Functional Pyrimidinyl Pyrazolate Pt(II) Complexes: Role of Nitrogen Atom in Tuning the Solid-State Stacking and Photophysics

Article abstract of DOI:10.1002/adfm.201900923

Pt(II) metal complexes are known to exhibit strong solid-state aggregation and are promising for realization of efficient emission in fabrication of organic light emitting diodes (OLED) with nondoped emitter layer. Four pyrimidine–pyrazolate based chelates, together with four isomeric Pt(II) metal complexes, namely: [Pt(pm2z)₂], [Pt(tpm2z)₂], [Pt(pm4z)₂], and [Pt(tpm4z)₂], are isolated and systematically investigated for their structure–property relationships for practical OLED applications. Detailed single molecular and aggregated structures are revealed by photophysical and mechanochromic measurements, grazing-incidence X-ray diffraction, and theoretical approaches. These results suggest that these Pt(II) emitters pack like a deck of playing cards under vacuum deposition, and their emission energy is not only affected by the single molecular designs, but notably influenced by their intermolecular packing interaction, i.e., Pt···Pt separations that are arranged in the order: [Pt(tpm4z)₂] > [Pt(pm4z)₂] > [Pt(tpm2z)₂] > [Pt(pm2z)₂]. Nondoped OLED with emission ranging from green to red are prepared, to which the best performances are recorded for [Pt(tpm2z)₂], giving maximum external quantum efficiency (EQE) of 27.5% at 10³ cd m^?2, maximum luminance of 2.5 × 10⁵ cd m^?2 at 17 V, and with stable CIE_x,y of (0.56, 0.44).

Full text of DOI:10.1002/adfm.201900923

FULL PAPER
Organic Light-Emitting Diodes
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Functional Pyrimidinyl Pyrazolate Pt(II) Complexes: Role
of Nitrogen Atom in Tuning the Solid-State Stacking and
Photophysics
Paramaguru Ganesan, Wen-Yi Hung,* Jen-Yung Tso, Chang-Lun Ko, Tsai-Hui Wang,
Po-Ting Chen, Hsiu-Fu Hsu,* Shih-Hung Liu, Gene-Hsiang Lee, Pi-Tai Chou,*
Alex K.-Y. Jen,* and Yun Chi*
computers, notepads, and cell phones.
Pt(II) metal complexes are known to exhibit strong solid-state aggregation and
are promising for realization of efficient emission in fabrication of organic light
emitting diodes (OLED) with nondoped emitter layer. Four pyrimidine–pyrazolate
based chelates, together with four isomeric Pt(II) metal complexes, namely:
[Pt(pm2z)2], [Pt(tpm2z)2], [Pt(pm4z)2], and [Pt(tpm4z)2], are isolated and system-
atically investigated for their structure–property relationships for practical OLED
applications. Detailed single molecular and aggregated structures are revealed
by photophysical and mechanochromic measurements, grazing-incidence X-ray
diffraction, and theoretical approaches. These results suggest that these Pt(II)
emitters pack like a deck of playing cards under vacuum deposition, and their
emission energy is not only affected by the single molecular designs, but notably
influenced by their intermolecular packing interaction, i.e., Pt···Pt separations
that are arranged in the order: [Pt(tpm4z)2] > [Pt(pm4z)2] > [Pt(tpm2z)2] >
[Pt(pm2z)2]. Nondoped OLED with emission ranging from green to red are
prepared, to which the best performances are recorded for [Pt(tpm2z)2], giving
maximum external quantum efficiency (EQE) of 27.5% at 103 cd m−2, maximum
luminance of 2.5 × 105 cd m−2 at 17 V, and with stable CIEx,y of (0.56, 0.44).
Their efficiencies have been improved
during the past two decades using suit-
able third-row transition-metal phosphors
as emitters. Among these emitters, Pt(II)
metal complexes have been attracting
intensive studies owing to their higher sta-
bility and intense luminescence at room
temperature (RT).[1] However, in contrast
to the Ir(III) emitters with d6-electronic
configuration and octahedral coordination
structure,[2] the Pt(II) emitters possesses
distinctive d8-configuration and square
planar geometry, which induced a greater
tendency in forming π–π-stacking interac-
tion between adjacent molecules in solid
state. As for application, this aggregation is
capable of tuning emission to lower energy
region of the visible spectra and even fur-
ther into the near-infrared (NIR), which
allows versatile prospective in applications
for this class of emitters.
Many aggregated Pt(II) complexes
have been synthesized, for which their structures can be dif-
ferentiated into two classes. One is best represented by those
bearing a linear Ptn architecture and with Pt···Pt distance of
less than 3.4 Å; the latter is the sum of van del wall radii of
1. Introduction
Organic light emitting diodes (OLED) have been spurred by
the fast advancement of modern display technologies of laptop
Dr. P. Ganesan, Prof. Y. Chi
Department of Chemistry
National Tsing Hua University
Hsinchu 30013, Taiwan
Dr. S.-H. Liu, Dr. G.-H. Lee, Prof. P.-T. Chou
Department of Chemistry and Instrumentation Center
National Taiwan University
Taipei 10617, Taiwan
E-mail: ychi@mx.nthu.edu.tw
E-mail: chop@ntu.edu.tw
Prof. W.-Y. Hung, J.-Y. Tso, C.-L. Ko
Institute of Optoelectronic Sciences
National Taiwan Ocean University
Keelung 202, Taiwan
Prof. A. K.-Y. Jen, Prof. Y. Chi
Department of Materials Science and Engineering
and Department of Chemistry
City University of Hong Kong
Hong Kong SAR
E-mail: wenhung@mail.ntou.edu.tw
E-mail: alexjen@cityu.edu.hk
Dr. T.-H. Wang, P.-T. Chen, Prof. H.-F. Hsu
Department of Chemistry
Tamkang University
New Taipei 25137, Taiwan
E-mail: hhsu@mail.tku.edu.tw
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201900923.
DOI: 10.1002/adfm.201900923
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carbon atom, symbolized the shortest possible Pt–Pt distance
without causing major distortion to the coordinative chelates.
This linear-chain arrangement has been reported in several
examples, such as [Pt(NH3)4][PtCl4] (Magnus Green Salt),[3]
[Pt(diimine)2(CN)2], diimine = bipyridine and biisoquinoline,[4]
of 38.8% for orange-emitting OLED using such thin film crys-
talline as the emitting layer.[15]
Knowing their tendency in forming crystalline thin film,
we also functionalized [Pt(fppz)2] by substituting pyridyl
with pyrazinyl fragment to give [Pt(fprpz)2], in anticipation
of obtaining further red-shifted emission.[16] Emission peak
maximum at 740 nm, quantum yield (QY) of 81% and life-
time of 313 ns were observed; all are attributable to the
metal-metal-to-ligand charge transfer (MMLCT) character in
the emitting triplet excited states. The NIR-emitting OLED
gave EQE of 24 1% and maximum radiance of (3.6 0.2) ×
105 mW sr−1·m−2 without the light out-coupling hemisphere
structure; affording the highest record among NIR OLED.
