Near-infrared (NIR) luminescent homoleptic lanthanide Salen complexes Ln4(Salen)4 (Ln = Nd, Yb or Er)

Article abstract of DOI:10.1039/c2ce06566e

The series of homoleptic tetranuclear [Ln₄(L) ₂(HL)₂(NO₃)₂(OH)₂] ·2(NO₃) (Ln = Nd, 1; Ln = Yb, 2; Ln = Er, 3; Ln = Gd, 4) have been self-assembled from the reaction of the Salen-

Full text of DOI:10.1039/c2ce06566e

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PAPER
Near-infrared (NIR) luminescent homoleptic lanthanide Salen complexes
Ln4(Salen)4 (Ln ¼ Nd, Yb or Er)†
a
a
Weixu Feng, Yao Zhang, Xingqiang Lu, ab Yani Hui,a Guoxiang Shi,a Dan Zou,a Jirong Song,a Daidi Fan,a
*
€
Wai-Kwok Wong and Richard A. Jonesd
c
*
Received 23rd November 2011, Accepted 2nd March 2012
DOI: 10.1039/c2ce06566e
The series of homoleptic tetranuclear [Ln4(L)2(HL)2(NO3)2(OH)2]$2(NO3) (Ln ¼ Nd, 1; Ln ¼ Yb, 2;
Ln ¼ Er, 3; Ln ¼ Gd, 4) have been self-assembled from the reaction of the Salen-type Schiff-base ligand
H2L with Ln(NO3)3$6H2O (Ln ¼ Nd, Yb, Er or Gd), respectively (H2L: N,N0-bis(salicylidene)
cyclohexane-1,2-diamine). The result of their photophysical properties shows that the strong and
characteristic NIR luminescence for complexes 1 and 2 with emissive lifetimes in microsecond ranges
1
3
are observed and the sensitization arises from the excited state (both LC and LC) of the Salen-type
Schiff-base ligand with the flexible linker.
Salen-type Schiff-base ligands,9 the photophysical properties of
1. Introduction
polynuclear lanthanide Salen complexes have not been
researched nearly as extensively, especially the limited single-
crystal X-ray diffraction study has been reported for those classic
complexes.10 Nonetheless, in the formation of the series of
binuclear triple-decker,11 trinuclear triple-decker,12 trinuclear
tetra-decker13 or pentanuclear tetra-decker lanthanide
complexes,14 different Salen-type Schiff-base ligands with rigid
linkers have been used, and their photophysical properties
should be further enhanced, despite the chromophores with rigid
linkers absorbing at longer wavelength. Moreover, the self-
assembly from LnX3 (Xꢀ ¼ OAcꢀ or CF3SO3ꢀ) with the Salen-
type Schiff-base ligands with the flexible linkers usually gives the
formation of polymeric lanthanide coordination complexes, not
the discrete polynuclear lanthanide complexes.15 In particular,
the recent report of discrete binuclear or tetranuclear homoleptic
lanthanide Salen complexes16 revealed that the pure Salen-type
Schiff-base ligand without the outer O2O2 moiety could coordi-
nate with the Ln3+ ions in quadridentate and bidentate modes,
while the self-assembly process is complicated, anion-dependent
and strictly relative to the detailed reaction conditions. To the
best of our knowledge, no report of the self-assembly of poly-
nuclear lanthanide complexes from flexible hexadentate Salen-
type Schiff-base ligands with the outer O2O2 moiety has been
documented. Herein, starting from the Salen-type Schiff-base
Recent startling interest for polynuclear Ln3+ complexes with the
unusual photophysical and magnetic properties, is stimulated by
the continuously expanding need for luminescent1 and magnetic
materials.2 However, the synthesis is often problematic due to the
high coordination numbers and the flexible coordination geom-
etries adopted by the Ln3+ ions.3 In fact, the challenge to resolve
the problem is strengthened because the construction of the
polynuclear Ln3+ complexes is distinctively affected by a variety
of factors, such as the character of organic ligands,4 the nature of
counter anions5 and the reaction conditions.6 From the view-
point of self-assembly of the polynuclear Ln3+ complexes with
good photophysical properties, this requires that the strong light-
harvesting of the organic chromophores, the effective energy
transfer from the chromophores to the Ln3+ ions and the mini-
mization of non-radiative processes of the Ln3+ are achieved,7
besides completely avoiding or decreasing the luminescent
quenching effect arising from OH-, CH- or NH-oscillators
around the Ln3+ ion.8
Compared to the amount of effort on the photophysical
behavior of 3d–4f heteronuclear complexes from compartmental
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
ligand H2L (H2L
¼
N,N0-bis(3-methoxy-salicylidene)cyclo-
hexane-1,2-diamine) with the flexible linker, the richness of its
coordination modes ((L)2ꢀ and (HL)ꢀ modes, as shown in
Scheme 1) endows the formation of series of homoleptic tetra-
nuclear [Ln4(L)2(HL)2(NO3)2(OH)2]$2(NO3) (Ln ¼ Nd, 1; Ln ¼
Yb, 2; Ln ¼ Er, 3; Ln ¼ Gd, 4) under similar reaction conditions.
