Near-infrared (NIR) luminescent hetero-tetranuclear Zn2Ln 2 (Ln = Nd, Yb or Er) complexes self-assembled from the benzimidazole-based HL and two rigid 4,4′-bipyridine ligands with different spacers

Article abstract of DOI:10.1016/j.saa.2013.06.122

(Chemical Equation Presented) Through the self-assembly of the benzimidazole-based ligand HL (HL = 2-(1H-benzo[d]imidazol-2-yl)-6- methoxyphenol) with Zn(OAc)₂·2H₂O, Ln(NO ₃)₃·6H₂O (Ln = Nd, Yb, Er or

Full text of DOI:10.1016/j.saa.2013.06.122

Accepted Manuscript
Near-infrared (NIR) luminescent hetero-tetranuclear Zn Ln (Ln = Nd, Yb or
2 2
Er) complexes self-assembled from the benzimidazole-based HL and two rigid
4,4’-bipyridine ligands with different spacers
Zhao Zhang, Weixu Feng, Peiyang Su, Han Liu, Yao Zhang, Zheng Wang,
Tiezheng Miao, Xingqiang Lü, Daidi Fan, Wai-Kwok Wong, Richard A. Jones
PII:
S1386-1425(13)00736-1
http://dx.doi.org/10.1016/j.saa.2013.06.122
SAA 10743
DOI:
Reference:
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo‐
lecular Spectroscopy
Please cite this article as: Z. Zhang, W. Feng, P. Su, H. Liu, Y. Zhang, Z. Wang, T. Miao, X. Lü, D. Fan, W-K.
Wong, R.A. Jones, Near-infrared (NIR) luminescent hetero-tetranuclear Zn Ln (Ln = Nd, Yb or Er) complexes
2
2
self-assembled from the benzimidazole-based HL and two rigid 4,4’-bipyridine ligands with different spacers,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa.
2013.06.122
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Near-infrared (NIR) luminescent hetero-tetranuclear
Zn2Ln2 (Ln = Nd, Yb or Er) complexes self-assembled
from the benzimidazole-based HL and two rigid
4,4’-bipyridine ligands with different spacers
Zhao Zhanga, Weixu Fenga*, Peiyang Sua, Han Liua, Yao Zhanga, Zheng Wanga,
Tiezheng Miaoa, Xingqiang Lüa,b*, Daidi Fana, Wai-Kwok Wongc, Richard A. Jonesd
aShaanxi Key Laboratory of Degradable Medical Material, Shaanxi Key Laboratory of
Physico-inorganic Chemistry, Northwest University, Xi’an 710069, Shaanxi, China
bFujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou
350002, Fujian, China
cDepartment of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong,
Hong Kong, China.
dDepartment of Chemistry and Biochemistry, The University of Texas at Austin, 1 University
Station A5300, Austin, TX 78712-0165, United States
To whom correspondence should be addressed. Tel./fax: +86-029-88302312
E-mail: lvxq@nwu.edu.cn and fwxdk@foxmail.com
1

Abstract
Through the self-assembly of the benzimidazole-based ligand HL (HL
=
2-(1H-benzo[d]imidazol-2-yl)-6-methoxyphenol) with Zn(OAc)2·2H2O, Ln(NO3)3·6H2O (Ln
= Nd, Yb, Er or Gd) and 4,4’-bipyridine ligand (bpy, 4,4’-bipyridine or bpe,
trans-bis(4-pyridyl)ethylene),
two
series
of
Zn2Ln2-arrayed
complexes
[Zn2Ln2(L)4(bpy)(NO3)6] (Ln = Nd, 1; Yb, 2; Er, 3 or Gd, 4) and [Zn2Ln2(L)4(bpe)(NO3)6] (Ln
= Nd, 5; Yb, 6; Er, 7 or Gd, 8) were obtained, respectively. The result of their photophysical
properties shows that the characteristic near-infrared (NIR) luminescence of Nd3+, Yb3+ or
1
3
Er3+ ion has been sensitized from the excited state (both LC and LC) of the mixed HL and
bipyridyl ligands in both complexes 1-3 and 5-7. Moreover, the change from bpy to bpe
bridging for the fine-tuning of whole molecular conjugations, attributing to the different
crossings of the two benzimidazole-based L- ligands, has the important influence on their NIR
luminescent properties.
Keywords: Hetero-tetranuclear Zn2Ln2-arrayed complexes; Mixed HL-bipyridine ligands;
Sensitization of NIR luminescence
2

1. Introduction
Much recent research interest has been devoted into near-infrared (NIR) luminescent
lanthanide (Ln = Nd, Yb or Er) complexes [1] with the long-lived (ms or μs) lifetime, the
large stokes’ shift and the characteristic narrow-line emission [2], which have potential
applications in functional devices for NIR organic light-emitting diodes (OLEDs) [3],
tele-communication [4] and bio-analysis [5]. However, due to the low absorption coefficients
and the low emissive rates of the parity forbidden f-f transitions [6], the introduction of
organic chromophores [7] including transition metal complexes [8] as the sensitizers of these
Ln3+ acceptors is necessarily emerging. From the viewpoint of efficient sensitization of
Ln3+-based NIR luminescence, the choice of suitable chromophores remains an actual
challenge, which requires the realization of the energy level’s match of the excited state of the
chromophores to the Ln3+ ions’ excited state besides the complete avoidance or decrease of
the luminescent quenching effect arising from OH-, CH- or NH-containing oscillators around
the Ln3+ ions [9].
It is of special notice that series of 3d Zn2+ complexes from adeninate [10],
hydroxylquinoline [11], imidazoledicarboxylate [12] or tetrazole-based ligands [13], as the
chromophores, have been used to effectively sensitize the NIR emission of the Ln3+ ions,
attributed to their strong light-harvesting in the near-UV and visible ranges, the efficient
energy transfer from the chromophores to the Ln3+ ions and the minimization of the
non-radiative processes of the Ln3+ ion. In this regard, our past research results have shown
that hetero-binuclear ZnLn [14] or hetero-trinuclear Zn2Ln [15] complexes could be obtained
from the pure Salen-type Schiff-base ligands, where the Zn2Ln-arrayed complexes with two
3

