Near-infrared (NIR) luminescent metallopolymers based on Ln 4(Salen)4 nanoclusters (Ln = Nd or Yb)

Article abstract of DOI:10.1039/c3tc31814a

A series of novel Wolf Type II Ln³⁺-containing metallopolymers PNBE-co-Ln₄(Salen)₄ (Ln = La, Nd, Yb, Er or Gd) are obtained from the controlled ring-opening metathesis polymerization (ROMP) of norbornene (NBE) and the homo

Full text of DOI:10.1039/c3tc31814a

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Materials Chemistry C
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Near-infrared (NIR) luminescent metallopolymers
based on Ln4(Salen)4 nanoclusters (Ln ¼ Nd or Yb)†
Cite this: J. Mater. Chem. C, 2014, 2,
1489
a
a
a
a
a
Weixu Feng, Yao Zhang, Zhao Zhang, Peiyang Su, Xingqiang Lu, Jirong Song,a
*
¨
Daidi Fan,a Wai-Kwok Wong,b Richard A. Jonesc and Chengyong Sud
A series of novel Wolf Type II Ln3+-containing metallopolymers PNBE-co-Ln4(Salen)4 (Ln ¼ La, Nd, Yb, Er or
Gd) are obtained from the controlled ring-opening metathesis polymerization (ROMP) of norbornene (NBE)
and the homoleptic tetranuclear [Ln4(L)2(HL)2Cl2(m3-OH)2]$2Cl (Ln4(Salen)4, Ln ¼ La, 1; Nd, 2; Ln ¼ Yb, 3; Ln
¼ Er, 4; Ln ¼ Gd, 5) self-assembled from the diallyl-modified flexible hexadentate Salen-type Schiff-base
ligand H2L (N,N0-bis(5-allyl-3-methoxysalicylidene)cyclohexane-1,2-diamine) with LnCl3$6H2O. The
results of their photophysical properties show that the strong and characteristic near-infrared (NIR)
luminescent Ln3+-centered emissions for PNBE-co-Ln4(Salen)4 (Ln ¼ Nd, 2; Yb, 3) are retained with
relatively higher intrinsic quantum yields than the corresponding monomers in solution, and the
concentration self-quenching of Ln3+-based materials could be effectively prevented in both solution
and solid states.
Received 13th September 2013
Accepted 4th November 2013
DOI: 10.1039/c3tc31814a
www.rsc.org/MaterialsC
linker, covalently coupled to the backbone or directly incorpo-
rated into the polymer backbone, respectively. In contrast to
1. Introduction
Wolf Type I Ln3+-containing metallopolymers, the other two
(Types II and III) exhibit relatively more efficient energy transfer
due to the direct electronic communication between the Ln3+
ion and the polymer backbone.
Ln3+-containing metallopolymers,1 as a unique class of hybrid
materials, possess the advantage of the benecial properties of
both the inorganic lanthanide ions and the attractive features
including mechanical strength, exibility, ease of processing
and low cost of organic polymers. This advantage makes them
ideal as emissive materials in luminescent applications,
including sensors,2 memory devices3 and polymer light-emit-
ting diodes.4 As a matter of fact, based on the arrangements of
Ln3+ ions relative to the polymer backbone, three types (Type I
to Type III) described by Wolf5 for the so metallopolymers have
also been realized, in which the Ln3+ metal center is either
tethered to the polymeric backbone by a saturated organic
On the one hand, compared to the amount of effort targeted at
the development of visible-emissive Ln3+-containing Wolf Type I,
II or III metallopolymers,1 there are few examples of NIR lumi-
nescent Ln3+-containing metallopolymers,6 which is quite
disproportional to the great need for NIR luminescent Ln3+ optical
materials.7 On the other hand, despite some strategies including
polycondensation, radical polymerization and electro-
polymerization developed for their preparation, to the best of our
knowledge, the obtained visible or NIR Ln3+-containing metal-
lopolymer systems are usually focused on the mononuclear Ln3+
complexes, and no systems built from Ln3+ nanoclusters, espe-
cially with NIR luminescence, have yet been reported. It can be
expected that more efficient energy transfer could be obtained
from the electronic communication between the multiple metal
centers from the Ln3+ nanoclusters besides that between the
metal center and the polymer backbone. In our recent report, we
successfully designed two series of anion-induced tetranuclear
Ln4(Salen)4 (ref. 8) or Ln4(Salen)2 (ref. 9) nanoclusters based on
the typical exible Salen-type Schiff-base ligand. It was founded
that the good NIR luminescent properties arose from multiple
Ln3+-center complexes through the both 1LC and 3LC excited
states of the exible ligand. Naturally, if introducing new
functions (for example, C]C–C groups) into the designed diallyl-
modied Salen-type Schiff-base ligand H2L (H2L ¼ N,N0-bis(5-allyl-
aShaanxi Key Laboratory of Degradable Medical Material, Shaanxi Key Laboratory of
Physico-inorganic Chemistry, Northwest University, Xi'an 710069, Shaanxi, China.
E-mail: lvxq@nwu.edu.cn; Tel: +86-29-88302312
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
dMOE Laboratory of Bioinorganic and Synthetic Chemistry/KLGH EI of Environment
and Energy Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou 510275, Guangdong, China
† Electronic supplementary information (ESI) available: The selected bond
lengths and angles for complex 3$4EtOH$6H2O in Table 1S, and the
intra-molecular N–H/O H-bond interaction for complex 3$4EtOH$6H2O in
Fig. 1S, the visible spectra of H2L and complex 5, and the visible spectra of
PNBE-co-5 in Fig. 2–3S, DR spectra of Ln4(Salen)4, visible emission and
excitation spectra of PNBE-co-5 and 5 and NIR emission and excitation spectra
of PNBE-co-3 from different monomer ratios in solid state in Fig. 4–6S. CCDC
957505. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3tc31814a
3-methoxysalicylidene)cyclohexane-1,2-diamine),
Clꢀ-induced
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tetranuclear [Ln4(L)2(HL)2Cl2(m3-OH)2]$2Cl (Ln ¼ La, 1; Ln ¼ Nd, 1H, –CH]), 5.31 (m, 2H, ]CH2), 4.67 (d, 2H, –CH2), 3.90 (s, 3H,
2; Ln ¼ Yb, 3; Ln ¼ Er, 4 or Ln ¼ Gd, 5) complex monomers –OMe).
should be formed and could be further copolymerized, thus the
rst examples of NIR luminescent Ln3+-containing metal- Synthesis of 5-allyl-2-hydroxy-3-methoxybenzaldehyde
lopolymers based on tetranuclear Ln3+ (Ln ¼ Nd, Yb or Er)
A solution of 2-propenyloxy-3-methoxybenzaldehyde (5.76 g,
complexes could be expected.
30 mmol) in 30 mL of absolute toluene and absolute N,N-
ꢁ
dimethylaniline in a sealed tube was heated at 215 C for 5 h.
Aer cooling to room temperature, the solvents were removed
under reduced pressure, and the residual oil was chromato-
2. Experimental
HPLC grade tetrahydrofuran (THF) was purchased from Fisher
Scientic and puried over solvent columns. Other solvents
graphed on a silica gel column, eluting with n-hexane–ethyl
acetate (4 : 1, v/v) to give the pale yellow solid product in ca. 51%
yield (2.95 g). Calc. for C11H12O3: C, 68.74; H, 6.29%; found: C,
68.72; H, 6.34%. IR (KBr, cmꢀ1): 3144 (b), 3007 (w), 2970 (w),
2841 (w), 1873 (w), 1659 (s), 1612 (m), 1593 (m), 1497 (s), 1464
(w), 1433 (m), 1395 (w), 1369 (w), 1269 (vs), 1186 (m), 1146 (m),
1072 (w), 993 (w), 934 (m), 901 (w), 853 (w), 814 (w), 743 (w), 669
(w), 598 (w), 542 (w), 498 (w). 1H NMR (400 MHz, CDCl3): d (ppm)
11.00 (s, 1H, –OH), 9.90 (s, 1H, –CHO), 6.99 (s, 1H, –Ph), 6.95 (s,
1H, –Ph), 5.97 (m, 1H, –CH]), 5.13 (m, 2H, ]CH2), 3.91 (s, 3H,
–OMe), 3.38 (s, 2H, –CH2).
