An unexpected electro-luminescent properties of scandium(III) heteroaromatic 1,3-diketonate complex

Article abstract of DOI:10.1016/j.ica.2014.02.013

A novel complex of Sc(III) with pyrazole substituted 1,3-diketone (1,3-bis(1,3-dimethyl-1H-pyrazol-4-yl)-1,3-propanedione) was synthesized and characterized by X-ray diffraction, nanotechnology-assisted laser desorption/ionisation time-of-flight mass spectroscopy (NALDI-TOF-MS) and NMR spectroscopy, including ⁴⁵Sc NMR. The first sample of organic light-emitting diode (OLED) based on heteroaromatic Sc(III) 1,3-diketonate as an active layer was fabricated and tested. The maximum intensity of yellowish-green electroluminescence (530-560 nm) was equal to 40 cd/m ² and turn-on voltage was about 4.5 V. Further device modification and ligand structure optimization can lead to a better performance of such materials for OLEDs.

Full text of DOI:10.1016/j.ica.2014.02.013

Inorganica Chimica Acta 414 (2014) 234–239
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
An unexpected electro-luminescent properties of scandium(III)
heteroaromatic 1,3-diketonate complex
Ilya Taydakov a, , Andrey Vaschenko , Yuri Strelenko , Alexey Vitukhnovsky
⇑
a
b
a
a
S.I. Vavilov Luminescence Laboratory, P.N. Lebedev Physical Institute, Russian Academy of Sciences, Leninsky Prospect 53, 119991 Moscow, Russian Federation
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47 , 119991 Moscow, Russian Federation
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel complex of Sc(III) with pyrazole substituted 1,3-diketone (1,3-bis(1,3-dimethyl-1H-pyrazol-4-
yl)-1,3-propanedione) was synthesized and characterized by X-ray diffraction, nanotechnology-assisted
laser desorption/ionisation time-of-flight mass spectroscopy (NALDI-TOF-MS) and NMR spectroscopy,
Received 4 November 2013
Received in revised form 4 February 2014
Accepted 10 February 2014
Available online 18 February 2014
45
including Sc NMR. The first sample of organic light-emitting diode (OLED) based on heteroaromatic
Sc(III) 1,3-diketonate as an active layer was fabricated and tested. The maximum intensity of yellow-
ish-green electroluminescence (530–560 nm) was equal to 40 cd/m2 and turn-on voltage was about
Keywords:
4.5 V. Further device modification and ligand structure optimization can lead to a better performance
Scandium(III) complexes
Electroluminescence
OLED
of such materials for OLEDs.
Ó 2014 Elsevier B.V. All rights reserved.
45
Sc NMR
1
. Introduction
spin–orbital interactions. Although this way (ligand-centered
luminescence) is not typical for lanthanide complexes, it becomes
the main route for high-luminescent complexes of non-transitions
Since Kido and co-workers reported [1] about the first electro-
luminescent device based on simple terbium complex Tb(acac)
in 1990, much efforts were made to develop effective lanthanide-
base light emitters for OLED during the last two decades. Com-
plexes of all lanthanides (except for La, Pm, Gd and Lu) were tested
for this application, and the best results were obtained for terbium
3
elements such as tris(8-hydroxyquinolinato)aluminum (Alq
to now Alq is one of the best green luminescent and electron-
transport material [7].
3
). Up
3
Scandium(III) has no f-electrons and electron configurations of
Sc3+ and Al ions are similar. Probably this fact was a starting
point to test Sc(III) complexes as new electroluminescent materi-
als. It was Bochkarev who firstly reported [8] that Tris(8-hydrox-
yquinolinato)scandium was better electroluminescent material
3+
(
green), europium and samarium (red–orange), ytterbium and er-
bium (near infra-red spectral region) compounds. These results
are summarized in several comprehensive reviews [2–5].
The simplified scheme of electroluminescence of lanthanide
complexes involves excitation of triplet and singlet levels of organ-
ic ligand by excitons and succeeding energy transfer to resonance
3
than Alq . Later some promising materials based on O,N- or S,N-do-
nor ligands [9] were developed by researches of Bochkarev’s group.
