Re-arrangements of 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol with different inorganic anions: Crystal structure and fluorescence properties

Article abstract of DOI:10.1016/j.poly.2017.05.025

Fluorescent materials have many interesting applications, but it remains difficult to control the luminescence of organic materials and in particular to retain the same luminescence in solution and in the solid state, a property of interest for various imaging applications. In this work, the influence of inorganic-anions on the crystal structure and physical–chemical properties of a series organic salts L⁺Cl^? (1), L⁺ClO₄^? (2), L⁺NO₃^?·H₂O (3), 2L⁺SO₄^2?·2H₂O (4) (L?=?4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol) has been investigated. Single crystal X-ray analyzes revealed that salt 1 formed a linear structure connected by H?O interactions, while salts 2–4 all formed 2D network structures through O?O interactions. The protonated L⁺s adopt face-to-face slipped π-stacked arrangement in salts 1 and 3, while display monomer arrangements in 2 and 4, the latter can be attribute to the entrapment of heavy anions in the crystal lattices. Optical-properties measurements of these four salts have shown L⁺ arrangements dependent on solid-state properties: salt 1 and 3 show broad emission bands (λ₁?=?478?nm, λ₂?=?487?nm) with a red shift of about 60?nm relative to its fluorescence in solvents, while 2 and 4 are similar to each other, exhibiting sharp bands (λ₃?=?453?nm, λ₂?=?433?nm) with the higher ?_PL values (?_PL1?=?0.08, ?_PL1?=?0.15, ?_PL1?=?0.06, ?_PL1?=?0.14). These results demonstrated the significant influence of inorganic-anions on the organic salts and the optical properties of organic materials could be modulated by entrapping different anions in lattice.

Full text of DOI:10.1016/j.poly.2017.05.025

Polyhedron 133 (2017) 203–212
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Re-arrangements of 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol with
different inorganic anions: Crystal structure and fluorescence properties
⇑
⇑
Ya-Wen Zhang, Jing-Wen Wang, Hong-Shuai Wu, Ming-Xin Wang, Yang-Hui Luo , Bai-Wang Sun
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluorescent materials have many interesting applications, but it remains difficult to control the lumines-
cence of organic materials and in particular to retain the same luminescence in solution and in the solid
state, a property of interest for various imaging applications. In this work, the influence of inorganic-
anions on the crystal structure and physical–chemical properties of a series organic salts L+Clꢀ (1),
L+ClOꢀ4 (2), L+NO3ꢀꢁH2O (3), 2L+SO42ꢀꢁ2H2O (4) (L = 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol) has
been investigated. Single crystal X-ray analyzes revealed that salt 1 formed a linear structure connected
by Hꢁ ꢁ ꢁO interactions, while salts 2–4 all formed 2D network structures through Oꢁ ꢁ ꢁO interactions. The
Received 13 April 2017
Accepted 18 May 2017
Available online 30 May 2017
Keywords:
Re-arrangements
Inorganic-anions
Organic salts
protonated L+s adopt face-to-face slipped
p-stacked arrangement in salts 1 and 3, while display monomer
arrangements in 2 and 4, the latter can be attribute to the entrapment of heavy anions in the crystal lat-
tices. Optical-properties measurements of these four salts have shown L+ arrangements dependent on
solid-state properties: salt 1 and 3 show broad emission bands (k1 = 478 nm, k2 = 487 nm) with a red shift
of about 60 nm relative to its fluorescence in solvents, while 2 and 4 are similar to each other, exhibiting
sharp bands (k3 = 453 nm, k2 = 433 nm) with the higher /PL values (/PL1 = 0.08, /PL1 = 0.15, /PL1 = 0.06,
Optical-properties
Schiff base
/PL1 = 0.14). These results demonstrated the significant influence of inorganic-anions on the organic salts
and the optical properties of organic materials could be modulated by entrapping different anions in
lattice.
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
the complex between an acid and a base [10–12]. The advantage
of an organic salt over neutral molecules in a co-crystal is the
Systematic structural variation of fluorescent organic molecules
plays a vital role in solid for the enhancement of emission proper-
ties. In recent years, the design and synthesis of organic solid-state
fluorescence materials as well as promising optoelectronic applica-
tions in lasers [1], sensors [2], organic light-emitting diodes [3] and
two-photon fluorescent materials [4] have attracted great atten-
tion. From the previous study, many organic fluorophores have
strong fluorescence in solution while losing this property in the
solid state [5,6].
