Single-molecule white-light of tris-pyrazolonate-Dy3+ complexes

Article abstract of DOI:10.1016/j.inoche.2019.107573

Based on the self-assembly of the pyrazolone ligand HPMIP (1-phenyl-3-methyl-4-(isobutyryl)-5-pyrazolone), DyCl₃·6H₂O and/or the 5-Br-2,2′-bpy (5-bromo-2,2′-bipyridine), two tris-pyrazolonate-Dy³⁺-complexes [Dy(PMIP)₃(H₂O)₂] (2) and [Dy(PMIP)₃(5-Br-2,2′-bpy)] (5) characteristic of dual-emissive emissions toward single-molecule white-light are obtained, respectively, and in dependence on the effective suppression from the oscillator-induced quenching by the involved 5-Br-2,2′-bpy, the color-compensation of the residual ligands-based strong emission and the Dy³⁺-centered (⁴F_9/2 → ⁶H_J/2 transitions) multiple emissions renders its Dy³⁺-complex 5 an efficient (Φ_em = 4.2%) single-molecule white-light.

Full text of DOI:10.1016/j.inoche.2019.107573

InorganicChemistryCommunications109(2019)107573
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Single-molecule white-light of tris-pyrazolonate-Dy3+ complexes
⁎
⁎
Jiaxiang Liua,1, Qi Shib,1, Yani Hea, , Guorui Fua, Wentao Lia, Tiezheng Miaoa, Xingqiang Lüa,
a School of Chemical Engineering, Northwest University, Xi'an 710069, Shaanxi, China
b School of Chemistry and Chemical Engineering, Xi'an University, Xi'an 710065, Shaanxi, China
G R A P H I C A L A B S T R A C T
Assisted by the ancillary ligand 5-Br-2,2′-bpy, the complex [Dy(PMIP)3(5-Br-2,2′-bpy)] (5) endows more effective suppression from the non-radiative deactivation
than that of [Dy(PMIP)3(H2O)2] (2), and its dual emissions with the residual ligands-based stronger emission, renders the Dy3+-complex 5 more efficient single-
molecule white-light.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Based on the self-assembly of the pyrazolone ligand HPMIP (1-phenyl-3-methyl-4-(isobutyryl)-5-pyrazolone),
DyCl3·6H2O and/or the 5-Br-2,2′-bpy (5-bromo-2,2′-bipyridine), two tris-pyrazolonate-Dy3+-complexes [Dy
(PMIP)3(H2O)2] (2) and [Dy(PMIP)3(5-Br-2,2′-bpy)] (5) characteristic of dual-emissive emissions toward single-
molecule white-light are obtained, respectively, and in dependence on the effective suppression from the os-
cillator-induced quenching by the involved 5-Br-2,2′-bpy, the color-compensation of the residual ligands-based
strong emission and the Dy3+-centered (4F9/2 → 6HJ/2 transitions) multiple emissions renders its Dy3+-complex
5 an efficient (Φem = 4.2%) single-molecule white-light.
Tris-pyrazolonate-Dy3+-complexes
Single-molecule white-light
Partial energy transfer
Organo-Dy3+-contributed white-light, due to the initiation of the
storing [3]. To date, although concerted efforts [4–7] have been de-
voted to Dy3+-based 1D to 3D coordination polymers capable of single-
phase white-light [8], their vacuum-deposition or solution-processable
film-forming infeasibility during device fabrication, restricted by the
non-sublimable and/or insoluble natures, actually limit the utilization
4
Dy3+-centered unique F9/2 → 6HJ/2 (J = 9, 11, 13 and 15) transitions
forward with the multiple visible (blue-yellow-red) spectral region,
attracts a particular attention to white-light-emitting diodes (WLEDs)
[1] apart from the potential applications in sensors [2] and energy
⁎ Corresponding authors.
E-mail addresses: 303482357@qq.com (Y. He), lvxq@nwu.edu.cn (X. Lü).
1 These authors contributed equally and should be first co-authors.
https://doi.org/10.1016/j.inoche.2019.107573
Received 28 August 2019; Received in revised form 4 September 2019; Accepted 7 September 2019
Availableonline09September2019
1387-7003/©2019ElsevierB.V.Allrightsreserved.

