Emitting-state properties of square-planar dithiocarbamate complexes of palladium(II) and platinum(II) probed by pressure-dependent luminescence spectroscopy

Article abstract of DOI:10.1139/V09-132

Temperature- and pressure-dependent Raman and luminescence spectra of four crystalline complexes of palla- dium(II) and platinum(II) with chelating diethyldithiocarbamate (EDTC) and pyrrolidine-A-dithiocarbamate (PDTC) li- gands are reported. The crystal

Full text of DOI:10.1139/V09-132

1
625
Emitting-state properties of square-planar
dithiocarbamate complexes of palladium(II) and
platinum(II) probed by pressure-dependent
luminescence spectroscopy
Caroline Genre, Genevi e` ve Levasseur-Th e´ riault, and Christian Reber
Abstract: Temperature- and pressure-dependent Raman and luminescence spectra of four crystalline complexes of palla-
dium(II) and platinum(II) with chelating diethyldithiocarbamate (EDTC) and pyrrolidine-N-dithiocarbamate (PDTC) li-
gands are reported. The crystal structure of [Pd(PDTC)2] was resolved at 120 K. Luminescence band maxima are observed
at approximately 14 500 cm–1 and 16 000 cm for the palladium(II) and platinum(II) complexes, respectively. Pressure
–1
–
1
–1
leads to blue shifts of the band maxima by +9 and +13 cm /kbar for [Pd(EDTC)2] and [Pd(PDTC)2], and +15 cm /kbar
for [Pt(EDTC)2]. These spin-forbidden d–d luminescence transitions have lifetimes of approximately 600 ms at tempera-
tures below 20 K. Luminescence intensities at room temperature are low, but they increase significantly with external pres-
sure. The experimental results show that strong increases of luminescence intensities caused by pressure are not limited to
complexes with monodentate ligands, a result providing insight on the coordinates with emitting-state distortions responsi-
ble for this effect.
Key words: palladium(II) complexes, platinum(II) complexes, high-pressure luminescence spectroscopy, high-pressure
Raman spectroscopy.
R e´ sum e´ : Cet article porte sur les propri e´ t e´ s spectroscopiques d’une s e´ rie de quatre compos e´ s du palladium(II) et du plati-
ne(II) avec les ligands ch e´ latants di e´ thyldithiocarbamate (EDTC) et pyrrolidine-N-dithiocarbamate (PDTC). La structure cris-
talline du complexe [Pd(PDTC)2] a` 120 K a en outre e´ t e´ r e´ solue. Les spectres Raman et de luminescence a` temp e´ rature et
pression variable ont e´ t e´ mesur e´ s pour les quatre compos e´ s. Les maxima des bandes de luminescence se trouvent a` environ
4500 cm–1 et 16000 cm pour les complexes de palladium(II) et platine(II). La pression induit un d e´ calage vers le bleu
–1
1
–
1
–1
–1
des maxima de luminescence de +9 cm /kbar pour [Pd(EDTC)2], de +13 cm /kbar pour [Pd(PDTC)2] et de +15 cm /kbar
pour [Pt(EDTC)2]. Ces transitions d–d interdites de spin ont des dur e´ es de vie de l’ordre de 600 ms a` basse temp e´ rature. Les
bandes de luminescence ont une faible intensit e´ a` temp e´ rature ambiante, mais celle-ci augmente de mani e` re importante avec
la pression. Les r e´ sultats exp e´ rimentaux montrent que l’augmentation de l’intensit e´ de luminescence a` haute pression n’est
pas limit e´ e aux complexes avec ligands monodentates.
Mots-cl e´ s : complexes du palladium(II), complexes du platine(II), spectroscopie de luminescence a` basse temp e´ rature et a`
haute pression, spectroscopie Raman a` haute pression.
Introduction
tural changes, such as mechanical grinding, reported re-
cently for a platinum(II) terdentate polypyridyl complex.3
While the observed variations of spectroscopic properties
such as the energies of band maxima are significant, more
controlled and preferably reversible structural changes are
desirable, as they lead to additional and detailed insight
through clear trends. External pressure is one experimental
approach to obtain such variations. Recent advances in
high-sensitivity detection using CCD cameras for both struc-
tural and spectroscopic studies have led to renewed interest
8
Square-planar complexes of d -configured metal ions such
as platinum(II) and palladium(II) have a rich variety of elec-
tronic excited states, which has made them attractive sys-
tems for studies of electronic structure. They show
intriguing and widely variable luminescence properties. Ap-
plications as luminescent sensors or organometallic light-
emitting devices have been proposed.