The sensitization and the energy transfer for the NIR lumines-
cence of the Ln3+ ions in the homoleptic tetranuclear Ln4(Salen)4
complexes are discussed.
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. E-mail: wkwong@hkbu.edu.hk
dDepartment of Chemistry and Biochemistry, The University of Texas at
Austin, 1 University Station A5300, Austin, TX 78712-0165, United States
† Electronic supplementary information (ESI) available. CCDC
reference numbers 854306–854308. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2ce06566e
3456 | CrystEngComm, 2012, 14, 3456–3463
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Scheme 1 Molecular structures and bonding modes of Salen-type Schiff-base ligands for polynuclear or polymeric lanthanide complexes.
1619 (s), 1588 (w), 1470 (s), 1418 (m), 1345 (w), 1251 (vs), 1196
(w), 1168 (w), 1144 (w), 1085 (m), 1036 (w), 984 (m), 953 (w), 894
(w), 849 (m), 776 (w), 731 (m), 668 (w), 616 (w), 595 (w), 568 (w),
516 (w), 467 (w), 422 (w). 1H NMR (400 MHz, CD3CN): d (ppm)
13.84 (s, 2H, –OH), 8.24 (s, 2H, –CH]N), 6.85 (d, 2H, –Ph), 6.78
(d, 2H, –Ph), 6.72 (t, 2H, –Ph), 3.86 (s, 6H, MeO–), 3.32 (m, 2H,
–Ch), 1.92 (m, 4H, –Ch), 1.58 (m, 4H, –Ch).
2. Experimental
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 spectropho-
tometer in the region 4000–400 cmꢀ1 using KBr pellets. 1H
NMR spectra were recorded on a JEOL EX270 spectrometer
with SiMe4 as internal standard in CD3CN at room tempera-
ture. 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 or MeOH as solvent. Electronic absorption spectra in
the UV/Vis region were recorded with a Cary 300 UV spec-
trophotometer, and steady-state visible fluorescence, PL exci-
tation 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
Synthesis of [Nd4(L)2(HL)2(NO3)2(OH)2]$2(NO3) (1). To
a stirred solution of H2L (0.115 g, 0.3 mmol) in absolute MeCN
(10 mL), Et3N (100 mL) and a solution of Nd(NO3)3$6H2O
(0.3 mmol, 0.117 g) in absolute MeOH (10 mL) were added,
respectively. The resultant mixture was refluxed for 2 h, and the
clear pale yellow solution was then cooled to room temperature
and filtered. Diethyl ether was allowed to diffuse slowly into the
filtrate at room temperature and the pale yellow microcrystal
products of 1 were obtained in a few weeks. For 1: Yield: 0.118 g,
67%. Anal. Calc. for C88H100N12O30Nd4: C, 44.36; H, 4.23; N,
7.05%; found: C, 44.15; H, 4.37; N, 7.01%. IR (KBr, cmꢀ1): 3402
(b), 3065 (w), 2938 (m), 2857 (w), 1648 (s), 1615 (m), 1556 (m),
1503 (m), 1452 (vs), 1384 (m), 1355 (w), 1283 (s), 1237 (w), 1221
(s), 1169 (m), 1144 (w), 1098 (w), 1075 (m), 1058 (w), 1019 (w),
946 (w), 902 (w), 847 (m), 783 (w), 742 (m), 659 (w), 605 (w), 561
quantum yield (quinine sulfate in dilute H2SO4 solution, Fem
¼
0.546). NIR emission and excitation in solution were recorded
by PTI QM4 spectrofluorometer with a PTI QM4 Near-
Infrared InGaAs detector.
1
(w), 491 (w), 470 (w), 433 (w). H NMR (400 MHz, CD3CN):
d (ppm) 14.58 (s, 4H), 14.17 (s, 4H), 10.10 (t, 4H), 9.40 (t, 4H),
8.61 (d, 4H), 8.36 (d, 4H), 7.59 (m, 4H), 6.53 (m, 4H), 5.48 (m,
12H), 4.66 (m, 12H), 4.02 (m, 4H), 3.80 (m, 4H), 1.70 (s, 6H),
1.31 (s, 6H), 1.16 (m, 3H), 1.03 (m, 3H), ꢀ0.10 (m, 4H), ꢀ0.42
(m, 4H), ꢀ0.81 (m, 4H), ꢀ1.23(m, 4H), ꢀ1.83 (m, 4H), ꢀ2.65(m,
4H), ꢀ2.90 (m, 4H), ꢀ5.68 (m, 4H). ESI-MS (in MeCN) m/z:
2320.78 (100%), [M ꢀ NO3]+; 1129.38 (45%), [M ꢀ (NO3)2]2+;
745.94 (4%), [M ꢀ (NO3)3 + (MeCN)]3+ and 554.22 (11%), [M ꢀ
(NO3)4 + (MeCN)2]4+; ESI-MS (in MeOH) m/z: 2320.78 (100%),
[M ꢀ NO3]+; 1129.38 (41%), [M ꢀ (NO3)2]2+, 742.94 (5%), [M ꢀ
Syntheses
H2L (H2L: N,N0-bis(3-methoxy-salicylidene)cyclohexane-1,2-
diamine). To a stirred solution of an equimolar mixture of cis-
and trans-1,2-diaminocyclohexane (6.0 mL, 50 mmol) in absolute
EtOH (20 mL), o-vanillin (15.0 g, 100 mmol) was added, and the
resulting mixture was refluxed for 5 h. After cooling to room
temperature, the insoluble yellow precipitate was filtered and was
recrystallized using absolute EtOH to give a pale yellow poly-
crystalline solid. Yield: 13.6 g, 71%. Calc. for C22H26N2O4: C
69.09, H 6.85, N 7.32%; found: C, 69.01, H, 6.94, N, 7.26%. IR
(KBr, cmꢀ1): 3455 (b), 3058 (w), 2933 (w), 2862 (w), 2597 (w),
(NO3)3 + (MeOH)]3+ and 549.71 (10%), [M ꢀ (NO3)4
+
(MeOH)2]4+.