Co-crystals deal with the design, synthesis, and evaluation of
solid-state supramolecular structures with tailored form and func-
tion [7], that is, from the designed chemistry of intermolecular
forces to materials with, hopefully, desired fine-tuning properties
strong hydrogen bonding, which controls the molecular assembly
tailoring the properties of solid-state materials. Consequently,
the molecular tailorability of promising molecules exhibiting fluo-
rescence has attracted considerable attention [13].
Triazole class is a rich N-containing heterocycle comprising of
three nitrogen and two carbon atoms [14], which has received
attention as a novel non-toxic chemical compound [15]. Hetero-
cyclic compounds especially 4-Amino-1,2,4-triazoles and their
condensed derivatives [16] have been used as excellent candidates
for the construction of medicine and pesticide intermediates [17].
The fluorescence properties of 4-Amino-1,2,4-triazoles is very
weak, however, after introducing aromatic aldehyde, the fluores-
cence efficiency increases due to the higher conjugate degree and
the presence of hydroxyl having a good coordination ability. The
photometric, electrochemical and thermochromic properties [18]
of triazol containing Schiff base and phenol moieties allow their
applications in potentiometric sensors [19], molecular electronic
devices [20], optical computers [21], imaging systems [22], molec-
ular memory storage devices [23] and as photodetectors in biolog-
ical systems [24].
by modulating the p-conjugated molecule structure [8,9]. Co-crys-
tals remain neutral while crystallizing together, whereas an
organic salt is usually formed due to a proton exchange within
⇑
Corresponding authors. Fax: +86 25 52090614.
E-mail
addresses:
peluoyh@sina.com (Y.-H. Luo), chmsunbw@seu.edu.
cn (B.-W. Sun).
http://dx.doi.org/10.1016/j.poly.2017.05.025
0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.

204
Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
Because of their interesting architectures and applications in
C–Hꢁ ꢁ ꢁ
p interactions have been detected, due to the separating
action of the perchlorate ions (Fig. 2d).
diverse areas, 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol (L,
Scheme 1) was used as organic ligand in our efforts. L can be coor-
dinated to metal ions in different modes, bidentatebridging or
monodentate, depending on the nature of the substituent group
at the triazole ring [25]. Besides, the complexes with different
nuclearity have been obtained depending on variables such as
the co-ligand, anion and the metal–ligand ratio [26]. The salt for-
mation of L with inorganic anions has not been investigated yet.
Hence, this paper discusses the arrangements of L molecules by
formation of its different salts and the influence of inorganic anions
in crystal structures and physical properties. Four acids, Hydro-
bromic acid (HCl), perchloric acid (HClO4), nitric acid (HNO3) and
sulfuric acid (H2SO4) were used as hydrogen bond donors, and
O–Hꢁ ꢁ ꢁO hydrogen bonds would be expected to be formed with
the hydroxyl group as hydrogen bond acceptors. In this work, we
obtained four organic salts: L+Clꢀ (1), L+ClO4ꢀ (2), L+NO3ꢀꢁH2O (3),
2L+ SO24ꢀꢁ2H2O (4). By entrapment of different anions in the lattice,
the stacking modes changed greatly, which afforded the different
properties in solid state. These salts were characterized by sin-
gle-crystal X-ray diffraction, molecular Hirshfeld surfaces and
physical measurements (differential scanning calorimetry (DSC),
thermogravimetric analyzes (TGA), infrared absorption spec-
troscopy (IR)). We further analyzed absorption spectra (UV–Vis,
diffuse reflectance spectra) and fluorescence of the four salts.
2.1.3. L+NO3ꢀꢁH2O (3)
Organic salt 3 crystallizes as a brown needlelike crystal and has
a monoclinic system and space group P21/c with Z = 4. In particular,
the asymmetric unit of 3 consists of one protonated L+, one NO3ꢀ
anion and a solvent site H2O, which is different from 1 and 2
(Fig. 3a). L+ and NOꢀ3 are connected by O–Hꢁ ꢁ ꢁO (distance of
2.719 Å for the hydroxyl group) hydrogen bonding interactions.
