Light-driven coordination anions-directed regulation of chromism in three metal complexes assembled by cyano-equipped dithienylethene ligand

Article abstract of DOI:10.1016/j.ica.2020.119666

Three coordination compounds were self-assembled successfully with Ag(I) ions and photo-responsive ligands featuring a cyano-equipped dithienylethene unit. Anions with different coordinating abilities were selected to construct distinct topologies, which are evidenced by X-ray crystallographic analysis. CF₃ COO^? and CF₃ SO₃^? are involved into coordination in 1 and 3, respectively, whereas in 2 BF₄^? present merel y as ionic guests counterbalancing the framework charge. Anions also show their subtle effects on coordinated atoms, interactions, and ligand configurations. The quite different structures of 1–3 indicate an interesting anion-directed structural self-assembly. Followed examinations on photochromism illustrated that the reversible photo-isomerizations of three complexes are all retained after complexzation with Ag(I) center. Various degrees of bathochromic shifts in absorptions were observed as compared with that of the metal-free ligand. Therefore, the photo-switching of these compounds could be modulated conveniently and finely by varying simply the coordination anions and then triggering by light irradiation. The discussions of structure-properties relationship concluded that anions exert their effects on coordination structure through adjusting the configurations of ligands in complexes. Strong coordinating anions facilitate a pronounced conformational change of ligand whereas un-coordinating anion leads to smaller variation. The distinct structural changes of ligand result finally the different perturbation of photochromic behavior.

Full text of DOI:10.1016/j.ica.2020.119666

InorganicaChimicaActa509(2020)119666
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Research paper
Light-driven coordination anions-directed regulation of chromism in three
metal complexes assembled by cyano-equipped dithienylethene ligand
⁎
⁎
Jing Hana,c, , Qing Lia, Zhong Yub, , Chun-yan Quana, Xiang Liua, Jia-cai Hana
a Department of Material Physics and Chemistry, Xi’an University of Technology, Xi’an, Shaanxi 710048, China
b Department of Chemistry, Xi’an University of Technology, Xi’an, Shaanxi 710048, China
c Department of Chemistry, University of California, Riverside, CA 92521, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
Three coordination compounds were self-assembled successfully with Ag(I) ions and photo-responsive ligands fea-
turing a cyano-equipped dithienylethene unit. Anions with different coordinating abilities were selected to construct
Dithienylethene
Photochromism
Metal complexes
Anion effect
distinct topologies, which are evidenced by X-ray crystallographic analysis. CF3 COO− and CF3 SO3 are involved
−
−
into coordination in 1 and 3, respectively, whereas in 2 BF4 present merel y as ionic guests counterbalancing the
framework charge. Anions also show their subtle effects on coordinated atoms, interactions, and ligand configura-
tions. The quite different structures of 1–3 indicate an interesting anion-directed structural self-assembly.
Followed examinations on photochromism illustrated that the reversible photo-isomerizations of three com-
plexes are all retained after complexzation with Ag(I) center. Various degrees of bathochromic shifts in absorptions
were observed as compared with that of the metal-free ligand. Therefore, the photo-switching of these compounds
could be modulated conveniently and finely by varying simply the coordination anions and then triggering by light
irradiation. The discussions of structure-properties relationship concluded that anions exert their effects on co-
ordination structure through adjusting the configurations of ligands in complexes. Strong coordinating anions
facilitate a pronounced conformational change of ligand whereas un-coordinating anion leads to smaller variation.
The distinct structural changes of ligand result finally the different perturbation of photochromic behavior.
Crystal engineering
1. Introductions
not only offers rich properties but also enables the regulation of the
photo-switching performance, such as interesting spin-crossover
Dithienylethene (DTE) architectures responding to external input of
light have stood out as excellent candidates for future applications as
diverse as high-density information storage, switching devices and ac-
tuators owing to their reversible electronic as well as geometrical
structure changes induced by chemical bond rearrangement during the
photo-transformation between a non-conjugated open-form and a π-
conjugated closed-form by alternate irradiation with UV and visible
light [1–3]. For practical applications, improving their photophysical
and photochemical properties has been especially concerned by opti-
mizing the substitutes on the thienyl rings [4–6], addition of metal ions
[7–10] and doping with composites [11–13]. Among these approaches,
addressing metal centers to form hybrid materials through coordination
with organic photo-responsive ligand could allow the combination of
the advantages of both inorganic and photochromic organic species at
the molecular level. Because metal complex plays the role of a photo-
sensitizer in reversibly regulating the lifetime of the metal-ligand’s
excited state [14], incorporating dithienylethene into a complex system
[15–18], improved luminescence [19–22], NLO response [23,24] and
porosity etc. [9,25,26], as demonstrated by several groups to date.
However, the existing explorations have mainly concentrated on the
effect of metal ions [7–10], the contribution of coordination anions on
photochromic behavior in solid state, from the crystal engineering point,
has never been investigated systemically for dithienylethene species
[27]. And anions were especially found to produce dramatic effects on
the extended structure of the networks amongst those secondary driving
forces, namely, pH value [28], temperature [29], reagent concentration
[30] and solvent media [31]. We have previously synthesized a series of
metal complexes with dithienylethene by designing dithienylethene li-
gands attached with various binding groups, including pyridine [32,33],
carboxyl acid [34,35] and cyanophenyl [36]. Our results showed some
successful examples of enhanced photo-switching in solid state after
metal complexation, such as improved photo-stimulus [37] and sup-
pressed side photoreaction [27] with only a few exceptions [38]. Our
recent work indicated BF4−, a un-coordinating anion, is favorable for the
⁎ Corresponding authors at: No. 5 South Jinhua Road, Xi’an, Shaanxi 710048, China.
