Halogen-bonding contacts determining the crystal structure and fluorescence properties of organic salts

Article abstract of DOI:10.1039/c7nj02034a

By using the ligands 4-(4-bromobenzylideneamino)-4H-1,2,4-triazole (L₁) and 4-(4-iodobenzylideneamino)-4H-1,2,4-triazole (L₂), four different organic salts HL₁⁺·Cl^- (1), L₁·HL₁

Full text of DOI:10.1039/c7nj02034a

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Halogen-bonding contacts determining the
crystal structure and fluorescence properties
Cite this:DOI: 10.1039/c7nj02034a
of organic salts†
Jing-Wen Wang, Chen Chen, Yao-Ja Li, Yang-Hui Luo * and Bai-Wang Sun
*
By using the ligands 4-(4-bromobenzylideneamino)-4H-1,2,4-triazole (L1) and 4-(4-iodobenzylideneamino)-
4H-1,2,4-triazole (L2), four different organic salts HL1 ÁClÀ (1), L1ÁHL1 ÁClO4 (2), HL2 ÁClÀ (3), and L2ÁHL2
Á
+
+
À
+
+
À
ClO4 (4) have been synthesized and structurally characterized through single-crystal X-ray diffraction (L1, L2,
2 and 3) and PXRD (1 and 4) measurements. The results revealed that ligands L1 and L2 themselves exhibit a
polymeric 1-D chain in a zigzag structure connected by C–BrÁÁÁN and C–IÁÁÁN halogen-bonding contacts,
respectively. L1 in salt 2 was connected into a 3-D structure through N–HÁÁÁO hydrogen-bonding contacts,
while the interleaved 3-D network of L2 in salt 3 was mainly connected by C–IÁÁÁCl halogen-bonding
contacts. As a result, the fluorescence properties of ligands L1 and L2 were reserved in halogen-bonding
connected salts 1 and 3, while there was a reduction in hydrogen-bonding connected salts 2 and 4. Thus,
the relationship between emission properties and halogen-bonding contacts can be proposed.
Received 7th June 2017,
Accepted 20th July 2017
DOI: 10.1039/c7nj02034a
rsc.li/njc
and variable size of the donor atom are four unique features of
XB, which offer new opportunities if compared to the well-
1. Introduction
As is well known, supramolecular engineering strategies rely on established use of HB.8,9 In addition, XB is capable of driving the
noncovalent intermolecular interactions such as van der Waals self-assembly of supramolecular networks10 and has recently been
interactions, hydrogen bonds, and halogen bonds.1–3 Although introduced into solid-state reactivity as well as cocrystallization
compared to the assembly of supramolecular networks with (superior to other supramolecular interactions),11–13 which incredibly
conventional noncovalent interactions (hydrogen bonding and opens a new door for chemists to discover the fantastic phenomena
pÁ Á Áp interactions), designed supramolecular crystal engineering in multi-component molecular solids. Thereafter, systematic studies
with halogen bonding (XB) has been less studied, a lot of revealed that halogen related interactions, such as XÁÁÁN (O, S), XÁÁÁp
emphasis has been put on halogen bonding and supramolecular and XÁÁÁX interactions, are ubiquitous, noncovalent interactions in
structures self-assembled using such interaction, particularly in supramolecular engineering. At present, halogen bonds are more
the last 15 years. Interestingly, lots of experimental observations widely studied; the formation of XB can develop great potential for
and phenomena, where we are now acknowledging the role high-value, functional materials, most notably in the fields of
played by XB were reported. Additionally, XB was at the core of biological systems,14 magnetic and conducting materials,15 non-
some important achievements in chemistry, being relevant to the linear optics (NLO),16 catalysis17 and molecular recognition and
work of R. Mulliken4,5 on the chemical bond and central to the assembly.18 However, research on halogen-bonded systems has
conformational studies of O. Hassel.6
been primarily focused on crystal engineering19,20 rather than
Halogen bonding (discovered in 1863)7 with strong n–s* on photoluminescence in the past few decades.21 This hinders
interactions and the analogies between XB and HB were valuable further rational design and preparation of photoluminescent
in the recent past to understand the features of XB, and the functional halogen-bonded organic salts. Therefore, the photo-
differences between the two interactions may help in the future luminescence characteristics of halogen-bonded solids and
to identify the specific properties of XB representing added their molecule-packing–property relationship currently remain
value. For instance, directionality, tunability, hydrophobicity, unknown.
