Anion-controlled structures and luminescent properties of three Cd(ii) complexes assembled by a 2-substituted 8-hydroxyquinoline ligand

Article abstract of DOI:10.1039/c3ce40348c

This work reports the self-assembly of three cadmium complexes 1-3 from one 2-substituted 8-hydroxyquinoline ligand (HL). The skeleton exhibits an unprecedented structural diversification and fabricates one mononuclear and two different tetranuclear Cd(ii) building units in response to the counteranions NO₃^-, OAc^- and I^-, respectively. The self-assembly behavior of the cadmium salts and the HL in MeOH was subsequently investigated using UV-vis spectroscopy. In the solid state, the supramolecular structures of 1-3 feature unique helical chains or a 3D network via non-covalent interactions, such as π...π stacking, C-H...π, hydrogen bonding and halogen-related interactions. As a result, the three Cd(ii) complexes exhibit disparate photophysical properties. This unique capability may provide a useful strategy to tune the optical properties of multinuclear materials, which could be exploited as important components for optoelectronic devices.

Full text of DOI:10.1039/c3ce40348c

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Anion-controlled structures and luminescent properties
of three Cd(II) complexes assembled by a 2-substituted
8-hydroxyquinoline ligand3
Cite this: CrystEngComm, 2013, 15,
7307
Guozan Yuan,* Lulu Rong, Xuelong Qiao, Li Ma and Xianwen Wei*
This work reports the self-assembly of three cadmium complexes 1–3 from one 2-substituted
8-hydroxyquinoline ligand (HL). The skeleton exhibits an unprecedented structural diversification and
fabricates one mononuclear and two different tetranuclear Cd(II) building units in response to the
counteranions NO32, OAc2 and I2, respectively. The self-assembly behavior of the cadmium salts and the
HL in MeOH was subsequently investigated using UV-vis spectroscopy. In the solid state, the
supramolecular structures of 1–3 feature unique helical chains or a 3D network via non-covalent
Received 26th February 2013,
Accepted 9th July 2013
…
…
interactions, such as p p stacking, C–H p, hydrogen bonding and halogen-related interactions. As a
result, the three Cd(II) complexes exhibit disparate photophysical properties. This unique capability may
provide a useful strategy to tune the optical properties of multinuclear materials, which could be exploited
as important components for optoelectronic devices.
DOI: 10.1039/c3ce40348c
www.rsc.org/crystengcomm
electron-transporting properties for organic light-emitting
devices (OLEDs).6 Similar to zinc(II), cadmium(II) also has a
Introduction
spectroscopically silent d10 electronic configuration.
Therefore, owing to the amiability to form bonds with
different donors simultaneously, the larger radius, and the
wide variety of coordination modes and coordination numbers
of the Cd(II) ions, much attention has been given to the
functional complexes involving Cd(II) ions in search of
molecular-based materials with interesting physical proper-
ties.7 With few notable exceptions, oligomeric (multinuclear)
Cd(II) 8-hydroxyquinolinate-based complexes with high lumi-
nous efficiencies have not yet been developed.8
Photoluminescent materials have been extensively explored
and realized for their diverse functionalities and applications
in lighting, display, sensing, and optical devices.1 Motivated by
the success of tris(8-hydroxyquinolinato)aluminum (Alq3) in
vacuum-deposited LEDs, organic chelate metal complexes
based on 8-hydroxyqunoline derivatives have, in particular,
attracted a lot of attention.2–4 Significantly, recent reports have
suggested that the fluorescent properties of the metal
derivatives of 8-hydroxyquionoline (q–H) in the solid state
are dependent on the character of the metal ion, the degree of
aggregation, the molecular and crystal structure, as well as the
intermolecular noncovalent interactions involved.5 Therefore,
the development of new synthetic methods and strategies for
the construction of well-defined heteroleptic molecular com-
