Synthesis and characterization of luminescent cyclometalated platinum(II) complexes of 1,3-bis-hetero-azolylbenzenes with tunable color for applications in organic light-emitting devices through extension of π conjugation by variation of the heteroatom

Article abstract of DOI:10.1002/chem.201301586

A series of luminescent cyclometalated platinum(II) complexes of N^C^N ligands [N^C^N=2,6-bis(benzoxazol-2'-yl)benzene (bzoxb), 2,6-bis(benzothiazol- 2'-yl)benzene (bzthb), and 2,6-bis(N-alkylnaphthoimidazol-2'-yl)benzene (naphimb)] has been synthesized and characterized. Two of the platinum(II) complexes have been structurally characterized by X-ray crystallography. Their electrochemical, electronic absorption, and luminescence properties have been investigated. In dichloromethane solution at room temperature, the cyclometalated N^C^N platinum(II) complexes exhibited rich luminescence with well-resolved vibronic-structured emission bands. The emission energies of the complexes are found to be closely related to the electronic properties of the N^C^N li Copyright

Full text of DOI:10.1002/chem.201301586

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DOI: 10.1002/chem.201301586
Synthesis and Characterization of Luminescent Cyclometalated Platinum(II)
Complexes of 1,3-Bis-Hetero-Azolylbenzenes with Tunable Color for
Applications in Organic Light-Emitting Devices through Extension of
p Conjugation by Variation of the Heteroatom
Alan Kwun-Wa Chan, Elizabeth Suk-Hang Lam, Anthony Yiu-Yan Tam,
Daniel Ping-Kuen Tsang, Wai Han Lam,* Mei-Yee Chan, Wing-Tak Wong, and
Vivian Wing-Wah Yam*[a]
Abstract: A series of luminescent cy-
clometalated platinum(II) complexes
of N^C^N ligands [N^C^N=2,6-bis-
ed. In dichloromethane solution at
room temperature, the cyclometalated
N^C^N platinum(II) complexes exhib-
ited rich luminescence with well-re-
solved vibronic-structured emission
bands. The emission energies of the
complexes are found to be closely re-
lated to the electronic properties of the
N^C^N ligands. By varying the elec-
tronic properties of the cyclometalated
ligands, a fine-tuning of the emission
energies can be achieved, as supported
by computational studies. Multilayer
organic light-emitting devices have
been prepared by utilizing two of these
platinum(II) complexes as phosphores-
cent dopants, in which a saturated
yellow emission with Commission In-
ternational de I’Eclairage coordinates
of (0.50, 0.49) was achieved.
ACHTUNGTRENNUNG(benzoxazol-2’-yl)benzene (bzoxb), 2,6-
bis(benzothiazol-2’-yl)benzene (bzthb),
and 2,6-bis(N-alkylnaphthoimidazol-2’-
yl)benzene (naphimb)] has been syn-
thesized and characterized. Two of the
platinum(II) complexes have been
structurally characterized by X-ray
crystallography. Their electrochemical,
electronic absorption, and lumines-
cence properties have been investigat-
Keywords: cyclometalation · densi-
ty functional theory calculation · lu-
minescence · organic light-emitting
diodes · platinum
Introduction
of these complexes exhibit solid-state polymorphism and in-
teresting photophysical properties.
Luminescent transition-metal complexes have attracted
much attention in recent years due to their capability to
show strong phosphorescence arising from the introduction
of heavy metal atoms to increase the spin–orbit coupling,
and thereby facilitate an efficient intersystem crossing to
harvest both the singlet and triplet excitons. Luminescent d8
platinum(II) complexes are one of the important classes of
metal complexes that have been extensively explored due to
their rich photophysical properties.[1–5] The d8 platinum(II)
complexes adopt a square-planar geometry that can accom-
modate a variety of bidentate[1,2] or tridentate[3] pyridyl ni-
trogen donor ligands. For example, terpyridines and 2,6-bis-
Apart from the pyridyl nitrogen donor ligands, there are
other interesting classes of cyclometalated pincer ligands.
