DOI: 10.1002/asia.200800211
Pt
IIComplexes with 6-(5-Trifluoromethyl-Pyrazol-3-yl)-2,2’-Bipyridine
Terdentate Chelating Ligands: Synthesis, Characterization, and Luminescent
Properties
Jing-Lin Chen,
[a]Sheng-Yuan Chang,
[a]Yun Chi,*
[a]Kellen Chen,
[a]Yi-Ming Cheng,
[b]Chun-Wei Lin,
[b]Gene-Hsiang Lee,
[b]Pi-Tai Chou,*
[b]Chen-Hao Wu,
[c]Ping-I Shih,
[c]and
Ching-Fong Shu*
[c]Introduction
Square-planar d8-PtII complexes have attracted a great deal
of interest because of their intriguing spectroscopic and pho-tophysical properties, as well as their promising applications in optical-power limiting,[1] electroluminescence,[2]
chemo-sensors,[3]anticancer medicine,[4]and photocatalysis.[5]These
PtIIcomplexes exhibit bright emission in solution, attributed
to a combination of ligand-centered pp* and metal-to-ligand charge transfer (MLCT) transitions.[6] Moreover,
owing to the higher tendency in forming Pt···Pt and p···p in-teractions in the condensed phase, the respective emission often shows a clear red shift relative to the emission record-ed in a dilute solution. These transitions are often denotrecord-ed as either metal–metal-to-ligand charge transfer (MMLCT)[7]
or excimeric ligand-to-ligand charge transfer emission.[8]
Among the chelating ligands documented in the literature, 2,2’-bipyridine (Figure 1) is the most popular neutral ligand, and it can interact with a metal atom using simple dative bonding involving N-donor atoms. On the other hand, 2-phenylpyridine is known for its capability of undergoing a cyclometalation reaction, by supplying the anionic bonding character with respect to the central metal atom.[9]In sharp
contrast, C-linked 2-pyridyl azoles, such as pyrazoles, are known to serve as both neutral chelating ligands[10]or
anion-ic chelates by deprotonation of the pyrazolyl-NH bond.[11]
The latter process is conceptually related to the cyclometala-tion. In fact, these functionalized C-linked 2-pyridyl pyra-zoles have served as excellent candidates in stabilizing and Abstract: A series of PtIIcomplexes
ACHTUNGTRENNUNG(fpbpy)Cl (1), PtACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(OAc) (2), Pt-ACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(NHCOMe) (3), Pt ACHTUNGTRENNUNG(fpbpy)-ACHTUNGTRENNUNG(NHCOEt) (4), and [Pt ACHTUNGTRENNUNG(fpbpy)-ACHTUNGTRENNUNG(NCMe)]ACHTUNGTRENNUNG(BF4) (5) with deprotonated
6-(5-trifluoromethyl-pyrazol-3-yl)-2,2’-bipyridine terdentate ligand are pre-pared, among which 1 is converted to complexes 2–5 by a simple ligand sub-stitution. Alternatively, acetamide com-plex 3 is prepared by hydrolysis of ace-tonitrile complex 5, while the back
con-version from 3 to 1 is regulated by the addition of HCl solution, showing the reaction sequence 1!5!3!1. Multi-layer OLED devices are successfully fabricated by using triphenyl-(4-(9-phenyl-9H-fluoren-9-yl)phenyl) silane (TPSi-F) as host material and with
doping concentrations of 1 varying from 7 to 100 %. The electrolumines-cence showed a substantial red-shifting versus the normal photoluminescence detected in solution. Moreover, at a doping concentration of 28 %, the device showed a saturated red lumines-cence with a maximum external quan-tum yield of 8.5 % at 20 mA cm2and a
peak luminescence of 47 543 cd m2 at 18.5 V.
Keywords: charge transfer · chelates · N ligands · platinum · OLEDs
[a] Dr. J.-L. Chen, S.-Y. Chang, Prof. Y. Chi, K. Chen Department of Chemistry
National Tsing Hua University Hsinchu 30013 (Taiwan) Fax: (+ 886) 3-572-0864 E-mail: [email protected]
[b] Dr. Y.-M. Cheng, C.-W. Lin, Dr. G.-H. Lee, Prof. P.-T. Chou Department of Chemistry and Instrumentation Center National Taiwan University
Taipei 10617 (Taiwan) Fax: (+ 886) 2-2369-5208 E-mail: [email protected]
[c] C.-H. Wu, P.-I. Shih, Prof. C.-F. Shu Department of Applied Chemistry National Chiao Tung University Hsinchu 30010 (Taiwan) E-mail: [email protected]
fine-tuning the chemical and photophysical properties of metal complexes.[12]
Parallel to the chemistry of bidentate cyclometalating che-lates, 6-phenyl-2,2’-bipyridine is probably the most widely inves-tigated terdentate cyclometalat-ing ligand for stabilization of the PtII–metal framework.[13]
Significant study has also been related to the system involving symmetrical 1,3-di(2-pyridyl)-benzene, for which the cyclo-metalated ligand is located at
the central position of the terdentate ligand.[14]Encouraged
by the success of these attempts, we decided to focus on dis-tinctive bidentate chelates, such as 2-pyridyl pyrazole,[15]and
terdentate ligands, like 6-(5-trifluoromethyl-pyrazol-3-yl)-2,2’-bipyridine, denoted as fpbpyH.[16] It is noted that the
electron-withdrawing CF3substituent of fpbpyH not only
in-creases the pyrazolic NH acidity and activity, but also mini-mizes the unwanted side reaction by blocking coordination to the adjacent nitrogen atom, allowing a stable and desig-nated terdentate bonding. The resulting tailor-made ligand
In this study, we focused on the preparation, characteriza-tion, and potential OLED application of a series of PtII
com-plexes derived from PtACHTUNGTRENNUNG(fpbpy)Cl (1). As shown in Scheme 1, PtIIcomplexes 2, 3, and 4 can be isolated by simple
substitu-tion of the chloride in 1 by acetate, acetamide, and propio-namide, respectively. Intriguing photophysical properties of complexes 1, 2, and 3, in both solution and solid phases were investigated. Moreover, the acetamide complex 3 can be obtained by in-situ hydrolysis of acetonitrile complex 5 in
basic media, and the release of acetamide by back-conver-sion to 1 upon addition of dilute HCl solution. This pattern of reactivity also revealed one additional case for the metal-catalyzed hydrolysis of acetonitrile to acetamide. As a result, the reaction sequence documented here can serve as a useful mechanistic model for the PtII-catalyzed hydrolysis
of organonitriles.[17]Finally, bright luminescence of 1 in the
solid state has also encouraged us to utilize this material as a dopant for the fabrication of red-emitting phosphorescent organic light-emitting diodes (OLED)s.