Scandium-based materials demonstrate bright yellow, green or
white ligand-centered electroluminescence and good electrical
conductivity .
3
+
f-levels of Ln ion by dipole–dipole Föster mechanism [6].
⁄
The f ? f transitions are resulted to the light emission, usually
in a narrow spectral range. The emission spectrum weakly depends
on the nature of ligand. Of course, this transfer may be accompa-
nied by a number of nonradiative processes both ligand and central
ion, and intermolecular energy conversion. All these ways of relax-
ation generally decrease the efficiency of the metal-centered light
generation.
To our best knowledge, Sc(III) complexes based on 1,3-diketo-
nato ligands were early not tested as electroluminescent materials.
Yttrium(III) 1,3-diketonates were reported [10] as dopants for iso-
structural Eu(III) complexes. They improved conductivity but did
not affect on the spectral characteristic of the emission.
A number of complexes with other rare earths ions and pyra-
zole substituted 1,3-diketones obtained by us were described and
discussed early [11].
Due to our continuous interest in synthesis and application of
novel 1,3-diketonato ligands bearing pyrazole moiety, here
we wish to report about synthesis and electroluminescent proper-
ties of scandium(III) tris(1,3-bis(1,3-dimethyl-1H-pyrazol-4-yl)-1,
3-propanedionate).
1
0
Moreover, light may be emitted by S ? S (fluorescence) and
1
0
T ? S (phosphorescence) relaxation pathways directly in the
ligand. The triplet–singlet relaxation is spin forbidden, but this
route can be realized in the presence of heavy atoms due to
⇑
Corresponding author. Tel.: +7 916 1381665.
E-mail address: taidakov@gmail.com (I. Taydakov).
http://dx.doi.org/10.1016/j.ica.2014.02.013
020-1693/Ó 2014 Elsevier B.V. All rights reserved.
0

I. Taydakov et al. / Inorganica Chimica Acta 414 (2014) 234–239
235
4
5
2
. Experimental
Sc NMR (DMSO-d
m(cm ): 1657, 1540, 1465.
6
) d (ppm): 96.9.
ꢀ1
IR
2.1. General information
Ligand was synthesized by the previously described method
2.3. X-ray crystal data
[
12]. All other reagents were purchased from Aldrich and used
without further purification. Elemental analysis was performed
on the Elementar CHNO(S) analyzer.
Single crystals of 2a or 2b suitable for X-ray diffraction were ob-
tained by slow diffusion of MeCN vapor into saturated solution of 2
in DMSO or DMSO–EtOH (1/1 by volume) at the room temperature.
Crystals are not stable in air at the room temperature and lose the
solvent with crystal degeneration easily.
Two different crystal modifications (solvatomorphs [14]) of
complex 2 were obtained from different solvents. Rhombohedral
crystals of 2 ꢂ 3MeCN solvate (2a) were obtained from MeCN–
DMSO mixture while monoclinic crystals of 2 ꢂ MeCN ꢂ EtOH (2b)
were formed in the presence of EtOH. All diffraction data were
collected on the Bruker SMART APEX II CCD diffractometer [k(Mo
Special reagents and ITO coated glass slides (resistance 10–
1
(
5
X
/sq) for OLED fabrication were purchased from Lumtec Corp.
Taiwan).
UV–Vis absorption spectra were recorded on the Perkin-Elmer
Lambda 45 instrument. Luminescent spectra were obtained on
the Perkin-Elmer SL-45 spectrofluorimeter equipped with a fiber
optics probe and xenon flash lamp.
Nanotechnology-assisted laser desorption/ionisation time-of-
flight mass spectrometry spectra (NALDI-TOF-MS) were obtained
on the Bruker Autoflex Speed instrument equipped with nitrogen
pulse laser (337 nm). Samples in form of 2 mg/mL solutions in
EtOH were dropped on the surface of the nano-structured target
Ka) = 0.071072 nm, x-scans]. The substantial redundancy in data
allows empirical absorption correction to be performed with SADABS
[15,16], using multiple measurements of equivalent reflections.
