Efficient near-infrared-emitting cationic iridium complexes based on highly conjugated cyclometalated benzo[g]phthalazine derivatives

Article abstract of DOI:10.1039/c5ra04511h

Two near-infrared- (NIR-) emitting cationic iridium(iii) complexes, [Ir(dpbpa)₂(Bphen)]⁺PF₆^- (1) and [Ir(dtbpa)₂(Bphen)]⁺PF₆^- (2), were rationally designed and synthes

Full text of DOI:10.1039/c5ra04511h

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Efficient near-infrared-emitting cationic iridium
complexes based on highly conjugated
Cite this: RSC Adv., 2015, 5, 42354
cyclometalated benzo[g]phthalazine derivatives†
b
a
b
Lijun Xin,ab Jie Xue, Gangtie Lei and Juan Qiao
*
*
Two near-infrared- (NIR-) emitting cationic iridium(III) complexes, [Ir(dpbpa)2(Bphen)]+PF6 (1) and
ꢀ
[Ir(dtbpa)2(Bphen)]+PF6 (2), were rationally designed and synthesized, where dpbpa, dtbpa, and Bphen
ꢀ
represent 1,4-diphenylbenzo[g]-phthalazine, 1,4-di(thiophen-2-yl)benzo[g]phthalazine and 4,7-dipheny-
1,10-phenanthroline, respectively. By using highly conjugated cyclometalated benzo[g]phthalazine
ligands, these two complexes exhibited a significantly large red shift in wavelength to the truly NIR
region with maximum peaks at 715 nm for 1 and 775 nm for 2. Complex 1 exhibited unexpectedly
improved quantum efficiency up to 6.1% in the solid films. Based on these solution-processable
phosphors, NIR organic-light-emitting devices (OLEDs) have been fabricated and demonstrated
negligible efficiency roll-off with nearly constant external quantum efficiency around 0.5% over a wide
range of current density of 1–100 mA cmꢀ2. Density functional theory calculations were performed to
discover that the newly cyclometalated benzo[g]phthalazine ligands have several areas of superiority
over the previous benzo[g]quinoline ligands in views of stronger Ir–N bonds, smaller chelate congestion,
higher electron-accepting ability, thus improving the overall phosphorescence of the corresponding
iridium complexes in the NIR region.
Received 14th March 2015
Accepted 5th May 2015
DOI: 10.1039/c5ra04511h
www.rsc.org/advances
NIR-OLEDs with the record maximum external quantum effi-
ciencies (EQEs) up to ꢁ14.5% at ꢁ700 nm,13 and ꢁ9% in the
750–800 nm range12 and 4% in the 850–900 nm range.11
Introduction
In recent years, near-infrared- (NIR-) emitting materials have
aroused rapidly growing attention because of their emerging
applications in night-vision and/or information-secured
displays, bio-imaging and diagnosis, chemosensors, optical
communication, and phototherapy, etc.1–6 To date, several kinds
of pure organic compounds and metal-complexes have been
reported as organic NIR emitting materials, such as organic
small-molecules,1,2 lanthanide complexes,5,6 conjugated poly-
mers,7–9 metalloporphyrins10–12 and other transition-metal
complexes.3,4 Compared with traditional uorescent materials,
phosphorescent transition-metal complexes offer several
important advantages as NIR dyes.3,4 For NIR organic-light-
emitting devices (OLEDs), transition-metal complexes can
harvest both singlet and triplet excitons as light and theoretically
achieve nearly 100% internal quantum efficiency. Most notably,
platinum metal complexes have demonstrated the most efficient
However, those NIR-OLEDs based on platinum metal-
loporphyrins generally suffer from severe efficiency roll-off with
increasing current density, which could be mainly ascribed to
the easy aggregation of the planar-square conguration and the
intrinsic long phosphorescence lifetime of the complexes.11–13 In
comparison, the other transition-metal complexes with octahe-
dral congurations (e.g., containing Ir3+, Os3+, Re3+) would help
to alleviate the efficiency roll-off in the corresponding OLEDs,
although with relatively low efficiencies in NIR region.14–20 Since
2009, we have designed and synthesized a series of NIR-emitting
iridium complexes based on benzo[g]quinoline and benzo[g]-
quinoxaline derivatives, which exhibited desirable emission
covering 690–1000 nm and reasonable quantum efficiency
around 3% in solutions.19–22 Of particular concern is that the
NIR-OLEDs based on those Ir(III) complexes demonstrated
negligible efficiency roll-off at high current density.19–21
However, it is still quite challenging to design high-efficiency
NIR emitting phosphors, since their photoluminescence
quantum yields (PLQY) tend to decrease with an increase in the
emission wavelength according to the energy gap law. In this
work, we continue our efforts in developing highly efficient NIR
emitting Ir(III) phosphors. To loosen the steric congestion from
the previous benzo[g]quinoline ligand and further improve the
overall phosphorescence of iridium complexes in NIR region, we
aKey Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of
Education, College of Chemistry, Xiangtan University, Hunan 411105, China. E-mail:
lgt@xtu.edu.cn
bKey Lab of Organic Optoelectronics and Molecular Engineering of Ministry of
Education, Department of Chemistry, Tsinghua University, Beijing 100084, China.