One recent focus of attention are luminescence properties
that show drastic variations as a consequence of small struc-
1
,2
4–7
in pressure as an attractive probe. In addition, theoretical
modeling of such effects has also obtained recent attention.8
Received 11 May 2009. Accepted 31 July 2009. Published on
the NRC Research Press Web site at canjchem.nrc.ca on
In this study, we present new d–d luminescence spectra for a
series of complexes of palladium(II) and platinum(II) with the
chelating diethyldithiocarbamate (EDTC) and pyrrolidine-N-
dithiocarbamate (PDTC) ligands. The temperature-dependent
luminescence spectra and lifetimes support the d–d assignment
for these transitions. Temperature- and pressure-dependent
spectra show quantitative variations that are compared to those
2
7 October 2009.
C. Genre, G. Levasseur-Th e´ riault, and C. Reber.1
D e´ partement de chimie, Universit e´ de Montr e´ al, Montr e´ al QC
H3C 3J7, Canada.
1Corresponding author (e-mail: christian.reber@umontreal.ca).
Can. J. Chem. 87: 1625–1635 (2009)
doi:10.1139/V09-132
Published by NRC Research Press

1
626
Can. J. Chem. Vol. 87, 2009
Table 1. Crystallographic data for [Pd(PDTC)2].
reported for monodentate ligands such as thiocyanate.5–7
Luminescence intensities increase with pressure for all com-
plexes presented here, but to a lesser extent than for com-
pounds with monodentate ligands, most likely due to the
chelating ligands that block excited-state distortions along
ligator atom – metal ion – ligator atom bending normal co-
ordinates. Raman spectra show continuous changes of fre-
quencies as a function of pressure, indicative of the absence
of structural phase transitions in the pressure range studied.
The spectra allow us to identify trends and to characterize
differences between structurally very similar compounds.
Compound
Empirical formula
M
[Pd(PDTC)2]
C10H16N2S4Pd
398.89
System
Triclinic
P1ꢀ
Space group
˚
a (A)
6.383(2)
7.573(2)
7.842(2)
72.424(4)
89.515(5)
85.997(4)
360.4(2)
1
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
Experimental
˚ 3
V (A )
All reagents and solvents were purchased from Sigma-
Aldrich and used without further purification.
Z
˚
l (A)
1.54178
15.627
200
–
1
m (Cu Ka, mm )
F (000)
Synthesis of [Pd(EDTC) ] and [Pt(EDTC) ]
2
2
q range (8)
42.10–65.15
1076
1070
0.099
0.241
Hydrated PdCl was dissolved in a minimum of DMSO.
2
Independent reflections
Independent reflections with I > 2s (I)
Two equivalents of Na(EDTC) in an ethanol solution were
then added, and a pale yellow powder immediately precipi-
tated. It was filtered and washed with diethyl ether. Single
crystals were obtained by slow evaporation of an acetone
solution of the complex. Elemental analysis calculated for
C H N S Pd (402.96): C 29.81, H 5.00, N 6.95, S 31.83;
2
2
R [F > 2s (F) ]
2
wR2 (F )
Note: Estimated standard deviations on the least-significant di-
gits are given in parentheses.
1
0
20
2 4
found: C 29.97, H 5.02, N 6.86, S 28.51.
tion of the data collection, the first 101 frames were recol-
lected to improve the decay correction analysis.
[
Pt(EDTC) ] was prepared in an analogous fashion from
2
hydrated PtCl , orange crystals were obtained. Elemental
2
analysis calculated for C H N S Pt (491.62): C 24.43, H
All non-H atoms were refined by full-matrix least-squares
with anisotropic displacement parameters. The H atoms were
1
0
20
2 4
4
2
.10, N 5.70, S 26.09; found: C 24.54, H 4.07, N 5.63, S
5.89.
˚
˚
generated geometrically (C–H 0.97 A and N–H 0.86 A) and
were included in the refinement in the riding model approx-
imation; their temperature factors were set to 1.2 times those
of the equivalent isotropic temperature factors of the parent
site. A final verification of possible voids was performed us-
Synthesis of [Pd(PDTC) ] and [Pt(PDTC) ]
2
2
[
Pd(PDTC) ] was prepared from hydrated PdCl2 and
2
NH (PDTC) as described above. Pale yellow crystals were
4
9
ing the VOID routine of the PLATON program.
grown from an acetone solution of the complex. Elemental
analysis calculated for C H N S Pd (398.93): C 30.11, H
1
0
16
2 4
4
2
.04, N 7.02, S 32.15; found: C 29.97, H 5.02, N 6.86, S
8.51.
Spectroscopic measurements
The solution absorption spectra of the complexes were re-
corded with a double-beam Varian Cary 5E spectrophotom-
eter. The luminescence and Raman spectra of the crystalline
samples were measured using a Renishaw 3000 imaging mi-
croscope system equipped with a CCD camera. The excita-
tion sources were the 488 and 514.5 nm lines of argon ion
lasers for the luminescence experiments and the 785 nm
line of a diode laser for the Raman experiments. All spectra
are unpolarized and corrected for spectrometer response.
