Synthesis, structures and properties of a new series of platinum-diimine-dithiolate complexes

Article abstract of DOI:10.1039/b709252k

The new square-planar platinum-diimine-dithiolate compounds [Pt(mesBIAN)SS] have been synthesised {mesBIAN = bis(mesityl)biazanaphthenequinone; SS = 1,2-dithiooxalate (dto) 1, maleonitriledithiolate (mnt) 2, 1,2-benzenedithiolate (bdt) 3, 3,4-toluenedithiolate (tdt) 4 and 1,3-dithia-2-thione-4,5-dithiolate (dmit) 5}, and the X-ray crystal structures of 3 and 5 determined. Cyclic voltammetry reveals that all the compounds form stable anions, and ESR spectroscopy of these anions shows that the SOMO is based upon the mesBIAN ligand; compounds 2-5 also show a reversible oxidation wave in their CV. Computational studies reveal that charge-transfer processes from orbitals that are combinations of metal and dithiolate ligand to a mesBIAN π*-based LUMO are responsible for the low energy absorptions seen in the UV/visible spectra of these compounds, and that the reverse process is responsible for the observed room-temperature solution luminescence of [Pt(mesBIAN)Cl₂] and 1, 2 and 5. Compounds 3 and 4, containing aromatic thiolates, were not found to luminesce under the same conditions. Resonance Raman experiments have shown the origin of band-broadening of the lowest-energy absorption band in the absorption spectra of 2-5 to be due to vibronic structure within one electronic transition. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709252k

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Synthesis, structures and properties of a new series of
platinum–diimine–dithiolate complexes†
Christopher J. Adams,*a Natalie Fey,a Matthew Parfitt,a Simon J. A. Popeb and Julia A. Weinstein*c
Received 19th June 2007, Accepted 25th July 2007
First published as an Advance Article on the web 9th August 2007
DOI: 10.1039/b709252k
The new square-planar platinum–diimine–dithiolate compounds [Pt(mesBIAN)SS] have been
synthesised {mesBIAN = bis(mesityl)biazanaphthenequinone; SS = 1,2-dithiooxalate (dto) 1,
maleonitriledithiolate (mnt) 2, 1,2-benzenedithiolate (bdt) 3, 3,4-toluenedithiolate (tdt) 4 and
1,3-dithia-2-thione-4,5-dithiolate (dmit) 5}, and the X-ray crystal structures of 3 and 5 determined.
Cyclic voltammetry reveals that all the compounds form stable anions, and ESR spectroscopy of these
anions shows that the SOMO is based upon the mesBIAN ligand; compounds 2–5 also show a
reversible oxidation wave in their CV. Computational studies reveal that charge-transfer processes from
orbitals that are combinations of metal and dithiolate ligand to a mesBIAN p*-based LUMO are
responsible for the low energy absorptions seen in the UV/visible spectra of these compounds, and that
the reverse process is responsible for the observed room-temperature solution luminescence of
[Pt(mesBIAN)Cl2] and 1, 2 and 5. Compounds 3 and 4, containing aromatic thiolates, were not found
to luminesce under the same conditions. Resonance Raman experiments have shown the origin of
band-broadening of the lowest-energy absorption band in the absorption spectra of 2–5 to be due to
vibronic structure within one electronic transition.
Within this context we have recently reported our preli-
minary studies on platinum compounds of the bis(mesityl)-
Introduction
biazanaphthenequinone ligand (mesBIAN).7 In this ligand the
ligating nitrogen atoms are exocyclic rather than incorporated
into the carbocyclic system, but otherwise it functions much
as an analogue of bipyridine and phenanthroline, forming air
stable platinum compounds [Pt(NN)(LL)] with a variety of co-
ligands L. An important feature of such compounds is that the
mesBIAN ligand has a very low-energy p* orbital that forms
the LUMO. This means that whilst [Pt(bipy)Cl2] is reduced at
−1.14 V vs. SCE,8 [Pt(mesBIAN)Cl2] is reduced at −0.60 V
on the same scale,7 and that the longest wavelength absorption
maximum in the UV/visible spectrum of [Pt(mesBIAN)Cl2] is
at 518 nm, compared to 393 nm for [Pt(bipy)Cl2].9 Such a low
energy LUMO significantly affects the photophysical properties
of the compounds; we have shown that whereas [Pt(bipy)Cl2]
does not luminesce from fluid solution because of the metal-
centered nature of its lowest excited state,10 [Pt(mesBIAN)Cl2]
emits at 777 nm from a mixed platinum/chlorine-to-diimine
charge-transfer excited state.7 This is, to the best of our knowledge,
the first platinum–diimine–dichloride compound to exhibit this
behaviour.
Unlike bipyridine and its derivatives, the presence of the methyl
groups in the 2 and 6 positions of the mesityl ring allows the
mesBIAN ligand to shield the axial positions of the platinum
atom, which might potentially hinder solvent interaction at these
sites and thus prevent solvent mediated non-radiative relaxation
of any excited states. Furthermore, the rigidity of the ligand might
lead to improved luminescence behaviour. We report herein on
a new series of [Pt(mesBIAN)(SS)] complexes (Fig. 1), where
SS represents a dithiolate ligand {SS = 1,2-dithiooxalate (dto)
1, maleonitriledithiolate (mnt) 2, 1,2-benzenedithiolate (bdt) 3,
The past twenty years have seen a great deal of interest in
the photophysical properties of compounds of the general form
[Pt(NN)(SS)], where NN is a bidentate N-heterocyclic ligand such
as bipyridine or phenanthroline (or a derivative thereof), and SS
is a bidentate dithiolate ligand. The reason for such interest is the
observation that these compounds photoluminesce, not only in
glassy media at low-temperature, but also in fluid solution at room
temperature.1 This indicates a long-lived excited state, the energy of
which might be utilised in applications that require photo-induced
charge separation; studies of the dithiolates as dyes for solar cells,2
for the sensitisation of the excited states of oxygen,3 as cation
sensors4 and as non-linear optical materials5 have been reported.
