Mixed-ligand Pt(ii) dithione-dithiolato complexes: Influence of the dicyanobenzodithiolato ligand on the second-order NLO properties

Article abstract of DOI:10.1039/c2dt11956k

The mixed-ligand dithiolene complex [Pt(Bz₂pipdt)(dcbdt)] (1) bearing the two ligands Bz₂pipdt = 1,4-dibenzyl-piperazine-3,2- dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by structural data showing that in the C₂S₂PtS₂C₂ dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the Pt-S bond distances involving the two ligands are similar, while the C-S bond distances in the C₂S₂ units exhibit a significant difference in Bz₂pipdt (dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region, which can be related to a HOMO-LUMO transition, where the dcbdt ligand (dithiolato) contributes mostly to the HOMO, and the Bz₂pipdt one (dithione) mostly to the LUMO. Thus this transition has ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes negative solvatochromism and molecular quadratic optical nonlinearity (μβ₀ = -1296 ×10^-48 esu), which was determined by the EFISH (electric-field-induced second-harmonic generation) technique and compared with the values of similar complexes on varying the dithiolato ligand (mnt = maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4, 5-dithiolato). Theoretical calculations help to elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of these complexes. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt11956k

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PAPER
Mixed-ligand Pt(II) dithione-dithiolato complexes: influence of the
dicyanobenzodithiolato ligand on the second-order NLO properties†
Davide Espa,a Luca Pilia,a Luciano Marchiò,b Flavia Artizzu,a Angela Serpe,a Maria Laura Mercuri,a
Dulce Simão,c Manuel Almeida,d Maddalena Pizzotti,e Francesca Tessoree and Paola Deplano*a
Received 17th October 2011, Accepted 21st December 2011
DOI: 10.1039/c2dt11956k
The mixed-ligand dithiolene complex [Pt(Bz2pipdt)(dcbdt)] (1) bearing the two ligands Bz2pipdt = 1,4-
dibenzyl-piperazine-3,2-dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized
and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two
ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by
structural data showing that in the C2S2PtS2C2 dithiolene core the four sulfur atoms define a square-planar
coordination environment of the metal where the Pt–S bond distances involving the two ligands are
similar, while the C–S bond distances in the C2S2 units exhibit a significant difference in Bz2pipdt
(dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region,
which can be related to a HOMO–LUMO transition, where the dcbdt ligand (dithiolato) contributes
mostly to the HOMO, and the Bz2pipdt one (dithione) mostly to the LUMO. Thus this transition has
ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes
negative solvatochromism and molecular quadratic optical nonlinearity (μβ0 = −1296 ×10−48 esu), which
was determined by the EFISH (electric-field-induced second-harmonic generation) technique and
compared with the values of similar complexes on varying the dithiolato ligand (mnt =
maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato). Theoretical calculations help to
elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of
these complexes.
π-delocalized system are required to promote NLO properties.
Introduction
Among these square-planar d8 metal–dithiolene asymmetric
Optical properties, in particular nonlinear (NLO) properties are
of interest for a number of photonic applications, including high
speed optical switching, telecommunications, optical data pro-
cessing and storage.1 Most often molecules with extensive
complexes at the molecular level, such as mixed-ligand com-
plexes with dithione–dithiolato ligands, and a non-centrosym-
metric crystal packing for a bulk material, are suitable to
generate second-order NLO properties.2 The potential of this
class of complexes in generating second order NLO activity is
currently under study by our group, through a systematic investi-
gation on varying the metal, the dithione and the dithiolato
ligand in order to reach a structure/property relationship useful to
prepare complexes with optimized properties.3 A combined
theoretical and experimental study on the triad [M(II)(Bz2pipdt)
(mnt)] (M(II) = Ni, Pd and Pt; Bz2pipdt = 1,4-dibenzyl-pipera-
zine-3,2-dithione; mnt = maleonitriledithiolato) has recently
pointed out the main factors which affect the second order NLO
activity.3b It was found that the most appealing candidate in the
metal triad was the platinum compound. This is related to: i) the
most extensive mixture of the dithione/metal/dithiolato orbitals
in the frontier orbitals (FOs); ii) the influence of the electric field
of the solvent on the FOs that maximizes the difference in dipole
moments between excited and ground state; and iii) the platinum
compound having the largest oscillator strength. Accordingly we
have selected platinum complexes to investigate the variation of
the optical properties with the dithiolato ligand, while keeping
aDipartimento di Chimica Inorganica ed Analitica, Università di
Cagliari, S.S. 554-Bivio per Sestu, I09042 Monserrato-Cagliari, Italy.
