Photoresponsiveness of Anthracene-Based Supramolecular Polymers Regulated via a σ-Platinated 4,4-Difluoro-4-bora-3a,4a-diaza- s-indacene Photosensitizer

Article abstract of DOI:10.1021/acs.inorgchem.9b02073

Anthracene and its derivatives have attracted tremendous interest in recent years because of their intriguing photoresponsive behaviors. Our research group has previously constructed anthracene-based supramolecular polymers, which display multicycle anthracene-endoperoxide photoswitching in a macroscopic manner. However, high-energy light excitation (λ = 365-460 nm) is required for anthracene-to-endoperoxide photooxygenation, giving rise to severe photodegradation problems. In this work, we have developed an effective approach to addressing this issue, by encapsulating a σ-platinated 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) photosensitizer into anthracene-based supramolecular polymeric systems. The platination effect enhances π-electron delocalization, while promoting intersystem crossing from singlet to triplet excited states. Accordingly, the σ-platinated BODIPY photosensitizer displays excellent ¹O₂ production capability, facilitating anthracene-to-endoperoxide transformation under low-energy irradiation conditions (λ = 520-590 nm). This leads to the breakup of supramolecular polymers and gels, which can be restored at room and elevated temperatures because of the reversible endoperoxide-to-anthracene deoxygenation process. Overall, the rational design of a σ-metalated photosensitizer opens up a new avenue to regulating the photoresponsiveness of supramolecular polymers under mild and nondestructive conditions.

Full text of DOI:10.1021/acs.inorgchem.9b02073

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Photoresponsiveness of Anthracene-Based Supramolecular
Polymers Regulated via a σ‑Platinated 4,4-Difluoro-4-bora-3a,4a-
diaza‑s‑indacene Photosensitizer
†
†
†
†
†
,‡
Yifei Han, Mingyang Liu, Ruolei Zhong, Zongchun Gao, Ze Chen, Mingming Zhang,*
,
†
and Feng Wang*
†CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and
Technology of China, Hefei, Anhui 230026, P. R. China
‡
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China
*
S Supporting Information
ABSTRACT: Anthracene and its derivatives have attracted
tremendous interest in recent years because of their intriguing
photoresponsive behaviors. Our research group has previously
constructed anthracene-based supramolecular polymers,
which display multicycle anthracene−endoperoxide photo-
switching in a macroscopic manner. However, high-energy
light excitation (λ = 365−460 nm) is required for anthracene-
to-endoperoxide photooxygenation, giving rise to severe
photodegradation problems. In this work, we have developed
an effective approach to addressing this issue, by encapsulating
a σ-platinated 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(
BODIPY) photosensitizer into anthracene-based supra-
molecular polymeric systems. The platination effect enhances π-electron delocalization, while promoting intersystem crossing
from singlet to triplet excited states. Accordingly, the σ-platinated BODIPY photosensitizer displays excellent O production
1
2
capability, facilitating anthracene-to-endoperoxide transformation under low-energy irradiation conditions (λ = 520−590 nm).
This leads to the breakup of supramolecular polymers and gels, which can be restored at room and elevated temperatures
because of the reversible endoperoxide-to-anthracene deoxygenation process. Overall, the rational design of a σ-metalated
photosensitizer opens up a new avenue to regulating the photoresponsiveness of supramolecular polymers under mild and
nondestructive conditions.
1
. INTRODUCTION
neously at room temperature. The quantitative anthracene−
endoperoxide photoswitching leads to macroscopic photo-
responsiveness of the supramolecular polymers (Scheme 1).
Despite the progress achieved so far, blue visible light (λ =
∼460 nm) or long-wave UV light (λ = ∼365 nm) is required
for the anthracene-to-endoperoxide photooxygenation of 1
Supramolecular polymers, representing the polymeric arrays of
monomeric units held together by directional and reversible
noncovalent bonds, are endowed with fascinating adaptive
and self-healing properties. Light is one of the attractive
external stimuli in terms of its remote and clean character,
which stimulates the incorporation of photoresponsive
building blocks into supramolecular polymers. In this respect,
anthracene and its monosubstituted derivatives represent
intriguing candidates capable of undergoing the well-known
1
2
5
c
(
Scheme 1). High-energy irradiation gives rise to an
7
3
unavoidable photodegradation issue. In this regard, it is
desirable to trigger photoresponsiveness under mild con-
ditions. A plausible approach to attaining this objective is to
encapsulate a photosensitizer into the supramolecular poly-
meric systems. Upon excitation of the photosensitizer via low-
energy visible light, reactive oxidized species can be generated
in situ, which, consequently, react with 1 and facilitate the
disassembly of supramolecular polymers.
