Single-component Eu3+-Tb3+-Gd3+-grafted polymer with ultra-high color rendering index white-light emission

Article abstract of DOI:10.1039/c6ra26724f

Through an approach of pre-coordination and following terpolymerization from vinyl-functionalized complex monomers [Ln(TTA)₃(4-VB-PBI)] (Ln = Eu, 2; Ln = Tb, 3 and Ln = Gd, 4; HTTA = 2-thenoyltrifluoroacetonate; 4-VB-PBI = 1-(4-vinylbenzyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole) with methyl methacrylate, the first example of a single-component Eu²⁺-Tb³⁺-Gd³⁺-grafted polymer Poly(MMA-co-2-co-3-co-4) was obtained. Moreover, control of an optimal dye content for suppressing Tb³⁺ to Eu³⁺ energy transfer and exciton formation provides new perspectives for hetero-Ln³⁺-grafted polymers as featured in Poly(MMA-co-2-co-3-co-4) with color-tuning to direct white-light (Commission International De L'Eclairage coordinates of x = 0.322, y = 0.331; corrected color temperature of 5979 K; and color rendering index up to 94), including a highly luminous efficiency of 17.8%.

Full text of DOI:10.1039/c6ra26724f

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Single-component Eu3+–Tb3+–Gd3+-grafted
polymer with ultra-high color rendering index
Cite this: RSC Adv., 2017, 7, 6762
white-light emission†
a
a
a
a
Lin Liu, Guorui Fu, Baoning Li, Xingqiang Lu, Wai-Kwok Wongb
*
¨
and Richard A. Jonesc
Through an approach of pre-coordination and following terpolymerization from vinyl-functionalized
complex monomers [Ln(TTA)3(4-VB-PBI)] (Ln ¼ Eu, 2; Ln ¼ Tb, 3 and Ln ¼ Gd, 4; HTTA ¼ 2-
thenoyltrifluoroacetonate; 4-VB-PBI
¼
1-(4-vinylbenzyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole) with
methyl methacrylate, the first example of
a
single-component Eu2+–Tb3+–Gd3+-grafted polymer
Poly(MMA-co-2-co-3-co-4) was obtained. Moreover, control of an optimal dye content for suppressing
Tb3+ to Eu3+ energy transfer and exciton formation provides new perspectives for hetero-Ln3+-grafted
polymers as featured in Poly(MMA-co-2-co-3-co-4) with color-tuning to direct white-light (Commission
International De L'Eclairage coordinates of x ¼ 0.322, y ¼ 0.331; corrected color temperature of 5979 K;
and color rendering index up to 94), including a highly luminous efficiency of 17.8%.
Received 13th November 2016
Accepted 30th December 2016
DOI: 10.1039/c6ra26724f
www.rsc.org/advances
a common multi-component strategy.13 Nonetheless, realiza-
tion of high luminous efficiencies for single-component homo-
or hetero-Ln3+-graed polymers remains a challenge. Especially
from the viewpoint of high-quality white-light, an ultra-high
color rendering index (CRI; >90) single-component system
ideal for fabricating white polymeric light-emitting diodes or
other optoelectronic devices has not been reported.
Introduction
White-light-emitting materials are receiving considerable
research attention because of their wide applications1 in solid-
state lighting, back-lighting for liquid-crystal display and full-
color at-panel display. Until now, even though several white-
light-emitting materials reliant on inorganic phosphors2 or
nano-crystals,3 organic dyes,4 polymers,5 transition-metallic
complexes6 and transition-metal-graed polymers7 have been
studied extensively, organo-lanthanide (Ln3+) sources are still of
special interest. This interest can be attributed to the unique-
ness of Ln3+ emitting specic, narrow line-like emission over
a long lifetime. In this context, in contrast to the distinctive
drawbacks of low thermal stability and/or phase separation for
Ln3+-complexes8 or their doping systems9 and the inherent
defect of poor mechanical properties for Ln3+-related metal–
organic frameworks (MOFs),10 Ln3+-graed polymers should be
considered.11 A polymeric matrix endows exceptional thermal
stability, good mechanical strength and excellent lm-forming
property. Moreover, a single-component strategy for Ln3+-graf-
ted polymers12 capable of white-light production renders easier
fabrication and better color rendition as compared with
White-light-emitting single-component homo-12a,12b or
hetero-Ln3+-graed polymers12c are regularly constructed
through post-coordination from the rst-prepared coordination
site-retained polymer skeletons with europium ion (Eu3+) and/
or terbium ion (Tb3+)-complex units. However, due to the
usual wrapping of coordination sites by polymeric backbones or
branched-chains, discerning the localized circumstance and
stoichiometry of Ln3+ within them is difficult. The solution can
have recourse to another feasible approach12d of pre-
coordination and copolymerization where, by establishing
Eu3+ and/or Tb3+-complex primary-color monomers with de-
nite micro-environments in advance, the chromophore content
is “ne-tuned” through control of the desired feed ratios of
monomers during copolymerization. An alternative more
worthy of consideration is smooth supplementation of the
color-compensatory deciency from terpolymerization of co-
monomers. Nevertheless, it still suffers from an unmanage-
able Tb3+ to Eu3+ energy transfer in Eu3+–Tb3+–graed poly-
mers12c or intangible aggregation-induced exciton formation in
Eu3+- or Tb3+-containing metallopolymers,12a,12b which is fatal to
white-light modulation14 and makes obtaining an ultra-high
CRI value difficult.12d,15 Our conceptual strategy of molecular
dispersal of all-incorporated chromophores within a polymeric
matrix should be promising. That is, the Ln3+-complex primary-
aSchool of Chemical Engineering, Shaanxi Key Laboratory of Degradable Medical
Material, Northwest University, Xi'an 710069, Shaanxi, China
bDepartment of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon
Tong, Hong Kong, China
cDepartment of Chemistry and Biochemistry, The University of Texas at Austin, 1
University Station A5300, Austin, TX 78712-0165, USA
† Electronic supplementary information (ESI) available. CCDC 1494131. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c6ra26724f
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color components are separated to effectively suppress Tb3+ to diffraction (PXRD) patterns were recorded on a D/Max-IIIA
Eu3+ energy transfer or exciton formation through an optimal diffractometer with graphite-monochromatized Cu Ka radia-
˚
graing concentration, and their simultaneous emission gives tion (l ¼ 1.5418 A). Thermal properties were characterized
rise to continuous broad spectra for direct white-light. Herein, using thermogravimetric (TG) analyses and differential scan-
with terpolymerization of vinyl-functionalized complex mono- ning calorimetry (DSC) on a NETZSCH TGꢀ2109 instrument
ꢁ
mers [Ln(TTA)3(4-VB-PBI)] (Ln ¼ Eu, 2; Ln ¼ Tb, 3 and Ln ¼ Gd, under nitrogen at a heating rate of 10 C min
4) with methyl methacrylate (MMA), the rst example of a pol-
.
