Influence of Molecular Dipoles on the Photoluminescence and
Electroluminescence of Dipolar Spirobifluorenes**
By Chih-Long Chiang, Shih-Min Tseng, Chin-Ti Chen,* Chao-Ping Hsu,* and Ching-Fong Shu*
1. Introduction
A general problem with using a fluorescent material in or-ganic light-emitting diodes (OLEDs) and other luminescent devices is concentration quenching in the solid state.[1]Device engineers adopt a physical method, such as doping of the fluo-rescent material in an appropriate host, to alleviate the prob-lem.[2]The success of such a physical method hinges on the mo-lecular isolation and, hence, reduction of the momo-lecular contact
(p–p interaction) of the fluorophore in the solid state. A re-cently reported exception, a donor–acceptor-substituted spiro-bifluorene, 2-diphenylamino-7-(2,2-dicyanovinyl)-9,9′-spirobi-fluorene (DCV; Scheme 1) is one of those rare red fluorophores that is very efficient for non-dopant-based red OLEDs.[3]Based on the single-crystal X-ray structure, DCV has a unique molecular structure with a non-planar bulge p-framework, thereby avoiding p–p stacking between DCV molecules.
More recently we have synthesized a molecular “dimer” ver-sion of DCV, 2DCV (Scheme 1),[4]which has twice the chro-mophore density and offers the possibility to enhance photolu-minescence (PL) or electroluphotolu-minescence (EL). In fact, 2DCV performed much worse than did DCV in terms of EL intensity and efficiency.[5]In this paper, similar results have been found for “monomeric” 2-diphenylamino-9,9′-spirobifluorene7-car-xoxaldehyde (CHO) when compared with “dimeric” 2CHO (Scheme 1). In contrast, newly prepared blue fluorophores 2-diphenylamino-7-(2,2-diphenylvinyl)-9,9′-spirobifluorene (DPV) and 2DPV (Scheme 1) performed rather similarly. The six molecules studied in the present work provide an ideal plat-form to study the factors influencing the luminescence yield, since each series of molecules is composed of a monomeric and
–
[*] Prof. C.-T. Chen, Prof. C.-P. Hsu, C.-L. Chiang, S.-M. Tseng Institute of Chemistry
Academia Sinica
Taipei, Taiwan 11529 (R.O.C.)
E-mail: [email protected]; [email protected] Prof. C.-F. Shu, C.-L. Chiang
Department of Applied Chemistry National Chiao Tung University Hsin-Chu, Taiwan 30050 (R.O.C.) E-mail: [email protected]
[**] This work was supported by Academia Sinica and the National Science Council of Taiwan.
This Full Paper investigates a series of strongly fluorescent donor–acceptor-substituted spirobifluorene compounds, red 2-di-phenylamino-7-(2,2-dicyanovinyl)-9,9′-spirobifluorene (DCV), green 2-diphenylamino-9,9′-spirobifluorene7-carxoxaldehyde (CHO), and blue 2-diphenylamino-7-(2,2-diphenylvinyl)-9,9′-spirobifluorene (DPV), together with their spiro-linked “dimeric” analogs, 2DCV, 2,2′-bis(diphenylamino)-9,9′-spirobifluorene-7,7′-dicarboxaldehyde (2CHO), and 2,2′-bis(diphenylamino)-7,7′-bis(2,2-diphenylvinyl)-9,9′-spirobifluorene (2DPV), respectively. The emission optical density and, hence, the intensity of photoluminescence (PL) or electroluminescence (EL) of the “dimeric” analogs is presumed to increase, which is beneficial for organic light-emitting diode (OLED) applications. The physical properties, including the dipole moments obtained from quan-tum chemistry calculations, emission solvatochromism, fluorescence quanquan-tum yield (Uf) as well as the EL of these six spirobi-fluorene compounds have been examined in detail. We found that Ufas well as OLED performance (EL efficiency and inten-sity) of the strongly dipolar DCV decrease significantly in the “dimeric” analog 2DCV, but less so in the moderately dipolar CHO and 2CHO, and only slightly in the weakly dipolar DPV and 2DPV. This is parallel to the intramolecular dipole moment, which is large for 2DCV, medium for 2CHO, and very small for 2DPV. Here, we show for the first time systematically that the luminescence intensity is closely correlated with the local electric field induced by the molecular dipole. A strong electric field may facilitate radiationless decay channels with a charge-transfer nature, leading to a high quenching rate. Consistent with this conclusion, which is derived from the red DCV/2DCV and green CHO/2CHO, our new blue fluorophore DPV with an essen-tially zero dipole moment has successfully achieved one of the best electrofluorescent blue OLEDs. At the same time, by doping the highly dipolar DCV into an isolated environment with the low-polarity Alq3as the host matrix, we obtained a very high performance of saturated yellow OLEDs as well, This is possibly due to the reduction of emission-quenching dipoles from the neighboring molecules. Our results have provided an important insight in designing luminescent materials, as follows: molecular dipole moments should be kept at a low magnitude to avoid quenching induced by a strong local electric field in the chromophore.
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a dimeric pair and different series have a different dipolar char-acter. The common bulky spirofluorene moiety can reduce the difference arising from intermolecular packing. We have dem-onstrated that the molecular dipole moment is a critical factor for the efficiency of luminescence, either PL or EL, taking ex-amples of these spirobifluorene fluorophores. The molecular dipole from each molecular half of spirofluorenes gives rise to a strong local electric field and quenches PL or EL, possibly by making charge-transfer channels available. Therefore, we can define a new insight in understanding the performance of fluo-rescent materials, and derive new designing principles for OLED materials.
2. Results and Discussion
2.1. Photophysical Properties
Dimeric 2DCV possesses a very similar energy, both in ab-sorption or emission wavelengths, to that of monomeric DCV (Table 1). This situation is similar in the green CHO/2CHO and blue DPV/2DPV pairs as well. This reflects the fact that an sp3-hybridized carbon between the molecular halves of 2DCV, 2CHO, or 2DPV renders little electronic coupling (p-conjugat-ed interaction).[6]At the same time, similar to other red
dop-ant-based OLED materials, the color changes from red via orange to yellow (Fig. 1) with diminishing dopant concen-tration of DCV or 2DCV. This is com-monly known as the solid-state solvation effect, induced by the doping molecules themselves.[2,7] It resembles the solvato-chromism observed in the emission spec-tra, as shown in Table 1 and Figure 2. A red-shift is observed for solutions with sol-vents of increasing polarity. Such a phe-nomenon usually happens in molecules with a large change in its dipole when electronically excited, and with a large ground-state dipole that provides a polar environment in the solid state.[8]In addition, the bathochromic shifts of the emis-sion from solution state to the condensed phase (solid state) were found to be varied for these spirobifluorene compounds: a very large shift for red DVC or 2DCV, a medium shift for green CHO or 2CHO, and virtually no shift for blue DPV or 2DPV (Table 1).
