Ángela Vivancos1, Adrián Jiménez-García1, Delia Bautista2, Pablo González-Herrero1. 1. Departamento de Química Inorgánica, Facultad de Química, Universidad de Murcia, Campus de Espinardo 19, 30100 Murcia, Spain. 2. Área Científica y Técnica de Investigación, Universidad de Murcia, Campus de Espinardo, 21, 30100 Murcia, Spain.
Abstract
The synthesis, electrochemistry, and photophysical properties of a series of bis-cyclometalated Pt(IV) complexes that combine the mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with either 1-phenylpyrazole or 2-arylpyridine (C∧N) are reported. The complexes (OC-6-54)-[PtCl2(C∧N)(trz)] bearing cyclometalating 2-arylpyridines present phosphorescent emissions in the blue to yellow color range, which essentially arise from 3LC(C∧N) states, and reach quantum yields of ca. 0.3 in fluid solutions and almost unity in poly(methyl methacrylate) (PMMA) matrices at 298 K, thus representing a class of strong emitters with tunable properties. A systematic comparison with the homologous C2-symmetrical species (OC-6-33)-[PtCl2(C∧N)2], which contains two equal 2-arylpyridine ligands, shows that the introduction of a trz ligand leads to significantly lower nonradiative decay rates and higher quantum efficiencies. Computational calculations substantiate the effect of the carbene ligand, which raises the energy of dσ* orbitals in these derivatives and results in the higher energies of nonemissive deactivating 3LMCT states. In contrast, the isomers (OC-6-42)-[PtCl2(C∧N)(trz)] are not luminescent because they present a 3LMCT state as the lowest triplet.
The synthesis, electrochemistry, and photophysical properties of a series of bis-cyclometalated Pt(IV) complexes that combine the mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with either 1-phenylpyrazole or 2-arylpyridine (C∧N) are reported. The complexes (OC-6-54)-[PtCl2(C∧N)(trz)] bearing cyclometalating 2-arylpyridines present phosphorescent emissions in the blue to yellow color range, which essentially arise from 3LC(C∧N) states, and reach quantum yields of ca. 0.3 in fluid solutions and almost unity in poly(methyl methacrylate) (PMMA) matrices at 298 K, thus representing a class of strong emitters with tunable properties. A systematic comparison with the homologous C2-symmetrical species (OC-6-33)-[PtCl2(C∧N)2], which contains two equal 2-arylpyridine ligands, shows that the introduction of a trz ligand leads to significantly lower nonradiative decay rates and higher quantum efficiencies. Computational calculations substantiate the effect of the carbene ligand, which raises the energy of dσ* orbitals in these derivatives and results in the higher energies of nonemissive deactivating 3LMCT states. In contrast, the isomers (OC-6-42)-[PtCl2(C∧N)(trz)] are not luminescent because they present a 3LMCT state as the lowest triplet.
Transition-metal complexes featuring long-lived emissive triplet
excited states are at the core of numerous technological, analytical,
biomedical, and synthetic developments, including chemosensing,[1,2] cell imaging,[3] photodynamic therapy,[4] photocatalysis,[5] and
light-emitting materials.[6−8] Over the past decades, most research
in this area has focused on luminescent Ir(III)[8−13] and Pt(II)[14−18] complexes with cyclometalating heteroaromatic ligands because of
the high tunability and adaptability of their excited states, whereas
Pt(IV) complexes have only started to be systematically explored as
strong emitters in recent years.[19−27]In previous contributions, we have shown that several types of
Pt(IV) complexes bearing cyclometalating 2-arylpyridines may exhibit
efficient and long-lived luminescence from essentially ligand-centered
triplet excited states (3LC) that possess a very low metal-to-ligand
charge-transfer (MLCT) admixture.[19,20,22,23,27] These characteristics make them promising candidates for applications
that take advantage of relatively long excited-state lifetimes, such
as sensing, singlet-oxygen sensitization, or photocatalysis. Although
small, the extent of the MLCT contribution to the emissive state has
been observed to fluctuate depending on the coordination environment,
causing variations in the radiative rates.[21,22,28] Thus, shorter Pt–C bonds from metalated
aryls or the presence of suitable π-donor ancillary ligands,
e.g., the fluoride ion, result in occupied dπ orbitals with
higher energies and greater MLCT admixtures, leading to higher radiative
rates.[21] However, a more critical factor
that influences the emission properties of cyclometalated Pt(IV) complexes
is the presence of thermally accessible ligand-to-metal charge-transfer
(LMCT) excited states originating from electronic promotions to dσ*
orbitals, which can provide the effective nonradiative deactivation
of the emissive excited state. Higher-energy LMCT states can be achieved
by introducing strong σ-donor ligands, which usually lead to
lower nonradiative rates and increased emission efficiencies.[23]N-Heterocyclic carbenes (NHCs) have emerged as
very valuable ligands for the design of highly efficient luminescent
complexes of late-transition-metal ions, such as Ir(III),[29,30,39,31−38] Pt(II),[40,41,50,42−49] or Au(III).[51] The beneficial effects
exerted by these ligands can be attributed to their exceptional σ-donor
capabilities,[52,53] which lead to strong ligand–field
splittings and an increased energy of nonemissive excited states that
arise from electronic transitions to dσ* orbitals, which could
otherwise become thermally populated and cause nonradiative deactivation
or even degradation via ligand–metal σ-bond labilization.
Consequently, the use of NHCs brings about improved stabilities and
emission efficiencies, which are particularly important for the development
of blue emitters. Diverse types of NHC ligands have been used to synthesize
luminescent complexes, including chelating dicarbenes (C*∧C*),[54−56] pyridyl-NHCs (N∧C*),[57,58] and cyclometalated aryl-NHCs (C∧C*).[43,50,59] Most incorporate normal Arduengo-type
NHC moieties, whereas the use of mesoionic NHCs is rather infrequent.[46,47,60−62] Although cyclometalating
aryl-NHCs have been demonstrated as chromophoric ligands in homoleptic
Ir(III) emitters,[37−39] mixed-ligand systems have also been developed in
which they act as supporting ligands while other chelating heteroaromatic
ligands, such as arylpyridines[34,63] or bipyridines,[32] are responsible for the emission.We have recently developed a synthetic method that allowed the
preparation of the first examples of Pt(IV) complexes bearing a cyclometalated
aryl-NHC ligand.[64] The reported complexes
combined a mesoionic carbene of the 1,2,3-triazolylidene subclass
(C∧C*) and either a monocyclometalating 2,6-diarylpyridine
or a dicyclometalating 2,6-diarylpyridine (C∧N∧CH or C∧N∧C, respectively)
and were found to display exceptionally intense phosphorescence in
poly(methyl methacrylate) (PMMA) matrices at 298 K, which originated
from a 3LC state involving the C∧N∧CH or C∧N∧C ligand.