ymethyl methacrylate (PMMA)-supported and Eu2+–Tb3+–Gd3+
-
Synthesis of the precursor 2-(pyridine-2-yl)-1H-benzo[d]
graed single-component polymer Poly(MMA-co-2-co-3-co-4) imidazole (HPBI)
was obtained. Through control of a safe graing concentration,
The benzimidazole-based organic precursor HPBI was synthe-
sized from the reaction of 1,2-diaminobenzene (2.75 g, 25
mmol) with 2-pyridinecarboxaldehyde (2.68 g, 25 mmol) in the
presence of p-toluenesulfonic acid (2.27 mmol, 0.40 g) accord-
ing to a well-established procedure.16 Yield: 4.10 g (84%). Calc.
for C12H9N3: C, 73.83; H, 4.65; N 21.52%. Found: C, 73.80; H,
4.69; N, 21.54%. 1H NMR (400 MHz, DMSO-d6): d (ppm) 13.13 (s,
1H, –NH), 8.75 (d, 1H, –Py), 8.33 (t, 1H, –Py), 8.01 (t, 1H, –Py),
7.71 (d, 1H, –Py), 7.54 (t, 2H, –Ph), 7.24 (t, 2H, –Ph).
color-tuning to white-light with an ultra-high CRI was also ex-
pected from integration of Eu3+-red-light, Tb3+-green-light and
ligand-centered Gd3+-incorporated blue-light. Moreover, its
comparison relative to the polymer blend Poly(MMA-co-2)
@Poly(MMA-co-3)@Poly(MMA-co-4) was also explored.
Experimental section
High-performance liquid chromatography (HPLC)-grade tetra-
hydrofulan (THF) or acetonitrile (MeCN) was purchased from
Fisher Scientic and puried over solvent columns prior to use.
Other solvents were used as received from Sigma-Aldrich and
Synthesis of the vinyl-functionalized ancillary ligand 4-VB-PBI
(4-VB-PBI ¼ 1-(4-vinylbenzyl)-2-(pyridin-2-yl)-1H-benzo[d]
imidazole)
˚
stored over 3 A-activated molecular sieves. MMA was washed
To the solution of HPBI (2.00 g, 10 mmol) in absolute DMSO (20
mL), tetraethylammonium bromide (0.20 g, 10 mmol) was
added, and the mixture was reacted at RT. Aer 2 h, an aqueous
solution (5 mL) of KOH (0.67 g, 12 mmol) was added dropwise,
and the resulting mixture stirred at RT for another 2 h. Solid 1-
(chloromethyl)-4-vinylbenzene (1.80 g, 12 mmol) was added and
the resultant mixture continuously stirred at RT overnight.
Then, the mixture was poured into deionized water (200 mL) to
give a white precipitate. The crude product was ltered and
further dissolved into absolute EtOH (30 mL) to give a white
polycrystalline solid by evaporation at RT. Yield: 2.40 g (76%).
Calc. for C21H17N3: C, 81.00; H, 5.50; N, 13.49%. Found: C,
79.46; H, 5.58; N, 13.42%. FT-IR (KBr, cmꢀ1): 3048 (w), 3000 (w),
2987 (w), 1744 (w), 1716 (w), 1610 (m), 1589 (m), 1568 (m), 1510
(m), 1463 (m), 1440 (s), 1407 (w), 1390 (s), 1350 (w), 1331 (m),
1275 (m), 1267 (w), 1259 (w), 1210 (w), 1165 (m), 1148 (w), 1111
(m), 1097 (m), 1083 (w), 1046 (m), 1017 (w), 992 (m), 979 (m), 910
(m), 824 (s), 791 (m), 774 (m), 764 (m), 738 (vs), 700 (m), 626 (m),
612 (m), 578 (w), 562 (w), 543 (w). 1H NMR (400 MHz, DMSO-d6):
d (ppm) 8.70 (d, 1H, –Py), 8.37 (d, 1H, –Py), 8.01 (t, 1H, –Py), 7.77
(m, 1H, –Py), 7.59 (m, 1H, –Ph), 7.52 (m, 1H, –Ph), 7.34 (d, 2H,
–Ph), 7.28 (m, 2H, –Ph), 7.10 (d, 2H, –Ph), 6.63 (m, 1H, –CH]C),
6.22 (s, 2H, –CH2), 5.73 (d, 1H, ]CH2), 5.19 (d, 1H, ]CH2). ESI-
MS (in CHCl3) m/z: 312.39 (100%), [M ꢀ H]+.
twice with aqueous NaOH (5%, wt) and twice with distilled
water, followed by drying over anhydrous MgSO4 and distilla-
tion over CaH2 under a N2 atmosphere at reduced pressure.
Azobis(isobutyronitrile) (AIBN) was puried by recrystallization
twice from absolute MeOH prior to use. Other chemicals were
commercial products of reagent grade and used without further
purication. Elemental analyses were performed on a Perki-
nElmer 240C elemental analyzer. Infrared spectra were recor-
ded on a Nicolet Nexus-670 FT-IR spectrophotometer with KBr
pellets in the region 4000–400 cmꢀ1 1H NMR spectra were
.
recorded on a Bruker Plus 400 spectrometer with SiMe4 as the
internal standard in DMSO-d6 at room temperature. Electro-
spray ionization mass spectrometry (ESI-MS) was performed on
a Finnigan LCQDECA XP HPLC-MSn mass spectrometer with
a mass to charge (m/z) range of 4000 using a standard electro-
spray ion source and CHCl3 or MeCN as solvent. Electronic
absorption spectra in the UV/vis region and diffuse reection
(DR) spectra were recorded with a Cary 300 UV spectropho-
tometer. Visible emission and excitation spectra were collected
by a combined uorescence lifetime and steady-state spec-
trometer (FLS-980) with a 450 W Xe lamp. Excited-state decay
times were obtained by the same spectrometer but with a mF900
Xe lamp. The luminescent absolute overall quantum yield (Fem
or FLLn) in solution or solid state was determined by the same
spectrometer using a 450 W Xe lamp and an integrating sphere.
Gel permeation chromatography (GPC) analyses of polymers
were performed using a Waters 1525 binary pump coupled to
a Waters 2414 refractive index detector with HPLC THF as the
Synthesis of a series of vinyl-containing complex monomers
[Ln(TTA)3(4-VB-PBI)] (Ln ¼ La, 1; Ln ¼ Eu, 2; Ln ¼ Tb, 3;
Ln ¼ Gd, 4)
eluant on American Polymer Standard linear mixed bed packing To a stirred MeOH solution (15 mL) of the ligand 2-thenoyltri-
columns (particle size, 10 mm). GPC was calibrated using poly- uoroacetonate (HTTA; 132 mg, 0.6 mmol) in the presence of an
styrene standards. X-ray photoelectron spectroscopy (XPS) was equimolar amount of NaOH (24 mg, 0.6 mmol), another
carried out on a PHI 5700 XPS system equipped with a dual Mg aqueous solution (10 mL) of LnCl3$6H2O (0.2 mmol; Ln ¼ La,
X-ray source and monochromatic Al X-ray source complete with 71 mg; Ln ¼ Eu, 73 mg; Ln ¼ Tb, 75 mg or Ln ¼ Gd, 74 mg) was
depth prole and angle-resolved capabilities. Powder X-ray added, and precipitation occurred immediately, respectively.
