Near-Infrared Emission Induced by Shortened Pt-Pt Contact: Diplatinum(II) Complexes with Pyridyl Pyrimidinato Cyclometalates

Article abstract of DOI:10.1021/acs.inorgchem.9b01754

Four diplatinum(II) complexes with the formula [Pt(pypm)(μ-Fⁿ)]₂ (2, 3a-c) bearing both a pyridine-pyrimidinate chelate and formamidinate bridge, where (pypm)H and FⁿH stand for 5-(pyridin-2-yl)-2-(trifluoromethyl)pyrimidi

Full text of DOI:10.1021/acs.inorgchem.9b01754

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Near-Infrared Emission Induced by Shortened Pt−Pt Contact:
Diplatinum(II) Complexes with Pyridyl Pyrimidinato Cyclometalates
Sheng Fu Wang,† Li-Wen Fu,† Yu-Chen Wei,‡ Shih-Hung Liu,‡ Jia-An Lin,‡ Gene-Hsiang Lee,‡
Pi-Tai Chou,*,‡ Jian-Zhi Huang,§ Chih-I Wu,§ Yi Yuan,∥ Chun-Sing Lee,*,∥ and Yun Chi*,†,∥
†Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua
University, Hsinchu 30013, Taiwan
‡Department of Chemistry and Instrumentational Center, National Taiwan University, Taipei 10617, Taiwan
§Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 10617 Taiwan
∥Department of Materials Science and Engineering, Department of Chemistry, City University of Hong Kong, Hong Kong SAR,
People’s Republic of China
S
* Supporting Information
ABSTRACT: Four diplatinum(II) complexes with the formula
[Pt(pypm)(μ-Fn)]2 (2, 3a−c) bearing both a pyridine-pyrimidinate
chelate and formamidinate bridge, where (pypm)H and FnH stand
for 5-(pyridin-2-yl)-2-(trifluoromethyl)pyrimidine and functional
i
formamidines with various substituents of Pr (n = 1), Ph (n = 2),
t
C6H4 Bu (n = 3), and C6H4CF3 (n = 4), were synthesized en route
from a mononuclear intermediate represented by [Pt(pypm)Cl-
(F1H)] (1). Single-crystal X-ray diffraction studies confirmed the
structure of 1 and 3a comprised of an individual “Pt(pypm)” unit
and two “Pt(pypm)” units with a Pt···Pt distance of 2.8845(2) Å,
respectively. Therefore, in contrast to the structured emission of
mononuclear 1 with the first vibronic peak wavelength at 475 nm, all
other diplatinum complexes with shortened Pt···Pt separation
exhibited greatly red shifted and structureless metal−metal to ligand charge transfer (MMLCT) emission that extended into
the near-infrared region in solid states. Their photophysical characteristics were measured under three distinctive morphological
states (i.e., crystals, sublimed powders, and vacuum-deposited thin films) by steady-state UV−vis spectroscopy, while retention
of Pt···Pt interactions in deposited thin films of 2 and 3a−c was confirmed using Raman spectroscopy, demonstrating lowered
Pt···Pt stretching at 80−200 cm−1. Most importantly, complexes 3a−c exhibited a gradual red shift with the trends crystals <
sublimed powders < vacuum-deposited thin films, a result of increased intermolecular π−π stacking interactions and Pt···Pt
interactions, while crystalline samples exhibited the highest luminescence among all three morphological states due to the fewest
defects in comparison to other morphologies. Finally, 3b was selected as a nondoped emitter for the fabrication of NIR-emitting
OLEDs, giving an electroluminescence peak at 767 nm and a maximum external quantum efficiency of 0.14% with negligible
roll-off.
From the prospect of material designs, NIR emitters may be
classified as either (i) organic or (ii) metal-based (i.e., mainly
transition-metal-based) materials,4−6 and their luminescence
efficiencies are both lower than those observed in their red-
emitting counterparts, mainly due to the increased non-
radiative decay rate as predicted by the energy gap law.7,8
However, among the transition-metal-based NIR emitters,
Pt(II) emitters possess a distinctive square-planar geometry,
which enables intermolecular π−π stacking interactions and
intimate Pt···Pt contacts.9−11 They next induce a drastic
change in electronic properties at the excited states, termed
metal−metal to ligand charge transfer (MMLCT) or excimeric
INTRODUCTION
■
Organic light-emitting diodes (OLEDs) have been widely used
in the fabrication of displays of mobile phones and solid-state
lighting luminaries. Many of the corresponding red, green, and
blue (RGB) emitters have already passed the harsh industrial
assessments for commercial applications. However, the
development of near-infrared (NIR)-emitting dyes and
OLED devices having emission peak wavelengths beyond
700 nm are still lagging behind as well as being relatively
unexplored.1−3 Lighting in the NIR region is virtually invisible
to human eyes and can exert greater penetration into human
and cell tissues than visible light. As a result, NIR-emitting
OLEDs are favored over traditional OLEDs to provide
illumination for artificial intelligence, security recognition,
biosensing, and therapeutic applications.
Received: June 13, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ππ* emission,12,13 which is in sharp contrast to the octahedral
Os(II) and Ir(III) emitters that show mixed metal to ligand
charge transfer (MLCT) and ligand-centered ππ* transition
characters.14−22 Furthermore, the MMLCT transition involves
the transfer of an electron from a filled Pt···Pt orbital of the
aggregated species (i.e., as dimer or oligomer) to a vacant π*
orbital of chelate, often with a lowered transition energy in
comparison to that of the corresponding monomer emission,
together with an enhanced emission efficiency and a shortened
radiative lifetime. As a proof of concept, Ma et al. reported a
series of Pt(II) dimers [Pt(C∧N)(μ-pz)]2, where (C∧N)H = 2-
(4,6-difluorophenyl)pyridine and (pz)H = pyrazole, 3-methyl-
5-tert-butylpyrazole, and 3,5-bis(tert-butyl)pyrazole.23,24 As
expected, these bridging pyrazolates have the capability to
hold the Pt(II) atoms in close proximity, while the bulky
substituents on pyrazolates force the Pt···Pt separation to turn
shorter in distance, inducing a substantial red shift in emission
energy gap.
Scheme 1. Transformation from Pt(II) Complex 1 to 2 and
Drawings of Pt(II) Derivatives 3a−c, While the Void Site of
the [Pt(pypm)(F1)] Intermediate Is Indicated as a Square
Other bridging chelates useful for making dimeric
diplatinum complexes include acetamide,25 pyridinethio-
late,26,27 pyridonato,28 tartarate,29 and their functional
analogues.30−32 The resulting diplatinum dimers were typically
prepared from the coupling of bridging ligands with the readily
accessible precursors [Pt(C∧N)(μ-Cl)]2 (or analogues), while
the tuning of emission was best executed by varying both π-
conjugation and electronic properties of C∧N cyclometalates.
Their syntheses and photophysical properties were recently
reviewed.33,34 Furthermore, these diplatinum complexes were
also found useful for fabrications of NIR-emitting OLEDs.35−38
In the present contribution, we intend to use 5-(pyridin-2-yl)-
2-(trifluoromethyl)pyrimidine39,40 and functionalized forma-
midinates41 as cyclometalating and bridging chelates, respec-
tively, in synthesizing relevant NIR-emitting complexes. Our
goal is to explore the structure−property relationship that
affects the photophysical properties, such as the corresponding
emission peak wavelengths and quantum yields.