chromophores endow the enhanced NIR luminescence. On the other hand, the Salen-type
Schiff-base ligands with the outer O2O2 moieties from MeO- groups are apt to bind both Zn2+
and Ln3+ ions, especially in the presence of anion or multidentate-ligand bridging, series of
ZnLn [16], Zn2Ln [17], Zn2Ln2 [18] or Zn4Ln2 [19] complexes could be constructed, in which
both two or more chromorphores and/or two Ln3+ acceptors with increased energy transfer
rates usually lead to the better NIR luminescent properties in contrast to those of
hetero-binuclear ZnLn complexes. Moreover, the use of flexible linkers while not the rigid
ones of the Salen-type Schiff-base ligands with the outer O2O2 moieties results in the larger
NIR quantum yields, due to the more effective intra-molecular energy transfer from the
3
1
1
ligand-centered both LC and LC not just LC excited state to the Ln3+ ions. As a matter of
fact, based on the success of hetero-binuclear [Zn(Ln)2Ln(Py)] (Py = pyridine) complexes [20],
the benzimidazole-based ligands (HLn) are proved also to bind both Zn2+ ion in the inner
cis-N2O2 core and Ln3+ ion in the outer O2O2 moiety, and the Zn2+ complexes ([Zn(Ln)2(Py)])
with two benzimidazole-based ligands sandwiched through the N,O-chelate mode can
sensitize the NIR emission of some Ln3+ ions. Evidently, if using multidentate bridges (such
as 4,4’-bipyridine ligands) in replacement with monodentate Py group at the axial position of
Zn2+ ion, hetero-polynuclear ZnxLny (x > 1 and/or y > 1) complexes based on the
benzimidazole-based ligands with two or more chromophores and/or two Ln3+ acceptors
should be obtained, and their improved NIR luminescent emissions are also expected. Herein,
two series of complexes [Zn2Ln2(L)4(bpy)(NO3)6] (Ln = Nd, 1; Yb, 2; Er, 3 or Gd, 4) and
[Zn2Ln2(L)4(bpe)(NO3)6] (Ln = Nd, 5; Yb, 6; Er, 7 or Gd, 8) were obtained from the
self-assembly
of
the
benzimidazole-based
ligand
HL
(HL
=
4

2-(1H-benzo[d]imidazol-2-yl)-6-methoxyphenol), Zn(OAc)2·2H2O, Ln(NO3)3·6H2O (Ln = Nd,
Yb, Er or Gd) and bpy or bpe, and the sensitization mechanism on the NIR luminescence of
the Ln3+ ions were also discussed.
2. Experimental Section
2.1 Materials and Methods
All chemicals were commercial products of reagent grade and were used without further
purification. 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
1
4000 - 400 cm-1 using KBr pellets. H NMR spectra were recorded on a JEOL EX270
spectrometer with SiMe4 as internal standard in CD3CN 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 CH3CN as solvent.
Electronic absorption spectra in the UV/Vis region were recorded with a Cary 300 UV
spectrophotometer, and steady-state visible fluorescence, PL excitation spectra on a Photon
Technology International (PTI) Alphascan spectrofluorometer 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, Ф = 0.546). NIR emission
em
and excitation in solution were recorded by PTI QM4 spectrofluorometer with a PTI QM4
Near-Infrared InGaAs detector.
5

2.2 Synthesis
2.2.1 Synthesis of the benzimidazole-based ligand HL
(HL = 2-(1H-benzo[d]imidazol-2-yl)-6-methoxyphenol)
The ligand HL was prepared according to the improved procedure from the literature
[20]. A solution of o-vanillin (3.04 g, 20 mmol) in absolute EtOH (10 ml) was added slowly
to the solution of o-phenylenediamine (2.16 g, 20 mmol) in absolute EtOH (10 ml) under a
nitrogen atmosphere, and the resulting mixture was stirred overnight at room temperature.
The insoluble orange precipitate was filtered off, and was added to absolute EtOH (20 ml)
and the mixture was refluxed for 12 h. After cooling to room temperature, the resulting
off-white microcrystalline precipitate was filtered off, washed with absolute EtOH and diethyl
ether. Yield: 1.46 g, 61%. Calc. for C14H12N2O2: C, 69.99; H, 5.03; N, 11.66%; found: C,
69.91; H, 5.07; N, 11.65%. IR (KBr, cm-1): 3337 (b), 1899 (w), 1836 (w), 1768 (w), 1625 (m),
1593 (m), 1532 (m), 1495 (s), 1475 (s), 1463 (s), 1450 (s), 1424 (s), 1391 (s), 1333 (m), 1304
(w), 1281 (m), 1262 (s), 1196 (w), 1181 (w), 1155 (w), 1088 (m), 1060 (s), 1009 (w), 981 (m),
954 (w), 930 (w), 909 (m), 863 (m), 831 (s), 788 (s), 745 (s), 717 (m), 651 (m), 633 (m), 603
1
(s), 571 (w), 555 (w), 520 (w), 501 (m), 440 (m), 419 (w). H NMR (400 MHz, CD3CN): δ
(ppm) 13.42 (s, 1H, -NH), 13.27 (s, 1H, -OH), 7.67 (d, 1H, -Ph), 7.45 (m, 2H, -Ph), 7.32 (m,
2H, -Ph), 7.10 (d, 1H, -Ph), 6.93 (t, 1H, -Ph), 3.83 (s, 3H, -OMe).
2.2.2 Syntheses of [Zn2Ln2(L)4(bpy)(NO3)6] (Ln = Nd, 1; L n = Yb, 2; Ln = Er, 3 or Ln = Gd,
4)
To a stirred solution of HL (95.6 mg, 0.4 mmol) in absolute THF (10 ml), solid
6

