Facile tuning of luminescent platinum(II) Schiff base complexes from yellow to near-infrared: Photophysics, electrochemistry, electrochemiluminescence and theoretical calculations

Article abstract of DOI:10.1002/chem.201302339

The photophysical and related properties of platinum(II) Schiff base complexes can be finely and predictably tuned over a wide range of wavelengths by small and easily implemented changes to ligand structure. A series of such complexes, differing only in

Full text of DOI:10.1002/chem.201302339

FULL PAPER
DOI: 10.1002/chem.201302339
Facile Tuning of Luminescent Platinum(II) Schiff Base Complexes from
Yellow to Near-Infrared: Photophysics, Electrochemistry,
Electrochemiluminescence and Theoretical Calculations
[
a]
[b]
[a]
[a]
Ellen F. Reid, Vernon C. Cook, David J. D. Wilson, and Conor F. Hogan*
Abstract: The photophysical and relat-
ed properties of platinum(II) Schiff
base complexes can be finely and pre-
dictably tuned over a wide range of
wavelengths by small and easily imple-
mented changes to ligand structure. A
series of such complexes, differing only
in the number and positioning of me-
thoxy substituents on the phenoxy ring,
were synthesised and their photophysi-
cal, electrochemical and electrochemi-
luminescent (ECL) properties investi-
gated. Theoretical calculations were
performed in order to gain further in-
sight into the relationship between
structure and properties in these mate-
rials. By positioning methoxy groups
para and/or ortho to either the imine
or the oxygen group on the ligand,
electron density could be directed se-
lectively toward the LUMO or HOMO
as required. This allowed the emission
colour (both photoluminescent and
electrochemiluminescent) to be tuned
over a wide range between 587 and
739 nm. The variation in orbital ener-
gies was also manifested in the posi-
tions of the absorption bands and the
redox properties of the complexes, as
well as in the NMR shifts for the unco-
ordinated ligands. All reported com-
plexes displayed intense electrochemi-
luminescence (ECL), which could be
initiated either by annihilation or co-
reactant pathways. The relationship be-
tween the electrochemical and photo-
physical properties and the efficiency
of the ECL is discussed. For two of the
complexes solid-state ECL could be
generated from electrodeposited layers
of the complex.
Keywords: density functional theo-
ry
· electrochemiluminescence ·
electrochemistry · luminescence ·
platinum Schiff base complexes
Introduction
been shown to be highly promising candidates for techno-
[9]
logical applications, in particular for OLEDs in which
their neutral charge, high quantum yields and the stability
afforded by the chelate effect make them particularly attrac-
tive. The lead compound in the work presented here, plati-
num(II) salophen (1), was first reported in 1991 by Shagisul-
Luminescent transition metal complexes continue to grow in
importance due to the diversity of applications for such ma-
[
1]
terials, such as organic light-emitting diodes (OLEDs),
[
2]
[3]
6
solar cells and sensors. Although d coordination com-
pounds, such as those based on ruthenium and iridium, have
[10]
[11]
tanova and co-workers.
Che et al.
later reported the
8
been extensively researched, square planar compounds (d
coordination), have received comparatively less attention.
properties of a variety of structural variants with a focus on
properties related to OLED applications.
[4]
Platinum complexes have received most attention in this
class of luminescent materials, with work on these com-
In order to gain insight into the structure–function rela-
tionship in these and similar materials, we have systematical-
ly varied the number and positions of methoxy groups on
the phenoxy ring of the salophen ligand, and studied the ef-
fects on the photophysical and electrochemical properties of
their platinum(II) complexes using a range of experimental
and theoretical techniques. We demonstrate that the
HOMO and LUMO levels can be independently adjusted
through strategic placement of the substituents on the ring.
This study was further motivated by the need for new
photoluminescent (PL) and electrochemiluminescent (ECL)
sensing materials possessing tuneable emission wave-
[5]
pounds extending back to the 1960s.
Schiff base ligand systems are attractive because their syn-
thetic chemistry is well developed, making it relatively easy
to modify their structure. The synthesis of Schiff-base “salo-
phen” complexes (salophen=N,N’-bis(salicylaldehyde)-1,2-
[6]
phenylenediimino) with metallic centres of ruthenium(II),
[7]
[8]
chromium
ACHTUNGTRENNUNG( III) and zinc(II) have been previously ex-
plored. Recently, platinum(II) Schiff base complexes have
[
a] Dr. E. F. Reid, Dr. D. J. D. Wilson, Dr. C. F. Hogan
Department of Chemistry, La Trobe Institute for Molecular Science
La Trobe University, Melbourne, Victoria 3086 (Australia)
E-mail: c.hogan@latrobe.edu.au
[12]
lengths. Current ECL systems are based almost exclusive-
[12e]
ly on ruthenium polypyridyl systems
that emit over a lim-
ited range, making them unsuitable for multiplexed detec-
tion. Furthermore, there is a great need for luminescent
sensing materials that emit in the biologically useful near-in-
frared region. The ECL properties of the systems described
here are also of interest in the field of light emitting electro-
[
b] Dr. V. C. Cook
CSIRO, Molecular Health Technologies
Clayton, Victoria (Australia)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302339.
Chem. Eur. J. 2013, 19, 15907 – 15917
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15907

[
13]
chemical cells (LECs). There have been few reported in-
vestigations of the ECL of luminescent platinum com-
a little water. The product is filtered off and washed with
excess solvent. Yields for complexes varied from 65 to 85%.
The H NMR spectra of the unbound ligands are relative-
ly straightforward, with all but the aromatic regions of
ligand b and c being clearly resolved. Of particular note are
the phenolic OH resonances, which are all clearly resolved
[14]
1
plexes and the ECL of platinum(II) Schiff base complexes
has not been previously investigated.
Results and Discussion
in CDCl and significantly shifted down-field due to interac-
3
tions with the imine group. This OH proton is effectively co-
ordinated between the phenolic oxygen and the imine nitro-
gen. The position of this peak is strongly influenced by the
positioning of the methoxy groups on the adjacent phenyl
ring (Figure S1 in the Supporting Information). In ligand b,
c and d in which the ring is substituted at one or both posi-
Synthesis and characterisation: The series of methoxy
OMe) ligands (a–g) were synthesised (Scheme 1), with
yields varying from 85 to 97% by mixing aromatic diamine
(
4
6
tions meta to the phenoxy oxygen (R and R ), the peak is
shifted down-field relative to a. Similarly, for e, f and g
there is a general up-field shift in this peak with substitution
3
at the positions ortho and/or para to the oxygen group (R
5
and R ). This variation in deshielding effect is mirrored in
the photophysical and electrochemical properties of com-
plexes prepared from these ligands, as discussed in the next
section. The substitutions have less effect on the HC=N
imine proton resonance with the exception that the presence
6
of a methoxy in the R position shifts the resonance down-
field. This is most likely due to interactions with the oxygen
lone pairs.
In the platinum complexes the imine proton is moved
down-field to at least 9.19 ppm. This peak has a reduced in-
tegration and a pair of broad satellites due to three-bond
1
95
coupling to the 34% naturally abundant Pt isotope. The
aromatic protons of the complexes are more clearly resolved
than in the aromatic protons in the free ligands.
Scheme 1. Synthesis of Schiff base ligands (a–g) and platinum(II) com-
plexes (1–7).
Photophysical properties: The spectroscopic properties of
all complexes are summarised in Table 1. The UV/Vis ab-
sorption spectra in dichloromethane of 1, 2 and 5 are depict-
ed in Figure 1. The bands in the high-energy region of the
absorption spectrum (<350 nm) can be attributed to ligand
based (p–p*) transitions. Each complex contains one intense
absorption peak at about 260 nm, close to where the free
with the corresponding aldehyde in methanol. In the origi-
nal report of the synthesis of the lead complex 1 by Shagi-
[10b]
sultanova and co-workers,
aqueous tetrachloroplatinate
was mixed with a KOH ligand solution in aqueous aceto-
[
10a]
ne.