In addition, the complex with a monocyclometalating C∧N∧CH ligand showed an intense luminescence in a
fluid solution, which was marginally enhanced compared to that of
similar bis-cyclometalated Pt(IV) complexes containing only C∧N ligands. However, no systematic evidence of the effects
of supporting C∧C* ligands on the emission efficiencies
of Pt(IV) complexes has been gathered so far.In this work, we present a family of bis-cyclometalated Pt(IV)
complexes bearing an aryl-1,2,3-triazolylidene ligand and cyclometalating
C∧N ligands of different energies for the lowest
π–π* transition, which exhibit strong phosphorescent
emissions in fluid solutions and can reach quantum efficiencies of
almost unity in PMMA matrices. Their emission properties are compared
to those of bis-cyclometalated complexes bearing only C∧N ligands with the aim of providing a clear and general demonstration
of the electronic effects of the C∧C* ligand.
Results and Discussion
Synthesis
Scheme shows the synthetic route to the targeted bis-cyclometalated
complexes (OC-6-54)-[PtCl2(C∧N)(trz)] (3a–e), where trz = cyclometalating
4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene
and C∧N = cyclometalating 1-phenylpyrazole (ppz, a), 2-(2,4-difluorophenyl)pyridine (dfppy, b),
2-phenylpyridine (ppy, c), 2-(p-tolyl)pyridine
(tpy, d), or 2-(2-thienyl)pyridine (thpy, e), based on the previously described methodology for (OC-6-54)-[PtCl2(dtpyH)(trz)] (dtpyH = monocyclometalating
2,6-di(p-tolyl)pyridine).[64] The reaction of dichlorido-bridged dimers [Pt2(μ-Cl)2(C∧N)2] (1a–e) with the in situ-generated silver carbene “AgI(trzH)”
led to the selective formation of complexes trans-C,C*-[PtCl(C∧N)(trzH)] (trans-C,C*-2a–e), where the carbene coordinates
in a trans-configuration to the metalated aryl of
the C∧N ligand. The photoisomerization to the corresponding cis-C,C*-2a–e complexes
was achieved by either irradiating acetone solutions of trans-C,C*-2b–e with visible light (λ
= 454 nm) or irradiating a MeCN solution of trans-C,C*-2a with UV light (λ = 310 nm). In contrast to
the related Pt(II) species cis-N,N-[PtCl(C∧N)(N∧CH)], which undergo a photochemical cyclometalation
of the coordinated N∧CH ligand to give a bis-cyclometalated
Pt(IV) hydride,[65] the cyclometalation of
the coordinated trzH ligand did not occur in any of the studied cases.
Scheme 1
(i) Ag2O, (ii)
[Pt2(μ-Cl)2(C∧N)2] (1), (iii) blue LEDs, and (iv) PhICl2.
(i) Ag2O, (ii)
[Pt2(μ-Cl)2(C∧N)2] (1), (iii) blue LEDs, and (iv) PhICl2.The photoisomerization process can be easily followed by 1H NMR spectrometry because the resonance of the proton ortho to the metalated carbon of the C∧N ligand is significantly
shielded in the cis-isomers due to the diamagnetic
current of the triazolylidene ring (e.g., 7.83 vs 6.31 ppm for trans- and cis-C,C*-2d, respectively).
Further confirmation of the isomerization was provided by the X-ray
diffraction study of complex cis-C,C*-2d (Figure ).
Figure 1
Structure of cis-C,C*-2d (thermal
ellipsoids at 50% probability). Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
1.9825(17); Pt–N1, 2.0669(15); Pt–C13, 1.9670(18); Pt–Cl,
2.4091(5); C1–Pt–N1, 81.13(7); and N1–Pt–C13,
174.67(6).
Structure of cis-C,C*-2d (thermal
ellipsoids at 50% probability). Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
1.9825(17); Pt–N1, 2.0669(15); Pt–C13, 1.9670(18); Pt–Cl,
2.4091(5); C1–Pt–N1, 81.13(7); and N1–Pt–C13,
174.67(6).The treatment of cis-C,C*-2a–e with PhICl2 led to the bis-cyclometalated complexes 3a–e, respectively, as the major products
in all cases. However, the trichlorido complex (OC-6-41)-[PtCl3(C∧N)(trzH)] (4a–e) was also formed as a minor product (Scheme ). This result differs
from the reported reactions of the related cis- or trans-N,N-[PtCl(C∧N)(N∧CH)] complexes with PhICl2, which exclusively led to bis-cyclometalated
Pt(IV) complexes.[19,24,66,67] Complexes 3 and 4 were obtained in different molar ratios depending on the C∧N ligand (Table S2). The most favorable
outcome was obtained with the dfppy ligand in a 95:5 ratio (3b:4b), whereas the tpy ligand led to the lowest
molar proportion of the bis-cyclometalated complex in a 60:40 ratio
(3d:4d). The formation of these mixtures
can be attributed to two competing processes that take place from
the pentacoordinate Pt(IV) intermediate arising from the formal addition
of a Cl+ ion to cis-C,C*-2a–e (Scheme ). The electrophilic metalation of the phenyl ring
of the trzH ligand (path A) leads to 3, whereas the coordination
of the Cl– ion released from the PhICl2 reagent in the vacant coordination site produces 4 (path
B). Apparently, the electrophilic metalation is less favored for the
present triazolylidene ligand compared to that for N-coordinated 2-arylpyridines. The fact that derivative 3b was obtained in a higher molar proportion can be explained by the
higher electrophilic character of the metal center in this case because
of the diminished electron-donating ability of the dfppy ligand.