plexes or multinuclear clusters supported by 8-hydroxyquino-
line ligands, with unique self-assembly structures and
properties and with enhanced and tunable photoluminescence
features, is a big challenge for chemists. Recently, zinc(II) has
been explored as a metal center to construct analogues of Alq3,
bis(8-hydroxyquinoline)zinc(II) (Znq2) complexes, which have
been demonstrated to be potential candidates to enhance the
Over the past decades, zero-, one-, two-, or three-dimen-
sional (0D, 1D, 2D, or 3D) coordination assemblies have been
studied extensively owing to their structural versatility and
distinctive properties, as well as their potential applications in
different fields of science.9–11 The observed properties are very
much influenced by subtle changes in the supramolecular
structure of the building entities, and the development of
synthetic strategies to achieve the desired product with
predefined properties is a long-term challenge. Numerous
routes have been investigated in recent times to predict and
control the supramolecular assembly. Some are controlled by
the structure-directing factors such as the inbeing of the metal
ions, the predesigned organic ligands,12 the solvent,13 the pH
value of the solution,14 the temperature,15 the counterion with
a different bulk or coordination ability,16 the template and
metal-to-ligand stoichiometry,17 while others are controlled by
the inter- and intramolecular noncovalent forces such as
School of Chemistry and Chemical Engineering, Anhui University of Technology,
Maanshan 243002, China. E-mail: guozan@ahut.edu.cn; xwwei@ahut.edu.cn;
Fax: +86 555 2311552; Tel: +86 555 2311807
Electronic supplementary information (ESI) available. CCDC numbers 923976–
3
923978 contain the supplementary crystallographic data for 1–3, respectively.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ce40348c
…
…
…
hydrogen bonding, p p stacking, metal metal, metal p, C–
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…
…
H
p, anion p and halogen-related interactions that also
fluorescent properties due to their different core and
supramolecular structures.
determine the supramolecular topology and dimensionality in
the solid state.18
Of particular interest to us are works on the elegant design
and synthesis of novel crystalline photoluminescent materials
whose core (or supramolecular) structures can be tuned.19 Our
goal is to achieve control over the molecular or supramolecular
structures and thereby the physical properties of the new
materials. The HL ligand ((E)-2-[2-(2,6-dichlorophenyl)ethenyl]-
8-hydroxyquinoline) is chosen for the following reasons: (1) in
the quinolinolate ligands, the HOMO-orbitals are mainly
located on the phenoxide side of the ligand, while the
LUMO-orbitals are mainly located on the pyridyl side.20 This
implies that the band gap of the 8-hydroxyquinoline-based
coordination complexes could be delicately tuned by the
introduction of electron withdrawing groups onto the 2-posi-
tion of the pyridine ring, thus modulating the emission
property; (2) the chlorine group on the HL ligand can facilitate
Experimental section
Materials and characterization methods
All of the chemicals are commercial available, and used
without further purification. Elemental analyses of C, H and N
were performed with an EA1110 CHNS-0 CE elemental
analyzer. The IR (KBr pellet) spectrum was recorded (400–
4000 cm21 region) on a Nicolet Magna 750 FT-IR spectrometer.
The powder X-ray diffraction (PXRD) data were collected on a
DMAX2500 diffractometer using Cu Ka radiation. The calcu-
lated PXRD patterns were produced using the SHELXTL-XPOW
program and single-crystal reflection data. All fluorescence
measurements were carried out on a LS 50B Luminescence
Spectrometer (Perkin Elmer, Inc., USA). The room-temperature
(RT) lifetime measurements were determined on a FLS920
time-resolved and steady-state fluorescence spectrometer
(Edinburgh Instruments). All UV-vis absorption spectrum were
recorded on a Lambda 20 UV-vis Spectrometer (Perkin Elmer,
Inc., USA).