However, this method gives a low variable yield pro-
ducing large amounts of colloidal platinum. Moreover, the
solubility of the product appears to be dependent on the
purity, which can make subsequent separations difficult. The
present work employs the method that was reported by Lu
[16]
ligand is found to absorb strongly.
Between 350 and
400 nm, two or three absorption bands for each of the com-
plexes are observed. These bands can be attributed to a mix-
[15]
et al. using a mixture of DMF and DMSO and the softer
base sodium acetate.
Table 1. Photoluminescence and absorbance spectroscopic data for dilute
The pure Schiff base platinum complexes are rather in-
soluble in common solvents, such as methanol and acetoni-
trile. Unreacted ligand increases the solubility of the com-
plexes making column chromatography and simpler recrys-
tallisation techniques difficult. These issues can be overcome
by using a slight excess of platinum in the reaction. Com-
plexes 1–7 were synthesised by adding potassium tetrachlor-
oplatinate to a hot solution of DMSO, sodium acetate and
the respective ligand (Scheme 1). This allows virtually all
the ligand to be reacted and converted in the respective
complexes, leaving unreacted platinum and other salts as
by-products in solution. The platinum complexes are either
crystallised from the reaction mixture or precipitated with
2 2
solution of 1–7 in CH Cl .
Complex
l
em
E
[eV]
s
F
em
l
abs
[a]
[b]
[
nm]
[%] [nm]
1
2
3
4
5
6
7
620
587
590
615
647
698
739
620
2.00
2.11
2.10
2.02
1.91
1.78
1.68
2.00
3.8
1.6
5.6
4.9
3.5
4.8
0.6
2.7
534, 498, 463, 381, 362, 321, 252
496, 401, 381, 361, 254
505, 475, 441, 381, 365, 315, 254
526, 505, 461, 407, 384, 334, 254
538, 516, 468, 395, 373, 351, 263
575, 545, 498, 387, 366, 322, 263
581, 548, 508, 396, 370, 335, 254
450, 270
2+
[
3
Ru ACHTUNGTRNENUG( bpy) ]
[
a] Responses corrected for variation in detector sensitivity with wave-
=hc/l, in which h is the Plank constant [eVs], c is the
speed of light and l is the wavelength of emission.
length; [b] E
s
15908
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Chem. Eur. J. 2013, 19, 15907 – 15917

Tuning of Luminescent Platinum(II) Schiff Base Complexes
FULL PAPER
Figure 2. Photoluminescence spectra for (from left to right) complexes 2,
4
, 5, 6 and 7 in CH
2 2
Cl at 298 K. The spectral profiles have been correct-
ed for variation in detector sensitivity with wavelength.
ꢀ
1
Figure 1. Absorption spectra of 1, 2 and 4 in CH
offset for clarity).
2
Cl
2
at 298 K (spectra
sions=1460 and 1490 cm ). Complexes 5–7 show a less dis-
tinct shoulder that is red-shifted by approximately 75 nm
from the primary emission peak. These observations are
consistent with an intra
state and are similar to the conclusions of Che et al.
tetradentate Schiff-base platinum complexes, similar in
ACTHUNGTRENNGNUl i ACHTUNGTRENNUNGg and component to the excited
1
II
[11b]
ture of metal-to-ligand
A
H
U
G
R
N
N
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
for
1
[17]
L(phenoxide)!p*
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
plexes exhibit three moderately intense absorption bands in
the 440–600 nm range. These are attributed to the metal-to-
ligand-charge-transfer (MLCT) processes; formally, one
electron from each of the metal orbitals (b -, e - and a -)
structure to 1.
The emission maxima (l ) of the complexes increase in
em
the order 2<3<4<1<5<6<7. Similar to the trend ob-
served for the absorbance data, the emission colour is red-
shifted or blue-shifted with respect to 1 depending on the
positions of the methoxy substituents on the bonded ligand.
This trend can be very satisfactorily explained in terms of
the extent to which electron density is directed toward the
HOMO or LUMO, as dictated by the positions of the me-
thoxy substituents. For example, 2, 3 and 4 are substituted at
positions ortho and/or para to the imine group (R and R )
and the emission energy increases according to the following
pattern: ortho<para<ortho and para. This is consistent
with a LUMO residing substantially on the imine moiety,
which is progressively destabilised on going from 4 to 3 to 2.
A similar argument applies to complexes 5, 6 and 7, which
are substituted at positions R and R , ortho and/or para to
the metal bonded oxygen. In this case the HOMO is pro-
gressively destabilised on going from 5 to 6 to 7, which has
the effect of decreasing the size of the HOMO–LUMO gap
and lowering the energy of the emission. This will be elabo-
rated on in the calculation section.
2
g
g
ig
2
ꢀ
moves into the anti-bonding p* orbital of the salophen
ligand. Similar observations have been made by Shagisulta-
[10b]
[11a,b]
nova et al.
and Che et al.
for these types of com-
plexes. There is a strong trend in the absorbance data across
the methoxy-salophen series, with the MLCT absorbance
bands progressively shifting from 496 to 581 nm on going
from 2 to 7. Note that 2, 3 and 4 are blue-shifted with re-
spect to 1 whereas 4, 5 and 6 are red-shifted. This is consis-
tent with increasing electron donating ability towards the
phenoxy group and metal centre for 5, 6 and 7, and toward
the imine in the case of 2, 3 and 4. This will be discussed fur-
ther in the calculations section.
6
4
3
5
All complexes are luminescent in anaerobic CH Cl solu-
2
2
tion at 298 K and display structured emission profiles
Figure 2). In general, the complexes showed moderately
high photoluminescence quantum yields (f ); for example,
(
em
2
3
+
all but two are more luminescent than Ru
identical experimental conditions.
A( bpy)
H
U
G
R
N
U
G
under
The phosphorescence in these complexes originates from
the transitions between the lowest triplet excited state,
Electrochemical properties: The electrochemical data for
complexes 1–7 are summarised in Table 2. The cyclic vol-
tammetric responses for 1, 3 and 4 in CH Cl at 298 K are
which has mixed [L(phenoxide)!p*
ACHTUNGTRENNUNG
p*(Schiff-base)] charge-transfer character, and the singlet
2
2
[11c]
ground state.
originate purely from a metal-to-ligand transition.
depicted in Figure 3. Complexes 1–4 all show a similar
[11,18]
Com-
quasi-reversible oxidation pattern with the forward wave in
+
/0
plexes 1–4 each exhibit three distinct peak maxima in their
the range of E =0.45 to 0.73 V versus [Cp Fe]
and the
pa
2
photoluminescence spectra, with an average vibronic pro-
reverse peak in the range of E =0.27 to 0.52 V versus
pc
[11a,17]
ꢀ1
+/0
gression
of 1470 cm . For example, complex 1 has
[Cp Fe] . Complexes 6 and 7 show similar quasi-reversible
2
peak maxima at 587, 642 and 710 nm (vibronic progres-
wave patterns with the forward wave at E =0.35 and
pa
Chem. Eur. J. 2013, 19, 15907 – 15917
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15909

C. F. Hogan et al.
Table 2. Electrochemical data for 1–7 at 298 K in CH
TBAPF supporting electrolyte.