˚
were used as received from Sigma Aldrich and stored over 3 A
activated molecule sieves. Other chemicals including norbor-
nene (NBE) and Grubbs II were commercial products of reagent
grade and were used without further purication. All manipu-
lations of air and water sensitive compounds were carried out
under dry N2 using the standard Schlenk line techniques.
Elemental analyses were performed on a Perkin-Elmer 240C
elemental analyzer. Infrared spectra were recorded on a Nicolet
Nagna-IR 550 spectrophotometer in the region 4000–400 cmꢀ1
using KBr pellets. 1H NMR spectra were recorded on a JEOL EX
400 spectrometer with SiMe4 as internal standard in CDCl3 or
DMSO-d6 at room temperature. 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 electrospray
ion source and MeCN as solvent. Both electronic absorption
spectra in solution and diffuse reectance (DR) spectra in the
solid state within the UV/Visible to NIR region were recorded
with a Cary 300 UV spectrophotometer, steady-state visible
uorescence and PL excitation spectra on a Photon Technology
International (PTI) Alpha scan spectrouorometer and visible
decay spectra on a pico-N2 laser system (PTI Time Master). The
quantum yield of the visible luminescence for each sample was
determined by the relative comparison procedure, using a
reference of a known quantum yield (quinine sulfate in dilute
H2SO4 solution, Fem ¼ 0.546). NIR emission and excitation in
solution or solid were recorded by a PTI QM4 spectrouorom-
eter with a PTI QM4 Near-Infrared InGaAs detector. Gel
permeation chromatography (GPC) analyses of the polymers
were performed using a Waters 1525 binary pump coupled to a
Waters 2414 refractive index detector with HPLC THF as the
eluant on American Polymer Standard 10 mm particle size,
linear mixed bed packing columns. The GPC was calibrated
using polystyrene standards. X-ray photoelectron spectroscopy
(XPS) was carried out on a PHI 5700 XPS system equipped with a
dual Mg X-ray source and monochromatic Al X-ray source
complete with depth prole and angle-resolved capabilities.
Synthesis of diallyl-modied Salen-type Schiff-base ligand H2L
(H2L ¼ N,N0-bis(5-allyl-3-methoxysalicylidene)cyclohexane-
1,2-diamine)
To a stirred solution of an equimolar mixture of cis- and trans-
1,2-diaminocyclohexane (600 mL, 5 mmol) in absolute EtOH
(15 mL), 5-allyl-2-hydroxy-3-methoxybenzaldehyde (1.92 g,
10 mmol) was added, and the resulting mixture was stirred
under N2 atmosphere at room temperature for 12 h. The
insoluble yellow precipitate was ltered and was recrystallized
using absolute diethyl ether to give a yellow polycrystalline
solid. Yield: 1.48 g, 64%. Calc. for C28H34N2O4: C, 72.70; H, 7.41;
N 6.06%; found: C, 72.63; H, 7.44; N, 6.02%. IR (KBr, cmꢀ1):
3320 (b), 3134 (m), 1630 (s), 1482 (m), 1462 (m), 1401 (vs), 1267
(s), 1165 (m), 1102 (m), 1045 (w), 988 (m), 943 (w), 918 (m), 863
(m), 801 (w), 738 (w), 688 (w), 604 (w), 571 (w), 534 (w), 485 (w),
443 (w), 432 (w). 1H NMR (400 MHz, DMSO-d6): d (ppm) 13.65 (d,
2H, –OH), 8.20 (d, 2H, –CH]N), 6.68 (d, 2H, –Ph), 6.59 (d, 2H,
–Ph), 5.87 (m, 2H, –CH]), 5.03 (m, 4H, ]CH2), 3.85 (d, 6H,
–OMe), 3.29 (m, 4H, –CH2), 3.23 (m, 2H, –Ch), 1.89 (m, 4H, –Ch),
1.70 (m, 2H, –Ch), 1.47 (m, 2H, –Ch).
Synthesis of [Ln4(L)2(HL)2Cl2(m3-OH)2]$2Cl complex
monomers (Ln ¼ La, 1; Ln ¼ Nd, 2; Ln ¼ Yb, 3; Ln ¼ Er, 4; Ln
¼ Gd, 5)
To a stirred solution of H2L (0.139 g, 0.3 mmol) in absolute
EtOH (5 mL), Et3N (100 mL) and a solution of LaCl3$6H2O
(0.3 mmol; Ln ¼ La, 0.106 g; Ln ¼ Nd, 0.108 g; Ln ¼ Yb, 0.116 g;
Ln ¼ Er, 0.115 g or Ln ¼ Gd, 0.112 g) in absolute EtOH (5 mL)
Synthesis of 2-propenyloxy-3-methoxybenzaldehyde
2-Propenyloxy-3-methoxybenzaldehyde was obtained as a pale were added. The resultant mixture was stirred under N2 atmo-
yellow liquid according to a well-established procedure10 from o- sphere at room temperature for 5 h, and the clear pale yellow
vanillin (6.08 g, 40 mmol), allyl bromide (12.0 mL, 140 mmol) solution was ltered. Diethyl ether was allowed to diffuse slowly
and anhydrous K2CO3 (11.06 g, 80 mmol). Yield: 6.9 g, 90%. into the ltrate at room temperature and the pale yellow
Calc. for C11H12O3: C, 68.74; H, 6.29%; found: C, 68.69; H, microcrystal products of 1–5 were obtained in a few weeks.
6.41%. 1H NMR (400 MHz, CDCl3): d (ppm) 10.42 (s, 1H, –CHO),
7.41 (d, 1H, –Ph), 7.18 (d, 1H, –Ph), 7.12 (t, 1H, –Ph), 6.08 (m, 52.23; H, 5.17; N, 4.35%; found: C, 52.28; H, 5.25; N, 4.29%. IR
For 1: yield: 0.133 g, 69%. Calc. for C112H132N8O18Cl4La4: C,
1490 | J. Mater. Chem. C, 2014, 2, 1489–1499
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(KBr, cmꢀ1): 3029 (m), 2347 (w), 2174 (w), 1662 (vs), 1462 (s), solution of NBE (70 mg, 0.75 mmol) in absolute CHCl3 (15 mL),
1401 (s), 1342 (w), 1280 (w), 1249 (m), 1209 (w), 1164 (m), 1106 Grubbs II initiator (1.5 mg, 0.2 mol mLꢀ1 of the solvent) was
(m), 1032 (w), 994 (m), 951 (w), 908 (w), 857 (w), 822 (w), 780 (w), added, and the resultant mixture was purged with N2 for 10 min
1
690 (w), 592 (m), 531 (w), 480 (w), 444 (w). H NMR (400 MHz, and sealed under a reduced N2 atmosphere. Aer the homo-
DMSO-d6): d (ppm) 13.30 (s, 2H, –NH), 8.42 (s, 8H, –CH]N), geneous solution was continuously stirred at room temperature
8.10 (s, 2H, OHꢀ), 6.80 (m, 8H, –Ph), 6.72 (m, 8H, –Ph), 5.90 (m, for 24 h, ethyl vinyl ether (100 mL) was added to quench the
8H, –CH]), 5.02 (m, 16H, ]CH2), 3.74 (t, 24H, –OMe), 3.24 (m, reaction. The viscousꢁmixture was diluted with absolute THF (20
16H, –CH2), 3.20 (m, 8H, –Ch), 1.84 (m, 16H, –Ch), 1.61 (m, 8H, mL) and dried at 45 C under vacuum to constant weight.
–Ch), 1.45 (m, 8H, –Ch). ESI-MS (in MeCN) m/z: 1252.43 (100%),
[M ꢀ (Cl)2]2+; 2540.30 (18%), [M ꢀ Cl]+.