…
the formation of diverse halogen bonds (such as C–Cl H–C,
…
…
C–Cl p, and C–Cl Cl–C interactions), which may affect the
supramolecular structure of the coordination complexes by
extending the multinuclear discrete subunits or low dimen-
sional entities into high-dimensional supramolecular net-
works, hence generating fascinating properties in the solid
state.19a,21 Herein, we report the self-assembly of three
cadmium complexes 1–3 from the 2-substituted 8-hydroxyqui-
noline HL ligand (Scheme 1). The skeleton exhibits an
unprecedented structural diversification and fabricates one
mononuclear and two different tetranuclear Cd(II) core
structures in response to the counteranions NO32, OAc2 and
I2, respectively. In the solid state, the supramolecular
structures of 1–3 feature unique helical chains or a 3D
Synthesis of compounds 1–3
A mixture of CdX2 (X = NO32, OAc2, I2, 0.02 mmol), HL (0.01
mmol), H2O (0.2 mL), DMF (0.2 mL), and MeOH (2 mL) in a
capped vial was heated at 80 uC for one day. Yellow blocklike
crystals of 1–3 suitable for single-crystal X-ray diffraction were
collected, washed with ether and dried in air. The products
were not significantly affected by a change in the molar ratio of
Cd(II) and HL (1 : 1, 2 : 1 or 1 : 2). Yield: 1, 3.0 mg, 75%; 2, 3.4
mg, 80%; 3, 3.5 mg, 78%.
Elemental analysis data and IR of 1. Anal (%). calcd for
C35H28Cl4N2O5Cd: C, 51.84; H, 3.48; N, 3.45. Found: C, 51.95; H,
3.62; N, 3.56. FTIR (KBr pellet): 3720.2(s), 3649.2(s), 3545.6(s),
3467.3(s), 3418.1(s), 3271.1(m), 2841.1(w), 2599.6(w), 2164.2(w),
2012.9(w), 1886.2(w), 1743.6(w), 1639.5(m), 1620.7(m),
1548.4(m), 1434.4(m), 1365.7(m), 1340.7(m), 1280.7(m),
1176.8(w), 1144.4(w), 1096.7(m), 971.1(m), 948.5(m), 828.4(w),
770.82(m), 740.7(m), 607.6(s), 571.6(s).
…
network via non-covalent interactions, such as p p stacking,
…
C–H p, hydrogen bonding and halogen-related interactions.
As a result, the three Cd(II) complexes exhibit disparate
Elemental analysis data and IR of 2. Anal (%). calcd for
C
108H74Cl12N6O12Cd4: C, 51.42; H, 2.96; N, 3.33. Found: C,
51.32; H, 2.93; N, 3.26. FTIR (KBr pellet): 3856.5(s), 3805.1(s),
3747.0(s), 3671.9(s), 3626.2(s), 3569.5(s), 3514.1(s), 3449.7(s),
3418.0(s), 3381.2(s), 2600.8(w), 1670.5(m), 1602.3(m),
1553.0(m), 1503.1(w), 1437.2(s), 1369.9(m), 1333.1(m),
1268.4(m), 1099.5(m), 966.8(w), 878.2(w), 829.2(w), 774.7(m),
746.4(m), 610.8(w), 576.5(w).
Elemental analysis data and IR of 3. Anal (%). calcd for
C
104H68Cl12I2N6O8Cd4: C, 46.98; H, 2.58; N, 3.16. Found: C,
46.80; H, 2.55; N, 3.13. FTIR (KBr pellet): 3858.2(m), 3801.9(m),
3748.3(s), 3714.1(s), 3635.0(s), 3556.6(s), 3451.3(s), 3416.7(s),
2600.2(w), 1634.7(s), 1617.8(s), 1551.8(s), 1503.9(w), 1435.6(s),
1369.2(m), 1332.3(s), 1274.2(m), 1148.4(w), 1100.4(s),
1065.5(m), 968.8(m), 948.6(m), 828.7(w), 773.3(m), 7443.5(m),
595.9(s), 5571.1(s).
Scheme 1 Synthesis of Cd(II) complexes 1–3 from ligand HL.