By peripheral modification of the dithiolate ligand, the precise
details of the photophysical properties can be fine tuned.6
A crucial feature in preparing luminescent compounds of
this type is to minimise the effect of radiationless deactivation
pathways. In practice, such pathways often occur via non-emissive
d–d (i.e. ligand field) excited states, so to avoid such radiationless
deactivation it is important to ensure that the energy of such
excited states is much higher than that of the excited state
responsible for the emission. Conversely, if the energy of the
charge-transfer excited state is too low, then it can fall victim to the
‘energy gap law’ and be quenched by efficient internal conversion
to the ground state.
aSchool of Chemistry, University of Bristol, Bristol, UK BS8 1TS
bSchool of Chemistry, Main Building, Cardiff University, Park Place, Cardiff,
UK CF10 3AT
cDepartment of Chemistry, University of Sheffield, Sheffield, UK S3 7HF
† CCDC reference numbers 651111 and 651112. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b709252k
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Fig. 1 Structures of the dithiolate compounds reported herein.
Fig. 2 The molecular structure of 3. Hydrogen atoms have been removed
for clarity.
3,4-toluenedithiolate (tdt) 4 and 1,3-dithia-2-thione-4,5-dithiolate
(dmit) 5}. The use of strong r-donor ligands such as dithio-
lates is a recognised strategy for creating luminescent platinum–
diimine compounds,11 and we show that in complexes which
also contain the mesBIAN diimine ligand (which gives them a
low lying LUMO), small HOMO–LUMO gaps and low-energy
charge transfer excited states are produced. These have been fully
characterised by experimental and computational methods.
Results and discussion
Synthesis and characterisation
The dithiolate compounds [Pt(mesBIAN)(SS)] 1–4 (Fig. 1) were
synthesised by a metathesis reaction between [Pt(mesBIAN)Cl2]
and the appropriate dimethyltin dithiolate [Me2Sn(SS)], with
SS = 1,2-dithiooxalate (dto) 1, maleonitriledithiolate (mnt) 2, 1,2-
benzenedithiolate (bdt) 3 and 3,4-toluenedithiolate (tdt) 4. Com-
pound 5 contains the 1,3-dithia-2-thione-4,5-dithiolate (dmit)
ligand, and was obtained from reaction of the same platinum-
containing starting material with the salt [Bu4N][Zn(dmit)2]
because of the instability of the dimethyltin compound of dmit.
Complexes 1–4 were simply crystallised from the appropriate
reaction mixture, and 5 required only a simple workup prior
to isolation. All new compounds are air stable as solids and as
solutions, and were fully characterised by elemental analysis and
NMR spectroscopy.
Fig. 3 The molecular structure of 5. Hydrogen atoms have been removed
for clarity.
The X-ray crystal structures of 3 and 5 were determined (Fig. 2
and 3). Important bond lengths and angles are given in Tables 1
and 2, and crystallographic data in Table 3. The compounds are, as
expected, approximately square planar about the platinum atom,
with deviation due to the small bite angle of the mesBIAN ligand
(about 80◦). The molecules are essentially planar apart from the
mesityl rings of the mesBIAN ligand, which lie approximately
perpendicular to the rest of the molecule, and which effectively
prevent p-interactions between neighbouring molecules. This may
prove useful in preventing the excimer emission and self-quenching
behaviour that many platinum–diimine compounds display in
solution.11 The Cambridge Structural Database (CSD)12 contains
six structures that contain the platinum–diimine–bdt unit, in all
of which the diimine is 2,2ꢀ-bipyridine or a derivative thereof.3,13
The metrical parameters of the platinum–bdt unit in the structure
of 3 are similar to those in these six structures; thus, for example,
˚
the platinum–sulfur distances of 3 {2.232(2) to 2.244(2) A} may be
3
˚
compared with those of [Pt(bipy)(bdt)] {2.244(2) and 2.250(2) A},
˚
and the platinum–nitrogen distances of 2.027(5) to 2.054(5) A
˚
in 3 with 2.050(4) and 2.049(5) A in [Pt(bipy)(bdt)]. The CSD
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◦
˚
Electronic absorption spectroscopy
Table 1 Selected bond lengths (A) and angles ( ) for 3. There are two
molecules of 3 in the asymmetric unit, the second of which is numbered
similarly to the first but with numbers increased by 100
The compounds are all bold colours. Solutions of 1 are red,
2 is blue, and 3, 4 and 5 are green. These colours are due to
strong absorption bands between 500 and 850 nm (e ≈ 2 ×
104 M−1 cm−1) (Table 4 and Fig. 4), which are the singlet electronic
transitions from a mixed dithiolate–metal HOMO to a mesBIAN
p* LUMO (see below), and which are therefore mixed metal–
ligand to ligand charge transfer (MMLLꢀCT) transitions. This
assignment is supported by the nature of their solvatochromism;
the lowest energy absorption band for all compounds moves to
lower energy as the polarity of the solvent is decreased.14,15 This
is illustrated graphically in Fig. 5, which plots the lowest energy
absorption maximum for each compound in a variety of solvents
against the solvent polarity parameter (as empirically determined
by Eisenberg and Cummings for [Pt(NN)(SS)] compounds).15 It
is apparent from Fig. 5 that compounds 1, 2 and 5 give less good
linear correlations to the scale than 3 and 4, particularly for the
data points pertaining to dichloromethane (polarity 0.765). 1, 2
and 5 have dithiolate ligands that have lone pairs of electrons and
protuberant p-bonds, and CH2Cl2 is the only solvent used that
does not possess either of these features.
