Bistable Solid-State Fluorescence Switching in Photoluminescent, Infinite Coordination Polymers

Article abstract of DOI:10.1002/chem.201701656

Photo-functional infinite coordinated polymers (ICPs) were synthesized that consist of the photochromic dithienylethene (DTE) and a luminescent bridging unit to give enhanced fluorescence in the solid state. We could fabricate well-ordered micropatterns of these ICPs by a soft-lithographic method, which repeatedly showed high contrast on–off fluorescence switching.

Full text of DOI:10.1002/chem.201701656

A Journal of
Accepted Article
Title: Bistable Solid-State Fluorescence Switching in Photoluminescent
Infinitely Coordinated Polymer
Authors: Jincheol Kim, Youngmin You, Seong-Jun Yoon, Jong H. Kim,
Boseok Kang, Sang Kyu Park, Dong Ryeol Whang, Jangwon
Seo, Kilwon Cho, and Soo Young Park
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To be cited as: Chem. Eur. J. 10.1002/chem.201701656
Link to VoR: http://dx.doi.org/10.1002/chem.201701656
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Bistable Solid-State Fluorescence Switching in Photoluminescent
Infinitely Coordinated Polymer
Jincheol Kim,[a] Youngmin You,[b] Seong-Jun Yoon,[a] Jong H. Kim,[c] Boseok Kang,[d] Sang Kyu Park,[a]
Dong Ryeol Whang,[a] Jangwon Seo,[a] Kilwon Cho,[d] and Soo Young Park*[a]
Abstract: Photo-functional infinitely coordinated polymers (ICPs)
consisting of the photochromic dithienylethene (DTE) and the
luminescent bridging unit to give enhanced fluorescence in the solid
state were synthesized. We could fabricate well-ordered
micropatterns of these ICPs by a soft-lithographic method, which
repeatedly showed high contrast on-off fluorescence switching.
Incorporation of photoluminescent ligands into the
coordination networks of ICPs provides a useful way to the
preparation of solid-state luminescent materials. The ability to
locate photoluminescent ligands within the multidimensional
space is advantageous over conventional organic polymers that
suffer from detrimental photoluminescence quenching due to
nonspecific inter-chain interactions in the solid state. Despite this
advantage, demonstration of fluorescent ICPs has seldom been
reported.[7] However, we could recently report fluorescent
micropatterns of ICPs using a soft-lithographic micromolding in
capillaries (MIMIC) technic, which is a simple yet very effective
method for nano-/micro-structures fabrications.[8] The ICP is
comprised of diamagnetic zinc ions and bridge ligands having 4-
pyridyl moieties at both termini. Fluorescence intensity of the
ICP was much stronger than that of the ligand solution, which
was attributed to the suppressed vibronic relaxation due to
coordination-induced restriction of intramolecular motions.
Having previously demonstrated the promising utility of
fluorescent ICPs, we sought to extend their capability by
incorporating the second photofunctional ligands into the ICP
backbones in this work. This approach is reminiscent of
statistical copolymers. We chose photochromic DTE compounds
as the second ligands to demonstrate an all-optical solid-state
fluorescence memory, because the photochromism of the DTE
compounds features many advantages, including high thermal
stability, good fatigue resistance, and excellent reversibility.[9]
The actual photochromism involves optical interconversion
between the open and closed forms of DTE, where the latter
exhibits strong absorption in the visible region ( > 104 M1 cm1).
Thus, fluorescence quenching by interligand energy transfer
Metalligand coordination provides
a unique strategy to
construct functional polymers, as the coordination geometry
directs facile formation of well-defined, multidimensional
networks.[1] Metalorganic frameworks (MOFs) are the most
prominent material class among the coordination polymers,
because their reticular inner structures render interesting
functions such as storage or separation of guest molecules.[2]
However, MOFs do not carry the full merit of the metalligand
coordination because the brittle structures are seldom
compatible with solution processes. In addition, the prevalent
solvothermal synthetic method to attain MOFs manifests a
severe limitation, i.e., difficulties for finding ligand structures that
can be incorporated into the polymeric structure. On the other
hand, ICPs with reversible coordination bonds are considered as
promising alternatives to MOFs, in the context of versatility and
processibility.[3] ICPs can readily be prepared under mild
condition, and their synthetic procedures feature excellent
flexibility in the selection of a wide range of metal ions and
bridging ligands. The latter feature is particularly beneficial
because it allows broad control over physical and chemical
properties. Indeed, several classes of ICPs have been prepared
and demonstrated for potential use in catalysis,[4] drug delivery,
[5] and sensing.[6]
(most likely by a Förster resonance energy-transfer mechanism
[9b-d, 10]
as reported before)
from the fluorescent ligand to the
closed form of the DTE ligand is anticipated. Photochromic
cycloreversion to the open form of DTE restores the strong
fluorescence, completing the fluorescence memory cycle. It
should be noted that the individual operation of writing, reading,
and erasing can be done in an all-optical manner with minimal
crosstalks. This can be led from the negligible absorption cross-
section of both closed and open forms of DTE at the
photoluminescence excitation wavelength of the aggregation-
induced enhanced emission (AIEE) type bridge ligand (430 nm).