For PNBE: yield: 92%. IR (KBr, cmꢀ1): 2924 (m), 2853 (s),
1944 (w), 1450 (s), 1432 (s), 1401 (w), 1370 (w), 1346 (w), 1303 (w),
For 2: yield: 0.125 g, 64%. Calc. for C112H132N8O18Cl4Nd4: C, 1180 (w), 1069 (w), 1028 (w), 996 (w), 941 (w), 735 (s), 698 (vs),
1
51.80; H, 5.12; N, 4.31%; found: C, 51.66; H, 5.24; N, 4.26%. IR 540 (w). H NMR (400 MHz, CDCl3): d (ppm) 5.33 (s, ]CH–),
(KBr, cmꢀ1): 3020 (m), 2360 (w), 2164 (w), 1667 (vs), 1460 (s), 2.42 (s, –CH), 1.81 (m, –CH2), 1.29 (m, –CH2), 1.03 (m, –CH2).
1403 (s), 1340 (w), 1280 (w), 1253 (m), 1210 (w), 1163 (m), 1112
(m), 1030 (w), 995 (m), 956 (w), 918 (w), 855 (w), 826 (w), 785 (w),
Preparation of copolymerized metallopolymers PNBE-co-
691 (w), 593 (m), 539 (w), 470 (w), 440 (w). ESI-MS (in MeCN) m/z:
1263.09 (100%), [M ꢀ (Cl)2]2+; 2561.64 (22%), [M ꢀ Cl]+.
[Ln4(L)2(HL)2Cl2(m3-OH)2]$2Cl (Ln ¼ La, Nd, Yb, Er or Gd, 1–5)
activated with Grubbs II
For 3: yield: 0.142 g, 70%. Calc. for C112H132N8O18Cl4Yb4: C,
49.60; H, 4.91; N, 4.13%; found: C, 49.55; H, 4.93; N, 4.07%. IR The homogeneous copolymerization of NBE and each of tetra-
(KBr, cmꢀ1): 3021 (m), 2344 (w), 2172 (w), 1666 (s), 1457 (s), 1402 nuclear complexes 1–5 activated with Grubbs II was also carried
(s), 1339 (w), 1279 (w), 1248 (m), 1211 (w), 1162 (m), 1103 (m), out in a Fisher–Porter glass reactor and protected by nitrogen.
1032 (w), 989 (m), 957 (w), 916 (w), 856 (w), 821 (w), 782 (w), 688 To
a solution of complex [Ln4(L)2(HL)2Cl2(m3-OH)2]$2Cl
(w), 591 (m), 533 (w), 472 (w), 455 (w). ESI-MS (in MeCN) m/z: (0.015 mmol, Ln ¼ La (1), 38 mg; Ln ¼ Nd (2), 39 mg; Ln ¼ Yb
1320.69 (100%), [M ꢀ (Cl)2]2+; 2676.84 (15%), [M ꢀ Cl]+.
(3), 41 mg; Ln ¼ Er (4), 40 mg or Ln ¼ Gd (5), 40 mg) in absolute
For 4: yield: 0.123 g, 61%. Calc. for C112H132N8O18Cl4Er4: C, CHCl3 (15 mL), Grubbs II initiator (1.5 mg, 0.2 mol mLꢀ1 of the
50.02; H, 4.95; N, 4.17%; found: C, 49.92; H, 5.02; N, 4.05%. IR solvent) and NBE (70 mg, 0.75 mmol) were added, and the
(KBr, cmꢀ1): 3019 (m), 2349 (w), 2170 (w), 1670 (vs), 1455 (s), resultant mixture was purged with N2 for 10 min and sealed
1401 (s), 1339 (w), 1281 (w), 1249 (m), 1214 (w), 1163 (m), 1108 under a reduced N2 atmosphere. Aer the homogeneous solu-
(m), 1030 (w), 995 (m), 953 (w), 918 (w), 854 (w), 824 (w), 782 (w), tion was continuously stirred at room temperature for 24 h,
687 (w), 592 (m), 530 (w), 481 (w), 442 (w). ESI-MS (in MeCN) m/z: ethyl vinyl ether (100 mL) was added to quench the reaction. The
1309.13 (100%), [M ꢀ (Cl)2]2+; 2653.72 (12%), [M ꢀ Cl]+.
viscous mixture was diluted with absolute THF (20 mL) and
For 5: yield: 0.119 g, 60%. Calc. for C112H132N8O18Cl4Gd4: C, dried in air. The resulted solid lm of PNBE-co-
ꢁ
50.78; H, 5.02; N, 4.23%; found: C, 50.75; H, 5.16; N, 4.13%. IR [Ln4(L)2(HL)2Cl2(m3-OH)2]$2Cl was collected and dried at 45 C
(KBr, cmꢀ1): 3025 (m), 2346 (w), 2175 (w), 1669 (vs), 1456 (s), under vacuum to constant weight.
1400 (s), 1338 (w), 1280 (w), 1249 (m), 1211 (w), 1164 (m), 1102
For PNBE-co-1 (50 : 1): FT-IR (KBr, cmꢀ1): 3082 (w), 3061 (w),
(m), 1037 (w), 993 (m), 953 (w), 914 (w), 855 (w), 821 (w), 784 (w), 3026 (m), 2941 (m), 2862 (s), 1944 (w), 1845 (w), 1803 (w), 1744
687 (w), 590 (m), 517 (w), 481 (w), 443 (w). ESI-MS (in MeCN) m/z: (w), 1637 (m), 1543 (w), 1506 (s), 1446 (s), 1406 (w), 1340 (w),
1289.11 (100%), [M ꢀ (Cl)2]2+; 2613.68 (19%), [M ꢀ Cl]+.
1296 (w), 1240 (w), 1161 (w), 1101 (w), 1031 (w), 964 (w), 908 (w),
850 (w), 825 (s), 729 (w), 698 (vs), 646 (w), 540 (w). 1H NMR (400
MHz, DMSO-d6): d (ppm) 13.29 (s, –NH), 8.42 (s, –CH]N), 8.31
(s, OHꢀ), 7.70 (m, –CH]), 6.81 (m, –Ph), 6.43 (m, –Ph), 5.90 (m,
–CH]), 4.92 (m, –CH]), 4.12 (m, –CH]), 3.72 (m, –OMe), 2.70
(m, –CH), 2.32 (m, –CH), 1.86 (m, –Ch), 1.62 (m, –Ch), 1.51 (m,
–Ch), 1.29 (m, –CH2), 0.90 (m, –CH2).
X-ray crystallography
'Single crystals of [Yb4(L)2(HL)2Cl2(m3-OH)2]$2Cl$4EtOH$6H2O
(3$4EtOH$6H2O) of suitable dimensions were mounted onto
thin glass bers. All the intensity data were collected on a Bruker
SMART CCD diffractometer (Cu-Ka radiation and l ¼ 1.54184 A)
˚
For PNBE-co-2 (50 : 1): IR (KBr, cmꢀ1): 3080 (w), 3056 (w),
3019 (m), 2939 (m), 2869 (s), 1946 (w), 1852 (w), 1801 (w), 1746
(w), 1633 (m), 1544 (w), 1510 (s), 1448 (s), 1404 (w), 1344 (w),
1295 (w), 1243 (w), 1165 (w), 1103 (w), 1036 (w), 961 (w), 906 (w),
855 (w), 823 (s), 721 (w), 696 (vs), 642 (w), 541 (w).
in F and u scan modes. Structures were solved by direct
methods followed by difference Fourier syntheses, and then
rened by full-matrix least-squares techniques against F2 using
SHELXL-97.11 All other non-hydrogen atoms were rened with
anisotropic thermal parameters. Absorption corrections were
applied using SADABS.12 Hydrogen atoms were placed in calcu-
lated positions and rened isotropically using a riding model.