Such solvatochromism results imply that the molecular di-pole moments of these three sets of spirobifluorene compounds (DCV/2DCV, CHO/2CHO, and DPV/2DPV) are substantially different from each other. Considering a rather similar transi-tion energy of dimers of 2DCV, 2CHO, or 2DPV to the corre-sponding monomers, DCV, CHO, or DPV, respectively, it begs the question why the blue spirobifluorenes (DPV/2DPV) be-have so differently from the green (CHO/2CHO) or red (DCV/2DCV) ones. Moreover, since the performance of OLEDs based on 2DPV and DPV is relatively similar, why does the performance of 2DCV and DCV OLEDs differ so much? We searched for an explanation based on the relatively large red-shift of the emission wavelength, which has a signifi-cantly smaller fluorescence quantum yield, as well as in the large difference in OLED performance of 2DCV or 2CHO. We found and verified that the key to the explanation is the ground-state molecular dipole moment of the dipolar spirobi-fluorene compounds, as detailed below.
2.2. Molecular Dipole Moments
The molecular dipole moments of these six spirobifluorenes were estimated by density functional theory (DFT) calcula-tions (Fig. 3). The red spirobifluorenes possess large molecular dipole moments (11–15 Debye) and those of the blue spirobi-fluorenes are very small (< 1 Debye), whereas those of green spirobifluorenes are in the middle range (5–7 Debye). This is consistent with the electron push–pull properties of the func-tional groups in the molecules. DCV has a strong electron with-drawing dicyanovinyl group, and in CHO, the formyl group has a medium strength of withdrawing p-electrons and, hence, there is a polarization effect in between the functionalties of di-cyanovinyl and diphenylvinyl. Qualitatively, this agrees with the intermediate dipole moment calculated for CHO/2CHO compared with those of DVC/2DCV and DPV/2DPV. The ground-state dipole of 2DCV can be considered as a simple N CN NC Ph Ph N CN NC Ph Ph NC CN N Ph Ph N Ph Ph N Ph Ph N Ph Ph 2DCV 2DPV N O Ph Ph N O Ph Ph O N Ph Ph 2CHO DCV CHO DPV
Scheme 1. Chemical structures of DCV, 2DCV, CHO, 2CHO, DPV, and 2DPV.
Table 1. Optical properties and energy levels of spirobifluorene
com-pounds Solution Solid state DE[c] [eV] HOMO/LUMO [eV/eV] kmaxab,kmaxfl[a]
[nm,nm] Uf[b] [%] kmaxfl, Uf [nm,%] DCV 460, 582 79,70,49 633, 33 2.41 5.73/3.32 2DCV 460, 587 31,14, 7 654, 6 2.39 5.75/3.36 CHO 400, 497 83,68,62 502, 42 2.83 5.61/2.78 2CHO 401, 495 49,46,30 508, 10 2.83 5.63/2.80 DPV 385, 468 60,60,56 467, 51 2.91 5.50/2.59 2DPV 385, 468 50,54,43 468, 21 2.89 5.53/2.64
[a] DPV and 2DPV were in chlorobenzene; CHO, 2CHO, DCV and 2DCV were in 1,4-dioxane. [b] In chlorobenzene, 1,4-dioxane, and ethyl acetate, respectively. [c] Band-gap energies estimated from the red edge of absorp-tion spectra.
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vector sum of the two dipoles of the two orthogonally linked fragments. When the molecular dipole moment (10.59 Debye) of DCV was used to model the dipole of a half fragment in 2DCV, then a simple vector summation (2 × 10.59 × cos 45°) leads to 14.95 Debye of 2DCV (Fig. 3, bottom), which is very
close to 14.50 Debye obtained by the same calculation for 2DCV. A similar vector analysis of dipole moment works as well for 2CHO. Both DPV and 2DPV do not possess a elec-tron-withdrawing functionality (the diphenylvinyl group is a very weak electron acceptor), and the calculated dipoles are not only small but also disoriented in a non-orthogonal direc-tion (Fig. 4).
The calculated dipole moment differences between the ground state and the first excited state are much larger for DCV and 2DCV (6.14 and 4.43 Debye, respectively) than those for DPV and 2DPV (1.66 and 1.35 Debye, respectively). These dipole moment results are parallel to the solvatochromic behavior of these fluorescent spirobifluorenes in solution (see previous section of photophysical properties). The solvatochro-mism observed for spirobifluorene compounds is qualitatively in agreement with the dipole moment changes calculated for the red, green, and blue spirobifluorenes and following the known principle.[8,9] In solution, fluorescence quantum yields decrease as the solvent polarity increases (Table 1). Such sol-vent polarity dependence is particularly obvious in the case of the red pair of DCV/2DCV, is relatively less dependent for the green pair of CHO/2CHO, and is much less so in the case of the blue pair of DPV/2DPV (Table 1). Such behavior is also consistent with their relative dipole moment values, which are estimated by calculation.
2.3. PL/EL Quantum Yield, Molecular Dipole Moment, and Electric Field
Solution fluorescence quantum yields drop from monomeric to dimeric species, with a 60–85 % decrease for the red DVC→2DCV, 50–70 % for the green CHO→2CHO, and only a 10–25 % decrease for the blue DPV→2DPV (Table 1). The sensitivity of fluorescence quantum yields to the polarity in its surrounding implies that the polar environment may facilitate a non-radiative decay process leading to fluorescence quench-ing. This effect is closely related to a field-induced fluorescence quenching of donor–acceptor-substituted diphenyloligoene fluorophores reported earlier,[10] where an external electric field with a strength of∼ 1 MV cm–1was applied to the fluoro-phore-doped polystyrene thin film. It was shown that under an external electric field, fluorescence quenching occurred on the fluorophore with large dipole moment changes between ground and the first excited states. Therefore, it was concluded that the external electric-field may lower the energy of a non-emissive intramolecular charge-transfer (ICT) state and be-come a quenching channel of the light emission.