J. Liu, et al.
InorganicChemistryCommunications109(2019)107573
in white organic/polymer light-emitting diodes (WOLEDs/WPLEDs). By
contrast, the circumvention of such problems can rely on small-mole-
cule organo-Dy3+-complex [9] or its doping [10] into an appropriate
polymeric matrix, where also through organo-Dy3+-within color-com-
pensations, the electric-driven white-lights can be realized for their
reliable WOLEDs/WPLEDs.
As a matter of fact, based on the sensitization of the Dy3+-centered
multiple (4F9/2 → 6HJ/2; J = 9, 11, 13 and 15) visible emissions, single-
molecule white-light can be expected for an organo-Dy3+ complex to-
ward simplified WOLEDs, advantageous of high thermal character, and
good white-light stability over those of its doping [10] and single-phase
coordination polymer systems [4–7]. On one hand, through the effec-
tive sensitization and the substantially complete energy transfer from
an appropriate chromophore, the concurrent emissions of both Dy3+
-
centered blue-light (λem = 480 nm) arisen from a magnetic dipole
transition (4F9 → 6H15/2) and yellow-light (λem = 572 nm) as a hy-
persensitive electric dipole transition (4F9 → 6H13/2) are definitely ob-
tained, and convincingly, in dependence on the crystal-field adjustment
ruled with the sensitivity to the electric-dipole-governed transition, the
dichromaticity-integrated color from blue to white and to yellow-light
can theoretically be achieved. Nonetheless, suffering from the in-
sensitivity of the magnetic-dipole-transited blue-light to the crystal-
field, the usual low ratio of the blue to yellow emission, enables a great
challenge to one organo-Dy3+ complex with an efficient simple-mole-
cule white-light. Clearly, the dichromatic-integration just with two
Dy3+-centered sharp visible emissions does not cover the broad
400–700 nm spectral region, giving another parallel challenge to single-
molecule white-light with the good color-rendering property from one
organo-Dy3+ complex. Alternatively, if the dual emissions from both
the sensitizer and the Dy3+ ion are adopted as the complementary
colors, simple-molecule white-light should be smoothly approached for
the specific organo-Dy3+ complex. In this context, after the exploration
on single-molecule white-light-emitting organo-Dy3+ complexes with
the chromophores from flexible carboxylate ([Dy(Lc)3(Phen)] [11]) to
diacetate ([Dy(ODA)(Phen)(H2O)4] [12]) ligands, the Dy3+-based β-
diketonate complexes ({[M[Dy(acac)4]} [13] or ([Zn2(Salen)2(DyxEu1-
x)2(hfac)2] [14]) with more favorable sensitization process, earn back
their place with relatively higher single-molecule white-light effi-
Scheme 1. Synthetic scheme of the two kinds of tris-pyrazolonate-Ln3+ com-
plexes [Ln(PMIP)3(H2O)2] (Ln = La, 1; Dy, 2 or Gd, 3) and [Ln(PMIP)3(5-Br-
2,2′-bpy)] (Ln = La, 4; Dy, 5 or Gd, 6).
THF-within solvents prefer to the production of [Ln(PMIP)3]2 com-
plexes [18]. As to the binary tris-pyrazolonate-Ln3+ complexes [Ln
(PMIP)3(5-Br-2,2′-bpy)2] (Ln = La, 4; Dy, 5 or Gd, 6), they were syn-
thesized from the direct self-assembly (Method A) of LnCl3·6H2O
(Ln = La, Dy or Gd), the HPMIP and the 5-Br-2,2′-bpy at a molar ratio
of 1:3:1 in the presence of NaOH, respectively. Convincingly, their high
yields with the 5-Br-2,2′-bpy involvement should be due to the effec-
tive suppression of [Ln(PMIP)3]2 intermediates [18]. On the other
hand, despite the success of their sequential synthesis (Method B) by
reacting [Ln(PMIP)3(H2O)2] (Ln = La, 1; Dy, 2 or Gd, 3) with the 5-Br-
2,2′-bpy as an alternative, the relatively lower yield and hyperthermic
treatment should be faced.
