Two tridentate pyridinyl-hydrazone zinc(II) complexes as fluorophores for blue emitting layers

Article abstract of DOI:10.1016/j.molstruc.2019.07.098

Two new complexes were obtained by reaction of zinc (II) acetate with 4-fluoro-N'-(pyridin-2-ylmethylene) benzohydrazide or with 4-(hexyloxy)-N'-(pyridin-2-ylmethylene)benzohydrazide ligands in pyridine. Both ligands have a pyridinyl-hydrazone moiety able

Full text of DOI:10.1016/j.molstruc.2019.07.098

Journal of Molecular Structure 1197 (2019) 672e680
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Two tridentate pyridinyl-hydrazone zinc(II) complexes as
fluorophores for blue emitting layers
,
*
Rosita Diana a, Barbara Panunzi a, Angela Tuzi b, Ugo Caruso b
a Department of Agriculture, University of Napoli Federico II, Portici, NA, Italy
b Department of Chemical Sciences, University of Napoli Federico II, Napoli, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new complexes were obtained by reaction of zinc (II) acetate with 4-fluoro-N'-(pyridin-2-
ylmethylene) benzohydrazide or with 4-(hexyloxy)-N'-(pyridin-2-ylmethylene)benzohydrazide ligands
in pyridine. Both ligands have a pyridinyl-hydrazone moiety able to act as a mono-negative tridentate
ligand toward zinc ion in 2:1 stoichiometric ratio, producing an octahedral environment. The derived
complexes are poor emitters in diluted solution. Instead, they exhibit intense blue fluorescence in the
solid state due to AIE (aggregation induced emission) effect. The crystalline complexes are bright yellow
in natural light and blue under UVevisible light with remarkable Stokes Shifts. Their photoluminescence
quantum yields are ranging 20e30%, making them very promising as fluorophore dopants for blue
emissive layers. One ligand and its complex were characterized by single crystal X-ray diffraction analysis
that revealed an almost planar conformation of two ligands coordinated to the metal and the presence of
Received 18 June 2019
Received in revised form
19 July 2019
Accepted 26 July 2019
Available online 27 July 2019
Keywords:
OLEDs
Zinc complex
PLQYs
significant
p-p stacking of molecules at about 3.4 Å.
© 2019 Published by Elsevier B.V.
1. Introduction
device stability and high cost of rare metals still represent a limit in
the industrialization of LEDs, especially for the blue emitters.
From our laboratories a large number of articles have been
published on metal complexes containing aroyl-hydrazones and/or
pyridinyl-hydrazones [20,26], specially zinc (II) based. Zinc as a low
cost and biocompatible metal [5,27e31], has been studied in
complexes with polydentate ligands for the specific redox and co-
ordination properties, and as a catalytic co-factor for many metal-
loenzymes [6e9]. Our interest in this class of complexes was
promoted by the ability to display photoluminescence activity in
the crystal phase. This property results related to the structural
features and intermolecular packing effects and highly tunable
according to substituents of the ligand. Due to the closed-shell ef-
fect, zinc ion can preserve or even enhance PL performances of the
ligands. In particular, we demonstrated that tuning in emission
wavelength of the zinc complexes can be obtained by insertion of
an electron withdrawing and/or of a pyridine moiety into the main
structure of hydrazone type ligands. This behavior is related to the
change in the push-pull conjugation pattern [12,21,22,32e34].
Moreover, we found that the lowest unoccupied molecular orbital
(LUMO) was often confined onto pyridine moiety which played an
exclusive role in determining the PL response. In our previous
contributions we prepared a number of polydentate hydrazones
zinc-complexes promising as emissive components in PL devices
Polydentate hydrazones are important scaffolds with a broad
range of multidisciplinary applications in coordination chemistry.
There are numerous reports dealing with aryl-hydrazone metal
complexes as catalytic systems [1e12], antimicrobial and antifungal
molecules [13e16], chemospecific reactants [17], DNA binding and
cleavage active tools [18,19].