Zn(OAc)2·2H2O (43.9 mg, 0.2 mmol) and absolute bpy (4,4′-bipyridine, 15.6 mg, 0.1 mmol)
were added separately, and the final mixture was heated under reflux for 5 h. A solution of
Ln(NO3)3·6H2O (0.2 mmol, Ln = Nd, 0.087 g; Ln = Yb, 0.089 g; Ln = Er, 0.088 g; Ln = Gd,
0.090 g) in absolute MeCN (4 ml) were added, sequentially. Then the mixture was refluxed
for another 3 h. The respective yellow clear solution was then cooled to room temperature and
filtered. Diethyl ether was allowed to diffuse slowly into the respective solution at room
temperature and pale yellow microcrystallines 1-4 were obtained in about three weeks,
respectively.
For 1: Yield: 0.136 g, 68%. Calc. for C66H52N16O26Zn2Nd2: C, 41.62; H, 2.75; N, 11.77%;
found: C, 41.53; H, 2.82; N, 11.78%. IR (KBr, cm-1): 3421 (w), 3132 (w), 1772 (w), 1658
(m), 1627 (m), 1609 (m), 1538 (m), 1479 (s), 1452 (m), 1403 (m), 1387 (s), 1311 (vs), 1287
(s), 1248 (s), 1203 (m), 1180 (m), 1164 (m), 1121 (w), 1093 (m), 1050 (s), 994 (m), 934 (w),
908 (w), 867 (m), 848 (m), 816 (w), 785 (m), 741 (s), 702 (m), 659 (m), 626 (w), 563 (w),
1
535 (w), 512 (m), 445 (m), 415 (w). H NMR (400 MHz, CD3CN): δ (ppm) 12.27 (s, 4H),
8.85 (d, 4H), 7.71 (s, 4H), 7.04 (m, 8H), 6.82 (m, 4H), 6.65 (m, 4H), 6.12 (s, 4H), 5.60 (s, 4H),
4.73 (m, 4H), -1.96 (s, 12H). ESI-MS (CH3CN) m/z: 1905.48 ([M-H]+, 100%).
For 2: Yield: 0.131 g, 67%. Calc. for C66H52N16O26Zn2Yb2: C, 40.40; H, 2.67; N, 11.42%;
found: C, 40.28; H, 2.86; N, 11.46%. IR (KBr, cm-1): 3468 (w), 1772 (w), 1661 (m), 1628 (s),
1570 (m), 1542 (m), 1530 (m), 1492 (m), 1480 (s), 1453 (m), 1424 (w), 1384 (vs), 1316 (s),
1283 (w), 1257 (w), 1240 (w), 1200 (m), 1180 (m), 1087 (vs), 990 (m), 912 (w), 869 (w), 837
(m), 788 (m), 745 (m), 719 (s), 662 (w), 635 (w), 604 (w), 547 (w), 515 (m), 460 (w), 443 (m),
420 (m). ESI-MS (CH3CN) m/z: 1963.08 (100%).
7

For 3: Yield: 0.124 g, 64%. Calc. for C66H52N16O26Zn2Er2: C, 40.64; H, 2.69; N, 11.49%;
found: C, 40.55; H, 2.77; N, 11.50%. IR (KBr, cm-1): 3440 (w), 1772 (w), 1732 (w), 1659 (m),
1637 (m), 1568 (m), 1537 (m), 1496 (m), 1478 (vs), 1463 (m), 1451 (vs), 1424 (m), 1387 (vs),
1335 (w), 1308 (m), 1283 (m), 1260 (s), 1239 (m), 1198 (m), 1185 (m), 1158 (m), 1090 (s),
1059 (s), 993 (m), 933 (w), 909 (w), 864 (w), 834(w), 787 (m), 778 (m), 764 (w), 745 (s), 719
(m), 653 (w), 633 (w), 603 (w), 551 (w), 502 (w), 459 (w), 446 (m), 421 (m). ESI-MS
(CH3CN) m/z: 1951.52 (100%).
For 4: Yield: 0.116 g, 60%. Calc. for C66H52N16O26Zn2Gd2: C, 41.06; H, 2.72; N, 11.61%;
found: C, 40.95; H, 2.80; N, 11.62%. IR (KBr, cm-1): 3455 (w), 1772 (w), 1660 (s), 1626 (m),
1544 (w), 1535 (w), 1496 (w), 1479 (m), 1450 (m), 1427 (s), 1384 (s), 1309 (s), 1286 (m),
1260 (s), 1242 (w), 1200 (w), 1182 (w), 1085 (vs), 987 (s), 912 (w), 863 (m), 834 (w), 792
(w), 745 (m), 718 (w), 602 (w), 526 (w), 460 (m), 420 (m). ESI-MS (CH3CN) m/z: 1931.50
(100%).
2.2.3 Syntheses of [Zn2Ln2(L)4(bpe)(NO3)6] (Ln = Nd, 5; Ln = Yb, 6; Ln = Er, 7; Ln = Gd, 8)
To a stirred solution of HL (95.6 mg, 0.4 mmol) in absolute EtOH (10 ml), solid
Zn(OAc)2·2H2O (43.9 mg, 0.2 mmol) and absolute bpe (trans-1,2-bis(4-pyridyl)ethylene, 18
mg, 0.1 mmol) were added separately, and the final mixture was heated under reflux for 5 h. A
solution of Ln(NO3)3·6H2O (0.2 mmol, Ln = Nd, 0.087 g; Ln = Yb, 0.089 g; Ln = Er, 0.088 g;
Ln = Gd, 0.090 g) in absolute MeCN (20 ml) were added sequentially. Then the mixture was
refluxed for another 3 h. The respective yellow clear solution was then cooled to room
temperature and filtered. Diethyl ether was allowed to diffuse slowly into the respective
8

