Tunable excitation-dependent emissions in mixed-ligand Cd(II) complexes

Article abstract of DOI:10.1016/j.poly.2019.07.028

Two Cd(II) coordination compounds, i.e. [Cd(HL)(phen)₂(H₂O)]·HL·3H₂O (1) and {[Cd(HMeL)₂(4,4′-bipy)(H₂O)]·H₂O}_n (2) and (where HL = N-carboxymethyl-N-phenyliminoacetato, HMeL = N

Full text of DOI:10.1016/j.poly.2019.07.028

Polyhedron 171 (2019) 338–343
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Tunable excitation-dependent emissions in mixed-ligand Cd(II)
complexes
a,
a
a
a
b,
Jinling Miao a, , Yong Nie , Huihui Du , Xiaojun Yang , Chuanhao Zhang , Hong Yan
⇑
⇑
⇑
a School of Chemistry and Chemical Engineering, Institute for Smart Materials & Engineering, University of Jinan, No. 336 Nanxinzhuang West Road, 250022 Jinan, PR China
b State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, 210023 Nanjing, PR China
a r t i c l e i n f o
a b s t r a c t
Two Cd(II) coordination compounds, i.e. [Cd(HL)(phen)2(H2O)]ꢀHLꢀ3H2O (1) and {[Cd(HMeL)2(4,40-bipy)
(H2O)]ꢀH2O}n (2) and (where HL = N-carboxymethyl-N-phenyliminoacetato, HMeL = N-carboxymethyl-
N-(m-methylphenyl)iminoacetato, 4,40-bipy = 4,40-bipyridine and phen = phenanthroline) have been syn-
thesized and characterized by elemental analysis, IR and single-crystal X-ray diffraction methods. The
photoluminescence properties and mechanisms, studied by means of solid state absorption, excitation/
emission spectra, time-resolved lifetime techniques, and density functional theory (DFT) calculations,
indicate that the emissions of both compounds originate from a combination of exciplex and charge-
transfer complex. Interestingly, both compounds exhibit excitation-dependent emissions, which can be
used to fine-tune their emission wavelengths.
Article history:
Received 9 May 2019
Accepted 21 July 2019
Available online 29 July 2019
Keywords:
Luminescence
Charge transfer
Exciplex
Coordination compound
X-ray structure
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
aromatic amine using the exciplex emission [8]; Wagner and
Zaworotko utilized exciplex emission to probe the structures of
Luminescent metal–organic complexes have attracted enor-
mous attention because of their potential applications in light-
emitting devices, luminescent sensors, biomedical imaging and
so on [1]. It has been revealed that the luminescence of such mate-
rials can arise from organic ligand(s), metal ion(s), ligand to metal
charge transfer (LMCT), metal to ligand charge transfer (MLCT), and
even guest molecules [2]. Compared to many examples of the
ligand-centered luminescence, the exciplex/excimer and charge
transfer emissions from complexes formed between ligands or
ligand/guest molecules are much less studied [3], although such
behavior has been frequently accessible in pure organic lumines-
cence [4]. In metal–organic materials, supramolecular interactions
some organic complexes [9]. White light-emitting materials could
also be achieved by using the exciplex emission [10]. Furthermore,
tunable emissions of materials would be realized by varying the
excitation wavelength when CTC and exciplex coexist and have
enough energy difference. Therefore, more luminescent com-
pounds exhibiting the exciplex/excimer and charge transfer emis-
sion are worth studying to render new functional metal–organic
materials.
On the other hand, the excitation-dependent emission phe-
nomenon has been attracting increasing current attention [11] as
it is a convenient way to fine-tune the emission property of lumi-
nescent materials, and also provides important examples of anti-
Kasha rules which state that the emissions always come from the
lowest excited state (S1 or T1). Most of the excitation-dependent
emission systems so far reported are the nanomaterials (such as
carbon dots), the corresponding metal coordination compounds
have been relatively much less studied, and pure organic materials
are emerging.
(pꢀ ꢀ ꢀ
p
and CAHꢀ ꢀ ꢀ
p interactions) drive organic chromophores in a
close proximity, which makes the formation of charge-transfer
complex (CTC) and exciplex easy. However, these species are usu-
ally unwanted because they often reduce the emission efficiency
[5]. More efforts have been made to prevent the formation of these
species [6]. However, it has been recently reported that the forma-
tion of exciplex could actually enhance the emission, in some cases
[7]. Moreover, these species have exhibited potential applications.