Pressure was applied to the solid samples by loading crystals
into a gasketed diamond-anvil cell (DAC, High-Pressure Di-
amond Optics) with paraffin oil as the pressure-transmitting
Powdered [Pt(PDTC) ] was prepared with the same
2
method, and orange crystals were grown by slow evapora-
tion of a dichloromethane solution of the complex. Elemen-
tal analysis calculated for C H N S Pt (487.57): C 24.63,
1
0
16
2 4
H 3.31, N 5.75, S 26.31; found: C 24.66, H 3.06, N 5.77,
S 25.84.
Single crystal X-ray diffraction
X-ray crystallographic data for [Pd(PDTC) ] were col-
2
lected from a single crystal sample, which was mounted on a
loop fiber. Data were collected using a Bruker Platform dif-
fractometer, equipped with a Bruker SMART 2K Charged-
Coupled Device (CCD) Area Detector using the program
SMART and normal focus sealed-tube source graphite mono-
medium. The ruby (R ) method was used to calibrate the
1
pressure.10 All pressure effects reported here are reversible:
upon gradual release of external pressure, all quantities re-
turn to their normal values at ambient pressure. For the tem-
perature-dependent measurements, the samples were cooled
in a gas-flow microcryostat system (Janis ST-500) with
liquid nitrogen or liquid helium.
chromated Cu K radiation. The crystal-to-detector distance
a
was 4.908 cm, and the data collection was carried out in
5
12 Â 512 pixel mode, utilizing 4 Â 4 pixel binning. The in-
itial unit cell parameters were determined by a least-squares
fit of the angular setting of strong reflections, collected by a
Luminescence lifetimes were measured using the 532 nm
line of a pulsed Nd:YAG laser, a 0.5 m single monochroma-
tor (Spex 500M, 600 lines/mm grating) and a Hamamatsu
9
.08 scan in 30 frames over four different parts of the recip-
rocal space (120 frames total). One complete sphere of data
˚
11,12
was collected, to better than 0.8 A resolution. Upon comple-
R928 photomultiplier.
Published by NRC Research Press

Genre et al.
1627
Fig. 1. View of a molecule of [Pd(PDTC)2] showing the labelling scheme. The hydrogen atoms are omitted for clarity.
˚
Table 2. Selected bond lengths (A) and
angles (8) for [Pd(PDTC)2].
Results
Crystal structure of [Pd(PDTC)2]
The single crystal structure of [Pd(PDTC) ] was deter-
[
Pd(PDTC)2]
2
mined by X-ray diffraction at 120 K. The compound crystal-
Pd—S1
Pd—S2
S1—C1
S2—C1
C1—N
N—C2
N—C5
S1–Pd–S2
S1–Pd–S1a
S1–Pd–S1a
S1–C1–S2
2.352(2)
2.343(2)
1.742(9)
1.726(9)
1.35(2)
1.47(2)
1.49(2)
75.69(8)
180
ꢀ
lizes in the P1 space group, with Z = 1. The palladium atom
lies at the inversion center of the cell. Table 1 provides fur-
ther information about the unit cell.
The complex molecule displays a square-planar coordina-
tion for palladium(II), with the metal ion situated exactly in
the plane of the four S donor atoms, as illustrated in Fig. 1.
˚
The Pd–S distances are 2.343(2) and 2.352(2) A, typical for
1
3,14
this kind of complex.
The S(1)–Pd–S(2) angle is
180
112.3(6)
7
5.69(8)8, the distortion from a perfect square geometry
being imposed by the chelating PDTC ligand, as summar-
ized in Table 2. The C1 and N atoms lie fairly close to the
coordination plane, consistent with the double-bond charac-
ter of the C1–N bond. The complex molecules globally
have planar structures, and are packed in parallel sheets, as
illustrated in Fig. 2. The shortest intermolecular metal–metal
Note: Estimated standard deviations in the
least-significant digits are given in parentheses.
a
Symmetry operation: –x, –y, –z.
˚
1
3
11
distance is 6.38 A, indicating that there is no Pd–Pd interac-
spin-forbidden A1g ? E band. For [Pd(SCN) ](n-Bu N) ,
g 4 4 2
the energy of the spin-forbidden transition is 16 450 cm
–1
tion. The C3 atom of each molecule is situated at a nearly
axial position above the Pd atom of the molecule translated
by a (0,0,1) vector, the distance between these two atoms
–1
–1
with a molar absorptivity 3 of 10 mol/L cm , while the
singlet–singlet transition occurs at 18 590 cm–1 and has a mo-
˚
–1
–1
being 3.85 A. The Pd–C3 bond forms a 75.48 angle with
lar absorptivity 3 of approximately 100 mol/L cm . The
the metal-coordination plane.
lowest-energy metal–ligand charge-transfer band has an en-
–1
Although numerous palladium(II) and platinum(II) bis-di-
thiocarbamato complexes are known, only a few have been
ergy of 29 000 cm , significantly higher in energy than the
d–d bands. For the platinum(II) complexes, the spin-
1
3–25
1
3
structurally characterized.