Furthermore, the solid-state memory retains the full advantage
of ICPs, as it can be reconfigured using solution processes to
prepare various bulk structures, such as films and micropatterns
(see Scheme 1). To the best of our knowledge, this is the first
report on bistable solid-state fluorescence memory based on
ICPs synthesized by non-solvothermal synthetic method. Herein,
we report the design and synthesis of novel ICPs containing zinc
ions, fluorescent (Z)-2,3-bis(4-(4-pyridinyl)phenyl)acrylonitrile (L1)
[a]
J. Kim, Dr. S. -J. Yoon, Dr. J. H. Kim, Dr. S. K. Park, Dr. D. R.
Whang, Dr. J. Seo, Prof. S. Y. Park*
Center for Supramolecular Optoelectronic Materials and
Department of Materials Science and Engineering
Seoul National University
Daehak-dong, Gwanak-gu, Seoul, Korea
E-mail: parksy@snu.ac.kr
Prof. Y. You
[b]
[c]
Division of Chemical Engineering and Materials Science
Ewha Womans University
Seoul 03760, Korea
Prof. Jong H. Kim
Department of Molecular Science and Technology,
Department of Applied Chemistry and Biological Engineering,
Ajou University
Suwon 443-749, Korea
B. Kang, Prof. K. Cho
Department of Chemical Engineering
Pohang University of Science and Technology
Pohang 790-784, Korea
[d]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201701656
Chemistry - A European Journal
COMMUNICATION
these species are 1.03%, 1.92%, 6.3%, and 5.35%, respectively,
for ICP1. To quantify ligands, each component is calculated by
counting elements such as N(1s) or F(1s) as shown in
supporting information. It should be noted that corresponding co-
monomer ratio for (L1 + L3) / Zn is determined to be 2.07 which
implies ICP1 possess 2D or 3D alternating (zinc-ligand)
backbone structures similar to our previous ICP comprising
solely L1 ligand. The authors argue that formation of the
multidimensional structure is allowed by the weakly coordinating
perchlorate ion of zinc.[8] ICP2 exhibits a co-monomer ratio for
(L2 + L3) / Zn = 0.94 and these values suggest that the ICPs
possibly contained 0D or 1D wire coordination structures (SI,
Table S1). Since the thiophene moiety does not compete with 4-
pyridine moiety for the coordination polymerization, it is most
likely that the lower dimensional characteristic of ICP2 is caused
by non-linear ligand geometry caused by substitution of
thiophene instead of pheynyl moiety which could hinder the
formation of higher multidimensional network. It is also
noteworthy to mention that the co-monomer ratios in these
pristine powders of ICP1 and ICP2 are lower than those in the
regenerated MIMIC patterns (vide infra) attributed to the different
crystallization condition as mentioned later.
Treatments of the ICP powders with an excess amount of
pyridine led to the complete de-coordination of the bridging
ligands. The coordination bonds would then be recovered by
selective removal of pyridine, due to reversible coordination. We
combined this depolymerization-repolymerization principle and
the MIMIC technic. Since ordering in supramolecular structures
and bulk morphology is crucial for attaining enhanced
optoelectronic properties, such as high carrier mobility and
fluorescence quantum yields of solid-state devices, the
combined approach is expected to provide a unique advantage
for fabrication of the microscale patterns with three dimensional
coordination structures.[8] To fabricate micropatterns, pyridine
solutions (1000 mg) containing 20 mg of ICP1 or ICP2 were
prepared. The solutions were injected into the rectangular
capillary channels which were formed by conformal contact
Scheme 1. Illustration of energy transfer concept of ICP1 and ICP2 (not the
proposed structure of ICP), and the schematic representation of the fabrication
of micropatterns of ICP by the MIMIC method. Inset images on the bottom
show the FE-SEM and AFM images of the micro-line/space patterns of ICP1.