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Synthesis of [Yb4(L)2(HL)2(NO3)2(OH)2]$2(NO3) (2). The
complex 2 was prepared in the same way as 1 except that
Yb(NO3)3$6H2O (0.3 mmol, 0.126 g) was used instead of
Nd(NO3)3$6H2O (0.3 mmol, 0.117 g). For 2: Yield: 0.109 g, 58%.
Anal. Calc. for C88H100N12O30Yb4: C, 42.31; H, 4.04; N, 6.73%;
found: C, 42.18; H, 4.17; N, 6.66%. IR (KBr, cmꢀ1): 3411 (b),
3065 (w), 2936 (m), 2861 (w), 1653 (s), 1618 (m), 1560 (m), 1505
(m), 1476 (vs), 1384 (m), 1360 (w), 1291 (s), 1236 (w), 1229 (s),
1168 (m), 1143 (w), 1100 (w), 1078 (m), 1060 (w), 1042 (w), 1022
(w), 949 (w), 903 (w), 847 (m), 782 (w), 738 (m), 695 (w), 660 (w),
613 (w), 561 (w), 502 (w), 482 (w), 448 (w). ESI-MS (in MeCN)
m/z: 2435.98 (100%), [M ꢀ NO3]+; 1186.98 (41%), [M ꢀ
(NO3)2]2+; 784.34 (6%), [M ꢀ (NO3)3 + (MeCN)]3+ and 583.02
(13%), [M ꢀ (NO3)4 + (MeCN)2]4+; ESI-MS (in MeOH) m/z:
2435.98 (100%), [M ꢀ NO3]+; 1186.98 (40%), [M ꢀ (NO3)2]2+;
748.00 (8%), [M ꢀ (NO3)3 + (MeOH)]3+ and 578.51 (14%), [M ꢀ
(NO3)4 + (MeOH)2]4+.
(3$3H2O) of suitable dimensions were mounted onto thin glass
fibers. All the intensity data were collected on a Bruker SMART
ꢀ
CCD diffractometer (Mo-Ka radiation and l ¼ 0.71073 A) in F
and u 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 SHELXL-
97.17 All other non-hydrogen atoms were refined with anisotropic
thermal parameters. Absorption corrections were applied using
SADABS.18 Hydrogen atoms were placed in calculated positions
and refined isotropically using a riding model. In each of
complexes 1–3, the three water solvates are disordered with each
O atom occupying two sets of positions, and the H atoms are not
added. Crystallographic data and refinement parameters for the
complexes are presented in Table 1. CCDC reference number
854306–854308 for 1$3H2O, 2$3H2O and 3$3H2O, respectively.
3. Results and discussion
Synthesis of [Er4(L)2(HL)2(NO3)2(OH)2]$2(NO3) (3). The
complex 3 was prepared in the same way as 1 except that
Er(NO3)3$6H2O (0.3 mmol, 0.124 g) was used instead of
Nd(NO3)3$6H2O (0.3 mmol, 0.117 g). For 3: Yield: 0.113 g, 61%.
Anal. Calc. for C88H100N12O30Er4: C, 42.71; H, 4.07; N, 6.79%;
found: C, 42.58; H, 4.17; N, 6.76%. IR (KBr, cmꢀ1): 3414 (b),
3066 (w), 2940 (m), 2860 (w), 1652 (s), 1618 (m), 1558 (m), 1504
(m), 1469 (vs), 1383 (m), 1357 (w), 1288 (s), 1240 (w), 1227 (s),
1169 (m), 1146 (w), 1100 (w), 1078 (m), 1060 (w), 1043 (w), 1025
(w), 964 (w), 949 (w), 903 (w), 844 (m), 783 (w), 740 (m), 695 (w),
659 (w), 611 (w), 561 (w), 502 (w), 481 (w), 443 (w). ESI-MS
(in MeCN) m/z: 2412.86 (100%), [M ꢀ NO3]+; 1175.43 (40%),
[M ꢀ (NO3)2]2+; 776.63 (3%), [M ꢀ (NO3)3 + (MeCN)]3+ and
577.24 (17%), [M ꢀ (NO3)4 + (MeCN)2]4+; ESI-MS (in MeOH)
m/z: 2412.86 (100%), [M ꢀ NO3]+; 1175.43 (42%), [M ꢀ
(NO3)2]2+; 773.63 (5%), [M ꢀ (NO3)3 + (MeOH)]3+ and 572.73
(15%), [M ꢀ (NO3)4 + (MeOH)2]4+.