E-mail addresses: hanj@xaut.edu.cn (J. Han), yuzhong@xaut.edu.cn (Z. Yu).
https://doi.org/10.1016/j.ica.2020.119666
Received 4 December 2019; Received in revised form 10 April 2020; Accepted 10 April 2020
Availableonline13April2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

J. Han, et al.
InorganicaChimicaActa509(2020)119666
Scheme 1. Synthesis of BM-4-CP-3-TP.
effective photo-isomerization when it’s self-assembled with cis-dbe into a
coordination framework [27].
2.3. Syntheses
As an extension of our research interests and in order to further un-
derstand the exact role of anions in the modulations of photochromism,
herein three Ag(I) salts with anions featuring distinct coordination affi-
1,2-bis-[2′-methyl-5′-(4″-cyanophenyl)-3′-thienyl] perfluoro-
cyolopentene (BM-4-CP-3-TP). BM-4-CP-3-TP was prepared using
modified literature method [39] (Scheme 1). Anal. IR (KBr pellet): 2360
(m), 2218 (m), 1595 (m), 1261 (m), 1101 (s), 983 (s), 823 (s), 743 (m).
1HNMR (CDCl3, 400 MHz, δ / ppm), 7.68 (d, 4H, J = 8.5 Hz, cyano-
phenyl H), 7.63 (d, 4H, J = 8.6 Hz, cyanophenyl H), 7.40 (s, 2H,
thienyl H), 2.00 (s, 6H, methyl). Its single crystal was obtained by re-
crystallization from THF solution at room temperature and it’s different
from that of the previous compound [39].
−
nities, i.e. CF3COO−, BF4 and CF3SO3−, were selected judiciously to
assemble with a photo-active dithienylethene ligand equipped with cyano
groups. The ligand (1,2-bis-[2′-methyl-5′-(4″-cyanophenyl)-3′-thienyl]
perfluorocyolopentene, hereafter referred as BM-4-CP-3-TP) was originally
structural characterized by liu [39] and it shows effective photochromism
in solution and film [40]. In this paper, three new complexes based on the
ligand were isolated and structurally characterized by X-ray crystal-
lography. And the relationships between crystal structures and solid state
photo-switching were investigated further from the anions aspect.
{[Ag2(BM-4-CP-3-TP)(CF3COO)2)]•(C6H6)2}n (1). Complex
1 was
prepared by mixing of AgCF3COO (0.0162 g, 73 µmol) and L (0.0183 g,
32 µmol) in 2 mL benzene. After stirring for 30 min, the resultant solution
was introduced into a 9 mm diameter glass tube and layered with n-pentane
(2 mL) as a diffusion solvent. After standing at room temperature for one
week, colorless crystals of 1 suitable for single crystal X-ray analysis were
obtained. Yield: 38.32%. Anal. Calcd. for C45H28Ag2F12N2O4S2 C, 46.25; N,
2.39; H, 2.42; S, 5.49; Found: C, 45.87; N, 2.84; H, 2.14; S, 5.84%. IR (KBr
pellet): 2223 (m), 1680 (s), 1434 (m), 1267 (m), 1132 (w), 1106 (w), 830
(w), 806 (w), 743(w). 1HNMR (CDCl3, 400 MHz, δ/ppm), 7.68 (d, 4H,
J = 8.2 Hz, cyanophenyl), 7.64 (d, 4H, J = 8.2 Hz, cyanophenyl), 7.40 (s,
2H, thienyl H) 7.37 (s, 6H, benzene), 1.99 (s, 6H, methyl).
2. Experimental
2.1. Material and methods
Unless otherwise indicated, all starting materials were obtained
from commercial suppliers and without further purification prior to
use. Infrared spectra were recorded as KBr disk on JASCO FT-IR 8000
spectrometers. Absorption spectra in solid state were measured by
diffuse reflection using the Kubelka-Munk method on a SHIMADZU UV-
3600 spectrometer, and barium sulfate was used as a reference.
Absorption spectra in solutions were measured using a Hitachi U-3900
spectrometer. Solvent were used as purchased and not degassed. The
concentration of solution is ca. 1 × 10−6 mol/L. PMMA film was
prepared by solubilizing ultrasonically 10 mg complex and 100 mg
PMMA in chloroform (1 mL), then spin-coating the homogeneous so-
lution on a glass substrate (20 mm × 20 mm × 1 mm) with a spin
rotation speed of 1500 rpm. Photo-irradiation was carried out using a
100 W Xe lamp in atmosphere, and light with appropriate wavelength
was isolated by passing the light through andover filter (254 FS25) and
RANYAN cut-off filter (BP 550–10 K). 1HNMR spectra were measured
on a Varian INOVA NMR spectrometer in CDCl3 solution at room
temperature (with Tetramethylsilane as internal reference).
Complex 1a. The benzene-eliminated 1a was obtained by heating 1
for 1 h at 190 °C. 1H NMR (CDCl3, 400 MHz, δ/ppm): 7.68 (d, 4H,
J = 8.0 Hz, cyanophenyl), 7.63 (d, 4H, J = 7.8 Hz, cyanophenyl), 7.40
(s, 2H, thienyl H), 1.99 (s, 6H, methyl).
Complex 1b. 1b was prepared by exposing 1a to benzene vapor for
24 h at 50 °C, filtrated and dried. 1H NMR (CDCl3, 400 MHz, δ/ppm), 7.69
(d, 4H, J = 8.0 Hz, cyanophenyl), 7.64 (d, 4H, J = 8.0 Hz, cyanophenyl),
7.40 (s, 2H, thienyl H), 7.37(s, 6H, benzene), 1.99 (s, 6H, methyl).
Complex 1c. 1c was prepared by exposing 1a to toluene vapor for
24 h at 50 °C, filtrated and dried. 1H NMR (CDCl3, 400 MHz, δ/ppm),
7.68 (d, 4H, J = 8.5 Hz, cyanophenyl), 7.63 (d, 4H, J = 8.5 Hz, cya-
nophenyl), 7.40 (s, 2H, thienyl H), 1.99 (s, 6H, methyl).