The structures were solved by a direct method and refined by
(
Bruker MSP 96 NALDI target) and dried. Spectra were collected
2
in a positive mode.
the full-matrix least-squares technique against F in the aniso-
TGA-DSC analysis was performed on the Netzsch STA 449C
tropic approximation for all non-hydrogen atoms. In the crystal
of solvate 2b for the disordered ligand (see Fig. 1 in Supplemen-
tary) the refinement was performed with additional constraints
on C–C, C–O and C–N bond lengths and anisotropic displacement
parameters of corresponding atoms.
instrument in Al
00 °C (293–1173 K). Heating rate was 10 K/min, mixture of air
40 mL/min) and Ar (20 mL/min) was used as an oxidation
2 3
O crucibles in the temperature range 20–
9
(
environment.
FTIR spectra were obtained on the Bruker Vector 22 instrument
in KBr pellets.
The positions of hydrogen atoms were calculated and refined
with the riding model in the isotropic approximation. In crystals
of solvate 2b the analysis of the Fourier electron density synthesis
has revealed that solvate molecules are strongly disordered and
with high probability the observed electron density maxima can
be attributed to superposition of ethanol and acetonitrile. We
failed to establish appropriate model for refining of the above
residual electron density corresponding to this molecules. That is
why we had to exclude unresolved solvents by means of SQUEEZE
procedure [17]. Taking into account that disordered ligand pre-
dominantly forms intermolecular interactions with superimposed
solvate it is reasonable to propose that observed disorder of the lat-
ter is the consequence of intermolecular interactions.
6
NMR study was performed in DMSO-d solutions at 298 K on
the Bruker DXR-500 instrument operating at 500.1, 125.8 and
1
13
45
1
(
21.5 MHz for H,
d = 0.00 ppm) was used for 1H and C NMR measurements as a
2
internal standard, and 1.5 M solution of Sc(NO * 5H O in D O
C
and
Sc nucleus respectively. TMS
13
3
)
3
2
4
5
at pH 1 was used as external standard (d = 0.0 ppm) [13] for Sc
NMR measurements.
Current–voltage–brightness characteristics of the organic LEDs
were measured on the Keithley (Keithley, USA) digital multimeter
and the TKA (Russia) luminance meter. The electroluminescence
and photoluminescence spectra were measured on the multi-chan-
nel spectrometer S2000 (Ocean Optics, USA).
All calculations were performed with the SHELXTL software pack-
age [16].
3
2.2. Synthesis and characterization of complex 2 (ScL )
Solution of ScCl
3
was prepared by treatment of Sc
2
O
3
(1.0 g,
2.4. OLED device fabrication
7
.25 mmol) with pure conc. HCl, evaporation of the reaction mass
to dryness and dissolution of the residue in deionized water in a
volumetric flask. Overall volume was brought to 10 mL.
In this study a series of multilayer OLED structures with the
ScL as an emissive layer was prepared. Conductive layer – PED-
3
OT:PSS (poly-ethylene dioxythiophene:poly-styrenosulphonate)
In 7 mL of ethanol 0.575 g (2.17 mmol) of HL was dissolved. To
this solution 0.5 mL (0.725 mmol) of ScCl
3
solution was added with
and a hole transport layer – TPD (N,N-diphenyl-N,N-bi(3-methyl-
0
0
stirring and pH was adjusted to 9 by the carefully addition of aque-
ous ammonia. The reaction mixture was held at 50 °C for 2 h,
cooled and the precipitate was filtered and washed successively
by small portions of water, 30% aqueous ethanol and ether and fi-
phenyl)-[1,1 -diphenyl]-4,4 -diamine) were deposited by the solu-
tion spin-coating technique. The electron transporter layer – TAZ
(3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole), Alq
3
ꢀ2
nally dried at 10 Torr and 45 °C to a constant weight. Yield of the
white crystalline powder is 0.27 g (53%).
Analytical sample was recrystallized from DMSO–CH
2 2
Cl mix-
ture, washed with CH Cl and dried to a constant weight under
2
2
ꢀ2
the diminished pressure (1 ꢁ 10 Torr).
Elemental analysis: Anal. Calc. for C39
45 12 6
H N O Sc: C, 56.93; H,
5
.51; N, 20.43; Sc, 5.46. Found: C, 57.07; H, 5.79; N, 20.67, Sc, 5.51%.