E-mail: deplano@unica.it
bDipartimento di Chimica Generale e Inorganica, Chimica Analitica,
Chimica Fisica, Università di Parma, Parco Area delle Scienze 17A,
I43100 Parma, Italy
cDepartamento de Engenharia Química, Instituto Superior Técnico, Av.,
Rovisco Pais, P-1049-001 Lisboa, Portugal
dDepartamento de Química, ITN/CFMCUL, P-2686-953 Sacavém,
Portugal
eDipartimento di Chimica Inorganica Metallorganica e Analitica
“Lamberto Malatesta”, Università di Milano, Unità di Ricerca
dell’INSTM, via G. Venezian 21, 20133 Milano, Italy
†Electronic supplementary information (ESI) available: List of bond dis-
tances [Å] and angles [°] and DFT optimized geometries (Tables S1,
S2); drawing, relative energy and percent compositions of the most sig-
nificant orbitals obtained by DFT calculations in vacuum and in DMF
(Tables S2, S3, S4, S5); Simulated visible spectra for [Pt(Bz2pipdt)
(dcbdt)] and [Pt(Bz2pipdt)(dmit)] (Figures S1 and S2). CCDC reference
number 848108. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt11956k
This journal is © The Royal Society of Chemistry 2012
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the dithione constant (Bz2pipdt). Full characterization, including
optical properties, for [Pt(Bz2pipdt)(dmit)] (dmit = 2-thioxo-1,3-
dithiole-4,5-dithiolato) and [Pt(Bz2pipdt)(mnt)] is available, and
in this work we report the study of [Pt(Bz2pipdt)(dcbdt)] where
dcbdt, dicyanobenzodithiolato, is a benzenoid-dithiolene where
the electron withdrawing properties of the benzodithiolato ligand
are improved by the presence of the cyano-substituents at the 4
and 5 positions of the aromatic ring. Among the square-planar
metal d8 dithiolene complexes, benzodithiolates have been
extensively investigated for their interesting electronic proper-
ties.4 In particular the assignment of the innocent or noninnocent
character of these ligands has been an object of debate. The
Jørgensen former definition, “ligands are innocent when they
allow oxidation states of the central atoms to be defined”,5 has
been pointed out later on by Ward and McCleverty.6 Through the
discussion of several examples, it was cleared up that the term
noninnocent is applied properly when it refers to a particular
combination of the metal and the ligand, where a significant
mixing between the metal-centered and ligand-centered orbitals
occurs, rather than to redox-active ligands alone. By taking into
account the series [Ni(LBu)2]0, 1−, 2− (LBu = 3,5-di-ter-buthyl-
benzene-1,2-dithiolato) and in particular structural results, it was
proposed that the ligand behaves as a normal dithiolato (2-)
implying the presence of the metal as formal Ni(II), Ni(III) and
Ni(IV) for the members of the series7 in contrast to the enedithio-
lato-complexes case.8 Later on, theoretical and experimental
studies from the Wieghardt group have shown that benzene-1,2-
dithiolato ligands may also behave as noninnocent ligands in
their complexes.9
The synthesis and characterization of 1 allows us to compare
the influence of the dithiolato ligand on the second-order NLO
properties of dithione-dithiolato platinum(II) complexes, with the
dithione being constant. In particular the selected ligands belong
to a different class of dithiolato: dcbdt belongs to the class of
benzodithiolato ligands; mnt to enedithiolato ones and dmit to
dithiolato bearing a heterocycle (which contains in this case
additional sulfur atoms) fused to the C2S2 moiety (see Chart 1
below). These complexes are useful to deepen the knowledge of
the nature of bonding and to elucidate the factors which affect
second-order NLO properties with a view to finding guidelines
for designing candidates with controlled properties.
were recorded with a Cary 5 spectrophotometer. Cyclic voltam-
mograms were carried out on an EG&G (Princeton Applied
Research) potentiostat–galvanostat model 273, by using a con-
ventional three-electrode cell consisting of a platinum wire
working electrode, a platinum wire as counter-electrode and Ag/
AgCl in saturated KCl solution as reference electrode. The
experiments were performed at room temperature (25 °C), in dry
and argon-degassed DMF containing 0.1 mol dm−3 Bu4NBF4 as
supporting electrolyte, at 25–200 mV s−1 scan rate.