Pt(1)–N(1)
Pt(1)–N(2)
Pt(1)–S(1)
Pt(1)–S(2)
N(1)–C(1)
N(1)–C(13)
N(2)–C(11)
N(2)–C(22)
S(1)–C(32)
S(2)–C(31)
2.027(5)
2.044(5)
2.2321(18) Pt(101)–S(101)
2.2439(17) Pt(101)–S(102)
1.297(7)
1.462(7)
1.298(8)
1.462(7)
1.751(7)
1.746(7)
Pt(101)–N(101)
Pt(101)–N(102)
2.053(5)
2.054(5)
2.2409(18)
2.2439(17)
1.289(8)
1.462(7)
1.288(7)
1.457(7)
1.744(6)
1.747(7)
N(101)–C(101)
N(101)–C(113)
N(102)–C(111)
N(102)–C(122)
S(101)–C(132)
S(102)–C(131)
N(1)–Pt(1)–N(2)
N(1)–Pt(1)–S(1)
N(2)–Pt(1)–S(1)
N(1)–Pt(1)–S(2)
N(2)–Pt(1)–S(2)
S(1)–Pt(1)–S(2)
79.9(2)
N(101)–Pt(101)–N(102)
79.2(2)
93.86(15) N(101)–Pt(101)–S(101)
172.12(15) N(102)–Pt(101)–S(101)
175.43(15) N(101)–Pt(101)–S(102)
96.04(15) N(102)–Pt(101)–S(102)
95.90(15)
174.69(15)
173.95(15)
94.99(15)
89.98(6)
90.37(6)
S(101)–Pt(101)–S(102)
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 5
Pt(1)–N(2)
Pt(1)–N(1)
Pt(1)–S(2)
Pt(1)–S(1)
N(1)–C(1)
N(1)–C(13)
N(2)–C(11)
N(2)–C(22)
2.042(4)
2.042(3)
2.2523(12)
2.2580(13)
1.302(6)
1.448(5)
1.291(6)
1.455(5)
S(1)–C(32)
S(2)–C(31)
C(31)–C(32)
C(31)–S(4)
C(32)–S(3)
S(3)–C(33)
S(4)–C(33)
C(33)–S(5)
1.743(5)
1.738(5)
1.357(6)
1.743(5)
1.749(5)
1.724(5)
1.734(5)
1.662(5)
N(2)–Pt(1)–N(1)
N(2)–Pt(1)–S(2)
N(1)–Pt(1)–S(2)
N(2)–Pt(1)–S(1)
N(1)–Pt(1)–S(1)
S(2)–Pt(1)–S(1)
C(32)–C(31)–S(2)
C(32)–C(31)–S(4)
S(2)–C(31)–S(4)
80.37(14)
93.38(11)
173.65(10)
174.33(10)
94.45(10)
91.75(4)
123.2(4)
116.1(4)
120.7(3)
C(31)–C(32)–S(1)
C(31)–C(32)–S(3)
S(1)–C(32)–S(3)
C(33)–S(3)–C(32)
C(33)–S(4)–C(31)
S(5)–C(33)–S(3)
S(5)–C(33)–S(4)
S(3)–C(33)–S(4)
122.2(4)
115.7(4)
122.1(2)
97.7(2)
97.6(2)
125.6(3)
121.4(3)
112.9(3)
Fig. 4 UV/visible spectra of 1–5 in CH2Cl2.
contains no platinum–diimine–dmit structures comparable to 5.
Comparison of the bond lengths and angles in the structure of 5
to those of 3 reveal no significant differences in the coordination
environment about the platinum atom and in the geometry of the
mesBIAN ligand.
The lowest energy absorption in 2–5 is apparently composed of
at least two separate bands of similar energy (Fig. 4); a deconvolu-
tion of the spectrum of 4 into three separate components is shown
in Fig. 6. These components appear to have slightly different
Table 3 Summary crystallographic data for 3·0.75CH2Cl2 and 5·2CHCl3
Compound
3·0.75CH2Cl2
5·2CHCl3
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C36.75H33.50Cl1.50N2PtS2
C35H30Cl6N2PtS5
815.54
100(2)
Monoclinic
P2(1)/c
14.971(3)
18.145(4)
24.787(5)
90.83(3)
1046.70
100(2)
Monoclinic
P2(1)/c
15.048(4)
11.847(4)
21.948(7)
94.33(2)
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
Volume/A
Z
6732(2)
8
4.440
3902(2)
4
4.305
l/mm−1
Reflections collected
Independent reflections
Final R indices [I >2 r(I)]
R indices (all data)
76 537
26 993
15 442 [R(int) = 0.1076]
R1 = 0.0442, wR2 = 0.0750
R1 = 0.0820, wR2 = 0.0819
8951 [R(int) = 0.0494]
R1 = 0.0377, wR2 = 0.0812
R1 = 0.0486, wR2 = 0.0850
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Table 4 Lowest absorption maxima (nm) and extinction coefficients (M−1 cm−1) in various solvents for 1–5
MeCN
DMSO
DMF
Acetone
DCM
thf
k
e
k
e
k
e
k
e
k
e
k
e
1
2
3
4
5
503
639
720
738
769
9384
496
630
733
753
772
10 049
13 447
19 813
20 423
5202
507
632
740
756
774
969
517
654
748
766
794
10 616
14 485
20 739
22 184
11 268
530
682
748
769
829
10 464
17 327
16 608
22 398
18 052
536
693
781
801
856
11 564
18 673
24 159
26 450
20 368
13 433
17 789
19 890
5153
13 604
19 902
20 491
9903
Fig. 5 The energy of the lowest energy transition in compounds 1–5 plotted against the solvent polarity, showing lines of best fit with R2 values. The
values for the polarity are taken from Eisenberg and Cummings,15 and are as follows: MeCN, 1; DMSO, 0.973; DMF, 0.901; acetone, 0.797; CH2Cl2,
0.765; thf, 0.494.
Table 5 Electrochemical data for 1–5
Compound
E◦ (red)/V
ꢀ
E◦ (ox)/V
ꢀ
1
2
3
4
5
−0.57
−0.63
−0.86
−0.85
−0.71
—
1.34
0.82
0.77
0.77
LUMO gap. As this gap is also related to the lowest energy
transition in the UV/visible spectrum, a correlation between the
two may be observed (Fig. 7).
