Series of O,N,O-tridentate ligands zinc(II) complexes with high solid-state photoluminescence quantum yield

Article abstract of DOI:10.1002/ejic.201400095

Four neutral complexes were synthesized by reaction of tridentate N-salicylidene-N′-aroylhydrazine ligands with zinc(II) acetate and pyridine as a further ligand. The O,N,O-chelating molecules N-4-[octyloxy(phenoxy)]salicylidene-N′-fluorobenzoylhydrazine (L¹), N-4-[octyloxy(phenoxy)]salicylidene-N′-(benzothiazolyl)hydrazine (L²), N-4-[octyloxy(phenoxy)]salicylidene-N′-cyanobenzoylhydrazine (L³), and N-4-[octyloxy(phenoxy)]salicylidene-N′-nitrobenzoylhydrazine (L⁴) gave dinuclear O-bridged complexes Zn₂Lⁱ₂Py_2n (n = 1 or 2 in a special case). The ligands and complexes were characterized by mass spectrometry and ¹H NMR, UV/Vis, photoluminescence (PL), and FTIR spectroscopy. The structures of the complexes were solved by single-crystal X-ray analysis. Nematogenic phase behavior was detected for some ligands by thermogravimetric analysis/differential scanning calorimetry (TGA-DSC) and optical observations. The solution and solid-state photoluminescence spectra were recorded and show that the ligands are roughly nonfluorescent, whereas all of the complexes show noteworthy emission in widely different spectral regions. The photoluminescence maxima of the complexes in the solid state are dependent on the acceptor group, and medium-to-high PL quantum yields were recorded for all of the crystalline complexes. Both periodic and molecular DFT calculations were performed to rationalize the absorbance of these complexes at the one-particle level. Neutral Zn^II complexes with tridentate ligands and pyridine were synthesized. Their X-ray structures reveal penta- and hexacoordination. The solid-state photoluminescence changed with the electron-withdrawing group, and good photoluminescence quantum efficiencies (PLQEs) were recorded. By DFT analysis, a first rationalization of the absorption redshift in the series was produced.

Full text of DOI:10.1002/ejic.201400095

FULL PAPER
DOI:10.1002/ejic.201400095
Series of O,N,O-Tridentate Ligands Zinc(II) Complexes
with High Solid-State Photoluminescence Quantum Yield
Fabio Borbone,[a] Ugo Caruso,[a] Mauro Causà,[b] Sandra Fusco,[a]
Barbara Panunzi,*[c] Antonio Roviello,[a] Rafi Shikler,[d] and
Angela Tuzi[a]
Keywords: Zinc / Schiff bases / Tridentate ligands / Luminescence
Four neutral complexes were synthesized by reaction of tri-
dentate N-salicylidene-NЈ-aroylhydrazine ligands with
zinc(II) acetate and pyridine as a further ligand. The O,N,O-
chelating molecules N-4-[octyloxy(phenoxy)]salicylidene-
NЈ-fluorobenzoylhydrazine (L1), N-4-[octyloxy(phenoxy)]sal-
icylidene-NЈ-(benzothiazolyl)hydrazine (L2), N-4-[octyloxy-
(phenoxy)]salicylidene-NЈ-cyanobenzoylhydrazine (L3), and
N-4-[octyloxy(phenoxy)]salicylidene-NЈ-nitrobenzoylhydraz-
ine (L4) gave dinuclear O-bridged complexes Zn2Li2Py2n (n =
1 or 2 in a special case). The ligands and complexes were
characterized by mass spectrometry and 1H NMR, UV/Vis,
photoluminescence (PL), and FTIR spectroscopy. The struc-
tures of the complexes were solved by single-crystal X-ray
analysis. Nematogenic phase behavior was detected for
some ligands by thermogravimetric analysis/differential
scanning calorimetry (TGA-DSC) and optical observations.
The solution and solid-state photoluminescence spectra were
recorded and show that the ligands are roughly nonfluores-
cent, whereas all of the complexes show noteworthy emis-
sion in widely different spectral regions. The photolumines-
cence maxima of the complexes in the solid state are depend-
ent on the acceptor group, and medium-to-high PL quantum
yields were recorded for all of the crystalline complexes. Both
periodic and molecular DFT calculations were performed to
rationalize the absorbance of these complexes at the “one-
particle” level.