We believe that the electric field, either external or local, has a strong influence on the spirobifluorene fluorophores in the OLEDs studied in this paper. Light-emitting materials experi-ence an external electric field of 0.1–1.5 MV cm–1(1–15 V ap-plied to the device with∼ 100 nm thickness), which is compar-able with those in Ref. [10]. Moreover, for dimeric donor– acceptor-substituted spirobifluorenes, the large ground state di-pole moment may generate a local electric filed that has an even stronger effect on a nearby molecule or internally to the
300 350 400 450 500 550 600 650 700 750 800 850 900 0.0 0.2 0.4 0.6 0.8
1.0 Photoluminescencethin film 633 nm Electroluminescence 0.5% 584 nm 3% 602 nm 100% 634 nm DCV Emi ssi o n Inte nsi ty (A.U .) Wavelength (nm) a) 300 350 400 450 500 550 600 650 700 750 800 850 900 0.0 0.2 0.4 0.6 0.8 1.0 Photoluminescence thin film 654 nm 2DCV Electroluminescence 0.5% 592 nm 5% 612 nm 100% 648 nm Emi ssi o n Inte nsi ty (A .U .) Wavelength (nm) b) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 2DPV DPV CIE 10 nm spacing CRT DCV 0.5% DCV 3% DCV 100% 2DCV 0.5% 2DCV 5% 2DCV 100% 480 nm 520 nm 560 nm 600 nm Y X c)
Figure 1. PL and EL spectra of OLEDs based on a) DCV, and b) 2DCV. The
1931 CIE (the Commission Internationale de l’Ecairage) color chromaticity diagram of all OLEDs, including those of DPV and 2DPV ones, are shown in (c).
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other molecular half. The strength of the local electric field near a dipole moment can be estimated by
E 1 4pe0r3 3 ~l ^r^r ~l ≈ ~l 4pe0r3 (1) where E is the electric field, 4pe0is 1.1 × 10–10J–1C2m–1, r is the distance to the dipole, l is the dipole moment in C m and 1 Debye = 3.34 × 10–30C m. In Equation 1 we have used the
field strength at the equatorial plane of the dipole as an esti-mate. For 2DCV, the edge-to-edge distance between the two molecular halves is about 5 Å.[3,5] Taking the calculated DFT value of 10 D for DCV as a model for the internal dipole mo-ment of the monomer, each molecular half of 2DCV experi-ences a local electric field from the opposite half of the mole-cule with a strength of∼ 12 MV cm–1(assuming half of a full charge separation), which is at least an order of magnitude
a) b) 400 450 500 550 600 650 700 750 800 850 900 9501000 0.0 0.2 0.4 0.6 0.8 1.0 DCV Em is s ion In tensity (A.U.) Wavelength (nm) Toluene 562 nm 1,4-Dioxane 582 nm Ethyl acetate 621 nm Cyanobenzene 670 nm 400 450 500 550 600 650 700 750 800 850 900 9501000 0.0 0.2 0.4 0.6 0.8 1.0 2DCV Emissi o n In te n s it y ( A .U .) Wavelength (nm) Toluene 567 nm 1,4-Dioxane 587 nm Ethyl acetate 626 nm Cyanobenzene 671 nm c) d) 400 450 500 550 600 650 700 750 800 850 900 9501000 0.0 0.2 0.4 0.6 0.8 1.0 CHO Toluene 460 nm 1,4-Dioxane 475 nm Ethyl acetate 489 nm Cyanobenzene 535 nm E m is s ion Inte nsity (A.U. ) Wavelength (nm) 400 450 500 550 600 650 700 750 800 850 900 9501000 0.0 0.2 0.4 0.6 0.8 1.0 2CHO Toluene 461 nm 1,4-Dioxane 476 nm Ethyl acetate 491 nm Cyanobenzene 535 nm Emi s s ion Inten s it y (A.U.) Wavelength (nm) e) f) 400 450 500 550 600 650 700 750 800 850 900 9501000 0.0 0.2 0.4 0.6 0.8 1.0 DPV Toluene 458 nm 1,4-Dioxane 460 nm Ethyl acetate 466 nm Cyanobenzene 495 nm Emi ssion I n te nsity (A .U .) Wavelength (nm) 400 450 500 550 600 650 700 750 800 850 900 9501000 0.0 0.2 0.4 0.6 0.8 1.0 2DPV Emi ssion I n ten s it y (A.U.) Wavelength (nm) Toluene 458 nm 1,4-Dioxane 459 nm Ethyl acetate 466 nm Cyanobenzene 491 nm
Figure 2. Fluorescence spectra of a) DCV, b) 2DCV, c) CHO, d) 2CHO, e) DPV, and f) 2DPV in toluene, 1,4-dioxane, ethyl acetate, and cyanobenzene,
respectively.
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stronger than the external electric field of OLEDs and it can be very influential on the fluorescence quenching of the mate-rial. For the monomeric DCV, there is no intramolecular elec-tric field but there is an intermolecular effect from the
neigh-boring molecules, of which the strength of the electric field can be estimated to be as high as 12 MV/cm, assuming that the mo-lecular separation distance is 5 Å. Such a value should be con-sidered as an upper limit of the possible electric field because of the possible cancellation of all the surrounding molecular di-poles. Considering that the strength of such an electric field falls off quickly as the third power of distance from the mole-cules, dopant usage can significantly boost the performance of DCV OLEDs but not as much for 2DCV ones, as indicated in our data.
For nonpolar blue spirobifluorenes, the solution Uf of the blue 2DPV is only about 10–25 % smaller than that of DPV in solutions (Table 1), which is consistent with the very small di-pole moments of both fluorophores, since they can not gener-ate a local electric field with influential strength. The nonradia-tive decay resulting from the higher degree of vibrational motion of the floppy diphenylvinyl groups of 2DPV and DPV is likely to play a more significant role in fluorescence quench-ing than the effects of dipole moments herein.
2.4. Solid State Fluorescence Quantum Yield and Single Crystal X-Ray Structure
In the solid state, a big drop of fluorescence quantum yield from monomeric to dimeric spirobifluorenes was observed, ∼ 82 % for red DCV→2DCV vs. ∼ 59 % for blue DPV→2DPV. From the single-crystal X-ray data reported by us pre-viously,[3,5]no intimate p-p contact was observed for DCV and 2DCV in the solid state, so we can mainly rule out the fluores-cence quenching due to non-emissive excitons arising from p-p stacking. The drop of fluorescence quantum yield observed for blue DPV→2DPV is quite large, considering their very small dipole moment numbers. We can only surmise that it is the strong molecular contact of 2DPV in the solid state that leads to fluorescence quenching. Unfortunately, we did not have
suit-2DCV 2 x 10.59 x cos45º = 14.97 Debye µ µ µsum= 2µ cos45º 2CHO 2 x 5.20 x cos45º = 7.35 Debye
Figure 3. Dipole moments, in Debye (D), of DCV, CHO, DPV, 2DCV,
2CHO, and 2DPV obtained by DFT/B3LYP calculation. The direction and the approximate magnitude of the dipole are illustrated with a yellow ar-row. Simple vector analysis of intramolecular dipoles and the net dipole moment of 2DCV and 2CHO is schematically presented in the bottom of the figure.
b
a
a
c
Figure 4. X-ray determined molecular structure and crystal packing diagram of CHO. The arrows in the packing diagram at the bottom indicate the
direc-tion from diphenylamino to formyl substituent. For clarity, molecules stacking in unit cells are marked with arrows having white, gray, or black shades.
