First examples of luminescent zinc(II)-bisquinoxalinato complexes: Synthesis, spectroscopic and theoretical studies

Article abstract of DOI:10.1016/j.ica.2012.01.008

Four novel homoleptic zinc(II) complexes were prepared in high yield from para-substituted 2,3-diphenyl-5-hydroxyquinoxaline ligands, LHⁿ, giving [Zn(Lⁿ)₂]. Density functional theoretical calculations were performed to pro

Full text of DOI:10.1016/j.ica.2012.01.008

Inorganica Chimica Acta 387 (2012) 145–150
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First examples of luminescent zinc(II)-bisquinoxalinato complexes: Synthesis,
spectroscopic and theoretical studies
⇑
Andrew J. Hallett , Simon J.A. Pope
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel homoleptic zinc(II) complexes were prepared in high yield from para-substituted 2,3-diphe-
nyl-5-hydroxyquinoxaline ligands, LHn, giving [Zn(Ln)2]. Density functional theoretical calculations were
performed to probe the influence of the variation in para-substitution on the ligands. The calculations
suggest that ligand-centred character appears to dominate the HOMO and LUMOs. Experimental electro-
chemical and spectroscopic characterisation showed that the subtle variations in absorption and emis-
sion wavelengths are due to ligand-dominated transitions that are influenced by electronic nature of
the para-substituted phenyl units in coordinated Ln, in both solution and the solid states.
Ó 2012 Elsevier B.V. All rights reserved.
Received 19 October 2011
Accepted 4 January 2012
Available online 11 January 2012
Keywords:
5-Hydroxyquinoxaline
Zn(II) complexes
Photophysical properties
DFT studies
OLEDs
1. Introduction
hydroxyquinoxaline ligands in which the 2- and 3-positions
contain para-substituted phenyl groups. The synthesis, character-
In recent years there has been an increasing interest in metal
complexes with luminescent properties for use in a multitude of
areas, in particular photovoltaics, OLEDs, responsive probes and
biological imaging [1]. These complexes contain chromophores,
which can be modified in order to tune the wavelength of emitted
light. 8-Hydroxyquinoline metal complexes are some of the most
reliable electro-transporting and emitting materials used in OLEDs
due to their thermal stability, high fluorescence and electron trans-
porting mobility [2]. Whilst there have been numerous reports on
metal ion complexes of Al(III) containing commercially available
8-hydroxyquinoline derivatives [1f,3], by comparison, relatively
few describe the incorporation of substituted hydroxyquinoline li-
gands into zinc(II) complexes. OLEDs based upon the benchmark
tris(8-hydroxyquinolinato)aluminium, Alq3, (Fig. 1) complex pos-
sess an electroluminescence efficiency (quantum yield) which is
limited to a maximum of 25% [4]. However, the zinc(II) analogue,
Znq2, (Fig. 1) has better injection efficiency, lower operating volt-
age, and higher quantum yield [5]. Therefore, zinc complexes of
this type have a great potential in electroluminescent (EL) devices.
Recently, it has been reported that the emission wavelength of
these complexes can be easily tuned by modifying the 8-hydroxy-
quinoline ligands in the 2-, 5- and 7-positions [6]. Here we report
the first examples of zinc(II) complexes bearing substituted
isation and spectroscopic properties are reported herein.
2. Experimental
2.1. Materials and Instruments
All reactions were performed with the use of vacuum line and
Schlenk techniques. Reagents were commercial grade and were
used without further purification. 1H NMR spectra and were run
on an NMR-FT Bruker 400 MHz spectrometer and were recorded
in d6-DMSO. 1H NMR chemical shifts (d) were determined relative
to internal TMS and are given in ppm. Mass spectra were carried
out by the staff at Cardiff University. High resolution mass spectra
were carried out by at the EPSRC National Mass Spectrometry Ser-
vice at Swansea University. UV–Vis studies were performed on a
Jasco V-570 spectrophotometer as DMF solutions (5 ꢀ 10ꢁ5 M).
