Color-tunable white-light of binary tris-β-diketonate-(Dy3+, Gd3+ x) complexes’ blend under single wavelength excitation

Article abstract of DOI:10.1016/j.inoche.2020.107814

Based on the Dy³⁺-centered yellow-light and the ligands-based blue-light of the iso-structural two complexes [Ln(acac)₃(5-Br-2,2′-bpy)] (Ln³⁺ = Dy³⁺ (2) or Gd³⁺ (3); Hacac = acetylacetone, 5-Br-2,2′-b

Full text of DOI:10.1016/j.inoche.2020.107814

InorganicChemistryCommunications113(2020)107814
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Color-tunable white-light of binary tris-β-diketonate-(Dy3+, Gd3+
)
x
complexes’ blend under single wavelength excitation
⁎
Qi Shia,1, Jiaxiang Liub,1, , Jia Wanga, Xiaohui Yanga, Xingmei Zhanga, Shuna Lia, Ping Suna,
⁎
Jin Chena, Beibei Lia, Xingqiang Lüb,
a School of Chemistry and Chemical Engineering, Xi’an University, Xi’an 710065, Shaanxi, China
b School of Chemical Engineering, Shaanxi Key Laboratory of Degradable Medical Materials, Northwest University, Xi’an 710069, Shaanxi, China
G R A P H I C A L A B S T R A C T
Based on the Dy3+-centered yellow-light and the ligands-based blue-light of the tris-β-diketonate-(Dy3+, Gd3+x)-mixed complex [Ln(acac)3(5-Br-2,2′-bpy)] (Ln3+
=
Dy3+, Gd3+x), the stoichiometrically dichromatic integration gives the color-tunable white-light in solution or solid-state.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Based on the Dy3+-centered yellow-light and the ligands-based blue-light of the iso-structural two complexes [Ln
(acac)3(5-Br-2,2′-bpy)] (Ln3+ = Dy3+ (2) or Gd3+ (3); Hacac = acetylacetone, 5-Br-2,2′-bpy = 5-bromo-2,2′-
bipyridine), respectively, the stoichiometric fluorescence titrations of their tris-β-diketonate-(Dy3+, Gd3+x)-
mixed complex, show that it is capable of the smooth color-tuning (yellow- to white- and to blue-light) under
Tris-β-diketonate-(Dy3+, Gd3+x) complex
Dichromatic white-light
Absorption
Fluorescence titrations
single wavelength excitation. Moreover, through the dichromatic integration, the binary tris-β-diketonate-(Dy3+
,
Gd3+x) complex exhibits the straightforward white-light in solid-state.
1. Introduction
(ISC; 1S → 1T) and the 1T → Dy3+* transfer for one specific organo-
Dy3+ chromophore. Especially in consideration of its two prominent
Contributing from the receptive “antenna” effect [1], the Dy3+
-
yellow-light (578 nm; 4F5/2
→
6H13/2) and blue-light (484 nm; 4F5/2
→
centered characteristic emissions (4F5/2
→
6HJ/2; J = 9, 11, 13 or 15)
6H15/2) line-like emission bands within the visible region characteristic
of ideal color-primary components toward straightforward white-light,
concerted efforts have been devoted to white-light-emissive organo-
[2] can be sensitized through the Laporte- and spin-allowed ligand-
centered transition (0S → 1S) followed by the inter-system crossing
⁎ Corresponding authors.
E-mail addresses: 981125020@qq.com (J. Liu), lvxq@nwu.edu.cn (X. Lü).
1 Both authors contributed equally to the study.
https://doi.org/10.1016/j.inoche.2020.107814
Received 14 January 2020; Received in revised form 23 January 2020; Accepted 24 January 2020
Availableonline27January2020
1387-7003/©2020ElsevierB.V.Allrightsreserved.