[Pt(Bz2pipdt)(dcbdt)] (1). (Bu4N)2[Pt(dcbdt)2] (25.74 mg) in
25 cm3 of CH3CN (green solution) was added drop-wise to a
dark red suspension of [Pt(Bz2pipdt)Cl2] (16,50 mg) in 25 cm3
CH3CN, under stirring and reflux (see Results and Discussion
section, where related references are quoted). After one day a
dark-blue solid was formed (17.82 mg; yield: 89%). This solid
was filtered, washed with diethyl ether and then dried. Well
formed crystals suitable for X-ray crystallography were obtained
through re-crystallization from DMF/diisopropyl ether. Analyti-
cal results are in accordance with the formula [Pt(Bz2pipdt)
(dcbdt)]. Anal. Found: C, 43.7; H, 2.9; N, 7.5, S, 17.7. Calc. for
C26H20N4PtS4: C, 43.9; H, 2.8; N, 7.9; S, 18.0. IR (KBr pellets)
2964 vw, 2854 vw, 2221vs, 1611vw, 1531vs, 1506 m 1453w,
1434w, 1404w, 1386 m, 1363vs, 1308 m, 1246w, 1218vs,
1114 m, 1051w, 1030w, 892w, 733w, 695w, 683vw, 622vw,
618vw, 548vw, 527w, 498vw, 448vw, 422vw.
EFISH experiments were performed using a freshly prepared
10−3M solution in DMF and working with a 1907 nm incident
wavelength, obtained by Raman shifting the 1064 nm emission
of a Q-switched Nd:YAG laser in a high pressure hydrogen cell
(60 bar). A liquid cell with thick windows in the wedge
configuration was used to obtain the Maker fringe pattern (har-
monic intensity variation as a function of liquid cell translation).
In the EFISH experiments the incident beam was synchronized
with a DC field applied to the solution, with 60 and 20 ns pulse
duration, respectively, in order to break its centro-symmetry.
From the concentration dependence of the harmonic signal with
respect to that of the pure solvent, the NLO responses were deter-
mined (assumed to be real because the imaginary part has been
neglected) from the experimental value γEFISH, through eqn (1):
μβλðꢀ2ω; ω; ωÞ
γEFISH
¼
þ γðꢀ2ω; ω; ω; 0Þ
ð1Þ
5kT
where γEFISH is the sum of a cubic electronic contribution
γ(−2ω; ω, ω, 0) and of a quadratic orientational contribution μβλ
(−2ω; ω, ω)/5kT, with μ being the ground state dipole moment,
and βλ the projection along the dipole moment direction of the
vectorial component βvec of the tensorial quadratic hyperpolariz-
ability working with the incident wavelength λ. Twelve succes-
sive measurements have been performed and the obtained values
averaged.
Experimental Section
General Comments
Reagents and solvents of reagent grade and spectroscopic grade
(DMF, CH3CN and CS2) have been used as received from
Aldrich. Microanalyses were performed by means of a Carlo
Erba CHNS Elemental Analyzer model EA1108.
I.R. spectra (4000–350 cm−1) were recorded on a Bruker
IFS55 FT-IR Spectrometer as KBr pellets. Electronic spectra
X-ray Crystallography
A summary of data collection and structure refinement for [Pt
(Bz2pipdt)(dcbdt)] is reported in Table 1. Single crystal data
were collected with a Bruker Smart APEXII area detector dif-
fractometer, Mo-Kα: λ = 0.71073 Å. The unit cell parameters
were obtained using 60 ω-frames of 0.5° width and scanned
from three different zones of the reciprocal lattice. The intensity
Chart 1
3486 | Dalton Trans., 2012, 41, 3485–3493
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Table 1 Summary of X-ray crystallographic data for Pt(Bz2pipdt)
(dcbdt)]
calculate the energies of the five lowest singlet electronic tran-
sitions. The dipole moment difference between the excited and
ground states (Δμge) was calculated by using the finite field
approach (external field strength of 0.0005 atomic units).29 All
the calculations were performed with the Gaussian 03
program.30 AOMix was used for determining atomic orbital
compositions employing Mulliken Population Analysis,31 and
SWizard32 was used to analyze the electronic transitions
obtained from TD-DFT calculations.
[Pt(Bz2pipdt)(dcbdt)]
Empirical formula
Formula weight
C26H20N4PtS4
711.79
Color, habit
Crystal size/mm
Crystal system
Space group
Black, prism
0.25 × 0.10 × 0.07
Orthorhombic
Pbcn
a/Å
b/Å
c/Å
α (°).
21.151(2)
16.300(1)
7.921(1)
90
Results and Discussion
β (°).
90
γ (°).