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able single crystals of DPV and 2DPV for X-ray crystallographic analysis to verify our conjecture. However, for green spiro-bifluorenes CHO→2CHO, a comparable ∼ 76 % drop (with ∼ 82 % drop of DCV→2DCV) of fluorescence quantum yield was observed as well. In that case, the drop of solid state fluorescence quan-tum yield could be attributed to the detri-mental intermolecular p-p contact found for 2CHO (see the discussion of single crystal X-ray structure of CHO and 2CHO below).
The single crystals of CHO or 2CHO were obtained from chloroform/dichloro-methane/hexanes solution and were ex-amined for X-ray diffraction structure analysis (see Figs. 4 and 5).[11]There is no intermolecular face-on p–p interaction in CHO crystals. The intermolecular con-tacts are either non-orthogonal edge-to-face (C–H···p) or unparallel edge-to-face-to-edge-to-face p–p interaction, such as a 3.34 Å contact distance between C19 and C45, which is the contact between two nonparallel (∼ 47°) benzene rings (Fig. 6 left) of fluo-rene and diphenylamino group. The clos-est contact found for the crystal of CHO is a 3.31 Å contact between the C34–
C35 double bond and that of adjacent molecule. This is a short-distance contact but it is an edge-to-edge sideways inter-action between two phenyl rings of neighboring molecules (Fig. 6 right), and it is unlikely to cause significant florescence quenching.
For the crystal structure of 2CHO, a direct face-on p-p inter-action within the distance of∼ 3.4–3.5 Å was observed between the phenyl ring of the donor (diphenylamino group) and the C14–O1 double bond of the formyl acceptor of neighboring molecule (Fig. 7). Furthermore, because of the symmetry taking place in crystal packing, such face-on p–p interaction of 2CHO occurs twice for each molecule in the crystal. Therefore, a more significant fluorescence quenching can be expected for
2CHO than 2CHO in solid state. This deduction is consistent with the fact that 2CHO has about 40–50 % smaller Ufs than those of CHO in solution but nearly 75 % smaller Ufs in the solid state (Table 1)
2.5. OLEDs and High-Performance Yellow DCV Dopant and Blue DPV Non-Dopant
Figure 8 displays the characteristics of the devices, namely current density, voltage, external quantum efficiency, and elec-troluminance of all spirobifluorene OLEDs fabricated in the present research. Corresponding OLED data are summarized in the Experimental section.
b
c
Figure 5. X-ray determined molecular structure and crystal packing diagram of 2CHO. The arrows
in the packing diagram at the bottom indicate the direction from the diphenylamino to formyl sub-stituent.
Figure 6. Left: an unparallel ring-to-ring contact between C19 and C45 (3.34 Å) of CHO. Right: an edge-to-edge sideways interaction between two phenyl
rings of neighboring molecules of CHO.
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Figure 9 illustrates the voltage dependency of external quan-tum efficiency and electroluminance. As mentioned earlier, di-meric 2DCV or 2CHO OLEDs performed much worse than the monomeric DCV or 2CHO OLEDs, whereas the perfor-mance of 2DPV OLEDs was just moderately inferior to that of DPV OLEDs. In addition, a typical roll-off of external quan-tum efficiency associated with the increase of applied voltage was observed for DCV/2DCV, CHO/2CHO, and DPV/2DPV OLEDs.
Consistent with this new insight derived from the red 2DCV and DCV, our new blue fluorophore DPV successfully achieved one of the best electrofluorescence blue OLEDs.[13] Non-dopant blue DPV OLEDs have outperformed other OLEDs with similar blue color purity (CIEx,y= 0.13–0.18,
0.16–0.24) in terms of EL efficiency and intensity: maximum efficiency of 3.4 %, 5.4 cd A–1, or 5.7 lm W–1, and the peak in-tensity of 33020 cd m–2or 1430 cd m–2at 20 mA cm–2 (Fig. 8 and Table 3). This can be ascribed to the practically zero dipole moment of DPV and, hence, few problems with electric-field-induced fluorescence quenching in the solid state. On the other hand, by doping the dipolar and bright DCV in an isolated en-vironment (avoiding the effect of dipole moments from the neighbors), very high performance of saturated yellow OLEDs was obtained. Its EL efficiency is as high as 3.4 %, 9.4 cd A–1, or 8.2 lm W–1, and the peak EL intensity is 37640 cd m–2and 1750 cd m–2 at 20 mA cm–2 (Fig. 8 and Table 3), which also outperforms most electrofluorescence yellow OLEDs.[14]
3. Conclusion
Owing to their unique structural feature, donor–acceptor-substituted spirobifluorenes including both monomeric and di-meric ones, provide an ideal model set in elucidating the influ-ence of molecular dipole moment on fluorescinflu-ence quenching in solution as well as in the solid state. EL and consequently the performance of OLEDs are strongly affected by the molec-ular dipole moment. In addition to the vibrational-motion-in-duced emission quenching and solid-state molecule–molecule contact quenching, which are well known sources of fluores-cence quenching, the intramolecular and intermolecular elec-tric field arising from the molecular dipoles is another detri-mental factor that should be avoided in the molecular design of
Figure 7. The face-on p–p interaction between formyl acceptor and the
phenyl ring of diphenylamino donor of 2CHO.
0 100 200 300 400 500 600 700 800 900 10001100 0.1 1 0.5 5 4 3 0.5% 3% 100%
Current Density (mA/cm2)
E x ter nal Q uant um Effi c ienc y ( % ) 2 DCV 5000 10000 15000 20000 25000 30000 35000 40000 El ec tr ol umi nanc e ( c d/m 2) 0 100 200 300 400 500 600 700 800 900 10001100 0.1 1 0.5 5 4 3
Current Density (mA/cm2)
E x ter n al Qu an tu m E ffi ci e n c y ( % ) 2 2DCV 0 5000 10000 15000 20000 25000 30000 35000 40000 0.5% 5% 100% El ec tr o lum in an c e (c d /m 2) 0 200 400 600 800 1000 1200 1400 0.1 1 5 4 3
Current Density (mA/cm2)
E x te rnal Qua n tu m E ffi ci e n cy ( % ) 2 0 5000 10000 15000 20000 25000 30000 35000 40000 CHO 2CHO El ec tr ol um in anc e (c d /m 2 ) 0 200 400 600 800 1000 1200 1400 1600 1800 0.1 1 0.5 5 4 3 DPV 2DPV
Current Density (mA/cm2)
E x ter n a l Q uantum Effi c ien c y ( % ) 2 0 5000 10000 15000 20000 25000 30000 35000 40000 E lec lu mi nan c e ( c d /m 2)
Figure 8. Electroluminescence characteristics (EL efficiency-current density-EL intensity) of OLEDs: ITO/NPB/DCV or 2DCV:Alq3(0.5, 3, 5, or 100 %)/
BCP/Alq3/Mg:Ag; ITO/NPB/CHO or 2CHO/BCP/Alq3/Mg:Ag and ITO/NPB/DPV or 2DPV/TPBI/LiF/Al [12].