{[Ag(BM-4-CP-3-TP)]BF4·(THF)}n (2). To
a benzene solution
(15 mL) of L (0.057 g, 0.1 mmol) was added AgBF4(0.019 g, 0.1 mmol).
The resultant solution was stirred for 30 min and the resulted pre-
cipitate were dissolved in THF followed by filling in a long and narrow
tube for slow evaporation. Colorless crystals were obtained after
standing at room temperature for 1 week. Yield: 54%. Anal. Calcd.
2.2. X-ray data collection and structure solutions and refinements
Diffraction data for BM-4-CP-3-TP and complexes 1–3 were col-
lected on BRUKER APEX-II CCD with graphite monochromated Mo-Kα
radiation (λ = 0.71073 Å). The intensity data were collected at 296 K
for all crystals, using the multi scan technique. The linear absorption
C
33H24AgBF10N2OS2 C, 47.33; N, 3.35; H, 2.89; S, 7.66; Found: C,
47.07; N, 3.65; H, 2.54; S, 7.86%. IR (KBr pellet): 2232 (m), 1600 (m),
1340 (m), 1274 (s), 1086 (w), 987 (w), 832 (w), 744 (w).
{[Ag2(BM-4-CP-3-TP)(CF3SO3)2(H2O)2]}n (3). Colorless crystals
of 3 were grown similarly to those of 1 using ligand (0.057 g, 0.1 mmol)
and AgCF3SO3 (0.026 g, 0.1 mmol). Yield: 45% (based on Ag). Anal.
Calcd. C31H20Ag2F12N2O8S4 C, 33.23; N, 2.50; H, 1.80; S, 11.45; Found:
C, 33.57; N, 2.35; H, 1.94; S, 11.33%. IR (KBr pellet) 2231 (m), 1267
(s), 1175 (m), 1107 (w), 1037 (w), 831 (w), 646 (w).
coefficient
μ for Mo-Kα radiation is 0.240, 1.043, 0.786 and
1.312 mm−1 for BM-4-CP-3-TP and complexes 1–3, respectively. The
structures were solved by direct methods followed by subsequent
Fourier calculations. The non-hydrogen atoms were refined aniso-
tropically. The final cycle of the full-matrix least squares refinement
was based on 1095, 5759, 1942 and 2220 reflections for four crystals,
respectively, converged with the unweighted and weighted agreement
factors of R = Σ||Fo|-|Fc||/Σ|Fo| and Rw=(Σw(Fo2-Fc2)2/Σw(Fo2)2)1/
2. The atomic scattering factors and anomalous dispersion terms were
taken from the International Tables for X-ray Crystallography, Vol. IV
[41]. All calculations were performed using SHELXS-97 or SHELXS-
2018/3 crystallographic software package [42,43].
3. Results and discussions
3.1. Infrared spectra analysis
The FT-IR spectra of complexes 1–3 are illustrated in Figs. S1–S3 to
2

J. Han, et al.
InorganicaChimicaActa509(2020)119666
prove the formation of these complexes. The typical C^N stretching
vibration peaks of cyanophenyl are found at 2221 cm−1 in ligand, and
appeared at 2223 cm−1, 2232 cm−1, and 2231 cm−1 in complexes 1–3,
respectively. The red-shifts of C^N stretches are attributed to the π-
back donation effect of cyano group [44,45], the s-donation as well as
the electrostatic interaction [46–48]. The different shifts of the C^N
frequency in 1 (+2 cm−1) and 2–3 (+11 cm−1) relative to the cor-
responding band in the free ligand may be ascribed to the subtle elec-
trostatic interaction originating from their different structures. In ad-
dition, the characteristic νs(C-F) peak of perfluorocyclopentene in
ligand at 1266 cm−1 are present at 1267 cm−1, 1274 cm−1 and
1267 cm−1 in complexes 1–3, respectively. These observations indicate
that BM-4-CP-3-TP is included into all three complexes.
facilitates efficient electronic delocalization between the photoactive
and the cyanophenyl moieties.
There are extensive intermolecular and intramolecular H contacts
exist between two adjacent ligand molecules in BM-4-CP-3-TP·THF and
two neighboring pairs of molecules are further interact with H bonds
(Fig. 1(b) and (c)) which lead to a comparable closer packing than the
referenced ligand where no H bonds present between two neighboring
pairs of molecules (Fig. S4). The detailed bond distances and angels of
both compounds are listed in Tables S2 and S3. The different packing
modes of BM-4-CP-3-TP·THF and the referenced compound may be
responsible for their different densities. The closer packing of BM-4-CP-
3-TP·THF results in a smaller density of 1.394 g cm−3 (1.526 g cm−3 for
referenced work).
On the other hand, in the spectrum of 1, the νs(CeO) peaks around
1680 cm−1 and 1434 cm−1 (1687 cm−1 and 1430 cm−1 for
AgCF3COO) as well as the νs(C-F) peak of 1132 cm−1 (1140 cm−1 for
AgCF3COO) indicate the presence of CF3COO-. Similarly, the typical
peak of νs(B-F) of 1073 cm−1 appeared at 1056 cm−1 in the spectrum
of 2, suggesting the incorporation of BF4- into complex 2. The stretching
vibrations of CF3SO3−, i.e. 1175 cm−1, 1037 cm−1 and 646 cm−1, are
In BM-4-CP-3-TP·THF, the dihedral angle between the mean planes
of cyclopentene ring and the two thiophene rings is 51.26°. The mean
torsion angle between thiophene and hexafluorocyclopentene rings
(C13-C13A-C10A-C9A) is 128.71°. In the referenced structure, the mean
torsion angle between thiophene and cyclopentene rings is −149.804°.