In this contribution, we proceed to investigate the structure–
property relationship of functional [Pt(fprpz)2] by relocating
the noncoordinating N-atom in pyrazinyl fragment, forming
either 2-pyrimidinyl or 4-pyrimidinyl substituted pyrazolate
Pt(II) complexes, c.f. Scheme 1. Interestingly, photophysical
properties such as emission peak wavelength, quantum yield
and response to the physical states and external mechano-
stimulus were affected not only by the location of noncoordi-
nating N-atom but also by their substituents, i.e., R = H and
tBu. Detailed analyses conclude that the variations are mainly
attributed to a change of intermolecular stacking (i.e., Pt···Pt
interaction) that constituted a special case for the aggregated
induced emission (AIE).[17] Moreover, fabrication of nondoped
phosphorescent OLED were also conducted and discussed in
the following sections.

[Pt(diBrbpy)(C CC6H4Et-4)2], diBrbpy
=
4,4-dibromo-2,2′-
bipyridine,[5] and [Pt(DECO)2]n, DECO = cyano-2-oximino-
N,N′-diethylaminoacetamide.[6] The corresponding Pt(II) units
are held together by noncovalent metallophilic Pt···Pt interac-
tion, as shown by the single crystal X-ray diffraction studies.
Alternatively, there are also a second class of Pt(II) complexes
that showed the reduced Pt···Pt interaction between a pair of
nearby Pt(II) fragments.[7] These “dinuclear” complexes can be
either physically assembled using bridging ligand(s),[8] sup-
ported via the metallophilic Pt···Pt interaction mentioned in
the linear chain type of Pt(II) complexes,[9] or even forced to
form the Pt(II) dimers upon addition of cucurbit[8]uril mac-
rocycles.[10] In general, these Pt(II) complexes would exhibit
bright “assembly-induced” phosphorescence, which may be
either independent or sensitive to their physical conditions and
external stimulation.
Concurrently, there is a growing interest in the series of
Pt(II) complexes with pyrazolate-containing chelates and in
studies of the photophysical effect induced by aggregation in
solid states.[11] One of interesting examples is ascribed to Pt(II)
complex [Pt(fppz)2], fppz = 3-(trifluoromethyl)-5-(2-pyridinyl)
pyrazolate, c.f. Scheme 1,[11b] which possessed a columnar
packing arrangement, as well as large red shift in both photo-
luminescence (PL) and electroluminescence (EL).[12] This complex
is also known for forming highly ordered crystalline structure
in vacuum deposited thin film, similar to that observed in
many Pt(II) porphyrin derivatives.[13] However, the record-high
emission of [Pt(fppz)2] (Φ ≈ 96%) allowed the fabrication of a
nondoped OLED with a remarkably high external quantum effi-
ciency (EQE) of 31.1%.[14] Moreover, the ordered morphology
in the vacuum deposited thin film has induced the horizontal
arranged emitting dipole ratio of 93%, giving a very high EQE
2. Results and Discussion
2.1. Syntheses and Characterization
The synthetic protocols of the pyrimidine pyrazole chelates
employed in the present studies are outlined in Schemes 2 and 3,
to which the detailed experimental procedures are elaborated in
supporting information. Preparation of all four pyrimidine ligands
demanded the acetyl derivatives as key intermediates. Hence,
reaction of 2-pyrimidinecarbonitrile (pm-2CN) with methyl
Scheme 2. Syntheses of chelates (pm2z)H and (tpm2z)H; experimental
conditions: i) MeMgBr, ether, 0 °C, 4 h. ii) Pivalic acid, K2S2O8, AgNO3,
H2SO4, iii) ethoxyethenyl-tri-n-butylstannane, Pd(PPh3)2Cl2, iv) 2 N HCl,
acetone, RT, v) CF3CO2Et, NaOEt, and vi) N2H4·H2O, p-TsOH, EtOH, reflux.
Scheme 1. Structural drawing of the studied Pt(II) complexes bearing
various pyrazolate-containing chelates.
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Scheme 3. Syntheses of chelates (pm4z)H and (tpm4z)H; experimental
conditions: i) paraldehyde, tBuO2H, CF3CO2H, FeSO4·7H2O, CH3CN,
ii) pm-4Ac, pivalic acid, K2S2O8, AgNO3, H2SO4, iii) CF3CO2Et, NaOEt,
and iv) N2H4·H2O, p-TsOH, EtOH, reflux.
magnesium bromide yielded 2-acetyl pyrimidine (pm-2Ac).
Alternatively, the t-butyl analogue of pm-2Ac (i.e., tpm-2Ac)
was obtained by reaction of 2-chloro pyrimidine (pm-2Cl) with
pivalic acid, followed by treatment with tri-n-butyl-1-ethoxyvi-
nylstannane to yield the ethoxy vinyl derivative, which could be
readily hydrolyzed to form the tpm-2Ac. As for 4-substituted
acetyl pyrimidine (pm-4Ac), it can be synthesized by the direct
acetylation of pyrimidine with paraldehyde, while the reaction of
pm-4Ac with pivalic acid, in presence of both K2S2O8 and AgNO3,
results in the formation of its tBu analogue, i.e., tpm-4Ac in good
yields.
Figure 1. Structural drawing of [Pt(pm4z)2] with ellipsoids shown at the
30% probability level; selected bond distances: Pt(1)-N(1) = 2.036(3),
Pt(1)-N(3) = 1.995(3) and Pt(1)···Pt(1) = 3.625 and N(4)···H(1) = 2.271
Å, and bond angles: N(1)-Pt(1)-N(3A) = 100.99(13), N(1)-Pt(1)-N(3) =
79.01(13)°.
of [Pt(pm4z)2] was successfully grown from acetone and hexane
at RT. It is notable that the red-emitting powder of [Pt(pm4z)2]
turned colorless upon dissolution in acetone and, eventu-
ally, formed yellow crystals, which are green emitting, by slow
addition of hexane. This variation of photophysical property
is apparently due to the weakening of intermolecular π–π
stacking and disruption of Pt···Pt interaction, upon dissolu-
tion in acetone and formation of single crystals.
With these acetyl substituted pyrimidines in hands, we
conducted the Claisen condensation with ethyl trifluoroac-
etate to form diketone intermediates. Subsequent cyclization
with hydrazine hydrate yielded the target pyrimidinyl pyrazole
chelates, c.f. (pm2z)H, (tpm2z)H, (pm4z)H, and (tpm4z)H.