of the final complex. Our interest in d10 metal ion such as
ZnII was first promoted by the large number of possible
coordination numbers and symmetry. We are aware of its
role both as a biological essential element[6] and as a key
element in many optoelectronic applications, such as
Introduction
Multidentate ligands containing N and O donor atoms
have been widely investigated for their role in stabilizing
transition metal complexes.[1] The coordination of transi-
tion metal ions with polydentate Schiff base ligands is of
particular interest, primarily for the variety of attainable
coordination numbers, nuclearities, configurations, and mo-
lecular architectures.[2] The activity of this kind of complex
in biological,[3] medical,[3c] and catalytic systems has been
extensively studied.[4] On the other hand, studies on their
optical properties such as their electro- and photolumines-
cence (PL) are more recent and rare.[5] Clearly, the choice
of metal center leads to considerable changes in the features
chemosensors
and
organic
light-emitting
diodes
(OLEDs).[7] The role of zinc(II) ions in the construction of
photoluminescent architectures is well known and pre-
viously attracted our attention.[8] This paper follows on di-
rectly from the work on N,O-chelate complexes of zinc(II),
for which we explored the PL properties of some mononu-
clear zinc complexes and found that the PL of the ligand
was significantly enhanced by the metal coordination. On
the other hand, we have previously proposed[9] a metal co-
ordination approach based on the use of N-salicylidene-NЈ-
aroylhydrazine O,N,O-tridentate ligands for the coordina-
tion of copper(II) and palladium(II) centers. This approach
led to highly stable dinuclear and mononuclear neutral non-
[a] Dipartimento di Scienze Chimiche, Università di Napoli
Federico II,
via Cintia, 80126 Napoli, Italy
E-mail: barbara.panunzi@unina.it
[b] Dipartimento di Ingegneria Chimica, dei Materiali e delle emissive complexes with the coordination sphere completed
Produzioni Industriali, Università di Napoli Federico II,
by pyridine (Py) molecules. On this basis, we assumed that
Piazzale V. Tecchio 80, 80125 Napoli, Italy
O,N,O-tridentate zinc(II) complexes could have both good
chemical stability and high PL quantum yields. In this pa-
per, we report the synthesis and characterization of four N-
4-[octyloxy(phenoxy)]salicylidene-NЈ-R-benzoylhydrazine
ligands (R is an electron-acceptor substituent), which have
the same O,N,O tridentate chelating core, and their ZnII
complexes obtained by reaction with zinc(II) acetate in pyr-
[c] Dipartimento di Agraria, Università di Napoli Federico II, via
Università 100,
80055 Portici (Napoli), Italy
www.international.unina.it
[d] Department of Electrical and Computer Engineering,
Ben-Gurion University of the Negev,
POB 653, Beer-Sheva 84105, Israel
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201400095.
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idine. The resulting complexes are dinuclear O-bridged spe-
cies with the general formula Zn2Li2Py2n (n = 1 or 2 in one
particular case). The ligands and complexes were charac-
1
terized by mass spectrometry and H NMR, UV/Vis, pho-
toluminescence, and IR spectroscopy. Single-crystal X-ray
analysis gave the structures of the complexes as obtained
from pyridine solution. A series of ab initio calculations
within the density functional theory were performed both
on molecular and crystalline models to understand the ef-
fects of the structural and chemical features on the spectro-
scopic responses of the complexes. The present theoretical
analysis will focus on the nature of the fluorophores and
their absorbance to give a first insight into the photophysics
of these compounds, and a full theoretical investigation of
the PL mechanisms will be the subject of a separate work.
A full examination of the photoluminescence properties
was performed. It is worth noting that these complexes
strongly emit in the solid state, and the PL emission max-
ima can be easily tuned by varying the electron-acceptor
group R.
Scheme 1. Molecular schemes of ligands (Li) and complexes
Zn2Li2Py2n.
Results and Discussion
the ZnO percentage from the thermogravimetric analysis
The four ligands N-4-[octyloxy(phenoxy)]salicylidene- (TGA) residue (Table 1). The 1H NMR spectra of the com-
NЈ-fluorobenzoylhydrazine (L1), N-4-[octyloxy(phenoxy)]- plexes in [D5]pyridine appears consistent with the assigned
salicylidene-NЈ-(benzothiazolyl)hydrazine (L2), N-4-[octyl- structure. In Figure 1, the enlargement from δ = 14 to
oxy(phenoxy)]salicylidene-NЈ-cyanobenzoylhydrazine (L3), 6 ppm shows that there are some diagnostic differences in
and N-4-[octyloxy(phenoxy)]salicylidene-NЈ-nitrobenzoyl- the 1H NMR spectra of L3 in [D5]pyridine and [D6]di-
hydrazine (L4) were obtained by a general procedure[9] from methyl sulfoxide ([D6]DMSO) and Zn2L3 Py2 in [D5]pyr-
2
the corresponding 4-R-benzohydrazide and 2-hydroxy-4- idine.
(octyloxy)benzaldehyde in pyridine. Their identification and
1
the evaluation of their purity were performed by H NMR
spectroscopy and differential scanning calorimetry (DSC).
A liquid crystal phase behavior was revealed by optical ob-
servations and the DSC analysis. In particular, L4 features
a 32 °C range of stability for the nematic phase with isotrop-
ization (Ti) at 206 °C, whereas the anisotropization of the
monotropic nematic phase was recorded at 127 °C in the
DSC cooling curve for L1. Ligands L2 and L3 showed no
mesophase. Dinuclear O-bridged complexes Zn2Li2Py2n
(Scheme 1) were obtained as the products of the reactions
between zinc(II) acetate and the four ligands L1–L4.
The reactions were performed in pyridine, and the com-
plexes were crystallized from pyridine/methanol for charac-
1
Figure 1. H NMR spectra of L3 in (a) [D5]pyridine and (b) [D6]-
terization. The elemental analyses were consistent with the
calculated values. The zinc content was also measured as DMSO and (c) Zn2L2 Py2 in [D5]pyridine from δ = 14 to 6 ppm.
3
Table 1. Thermodynamic data for the complexes.
[b]
[c]
[d]
[e]
[f]
Tpy [°C][a]
%pycalc
%pyexp
Td
%ZnOcalc
%ZnOexp
Zn2L1 Py2
Zn2L22Py2
Zn2L32Py2
180
171
217
196
14.95
13.91
14.75
14.22
14.85
13.41
15.05
13.99
334
370
325
340
15.39
14.33
15.18
14.63
15.68
14.65
15.07
14.23
Zn2L42Py2
2
[a] Maximum of endothermic signal related to pyridine loss. [b] Calculated pyridine weight percentage. [c] Experimental pyridine weight
percentage. [d] Decomposition temperature, calculated as the 5% weight loss temperature in N2. [e] Zinc residue calculated as ZnO. [f]
Experimental zinc residue as ZnO.