Hence, two novel triazolyl Schiff base ligands L1 and L2
(Scheme 1) are selected for their unique p-conjugated system
School of Chemistry and Chemical Engineering, Southeast University,
Nanjing 211189, P. R. China. E-mail: peluoyh@sina.com, chmsunbw@seu.edu.cn;
and strong halogen-bonding. Four different molecular saltÀs
Fax: +86-25-52090614; Tel: +86-25-52090614
HL1 ÁClÀ (1), L1ÁHL1 ÁClO4 (2), HL2 ÁClÀ (3), L2ÁHL2 ÁClO4
(4) have been synthesized and structurally characterized
by single-crystal X-ray diffraction (L1, L2, 2 and 3) and PXRD
+
+
À
+
+
† Electronic supplementary information (ESI) available: CCDC 1519856–1519859.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7nj02034a
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Scheme 1 The molecular structure of ligands L1 and L2 and salts 2 and 3.
(1 and 4). By halogen bonding interaction with different anions as a white solid. The white solid was collected by filtration,
in the lattice, the stacking modes change greatly, which affords washed with ethanol and air-dried at room temperature (yield
different fluorescence properties in the solid state. These 65%). Elemental analysis anal. calcd (%): C, 43.05; N, 22.32; H,
ligands and salts are also characterized through thermogravimetric 2.81. Found: C, 43.07; N, 22.30; H, 2.82. IR (KBr, cmÀ1): 3438,
analyses (TGA), Raman spectroscopy and molecular Hirshfeld 3120, 1614, 1539, 1501, 1486, 1282, 824, 759.
surfaces. Meanwhile, their fluorescence properties in solution
Synthesis of ligand L2 4-(4-iodobenzylideneamino)-4H-1,2,4-
and in the crystalline state with halogen-bonding contacts were triazole. The synthetic routes of compound L2 are illustrated in
investigated. This research may also help in anticipating future Scheme S2 (ESI†), and the synthesis method is similar to that of
directions.
compound L1. 10 mmol 4-iodobenzaldehyde (2.32 g) and 10 mmol
4-amino-4H-1,2,4-triazole (0.84 g) were dissolved in 80 mL ethanol,
and then the resulting solution was refluxed at 80 1C for about 3 h.
Concentration by rotary evaporation gave L2 as a yellow solid. The
yellow solid was collected by filtration, washed with ethanol and
air-dried at room temperature (yield 65%). Elemental analysis anal.
calcd (%): C, 36.26; N, 18.80; H, 2.37. Found: C, 36.27; N, 18.82; H,
2.35. IR (KBr, cmÀ1): 3437, 3117, 1615, 1583, 1533, 1483, 1164,
854, 819.
Ligands L1 and L2 were dissolved in a small amount of
methanol solvent. The resulting clear solutions evaporated at
room temperature after four days and single crystals of color-
less and blocky L1 crystals and light yellow blocky L2 crystals
suitable for X-ray diffraction were obtained.
2. Experimental section
2.1. Materials and physical measurements
All reagents were obtained commercially, and used without
further purification. Elemental analyses for carbon, hydrogen
and nitrogen were performed using a Vario-EL III elemental
analyzer. The infrared spectra (IR) were recorded on a SHIMADZU
IR Prestige-21 FTIR-8400S spectrometer as KBr pellets in the
range of 4000–400 cmÀ1. Thermogravimetric analysis (TGA) was
performed using a NETZSCH TG 2009 F3 system at a heating rate
of 20 K minÀ1 under an atmosphere of dry nitrogen flowing at
20 cm3 minÀ1 over the range from 50 to 500 1C. The UV-vis
absorption spectra were recorded on a Shimadzu UV-3600
spectrometer in the range of 200–800 nm. The fluorescence
spectra were obtained on a Horiba FluoroMax 4 spectrofluorometer.
