Electrochromic second-order NLO chromophores based on MII (M = Ni, Pd, Pt) complexes with diselenolato-dithione (donor-acceptor) ligands

Article abstract of DOI:10.1039/c2dt31537h

The donor-acceptor type mixed-ligand complexes [M(Bz₂pipdt) (dsit)]; dsit = 2-thioxo-1,3-dithiole-4,5-diselenolato (donor); Bz ₂pipdt = 1,4-dibenzyl-piperazine-2,3-dithione (acceptor); M(ii) = Ni (1), Pd (2), and Pt (3) were prepared

Full text of DOI:10.1039/c2dt31537h

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PAPER
Electrochromic second-order NLO chromophores based on MII
(M = Ni, Pd, Pt) complexes with diselenolato–dithione (donor–acceptor) ligands†
Davide Espa,a Luca Pilia,a,b Luciano Marchiò,c Maddalena Pizzotti,d Neil Robertson,b Francesca Tessore,d
M. Laura Mercuri,a Angela Serpea and Paola Deplano*a
Received 12th July 2012, Accepted 1st August 2012
DOI: 10.1039/c2dt31537h
The donor–acceptor type mixed-ligand complexes [M(Bz2pipdt)(dsit)]; dsit = 2-thioxo-1,3-dithiole-4,5-
diselenolato (donor); Bz2pipdt = 1,4-dibenzyl-piperazine-2,3-dithione (acceptor); M(II) = Ni (1), Pd (2),
and Pt (3) were prepared and characterized to investigate the variation of the properties by substituting
selenium for sulfur in the donor ligand dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato of the corresponding
known complexes. Both these classes of complexes exhibit large negative second-order polarizabilities,
amongst the highest values determined so far for metal-complexes, and are potential candidates for redox
switchability of the molecular first hyperpolarizability due to the bleaching/restoring of the
solvatochromic peak for mono-reduction/oxidation. DFT and TD-DFT calculations on 1–3 allow one to
correlate geometries and electronic structures and are in agreement with the observed minor changes
following the substitution of selenium for sulfur atoms in the dichalcogenolato ligand. The observed
differences can be ascribed to the increased size of the selenium atom leading to increased M–X distances
and dipolar moments of the ground state, which are highest for the Pd-derivative in the triad.
having different electron withdrawing/donating capability induce
a redistribution of the π-electrons in such a way that one of the
Introduction
Complexes of d8-metals (most often Pt(II)) with non-innocent
ligands based on donor–acceptor systems have been deeply
ligands can be described as a dithione (acceptor), the other one
as a dithiolato (donor).4 These complexes behave as second-
investigated for their peculiar electronic properties and are of
current interest for their possible use as potential photocatalysts,1
in solar energy harnessing2 and as second-order-non-linear chro-
mophores.3 In square-planar d8-metal dithiolene complexes,
terminal groups attached to the dithiolene core (C2S2MC2S2) and
order non-linear chromophores and show a solvatochromic peak
in the visible–near infrared spectral region and remarkably high
negative molecular first hyperpolarizability.5 The solvatochromic
peak is relatable to a HOMO–LUMO transition, where the
HOMO is formed by a mixture of metal and dithiolate orbitals
while the dithione orbitals give a predominant contribution to
the LUMO. A mixed metal/ligand to ligand charge transfer char-
acter (MMLL′CT) has been assigned to this transition.6 A sys-
tematic study on varying the metal,6 the dithione and the
dithiolato7a,b ligands is in course to highlight the role that each
of them plays in affecting the properties of these complexes also
with the goal to find the most appealing candidate for second-
order NLO applications. Results of a combined theoretical and
experimental study previously performed on the triad [M(II)-
(Bz2pipdt)(mnt)] (M(II) = Ni, Pd and Pt; Bz2pipdt = 1,4-dibenzyl-
piperazine-2,3-dithione, mnt = maleonitriledithiolato) suggested
the platinum compound as the most appealing candidate.6
Accordingly we have selected platinum complexes, keeping con-
stant the dithione, Bz2pipdt, and changing the dithiolato ligand:
L = mnt; dcbdt = dicyanobenzodithiolato; dmit = 2-thioxo-1,3-
dithiole-4,5-dithiolato, see Chart 1.