In addition to the biological activity, complexes from poly-
dentate hydrazone has been investigated intensively in the last
years for the wide possibility of coordination numbers, crystal
structure and sometimes unusual symmetries [20e22]. In recent
years, the optical and electro-optical properties of polydentate
chelates were examined. The specific goal is to achieve light ab-
sorption/emission performance for targeted technological appli-
cations. In particular, their photoluminescence (PL) performances
have been widely investigated as neat solids or as dopants in the
construction of the emitting layers for LEDs and solar cells [23,24].
A lot of emissive heavy-metal complexes have been proposed to
achieve high PL performance [4,25]. Despite recent progress, low
* Corresponding author.
E-mail address: ugo.caruso@unina.it (U. Caruso).
https://doi.org/10.1016/j.molstruc.2019.07.098
0022-2860/© 2019 Published by Elsevier B.V.

R. Diana et al. / Journal of Molecular Structure 1197 (2019) 672e680
673
for their tunable color emission and high PL quantum yields
(PLQYs).
alkoxy substituents in order to get the desirable blue emission in
the zinc complexes. Fluorine in organic molecules has unique
charge density distribution, thermal and oxidative stability. Fluo-
rinated compounds have applications in photovoltaic devices as
blue emissive materials [42]. Thanks to its high electronegativity
and electron-withdrawing ability this substituent would generally
lower the energy of both LUMO and HOMO energy levels, with a
more pronounced effect on HOMO energy. This higher band gap
typically produce blue emission [21,22,33,42,43]. On the other
hand, despite the inductive effect due to the electronegative oxy-
gen, the alkoxy substituents scarcely affects the electronic pattern
of the aromatic ring. Conversely it can exert an effect on the crystal
packing. It is well known the optical properties of the materials in
the solid state strictly depend on the crystal packing. The addition
of short or long alkoxy chains can result a tuning in PL perfor-
mances in the solid state [22,33]. In some organic dyes it has been
demonstrated that the derivatives with longer alkoxy chains
exhibited blue-shifted fluorescence in the crystalline state respect
to shorter or unsubstituted derivatives [11,44].
In this work we synthetized two complexes 3 and 4 by reaction
of zinc (II) acetate with 4-fluoro-N'-(pyridin-2-ylmethylene)ben-
zohydrazide (1) [35] or with 4-(hexyloxy)-N'-(pyridin-2-
ylmethylene)benzohydrazide (2) ligands in pyridine (Scheme 1).
Both the ligands are mono-negative scaffolds which acts as tri-
dentate toward the bivalent ion. Two ligands chelate the metal in
an octahedral environment were the pyridinic nitrogen participates
to the coordination.
The complexes are weakly emitters in diluted solution. Contrary
to previous literature indications for similar unsubstituted struc-
tures [36] the main strength of the complexes are the relevant
emission in the crystalline arrangement, as a result of the increase
in the push-pull efficiency due to an almost planar conformation of
the ligands coordinated to the Zn(II) ion [37e41]. The crystalline
complexes are blue emitters in the solid state with PLQYs above
20% and large Stokes Shifts.
Photophysical measurements were performed on the complexes
dissolved in dichloromethane or spin-coated on quartz slides. Op-
tical data are summarized in Table 1. Both the complexes 3 and 4
show weak emission in diluted solution with low PLQYs (<5%) and
no relevant solvatochromism in dependence of solvent polarity.
Stokes shifts values in solution are about 120 nm for both samples
(see Fig. 1). Not unexpectedly [45], we found that compounds 3 and
4 exhibit intense fluorescence in the solid-state, due to their AIE
behavior. Under these conditions, fluorophores weakly emissive in
dilute solution strongly emit in the aggregate state due to their
structural features [45]. The X-ray diffraction analysis of the
structural peculiarity of one of the complexes compared with its
own ligand is reported below.