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Under vigorous stirring at RT for 12 h, each of the off-white (1.5 mol% of MMA) was carried out in a Fisher-Porter glass
precipitates was separated by ltration, and washed with reactor and protected by a reduced N2 atmosphere. A mixture of
deionized water and absolute CH2Cl2. Aer drying at 45 ꢁC MMA (9.5 mmol, 1 mL) and one of the complex monomers 1–4
under vacuum, each of the solid products was added to a mixed at a stipulated feed molar ratio (100 : 1, 200 : 1, 400 : 1 or
solvent (30 mL; EtOH and petroleum ether, v/v ¼ 3 : 1), and 600 : 1) in the presence of AIBN initiator (1.5 mol% of MMA)
solid 4-VB-PBI (62 mg, 0.2 mmol) was added, then the resultant was dissolved in dry THF (30 mL), and the respective resultant
mixture was continuously stirred at RT for another 12 h. The mixture heated to 60 ꢁC with continuous stirring for 48 h under
respective clear-yellow solution was ltered, and le to stand at a reduced N2 atmosphere. All the reaction mixtures remained
RT for several days to give the pale-yellow microcrystalline clear throughout the polymerization. Aer cooling to room
products of complex monomers 1–4, respectively.
temperature, each viscous mixture was diluted with dry THF (20
For 1: yield: 167 mg (75%). Calc. for C45H29F9LaN3O6S3: C, mL) and precipitated with absolute diethyl ether (50 mL) thrice.
48.53; H, 2.62; N, 3.77%. Found: C, 48.29; H, 2.69; N, 3.75%. FT- The resulting solid products were collected by ltration and
IR (KBr, cmꢀ1): 3093 (w), 2989 (w), 2901 (w), 1614 (m), 1596 (s), dried at 45 C under vacuum to constant weight, respectively.
ꢁ
1535 (s), 1507 (m), 1476 (m), 1457 (m), 1411 (s), 1355 (m), 1298
For Poly(MMA-co-1) (200 : 1): yield: 87%. FT-IR (KBr, cmꢀ1):
(s), 1288 (sh), 1246 (m), 1229 (m), 1182 (s), 1134 (vs), 1122 (m), 2992 (w), 2951 (w), 1728 (vs), 1681 (w), 1480 (m), 1448 (m), 1436
1083 (w), 1060 (m), 1034 (w), 1010 (w), 989 (w), 932 (m), 913 (w), (m), 1387 (m), 1369 (w), 1270 (m), 1242 (m), 1193 (m), 1148 (s),
859 (m), 836 (w), 785 (s), 746 (s), 717 (s), 692 (w), 680 (m), 640 1074 (w), 1065 (m), 1057 (w), 988 (w), 967 (w), 913 (w), 842 (w),
(m), 605 (w), 578 (w), 542 (w), 532 (w). 1H NMR (400 MHz, 812 (w), 803 (w), 750 (m), 720 (w), 673 (w), 656 (m), 630 (m). 1H
DMSO-d6): d (ppm) 8.69 (d, 1H, –Py), 8.38 (d, 1H, –Py), 8.01 (t, NMR (400 MHz, DMSO-d6): d (ppm) 8.70 (b, 1H, –Py), 8.37 (b,
1H, –Py), 7.62 (s, 7H, –Py and –Th), 7.58 (t, 1H, –Ph), 7.52 (t, 1H, 1H, –Py), 8.02 (b, 2H, –Py), 7.76 (b, 4H, –Ph and –Th), 7.54 (d, 3H,
–Ph), 7.34 (d, 2H, –Ph), 7.28 (m, 2H, –Ph), 7.10 (d, 5H, –Ph and –Ph), 7.28 (b, 4H, –Ph), 6.97 (m, 6H, –Ph), 6.18 (b, 5H, –CH2 and
–Th), 6.62 (m, 1H, –CH]C), 6.22 (s, 2H, –CH2), 6.15 (s, 3H, –CH), 3.55 (s, 260H, –COOMe), 2.85 (b, 1H, –CH), 1.82 (m, 175H,
–CH), 5.74 (d, 1H, ]CH2), 5.19 (d, 1H, ]CH2). ESI-MS (in –CH2), 1.32 (b, 2H, –CH2), 0.93 (m, 260H, –CH3).
MeCN) m/z: 1114.82 (100%), [M ꢀ H]+.
For Poly(MMA-co-2) (100 : 1, 200 : 1, 400 : 1 or 600 : 1): yield:
For 2: yield: 176 mg (78%). Calc. for C45H29F9EuN3O6S3: C, 84% (100 : 1); 88% (200 : 1); 91% (400 : 1); 93% (600 : 1). FT-IR
47.96; H, 2.59; N, 3.73%. Found: C, 47.83; H, 2.67; N, 3.64%. FT- (KBr, cmꢀ1): 2994 (w), 2951 (w), 1728 (vs), 1680 (w), 1483 (m),
IR (KBr, cmꢀ1): 3096 (w), 2988 (w), 2902 (w), 1615 (m), 1597 (s), 1449 (m), 1436 (m), 1387 (m), 1369 (w), 1270 (m), 1242 (m), 1192
1537 (s), 1505 (m), 1478 (m), 1458 (m), 1411 (s), 1354 (m), 1302 (m), 1149 (s), 1084 (w), 1065 (m), 1052 (w), 987 (w), 965 (w), 911
(s), 1290 (sh), 1246 (m), 1230 (m), 1184 (s), 1134 (vs), 1121 (m), (w), 841 (w), 813 (w), 805 (w), 750 (m), 722 (w), 672 (w), 653 (m),
1083 (w), 1061 (m), 1036 (w), 1015 (w), 1010 (sh), 989 (w), 934 630 (m).
(m), 914 (m), 859 (m), 836 (w), 785 (s), 746 (s), 717 (s), 693 (w),
For Poly(MMA-co-3) (100 : 1, 200 : 1, 400 : 1 or 600 : 1): yield:
681 (m), 642 (m), 605 (w), 581 (m), 545 (w), 535 (w). ESI-MS (in 82% (100 : 1); 86% (200 : 1); 90% (400 : 1); 92% (600 : 1). FT-IR
MeCN) m/z: 1127.88 (100%), [M ꢀ H]+.