aDipartimento di Scienze Chimiche e Geologiche, Università di
Cagliari, Unità di Ricerca dell’INSTM, S.S. 554-Bivio per Sestu, I09042
Monserrato-Cagliari, Italy. E-mail: deplano@unica.it;
Fax: +0706754456; Tel: +0706754680
bEaStChem and School of Chemistry, University of Edinburgh,
King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, UK
cDipartimento di Chimica Generale ed Inorganica, Chimica Analitica,
Chimica Fisica, Università di Parma, Parco Area delle Scienze 17A,
I43100 Parma, Italy
dDipartimento di Chimica, Università di Milano, Unità di Ricerca
dell’INSTM, via Golgi 19, 20133 Milano, Italy
†Electronic supplementary information (ESI) available: Fig. S1. Solvato-
chromic behaviour of [Pt(Bz2pipdt)(dsit)]; Fig. S2–S5. Electro-
chemical measurements of [Nit(Bz2pipdt)(dsit)]; Table S1. Selected
experimental and calculated bond distances (Å) and angles (°) for [Ni-
(Bz2pipdt)(dsit)], [Pd(Bz2pipdt)(dsit)] and [Pt(Bz2pipdt)(dsit)]. Tables
S2–S4. Spatial plots of relevant MOs of [M(Bz2pipdt)(dsit)] in DMF
(M = Ni, Pd, Pt). Table S5. TD-DFT calculated energies and compo-
sitions of the lowest lying singlet electronic transitions of [M(Bz2pipdt)-
(dsit)] (M = Ni, Pd, Pt) complexes in DMF. Table S6. Comparison of
HOMO–LUMO energy and energy gap between [Pt(Bz2pipdt)(dmit)]
and [Pt(Bz2pipdt)(dsit)]. Table S7. Calculated ground state dipole
moments (μg) and Δμge of [M(Bz2pipdt)(dsit)] as former Table S6.
CCDC 884915 and 884916. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt31537h
All these complexes, which are redox active, show large nega-
tive second-order polarizability. The observed experimental
sequence of μβ0 (×10−48 esu) = −822 (mnt); −1296 (dcbt);
−2011 (dmit) points to the crucial role of the dithiolato ligand in
affecting the NLO properties.7b Theoretical studies helped to
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[Pd(Bz2pipdt)(dsit)] (2). Compound 2 has been prepared as
reported above for the corresponding Ni complex using the fol-
lowing amounts: 70.01 mg of (Bu4N)2[Ni(dsit)2] in 25 cm3
CH3CN and 156.18 mg of [Pd(Bz2pipdt)Cl2] in 25 cm3 of
CH3CN; 71.43 mg of a green solid have been obtained (yield:
82%). Analytical results are in accordance with the formula
[Pd(Bz2pipdt)(dsit)]. Calculated for C21H18N2PdS5Se2: C, 34.88;
H, 2.51; N, 3.87; S, 21.84. Found: C, 35.0; H, 2.6; N, 3.8; S, 21.2.
IR spectra, KBr pellets, wavenumbers (cm−1): 3081 vw, 3058
vw, 3030 vw, 2954 vw, 2941 vw, 2917 vw, 1516 vs, 1450 m,
1435 m, 1365 s, 1342 s, 1303 vw, 1268 m, 1237 vw, 1184 w,
1110 vw, 1056 vs, 1029 vw, 979 vw, 882 vw, 797 vw, 727 m,
696 m, 665 vw, 627 vw, 569 vw, 546 vw, 507 m, 445 m, 422 s.