Q. Shi, et al.
InorganicChemistryCommunications113(2020)107814
(acac)3(5-Br-2,2′-bpy)] (3), can be definitely assigned to the major
species [M−H]+, respectively. These observations confirm that each
binary tris-β-diketonate-Ln3+ species of the three iso-structural com-
plexes 1–3, can keep stable in the respective solution.
Dy3+ materials promising for optoelectronic devices [3] and sensing
platforms [4]. In principle, through the effective sensitization and the
subsequently complete energy transfer for one specific organo-Dy3+
complex, one feasible strategy to direct white-light seems highly at-
tractive after the simultaneous emissions of both Dy3+-centered blue-
The photo-physical properties of the binary tris-β-diketonate-Ln3+
complex 2–3 were examined in dilute MeCN solutions at room tem-
perature or 77 K, and summarized in Figs. 1 and 1S. In contrast to the
strong absorption bands (Fig. 1S) limited to the λab < 350 nm range
for the two (Hacac and 5-Br-2,2′-bpy) free ligands, the complex 2–3
display the similar ligands-based (248–250 and 284–287 nm) while
significantly broadened (200–400 nm) absorption spectra, in which, the
lower strong one (284–287 nm) should be assigned to the ligands-based
π-π*-transitions. Moreover, upon Ln3+coordination, the molar ab-
sorption coefficients of the complexes 2–3 at the lower energy ab-
sorption peaks are almost three orders of magnitudes larger than those
of the two (Hacac and 5-Br-2,2′-bpy) free ligands, also indicative of
their tris-β-diketonate-Ln3+ component with the enhanced π-conjuga-
tion effect. For the Dy3+-based complex 2, upon photo-excitation of the
chromohphores at the range of 200–410 nm (λex = 340 nm), as shown
in Fig. 1, the strong Dy3+-characteristic line-like emissions (484 nm
(4F9/2 → 6H15/2 transition), 578 nm (4F5/2 → 6H13/2 transition), 629 nm
light (λem = 478 nm; 4F9
→
6H15/2 magnetic dipole transition) and
yellow-light (λem = 572 nm; 4F9
→
6H13/2 electric dipole transition)
with a comparable intensity [5]. Nonetheless, due to the ligands-field
adjustment [6] just hyper-sensitive to the electric-dipole-governed
transition while insensitive to the magnetic dipole transition, the
dominated yellow-light as a universal basis, renders the Dy3+-exclusive
white-light greatly challenging [7] to one certain organo-Dy3+ com-
plex. Meanwhile, the dichromatic-integration just with the two Dy3+
-
centered sharp visible emissions, does not cover the broad 400–700 nm
spectral region, also suffering from the inferior color-rendering prop-
erty [8]. By contrast, through the ligands-based residual blue-light
color-compensated with the Dy3+-dominated yellow-light, single-mo-
lecule dichromatic white-light [9] could be smoothly approached for
the specific organo-Dy3+ complex. However, this alternative strategy
still suffers from an intrinsic obstacle of having unsatisfactory white-
light efficiency (Φ < 5%) caused by the partial energy transfer.
Convincingly, the circumvention of such problem can rely on (Dy,
Gd)-[10], (Dy, Eu)-[11], (Dy, Eu, Gd)-[12], (Dy, Sm, Gd)-[13], (Dy, La,
Sm)-[14], (Dy, Eu, Tb)-blended [15] metal-organic frameworks (MOFs)
or coordination polymers, where in dependence on a precise control of
the Ln3+-mixed stoichiometry, different color-compensatory contribu-
tions could be well balanced for efficient and high-quality white-lights.
However, despite their potential applications [3a] for white light-
emitting diodes (WLEDs), the detrimental deficiency of their processing
(vacuum-deposition or solution-process) inability, limits the utilization
in flexible white organic/polymer light-emitting diodes (WOLEDs/
WPLEDs) [3b–3d]. As a matter of fact, from the perspective of film-
forming capability necessary, the Ln3+-mixed complexes’ blending to-
ward direct white-light, does conceptually take effect on WOLEDs/
WPLEDs. Herein, in light of the color-tunable white-light of (Eu, Tb)-
mixed complexes [16] disadvantageous of unmanageable Tb3+-to-Eu3+
energy transfer, the (Dy, Gd) complexes’ blending should be more
worthy of motivation, because the patent absence of Dy3+-to-Gd3+
energy transfer is unambiguous, due to the extremely higher energy
level of Gd3+ ion [17]. Moreover, based on the yellow-plus-blue in-
tegration, one certain (Dy3+, Gd3+x) complexes’ blending is also cap-
able of direct white-light. Interestingly, with the judicious adjustment
of the (Dy3+, Gd3+x)-mixed ratio, the dichromatic white-light endowed
from the two isomeric Ln3+-complexes, can be easily realized under
single wavelength excitation.
(4F5/2
→
6H11/2 transition) and 665 nm (4F5/2
→
6H9/2 transition)) and
the weak while detectable emission peaking at λem = 388 nm, are
concurrently observed. For the dual-emissive complex 2 featuring a
bright yellow-light with the CIE (Commission International De L’E-
clairage) chromatic coordinate × = 0.400 and y = 0.401, the ligands-
based residual (λem = 388 nm) emission should be assigned to the
intra-ligands π-π* transition, and the hyper-sensitive peak at 578 nm
4
from the F5/2
→
6H13/2 transition should be resulted from its low
molecular symmetry [19]. Moreover, its dual-emitting nature, can
further be confirmed with the lifetimes-decayed combination of the
ligands-based fluorescence (τ = 1.13 ns; λem = 388 nm) and the Dy3+
-
centered phosphorescence (τ = 3.2 μs at λem = 578 nm; 4F5/2 → 6H13/2
transition) from the same chromophores. As to the Gd3+-based complex
3 in solution at room temperature, it displays the typically ligands-
based fluorescence (λem = 394 nm, τ = 1.02 ns and Φem = 4.7%) also
shown in Fig. 1, exhibiting a blue-light with the CIE chromatic co-
ordinate × = 0.203 and y = 0.202. In contrast, the Gd3+-centered
complex 3 in solution at 77 K, shows the 0–0 transition phosphores-
cence (λem = 433 nm andτ = 0.98 μs; also Fig. 1), from which, the
triplet (3π-π*) energy level of 23095 cm−1 is obtained. With regard to
the singlet (1π-π*) energy level, it (27855 cm−1) can be reasonably
estimated from the lower wavelength (359 nm) of its UV–visible ab-
sorbance edge, and thus, the first energy gap ΔE1 (1π-π* - 3π-π*,
4760 cm−1; Fig. 2S) near to the desirable 5000 cm−1, endows a rela-
tively effective ISC process according to the Reinhoudt’s empirical rule
[20]. Importantly, further checking the energy level match between the
As shown in Scheme 1, the N,N’-chelate ancillary ligand 5-Br-2,2′-
bpy was synthesized in the yield of 67% from the well-established Stille
coupling reaction between 2,5-dibromopyridine and 2-(tributylstannyl)
pyridine in the presence of Pd(PPh3)4 (0) as the literature [18]. Further
through the one-pot reaction of LnCl3⋅6H2O (Ln = La, Dy, or Gd), the
N,N’-chelate ancillary ligand 5-Br-2,2′-bpyand the β-diketone ligand
Hacac treated with an equimolar amount of anhydrous NaOH, the
series of binary tris-β-diketonate-Ln3+ complexes [Ln(acac)3(5-Br-2,2′-
bpy)] (Ln = La (1); Dy (2); or Gd (3)) were self-assembled in receptive
yields of 68–75%, respectively.
4
ligands-based 3π-π* energy and the lowest excited state F5/2
(20830 cm−1) of Dy3+ ion, the second energy gap ΔE2 (2265 cm−1; 3π-
π*-4F5/2) is beyond the ideal 2500-–4500 range ruled by the Latva’s
empirical rule [21], and thus, the allowed back energy transfer should
be the reason to the dual emissions of the Dy3+-based complex 2.
Noticeably, the absolute quantum efficiency (Φem) of the complex 2,
characteristic of the Dy3+-endowed yellow-light, is up to 5.8%, which
is at the top level among previously reported organo-Dy3+-complexes
[3b–3d,8–9], which should be mainly due to the strengthened optical
absorbance and the effective suppression from the oscillator-induced
quenching [22] or the non-radiative deactivation by the 5-Br-2,2′-bpy-
ancillary involvement.
All of the three complexes 1–3, soluble in common organic solvents,
were well characterized by EA, FT-IR, 1H NMR and ESI-MS. Especially
in the 1H NMR spectrum of the anti-ferromagnetic [La(acac)3(5-Br-2,2′-
bpy)] (1), the combined proton resonances (δ = 8.82–1.73 ppm) of
both the deprotonated ligand (acac)− and the ancillary ligand 5-Br-
2,2′-bpy are observed with a stipulated molar ratio of 3:1. Meanwhile,
the presence of the typically enolic eCH]C proton singlet peak at
In consideration of the relatively higher efficiencies for the yellow-
light of the Dy3+-based complex 2 and the blue-emitting of the isomeric
Gd3+-based complex 3, it is particular interest on their binary tris-β-
diketonate-(Dy3+, Gd3+x) complexes’ blend toward the dichromatic
white-light modulation as desirable. After adding different amounts of
δ = 5.19 ppm of the (acac)− ligand should be resulted from the La3+
-
coordination. Moreover, based on the ESI-MS results of the complexes
1–3, a strong mass peak at m/z 672.02 for [La(acac)3(5-Br-2,2′-bpy)]
(1), 695.91 for [Dy(acac)3(5-Br-2,2′-bpy)] (2)or 691.04 for [Gd
the Gd3+-based complex
3
to the Dy3+-based complex 2, the
UV–visible absorption and fluorescence titrations in MeCN solution
2