E-mail: qjuan@mail.tsinghua.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra04511h
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supporting electrolyte was tetra-n-butylammonium perchlorate
(0.1 M), and ferrocene was selected as the internal standard.
Materials and synthesis
All reagents and starting materials are commercially available
and were used without further purication. All procedures
involving IrCl3$xH2O were carried out under a nitrogen
atmosphere.
Synthesis of 1,4-diphenylbenzo[g]phthalazine (dpbpa).
dpbpa was prepared according to the literature methods.23,24
Briey, t-BuOK was slowly added into the hot ethanolic solution
of 1,4-diphenylbutane-1,4-dione (8 mmol, 1.9 g) and phthala-
dehyde (8 mmol, 1.0 g) and the solution was stirred for 8 h at
room temperature. Upon cooling, the mixture was ltered and
the residue was washed with methanol to afford naphthalene-
2,3-diylbis(phenylmethanone) (2.4 g). Then, a hot methanolic
solution of naphthalene-2,3-diylbis(phenylmethanone) was
dropped in excess hydrazine hydrate and the mixture was
reuxed at 70 ꢂC for 1 hour. Aer being cooled to room
temperature, the mixture was ltered and the residue was
washed with methanol to give bright yellow solid as the ligand
Scheme 1 The synthetic route of the cyclometalated ligand dpbpa,
dtbpa and the molecular structures of the iridium(III) complexes.
designed two novel cyclometalated benzo[g]phthalazine deriva-
tives, 1,4-diphenylbenzo[g]-phthalazine (dpbpa) and 1,4-di(thio-
phen-2-yl)benzo[g]phthalazine (dtbpa) and synthesized the
corresponding iridium complexes with the diimine ancillary
ꢀ
ligand (Scheme 1): [Ir(dpbpa)2(Bphen)]+PF6 (1) and [Ir(dtbpa)2
ꢀ
(Bphen)]+PF6 (2) (Bphen ¼ 4,7-diphenyl-1,10-phenanthroline).
Herein, we systematically studied their molecular and electronic
structures, photophysical and electrochemical properties, and the
NIR-OLEDs performances, particularly in comparison with the
previous benzo[g]quinoline based complexes.22 The density-
functional theory calculations were also performed to gain a
better understanding the relationship between the structure of
cyclometalated ligand and NIR emitting properties of these
materials. It was found that the newly cyclometalated benzo[g]-
phthalazine ligands have several superiority over the previous
benzo[g]quinoline ligands in views of easy synthesis, stronger Ir–
N bonds, smaller chelate congestion and higher electron-
accepting ability, thus providing the corresponding complexes
with truly NIR emission up to 788 nm and notable PLQY up to
6.1% due to the desirable MLCT (metal-to-ligand charge transfer)
mixed with ILCT (intraligand charge transfer).
1
dpbpa (2.4 g, yield 90%). H NMR (400 MHz, CDCl3): d 8.71 (s,
2H), 8.07 (dd, J ¼ 6.4, 3.0 Hz, 2H), 7.98–7.83 (m, 4H), 7.72–7.55
(m, 8H). ESI-MS (m/z): 333 [M + H]+. HRMS (ESI+): calcd for
C
24H16N2: 333.1386 [M + H]+, found 333.1381 [M + H]+. Anal.
calcd (%) for C24H16N2: C, 86.72; H, 4.85; N, 8.43. Found: C,
86.21; H, 4.48; N, 8.68%.
Synthesis
of
1,4-di(thiophen-2-yl)benzo[g]phthalazine
(dtbpa). The synthesis of dtbpa was similar to that of dpbpa
except that 1,4-diphenylbutane-1,4-dione was replaced with 1,4-
di(thiophen-2-yl)butane-1,4-dione. The target ligand dtbpa was
received as yellow solid (3 g, yield 90%). 1H NMR (600 MHz,
DMSO-D6) d 9.29 (s, 2H), 8.46 (dd, J ¼ 6.4, 3.3 Hz, 2H), 8.17–8.10
(m, 2H), 8.00–7.90 (m, 2H), 7.82 (dd, J ¼ 6.5, 3.2 Hz, 2H), 7.43
(dd, J ¼ 5.0, 3.7 Hz, 2H). ESI-MS (m/z): 345 [M + H]+. HRMS
(ESI+): calcd for C20H12N2S2: 345.0515 [M + H]+, found: 345.0514
[M + H]+. mp: 258–260 ꢂC. Anal. calcd (%) for C20H12N2S2: C,
69.74; H, 3.51; N, 8.13. Found: C, 69.37; H, 3.45; N, 7.78%.