(KBr, cmꢀ1): 2994 (w), 2951 (w), 1727 (vs), 1681 (w), 1483 (m),
For 3: yield: 161 mg (71%). Calc. for C45H29F9TbN3O6S3: C, 1448 (m), 1435 (m), 1387 (m), 1369 (w), 1272 (m), 1242 (m), 1193
47.67; H, 2.58; N, 3.71%. Found: C, 47.53; H, 2.71; N, 3.65%. FT- (m), 1149 (s), 1075 (w), 1065 (m), 1057 (w), 986 (w), 967 (w), 914
IR (KBr, cmꢀ1): 3093 (w), 2975 (w), 2902 (w), 1611 (m), 1598 (s), (w), 842 (w), 810 (w), 805 (w), 750 (m), 720 (w), 673 (w), 654 (m),
1537 (s), 1507 (m), 1476 (m), 1459 (m), 1412 (s), 1356 (m), 1304 630 (m).
(s), 1291 (w), 1246 (m), 1230 (m), 1186 (s), 1134 (vs), 1125 (m),
For Poly(MMA-co-4) (200 : 1): yield: 85%. FT-IR (KBr, cmꢀ1):
1082 (w), 1061 (m), 1036 (w), 1014 (w), 1010 (w), 992 (w), 932 (m), 2990 (w), 2952 (w), 1726 (vs), 1681 (w), 1482 (m), 1448 (m), 1436
914 (w), 859 (m), 836 (w), 785 (s), 768 (m), 746 (s), 719 (s), 693 (m), 1386 (m), 1370 (w), 1271 (m), 1242 (m), 1192 (m), 1149 (s),
(w), 680 (m), 641 (m), 605 (w), 581 (m), 543 (w), 531 (w). ESI-MS 1076 (w), 1066 (m), 1057 (w), 988 (w), 967 (w), 913 (w), 843 (w),
(in MeCN) m/z: 1134.84 (100%), [M ꢀ H]+.
810 (w), 803 (w), 750 (m), 721 (w), 673 (w), 654 (m), 631 (m).
For 4: yield: 159 mg (70%). Calc. for C45H29F9GdN3O6S3: C,
47.74; H, 2.58; N, 3.71%. Found: C, 47.61; H, 2.69; N, 3.66%. FT-
IR (KBr, cmꢀ1): 3094 (w), 2988 (w), 2901 (w), 1618 (m), 1598 (s),
1536 (s), 1505 (m), 1477 (m), 1459 (m), 1428 (w), 1411 (s), 1354
(m), 1303 (s), 1291 (sh), 1246 (m), 1230 (m), 1184 (s), 1133 (vs),
Synthesis of polymers' blend Poly(MMA-co-2)@Poly(MMA-co-
3)@Poly(MMA-co-4) with different doping mass ratios
(1 : 7 : 4, 1 : 7 : 8 or 1 : 7 : 12)
1126 (m), 1083 (w), 1061 (m), 1035 (w), 1014 (w), 1010 (sh), 989 A mixture of Poly(MMA-co-2), Poly(MMA-co-3) and Poly(MMA-
(w), 934 (m), 914 (w), 859 (m), 835 (w), 785 (s), 768 (m), 746 (s), co-4) obtained from AIBN-initiated copolymerization of MMA
717 (s), 690 (w), 681 (m), 643 (m), 601 (w), 580 (m), 547 (w), 534 and one of the complex monomers 2–4 at the same stipulated
(w). ESI-MS (in MeCN) m/z: 1133.17 (100%), [M ꢀ H]+.
feed molar ratio (200 : 1) was dissolved in absolute CHCl3 (30
mL) with one of the mass ratios (1 : 7 : 4, 1 : 7 : 8 or 1 : 7 : 12) to
form a clear solution, and each of the resulting solutions was
stirred under a reduced N2 atmosphere at RT for 36 h. Aer
precipitation with absolute diethyl ether (50 mL) thrice, each of
Synthesis of homo-Ln3+-graed polymers poly(MMA-co-
[Ln(TTA)3(4-VB-PBI)]) (Ln ¼ La, 1; Eu, 2; Tb, 3; Gd, 4)
The homogeneous copolymerization of MMA and each of the the resultant solid products was collected by ltration and dried
vinyl-containing complex monomers 1–4 activated with AIBN at 45 ꢁC under vacuum to constant weight. For Poly(MMA-co-2)
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@Poly(MMA-co-3)@Poly(MMA-co-4): yield: 95% (1 : 7 : 4); 94% vinyl-containing complex monomers [Ln(TTA)3(4-VB-PBI)] (Ln ¼
(1 : 7 : 8) or 96% (1 : 7 : 12). FT-IR (KBr, cmꢀ1): 2993 (w), 2951 La, 1; Ln ¼ Eu, 2; Ln ¼ Tb, 3 or Ln ¼ Gd, 4) was obtained,
(w), 1728 (vs), 1482 (w), 1448 (m), 1436 (m), 1387 (w), 1270 (m), respectively. The ancillary ligand 4-VB-PBI and its four complex
1242 (m), 1193 (m), 1148 (s), 1062 (w), 989 (w), 968 (w), 917 (w), monomers 1–4 were well-characterized by EA, FT-IR, 1H NMR and
841 (w), 828 (w), 816 (w), 811 (w), 797 (w), 783 (w), 776 (w), 750 ESI-MS. In the FT-IR spectra of 1–4, the similar and combined
(m), 732 (w), 722 (w), 701 (w), 685 (w), 673 (w), 660 (w), 643 (m), absorption characteristics of the coordinated precursor 4-VB-PBI
627 (m).
and (TTA)-ligand are exhibited, and the remaining two charac-
teristic absorptions at 1611–1618 and 1121–1126 cmꢀ1 assigned to
the respective n(]CH) vibration and the g(]CH) vibration of the
active vinyl group are also observed. With respect to the 1H NMR
spectrum (Fig. 1S†) of anti-ferromagnetic La3+-based complex
monomer 1, besides a stipulated proton molar ratio (3 : 1) of
(TTA)ꢀ to 4-VB-PBI, the proton resonances (d ¼ 6.62, 5.74 and 5.19
ppm) of the terminal functional vinyl group remain identical to
those (d ¼ 6.63, 5.73 and 5.19 ppm) of 4-VB-PBI despite the
coordination of La3+. Moreover, ESI-MS spectra of the series of
complex monomers 1–4 display similar patterns and exhibit
a strong mass peak at m/z 1114.82 (1), 1127.88 (2), 1134.84 (3) or
1133.17 (4) assigned to the major species [M ꢀ H]+ of complex
monomers 1–4, respectively, indicating that the respective discrete
tris-b-diketonate mononuclear unit is retained in the corre-
sponding dilute MeCN solution.