Raman spectra were recorded using a Raman microscope (Kaiser
Optical Systems, Inc., Ann Arbor, MI, USA) with 785 nm laser
excitation. The spectra were obtained for one 2 min exposure
of the CCD detector in the wave-number range. X-ray powder
diffraction was recorded on a D8 ADVANCE XRD (Bruker,
Germany) with Cu Ka radiation (l = 1.54056 Å) at 40 mA and
45 kV. The sample was packed into a glass holder and diffraction
patterns were collected over a 2y range of 5–50, at a scan rate
2.3. Synthesis of organic salts 1–4
Synthesis of salts 1 and 2. The preparation of the salt was
performed in solution by crystallization and pure methanol
was used as the solvent. Salt 1 was obtained by dissolving a
3 : 5 stoichiometric ratio of L1 (75.33 mg, 0.3 mmol) and HCl
(47.97 mg, 0.5 mmol) in methanol under stirring at room tem-
perature for twenty minutes and the resulting homogeneous
solution was kept undisturbed at ambient temperature for slow
evaporation. Elemental analysis anal. calcd (%): C, 37.59; N, 19.49;
H, 2.80. Found: C, 37.60; N, 19.47; H, 2.82. IR (KBr, cmÀ1): 3345,
3081, 2751, 2677, 1612, 1587, and 1088. Salt 2 was obtained in a
similar way to salt 1 but using HClO4 instead of HCl. Elemental
analysis anal. calcd (%): C, 35.84; N, 18.60; H, 2.49. Found:
of 31 minÀ1
.
2.2. Synthesis of ligands L1 and L2
Synthesis of ligand L1 4-(4-bromobenzylideneamino)-4H- C, 35.81; N, 18.62; H, 2.47.
1,2,4-triazole. Ligands L1 and L2 were synthesized according
Synthesis of salts 3 and 4. Salt 3 was obtained by dissolving a
to the summary of the existing synthetic method, previously 3 : 5 stoichiometric ratio of L2 (89.42 mg, 0.3 mmol) and HCl
reported.22–26 The synthetic routes of compound L1 are illustrated (47.97 mg, 0.5 mmol) in methanol under stirring at room
in Scheme S1 (ESI†). 10 mmol 4-bromobenzaldehyde (1.85 g) and temperature for twenty minutes and the resulting homogeneous
10 mmol 4-amino-4H-1,2,4-triazole (0.84 g) were dissolved in solution was kept undisturbed at ambient temperature for slow
80 mL ethanol, and then the resulting solution was refluxed at evaporation. Elemental analysis anal. calcd (%): C, 32.31; N,
80 1C for about 3 h. Concentration by rotary evaporation gave L1 16.75; H, 2.41. Found: C, 32.34; N, 16.73; H, 2.40. Salt 4 was
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Table 1 Crystal data and structural refinement for compounds L1 and L2 and crystals 2 and 3
Compounds
L1
L2
2
3
Formula
Formula weight
C9H7N4Br
251.10
C9H7N4I
298.09
C18H15N8O4ClBr2
602.65
C9H8N4ClI
334.54
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
Triclinic
P1
Monoclinic
P21/c
%
a/Å
b/Å
c/Å
a/1
b/1
g/1
4.2810(9)
11.572(2)
19.146(4)
90.00
93.49(3)
90.00
946.7(3)
4
1.762
4.4720(9)
11.701(2)
19.032(4)
90.00
95.38(3)
90.00
991.5(3)
4
1.997
8.000(16)
9.886(2)
14.320(3)
95.05(3)
97.80(3)
105.59(3)
1071.5(4)
2
9.5620(19)
6.6530(13)
17.748(4)
90.00
93.05(3)
90.00
1127.5(4)
4
1.971
V, Å3
Z
D calc (Mg mÀ3
T/K
)
1.331
293(2)
3.953
293(2)
4.304
293(2)
3.194
293(2)
3.049
m (mmÀ1
)
Cryst dimensions
No. of reflns collected
No. of unique reflns
No. of params
0.2 Â 0.1 Â 0.1
0.25 Â 0.3 Â 0.1
0.2 Â 0.2 Â 0.1
0.2 Â 0.1 Â 0.1
1733
1811
3893
2062
953
127
1.006
0.0546, 0.0903
0.1221, 0.1055
1519856
1191
127
1.002
0.0435, 0.0934
0.0797, 0.1052
1519857
2061
298
1.087
0.0896, 0.2170
0.1625, 0.2495
1519859
1618
136
1.008
0.0429, 0.1028
0.0588, 0.1101
1519858
Goodness-of-fit on F2
R1, wR2 (I 4 2s(I))
R1, wR2 (all data)
CCDC No.