All corresponding Pt(II) complexes [Pt(pm2z)2], [Pt(tpm2z)2],
[Pt(pm4z)2] and [Pt(tpm4z)2] were next synthesized by treat-
ment of [PtCl2(DMSO)2] with these pyrazole ligands with addi-
tion of Na2CO3 in refluxing 2-methoxyethanol. The resulting
Pt(II) complexes exhibited poor solubility in 2-methoxyethanol
(and other solvents as well) and were immediately precipitated
out of the solution even during reflux. For workup, all precipi-
tates were collected by filtration, washing with water and ether
in sequence and, finally, were sublimed under high vacuum to
produce the analytically pure samples. It is notable that all these
chelates contain a trifluoromethyl (CF3) substituent on pyrazole,
giving increased electron deficiency at the Pt(II) metal center
upon formation of Pt(II) complexes. Moreover, upon further
consideration of its reduced van der Waals attraction and smaller
size (only slightly larger than methyl group), the complexes may
result in an improved intermolecular stacking. Overall, this CF3
group is probably a key player in formation of the well-stacked
(crystalline) thin film of these highly emissive Pt(II) complexes.
Single crystal X-ray structural determination of [Pt(pm4z)2]
was next executed. Molecules in crystal lattices constituted an
infinite 1D chain-like arrangement along the b-axis of unit cells.
However, Figure 1 shows the packing structure involving only
three molecules that are separated by an elongated Pt···Pt
distance of ≈3.625 Å. The pm4z chelates in Pt(II) complex are
arranged in the mutual trans-orientation, while the adjacent
Pt(II) complexes are rotated around the Pt–Pt axis by 102.81°,
followed by a second 77.19° (i.e., 180–102.81°) rotation to regen-
erate the third molecule with identical orientation, so that the
pyrazolate and pyrimidine fragments of the pm4z chelate are
sandwiched by their adjacent counterparts of Pt(II) complexes.
Overall, this single crystal X-ray structural analysis confirmed
that the observed green emission from single crystals originates
from the isolated molecules, while the red-emitting, sublimed
powders (or even vacuum deposited thin film) may possess a
distinctive morphology with an increased Pt···Pt stacking inter-
action. Interestingly, no acetone molecule can be located on the
electron density map of single crystal X-ray structural analyses,
probably due to its nonexistence or in disordered arrangement.
1
These Pt(II) complexes were characterized using H and 19F
NMR spectroscopies, mass spectrometry and elemental anal-
yses. The 1H and 19F NMR spectra were generally taken at higher
temperature (80 °C) because of their poor solubility in common
organic solvents such as dimethyl sulfoxide (DMSO) and tol-
uene. However, [Pt(pm4z)2] represents one exception as it is
reasonably soluble in acetone at RT. A characteristic downfield
signal at δ 9.8–10.50 was observed in the 1H NMR spectrum of
all Pt(II) complexes. These unique signals were assigned to the
CH unit next to the coordinated nitrogen atom of pyrimidinyl
2.2. Photophysical Properties
Figure 2 displays the UV–vis absorption and emission spectra
of these Pt (II) metal complexes as vacuum deposited thin film,
while the pertinent numerical data are summarized in Table 1.
The data in solution were not reported due to their poor solu-
bility in majority of organic solvents. For the thin film spectra,
intense absorption bands appeared in the higher-energy region
(<350 nm) are attributed to the ligand-centered π–π* pyrazolate

fragment, confirming the existence of interligand C H···N
bonding to the nearby pyrazolate, as observed in many related
pyrazolate complexes.[18] Due to improved solubility, the crystal
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7
information, we thus propose a qualitative order of solid-state
stacking interaction within these closely related Pt(II) complexes:
[Pt(tpm4z)2] > [Pt(pm4z)2] > [Pt(tpm2z)2] > [Pt(pm2z)2].
Pt(pm2z)2
1.0
0.8
0.6
0.4
0.2
0.0
6
5
4
3
2
1
0
Pt(tpm2z)2
Pt(pm4z)2
Pt(tpm4z)2
The MMLCT transition can be further confirmed by their
structureless emission profile with peak maximum at 542, 603,
654, and 662 nm for [Pt(pm2z)2], [Pt(tpm2z)2], [Pt(pm4z)2], and
[Pt(tpm4z)2] respectively. Furthermore, relocating the pyrazolate
unit from the 2nd to 4th position of pyrimidine substantially
red shifted its MMLCT emission by 112 and 59 nm from the
parent and t-butyl substituted pm2z derivatives. Moreover, addi-
tion of t-butyl group to the pyrimidinyl fragment of [Pt(pm2z)2]
induced a red shift of 61 nm in forming [Pt(tpm2z)2], while
there is only a small variation in the emission maximum of
8 nm upon changing from [Pt(pm4z)2] to [Pt(tpm4z)2]. This
observation is in agreement with existence of the already
strengthened Pt···Pt interaction in [Pt(pm4z)2].
300
400
500
600
700
800
Wavelength (nm)
Figure 2. Absorption and emission spectra of various vacuum deposited
Figure 3 displays various photographic images of [Pt(pm2z)2]
recorded in ambient and under UV irradiation as well as the
emission spectra recorded using spectrophotometer. The
vacuum deposited thin film gave a broadened emission profile
with maximum at 542 nm. In sharp contrast, the sublimed,
powdery sample appears as yellow in ambient and exhibits a
sky-blue emission, and feature of vibronic progression with
peak maximum at 443, 470, and 499 nm. This change in emis-
sion pattern is probably due to the improved Pt···Pt interac-
tion that existed in the vacuum deposited thin film versus the
sublimed powder, for which the change of Pt···Pt interaction
in powder can be verified by applying the mechanical stimulus
to the sublimed powder in giving the ground powder. As can be
seen, its yellow emission with maximum at 565 nm is further
red-shifted versus the vacuum deposited thin film, reflecting
formation of the strongest Pt···Pt interaction in the ground
powder. Similar modulation of luminescent properties by
aggregation of Pt(II) complexes has also been documented.[9g,21]
In general, thin film deposition demands a slow transfer of
vaporized molecule onto the cold substrate surface, while subli-
mation involves a fast transport of larger quantity of samples, to
which the vacuum deposition and sublimation are under ther-
modynamic and kinetic control respectively, causing substantial
difference in intermolecular packing interaction.
Pt(II) thin films on quartz glass.