Moreover, due to the coexistence of a chloride and an
amidinic N−H fragment in close vicinity, complex 1 should
have a high tendency to afford the unsaturated intermediate
“[Pt(pypm)(F1)]” upon removal of the “HCl” entity. In fact,
this transformation can be achieved by heating of 1 with Ag2O
in refluxing 1,2-dichloroethane, giving the formation of the
dark red Pt(II) dimer [Pt(pypm)(μ-F1)]2 (2) from the in situ
generated, unsaturated [Pt(pypm)(F1)]. In sharp contrast,
treatment of [Pt(pypm)(μ-Cl)]2 with N,N′-diarylformami-
dines, such as F2H, F3H, and F4H with the aryl group being
t
phenyl, C6H4 Bu and C6H4CF3, respectively, afforded the dark
red Pt(II) dimers [Pt(pypm)(μ-Fn)]2 (n = 2−4, 3a−c) without
the isolation of the corresponding yellow intermediate. We
hypothesize that such a discrepancy is caused by the relatively
electron deficient aryl substituents on amidines, which
increased the acidity of its N−H fragment and promoted a
faster and concomitant removal of “HCl” in the presence of the
less reactive chloride scavenger K2CO3. Moreover, formation
of the bridging amidinate, rather than the chelating amidinate,
is probably caused by the lack of an aryl (or alkyl) substituent
at the central carbon atom of the amidinate.55−57
RESULTS AND DISCUSSION
■
Synthesis and Characterization. 5-(Pyridin-2-yl)-2-
(trifluoromethyl)pyrimidine (pypm)H was selected for its
close resemblance to the chelate 5-(pyridin-2-yl)-3-(trifluor-
omethyl)-1H-pyrazole (pypz)H
All of these Pt(II) complexes were characterized by mass
spectrometry, NMR spectroscopy, and elemental analyses.
Particularly, complexes 1 and 3a were subjected to single-
crystal X-ray analyses to unambiguously confirm their
structures. Figure 1 illustrates the molecular geometry and
labeling scheme of 1, together with selected bond lengths and
angles. This molecule consists of a pypm cyclometalate, to
which the chloride and bent amidine are trans to the
pyrimidinato and pyridyl fragments, respectively. Furthermore,
the amidine group is oriented in such a way as to direct its N−
H fragment to the central Pt(II) atom, with a nonbonding
N(4)···N(5) distance of 2.328(3) Å; all of these structural
features are akin to those observed in the relevant Pt(II)
amidine complexes.58,59
for which its functional derivatives have been extensively
employed in the assembly of Pt(II)-based phosphors.42−44 In
the present study, the preparation of mononuclear [Pt(pypm)-
Cl(F1H)] (1) demanded the in situ generation of both the
hypothetical Pt(II) dimer [Pt(pypm)(μ-Cl)]2 and N,N′-
diisopropylformamidine: i.e., F1H. Accordingly, the [Pt-
(pypm)(μ-Cl)]2 dimer was obtained by heating K2PtCl4 with
(pypm)H in a mixture of water and ethoxyethanol,45−49 while
the amidine F1H was prepared in situ from the coupling of
diisopropylamine and triethyl orthoformate.50 Without further
purification, heating of [Pt(pypm)(μ-Cl)]2 and F1H in 1,2-
dichloroethane afforded the expected yellow Pt(II) complex 1
(cf. Scheme 1). It is notable that the formation of 1 proceeded
with the amidine-induced cleavage of dinuclear [Pt(pypm)(μ-
Cl)]2. Similar donor-induced dissociation of dimers has been
well documented in the literature.51−54
Next, the cture of 3a is depicted in Figure 2, consisting of
two essentially planar [Pt(pypm)] units, together with two cis-
oriented, bridging amidinates. However, the pypm chelate on
each [Pt(pypm)] unit is arranged in an antiparallel fashion to
B
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
due to their different intermolecular packing interactions
imposed during sample preparation. It is notable that all
studied Pt(II) complexes showed no detectable emission in
solution at room temperature, which is attributed to the fast
collisional quenching by solvents, for having flexible amidine
pendant in 1 and, then, lowered the NIR emission efficiency of
Pt(II) dimers 2 and 3a−c, respectively.
UV−visible absorptions in CH2Cl2 solution/thin film and
solid-state emission spectra of Pt(II) monomer 1 and dimer 2
are shown in Figure 3 and Figure S1. The corresponding
Figure 1. Structure of [Pt(pypm)Cl(F1H)] (1) with thermal
ellipsoids shown at 50% probability. Selected bond distances and
interaction (Å): Pt−C(6) = 1.950(3), Pt−N(1) = 2.037(2), Pt−N(4)
= 2.018(2), Pt−Cl(1) = 2.3901(7), and N(4)···N(5) = 2.328(3).
Selected bond angles (deg): N(1)−Pt−N(4) = 175.33(10) and
C(6)−Pt(1)−Cl(1) = 172.42(8).
maximize the π−π stacking interactions. The observed
intramolecular Pt···Pt distance of 2.8845(2) Å is similar to
that observed in many structurally characterized dimeric Pt(II)
complexes60 but is significantly longer than the closest
intermolecular Pt···Pt contact of ∼6.790 Å. In addition, this
intramolecular Pt···Pt separation is also longer than the
intraligand N···N nonbonding contact of amidinates (i.e.,
N(4)−N(5A) = 2.339(2) Å) and, as a result, the intra-
molecular pypm to pypm chelate separation, which is defined
by the spatial distance between the C(6) atom of the
pyrimidinato fragment and the N(1A) atom of an adjacent
pyridine, is calculated to be 3.570(2) Å. Finally, due to the
adoption of this clamshell geometry, the intramolecular center
to center distances between the adjacent pyrimidinato and
pyridyl rings (i.e., 4.144 Å) are notably longer than the
corresponding intermolecular pypm to pypm separation (i.e.,
3.758 Å).
Figure 3. UV−vis absorption recorded in CH2Cl2 at room
temperature and emission spectra of crystalline samples and sublimed
powders of Pt(II) complexes 1 and 2.
numerical data are depicted in Table 1. For both complexes 1
and 2, the absorptions below 400 nm are primarily due to ππ*
transitions originating from pypm cyclometalate and amidine
ligand. In comparison, the broadened absorption bands of 2
with multiple peak wavelengths over the region of 400−650
nm are attributed to mixed 1MMLCT and 3MMLCT
transitions, enhanced by the strong spin−orbit coupling due
to the diplatinum unit. In fact, the lack of this lower energy
absorption band in 1 also provided critical evidence to support
this assignment.
Photophysical Properties. The UV−visible absorption
spectra of all studied Pt(II) complexes were measured in
CH2Cl2 solution, while emission spectra were recorded using
three physically distinctive samples, namely crystals (cr),
sublimed powders (su), and vacuum-deposited thin films (th),
Figure 2. (a) Structural drawing and (b) packing diagram of [Pt(pypm)(μ-F2)]2 (3a) with thermal ellipsoids shown at 50% probability. Selected
bond distances and interactions (Å): Pt(1)−N(1) = 2.023(2), Pt(1)−N(4) = 2.020(2), Pt(1)−N(5) = 2.129(2), Pt(1)−C(6) = 1.960(2), C(6)···
N(1A) = 3.570(2), Pt(1)···Pt(1A) = 2.8845(2), and N(4)···N(5A) = 2.339(2). Selected bond angles: N(1)−Pt(1)−N(4) = 176.43(8) and N(5)−
Pt(1)−C(6) = 176.80(9).
C
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Photophysical Data of the Studied Pt(II)-Based Monomer 1 and Dimers 2 and 3a−c
b
b
b
em λmax/nm
Φ/%
τ/ns
a
abs λmax/nm (ε/103 M−1 cm−1
)
cr
su
th
cr
su
th
cr
su
th
1
2
3a
3b
3c
265(21), 359(4)
475, 512, 552
477, 511, 552
476, 511, 537
8.4
2.8
24.0
12.4
12.4
7.3
1.3
4.8
8.2
7.0
5.5
<1.0
<1.0
<1.0
<1.0
752
41
177
185
202
578
40
125
167
187
387
26
41
31
84
256(42), 436(3.5), 524(3.8)
262(52), 325(36), 417(4.7), 528(2.8)
263(58), 321(40), 428(4.7), 530(3.1)
271(29), 331(27), 519(1.5)
721
688
692
698
729
720
711
706
732
758
769
726
a
b
Data recorded at 1.0 × 10−5 M in CH2Cl2. PL data were recorded as crystalline samples (cr), sublimed fine powders (su), and vacuum-deposited
thin films (th).