Synthesis of complexes 1 and 2. These two complexes were
Experimental section
General information and characterization
1H NMR spectra were collected using JEOL JNM-ECA400NMR
(400 MHz) NMR spectrometers with tetramethylsilane as the prepared in a conventional two-step procedure according to our
internal standard. Mass spectra were recorded on a Thermo- previous work.20
Electron Corporation Finnigan LTQ mass spectrometer
[Ir(dpbpa)2(Bphen)]+PF6ꢀ (1): black solid. (0.81 g yield 40%).
(ESI-MS) and LCMS-IT/TOF (HRMS). Elemental analysis was 1H NMR (600 MHz, CDCl3): d 9.60 (d, J ¼ 10.4 Hz, 2H), 8.75 (s,
performed on an Elementar Vario EL CHN elemental analyzer. 2H), 8.60 (d, J ¼ 7.9 Hz, 2H), 8.48 (d, J ¼ 5.2 Hz, 2H), 8.29 (d, J ¼
Steady-state absorption and photoluminescence (PL) spectra 8.4 Hz, 2H), 8.09 (d, J ¼ 8.5 Hz, 2H), 8.01 (s, 2H), 7.81–7.78 (m,
were carried out on an Agilent 8453 UV-vis spectrophotometer 2H), 7.75–7.71 (m, 2H), 7.67 (d, J ¼ 5.2 Hz, 2H), 7.60–7.55 (m,
and an Edinburgh FLSP920 spectrouorometer, respectively. 7H), 7.49 (t, J ¼ 7.5 Hz, 3H), 7.45 (d, J ¼ 7.6 Hz, 4H), 7.35 (dt, J ¼
The photoluminescence quantum yield (PLQY) was measured 29.2, 7.5 Hz, 8H), 7.09 (t, J ¼ 7.4 Hz, 2H), 6.76 (d, J ¼ 7.5 Hz, 2H).
in degassed CH2Cl2 solutions and 5 wt% doped poly(methyl ESI-MS (m/z): 1187 [M ꢀ PF6]+. HRMS (ESI+): calcd for
methacrylate) (PMMA) lms with an absolute PL quantum yield IrC72H46N6+: 1187.3420 [M ꢀ PF6]+, found 1187.3416 [M ꢀ PF6]+.
spectrometer (C9920-02, Hamamatsu photonics k. k.). Cyclic Anal. calcd (%) for IrC72H46N6PF6: C, 64.91; H, 3.48; N, 6.31.
voltammetry measurements were performed on a Princeton Found: C, 63.36; H, 3.20; N, 5.70%.
Applied Research potentiostat/galvanostat model 283 voltam-
[Ir(dtbpa)2(Bphen)]+PF6ꢀ (2): black solid. (1.47 g, yield 50%).
metric analyzer in anhydrous acetonitrile solutions (10ꢀ3 M) at 1H NMR (600 MHz, CD2Cl2): d 9.44 (d, J ¼ 19.4 Hz, 2), 9.10 (d, J ¼
a scan rate of 150 mV sꢀ1 at room temperature. The solutions 14.3 Hz, 2H), 8.52–8.44 (m, 2H), 8.34 (d, J ¼ 8.4 Hz, 2H), 8.20 (d,
were bubbled with argon for 15 min before measurements. The J ¼ 8.4 Hz, 2H), 8.11 (d, J ¼ 7.6 Hz, 2H), 7.84–7.80 (m, 4H), 7.79–
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7.75 (m, 4H), 7.70 (t, J ¼ 5.1 Hz, 2H), 7.64–7.60 (m, 6H), 7.58– orange to red emitting iridium complexes.4,28–33 For iridium
7.54 (m, 4H), 7.40–7.35 (m, 2H), 7.23–7.16 (m, 2H), 6.85–6.80 complexes, expanding p-conjugation length in the cyclo-
(m, 2H). ESI-MS (m/z): 1211 [M ꢀ PF6]+. HRMS (ESI+): calcd for metalated C^N ligands has been demonstrated as one of the
+
C
64H38IrN6S4 : 1211.1669 [M ꢀ PF6]+, found 1211.1174 [M ꢀ most effective means to reduce the emission energy into NIR
PF6]+. Anal. calcd (%) for IrC64H38N6S4PF6: C, 56.67; H, 2.82; N, region.3,4 However, it is intrinsically quite challenging to design
