A Dynamic Tetracationic Macrocycle Exhibiting Photoswitchable Molecular Encapsulation

Article abstract of DOI:10.1021/jacs.8b10526

Designing macrocycles with appropriate molecular recognition features that allow for the integration of suitable external stimuli to control host-guest processes is a challenging endeavor which enables molecular containers to solubilize, stabilize, and separate chemical entities in an externally controllable manner. Herein, we introduce photo- and thermal-responsive elements into a semi-rigid tetracationic cyclophane, OPVEx²Box⁴⁺, that is composed of oligo(p-phenylenevinylene) pyridinium units and the biphenylene-bridged 4,4-bipyridinium extended viologens and adopts a rectangle-like geometry. It transpires that when the photoactive oligo(p-phenylenevinylene) pyridinium unit is incorporated in a macrocyclic scaffold, its reversibility is dramatically improved, and the configurations of the cyclophane can go back and forth between (EE)- and (EZ)-isomers upon alternating blue light irradiation and heating. When the macrocycle is found in its (EE)-configuration, it is capable of binding various π-electron-rich guests - e.g., anthracene and perylene - as well as π-electron-deficient guests - e.g., 9,10-anthraquinone and 5,12-tetracenequinone - through charge-transfer and van der Waals interactions. When irradiated with blue light, the (EE)-isomer of the cyclophane can be transformed successfully to the (EZ)-isomer, resulting in the switching off of the binding affinity for guest molecules, which are bound once again upon heating. The use of light and heat as external stimuli to control host-guest interactions involving a multi-responsive host and various guests provides us with a new opportunity to design and construct more-advanced molecular switches and machines.

Full text of DOI:10.1021/jacs.8b10526

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Article
A Dynamic Tetracationic Macrocycle Exhibiting
Photoswitchable Molecular Encapsulation
Huang Wu, Yong Chen, Long Zhang, Ommid Anamimoghadam, Dengke Shen, ZHICHANG
LIU, Kang Cai, Cristian Pezzato, Charlotte L. Stern, Yu Liu, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10526 • Publication Date (Web): 26 Nov 2018
Downloaded from http://pubs.acs.org on November 26, 2018
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A Dynamic Tetracationic Macrocycle Exhibiting
Photoswitchable Molecular Encapsulation
‡
,†
‡
†
†
†
Huang Wu, Yong Chen, Long Zhang, Ommid Anamimoghadam, Dengke Shen, Zhichang
⊥
Liu, Kang Cai, Cristian Pezzato, Charlotte L. Stern, Yu Liu* and J. Fraser Stoddart*
†
†
†
,‡,§
,†, ,║
∇
‡College of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, 94 Weijin Road,
Nankai District, Tianjin 300071, P.R. China
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
⊥
School of Science, Westlake University, 18 Shilongshan Road, Hangzhou 310024, China
§Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 92 Weijin Road, Nankai District,
Tianjin 300072, P. R. China
∇School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
║Institute for Molecular Design and Synthesis, Tianjin University, 92 Weijin Road, Nankai District, Tianjin
3
00072, P.R. China
*
E-mail: yuliu@nankai.edu.cn
* E-mail: stoddart@northwestern.edu
MAIN TEXT
*
Correspondence Address
*Correspondence Address
Professor Yu Liu
Department of Chemistry
Nankai University
Professor J Fraser Stoddart
Department of Chemistry
Northwestern University
9
4 Weijin Road
2145 Sheridan Road
Tianjin 300071 (CHN)
Tel: (+86)-2350-3625
Evanston, IL 60208-3113 (USA)
Tel: (+1)-847-491-3793
E-Mail: yuliu@nankai.edu.cn
E-Mail: stoddart@northwestern.edu
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ABSTRACT: Designing macrocycles with appropriate molecular recognition features that
allow for the integration of suitable external stimuli to control host−guest processes is a
challenging endeavor which enables molecular containers to solubilize, stabilize and separate
chemical entities in an externally controlled manner. Herein, we introduce photo- and thermal-
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responsive elements into a semi-rigid tetracationic cyclophane, OPVEx Box , that is
composed of oligo(p-phenylenevinylene) pyridinium units and the biphenylene-bridged 4,4-
bipyridinium extended viologens, and adopts a rectangle-like geometry. It transpires that the
reversibility of the photoactive oligo(p-phenylenevinylene) pyridinium unit is dramatically
improved when incorporated in a macrocyclic scaffold, and the configurations of the
cyclophane can go back and forth between (EE)- and (EZ)-isomers upon alternating blue light
irradiation and heat. When the macrocycle is found in its (EE)-configuration, it is capable of
binding various π-electron-rich guestse.g., anthracene and peryleneas well as π-electron-
deficient guestse.g., 9,10-anthraquinone and 5,12-tetracenequinonethrough charge
transfer and van der Waals interactions. When irradiated with blue light, the (EE)-isomer of the
cyclophane can be transformed successfully to the (EZ)-isomer, resulting in the switching off
of the binding affinity for guest molecules, which are bound once again upon heating. The use
of light and heat as external stimuli to control host−guest interaction involving a multi-
responsive host and various guests provides us with a new opportunity to design and construct
more advanced molecular switches and machines.