Platinum(II) complexes containing tridentate cyclometalat-
ed pincer ligands (both in N^C^N and C^N^N coordination
modes) have been demonstrated to display a variety of
emissive excited states, including metal-to-ligand charge
transfer (MLCT), intraACTHNUTRGENNUlG iACHUTTGNRNENUGgand (IL) (p–p*), excimeric, and
metal-metal-to-ligand charge-transfer (MMLCT) excited
states.[5] The relative energy of these excited states is strong-
ly affected by subtle changes in the local environment in
which the platinum(II) complex is located. Constable and
coworkers first reported the coordination of 6-phenyl-2,2’-
bipyridine to a platinum(II) metal center as tridentate
C^N^N ligand.[6] The luminescent properties of this system
were further explored by Che and coworkers.[7a] A compre-
ACHTUNGTRENNUNG
(benzimidazol-2-yl)pyridines[4] (bzimpy) have been coordi-
nated to the platinum(II) metal center as tridentate N-
donor ligands and have been extensively explored;[4c–e] some
hensive investigation of
a cyclometalated platinum(II)
C^N^N systems was conducted with the use of strong-field
ancillary ligands such as phosphine[7b,c] and alkynyl moie-
ty,[7d] with the extension of p conjugation of the cyclometa-
lated ligand[7e] to enhance the luminescence.
On the other hand, cyclometalated platinum(II) N^C^N
complexes of 1,3-dipyridylbenzene have been reported by
Williams and coworkers and have been shown to display
strong phosphorescence and electrophosphorescence when
compared to the C^N^N system.[8a] Such platinum(II)
[a] A. K.-W. Chan, E. S.-H. Lam, Dr. A. Y.-Y. Tam, D. P.-K. Tsang,
Dr. W. H. Lam, Dr. M.-Y. Chan, Prof. Dr. W.-T. Wong,
Prof. Dr. V. W.-W. Yam
Department of Chemistry, The University of Hong Kong
Pokfulam Road (Hong Kong)
Fax : (+852)2857-1586
E-mail: chsue@hku.hk
wwyam@hku.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301586.
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FULL PAPER
N^C^N system displayed strong green phosphorescence in
CH2Cl2 solution at room temperature with a photolumines-
cence quantum yield (fem) of 0.4 to 0.6, in which the emis-
sion bands were assigned to be originated from the 3IL
spectively, and isophthaloyl dichloride, whereas the naphimb
N^C^N ligands were synthesized by the reaction between
2,3-diaminonaphthalene and isophthalaldehyde in the pres-
ence of sodium metabisulphite as the oxidizing agent, fol-
lowed by alkylation. However, the ring condensation reac-
tion between 2-aminophenol and isophthaloyl dichloride re-
quired the use of methanesulfonic acid to activate the amide
intermediate to give the desired product. The corresponding
cyclometalated chloroplatinum(II) complexes were synthe-
sized by the reaction with K2PtCl4 in glacial acetic acid,
which was heated to reflux, to give yellow to orange crystals.
The 9-(prop-2-ynyl)-9H-carbazole has also been successfully
incorporated into the platinum(II) center by reacting with
the corresponding cyclometalated chloroplatinum(II) com-
plexes in the presence of sodium hydroxide in a mixture of
methanol and dichloromethane (1:1 v/v) to yield the desired
platinum(II) complexes. The identities of all the newly syn-
thesized cyclometalated platinum(II) complexes have been
A
ACHTUNGTRENNUNG
(N^C^N)] excited state.[8a] Extension of the work has
been done by the same group by utilizing different N^C^N
ligands to optimize the photoluminescence quantum yield of
the platinum(II) system.[8b–d] A class of pincer platinum(II)
complexes of 5-methyl-1,3-bis(benzothiazol-2-yl)benzene
(bzthb) that shows yellow phosphorescence in solution has
also been reported.[9] By replacing the pyridine rings with
azaindoles in the N^C^N ligand, the complexes only emit in
low-temperature glass in spite of the relief of angle strain.[10]
Recently, our group has reported a new class of lumines-
cent cyclometalated platinum(II) complexes with 2,6-bis(N-
alkylbenzimidazol-2’-yl)benzene (bzimb) as the N^C^N
ligand and demonstrated that they show green electrophos-
phorescence in multilayer phosphorescent organic light-
emitting devices (PHOLEDs) with a high current efficiency
and an external quantum efficiency (EQE) of 38.9 cdAꢀ1
and 11.5%, respectively.[11a] Although these complexes are
unique in that they give rise to intense green phosphores-
cence,[11] it is envisaged that the utilization of various types
of cyclometalated N^C^N ligands with slightly different
electronic properties could lead to a gradual variation in the
frontier molecular orbital energies to give tunable emission
colors. The ancillary chloro ligand can be further replaced
by the s-donating alkynyl ligand to give cyclometalated plat-
inum(II) alkynyl complexes with diverse luminescence prop-
erties. Herein, we report the synthesis, characterization,
electrochemistry, and photophysical studies of a new class of
luminescent cyclometalated N^C^N platinum(II) com-
plexes, that is, complexes 1–8, with 2,6-bis(benzoxazol-2’-yl)-
benzene (bzoxb), 2,6-bis(benzothiazol-2’-yl)benzene (bzthb),
and 2,6-bis(N-alkylnaphthoimidazol-2’-yl)benzene (naph-
imb) as the cyclometalated N^C^N ligands. A hole-trans-
porting moiety, 9-(prop-2-ynyl)-9H-carbazole has also been
successfully incorporated into the platinum(II) metal center
to give another new class of luminescent cyclometalated
N^C^N platinum(II) alkynyl complexes, that is, complexes,
9–14. Two of these complexes have been employed to fabri-
cate PHOLED devices and both give satisfactory perform-
ance with a saturated yellow emission and Commission In-
ternational de I’Eclairage (CIE) coordinates of (0.50, 0.49).