1
H NMR (DMSO-d
6
) d (ppm): 7.95 (br. s, 6H, Pyr–H), 5.9 (s, 3H,
), 2.30 (s, 18H, CH ).
) d (ppm): 177.83 (C@O), 149.96 (C(3)Pyr),
32.09 (C(5)Pyr), 126.61 (C(4) Pyr), 95.54 (CH@), 38.19 (N–CH ),
3.73 (CH ).
C@CH), 3.70 (s, 18H, N–CH
3
3
1
3
C NMR (DMSO-d
6
1
1
3
3
Fig. 1. Synthesis of complex 2.

2
36
I. Taydakov et al. / Inorganica Chimica Acta 414 (2014) 234–239
(
Tris(8-hydroxy-quinolinato)aluminum) and complex
2 were
deposited by thermal evaporation.
ꢀ
5
The evaporation was performed at 10 mbar using resistively
heated tungsten boats. Deposition rate was kept at 0.05–0.2 nm/s
and controlled by the Inficon IC-6000a calibrated microquartz
deposition controller. Aluminum cathode was deposited at
ꢀ
6
5
1
ꢁ 10 mbar at a rate 1–2 nm/s. The emitting area was about
2
0 mm .
3
. Results and discussion
3
3.1. Synthesis of complex 2 (ScL )
Complex 2 was synthesized according to the following scheme
Complex 2 had low solubility in common solvents such as eth-
(
Fig. 1):
anol or dichloromethane and was purified by successively washing
with ethanol and water from starting materials and other impuri-
ties. It can be additionally recrystallized from large amount of hot
EtOH, but solvent vapor diffusion method is better one to obtain
good crystals.
3.2. Crystal structures of 2a and 2b
Crystal data and structure refinement parameters for 2a and 2b
are summarized in Table 1.
Fig. 2. The general view of complex 2a in representation of atoms by thermal
ellipsoids (p = 50%). Hydrogen atoms have been omitted for clarity.
The coordination polyhedron for sixfold coordinated Sc atom in
both solvatemorphs 2a and 2b is a distorted trigonal antiprism. In
the crystal of 2a molecule occupies the 3-fold axis while 2b all
atoms are located in general position. The Sc–O bond distances in
optimal conditions for MS determination of pyrazole substituted
1
,3-diketonates of rare earths [19]. Unfortunately, most of these
compounds are unstable under laser irradiation even in the pres-
ence of different matrixes. More stable and predictable results
were obtained without matrix. Stainless steel (standard type) or
nano-structured targets are both suitable for the analysis, but bet-
ter spectra were obtained with the last one.
2
a and 2b (for ordered ligands) vary in the narrow range
2
.078(1)–2.095(1) Å and almost equal to those found out in the
3
Sc(tfacac) (Htfacac – trifluoroacetylacetone) complex [18]. The
analysis of crystal packing have revealed that all intermolecular
contacts corresponds to the normal Van-der-Waals interactions
Primary ion formation mechanisms in NALDI method are differ
from electron impact (EI) ionization. Two main processes are pre-
dominated – (i) proton transfer, that lead to formation of proton-
of C–Hꢃ ꢃ ꢃ
p
, C–Hꢃ ꢃ ꢃN and Hꢃ ꢃ ꢃH types (see Figs. 2 and 3 in Supple-
mentary) The molecular structure of 2a is shown in Fig 2.
+
3.3. NALDI-MS and TGA studies
ated ions such as [M+H] or (ii) cationization in which ions such
+
+
+
+
4
as [M+Cat] (Cat = NH , Na , K , etc.) are formed. Specific pathway
Application of mass-spectroscopy for the identification of com-
plexes is very attractive task. We have systematically studied an
of the ionization is thermodynamically and kinetically controlled,
but formation of complex ions rather then simple molecular ion
is a common phenomenon in NALDI (and MALDI) mass spectrom-
etry [20].
Table 1
Crystal data and structure refinement parameters for 2a and 2b.