2. Results and discussion
The ligands were obtained by condensation reaction between a
substituted benzoyl hydrazine and pyridine-2-carboxaldehyde
(picolinaldehyde). These ligands, as other systems containing
NeH groups in conjugated rings or chains, are reported to exhibit
tautomeric behavior in solution and can coordinate in neutral or
monoanionic mode (enolate). As expected, two ligands coordinate
to the zinc (II) ion in both complexes (see Scheme 1). Ligands
themselves have poor emission in solution or in solid state,
becoming emissive after the structural freezing in
conformation imposed by coordination [22,33].
a planar
The aroyl-hydrazone ligands were designed with fluorine or
Scheme 1. Synthetic route to the target compounds.

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R. Diana et al. / Journal of Molecular Structure 1197 (2019) 672e680
Table 1
Optical data of complexes.
Compound
labs.sol (nm)a
lem.sol (nm)b
labs.film (nm)c
lem.film (nm)d
PLQY%e
3
4
390
382
507
501
333
351
473
472
23
26
5
2
a
Wavelength of UVeVisible absorbance maxima in dichloromethane solution.
Wavelength of emission maxima in dichloromethane solution.
Wavelength of UVeVisible absorbance maxima on thin film obtained from the crystalline sample.
Wavelength of emission maxima on thin film obtained from the crystalline sample.
PL quantum yield on thin film obtained from the crystalline sample.
b
c
d
e
In order to investigate the type of molecular packing leading to
the good PL performance, ligand 1 and its complex 3 were analysed.
Both of them show
arrangement demonstrating that the presence of
p
-stacked molecules in the crystalline
interactions
p-p
do not necessarily result in fluorescence quenching [22,33,46].
The complex 3 crystallizes in the monoclinic P21/c space groups
with one molecule contained in the independent unit. The ligand 1
crystallizes in the triclinic P-1 space group with one molecule and
one H2O solvent crystallization molecule contained in the inde-
pendent unit. Crystal data and structural details are reported in
Table 2. All bond lengths and angles are in the normal range, a
selection of them is reported in Table 3. The ORTEP view of the
molecular structure of 1 and 3 are reported in Fig. 3 and Fig. 4.
The ligand 4-fluoro-N'-(pyridin-2-ylmethylene) benzohy-
drazide belongs to a class of compounds that exhibit keto-enol
tautomerism. The analysis of 1 bond lengths pattern confirms the
presence of the cheto-tautomer, with the carbonyl C7eO1 bond
distance of 1.224 (3) Å typical of a double bond. The molecule as-
sumes a fairly all planar conformation with a didhedral angle of 11.7
(1) ꢀ between the mean planes of fluorophenyl and pyridyl groups.
Fig. 1. Absorption (blue curves) and emission (red curves) of 3 (solid line) and 4 (dot
line) in dichloromethane.
In the crystal packing (Fig. 5) sheets of
p-p stacked molecules at
Thin films of 3 and 4 were deposed as shuttered crystals on
quartz slides and used to evaluate the PL performances of the
complexes, including PLQYs. The solid samples emit in the blue
region of the visible spectrum and are bright yellow under natural
light, as distinctly perceptible even to the naked eye (see Fig. 2).
The large Stokes Shifts (121 nm and 140 nm respectively for 3
and 4 from absorbance or excitation to emission maximum wave-
length) is a relevant parameter for the efficiency of a PL device. A
large (more than 100 nm) Stokes shift eliminates spectral overlap
between absorption and emission phenomena. Accordingly, the
detection of the fluorescence improves both in the intensity and in
the color purity. PLQYs were measured on the blue emissive layers
of the neat compounds obtaining 23% and 26% respectively for 3
and 4. In some other articles [10e12] we found that luminescence
was largely enhanced when the fluorophore was added as a dopant
to a polymeric matrix, and this is a commonly employed technique
for producing the emitting layers of PL devices. Dopant fluo-
rophores with PLQYs ranging 20e30% are usually efficiently
employed in emitting layers for LEDs. This makes 3 and 4 promising
fluorophores, easy to synthesize and low-cost, for actual
applications.
about 3.5 Å are joined through strong hydrogen bonds that involve
the water crystallization molecules (Table 4).