For PNBE-co-3 (30 : 1, 50 : 1 or 100 : 1): IR (KBr, cmꢀ1): 3083
(w), 3064 (w), 3021 (m), 2936 (m), 2871 (s), 1951 (w), 1851 (w),
1810 (w), 1746 (w), 1632 (m), 1541 (w), 1504 (s), 1450 (s), 1410
(w), 1344 (w), 1291 (w), 1245 (w), 1161 (w), 1106 (w), 1033 (w), 966
(w), 903 (w), 851 (w), 825 (s), 723 (w), 695 (vs), 642 (w), 541 (w).
For PNBE-co-4 (50 : 1): IR (KBr, cmꢀ1): 3081 (w), 3063 (w),
Preparation of PNBE activated with Grubbs II
The homogeneous polymerization activated with Grubbs II for
comparison was carried out in a Fisher–Porter glass reactor and 3022 (m), 2941 (m), 2869 (s), 1949 (w), 1856 (w), 1812 (w), 1744
protected by nitrogen according to the typical procedure.13 To a (w), 1635 (m), 1543 (w), 1501 (s), 1453 (s), 1416 (w), 1349 (w),
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1299 (w), 1241 (w), 1163 (w), 1104 (w), 1036 (w), 961 (w), 902 (w),
854 (w), 821 (s), 725 (w), 693 (vs), 641 (w), 543 (w).
For PNBE-co-5 (50 : 1): IR (KBr, cmꢀ1): 3079 (w), 3058 (w),
3021 (m), 2949 (m), 2871 (s), 1951 (w), 1855 (w), 1819 (w), 1751
(w), 1641 (m), 1548 (w), 1509 (s), 1451 (s), 1415 (w), 1342 (w),
1291 (w), 1239 (w), 1161 (w), 1109 (w), 1041 (w), 965 (w), 903 (w),
851 (w), 823 (s), 723 (w), 695 (vs), 643 (w), 545 (w).
3. Results and discussion
Synthesis and characterization of ligand H2L and
[Ln4(L)2(HL)2Cl2(m-OH)2]$2Cl (Ln ¼ La, 1; Ln ¼ Nd, 2; Ln ¼ Yb,
3; Ln ¼ Er, 4; Ln ¼ Gd, 5) nanocluster monomers
As shown in Scheme 1, 5-allyl-2-hydroxy-3-methox-
ybenzaldehyde was obtained from o-vanillin with the allyl
bromide and K2CO3 and the subsequent para Claisen
Fig. 1 1H NMR spectra for metallopolymer PNBE-co-1 (top) in DMSO-
rearrangement.10 Reaction of 5-allyl-2-hydroxy-3-methoxy- d6, PNBE (middle) in CDCl3 and La4(Salen)4 complex 1 (bottom) in
DMSO-d6.
benzaldehyde and an equimolar mixture of cis- and trans-1,2-
diaminocyclohexane afforded the diallyl-modied Salen-type
Schiff-base ligand H2L in the good yield of 64%. Further
slightly spread shi (d from 8.42 to 1.45 ppm) of the proton
resonances of the ligands relative to that of the free H2L ligand
(d from 8.20 to 1.47 ppm) is observed. Furthermore, the proton
through the reaction of equimolar amounts of the deprotonated
Salen-type Schiff-base L2ꢀ ligand and LnCl3$6H2O (Ln ¼ La, Nd,
Yb, Er or Gd) in absolute EtOH, series of tetranuclear Ln4(Sa-
resonances (d ¼ 5.90, 5.02 and 3.24 ppm) of the functional allyl
len)4 complexes [Ln4(L)2(HL)2Cl2(m3-OH)2]$2Cl (Ln ¼ La, 1; Nd,
groups in complex 1 remain the same as those (d ¼ 5.87, 5.03
2; Ln ¼ Yb, 3; Ln ¼ Er, 4 or Ln ¼ Gd, 5) were isolated as yellow
and 3.29 ppm) of the ligands despite the coordination of La3+
ions. The ESI-MS spectra of series of complexes 1–5 in MeCN
display the corresponding similar patterns and exhibit the
strong mass peak at m/z 1252.43 (1), 1263.09 (2), 1320.69 (3),
1309.13 (4) or 1289.11 (5) assigned to the major species
[Ln4(L)2(HL)2Cl2(m3-OH)2]2+ of complexes 1–5. These observa-
tions further indicate that the discrete homoleptic Ln4(Salen)4
units are retained in the respective dilute MeCN solution.
microcrystalline solids.
The diallyl-modied Salen-type Schiff-base ligand H2L and
the series of tetranuclear Ln4(Salen)4 complexes 1–5 were well
1
characterized by EA, FT-IR, H NMR and ESI-MS. In the room
temperature 1H NMR spectrum of the tetranuclear La3+ complex
1 (Fig. 1), besides the appearance of intra-molecular resonance-
assisted hydrogen bonded (RAHB) N–H/O proton (d ¼
13.30 ppm) and coordinated m3-OHꢀ proton (d ¼ 8.10 ppm)
resonances and disappearance of the RAHB O–H/N proton
resonances (d ¼ 13.65 ppm) of the diallyl-modied Salen-type
Schiff-base ligand H2L, due to the coordination of La3+ ions, a
Table
1 Crystal data and structure refinement for complex
3$4EtOH$6H2O
Compound
3$4EtOH$6H2O
C120H168N8O28Cl4Yb4
3004.58
Empirical formula
Formula weight
Crystal size/mm
T/K
0.30 ꢂ 0.25 ꢂ 0.22
153(2)
Crystal system
Space group
Monoclinic
P21/n
14.70010(10)
23.6787(2)
20.1012(2)
90
102.1750(10)
90
6873.46(10)
2
˚
a/A
˚
b/A
˚
c/A
a/ꢁ
b/ꢁ
g/ꢁ
˚
V/A
Z
r/g cmꢀ3
1.452
6.103
m/mmꢀ1
F(000)
3032
Data/restraints/parameters
Quality-of-t indicator
Final R indices [I > 2s(I)]
41 855/4/740
1.069
R1 ¼ 0.0514
wR2 ¼ 0.1394
R1 ¼ 0.0558
wR2 ¼ 0.1438
Scheme 1 Reaction scheme for the synthesis of the diallyl-modified
Salen-type Schiff-base ligand H2L and the homoleptic cyclic tetra-
nuclear Ln4(Salen)4 complexes 1–5.
R indices (all data)
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Fig.
2 Perspective drawing of the cationic part in complex
3$4EtOH$6H2O; free anions, H atoms and solvates are omitted for
clarity.
Fig. 3 UV-visible absorption spectra of the ligand H2L and complexes
2–5 in MeCN solution at 1 ꢂ 10ꢀ5 M at room temperature.
The solid state structure of 3$4EtOH$6H2O as the represen-
tative of complexes 1–5 was determined by X-ray single-crystal
diffraction analysis. Crystallographic data and selected bond
lengths and angles for the complexes are presented in Tables 1
and 1S,† respectively. Complex 3$4EtOH$6H2O crystallizes in
the monoclinic space group P21/n. For complex 3$4EtOH$6H2O,
the structural unit is composed of one cation
[Yb4(L)2(HL)2Cl2(m3-OH)2]2+, two free Clꢀ anions, four solvates
EtOH and six solvates H2O. As shown in Fig. 2, for the cationic
[Yb4(L)2(HL)2Cl2(m3-OH)2]2+ part, two equivalent Yb2(L)(HL)
moieties are bridged by two m-O phenoxide atoms (O6 and O6a)
of two Salen-type Schiff-base (HL)ꢀ ligands and two O atoms (O9
and O9a) of two coordinated m3-OHꢀ groups, resulting in the
formation of a homoleptic cyclic tetranuclear (Yb4(Salen)4) host
structure. In each of the two equivalent Yb2(L)(HL) moieties,
two Yb3+ (Yb1 and Yb2) ions with different coordination envi-
ronments also linked by two m-O phenoxide atoms (O2 and O3)
of one Salen-type Schiff-base (L)2ꢀ ligand and one O atom (O9)
Fig. 4 NIR emission and excitation spectra of complexes 2–3 in MeCN
solution at 1 ꢂ 10ꢀ5 M at room temperature.
