DOI: 10.1002/asia.200800468
Syntheses, Photophysics, and Application of Iridium
ACHTUNGTRENNUNG(III) Phosphorescent
Emitters for Highly Efficient, Long-Life Organic Light-Emitting Diodes
Tsang-Chi Lee,
[a]Chiung-Fang Chang,
[a]Yuan-Chieh Chiu,
[a]Yun Chi,*
[a]Tzu-Ying Chan,
[b]Yi-Ming Cheng,
[b]Chin-Hung Lai,
[b]Pi-Tai Chou,*
[b]Gene-Hsiang Lee,
[b]Chen-Han Chien,
[c]Ching-Fong Shu,*
[c]and Jens Leonhardt
[d]Introduction
Organometallic complexes possessing a third-row transition-metal element are crucial for the fabrication of highly effi-cient organic light-emitting diodes (OLEDs).[1] The strong
spin-orbit coupling induced by a heavy metal ion such as iridiumACHTUNGTRENNUNG(III) promotes efficient intersystem crossing from the singlet to the triplet excited state manifold, which then facil-itates strong electroluminescence by the harnessing of both singlet and triplet excitons induced by charge recombina-tion. An internal phosphorescence quantum efficiency (hint)
of ~ 100 % could be achieved, hence, these heavy metal con-taining emitters would be superior to their fluorescent coun-terparts in the fabrication of OLEDs. As a result, there has been a continuous trend of shifting research endeavors to these heavy transition-metal based complexes.
In fact, the tri-substituted (or homoleptic) IrIIIcomplexes
with formula [IrACHTUNGTRENNUNG(C^N)3], (C^N)H = 2-(4’,6’-difluorophenyl)
Abstract: Rational design and synthesis of IrIIIcomplexes (1–3) bearing two
cy-clometalated ligands (C^N) and one 2-(diphenylphosphino)phenolate chelate (P^O) as well as the corresponding IrIII
derivatives (4–6) with only one (C^N) ligand and two P^O chelates are re-ported, where (C^NH) = phenylpyri-dine (ppyH), 1-phenylisoquinoline (piqH), and 4-phenylquinazoline (nazoH). Single crystal X-ray diffrac-tion studies of 3 reveal a distorted octa-hedral coordination geometry, in which two nazo ligands adopt an eclipsed configuration, with the third P^O ligand located trans to the phenyl group of both nazo ligands, confirming the general skeletal pattern for 1–3. In sharp contrast, complex 4 reveals a
trans-disposition for the PPh2 groups,
along with the phenolate groups resid-ing opposite the unique cyclometalated ppy ligand, which is the representative structure for 4–6. These IrIIIcomplexes
exhibit green-to-red photolumines-cence with moderate to high quantum efficiencies in the degassed fluid state and bright emission in the solid state. For 1–6, the resolved emission spec-troscopy and relaxation dynamics are well rationalized by the computational approach. OLEDs fabricated using 12 wt. % of 3 doped in CBP and with
BCP as hole blocking material, give bright electroluminescence with lmax=
628 nm and CIExy coordinates (0.65,
0.34). The turn-on voltage is 3.2 V, while the current efficiency and the power efficiency reach 11.2 cd A1 and
4.5 lm W1 at 20 mA cm2. The
maxi-mum efficiency reaches 14.7 cd A1and
6.8 lm W1 upon switching to TPBI as hole blocking material. For evaluating device lifespan, the tested device incor-porating CuPc as a passivation layer, 3 doped in CTP as an emitting layer, and BAlq as hole blocking material, shows a remarkably long lifetime up to 36 000 h at an initial luminance of 500 cd m2.
Keywords: density functional calcu-lations · iridium · luminescence · OLEDs · photophysics
[a] T.-C. Lee, C.-F. Chang, Y.-C. Chiu, Prof. Y. Chi Department of Chemistry
National Tsing Hua University Hsinchu 300 (Taiwan) Fax: (+ 886) 3-572-0864 E-mail: [email protected]
[b] T.-Y. Chan, Dr. Y.-M. Cheng, C.-H. Lai, Prof. P.-T. Chou, Dr. G.-H. Lee
Department of Chemistry National Taiwan University Taipei 106 (Taiwan) Fax: (+ 886) 2-2369-5208 E-mail: [email protected] [c] C.-H. Chien, Prof. C.-F. Shu
Department of Applied Chemistry National Chiao Tung University Hsinchu 30010 (Taiwan) E-mail: [email protected] [d] J. Leonhardt
Sensient Imaging Technologies GmbH ChemiePark Bitterfeld-Wolfen, Areal A
cence in both fluid and solid states and hence are highly de-sirable for the fabrication of phosphorescent OLEDs. Un-fortunately, many cyclometalating ligands (C^N)H do not react with IrIII reagents to give the designated homoleptic
IrIII complexes [IrACHTUNGTRENNUNG(C^N)
3]. Alternatively, researchers have
turned to develop a distinctive class of heteroleptic com-plexes with formula [IrACHTUNGTRENNUNG(C^N)2ACHTUNGTRENNUNG(L^X)], L^X= ancillary
anionic chelate, for which much higher product yields have been achieved.[5]
As for the photophysical properties, it has been reported that a wide range of anionic ancillary L^X ligands can be in-corporated into these complexes [(C^N)2IrACHTUNGTRENNUNG(L^X)], in which
L^X = acetylacetonate (acac),[6] N-methylsalicyliminate
(sal),[7] picolinate (pic) and analogues,[8] carbamate,[9] and
even 2-pyridyl azolate (pyaz) ligands. Note the azolates can be pyrazolate, triazolate, or tetrazolate.[10]These heteroleptic
complexes retain the main characteristics of the parent frag-ment [IrACHTUNGTRENNUNG(C^N)2] to a large degree. A small change in the
emission peak wavelength was noted, which varied accord-ing to the intrinsic nature of the ancillary L^X ligand. For instance, the observed red shift of emission lmax is in the
order of pic < sal ~ acac, which is proportional to their rela-tive electron donor strengths, resulting in a reduction of the energy gap.[11]
As for the 2-pyridyl azolate (pyaz) class of L^X ligands, a similar dependence on the azolate fragments and the prop-erties of the substituents on the azolates is also noted.[5b, 12]
We can thus select the pyaz ligands, ranging from the CF3
substituted pyrazole (fppzH) and triazole (fptzH) or the more electron rich tert-butyl substituted pyrazole (bppzH), in an attempt to conduct subtle color tuning. The donor strengths of pyaz ligands are expected to follow the trend fptz < fppz < bppz, therefore, it is not surprising that the fptz substituted complex displays the most blue-shifted emission signal, while the bppz substituted complexes show relatively red-shifted emission from the IrIIIcomplexes possessing the
same class of cyclometalated C^N ligands.
Despite the above superiority in color tuning, unfortu-nately, most of the L^X ligands, such as the acac ligand, are weak-field ligands. As a result, the chemical stabilities of the resulting complexes, as well as the relative energy gap for the metal centered dd transition, could not be as large as those incorporating strong-field ligands. This case is particu-larly true for the red-orange emitting complex [(pq)2
Ir-ACHTUNGTRENNUNG(acac)], pqH=phenylquinoline, for which an inferior device lifetime arising from the influence of protons from PE-DOT:PSS has been reported,[13]making such a series of acac
substituted materials less desirable for industrial OLED ap-plication.