NALDI spectrum of complex 2 is relatively simple. The most in-
2
a
2b
+
tense signal (m/z = 536.788, 100%) was attributed to [ScL
Other signal with m/z = 846.101 (4.5%) was referred to [ScL
2
]
ion.
+Na]+
Empirical formula
Formula weight
T (K)
Crystal system
Space group
C
45
H
54
N
15
O
6
Sc
C
42
H
54
N
13
O
7
Sc
3
945.99
100
rhombohedral
R3
897.94
100
monoclinic
fragment. The formation of more heavy ions was not observed un-
der these conditions. No simple molecular ion was detected in the
spectrum.
P2 /n
1
0
Z(Z )
3(1/3)
4
The first sharp endothermic peak on TGA-DSC curve at 297.3 °C
corresponds to the melting of complex 2. Exothermic decomposi-
tion of the compound 2 started at 419.5 °C and finished at
a (Å)
b (Å)
c (Å)
b (°)3
V (Å )
19.1881(14)
19.1881(14)
11.1360(8)
90.00
3550.8(4)
1.327
11.8292(6)
17.3915(9)
21.7841(11)
93.0564(11)
4475.2(4)
1.333
7
25.1 °C. Weight of the residue (Sc
2
O
3
) was 7.89% at 900 °C (theo-
retical mass 8.38%).
D
l
calc (g cmꢀ3)
(cm
F(000)
ꢀ
1
)
2.2
1494
2.29
1896
3.4. NMR studies
2
h maximum (°)
57
58
35083
Reflections collected (Rint
)
6836 (0.0261)
High sensitivity (0.3 relative to H) of the 45Sc nuclei and mod-
1
(
0.0355)
ꢀ28
2
erate quadrupole moment (ꢀ0.22 ꢁ 10
m ) make it potentially
possible to use Sc NMR for the investigation of its coordination
compounds.
Independent reflections
Observed reflections with I > 2
Number of parameters/restraints
3873
3406
208/1
0.0327
0.0771
1.034
0.287/ꢀ0.265
11875
8578
588/141
0.0553
0.1502
1.056
4
5
r
(I)
R
1
3
Since coordination bonds in the ScL are considerably covalent,
wR
Goodness of fit (GOF) on F
Largest difference peak and hole, e (nm)
2
no ligand exchange was obtained even in the DMSO solutions. An-
other favorable factor is a relatively high symmetry of complex 2 in
the solution. But actually we could not obtain sufficiently narrow
2
ꢀ
4
1.259/ꢀ0.453

I. Taydakov et al. / Inorganica Chimica Acta 414 (2014) 234–239
237
signals. After Lorentzian approximation by the Bruker TopSpin
software the width of the signal at the half of height was estimated
as 4440 ± 26 Hz.
The chemical shift of the 45Sc NMR signal (96.9 ± 0.1 ppm) for
the compound 2 was measured relatively to 1.5 M Sc(NO
3
)
3
(d = 0.0) aqueous solution. Practically the same value (89 ppm)
was reported [13] earlier for Sc(acac)
3
in benzene or CHCl
3
solutions.
Therefore one can assume that the new ligand 1 is slightly
stronger electron acceptor than acetylacetone (Hacac). Unfortu-
3
nately, Sc(acac) seems to be the only one complex with 1,3-dike-
4
5
tone for which Sc NMR chemical shift was reported previously, so
there are no enough data for any reliable correlations.
1
13
The H and C NMR spectra were measured by a common way
and fully matched to the structure of the complex ScL
3
.
3
.5. Photophysical study of 2
Fig. 3. UV–Vis absorption spectra of HL (1, blue line) and complex 2 (2, black line)
in acetonitrile. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
The UV–Vis absorption spectra of HL (1) and complex 2 (2) in
ꢀ5
acetonitrile (solutions with concentrations of 4.4 ꢁ 10
and
ꢀ5
1
.7 ꢁ 10 M, respectively were used) are shown in Fig. 3.
Complex 2 is demonstrated high UV-light absorption in the
wide spectral range (200–380 nm). Absorption maxima in ScL
3
spectrum are red-shifted relative to the spectrum of free ligand
and characterized by high extinction coefficient (26470 at
3
50 nm).