Photophysical data were obtained on a JobinYvon–Horiba Fluoro-
log spectrometer fitted with a JY TBX picosecond photodetection
module as MeOH solutions or in the solid state. Emission spectra
were uncorrected and excitation spectra were instrument cor-
rected. The pulsed source was a Nano-LED configured for 372 nm
output operating at 500 or 100 kHz. Luminescence lifetime profiles
were obtained using the JobinYvon–Horiba FluoroHub single pho-
ton counting module and the data fits yielded the lifetime values
using the provided DAS6 deconvolution software. The ligands
LHn (n = 1–4) were prepared as previously reported [7].
⇑
Corresponding author. Tel.: +44 029 20879316; fax: +44 029 20874030.
E-mail address: hallettaj@cardiff.ac.uk (A.J. Hallett).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.008

146
A.J. Hallett, S.J.A. Pope / Inorganica Chimica Acta 387 (2012) 145–150
2.4.3. [Zn(L3)2]
Prepared similarly from Zn(OAc)2ꢂ2H2O (32 mg, 0.14 mmol) and
LH3 (95 mg, 0.29 mmol). Yield 86 mg, 0.24 mmol (82%). 1H NMR
3
(400 MHz, d6-DMSO) dH = 7.66 (2H, app. t, JHH = 8.3 Hz), 7.22 (2H,
3
3
d, JHH = 8.4 Hz), 7.11 (6H, app. t, JHH = 8.2 Hz), 6.97 (4H, d,
3JHH = 7.9 Hz), 6.91 (4H, br s), 6.45 (4H, d, JHH = 7.8 Hz) ppm. ES
3
MS found m/z 717.2, calculated m/z 717.2 for [M+H]+. HR MS found
m/z 715.2055, calculated m/z 715.2046 for [C44H35O2N4Zn1]+.
UV–Vis (DMF): kmax (e
dm3 molꢁ1 cmꢁ1) = 294(16600), 353(3500),
475(400) nm.
2.4.4. [Zn(L4)2]
Fig. 1. The structures of Alq3 and Znq2.
Prepared similarly from Zn(OAc)2ꢂ2H2O (17 mg, 0.08 mmol) and
LH4 (57 mg, 0.16 mmol). Yield 53 mg, 0.07 mmol (88%). 1H NMR
2.2. Electrochemistry
3
(400 MHz, d6-DMSO) dH = 7.63 (2H, app. t, JHH = 8.5 Hz), 7.14 (4H,
3
3
d, JHH = 8.6 Hz), 7.06 (2H, d, JHH = 8.1 Hz), 6.94 (6H, m), 6.82 (4H,
Electrochemical studies were carried out using a Parstat 2273
potentiostat in conjunction with a three-electrode cell. The auxil-
iary electrode was a platinum wire and the working electrode a
platinum (1.0 mm diameter) disc. The reference was a silver wire
separated from the test solution by a fine-porosity frit. Solutions
(10 ml DMF) were 1 ꢀ 10ꢁ3 mol dmꢁ3 in the test compound and
0.1 mol dmꢁ3 in [NBu4][PF6] as the supporting electrolyte. Solu-
tions were de-oxygenated with a stream of N2 gas and were main-
tained under a positive pressure of N2 during all measurements.
3
3
d, JHH = 8.6 Hz), 6.25 (4H, d, JHH = 8.5 Hz), 3.74 (6H, s), 3.46 (6H,
s) ppm. ES MS found m/z 803.2, 819.2 and 844.1, calculated m/z
803.2, 819.3 and 844.2 for [M+Na]+, [M+K]+ and [M+MeCN+Na]+
respectively. HR MS found m/z 801.1656, calculated m/z 801.1662
for [C44H34O6N4Na1Zn1]+. UV–Vis (DMF): kmax dm3 molꢁ1 cmꢁ1) =
(e
301(11800), 329(8400), 377(5000), 479(550) nm.