■
INTRODUCTION
Allostery, triggered by external stimuli, enables tight regulation of protein functions in the
1
cell environment. Classical stereochemical models among the proteins are the tetrameric
hemoglobins, which can go back and forth between relaxed and tense states upon catching or
2
releasing oxygen. In order to mimic these complicated allosteric processes in nature, the design
3
and synthesis of well-crafted artificial macrocycles have long been a worthwhile goal in
supramolecular chemistry. Some of the intensively studied macrocyclic compounds, such as
4
5
6
7
crown ethers, cyclodextrins, calixarenes, and pillararenes, possess various conformations
featuring an optimal fit for the binding of diverse guest molecules. Positive allosterism8,
however, permits tuning “on / off” the behavior of macrocyclic hosts by external stimulifor
example, a reversible switch from prominently bound to unbound or active to inactive
9
10
11
structures, which is important for developing new switchable catalysts and sensors , and
1
2
also possesses potential applications in separation and purification technologies. A further
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series of dynamic molecular macrocycles,13 cages,14 and tweezers15 have been developed,
which respond to pH , temperature, redox,14a interfacial tension,15b light, and several other
1
3b
13d
16
kinds of external stimuli. Among these external stimuli, light is the ideal candidate, owing to
its noninvasive, clean, and remote-controlling nature, as well as its easy to operate properties.17
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On the other hand, the concept of photocontrolled molecular encapsulation is generally based
on light-responsive guest molecules and, less frequently, host macrocycles, because the latter
often suffer from tedious synthesis and also the tendency to hinder the isomerization of light
19
active species by ring strain or molecular crowding. Photoactive host molecules, however,
have much wider scope for applications than that of the guest molecules in various fields. It is
necessary, therefore, to identify eligible systems to break the logjam.13a
In addition, the rigid tetracationic cyclophane, cyclobis(paraquat-p-phenylene)20
4
+
(
CBPQT ), consisting of two π-electron-deficient bipyridinium units linked by p-xylylene
bridges, has attracted a lot of attention, not only because of the well-defined inner cavity
21
suitable for the encapsulation of π-electron-rich substratese.g., 1,5-dioxynaphthalene units
2
2
and tetrathiafulvalene derivatives , but also because it serves as a vital building block for
2
3
constructing mechanically interlocked molecules (MIMs). In order to modulate the size of the
4
+
cavity in CBPQT , a series of “horizontal” p-phenylene connectors have been installed
4
+
between the bipyridinium units. For example, ExBox comprises two π-electron-poor 1,4-
phenylene-bridged extended bipyridinium units, which endow it with the ability to bind a series
of neutral polycyclic aromatic hydrocarbons of different sizes and shapes through
donor−acceptor interactions.12a Then, Ex Box , containing two biphenylene-bridged
2
4+
bipyridinium units, is capable of forming one-to-one complexes with polyether
2
4
macrocyclese.g., 1,5-dinaphtho[38] crown-10or binding various long oligoacenes e.g.,
tetracene, tetraphene, and chrysene. By taking advantage of these outstanding host−guest
properties and the semi-rigid structures of this extended-box family, we have incorporated
2
4+
photoactive elements into the semi-rigid organic cyclophane, (EE)-OPVEx Box , which
adopts (Scheme 1) a rectangle-like geometry and is composed of the photoactive oligo(p-
2+
phenylenevinylene) bipyridinium (OPV ) unit and the biphenylene-bridged 4,4-bipyridinium
2
2+
extended viologen (Ex BIPY ), connected end-to-end by p-xylylene linkers. The reversibility
of the photo- and thermal-responsive OPV2+ unit is improved dramatically, when it is
incorporated into a macrocyclic scaffold, benefiting as it does from the ring strain in its (EZ)-
configuration following blue light irradiation. The (EE)- and (EZ)- configurations of
2
4+
OPVEx Box can be regulated reversibly by alternating blue light irradiation and heat, as
confirmed by NMR, UV-Vis absorption, fluorescence spectroscopy and also by single-crystal
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X-ray diffraction. Moreover, on account of the distinct electronic character of (EE)-
2
4+
OPVEx Box , it is capable of binding π-electron-riche.g., anthracene and peryleneas well
as π-electron-deficiente.g., 9,10-anthraquinone (9,10-AQ) and 5,12-tetracenequinone (5,12-
TQ) guest molecules, through charge-transfer and van der Waals interactions. Most
importantly of all, the corresponding guest molecules of diverse sizes, shapes, and electronic
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compositions can be taken up and release reversibly from the cavity of the OPVEx Box by
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alternating irradiation with blue light and heating, as indicated by H NMR and UV-Vis
absorption spectroscopies.