Fig. 6 Deconvolution of the low-energy absorption envelope of 4 (bold
line) into three separate Gaussian components (at 12 706, 13 750 and
15 135 cm−1).
solvatochromism, as their resolution increases with decreasing
solvent polarity. This effect is also apparent in the spectra of
analogous 4,4ꢀ-bis(tert-butyl)-2,2ꢀ-bipyridine compounds.15
Electrochemistry and ESR spectroscopy
All compounds 1–5 display a fully reversible one-electron reduc-
tion at a potential between −0.5 and −1 V, and 2–4 also show a
reversible oxidation wave between 0.75 and 1.4 V (Table 5). As the
reduction potential is related to the energy of the LUMO of the
complex, and the oxidation potential to the energy of the HOMO,
then the difference between them should relate to the HOMO–
Fig. 7 Correlation of DE, the difference in oxidation and reduction poten-
tials for 2–5 (×), with m, the energy of the corresponding HOMO–LUMO
transition.
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ESR spectra of the anions 1−, 2−, 4− and 5− were recorded on
2 : 1 thf–CH2Cl2 solutions of the compounds chemically reduced
in-situ in an ESR tube. As fluid samples they all show a single broad
peak which overlaps with the satellites caused by coupling to 195Pt.
As frozen glasses a well defined rhombic spectrum is observed,
with 195Pt satellites on all three g components, and coupling to
the two nitrogen atoms of the mesBIAN ligand on the high field
component. Fig. 8 shows the spectrum of 4− as a representative
example, and g and A values for all complexes are presented in
Table 6. Assignment of these values follows the methodology of
McInnes and co-workers8 which also allows calculation of the
values of a2 and b2, the coefficients of the 5dyz and 6pz orbitals
in the SOMO. ESR spectroscopy on samples of compound 3
reduced under identical circumstances did not provide a spectrum
showing a single paramagnetic species, possibly due to molecular
aggregation within the sample.‡
Fig. 8 The experimental (top) and simulated (bottom) ESR spectrum of
4−, recorded in frozen 2 : 1 thf–CH2Cl2 solution at 110 K.
The hyperfine coupling constants of the anions 1−, 2−, 4− and
5−, and of [Pt(mesBIAN)Cl2]−, indicate a contribution of between
7% and 14% from metal orbitals to the SOMO (Table 6). For
2−, 4−, 5− and [Pt(mesBIAN)Cl2]− the majority of this (between
4 and 8%) is from the 5dyz orbital and the remainder from the
6pz. The hyperfine coupling constants A for 1− contrast with those
for the remaining compounds. At 50 × 10−4 cm−1, the magnitude
of the isotropic coupling constant <A> is much smaller than
for the other species, for which it is around 75 × 10−4 cm−1.
Also, uniquely for 1− the anisotropic hyperfine coupling constants
indicate a value of a2 which is lower than b2, indicating a larger
contribution to the SOMO from the 6pz orbital than from the 5dyz.
Taken together, these facts would seem to indicate that whilst the
metal contribution to the SOMO of 1− is of the same magnitude
as in the other compounds, the precise composition is different.
For all compounds, the total contribution of metal orbitals to the
SOMO calculated in this way agrees to within 5% of the value
from computational modelling of the anions (Table 6).
‡ Throughout this paper, we use a coordinate system with the y-axis
bisecting the diimine and dithiolate ligands (i.e. the C2 rotational axis),
the z-axis perpendicular to the plane of the molecule, and the x axis
perpendicular to y and z.
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Computational studies
atom). This may account for the different LUMO constitution of
this compound apparent from the ESR spectrum. The LUMO + 1
in all cases is also essentially a mesBIAN p* orbital, of a2
symmetry.
The nature of the HOMO varies between compounds. In
[Pt(mesBIAN)Cl2] and 1, it is a p* orbital of a2 symmetry, with
major contributions from the metal dxz orbital and the chlorine or
sulfur atoms respectively. It is metal–ligand antibonding. In 2–5 it
is of b1 symmetry, comprised mainly of a dithiolate p* orbital (70–
80%), which again is out of phase with a small metal contribution.
The equivalent b1(p*) orbital is the HOMO-2 of 1 and HOMO-5
of [Pt(mesBIAN)Cl2].
The computed electronic transitions (Table 8) shed light upon
the UV/vis spectra of the compounds. We have correlated the
computed and observed transitions by neglecting computed
transitions with oscillator strengths f lower than 0.1; calculations
of transitions often give many transitions with very low oscillator
strengths, and matching computed and observed transitions can
be problematic. In all cases, the lowest energy transition visible
In an attempt to more thoroughly understand the electronic
properties of 1–5, and of [Pt(mesBIAN)Cl2], we have used a
standard DFT approach to optimise their ground state geometries
and hence visualise their frontier orbitals (Table 7 and Fig. 9), as
well as TD-DFT to aid in the interpretation of allowed UV/visible
transitions (Table 8).16 All calculations used a dielectric continuum
to approximate dichloromethane solvation (see the Experimental
for details of calculations).
The compounds all have a LUMO of b1 symmetry (in the C2v
point group) and common origin, being based mainly (between
80 and 90%) upon the p* system of the mesBIAN ligand with
contributions from the metal dyz orbitals (between 4 and 10%), in
accord with the interpretation of the ESR spectra of their anions.
