Photophysical Properties of Luminescent Zinc(II)?Pyridinyloxadiazole Complexes and their Glassy Self-Assembly Networks

Article abstract of DOI:10.1002/ejic.201800344

Three multifunctional ligands containing 2,5-disubstituted-oxadiazole units including an ortho/meta/para-substituted pyridine moiety have been synthesized. The reactions of the ligands with zinc(II) acetate in pyridine gave crystalline complexes with the

Full text of DOI:10.1002/ejic.201800344

https://doi.org/10.1002/ejic.201800344
Full Paper
Luminescent Transparent Films
Photophysical Properties of Luminescent Zinc(II)–
Pyridinyloxadiazole Complexes and their Glassy Self-Assembly
Networks
Barbara Panunzi,[a] Simona Concilio,[b] Rosita Diana,*[c] Rafi Shikler,[d] Shiran Nabha,[d]
Stefano Piotto,[e] Lucia Sessa,[e] Angela Tuzi,[c] and Ugo Caruso[c]
Abstract: Three multifunctional ligands containing 2,5-disub- in the coordination of the metal was observed in all cases. The
stituted-oxadiazole units including an ortho/meta/para-substi- spectroscopic behaviour of the complexes was explored and
tuted pyridine moiety have been synthesized. The reactions of compared. In particular, medium-to-high photoluminescent
the ligands with zinc(II) acetate in pyridine gave crystalline com- quantum yields were observed for thin films produced from
plexes with the general formula [Zn(Li)2py2]. On the other hand, both crystalline and glass samples. Molecular and periodic
zinc-assisted self-assembly of the ligands in the absence of pyr- calculations were performed by using DFT to rationalize the
idine led to stable glassy networks with the general stoichio- photophysical behaviour.
metry [Zn(Li)2]. The direct involvement of the pyridine moiety
Introduction
current interest because of the advantages of excellent uni-
formity, high transparency, easy processability into desired
shapes and low production cost.
The scientific literature is littered with reports of the various
biological and pharmacological activities of 2,5-disubstituted-
1,3,4-oxadiazoles, from antimicrobial[1] to anti-inflammatory,[2]
antimitotic[3] and neurological[4] activities. In addition to these,
a number of researchers have reported catalytic[5] and electro-
and photoluminescent activities of the same moiety in poly-
mers,[6] nanocomposites[7] and transition-metal complexes.[8]
During the past decades, interest has been focused on employ-
ing heterocyclic rings containing multiple potential active coor-
dination sites for the formation of one- to three-dimensional
networks with desirable structural features and properties by
metal-ion-assisted self-assembly.[9] Actually, the design of mate-
rials through self-assembly allows the easy transfer of desired
properties from a molecular level to the macro-level to obtain
materials with unprecedented performances. The application of
these materials clearly takes advantage of the manipulation of
the macroscopic order in films or bulk. Special glasses are of
In the last years, our research group has focused interest on
N,O ligands apt to coordinate transition metals as stable struc-
tures with specific properties.[10] Zinc chelate complexes have
attracted our interest[11] for their structural features and optical
properties. In particular, we have studied how the insertion of
a pyridine moiety into the main structure of a O,N,O-chelating
aroylhydrazone ligand affects the properties of derived zinc
complexes. In that case, the additional coordination site leads
to the formation of mono-, di- and polymeric crystalline ar-
rangements depending on the position of the pyridine nitro-
gen.
As a part of our continued research into the relationship be-
tween structure and properties in transition-metal complexes
for various optoelectronic applications, herein we report the
reactions of the bent multifunctional ligands 2-[5-(pyridin-n-yl)-
1,3,4-oxadiazol-2-yl]phenol with ZnII. Ligands will be named on
the basis of the position of the nitrogen in the pyridine frag-
ment, that is, Lo (L ortho) when n = 2, Lm (L meta) when n = 3
and Lp (L para) when n = 4. As expected, zinc can drive various
coordination modes.[10a,12] The employment of the ligands in
pyridine produced crystalline complexes with the general for-
mula [Zn(Li)2py2], the coordination sphere being completed by
the nitrogen of two solvent molecules. The same reaction per-
formed in the absence of pyridine produced disordered coordi-
nation networks with the same [Zn(Li)2] building block. In this
case, the zinc centres are connected to each other by the li-
gands, and the active units are frozen in the glassy state after
removal of the solvent. This has proven to be an easy way to
obtain coordination polymers (CPs) by the zinc-driven self-
[a] Department of Agriculture, University of Napoli Federico II,
Via Università 100, 80055 Portici NA, Italy
[b] Department of Industrial Engineering, University of Salerno,
Via Giovanni Paolo II 132, 84084 Fisciano SA, Italy
[c] Department of Chemical Sciences, University of Napoli Federico II,
Via Cintia, 80126 Napoli, Italy
E-mail: rosita.diana@libero.it
http://www.scienzechimiche.unina.it/ricerca/dream
[d] Department of Electrical and Computer Engineering, Ben-Gurion University
of the Negev,
POB 653, Beer-Sheva 84105, Israel
[e] Department of Pharmacy, University of Salerno,
Via Giovanni Paolo II 132, 84084 Fisciano SA, Italy
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201800344.