solution at room temperature and pale yellow microcrystallines 5-8 were obtained in about
three weeks, respectively.
For 5: Yield: 0.144 g, 75%. Calc. for C68H54N16O26Zn2Nd2: C, 42.31; H, 2.82; N, 11.61%;
found: C, 42.20; H, 2.88; N, 11.63%. IR (KBr, cm-1): 3122 (m), 2427 (w), 2347 (w), 1649 (m),
1620 (m), 1534 (w), 1479 (s), 1400 (vs), 1387 (vs), 1312 (m), 1240 (m), 1203 (w), 1179 (w),
1163 (w), 1118 (w), 1092 (w), 1053 (m), 1028 (w), 994 (w), 865 (w), 844 (m), 820 (w), 782
1
(m), 743 (m), 655 (m), 625 (m), 564 (w), 507 (w), 478 (w), 457 (w), 441 (w), 419 (w). H
NMR (400 MHz, CD3CN): δ (ppm) 13.43 (s, 2H), 13.21 (s, 4H), 8.92 (d, 4H), 7.83 (s, 4H),
7.15 (m, 8H), 6.97 (m, 4H), 6.76 (m, 4H), 6.05 (s, 4H), 5.72 (s, 4H), 4.11 (m, 4H), -2.24 (s,
12H). ESI-MS (CH3CN) m/z: 1931.52 ([M-H]+, 100%).
For 6: Yield: 0.145 g, 73%. Calc. for C68H54N16O26Zn2Yb2: C, 41.08; H, 2.74; N, 11.27%;
found: C, 40.96; H, 2.82; N, 11.30%. IR (KBr, cm-1): 3436 (w), 3200 (w), 2429 (w), 2342 (w),
1836 (w), 1650 (m), 1623 (m), 1540 (w), 1458 (s), 1396 (vs), 1383 (vs), 1319 (m), 1244 (m),
1198 (w), 1177 (m), 1105 (w), 1056 (m), 1031 (w), 995 (w), 866 (w), 846 (w), 828 (w), 785
(w), 745 (m), 658 (m), 590 (s), 548 (m), 525 (w), 444 (w),425 (w). ESI-MS (CH3CN) m/z:
1989.12 ([M-H]+, 100%).
For 7: Yield: 0.138 g, 70%. Calc. for C68H54N16O26Zn2Er2: C, 41.32; H, 2.75; N, 11.34%;
found: C, 41.21; H, 2.82; N, 11.37%. IR (KBr, cm-1): 3413 (w), 3133 (w), 2434 (w), 2370 (w),
2347 (w), 1651 (s), 1623 (s), 1566 (m), 1547 (m), 1480 (s), 1396 (vs), 1381 (vs), 1316 (m),
1291 (w), 1238 (m), 1201 (w), 1176 (w), 1164 (m), 1113 (w), 1093 (w), 1055 (m), 1028 (w),
994 (w), 866 (w), 847 (w), 829 (w), 783(m), 746 (s), 680 (w), 661 (w), 618 (w), 550 (w), 516
(w), 480 (w), 464 (w), 444 (w), 433 (w), 416 (w). ESI-MS (CH3CN) m/z: 1977.56 ([M-H]+,
9

100%).
For 8: Yield: 0.133 g, 68%. Calc. for C68H54N16O26Zn2Gd2: C, 41.74; H, 2.78; N, 11.45%;
found: C, 41.63; H, 2.85; N, 11.46%. IR (KBr, cm-1): 3432 (w), 3139 (w), 2427 (w), 1649 (m),
1625 (s), 1565 (m), 1545 (m), 1480 (m), 1396 (vs), 1379 (vs), 1313 (m), 1281 (w), 1239 (m),
1201 (w), 1179 (w), 1165 (w), 1093 (w), 1057 (s), 1031 (w), 994 (m), 864 (w), 846 (m), 830
(w), 785 (m), 743 (s), 681 (w), 661 (m), 618 (w), 564 (w), 511 (w), 474 (w), 458 (w), 444 (w),
420 (m). ESI-MS (CH3CN) m/z: 1957.54 ([M-H]+, 100%).
2.3 X-ray crystallography
Single crystals of 1·3H2O and 5·2EtOH of suitable dimensions were mounted onto thin
glass fibers. All the intensity datas were collected on a Bruker SMART CCD diffractometer
(Mo-Kα radiation and λ = 0.71073 Å) in Φ and ω scan modes. Structures were solved by
Direct methods followed by difference Fourier syntheses, and then refined by full-matrix
least-squares techniques against F2 using SHELXTL [21]. All other non-hydrogen atoms were
refined with anisotropic thermal parameters. Absorption corrections were applied using
SADABS [22]. All hydrogen atoms were placed in calculated positions and refined
isotropically using a riding model. Crystallographic data and refinement parameters for the
complexes are presented in Table 1. Relevant atomic distances and bond angles are collected
in Table 2. CCDC reference number 934710 (for 1⋅3H2O) and 929956 (for 5⋅2EtOH).
3. Results and discussion
3.1 Synthesis and characterization
10

Reaction of o-phenylenediamine with o-vanillin in 1:1 molar ratio in absolute EtOH at
ambient temperature and subsequent refluxing in absolute EtOH, endowed the off-white
benzimidazole-based ligand HL in 61% yield. Further through the self-assembly of HL,
Zn(OAc)2·2H2O, Ln(NO3)3·6H2O (Ln = Nd, Yb, Er or Gd) with bpy or bpe in the molar ratio
of 4:2:2:1, two series of eight hetero-tetranuclear Zn2Ln2 complexes [Zn2Ln2(L)4(bpy)(NO3)6]
(Ln = Nd, 1; Yb, 2; Er, 3 or Gd, 4) and [Zn2Ln2(L)4(bpe)(NO3)6] (Ln = Nd, 5; Yb, 6; Er, 7 or
Gd, 8) were obtained, respectively, as shown in Scheme 1.
[Scheme 1 inserted here]
The benzimidazole-based ligand HL and two series of Zn2Ln2-arrayed complexes 1-4
1
and 5-8 complexes were well characterized by EA, FT-IR, H NMR and ESI-MS. In the
FT-IR spectra, the characteristic strong absorptions of the ν(C=N) vibrations [23] of
benzimidazole rings at 1661-1658 and 1637-1626 cm-1 for complexes 1-4 or 1651-1649 and
1625-1623 cm-1 for complexes 5-8, respectively, are slightly blue-shifted by the range of
33-36 and 33-44 cm-1 or 24-26 and 30-33 cm-1 relative to those (1625 and 1593 cm-1) of the
free benzimidazole-based ligand HL upon the coordination of the metal ions. Moreover, two
additional strong absorptions at 1387-1384 and 1316-1308 cm-1 for complexes 1-4 or
1387-1379 and 1319-1311 cm-1 for complexes 5-8 were observed, which are typical for the
-
1
bidentate NO3 groups [23], respectively. As to the room temperature H NMR spectra in
CD3CN of both complexes 1 and 5, a large shift (δ from 12.27 to -1.96 for 1; 13.43 to -2.24
ppm for 5) of the photon resonances of the two mixed ligands (L- and bpy or bpe) to high
fields is observed due to the Nd3+-induced shift. The down-field signals (δ 12.27 and 8.85
ppm for 1; δ 13.43, 13.21 and 8.92 ppm for 5) are assigned to the protons of complexed
11