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The spectrum of L3 in [D5]pyridine (Figure 1, a, solvent X-ray Crystallography
signals at δ = 8.60, 7.64, and 7.25 ppm) shows well-sepa-
Three of the complexes crystallized by evaporation from
pyridine at room temperature show a pentacoordinate zin-
c(II) ion. For L2, these conditions afforded only the species
rated signals for the aromatic and phenolic (OH) protons.
No signal was recorded for the NH group, probably owing
to solvent exchange. In Figure 1 (b) (L3 in [D6]DMSO), the
signals at δ = 11.42 and 12.16 ppm can be assigned to the
protons of the OH and NH groups, respectively, and some
of the aromatic signals overlap. Different chemical shifts
with respect to those of the free ligand are evident for the
complex (Figure 1, c).
Zn2L2 Py4 with an octahedral coordination. The molecular
2
structures of Zn2L2Py2 (L = L1, L3, or L4) and Zn2L2 Py4
2
are shown in Figures 3, 4, S2, and S3. The crystal data and
refinement details are reported in Table S1. In all of the
reported structures, the bond lengths are in the normal
range and in accordance with those of analogous com-
pounds.[9] In all of the complexes, the ligand assumes an
almost planar conformation and coordinates to the ZnII ion
through two oxygen atoms and one nitrogen atom. The co-
ordination sphere of the ZnII ion is completed by one or
two pyridine molecules to afford two different geometric
environments. All complexes exhibit pentacoordination ex-
The FTIR spectra of the ligands and complexes in KBr
were recorded to provide a useful comparison. The FTIR
spectra of L3 and the corresponding zinc complex
Zn2L3 Py2 show the expected signals pertaining to the CN
2
stretching at 2225 cm–1, the aromatic C–H stretching at
2921 cm–1, and aliphatic C–H stretching at 2856 cm–1 (Fig-
ure S1). The signal pertaining to the NH stretching at ca.
3240 cm–1 and the broad signal centered at 3430 cm–1 for
OH stretching of the ligand were not found in the spectrum
of the complex, which confirms the tautomerization of the
amidic group and deprotonation of the phenolic hydroxy
group. The ligand shows two intense peaks at 1650 and
1633 cm–1 owing to amidic C=O stretching and N–H bend-
ing, respectively, and a weaker third peak at 1608 cm–1 ow-
ing to the iminic C=N group. The last one is shifted to
lower frequency and overlaps with the azomethine C=N
band (strong 1590 cm–1) in the complex spectrum. A highly
intense broad band centered at 1276 cm–1 is due to both C–
O–C and phenol C–O stretching of L3. For the complex,
only the C–O–C stretching vibrations were recorded at
lower frequencies (1212 cm–1). Finally, the weak bands in
the region of 500 cm–1 for the complex can be attributed to
M–O and M–N bonds, as reported for analogous com-
pounds.[10] Concerning the thermal behavior, decomposi-
tion occurs before melting for all of the complexes. Only a
broad endothermic signal above 170 °C (data in Table 1)
was detected by DSC. This signal can be attributed to the
loss of the pyridine molecule coordinated to the zinc center
by comparison of the DSC and TGA curves (data related
to pyridine weight loss in Table 1). As an example, we re-
cept Zn2L2 Py4, in which the ZnII ion is hexacoordinate.
2
Figure 3. ORTEP view of Zn2L1 Py2 with thermal ellipsoids drawn
2
at the 30% probability level. Z1–O1 2.065(4), Zn–O2 2.172(3), Zn–
O2i 1.979(3), Zn1–N2 2.014(4), Zn1–N3 2.055(4), Zn1···Zn1i
3.2234(15) Å, Zn1–O2–Zn1i 101.8(1)°, C3–C4–C7–N1 –9.5(8)°; i
= –x + 1, –y + 1, –z + 1.
port the TGA and DSC curve for Zn2L1 Py2 in Figure 2.
2
Figure 4. ORTEP view of Zn2L2 Py4 with thermal ellipsoids drawn
2
at the 30% probability level. H atoms and the minor part of the
disordered alkyl groups are not shown for clarity. Z1–O1 1.992(6),
Zn–O2 2.016(6), Zn–O2i 2.046(5), Zn1–N3 2.071(5), Zn1–N4
2.262(7), Zn1–N5 2.281(6), Zn1···Zn1i 3.113(2) Å, Zn1–O2–Zn1i
100.1(2)°, S1–C7–C8–N1 176.1(5)°; i = –x + 2, –y + 1, –z).
A small deviation from planarity of the ligand was ob-
served in Zn2L1 Py2 and Zn2L3 Py2 with a slight tilt of the
2
2
aromatic R group. All of the complexes consist of centro-
symmetric dimers, in which the phenoxo O atom bridges
two ZnII ions across an inversion center to form a coplanar
Zn2(μ-O)2 group. The Zn···Zn distances [3.113(2) to
3.2234(15) Å] and the Zn–O–Zn bridging angles [100.89(9)
to 101.8(1)°] are in the range observed for hexa- and penta-
coordinate ZnII dinuclear complexes with a Zn2(μ-O)2
group.[11]
Figure 2. (a) TGA and (b) DSC plots for Zn2L1 Py2.
2
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The geometry around the ZnII ion is distorted square pyridine spacer and the aliphatic chains of the adjacent
pyramidal in the complexes Zn2L2Py2 (L = L1, L3, or L4), molecules (Figure S4). In the crystal packing of Zn2L2 Py4
2
as confirmed by the Addison[12] distortion parameter τ in (Figure S5), parallel sheets of molecules stacked by layers
the range 0.26–0.32 (0 for square pyramid, 1 for trigonal of pyridine groups are formed.
bipyramid). The basal plane is formed by three O atoms
(two phenoxo and one carbonyl) and one N hydrazone
Photophysical Characterization
atom, and the apical position is occupied by the pyridine N
atom. The ZnII ion deviates [0.424(1) to 0.468(1) Å] from
the basal plane and is shifted towards the pyridine N atom.
