Ternary Cocrystals with Large Soft Cavities: A 1,4-diiodotetrafluorobenzene (DITFB)?4-Biphenylpyridine N-oxide (BPNO) Host Assembled by Inclusion of Planar Aromatic Guests

Article abstract of DOI:10.1002/cplu.202000779

A large soft-cavity host composed of 1,4-diiodotetrafluorobenzene (DITFB) and 4-biphenylpyridine N-oxide (BPNO) is assembled under the mediation of a planar aromatic guest molecule (pyrene or perylene) through C?I???^?O?N⁺ halogen bonds and π-hole???π bonds. Single-crystal X-ray diffraction reveals that guest molecules can be completely encapsulated in the four-layer host cavity to assemble ternary host-guest cocrystals; namely, Pyr@DITFB ? BPNO and Per@DITFB ? BPNO. The luminescence of these ternary cocrystals originates from their discrete guest molecules, which exhibit pure-blue and yellow emissions, respectively, that are localized at 425 nm and in the range of 485 to 578 nm, respectively. In addition, the contribution of different fragments to the stabilization of the crystal structure is estimated by computational chemistry. These cocrystals have significant potential for use in optical applications or materials, such as photonics or organic light-emitting diodes, respectively, that require to avoid the aggregation between luminophores.

Full text of DOI:10.1002/cplu.202000779

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Ternary Cocrystals with Large Soft Cavities: A 1,4-
diiodotetrafluorobenzene (DITFB)·4-Biphenylpyridine N-
oxide (BPNO) Host Assembled by Inclusion of Planar
Aromatic Guests
Wen Juan Liang,[a] Hui Wang,*[b] Xue Chen,[c] Ting Ting Zhang,[b] Yun Feng Bai,[a]
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11 Feng Feng,*[a] and Wei Jun Jin*[a, c]
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A large soft-cavity host composed of 1,4-diiodotetrafluoroben-
zene (DITFB) and 4-biphenylpyridine N-oxide (BPNO) is as-
discrete guest molecules, which exhibit pure-blue and yellow
emissions, respectively, that are localized at 425 nm and in the
range of 485 to 578 nm, respectively. In addition, the contribu-
tion of different fragments to the stabilization of the crystal
structure is estimated by computational chemistry. These
cocrystals have significant potential for use in optical applica-
tions or materials, such as photonics or organic light-emitting
diodes, respectively, that require to avoid the aggregation
between luminophores.
sembled under the mediation of a planar aromatic guest
molecule (pyrene or perylene) through CÀ I···À OÀ N+ halogen
bonds and π-hole···π bonds. Single-crystal X-ray diffraction
reveals that guest molecules can be completely encapsulated in
the four-layer host cavity to assemble ternary host-guest
cocrystals; namely, Pyr@DITFB·BPNO and Per@DITFB·BPNO. The
luminescence of these ternary cocrystals originates from their
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Introduction
cavities using σ-hole and π-hole bonds.[42–45] Such soft or flexible
cavities can capture different types of guest molecules in
response to environmental changes, providing them with the
potential for use in various applications. In terms of optical
materials, the soft-cavity host assembled by 1,4-diiodotetrafluor-
obenzene (DITFB) and 4-phenylpyridine N-oxide (PPNO) can
encapsulate polyaromatic hydrocarbon (PAH) guest molecules of
different sizes and shapes to generate a ternary host-guest
cocrystal.[42–44] This host cavity can act as a lattice unit to isolate
the PAH guest, allowing it to effectively avoid the luminescent
quenching of guest molecules in the solid phase. Similar cavities
or cages include zeolite-like hydrogen-bonded frameworks and
small-molecule ionic isolation lattice materials.[46,47] However, it is
unclear whether a larger soft cavity can also act as a lattice unit
to isolate the light-emitting guest molecules; specifically, to avoid
the luminescent quenching of guest molecules.
In this work, a larger soft-cavity host is assembled using
DITFB and 4-biphenylpyridine N-oxide (BPNO) under the media-
tion of pyrene (Pyr) or perylene (Per) molecules as planar guests.