For example, Maji and co-workers studied the detection of ions and
Aromatic iminocarboxylic acids ArN(CH2COOH)2 are excellent
ligands towards metal complexes as they exhibit multiple coordi-
nation modes [12]. The electron-rich ArN moieties of these ligands
act as electron donors when suitable electron acceptors are pre-
sent. An analysis of the literature shows that when auxiliary
ligands such as phenanthroline, 4,40-bipyridine, or 2,20-bipyridine
are present, the corresponding complexes of Zn(II) and Cd(II)
⇑
Corresponding authors.
E-mail addresses: chm_miaojl@ujn.edu.cn (J. Miao), chm_niey@ujn.edu.cn
(Y. Nie), hyan1965@nju.edu.cn (H. Yan).
https://doi.org/10.1016/j.poly.2019.07.028
0277-5387/Ó 2019 Elsevier Ltd. All rights reserved.

J. Miao et al. / Polyhedron 171 (2019) 338–343
339
exhibit much longer wavelength emission than those of the ligands
[12b,c,e–g], however, such phenomena and the emission mecha-
nisms have not been understood comprehensively. Recently we
reported such a zinc coordination compound with 4,40-bipyridine
as the auxiliary ligand (acceptor) which exhibits reversible ther-
mochromism and exciplex emission [13]. In this paper we utilized
N-phenyliminodiacetic acids as the main ligands, and phenanthro-
line/4,40-bipyridine as the auxiliary ligands, to obtain two
metal–organic complexes, [Cd(HL)(phen)2(H2O)]ꢀHLꢀ3H2O (1) and
{[Cd(HMeL)2(4,40-bipy)(H2O)]ꢀH2O}n (2). It is found that the emis-
sions of both compounds can be tuned by varying the excitation
light. The luminescent mechanisms were studied by a combination
of solid state absorption and excitation/emission spectra, fluores-
cence lifetime techniques and DFT calculations, which indicate that
the emissions arise from both exciplex and charge-transfer
complexes.
Anal. Calc. for C32H36CdN4O10, C, 51.31; H, 4.84; N, 7.48; found: C,
51.34; H, 4.77; N, 7.44; IR (KBr, cmꢁ1): 3434, 2956, 2922, 1728,
1609, 1582, 1410, 1328, 774, 678.
2.4. Single-crystal X-ray crystallography
Data collection was performed with suitable crystals on an
Oxford Diffraction Gemini
E
diffractometer with graphite-
scan mode).
monochromated Mo K radiation (k = 0.71073 Å,
a
x
The structures were solved by Direct Methods and expanded using
Fourier difference techniques with the SHELXL program package [15].
The non-hydrogen atoms were refined anisotropically by full-
matrix least-squares calculations on F2. The details of the data col-
lection and refinement of the complexes are shown in Table S1,
and the selected bond lengths and angles are presented in Table S2.
3. Results and discussion
2. Experimental
3.1. Crystal structures
2.1. General methods
The brown crystals of 1 and pale yellow crystals of 2 were
obtained by the reactions of iminocarboxylic acids (H2L/H2MeL),
4,40-bipy/phen and Cd(NO3)2 in water–ethanol media, respectively.
According to the single crystal X-ray structural analyses, 1 is a dis-
crete mononuclear complex with phen as the auxiliary ligand
(Fig. 1). The Cd center in 1 coordinates to two phen, a HL moiety
and an aqua ligand, adopting a slightly distorted octahedral geom-
etry. The HL ligand coordinates to the Cd(II) in a monodentate
manner via one oxygen atom, and the other HL moiety acts as a
counter anion. The bond angles around the uncoordinated N1 atom
(119.0°, 119.8° and 120.2°) and N2 atom (119.2°, 119.7° and
120.8°) are all near to 120°, and the NAC bond lengths (N1AC5
1.392 Å, N2AC15 1.382 Å) are between those of a single (1.47–
1.50 Å) and a double bond (1.34–1.38 Å) [16], which indicates that
both the N1 and N2 atoms adopt an sp2 hybridization and are con-
jugated to the phenyl ring, making the HL moiety electron-rich in
nature.