3. Results and discussion
Potentials are quoted versus the [Fe(
g
5-C5H5)2]+/[Fe(
g
5-C5H5)2]
couple (Eo = + 0.45 V in DMF) [8] as the internal standard.
0
3.1. Synthesis and characterisation
Four
para-substituted
2,3-diphenyl-5-hydroxyquinoxaline
ligands, LH1–4 (1 = p-H, 2 = p-Br, 3 = p-Me, 4 = p-OMe) were pre-
pared in a two-step reaction (Scheme 1) as previously reported
[7]. Firstly, 2-amino-3-nitrophenol was reduced by heating to
reflux in acidic EtOH for 14 h in the presence of zinc dust, giving
the corresponding 2,3-diamino compound. Subsequent condensa-
tion with a range of substituted diones in refluxing EtOH for 16 h
gave the desired ligands as yellow powders.
2.3. DFT studies
All calculations were performed on the Gaussian 03 program
[9]. Geometry optimisations were carried out without constraints
using the B3PW91 functional. The LANL2DZ basis set was used
for the Zn centres, and was invoked with pseudo-potentials for
the core electrons, a 6–31G(d,p) basis set for all coordinating atoms
with a 6–31G basis set for all remaining atoms. All optimisations
were followed by frequency calculations to ascertain the nature
of the stationary point (minimum or saddle point).
The neutrally charged homoleptic complexes [Zn(Ln)2] (n =
1–4) were isolated, following addition of base to a reaction mixture
composition of 2:1 ligand to Zn(OAc)2ꢂ2H2O in 2-methoxyethanol,
in excellent yields of 82–91%. The complexes were isolated by
addition of water to the reaction mixture as bright orange/red
powders. They were characterised in the solution state using 1H
NMR, UV–Vis. and luminescence spectroscopy and cyclic voltam-
metry. EI mass spectrometry revealed the parent ions [M]+ for
[Zn(L1)2], negative ion revealed [M+Cl]ꢁ for [Zn(L2)2] and ES mass
spectrometry revealed the protonated parent ions [M+H]+ for
[Zn(L3)2] and [Zn(L4)2].
2.4. Synthesis
2.4.1. [Zn(L1)2]
Zn(OAc)2ꢂ2H2O (25 mg, 0.11 mmol), LH1 (69 mg, 0.23 mmol),
Na2CO3 (122 mg, 1.15 mmol) and 2-methoxyethanol (8 ml) were
heated at 110 °C for 14 h. The mixture was cooled to room temp and
the product precipitated by the slow addition of water (25 ml). The or-
ange powder was filtered, washed with water and dried in vacuo. Yield
66 mg, 0.10 mmol (88%). 1H NMR (400 MHz, d6-DMSO) dH = 7.71 (2H,
app. t {coincident dd}, 3JHH = 8.1 Hz), 7.36–7.28 (14H, m), 7.15 (2H, d,
3JHH = 8.3 Hz), 6.99 (2H, d, 3JHH = 7.5 Hz), 6.87 (6H, m) ppm. EI MS found
m/z 660.1, calculated m/z 660.1 for [M]+. HR MS found m/z 658.1344,
3.2. Density functional theory (DFT) studies
In order to understand the nature of the electronic transitions
within this class of complex, DFT calculations (computed using
the B3PW91 hybrid orbital) were undertaken. In these examples,
an assessment of the frontier orbitals provided a qualitative insight
into the HOMO (highest occupied molecular orbital) and LUMO
(lowest unoccupied molecular orbital) energy levels.
Firstly, for the quinoxalinato complexes described here the low-
est energy configurations show that the predicted structures have
one of the two phenyl rings twisted out of the plane for both li-
gands in each case. The calculated energy levels of both the HOMO
calculated m/z 658.1342 for [C40H26O2N4Zn1]+. UV–Vis (DMF): kmax
(e
dm3 molꢁ1 cmꢁ1) = 302(44500), 320(38150), 465(2400) nm.