6.20. Found: C, 56.51; H, 2.60; N, 6.12%.
high-efficiency NIR emitting phosphors in accordance with the
energy gap law. In our previous work, we designed cyclo-
metalated benzo[g]quinoline and benzo[g]quinoxaline ligands
with fused aromatic rings and realized efficient NIR-emitting
iridium complexes.19–22 To loosen the steric congestion from
those bulky ligands and further improve the PLQY of the
complexes in NIR region, we designed two novel cyclometalated
ligands based on benzo[g]phthalazine and synthesized the
corresponding iridium complexes with the diimine ancillary
ligand. Scheme 1 depicted the synthesis of the benzo[g]phtha-
lazine ligands and the molecular structures of the correspond-
ing complexes 1 and 2. The ligand dpbpa and dtbpa were readily
obtained by two steps with a high yield of 90% according to the
literature methods.23,24 The iridium complexes were obtained
via treatment of the corresponding chloride-bridged dimer
precursor with Bphen as the ancillary ligand, followed by a
counterion exchange reaction from Clꢀ to PF6ꢀ. The NMR
spectra of the chloride-bridged dimer and the nal complexes
are both consistent with the cyclometalated ligands being in a
C–C cis, N–N trans conguration.
Quantum chemical calculation
All calculations were carried out with Gaussian 03 soware
package using a spin-restricted formalism.25 The ground and
excited electronic states of the complexes were calculated using
density functional theory (DFT) and time-dependent DFT
(TD-DFT) at the B3LYP level.26,27 The standard valence double-x
polarized basis sets were used for C, H and N (6-31G*) and Ir
(LANL2DZ). An effective core potential (ECP) replaces the inner
core electrons of Ir, leaving the outer core [(5s)2(5p)6] electrons
and the (5d)6 valence electrons of Ir(III). The geometries of the
singlet ground state (S0) were fully optimized without symmetry
constraints at the N–N trans type, which was reported as the
most stable structural isomer for iridium(III) complexes with the
diimine ligand. The electron density diagrams of molecular
orbitals were obtained with the ChemBioOffice 2010 soware.
Fabrication and characterization of OLEDs
Before device fabrication, the prepatterned indium tin oxide
(ITO) glass substrates were carefully cleaned and treated by an
Photophysical properties
UV–ozone for
6
min. Then 40–50 nm poly(3,4-
ethylenedioxythiophene) (PEDOT:PSS, Bayer Baytron P8000)
used as a hole-injecting layer was spin-coated on the ITO
substrate and baked at 200 ꢂC for 10 min in a nitrogen glovebox.
The emitting layer was spin-coated onto the PEDOT:PSS-coated
substrate from 1,2-dichloroethane solutions and then annealed
Photophysical properties of complexes 1 and 2 were studied in
degassed CH2Cl2 solutions and thin lms. Fig. 1 depicts both
UV-vis absorption and PL spectra of these two complexes in
degassed CH2Cl2 solutions at room temperature. These two
complexes exhibit similar absorption peaks from 300 to 600 nm.
The intense short-wavelength absorption bands below 350 nm
could be ascribed to spin-allowed intraligand p / p* transi-
tions, while the relatively long-wavelength absorption above 350
nm could be assigned to mixed transitions of spin-allowed
1MLCT and spin-forbidden 3MLCT (singlet and triplet metal-
to-ligand charge transfer). Because of thiophen-2-yl instead of
phenyl in the cyclometalated ligands, those charge transfer
transitions of complex 2 give a little bit red-shied absorption
bands around 454, 535 nm versus complex 1. As shown in Fig. 1,
the whole PL spectra of these two complexes fall in the NIR
region, with a signicantly red-shi around 100 nm compared
with the red-emitting iridium complexes (625 nm) with
substituted cyclometalated phenylphthalazine ligand.33
Complex 1 exhibits a clearly resolved emission proles with two
strong emission peaks at 715 nm and 788 nm. In comparison,
complex 2 further gives over 60 nm red-shied emission peaks
at 775 nm due to the thiophen-2-yl instead of phenyl in the
cyclometalated ligands.