The CÀ I···À OÀ N+ halogen bonds, π-hole···π bonds and other
weak interactions resulted in the formation of a ternary host-
guest cocrystal. The constructed soft cavity can adaptively
capture the Pyr or Per molecules, and also act as a lattice unit to
separate them. The structures of two ternary host-guest
cocrystals are compared with each other and with the ternary
cocrystals fabricated by DITFB and PPNO. Additionally, the
binding strengths of the main noncovalent interactions within
the ternary cocrystals are measured by computational chemistry,
and their luminescence properties are examined.
Supramolecular host-guest chemistry has attracted significant
interest in the fields of material science, molecular recognition,
gas storage, drug delivery, and biomedical engineering.[1–8] At
present, hosts with an intrinsic cavity, capsule or cage-type
structure have been constructed based on various noncovalent
interactions to recognize or capture guest molecules among
various other applications.[9–11] Such hosts include metal-organic,
covalent-organic, hydrogen-bonded organic, and halogen-
bonded organic frameworks, as well as porous organic polymers
and inclusion crystals.[12–31] Among the aforementioned non-
covalent interactions, σ-hole and π-hole bonds are excellent
supramolecular interactions owing to their high directionality,
tunable binding strength, and hydrophobicity.[32–41]
Due to the lack of flexibility and applicability of rigid cavities,
recent efforts have been made to construct soft or flexible host
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[a] Dr. W. J. Liang, Prof. Y. F. Bai, Prof. F. Feng, Prof. W. J. Jin
School of Chemistry and Chemical Engineering
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Shanxi Datong University
Datong, Shanxi 037009 (P. R. China)
E-mail: feng-feng64@263.net
wjjin@bnu.edu.cn
[b] Assoc. Prof. H. Wang, Assoc. Prof. T. T. Zhang
College of Chemistry and Material Science
Shanxi Normal University
Linfen, Shanxi 041004 (P. R. China)
E-mail: wanghui@sxnu.edu.cn
[c] X. Chen, Prof. W. J. Jin
College of Chemistry
Beijing Normal University
Beijing 100875 (P. R. China)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.202000779
This article is part of a Special Collection dedicated to the 4th International
Symposium on Halogen Bonding (ISXB-4)
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Results and Discussion
2.954, 2.984 Å) hydrogen bonds. Additionally, it was observed
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that four BPNO molecules from adjacent building motifs from an
octagonal unit, as indicated by the cyan shaded area in
Figure 1a.
Crystal structures and bonding models
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Due to its highly polarized N-oxide moiety,[48–54] the halogen-
bond acceptor BPNO was selected to interact with halogen-bond
donor DITFB. This interaction was predicted to give a binary host
cocrystal with a larger cavity structure than that of the
DITFB·PPNO cocrystal,[42] so as to better accommodate the larger
π-electron-rich aromatic guests (e.g., Pyr and Per). To obtain the
crystal structure and compare their differences, four crystals
suitable for XRD analysis were achieved by slowly evaporating
the solvent at room temperature. That are as follows: a unary
BPNO (C17H13NO) crystal, a binary DITFB·BPNO (C40H26F4I2N2O2)
cocrystal, and ternary Pyr@DITFB·BPNO (C62H36F8I4N2O2) and
Per@DITFB·BPNO (C66H38F8I4N2O2) cocrystals. The major crystal
structure data of these crystals are summarized in Table 1.
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Binary DITFB·BPNO cocrystal
When the DITFB molecule was introduced into the BPNO lattice,
the conformation of the BPNO molecule changes greatly,
exhibiting two conformations bearing corresponding torsion
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°
°
°
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angles of either 24.5 and 38.8 or 26.1 and 26.2 . The produced
binary cocrystal is monoclinic with a Pc space group and exhibits
a 1:2 stoichiometry of DITFB to BPNO.