→ pyrimidine transitions as supported by their higher extinc-
tion coefficients. Similar transition characters and assignments
are well documented in literature.[19] Their poor solubility
also implicates strong aggregation which was well supported
by the additional band observed around 400–512 nm arising
due to the MMLCT transition.[20] In general, aggregation may
induce a change from an array of isolated single Pt(II) mole-
cule to a continuous chain-like arrangement via formation of
Pt···Pt interactions. With this expectation, the lowest energy
MMLCT band of [Pt(pm2z)2] at 400 nm (or lack of lower energy
MMLCT absorption band) may suggest a relatively diminished
Pt···Pt interaction. In contrast, after introducing the t-butyl
substituents in forming [Pt(tpm2z)2], its MMLCT absorption
clearly shifted to 474 nm, showing strengthened intermolec-
ular Pt···Pt nonbonding interaction, despite of having bulky
t-butyl substituents. This stacking induced red-shift in MMLCT
absorption band is further confirmed by the occurrence of
enlarged S1 energy gap of the t-butyl substituted [Pt(tpm2z)2] in
DMSO in comparison to its parent complex [Pt(pm2z)2], which
is revealed by the time-dependent density functional theory
(TDDFT) calculation discussed in theoretical section, vide infra.
Alternatively, [Pt(pm4z)2] and [Pt(tpm4z)2] showed the MMLCT
absorption band at 501 and 512 nm, respectively, which are obvi-
ously further red shifted in comparison to their pm2z analogues
[Pt(pm2z)2] and [Pt(tpm2z)2]. This observation is in sharp con-
trast to the S0-S1 energy gap of the isolated Pt(II) complexes in
TDDFT calculation, which exhibited an energy gap difference of
less than 8 nm between [Pt(pm2z)2] and [Pt(pm4z)2] and between
t-butyl substituted [Pt(tpm2z)2] and [Pt(tpm2z)2]. With these
Alternatively, both vacuum deposited thin film and sub-
limed powder of third Pt(II) complex [Pt(pm4z)2] exhibit rela-
tively stronger Pt···Pt interaction, which are evidenced by
their identical, red-orange emission. However, the sublimed red
powder turned yellow upon dissolving in acetone, followed by
reprecipitation or evaporation of solvent, and the resulted solid
sample exhibits a greenish-blue emission under UV (Figure 4).
Concomitant with this notable change in morphology, its
Table 1. Photophysical properties of Pt(II) complexes as sublimed fine powder and deposited thin film.
Abs λmax [ε × 10−4, m−1 cm−1 a)
]
PL λmax [nm]b)
(443, 470, 499); [542]
(589); [603]
Φ [%]b)
(25); [75]
(77); [86]
(52); [78]
(41); [61]
τ
obs [ns]b)
FWHM [cm−1
]
[Pt(pm2z)2]
[Pt(tpm2z)2]
[Pt(pm4z)2]
[Pt(tpm4z)2]
267 (4.01), 400 (1.56)
266 (4.43), 474 (1.49)
262 (6.58), 501 (2.06)
241 (4.09), 512 (1.62)
(563); [486]
(86); [327]
(201); [273]
(149); [240]
(2699); [2993]
(2230); [2599]
(2278); [2337]
(3080); [2492]
(636); [654]
(648); [662]
a)UV–vis spectra were recorded using vacuum deposited thin film on quartz glass; b)PL data measured as sublimed powder and vacuum deposited thin film at RT are
depicted in parentheses and square bracket, respectively.
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Figure 4. a) Photographs in various states of [Pt(pm4z)2]: i) vacuum
deposited thin film; ii) sublimed powder; iii) after acetone rinse, taken
under ambient and UV excitation at λmax = 365 nm. b) Photolumines-
cence of [Pt(pm4z)2] as deposited thin film (magenta), sublimed powder
(orange), and after acetone rinse (cyan).
Figure 3. a) Photographs in various states of [Pt(pm2z)2]: i) vacuum
deposited thin film; ii) sublimed powder; iii) ground powder, taken under
ambient and UV excitation at λmax = 365 nm). b) Photoluminescence of
[Pt(pm2z)2] as deposited thin film (cyan), sublimed powder (blue), and
ground powder (orange).
emission maximum is now shifted from 636 to 480 nm, together
with the occurrence of a less intense shoulder at 501 nm, which
is attributed to the disruption of π–π stacking and weakening of
Pt···Pt contact. Change of emission can be similarly achieved
upon exposing to solvents such as THF, ethyl acetate, and
dimethylformamide (DMF). Furthermore, [Pt(pm4z)2] exhibits
relatively poor solubility in acetonitrile and toluene and, hence,
they did not afford a prompt change in emission, but required a
much longer induction period (i.e., ≈4 h). In sharp contrast, dis-
ruption of Pt···Pt stacking was not observed upon exposure to
CH2Cl2 or methanol even for a long period of time and at higher
temperature. This solvatochromic effect is fully reversible, as
the yellow powder can be reverted back to its red form upon
applying mechanical stress or subjected to vacuum sublimation.
Interestingly, these morphological changes in response to
solvent and mechanical stimulus are relatively less notable
in their t-butyl substituted congeners [Pt(tpm2z)2] and
[Pt(tpm4z)2]. This may be related to the notable enhancement
of Pt···Pt stacking interaction upon introduction of t-butyl
groups. Photographs of these samples and emission profiles are
depicted in Figures S1 and S2 in the Supporting Information.
films of [Pt(pm2z)2] and [Pt(pm4z)2] showed single-crystal-like
grazing-incidence X-ray diffraction (GIXD) patterns, while those
of t-butyl derivatives [Pt(tpm2z)2] and [Pt(tpm4z)2] exhibited
less ordered, semicrystalline patterns (Figure S3, Supporting
Information). Their lattice parameters are listed in Table 2. Sev-
eral remarks can be delineated from the recorded data. First,
these thin films crystalized in the monoclinic system (a ≠ b ≠ c,
α = γ = 90°) with β, i.e., tilting angle of a-axis versus bc plane,
being 107°, 97°, 97°, and 97° for [Pt(pm2z)2], [Pt(tpm2z)2],
[Pt(pm4z)2], and [Pt(tpm4z)2], respectively. Moreover, the b-axis
and β-angle of [Pt(pm2z)2] are significantly different from other
Pt(II) complexes, exhibiting a distinctive packing orientation.
Next, their electron density maps (EDMs) were derived from
the 2D GIXD patterns,[22] which can be utilized to decipher
and confirm their packing orientations in crystallites. Figure 5
shows electron density maps of all studied Pt(II) complexes
along the ab and ac planes, to which the tilting of molecular
planes versus the substrate surface is revealed by the side views
along the ac plane. As can be seen in their packing illustrations,
the bc planes are tilted 19° for [Pt(pm2z)2] and 7° for Pt(II) com-
plexes [Pt(tpm2z)2], [Pt(pm4z)2], and [Pt(tpm4z)2], which exhibit
similar packing along their ab plane, i.e., adjacent Pt(II) mol-
ecules are offset by one-half of unit cell along the a axis. On the
other hand, [Pt(pm2z)2] showed a different in-plane packing, to
which the molecules are packed on top of each other along the
a axis. This feature can be also visualized from the ac plane of
electron density map.
2.3. Solid-State Packing
All these Pt(II) complexes were subject to studies using syn-
chrotron radiation facility, to which the vacuum deposited thin
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Table 2. Crystallographic data of vacuum deposited Pt(II) thin films.