4
[4 + 4] and/or [4 + 2] photodimerization reactions. For 9,10-
disubstituted anthracenes, anthracene-to-endoperoxide photo-
switching takes place upon light irradiation. Our research
group has previously constructed one-dimensional supra-
molecular polymers on the basis of 9,10-disubstituted
5
5
c
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) rep-
anthracene monomer 1 (Scheme 1). The supramolecular
polymerization process is synergistically driven by multiple
hydrogen-bonding and π−π-stacking interactions because of
8
resents a fascinating class of visible-light photosensitizers. The
parent BODIPY chromophore displays intense absorbance in
6
the presence of two amide units on 1. Photooxygenation of 1
to its endoperoxide form (1-O ; Scheme 1) takes place upon
Received: July 11, 2019
2
light irradiation, while backward conversion occurs sponta-
©
XXXX American Chemical Society
A
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Scheme 1. Schematic Representation for Regulating the Photoresponsiveness of a One-Dimensional Supramolecular Polymer
Derived from 1 via the σ-Platinated BODIPY Photosensitizer 2
the green region (λ = ∼500 nm; Scheme 2). To extend
absorbance in the low-energy region, a feasible approach is to
BODIPY, this strategy has received much less attention
11
compared to the aforementioned ones.
Herein, σ-platinated BODIPY 2 (Scheme 1) has been
employed as an efficient photosensitizer to regulate the
photoresponsiveness of one-dimensional supramolecular poly-
mers derived from 1. The design principle is based on the
following considerations. First, both 1 and 2 feature a similar
Scheme 2. Plausible Approaches to Extending the
Absorption Wavelength of a BODIPY Chromophore
12
platinum acetylide unit, which facilitates the embedment of a
photosensitizer into the rod segment of supramolecular
polymers. Second, d−p orbital overlap takes place between
13
platinum(II) and the 2,6-diacetylene BODIPY ligand on 2.
The enhancement of π-electron delocalization results in a red
13
shifting of the absorbance signal. Third, 2 tends to undergo
intersystem crossing (ISC) upon light excitation, which
1
14
provides O to the neighboring monomer 1. Accordingly,
2
photooxygenation of 1 is operated via low-energy visible-light
irradiation. This leads to the breakup of supramolecular
polymers, which can be restored at room temperature because
of spontaneous conversion from 1-O to 1 (Scheme 1). Hence,
2
employment of a σ-platinated photosensitizer represents an
effective approach to modulating the responsiveness of
supramolecular systems under nondestructive photoirradiation
conditions.
2. RESULTS AND DISCUSSION
incorporate π-conjugation units at the methyl or α position of
the pyrrole moieties (Scheme 2). An alternative protocol
Synthesis of σ-Platinated BODIPY 2. The synthetic
9
route toward 2 is quite straightforward (Scheme 3). Briefly, the
BODIPY skeleton 3 is obtained via condensation reaction
between benzaldehyde and 2,4-dimethylpyrrole, which is
further iodinated to provide the intermediate 4. 4 is reacted
involves the fusing of π-aromatic rings to the BODIPY
10
chromophore (Scheme 2). Although metalation can also be
regarded as a plausible way to modulate the absorbance of
B
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Scheme 3. Synthetic Route toward the σ-Platinated
BODIPY Photosensitizer 2
0.39 ns for the aerated sample (Figure S5b). For the deaerated
sample, a longer emission lifetime of 32.9 μs is detected
a
(
Figure S5d), supporting the presence of phosphorescent
emission. Accordingly, ISC takes place for 2 from singlet to
triplet excited states, leading to dual-emission character.
We then resort to density functional theory (DFT) and
time-dependent DFT (TD-DFT) computations to understand
the spectroscopic difference between 2 and 5 (Figure 2). For
a
DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
with (trimethylsilyl)acetylene to form 5 via the Sonogashira
reaction, followed by deprotection of the trimethylsilyl (TMS)
unit. The final step involves a copper-catalyzed coupling
reaction between the deprotected form of 5 and trans-
[
PtI (PEt ) ]. The chemical structure of 2 is verified by
2 3 2
1
13
31
means of H, C, and P NMR and matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) spectra
3
1
(
Figures S1−S4). The P NMR spectrum of 2 shows one
Figure 2. Energy-level diagram of 2 and 5 via TD-DFT computation.