4,4’bipyridine linkers, shifted down-field relatively compared with free bpy (δ 7.82 and 8.69
ppm) or bpe (δ 7.48, 7.73 and 8.74 ppm). The signals of the coordinated ligands (δ from 7.71
to -1.96 ppm for 1 and δ from 7.83 to -2.24 ppm for 5) are significantly spread relative to
those (δ from 7.67 to 3.83 ppm) of the free benzimidazole-based ligand HL. The ESI-MS
spectra of the two series of complexes 1-4 and 5-8 in MeCN display the similar patterns and
exhibit the strong mass peak at m/z 1905.48 (1), 1963.08 (2), 1951.52 (3) or 1931.50 (4) and
1931.52 (5), 1989.52 (6), 1977.56 (7) or 1957.54 (8), assigned to the major species
{[Zn2Ln2(L)4(bpy)(NO3)6]-H}+ of complexes 1-4 and {[Zn2Ln2(L)4(bpe)(NO3)6]-H}+ of
complexes 5-8, respectively. These observations further indicate that each of the discrete
tetranuclear molecules 1-4 and 5-8 is retained in the respective dilute MeCN solution.
3.2 Molecular Structures of Complexes 1·3H2O and 5·2EtOH
The molecular structures of 1·3H2O as the representative of complexes 1-4 and 5·2EtOH
as the representative of complexes 5-8 were determined by X-ray single-crystal diffraction
analysis. Crystallographic data for the two complexes are presented in Table 1, and selected
bond lengths and angles are given in Table 2. For complex 1·3H2O, it crystallizes in the
monoclinic space group of C2/c. As shown in Figure 1, two crystallographically equivalent
Zn-Nd moieties lie about the inversion center, and one bpy bridges the two Zn2+ ions. Each
Zn2+ (Zn1 or Zn1a) ion lies in a common five-coordinate environment and has a distorted
square pyramidal geometry, as shown by the same calculated τ value [24] of 0.496, where the
inner N2O2 core (N2, O2, N3 and O3) from two deprotonated benzimidazole-based L- ligands
with the same N’O-chelate mode comprises the basal plane, and one terminal N (N5) atom
12

from the coordinated bpy at the apical position. For each of the two Nd3+ (Nd1 or Nd1a) ions,
it is ten-coordinated, composed of the outer O2O2 cavity (O1, O2, O3 and O4) from two
deprotonated benzimidazole-based L- ligands and six O atoms from three bidentate NO3
-
anions. The bond lengths (1.964(19)-2.094(14) Å) of Zn-N (the benzimidazole-based L-
ligand) are slightly shorter than that (2.110(13) Å) of Zn-N (N5 of bpy) bond while between
those (2.062(10)-2.135(15) Å) of Zn-O (phenoxo, O2 and O3 atoms from two
benzimidazole-based L- ligands) bonds. The ten Nd-O bond lengths depend on the nature of
the
O
atoms: they vary from 2.407(11)-2.588(15) Å, and the bond lengths
(2.407(11)-2.421(13) Å) from the phenoxo O atoms (O2 and O3) are slightly shorter than
-
those (2.534(14)-2.588(15) or 2.427(16)-2.584(19) Å) from -OMe groups or NO3 anions. For
the tetranuclear Zn2Nd2-arrayed complex 1·3H2O, the same Zn-Nd distance of each moiety
with two benzimidazole-based L- ligands sandwiched is 3.655(3) Å, slightly longer than that
(3.636(3) Å) of hetero-binuclear complex [ZnNd(L)2(Py)(NO3)3] from the same
benzimidazole-based L- ligand [20], which should be resulted from the larger crossing
(118.4(2)° of the dihedral angle between the two benzimidazole rings) of the two
benzimidazole-based L- ligands in each moiety. It is worth noting that in the formation of
Zn2Nd2-arrayed structure of complex 1·3H2O, the occupation of bpy at the axial positions of
two Zn2+ ions, is considered to completely avoid the further coordination of OH-, CH- or
NH-containing solvents around the two Nd3+ ions, which is comparable of that of complex
[Zn2Nd2(Salen)2(bpy)(NO3)6] from the Salen-type Schiff-base ligand [18c], where the similar
Zn···Zn separation of 11.243(4) Å from bridged bpy in complex 1·3H2O while the larger
dihedral angles (112.9(2)-119.2(2)°) between the two benzimidazile rings and bpy are
13

observed. The three solvate H2O molecules are not bound to the framework, and they exhibit
no observed interactions with the host structure. A detailed examination of the crystal packing
of complex 1·3H2O reveals that two kinds of complementary intermolecular C23-H23···O12
(3.465(23) Å and 158°) and N1-H1···O13 (2.884(23) Å and 157°) hydrogen bonding, one
kind of complementary intermolecular C18-H18···π (3.585(23) Å and 127°) hydrogen
bonding and two kind of weak intermolecular π···π interactions (3.479(13)-3.891(15) Å) exist
between every two adjacent molecules, thus generating to a 1D polymeric chain (shown in
Figure 1S). These chains that run parallel are linked with each other through multiple
supramolecular forces involving C5-H5···O6 (3.207(31) Å and 169°), C11-H11···O11
(3.430(42) Å and 149°), N4-H4···O7 (3.089(30) Å and 145°) hydrogen bonds and weak π···π
interactions (3.905(15) Å), resulting in a 2D sheet as shown in Figure 2S.
[Tables 1-2 and Figure 1 inserted here]
The replacement of 4,4’-bipyridine bridge from bpy to bpe does not lead to a significant
change of the hetero-tetranuclear host structure of complex 5·2EtOH. For complex 5·2EtOH,
it crystallizes in the monoclinic space group of P2(1)/c, where one hetero-tetranuclear
Zn2Nd2-arrayed [Zn2Ln2(L)4(bpe)(NO3)6] host structure and two solvate EtOH molecules
comprise the repeating unit. As shown in Figure 2, in the two crystallographically equivalent
Zn-Nd moieties, in addition to the axial position of each five-coordinate Zn2+ (Zn1 or Zn1a)
ion occupied by the terminal N (N5 or N5a) atom from the bpe bridge, the similar
coordination environments for each Zn2+ (Zn1 or Zn1a) and each Nd3+ (Nd1 or Nd1a) ion to
those in 1·3H2O are observed. The use of bpe endows the observation of relatively longer
Zn···Zn separation of 13.607(8) Å and a distorted trigonal bipyramidal geometry (shown by
14