The two series of tris-pyrazolonate-Ln3+ complexes [Ln
(PMIP)3(H2O)2] (Ln = La, 1; Dy, 2 or Gd, 3) and [Ln(PMIP)3(5-Br-2,2′-
bpy)2] (Ln = La, 4; Dy, 5 or Gd, 6) were well-characterized by EA, FT-
IR, 1H NMR and ESI-MS. Especially in the FT-IR spectra of complexes
4–6, besides the absence of a broad absorption in the region of
3000–3700 cm−1 as compared with those of complexes 1–3, the addi-
tional strong absorptions (767–773 cm−1) assigned to the ν(C-Br) vi-
bration are also observed, implying that no water molecules surround
the Ln3+ ions for the three complexes 4–6 due to the involvement of
one 5-Br-2,2′-bpy. X-ray quality single-crystals were obtained for the
representative complex [La(PMIP)3(H2O)2]·CH2Cl2 (1·CH2Cl2) with
crystallographic data in Tables 1–2S. Complex 1·CH2Cl2 crystallizes in
the monoclinic P2(1)/c space group. As shown in Fig. 1, three pyr-
azolonate (PMIP)− ligands with the same O^O-chelate mode (O1^O2,
O3^O4 or O5^O6) and two coordinated H2O (O7 and O8) coordinate to
the central La3+ ion (La1) in a square anti-prismatic pattern, featuring
the typical dihydrate tris-pyrazolonate-Ln3+ complexes [19]. The two
coordination H2O molecules occupy in the trans-position (148.01(18)°
of O7-La1-O8) within the square place, confirming the hyperthermic
desirability during the replacement by the 5-Br-2,2′-bpy. In the 1H
NMR spectrum of [La(PMIP)3(5-Br-2,2′-bpy)] (5), besides the addi-
tional proton resonance (δ = 7.49–8.83 ppm) of the 5-Br-2,2′-bpy, the
stipulated proton molar ratio (3:1) of (PMIP)− to 5-Br-2,2′-bpy further
verifies its specific binary tris-pyrazolonate-La3+ component.
4
ciencies. However, considering the lowest excited energy level of F9/2
at 20830 cm−1 of Dy3+ ion more close to that (20,545 cm−1) of 5D4 for
Tb3+ ion than Eu3+ ion (17,286 cm−1 of 5D0), typical pyrazolonate
ligands with relatively higher 3π-π* energy levels than those of common
β-diketonate ligands, should endow much more opportunities [15] to
effectively sensitize the Dy3+-centered multiple visible emissions.
Herein, selecting the pyrazolone ligand HPMIP with the 3π-π* energy
level (23,000 cm−1) [16] as the chromophore in its [Dy(PMIP)3(H2O)2]
(2), the residual emission of the sensitizer is well color-compensated
with the visible emission of Dy3+ ion, a deserved single-molecule
white-light for [Dy(PMIP)3(H2O)2] (2) is firstly observed through that
dual-emissive strategy. Moreover, starting from the prevention of the
Dy3+ ion from a direct oscillator’ coordination, the binary tris-pyr-
azolonate-Ln3+ complexes [Dy(PMIP)3(5-Br-2,2′-bpy)2] (5) with 5-Br-
2,2′-bpy as the ancillary ligand is constructed, and its single-molecule
white-light with an enhanced luminescent efficiency is also expected.