Furthermore, the polycrystalline samples of Pt(II) complex
1 showed a structured emission profile with the first peak
maximum at 475 nm and two lower-energy vibronic bands at
512 and 552 nm, which suggested the dominant ligand-
centered ππ* transition character. The lowered emission
quantum yield (∼8%) confirmed the existence of effective
quenching pathways, which agreed with the existence of many
loosely attached functional appendages on the molecule. Upon
a switch from crystals to sublimed powders, the highest energy
0−0 emission reveals a notable decreased in relative intensity
due to a change in local environment and hence intermolecular
interaction. In sharp contrast, the dimer complex 2 exhibits a
much broadened and red shifted MMLCT emission, showing
that the intermolecular Pt···Pt contact has significantly affected
the emission peak wavelength. In addition, complex 2
exhibited a relatively minor change in photophysical properties
in all three morphological states: i.e., as crystals, sublimed
powders, and deposited thin films (cf. Figure 3 and Table 1).
Next, UV−visible absorption and emission spectral data of
the three aryl-substituted Pt(II) dimers 3a−c are depicted in
Figure 4, Figure S2, and Table 1. Their photophysical
poorer the emission efficiency . This red shifting becomes
more noticeable for both the sublimed samples (λmax: 3a, 720
nm; 3b, 711 nm; 3c, 706 nm) and vacuum-deposited thin films
(λmax: 3a, 758 nm; 3b, 769 nm; 3c, 726 nm), together with a
reduction in emission quantum yield especially for vacuum-
deposited thin films, revealing the consequence of the energy
gap law.7,8 Since the emission efficiencies of thin films for 2
and 3a−c are too weak to be detected, we assign the PLQY of
these samples to be less than 1%, which is the instrumental
detection limit on the basis of an integrated sphere for a solid
sample. The detected red shift suggests the formation of more
compressed Pt···Pt contacts, a greater number of π−π stacking
interactions, and enhanced MMLCT transition character for
the samples prepared, which follows the qualitative order
vacuum‐deposited thin film > sublimed powder
> crystalline samples
The dependence of the photophysical properties vs the
morphological arrangement was also investigated by powder X-
ray diffraction studies (XRD). As shown in Figure S3, all
studied fine crystals of 3a−c revealed intense and sharp
diffraction signals that correspond to the ordered packing
structure with larger grain sizes. However, the sublimed
powders and vacuum-deposited thin films showed progres-
sively reduced diffraction signals, indicating that these Pt(II)
complexes have poor crystallinity in both states. Especially in
vacuum-deposited thin films, the latter are even worse than
that of an amorphous arrangement. This could explain the
dramatic changes in the quantum yields for diplatinum
complexes 2 and 3a−c in different solid morphologies. We
attribute this phenomenon to the nonradiative process induced
by the defects under amorphous conditions, since a triplet
exciton is prone to be quenched owing to the longer diffusion
length in comparison to that of a singlet exciton.61,62 This
could also rationalize the morphological-dependent PLQY of
all title complexes, among which the crystalline samples exhibit
the highest PLQY because of their perfectly packed structures,
while both the sublimed powder and vacuum-deposited thin
film show much lower PLQYs due to the lack of an ordered
packing arrangement.
In order to further evaluate the relationship between
emission peak wavelengths and quantum yields, we repeated
the PL measurement in a PMMA matrix with a doping level of
2 wt % at room temperature, with the aim that the rigid
PMMA polymer can restrict both the rotational and vibration
motion of Pt(II) emitters, giving improved photophysical
properties. The corresponding spectra are shown in Figure S4
in the Supporting Information, while the numerical data are
presented in Table 2. As can be seen, the majority of spectral
features were akin to those obtained in crystalline samples,
Figure 4. UV−vis absorption recorded in CH2Cl2 and solid-state
emission of crystalline samples of Pt(II) dimers 3a−c.
properties are similar to those of aforementioned Pt(II)
complex 2 with a slight variation in emission peak position.
Moreover, the emission peak of the crystalline samples 3a−c
appears in a narrow range between 688 and 698 nm, among
which the phenyl derivative 3a showed the highest emission
efficiency of 24% in comparison to the other Pt(II) complexes
(2, 2.8%; 3b, 12.4%; 3c, 12.4%). Moreover, 3a exhibits the
greatest blue-shifted emission maximum among all these
complexes, as shown in Table 1. Hence, the variation of
emission quantum yield correlates with their relative peak
emission wavelength: i.e., the longer the wavelength, the
D
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
pypm chelate centered reduction process. Moreover, the redox
couples of 3a−c were reversible, among which the phenyl
derivative 3a showed the redox peaks at 0.34 and −2.06 V;
both of the positive shifts in comparison to those of 2 are
attributed to a greater electron withdrawing character of the
phenyl groups on bridging amidinates in comparison to that of
the isopropyl substituents. After introduction of a 4-tert-butyl
substituent in 3b, its redox peaks are cathodically shifted to
0.27 and −2.08 V, while the electron-withdrawing CF3
substituents caused an opposite shift to 0.55 and −1.92 V
for 3c. Both observations are consistent with the anticipated
electronic character of these functional aryl substituents.
Finally, in all these Pt(II) complexes, the changes in oxidation
potentials are found to be much greater than the correspond-
ing changes in reduction potentials. These results indicate the
greater influence of amidinates on the intramolecular Pt···Pt
bonding but less on the pypm chelate, as all reduction
processes occur at the pypm cyclometalate, for which the long
separation from bridging amidinates has less influence from
their functional substituents.
Table 2. Emission Data of 2 wt % of the Studied Pt(II)
Complexes in PMMA Matrix at Room Temperature
a
a
em λmax/nm
Φ/%
τobs/ns
kr/105 s−1
knr/105 s−1
1
2
3a
3b
3c
474, 512, 547
22.6
8.0
12.1
20.1
13.5
1911
41.4
177.5
165.4
336.4
1.18
1.93
6.81
12.2
4.01
4.05
222
49.5
48.3
25.7
715
698
699
676
a
kr = QY/τobs and knr = (1 − QY)/τobs
.
except that the emission peak maximum of 3c in a PMMA
matrix is blue-shifted to 676 nm in comparison to that
observed for the crystalline sample (698 nm), which can be
attributed to the media effect. Moreover, the observed
nonradiative rate constants of Pt(II) complexes 2 and 3a−c
are found to be proportional to the emission wavelengths,
which, in a qualitative manner, confirms the prediction of
energy gap law.
Electrochemical Properties. The electrochemical proper-
ties of Pt(II) complexes 1, 2, and 3a−c were studied using
cyclic voltammetry, and their CV traces and numeric data are
depicted in Figure 5 and Table 3. First, Pt(II) complex 1
Raman Spectroscopy. To investigate the possible intra-
molecular Pt···Pt interaction of diplatinum complexes, we
measured the ground-state Raman spectra and conducted
calculation on Pt−Pt stretching vibrations in the region
between 80 and 200 cm−1 (Figure 6 and Figure S5). Since
Figure 5. Cyclic voltammograms measured in CH2Cl2 for anodic
sweep and in THF for cathodic sweep.
Table 3. Electrochemical Data of All Studied Pt(II)
Complexes
Figure 6. Ground-state Raman spectra of 1, 2, and 3a−c of vacuum-
deposited thin films with a thickness of 1000 Å and excitation
wavelength λex = 532 nm.
a
a
Eo1/x2/V [ΔEp/V]
Er1e/2/V [ΔEp/V]
1
2
3a
3b
3c
1.32 [irr]
−2.25 [0.17]
−2.36 [irr]
−2.06 [0.15]
−2.08 [0.12]
−1.92 [0.05]
0.14 [0.15]
0.34 [0.10]
0.27 [0.09]
0.55 [0.08)
crystalline and powder samples exhibit severe luminescence
interference to the Raman signals, we choose the vapor-
deposited thin film, which is almost nonemissive under the
conditions of Raman measurements. In general, there are two
major peaks, one occurring at around 100 cm−1 and the other
at 150 cm−1 in the Raman spectra. The broadened peak profile
results from the superposition of multiple low-frequency
Raman transitions correlated to the Pt···Pt stretching vibration
(see Figures S6−S9 for the calculated vibrational modes). This
assignment is also supported by the absence of low-frequency
bands for monomer 1. The calculated Raman spectra indicate
that different substituents give rise to a distinctive distribution
of transitions, which show a higher energy peak at 150 cm−1 in
2 and 3a but are not observable for complexes 3b,c (Figure S5,
Raman spectra). In brief, this result, together with the single-
crystal X-ray data of 3a, confirms the existence of non-
a
E1/2 (V) refers to [(Epa + Epc)/2], where Epa and Epc are the anodic
and cathodic peak potentials, respectively, referenced to the ferrocene
redox couple (Fc/Fc+), ΔEp = Epa − Epc is for the reversible process,
and “irr” denotes an irreversible process. The oxidation and reduction
experiments were conducted in CH2Cl2 and THF, respectively.