Absorption
Also, Zn2L2 Py4 forms a dimer through a phenoxo O atom
The four complexes feature a donor–π-acceptor structure
2
bridge; however, at variance with the other complexes, the in which the conjugation extends to the whole molecule
geometry around the ZnII ion is octahedral with two pyr- upon complexation,[9] and the alkoxy chain acts as the elec-
idine ligands at the apical positions (Figure 4). The Zn···Zn tron donor group. The absorption spectra show a progress-
distance is 3.113(2) Å, and the Zn–O–Zn bridging angle is ive redshift across the L1–L4 series (Table 2). This effect can
100.1(2)°. The apical Zn–N(pyridine) distance [2.262(7) and be ascribed to the increasing electron-acceptor character of
2.281(6) Å] is longer than those of the pentacoordinate the terminal group on the phenyl ring (NO2 ϾCNϾF),
complexes, and this may account for the observed slow de- which leads to a progressive reduction of the gap between
gradation of the crystals, probably by loss of the coordi- the highest occupied molecular orbital (HOMO) and the
nated pyridine. The crystal packing in all of the complexes lowest unoccupied molecular orbital (LUMO), mainly by
is dominated by normal van der Waals and weak intermo- destabilization of the HOMO (as described below). For L2,
lecular interactions, and there is no π–π aromatic stacking. the phenyl ring bearing the acceptor was replaced by the
In all of the complexes, the dimeric molecule is almost com- benzothiazole group, which contains the thiazole ring. Elec-
pletely planar, and the pyridine ligands act as spacers be- tron-rich rings such as thiophene and thiazole are known
tween the molecules. In Zn2L1 Py2 and Zn2L4 Py2, the as both auxiliary donors and acceptor groups when they
2
2
shortest intermolecular distances involve pyridine and the are linked to a strong acceptor in a push–pull molecule
phenoxo groups of adjacent molecules in a very similar way (which leads to strong enhancement of molecular hyperpo-
(Figure 5). In Zn2L3 Py2, a herring-bone-type crystal pack- larizabilities).[13] In this case in which no acceptor is pres-
2
ing is observed, and the shortest distances are between the ent, the electron-withdrawing function is played by the
benzothiazole group itself.
Photoluminescence
The coordination to a transition metal ion can enhance
or quench the fluorescence of ligands. Enhanced fluores-
cence is of much interest for the photochemical applications
of the resulting complexes. This phenomenon is usually due
to the increased rigidity of the molecular structure, which
leads to a decreased probability of nonradiative transitions
by electrons in the excited states.[7] This behavior was clearly
evidenced in the compounds of the present study. As the
emission intensity of the ligands was too low both in the
solid state and in several solvents, photoluminescence mea-
surements were only performed on the four complexes. All
of the complexes showed PL emission both in the solid state
and in solution; however, the emission was scarce at low
concentration. The UV/Vis and PL data are summarized in
Figure 5. Partial packing of Zn2L1 Py2 showing the pyridine face-
to-face interactions. The minimum intermolecular distance is illus-
trated. H atoms are omitted for clarity.
2
Table 2. Another interesting result is the ability to tune the
PL of the complexes to obtain remarkable differences in the
Table 2. Optical data for the complexes.
λabs-sol [nm][a]
λabs-film [nm][b]
λem-sol [nm][c]
λem-film [nm][d]
PLQE[e]
PLQE error[f]
Zn2L1 Py2
Zn2L22Py2
Zn2L32Py2
394(415)
424
310(420)
444
364
406
390
250(408)
455
515
525
530
434
570
520
612(630)
28.5
33.8
45.0
8.8
Ϯ3.3
Ϯ5.6
Ϯ7.0
Ϯ1.0
Zn2L42Py2
2
[a] Wavelength of UV/Visible absorbance maxima in pyridine solution with the less-intense maximum in parentheses. [b] Wavelength of
UV/Vis absorbance maxima of thin film with the less-intense maximum in parentheses. [c] Wavelength of PL maxima in pyridine solution
with the less-intense maximum in parentheses. [d] Wavelength of PL maxima of thin film with the less-intense maximum in parentheses.
[e] PL quantum yield (%) of thin film. [f] Error in PL quantum yield.
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maximum wavelength of emission by varying the R group response for the different crystalline and molecular models.
(see Table 2). The solid-state normalized PL spectra of the These are the crystalline model (cry) at optimized geometry,
four complexes virtually cover the entire visible spectrum the insulated molecules (mol) with crystalline geometry, the
between 400 and 700 nm (Figure 6). This effect can be bet- molecules without the hydrophobic C8 chain (s-mol), and
ter appreciated by looking at the photos of thin films of the the molecules without the C8 chain and without the coordi-
four complexes irradiated by a UV lamp (365 nm). The four nated pyridine molecules (s-mol-npy). The sequence of
different colors, ranging from blue to orange-red, produced these models was realized to test the effects of the different
upon irradiation at this single wavelength are shown in Fig- molecular features on the chromophore. For ligand L2 only,
ure 7. The thin films in the photos were obtained by spin- the gap was calculated for model complexes Zn2L2 Py2 and
2
ning a dispersion of the complex and low molecular weight Zn2L2 Py4. As expected, the value for Zn2L2 Py2 (377 nm)
2
2
polystyrene in acetone/dichloromethane (1:2). The same was between those of models without pyridine ligands and
samples were used to measure the PL quantum yields with with four pyridine ligands (368 and 380 nm, respectively).
a laser at 376 nm. This wavelength is located at the higher
end of the absorbance of the molecule and is far away from
its emission to ensure there is no overlap between the exci-
tation and emission spectra.