Table 2 The photophysical properties of the H2L, complexes 2–5 at 2 ꢂ 10ꢀ5 M in absolute MeCN solution and metallopolymer PNBE-co-
Ln4(Salen)4 (Ln ¼ Nd, Yb, Er or Gd) in CHCl3 solution at 0.3 g Lꢀ1 at room temperature or 77 K
Absorption
Excitation
Emission
lab/nm
Compound
[log(3/dm3 molꢀ1 cmꢀ1)]
lex/nm
lem/nm (s, F ꢂ 103)
H2L
2
3
4
5
226(0.68), 262(0.31), 336(0.07)
233(1.71), 271(0.73), 347(0.24)
233(1.74), 271(0.72), 348(0.25)
232(1.77), 272(0.77), 348(0.26)
234(1.68), 274(0.76), 348(0.26)
304, 356(sh)
375, 402(sh)
370(sh), 398
400
329(sh), 396
398
362(1.22 ns, 0.23), 478(1.57 ns, 0.25)
526(w); 905(1.67 ms), 1090(1.71 ms), 1363(1.73 ms)
528(w); 978(16.32 ms)
532(w); —a
530(0.81 ns, 0.74)
511(3.6 ns, 77 K), 569(7.8 ms, 77 K)
—
PNBE
230(2.79)
—
PNBE-co-2
PNBE-co-3
PNBE-co-4
PNBE-co-5
226(3.30), 276(0.48), 346(0.21)
228(1.31), 270(0.57), 346(0.21)
233(1.05), 275(0.48), 345(0.21)
227(1.20), 269(0.52), 343(0.19)
312(sh), 428
316(sh), 432
—
312, 430
431
901(1.76 ms), 1082(1.81 ms), 1356(1.79 ms)
980(17.03 ms)
—
545(0.75 ns, 0.68)
525(2.1 ns, 77 K), 573(8.2 ms, 77 K)
a
a
The emission is too weak to be detected.
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Scheme
2 Reaction scheme for the synthesis of PNBE-co-
Ln4(Salen)4.
Fig. 6 UV-visible absorption spectra of the metallopolymers PNBE-
co-Ln4(Salen)4 in CHCl3 solution at 0.3 g Lꢀ1 at room temperature.
Table 3 GPC data of the samples PNBE and PNBE-co-Ln4(Salen)4
Sample
Monomers
NBE/monomer
Mna/g molꢀ1
PDIb
PNBE
NBE
—
20 199
10 143
12 792
9266
12 245
13 383
12 681
12 524
2.03
2.27
2.16
1.43
2.18
2.55
2.14
2.26
PNBE-co-1
PNBE-co-2
PNBE-co-3
PNBE-co-3
PNBE-co-3
PNBE-co-4
PNBE-co-5
NBE and 1
NBE and 2
NBE and 3
NBE and 3
NBE and 3
BNE and 4
NBE and 5
50
50
30
50
100
50
50
a
Mn is number average molecular weight. b PDI ¼ Mw/Mn, where Mw is
weight average molecular weight.
Fig. 7 NIR emission and excitation spectra of metallopolymers PNBE-
co-Ln4(Salen)4 (Ln ¼ Nd, 2 or Ln ¼ Yb, 3) in CHCl3 solution at 0.3 g Lꢀ1
at room temperature.
(O5a of MeO– group and O6a of m-O phenoxide atom) from the
Salen-type Schiff-base (HL)ꢀ ligand and two O atoms (O9 and
O9a) of two coordinated m3-OHꢀ groups, while the outer Yb3+
ion (Yb2) is nine-coordinate, in addition to the seven oxygen
atoms from the two outer O2O2 moieties of the two Salen-type
Schiff-base ligands, it saturates its coordination environment
with one O atom (O9) from the coordinated m3-OHꢀ group and
one Clꢀ anion (Cl1). Three unique Yb/Yb distances with nano
˚
sizes are different at 3.4375(5), 3.6879(4) and 3.7947(4) A for
Yb/Yb2, Yb1/Yb1a and Yb1/Yb2a, respectively, in which
each of the Yb1/Yb2 separations in the equivalent Yb2L(HL)
moieties is slightly shorter than that (Yb1/Yb1a or Yb1/Yb2a
separation) between two equivalent Yb2L(HL) moieties. As to
the charge balance to the cationic [Yb4(L)2(HL)2(m3-OH)2Cl2]2+
Fig. 5 XPS data of the series of metallopolymers PNBE-co-Ln4(Salen)4
(Ln ¼ Nd, Yb, Er or Gd).
of the coordinated m3-OHꢀ group. The unique inner Yb3+ ion part, it should be balanced by the protonation of one (N4 or
(Yb1) is eight-coordinate and bound by the N2O2 core of the N4a) of the imino nitrogen atoms for two of the four deproto-
Salen-type Schiff-base (L)2ꢀ ligand in addition to two O atoms nated Salen-type Schiff-base (L)2ꢀ ligands, which endows the
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Fig. 8 NIR emission and excitation spectra of metallopolymers PNBE-
co-3 from different monomer ratios (30 : 1, 50 : 1 and 100 : 1) of NBE
to 3 in CHCl3 solution at 0.3 g Lꢀ1 at room temperature.
Fig. 9 DR spectra of metallopolymers PNBE-co-Ln4(Salen)4 (Ln ¼ Nd,
2; Yb, 3; Er, 4 or Gd, 5) in the solid state at room temperature.
˚
formation of two strong intramolecular N4–H4/O7 (2.615(8) A
and 135.6(4)ꢁ) H-bond interactions shown in Fig. 1S.† The two
free Clꢀ anions, the four solvate EtOH and six solvate H2O
molecules of complex 3$4EtOH$6H2O are not bound to the
framework and they exhibit no observed interactions with the
host structure. It is worth noting that the homoleptic tetranu-
clear Ln4(Salen)4 host structure is not only distinctively different
from the reported structures of binuclear triple-decker,14 tri-
nuclear triple-decker,15 trinuclear tetra-decker,16 pentanuclear
tetra-decker Ln3+ complexes17 based on the simple Salen-type
Schiff-base ligands with the rigid not the exible linkers, but
also different from that of the reported polymeric18 or Tb4(Sa-
len)6 complex,19 where the typical exible Salen-type Schiff-base
Fig. 10 NIR emission and excitation spectra of metallopolymers
ligands without the outer O2O2 moieties were used. On the PNBE-co-Ln4(Salen)4 and Ln4(Salen)4 (Ln ¼ Nd, 2 or Ln ¼ Yb, 3) in the
solid state at room temperature.
other hand, the formation of the homoleptic tetranuclear
Ln4(Salen)4 framework of complexes 1–ꢀ5 appears to be Clꢀ-
dependent, which is comparable to NO3
or Clꢀ-dependent9
8
˚
the bond lengths of 1.231(2)–1.283(2) A for the typical C]C
Ln4(Salen)4 from no diallyl-modied exible Salen-type Schiff-
base ligand with both the inner N2O2 core and the outer O2O2
moiety, although the functional two terminal allyl groups (with
bonds of allyl groups) in complex 3$4EtOH$6H2O are
introduced.
Table 4 The photophysical properties of PNBE-co-Ln4(Salen)4 (Ln ¼ Nd, Yb, Er or Gd) and complexes 2–5 in the solid state at room temperature
Absorption
Excitation
Emission
Compound
lab/nm
lex/nm
lem/nm (s)
PNBE-co-2
PNBE-co-3
PNBE-co-4
PNBE-co-5
2
3
4
5
228, 286, 374, 742, 803, 874
232, 284, 376, 986
232, 283, 375, 656, 804, 975
230, 284, 373
212, 270, 360, 584, 744, 804, 872
214, 268, 358, 972
219, 270, 352, 522, 656, 806, 979
218, 269, 354
428
423
—
436
402
395
—
886(1.25 ms), 1066(1.30 ms), 1341(1.27 ms)
978(10.26 ms)
—
a
565(0.38 ns)
862(1.07 ms), 1066(1.11 ms), 1340(1.20 ms)
978(9.83 ms)
a
—
397
550(0.31 ns)
a
The emission is too weak to be detected.