In addition, water molecules play the major role in the
supramolecular 3D organization of the structure. Through their
oxygen atoms acting as multihydrogen-bonded acceptors, the
water molecules link neighboring L+ and NO3ꢀ units of the structure
by means of N–Hꢁ ꢁ ꢁO and O–Hꢁ ꢁ ꢁO hydrogen bonds. The adjacent
1D chains in one plane are connected with each other by R42ð12Þ.
pramolecular synthons through
p–p (distance of 2.812 and
2.891 Å) intermolecular interactions between oxygen atoms of
NOꢀ3 and H2O molecules into 2D structure (Fig. 3b). The packing
of 3 are stacked in a parallel fashion, in which the rings adopt a
face-to-face slipped p-stacked arrangement. The closest plane sep-
aration between the neighboring moieties is 3.398 Å, which is
shorter than 2 could due to the addition of water molecules
(Fig. 3c).
2.1.4. 2L+ SO42ꢀꢁ2H2O (4)
2. Results and discussion
Organic salt 4, brown needlelike crystals, crystallizes in mono-
clinic Cc space group with Z = 4. The ASU consists of an entire
molecule of two L+ cations, one SO42ꢀ anion and two H2O molecules
(Fig. 4a), which is similar to 3. The hydrogen bonds linked to the
cation, which have N1 and N5 as the donors, are attached to one
sulfate anions and have length of 2.630 Å. The other hydrogen
bond of a dimer unit has a donor–acceptor distance of 2.635 Å
(Fig. 4b). Each of the dimeric units is stacked through the Oꢁ ꢁ ꢁO
interactions, in which L molecules are widely separated and no
2.1. Crystal structures
The crystal structures of 1–4 were elucidated by single crystal
X-ray diffraction and the crystallographic data are presented in
Table 1
2.1.1. L+Clꢀ (1)
Organic salt 1 crystallizes as brown cuboid-shaped crystals. The
p p interaction between the two ring fluorophores (the closest
ꢀ
structural determination shows 1 forms a 1:1 (L+: Clꢀ) salt in the
distance between two parallel rings is 5.392 Å), which is different
ꢀ
Triclinic P1 space group with Z = 2. Fig. 1a shows the asymmetric
from crystal 3 may due to the heavy atoms of SO24ꢀ (Fig. 4c).
unit (ASU) of 1 consists of an entire molecule of L+ cation and
one Clꢀ anion, and selected bond distances and angles are tabu-
lated in Table 2. In the structure, the L+ and Clꢀ associate through
the O–Hꢁ ꢁ ꢁCl (distance of 3.125 Å) and N–Hꢁ ꢁ ꢁCl (distance of
3.024 Å) hydrogen bonds to form the infinite 1D chains (Fig. 1b).
2.2. Hirshfeld surface calculation
Molecular Hirshfeld surface is a powerful tool for gaining addi-
tional insight into crystal structure and polymorph comparison by
color-coding short or long contacts. 3D color-coding scheme can
give intermolecular interactions, and it can be resolved into 2D fin-
gerprint plots, which give a quantitatively summary of the nature
and type of intermolecular contacts experienced by the molecules
in the crystal [27]. The 3D Hirshfeld surfaces and 2D fingerprint
plots of four organic salts are shown in Fig. 5, which clearly show
the similarities and differences of the influences of different anions
on the intermolecular interactions of the L molecule. For salt 1, the
large red region in the middle of the dnorm surface corresponding
to both donor and acceptor of the O–Hꢁ ꢁ ꢁCl hydrogen bond, repre-
sents the ligand molecule connecting with Cl into the linear struc-
ture. For 2–4, the closest intermolecular interactions are all found
to be O–Hꢁ ꢁ ꢁO and N–Hꢁ ꢁ ꢁO interactions and have a most signifi-
cant contribution to the total Hirshfeld surfaces, indicating the for-
mation of hydrogen bonds. The hotspots on the 2D fingerprint
plots are represent the close intermolecular interactions. The dif-
ferences in the 3D dnorm and 2D fingerprint plots of salts 1–4 indi-
cate visually that the introduction of anions exerts a significant
influence on the types and numbers of intermolecular interactions.
Then, the problem arises how intermolecular interactions vary
quantitatively. To answer it, relative contributions of different
As shown in Fig. 1c, the two neighboring chains are linked by
p–
p
interactions between the rings, and the plane separation of the
double-decker is 3.563 Å.
