Phosphorescent cocrystals assembled by 1,4-diiodotetrafluorobenzene and fluorene and its heterocyclic analogues based on C-I???π halogen bonding

Article abstract of DOI:10.1021/cg300515a

Three phosphorescent cocrystals were prepared by 1,4- diiodotetrafluorobenzene and fluorene (cocrystal 1) and its heterocyclic analogues, dibenzofuran (cocrystal 2) and dibenzothiophene (cocrystal 3), based on C-I???π halogen bonding, and C-H???π, C-H???I, or C-H???F hydrogen bonding as well as F???F and S???S contacts. They were well characterized by X-ray crystallography, infrared, Raman spectroscopy, and differential scanning calorimetry and thermogravimetric analysis. The calculated halogen and hydrogen bonding energies indicate that the synergistic double C-I???π in cocrystal 3 or π??? _XDI_HA???H patterns in cocrystal 2 really exist. 1,4-DITFB is a dual functional synthon: the cement to link luminescence molecules and a heavy atom perturber to enhance phosphorescence of emitters by spin-orbital coupling. Three cocrystals phosphoresce distinctively with well defined vibrational bands at 496 (0-0) and 531 nm (max) for 1, 496 (0-0) and 529 nm (max) for 2, and 520 (0-0) and 564 nm (max) for 3, respectively. All the decays obey a monoexponential law with lifetimes 0.34, 0.51, and 2.50 ms, respectively. The phosphorescence spectra of fluorene, dibenzofuran, or dibenzothiophene in cocrystals are largely red-shifted by approximately 50-90 nm with respect to the free monomer in β-cyclodextrin aqueous solution, indicating modulatory phosphorescence emission by the molecular structure of emitters per se, weak bonding, and the cocrystal environment.

Full text of DOI:10.1021/cg300515a

Article
pubs.acs.org/crystal
Phosphorescent Cocrystals Assembled by 1,4-
Diiodotetrafluorobenzene and Fluorene and Its Heterocyclic
Analogues Based on C−I···π Halogen Bonding
Hai Yue Gao, Xiao Ran Zhao, Hui Wang, Xue Pang, and Wei Jun Jin*
College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China
S
* Supporting Information
ABSTRACT: Three phosphorescent cocrystals were prepared
by 1,4-diiodotetrafluorobenzene and fluorene (cocrystal 1) and
its heterocyclic analogues, dibenzofuran (cocrystal 2) and
dibenzothiophene (cocrystal 3), based on C−I···π halogen
bonding, and C−H···π, C−H···I, or C−H···F hydrogen
bonding as well as F···F and S···S contacts. They were well
characterized by X-ray crystallography, infrared, Raman
spectroscopy, and differential scanning calorimetry and
thermogravimetric analysis. The calculated halogen and
hydrogen bonding energies indicate that the synergistic double
C−I···π in cocrystal 3 or π···XDIHA···H patterns in cocrystal 2
really exist. 1,4-DITFB is a dual functional synthon: the
cement to link luminescence molecules and a heavy atom
perturber to enhance phosphorescence of emitters by spin−orbital coupling. Three cocrystals phosphoresce distinctively with
well defined vibrational bands at 496 (0−0) and 531 nm (max) for 1, 496 (0−0) and 529 nm (max) for 2, and 520 (0−0) and
564 nm (max) for 3, respectively. All the decays obey a monoexponential law with lifetimes 0.34, 0.51, and 2.50 ms, respectively.
The phosphorescence spectra of fluorene, dibenzofuran, or dibenzothiophene in cocrystals are largely red-shifted by
approximately 50−90 nm with respect to the free monomer in β-cyclodextrin aqueous solution, indicating modulatory
phosphorescence emission by the molecular structure of emitters per se, weak bonding, and the cocrystal environment.
S1 to T1 and T1 to S0 or S1 faster; consequently,
phosphorescence of the materials is enhanced.