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Table 1 Crystal data and structure refinement for 1–3
Identification code
1
2
3
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
C35H28Cl4N2O5Cd
809.98
296(2)
0.71073
Monoclinic
P21/c
C108H74Cl12N6O12Cd4
2522.78
296(2)
0.71073
Monoclinic
C104H68Cl12I2N6O8Cd4
2658.58
58(2)
0.71073
Monoclinic
C2/c
Space group
P21/c
Unit cell dimensions
a = 9.090(3) Å
b = 26.329(8) Å
c = 14.854(5) Å
alpha = 90u
beta = 95.5u
gamma = 90u
3538.5(19), 4
1.507
a = 14.7666(10) Å
b = 24.2437(17) Å
c = 14.9700(10) Å
alpha = 90u
a = 15.880(2) Å
b = 24.478(3) Å
c = 26.637(7) Å
alpha = 90u
beta = 106.3u
gamma = 90u
9940(2), 4
1.734
1.845
5056
beta = 112u
gamma = 90u
4968.1(6), 2
1.686
1.234
2512
Volume (Å3), Z
Density (calculated) (Mg m23
)
)
Absorption coefficient (mm21
F(000)
0.962
1600
h range for data collection (u)
Reflections collected
Independent reflections
Completeness to theta
Data/restraints/parameters
Goodness-of-fit on F2
2.38 to 27.48
21 813
8039 [Rint = 0.0293]
27.48/98.9%
8039/0/414
2.36 to 27.48
30 702
11 286 [Rint = 0.0387]
27.48/99.0%
11 286/710/671
1.049
R1 = 0.0504, wR2 = 0.1256
R1 = 0.0916, wR2 = 0.1464
1.060 and 20.692
1.57 to 27.55
22 554
11 258 [Rint = 0.0420]
27.55/98.2%
11 258/582/603
1.111
R1 = 0.0538, wR2 = 0.1432
R1 = 0.1071, wR2 = 0.2035
1.715 and 20.901
1.033
Final R indices [I . 2sigma(I)]
R indices (all data)
R1 = 0.0448, wR2 = 0.1212
R1 = 0.0648, wR2 = 0.1341
1.158 and 20.609
Largest diff. peak and hole (e Å23
)
band. No differences were observed in the absorption spectra
after the 1 : 2 ratio of Cd2+/HL, so this experiment clearly
indicates the formation of (CdL2)n. By following the same
procedure, the formation of (Cd2L3)n for 2 and 3 was also
determined by UV-vis spectroscopic titrations (Fig. 1a and 1b).
X-ray crystallography
The single-crystal XRD data for compounds 1–3 were all
collected on a Bruker APEX area-detector X-ray diffractometer
with Mo Ka radiation (l = 0.71073 Å) or Cu Ka radiation (l =
1.54178 Å). The empirical absorption correction was applied by
using the SADABS program (G. M. Sheldrick, SADABS, program
for empirical absorption correction of area detector data;
Structural description
[CdL2]?2H2O?2MeOH (1). Single-crystals of complex 1 were
readily obtained in good yields by heating Cd(NO3)2 and HL in
a mixture of DMF, MeOH and water. Complex 1 crystallizes in
the monoclinic space group P21/c with Z = 4. The asymmetric
unit contains one formula unit, that is, one unique Cd(II)
center, two L ligands, two coordinated H2O, and one methanol
molecule. The Cd(II) center adopts an octahedral geometry
with the equatorial plane occupied by the NO3 donors of two L
ligands and one H2O molecule, and the apical position is
occupied by one quinoline nitrogen atom and one H2O
¨
¨
University of Gottingen, Gottingen, Germany, 1996). The
structure was solved using the direct method, and refined by
full-matrix least-squares on F2 (G. M. Sheldrick, SHELXTL97,
program for crystal structure refinement, University of
¨
Gottingen, Germany, 1997). The crystal data and the details
of the data collection are given in Table 1, whereas the selected
bond distances and angles are presented in Tables S1–S3, ESI.