Q. Shi, et al.
InorganicChemistryCommunications113(2020)107814
Scheme 1. Synthetic scheme of the ancillary ligand 5-Br-2,2′-bpy and its binary tris-β-diketonate-Ln3+ complexes 1–3 (Ln3+ = La3+ (1); Ln3+ = Dy3+ (2) or Ln3+
= Gd3+ (3)).
Fig. 1. The excitation and visible emission spectra of the complexes [Dy
(acac)3(5-Br-2,2′-bpy)] (2) and [Gd(acac)3(5-Br-2,2′-bpy)] (3) in dilute MeCN
Fig. 2. The UV–visible absorption titration at room temperature of the binary
tris-β-diketonate-Dy3+ complex 2 in MeCN solution with its isomeric binary tris-
β-diketonate-Gd3+ complex 3 in MeCN solution.
solution (1 × 10−5 M) at room temperature or 77 K.
were examined at room temperature, respectively. As shown in Fig. 2,
the UV–visible absorption titration of the binary tris-β-diketonate-
(Dy3+, Gd3+x) complexes’ blending system, displays that both of the
characteristic emissions (4F5/2→6HJ/2; J = 9, 11, 13 or 15) are si-
multaneously observed and cover the broad-ranging 350–750 nm.
Moreover, for the binary tris-β-diketonate-(Dy3+, Gdx3+) complexes, the
gradual waxing of the combined (350–450 nm) emission intensity,
two absorbance at 250 nm and 284 nm of the Dy3+-based complex 2 in
3+
solution monotonously increase with the increasing ((Dy3+, Gd ) to
0.02
3+
(Dy3+, Gd )) of the concentration of the Gd3+-based complex 3.
1.04
trades off the gradual waning of the two (572 and 478 nm) Dy3+
-
Whereas besides the absence of an isosbestic point, each absorbance is
almost proportional to the (Dy3+, Gd3+)-mixed total (1 + x) content,
which strictly adhered to the Lambert-Beer law [23] with the R of 0.993
or 0.996, should be arisen from the physical coexistence of two isomeric
complexes 2 and 3 without intermolecular interactions in dilute solu-
characteristic dominated and ratio-fixed emission intensities. Interest-
ingly, after the introduction of the Gd3+-based complex 3, their in-
tegrated colors change from yellow-light (CIE chromatic co-
3+
ordinates × = 0.385, y = 0.386; (Dy3+, Gd )) to white-lights (CIE
0.02
chromatic coordinates × = 0.258–0.337, y = 0.250–0.324, CCTs of
tions. As to the fluorescence titration (λex = 340 nm) shown Table 1S
3+
3+
3628–4153 K and CRIs of 90–92; (Dy3+, Gd ) to (Dy3+, Gd )) and
0.08
0.92
and in Fig. 3, during the amount increasing ((Dy3+, Gd ) to (Dy3+
,
3+
0.02
to blue-lights (CIE chromatic coordinates
×
=
0.230–0.250,
Gd31+.04)) of the Gd3+-based complex 3, its emission and the ligands’
residual emission of the Dy3+-based complex 2 are combined in the
350–450 nm range, which, together with the Dy3+-centered
y = 0.207–0.229; (Dy3+, Gd ) to (Dy3+, Gd )). And during the
3+
3+
0.98
1.04
linear shifting of the color-point beginning from the yellow-light of the
Dy3+-based complex 2 and ending at the blue-light of the Gd3+-based
3