In this paper, we report a systematic design, synthesis, and characterization of heteroleptic iridiumACHTUNGTRENNUNG(III) complexes pos-sessing a functionalized 2-(diphenylphosphino)phenolate (P^O) chelate. We believe that this new class of P^O ligand, owing to the synergy of the p-accepting diphenyl-phosphino unit and electron donating phenolate fragment,[14]
acter is expected to exhibit certain unusual characteristics, such as better thermal stabilities, higher emission efficien-cies, and enhanced intramolecular pp stacking. Details are elaborated in the following sections.
Results and Discussion
Synthesis and CharacterizationThe chloride bridged dimers [(C^N)2IrACHTUNGTRENNUNG(m-Cl)]2, where
(C^N)H stands for 2-phenylpyridine (ppyH), 1-phenyliso-quinoline (piqH), or 4-phenylquinazoline (nazoH), were synthesized from the direct reaction employing IrCl3·hydrate
mixed with two equiv of (C^N)H ligand in refluxing me-thoxyethanol. Subsequent treatment of [(C^N)2IrACHTUNGTRENNUNG(m-Cl)]2
with a stoichiometric amount of P^OH ligand in the pres-ence of excess of Na2CO3as proton scavenger gave isolation
of the heteroleptic IrIII complexes [(C^N)
2IrACHTUNGTRENNUNG(P^O)] (1–3).
On the other hand, according to the synthetic strategy that produced the mono-cyclometalated IrIII complexes
[(C^N)IrACHTUNGTRENNUNG(L^X)2],[15] treatment of IrCl3·hydrate with a
stoi-chiometric amount of (C^N)H ligand, followed by addition of two equiv of P^OH afforded the anticipated IrIII
com-plexes bearing two P^O ligands [(C^N)IrACHTUNGTRENNUNG(P^O)2] (4–6) in
moderate to low yields. Their chemical structures are depict-ed in Scheme 1.
The basic photophysical properties of [(C^N)2IrACHTUNGTRENNUNG(P^O)]
complexes are, to a certain extent, analogous to the IrIII
complexes that possess similar cyclometalated ligands, de-spite them possessing distinctive ancillary ligands. Interest-ingly, introduction of two P^O ligands resulted in a signifi-cant variation of emission characteristics, as shown by the
photophysical data of [(C^N)2IrACHTUNGTRENNUNG(P^O)] (1–3) versus those of
the [(C^N)IrACHTUNGTRENNUNG(P^O)2] counterparts (4–6). These differences
will be elaborated in the section dealing with the photophys-ical data. In addition, all P^O chelated complexes 1–6 are highly soluble in chlorinated solvents and show negligible decomposition upon a raise in temperature. Detailed charac-terizations were conducted using MS, NMR, and elemental analysis (see Experimental Section), while complex 3 and 4 were further identified using single crystal X-ray analysis to establish their solid-state structure.
From the X-ray structural determination, complex 3 pos-sesses a typical heteroleptic arrangement with two cyclome-talating nazo chelates and one PPh2 substituted phenolate
ligand (Figure 1). The nazo chelates adopt a mutually eclipsed configuration with their coordinated nitrogen atoms N(1) and N(3) and carbon atoms C(1) and C(15), being lo-cated in trans- and cis-orientation, respectively. Moreover, the third P^O chelate resides opposite to the carbon atoms of both nazo ligands. The overall ligand arrangement is akin to those of the chloride-bridged dimer [(ppy)2IrACHTUNGTRENNUNG(m-Cl)]2, as
well as other heteroleptic complexes such as [(dpqx)2
Ir-ACHTUNGTRENNUNG(fppz)],[16]suggesting that the P^O chelate in our case is
co-ordinated to the IrIIIcenter by a stereoselective replacement
of chloride ligands in its precursor [(nazo)2IrACHTUNGTRENNUNG(m-Cl)]2.
More-over, the elongated IrC(1) distance (2.034(3) ) versus the IrC(15) bond (1.984(3) ) shows the notable trans-effect imposed by the PPh2 fragment, while twisting of nazo
li-gands is caused by the repulsion between the pair of hydro-gen atoms located on carbon atoms C(5), C(9) and C(19), C(23), respectively. Formation of such twisted cyclometalat-ed ligands has been well establishcyclometalat-ed for complexes with either piq or nazo chelates in recent literature.[17]
Moreover, detailed analysis of the structural data revealed the presence of two sets of pp stacking interactions
(Fig-ure 1 b). The first one is ascribed to the pp stacking between one phenyl group of the Ph2P segment and the adjacent
nazo chelate, for which the centroid–centroid contact is cal-culated to be 3.942 . The second nazo ligand is parallel to the nazo chelate of the adjacent molecule where an even stronger pp stacking was observed with the centroid–cent-roid contact being reduced to 3.698 . In sharp contrast to the aforementioned case, we expected this intramolecular PPh2-nazo pp stacking interaction, observed in a crystal
lat-tice, would be easily disrupted upon dissolution in organic solvents.
Figure 2 showed the crystal structure of 4, for which its structural motif exhibits only one cyclometalated ppy and two phenolate chelates bearing PPh2substitution. It is
nota-ble that oxygen atoms O(1) and O(2) occupied a cis-disposi-tion, while the phosphorus atoms P(1) and P(2) adopted a trans-disposition, which is consistent with the large JP,P
cou-pling constant (359 Hz) observed in the 31P NMR spectrum.
A similar trans arrangement of phosphine ligands has been observed in the related IrIII complexes such as
trans-[Ir-ACHTUNGTRENNUNG(ppy)2ACHTUNGTRENNUNG(PPh3)2]+ and [IrACHTUNGTRENNUNG(ppy)ACHTUNGTRENNUNG(PPh3)2(H)L]0, +, L = MeCN,
CO, CN;[18]all of which showed emission in the sky-blue to
blue region partially owing to the stronger electron accept-ing character of phosphine that stabilize the metal dp
orbi-tals (vide infra). Moreover, the IrO(1) distance (2.086(3) ) is shorter than the IrO(2) bond (2.140(3) ). This result is again attributed to the weakened trans-effect exerted by the nitrogen donor versus the carbon donor atom. Notably, these IrO distances, as well as the IrP dis-tances in 4, are both shorter than the respective IrO and IrP distances observed in 3. This discrepancy may be ascri-bed to the depletion of electron density at the central IrIII
cation by double phosphine coordination, resulting in the enhancement of the metal–ligand bond strength in 4. Finally,
Figure 1. a) ORTEP diagram of 3 with thermal ellipsoids shown at 30 % probability level; selected bond lengths (): Ir-C(1) = 2.034(3), Ir-C(15) = 1.984(3), N(1) = 2.057(3), N(3) = 2.030(3), P(1) = 2.3540(8), Ir-O(1) = 2.159(2) and selected bond angles (8): C(15)-Ir-N(3) = 79.43(12), C(1)-Ir-N(1) = 78.21(11), O(1)-Ir-P(1) = 81.60(6). b) Stacking diagram showing the centroid–centroid contacts between nazo ligands (dash, 3.698 ) and between the dpp ligand and nazo chelate (dot line, 3.942 ).