■
RESULTS AND DISCUSSION
2
4+
The synthesis of (EE)-OPVEx Box is shown in Scheme 1. Briefly, the Knoevenagel
condensation between terephthalaldehyde and 4-picoline in acetic anhydride was carried out
under reflux for 24 h to obtain the 1,4-BPEB in 60% yield. Subsequently, an S 2 reaction
between an excess of p-xylylene dibromide and 1,4-BPEB in MeCN under reflux for 24 h, led
N
to the formation of BrOPV•2PF
6
after counterion exchange in a yield of 82%. Finally,
2
equimolar amounts of BrOPV•2PF
6
and 4,4′-(4,4′-biphenylene)bipyridine (Ex BIPY) in
presence of tetrabutylammonium iodide (TBAI) in MeCN under reflux for 3 days in dark
2
conditions, resulting in the isolation of the macrocycle (EE)-OPVEx Box•4Cl after work-up
by precipitation of the crude product with tetrabutylammonium chloride (TBACl). The desired
2
product (EE)-OPVEx Box•4PF
6
was obtained after purification by reverse-phase column
PF / H O) in 40% yield. The
chromatography, followed by counterion exchange (NH
4
6
2
1
13
1
1
compound was characterized by H NMR (Figure S10), C NMR (Figure S11), 2D H- H
COSY NMR (Figure S16), 2D ROESY NMR (Figure S17) spectroscopies and also by high-
resolution mass spectrometry (HRMS) (Figure S38). As a result of the asymmetric constitution
2
4+
of the (EE)-OPVEx Box , the methylene protons were split into two sets of peaks (H
5.69 / 5.61 ppm), they were assigned by the analysis of both the one and two-dimensional
NMR spectra. HRMS revealed the presence of the [M – PF
]+ and [M – 2PF ]2+ species (m/z =
235.2813 and 545.1585, respectively), which are consistent with the theoretical calculations.
e
/ H
h
, δ
=
6
6
1
2
The solid-state (super)structure of (EE)-OPVEx Box•4PF
6
which was determined by the
single-crystal X-ray diffraction analysis of a yellow single crystal obtained by slow vapor
2
4+
diffusion of Et
2
O into a MeCN solution of (EE)-OPVEx Box after 4 days, reveals an
asymmetric, box-like geometry (Figure 1a), with average dimensions of 19.1 × 7.1 Å (Figure
1
b). The distances between the nitrogen atoms are 15.7 × 6.3 Å (Figure 1b and 1c) and the
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torsional angles of adjacent aromatic rings in the Ex BIPY unit are ∼26°, ∼36°, and ∼25°,
2
4+
respectively (Figure 1a). The solid-state superstructure (Figure 1d) of (EE)-OPVEx Box
reveals that it crystallizes in the monoclinic space group P2 /c with the molecules packing in a
herringbone fashion. There is only partial π···π stacking between the Ex BIPY unit and the
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2+
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+
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OPV unit with a distance of 3.8 Å between adjacent cyclophanes, and the residual pyridinium
2
2+
2+
in both the Ex BIPY and the OPV units aptly stack with the p-xylylene ring in the closest
adjacent cyclophanes via the π···π interaction with a distance of 3.7 Å, which may arise from
the balance between Coulombic repulsions and the maximization of π-overlap in the entire
superstructure.25
2
4+
26
Because of the presence of the stilbene moiety in (EE)-OPVEx Box , it was anticipated
that it might undergo (E)→(Z) isomerization upon light irradiation. The UV-Vis absorption
2
4+
spectrum (Figure S1) of (EE)-OPVEx Box revealed an absorption peak up to 470 nm,
suggesting it might undergo the photoisomerization upon irradiated with blue light. The
2
4+
1
photoisomerization of (EE)-OPVEx Box was first investigated by H NMR spectroscopy.
1
The H NMR spectrum (Figure 2 and S20) of a freshly prepared CD
3
CN solution of (EE)-
2
4+
OPVEx Box showed two distinct sets of proton resonances upon blue light (450 – 460 nm)
2
4+
irradiation, indicative of (EE)-OPVEx Box undergoing isomerization. The resonances of the
protons H and H residing on the C=C double bond were split into two sets: the original set was
assigned to the unreacted species while the other set of proton resonances (Figure 2b), H (d, J
k
l
o
2
4+
=
12.9 Hz) and H
p
(d, J = 12.8 Hz) were attributed to the transformation of OPVEx Box from
the (EE)- to the (EZ)-isomers. The intensity of the proton signals (Figure S20) of (EE)-
2
4+
2
4+
OPVEx Box gradually decreased, accompanied by the increase of the (EZ)-OPVEx Box
proton resonances with prolonging irradiation. According to the relative integrals of the protons,
the ratio of the (EE)- and the (EZ)-isomers was calculated to be 15:85 in the photostationary
state (PSS). The non-quantitative conversion from the (EE)- to the (EZ)-isomers under
irradiation with blue light is probably a result of overlapping of the absorbance spectra of the
2
+
(
EE)- and the (EZ)-isomers (Figure S1). In order to confirm the isomerization of the OPV unit,
2+
1
a reference compound (EE)-MeOPV was synthesized (Scheme 1), and characterized by H
1
3
1
1
NMR (Figure S8), C NMR (Figure 9), 2D H- H COSY NMR (Figure S14), 2D NOESY
NMR (Figure S15) spectroscopies and also by high-resolution mass spectrometry (HRMS)
1
(
Figure S36). H NMR spectroscopic changes in the chemical shifts of the protons residing on
2+
the C=C double bond associated with the (EE)→(EZ) isomerization in MeOPV (Figure S23)
2
4+
are similar to those observed in the case of OPVEx Box . Upon irradiating the MeCN solution
2
+
of the (EE)-MeOPV for 10 s, the conversion yield (Figure S23) from the (EE)- to the (EZ)-
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4+
isomers is 90%, which is a little higher than that (85%) for (EE)-OPVEx Box . The (ZZ)-
2
4+
isomer was not detected in the light promoted isomerization of both OPVEx Box and
2
+
MeOPV , a situation which can possibly be ascribed to the (EZ)-isomers following irradiation
showing (Figure 3a and S1) very week absorption for the blue light (450 – 460 nm). On heating
2
4+
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the solution of the photostationary state (EZ)-OPVEx Box at 70 ºC for 16 h, the H NMR
spectra (Figure 2c and S21) indicated almost quantitative conversion of the (EZ)-isomer into
the (EE)-isomer
.