INTRODUCTION
■
Halogen bonding (XB) is a kind of intermolecular noncovalent
interaction between the electron-deficient, polarizable halogen
atom (Lewis acid, XB donor) and the electron-rich atom, anion,
or π-electron system (Lewis base, XB acceptor).1 In nature, XB
is the attractive interaction between a region of positive
electrostatic potential in a covalently bonded halogen atom and
a negative site on an atom or group. Of course, electrostatic
effects, polarization, charge transfer, and dispersion may all
contribute to XB.2−6 Because of its highly directional,7 as well
as fully hydrophobic, characteristics,8 halogen bonding is
developing into a common tool for the design and construction
of supramolecular assemblies, and it is particularly becoming a
popular means in the crystal engineering field.9−14 Also,
halogen bonding has become an important tool in molecular
recognition,15 design of drugs,16 and exploration of biological
interaction.17,18
1,4-DITFB is an excellent halogen bond donor because of
the incorporation of strongly electron withdrawing groups
attached to an I atom, resulting in the presence of a σ-hole
acting as an XB donor site.19 And frequently it is considered as
a functional connective molecule in designing crystalline
materials relevant to halogen bonding. It can also act as a
heavy atom perturber in designing luminescent crystal
materials, making the intersystem crossing rate kISC from both
Fluorene and its heterocyclic analogues, such as carbazole,
dibenzofuran, and dibenzothiophene, etc., have peculiar
extensive π-conjugation electronic structure and highly emissive
electron-conducting properties, which make them promising
candidates for use in such diverse technologies as organic light-
emitting diodes, photovoltaics, photorefractive electrolumines-
cent devices, etc.20−23 Generally, the purely organic materials
emitting phosphorescence with long lifetime are very rare, and
the few that are known are very inefficient emitters.24
Moreover, crystallized organic luminescence materials can
make the luminogens emitted in highly phosphorescently
efficient at room temperature.25 However, from the viewpoint
of crystal engineering, the combination of different function-
alized synthons in a cocrystal should be very important in
improving the function of crystalline materials. So, the
molecular cocrystals, in principle, could be made by 1,4-
DITFB and fluorene and its heterocyclic analogues with
predictable structures and improvable functions. The present
work reports the crystallographic properties of three supra-
Received: April 16, 2012
Revised: July 18, 2012
Published: July 18, 2012
© 2012 American Chemical Society
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157, 299, 499, 747, 1033, 1242, 1496, 1612 cm−1. Elemental analysis:
(%): Calcd. for C18H8F4I2O: C, 37.92; H, 1.41. Found: C, 37.93; H,
1.39.
molecular cocrystals assembled by fluorene and its other two
heterocyclic analogues with 1,4-DITFB, fluorine/1,4-DITFB
(cocrystal 1), dibenzofuran/1,4-DITFB (cocrystal 2), and
dibenzothiophene/1,4-DITFB (cocrystal 3) based on a C−
I···π halogen bond and C−H···π, C−H···I, and C−H···F
hydrogen bonding, as well as F···F and S···S contacts. The
differences of the three cocrystals in the crystal structures and
phosphorescence behavior are also discussed. These phosphor-
escent organic cocrystals are easily synthesized by evaporation
of organic solvent or precipitation in water phase, and it could
be expected that they are useful in fabricating phosphorescent
devices. Particularly, the S···S contact in cocrystal 3 should be
conducive to designing new materials with both long lifetime
luminescent and electron transport capacity.
Cocrystal 3. Dibenzothiophene and 1,4-DITFB in a 1:2 molar ratio
were dissolved in 4:1 acetone/chloroform mixed solvent in a glass vial.
The slow evaporation of the solvent at room temperature for 3 weeks
yielded the colorless crystals. The product was isolated prior to full
evaporation of the solvent to guarantee the crystal purity. IR (ATR):
νmax 1466, 981, 945, 761, 748 cm−1. Raman (solid): νmax 159, 283, 408,
500, 703, 1023, 1134, 1162, 1228, 1317, 1475, 1556, 1599 cm−1.
Elemental analysis: (%) Calcd. for C24H8F8I4S: C, 29.18; H, 0.82.
Found: C, 29.31; H, 0.83.
Crystallography. XRD data of these cocrystals were collected at
293 K with a Bruker SMART APEX or Smart Apex II diffractometer
using Mo Kα X-ray radiation (λ = 0.71073 Å) and a graphite
monochromator. Empirical absorption corrections were applied using
a multiscan method. The structures were resolved by a direct method
and refined by full-matrix least-squares on F2 using the SHELX
program with anisotropic thermal parameters for all non-hydrogen
atoms. H atoms were placed by calculation positions. The most
relevant crystallographic data of the structure reported in this paper are
shown in Table 1.
EXPERIMENTAL SECTION
■
Fluorene (98%), iodobenzene (98%), and bromocyclohexane (98%)
were purchased from Alfa Aesar Co. (WardHill, MA, USA).
Dibenzofuran (98%) was purchased from J&K Scientific Ltd.
Dibenzothiophene (98%) and 1,4-diiodotetrafluorobenzene (98%)
were purchased from Sigma-Aldrich. Acetone and chloroform were
purchased from Beijing Company of China National Medicals. β-
Cyclodextrin (99%) was purchased from Yunan Yongguang Cyclo-
dextrin Co. Ltd., Guangdong, China. All of the other starting materials
were commercially available reagents of analytical grade and used
without further purification. The chemical structures of fluorene and
its heterocyclic analogues are shown in Scheme 1.