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high performance fluorophores for OLEDs. This is quite a challenge for the red (or long-wavelength) light-emitting mate-rials because most of the molecular structure of red fluoro-phore with an appreciable intensity of electrofluorescence are based on a donor–acceptor-substituted p-conjugated system and they are dipolar in nature.[1] Unlike dimeric 2DCV, the outstanding performance of a DCV-based non-dopant OLED is one rare exception. Only modest solid-state fluorescence quenching was found for DCV, and it is due to the bulky and rigid molecular framework of spirobifluorene that prevents the molecules from close contact and hence reduces the adverse p-p interaction as well as the strength of the intermolecular electric field.
From the dipole moment properties of these fluorescent ma-terials, we arrived at a convincing explanation of why the di-meric DCV (or 2CHO) exhibit a significantly worse OLEDs performance than the monomeric DCV (or 2CHO) OLED.
Consequently, in addition to the high performance of non-dop-ant red DCV OLEDs, we have successfully achieved one of the best electrofluorescent yellow OLEDs using DCV as dopant material. We have also achieved one of the best electrofluores-cent blue OLEDs using an efficient and bright DPV non-dop-ant material that has a practically zero dipole moment and hence a very limited electric-field-induced fluorescence quenching in the solid state.
4. Experimental
General Methods: Both solution and solid-state fluorescence
quan-tum yields (Ufs) of the donor–acceptor-substituted sprirobifluorenes
were determined by the integrating-sphere method described by de Mello et al. [3,15]. The ionization potentials (or HOMO energy levels) of six spirobifluorene compounds were determined by low energy photo-electron spectrometry (Riken-Keiki AC-2). From our previous data, we found that the precision of the AC-2 measurement is about ±0.05 eV. LUMO energy levels were estimated by subtracting the ener-gy gap (DE) from the HOMO enerener-gy levels. DE was determined by the on-set absorption energy from the absorption spectra of the materials.
Materials: The synthesis and characterization of DCV, 2DCV, CHO,
2CHO, and 2DPV have been reported previously [3,4]. Blue fluoro-phore DPV was previously unknown and the details of the synthesis are reported in the present paper. Other materials involved in the fab-rication of dopant or non-dopant OLEDs, tris(9-hydroxyquinoline)alu-minum (Alq3), 1,4-bis(1-naphylphenylamino)biphenyl (NPB), 2,2
′,2″-(1,3,5-phenylene)-tris[1-phenyl-1H-benzimidazole] (TPBI) were pre-pared in the laboratory using published methods and were subjected to gradient sublimation prior to use. Hole-blocking material, 2,9-dimeth-yl-4,7-diphenyl-1,10-phenanthroline, bathocuproine (BCP), was pur-chased from Aldrich and used as received.
Synthesis of 2-Diphenylamino-7-(2,2-diphenylvinyl)-9,9 ′-spirobifluo-rene (DPV): Under a nitrogen atmosphere, NaH (0.26 g, 60 wt % in
oil, 6.20 mmol) was added to a diethoxydiphenylmethylphosphonate (1.88 g, 6.02 mmol) dry THF solution (10 mL), and the mixture was stirred for 1 h at 55 °C. After cooling, CHO (1.50 g, 2.93 mmol) was added, then the reaction solution was heated to refluxing temperature for 16 h. After cooling to room temperature, the reaction mixture was added to water, extracted with ethyl acetate, and dried over MgSO4.
The solution was concentrated under reduced pressure and subjected to flash column chromatography (silica gel, dichloromethane/hexanes: 1/5). A yellow solid was obtained with a yield of 88 % (1.7 g).1H NMR
(400 MHz, CDCl3, d): 7.66 (d, 2H, J = 7.6 Hz), 7.57 (d, 1H, J = 8.3 Hz), 7.49 (d, 1H, J = 8.0 Hz), 7.29 (td, 2H, J = 7.5 Hz, J = 0.9 Hz ), 7.17-7.22 (m, 5H), 6.97–7.11 (m, 11H), 6.92–6.97 (m, 3H), 6.85–6.97 (m, 6H), 6.78 (s, 1H), 6.71 (d, 1H, J = 7.6 Hz), 6.48 (d, 1H, J = 2.0 Hz), 6.11 (s, 1H).13C NMR (125 MHz, CDCl 3, d): 150.3, 148.6, 148.4, 147.5, 147.4, 143.2, 141.9, 141.5, 140.0, 139.9, 136.5, 136.2, 129.8, 129.4, 129.0, 128.3, 128.1, 127.5, 127.4, 127.3, 127.2, 124.8, 124.0, 123.8, 122.5, 120.5, 119.9, 118.8, 65.6. FAB-MS: calcd 661.8, m/z = 661.3 (M+). Anal. calcd for C51H35N: C 92.35, H 5.49, N 2.49; found: C 92.55, H
5.33, N 2.12.
Theoretical Calculation: For theoretical dipole moment (l)
calcula-tion, donor–acceptor-substituted spirobifluorene molecules were opti-mized by applying DFT with the hybrid B3LYP functional [16] and 6-31G* basis set. With the optimized structure, we calculated the ground state dipole moments with the Hartree-Fock (HF), DFT (B3LYP) models and the 6-31G* basis set. Excited state dipole moments were calculated using Configuration Interaction Singles (CIS)/6-31G*. In all cases the first excited states were highly optically active, as indicated by the large oscillator strengths (> 0.9). Therefore we used the first CIS excited state to model the observed light-emitting states. We avoided using time-dependent DFT because of its problem in describing states with a charge-transfer (CT) nature [17], which may arise from the donor–acceptor moieties and because such CT states
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0.1 1 5 4 3 2 DCV 2DCV Voltage (V) Ex ter nal Quantu m Effi c ienc y ( % ) 5000 10000 15000 20000 25000 30000 35000 40000 El ec trol umi n es c e nc e ( c d/ m 2 ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0.1 1 5 4 3 2 CHO 2CHO Voltage (V) Ex ter nal Quantu m Effi c ienc y ( % ) 5000 10000 15000 20000 25000 30000 35000 40000 El ec trol umi nes c e nc e ( c d/ m 2 ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0.1 1 5 4 3 DPV 2DPV Voltage (V) E x ter na l Q ua nt um E ffi c ien c y (% ) 2 5000 10000 15000 20000 25000 30000 35000 40000 Ele c trol umi na nc e ( c d/ m 2 )
Figure 9. Electroluminescence characteristics (EL efficiency-voltage-EL
inten-sity) of OLEDs: ITO/NPB/DCV or 2DCV/BCP/Alq3/Mg:Ag, ITO/NPB/CHO
or 2CHO/BCP/Alq3/Mg:Ag, and ITO/NPB/DPV or 2DPV/TPBI/LiF/Al.