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assembly of dyes into supramolecular glassy, rigid and some- Results and Discussion
what strong polymers, and an efficient approach to merging
the photoluminescent (PL) behaviour of the fluorophores with The ligands Li, depicted in Scheme 1, were synthesized accord-
the mechanics of the macromolecular materials.[13] The simulta- ing to a known procedure[16] from the corresponding hydraz-
neous presence of the monodentate pyridinyl nitrogen and bi- ino(pyridin-n-yl)methanone and 2-hydroxybenzoic acid in poly-
dentate N,O-chelating sites in the organic ligands have proven phosphoric acid. Identification and evaluation of the degree of
to be very useful for the formation of self-assembled CPs.[14] purity was assessed by elemental analysis and 1H NMR spectro-
Nevertheless, the obtainment of amorphous and transparent scopy. The phase behaviour was examined by optical observa-
materials is not common. This would be of great importance tion and DSC analysis. Ligand Lo was obtained as white crystals
for application in photonics and optoelectronics, for which the that melt at 149 °C. For the known compounds Lm and Lp, the
presence of homogeneous transparent thin films is essential for data are in good agreement with the literature.[16,17] The reac-
the geometry and efficiency of devices.[15]
tion of zinc(II) acetate with ligands Li in pyridine gave mono-
nuclear pale-to-bright yellow microcrystalline complexes with
general formula [Zn(Li)2py2] (Scheme 1). The same reactions
performed in chloroform/ethanol produced yellow non-crystal-
line solids with general formula [Zn(Li)2] that can be converted
into [Zn(Li)2py2] by dissolving them in pyridine.
To measure the photophysical properties of the new com-
pounds in the solid state, thin films were produced from both
crystalline and amorphous complexes. In particular, thin films
of [Zn(Li)2py2] were obtained by spin-coating a dispersion of
shattered crystals of complexes in a polystyrene matrix, and
thin films of [Zn(Li)2] were obtained by spin-casting chloroform
solutions of the complexes.
As expected, compounds Li act as O,N mono-negative bi-
dentate ligands coordinating to the metal through the phenolic
The spectroscopic characteristics of the complexes, including oxygen and one nitrogen atom of the oxadiazole. Single-crystal
the photoluminescent quantum yields (PLQYs), were analysed X-ray analysis of the meta derivative obtained from pyridine
and compared, and the collected data discussed in relation to revealed the coordination sphere of the metal to be completed
the results of DFT calculations. The ground and first excited by two pyridine molecules in a hexa-coordinate mononuclear
electronic states were investigated by DFT approaches at the environment. The complexes obtained in chloroform/ethanol
B3LYP level of theory, whereas the excited singlet states were do not have a crystalline appearance. Plausibly, in the absence
examined by using time-dependent (TD-)DFT. Good agreement of free pyridine molecules, [Zn(Li)2] fragments can connect to
was obtained between the theoretical and experimental ab- each other through the pyridinic nitrogen of the difunctional-
sorptions for all complexes. In addition, the electron density ized ligand, acting as an additional coordinative site for the
calculations on the meta complex [Zn(Lm)2py2] helped to ration- metal. Owing to the free rotation of the pyridine ring in solu-
alize the extremely high value of the PLQY.
tion, the ligand occupies statistic conformations producing a
Scheme 1. Synthesis of the ligands Lo, Lm and Lp, the zinc-assisted self-assembled [Zn(Lo)2], [Zn(Lm)2] and [Zn(Lp)2], and the metal complexes [Zn(Lo)2py2],
[Zn(Lm)2py2] and [Zn(Lp)2py2].
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disordered glassy supramolecular network when the additional N atom of the ligand is not involved in coordination or weak
Zn–N bridges freeze upon precipitation. This interpretation is intermolecular interactions. In the X-ray crystal analysis, the dif-
supported by powder XRD data; the PXRD patterns obtained ference Fourier maps clearly show that the 3-pyridyl group is
from powder samples of [Zn(Li)2] show that the samples are disordered in two positions related to a 180° rotation around
largely amorphous.
the C4–C6 bond. The occupancy factor refined to 0.49/0.51 indi-
cates that there is no preference for the 3-pyridyl N position.
The crystal packing is stabilized by face-to-edge aromatic inter-
actions and π-stacking of the aromatic groups is observed (Fig-
ure 2 and Figure 3). A similar assembly of parallel sheets of
polymeric chains was found in tridentate meta-pyridinyl-
hydrazone derivatives.[10a] In those cases, the assembly of crys-
tals into packed polymeric structures led to an enhancement of
the PL emission of meta-substituted complexes.
Crystal Structures
Essential information on the structures of the pyridinic deriva-
tives was obtained by X-ray diffraction analysis and employed
as a support to the interpretation of the photophysical proper-
ties in the solid state. In particular, ligand Lo and [Zn(Lm)2py2]
were obtained as single crystals from chloroform/hexane and
pyridine, respectively. The X-ray structure of the ligand Lo and
the crystal data are reported in Supporting Information.
The X-ray structure of the complex [Zn(Lm)2py2] is presented
in Figure 1. The compound crystallizes in the triclinic P1¯ space
group with one-half molecule contained in the asymmetric unit.
The molecule sits on a crystallographic inversion centre and has
Ci symmetry. All the bond lengths and angles are in the ex-
pected range and in agreement with similar compounds.[18] The
crystal data and structural details are reported in Table 4.
Figure 2. Crystal packing of [Zn(Lm)2py2] viewed along the c axis.
Figure 1. Ortep view of [Zn(Lm)2py2] with thermal ellipsoids drawn at the
30 % probability level. Only one position of the disordered 3-pyridyl group is
reported. Selected bond lengths [Å]: Zn1–O2 = 2.021(4), Zn1–N3 = 2.143(6),
Zn1–N4 = 2.230(5), N2–N3 = 1.398(7), N2–C6 = 1.301(9), N3–C7 = 1.292(8),
O2-C9 = 1.310(8); i = –x, –y + 1, –z.
In [Zn(Lm)2py2] two mono-negative Lm ligands coordinate to
ZnII to afford a mononuclear neutral complex with a crystallo-
graphic inversion centre. The ligands are O,N-bidentate and co-
ordinate to the metal through the phenolate oxygen atom and
an N atom of the oxadiazole ring. Two pyridine molecules com-
plete the six-coordination sphere of the Zn metal centre, which
has an octahedral environment. The two pyridine molecules oc-
Figure 3. Partial packing with π-stacking of [Zn(Lm)2py2] molecules.