[Pt(Bz2pipdt)(dsit)] (3). Compound 3 has been prepared as
reported above for the corresponding Ni complex using the fol-
lowing amounts: 112.89 mg of (Bu4N)2[Ni(dsit)2] in 25 cm3
CH3CN and 50.00 mg of [Pt(Bz2pipdt)Cl2] in 25 cm3 of
CH3CN; 62.77 mg of a green solid have been obtained (yield:
89%). Well formed crystals suitable for X-ray crystallography
have been obtained through re-crystallization from DMF–diethyl
ether. Analytical results are in accordance with the formula [Pt-
(Bz2pipdt)(dsit)]. Calculated for C21H18N2PtS5Se2: C, 31.07; H,
2.24; N, 3.45; S, 19.75. Found: C, 30.8; H, 2.1; N, 3.3; S, 18.8.
IR spectra, KBr pellets, wavenumbers (cm−1): 3062 vw,
2920 vw, 1515 vs, 1454 m, 1429 m, 1332 vs, 1307 vw, 1267 w,
1190 vw, 1059 vs, 1028 vw, 977 vw, 734 w, 698 w, 633 w,
511 w, 500 w, 424 w.
Chart 1
explain the high negative second-order polarizability values,
which are relatable to a large difference in dipole moments
between the excited and the ground state enhanced by the elec-
tric field of the solvent and the large oscillator strength for the
charge transfer transition which falls at relatively low energy.
These factors provide the largest second-order polarizability
value to [Pt(Bz2pipdt)(dmit)]. Building on these studies we have
selected dsit = 2-thioxo-1,3-dithiole-4,5-diselenolato where sele-
nium substitutes sulfur in the ligand dmit to further develop the
role of the donor, and report here the synthesis, characterization
and properties of the triad [M(Bz2pipdt)(dsit)], M(II) = Ni (1),
Pd (2), and Pt (3). To the best of our knowledge X-ray diffracto-
metric studies on 1 and 3 provide the first structural data on
d8-metal donor/acceptor systems based on dithione/diselenolato
ligands. Structural data so far available on d8-metal mixed-S/
Se-dichalcogenolene complexes are based on ligands with similar
electron-donating properties (extensive π-electron delocalisation
inside the C2S2MSe2C2 core).8 Structural data on donor/acceptor
systems bearing diselenolato ligands have been previously
reported for nickel and platinum diimine–diselenolato complexes.9
Despite the greater size and polarizability of the Se atom com-
pared with the S one, and the crucial contribution of chalcogen
atoms of the dichalcogenolato to the HOMO, minor changes in
their properties follow the substitution of the chalcogen atoms.
Microanalyses were performed by means of a Carlo Erba
CHNS Elemental Analyzer model EA1108.
Spectroscopic and electrochemical measurements
IR spectra (4000–350 cm−1) were recorded on a Bruker IFS55
FT-IR Spectrometer as KBr pellets. Electronic spectra were
recorded with a Cary 5 spectro-photometer. Cyclic voltammo-
grams 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 the counter-electrode and
Ag/AgCl in saturated KCl solution as the reference electrode.
The experiments were performed at room temperature (25 °C),
in dry and argon-degassed DMF containing 0.1 mol dm−3
Bu4NPF6 as the supporting electrolyte, at 25–200 mV s−1 scan
rate.
Experimental section
Chemicals
Reagents and solvents of Reagent Grade and Spectroscopic
Grade (DMF, CH3CN and CS2) have been used as received from
Aldrich.