All of them have a planar
forbidden A1g ? Eg transition is not observed due to its
weak intensity and possible overlap with the onset of the
spin-allowed d–d band. For [Pt(SCN)4](n-Bu4N)2, this transi-
centrosymmetric [MS4] core unit. Due to the shape of the di-
thiocarbamate ligands, the S–M–S angles are not exactly 908
but have values of approximately 758 for both platinum(II)
and palladium(II) complexes. The M–S distances are within
–1
tion has an energy of 19 460 cm and a molar absorptivity of
–
1
–1 11
less than 10 mol/L cm . The spin-allowed d–d band of this
˚
–1
–1
–1
a 2.31–2.33 A range for both metals. Structures for palladiu-
complex has a maximum at 21 960 cm (3 > 100 mol/L cm ),
1
1
m(II) and platinum(II) complexes with identical ligands are
isomorphous. None of the structurally characterized palladiu-
m(II) or platinum(II) dithiocarbamate complexes report any
significant metal–metal interactions. The structural data pre-
sented here are in perfect agreement with structures reported
for other dithiocarbamate complexes.
It is interesting to note that the complexes with EDTC li-
gands are packed orthogonally to one another, whereas the
structures of the PDTC complexes consist of parallel sheets
of molecular units.
similar to the A1g ? Eg transitions of both platinum(II)
dithiocarbamate complexes. The absorption spectra of the
title complexes are very similar to those of other square-
8
planar d complexes with sulfur ligator atoms.
Temperature-dependent luminescence and Raman
spectroscopy
The luminescence spectra of crystals of the title com-
pounds are broad, spanning up to 4500 cm–1. The band max-
ima are at approximately 14 500 cm–1 for the palladium(II)
complexes and 16 000 cm–1 for the platinum(II) complexes,
values comparable to those reported in the literature for
other square-planar complexes of these two metals.1
Absorption spectroscopy
1,12,26–30
The absorption spectra of the title complexes in solution
have been recorded and are given as Supplementary data. In
the region of the lowest-energy absorption bands, the spin-al-
It is commonly observed that the luminescence energies of
square-planar platinum(II) complexes are higher than those
1
1
11,27
lowed d–d A1g ? E transition is higher in energy than the
of their palladium(II) analogs.
g
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1
628
Can. J. Chem. Vol. 87, 2009
Fig. 2. View of the crystal packing of [Pd(PDTC)2]. The hydrogen atoms are omitted for clarity.
Figures 3 and 4 display representative temperature-de-
pendent luminescence spectra of the four complexes on
identical wavenumber scales. Their luminescence intensities
clearly increase as temperature is lowered due to a lower
rate of non-radiative relaxation at low temperature. The lu-
complexes show a decrease in energy of the luminescence
band maxima when the sample is heated from 70 K
to room temperature, with a similar red shift of about
–1
– 4 cm /K.
Figure 4 displays the temperature-dependent luminescence
spectra of [Pt(EDTC) ] and [Pt(PDTC) ]. The luminescence
minescence lifetimes of [Pd(PDTC) ] at three wavelengths
2
2
2
have been measured at different temperatures as shown in
intensity of those compounds also decreases with tempera-
ture. The temperature-dependent emission lifetimes of
Fig. 5. The luminescence lifetime of [Pd(PDTC) ] at 8 K is
2
–1
6
50 ms, similar to the value of 947 ms observed for
[Pt(PDTC) ] have been recorded at 16 667 cm , close to
2
1
1,12
[Pd(SCN) ](n-Bu N) at 5 K.
This indicates that the na-
the luminescence band maximum. They decrease with in-
creasing temperature, from 700 ms at 77 K to 145 ms at
200 K. These lifetimes are somewhat longer than those for
[Pt(SCN) ](n-Bu N) , where values of 320 ms at 5 K and
4
4
2
ture of the luminescence transition is similar, confirming our
d–d assignment. Much shorter lifetimes are observed at
higher temperature. At 150 K, the lifetime is 0.1 ms for
4
4
2
11
[
[
Pd(PDTC) ], even shorter than the value of 19 ms for
0.3 ms at 200 K have been reported. In contrast to the
palladium(II) complexes, the platinum(II) complexes studied
here show a blue shift of their luminescence band maxima
with increasing temperature. The energy of the band maximum
for [Pt(EDTC) ] changes from approximately 16 300 cm at
5 K to approximately 16 600 cm at 200 K, a blue shift
2
11
Pd(SCN) ](n-Bu N) observed at this temperature, a con-
4
4
2
sequence of efficient non-radiative relaxation processes.