Synthesis and characterization
From reaction of the deprotonated L2ꢀ and equimolar amount of
Ln(NO3)3$6H2O (Ln ¼ Nd, Yb, Er or Gd) in MeOH–MeCN,
series of four homoleptic [Ln4(L)2(HL)2(NO3)2(OH)2]$2(NO3)
complexes (Ln ¼ Nd, 1; Ln ¼ Yb, 2; Ln ¼ Er, 3; Ln ¼ Gd, 4) are
obtained. Similar to the good solubility of the Salen-type Schiff-
base ligand H2L in common organic solvents except for water,
the four complexes 1–4 are also soluble in absolute MeCN or
MeOH, which should be due to the use of the Salen-type Schiff-
base ligand H2L with the flexible linker and the charge of the two
components ([Ln4(L)2(HL)2(NO3)2(OH)2]2+ and NO3ꢀ) in each
of the four complexes.
The series of four complexes 1–4 were well characterized by
1
EA, FT-IR, H NMR and ESI-MS. In the FT-IR spectra, the
characteristic strong absorptions of the n(C]N) vibration at
1648–1653 cmꢀ1 for complexes 1–4, are slightly blue-shifted by
the range of 29–34 cmꢀ1 relative to that of the free Salen-type
Schiff-base ligand H2L (1619 cmꢀ1) upon coordination of the
Ln3+ ions. The two additional strong characteristic absorptions
at 1452–1476 cmꢀ1 and 1283–1291 cmꢀ1 for the series of
complexes 1–4 were observed, which are tentatively attributed to
Synthesis of [Gd4(L)2(HL)2(NO3)2(OH)2]$2(NO3) (4). The
complex 4 was prepared in the same way as 1 except that
Gd(NO3)3$6H2O (0.3 mmol, 0.121 g) was used instead of
Nd(NO3)3$6H2O (0.3 mmol, 0.117 g). For 4: Yield: 0.117 g, 64%.
Anal. Calc. for C88H100N12O30Gd4: C, 43.41; H, 4.14; N, 6.90%;
found: C, 43.26; H, 4.25; N, 6.85%. IR (KBr, cmꢀ1): 3414 (b),
3065 (w), 2938 (m), 2860 (w), 1651 (s), 1617 (m), 1556 (m), 1504
(m), 1457 (vs), 1384 (m), 1365 (w), 1289 (s), 1243 (w), 1224 (s),
1170 (m), 1144 (w), 1098 (w), 1075 (m), 1058 (w), 1044 (w), 1023
(w), 961 (w), 947 (w), 903 (w), 855 (m), 782 (w), 740 (m), 692 (w),
660 (w), 608 (w), 560 (w), 498 (w), 477 (w), 434 (w). ESI-MS (in
MeCN) m/z: 2372.82 (100%), [M ꢀ NO3]+; 1155.41 (39%), [M ꢀ
(NO3)2]2+; 763.29 (7%), [M ꢀ (NO3)3 + (MeCN)]3+ and 567.23
(13%), [M ꢀ (NO3)4 + (MeCN)2]4+; ESI-MS (in MeOH) m/z:
2372.82 (100%), [M ꢀ NO3]+; 1155.41 (43%), [M ꢀ (NO3)2]2+;
760.28 (5%), [M ꢀ (NO3)3 + (MeOH)]3+ and 562.75 (15%), [M ꢀ
(NO3)4 + (MeOH)2]4+.
n(NO3ꢀ). As to the room temperature H NMR spectrum in
1
CD3CN of complex 1, large shifts (d from 14.58 to ꢀ5.68 ppm
and 14.17 to ꢀ2.90 ppm) of two sets of the proton resonances of
the L2ꢀ ligands endowed from the observed (L)2ꢀ and (HL)ꢀ
modes in the molecular structure are observed due to the Nd3+-
induced shift, significantly spread in relation to those of the free
H2L ligand (d from 13.84 to 1.58 ppm). The ESI-MS spectra
of the series of four complexes (1–4) in MeCN or MeOH
display similar patterns and exhibit two strong mass peaks
assigned to the major species {[Ln4(L)2(HL)2(NO3)2(OH)2]$
NO3}+ and [Ln4(L)2(HL)2(NO3)2(OH)2]2+, and two fragments to
[Ln4(L)2(HL)2(NO3)(OH)2(S)]3+ and [Ln4(L)2(HL)2(OH)2(S)2]4+
(S ¼ MeCN or MeOH) for complexes 1–4, respectively. These
observations further indicate that the discrete homoleptic tetra-
nuclear unit retains in the respective dilute MeCN or MeOH
solution.