One noticeable anomaly is that the dithiooxalate complex 1 has a
smaller contribution to this MO from the donor sulfur atoms of
the dithiolate ligand (0.5% from each sulfur atom) than any other
compound (which have contributions of 3 to 4% from each sulfur
Table 7 Calculated compositions of orbitals involved in visible transitions
Contribution to orbital (%)
Pt
mesBIAN
Total
Ligand
Total
Compound
Orbital
Symmetry
Total
From each S atom
[Pt(mesBIAN)Cl2]
132 (HOMO − 5)
137 (HOMO)
138 (LUMO)
b1
a2
b1
a2
50.2
41.2
9.7
15.8
31.3
88.2
98.7
16.9, 17.0
13.8, 13.6
1.0, 1.0
0, 0
—
—
—
—
139 (LUMO + 1)
1.2
1
148 (HOMO − 2)
150 (HOMO)
151 (LUMO)
b1
a2
b1
a2
39.5
33.3
8.5
11.7
8.6
87.6
98.9
48.8
58.1
3.9
0
18.8
24.9
0.5
0
152 (LUMO + 1)
1.01
2
3
4
155 (HOMO)
156 (LUMO)
b1
b1
13.5
9.6
10.5
83.9
76.0
6.5
26.6
2.9
156 (HOMO)
157 (LUMO)
b1
b1
11.2
10.8
13.5
80.7
75.4
8.4
24.0
3.5
159 (HOMO − 1)
160 (HOMO)
161 (LUMO)
b1
b1
b1
22.9
10.2
10.8
3.7
13.3
80.6
73.4
76.5
8.6
28.8, 20.1
19.3, 26.6
3.5, 3.6
5
166 (HOMO − 3)
169 (HOMO)
170 (LUMO)
b1
b1
b1
b1
18.4
6.0
10.1
0.2
6.0
9.7
81.2
0.1
75.6
84.3
8.6
7.2
18.9
3.5
0
172 (LUMO + 2)
99.8
Fig. 9 Frontier orbitals of [Pt(mesBIAN)Cl2], illustrating the general form of these orbitals in 1–5. From left to right: orbital 132 (b1 symmetry), 137
(HOMO, a2), 138 (LUMO, b1), 139 (a2).
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Table 8 Observed and computed low-energy absorptions for 1–5
Observed
Calculated
Compound
Energy/eV
Energy/eV
State no.
f
Constitution
[Pt(mesBIAN)Cl2]
2.40
2.56
2.34
2.55
1.82
1.66
1.61
1.50
2.82
3.36
2.57
3.03
1.92
1.73
1.68
1.57
2.77
8
13
5
10
1
1
1
0.17
0.26
0.25
0.19
0.26
0.30
0.26
0.28
0.21
132–138 (79%), 137–139 (5%)
137–139 (83%),
148–151 (83%)
150–152 (90%)
155–156 (84%)
156–157 (79%)
160–161 (71%), 159–161 (11%)
169–170 (82%)
1
2
3
4
5
1
8
169–172 (75%), 166–170 (16%)
in the UV/visible spectrum is predicted to be between two
orbitals of b1 symmetry; that is from HOMO to LUMO in 2–
5, from HOMO-2 to LUMO in 1, and from HOMO-5 to LUMO
in [Pt(mesBIAN)Cl2]. This is from the occupied mixed metal–
dithiolate p* orbital to the empty mesBIAN p* orbital, and is
thus mixed metal–ligand to ligand charge transfer (MMLLꢀCT)
in nature. Transitions from the dithiolate (or chloride) ligand
to mesBIAN move charge along the molecular y-axis, creating
a substantial transition dipole moment in this direction and a
very small transition dipole moment in the perpendicular x and
z directions. Given this, the properties of the C2v point group
mean that the overlap integral <we|ly|wg> is only appreciable
when wg and we are both orbitals of the same symmetry. This
explains why transitions from the HOMO to other orbitals in the
mesBIAN manifold which do not have b1 symmetry have very
small computed oscillator strengths and are hence not likely to
be observed experimentally. Compared with the experimentally
observed transitions, the calculated values are generally between
0.05 to 0.1 eV too large, while for compound 1 there is a much
larger difference (0.2 eV). For 2–5, the calculated transition energy
is within the broad envelope of the observed transition, generally at
a point midway between the two discernable peaks that contribute
to the envelope.
The calculations also predict that for [Pt(mesBIAN)Cl2] and
for 1 there is a second strong transition within 80 nm of that of
lowest energy. In both cases this is from HOMO to LUMO + 1,
again mixed metal–dithiolate to mesBIAN p* MMLLꢀCT in
character and strongly symmetry allowed (both orbitals having a2
symmetry). This would explain the characteristic double peak seen
at around 515 nm in the spectra of these compounds. In contrast,
for the remaining compounds the calculations do not predict any
other strong absorptions within 250 nm of that at the lowest energy.
We therefore surmised that the observed broadness of this lowest
energy absorption band in 2–5 is probably due to poorly resolved
vibrational structure, as described for [M(baba)(mnt)] (M = Ni
or Pd, baba = bisacetylbisaniline)17 and [Pt(bipy)(pz)2] (pz =
pyrazolate)18 complexes, which is not captured by our TD-DFT
studies, rather than the presence of more than one transition.
This hypothesis was confirmed by resonance Raman spec-
troscopy on [Pt(mesBIAN)Cl2] and 4. The Raman spectrum
of [Pt(mesBIAN)Cl2], obtained using 514.5 nm excitation in
resonance with the lowest allowed electronic transition of this
compound, is shown in Fig. 10. It is dominated by a band
at 1570 cm−1, which we assign to CN stretching vibrations of
the coordinated mesBIAN ligand by comparison with data on
related metal–diazabutadiene complexes.17,19 An enhancement of
Fig. 10 Resonance Raman spectrum of polycrystalline [Pt(mesBIAN)-
Cl2] under 514.5 nm excitation.
the m(CN) mode would also be consistent with our assignment
of the electronic transition at 514 nm, and the fact that the
LUMO involved has a large component on the CN fragment
of the mesBIAN ligand (Fig. 9 and Table 7, transition between
orbitals 132 and 138). The lack of Raman bands characteristic of
the phenyl ring in the region 1080–1100 cm−1 indicates that the
mesityl substituents on the mesBIAN ligand are not involved in
the electronic transition of interest, and is also consistent with the
calculated composition of the frontier orbitals.