In the coordination to the metal, the ligand adopts the depro-
tonated enolic form acting as a mononegative enolate ion and a
mononuclear complex forms, consisting of one Zn (II) cation and
two 4-fluoro-N'-(pyridin-2-ylmethylene) benzohydrazide anions.
In the complex 3 each ligand acts as ONN tridentate through the
Oeno, the Nim, and the Npy atoms that occupy a meridional plane in a
distorted octahedral geometry around Zn atom. The apical posi-
tions are occupied by the pyridine nitrogen and the enolate oxygen
atoms of one ligand [N1A-Zn1-O1A ¼ 149.06 (11) ꢀ]. The great
distorsion from regular octahedral geometry is mainly due to the
formation of four planar 5-membered chelateꢀ rings with the
ꢀ
chelate bite angles in the range from 73.55 (11) to 75.65 (11)
.
Bond distances ZneO and ZneN in 3 (Table 2) are in the range
found for similar octahedral Zn complexes. The Zn-Npy distances
are longer than Zn- Nim ones, showing a weaker coordination
ability of pyridyl-N towards the metal. The pattern of C-Oeno and C-
Nim bond lengths (see Table 3) is in agreement with a partial double
bond character, suggesting that extended conjugation in the ligand
occurs after the complexation. A fairly planar shape of the ligand is
Fig. 2. Thin films obtained from 3 (above) and from 4 (bottom) in natural (on the left) and under UVevisible 365 nm commercial lamp (on the right).

R. Diana et al. / Journal of Molecular Structure 1197 (2019) 672e680
675
Table 2
Crystal data and structure refinement details for 3 and 1.
3
1
CCDC number
Empirical formula
Formula weight
T (K)
1923052
1923053
C13 H10 F N3 O$H2O
261.26
C
26 H18 F2 N6 O2 Zn
549.83
296 (2)
173 (2)
l
(Å)
0.71073
0.71073
Crystal system
Space group
a (Å)
Monoclinic
P21/c
10.451 (3)
13.397 (4)
19.194 (6)
90
119.933 (16)
90
2328.9 (13)
4
Triclinic
P -1
6.217 (2)
6.9640 (10)
15.097 (4)
97.489 (15)
99.27 (2)
106.210 (13)
608.7 (3)
b (Å)
c (Å)
a (ꢀ)
b
(ꢀ)
(ꢀ)
g
V (Å3)
Z
2
Dcalc (Mg/m3)
1.568
0.109
1.425
0.109
m
(mmꢁ1
)
F (000)
q
1120
272
range (ꢀ)
2.806e27.500
20513/5159 [R (int) ¼ 0.0754]
5159/0/334
1.114
2.782e27.493
7962/2759 [R (int) ¼ 0.0641]
2759/0/184
1.029
Reflections collected/unique [R (int)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R, wR indices [I > 2s(I)]
Final R, wR indices (all data)
0.0481, 0.0831
0.1366, 0.1139
0.389/ꢁ0.458
0.0621, 0.1381
0.1179, 0.1623
0.264/ꢁ0.261
Largest diff. peak/hole (eAꢁ3
)
Table 3
Selected bond lengths (Å) and angles (ꢀ) for 3 and 1 with e.s.d.‘s in parentheses.
3
Zn1eN1A
Zn1eN1B
Zn1eN2A
Zn1eN2B
Zn1eO1A
Zn1eO1B
C6A-N2A
N1A-Zn1-O1A
N1BeZn1eO1B
N1A-Zn1-N2A
1
2.331 (3)
2.253 (3)
2.059 (3)
2.062 (3)
2.076 (3)
2.079 (3)
1.266 (5)
149.06 (11)
148.54 (11)
73.55 (11)
C6BeN2B
C7A-O1A
C7BeO1B
C7A-N3A
C7BeN3B
N2A-N3A
N2BeN3B
N1BeZn1eN2B
N2A-Zn1-O1A
N2BeZn1eO1B
1.276 (5)
1.272 (4)
1.267 (4)
1.330 (5)
1.334 (5)
1.358 (4)
1.361 (4)
73.73 (11)
75.65 (11)
75.49 (12)
N2eC6
N3eC7
1.278 (3)
1.353 (3)
N2eN3
C7eO1
1.374 (3)
1.224 (3)
Fig. 3. ORTEP view of the molecular structure of ligand 1 with thermal ellipsoids drawn at 30% probability level. The water crystallization molecule is also shown.
found, with a dihedral angle between mean planes of fluorophenyl
stacking interactions of fluorophenyl and pyridinyl rings at about
3.4 Å (Fig. 6).