Figure 2. ORTEP diagram of 4 with thermal ellipsoids shown at 30 % probability level ; selected bond lengths (): Ir-C(1) = 2.016(3), Ir-N(1) = 2.021(3), O(1) = 2.086(3), O(2) = 2.140(3), P(1) = 2.3208(9), Ir-P(2) = 2.316(1) and selected bond angles (8): C(1)-Ir-N(1) = 80.68(14), O(1)-Ir-P(1) = 82.59(7), O(2)-Ir-P(2) = 82.49(7), P(1)-Ir-P(2) = 176.02(4). The centroid–centroid contacts from dpp ligand to the pyridyl and phenyl group of ppy chelate being 3.892 and 3.891 , respectively.
the ppy ligand is sandwiched between the phenyl groups on each of the two P^O chelates (see dash lines, Figure 2). Their reduced centroid–centroid contacts of 3.891–3.892 symbolized the formation of a non-negligible intra-molecu-lar pp stacking interaction, which is analogous to those re-ported in the isoelectronic ReIand OsIIcomplexes with two
trans-substituted phosphine ligands.[19]This interaction is
ex-pected to reveal red-shifted singlet pp* and metal-to-ligand charge transfer (MLCT) bands in the UV/Vis absorption spectra, prolonged lifetime of 3MLCT, and blue-shifted 3MLCT transition versus the congeners without such
inter-action.[19a] The analogous correlation can not be delineated
in the present system owing to the lack of related dialkyl-phosphino derivatives.
Photophysical Properties
The absorption spectra of 1–6 in CH2Cl2solution are
depict-ed in Figure 3. For the pair of ppy complexes 1 and 4, strong absorption bands in the UV region are reasonably assigned to the ligand-centered pp* transition involving ppy ligands. Similar to that of 4, for which its MLCT transition appears at 348 nm, complex 1 exhibits respective MLCT absorption at a slightly lower energy region of 356 nm. Moreover, for
complex 1, it is reasonable to assign the lowest energy shoulder extending into the visible region to spin-orbit cou-pling enhanced 3pp* and 3MLCT transitions, whereas for
complex 4, such absorption to the triplet manifold is ob-scure. This difference could arise from the possession of a single cyclometalated ligand in 4 and the reduced MLCT contribution (vide infra), such that the corresponding transi-tion probabilities, and hence the absorptransi-tion extinctransi-tion coef-ficients are decreased.
Like their analogues 1 and 4, the other pairs of piq com-plexes 2 and 5, as well as nazo comcom-plexes 3 and 6 exhibit the spin allowed pp* absorption at around 423, 464, 411, and 441 nm, respectively. The lower energy absorption band at 457 and 535 nm for 2 and 3, are tentatively assigned to the singlet MLCT transition, while the next lower energy absorptions are attributed to the spin-orbit coupling en-hanced 3pp* and 3MLCT transitions. Analogous to the
aforementioned differences between 1 and 4, their counter-parts 5 and 6 bearing dual P^O chelates exhibited only the singlet MLCT absorption at 411 and 441 nm, but failed to show the notable 3pp* and 3MLCT bands arising from the
reduced extinction coefficients discussed earlier.
Figure 3 also depicts emission spectra of the titled com-plexes in CH2Cl2, while pertinent photophysical data are
listed in Table 1. Moderate to highly intensive luminescence (Fp~ 0.015–0.67, see Table 1) was observed for all complexes
in degassed CH2Cl2solution. The entire emission band
origi-nating from a triplet state manifold was ascertained by the O2 quenching rate constant as high as 1.5–2.0 10
9m1s1.
For clarity, the data are categorized into two classes, namely series I and II, anchored by single P^O and dual P^O che-lates, respectively. As a result, the emission peak wavelength is in the order of 1 (515 nm) < 2 (652 nm) < 3 (657 nm) for series I, and 4 (510 nm) < 5 (620 nm) < 6 (690 nm) for series II. Thus, it is obvious that the emission gap as a function of ligand chromophore follows the trend ppy > piq > nazo.[16]
Since pyridine (in ppy), isoquinoline (in piq), and quinazo-line (in nazo) contribute to the LUMO for both the I and II series of complexes (vide infra), the results are well rational-ized by the decrease of the pp* energy upon either elonga-tion of p electron conjugaelonga-tion (ppy vs piq) or the additive electron withdrawing nitrogen atom in the fused heterocycle (piq vs nazo).[20] This viewpoint is also supported by the
electrochemical data, of which the reduction potential plays a major role to account for the emission gap in both the I and II series of complexes.
On the other hand, a certain correlation between the I and II series was also noted in terms of emission energy gap and other photophysical characteristics. Firstly, for the com-pounds bearing the same ligand chromophore, it is worth-while to note that the trends of the emission wavelengths follow 2 > 5, but 3 < 6. The blue shift observed for 5 versus 2, from our viewpoint, could arise from the increase of ligand-centered pp* character at excited states and a con-comitant decrease of MLCT contribution (22.02 % vs 14.05 %, vide infra) as shown in Table 2; the latter tends to possess a much reduced energy gap versus the respective
Figure 3. a) Absorption and b) photoluminscence spectra of complex 1–6 recorded in CH2Cl2solution at room temperature.
pp* transition. In contrast, complexes 3 and 6 show the HOMO principally located on the phenyl group of nazo chelates in 3 and phenolate fragment of P^O ligand in 6. Since the phenyl group possesses a much lowered energy level compared with that of the phenolate, switching of the HOMO leads to a reduction of emission energy gap and bathochromic shift for 6. Secondly, based on the same chro-mophore ligand, that is, ppy, piq, or nazo, the deduced radia-tive decay rate constant reveals a trend of I > II, such as 1 > 4, 2 > 5, and 3 > 6. The result is in agreement with the con-clusion drawn in the discussion of absorption data, in which the MLCT contribution to the lowest triplet state is reduced in the II complexes, resulting in a weaker spin-orbit coupling and hence a smaller radiative decay rate constant for the phosphorescence.[21] In sharp contrast, no apparent
correla-tion was observed for the non-radiative decay rate constant (knr) and hence the emission quantum yields, which does not
seem unreasonable because both chromophore and ancillary ligands vary simultaneously in series I and II. Nevertheless, as shown in Table 1, the emission quantum yield seems to increase for blue-green (1 and 4) to orange-red emission (2
and 5) and then decreases in the deep red region (3 and 6). The result can be rationalized, in a qualitative manner, by the fact that higher energy emission (e.g., 1, 4) is increasing-ly subject to radiationless channels associated with those high-energy states of shallow potential such as dpds*and/or
pds* transitions with repulse potential energy surfaces,[22]
while deep red emission (e.g., 3 and 6) is normally quenched by certain high-frequency vibration modes, i.e., the opera-tion of energy gap law.[23]
To gain more fundamental insight into the above experi-mental results, we then performed theoretical calculations to investigate the underlying photophysical properties of com-plexes 1–6. The results are summarized in Table 2 and Figure 4. Owing to the similarity between 2 and 3 as well as 5 and 6 (vide infra), Figure 4 only depicts the frontier orbi-tals for 1, 2, 4, and 5. As revealed in Figure 4 and Table 2, the lowest-energy singlet excited state (S1) is dominated by
the HOMO!LUMO transition for compounds 1–6. For complex 1, the HOMO!LUMO transition is mainly ascri-bed to the metal dporbital and the p orbital of the
pheno-late of the P^O moiety!pyridine in ppy, namely, MLCT and ligand-to-ligand charge transfer (LLCT) transitions, re-spectively. On the other hand, the same transition of 2 and 3 can be ascribed to an intra-ligand charge transfer (phenyl! isoquinoline or quinazoline in piq or nazo; ILCT) mixed with MLCT. As for the S0!T1transition of 1–3, the
excita-tion character is relatively complicated and is composed of more than one type of transition involving, for example, HOMO-1!LUMO and HOMO-2!LUMO (see Table 2). We thus intended not to analyze each transition but simply assigned the first triplet state of 1–3 to a mixing character of ILCT, LLCT, and MLCT.