90
2730.9(5)
4
As shown in Scheme 1:
V/Å3
Z
T/K
293(2)
1.731
5.466
ρ (calc), Mg m−3
μ/mm−1
θ range/°
No. of reflections/unique
GooF
R1
1.58 to 28.63
36999/3513
1.004
0.0286
Scheme 1
wR2
0.0561
a R1 = ΣkFo| − |Fck/Σ|Fo|, wR2 = [Σ[w(Fo2−Fc2)2]/Σ[w(Fo2)2]]12, w =
1/[σ2(Fo2) + (aP)2 + bP], where P = [max(Fo2,0) + 2Fc2]/3
[Pt(Bz2pipdt)(dcbdt)], has been obtained by reacting [Pt
(Bz2pipdt)Cl2] with (Bu4N)2[Pt(dcbdt)2],33 following a pro-
cedure which has shown to work well for the preparation of the
corresponding [Pt(Bz2pipdt)(dmit)] complex.3a In these cases,
the desired dianion, which is less stable when uncoordinated, is
provided by salts of platinum (or also nickel) dianionic com-
plexes, and Bu4NCl and degradation products containing the
metal cations are formed. The use of alkaline salts of dithiolato
as direct anion source is preferable, when they have been pre-
viously isolated and their stability allows it (for example in the
Na2mnt case).3b The starting reagent [Pt(Bz2pipdt)Cl2] was pre-
pared by mixing, in CH3CN under reflux, equimolar amount of
Bz2pipdt and [Pt(DMSO)2Cl2].34 On slow evaporation of the
solvents, well formed dark-blue crystals precipitated in high
yield (∼90%). These crystals were suitable for X-ray diffracto-
metric characterization, which shows that 1 crystallizes in the
orthorhombic Pbcn space group. A summary of the X-ray crystal-
lographic data and selected bond lengths and angles for 1 are
reported in Tables 1 and 2. The molecular drawing of 1 is
depicted in Fig. 1, which shows that four sulfur atoms from the
two different ligands define the coordination environment of the
metal, which is in a slightly distorted square-planar geometry.
As reported in Table 2, the Pt–S bond distances involving the
dcbdt ligand are only slightly shorter than those originating from
Bz2pipdt.
data were integrated from several series of exposures frames
(0.3° width) covering the sphere of reciprocal space.10 An
absorption correction was applied using the program SADABS11
with min. and max. transmission factors of 0.652 and 1.000,
respectively. The structure was solved by direct methods
(SIR2004)12 and refined on F2 with full-matrix least squares,13
using the Wingx software package.14 Non hydrogen atoms were
refined anisotropically and the hydrogen atoms were placed at
their calculated positions. Graphical material was prepared with
the Mercury 2.015 and ORTEP3 for Windows16 programs.
CCDC 848108 contains the supplementary crystallographic data
for this paper.†
Computational details
The electronic properties of the [Pt(Bz2pipdt)(dcbdt)] and [Pt
(Bz2pipdt)(dmit)] complexes were investigated by means of DFT
methods.17 The molecular structures of the complexes were opti-
mized starting from the experimental geometries derived by the
X-ray structural characterization using no symmetry constraints.
The Becke three-parameter exchange functional with Lee–Yang–
Parr correlation functional (B3LYP)18,19 was employed together
with the 6-311G(d,p) basis set20,21 for the C, H, N, and S atoms.
The Pt atom was treated with SDD valence basis set,22–24 and
with the MWB60 effective core potentials in order to take into
account relativistic effects.
At variance, the C–S bond distances of dcbdt are significantly
longer than those of Bz2pipdt (1.751(4) vs. 1.688(4) in Å),
which is in accordance with the different natures of the ligands
with prevailing dithiolato character for dcbdt and dithione one
for Bz2pipdt.
In Fig. 2 the crystal packing of 1, projected along the c axis is
shown. The Pt atoms are located between two hexa-atomic rings
of the Bz2pipdt ligand, and the distance between the centroid of
the ligand and the Pt atom is of approximately 4 Å.
To address DMF solvation effects, the conductor-like screen-
ing model as developed in the polarisable continuum model
(CPCM),25 has been applied with united atom topological model
atomic radii (DMF parameters: ε = 37.21, molar volume =
77.41, density = 0.0079, solvent radius = 2.647 Å).26 Single
point CPCM calculations of the complexes were performed
using gas-phase optimized geometries. TD-DFT27,28 was used to
Most significant vibrations (IR spectra) are found at approxi-
mately 1531 cm−1, (the typical C–N vibrations of thioamidic
groups) while dcbdt exhibits peaks due to ν(CN) at 2221 cm−1
.