In compound 2, a coordination polymer, each seven-coordinate
metal center binds two HMeL moieties, two pyridine and an aqua
ligand (Fig. 1). Each HMeL ligand coordinates to the metal center in
a bidentate way and the two HMeL moieties point to the opposite
directions relative to the metal ion. The bond angles around the
uncoordinated N1 (120.2°, 118.0° and 121.8°) and N2 atoms
(121.4°, 118.8° and 119.2°), and the N1AC5 (1.383 Å) and
N2AC16 (1.392 Å) bond lengths suggest that both N atoms are con-
jugated to the phenyl rings, as in compound 1.
The ligands H2L and H2MeL were prepared by the reactions of
aniline or m-methylaniline with chloroacetic acid under basic
conditions, respectively, according to the literature [14]. Cd(NO3)2ꢀ
4H2O, 4,40-bipy and phenꢀH2O were commercially purchased and
used without further purification. Elemental analyses were per-
formed on an Elementar Vario EL III Analyzer. PXRD patterns were
recorded on a Bruker D8 diffractometer. IR spectra were measured
on a Perkin Elmer Spectrum RX I spectrometer. UV–Vis diffuse
reflectance spectra were recorded on a Shimadzu UV-3101PC spec-
trometer with an integration sphere attachment, and BaSO4 was
used as the reference. Excitation and emission spectra were mea-
sured on an Edinburgh FLS 920 fluorimeter, using a front-face solid
sample configuration. The fluorescence decay curves were mea-
sured by means of the time-correlated single-photon-counting
technique with a hydrogen lamp as excitation light source, and
the data were analyzed using the software supplied by Edinburgh
Instruments. This software allows the simulation of the multiexpo-
nential decay curves:
s1
s2
RðtÞ ¼ A þ B1eꢁt= þ B2eꢁt= þ ꢀ ꢀ ꢀ
where B is pre-exponential factor,
s
is the characteristic lifetime,
and A is an additional background. The results are evaluated using
global x2 and local residuals.
2.2. Synthesis of [Cd(HL)(phen)2(H2O)]ꢀHLꢀ3H2O (1)
A solution of Cd(NO3)2ꢀ4H2O (0.156 g, 0.5 mmol) in 20 mL of
water/ethanol (1:1, V/V) was added dropwise to a solution of H2L
(0.208 g, 1 mmol) in 20 mL of water/ethanol (1:1, V/V). After stir-
ring at 80 °C for 30 min, phenanthroline monohydrate (0.194 g,
1 mmol) was added to the reaction mixture which was further stir-
red for 1 h. The resulting mixture was filtered while hot to produce
a yellowish filtrate, and brown crystals (0.288 g, 60% yield based on
Cd) suitable for X-ray diffraction started to grow once the filtrate
was cooled to room temperature. Anal. Calc. for C44H44N6O12Cd,
C, 54.98; H, 4.61; N, 8.74; found: C, 54.84; H, 4.40; N, 8.79; IR
(KBr, cmꢁ1): 3436, 2924, 1698, 1600, 1508, 1427, 1194, 846, 730.
3.2. Fluorescence spectroscopy
The phase purity of 1 and 2 was confirmed by the comparison of
the PXRD patterns of the as-synthesized samples with those simu-
lated from the single crystal X-ray data (Fig. S1). The photolumi-
nescent emission spectra of solid 1 and 2 were studied at
ambient temperature and the results are shown in Fig. 2. Both
complexes show broad structureless emission bands. Interestingly,
the emission maxima of 1 change from 556 nm to 625 nm gradu-
ally upon varying the excitation wavelengths from 300 to
520 nm (Fig. 2a and Table 1), exhibiting obvious excitation-depen-
dent emissions. As a result, the emission colors are tuned from
green yellow to red. The excitation spectra of 1 (Fig. 2b), obtained
by setting 556 and 625 nm as the emission wavelengths, respec-
tively, are much different, especially in the relative intensity of
each peak/wavelength. Obviously, the emission intensity at
556 nm reaches the maximum by excitation at 394 nm. The emis-
sion at 625 nm displays the strongest intensity upon excitation at
2.3. Synthesis of {[Cd(HMeL)2(4,40-bipy)(H2O)]ꢀH2O}n (2)
Pale yellow crystals of 2 suitable for X-ray diffraction were
obtained by similar procedures to those described for 1, with
H2MeL (0.474 g, 2 mmol), Cd(NO3)2ꢀ4H2O (0.310 g, 1 mmol) and
4,40-bipy (0.160 g, 1 mmol). Yield: 0.378 g, 50% yield based on Cd.