Results and Discussion
Synthesis and Characterization
The required terdentate chelate fpbpyH was obtained by a Claisen condensation reaction employing 6-acetyl-2,2’-bipyr-idine and ethyl trifluoroacetate, followed by treatment with an excess of hydrazine hydrate in refluxing ethanol accord-ing to procedures reported in the literature.[16, 18]This ligand
has a high potential for forming the anionic tridentate che-late by facile deprotonation during the reaction with a tran-sition-metal reagent in basic media.
The key starting material, that is, complex 1, was synthe-sized by heating the fpbpyH ligand and K2PtCl4reagent in a
1:1 mixture of acetonitrile and water. Single red crystals were obtained by cooling the saturated mixture in DMSO and acetone. Its molecular composition was initially con-firmed by the detection of all aromatic fpbpy signals in the
1H NMR spectrum. Moreover, the addition of concentrated
Abstract in Chinese:
Figure 1. Chemical structures of some common chelating ligands.
Scheme 1. Schematic for PtIIcomplexes 2, 3, and 4 isolated by simple substitution of the chloride in 1 by
ace-tate, acetamide, and propionamide, respectively. i) AgOAc in DMF, 1008C, ii) NH2COR (R = Me or Et),
HCl to 1 failed to protonate the noncoordinated pyrazolyl-nitrogen atom on 1. This result is in sharp contrast to the re-lated bidentate ligands, 2-pyridyl pyrazole and 1-isoquinolin-yl indazole, which are capable of forming PtII complexes
with the neutral chelating ligand.[19]
Crystal structural analysis shows that complex 1 adopts a distorted square-planar geometry as shown in Figure 2. The NPtN bond angles, N(1)PtN(2) = 80.0(4)8, N(2)Pt N(3) = 80.6(4)8, N(1)PtN(3) = 160.6(3)8 (see Table 1)
de-viate slightly from the idealized values of 908 and 1808 as a consequence of the internal constraint imposed by the fpbpy ligand. These results are common for PtII complexes with a
terdentate-cyclometalated chelate.[20]All the PtN bond
dis-tances, PtN(1) = 2.025(9) , PtN(2) = 1.931(9) , and Pt N(3) = 1.987(9) , are comparable to those found in other PtII complexes possessing a bipyridyl group.[21] The central
PtN distance of the ligand fpbpy is shorter than those of the outer nitrogen atoms, mainly because of the steric demand as documented in the literature.[18] The PtCl(1)
distance of 2.314(3) is comparable to that of the other
PtII-chloride complexes.[22] Furthermore, complex 1
exhibit-ed a head-to-tail stacking along the b axis of the crystal lat-tice, giving a zigzag [Pt]n metal chain with intermolecular
Pt···Pt distances of 3.385 and a PtPtPt angle of 153.598. This stacking pattern is similar to those observed in other linear-chain PtII complexes that show intermolecular Pt···Pt
interaction and strong p···p stacking.[23]However, there is a
reduced degree of p···p stacking between the fpbpy ligands in 1 arising from the alternating arrangement of this chelate. Reactivity studies showed that treatment of 1 with silver acetate resulted in the formation of the acetate-substituted derivative 2, for which the Ag+ cation served as the chloride
scavenger, allowing the acetate to occupy the fourth coordi-nation site of the PtII cation. Similarly, treatment of 1 with
acetamide or propionamide in the presence of Na2CO3
af-forded the respective amido complexes 3 and 4 in refluxing ethanol solution. Additional support was given by the
1H NMR spectral analysis, from which the detection of
methyl and ethyl signals for 3 and 4 confirmed their exis-tence, while the unique NH proton of the acetamide ligand occurred at d 5.58 (3) and d 5.60 (4), which are within the
1H NMR spectral range (d 4.99–5.78) reported for other
PtII–acetamide complexes.[24]
The single crystals of 3 could be obtained by slow cooling of the supersaturated solution in DMSO to room tempera-ture. The solid-state structure of 3 is studied by X-ray dif-fraction analysis. Figure 3 shows only one crystallographical-ly independent molecule in the asymmetric unit. As shown Figure 2. ORTEP drawing of 1 showing atom-labeling scheme with 30 %
thermal ellipsoids and the Pt···Pt interaction in the crystal lattices.
Table 1. Selected bond length () and angles (8) for complexes 1 and 3. Complex 1 PtN(1) 2.025(9) Pt-N(2) 1.931(9) PtN(3) 1.987(9) Pt-Cl(1) 2.314(3) Pt···Pt 3.385 N(2)PtN(1) 80.6(4) N(2)-Pt-N(3) 80.0(4) N(1)PtCl(1) 97.5(3) N(3)-Pt-Cl(1) 102.0(3) N(3)PtN(1) 160.6(3) N(2)-Pt-Cl(1) 178.0(3) Complex 3 Pt(1)N(1) 2.043(6) Pt(1)N(2) 1.951(6) Pt(1)N(3) 1.996(5) Pt(1)N(5) 2.012(6) Pt(1)···Pt(1) 4.587 4.968 N(2)Pt(1)N(1) 79.9(2) N(2)Pt(1)N(3) 80.4(2) N(5)Pt(1)N(1) 104.2(2) N(3)Pt(1)N(5) 95.5(2) N(3)Pt(1)N(1) 160.3(2) N(2)Pt(1)N(5) 173.5(2)
Figure 3. (a) ORTEP drawing of one crystallographic independent mole-cule of 3 with 30 % thermal ellipsoids, and (b) the side view that depicted the accompanying packing diagram in the crystal lattices.