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Table 2 Selected Experimental and calculated (see related section)
bond distances (Å) and angles (°) for [Pt(Bz2pipdt)(dcbdt)]
Exp.
Calc.
Pt–S (dcbdt)
2.273(1)
2.278(1)
1.751(4)
1.688(4)
2.314
2.314
2.339
2.339
1.753
1.753
1.702
1.702
1.411
1.460
1.363
1.363
88.95
86.52
104.05
104.05
105.84
105.84
121.48
121.48
120.78
120.78
Pt–S (Bz2pipdt)
S–C (dcbdt)
S–C (Bz2pipdt)
C–C (dcbdt)
C–C (Bz2pipdt)
C–N (Bz2pipdt)
1.390(7)
1.484(8)
1.323(4)
Fig. 3 Solvatochromic behaviour of 1 in DMF/CS2 mixtures ranging
from DMF 100% to 20%. The energy of the solvatochromic peak of 1
versus the solvent polarity parameter for solutions from DMF 100% to
20% follows a straight line with slope 1.2.
S–M–S (dcbdt)
S–M–S (Bz2pipdt)
M–S–C (dcbdt)
89.88(5)
88.27(5)
103.7(1)
M–S–C (Bz2pipdt)
S–C–C (dcbdt)
105.8(1)
121.3(1)
119.9(1)
CC stretching vibrations, which are a convenient marker for the
oxidation level of a given coordinated dithiolene ligand in the
case of nonbenzenoid-dithiolenes,35 do not work well in the case
of benzenoid ligands. In fact in benzenoid dithiolene complexes
the chelate ring C–C distance, differently to the ene-dithiolato
cases, undergoes only small variation with the charge in the o-
phenylene moiety, while C–S distances in the C2S2 units are
more dependent on the charge of the ligand.36
Complex 1 is characterized in the visible region by a broad
absorption at 711 nm in DMF with medium molar absorption
coefficient [10.6 × 103 M−1 cm−1] and a negative solvatochro-
mism, as shown in Fig. 3. The energy of the solvatochromic
peak shows a linear dependence on the solvent polarity par-
ameter proposed by Cummings and Eisenberg for d8-metal
diimine-dithiolato complexes.37 The solvatochromic shift deter-
mined as the gradient of this plot falls in the range found for d8-
metal diimine-dithiolato complexes which exhibit relatively high
values of molecular hyperpolarizability.2c
S–C–C (Bz2pipdt)
This solvatochromic peak has been assigned for both these
classes of complexes to a mixed-metal-ligand-to-ligand charge
transfer (MMLL′CT) HOMO → LUMO transition.36,37 EFISH
(electric field induced second harmonic generation) experiments
have been performed using a freshly prepared 10−3 M solution
in DMF and working with a 1907 nm incident wavelength,
obtained by Raman shifting the 1.064 nm emission of a Q-
switched Nd:YAG laser in a high pressure hydrogen cell (60
bar), as described in detail in the Experimental Section. The set
up used allows the determination of the scalar product μβλ(μ =
ground state dipole moment; βλ = projection of the vectorial
component of the quadratic hyperpolarizability tensor along the
dipole moment axis). The μβλ values determined at 1907 nm
incident wavelength have been extrapolated to zero frequency
applying the equation: β0 = βλ[1−(2λmax/λ)2] [1−(λmax/λ)2.
These values are collected in Table 3.
Fig. 1 Ortep drawing of [Pt(Bz2pipdt)(dcbdt)] (1) with thermal ellip-
1
soids drawn at the 30% probability level. Symmetry code ′ = −x; y;
−
2
z.
Cyclic voltammetric experiments performed on DMF sol-
utions of the complex exhibit two reversible reduction waves
and one irreversible oxidation wave. CV data are collected in
Table 4.
The reduction processes, see the CV scans in Fig. 4, show
reduction values close to those found for other mixed-ligand
complexes bearing R2pipdt ligands. Instead the oxidation poten-
tial (Ea) falls between those of dmit and of mnt, suggesting a
Fig. 2 Crystal packing of [[Pt(Bz2pipdt)(dcbdt)] projected along the c
axis. The Pt atoms are located between two hexa-atomic rings of the
Bz2pipdt ligand.
3488 | Dalton Trans., 2012, 41, 3485–3493
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Table 3 Optical and EFISH results for [Pt(Bz2pipdt)(L)] (L = dcbdt;
mnt; dmit) complexes
μβa,b/10−48
a
Complex
[Pt(Bz2pipdt)(L)] nm
λmax
/
ε (×10−3mol−1 esu μβ0/10−48
dm3cm−1
)
esu
Ref.