340
J. Miao et al. / Polyhedron 171 (2019) 338–343
Fig. 1. Crystal structures of 1 and 2 (hydrogen atoms are omitted for clarity).
Fig. 2. Solid state emission (a, c) and excitation (b, d) spectra of crystalline 1 (a, b) and 2 (c, d).
Table 1
Photophysical data of 1.
kex (nm)
kem (nm)
Lifetime contribution (%)
1 = 0.98 ns
s
s2 = 5.51 ns
s3 = 23.17 ns
300
400
420
460
480
520
556
585
592
598
602
625
9.15
20.65
26.02
33.41
35.76
32.69
31.90
70.20
56.83
37.16
6.59
1.67
0
17.15
29.43
57.65
65.64
68.10
510 nm. These results demonstrate that the emissions of 1 result
from several electronic transitions. The emission properties of 2
(Fig. 2c) are much different from those of 1. The emission color
keeps essentially green-yellow. There is only a slight bathochromic
shift of 7 nm upon changing the excitation wavelengths from 300
to 460 nm (Table 2). When the excitation wavelength is further

J. Miao et al. / Polyhedron 171 (2019) 338–343
341
Table 2
Photophysical data of 2.
kex (nm)
kem (nm)
Lifetime contribution (%)
s
1 = 1.72 ns
s2 = 6.22 ns
s3 = 33.65 ns
300
400
440
460
540
540
542
547
0
0
0
100
2.12
5.53
12.75
97.88
88.50
67.30
5.97
19.95
increased, 2 keeps emission silent, in agreement with the excita-
tion spectrum (Fig. 2d) which shows that the emission of 2 can
not be induced by light with a wavelength longer than 460 nm.
Though the emission behaviors of 1 and 2 are different, both of
them have far red-shifted emission maxima when compared with
those of the ligands. As shown in Fig. S2, H2L, H2MeL and 4,40-
bipyridine exhibit emission maxima at 351, 364, and 367 nm,
respectively. Phenanthroline monohydrate shows a vibrational
emission spectrum with three peaks at 363, 379 and 401 nm
(Fig. S2), the same as that reported for phen in the literature
[17]. Compared with those of the ligands, the emission bands of
1 and 2 are red-shifted by at least 155 and 173 nm, respectively,
which is rare in Cd(II) coordination compound [2b].
3.3. Mechanism of emission
To get insights into the photoluminescence mechanisms, den-
sity functional theory (DFT) calculations were carried out to inves-
tigate the electronic structures of 1 and 2, and the selected
molecular orbitals are listed in Fig. S4. The results show the elec-
tron density distribution of the HOMO is located on the ArN moi-
eties, whereas the electron density distribution of the LUMO is
governed by phen and 4,40-bipy, respectively, and the Cd(II) ion
does not notably contribute to the frontier orbitals (Fig. S4), in both
1 and 2. Moreover, it was reported that the complex [Cd(L)(H2O)]ꢀ
H2O, without auxiliary ligand as compared to 1 and 2, exhibits an
emission peak at 464 nm [12b]; and the coordination compound of
Zn(II) and 4,40-bipy has even higher energy emission centered at
398 and 422 nm than that of [Cd(L)(H2O)]ꢀH2O [18]. Moreover,
Maji and George et al. reported that the excited N,N0-dimethylani-
line-phen charge-transfer complex exhibits an emission maximum
at 576 nm [8b], similar to those of 1. Therefore, the emission bands
of 1 and 2 should be assigned to the transitions between the ArN
and phen/4,40-bipy moieties, in which the ArN moieties act as
electron donor (D) and phen/4,40-bipy as electron acceptor (A) in
the photoluminescence process.