The HOMO of complexes 4–6 are composed of the dp
or-bital of IrIII together with the p orbital of the phenolate
ligand. Other occupied orbitals involved in the first triplet state of 4–6, for example, HOMO-2, reside in the IrIIIatom
and phenyl moiety of the ppy ligand in 4, piq ligand in 5, and nazo ligand in 6. The LUMO of 4–6 are mainly ascribed to the p* orbital of pyridine in ppy of 4, isoquinoline in piq of 5, and quinazoline in nazo of 6. It is thus reasonable to ascribe the lowest singlet excited state of 4–6 to a LLCT state mixed with MLCT and the triplet excited state of 4–6 to a mixed LLCT and MLCT plus an increased proportion of ILCT character. Moreover, although the %MLCT ob-tained in this approach is qualitative, the calculated value
Table 1. Photophysical and electrochemical properties of IrIIIcomplexes 1–6 in CH
2Cl2at room temperature.
UV/Vis lmax[e 103, M1cm1][a] em lmax[nm] F [%] tobs[ms] kr knr E1/2ox[DEp][b]/V E1/2red[DEp][b]/V
1 258 (74), 300 (25), 356 (10), 390 (6.6), 424 (3.6) 515 8.9 0.11 8.3 105 8.4 106 0.40 (110) 2.76[d] 2 289 (70), 345 (37), 423 (12), 457 (11) 652 25.0 1.55 1.6 105 5.0 105 0.41 (140) 2.27 (130), 2.59 (150) 3 287 (55), 342 (38), 464 (13), 535 (5.8) 657 18.0 0.96 1.8 105 8.9 105 0.53 (130) 1.85 (100), 2.16 (130) 4 254 (55), 296 (12), 348 (8.5) 510 1.8 0.05 4.1 105 2.2 107 0.35[c] 2.66[d] 5 276 (37), 341 (18), 411 (6.2) 591, 620 67.8 5.09 1.3 105 6.3 104 0.36[c] 2.33 (70) 6 294 (31), 348 (25), 441 (7.0), 550 (0.9) 690 1.5 0.10 1.5 105 9.8 106 0.41[c] 2.00 (110)
[a] The systematic error of the absorption coefficients measurement is ~ 20 %. [b] E1/2refers to [(Epa+Epc)/2] where Epaand Epcare the anodic and
cathodic peak potentials referenced to the Fc+/Fc couple. DE
p=j EpaEpcj was reported in mV, and the oxidation and reduction experiments were
con-ducted in CH2Cl2and THF solution, respectively. [c] Epa. [d] Epc.
Table 2. The excitation energies, oscillation strengths, MLCT % of com-plexes 1–6.[a]
States lcal[nm] Assignments F MLCT
1 S1 407.0 HOMO!LUMOACHTUNGTRENNUNG(+81 %), HOMO-1!LUMOACHTUNGTRENNUNG(+12%) 0.0058 13.97 % T1 457.2 HOMO-1!LUMOACHTUNGTRENNUNG(+41%), HOMO-2!LUMO + 1ACHTUNGTRENNUNG(+21%) 22.02 % 2 S1 485.0 HOMO!LUMOACHTUNGTRENNUNG(+87 %) 0.0058 13.54 % T1 568.9 HOMO-2!LUMO + 1ACHTUNGTRENNUNG(+70%), HOMO-1!LUMO + 1ACHTUNGTRENNUNG(15%) 22.25 % 3 S1 540.9 HOMO!LUMOACHTUNGTRENNUNG(+83 %), HOMO!LUMO + 1ACHTUNGTRENNUNG(13 %) 0.0062 10.53 % T1 564.9 HOMO!LUMOACHTUNGTRENNUNG(+47 %), HOMO-1!LUMOACHTUNGTRENNUNG(+14%), HOMO!LUMO + 1ACHTUNGTRENNUNG(14 %), HOMO-2!LUMO + 1ACHTUNGTRENNUNG(+13%) 12.25 % 4 S1 434.2 HOMO!LUMOACHTUNGTRENNUNG(+96 %) 0.0005 11.97 % T1 446.4 HOMO-2!LUMOACHTUNGTRENNUNG(+55 %), HOMO-4!LUMOACHTUNGTRENNUNG(+23 %), HOMO!LUMOACHTUNGTRENNUNG(+10 %) 18.02 % 5 S1 520.4 HOMO!LUMOACHTUNGTRENNUNG(+97 %) 0.0002 7.57 % T1 562.6 HOMO-2!LUMOACHTUNGTRENNUNG(+54 %), HOMO-3!LUMOACHTUNGTRENNUNG(+45 %) 14.09 % 6 S1 600.5 HOMO!LUMOACHTUNGTRENNUNG(+97 %) 0.0001 6.05 % T1 602.8 HOMO!LUMOACHTUNGTRENNUNG(+95 %) 13.02 %
follows the trend of 1 > 4, 2 > 5, and 3 > 6 (see Table 2), con-sistent with the conclusions made from both absorption and emission studies (vide supra).
The calculated energy gap, in terms of wavelength, is blue-shifted from that of the emission data for all complexes studied. We believe that this discrepancy mainly arises from the neglect of the solvent effect or nodeless pseudopoten-tial.[24]Nevertheless, in terms of the S
0-T1energy gap, which
is revealed by the phosphorescence peak frequency experi-mentally, the computational results show the trend of 1 > 2 > 3 and 4 > 5 > 6 (see Table 2), consistent with those obtained experimentally (Table 1). In fact, the results also predict that the phosphorescence emission, in terms of peak wave-length, should be in the approximate order of 4 ~ 1 < 5 < 2 < 3 < 6, again in good agreement with the experimental data. However, we also have to point out that during this ap-proach, only the scalar relativistic effect is taken into ac-count, while the spin-orbit coupling effect on the excitation energies is neglected in almost all TDDFT calculations for heavy elements. The neglect of the spin-orbit coupling pa-rameter may cause appreciable error in estimating the S1-T1
gap. The excitation wavelength of the S1state of, for
exam-ple, 6 is calculated to be nearly the same as that of the T1
state.[25]
Electrochemistry
The electrochemical behavior of these IrIIImetal complexes
was investigated by cyclic voltammetry using ferrocene as the internal standard. These results are also listed in Table 1. Interestingly, during the anodic scan in CH2Cl2, all Ir
III
metal complexes 1–6 exhibited either quasi-reversible or ir-reversible oxidation in the range of 0.35–0.53 (Figure 5). Complexes 4–6 gave lower oxidation signals in the region 0.35–0.41 V with respect to their counterparts 1–3. This could arise from the presence of two P^O chelates in 4–6, for which, in addition to the IrIIImetal oxidation, the
exces-sive donation of the electron from the phenolate fragments to the HOMO gave the reduced oxidation potentials. In contrast, the cyclometalated ligands failed to show a notable influence on the oxidation potential of 4–6 because they ex-hibited no essential contribution to the HOMO, which is in
good agreement with the above theoretical calculation, con-cluding that the HOMO is mainly ascribed to the phenolate moiety in the P^O chelate.