in Table 1, all metrical parameters are very similar to those of 1, which shows virtually identical PtN bond distances, as well as closely related bite angles, to the fpbpy ligand, while the accompanying PtNacetamide distances occur in the range
of 2.012(6)–2.010(6) , which are comparable to those of the PtII–amidate complexes reported.[25]Moreover, 3
exhibit-ed head-to-tail and zigzag stacking along the a axis, with two pairs of alternating Pt···Pt nonbonding distances (3.569 and 5.683 for distances between Pt(2) atoms, and 4.587 and 4.968 for distances between Pt(1) atoms) which are significantly longer than that of 1, but are akin to those ob-served in other stacked PtII complexes.[26] It is notable that
the shortest Pt–Pt distance in 3 is far longer than the theo-retical limit of 3.5 , which is the maximum distance for ef-fective overlap between Pt-dz2 and -pz orbitals. As a result,
complex 3 showed the existence of only weakly bonded dimers, if there is such nonbonding interaction at all in the solid state. On the other hand, in sharp contrast to that of 1, the fpbpy chelate of each Pt molecule exhibits a favorable side-to-side stacking with one another (see Figure 3 b). The inter-planar distances are estimated in the range of 3.3– 3.5 , which is below the upper limit of 3.8 for typical p···p interactions detected for aromatic compounds.[27]
It is worthwhile to note that the chloride complex 1 react-ed slowly with Na2CO3in refluxing acetonitrile. The
suspen-sion first turned into a transparent solution, showing the de-pletion of the starting material and the formation of some soluble intermediate. Then, a trace amount of precipitate began to deposit from the solution. The result is consistent with the sequential conversion from the intermediate, which is highly soluble in acetonitrile, to the less soluble acetamide complex 3. Finally, the solution turned into a turbid yellow suspension, which showed the complete formation of the end product 3. For further investigation and confirmation of this stepwise transformation, reaction of 1 with one equiv of AgBF4 in anhydrous acetonitrile was conducted. A highly
soluble, orange–yellow complex identified as the acetonitrile complex [PtACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(NCMe)]ACHTUNGTRENNUNG(BF4) 5 was afforded. Complex
3 is attainable by treatment of 5 with a slight excess of Na2CO3 in mixed acetonitrile and water, confirming the
status of 5 as the intermediate for the aforementioned trans-formation of 1 to 3 (Scheme 2). For the final confirmation, the propionamide complex 4 was also accessible by treat-ment of 1 with AgBF4, followed by the addition of both
pro-pionamide and Na2CO3 in DMF at elevated temperature,
while both 3 and 4 could be reverted to 1 by treatment with HCl (2 m) upon reflux, showing the sequential ligand trans-formation from chloride, nitrile, acetamide, and back to the chloride.
Photophysical Properties
The UV/Vis absorption and emission spectra of complexes 1, 2, and 3 in CH2Cl2 at 298 K are shown in Figure 4. All
pertinent spectroscopic data are tabulated in Table 2. To gain further insight into the photophysical behavior of all titled complexes, density functional theory (DFT) was also applied to access molecular-orbital information. As a result, those HOMO and LUMO that are mainly involved in the lowest-lying transition are depicted in Figure 5 and the de-scription of the energy gap of each transition is listed in Table 3. It has been noticed that TDDFT (time-dependent density functional theory) yields substantial errors for the excitation energies of charge-transfer excited states, when local functionals, such as LDA or GGA, are used.[28]
Never-theless, it has been widely recognized as an efficient method to explore the electronic structures of certain organometallic complexes. Furthermore, the continuum-solvation model of CH2Cl2(IEFPCM) used in this approach might also
contrib-ute to some errors, which leads to a further deviation of the results obtained from the calculations as compared to those from experiment. However, after a qualitative comparison of Tables 2 and 3, the calculated energy gaps agree satisfac-torily with the experimentally obtained photophysical data, suggesting that the TDDFT calculations are able to predict the photophysical behavior of these PtII complexes to a
cer-tain degree.
For complex 1, there are several absorption bands ranging from 250–370 nm with intense molar extinction coefficients e > 104m1cm1 and a relatively weaker transition (e ~ 2.4
103m1cm1) at longer wavelengths (maximized at
~ 400 nm). With reference to previous work on PtII
com-plexes possessing the C^N^N and N^N^N terdentate li-gands,[22, 29] the absorptions at 250–370 nm most likely
origi-nate from the ligand-centered1pp* transitions. Further
sup-port is given by the close matching of several absorption bands in the high-energy region ( 370 nm) in complexes 1, 2, and 3 with the same ligand, fpbpy. Moreover, as shown in the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of com-plex 1 (see Figure 5), the elec-tron density shifted from the central metal ion to the ligands, indicating a metal-to-ligand charge transfer (MLCT) tion of the lowest singlet transi-tion. Also, the MLCT character for the lowest singlet transition of 1 was calculated to be 20.9 % (see Table 3). Therefore, the Scheme 2. Schematic for complex 3 obtained by treatment of 5 with slight excess of Na2CO3in mixed
broad-band absorption at ~ 400 nm can be reasonably as-signed as the metal-to-ligand charge transfer transition (1MLCT) from the d
porbital of the (5d8) Pt-metal center to
the unoccupied p* orbital of the ligand (fpbpy), mixed with the intra-ligand pp* transition inside fpbpy. The frontier orbi-tals of the HOMO and LUMO in complex 2 (not shown here) are essentially similar to that of 1. This can be further supported by the closely matched absorp-tion and emission spectra be-tween 1 and 2. Accordingly, for 2, the strong absorption bands ( 370 nm) are thus assigned to the ligand-centered1pp*
transi-tions, and the comparatively weak absorption at around 410 nm is attributed to the ligand-centered1pp* transitions
mixed with a certain degree of MLCT. On the same basis, the intense absorption bands of 3 at 250–370 nm can thus be as-signed as the intra-ligand 1pp*
transitions of the fpbpy ligand. Moreover, the relatively weaker absorption at 418 nm can be attributed to the spin-allowed 1MLCT transitions
(17.5 %) mixed with the ligand-centered pp* transition (see Table 3). However, it is interesting to note that the predomi-nant ligand-centered pp* transition is ascribed to an inter-ligand charge transfer transition from acetamide to the bi-pyridyl fragment of fpbpy ligand according to the result from the molecular-orbital analyses (see Figure 5).
In a dilute solution of CH2Cl2, 1 (8.46 10 6
m), 2 (7.33 106m), and 3 (9.0 106m) exhibited emission with a peak
wavelength at 560 nm, 557 nm, and 604 nm, respectively. The radiative rate constant (kr) is defined by kr=kobs Fem,
in which, kobsand Fem denote the observed decay rate
con-stant and emission quantum yield, respectively. Accordingly, by using the data listed in Table 2, kris calculated to be 7.5
104s1
for 1, 1.1 104s1
for 2, and 4.4 104s1
for 3. This result leads to the unambiguous confirmation that the emis-sion is phosphorescence. The theoretical calculation also showed satisfying results in predicting the T1–S0 transition
gap of 1, which closely fits the emission band, with 19.4 % MLCT character (see Table 3). However, with regard to the spectral features, complexes 1 and 2 revealed emission with Figure 4. UV/Vis absorption and emission spectra of 1 (-&-), 2 (-~-), and 3 (-!-) recorded in a dilute CH
2Cl2
solution at RT (see text). The gray lines with the open symbols (-&-~-!-) depicted the emission spectra of the
respective single crystal samples. Note that the emission intensity is in arbitrary units.
