Near-infrared photothermal conversion of stable radicals photoinduced from a viologen-based coordination polymer

Article abstract of DOI:10.1039/d0cc02193h

Upon simple UV-visible irradiation, a viologen-based coordination polymer was transformed to a highly stable radical, protected against quenching by a densely packed structure. The photo-generated radical exhibited a remarkable near-infrared (NIR) phototh

Full text of DOI:10.1039/d0cc02193h

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DOI: 10.1039/D0CC02193H
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Near-infrared Photothermal Conversion of Stable Radicals
Photoinduced from a Viologen–based Coordination Polymer
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
b
a
Shanshan Wang, Shuning Li , Junyu Xiong, Zhengguo Lin, Wei Wei and Yanqing Xu *
DOI: 10.1039/x0xx00000x
www.rsc.org/
1
5
Upon simple UV-visible irradiation, a viologen–based coordination electricity . Recent years have witnessed a rapid development
polymer was transformed to a highly stable radical, protected of viologen-containing coordination polymers, of which many
against quenching by the densely packed structure. The photo- are stimulus-induced with the formation of monocationic
2
0-34
20, 23
22, 23
generated radical exhibited remarkable near-infrared (NIR) radicals
photothermal effect with good recyclability and high conversion properties
efficiency. To our knowledge, this is the first example of crystalline viologen-based coordination polymers have new absorption
and their electronic
, magnetic
and optical
3
3, 34
are dramatically changed. In fact, some chromic
3
5, 36
viologen-based coordination polymer functioned as NIR bands in the NIR region
, offering the possibility suggesting
photothermal material by forming stable radicals.
a potential application in NIR photothermal conversion.
Unfortunately, most of them exhibit reversible chromic
Photothermal materials1-5 can convert absorbed light into heat
by a nonradiative relaxation process . For most applications
22-24, 27-30
behaviors,
and the generated radicals can be readily
6
, 7
quenched by oxygen molecules, in particular at high
temperature. Thus, most radicals in viologen-based
coordination polymers lack enough thermal stability for
8
9
such as photothermal therapy , diagnosis and night vision
1
0
sensors , photothermal materials with absorption in the near-
infrared (NIR) region from 780 nm to 1300 nm have attracted
increasing interest, because NIR light is a preferred source to
penetrate biological tissues and realize dark field viewing .
Conventional NIR-absorbing organic molecules may not only
require tedious synthesis but also readily suffer from
photobleaching under irradiation. These disadvantages lead
to high costs and potential decay of photothermal
performance. Thus, it is essential to develop new agents with
easy synthesis and efficient photothermal capacity. Very
Recently, Yin and co-workers reported the first example of
crystalline coordination polymer as NIR photothermal
material, which contains stable perylenediimide radical anions
photothermal
conversion.
Strikingly,
viologen-based
coordination polymers have not been investigated in NIR
photothermal transfer.
In this work, a viologen-based coordination polymer
3
,4
11
3 3 3 6 3 6 1/3 n
{[La (bcbp) (NO ) O]·[La(NO ) ] } , (1, bcbp = N,N′-bis(3-
1
2
methyl-4-carboxylatophenyl)-4,4′-bipyridinium) with 2-fold
interpenetrated framework was synthesized by a solvothermal
method. Upon simple UV-visible irradiation without
introducing any electron donors, 1 formed the colored radicals
that can stay intact in air even by continuously heating at 100
o
C. Such thermal stability is probably due to the densely
packed structure that protects the photo-generated viologen
radicals from air oxidation, together with the effective
structural separation from polymer network. Based on the
superior stability and distinct NIR absorption, the viologen
radicals exhibited remarkable NIR photothermal effect with
good recyclability and high conversion efficiency. It firstly
shows that viologen-based coordination polymer can be
employed as NIR photothermal material by stabilizing organic
radicals.
Compound 1 was synthesized as yellow crystals by
solvothermal methods from lanthanum nitrate and ligand
2 6
H bcbp·2PF in a mixed solution of acetonitrile and methanol
(ESI†). Single-crystal X-ray analysis reveals that 1 crystallizes in
the trigonal R32 space group. The asymmetric unit contains
two types of La(III) ions, two and two-third nitrate ions, one
1
3
generated through photo-induced electron transfer . Their
work demonstrates that ordered coordination polymers are
promising candidates as high-efficiency NIR photothermal
materials, for the improved stability and simple synthesis,
although additional electron donors of organic amines are still
needed in this system to post-modify the hybrid material.
It has long been known that viologen moiety can accept an
electron from donors to form the highly colored radical by an
1
4-16
17, 18
19
external stimulus such as light
, heat
, pressure or
a. Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key
Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of
Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail:
xyq@bit.edu.cn.
b. Department of Chemistry, Capital Normal University, Beijing 100048, China.
†Electronic Supplementary Information (ESI) available: Synthetic procedures; X-ray
crystallographic details. CCDC 1991648. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Fig. 2. (a) Time-dependent UV-vis adsorption spectra of 1 under irradiation.