In order to detect additional packing features and to analyse
close intermolecular contacts in 3 and 1, we have examined the
and pyridyl groups of 17.3 (2)ꢀ (in A-labeled molecule) and 6.06 (17)
ꢀ
(in B-labeled molecule). In C1 the crystal packing is stabilized by
weak intermolecular Car-H … Namide interactions and by
p … p

676
R. Diana et al. / Journal of Molecular Structure 1197 (2019) 672e680
Fig. 6. Crystal packing of 3 viewed along b axis showing
p … p stacking of molecules.
Contacts shorter than the sum of vdW radii are drawn as cyan dashed lines.
The two-dimensional full fingerprint plot and the Hirshfleld
surface mapped over dnorm of 3 and 1 are reported in Fig. 7 and
Fig. 8 respectively. In the Hirshfeld fingerprint plot of 3 (Fig. 8 left)
the intermolecular N/H/H/N (7.9%) interactions are displayed as
a pair of sharp spikes at about di þ de ¼ 2.5 Å, symmetrically
disposed with respect to the diagonal. The weak Car-H … Namide
hydrogen bond can be observed as red spots in the Hirshfleld sur-
face mapped over dnorm (Fig. 8 right). Significant C/C contacts
(7.6%) individuated by the green area at about di þ de ¼ 3.4e3.6 Å
Fig. 4. ORTEP view of the molecular structure of complex 3 with thermal ellipsoids
drawn at 30% probability level.
confirm the presence of p$$$p stacking interactions. As expected, a
large part of H/H (37.9%) interactions are found (diagonal blue
strip that ends at about di ¼ de ¼ 1.08 Å) and C/H/H/C (19.2%)
contacts (two broad symmetrical zones at about di þ de ¼ 2.8 Å).
The two-dimensional full fingerprint plot of the ligand 1 (Fig. 8
Hirshfeld surfaces and two-dimensional fingerprint plots using
CrystalExplorer 17.5 [47]. The electrostatic potentials were calcu-
lated using TONTO, integrated within CrystalExplorer, using wave-
functions at the B3LYP/6-31G (d,p) level.
Fig. 5. Crystal packing of 1 showing sheets of
p-stacked molecules. Shortest intermolecular distances are drawn in green; hydrogen bonds are drawn in cyan.
Table 4
Hydrogen bonding geometry for 3 and 1 (e.s.d.‘s in parentheses).
3
ꢀ
ꢀ
DeH/A
C4AeH4/N3B
1
d (DeH) Å
0.93
d (H$$$A) Å
2.58
d (D/A) Å
3.283 (5)
<(DeH/A)
132.8
(i)
DeH/A
d (DeH) Å
1.00 (3)
0.88 (3)
0.95 (4)
0.95 (4)
0.93 (4)
d (H$$$A) Å
2.34 (3)
1.95 (3)
2.55 (4)
1.96 (4)
1.96 (4)
d (D/A) Å
3.181 (3)
2.821 (3)
3.150 (3)
2.874 (3)
2.858 (3
<(DeH/A)
140 (2)
170 (3)
122 (3)
159 (3)
163 (3)
C6eH6/O2
N3eH3A … O2
O2eH1O /N2(ii)
O2eH1O/O1(ii)
O2eH2O/N1(iii)
Symmetry code: (i) ¼ -x, y ꢁ1/2, -z þ 1/2; (ii) ¼ x-1, y, z; (iii) ¼ -x, -y, -zþ1.