Thermal and Other Characterizations
cupy the axial positions and the two ligands are in the equato- The elemental analyses of the complexes [Zn(Li)2py2] were con-
rial positions. As already found for the Lo structure, in the sistent with the calculated values. The zinc content was also
[Zn(Lm)2py2] complex, the ligands arrange themselves in a pla- measured as ZnO % from the residues of TGA analyses of both
nar conformation that favours π interactions in the solid state the pyridine-containing complexes and the amorphous sam-
(Figure 2 and Figure 3). In the complex, the mean plane of the ples. As expected, the TGA curves for the complexes [Zn(Li)2py2]
ligand molecule is tilted 17.9(2)° with respect to the coordina- show the same general pattern, with one step for the loss of
tion plane defined by the O2, N3 and Zn1 atoms. The pyridine the two coordinated pyridine units. For the [Zn(Li)2] samples,
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Table 1. Thermodynamic data for the complexes [Zn(Li)2py2] and [Zn(Li)2].
Compound
Tpy [°C][a]
pycalcd. [%][b]
pyexp [%][c]
Td [°C][d]
ZnOcalcd. [%][e]
ZnOexp [%][f]
[Zn(Lo)2py2]
[Zn(Lm)2py2]
[Zn(Lp)2py2]
[Zn(Lo)2]
[Zn(Lm)2]
[Zn(Lm)2]
164
160
156
–
–
–
22.12
22.12
22.12
–
–
–
22.48
22.50
22.81
–
–
–
360
371
372
370
375
375
11.38
11.38
11.38
14.61
14.61
14.61
10.89
10.90
10.65
14.80
14.20
14.50
[a] Maximum of the 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.
the TGA curves recorded from 50 to 900 °C also show the same much affected by this equilibrium and the absorbance and
general pattern, with the only difference that no weight loss emission maxima of the pyridinic derivatives (see Table 2) are
was observed below 350 °C, thereby confirming that the very similar both in pyridine and in DMSO. This can be ascribed
[Zn(Li)2py2] samples turn into [Zn(Li)2] after the pyridine is lost. to the key role of the coordination core [Zn(Li)2] in determining
As an example, the TGA curves of samples [Zn(Lo)2py2] and the electronic transitions involved in absorbance and emission
[Zn(Lo)2] are shown in Figure 4. For all the complexes, decompo- processes.
sition occurs before melting, the only signals detectable in the
DSC pattern being the two peaks related to the loss of the two
pyridine molecules, typically from 140 to 180 °C. Quantitative
data related to the pyridine weight loss are reported in Table 1.
Figure 5. 1H NMR spectra of (a) Lo, (b) [Zn(Lo)2py2] and (c) [Zn(Lo)2] in
[D6]DMSO.
Photophysical Characterization
Photophysical measurements were performed both in solution
and in the solid state. The [Zn(Li)2py2] and [Zn(Li)2] complexes
were dissolved in pyridine and DMSO to record the absorption
Figure 4. TGA curves for (a) [Zn(Lo)2py2] and (b) [Zn(Lo)2].
In Figure 5, the 1H NMR spectra of Lo (trace a) and complexes and emission spectra. The [Zn(Li)2py2] samples dissolved in
[Zn(Lo)2py2] and [Zn(Lo)2] (traces b and c, respectively) in DMSO show very similar spectra to those recorded in pyridine
[D6]DMSO are compared. The spectra of the [Zn(Li)2] complexes solution. The UV/Vis and fluorescence data are summarised in
recorded in pyridine exactly coincide with those of the Table 2. The absorption and emission maxima of the [Zn(Li)2py2]
[Zn(Li)2py2] derivatives, which confirms that [Zn(Li)2] turn into and [Zn(Li)2] complexes are very similar, both in solution and in
[Zn(Li)2py2] by dissolving them in pyridine. The NMR patterns the solid state. In solution, the absorption spectra show a maxi-
of the ligands show well-separated signals for the aromatic pro- mum in the range from 372 to 379 nm for both species. The
tons and some diagnostic differences in the chemical shifts with emission maxima of the same solutions range between 500 and
respect to the coordinated ligand. The signal due to phenol was 550 nm with remarkable Stokes shifts. All the complexes show
not observed in [D6]DMSO. The 1H NMR spectra of the com- low emission in dilute solution, whereas bright luminescence is
plexes with or without pyridine appear consistent with the as- observed in the solid state. We then focused our attention on
signed structure. The patterns of the complexes are similar ex- solid-state films. Film samples were obtained in two ways. Thin
cept for coordinated pyridine signals appearing at 8.61, 7.71 films of [Zn(Li)2py2] were obtained by dispersing shattered crys-
and 7.41 ppm for [Zn(Lo)2py2]. It should be noted that the talline samples in polystyrene and spin-coating the dispersion
broader signals observed for [Zn(Lo)2py2] can conceivably be onto quartz slides. On the other hand, glassy thin films of
ascribed to an equilibrium involving the coordination and deco- [Zn(Li)2] complexes were obtained by casting chloroform solu-
ordination of pyridine ligands to the [Zn(Lo)2] fragment in tions onto quartz slides. The thicknesses of the glassy films of
DMSO. Anyway, the photophysical properties in solution are not [Zn(Li)2] were measured on the quartz substrates. For the sam-
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Table 2. Optical data of the complexes [Zn(Lo)2py2] and [Zn(Lo)2].