showed a positively shifted oxidation peak potential at 1.32 V
and reversible reduction peak at −2.25 V, confirming its
mononuclear nature. In contrast, Pt(II) complex 2 exhibited
quasi-reversible oxidation and irreversible reduction peaks at
0.14 and −2.36 V, for which the negatively shifted oxidation
potential is consistent with the formation of a diplatinum unit,
while the small variation of reduction potential indicated a
E
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. (a) Current density−voltage−radiance and (b) EQE-current density characteristics of a complex 3b based device (insert: EL spectrum at
50 mA cm−2).
negligible Pt···Pt interactions for 2 and 3a−c, similar to the
Pt···Pt interactions documented in the literature.63−65
characterized using spectroscopic methods and single-crystal
X-ray structure determinations. Complexes 1 and 3a were
exploited as paradigms for structural analysis; they consist of
one and two quasi-planar “Pt(pypm)” units, together with a
single formamidine and two bridging formamidinates,
respectively. Furthermore, complex 3a reveals a shortened
Pt···Pt contact and a structureless MMLCT emission with a
peak wavelength at 688 nm as crystalline samples. In contrast
to 2 with isopropyl-substituted formamidinates F1, all other
diplatinum complexes 3a−c bearing aryl-substituted amidi-
nates, i.e. F2−F4, exhibited red-shifted emission on changing
the morphological states from crystalline solids to sublimed
powders and vacuum-deposited thin films. This observation is
due to the enhanced π−π stacking interactions and Pt···Pt
contacts, as their packing density is probably arranged in an
ascending order: crystalline samples < sublimed powders <
vacuum-deposited thin films. The existence of Pt···Pt contacts
in the ground state is also confirmed by Raman spectroscopy,
showing Pt···Pt stretching from 80 to 200 cm−1. An OLED
with nondoped 3b realized electroluminescence with a peak
wavelength at 767 nm and maximum EQE of 0.14%.
Importantly, despite the existence of a dimeric structure with
notable Pt···Pt contacts, its performance was significantly lower
than that of the Pt(II) complex [Pt(fprpz)2],10 for which a
maximum EQE of over 24% was reported for an NIR OLED
device with an emission peak maximum at 740 nm. This
seemingly contrary result implies that the extensively
delocalized Pt···Pt interactions and high crystallinity are the
key factors that boost the NIR emission efficiencies.
Theoretical Analyses of UV−Vis Transition Proper-
ties. As evidenced by the low-frequency Raman spectra and
the steady-state UV−vis spectra, the transition characteristics
of 1, 2 and 3a−c were next validated by time-dependent
density functional theory (TDDFT) calculations, shown in
Figures S10−S14. The S0−S1 transition of 1 showed typical
absorption transition characteristics involving both a ligand-
centered ππ* transition mixed with some MLCT and
interligand chloride to pypm chelate (XLCT) charge transfer
contribution. The orbitals involved in the S0−S1 transition in 2
2
2
and 3a−c showed a large contribution of dz −dz orbitals,
leading to a small vibronic progression, which is apparently
lacking for complex 1. The deviation of absorption energy
between calculated and experimental results arises, in part,
from state mixing between singlet and triplet states, which is
not considered in the calculation and the incomplete basis set
being applied.
Electroluminescent Properties. To determine the
electroluminescent (EL) properties of the series of Pt(II)
dimer complexes, complex 3b was selected as an emitter to
fabricate NIR OLEDs due to its shortest observed emission
lifetime in a thin film. The device was fabricated with a
multilayered architecture of indium tin oxide (ITO)/
dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarboni-
trile (HATCN, 10 nm)/1,1-bis[4-[N,N′-di(p-tolyl)amino]-
phenyl]cyclohexane (TAPC, 40 nm)/4,4′,4″-tris(N-carbazol-
yl)-triphenylamine (TCTA, 10 nm)/complex 3b (20 nm)/
4,7-diphenyl-1,10-phenanthroline (BPhen, 10 nm)/1,3,5-tris-
(1-phenyl-1H-benzo[d]imidazole-2-yl)benzene (TPBi, 60
nm)/8-hydroxyquinolatolithium (Liq, 2 nm)/Al (100 nm).
As shown in Figure 7, the complex 3b based nondoped device
gives emission with an emission peak maximum at 767 nm,
which is consistent with the PL peak maximum at 769 nm
recorded as a vacuum-deposited neat film. Moreover, the
maximum external quantum efficiency (EQE) is measured to
be only 0.14%, which is consistent with the poor PLQY
recorded in the deposited thin films. Despite this, the as-
fabricated NIR OLED device exhibits a very small efficiency
roll-off at high current density (only dropping by 0.01% even at
a current density of 300 mA cm−2), which could be attributed
to the shortened emission lifetime of 3b.
EXPERIMENTAL SECTION
■
General Information and Materials. All reactions were
performed under a nitrogen atmosphere. Solvents were distilled
from appropriate drying agents prior to use. Commercially available
reagents were used without further purification. 5-(Pyridin-2-yl)-2-
(trifluoromethyl)pyrimidine (pypm)H was prepared using Pd-
catalyzed C−C coupling of 5-bromo-2-(trifluoromethyl)pyrimidine
and 2-(tributylstannyl)pyridine.39,66 Mass spectra were obtained on a
JEOL AccuTOF GCX instrument operating in field desorption (FD)
mode. 1H and 19F NMR spectra were recorded on a Bruker AVANCE
500 instrument or a Varian 400-MR instrument. Elemental analysis
was carried out with a Heraeus CHN-O rapid elemental analyzer.
Synthesis of N,N′-Dialkyl- and N,N′-Diarylformamidines.67
A mixture of diisopropylamine or functionalized aniline (50 mmol)
and triethyl orthoformate (25 mmol) was heated to 160 °C for 2 h.
After it was cooled to room temperature, the mixture was dried under
vacuum and the residue was recrystallized in a mixture of CH2Cl2 and
Conclusion. In summary, sky blue emitting mononuclear
Pt(II) complex 1 and NIR-emitting diplatinum complexes 2
and 3a−c bearing a pypm cyclometalate were synthesized and
F
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
hexane to give the desired compound. This compound was relatively
unstable and was used for subsequent reaction without further
purification.
1092.1. Anal. Calcd for C34H40F6N10Pt2: C, 37.37; H, 3.69; N, 12.82,
Found: C, 37.26; H, 3.61; N, 12.87.
Synthetic Procedure for [Pt(pypm)(μ-F2)]2 (3a), [Pt(pypm)(μ-
F3)]2 (3b), and [Pt(pypm)(μ-F4)]2 (3c). A mixture of K2PtCl4 (1.0
mmol) and 5-(pyridin-2-yl)-2-(trifluoromethyl)pyrimidine (pypmH,
1.1 mmol) in a mixture of water and ethoxyethanol was heated to
reflux for 36 h. After it was cooled to room temperature, the solution
was treated with 30 mL of water. The solid was filtered, thoroughly
washed with water, and dried under vacuum to give a black solid. To
this black solid were added respectively N,N′-diarylformamidine
(F2H, F3H, and F4H, 1.3 mmol), K2CO3 (3 mmol), and 20 mL of 1,2-
dichloroethane; the mixture was refluxed for 24 h. After that, the
solution was evaporated to dryness and the residue was extracted with
CH2Cl2 (50 mL). It was next washed with water (50 mL), dried over
anhydrous Na2SO4, filtered, and concentrated to dryness. The crude
product was purified by column chromatography with CH2Cl2 as
eluent to give a dark red solid. It was further purified by
recrystallization from CH2Cl2 and hexane as well as vacuum
sublimation. Crystals of 3a suitable for X-ray diffraction were
obtained from a mixture of CH2Cl2 and methanol at room
temperature.