Scheme 2
The isolation of complexes 3 from the above mixtures
was only possible after several recrystallizations, resulting in low
to moderate yields (from 13% for 3d to 50% for 3b). Complexes 4 could not be obtained in pure
forms except for the tpy derivative 4d; nevertheless,
the 1H NMR spectra of enriched fractions allowed us to
unequivocally establish their identities. In the case of the tpy derivative 4d, the considerable shielding of the proton ortho to the metalated tolyl carbon (6.71 ppm, JHPt = 33 Hz) indicates that it is directed toward the triazolylidene
moiety and is affected by its ring current, implying that the mutual
disposition of the tpy and carbene ligands is retained after the oxidative
addition of PhICl2. In view of this configuration, we considered
forcing the metalation of the phenyl group of the carbene ligand in
complexes 4a and 4c–e at a high temperature in the presence of a base. Thus, by heating
mixtures of 3 and 4 at 130 °C in 1,2-dichlorobenzene
in the presence of Na2CO3, complexes 4 produced the corresponding complexes 3, which could
then be isolated in improved yields (38–64%).The 1H NMR spectra of complexes 3 corroborated
the presence of two metalated aryl groups, each of which gives a considerably
shielded resonance flanked by 195Pt satellites arising
from the proton ortho to the metalated carbon, which
is affected by the diamagnetic current of an orthogonal ring. The
crystal structures of 3d and 3e (Figures and 3, respectively) are compatible with the NMR data, further
confirming that the metalated aryls are mutually cis, while the carbene and pyridine moieties are trans to each other.
Figure 2
Thermal ellipsoid representation (50% probability) of the crystal
structure of 3d. Hydrogen atoms and solvent molecules
are omitted. Selected bond distances (Å) and angles (°)
are as follows: Pt–C1, 2.011(2); Pt–N1, 2.0866(19);
Pt–C20, 1.987(2); Pt–C41, 2.019(2); Pt–Cl1, 2.4361(6);
Pt–Cl2, 2.4295(6); C1–Pt–N1, 80.84(8); C20–Pt–N1,
173.32(8); and C20–Pt–C41, 80.80(10).
Figure 3
Thermal ellipsoid representation (50% probability) of the crystal
structure of 3e. Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
1.992(2); Pt–N1, 2.1031(18); Pt–C20, 1.983(2); Pt–C41,
2.024(2); Pt–Cl1, 2.4227(5); Pt–Cl2, 2.4178(6); C1–Pt–N1,
80.16(8); C20–Pt–N1, 172.66(8); and C20–Pt–C41,
80.77(9).
Thermal ellipsoid representation (50% probability) of the crystal
structure of 3d. Hydrogen atoms and solvent molecules
are omitted. Selected bond distances (Å) and angles (°)
are as follows: Pt–C1, 2.011(2); Pt–N1, 2.0866(19);
Pt–C20, 1.987(2); Pt–C41, 2.019(2); Pt–Cl1, 2.4361(6);
Pt–Cl2, 2.4295(6); C1–Pt–N1, 80.84(8); C20–Pt–N1,
173.32(8); and C20–Pt–C41, 80.80(10).Thermal ellipsoid representation (50% probability) of the crystal
structure of 3e. Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
1.992(2); Pt–N1, 2.1031(18); Pt–C20, 1.983(2); Pt–C41,
2.024(2); Pt–Cl1, 2.4227(5); Pt–Cl2, 2.4178(6); C1–Pt–N1,
80.16(8); C20–Pt–N1, 172.66(8); and C20–Pt–C41,
80.77(9).We also attempted the preparation of bis-cyclometalated complexes
with a trans-arrangement of the carbene and aryl
moieties to compare their photophysical properties with those of the
isomeric complexes 3. The reaction of trans-C,C*-2a with PhICl2 afforded a mixture from which
complex (OC-6-42)-[PtCl2(ppz)(trz)] (5a; Scheme ) could be isolated in a 23% yield, while the other products could
not be identified. In the case of trans-C,C*-2d, the same reaction gave a mixture of the desired complex
(OC-6-42)-[PtCl2(tpy)(trz)] (5d) and the trichlorido complex (OC-6-43)-[PtCl3(tpy)(trzH)] (6d) in a ca. 18:82 molar ratio;
the mixture could be separated thanks to their different solubilities
in MeOH, and the complexes were isolated in 11 and 67% yields, respectively.
The 1H NMR spectrum of complex 6d shows that
the protons ortho to either the metalated p-tolyl or the coordinated N atom are not shielded by the
triazolylidene ring, implying that the carbenic carbon is not coplanar
with the tpy ligand. Reasonably, the cationic Pt(IV) intermediate
complex isomerizes to avoid the trans-arrangement
of the tolyl and triazolylidene groups, which explains the low yield
in 5d. A similar result was previously found upon the
oxidation of an analogous complex bearing monocyclometalating 2,6-di(p-tolyl)pyridine with PhICl2.[64]
Scheme 3
The crystal structures of 5a and 5d (Figures and 5, respectively) confirmed the expected ligand arrangement.
In both cases, the Pt–C* bond is nearly 0.1 Å longer than
those in complexes 3 because of the high trans-influence exerted by the metalated aryl ring of the C∧N ligand. Additionally, significantly longer Pt–C1 bond lengths
were found, e.g., 2.044 Å for 5d vs 2.011 Å
for 3d, as a consequence of the higher trans-influence of the triazolylidene ring relative to that of the chlorido
ligand.
Figure 4
Thermal ellipsoid representation (50% probability) of the crystal
structure of 5a. Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
2.050(2); Pt–N1, 2.006(2); Pt–C20, 2.078(2); Pt–C41,
2.025(2); Pt–Cl1, 2.4227(6); Pt–Cl2, 2.3177(6); C1–Pt–N1,
80.09(9); and C20–Pt–C41, 80.71(10).
Figure 5
Thermal ellipsoid representation (50% probability) of the crystal
structure of 5d. Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
2.0444(15); Pt–N1, 2.0363(13); Pt–C28, 2.0832(15); Pt–C12,
2.0285(15); Pt–Cl1, 2.4196(4); Pt–Cl2, 2.3293(4); C1–Pt–N2,
80.79(6); and C12–Pt–C28, 80.43(6).