Inset: Photographs of crystals before and after UV-vis light irradiation. (b)
EPR spectra of 1 before and after irradiation.
with a slight enhancement in the UV-vis spectrum after
irradiation (Fig. S3, ESI†). This phenomenon implies the highly
ordered structure of 1 may facilitate the process of photo-
induced electron transfer.
Electron paramagnetic resonance (EPR) measurements were
performed to confirm the involvement of photoinduced
radicals in the photochromic process. As shown in Fig. 2b and
Fig. S4 (ESI†), after irradiation strong EPR signals at g=2.0022
Fig. 1. Crystal structure of 1: (a) The coordination environment of Zn(II) ions
in 1 with 30% thermal ellipsoids (all hydrogen atoms and solvent molecules
are omitted for clarity); (b) La
unit of La node; (d) The 3D packing structure with 2-fold interpenetration
filled by La(NO
3
core and bcbp ligand; (c) Basic structural
3
3
-
3 6
) guest (green ball).
zwitterionic bcbp ligand, and one-third oxide ion (Fig 1a). La1
adopting a common nine-coordinate geometry connects one
2 6
for 1R and g=2.0019 for H bcbp·2PF were observed,
respectively. These values are close to that of a free electron
carboxylate group of bcbp ligand, two nitrate ions and a μ
oxygen ion to form an unusual trinuclear La unit with a La…La
distance of 4.339 Å (Fig 1b). Each La unit is bridged by six bcbp 49
ligands, and each bcbp is linked by two La
3
(
2.0023) and similar to those found in viologen complexes,
26-32, 42, 46-
which confirm the existence of viologen free radicals.
3
Thus, the photochromic behavior of 1 could be ascribed to
the photoinduced electron transfer and the generation of
viologen radicals. To get better insight into the electron
transfer mechanisms, X-ray photoelectron spectroscopy (XPS)
of 1 was also measured before and after irradiation (Fig. S5,
ESI†). The O 1s peak shows a slight increase in the relative
intensity at the high-energy region after irradiation, suggesting
the electron loss of some oxygen atoms from carboxylate
3
units, thus forming
1
2
3
a 3D framework with a 4 6 pcu topology (Fig 1c).
Furthermore, two identical but independent sets of networks
interpenetrate to give a 2-fold interpenetrated framework (Fig
1
d). To be noted, the framework is positively charged, which is
3
-
balanced by mononuclear La(NO
)
3 6
guest moieties filled in
the channels, which have been structurally described
previously3 . To achieve the charge balance of whole
structure, after careful analysis, the site occupancy of La2 in
7-39
and/or nitrate, which indicates O atoms could have served as
14, 23, 50
electron donors upon irradiation.
The N 1s spectrum
3
-
3 6
La(NO ) guest moieties lying on a 3-fold axis is determined to
shows two main bands at 401.9 and 406.7 eV, which are
be 1/9 in the crystal structure. Because of the closely packed
structure, there is almost no solvent-accessible volume, which
attributable to the pyridinium and nitrate
respectively. After irradiation, the peaks of the two main
bands are lowered, while a new band at lower energy around
N atoms,
51
4
0
has been estimated by PLATON program . To further confirm
this issue, N adsorption–desorption measurement was
2
4
00.0 eV appears, indicating the formation of monocationic
performed and it did not show obvious interior porosity (Fig.
S2, ESI†).
viologen radical. To further understand the photoinduced
electron transfer process, density functional theory
calculations have been carried out. As shown in Fig. S9, the
majority of the electron distribution of the highest occupied
molecular orbital (HOMO) is located on the carboxyl group,
while that of the lowest unoccupied molecular orbital (LUMO)
is mainly centered on the pyridinum cation. Such electron
distributions on HOMO and LUMO, together with the XPS
results, indicate that the electron-rich benzenecarboxylate and
electron-deficicient pyridinium groups indeed can act as the
As observed in many viologen-based coordination
polymer,2
5, 41
1 undergoes a photochromic transformation with
exposure to UV-vis light at room temperature. Upon irradiated
by a 300 W xenon lamp at room temperature in air, bright
yellow crystals of 1 showed a visible color change to afford
dark brown sample, denoted by 1R. Time-dependent UV-vis-
NIR measurements have been used to explore the
photochromic process (Fig. 2a). In the solid UV-vis-NIR diffuse
reflectance spectra, the pristine 1 shows intense absorption
bands around 380 nm, with shoulder around 450 nm. After
irradiation, 1R exhibits much more strong absorption in the
visible region at around 600 nm, and some new broad
absorption bands at around 717, 948 and 1103 nm in NIR
29
electron donor and acceptor, respectively .
During the coloration process, the UV-vis-NIR and EPR
spectra of 1 and 1R are obviously different, but infrared
spectroscopy and powder X-ray diffraction results both have
no appreciable change (Fig. S6 and S7, ESI†), suggesting no
clear structural transformation or photolysis before and after
irradiation. Besides, thermogravimetric analysis shows that 1
region, which is comparable with other viologen-based
chromic products.4
2-45
Whereas for solids of ligand
2 6
H bcbp·2PF , it only shows a less pronounced color change
2
| J. Name., 2012, 00, 1-3
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can be stable up to ca. 250 °C, indicating a good thermal
stability for practical application of this crystalline material
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DOI: 10.1039/D0CC02193H
(Fig. S8, ESI†).