the same calculated τ value of 0.515) of the two five-coordinate Zn2+ ions, which decreases
the dihedral angle (75.4(2)°) between the two benzimidazole rings in the both Zn-Nd moieties
and dihedral angles (46.9(2)-111.1(2)°) between the two benzimidazile rings and bpe in
complex 5·2EtOH. Furthermore, the use of bpe leads to the different crystal packing in
complex 5·2EtOH. As shown in Figure 3S, weak intermolecular π···π interactions (3.666(11)
Å) connect every two adjacent molecules to form a polymeric 1D chain. These chains run
parallel and are linked with each other through two kinds of intermolecular C33-H33···O9
(3.349(22) Å and 144°) and N7-O9···π (4.258(22) Å and 106.9°) hydrogen bonds, leading to
the formation of a 2D sheet as shown in Figure 4S. A 3D framework is obtained from the
intermolecular N1-H1···O13 (3.038(24) Å and 144°) hydrogen bonding interactions as shown
in Figure 5S
[Figure 2 inserted here]
3.3 Photophysical Properties of Complexes 1-4 and 5-8
The photophysical properties of HL and two series of complexes 1-4 and 5-8 have been
examined in dilute MeCN at room temperature or 77 K, and summarized in Table 3 and
Figures 3-7. As shown in Figure 3, the almost identical ligand-centered solution absorption
spectra (216-220, 296-298 and 332-334 nm) of complexes 1-4 in the UV-visible region are
observed, and the lowest energy absorptions are red-shifted by 16-18 nm upon coordination to
Zn2+ and Ln3+ ions as compared to that (316 nm) of the free benzimidazole-based ligand HL.
It is noticed that the molar absorption coefficients of complexes 1-4 in all the lowest energy
bands are about three while not four orders of magnitude larger than that of the HL ligand
15

despite the four coordinated L- ligands involved. Moreover, the lowest energy absorptions
(332-334 nm) for complexes 1-4 are blue-shifted by 5-8 nm in comparison with those
(225-231, 298-301 and 339-340 nm) of the previous Zn-Ln complexes [ZnLn(L)2(Py)(NO3)3]
from the same HL ligand [20]. The decreased electronic conjugation of the whole molecule
system, assigned to the larger dihedral angle (118.4(2)°) between the two benzimidazole rings
of two benzimidazole-based L- ligands in complex 1·3H2O, is evidently in disagreement with
the comparison between the Zn2Ln2 and ZnLn complexes from the almost planar Salen-type
Schiff-base ligand with the use of bpy and Py [18c], respectively. For complexes 1-3, the
similar residual visible emission (λem = 428 nm, τ < 1 ns and Φem < 10-5, almost undetectable)
in dilute absolute MeCN solution at room temperature are observed, while photo excitation at
the range of 270-360 nm (λex = 340 nm), as shown in Figure 4, gives rise to the characteristic
4
2
emissions of Nd3+ ion (4F3/2 → IJ/2, J = 9, 11, 13) for complex 1, the Yb3+ ion (2F5/2 → F7/2) for
4
complex 2 and the Er3+ ion (4I13/2 → I15/2) for complex 3 in the NIR region, respectively. For
4
4
4
complex 1, the emissions at 908, 1064 and 1326 nm can be assigned to F3/2 → I9/2, F3/2
→
4
4I11/2 and 4F3/2 → I13/2 transitions of the Nd3+ ion, respectively. As to complexes 2-3, the strong
2
2
emission at 978 nm and the weak emission at 1540 nm can be attributed to the F5/2 → F7/2)
4
4
transition of the Yb3+ ion and the I13/2 → I15/2 transition of the Er3+ ion, respectively. Further
based on the results of time-resolved luminescent experiments, the three luminescent decay
curves can be fitted mono-exponentially with time constants of microseconds (1.24 μs for 1,
12.96 μs for 2 and 1.68 μs for 3), manifesting the existence of one luminescent center in the
respective solution, and the intrinsic quantum yields ФLn (0.50% for 1, 0.65% for 2 and 0.21%
for 3) of the Ln3+ emissions may be estimated by ФLn = τ /τ , where τ is the observed
obs
0
obs
16

emission lifetime and τ is the “neutral lifetime” viz 0.25 ms, 2.0 and 8.0 ms for the Nd3+, Yb3+
0
and Er3+ ions [25], respectively. For complex 4, it does not exhibit NIR luminescent emission
under the same condition, while the typical ligand-centered fluorescence (λem = 428 nm τ =
1.54 ns and ϕem = 0.33×10-3) in the visible region is observed, as shown in Figure 5. It is
worth noting that the similar excitation spectrum (λex = 340 nm) monitored at the respective
strongest NIR emission peak (1064 nm for 1, 978 nm for 2 or 1540 nm for 3) to that at the
visible emission peak (428 nm) of complex 4, clearly shows that both the NIR emissions are
originated from the same π-π* transitions of the ligands for complexes 1-3. This also
suggests that the energy transfer from the antenna to the Ln3+ ions takes place efficiently [26].
[Table 3 and Figures 3-4 inserted here]
It is of special interest to explore the mechanism of energy transfer process on the NIR
luminescent complexes 1-3. As a reference compound, complex 4 allows the further study of
the antenna luminescence in the absence of energy transfer, because the Gd3+ ion has no
energy levels below 32000 cm-1, and therefore cannot accept any energy from the antennae
excited state [27]. In dilute MeCN solution at 77 K, complex 4 displays the strengthened
antennae fluorescence, showing the higher luminescent intensity (λem = 413 nm and 464 nm)
and the distinctively longer luminescence lifetimes (2.35 ns and 3.84 ms). This result shows
that the sensitization of the NIR luminescence for complexes 1-3 should arise from both the
1LC (24213 cm-1) and the 3LC (21552 cm-1) excited state of the mixed HL-bpy ligands at low
temperature while not the simple intra-ligand 1π-π* to 3π-π* transition of each ligand [28]. If
the antennae luminescence lifetime of complex 4 is to represent the excited-state lifetime in
the absence of the energy transfer, the energy transfer rate (kET) in the complexes 1-3 can thus
17