2
Cl
2
with 0.1m
Similar to the photophysical properties discussed above,
noticeable trends are apparent between the electrochemical
properties and the charge donating characteristics of the
ligand as determined by the placement of the methoxy sub-
stituents. The electrochemical data presented in Table 2 indi-
cate that complexes 4, 3 and 2 become progressively more
difficult to reduce as more electron density is directed
toward the imine moieties whereas the reduction potentials
for 5, 6 and 7 are relatively invariant. Although the trend in
the oxidation data is less obvious, it is clear that the red-
shifted complexes are oxidised at lower potentials than the
corresponding blue-shifted complexes. The HOMO–LUMO
gap may be estimated by the difference in the ground state
6
[
a]
Complex
Potential [V] (vs. Fc)
oxidative
E
p,a(I)ꢀEp,c(II) [V]
[
b]
reductive
(HOMO–LUMO) [eV]
process (I)
processes (II)
E
pa
E
pc
E
pc
E
pa
1
2
3
4
5
6
7
0.73
0.45
0.63
0.68
0.35
0.46
0.53
0.30
0.27
0.36
0.52
0.19
0.22
0.33
ꢀ1.98
ꢀ2.11
ꢀ2.05
ꢀ1.97
ꢀ1.80
ꢀ1.75
ꢀ1.83
ꢀ1.20
ꢀ1.17
ꢀ1.15
ꢀ1.07
–
2.72
2.58
2.68
2.64
2.18
2.33
2.28
ꢀ0.98
–
[
a] E1/2 ferrocene (Fc) was 0.35 V versus Ag/AgCl; [b] the values for the
electrochemical HOMO–LUMO gap are numerically the same as the
values for DGann for the annihilation reaction between the oxidised and
reduced species as discussed later.
[19]
redox potentials.
The results in Table 2 further indicate
that there is a similar trend between the electrochemically
estimated “gap” and the energy corresponding to the emis-
sion maxima. In general, although the quasi-reversible
nature of the electron transfer processes complicates their
interpretation, the electrochemical data suggest that the op-
tical and electrochemical properties of these complexes are
similarly tuned by varying the ligand structure.
Spectroelectrochemistry: To probe the nature of the oxi-
dised species spectroelectrochemical measurements were
performed. Figure 4 shows the changes in the visible absorp-
Figure 3. Cyclic voltammograms for 1 mm solutions of 1 (top), 3 (middle)
ꢀ
1
2 2 6
and 4 (bottom) in CH Cl , 0.2 Vs scan-rate, 0.1m TBAPF supporting
electrolyte.
0
.53 V and the reverse peak at E =0.19 and 0.33 V, respec-
pc
tively. Complex 5 shows an irreversible oxidation wave at
Figure 4. Changes in UV–visible absorbance spectrum during electrolysis
of 0.5 mm solution of 1 in CH Cl /0.1m TBAPF . The potential of the Pt
2 2 6
gauze electrode was held at constant potential of 1.0 V versus Fc. during
the course of the oxidation.
E =0.68 V. Controlled potential coulometry was performed
pa
to determine the number of electrons transferred per mole-
cule for the anodic wave observed in the cyclic voltammo-
gram. When bulk electrolysis was performed at potentials
2
00–300 mV less positive than E , no electrons were count-
tion spectrum while electrochemically oxidising a 0.5 mm so-
lution of complex 1. The potential was maintained at 1.00 V
1
/2
ed. When the potential of electrolysis was set 200 mV more
positive than E , two electrons were found to be trans-
versus ferrocene (ꢁ250 mV more positive than E ) and
1/2
p
ferred per molecule based on the charge passed per mole of
complex. Thus the quasi-reversible oxidation processes can
a spectrum acquired every 30 s until no further spectroscop-
ic changes were observed. Three clear isosbestic points at
493, 392 and 352 nm are apparent indicating that no inter-
mediate products are generated during the electrolysis. The
observed spectral changes due to the oxidative process are
fully reversible with the original species easily regenerated
through bulk electrolysis at a potential more negative than
the oxidation peak. Several absorbance bands are complete-
ly lost during the electrolysis from both the MLCT region
2
+
4+
be formally attributed to the Pt /Pt couple in each case.
The reductive pattern is similar for each complex, with an ir-
reversible peak observed on the negative going scan and an
associated anodic peak at more positive potentials on the
return scan (except for 5 and 7, which did not display an
anodic peak on the return scan). These irreversible reduc-
tions are presumed to be ligand centred.
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Chem. Eur. J. 2013, 19, 15907 – 15917

Tuning of Luminescent Platinum(II) Schiff Base Complexes
FULL PAPER
(
498 and 534 nm) and intra
A
H
U
G
R
N
U
li
A
H
U
G
R
N
U
gand charge transfer region
excited state, E (~2.00 eV) therefore ECL is possible. Simi-
s
II
(
381 nm). This suggests that that the HOMO has both met-
larly it can be shown that all of these Pt salophen systems
[22]
allic and ligand characteristics. The complete loss of two of
the three MLCT bands suggests this process to be primarily
localised on the platinum centre. Che et al.
lar Schiff-base platinum complexes to have an emissive trip-
let excited state with mixed MLCT and ILCT characteristics.
These observations are also consistent with the theoretical
calculations presented later.
are energy sufficient annihilation ECL systems.
As shown by the data in Table 3, the most intense ECL
emitter, under annihilation conditions is complex 1, followed
closely by 2 and 3. Complexes 1 and 2 give ECL intensities,
which are significantly more intense than the benchmark
[11b]
reported simi-
2
+
ECL emitter, [Ru ACHTUNGTERNN(UGN bpy) ] , tested under the same condi-
3
tions. Although these three complexes have the most nega-
tive reduction potentials of all the complexes, there is no ob-
vious correlation between annihilation reaction exergonicity
and ECL intensity as might be expected from Marcus
Electrogenerated chemiluminescence (ECL): Annihilation
ECL was observed for all the complexes by pulsing the po-
tential of the working electrode sequentially past the oxida-
tion and then the reduction potential of the complex. The
generation of the excited state, [Pt salophen] * via annihi-
lation ECL involves electron transfer reaction between the
oxidised and reduced forms of the species. In the present
case, the mechanism is assumed to involve two sequential
electron transfers [Eq. (3) and (4)], the second of which re-
[23,24]
theory.
Moreover, all of the annihilation reactions are
energy sufficient for ECL by at least 0.27 eV. It may be sur-
mised therefore, that a large DGann is a necessary but not
sufficient condition for intense ECL.
II
2+
[20]
Co-reactant ECL was also observed for all complexes in
the presence of tripropylamine (TPrA) with light emission
occurring in each case, at the potential corresponding to the
oxidation of the platinum complex (Figure 5). The co-reac-
tant (TPrA) is oxidised in the same potential step as the
[21]
sults in formation of the excited state product.
Pt Salꢂ ! ½Pt Salꢂ þ 2 eꢀ
II
IV
2þ
2+
½
½
½
½
½
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
Pt species, the oxidised TPrA then generates a product
4
+
that reacts with Pt to generate the excited state, which
subsequently produces light.
II
ꢀ
II
ꢀ
Pt Salꢂ þ e ! ½Pt Salꢂ
The mechanism, outlined below is based on the ECL ex-
IV
2þ
II
ꢀ
III
þ
II
Pt Salꢂ þ ½Pt Salꢂ ! ½Pt Salꢂ þ ½Pt Salꢂ
2+
[25]
periments of [Ru
A
H
U
G
R
N
U
G
and other transition
3
III
þ
II
ꢀ
II
II
Pt Salꢂ þ ½Pt Salꢂ ! ½Pt Salꢂ* þ ½Pt Salꢂ
II
II
Pt Salꢂ* ! ½Pt Salꢂ þ hn
In order to determine whether annihilation ECL is possi-
ble, the free energy available from the electron transfer re-
action between the oxidised and reduced species (DGann
)
can be estimated from the electrochemical data (Table 3)
using the following relationship (entropic contributions ne-
glected):
ꢀ
DGann ¼ E ðIÞꢀE ðIIÞ
ð6Þ
pa
pc
This may be compared with E , the energy of the excited
s
state (Table 1). Taking complex 1 as an example, ꢀDG
ann
(
2.72 eV) is in excess of the energy required to populate the
Table 3. Electrochemiluminescence (ECL) data.