Thermal ellipsoid representation (50% probability) of the crystal
structure of 5a. Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
2.050(2); Pt–N1, 2.006(2); Pt–C20, 2.078(2); Pt–C41,
2.025(2); Pt–Cl1, 2.4227(6); Pt–Cl2, 2.3177(6); C1–Pt–N1,
80.09(9); and C20–Pt–C41, 80.71(10).Thermal ellipsoid representation (50% probability) of the crystal
structure of 5d. Hydrogen atoms are omitted. Selected
bond distances (Å) and angles (°) are as follows: Pt–C1,
2.0444(15); Pt–N1, 2.0363(13); Pt–C28, 2.0832(15); Pt–C12,
2.0285(15); Pt–Cl1, 2.4196(4); Pt–Cl2, 2.3293(4); C1–Pt–N2,
80.79(6); and C12–Pt–C28, 80.43(6).
Photophysical Properties
The electronic absorption
spectra of 3a–e, 5a,
and 5d were registered in a CH2Cl2 solution at 298 K (Table and Figure ). Structured absorption bands are observed for complexes 3a–e in the 250–360 nm range that can be
ascribed to essentially 1LC transitions involving the cyclometalating
ligands.[20,23,24] The lowest-energy
band resembles those observed for complexes [PtMe(Cl)(C∧N)2];[23] its lowest maximum
shifts from 320 to 354 nm along the sequence 3a → 3e in accordance with the decreasing energies of the lowest
π–π* transition of the C∧N ligands
and can therefore be ascribed to a primarily 1LC(C∧N) excitation. The absorption spectra of 5a and 5d differ from their respective isomeric complexes 3a and 3d mainly in regard to the lowest-energy
feature, which appears to enclose additional absorptions. This is
particularly evident for 5d, whose lowest-energy feature
is significantly red-shifted with respect to that of 3d. We attribute these differences to LMCT transitions on the basis
of TDDFT calculations (see below).
Table 1
Electronic Absorption Data for the
Studied Complexes in a CH2Cl2 Solution (ca.
5 × 10–5 M) at 298 K
Electronic absorption spectra of complexes 3a–e, 5a, and 5d in a CH2Cl2 solution at 298 K.
Electronic absorption spectra of complexes 3a–e, 5a, and 5d in a CH2Cl2 solution at 298 K.The photoluminescence of 3a–e was
examined in a deaerated CH2Cl2 solution and
poly(methyl methacrylate) (PMMA) matrices (2 wt %) at 298 K. Air-equilibrated
samples were also examined to evaluate the luminescence quenching
of these complexes by atmospheric oxygen. The obtained emission data
are compiled in Table , and the spectra in the deaerated CH2Cl2 solution
are shown in Figure . In all cases, the excitation spectra match the corresponding absorption
profiles (Figure S29). The excitation and
emission spectra in PMMA are almost identical to those in CH2Cl2 (Figure S30). In the absence
of molecular oxygen, complexes 3b–e show intense emissions in both media, whereas 3a is
not emissive in CH2Cl2 and only weakly so in
PMMA. In all cases, the bands are vibronically structured, and the
energy of the highest-energy peak correlates with the triplet emission
of the respective N∧CH ligand (ppzH, 378 nm;[68] dfppyH, 424 nm;[69] ppyH, 430 nm;[70] tpyH, 437 nm;[69] and thpyH, 485 nm[70]). Therefore, the C∧N ligand is the chromophoric
one in all cases, while the cyclometalating trz acts as a supporting
ligand. The radiative lifetimes range from tens to hundreds of microseconds,
which is consistent with triplet emissive states of an essentially
ligand-centered character (3LC). The complexes bearing
ppy-based ligands (3b–d) are the
most efficient emitters, with quantum yields around 0.3 in solution
that increase to almost unity in PMMA matrices; the latter are the
highest values ever observed for Pt(IV) complexes. An analysis of
their radiative and nonradiative rate constants (kr and knr, respectively) shows
that the large increases in the quantum yields in PMMA are a consequence
of the inhibition of molecular motion, which leads to dramatic decreases
in the value of knr. The significantly
weaker emission of the ppz derivative 3a can be attributed
to the thermal population of a nonemissive 3LMCT excited
state because of the high energy of the 3LC(ppz) state;
a similar behavior has been reported for the tris-cyclometalated complexes fac-[Ir(ppz)3][37] and fac-[Pt(ppz)3]+.[19] The lower efficiency of the thpy complex 3e compared with those of 3b–d can
be explained by its lower emission energy, which must result in an
increased nonradiative deactivation via a vibrational overlap with
the ground state.
Table 2
Emission Data of Complexes 3a–e
complex
mediuma
λem (nm)b
Φc
τ (μs)d
kr × 10–3 (s–1)e
knr × 10–3 (s–1)f
3a
PMMA
417, 467, 487
0.044
46
0.96
21
3b
CH2Cl2
436, 466, 494 (sh), 554 (sh)
0.27
133
2.0
5.5
PMMA
435, 464, 493
0.97
295
3.3
0.088
CH2Cl2 (air)
436, 465, 495
0.012
4.3
2.8
233
PMMA (air)
436, 465, 494
0.23
88 (48%), 207 (52%)
3c
CH2Cl2
447, 478, 504 (sh)
0.31
110
2.8
6.2
PMMA
446, 478, 504 (sh)
0.93
258
3.6
0.26
CH2Cl2 (air)
448, 478, 503 (sh)
0.009
3.4
2.6
291
PMMA (air)
447, 478, 504 (sh)
0.23
80 (41%), 178 (59%)
3d
CH2Cl2
453, 485, 514 (sh)
0.26
140
1.8
5.3
PMMA
452, 484, 511 (sh)
0.86
319
2.7
0.43
CH2Cl2 (air)
453, 484, 513 (sh)
0.007
2.5
2.8
397
PMMA (air)
453, 484, 513 (sh)
0.20
88 (52%), 213 (48%)
3e
CH2Cl2
513, 530, 552
0.046
45
1.0
21
PMMA
512, 528, 551
0.49
304
1.6
1.7
PMMA (air)
513, 530, 552
0.11
86 (41%), 196 (39%)
Under the exclusion of oxygen, except
where noted.
The most intense peak is italicized.
Quantum yield.
Emission lifetime; relative amplitudes
are given in parentheses for biexponential decays.
Radiative rate constant, kr = Φ/τ.
Nonradiative rate constant, knr = (1 – Φ)/τ.
Figure 7
Emission spectra of complexes 3b–e in deaerated CH2Cl2 solutions at 298 K.