Moreover, this interconversion can be repeated through several cycles without
any appreciable light fatigue being evident (Figure S22) upon alternating blue light irradiation
2
+
and heating. In the case of MeOPV , however, the (EZ)-isomer could not be transformed
completely (Figure S24) into the (EE)-isomer, even after heating at 70 ºC up to 144 h. The ratio
of the (EE)- to the (EZ)-isomers was calculated to be 75:25 in the thermal steady state.
2
4+
2+
Comparing OPVEx Box with MeOPV , it can be concluded that the ring effect improves
significantly the conversion yield and the reaction rate of the reverse (EZ)→(EE) isomerization
upon heating.
2
4+
2+
The isomerizations exhibited by OPVEx Box and MeOPV were also studied by UV-Vis
absorption and fluorescence spectroscopy. The UV-Vis absorption spectrum (Figure 3a) of the
2
4+
(
EE)-OPVEx Box exhibits two strong absorption peaks at 359 and 406 nm, which can be
2
2+
2+
assigned to the characteristic absorptions of the Ex BIPY and OPV units, respectively. After
2
4+
a MeCN solution of (EE)-OPVEx Box had been irradiated with blue light (450 – 460 nm) for
1
2 s, the intensity of the band at 406 nm (Figure 3a and 3b), corresponding to the absorption of
2+
the OPV unit, decreased gradually, and was accompanied by a slight increase in the intensity
of the absorption band at 220 − 350 nm. These changes can be directly ascribed to the
2
4+
transformation of OPVEx Box from the (EE)- to (EZ)-isomers. When the solution of the
2
4+
photostationary state of (EZ)-OPVEx Box was heated at 70 ºC for 10 min, the UV-Vis
absorption spectrum (Figure 3c) all but recovered to that of the (EE)-isomer. On the other hand,
2
4+
when exciting at 359 nm, the fluorescence spectrum (Figure S5) of (EE)-OPVEx Box reveals
2
4+
an emission at λmax = 485 nm. The fluorescence intensity of OPVEx Box was quenched
gradually (Figure S6) upon irradiation with blue light (450 – 460 nm) during 12 s. The changes
2
4+
of the fluorescence spectrum also indicate the conversion of OPVEx Box from the (EE)- to
EZ)-isomers upon irradiation with blue light. Moreover, this interconversion could be repeated
(
through several cycles without any noticeable fatigue (Figure 3c and 3d), an observation which
1
is consistent with that (Figure S22) from the H NMR spectroscopy. These two experiments
demonstrated jointly the prominent photo- and thermal-switched configurational
2
4+
interconversion properties of OPVEx Box . The changes in the UV-Vis absorption (Figure
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S2) and fluorescence (Figure S7) spectra associated with the photoisomerization of MeOPV2+
2
4+
are analogous to those observed for OPVEx Box
.
The absorption peak at 392 nm and the
fluorescence peak at 474 nm decreased gradually upon irradiation with blue light (450 – 460
nm), as a result of the conversion from the (EE)- into the (EZ)-isomers. when the MeCN
solution of the photostationary state (EZ)-MeOPV2+ was heated at 70 ºC for 50 min, the
absorption spectrum, however, did not revert (Figure S3a) to its original state, demonstrating
the (EZ)-isomer cannot be completely converted into the (EE)-isomer. In three cycles of
alternating blue light irradiation and heating, the absorption peak at 392 nm became (Figure
S3b) lower and lower and was accompanied by the appearance of a new peak at 274 nm,
indicating that some photochemical side-reactions were occurring during light irradiation.27
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2
2
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2
2
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4
4
4
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5
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0
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2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
4+
Considering the photophysical properties and the isomerization of both OPVEx Box and
2+
2+
MeOPV , it can be concluded that the photoisomerization of the OPV unit from the (EE)- to
the (EZ)-configurations was not barred within the macrocyclic scaffold. On the contrary, the
incorporation into the cyclophane improved drastically the reversibility of the OPV2+ unit,
especially in the conversion yield and the reaction rate from the (EZ)- into the (EE)-isomers
upon heating.