The other vibrational bands observed at 533, 656, 1149, 1161,
1200, 1311, 1416, 1438, 1604 and 1787 cm−1 are also assigned to
the coordinated mesBIAN ligand. The low-frequency bands at 365
and 440 cm−1 are tentatively attributed to m(Pt-Cl)20 and m(Pt–N)21
respectively, while the band at 226 cm−1 is likely to be due to an
intra-mesBIAN vibration, since vibration in this region has been
reported for coordinated 2,2ꢀ-bipyridine.22
The resonance Raman spectrum of polycrystalline [Pt-
(mesBIAN)(tdt)] 4 was obscured by the emission background,
and the only prominent bands detected were those at 456, 531 and
1095 cm−1. Both the 456 cm−1 band and that at 531 cm−1 have
counterparts in the Raman spectrum of [Pt(mesBIAN)Cl2], and
can therefore be attributed to the Pt–mesBIAN framework. The
spectrum is dominated by the vibration at 1095 cm−1, which is not
present in the rR spectrum of [Pt(mesBIAN)Cl2], is characteristic
of intra-phenyl ring vibrations, and is therefore attributed to the
tdt ligand. This interpretation is consistent with the proposed
MMLLꢀCT nature of the lowest allowed electronic transition in 4.
The multi-Gaussian deconvolution of the lowest absorption
envelope of 4 in CH2Cl2 shown in Fig. 6 reveals contributions
4452 | Dalton Trans., 2007, 4446–4456
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Table 9 Calculated energy difference between the ground state and the
lowest triplet excited state for all compounds, and the corresponding
predicted and observed emission maxima
Emission maximum/nm
Compound
DE/eV
Predicted
Observed
[Pt(mesBIAN)Cl2]
1.57
1.50
1.29
1.03
0.99
0.94
791
829
964
1199
1254
1325
777
820
1010
—
—
—
1
2
3
4
5
Fig. 11 Emission spectrum of 2 in degassed CH2Cl2 solution at room
temperature.
Emission maxima in CH2Cl2 solution at RT.7
One of the applications of the long-lived triplet excited states
of transition metal complexes is in the production of singlet
oxygen, 1O2, by triplet–triplet energy transfer23 to the ground state
of molecular oxygen. We observed that the phosphorescence of
[Pt(mesBIAN)(mnt)] 2 was quenched in the presence of oxygen.
Emission quenching was accompanied by the generation of
singlet oxygen, as indicated by the appearance of a characteristic
from three transitions, at 15 093, 13 786 and 12 702 cm−1. Although
care should be taken while interpreting the results of such
deconvolutions, the pattern obtained would account for 1307
and 1084 cm−1 vibrational progressions, which matches (within
experimental error) the intra-mesBIAN vibration at 1311 cm−1,
and the intra-tdt vibration at 1095 cm−1, seen in the resonance
Raman spectra of [Pt(mesBIAN)Cl2] and 4 respectively. Thus, we
conclude that the broadness of the low-energy absorption band
is due to an unresolved vibrational progression originating from
vibrational modes of both ligands.
3
O2(1Dg) → O2 phosphorescence at k >1200 nm. The yield of
formation of 1O2, ␾(1O2), was determined by measuring its phos-
phorescence intensity using an optically matched solution of
phenalenone as a reference sensitizer.24 Consistent with its rela-
tively long excited state lifetime, 3 produces 1O2 in aerated CH2Cl2
with a yield of 45%. The emission decay in the region >1200 nm
was monoexponential, with a rate constant of 1.24 × 104 s−1 (i.e.
Luminescence studies
Our preliminary studies reported upon the emission of
[Pt(mesBIAN)Cl2] and of 1 from fluid solution at room tem-
perature (at 777 and 820 nm respectively).7 The purpose of
the work reported herein was to try and extend the class of
dithiolate compounds exhibiting this property. Assuming that
emission occurs from the lowest-lying excited state, we have used
the calculated energy difference between the singlet ground state
and the lowest triplet state to estimate the wavelength of emission
in each case.§ These values are shown in Table 9, and they suggest
that compounds 2–5 would emit at much longer wavelengths than
[Pt(mesBIAN)Cl2] and 1. Indeed, we were able to obtain emission
from 2 in degassed CH2Cl2 (Fig. 11) and MeCN solutions at room
temperature, from a broad band centred at 1010 nm in the near-
IR region, following excitation at a range of energies between
350 and 450 nm. The lifetime (kex = 355 nm, kem = 1000 nm)
of the emission was relatively long-lived, single exponential in
character, and mildly solvent dependent, with s = 287 and 259 ns
in dichloromethane and acetonitrile respectively. Unfortunately,
we were unable to confidently observe luminescence at room
temperature from degassed solutions of complexes 3–5, although
5 looked the most promising with a very weak broad band,
again around 970–1100 nm, which possessed a lifetime <10 ns.
The emission was extremely weak, which precluded any further
investigations. No emission was detected for the derivatives of
aromatic thiolates 3 and 4.
1
a lifetime of ca. 80 ls), very similar to that tabulated for O2 in
CH2Cl2,25 and to that detected for 1O2 produced by irradiation of
a solution of the phenolenone reference (1.4 × 104 s−1), confirming
that the emission signal detected was indeed originating from 1O2.
Conclusions
We have shown that platinum–dithiolate compounds containing
the mesBIAN ligand are readily synthesised and are stable.
The dark colours of the compounds are caused by MMLLꢀCT
transitions which have a common charge transfer basis in all
compounds, irrespective of the energy of the orbitals involved.
This means that, whilst the appropriate transition is HOMO to
LUMO in 2–5, the originating b1 symmetry orbital lies deeper in
the occupied manifold for 1 and for [Pt(mesBIAN)Cl2].