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complexes 2–3, the respective NIR luminescent decay curves
obtained from time-resolved luminescent experiments can be
tted mono-exponentially with time constants of microseconds
(1.71 ms for 2 at 1090 nm and 16.32 ms for 3 at 978 nm), and the
intrinsic quantum yield FLn (0.68% for 2 or 0.82% for 3) of the
Ln3+ emission may be estimated by FLn ¼ sobs/s0, where sobs is
the observed emission lifetime and s0 is the “natural lifetime”,
viz. 0.25 ms and 2.0 ms for the Nd3+ and Yb3+ ions, respec-
tively.25 As to the relatively higher quantum efficiency of 3
(0.82%) than that of 2 (0.68%), although there is a slightly larger
energy gap (2F5/2, 10 225 cmꢀ1, DE ¼ 7350 cmꢀ1) of Yb3+ ion in
complex 3 than that (4F3/2, 9174 cmꢀ1, DE ¼ 8401 cmꢀ1) of the
Nd3+ ion in complex 2, the excited state of the Nd3+ ion in
complex 2 is more sensitive to quenching by the O–H oscillators
of the coordinated m3-OHꢀ groups and the distant C–H or N–H
oscillators of the Salen-type Schiff-base ligand H2L around the
Nd3+ ions,26 besides the quantity of accepting levels of the Nd3+
ion while only one for the Yb3+ ion. It is worth noting that the
relatively larger NIR intrinsic quantum yields for both
complexes 2–3 than those of binuclear triple-decker14 or trinu-
clear triple-decker lanthanide (Nd3+ or Yb3+) complexes15 based
on the typical Salen-type Schiff-base ligand with the rigid linker,
should result from the sensitization of the NIR luminescence
from both the 1LC and the 3LC excited states of the exible
Schiff-base ligand H2L for complexes 2–3, and the decrease of
the luminescent quenching effect by the coordinated MeOH
solvates around the Ln3+ ions for the triple-decker Ln3+
complexes.
Photophysical properties of series of Ln4(Salen)4 nanocluster
monomers in solution
The photophysical properties of the ligand H2L and complexes
2–5 have been examined in dilute MeCN solution at room
temperature or 77 K, and are summarized in Table 2 and Fig. 3,
4 and 2S.† As shown in Fig. 3, similar ligand-centered solution
absorption spectra (232–234, 271–274 and 347–348 nm) of
complexes 2–5 in the UV-visible region are observed, which are
red-shied upon coordination of the Ln3+ ions compared to
those (226, 262 and 336 nm) of the ligand H2L in MeCN. For
complexes 2–5, the similar weak visible emissions (ca. lem
¼
530 nm and s < 1 ns) with low quantum yields (Fem < 10ꢀ5) are
observed in dilute MeCN solution at room temperature. In
addition to the residual weak visible emission, as shown in
Fig. 4, photo excitation of the chromophore in the range of 250–
500 nm (lex ¼ 375 nm for 2 or 398 nm for 3) gives rise to the
characteristic ligand-eld splitting emissions of the Nd3+ ion
(lem ¼ 905, 1090 and 1363 nm; 4F3/2 / 4IJ/2, J ¼ 9, 11, 13) or the
2
Yb3+ ion (lem ¼ 978 nm; F5/2
/
2F7/2) in the NIR range,
respectively. However, unlike complexes 2 or 3, the character-
istic NIR emission of the Er3+ ion for complex 4 cannot be
observed. The free ligand H2L or complex 5 also just display the
typical strong luminescence of the Salen-type Schiff-base ligand
in the visible range, as shown in Fig. 2S.† The excitation spectra
of complexes 2–3, monitored at the respective NIR emission
peak (1090 nm for 2 or 978 nm for 3) are similar to those
monitored at their respective residual visible emission peak,
which clearly demonstrates that both the NIR and visible
emissions for complexes 2–3 originated from the same p–p*
transitions of the ligand H2L, and the energy transfer from the
chromophore to the Ln3+ ions takes place efficiently.20 As a
suitable reference compound,21 the Gd3+ complex 5 in dilute
MeCN solution at 77 K, compared with that (lem ¼ 530 nm, s ¼
0.81 ns and f ¼ 0.74 ꢂ 10ꢀ3) at room temperature under the
same conditions, exhibits strengthened uorescence, with
higher luminescent intensity (lem ¼ 511 nm and 569 nm) and
distinctively longer luminescence lifetimes (3.6 ns and 7.8 ms).
This result demonstrates that the sensitization of the NIR
luminescence for complexes 2–3 should arise from both the 1LC
Synthesis and characterization of metallopolymers PNBE-co-
Ln4(Salen)4
Based on the success of homo-polymerizations27 of norbornene
(NBE) or its derivatives and their copolymerizations28 with many
C]C-containing functional monomers using the typical ring-
opening metathesis polymerization (ROMP) with a series of
ruthenium–carbene complexes as the catalysts, the copolymer-
izations of the series of Ln4(Salen)4 monomers (1–5) containing
active allyl (–CH2–CH]CH2) groups with NBE were carried out
in chloroform at room temperature using Grubbs' II initiator
(Scheme 2). As expected, all the copolymerizations were
complete aer 48 h, where, in order to tailor the solubility of the
polymeric materials, a molar ratio of 50 : 1 between NBE and
each of Ln4(Salen)4 monomers (1–5) was used. It is worth noting
that the Ln4(Salen)4 unit with eight functional allyl groups has
been incorporated in the respective metallopolymer, where each
of the four Ln3+ ions is coupled to the polymer backbone, thus
the only Wolf Type II metallopolymers based on the tetranuclear
Ln4(Salen)4 units have been constructed to date.1 The obtained
metallopolymers PNBE-co-Ln4(Salen)4 (Ln ¼ La, Nd, Yb, Er or
Gd, 1–5) with Wolf Type II arrangement were well characterized
3
(19 569 cmꢀ1) and the LC (17 575 cmꢀ1) excited states of the
Schiff-base ligand H2L at low temperature.22 Moreover, the
energy transfer rate (kET) in the complexes 2–4 can thus be
calculated from kET ¼ 1/sq ꢀ 1/su,23 where sq is the residual
lifetime of the luminescent emission undergoing quenching by
the respective Ln3+ ion, and su is the unquenched lifetime in the
reference complex 5, so the energy transfer rates for the Ln3+
ions in complexes 2–4 may all be estimated to be above 5 ꢂ 108
sꢀ1, which should also be the reason for the effective energy
transfer for complexes 2–4. From the viewpoint of energy level
matching, in spite of the effective energy transfer also taking
place in complex 4, the larger energy gap between the energy-
1
1
by FT-IR, H NMR, GPC and XPS. The H NMR spectrum anal-
ysis of PNBE-co-1 (Ln ¼ La, also shown in Fig. 1) showed that the
combined photon resonances (d ¼ 13.29 to 0.90 ppm) of the
coordinated diallyl-modied Salen-type Schiff-base ligand H2L
and NBE were observed, in which the photon resonances of two
trans-mode –CH] of the allyl groups from 1 or PNBE appeared
3
donating LC level (17 575 cmꢀ1) and the emitting level (4I13/2
)
of Er3+ ion than those of complexes 2–3 results in the great non-
radiative energy loss during the energy transfer, which should
be the reason for the weak and unobservable luminescence in
the range of 800–1800 nm for complex 4.24 Moreover, for
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at 7.70 and 5.90 ppm or 4.91 and 4.12 ppm, respectively. As polymer matrixes, results in the relatively larger intrinsic
shown in Table 3, the number-average molecular weights (Mn) quantum yield FLn (0.72% for PNBE-co-2 or 0.85% for
and polydispersity indexes (PDI ¼ Mw/Mn) of these metal- PNBE-co-3) of the Ln3+ emission compared with that of the
lopolymers are in the ranges 10 143–12 792 g molꢀ1 and 2.14– respective monomer, possibly due to the molecular extension of