Complex
l
ECL
Relative ECL intensity
[
a]
[b]
[nm]
ECLann
ECLTPrA
1
2
3
4
5
6
7
620
595
591
615
646
696
739
620
489.6
229.0
73.5
0.6
0.6
1.0
0.1
0.2
1.0
2.0
49.5
35.6
1.9
3.9
100.0
Figure 5. Complex 1 (1 mm) in CH
2
Cl
2
, 0.1m TBAPF
6
supporting electro-
2
+
[
Ru
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpy)
3
]
100.0
ꢀ1
lyte, 0.2 Vs scan-rate. Top: solution phase response in the absence of
co-reactant. Middle: solution phase cyclic voltammetric (black) and ECL
(grey) response in the presence 1 mm TPrA co-reactant. Bottom: solid-
state cyclic voltammetric (black) and ECL (grey) response in the pres-
ence of 1 mm TPrA co-reactant.
[
a] Annihilation ECL integrated intensities corrected for charge passed
during forward step and compared to the same experiment with [Ru-
2
+
2+
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpy)
3
]
in CH
Cl
2 2 3
Cl ; [b] co-reactant ECL compared to [Ru ACHTNURGTENNUNG( bpy) ]
/TPrA in CH
2
2
.
Chem. Eur. J. 2013, 19, 15907 – 15917
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15911

C. F. Hogan et al.
metal/TPrA systems, except that two presumably sequential
reductions [Eq. (9)] are required:
II
IV
ꢀ
½
Pt Salꢂ ! ½Pt Salꢂ þ 2 e
ð7Þ
ð8Þ
TPrA ! TPrAC ! TPrAC þ Hþ
þ
IV
II
½
Pt Salꢂ þ 2 TPrAC ! ½Pt Salꢂ* þ products
ð9Þ
II
II
½
Pt Salꢂ* ! ½Pt Salꢂ þ hv
ð10Þ
The most intense ECL emitter under co-reactant condi-
tions is complex 5, followed closely by 6. This appears to be
the inverse of the case with the annihilation ECL intensities,
as these two complexes have the least negative reduction
potentials. As pointed out by Lee et al. an important re-
quirement for efficient co-reactant ECL is that the reduc-
tion potential for the reductant (TPrAC in this case) be more
[26]
Figure 6. ECL spectra for 1 mm solutions of complexes (left to right) 2, 1,
5
and 6 in CH
2 2 6
Cl containing 0.1m TBAPF as supporting electrolyte and
1
0 mm TPrA co-reactant.
negative than the reduction potential (~LUMO energy
level) for the complex. This is of course necessary in order
for the reaction in Equation (9) to be energy sufficient.
es after approximately 10 scans. Under these conditions the
formation of an insoluble film at the surface of the glassy-
carbon working electrode is observed. The electrochemical
properties of the insoluble film are conserved when trans-
ferred into blank dichloromethane/electrolyte solution.
Peaks associated with the oxidation and reduction of the
film were also observed in aqueous systems of either phos-
phate buffer (pH 7.0) or sulfuric acid (0.1m). The film was
stable for in excess of 50 repetitive scan cycles in either
aqueous or organic media. The film formation for 1 was sug-
o
Given that E (TPAC) is approximately ꢀ2.1 V versus ferro-
[27]
cene, it can readily be understood from the reduction po-
tential data in Table 2 why complexes 5 and 6, which have
the least negative potentials, produce more intense co-reac-
tant ECL relative to the other complexes.
There is also a relationship between the oxidation poten-
tial for the complex and ECL intensity, with the two most
intense emitters having the lowest oxidation E . This may
1
/2
[10b]
be explained by the fact that more easily oxidised species
are much less likely to undergo parasitic side reactions,
which degrade ECL efficiencies, while noting that the oxida-
tion potential of the complex is still positive enough to ef-
fectively oxidise TPrA in each case.
gested by Shagisultanova et al.
to be due to the genera-
tion of a polymeric or partially oxidised species in which the
partially vacant d orbital of the oxidised metal may accept
a p electron from the phenyl rings of the salophen ligand of
a neighbouring molecule. As shown in Figure 5c, ECL activ-
ity was also observed for the film in organic media. When
the modified electrode was electrochemically cycled in the
presence of TPrA as co-reactant, emission was detected at
the potential corresponding to the oxidation of the solid.
Spectral analysis of the luminescence (Figure S4 in the Sup-
porting Information) showed that it was slightly red-shifted
compared to the emission produced by the complex in solu-
tion. The ability to electrodeposit these materials while con-
serving their properties may be useful for technological ap-
plications.
Another important determinant of ECL efficiency is pho-
toluminescence quantum yield, since this represents the
upper limit of efficiency for the overall ECL process. How-
ever, no correlation is observed between the PL and ECL
efficiencies of the complexes. This and the previous observa-
tions highlight the complexity of ECL systems and the diffi-
culty associated with predicting ECL ability on the basis of
electrochemical or spectroscopic characteristics, as pointed
[20a,28]
out by other workers.
Figure 6 shows the ECL spectra for complexes 1, 2, 5 and
6
obtained using the co-reactant method. Emission maxima
(
Table 3) were found to closely match the photolumines-
Theoretical calculations: The TD-DFT results generally sup-
port the assignment of absorption bands from the experi-
mental data. The assignment of transitions as being either
pure MLCT or ILCT would not be justified due to the deloc-
alisation of the MOs, and in particular the HOMO, of these
complexes. Moreover, the presence of spin-orbit coupling in
heavy metal complexes such as these further obfuscates the
separation between singlet and triplet states, as well as CT
states. The absorption bands at lower energy (above
400 nm) are generally associated with a greater degree of
MLCT, although inclusion of spin-orbit coupling would be
required to accurately model this region of the spectrum.
cence spectra; therefore, the same excited state is populated
regardless of whether optical or electrochemical excitation
is employed. Minor differences in shape between the ECL
and PL spectra are due to differences in instrument resolu-
tion and concentration effects.
An interesting and potentially useful aspect of the electro-
chemical properties of some of the complexes studied here
is illustrated in Figures S2–S5 in the Supporting Information.
These figures show the consistent growth in anodic current,
which occurs upon successive scanning of complexes 1 and 3
at concentrations >1 mm. The growth in the current stabilis-
15912
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15907 – 15917

Tuning of Luminescent Platinum(II) Schiff Base Complexes
FULL PAPER
The influence of the substituents on the aromatic ligand
system may be rationalised in terms of mesomeric (reso-
nance) and inductive effects. The mesomeric effect is direct-
ly related to the sharing of p electrons between the aromatic
core and the substituent, which is influenced by the topology
of electron density of the relevant MO. If the substituent is
attached in a position of little electron density, such as at
a node, or where the coefficient of the linear combination of
atomic orbitals (LCAO) is small, then the effect of the sub-
stituent on the MO will be weak. Conversely, if the attach-
ment is at a point of significant electron density, then the in-
teraction will be stronger and the MO more readily effected.
For a methoxy substituent the mesomeric effect is dominant
over inductive effects, and so the following discussion is
based on mesomeric effects.
Substituent effects have been examined by comparing the
electron density of each complex with the unsubstituted
complex, 1. The nature of the substituent has a small, yet
important effect on the energy and nature of the frontier
MOs of the complexes. Similar topologies were found for
the HOMO and LUMO of each platinum complex 1–7. The
HOMO and LUMO for 1 are illustrated in Figure 7, with
the substituent methoxy-phenol ring interaction is in all
cases antibonding with respect to the CꢀO(Me) bond.
On this basis, it is possible to predict methoxy substituent
effects in these complexes (in comparison with complex 1).
4
6
Substitution at the R and/or R positions will impact the
energy of the HOMO much more than the LUMO, and sub-
3
5
stitution at the R and/or R positions will have a greater
impact on the energy of the LUMO over the HOMO.