To avoid the convergence difficulties, the energy levels in the diagram
are obtained via DFT optimization in the gas phase. The electronic
transition energy, oscillation strength, and oscillation contribution are
obtained from TD-DFT computation in chloroform.
signal at 9.00 ppm in chloroform-d (Figure S3). The
equivalence of the phosphorus atoms validates the trans
configuration of the triethylphosphine units.
Spectroscopy of σ-Platinated BODIPY 2. UV−vis
spectroscopy measurement is then performed for 2. In
trichloromethane (CHCl ), it possesses a broad intense band
3
4
from 500 to 700 nm (abbreviated as band I, ε = 5.96 × 10
dm mol cm for the maximum absorption at 632 nm),
the simulated UV−vis spectra of both compounds (Figures S6
and S7), the electronic transitions from highest occupied
molecular orbital (HOMO) to lowest unoccupied molecular
orbital (LUMO) are predominant (94.4% for 5 and 96.2% for
3
−1
−1
together with a relatively weak absorbance at 350−500 nm
4
3
−1
−1
(
abbreviated as band II, ε = 1.89 × 10 dm mol cm at 417
nm; Figure 1a). Both bands are bathochromic-shifted from
2
5
), as evidenced by the strong oscillator strength (f = 0.80 for
and 0.72 for 2; Figure 2). The electron densities from
HOMO to LUMO are distributed over the diacetylene
BODIPY unit (Figure 2). Hence, it is assigned to the transition
from π to π* orbitals, which exemplify the intense absorbance
of band I for both compounds 2 and 5 (Figure 1a).
For 5, the HOMO and LUMO energy levels are calculated
to be −5.561 and −2.753 eV, respectively (Figure 2). In
comparison, σ-platination destabilizes both the HOMO and
LUMO energy levels (for 2, −5.082 eV for the HOMO energy
level and −2.517 eV for the LUMO energy level; Figure 2).
Evidently, the HOMO orbital is perturbed to a larger extent
than the LUMO one. The result is primarily ascribed to the
energy matching between the 5d orbital of platinum(II) and
the HOMO of the diacetylene BODIPY unit. As a result, the
HOMO−LUMO energy gap decreases from 5 to 2. The
calculation is consistent with the experimental data, in which
band I of 2 displays 75 nm more bathochromic shift than 5
(Figure 1a).
Figure 1. (a) UV−vis and (b) emission spectra of 2 (red lines) and 5
−
5
(
black lines) in CHCl (c = 1 × 10 M for each compound).
3
15
those of TMS-substituted BODIPY 5 (λmax = 557 nm for band
I, and λmax = 394 nm for band II). Meanwhile, 2 also displays a
more red-shifted emission signal than that of 5 (λmax = 670 and
5
75 nm in CHCl , respectively; Figure 1b). The relative
3
quantum yield (Φ ) of 2 is determined to be 0.033, which is
According to TD-DFT computation, bands II of both
compounds originate from the electronic transition between
low-lying HOMO-n and LUMO. For 5, it mainly comprises
HOMO−6 to LUMO, as well as HOMO−4 to LUMO
(compositions for the former and latter transitions are 86.6%
and 88.0%, respectively; Figure 2). In terms of 2, it is assigned
F
significantly lower than that of 3 (Φ = 0.527). The result
F
suggests emission quenching upon σ-plantination of BODIPY.
The emission lifetime is determined to be 5.75 ns for 5 in
chloroform (Figure S5a), validating the fluorescent emission
character. In terms of 2, the fluorescent emission lifetime is
C
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to the HOMO−2-to-LUMO transition (the composition is
Figures 3c and S8c). In stark contrast, the epoxidation product
of DMA can hardly be probed for 5 under the same
9
2.9%; Figure 2).
Visible-Light Photosensitization Capability of σ-
circumstances (Figures 3c and S8b), denoting the absence of
1
Platinated BODIPY 2. The dual-emission character of 2
suggests that it can be excited and intersystem crossed to the
triplet excited state. Successive energy transfer can take place
between 2 and the surrounding molecular oxygen (Figure 3a).