Synthesis of Eu3+–Tb3+–Gd3+-graed polymers Poly(MMA-co-2-
co-3-co-4) with a stipulated feed molar ratio of 200 : 1[2/3/4] ¼
1 : 7 : 4, 1 : 7 : 8 or 1 : 7 : 12
Poly(MMA-co-2-co-3-co-4) with a stipulated feed molar ratio and
different monomer-mixed molar ratios (200 : 1[2/3/4] ¼ 1 : 7 : 4,
1 : 7 : 8 or 1 : 7 : 12) was prepared in the same way as shown for
Poly(MMA-co-2) (200 : 1) except that complex monomers 2–4
with different mixed molar ratios ([2/3/4] ¼ 1 : 7 : 4, 1 : 7 : 8
or 1 : 7 : 12) were used instead of complex monomer 2. For
Poly(MMA-co-2-co-3-co-4) (200 : 1[2/3/4] ¼ 1 : 7 : 4, 1 : 7 : 8 or
1 : 7 : 12): yield: 86% (200 : 1[2/3/4] ¼ 1 : 7 : 4); 87% (200 : 1[2/3/
4] ¼ 1 : 7 : 8) or 85% (200 : 1[2/3/4] ¼ 1 : 7 : 12). FT-IR (KBr,
cmꢀ1): 2989 (w), 2953 (w), 2902 (w), 1730 (vs), 1484 (m), 1448
(m), 1436 (m), 1393 (m), 1387 (m), 1271 (m), 1242 (m), 1193 (m),
1148 (s), 1075 (w), 1066 (m), 1059 (w), 989 (w), 967 (w), 911 (w),
843 (w), 812 (w), 800 (w), 751 (m), 720 (w), 673 (w), 648 (m), 629
(m). XPS quantitative results of Ln3+ molar ratios: Eu3+/Tb3+/
Gd3+ ¼ 1 : 6.97 : 3.93 vs. 1 : 7 : 4; 1 : 7.02 : 7.96 vs. 1 : 7 : 8;
1 : 6.98 : 11.95 vs. 1 : 7 : 12.
Molecular structure of [Gd(TTA)3(4-VB-PBI)] (4) as a repre-
sentative of complex monomers 1–4 was obtained by single-
crystal X-ray diffraction analysis, with crystallographic data
shown in Tables 1 and 2S.† Complex monomer 4 crystallizes in
ꢀ
the space group P1 with the asymmetric unit consisting of one
Gd3+, three deprotonated (TTA)ꢀ ligands and one ancillary
ligand 4-VB-PBI. As shown in Fig. 1, the central Gd3+ (Gd1) is
eight-coordinate, whereby three deprotonated (TTA)ꢀ ligands
with a similar O,O0-chelate mode and one ancillary ligand 4-VB-
PBI with a N,N0-chelate mode coordinate to one Gd3+ (Gd1) in
a square anti-prismatic geometry, resulting in formation of
a typical tris-b-diketonate binary mononuclear host structure.17
Results and discussion
Synthesis and characterization of vinyl-containing complex
monomers 1–4
As shown in Scheme 1, the vinyl-functionalized ancillary ligand 4-
VB-PBI was synthesized from the nucleophilic replacement reac-
tion of 1-(chloromethyl)-4-vinylbenzene with the benzimidazole-
based precursor HPBI in the presence of KOH in a yield of 76%.
Furthermore, through self-assembly of the deprotonated b-diket-
onate ligand (TTA)ꢀ, LnCl3$6H2O (Ln ¼ La, Eu, Tb or Gd) and the
ancillary ligand 4-VB-PBI in a molar ratio of 3 : 1 : 1, a series of
˚
The six Gd–O bond lengths (2.334(4)–2.367(4) A) are slightly
˚
shorter than those (2.525(5)–2.602(6) A) of the two Gd–N bonds,
and the coordination of Gd3+ endows almost co-planar charac-
teristics of the thiophene ring with the O,O0-chelate ring for
each deprotonated (TTA)ꢀ ligand and the benzimidazole ring,
and the pyridine ring with the N,N0-chelate ring for the ancillary
ligand 4-VB-PBI. These observations, together with involvement
Scheme 1 Reaction scheme for the synthesis of the vinyl-function-
alized complex monomers 1–4 and Eu3+–Tb3+–Gd3+-grafted poly- Fig. 1 Perspective drawing of complex monomer [Gd(TTA)3(4-VP-
mer Poly(MMA-co-2-co-3-co-4).
PBI)] (4); H atoms have been omitted for clarity.
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of the CF3-group in each (TTA)ꢀ ligand, promote spin–orbit s ¼ 3.4 ns and FeLm ¼ 3.5%) assigned to the intra-ligand p–p*
coupling to effectively sensitize Ln3+-based luminescence. It is transition at RT and belonging to the perturbation lumines-
worth noting that the retained vinyl functional group charac- cence of Gd3+ to the ligand, complex monomer 4 exhibits 0–0
˚
teristic of a typical C]C bond length of 1.2996(12) A from 4-VB- transition phosphorescence (lem ¼ 480 nm and s ¼ 7.6 ms) at 77
PBI in complex monomer 4 renders each of the complex K, from which the triplet (3p–p*) energy level at 20 833 cmꢀ1 is
monomers active for the following copolymerization.
obtained with regard to the singlet (1p–p*) energy level
(25 974 cmꢀ1) estimated by the lower wavelength of its
UV-visible absorbance edge. Therefore, in addition to the
slightly larger energy gap DE1 (1p–p* to 3p–p*, 5141 cmꢀ1) than
5000 cmꢀ1 endowing the effective intersystem crossing process
according to Reinhouldt's empirical rule,20 the ligands-based
3p–p* energy level (20 833 cmꢀ1) lies above the energies of
the main emitting levels of 5D0 (17 286 cmꢀ1) for Eu3+ shown in
Photophysical properties and energy-transfer process of vinyl-
containing complex monomers 2–4 in solution
The photophysical properties of complex monomers 2–4 were
investigated in dilute MeCN solution at RT or 77 K, and
summarized in Fig. 2 and 2S.† The similar and combined
ligands-centered absorption spectra of 202–204, 260–264 and
324–326 nm for complex monomers 2–4 in the UV-visible region
are observed (Fig. 2S†), where the lowest energy absorptions
should be attributed to the p–p* transitions of (TTA)ꢀ and 4-VB-
PBI ligands. For complex monomer 2, photo-luminescence (lex
¼ 380 nm) exhibits just the Eu3+-centered characteristic emis-
sions (5D0 / 7FJ, J ¼ 0–4) shown in Fig. 2, giving a bright color-
pure red-light with a Commission International De L'Eclairage
(CIE) chromatic coordinate (0.665, 0.334). However, use of Tb3+
in the replacement endows complex monomer 3 dual emissions
(CIE chromatic coordinate 0.174, 0.187) composed of both
5
Fig. 3S,† and the energy gap DE2 (3p–p* to D0) of 3547 cmꢀ1
falls well within the ideal 2500–4500 cmꢀ1 range from
Latva's empirical rule,21 conrming efficient sensitization of the
Eu3+-based complex monomer 2. However, for complex mono-
5
mer 3, the small energy gap DE2 (3p–p* to D4, 288 cmꢀ1) for
Tb3+ allows for back energy transfer,22 giving rise to its dual
emissions due to intra-molecular partial energy transfer.