A similar building motif formed by BPNO was observed in
the binary cocrystal, which was further connected by the bridged
DITFB molecule to form infinite 1D chains based on CÀ I···À OÀ N+
halogen bonds and CÀ H···π (d=2.839, 2.845 and 2.900 Å), CÀ H···I
(d=3.154 Å) and CÀ H···F (d=2.588 Å) hydrogen bonds, as shown
in Figure 1b. The resultant halogen-bond lengths of
C18À I1···À O1À N1+ and C21À I2···À O2À N2+ [for the O1 and O2
atoms, respectively, at (x, y, z)] are 2.823(4) and 2.710(4) Å,
respectively. These values are 19.3% and 22.6% shorter than the
sum of the van der Waals radii of the iodine and oxygen atoms,
respectively. The adjacent 1D chains are connected to each other
by other CÀ H···π (d=2.810, 2.879 and 2.881 Å) hydrogen bonds
to generate a 2D sheet (Figure 1b). The levels of this split-level,
2D network cross-link together by the π-hole···π bonds of DITFB
and BPNO and additional CÀ H···π (d=2.810–2.881 Å) and CÀ H···O
Unary BPNO crystal
Single-crystal XRD revealed that the BPNO crystal has
a
monoclinic P21/c space group with one molecule in the
asymmetric unit. The BPNO molecule adopts a nearly planar
°
geometry with a small torsion angle of 3.3 between the middle
°
phenyl and pyridine N-oxide unit and an angle of 0 between
the middle phenyl and the peripheral phenyl. The adjacent BPNO
molecules interacted with each other through weak CÀ H···π
hydrogen bonds (d=2.933–2.945 Å) to construct a building motif
(Figure 1a), which is further propagated to a 1D chain and 2D
network by CÀ H···O (d=2.340, 2.529 Å) and other CÀ H···π (d=
(d=2.384, 2.679 Å) hydrogen bonds to construct
supramolecular structure (cf. Figure S1). Furthermore, the π-hole
a 3D
Table 1. Crystal data and structure refinement of BPNO, DITFB·BPNO, Pyr@DITFB·BPNO, and Per@DITFB·BPNO crystals.[a]
Compound
BPNO
DITFB·BPNO
Pyr@DITFB·BPNO
Per@DITFB·BPNO
CCDC no.
Formula
Weight
Crystal system
Space group
a/Å
2018346
C17H13NO
247.28
monoclinic
P21/c
7.8148(3)
5.5433(2)
27.9825(8)
90
95.581(3)
90
1206.45(7)
1.361
2018347
C40H26F4I2N2O2
896.43
monoclinic
Pc
8.3283(2)
9.6376(2)
21.0236(4)
90
95.010(2)
90
1681.01(6)
1.771
2018348
C62H36F8I4N2O2
1500.53
monoclinic
P2/n
13.9671(3)
19.2085(3)
20.0437(4)
90
96.364(2)
90
5344.33(18)
1.865
2018349
C66H38F8I4N2O2
1550.58
triclinic
P-1
11.3495(3)
14.8321(4)
18.2549(4)
67.449(2)
77.942(2)
80.450(2)
2762.84(13)
1.864
b/Å
c/Å
°
α/
°
β/
γ/
°
V/Å3
1calcd/g·cmÀ 1
Z
4
2
4
2
T/K
100.00(10)
0.668
520.0
99.99(10)
15.224
876.0
100.0(3)
18.961
2888.0
40426
99.98(10)
18.365
1496.0
32761
11056
0.0398
0.0388
0.1108
0.0442
μ/mmÀ 1
F(000)
Reflections collected
Independent reflections
Rint
R1 (I>2σ(I))
wR(F2) (I>2σ(I))
R1 (all data)
wR(F2) (all data)
Goodness-of-fit on F2
7321
2343
18972
5587
10633
0.0224
0.0446
0.1279
0.0546
0.1366
1.061
0.0336
0.0347
0.0910
0.0363
0.0926
1.076
0.0493
0.0490
0.1441
0.0574
0.1502
1.105
0.1177
1.113
[a] BPNO=4-biphenylpyridine N-oxide crystal; DITFB=1,4-diiodotetrafluorobenzene; DITFB·BPNO=binary cocrystal formed by DITFB and BPNO;
Pyr@DITFB·BPNO and Per@DITFB·BPNO=ternary cocrystals formed by DITFB, BPNO and pyrene (Pyr) or perylene (Per), respectively.
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Figure 2. Structural details of the Pyr@DITFB·BPNO cocrystal: (a) The
CÀ I···À OÀ N+ halogen bonds between DITFB and BPNO molecules, (b, c) the
π-hole···π bonds between interlamellar DITFB and BPNO molecules, and (d)
the π-hole···π bonds between two reversed BPNO molecules (distances in Å).
z)/(x, yÀ 1, z) and (x, y, z)] are 2.772(4), 2.772(4), 2.779(4), and
2.781(4) Å, respectively. These values are approximately 20.6%
shorter than the sum of the van der Waals radii of the iodine and
oxygen atoms, indicating very strong CÀ I···À OÀ N+ halogen
bonds.