The temperature-dependent luminescence spectra of
–1
[
Pd(EDTC) ] have been recorded between 20 K and 140 K.
2
2
–1
1
In this temperature range, the emission maximum is located
–1
–1
–
1
of +2.5 cm /K. A similar blue shift of +3.3 cm /K is
at approximately 12 800 cm . This is quite different from
–
1
observed for [Pt(PDTC) ], with its emission maxima shifting
the value of 14 500 cm observed at room temperature.
2
from 15 800 cm–1 at 15 K to 16 500 cm at 250 K.
–1
–
1
The band at 12 800 cm is only observed at low tempera-
ture, while another band emerges when the sample is heated
above approximately 70 K, as illustrated in Fig. 3b, where
the onset of the higher energy band is clearly visible in the
spectrum measured at 70 K. A possible explanation for this
could be the occurrence of a structural phase transition.
Temperature-dependent Raman spectra of all four com-
plexes have been recorded between 80 K and room temper-
ature, given as Supplementary data. There are no major
modifications in the shape of the spectra or major shifts of
the Raman peaks for any of the compounds, suggesting that
none of them undergoes a phase transition in this tempera-
ture range.
The luminescence band maximum for [Pd(PDTC) ] is at ap-
2
–
1
proximately 14 100 cm . The spectra of both palladium(II)
Published by NRC Research Press

Genre et al.
1629
Fig. 3. Representative temperature-dependent luminescence spectra
of (a) [Pd(PDTC)2] at 15, 26, 42, and 70 K (top to bottom) and
Fig. 4. Representative temperature-dependent luminescence spectra
of (a) [Pt(PDTC)2] at 78, 125, 200, and 250 K (top to bottom) and
(b) [Pt(EDTC)2] at 60, 150, and 200 K (top to bottom).
(
b) [Pd(EDTC)2] at 20, 50, 70, and 140 K (top to bottom).
ima and shifts are summarized in Table 3. These blue shifts
are comparable to those observed for palladium(II) and plat-
Pressure-dependent luminescence and Raman
spectroscopy
inum(II) tetrathiocyanate and tetraselenocyanate com-
plexes.1
1,31
They are clearly different from the red shifts
Figures 6 and 7 show representative pressure-dependent
luminescence spectra of the title compounds. The lumines-
cence intensities show a fast and continuous increase with
pressure, followed by a decrease above a threshold pressure.
For the palladium(II) complexes, this effect is accompanied
by a blue shift of the luminescence band maxima by +9 cm–
–1
observed for the well-studied tetracyanoplatinate(II) com-
plexes where the metal–metal interaction between the
stacked [Pt(CN)4]2– chromophores
32,33
is enhanced by pres-
sure, leading to strong red shifts of the luminescence max-
ima. No such interactions are present in the dithiocarbamate
complexes, as shown by the pressure effects reported here.
1/
kbar for [Pd(EDTC) ] and +13 cm /kbar for [Pd(PDTC) ].
2 2
This blue shift is easily seen by comparing the spectra of
Pd(PDTC) ] at 21 kbar and 42 kbar in Fig. 6a. The lumines-
Pressure-dependent Raman spectra of the four title com-
plexes are shown in Figs. 8 and 9. The 100–600 cm–1 region
contains the vibrational modes involving the metal ions and
ligator atoms. The ambient-pressure Raman spectra of
[Pd(EDTC) ] and [Pt(EDTC) ] and those of [Pd(PDTC) ]
[
2
–1
cence of [Pt(EDTC) ] displays a blue shift of +15 cm /kbar.
2
Although several attempts were made, it was not possible to
establish a clear trend for the pressure-induced shift of the
2
2
2
emission maximum for [Pt(PDTC) ]. All luminescence max-
and [Pt(PDTC) ] are similar in shape and number of peaks.
2
2
Published by NRC Research Press

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630
Can. J. Chem. Vol. 87, 2009
Fig. 5. Temperature-dependent luminescence lifetimes for
Pd(PDTC)2] measured at different wavelengths across the emission
spectrum. The excitation wavelength is 532 nm.
Fig. 6. Representative pressure-dependent luminescence spectra of
(a) [Pd(PDTC)2] at 28, 32, 21, 42, and 16 kbar (top to bottom) and
(b) [Pd(EDTC)2] at 39, 37, 48, 33, and 15 kbar (top to bottom). The
square and asterisk markers denote the ruby luminescence used for
pressure calibration and the Raman peak of the diamond anvils, re-
spectively.