R. Diana et al. / Journal of Molecular Structure 1197 (2019) 672e680
677
Fig. 7. Left: Two-dimensional full fingerprint plot of 3 (H/H (37.9%), C/H/H/C (19.2%), N/H/H/N (7.9%), C/C (7.6%). Right: Hirshfleld surface mapped over dnorm with one H-
bonded molecule and one -stacked molecule displayed.
p
Fig. 8. Left: Two-dimensional full fingerprint plot of 1 (O/H/H/O (13.9%), C/H/H/C (15.2%). Right: Hirshfleld surface mapped over dnorm with one H-bonded H2O crystallization
molecule displayed.
left) displays the significant pair of sharp spikes at about
di þ de ¼ 2.0 Å symmetrically disposed with respect to the diagonal,
due to O/H/H/O (13.9%) and to N/H/H/N (5.7%) hydrogen
bonds contacts. Significant C/C contacts (7.0%, green area at
ligand 1 the major part of EPS surface displays the white color of
zero potential, with very strictly located negative potential (red
regions) at Nimine, Ocarbonyl and F atoms and positive potential (bue
regions) at Npy atom. In 1 the red-coloured region, rich in electrons,
corresponds to the electron-withdrawing fluoro-aryl group and
extends towards the metal. The blue region that is poor in electrons
corresponds to the pyridine electron-donor group, thus confirming
the push-pull efficiency of ligand after coordination.
di þ de ¼ 3.6 Å) are due to
p$$$p stacking interactions. Other sig-
nificant contacts are H/H (35.9%), C/H/H/C (15.2%) and F/H/
H/F (10.6%). The red spots on the Hirshfeld surface mapped over
d
norm (Fig. 8 rigth) individuate the strong intermolecular hydrogen
bonds that involve the water crystallization molecules.
In the Hirshfeld surfaces mapped over the electrostatic potential
(ESP) of 3 and 1 (Fig. 9) the negative potential (electron-rich re-
gions) is displayed as red surface, positive potential (electron-poor
regions) as blue and white regions display zero potential. In the
3. Experimental
All starting products were commercially available. Both ligands
1 and 2 were prepared according to a known procedure [33].

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R. Diana et al. / Journal of Molecular Structure 1197 (2019) 672e680
C
19H23N3O2: C, 70.13; H, 7.12; N, 12.91; found: C, 70.18; H, 7.11; N,
12.89. UV/Vis: lmax 335 nm. 1H NMR (1,1,2,2-TCE d2, 25 ꢀC,
400 MHz):
d (ppm): 0.90 (d, 3H); 1.41 (m, 4H); 1.51 (m, 2H); 1.80 (m,
2H); 4.00 (t, 2H); 7.10 (d, 2H); 7.86 (m, 5H); 8.56 (m, 1H), 11.00 (s,
13
1H). C NMR (1,1,2,2-TCE d2, 25 ꢀC, 400 MHz):
d (ppm): 13.98,
22.69, 25.60, 29.25, 31.80, 68.69, 102.37, 117.35, 121.00, 122.83,
128.65, 137.54, 144.88, 150.00, 153.60, 162.97, 163.27 ppm.
3.2. Synthesis of complexes: general procedure
The synthesis of 3 has been reported as an example. In 10 mL of
pyridine, 0.486 g of ligand 1 (2.00 mmol) were dissolved. Under
stirring at room temperature 0.219 g of zinc acetate dihydrate
(1.00 mmol) were added. The reaction was allowed for 4 h and then
a yellow crystalline solid was formed, collected by filtration under
vacuum and washed with fresh methanol. Recrystallization from
pyridine gave the pure product. Bright yellow crystals suitable for
X-ray analysis were isolated from the recovered material. Yield:
80%. No melting point was detected till decomposition. Td: 335 ꢀC.