2.1.2. L+ClOꢀ4 (2)
Organic salt 2 crystallizes as brown needlelike crystals. And it
forms in the monoclinic P21/c space group with Z = 4. The struc-
tural determination shows that 2 forms a 1:1 (L+: ClO4ꢀ) salt and
has an entire molecule of L+ ClOꢀ4 in the ASU (Fig. 2a). L+ cation
and ClOꢀ4 anion are connected through N–Hꢁ ꢁ ꢁO (distance of
2.672 Å) and O–Hꢁ ꢁ ꢁO (distance of 2.568 Å) hydrogen bonds
(Fig. 2b). As illustrated in Fig. 2c, L+ and ClO4ꢀ associate alternately
through Oꢁ ꢁ ꢁO interactions (distance of 2.897 Å) to construct R22ð6Þ.
pramolecular synthons. Despite the abundance of aromatic rings
in the crystal structure, no significant intermolecular
pꢁ ꢁ ꢁp or
N
N
N N
HO
Scheme 1. Chemical structure of L.

Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
205
Table 1
Crystal data and structure refinement for salts 1–4.
Salt
1
2
3
4
Formula
C9H7ClN4O
222.64
C9H9ClN4O5
288.65
monoclinic
P21/c
7.1870(14)
16.366(3)
10.661(2)
90.00
95.43(3)
90.00
1248.3(4)
4
1.536
293(2)
0.329
C6H9N2O2
269.23
monoclinic
P21/c
7.1440(14)
18.168(4)
9.3760(19)
90.00
104.84(3)
90.00
1176.3(4)
4
1.520
293(2)
0.126
C18H22N8O8S
510.50
monoclinic
Cc
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
ꢀ
P1
6.4952(13)
7.5138(15)
10.839(2)
97.30(3)
93.46(3)
102.79(3)
509.54(19)
2
6.7700(14)
20.084(4)
16.848(3)
90.00
101.50(3)
90.00
2244.9(8)
4
1.510
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (mg mꢀ3
T (K)
)
1.451
293(2)
0.352
293(2)
0.208
l
(mmꢀ1
)
Cryst dimensions
0.2 ꢂ 0.25 ꢂ 0.32
0.3 ꢂ 0.25 ꢂ 0.3
0.3 ꢂ 0.3 ꢂ 0.4
0.3 ꢂ 0.2 ꢂ 0.25
No. of reflections collected
No. of unique reflections
No. of parameters
2263
2298
2108
2266
1942
137
1.093
967
172
0.973
1381
172
1.004
1466
316
1.002
Goodness-of-fit (GOF) on F2
R1, wR2 (I > 2
R1, wR2 (all data)
CCDC NO.
r
(I))
0.0533, 0.1610
0.0603, 0.1679
1524375
0.0904, 0.1664
0.2150, 0.2116
1524377
0.0661, 0.1644
0.1022, 0.1848
1524379
0.0548, 0.1059
0.1002, 0.1243
1524378
Fig. 1. (a) Molecular structure of salt 1, (b) Hydrogen bonds between the L+ and Clꢀ molecules, (c) Double-decker structure of 1, the plane separation is highlighted, (d)
Spacefill model of 1 (L+ and Clꢀ are indicated by blue and green). (Color online.)
intermolecular interactions to the Hirshfeld surface are summa-
rized in Fig. 6.
Table 2
Geometrical parameters for the hydrogen bonds in salts.
Salt
D–H (Å)
Hꢁ ꢁ ꢁA (Å)
D–A (Å)
\D–Hꢁ ꢁ ꢁA (°)
2.3. Thermal behavior
1
O1–H1ꢁ ꢁ ꢁCl1
N4–H4ꢁ ꢁ ꢁCl1
2
O1–H1Aꢁ ꢁ ꢁO5
N2–H2ꢁ ꢁ ꢁO4
3
O1–H1Aꢁ ꢁ ꢁO5
N1–H1Bꢁ ꢁ ꢁOW
4
0.820
0.861
2.310
2.179
3.125
3.024
172.8
167.2
Thermal studies over the Schiff base ligand and their salts,
through the thermogravimetric (TGA-DTG) (Fig. 7) and differential
scanning calorimetric (DSC) (Fig. 1S) techniques, were investi-
gated. And the obtained weight loss data of L and four salts are tab-
ulated in Table 3. From the curves, it can be seen that the addition
of HCl, HClO4, HNO3 and H2SO4 can affect the pyrolysis. The data
indicate that the ligand and salts 1–4 start decomposition at 272,
233, 235, 240 and 230 °C, respectively. The points of salts 1–4
are lower than L, which may attribute to the hydrogen bonds
0.851
0.860
1.719
1.813
2.568
2.672
175.2
179.1
0.820
0.861
1.899
1.814
2.719
2.643
178.3
161.2
O1–H1Aꢁ ꢁ ꢁOW1
0.821
0.859
1.822
1.777
2.635
2.630
170.0
169.2
N1–H1Bꢁ ꢁ ꢁO4

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Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
Fig. 2. (a) Molecular structure of salt 2, (b) Hydrogen bonds between the L+ and ClO4ꢀ molecules, (c) Supramolecular synthons around the L+ unit and the ClOꢀ4 molecule, (d)
Stacking structure of 2 (L+ and ClO4ꢀ are indicated by green and blue). (Color online.)
