Probing the Effects of the Number and Positions of ?OCH3 and ?CN Substituents on Color Tuning of Ir(III) Complex Derivatives through a Joint Computational and Experimental Study

Article abstract of DOI:10.1002/cphc.201800928

We performed a joint theoretical and experimental study on sixteen Ir(III) complexes bearing a similar molecular platform of bis(2-phenylbenzothiozolato-N,C^2’) iridium(III) (acetylacetonate) by grafting ?OCH₃ group and/or ?CN group on different positions of the C-related arene moiety of the (Formula presented.) ligand (C-ring). Our results reveal that the introduction of ?CN renders an overall drop in the FMO energy levels while a reverse increase is observed for ?OCH₃. The ortho- and para-sites of the C-ring are more effective substitution positions to modulate the HOMO energy level due to the fact that the electronic density of HOMO mainly locates at them while the meta-site would induce a stronger impact on LUMO since the electronic density of LUMO mainly distributes over the position. Utilizing the synergistic effects of the substituents and the substituted positions, a wide color-tuning range from 479 nm to 637 nm was achieved, which covers nearly the whole window of visible spectrum. In particular, the tri-substituted Ir35mo4cn complex (λ_{em max}=637 nm) may be a potential candidate for high efficiency red OLEDs materials due to its greatly enhanced absorption processes, relatively higher ³MLCT (%), lower ΔE_S1–T1, enlarged separation between ³MLCT/π–π* and ³MC d–d states, and good hole and particle-transporting performances. Finally, six representative complexes were synthesized and their spectra were determined, which confirm the reliability of our computational strategy.

Full text of DOI:10.1002/cphc.201800928

Accepted Article
Title: Probing effects of the number and positions of -OCH₃ and -CN
substituents on color tuning of Ir (III) complex derivatives through
a joint study of computation and experiment
Authors: Xiao Qin, Ming Li, Minghui Xiang, Yi Luo, Yan Jiao, Rongao
Yuan, Ning Wang, Zhiyun Lu, and Xuemei Pu
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
Probing effects of the number and positions of -OCH and -CN
3
substituents on color tuning of Ir (III) complex derivatives through
a joint study of computation and experiment
Xiao Qin,‡ [a] Ming Li,‡ [a] Minghui Xiang, Yi Luo, Yan Jiao, Rongao Yuan, Ning Wang, Zhiyun
[a]
[a]
[a]
[b]
[a]
[a]
[a]
Lu* and Xumei Pu*
Abstract: We performed a joint theoretical and experimental study on
because the emission color of the Ir(III) complex was considered
[
4]
sixteen Ir (III) complexes bearing a similar molecular platform of bis(2-
to correlate highly with the conjugation length of its C^N ligand ;
2’
[5]
phenylbenzothiozolato-N,C ) iridium(III) (acetylacetonate) by grafting
OCH group and/or -CN group on different positions of the C-related
2) modification of chemical structure of the ancillary ligand ; 3)
-
3
grafting of substituent(s) at the C^N ligand while fixing the ligand
skeleton[3c]. Nevertheless, it has been reported recently that the
emission color of Ir (III) complexes do not always correspond to
the extent of π-conjugation in their C^N ligands [6]. The second
strategy could work effectively only when the ancillary ligand
contributes substantially to the frontier molecular orbitals (FMOs)
arene moiety of the C^N ligand (C-ring). Our results reveal that the
introduction of -CN renders an overall drop in the FMO energy levels
3
while a reverse increase is observed for -OCH . The ortho- and para-
sites of the C-ring are more effective substitution positions to
modulate the HOMO energy level due to the fact that the electronic
density of HOMO mainly locates at them while the meta-site would
induce a stronger impact on LUMO since the electronic density of
LUMO mainly distributes over the position. Utilizing the synergistic
effects of the substituents and the substituted positions, a wide color-
tuning range from 479 nm to 637 nm was achieved, which covers
nearly the whole window of visible spectrum. In particular, the tri-
substituted Ir35mo4cn complex (λem max = 637 nm) may be a potential
candidate for high efficiency red OLEDs materials due to its greatly
[
5]
of the complex . Compared to the former two strategies, the third
one would be more effective to gain in-depth understanding of
structure-property relationships for the Ir (III) complexes, through
which rational molecular design of the iridium complex with
desirous emission color could be actualized. More importantly,
through the third strategy, a library of phosphorescent compounds
bearing similar molecular platform but showing a wide color-
[
7]
tuning range could be easily constructed , thereupon highly
efficient energy-transfer luminogen pairs[8] could be achieved for
further opto-electronic applications[9].
3
enhanced absorption processes, relatively higher MLCT (%), lower
3
3
△ES1-T1, enlarged separation between MLCT/π-π* and MC d-d
states, and good hole and particle-transporting performances. Finally,
six representative complexes were synthesized and their spectra
were determined, which confirm the reliability of our computational
strategy.
Recently, 2-phenylbenzothiazole (bt) has been found to be a
good candidate in the construction of high-performance yellow-
emitting iridium phosphors. Afterwards, many efforts have been
devoted to the fine-tuning of the emission color, phosphorescence
efficiency and charge-carrier injection/transportation of bt-based
iridium complexes via substituent effects [10]. Nevertheless, with
respect to the color-tuning range of bt-based iridium complexes
via substituent effects, quite limited progress has been achieved.
1. Introduction
For example, the incorporation of different substituents like -CH
3
,
Cyclometalated Ir (III) complexes have attracted considerable
-
OCH , -CF , -F, -Cz (carbazolyl), and -Ph N (diphenylamino),
3
3
2
interests due to their applications in phosphorescent organic light-
emitting diodes (PhOLEDs) [1]. They can harvest both singlet and
triplet excitations for emissions and theoretically could realize an
internal quantum efficiency up to 100% [2]. To achieve full-color
flat-panel displays, much effort has been devoted to develop
highly efficient phosphors that can cover the entire visible light
into the 5-, 6- or 7-sites of the benzothiazole ring of the bt-based
iridium chelate was found to result in insignificant spectral tuning,
although in some cases, it may result in significant alteration in
[10d,
10h]. On the
phosphorescent quantum yield of the chelates
[10b]
and Jang[10i]
other hand, according to the reports from Lasker
the grafting of electron-donating (-CH , -OCH
electron-withdrawing (-CF , -F) groups at the meta- or para-site of
,
3
3 3 3
, -Si(CH ) ) or
region [3]
.
3
Generally, the phosphorescence color of Ir (III) complexes
could be fine-tuned through the following three strategies: 1)
alteration of framework of the cyclometalated (C^N) ligand
the C-ring of the bt ligand only give rise to less than 17 nm blue-
shift or 13 nm red-shift of the emission maximum (λem max) of the
resultant complexes. The key to the problem lies in understanding
correlations between the nature of substituents, the number of
substituents, the substituted positions and photophysical
properties.