ꢂ
at 80 C for 30 min to remove the solvent residue. Aerwards,
the substrates were transferred into an evaporation chamber
integrated with the glovebox for the deposition of electron-
transporting layer and metal cathode. 2,20,200-(1,3,5-Benzene-
triyl)tris-(1-phenyl-1H-benzimidazole) (TPBI) was evaꢀp1orated as
˚
the electron-transporting layer at a rate of 0.5–1 A s under a
pressure of 4 ꢃ 10ꢀ4 Pa. Finally, the Cs2CO3/Al bilayer cathode
ꢀ1
was evaporated at rates of 0.2 A s and 5–10 A sꢀ1, respectively,
under a base pressure of 4 ꢃ 10ꢀ4 Pa. The current density–
voltage characteristics were measured with a Keithley 4200
semiconductor characterization system, and optical power was
recorded on a Newport 1936-C power meter coupled to a cali-
brated Newport 918D-UV-OD3 detector with a spectral response
range from 200 to 1100 nm. The EQE (hext) was derived on the
basis of the generally-adopted methods.20,21 Electro-
phosphorescent spectra were collected with a Jobin Yvon
FluoroMax-3 uorospectrophotometer. All measurements were
carried out in air at room temperature with encapsulation.
˚
˚
The detailed photophysical properties of these two
complexes were summarized in Table 1. In degassed CH2Cl2
solutions, complex 1 exhibits modest PLQY of 3.1% and a
relatively long emission lifetime of 0.79 ms. We deduced the
Results and discussion
Design, synthesis and characterization
Substituted quinoline, isoquinoline and phthalazine derivatives radiative decay rate (kr) of complex 1 to be 3.9 ꢃ 104 sꢀ1, which
are well-known cyclometalated ligands to obtain highly efficient is comparable to that of our previously reported NIR-emitting
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reversible oxidation process arose at half-wave potentials of
1.02 V for 1 and 0.99 V for 2, respectively. From the onset of
oxidation/reduction potentials, the HOMO, LUMO, and HOMO–
LUMO gap levels of these two complexes were obtained. The
HOMO levels of 1 and 2 are ꢀ5.82 and ꢀ5.79 eV, respectively,
indicating that the thiophen-2-yl replacing phenyl in the
cyclometalated ligand could destabilize the HOMO level of
complex 2. That is because the HOMO orbitals of the cyclo-
metalated iridium complexes generally reside on the iridium
center and the coordinated phenyl moiety, thus the replace-
ment of the phenyl ring by the electron-rich thiophen-2-yl
moiety could help destabilize HOMO level. However, the
LUMO levels of these two complexes are very close (ꢀ3.40 vs.
ꢀ3.41 eV). Thus complex 2 have a little smaller energy gap (2.38
eV) than 1 (2.42 eV) due to the electronegativity of dtbpa is
stronger than dpbpa. These results were further supported by
the DFT and TDDFT calculations (see below).
Fig. 1 The room-temperature UV-vis absorption (left) and photo-
luminescence spectra (right, corrected to 850 nm corresponding to
the instrument limit) of complexes 1 and 2 in degassed CH2Cl2 solu-
tions (10ꢀ5 M). The excitation wavelength was 440 nm.
Theoretical calculations
To gain further insight into the above photophysical and elec-
trochemical properties of these two complexes, DFT and TD-
DFT calculations were used to assess the frontier orbitals and
electronic structures of these two complexes, particularly
compared with the analogous 1,4-diphenylphthalazine based
iridium complexes based on cyclometalated benzo[g]quinoline
so the kr is estimated to be 8.78 ꢃ 104 sꢀ1. When doped in
PMMA lms (5 wt%), both complexes exhibit similar emission
prole to those in solutions, but a little blue-shied (ꢁ10 nm)
emission peaks and markedly improved PLQYs due to an
increased rigidity in solid lms and the polarity difference of
the solvent/medium (Fig. S1 in the ESI†). The PLQYs of the
PMMA doped lms are signicantly increased compared with
those in solutions. In particular, the PLQY of complex 1 reaches
6.1% in the 5 wt% doped PMMA lm, nearly twice than that in
solution. It could be explained by the efficient suppression of
non-radiative decay pathways in the rigid polymer matrix.
ꢀ
complex [Ir(dppa)2(Bphen)]+PF6 ꢀand benzo[g]quinoline based
complex [Ir(pbq-g)2(Bphen)]+PF6 (3).22 As shown in Fig. 3, in
comparison with [Ir(dppa)2(Bphen)]+PF6ꢀ, complex 1 and 2
show signicantly destabilized HOMO orbitals (by 0.10 and
0.17 eV) and stabilized LUMO orbitals (by 0.06 and 0.14 eV),
thus giving expected narrowed energy gaps for NIR-emission.