Upon switching to the cathodic sweep in THF, only one irreversible reduction peak was observed for the ppy com-plexes 1 and 4, consistent with the largest energy gap for the ppy ligand as well as the green phosphorescence observed for both of them. On the other hand, two reversible reduc-tion processes, with potentials ranging from 1.85 to 2.59 V, were detected for complexes 2 and 3, while only one reversible reduction signal was detected at 2.33 and 2.0 V for 5 and 6, respectively. As revealed in previous studies,[26] the reversible reductions occur primarily on the
stronger electron accepting heterocyclic portion of the cyclo-metalated C^N ligands. Therefore, replacing the piq frag-ment with a nazo moiety, to afford 3 and 6, would signifi-cantly lower their reduction potentials. This was demonstrat-ed by the observdemonstrat-ed potentials of 3 (1.85, 2.16 V) vs those of 2 (2.27, 2.59 V) and 2.0 V of 6 vs that of 2.33 V of 5. Moreover, the detection of two reduction peaks in 2 and 3 is attributed to the presence of two cyclometalated ligands within the coordination sphere, thus the second reduction is strongly influenced by the preceding negative charge resid-ing on the first cyclometalated ligand.[27]
OLEDs Characterization
To demonstrate their capabilities in exhibiting decent elec-troluminescence, we first fabricated red phosphorescence devices based on dopants 2 and 3, denoted as device A and B, respectively. A multilayer structure of ITO/NPB (40 nm)/ CBP:12 wt. % of dopant (30 nm)/TPBI (10 nm)/AlQ3
(30 nm)/LiF (10 )/Al (150 nm) is employed. This device ar-chitecture resembles those reported in literature,[28] for
which the abbreviations NPB, CBP, TPBI, and AlQ3 stand
for bis[N-(1-naphthyl)-N-phenylamino] biphenyl, 4,4’-N,N’-dicarbazolyl-1,1’-bipheny, 1,3,5-tris(N-phenyl benzimi-dizol-2-yl)benzene, and tris(8-hydroxyquinolinato) alumi-num (III), and act as hole-transporting, host, hole-blocking, and electron-transporting materials, respectively. Their rela-tive energy alignment is shown in Figure 6,[29]while all
cru-cial performance characteristics are collected in Table 3.
Both devices exhibited a relatively low turn-on voltage of 3.2 V and decent EL efficiencies. At a current density of 20 mA cm2, the external quantum efficiency, luminous effi-ciency, and power efficiency were 12.4 %, 14.7 cd A1, and
6.8 lm W1 for the 3-based device, respectively, and 11.8 %, 17.4 cd A1, and 6.6 lm W1 for the 2-based device,
respec-tively. As shown in Figure 7, the 3-doped device showed a notable bathochromic shift (lmax=628 nm) compared to that
of the device based on dopant 2 with lmax=618 nm, giving a
corresponding Commission Internationale de L’Eclairage (CIEx,y) chromaticity coordinates of (0.65, 0.34) and (0.63,
0.37), both are very close to the NTSC red standard (0.66, 0.33). The better red color purity of 3 over 2 is obviously a result of its lowered LUMO energy level resulting from the relatively more stabilized p* orbital of nazo ligands.
Furthermore, our red-emitting devices exhibited a much lower efficiency roll-off at high brightness. Particularly for the 3-based device, the EL efficiencies remained above 10.1 %, 11.9 cd A1, and 4.3 lm W1 at 100 mA cm2; while
the external quantum efficiency can still be maintained at a level as high as 7.8 % upon increasing to 300 mA cm2,
which are quite respectable for the best red-emitting PhO-LEDs. Finally, bright luminescence of over 57 369 cd m2and
46 521 cd m2were observed for devices based on 3 and 2 at
the driving voltage of 13.5 and 14.0 V, respectively. The better performance can be attributed to a shorter phosphor-escence radiative lifetime,[4, 30]together with the presence of Figure 6. Schematic energy alignment of devices A and B that employed 12 wt. % of 2 and 3 as phosphorescent dopant; energy levels are taken from the following literature citations: NPB and TPBI, CBP, and AlO3
and LiF/Al.
Table 3. Performance data for OLEDs fabricated using complexes 2 and 3 as dopant.
Entry A B
Dopant 2 3
Max. luminance[a] 46 521 (14.0) 57 369 (13.5)
E.Q.E. [%][b] 11.8 (8.8) 12.4 (10.1) Luminance [cd m2][b] 3472 (12 952) 2917 (11 791) L.E. [cd A1][b] 17.4 (13.0) 14.7 (11.9) P.E. [lm W1][b] 6.6 (4.0) 6.8 (4.3) lmax[(nm)/ACHTUNGTRENNUNG(CIExy)][c] 618 (0.63, 0.37) 628 (0.65, 0.34) Turn-on (V) 3.2 3.2
[a] Values in the parentheses are the applied driving voltage. [b] Data
re-corded at 20 mA cm2, values in the parentheses are recorded at
100 mA cm2. [c] Measured at the driving voltage of 8 V.
unique P^O chelate that may suppress the solid-state aggre-gation and stabilize the metal complexes using a PPh2
frag-ment.
To investigate their potential as dopants suitable for in-dustrial applications, we then tested the lifetime of 3-based OLEDs with two slightly different architectures. Devices C possessed the layered configuration of ITO/CuPc (7 nm)/ NPB (30 nm)/EL (25 nm)/AlQ3 (60 nm)/LiF (0.3 )/Al
(150 nm), for which CuPc was selected as buffer to adjust the surface smoothness of ITO and provide good adhesion to the NPB charge transport material, while two types of EL, namely, CBP + 12 % 3 and (4,4’-bis(carbazol-9-yl)ter-phenyl) CTP + 12 % 3, were used to evaluate the influence of host materials on the OLED device lifetime. The respec-tive behavior of hole blocking materials was next tested with the series of devices D, which possess the analogous configuration of ITO/CuPc (7 nm)/NPB (30 nm)/CTP + 12 % 3 (25 nm)/HBM (10 nm)/AlQ3 (60 nm)/LiF (0.3 )/Al
(150 nm). Note that three hole blocking materials (HBM), namely, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), TPBI, and aluminumACHTUNGTRENNUNG(III) bis(2-methyl-8-quinolina-to)(4-phenylphenolate) (BAlq), were alternatively selected for this study.
Allowing the initial luminance to be 500 cd m2, the life-time data of all OLED devices C and D were determined by measuring the elapsed time for the luminance to decay
to half of its initial value. These estimated lifetime data are summarised in Table 4. It is notable that the device C2 with CTP as host material shows a better lifetime, of over 33 000 h, than the device C1 employing CBP as the host ma-terial (18 000 h). Moreover, the lifetime was also strongly af-fected by the hole blocking materials (HBM), as shown in the series of devices D. Although both TPBI and BCP showed a drastic reduction of lifetime, the lifetime of more than 36 000 h was registered by using BAlq, the hole block-ing material. The result thus revealed a strong influence im-posed by the inherent characteristics of both host and hole blocking materials. Taking these OLED lifetime data into account, together with the fact that the PPh2group of P^O
chelate is known to be a strong-field ligand, it is reasonable to expect that dopant 3 could render a bright prospect toward commercial application and should attract a broad spectrum of interest in the field of OLEDs.
Figure 7. a) EL spectra; b) CIE coordinates; c) external quantum efficiency; d) dependence of current density and luminance versus driving voltage for devices A and B.
Table 4. Lifetime data of PhOLEDs doped with 12 wt. % of 3.
Devices Host HBM[a] lifetime [h]
C1 CBP – 18 334
C2 CTP – 33 053
D1 CTP TPBI 1197
D2 CTP BCP 1047
D3 CTP BAlq 36 303
Conclusions
In summary, we report here the design concept as well as preparation of a series of emissive IrIIIcomplexes employing
ancillary P^O chelate, for which its PPh2 fragment is well
known for the excellent p-accepting characteristics and clas-sified as a strong-field ligand, while the phenolate segment is more electron donating and capable of facilitating stable metal–ligand bonding. Moreover, the corresponding photo-physical properties and DFT calculations suggest that this P^O ligand would only exert a secondary influence, that is, provide a stable and rigid coordination framework, while its participation could fine-tune the light-emitting electronic transition according to the observed photophysical data. Moreover, OLEDs fabricated using 12 wt. % of 3 doped in CBP together with TPBI as hole blocking material gave bright electroluminescence, for which the current and power efficiencies were 14.7 cd A1and 6.8 lm W1 at 20 mA cm2, and giving the CIExy chromaticity of 0.65, 0.34 at a driving
voltage of 8 V. Finally, one set of the tentatively tested devi-ces employing CuPc as passive layer, CTP + 12 wt. % of 3 as emitting layer, and BAlq as hole blocking material, shows a remarkable lifetime up to 36 000 h at an initial luminance of 500 cd m2. Based on these findings, the aforementioned IrIII
metal complexes, particularly nazo derivative 3 can be a promising candidate for real optoelectronic applications, after conducting minor modifications to the molecular struc-ture.
Experimental Section
General Procedures
All reactions were performed under nitrogen. Solvents were distilled from appropriate drying agents prior to use. Commercially available re-agents were used without further purification unless otherwise stated. Reactions were monitored by TLC with Merck pre-coated glass plates (0.20 mm with fluorescent indicator UV254). Compounds were visualized
with UV light irradiation at 254 nm and 365 nm. Flash column chroma-tography was carried out using silica gel from Merck (230–400 mesh). Mass spectra were obtained on a JEOL SX-102 A instrument operating in electron impact (EI) mode or fast atom bombardment (FAB) mode.
1H and 13C NMR spectra were recorded on Varian Mercury-400 or
INOVA-500 instruments. The IrIIIcomplexes [(ppy)
2IrACHTUNGTRENNUNG(m-Cl]2, [(piq)2
IrACHTUNGTRENNUNG(m-Cl]2, and [(nazo)2IrACHTUNGTRENNUNG(m-Cl]2 were obtained from treatment of IrCl3·3H2O
with phenylpyridine (ppyH), 1-phenylisoquinoline (piqH), and 4-phenyl-quinazoline (nazoH), respectively;[31] while 2-diphenylphosphinophenol
(P^OH) was prepared according to the literature methods.[32]
Synthesis and Characterization
Preparation of [(ppy)2IrACHTUNGTRENNUNG(P^O)] (1). To a 25 mL flask was added
[(ppy)2IrACHTUNGTRENNUNG(m-Cl)]2 (107 mg, 0.1 mmol), 2-(diphenylphosphino)phenol
(P^OH, 61 mg, 0.22 mmol), Na2CO3(106 mg, 1.0 mmol), and
2-methoxy-ethanol (10 mL). The mixture was heated at 120 8C for 1.5 h, and quenched by addition of deionized water (15 mL) after cooling. The pre-cipitate was filtered and dried under high vacuum. Purification was car-ried out by silica gel column chromatography eluting with ethyl acetate. Recrystallization from a mixture of CH2Cl2 and hexane gave a pale
yellow crystalline solid [(ppy)2IrACHTUNGTRENNUNG(P^O)] (1 80 mg, 0.05 mmol) in 51 %
yield.
Spectral data for 1.1H NMR (400 MHz, CDCl
3, 298 K, TMS): d = 8.37 (d, JH,H=5.2 Hz, 1 H), 8.06 (d, JH,H=5.6 Hz, 1 H), 7.81–7.74 (m, 3 H), 7.59– 7.53 (m, 2 H), 7.43 (td, JH,H=7.8, 1.6 Hz, 1 H), 7.39–7.30 (m, 4 H), 7.27 (d, JH,H=7.2 Hz, 1 H), 7.24–7.19 (m, 2 H), 7.03 (dd, JH,H=7.6, 6.0 Hz, 1 H), 6.97 (t, JH,H=6.0 Hz, 1 H), 6.88–6.82 (m, 3 H), 6.82–6.76 (m, 4 H), 6.62 (t, JH,H=6.4 Hz, 2 H), 6.53 (t, JH,H=8.4 Hz, 2 H), 6.42 (t, JH,H=6.0 Hz, 1 H), 6.07 ppm (dd, JH,H=6.8, 4.4 Hz, 1 H);31P-{1H} NMR (202 MHz, CDCl3,
298 K, TMS): d = 12.30 ppm (s, 1P); MS (FAB,192Ir): m/z (%) calcd for
C40H30IrN2OP: 777.87 [M]+, 623.68 [M-ppy]+; found: 778 (100), 624(24);
elemental analysis: calcd (%) for C40H30IrN2OP: C 61.76, H 3.89, N 3.60;
found: C 61.45, H 4.24, N 3.68.
Preparation of [(piq)2IrACHTUNGTRENNUNG(P^O)] (2). Following the procedure described
for 1, a mixture of [(piq)2IrACHTUNGTRENNUNG(m-Cl]2 (64 mg, 0.05 mmol), P^OH (31 mg,
0.11 mmol), and Na2CO3(53 mg, 0.5 mmol) was refluxed for 1.5 h to
pro-vide a red solid (27 mg, 0.03 mmol, 31 %). Spectral data for 2.1H NMR (400 MHz, CDCl
3, 298 K, TMS): d = 8.88 (d, JH,H=8.0 Hz, 1 H), 8.55 (d, JH,H=8.0 Hz, 1 H), 8.37 (d, JH,H=6.5 Hz, 1 H), 8.17 (d, JH,H=8.5 Hz, 1 H), 8.10 (d, JH,H=8.5 Hz, 1 H), 7.99 (d, JH,H= 6.5 Hz, 1 H), 7.76 (t, JH,H=8.3 Hz, 3 H), 7.71–7.66 (m, 3 H), 7.59–7.55 (m, 2 H), 7.45 (t, JH,H=7.3, 1 H), 7.42 (t, JH,H=6.8 Hz, 1 H), 7.35 (t, JH,H= 8.0 Hz, 2 H), 7.20 (t, JH,H=7.0 Hz, 1 H), 7.17 (d, JH,H=6.5 Hz, 1 H), 7.01 (t, JH,H=8.0 Hz, 1 H), 6.97 (t, JH,H=8.0 Hz, 1 H), 6.91 (t, JH,H=7.0 Hz, 1 H), 6.81 (d, JH,H=7.0 Hz, 1 H), 6.80 (d, JH,H=7.0 Hz, 1 H), 6.76 (t, JH,H= 7.8 Hz, 1 H), 6.65 (d, JH,H=8.0 Hz, 1 H), 6.60 (t, JH,H=7.3 Hz, 1 H), 6.48– 6.45 (m, 4 H), 6.39–6.37 ppm (m, 2 H);31P-{1H} NMR (202 MHz, CDCl 3,
298 K, TMS): d = 13.31 ppm (s, 1P); MS (FAB,192Ir): m/z (%) calcd for
C48H34IrN2OP: 877.99 [M] +
; found: 878 (100); elemental analysis: calcd (%) for C48H34IrN2OP: C 65.66, H 3.90, N 3.19; found: C 65.33, H 4.12,
N 3.06.
Preparation of [(nazo)2IrACHTUNGTRENNUNG(P^O)] (3). Following the procedure described
for 1, a mixture of [(nazo)2IrACHTUNGTRENNUNG(m-Cl]2 (64 mg, 0.05 mmol) and P^OH
(31 mg, 0.11 mmol) was refluxed for 2 h to provide a red solid (57 mg, 0.06 mmol, 65 %).
Spectral data for 3.1H NMR (400 MHz, CDCl
3, 298 K, TMS): d = 9.21 (s, 1 H), 8.80 (d, JH,H=7.2 Hz, 1 H), 8.65 (s, 1 H), 8.44 (d, JH,H=8.4 Hz, 1 H), 8.28 (d, JH,H=8.4 Hz, 1 H), 8.17 (d, JH,H=8.0 Hz, 1 H), 7.93 (t, JH,H= 7.4 Hz, 2 H), 7.84 (t, JH,H=7.6 Hz, 1 H), 7.78–7.68 (m, 4 H), 7.59 (t, JH,H= 7.8 Hz, 1 H), 7.44–7.38 (m, 4 H), 7.16 (t, JH,H=8.8 Hz, 1 H), 7.03–6.91 (m, 3 H), 6.90–6.85 (m, 2 H), 6.81 (t, JH,H=7.6 Hz, 1 H), 6.60 (t, JH,H=7.4 Hz, 1 H), 6.49–6.42 (m, 5 H), 6.36 ppm (dd, JH,H=8.4, 4.0 Hz, 1 H);31P-{1H} NMR (202 MHz, CDCl3, 298 K, TMS): d = 12.30 ppm (s, 1P); MS (FAB, 192Ir): m/z (%) calcd for C
46H32IrN4OP: 879.96 [M] +
, 674.73 [M-nazo]+
;
found: 880 (40), 675(20); elemental analysis: calcd (%) for
C46H32IrN4OP: C 62.79, H 3.67, N 6.37; found: C 62.52, H 3.93, N 6.57.
Preparation of [(ppy)IrACHTUNGTRENNUNG(P^O)2] (4). To a 50 mL flask was added
IrCl3·3H2O (100 mg, 0.29 mmol), ppyH (47 mg, 0.3 mmol) and
2-methox-yethanol (35 mL). The mixture was heated at 120 8C for 1.5 h, followed by addition of 2-(diphenylphosphino)phenol (P^OH, 167 mg, 0.6 mmol) and Na2CO3(302 mg, 2.9 mmol), and heating was resumed for another
12 h. After cooling, the mixture was concentrated to 1/3 of its original volume and quenched by addition of deionized water (15 mL), giving yellow precipitate which was collected by filtration. Further purification was carried out by silica-gel column chromatography eluting with a 1:1 mixture of ethyl acetate and hexane. Recrystallization from mixed CH2Cl2 and methanol gave yellow crystals [(ppy)IrACHTUNGTRENNUNG(P^O)2] (61 mg,
0.07 mmol) in 24 % yield.
Spectral data for 4.1H NMR (400 MHz, CDCl
3, 298 K, TMS): d = 8.50 (d,
JH,H=5.3 Hz, 1 H), 8.41–8.37 (m, 2 H), 8.21–8.15 (m, 2 H), 7.40–7.30 (m,
7 H), 7.24–7.00 (m, 7 H), 6.95–6.92 (m, 2 H), 6.75 (d, JH,H=8.0 Hz, 1 H),
6.69–6.57 (m, 8 H), 6.47–6.44 (m, 2 H), 6.26–6.20 ppm (m, 4 H);31P-{1H}
NMR (202 MHz, CDCl3, 298 K, TMS): d = 14.97 (d, JP,P=359 Hz, 1P),
12.39 ppm (d, JP,P=359 Hz, 1P); MS (FAB, 192Ir): m/z (%) calcd for
C47H36IrNO2P2: 900.96 [M]
+; found: 901 (100); elemental analysis: calcd
(%) for C47H36IrNO2P2: C 62.66, H 4.03, N 1.55; found: C 62.69, H 4.35,
N 1.31.
Preparation of [(piq)IrACHTUNGTRENNUNG(P^O)2] (5) and [(nazo)IrACHTUNGTRENNUNG(P^O)2] (6). Following
the procedure described for 4, treatment of IrCl3·3H2O and piqH (or
nazoH), then with P^OH in a molar ratio of 1:1:2 provides the orange complexes [(piq)IrACHTUNGTRENNUNG(P^O)2] (5) and [(nazo)IrACHTUNGTRENNUNG(P^O)2] (6) in 30 % and 5 %
Spectral data for 5.1H NMR (400 MHz, CDCl 3, 298 K, TMS): d = 8.46 (d, JH,H=6.4 Hz 1 H), 8.36 (td, JH,H=7.6, 1.6 Hz, 2 H), 8.19 (td, JH,H=7.0, 1.6 Hz, 2 H), 8.02 (d, JH,H=8.8 Hz, 1 H), 7.54 (d, JH,H=7.6 Hz, 1 H), 7.47– 7.30 (m, 10 H), 7.23–7.20 (m, 2 H), 7.15 ~ 7.00 (m, 4 H), 6.93 ~ 6.88 (m, 2 H), 6.70–6.64 (m, 2 H), 6.50–6.39 (m, 7 H), 6.22–6.13 ppm (m, 4 H);31 P-{1H} NMR (202 MHz, CDCl 3, 298 K, TMS): d = 15.32 (d, JP,P=355 Hz,
1P), 12.79 ppm (d, JP,P=355 Hz, 1P); MS (FAB,192Ir): m/z (%) calcd for
C51H38IrNO2P2: 951.02 [M]
+; found: 951 (100); elemental analysis: calcd
(%) for C51H38IrNO2P2: C 64.41, H 4.03, N 1.47; found: C 64.87, H 4.28,
N 1.45.