Selected spectroscopic data of N,N′-bis(isopropyl)formamidine
1
(F1H): yield 64%; H NMR (400 MHz, CDCl3, 273 K) δ 7.22 (s,
1H), 3.51−3.41 (m, 2H), 1.26 (d, J = 6.6 Hz, 12H).
Selected spectroscopic data of N,N′-diphenylformamidine (F2H):
yield 80%; 1H NMR (400 MHz, CDCl3, 273 K) δ 8.19 (s, 1H), 7.36−
7.28 (m, 4H), 7.14−7.01 (m, 6H).
Selected spectroscopic data of N,N′-bis(4-tert-butylphenyl)-
1
formamidine (F3H): yield 86%; H NMR (400 MHz, CDCl3, 273
K) δ 8.17 (s, 1H), 7.33 (d, J = 8.5 Hz, 4H), 6.98 (d, J = 8.3 Hz, 4H),
1.32 (s, 18H).
Selected spectroscopic data of N,N′-bis(4-trifluoromethylphenyl)-
formamidine (F4H): yield 93%; 1H NMR (400 MHz, CDCl3, 273 K)
δ 8.19 (s, 1H), 7.59 (d, J = 8.2 Hz, 4H), 7.16 (d, J = 7.7 Hz, 4H).
Synthetic Procedure for [Pt(pypm)Cl(F1H)] (1). A mixture of
K2PtCl4 (1.0 mmol) and 5-(pyridin-2-yl)-2-(trifluoromethyl)-
pyrimidine (pypmH, 1.1 mmol) in a mixture of water and
ethoxyethanol was heated to reflux for 36 h. After it was cooled to
room temperature, the solution was treated with 30 mL of water. The
solid was filtered, thoroughly washed with water, and dried under
vacuum to give a black solid. To this black solid were added N,N′-
diisopropylformamidine (F1H, 1.3 mmol), K2CO3 (3 mmol), and 20
mL of 1,2-dichloroethane; the mixture was refluxed for 24 h. After
that, the solution was evaporated to dryness and the residue was
extracted into CH2Cl2 (50 mL). This solution was next washed with
water (50 mL), dried over anhydrous Na2SO4, filtered, and
concentrated to dryness. The obtained solid was purified by column
chromatography with a 1/1 mixture of hexane and ethyl acetate as
eluent to give a yellow solid in 55% yield. It was recrystallized from
CH2Cl2 and hexane and purified by vacuum sublimation. Crystals
suitable for X-ray diffraction were obtained from a mixture of CH2Cl2
and methanol at room temperature.
Selected spectroscopic data of 3a: yield 65%; 1H NMR (500 MHz,
DMSO-d6, 273 K) δ 8.46 (s, 2H), 8.27 (s, 2H), 7.86−7.78 (m, 4H),
7.68 (d, J = 5.7 Hz, 2H), 7.44 (d, J = 7.8 Hz, 4H), 7.32 (d, J = 7.8 Hz,
4H), 7.23 (t, J = 7.8 Hz, 4H), 7.14 (t, J = 7.8 Hz, 4H), 7.06 (t, J = 7.4
Hz, 2H), 6.93 (t, J = 7.3 Hz, 2H), 6.61 (td, J = 6.2, 2.2 Hz, 2H); 19F
NMR (470 MHz, DMSO-d6, 273 K) δ −68.94 (s, 6F); MS (FD)
calcd for C46H32F6N10Pt2 [M+] m/z 1228.2, found 1228.3. Anal.
Calcd for C46H32F6N10Pt2: C, 44.96; H, 2.62; N, 11.40, Found: C,
45.07; H, 2.52; N, 11.36.
Selected X-ray structural data of 3a: C47H34Cl2F6N10Pt2; Mr =
1313.92; monoclinic; space group C2/c, a = 21.4155(7) Å, b =
15.8054(6) Å, c = 15.3793(5) Å; β = 119.1925(11)°; V = 4544.4(3)
Å3; Z = 4; ρcalcd = 1.920 Mg m−3; F(000) = 2520; crystal size 0.220 ×
0.203 × 0.187 mm3; T = 150(2) K; μ = 6.339 mm−1; 21360
reflections collected, 6619 independent reflections (Rint = 0.0269),
maximum and minimum transmission 0.7460 and 0.5995; restraints/
parameters = 32/326, GOF = 1.071, final R1(I > 2σ(I)) = 0.0198 and
wR2(all data) = 0.0467; largest diffraction peak and hole 0.904 and
−0.632 e Å−3.
1
Selected spectroscopic data of 1: H NMR (400 MHz, DMSO-d6,
273 K) δ 9.69 (d, J = 5.7 Hz, 1H), 8.95 (s, 1H), 8.31 (d, J = 7.9 Hz,
1H), 8.24 (t, J = 7.6 Hz, 1H), 7.75 (d, J = 12.6 Hz, 1H), 7.66 (t, J =
6.5 Hz, 1H), 6.77 (td, J = 10.4, 3.7 Hz, 1H), 3.83−3.75 (m, 1H),
3.60−3.53 (m, 1H), 1.31 (d, J = 6.6 Hz, 3H), 1.27 (d, J = 6.6 Hz,
3H), 1.09 (d, J = 6.5 Hz, 3H), 1.00 (d, J = 6.5 Hz, 3H); 19F NMR
(376 MHz, DMSO-d6, 273 K) δ −69.23 (s, 6F); MS (FD) calcd for
C17H21ClF3N5Pt [M+] m/z 582.1, found 583.1. Anal. Calcd for
C17H21ClF3N5Pt: C, 35.03; H, 3.63; N, 12.01, Found: C, 35.40; H,
3.55; N, 12.08.
Selected spectroscopic data of 3b: yield 60%; 1H NMR (500 MHz,
DMSO-d6, 273 K) δ 8.44 (s, 2H), 8.18 (s, 2H), 7.86 (d, J = 5.8 Hz,
2H), 7.83−7.77 (m, 4H), 7.28 (d, J = 8.6 Hz, 4H), 7.19 (d, J = 8.6
Hz, 4H), 7.14 (d, J = 8.6 Hz, 4H), 7.09 (d, J = 8.6 Hz, 4H), 6.69 (td, J
= 6.0, 2.5 Hz, 2H), 1.22 (s, 18H). 1.17 (s, 18H); 19F NMR (470
MHz, DMSO-d6, 273 K) δ −68.91 (s, 6F); MS (FD) calcd for
C62H64F6N10Pt2 [M+] m/z 1452.5, found 1452.6. Anal. Calcd for
C62H64F6N10Pt2: C,51.24; H, 4.44; N, 9.64, Found: C, 51.60; H, 4.34;
N, 9.56.
Selected X-ray data of 1: C17H21ClF3N5Pt; Mr = 582.93;
orthorhombic; space group Pbca; a = 9.9600(4) Å, b = 18.9150(8)
Å, c = 20.7628(9) Å; V = 3911.6(3) Å3; Z = 8; ρcalcd = 1.980 Mg m−3;
F(000) = 2240; crystal size 0.291 × 0.058 × 0.029 mm3; T = 150 (2)
K; μ = 7.350 mm−1; 24704 reflections collected, 5690 independent
reflections (Rint = 0.0346), maximum and minimum transmission
0.7460 and 0.5409; restraints/parameters = 0/252, GOF = 1.099, final
R1(I > 2σ(I)) = 0.0214 and wR2(all data) = 0.0422; largest
diffraction peak and hole 1.660 and −1.234 e Å−3.
Selected spectroscopic data of 3c: yield 64%; 1H NMR (500 MHz,
DMSO-d6, 273 K) δ 8.57 (s, 2H), 8.51 (s, 2H), 8.33 (d, J = 5.7 Hz,
2H), 7.91−7.81 (m, 4H), 7.71−7.64 (m, 8H), 7.58 (d, J = 8.7 Hz,
4H), 7.45 (d, J = 8.6 Hz, 4H), 7.09−7.04 (m, 2H); 19F NMR (470
MHz, DMSO-d6, 273 K) δ −60.09 (s, 6F), −60.27 (s, 6F), −69.08 (s,
6F); MS (FD) calcd for C50H28F18N10Pt2 [M+] m/z 1500.2, found
1500.3. Anal. Calcd for C50H28F18N10Pt2: C,40.01; H, 1.88; N, 9.33.
Found: C, 40.35; H, 1.99; N, 9.26.