Computational studies have also been performed to provide
insights into the nature of the excited states that led to the
luminescence properties of these complexes, which would
provide guiding principles on the design of such classes of
complexes with tunable emission energies.
1
confirmed by H NMR spectroscopy, FAB mass spectrome-
try, IR spectroscopy, and satisfactory elemental analyses.
Figure 1 shows the chemical structures of complexes 1–14.
Results and Discussion
Synthesis and characterization: Cyclometalated N^C^N li-
gands were prepared from ring condensation reactions. The
bzoxb and bzthb N^C^N ligands were cyclized by the reac-
tion between 2-aminophenol or (2-aminothiophenol), re-
Figure 1. Molecular structures of complexes 1–14.
Chem. Eur. J. 2013, 19, 13910 – 13924
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W. H. Lam, V. W.-W. Yam et al.
X-ray crystal structures: Single crystals of complexes 11 and
13 were obtained by diffusion of diethyl ether vapor into a
concentrated dichloromethane solution of the complex, and
their structures were determined by X-ray crystallography.
The perspective views of complexes 11 and 13 are depicted
in Figure 2. Crystal structure determination data are sum-
distortions can be attributed to the steric demand of the
bzthb N^C^N ligand and the naphimb N^C^N ligand, re-
ꢀ
spectively. The bond lengths of Pt1 C1 for complex 11 and
ꢀ
Pt1 C37 for complex 13 are 2.06 and 2.08 ꢁ, respectively,
which are comparatively longer than that of the alkynylpla-
tinum(II) bzimpy complexes[4d] because of the stronger trans
influence exerted by the anionic phenyl ring of the N^C^N
ꢀ
ligand, which would weaken the Pt C bond. The interplanar
angle between the plane of the 9-(prop-2-ynyl)-9H-carb-
AHCTUNGTREGaNNUN zole ligand and that of the platinum N^C^N unit is 82.48
for complex 11 and 80.88 for complex 13 to minimize steric
hindrance between the propynylcarbazole ligand and the
N^C^N moiety. The crystal packings of complexes 11 and
13 show a head-to-tail stacking between pairs of complex
molecules (Figures 3 and 4). However, it is observed that
the two naphthalene planes in the naphimb N^C^N ligand
were twisted with respect to the N1-Pt1-N4 plane by 12.98 in
complex 13 (Figure 4b), making the PtACHTNUTRGNEUNG(N^C^N) motif no
longer coplanar. The shortest Pt···Pt distances between adja-
Figure 2. Perspective drawings of complex 11 (top) and complex 13
(bottom) with atomic numbering. Hydrogen atoms, counterions, and sol-
vent molecules are omitted for clarity. Thermal ellipsoids were shown at
the 30% probability level.
marized in Tables S1a and S2a in the Supporting Informa-
tion, whereas the selected bond lengths and bond angles are
tabulated in Tables S1b and S2b in the Supporting Informa-
tion for complexes 11 and 13, respectively. Similar to the
other cyclometalated N^C^N and C^N^N platinum(II)
complexes,[7,8,11] the platinum(II) metal center adopts a dis-
torted square-planar geometry. The N-Pt-N and N-Pt-C
angles (N2-Pt1-N3 156.5, N2-Pt1-C28 78.3, and N3-Pt1-C28
78.38 for complex 11 and N4-Pt1-N1 157.9, N4-Pt1-C17 78.9,
and N1-Pt1-C17 79.18 for complex 13) are found to deviate
from the idealized value of 180 and 908, respectively. The
Figure 3. Crystal packing diagrams of complex 11 showing the head-to-
tail configuration (a) and the tetrameric structure (b).
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Synthesis and Characterization of Luminescent Cyclometalated Platinum(II) Complexes
FULL PAPER
Table 1. Electrochemical data for complexes 1–14.