Elemental analysis calculated (%) for C26H18F2N6O2Zn: C, 56.80; H,
3.30; N, 15.28; found: C, 56.78; H, 3.40; N, 15.19. By thermogravi-
metric analysis under nitrogen flow the Zn content was determined
as the residue at 800 ꢀC: Zn calcd: 11.89%, found: 11.46%. 1H NMR
(1,1,2,2-TCE d2, 25 ꢀC, 400 MHz):
1H); 7.46 (d, 1H); 7.76 (t, 1H); 8.02 (m, 1H), 8.16 (d, 2H), 8.65 (s, 1H).
13C NMR (1,1,2,2-TCE d2, 25 ꢀC, 400 MHz):
(ppm): 115.50, 116.05,
d (ppm): 7.04 (m, 2H); 7.21 (m,
d
123.80, 125.49, 130.84, 136.00, 145.00, 152.50, 163.00, 165.77,
166.07 ppm.
Complex 4: Yield: 77%. No melting point was detected till
decomposition. Td: 300 ꢀC. Elemental analysis calculated (%) for
C38H44N6O4Zn: C, 63.91; H, 6.21; N, 11.77; found: C, 63.18; H, 6.16;
N, 11.90. By thermogravimetric analysis under nitrogen flow the Zn
Fig. 9. Hirshfeld surface mapped over the electrostatic potential for 3 and 1. Red
surfaces indicate the negative potential, blue the positive potential, white the zero
potential.
content was determined as the residue at 800 ꢀC: Zn calcd: 9.15%,
1
found: 9.00%. H NMR (1,1,2,2-TCE d2, 25 ꢀC, 400 MHz):
d (ppm):
6.88 (m, 2H); 7.13 (m, 1H); 7.37 (m, 1H); 7.71 (t, 1H); 7.98 (d, 2H);
8.08 (d, 1H), 8.53 (s, 1H). 13C NMR (1,1,2,2-TCE d2, 25 ꢀC, 400 MHz):
Optical observations were performed by using a Zeiss Axioscop
polarizing microscope with a FP90 Mettler heating stage. Phase
transition temperatures and enthalpies were measured using a DSC
scanning calorimeter PerkinElmer Pyris 1 at a scanning rate of
10 ꢀC/min, under nitrogen flow. Thermogravimetric analyses were
performed in air by a TA Instruments SDT 2960 Simultaneous DSC-
TGA. Decomposition temperature calculated as 5% weight loss. The
zinc content was measured as ZnO residue from TGA analysis. The
structure of the complexes was confirmed by 1H NMR and 13C NMR
recorded in 1,1,2,2-TCE d2 using a Bruker Spectrometer operating at
400 MHz. UVeVisible and fluorescence spectra were recorded by
JASCO spectrometers.
The same synthetic procedure was applied for the ligands, ob-
tained by condensation reaction of the correspondent benzohy-
drazide with picolinaldehyde in ethanol.
The complexes were obtained following the same synthetic
route, by deprotonation of the ligands and coordination to zinc (II)
cation in a basic solvent (pyridine).
d
(ppm): 14.11, 22.70, 25.60, 29.60, 31.80, 68.70, 114.50, 116.35,
121.40, 123.83, 129.65, 136.54, 145.00, 152.00, 161.00, 163.97,
166.27 ppm.
3.3. Photoluminescence measurements
Thin films were obtained by the spin-coating technique of a
dispersion of shuttered crystalline complexes in 1:1 dichloro-
methane/hexane (20 mg of complex dissolved in 1 mL) and ther-
mally annealed at 70 ꢀC for 10 min. The spin coater SCS P6700
apparatus operated at 800 RPM.
Photoluminescence quantum efficiency measurements were
conducted in accordance with a known methodology [48]. The
samples were placed within an integrating sphere that equipped
with an optical fiber connection (StellarNet F400 e UV VIS SR). For
excitation source a diode laser at 376 nm (Toptica Photonics) was
employed and the light were analysed using StellarNet spectrom-
eter (black-comet 50).
