Synthesis, Crystal Structure, and Prediction of Hole Mobilities of 2,7′-Ethylenebis(8-hydroxyquinoline) He-Ping Zeng,* Xin-Hua OuYang, Ting-Ting Wang, Guo-Zan Yuan, Guang-Hui Zhang, and Xin-min Zhang
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1697-1702
Institute of Functional Molecule, South China UniVersity of Technology, Guangzhou 510641, People's Republic of China ReceiVed April 6, 2006; ReVised Manuscript ReceiVed May 16, 2006
ABSTRACT: A novel bis(8-hydroxyquinoline) compound based on formylquinolinol has been synthesized and characterized by X-ray single-crystal diffraction, IR, UV, 1H NMR, MS, and elemental analysis. It is reported that the formylation of 8-hydroxyquinoline gives substitution at the 7-position by X-ray structure analysis. In comparison with the spectrum for 8-hydroxyquinoline, the fluorescence spectrum of this novel bis(8-hydroxyquinoline) species shows a red shift. The optimized geometrical structures of the neutral and cationic states on 2,7′-ethylenebis(8-hydroxyquinoline) were optimized at the DFT B3LYP/cc-pvdz level. The results are in good agreement with the reference and experimental values. The hole mobility of 2,7′-ethylenebis(8-hydroxyquinoline) has been predicted by the reorganization energy and electron-transfer coupling matrix elements using quantum mechanics (QM), and the value is 1.612 cm2/(V s). Introduction Since small-molecule-based heterojunction organic lightemitting diodes (OLED) were reported by Tang et al. in 1987, organic materials based on 8-hydroxyquinoline for OLED have attracted extensive interest because of their high efficiency and potentially low production cost.1,2 One of the most attractive features of OLED is the ability to easily tune their emission wavelengths by modifying the substituents on the organic materials.3,4 Matsumura and Burrows reported that electronwithdrawing groups (EWGs) such as fluoro,5 chloro,5 and cyano6 groups at the 5- or 7-position of the benzene ring resulted in almost negligible emission shifts, while strong EWGs such as sulfonamide (-SO2NR2) resulted in significantly blue-shifted emission.7 Here a novel bis(8-hydroxyquinoline) species has been synthesized, in which the 8-hydroxyquinoline moiety is substituted at the 7-position, and the fluorescence spectrum of this compound shows a red shift compared with that of 8-hydroxyquinoline. On the other hand, for hole-transport materials, good electrondonating properties, low ionizing energy, and high hole mobility are important. With the alteration of the geometric and electronic properties of the molecules, the fundamental physical processes such as hole mobility, which is important in optimizing the performance of OLED, changed. The hole-transport mobility increases when the theoretically evaluated reorganization energy decreases. High mobilities can reduce the resistance of the device, leading to high power efficiency. Deng et al.8 developed a model to predict accurate values of the reorganization energies and absolute hole mobilities for organic crystals of the compounds. In the present study, we have also employed this model to estimate the hole mobility for the crystal of 2,7′-ethylenebis(8-hydroxyquinoline) by calculating the reorganization energy and electron-transfer coupling matrix elements. Results and Discussion Synthesis and Crystal Structure of 2,7′-Ethylenebis(8hydroxyquinoline). Yoneda9 reported the crystal structure of 2,2′-ethylenebis(8-hydroxyquinoline). Recently, we have established a facile synthetic route, as shown in Scheme 1, to