3b
L = mnt
L = dcbdt
L = dmit
685
711
827
10.1
−1950
−822
−3390
−1296
−10000
−2011
10.6
This
work
3a
14.8
a in DMF; b the uncertainty of the measure is between
5.5%.
3 and
Table 4 Cyclic voltammetric Data. Measured at Pt electrode in DMF),
0.1 M Bu4NPF6, scan rate 100 mV s−1 (Reference Electrode Ag/AgCl)
Fig. 4 Cyclic voltammograms of 1 recorded at 293 K with different
scan rates in a DMF solution containing 0.1 M TBABF4.
Ea(V)a 0 →
[Pt(Bz2pipdt)(L)] 1+
E11/2(V)b 0 ⇆
1−
E21/2(V)b 1− ⇆
2−
L = mnt3b
L = dcbdt
L = dmit3a
+1.09
+1.04
+0.86
−0.37
−0.38
−0.39
−0.87
−0.86
−0.84
both compounds, the most relevant difference between the exper-
imental and optimized geometries is in the orientation of the per-
ipheral benzyl groups of the Bz2pipdt ligands.
As usually found in square-planar metal d8-dithiolene com-
plexes, the four sulfur atoms are forced into close proximity in
the ligand cage as a consequence of the metal coordination and a
significant intra- and inter-ligand repulsive interaction of the
S⋯S type occurs.4a As a result, ligand orbitals are significantly
destabilized as compared to the metal’s d-orbitals which are
found at lower energies. This situation gives rise to the so called
inverted bonding scheme (in a normal bonding scheme the metal
d orbitals lie at higher energies compared to the ligand orbi-
tals).4a–c
a irreversible anodic broad wave; b reversible reduction waves (linear
dependence of the peak currents (ip) from the square root of the voltage
scan rate with ipred/ipox close to the unity).
similar sequence for the ease of the one electron removal
process. Electrochemical results are consistent with spectro-
scopic findings and with theoretical results (see related discus-
sion in the next section). It is found that the ligand acting as
dithione gives a prevalent contribution to the LUMO (relatable
to reduction processes) and the ligand acting as dithiolato to the
HOMO (relatable to the oxidation process).
The very similar E1/2 values for the reduction processes and
the oxidation potential sequence are in accordance with the ener-
getic sequence of the FOs. Moreover the redox activity, which is
related to the frontier orbitals involved in the CT transition
responsible of the solvatochromic peak, suggests these com-
plexes may exhibit redox-switching of molecular first hyperpo-
larizabilities.38 A suitable processing of the material (for
example processing it into an electro-conducting thin-film)39 is
required for its possible application in optical devices.
The electronic structures of both complexes were evaluated
both in a vacuum and by taking into account the contribution of
the solvent (DMF) by means of the CPCM method. In Tables 5,
6, and S4–S7† the energies along with the contour plots of the
frontier orbitals of [Pt(Bz2pipdt)(dcbdt)] and [Pt(Bz2pipdt)
(dmit)] are reported, whereas those of the mnt derivative are
available in ref. 3b. The HOMO and LUMO are both formed
from the out-of-plane antibonding interactions between the
metal’s dxz orbital and a ligand based C2S2 orbital (with C–C π
and C–S π* character). By inspecting the shape of the HOMO
and LUMO, it is evident that the former is mainly located on the
dithiolato ligand, whereas the latter is on the dithione one.
This picture is more clearly evidenced by analyzing the contri-
bution of three molecular fragments (the two ligands and the
metal) to the molecular orbitals, Fig. 5 and Tables 5 and 6. In
fact at least 70% of the HOMO is due to the dithiolato ligand,
whereas the dithione ligand mostly contributes to the LUMO.