Figure 8. HOMO–LUMO gaps expressed as wavelength in nano-
meters [nm] for different models of Zn2L1 Py2 to Zn2L4 Py2 indi-
2
2
cated as a1–a4: crystalline (squares, cry), insulated molecules
(circles, mol), molecules without C8 chain (triangles, s-mol), and
molecules without C8 chain and without pyridine molecules
(crosses, s-mol-npy).
Figure 6. Normalized PL spectra of (a) Zn2L1 Py2, (b) Zn2L3 Py2,
2
2
(c) Zn2L2 Py2, and (d) Zn2L4 Py2 as thin films.
2
2
The C8 chain has no effect on the chromophore, because
the mol and s-mol gaps are the same for all of the com-
pounds. The s-mol and s-mol-npy results show some differ-
ences, which suggests that the coordinated pyridine mol-
ecules play some role in the chromophore. The crystalline
environment enhances the redshift along the Zn2L2Py2 (L
= L1–L4) series, as shown by the differences between the
cry and the mol HOMO–LUMO gaps. This is in accord-
ance with the larger difference in optical activity in the solid
state, and requires further discussion. The HOMO and
Figure 7. Photos of (I) Zn2L1 Py2, (II) Zn2L3 Py2, (III) Zn2L2 Py2,
2
2
2
and (IV) Zn2L4 Py2 as thin films under a UV lamp (365 nm).
2
The PL quantum yields are reported in Table 2. The
great variability of the error is due to nonhomogeneous sur-
faces of the different-sized crystals. Even taking into ac-
LUMO for Zn2L1 Py2 with the s-mol model (the insulated
2
molecule with the crystalline equilibrium geometry without
the C8 chain) are shown in Figure 9. A preliminary analysis
in terms of one-electron molecular orbitals, as performed
in the present study, can give a first interpretation of the
molecular photophysics, even if an electronic transition is a
collective molecular transformation, and the electronic cor-
relation is very important. As shown in Figure 9, the
HOMO lies on the molecular plane without the involve-
ment of the Zn centers, and the lowest unoccupied (LUMO)
resides on the pyridine molecule; therefore, the HOMO and
LUMO clearly react in a very different way to intra- and
intermolecular electrostatic effects. This analysis confirms
that the Zn atoms are not involved in the electronic transi-
tions, as usually happens for a d10 transition metal. Even if
count the error, the value for Zn2L3 Py2 should be consid-
2
ered quite high overall, as should the results for Zn2L1 Py2
2
and Zn2L2 Py2. Effectively, these values can be placed
2
among the highest so far reported for analogous solid-state
zinc complexes such as ZnQ2·2H2O (36.6%; Q = 8-hydroxy-
quinoline)[7e] and Zn(SH1)2 (32.8%; SH1 = N,O-donor
Schiff base),[7h] which makes them promising as emissive
materials in luminescent devices, both as crystalline com-
pounds or grafted onto preformed polyvinylpyridine.[9]
Computational Studies
For the complete set of absorption and emission data, the valence orbitals of the metal center are not involved in
the same redshift in the series of compounds from the HOMO and LUMO of the chromophore, its role is not
Zn2L1 Py2 to Zn2L4 Py2 can be observed in the HOMO– purely electrostatic. This was confirmed by studying the
2
2
LUMO gaps (Figure 8), as a first indication of the optical electronic gaps of various simplified models for Zn2L1 Py2.
2
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The ligand alone has a gap of 781 nm, whereas the com-
plete chromophore (two ligands, two Zn ions, and two pyr-
idine molecules) has a gap of 395 nm, which is reasonably
close to the observed absorption. If the contribution of the
transition metal ion was purely electrostatic owing to point
charges in the range 0–2 electrons, a gap in the range 950–
1300 nm would be obtained, which is totally different from
the experimental absorption and fluorescence values. To
elucidate the intramolecular electrostatic effects on the
chromophore, a model molecule a0 was created from the
Zn2L1 Py2 molecule by substitution of the fluorine atom
2
with a hydrogen on the aromatic radical R, as shown by the
molecular scheme in Figure 10a. Two point charges were
arranged at a distance of 2 Å from the aromatic rings. The
value of the point charges was varied in the range [+1.0,
–1.0] electronic charge. As shown in Figure 10 (b), there is
a redshift in the HOMO–LUMO gap as the charge becomes
more negative, as happens in the molecular series Zn2L2Py2
(Figure 8) as the substituents on the terminal aromatic rings
become more electronegative. To better understand the ef-
fect of the crystalline environment on the chromophore, we
analyzed the electrostatic intermolecular effects.
Figure 10. Molecular electrostatic model derived for Zn2L2Py2 (L
= L1–L4). (a) Stick-and-ball plot of the molecular model; red, grey,
blue, and pink balls indicate the C, O, N, H, and Zn atomic posi-
tions, respectively. The light gray balls labeled q indicate the posi-
tion of the point charges as electrostatic models of the substituents.
The values of the charge of q were varied in the range [+1.0, –1.0]
electronic units. (b) The effect of the charge on the HOMO–LUMO
gap expressed in nm.
molecular plane, so the crystalline environment affects
mainly the HOMO region. For Zn2L1 Py2, a positive region
2
dominates, which indicates that the crystalline environment
stabilizes the HOMO orbital; this results in a blueshift of
the gap. For Zn2L4 Py2, negative lobes dominate and desta-
2
bilize the HOMO and lead to a redshift. We want to stress
that this is strictly a semiquantitative analysis of the effect
of the crystalline environment on the chromophore re-
sponse. A more accurate and quantitative theoretical analy-
sis will be the subject of a further study. A preliminary set
of time-dependent density functional theory (TD-DFT) cal-
culations was performed to prove the validity of the above
one-particle analysis of the electronic structure of the
Zn2L2Py2 (L = L1–L4) chromophores.