[
Changes of the band shapes and Raman shifts occur as pres-
sure is increased on the crystalline samples. A precise study
of the evolution of each Raman band shows that most of
them are shifted towards higher energies as pressure is in-
creased, indicative of shorter and stronger chemical bonds
at high pressure. The Raman band maxima of all four com-
plexes show linear shifts with pressure. The shape of the
peaks is also influenced by pressure. It is interesting to note
that the complexes with the two different ligands show a
–
1
different behavior. The bands at 160–175 cm and at
4
00 cm–1 are split and enlarged at high pressure for both
EDTC complexes, but not for the PDTC complexes. This
difference could indicate a possible structural phase transi-
tion in the EDTC complexes, most likely involving slight
changes in packing. The pressure-dependent luminescence
spectra of the EDTC complexes in Figs. 6b and 7b do not
show evidence for a structural phase transition, as the bands
are likely too large to be influenced by small changes in the
packing of the chromophores. In general, it appears that the
EDTC complexes are more susceptible to structural phase
transitions, as illustrated by the pressure-dependent Raman
spectra and the temperature-dependent luminescence spectra
in Fig. 3b.
Table 3. Luminescence band maxima Emax at ambient pressure and
pressure-induced shifts of the luminescence maxima DEmax/Dp.
Emax (cm–1)
–1
Compound
DEmax/Dp (cm /kbar)
+9
+13
+15
Pd(EDTC)2]a
[
[
[
[
14400
14300
16000
15800
12390
14440
a
Pd(PDTC)2]
Pt(EDTC)2]
Pt(PDTC)2]a
a
Not measurable
b
Discussion
[Pd(SCN)4](n-Bu4N)2
+29
+24
[
Pt(SCN)4](n-Bu4N)2b
Assignment of the luminescence transitions
The absorption spectra of the title complexes all show
bands with molar absorptivities of 2000 mol/L cm or
a
b
This work.
Reference 11.
–1
–1
less, which can be attributed to d–d transitions. [Pd(EDTC) ]
2
–1
in particular has a very weak band at 16 050 cm , corre-
ature, again typical for forbidden d–d transitions gaining vi-
1
3
26,27,34
sponding to the symmetry and spin-forbidden A1g ? Eg
transition. This is a strong indication that the observed lumi-
nescence bands also are of a d–d nature. The oscillator
strength of the corresponding absorption bands in thio-
cyanate and halide complexes with better resolved bands
than observed for the title complexes increases with temper-
bronic intensity.
The comparison of the luminescence
spectra of the dithiocarbamate complexes with those of
[Pd(SCN) ](n-Bu N) and [Pt(SCN) ](n-Bu N) , which also
4
4
2
4
4
2
1
1,26,31
show d–d transitions,
supports this assignment. The
observed luminescence lifetimes at low temperatures for the
dithiocarbamate complexes are between 300 ms and
Published by NRC Research Press

Genre et al.
1631
Fig. 7. Representative pressure-dependent luminescence spectra of
Fig. 8. Representative pressure-dependent Raman spectra of
(
(
a) [Pt(PDTC)2] at 19, 24, 27, and 12 kbar (top to bottom) and
b) [Pt(EDTC)2] at 21, 28, 12, and 4 kbar (top to bottom).
(a) [Pd(PDTC)2] at 44, 42, 38, 32, 20, 16, and 12 kbar and 1 bar
(top to bottom) and (b) [Pd(EDTC)2] at 32, 29, 23, 17, 11, and 2
kbar and 1 bar (top to bottom).
1
000 ms, similar to those reported for the thiocyanate com-
dimensionless normal coordinate offset D are the only ad-
justable parameters. Vibrational frequencies are known from
the Raman spectra, and the width of the vibronic lines G is
set to a value somewhat lower than observed to better illus-
trate the progression. The 7 K luminescence spectrum of
11
plexes. The luminescence maxima of both kinds of com-
plexes can also be compared. They are observed between
1
and between 12 400 cm and 12 800 cm for palladium(II)
tetrathiocyanate and tetraselenocyanate complexes, close to
the band maxima of the title complexes summarized in
Table 3. The width at half height of the luminescence bands
4 200 cm–1 and 14 800 cm for all platinum(II) complexes
–1
–
1
–1
[
Pd(EDTC) ] is displayed in Fig. 10a, along with a series of
2
calculated spectra. The average interval between the mem-
bers of the progression in the experimental spectrum is
–1
of the dithiocarbamate complexes is between 2500 cm and
–1
3
90 cm , close but not identical to the observed frequency
–1
4000 cm , again similar to the thiocyanate compounds.