3.1. Synthesis of ligands: general procedure
3.4. X-ray crystallography
The synthesis of 2 has been described as an example. Com-
mercial 4-(hexyloxy)benzohydrazide (0.236 g, 1.00 mmol) was
dissolved in boiling ethanol (5 mL) and then picolinaldehyde
(0.107 g, 1.00 mmol) dissolved in ethanol (5 mL) added. After
40 min of stirring at room temperature, the solution was cooled by
using an ice bath. A whitish crystalline solid formed, which was
collected by filtration and washed with cold ethanol. Yield: 75%.
Tm: 115 ꢀC; Td: 310 ꢀC. Elemental analysis calculated (%) for
Single crystals of 3 and 1 suitable for X-ray crystal structure
analysis were obtained from slow evaporation of pyridine (3) and of
CH2Cl2/EtOH (1:1) (1) solutions at room temperature. One selected
crystal was mounted at ambient temperature (3) and in flowing N2
at 173 K (1) on a Bruker-Nonius KappaCCD diffractometer equipped
with 700 Oxford Cryostream apparatus (graphite monochromated
MoKa radiation,
l
¼ 0.71073 Å, CCD rotation images, thick slices, 4
and scans to fill asymmetric unit). Semiempirical absorption
u

R. Diana et al. / Journal of Molecular Structure 1197 (2019) 672e680
679
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corrections (SADABS) were applied. Both structures were solved by
direct methods (SIR97 program [49]) and anisotropically refined by
the full matrix least-squares method on F2 against all independent
measured reflections using SHELXL-2018/3 [50] and WinGX soft-
ware [51]. In 1 crystallization H2O is present. The H atoms bound to
N (amide) and to O (water) in 1 were located in difference Fourier
maps and their coordinates freely refined (Uiso(H) equal to 1.2Ueq of
the carrier atom). All the other hydrogen atoms in 1 and 3 were
introduced in calculated positions and refined according to a riding
model (CeH distances equal to 0.93e0.95 Å and Uiso(H) equal to
1.2Ueq of the carrier atom). Crystal data and structure refinement
details are reported in Table 1. Selected bond lengths and angles are
reported in Table 2. Hydrogen bonds geometry is reported in
Table 3. The figures were generated using ORTEP-3 [52] and Mer-
cury CSD 4.0 [53] programs.
Crystal data were deposited at Cambridge Crystallographic Data
Centre with assigned number CCDC 1923052 (3) and 1923053 (1).
These data can be obtained free of charge from www.ccdc.cam.ac.
uk/data_request/cif.
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U. Caruso, Photophysical properties of luminescent zinc(II)‒Pyridinylox-
adiazole complexes and their glassy self-assembly networks, Eur. J. Inorg.
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4. Conclusions
Two zinc (II) complexes were obtained from pyridinyl-
hydrazone type ligands. The hydrazones were designed with fluo-
rine or alkoxy substituents to get the desirable blue emission in the
solid state which still represent a goal in the production of blue
LEDs. The ligands act as mono-negative tridentate chelates toward
zinc ion, producing two octahedral complexes with ligand to metal
2:1 stoichiometric ratio. The complexes are poor emitters in diluted
solution but exhibit intense blue fluorescence in the crystalline
state due to their AIE feature. In order to get information about the
crystalline packing, complex 3 and its ligand 1 were explored by
single crystal X-ray diffraction analysis. The crystallographic anal-
ysis revealed a planar conformation of coordinated 1 and the
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presence of significant
p-p stacking and led to a comparison be-
tween the electrostatic potentials. Large Stokes Shifts (above
100 nm) have been recorded on the solid samples and PLQYs above
20%. These results make the complexes very promising as easy and
low-cost fluorophore dopants for blue emissive layers.
[21] F. Borbone, U. Caruso, S. Concilio, S. Nabha, B. Panunzi, S. Piotto, R. Shikler,
A. Tuzi, Mono-, di-, and polymeric pyridinoylhydrazone ZnII complexes:
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Acknowledgments
This research was supported by the Italian Ministry of Educa-
tion, University and Research (MIUR) [grant number
300395FRB17].
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R. Shikler, A. Tuzi, Color tuning and noteworthy photoluminescence quantum
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