2.4.2. [Zn(L2)2]
Prepared similarly from Zn(OAc)2ꢂ2H2O (16 mg, 0.07 mmol) and
LH2 (69 mg, 0.15 mmol). Yield 64 mg, 0.67 mmol (91%). 1H NMR
(400 MHz, d6-DMSO) dH = 7.72 (2H, app. t, JHH = 8.2 Hz), 7.63
3
3
3
(2H, d, JHH = 8.3 Hz), 7.51 (4H, d, JHH = 8.4 Hz), 7.19 (4H, d,
and HOMO-1 are sufficiently similar (DE < 0.2 eV) to be considered
3JHH = 8.5 Hz), 7.13 (2H, d, JHH = 8.4 Hz), 7.09–6.92 (8H, m) ppm.
isoenergetic in all cases. The same is true of the LUMO and
LUMO+1. Both the HOMO and LUMO are located primarily on the
quinoxalinato ligand with little or no orbital coverage on the metal
centre. The HOMO lies exclusively on one of the quinoxalinato
ligands with the HOMO-1 (a mirror image) on the second
3
Negative ion MS found m/z 1010.7, calculated m/z 1011.1 for
[M+Cl]ꢁ. HR MS found m/z 1004.7473, calculated m/z 1004.7462
for [C40H22O2N4Br4Cl1Zn1]ꢁ. UV–Vis (DMF): kmax dm3 molꢁ1
(e
cmꢁ1) = 300(54500), 320(57300), 363(16350), 459(2650) nm.

A.J. Hallett, S.J.A. Pope / Inorganica Chimica Acta 387 (2012) 145–150
147
Scheme 1. Synthetic route to ligands LHn and complexes [Zn(Ln)2] (n = 1–4).
quinoxalinato ligand. As these are isoenergetic, they have been de-
picted together (Fig. 2). Again, LUMO and LUMO+1 are mirror
images and are depicted as one.
The HOMO orbitals are more delocalised over the phenolato
moieties while the LUMOs are over the pyrazine moieties, although
this effect is not as exaggerated as for quinoline complexes [10].
There is no direct contribution from the substituted phenyl rings
to the HOMO coverage, however, the nature of the para-substitu-
ent does appear to impart an influence on the overall energy level.
For example, the complex of the electron withdrawing para-Br li-
gand [Zn(L2)2] has the lowest HOMO energy level (E = ꢁ5.66 eV)
whereas the complex of the electron donating para-OMe
[Zn(L4)2] has the highest (E = ꢁ5.12 eV). Similarly, the LUMO ener-
gies also reflect this variation (para-Br ELUMO = ꢁ2.67 eV; para-OMe
ELUMO = ꢁ2.07 eV). This calculated data suggests that variation of
the remote para substituent can lead to a degree of tunable optical
Fig. 2. Graphical representation of the frontier orbitals of [Zn(Ln)2] (n = 1–4 left to right). Bottom, HOMOs; top, LUMOs (energy levels are in eV).

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A.J. Hallett, S.J.A. Pope / Inorganica Chimica Acta 387 (2012) 145–150
properties within this series of complexes. Interestingly, the calcu-
lated HOMO–LUMO bandgaps do not vary a great deal (2.99–
3.07 eV).
3.3. Cyclic voltammetry
An investigation into the electrochemical behaviour of the four
complexes was undertaken in deaerated DMF. Electrochemical
studies were performed in order to approximate the HOMO energy
levels for each complex. The cyclic voltammograms, measured at a
platinum disc electrode (scan rate 200 mV sꢁ1, 1 ꢀ 10ꢁ3 M solu-
tions, 0.1 M [NBu4][PF6] as a supporting electrolyte) of the com-
plexes [Zn(Ln)2] (n = 1–4) each showed one oxidation, which
were not fully reversible (Table 1). One or two irreversible reduc-
tions were also observed which can be assigned to the reduction
of the quinoxaline moiety [11].