Preparation
[Ni(Bz2pipdt)(dsit)] (1). (Bu4N)2[Ni(dsit)2] (50.00 mg) in
25 cm3 of CH3CN (green solution) was added dropwise to a
blue solution of [Ni(Bz2pipdt)Cl2] (94.85 mg) in 25 cm3
CH3CN, under stirring and reflux. After one day a blue solid was
formed (48.44 mg; yield: 85%). This solid was filtered, washed
with diethyl ether and then dried. Well formed crystals suitable
for X-ray crystallography have been obtained through re-crystal-
lization from DMF–diethyl ether. Analytical results are in
accordance with the formula [Ni(Bz2pipdt)(dsit)]. Calculated for
C21H18N2NiS5Se2: C, 37.35; H, 2.69; N, 4.15; S, 23.74. Found:
C, 37.6; H, 2.6; N, 4.1; S, 22.4. IR spectra, KBr pellets, wave-
numbers (cm−1): 3055 vw, 2933 vw, 1509 s, 1447 w, 1431 m,
1342 s, 1307 vw, 1261 m, 1238 vw, 1180 m, 1060 vs, 975 vw,
959 vw, 905 vw, 882 vw, 859 vw, 797 vw, 732 m, 689 m, 662 w,
631 w, 573 w, 503 m, 441 m, 422 w.
Spectroelectrochemistry measurements have been performed
at room temperature by the optically transparent thin layer
electrochemistry technique (OTTLE) in dry and argon-degassed
DMF containing 0.1 mol dm−3 Bu4NPF6 as the supporting elec-
trolyte using a 0.5 mm quartz cell at −0.50 and +0.1 V for the
reduction and oxidation process, respectively. The UV-Vis-NIR
spectra have been recorded with
spectrophotometer.
a
Jasco V-670
NLO measurements
EFISH10a–c experiments have been performed using a freshly
prepared 10−3 M solution in DMF (chosen in order to avoid any
possible aggregation of the chromophores and to guarantee
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enough solubility to ensure the accuracy of the measurement)
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 has been
used to obtain the Maker fringe pattern (harmonic intensity
variation as a function of liquid cell translation). In the EFISH
experiments the incident beam has been synchronized with a
DC field applied to the solution, with 60 and 20 ns pulse dur-
ation 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 have
been determined (assumed to be real because the imaginary part
has been neglected) from the experimental value γEFISH, through
eqn (1):
reciprocal space.11 An absorption correction was applied using
the program SADABS12 with min. and max. transmission
factors of 0.773 and 1.000 (1), 0.601 and 1.000 (3). The struc-
tures were solved by direct methods (SIR200413) and refined on
F2 with full-matrix least squares (SHELXL-9714), using the
Wingx software package.15
For both compounds, non-hydrogen atoms were refined aniso-
tropically and the hydrogen atoms were placed at their calculated
positions. Graphical material was prepared with the Mercury
2.016 and ORTEP3 for Windows17 programs. CCDC 884915
and 884916 contain the supplementary crystallographic data for
this paper.
Computational details
The electronic properties of the [M(Bz2pipdt)(dsit)] complexes
(M = Ni, Pd, Pt) were investigated by means of DFT methods.18
The molecular structures of the complexes were optimized start-
ing from the X-ray experimental geometry of [Pt(Bz2pipdt)-
(dsit)] using no symmetry constraints. The Becke
three-parameter exchange functional with the Lee–Yang–Parr
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, μ is the ground state dipole moment, and
βλ is the projection along the dipole moment direction of the
vectorial component βvec of the tensorial quadratic hyperpolari-
zability working with the incident wavelength λ. The EFISH
experiment has been performed as follows. First the second
order response of the pure solvent, then the second order
response of the chromophore solution, and finally the solvent
again have been recorded. The EFISH values reported in