The CCDC numbers of 1–3 are 923976, 923977 and 923978,
3
respectively.
molecule (Fig. S1, ESI ). The bond lengths around Cd(II) are
3
2.359(4)–2.365(3) Å for Cd–O, and 2. 397(3) and 2.438(3) Å for
Cd–N, respectively. The two crystallographically unique
ligands have dihedral angles (ca. 58.6u and 66.8u for 1)
between the quinoline and 2,6-dichlorophenyl moieties which
indicate that the ligands are non-coplanar in 1. There are weak
Results and discussion
Coordination studies in solution
In order to further investigate the interaction mode of the HL
ligand with the Cd2+ ion, the coordination reaction of HL with
the Cd2+ ion was monitored through a UV-vis spectroscopic
titration. The absorption bands of HL in MeOH dramatically
change upon the addition of the Cd2+ ions. As depicted in
Fig. 1a, the band of 1 showed a significant bathochromism
shift from 283 to 297 nm. The peak at 329 nm was found to
become weak and disappeared in the end in the presence of
increasing amounts of the Cd2+ ion. The new band at 420 nm
is attributed to the charge transfer from the metal to ligand
…
…
nonclassical C–H O and C–H Cl intramolecular hydrogen
bonds in 1 between the phenolato oxygen (or 2,6-dichlorophe-
…
nyl chlorine) atoms and the C–H group of ethenyl (C O =
…
…
3.226(4) and 3.357(5) Å, C–H O = 148 and 156u; C Cl =
…
3.165(4) Å, C–H Cl = 109u), which play a significant role in the
construction of the mononuclear Cd(II) unit (Fig. S2, ESI ).
3
Interestingly, a kind of meso-helical chain (P + M) along the
…
b axis constructed via a C–H Cl interaction between the –Cl
atom of dichlorophenyl and the C–H group of the adjacent
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Fig. 2 a) A view of the 1D intertwined supramolecular helical chains in 1 along
…
the b axis; b) the 2D supramolecular structure of 1 mediated by C–H p and C–
…
Cl p interactions; c) the 3D network of 1.
monoclinic space group P21/n, with one half of a formula unit
in the asymmetric unit, that is, two Cd(II) atoms, three L
ligands, one coordinated acetate anion and one methanol
molecule. The coordination environment of the Cd2+ ions in
complex 2 is shown in Fig. 3, and it can be seen that there are
two crystallographically independent Cd(II). Cd1 is six-coordi-
nated to four phenolato oxygen atoms (one O1, one O2, and
two O3) and two nitrogen atoms (N1 and N2) from four L
ligands displaying a distorted octahedron geometry. Cd2 is
also six-coordinated with a distorted octahedron geometry. In
addition to O1, O2, O3 and N3 from three L ligands, the other
two coordination atoms for Cd2 are O4 and O5 from one
acetate anion. The bond lengths and angles around Cd(II) are
2.324(4)–2.392(4) Å for Cd–N, 2.200(3)–2.435(3) Å for Cd–O,
78.18(11)–169.28(12)u for O–Cd–O, 71.92(11)–150.99(13)u for
O–Cd–N and 103.55u for N–Cd–N, respectively. However, the
bridging of the six oxygen atoms of the six L ligands makes
2Cd1 and 2Cd2 involved in a double-open cubane-like unit
(Cd4O6) (Fig. 3). To the best of our knowledge, the double-open
cubane-type Cd4O6 cluster of 2 reported here is the first
example of a tetranuclear Cd(II) oxygen cluster, although a
Fig. 1 UV-vis titrations of HL (10 mM) in methanol with Cd(NO3)2 (a), Cd(OAc)2
(b) and CdI2 (c), respectively. Each spectrum was acquired 5 min after the Cd2+
addition.
ligand (3.744 Å) is observed in the solid state of this
compound. As shown in Fig. 2a, the two 21 helices have an
identical pitch of 26.329(8) Å (equal to the b axis length). To
the best of our knowledge, such helical chains constructed by
…
C–H Cl interactions are very rare. Additionally, it is notable
that each cluster of 1 involves abundant intermolecular C–
…
H
p (between the C–H group of the quinoline units and the
…
pyridine ring of the adjacent ligand, 3.402(4) Å) and C–Cl
p
(between the C–Cl group of dichlorophenyl and the pyridine
ring of the adjacent ligand, 3.716(2) and 3.382(2) Å) interac-
tions in the ac plane (Fig. 2b). By the coactions of the three
kinds of noncovalent interactions, the structure extends into a
3D network (Fig. 2c).