Generally it is found that platinum–diimine compounds that
luminesce in solution have a charge-transfer state as the lowest
excited state, and two strategies are commonly employed to
generate emissive platinum(II) complexes. The first is to use strong
r-donor ligands (often alkynyls26) opposite the diimine, which has
the effect of raising the energy of the empty metal dxy orbital
so that excited states involving it are high energy. The second
strategy is to lower the energy of the diimine LUMO below
that of the dxy orbital, normally via substitution with electron
withdrawing groups. In the compounds herein, both of these
strategies have been combined—strong-donor dithiolate ligands,
and a diimine with a low-energy LUMO. Calculations predict
low energy differences between the lowest energy triplet excited
state and the ground state, with a very good match for the
emission energies obtained for the parent [Pt(mesBIAN)Cl2] and
the thiolate complexes 1 and 2. Thus, the present work extends
§ TD-DFT calculations could also be used to predict emission wavelengths
and orbital compositions, but the reliable assignment of transitions is again
problematic, especially where experimental reference data is limited. The
spin-forbidden triplet–singlet transitions associated with emissions only
acquire intensity due to spin–orbit coupling effects, which are not captured
by the current implementations of TD-DFT.
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the range of Pt(II)-based NIR emitters. The ‘energy-gap law’
predicts that the rate of vibrational non-radiative decay of an
excited state increases as its energy relative to the ground state
decreases,27 meaning that the excited states of compounds 3–
5, which possess small HOMO–LUMO gaps, are apparently
deactivated too efficiently to allow emission.
(d, ppm.): 2.42 (12H, s, Me); 2.52 (6H, s, Me); 6.71 (2H, dd, 3J =
4
6 Hz, J = 2 Hz); 7.16 (4H, s); 7.21 (1H, s); 7.36–7.42 (4H, m);
8.14 (2H, d, 3J = 8 Hz).
[Pt(mesBIAN)(tdt)] 4. 0.049
g (0.072 mmol) of [Pt-
(mesBIAN)Cl2] and 0.025 g (0.083 mmol) of [Me2Sn(tdt)] were
stirred overnight in 10 ml of dichloromethane. The solution was
then filtered through a small pad of Celite, and hexane slowly
added until solid material became apparent. Storage at −5 ◦C
yielded the product as black–green needles, which were isolated
by filtration, washed with hexane, and dried to give 0.036 g
(0.047 mmol, 65%) of product. Elemental analysis: Calc. C, 58.02;
H, 4.47; N, 3.66%. Found: C, 58.48; H, 4.66; N, 3.78%. 1H NMR
(d, ppm.): 2.19 (3H, s, Me); 2.41 (12H, s, Me); 2.51 (6H, s, Me);
6.54 (1H, dd, 3J = 8 Hz, 4J = 2 Hz); 6.84 (2H, dd, 3J = 7, 5 Hz);
7.16 (4H, s); 7.21 (1H, s); 7.27 (1H, d, 3J = 8 Hz); 7.38 (2H, t, 3J =
7 Hz); 8.13 (2H, d, 3J = 8 Hz).
Experimental
[Pt(mesBIAN)Cl2] and [Pt(mesBIAN)(dto)]
1
were pre-
pared as reported.7 The dimethyltin dithiolate compounds
[Me2Sn(bdt)], [Me2Sn(tdt)]28 and [Me2Sn(mnt)]29 and the reagent
[Bu4N]2[Zn(dmit)2]30 were prepared by literature methods. Prepa-
rations of new compounds were carried out under nitrogen using
dried solvents. NMR spectra were obtained in CDCl3 on a JEOL
ECP300 spectrometer and referenced to SiMe4. ESR spectra were
recorded on a Bruker ESP300E spectrometer calibrated with dpph,
and simulated using Bruker Simfonia. Radicals were generated
for ESR study by adding the reductant ([Co(g-C5Me5)2]) to a
frozen 2 : 1 thf–CH2Cl2 solution of the appropriate compound and
allowing the system to warm in the ESR cavity until a signal was
observed. Microanalysis was carried out by the microanalytical
laboratory of the University of Bristol School of Chemistry.
Electrochemical studies were carried out using an EG&G model
273A potentiostat linked to a computer using EG&G Model 270
Research Electrochemistry software in conjunction with a three-
electrode cell. The auxiliary electrode was a platinum wire and
the working electrode a platinum disc (1.6 mm diameter). The
reference electrode was an aqueous saturated calomel electrode
separated from the test solution by a fine porosity frit and an
agar bridge saturated with KCl. Solutions in CH2Cl2 were 1.0 ×
[Pt(mesBIAN)(dmit)] 5. 0.113
g (0.166 mmol) of [Pt-
(mesBIAN)Cl2] and 0.065 g (0.090 mmol) of [nBu4N]2[Zn(dmit)2]
were stirred overnight in 10 ml of dichloromethane. The resulting
solution was then washed three times with saturated NaCl
solution, then once with water, and then dried with magnesium
sulfate. It was then filtered to remove the drying agent, and the
volume of the solution reduced to approximately 20 ml on the
rotary evaporator. Hexane was slowly added until solid material
became apparent, and subsequent storage at −5 ◦C yielded the
product as a microcrystalline black–green solid, which was isolated
by filtration, washed with hexane, and dried to give 0.058 g
(0.064 mmol, 38%) of product. Elemental analysis (as a CH2Cl2
solvate): Calc: C, 45.74; H, 3.39; N, 3.14%. Found: C, 45.71; H,
3.24; N, 2.94%. 1H NMR (d, ppm.): 2.41 (12H, s, Me); 2.51 (6H, s,
Me); 6.76 (2H, d, 3J = 7 Hz); 7.15 (4H, s); 7.43 (2H, t, 3J = 8 Hz);
8.18 (2H, d, 3J = 8 Hz).
10−3 mol dm−3 in the compound and 0.1 mol dm−3 in [NBun ][PF6]
4
as the supporting electrolyte. Under these conditions, Eoꢀ for the
one-electron oxidation of [Fe(g-C5H5)2], added to the test solutions
as an internal calibrant, is 0.47 V.