(Supporting Information). In addition, the frontier molecular
orbitals involved in the lower-lying transitions are depicted in
Figure 6 and Figures S4–S7 in the Supporting Information.
The calculated S0 → S1 optical transition are recorded to be:
[Pt(pm2z)2]: 370.6 nm, [Pt(tpm2z)2]: 358.6 nm, [Pt(pm4z)2]:
378.6 nm, and [Pt(tpm4z)2]: 365.1 nm. Moreover, after geom-
etry optimization of the T1 excited state, the calculated T1 →
S0 emission transitions turned to [Pt(pm2z)2]: 531.6 nm,
[Pt(tpm2z)2]: 517.5 nm, [Pt(pm4z)2]: 495.9 nm, and [Pt(tpm4z)2]:
486 nm. Furthermore, the S0 → S1 transition is mainly derived
from the highest occupied molecular orbital (HOMO) → lowest
unoccupied molecular orbital (LUMO) transition (>97%), while
the T1 → S0 transition possesses a reduced contribution form
the LUMO → HOMO process (<56%). For both S0 and T1 states
of these Pt(II) complexes, the electron density distributions of
HOMO are mainly localized at the central Pt(II) atom (≈36%)
and the pyrazolates, while the electron density distributions of
LUMO are mainly delocalized in the pyrimidinyl fragments
and with minor contribution from the Pt(II) atom (≈5%). As a
result, both the S0 → S1 optical absorption and T1 → S0 emis-
sion of these Pt(II) complexes in monomer are assigned to
metal-to-ligand charge transfer transition (MLCT) in ≈22–18%
contributions, together with ligand-to-ligand charge transfer
(LLCT) and intraligand charge transfer (ILCT) transition
processes.
We also perform the calculation on S1 and T1 of both the
dimeric and trimeric stacking array. In this approach, the geom-
etry of dimer and trimer is taken originally from truncation of the
single crystal structural data of [Pt(pm4z)2] (Figure 1), followed
by full structure optimization in the excited state. The obtained
transition characters are listed in Table 4 and Tables S6–S7
in the Supporting Information, while the frontier molecular
orbitals involved in the lower-lying transitions are depicted in
Figure 7 and Figures S8–S9 in the Supporting Information.
The calculated Pt···Pt distance is 4.40 Å in dimer and 4.05 Å
in trimer. This value is a little far from the experimentally
observed Pt···Pt distance 3.625 Å. This is not surprising in
absence of the constraint of lattice energy, as two individual
monomers would feel the repulsive force from each other and
hence attempt to locate a stable geometric configuration in the
global minimum with a longer Pt···Pt distance 4.40 Å. As for
the trimeric structure, the middle monomer senses the repul-
sive forces in the opposite direction by the two neighboring
monomers, forming a squeezed sandwich structure with a
Pt···Pt distance (4.05 Å) shorter than that of the dimer (4.40 Å).
Based on this trend, it is expected that increasing linearly
aligned Pt(II) complexes, similar to the exertion of the lattice
energy, should further shorten the Pt···Pt distance close to
that in crystal structure.
[Pt(pm2z)2] [Pt(tpm2z)2] [Pt(pm4z)2] [Pt(tpm4z)2]
a [Å]
22.2
20.6
7.1
90
25.9
15.2
6.6
90
24.2
11.2
7.4
90
26.1
15.6
6.6
90
b [Å]
c [Å]
α [°]
109
90
97
97
97
β [°]
90
90
90
γ [°]
Pt···Pt [nm]
0.41
0.33
33.4
9.4
8.0
31
0.33
0.33
23.8
7.4
4.2
19
0.37
0.34
49.1
5.9
6.5
42
0.33
0.33
18.1
5.8
3.8
15
π–π [nm]
Grain size along [200] [nm]
Grain size along [020] [nm]
Grain size along [002] [nm]
Packed molecules along [100]
Packed molecules along [010]
Packed molecules along [001]
10
6
6
8
24
14
18
12
Tilting angle of molecule to
17
0
16
0
the substrate normal [°]
Moreover, for both [Pt(pm2z)2] and [Pt(pm4z)2], molecular
planes are tilted at 17° and 16° against substrate normal, while
their t-butyl counterparts [Pt(tpm2z)2] and [Pt(tpm4z)2] aligned
in a parallel manner with respect to the substrate normal. Since
all samples showed similar π–π distance of 0.33 nm between
the adjacent plane of stacked molecules, the enlarged tilting
angle of [Pt(pm2z)2] and [Pt(pm4z)2] will produce a larger
Pt···Pt separation of 0.41 and 0.37 nm in comparison
to 0.33 nm that was observed for both t-butyl substituted
[Pt(tpm2z)2] and [Pt(tpm4z)2]. Hence, combining the influence
of both the tilting angles of c axis and molecular plane versus
substrate normal, their stacking overlap (or Pt···Pt distance)
follows an order of [Pt(tpm2z)2] ≈ [Pt(tpm4z)2] >> [Pt(pm4z)2]
> [Pt(pm2z)2]. With this understanding, increase of MMLCT
interaction in thin film of [Pt(pm4z)2] versus [Pt(pm2z)2], and
in the t-butyl substituted derivatives [Pt(tpm2z)2]/[Pt(tpm4z)2]
versus parent complexes [Pt(pm2z)2]/[Pt(pm4z)2] can be
expected.
Finally, the grain sizes along each axis and the numbers of
packed molecules within crystallite are also calculated using
Scherrer equation[23] and pertinent data are listed in Table 2.
The numbers of packed molecules along stacking axis [002]
for [Pt(pm2z)2] are considerably larger than all other Pt(II)
complexes, showing the largest solid-state stacking interaction
among all derivatives.
The calculated S1 → S0 and T1 → S0 transitions for dimer
and trimer are recorded to be 429.5 and 497.2 nm, and 435.5
and 510.2 nm, which are lowered in energy compared to those
of their monomeric counterpart. For their S1 and T1 excited
states, the electron density distributions of HOMO are mainly
localized at the Pt(II) atoms (31–37% in total) and adjacent
pyrazolate fragments, while the electron density distributions
of LUMO are delocalized over the pyrimidinyl fragments
and with less contribution from Pt(II) atoms (4.2–5.8% in
total). As a result, the S1 → S0 and T1 → S0 transitions are
2.4. Theoretical Investigation
We then executed the TDDFT calculation to gain further
insights into the photophysical fundamental. However, due to
the poor solubility in all common organic solvents, we choose
DMSO, which is the solvent employed in majority of our NMR
measurement, in constructing our simulated model. For mon-
omer, the calculated optical and emission transition characters
of all Pt(II) complexes are listed in Table 3 and Tables S2–S5
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Figure 5. From left to right: EDMs along ab plane, ac plane (middle), molecular packing illustrations along ab, ac planes and perspective views of
a) [Pt(pm2z)2], b) [Pt(tpm2z)2], c) [Pt(pm4z)2], and d) [Pt(tpm4z)2]. Green arrow: direction of π–π interaction, red arrow: direction of Pt···Pt interaction.