obtained using HClO4 instead of HCl. Elemental analysis anal.
calcd (%): C, 31.03; N, 16.09; H, 2.17. Found: C, 31.05; N, 16.07;
H, 2.18.
2.4. X-ray crystallographic studies
The single-crystal X-ray diffraction data of compounds L1 and
L2 and crystals 2 and 3 were collected at 293 K with graphite-
monochromated Mo-Ka radiation (l = 0.071073 nm) equipped
with a Rigaku SCXmini diffractometer.27,28 The lattice para-
meters were integrated using vector analysis and refined from
the diffraction matrix; the absorption correction was carried
out by using the Bruker SADABS program using the multi-scan
method. The crystallographic data, data collection, and refinement
parameters for compounds L1 and L2 are given in Table 1. The
structures were solved using full-matrix least-squares methods on
all F2 data, and the SHELXS-2014 and SHELXL-2014 programs
were used for structure solution and refinement, respectively.29,30
All non-hydrogen atoms were refined anisotropically and hydrogen
atoms were geometrically fixed. The molecular graphics were
prepared by using Mercury.31
Fig. 1 The crystal packing of ligands L1 and L2 and crystals 2 and 3.
halogen bonding connections are similar to those in L1, but the
halogen bonding C–IÁ Á ÁN is measured with distances of 3.210 Å
(IÁ Á ÁN), which implies that the halogen-bonding interactions of I
are stronger than those of Br, resulting in the molecules of L2
adopting different stacking modes through weak pÁ Á Áp inter-
molecular interactions forming a stacked 3-D layered structure.
Crystal structural determinations reveal that 2 crystallizes in
3. Results and discussion
3.1. Crystal structures of ligands L1 and L2 and salts 2 and 3
%
the triclinic P1 space group; the asymmetric unit of 2 is composed
À
Shown in Fig. 1 are the crystal connecting motifs of ligands L1 of one protonated L1, one L1Àmolecule and one ClO4 anion. L1
and L2, and salts 2 and 3. L1 and L2 were crystallized in the molecules connect with ClO4 anions into a 3-D frame structure
monoclinic P21/n space group, but the halogen-bonding inter- through hydrogen-bonding and halogen-bonding interactions. In
À
actions were different. The adjacent L1 molecules are connected addition, because ClO4 anions are an oxo-acid, hydrogen bonds
into a polymeric 1-D chain structure via the C–BrÁ Á ÁN halogen play a major role in comparison to halogen bonds. The salt 2
bonding interactions with the distances of BrÁ Á ÁN being 3.296 Å molecules are connected by N4–HÁ Á ÁN8 interaction to form a
(Scheme 2). The chains are parallel to each other in a zigzag tail–tail type interaction with one L1 molecule connecting to a
À
structure and different zigzag structures are linked in a parallel ClO4
anion via C-Br1Á Á ÁO4 halogen-bonding interaction
fashion to form a similar planar structure by pÁÁÁp intermolecular (Scheme 2). Salt 3 crystallizes in the monoclinic P21/c space
interactions with a line separation of 2.889 Å. For crystal L2, the group and the asymmetric unit is composed of one protonated
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Scheme 2 The intermolecular hydrogen bonding and halogen bonding interactions have been marked.