X-ray crystallography
Single crystals of [Nd4(L)2(HL)2(NO3)2(OH)2]$2(NO3)$
3H2O (1$3H2O), [Yb4(L)2(HL)2(NO3)2(OH)2]$2(NO3)$3H2O
X-ray quality crystals of 1$3H2O, 2$3H2O or 3$3H2O of the
series of four complexes 1–4 were obtained, and the table of
selected crystal properties are given in Table 1. For complex
(2$3H2O)
and
[Er4(L)2(HL)2(NO3)2(OH)2]$2(NO3)$3H2O
3458 | CrystEngComm, 2012, 14, 3456–3463
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Table 1 Crystal data and refinement for complexes 1$3H2O, 2$3H2O and 3$3H2O
Compound
1$3H2O
2$3H2O
3$3H2O
Empirical formula
Formula weight
Crystal system
Space group
C88H106N12O33Nd4
2436.81
Triclinic
C88H106N12O33Yb4
2552.01
Triclinic
C90H106N12O33Er4
2528.89
Triclinic
ꢁ
ꢁ
ꢁ
P1
P1
P1
ꢀ
a/A
14.4121(9)
14.7205(9)
14.7927(10)
110.087(2)
104.020(2)
108.7390(10)
2562.5(3)
14.4240(12)
14.4630(11)
14.633(2)
111.254(2)
104.137(2)
107.6990(10)
2485.0(4)
14.4573(13)
14.5690(13)
14.707(2)
111.567(2)
102.880(2)
107.5400(10)
2544.9(5)
ꢀ
b/A
ꢀ
c/A
a (ꢁ)
b (ꢁ)
g (ꢁ)
3
ꢀ
V/A
Z
1
1.579
1
1.705
1
1.650
r/g cmꢀ3
Crystal size/mm
m(Mo-Ka)/mmꢀ1
Data/restraints/parameters
Quality-of-fit indicator
Final R indices [I > 2s(I)]
0.32 ꢂ 0.27 ꢂ 0.23
0.33 ꢂ 0.26 ꢂ 0.22
0.30 ꢂ 0.26 ꢂ 0.24
2.076
8717/1/619
1.095
3.814
8624/1/618
1.044
3.347
8852/1/618
1.020
R1 ¼ 0.0752
wR2 ¼ 0.1978
R1 ¼ 0.1150
wR2 ¼ 0.2431
R1 ¼ 0.0390
wR2 ¼ 0.0956
R1 ¼ 0.0532
wR2 ¼ 0.1044
R1 ¼ 0.0394
wR2 ¼ 0.0990
R1 ¼ 0.0576
wR2 ¼ 0.1114
R indices (all data)
1$3H2O, the structural unit is composed of one cation
[Nd4(L)2(HL)2(NO3)2(OH)2]2+, two free NO3ꢀ anions, and three
solvate H2O molecules. As shown in Fig. 1, for the cationic
[Nd4(L)2(HL)2(NO3)2(OH)2]2+ part lying about an inversion
centre, two equivalent Nd2L(HL) moieties are bridged by two m-
O phenoxide atoms (O6 and O6a) of two Salen-type Schiff-base
(HL)ꢀ ligands with O4 tetradentate mode (as shown in Scheme 1)
and two O atoms (O9 and O9a) of two coordinated m3-OHꢀ
groups, resulting in the formation of a homoleptic cyclic tetra-
nuclear (Nd4(Salen)4) host structure. In each of two equivalent
Nd2L(HL) moieties, two Nd3+ (Nd1 and Nd2) ions with different
coordination environments are also linked by two m-O phenoxide
atoms (O2 and O3) of one Salen-type Schiff-base (L)2ꢀ ligand
with N2O4 hexadentate mode and one O atom (O9) of the
coordinated m3-OHꢀ group. The unique inner Nd3+ ion (Nd1) is
eight-coordinate and bound by the N2O2 core of the Salen-type
Schiff-base (L)2ꢀ ligand in addition to two O atom (O5a of MeO–
group and O6a of m-O phenoxide atom) from the Salen-type
Schiff-base (HL)ꢀ ligand and two O atoms of two coordinated
m3-OHꢀ groups. Meanwhile the outer Nd3+ ion (Nd2) is nine-
coordinate; in addition to the seven oxygen atoms from the two
outer O2O2 moieties of the two Salen-type Schiff-base ligands,
where four O atoms (two of MeO– groups and two of phenoxide
atoms) are from the Salen-type Schiff-base (L)2ꢀ ligand and three
O atoms (one of MeO– groups and two of phenoxide atoms)
from the Salen-type Schiff-base (HL)ꢀ ligand, it completes its
coordination environment with one O atom from the mono-
ꢀ
dentate NO3 anion and one O atom from the coordinated m3-
OHꢀ group. The unique Nd/Nd distances are different, with the
ꢀ
distances of 3.6407(10) and 3.8757(11)–3.9782(8) A for Nd1/
Nd2, Nd1/Nd1a and Nd2/Nd1a, respectively, in which each
of the Nd1/Nd2 separations in the equivalent Nd2L(HL)
moieties is slightly shorter than that (Nd1/Nd1a or Nd2/Nd1a
separation) between two equivalent Nd2L(HL) moieties. For the
inner Nd3+ ion (Nd1 or Nd1a), the Nd–O bond length (2.414(7)–
ꢀ
ꢀ
2.419(6) A; O atoms from coordinated m3-OH group) or the
ꢀ
Nd–N bond lengths (2.477(8)–2.553(9) A) are in the range of the
ꢀ
Fig. 1 Perspective drawing of the cationic part in complex 1$3H2O; free