Table 3. The calculated wavelengths, transition probabilities, and main charge characters of the lowest optical absorption and emission transitions
for the Pt(II) complexes [Pt(pm2z)2], [Pt(tpm2z)2], [Pt(pm4z)2], and [Pt(tpm4z)2] in DMSO.
Complex
State
f
Main Assignments
MLCT
15.99%
30.14%
17.50%
15.78%
30.01%
14.53%
21.56%
30.85%
17.46%
19.09%
30.23%
15.45%
λ [nm]
421.1
370.6
531.6
409.1
358.6
517.5
421.7
378.6
495.9
407.2
365.1
486
[Pt(pm2z)2]
0
S0 → T1
S0 → S1
T1 → S0
S0 → T1
S0 → S1
T1 → S0
S0 → T1
S0 → S1
T1 → S0
S0 → T1
S0 → S1
T1 → S0
HOMO → LUMO (46%)
HOMO → LUMO (98%)
LUMO → HOMO (50%)
HOMO → LUMO (43%)
HOMO → LUMO (98%)
LUMO → HOMO (52%)
HOMO → LUMO (66%)
HOMO → LUMO (97%)
LUMO → HOMO (56%)
HOMO → LUMO (60%)
HOMO → LUMO (97%)
LUMO → HOMO (52%)
0.0354
0
[Pt(tpm2z)2]
[Pt(pm4z)2]
[Pt(tpm4z)2]
0
0.0387
0
0
0.0674
0
0
0.0955
0
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Figure 6. Frontier molecular orbital HOMO and LUMO of the ground
state S0 for all studied Pt(II) complexes. “Pt” indicates the relative elec-
tron density distribution at the Pt atoms.
Figure 7. Frontier molecular orbital HOMO and LUMO of the excited state
T1 and S1 for the Pt(II) complexes [Pt(pm4z)2] dimer and trimer. “Pt” and
“Pt5dz2” indicate the relative electron density distribution at each Pt atom.
mainly assigned to MMLCT (≈30% for S1 and ≈16% for T1,
respectively) mixed with certain LLCT and ILCT characters.
The MMLCT transition would exert the heavy atom effect in
enhancing the spin–orbit coupling that, in turn, induces the
fast electron flipping and facilitates both the S1 → T1 (or Tm,
m > 1) intersystem crossing and T1 → S0 phosphorescence.
It eventually produces phosphorescence with quantum yield
>50% (vide supra, Table 1) and reduced phosphorescence
radiative lifetime (≈0.35 µs) in vacuum deposited thin film.
Particularly, the Pt···Pt (for dimer) and Pt···Pt···Pt (for
trimer) interactions are mainly conveyed via their 5dz2 orbitals
(in HOMO) and resulting in a linear array of stacked structure
for [Pt(pm4z)2].
Table 4. The calculated wavelengths, transition probabilities, and charge
transfer character of the lowest emission for the monomer of Pt(II) com-
plex [Pt(pm4z)2] in DMSO.
2.5. Electroluminescence
To investigate their EL properties, the following nondoped
PhOLED were employed: ITO/ 4% ReO3:SimCP (60 nm)/SimCP
(15 nm)/emitting layer (EML) (20 nm)/PO-T2T (50 nm)/Liq
(0.5 nm)/Al (100 nm), for which 3,5-di(N-carbazolyl)tetraphenyl-
silane (SimCP)[24] and ((1,3,5-triazine-2,4,6-triyl)-tris(benzene-
3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T)[25] were used
as the hole-transporting layer (HTL) and electron-transporting
State
f
0
Main Assignments
MLCT
17.95%
30.01%
16.29%
30.08%
λ [nm]
497.2
429.5
510.2
435.5
Dimer
Trimer
T1 → S0
S1 → S0
T1 → S0
S1 → S0
LUMO → HOMO (51%)
LUMO → HOMO (95%)
LUMO → HOMO (29%)
LUMO → HOMO (49%)
0.1035
0
0.1966
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Figure 8. a) Schematic OLED structure with Pt(II) complexes as EML, b) current density–voltage–luminance (J–V–L) characteristics, c) EQE and power
efficiencies (PE) as a function of luminance, and d) EL spectra, respectively.
layer (ETL), to achieve the optimized carrier transport. Rhenium
oxides (ReO3) in SimCP are applied as the Ohmic contact,[26]
while 8-hydroxyquinolinolatolithium (Liq) and Al are used as
the electron injection layer (EIL) and cathode, respectively. The
device structure and the energy levels diagram are presented
in Figure 8a. In this study, we have optimized the EML thick-
ness and found that 20 nm of EML was the best choice for cur-
rent device, which efficiently confined the exciton density at the
recombination zone to maximize the performance. Figure 8b–d
depicts the current density–voltage–luminance (J–V–L)
characteristics, device efficiencies, and EL spectra, respectively.
The key data are summarized in Table 5. All devices exhibited
pure emission from phosphors and no residue emission from
the adjacent layers. The emission peak maxima were located
at 577, 603, 630, and 641 nm for [Pt(pm2z)2], [Pt(tpm2z)2],
[Pt(pm4z)2] and [Pt(tpm4z)2] respectively, revealing spectral fea-
tures similar to their thin film photoluminescence (c.f. Table 1
and Figure 2). All devices displayed a low turn-on voltage of
2.4–2.6 V. The yellow device derived from [Pt(pm2z)2] achieved a
maximum EQE of 24.9%, current efficiency (CE) of 74.0 cd A−1
and power efficiency (PE) of 58.5 lm·W−1 with Commission
Internationale de l’éclairage (CIE) coordinates of (0.48, 0.50).
By introducing t-butyl substituents in forming [Pt(tpm2z)2],
the orange device achieved a maximum EQE of 27.5%, CE of
64.2 cd A−1 and PE of 37.4 lm W−1 with CIE coordinates of
(0.56, 0.44). In addition, the efficiency roll-off is relatively low,
exhibiting EQE of 27.5% at 1000 cd m−2 (5.6 V), in which the
EQE value was the highest among those obtained by non-
doped orange devices. The red device of [Pt(pm4z)2] reveals
a maximum luminance (Lmax) of 104 830 cdm−2 at 16.2 V
(2919 mA cm−2) and CIEx,y coordinates of (0.61, 0.39). The max-
imum efficiencies were measured to be 23.8%, 31.8 cd A−1, and
29.4 lm W−1, all of them are higher than those obtained using
[Pt(tpm4z)2] as emitter, c.f. 19.7%, 17.3 cd A−1, and 9.4 lm W−1
with comparable CIEx,y coordinates of (0.65, 0.35), which is due
to low quantum yields in neat films (Φ = 61%, see Figure S10
in the Supporting Information).