(scale bar = 20 m)
(Z)-2-(4-(4-pyridineyl)phenyl)-3-(5-(pyridine-4-yl)thiophen-
2-yl) acrylonitrile (L2) ligands, and a photochromic 1,2-bis(2-
methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene (L3) ligand
(Supporting Information (SI), Scheme S1-S3).
The AIEE-active (L1) and photochromic DTE (L3) ligands
were synthesized according to the method described previously
in the literature.[11] To maximize the interligand energy transfer
efficiency, we also synthesized AIEE-active L2 ligand by
substituting the phenyl ring in L1 with thiophene, which is
responsible for a red-shifted fluorescence emission of ICP2. In
these ligand structures, Lewis-basic moieties were kept identical
to be 4-pyridine to guarantee even incorporation of the ligands
into the polymer networks. All compounds were fully
characterized by 1H and 13C NMR spectroscopy, MS
spectrometry, and elemental analysis (SI).
between
a negatively pre-patterned poly(dimethylsiloxane)
(PDMS, Sylgard 184, Dow Corning) mold and a glass substrate.
At the inner walls of the capillaries, pyridine solvent was
selectively absorbed by the PDMS mold, regenerating and
furnishing ICPs inside the mold channels.[8] Peeling off the mold
from the glass substrate after full pyridine absorption into PDMS
mold produced micro-patterned coordination polymer wires
arrays (Scheme 1).
Field-emission scanning electron microscope (FE-SEM) and
atomic force microscope (AFM) images reveal the flat surface of
the ICP micropatterns with negligible shrinkage (SI, Figure S2).
The patterned ICPs were insoluble in dichloromethane and
chloroform, whereas ligand micropatterns prepared through the
identical MIMIC technique (i.e., no zinc ions) were dissolved out
by those solvents (SI, Figure S3). Unidirectional property of a
patterned wire is further evaluated from the strong birefringence
observed using an optical microscope under a cross-polarized
condition (SI, Figure S3). Assays based on XPS data revealed
retention of multidimensional structures (i.e., 2D net; SI, Table
S1). Interestingly, the ligand/Zn ratios (2.65 for ICP1) for the
MIMIC micropatterns were significantly larger than that of the as-
ICP1 was prepared by slowly adding 1 mL of 200 M
Zn(ClO4)2 (MeOH) into
a stirred 10 mL CH2Cl2 solution
containing 100 M L1 and 100 M L3. The mixture was then
stirred for additional 12 h. The supernatant was removed by
decantation, and the powders were collected by filtration and
thoroughly washed with dichloromethane and water to remove
residual ligands and zinc ions. ICP2 was obtained through the
identical procedure, employing 100 M L2 instead of L1. FTIR
data for ICP pellets display absorption bands at 1614 cm1 due
to zinc-coordinated pyridine, which is distinct from that (1595
cm1) of free pyridine (SI, Figure S1).[12] Incorporation of the zinc
ions in the ICP backbones was verified by XPS analysesas
reported previously.[8] The XPS data display sharp peaks due to
Zn(2p), S(2p), F(1s), and N(1s), where the relative abundance of
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Figure 2. PL spectra of a) ICP1 and c) ICP2 in the open (open circles) and
closed (filled circles) DTE states, and photoluminescence intensities of b)
ICP1 and d) ICP2 upon repeative exposure of the photoexcitation beam (ex
=
430 nm). Inset graphs show the photochromic modulation of
photoluminescence of ICP1 and ICP2 films. e) photo-switchable fluorescence
images of the micropatterns of ICP1 (blue) and ICP2 (orange) (Scale bar = 40
m).