Under the exclusion of oxygen, except
where noted.The most intense peak is italicized.Quantum yield.Emission lifetime; relative amplitudes
are given in parentheses for biexponential decays.Radiative rate constant, kr = Φ/τ.Nonradiative rate constant, knr = (1 – Φ)/τ.Emission spectra of complexes 3b–e in deaerated CH2Cl2 solutions at 298 K.Air-equilibrated samples of 3b–d showed measurable luminescence, with quantum yields around 0.01
in CH2Cl2 and 0.20 in PMMA. However, no emission
could be detected from 3a in any medium, and 3e was emissive only in PMMA in the presence of atmospheric oxygen.
Lifetimes dropped to a few microseconds in CH2Cl2. The calculated kr values in this medium
remained the same order of magnitude as those obtained from deaerated
samples, but knr values increased by two
orders of magnitude as a consequence of oxygen quenching. Much longer
lifetimes were observed for samples in PMMA matrices; however, they
could only be fitted to biexponential decays, which is probably because
of the inhomogeneous oxygen distribution. The present data show that
complexes of this kind could be used for luminescence-based applications
in the presence of atmospheric oxygen as well as for the development
of oxygen sensors.For comparison purposes, the luminescence of C2-symmetrical complexes (OC-6-33)-[PtCl2(C∧N)2] (Chart ) with C∧N = ppz,[19] dfppy,[19] ppy,[66] tpy[19] and thpy[20] was also studied in a deaerated CH2Cl2 solution and PMMA matrices (2 wt %) (Table ). These compounds show moderate
or weak emissions except for the thpy derivative, which was not emissive
in solution, and the ppz derivative, which did not show an emission
in any medium. The observed emission spectra are almost identical
in shape to those of the corresponding complexes 3, although
they are slightly red-shifted (Figures S31 and S32). Single-exponential decays were observed for the ppy-based
derivatives in the CH2Cl2 solution, whereas
double-exponential decays were obtained in PMMA. Where possible, comparisons
with complexes 3 show that lifetimes are significantly
shorter for (OC-6-33)-[PtCl2(C∧N)2]. In all cases, the measured quantum yields are much
lower than those of complexes 3, implying that the replacement
of one of the C∧N ligands by a cyclometalated trz
mainly results in an enhancement of the emission efficiencies. This
beneficial effect is primarily reflected in the knr values, which are generally one order of magnitude
lower for complexes 3 relative to the value of the corresponding
(OC-6-33)-[PtCl2(C∧N)2] complex, whereas variations in the value of kr are much less significant. Reasonably, the stronger
σ-donating ability of the carbene compared with that of the
C∧N ligand pushes the metal dσ*-orbitals to
higher energies in complexes 3, implying both that the
deactivating LMCT states lie at higher energies and that their thermal
population from the emitting state is more difficult than those in
(OC-6-33)-[PtCl2(C∧N)2] complexes, leading to lower nonradiative decay rates.
Chart 1
Table 3
Emission Data of C2-Symmetrical Complexes (OC-6-33)-[PtCl2(C∧N)2]
C∧N
mediuma
λem (nm)b
Φc
τ (μs)d
kr × 10–3 (s–1)e
knr × 10–3 (s–1)f
dfppy
CH2Cl2
440, 470, 506 (sh)
0.058
44
1.3
21
PMMA
439, 469, 500 (sh)
0.21
76.6 (29%), 218 (71%)
ppyg
CH2Cl2
450, 481, 509 (sh)
0.11
27
4.1
33
PMMA
450, 481, 509 (sh)
0.18
57.0 (28%), 157 (72%)
tpy
CH2Cl2
457, 489, 524 (sh)
0.15
40
3.8
21
PMMA
457, 487, 521 (sh)
0.22
84.7 (20%), 208 (80%)
thpy
PMMA
515, 530 (sh), 553
0.054
111
0.5
8.5
Under the exclusion of oxygen.
The most intense peak is italicized.
Quantum yield.
Emission lifetime; relative amplitudes
are given in parentheses for biexponential decays.
Radiative rate constant, kr = Φ/τ.
Nonradiative rate constant, knr = (1 – Φ)/τ.
Data in the CH2Cl2 solution from ref (21).
Under the exclusion of oxygen.The most intense peak is italicized.Quantum yield.Emission lifetime; relative amplitudes
are given in parentheses for biexponential decays.Radiative rate constant, kr = Φ/τ.Nonradiative rate constant, knr = (1 – Φ)/τ.Data in the CH2Cl2 solution from ref (21).In contrast to complexes 3, the isomeric 5a and 5d, where the carbene moiety is trans to the metalated aryl of the C∧N ligand, are not
emissive in either the CH2Cl2 solution or PMMA
films at room temperature. A similar behavior was previously observed
for the homologous unsymmetrical (OC-6-32)-[PtCl2(C∧N)2] complexes (Chart ), which was attributed to a
thermally accessible and nonemissive 3LMCT excited state
that provides an effective nonradiative deactivation pathway.[21]
Electrochemistry
The redox properties of the bis-cyclometalated
complexes 3a–e, 5a,
and 5d were examined by means of cyclic voltammetry in
a MeCN solution. The voltammograms are depicted in Figure , and the potentials of the
most important redox processes and highest occupied and lowest unoccupied
molecular orbital (HOMO and LUMO, respectively) energy estimations
are compiled in Table . An irreversible oxidation peak was observed in the range from 1.71
to 2.03 V vs SCE except for the dfppy derivative 3b,
where the oxidation must fall outside the accessible potential range.
The associated HOMO energies vary according to the sequence 3c < 3a < 3d < 3e and agree with previously determined C∧N-based
π-orbital energies in cyclometalated Pt(IV) complexes.[19,20,27] The isomeric pairs 3a/5a and 3d/5d possess identical HOMO energies, suggesting
that the HOMO is also primarily a π-orbital of the C∧N ligand in complexes 5a and 5d. The first
reduction is irreversible for all complexes and is visible in the
range from −1.54 to −1.69 V vs SCE for 3a–e, whereas for 5a and 5d it appears at distinctively less negative potentials (−1.41
or −1.43 V vs SCE, respectively). The LUMO energies are almost
identical for 3a, 3c, and 3d and somewhat lower for 3b and 3e. Since
the ppz-, ppy-, and tpy-based LUMOs have been previously shown to
have higher energies,[19] it is likely that
the LUMO in derivatives 3a, 3c, and 3d is a π*-orbital of the trz ligand, as predicted by
the DFT calculations for 3d (see below). The LUMO energies
found for 3b and 3e are compatible with
a dfppy- and thpy-based orbital, respectively. In contrast, the significantly
lower LUMO energies found for 5a and 5d imply
that the LUMO is no longer ligand-localized in these complexes. Instead,
it is assigned as a dσ*-orbital on the basis of DFT calculations
(see below). In all cases, the first reduction is followed by a reversible
or quasi-reversible wave in the E1/2 range
from −1.95 to −2.16 V vs SCE, and additional irreversible
reductions were also observed. The reversible wave was observed at
identical E1/2 values for each of the
pairs 3a/5a and 3d/5d, suggesting that the
species produced as a consequence of the first irreversible reduction
is not dependent on the ligand arrangement; however, its identity
cannot be unambiguously established.