Interestingly, the photochromic behavior of 1 is irreversible
and the photogenerated radicals in 1R are remarkably stable
under ambient conditions. Experiments demonstrated that the
brown crystals of 1R remained intact in air at room
temperature for several months, and the reverse bleached
o
reaction cannot occur even by continuously heating at 100 C.
The EPR signal of 1R placed at room temperature for two
months is almost identical to that of the freshly irradiated
sample (Fig. 2b), indicating an extraordinary stability of the
viologen-based radicals. Generally speaking, the photo-
generated radicals can be easily quenched by O
bleaching process under air atmosphere.
2
via a
Zhang et al.
recently demonstrated that porous viologen radicals even can
quickly detect O by color change, because internal channels
provid a pathway for O
species can be obtained via embedding the viologen moiety
2
4, 29,25
Fig. 3. (a) Illustration of the radical formation and photothermal conversion
of 1R. (b) Photothermal conversion curves of 1R film on quartz glass under
2
2
5
2
.
By contrast, long lived radical
-2
laser irradiation (2 W cm ). (c) Temperature rises of 1R film at different NIR
laser intensities. Inset shows temperature rise (ΔT) as a function of NIR
-2
4
2
laser power. (d) Photothermal cycling curve of 1R film (1.5 W cm ).
into
a
condensed coordination network.
Another
stabilization factor may be the distance of electron donor and
acceptor. The closest distance between the O atom of
carboxylate group and N atom of bipyridinium in 1 is up to 4.7
Å, while this separation in the crystal structure of N,N′-bis(4-
carboxylatophenyl)-4,4′-bipyridinium, an analogue of ligand
bcbp, is only 3.5 Å.52 This is probably why the tightly packed
structure of pure ligand cannot avoid the rapid bleaching
process of radicals. Thus, the superior atmospheric and
thermal stability of radicals in 1R is not only derived from the
least five times, demonstrating the high stability and durability of
1
R at high temperatures. Taking the relatively low absorbance value
(A
808nm = 0.143) into consideration, the photothermal effect of the
photoactivated 1R is surprising, which implies its high conversion
efficiency for NIR. On the basis of the cooling curve of 1R film (Fig.
S10, ESI†), the conversion efficiency η can be calculated to be 77%
(ESI†), which is a result of the active non-radiative pathways for
viologen radicals. The temperature increase of 1R is large enough in
bio-imaging and biomedical applications such as photothermal
2
densely packed structure preventing the diffusion of O , but
also due to the effective structural separation, which hinder
the electron transfer in bleaching process.
4
,56
therapy.
To our knowledge, this is the first reported NIR
photothermal conversion based on viologen-based coordination
polymer by using stable radicals.
Extending the absorption spectra to NIR region is very important
In summary, a viologen-based coordination polymer with 2-fold
interpenetrated framework was synthesized by a solvothermal
method. After irradiation with UV-vis light, the photogenerated
radicals have extraordinary thermal and atmospheric stability, due
to protection of the densely packed structure against quenching.
Noticeably, the photo-irradiated sample exhibited remarkable NIR
photothermal effect with good recyclability and high conversion
efficiency. This work not only extends the applicaiton of the
viologen-based derivation, but also paves the way toward novel
NIR-active hybrid materials by stabilizing organic radical.
for many applications.8
-13, 53-56
According to the previous literatures,
the broad band at around 600-700 nm is characteristic of viologen
4
2,43
radical absorption,
while longwave NIR absorption at around
9
48 and 1103 nm can be attributed to the radical–radical
4
3-45
interactions of viologen moieties.
Encouraged by the distinct
NIR absorption and extraordinary stability, we further investigated
the properties of 1R as a novel NIR photothermal material (Fig. 3a).
−2
Under irradiation with NIR laser (808 nm, 2 W cm ), crystals of 1R
were heated to 110 °C within 10 s (Supplementary Movie 1),
preliminarily indicating a remarkable photothermal effect. To
accurately measure the photothermal conversion, crystals of 1R
were ground by pestle into powders and then fixed on a piece of
quartz glass to form a uniform film. Under irradiation with 808 nm
This study was supported by the National Natural Science
Foundation of China (Project No. 11474204 and 21271025).
-2
laser light (2 W cm ), the temperature of the quartz glass was
sharply increased to as high as 145 °C, with a temperature rise of
over 120 °C within 200 s (Supplementary Movie 2). By contrast,
under identical irradiation conditions, the temperature of blank
Conflicts of interest
There are no conflicts to declare.
quartz glass rose only 2.0 °C, and the temperature rise of a piece of References
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DOI: 10.1039/D0CC02193H
Near-infrared Photothermal Conversion of Stable Radicals
Photoinduced from a Viologen–based Coordination Polymer
a
a
a
a
b
Shanshan Wang, Shuning Li, Junyu Xiong, Zhengguo Lin, Wei Wei and Yanqing
Xu*a
A highly stable radical photoinduced from a viologen-based coordination polymer exhibited
remarkable near-infrared photothermal effect with good recyclability.