Compound
λabs–sol [nm][a]
λabs–film [nm][b]
λem–sol [nm][c]
λem–film [nm][d]
PLQY [%][e]
PLQY error[f]
[Zn(Lo)2py2]
[Zn(Lm)2py2]
[Zn(Lp)2py2]
[Zn(Lo)2]
[Zn(Lm)2]
[Zn(Lm)2]
379i, 375ii
378i, 373ii
385i, 380ii
374ii
388
372
370
365
372
375
507i, 525ii
500i, 513ii
533i, 558ii
522ii
490
481
484
500
497
518
23
75
16
17
44
12
5.0
2.0
2.0
2.0
4.0
0.8
372ii
506ii
378ii
550ii
[a] Wavelength of UV/Visible absorbance maxima in pyridine (i) or DMSO (ii) solution. [b] Wavelength of UV/Visible absorbance maxima of thin film. [c]
Wavelength of PL maxima in pyridine solution. [d] Wavelength of PL maxima of thin film. [e] PL quantum yield. [f] Error in PLQY.
ple [Zn(Lm)2], we registered a thickness of 1800 nm and an aver- electronic transitions, as usually happens for d10 metals. The
age roughness of 20 nm, and for the samples [Zn(Lo)2] and predicted absorption maxima are in good agreement with the
[Zn(Lp)2], thicknesses of 1900 and 2020 nm were measured, re- experimental values (Table 3). The possibility of developing
spectively, with average roughnesses of 540 and 610 nm. These novel materials for optoelectronics applications is generally lim-
self-assembling materials can be easily processed into fully ited to complexes with small reorganization energies. The reor-
transparent, homogeneous, stable, strong films. The glassy thin ganization energy is the difference between the energy re-
films under natural light and irradiated under a 365 nm UV quired for the neutral molecule to relax from the ion geometry
lamp are shown in Figure 6. Broad absorption bands were ob- to the neutral geometry and the energy required for the ion to
served with maxima ranging from 365 to 388 nm. Emission in relax from the neutral geometry to the ion geometry. All the
the solid state was observed for all the samples with maxima complexes exhibit low hole and electron reorganization ener-
ranging from 481 to 518 nm, again with large Stokes shifts. The gies (HRE and ERE, respectively). The complex [Zn(Lm)2py2] has
PLQYs of the thin films were measured with a laser at 376 nm a very interesting value of 0.16 eV, lower than similar com-
to guarantee no overlap between the excitation and emission plexes.[10a] DFT calculations permit investigation of the first
spectra. The PLQYs for the pyridinic complexes are higher than oxidation potential to explain the electron-donating properties
those observed for the glassy films. In particular, the value for of the compounds. In a redox reaction, the electron is with-
the [Zn(Lm)2] film is high compared with the average reported drawn from its HOMO. The observed trend in oxidation poten-
in the literature for solid-state zinc complexes. Moreover, the tial for the three complexes indicates destabilization of the
value measured for [Zn(Lm)2py2] is among the highest reported HOMO. Single-point calculations were conducted to determine
in the literature for solid-state zinc complexes[19] and compara- the electron and hole reorganization energies (see Table 3). The
ble to
a
zinc meta derivative reported in previous HRE is smaller than the ERE, which indicates a preference to
works.[10a,11,12,20] This value is definitely suitable for a lighting operate as hole carrier. These values suggest that the mecha-
device.
nism for triplet exciton formation and subsequent phosphores-
Figure 6. Glassy films of [Zn(Li)2] under natural light and irradiated under
365 nm UV radiation. The letters o, m and p have been placed under the
relevant glass.
Computational Studies
The results of calculations on all the complexes as mononuclear
species in pyridine showed that the HOMO and LUMO orbitals
lie on the ligand (see Figure 7<yigr7 pos="x22">) and that the
pyridines are not involved, in contrast to previously synthesized
analogous complexes.[10a] The zinc is not directly involved in
Figure 7. Frontier molecular orbital surfaces calculated at the B3LYP/6-
31G(d,p) level for (a) [Zn(Lo)2py2], (b) [Zn(Lm)2py2] and (c) [Zn(Lp)2py2] with
isovalue = 0.03.
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Table 3. Results of theoretical calculations.
Compound
Oxidation
Reduction
Hole reorganization
energy [eV][c]
Electron reorganization
energy [eV][d]
λmax
Triplet reorganization
HOMO
[eV][g]
LUMO
[eV][g]
potential [eV][a] potential [eV][b]
[nm][e]
energy [eV][f]
[Zn(Lo)2py2]
[Zn(Lm)2py2]
[Zn(Lp)2py2]
0.88
1.01
1.06
–1.97
–1.92
–1.78
0.14
0.12
0.12
0.18
0.16
0.18
390
393
402
0.548
0.524
0.760
–5.41
–5.29
–5.59
–2.56
–2.36
–2.75
[a] Oxidation potentials in crystals. [b] Redox potentials in crystals. [c] Hole reorganization energies in crystals. [d] Electron reorganization energies in crystals.
[e] Predicted UV/Vis absorption maxima in solution. [f] Triplet reorganization energies in crystals. [g] Frontier orbital energies in crystals.
Figure 8. Spatial representations of the HOMO and LUMO of the [Zn(Lm)2py2] crystal.
cent light emission probably involves an initial hole-injection Experimental Section
step to form the radical cation.
General Procedures: 2-Pyridinecarbohydrazide, nicotinic hydraz-
ide, isonicotinic hydrazide, 2-hydroxybenzoic acid, polyphosphoric
The meta complex [Zn(Lm)2py2] exhibits the lowest triplet
reorganization energy among the series. This aspect can be an
acid and zinc(II) acetate dihydrate were purchased from Aldrich.
important element for further optimization of materials for
Optical observations were performed by using a Zeiss Axioscop po-
larizing microscope equipped with an FP90Mettler heating stage.
Phase-transition temperatures and enthalpies were measured by us-
ing a Perkin–Elmer Pyris 1DSC scanning calorimeter at a scanning
rate of 10 °C min–1 under a flow of nitrogen. Thermogravimetric
analyses were performed in air by using a TA Instruments SDT 2960
Simultaneous DSC-TGA. The structures of ligands, intermediates and
complexes were confirmed by 13C and/or 1H NMR spectroscopy.
The spectra were recorded in [D6]DMSO by using Bruker DRX spec-
trometers operating at 300 or 400 MHz. Mass spectra were recorded
by using a Q-TOF premier instrument (Waters, Milford, MA, USA)
equipped with an electrospray ion source and a hybrid quadrupole
time-of-flight analyser. Mass spectra were acquired in positive ion
mode over the 250–800 m/z range. Instrument mass calibration was
thermally activated delayed fluorescence (TADF) processes.