Figure 1. 2D networks of (a) the unary BPNO crystal constructed by CÀ H···π
and CÀ H···O hydrogen bonds and (b) the binary DITFB·BPNO cocrystal
constructed by CÀ I···À OÀ N+ halogen bonds and CÀ H···π and CÀ H···F hydro-
gen bonds (distances in Å).
The 1D zigzag chains assembled in the closed cage-type
cavity structure along the ab-plane direction through the π-
hole···π bonds of adjacent DITFB and BPNO molecules, the
interactions between two inversely arranged BPNO molecules,
the π-hole···π-hole stackings of two parallel DITFB units, and the
CÀ H···F hydrogen bonds (Figures 2b–2d, 3). The Pyr molecule
acting as a guest is vertically encapsulated in the host cavity
structure along the b-axis direction through CÀ H···π hydrogen
bonds. Thus, it can be seen that the structure of the
Pyr@DITFB·BPNO cocrystal is very similar to that of the
Pyr@DITFB·PPNO cocrystal.[44] While there are some differences
between Pyr@DITFB·BPNO and Pyr@DITFB·PPNO cocrystals, one
is the spatial dimension of the closed cavity and the other is the
bonding models between the host cavity and guest molecules.
The closed cavity constructed by BPNO (14.4 Å×5.7 Å×10.5 Å) is
much larger than that constructed by PPNO (11.6 Å×5.7 Å×
10.5 Å), and the distance of the isolated Pyr guest molecules
becomes larger in the Pyr@DITFB·BPNO cocrystal than it was in
the Pyr@DITFB·PPNO cocrystal (Figure S2). This results in only
weak CÀ H···π hydrogen bonds existing between BPNO and Pyr in
the Pyr@DITFB·BPNO cocrystal, with no CÀ H···F hydrogen bonds
being observed between the Pyr and DITFB units.
bonding lengths of C20···C12 and C23···C3 [for the C12 and C3
atoms at (1+x, 1À y, zÀ 0.5)] are 3.399(9) and 3.383(9) Å,
respectively, which are 8.1% and 8.6% shorter than the sum of
the van der Waals radii of the two benzene rings.
We noted that the bonding lengths of the CÀ H···π hydrogen
bonds (d=2.810–2.900 Å) in the binary DITFB·BPNO cocrystal
were shorter than those in the unary BPNO crystal (d=2.933–
2.986 Å), meaning that the introduction of DITFB strengthens the
CÀ H···π hydrogen bonds of the BPNO molecules. Additionally,
although the structure of BPNO is very similar to that of PPNO, a
supramolecular cavity network was not formed in the binary
DITFB·BPNO cocrystal, but was formed in the DITFB·PPNO
cocrystal.[42] To verify whether the cavity structure could be
formed by DITFB and BPNO under the mediation of planar π-
electron-rich aromatic guests, the ternary cocrystals were
prepared.
Ternary Pyr@DITFB·BPNO cocrystal
XRD analysis revealed that the Pyr@DITFB·BPNO cocrystal has
the P2/n space group of the monoclinic system with a 2:2:1
stoichiometry of DITFB:BPNO:Pyrene. In the cocrystal, the BPNO
molecules exhibit two types of conformations, bearing corre-
Ternary Per@DITFB·BPNO cocrystal
°
°
°
sponding torsion angles of either 11.1 and 28.6 or 22.7 and
°
22.0 . Each iodine atom of DITFB interacts with the N-oxide
The Per@DITFB·BPNO cocrystal possesses a 2:2:1 stoichiometry
of DITFB:BPNO:Per, and exhibits a cage-type cavity (15.0 Å×
5.7 Å×10.5 Å) similar to that of the Pyr@DITFB·BPNO cocrystal.
The main differences between the two cocrystals are their crystal
systems, space groups, bonding modes, and arrangement modes
oxygen of BPNO to generate a 1D zigzag chain via CÀ I···À OÀ N+
halogen bonds, as shown in Figure 2a. The resultant
C1À I1···À O2À N2+, C4À I2···À O1À N1+, C7À I3···À O1À N1+, and
C10À I4···À O2À N2+ separations [for the O1 and O2 atoms at (x, y,
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of their guest molecules. Specifically, the Per@DITFB·BPNO
cocrystal has a triclinic system with a P-1 space group. The
bonding modes in the host structure are CÀ I···À OÀ N+ halogen
bonds and CÀ H···I hydrogen bonds. The resultant lengths of the
C6À I1···À O1À N1+, C3À I2···À O2À N2+, C10À I3···À O2À N2+, and
C7À I4···À O1À N1+ bonds [for the O1 and O2 atoms, respectively,
at (x, y, z) and (x, y, z)/(x+1, y, zÀ 1)] are 2.720(3), 2.757(3),
2.780(3), and 2.724(3) Å, respectively. These values are 20.6%–
22.3% shorter than the sum of the van der Waals radii of
interacting atoms (Figure S3a), also indicating very strong
CÀ I···À OÀ N+ halogen bonds. It can be noted in terms of bond
length that the CÀ I···À OÀ N+ halogen bonds in the Per@DITFB·BP-
NO cocrystal are slightly stronger than those in the Pyr@-
DITFB·BPNO cocrystal. Additionally, owing to the larger size of
the host cavity, the Per molecule, which is larger than Pyr, can be
completely and obliquely encapsulated in the four-layered host
cavity by the CÀ H···F and CÀ H···I hydrogen bonds as well as the
CÀ H···π bonds (Figures S4a and S4b). This is largely different from
the encapsulation exhibited by the Per@DITFB·PPNO cocrystal.[44]
CÀ I···À OÀ N+ halogen bonds (2.823 Å, Figure 1b) and CÀ H···I
hydrogen bonds (3.154 Å, Figure 1b) is À 30.4 kJ·molÀ 1.
For the ternary Pyr@DITFB·BPNO cocrystal, the interaction
energies of the CÀ I···À OÀ N+ halogen bonds (Figure 2a) are
between À 25.0 and À 30.8 kJ·molÀ 1. The π-hole···π bonding
energies for the DITFB and BPNO stacking units (Figures 2b and
2c) are À 54.8 and À 53.2 kJ·molÀ 1, respectively, and that for the
stacking unit of two reversed BPNO units (Figure 2d) is
À 83.7 kJ·molÀ 1.
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In addition, for the ternary Per@DITFB·BPNO cocrystal, the
CÀ I···À OÀ N+ halogen bonding strength (À 26.6–30.4 kJ·molÀ 1,
Figure S3a) is similar to that observed in the Pyr@DITFB·BPNO
cocrystal, while the π-hole···π bonding strength for the stacking
unit of DITFB and BPNO and for two reversed BPNO units (À 51.3
and À 76.7 kJ·molÀ 1, respectively, Figures S4b and S4c) were
weaker than those in the Pyr@DITFB·BPNO cocrystal.
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Photoluminescence behavior
The luminescent properties of the unary BPNO crystal and the
cocrystals were investigated. The unary BPNO crystal emits
fluorescence at 420 nm, but it is extremely weak with a lifetime
of 1.79 ns (Figure S6) and a quantum yield of only 0.4%
(Table S3). Similarly, the luminescence of the binary DITFB·BPNO
cocrystal is also very weak, but its lifetime is 1.10 ns (Figure S7)
and its quantum yield is 0.7% (Table S3).
Figure 4 shows photos of the ternary cocrystals, Per and Pyr
crystals under ambient light and UV-365 nm light. Their
luminescence spectra are shown in Figure 5. As shown in
Figure 4, for the ternary Per@DITFB·BPNO and Pyr@DITFB·BPNO
cocrystals, only the fluorescence of the guest monomer is
detected under excitation by 365-nm and 389-nm UV light,
respectively. Their fluorescence lifetimes are 4.78 ns and 0.47 ns,
respectively, and their absolute quantum yields are 13.3% and
2.2%, respectively (Figure S8 and S9). We also noted that the
fluorescence of the Per@DITFB·BPNO cocrystal is localized over a
wide wavelength range, from 485 to 578 nm (Commission
Quantum chemistry calculations
Calculations were carried out to estimate the contributions of the
different fragments of the lattice to the stabilization of the crystal
structures.[55] The selected interaction units and their geometry
parameters for calculating interaction energies were extracted
from the crystal structure data, as shown in Figures 1–3 and S3–
S4. The results are listed in Table S2.