O during the photoirradiating process. Although the iodo-
2
substituted BODIPY 4 displays excellent photosensitization
1
capability at 525 nm, it is unable to generate O under 590 nm
2
irradiation (Figure S9c). The phenomena are highly
reasonable, considering that both 4 and 5 possess weak
2
3
−1
−1
absorption at 590 nm (ε = 2.00 × 10 dm mol cm for 4
3
3
−1
−1
and 1.60 × 10 dm mol cm for 5 in CHCl ). Hence, σ-
3
platination represents an efficient protocol to enhance the
photosensitization capability of BODIPY under low-energy
irradiation conditions.
On this basis, light irradiation is further performed for the
mixture of 6 (0.10 mM) and 2 (2 mol %) (Figure 3d).
17
Compound 6 possesses a 9,10-platinated anthracene unit,
which is structurally similar to that of 1. Accordingly, 6 is
capable of undergoing photosensitized oxygenation and can
serve as a model system to evaluate the photoresponsiveness of
1
. As can be seen, absorbance of the anthracene unit on 6
gradually declines upon 590 nm light irradiation. In the
meantime, the aromatic H NMR resonances (8.68 and 7.46
1
ppm) weaken, while a new set of upshifted resonance peaks
emerge (7.80 and 7.30 ppm; Figure S10). The results suggest
the photooxygenation of 6 to the 9,10-endoperoxide product.
The conversion rate from 6 to 6-O is determined to be 0.021
2
−
1
s
with 2 mol % of the photosensitizer 2, which shows a
further increase at the higher amount of 2 (k = 0.043, 0.056,
−
1
and 0.079 s for 4, 6, and 8 mol % of 2, respectively; Figures
e and S13). On the contrary, no photooxygenation reaction
3
takes place for 6 in the absence of 2 (Figure S11).
Supramolecular Polymerization and Gelation Behav-
iors of 1 in the Presence of 2. As previously demonstrated,
Figure 3. (a) Schematic representation for the photosensitization of 2
1
to generate O in situ. Time-dependent UV−vis spectra of (b) DMA
2
(
c = 0.20 mM for DMA and 0.004 mM for 2 in CHCl ) under 525
3
1
is capable of forming one-dimensional supramolecular
−
2
nm irradiation (12.3 mW cm ) and (d) 6 (c = 0.20 mM for 6 and
.004 mM for 2 in CHCl ) upon 590 nm irradiation (6.63 mW
5c
polymers in a dilute methylcyclohexane (MCH) solution.
In concentrated MCH, supramolecular gels are prone to
formation because of the entanglement of one-dimensional
0
3
−
2
cm ). (c) Linear fitting of the photooxygenation of DMA (c = 0.20
and ■) 525
and 590 nm irradiation with the presence of 2; (▼ and ●) 525 and
90 nm irradiation with the presence of 5. (e) Linear fitting of the
mM for DMA and 0.004 mM for 2 or 5 in CHCl ): (
▲
3
5c
polymers to form fibrous networks. Because 2 is totally not
soluble in pure MCH, a small amount of chloroform is
required to solubilize the photosensitizer. The polar chlori-
nated solvent is capable of weakening intermolecular hydrogen
bonds in supramolecular polymers derived from 1. Hence,
prior to a study of the photoresponsiveness of 1/2, the
supramolecular polymerization and gelation capability of 1
should be reexamined with the presence of photosensitizer 2.
As widely documented, circular dichroism (CD) spectros-
copy represents a powerful technique to probe the formation
5
photooxygenation of 6 (c = 0.20 mM in CHCl ) upon 590 nm
3
−
2
irradiation (6.63 mW cm ): c = (■) 0.004, (●) 0.008, (▲) 0.012,
and (▼) 0.016 mM for 2.
In this regard, the photosensitization capability of 2 is
examined by employing 9,10-dimethylanthracene (DMA) as
1
16
the O scavenger. Briefly, DMA (c = 0.20 mM) and 2 mol %
2
2
are mixed together in CHCl . Upon green-light irradiation
3
18
[
light-emitting-diode (LED) lamp, 12 W, λ = 525 nm], the
characteristic absorption bands of DMA (λmax = 360, 380, and
01 nm) decline and completely disappear within 210 s
Figure 3b), suggesting the consumption of DMA. The time-
dependent absorbance curve conforms to the pseudo-first-
of supramolecular polymers. The supramolecular chirality
signals are only present for long-range-ordered polymeric
assemblies and absent for monomeric and small-sized
oligomeric structures. To induce helical bias at the supra-
molecular level, optically active (S)-3,7-dimethyloctyl groups
are introduced to the peripheral side chains of 1 (Scheme 1).
4
(
1
−1
order kinetics, providing a O generation rate of 0.025 s for
2
−
4
−6
2
5
(Figure 3c). Notably, the value is 2.8-fold higher than that of
under the same conditions (k = 0.0091 s ; Figures 3c and
For the mixture of 1 and 2 (c = 1 × 10 M for 1 and 2 × 10
−
1
M for 2), the Cotton effect is absent in pure chloroform
(Figure 4a), suggesting the dominance of a molecularly
dissolved state. In sharp contrast, the bisignate Cotton effect
S8a), despite the fact that the molar absorption coefficient of 2
is significantly lower than that of 5 (at 525 nm, ε = 6.00 × 10
dm mol cm for 2 vs 2.83 × 10 dm mol cm for 5).
Additionally, the photosensitization experiment is also
performed via low-energy amber-light irradiation (LED lamp,
3
3
−1
−1
4
3
−1
−1
is present for 1/2 in MCH/CHCl (97/3, v/v). The negative
3
maximum is located at 487 nm, while the positive maxima are
at 473 and 450 nm, respectively (at 283 K, Δε = −52.5 L
−
1
−1
1
5
2 W, λ = 590 nm). Because 2 features intense absorption at
mol cm and g = −0.0023 for 487 nm; Figure 4a). With
4
3
−1
−1
90 nm (ε = 3.18 × 10 dm mol cm nm in CHCl ), the
reference to the UV−vis spectrum (Figure 4b), the CD signals
3
1
−1
1
O generation rate shows a further increase (k = 0.034 s ;
originate from the L transition band of the anthracene unit
2
a
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(
1
590 nm, 12 W) irradiation, the CD signals of 1/2 (c = 1 ×
−
4
−6
0
M for 1 and 2 × 10 M for 2) progressively decline and
vanish within 40 min (Figure 5a). The disappearance of
−
4
Figure 4. (a) CD and (b) UV−vis spectra of 1/2 (c = 1 × 10 M for
1
and 2 × 10− M for 2 at 283 K) in (black line) CHCl and (blue
6
3
line) MCH/CHCl (97/3, v/v). (c) Nonlinear fitting of the melting
3
−
4
curves: (red line) 1 (c = 1 × 10 M) in pure MCH; (blue line) 1/2
Figure 5. (a) CD and (b) UV−vis spectral changes of 1/2 (c = 0.1
c = 1 × 10− M for 1 and 2 × 10 M for 2) in MCH/CHCl (97/3,
4
−6
(
3
mM for 1 and 0.002 mM for 2 in MCH/CHCl (97/3, v/v) upon 590
3
v/v). (d) Supramolecular gels of 1/2 (c = 12.8 mM for 1) and 1 (c =
3.7 mM for 1) in MCH/CHCl (90/10, v/v).
nm irradiation. (c) CD spectral changes of the mixture of 1-O and 2
2
1
3
upon heating at 318 K. (d) Photoresponsiveness of supramolecular
gels from 1/2 and 1.
on 1. The supramolecular chirality transfers from the alkyl
periphery to the inner anthracene unit, supporting the
formation of long-range-ordered supramolecular polymers in
supramolecular chirality reveals the disassembly of supra-
molecular polymeric structures. Meanwhile, the visible-light
absorbance of 1 dramatically declines, accompanied by the
presence of an isosbestic point at 357 nm (Figure 5b). The
phenomenon suggests the conversion of 1 to its epoxidized
apolar MCH/CHCl (97/3, v/v).
3
On this basis, we sought to understand the supramolecular
polymerization thermodynamics by performing temperature-
dependent CD measurements in MCH/CHCl (97/3, v/v; c =
3
−
4
−6
structure (1-O ). Remarkably, no spectroscopic changes take
1
× 10 M for 1 and 2 × 10 M for 2). When the CD
2
place for 1 in the absence of photosensitizer 2 (Figure S16).
The results unambiguously support the crucial role of a σ-
platinated photosensitizer for the disassembly of supra-
molecular polymers under mild irradiation conditions.
intensity at 486 nm versus temperature is monitored, a
nonsigmoidal curve is obtained (Figures 4c and S14). The
result unambiguously suggests the involvement of a coopera-
tive nucleation−elongation supramolecular polymerization
6
mechanism. The quantitative thermodynamic parameters for
On this basis, supramolecular repolymerization is inves-
the 1/2 supramolecular polymerization process can be further
acquired by fitting the cooling curve by the Meijer−
Schenning−van der Schoot mathematical model (see the
tigated for the resulting mixture of 1-O
reported, the endoperoxide-to-anthracene conversion from 1-
to 1 occurs spontaneously at room temperature and
2
and 2. As previously
O
2
5c
6
d
Experimental Section). Specifically, the enthalpy release (h )
accelerates at elevated temperature. As shown in Figure 5c,
the CD signal is restored in MCH/CHCl (97/3, v/v) upon
heating of the sample at 318 K for 12 min. The re-formation of
supramolecular chirality is attributed to the quantitative
e
−1
is determined to be −85.5 kJ mol for 1/2 in MCH/CHCl3
97/3, v/v; Figure 4c). The h value is relatively lower than
that of 1 in pure MCH (h = −95.5 kJ mol ; Figure 4c).
Moreover, the critical elongation temperature (T ) of 1/2 is
3
(
e
−
1
e
endoperoxide-to-anthracene transition of 1-O . The reversible
e
2
determined to be 297.2 K, which is 15.8 K lower than that of 1
photooxygenation process of 1/2 can be macroscopically
visualized by “gel−sol” transition experiments (Figure 5d).
Briefly, when 1/2 [c = 12.8 mM for 1 and 0.67 mM for 2 in
in MCH (T = 313.0 K). Overall, it can be concluded that one-
e
dimensional supramolecular polymers still form for 1/2 in
MCH/CHCl3 (97/3, v/v), although their stability is
influenced by the small amount of chlorinated solvent. The
nanofibers obtained from 1/2 can be directly confirmed by the
transmission electron microscopy (TEM) measurements
MCH/CHCl (90/10, v/v)] is irradiated for 60 min (590 nm),
3
the supramolecular gel collapses to a dark-green fluid (Figure
5d), which converts back to a supramolecular gel upon
standing at room temperature. In stark contrast, no “gel-to-sol”
conversion takes place for 1 in the absence of photosensitizer 2
under the same amber-light irradiation conditions (Figure 5d).
(Figure S15). Additionally, 1/2 is prone to forming supra-
molecular gels in concentrated MCH/CHCl (90/10, v/v;
3
critical gelation concentration = 12.1 mM). The dark-green
color of 1/2 (Figure 4d) originates from the intense visible-
light absorption of the σ-platinated BODIPY photosensitizer 2.
Regulating the Photoresponsiveness of Supramolec-
ular Polymers. Finally, we turned to regulating the photo-
responsiveness of supramolecular polymers derived from 1 via
the σ-platinated BODIPY photosensitizer 2. Upon amber-light
3
. CONCLUSION
In summary, σ-platinated BODIPY 2 has been designed as an
efficient photosensitizer to regulate the photoresponsiveness of
anthracene-based supramolecular polymers. For photosensi-
tizer 2 itself, the metalation effect enhances π-electron
delocalization of the BODIPY unit while promoting ISC
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from singlet to triplet excited states. As a result, 2 displays
Synthesis of σ-Platinated BODIPY 2. trans-Diiodobis-
triethylphosphine)platinum(II) (380 mg, 0.55 mmol) and CuI (5
mg, 0.025 mmol) were added to a 50 mL Schlenk flask and cooled to
−78 °C. The deprotected form of 5 (91 mg, 0.25 mmol) in THF (10
mL) was injected into the flask, followed by the successive addition of
1
(
excellent O production capability under green- and amber-
2
light irradiation conditions. For the 1/2 mixed system, the in
1
situ generated O2 further reacts with 1 to form the
endoperoxide product 1-O , avoiding the harsh conditions
2
(
i-Pr) NH (4 mL). The resulting solution was stirred at −78 °C for 1
2
(
blue visible light or long-wave UV light) required for the
h and then stirred at room temperature overnight. The dark-green
conventional photooxygenation process. Accordingly, supra-
molecular polymers and gels break up upon low-energy visible-
light excitation, which can be restored at room and elevated
temperatures because of the reversible endoperoxide-to-
anthracene deoxygenation process. Therefore, the current
study opens up an effective approach to modulate the
photoresponsiveness of supramolecular polymeric systems
under mild conditions.
product was purified by a silica gel column (eluent: petroleum ether/
ethyl acetate = 9/1, v/v) to afford 2 as a dark-green powder (130 mg,
1
yield 35%). H NMR (400 MHz, CDCl , 298 K): δ 7.48−7.47 (3H,
3
C
H
), 7.29−7.28 (2H, C H ), 2.62 (s, 6H, CH ), 2.18−2.11 (m,
6
5
6 5 3
2
4H, PCH CH ), 1.38 (s, 6H, CH ), 1.15−1.07 (m, 36H,
2
3
3
13
PCH CH ). C NMR (100 MHz, CDCl , 298 K): δ 156.9, 140.3,
2
3
3
1
2
39.9, 135.6, 130.9, 129.1, 128.9, 128.3, 121.1, 95.9, 95.8, 95.6, 91.5,
3
1
9.8, 16.9, 16.7, 16.5, 13.8, 13.3, 8.5, 8.4, 8.3. P NMR (162 MHz,
1
+
CDCl ): δ 9.00 (s, J
= 2345.8 Hz). MALDI-TOF ([M] ): m/z
3
Pt−P
1486.7575.
4
. EXPERIMENTAL SECTION
Materials and Methods. All chemical reagents and solvents
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02073.
■
utilized in the preparation or testing procedure are commercially
available and were received without further purification or isolation.
BODIPY-based compounds 3−5 and trans-diiodobis-
*
S
(
triethylphosphine)platinum(II) were synthesized according to the
19
1
13
31
previously reported literature. H, C, and P NMR measurements
were recorded on a Varian Unity INOVA-300 spectrometer with
tetramethylsilane as the internal standard. A Bruker Autoflex Speed
spectrometer was exploited with trans-2-[3-(4-tert-butylphenyl)-2-
methyl-2-propenylidene]malononitrile as the basic matrix. A
Shimadzu UV-1800 spectrometer and a FluoroMax-4 spectrofluor-
ometer were utilized to record the spectra of UV−vis absorption and
fluorescent emission. For quantum yield measurements, rhodamine B
Synthesis, characterization, spectroscopic data, and other
materials (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: mingming.zhang@xjtu.edu.cn (M.Z.)
E-mail: drfwang@ustc.edu.cn (F.W.)
■
20
(
Φ = 65% in ethanol) was employed as the standard reference. The
F
*
emission lifetime measurements were acquired with a 1 MHz LED
laser with an excitation wavelength at 455 nm (NanoLED-455) and
analyzed with DataStation v6.6 (Horiba Scientific). Supramolecular
chirality signals for supramolecular assemblies were recorded on a
Jasco J-1500-type CD spectrometer. During temperature-variable CD
measurements, a PFD-425S/15 Peltier-type temperature controller
was employed to precisely control the testing temperature of the
cuvette. The cooling rate for cooling curve measurements is set as 1.0
ORCID
Mingming Zhang: 0000-0003-3156-7811
Feng Wang: 0000-0002-3826-5579
Author Contributions
All authors have given approval to the final version of the
manuscript.
−
1
K min . For light irradiation experiments, 590 and 525 nm LED
lamps (12 W) were exploited as light sources, by keeping the distance
between the LED and cuvette at 10 cm. TEM images of 1 and 1/2
were performed on a Tecnai G2 Spirit BioTWIN electron
microscope.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (Grants 21922110, 21674106, and
■
DFT and TD-DFT Computation. DFT computations were
21
performed by the Gaussian 09, D.01 version, software package.
During optimization, nonmetallic atoms were described by the
B3LYP/6-31G(d) computational level, while all platinum(II) atoms
were described by the Lanl2dz effective core potential. The optimized
geometries of 2 and 5 were utilized directly to compute feasible
2
1871245), the Fundamental Research Funds for the Central
Universities (Grant WK3450000004), the CAS Youth
Innovation Promotion Association (Grant 2015365), and
State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University (Grant 20182009).
22
electronic transitions in CHCl via TD-DFT.
3
Determination of the Assembling Thermodynamics of
Supramolecular Polymers. To gain deeper insight into the
supramolecular polymerization behaviors of 1 and 1/2 in different
solvents, temperature-variable CD spectral measurements were
performed. The resulting nonsigmoidal curves for the supramolecular
polymerization process are divided into elongation and nucleation
regimes, which are fitted by the Meijer−Schenning−van der Schoot
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