Table 2. UV/Vis absorption and emission data of 1 and 3 in CH2Cl2at RT.
Abs lmax[nm] (e 103m1cm1) Em lmax[nm] t [ns] Fem kr
1 273 (26.4), 317 (13.3), 343 (9.2), 363 (8.8), 400 (2.4) 524, 560 ACHTUNGTRENNUNG(675)[a] 173 ACHTUNGTRENNUNG(113)[b] 1.3 102 ACHTUNGTRENNUNG(0.2)[b] 7.3 104 ACHTUNGTRENNUNG(1.7106)[b] 2 270 (27.5), 314 (14.0), 343 (10.0), 365 (8.9), 400 (2.1) 522, 557 ACHTUNGTRENNUNG(656)[a] 187 ACHTUNGTRENNUNG(26.0)[a] 2.1 103 (–)[c] 1.1 104 3 274 (27.1), 324 (15.7), 418 (2.9) 604 (505, 543, 582, 636)[a] 2.45 ACHTUNGTRENNUNG(292)[a] 1.0 102 (–)[c] 4.4 104
[a] The data in the parentheses are obtained from the single crystalline samples. [b] The solid-state photophysical data of 1 was measured using a vacuum-deposited thin film sample. [c] Sample decomposed during preparation of thin film.
notable vibronic progressions as opposed to a featureless pattern in 3, despite complexes 1–3 all possessing the same fpbpy ligand and a similar MLCT percentage at the triplet manifold, that is, 19.4 % for 1, 18.5 % for 2, and 19.8 % for 3. This drastic difference again evidenced the intrinsic differ-ences in their frontier orbitals, being intra-ligand for 1 and 2 versus inter-ligand charge transfer for 3, involved in the lowest lying transition.
As for the red-shifted lowest lying absorption of 3 with re-spect to 1 and 2, one plausible explanation might lie in the difference in bonding strength of the chloride or acetate versus acetamide to the central PtIIcation. The anion could
probably contribute its electron densities to the d orbital of the central PtII-metal ion, leading to a stronger dative
inter-action. Consequently, the orbital energy of the central PtII
ion could be increased arising from the conveying of elec-tron density from this anion, and thereby decreasing the electron-transition gap. Accordingly, the donor strength of acetamide in 3 is believed to be stronger than that between the chloride (acetate) and PtII in 1 (or 2), resulting in a
smaller energy gap in 3.
It is also believed that the p-conjugation may also play a certain role to account for the red-shifted emission of 3 with respect to 1 and 2. As shown in Figure 5, for 3, the frontier orbitals have been extensively extended to the acetamide. Particularly, both the HOMO-5 and HOMO in 3 clearly in-dicate the involvement of the nitrogen and the carbonyl group of acetamide. With such an enlarged conjugated p system, the HOMO–LUMO gap decreases, such that the emission of 3 is red-shifted with respect to 1. For further confirmation, Figure 6 depicts a plot of the partial density of states, in which the contributions of different moieties to specific frontier orbitals are illustrated. The large percentage of the acetamide contribution in the HOMO assures its piv-otal role in the pp* transition, and renders firm support for its photophysical properties. As for complex 1 (or 2), the electron-density distribution in the HOMO (see Figure 5) indicates nearly no participation of the chloride (or acetate, not shown here because of the similarity) to the overall con-jugated system. As a result, its influence to the electron den-sity of the PtII ion is apparently smaller than that in 3.
Ac-cordingly, Figure 6 further illustrates the reduced electron-density contribution of the chloride to the HOMO.
As shown in Figure 7, the emission profile of 1 exhibits apparent concentration dependences. Upon increasing the
concentration of 1 in CH2Cl2 from 8.46 10
6m to 1.21
104m, in addition to the ~ 560 nm emission peak (defined as
the F1 band), a lower energy emission band maximum at
~ 660 nm (defined as the F2 band) gradually increases,
ac-companied by a decrease of the F1 band. The excitation
spectra monitored at either F1 (e.g., 520 nm) or F2 band
(e.g., 700 nm) are identical to each other (not shown here) and to the absorption spectrum, indicating that both emis-sion bands originate from the same ground-state species. Knowing there exists a higher tendency for forming multiple Pt···Pt interactions in the condensed phase, it is thus reason-able to ascribe the F2band to originate from either metal–
metal-to-ligand charge transfer (MMLCT)[7] or excimeric
ligand-to-ligand charge transfer emission.[8]Further evidence
is given by the emission spectrum of 1 recorded using a single-crystal sample (see the Experimental Section). Com-plex 1 exhibits a well-aligned head-to-tail stacking along the b axis of crystal lattices, with intermolecular Pt···Pt distances Table 3. The calculated energy levels and orbital-transition analyses of 1
and 3 in the continuum-solvation model of CH2Cl2(IEFPCM model, see
Experimental Section).
State lcal[nm] f Assignment MLCT %
1 T1 512.9 HOMO!LUMO (93 %) HOMO-5!LUMO (7 %) 19.4 S1 448.4 0.0022 HOMO!LUMO (88 %) HOMO-1!LUMO (6 %) 20.9 3 T1 519.8 HOMO!LUMO (95 %) HOMO-5!LUMO (5 %) 19.8 S1 465.4 0.0342 HOMO!LUMO (93 %) 17.5
Figure 6. The spectrum of partial density-of-states of complexes 1 (upper) and 3 (bottom).
Figure 7. The emission spectra of 1 excited at 284 nm in CH2Cl2solution
at RT by varying the concentrations from (a) 1.21 104
m(-*-), (b) 9.63
105m, (c) 7.20 105m, (d) 5.76 105m, (e) 4.43 105m, (f) 2.23 105m,
(g) 1.15 105
m,and (h) 8.46 106m(solid line). All emission spectra are normalized at 565 nm.
as short as 3.385 (as mentioned previously). As a result, 1 as a single-crystal sample illustrated a stacking perturbed emission maximum at 675 nm, the peak of which is consis-tent with that of the excimeric-like emission, that is, the F2
band of 660 nm, for a solution of 1 in CH2Cl2. The
concen-tration-dependent emission features have also been ob-served in 2 (not shown here), supporting its similar stacking framework as that of complex 1. Conversely, in the case of 3, the emission feature remained unchanged up to 1 104m in CH2Cl2. This can plausibly be rationalized by the
relative-ly small tendency for self-association in 3, as it forms onrelative-ly dimers in the crystal lattices (see Figure 3), while complex 1 (or 2) showed strong aggregation to afford the [Pt]n
long-chain arrangement. Thus, as opposed to the largely red-shift-ed emission of 1 (the F2 band) in crystal versus that
ob-served in CH2Cl2solution (the F1band), the emission of 3 in
solid crystal revealed a slightly blue-shifted emission (as compared to the emission in solution), possessing notable vi-bronic progression possibly caused by the space confine-ment, that is, the rigidity of molecular geometry.