That result suggests that the phenyl ring fused at g site in
benzo[g]phthalazine ligands does effectively extend the p
conjugated system of the parent phthalazine and the electron-
rich thiophen-2-yl moieties could further help narrow the
Electrochemical properties
The electrochemical properties of these two complexes were energy gap, which are in good agreement with the above pho-
investigated by cyclic voltammetry in anhydrous CH3CN solu- tophysical and electrochemical results.
tion. As shown in Fig. 2, both complexes are electrochemically
Meanwhile, complexes 1 and 2 have nearly equal Ir–C and
active and undergo similar electrochemical characteristics with Ir–N bond lengths to that of [Ir(dppa)2(Bphen)]+PF6ꢀ, indicating
one reversible oxidation process and two reversible reduction the benzo[g]phthalazine ligands does not bring extra steric
processes in CH3CN solution. Two reversible reduction hindrance as benzo[g]quinoline ligand in complex 3. The
˚
processes occurred at half-wave potentials of ꢀ1.40 V and calculated Ir–C bond lengths of complex 1 and 2 (2.01 A and 2.00
˚
˚
ꢀ1.55 V and for 1, ꢀ1.39 V and ꢀ1.55 V for 2, while the A, respectively) are almost same to those of complex 3 (2.00 A)
Table 1 Photophysical and electrochemical characteristics of complexes 1 and 2 (compared with [Ir(pbq-g)2(Bphen)]+PF6 3)
ꢀ
Room temperature emissionb
Absorptiona
Electrochemical datac
CH2Cl2 solution
PMMA solid
l [nm] (3 [ꢃ104 Mꢀ1 cmꢀ1])
l (nm) FPL (%) s (ms) l (nm)
FPL (%) Eox1/2 (V) Ered1/2 (V)
Eg (eV)
d
1
2
350 (3.6), 419 (1.8), 448 (1.5), 516 (0.6) 715, 788 3.1
321 (4.2), 421 (1.4), 454 (1.3), 535 (0.9) 775 2.9
0.79
0.33
1.86
708, 778 6.1
760 4.1
700, 764 4.0
1.02
0.99
0.99
ꢀ1.40, ꢀ1.55 2.42
ꢀ1.39, ꢀ1.55 2.38
3 (ref. 22) 309 (7.7), 383 (1.9), 436 (1.1), 493 (0.5) 698, 760 3.5
ꢀ1.60, ꢀ1.74 2.60
a
In CH2Cl2 solutions (10ꢀ5 M). 3 denotes the molar extinction coefficients. b In degassed CH2Cl2 (1 ꢃ 10ꢀ5 M) solutions. FPL was measured using an
absolute PL quantum yield measurement system. c Collected in CH3CN solutions (10ꢀ3 M). The data were versus Fc+/Fc (Fc is ferrocene). d Eg ¼ Eox
1/2
1/2
ꢀ Ered
.
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phosphorescence. Moreover, the dihedral angle between cyclo-
metalated ligands and diimine ancillary ligand of complex 1 and
2 (89ꢂ and 89ꢂ, respectively) is about 10ꢂ greater than that of
complex 3 (79ꢂ), which help loosen the steric hindrance toward
the N chelating to the metal, thus improving the molecular
stability and the whole phosphorescence of the complexes.33
Complexes 1 and 2 show similar HOMO and LUMO cong-
urations. The HOMO orbital resides on the iridium center and
the coordinated phenyl or thiophen-2-yl moiety of the cyclo-
metalated ligands, and the LUMO orbital dominates the benzo
[g]phthalazine groups of the cyclometalated ligands. The
calculated HOMO and LUMO energy levels are in accordance
with the experimentally values from the CV measurements. To
note, the ancillary ligand of Bphen has little contribution to the
rst two LUMOs of these two complexes. In contrast, for the
cationic iridium complexes based on parent phthalazine and
benzo[g]quinoline, their LUMOs are dominantly located on the
ancillary ligand of Bphen. That indicates that the cyclo-
metalated benzo[g]phthalazine ligand has better electron-
accepting capability than the benzo[g]quinoline ligand and
Bphen, which in good agreement with the relatively lowered the
reduction potential (ꢁ0.2 eV) of the corresponding complexes.22
Table 2 lists the vertical excitation energies and molecular
orbitals involved in the excitations for the rst two triplets. All
Fig. 2 Cyclic voltammograms of complexes 1 and 2 in anhydrous
CH3CN solutions (10ꢀ3 M) at a scan rate of 150 mV sꢀ1
.
and [Ir(dppa)2(Bphen)]+PF6ꢀ. However, the calculated Ir–N bond
˚
˚
lengths (2.06 A and 2.07 A, respectively) are clearly shorter than
˚
that of complex 3 (2.14 A), indicating a stronger Ir–N bond in the
complexes based on cyclometalated benzo[g]phthalazine deriv-
atives. Such stronger Ir–N bonding results in stronger bonding
between metal and ligand, which might lead to efficient mixing
of the singlet and triplet excited states and be benecial for
Fig. 3 The molecular sꢀtructures, calculated bonding lengths, and the energy levels and isocontour plots of the frontier orbitals of complexes
[Ir(dpbpa)2(Bphen)]+PF6 (1) and [Ir(dtbpa)2(Bphen)]+PF6 (2) compared with [Ir(dppa)2(Bphen)]+PF6 and [Ir(pbq-g)2(Bphen)]+PF6 (3).22 All the
molecular surfaces correspond to an isocontour value of |J| ¼ 0.03.