Such optimization is usually a compromise between the oppos-
ing requirements of spatial orbital separation (to minimize the
energy gap between singlet and triplet lowest excited states)
and sufficient overlap to gain a large transition dipole moment.
We successfully determined the crystallographic structure of
[Zn(Lm)2py2], and the frontier orbital analysis of the crystals re-
vealed the intersection of the HOMO and LUMO (see Figure 8).
This partial overlap can, in part, justify the high PLQYs of the
meta complexes, as already observed in similar compounds.[10a]
Conclusions
achieved by a separate injection of 1.0 m
M NaI in 50 % CH3CN.
The self-assembly of two new bent multifunctional ligands,
namely 2-[5-(pyridin-n-yl)-1,3,4-oxadiazol-2-yl]phenol (Li), with
zinc(II) has been studied for the rational construction of ZnII
UV/Vis and fluorescence spectra were recorded with JASCO FP-750
spectrometers. Thin films of [Zn(Li)2py2] were obtained by the spin-
coating of a dispersion of shattered crystalline complexes and low-
coordination complexes and coordination polymers. The results weight commercially available polystyrene in acetone/dichloro-
methane (1:2) using an SCS P6700 spin-coater operating at 800 rpm.
Thin films of [Zn(Li)2] were obtained by casting a solution of the
complex in chloroform (about 30 % by weight). The film thicknesses
were measured by using a KLA Tencor P-10 surface profilometer.
Measurements were performed on films deposited on quartz sub-
strates. Photoluminescence quantum yields (PLQYs) were calculated
according to the methodology outlined by de Mello et al.[21] The
experimental setup consisted of a hollow integrating sphere, con-
structed such that its inner surface is coated with a diffusely reflect-
ing material, a laser excitation source and an optical fibre that con-
nects the sphere and a spectrometer. Each individual sample was
reported herein demonstrate that the asymmetric ligands Lo,
Lm and Lp, which contain mono- and bidentate chelating bind-
ing sites, can be very useful organic ligands for the construction
of zinc crystalline complexes as well as amorphous CPs. The
photophysical behaviour of the new materials was studied;
bright luminescence was observed in the solid state for all com-
plexes, with PLQY values up to 75 % for [Zn(Lm)2py2]. Theoreti-
cal investigation of the frontier orbitals suggests that in this
case a partial intersection of the HOMO and LUMO could, in
part, justify this high PLQY.
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placed within
6
inches of an RT-060-SF integrating sphere
7.70 (dd, 2 H), 7.78 (d, 1 H), 8.08 (m, 1 H), 8.23 (d, 1 H), 8.60 (d, 2 H)
(Labsphere Inc.). A diode laser at 376 nm (Toptica Photonics) was
used as an excitation source and the transmitted and emitted light
were collected and analysed by using an optic-fibre-coupled
CDS-2600 spectrometer (Labsphere Inc.) with a detection range of
300–1080 nm.
8.83 (d, 1 H) ppm. MS (ESI, +ve): m/z = 714.1 [M + H]+.
1
[Zn(Lm)2py2]: Yield 70 %. H NMR ([D6]DMSO, 25 °C, 400 MHz): δ =
6.58 (t, 1 H), 6.78 (d, 1 H), 7.29 (t, 1 H), 7.40 (t, 1 H), 7.67 (d, 1 H),
7.72 (dd, 2 H), 7.88 (d, 1 H), 8.48 (d, 1 H), 8.60 (d, 2 H), 8.82 (t, 1 H),
9.29 (s, 1 H) ppm. MS (ESI, +ve): m/z = 714.1 [M + H]+.
1
Ligands: The same synthetic procedure was applied to all ligands.
Here we report the synthesis of Lo. 2-Pyridinecarbohydrazide
(1.37 g, 10.00 mmol) and 2-hydroxybenzoic acid (1.37 g,
10.00 mmol) were dispersed in polyphosphoric acid (20 mL) and
the mixture was stirred overnight at 150 °C. After this time, the
suspension was cooled to room temperature and poured into ice/
water. After the pH was regulated at about neutral with sodium
hydrogen carbonate, a whitish solid was formed, which was col-
lected by filtration under vacuum and washed with water. The prod-
uct was crystallised in acetone.
[Zn(Lp)2py2]: Yield 78 %. H NMR ([D6]DMSO, 25 °C, 400 MHz): δ =
6.57 (s, 1 H), 6.74 (d, 1 H), 7.27 (t, 1 H), 7.44 (t, 1 H), 7.71 (dd, 2 H),
7.86 (dd, 1 H), 8.03 (d, 2 H), 8.65 (d, 2 H), 8.84 (m, 1 H) ppm. MS
(ESI, +ve): m/z = 714.1 [M + H]+.
X-ray Diffraction Analysis: Single crystals of [Zn(Lm)2py2] and Lo
suitable for X-ray structural analysis were obtained by slow evapora-
tion of pyridine ([Zn(Lm)2py2]) and chloroform/hexane (Lo) solutions
at room temperature. A selected crystal of [Zn(Lm)2py2] and Lo were
each mounted in flowing N2 at 173 K in a Bruker-Nonius Kappa
CCD diffractometer equipped with an Oxford Cryostream apparatus
(graphite-monochromated Mo-Kα radiation, λ = 0.71073 Å, CCD ro-
tation images, thick slices, ꢀ and ω scans to fill the asymmetric
unit). Semi-empirical absorption corrections (SADABS[22]) were ap-
plied. The two structures were solved by direct methods using the
SIR97 program[23] and anisotropically refined by the full-matrix
least-squares method on F2 against all independently measured re-
flections using the SHELXL-2016[24] and WinGX software.[25] All
hydrogen atoms were geometrically positioned with C–H = 0.95 Å
and refined according to the riding model with Uiso set at 1.2Ueq of
the carrier atom, except for the hydroxy H atom of Lo ligand, which
Lo: Yield: 80 %. M.p. 149 °C. 1H NMR ([D6]DMSO, 25 °C, 400 MHz):
δ = 7.04 (t, 1 H), 7.12 (d, 1 H), 7.49 (t, 1 H), 7.70 (t, 1 H),
7.91 (d, 1 H), 8.09 (t, 1 H), 8.25 (d, 1 H), 8.82 (d, 1 H), 10.36 (s, 1
H) ppm. MS (ESI, +ve): m/z = 240.1 [M + H]+. UV/Vis: λmax (εmax) =
316
.