The oxidation potential varies very little across the series of
complexes (0.95–1.00 V): the difference in oxidation potentials is
presumably due to the subtle donor variations of the substituted
quinoxalinato ligands. The EHOMO values were calculated using
the reported equations [12] and the resultant values, ranging from
ꢁ5.28 to ꢁ5.33 eV, are detailed in Table 1.
Fig. 3. UV–Vis spectrum of [Zn(L1)2] (blue), [Zn(L2)2] (purple), [Zn(L3)2] (red) and
[Zn(L4)2] (green) recorded as 5 ꢀ 10ꢁ5 M DMF solutions. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
3.4. Photophysical properties
The UV–Vis absorption spectra of the complexes were mea-
sured as aerated DMF solutions. The complexes all absorb in the
visible region (Fig. 3, Table 1): the features are in the range 300–
550 nm and are typically broad in appearance. The absorption
properties are dominated by a broad visible absorption around
complexes can be varied by modification at the 2-position of the
8-hydroxyquinoline rings [14]. Here we have functionalised the
quinoxaline group in the 2- and 3-positions. The emission maxima
are in the range 451–480 nm showing a degree of tunability within
the series of complexes as well as slight differences from the free
ligands, which are emissive in the region 438–474 nm [7]. The
complexes [Zn(L1)2] and [Zn(L2)2] show slightly blue shifted emis-
sion compared to the ligands whereas [Zn(L3)2] and [Zn(L4)2] are
red shifted. As it is not possible for d10 zinc complexes to undergo
low energy charge transfer or participate in metal-centred transi-
tions, the excited states of these Zn(II) complexes are classically
ligand-centred (LC) in nature [15]. Therefore, the emission from
[Zn(Ln)2] is believed to arise from an excited state centred on the
quinoxalinato ligand in each case. Related quinolinato complexes
of Al(III) have shown that there is also significant CT character that
originates from intra-ligand phenoxide-to-pyridine transitions
[16]. Our DFT studies suggest that a similar electronic character
is inherent within the Zn complexes described here. The calcula-
320–400 nm attributed to IL
p–
p⁄ transitions, possibly including
phenoxide-to-pyrazine charge transfer. The electronic nature of
the para-substituted groups subtly influences the positioning of
this absorption band. Indeed, complexes of the relatively electron
poor Ph and para-Br substituted ligands, [Zn(L1)2] and [Zn(L2)2]
respectively, possess similar absorption maxima (ꢃ320 nm),
whereas the complex of the electron-donating para-OMe substi-
tuted ligand, [Zn(L4)2], showed a significant bathochromic shift
(377 nm). The complexes each exhibit a relatively weak absorption
band that tails into the visible region. Since Zn(II) to quinoxaline
ring charge transfer (MLCT) is ruled out, the relatively weak
absorption bands from 400 to 550 nm are assigned to intra-ligand
charge transfer (1ILCT) transitions [13].
Luminescence measurements were conducted on MeOH
solutions of the complexes (Table 1, Fig. 4). It has been reported
that the luminescent behaviour of zinc(II) hydroxyquinolinato
Table 1
Absorption, emission and electrochemical properties of the zinc(II) complexes.