synthesize 2,7′-ethylenebis(8-hydroxyquinoline). Compound 1 was isolated from 8-hydroxyquinoline after a Reimer-Tiemann reaction. It was identified as formylquinolinol by its melting point and mass spectrum. The spectroscopic/spectrometric (1H NMR, IR, MS) and elemental analysis data unambiguously support the structure assignment of the compound 4. In addition, an X-ray crystal structure analysis (Figure 1) clearly shows the conformation of the bis(8-hydroxyquinoline) compound. Thus, we located the position of formylation at the 7-position during the ReimerTiemann reaction. In this molecular structure, the bond lengths of C(1)-C(2) and C(2)-C(3) are 1.335(2) and 1.456(2) Å, respectively, which are significantly shorter than the bond length of a classical C-C bond. The hydrogen bonds play important roles in the crystal packing of compound 4. In Figure 2, atoms N(1) and N(2) act as intramolecular hydrogen bond acceptors to form O-H‚‚‚N (H‚‚‚N ) 2.28(1) and 2.21(2) Å, O-H‚‚‚N ) 114.5(1) and 116.6(2)°). The existence of the two interlayer H bonds extends the two-dimensional structure further into a three-dimensional array. With a neighboring molecule, in Figure 3, O(1) and O(2A) participate in O-H‚‚‚O contacts (H‚‚‚O ) 2.18 Å, O(1)-H(1A)‚ ‚‚O(2)#1 ) 140.0°; symmetry code #1 corresponds to x, y + 1, z). In the meantime, O(2) and O(2B) are involved in a cyclic hydrogen bonding mode with N(2) and N(2B) atoms (H‚‚‚N ) 2.60 Å, O(2)-H(2A)‚‚‚N(2)#2 ) 122.6°; symmetry code #2 corresponds to -x + 2, -y, -z). The H‚‚‚O and H‚‚‚N distances in compound 4 fall well within the estimated range of 2.162.65 Å. Fluorescence Properties of Compound 4. Figure 4 shows the luminescence spectra of 8-hydroxyquinoline and 7-formyl8-hydroxyquinoline (1), which were recorded in CHCl3 at room temperature. Excited by 310 and 376 nm light, respectively, compound 2 exhibits two broad emission bands (λmax ) 387, 488 nm) which might correspond to the singlet and triplet states.10,11 Figure 5 shows the luminescence spectra of compounds 1 and 4, which were recorded in CHCl3 at room
10.1021/cg060197s CCC: $33.50 © 2006 American Chemical Society Published on Web 06/14/2006
1698 Crystal Growth & Design, Vol. 6, No. 7, 2006 Scheme 1.
Zeng et al. Synthetic Route
where λ is the reorganization energy, V is the coupling matrix element, kB is the Boltzmann constant, and T is the temperature. The diffusion coefficient can be evaluated from the hopping rates as
D)
1
r2WiPi ∑ 2n i
(2)
where n ) 3 is the dimensionality, Wi is the hopping rate due to charge carrier to the ith neighbor, ri is the distance to neighbor i, and P is the relative probability for charge carrier to a particular ith neighbor
Pi ) Wi/
Figure 1. Molecular structure and labeling scheme for compound 4.
∑i Wi
(3)
Summing over all possible hops leads to the diffusion coefficient in eq 2. The drift mobility of hopping, µ, is then evaluated from the Einstein relation
µ)
e D kBT
(4)
where e is the electronic charge. Considering now the structural disorder present at higher temperature, the coupling matrix element V becomes a function of distance between two adjacent molecules, leading to
Figure 2. Intramolecular hydrogen bonds of compound 4.
temperature. In comparison with the spectrum for compound 1, the fluorescence spectrum of the compound 4 shows a red shift. Prediction of the Hole Mobility for 2,7′-Ethylenebis(8hydroxyquinoline). To describe the hole transport mobility of 2,7′-ethylenebis(8-hydroxyquinoline), we employed the model and a suite of expressions put forward by Deng and Goddard.8 They considered an incoherent hopping model in which charge can transfer only between adjacent molecules. A similar approach was used successfully to describe the conduction properties of organic superconductors. With each hopping event viewed as a nonabiabatic electro-transfer reaction, standard Marcus theory was used to express the rate of charge motion between neighboring molecules, W. The expressions are listed as follows:
W(r) )
( ) (
V(r)2 π p λkBT
1/2
exp -
λ 4kBT
)
(5)
Here we neglect the angular changes between the dimers. The relative probability in eq 3 becomes
P(r) ) N(r)W(r)/
∫0∞N(r)W(r) dr
(6)
where N(r) is the number of near neighbors at r
N(r) ) Fg(r)4πr2 dr
(7)
and g(r) is the probability density of having this type of neighbor at a distance r. The resulting diffusion coefficient becomes
D)
∫0rr2W(r)P(r) dr
1 2n
(8)
Accordingly, the elementary hopping step in molecular wires is characterized by four energies: E (neutral in neutral
2,7-Ethylenebis(8-hydroxyquinoline)
Crystal Growth & Design, Vol. 6, No. 7, 2006 1699
Figure 3. Intermolecular hydrogen bond network of compound 4.