L2 molecule and one ClÀ anion (Scheme 1). For salt 3, XB plays a experimental X-ray powder diffraction patterns compared to
significant role in crystal formation and the crystal structure. those simulated from single crystals of 2 and 3.
The ClÀ ion is five-connected being bonded to three carbon
3.2. Thermal study
atoms [C1, C2 and C3] and one nitrogen atom [N1] and one
iodine atom [I]. The ClÀ ion, which plays a bridging role,
The thermal behaviors of ligands L1 and L2 and salts 1–4 were
connects the two protonated L2 cations into a 1-D tail-face chain
structure by infinite IÁÁÁCl halogen-bonding interaction (distance
of 3.344 Å) and N1–HÁÁÁCl hydrogen-bonding interaction (distance
of 3.035 Å). At the same time, the ClÀ ion through weak C–HÁÁÁCl
hydrogen bonding results in 2-D planes which are further inter-
leaved into a 3-D network structure (C1–HÁÁÁCl = 3.376 Å and
C2–HÁÁÁCl = 3.544 Å and C3–HÁÁÁCl = 3.574 Å). The geometrical
parameters for the halogen bonds in ligands L1 and L2 and salts 2
and 3 are shown in Table S1 (ESI†). Both aromatic pÁÁÁp stacking
and halogen-bonding or hydrogen-bonding interactions together
stabilize the structure.
investigated on a Proteus Thermal analysis system (from
NETZSCH TG 2009 F3) and the heating rate was around
20 K minÀ1 with the temperature range of 50–500 1C. Fig. 3 shows
their TGA traces. The decomposition temperatures of ligands L1
and L2 are 250 1C and 262 1C, respectively, implying that the
intermolecular halogen bonds of L2 are stronger than those of
L1. Salt 1 shows two steps of mass loss, the first mass loss step
is in the temperature range of 138–200 1C (mass loss 15.47%),
and the second decomposition is at a temperature of 254 1C
with a loss of 58.23%, indicating the decomposition of ligand
L1, which is similar to crystal 2. Salt 3 shows a mass loss of
14.64% in the temperature range of 149–200 1C and further
weight loss corresponds to the decomposition of L2 at 262 1C.
For salt 2, due to serious hygroscopicity, the first weight loss is
For salts 1 and 4, only crystalline powders were obtained,
PXRD measurements (Fig. 2) revealed that the crystal structures
of 1 and 4 are isostructural to 2 and 3, due to the very similar
Fig. 2 PXRD patterns of salts 1–4.
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Fig. 3 TGA profiles of ligands L1 and L2 and salts 1–4 in the temperature range of 50–500 1C.
accompanied by the loss of free water molecules (mass loss Hirshfeld surface by using a red–blue–white color scheme: red
4.14%) and the next steps of weight loss above 264 1C are due to regions represent closer contacts; blue regions represent longer
the disintegration and decomposition of the ligand L1 molecules. contacts; and white regions represent the distance of contacts
For 4, there is an obvious large weight loss above 50–200 1C (mass equal to vdW separation.33,34 The 3-D dnorm surfaces can be
loss 42.13%). The second step of weight loss above 263 1C may be resolved into 2-D fingerprint plots, and the 2-D fingerprint plots
due to the decomposition of L2 molecules, perchlorate anions are displayed by using the standard 0.4–2.6 Å view with the de
and the collapse of the lattice structure. It can be seen that the and di distance scales displayed on the graph axes.
characteristic decomposition temperatures of ligands L1 and L2
For L1 and L2, the red regions on the dnorm surface are
are reserved in halogen-bonding connected salts 1 and 3, while mainly concentrated around BrÀ and IÀ, corresponding to the
they increase in hydrogen-bonding connected salts 2 and 4.