Nd–O bond lengths (2.382(6)–2.569(7) A) with O atoms from
the phenoxo groups. For the outer Nd3+ ion (Nd2 or Nd2a), the
anions, H atoms andꢁ solvates are omitted for clarity. Selected bond
ꢀ
length (A) and angles ( ): Nd1–N1, 2.553(9); Nd1–N2, 2.477(8); Nd1–O2,
nine Nd–O bond lengths also depend on the nature of the oxygen
ꢀ
atoms, varying from 2.361(7) to 2.712(7) A, in which the bond
ꢀ
lengths (2.653(8)–2.712(7) A) from the oxygen atoms of MeO–
2.382(6); Nd1–O3, 2.434(6); Nd1–O5a, 2.569(7); Nd1–O6a, 2.437(7);
Nd1–O9, 2.414(7); Nd1–O9a, 2.419(6); Nd2–O1, 2.706(7); Nd2–O2,
2.441(6); Nd2–O3, 2.433(6); Nd2–O4, 2.712(7); Nd2–O6, 2.526(7); Nd2–
O7, 2.361(7); Nd2–O8, 2.653(8); Nd2–O9, 2.445(6) and Nd2–O11,
2.489(9). Nd1/Nd2, 3.6407(10); Nd1/Nd2a, 3.9781(8); Nd1/Nd1a,
3.8757(11) and Nd2/Nd1a, 3.9782(8). N1–Nd1–N2, 64.2(3); O2–Nd1–
O3, 68.2(2); O1–Nd2–O4, 140.2(2); O2–Nd2–O3, 67.3(2); O7–Nd2–O8,
63.0 (2) and O9–Nd2–O6, 68.7(2). Symmetry transformation used to
generate equivalent atoms: a: ꢀx + 1, ꢀy + 2, ꢀz + 1.
ꢀ
groups are distinctively longer than those (2.361(7)–2.526(7) A or
ꢀ
ꢀ
2.489(9) A and 2.445(6) A) from the phenoxo oxygen atoms,
ꢀ
monodentate NO3 anion or coordinated m3-OHꢀ group. It is
interesting to note the presence of an ‘‘apical’’ triply bridged m3-
OHꢀ group for each of the two central O atoms (O9 or O9a),
which could be shown from the reasonable directionality of the
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interactions with three Nd3+ ions. Furthermore, as to the cationic
[Nd4(L)2(HL)2(NO3)2(OH)2]2+ part, the charge should be
balanced by the protonation of one (N4 or N4a) of the imino
nitrogen atoms for two of the four deprotonated Salen-type
Schiff-base (L)2ꢀ ligands, which endows the formation of two
strong intramolecular H-bond interactions with the short N4–
ꢀ
H4/O7 distance (2.575(17) A) shown in Fig. 1s.† The three
solvate H2O molecules and the two free NO3ꢀ anions of complex
1$3H2O are not bound to the framework and they exhibit no
observed interactions with the host structure.
It is worth noting that the homoleptic cyclic tetranuclear
(Nd4(Salen)4) host structure in complex 1$3H2O is distinctively
different from the reported structures of binuclear triple-
decker,11 trinuclear triple-decker,12 trinuclear tetra-decker13 or
pentanuclear tetra-decker lanthanide complexes14 based on the
Salen-type Schiff-base ligands with the rigid linkers, which
should be due to the use of the flexible Salen-type Schiff-base
ligand H2L with the outer O2O2 moiety for complex 1$3H2O. On
the other hand, the formation of homoleptic cyclic tetranuclear
(Nd4(Salen)4) framework in complex 1$3H2O appears to be
NO3ꢀ anion-dependent since the use of other LnX3 (Xꢀ ¼ OAcꢀ
or CF3SO3ꢀ) in the self-assembly with the Salen-type Schiff-base
ligands with the flexible linkers usually results in the formation of
polymeric lanthanide coordination complexes.15 It is of special
interest to compare the self-assembly of complex 1$3H2O with
that of the reported Tb4(Salen)6 complex.16 Although the similar
tetranuclear framework is obtained necessarily from the Salen-
type Schiff-base ligands with flexible linkers, the character of the
Salen-type Schiff-base ligand H2L with both the inner N2O2 core
and the outer O2O2 moiety in complex 1$3H2O, instead of the
pure Salen-type Schiff-base ligand without the outer O2O2
moiety in the reported Tb4(Salen)6 complex16 endows the
Fig. 3 Perspective drawing of the cationic part in complex 3$3H2O; free
anions, H atoms and solvates are omitted for clarity.
lanthanide contraction causes the slight variation of the detailed
structures.
Photophysical properties
The photophysical properties of the ligand H2L and complexes
1–4 have been examined in dilute MeCN solution at room
temperature or 77 K, and summarized in Table 2 and Fig. 4–6.