For the unary BPNO crystals, the energy of the shorter
CÀ H···O hydrogen bond (2.340 Å, Figure 1a) is À 18.4 kJ·molÀ 1,
and the energy for the longer one (2.529 Å, Figure 1a) is
À 12.8 kJ·molÀ 1. The energy of the building motif constructed by
the CÀ H···π hydrogen bonds is À 48.9 kJ·molÀ 1.
For the binary DITFB·BPNO cocrystal, the halogen bonding
strength estimated for the shorter CÀ I···À OÀ N+ bonds (2.710 Å,
Figure 1b) is À 29.5 kJ·molÀ 1. The sum of the energies of the
Figure 3. The cavity-type structure of host-guest Pyr@DITFB·BPNO cocrystal. (a) The host-guest structure units along the bc-plane direction with Pyr molecules
trapped in the cavity by CÀ H···π hydrogen bonds. (b) 3D supramolecular host-guest units along the ab-plane direction.
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In addition, there was no new emission band observed for
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the Per@DITFB·BPNO cocrystal in the phosphorescence mode
(set delay time 0.1 ms). Only a very weak band overlapped with
the fluorescence spectrum was noted, which is likely a weak
delayed fluorescence (Figure 5a) or is caused by a small amount
of transmitted light from the xenon lamp. In addition, the
microsecond-scale lifetime is not captured. Therefore, the
cocrystal had no phosphorescence. For the Pyr@DITFB·BPNO
cocrystal, the phosphorescence spectrum at a longer wavelength
domain was detected with a phosphorescence lifetime of
0.213 ms (Figure S9).
Therefore, the luminescence of Per and Pyr are derived from
their separate monomers in the ternary cocrystal, which is
different from their respective unary crystals (Figure 4). The two
examples show that the soft-cavity system studied in this work
can isolate PAH guest molecules, allowing them to discretely
exist in the solid material system without mutual disturbance.
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Figure 4. Photos of (a, b) ternary Per@DITFB·BPNO cocrystal, (c) single Per
crystal, (d, e) ternary Pyr@DITFB·BPNO cocrystal, and (f) single Pyr crystal.
The photos in panels (a) and (d) were taken under ambient light, while those
in panels (b), (c), (e), and (f) were taken under UV light (365 nm).
Conclusion
The soft-cavity host structures described here have significant
potential in the development of solid fluorescent materials,
because they can act as small-molecule isolation lattices to
inhibit fluorescence quenching. In this study, monomeric BPNO,
binary DITFB·BPNO, and ternary Pyr@DITFB·BPNO and Per@-
DITFB·BPNO single crystals were assembled to compare their
structures and properties. We observed that the supramolecular,
cavity-type network was not formed in the binary DITFB·BPNO
cocrystal. However, under the mediation of π-electron-rich
aromatic guest molecules (Pyr or Per), the larger soft-cavity host
was successfully assembled from DITFB and BPNO through
CÀ I···À OÀ N+ halogen bonds, π-hole···π bonds, and other weak
interactions. This resulted in the formation of a ternary host-
guest cocrystal.
It is worth noting that the thickness of the host cavity does
not change with the size of the guest molecule. Instead, the
isolated Pyr or relatively large Per molecules can be encapsulated
completely in the four-layer host cavity. The luminescence of the
Pyr@DITFB·BPNO and Per@DITFB·BPNO host-guest cocrystals
were localized in the range of 485 to 578 nm (CIE: 0.4886, 0.4351)
and at 425 nm (CIE: 0.1612, 0.0715), respectively, and had
originated from their discrete monomers. Furthermore, com-
pared to what was observed for the guest molecules within the
ternary Pyr/Per@DITFB·PPNO cocrystals[44] the distance between
the isolated guest molecules in the present work were larger and
their contactless optical interactions were weaker, leading to
improvements in their luminescence behaviors.
Figure 5. Normalized total luminescence spectra (blue) and the spectra (red)
measured in phosphorescence mode (red) (setting delay time as 0.1 ms) of
(a) Per@DITFB·BPNO and (b) Pyr@DITFB·BPNO.