–1
of 381 cm , corresponding to the range of the M–S stretch-
4
ing vibrations. This discrepancy is possibly a manifestation
of a missing mode effect.2
9,37
The consequence of a varia-
Vibronic structure in low-temperature luminescence
spectra
The low-temperature luminescence spectra of [Pd(EDTC)2]
tion of the offset D by ±10ꢀ is shown by the dotted calcu-
lated spectra in Fig. 10a. It is obvious that these spectra do
not reproduce the band width and intensities within the pro-
gression as well as the best calculated spectrum shown as a
solid line. The low value for D leads to a calculated spec-
trum narrower than the experimental spectrum; the high
value results in a calculated band larger than the experimen-
tal spectrum, indicating that offsets are defined to within
better than 10ꢀ by the simple model used here. The com-
and [Pd(PDTC) ] display vibronic progressions. The inten-
2
sity distributions provide insight on the structural changes
which the complex molecules undergo when emitting a pho-
ton. The simplest model to analyze vibronic progressions is
based on harmonic potential energy curves along a single
configurational coordinate with identical vibrational frequen-
cies for the ground and emitting states. The theoretical equa-
tions used to calculate spectra have been published and
parison of the luminescence spectrum of [Pd(PDTC) ] meas-
2
3
5–37
discussed in detail before.
The energy of the electronic
ured at 6 K and a calculated spectrum is shown in Fig. 10b.
origin of the luminescence band maximum E00 and the
The parameter values obtained for the palladium(II) dithio-
Published by NRC Research Press

1
632
Can. J. Chem. Vol. 87, 2009
Fig. 9. Representative pressure-dependent Raman spectra of
Fig. 10. Comparison of experimental and calculated luminescence
spectra. (a) [Pd(EDTC)2] at 7 K with calculated spectrum given as
a solid line. The dotted and dash–dotted calculated spectra were
obtained by changing the D value in Table 4 by ±10ꢀ.
(
(
a) [Pt(PDTC)2] at 28, 20, 18, 13, 9, and 3 kbar (top to bottom) and
b) [Pt(EDTC)2] at 30, 28, 23, 21, and 4 kbar (top to bottom).
(
b) [Pd(PDTC)2] at 6 K with calculated spectrum shown as a solid
line.
carbamate complexes are summarized in Table 4 and com-
11
pared to literature values for [Pd(SCN) ](n-Bu N) . The
4
4
2
low-temperature spectrum of [Pd(SCN) ](n-Bu N) displays
4
4
2
better resolved vibronic structure than the spectra in Fig. 10.
Four different vibrational modes contribute to the observed
progressions. The normal coordinate offset D for the totally
symmetric (a ) mode is lower for [Pd(SCN) ](n-Bu N) than
higher pressure reduces most interatomic distances, as is in-
tuitively expected. The frequencies of the Raman bands vary
linearly with pressure, and there is no sign of an abrupt
change that would indicate a phase transition in the studied
pressure range.
1g
4
4
2
for the two palladium(II) dithiocarbamate complexes. This is
due to the fact that other vibrational modes are identified in
the vibronic structure of [Pd(SCN) ](n-Bu N) , allowing the
4
4
2
emitting-state distortion of the molecule to be distributed on
several normal coordinates. In contrast, the low-temperature
spectra of the dithiocarbamate complexes in Fig. 10 are not
sufficiently resolved to determine multiple offsets, and only
a single coordinate is used in our analysis, leading to the re-
sults in Table 4.
The luminescence spectra of the palladium(II) and
platinum(II) dithiocarbamate complexes also show a pres-
sure-dependence. Their luminescence energies are shifted
towards higher values with pressure, as is the case for
1
1,12
[Pd(SCN) ](n-Bu N) and [Pt(SCN) ](n-Bu N)
with
4
4
2
4
4
2
numerical values given in Table 3. These blue shifts are
linear with pressure, and the width of the luminescence
bands does not vary significantly over the studied pressure
range. The sign of the shift is due to the s-antibonding
dx Ày LUMO whose energy increases sharply with decreas-
Pressure effects on luminescence and Raman spectra
A detailed study of the pressure-dependent Raman spectra
of the four title complexes shows that a vast majority of the
frequencies increase when pressure is imposed on the sam-
ples. This is an indication that submitting the crystals to a
2
2
ing metal–S distances. The HOMO is p-antibonding, and
its energy increases to a lesser extent as bond lengths de-
Published by NRC Research Press

Genre et al.
1633
Table 4. Parameters used to calculate the luminescence spectra of [Pd(EDTC)2], [Pd(PDTC)2],
and [Pd(SCN)4](n-Bu4N)2.