P
might interfere with the optically active states we investigated. Even though the CIS results might be less accurate than TDDFT, the qualitative trends in the CIS results can still offer correct physical in-sights. The results of the calculation are summa-rized in Table 2. All calculations were preformed with a developmental version of Q-Chem [18].
X-Ray Crystallography Studies: Data collection
was carried out on a Bruker X8APEX CCD diffrac-tometer at 100 K for CHO and 2CHO single crys-tals. Mo radiation (k·= 0.71073 Å) was used for both crystals. The unit cell parameters were ob-tained by a least-square fit to the automatically cen-tered settings for reflections. Intensity data were collected by using the x/2h scan mode. Corrections were made for Lorentz and polarization effects. The structures were solved by direct methods
SHELX-97 [19]. All non-hydrogen atoms were
lo-cated from the difference Fourier maps and were refined by full-matrix least-squares procedures. Hy-drogen atoms were calculated and refined with an overall isotropic temperature factor. Calculations and full-matrix least-squares refinements were per-formed utilizing the WINGX program package [20] in the evaluation of values of R (Fo) for reflections
with I > 2r(I) and Rw (Fo), where R = R||Fo| – |Fc||/R|Fo| and
Rw= [R{w(Fo2– Fc2)2}/R{w(Fo2)2}]1/2. Intensities were corrected for
ab-sorption.
OLED Fabrication and EL Characterization: The fabrication of
OLEDs and their EL characterization have been described elsewhere [13b]. The current, voltage, and light intensity (I–V–L) measurements were made simultaneously using a Keithley 2400 programmable source meter and a Newport 1835C Optical meter equipped with a Newport 818-ST silicon photodiode. The device was placed close to the photo-diode such that all the forward light went to the photophoto-diode. The effec-tive size of the emitting diode was 3.14 mm2, which is significantly
smaller than the active area of the photodiode detector, a condition known as “under filling” to satisfy the measurement protocol [21]. Only light emitting from the front face of the devices was collected and used in subsequent calculations of external quantum efficiency accord-ing to the method described earlier [21]. The luminous flux (lm) has been defined previously [22], and we adopted it to meet the purpose of our research. EL characteristics of OLEDs are summarized in Table 3.
Energy-Level of Spirobifluorenes and their OLED Layer Structure Analysis: Together with other materials adopted in the OLED
fabrica-tion, the relative energy-level alignments and layer thickness of OLEDs are schematically depicted in Scheme 2 [23]. Since the HOMO energy levels of DCV/2DCV or CHO/2CHO are rather close (0.2– 0.3 eV difference) to that of electron-transporting Alq3in OLEDs, a
hole blocking layer of BCP was deployed in either DCV/2DCV or
CHO/2CHO OLEDs to prevent the charge-recombination occurring on Alq3that emits undesired green light. A large energy gap (∼ 0.8 eV
difference) between the LUMO levels of NPB and DCV or CHO (2DCV or 2CHO) can effectively confine the electrons inside the light-emitting layer of CHO or DCV (2DCV or 2CHO) and facilitates charge recombination that leads to the EL of dopant or non-dopant OLEDs. Within this context, the same device configuration, except for a thinner NPB layer (5 nm), was adopted for the fabrication of non-dopant CHO and 2CHO OLEDs (bottom left scheme of Scheme 2). In order to enhance the performance of OLEDs, blue DPV or 2DPV OLED was fabricated in different configuration: ITO/NPB(10 nm)/ DPV or 2DPV (40 nm)/TPBI (50 nm)/LiF(1 nm)/Al (see the bottom right scheme of Scheme 2). There is no need of BCP here owing to the relatively large difference (∼ 0.7 eV difference) of HOMO energy lev-els between TPBI and DPV (or 2DPV).
Received: February 1, 2007 Revised: October 6, 2007
–
[1] C.-T. Chen, Chem. Mater. 2004, 16, 4389.
[2] a) C. W. Tang, S. A. VanSlyke, C. H. Chen, J. Appl. Phys. 1989, 65, 3610. b) V. Bulovic, A. Shoustikov, M. A. Baldo, E. Bose, V. G. Ko-zlov, M. E. Thompson, S. R. Forrest, Chem. Phys. Lett. 1998, 287, 455. c) C. H. Chen, J. Shi, C. W. Tang, Macromol. Symp. 1997, 125, 1. [3] C.-L. Chiang, M.-F. Wu, D.-C. Dai, Y.-S. Wen, J.-K. Wang, C.-T. Chen,
Adv. Funct. Mater. 2005, 15, 231.
[4] C.-L. Chiang, C.-F. Shu, C.-T. Chen, Org. Lett. 2005, 7, 3717. [5] C.-L. Chiang, M.-F. Wu, C.-F. Shu, C.-T. Chen, Proc. SPIE-Int. Soc.
Opt. Eng. 2005, 5632, 80.
[6] N. Johansson, D. A. dos Santos, S. Guo, J. Cornil, M. Fahlman, J. Sal-beck, H. Schenk, H. Arwin, J. L. Brédas, W. R. Salanek, J. Chem.
Phys. 1997, 107, 2542.
[7] V. Bulovic, R. Deshpande, M. E. Thompson, S. R. Forrest, Chem.
Phys. Lett. 1999, 308, 317.
[8] Examples see: C. Reichardt, Solvents and Solvent Effects in Organic
Chemistry, VCH, Weinheim 1988.
[9] C.-T. Chen, S.-Y. Liao, K.-J. Lin, C.-H. Chen, T.-Y. J. Lin, Inorg.
Chem. 1999, 38, 2734.
[10] T. Nakabayashi, Md. Wahadoszamen, N. Ohta, J. Am. Chem. Soc.
2005, 127, 7041.