Absorbance in DMSO: λabs,max
=
291, 316 nm
(2.6 × 104 dm3 mol–1 cm–1). Fluorescence in DMSO: λem,max = 410,
484 nm.
Lm: Yield: 74 %. 1H NMR ([D6]DMSO, 25 °C, 400 MHz): δ = 7.05
(t, 1 H), 7.11 (d, 1 H), 7.49 (t, 1 H), 7.67 (t, 1 H), 7.94 (d, 1 H), 8.45
(d, 1 H), 8.82 (d, 1 H), 9.26 (s, 1 H), 10.37 (s, 1 H) ppm. UV/Vis: λmax
was located in difference Fourier maps and freely refined with Uiso
=
(εmax
) = Absorbance in DMSO: λabs,max = 290, 313 nm
1.2Ueq of the carrier atom. The crystal data and structure refinement
details are reported in Table 4. The figures were generated by using
the ORTEP-3[26] and Mercury CSD 3.9[27] programs.
(1.8 × 104 dm3 mol–1 cm–1). Fluorescence in DMSO: λem,max = 384,
393 nm.
1
Lp: Yield: 80 %. H NMR ([D6]DMSO, 25 °C, 400 MHz): δ = 7.05 (t, 1
Table 4. Crystal data and structure refinement parameters for [Zn(Lm)2py2].
H), 7.10 (d, 1 H), 7.49 (m, 1 H), 7.95 (d, 1 H), 8.00 (dd, 2 H), 8.85
(d, 2 H), 10.41 (s, 1 H) ppm. UV/Vis: λmax (εmax) = Absorbance in
DMSO: λabs,max = 296, 319 nm (2.9 × 104 dm3 mol–1 cm–1). Fluores-
cence in DMSO: λem,max = 374; 429; 484 nm.
Empirical formula
Formula mass
T [K]
C36H26N8O4Zn
700.02
173(2)
λ [Å]
0.71073
Triclinic
Complexes: The same synthetic procedure was applied to all com-
plexes. As an example, we report here the synthesis of [Zn(Lo)2py2].
Ligand Lo (0.239 g, 1.00 mmol) was dissolved in pyridine (10 mL).
Under stirring at 100 °C, zinc acetate dihydrate (0.109 g, 1.00 mmol)
was added. The reaction was allowed to continue for 1 h and a
yellow solid formed after cooling, which was collected by filtration
under vacuum and washed with fresh methanol. Recrystallization
from pyridine gave the pure product.
Crystal system
Space group
a [Å]
b [Å]
c [Å]
P1
¯
8.3910(9)
9.3020(16)
10.413(3)
84.418(18)
72.262(12)
87.268(13)
770.3(2)
α [°]
ꢁ [°]
γ [°]
V [Å3]
The related complex [Zn(Lo)2] was obtained by the same procedure,
dissolving Lo in chloroform (8 mL) and the zinc salt in ethanol
(2 mL). The yellow precipitate obtained upon cooling was washed
in ethanol and employed as a crude product.
Z
1
Dcalcd. [Mg m–3
]
1.509
0.854
360
μ [mm–1
F(000)
θ range [°]
Reflections collected/unique
[R(int)]
]
3.144 to 25.195
6630/2717
0.1166
2717/0/224
0.991
0.0742, 0.1500
0.1477, 0.1857
0.427/–0.498
[Zn(Lo)2]: Yield 76 %. 1H NMR ([D6]DMSO, 25 °C, 400 MHz): δ = 6.61
(s, 1 H), 6.78 (d, 1 H), 7.31 (t, 1 H), 7.67 (t, 1 H), 7.78 (d, 1 H), 8.08
(m, 1 H), 8.23 (d, 1 H), 8.83 (d, 1 H) ppm. MS (ESI, +ve): m/z = 556.1
[M + H]+.
Data/restraints/parameters
Goodness-of-fit on F2
Final R, wR indices [I > 2σ(I)]
Final R, wR indices (all data)
Largest diff. peak/hole [e A–3
[Zn(Lm)2]: Yield 72 %. 1H NMR ([D6]DMSO, 25 °C, 400 MHz): δ = 6.58
(t, 1 H), 6.78 (d, 1 H), 7.29 (t, 1 H), 7.67 (d, 1 H), 7.88 (d, 1 H), 8.48
(d, 1 H), 8.82 (t, 1 H), 9.29 (s, 1 H) ppm. MS (ESI, +ve): m/z = 556.1
[M + H]+.
]
CCDC 1828356 (for Lo), and 1828357 (for [Zn(Lm)2py2]) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre.
[Zn(Lp)2]: Yield 70 %. 1H NMR ([D6]DMSO, 25 °C, 400 MHz): δ = 6.57
(s, 1 H), 6.74 (d, 1 H), 7.27 (t, 1 H), 7.86 (dd, 1 H), 8.03 (d, 2 H), 8.84
(m, 1 H) ppm. MS (ESI, +ve): m/z = 556.1 [M + H]+.
1
[Zn(Lo)2py2]: Yield 75 %. H NMR ([D6]DMSO, 25 °C, 400 MHz): δ =
Calculations: All quantum calculations were carried out by using
6.60 (s, 1 H), 6.79 (d, 1 H), 7.31 (t, 1 H), 7.40 (t, 1 H), 7.67 (t, 1 H),
the Jaguar program[28] as implemented in the Schrödinger Material
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Science Suite.[29] Geometry optimizations were performed by using
the B3LYP functional and the 6-31G** basis set, in which the transi-
tion metals are represented by using the Los Alamos LACVP** basis,
which includes relativistic effective core potentials. The energies of
the optimized structures were re-evaluated by performing addi-
tional single-point calculations on each optimized geometry using
Dunning's correlation-consistent triple-ꢀ basis set cc-pVTZ(-f),
which includes a double set of polarization functions. The results
of vibrational frequency calculations based on analytical second de-
rivatives at the B3LYP/6-31G**(LACVP**) level of theory were used
to confirm proper convergence to local minima and to derive the
zero-point energies (ZPEs) and entropy corrections at room temper-
ature, at which temperature the unscaled frequencies were used.