Fig. 3. (a) Molecular structure of salt 3, (b) Supramolecular synthons around the L+, NO3ꢀ and H2O, (c) The packing of 3 (L+, NO3ꢀ and H2O are indicated by green, blue, red).
(Color online.)
and weak contacts between the triazole rings in their crystals. For 1
and 2, the stage occurred from 200 to 400 °C with over 80% mass
loss, corresponding to the release of the p-Hydroxybenzaldehyde
from Schiff base. During this process the C@N of Schiff base may
be completely broken. Salts 3 and 4 show two steps of mass loss,
the first pronounced mass loss step is in temperature range of
100–130 °C (the yellow square), which refer to the decomposition
of salts and lattice water molecule (mass loss 10%), the second
decomposition is in temperature range of 200–400 °C with loss
of 40%, indicating the decomposition of L, which is the same to 2,

Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
207
Fig. 4. (a) Molecular structure of salt 4, (b) Dimeric units of L+, NO3ꢀ and H2O, (c) 2D chain motif of 4, (d) The packing motif of 4(L+, SO42ꢀ and H2O are indicated by red (blue),
green, yellow (purple)). (Color online.)
Fig. 5. Hirshfeld surfaces (top) and fingerprint plots (bottom) for salts 1–4.

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Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
phenolic ring and associated conjugated segment up to
azomethine group of these compounds.
As typical -conjugated molecules, L and salts have displayed a
p
potential value in the field of optics. Therefore, the photophysical
properties of the four salts have been further investigated and
compared to disclose the influence of different anions on the opti-
cal properties of L, which would be beneficial for exploring new
materials. In order to better understand the influence of inter-
molecular interactions and stacking models on the photophysical
properties of crystals, fluorescence emission spectra were
exploited to investigate the optical-properties of the crystals.
The diffuse reflectance spectra (DRS) as pure solid powders to
avoid matrix and environment effects [31], each exhibit bands
exclusively in the UV region (Fig. 9b), which is typical of the enol
form. Similarly, the solids (four salts and L) show broad bands with
fine peaks (225 nm, 300 nm, 363 nm) arising from triazole and
phenolic ring, corresponding to the spectra of compounds in the
solvents. However, for the four salts, the main absorption peaks
red shift compared with the L due to intermolecular hydrogen
bond interactions. Thus we may conclude that the formation of
hydrogen bond interactions lead to red shift.
The fluorescence emission spectra of the five compounds in
methanol are shown in Fig. 10a. All the compounds exhibited emis-
sions with maximum peaks (kmax) at around 420 nm in solution.
By comparing the absorption spectra and fluorescence emission
spectra of the five compounds in solution, it can be seen that the
absorption and fluorescent emissions of the four salts are probably
assigned to the intraligand fluorescent emission because similar
behaviors are also observed for the free ligand in methanol solu-
tion. In present study, metal–ligand complexes have been synthe-
sized and characterized. The conclusion showed that the
fluorescence peak of these complexes were same as L and the
red-shift is based on metal-to-ligand charge transfer [26]. How-
ever, the emissions of 1–4 in solution are slightly blue-shifted,
which are attributed to intermolecular hydrogen bond interactions
of L and anions. The blue shift of photoluminescence suggests that
ligand in salts are in higher conjugated level, which increases with
the enhancement of conjugated structures [32].
Fig. 6. The percentage contributions from individual intermolecular interactions to
the Hirshfeld surfaces of salts 1–4.
3 and L. Notably, the DSC curves of 2, 3 and 4 show obvious peaks
at the temperature region from 130 to 150 °C, which may be rela-
tive to the melting point of acids. But the curve of 1 at this region is
flat, probably due to the linear structure.
2.4. FT-IR spectroscopy
The FT-IR spectrum of L and salts 1–4 shows several character-
istic stretching vibration modes due to O–H, N–H, CH@N, C–N and
N–N bands, as shown in Fig. 8. There are distinct differences in the
band characteristics between L and four salts. FT-IR spectrum of L
shows additional bands, ꢃ3420, ꢃ1598, ꢃ1505 and ꢃ1050 cmꢀ1
corresponding to O–H, CH@N, C–N and N–N bonds respectively.