Crystallography†
Many of the details of the crystallographic structure determina-
tions of 3 and 5 are given in Table 3. The structure of 3 contains two
independent molecules of 3 in the asymmetric unit; these have been
given the same numbering scheme but 100 apart (e.g. Pt(1) in one
molecule corresponds to Pt(101) in the other, etc.). There are also
two molecules of CH2Cl2, one of which is disordered equally over
two sites related by a crystallographic inversion centre. The largest
residual electron density peaks remaining are near this disordered
solvent molecule. The structure of 5 contains two molecules of
CHCl3, one of which shows twofold orientational disorder. This
structure has the largest residual electron density peaks near the
atom Pt(1).
Syntheses
[Pt(mesBIAN)(mnt)] 2. 0.046
g (0.067 mmol) of [Pt-
(mesBIAN)Cl2] and 0.020 g (0.068 mmol) of [Me2Sn(mnt)] were
stirred overnight in 50 ml of dichloromethane. The solution was
then filtered, and reduced in volume on the rotary evaporator
to approximately 20 ml. An equal volume of hexane was added,
and overnight refrigeration yielded metallic-purple needles, which
were isolated by filtration, washed with hexane, and dried to give
0.036 g (0.048 mmol, 72%) of product. Elemental analysis (as
a dichloromethane solvate): Calc. C, 50.24; H, 3.61; N, 6.70%.
1
Found: C, 49.84; H, 3.65; N, 6.55%. H NMR (d, ppm.): 2.36
(12H, s, Me); 2.51 (6H, s, Me); 6.84 (2H, d, 3J = 7 Hz); 7.16 (4H,
s); 7.49 (2H, t, 3J = 8 Hz); 8.22 (2H, d, 3J = 8 Hz).
Computational studies
[Pt(mesBIAN)(bdt)] 3. 0.105
g
(0.154 mmol) of [Pt-
All calculations were performed with Gaussian 03 (Revision
C.02)31 and used the popular B3LYP density functional.32 The
Stuttgart–Dresden (SDD) basis set33 was used with a relativistic
effective core potential for Pt and all ligand atoms (C, H, N,
O, S, Cl) were described by the all-electron 6-31G* basis set.
In agreement with results reported by other groups, we found
that changing the basis set or the density functional had little
effect on the calculated transitions and orbital compositions.34
All calculations were performed in a dichloromethane (e = 8.93)
(mesBIAN)Cl2] and 0.049 g (0.169 mmol) of [Me2Sn(bdt)] were
stirred overnight in 10 ml of dichloromethane. The solution was
then filtered through a small pad of Celite, and hexane slowly
added until solid material became apparent. Storage at −5 ◦C
yielded the product as black–green needles, which were isolated
by filtration, washed with hexane, and dried to give 0.077 g
(0.102 mmol, 66%) of product. Elemental analysis: Calc. C, 57.51;
H, 4.29; N, 3.73%. Found: C, 57.20; H, 4.51; N, 3.76%. 1H NMR
4454 | Dalton Trans., 2007, 4446–4456
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continuum solvation field, using the default polarisable continuum
model PCM35 as implemented in G03 with radii from the UFF
force field (explicit hydrogens). To avoid problems with cavity
generation and slow convergence, solvated calculations were run
without symmetry, and some scrf solvation parameters were
changed from their default settings (ofac = 0.8, rmin = 0.5).
Orbital symmetry labels were confirmed by optimisation and
TD-DFT in vacuo with C2v symmetry; despite optimisations
without symmetry, we observed no significant departure from
these geometries. Typically, TD-DFT calculations36 were used
to calculate the 10–15 lowest singlet transitions. Only singlet
transitions with calculated oscillator strengths >0.1 have been
considered in this work. Due to the size of the complexes (up to 76
atoms, no symmetry), loose geometry optimisation criteria were
used. Molecular orbital compositions were calculated as described
by Liu et al.37
AR-coated silicon (II-IV Inc) was placed between the diode and
the cell to act as a cut-off filter for light below 1100 nm. The 403HS
power supply bias voltage was operated at 450 V. The amplifier
output was AC coupled to the digitizer. The output was displayed
on a Tektronix TDS 380 digitizing oscilloscope. Data processing
was performed on an IBM PC using in-house developed software.
The quantum yield of 1O2 production was determined by
comparing the slopes of the linear plots of initial emission intensity
vs. laser energy for optically matched solutions (kexc 355 nm) of the
compound under study and that of the standard (phenolenone,
φ(1O2) = 95%).
Acknowledgements
We thank the University of Bristol (CJA and NF), the EPSRC
and the Royal Society (JAW) for funding, and Johnson-Matthey
for a loan of platinum salts. JAW is grateful to Dr S. F. Parker
and Dr R. Jeans for assistance with the Raman measurements,
to Dr S. Navaratnam and Dr R. Edge for their help with singlet
oxygen measurements, and to Daresbury Laboratory, CCLRC, for
the associated grant. SJAP wishes to thank Cardiff University for
funding.
Emission spectroscopy
Emission experiments were performed at room temperature,
in CH2Cl2 solutions degassed by freeze–pump–thaw methods.
Emission spectra were recorded on a home built system (Sheffield)
comprising a cw Ar ion laser (2 mW) or He–Ne laser (1 mW),
Bentham TMC600 spectrograph, and Andor iDUS CCD camera.
The spectrograph and CCD detection system was calibrated with
a Bentham CL2 tungsten calibration lamp. The emission spectra
were corrected for the overall system sensitivity. Emission kinetic
decay were registered on an Edinburgh Instruments MiniTau
machine with R928 PMT, a cut-off filter >750 nm, and a
picosecond 410 nm diode laser with a 75 ps pulse duration
as an excitation source. Steady-state and time-resolved emission
experiments were also performed in Cardiff, on a JobinYvon–
Horiba Fluorolog spectrometer fitted with a JY TBX picosecond
photodetection module and a Hamamatsu R5509-73 detector
(cooled to −80 ◦C using a C9940 housing). The pulsed laser source
was a Continuum Minilite Nd:YAG configured for 355 nm output.
Lifetimes were obtained using the JY–Horiba FluorHub single
photon counting module.
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4456 | Dalton Trans., 2007, 4446–4456
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