In an effort to investigate thoroughly the reason for the ring effect improving the reversibility
of the OPV2+ unit, the (EZ)-OPVEx Box was isolated (Scheme S6) by reverse-phase
2
4+
1
13
preparative HPLC after blue light irradiation. The H NMR (Figure S12), C NMR (Figure
2
4+
S13) spectra, and the HRMS (Figure S39) of (EZ)-OPVEx Box all indicated the existence of
a single compound, in which the complete assignment of all the proton resonances were
1
1
achieved by 2D H- H COSY NMR (Figure S18) and 2D ROESY NMR (Figure S19)
spectroscopy. Specific information on the configuration was obtained from the single-crystal
X-ray diffraction. The X-ray structural analysis shows that the solid-state structure (Figure 4)
2
4+
of the (EZ)-OPVEx Box is distinctly different from the rectangle-like structure adopted by
2
4+
2+
(EE)-OPVEx Box . The configuration of the OPV unit changes from a linear- to a “seven”-
like geometry, as a result of the photoisomerization causing the (EE)-isomer to be converted
into the (EZ)-isomer after blue light irradiation. The distance (Figure 4b) between the two
2
+
nitrogens in the OPV unit (N
3
−N
4
) is 12.9 Å, which is shorter than the distance (15.7 Å) in
2
4+
2+
(
EE)-OPVEx Box . The isomerization of the OPV unit affects the conformations adopted
by every component in the cyclophane, because of the ring effect. The two p-xylylene units are
°
tilted (Figure 4a) with respect to each other with a dihedral angle of 86 , which is quite different
2
4+
2
2+
from the almost parallel arrangement in (EE)-OPVEx Box . The Ex BIPY unit is compelled
to bend into the cavity of the cyclophane, with a distance between the two nitrogens (N
1
−N )
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2
4+
of 15.1 Å (Figure 4b), which is also shorter than that (15.7 Å) in (EE)-OPVEx Box . The
2
2+
strain arising from the curving Ex BIPY unit maybe the reason for the reversibility of the
2
+
2
4+
OPV unit being improved when it is incorporated in the macrocycle, OPVEx Box . The
distance between N
1
−N
3
and N
2
−N increases to 7.2 Å and 6.7 Å, respectively (Figure 4a), from
4
2
4+
2
2+
2+
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the initial 6.3 Å in (EE)-OPVEx Box . On the other hand, the Ex BIPY and OPV units in
2
4+
(EZ)-OPVEx Box do not adopt a face-to-face arrangement (Figure 4b), which is different
2
4+
from the aspectant arrangement in (EE)-OPVEx Box . The solid-state superstructure of (EZ)-
2
4+
OPVEx Box reveals a layered stacking, in which the molecules crystallize in the monoclinic
2
4+
space group C2/c. The adjacent (EZ)-OPVEx Box adopts (Figure 4c) a face-to-face stacking
2
2+
2+
geometry incorporating π···π interactions between the adjacent Ex BIPY and (EZ)-OPV
units with a distance of 3.4 Å along the c-axis. The three-dimensional (3D) superstructure was
achieved by the two-level assembliesfirstly, the adjacent layers through π···π interactions
2
+
between two (EZ)-OPV units with a distance of 3.6 Å to form a bilayer (Figure 4d), and then
a further assembly sustained by electrostatics and hydrogen bonding interactions.
2
4+
The constitution of the OPVEx Box is based on the family of well known “extended
2
8
viologen cyclophanes” , which possess high affinity for binding polycyclic aromatic
hydrocarbons, as a result of charge transfer interactions between the π-electron-poor viologen
2
4+
and π-electron-rich aromatic rings. In the case of (EE)-OPVEx Box
,
however, one bipyridium
ring is bridged by a biphenylene subunit, while another is linked by a p-divinylbenzene subunit.
These longer linkers may be responsible for the electronical distortions in the “extended
24
2
4+
viologen” units. It is possible, therefore, for (EE)-OPVEx Box to exploit multiple modes
of binding different guests with various electronic configurations. In order to confirm the
2
4+
binding diversity of (EE)-OPVEx Box
,
two π-electron-rich guestse.g., anthracene and
peryleneand two π-electron-deficient guestse.g., 9,10-AQ and 5,12-TQwere selected as
representative examples. High-quality single crystals of their 1:1 complexes were obtained by
2
4+
slow vapor diffusion of the iPr O into a solution of (EE)-OPVEx Box containing excesses of
2
guest molecules in MeCN over a period of one week. The X-ray diffraction analyses of these
crystals reveal (Figure 5) the various binding modes and diverse superstructures of the
complexes.
In the case of the π-electron-rich guests, the colors of the crystals reflect directly the charge
transfer interaction between the host and guests, a situation which could also be indicated
(Figure S4) by the corresponding charge-transfer bands in absorption spectra. The crystals of
2
4+
the anthracene⊂(EE)-OPVEx Box complex are deep yellow (Figure S4a, inset), while those
2
4+
of the perylene⊂(EE)-OPVEx Box complex are orange (Figure S4b, inset), making it
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convenient to detect them visually when the electron-rich guests are complexed inside the
cavity of the cyclophane. The long axis of the anthracene is (Figure 5a and S42) almost aligned
along the one side of the cyclophane, and it is positioned closer to one end of the cyclophane
than the other, a situation which can be achieved with the assistance of change-transfer
interactions at the end of the cyclophane and favorable van der Waals interactions in the center.
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0
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0
2
4+
The binding mode of perylene inside the cavity of (EE)-OPVEx Box is analogous to that of
anthracene. The long axis of perylene almost adopts (Figure 5b and S43) a parallel arrangement
2
4+
inside the (EE)-OPVEx Box . Anthracene and perylene show analogous stacking in the solid-
2
4+
state superstructure. A 2:1 ratio is observed between anthracene and (EE)-OPVEx Box in the
crystal, wherein one anthracene molecule is located inside the cavity of the cyclophane and the
other is located outside (Figure S42). Similarly, perylene co-crystallizes with (EE)-
2
4+
OPVEx Box in a ratio of 7:4, wherein one perylene molecule is positioned inside the cavity
of the host and the remaining perylene molecules were accommodated (Figure S43) in the space
between two host−guest complexes. Nevertheless, the binding mode between (EE)-
2
4+
OPVEx Box and electron-deficient guests is different from that of the electron-rich guests.