Salts 1–4 show boarder absorption peaks around 3400 cmꢀ1 attri-
bute to stretching vibration of O–H groups due to stronger hydro-
gen bond interactions than in L. In four salts, the N–H stretching
frequency of the salts was used as a probe to identify the proton
displacement. The acidꢀbase interaction involving the proton
transfer gives rise to a hydrogen bond, and it appears as a broad
and medium-strong band at about 3080 cmꢀ1 from the Fig. 8. This
band is due to the N–H+ stretching vibration of the salt, occurring
in the 3300–2370 cmꢀ1 region [28]. In all salts, the imine bands
appeared in the region from 1603 to 1620 cmꢀ1 [29a] and the
Compared with the similar photophysical properties with max-
imum fluorescence peaks (kmax) around 420 nm in methanol solu-
tion, the fluorescence spectra of all the salts 1–4 in the solid state
display strong and red-shifted emissions (Fig. 10b). The emissions
in solution should be related to the conjugated configuration and
the corresponding intramolecular rotational motions around the
double bonds beside the conjugate moieties. These intramolecular
motions can lead to fast non-radiative relaxation in solution [33].
In addition, in the crystalline state, intramolecular vibrational
relaxation is effectively inhibited, leading to the closure of the
non-radiative decay channel, which would cause the strong shifted
emissions in the crystalline state compared with those in solution
[34].
triazole (1510 cmꢀ1
) C–N stretching bands are shown in
ꢃ1510 cmꢀ1 [29b]. This was evident from
a comparison of
intensities of the N–N peak (1050 cmꢀ1). A ratio of 0.5 was
noticed for four salts, which is result of hydrogen bonds between
L and anions [29b]. These differences provide a proof of the
formation of hydrogen bonds.
2.5. Absorption spectra and fluorescence emission spectra
Organic salts 1 and 3 exhibit a broader emission band located at
470–450 nm, which are ascribed to phenolic ring and triazole flu-
orophores. The strongest peaks of 2 and 4 are blue-shifted to 455
and 435 nm, which indicates that different types of electronically
excited states exist for these salts [35]. It also implies different
anions have different influence on the fluorescence properties of
L in crystalline. The vibrational fine structures appear in the fluo-
rescence spectrum of 2 and 4, which is the similar image of the
absorption spectrum of 2 and 4 in the range from 380 to 600 nm
and can be ascribed to the transitions from the lowest vibrational
level of the singlet first excited electronic state (S1) to any of vibra-
tional levels of the singlet ground state (S0). It is because phenolic
ring and triazole fluorophores adopt monomer arrangement with
the closest centroid long distance (6.658 Å for salt 2 and 5.392 Å
for 4), Ls are widely separated by means of entrapment of heavy
The UV–Vis spectroscopic measurements were done for explor-
ing the structural differences of four organic salts and their raw
material in methanol solution (concentration of 10–4 g/ml) by cor-
relating the absorption spectra recorded (Fig. 9a). The four salts are
all shown maximum absorption around 310 nm and other absorp-
tion peaks around 203 nm and 230 nm, which are almost identical
with ligand except the increase of intensity. Previous studies
revealed that triazolic chromophore absorbs around 205 nm [30].
Hence, the absorption peaks of all our selected crystals (227 nm)
are attributable to
p
p⁄ transition of triazolic moiety. The corre-
ꢀ
sponding redshift observed in the spectra of all these compounds
in comparison to triazole might be due to the attached aromatic
system. The intense absorption band appearing around 308 nm
in the spectra can be attributed to the
p
p⁄ transitions of the
ꢀ

Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
209
0.5
100
80
60
40
20
0
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
0
0
0
0
0
100
100
100
100
100
200
200
200
200
200
300
400
500
500
500
500
500
600
600
600
600
600
700
700
700
700
700
Temperature/
0.5
100
80
60
40
20
0
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
300
400
Temperature/
1
100
80
60
40
20
0
0
-1
-2
-3
-4
-5
-6
-7
300
400
Temperature/
1
100
80
60
40
20
0
-1
-2
-3
-4
-5
-6
-7
-8
300
400
Temperature/
0.2
0.0
100
80
60
40
20
-0.2
-0.4
-0.6
-0.8
-1.0
300
400
Temperature/
Fig. 7. TGA-DTA profiles of L and four salts.
