
Natural Science Foundation of China (Grant No. 21471029)) for
experimental measurements. PhD startup fund of Shenyang
Normal University is gratefully acknowledged (054/
91800161006).
Notes and references
Fig. 5 Emission spectra and the relative 5D4
/ /
7F5 or 5D0 7F2
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