[11] Crystal data for CHO: C38H25NO: Fw = 511.59, monoclinic, P21/c,
Z = 4, F(000) = 1072. Cell dimensions: a = 10.3824(2) Å, b = Table 2. Calculated ground state dipole moments and differences in
per-manent dipole moments of the first excited state and ground state [a]. Ground state dipole First excited state dipole Difference dipole moment
lgrDFT lgrHF lexCIS |lexCIS–lgrHF|
DCV 10.59 10.40 16.50 6.14 CHO 5.20 5.10 9.56 4.46 DPV 0.65 0.46 2.07 1.66 2DCV 14.50 14.05 18.41 4.43 2CHO 7.07 6.79 9.97 3.13 2DPV 0.47 0.42 1.75 1.35
[a] in Debye. B3LYP/6-31G* and HF/6-31G* were used for ground state, and CIS/6-31G* for excited state calculations.
Table 3. Electroluminescence characteristics of dopant and non-dopant OLEDs of DCV, 2DCV,
CHO, 2CHO, DPV, and 2DPV. Doping level of devices [wt. %] Max. luminance and voltage [cd m–2, V] Luminance, Efficien-cy, Voltage [cd m–2, %, V] [e] Max. efficiency [%, cd A–1, lm W–1] kmaxel [nm] CIE 1931 chromaticity [x, y] DCV 0.5 [a] 37640, 15 1750,3.1, 9.1 3.4, 9.4, 8.2 584 0.50, 0.48 DCV 3 [a] 20950, 15 1080,2.4, 9.1 2.7, 6.3, 4.1 602 0.55, 0.44 DCV 100 [b] 8190, 15 3103,1.4,6.3 1.5, 1.6, 1.1 634 0.65, 0.35 2DCV 0.5 [a] 8080, 15 1210,2.3,11.3 2.4, 6.4, 4.5 592 0.52, 0.47 2DCV 5 [a] 4290, 15 430,1.2,9.1 1.6, 3.0, 2.3 612 0.59, 0.41 2DCV 100 [b] 2470, 15 90,0.66,6.6 0.69,0.47,0.22 648 0.67, 0.33 CHO 100 [c] 33470, 15 1430, 2.6, 7.3 2.7, 7.3, 3.8 502 0.19, 0.50 2CHO 100 [c] 14600, 15 410, 0.7, 7.5 0.8, 2.4, 0.9 506 0.20, 0.53 DPV 100 [d] 33020, 14 910,2.9,4.7 3.4,5.4, 5.7 478 0.14, 0.22 2DPV 100 [d] 25470, 15 780,2.2,5.1 2.8,4.9, 5.1 480 0.15, 0.25
[a] Devices have the configuration of ITO/NPB/DCV or 2DCV: Alq3/BCP/Alq3/Mg:Ag (top right diagram in Scheme 2). [b] ITO/NPB/DCV or 2DCV/BCP/Alq3/Mg:Ag (top left diagram in Scheme 2). [c] ITO/NPB/CHO or 2CHO/BCP/Alq3/Mg:Ag (bottom left diagram in Scheme 2). [d] Devices have the configuration of ITO/NPB/DPV or 2DPV/TPBI/LiF/Al (bottom right dia-gram in Scheme 2). [e] At current density of 20 mA cm–2
FULL
P
28.2780(6) Å, c = 8.9198(2) Å, a = 90°, b = 91.7920(10)°, c = 90°,
V = 2617.51(10) Å3, 2hmax= 50.0o, qcalcd= 1.298 g cm–3. Of 20425
re-flections, 4628 were independent, 362 parameters, R (Fo) = 0.0301
(for reflections with I > 2r(I)), Rw(Fo) = 0.0640 (for reflections with I
> 2r(I)). The GoF on F2was equal 0.854. Crystal data for 2CHO: C51H34N2O2: Fw = 706.80, orthorhombic, Pbcn, Z = 4,
F(000) = 1480. Cell dimensions: a = 14.9404(4) Å, b = 11.3117(3) Å, c
= 21.3320(6) Å, a = 90°, b = 90°, c = 90°, V = 3605.14(17) Å3,
2hmax= 50.0o, qcalcd= 1.302 g cm–3. Of 28706 reflections, 3017 were
in-dependent, 250 parameters, R (Fo) = 0.0395 (for reflections with I >
2r(I)), Rw(Fo) = 0.0833 (for reflections with I > 2r(I)). The GoF on
F2was equal 0.925. CCDC-645601 and CCDC-645602 contain the
sup-plementary crystallographic data of CHO and 2CHO, respectively. These data can be obtained free of charge via www.ccd.cam.ac.uk/ conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or [email protected]).
[12] The layer thickness of each OLED is described in Scheme 2 in the Ex-perimental section.
[13] a) Y.-H. Kim, D.-C. Shin, S.-H. Kim, C.-H. Ko, H.-S. Yu, Y.S. Chae, S.-K. Kwon, Adv. Mater. 2001, 13, 1690. b) L.-H. Chan, R.-H. Lee, C.-F. Hsieh, H.-C. Yeh, C.-T. Chen, J. Am. Chem. Soc. 2002, 124, 6469. c) K. Danel, T.-H. Huang, J. T. Lin, Y.-T. Tao, C.-H. Chuen, Chem.
Ma-ter. 2002, 14, 3860. d) J. Shi, C. W. Tang, Appl. Phys. Lett. 2002, 80, 3201.
e) M. Guan, Z. Q. Bian, Y. F. Zhou, F. Y. Li, Z. J. Li, C. H. Huang,
Chem. Commun. 2003, 2708. f) K. R. J. Thomas, J. T. Lin, Y.-T. Tao,
C.-H. Chien, Adv. Funct. Mater. 2003, 13, 445. g) W.-J. Shen, R. Dodda, C.-C. Wu, F.-I. Wu, T.-H. Liu, H.-H. Chen, C. H. Chen, C.-F. Shu,
Chem. Mater. 2004, 16, 930. h) C.-H. Chen, F.-I. Wu, C.-F. Shu, C.-H.
Chien, Y.-T. Tao, J. Mater. Chem. 2004, 14, 1585. i) F.-I. Wu, C.-F. Shu, T.-T. Wang, E. W.-G. Diau, C.-H. Chien, C.-H. Chuen, Y.-T. Tao, Synth.
Met. 2005, 151, 285. j) K. R. J. Thomas, M. Velusamy, J. T. Lin, C.-H.
Chien, Y.-T. Tao, J. Mater. Chem. 2005, 15, 4453. k) J. Y. Shen, C. Y. Lee, T.-H. Huang, J. T. Lin, Y.-T. Tao, C.-H. Chien, C. Tsai, J. Mater.