be calculated from kET = 1/τq – 1/τu [29], where τq is the residual fluorescence lifetime, and τu
is the fluorescence lifetime in the reference complex 4, so the energy transfer rates for the
Nd3+, Yb3+ and Er3+ ions in complexes 1-3 may all be estimated to be above 5×108 s-1, which
could well imply the reason to the effective energy transfer for complexes 1-3. Furthermore,
from the viewpoint of the energy level match, in spite of the effective energy transfer taking
place in complex 3, the larger energy gap between the energy-donating 3LC (21552 cm-1) and
the emitting level (4I13/2, 6494 cm-1) of Er3+ ion than those (4F3/2, 11013 cm-1 of the Nd3+ ion
2
and F5/2, 10225 cm-1 of Yb3+ ion) of complexes 1-2 results in the great non-radiative energy
loss during the energy transfer, which should be the reason to the weaker NIR luminescence
for complex 3 (ФLn = 0.21%). As the reason to the higher intrinsic quantum yield ФLn (0.65%)
of the Yb3+ emission for 2 than that (0.50%) of the Nd3+ emission for 1, the excited state of
the Nd3+ ions in 1 is more sensitive to quenching by the nearby and distant C-H oscillators of
HL and bpy [30] besides the less opportunities of non-radiative migration from the single
2
emitting transition (2F5/2 → F7/2) of the Yb3+ ion in 3, although the energy gap (4F3/2, 11013
cm-1) of the Nd3+ ion in 2 is smaller than that (2F5/2, 10225 cm-1) of Yb3+ ion in 3.
[Figure 5 inserted here]
Changing the bridge from bpy to more rigid bpe does not extend the conjugation of the
whole molecule systems of complexes 5-8 as expected. On the contrary, the decrease of
dihedral angle (from 118.4(2)° to 74.2°) between the two benzimidazole rings of the two
benzimidazole-based L- ligands for complex 5·2EtOH arisen from the coordination geometry
change of the five-coordinate Zn2+ ions, should be responsible for the lower conjugation,
which can be shown the slight blue-shift of the lowest energy absorptions (327-330 nm,
18

shown in Figure 6) for complexes 5-8 in comparison with those (332-334 nm) of complexes
1-4. Similarly, for complexes 5-7, photo excitation at the range of 250-400 nm (λex = 351 nm),
4
as shown in Figure 7, leads to the characteristic emissions of Nd3+ ion (4F3/2 → IJ/2, J = 9, 11,
2
4
13) for complex 5, the Yb3+ ion (2F5/2 → F7/2) for complex 6 and the Er3+ ion (4I13/2 → I15/2) for
complex 7 in the NIR region, respectively. Similarly, through the further investigation on the
emission of the reference complex 8, especially at 77 K, the existence of both nanosecond
(2.26 ns, λem = 405 nm) and microsecond (3.42 ms, λem = 455 nm) luminescence lifetimes also
shows the sensitization procedure of the NIR luminescence for the bpe-bridged complexes
5-7 should resulted from both the 1LC (24691 cm-1) and the 3LC (21978 cm-1) excited state of
the mixed HL-bpe ligands at low temperature. Moreover, the smaller energy level gap
between the energy-donating 3LC (21978 cm-1) and the emitting level (4F3/2, 11013 cm-1 of the
Nd3+ ion or 2F5/2, 10214 cm-1 of Yb3+ ion) in complex 5 or 6 than that (4I13/2, 6451 cm-1) of Er3+
ion of complex 7 (Φ = 0.13%) leads to the stronger NIR luminescence for complexes 5-6
em
(ФLn = 0.61% for 5 and ФLn = 0.43% for 6). It is worth noting that the NIR intrinsic quantum
yields ФLn (Ln = Nd, Yb or Er) of both complexes 1-3 and 5-7 are slightly lower than those of
hetero-binuclear [ZnLn(L)2(Py)(NO3)3] complexes [20], respectively, which should be due to
the distinctive decrease of the conjugation of the whole Zn2Ln2-arrayed molecule system in
the presence of bpy or bpe, although two chromophores and two acceptors are involved
besides the similarly complete avoidance of the luminescent quenching effect arising from
OH-, CH- or NH-containing oscillators around the two Ln3+ ions. This is evidently
disagreeable to the comparison between the Zn2Ln2 and ZnLn complexes from the almost
planar Salen-type Schiff-base ligand with the use of bpy or bpe and Py [18c], respectively. As
19

to the higher NIR intrinsic quantum yields Ф for 1-3 from bpy than those of 5-8 from bpe,
Ln
3
respectively, attributing to the more suitable energy match between the energy-donating LC
and the emitting level of the respective Ln3+ ion, it can be shown from the larger crossing of
the two benzimidazole-based ligands in complexes 1-3.
[Figures 6-7 inserted here]
4. Conclusions
In conclusion, two series of Zn2Ln2-arrayed complexes [Zn2Ln2(L)4(bpy)(NO3)6] (Ln =
Nd, 1; Yb, 2; Er, 3 or Gd, 4) and [Zn2Ln2(L)4(bpe)(NO3)6] (Ln = Nd, 5; Yb, 6; Er, 7 or Gd, 8)
were obtained from the self-assembly of the benzimidazole-based HL with Zn(OAc)2·2H2O,
Ln(NO3)3·6H2O and 4,4’-bipyridine ligand (bpy or bpe), respectively, where two
chromophores and two Ln3+ acceptors are involved besides the similarly complete avoidance
of the luminescent quenching effect arising from OH-, CH- or NH-containing oscillators
around the two Ln3+ ions. The result of their photophysical properties shows that the
characteristic near-infrared (NIR) luminescence of Nd3+, Yb3+ or Er3+ ion has been sensitized
1
3
from the excited state (both LC and LC) of the mixed HL and bipyridyl ligands in both
complexes 1-3 and 5-7, and the change from bpy to bpe bridging for the difference of whole
molecular conjugations, endows the fine-tuning of their NIR luminescent properties. While in
facilitating the NIR sensitization, the specific design of hetero-polynuclear complexes from
the benzimidazole-based ligands and other second linkers is now under way.
Acknowledgements
This work is funded by the National Natural Science Foundation (912222201, 21173165,
20

20871098), the Program for New Century Excellent Talents in University from the Ministry
of Education of China (NCET-10-0936), the State Key Laboratory of Structure Chemistry
(20100014), the Provincial Natural Foundation (2011JQ2011) of Shaanxi, 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.
Appendix A. Supplementary data
The crystal packing diagrams for complexes 1·3H2O and 5·2EtOH are shown in Figures
1-5S of Supporting information. The crystallographic data for complexes 1·3H2O and
5·2EtOH have been deposited at the Cambridge Crystallographic Data Center, CCDC-934710
(for 1·3H2O) and CCDC-929956 (for 5·2EtOH). The data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge CB21EZ, UK.
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25