To gain more insights into the excimer formation, the re-laxation dynamics of monomer and excimer were obtained for an aerated solution of 1 in CH2Cl2 (1.1 10
4
m). By monitoring the emission at 500 nm, the lifetime of monomer is fitted to a single exponential to give a decay of ~ 126 ns, while the relaxation of the excimer monitored at 700 nm is composed of a rise and decay component of 112 ns and 117 ns, respectively. The rise component, considering the un-certainty arising from the spectral fitting, is the same as the decay of the monomer. This result firmly supports the pre-cursor–successor relationship between the monomer and the excimer in the excited state.
Furthermore, assuming that the concentration of 1 in the excited state is relatively small when compared to that of the ground state, the mechanism of excimer formation can be expressed using a pseudo first-order approximation: A*k1½A
!ðA AÞ* k3
! A þ A ð1Þ
A* k2
! A ð2Þ
Thus, the time-dependent concentrations of excited mono-mer, A*, and excimono-mer, (AA)*, can be expressed as
A*ðtÞ ¼ A*ð0Þ exp kð ð 1½AÞ þ k2Þ ð3Þ
ðA AÞ*ðtÞ ¼ k1½AA*ð0Þ k1½A þ k2þ k3
expðk3tÞ exp kð ð 1½A þ k2ÞÞt
ð Þ
ð4Þ
where k1 denotes the bimolecular rate constant of excimer
formation, k2stands for the overall relaxation rate constant
except for the contribution from excimer formation, and k3
represents the relaxation rate constant of excimer. Note that Equation (1) was derived under the assumption that the re-verse process involving (AA*)!A* + A is energetically prohibited.
As a result, the plot of observed decay rate constant of the excited monomer (A*) as a function of concentration [A] reveals a straight line (see Figure 8). This result is ex-pected because the observed decay rate constant of the mo-nomer, in theory, is the sum of k1[A] + k2 [Eq. (3)]. The
slope and intercept are then deduced to be 7.2 1010m1
s1 (k1) and 7.1 106s1(k2), respectively.
The efficiency of the excimer formation, Feff, is defined
as:
Feff ¼
k1½A
kobs
ð5Þ
With the known emission quantum yield, Fmon, of
mono-meric 1, the emission quantum yield of the excimer, Fex, can
be expressed as: Fex¼ Fmon 1 Feff Iex Feff Imon ð6Þ
where Imon and Iex denote the integrated emission intensity
of excited monomer and excimer, respectively.
As a simple approach, by knowing k1, kobs (monomer),
and for example, [A] = 1.1 104m, Feffis then calculated to be 10.6 %. Fmonhas been determined to be 0.01 in dilute
so-lution (see Table 2). Under [A] = 1.1 104m, the
deconvo-luted Iex/Imon is calculated to be 0.28. As a result, Fex is
de-duced to be 2.3 %. The observed rate constant for the exci-mer emission is measured to be 8.5 106s1
. Therefore, the radiative decay rate constant of the excimer emission is de-duced to be 2.0 105s1and is larger than that (7.3 104s1)
of the monomer emission. This result is fundamentally intri-guing, indicating that the contribution of the metal-dp
orbi-tal increases upon excimer formation. Theoretically, this can be rationalized by the formation of PtII–PtII d
z2 interaction in the excited state, resulting in a raise of the dz2 energy
Figure 8. The plot of observed decay rate constant of excited monomer 1 as a function of the concentration. The slope and intercept of the fitted results are 7.2 109and 7.1 106, respectively.
level (HOMO), and hence the reduction of the energy gap, as well as the increase of the MLCT contribution. This pro-posed mechanism is consistent with the packing arrange-ment of 1 revealed in the single crystal, in which the p stack-ing between two adjacent complexes is obscure in compari-son to the major PtII–PtIIinteraction.
OLED Fabrication
The PtII complex 1 exhibits excellent thermal stability and
good phosphorescence efficiency, which are desirable for light-emitting diode applications. To study the device per-formances of 1, the multilayer devices of the configuration ITO/NPBACHTUNGTRENNUNG(30 nm)/mCPACHTUNGTRENNUNG(10 nm)/TPSi-F:1ACHTUNGTRENNUNG(40 nm)/BCP-ACHTUNGTRENNUNG(10 nm)/TPBIACHTUNGTRENNUNG(30 nm)/LiFACHTUNGTRENNUNG(1 nm)/Alnm)/BCP-ACHTUNGTRENNUNG(100 nm) were prepared. The abbreviations NPB, mCP, TPSi-F, BCP, and TPBI stand for 4,4’-bis(N-(1-naphthyl)-N-phenylamino)biphenyl, 1,3-bis(9-carbazolyl)benzene, triphenyl-(4-(9-phenyl-9H-fluoren-9-yl)phenyl) silane,[30]
2,9-dimethyl-4,7-diphenyl-1,10-phe-nanthroline, and 1,3,5-tris(N-phenyl benzimidazol-2-yl)ben-zene, respectively, while doping concentrations of 1 are 7 %, 14 %, 28 %, 50 % and 100 %. Very bright emission was ob-served for all doping concentrations. Table 4 summarizes the selected performance data for these OLED devices. The I-V-L curves, plotted in Figure 9, show a trend of increasing current density with increasing concentration of 1. This phe-nomenon implies that the charges may be injected directly to the PtIIcomplex; the dopant then serves as an additional
channel to transport charges by hopping between the dopant sites.[31]
Figure 10 a shows the electroluminescence (EL) emission profiles at various doping concentrations. The relative inten-sity of the peaks at 528 and 572 nm decreases with increas-ing concentrations. At the same time, the peak intensity at higher wavelengths increases. According to the previous photoluminescence (PL) data, it appears to us that the lumi-nescence at 528 and 572 nm is derived from the monomeric species, while the emission at the longer wavelengths origi-nates from the aggregated forms and/or excimers. Arising from the reduction of the monomeric species at the higher concentrations, the full width at half maximum (FWHM) of the EL signal decreased from 153 nm to 107 nm.