Table 3
Thermodynamic properties of salts 1–4.
Compound
Decomposition
Major weight loss transition (°C)
Residual weight loss at 600 (°C) (%)c
temperature (°C)
a
b
T10
T30
L
272
233
235
240
230
250–315
200–290
210–250
220–280
205–370
15.9
6.1
3.2
10.6
24.4
1
2
3
4
118
125
a
b
c
Temperature at which 10% of weight loss was observed by TGA-DTG.
Temperature at which 40% of weight loss occurred in TGA-DTG.
Residual weight observed by TGA-DTG at 600 °C in N2.

210
Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
anions. The red-shifted and broadening fluorescence spectra of 1
and 3 can be ascribed to the benzene and triazole aggregate
because the phenolic and triazole fluorophores form a face-to-face
slipped
pꢀp stacked aggregate.
Comparing the fluorescence spectra of the four organic salts in
solution and in the solid state indicates that intermolecular inter-
actions and stacking of molecules play an important role in affect-
ing the fluorescence emissions of the four salts. Therefore, to better
understand the role of anions in affecting the optical properties in
the crystalline state and explore the relationship between the solid
state emissions and molecular stacking modes, comparisons of the
fluorescence spectra of the salts in the solid state are necessary.
These optical properties of salts with different inorganic anions
are expected to be useful for the utilization of organic salt-based
optoelectronic devices [36].
To evaluate the solid-state fluorescence intensity of the four
salts, their emission quantum yields (/PL) were investigated
(Table 4). Compared with that of 1 and 3, the higher /PL values
of 2 and 4 could be found, which may be attributed to the suppres-
sion of
p p stacking between phenolic and triazole fluorophores
ꢀ
Fig. 8. FT-IR spectra for L and salts 1–4.
by entrapment of heavy anions in the lattices [37].
1.0
1.0
0.8
0.6
0.4
0.2
0.0
(b)
L
1
2
3
4
L
1
2
3
4
(a)
0.8
0.6
0.4
0.2
0.0
200
250
300
350
200
300
400
500
600
Wavelength/nm
Wavelength/nm
Fig. 9. (a) UV–Vis absorbance (concentration of 10ꢀ4 g/ml) and (b) diffuse reflectance absorption spectra of L and four salts.
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
4
(b)
(a)
L
1.0
1
2
3
4
0.8
0.6
0.4
0.2
400
450
500
550
600
650
400
450
500
550
600
650
700
Wavelength/nm
Wavelength/nm
Fig. 10. Fluorescence spectra of (a) L and four salts in solution and (b) 1–4 in solid-state.

Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
211
Table 4
resulting solution was kept string for approximately an hour then
left to evaporate at room temperature for several days.
Photophysical properties of the four salts.
Salt
Solution
Solid
4.4. X-ray crystallographic study
Abs (nm)
PL (nm)
PL (nm)
/
(%)
PL
1
2
3
4
307
307
306
308
423
412
413
421
478
455
487
435
0.08
0.15
0.06
0.14
The single-crystal X-ray diffraction data of salts 1–4 was col-
lected at a temperature of 293 K using graphite-monochromated
Mo K
tometer (
a
radiation (k = 0.071073 nm) on a Rigaku SCXmini diffrac-
-scan technique) [39]. The lattice parameters were inte-
x
grated using vector analyzis and refined from the diffraction
matrix. The absorption correction was carried out by the Bruker
3. Conclusion
SADABS program with a multi-scan method. The crystallographic
data, data collection and refinement parameters of four salts are
summarized in Table 1. The structures were solved by full-matrix
least-squares methods on all F2 data, and used SHELXS-97 and
In summary, the molecular interactions and stacking modes of
ligand 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol have been
successfully altered by introducing of different inorganic anions.
The obtained organic salts were changing from linear structure to
2D networks. As discussed before, salts in crystalline state were
coordinated with N atomy of triazole in L [26]. Moreover, the stack-
ing modes of the protonated ligand have shown a close relation-
ship with the solid state fluorescence emissions properties: the
SHELXL-97 programs [40] for structure solution and refinement,
respectively. All non-hydrogen atoms were refined anisotropically
and hydrogen atoms were geometrically fixed.