Chem. 2005, 15, 2455. l) F.-I. Wu, P.-I. Shih,
M.-C. Yuan, A. K. Dixit, C.-F. Shu, Z.-M. Chung, E. W.-G. Diau, J. Mater. Chem. 2005,
15, 4753. m) S. Tao, Z. Peng, X. Zhang,
P. Wang, C.-S. Lee, S.-T. Lee, Adv. Funct.
Ma-ter. 2005, 15, 1716. n) X. R. Wang, J. S. Chen,
H. Yiou, D. G. Ma, R. G. Sun, Jpn. J. Appl.
Phys. Part 1 2005, 44, 8480. o) R. J. Tseng,
R. C. Chiechi, F. Wudl, Y. Yang, Appl. Phys.
Lett. 2006, 88, 093512. p) C. Tang, F. Liu, Y.-J.
Xia, J. Lin, L.-H. Xie, G.-Y. Zhong, Q.-L. Fan, W. Huang, Org. Electron. 2006, 7, 155. q) R. C. Chiechi, R. J. Tseng, F. Marchioni, Y. Yang, F. Wudl, Adv. Mater. 2006, 18, 325. r) H.-C. Li, Y.-P. Lin, P.-T. Chou, Y.-M. Cheng, R.-S. Liu,
Adv. Funct. Mater. 2007, 17, 520. s) C. J.
Ton-zola, A. P. Kulkarni, A. P. Gifford, W. Ka-minsky, S. A. Jenekhe, Adv. Funct. Mater.
2007, 17, 863.
[14] a) Z. Y. Xie, L. S. Hung, S. T. Lee, Appl.
Phys. Lett. 2001, 79, 1048. b) X. Q. Lin, B. J.
Chen, X. H. Zhang, C. S. Lee, H. L. Kwong, S. T. Lee, Chem. Mater. 2001, 13, 456. c) K. R. J. Thomas, J. T. Lin, Y.-T. Tao, C.-H. Chuen, Chem. Mater. 2002, 14, 2796. d) K. R. J. Thomas, J. T. Lin, Y.-T. Tao, C. H. Chuen, J. Mater. Chem. 2002, 12, 3516. e) K. R. J. Thomas, J. T. Lin, Y.-T. Tao, C. H. Chuen, J. Mater. Chem. 2002, 12, 3852. f) W.-F. Xie, C.-N. Li, S.-Y. Liu, Chin. Phys.
Lett. 2003, 20, 956. g) T.-H. Liu, Y.-S. Wu,
M.-T. Lee, H.-H.Chen, C.-H. Liao, C. H. Chen, Appl. Phys. Lett. 2004, 85, 4304. h) H.-Y.Chen, Chi, Y. C.-S. Liu, J.-K. Yu, Y.-M. Cheng, K.-S. Chen, P.-T. Chou, S.-M. Peng, G.-H. Lee, A. J. Carty, S.-J. Yeh, C.-T. Chen, Adv.
Mater. 2005, 17, 567. i) F. Nüesch, D. Berner, E. Tutiš, M. Schaer, C.
Ma, X. Wang, B. Zhang, L. Zuppiroli, Adv. Funct. Mater. 2005, 15, 323. j) J. Lee, Y.-Y. Yuan, Y. Kang, W.-L. Jia, Z.-H. Lu, S. Wang, Adv.
Funct. Mater. 2006, 16, 681. k) Y.-S. Wu, T.-H. Liu, H.-H. Chen, C. H.
Chen, Thin Solid Films 2006, 496, 626.
[15] J. C. de Mello, H. F.Wittmann, R. H. Friend, Adv. Mater. 1997, 9, 230. [16] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
[17] A. Dreuw, M. Head-Gordon, J. Am. Chem. Soc. 2004, 126, 4007. [18] J. Kong, C. A. White, A. I. Krylov, C. D. Sherrill, R. D. Adamson,
T. R. Furlani, M. S. Lee, A. M. Lee, S. R. Gwaltney, T. R. Adams, C. Ochsenfeld, A. T. B. Gilbert, G. S. Kedziora, V. A. Rassolov, D. R. Maurice, N. Nair, Y. Shao, N. A. Besley, P. E. Maslen, J. P. Dombros-ki, H. Daschel, W. Zhang, P. P. Korambath, J. Baker, E. F. C. Byrd, T. Van Voorhis, M. Oumi, S. Hirata, C.-P. Hsu, N. Ishikawa, J. Florian, A. Warshel, B. G. Johnson, P. M. W. Gill, M. Head-Gordon, J. A. Po-ple, J. Comput. Chem. 2000, 21, 1532.
[19] G. M. Sheldrick, SHELXL-97, University of Göttingen, Germany
1997.
[20] L. J. Farrugia, J. Appl. Crystallogr. 1999, 32, 837.
[21] S. R. Forrest, D. D. C. Bradley, M. E. Thompson, Adv. Mater. 2003,
15, 1043.
[22] G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods,
Quantitative Data and Formulae, Wiley, New York, NY 1982, p. 259.
[23] In this study, we estimated the HOMO energy level of the material by AC-2 measurement instead of cyclic voltammetry (CV). This is be-cause the samples subjected to the measurement are in solid state for AC-2 rather than in solution for CV. Solid-state samples are consistent with the form in OLEDs. It is well-known now that the solution CV-determined HOMO energy levels are different from those CV-determined by solid-state photoemission spectroscopy, such as AC-2. B. W. D’An-drare, S. Datta, S. R. Forrest, P. Djurovich, E. Polikarpov, M. E. Thompson, Org. Electron. 2005, 6, 11.
3.7 6.4~6.5 2.9~3.0 BCP Alq3 5.95 3.25 5.73~5.75 3.32~3.36 5.6 2.5 4.7 5.1 ~ DCV or 2DCV NPB ITO BCP Alq3 5.73~5.75 3.32~3.36 doped in NPB ITO Alq3
Non-Dopant-Based Red OLEDs Dopant-Based Yellow or Orange OLEDs
50 nm DCV and 2DCV: 40 nm in non-dopant devices
NPB:
Alq3: 30 nm for electron-transporting layers; 40 nm for the host layer of dopants or 2DCV BCP: 10 nm Mg:Ag DCV Mg:Ag Al LiF TPBI NPB ITO DPV or 2DPV 1 nm 40 nm 50 nm 10 nm 4.1 6.2 2.7 5.60~5.64 2.90~2.95 NPB ITO CHO or 2CHO BCP Alq3 Mg:Ag 40 nm 10 nm 30 nm 5 nm
Non-Dopant-Based Green and Blue OLEDs
2.59~2.64
5.50~5.53
Scheme 2. Relative energy-level alignments and layer thickness of OLEDs.