The 9,10-AQ resides in the center of the cyclophane, accompanied (Figure 5c and S44) by a tilt
angle of 24°, which is caused by the π-electron-poor central quinone substructure in 9,10-AQ
that is prone to overlap with the π-electron-rich biphenylene and divinylbenzene units in the
2
4+
center of (EE)-OPVEx Box . On the other hand, 5,12-TQ is (Figure 5d and S45) completely
aligned along its long axis inside the center of the cavity, and the π-electron-poor quinone
2
4+
subunit is 1.3 Å distant from the central position of (EE)-OPVEx Box , achieving maximized
2
4+
π-π overlap between host and guest. And both the 9,10-AQ⊂(EE)-OPVEx Box and 5,12-
2
4+
TQ⊂(EE)-OPVEx Box are crystallized in 1:1 host-guest complexes instead of forming co-
crystals between the complexes and excesses of guests. In the case of the 9,10-AQ⊂(EE)-
2
4+
2
2+
OPVEx Box , the complexes are integrated by π···π interaction between the Ex BIPY and
OPV2+ units to form (Figure S44) a layered superstructure, while the 5,12-TQ⊂(EE)-
2
4+
OPVEx Box relies on ion dipole interactions between the positive charge pyridinium units in
2
4+
(EE)-OPVEx Box and the carbonyl group in complexed 5,12-TQ to form (Figure S45) a
sandwich superstructure, revealing the change of the guest molecules is a feasible way to
modulate crystal superstructures.
2
4+
In order to elucidate the binding affinity of (EE)-OPVEx Box towards guest molecules in
2
4+
1
solution, their complexes with (EE)-OPVEx Box were investigated by H NMR spectroscopy
1
2
4+
and HRMS. The H NMR spectra (Figure 6) of the complexes formed by (EE)-OPVEx Box
with both the π-electron-rich guestse.g., anthracene and peryleneand π-electron-poor
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guestse.g., 9,10-AQ and 5,12-TQall show characteristic upfield shifts. The resonances
corresponding to protons H
a
, H
b
on the biphenylene, H
k
, H , H
l m
on the divinylbenzene, and H ,
c
2
4+
H
j
on the pyridinium rings in (EE)-OPVEx Box , and the protons in the guest molecules all
show an obvious upfield shift. These results reveal the guest molecules are included inside the
cavity of the cyclophane. The specific changes in chemical shift for the individual protons of
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guest molecules were investigated in detail. The chemical shifts of the protons (H
in anthracene underwent (Figure 6b) appreciable upfield shifts (Δδ = 0.07, 0.06 and 0.06 ppm
for H , H and H , respectively). The proton signals of perylene displayed (Figure 6c) significant
upfield shifts (Δδ = 0.15, 0.14 and 0.22 ppm for H , H and H , respectively). In addition, the
π-electron-poor guests 9,10-AQ and 5,12-TQ were also investigated. The chemical shifts of the
protons (H and H ) in 9,10-AQ exhibited (Figure 6d) appreciable upfield shifts (Δδ = 0.10 and
.14 ppm for H and H , respectively). The resonances for the each proton in 5,12-TQ, however,
display considerable upfield shifts (Figure 6e), in which the protons H and H exhibited the
largest upfield shifts (Δδ = 1.10 and 1.06 ppm for H and H , respectively), while the other
protons displayed upfield shifts in the range of 0.41 − 0.44 ppm, which can be attributed to
additional shielded of protons H and H by the p-xylylene at two ends of the cyclophane. The
Job plots for anthracene (Figure S26b), perylene (Figure S28b), 9,10-AQ (Figure S30b) and
α
, H
β
and H
γ
)
α
β
γ
α
β
γ
α
β
0
α
β
α
β
α
β
α
β
2
4+
5
,12-TQ (Figure S32b) with (EE)-OPVEx Box all show a maximum at a molar fraction of
.5, indicating a stoichiometric ratios of 1:1. The association constant between (EE)-
0
2
4+
3
-1
OPVEx Box and perylene (1.11×10 M ) (Figure S27 and S28a) is significantly higher than
-1
3
-1
that (135 M ) observed for anthracene (Figure S25 and S26a), and the K value (4.01×10 M )
a
2
4+
for the complexation of (EE)-OPVEx Box with 5,12-TQ (Figure S31 and S32a) is also higher
-1
than that (148 M ) with 9,10-AQ (Figure S29 and S30a). These significant increase in the
association constants suggested that the aromatic core of guest molecules possesses a
significant influence on the association constant, with the larger π-systems resulting in the
greater π-π interactions between host and guests.29 Moreover, the formation of the 1:1
complexes was also confirmed by HRMS. The HRMS spectra of the anthracene⊂(EE)-
2
4+
2
4+
OPVEx Box (Figure S40) and perylene⊂(EE)-OPVEx Box (Figure S41) show molecular
ion peaks at 1413.36 and 1487.37, assigned to [anthracene⊂(EE)-OPVEx Box – PF
]+ and
2
4+
6
2
4+
]+, respectively.