Thus, in this study, we designed 16 novel complexes by
3
grafting different numbers of -CN and -OCH at ortho-, meta-
and/or para-site of the C-ring to perform a systematic study on the
influence of different positions and the number of the two
substituents on the emitting color and phosphorescence
[
a]
Dr. Xiao Qin, Dr. Ming Li, Dr. Minghui Xiang, Dr. Yi Luo, Dr. Yan
Jiao, Dr. Ning Wang, Prof. Zhiyun Lu and Prof. Xumei Pu
Department of Chemistry, Sichuan University, Chengdu 610064,
People’s Republic of China
E-mail: xmpuscu@scu.edu.cn
[
b]
Stu. Rongao Yuan
Sichuan Chengdu NO.7 High School, Chengdu 610041, People’s
Republic of China
‡
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
2
efficiency. Herein, we selected Ir(bt) (acac) (labelled as Irbt) as
the parent complex. Although the complex may have more than
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
one diastereomers, according to the X-ray crystallographic
analysis results of Irbt[11] and its derivatives[12], their C^N ligands
were all observed to adopt the configuration with the Cbt-donor
trans- to the O-donor of acac while the Nbt-donor trans- to each
other. Thus, the diastereomer is one preferred configuration and
served as the parent complex in the work.The chemical structures
of the sixteen investigated Irbt derivatives are shown in Table 1.
Owing to the strong electron-withdrawing nature of -CN group, the
multi-CN substituion at the C-ring of the C^N ligand is expected
to result in drastically decreased electronic density of the
coordinative C-atom, which is unfavourable for the synthesis of
the chelates. Therefore, we did not design multi-CN substituted
complexes in the work. The ground and excited state geometries,
FMOs and the electronic absorption and emission properties as
well as the phosphorescence efficiency were studied with the aid
of the computational method. Then, experimental methods were
utilized to validate the computational results for some
representative complexes. Our objective is to elucidate the
relationship between the substituents (positions and number) and
the optical properties, in turn developing a facile and effective
color-tuning strategy to help experimental workers synthetize
highly efficient OLED materials with desirous color.
2.1 Predicted geometries in the ground and lowest
triplet excited states
The schematic structures of the studied complexes are
depicted in Table 1 and the optimized geometry structures of all
the complexes in the ground state (S
0
)
at the
B3LYP/LANL2DZ+6-31G* level are shown in Figure. 1 and Figure.
S5, along with the number of some key atoms. The optimized
geometry parameters of all the complexes in the ground-state (S
0
)
and lowest-lying triplet states (T ) are shown in Table 2. For the
1
parent compound Irbt, the calculated geometries are close to the
experimental ones[ , only presenting minor deviations of 0.021-
0.059 Å, which confirm the reliability of the computation level.
Compared with the parent complex Irbt, the mono-substituted
complexes by -CN group almost show shortened bond lengths.
The shortened C-Ir bond should be attributed to the decreased
electronic density of the C-ring of the C^N ligand upon the
substitution of the electron-withdrawing –CN group. This means
that a closer distance between the C-ring and the Ir atom than that
in the parent complex Irbt is required to realize effective
coordination. In this case, the difficulty in the synthesis may be
increased due to the decreased electronic density of the
13]
3
coordinative atoms on the ligand. Whereas for the -OCH mono-
substituted complexes, there were no significant changes. In
addition, for the ortho-substituted Ir5mo and Ir5cn, we noticed
Table 1. Chemical structures and substituents used to design new mono-
substituted, di-substituted and tri-substituted objective complexes.
9
that the Ir-C /Ir-C22 distances are elongated obviously with
respect to the parent Irbt, which may be explained by the larger
steric hindrance between the ortho-substituents and the adjacent
C^N ligands. In addition, the bond lengths of the Ir-O, Ir-N and Ir-
C bonds do not present significant differences between the di-
substituted complexes and the tri-substituted ones.
Compared with the S
observed in the T states. As shown in Table 2, the Ir-C
and Ir-N / Ir-N bond lengths in the T state are shortened for most
complexes under study, implying a strengthened interaction in the
excited state. However, some exceptions are also observed for
Ir2cn, Ir3cn, Ir4cn, Ir4mo and Ir3cn4mo, in which one Ir-N bond
is significantly shortened while the other Ir-N bond is significantly
elongated, which may be attributed to the space directions of the
ligands.
0
states, some structural changes were
1
9
/ Ir-C22
complex
R
H
1
R
2
R
3
R
4
1
2
1
Irbt
H
H
H
H
H
H
H
H
H
Ir2cn
CN
H
Ir3cn
CN
H
Ir4cn
H
CN
H
Ir5cn
H
H
CN
H
Ir2mo
OCH
H
3
H
H
Ir3mo
OCH
H
3
3
H
H
Ir4mo
H
OCH
H
3
H
Ir5mo
H
H
OCH
OCH
H
3
3
Ir35mo
Ir24mo
Ir3cn4mo
Ir3mo4cn
Ir35mo4cn
Ir24mo3cn
Ir345mo
H
OCH
H
H
OCH
H
3
OCH
OCH
CN
CN
OCH
OCH
3
3
CN
OCH
OCH
CN
OCH
H
H
3
3
H
H
OCH
H
3
3
OCH
H
3
3
3
3
OCH
Figure 1. Optimized geometry structure of the parent complex Irbt at the B3LYP
2. Results and Discussion
/LANL2DZ+6-31G* level.
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
2
.2 Frontier molecular orbitals
impact on this particular MO while the group introduced to a
position, where a MO has a node, would only bring weak
influence[7a, 14]. Thus, the substitutions at the para- and ortho- sites
of the C-ring should induce larger perturbance on the electronic
structure of HOMO than the meta-site, while the substitution at
meta-site would play more significant role in influencing LUMO
than the para- and ortho- sites. This deduction is further validated
from the contours plots of all the complexes. As reflected by
Figure. 3, for the ortho- and para-sites methoxy-monosubstituted
(Ir3mo and Ir5mo) and cyano-monosubstituted (Ir3cn and Ir5cn)
complexes, substantial π-electron density contributions from
As known, the spectral properties are closely related to the
frontier molecular orbitals (FMOs), in particular for the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO). Hence, we calculated the FMO energy
levels, their compositions and distribution of electronic densities
for all the studied complexes, as shown in Figure. 2, Figure. 3 and
Tables S1-S16. For the parent compound Irbt, it can be seen from
Figure. 2 that its HOMO is mainly located on the t2g-d orbital of the
Ir atom (42%) and the π orbital of phenyl (C-ring) of the C^N ligand
(
43%); while its LUMO is primarily distributed on the π* orbitals of
3
either -OCH or -CN could be observed in their HOMOs; on the
the entire C^N ligand (97%) including both the C-ring and the N-
ring. Further inspection is shown that the electronic density of
HOMO mainly distributes on the ortho- and para-sites (e.g., 3- and
contrary, for the meta-site methoxy-monomodified (Ir2mo and
Ir4mo) and cyano-monografted (Ir2cn and Ir4cn) complexes,
their -OCH or -CN group contributes more effectively to LUMOs
3
5-sites) of C-Ir bond while that of LUMO mainly centers on meta-
than HOMOs.
site (e.g., 2- and 4-sites). Hence the grafting of -OCH or -CN
3
For the mono-substituted ones, in comparison with the parent
group at these positions of the C-ring perturbs not only HOMO but
also LUMO, should lead to larger color-tuning range than the
other positions. For the other derivatives, their electronic
distributions on FMOs are analogous: HOMOs are basically
comprised of a mixture of the C-rings (37~47%) and iridium
orbitals (31~44%) and LUMOs are mainly located on the whole
C^N ligands (95~97%).