ꢀ
ꢀ
ꢀ
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the complexes exhibit degenerate T1 and T2 states in energy. (PVK): 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD,
[Ir(dppa)2(Bphen)]+PF6ꢀ based on the parent phthalazine ligand 30 wt%): 1 or 2 (10–20 wt%) (90 nm)/TPBI (30 nm)/Cs2CO3
has the highest triplet energies about 2.19 eV and correspond to (2.3 nm)/Al (150 nm). The PVK–PBD blend was used as the host
the transitions at 565 and 563 nm. In comparison, complexes 1 material because of its desirable bipolar character and good lm-
and 2 show signicantly reduced triplet energies (by 0.44 and forming capability through solution process. TPBI was used as
0.58 eV), which correspond to a very large red shi around 140 the electron-transporting and exciton-blocking layer. The doping
nm due the highly conjugated cyclometalated benzo[g]phtha- concentration of complexes 1 and 2 varied from 10 to 20 wt% to
lazine ligands. Their electron transitions correspond to 705 nm optimize the device performance. As shown in Fig. 4a, the emis-
for 1 and 768 nm for 2, which are very close to the experimen- sion from the host around 415 nm can be nearly quenched when
tally observed emission peaks. All these electron transitions the doping concentration increased to 20 wt% and the intense
have a multi-congurational characteristic and dominantly NIR emission was detected at 715 nm, 788 nm for the devices with
originate from triplet metal-to-ligand charge transfer (3MLCT, complex 1 and 791 nm for the devices with complex 2. Interest-
dp(Ir) / p*C^N) and intraligand charge transfer (3ILCT, pC^N
/
ingly, for the devices with complex 1, the emission band at the
p*C^N) as well as that of [Ir(dppa)2(Bphen)]+PF6ꢀ. In contrast, long wavelength of 788 nm exhibited much stronger intensity
the analogous iridium complexes (3) based benzo[g]quinoline than that at short wavelength of 715 nm, which is very different
still have considerable contribution from 3MLCT (dp(Ir) / from the PL spectra of complex 1.
p*N^N) and triplet ligand-ligand-charge-transfer (3LLCT, pC^N
Furthermore, the emission intensity at long wavelength
/ p*N^N), thus in most cases rendering relatively lower PLQY increases as the doping concentration increases. That might be
than the mixed MLCT and ILCT emitting materials. That mixing ascribed to the optical effects in devices or the conned radia-
of 3MLCT and 3ILCT in complexes 1 and 2 might play an tion eld by the metal cathode.35 For the devices with complex 2,
important role for nice PLQY because this intersystem crossing there is a small peak at 580 nm at 10 wt% doping concentration,
can be enhanced for the systems involved notable MLCT which could be tentatively attributed to the emission from the
participation.34
possible interface exciplex between PVK and TPBI. That
suggests complex 2 with electron-rich thiophen-2-yl moieties
might contribute to hole transport and help move the recom-
bination zone approaching the interface between host and
TPBI. The underlying mechanism needs further investigation.