Internationale de l'Eclairage, CIE: 0.4886, 0.4351, Figure S10),
while that of the Pyr@DITFB·BPNO cocrystal is localized at
425 nm (CIE: 0.1612, 0.0715). All of the fluorescence observed for
the cocrystals was due to their discrete monomers. The
fluorescence of the Pyr crystal is cyan (Figure 4f) with an emission
wavelength of 470 nm (CIE: 1809, 2475),[44] and the fluorescence
of the Per crystal is red (Figure 4c) with an emission wavelength Experimental Section
of 606 nm (CIE: 0.5325, 0.4595).44 The fluorescence of the
Pyr@DITFB·BPNO (present work) and Pyr@DITFB·PPNO[44] cocrys-
tals are different from one another, with the former displaying a
pure-blue color (425 nm) and the latter purple-pink (397 nm, CIE:
0.2016, 0.0614).
Materials and chemicals
1,4-diiodotetrafluorobenzene (DITFB, 98%) was purchased from
Sigma-Aldrich. Pyrene (Pyr, 98%) and perylene (Per, 98%) were
purchased from J&k Scientific Ltd. 4-Bromopyridine, [1,1’-biphenyl]-
4-ylboronic acid, Pd(PPh3)4, and 4-([1,1’-biphenyl]-4-yl)pyridine, and
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3-chloroperoxybenzoic acid (mcpba) were purchased from Bide
Pharmatech Ltd., of China. All other organic solvents, including
ethanol, methanol (MeOH), acetone, chloroform, dichloromethane
(DCM), and ethyl acetate (EA), and Na2CO3 were of analytical grade
and obtained commercially from the Beijing Company of China
National Medicals.
Ternary Pyr/Per@DITFB·BPNO cocrystals: DITFB, BPNO and Pyr or
Per in a 2:2:1 molar ratio were dissolved in the chloroform in a
glass vial, which was sealed using Parafilm with pinholes and kept
in the dark at room temperature to slowly evaporate the solvent.
Well-formed crystals suitable for XRD measurement appeared
within approximately one to two weeks.
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Synthesis of BPNO
Single crystal X-ray diffraction analysis
4-([1,1’-Biphenyl]-4-yl)pyridine-N-oxide (4-biphenylpyridine N-oxide,
BPNO) was synthesized according to the pathway shown in
Scheme 1 and the detail procedure is subsequently described.
All the single-crystal XRD data of the cocrystals were collected
using a SuperNova Rigaku Oxford Diffraction diffractometer with
dual X-ray sources (Cu and Mo, CuÀ Kα radiation, λ=1.5418 Å, were
selected for the experiments). Using Olex2,[56] the structure was
solved with the ShelXT[57] structure solution program using direct
methods and refined with the olex2.refine[58] refinement package
using Gauss-Newton minimization. Hydrogen atoms were added
according to the theoretical analysis.
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4-([1,1’-Biphenyl]-4-yl)pyridine: A mixture of 4-bromopyridine (3 g,
18.9 mmol), [1,1’-biphenyl]-4-ylboronic acid (4.5 g, 22.7 mmol), Pd-
(PPh3)4 (1.09 g, 0.945 mmol), and Na2CO3 (4 g, 37.8 mmol) in 1,4-
°
dioxane (40 mL) and H2O (5 mL) was heated at 90 C for 18 h under
a N2 atmosphere. The reaction mixture was cooled to room
temperature and filtered through a celite pad. The resulting filtrate
was concentrated under reduced pressure to give the crude
mixture, which was diluted with EA (50 mL), washed with water and
brine, dried over Na2SO4, and filtered. The filtrate was concentrated
under vacuum to give a crude product, which was purified by flash
column chromatography (silica gel, eluting with 5% to 8% MeOH/
DCM) to afford 4-([1,1’-biphenyl]-4-yl)pyridine (3.2 g, 73%) as a
Deposition numbers 2018346 (for BPNO), 2018347 (for DITFB·BP-
NO), 2018348 (for Pyr@DITFB·BPNO) and 2018349 (for Per@-
DITFB·BPNO) contain(s) the supplementary crystallographic data
for this paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinformationszen-
trum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/
structures.
white solid. HPLC/UV purity: 100%; LC-MS (ESI): 232.2 (M+1)+. H
NMR (DMSO-d6): (ppm) 8.65–8.66 (dd, J=6 Hz, 1.6 Hz, 2H), 7.91–
7.93 (d, J=8.4 Hz, 2H), 7.82–7.84 (d, J=8.4 Hz, 2H), 7.74–7.78 (m,
4H), 7.48–7.52 (m, 2H), 7.39–7.42 (m, 1H).