Oxidation E1/2 (Epa) [V]
versus SCE[a,b]
(+1.09), (+1.59)
(+1.09), (+1.53)
(+1.05), (+1.47), (+1.73)
(+0.97), (+1.47), (+1.69)
(+0.91), (+1.48), (+1.62)
(+0.93), (+1.44), (+1.80)
(+0.96), (+1.43), (+1.83)
(+0.91), (+1.43), (+1.77)
(+1.01), +1.36
(+0.90), +1.31
(+0.84), +1.35
(+0.72), +1.29
(+0.90), +1.31, (+1.80)
(+0.92), +1.36, (+1.77)
Reduction Epc [V]
versus SCE[c]
1
2
3
4
5
6
7
8
N
ꢀ1.52
ꢀ1.57
ꢀ1.48
ꢀ1.47, (ꢀ1.95)
ꢀ1.41, (ꢀ1.93)
ꢀ1.69
ꢀ1.71
ꢀ1.61
9
ꢀ1.60
10
11
12
13
14
ꢀ1.53
ꢀ1.51, (ꢀ1.97)
ꢀ1.41, (ꢀ1.95)
ꢀ1.69
ꢀ1.70
[a] E1/2 refers to the quasi-reversible anodic peak calculated by E1/2
=
(Epa+Epc)/2; Epa and Epc are the peak anodic and peak cathodic poten-
tials, respectively. [b] Values in parentheses refer to the anodic peak po-
tential (Epa) for irreversible oxidation waves. [c] Epc refers to the cathodic
peak potential for the irreversible reduction waves.
Figure 4. Crystal packing diagrams of complex 13 showing the head-to-
tail configuration (a) and the tetrameric structure (b).
Figure 5. Cyclic voltammogram of complex 11 in CH2Cl2 in the presence
of 0.1m [nBu4N]ACHTNUTRGNEUNG[PF6].
cent molecules are found to be 5.45 ꢁ for complex 11 and
4.86 ꢁ for complex 13, indicating that there are no signifi-
cant Pt···Pt interactions between the two metal centers. The
interplanar distance between the aryl ring in the N^C^N
ligand of two adjacent molecules is found to be 3.46 ꢁ for
complex 11 and 3.66 ꢁ for complex 13, indicative of the
presence of p–p interactions.
centered reduction, which is found to be sensitive towards
the nature of the N^C^N ligands. The [Pt(bzoxb)Cl] com-
AHCTUNGTRENNUNG
plexes 1–3 are found to display a more negative reduction
potential when coordinated to bzoxb ligand with more s-do-
nating para substituents, that is, 3 (ꢀ1.48 V)>1 (ꢀ1.52 V)>
2 (ꢀ1.57 V). Similar trends were observed for the [Pt-
(bzthb)Cl] complexes 4–5, that is, 5 (ꢀ1.41 V)>4 (ꢀ1.47 V),
Electrochemistry: Cyclic voltammetry has been performed
and the [PtACHNUTGRTNEG(NU naphimb)Cl] complexes 6–8, that is, 8
to study the electrochemical behavior of the platinum(II)
complexes in dichloromethane (0.1m [nBu4N]ACTHNUTRGNENU[G PF6]). The
electrochemical data are summarized in Table 1 and the
cyclic voltammogram of complex 11 is shown in Figure 5.
The cyclic voltammograms of the cyclometalated chloropla-
tinum(II) complexes 1–8 in dichloromethane (0.1 M
(ꢀ1.61 V)>6 (ꢀ1.69 V)>7 (ꢀ1.71 V). The trend could be
attributed to the inductive effect of s-donating substituents
at the 5-position of the aryl ring of the N^C^N ligand,
which destabilizes the lowest unoccupied molecular orbital
(LUMO) level, leading to a more negative reduction poten-
tial.
[nBu4N]
[PF6]) displayed irreversible oxidation waves in the
For the cyclometalated platinum(II) alkynyl complexes 9–
14, irreversible oxidation waves and a quasi-reversible oxi-
dation couple were observed. The quasi-reversible oxidation
couple at about +1.38 V versus SCE was assigned as the ox-
idation of the carbazole moiety.[12b] Meanwhile, the irreversi-
range from +0.91 to +1.83 V versus standard calomel elec-
trode (SCE) and a quasi-reversible reduction couple at
about ꢀ1.41 to ꢀ1.71 V versus SCE. The quasi-reversible re-
duction couple was tentatively assigned as a N^C^N ligand-
Chem. Eur. J. 2013, 19, 13910 – 13924
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