use the unrestricted formalism (UB3LYP), but spin contamination is minimal (less than 0.01). Thus, the coupling matrix element for a given electronic level is related to the energetic splitting of that level in the dimer as compared to the isolated neutral molecule. For these organics the highest occupied molecular orbital (HOMO) of the isolated molecule is a π orbital delocalized over the molecule (with energy ), which for the neutral dimer splits into two levels denoted as HOMO and HOMO-1. The coupling matrix element, V, is given by
V)
Figure 4. Fluorescence spectra of 8-hydroxyquinoline (a) and compound 1 (b).
Figure 5. Fluorescence spectra of compounds 1 (a) and 4 (b).
W)
( ) (
V2 π p λkBT
1/2
exp -
λ 4kBT
)
(1)
geometry), E* (neutral in ion geometry), E+ (ion in ion geometry), and E*+ (ion in neutral geometry). By definition
λ ) λ1 + λ2 ) (E*+ - E+) + (E* - E)
(9)
In addition to reorganization energy, the vertical ionization potential (VIP) has also been determined:
VIP ) E*+ - E
(10)
The QM calculations to determine these quantities used the B3LYP flavor of density functional theory (DFT) with the ccpvdz basis set. All calculations on open-shell (ionized) states
1 (E - EHOMO-1)2 - (2 - 1)2 2x HOMO
(11)
In this case the isolated molecules are identical (and in equivalent sites in the crystal), so that eq 11 becomes
1 V ) (EHOMO - EHOMO-1) 2
(12)
The neutral and cationic states of 2,7′-ethylenebis(8-hydroxyquinoline) were optimized at the DFT level using the B3LYP functional12 with the cc-pvdz basis set. The single-point energy and MO calculations of neutral and cationic species were performed at the B3LYP/cc-pvdz level of theory using the Gaussian 2003 program suite.13 4,4′-Bis(phenyl-m-tolylamino)biphenyl (TPD) has also been calculated to compare with the data calculated at the B3LYP/6-31G(d) level by Lin14 et al. Optimized Geometry. The chemical structure of 2,7′ethylenebis(8-hydroxyquinoline) is shown in Figure 6. Tables 1 and 2 give bond lengths, torsional angles, and dihedral angles in the optimized geometries of TPD and 2,7′ethylenebis(8-hydroxyquinoline) in its neutral and cationic states. From Table 2, it can be seen that 2,7′-ethylenebis(8hydroxyquinoline) shows little geometry relaxation, with the greatest changes in C1-C12 bond length on the order of 0.031 Å between the neutral and cationic states. The values in parentheses are the experimental values measured by X-ray crystallography. The largest errors in bond length and bond angle between theoretical predictions and experimental measurements are 0.018 Å and 1.5°, respectively. The torsional angles show that all the atoms in the neutral state of 2,7′-ethylenebis(8-hydroxyquinoline) stay almost planar; this is also the case for the cation. The shortening of the interring distances in the cationic state can easily be seen from the HOMO (-5.47 eV) of 2,7′-ethylenebis(8-hydroxyquinoline) (Figure 7). There is an antibonding interaction between the π orbitals on the two 8-hydroxyquinoline groups. Hence, in comparison with the neutral state, removing an electron from the HOMO leads to a shortening of the inter-ring distance in the cationic state. The LUMO (-2.14 eV) of 2,7′-ethylenebis(8-hydroxyquinoline) shows that the shortening of the inter-
1700 Crystal Growth & Design, Vol. 6, No. 7, 2006
Zeng et al.