[8] a) G. Landi, W. R. Fahrner, S. Concilio, L. Sessa, H. C. Neitzert, IEEE J.
Electron Devices. Soc. 2014, 2, 179–181; b) P. Kathirgamanathan (South
Bank University), WO2000026323, 2000; c) M. E. Thompson, Z. Liu, C. Wu
(University Of Southern California), WO20120160742012.
[9] a) S. P. Gavrish, Y. D. Lampeka, P. Lightfoot, Cryst. Growth Des. 2015, 15,
2024–2032; b) M. Tominaga, T. Kawaguchi, K. Ohara, K. Yamaguchi, H.
Masu, I. Azumaya, CrystEngComm 2016, 18, 266–273.
[10] a) F. Borbone, U. Caruso, S. Concilio, S. Nabha, B. Panunzi, S. Piotto, R.
Shikler, A. Tuzi, Eur. J. Inorg. Chem. 2016, 818–825; b) F. Borbone, U.
Caruso, S. Concilio, S. Nabha, S. Piotto, R. Shikler, A. Tuzi, B. Panunzi,
Inorg. Chim. Acta 2017, 458, 129–137; c) f. Borbone, A. Tuzi, B. Panunzi,
S. Piotto, S. Concilio, R. Shikler, S. Nabha, R. Centore, Cryst. Growth Des.
2017, 17, 5517–5523; d) B. Panunzi, F. Borbone, A. Capobianco, S. Con-
cilio, R. Diana, A. Peluso, S. Piotto, A. Tuzi, A. Velardo, U. Caruso, Inorg.
Chem. Commun. 2017, 84, 103–108.
Supporting Information (see footnote on the first page of this
article): Additional crystallographic material.
[11] M. Argeri, F. Borbone, U. Caruso, M. Causà, S. Fusco, B. Panunzi, A. Rovi-
ello, R. Shikler, A. Tuzi, Eur. J. Inorg. Chem. 2014, 5916–5924.
[12] F. Borbone, U. Caruso, M. Causà, S. Fusco, B. Panunzi, A. Roviello, R. Shi-
kler, A. Tuzi, Eur. J. Inorg. Chem. 2014, 2695–2703.
Acknowledgments
The authors would like to thank the Italian Ministry for Research
MIUR (Grant Number 300395FRB16) for financial support.
[13] A. Lavrenova, D. W. Balkenende, Y. Sagara, S. Schrettl, Y. C. Simon, C.
Weder, J. Am. Chem. Soc. 2017, 139, 4302–4305.
[14] a) X. Lin, I. Telepeni, A. J. Blake, A. Dailly, C. M. Brown, J. M. Simmons, M.
Zoppi, G. S. Walker, K. M. Thomas, T. J. Mays, J. Am. Chem. Soc. 2009,
131, 2159–2171; b) X. Feng, R. Li, L. Wang, S. W. Ng, G. Qin, L. Ma, Cryst-
EngComm 2015, 17, 7878–7887; c) M. Du, X.-J. Zhao, J. Mol. Struct. 2003,
655, 191–197; d) Z. Fang, W. Yang, J. He, K. Ding, X. Wu, Q. Zhang, R. Yu,
C. Lu, CrystEngComm 2012, 14, 4794–4800; e) X.-W. Wu, F. Pan, D. Zhang,
G.-X. Jin, J.-P. Ma, CrystEngComm 2017, 19, 5864–5872.
Keywords: Zinc · Self-assembly · Heterocycles ·
Luminescence
[15] a) V. Bugatti, P. Iannelli, S. P. Piotto, S. Bellone, M. Ferrara, H. C. Neitzert,
A. Rubino, D. Della Sala, P. Vacca, Synth. Met. 2006, 156, 13–20; b) Z. Sun,
A. Martinez, F. Wang, Nat. Photonics 2016, 10, 227; c) S. Concilio, V. Bu-
gatti, P. Iannelli, S. P. Piotto, Int. J. Polym. Sci. 2010, 581056; d) A. Carella,
R. Centore, L. Mager, A. Barsella, A. Fort, Org. Electron. 2007, 8, 57–62; e)
P. Morvillo, R. Diana, R. Ricciardi, E. Bobeico, C. Minarini, J. Sol-Gel Sci.
Technol. 2015, 73, 550–556.
[16] M. Casalboni, U. Caruso, A. De Maria, M. Fusco, B. Panunzi, A. Quatela, A.
Roviello, F. Sarcinelli, A. Sirigu, J. Polym. Sci., Part A: Polym. Chem. 2004,
42, 3013–3022.
[17] A. F. Li, Y. B. Ruan, Q. Q. Jiang, W. B. He, Y. B. Jiang, Chem. Eur. J. 2010,
16, 5794–5802.
[18] A. Carella, A. Castaldo, R. Centore, A. Fort, A. Sirigu, A. Tuzi, J. Chem. Soc.
Perkin Trans. 2 2002, 1791–1795.
[19] a) R. Wang, Y. Cao, D. Jia, L. Liu, F. Li, Optical Mater. 2013, 36, 232–237;
b) O. Kotova, K. Lyssenko, A. Rogachev, S. Eliseeva, I. Fedyanin, L. Lepnev,
L. Pandey, A. Burlov, A. Garnovskii, A. Vitukhnovsky, J. Photochem. 2011,
218, 117–129.
[20] F. Borbone, U. Caruso, S. D. Palma, S. Fusco, S. Nabha, B. Panunzi, R.
Shikler, Macromol. Chem. Phys. 2015, 216, 1516–1522.
