Substituent-Modulated Assembly Formation: An Approach to Enhancing the Photostability of Photoelectric-Sensitive Chalcogenide-Based Ion-Pair Hybrids

Article abstract of DOI:10.1021/acs.inorgchem.6b03061

A series of electronically active viologen dications (RV) with tunable substituent groups were utilized to hybridize with [Ge₄S₁₀]^4- (T2 cluster) to form the hybrids of T2@RV. These hybrids exhibited variable supermolecula

Full text of DOI:10.1021/acs.inorgchem.6b03061

Communication
pubs.acs.org/IC
Substituent-Modulated Assembly Formation: An Approach to
Enhancing the Photostability of Photoelectric-Sensitive
Chalcogenide-Based Ion-Pair Hybrids
Jian Lin,† Zhixing Fu,† Jiaxu Zhang,† Yujia Zhu,† Dandan Hu,† Dongsheng Li,‡ and Tao Wu*,†
†College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China
‡College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key
Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, Hubei
443002, China
S
* Supporting Information
indicator or stabilizer.9−15 Moreover, it is very easy to
ABSTRACT: A series of electronically active viologen
systematically construct inorganic−organic hybrids through
alteration of the substituted group of viologen.16−18 Actually,
dications (RV) with tunable substituent groups were
utilized to hybridize with [Ge4S10]4− (T2 cluster) to form
viologens are likely to hybridize with metal chalcogenides
because of their strong ion-pair interactions.19,20 For example,
the hybrids of T2@RV. These hybrids exhibited variable
supermolecular assembly formation, tunable optical
absorption properties, and different photoelectric response
under the influence of different RV dications. Raman
testing and time-dependent photocurrent response in-
dicated that the photosensitivity and photostability of
T2@RV could be integrated while choosing suitable RV
dications. Current research provides a general method to
build a tunable hybrid system based on crystalline metal
chalcogenide compounds through the replacement of
photoinactive cationic organic templates with photoactive
ones with different substituent groups.
influenced by electronically active viologen, hybrid chalcogeni-
dometalates show dramatic red shifts in their absorption spectra
as well as improved stability compared with traditional-metal
chalcogenides.19,21
Generally, the inorganic parts in the reported hybrid CMCs
are either electrically neutral or negatively charged. They are
usually protected by covalently bonded organic ligands, which
are detrimental to the electron-related process. However, for the
negatively charged metal chalcongenide nanocluster without a
covalent organic ligand, they are vulnerable to external attacks,
such as light illumination and/or oxidation.22,23 In order to
simultaneously reserve the electric sensitivity and stability of
electronegatively charged metal chalcogenide clusters, well-
designed photoactive countercations are necessary. As one of the
well-known negatively charged semiconductor clusters with high
stability and solubility in water, [Ge4S10]4− was ever selected as a
secondary building unit for the construction of many
hybrids.24−28 Recently, Dai’s group reported three hybrid
thiogermanates based on methylviologen cations
([MV]2(Ge4S10)·Sol (denoted as T2@MV), which show
solvent-induced color changes and switchable fluorescent
emission as well as photoelectric responsive properties.29
However, the photoelectric response of T2@MV exhibits a
clear decay even in the 400 s interval irradiation. The
combination of the photoelectric sensitivity of hybrids and
photostability is of fundamental importance to endow them with
potential applications in optoelectronic devices.
norganic−organic hybridization is one of the most effective
Imethods to functionalize inorganic and/or organic units
through synergistic interactions. During the past decades, a large
number of hybrid crystalline metal chalcogenides (CMCs) have
been synthesized through a hybridization strategy. Such hybrid
CMCs with structural diversity and tunability of the composition
and electronic structure exhibit potential and practical
applications in photoluminescence, as catalysts, as sensors, in
ion conducting, and so on.1−8 It should be noted that most of the
CMCs with variable geometry structures as well as tunable
electronic structures contain organic templates and/or structure-
directing agents (SDAs). These templates in the CMCs only
functioned as either charge-balancing or space-filling agents.
Poor electric activities of SDAs to some extent deteriorate the
photoelectric properties of these solid-state CMCs. Intentionally
integrating electronically active organic components into CMCs
is an alternative method to further ameliorate their electric
properties. In order to stabilize the negatively charged inorganic
components and integrate the organic functional unit, positively
charged electronically active organic molecules are welcomed in
most of the electron-rich CMCs. Viologens are well-known
electronically active organic molecules that are frequently used as
a cation electron acceptor to prepare inorganic−organic hybrids.
These hybrids generally exhibit some novel properties, such as
electro/photochromism, ferroelectricity, and electron-transfer
In this manuscript, we applied electron-poor viologen
dications with different substituent groups (denoted as RV; the
synthetic method is shown in Scheme S1 and the Supporting
Information, SI) to construct a series of [Ge4S10]4− cluster (T2)-
based hybrid compounds (denoted as T2@RV). Under the
influence of steric hindrance from RV with different substituent
groups (MV = methyl viologen, EV = ethyl viologen, n-BV =
butyl viologen, AV = amyl viologen, and BV = benzyl viologen),
Received: December 18, 2016
Published: February 28, 2017
© 2017 American Chemical Society
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T2@RV compounds exhibit different assembly formation and
diverse photoelectric sensitivity and photostability. Suitable
substituent groups in T2@RV finally give rise to combined
photoelectric sensitivity and photostability in hybrid chalcoge-
nidometalates.
1b,c). Abundant free space in the sublattice framework is filled
with five other types of sublattice frameworks (Figure 1d). The
S···N distance in the two closest sublattices is 4.327 Å (Figure
S4).
When the alkyl chain length of RV increases or a more rigid
benzene ring is added on the RV, the hybrids of T2@AV and
T2@BV are obtained, respectively. Direct mixing of an aqueous
solution of the T2 cluster and an AV dication salt can lead to the
formation of crystals of T2@AV in a few seconds. By contrast,
the reaction of T2 and BV ions in the aqueous solution is much
faster, which only produces crystalline powders. Crystals of T2@
BV are obtained by a diffusion method (see the SI). Hybrids of
T2@AV and T2@BV crystallize in the same tetragonal space
group (P42/n) with T2@n-BV. They also exhibit an assembly
formation similar to that of T2@n-BV. The anionic T2
nanocluster is surrounded by four AV and BV molecular
dications in T2@AV and T2@BV, respectively. Also, the S···N
distances between the T2 cluster and AV and BV dications are
3.493 and 3.773 Å, respectively (Figures S5a and S6a). The T2
clusters could also interact with AV and BV molecular cations to
form a 3D supermolecular framework (Figures S5 and S6).
Densely packed ion-pair hybrids of T2@AV and T2@BV could
be realized by interpenetrating six sublattices of the 3D
supermolecular framework. In contrast to T2@n-BV, the S···N
distance between the two closest sublattices in T2@AV and
T2@BV is further increased (Figures S5e and S6e). Although the
ion-pair interactions between neighboring sublattice frameworks
in T2@AV and T2@BV are much weaker than those in T2@n-
BV, the long-alkyl-chain and rigid substituent in AV and BV
could provide more steric hindrance to anchor the flexible
sublattice framework, which may be responsible for the good
quality of the crystallinity.
Crystalline ion-pair hybrids of T2@RV were prepared at room
temperature (see the SI). The structure refinement results are
summarized in Table S3. Single-crystal X-ray diffraction (XRD)
analyses show that T2@RV consists of a discrete [Ge4S10]4−
cluster and two RV dications. Because of different substituent
groups on the RV, T2@RV exhibits variable assembly formation.
For hybrids of (MV)2·[Ge4S10] (now denoted as T2@MV-1 and
T2@MV-2), they have structure packing patterns similar to that
reported in the literature.29 As shown in Figure S1, two MV2+
cations look like “electron locks” around the corner of the T2
nanocluster in T2@MV-1, and strong ion-pair interactions exist
between the [Ge4S10]4− cluster and MV2+ because the S···N
distance (3.479 Å) is short. However, such MV2+ cations show
weak interaction (the S···N distance is 4.001 Å) with neighboring
T2 clusters in the crystal lattice. Compared with T2@MV-1,
T2@MV-2 exhibits lower structure symmetry and weaker ion-
pair interactions (Figure S2). With an increase of the alkyl chain
length in RV, T2@EV and T2@n-BV exhibit poor crystallinity
during the supermolecular self-assembly process. Single-crystal
structure analysis indicates that T2@EV shows low symmetry
and weak ion-pair interaction as well (Figure S3). As shown in
Figure 1, a negatively charged T2 cluster is surrounded by four n-
BV dications. The S···N distance between each terminal sulfur
atom in the T2 cluster and the positively charged nitrogen center
in n-BV is 3.454 Å (Figure 1a). Four such n-BV dications further
interact with four other T2 clusters to pack into a 3D
supermolecular framework through ion-pair interactions (Figure
The phase purity of T2@RV was examined by powder XRD
measurements by comparing the XRD patterns of as-synthesized
samples with the simulated ones from single-crystal data (Figure
S7). UV−vis diffuse-reflectance spectroscopy (DRS) measure-
ments were also performed to determine the optical band gap.
Owing to the influence by different substituents, the optical
absorption edge of RV varied from 2.75 to 3.35 eV (Figure S8).
The optical absorption edge of T2@RV exhibits a large red shift
compared with the pristine T2@PR (PR is the abbreviation of
piperidine), which may be explained by the introduction of
electronically active viologen and charge-transfer-related absorp-
tion (Figure 2). The charge-transfer band gap of T2@RV
decreased from 2.50 to 1.75 eV with an increase of the alky chain
length of RV. For T2@BV, the band gap is 2.25 eV.
Besides the optical absorption properties, the photoelectric
conversion properties as well as the photostability of T2@RV
were also investigated in a photoelctrochemical cell with a three-
electrode setup (a more detailed description is given in the SI).
As presented in Figure S9, the background photocurrent of
indium−tin oxide (ITO) is 0.04 μA/cm3, which is far below the
value of a T2@RV-modified ITO working electrode. While the
biased potential is set at 0.5 V, the T2@MV-1-decorated
photoelectrode generates a good photocurrent, comparable with
the results reported in the literature (Figure 3).29 However, the
photocurrent density of T2@MV-1 exhibits an observable
decrease after 30 cycles in 600 s. After long-time irradiation,
the photocurrent density of T2@MV-1 was reduced to one-third
of its original value (inset in Figure 3). As for the T2@AV-
modified photoelectrode, it exhibits photostability similar to that
of T2@MV-1 (Figure S10). When the alkyl group was replaced
by the benzyl group, the photostability of T2@BV was
Figure 1. (a) Interaction between the T2 cluster and four n-BV
dications. (b) Packing structure of an ion-pair hybrid of T2@n-BV
viewed from the b axis. (c) One type of supermolecular sublattice
framework of T2@n-BV viewed from the [110] direction. (d) 6-fold
crossed supermolecular framework of T2@n-BV viewed from the [110]
direction.
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clusters, higher voltage needs to be applied on the T2@RV-
decorated photoelectrode in order to produce the appropriate
photocurrent. Nevertheless, the relatively short alkyl group, such
as the ones appearing in T2@MV-1, T2@EV, and T2@n-BV,
cannot provide enough steric hindrance to resist photo-
degradation. A moderate charge-transfer pathway as well as the
effective protection on the optically active units throughout the
whole crystal lattice is very helpful for the real application of light-
stimuli-responsive materials. In our case, a hybrid ion-pair
compound of T2@BV is a potential candidate as a light-sensitive
material because it can not only provide a moderate photocurrent
but also offer well protection on the optically active unit
[Ge4S10]4− clusters.
In summary, a series of ion-pair hybrids of T2@RV have been
synthesized by integrating electronically active organic viologen
dications with [Ge4S10]4− cluster-based chalcogenidometalates.
Under the influence of RV dications with varied substituent
groups, T2@RV compounds exhibit different supermolecular
assembly formation and tunable optical absorption properties
and different photosensitivity and photostability. This work
provides a general method to stabilize and explore optically active
units in a variety of chalcogenide cluster-based anionic structures
through the replacement of electronically inactive organic
templates by active viologen dications derivatives.
Figure 2. Tauc plots of ion-pair hybrids T2@PR and T2@RV derived
from UV−vis DRS.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b03061.
X-ray crystallographic data in CIF format (CIF)
General methods, tables for the results of single-crystal
data and elemental analysis, extra figures, powder XRD,
absorption as well as photocurrent response, and Raman
spectra (PDF)
Figure 3. Photocurrent responses of T2@MV-1 (black line) and T2@
BV (red line) in light on-off irradiation with applied potentials at 0.5 and
0.8 V, respectively. Inset: Photoelectric response of T2@MV-1 after
long-time irradiation.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wutao@suda.edu.cn.
■
significantly improved. As shown in Figure 3, the photocurrent
repeatability of T2@BV was proven by not only the 30 cycles of
light on−off processes but also the continuous long-time
irradiation. By increasing the applied potential on the T2@BV-
modified photoelectrode from 0.5 to 0.8 V, the photocurrent
density of T2@BV increased as well (Figure S10b). Moreover,
the photostability of the T2@RV samples were also verified by
Raman testing. As shown in Figure S11a, the Raman signals of
T2@PR, T2@AV, and T2@BV are clearly detectable under the
785 nm laser irradiation (class IIIB laser, 5−500 mW). The
signals from an optically active inorganic T2 cluster in the short-
wavelength region are distinguished from the ones from the AV
and BV organic dications in the high-energy region.24 However,
the Raman signal of T2@MV-1 totally disappeared under
excitation of the 785 nm laser (Figure S11b). An obvious color
change is also observed, indicating that it decomposed instantly
upon exposure to laser irradiation (inset in Figure S11b).
ORCID
Dongsheng Li: 0000-0003-1283-6334
Tao Wu: 0000-0003-4443-1227
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Natural
Science Foundation of China (Grants 21271135 and 21671142),
Jiangsu Province Natural Science Fund for Distinguished Young
Scholars (BK20160006), a start-up fund (Q410900712) from
Soochow University, the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions, Young Thousand
Talented Program, and a graduate student fund
(KYZZ15_0323) from Scientific Research Innovation Projects
of Jiangsu Province.
On the basis of the above results, it is obviously observed that
the photosensitivity and photostability of hybrids of T2@RV are
greatly influenced by the structure diversity. The potential
charge-transfer efficiency can be regulated by the potential
charge-transfer pathway, i.e., the nearest S···N distance in the
T2@RV, which is influenced by the length of the substituent
groups. For a longer S···N distance between the RV and T2
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