Table 2 are the average of twelve successive measurements
performed on the same sample, each compared to the previous
and to the following solvent. All experimental EFISH βλk0
values are defined according to the “phenomenological”
convention.10d
Table 1 Summary of X-ray crystallographic data for [Ni(Bz2pipdt)
(dsit)] (1) and [Pt(Bz2pipdt)(dsit)] (3)
[Ni(Bz2pipdt)(dsit)] (1) [Pt(Bz2pipdt)(dsit)] (3)
Empirical formula
Formula weight
Color, habit
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C21H18N2NiS5Se2
675.30
Brown, plate
0.13 × 0.11 × 0.05
Monoclinic
P21/n
9.256(1)
23.119(2)
11.503(1)
90
C21H18N2PtS5Se2
811.68
Brown, block
0.18 × 0.17 × 0.11
Monoclinic
P21/n
9.267(2)
23.189(5)
11.555(3)
90
β (°)
98.741(2)
90
2432.9(4)
4
293(2)
1.844
98.069(4)
90
2459(1)
4
293(2)
2.193
Data collection and structure determination
γ (°)
V (Å3)
A summary of data collection and structure refinement for [Ni-
(Bz2pipdt)(dsit)] (1) and [Pt(Bz2pipdt)(dsit)] (3) is reported in
Table 1. Single crystal data were collected with a Bruker Smart
APEXII area detector diffractometer, 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 recipro-
cal lattice. The intensity data were integrated from several series
of exposures frames (0.3° width) covering the sphere of
Z
T (K)
ρ(calc) (Mg m−3
)
μ (mm−1
θ range (°)
)
4.232
1.76 to 26.16
9.107
1.76 to 27.66
32 475/5703
1.017
0.0426
0.0909
No. of rflcn/unique 29 067/4861
GooF
R1
1.006
0.0419
0.0728
wR2
Table 2 Optical and EFISH* results for [M(Bz2pipdt)(dsit)] complexes. Values for corresponding dmit7 are reported for comparison reasons
Complex [M(Bz2pipdt)(dsit)]
1–3 [M(Bz2pipdt)(dmit)] 1a–3a
λmax a (nm)
εa (×10−3 mol−1 dm3 cm−1
)
μβa,b (10−48 esu)
μβ0 (10−48 esu)
Ni
Pd
Pt
1
1a
2
2a
3
3a
840
858
812
819
803
827
9.2
10.6
6.0
11.3
9.4
−5330
−6100
−4950
−5360
−4400
−1000
−962
−926
−1114
−1146
−1053
−2011
14.8
a In DMF. b Average value of twelve successive measurements, the uncertainty of the measure is between 3 and 5.5%.
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correlation functional (B3LYP)19,20 was employed together with
the 6-311G(d,p) basis set21,22 for the C, H, N, and S atoms. The
Ni, Pd, Pt, and Se atoms were treated with the SDD valence
basis set23–25 and with the MDF10 (Ni), MWB28 (Pd, Se), and
MWB60 (Pt) effective core potentials. An additional d polariz-
ation function was employed for selenium.26 Single point calcu-
lations were performed using the same basis set employed for
the calculation for the metal atoms and Se, whereas C, H, N, and
S atoms were treated with the 6-311G(d,p) basis set.27 To
address DMF solvation effects, the conductor-like screening
model as developed in the polarisable continuum model
(CPCM)28 has been applied with united atom topological model
atomic radii.29 Single point CPCM calculations of the complexes
were performed using gas-phase optimized geometries.
TD-DFT30,31 was used to calculate the energies of the five
lowest singlet electronic transitions. The dipole moment differ-
ence between the excited and ground states (Δμge) was calculated
by using the finite field approach (external field strength of
0.0005 atomic units). All the calculations were performed with
the Gaussian 03 program suite.32 GaussView was used to gener-
ate molecular orbitals.33 AOMix was used for determining
atomic orbital compositions employing Mulliken population
analysis,34 and SWizard35 was used to analyze the electronic
transitions obtained from TD-DFT calculations.
Fig. 1 Ortep drawing of [Pt(Bz2pipdt)(dsit)] (3) with thermal ellip-
soids drawn at the 30% probability level.
Fig. 2 Crystal packing of [Pt(Bz2pipdt)(dsit)] (3). Partial π-stacks are
depicted by dashed bonds.