4
For complexes 5–7, methoxy substitution occurs at R
6
and/or R (ortho and para to the platinum-coordinated
4
6
oxygen). At both the R and R positions in the aromatic
ring there is significant contribution to the electron density
in the HOMO (non-zero LCAO) and so substituent meso-
meric effects will be significant. Since the CꢀO(Me) interac-
tion is actually antibonding, the HOMO is destabilised rela-
tive to complex 1, as is observed in the MO energies plotted
in Figure 8. For complexes 2–4, methoxy substitution occurs
3
5
at R and/or R (meta to the platinum-coordinated oxygen),
where there is little electron density (a node with LCAO co-
efficients almost zero). Since there is little contribution to
the HOMO from the ring carbons at R and R , the meso-
meric effect is weak and the HOMO energy for 2–4 may be
expected to be similar to that of 1, which is noted in
Figure 8.
3
5
An analogous trend is noted for the LUMO of these com-
plexes, which contain a significant contribution (non-zero
Figure 7. Isodensity surfaces of the HOMO (left) and LUMO (right) of
complex 1. Calculated at the mPW1PW91/SDD, TZVP level of theory in-
clusive of CH Cl solvation effects (SCRF, IEFPCM).
2 2
MOs of the remaining complexes included in the Supporting
Information (Figure S6). In all complexes the HOMO is not
purely metal-based but delocalised over the complex. This is
consistent with results from the spectroelectrochemistry ex-
periments, which suggested that the HOMO has both metal-
lic and ligand character. A notable trend is that the plati-
num contribution to the HOMO is greater in complexes 1–4
(
22–24%) than for 5–7 (10–15%). For all complexes, the
LUMO is entirely ligand based, with less than 0.2% contri-
bution from the metal. Fragment contributions to frontier
MOs are plotted in Figure S7 in the Supporting Information.
Two factors are critical to understanding the observed
trends in photophysical properties of the seven complexes.
Firstly, the HOMO of all complexes contains almost zero
3
5
contribution from carbon atoms at R and R (LCAO is
zero; a node passes through these positions) but contains
4
6
a significant contribution from carbon atoms at R and R
significant electron density is present at these positions).
(
The reverse is observed in the LUMO of all complexes. This
is illustrated in Figure 7 for the case of complex 1 (Figure S6
in the Supporting Information for complexes 2–7). Secondly,
Figure 8. Plot of HOMO/LUMO energies (top) and correlation between
calculated HOMO–LUMO gap and emission energy (Eem).
Chem. Eur. J. 2013, 19, 15907 – 15917
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15913

C. F. Hogan et al.
4
LCAO coefficients) from the ring carbon atoms at the R
of OMe substituents on the phenoxy ring of the salophen
ligand in a series of seven complexes, we have demonstrated
the ability to systematically and independently modulate the
energies of the HOMO or the LUMO in these systems. Be-
cause OMe is electron-donating and ortho/para directing it
can be used to destabilise the imine-localised LUMO by
6
and R positions. For complexes 2–4, methoxy substitution
4
6
at R and/or R will significantly affect the energy of the
LUMO, which is substantially localised on the imine group.
Since the CꢀO(Me) interaction is antibonding, substitution
4
6
at R and R will destabilise the LUMO energy relative to
3
5
3
5
complex 1. For complexes 5–7, substitution at R and/or R
where there is little electron density) does not have a sub-
placement at positions R and/or R , whilst having virtually
no impact on the HOMO. Similarly, placement at positions
(
6
4
stantial mesomeric effect, and so the energy of the LUMO
remains similar to that of 1.
R /R , ortho/para to the PtꢀO localised HOMO, destabilises
this orbital without impacting on the LUMO. The facility to
independently tune the HOMO and LUMO is desirable for
a variety of reasons, for example the need to modulate emis-
sion colour while maintaining constant oxidising power in
ECL-based sensing.
To confirm this analysis, calculations were performed for
hypothetical complexes, 2Me, 7Me, 2CN and 7CN, similar
in structure to 2 and 7 but with CN and methyl substituents
replacing OMe. The dominant electron donating/withdraw-
ing effects are: OMe is a mesomeric electron-donor and is
ortho/para directing, Me is an inductive electron-donor and
is ortho/para directing, and CN is a mesomeric electron-
withdrawing group and is meta directing.
If we first compare the electron-donating and ortho/para
directing OMe and Me groups, it is noted that the HOMO–
LUMO gap of 2Me is greater than that of 7Me; however,
the effect on the HOMO–LUMO gap is much less for Me
DFT calculations are in very satisfactory agreement with
the experimental photophysical results and strongly support
the interpretation of the data. Furthermore, calculations
show the feasibility of employing other substituents with dif-
ferent properties to modulate the properties of the com-
plexes in differing ways. For example, the meta-directing,
mesomeric electron withdrawing CN group will have an
effect equal but opposite to OMe, whereas a methyl group,
which is an inductive rather than mesomeric electron-donor,
will direct electron density in a similar way to OMe but with
smaller effect. Obviously, the effect could also be amplified
by choosing a stronger mesomeric electron withdrawing sub-
(
inductive e-donor) than for OMe (mesomeric e-donor).
Since resonance effects are not significant for Me (as with
OMe), destabilisation of the LUMO of 2 and the HOMO of
will not occur to the same extent as is calculated for OMe.
7
In fact, for 2Me the substituent effect (relative to 1) is
greater for the HOMO rather than the LUMO, which re-
sults in the HOMO–LUMO gap of 2Me being smaller than
that of 1.
stituent, such as NH , allowing the colour to be tuned over
an even wider range of wavelengths.
2
The electrochemical properties of the complexes in gener-
al mirror the photophysical results, with the redox levels
being tuned in a similar way. However, the quasi-reversible
nature of the redox processes tends to obscure the correla-
With the meta-directing CN substituents, which are pre-
dominantly mesomeric electron-withdrawing, the effects on
the HOMO and LUMO energies are quite substantial and
are opposite in effect to the OMe substituents. That is, the
HOMO–LUMO gap for 2CN is approximately equal to that
of 7 (less than for 1), whereas the energy gap for 7CN is
similar to that of 2 (greater than for 1). This is to be expect-
II
tion. This is the first report of ECL from Pt Schiff base
complexes; all of the complexes show intense ECL through
both annihilation and co-reactant pathways. Complexes 6
and 7 are of particular note because near-infrared emitting
electrochemiluminophores are rare. The determinants of
ECL efficiency are found to be complex and interrelated
and different factors appear to dictate the efficiency of anni-
hilation and co-reactant ECL. In most cases the ECL inten-
sity for the compounds reported here does not exceed that
4
6
ed, as substitution at R /R for the meta-directing CN group
will mostly impact the LUMO energy, whereas substitution
3
5
at R /R will impact the HOMO energy (opposite to that of
OMe).
2
+
These results confirm our analysis, that by changing sub-
stituents we can modify the energies of the HOMO and
LUMO orbitals and thus serve to “fine-tune” the HOMO–
LUMO gap. With an understanding of substituent mesomer-
ic and inductive effects and frontier MOs of the metal–
ligand complex, emission colour tuning may be achieved.
This “tuning” is reflected in the electrochemical and photo-
physical measurements.
of the Ru
A
H
U
G
R
N
U
G
standard, however, the ability to tune the
3
colour of the electrogenerated emission is unsurpassed in
ECL systems.
Experimental Section
Instrumentation and apparatus: The electrochemical measurements were
carried out using an Eco Chemi m-Autolab type II potentiostat or a CH
instrument model 660B electrochemical workstation. Electrochemilumi-
nescence (ECL) experiments were performed using the m-Autolab with
a custom built light-tight Faraday cage and a photomultiplier tube (Elec-
tron tubes Ltd., model 98285B), biased at 600 V, which was coupled to
a transimpedance amplifier (AMES). All solution-phase electrochemical
experiments were performed in a quartz bottomed cell. The working
electrode was a 3 mm in diameter glassy carbon (GC) disk electrode (CH
Instruments), the counter electrode a platinum wire and the reference
Conclusion
The emission colour and other properties of platinum(II)
salophen complexes may be readily and predictably tuned
over a wide range by small, easily implemented variations to
the ligand structure. By methodically varying the positions
15914
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15907 – 15917

Tuning of Luminescent Platinum(II) Schiff Base Complexes
FULL PAPER
electrode was a nonaqueous silver wire electrode (CH Inst; blank elec-
trolyte filling solution) for organic media. A silver/silver chloride refer-
ence electrode (filling solution; 3m KCl) was used for aqueous systems.
stored over sieved prior to use. All synthetic chemicals used in the syn-
thesis were commercial products of reagent grade and were used without
further purification.