Figure 8
Cyclic voltammograms of complexes 3a–e, 5a, and 5d in MeCN at 100 mV
s–1.
Table 4
Electrochemical Dataa and HOMO and LUMO Energy Estimationsb for Complexes 3a–e, 5a, and 5d
complex
Ep,ac
Ep,cd
E1/2e
EHOMO
ELUMO
ΔEHOMO–LUMO
3a
1.98
–1.67, –2.59
–2.15
–6.54
–3.15
3.39
3b
–f
–1.56, –2.25
–1.95
–3.25
3c
2.03
–1.65, –2.30, –2.48
–2.01
–6.89
–3.15
3.74
3d
1.93
–1.69, –2.30, –2.53
–2.06
–6.48
–3.13
3.35
3e
1.71, 1.91
–1.54, –2.27, –2.44
–1.97
–6.32
–3.20
3.12
5a
1.90
–1.41, –2.60
–2.16
–6.50
–3.48
3.02
5d
1.86
–1.43, –2.30, –2.53
–2.06
–6.44
–3.42
3.02
In volts versus SCE, registered
in a 0.1 M solution of (Bu4N)PF6 in dry MeCN
at 100 mV s–1.
In electronvolts.
Irreversible anodic peak potentials.
Irreversible cathodic peak potentials.
For the reversible wave.
Outside the solvent window.
Cyclic voltammograms of complexes 3a–e, 5a, and 5d in MeCN at 100 mV
s–1.In volts versus SCE, registered
in a 0.1 M solution of (Bu4N)PF6 in dry MeCN
at 100 mV s–1.In electronvolts.Irreversible anodic peak potentials.Irreversible cathodic peak potentials.For the reversible wave.Outside the solvent window.
Computational Study
DFT and TDDFT calculations were
performed for complexes 3d, (OC-6-33)-[PtCl2(tpy)2], and 5d (see the Supporting Information for details). Frontier
orbital energies and their main characters are presented in Figure . In the three cases,
the HOMO is essentially a π-orbital of the tpy ligand(s), with
some contribution from metal dπ*-orbitals (4% for 3d, 7% for (OC-6-33)-[PtCl2(tpy)2], and 3% for 5d). The LUMO and LUMO + 1 in 3d are π-orbitals of the trz and tpy ligands, respectively, and
those in (OC-6-33)-[PtCl2(tpy)2] correspond to π*-orbitals delocalized over the two tpy ligands.
In contrast, the LUMO in 5d is essentially a dσ*-orbital
that is mostly distributed along the N–Pt–Cl axis and
lies at a noticeably lower energy with respect to the trz-based LUMO
of 3d, which agrees with the electrochemical results.
Notably, the lowest molecular orbital with a primarily dσ* character
in complexes 3d and (OC-6-33)-[PtCl2(tpy)2] is LUMO + 2, which has a significantly
higher energy for the trz complex, implying that a major effect of
the carbene is to increase the ligand-field splitting.
Figure 9
Orbital energy diagrams from DFT calculations for complexes 3d, (OC-6-33)-[PtCl2(tpy)2], and 5d.
Orbital energy diagrams from DFT calculations for complexes 3d, (OC-6-33)-[PtCl2(tpy)2], and 5d.The TDDFT calculations reveal a ligand-to-ligand charge transfer
(LLCT) from the tpy ligand to the trz ligand, π(tpy)−π*(trz),
and a LC transition within the tpy ligand, π(tpy)−π*(tpy),
as the lowest singlet excitations in complex 3d (S1 and S2, respectively), whereas in (OC-6-33)-[PtCl2(tpy)2] only LC(tpy) excitations
are predicted to contribute to the lowest-energy absorptions. The
three lowest singlet excitations in complex 5d are predicted
to be weak and involve transitions from π(tpy), π(trz),
or p(Cl) orbitals to dσ*-orbitals that can hence be designated
as LMCT or LMCT/XMCT; a more intense excitation of primarily LC(tpy)
character is predicted at a higher energy (S4). Therefore,
the presence of low-energy LMCT absorptions explains the red-shifted
lowest-energy feature in the absorption spectrum of 5d.The lowest triplet excitation energies are represented in Figure . The first triplet
(T1) corresponds to an essentially LC(tpy) transition in
complexes 3d and (OC-6-33)-[PtCl2(tpy)2], with a somewhat lower energy for the latter
complex that is in agreement with the observed variation in emission
energies. The lowest 3LMCT excitations are T4 in 3d and T3 in (OC-6-33)-[PtCl2(tpy)2]; the energy difference with respect to
T1 is higher for 3d (0.78 eV) than for (OC-6-33)-[PtCl2(tpy)2] (0.64 eV),
implying that, consistent with the observed lower k values, the thermal population from
the emitting state should be less favorable for the carbene complex.
In the case of 5d, the first triplet excitation is a
LMCT transition, which explains the lack of emission of this complex.
Figure 10
Lowest triplet excitation energies from TDDFT calculations at the
ground-state geometry. The red lines correspond to essentially LMCT
excitations.