Results and discussion
Synthesis
CC bond distances involving the dsit ligand are shorter than
those originating from Bz2pipdt, in agreement with the prevail-
ing diselenolato character for dsit and dithione character for
Bz2pipdt. This is also reflected in the C–S distances of the
Bz2pipdt ligands (range 1.673(5)–1.700(6) Å), as they are in
agreement with a dithione structure.5–7,36 The crystal packing of
3 is reported in Fig. 2. The Pt atoms are located between one of
the CH2 groups and an aromatic CH group of two symmetry
related Bz2pipdt ligands. Moreover, the presence of an extensive
net of π-stacks between the aromatic rings of the Bz2pipdt
ligands determines the formation of layers that are parallel to the
ac plane. Those layers interact with each other by means of very
weak π–π interactions of the Bz2pipdt ligands with the minimum
distance occurring between C(101) and C(61)′ atoms (3.83(1) Å,
symmetry code′ = −x; −y; −z).
[M(II)(Bz2pipdt)(dsit)] [M(II) = Ni (1); Pd (2); Pt (3)] have been
obtained by reacting [M(II)(Bz2pipdt)Cl2] with (Bu4N)2[Ni(II)-
(dsit)2]36 following the same procedure used for the preparation
of the corresponding dmit complexes,7a as shown in Scheme 1.
Salts of nickel dianionic complexes provide the required
dianion, which when uncoordinated is less stable, to form the
desired mixed-ligand complexes in high yields (80–90%) with
respect to the [M(Bz2pipdt)Cl2] reagent, accompanied by
Bu4NCl and degradation products containing the nickel cations.
On slow evaporation of the solvent, well formed dark crystals
precipitated.
X-ray structures
Compounds 1 and 3 crystallize in the monoclinic P21/n space
group and are isostructural, for this reason only the molecular
structure of 3 will be described in detail. The molecular drawing
of 3 is depicted in Fig. 1. In 3 two sulfur and two selenium
atoms from the Bz2pipdt and dsit ligands, respectively, define the
coordination environment of the metals, which is in a slightly
distorted square planar geometry. As reported in Table S1,† the
Visible absorption spectroscopy and EFISH data
Complexes 1–3 are characterized by a broad absorption in the
visible region with medium molar absorption coefficients
reported in Table 2, and a negative solvatochromism, as shown
in Fig. S1† as an example for 3. The energy of the solvatochro-
mic peaks 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.38a This solvato-
chromic peak has been assigned for both these classes of
complexes to a mixed-metal-ligand-to-ligand charge transfer
Scheme 1
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(MMLL′CT) HOMO → LUMO transition.6,37,38b EFISH (elec-
tric 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 1064 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 by
applying the equation β0 = βλ[1 − (2λmax/λ)2][1 − (λmax/λ)2].
These values are collected in Table 2.
Electrochemistry
Fig. 4 Spectro-electrochemical experiments on [Pt(Bz2pipdt)(dsit)] in
0.1 M Bu4NPF6–DMF at room temperature (298 K). The potential was
held at −0.5 V vs. Ag/AgCl.
Cyclic voltammetric experiments performed on DMF solutions
of the complex exhibit two reversible reduction waves and one
irreversible oxidation wave as reported in Table 3.
The reduction processes, see CV scans in Fig. 3 as a represen-
tative example for 1 and Fig. S2 and S3,† show reduction values
close to those found for other mixed-ligand complexes bearing
R2pipdt ligands.6–8
The oxidation potential (Ea), which is sensitive to the nature
of the donor dichalcogenolato ligand, falls between those of
dmit7a and of mnt,39 suggesting a similar sequence for the ease
of one electron removal which can be related to the donor
capacity of the ligand.