Figure 9. Plots of the (a) HOMO and (b) LUMO for the Zn2L1 Py2
mol model. Isosurfaces corresponding to 0.02 atomic units
(bohr–3), are reported. Red and blue indicate positive and negative
wavefunction values.
2
Figure 11. Isosurfaces of the difference of the electrostatic potential
between the crystal and insulated molecule calculation for
Zn2L1 Py2 (a1, left) and Zn2L4 Py2 (a4, right). The 0.02 V isosur-
face is shown. Red and blue correspond to 0.02 V and –0.02 V,
respectively.
2
2
In Figure 11, the differences between the crystalline and
the insulated molecule electrostatic potentials are shown for
The electronic transitions are intrinsically collective phe-
Zn2L1 Py2 and Zn2L4 Py2 to compare the behavior of the nomena and a description in terms of a one-electron jump
2
2
series extremes. These differences are stronger around the from an occupied to a free molecular orbital is only a very
Eur. J. Inorg. Chem. 2014, 2695–2703
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approximate approach. Some results of the TD-DFT calcu-
lations on the s-mol models are shown in Figure 12.
Experimental Section
General Procedures: All hydrazides except 4-fluorobenzohydrazide
(Aldrich) were synthesized according to a general procedure, which
starts with the synthesis of the corresponding ethyl ester from the
chloride, followed by the reaction of the chloride with hydrazine in
water/ethanol.[9]
2-Hydroxy-4-(octyloxy)benzaldehyde was prepared according to a
known procedure from 2-hydroxy-4-benzaldehyde and n-octyl
bromide.[14]
Ligands L1–L4: The same synthetic procedure was applied for all
ligands. We report the synthesis of L1 as an example. 2-Hydroxy-
4-(octyloxy)benzaldehyde (0.500 g, 1.99 mmol) and 4-fluoro benzo-
hydrazide (0.292 g, 1.89 mmol) were dissolved in pyridine (10 mL).
After 3 h at room temperature, methanol was added, and the whit-
ish solid precipitate was recovered by filtration, washed with meth-
anol, dried, and used as the crude product, yield 85%, m.p. 136 °C.
Ti: 127 °C (monotropic). H NMR (200 MHz, [D6]DMSO, 25 °C):
δ = 0.84 (t, 3 H), 1.25 (m, 10 H), 1.68 (m, 2 H), 3.96 (t, 2 H), 6.46
(s, 1 H), 6.48 (d, J = 8.6 Hz, 1 H), 7.35 (m, 2 H), 7.98 (m, 1 H),
Figure 12. Comparison of the DFT HOMO–LUMO gap with the
TD-DFT first excitations for insulated s-mol models (chromophore
without the C8 tale), indicated as a1–a4. The HOMO–LUMO gaps
(squares, s-mol) correlate both with the TD-DFT first excitation
(circles, tddft) and with the TD-DFT spectroscopic λmax (triangles,
tddft λmax).
1
8.51 (s, 1 H), 11.54 (s, 1 H), 11.98 (s, 1 H) ppm. UV/Vis: λmax (εmax
,
dm3 mol–1 cm–1) = 334 (2.4ϫ104) nm.
There is a substantial agreement between the HOMO–
LUMO gap and the first TD-DFT excitation; the latter is
redshifted by ca. 50 nm. This is because TD-DFT takes into
account the excited-state electronic relaxation, which gives
more stable excited states and a lower first excitation energy.
TD-DFT also gives realistic excitation probabilities and al-
lows the estimation of the entire absorption spectrum with
the evaluation of the spectroscopic λmax. The TD-DFT cal-
culated λmax for the insulated (s-mol) Zn2L2Py2 molecules
correlates very much with the HOMO–LUMO gap: this
confirms the validity of the above analysis on the electronic
structure of the chromophore.
L2: Yield 87 %, m.p. 157 °C. 1H NMR (200 MHz, [D6]DMSO,
25 °C): δ = 0.84 (t, 3 H), 1.25 (m, 10 H), 1.68 (m, 2 H), 3.97 (t, 2
H), 6.47 (s, 1 H), 6.51 (d, 1 H), 7.40 (d, J = 8.6 Hz, 1 H), 7.64 (m,
2 H), 8.22 (dd, J1 = 7.0 Hz, J2 = 15.4 Hz, 2 H), 8.76 (s, 1 H), 11.37
(s, 1 H), 11.86 (s, 1 H) ppm. UV/Vis: λmax (εmax, dm3 mol–1 cm–1) =
336 (1.7ϫ 104) nm.
L3: Yield 85%. Solid-solid transition: 93 °C, m.p. 157 °C. 1H NMR
(200 MHz, [D6]DMSO, 25 °C): δ = 0.84 (t, 3 H), 1.25 (m, 10 H),
1.68 (m, 2 H), 3.96 (t, 2 H), 6.47 (s, 1 H), 6.50 (d, J = 8.6 Hz, 1
H), 7.43 (d, J = 8.6 Hz, 1 H), 8.04 (m, 4 H), 8.54 (s, 1 H), 11.42
(s, 1 H), 12.16 (s, 1 H) ppm. UV/Vis: λmax (εmax, dm3 mol–1 cm–1) =
342 (5.3ϫ 104) nm.