Q. Shi, et al.
InorganicChemistryCommunications113(2020)107814
2. Conclusions
In summary, through the fluorescence titration of the yellow-light-
emitting complex [Dy(acac)3(5-Br-2,2′-bpy)] (2) with its isomeric blue-
light-emissive complex [Gd(acac)3(5-Br-2,2′-bpy)] (3), the dichromatic
balance is strictly dependent on the (Dy3+, Gd3+x)-mixed content,
exhibiting the smoothly adjustable integration colors from yellow-lights
to white-lights and blue-lights. This result renders the binary tris-β-di-
ketonate-(Dy3+, Gd3+x) complexes’ blending a new platform to solid-
state white-light-emitting materials, whose performance to be improved
by the future structural optimization, is now under way.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Fig. 3. The fluorescence titration upon λex = 340 nm at room temperature of
the binary tris-β-diketonate-Dy3+ complex 2 in MeCN solution with its isomeric
binary tris-β-diketonate-Gd3+ complex 3 in MeCN solution.
Acknowledgements
This work is funded by the National Natural Science Foundation
(21373160, 21173165, 51704240), the Education Committee of
Shaanxi Province (17JK1125), the Xi’an Science and Technology Plan
Innovation Funds (2017CDWL-10, 2017CDWL-21, 2017CDWL-23,
2017CDWL-24, 2017CDWL-27, 2017CDWL-28) and the Training
Program of Innovation and Entrepreneurship for Undergraduate of
Xi’an University (2019058) in China.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.inoche.2020.107814.
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