Photophysical Measurements. Steady-state UV−vis absorption
and emission spectra were recorded using the Hitachi (U-4100)
spectrophotometer and the Edinburgh (FS920) fluorometer,
respectively. The nanosecond time-resolved studies were performed
by an Edinburgh FL 900 time-correlated single photon-counting
(TCSPC) system with a pulsed hydrogen- or nitrogen-filled lamp as
the excitation light source and with an InGaAs detector. Data were
fitted with sum of exponential functions using a nonlinear least-
squares procedure in combination with the convolution method. The
Raman spectra were collected with an Andor IDus 420 CCD
instrument with excitation wavelength 532 nm. All photophysical
measurements in this study were performed at room temperature
(298 K).
Conversion of [Pt(pypm)Cl(F1H)] (1) to [Pt(pypm)(μ-F1)]2 (2).
A mixture of 1 (1.0 mmol) and Ag2O (1.1 mmol) in 20 mL of 1,2-
dichloroethane was heated to reflux for 24 h. After that, the solution
was evaporated to dryness and the residue was extracted into CH2Cl2
(50 mL). This solution was next washed with deionized water (50
mL), dried over anhydrous Na2SO4, filtered, and concentrated to
dryness under vacuum. The crude product was purified by column
chromatography with pure CH2Cl2 as eluent to give a red solid in 70%
yield. It was recrystallized from CH2Cl2 and hexane and further
purified by vacuum sublimation.
1
Selected spectroscopic data of 2: H NMR (400 MHz, DMSO-d6,
273 K) δ 8.83 (d, J = 5.9 Hz, 2H), 8.35 (s, 2H), 8.17 (s, 2H), 7.78
(tdd, J = 7.8, 1.4, 1.3 Hz, 2H), 7.57 (d, J = 8.0 Hz, 2H), 7.28 (t, J =
6.6 Hz, 2H), 4.12−4.05 (m, 2H), 4.00−3.93 (m, 2H), 1.56 (d, J = 6.8
Hz, 6H), 1.50 (d, J = 6.8 Hz, 6H), 1.39 (d, J = 6.7 Hz, 6H), 1.32 (d, J
= 6.7 Hz, 6H); 19F NMR (376 MHz, DMSO-d6, 273 K) δ −68.26 (s,
6F); MS (FD) calcd for C34H40F6N10Pt2 [M+] m/z 1092.3, found
G
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(9) Li, K.; Ming Tong, G. S.; Wan, Q.; Cheng, G.; Tong, W.-Y.; Ang,
W.-H.; Kwong, W.-L.; Che, C.-M. Highly Phosphorescent Platinum-
(II) Emitters: Photophysics, Materials and Biological Applications.
Chem. Sci. 2016, 7, 1653−1673.
(10) Tuong Ly, K.; Chen-Cheng, R.-W.; Lin, H.-W.; Shiau, Y.-J.;
Liu, S.-H.; Chou, P.-T.; Tsao, C.-S.; Huang, Y.-C.; Chi, Y. Near-
infrared organic light-emitting diodes with very high external quantum
efficiency and radiance. Nat. Photonics 2017, 11, 63−68.
(11) Cheng, G.; Wan, Q.; Ang, W.-H.; Kwong, C.-L.; To, W.-P.;
Chow, P.-K.; Kwok, C.-C.; Che, C.-M. High-Performance Deep-Red/
Near-Infrared OLEDs with Tetradentate [Pt(O∧N∧C∧N)] Emitters.
Adv. Opt. Mater. 2019, 7, 1801452.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01754.
■
S
General information on calculations and device
fabrication, as well as the results of the calculated
Raman transition and TD-DFT calculations of the
studied Pt(II) metal complexes (PDF)
Accession Codes
(12) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Solvent-Induced
Aggregation through Metal···Metal/π···π Interactions: Large Solvato-
chromism of Luminescent Organoplatinum(II) Terpyridyl Com-
plexes. J. Am. Chem. Soc. 2002, 124, 6506−6507.
CCDC 1920736−1920737 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(13) Lu, W.; Chan, M. C. W.; Zhu, N.; Che, C.-M.; Li, C.; Hui, Z.
Structural and Spectroscopic Studies on Pt···Pt and π−π Interactions
in Luminescent Multinuclear Cyclometalated Platinum(II) Homo-
logues Tethered by Oligophosphine Auxiliaries. J. Am. Chem. Soc.
2004, 126, 7639−7651.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for P.-T.C.: chop@ntu.edu.tw.
*E-mail for C.-S.L.: apcslee@cityu.edu.hk.
*E-mail for Y.C.: yunchi@cityu.edu.hk.
■
(14) Chi, Y.; Chou, P.-T. Contemporary progresses on neutral,
highly emissive Os(II) and Ru(II) complexes. Chem. Soc. Rev. 2007,
36, 1421−1431.
(15) Chi, Y.; Chou, P.-T. Transition Metal Phosphors with
Cyclometalating Ligands; Fundamental and Applications. Chem. Soc.
Rev. 2010, 39, 638−655.
ORCID
(16) You, Y.; Nam, W. Photofunctional triplet excited states of
cyclometalated Ir(III) complexes: beyond electroluminescence. Chem.
Soc. Rev. 2012, 41, 7061−7084.
Pi-Tai Chou: 0000-0002-8925-7747
Chun-Sing Lee: 0000-0001-6557-453X
Yun Chi: 0000-0002-8441-3974
(17) Xiang, H.; Cheng, J.; Ma, X.; Zhou, X.; Chruma, J. J. Near-
infrared phosphorescence: materials and applications. Chem. Soc. Rev.
2013, 42, 6128−6185.
Notes
The authors declare no competing financial interest.
(18) Henwood, A. F.; Zysman-Colman, E. Lessons learned in tuning
the optoelectronic properties of phosphorescent iridium(iii) com-
plexes. Chem. Commun. 2017, 53, 807−826.
ACKNOWLEDGMENTS
■
(19) Ma, D.; Tsuboi, T.; Qiu, Y.; Duan, L. Recent Progress in Ionic
Iridium(III) Complexes for Organic Electronic Devices. Adv. Mater.
2017, 29, 1603253.
This work was supported by funding from the 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.
We also thank the National Center for High-performance
Computing (NCHC) of Taiwan for the valuable computer
time and facilities.
(20) Li, T.-Y.; Wu, J.; Wu, Z.-G.; Zheng, Y.-X.; Zuo, J.-L.; Pan, Y.
Rational design of phosphorescent iridium(III) complexes for
emission color tunability and their applications in OLEDs. Coord.
Chem. Rev. 2018, 374, 55−92.
(21) Wu, X.; Zhu, M.; Bruce, D. W.; Zhu, W.; Wang, Y. An
Overview of Phosphorescent Metallomesogens based on Platinum
and Iridium. J. Mater. Chem. C 2018, 6, 9848−9860.
(22) Zhang, Y.; Wang, Y.; Song, J.; Qu, J.; Li, B.; Zhu, W.; Wong,
W.-Y. Near-Infrared Emitting Materials via Harvesting Triplet
Excitons: Molecular Design, Properties, and Application in Organic
Light Emitting Diodes. Adv. Opt. Mater. 2018, 6, 1800466.
(23) Ma, B.; Li, J.; Djurovich, P. I.; Yousufuddin, M.; Bau, R.;
Thompson, M. E. Synthetic Control of Pt···Pt Separation and
Photophysics of Binuclear Platinum Complexes. J. Am. Chem. Soc.
2005, 127, 28−29.
REFERENCES
■
(1) Zampetti, A.; Minotto, A.; Cacialli, F. Near-Infrared (NIR)
Organic Light-Emitting Diodes (OLEDs): Challenges and Oppor-
tunities. Adv. Funct. Mater. 2019, 29, 1807623.
(2) Ding, F.; Fan, Y.; Sun, Y.; Zhang, F. Beyond 1000 nm Emission
Wavelength: Recent Advances in Organic and Inorganic Emitters for
Deep-Tissue Molecular Imaging. Adv. Healthcare Mater. 2019, 8,
1900260.