These high device performances can be attributed to the
combined contribution from both high emission quantum
yield of thin-film as well as the aligned horizontal orientation of
the emission dipoles. It has been reported that the well aligned
transition dipole moment in the plane of the substrate can
increase the light out-coupling efficiency of the OLED compared
to devices with randomly oriented emitting dipole moment.[27]
Table 5. EL data of OLED fabricated using respective Pt(II) metal-based emitters.
max [cd m−2
]
Imax [mA cm−2
2193
]
CEmax [cd A−1
74.0
]
PEmax [lm W−1
]
EML
Vona) [V]
2.6
EQEmax [%]
24.9
at 103 nitb) [%, (V)]
22.7% (7.0)
CIE [x,y]
0.48, 0.50
0.56, 0.44
0.61, 0.39
0.65, 0.35
L
[Pt(pm2z)2]
[Pt(tpm2z)2]
[Pt(pm4z)2]
[Pt(tpm4z)2]
316400 (16.6 V)
250530 (17 V)
104830 (16.2 V)
50510 (19.2 V)
58.5
37.4
29.4
9.4
2.6
1697
27.5
64.2
27.5% (5.6)
2.6
2919
23.8
31.8
16.0% (6.2)
2.4
1405
19.7
17.3
17.4% (8.8)
a)Turn-on voltage at which emission became detectable; b)The values of EQE of device at 1000 cd m−2; the numbers in parentheses stand for the corresponding driving
voltages.
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Figure 9. Angle-dependent PL intensity of p-polarized light at peak emission for a) [Pt(pm2z)2], b) [Pt(tpm2z)2], c) [Pt(pm4z)2], and d) [Pt(tpm4z)2]
(circle) compared to simulated profiles (lines) with a different ratio of horizontal dipoles (Θ) [dashed line for fit, solid line for Θ = 1 (fully horizontal),
Θ = 0. 9, and Θ = 0.67 (isotropic)].
In order to analyze the orientation of the transition dipole
moment of the emitters, we measured the angular dependence
of the p-polarized PL intensity, for which all simulated data are
shown in Figure 9 for scrutiny. Angle-dependent PL spectra at
various degrees are also shown in Figure S11 in the Supporting
Information. As can be seen, all horizontal dipole ratio (Θ) of
those Pt(II) complexes are greater than 80%, which consistent
with the observed high efficiency of the OLED devices. Hence,
the high EQE of 27.5% observed for the emitter [Pt(tpm2z)2] at
1000 cd·m−2 is attributed to both the high PLQY (Φ) of 86%
and horizontal dipole ratio (Θ) of 85%.
Last but not least, we also like to point out that the recent
studies on [Pt(fppz)2] and analogues have already confirmed the
high stability for this class of nondoped OLED devices.[28] Their
robustness can be rationalized by the well organized and tightly
stacked alignment of Pt(II) emitters upon deposition into EML
thin film layer; hence, the fabricated OLED device is no longer
subject to the commonly occurred exciton-polaron-induced
aggregation degradation. The durability of such materials in
OLED applications is thus very promising.
pattern and photophysical properties. In addition to the mole-
cular identity, the stacking patterns of the titled Pt(II) com-
plexes, as revealed by GIXD, in terms of tilting angle and
stacking distances play a crucial role in manipulating the emis-
sion properties. Therefore, harnessing the stacking pattern in
solid states and hence, tuning the intermolecular Pt···Pt inter-
action is of prime importance to promote the Pt(II) emitters in
fabrication of efficient OLED. We believe that this strategy of
packing molecules as a deck of poker cards can be far-reaching
via ingenious ligand design in combination with the increasing
power of the computational approaches to achieve exceedingly
broad emission tuning from blue to near infrared region.
4. Experimental Section
General Procedures: Solvents were dried over appropriate drying
agents and commercially available reagents were used without further
purification. All reactions were conducted under N2. Reactions were
monitored by precoated TLC plates (0.20 mm with fluorescent indicator
F254). Mass spectra were obtained on a JEOL SX-102A instrument
operating in electron impact (EI) or fast atom bombardment (FAB)
mode. 1H, 13C, and 19F NMR spectra were recorded on a Varian
Mercury-400 or an INOVA-500 instrument. Elemental analysis was
carried out with a Heraeus CHN-O-Rapid Elemental Analyzer.
3. Conclusion
General Procedures: Synthesis of [Pt(pm2z)2]—A mixture of
[Pt(DMSO)2Cl2] (0.40 g, 0.95 mmol), (pm2z)H (0.42 g, 1.98 mmol) and
Na2CO3 (0.60 g, 5.68 mmol) in 40 mL of 2-methoxyethanol was refluxed
for 12 h. The resulting precipitate was filtered and washed with deionized
water, diethyl ether to afford a light yellow solid (0.47 g, 70%). Other
Pt(II) complexes, namely: [Pt(tpm2z)2], [Pt(pm4z)2], and [Pt(tpm4z)2]
In this study, based on the strategic design and synthesis of
pyrimidinyl pyrazolate Pt(II) complexes, in which the noncoor-
dinating N-atom is placed at various positions, we are able to
tune the solid-state stacking properties, and further shed light
on the relationship among structure, molecular aggregation
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substrates having a sheet resistance of 15 Ω sq−1. Prior to use the ITO
surface was cleaned ultrasonically; i.e., with acetone, methanol, and
deionized water in sequence and finally with UV-ozone. The deposition
rate of each organic material was ca. 1–2 Å s−1. The J–V–L characteristics
of the devices were measured simultaneously in a glove box using a
Keithley 2614B source meter equipped with a calibration Si-photodiode.
EL spectra were measured using a photodiode array (Ocean Optics
USB2000+).
Measurement and Simulation of Angle-Dependent PL Spectra: The
angular dependent p-polarized fluorescence intensity was measured
in an attempt to analyze the transition dipole moment of emitters.
The experimental setup was similar to that described in literature,[31]
which composed of a motorized rotation stage, a fused silica-based
half-cylindrical len, a longpass filter to stop the excitation beam, a
polarizer, and a fiber-guided spectrometer (Ocean Optics USB4000)
to collect the polarized emission. A continuous-wave Nd:YAG laser
(355 nm, 10 kHz) was used as the excitation source which was fixed at
an incident angle of 45°. The sample was deposited on top of a quartz
substrate and encapsulated under N2 atmosphere. Intensity of the
p-polarized emission was recorded from 0° to 90° in steps of 5°. The
data were analyzed using SETFOS 4.5 (Fluxim AG, Switzerland) to fit
the ratio of horizontal dipoles (Θ). A completely random orientation in
tris-(8-hydroxyquinoline)aluminum (Alq3) with Θ = 0.66 was employed
for system calibration. As shown in Figure S12 in the Supporting
Information, the refractive index n and extinction coefficient k were
determined by ellipsometry (J. A. Woollam α-SE) and analyzed using
Woollam CompleteEASE software.
were prepared under similar experimental condition in 81%, 68%, and
66%, respectively.