1
L4: Yield 83%, m.p. 174 °C. Ti: 206 °C. H NMR (200 MHz, [D6]-
DMSO, 25 °C): δ = 0.83 (t, 3 H), 1.23 (m, 10 H), 1.66 (m, 2 H),
3.95 (t, 2 H), 6.45 (s, 1 H), 6.47 (d, J = 8.6 Hz, 1 H), 7.43 (d, J =
8.6 Hz, 1 H), 8.13 (d, J = 8.0 Hz, 2 H), 8.35 (d, J = 8.0 Hz, 2 H),
Conclusions
8.55 (s, 1 H), 11.39 (s, 1 H), 12.02 (s, 1 H) ppm. UV/Vis: λmax (εmax
,
dm3 mol–1 cm–1) = 348 (2.2ϫ 104) nm.
Four N-salicylidene-NЈ-aroylhydrazine O,N,O-tridentate
ligands and their dinuclear ZnII complexes with pyridine
were synthesized and characterized by spectroscopic and
thermal analysis techniques. In all cases, the characteriza-
tion included single-crystal X-ray diffraction, which re-
vealed both penta- and hexacoordination for the ZnII ions.
Remarkably, the complexes showed medium-to-high PL
efficiency in the crystal state, whereas a considerable depen-
dence of the maximum photoluminescence wavelength on
the electron-withdrawing strength of the ligand substituent
was revealed. By DFT analysis, a first rationalization of the
redshift in the chromophore series was produced. A more
detailed theoretical analysis of the transition probabilities
both in absorbance and in fluorescence will be the subject
of a specific study. Owing to their good performance as
solid-state emitters covering the whole visible spectrum,
these complexes can be considered as good candidates for
use in the fabrication of optoelectronic devices (OLEDs).
This possibility is currently under investigation along with
that of further chemical modifications to tune the emission
maxima and PL efficiency.
Zn2Ln Py2 (n = 1–4): The same synthetic procedure was applied
2
for all of the complexes. As an example, we report the synthesis
of Zn2L1 Py2. Ligand L1 (2.00 g, 5.19 mmol) and zinc(II) acetate
2
dihydrate (1.14 g, 5.19 mmol) were dissolved in pyridine (10 mL).
After 3 h at room temperature, methanol was added, and the pre-
cipitated yellow solid was recovered by filtration, washed with
methanol, dried, and crystallized from pyridine/methanol, yield
78%. Td: 334 °C. C54H60F2N6O6Zn2 (1057.86): calcd. C 61.31, H
5.72, N 7.94, Zn 12.36; found C 61.70, H 5.66, N 7.55, Zn 12.18.
1H NMR (200 MHz, [D5]pyridine, 25 °C): δ = 0.84 (t, 3 H), 1.21
(m, 6 H), 1.42 (t, 2 H), 1.77 (m, 2 H), 4.06 (t, 2 H), 6.53 (dd, J1 =
2.6 Hz, J2 = 9.0 Hz, 1 H), 7.03 (d, J = 2.6 Hz, 1 H), 7.20 (m, 2 H),
7.30 (d, J = 9.0 Hz, 1 H), 7.57 (m, 1 H), 8.71 (s, 2 H), 8.76 (m, 4
H), 9.14 (s, 1 H) ppm. UV/Vis: λmax (εmax, dm3 mol–1 cm–1) = 394
(6.1ϫ 104) nm.
2
Zn2L2 Py2: Yield 80%. Td: 370 °C. C66H70N10O6S2Zn2 (1294.22):
calcd. C 61.25, H 5.45, N 10.82, Zn 10.10; found C 61.24, H 5.57,
1
N 10.69, Zn 10.18. H NMR (200 MHz, [D5]pyridine, 25 °C): δ =
0.84 (t, 3 H), 1.19 (m, 6 H), 1.39 (t, 2 H), 1.75 (m, 2 H), 4.03 (t, 2
H), 6.53 (dd, J1 = 2.4 Hz, J2 = 8.6 Hz, 1 H), 7.02 (d, J = 2.4 Hz,
1 H), 7.19 (m, 2 H), 7.32 (d, J = 8.6 Hz, 1 H), 7.42 (m, 2 H), 7.56
Eur. J. Inorg. Chem. 2014, 2695–2703
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(m, 1 H), 8.02 (d, J = 7.8 Hz, 1 H), 8.30 (d, J = 7.8 Hz, 1 H), 8.71
the phenoxy alkyl chain were found with refined occupancy factors
(s, 2 H), 9.04 (s, 1 H) ppm. UV/Vis: λmax (εmax, dm3 mol–1 cm–1) = of 0.73 and 0.25. Some restraints were introduced on the C–C bond
424 (7.5ϫ 104) nm.
lengths (by using the DFIX instruction of SHELXL) in the last
stage of refinement. A slow degradation of the Zn2L2 Py4 crystals
3
2
Zn2L2 Py2: Yield 76%. Td: 325 °C. C56H60N8O6Zn2 (1071.90):
was observed, probably because of the loss of coordinated pyridine.
calcd. C 62.75, H 5.64, N 10.45, Zn 12.20; found C 62.99, H 5.52,
1
CCDC-975031 (for Zn2L1 Py2), -975032 (for Zn2L3 Py2), -975033
N 10.12, Zn 12.12. H NMR (200 MHz, [D5]pyridine, 25 °C): δ =
2
2
(for Zn2L2 Py4), and -975034 (for Zn2L4 Py2) contain the supple-
0.84 (t, 3 H), 1.19 (m, 6 H), 1.40 (t, 2 H), 1.76 (m, 2 H), 4.04 (t, 2
H), 6.53 (dd, J1 = 2.4 Hz, J2 = 8.6 Hz, 1 H), 7.02 (d, J = 2.4 Hz,
1 H), 7.19 (m, 2 H), 7.35 (d, J = 8.6 Hz, 1 H), 7.56 (m, 1 H), 7.79
(d, J = 8.4 Hz, 2 H), 8.71 (s, 2 H), 8.75 (d, J = 8.4 Hz, 2 H), 9.10
(s, 1 H) ppm. UV/Vis: λmax (εmax, dm3 mol–1 cm–1) = 420 (5.2ϫ 104)
nm.