The torsion angle between thiophene and hexafluorocyclopentene rings
of BM-4-CP-3-TP·THF is significantly smaller than that of referenced
-
witnessed in the spectrum of 3, demonstrating the existence of CF3SO3 .
structure, whereas the reactive distance of BM-4-CP-3-TP·THF (3.65 Ǻ)
is longer than that of referenced structure (3.60 Ǻ).
These findings indicate that the respective anions are included in the
frameworks of the three complexes.
3.2.2. Crystal structure of complex 1
3.2. Structural characterizations
Complex 1 is revealed as a 2D layer constructed by bridging ligand
and 1D AgCF3COO chain. Benzene acts as guest intercalating between
the adjacent layers. In one unit, there are two crystallographically in-
dependent Ag(I) centers with different coordination environments. Ag1
is four-coordinated to three O atoms from three CF3COO- anions (Ag1-
The crystal structures of BM-4-CP-3-TP and complexes 1–3 are de-
termined by X-ray single crystal diffractions. Details of the X-ray data
are summarized in Table 1. Selected bond lengths and bond angles are
given in Table S1.
O
= 2.279(5)-2.477(7) Å) and one N atom from ligand (Ag1-
N2C = 2.297(6) Å), showing a distorted tetrahedral structure. The
structural distortion parameter τ4 is 0.718 indicating of complex 1 a
distorted trigonal pyramid (0.85 for prefect trigonal pyramid). The
bottom plane of the trigonal pyramid is comprised of O1, O4 and N2C
atoms with silver center lies out of the plane by the distance of
0.2033 Å. Ag2 is however surrounded by one O atom from CF3COO-
anion and one N atom from ligand with N1-Ag2-O2 angle of 150.4(2)°,
displaying a linear arrangement (Fig. 2(a)).
3.2.1. Crystal structure of BM-4-CP-3-TP
For BM-4-CP-3-TP, it was firstly prepared in 2008 and its single
crystals were crystallized from hexane [39]. In the present study,
crystallization from THF led to the isolation of a new solvated form,
which is different from the reported structure.
Fig. 1(a) depicts the crystal structure of BM-4-CP-3-TP·THF de-
termined by X-ray crystallographic analysis. The THF molecules, using
as crystallization solvent, are contained in the crystals and are dis-
ordered. The two thienyl rings are fixed in a photo-reactive anti-parallel
conformation with the dihedral angel of 58.86° between two thienyl
rings. The distance between reactive carbon atoms, C11 and C11A, is
3.65 Å. This anti-parallel conformation and a C···C distance less than
4.2 Å are mandatory for observing the photo-isomerization of the dia-
rylethene unit in solid state. The cyanophenyl and thienyl rings are
almost coplanar with the dihedral angel of 6.75° whereas in the re-
ference crystals the dihedral angel is 22.43° [39]. This characteristic
Two Ag1 ions are linked by two anions to form a four-membered
ring. The neighboring Ag1 and Ag2 are bridged via one anion through
two monodentated O atoms. The above arrangement leads to a Ag4
cluster. These Ag4 clusters are further interconnected each other by
Ag…O interactions to generate a 1D chain along b-direction (Ag2-
O2B = 2.6592(52) Å). Ag1…Ag2 distance is 3.1004 (9) Å, which is
longer than the Ag-Ag separation in silver metal (2.89 Å) but shorter
than twice the van der Waals radius of silver ions (3.44 Å), indicating
weak Ag…Ag interaction [49]. From the perspective of anion, CF3COO−
Table 1
Crystal data of BM-4-CP-3-TP·THF and three complexes.
Compound
BM-4-CP-3-TP·THF
1
2
3
Formula
C
33H24F6N2OS2
C45H28Ag2F12N2O4S2
1168.54
C33H24AgBF10N2OS2
837.34
C31H20Ag2F12N2O8S4
1120.47
Formula weight
Temperature/K
Crystal system, group
a/Å
642.66
296(2)
296(2)
296(2)
296(2)
Monoclinic, C2/c
20.26(2)
13.972(15)
10.816(13)
90
Triclinic, P-1
10.923(2)
11.512(2)
20.311(4)
74.918(3)
84.284(3)
67.800(3)
2,1.700
Monoclinic, P2(1)/c
13.923(9)
12.083(8)
21.164(13)
90
Monoclinic, Cc
38.457(18)
9.260(4)
b/Å
c/Å
11.343(5)
90
α/(deg.)
β/(deg.)
90.432
103.077(13)
90
102.569(9)
90
γ/(deg.)
90
Z, Dc (g/m3)
4,1.394
4,1.604
4,1.888
µ (mm−1
)
0.240
1.043
0.786
1.312
Reflections
2697/1095
1.036
9034/5759
1.010
6165/1942
0.901
3444/2220
1.048
Gof-fit on F2
R
all/Rget
0.2109/0.1056
0.2962/0.2381
0.1019/0.0667
0.2345/0.1961
0.2771/0.0905
0.2948/0.1936
0.1368/0.0996
0.2652/0.2444
wR2all/wR2get
3

J. Han, et al.
InorganicaChimicaActa509(2020)119666
Fig. 1. Crystal structure of BM-4-CP-3-TP·THF (a) and its packing structures showing (b) H bonds and (c) interactions between molecules.
in 1 exhibits two kinds of coordination modes, i.e. µ2-η1:η1-bridging
mode and µ2-η2-bridging mode to join Ag centers, respectively.
In complex 1 the ligand acts as a bis-monodentate bridging linker
through N atoms to spread the above mentioned 1D chain to a 2D layer.
Benzene is intercalated within neighboring layers as guest. The nearest
distance between Ag and benzene is 2.7767 Å, which is longer than the
longest length of Ag-C coordination bond in published literatures
(2.69 Å) indicating a cation-π interactions between the benzene and 2D
layer [50]. Thus, in addition to filling the void spaces in the framework
of 1, the presence of guest benzenes can also contribute to the packing
of 1 and enhance its stability.