[Cd4(L)6(OAc)2]?2MeOH (2). When the 8-hydroxyquinoline
HL ligand was reacted with Cd(OAc)2 under the same reaction
conditions, one double cubane tetranuclear compound,
namely, [Cd4(L)6(OAc)2]?2MeOH (2), was obtained. The sin-
gle-crystal structure analysis reveals that 2 crystallizes in the
Fig. 3 Views of the coordination geometries of Cd(II) atoms in 2 (H atoms
omitted for clarity).
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…
Fig. 4 A view of the 1D helical chains fabricated via the C–Cl Cl–C
intermolecular interaction in 2 along the b axis.
number of face-sharing double cubane structures of other
metal complexes have been reported.22
There are intramolecular C–H Cl (between the 2,6-dichlor-
Fig. 6 a) A view of the coordination geometries of the Cd(II) atoms in 3; b) the
…
…
C–H p interactions in 3 along the c-axis; c) the 2D network of 3 mediated by C–
…
H
Cl interactions; d) the 3D network of 3.
ophenyl chlorine atoms and the C–H group of ethenyl,
…
2.993(5)–3.205(7) Å) and p p interactions (face-to-face dis-
tance: 3.550(5) Å) in 2, which play a significant role in the
oxygen atoms from the three L ligands with an average Cd–O
bond distance of 2.276(5) Å, a Cd–I bond distance of 2.716(8) Å
and a Cd–N bond distance of 2.369(6) Å. The bond angles
around Cd1 are in the range of 70.15(17)–140.66(17)u. While
the two Cd2 centers adopt a distorted octahedral geometry
with the equatorial plane occupied by the NO3 donors of three
L ligands and the apical position occupied by one nitrogen and
one oxygen atom of two ligands. The average Cd–O and Cd–N
distances around Cd2 are 2.247(5) and 2.367(5) Å, respectively.
The bond angles around Cd2 range from 70.85(17) to 146.8(2)u.
The intramolecular (or intermolecular) interactions play a
significant role in the packing of the crystals. Three pairs of
construction of the tetranuclear Cd(II) units (Fig. S8, ESI ).
3
…
…
Through the C–Cl Cl–C intermolecular interaction (Cl1 Cl6
= 3.215(2) Å), the tetranuclear Cd(II) units are further linked
into a helical chain along the b axis. The helical chain is thus
generated around the crystallographic 2(1) axis with a pitch of
16.712(4) Å, which is equal to the b axis length (Fig. 4).
Additionally, halogen bonds involving Cl2 and Cl3 from
…
different tetranuclear units (Cl2 Cl3 = 3.810(2) Å) link the
tetranuclear Cd(II) units into a supramolecular chain along the
…
c axis. Based on the two kinds of C–Cl Cl–C intermolecular
interactions, the tetranuclear Cd(II) units are connected into a
2D network (Fig. 5).
ligands with offset face-to-face p–p stacking (Fig. S11, ESI ) are
3
[Cd4(L)6I2]?2MeOH (3). In order to further investigate the
influence of the counterions on the formation and structures
of the complexes, the 8-hydroxyquinoline HL ligand was
reacted with CdI2 under the same conditions, and one
tetranuclear compound, namely, [Cd4(L)6I2]?2MeOH (3), was
obtained. As shown in Fig. 6a, there are two crystallographi-
cally independent Cd(II) in the tetranuclear centrosymmetric
complex 3. The two terminal Cd1 atoms are five-coordinated
by one iodine atom, one nitrogen atom and three hydroxyl
arranged around four Cd(II) atoms to form a tetranuclear
…
cluster. As shown in Fig. 6c, through C(8)–H(8A) Cl(4)
(3.742(8) Å) intermolecular interactions, the neutral molecular
units in 3 are linked into 2D layers in the ab plane, which are
further assembled into a 3D supramolecular network (Fig. 6d)
…
via an interlayer hydrogen bond C(14)–H(14A) p (between the
–CH3 group of dichlorophenyl and the adjacent dichlorophe-
nyl ring, 3.640(15) Å) interaction (Fig. 6b), contributing to the
additional stability of the structure.