Figure 6. Chemical structures of TPD and 2,7′-ethylenebis(8-hydroxyquinoline). Table 1. Bond Lengths, Bond Angles, and Torsional Angles of the Neutral and Cationic States of TPD neutral
cation
N-C4 N-C5 N-C7 C1-C1′
1.419 (1.419) 1.422 (1.422) 1.424 (1.423) 1.482 (1.480)
1.386 (1.387) 1.432 (1.431) 1.433 (1.432) 1.456 (1.455)
C4-N-C7 C4-N-C5 C5-N-C7
120.0 120.0 119.8
121.1 121.1 117.9
C3-C4-N-C5 C6-C5-N-C4 C8-C7-N-C4 C2′-C1′-N-C1-C2
40.3 (41.5) 42.1 (40.9) 44.2 (42.7) 34.4 (34.8)
26.2 (25.8) 49.7 (48.8) 50.4 (49.3) 18.8 (22.4)
Table 2. Bond Lengths, Bond Angles, and Torsional Angles of the Neutral and Cationic States of 2,7-Ethylenebis(8-hydroxyquinoline) neutral
cation
C1-C2 C1-C12 C2-C3 C12-C18 C12-C13 C3-C4 C3-N1
1.353 (1.335) 1.460 (1.454) 1.463 (1.456) 1.398 (1.383) 1.431 (1.419) 1.432 (1.423) 1.334 (1.332)
1.377 1.429 1.436 1.422 1.438 1.435 1.355
C2-C1-C12 C1-C12-C13 C1-C12-C18 C18-C12-C13 C1-C2-C3 C2-C3-C4 C2-C3-N1 C4-C3-N1
126.6 (127.9) 123.6 (122.5) 119.0 (120.4) 117.4 (117.2) 126.3 (125.3) 123.2 (121.7) 115.8 (116.9) 120.9 (121.4)
126.2 124.6 118.2 117.2 125.8 124.4 114.7 120.9
C2-C1-C12-C18 C2-C1-C12-C13 C12-C1-C2-C3 C1-C2-C3-N1 C1-C2-C3-C4
180.0 (177.5) 0.0 (-3.7) 180.0 (178.5) 180.0 (180.0) 0.0 (-1.4)
180.0 0.0 180.0 180.0 0.0
ring distance in the anionic state is due to the bonding interactions between the π orbitals on the two 8-hydroxyquinoline groups. Reorganization Energy and Hole Mobility. The hole mobility (µ) mainly depends on the monomer reorganization energy (λ) and the coupling matrix element (V) between dimers. The reorganization energies λ of TPD and 2,7′-ethylenebis(8hydroxyquinoline) are collected in Table 3. The calculated reorganization energy λ (0.29 eV) for TPD, which is amorphous, is in good agreement with the data calculated by Bredas et al.15 In comparison with the experimental values, the calculated values of the TPD are bigger than the experimental values by a factor of about 5. The results above indicate that the method we chose is suitable for calculating the hole mobility of the compound. Here we employed the four possible dimers which contribute more to hole mobility, T1, T2, T3, and T4, as shown in Figure 8, to calculate the reorganization energy of 2,7′-ethylenebis(8-
hydroxyquinoline). The geometries of the dimers were optimized at the B3LYP/cc-pvdz level with the center of mass distance and cross-angle plane fixed. The diffusion coefficients and hole mobilities of TPD and 2,7′-ethylenebis(8-hydroxyquinoline) are given in Table 4. The calculated value of TPD in this paper is 4.895 × 10-3 cm2/(V s), which is larger than the experimental value by a factor of ∼5. The hole mobility of 2,7′-ethylenebis(8-hydroxyquinoline) is 1.612 cm2/(V s). The results show that 2,7′-ethylenebis(8-hydroxyquinoline), which is a planar molecule, is favorable for the hole transport and electron transport. From Table 4, we can see that the dimers T2 and T3 make large contributions to the hole mobility. The coupling matrix element of each dimer is small when the center of mass distance becomes large; conversely, the coupling matrix element is large. This indicates that the hole mobility is quite anisotropic and is dominated by hole transfer within the layers. Conclusion A novel bis(8-hydroxyquinoline) species based on formylquinolinol was synthesized and characterized by X-ray single-crystal diffraction, IR, UV, 1H NMR, MS, and elemental analysis. The formylation of 8-hydroxyquinoline resulted in substitution at the 7-position by X-ray structure analysis. The fluorescence spectrum of the