Synthesis, characterization and photophysical property of
homo-Ln3+-graed polymers Poly(MMA-co-1), Poly(MMA-co-
2), Poly(MMA-co-3) and Poly(MMA-co-4)
ligands-based residual blue-light emission at 434 nm and Tb3+
-
7
centered characteristic green-light (5D4 / FJ, J ¼ 6, 5, 4, 3).
Moreover, the outstanding color-pure red-light 2 can also be
testied by a long Eu3+-centered (7F2, lem ¼ 613 nm) lifetime of
312 ms and an inspiring FLEu of 47.0% within all the reported
(TTA)3–Eu3+-complexes.18 With regard to 3, two-centered species
decay with a uorescence lifetime (2.7 ns) of the ligands-based
residual emission and a phosphorescence lifetime 143 ms of
Tb3+, respectively, whereas its large optical absorbance of the
(TTA)ꢀ and 4-VB-PBI involved renders an attractive FTLb of 6.8%
for 3.
In consideration of the congenital defect of low thermal stability
of complex monomers and the excellent performance23 of the
PMMA matrix with low cost, transparency and good mechanical
property, each of the complex monomers 2–4 with an active
terminal vinyl group was copolymerized with MMA with one
stipulated feed molar ratio (100 : 1, 200 : 1, 400 : 1 or 600 : 1) in
the presence of AIBN to obtain the series of homo-Ln3+-graed
polymers poly(MMA-co-[Ln(TTA)3(4-VB-PBI)) (Ln ¼ Eu, Tb or
Gd), respectively. To elucidate their AIBN-assisted free-radical
copolymerization,24 the representative polymer Poly(MMA-co-
1) with a stipulated feed molar ratio of 200 : 1 from the iso-
structural and anti-ferromagnetic La3+-based complex mono-
mer 1 was obtained for comparison. With respect to the 1H
NMR spectrum of Poly(MMA-co-1) also shown in Fig. 1S,†
besides the combined proton resonances (d ¼ 8.70–0.93 ppm) of
the polymerized complex monomer 1 and MMA, the original
three proton resonances of the characteristic vinyl group are
unambiguously replaced by two new up-shied proton reso-
nances (d ¼ 2.85 and 1.32 ppm) of the –CH and –CH2 groups,
conrming the covalent-bonding of complex monomers into
the PMMA backbone.25 Moreover, GPC results (Table 3S†) show
that all the polydispersity indices (PDI ¼ Mw/Mn) of these
polymers are in the relatively narrow range of 1.17–1.38 due to
AIBN-initiated radical copolymerization. It is worth noting that
there is an almost linear relationship between the Mn value and
the feed molar ratio (100 : 1, 200 : 1, 400 : 1 or 600 : 1) for
both Poly(MMA-co-2) and Poly(MMA-co-3), also suggesting
a random distribution of complex monomers along the poly-
meric backbone. Furthermore, the PXRD pattern (Fig. 3) of
the representative polymer Poly(MMA-co-2) (200 : 1) endows
only PMMA-based amorphous peaks, also indicating the
low-concentration homogeneous distribution26 of the complex
To further address the sensitization mechanism of complex
monomers 2–3, their iso-structural Gd3+-based complex mono-
mer 4, as a suitable reference, endows study of chromophore
luminescence in the absence of energy transfer because Gd3+
has no energy levels below 32 000 cmꢀ1, and so cannot accept
any energy from the excited state of the chromophore.19
Different from the typical uorescent blue-light (lem ¼ 432 nm,
Fig. 2 Emission and excitation spectra (lex ¼ 380 nm at RT or lex
339 nm at 77 K) of 2–4 in MeCN solution at 2 ꢂ 10ꢀ5 M.
¼
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Fig. 3 PXRD patterns of PMMA, Poly(MMA-co-2) (200 : 1), polymers'
blend Poly(MMA-co-2)@Poly(MMA-co-2)@Poly(MMA-co-2) (1 : 7 : 8)
and Eu3+–Tb3+–Gd3+-grafted polymer Poly(MMA-co-2-co-3-co-4)
(200 : 1(2/3/4 ¼ 1 : 7 : 8)) in the solid state.
Fig. 5 DR spectra of Poly(MMA-co-2) (200 : 1), Poly(MMA-co-2)
(200 : 1), polymers' blend Poly(MMA-co-2)@Poly(MMA-co-2)@Poly-
(MMA-co-2) (1 : 7 : 8) and polymer Poly(MMA-co-2-co-3-co-4)
(200 : 1(2/3/4 ¼ 1 : 7 : 8)) in the solid state at room temperature.
Fig. 6 Emission and excitation spectra (lex ¼ 341 nm) of homo-Ln3+
-
grafted polymers with different feed molar ratios (100 : 1, 200 : 1,
400 : 1 and 600 : 1) in the solid state at room temperature; (6a) for
Poly(MMA-co-2); (6b) for Poly(MMA-co-3).
Fig. 4 TG curves of PMMA, complex monomer 2 or polymer Poly(-
MMA-co-2) (200 : 1) and TG and DSC (inset) curves of polymer Pol-
y(MMA-co-2-co-3-co-4) (200 : 1(2/3/4 ¼ 1 : 7 : 8)) in the solid state.
detected in the corresponding samples. Interestingly, different
from the ligands-centered emission (lem ¼ 434 nm shown in
Fig. S4†) for Poly(MMA-co-4) (200 : 1), both Poly(MMA-co-2) and
Poly(MMA-co-3) with different feed molar ratios exhibit the
monomer 2. TG analysis (Fig. ꢁ4) of Poly(MMA-co-2) (200 : 1)
exhibits a slight increase of 19 C for the Tonset in comparison
with pure PMMA, and decomposition with maxima around the
higher temperature interval (385–397 ꢁC) than that (310–324 ꢁC)
of complex monomer 2, showing that the thermal stability of
Poly(MMA-co-2) (200 : 1) is signicantly improved through
copolymerization.