Spectral data for 6.1H NMR (400 MHz, CDCl
3, 298 K, TMS): d = 9.17 (s, 1 H), 8.42–8.38 (m, 2 H), 8.24–8.23 (m, 2 H), 7.98 (d, JH,H=8.4 Hz, 1 H), 7.77 (dd, JH,H=8.0, 0.8 Hz, 1 H), 7.63 (td, JH,H=6.6, 1.2 Hz, 1 H), 7.46– 7.34 (m, 9 H), 7.20–7.12 (m, 5 H), 7.06–7.00 (m, 1 H), 6.93–6.92 (m, 1 H), 6.74–6.71 (m, 2 H), 6.51–6.43 (m, 7 H), 6.22–6.17 ppm (m, 4 H);31P-{1H} NMR (202 MHz, CDCl3, 298 K, TMS): d = 15.31 (d, JP,P=346 Hz, 1P),
13.19 ppm (d, JP,P=346 Hz, 1P); MS (FAB, 192Ir): m/z (%) calcd for
C50H37IrN2O2P2: 952.01 [M] +
; found: 952 (100); elemental analysis: calcd (%) for C50H37IrN2O2P2: C 63.08, H 3.92, N 2.94; found: C 62.72, H 3.65,
N 2.91.
Measurement of Photophysical Data
Steady-state absorption and emission spectra were recorded by a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920) fluorimeter, spectively. Both the wavelength-dependent excitation and emission re-sponse of the fluorimeter were calibrated. A configuration of front-face excitation was used to measure the emission of the solid sample, in which the cell was made by assembling two edge-polished quartz plates with various Teflon spacers. A combination of appropriate filters was used to avoid the interference from the scattering light. Lifetime studies were performed by an Edinburgh FL 900 photon-counting system with a hy-drogen-filled/or a nitrogen lamp as the excitation source. Data were ana-lyzed using the nonlinear least squares procedure in combination with an iterative convolution method. The emission decays were analyzed by the sum of exponential functions, which allows partial removal of the instru-ment time broadening and consequently renders a temporal resolution of ~ 200 ps.
Electrochemical Measurement Cyclic voltammetry (CV) measure-ments were performed using a BAS
100 B/W electrochemical analyzer.
The oxidation and reduction measure-ments were recorded using Pt wire and a Au disk coated with Hg as work-ing electrodes, respectively. Experi-ments were performed in anhydrous CH2Cl2 and anhydrous THF
contain-ing 0.1 m (TBA)PF6 as the supporting
electrolyte, at a scan rate of 50 m V s1.
The potentials were measured against a Ag/AgCl (0.01 m AgNO3) reference
electrode with ferrocene as the inter-nal standard.
X-ray Structural Analysis
Single crystal X-ray diffraction data of complexes 3 and 4 were measured on a Bruker Smart CCD diffractometer using lACHTUNGTRENNUNG(MoKa) radiation (l =
0.71073 ). The data collection was executed using the SMART program. Cell refinement and data reduction were made with the SAINT program. The structure was determined using the SHELXTL/PC program and re-fined using full-matrix least squares. All non-hydrogen atoms were refined
anisotropically, whereas hydrogen atoms were placed at the calculated positions and included in the final stage of refinements with fixed param-eters. The respective crystallographic refinement parameters are summar-ized in Table 5.
CCDC 717863 and CCDC 717864 contain the supplementary crystallo-graphic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre at www.ccdc.cam. ac.uk/data_request/cif.
Computational Methodology
Time-dependent PBE0 calculations are based on the geometry optimized structures at the PBE0 level. The basis set for the geometry optimization and the excitation energy calculation are both a double-z quality basis set consisting of Hay and Wadts quasi-relativistic effective core poten-tials (LANL2DZ) for an IrIIIatom;[33]a 6-31G* basis set was employed
for the H, C, N, O, and P atoms. Typically, the lowest triplet and singlet roots of the nonhermitian eigenvalue equations were obtained to deter-mine the vertical excitation energies. Oscillator strengths were deduced from the dipole transition matrix elements (for singlet states only). All the calculations were performed with the Gaussian 03 package.[34]
Fabrication of Light-Emitting Devices
The EL devices were fabricated by vacuum deposition of the materials at 106torr onto a clean glass that was pre-coated with a layer of indium tin
oxide with a sheet resistance of 25 W/&. Various organic layers were de-posited sequentially at a rate of 1–2 s1. Phosphorescent dopant was
co-evaporated along with CBP by two independent source reservoirs. A thin layer of LiF (1 nm) and a thick layer of Al (150 nm) were sequen-tially deposited at the cathode. The active area of the emitting diode was 9.00 mm2. The current-voltage-luminance of the devices was measured in
ambient conditions with a Keithley 2400 Source meter and a Newport 1835C Optical meter equipped with an 818ST silicon photodiode. The EL spectrum was obtained using a Hitachi F4500 spectrofluorimeter. Lifetime Measurements
All devices for lifetime measurements were prepared by thermal evapo-ration in a high vacuum system with a pressure better then 5 104Pa
without breaking the vacuum. ITO-coated glass plates with a surface re-sistivity 15 W/& were used as substrate. They were ultrasonically
Table 5. Crystal data and structure refinement parameters for complexes 3 and 4.
complex 3·2 CHCl3·1/2 C6H14 4·THF
Empirical formula C51H41Cl6IrN4OP C51H44IrNO3P2
Formula mass [g mol1] 1161.75 973.01
T [K] 150(2) K 150(2) K
Crystal system triclinic monoclinic
Space group P1¯ P21/c a [] 9.6916(5) 9.3059(6) b [] 12.7650(6) 27.004(2) c [] 20.348(1) 16.545(1) a [8] 102.604(1)8 908 b [8] 100.826(1)8 98.440(1)8 g [8] 102.647(1)8 908 Volume, Z 2322.5(2) 3, 2 4112.7(5) 3, 4 1calcd[Mg m3] 1.661 1.571 m [mm1] 3.298 3.370 FACHTUNGTRENNUNG(000) 1154 1952 Crystal dimensions [mm] 0.40 0.17 0.17 0.25 0.12 0.05 Reflections collected 29 994 26 613
Independent reflections 10 631 [RACHTUNGTRENNUNG(int) =0.0382] 9422 [RACHTUNGTRENNUNG(int) =0.0481]
Max. and min. transmission 0.6040 and 0.3521 0.7293 and 0.4862
Data/restraints/parameters 10 631/0/578 9422/11/518
Goodness-of-fit on F2 1.059 1.019
Final R indices [I 2 s(I)] R1=0.0311, wR2=0.0686 R1=0.0334, wR2=0.0704
R indices (all data) R1=0.0349, wR2=0.0701 R1=0.0475, wR2=0.0763
cleaned at first and then treated by oxygen plasma for work function tuning. During the evaporation process the deposition rates were moni-tored by several controllers, which were calibrated by a Dektak 6 m sur-face profiler from Veeco. These devices were encapsulated with an UV curing adhesive in a nitrogen atmosphere before lifetime testing. The OLED lifetime test system Polaronix M6000 from Mac Science was used for lifetime measurement in an accelerated mode constant current mode at room temperature.
Acknowledgements
This work was funded by the National Science Council of Taiwan, R.O.C. under grants: NSC 93-2113M-007-012 and NSC 93-2752M-002-002-PAE. We are also grateful to the National Center for High-performance Com-puting for computer time and facilities.
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Received: December 15, 2008 Revised: January 29, 2009 Published online: March 17, 2009