Experiments conducted in organic media were referenced to the formal
potential of the ferrocene/ferrocenium couple measured in situ for each
complex. Electrochemical experiments in organic media were performed
Ligand synthesis: The Schiff base ligands were synthesised by dissolving
4,5-dimethyl-1,2-phenylenediamine (100 mg) in methanol (5 mL) and
slightly more than 2 molar equivalence of the respective aldehydes in
methanol (3 mL) and mixing the two. The resulting solutions were left,
overnight, yielding crystals of the product, which were filtered off,
in 0.1m tetra-n-butylammonium hexafluorophosphate (TBAPF
chemical grade) in anhydrous dichloromethane (ꢃ99.8% with amylene
stabiliser) with a complex concentration of 1.0ꢁ10 m. Aqueous solu-
6
; electro-
ꢀ
3
washed with methanol and dried in vacuo.
ꢀ
1
tions were prepared using deionised water (18 MWcm ). All scans were
1
Salophen (a): Yield: 93%; m.p. 1338C; H NMR (200 MHz, [D
6
]DMSO):
ꢀ
1
conducted at 0.2 Vs unless otherwise stated and carried out at ambient
temperatures (20ꢄ28C). All solutions were deoxygenated using grade 5
nitrogen prior to electrochemical experimentation. The GC working elec-
ꢀ
d=13.17 (s, 2H, OH), 8.62 (s, 2H, HC=N), 7.38–7.33 (m, 4H, aromatic
CH), 7.04 (d, 2H, aromatic CH), 7.03 (s, 2H, N CH), 6.91 (td, 2H, aro-
ꢀ
ꢀ
+
matic CH), 2.33 ppm (s, 6H,
ꢀ
ꢀCH3
); EIMS: calcd for [C22
H
20
N
2
O
2
ꢀH]
trodes were polished prior to each experiment using BUEHLER Micro-
3
44.15; found: 344.1.
ꢂ
cloth Polishing cloth with an aqueous slurry of 0.3 mm alumina.
1
4
,6-(OMe)
2
-salophen (b): Yield: 85%; m.p. 2048C; H NMR (200 MHz,
Spectro-electrochemical measurements were performed using Varian
Cary UV/Vis spectrometer and a CH660B potentiostat with a 1 mm
path-length thin layer quartz spectroelectrochemical cell. A platinum
gauze working electrode, platinum wire counter electrode and silver wire
reference electrode was employed. Bulk electrolysis (coulometric) ex-
periments were performed in a 20 cm cell using a platinum gauze work-
ing electrode, silver wire reference and a platinum counter electrode sep-
arated from the main solution via a porous frit. The solution was stirred
ꢀ
[
D
6
]DMSO): d=14.62 (s, 2H, OH), 8.90 (s, 2H, HC=N), 7.03 (s, 2H, N
CH), 6.12 (d, 2H, CH aromatic), 5.82 (d, 2H, CH aromatic), 3.82 (s,
6
ꢀ
ꢀ
H, OCH
calcd for [C26
-OMe-salophen (c): Yield: 94%; m.p. 1818C; H NMR (200 MHz,
]DMSO): d=13.72 (s, 2H, OH), 8.53 (s, 2H, HC=N), 7.01 (s, 2H, Nꢀ
ꢀ
3
), 3.80 (s, 6H, OCH
H N O
28 2 6
ꢀ
3
), 2.31 ppm (s, 6H,
ꢀCH3); EIMS:
+
ꢀH] 464.19; found: 464.2.
1
4
3
[
D
6
CH), 7.24 (d, 2H, CHꢀaromatic), 6.54 (d, 2H, CHꢀaromatic), 6.45 (d,
2
H, CHꢀaromatic), 3.83 (s, 6H, ꢀOCH
3
), 2.32 ppm (s, 6H, ꢀCH
3
);
(
magnetic stirrer) during electrolysis.
+
EIMS: calcd for [C24
H
24
N
2
O
4
ꢀH] 404.17; found: 404.2.
NMR spectra were obtained with a Bruker BioSpin Av200 with an oper-
ating frequency of 200.13 MHz. Accurate mass electrospray mass spectra
were recorded with a Micromass Q-TOF II mass spectrometer using
a cone voltage of 50 V and a capillary voltage of 3.0 kV. ECL spectra
were obtained using a QE65000 Scientific-grade Spectrometer, incorpo-
rating a Hamarnatsu S7031-1006 FFT-CCD detector. UV/Vis spectra
were recorded using a Varian Cary UV/Vis spectrometer with Eclipse
software. Emission spectra at ambient temperature were obtained on
a Varian Cary Eclipse fluorescence spectrometer and were corrected. All
solutions for photophysical measurements used a 10 mm path-length seal-
able quartz cell and were degassed with nitrogen (15 min) prior to experi-
mentation.
1
6
-OMe-salophen (d): Yield: 97%; m.p. 1968C; H NMR (200 MHz,
[
D
6
]DMSO): d=14.08 (s, 2H, OH), 9.10 (s, 2H, HC=N), 7.02 (s, 2H, Nꢀ
CH), 7.25 (t, 2H, CHꢀaromatic), 6.62 (d, 2H, CHꢀaromatic), 6.33 (d,
2
H, CHꢀaromatic), 3.85 (s, 6H, ꢀOCH
3
), 2.33 ppm (s, 6H, ꢀCH
3
);
+
EIMS: calcd for [C24
H
24
N
2
O
4
ꢀ H] 404.17; found: 404.2.
1
3
[
8
-OMe-salophen (e): Yield: 76% ; m.p. 1588C; H NMR (200 MHz,
D ]DMSO): d=13.29 (s, 2H, OH), 8.61 (s, 2H, HC=N), 7.02–6.80 (m,
6
H, aromaticꢀCH), 3.90 (s, 6H, ꢀOCH
), 2.33 ppm (s, 6H, ꢀCH
3
);
3
+
EIMS: calcd for [C24
H
24
N
2
O
4
ꢀH] 404.17; found: 404.2.
1
5
[
-OMe-salophen (f): Yield: 92% ; m.p. 1288C; H NMR (200 MHz,
D
6
]DMSO): d=12.68 (s, 2H, OH), 8.59 (s, 2H, HC=N), 7.03 (s, 2H, Nꢀ
CH), 3.79 (s, 6H, ꢀOCH
), 2.33 ppm (s, 6H, ꢀCH
); EIMS: calcd for
3
3
Emission quantum yields were determined using either the single point
+
[
C
24
H
24
N
2
O
4
ꢀH] 404.17; found: 404.2.
(
3–7) or multiple-point (1 and 2) method. A degassed dichloromethane
1
3
,5-(OMe)
2
-salophen (g): Yield: 87% ; m.p. 848C; H NMR (200 MHz,
[D ]DMSO): d=12.84 (s, 2H, OH), 8.56 (s, 2H, HC=N), 6.98 (s, 2H,
solution of [Ru
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpy)
ref =0.029). Yields were calculated by F
multiple point) or F
3
]PF
6
(bpy=2,2’-bipyridine) was used as a standard
2
2
(
(
F
x
=Fref.(Grad
x
/Gradref.)(h
x
/h ref
)
6
HC=N), 6.61 (d, 2H, aromaticꢀCH), 6.46 (d, 2H, aromaticꢀCH), 3.87 (s,
x
=Fref.(I
x
A
ref./Iref.
A
x
) (single point), in which the sub-
6
H, ꢀOCH
calcd for [C26
Complex synthesis
Pt-salophen] (1): K
3
), 3.79 (s, 6H, ꢀOCH
3
), 2.34 ppm (s, 6H, ꢀCH
3
); EIMS:
scripts ref. and x denote the reference and unknown, respectively, F is
the fluorescence quantum yield, Grad is the gradient from the plot of in-
tegrated fluorescence intensity versus absorbance, h is the refractive
index of the solvent, I is the integrated emission spectra, and A is the ab-
sorbance at a particular wavelength. Estimated uncertainty is ꢄ10%.
+
H
28
N
2
O
6
ꢀH] 464.19; found: 464.1.
[
2
PtCl
4
(0.0688 g) was added to an 808C mixture of
the ligand Salophen (0.0524 g) and sodium acetate (0.0274 g) in DMSO
(4 mL). The mixture was heated to 1108C for 3 h with stirring. The reac-
tion mixture was then cooled to room temperature. Small red crystals of
product were filtered off and washed with DMSO (2 mL), methanol
(4 mL) and ether (8 mL). The product was then dried, overnight at
Computational methods: Density functional theory (DFT) calculations
[
29]
were carried out within the Gaussian09 suite of programs.
Ground
[
30]
state geometries were optimised in the absence of solvent with B3LYP
and mPW1PW91 functionals in conjunction with the 6–31+G(d) basis
set for non-metal atoms and the LANL2DZ basis set and core poten-
tial for platinum. Only mPW1PW91 results are presented since it has
been shown previously that this functional yields reliable results. Sym-
metry of the optimised ground state structures is C for all systems
except complex 2 (C ). Final single-point energy calculations were carried
out at the 6–31+G(d)/LANL2DZ optimised geometries using the SDD
[
31]
[
32]
1
1008C. Yield: 0.0719 g (85%); m.p. >3208C; H NMR (200 MHz,
[
33]
[D ]DMSO): d=9.38 (s, 2H, HC=N, (with Pt satellites)), 8.18 (s, 2H, Nꢀ
6
[
34]
CꢀCH), 7.80 (dd, 2H, CHꢀaromatic), 7.53 (dt, 2H, CHꢀaromatic), 7.07
(d, 2H, CHꢀaromatic), 6.75 (t, 2H, CHꢀaromatic), 2.32 ppm (s, 6H, ꢀ
1
+
2
CH ); HRMS (ESI): m/z calcd for [C H N O PtꢀH] 537.1073; found:
3
22 18
2
2
537.1056; elemental analysis calcd (%) for C H N O Pt: C 49.16, H
2
2
18
2
2
[
33a,35]
[36]
basis and core potential (MWB)
for Pt and the TZVP basis set
3.38, N 5.21; found: C 49.03, H 3.53, N 5.12.
[Pt-4,6-(OMe) -salophen] (2): K PtCl (0.0487 g) was added to an 808C
mixture of the ligand 4,6-(OMe) -Salophen (0.0508 g) and sodium acetate
0.0260 g) in DMSO (4 mL). The mixture was heated to 1108C for 3 h
with stirring. The reaction mixture was then cooled to room temperature.
Some small crystals had formed. Water (4 mL) was added to precipitate
the remainder of the platinum complex. The product was filtered of and
washed with 50% aqueous DMSO (1 mL), methanol (4ꢁ1 mL) and
[
37]
for all other atoms. The polarisable continuum model (PCM) self-con-
sistent reaction field (SCRF) was used to model solvent effects at the
gas-phase optimised geometries with a solvent of dichloromethane, con-
sistent with the experimental system. HOMO and LUMO energies were
calculated using DFT MOs. Excitation energies to singlet and triplet ex-
A
H
U
T
E
U
G
A
H
U
G
R
N
N
2
2
4
2
(
[
38]
cited states were investigated with TD-DFT with 40 states calculated.
ꢀ
8
An SCF convergence criteria of 10 a.u. was employed throughout. Mo-
[
39]
lecular orbital analysis was carried out with the AOMix program.
ether (4ꢁ2 mL). The product was then dried at 1008C. Yield: 0.0545 g
1
Materials: All electrochemical reagents were of analytical grade or
(76%); m.p. >3208C; H NMR (200 MHz, [D
6
]DMSO): d=9.23 (s, 2H,
higher and were purchased from Sigma–Aldrich. Electrochemical grade
HC=N, (with Pt satellites)), 7.89 (s, 2H, NꢀCꢀCH), 6.21 (d, 2H, CHꢀar-
TBAPF
6
electrolyte was used and organic solvents were distilled and
omatic), 5.94 (d, 2H, CHꢀaromatic), 3.90 (s, 6H, ꢀOCH
3
), 3.80 (s, 6H, ꢀ
Chem. Eur. J. 2013, 19, 15907 – 15917
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15915

C. F. Hogan et al.
+
OCH
3
), 2.32 ppm (s, 6H, ꢀCH
3
); HRMS (ESI): m/z calcd for
PtꢀNa] 679.1315; found: 679.1329; elemental analysis calcd
HRMS (ESI): m/z calcd for [C26
657.1435; elemental analysis calcd (%) for C26
3.99, N 4.26; found: C 46.91, H 4.01, N 4.18.
H
26
N
2
O
6
PtꢀH] 657.1439; found:
Pt: C 47.49, H
+
[
C
26
H
26
N
2
O
6
26 2 6
H N O
(
4
%) for C26
.32.
Pt-4-OMe-salophen] (3): K
ture of the ligand 4-(OMe)-Salophen (0.0504 g) and sodium acetate
0.0233 g) in DMSO (4 mL). The mixture was heated to 1108C for 3 h
26 2 6
H N O Pt: C 47.49, H 3.99, N 4.26; found: C 47.73, H 4.05, N
[
2 4
PtCl (0.0538 g) was added to an 808C mix-
(
with stirring. The reaction mixture was then cooled to room temperature.
Small red crystals of product were filtered off and washed with methanol
Acknowledgements
(
1
4 mL) and ether (8 mL). The product was then dried, overnight, at
C.F.H. acknowledges the Australian Research Council (ARC) for a dis-
covery project grant (DP1094179). The authors acknowledge support
from the National Computational Infrastructure National Facility
(NCINF), Victorian Partnership for Advanced Computing (VPAC), Vic-
torian Life Science Computing Initiative (VLSCI) and the high perfor-
mance computing facility of La Trobe University.
1
008C. Yield: 0.0555 g (75%); m.p. >3208C; H NMR (200 MHz,
]DMSO): d=9.19 (s, 2H, HC=N, (with Pt satellites)), 8.11 (s, 2H, Nꢀ
[
D
6
CꢀCH), 7.68 (d, 2H, CHꢀaromatic), 6.57 (d, 2H, CHꢀaromatic), 6.44
(
dd, 2H, CHꢀaromatic), 3.81 (s, 6H, ꢀOCH
3
), 2.33 ppm (s, 6H, ꢀCH
3
);
+
HRMS (ESI): m/z calcd for [C24
H
22
N
2
O
4
PtꢀNa] 619.1104; found:
6
3
19.1115; elemental analysis calcd (%) for C24
.71, N 4.69; found: C 48.46, H 3.77, N 4.83.
PtCl (0.0538 g) was added to an 808C mix-
ture of the ligand 6-(OMe)-Salophen (0.0507 g) and sodium acetate
0.0238 g) in DMSO (4 mL). The mixture was heated to 1108C for 3 h
22 2 4
H N O Pt: C 48.24, H
[
Pt-6-OMe-salophen] (4): K
2
4
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lyst 1999, 124, 1463–1466.
(
with stirring. The reaction mixture was then cooled to room temperature.