Table 1 Crystal data and refinement for 1·3H2O and 5·2EtOH
Compound
1·3H2O
5·2EtOH
Empirical formula
C66H64N16Nd2O32Zn2 C72H66N16O28Zn2Nd2
Formula weight
2012.55
Monoclinic
C2/c
2022.63
Monoclinic
P2(1)/c
21.506(13)
11.461(7)
20.955(14)
90
Crystal system
Space group
A/Å
21.959(9)
19.270(8)
26.659(11)
90
b/Å
c/Å
α/˚
β/˚
90.092(7)
90
95.469(16)
90
γ/˚
V/Å3
11281(8)
4
5141(6)
2
Z
ρ/g·cm-3
1.185
1.307
Crystal size/mm
0.42 × 0.32 × 0.23
0.43 × 0.35 × 0.29
1.393
1.525
μ(Mo-Kα)/mm-1
Data/restraints/parameters
12723/6/498
9954/31/530
Quality-of-fit indicator
No. Unique reflections
No. Observed reflections
Final R indices [I>2σ(I)]
1.061
0.910
12723
9954
31824
26582
R1 = 0.1486
W R2 = 0.3621
R1 = 0.3403
W R2 = 0.4624
R1 = 0.1132
W R2 = 0.2512
R1 = 0.3068
W R2 = 0.3530
R indices (all data)
26

Table 2 Interatomic distances (Å) and bond angles (˚) with esds for complexes 1·3H2O and
5·2EtOH
1·3H2O
5·2EtOH
Zn(1)-N(2)
Zn(1)-N(3)
Zn(1)-N(5)
Zn(1)-O(2)
Zn(1)-O(3)
1.964(19)
2.094(14)
2.110(13)
2.062(10)
2.135(15)
N(2)-Zn(1)-N(3)
N(2)-Zn(1)-N(5)
N(2)-Zn(1)-O(2)
N(2)-Zn(1)-O(3)
109.2(6)
91.5(6)
87.4(5)
162.5(5)
Zn(1)-N(2)
Zn(1)-N(3)
Zn(1)-N(5)
Zn(1)-O(2)
Zn(1)-O(3)
2.040(20)
1.985(16)
2.125(9)
N(2)-Zn(1)-N(3)
N(2)-Zn(1)-N(5)
N(2)-Zn(1)-O(2)
N(2)-Zn(1)-O(3)
108.3(7)
93.4(6)
88.5(5)
163.1(6)
2.073(11)
2.130(12)
O(1)-Nd(1)-O(4)
O(2)-Nd(1)-O(3)
O(5)-Nd(1)-O(7)
O(8)-Nd(1)-O(10)
153.7(5)
65.0(4)
49.2(5)
52.0(6)
O(1)-Nd(1)-O(4)
O(2)-Nd(1)-O(3)
O(5)-Nd(1)-O(7)
O(8)-Nd(1)-O(10)
174.8(4)
65.7(4)
51.0(6)
51.5(5)
Nd(1)-O(1)
Nd(1)-O(2)
Nd(1)-O(3)
Nd(1)-O(4)
Nd(1)-O(5)
Nd(1)-O(7)
Nd(1)-O(8)
2.534(14)
2.421(13)
2.407(11)
2.588(15)
2.514(13)
2.584(19)
2.527(17)
Nd(1)-O(1)
Nd(1)-O(2)
Nd(1)-O(3)
Nd(1)-O(4)
Nd(1)-O(5)
Nd(1)-O(7)
Nd(1)-O(8)
2.552(12)
2.393(12)
2.310(12)
2.821(12)
2.501(16)
2.450(20)
2.520(14)
O(11)-Nd(1)-O(13) 49.9(5)
O(11)-Nd(1)-O(13) 51.0(5)
Nd(1)-O(10) 2.427(16)
Nd(1)-O(11) 2.559(16)
Nd(1)-O(13) 2.555(13)
Nd(1)-O(10) 2.540(14)
Nd(1)-O(11) 2.575(16)
Nd(1)-O(13) 2.520(18)
27

Table 3 The photophysical properties of HL and complexes 1-8 at 2×10-5 M in absolute
MeCN solution at room temperature or 77 K
Compound
Absorption
Excitation
Emission
λ /nm [log(ε/dm3mol-1cm-1)]
λ /nm
ex
λ
em/nm (τ, Φ ×103)
ab
HL
1
219(0.45), 301(0.26), 316(0.17)
220(1.58), 296(0.82), 334(0.61)
220(1.67), 298(0.83), 332(0.61)
216(1.43), 298(0.74), 332(0.61)
220(1.58), 296(0.79), 332(0.58)
364
340
340
338
343
434(s)
427(w), 908(1.23, μs), 1064(1.24 μs), 1326(1.26 μs)
428(w), 978(12.96 μs)
2
3
430(w), 1540(1.68 μs)
4
428(m, 1.54 ns, 0.33)
413(s, 2.35 ns, 77K), 464(s, 3.84 ms, 77K)
422(w), 908(1.05, μs), 1063(1.08 μs), 1325(1.11 μs)
423(w), 979(12. 22 μs)
5
6
7
8
217(1.01), 300(0.59), 330(0.37)
217(1.09), 298(0.57), 328(0.36)
216(1.18), 299(0.56), 327(0.38)
214(0.94), 300(0.58), 330(0.36)
351
351
351
353
422(w), 1550(1.44 μs)
420(m, 1.12 ns, 0.16)
405(s, 2.26 ns, 77K), 455(s, 3.42 ms, 77K)
28

Captions to Scheme and Figures
Scheme 1 Controlled self-assembly of two series of hetero-tetranuclear Zn2Ln2-arrayed
complexes 1-4 and 5-8
Figure 1 Perspective drawing of complex 1·3H2O with 15% probability ellipsoids, H atoms
and solvates are omitted for clarity
Figure 2 Perspective drawing of complex 5·2EtOH with 15% probability ellipsoids, H atoms
and solvates are omitted for clarity
Figure 3 UV-Visible absorption spectra of HL and complexes 1-4 in MeCN solution at
2×10-5 M at room temperature
Figure 4 NIR emission and excitation spectra of complexes 1-3 (λ = 340 nm for 1, λ = 340
ex
ex
nm for 2 and λ = 338 nm for 3) in MeCN solution at 2×10-5 M at room temperature
ex
Figure 5 Visible emission and excitation spectra of HL (λ = 364 nm), complexes 4 (λ
=
ex
ex
343 nm) and 8 (λ = 353 nm) in MeCN solution at 2×10-5 M at room temperature
ex
Figure 6 UV-Visible absorption spectra of HL and complexes 5-8 in MeCN solution at
2×10-5 M at room temperature
29