BrÁ Á ÁN and IÁ Á ÁN halogen bonding interactions, and the NÁ Á ÁH
and CÁ Á ÁC hydrogen bonding interactions also show a weak red
region in L1 and L2. For salt 2, the red regions mainly represent
the significant NÁ Á ÁH, OÁ Á ÁH and BrÁ Á ÁO interactions and these
interactions comprise 14.2%, 22.6% and 4.7% to the total
Hirshfeld surface, respectively. For 3, IÁ Á ÁCl and ClÁ Á ÁH account
for a large proportion of the red region in the total Hirshfeld
surface, comprising 2.6% and 20.3%, respectively (all kinds of
3.3. Hirshfeld surface calculations
The Hirshfeld surface serves as a powerful tool for describing
the surface characteristics of the molecules and identifying
intermolecular interactions.32 The 3-D Hirshfeld dnorm surfaces
and 2-D fingerprint plots of ligands L1 and L2 and salts 2 and 3
are shown in Fig. 4. The 3-D dnorm values are mapped onto the
Fig. 4 Hirshfeld dnorm surfaces and 2-D fingerprint plots of ligands L1 and L2, and salts 2 and 3.
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halogen bonds, and does not appear in ligand L1. For L2, a
visible difference has been displayed in Fig. 6b between 3200
and 2995 cmÀ1. This may be due to the inorganic anion by
halogen-bonding with I affecting the fluorescence intensity of
L2. The similar Raman profiles of ligands L1 and L2 and
halogen-bonding connected salts 1 and 3 indicate that salt
formation by halogen-bonding interaction has little effect on
the vibrations of ligand functional groups, which also was
confirmed by the IR spectra (Fig. S1, ESI†).
3.5. Fluorescence spectra
To further investigate the effect of halogen-bonding contacts on
organic salts, fluorescence emission spectra studies were carried
out. The emission spectra of the six compounds in methanol are
shown in Fig. 7 and the excitation wavelength is 285 nm. By
comparing the fluorescence emission spectra of the four salts
and the corresponding ligand in solution, which is ligand L1,
salts 1 and 2 exhibit emissions with maximum peaks (lmax) at
around 372 nm in solution, and for ligand L2, salts 3 and 4 at
370 nm have a maximum emission peak; it can be seen that the
fluorescence emissions of salts 1 and 3 are probably assigned to
the intra-ligand fluorescence emission because similar behaviors
are also observed for the free ligands L1 and L2, respectively, and
halogen bonding contacts account for the major part among the
salts and ligands in methanol solution. Besides, this can be
Fig. 5 The percentage contributions from the individual intermolecular
interactions to the Hirshfeld surface of ligands L1 and L2 and salts 2 and 3.
interactions are quantitatively summarized in Fig. 5). The above
analysis shows that the halogen bond, similar to the hydrogen
bond, plays an important role in the formation of crystal 3, and
halogen bonds account for more than hydrogen bonds. These
results are in agreement with the X-ray crystallographic analysis
of hydrogen-bonding and halogen-bonding interactions.
3.4. Raman spectroscopy
To investigate the peak change of compounds L1 and L2 after further confirmed using UV-visible absorption spectra in solution
salt formation, we compared the Raman and IR spectra of as reported in this paper (Fig. S2, ESI†).
compounds L1 and L2 and salts 1–4. Fig. 6a shows Raman
Compared with the similar maximum fluorescence peaks
spectra in the region of 3300–40 cmÀ1. For L1, its characteristic (lmax) in methanol solution, the fluorescence spectra of all the
peaks are around 3125, 1507, 881, and 373 cmÀ1. However, salts salts in the solid state displayed strong and red-shifted emissions
1 and 2 have almost no influence on the characteristic peaks of using the excitation wavelength at 373 nm (Fig. 8). Intermolecular
L1 apart from the disappearance of the peak at 1507 cmÀ1 due vibrational relaxation is effectively inhibited, which would cause
to the formation of halogen bonds and hydrogen bonds. strong shifted emissions in the solid state compared with those
Furthermore, salt 2 with a 3-D framework, has a distinct in solution.35 Furthermore, after introducing the conjugated
difference, as has been highlighted by rectangles in Fig. 6. aromatic aldehyde structure into 4-amino-1,2,4-triazole, the
The peak at around 453 cmÀ1 for 2 may be attributed to BrÁ Á ÁO fluorescence efficiency increases with increasing conjugation
Fig. 6 Raman spectra of complexes L1 and L2 and salts 1–4. The differences are highlighted.