[
perylene⊂(EE)-OPVEx Box – PF
6
The remarkable photo- and thermal-induced isomerization and abundant host−guest
2
4+
interactions of OPVEx Box led us to investigate the photo- and thermal-controlled uptake
2
4+
and release of guest molecules. Upon exposure of the 9,10-AQ⊂(EE)-OPVEx Box complex
to blue light (450 – 460 nm), the (EE)→(EZ) isomerization of the host can be driven, and the
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chemical shifts of the 9,10-AQ protons (H
α
, and H ) in host−guest complex undergo (Figure
β
7c-g) a downfield shift gradually upon irradiation with blue light, indicating that the 9,10-AQ
2
4+
is expelled increasingly away from the cavity of OPVEx Box . Upon irradiation with blue
light (450 – 460 nm) for 10 s (Figure 7g), the chemical shifts of H
α
and H were identical with
β
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9
0
those of the reference H NMR spectrum (7a) containing 15% host (Figure 7a), indicating that
about 85% of the guest molecules have been released. The release ratio is identical with the
2
4+
2
4+
isomerization yield (85%) of (EE)-OPVEx Box , which shows that the (EZ)-OPVEx Box
has little or no binding affinity for 9,10-AQ. When heating the photostationary state mixture in
CD CN solution at 70 ºC for 16 h, the chemical shifts of protons in 9,10-AQ recover to their
original values (Figure 7h), revealing 9,10-AQ is bound inside the cavity of (EE)-
3
2
4+
OPVEx Box once again. The efficient release and uptake of π-electron-poor guests can also
2
4+
be confirmed by irradiating and heating the CD CN solution of 5,12-TQ⊂(EE)-OPVEx Box
3
complex. The chemical shifts of the 5,12-TQ protons are shifted (Figure S33) downfield upon
exposure to blue light and subsequently recover their initial values upon heating. In the case of
1
the π-electron-rich guests, similar release and uptake processes were observed in H NMR
spectra upon alternating blue light irradiation and heating. Upon irradiation of the CD CN
3
2
4+
solution of the anthracene⊂(EE)-OPVEx Box complex with blue light (450 – 460 nm), the
signals for anthracene protons undergo (Figure S34) a downfield shift, where almost 85%
anthracene is released during 12 s (Figure S34h), revealing that the light-controlled guest
release process is also functioning with π-electron-rich guests. On heating the CD CN solution
3
2
4+
of the anthracene⊂(EE)-OPVEx Box complex after irradiation, the chemical shifts of the
anthracene protons return (Figure S34i) to the initial values. In order to confirm the
photocontrolled π-electron-rich guests encapsulated process, the perylene⊂(EE)-
2
4+
1
OPVEx Box complex was also investigated by H NMR spectroscopy. Downfield shifts
observed (Figure S35) for the signals of perylene protons upon blue light irradiation and the
chemical shifts of the protons in perylene returned to their original values after heating. The
photocontrolled release process of π-electron-rich guests were also demonstrated (Figure S4)
by UV-Vis absorption spectroscopy. The CT band between host−guest complexes and the host
or guests decrease dramatically after blue light irradiation, owing to the isomerization of the
2+
OPV unit, leading to the release of the guest molecules. All of these results prove that the
2
4+
isomerization of OPVEx Box is not affected by guest molecules, which allow for reversible
capture and release of both π-electron-poor and π-electron-rich guests by heating and blue light
irradiation, respectively. The intrinsic reason for the photocontrollable guest releases may be
2
4+
attributed to the destruction of the preorganized (box-like) structure of the (EE)-OPVEx Box
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following blue light irradiation, which results in the width of the cyclophane being broaden, its
length being shorten, disfavoring effective binding between host and guests. More important is
2
+
the fact that the (EZ)-OPV unit protrudes outside the initial plane after blue light irradiation,
hindering the host and guests molecule to form the stable sandwich-like structure.
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■ CONCLUSION
2
4+
A semi-rigid photo- and thermal-responsive tetracationic organic cyclophane, OPVEx Box ,
has been shown to be converted reversibly between (EE)- and (EZ)-configurations by
alternating blue light irradiation and heating. The reaction rate and conversion yield of the
reverse isomerization were found to be significantly improved within the macrocyclic scaffold
in comparison with non-cyclic derivatives of photoactive oligo(p-phenylenevinylene)
bipyridinium units. The unique electronic character of the (EE)-isomer renders the cyclophane
capable of binding both π-electron-rich and π-electron-deficient guest molecules through
charge-transfer and van der Waals interactions. All the guest moleculese.g., anthracene,
perylene, 9,10-AQ and 5,12-TQcan be released and bound reversibly by irradiating and
heating in alternating fashion. This photo- and thermal-controlled molecular encapsulation
strategy, not only provides the incentive to design more advanced photoresponsive host−guest
systems and switchable molecular machines, but also possesses the potential for applications in
the fields of active ingredients stabilization, controllable drugs release, and toxins separation.