For a further comparison, the thin film samples of 1 doped in TPSiF were prepared and their PL characteristics measured. The concentration dependent red-shifting was also displayed in the corresponding PL spectra (Figure 10 b),
further substantiating the self-aggregation of the dopant. This tendency has been illustrated by the well-aligned head-to-tail stacking arrangement and the exceedingly short inter-molecular Pt···Pt distances of 1. In fact, the phenomenon is typical for the square-planar PtIIcomplexes, for which
inter-molecular p···p and Pt···Pt interactions often result in molec-ular stacking and the formation of aggregates and exci-mers.[32]Therefore, the detected luminescence would display
features involving both 3MLCT
and 3MMLCT excited states,
which are generated from the monomer and the oligomeric counterparts, respectively.[20b, 33]
Moreover, among the various dopant concentrations, the best device was achieved at the
28 wt % doping level
(Figure 11), which rendered a turn-on voltage of 6.7 V (at Table 4. Performance characteristics for ITO/NPB/mCP/TPSi-F:1/BCP/TPBI/LiF/Al.
Conc. (%) Max lum.[a]
ACHTUNGTRENNUNG[cd m2(V)] E.Q.E. [%][b] Luminous eff. ACHTUNGTRENNUNG[cdA1][b] Power eff. ACHTUNGTRENNUNG[lm W1][b] E. L. lmax(C.I.E.)[c] 7 % 12021 (22.5) 3.3 (2.4) 6.9 (5.1) 1.4 (0.9) 530, 618 (0.50, 0.46) 14 % 28289 (21.0) 6.6 (4.9) 13.8 (10.3) 3.1 (1.9) 530, 618 (0.52, 0.47) 28 % 47543 (18.5) 8.5 (6.7) 18.5 (14.6) 4.9 (3.2) 530, 618 (0.53, 0.46) 50 % 30056 (15.5) 7.6 (5.5) 11.4 (9.0) 3.9 (2.5) 532, 626 (0.56, 0.42) 100 % 26888 (13.0) 6.5 (4.9) 10.1 (7.6) 4.2 (2.6) 630 (0.59, 0.41) [a] Values in the parentheses are the applied driving voltage. [b] Data collected under 20 mA cm2, while
values in the parentheses are the data collected under 100 mA cm2. [c] Measured at the driving voltage of 8 V.
Figure 9. I-V-L characteristics of OLED devices ITO/NPB/mCP/TPSi-F:1/BCP/TPBI/LiF/Al as a function of dopant concentration.
Figure 10. (a) EL spectra of OLED devices ITO/NPB/mCP/TPSi-F:1/ BCP/TPBI/LiF/Al as a function of dopant concentration, and (b) PL spectra of 1 embedded to the TPSi-F matrix with variable dopant concen-trations.
1 cd m2) and maximum external quantum efficiency (EQE)
of 8.62 at 11.5 V, CIE coordinates of (0.53, 0.46) at 8 V, and maximum brightness of 47 543 cd m2at a driving voltage of
18.5 V. At higher dopant concentrations, the EQE decreases, probably arising from triplet–triplet annihilation.[15a]Its
per-formance data are nevertheless very encouraging, showing characteristics comparable to the best PtII-based OLEDs
documented in literature,[34]and giving a peak h
ext of 8.5 %
(corresponding to a luminance efficiency of 18.5 cd A1),
to-gether with a brightness of 3700 cd m2at a current density of 20 mA cm2. Even at a higher current density of
100 mA cm2, 80 % of the peak efficiency (6.7 %) could still
be sustained, together with a fairly bright phosphorescence of approximately 14 570 cd m2. We attribute this superiority
to the higher emission QE and its shorter triplet lifetime. The latter helped to minimize the degree of exciton quench-ing through triplet–triplet annihilation occurrquench-ing at high cur-rent density.
Conclusions
In summary, facile syntheses are established for PtII
com-plexes that possess the common terdentate chelating anion fpbpy and a monodentate anion occupying the fourth coor-dination site. From structural and spectroscopic characteri-zation, it is demonstrated that the inter-conversion between the chloride complex 1 and the acetamide complexes 3 and 4 can be achieved by organonitrile coordination, base-in-duced organonitrile hydrolysis, and release of the respective acetamide ligand by addition of HCl. In dilute solution, complexes 1, 2, and 3 exhibit orange (560 nm), orange (557 nm), and red (604 nm) phosphorescence, respectively, in which complexes 1 and 2 show strong concentration-de-pendent emission originating from the excimeric type associ-ation in the excited state, which is manifested in a strong Pt–Pt interaction. For complex 1, the moderately strong deep-red emission with quantum yield of ~ 0.2 and short life-time of ~ 113 ns may find useful application in the field of
red-emitting OLEDs. Work focusing on this aspect is cur-rently in progress.
Experimental Section
General Information and Materials
All reactions were performed under a nitrogen atmosphere using anhy-drous solvents or solvents treated with an appropriate drying reagent. 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 19F NMR spectra were recorded on Varian Mercury-400 or
INOVA-500 instruments. Elemental analyses were conducted at the NSC Regional Instrumentation Center at National Chiao Tung University. Steady-state absorption and emission spectra were recorded by a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920) fluorimeter, re-spectively. Solution samples, unless otherwise specified, were degassed by three freeze-pump-thaw cycles. A confocal microscope (Witec a SNOM) connected with an intensified charge-coupled detector (PI-MAX) by the optical fiber was used to measure the emission spectra of single crystal samples. Coumarin 480 (F = 0.93 in EtOH)[35]was used as a reference to
determine the luminescence quantum yields of the studied compounds in solution, using Equation (7):
Fs¼ Fr h2 sArIs h2 rAsIr ð7Þ
in which Fsand Frare the quantum yields of the unknown and reference
samples, respectively, h is the refractive index of the solvent, Arand As
are the absorbance of the reference and the unknown samples at the ex-citation wavelength, and Isand Irare the integrated areas under the
emis-sion spectra of interest, respectively. An integrating sphere (Lab sphere) was applied to measure the quantum yield in the solid state. The solid thin film was prepared by direct vacuum deposition and was excited by an argon-ion laser at 363 nm. The resulting luminescence was acquired with an intensified charge-coupled detector for subsequent quantum yield analyses according to a reported method.[36]
Synthesis
PtACHTUNGTRENNUNG(fpbpy)Cl (1) (Method A): A mixture of K2PtCl4(300 mg, 0.72 mmol)
and 6-(5-trifluoromethyl-pyrazol-3-yl)-2,2’-bipyridine (fpbpyH) (210 mg, 0.72 mmol) in a 1:1 mixture of water and acetonitrile (30 mL) was vigo-rously refluxed for 48 h, giving a red-violet precipitate. After cooling the suspension to room temperature, the precipitate (1, 319 mg, 0.61 mmol, 85 %) was filtered, washed with diethyl ether, and dried under vacuum. X-ray crystals were afforded by cooling the saturated solution of 1 in a 1:3 mixture of DMSO and acetone.