Table 1. Cocrystal Data and Structure Refinements
cocrystal 1
cocrystal 2
cocrystal 3
C24H8F8I4S
formula
Mr
C19H10F4I2
568.07
monoclinic
C2
C18H8F4I O
2
570. 04
triclinic
987.96
cryst syst
space group
a/Å
triclinic
P1
̅
P1
̅
Scheme 1. Molecular Structures of Fluorene and Its Two
Heterocyclic Analogues
8.587(1)
7.114(1)
14.78(1)
90
5.425(2)
5.769(1)
13.91(1)
90.58(0)
101.0(0)
101.5 (1)
418.3(2)
1
6.146(2)
9.385(2)
12.21(1)
74.32(1)
89.80(0)
82.23(1)
671.8(3)
1
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
99.20(1)
90
891.6(1)
2
Z
ρcalc/(g cm−3
)
2.116
2.263
2.442
μ(Mo KR)/mm−1
3.564
3.803
4.785
F(000)
532
266
452
T/K
296(2)
150(2)
296(2)
hkl range
−10 ≤ h ≤ 10 −6 ≤ h ≤ 4
−7 ≤ h ≤ 7
−10 ≤ k ≤ 11
−10 ≤ l ≤ 14
3215
−8 ≤ k ≤ 8
−17 ≤ l ≤ 10
2026
−6 ≤ k ≤ 6
−16 ≤ l ≤ 15
2157
measured refl
unique refl/Rint
1379/0.0115
none
1487/0.0247
2399/0.0302
absorption
correction
semiempirical
from
equivalent
semiempirical
from
equivalent
T
min/Tmax
0.6544/0.4944 0.8326/0.4498
0.0208, 0.0488 0.0535, 0.1328
0.2636/0.2385
0.0503, 0.1361
R1, wR2 [I >
2σ(I)]
final R1, wR (F2)
Preparation of Cocrystals. Cocrystal 1. Fluorene and 1,4-DITFB
in an equimolar ratio were dissolved in a 4:1 acetone/chloroform
mixture solvent in a glass vial. The evaporation of the solvent at room
temperature for three weeks yielded the colorless crystals. The product
was isolated prior to full evaporation slowly of the solvent to guarantee
the crystal purity. IR (ATR): νmax 1466, 978, 945, 760, 747 cm−1.
Raman (solid): νmax 158, 281, 499, 742, 786, 1018, 1233, 1477, 1611
cm−1. Elemental analysis: (%): Calcd. for C19H10F4I2: C, 40.17; H,
1.77. Found: C, 40.13; H, 1.83.
Cocrystal 2. Dibenzofuran and 1,4-DITFB in an equimolar ratio
were dissolved in 4:1 acetone/chloroform mixed solvent in a glass vial.
The slow evaporation of the solvent at room temperature for 2 weeks
yielded the colorless crystals. The product was isolated prior to full
evaporation of the solvent to guarantee the crystal purity. IR (ATR):
νmax 1467, 1192, 978, 945, 839, 753, 721 cm−1. Raman (solid): νmax
0.0227, 0.0510 0.0575, 0.1367
0.0577, 0.1437
1.117/−1.272
1.042
values (all data)
ρfin(max/min)/(e
0.320/−0.562
4.640/−1.322
Å−3
)
goodness of fit on
1.111
1.058
F2
Spectroscopic Measurements. All phosphorescence spectra
were recorded on a Cary Eclipse spectrophotometer (Varian)
equipped with a flashed xenon lamp and with a 1 mm × 10 mm
quartz cuvette at a 30/60° angle for the powder samples or ground
cocrystals. The excitation wavelengths for three cocrystals were set at
280 nm, 330 nm, or 380 nm, respectively. Both excitation and
emission bandpasses were set at the adaptive width (5/2.5 nm for 1,
10/5 nm for 2, and 20/10 nm for 3). The delay time, gate time, and
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Figure 1. Crystal structures of cocrystal 1. (a) Crystal cell unit. (b) Infinite chain stacking structures of cocrystal 1 formed by C−I···π and C−H···π.
The red chain is a halogen bonding infinite chain. (c) Detailed diagram of bonding. (d) Packing diagram for cocrystal 1. Color code: C, gray; H,
white, F, green; I, purple. Part d was obtained by the horizontal flip of part b in the clockwise direction and extension of the fluorene to be adjacent
another one by C−H10B···π bonding. (Two acidic H atoms in the same fluorene molecule are marked as H10A and H10B, respectively.)
total decay time were set as 0.1, 0.5, and 50 ms, respectively. The
phosphorescence decay was measured under the same conditions.
A 10 mm × 10 mm quartz cuvette was used for β-CD aqueous
systems by setting both ex/em slits at 20/20 nm; the delay time, gate
time, and total decay time were set as 0.1, 0.5, and 50 ms, respectively.
The excitation wavelengths were set at 260 nm (for fluorene), 290 nm
(for dibenzofuran), and 280 nm (for dibenzothiophene), respectively.