Electrochemical results are consistent with theoretical results
(see related discussion in next section), which show that the
ligand acting as dithione gives a prevalent contribution to the
LUMO (relatable to reduction processes) and the ligand acting
as dichalcogenolato 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
energetic sequence of the FOs. Moreover, the reversible nature
of the first reduction process, verified by the variable scan rate
cyclic voltammetry (see Fig. S4 and S5†), allowed a spectroelec-
trochemical investigation. The formation of the monoreduced
species (Fig. 4) is accompanied by a change of the electronic
spectrum with depletion of the solvatochromic peak, which can
be restored during the oxidation process. A reduction in intensity
and a small shift to higher energy of the HOMO–LUMO tran-
sition at 827 nm are typical for monoreduced metal-bis-1,2-
dithiolene complexes.4
Table 3 Cyclic voltammetric data. Measured at the Pt electrode in
DMF, 0.1 M Bu4NPF6, scan rate 100 mV s−1 (reference electrode
Ag/AgCl)
[M(Bz2pipdt)-
(dsit)]
Ea (V)a
0 → 1+
E11/2 (V)b
0 ⇄ 1−
E21/2 (V)b
1− ⇄ 2−
Ni
Pd
Pt
+0.86
+0.69
+0.65
−0.39
−0.41
−0.49
−0.84
−0.91
−1.04
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 unity).
These results suggest that these redox active chromophores
can be of interest for redox switching of the nonlinear optical
response40 useful for applications in optical devices although
suitable processing of these complexes41 is required for the
realization of a molecular optical switch.
Theoretical studies
The electronic properties of complexes were investigated by
means of DFT methods in order to evaluate the influence of the
Se atom of dsit in place of the S of the corresponding dmit com-
plexes previously investigated7b and of the metal center (Ni, Pd,
and Pt). The molecular structures of the complexes of formula
[M(Bz2pipdt)(dsit)] were optimized starting from the experimen-
tal geometry of [Pt(Bz2pipdt)(dsit)]. In general there is good
agreement between the DFT-optimized and experimental geome-
tries, even though, for all complexes, the M–S/Se bond distances
are slightly overestimated, see Table S1.† The electronic struc-
tures of the complexes were evaluated both in vacuum and by
Fig. 3 Cyclic voltammograms of 1 recorded at 298 K with different
scan rates in a DMF solution containing 0.1 M Bu4NPF6.
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Table 4 Spatial plots of HOMO and LUMO (isovalue = 0.04) of [M(Bz2pipdt)(dsit)] (M = Ni, Pd, Pt) in the gas-phase and in DMF (B3LYP/6-311G
(d,p)-SDD), together with the contribution of different fragments to these orbitals
Ni
Pd
Pt
E (eV)
E (eV)
E (eV) Fragment composition (%)
Fragment composition (%)
Fragment composition (%)
Ni
Gas-phase
Bz2pipdt
dsit
Pd
Bz2pipdt
dsit
Pt
Bz2pipdt
dsit
Gas-phase
Gas-phase
LUMO
HOMO
HOMO–LUMO gap
−3.66
−4.97
1.31
4.4
3.6
72.5
25.1
23.1
71.4
−3.73
−4.92
1.19
3.5
3.5
74.9
22.9
21.7
73.5
−3.69
−4.99
1.30
6.0
4.2
71.2
26.3
22.8
69.5
DMF-phase
DMF-phase
DMF-phase
LUMO
HOMO
HOMO–LUMO gap
−3.68
−5.34
1.66
2.8
6.4
87.2
13.9
10.0
74.8
−3.75
−5.30
1.55
2.4
5.9
89.1
11.6
8.5
82.5
−3.74
−5.32
1.58
4.3
7.9
85.2
16.8
10.6
75.3
Fig. 5 Percentage contributions of different atoms to frontiers molecular orbitals of [M(Bz2pipdt)(dsit)] (HOMO, below and LUMO, above). CPCM
(DMF) B3LYP/6-311G(d,p)-SDDv.
taking into account the contribution of the solvent (DMF) by
means of the CPCM method. The energies along with the
contour plots of the frontier orbitals of [M(Bz2pipdt)(dsit)] cal-
culated in DMF and evaluated in the gas-phase for the sake of
completeness are reported in Tables 4 and S2–S4.† In all cases
the HOMO and LUMO are both formed from the out-of-plane
antibonding interactions between the metal’s dxz orbital and
ligand based C2X2 (X = S and Se) orbitals (with C–C π and C–
X π* character). As previously found for [Pt(Bz2pipdt)(dithiola-
to)],7b the HOMO is mainly located on the dichalcogenolato
ligand, whereas the LUMO mainly derives from the dithione
Fig. 6 Energy levels of frontier HOMO and LUMO of [M(Bz2pipdt)-
ligand. The contribution of three molecular fragments (metal, dsit,
Bz2pipdt) to the frontier molecular orbitals is depicted in Fig. 5.