When the doping concentration increases to 20 wt%, the visible
Electrophosphorescent devices
The NIR-emitting OLEDs based on complexes 1 and 2 were
fabricated by spin-coating and vacuum thermal evaporation, with
the conguration of ITO/PEDOT:PSS (40 nm)/poly(vinylcarbazole)
ꢀ
Table 2 The first two triplet states for complexes 1 and 2 (compared with [Ir(dppa)2(Bphen)]+PF6 and [Ir(pbq-g)2(Bphen)]+PF6 3) calculated
using the TDDFT approach
Complexes
States (E (eV), la (nm))
T1 (2.194, 565)
Dominant excitationsb
Character
[Ir(dppa)2(Bphen)]+PF6
H / L+2 (0.54)
p(Bppa)/dp(Ir) / p*(Bppa)
p(Bppa)/dp(Ir) / p*(Bppa)
p(Bppa)/dp(Ir) / p*(Bppa)
p(Bppa)/dp(Ir) / p*(Bppa)
p(Bppa)/dp(Ir) / p*(Bppa)
p(Bppa)/dp(Ir) / p*(Bppa)
p(dpbpa)/dp(Ir) / p*(dpbpa)
p(dpbpa)/dp(Ir) / p*(dpbpa)
p(dpbpa)/dp(Ir) / p*(dpbpa)
p(dpbpa)/dp(Ir) / p*(dpbpa)
p(dpbpa)/dp(Ir) / p*(dpbpa)
p(dpbpa)/dp(Ir) / p*(dpbpa)
p(dtbpa)/dp(Ir) / p*(dtbpa)
p(dtbpa)/dp(Ir) / p*(dtbpa)
p(dtbpa)/dp(Ir) / p*(dtbpa)
p(dtbpa)/dp(Ir) / p*(dtbpa)
p(dtbpa)/dp(Ir) / p*(dtbpa)
p(dtbpa)/dp(Ir) / p*(dtbpa)
p(pbq-g)/dp(Ir) / p*(pbq-g)
p(pbq-g)/dp(Ir) / p*(pbq-g)/p*(Bphen)
p(pbq-g)/dp(Ir) / p*(pbq-g)
p(pbq-g)/dp(Ir) / p*(pbq-g)
p(pbq-g)/dp(Ir) / p*(pbq-g)/p*(Bphen)
p(pbq-g)/dp(Ir) / p*(pbq-g)
ꢀ
Hꢀ1 / L+3 (0.27)
Hꢀ2 / L+2 (0.11)
H / L+3 (0.47)
Hꢀ1 / L+2 (0.34)
Hꢀ2 / L+3 (0.12)
Hꢀ1 / L (0.50)
Hꢀ2 / L+1 (0.27)
H / L+1 (0.22)
T2 (2.199, 563)
T1 (1.758, 705)
T2 (1.758, 705)
T1 (1.613, 768)
T2 (1.622, 764)
T1 (1.789, 693)
T2 (1.791, 692)
1
Hꢀ1 / L+1 (0.48)
Hꢀ2 / L (0.28)
H / L (0.24)
2
H / L (0.56)
Hꢀ1 / L+1 (0.39)
Hꢀ2 / L (0.04)
H / L+1 (0.48)
Hꢀ1 / L (0.46)
Hꢀ2 / L +1 (0.05)
Hꢀ2 / L+2 (0.24)
Hꢀ1 / L+3 (0.20)
Hꢀ1 / L+1 (0.16)
Hꢀ1 / L+2 (0.32)
Hꢀ2 / L+3 (0.15)
Hꢀ2 / L+1 (0.12)
3 (ref. 22)
Data in parentheses are excitation energies and corresponding wavelengths. b H and L denote HOMO and LUMO, respectively; data in parentheses
a
are the contributions of corresponding excitations.
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the maximum forward light output of 1112 and 572 mW mꢀ2 at
20 V for complexes 1 and 2, respectively (Fig. 4b and c). It is
worth noting that the devices exhibited negligible efficiency roll-
off with increasing current density. Even at 100 mA cmꢀ2, the
value of EQE still remained at 0.50% and 0.34% for the devices
with complexes 1 and 2, respectively. Just like our previously
reported NIR-emitting iridium complexes,19–21 this result again
corroborated that iridium complexes would endow the NIR-
OLEDs with the unique characteristics of negligible efficiency
roll-off.
Conclusions
In summary, we have designed and synthesized two novel NIR-
emitting cationic iridium complexes based on highly conju-
gated cyclometalated benzo[g]phthalazine ligands, with bath-
ochromically shied emission up to 775 nm and notable PLQY
up to 6.1% in solid lms. Density functional theory calculations
have been carried out to reveal that the newly cyclometalated
benzo[g]phthalazine ligands have several advantages over the
previous benzo[g]quinoline ligands, including much stronger
Ir–N bonds, less chelate congestion, better electron-accepting
ability, thus providing the corresponding complexes with
satisfactory NIR emissions due to the desirable MLCT mixed
with ILCT. Employing these two complexes, the NIR-OLEDs
have been fabricated and showed nearly at efficiency-current
density characteristics with the EQE around 0.5% over a wide
range of current density of 1–100 mA cmꢀ2. Such unique char-
acteristics allow great potential of NIR-emitting iridium
complexes for practical applications. To further improve the
device efficiency, the synthesis of the corresponding neutral
complexes and optimization of the device structures are
underway.
Acknowledgements
We thank the National Nature Science Foundation of China (no.
51473086 and 21161160447) and the National Key Basic
Research and Development Program of China (no.
2011CB808403) for nancial support. We especially thank
Professor Hua Ruimao and Senior engineer Jianhua Cao for
their help to synthesize the benzo[g]phthalazine ligands.
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