The photophysical properties of polymers Poly(MMA-co-2) or
Poly(MMA-co-3) with different feed molar ratios (100 : 1, 200 : 1,
400 : 1 and 600 : 1) and Poly(MMA-co-4) (200 : 1) were investi-
gated in solid state, and summarized in Fig. 5, 6 and S4.† As
shown in Fig. 5, a similar DR spectrum of the PMMA-supported
polymer from 2 or 3 at a stipulated feeding molar ratio of 200 : 1
exhibits relatively broader absorption bands than those of 2–4
in solution, where the absorptions at 216–220, 282–284 and
345–349 nm in the UV-visible region should be assigned to
electronic transitions from organic moieties of PMMA and the
coordinated ligands. Due to the characteristic absorptions of
Eu3+ or Tb3+ commonly appearing above 1000 nm,27 they are not
corresponding dual-emitting behaviours (Fig. 6) with lex
¼
341 nm, which are similar to that of complex monomer 3 in
solution but distinctively different from just Eu3+-centered
color-pure red-light of complex monomer 2. For Poly(MMA-co-
2), through involvement of low Eu3+-graing concentration
(400 : 1 or 600 : 1), the emission spectrum contains a domi-
nating and uncharacteristically broad emission band centered
at 412 nm in addition to the peaks associated with Eu3+
-
centered emissions, as shown in Fig. 6a. The result of the new
band at 412 nm with signicant intensity unlikely to originate
from pure PMMA (lem ¼ 390 nm; Fig. 4S†) and distinctively
blue-shied by 22 nm relative to the expected ligands-based
emission (lem ¼ 434 nm; Fig. 4S†) as Poly(MMA-co-4) (200 : 1),
indicates the presence of another species. Actually, when
complex monomers of 2 are cast into the PMMA matrix at a feed
molar ratio of 400 : 1 or 600 : 1, an intimate spatial wrapping of
the exible PMMA critically connes complex monomers with a
high local concentration routinely. In this case, bridging or p–p
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stacking between complex monomers is probably essential for
generation of the new emission.28 Noticeably, if graing the
complex monomers 2 at excess concentration (100 : 1), the
emission of Poly(MMA-co-2) regresses to the Eu3+-centered red-
light as expected. However, one dissimilar but complementary
weak emission composed of ligands-based blue-light (432 nm)
and unforeseen red-shied blue-light (468 nm) due to
aggregation-induced exciton formation29 is observed, from
which the Poly(MMA-co-2) is characteristic of a highly conju-
gated polymer. In contrast, both the ligands-based blue-light
centered at 432 nm and the Eu3+-centered red-light are undis-
turbed for Poly(MMA-co-2) with a feed molar ratio of 200 : 1, so
this graing concentration could be supposed to be an optimal
dye content despite incomplete energy transfer from the poly-
meric backbone to Eu3+. Returning to Poly(MMA-co-3) with
different feed molar ratios, all the dual emissions (Fig. 6b) seem
similar to those of Poly(MMA-co-2), where the ligands-based
blue-light (lem ¼ 434 nm) and the Tb3+-centered green-light
are consolidated at a feed molar ratio of 200 : 1 or 100 : 1
without observation of clustering of emitters due to the lower
quantum efficiency of complex monomer 3. In addition,
attributed to the PMMA-matrixed substrate with a larger
refractive index30 and recombination of the charge carrier at
Eu3+- or Tb3+-related trap sites31 due to molecular extension for
covalently-bonded polymers, the signicantly improved lumi-
nescent properties for Poly(MMA-co-2) (200 : 1) or Poly(MMA-co-
3) (100 : 1 and 200 : 1) are further conrmed by longer lifetimes
(901 ms for Poly(MMA-co-2); 714–721 ms for Poly(MMA-co-3)) and
the relatively larger overall quantum yields (Fem ¼ 58.4% for
Poly(MMA-co-2); Fem ¼ 17.3–21.5% for Poly(MMA-co-3)) than
those of (47.0% or 6.8%) the corresponding complex monomer
2 or 3 in solution, respectively. For this point, by decreasing the
graing content of the incorporated chromophore at a certain
level, the clustering of Eu3+ or Tb3+-based emitters within could
be effectively suppressed.
Fig. 7 Emission spectra (left) and corresponding CIE coordinates
(right) of polymers' blends Poly(MMA-co-2)@Poly(MMA-co-2)@Pol-
y(MMA-co-2) with different doping mass ratios (1 : 7 : 4, 1 : 7 : 8 and
1 : 7 : 12) in the solid state at room temperature upon excitation: (7a)
1 : 7 : 4; (7b) 1 : 7 : 8; (7c) 1 : 7 : 12.
to-Eu3+-containing energy transfer between the interfaced
chromophores. This trichromatic integration is responsible for
the resultant emitting colors from white-light (points a–b) to
purplish-white (points c–e) with variation of the excitation
wavelength from 330 to 370 nm. However, the two white-light
points (0.367–0.371, 0.320–0.328) are critically deviated from
the equal energy point (0.333, 0.333), arising from the deciency
Synthesis, characterization and color-tunable white-light of
polymers' blend of Poly(MMA-co-2)@Poly(MMA-co-3)
@Poly(MMA-co-4) and hetero-Ln3+-graed polymer
Poly(MMA-co-2-co-3-co-4)
of the ligand-based blue-light. With an increase of relative Gd3+
-
In consideration of the dual-emitting character of both Poly(-
MMA-co-2) (200 : 1) and Poly(MMA-co-3) (200 : 1) and the uo-
rescent blue-light (lem ¼ 434 nm) of Poly(MMA-co-4) (200 : 1),
their simple blending can be expected to be capable of white-
light production through simultaneous emission from the
three chromophores within. However, as shown in Fig. 7,
emission curves for Poly(MMA-co-2)@Poly(MMA-co-3)@Poly-
(MMA-co-4) with different doping mass ratios (1 : 7 : 4, 1 : 7 : 8
and 1 : 7 : 12) do not show the simple addition spectra as ex-
content in the polymers' blend with a doping mass ratio of
1 : 7 : 8, it is also believed that efficient Tb3+ to Eu3+ energy
transfer takes place by similar domination of the Eu3+-centered
emission intensity shown in Fig. 7b. Noticeably, although all the
emissive colors (points f–j) almost fall within the white-light
region at the whole excitation range of 330–370 nm, the
closest to the standard white-light (0.333, 0.333) is located at
point g (0.323, 0.298) upon a excitation wavelength of 340 nm.
Moreover, besides the slightly higher color corrected tempera-
ture (CCT) of 6937 K and the slightly lower CRI of 77 beyond
the solid-state lighting requirements (CCT between 2500–6500
and CRI above 80),32 a quantum yield down to 10.5% is
also observed. Furthermore, the three-centered species decay
with lifetimes of 4.6 ns from ligands-based blue-light, 934 ms
from Eu3+-centered red-light and 478 ms from Tb3+-centered
pected. At a doping mass ratio of 1 : 7 : 4, although the Gd3+
-
involved ligands-based blue-light (lem ¼ 434 nm), Eu3+-centered
red-light (lem ¼ 613 nm) and Tb3+-centered green-light (lem
¼
546 nm) are simultaneously emissive, as shown in Fig. 7a, the
distinctive suppression of ligands-based blue-light, together
with the larger balanced emission intensity of 613 nm (7F2 of
Eu3+) than that of 546 nm (7F5 of Tb3+), shows an efficient Tb3+
-
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green-light conrm that the white-light should unambiguously
originate from both uorescence and phosphorescence, and
especially, the distinctively decreased Tb3+-centered phospho-
rescent lifetime (478 ms) than that (714 ms) of Poly(MMA-co-3)
with the same feed molar ratio of 200 : 1 veries that energy
transfer from Tb3+ to Eu3+ centers indeed takes place. Inter-
estingly, providing further excess Poly(MMA-co-4) in the blend
(1 : 7 : 12), as shown in Fig. 7c, the domination of ligands-based
blue-light, together with an unbalanced trichromatic integra-
tion, envisions all the resulting emissions beyond the white-
light region.