(24) Chaaban, M.; Zhou, C.; Lin, H.; Chyi, B.; Ma, B. Platinum(ii)
binuclear complexes: molecular structures, photophysical properties,
and applications. J. Mater. Chem. C 2019, 7, 5910−5924.
(25) Yoshida, M.; Yashiro, N.; Shitama, H.; Kobayashi, A.; Kato, M.
A Redox-Active Dinuclear Platinum Complex Exhibiting Multicolored
Electrochromism and Luminescence. Chem. - Eur. J. 2016, 22, 491−
495.
(26) Tzeng, B.-C.; Fu, W.-F.; Che, C.-M.; Chao, H.-Y.; Cheung, K.-
K.; Peng, S.-M. Structures and photoluminescence of dinuclear
platinum(II) and palladium(II) complexes with bridging thiolates and
2,2′-bipyridine or 2,2′:6′,2″-terpyridine ligands. J. Chem. Soc., Dalton
Trans. 1999, 1017−1023.
(27) Kato, M.; Omura, A.; Toshikawa, A.; Kishi, S.; Sugimoto, Y.
Vapor-Induced Luminescence Switching in Crystals of the Syn Isomer
of a Dinuclear (Bipyridine)platinum(II) Complex Bridged with
(3) Zhu, S.; Tian, R.; Antaris, A. L.; Chen, X.; Dai, H. Near-Infrared-
II Molecular Dyes for Cancer Imaging and Surgery. Adv. Mater. 2019,
31, 1900321.
(4) Guo, Z.; Park, S.; Yoon, J.; Shin, I. Recent progress in the
development of near-infrared fluorescent probes for bioimaging
applications. Chem. Soc. Rev. 2014, 43, 16−29.
(5) Barbieri, A.; Bandini, E.; Monti, F.; Praveen, V. K.; Armaroli, N.
The Rise of Near-Infrared Emitters: Organic Dyes, Porphyrinoids,
and Transition Metal Complexes. Top. Curr. Chem. 2016, 374, 47.
(6) Zhao, Q.; Sun, J. Z. Red and near infrared emission materials
with AIE characteristics. J. Mater. Chem. C 2016, 4, 10588−10609.
(7) Englman, R.; Jortner, J. The energy gap law for radiationless
transitions in large molecules. Mol. Phys. 1970, 18, 145−164.
(8) Caspar, J. V.; Meyer, T. J. Application of the energy gap law to
nonradiative, excited-state decay. J. Phys. Chem. 1983, 87, 952−957.
H
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Pyridine-2-Thiolate Ions. Angew. Chem., Int. Ed. 2002, 41, 3183−
3185.
(28) Chakraborty, A.; Yarnell, J. E.; Sommer, R. D.; Roy, S.;
Castellano, F. N. Excited-State Processes of Cyclometalated Platinum-
(II) Charge-Transfer Dimers Bridged by Hydroxypyridines. Inorg.
Chem. 2018, 57, 1298−1310.
(29) Ohno, K.; Kusano, Y.; Kaizaki, S.; Nagasawa, A.; Fujihara, T.
Chromism of Tartrate-Bridged Clamshell-like Platinum(II) Complex:
Intramolecular Pt−Pt Interaction-Induced Luminescence Vapo-
chromism and Intermolecular Interactions-Triggered Thermochrom-
ism. Inorg. Chem. 2018, 57, 14159−14169.
(30) Wang, Z.; Jiang, L.; Liu, Z.-P.; Gan, C. R. R.; Liu, Z.; Zhang, X.-
H.; Zhao, J.; Hor, T. S. A. Facile formation and redox of benzoxazole-
2-thiolate-bridged dinuclear Pt(ii/iii) complexes. Dalton Trans. 2012,
41, 12568−12576.
(31) Sicilia, V.; Fornies, J.; Casas, J. M.; Martin, A.; Lopez, J. A.;
Larraz, C.; Borja, P.; Ovejero, C.; Tordera, D.; Bolink, H. Highly
Luminescent Half-Lantern Cyclometalated Platinum(II) Complex:
Synthesis, Structure, Luminescence Studies, and Reactivity. Inorg.
Chem. 2012, 51, 3427−3435.
(32) Xiong, W.; Meng, F.; You, C.; Wang, P.; Yu, J.; Wu, X.; Pei, Y.;
Zhu, W.; Wang, Y.; Su, S. Molecular isomeric engineering of naphthyl-
quinoline-containing dinuclear platinum complexes to tune emission
from deep red to near infrared. J. Mater. Chem. C 2019, 7, 630−638.
(33) Yoshida, M.; Kato, M. Regulation of metal−metal interactions
and chromic phenomena of multi-decker platinum complexes having
π-systems. Coord. Chem. Rev. 2018, 355, 101−115.
(34) Puttock, E. V.; Walden, M. T.; Williams, J. A. G. The
luminescence properties of multinuclear platinum complexes. Coord.
Chem. Rev. 2018, 367, 127−162.
(35) Xiong, W.; Meng, F.; Tan, H.; Wang, Y.; Wang, P.; Zhang, Y.;
Tao, Q.; Su, S.; Zhu, W. Dinuclear platinum complexes containing
aryl-isoquinoline and oxadiazole-thiol with an efficiency of over 8.8%:
in-depth investigation of the relationship between their molecular
structure and near-infrared electroluminescent properties in PLEDs. J.
Mater. Chem. C 2016, 4, 6007−6015.
(36) Zhang, Y.-M.; Meng, F.; Tang, J.-H.; Wang, Y.; You, C.; Tan,
H.; Liu, Y.; Zhong, Y.-W.; Su, S.; Zhu, W. Achieving near-infrared
emission in platinum(ii) complexes by using an extended donor-
acceptor-type ligand. Dalton Trans. 2016, 45, 5071−5080.
(37) Su, N.; Meng, F.; Wang, P.; Liu, X.; Zhu, M.; Zhu, W.; Su, S.;
Yu, J. Near-infrared emission from binuclear platinum (II) complexes
containing pyrenylpyridine and pyridylthiolate units: Synthesis,
photo-physical and electroluminescent properties. Dyes Pigm. 2017,
138, 162−168.
(38) Yang, S.; Meng, F.; Wu, X.; Yin, Z.; Liu, X.; You, C.; Wang, Y.;
Su, S.-J.; Zhu, W. Dinuclear Platinum(II) Complex Dominated by a
Zig-Zag-Type Cyclometalated Ligand? An New Approach to Realize
High-efficiency Near Infrared Emission. J. Mater. Chem. C 2018, 6,
5769−5777.
(39) Liu, X.; Zhang, S.; Jin, Y.-M.; Lu, G.-Z.; Jiang, L.; Liang, X.; Xu,
Q.-L.; Zheng, Y.-X. Syntheses, crystal structure and photophysical
property of iridium complexes with 1,3,4-oxadiazole and 1,3,4-
thiadiazole derivatives as ancillary ligands. J. Organomet. Chem. 2015,
785, 11−18.
K.-Y.; Chi, Y. Functional Pyrimidinyl Pyrazolate Pt(II) Complexes:
Role of Nitrogen Atom in Tuning the Solid-State Stacking and
Photophysics. Adv. Funct. Mater. 2019, 29, 1900923.
(45) Kourkoulos, D.; Karakus, C.; Hertel, D.; Alle, R.; Schmeding,
S.; Hummel, J.; Risch, N.; Holder, E.; Meerholz, K. Photophysical
properties and OLED performance of light-emitting platinum(ii)
complexes. Dalton Trans. 2013, 42, 13612−13621.
(46) Poloek, A.; Lin, C.-W.; Chen, C.-T.; Chen, C.-T. High colour
rendering index and colour stable hybrid white efficient OLEDs with a
double emitting layer structure using a single phosphorescence dopant
of heteroleptic platinum complexes. J. Mater. Chem. C 2014, 2,
10343−10356.
(47) O’Brien, C.; Wong, M. Y.; Cordes, D. B.; Slawin, A. M. Z.;
Zysman-Colman, E. Cationic Platinum(II) Complexes Bearing Aryl-
BIAN Ligands: Synthesis and Structural and Optoelectronic
Characterization. Organometallics 2015, 34, 13−22.