Fig. 3. Solid state absorption spectra of 1 (a), 2 (b) and the corresponding ligands.
in materials [19]. The ligands H2L, H2MeL and phenanthroline
monohydrate show absorption bands in the UV region and that
of 4,40-bipy is below 450 nm, whereas 1 exhibits a new strong
absorption band tailing to 700 nm, which confirm the existence
of CTC. The CT absorption band of 2 is much weaker than that of
1. These results are consistent with those of the emission and exci-
tation spectra mentioned above, which show that 1 can be excited
effectively by light with a wavelength longer than 460 nm, but 2
can not be excited under the same conditions.
The transition between a donor (D) and an acceptor (A) usually
gives rise to the charge-transfer emission, as well as that of exci-
plex when D and A are both aromatic species [19]. Exciplex and
CTC generally coexist and are connected in the excited state
[19a,20]. Thus it is difficult to distinguish these emissions.
However the existence of each emissive species could be
explored by choosing the excitation wavelength, i.e. direct
excitation of one of the moieties (donor or acceptor) would yield
the exciplex (A–D)*EX as the main species (Eq. (1)), and the
The reason of the different absorption behaviors of 1 and 2 can
be ascribed to the aromatic stacking interactions in 1 and 2 as
shown in Fig. 4, as the intensity of CT absorption is highly governed
by the aromatic stacking interactions, i.e.
actions between the chromophores in metal–organic structures
[8a]. It was found the interactions exist
p
ꢀ ꢀ ꢀ
p
and CAHꢀ ꢀ ꢀp inter-
excited CT ((Adꢁ–Dd+)*CT) would arise mainly when the CT ((Adꢁ
–
p
ꢀ ꢀ ꢀ
p
and/or CAHꢀ ꢀ ꢀ
p
Dd+
)
CT) absorption band is excited (Eq. (2)) [19b,20a].
between the chromophores in both complexes. As shown in
v1
A þ D hꢀ Aꢂ þ DꢀðA ꢁ DÞEꢂX
ð1Þ
ð2Þ
Fig. 4a, two phen (A) and PhN (I) moieties from four molecules of
1
are connected via
p
ꢀ ꢀ ꢀ
p
interactions (the Aꢀ ꢀ ꢀA
centroidꢀ ꢀ ꢀcentroid distance 3.69 Å, Aꢀ ꢀ ꢀI centroidꢀ ꢀ ꢀcentroid
ꢀ
ꢁ
ꢀ
ꢁꢂ
h
v2
A þ Dꢀ Adꢁ ꢁ Ddþ
Adꢁ ꢁ Ddþ
distance 3.67 Å), which are further connected by two phen (B)
ƒƒƒ!
CT
CT
moieties, through
distance 3.57 Å) and the CAHꢀ ꢀ ꢀ
distance 3.75 Å) to form a 1D chain structure. The 1D chains are
linked into a 2D layer by the HL moieties (II) through CAHꢀꢀ
pꢀ ꢀ ꢀ
p
interaction (the Bꢀ ꢀ ꢀB centroidꢀ ꢀ ꢀcentroid
p
interactions (the C26ꢀ ꢀ ꢀcentroid
To verify the existence of CTC, the solid state absorption of 1, 2
and the ligands were studied (Fig. 3). The emergence of a new
absorption band in the low energy region is the feature of a CTC
p

342
J. Miao et al. / Polyhedron 171 (2019) 338–343
Fig. 4.
pꢀ ꢀ ꢀp
and/or CAHꢀ ꢀ ꢀ
p
interactions in 1 and 2 (only chromophores are displayed for clarity), (a) 1D chain and (b) 2D layer formed in 1, (c) zigzag chain formed in 2.
interactions (C43ꢀ ꢀ ꢀcentroid distance 3.67 Å, C22ꢀ ꢀ ꢀcentroid
ArN moieties and/or phen are excited in such situations. As illus-
trated in Eq. (1), s3 attributes to the emission of the exciplex. The
distance 3.53 Å, Fig. 4b).