Figure 1. 2D GIXD of coordination polymer patterns of ICP1 in a) the vertical
direction and b) horizontal direction. c) 1D out-of-plane (green line) and 17°
direction (red line) X-ray profile extracted from lines of (a). d) 1D out-of-plane
(green line) and in-plane (red line) X-ray profile extracted from (b). e)
Representation of a possible coordination geometry which can be predicted
from the 2D GIXD results. Nitrogen and zinc are represented by blue and
orange atoms. Counteranions and hydrogen are omitted for clarity.
crystalline feature (Figure 1b and 1d). The notable difference
appears at 1.75 Å-1 at the in-plane direction (a d-spacing of 3.8
Å), which can be attributed to 훑-훑 stacking of the conjugated
ligands. Considering all the information described above and the
supporting information as well as the calculation results using
the principles of quantum chemistry (SI, Figure S5), we could
suggest a most probable structure of patterned ICP 1. As shown
in Figure 1e, photochromic ligand L3 is placed between the
parallelogram structure of ligand L1. In case of ICP2, we were
unable to get the strong diffraction peak from GIXD. This should
originate from the lower multidimensional feature compared to
ICP1 which is consistent with the lower co-monomer ratio from
XPS data as we mentioned previously. Because ICP2 has the
virtually similar length of ligand molecule, insolubility of pattern,
and birefringence in the pattern of ICP1, we may suppose the
lower multidimensional characteristic is caused by non-linear
geometry of ligand L2.
Before the investigation of the fluorescence switching
properties, we performed the structure analyses on the ICPs in
the regenerated film state. We prepared 2 wt% ICP1 and ICP2
solutions in pyridine and fabricated photochromic fluorescent
ICP films by spin-coating (700 rpm for 60 s). FTIR spectra for
the films display strong absorption peaks at 1616 cm1,
indicating retention of Znligand bonds (SI, Figure S7).[12] XPS
analyses for the Zn(2p), S(2p), F(1s) and N(1s) elements in
ICP1 and ICP2 reveal comonomer ratios of (AIEE ligands + DTE
ligand) / zinc ions to be 2.40 and 1.15, respectively. These
values correspond to multidimensional structures that involve 3D
prepared powders (2.01) which suggests that the micro-
patterned ICPs possess higher dimensional coordination
structures. This presumably originates from controlled
repolymerization inside the capillary channels, providing longer
crystallization time in a confined space to give highly-oriented
coordination polymer.
We measured 2D grazing incidence X-ray diffraction (2D
GIXD) of the patterned ICP1 to obtain the detailed structural
information. Different from the previously reported jungle gym-
like cubic structure composed solely of a fluorophore (L1) and
zinc,[8] the patterned ICP1 showed crystalline anisotropy
depending on measuring directions. When the X-ray beam is
injected for a perpendicular direction to the line patterns (Figure
1a), strong (00k) diffraction peaks appear along the out-of-plane
direction up to the 5th order, which corresponds to the d-spacing
value of 21.7 Å. This value matches well with the length of ligand
L1 (SI, Figure S4). Furthermore, the 2D GIXD pattern at the
perpendicular direction shows the unique oblique parallelogram
structure with an angle of ~17° with respect to the substrate. The
1D diffraction intensity profile extracted at ~17° direction displays
two peaks at q = 0.48 and 0.94 Å -1, in which the length value of
13.1 Å calculated from these peaks is shorter than that of the
used ligands. The 2D GIXD pattern of the patterned ICP1
measured at the horizontal direction showed apparently different
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10.1002/chem.201701656
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= 0.69) calculated by η = 1 – off / on (off and on are the average
fluorescence lifetimes of the closed and open form of ICPs). The
Förster radii calculated for the former and the latter pairs are R0
= 14.82 Å and 12.58 Å for ICP1 and ICP2, respectively. Since
the distances between L3 and L1 (or L2) in ICPs (R = ~3.6 Å,
calculated using B3LYP/6-31G**) would be short enough
compared to the Förster radii (R/R0 = ~0.26), the occurrence of
highly efficient interligand energy transfer is anticipated. It
should be noted that we could not decide the specific value of
the orientation factor explicitly even we could obtain the
suggested structure of ICPs through XPS and GIXD. Therefore,
we assumed our ICPs has random orientation. The Förster radii
calculated for the former and the latter pairs are R0 = 14.82 Å
and 12.58 Å for ICP1 and ICP2, respectively. Since the
distances between L3 and L1 (or L2) in ICPs (R = ~3.6 Å ,
calculated using B3LYP/6-31G**) would be short enough
compared to the Förster radii (R/R0 = ~0.26), the occurrence of
highly efficient interligand energy transfer is anticipated.
or 2D nets for ICP1 and 1D or 2D nets for ICP2 (SI, Table S2)
which are similar to but somewhat different from those of the
micropatterns. It is noted that the co-monomer ratios in these
thick films are slightly lower than those in the micro-patterned
structures, probably due to the faster repolymerization process
compared to that in the confined environment (inside of the
PDMS mold channels).