(48) Zhou, C.; Tian, Y.; Yuan, Z.; Han, M.; Wang, J.; Zhu, L.;
Tameh, M. S.; Huang, C.; Ma, B. Precise Design of Phosphorescent
Molecular Butterflies with Tunable Photoinduced Structural Change
and Dual Emission. Angew. Chem., Int. Ed. 2015, 54, 9591−9595.
(49) Ricciardi, L.; La Deda, M.; Ionescu, A.; Godbert, N.; Aiello, I.;
Ghedini, M. Anionic cyclometallated Pt(ii) square-planar complexes:
new sets of highly luminescent compounds. Dalton Trans. 2017, 46,
12625−12635.
(50) Dunsford, J. J.; Tromp, D. S.; Cavell, K. J.; Elsevier, C. J.;
Kariuki, B. M. N-alkyl functionalised expanded ring N-heterocyclic
carbene complexes of rhodium(i) and iridium(i): structural
investigations and preliminary catalytic evaluation. Dalton Trans.
2013, 42, 7318−7329.
́
́
(51) Rodríguez-Castro, A.; Fernandez, A.; Lopez-Torres, M.;
́
́
Vazquez-García, D.; Naya, L.; Vila, J. M.; Fernandez, J. J.
Mononuclear cycloplatinated complexes derived from 2-tolylpyridine
with N-donor ligands: Reactivity and structural characterization.
Polyhedron 2012, 33, 13−18.
(52) Chepelin, O.; Ujma, J.; Barran, P. E.; Lusby, P. J. Sequential,
Kinetically Controlled Synthesis of Multicomponent Stereoisomeric
Assemblies. Angew. Chem., Int. Ed. 2012, 51, 4194−4197.
(53) Stacey, O. J.; Platts, J. A.; Coles, S. J.; Horton, P. N.; Pope, S. J.
A. Phosphorescent, Cyclometalated Cinchophen-Derived Platinum
Complexes: Syntheses, Structures, and Electronic Properties. Inorg.
Chem. 2015, 54, 6528−6536.
(54) Zhao, J.; Feng, Z.; Zhong, D.; Yang, X.; Wu, Y.; Zhou, G.; Wu,
Z. Cyclometalated Platinum Complexes with Aggregation-Induced
Phosphorescence Emission Behavior and Highly Efficient Electro-
luminescent Ability. Chem. Mater. 2018, 30, 929−946.
(55) Barker, J.; Kilner, M.; Gould, R. O. Synthesis and study of
monomeric and dimeric amidine complexes of nickel(II) and
platinum(II). Crystal structure of bis(N,N’-diphenylbenzamidino)-
platinum(II), [Pt{PhNC(Ph)NPh}2]. J. Chem. Soc., Dalton Trans.
1987, 2687−2694.
(56) Rai, V. K.; Nishiura, M.; Takimoto, M.; Hou, Z. Guanidinate
ligated iridium(iii) complexes with various cyclometalated ligands:
synthesis, structure, and highly efficient electrophosphorescent
properties with a wide range of emission colours. J. Mater. Chem. C
2013, 1, 677−689.
(57) Rai, V. K.; Nishiura, M.; Takimoto, M.; Hou, Z. Substituent
effect on the electroluminescence efficiency of amidinate-ligated
bis(pyridylphenyl) iridium(iii) complexes. J. Mater. Chem. C 2014, 2,
5317−5326.
(40) Xu, Q.-L.; Liang, X.; Jiang, L.; Zhao, Y.; Zheng, Y.-X. Crystal
structure, photoluminescence and electroluminescence of three bluish
green light-emitting iridium complexes. Dalton Trans. 2016, 45,
7366−7372.
(41) Junk, P. C.; Cole, M. L. Alkali-metal bis(aryl)formamidinates: a
study of coordinative versatility. Chem. Commun. 2007, 1579−1590.
(42) Chi, Y.; Tsai, H.-Y.; Chen, Y.-K. Pt(II) Complexes with
Azolate-containing Bidentate Chelate: Design, Photophysics, and
Application. J. Chin. Chem. Soc. 2017, 64, 574−588.
(58) Cotton, F. A.; Matonic, J. H.; Murillo, C. A. Synthesis and
Structural Studies of Platinum Complexes Containing Monodentate,
Bridging, and Chelating Formamidine Ligands. Inorg. Chem. 1996, 35,
498−503.
(59) Cornacchia, D.; Pellicani, R. Z.; Intini, F. P.; Pacifico, C.;
Natile, G. Solution Behavior of Amidine Complexes: An Unexpected
cis/trans Isomerization and Formation of Di- and Trinuclear
Platinum(III) and Platinum(II) Species. Inorg. Chem. 2009, 48,
10800−10810.
́
(43) Cebrian, C.; Mauro, M. Recent advances in phosphorescent
platinum complexes for organic light-emitting diodes. Beilstein J. Org.
Chem. 2018, 14, 1459−1481.
(44) Ganesan, P.; Hung, W.-Y.; Tso, J.-Y.; Ko, C.-L.; Wang, T.-H.;
Chen, P.-T.; Hsu, H.-F.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Jen, A.
I
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(60) Leopold, H.; Tenne, M.; Tronnier, A.; Metz, S.; Munster, I.;
̈
Wagenblast, G.; Strassner, T. Binuclear C∧C* Cyclometalated
Platinum(II) NHC Complexes with Bridging Amidinate Ligands.
Angew. Chem., Int. Ed. 2016, 55, 15779−15782.
(61) Mikhnenko, O. V.; Kuik, M.; Lin, J.; van der Kaap, N.; Nguyen,
T.-Q.; Blom, P. W. M. Trap-Limited Exciton Diffusion in Organic
Semiconductors. Adv. Mater. 2014, 26, 1912−1917.
(62) Mikhnenko, O. V.; Blom, P. W. M.; Nguyen, T.-Q. Exciton
diffusion in organic semiconductors. Energy Environ. Sci. 2015, 8,
1867−1888.
(63) Xia, B.-H.; Zhang, H.-X.; Che, C.-M.; Leung, K.-H.; Phillips, D.
L.; Zhu, N.; Zhou, Z.-Y. Metal−Metal Interactions in Hetero-
bimetallic d8−d10 Complexes. Structures and Spectroscopic Inves-
tigation of [M’M″(μ-dcpm)2(CN)2]+ (M’ = Pt, Pd; M″ = Cu, Ag, Au)
and Related Complexes by UV−vis Absorption and Resonance
Raman Spectroscopy and ab Initio Calculations. J. Am. Chem. Soc.
2003, 125, 10362−10374.
(64) Clark, R. J. H.; Michael, D. J. Resonance raman spectra of metal
II/IV dimer chain complexes of platinum and palladium: Analysis of
the component structure to the band assigned to the symmetric XMX
chain stretching mode (X = Cl or Br). J. Mol. Struct. 1988, 189, 173−
185.
́
̌
́
̌
̌
(65) Hofbeck, T.; Lam, Y. C.; Kalbac, M.; Zalis, S.; Vlcek, A.; Yersin,
H. Thermally Tunable Dual Emission of the d8 - d8 Dimer [Pt2(μ-
P2O5(BF2)2)4]4−. Inorg. Chem. 2016, 55, 2441−2449.
(66) Chang, C.-H.; Wu, Z.-J.; Chiu, C.-H.; Liang, Y.-H.; Tsai, Y.-S.;
Liao, J.-L.; Chi, Y.; Hsieh, H.-Y.; Kuo, T.-Y.; Lee, G.-H.; Pan, H.-A.;
Chou, P.-T.; Lin, J.-S.; Tseng, M.-R. A New Class of Sky-Blue-
Emitting Ir(III) Phosphors Assembled Using Fluorine-Free Pyridyl
Pyrimidine Cyclometalates: Application toward High-Performance
Sky-Blue- and White-Emitting OLEDs. ACS Appl. Mater. Interfaces
2013, 5, 7341−7351.
(67) Archibald, S. J.; Alcock, N. W.; Busch, D. H.; Whitcomb, D. R.
Synthesis and Characterization of Functionalized N,N’-Diphenylfor-
mamidinate Silver(I) Dimers: Solid-State Structures and Solution
Properties. Inorg. Chem. 1999, 38, 5571−5578.
J
DOI: 10.1021/acs.inorgchem.9b01754
Inorg. Chem. XXXX, XXX, XXX−XXX