As shown in Fig. 4, the similar ligand-centered solution
absorption spectra (225–227, 267–268 and 339–342 nm) of
complexes 1–4 in the UV-visible region are observed and are red-
shifted upon the coordination of the Ln3+ ions as compared to
that (221, 259 and 332 nm) of the ligand H2L or (L)2ꢀ in MeCN
or in MeOH (shown in Fig. 4s†). The molar absorption coeffi-
cients of complexes 1–4 in all the lowest energy bands are almost
four orders of magnitude larger than that of the ligand H2L due
to the involvement of four chromophores. For complexes 1–3,
the similar residual visible emission bands (ca. 505 nm and s < 1
ns, almost undetectable) and low quantum yields (Fem < 10ꢀ5) in
dilute absolute MeCN solution at room temperature are
observed. While photoexcitation of the antennae at the range of
230–500 nm (lex ¼ 402 nm for 1 or 385 nm for 2), as shown in
Fig. 5, gives rise to the characteristic emissions of the Nd3+ ion
(4F3/2 / 4IJ/2, J ¼ 9, 11, 13) and the Yb3+ ion (2F5/2 / 2F7/2) in
the NIR region. For complex 1, the emissions at 909, 1085 and
ꢀ
unusual formation of NO3 anion-dependent homoleptic cyclic
tetranuclear (Nd4(Salen)4) framework in complex 1$3H2O.
Single-crystal X-ray diffraction analyses indicated that both
complex 2$3H2O and 3$3H2O are isomorphous with complex
1$3H2O, as shown in Fig. 2 and 3, and the similar strong intra-
molecular H-bond interactions with the short N4–H4/O7
ꢀ
ꢀ
distances (2.605(11) A for 2$3H2O and 2.602(11) A for 3$3H2O)
are observed in Fig. 2s and 3s,† respectively. The effect of
1365 nm can be respectively assigned to 4F3/2 / 4I9/2, 4F3/2
/
4I11/2 and 4F3/2 / 4I13/2 transitions of the Nd3+ ion, and the
2
emission at 1022 nm can be attributed to the F5/2 / 2F7/2
transition of the Yb3+ ion for complex 2. Moreover, unlike
complex 1 or 2, the characteristic NIR emission of the Er3+ ion
for complex 3 is too weak to be detected. The ligand H2L or the
complex 4 also does not exhibit the NIR luminescence under the
same conditions, and just have the typical luminescence (lem
¼
361, s ¼ 1.21 ns and f ¼ 0.23 ꢂ 10ꢀ3; lem ¼ 479 nm, s ¼ 1.56 ns
and f ¼ 0.25 ꢂ 10ꢀ3 for the ligand H2L or lem ¼ 505 nm, s ¼ 0.74
ns and f ¼ 0.64 ꢂ 10ꢀ3 for 4) of the Salen-type Schiff-base ligand
in the visible region, as shown in Fig. 6. It is worth noting that for
complex 1 or 2, the similar excitation spectrum (lex ¼ 385 and
403 nm for 1 or lex ¼ 402 nm for 2) monitored at the respective
strongest NIR emission peak (lem ¼ 1085 nm for 1 or lem
¼
Fig. 2 Perspective drawing of the cationic part in complex 2$3H2O; free
anions, H atoms and solvates are omitted for clarity.
1022 nm for 2) or the visible emission peak (lem ¼ 505 nm) of
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Table 2 The photophysical properties of the H2L and complexes 1–4 at 1 ꢂ 10ꢀ5 M in absolute MeCN solution at room temperature or 77 K
Absorption
lab/nm (log[3/dm3 molꢀ1 cmꢀ1])
Excitation
lex/nm
Emission
lem/nm (s, F ꢂ 103)
Compound
H2L
221 (0.24), 259 (0.11), 332 (0.03)
227 (0.74), 268 (0.31), 342 (0.11)
225 (0.76), 268 (0.33), 339 (0.11)
226 (0.82), 267 (0.35), 339 (0.12)
226 (0.83), 267 (0.35), 339 (0.12)
303, 321, 370
385, 403(sh)
361 (1.21 ns, 0.23), 479 (1.56 ns, 0.25)
909 (1.39 ms), 1085 (s, 1.41 ms), 1365a
1022 (13.22 ms)
—
505 (0.74 ns, 0.64)
487 (2.4 ns, 77 K), 543 (6.2 ms, 77 K)
1
2
3
4
307(sh), 357, 402(sh)
—
290(sh), 350, 397
b
b
a
Due to the limitations of the instrument, we were unable to measure the lifetime of the NIR luminescence above 1300 nm. b The emission is too weak to
be detected.
Fig. 6 Visible emission and excitation spectra of the ligand H2L and
complex 4 in MeCN solution at 1 ꢂ 10ꢀ5 M at room temperature.
Fig. 4 UV-Vis spectra of the ligand H2L and complexes 1–4 in MeCN
solution at 1 ꢂ 10ꢀ5 M at room temperature.
obtained from time-resolved luminescent experiments can be
fitted mono-exponentially with time constant of microseconds
(1.41 ms for 1 at 1085 nm and 13.22 ms for 2 at 1022 nm), and the
intrinsic quantum yield FLn (0.564% for 1 or 0.661% for 2) 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, respectively,20
The relatively smaller NIR intrinsic quantum yields of complexes
1 and 2 compared with those of trinuclear triple-decker12 or
pentanuclear tetra-decker lanthanide (Nd3+ or Yb3+) complexes14
based on the Salen-type Schiff-base ligand with the rigid linker
should be as a result of the partially luminescent quenching effect
by the OH-vibrators from coordinated m3-OHꢀ groups around
the Ln3+ ions, in spite of the involvement of more chromophores.