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Fig. 7 Fluorescence emission spectra of ligands L1 and L2 and salts 1–4 in methanol solution (2.5 Â 10À5 mol LÀ1).
Fig. 8 Fluorescence emission spectra of ligands L1 and L2 and salts 1–4 in the solid state.
of p electrons and molecular flatness, and the fluorescence fluorescence properties of ligands L1 and L2 are reserved in
spectrum moves in the long wavelength direction. Salts 1 and halogen-bonding connected salts 1 and 3, while change and
2 have a maximum emission peak at 435 nm and 456 nm, reduction in hydrogen-bonding connected salts 2 and 4.
respectively, which shows a slight red-shift compared to that of
ligand L1 at 432 nm. Salts 3 and 4 show a visible change
compared to ligand L2 and salt 4 even has two fluorescence
4. Conclusions
emission peaks, which implies that different anions have different
influences on the fluorescence properties of the ligand in the solid In summary, HL1 ÁClÀ (1), L1ÁHL1 ÁClO4 (2), HL2 ÁClÀ (3), and
+
+
À
+
+
À
state. The connection and strength are also different; from the L2ÁHL2 ÁClO4 (4), based on the new ligands 4-(4-bromo-
above analysis, it can be seen that the halogen bond plays a major benzylideneamino)-4H-1,2,4-triazole (L1) and 4-(4-Iodobenzyl-
role in salts 1 and 3, in the same way as the ligand, while the ideneamino)-4H-1,2,4-triazole (L2), have been successfully
hydrogen bond occupies a larger proportion in 2 and 4. In synthesized and the influence of halogen-bonding contacts
addition, the red-shifted and reduced intensity of the fluorescence on the crystal structure and the fluorescence properties of these
of the salts can be ascribed to the increase of conjugation of p organic salts has been investigated. Compounds L1 and L2
electrons, where the ligands with anions are linked mainly by exhibit a polymeric 1-D chain in zigzag structures via C–BrÁ Á ÁN
halogen bonds in 1 and 3 and through hydrogen bonds in 2 and 4. and C–IÁ Á ÁN halogen-bonding interactions. For salt 2, N–HÁ Á ÁO
Comparing the fluorescence spectra of the ligand and the corres- hydrogen-bonding contacts play a major role, resulting in a 3-D
ponding salts in solution and in the solid state indicates that the framework structure. Each ClÀ ion connects to three carbon atoms,
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one nitrogen atom and the one iodine atom; meanwhile, the ClÀ
ion using C–IÁÁÁCl halogen-bonding results in an interleaved 3-D
network structure of 3. The results of the luminescence properties
reveal that compared with hydrogen-bonding contacts, halogen-
bonding contacts can lead to preservation of the emission intensity
of the ligand in salts 1 and 3, while reducing the emission intensity
in hydrogen-bonding connected salts 2 and 4. This work may
provide a better understanding of the role of halogen-bonding
contacts in tuning the photo-physical properties of organic salts.
G. Terraneo, CrystEngComm, 2011, 13, 6593; (d) R. Liantonio,
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This research was based on work supported by the Fundamental
Research Funds for the Central Universities (No. 3207047407),
Natural Science Foundation of China (Grant No. 21371031 and
21628101), International S&T Cooperation Program of China (No.
2015DFG42240) and the Priority Academic Program Development
(PAPD) of Jiangsu Higher Education Institutions.
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