3
The excitation spectrum (1) of ScL crystal (kem = 467 nm) and
the emission spectra of 2 in crystals (2) are shown in Fig. 4.
3
+
As far as Sc has no f-electrons, only the ligand-centered lumi-
nescence can be observed. Weak greenish-blue emission is charac-
terized by relatively wide band of molecular luminescence with
maximum at 467 nm. Emission spectra of bulk crystalline material
and thin film, deposited by high vacuum evaporation on glass slide
(
see below, Fig. 6), are virtually identical. One can assume that
complex 2 is not decomposed during high vacuum deposition. No
phosphorescence was detected at the room temperature.
3.6. Electroluminescent properties of 2
3
Fig. 4. The excitation spectrum (1, blue line) of ScL crystal (kem = 467 nm) and
normalized emission (kex = 337 nm) spectra (2, black line). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
Structures of three different multilayer devices that were fabri-
cated (designated as devices 1–3), and chemical structures of the
materials used in the present study are shown in Fig. 5.
Fig. 5. Structures of OLEDs (devices 1–3).

2
38
I. Taydakov et al. / Inorganica Chimica Acta 414 (2014) 234–239
It is worth noting that volt–current dependencies (Fig. 7) of de-
vices 1 and 2 are similar while device 3 exhibited more sloping
characteristic. This may be attributed to the better hole blocking
ability of TAZ than Alq
. Conclusion
In summary, scandium(III) complex with novel ligand 1,3-
3
.
4
bis(1,3-dimethyl-1H-pyrazol-4-yl)-1,3-propanedionate was syn-
thesized and characterized by X-ray single crystal diffraction, ele-
1
13
45
mental analysis, NALDI-TOF-MS and NMR on H, C and Sc
nucleus. It was demonstrated that this compound formed solvates
while being crystallized from mixed solvents (DMSO–EtOH–
MeCN).
The first sample of OLED device with heteroaromatic Sc(III) 1,3-
diketonato complex as the light-emitting layer was fabricated by
vacuum deposition/spin coating technique and successfully tested.
This OLED emits yellowish light (maximum at 530–560 nm) with
3
Fig. 6. PL spectrum of ScL film (1, green line) and EL spectra of devices 1 (2, black
line), 2 (3, blue dotted line) and 3 (4, red line). (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article.)
2
40 cd/m overall brightness.
The possible mechanism of PL can be explained by emission
⁄
from
p
to
p
states of organic ligand. Thus, 1,3-diketonates of Sc(III)
with heteroaromatic or aromatic ligands are promising candidates
for the fabrication of multilayer electro-optical devices. Further de-
vice modification and optimization can lead to a better perfor-
mance of this material in OLEDs.
Acknowledgments
We are grateful to Prof. Konstantin Lyssenko (X-Ray Structural
Centre Chemistry and Material Sciences Division of A.N.Nesmeya-
nov Institute of Organoelement Compounds RAS) for the perform-
ing of X-ray diffraction experiments and fruitful discussion, and Dr.
Roman Borisov (Laboratory of Mass-spectrometry of A.V.Topchiev
Institute of Petrochemical Synthesis RAS) for the performing of
NALDI-TOF experiments. This work was partially supported by
the Russian Fund for Basic Research (grant No. 12-0300839-A).
Appendix A. Supplementary material
Fig. 7. Volt–current dependencies for devices 1 (1, black line), 2 (2, blue line) and 3
(
3, red line). (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
CCDC 917057 (2a) and No. 917058 (2b) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.ica.2014.02.013.
Volt–current dependencies and electroluminescence (EL) spec-
tra of OLEDs structures with ScL as an emitting layer along with
3
photoluminescence (PL) spectra of 2 are shown in Fig. 6.
The devices exhibited bright yellowish luminance with the
2
maximum brightness of about 40 cd/m . The turn on voltage was
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4
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3
3
3
3
We also found no dependence of wavelength of EL peak on volt-
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be responsible for observed spectral shift. Moreover we found a
slight broadening of emission spectra of 155 nm of devices 2 and
3
1
and 3.
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vice 2 as Alq PL peak is in the same spectral region (530–540 nm).
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[
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[
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[
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