The influence of thiolate ligands on the luminescence properties of cycloplatinated(II) complexes

Article abstract of DOI:10.1039/c7dt03599c

Complexes [Pt(C^{^}N)(PPh₃)Cl] (C^{^}N = bzq (7,8-benzoquinolinyl, A) and ppy (2-phenylpyridinyl, B)) were reacted with various thiolate ligands to afford complexes [Pt(C^{^}N)(PPh₃)(κ¹-S-SR)], C

Full text of DOI:10.1039/c7dt03599c

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Influence of Thiolate Ligands on Luminescent Properties of
Cycloplatinated(II) Complexes
Mahboubeh Jamshidi,a Mojgan Babaghasabha,b Hamid R. Shahsavarib* and S. Masoud
Nabavizadeha*
aDepartment of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran.
bDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Yousef
Sobouti Blvd., Zanjan 45137ꢀ66731, Iran.
*Email: shahsavari@iasbs.ac.ir. (H.R.S.); nabavizadeh@shirazu.ac.ir. (S.M.N).
Abstract:
Complexes [Pt(C^N)(PPh3)Cl] (C^N = bzq (7,8ꢀbenzoquinolinyl, A) and ppy (2ꢀ
phenylpyridinyl, B)) were reacted with various thiolate ligands to afford complexes
[Pt(C^N)(PPh3)(к1ꢀSꢀSR)], C^N = bzq, R = SPh (thiophenolate, 1a); C^N = ppy, R = SPh (1b);
C^N = bzq, R = Spy (pyridineꢀ2ꢀthiolate, 2a); C^N = ppy, R = Spy (2b); C^N = bzq, R = SpyN
(
pyrimidineꢀ2ꢀthiolate, 3a); C^N = ppy, R = SpyN (3b). The complexes 1-3 were characterized by
NMR spectroscopy, and the solidꢀstate structures of 1a and 2a which determined by Xꢀray
diffraction methods. Replacing chloride ligand with electronꢀrich thiolates change the lowestꢀ
energy singlet and triplet excited states to ones that feature charge transfer from the thiolate
(mixed with some metal character) to the C^N ligand, which was supported by TDꢀDFT
calculations. All complexes are emissive at 298 K in solid state except 2b and 3b, which are only
emissive at 77 K having less rigid structure compared to others. The emission of 1a and 1b
3
originates from a lowꢀenergy excited state of dPt/πSR → π*C^N while 3a exhibits LC/3MLCT
transition. For 1a and 1b, the radiative rate and the quantum efficiency are higher in rigid
environment such as solid compared to polymer and solution. Decreasing the rigidity of
environment leads to flexibility of rotation of the –SR around the axis of the Pt–S bond. So the
geometry can be easily changed after radiation and the lowest lying triplet excited state would
have the effective contribution of dd* transition, which opens a nonradiative pathway at room
temperature.
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Introduction
Thiolates can act as auxiliary ligands in transition metal complexes (especially in
cycloplatinated compounds) and play important roles in the facility of tuning both steric and
electronic properties of these complexes.1ꢀ7 Our recent research revealed that the cyclometalated
platinum complexes including thiolate ligands can be an effective target in photophysical
studies,8ꢀ10 fundamental reactions such as C–S bond coupling,11 C−H reductive elimination12 and
protonation reaction13 or considered as promising anticancer agents for drug development.14
However, despite the wide investigation on synthesis of binuclear cycloplatinated
complexes containing diꢀµꢀthiolate bridges, which introduced as halfꢀlantern compounds12, 15ꢀ20
and bidentate platinum complexes containing dithiolate ligands,21, 22 examples of mononuclear
cyclometalated platinum with a nonꢀbridged thiolate ligand are rare.11, 13, 14 The interest for
synthesis and photophysical investigation of these kind of complexes comes from the potential of
these complexes to efficiently emit in solid state due to the rigidity of some complexes induced
by π⋯π interactions and a short phosphorescence decay time,8ꢀ10 since for many technological
applications, such as OLED and sensors, the luminescent molecules must be embedded in a solid
matrix.23, 24 Coupling of strong σꢀdonating thiolate ligands would make the metal center more
electronꢀrich (according to the electron donor ability of the aromatic ring coordinated to S
atom),11 which boosts the luminescence properties of the complexes by raising of the energy of
the dd* states, thus increasing the population of the thermally activated excited state.25
As part of our ongoing research into the chemistry of platinum thiolate complexes,11ꢀ14
the aim of present work is to introduce a new class of emissive cycloplatinated(II) phosphine
complexes with a nonꢀbridging thiolate ligand. Also, the influence of a thiolate ligand on the
excited states of the cycloplatinated complexes compared to parent complexes, interpretation of
absorption and emission spectra at 298 and 77 K, and the effect of the rigidity of environment are
described in detail. For the better understanding of the relationship between structure and
photophysical properties, we have employed density functional theory (DFT) and timeꢀ
dependent DFT (TDꢀDFT) calculations.
2

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Results and discussion
Synthesis and Structural Characterization of Complexes
The new chemistry to prepare cycloplatinated(II) complexes is illustrated in Scheme 1.
The reaction of platinum precursor complexes [Pt(C^N)(PPh3)Cl], C^N = bzq (7,8ꢀ
benzoquinolinyl, A)26 and ppy (2ꢀphenylpyridinyl, B),27 with one equivalent of in situ freshly
prepared sodium thiolate ligands (Na+SRꢀ), formed by the reaction of appropriate thiol ligands
with sodium metal in ethanol, gave [Pt(C^N)(PPh3)(к1ꢀSꢀSR)], C^N = bzq, R = SPh
(
thiophenolate, 1a); C^N = ppy, R = SPh (1b);13 C^N = bzq, R = Spy (pyridineꢀ2ꢀthiolate, 2a); C^N
= ppy, R = Spy (2b);13 C^N = bzq, R = SpyN (pyrimidineꢀ2ꢀthiolate, 3a); C^N = ppy, R = SpyN
(3b). Complexes 1-3 were deduced from common analytical methods and their formulations
were confirmed by good agreement of elemental analyses with calculated values. The integrity of
these complexes in solution were characterized by (1H and 31P{1H}) means of NMR
spectroscopy, and schematic labeling is given in Scheme 2. The Experimental Section provides
numerical NMR data while the spectra are given in the Supporting Information (Figures S1ꢀS8).
Moreover, the solid state structure of 1a and 2a were determined by singleꢀcrystal Xꢀray
diffraction.
The 1H NMR spectra of 1-3 display signals for only one kind of cyclometalated, PPh3 and
SPh/Spy/SpyN ligands, corresponding to “Pt(C^N)(PPh3)(к1ꢀSꢀSR)” fragments.11, 13, 14 Also, the
31P{1H} NMR spectra of 1-3 show a sharp singlet resonance flanked by platinum satellites,
which confirms the coordination of only one phosphine ligand to the platinum center. The large
value of the coupling constant between platinum and phosphorus (1JPtP = 4296–4376 Hz)
confirms a PPh3 ligand is located in trans position of the nitrogen atom of the cyclometalated
moiety. Moreover, the other probable isomer (PPh3ꢀtransꢀC(sp2)) can be ruled out by regarding
to the transphobia effect.11, 13, 14 This effect has been used to elucidate the stable geometry of
squareꢀplanar d8 transitionꢀmetal complexes. In these thiolate complexes the PPh3 ligand
contains greater trans influence and softer ancillary atom than thiolate ligands is more suitable to
locate in cis position of Pt–C bond of cyclometalating ligands.13, 14
The structures of 1a, and 2a were established by single crystal Xꢀray analysis, and the
structure of 1b has been reported previously.13 Crystallographic data for 1a and 2a are collected
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in Table S1 and selected bond distances and angles are quoted in Table S2. The perspective
drawing of 1a and 2a are demonstrated in Figure 1.
C
N
PPh3
Cl
C
N
PPh3
EtOH/Na
RSH
A
B
bzq
ppy
Pt
Pt
S
R
R
C
N
SPh
Spy
S
S
S
1a Ph
bzq
ppy
1b Ph
2a py
2b py
3a pyN bzq
3b pyN ppy
ppy
bzq
ppy
N
ꢀ
ꢀ
N
N
C
SR
N =
N
N
bzq
SpyN
Scheme 1. The synthetic route for the formation of 1-3.
Single crystal Xꢀray diffraction studies of 1a and 2a revealed monomeric fourꢀcoordinate
distorted square planar structures around the platinum(II) center. The small bite angle of C^N
chelate ligand (80.84(9) for 1a and 80.76(9) for 2a) which caused the distortion from square
planar geometry is comparable to other bzq chelate Pt(II) complexes.28ꢀ31 The distances of the
Pt–S bonds are almost equal in all complexes (~ 2.36 Å). It is notable that the aromatic ring
including the к1ꢀSꢀSR moiety is approximately perpendicular to the metal plane (Figure S9) and
the dihedral angle between the metal plane and the aryl ring of thiolate ligand is 82.88° for 1a,
83.40° for 1b13 and 63.24° for 2a. Consequently, as is shown in Figure S10, the aromatic ring of
the thiolate ligand tends to form intramolecular π⋯π stacking to one of the phenyl groups of
PPh3 ligand and the distance between two centroids are 3.629 (1a), 3.837 (1b)13 and 3.597 Å
(2a). Additionally, intermolecular dimer π⋯π stacking between two bzq groups in 1a and 2a (in
a headꢀtoꢀtail fashion) formed supramolecular packing (Figure S10).
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Figure 1. Representations of the Xꢀray crystal structure of 1a (left) and 2a (right). Ellipsoids are
drawn at the 40% probability level, and hydrogen atoms and a CH2Cl2 solvent molecule in 1a
have been omitted for clarity.
Photophysical properties of complexes
All synthesized complexes are stable in the solid state and solution media. Complexes 1-3
exhibit good solubility in CH2Cl2. Photophysical properties of these complexes and precursor
complexes (A and B) were examined by using their absorption and luminescence spectra in solid
state, fluid solution (CH2Cl2). Complexes 1a and 1b were examined in PMMA (poly(methyl
methacrylate)) and PS (polystyrene) polymers at 298 and 77 K.
Effect of replacement of Cl by SPh
The replacement of the chloride ligand in A or B (with green color) by the thiophenolate
(SPh) has an observable effect on the absorption spectra, producing final products (1a and 1b)
with intense orange color. In Figure 2 (left), the absorption spectrum of 1b in solid state is shown
together with starting chloro complex (B). Compounds 1b and B have the same set of intense
bands in the UV region (λ <345 nm), but some different bands with different molar absorptivity
observed in the visible region, causing different color to be observed. The longestꢀwavelength,
spinꢀallowed broad band in 1b appears at 502 nm, while in the parent complex (B) a wellꢀ
defined band centered around 478 nm is observed. The appearance of the longꢀwavelength band
in this region related to change of chloride ligand with thiolate ligand can be rationalized in term
of a new lowꢀenergy chargeꢀtransfer state as supported by DFT calculation. In both complexes,
the lowest energy excited states arise from HOMO→LUMO transitions. This transition in B has
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1
a mixture character attributed to [dPt/πCl/πppy → π∗ppy] character (Figure 2 (right)). In 1b,
substituting the electronꢀrich sulfur atom leads to increasing HOMO’s energy. This causes a
remarkable red shift in lowest energy band with contributing the thiolate and the metal atom in
HOMO level, in contrast to the parent chloride complex. Meanwhile, the LUMO can be expected
to remain localized largely on the ppy (π∗) ligand. Thus, it can be anticipated the lowꢀenergy
1
1
excited state transition in 1b has an ascribed mixture character [dPt/πSPh → π*ppy]. The LC(ppy)
transition is therefore faded, which is supported by TDꢀDFT calculations (vide infra).9, 13, 32, 33
The same trend can be seen in emission spectra. Complex B displays vibronically
structured bands in the green region (λ= 483 nm) with a luminescence lifetime of 5.7 ꢁs, which is
3
proposed as a mixture of LC/3MLCT emission.34 The emission of 1b is shifted to the red and
shows a broad emission (λmax = 577 nm) with a shorter lifetime of 1.4 μs in the orange region of
the spectrum. This makes clear the profound effect of the substitution of chloride ligand by
3
thiophenolate ligand and changes the character of excited state to [(M+L′)LCT] (dPt/πSPh
→
π*ppy).9 The same data were also obtained for A and 1a in both absorption and emission spectra
(Table S3 and Table 1).
Absorption Spectra
The absorption spectra of 1-3 in CH2Cl2 solution, are shown in Figure 3, and numerical
UVꢀvis data are compiled in Table S3.
In the UVꢀvis spectra of 1a, 2a and 3a (bzq complexes), two intense high energy
absorption bands with large absorption coefficients (ε > 20000 Mꢀ1cmꢀ1) were observed at around
λ ≤ 300 nm (Figure 3a) which are attributed to π−π* intraligand transitions (1IL) located on the
cyclometalated (L = bzq), the SR ligand and PPh3 ligand.35 Electronic transitions with moderate
intensity observed at 348 nm (ε > 6000 Mꢀ1cmꢀ1) in 2a and 3a, refer to n−π* transition in py and
pyN moiety of the thiolate ligands. This assignment is on the basis of the similarity of the
absorption bands of the complexes in shape and extinction coefficient compare with absorption
bands for HSpy in CH2Cl2 solution (at 292 and 375 nm with ε= 14340 and 5430 Mꢀ1cmꢀ1), and
two absorption bands (at 282 and 359 nm with ε= 16060 and 6390 Mꢀ1cmꢀ1) for Na+Spyꢀ in
36
MeOH solution
which refer to π−π* and n−π* transitions, respectively. However, the
absorption spectrum of the thiophenolate complex (1a) was characterized by an absorption band
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located at 303 nm in CH3CN (ε = 13600 Mꢀ1cmꢀl) as a π–π* transition.37 New low energy bands
in 1a, 2a and 3a (λ > 365 nm) in Figure 3a relative to the chloro parent complex (A) show new
1[dPt/πSR → π*bzq] transitions.13 The order of low energy λmax, SPh > Spy > SpyN is an evidence
of contribution of SR ligand in the lowest excited state and is attributed to the effect of the
increasingly electronꢀdonating substituent in destabilizing the HOMO which is supported by TDꢀ
DFT calculations (Table S4 and Table S5). Figure 3b shows that the spectra of 1b, 2b and 3b
(ppy complexes) are similar in shape, but blueꢀshifted compared to the bzq analogous with
decreased absorption coefficients which indicate the existence of a less extended πꢀconjugated
system in ppy complexes.38 Dependency of all excited states on C^N ligands are clearer when we
compare the spectra of 1a and 1b (Figure S11). This dependency on the nature of the C^N
ligands are due to incorporation of C^N ligands in LUMO states (Table S5). Solid state diffuse
reflectance UVꢀvis spectra of the complexes displays a similar pattern to those observed in
solution (Figure S12).
τ = 5.7 ꢀ
s
PPh3
S
PPh3
Cl
Pt
τ = 1.4 ꢀ
s
Pt
N
N
90 nm
502 nm
478 nm
200
300
400
500
600
700
λ / nm
Figure 2. Normalized absorption (dash lines) and emission spectra (solid lines) of B (green
lines) and 1b (orange lines) in solid state at 298 K (left). Frontier orbitals of B and 1b calculated
by DFT in gas phase (right).
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6
5
4
3
2
1
0
(a)
0.3
0.2
0.1
0.0
1a
2a
3a
400
450
500
550
250
300
350
400
450
500
550
Wavelength / nm
6
5
4
3
2
1
0
(b)
1b
2b
3b
0.2
0.1
0.0
400
450
500
550
250
300
350
400
450
500
550
Wavelength / nm
Figure 3. Absorption spectra of (a) 1a, 2a and 3a; (b) 1b, 2b and 3b in CH2Cl2 at 298 K (the
insets show the expansion of the region of the lowꢀenergy bands).
Emission Spectra
Table 1 provides a summary of the emission maxima (λmax), lifetimes (τ), and
photoluminescence quantum yields (ФPL) of complexes. Excitation and emission spectra of 1a,
2a, 3a and 1b at 298 K in the solid state are presented in Figure 4.
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Table 1. Emission data for A, B, 1-3.
b
Comp.
Media
Temp.
298 K
λem [nm]
τem (ꢁs)b ФPL
knr [sꢀ1]
kr= ∅/τ [sꢀ1]
[%]
Solid
Solid
Solid
480, 508, 578
3.96
5.7
3
5
7.6 × 103
6.2 × 104
3.4 × 105
1.3 × 105
2.5 × 104
7.4 × 104
7.6 × 104
2.5 × 105
1.4 × 105
6.2 × 104
1.2 × 106
4.5 × 105
3.6 × 104
6.0 × 105
ꢀ
6.1 × 105
4.5 × 105
7.2 × 103
A
B
298 K 483, 516max, 560sh
298 K
77 K
572
570
1.27
5.86
1.61
13.54
1.45
1.43
6.91
43
79
4
1a
PMMA 298 K
77 K
584
564
≈ 100
11
36
95
PS
298 K
298 K
77 K
583
Solid
577
1b
575, 602sh
PMMA 298 K
565
566
1.70
2.10
3
1.8 × 104
5.7 × 104
5.7 × 105
4.2 × 105
PS
298 K
77 K
12
550
512
513
9.24
0.69
9.50
0.55
11.22
6.00
8.55
98
22
98
3
1.1 × 105
3.2 × 105
1.0 × 105
5.4 × 104
3.3 × 104
ꢀ
2.2 × 103
11.1 × 105
2.1 × 103
16.6 × 105
5.6 × 104
ꢀ
Solid
Solid
298 K
77 K
2a
3a
298 K 474max, 504, 540sh
77 K
77 K
77 K
510max, 534, 575sh
534max, 554
541
37
ꢀ
2b a
3b a
Solid
Solid
ꢀ
ꢀ
ꢀ
aComplexes 2b and 3b are not emissive at 298 K. bLifetime and quantum yield were measured in
neat powder or polymer film.
At ambient temperature (see Figure 4), 2a shows unstructured green emission and 3a
shows vibronic blueꢀgreen emission, whilst, 1a and 1b exhibit broad efficient orange emission
with peak maxima at 577 nm and 572 nm, respectively.
In a series of bzq complexes, 1a, 2a and 3a: a) the emission maxima are redꢀshifted in the
λmax order SpyN (3a, 474 nm) < Spy (2a, 512 nm) < SPh (1a, 572 nm) which are in agreement
with the theoretical wavelengths of the lowest lying triplet excited states (Table S4). b) The long
decay time which is indicative of phosphorescence emission increases from 0.55 ꢁs in 3a (SpyN)
to 0.69 ꢁs in 2a (Spy) and 1.27 ꢁs in 1a (SPh). c) The quantum yields follow the same trend as
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decay times (SPh (43%) > Spy (22%) > SpyN (3%)) which confirm the same trend in radiative
rate constants in these complexes.
1a
2a
3a
1b
300
400
500
600
nm
700
800
Wavelength
/
Figure 4. Normalized excitation (dash lines) and emission (solid lines) spectra of the complexes
in the solid state at 298 K.
The emission spectrum of 1b is approximately identical with that of 1a which proposed a
common excited state for both complexes and higher decay times compare to other complexes.
Unlike literature reports,29, 38 1a with higher π conjugated ligand (bzq) shows 5 nm blue shift
compared to 1b. This small shift (found also in the precursors A and B) suggests the influence of
1
1
intramolecular π⋯π interactions between in which induces red shift on IL and MLCT
transitions in absorption spectra, but blueꢀshift of 3MLCT excited state in the emission spectra.39
It is noteworthy that in less rigid and diluted media such as polymers (where no such interactions
is expected), the opposite trend occurs (see Table 1).
In 3a, the vibronic structured emission band together with the similarity of emission to the
3
emission of parent complex (A) exhibit the phosphorescence from mixture of LC (ligand
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centered, π→π∗ (bzq))/3MLCT (metal to ligand charge transfer, Pt → bzq) and negligible
effective of SpyN ligand in the excited state.
Complexes 2b and 3b are not emissive at room temperature. According to DFT
calculations of optimized geometries in ground (S0) and excited states (T1) shown in Figures 5
and S13, the geometries of 2b and 3b undergo a large distortion from square planar in S0
geometry to distorted tetrahedral geometry in the triplet geometry, (the R group of SR ligands
position parallel to ppy ligand). As a result, the contribution of MC (dd*) character in the lowest
lying excited states (originates from transition between HSOMO and LSOMO) shows a
considerable increase, which opens a nonradiative deactivation pathway at 298 K.
According to experimental results, we assumed that the geometry change is negligible in
other emissive complexes in rigid matrixes due to π⋯π interaction between the R group of SR
and one phenyl group of PPh3 ligand causes less flexibility. Thus, to understand the character of
emission of these complexes in the absence of probable thermally activated pathways, we
restricted the geometry change of these complexes and calculated lowest lying triplet excited
states. The results show that the first triplet exited states originate from transition between
HSOMO and LSOMO where HSOMO and LSOMO have the same character as LUMO and
HOMO, respectively (Figure 6). As supported by TDꢀDFT calculations, a lowꢀenergy excited
state of dPt/πSR → π*C^N character may thus be anticipated.
Upon cooling to 77 K, the decay times increase to 5ꢀ12 ꢁs by decrease of nonradiative
rate constants. For 1a, 1b, and 2a, the emission spectra at 77 K are close to those at 298 K
(Figure 7a). Lack of a clear vibronic structure even at 77 K in these complexes, suggests that the
3
emission is unlikely to be originated from LC transition and a lowꢀenergy excited state of
dPt/πSR → π*C^N character may thus be anticipated.
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Figure 5. DFT optimized S0 (left) and T1 (right) geometries of 2b with corresponding
contribution of different fragments in frontier molecular orbitals obtained in S0 and T1 states (H
atoms are omitted for clarity of presentation).
Figure 6. Frontier orbitals of complexes 1a, 2a, and 1b which are responsible for the first triplet
excited state in gas phase.
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(a)
1a
2a
3a
500
600
700
Wavelength / nm
(b)
1b
2b
3b
500
600
700
Wavelength / nm
Figure 7. Normalized emission spectra of the complexes (a) 1a, 2a and 3a; (b) 1b, 2b and 3b in
the solid state at 77 K.
Compounds 2b and 3b show emission bands at λmax 534 and 541 nm at 77 K, respectively
(Figure 7b), where the thermally activated population of the MC states cannot occur due to the
limitation of geometry change compared to room temperature. All thiolate complexes are not
emissive in solution at 298 K, because the emission properties are crucially determined by the
different molecular rigidities of the compounds. Studied complexes can easily undergo
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geometrical distortions upon excitation such as torsional distortions away from square planarity
to a tetrahedral symmetry in non rigid environment. This can lead to a stabilization of the dd*
state(s) and increasing of their thermal population. However, in a more “rigid” environment,
such distortions that lead to related energy stabilizations of the quenching states are less
distinct.40
Effect of rigidity on emission efficiency
To investigate the influence of rigidity of environment on emission properties of
complexes which effects on the different interactions, measurement of emission of 1a and 1b has
been done in PS and PMMA films (Figure S14). Quantum yield of 1a decreases in the order of
solid (43%) > PS Film (11%) > PMMA Film (4%). The same trend is seen for complex 1b, solid
(36%) > PS (12%) > PMMA (3%). The decrease of ФPL in comparison to solid state in these
complexes is accompanied by a slight increase of the emission lifetime in PS and PMMA
matrixes, respectively. These features afford in the polymer matrixes, a decrease of about one
order of magnitude of the radiative rate constant. Three main factors for interpreting the more
quantum efficiency in a high rigid environment such as solid are listed below: (1) in a rigid
media, complexes benefit from the restriction of the vibrational motions responsible for
nonradiative decay. (2) Additionally, intermolecular π⋯π stacking operating in solid state can
lead to more emissive excited states.23 (3) Besides, intramolecular π⋯π stacking between phenyl
groups of PPh3 and SPh ligands limits flexibility of rotation of the –SPh containing aryl ring
about the axis of the Pt–S bond, which causes geometry change in the excited state as mentioned
before.
Although the emission efficiency for other complexes in polymer films and solution were
negligible, heterocyclic thiol derivatives such as Spy and SpyN molecules have distinct
properties that can be remarkably different from those displayed by the normal aromatic thiols
such as thiophenol.41 So the uncoordinated nitrogen atom in pyridyl or pyrimidin may interact
rapidly and reversibly with available protons in the polymer or solvent and quench the emission.3
14

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Conclusion
The present study extends the information on the structures and optical properties of
cyclometalated platinum(II) complexes containing thiolate ligands. We have reported the
synthesis of cycloplatinated thiolate complexes (1-3) by the salt metathesis reaction on the
corresponding chloro precursor complexes (A and B) with in situ prepared Na+SRꢀ. This reaction
changes the lowestꢀenergy singlet and triplet excited states, to ones that feature charge transfer
from the thiolate (mixed with some metal character) to the C^N ligand. This is supported by TDꢀ
DFT calculations and by the observation that the decreasing energies of the bands correlate with
the electronꢀdonating ability of the thiolate.
All complexes are emissive at ambient temperature in solid state with the exception of 2b
and 3b, which are only emissive at low temperature. Emission spectra of 1a and 1b are
approximately identical and originate from a lowꢀenergy excited state of dPt/πSR → π*C^N.
3
Complex 3a exhibits the phosphorescence from mixture of LC (π→π∗ (bzq))/3MLCT (Pt →
bzq).
Upon excitation studied complexes can easily undergo geometrical distortions from
square planar to a distorted tetrahedral geometry in non rigid environment. This can lead to a
stabilization of the dd* state(s) and increasing the contribution of MC (dd*) character in the
lowest lying excited states increase which opens a nonradiative deactivation pathway at 298 K.
Experimental
General procedures and materials
All the reactions were carried out in the common solvents and all solvents were purified
and dried according to standard procedures.42 The microanalyses were performed using a vario
EL CHNS elemental analyzer and also all the melting point values were measured by a Buchi
31
510. Multinuclear (1H, and P{H}) NMR spectra were recorded on a Bruker Avance DPX 400
MHz spectrometer at 293 K. All chemical shifts are reported in ppm (part per million) relative to
1
their corresponding external standards (SiMe4 for H and 85% H3PO4 for 31P{H}) and all the
coupling constants (J values) are given in Hz. The UVꢀvis absorption spectra were carried out in
a PerkinElmer Lambda 25 spectrophotometer in a cuvette with a 1 cm and/or 1 mm path length.
Diffuse reflectance UVꢀvis (DRUV) data of the pressed powders were recorded on UV/VIS/NIR
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spectrometer Cary 5ꢀE (Varian). Luminescence spectra were measured with a Horiba Jobin Yvon
Fluorolog 3 steadyꢀstate fluorescence spectrometer. For decay time measurements, a PicoQuant
FBꢀ375 pulsed diode laser (λexc = 378 nm, pulse width 100 ps) was applied as the excitation
source. The emission signal was detected with a cooled photomultiplier attached to a FAST
ComTec multichannel scalar card with a time resolution of 250 ps. Absolute measurements of
the photoluminescence quantum yield at ambient temperature under Ar and at 77 K were
performed with a C9920ꢀ02 (Hamamatsu Photonics) system equipped with a Spectralon®
integrating sphere. Polymer films containing about 0.1 weight% of the Pt complexes were
obtained by dissolving the emitter and polymer in dichloromethane and spinꢀcoating the
solutions onto quartz glass substrates. Poly(methyl methacrylate) (PMMA) and polystyrene (PS)
films were measured under continuous flushing with nitrogen. 7,8ꢀBenzoquinoline (Hbzq), 2ꢀ
phenylpyridine (Hppy), thiophenol (HSPh), pyridineꢀ2ꢀthiol (HSpy) and pyrimidineꢀ2ꢀthiol
(HSpyN) were purchased from Aldrich or Acros. Complexes [Pt(bzq)(PPh3)Cl], A,26
[Pt(ppy)(PPh3)Cl], B,27 [Pt(ppy)(PPh3)(к1ꢀSꢀSPh)], 1b,13 [Pt(ppy)(PPh3)(к1ꢀSꢀSpy)], 2b,13 were
prepared as reported in literature. The NMR labeling for the ligands are shown in Scheme 2 for
clarifying the chemical shift assignments. Additional data for A: 1H NMR (400 MHz, CD2Cl2, 20
°C, δ): 10.01 (ddd, 3JHH = 4.7, 4JPH = 1.4, 1H, H2), 8.42 (dd, 3JHH = 8.0, 4JHH = 1.3, 1H, H4), 7.87ꢀ
7.79 (m, 6H, Ho of PPh3), 7.78 (d, 3JHH = 8.7, 1H, H6), 7.70 (ddd, 3JHH = 5.4, 5JPH = 1.5, 1H, H3),
3
7.66 (d, JHH = 8.7, 1H, H7), 7.51ꢀ7.44 (m, 4H, H9 and Hp of PPh3), 7.43ꢀ7.36 (m, 6H, Hm of
PPh3), 6.91 (m, 3JPtH = not resolved, 1H, H11), 6.82 (ddd, 3JHH = 3.0 and 3.1, 5JPH = 0.8, 1H, H10);
31P{1H} NMR (162 MHz, CD2Cl2, 20 °C, δ): 22.51 (s, 1JPtP = 4329, 1P).
[Pt(bzq)(PPh3)(к1-S-SPh)], 1a.
Complex A (100 mg, 0.15 mmol) was added to an ethanolic solution of sodium thiophenol
ligand (NaC6H5S) under inert atmosphere condition [NaC6H5S prepared by dissolving of sodium
(4.6 mg, 0.20 mmol) in 10 mL of absolute ethanol and was treated with thiophenol (15.2 ꢀL,
0.15 mmol)]. Orange solution was formed and after stirring for 5 h at room temperature a
yellowꢀorange solid precipitated which was separated, washed with ethanol (3 × 2 mL) and cold
◦
acetone (2 × 2 mL) and dried yielding complex 1a. Yield: 67 mg, 61%; m.p = 240 C. Anal.
Calcd for C37H28NPPtS (744.74): C, 59.67; H, 3.79; N, 1.88; Found: C, 59.42; H, 3.72; N, 1.92.
3
3
1H NMR (400 MHz, dmsoꢀd6, 20 °C) δ: 10.06 (m, JPtH = not resolved, 1H, H2), 8.67 (d, JHH
=
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3
3
3
8.0, 1H, H4), 7.90 (d, JHH = 8.7, 1H, H6), 7.83 (d, JHH = 8.7, 1H, H7), 7.77 (t, JHH = 6.4, 1H,
H3), 7.73ꢀ7.64 (m, 6H, Ho of PPh3), 7.58 (d, 3JHH = 7.8, 1H, H9), 7.49ꢀ7.41 (m, 3H, Hp of PPh3),
3
3
7.41ꢀ7.32 (m, 6H, Hm of PPh3), 7.18 (d, JHH = 7.7 ,2H, H2′), 6.99 (t, JHH = 7.6, 1H, H10), 6.83
(dd, 3JHH = 7.4, 4JPH = 2.5, 3JPtH = not resolved, 1H, H11), 6.76ꢀ6.69 (m, 2H, H3′), 6.65 (t, 3JHH
7.2, 1H, H4′); 31P{1H} NMR (162 MHz, dmsoꢀd6, 20 °C) δ: 23.02 (s, 1JPtP = 4296, 1P).
=
H3
H3
H4
H2
H4
H2
H6
H7
N
N
H5
H6
Pt
Pt
H7
H9
H9
H11
H8
H10
ppy
bzq
H3'
H4'
H4'
H4'
H3'
H2'
H5'
H6'
H3'
H5'
H4'
N
S
N
S
N
S
H2'
Pt
Pt
Pt
Spy
SpyN
SPh
Scheme 2. Numerical Scheme for 1H NMR assignment.
The following complexes were made similarly using the appropriate precursor complexes
and ligands.
[Pt(bzq)(PPh3)(к1-S-Spy)], 2a.
◦
Yield: 104 mg, 67%; m.p = 256 C. Anal. Calcd for C36H27N2PPtS (745.73): C, 57.98; H, 3.65;
1
N, 3.76; Found: C, 57.94; H, 3.66; N, 3.87. H NMR (400 MHz, dmsoꢀd6, 20 °C) δ: 10.00 (m,
3JPtH = not resolved, 1H, H2), 8.70 (dd, 3JHH = 8.1, 4JHH = 1.2, 1H, H4), 7.92ꢀ7.79 (m, 4H, H6, H7,
H3 and H6′), 7.76ꢀ7.67 (m, 6H, Ho of PPh3), 7.57 (d, 3JHH = 7.8, 1H, H9), 7.46ꢀ7.40 (m, 3H, Hp of
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PPh3), 7.39ꢀ7.31 (m, 6H, Hm of PPh3), 7.01ꢀ7.95 (m, 2H, H10 and H3′), 6.92 (td, 3JHH = 7.6, 4JHH
= 1.9, 1H, H4′), 6.81 (dd, 3JHH = 7.6, 4JPH = 2.4, 3JPtH = 38.1, 1H, H11), 6.58 (ddd, 3JHH = 4.9, 4JHH
= 1.1, 1H, H5′); 31P{1H} NMR (162 MHz, dmsoꢀd6, 20 °C) δ: 22.10 (s, 1JPtP = 4342, 1P).
[Pt(bzq)(PPh3)(к1-S-SpyN)], 3a.