As shown in Scheme 1, the pyrazolone ligand HPMIP was synthe-
sized from the nucleophilic replacement reaction [16] of 1-phenyl-3-
methyl-pyrazolone-5 (PMP) and isobutyryl chloride. The N,N-chelate
ancillary ligand 5-Br-2,2′-bpy was synthesized by the Stille coupling
reaction [17] from 2,5-dibromopyridine and 2-(tributylstannyl)pyr-
idine in the presence of Pd(PPh3)4. Based on the reaction of LnCl3·6H2O
(Ln = La, Dy or Gd) with the HPMIP at a molar ratio of 1:3 in the
presence of NaOH, the series of dihydrate tris-pyrazolonate-Ln3+
complexes [Ln(PMIP)3(H2O)2] (Ln = La, 1; Dy, 2 or Gd, 3) were rou-
tinely obtained, respectively. Worthy of notice, the formation of the
complexes [Ln(PMIP)3(H2O)2] (Ln = La, 1; Dy, 2 or Gd, 3) as the main
products is strictly relative to the water and/or alcohol medium, while
Photophysical properties of complexes [Ln(PMIP)3(H2O)2] (Dy, 2 or
Gd, 3) and [Ln(PMIP)3(5-Br-2,2′-bpy)2] (Dy, 5 or Gd, 6) have been
examined in MeCN solution at room temperature or 77 K, and sum-
marized in Figs. 2–3 and 1–2s. As shown in Fig. 1s, similar to the
(PMIP)−-centered absorption spectra of 235–237 and 263–266 nm for
complexes 2–3 in the UV–visible region, complexes 5–6 exhibit the
combined HPMIP and 5-Br-2,2′bpy absorptions at 250–252 and
2

J. Liu, et al.
InorganicChemistryCommunications109(2019)107573
270–273 nm, where the lower energy absorption (263–266 nm or
270–273 nm) should be assigned to the intraligand π-π* transition. For
complex 2, in addition to the weak ligand-based emission
(λem = 383 nm) shown in Fig. 2, photo-excitation upon λex = 336 nm
gives rise to the Dy3+-characteristic line-like emissions at 484, 578, 628
and 665 nm (4F9/2 → 6HJ/2; J = 9, 11, 13 and 15). On one hand, the
presence of the residual ligand-based fluorescent luminescence
(τ = 1.02 ns at 383 nm) indicates an incomplete chromophore-to-Dy3+
energy transfer. And within the two strong Dy3+-centered emissions
(484 and 578 nm) featuring
a typical phosphorescent nature
(τ = 0.98 μs at 578 nm), there has a much stronger intensity (a ratio of
4
4
2.4) of the F9/2 → 6H13/2 (electric-dipole) transition than that F9/2
→
6H15/2 (magnetic-dipole) transition. Therefore, the dominated Dy3+
-
centered emissions especially with the hypersenstitive 578 nm, render
the dual-emissive complex 2 a single-molecule white-light (Φem = 1.4%
and CIE chromatic coordinate x = 0.374 and y = 0.378, CCT = 3450 K
and CRI = 96). As to complex 5 with the 5-Br-2,2′-bpy involvement,
besides the similar dual-emissive character to that of complex 2, the
residual ligand-based emission (380 nm; τ = 1.34 ns) is strong enough,
also as shown in Fig. 2, and almost comparable to the Dy3+-centered
hypersensitive emission (τ = 11.3 μs) at 578 nm, also giving a single-
molecule white-light (CIE chromatic coordinate x = 0.287 and
y = 0.252; CCT of 3506 K and CRI of 95) while an enhanced quantum
yield (Φem = 4.2%) for complex 5. In comparison, by the replacement
of the two coordinated H2O molecules in complex 2 with the 5-Br-2,2′-
bpy for complex 5, the slightly decreased emissive intensity ratio of 2.0
Fig. 1. Perspective drawing of [La(PMIP)3(H2O)2]·CH2Cl2 (1·CH2Cl2), H atoms
and solvates CH2Cl2 are omitted for clarity.
4
4
between the hypersenstitive F9/2 → 6H13/2 and the F9/2 → 6H15/2
transition, suggests the tiny influence on the asymmetric coordination
sphere of the central Dy3+ ion within. However, the distinctively longer
phosphorescent lifetime (τ = 11.3 μs) centered at the Dy3+-centered
hypersensitive emission (578 nm) for complex 5 than that (τ = 0.98 μs)
of complex 2, should contribute from the avoidance of vibrational
quenching [20] in the inner coordination sphere of the Dy3+ ion be-
sides the Br-endowed heavy-atom effect.