Computational Methods. All calculations were carried out with
the GAUSSIAN0926,27 quantum chemistry package in the electronic
ground state using dispersion-corrected density functional theory
(DFT-D). The structures of all the monomers, dimers, and trimers
were determined by XRD data and were used for a single point energy
prediction by computational chemistry. The energies were obtained
using B97D28 with the 6-311++G** basis set for H, C, O, S, F atoms
and 6-311G** for I atom. The interaction energy (DE) of each
complex is calculated as the difference between the total energy of the
complex and the sum of the total energies of the nucleophile and the
acceptor. The basis set superposition error (BSSE) is also taken into
account by means of the Boys−Bernardi counterpoise (CP) technique.
Differential Scanning Calorimetry and Thermogravimetric
Analysis. Differential scanning calorimetry (DSC) was completed for
the cocrystal powder samples at a rate of 10 °C/min using a
differential scanning calorimeter (DSC 1, Mettler Toledo Co. Ltd.,
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Switzerland) under a nitrogen atmosphere. The range of temperature
was 50−120 °C or 50−150 °C.
1c. All these distances are less than the sum of the van der
Waals radii (Bondi radii, I 1.98 Å; Pauling radii, I 2.15 Å; half of
the aromatic ring thickness, 1.85; Bondi radius, H 1.20 Å;
Pauling radius, H 1.20 Å; Bondi radius, C 1.70 Å; Pauling
radius, C 1.72 Å).29,30 It can be seen that the halogen bonding
is more apt to be linear than hydrogen bonding. As shown in
Figure 1b, the fluorene acts as a halogen bonding acceptor
interacting with the 1,4-DITFB molecule in a C−I···π pattern
and a hydrogen bonding donor interacting with another
adjacent fluorene molecule in a C−H···π pattern. So, the
fluorene molecule has a dual bonding function in cocrystal 1.
There are no π···π stacks between 1,4-DITFBs or fluorenes.
The 3D structure of cocrystal 1 (Figure 1d) is extended by the
C−H10B···π bonding between fluorene and another adjacent
one (Two acidic H atoms in the same fluorene molecule are
marked as H10A and H10B, respectively).
Cocrystal 2. The nonresolvability of dibenzofuran molecules
along its short axis orientation in the cocrystal results in a
disorder structure, and there are two molecules in one cell unit,
just one dibenzofuran and one 1,4-DITFB in 1:1 stoichiometry,
as shown in Figure 2a. The cocrystal has a triclinic space group.
The 3D structure of the cocrystal can be described as follows.
First, the column-like infinite zigzag chain is constructed by C−
I···π halogen bonding in an over-the-bond pattern of I atom to
the π-system (expressed in blue in Figure 2b), and it also has
C−H···I hydrogen bonding between the same I atom in the
1,4-DITFB molecule and one H atom of the dibenzofuran
molecule in the adjacent chain to connect the chains together,
as shown in Figure 2b. The resulting I···C distances are 3.434
and 3.517 Å, or 3.546 and 3.390 Å with ∠C−I···C bonding
angles of 176.9 and 156.8°, or 166.8 and 167.8°, respectively.
The side labeled with bonding length 3.517 and 3.434 Å is
named as side A of the dibenzofuran plane. And the side labeled
with bonding length 3.546 and 3.390 Å is named as side B. The
H···I distance is 3.080 Å with ∠C−H···I bonding angle of
170.4°. If a dibenzofuran molecule is given a reversed
orientation, the I···C or H···I distances and bonding angles in
the sequentially resulting cocrystal structure are the same as the
original ones. As shown in Figure 2c, the H atom on the lateral
side of the dibenzofuran molecule contacts the F atom of 1,4-
DITFB in the adjacent chain to form a C−H···F interaction,
which should contribute to maintaining the stability of the 3D
structure of the cocrystal. The shortest C−H···F contact is
2.506 Å with a bonding angle of 160.5°.
Cocrystal 3. It has a triclinic space group with one
dibenzothiophene molecule and two 1,4-DITFB molecules in
a 1:2 stoichiometry in one cell unit (Figure 3a). The
dibenzothiophene molecule is nonresolvable along its short
axis orientation, similar to the above dibenzofunan/1,4-DITFB
or carbazole/1,4-DITFB cocrystals reported previously.31 As
shown in Figure 3b, the C−I···π bonding can be divided into
two groups. In group I, the resulting I···C distances are 3.502,
3.596, 3.556, and 3.607 Å, with bonding angles of 168.8, 162.3,
172.6, and 160.4°, respectively. In group II, the bonding
distances are 3.411, 3.655, 3.613, 3.487, and 3.536 Å, with
bonding angles of 170.6, 148.3, 159.5, 170.4, and 158.8°,
respectively. If a dibenzothiophene molecule is given a reversed
orientation, the bonding distances and bonding angles in the
sequentially resulting cocrystal structure are the same as the
original ones. Just as in Figure 3b, the column-like infinite chain
(expressed in yellow) is formed by a dibenzothiophene
molecule as a nodal section with the alternative bonding of
groups I and II.