For ligand, it could coordinate with metal ions via its thienyl S
atoms and/or other coordination atoms of functional groups to form
diverse coordination structures. However, thienyl S atoms are scarcely
involved into coordination sphere in previous dithienylethene-based
complexes [38,51]. In this study, the bond length of Ag1-S1 is 2.689(4)
Ǻ, which is well within the range of Ag-S distances in Ag-thiophene
complexes [52–55]. It represents the first example of Ag ions binding to
S donor of dithienylethene based on perfluorocyclopentene. The Ag-N
distances in complex 2 is 2.158(12) Ǻ and 2.192(12) Ǻ, respectively,
which is significantly shorter than the distance between Ag ions and S
donor. In fact, these bonding parameters are inconsistent with con-
ventional expectation that the soft silver metal forms a stronger bond
with the softer sulfur atom rather than the harder nitrogen atom.
Complex 2 exhibits a 1D ladder (or double chain) structure with the
neighboring ligands arranged in a syn-anti fashion. The cyanophenyl
planes of ligands in two inter-connected 1D chains are parallel each
other and stack in an offset or slipped fashion. The plane – plane dis-
tances between two adjacent benzene rings are 3.56 Å and 3.65 Å, re-
spectively. The corresponding centroid-centroid distances (3.98 Å and
4.13 Å) and displacement angle (26.6° and 27.8°) exceed the maximum
parameters for π-π interactions (3.8 Å and 20°), indicating no obvious
π-π interactions [56].
For ligand, the two thienyl rings adopt anti-parallel fashions with
dihedral angel of 61.23°. The two C atoms that are about to bind upon
photo-cyclization (C15 and C25) keep 3.59 Å apart. The mean torsion
angles between thiophene and hexafluorocyclopentene rings (C(17)-
C(21)-C(22)-C(23) and C(21)-C(17)-C(14)-C(13)) are −134.1° and
−129.13°, respectively. The cyanophenyl and thienyl rings are almost
coplanar in free ligand, whereas this dihedral angel increase greatly
from 6.75° to 40.07° after incorporation of Ag(I) ions. The bond length
of C^N in complex 1 (1.141(8) Å) is slightly longer than that in free
ligand. The change in bond length is also reflected as absorption shift in
IR spectrum.
In complex 2, the ligand acts as a linker to bind Ag(I) ions. Ligand
retains its anti-parallel fashions of two thienyl rings with dihedral angel
of 68.24°. The reactive carbon distance for photo-cyclization between
C11 and C21 is 3.79 Å, still close for a ring-closure from 1,3,5-hexa-
triene to cyclohexadiene. The mean torsion angles between thiophene
and hexafluorocyclopentene rings (C(13)-C(17)-C(18)-C(19) and C(17)-
C(13)-C(10)-C(9)) are 127.48° and 125.81°, respectively. Different from
complex 1, the cyanophenyl and thienyl rings are almost coplanar in 2,
which is similar to free ligand. This feature favors interactions between
the metal ion and the diarylethene groups.
3.2.3. Crystal structure of complex 2
Fig. 3 shows the crystal structure of complex 2 characterized by X-
ray single crystal diffraction. 2 was synthesized using the same ligand
-
and solvent as complex 1 only different in anion (BF4 ). In one asym-
metric unit, only one crystallographically independent Ag(I) center
existed and is confined by two N atoms from distinct ligands as well as
one S donor from another ligand, displaying a Y-shape trigonal ar-
rangement (N2C-Ag1-N1A = 135.0(5)°, N2C-Ag1-S1 = 114.8(3)°,
N1AC-Ag1-S1 = 104.4(3)°).
Fig. 2. Crystal structures of complex 1 showing (a) chain structure, (b) Ag…O interactions and (c) cation-π interactions between Ag ion and guest benzene. H atoms
are omitted for clarity. Symmetry code:A:1-x,2-y,2-z; B:1-x,3-y,2-z;C:2-x,2-y,1-z.
4

J. Han, et al.
InorganicaChimicaActa509(2020)119666
Fig. 3. Crystal structures of complex 2 showing (a) coordination structure, (b) 1-D ladder. Only the H atoms involved in H bond are displayed. Symmetry code:A:1-
x,2-y,2-z; B:1-x,3-y,2-z;C:2-x,2-y,1-z.
Fig. 4. Crystal structure of complex 3. Only the H atoms involved in coordination are displayed for clarity. Symmetry code: A: 0.5-x,0.5-y,1-z; B:-x,y,-0.5-z;
−
As an un-coordinating anion, BF4 is revealed as ionic guests
counterbalancing the framework charge. It is sandwiched between two
neighboring 1D ladders via weak intramolecular hydrogen bond be-
tween the F atoms of anion and H atoms of ligand. THF molecules,
using as crystallization solvent, are also contained in complex 1 acting
as guest intercalated into the adjacent ladders.
Table 2
Summary of ligand and complexes 1–3.
ligand
1
2
3
CF3COO−
40.07
BF4
CF3SO3
22.22
45.73
−
−
anions
dihedral angle A (°)
dihedral angle B (°)
6.75
3.48/12.49
59.72
51.26
49.28
51.78
55.77
Reactive C distance (Å)
coordination atoms
weak interactions
3.65
3.65
3.79
3.57
3.2.4. Crystal structure of complex 3
Only N atom
Ag…O
Ag…C
Ag…Ag
619
N and S atoms
H bond
Only N atom
Ag…Ag
Fig. 4 illustrates the 1D dinuclear zig-zag chain of 3, with neigh-
boring ligands packed in a syn-anti fashion. The crystallographically
independent Ag(I) center is four-coordination and adopts a [AgN1O3]
λ
max(closed-form, nm)
607
35
625
30
623
25
coordination environment. The τ4 value of 0.28 suggests a see-saw
time to PSS (min)
40
-
structure. Two
O
atoms are from the distinct CF3SO3 (Ag1-
A: dihedral angle between cyanophenyl and thiophene rings. B: dihedral angle
between cyclopentene and thiophene rings.
O1 = 2.579(12) Å and Ag1-O3A = 2.557(10) Å) and the third O atom
belongs to H2O (Ag1-O1W = 2.200(10) Å).
CF3SO3− is a moderately coordinating anion and only a few reports
are engaged in the coordinated CF3SO3 [57,58]. The bond distances of
5

J. Han, et al.
InorganicaChimicaActa509(2020)119666
Fig. 5. Time-dependent UV–Vis spectra of BM-4-CP-3-TP in solid state (a) irradiation with 254 nm light and (b) irradiation with ≥550 nm light.
Fig. 6. Time-dependent UV–Vis spectra of complex 1 in solid state (a) irradiation with 254 nm light and (b) irradiation with ≥550 nm light.
Fig. 7. UV–Vis spectra of 2 in solid state (a) and in PMMA films (b).
Ag-O (anions) in 3 are much longer than those in 1, demonstrating the
weaker coordination ability of CF3SO3- in comparison to CF3COO-. In 3,
CF3SO3- shows µ2-η1:η1-bridging mode to link two Ag(I) ions forming an
eight-membered dinuclear unit. These dinuclear units are further con-
nected by bridging ligands to yield 1D chain. The distance between two
Ag ions is 3.359 Å, which is slightly shorter than the twice van der
Waals radius of silver ions (3.44 Å), indicating weak Ag…Ag interaction.
As for ligand, it still retains its anti-parallel arrangement of two
thienyl rings with dihedral angel of 58.80° and C···C separation of the
photoactive carbon atoms of 3.57 Å. Thus, the structural requirement
for photo-cyclization in solid state is fulfilled. The mean torsion angles
between thiophene and hexafluorocyclopentene rings (C(13)-C(13B)-
C(10B)-C(9B)) is −136.93°. In order to coordinate with Ag(I) ions, li-
gand rotate its structure with changing considerably the dihedral angle
between cyanophenyl and thienyl rings from 6.748° in ligand to 22.22°
in complex 3.
3.2.5. Anions effect
During the coordination of ligand with metal ions, ligand con-
formations are elaborately modulated through non-covalent interac-
tions in three complexes. Anions play an especially un-neglectable role
in regulating the configurations of ligands to satisfy the coordination
spheres. Complexes 1–3 were constructed by same building blocks,
metal ions and solvents however anions with different coordination
6

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InorganicaChimicaActa509(2020)119666
Fig. 8. UV–Vis spectra of 3 in solid state (a) and in PMMA films (b).
abilities. As is expected, three complexes possess different structures.
The structural adjustment of ligands in complexes 1–3 are mainly re-
flected in the following two respects:
our previous results showed that these weak non-covalent interactions
have a non-significant effect on ring-closure due to their minor sup-
pression to the rotation of thiophene rings [27,35]. Therefore, photo-
switching of three metal complexes are further investigated in solid state.
UV–vis absorption spectra for complexes 1–3 are illustrated in
Figs. 6–8. As anticipation, the photo-switching of the ligand in these
complexes is effective. Irradiating the three powdered complexes with
UV light (254 nm) resulted in appearances of broad absorption bands in
the visible region corresponding to ring-closed isomer with color
changed from colorless to blue, indicating the successful forward photo-
reactions (ring-closing). Exposing the colored complexes to visible light
(> 550 nm) drove the reverse backward photo-reactions (ring-opening)
and regenerated both the original color and the absorption spectra.
In the photo-cyclization, chemical bond rearrangement requires the
rotation of thiophene rings to form a new bond. However, this change in
orientation and volume from the rotation might cause perturbation upon
others due to ligand being packed in rigid and infinite frameworks. Less
favorable, the cyanophenyl of ligand participates in not only coordina-
tion bonds but also intermolecular H bonds, which may further restrict
the free rotation of thiophene rings. Even so, all three complexes dis-
played reversible and effective photochromism in solid phase, indicating
a cooperative effect of elastic coordinative behavior of metal ions [59].
Similarities in spectral and color changes of the three complexes were
observed upon comparison with the characteristics of the corresponding
metal-free ligand (λmax = 607 nm). Three complexes in PSS showed red-
shifts of maximum absorption bands of 12 nm, 18 nm and 16 nm, re-
spectively, indicating a pronounced coordination effect (λmax is 619 nm,
625 nm and 623 nm for complexes 1–3, respectively). In addition, much
faster conversion rates for complexes 2 and 3 in solid phase were ob-
served as compared to free ligand (the irradiation time to achieve the PSS
is 35 min for ligand, Table 2). Therefore, metal coordination doesn’t
prohibit the photo-responsive performance of ligand but exerts an ad-
vantageous effect on the regulation of optical properties.
1) The dihedral angles between cyanophenyl and thiophene rings.
The angel is 6.75° in free ligand, and it increase to 40.07° and 22.22° in
complexes 1 and 3, respectively where the anticipated anions is strongly
coordinating CF3COO− and moderately coordinating CF3SO3−, respec-
tively. Whereas in complex 2 using non-coordination BF4−, the dihedral
angles are 3.48° and 12.49°, which is closer to that in free ligand.
Stronger anion promotes bigger changes of dihedral angles between cy-
anophenyl and thiophene rings of ligand. In complexes, the interactions
between metal ions and diarylethene groups are greatly affected by the
dihedral angles between cyanophenyl and thiophene rings. So it can be
anticipated that the absorption characteristics of the three complexes
maybe different due to the anion-directed structural differences.
2) The dihedral angles between cyclopentene and thiophene rings.
The angles of complexes 1–3 also change in varying degrees in com-
parison with the corresponding value in ligand. The angle is 51.26° in
ligand and changed to 49.28° and 51.78° in complex 1, 59.72° and 55.77°
in complex 2 and 45.73° in complex 3, respectively. This angle plays a
crucial role in the reactive carbons distance. The value of the angle in
complex 3 is the smallest one of the three and accordingly complex 3 has
the shortest reactive carbon distance (Table 2). And complex 2 possess
the biggest dihedral angle and the longest separation of reactive carbons.