compound Irbt, the introduction of -OCH
to a rise in the HOMO and LUMO energy levels, in which the -
OCH grafted at 3,5-sites centered by the electronic densities of
HOMO would induce larger rise in the energy level of HOMO than
that of the LUMO while -OCH substituted at 2,4-sites centered by
3
to the C-ring would lead
3
3
the electronic density of LUMO would cause larger rise in the
energy level of LUMO than that of HOMO. Consequently, the
A detailed inspection on the electronic structure of the FMOs
of the parent Irbt at the C-ring reveals that for HOMO, the electron
density of its C-ring is mainly centered on the ortho- and para-
sites with respect to the iridium atom, but displays nodes at the
two meta-sites; while the LUMO chiefly resides on the two meta-
sites relative to the iridium ion, showing two nodes at the ortho-
and para-sites (vide Figure. 2 and Figure. 3). It should be
expected that the substituent grafted at a position, where a MO
has significant electron density, would generally induce a strong
3
ortho- or para-OCH grafted complexes Ir5mo and Ir3mo present
smaller energy gap than the parent Irbt while an opposite trend is
observed for Ir2mo and Ir4mo. In addition, it can be seen from
Figure. 3 that the contributions of the electronic density from the
methoxy substituents to HOMOs are observed for Ir5mo and
Ir3mo and the contributions to LUMOs are observed for Ir2mo
and Ir4mo, which should be associated with higher HOMO levels
of Ir5mo and Ir3mo and higher LUMO levels of Ir2mo and Ir4mo.
Since 1% LUMO is distributed at the ortho-site (e.g., 5-site), the
Table 2. Selected bond lengths (Å) from the optimized geometry for the objective complexes in the ground state S
B3LYP/LANL2DZ+6-31G* level together with the experimental values for Irbt.
0 1
and the lowest lying tripletsstate T at
Ir2cn
Ir3cn
Ir4cn
Ir5cn
Ir2mo
Ir3mo
Ir4mo
Ir5mo
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
Ir-O
1
2.191
2.191
2.079
2.079
2.007
2.007
2.185
2.194
2.032
2.101
2.012
1.988
2.188
2.188
2.085
2.085
2.005
2.005
2.182
2.194
2.015
2.112
2.012
2.002
2.189
2.189
2.084
2.084
2.010
2.010
2.185
2.192
2.045
2.106
2.014
1.987
2.178
2.178
2.080
2.080
2.020
2.020
2.175
2.175
2.069
2.069
2.000
2.000
2.205
2.205
2.086
2.086
2.008
2.008
2.217
2.217
2.086
2.086
1.982
1.982
2.205
2.204
2.087
2.087
2.012
2.012
2.216
2.216
2.088
2.088
1.979
1.979
2.204
2.204
2.087
2.087
2.011
2.011
2.203
2.204
2.023
2.109
2.016
2.008
2.195
2.195
2.080
2.080
2.028
2.028
2.196
2.206
2.092
2.063
1.983
2.013
Ir-O
2
Ir-N
Ir-N
Ir-C
1
2
9
Ir-C22
Ir35mo
Ir24mo
Ir3cn4mo
Ir3mo4cn
Ir35mo4cn
Ir24mo3cn
Ir345mo
Irbt
[13]
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
S
0
T
1
exp
Ir-O
1
2
2.196
2.195
2.080
2.079
2.028
2.030
2.199
2.207
2.090
2.065
1.982
2.014
2.201
2.201
2.086
2.086
2.009
2.009
2.204
2.203
2.105
2.026
2.008
2.013
2.188
2.188
2.087
2.087
2.006
2.006
2.190
2.184
2.109
2.021
2.007
2.012
2.191
2.191
2.085
2.085
2.013
2.013
2.195
2.195
2.088
2.088
1.982
1.982
2.180
2.182
2.076
2.074
2.034
2.029
2.185
2.188
2.064
2.085
2.018
1.984
2.189
2.189
2.086
2.086
2.003
2.003
2.188
2.192
2.019
2.105
2.008
2.008
2.194
2.194
2.079
2.079
2.028
2.028
2.210
2.210
2.084
2.084
1.995
1.995
2.204
2.205
2.086
2.086
2.011
2.011
2.205
2.206
2.109
2.039
1.989
2.012
2.158
2.146
2.059
2.049
1.990
1.990
Ir-O
Ir-N
Ir-N
Ir-C
1
2
9
Ir-C22
0 1
Changes in bond lengths, upon going from S to T geometry, are shown in italic (>0.02 Å) and bold-faced (<-0.02 Å) fonts.
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
rise of LUMO is more significant in Ir5mo than Ir3mo. As a result,
the energy gap of HOMO-LUMO is larger for Ir5mo than Ir3mo.
For the electron-withdrawing CN, its substitution at the C-ring
would drop the HOMO and LUMO levels, in which the -CN grafted
at 3, 5-sites would induce larger drop in the energy level of HOMO
than that of the LUMO, while -CN substituted at the 2, 4-sites
would lead to larger drop in the energy level of LUMO than that of
the HOMO. Consequently, the energy gaps of HOMO-LUMO are
increased for the Ir3cn and Ir5cn while ones are decreased for
the Ir2cn and Ir4cn. The lower-lying HOMOs in Ir5cn and Ir3cn
and LUMOs in Ir2cn and Ir4cn are also ascribed to the
contribution of the electronic density from the CN substituent.
In comparison with mono-substituted Ir3mo and Ir5mo, the di-
substituted complex Ir35mo shows a synergetic substitution
effect on the HOMO energy level, leading to a higher HOMO
energy level than Ir3mo and Ir5mo. Consequently, the HOMO-
g
LUMO energy gap E was declined by 0.39 eV for Ir35mo with
respect to the parent compound, which is larger than those of
Ir3mo (0.31eV) and of Ir5mo (0.13 eV). Similarly, compared to
the mono-substituted Ir2mo and Ir4mo, Ir24mo also shows
synergetic substitution effect with a slightly elevated HOMO but a
g
much higher LUMO level, hence leading to 0.23 eV enlarged E .
As revealed above, the introduction of electron-withdrawing CN at
the para-position centered by the electron density of HOMO would
induce more significant drops in the HOMO energy level while the
3
introduction of the electron-denoted OCH at the meta-position
centered by the electron density of LUMO would cause more
significant rises in the LUMO energy level. As a result, the energy
gap between HOMO and LUMO of Ir3cn4mo is tuned to be as
large as 3.71 eV, significantly higher than Ir3cn and Ir4mo and
close to Ir24mo. Similarly owing to the synergetic effects of CN
and OCH
smaller than those of Ir3mo and Ir4cn as well as Ir35mo. In a
whole, the synergetic effects of the -OCH and -CN substitutions
on the modulation of E is more effective than that of di-OCH
substituted ones owing to the fact that the concurrent introduction
of -OCH and -CN can influence HOMO and LUMO energies
simultaneously, while di-OCH substitution mainly present
significant effect on only HOMO or LUMO energy, not both.
3 g
, E of Ir3mo4cn is narrowed to be 3.00 eV, significantly
3
g
3
Figure 2. Contour plots of the HOMO and LUMO distribution of parent
compound Irbt.
3
3
Figure 3. Energy levels, energy gaps (in eV) and orbital composition distributions of HOMO and LUMO of the objective complexes, calculated at the level
B3LYP/LANL2DZ+6-31G* in the CH
2
Cl
2
media. Red and green circles denote the -OCH3 and -CN substitutions, respectively.
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
Compared to the di-substituted Ir35mo, tri-substituted
Ir345mo introduces one OCH
above, the substitution by OCH
3
at the 4-position. As revealed
at the 4-position would increase
3
the energy gap of HOMO-LUMO. Consequently, Ir345mo
presents larger E than Ir35mo. Owing to the narrowed E by the
substitution of CN at 4-position, the tri-substituted Ir35mo4cn
exhibits smaller E (2.94 eV) than Ir35mo (3.08 eV), which is also
the smallest E and may be the reddest substance among all the
g
g
g
g
derivatives under study. Similarly, compared with di-substituted
Ir24mo, the tri-substituted Ir24mo3cn with an additional ortho-CN
at the 3-position exhibits a 0.36 eV enlarged E
enlargest E (viz 3.83 eV) and may be the bluest substance
among all the complexes under study.
g
, which is also the
g
2
.3 Predicted absorption spectra
The absorption spectrum is an important indicator for
evaluating energy transfer efficiency of the OLED device. Thus,
we calculated the spectrums for all the compounds in CH
2
Cl
2
solvent using TD-B3LYP method, based on the optimized S
0
structures. Figure. 4 shows simulated absorption spectra. The
calculated transition energy (eV), oscillator strength (ƒ), major
contribution and transition characters are summarized in Table
S18. The calculated absorption wavelengths are 432 nm and 390
nm for Ir4mo, 454 nm, 335 nm and 310 nm for Irbt, and 499 nm,
Figure 4. Simulated absorption spectra for the objective complexes in the
CH Cl media.
2 2
407 nm and 324 nm for Ir3mo, which well correspond to the
experimental values of 424 nm, 387 nm for Ir4mo, 447 nm, 327
nm and 313 nm for Irbt, and 522 nm, 420 nm, and 323 nm for
Ir3mo (vide Table S18). The excellent agreement between the
calculated results and experimental values further confirms the
reliability of our computational method.
and Ir5mo display a drop in the absorption strength, which may
be associated with the steric hindrance and the reduced
conjugation length of the C^N ligands.
It can be seen from Figure. 4 that the absorption spectra of all
these compounds exhibit two major absorption bands, i.e., a
strong band with λabs max of 300~360 nm at high energy region,
and a weak one with λabs max of 400~530 nm at low energy region.
2
.4 Predicted phosphorescence spectra
In order to reliably predict the phosphorescent emission colors,
TDDFT was used to calculate the emission energy at the PCM-
B3LYP level of theory in the dichloromethane. The calculated
results are listed in Table 3, along with some available
experimental data for Irbt, Ir3mo, Ir4mo, Ir24mo, Ir35mo and
Ir345mo. It was reported by some experimental works that
complexes of Ir4mo[10b], Irbt[12a] and Ir3mo[10a] emit green, yellow
and red light, respectively and their emission peaks are located at
around 544 nm, 557 nm and 610 nm, respectively. Compared with
the experimental results, the TDDFT method will reproduce the
change trend in the experimental value for the six compounds
despite of some differences in the absolute values. As can be
seen from Table 3, in comparison with the parent complex Irbt,
the mono-substituted Ir2cn, Ir4cn, Ir3mo and Ir5mo show red-
shifted emissions with λem maxs of 537 nm, 548 nm, 582 nm and
0 n
The band of 300-360 nm stems from S -S transitions with large
transition oscillator strength, presenting LC-featured transition (π-
π*). The very similar λabs max in the band of 300~360 nm for all
these compounds implies that the conjugations of the C^N ligands
are similar among all these complexes. The weak bands with λabs
0 1
max of 400~530 nm are ascribed to be the lowest-lying S →S
transition, in which the HOMO→LUMO transition predominates
over configurations (92~98%) and presents a mixed MLCT and
LC character. Accordingly, the drastically different λabs max of
400~530 nm among all these compounds in this lower-energy
band suggests that the substitution of the C-ring would induce a
rather significant impact on the band gap of the complexes.
Among the absorption bands, Ir24mo3cn shows the bluest λabs
max of 401nm and Ir35mo4cn displays the reddest λabs max of 534
nm. In addition, it can be observed that the substitutions also
significantly influence the absorption strength. Compared to the
parent compound, the absorption strength is enhanced by the
substitution for most of the derivatives, in particular for Ir3cn,
Ir3cn4mo and Ir24mo3cn. Their absorption intensities are
approximately three times than that of the parent Irbt, consistent
with the report in previous literature[15]. However, derivatives Ir2cn
531 nm, respectively; while Ir3cn, Ir5cn, Ir2mo and Ir4mo display
blue-shifted emissions with λem maxs of 493 nm, 497 nm, 495 nm
and 503 nm, respectively. The calculated results indicate that their
T1→S0 deactivation processes are dominated by the
LUMO→HOMO transition, presenting feature of mixed LC and
MLCT transition, as reflected by Table S18. The observations
further confirm the high correlation between the emission wave
g
length and the E data. For example, in agreement with the
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
2
2
S1
3
ꢃ
3
3
〈
휓
|HS0|휓T1〉 .휇
ꢁ6휋 ꢁꢂ 푛 퐸
T1
s1
smallest E
g
, Ir3mo shows the most red-shifted emission, which
ꢀ푟 ≈ 훾
, γ =
(2)
(
△E
S1−T1
)
2
ꢄℎ휀
0
was reported to emit orange-red light experimentally.
Where μs1 is the transition electric dipole moment in the S
transition, ET1 represents the emission energy in cm , n, h, and ε
are the refractive index, Plank’s constant, and the permittivity in
vacuum, respectively. In terms of equation (2), large μs1 and small
0
-S
1
For the di-substituted ones, Ir35mo and Ir3mo4cn show
further red-shifted emissions of 599 nm and 624 nm compared to
the corresponding mono-substituted complex, while Ir24mo and
Ir3cn4mo exhibit further blue-shifted emissions of 492 nm and
-1
0
△ES1-T1 values would favor the high quantum yield.
4
87 nm, respectively. In addition, it is noted that the concurrent
introduction of -CN and -OCH should be more effective in the
color-tuning range than that of the two -OCH . Utilizing the
synergetic effect of -CN and -OCH , the tri-substituted Ir35mo4cn
As revealed, the direct involvement of the Ir (III)-d orbital would
enhance the first-order SOC in the T -S transition, which would
1 0
3
3
induce a drastic decline of the radiative lifetime and avoid the
nonradiative process. Hence, a large 3MLCT contribution will
increase the quantum yield (ΦPL). We calculated MLCT% of all
3
shows a 125 nm red-shifted λem max compared to the parent Irbt
and the emission color falls into saturated red region while
complex Ir24mo3cn displays 33 nm blue-shifted
exhibiting greenish-blue phosphorescence. The results indicate
that a library of full-color emissive chelates with a 158 nm wide
color-tuning range could be constructed by simple grafting of -CN
[
17]
the derivatives under study following chou’s work , as shown in
em max
λ ,
3
Table 4. It can be seen that MLCT% is affected by the different
substituents and their grafted positions. Specially, the introduction
3
of -CN at the meta-site leads to an obvious increase of the MLCT
3
contribution. For example, the MLCT% percent for Ir35mo4cn,
3
and -OCH at ortho-, meta- and/or para-site of the C-ring of the
Ir3mo4cn, Ir4cn and Ir2cn are higher (3.2%~7.2%) than the
parent complex Irbt, which imply high ΦPL values for these red-
C^N ligands, which provides a facile strategy for readily fine-tuned
emission color covering the entire visible region of iridium
complexes.
shifted materials[18]
In addition, the S
.
1
1
-T intersystem crossing (ISC) rate derived
from the SOC interaction plays a key role in the phosphorescent
process [19]. As known, the ISC rate decreases exponentially as
the singlet-triplet splitting energy increases[20]. Therefore, a
Table 3. The calculated triplet energies and the predicted emission colors of the
objective complexes.
complex
E
TD / nm
E
TD / eV
Exp./ nm
minimal △ES1-T1 would facility enhancing the transition moment
Irbt
Ir2cn
Ir3cn
Ir4cn
Ir5cn
Ir2mo
Ir3mo
Ir4mo
512
537
493
548
497
495
582
503
531
599
492
487
624
637
479
564
2.42
2.31
2.51
2.26
2.50
2.50
2.13
2.47
2.33
2.07
2.52
2.55
1.99
1.95
2.59
3.93
557
r
and the ISC rate, in turn leading to an increasing k . It is noted that
Ir35mo4cn and Ir3mo4cn have the moderate μs1 and relatively
smaller △ES1-T1 values, indicating that they may be potential red-
r
emitting OLED materials with the large k values.
544a
a
602
54422
Table 4. Calculated emitting energy (ET1, in eV), metal ligand charge transfer
3
Ir5mo
character ( MLCT) (%) and singlet-triplet splitting energy (ΔΕS1–T1, in eV), along
Ir35mo
Ir24mo
Ir3cn4mo
Ir3mo4cn
Ir35mo4cn
Ir24mo3cn
Ir345mo
a
604a
529a
with the Transition Electric Dipole Moment (μs1, D) for the objective
complexes.
complex
E
T1
3MLCT
△E
S1-T1
μ
s1
Irbt
Ir2cn
Ir3cn
Ir4cn
Ir5cn
Ir2mo
Ir3mo
Ir4mo
2.42
2.31
2.51
2.26
2.50
2.50
2.13
2.47
2.33
2.07
2.52
2.55
1.99
1.95
2.59
2.20
24.82
31.98
20.72
29.93
28.96
17.16
26.04
23.93
29.37
24.03
21.78
18.60
28.90
28.05
18.90
24.30
0.31
0.34
0.35
0.35
0.42
0.32
0.35
0.40
0.33
0.36
0.44
0.45
0.37
0.37
0.50
0.40
3.36
3.38
3.18
3.32
1.37
3.20
3.68
3.34
0.32
0.61
3.55
3.52
1.25
1.00
3.33
1.09
579a
derived from the experimental determination in the work
2
.5 Phosporescence quantum efficiency calculated
Ir5mo
Ir35mo
Ir24mo
Ir3cn4mo
Ir3mo4cn
Ir35mo4cn
Ir24mo3cn
Ir345mo
As accepted, the quantum yield ΦPL from an emissive excited
state to the ground state is linked to radiative (k
r
) and nonradiative
(
k
nr) rates, which can be expressed by the following equation (1):
푘r
훷PL = k
τ
r em
=
(1)
푘r+푘nr
Where τem is emission decay time. Clearly, a large k
nr are required for a high quantum yield.
r
and a small
k
2.5.2 Comparision of Nonradiative Rate (knr)
As revealed, the metal-centered 3MC d-d is one main
deactivation pathway for the phosphorescent emission of
2
r
.5.1 Comparision of Radiative Rate (k )
[
21]
Theoretically, k
r
is proportional to the spin-orbit coupling
transition-metal complexes . Herein, we calculated the lowest
3
MLCT/π-π* and 3MC d-d states for all the studied complexes
(
SOC) and inversely proportional to the energy difference
[
16]
between the two states, as expressed by equation (2)
:
using unrestricted triplet optimizations in order to estimate their
3
different quantum yields. The MLCT/π-π* excited states were
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
obtained through performing an unrestricted triplet optimization
based on the optimized ground-state geometry (vide
Computational Method section). The electronic configurations of
MC d-d states were performed following the literature
methodology[22].
It was reported that the lowest triplet metal-to-ligand charge
3
transfer ( MLCT/π-π*) excited state can be rapidly converted to a
3
short-lived triplet metal-centred ( MC d-d) in a thermal inactivation,
3
[23]
3
in which no photochemistry occurs . Then, the MC d-d state
decay back to the ground state through a very fast nonradiative
way[22b]. The 3MLCT→ MC conversion was closely associated
3
A comparison of Table 2 with Table 5 shows that the geometric
3
3
discrepancy between the S
0
state and the MLCT/π-π* one is
with the relative energy difference between the MLCT/π-π* and
3
[24]
3
relatively small, while the distortion of the geometry from the S
0
MC d-d excited states . Therefore, a high-lying MC d−d state
3
and a large energy gap between 3MLCT/π−π* and 3MC d−d
state to the MC d-d one is rather obvious. For instance, the Ir-N
1
/
Ir-N
2
bond lengths in the 3MC d-d states are elongated
states is generally required for high quantum yield. It can be seen
3
significantly by 0.325-0.565 Å, compared to their ground states.
The structural distortion in the 3MC d-d state induced by the
elongated Ir-N bonds will increase the non-radiative probability.
from Figure. 5 that the energy of MC d-d state is higher than that
3
of the lowest lying MLCT/π-π* state for most studied complexes
except for Ir5cn. Compared to the parent compound Irbt, the
substitutions of -CN in the meta-positions like Ir35mo4cn,
Ir3mo4cn, Ir4cn and Ir2cn compounds obviously stabilize their
3
Accordingly, arrival at the MC state will decrease the quantum
yield, which is controlled by the competition between the radiative
3
(
k
r
) and the nonradiative (knr) rate.
MLCT/π-π*states, in particular for Ir35mo4cn and Ir3mo4cn. In
3
Table 5. Selected bond length (in Å) calculated for the objective complexes in the metal-centred ( MC) triplet excited states.