The HOMO is at least 75% derived from the dsit ligand, whereas
the dithione ligand contributes at least 85% to the LUMO.
(dsit)] (1–3) complexes (M = Ni, Pd, Pt) calculated in the gas-phase and
in DMF (CPCM).
Furthermore, the degree of localization of the HOMO and
LUMO on the dsit and Bz2pipdt ligands, respectively, varies in
the order Pt < Ni < Pd (see Tables 4 and S2–S4†). In Fig. 6
a comparison between the HOMO–LUMO energy gap calculated
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for the gas phase and in DMF for the three complexes is
reported. It can be appreciated how the presence of a polar
solvent such as DMF imparts considerable stabilization on the
HOMO, whereas the LUMO is only marginally affected. The
influence of the selenium atom in dsit can be evaluated by com-
paring the properties of the frontier orbitals of [Pt(Bz2pipdt)-
(dsit)] and [Pt(Bz2pipdt)(dmit)].7b The fragmental composition
of the HOMO and LUMO orbitals is very similar, but the
HOMO is stabilized by 0.06 eV in [Pt(Bz2pipdt)(dsit)] when
compared to [Pt(Bz2pipdt)(dmit)] (see Table S6†).
This stabilization can be ascribed to the presence of the Se atom
in place of S. The greater Pt–Se bond distances with respect to Pt–
S (∼0.1 Å) can have a stabilizing influence on the HOMO accord-
ing to the fact that this orbital is mainly localized on the dsit/dmit
ligand system and is characterized by π-antibonding character.
According to the shapes of the frontier orbitals, and as found
for strictly related complexes,5–7 it can be inferred that the
HOMO–LUMO transition will have mixed metal–ligand to
metal–ligand character (MMLL′CT). Vertical excitation energies
of the three complexes 1–3 were determined using time-
dependent density functional theory (TD-DFT) in DMF as the
solvent. The first five singlet–singlet excitation energies are
reported in Table S5.† For all complexes, the first transition is
characterized by the greater oscillator strength and is mainly
comprised of a HOMO to LUMO excitation, supporting its
assignment as the MMLL′CT transition. There is a qualitative
agreement between the calculated first singlet–singlet transition
in DMF and the experimental results, even though the calcu-
lation overestimates the λmax by more than 100 nm.
Both these classes of complexes exhibit large negative
second-order polarizabilities, amongst the highest values deter-
mined so far for metal-complexes, and, due to the bleaching/
restoring of the solvatochromic peak for mono-reduction/
oxidation, are potential candidates for redox switchability of the
second-order NLO response, given suitable processing of these
complexes.
It is noteworthy that the employment of the CPCM model
reveals the role of the solvent in defining the electronic proper-
ties of the complexes. In particular, the presence of a polar
solvent stabilizes the HOMO, which is mainly localized on the
diselenolato ligand, whereas it does not affect significantly the
LUMO, which is mainly localized on the dithione, in agreement
with experimental data.
Acknowledgements
Università di Cagliari and the EU Marie Curie programme (for
LP) are acknowledged for financial support.
Notes and references
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ð2Þ
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ðEmax
Þ
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versus Pt-dsit derivatives may explain the observed sequence.
Conclusions and perspectives
New diselenolato–dithione M(II) (M = Ni, Pd, Pt) redox-active,
electrochromic chromophores showing second-order NLO
response have been synthesized and studied experimentally and
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