General Procedures: Spectral Data of [Pt(pm2z)2]—1H NMR (500 MHz,
d6-DMSO, 353 K): δ 10.41 (dd, J = 5.9, 2.1 Hz, 2H), 9.17 (dd, J = 4.9,
2.1 Hz, 2H), 7.82–7.79 (m, 2H), 7.28 (s, 2H). 19F NMR (471 MHz,
DMSO, 353 K): δ -59.64 (s, 6F). MS(FAB), m/z 621.1 [M]+. Anal. Calcd.
for C16H8N8: C, 30.93; H, 1.30; N, 18.03. Found: 30.89; H, 1.30; N, 17.89.
General Procedures: Spectral Data of [Pt(tpm2z)2]—1H NMR
(500 MHz, d7-toluene, 353 K): δ 10.27 (d, J = 6.2 Hz, 2H), 7.15 (s, 2H),
6.39 (d, J = 6.3 Hz, 2H), 1.14 (s, 18H). 19F NMR (471 MHz, d7-toluene,
353 K): δ -60.74 (s, 6F). MS(FAB), m/z 733.2 [M]+. Anal. Calcd. for
C24H24N8: C, 39.30; H, 3.30; N, 15.28. Found: 39.32; H, 3.52; N, 15.20.
General Procedures: Spectral Data of [Pt(pm4z)2]—1H NMR (400 MHz,
d6-DMSO, 353 K): δ 9.93 (d, J = 1.0 Hz, 2H), 8.86 (d, J = 5.2 Hz, 2H),
7.80 (dd, J = 5.2, 1.2 Hz, 2H), 7.16 (s, 2H). 19F NMR (471 MHz,
d6-DMSO, 353 K): δ -59.67 (s, 6F). MS(FAB), m/z 621.1 [M]+. Anal. Calcd.
for C16H8N8: C, 30.93; H, 1.30; N, 18.03. Found: 30.93; H, 1.34; N, 18.09.
General Procedures: Selected Crystal Data of [Pt(pm4z)2]—C16H8F6N8Pt;
M = 621.39; monoclinic; space group = P21/c; a = 10.8827(4) Å,
b = 7.2509(2) Å, c = 22.2865(7)) Å, β = 101.2077(10)°; T = 150(2) K;
λ(Mo Kα) = 0.71073 Å; V = 1725.08(10) Å3; Z = 4; ρcalcd = 2.393 mg·m−3
F(000) = 1168; µ = 8.221 mm−1; crystal size = 0.293 × 0.127 ×
0.042 mm3; 13 850 reflections, 3954 independent reflections (Rint
;
=
0.0211), restraints / parameter = 24 / 293; GOF = 1.080, final R1
[I > 2σ(I)] = 0.0265, and wR2(all data) = 0.0499; largest diff. peak and
hole = 0.884 and −0.905 e Å−3
.
General Procedures: Spectral Data of [Pt(tpm4z)2]—1H NMR
(500 MHz, d7-DMF, 353 K): δ 10.75 (d, J = 1.0 Hz, 2H), 8.89 (s, 2H),
7.36 (s, 2H), 1.48 (s, 18H). 19F NMR (471 MHz, d7-DMF, 353 K):
δ -61.98 (s, 6F). MS(FAB), m/z 733.2 [M]+. Anal. Calcd. for C24H24N8: C,
39.30; H, 3.30; N, 15.28. Found: 39.40; H, 3.52; N, 15.33.
Supporting Information
Grazing-Incidence X-Ray Diffraction (GIXD) Measurement: GIXD
was performed at beamline 01C2 of Taiwan Light Source at National
Synchrotron Radiation Research Center (NSRRC), Taiwan. All studied thin
films were deposited on the Si substrate, and the GIXD data were collected
using a 2D image plate (Mar345) and 12 keV X-ray beam with wavelength
of 1.033 Å. The incidence angle of X-ray beam was ≈0.2° for GIXD.
Electron Density Map: Fourier reconstruction of the electron density
maps were carried out using the general formula for 2D electron density
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was supported by the funding from Ministry of Science
and Technology (MOST), featured areas research program within the
framework of the Higher Education Sprout Project administrated by
Ministry of Education (MOE) of Taiwan, and City University of Hong Kong,
Hong Kong SAR. The authors are also grateful to the National Center
for High-performance Computing (NCHC) and National Synchrotron
Radiation Research Center for the computer time and facilities.
as ρ (x,y)=
I(hk)exp[2πi(hx + ky)+ iϕ(hk)], where ϕ(hk) are phases
∑
hk
of structure factor (equal to 0 or π) and I(hk) is intensity of Bragg
peaks obtained from the GIXD patterns after background subtraction
and correlation of Lorentz and polarization factors, respectively. The
correct phase can be determined on the basis of physical merits of
the reconstructed electron density map, aided by other information
on the system coming from, e.g., other spectra or molecular simulation.
The data employed for reconstruction of electron density maps are listed
in Table S1a–d in the Supporting Information.
Conflict of Interest
Computational Method: All calculations were performed with the
Gaussian 16 program package.[29] The geometry optimization of ground
states of the four Pt(II) complexes were simulated with density functional
theory (DFT) at the hybrid functional PBE1PBE-D3/LANL2DZ (Pt) and
PBE1PBE-D3/6-31g(d,p) (H, C, N, F) levels using DMSO as the solvent.[30]
The solvent effect is based on the polarizable continuum model (PCM),
which is supported implemented in the Gaussian 16 program. The
optimized structures of the monomer for the four Pt(II) complexes were
used to calculate the five lowest singlet (S0 → S5) and triplet optical
electronic transitions (S0 → T5) using the time-dependent density
functional theory (TDDFT) method. The optimized excited state of T1 and
S1 for [Pt(pm4z)2] dimer and trimer were also performed. For both singlet
and triplet optical and emission transitions, Mulliken population analysis
(MPA) was applied to obtain the electron density distribution of each
atom in specific molecular orbital of the Pt(II) complexes as well as to
calculate the metal-to-ligand charge transfer (MLCT) in each assignment
during the singlet and triplet optical and emission transitions.
The authors declare no conflict of interest.
Keywords
aggregation, organic light emitting diodes, phosphorescent, platinum,
stacking interaction
Received: January 29, 2019
Revised: April 14, 2019
Published online:
OLED Fabrications: All chemicals were purified through vacuum
sublimation prior to use. The OLED were fabricated through vacuum
deposition of the materials at 10−6 torr onto the ITO-coated glass
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