Complex kmax
(e
/Mꢁ1 cmꢁ1) (nm)a
Eox
HOMO
(eV)c
Ebandgap
LUMO
(eV)e
kem
(nm)
s
(ns) aeratedf
kem
(solid)
(nm)
s (ns)
(V)b
(eV)d
solidf
[Zn(L1)2] 302(44500),
0.98 ꢁ5.31
1.00
ꢁ5.33
2.05
2.09
2.07
2.11
ꢁ3.26
ꢁ3.24
ꢁ3.22
ꢁ3.17
451 <1 (70%), 2.6 (30%)
623
618
613
606
<1
<1
<1
<1
320(38150),465(2400)
[Zn(L2)2] 300(54500), 320(57300),
363(16350), 459(2650)
458, 480 <1 (62%), 2.1 (38%),<1 (52%)g, 2.8 (48%)g
476 <1 (62%), 1.9 (38%)
[Zn(L3)2] 294(16600),
0.96 ꢁ5.29
0.95 ꢁ5.28
353(3500),475(400)
[Zn(L4)2] 301(11800), 329(8400),
377(5000), 479(550)
456 <1 (31%), 1.9 (69%)
a
b
c
d
e
f
Absorption spectra measured as DMF solutions (5 ꢀ 10ꢁ5 mol dmꢁ3).
Oxidation potentials measured as DMF solutions at 200 mV sꢁ1 with 0.1 M [NBu4][PF6] as supporting electrolyte calibrated with FeCp2/FeCp2
.
+
+
The HOMO energy level was calculated using the equation –HOMO (eV) = Eox ꢁ EFc/Fc + 4.8.
Ebandgap was determined from the absorption edge of the zinc complexes.
The LUMO energy level was calculated using the equation LUMO (eV) = HOMO + Ebandgap
Excitation at 372 nm.
Emission at 480 nm.
.
g

A.J. Hallett, S.J.A. Pope / Inorganica Chimica Acta 387 (2012) 145–150
149
1, Fig. 5). Each of the complexes absorbs from 350 to 550 nm and
are emissive in the region 606–623 nm (with no emission in the
range 450–480 nm), again noting a degree of tunability with
variation of the para-substituents on the phenyl rings. Lumines-
cence lifetimes are short (
states.
s = < 5 ns) in both the solid and solution
4. Conclusions
This paper describes the first examples of homoleptic Zn(II)
complexes containing functionalised quinoxalinato-based ligands.
Variation of the para-phenyl substituents in the 2- and 3-positions
of the quinoxaline ring provides the means for moderately tuning
the electronic characteristics of the complexes. Complexes are
generally emissive in the 450–480 nm region in solution and
600–620 nm in the solid state. Experimentally, a clear trend is
evident in the complexes in the solid state whereby an electron-
withdrawing group (L2) red-shifts the emission peak, but an elec-
tron donating group blue-shifts (L4). The wavelength variations
within this series demonstrate an ability to tune the emission char-
acter of the complexes as a function of the remote para substituent
of the ligands.
Supporting DFT calculations suggest that both the HOMO and
LUMOs are completely localised on the coordinated quinoxalinato
ligands. Spectroscopically, this facilitates a degree of tuning in the
electronic properties of the complexes, whilst the photophysical
properties of the complexes appear to be dominated by ligand-cen-
tred character.
Fig. 4. Excitation (dashed) and emission (solid) spectra of [Zn(L1)2] (blue), [Zn(L2)2]
(red), [Zn(L3)2] (pink) and [Zn(L4)2] (green) recorded as MeOH solutions. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
tions propose that the HOMO in each case comprises significant
phenolate character; in each case the LUMO has significant pyra-
zine-localised electron density.
Acknowledgements
The complex [Zn(L2)2] (p-Br) shows emission bands at 458 and
480 nm. This dual emissive behaviour has been previously re-
ported for 2-(2-hydroxyphenyl)benzoxazolate zinc complexes
The EPSRC are thanked for funding. Dr. Robert Jenkins, Mr. Ro-
bin Hicks and Mr. David Walker are thanked for running low reso-
lution mass spectra. The staff of the EPSRC MS National Service at
the University of Swansea are also gratefully acknowledged.
and is due to the
p–
p⁄ transition as well as short-lived excimer
formation in the excited state [17].
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Fig. 5. Excitation (dashed) and emission (solid) spectra of [Zn(L1)2] (blue), [Zn(L2)2]
(red), [Zn(L3)2] (pink) and [Zn(L4)2] (green) recorded as solids. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web
version of this article.)

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