As a reference compound, complex 4 allows the further study
of the antennae luminescence in the absence of energy transfer,
because the Gd3+ ion has no energy levels below 32 000 cmꢀ1, and
therefore cannot accept any energy from the antennae excited
state.21 In dilute MeCN solution at 77 K, complex 4 displays the
Fig. 5 NIR emission and excitation spectra of complexes 1 and 2 in
MeCN solution at 1 ꢂ 10ꢀ5 M at room temperature.
strengthened antennae fluorescence compared with that (lem
¼
505 nm, s ¼ 0.74 ns and f ¼ 0.65 ꢂ 10ꢀ3) at room temperature
under the same conditions, which shows higher luminescent
intensity (lem ¼ 487 nm and 543 nm) and distinctively longer
luminescence lifetimes (2.4 ns and 6.2 ms). This result shows that
the sensitization of the NIR luminescence for complexes 1–2
should arise from both the 1LC (20534 cmꢀ1) and the 3LC (18416
complex 4, clearly show both the NIR emissions originate from
the same p–p* transitions of the H2L Schiff-base ligand for
complexes 1 and 2, suggests that the energy transfer from the
antenna to the Ln3+ ions takes place efficiently.19 Moreover, for
complexes 1 and 2, the respective NIR luminescent decay curves
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cmꢀ1) excited state of the Schiff-base ligand H2L at low
temperature. 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
and 2 can thus be calculated from kET ¼ 1/sq ꢀ 1/su,22 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 4, so the energy transfer rates
for the Nd3+ and Yb3+ ions in complexes 1 and 2 may all be
estimated to be above 5 ꢂ 108 sꢀ1, which could well imply the
reason for the effective energy transfer for complexes 1 and 2.
Furthermore, from the viewpoint of the energy level match, in
spite of the effective energy transfer also taking place in complex
emissive lifetimes in the microsecond range, arises from the
excited state (both 1LC and 3LC) of the ligand due to the effective
intramolecular energy transfer in complexes 1 and 2. Moreover,
the energy level match between the excited states (3LC) of the
chromophores to the corresponding Ln3+ ion’s exciting state is
required for the enhancement of NIR luminescence, in addition
to avoiding or decreasing the luminescent quenching effect
arising from OH-, CH– or NH-oscillators around the Ln3+ ion.
The specific design of polynuclear complexes from the flexible
Salen-type Schiff-base ligands in facilitating the NIR sensitiza-
tion is now under way.
Acknowledgements
3
3, the larger energy gap between the energy-donating LC level
(18 416 cmꢀ1) and the emitting level (4I13/2) of Er3+ ion than those
of complexes 1 and 2 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 3.23 As to the relatively higher quantum
efficiency of 2 (0.661%) than that of 1 (0.564%), besides the
smaller energy gap (2F5/2, 9785 cmꢀ1) of Yb3+ ion in complex 2
than that (4F3/2, 9217 cmꢀ1) of the Nd3+ ion in complex 1, the
excited state of the Nd3+ ion in complex 1 is more sensitive to
quenching by the distant C–H or N–H oscillators of the Salen-
type Schiff-base ligand H2L and the O–H oscillators of the
coordinated m3-OHꢀ groups around the Ln3+ ions.24 Further-
more, the sensitivity of the NIR luminescent intensities to
different solvents for complexes 1 and 2 becomes apparent when
the determined solvent is changed from MeCN to CD3CN or
CD3OD. In CD3CN or CD3OD, the typical emission bands of
Nd3+ ion assigned to the 4F3/2 / 4IJ/2 (J ¼ 9, 11 or 13) transi-
tions for complex 1 and the emission band of Yb3+ ion attributed
to the 2F5/2 / 2F7/2 transition for complex 2 are also observed,
which indicates that the two complexes with the discrete homo-
leptic tetranuclear units can be stabilized in different solvents
(MeCN or MeOH), consistent with those found from the results
of ESI-MS. Moreover, for either complex 1 or 2, the relative
strongest emission intensities (ca. 1085 nm for complex 1 and ca.
1022 nm for complex 2) in CD3CN and CD3OD are higher
(almost 1.8 times for 1 and 1.5 times for 2 in CD3CN; 2.6 times
for 1 and 3.7 times for 2 in CD3OD) than those for the two
complexes in MeCN when using solutions with their concentra-
tions adjusted to give the same absorbance values at ca. 276 nm.
The observed changes following the deuteration of solvents may
be relative to an interaction of the solvents (MeCN or MeOH
from the dissociated fragments with the loss of coordinated
This work is funded by the National Natural Science Foundation
(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 of China, the State Key
Laboratory of Structure Chemistry (20100014), the Education
Committee Foundation of Shaanxi Province (11JK0588), 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, adminis-
tered by the American Chemical Society (47014-AC5).
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