The contribution of different fragments (the two ligands and the
central metal) to the complexes’ frontier orbitals is consistent
with π*-back-donation theory.3b,40 It can also be appreciated
how the presence of a relatively polar solvent such as DMF
influences the composition of these frontier orbitals; in fact, in
DMF, the HOMO and LUMO become even more localized on
the dithiolato and dithione ligands, respectively. Furthermore, the
presence of DMF also influences their energy levels since the
HOMO–LUMO gap increases by 0.3 eV for both compounds as
Theoretical studies
The electronic properties of the [Pt(Bz2pipdt)(dcbdt)] complex
as well as those of the [Pt(Bz2pipdt)(dmit)] were investigated by
means of DFT methods and compared with those previously
reported for [Pt(Bz2pipdt)(mnt)].3b For comparison purposes the
choice of the level of theory and method employed in this work
was consistent with that adopted for [Pt(Bz2pipdt)(mnt)]. The
molecular structures of both complexes were optimized starting
from the X-ray experimental geometries (Tables S1, S2 and S3
in the Supplementary Material†). In general there is good agree-
ment between the DFT-optimized and experimental geometries,
even though the coordination bond lengths are slightly overesti-
mated by the calculations (∼0.04 Å), Tables 2, S2 and S3.† For
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Table 5 Spatial plots of HOMO and LUMO (isovalue = 0.04) of [Pt(Bz2pipdt)(dcbdt)] in the gas phase and in DMF (B3LYP/6-311G(d,p)-SDD),
together with the contribution of different fragments to these orbitals
Fragment composition (%)
E (eV)
Pt
Bz2pipdt
dcbdt
15.3
Gas phase
LUMO
−3.85
−5.47
1.62
6.3
78.4
HOMO
8.4
23.3
68.3
HOMO–LUMO gap
DMF
LUMO
−3.72
4.6
88.2
17.6
7.2
HOMO
−5.64
13.7
68.7
HOMO–LUMO gap
1.92
Fig. 5 a) Percentage contributions of different atoms to frontiers molecular orbitals22 of [Pt(Bz2pipdt)(dcbdt)] (HOMO, below and LUMO, above)
on the left side. b) Percentage contributions of different atoms to frontiers molecular orbitals22 of [Pt(Bz2pipdt)(dmit)] (HOMO, below and LUMO,
above) on the right side. CPCM (DMF) B3LYP/6-311G(d,p)-SDD.
a consequence of the HOMO stabilization, and in the case of [Pt
(Bz2pipdt)(dcbdt)], also of the LUMO destabilization.
The shapes of the frontier orbitals, and the data reported in
Tables 5 and 6, suggest that the HOMO–LUMO transition will
have mixed metal–ligand to metal–ligand character (MMLL′
CT). Related data on [Pt(Bz2pipdt)(mnt)] are available in ref. 3b.
The HOMO–LUMO energy gap in [Pt(Bz2pipdt)(L)] follows the
order: mnt > dcbdt > dmit either in vacuum or in the presence of
3490 | Dalton Trans., 2012, 41, 3485–3493
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Table 6 Spatial plots of HOMO and LUMO (isovalue = 0.04) of [Pt(Bz2pipdt)(dmit)] in the gas phase and in DMF (B3LYP/6-311G(d,p)-SDD),
together with the contribution of different fragments to these orbitals
Fragment composition (%)
E (eV)
Pt
Bz2pipdt
dmit
23.9
Gas phase
LUMO
−3.70
−4.96
1.26
5.5
70.6
HOMO
3.1
27.2
69.7
HOMO–LUMO gap
DMF
LUMO
−3.74
4.4
6.0
84.6
17.0
11.0
77.0
HOMO
−5.26
HOMO–LUMO gap
1.52
the solvent and this can be explained by considering that the
HOMO is an antibonding MO mainly localized on the dithiolato
ligands.
Table 7 Low-lying singlet excited states of [Pt(Bz2pipdt)(dcbdt)]
computed by TD-DFT/CPCM(DMF) with the orbitals involved in the
excitations, vertical excitation energies (ΔE), oscillator strength (f), and
character of the CT transitions. The principal singlet transition
responsible for the main absorption band in the visible region is shown
in bold
The shape of the HOMO in the three complexes showing an
increase of the antibonding character (M–S; C–Sthiol in mnt
derivative; S–M; C–Sthiol and C11–C12 in dcbdt derivative;
M–S, C–Sendo and C–Sexo in dmit derivative) on going along the
series mnt, dcbdt, and dmit is in agreement with the increase of
the corresponding HOMO energies. Vertical excitation energies
of [Pt(Bz2pipdt)(dcbdt)] and [Pt(Bz2pipdt)(dmit)] were deter-
mined using time-dependent density functional theory
(TD-DFT) as described in the Experimental section. These cal-
culations were performed in three different conditions: 1) in the
gas phase, 2) in the presence of chloroform as solvent, and 3) in
the presence of DMF as solvent. A comparison between the
energies of the transitions in chloroform and in DMF was per-
formed in order to investigate the solvatochromic behavior of