In addition, in complex 2, anion doesn’t take part in coordination
whereas the thiophene group of ligand is found to coordinate with metal
ions through S atoms. This coordination feature may also contribute to
the longer reactive carbon distance in complex 2.
3.3. Photochromism of BM-4-CP-3-TP and complexes 1–3 in solid state
The photochromism of BM-4-CP-3-TP in hexane and thin film were
reported previously [40]. The maximum absorption of thin films of pure
compound deposited onto quartz in PSS is 591 nm. Because dithieny-
lethenes have the unique ability to undergo reversible photo-switching
in solid phase and X-ray diffraction shows a favorable structure of BM-
4-CP-3-TP for solid-state photochromism, i.e., the anti-parallel con-
formation and the short distance between the reacting carbon atoms,
the powder state photochromism of ligand is expected and examined.
As presented in Fig. 5, the photo-isomerizations between the open and
closed forms of ligand occurred reversibly in solid state with the ab-
sorption attributable to closed-ring isomer maintaining at 607 nm. This
maximum absorption is different from that of previous report (591 nm),
indicating that the solid structure of BM-4-CP-3-TP investigated herein
is different from that of deposited onto quartz in literature. It takes
35 min of photo-irradiation for ligand to achieve the PSS and 15 min for
photo-reversion in solid state.
Because thin films based on polymers can be used to tune a wide
range of surface properties, which is beneficial to the practical appli-
cations in storage memory, all complexes were further shaped into
PMMA films. These films all exhibit expected reversible photochromism
triggered by alterative irradiation with UV light and visible light
(Fig. 7(b), 8(b) and S5), indicating their promising potential for future
applications.
3.4. Discussions of structure-property relationship
Although all complexes displayed efficient photochromism in solid
state indicating metal coordination doesn’t suppress both ring-closure
and ring-opening, there are some differences in their absorptions of
worthy discussing.
Although there are H bonds and π-π interactions in complexes 1–3,
Firstly, the maximum absorptions of three complexes in closed-form
7

J. Han, et al.
InorganicaChimicaActa509(2020)119666
8°, Table 2) between cyclopentene and thiophene rings related to the
free ligand (6° for 3 and 2° for 1, respectively). This dissimilarity as
ligand is also responsible for its as much as 18 nm red-shifts in ab-
sorption.
Secondly, the time to reach PSS is varied in three complexes. This
may closely related to the interactions existed in complexes because the
non-covalent bonds, such as metal-coordinating bonds, hydrogen
bonds, Van de Waals contacts and aromatic-aromatic interactions are
considered to have a role in the photo-induced isomerizations when
dithienylethenes are self-assembled into coordination networks. As
shown in Table 2, there are Ag…O interactions, Ag…π interactions and
weak Ag…Ag interactions except coordinating bonds in complex 1
whereas only either hydrogen bonds or Ag…Ag interactions exist in
complex 2 and 3. So, complex 1 has the most complicated interactions
among three complexes, which led to comparably the greatest sup-
pression of photo-cyclization and the longest time to reach PSS
(40 min). In complex 2, besides N atoms, S atoms are also involved in
coordination, which resulted in a longer time to PSS (30 min) than that
for complex 3 (25 min).
Therefore, incorporation of metal ions into photochromic building
blocks, especially selection of anions with different coordinating abil-
ities, is a smart strategy to regulate the photophysical and photo-
chemical properties on the premise of retaining the reversible photo-
chromism. The absorption properties of complexes can be regulated
finely by varying merely the type of anions.
3.5. Guest desorption-readsorption properties of complex 1
Complex 1 is revealed as a guest-incorporated structure and thus its
guest desorption-readsorption properties were further investigated. It’s
known that the reversible structure transformation between two iso-
mers of dithienylethenes during photo-isomerizations will cause a vo-
lume change. It will shrink in photo-cyclization with open-isomer
transferred to closed-isomer and expand in photo-reversion with closed-
isomer returned to original open-isomer. Previous studies showed dia-
rylethenes were incorporated into MOFs to affect the porosity and then
gas sorption properties [24,60] as well as anions uptake and release
triggered by light [61]. However, whether the photo-induced volume
change of the framework 1 originating from ligand shrinkage could lead
to desorption of guests still remains great challenge, since the geometry
of solvent is much larger than the reported gas molecules and ions.
Unfortunately, our attempts to liberate the guests by light-stimulus
are not successful. 1HNMR examination results showed that after photo-
irradiation of complex 1 with UV light, the benzene signals didn’t dis-
appear or weaken in the 1HNMR spectrum, suggesting unsuccessful
guest-desorption by light excitation. This observation indicated that the
conformational change of framework during the photo-reaction is not
sufficiently enough to induce the liberation of guest in the present
system. In this work, single crystals of complex 1 are not stable enough
to keep their crystallinity after photo-cyclization. So the exact volume
change of complex 1 is unavailable. According to the previous reported
single-crystal to single-crystal photo-cyclization, the volume change of
ligand is as small as 3 Å3 [62]. During light irradiation, the shrinkage of
ligand driven by the photo-cylization will lead to the corresponding
shrinkage of the 2D layers in complex 1 (Fig. S6). However, this
shrinkage of 2D layers occurs in the bc plane and may has a little impact
on the spaces between two neighboring layers where the guests are
nested. In other words, the spaces for accommodation of the guests
didn’t change significantly. In addition, the cation-π interactions be-
tween guest benzene and host framework may have an extra dis-
advantageous role in the photo-responsive guest desorption.
Fig. 9. 1H NMR spectra of 1 and its derivatives in CDCl3 at room temperature:
(a) 1, (b) 1a, and (c) 1b.
are different. Complex 2 present the longest absorption at 625 nm
followed by complex 3 at 623 nm and complex 1 at 619 nm (Table 2).