In 2 there are no
p
ꢀ ꢀ ꢀ
p
interactions, but edge-to-face CAHꢀ ꢀ ꢀ
p
p
contribution change of s1 is opposed to that of s3, it is dominant
when 1 is excited with a wavelength longer than 460 nm, obvi-
ously in such situations the CT absorption bands are excited, so
s1 should be assigned to the emissions from CTC. As to s2, it’s con-
tribution first increases and then decreases, reaching the maxima
with an excitation light at 460 nm. The energy of 460 nm light
can not excite the ArN and phen moieties, thus s2 should come
interactions between the chromophores exist. The C18AHꢀ ꢀ ꢀ
(C18ꢀ ꢀ ꢀcentroid distance 3.58 Å) and C29AHꢀ ꢀ ꢀ
p
(C29ꢀ ꢀ ꢀcentroid
distance 3.54 Å) interactions between the benzene and the pyri-
dine rings form a zigzag chain (Fig. 4c). Clearly the D–A interac-
tions in 1 are much stronger than those in 2. Such difference in
the intermolecular interactions leads to the difference in the
absorption spectra that 1 has a strong CT absorption but 2 has only
a weak one.
from a different type of CT emission from that of s1. These results
indicate the emissions of 1 are originated from three components
including exciplex and CTCs, these components have different
emission wavelengths but are partially overlapped which brings
about the tunable emissions of 1.
The fitting results for the fluorescence decay curves of 2 are
shown in Figs. S11–14 and Table 2. Triexponential functions with
To gain further information on the emissive species involved in
the photophysical processes of 1 and 2, the time-resolved fluores-
cence spectra were measured by changing the excitation wave-
length, and the corresponding emission maxima were used as
emission wavelength. The fluorescence decay curves clearly show
the emissions of both 1 and 2 are from multiple components
(Fig. 5). It was found that almost all the fluorescence decay curves
of 1 could be analyzed with three components, and the lifetimes
s1 = 1.72 ns, s2 = 6.22 ns, s3 = 33.65 ns, respectively, are needed to
fit the curve with excitation wavelength longer than 440 nm. The
emission excited at 400 nm need a biexponential function to be
are
s
1 = 0.98,
s
2 = 5.51,
s
3 = 23.17 ns, respectively (Figs. S5–10).
fit with identical lifetimes of s2 and
at 300 nm need a monoexponential function, and the value of the
lifetime is the same as that of 3. Considering the excitation wave-
lengths, s3 should belong to the emission of exciplex. It is obvious
that s3 is dominant in all the situations, which implies the emis-
sions of 2 mainly come from the exciplex formed between the
ArN and 4,40-bipy moieties. Comparing the emission behaviors of
s3, and the fit of that excited
The contributions of each lifetime change regularly with the
increasing excitation wavelengths (Table 1). The contributions of
the longest lifetime s3 decrease gradually from the dominant
70.20% (excitation at 300 nm) to zero (excitation at 520 nm). When
the wavelength of the excitation light is shorter than 420 nm,
dominant. According to the absorption and excitation spectra, the
s
s3 is
Fig. 5. Fluorescence decay curves of 1 (a) and 2 (b) with different excitation wavelengths (the emission maxima were used for each curve).

J. Miao et al. / Polyhedron 171 (2019) 338–343
343
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covering green to red region compared with those of the ligands.
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in the excited states for both compounds. The aromatic stacking
interactions in the solid state structures position the electron
donors (the ArN moieties) and acceptors (the auxiliary ligands) in
a close proximity, promoting the emission behavior of these metal
complexes. It is due to the multiple components involved that the
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The authors thank the Natural Science Foundation of Shandong
Province (ZR2009BL008) and the Science and Technology Program
of University of Jinan (grant XKY1906) for support of this work. We
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Declaration of Competing Interest
(b) M.S. Wang, S.P. Guo, Y. Li, L.Z. Cai, J.P. Zuo, G. Xu, W.W. Zhou, F.K. Zheng, G.
C. Guo, J. Am. Chem. Soc. 131 (2009) 13572;
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Eremenko, W. Linert, J. Lumin. 202 (2018) 370;
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
(e) Y. Wang, I.A. Zhang, B.H. Yu, X.F. Fang, X. Su, Y.M. Zhang, T. Zhang, B. Yang,
M.J. Li, S.X.A. Zhang, J. Mater. Chem. C 3 (2015) 12328;
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Appendix A. Supplementary data
CCDC 1533758 and 1533759 contains the supplementary crys-
tallographic data for 1 and 2, respectively. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/re-
trieving.html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data to this
article can be found online at https://doi.org/10.1016/j.poly.2019.
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