Lowest triplet excitation energies from TDDFT calculations at the
ground-state geometry. The red lines correspond to essentially LMCT
excitations.For further insight, a geometry optimization of the lowest triplet
excited state (T1) was carried out for the three studied
complexes. The corresponding spin density distribution (Figure ) matches the topology
of a π–π* excitation within the tpy ligand in 3d or one of the tpy ligands in (OC-6-33)-[PtCl2(tpy)2], which is consistent with an essentially 3LC(tpy) emitting state in these complexes, and the associated
geometry variations relative to the ground state are mostly limited
to the affected ligand (Table S13). The
computed adiabatic energy differences with respect to the ground state
are 2.78 eV for 3d and 2.75 eV for (OC-6-33)-[PtCl2(tpy)2], which are a good match
with the observed emission energies. The natural spin densities on
the Pt atom are 0.0187 for 3d and 0.0221 for (OC-6-33)-[PtCl2(tpy)2], indicating
a small degree of metal orbital contribution and therefore a certain
MLCT admixture in the essentially LC emitting state[21] that is slightly higher for (OC-6-33)-[PtCl2(tpy)2]. This fact is consistent with both the
increased metal orbital contribution to the HOMO in the latter complex
and its lower emission energy that imply a higher energy of metal
dπ orbitals, probably because the arylpyridine is a weaker π-acceptor
than the cyclometalated trz.[52] In the case
of 5d, the spin density distribution in the relaxed T1 state clearly corresponds to a LMCT state involving an electronic
transition to a dσ* orbital, which causes severe geometry distortions
that mostly result from Pt–ligand bond elongations (Table S14).
Figure 11
Spin-density distributions (0.001 e bohr–3) of
the optimized lowest triplet excited states of 3d, (OC-6-33)-[PtCl2(tpy)2], and 5d.
Spin-density distributions (0.001 e bohr–3) of
the optimized lowest triplet excited states of 3d, (OC-6-33)-[PtCl2(tpy)2], and 5d.
Conclusions
Mixed-ligand Pt(IV) derivatives containing cyclometalating trz
and C∧N ligands of different energies for the lowest
π–π* transition have been synthesized by the oxidative
addition of PhICl2 to Pt(II) precursors of the type cis- or trans-C,C*-[PtCl(C∧N)(trzH)]. The electrophilic metalation of the pendant phenyl group of the trz ligand upon oxidation
proved to be more difficult in comparison to analogous reactions involving
2-arylpyridines and compete with the coordination of a chlorido ligand.
The complexes (OC-6-54)-[PtCl2(C∧N)(trz)] that contain cyclometalating 2-arylpyridines (3b–e) exhibit strong phosphorescent emissions that
originate from 3LC states primarily localized on the C∧N ligand, which can reach quantum yields of ca. 0.3
in a fluid solution and almost unity in PMMA matrices; the latter
are the highest efficiencies ever observed for Pt(IV) complexes. Therefore,
they constitute a class of strongly emissive compounds whose emission
energies can be tuned by varying the C∧N ligand.
A comparison between the photophysical properties of 3 and those of the homologous C2-symmetrical
complexes (OC-6-33)-[PtCl2(C∧N)2] showed that the replacement of one of the C∧N ligands for trz results in lower nonradiative decay rates and higher
quantum efficiencies. The computational results substantiate a higher
energy of dσ* orbitals and deactivating 3LMCT states
in complexes 3, which are attributed to the strong σ-donor
character of the trz ligand. In contrast, the isomeric complexes (OC-6-42)-[PtCl2(C∧N)(trz)]
(5), featuring a trans arrangement of
the carbene and aryl moieties, are not emissive because they present
a 3LMCT state as the lowest triplet, which involves a low-lying
dσ* orbital along the N–Pt–Cl axis.
Experimental Section
General Considerations
Unless otherwise noted, procedures
were performed at room temperature under atmospheric conditions using
synthesis-grade solvents. Reactions involving silver reagents were
conducted under a N2 atmosphere in the dark. The dichlorido-bridged
dimers 1a(71) and 1b–e,[28] PhICl2,[72] and the triazolium iodide salt[73] were synthesized following published procedures.
NMR spectra were registered on Bruker Advance 300, 400, or 600 MHz
spectrometers. Chemical shifts (δ) are given in parts per million
downfield from tetramethylsilane. Elemental analyses were determined
using a LECO CHNS-932 microanalyzer. The irradiation of trans-C,C*-2a was carried out using a 36 W Philips UVB Narrowband
lamp centered at 310 nm. Complexes trans-C,C*-2b–e were irradiated with Blue LEDs following
the previously described experimental setup.[74]
General Procedure for the Synthesis of trans-C,C*-[PtCl(C∧N)(trzH)] (trans-C,C*-2)
The triazolium salt (100 mg, 0.29 mmol)
and Ag2O (37 mg, 0.16 mmol) were suspended in CH2Cl2 (10 mL), and the mixture was stirred for 14 h. The
suspension was filtered through Celite, and [Pt2(μ-Cl)2(C∧N)2] (1) (0.15
mmol) was immediately added to the filtrate. The mixture was stirred
in the dark for 1 h and filtered through Celite. The filtrate was
evaporated to dryness, and the residue was washed with Et2O (3 × 5 mL) and vacuum-dried to give trans-C,C*-2.
General Procedure for the Synthesis of cis-C,C*-[PtCl(C∧N)(trzH)] (cis-C,C*-2)
A deaerated solution of trans-C,C*-2 in acetone (20 mL) was irradiated with blue LEDs for
16–72 h under a N2 atmosphere. The solvent was evaporated
to dryness, and the residue was washed with Et2O (3 ×
3 mL) and vacuum-dried to give cis-C,C*-2. For the ppz-derivative 2a, MeCN (5 mL) and UV light
were used.
General Procedure for the Synthesis of (OC-6-54)-[PtCl2(C∧N)(trz)] (3)
To
a solution of cis-C,C*-2 in CH2Cl2 (5 mL) was added PhICl2, and the mixture was stirred
for 30 min. Partial evaporation under reduced pressure (2 mL) and
the addition of Et2O (20 mL) led to the precipitation of
a pale-yellow solid, which was filtered off and vacuum dried. The
obtained product was placed in a Carius tube with 1,2-dichlorobenzene
(1 mL) and Na2CO3 (30 mg), and the suspension
was heated at 130 °C for 16 h under a N2 atmosphere.
After cooling to room temperature, Et2O (10 mL) was added,
and the resulting suspension was filtered. The collected solid was
extracted with CH2Cl2 (5 × 5 mL). The partial
evaporation of the resulting solution under reduced pressure (2 mL)
and the addition of Et2O (10 mL) led to the precipitation
of a solid, which was filtered off and vacuum dried to give 3.