4.5. Hirshfeld surface calculations
face-to-face
p-stacked geometries of phenolic ring and triazole
resulted in the aggregation-induced red-shifted emissions in salts
1 and 3, when compared with the monomer arrangement of
ligands in 2 and 4. These results demonstrate that the control of
molecular orientation and stacking model play a crucial role in
tuning their optical-properties via a salt strategy is an effective
way for exploring highly fluorescent derivatives. This strategy
can be also employed to tune the emission colors of organic salt
chromophores for other luminescent systems, providing a power-
ful way for designing various new types of organic luminescent
materials. Therefore, it may be a useful compound for imaging
and other applications [36].
Molecular Hirshfeld surfaces [41] calculations were conducted
by using the CrystalExplorer [42] program, which is a novel
method of analyzing intermolecular interactions [27c]. When the
cif files of salts 1–4 were input to the CrystalExplorer program,
bond lengths of hydrogen were adjusted to typical standard neu-
tron values which can be fully automated. (C–H = 1.083 Å, N–
H = 1.009 Å). In this work, the Hirshfeld surfaces were generated
using a standard (high) surface resolution. The 3D dnorm surfaces
mapped over a fixed color scale of ꢀ0.46 (red) to 1.9 Å (blue), 2D
fingerprint plots [27] were displayed by using the standard 0.6–
2.8 Å view with the de and di distance scales displayed on the
graph’s axes.
4. Experimental
4.6. Physical measurements
4.1. Materials
The four organic salts were characterized by differential scan-
ning calorimetry (DSC) and thermogravimetric analyzes (TGA)
were processed using a MettlerToledo TGA/DSC STARe System at
a heating rate of 10 K minꢀ1 under an atmosphere of dry N2 flow-
ing at 20 cm3 minꢀ1 over a range from 50 to 500 °C, samples were
placed in open aluminum oxide crucibles annealed at 1100 °C. The
TGA/DSC dates were analyzed by using STARe Software. Infrared
spectra of salts 1–4 in the form of potassium bromide pellets were
recorded on a SHIMADZU IR prestige-21 FTIR-8400S spectrometer
with the spectral range 3800–400 cmꢀ1. UV–Vis absorption spectra
were recorded on a Shimadzu UV-3600 spectrometer. Diffuse
reflectance spectra were obtained with a Varian Cary 5E spectrom-
eter using barium sulfate (BaSO4) as a reference. Fluorescence
microscopy images were obtained on an Olympus BX51 imaging
system. Fluorescence spectra were obtained on a Horiba Flu-
oroMax 4 spectrofluorometer.
p-Hydroxybenzaldehyde, 4-amino-4(H)-1,2,4-triazoles, hydro-
bromic acid (HCl), perchloric acid (HClO4), Nitric acid (HNO3) and
sulfuric acid (H2SO4), were all commercially available from Sigma
Aldrich and used as received without further purification. Metha-
nol, and ethanol were commercially available from Sinopharm
Chemical Reagent Company, Ltd. and used as received without fur-
ther purification.
4.2. Preparation of 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol (L)
The ligand has been reported by others [38] and was prepared
according to the following literature procedure: a mixture of 4-
amino-1,2,4-triazole (10 mmol), p-Hydroxybenzaldehyde (10 mmol)
and H2SO4 (4 mL) were dissolved in ethanol (50 mL) and were
refluxed for 4–10 h. The reaction was monitored through thin layer
chromatography. The resulting solvent was evaporated on a rotary
evaporator under reduced pressure and residue was washed with
ethanol then dried under vacuum. The product compound was
recrystallized in ethanol to afford a yellow solid ligand.
Acknowledgements
This work has been supported by the Natural Science Founda-
tion of China (Grant Nos. 21371031 and 21628101), International
S&T Cooperation Program of China (No. 2015DFG42240) and the
Priority Academic Program Development (PAPD) of Jiangsu Higher
Education Institutions.
4.3. Preparation of organic salts (1–4)
The salts of 4-[(4H-1,2,4-triazol-4-ylimino)methyl]phenol (L)
with HCl, HNO3, HClO4 and H2SO4 were conducted in a slow evap-
oration crystallization experiment. Organic salts 1–4 were all pre-
pared by mixing the corresponding reactants in 1:1 (2 mmol, HCl,
0.07 g; HClO4, 0.2 g; HClO4, 0.13 g; H2SO4, 0.20 g; L, 0.38 g) molar
ratio into a 20 ml methanol/water (1:1 v/v) solution, and the
Appendix A. Supplementary data
CCDC 1524375, 1524377, 1524379 and 1524378 contain the
supplementary crystallographic data for salt 1–4. These data can

212
Y.-W. Zhang et al. / Polyhedron 133 (2017) 203–212
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