2.27, respectively, and the relatively lower molecular weight the two metallopolymers, which also shows that more efficient
would enhance the solubility of the metallopolymers and energy transfer occurs because of the additional electronic
facilitate the formation of lms. For three different polymeri- communication between the metal centers. It is of special
zations carried out with NBE to complex 3 of 30 : 1, 50 : 1 and interest to note that there is a linear relationship (shown in
100 : 1, a linear relationship between the molecular weight Fig. 8) between the NIR emission intensity and concentration
versus NBE to complex 3 molar ratio suggests a random distri- (30 : 1, 50 : 1 and 100 : 1) of monomer 3 in PNBE-co-3. This
bution of NBE along the backbone.29 The XPS data were used to indicates that not only does the metallopolymer backbone not
determined the PNBE-co-Ln4(Salen)4 (Ln ¼ Nd, 2; Ln ¼ Yb, 3; Ln interfere with the NIR emission properties of the Ln4(Salen)4
¼ Er, 4 or Ln ¼ Gd, 5 in a molar ratio of 50 : 1 between NBE and monomers, but the metallopolymer backbone actually prevents
each of Ln4(Salen)4 monomers (2–5)) metallopolymer lms self-quenching of the monomers which is a common occur-
composition and metal coordination environment (Fig. 5), in rence for Ln3+-based doped hybrid materials.31
which the Ln3+ 3d5/2 and/or Ln3+ 3d3/2 (Ln ¼ Nd, Er or Gd)
For the metallopolymers PNBE-co-Ln4(Salen)4 (Ln ¼ Nd, 2;
peaks30 at 982.0 and 1004.5 eV, 1358.5 eV, 1187.9 and 1219.5 eV, Yb, 3; Er, 4 or Gd, 5) in the solid state, their photophysical
and the Yb3+ 4d peak30 at 185.0 eV, respectively, are observed, properties have also been examined at room temperature, and
corresponding well to the expected binding energy values for these are summarized in Table 4 and Fig. 9, 10 and 4S–6S.† As
Ln3+ bound to oxygen. Quantitative XPS analyses revealed that shown in Fig. 9, the DR spectra of the four metallopolymers
the metal contents (3.6%, 4.8%, 4.6% and 4.1%) of the four exhibit two broad absorption bands, in which one in the UV
PNBE-co-Ln4(Salen)4 materials were only 45–53% of the theo- region can be assigned to electronic transitions from the poly-
retical values, which suggests about one in every hundred mer backbone, and the other in the visible to NIR region
Ln4(Salen)4 units along each polymer backbone in spite of the corresponds to the characteristic transitions of the respective
feed molar ratio of 50 : 1.
Ln3+ ion from the ground level 4I9/2, 2F7/2 or 4I15/2 to the higher
energy levels for Nd3+
,
Yb3+
,
or Er3+ ion,34 respectively. In
32
33
contrast, the absorption bands of Gd3+ ion in the corresponding
sample were not discerned, because the main absorption of
Gd3+ ion commonly appears above 1000 nm.35 Compared with
Photophysical properties of metallopolymers PNBE-co-
Ln4(Salen)4 in solution and solid states
Due to the good solubility of PNBE and PNBE-co-Ln4(Salen)4 in the similar patterns of two broad absorptions for the four
CHCl3, their photophysical properties have been examined in Ln4(Salen)4 (Ln ¼ Nd, 2; Yb, 3; Er, 4 or Gd, 5) monomers in the
dilute CHCl3 solution at room temperature or 77 K, and these solid state shown in Fig. 4S,† each red-shied while the lower
are summarized in Table 2 and Fig. 6, 7 and 3S.† As shown in absorption intensity absorption band in the UV region for the
Fig. 6, all the absorption spectra of the PNBE-co-Ln4(Salen)4 are metallopolymers relative to that assigned to electronic transi-
the combination of both monomers, displaying the distinctively tions from the Schiff-base ligand suggests that the Ln3+
broad band characteristic of the extended metallopolymer complexes have been graed into the polymer matrixes in an
structure. Moreover, each band is red-shied compared to that extended conjugation fashion with the low concentration.36
of Ln4(Salen)4. For PNBE-co-Ln4(Salen)4 (Ln ¼ Nd, 2; Ln ¼ Yb, 3 Upon excitation of the chromopore's absorption band, for both
or Ln ¼ Er, 4), the visible emissions are almost completely PNBE-co-Ln4(Salen)4 and Ln4(Salen)4 (Ln ¼ Nd, 2; Ln ¼ Yb, 3 or
quenched, while as shown in Fig. 7, photo excitation in the Ln ¼ Er, 4) in the solid state, all the visible emissions are almost
range of 300–500 nm (lex ¼ 428 nm for PNBE-co-2 or 432 nm for completely quenched, while as shown in Fig. 10, similar char-
4
PNBE-co-3) gives rise to the characteristic NIR emissions of the acteristic NIR emissions of the Nd3+ ion (4F3/2 / IJ/2) or the
4
2
Nd3+ ion (4F3/2 / IJ/2) or the Yb3+ ion (2F5/2 / F7/2), respec- Yb3+ ion (2F5/2 / 2F7/2) to those in the solution are observed. For
tively, indicating more efficient energy transfer. All the excita- PNBE-co-5 in the solid state, weakened uorescence (lem
tion spectra are less broad and red shied relative to those of 565 nm) in the visible range is observed relative to that (lem
¼
¼
the Ln4(Salen)4 monomers, suggesting that the energy transfer 550 nm) of 5, as shown in Fig. 5S.† It is worth noting that all the
of the two extended metallopolymers takes place from a local- excitation bands of the metallopolymers are relatively narrower,
ized excited state. Unlike the unobserved emissions in both which should be attributed to the change of the environment
visible and NIR ranges of PNBE-co-4, PNBE-co-5 just displays the surrounding the Ln3+ ions in the copolymer matrix materials.37
typical strong ligand-centered luminescence (lem ¼ 545 nm) in Further through the measurement of the luminescent lifetimes
the visible range, as shown in Fig. 3S.† The luminescent life- for the metallopolymers, both the NIR emissive lifetimes and
times of PNBE-co-Ln4(Salen)4 (Ln ¼ Nd, 2 or Ln ¼ Yb, 3) are also the visible lifetimes are lower aer being graed into the matrix,
tted well by a single-exponential function in agreement with which may be attributed to the strong effect of the molecular
those of Ln4(Salen)4 monomers, which shows that the Ln3+ ions high energy vibration of the main exible framework groups
in the metallopolymers occupy the same average local envi- (PNBE) on non-radiative transitions of Ln3+ ions.38 Moreover, an
ronment. However, the slight increase of lifetimes of the two almost linear relationship (shown in Fig. 6S†) between the NIR
NIR luminescent metallopolymers aer being graed into the emission intensity and concentration (30 : 1, 50 : 1 and 100 : 1)
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Journal of Materials Chemistry C
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of monomer 3 for PNBE-co-3 shows the formation of uniform
metallopolymer hybrid materials is also helpful for preventing
the concentration self-quenching even in the solid state. The
NIR quantum yields of Nd3+ and Yb3+ ions in the hybrid mate-
rials are typically below 0.1% which is lower than the detection
limit of the setup.
6 (a) A. Meyers, A. Kimyonok and M. Weck, Macromolecules,
2005, 38, 8671; (b) R. Pizzoferrato, T. Ziller, R. Paolesse,
F. Mandoj, A. Micozzi, A. Ricci and C. Losterzo, Chem.
Phys. Lett., 2006, 426, 124.
7 (a) J. Kido and Y. Okamoto, Chem. Rev., 2002, 102, 2357; (b)
M. A. Katkova and M. N. Bochkarev, Dalton Trans., 2010,
39, 6599.