[1] K. K. Jha, A. Samad, Y. Kumar, M. Shaharyar, R. L. Khosa, J. Jain, V. Kumar,
P. Singh, Eur. J. Med. Chem. 2010, 45, 4963–4967.
[2] a) F. A. Omar, N. M. Mahfouz, M. A. Rahman, Eur. J. Med. Chem. 1996, 31,
819–825; b) E. Palaska, G. Şahin, P. Kelicen, N. T. Durlu, G. Altinok, Farm-
aco 2002, 57, 101–107; c) T. Ramalingam, P. Sattur, Eur. J. Med. Chem.
1990, 25, 541–544; d) K. Raman, H. K. Singh, S. K. Salzman, S. S. Parmar,
J. Pharm. Sci. 1993, 82, 167–169.
[3] K. L. Rai, N. Linganna, Farmaco 2000, 55, 389–392.
[4] M. Harfenist, D. J. Heuser, C. T. Joyner, J. F. Batchelor, H. L. White, J. Med.
Chem. 1996, 39, 1857–1863.
[5] G. Prakash, R. Ramachandran, M. Nirmala, P. Viswanathamurthi, J. San-
martin, Inorg. Chim. Acta 2015, 427, 203–210.
[6] a) D. Acierno, E. Amendola, S. Bellone, S. Concilio, L. Ferrara, P. Iannelli,
H. C. Neitzert, A. Rubino, F. Villani, J. Non-Cryst. Solids 2004, 338–340,
278–282; b) D. Acierno, E. Amendola, S. Bellone, S. Concilio, P. Iannelli,
H. C. Neitzert, A. Rubino, F. Villani, Macromolecules 2003, 36, 6410–6415;
c) S. Concilio, V. Bugatti, P. Iannelli, S. P. Piotto, Int. J. Polym. Sci. 2010,
581056; d) H. C. Neitzert, S. Cuccurullo, S. Concilio, P. Iannelli, J. Sensors
2016, 1642681; e) H. C. Neitzert, M. Ferrara, S. Bellone, A. Rubino, S.
Concilio, P. Iannelli, P. Vacca, L. Ferrara in Proceedings of SPIE - The Interna-
tional Society for Optical Engineering, Vol. 5840 PART I, Sevilla (Spain),
2005, pp. 139–146; f) H. C. Neitzert, M. Ferrara, A. Rubino, S. Concilio, P.
Iannelli, P. Vacca, L. Ferrara, C. Minarini, J. Non-Cryst. Solids 2006, 352,
1695–1699; g) M. Petrosino, A. Rubino, S. Concilio in 3rd International
Conference on Signals, Circuits and Systems, SCS 2009, Medenine (Tunisia),
2009.
[21] J. C. de Mello, H. F. Wittmann, R. H. Friend, Adv. Mater. 1997, 9, 230–232.
[22] SADABS, Bruker AXS Inc., Madison, Wisconsin (USA) 2001.
[23] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacovazzo, A.
Guagliardi, A. G. Moliterni, G. Polidori, R. Spagna, J. Appl. Crystallogr.
1999, 32, 115–119.
[24] G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3–8.
[25] L. J. Farrugia, J. Appl. Crystallogr. 1999, 32, 837–838.
[7] a) C. Borriello, S. Concilio, C. Minarini, P. Iannelli, T. Di Luccio in AIP Confer-
ence Proceedings, Vol. 1459, 1st ed., Ischia (Italy), 2012, pp. 151–153; b)
C. Borriello, S. Concilio, C. Minarini, S. Piotto, T. Di Luccio, Polym. Compos.
2013, 34, 1471–1476; c) A. Bruno, C. Borriello, T. Di Luccio, L. Sessa, S.
Concilio, S. A. Haque, C. Minarini, Optical Mater. 2017, 66, 166–170; d) A.
Bruno, C. Borriello, T. D. Luccio, G. Nenna, L. Sessa, S. Concilio, S. A.
Haque, C. Minarini, J. Nanopart. Res. 2013, 15, 2085; e) D. Acierno, S.
Concilio, A. Diodati, P. Iannelli, S. P. Piotto, P. Scarfato, Liquid Crystals
2002, 29, 1383–1392; f) H. C. Neitzert, S. Concilio, P. Iannelli, P. Vacca in
AIP Conference Proceedings, Vol. 1199, Rio de Janeiro (Brazil) ( 2009, pp.
139–140.
[26] L. J. Farrugia, J. Appl. Crystallogr. 1997, 30, 565–565.
[27] C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P. McCabe, E.
Pidcock, L. Rodriguez-Monge, R. Taylor, J. van de Streek, P. A. Wood, J.
Appl. Crystallogr. 2008, 41, 466–470.
[28] A. D. Bochevarov, E. Harder, T. F. Hughes, J. R. Greenwood, D. A. Braden,
D. M. Philipp, D. Rinaldo, M. D. Halls, J. Zhang, R. A. Friesner, Int. J. Quan-
tum Chem. 2013, 113, 2110–2142.
[29] Schrödinger, Release 2018–1 (Jaguar), Schrödinger, LLC, New York, 2018.
Received: March 19, 2018
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Luminescent Transparent Films
B. Panunzi, S. Concilio, R. Diana,*
R. Shikler, S. Nabha, S. Piotto,
L. Sessa, A. Tuzi, U. Caruso .............. 1–9
Photophysical Properties of Lumi-
nescent Zinc(II)–Pyridinyloxadiazole
Complexes and their Glassy Self-As-
sembly Networks
The self-assembly of two multifunc- plexes, with a PLQY value of 75 % for
tional ligands (Li) with zinc(II) has been [Zn(Lm)2py2]. The results of a DFT in-
exploited for the realization of ZnII vestigation suggest that a partial inter-
complexes and coordination polymers section of the HOMO and LUMO could
(CPs). Bright luminescence was ob- justify this high PLQY.
served in the solid state for all com-
https://doi.org/10.1002/ejic.201800344
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim