Towards an efficient blue emission cationic Ir(III) complex with azole-type ancillary ligands: A joint theoretical and experimental study

Article abstract of DOI:10.1039/c5nj03045e

Pyridine-azole moieties have proved to be an attractive building block for multifunctional cationic Ir(iii) complexes, however, few highly efficient blue materials have been demonstrated and the deep structure-property relationships need to be revealed. H

Full text of DOI:10.1039/c5nj03045e

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PAPER
Towards efficient blue emission cationic Ir(III) complex with azole-type
ancillary ligands: a joint theoretical and experimental study
Shao-Fen Huang, Hai-Zhu Sun, Guo-Gang Shan*, Yong Wu, Min Zhang* and Zhong-Min Su*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
Pyridineꢀazole moieties have been proved to be an attractive bluding block for multifunctional cationic
Ir(III) complexes, however, few highly efficient blue materials are demonstrated and the deep structrueꢀ
properties relationships need to be revealed. Herein, a series of cationic [Ir(dfppz)2(N^N)][PF6]
complexes (1–4) based on azoleꢀtype ancillay ligand, namely, 1,1'ꢀdiphenylꢀ1H,1'Hꢀ[2,2'] biimidazolyl
10 (Phbid), 2ꢀ(1ꢀphenylꢀ1Hꢀimidazolꢀ2ꢀyl)ꢀpyridine (Bpyim), 2ꢀ(1ꢀmethylꢀ1Hꢀimidazolꢀ2ꢀyl)ꢀpyridine
(Mpyim), and 2ꢀ(1,5ꢀdimethylꢀ1Hꢀ[1,2,4]triazoleꢀ3ꢀyl)ꢀpyridine (Mpytz), respectively, have been
prepared and their photophysical, electrochemical and charge transporting properties are investigated. The
reported complexes exhibit strong perceived green to blue emission as well as excellent redox
reversibility at room temperature. The comprehensive density functional theory calculations are
15 performed to provide insight into the electronic structures of 1–4 and disclose the ancillary ligand effect
on the emission behavior in detail. The simple methyl group modification endow the trizoleꢀtype ancillary
with exceedingly large πꢀπ* energy gap, resulting blue emitting complex 4, namely
[Ir(dfppz)2(Mpytz)][PF6] with peak value at 462 nm. Meanwhile, deipite the significant 3LC character, the
high efficiency with quantum yield of 31.6% of 4 is realized in neat film, which is higher than those of
20 reported cationic Ir(III) complexes with similar emissions. Addionaly, the calculation results also suggest
that complexes 2–4 possess better electronꢀtransporting abilities in comparison with that of 1.
cationic Ir(III) complexes through ingenious ligand control to
achieve high efficiency and modify the emission color
50 ultimately.27, 28 Increasing steric hindrance of ligands has been
proved to be an effective way to improve quantum efficiency.29, 30
As for emission color, the green, yellow, orange and red emitting
cationic Ir(III) complexes have been exploited widely.31ꢀ34 In
contrast, the number of blue emitting cationic Ir(III) complexes is
55 rather limited, despite the efficient blue ones are essential
components for fullꢀcolor displaying and solidꢀstate lighting.11, 35
Moreover, it is noted that the synthesized cationic Ir(III)
complexes should have excellent chargeꢀtransporting ability and
high phosphorescence quantum efficiency to realize stable and
60 efficient lighting. Otherwise, the LECs will not work or exhibit
low efficiency, which is relation with the structural feature of
LECs as revealed in the previous reports.36 Thereby, the
exploitation of more efficient cationic blue Ir(III) complexes, and
investigation of the relationships between the structure and
65 properties are highly desired.
Introduction
Phosphorescent transitionꢀmetal complexes have attracted
enormous interest during the past decade because of their
25 potential applications in academic research and practical
application.1ꢀ3 Such phosphors can harvest both singlet and triplet
excitons due to heavy atom induced spinꢀorbit coupling effect,
thus leading to high phosphorescence quantum efficiency.4ꢀ8
Among all kinds of phosphorescent complexes, cyclometalated
30 Ir(III) complexes have emerged as the most promising candidates
as a result of their synthetic versatility, good photoꢀ and thermal
stabilities, relatively short excitedꢀstate lifetime, convenient
emission wavelength tunability as well as high efficiency.9ꢀ13 Up
to now, the reported Ir(III) complexes can be generally classified
35 into two categories, namely neutral Ir(III) complexes and cationic
Ir(III) complexes. Recently, many efforts have been devoted to
investigating the latter ones with the formula of
Ir(III)(C^N)2(N^N)+ , which are balanced by the counter anions
Previous works indicate that the imidazoleꢀtype moieties as
ancillary ligands can facilitate the construction of the efficient
cationic Ir(III) complexes with higher emission energies.37ꢀ40 In
addition, such units are easily synthesized and modified via
70 grafting the functional groups. Imidazoleꢀtype moieties are thus
ꢀ
ꢀ
such as PF6 , BF4 , Clꢀ, on account of their wide applications in
40 chemical and biological sensors, photoꢀdriven water splitting and
optoelectronic devices in particular.14ꢀ20 Thanks to the
intrinsically ionic character of cationic Ir(III) complexes, they
have been successfully used as emitting layer in solidꢀstate lightꢀ
emitting electrochemical cells (LECs) with advantages of
45 solutionꢀprocess, singleꢀlayer and airꢀstable cathodes.21ꢀ26
used as
a promising building block to synthesize Ir(III)
complexes with tunable photophysical and chargeꢀtransfer
properties. Recently, we developed a series of multifunctional
Benefiting from appealing characteristics of LECs, extensive
researches have been conducted on the design and synthesis of
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Scheme 1. Chemical structures of studied ancillary ligands and synthetic routes of complexes 1–4.
cationic Ir(III) complexes based on 1,2,4ꢀtriazoleꢀpyridine type
ancillary ligands.41ꢀ43 These complexes exhibit high
phosphorescence quantum efficiencies in the neat films, however,
the emission colors are not in region of blueꢀemitting, even 1ꢀ
Experimental
5
General procedure
(2,4ꢀdifluroꢀphenyl)ꢀ1Hꢀpyrazole (dfppz) showing the large πꢀπ* 40 Commercial reagents were used as received unless specially
energy gap is used as cyclometalated ligand. The possible reason
is that the introduced moieties on 1,2,4ꢀtriazoleꢀpyridine ancillary
10 ligands such as phenyl ring possess extended conjugation, giving
smaller energy gap and redꢀshifted emission of the resulting
stated. The solvents for syntheses were freshly distilled over
appropriate dying reagents. All experiments were performed
under a nitrogen atmosphere using standard Schlenk techniques.
1H NMR spectra were recorded on Bruker Avance 500 MHz
complexes. It is supposed that if the conjugation can be reduced, 45 spectrometer with tetramethylsilane as internal standard. UVꢀVis
the blueꢀemitting cationic Ir(III) dyes will be constructed. Herein
a simple methyl unit was chosen to modify 1,2,4ꢀtriazoleꢀpyridine
15 ancillary ligands as its weak conjugation and electronꢀdonating
ability. Keeping this in mind, a new ancillary ligand, 2ꢀ(1,5ꢀ
absorption spectra were recorded on a Cary 500 spectrometer.
Photoluminescence spectra of all complexes were obtained
through the Fꢀ4600 FL spectrophotometer. The PLQYs of the
neat film were measured in an integrating sphere. The
dimethylꢀ1Hꢀ
[1,2,4]triazoleꢀ3ꢀyl)ꢀpyridine
(Mpytz)
was 50 thermogravimetric analyses (TGA) were performed on a Perkinꢀ
synthesized, as shown in scheme 1. Using dfppz as
cyclometalated ligand, complex [Ir(dfppz)2(Mpytz)][PF6](4) is
20 readily prepared. In addition, we also employ other imidazoleꢀ
based ancillary ligands, 1,1'ꢀdiphenylꢀ1H,1'Hꢀ[2,2'] biimidazolyl
(Phbid), 2ꢀ(1ꢀphenylꢀ1Hꢀimidazolꢀ2ꢀyl)ꢀpyridine (Bpyim) and 2ꢀ
(1ꢀmethylꢀ1Hꢀimidazolꢀ2ꢀyl)ꢀpyridine (Mpyim), to explore their
changes of emission color, excitedꢀstate properties and the
25 chargeꢀtransporting abilities.
In this paper, the synthesis, characterization, photophysical and
electrochemical properties of studied cationic Ir(III) complexes
1–4, have been described in detail. All complexes show strong
phosphorescence at room temperature with the emission
30 wavelength in range from 517 to 460 nm. The photophysical
behaviors are interpreted with the help of the comprehensive
density functional theory calculations. The obtained results
further demonstrate that they show comparable chargeꢀ
transporting properties. Importantly, it shows that complex 4 with
35 Mpytz ligands are blueꢀemitting material in its neat film with
PLQY of 31.6 %, higher than reported cationic Ir(III) complexes
with similar emissions in the films.
Elmer TGꢀ7 analyzer heated from R.T. to 800 °C in flowing
nitrogen.
Synthesis
The ligands used in this work were expediently synthesized
41
55 according to the reported literatures procedure.37,
The
1
corresponding H NMR data for all ligands are presented in the
electronic supplementary information (ESI).† All complexes
were obtained by analogy reactions of reported precedent.37
Synthesis of complexes
60 Hdfppz (0.90 g, 5.00 mmol) and IrCl3·3H2O (0.70 g, 2.00 mmol)
were dissolved in a mixture of 2ꢀethoxyethanol/water (16 mL, 3:1
V/V) solution. The reaction mixture was degassed repeatedly and
refluxed for 20 h while protected from light and oxygen. The
reaction was cooled down to room temperature and 5 mL of
65 water was added. The resulted solid was filtered and washed with
25 mL water, 25 mL ethanol and 25 mL diethyl ether (Et2O), then
dried in vacuo. The crude dichloroꢀbridged diiridium complex
was used without any further purification. A suspension of ꢀꢀ
dichloro bridged iridiumꢀdimer (0.15 g, 0.10 mmol) and desired
2
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ancillary ligand (0.27 mmol) in 16 mL ethaneꢀ1,2ꢀdiol was
degassed by multiple vacuum and N2 purging cycles and stirred at
SHELXLꢀ97 program in WINGX.42 The hydrogen atoms of
aromatic rings were included in the structure factor calculation at
150 °C for 20 h, protected from light. The reaction mixture was 50 idealized positions by using a riding model. Anisotropic thermal
cooled down to room temperature and diluted with water
subsequently. An aqueous solution of NH4PF6 was added to
afford the desired precipitate. The resulted solid was filtered and
further purified via silica gel column chromatography and
recrystallized from dichloromethane and petroleum ether.
parameters were used to refine all nonꢀhydrogen atoms except for
part of nitrogen and carbon atoms. The solvent molecules are
highly disordered, and attempts to locate and refine the solvent
peaks were unsuccessful. Contributions to scattering due to these
55 solvent molecules were removed by using the SQUEEZE routine
of PLATON; structures were then refined again using the data
generated.
5
[(dfppz)2Ir(phbid)] PF6 (1)
1
10 A white solid in 65% yield. H NMR (d6ꢀDMSO, 500 MHz) δ
(ppm): 8.62 (s, 2H), 8.39 (d, J = 8.0 Hz, 1H), 8.23 (t, J = 8.0 Hz,
1H), 8.03 (d, J = 5.5 Hz, 1H), 7.70 (s, 1H), 7.59 (t, J = 2.5 Hz,
1H), 7.27 (d, J = 2.5 Hz, 1H), 7.24 (d, J = 2.5 Hz, 1H), 7.13–7.05
(m, 2H), 6.79–6.77 (m, 2H), 6.69 (s, 1H), 5.71–5.70 (m, 1H),
15 4.25 (s, 3H). MS (MALDIꢀTOF): m/z 837.2 (M–PF6).
[(dfppz)2Ir(bpyim)] PF6 (2)
Theoretical calculations
The geometrical structures of the ground and the lowest triplet
60 states (T1) for all complexes were fully optimized with C1
symmetry constraints by using the restricted closedꢀshell and
openꢀshell B3LYP methods, respectively. The “Doubleꢀξ” quality
basis sets were employed for C, H, N atoms (6ꢀ31G*) and Ir atom
(LANL2DZ). The effective core potential (ECP) replaced the
65 inner core electrons of iridium leaving the outer core (5s)2(5p)6
electrons and the (5d)6 valence electrons of iridium(III). The
expectation values calculated for S2 were smaller than 2.05 in the
spinꢀunrestricted calculation. To improve the influence of triplet
instabilities in timeꢀdependent DFT (TDꢀDFT), TammꢀDancoff
70 Approximation (TDA) in TDꢀDFT calculations was performed in
the presence of the solvent (CH3CN) at the optimized T1
geometry.45, 46All calculations reported here were carried out with
the Gaussian 09 software package.47
A slight yellow solid in 52% yield. 1H NMR (d6ꢀDMSO, 500
MHz ) δ (ppm): 8.66 (t, J = 3.3 Hz, 2H), 8.03 (t, J = 8.8 Hz, 1H),
7.98 (t, J = 4.5 Hz, 1H), 7.92 (d, J = 1.5 Hz, 1H), 7.75 (s, 5H),
20 7.53–7.50 (m, 2H), 7.40 (d, J = 2.5 Hz, 1H), 7.11–7.10 (m, 2H),
6.93–6.90 (m, 2H), 6.84–6.82 (m, 2H), 5.69–5.66 (m, 2H). MS
(MALDIꢀTOF): m/z 802.2 (M–PF6).
[(dfppz)2Ir(mpyim)] PF6 (3)
A slight yellow solid in 55% yield. H NMR (d6ꢀDMSO, 500
1
25 MHz ) δ (ppm): 8.62 (s, 2H), 8.39 (d, J = 8.0 Hz, 1H), 8.23 (t, J =
8.0 Hz, 1H), 8.03 (d, J = 5.5 Hz, 1H), 7.70 (s, 1H), 7.59 (t, J = 2.5
Hz, 1H), 7.27 (d, J = 2.5 Hz, 1H), 7.24 (d, J = 2.5 Hz, 1H), 7.13–
7.05 (m, 2H), 6.79–6.77 (m, 2H), 6.69 (s, 1H), 5.71–5.70 (m,
1H), 4.25 (s, 3H). MS (MALDIꢀTOF): m/z 740.2 (M–PF6).
30 [(dfppz)2Ir(mpytz)] PF6 (4)
Electrochemistry
75 Cyclic voltammetry (CV) was performed on a BAS 100 W
instrument with a scan rate of 100 mV sꢀ1 in acetonitrile using
tetrabutylammonium hexafluorophosphate (0.1 M) as the
supporting electrolyte and ferrocene as the internal standard. The
analytic system adopted the threeꢀelectrode configuration with a
80 glassy carbon electrode as the working electrode, an aqueous
saturated calomel electrode as the operating reference electrode
and a platinum wire as the counter electrode.
1
A white solid in 38% yield. H NMR (d6ꢀDMSO, 500 MHz ) δ
(ppm): 8.67 (t, J = 3.3 Hz, 2H), 8.37 (d, J = 8.0 Hz, 1H), 8.28 (t, J
= 7.8 Hz, 1H), 7.92–7.88 (m, 2H), 7.67 (t, J = 6.8 Hz, 1H), 7.34
(d, J = 2.5 Hz, 1H), 7.20–7.10 (m, 2H), 6.88 (t, J = 2.5 Hz, 1H),
35 6.80 (t, J = 2.8 Hz, 1H), 5.64–5.62 (m, 1H), 5.54 (t, J = 3.8 Hz,
1H), 3.25 (s, 3H), 2.52 (s, 3H). MS (MALDIꢀTOF): m/z 725.1
(M–PF6).
X-ray crystallography
Results and discussions
Single crystals suitable for Xꢀray diffraction analyses were
40 successfully gained except for complex 3 through slow vapor
diffusion from the corresponding complexes solution in a mixture
of CH2Cl2/Et2O/CH3OH (5:3:2, in volume). The singleꢀcrystal Xꢀ
monoꢀchromated Mo Kα radiation (λ = 0.71069˚) at 293 K. The
ray diffraction data for complexes 1, 2 and 4 were collected on a
45 Bruker Apex CCD II areaꢀdetector diffractometer with graphite
structures were all solved by Direct Method of SHELXSꢀ97 and
refined by fullꢀmatrix leastꢀsquare techniques using the
Synthesis of the studied complexes
85 The studied complexes were synthesized in moderate yields of
38% to 65%. The products were prepared through traditional two
steps method. The column chromatograph method were applied
to purified the crude products. The products were characterized
1
by HꢀNMR and the structures of 1, 2 and 4 were confirmed by
90 single crystal structures.
Fig. 1. ORTEP diagram of complexes 1, 2 and 4 with thermal ellipsoids shown at the 30% probability level. The relevant solvent molecules, counter
anions (PF6ꢀ) are omitted for clarity.
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X-ray single crystal structure
45 their absorption spectra, the theoretical calculations based on
optimized geometries have been undertaken. The corresponding
calculated excited energies, dominant orbital excitations, and
oscillator strength, configurations and relevant orbital
distributions of 1 have been shown in Fig. 3 and data listed in
50 Table 2. The data for 2–4 are shown in the ESI.† Taking complex
1 as an example, the simulated absorption spectrum of complex 1
is depicted in Fig. 3 (a), which is consistent well with the
experimental spectrum profile. The occupied molecular orbitals
participating in main excitations, HOMO (H), HOMOꢀ1, HOMOꢀ
The single crystal structures of complexes 1, 2 and 4 are
illustrated in Fig. 1 and the detailed data are listed in Table S1–
S2†. It is obvious that these complexes show almost octahedral
geometry with minor distortion coordinating with two
cyclometalated ligands (dfppz) and one auxiliary ligand. All
complexes examined show small bite angles, causing a slightly
distorted octahedral geometry. Taking complex 1 as an example,
two dfppz ligands exhibit a transꢀarrangement of the Nꢀdonors
10 and cisꢀarrangement of the C atoms which is unexceptionally
consistent with the reported ionic Ir(III) complex.11, 29 The Ir–N
bond lengths of cyclometalated ligands lie between 2.025(7) and
2.017(8) which are slightly longer than Ir–C distances 2.013(9)
and 2.007(9). Owing to the strong Ir–C interaction of dfppz
15 ligands, the ancillary ligand presents remarkable lengthened Ir–N
distances of 2.137(7) and 2.140(7) compared to those transꢀ
orientated Ir–N bonds of dfppz. We can find the similar
phenomena for other two complexes in Table S2†.
5
55
3
and HOMOꢀ4 are predominantly localized on the
cyclometalated ligand and iridium center, while HOMOꢀ2 locates
on ancillary ligand and iridium center. As for the lowest
unoccupied orbital (LUMO), it resides on ancillary ligand
together with contribution from iridium center in part. Energy
60 region from 250 to 300 nm including bands 1 and 2 ascribe
primarily to the excitation from Hꢀ5→L+1, Hꢀ3→L+1 and Hꢀ
5→L, respectively. Consequently, the high energy absorption
bands are mainly assigned to the π–π* transitions involving both
cyclometalated and ancillary ligands. The calculated absorption
65 bands 3 and 4 spreading from 310 to 400 nm are predominantly
originated from H→L+1, Hꢀ4→L, Hꢀ2→L, characterized by
MLCT, LLCT as well as additional IL states feature. These
related attribution results agree with the experimental ones
perfectly. Moreover, the similar assignments for the rest
70 complexes can be obtained from our TDꢀDFT results as shown in
Tables S3ꢀS5†.
Photophysical properties
20 The absorption and emission spectra of 1–4 in CH3CN solution at
room temperature were recorded and shown in Fig. 2. The
numerical results are summarized in Table 1. As depicted in Fig.
2 (a), the intensive absorption ranging from 250 to 290 nm of
them are mainly assigned to spinꢀallowed 1π–π* transition of
25 ligands. While the following weak absorption bands that occur at
lower energy region extending from 290 to 400 nm can be
1
ascribed to an association of MLCT (metal to ligand chargeꢀ
Table 2. Calculated excited energies of the excited state (ES), dominant
orbital excitations, and oscillator strength (f) of complex 1 in CH3CN
solution from TDꢀDFT calculation.
transfer), 3MLCT, 1LLCT (ligandꢀtoꢀligand charge transfer),
3LLCT, and ligandꢀcentered 3πꢀπ* (3LC) transitions.48,
49
30 According to the spectral profiles, the absorption spectra can be
categorized into three classes. Complexes 2 and 3 with same 1Hꢀ
imidazolylpyrindine fragment but various substitutes show
similar absorption patterns with different intensity. The
absorption spectra of 1 and 4 exhibit different profiles compared
35 with those of 2 and 3. The results indicate that the N^N fragments
play a more important role on electron transition processes, while
the substitutions on ancillary ligands have small effect on that.
Major
contributions
1
ES
eV/nm
f
Character
S2
5
Band 1
4.85/253
0.26
Hꢀ5→L+1 (37%)
LLCT/MLCT
Hꢀ3→L+1 (12%)
Hꢀ1→L+2 (11%)
Hꢀ5→L (79%)
Hꢀ2→L (14%)
H→L+1 (88%)
Hꢀ4→L (43%)
Hꢀ1→L (52%)
IL
IL
Band 2
Band 3
S8
4.27/290
0.24
IL/ LCT
IL/MLCT
IL/MLCT
LLCT/MLCT
LLCT
S4
S3
3.98/312
3.97/313
0.05
0.06
Ban
4
S
3.86/322
0.03
Hꢀ2→L (75%)
IL/MLCT
IL/LLCT/ML
CT
MLCT/LLCT
Hꢀ5→L (16%)
H→L (96%)
S1
3.41/364
0.01
75 H and L denote HOMO and LUMO, respectively.
Fig. 2 (b) presents the photoluminescence spectra of 1–4
detected in acetonitrile at 298 K. Through ancillary ligands
modification, complexes 1–4 possess progressively blueꢀshifted
emissions with peak values at 517, 495, 487 and 460 nm,
80 respectively, and are characterized by somewhat structured
profiles with high energy shoulder except for 1. Generally, the
observed emissions for cationic Ir(III) complexes are assigned to
Fig. 2. (a) Absorption spectra and (b) photoluminescent spectra of
40
complexes 1–4 in acetonitrile (10–5 M) at 298 K.
Table 1. Photophysical and electrochemical characteristics for 1–4.
3MLCT, LLCT and LC πꢀπ excitedꢀstates or the mixed ones,
which can be described as follows: ΦT = aΦ(3MLCT) +
85 bΦ(3LLCT) + cΦ(3LC). Thereby, the emission spectra of cationic
Ir(III) complexes commonly depend on the normalized
coefficients of a, b and c. The broad and featureless emission is
3
3
a, b
λPL, max (nm)
517, 511
495, 495
487, 489
460, 462
Φb (%)
45.2
τb(ꢁs)
5.7
Eox c (V)
1.12
Ered c (V)
ꢀ2.28
1
2
3
4
41.0
2.4
1.09
ꢀ2.21
30.8
2.1
1.05
ꢀ2.26
3
3
observed for the complexes with MLCT and LLCT characters,
whereas 3LC character usually results in the vibronically
90 structured emission spectra. Thus, we tendentiously attribute the
31.6
0.4
1.18
ꢀ2.26
Measured in CH3CN (10–5 M) at 298 K. b Measured in the neat film.
a
c
Measured by CV with ferrocene as the standard.
To better understand the nature of the excited states involved in
3
3
origin of the emission of 1 to MLCT and LLCT characters, and
4
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Fig. 3. (a) TDꢀDFT simulated and experimental absorption spectra of complex 1 in CH3CN. (b) The shape of frontier electronic levels and selected
frontier molecular orbitals involved in crucial electronic excitations of complex 1. H and L denote HOMO and LUMO, respectively.
3
that the excitedꢀstates of 2–4 could have, in addition to MLCT 45 complexes 1–3. The net result of these finding will further lead to
3
3
5
and LLCT, different degrees of LC characters. Upon cooling
the solutions down to 77 K, complex 2–4 show much more
rigidochromic blueꢀshifts with respect to their emission at room
temperatures. As for complex 1, the broad and structureless
photoluminescent spectrum is replaced by vibronical structured
the blueꢀshifted emission. Based on these experimental and
theoretical results, it can conclude that if the energy gap of
[Ir(CN)2]+ is slightly large or close to that of ancillary ligand, the
ancillary ligand can participate in the excitedꢀstate efficiently.
50 Thereby, the emission colors of such complexes can be adjusted
by the ancillary ligands. Once the energy gap of ancillary ligand
is much large than that of [Ir(CN)2]+, the [Ir(CN)2]+ꢀcontrolled
10 one accompanied by a considerable degree of blueꢀshift, which
3
further demonstrates its excited states characters of MLCT and
3LLCT. Combining the emission spectra at 298 K with those at
emission will be observed with relatively large LC excitedꢀstate
3
77 K (see Fig. S1), the results suggest that the emissions of 2–4
show a relatively large LC component than that of 1. Further 55 complexes with large πꢀπ* gap can be achieved by employing the
character. As a result, tuning the emission color of cationic Ir(III)
3
15 evidences are supported by TDꢀDFT calculations approach and
the obtained results are presented in Table S6†. Based on the
theoretical data and photophysical characterizations, it is believed
various cyclometalated ligands, which may simplify the material
designs and the synthetic task.
3
that their emissions should originate from MLCT and 3LLCT
and 3LC characters with different contributed proportion. The
20 differences in electron density between T1 and the ground state S0
for 1–4 exhibited here further confirm the transition characters
mentioned above (see Fig. 4).
To deep understand the effect of ancillary ligands on their
emission behaviors, the formation of the frontier molecular
25 orbitals of 1–4 by combining the valence orbitals of
cyclometalated ligand and iridium atom, [Ir(CN)2]+ and those of
ancillary ligands, have been studied. As shown in Fig. S5, it is
clear that the ancillary ligands of 1–3 contributed to their frontier
molecular orbitals involved in emissions. The emission
30 wavelengths of 1–3 are thus related to the energy gaps of their
ancillary ligands. According to previous reports, if the ancillary
ligand participates in the emissive excitedꢀstate, the emission
color can be controlled by modifying the energy gap of ancillary
ligand. The much larger energy gap usually leads to blueꢀshifted
35 emission. From the view of the structure point, the conjugated
phenyl rings in 1 and 2 may decrease the energy gaps of ancillary
ligands, and hence the lowꢀenergy emissions of them with respect
to that of 3. Our calculated data further support this point (see Fig.
S5). However, for complex 4, the lowest lying excited state is
40 mainly dominated by the part of [Ir(CN)2]+ due to exceedingly
large πꢀπ* gap of ancillary ligand. This may explain why 4
possesses much more 3LC component than others. It is noted that
the higherꢀenergy unoccupied orbital of [Ir(CN)2]+ (LUMO+1)
contributes the emission of 4, which is not observed for
Fig. 4. Difference electron density computed by subtracting the electron
60 densities of the T1 and S0 states for complexes 1–4. The charge goes from
the green to the red areas.
In the realꢀworld application, the phosphorescent Ir(III)
complexes are utilized as luminescent materials in their solidꢀ
state, for example, as thin films in the fabrication of the
65 optoelectronic devices. However, the strong intermolecular
interactions between the neighboring chromophores always result
in larger redꢀshifted emission and decreased PLQY, which limits
the development of the efficient blueꢀemitting materials. In
addition, the efficiency of LECs can be evaluated from PLQY in
70 the neat films of cationic Ir(III) complex in principle. To further
investigate the photophysical properties in aggregation states,
their emission spectra (see Fig. 5) and PLQYs in neat films were
recorded, showing the considerable PLQYs. Encouragingly,
complex 4 exhibits strong emission with PLQY of 30.6% and
75 slightly redꢀshifted emission within 2 nm compared to the
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C5NJ03045E
photoluminescence in solution. Additionally, complex 1 exhibits
the highest PLQY of 45.2% among all complexes, which might
be attributed to the steric hindrance of phenyl groups on ancillary
ligands and this hypothesis was supported by the decreasing
PLQY along with the decreasing steric hindrance. All complexes
exhibit microsecond timescale indicating their phosphorescent
nature. Moreover, it is noteworthy that complex 4 possesses
relative short lifetime (0.4 ꢁs), which may reduce the tripletꢀ
triplet annihilation in neat film and benefit it for the applications
complexes with high thermal stabilities than biimidazole ones.
Charge-transfer properties
45 It is wellꢀknown that the electron injection and transport
properties of the materials play an important role on the device
performance, especially singleꢀlayer devices such as LECs.57
Both ionization potentials (IP) and electron affinities (EA) are the
5
main factors in evaluating the efficiency of holes and electrons
59
50 injection.58,
Herein, the theoretical calculation has been
10 in LECs.53
performed to explore the chargeꢀtransfer properties of complexes
1–4. The calculated values of IP, EA together with hole
extraction potential (HEP) and electron extraction potential (EEP)
for them are listed in Table 3.
55 Table 3. Vertical ionization potential (IP), vertical electron affinity (EA),
extraction potential (HEP and EEP) and intramolecular reorganization
energy (λhole and λelectron).
IP (v)
9.24
9.40
9.50
9.72
HEP (v)
9.13
9.30
9.40
9.53
EA (v)
3.06
3.41
3.52
3.38
EEP(v)
3.83
3.97
3.93
3.86
λhole
0.11
0.10
0.10
0.19
λelectron
0.77
0.56
0.41
0.48
1
2
3
4
The lower IP and higher EA are beneficial to injection of holes
and electrons, respectively. As observed from Table 3, complex 1
60 exhibit smaller IP and EA value than others, indicating that 1 may
have good holesꢀinjection ability rather than electronꢀinjection
ability.
3 exhibits improved electronꢀinjection ability in
Fig. 5. Photoluminescent spectra of 1–4 in neat films.
comparison with those of complex 1, 2 and 4, due to its higher
EA value. Additionally, the charge transport rate strongly
65 depends on the reorganization energy (λ) according to the Marcus
electron transfer regime.60, 61 As reported in the previous works,
the reorganization energy is determined by the inner
reorganization energy (λi) and the external reorganization energy
(λe). Usually, λi is dominant in λ because that the value of λe is
70 very small. To study the chargeꢀtransfer rates of complexes 1–4,
their λi for the hole and electron were evaluated on basis of the
following expressions, respectively.
Electrochemical properties
Electrochemical properties of complexes 1–4 were measured
15 using cyclic voltammetry (Fig. S6†) and the redox potentials are
presented in Table 1. All complexes exhibit reversible oxidation
processes in CH3CN, which is assigned to the oxidation of
iridium metal cationic site [Ir(III)→Ir(IV)].54 The complexes
using in LECs require superior redox reversibility to transport
20 both hole and electron. Although these complexes have identical
cyclometalated ligand, their oxidation potentials reveal different
values with 1.45, 1.40, 1.36 and 1.50 V, respectively, which
demonstrates the effect of ancillary ligand on oxidation potential.
According to the reported work, the HOMO mainly distributes on
λhole = λ+ + λ0 = [E+(M0) – E+(M+)] + [E0(M+) – E0(M0)] = IP(v) – HEP
λelectron = λꢀ + λ0 = [Eꢀ(M0) – Eꢀ(Mꢀ)] + [E0(Mꢀ) – E0(M0)] = EEP – EA(v)
75
Clearly, they show almost identical hole reorganization
energies (λhole), suggesting that the variation of ancillary ligand
has little different on their λhole but it significantly changes the
electron and hole injection abilities. The electron reorganization
energies (λelectron) of complexes 2–4 are obviously lower than that
25 πC^N and d orbitals of Ir center while the LUMO is on ancillary
56
ligand.55,
The reduction processes are reversible for all
complexes except for 1. The reduction irreversibility of complex
1 might be attributed to ancillary ligand Phbid and the similar
phenomena were observed in previous literature.37 As the
30 conjugation length decrease from complex 2 to 3, the reduction
potentials are shifted cathodically (by 170 mV), which indicates
that the LUMO of complex 2 is tremendously stabilized by the
phenyl moiety.
80 of complex 1. This result suggests that 2–4 possess better
electronꢀtransporting abilities with respect to 1.
Conclusion
Thermal properties
A series of [Ir(dfppz)2(N^N)][PF6] complexes chelated with
dfppz and different ancillary ligands have been successfully
85 prepared and fully characterized by spectroscopic and
electrochemical methods. The single crystal structures of
complexes 1, 2 and 4 have been elucidated. The use of different
ancillary ligands successfully tunes the emission of studied
complexes from green to blue. DFT calculations have been
90 performed to rationalize the experimental spectroscopy and
deeply understand their intrinsic chargeꢀtransfer properties.
Different from complexes 1–3 with imidazoleꢀtype ligands, the
35 The thermal stabilities of complexes 1–4 were evaluated by TGA
under a steam of nitrogen with a scanning rate of 10 °C minꢀ1.
The obtained results suggest that complexes 2–4 exhibit good
thermal stabilities with high decomposition temperatures (Td,
corresponding to 5% weight loss) of 331, 344 and 357 °C,
40 respectively, while complex 1 shows a relatively poor thermal
properties with Td of 230 °C. Generally, triazoleꢀbased ancillary
ligand are proved to be better candidates to construct Ir(III)
6
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DOI: 10.1039/C5NJ03045E
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65
3
relatively large LC feature. Despite the presence of significant
5
3LC character, the high efficiency in neat film of 4 is realized,
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Acknowledgments
The authors gratefully acknowledge the financial support from
NSFC (21303012, 21273030, 51203017 and 21131001), the
15 Science and Technology Development Planning of Jilin Province
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Notes and references
Institute of Functional Material Chemistry, faculty of chemistry,
Northeast Normal University, Changchun, 130024 Jilin, People’s
20 Republic
of
China.
Eꢁmail:
shangg187@nenu.edu.cn;
mzhang@nenu.edu.cn, zmsu@nenu.edu.cn
†Electronic Supplementary Information (ESI) available: [Synthesis and
characterization of the ligands used in this work, Xꢀray single crystal data,
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DOI: 10.1039/C5NJ03045E
Highly efficient blue emitting cationic Ir(III) complex based on
1,2,4-triazole-pyridine ligand modified by simple methyl moiety was reported.