these complexes. The first five singlet–singlet excitation energies
are reported in Tables 7 and 8. The first allowed transition is
characterized by the greater oscillator strength and is mainly
composed of a HOMO to LUMO excitation, supporting its
assignment as MMLL′ CT transition for both complexes. These
results are in agreement with previous calculations performed for
the [Pt(Bz2pipdt)(mnt)] complex.3b The negative solvatochro-
mism of these complexes is also confirmed by comparing the
calculated absorption spectra in DMF and chloroform, Figures
ΔE (eV)/
λ(nm)
State Composition
f
Character
1
2
3
4
HOMO →
1.68/739
0.3500 dcbdt/Pt/Bz2pipdt
→ Bz2pipdt
LUMO, 79%
(MMLL CT)
HOMO-1 →
LUMO, 91%
1.80/689
2.26/549
2.73/454
0.0009 dcbdt/Pt/Bz2pipdt
→ Bz2pipdt
(MMLL CT)
0.0008 Pt → Bz2pipdt
HOMO-2 →
LUMO, 97%
(MLCT)
0.0276 Bz2pipdt →
Bz2pipdt
HOMO-3→
LUMO, 94%
(ILCT)
S1 and S2.† Basically, the calculated first singlet–singlet tran-
sition in DMF is in reasonable agreement with the experimental
results. In fact, the absorption spectra of [Pt(Bz2pipdt)(dcbdt)]
recorded in DMF exhibits a λmax of 711 nm to be compared to
the value of 738 nm derived by the TD-DFT calculations. A
worse agreement is found for [Pt(Bz2pipdt)(dmit)] since the
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Table 8 Low-lying singlet excited states of [Pt(Bz2pipdt)(dmit)]
computed by TD-DFT/CPCM(DMF) with the orbitals involved in the
excitations, vertical excitation energies (ΔE), oscillator strength (f), and
character of the CT transitions. The principal singlet transition
responsible for the main absorption band in the visible region is shown
in bold
employment of the CPCM model reveals the role of the solvent
in defining the electronic properties of the complexes. In particu-
lar, the presence of a polar solvent stabilizes the HOMO, which
is mainly localized on the dithiolato ligand, whereas it destabi-
lizes the LUMO, which is mainly localized on the dithione one.
These results are in agreement with experimental electronic
spectra and electrochemical data for the complexes under study.
All these complexes show large negative second-order polariz-
ability values, amongst the highest reported so far for metal com-
plexes. The factors affecting these high negative second-order
polarizability values are related to the high difference in dipole
moments between excited and ground state enhanced by the
electric field of the solvent, the large oscillator strength for the
charge transfer transition, and the relatively low energy gap. In
particular, in [Pt(Bz2pipdt)(dmit)] the more extensive antibond-
ing nature of the HOMO induces a smaller energy gap providing
a larger β0 value. The redox activity of these complexes, which
is related to the frontier orbitals involved in the CT transition
responsible of the solvatochromic peak, suggests these com-
plexes deserve to be investigated further for possible redox-
switching of molecular first hyperpolarizability.38
ΔE (eV)/
λ(nm)
State Composition
f
Character
1
2
3
4
HOMO →
1.30/951 0.3410 dmit/Pt/Bz2pipdt →
Bz2pipdt
(MMLL CT)
1.87/662 0.0023 dmit/Pt/Bz2pipdt →
Bz2pipdt
(MMLL CT)
2.24/555 0.0005 Pt → Bz2pipdt
LUMO, 73%
HOMO-1 →
LUMO, 89%
HOMO-4 →
LUMO, 98%
(MLCT)
2.31/537 0.0023 dmit/Pt/Bz2pipdt →
dmit/Pt/Bz2pipdt
HOMO→
LUMO+2, 93%
(MMLL CT)
experimental λmax in DMF is 827 nm, whereas the calculations
yield a value of 951 nm. Nevertheless the trend observed for the
three complexes [Pt(Bz2pipdt)(mnt)], [Pt(Bz2pipdt)(dcbdt)] and
[Pt(Bz2pipdt)(dmit)] agrees well with the experimental results.
In fact the electron withdrawing properties of the dithiolato
ligands and the amount of mixing of their π orbitals in the
HOMO are in agreement with the observed electrochemical and
optical trends (oxidation process and λmax at decreasing poten-
tials and decreasing energy respectively for mnt > dcbdt > dmit
derivatives).
Acknowledgements
This work has been performed through cooperation across
COST Action D35, WG11 “Multifunctional and Switchable
Molecular Materials”. Università di Cagliari and FCT (Portugal)
under contract PTDC/QUI-QUI/101788/2008 are also acknowl-
edged for financial support.
The combined experimental and theoretical results help to elu-
cidate the factors affecting NLO properties. By taking into
account a simplified two-state model41 the second-order polariz-
ability, is given by:
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3ΔμgeðμgeÞ
β /
ð2Þ
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