To a solution of cis-C,C*-2d (73 mg, 0.098 mmol) in CH2Cl2 (5
mL) was added PhICl2 (30 mg, 0.109 mmol), and the mixture
was stirred at room temperature for 30 min. The mixture was concentrated
under reduced pressure (2 mL). To the mixture was added Et2O (20 mL), whereupon a pale-yellow precipitate formed that was filtered
off. Extraction with MeOH (2 × 1 mL) and evaporation to dryness
led to an analytically pure sample of 4d. 1H NMR (400 MHz, CD2Cl2): δ 9.72–9.67
(1H, Harom), 7.95–7.80 (4H, Harom), 7.64–7.59
(1H, Harom), 7.53–7.46 (1H, Harom), 7.43–7.32
(3H, Harom), 7.07–7.02 (1H, Harom), 6.71
(JPtH = 33 Hz, 1H, Harom),
4.26 (3H, NCH3), 3.36–3.27 (1H, CH2),
2.82–2.72 (1H, CH2), 2.37 (3H, CH3),
2.14–1.89 (2H, CH2), 1.76–1.60 (1H, CH2), 1.40–1.30 (1H, CH2), 0.95–0.85
(3H, CH3) (signal multiplicity could not be determined
for this complex because its very low solubility resulted in a poorly
resolved spectrum). Anal. Calcd for C25H27Cl3N4Pt·CH2Cl2: C, 40.56;
H, 3.80; N, 7.28. Found: C, 40.66; H, 3.79; N, 7.37.
(OC-6-42)-[PtCl2(ppz)(trz)] (5a)
To a solution of trans-C,C*-2a (100 mg, 0.170 mmol) in CH2Cl2 (10 mL) was added PhICl2 (47 mg, 0.170 mmol), and the
mixture was stirred for 30 min. The partial evaporation of the solvent
under reduced pressure (2 mL) and the addition of Et2O
(20 mL) led to the precipitation of a white solid, which was filtered
off, washed with MeOH (3 × 1 mL) and Et2O (2 ×
3 mL), and vacuum dried to give 5a. Yield: 25 mg, 23%. 1H NMR (600 MHz, CD2Cl2): δ 8.17
(dd, JHH = 7.5, 1.2 Hz, 1H, Harom), 8.06 (d, JHH = 2.9 Hz, 1H, Harom), 7.65 (dd, JHH = 7.9, 1.5 Hz, 1H, Harom), 7.44–7.36 (m, 3H, Harom), 7.11–7.05
(m, 2H, Harom), 6.87 (td, JHH = 7.6, 1.5 Hz, 1H, Harom), 6.47 (t, JHH = 2.7 Hz, 1H, Harom), 6.39 (dd with satellites, JHH = 7.9, 1.2 Hz, JPtH = 43 Hz, 1H, Harom), 4.25 (s, 3H, NCH3), 3.45
(ddd, JHH = 14.3, 9.9, 6.1 Hz, 1H, CH2), 3.34 (ddd, JHH = 14.3, 9.9,
6.0 Hz, 1H, CH2), 1.85–1.76 (m, 2H, CH2), 1.56–1.50 (m, 2H, CH2), 0.99 (t, JHH = 7.4 Hz, 3H, CH3). 13C APT NMR
(151 MHz, CD2Cl2): δ 150.8 (C), 149.2
(C), 142.4 (C), 142.1 (C), 141.3 (C), 138.7 (JCPt = 45 Hz, CH), 134.3 (CH), 132.6 (CH), 130.1 (JCPt = 46 Hz, CH), 128.4 (JCPt = 24 Hz, CH), 127.8 (JCPt = 19 Hz, CH),
126.7 (CH), 126.1 (CH), 122.6 (C), 116.4 (JCPt = 25 Hz, CH), 112.7 (CH), 109.0 (JCPt = 25 Hz, CH), 37.0 (NCH3), 32.5 (CH2), 23.9
(CH2), 23.1 (CH2), 14.2 (CH3). Anal.
Calcd for C22H23Cl2N5Pt·0.25CH2Cl2: C, 41.45; H, 3.67; N, 10.86. Found: C, 41.39;
H, 3.91; N, 11.04.
(OC-6-42)-[PtCl2(tpy)(trz)] (5d) and (OC-6-43)-[PtCl3(tpy)(trzH)]
(6d)
To a solution of trans-C,C*-2d (160 mg, 0.261 mmol) in CH2Cl2 (15 mL) was added PhICl2 (79 mg, 0.287 mmol), and the
mixture was stirred for 30 min. The partial evaporation of the solvent
under reduced pressure (2 mL) and the addition of Et2O
(20 mL) led to the precipitation of a pale-yellow solid, which was
collected by filtration. The 1H NMR spectrum of this material
revealed a mixture of complexes 5d and 6d in a 1:4 molar ratio, which was stirred in MeOH (20 mL) for 10 min
to give a suspension. The insoluble white solid was collected by filtration,
washed with Et2O (2 × 3 mL), and vacuum-dried to give 5d. The filtrate was evaporated to dryness, and the pale-yellow
residue was washed with Et2O (3 × 3 mL) and vacuum-dried
to give 6d.
Authors: Jaesang Lee; Hsiao-Fan Chen; Thilini Batagoda; Caleb Coburn; Peter I Djurovich; Mark E Thompson; Stephen R Forrest Journal: Nat Mater Date: 2015-10-19 Impact factor: 43.841
Authors: Yanouk Cudré; Felipe Franco de Carvalho; Gregory R Burgess; Louise Male; Simon J A Pope; Ivano Tavernelli; Etienne Baranoff Journal: Inorg Chem Date: 2017-09-18 Impact factor: 5.165
Authors: David C Powers; Diego Benitez; Ekaterina Tkatchouk; William A Goddard; Tobias Ritter Journal: J Am Chem Soc Date: 2010-10-13 Impact factor: 15.419
Authors: Pit J Boden; Patrick Di Martino-Fumo; Tobias Bens; Sophie T Steiger; Daniel Marhöfer; Gereon Niedner-Schatteburg; Biprajit Sarkar Journal: Chemistry Date: 2022-07-13 Impact factor: 5.020
Scheme 1
Figure 1
Scheme 2
Figure 2
Figure 3
Scheme 3
Figure 4
Figure 5
Figure 6
Figure 7
Chart 1
Figure 8
Figure 9
Figure 10
Figure 11