In contrast, with appropriate feed molar ratios (200 : 1[2/3/
4] ¼ 1 : 7 : 4, 1 : 7 : 8 and 1 : 7 : 12) via doping mass ratios for
the copolymerization of MMA and complex monomers 2–4, one
can expect molecular dispersal of different chromophores in
a
polymeric matrix. More importantly, the low graing
concentration (200 : 1) seems to suggest that the Eu3+ and Tb3+
-
based chromophores involved are effectively separated in the
PMMA matrix to inhibit possible Tb3+ to Eu3+ energy transfer. In
particular, blockade of intramolecular energy transfer does not
interfere with the sensitization of different chromophores
within, from which, their simultaneous emissions can also be
expected to be capable of white-light emission. For the repre-
sentative one (200 : 1[2/3/4] ¼ 1 : 7 : 8) in Eu3+–Tb3+–Gd3+-graf-
ted polymers Poly(MMA-co-2-co-3-co-4) (200 : 1[2/3/4] ¼ 1 : 7 : 4,
1 : 7 : 8 and 1 : 7 : 12), similar PXRD, TG and DR results (also in
Fig. 3–5) to those of Poly(MMA-co-2) (200 : 1) are exhibited due
to the iso-structural character of complex monomers 2–4.
Especially, its glass-transition temperature (Tg) from the DSC
Fig. 8 Emission spectra (left) and corresponding CIE coordinates
(right) of Eu3+–Tb3+–Gd3+-grafted polymers Poly(MMA-co-2-co-3-
co-4) in the solid state at room temperature upon excitation: (8a)
200 : 1(2/3/4 ¼ 1 : 7 : 4); (8b) 200 : 1(2/3/4 ¼ 1 : 7 : 8); (8c) 200 : 1(2/3/
4 ¼ 1 : 7 : 12).
ꢁ
analysis (also in Fig. 4) is about 158 C, and much higher than
those (50–130 ꢁC) of common organic polymers33 including
PMMA (105–128 ꢁC)34 upon involvement of polymerized
complex monomers. Such a high value of Tg should be desirable
for Eu3+–Tb3+–Gd3+-graed polymers used as emissive materials
for optoelectronic devices. Moreover, their reasonable PDI
values of 1.21–1.24 and Mn values of 19 664–19 725 g molꢀ1
(Table 3S†) are also obtained from the rational AIBN-initiated
radical copolymerization of 2–4 and MMA. XPS quantitative
analysis also veries the stoichiometric atomic molar ratios
(1 : 6.97 : 3.93, 1 : 7.02 : 7.96 and 1 : 6.98 : 11.95) of Eu3+ to Tb3+
and to Gd3+ as desired feeds ([2/3/4 ¼ 1 : 7 : 4, 1 : 7 : 8 and
1 : 7 : 12). As expected, all the polymers Poly(MMA-co-2-co-3-co-
deviated from 341 nm (lex). Nonetheless, the results of almost
constant Eu3+- or Tb3+-centered phosphorescent lifetime (903
ms or 718 ms) as comparable with that (901 ms or 714 ms) of
Poly(MMA-co-2) (200 : 1) or Poly(MMA-co-3) (200 : 1) should be
the reason for their uniform dispersal in the PMMA chain.
Thus, through contact in the interface mainly between PMMA
and chromophores, Tb3+ to Eu3+ centers' energy transfer is
effectively inhibited. Aer supplementation of insufficient blue-
light, the perfect trichromatic incorporation renders all the
resultant emissions characteristic of white-light as desired for
Poly(MMA-co-2-co-3-co-4) (1 : 7 : 8), as shown in Fig. 8b. In
contrast to Poly(MMA-co-2-co-3-co-4) (1 : 7 : 4), the highest-
quality white-light (point H; 0.322, 0.331) is more close to the
ideal white-light, and the CCT of 5979 K between 2500–6500 and
an ultra-high CRI of 94 are estimated. More importantly, the
optimal white-light is produced at a relatively shorter excitation
wavelength of 345 nm besides the similar resource of both
uorescence (6.3 ns) and phosphorescence (897 ms of Eu3+-
center and 717 ms of Tb3+-center). This wavelength is very near
to 341 nm (lex) of the three chromophores within, affording
a large quantum yield up to 17.8%, which is the highest among
4) exhibit the simultaneous emissions (Fig. 8) of the Eu3+
-
centered red-light at 613 nm, Tb3+-centered green-light at
546 nm and the blue-light at 434 nm from the Gd3+-involved
ligands, respectively. For Poly(MMA-co-2-co-3-co-4) (1 : 7 : 4), as
shown in Fig. 8a, even with scarce blue-light, its combination
with prominent Eu3+-centered red-light and Tb3+-centered
green-light gives rise to the emitting colors (points A–E) falling
well within the white-light region upon excitation from 345 nm
to 365 nm. Of particular note, excitation at 360 nm is found to
produce a high-quality white-light emission (point D): a CIE
coordinate of x ¼ 0.346, y ¼ 0.319; a CCT of 4769 K and the CRI
up to 85. To our dismay, a relatively lower overall white-light
quantum yield (9.6%) comparable with that (10.5%) of the
optimal point g for the polymers' blend is also obtained because
it is saturated from a longer excitation wavelength (360 nm)
all reported white-light-emitting polymeric systems with Ln3+
-
doping9 or Ln3+-graing12a,12b,12c,15 from the RGB trichromatic
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strategy, and comparable with the best (18.4% or 20.4%) of
Ln3+-related polymers12d or MOFs35 by the dichromatic strategy.
Similarly, further increase in the relative Gd3+-content in
formation of Poly(MMA-co-2-co-3-co-4) (1 : 7 : 12), the excess
Gd3+-incorporated blue-light leads to the low-quality white-light
shown in Fig. 8c, where a distinct tendency approaching bluish-
purple of their CIE coordinates (0.241–0.285, 0.184–0.253;
points L–Q) upon excitation from 340 to 365 nm is exhibited.
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Conclusions
For the rst example of the single-component Eu3+–Tb3+–Gd3+
-
graed polymer Poly(MMA-co-2-co-3-co-4), the color-tuning to
direct high-quality white-light (CIE coordinate of x ¼ 0.322, y ¼
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¨
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The authors acknowledge funds from the National Natural
Science Foundation (21373160, 91222201, 21173165), the
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