¨
8 W. Feng, Y. Zhang, X. Lu, Y. Hui, G. Shi, D. Zou, J. Song,
4. Conclusion
D. Fan, W.-K. Wong and R. A. Jones, CrystEngComm, 2012,
14, 3456.
Based on homoleptic tetranuclear [Ln4(L)2(HL)2(m3-
OH)2Cl2]$2Cl (Ln4(Salen)4, Ln ¼ La, 1; Nd, 2; Ln ¼ Yb, 3; Ln ¼
Er, 4; Ln ¼ Gd, 5) self-assembled from the diallyl-modied
exible hexadentate Salen-type Schiff-base ligand H2L with
LnCl3$6H2O, the rst Wolf Type II metallopolymers PNBE-co-
Ln4(Salen)4 (Ln ¼ La, Nd, Yb, Er or Gd) are obtained via
controlled ROMP copolymerization with NBE, where the
pure NIR luminescent Ln3+-centered emissions are retained
with relatively higher quantum yield in solution, and the
self-quenching of Ln3+-based materials could be effectively
prevented. This study suggests that they are excellent
candidates for use in high color purity NIR PLEDs and efforts
towards the realization of this goal are currently being
carried out.
¨
9 W. Feng, Y. Zhang, Z. Zhang, X. Lu, H. Liu, G. Shi, D. Zou,
J. Song, D. Fan, W.-K. Wong and R. A. Jones, Inorg. Chem.,
2012, 51, 11377.
10 E. Kim, J. Y. Kim and H. Rhee, Bull. Korean Chem. Soc., 2004,
25, 1720.
11 G. M. Sheldrick, SHELXL-97: Program for crystal structure
renement, Gottingen, Germany, 1997.
12 G. M. Sheldrick, SADABS, University of Gottingen, 1996.
¨
¨
13 K. Feng, C. Zuniga, Y.-D. Zhang, D. Kim, S. Barlow,
´
S. R. Marder, J. L. Bredas and M. Weck, Macromolecules,
2009, 42, 6855.
14 P. F. Yang, S. Chen, P. Chen, J. W. Zhang and G. M. Li,
CrystEngComm, 2011, 13, 36.
15 X. P. Yang, R. A. Jones, M. M. Oye, A. L. Holmes and
W.-K. Wong, Cryst. Growth Des., 2006, 6, 2122.
16 X. P. Yang and R. A. Jones, J. Am. Chem. Soc., 2005, 127, 7686.
17 X. P. Yang, R. A. Jones and W.-K. Wong, Dalton Trans., 2008,
1676.
18 X. P. Yang, R. A. Jones, J. H. Rivers and W.-K. Wong, Dalton
Trans., 2009, 10505.
19 X. P. Yang, R. A. Jones and W.-K. Wong, Chem. Commun.,
2008, 3266.
Acknowledgements
This work is funded by the National Natural Science Foun-
dation (21373160, 912222201, 21173165, 20871098), the
Program for New Century Excellent Talents in University from
the Ministry of Education of China (NCET-10-0936), the
research fund for the Doctoral Program (20116101110003) of
Higher Education, the Education Committee Foundation of
Shaanxi Province (11JK0588), Graduate Innovation and
Creativity Fund (YZZ12038) of Northwest University, Hong
Kong Research Grants Council (HKBU 202407 and FRG/06-07/
II-16) in P. R. of China, the Robert A. Welch Foundation
(Grant F-816), the Texas Higher Education Coordinating
Board (ARP 003658-0010-2006) and the Petroleum Research
Fund, administered by the American Chemical Society
(47014-AC5).
¨
20 S. Comby and J.-C.-G. Bunzli, Handbook on the Physics and
Chemistry of Rare Earths, ElsevierScience B. V., Amsterdam,
2007.
21 W. T. Carnall, P. R. Fields and K. Rajnak, J. Chem. Phys.,
1968, 49, 4443.
22 W.-K. Lo, W.-K. Wong, W.-Y. Wong, J. Guo, K.-T. Yeung,
Y.-K. Cheng, X. Yang and R. A. Jones, Inorg. Chem., 2006,
45, 9315.
23 D. L. Dexter, J. Chem. Phys., 1953, 21, 836.
¨
24 J.-C.-G. Bunzli, S. Comby, A. S. Chauvin and
C. D. B. Vandevyver, J. Rare Earths, 2007, 25, 257.
25 M. L. Mercyri, P. Deplano, L. Pilia, A. Serpe and E. Artizzu,
Coord. Chem. Rev., 2010, 254, 1419.
References
1 J. M. Stanley and B. J. Holliday, Coord. Chem. Rev., 2012, 256,
¨
26 J.-C.-G. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048.
1520.
2 J. M. Bryson, K. M. Fichter, W. J. Chu, J. H. Lee, J. Li, 27 W. J. Sommer and M. Weck, Coord. Chem. Rev., 2007, 251,
L. A. Madsen, P. M. Mclendon and T. M. Reineke, Proc.
Natl. Acad. Sci. U. S. A., 2009, 106, 16913.
3 Q. Ling, Y. Song, S. J. Ding, C. Zhu, D. S. H. Chan,
860.
28 S. K. Yang, A. V. Ambade and M. Weck, Chem. Soc. Rev., 2011,
40, 129.
D.-L. Kwong, E.-T. Kang and K.-G. Neoh, Adv. Mater., 2005, 29 P. Gumbley, D. Koylu and S. W. Thomas, III, Macromolecules,
17, 455.
2011, 44, 7956.
4 (a) Z.-G. Zhang, J.-B. Yuan, H.-J. Tang, H. Tang, L.-N. Wang 30 F. Mercier, C. Alliot, L. Bion, N. Thromat and P. Toulhoat,
and K.-L. Zhang, J. Polym. Sci., Part A: Polym. Chem., 2009, J. Electron Spectrosc. Relat. Phenom., 2006, 150, 21.
47, 210; (b) H. Xu, R. Zhu, P. Zhao and W. Huang, J. Phys. 31 (a) J. Feng and H. J. Zhang, Chem. Soc. Rev., 2013, 42, 387; (b)
Chem. C, 2011, 115, 15627.
L. D. Carlos, R. A. S. Ferreira, V. de Z. Bermudez, B. Julian-
5 M. O. Wolf, Adv. Mater., 2001, 13, 545.
Lopez and P. Escribano, Chem. Soc. Rev., 2011, 40, 536.
1498 | J. Mater. Chem. C, 2014, 2, 1489–1499
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
32 Y. Hasegawa, Y. Kimura, K. Murakoshi, Y. Wada, J. H. Kim, 35 V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani, M. Gorka and
¨
N. Nakashima, T. Yamanaka and S. Yanagida, J. Phys. Chem.,
1996, 100, 10201.
F. Vogtle, J. Am. Chem. Soc., 2002, 124, 6461.
36 J. Feng, J. B. Yu, S. Y. Song, L. N. Sun, W. Q. Fan, X. M. Guo,
33 L. N. Sun, J. B. Yu, G. L. Zhang, H. J. Zhang, Q. G. Meng,
S. Dang and H. J. Zhang, Dalton Trans., 2009, 2406.
C. Y. Peng, L. S. Fu, F. Y. Liu and Y. N. Yu, Eur. J. Inorg. 37 L. N. Sun, H. J. Zhang, J. B. Yu, S. Y. Yu, C. Y. Peng, S. Dang,
Chem., 2006, (19), 3962. X. M. Guo and J. Feng, Langmuir, 2008, 24, 5500.
34 J. A. Capobianco, F. Vetrone, J. C. Boyer, A. Speghini and 38 W. Q. Fan, J. Feng, S. Y. Song, Y. Q. Lei, L. Zhou, G. L. Zheng,
M. Bettinelli, J. Phys. Chem. B, 2002, 106, 1181.
S. Dang, S. Wang and H. J. Zhang, Nanoscale, 2010, 2, 2096.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 1489–1499 | 1499