Irbt
Ir2cn
Ir3cn
Ir4cn
Ir5cn
Ir2mo
Ir3mo
Ir4mo
Ir-O
Ir-O
1
2
2.141
2.165
2.489
2.650
2.021
2.019
2.155
2.116
2.590
2.427
2.022
2.035
2.120
2.136
2.497
2.567
2.036
2.026
2.149
2.116
2.634
2.480
2.022
2.034
2.155
2.127
2.627
2.405
2.031
2.030
2.168
2.140
2.594
2.453
2.021
2.026
2.165
2.164
2.571
2.562
2.005
2.005
2.135
2.166
2.490
2.652
2.026
2.020
Ir-N
Ir-N
Ir-C
1
2
9
Ir-C22
Ir5mo
Ir35mo
Ir24mo
Ir3cn4mo
Ir3mo4cn
Ir35mo4cn
Ir24mo3cn
Ir345mo
Ir-O
Ir-O
1
2
2.164
2.164
2.478
2.614
2.009
2.021
2.174
2.176
2.579
2.519
2.011
2.000
2.135
2.170
2.446
2.592
2.030
2.022
2.124
2.124
2.523
2.523
2.034
2.034
2.128
2.128
2.547
2.545
2.041
2.041
2.145
2.159
2.473
2.617
2.013
2.026
2.125
2.125
2.502
2.501
2.037
2.037
2.161
2.168
2.453
2.651
2.011
2.022
Ir-N
Ir-N
Ir-C
1
2
9
Ir-C22
contrast, the 3MC d-d states are hardly influenced by the
substitution. Consequently, the Ir35mo4cn, Ir3mo4cn, Ir4cn and
Ir2cn derivatives present large energy gaps between their
3MLCT/π-π* and 3MC d-d states, especially for Ir35mo4cn,
implying that their radiationless pathways would be less efficient
and the high emission yields probably be achieved.
2
.6 Predicting the performances of the complexes as
OLEDs
An ideal device performance of OLEDs is largely depended on
excellent electrical carrier-injection and transport abilities as well
as the balance between holes and electron transports. To
estimate the energy barriers of the injection of holes and the
electron transport, we calculated ionization potentials (IP) and
electron affinities (EA), which are also closely associated with
HOMO and LUMO, respectively. For photoluminescent materials,
a larger EA (smaller IP) suggests that it is easier to inject electrons
(
holes) into the emitting materials from the electron (hole)
transporting layer[25]. The calculated vertical IP (IP
), adiabatic IP
introduction of electron-donating -OCH groups to the C-ring
v
3
results in smaller IP values and EA values compared with the
parent complex, irrespective of the number of substituent and the
positions, while an opposite trend is observed for the substitution
of CN. The variation trend is consistent with ones of HOMO and
Figure 5. Energy level diagram of the objective complexes in the 3MLCT and
MC excited states, respectively, along with the normalized S levels.
0
3
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
B
LUMO energies. Thus, it is reasonable to assume that the -OCH
3
Where ꢈA and ꢈA are the energies of A and B at the optimized
A
substitution will facilitate the hole-injection but disfavor the
electron injection while the substitution of -CN would cause
reverse results. It is worth to note that Ir35mo4cn and Ir3mo4cn
present low IP and high EA values, indicating that the introduction
geometry of A, respectively; similarly, ꢈB and ꢈB are the energies
of A and B at the optimized geometry of B, respectively.
In addition, based on the M geometry, the hole extraction
potential (HEP), which represents the energy difference between
+
+
of -OCH
3
and -CN at the para- and meta- positions of C-ring of
M (neutral molecule) and M (cationic), is calculated. And in this
the C^N ligands can effectively improve the carrier injection ability. way, the electron extraction potential (EEP) can be expressed
According to Marcus theory[ , the charge (hole and electron)
26]
similar to that for HEP. As shown in Figure. 6, the reorganization
transfer rate ket can be expressed by the following equation (3):
energy for hole (λ
h
) and electron transport (λ
e
) can be expressed
by the following equation (5) and equation (6):
ꢀet = 퐴푒푥푝(ꢅ휆⁄4ꢀ 푇)
(3)
B
in which λ is the reorganization energy and A is prefactor related
to the electronic coupling between adjacent molecules. T and ꢀ퐵
are temperature and Boltzmann constant, respectively. Due to the
limited intermolecular charge transfer range in the solid state, the
mobility of charges was considered to be mainly associated with
the internal reorganization energy 휆 for OLED materials[27]. A
small 휆 value is necessary for an efficient charge transport
process. Hence, in order to evaluate the charge transfer rate and
the balance performance, the reorganization energy (λ) was
calculated for the sixteen compounds, which is caused by the
change of the internal nuclear coordinates from the reactant A to
the product B and vice versa (vide Figure. 6). It can be evaluated
in terms of the following equation (4):
휆h = Iꢊ ꢅ ꢋꢈꢊ
(5)
(6)
v
휆e = ꢈꢈꢊ ꢅ ꢈꢌv
As emitting layer materials, it needs to achieve hole and
electron injection balance. Table 6 shows that the reorganization
energies for the electron transport (λ
e
) are slightly larger than the
ones from the hole transport (λ ) for most compounds, implying
h
that the hole-transporting performance of these complexes is
slightly better than performances of their electron transports. It is
observed that the energy differences between λ and λ are 0.01
h e
eV for Ir3mo, 0.00 eV for Ir3mo4cn and 0.06 eV for Ir35mo,
smaller than the other complexes (0.02-0.14 eV), indicating that
the hole and electron transfer balance could be achieved easily in
the emitting layer for these complexes. In a whole, these energies
further suggest that Ir3mo4cn is one potential red-light OLED
material for high-performance.
A
B
B
휆i = 휆ꢂ ꢆ 휆 = ꢇꢈ ꢅ ꢈ ꢉ ꢆ ꢇꢈ ꢅ ꢈ ꢉ
(4)
ꢁ
A
A
B
Table 6. Ionization Potentials, Electron Affinities, Extraction Potentials, Internal Reorganization Energies, and ∆λ = |휆ℎ ꢅ 휆ꢍ|(in eV) for the objective complexes
calculated at the B3LYP/LANL2DZ+6-31G* level.
HOMO
IP
v
IP
a
HEP
LUMO
EA
v
EA
a
EEP
λ
h
λ
e
△λ
Irbt
Ir2cn
Ir3cn
Ir4cn
Ir5cn
Ir2mo
Ir3mo
Ir4mo
–5.28
–5.66
–5.72
–5.67
–5.71
–5.24
–4.98
–5.25
–5.03
–4.76
–5.20
–5.66
–5.35
–5.24
–5.63
–4.96
6.20
6.72
6.82
7.06
6.65
6.04
5.83
6.27
5.88
5.55
5.98
6.69
6.34
6.19
6.67
5.78
6.11
6.64
6.76
6.77
6.59
5.95
5.72
5.99
5.77
5.42
5.90
6.09
6.24
6.00
6.48
5.64
6.03
6.56
6.69
6.93
6.53
5.85
5.61
5.90
5.66
5.28
5.83
6.57
6.13
5.82
6.40
5.49
–1.81
–2.32
–2.12
–2.35
–2.10
–1.66
–1.82
–1.65
–1.69
–1.68
–1.50
–1.95
–2.35
–2.30
–1.80
–1.73
0.47
1.14
1.11
1.32
0.80
0.25
0.46
0.27
0.32
0.32
0.12
1.47
1.17
1.10
0.68
0.42
0.56
1.24
1.20
1.32
0.90
0.36
0.56
0.38
0.42
0.42
0.24
1.58
1.27
1.22
0.87
0.53
0.65
1.35
1.29
1.41
0.99
0.48
0.67
0.50
0.52
0.53
0.36
1.16
1.38
1.35
1.05
0.64
0.17
0.16
0.13
0.13
0.12
0.19
0.22
0.37
0.22
0.27
0.15
0.12
0.21
0.37
0.27
0.29
0.18
0.21
0.18
0.09
0.19
0.23
0.21
0.23
0.20
0.21
0.24
0.24
0.21
0.25
0.37
0.22
0.01
0.05
0.05
0.04
0.07
0.04
0.01
0.14
0.02
0.06
0.09
0.12
0.00
0.12
0.10
0.07
Ir5mo
Ir35mo
Ir24mo
Ir3cn4mo
Ir3mo4cn
Ir35mo4cn
Ir24mo3cn
Ir345mo
2
.7 Validation by absorption and emission
experiments
In order to validate the reliability of the molecular design
strategy above, we selected six representative compounds to
experimentally synthetize and measure their absorption and
emission spectra, as shown in Figure. 7. In line with the
computational predictions, all the six compounds exhibit two
distinguishable absorption bands in the UV-Vis region, i.e., a
strong one with absorption maximum (λabs max) of 310~330nm, and
a relatively weak one with λabs max of 420~510nm. Also similar to
the calculated results, these complexes show analogous λabs max
in the higher-energy bands while their λabs max in the lower-energy
Figure 6. Schematic description of the inner reorganization energy.
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
ones are quite different. Compared to the parent compound Irbt
with λabs max = 447nm, the mono-substituted complexes Ir2mo with
bearing not only a para-OCH
most red-shifted PL among all the six complexes (λem max = 604
nm) while Ir24mo bearing two meta-OCH displays the most blue-
shifted PL emission band with λem max of 529 nm. The observation
further confirms that the two -OCH groups display a synergistic
substituent effect on the tuning direction of emission color. The
experimental results further confirm that the color-tuning strategy
derived from the molecular design above is effective. The simple
3 3
but also an ortho-OCH shows the
a meta-OCH
3
and Ir3mo with a para-OCH
3
, display opposite color
3
tuning direction. λabs max values are 438 nm for Ir2mo and 506 nm
for Ir3mo, displaying blue-shift 9 nm and red-shift 59 nm,
respectively. Based on the two mono-substituted complexes, the
di-substituted complexes Ir24mo (λabs max of 424 nm) and Ir35mo
3
(λabs max of 484 nm) were further blue-shifted and red-shifted,
respectively. However, the absorption band of the tri-substituted
Ir345mo is similar to that of the di-substituted Ir35mo. The
absorption spectra are consistent with the calculated results.
The emission experiment shows that under photoexcitation, all
3
exchange of the substitution positions of -OCH in Ir35mo and
Ir24mo would result in a 75 nm shift between the λem max, further
confirming the reliability of the computational results.
these
phosphorescence with structureless emission bands (vide Figure.
and Table 7). Consistent with the computational results, the
single-substituted at the para- site by -OCH (viz., Ir3mo) indeed
lead to bathochromic shift of phosphorescence with respect to the
parent compound Irbt while the meta-OCH (Ir2mo) substitution
complexes
emit
intense
room-temperature
3
. Conclusions
7
3
In the work, DFT/TDDFT method were employed to study
3
sixteen Ir (III) complexes by attaching the -OCH and -CN groups
3
with different numbers (mono-, di-, and tri-substituted) to different
positions of the C-ring of the C^N ligand (ortho-, para- or meta-
sites with respect to the Ir atom). The predicted results were
further validated by the adsorption and emission experiments for
the six representative complexes. Our objective is to decipher the
relationship between the substituents and the photophysical
property of the Ir-complexes, in turn developing a molecular-
design strategy for rational fine-tuning of phosphorescence color
of Ir (III) complexes. The consistency between the experimental
results and the computational ones confirms the reliability of the
molecule-design strategy. Some valuable observations were
induces blue-shifted photoluminescence (PL). The Ir35mo
obtained from the work: 1) The introduction of -CN and -OCH
3
would lead to a decreased and increased FMO energy levels,
respectively, irrespective of substituted positions; 2) the
substituents (either electron-denoting -OCH
withdrawing -CN one) substituted at the ortho- and para-sites (e.g.,
-,5-sites) of the C-ring could more significantly modulate the
3
group or electron-
3
HOMO energy levels than the other positions since the electronic
densities of HOMO are mainly resided at the two positions, but
their substitutions at the meta-site centred by the electronic
densities of LUMO would play a stronger role in influencing the
LUMO energy levels; 3) compared with the mono-substituted
complexes, a wide color-tuning range can be obtained utilizing the
synergetic effect of di-, and tri-substituted -OCH
at different sites. Excitingly, Ir35mo4cn bearing not only one
meta-CN but also ortho- and para-OCH shows the most red-
shifted PL (λem max = 637 nm), while Ir24mo3cn bearing both one
para-CN and two meta-OCH displays the most blue-shifted PL
3
and -CN groups
3
3
emission band with λem max of 479 nm; The color-tuning range
could be up to 158 nm through the synergetic effect of simple -
OCH and -CN groups substituted at the C-ring of the Ir(III)
3
complex. 4) Ir3mo4cn may be potential candidates for high
Figure 7. Normalized UV-Vis absorption spectra and PL emission spectra of the
3
objective complexes in dilute CH
2
Cl
2
solutions.
efficiency red OLEDs materials due to high MLCT (%), low △ES1-
3
3
T1, enlarged separation between MLCT/π-π* and MC d-d states,
and good hole and particle-transporting performance.
Table 7. UV-Vis absorption and PL emission data of the objective complexes
in dilute CH
2
Cl
2
solutions.
In a whole, the quantum mechanics calculation could reliably
predict the geometric and electronic structures as well as the
spectral property. The substitution at positions centred by the
electrical density is a simple and efficient way to modify the
energy gap, in turn color-tuning and quantum efficiency. The
molecular strategy derived from our work provides useful
guidelines for exploring high-performance blue and red OLEDs..
complex
λ
abs max(nm)
λem max(nm)
Irbt
313,327,358,447,488
333,357,382,438
322,417,460,506
328,406,477,484
332,351,379,424
329,397,453,487
557
544
602
604
529
579
Ir2mo
Ir3mo
Ir35mo
Ir24mo
Ir345mo
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
4. Experimental Section
4
.1 Computational Methods
In this work, geometrics of all complexes in the ground state and the
lowest triplet excited one were fully optimized using B3LYP[28] and
unrestricted B3LYP(UB3LYP) in CH Cl media within the framework of the
2 2
polarizable continuum model (PCM)[29], respectively. There were no
symmetry constrain on these complexes. Vibrational frequencies were
calculated at the same theoretical level to confirm that each configuration
is a minimum on the potential energy surface. In addition, electronic
configurations of the triplet metal-centered (3MC) d-d states were
[
22]
optimized following Thummel’s work , in which the optimization starts
with a distorted molecular geometry by elongating the metal-ligand bonds.
In order to predict the phosphorescent spectra reliably, five different
functionals (viz., B3LYP, PBE0[30], M052X[31], M062X[32] and BP86[33]) were
tested to calculate the emission spectrum of the parent compound (Irbt).
The results indicated that B3LYP is more accurate in reproducing the
experimental data (Table S17) with respect to the other four methods.
Therefore, B3LYP is selected in the calculation of emission spectra. Based
on the optimized structures of the ground states and the triplet ones, the
2 2
absorption and emission properties in the CH Cl media were calculated
[
34]
by the time-dependent density functional theory (TDDFT)
framework of PCM.
within the
In all the calculations, a “double-ζ” quality basis set LANL2DZ[35] along
with the pseudopotential was employed for the iridium atom, in which a
relativistic effective core potential (ECP) on Ir replaces the inner core
2
6
6
electrons, leaving the outer core 5s 5p and 5d as the valence electrons
of Ir(III). The 6-31G* basis set was used for non-metal atoms in the
gradient optimizations. All calculations were performed using the program
of Gaussian 09 software package[36]. Chemcraft 1.6[37] was used for
structures and manipulations. The UV-Vis spectra was simulated via the
Scheme 1. The synthetic routes of the objective Ir (III) complexes. Reagents
and reaction conditions for the synthetic procedures: (i) SOCl
aminothiophenol, NMP, 100 °C; (iii) IrCl •3H2O, 2-ethoxyethanol-H
reflux; (iv) acetylacetone, 2-ethoxyethanol, Na CO , reflux.
2
, reflux; (ii) o-
Swizard program (revision 5.0)[38]
.
3
2
O=3/1,
2
3
4
.2 Experimental Methods
dichloro-bridged iridium(III) complexes (4a ～ 4f). Then the objective
chelates of 5a～5f were prepared by refluxing the chloride-bridged dimers
In order to validate the computational results, we selected six
4a～4f (0.1 mmol), acetylacetone (0.3 mmol) and sodium carbonate (1
representative Ir (III) complexes (Irbt, Ir2mo, Ir3mo, Ir35mo, Ir24mo and
Ir345mo) to synthesize. Scheme 1 shows their synthetic routes. H NMR
spectra of four unreported complexes (Ir2mo, Ir35mo, Ir24mo and
Ir345mo) were measured on a Bruker AVANCE-400 spectrometer in
1
mmol) in 10 mL of 2-ethoxyethanol under argon for 12 h. After being cooled
down, the precipitate was collected and purified from flash
chromatography through silica column using CHCl as eluent, followed by
3
recrystallization from a mixture of dichloromethane and methanol for more
than three times.
CDCl
3
using TMS as internal standard. The UV-Vis absorption spectra
Cl solution at 298 K using a Hitachi
were measured in 5×10−6 mol L−1 CH
2
2
2’
Bis(2-phenylbenzothiozolato-N,C )iridium(III)(acetylacetonate) (5a)
[39]:
orange solid. Yield: 46%.
Bis[2-(3-methoxyphenyl)benzothiazolato-
U-4100 UV-Vis-NIR scanning spectrophotometer. The PL emission
spectra were recorded under excitation of 400 nm in the same solvent and
temperature, using a Perkin-Elmer LS55 fluorescence spectrophotometer.
The reactants 1a～1f were commercially available and used without
further purification. All the solvents were of analytical grade and freshly
distilled prior to use.
The synthesis of cyclometallate ligands(3a～3f). 1a～1f (10 mmol)
was refluxed with 20 mL of thionyl chloride for 3 hours, then excessive
thionyl chloride was removed in vacuo. The residue was dissolved in 20
mL of N-methylpyrrolidinone (NMP), then the resultant solution was added
dropwise into a solution of 10 mmol of o-aminothiophenol in 20 mL of NMP
under inert atmosphere, followed by refluxing under stirring for 1 h. After
N,C ]iridium(III)(acetylacetonate) (5c) [39]: red solid. Yield: 38%.
2'
Bis[2-(2-methoxyphenyl)benzothiazolato-
N,C2' ]iridium(III)(acetylacetonate) (5b): orange solid. Yield: 43%. 1
H
NMR (CDC1
.88-7.90 (m, 2H, ArH), 7.38-7.41 (m, 4H, ArH), 6.62 (t, J = 8.0 Hz, 2H,
ArH), 6.36 (d, J = 8.4 Hz, 2H, ArH), 5.99 (d, J = 7.6 Hz, 2H, ArH), 5.09 (s,
H, acacH), 3.98 (s, 6H, OCH H), 1.73 (s, 6H, acacH).
3
, 400 MHz, δ/ppm, vide Figure. S1): 8.09-8.12 (m, 2H, ArH),
7
1
3
Bis[2-(3,5-dimethoxyphenyl)benzothiazolato-
N,C2' ]iridium(III)(acetylacetonate) (5d): red solid. Yield: 32%. 1H NMR
3
(CDC1 , 400 MHz, δ/ppm, vide Figure. S2): 8.00 (d, 2H, J = 7.2 Hz, ArH),
being cooled down, the reactant was poured into water followed by
neutralization with 7 mol·L-1 aqueous ammonia to pH = 8~9. The
precipitate was filtered and recrystallized from alcohol to afford the
corresponding cyclometallate ligands (3a～3f).
7.83 (d, 2H, J = 6.8 Hz, ArH), 7.31~7.37 (m, 4H, ArH), 6.89 (d, 2H, J = 2.0
Hz, ArH), 5.84 (d, J = 2.0 Hz, 2H, ArH), 5.05 (s, 1H, acacH), 3.79 (s, 6H,
OCH H), 2.67 (s, 6H, OCH H), 1.72 (s, 6H, acacH).
3 3
Bis[2-(2,4-dimethoxyphenyl)benzothiazolato-
The synthesis of the iridium complexes(5a～5f). 3a～3f (2.4 mmol)
N,C2' ]iridium(III)(acetylacetonate) (5e): orange solid. Yield: 40%. 1
H
3 2
was refluxed with IrCl ·3H O (1 mmol) in a mixture of 2-ethoxyethanol and
water (3:1) under argon for 24 h, then the precipitate was filtered and
washed with 10 mL of 1 mol·L-1 HCl and 3 × 15 mL methanol in sequence,
NMR (CDC1
ArH), 7.83 (d, 2H, J = 7.6 Hz, ArH), 7.38 (t, 2H, J = 6.8 Hz, ArH), 7.31 (t,
H, J = 6.8 Hz, ArH), 5.96 (d, J = 2.0 Hz, 2H, ArH), 5.51 (d, J = 2.0 Hz, 2H,
3
, 400 MHz, δ/ppm, vide Figure. S3): 8.05 (d, 2H, J = 8.0 Hz,
2
then dried over vacuo to afford the corresponding crude product of
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ChemPhysChem
10.1002/cphc.201800928
ARTICLE
ArH), 5.11 (s, 1H, acacH), 3.95 (s, 6H, OCH
3
H), 3.32 (s, 6H, OCH
3
H), 1.75
528-537; i) J. H. Jang, H. J. Park, J. Y. Park, H. U. Kim, D. H. Hwang,
Org. Electron. 2018, 60, 31-37.
(
s, 6H, acacH).
Bis[2-(3,4,5-trimethoxyphenyl)benzothiazolato-
N,C2' ]iridium(III)(acetylacetonate) (5f): red solid. Yield: 29%. 1H NMR
CDC1 , 400 MHz, δ/ppm, vide Figure. S4): 7.91 (d, 2H, J = 8.0 Hz, ArH),
.81 (d, 2H, J = 7.6 Hz, ArH), 7.26~7.35 (m, 4H, ArH), 7.12 (s, 2H, ArH),
.94 (s, 1H, acacH), 3.88 (s, 6H, OCH H), 3.67 (s, 6H, OCH H), 2.64 (s,
H, OCH H), 1.70 (s, 6H, acacH).
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