Thermogravimetric analysis (TGA) curves of the cocrystal powder
samples were obtained using a thermogravimetric analyzer (TGA/
DSC 1/1100, Mettler Toledo Co. Ltd., Switzerland) under a nitrogen
atmosphere with a flow rate of 20 mL/min. Each sample was placed in
a polytetrafluoroethylene crucible, and an empty one was used as
reference. The range of temperature used was 50−300 °C with a
heating rate of 10 °C/min to get the mass loss against heating
temperature curves.
RESULTS AND DISCUSSION
■
Cocrystal Structures and Intermolecular Interactions.
XRD analysis revealed a 1:1 stoichiometry for both 1
(fluorene/1,4-DITFB) and 2 (dibenzofuran/1,4-DITFB) and
1:2 for 3 (dibenzothiophene/1,4-DITFB), and their crystal
structures are shown in Figures 1−3. Multiple interesting
intermolecular interactions, including C−I···π halogen bonding,
C−H···π, C−H···I, and C−H···F hydrogen bonding, and F···F
and S···S interactions as well, are observed in these cocrystals,
and the main bonding parameters are summarized in Table 2.
Table 2. Bonding Properties and Geometrical Parameters of
Cocrystals
a
cocrystals
interactions
d/Å
θ/deg
θ/deg
1
C−I···5C
C−I···6C
C−H···6C
C−H···7C
C−I···6C
C−I···7C
C−I···12C
C−I···13C
C−H··· I
C−H···F
C−I···9C
C−I···10C
C−I···13C
C−I···14C
C−I···7C
C−I···8C
C−I···12C
C−I···16C
C−I···17C
C−F···F−C
S···S-plane
3.409(10)
3.534(0)
2.707(1)
2.837(0)
3.434(1)
3.517(1)
3.546(1)
3.390(1)
3.080(0)
2.506(1)
3.502(1)
3.596(1)
3.556(1)
3.607(1)
3.411(1)
3.655(2)
3.613(1)
3.487(1)
3.536(37)
2.669(1)
3.465(1)
93.4(0)
88.2(1)
83.3(2)
78.3(0)
85.5(0)
82.2(1)
80.6(0)
86.8(3)
170.4(0)
160.5(0)
82.6(0)
76.1(0)
80.3(1)
77.8(0)
85.4(8)
75.9(0)
77.5(6)
83.2(1)
80.6(11)
171.3(0)
164.3(0)
142.4(0)
135.1(1)
176.9(0)
156.8(1)
166.8(1)
167.8(1)
170.4(1)
160.5(0)
168.8(0)
162.3(1)
172.6(0)
160.4(0)
170.6(1)
148.3(1)
159.5(2)
170.4(0)
158.8(5)
167.0(1)
2 A
B
3 I
II
89.1(1)
a
θ/deg means the angles C−I···molecular plane; A/B mean two sides
of dibenzofuran in cocrystal 2. Groups I and II in cocrystal 3, cf. Figure
3b.
Cocrystal 1. As shown in Figure 1a, it crystallizes in the
monoclinic space group C2 with four molecules in one unit cell.
The infinite zigzag chain is constructed by C−I···π halogen
bonding in an over-the-bond pattern of the I atom to the π-
system (expressed in red in Figure 1b), and C−H···π between
the H10A site at one fluorine and the π-system of another
adjacent one connects the chains together (also the infinite
fluorene molecular chains connected by the C−H···π can be
regarded as the main chain). The resulting I···C distances are
3.409 and 3.534 Å with ∠C−I···C bonding angles of 171.3 and
164.3°, respectively. The H···C distances are 2.707 and 2.837 Å
with ∠C−H···C bonding angles of 142.4 and 135.1°,
respectively. The detail of the bonding is described in Figure
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Figure 2. Crystal structures of cocrystal 2. (a) Crystal cell unit. (b) Infinite main chain stacking structure of cocrystal 2 formed by C−I···π. The blue
chain is a halogen bonding infinite chain. (c) Side packing diagram of part b. Color code: C, gray; F, green; I, purple; H, white; O, red.
For the 3D structure, the adjacent and parallel chains are
connected mainly through S···S interactions and F···F contacts,
as well as alternative bonding of groups II and I, as shown in
Figure 3c. The shortest S···S contact is 3.465 Å with the angle
of 89.1°, almost vertical to the dibenzothiophene plane. The
S···S interactions should be interesting, just like the trans-
annular conjugation of the σ-type observed in the phenathrene/
1,4-DITFB cocrystal32 or σ-, π-type or intermediate charge
transfer.33,34 Together with S···S interactions, the F···F contacts
between 1,4-DITFB molecules with the distance of 2.669 Å
(less than the sum of the van der Waals radii) and bonding
angle of 167.0° also are necessary for the compact structure.