Parameter
Vibrational frequency (cm )
[Pd(EDTC)2]a
390
3.85
15050
110
[Pd(PDTC)2]a
358
4.61
17500
85
[Pd(SCN)4](n-Bu4N)2b
274 (a1g)
3.55
–1
D (dimensionless)
–
1
E00 (cm )
14343
6
–
1
G (cm )
a
This work.
b
Reference 11.
Fig. 11. One-dimensional harmonic potential energy curves of the
ground and excited states at atmospheric pressure (solid lines) and
high pressure (dotted lines). DEact represents the activation energy
barrier for non-radiative relaxation in a qualitative model view.
effect can be qualitatively rationalized with the model
shown in Fig. 11, based on the same potential energy curves
used for the calculations in Fig. 10. The main effect of high
pressure is a decrease of the offset D along the coordinate
Q, leading to a higher activation energy barrier for non-
radiative relaxation, as illustrated by the dotted potential
energy curves denoting the ground and emitting states at
high pressure. While this model rationalizes the observed
luminescence intensity increase, it cannot account quantita-
tively for the observed behavior, as many factors, such as
the different influence of several normal coordinates with
nonzero offsets D, the change of the potential energy curves
with increasing pressure, and all energy-transfer processes
are neglected. The effects of multiple modes can be ana-
lyzed from the well-resolved luminescence transition of
11
[
M(SCN) ](n-Bu N) (M = Pd, Pt), and the offset along
4 4 2
some of these modes could be blocked by the rigidity of the
chelate ligand, thus not allowing a shift of the wells along
this coordinate for the title compounds. Pressure appears to
have a smaller effect on the chelate compounds and causes
a smaller enhancement of their luminescence intensities.
This reasoning is further supported by the values of the lu-
minescence energy shifts of the dithiocarbamate compounds,
which are also lower than for the thiocyanate complexes.
The pressure-dependent Raman spectra support this
analysis, since the frequency of the totally symmetric
metal–ligand stretching mode shifts less in the dithiocarba-
mate than in the tetrathiocyanate compounds. The precise
–1
values of the pressure-induced shifts are +0.17 cm /kbar
–
1
and +0.49 cm /kbar for [Pd(EDTC) ] and [Pd(PDTC) ],
2
2
–1
respectively, lower than the value of +0.68 cm /kbar for
Pd(SCN) ](n-Bu N) .
the values are +0.29 cm /kbar for [Pt(EDTC) ] and
0.47 cm /kbar for [Pt(PDTC) ], again lower than the value
11,12
[
For the platinum(II) complexes,
4
4
2
–1
2
–1
+
2
–1
11
of +0.60 cm /kbar for [Pt(SCN) ](n-Bu N) . It is interest-
4
4
2
ing to note the different evolution of the Raman spectra of
the EDTC and the PDTC complexes of both metal ions with
pressure. Both EDTC complexes exhibit a splitting of their
–1
1
60–175 cm band, corresponding to the metal–sulfur bend-
–1
ing mode. A band at approximately 400 cm also shows a
pressure-induced splitting. The frequency of the bending
mode shifts by +0.19 cm /kbar and +0.17 cm /kbar for pal-
ladium(II) and platinum(II), respectively, less than in the thi-
ocyanate complexes, where shifts of +0.72 cm /kbar
and +0.78 cm /kbar are observed. In the PDTC complexes,
–1
–1
–1
–1
–1
this frequency shifts by +0.83 cm /kbar for palladium(II)
–1
and by +0.80 cm /kbar for platinum(II), closer to the values
enhancement of the luminescence intensity is due to less ef-
observed for complexes with monodentate ligands. It can be
assumed that the metal–sulfur bending mode is more affected
by pressure in the EDTC compounds than in their PDTC
ficient non-radiative processes at high pressure, as docu-
mented by lifetimes for thiocyanate complexes.1
1,12
This
Published by NRC Research Press

1
634
Can. J. Chem. Vol. 87, 2009
analogues or in [M(SCN) ](n-Bu N) (M = Pd, Pt). The or-
(9) Spek, A. L. Platon, 2000 version; University of Utrecht:
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4
4
2
thogonal structure of the EDTC complexes seems more
easily perturbed by pressure than the parallel sheet structure
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more prone to be disordered than the pyrrolidine cycle.
When pressure is increased on the crystal, there is a higher
probability of them arranging differently, thus provoking the
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Supplementary data for this article are available on the
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from the Depository of Unpublished Data, Document Deliv-
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K1A 0R6, Canada. DUD 5278. For more information on ob-
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Acknowledgements
The authors thank Francine B e´ langer-Gari e´ py for help
with the crystal structure determination and Kari A. Frant-
zen for preliminary measurements. Financial support from
the Natural Sciences and Engineering Research Council of
Canada (NSERC) is gratefully acknowledged.
1
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