This phenomenon can be explained by the structural distinctions of the
three complexes. As described previously, the interactions between
metal ions and ligands are greatly affected by the dihedral angles be-
tween cyanophenyl and thiophene rings. A smaller dihedral angle will
facilitate the metal–ligand interactions and exert a more signally in-
fluence on the photochromism. Complex 2 has the smallest dihedral
angle and thus the strongest electronic delocalization. In experimental,
it illustrate consistently the most significantly red-shifts in absorption
(18 nm). Besides, complex 2 has the biggest dihedral angle change (c.a.
But for all this, it offers a possible strategy to allow the photo-
triggered guest release which is however generally realized by heating
or pressure, and our related researches will be addressed in later pro-
gram.
8

J. Han, et al.
InorganicaChimicaActa509(2020)119666
Fig. 10. Time-dependent UV–Vis spectra of complex 1a in solid films (a) irradiation with 254 nm light and (b) irradiation with ≥550 nm light.
3.5.1. Elimination of guests by heating
4. Conclusions
The elimination of guest for 1 was then performed by heating.
According to thermogravimetric (TG) analysis, a weight loss corre-
sponding to the liberation of two mol benzene molecules (13.25%,
calcd.13.37%) occurred below 80 °C (Fig. S7) and the second weight
loss of 19.11% is assigned to the anions (calcd.19.34%). 1HNMR ana-
lysis was used to provide the additional evidence of the complete re-
lease of guests. As shown in Fig. 9a, the chemical shift at 8.05 ppm is
assigned to the H signals of phenyl of ligand and the 1HNMR resonance
observed at 7.35 ppm indicated the presence of benzene in 1. Whereas,
the 1HNMR resonances attributed to benzene vanished completely after
heating, supporting the thorough elimination of benzene in 1 (Fig. 9b).
Therefore, guest-free 1a could be obtained by heating of 1 under
monitoring with TG analyzer.
A cyano-functionalized dithienylethene was synthesized and em-
ployed as photo-responsive receptor to construct three new hybrid
systems by incorporation of Ag (I) salts. The key design aspect is to
tailor the photo-induced absorption properties of the resulted com-
plexes by selecting anions with different coordinating abilities to col-
laborate with the metal ions and organic component. The three com-
plexes derived from different anions exhibit expectedly distinct
coordination structures, ligand configuration and non-covalent inter-
actions. The Ag(I) coordination induces some perturbation of the pho-
tochromic behavior, especially, anions enable successfully the fine
tuning of the absorptions of ligand. The present study thus demon-
strates that combining inorganic metal salts with organic photochromic
compound to form hybrid materials by metal coordination and judi-
cious anions selection at molecular level could realize the convenient
regulation of reversible photo-responsive isomerizations. The finding
will contribute to design and elegant modulation of photochromic
compounds through crystal engineering based approach.
The structure of 1a was further examined. Due to the loss of crys-
tallinity XRPD of complexes 1 and 1a are not successful. Our previous
study showed that IR can provide useful information of structural
changes, e.g. the exact coordination modes of COO− [34]. The IR
spectrum of 1a was then measured whereas the vibration peaks as-
signed to CF3COO− remains almost unchanged indicating the anion
keep its coordination mode upon the elimination of guest molecule (Fig.
S8).
CRediT authorship contribution statement
Jing Han: Conceptualization, Funding acquisition, Methodology,
Software. Writing - original draft, Writing - review & editing. Qing Li:
Investigation. Zhong Yu: Conceptualization, Supervision, Funding ac-
quisition. Chun-yan Quan: Investigation. Xiang Liu: Investigation.
Jia-cai Han: Investigation.
3.5.2. Reincorporation of guests
Exposing the de-solvated 1a to benzene vapor yielded 1b. As shown
in Fig. 9c, the 1HNMR resonance of benzene (7.35 ppm) was witnessed
again in 1b, and the estimated ratio of re-adsorpted benzene is c.a.
55.2% based on integration approach. These evidences suggested that
the guest molecules in 1 can be removed by heating and included again
by absorption.
Declaration of Competing Interest
Toluene was further selected as new target for absorption due to its
similar chemical structure as benzene yet slightly larger geometry.
However, no resonance signals attributed to toluene was observed in
the 1HNMR spectrum of 1c (Fig. S9), suggesting the declined accom-
modation of toluene in 1a. As a conclusion, the solvated benzene in 1
can be released, reincorporated but not replaced by toluene.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgments
This work is supported by a grant for Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education Ministry
(Yu Zhong), Natural Science Foundation of China (No 51679190),
Shaanxi Provincial key research and development Projects (Nos
2018PT-27, 2018 GY-125 and 2019 NY-201) and China Scholarship
Council (CSC201808615022).
3.5.3. Photochromism of the guest-desorbed 1a
The photo-isomerization of 1a was investigated in solid films.
Fig. 10 illustrated the UV spectra of 1a and, interestingly, it showed
reversible spectral changes with color transformation between colorless
and blue. The maximum absorption at 607 nm of PSS is as same as that
of 1. This observation indicates that the dithienylethene backbone in 1a
didn’t collapse after guest liberation. Our previous example showed the
maintained photochromism upon heating to lose of the coordinated
solvent [34]. Herein, the present result exhibits the other effective
sustainability of photochromism after the release of guest.
Appendix A. Supplementary data
Details about additional structure graphics, IR, 1HNMR, and TGA.
Crystallographic data for the structure in this paper has been deposited
9

J. Han, et al.
InorganicaChimicaActa509(2020)119666
with the Cambridge Crystallographic Data Centre, CCDC, 12 Union
Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free
of charge on quoting the depository number CCDC 1953339, 1953341,
1953342, 1953343 (E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk). Supplementary data to this article can be found
online at https://doi.org/10.1016/j.ica.2020.119666.
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