(1) (Method B): To a suspension of PtACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(NHCOMe) (3) (40 mg, 0.071 mmol) in a 3:1 mixture of acetonitrile and water (20 mL), was added a small amount of HCl (2 m) to afford a clear solution. The solu-tion was refluxed for 6 h, giving a red-violet suspension. After cooling to room temperature, the precipitate (1, 32 mg, 0.061 mmol, 86 %) was fil-tered, washed with diethyl ether, and dried under vacuum.
Spectral data of 1: 1H NMR (400 MHz, [D
6]DMSO, 298 K, TMS): d =
8.95 (d, JH,H=5.6 Hz, 1 H), 8.49 (d, JH,H=7.6 Hz, 1 H), 8.39 (td, JH,H=7.8,
1.6 Hz, 1 H), 8.30–8.22 (m, 2 H), 7.99 (dd, JH,H=7.6, 1.2 Hz, 1 H), 7.86 (td,
JH,H=6.6, 1.3 Hz, 1 H), 7.29 ppm (s, 1 H);19F NMR (470 MHz, [D7]DMF,
298 K): d =60.47 ppm (s, 3F, CF3); MS (FAB,195Pt): m/z (%) calcd for
C14H8ClF3N4Pt: 519.00 [M]+; found: 519.00; elemental analysis: calcd
(%) for C14H8ClF3N4Pt: C 32.35, H 1.55, N 10.78; found: C 32.44, H 1.74,
N 10.32.
PtACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(OAc) (2): A mixture of 1 (50 mg, 0.096 mmol) and silver ace-tate (65 mg, 0.389 mmol) in DMF (15 mL) was stirred at 110 8C for 50 min to give an orange suspension. After cooling to room temperature, the solution was filtered, the filtrate was concentrated under vacuum, and the residue was dissolved in a small amount of DMF. Large excess of diethyl ether was added to induce the precipitation. The orange precipi-Figure 11. Quantum efficiency of OLED devices
tate (2, 42 mg, 0.077 mmol, 80 %) was collected, washed with diethyl ether, and dried under vacuum.
Spectral data of 2:1H NMR (500 MHz, [D 6]DMSO, 298 K, TMS): d = 8.51 (d, JH,H=8.0 Hz, 1 H), 8.49 (d, JH,H=5.0 Hz, 1 H), 8.42 (td, JH,H=7.9, 1.3 Hz, 1 H), 8.28 (t, JH,H=8.0 Hz, 1 H), 8.22 (d, JH,H=7.8 Hz, 1 H), 7.99 (d, JH,H=7.5 Hz, 1 H), 7.86 (td, JH,H=6.9, 1.5 Hz, 1 H), 7.30 (s, 1 H), 2.05 ppm (s, 3 H); 19F NMR (470 MHz, [D 6]DMSO, 298 K): d =
59.23 ppm (s, 3F, CF3); MS (FAB, 195Pt): m/z (%) calcd for
C16H11F3N4O2Pt: 544.05 [M + 1]+; found: 544.00; elemental analysis:
calcd (%) for C16H11F3N4O2Pt: C 35.37, H 2.04, N 10.31; found: C 34.94,
H 2.37, N 10.59.
PtACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(NHCOMe) (3) (Method A): A mixture of 1 (50 mg, 0.096 mmol), acetamide (57 mg, 0.965 mmol), and Na2CO3 (102 mg,
0.962 mmol) in ethanol (30 mL) was refluxed overnight. After cooling to room temperature, the solvent was removed under vacuum, and the resi-due was dissolved in a small amount of DMF. The crystalline product (3, 39 mg, 0.072 mmol, 75 %) was recrystallized from a mixture of DMF and diethyl ether. Single crystals suitable for X-ray diffraction analysis were obtained by placing the saturated solution of 3 in DMSO at room tem-perature for several days.
(3) (Method B): A mixture of 1 (60 mg, 0.116 mmol) and Na2CO3(13 mg,
0.123 mmol) in a 1:1 mixture of water and acetonitrile (20 mL) was re-fluxed overnight, giving a yellow precipitate. After cooling the suspension to room temperature, the precipitate (3, 60 mg, 0.107 mmol, 92 %) was filtered, washed with diethyl ether, and then dried under vacuum. (3) (Method C): A mixture of 5 (55 mg, 0.090 mmol) and Na2CO3(13 mg,
0.123 mmol) in a 3:1 mixture of acetonitrile and water (20 mL) was re-fluxed overnight, giving a yellow precipitate. After cooling the suspension to room temperature, the precipitate (3, 42 mg, 0.075 mmol, 83 %) was filtered, washed with diethyl ether, and dried under vacuum.