Calculations of Bonding Energies. The interaction units
and their geometry parameters extracted from single cocrystal
structure data for calculating energies are displayed in Figure 4,
and the calculated bonding energies are listed in Table 3. The
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Figure 3. Crystal structures of cocrystal 3. (a) Crystal cell unit. (b) Infinite main chain stacking structures of cocrystal 3 formed by C−I···π. The
yellow chain is the halogen bonding infinite chain. (c) Packing diagram for cocrystal. Color code: C, gray; F, green; I, purple; H, white; S, yellow.
BSSE-corrected energy of C−H···π between adjacent fluorenes
in cocrystal 1 is −26.2 kJ·mol−1, greater than the C−I···π
energies in all three cocrystals here, but very close to the C−
I···π energy in the carbazole/1,4-DITFB cocrystal.31 Among
three cocrystals, the maximum C−I···π energy is −18.0
kJ·mol−1 in cocrystal 2, which indicates an apparent synergistic
effect of the π···XDIHA···H pattern compared with −11.3
kJ·mol−1 calculated per molar (DI···A) unit (which does not
exist individually) in cocrystal 3. Next, the C−I···π energy is
−33.9 kJ·mol−1 calculated per molar 2(DI···A) unit in cocrystal
3, also indicating a synergy of double C−I···π bonding. The
minimum C−I···π energy is −14.5 kJ·mol−1 in cocrystal 1,
maybe due to the result balanced by strong C−H···π bonding.
All the C−I···π energies display the strong halogen bonding,
being ca. 2−4 times greater than that in solution between 2-
C3F7I or 1-C3F7I and toluene-d8, where the calculated
complexation energies are −6.4 or −5.4 kJ·mol−1, respec-
tively.35
The O or S atom in dibenzofuran or dibenzothiophene also
could act as XB acceptor to form XB. But why not herein? Does
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Figure 4. C−I···π halogen bonding, C−H···π or C−H···I hydrogen bonding units from single cocrystal structure data, and optimized C−I···O or C−
I···S bonding units for calculation of energies (B97D with the 6-311++G** basis set for H, C, O, S, F atoms and 6-311G** for I atom).
it mean that the C−I···π halogen bond is easier to be formed
than C−I···O or C−I···S in the corresponding system? The
calculated binding energy of C−I···O or C−I···S based on the
fully optimization structure of the C−I···O/S complex
predicted in Figure 4 is −14.8 or −16.2 kJ·mol−1, respectively.
All of them are slightly less than that of C−I···π (−18.0
kJ·mol−1 in cocrystal 2) or half of 2(C−I···π) (−33.9 kJ·mol−1
in cocrystal 3). So, this may be the reason or one of the reasons
why the C−I···O or C−I···S interaction does not occur in the
prepared cocrystals. In addition, as shown in Figure 4, the C−
I···O and C−I···S bonding angles are 180 and 177°,
respectively. But, the angles of C−I···O and C−I···S are 180
and 112° with respect to the molecular plane. This means that
the O atom in dibenzofuran keeps the approximate sp2 hybrid
state, while the S atom in dibenzothiophene keeps the sp3
hybrid state. So, the S−S contacts can occur in cocrystal 3 and
dibenzothiophene has longer phosphorescence lifetime due to
little contribution of lone pair electrons of the S atom to π-
conjugation.
XRPD. For confirming the phase purity and homogeneity of
the cocrystals, powder XRD patterns were carried out at room
temperature. As shown in Figure S1 of the Supporting
Information, the peak positions of experimental patterns
(color curve) are in good agreement with curves simulated
from single cocrystal XRD data (black curve), demonstrating a
good phase purity of the bulk crystal products. The few
discrepancies in intensity between simulated and experimental
values may be the consequence of preferred orientations of the
cocrystal powder samples.
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the DSC scans give comparable enthalpies (ΔH) as −60.6,
−70.9, and −55.6 J·g−1, respectively. The other data of the
cocrystals and raw materials are listed in Table S1. The starting
decomposed temperature as well as the rate of thermal
decomposition of the cocrystals can be obtained by TGA
curves, as shown in Figure 5, and they are 94.0 °C/−99.2%,
113.3 °C/−96.7%, and 113.4 °C/−95.4%, respectively. The
region is considered as the main decomposition step for these
samples because the mass loss corresponds to 95.4% and 99.2%
of the total mass loss.