Small red crystals of product were filtered off and washed with DMSO
(
(
(
2 mL), 50% aqueous DMSO (2 mL), methanol (4 mL) and ether
8 mL). The product was then dried, overnight, at 1008C. Yield: 0.0485 g
1
65%); m.p. >3208C; H NMR (200 MHz, [D
6
]DMSO): d=9.51 (s, 2H,
HC=N, (with Pt satellites)), 7.99 (s, 2H, NꢀCꢀCH), 7.39 (t, 2H, CHꢀaro-
matic), 6.69 (d, 2H, CHꢀaromatic), 6.32 (d, 2H, CHꢀaromatic), 3.93 (s,
6
H, ꢀOCH
3
), 2.35 ppm (s, 6H, ꢀCH
3
); HRMS (ESI): m/z calcd for
+
[
C
24
H
22
N
2
O
4
PtꢀH] 597.1227; found: 597.1229; elemental analysis calcd
(
4
%) for C24
.78.
Pt-3-OMe-salophen] (5): K
ture of the ligand 3-(OMe)-Salophen (0.0516 g) and sodium acetate
0.0250 g) in DMSO (4 mL). The mixture was heated to 1108C for 3 h
H
22
N
2
O
4
Pt: C 48.24, H 3.71, N 4.69; found: C 48.42, H 3.79, N
[
2 4
PtCl (0.0540 g) was added to an 808C mix-
(
[
4] a) T. Abe, T. Itakura, N. Ikeda, K. Shinozaki, Dalton Trans. 2009,
11–715; b) S. C. F. Kui, I. H. T. Sham, C. C. C. Cheung, C.-W. Ma,
B. Yan, N. Zhu, C.-M. Che, W.-F. Fu, Chem. Eur. J. 2007, 13, 417–
35; c) K. M.-C. Wong, V. W.-W. Yam, Coord. Chem. Rev. 2007, 251,
with stirring. The reaction mixture was then cooled to room temperature.
Small red crystals of product were filtered off and washed with DMSO
(
(
(
7
2 mL), 50% aqueous DMSO (2 mL), methanol (4 mL) and ether
4
8 mL). The product was then dried, overnight, at 1008C. Yield: 0.0633 g
1
2477–2488; d) K. M.-C. Wong, W.-S. Tang, X.-X. Lu, N. Zhu, V. W.-
W. Yam, Inorg. Chem. 2005, 44, 1492–1498.
83%); m.p. >3208C; H NMR (200 MHz, [D
6
]DMSO): d=9.37 (s, 2H,
HC=N, (with Pt satellites)), 8.22 (s, 2H, NꢀCꢀCH), 7.40 (dd, 2H, CHꢀar-
[
[
5] L. Ancarani Rossiello, J. Chem. Phys. 1969, 51, 5191–5192.
6] J. A. Miller, W. Jin, S. T. Nguyen, Angew. Chem. 2002, 117, 3953–
3957; Angew. Chem. Int. Ed. 2002, 41, 2953–2956.
omatic), 7.12 (dd, 2H, CHꢀaromatic), 6.68 (t, 2H, CHꢀaromatic), 3.82 (s,
6
H, ꢀOCH
3
), 2.35 ppm (s, 6H, ꢀCH
3
); HRMS (ESI): m/z calcd for
+
[
C
24
H
22
N
2
O
4
PtꢀH] 597.1227; found: 597.1222; elemental analysis calcd
[
[
[
7] R. L. Paddock, S. T. Ngyuen, J. Am. Chem. Soc. 2001, 123, 11498–
(
3
%) for C24
.85, N 4.30.
Pt-5-OMe-salophen] (6): K
ture of the ligand 5-(OMe)-Salophen (0.0500 g) and sodium acetate
0.0231 g) in DMSO (4 mL). The mixture was heated to 1108C for 12 h
22 2 4 2
H N O Pt.3H O: C 44.24, H 4.33, N 4.30; found: C 44.29, H
1
1499.
8] G. A. Morris, H. Zhou, C. L. Stern, S. Y. Nguyen, Inorg. Chem.
001, 40, 3222–3227.
9] W.-L. Tong, L.-M. Lai, M. C. Chan, Dalton Trans. 2008, 1412–1414.
[
2 4
PtCl (0.0530 g) was added to an 808C mix-
2
(
[
[
10] a) M. E. Ivanova, G. A. Shagisultanova, Russ. J. Phys. Chem. A
991, 65, 1563; b) G. A. Shagisultanova, I. E. Popenko, A. M. Timo-
with stirring. The resulting dark red solution was cooled to room temper-
ature. Water (1 mL) was added precipitating out the product. The prod-
uct was filtered and washed with methanol (4 mL) and ether (8 mL). The
product was then dried, overnight, at 1008C. Yield: 0.0604 g (68%); m.p.
>
1
nov, Russ. J. Inorg. Chem. 1991, 36, 3096.
11] a) Y.-Y. Lin, S.-C. Chan, M. C. W. Chan, Y.-J. Hou, N. Zhu, C.-M.
Che, Y. Liu, Y. Wang, Chem. Eur. J. 2003, 9, 1263–1272; b) C.-M.
Che, S.-C. Chan, H.-F. Xiang, M. C. W. Chan, Y. Liu, Y. Wang,
Chem. Commun. 2004, 1484–1485; c) C.-M. Che, C.-C. Kwok, S.-W.
Lai, A. F. Rausch, W. J. Finkenzeller, N. Zhu, H. Yersin, Chem. Eur.
J. 2010, 16, 233–247.
1
3208C; H NMR (200 MHz, [D
6
]DMSO): d=9.31 (s, 2H, HC=N), 8.11
(
s, 2H, NꢀCꢀCH), 7.27 (d, 2H, CHꢀaromatic), 7.23 (dd, 2H, CHꢀaro-
matic), 7.02 (d, 2H, CHꢀaromatic), 3.75 (s, 6H, ꢀOCH
), 2.29 ppm (s,
); HRMS (ESI): m/z calcd for [C24
3
+
6
H, ꢀCH
3
H
22
N
2
O
4
PtꢀNa] 619.1104;
found: 619.1091; elemental analysis calcd (%) for C24
H 3.71, N 4.69; found: C 48.38, H 4.04, N 4.52.
H N O Pt: C 48.24,
22 2 4
[
12] a) E. H. Doeven, E. M. Zammit, G. J. Barbante, P. S. Francis, N. W.
Barnett, C. F. Hogan, Chem. Sci. 2013, 4, 977–982; b) T. Joshi, G. J.
Barbante, P. S. Francis, C. F. Hogan, A. M. Bond, G. Gasser, L. Spic-
cia, Inorg. Chem. 2012, 51, 3302–3315; c) E. H. Doeven, E. M.
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A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[Pt-3,5-
mixture of the ligand 3,5-(OMe)
0.0260 g) in DMSO (4 mL). The mixture was heated to 1108C for 3 h
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OMe)
2
-salophen] (7): K
2
PtCl
4
(0.0493 g) was added to an 808C
2
-salophen (0.0506 g) and sodium acetate
(
with stirring. The reaction mixture was then cooled to room temperature.
Small red crystals of product were filtered off and washed with DMSO
(
overnight, at 1008C. Yield: 0.0525 g (81%); m.p. >3208C; H NMR
(
4 mL), methanol (5 mL) and ether (8 mL). The product was then dried,
1
6
200 MHz, [D ]DMSO): d=9.29 (s, 2H, HC=N, (with Pt satellites)), 8.14
(
s, 2H, NꢀCꢀCH), 6.83 (d, 2H, CHꢀaromatic), 6.78 (d, 2H, CHꢀaromat-
[13] G. J. Barbante, C. F. Hogan, A. Mechler, A. B. Hughes, J. Mater.
Chem. 2010, 20, 891–899.
ic), 3.81 (s, 6H, ꢀOCH
), 3.76 (s, 6H, ꢀOCH
), 2.32 ppm (s, 6H, ꢀCH
3
);
3
3
15916
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15907 – 15917

Tuning of Luminescent Platinum(II) Schiff Base Complexes
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Received: June 19, 2013
Published online: October 2, 2013
Chem. Eur. J. 2013, 19, 15907 – 15917
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15917