Anion effects on molecular structure
On the basis of the above results, the roles of the anions in
determining the structures of the three cadmium complexes
are unambiguously exhibited. The nature (coordinating ability,
size, and geometry) of the anions is the primary reason.
Compared with OAc2 and I2, the coordinating ability of NO3
2
is worst in the construction of the cadmium complex. The
NO32 ion cannot coordinate to the cadmium center in the self-
assembly of the mononuclear complex 1. The OAc2 ion has a
larger size and two coordinated oxygen atoms in comparison
with the I2 ion. This induces the six-coordinated Cd(II) center
in 2 to adopt a distorted octahedron geometry. The two
terminal Cd atoms in 3 are five-coordinated by one iodine
atom and four NO3 atoms from three ligands, and adopt a
distorted trigonal bipyramid geometry. Additionally, the
Fig. 5 The 2D supramolecular structure of 2.
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The fluorescent properties of compounds 1, 2 and 3 were
investigated in the solid state at room temperature (Fig. 8a). 1–
3 exhibit intense photoluminescences with emission maxima
at 580, 573, and 566 nm, respectively, upon excitation at 365
nm. Compared with the free ligand HL (Em: 465 nm), the
bright and extensive yellow emissions of complexes 1–3
predominantly originate from the metal-to-ligand charge
transfer (MLCT) transition and exhibit a remarkable red
shift.17a However, differences in the optical properties of 1–3
in the solid state are observed. The mononuclear complex 1 is
red shifted in comparison with the two tetranuclear complexes
2 and 3.
To further understand the fluorescent properties of 1–3,
their lifetimes were investigated in the solid state. The t values
of 1–3 are 5, 4 and 3 ns, respectively. The lifetimes of 1–3 are
different and shorter than that of the zinc complex Zn2L4
fabricated from HL.19a The above different fluorescent proper-
ties of complexes 1–3 can be explained by various molecular
packing characteristics, which are consistent with the recently
reported correlations between the molecular density of the
packing and the length of the interligand contacts of the
neighboring clusters in the crystal structures and the photo-
physical properties of the mer-Alq3 polymorphs (as a result of
different dispersive and dipolar interactions, as well as
different p–p orbital overlaps).5a
Fig. 7 The PXRD patterns of complexes 1–3.
…
noncovalent forces such as hydrogen bonding, p p stacking,
…
C–H p, and halogen-related interactions also determine the
self-assembly of the three supramolecular structures.
Fluorescent properties in the solid state
To prove that the crystal structures of trimeric 1–3 are truly
representative of their bulk materials, powder X-ray diffraction
(PXRD) experiments were carried out on the as-synthesized
samples. As we can see from Fig. 7, the PXRD experimental
patterns of 1–3 are in good agreement with the simulated
patterns.
Conclusions
In the present work, we have demonstrated the self-assembly
of three cadmium complexes built from a 2-substituted
8-hydroxyquinoline ligand and Cd(II) ions. In the solid state,
the building units of 1–3 exhibit a remarkable dependence on
the counter anions. One mononuclear and two different
tetranuclear Cd(II) core structures were fabricated in response
2
to the counteranions NO3
,
OAc2 and I2, respectively.
Additionally, the three Cd(II) complexes exhibit disparate
photophysical properties due to their different core and
supramolecular structures. This unique capability may provide
a useful strategy to tune the optical properties of multinuclear
materials, which could be exploited as important components
for optoelectronic devices.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21201002 and 21271006),
the Anhui Provincial Natural Science Foundation
(1308085QB22). the Provincial natural science research pro-
gram of higher education institutions of Anhui province
(KJ2013Z028), the National Training Programs of Innovation
and Entrepreneurship for Undergraduates (201210360087),
and the SRTP Project of Anhui University of Technology
(2013027Y).
Fig. 8 a) The emission spectra of complexes 1–3 in the solid state; b) the
emission decay trace of 1–3 in the solid state.
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