The open and closed forms of the DTE photochrome for
fluorescence switching in ICP patterns were generated by
irradiating visible ( > 550 nm) and UV ( = 365 nm) light,
respectively. The cyclization yields for the photo-stationary state
(PSS) under irradiation with 365 nm light was estimated to be
38% and 34% for ICP1 and ICP2, respectively, as determined
1
by H NMR spectroscopy (SI, Figure S7). The photochromism
for the open and closed states of ICPs were completely
o→c
reversible with distinct quantum yields (ICP1: Φpc
= 0.027,
c→o
o→c
c→o
In summary, we have designed and synthesized novel
Φpc
= 0.0013; ICP2: Φpc
= 0.011, Φpc
= 0.001), similar
to those of reported DTE compounds.[9a, 13] Strong fluorescence
reconfigurable ICPs which are capable of optical switching of
solid-state fluorescence. The ICPs contained two types of
bridging ligands with different function, i.e., photochromic and
AIEE-fluorescent ligands. Well-ordered crystalline micropatterns
were fabricated by taking advantage of the MIMIC method and
the reversible nature of the coordination bonds. The
multidimensional superstructures in the ICP micropatterns were
analysed using various spectroscopic techniques, including 2D
GIXD. The ICP micropatterns exhibited high contrast, reversible
modulation of photoluminescence emission, enabling true solid-
state all-optical memory. Steady-state and transient PL studies
indicated that the photoluminescence modulation was due to the
interligand energy transfer.
emission was observed with the open forms of the ICPs (ICP1:

em = 470 nm, PLQY = 0.18; ICP2: em = 577 nm, PLQY = 0.19)
when photoexcitation at 365 nm was provided, whereas
fluorescence was barely detected with closed form of the ICPs
(Figure 2). The large fluorescence quenching in the closed forms
is likely attributed to the fluorescence resonance energy transfer
(FRET) from the AIEE ligand to the closed form of the
photochromic ligand having significantly low energy absorption
band (vide infra; ligand centered absorption of L3; abs = 613
nm; SI, Figure S8). Corresponding fluorescence on/off ratios for
ICP1 and ICP2 are 24 and 47, respectively. Most intriguingly, as
shown in the graphs of Figure 2b and 2d, the fluorescence
intensities of these two isomeric states could remain completely
unaltered, if the excitation is made by the non-destructive read-
out beam (ex = 430 nm). These results demonstrate that the Acknowledgements
ICP micropatterns are promising for optical memory materials
(Figure 2e).
To gain information about the energy transfer processes,
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP;
No. 2009-0081571[RIAM0417-20150013])
steady-state and transient PL were studied. UVvis absorption
and fluorescence spectra of 20 M ligand solutions were
recorded, which revealed fairly good spectral overlaps between
the absorption spectrum of the closed form of L3 and the
fluorescence spectra of L1 and L2 ligands (SI, Figure S9).
Spectral overlap integrals for the pair of L2 and L3 is 4.73 ×
1014 M-1 cm-1 nm4, while that of L1 and L3 is relatively small (1.4
× 1014 M-1 cm-1 nm4). This ordering is consistent with the
fluorescence on/off ratios of the ICP micropatterns. We also
monitored the occurrence of energy transfer by recording
fluorescence decay traces of the ICP films in their open and
closed forms (SI, Figure S10). The decay traces followed a
multi-exponential kinetics due to heterogeneity of the ligands in
the ICP networks. We found that the average fluorescence
lifetimes () of ICP1 decreased from 0.83 ns to 0.26 ns upon
photocyclization, while that of ICP2 displayed a larger decrease
(1.37 ns to 0.33 ns). The rate constants for energy transfer were
calculated using the  values (kET = 1/closed  1/open; kET = 2.64
×109 s1 and 2.30×109 s1 for ICP1 and ICP2), assuming that
the photophysical processes inherent to L1 and L2 were
unaffected by the photochromism of L3. It is noted that the rate
constants for energy transfer are much larger than those for
radiative and non-radiative decay rates (SI, Figure S10).The
faster energy transfer in ICP2 can be explained on the basis of
the higher FRET efficiency (ηICP2 = 0.76) than that of ICP1 (ηICP1
Keywords: photochromism • coordination polymers •
nanostructures • fluorescence • micromolding
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