To further address the single-molecule white-light sensitization
mechanism on the two Dy3+-complexes 2 and 5, the emissive behaviors
of their isomeric Gd3+-complexes 3 and 6 as the references are explored
at room temperature or 77 K, respectively. Contrary to the ligand-based
fluorescence (λem = 383 nm and τ = 1.46 ns for 3; λem = 380 nm and
τ = 1.68 ns for 6) at room temperature, as shown in Fig. 2s, each of the
Gd3+-complexes 3 and 6 exhibits the typical 0–0 transition phosphor-
escence (λem = 435 nm and τ = 1.12 μs for 3; λem = 456 nm and
τ = 17.6 μs for 6) at 77 K, from which, the corresponding triplet (3π-π*)
energy level of 23,000 cm−1 for 3 or 21,930 cm−1 for 6 is obtained,
respectively. The lower 3π-π* energy level of complex 6 than that of
complex 3, should be arisen from the increased molecular conjugation
effect of the 5-Br-2,2′-bpy inducement. With regard to the singlet (1π-
π*) energy levels estimated by the UV–visible absorbance edge, the
usage of 5-Br-2,2′-bpy also renders a slightly lower energy-state
(355 nm) for complex 6 than that (350 nm) of complex 3. From the
chromophore-to-Dy3+ energy transfer of the two Dy3+-complexes 2
and 5, their larger 1π-π*-3π-π* energy gaps (5571–6239 cm−1) than
5000 cm−1, as shown in Fig. 3, endow the effective intersystem crossing
process according to the Reinhoudt's empirical rule [21]. Moreover,
further by checking the second energy gap between every 3π-π* energy
level and the lowest excited one (4F9/2; 20,830 cm−1) of Dy3+ ion, all
the two 3π-π*-4F9/2 energy gaps (2170 cm−1 for 2 or 1100 cm−1 for 5)
are small enough to allow for the back energy transfer [20]. And thus,
the dual emissions consisting of the ligand-based residual emission and
Fig. 2. Normalized emission spectra and corresponding CIE chromatic co-
ordinates (inset) of [Dy(PMIP)3(H2O)2] (2) upon λex = 336 nm and [Dy
(PMIP)3(5-Br-2,2′-bpy)] (5) upon λex = 334 nm in dilute MeCN solution at
room temperature.
4
the Dy3+-centered F9/2 → 6HJ/2 (J = 9, 11, 13 and 15) multiple
emissions, are confirmed for the two Dy3+-complexes 2 and 5. Whereas
even though the stronger residual ligands-based emission for complex 5
than that of complex 2 is induced by the smaller 3π-π*-4F9/2 energy gap,
their residual ligands-based emission (λem = 380 nm) is color-com-
pensated enough to give the desirable single-molecule white-lights.
Worthy of noting, the single-molecule white-light efficiency
Fig. 3. Schematic energy level diagram and possible energy transfer process for
[Dy(PMIP)3(H2O)2] (2) and [Dy(PMIP)3(5-Br-2,2′-bpy)] (5) with single-mole-
cule white-lights.
3

J. Liu, et al.
InorganicChemistryCommunications109(2019)107573
(Φem = 4.2%) of complex 5 is three-times larger than that of complex 2
or reported Dy3+-complexes [11–15], which should mainly be due to
the effective suppression [20] from the oscillator-induced quenching or
the non-radiative deactivation by the involved 5-Br-2,2′-bpy in addi-
tion to the strengthened optical absorbance and the suitable excitation.
In conclusion, using the pyrazolone HPMIP ligand as the O^O-che-
late main ligand and/or 5-Br-2,2′-bpy as the N^N-chelate ancillary li-
gand, two tris-pyrazolonate-Dy3+ complexes [Dy(PMIP)3(H2O)2] (2)
and [Dy(PMIP)3(5-Br-2,2′-bpy)2] (5) characteristic of dual-emissive
emissions toward single-molecule white-lights are obtained, respec-
tively. Moreover, in dependence on the effective suppression of the
oscillator-induced quenching for complex 5, the residual ligands-based
emission (λem = 380 nm) is strong enough and color-compensated with
the Dy3+-centered (4F9/2 → 6HJ/2 transitions) multiple emissions for its
desirable more efficient single-molecule white-light.
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