Table 3. Calculated C−I···π Halogen Bonding, C−H···π or
C−H···I Hydrogen Bonding, and Optimized C−I···O or C−
I···S Bonding Unit Energies
a
cocrystal
bonding unit
ΔE (kJ·mol−1
)
ΔECP (kJ·mol−1
)
1
DH ···A (C−H···π)
DI···A (C−I···π)
DI···A (C−I···π)
DH···A(I) (C−H···I)
DI···A (C−I···O)
DI···A (C−I···π)
2DI···A 2(C−I···π)
DI···A (C−I···S)
−29.0
−16.3
−20.1
−10.8
−16.7
−13.4
−46.2
−17.8
−26.2
−14.5
−18.0
−9.25
−14.8
−11.3
−33.9
−16.2
2
3
Phosphorescence Spectra and Decays. It is well-known
that fluorescence could be easily observed for fluorene and its
heterocyclic analogues. However, the phosphorescence emis-
sion is difficult to be obtained due to the spin-forbidden S0−T1
and S1−T1 transitions. Introduction of a 1,4-DITFB molecule
into the cocrystals would affect the luminescence characteristic
of fluorene and its heterocyclic analogues. 1,4-DITFB plays two
main roles in cocrystals: the cement to assemble fluorene and
its heterocyclic analogue molecules to special crystalline
materials, and the heavy atom perturber to induce phosphor-
escence emission by spin−orbital coupling. As shown in Figure
6 and Table 4, the cocrystal 1 emits strong phosphorescence.
The phosphorescence emission spectrum goes from 490 to
near 580 nm with well defined vibrational bands at 496 (0−0),
531 (max), 569, and 610 nm at the excitation wavelength of
280 nm. The phosphorescence of cocrystal 2 is obviously
weaker with well defined vibrational bands at 496 (0−0), 529
(max), 572, and ∼597 nm at the excitation wavelength of 330
nm. The phosphorescence of cocrystal 3 is the weakest, with
resolvable bands at 520 (0−0), 564 (max), 618, 669, and 748
nm at the excitation wavelength of 380 nm. The S···S
interaction (assigned to π−π conjugation by σ-type overlap of
π-orbitals of two different S atoms)34 may make the gap
between triplet and singlet ground states narrower and
sequentially phosphorescence weaker compared with the case
of cocrystal 1 or 2. But it may provide the cocrystal additional
function, such as electron transport.39,40
a
Note: the DI stands for halogen bonding donor, 1,4-DITFB, or DH
stands for hydrogen bonding donor, fluorene or dibenzofuran. A
stands for acceptor fluorene, dibenzofuran, and dibenzothiophene or I.
ΔE: single point energy by B97D/6-311++G(d,p)/6-311(d,p). ΔECP
BSSE-corrected energies of ΔE.
:
ATR-IR and Raman Spectra. Interactions between the
halogen bonding donor and acceptor in the cocrystals were
investigated by FT-IR and Raman spectroscopy. As shown in
Figure S2, FT-IR shows several significant differences of bound
1,4-DITFB compared with the free one. For free 1,4-DITFB,
the fundamental band of a ring sextant νC−C stretch generally
occurs in 1450−1600 cm−1 in FT-IR spectra. Herein it occurs
at 1474 cm−1 (free) and 1466 or 1467 cm−1 (bound),
respectively, red-shifted by 7−8 cm−1.36,37 The band at 158,
157, or 159 cm−1 in the Raman spectrum is due to a combined
symmetrical C−I stretch and ring elongation and increases by
2−4 cm−1 compared with 155 cm−1 for free 1,4-DITFB.38 All of
them emphasize the presence of distinct interactions in the
cocrystals and are in accord with a rule that the vibrational
bands of the halogen-bonding donor in a halogen-bonded
system exhibit a shift to lower wavenumber.
Differential Scanning Calorimetry Thermogram and
Thermogravimetric Analysis. Figure 5 illustrates DSC
thermogram and TGA curves of the three cocrystal powder
samples. A sharp increase can be observed in the enthalpy
relaxation as the cocrystals are heated to a temperature above
the glass transition temperature. The areas of the endothermic
peaks indicating melting points at Tp = 94.8 °C (cocrystal 1),
Tp = 99.2 °C (cocrystal 2), and Tp = 85.9 °C (cocrystal 3) in
The cocrystals, as participation of microparticles with the
same stoichiometry, also can be assembled very easily in
aqueous solutions. The phosphorescence spectral and decay
properties of the microparticles are the same with the cocrystals
assembled by evaporation of organic solvent (cf. Figure S3).
Figure 5. DSC (left) and TGA (right) curves of the three cocrystals.
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Figure 6. Phosphorescent excitation and emission spectra, and decay curves of cocrystals.