Spectral data of 3:1H NMR (400 MHz, [D 6]DMSO, 298 K, TMS): d = 9.48 (d, JH,H=5.0 Hz, 1 H), 8.42 (d, JH,H=7.6 Hz, 1 H), 8.30 (td, JH,H=7.8, 1.6 Hz, 1 H), 8.22–8.15 (m, 2 H), 7.91 (dd, JH,H=5.8, 3.0 Hz, 1 H), 7.70 (td, JH,H=6.7, 1.5 Hz, 1 H), 7.25 (s, 1 H), 5.58 (s, 1 H), 2.00 ppm (s, 3 H); 19F NMR (470 MHz, [D 7]DMF, 298 K): d =60.33 (s, 3F, CF3); MS (FAB, 195Pt): m/z (%) calcd for C 16H12F3N5OPt: 542.06 [M] +; found: 542.00;
ele-mental analysis: calcd (%) for C16H12F3N5OPt·H2O: C 34.29, H 2.52,
N 12.50; found: C 34.24, H 2.78, N 12.21. PtACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(NHCOEt) (4): A mixture of 1 (41 mg, 0.079 mmol), propiona-mide (91 mg, 1.245 mmol), and Na2CO3(150 mg, 1.415 mmol) in
etha-nol (15 mL) was refluxed for 24 h, and then cooled to room temperature. The solvents and excess propionamide were removed under vacuum, and the residue was dissolved in small amount of DMF. Large excess of water was added to induce precipitation. The yellow precipitate was collected, washed with diethyl ether, and dried under vacuum. The crystalline product (4, 19 mg, 0.034 mmol, 43 %) was ob-tained from a mixture of DMF and di-ethyl ether. Spectral data of 4: 1H NMR (500 MHz, [D6]DMSO, 298 K, TMS): d = 9.75 (d, JH,H=5.0 Hz, 1 H), 8.48 (d, JH,H=7.5 Hz, 1 H), 8.36 (td, JH,H=7.9, 1.3 Hz, 1 H), 8.27–8.23 (m, 2 H), 7.98 (dd, JH,H=6.0, 2.5 Hz, 1 H), 7.77 (td, JH,H=6.8, 1.5 Hz, 1 H), 7.32 (s, 1 H), 5.60 (s, 1 H), 2.28 (q, JH,H=7.5 Hz, 2 H), 1.08 ppm (t, JH,H=7.3 Hz, 3 H). 19F NMR (470 MHz, [D 6]DMSO, 298 K): d =59.24 ppm (s, 3F, CF3);
MS (FAB, 195Pt): m/z (%) calcd for
C17H14F3N5OPt: 557.08 [M + 1] +;
found: 557.00; elemental analysis: calcd (%) for C17H14F3N5OPt·H2O:
C 35.55, H 2.81, N 12.19; found: C 35.47, H 3.08, N 12.11.
[PtACHTUNGTRENNUNG(fpbpy)ACHTUNGTRENNUNG(NCMe)]ACHTUNGTRENNUNG(BF4) (5): A mixture of 1 (60 mg, 0.115 mmol) and
AgBF4 (23 mg, 0.118 mmol) in anhydrous acetonitrile (15 mL) was
re-fluxed for 4 h to give a yellow suspension. The solution was filtered and the filtrate was concentrated to approximately 3 mL. Yellow crystalline material (5, 57 mg, 0.093 mmol, 80 %)was obtained by a slow diffusion of diethyl ether into the acetonitrile solution.
Spectral data of 5:1H NMR (400 MHz, [D 6]DMSO, 298 K, TMS): d = 9.24 (d, JH,H=5.6 Hz, 1 H), 8.68 (d, JH,H=8.0 Hz, 1 H), 8.57–8.46 (m, 3 H), 8.23 (d, JH,H=7.6 Hz, 1 H), 8.05 (t, JH,H=6.8 Hz, 1 H), 7.54 (s, 1 H), 2.06 ppm (s, 3 H);19F NMR (470 MHz, [D 6]DMSO, 298 K): d =60.09 (s,
3F, CF3),148.22 ppm (s, 4F, BF¯ ); MS (FAB,4 195Pt): m/z (%) calcd for
C16H11F3N5Pt: 525.06 [M-BF4]+; found: 525.00; elemental analysis: calcd
(%) for C16H11BF7N5Pt·2 H2O: C 29.65, H 2.33, N 10.80; found: C 29.92,
H 2.30, N 10.60. X-ray Diffraction Studies
Single crystal X-ray diffraction data of 1 and 3 were measured on a Bruker SMART Apex CCD diffractometer using (MoKa) radiation (l =
0.71073 ). The data collection was executed using the SMART program. Cell refinement and data reduction were performed with the SAINT pro-gram. The structure was determined using the SHELXTL/PC program and refined using full-matrix least squares. Their crystallographic refine-ment parameters are summarized in Table 5. CCDC 93182 and 693183 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallograph-ic Data Centre at www.ccdc.cam.ac.uk/data_request/cif.
DFT Calculation Method
Molecular orbital and time-dependent calculations are based on the structures from the X-ray crystal studies at B3LYP level in the continuum solvation model of CH2Cl2(IEFPCM). The basis set is a double-z quality
basis set consisting of Hay and Wadts quasi-relativistic effective core po-tentials (LANL2DZ) for PtIIatom;[37]a 6-31G* basis set was employed
for the H, C, N, F, and O atoms. Typically, the lowest three triplet and singlet roots of the nonhermitian eigenvalue equations were obtained to determine the vertical excitation energies. Oscillator strengths were
de-Table 5. Crystal data and refinement parameters for complexes 1 and 3.
1 3
Empirical formula C17H14ClF3N4OPt C18H18F3N5O2PtS
Formula weight 577.86 620.52
Temperature [K] 150(2) 220(2)
Crystal system Monoclinic Triclinic
Space group P21/m P1¯ a [] 9.3098(7) 7.8258(5) b [] 6.5917(5) 13.7404(8) c [] 15.1612(11) 19.1996(11) a [8] 87.338(1) b [8] 106.399(2) 78.825(1) g [8] 88.222(1) V [3] 892.55(12) 2022.7(2) Z 2 4 1calcd[g cm3] 2.150 2.038 m [mm1] 8.055 7.093 FACHTUNGTRENNUNG(000) 548 1192 Crystal size [mm3] 0.20 0.15 0.02 0.40 0.12 0.07 Reflns. collected 5436 21419
Independent reflections 1713 [RACHTUNGTRENNUNG(int) = 0.0459] 9241 [RACHTUNGTRENNUNG(int) =0.0418]
Max., min. transmission 0.8555, 0.2957 0.6366, 0.1636
Data/restraints/parameters 1713/0/168 9241/0/548
GOF 1.202 1.088
Final R indices [I > 2s(I)] R1=0.0401, wR2=0.0918 R1=0.0456, wR2=0.0956
R indices (all data) R1=0.0428, wR2=0.0930 R1=0.0601, wR2=0.1093
duced from the dipole-transition matrix elements (for singlet states only). All the calculations were performed with the Gaussian 03 package.[38]
Compositions of molecular orbitals, overlap populations between molecu-lar fragments, and density-of-states spectra were calculated using the AOMix program.[39]
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 square1. Various organic layers
were deposited sequentially at a rate of 1–2 s1. Phosphorescent dopant
was co-evaporated with TPSi-F by two independent sources. A thin layer of LiF (1 nm) and a thick layer of Al (150 nm) were sequentially deposit-ed as 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 Op-tical meter equipped with an 818ST silicon photodiode. The EL spectrum was obtained using a Hitachi F4500 spectrofluorimeter.
Acknowledgements
This work was supported by the National Science Council and Ministry of Economic Affairs of Taiwan.
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Received: May 21, 2008 Published online: September 2, 2008