2
2
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Theoretical Calculations: Periodic and molecular theoretical calcu-
lations were performed by the hybrid density functional method:
the B3LYP functional of Becke was applied.[18] A 6-31G(dp)
Gaussian basis[19] set, which gives reasonable results for organic
molecular crystals, was used.[20] The dispersion forces were treated
by the Grimme D2 method,[21] and the parameters optimized for
molecular crystals were adopted.[20c] The treatment of the disper-
sion forces with a dumped Lennard–Jones potential as proposed
by Grimme[21] is very useful in molecular crystals to obtain realistic
optimized geometries: without including dispersion forces, the unit
cell expands up to 10%, which makes it impossible to study the
molecular stacking theoretically. All DFT calculations were per-
formed with the CRYSTAL program,[22] which applies the linear
combination of atomic orbitals for periodic systems in one, two,
and three dimensions and allows the same computational methods,
approximations, and accuracy for crystals and molecular models.
4
Zn2L2 Py2: Yield 77%. Td: 340 °C. C54H60N8O10Zn2 (1111.88):
calcd. C 58.33, H 5.44, N 10.08, Zn 11.76; found C 58.88, H 5.97,
N 9.99, Zn 11.48. 1H NMR (200 MHz, [D5]pyridine, 25 °C): δ =
0.84 (t, 3 H), 1.21 (m, 6 H), 1.42 (t, 2 H), 1.77 (m, 2 H), 4.06 (t, 2
H), 6.53 (dd, J1 = 2.4 Hz, J2 = 8.6 Hz, 1 H), 7.03 (d, J = 2.4 Hz,
1 H), 7.19 (m, 2 H), 7.37 (d, J = 8.6 Hz, 1 H), 7.56 (m, 1 H), 8.35
(d, J = 8.6 Hz, 2 H), 8.71 (s, 2 H), 8.79 (d, J = 8.6 Hz, 2 H), 9.11
(s, 1 H) ppm. UV/Vis: λmax (εmax, dm3 mol–1 cm–1) = 444 (3.4ϫ 104)
nm.
Physical Measurements: Optical observations were performed by
using a Zeiss Axioscop polarizing microscope equipped with a
FP90 Mettler heating stage. Phase-transition temperatures and en-
thalpies were measured with a Perkin–Elmer Pyris DSC instrument
at a scanning rate of 10 °C/min under nitrogen flow. Thermogravi-
metric analyses were performed in air with a TA Instruments SDT
2960 Simultaneous DTA-TGA instrument. The structures of the
ligands, intermediates, and complexes were confirmed by 1H NMR
spectroscopy. The spectra were recorded with samples in [D6]-
DMSO or [D5]pyridine by using a Varian Spectrometer operating
at 200 MHz. UV/Vis, fluorescence, and FTIR (KBr) spectra were
recorded with JASCO spectrometers. Thin films were obtained by
the spin-coating technique of a dispersion of shuttered crystalline
complexes and low weight commercially available polystyrene in
acetone/dichloromethane (1:2) with a SCS P6700 spin coater op-
erating at 800 RPM.
Molecular time-dependent DFT calculations were performed by
[23]
the approach introduced by Casida
with the Gaussian09 suite
of programs.[24]
Supporting Information (see footnote on the first page of this arti-
cle): FTIR spectra and additional crystallographic material.
Acknowledgments
Thanks are due to Dr. Mario Argeri (University of Eastern Pied-
mont, Alessandria, Italy) for the preliminary TD-DFT calculations.
Quantitative photoluminescence efficiency measurements were per-
formed with a de Mello[15] type setup with a laser at 376 nm. The
detector for this measurement was an 813-855-8687 model from
Stellar Net Inc. connected to an integrating sphere (Lab-sphere).
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2
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ture. Selected crystals of the complexes were mounted in flowing
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2
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Zn2L2 Py2), and the date were collected with a Bruker-Nonius
KappaCCD diffractometer equipped with a graphite-monochro-
mated Mo-Kα radiation source (λ = 0.71073 Å, CCD rotation
images, thick slices, φ and ω scans to fill asymmetric unit). Semiem-
pirical absorption corrections (SADABS) were applied. All of the
structures were solved by direct methods (SIR97 package)[16] and
anisotropically refined by the full-matrix least-squares method on
F2 against all independent measured reflections (SHELXL program
of the SHELX97 package).[17] All hydrogen atoms were introduced
in calculated positions and refined according to a riding model with
C–H distances in the range 0.95–0.99 Å and with Uiso(H) equal to
1.2Ueq or 1.5Ueq(Csp3) of the carrier atom. The final refinements
converged to R1 = 0.0624, wR2 = 0.1401 (Zn2L1 Py2); R1 = 0.0887,
2
wR2
= = 0. 0489, wR2 = 0.0885
0.2181 (Zn2L2 Py4); R1
2
(Zn2L3 Py2); and R1 = 0.0641, wR2 = 0.1033 (Zn2L4 Py2). In
2
2
Zn2L2 Py4, two different positions for the C17 and C18 atoms of
2
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Received: January 24, 2014
Published Online: April 30, 2014
Eur. J. Inorg. Chem. 2014, 2695–2703
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