■
ASSOCIATED CONTENT
Supporting Information
Detailed synthetic procedures, crystallographic, and spectroscopic (NMR, HRMS, UV-Vis
absorption, and fluorescence) characterization for the (EE)- and the (EZ)-isomers of
2
OPVEx Box•4PF
6
and the complexes formed by anthracene, perylene, 9,10-AQ and 5,12-TQ
2
with the (EE)-OPVEx Box•4PF
6
. This information is available free of charge via the internet
at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
E-mail: yuliu@nankai.edu.cn
*
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*
E-mail: stoddart@northwestern.edu
Notes
The authors declare no competing financial interest.
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■
ACKNOWLEDGMENTS
This research was conducted as part of the Joint Center of Excellence in Integrated
Nanosystems (JCIN) at King Abdulaziz City for Science and Technology (KACST) and
Northwestern University (NU). The authors thank both KACST and NU for their continued
support of this research. The authors thank National Nature Science Foundation of China
(NNSFC, Grant Nos. 21432004, 21672113, 21772099, 21861132001 and 91527301) for the
financial support. This work was also supported by the China Scholarship Council.
■
REFERENCES
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Captions to Scheme and Figures
2
Scheme 1. Synthesis of (EE)-MeOPV•2PF
6
and (EE)-OPVEx Box•4PF
6
.
2
4+
Figure 1. Solid-state (super)structure of (EE)-OPVEx Box
obtained from X-ray
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crystallography on single crystals of (EE)-OPVEx Box•4PF
6
. (a) Space-filling representation
2
2+
showing the torsional angles of the Ex BIPY unit (the torsional angles between adjacent
2
4+
aromatic rings are ∼26°, ∼36°, and ∼25°, respectively) in (EE)-OPVEx Box ; (b − c) Stick
representations of different side views showing the distances defining the box-like geometry;
2
4+
(
d) Solid-state superstructure of (EE)-OPVEx Box revealing a herringbone type of packing
and two kinds of π···π interactions. The hydrogen atoms, counterions and solvent molecules
are omitted for the sake of clarity.
1
2
Figure 2. H NMR spectra (500 MHz, CD
3
CN, 298 K) of (EE)-OPVEx Box•4PF (a) before
6
and (b) after blue light irradiation (450 – 460 nm, 12 W, 12 s); (c) Spectrum after heating the
2
solution of (b) for 16 h at 70 °C; (d) Spectrum of (EZ)-OPVEx Box•4PF
6
.
Figure 3. (a) UV-Vis absorption spectra (10 μM in MeCN, 298 K) and (b) absorbance intensity
2
4+
changes at 406 nm of OPVEx Box upon irradiation with blue light (450 – 460 nm, 1 W, 16
s); (c) UV-Vis absorption spectra (10 μM in MeCN, 298 K) and (d) absorbance intensity
2
4+
changes at 406 nm of OPVEx Box upon alternating blue light irradiation (450 – 460 nm, 1
W, 12 s) and heating (70 ºC , 10 min).
2
4+
Figure 4. Solid-state (super)structure of (EZ)-OPVEx Box
obtained from X-ray
2
crystallography on single crystals of (EZ)-OPVEx Box•4PF
6
. (a − b) Stick representations of
2
4+
different side views of (EZ)-OPVEx Box showing the distances and angles associated with
the geometry after blue light irradiation; (c) The solid-state superstructure showing the face-to-
face stacking between the adjacent Ex BIPY and (EZ)-OPV2+ units in adjacent (EZ)-
2
2+
2
4+
molecules; (d) Space-filling representations of the (EZ)-OPVEx2Box4+
OPVEx Box
superstructure revealing a bilayer type of packing involving two kinds of π···π interactions. The
hydrogen atoms, counterions and solvent molecules are omitted for the sake of clarity.
Figure 5. Structural formulas of the guests and the corresponding K
a
values for binding inside
CN (left), the side-on (middle) and
plan (right) views of the stick / space-filling representations of the solid-state superstructures
2
4+
1
(
EE)-OPVEx Box obtained by H NMR titrations in CD
3
2
4+
of the complexes of (EE)-OPVEx Box with (a) anthracene, (b) perylene, (c) 9,10-AQ and (d)
,12-TQ. The hydrogen atoms, counterions, and solvent molecules are omitted for the sake of
clarity.
5
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Figure 6. Partial H NMR spectra (600 MHz, CD
3
CN, 298 K) of (a) (EE)-OPVEx Box and
1
its complexes with (b) anthracene, (c) perylene, (d) 9,10-AQ and (e) 5,12-TQ. The H NMR
1
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3
CN are shown above the H NMR spectra of the
2
4+
individual complexes (blue for the (EE)-OPVEx Box and red for the bound-guest signals) in
b − e).
Figure 7. Partial H NMR spectra (600 MHz, CD
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(
1
3
CN, 298 K) of (a) 9,10-AQ with 15% (EE)-
2
4+
2
4+
OPVEx Box ; and 9,10-AQ⊂(EE)-OPVEx Box complex (b) before and (c−g) after blue
light (450 – 460 nm, 12 W) irradiation for (c) 2 s, (d) 4 s, (e) 6 s, (f) 8 s, and (g) 10 s; (h)
Spectrum after heating the solution of (g) at 70 °C for 16 h. The resonance peaks for the protons
in 9,10-AQ are indicated in red color.
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