Table 4. Phosphorescent Characteristics of Fluorene, Dibenzofuran, or Dibenzothiophene
excitation wavelengths/nm
emission wavelengths/nm
in cocrystals
mean lifetime/ms
in cocrystals
in β-CD
in β-CD
in cocrystals
in β-CD
a
fluorene
280
330
380
260
290
280
496 (0−0), 531 (max), 569, 610
496 (0−0), 529 (max), ∼572, ∼597
520 (0−0), 564 (max), 618, 669, 748
∼423 (0−0), 440 (max)
∼429 (0−0), 450 (max)
∼436 (0−0), 467 (max)
0.34
0.51
2.50
21.8/0.8
27.4
dibenzofuran
dibenzothiophene
25.7
a
Note: the mean phosphorescent lifetime of fluorene is 0.8 ms using iodobenzene as heavy atom perturber in β-CD, while the lifetime of
dibenzofuran or dibenzothiophene is not detectable under the same conditions, because of too weak a phosphorescent intensity.
The phosphorescence spectral positions of fluorene,
dibenzofuran, and dibenzothiophene display no remarkable
difference from those under similar conditions.41,42 However,
both the 0−0 and the maximum bands of fluorene,
dibenzofuran, or dibenzothiophene in cocrystals are largely
red-shifted by approximately 50−90 nm with respect to the free
monomer emission in β-cyclodextrin (β-CD) aqueous solution
induced by bromocyclohexane (Br-CH) or iodobenzene at
room temperature, as shown in Figure 7, also compared to that
on solid support induced by heavy metal ions.43 In the β-CD
cavity, fluorene, dibenzofuran, or dibenzothiophene is free, and
the main force driving them into the cavity together with Br-
CH to form a ternary inclusion complex is a hydrophobic or
vdW interaction.44 However, stronger chemically bonding
interactions between molecules occur in cocrystals, as described
in the structure section above, and these interactions should
decrease the triplet state level of fluorene, dibenzofuran, or
dibenzothiophene. The results signify that the phosphorescence
emission can be modulated by not only the molecular structure
of emitters but also the weak bonding and cocrystal
environment.
The ex/em slits are 5/2.5 nm for cocrystal 1, 10/5 nm for
cocrystal 2, and 20/10 nm for cocrystal 3. The positions of the
vibrational bands labeled in Figure 6 refer to those before
separation, and the vibration bands separated by assuming a
Gaussian distribution of each band are listed in Table 4.
The phosphorescence decays were also studied, and the
results are shown in Figure 6 and Table 4. The phosphor-
escence decays of three cocrystals obey a monoexponential law
with the lifetime of 0.34 ms for cocrystal 1, 0.51 ms for
cocrystal 2, and 2.50 ms for cocrystal 3, respectively. In
suspension, microparticles of three cocrystals have the same
phosphorescent lifetimes, while the mean phosphorescent
lifetime of fluorene increases to 0.8 ms using iodobenzene as
heavy atom perturber in β-CD. The phosphorescent lifetime of
dibenzofuran or dibenzothiophene is not detectable under the
same conditions, as the phosphorescent intensity is too weak.
However, the mean phosphorescent lifetimes of fluorene,
dibenzofuran, or dibenzothiophene using Br-CH as heavy atom
perturber in β-CD are largely increased to more than 21.0 ms.
The very similar phosphorescent lifetimes of three phosphor-
ophores in β-CD using Br-CH as heavy atom perturber indicate
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ASSOCIATED CONTENT
■
S
* Supporting Information
The experimental method and powder XRD, ATR-IR, and
Raman spectra of the cocrystals or 1,4-DITFB; table of DSC
and TGA data of the cocrystals or their raw materials; the
experimental method, emission spectra, and decay curves of
phosphorescence of the cocrystal suspended microparticles in
water phase; and X-ray crystallographic information files (CIF)
and checkCIF reports. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wjjin@bnu.edu.cn. Tel/Fax: (+86)10-58802146.
Notes
Figure 7. Phosphorescence emission spectra of fluorene and its
heterocyclic analogues induced by bromocyclohexane (Br-CH) in β-
CD aqueous solution. Conditions: [fluorine] and [dibenzofuran], 5.0
× 10−5 mol L−1; [dibenzothiophene], 1.0 × 10−4 mol L−1; [Br-CH],
2.0 × 10−5 mol L−1; [β-CD], 8.0 × 10−3 mol L−1. Excitation
wavelength of 260 nm, 290 nm, and 280 nm, respectively, with the ex/
em slits 20/20 nm. Also, replacement of Br-CH with iodobenzene as
heavy atom perturber results in the same phosphorescent spectra but
different lifetime.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (No. 90922023) for support. The authors also thank Dr.
Xue Bin Deng for his help in resolution of XRD data of
cocrystals.
REFERENCES
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CONCLUSIONS
■
Halogen bonding provides a new strategy to assemble
controllable functional cocrystals. Cocrystals 1, 2, and 3 are
successfully constructed by 1,4-diiodotetrafluorobenzene and
fluorene and its heterocyclic analogues based on C−I···π
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