1
Luminescence measurements
Steady fluorescence spectra of the samples and decay profiles of
their luminescence were measured using an FLS980 Spectrometer
(Edinburgh Instruments Ltd.). The fluorescence lifetimes (τ) were
obtained by fitting the decay curves. The τ of the samples were
obtained by deconvolution fitting (fluorescence) or tail fitting
(phosphorescence) of the decay curve with a multi-exponential
decay function of I(t)=A1exp(À t/τ1)+A2exp(À t/τ2)+···+Aiexp(À t/
τi), where Ai and τi represent the amplitudes and lifetimes of the
individual components of the multi-exponential decay profiles. The
mean lifetime was estimated as <τ> =A1τ1 +A2τ2 +···+Aiτi (where
Ai represents the normalized coefficient defined as Ai =Ai/�Ai (i=
1~n)).
4-([1,1’-Biphenyl]-4-yl)pyridine-N-oxide (BPNO): To a solution of 4-
([1,1’-biphenyl]-4-yl)pyridine (3.2 g, 13.8 mmol) in DCM (30 mL) was
added 3-chloroperoxybenzoic acid (mcpba, 3.35 g, 16.5 mmol, 85%
°
) dropwise at 0 C (icy water) under N2. Then, the mixture was
stirred at room temperature for 18 h. The reaction mixture was
quenched with ice water (100 mL), and then extracted with DCM
(60 mL×3). The organic layer was washed with aqueous NaHCO3
(50 mL×3), water (50 mL), and brine (50 mL), dried over Na2SO4,
concentrated, and purified by flash column chromatography (silica
gel, eluting with 8% to 10% MeOH/DCM) to afford 4-([1,1’-
biphenyl]-4-yl)pyridine-1-oxide (2.5 g, 74%) as a white solid. HPLC/
UV purity: 100%; LC-MS (ESI): 248.3(M+1)+. 1H NMR (DMSO-d6):
(ppm) 8.29–8.30 (dd, J=8 Hz, 2.8 Hz, 2H), 7.79–7.90 (m, 6H), 7.42
(dd, J=8.0 Hz, 2.8 Hz, 2H), 7.48–7.51 (m, 2H), 7.38–7.42 (m, 1H).
The absolute quantum yields of the three oligomers at room
temperature were estimated by using an integrating sphere (F-
M101, Edinburgh) accessory in an FLS980 fluorescence spectrom-
eter (Edinburgh Instruments Ltd.).
Growth of Cocrystals
Computational methods
BPNO Crystal: 0.1 mmol BPNO was dissolved in 5 mL of chloroform
in a glass vial, which was sealed using Parafilm with pinholes and
kept in the dark at room temperature to slowly evaporate the
solvent. Well-formed crystals suitable for X-ray diffraction (XRD)
measurements appeared within approximately one to two weeks.
All calculations were carried out with the GAUSSIAN 09 quantum
chemistry package[59] in the electronic ground state using density
functional theory (DFT). The structures of all the monomers and
binary and ternary cocrystals were determined from the XRD data
and calculated by the M06-2X method.[60] The Lanl2dz ECP basis set
was used to describe the I atoms, while aug-cc-pVDZ was applied
for all the other atoms. The interaction energies were obtained by a
single point energy prediction based on the structures of single
Binary DITFB·BPNO cocrystal: DITFB and BPNO in a 1:1 molar ratio
were dissolved in 10 mL of acetone or 1:1 ethanol-acetone in a
glass vial, which were sealed using Parafilm with pinholes and kept
in the dark at room temperature to slowly evaporate the solvent.
Well-formed crystals suitable for XRD measurement appeared
within approximately one to two weeks.
~
crystals, and the interaction energy ( E) of each complex was
defined as the difference between the total energy of the complex
and the sum of the total energies of the monomers.
Acknowledgements
The authors thank the Science and Technology Innovation Project
of Shanxi Colleges and Universities (No. 2020L0476), Applied Basic
Scheme 1. Synthesis pathway of BPNO.
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doi.org/10.1002/cplu.202000779
ChemPlusChem
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9
Keywords: cocrystals · halogen bonds · host-guest complexes ·
soft cavities · supramolecular chemistry
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