Yield: 64 mg, 58%; m.p = 281 ◦C. Anal. Calcd for C35H26N3PPtS (746.72): C, 56.30; H, 3.51; N,
1
5.63; Found: C, 56.24; H, 3.58; N, 5.56. H NMR (400 MHz, dmsoꢀd6, 20 °C) δ: 10.10 (m, 3JPtH
= not resolved, 1H, H2), 8.72 (d, 3JHH = 7.8, 1H, H4), 7.97 (d, 3JHH = 4.7, 2H, H4′), 7.92ꢀ7.82 (m,
3H, H6, H7, H3), 7.77ꢀ7.68 (m, 6H, Ho of PPh3), 7.56 (d, 3JHH = 8.0, 1H, H9), 7.45ꢀ7.384 (m, 3H,
Hp of PPh3), 7.378ꢀ7.30 (m, 6H, Hm of PPh3), 6.96 (t, 3JHH = 7.5, 1H, H10), 6.81 (ddd, 3JHH = 7.3,
4JPH = 2.8, 4JHH = 0.7, 3JPtH = not resolved, 1H, H11), 6.66 (t, 3JHH = 4.8, 1H, H5′); 31P{1H} NMR
(162 MHz, dmsoꢀd6, 20 °C) δ: 21.48 (s, 1JPtP = 4376, 1P).
[Pt(ppy)(PPh3)(к1-S-SpyN)], 3b.
Yield: 60 mg, 54%; m.p = 263 ◦C. Anal. Calcd for C33H26N3PPtS (722.70): C, 54.84; H, 3.63; N,
5.81; Found: C, 54.91; H, 3.56; N, 5.76. 1H NMR (400 MHz, dmsoꢀd6, 20 °C) δ: 9.83 (m, 3JPtH
=
3
3
3
not resolved, 1H, H2), 8.18 (d, JHH = 7.8, 1H, H5), 8.10 (t, JHH = 7.8, 1H, H4), 7.98 (d, JHH
=
4.7, 2H, H4'), 7.80 (d, 3JHH = 7.8, 1H, H6), 7.73ꢀ7.64 (m, 6H, Ho of PPh3), 7.50ꢀ7.29 (m, 10H, H3,
Hp and Hm of PPh3), 6.96 (t, 3JHH = 7.5, 1H, H7), 6.66 (t, 3JHH = 4.8, 1H, H8), 6.63 (m, 3JPtH = not
3
resolved, 1H, H9), 6.52 (t, JHH = 7.2,1H, H5'); 31P{1H} NMR (162 MHz, dmsoꢀd6, 20 °C) δ:
22.27 (s, 1JPtP = 4364, 1P).
Crystal Structure Determination and Refinement
Diffraction data were collected on a SuperNova Xꢀray diffraction system equipped with a
CuKα radiation source (λ = 1.54184 Å). The data were collected at 123 K using an Oxford
Instruments Cryojet Cooler for 2a or at 294 K for 1a. The structures were solved by Intrinsic
Phasing using the ShelXT (Sheldrick, 2015) structure solution program43 and refined by Least
Squares using version 2014/7 of ShelXL (Sheldrick, 2015).44 All nonꢀhydrogen atoms were
refined anisotropically. Hydrogen atom positions were calculated geometrically and refined
using the riding model. Details of crystal and structure refinement are listed in Table S1.
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Computational methods
The DFT optimized S0 and T1 geometries of all complexes were calculated theoretically
at the B3LYP/6–31G(d) level (LANL2DZ for Pt). TDꢀDFT calculations on the optimized ground
state singlet geometries were performed to gain insight in nature of singlet transitions of the
complexes besides the experimental observations.45 In order to estimate the phosphorescence
wavelengths, TDꢀDFT computations were done on Xꢀray structure of complexes. Percentages
compositions of molecular orbitals and theoretical absorption spectra were plotted using
Chemissian program.46
Electronic supplementary information (ESI) available: NMR spectra, crystallographic and
computational details. Crystallographic data (excluding structure factors) for the structural
analysis has been deposited with the Cambridge Crystallographic Data Centre, No. CCDCꢀ
1547213 (1a), CCDCꢀ1547214 (2a).
Acknowledgments
This work was supported by the Institute for Advanced Studies in Basic Sciences
(IASBS) Research Council, Shiraz University and the Iran National Science Foundation (Grant
no. 95834232). M. Jamshidi gratefully acknowledges Professor Hartmut Yersin for valuable
discussions and guidance and providing facilities during her sabbatical leave at Regensburg
University. She also thanks MSRT for financial support. Thanks, are also due to Mr. A. Biglari,
the operator of Bruker NMR instrument at IASBS, for recording the NMR spectra.
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Table of Contents Entry
Influence of Thiolate Ligands on Luminescent Properties of
Cycloplatinated(II) Complexes
The replacement of chloride ligand in cycloplatinated(II) complexes with various thiolate ligands
leads to a profound change in the nature of lowest-energy singlet and triplet excited states.
τ = 5.7 ꢀs
PPh3
PPh3
Pt
τ = 1.4 ꢀs
Pt
S
Cl
N
N
90 nm
502 nm
478 nm
200
300
400
500
600
700
λ / nm