bis(8-hydroxyquinoline) compound shows a red shift, in comparison with 8-hydroxyquinoline. The neutral and cationic states of 2,7′-ethylenebis(8-hydroxyquinoline) were optimized at the DFT B3LYP/cc-pvdz level of theory. MO calculations were performed at the same level of theory. By calculation of the reorganization energy (λ) and hole mobility for the compound, it can be found that the reorganization energy for hole transport (λ) is controlled by the HOMO. Therefore, the major contributor to the HOMO in the substituent moiety of the compound determines the magnitude of λ with some modification due to the presence of the other moieties. On the basis of the reorganization energy, compounds with desired transport properties can be designed by theoretical prediction to find suitable materials for OLED devices. Experimental Section Reimer-Tiemann Reaction of 8-Hydroxyquinoline. 8-Hydroxyquinoline (15 g), ethanol (60 mL), and aqueous sodium hydroxide (30 g in 60 mL of water) were refluxed while chloroform (27 g) was added dropwise over 1 h after refluxing for 20 h; ethanol and excess chloroform were distilled off. The residue was dissolved in water (600 mL), and the solution was acidified with hydrochloric acid. The solid that separated was dried and then continuously extracted with ethyl acetate. 7-Formyl-8-hydroxyquinoline as straw-colored needles was obtained by recrystallization from chloroform. Yield: 2.6 g, 14.5%. Mp: 174-176 °C. MS (m/z): 173 (M+, 100%). Synthesis of Compound 4.17 A mixture of 2-methyl-8-hydroxyquinoline (1.64 g, 0.01 mmol), 7-formyl-8-hydroxyquinoline (1; 1.80 g, 0.01 mol), and acetic anhydride (15 mL) was stirred and heated at 125 °C for 40 h under nitrogen. After this mixture was cooled, it was subsequently poured into ice-water and stirred overnight. The yellow solid obtained was filtered and washed with water. Yield: 0.72 g,
2,7-Ethylenebis(8-hydroxyquinoline)
Crystal Growth & Design, Vol. 6, No. 7, 2006 1701
Figure 7. HOMO and LUMO of TPD and 2,7′-ethylenebis(8-hydroxyquinoline) in the cationic and neutral states. Table 3. QM Energies (hartree) for Each Structure Calculated (UB3LYP/cc-pvdz)
E (eV) E* (eV) E+ (eV) E*+ (eV) VIP (eV) λ (eV) this work ref 16
TPD
2,7′-ethylenebis(8-hydroxyquinoline)
-42 910.43 -42 910.28 -42 904.70 -42 904.56 5.87
-28 044.19 -28 037.54 -28 037.42 -28 044.06 6.77
0.29 0.28
0.25
19.2%. A purified sample of 2 was obtained by recrystallization from DMF and had a melting point of 203-205 °C. MS (EI; m/z): 398.0 [M+], 357.0 [(M - 42)+], 314.0 [(M - 82)+, 100%]. FTIR (KBr; ν, cm-1): 1740.14, 1619.92, 1574.89, 1508.83. Compound 2 (0.001 mol, 0.40 g) was dissolved in DMF (20 mL) by heating at 120 °C. Aqueous hydrochloric acid (35%, 7 mL) was added to the solution, and the mixture was heated at 120-130 °C for 2 h. The precipitated orange solid was filtered off, washed with water, and dried to afford compound 3. Yield: 0.38 g, 97.4%.
Figure 8. Crystal structure of 2,7′-ethylenebis(8-hydroxyquinoline). A flask was charged with a mixture of compound 3 (0.6 mmol, 0.24 g) and DMF (10 mL). Triehylamine (2 mmol, 0.20 g) was added to the solution at the boiling point, and a red solution was obtained. This solution was subsequently poured into ice-water (50 mL). The yellow solid that was obtained was filtered off, washed with water, and dried
1702 Crystal Growth & Design, Vol. 6, No. 7, 2006
Zeng et al.
Table 4. Hole Mobilities of the Compounds
(3) (a) Yu, J.; Chen, Z.; Sakuratani, Y.; Suzuki, H.; Tokita, M.; Miyata, S. Jpn. J. Appl. Phys. 1999, 38, 6762. (b) Kido, J.; Iizumi, Y. Chem. Lett. 1997, 963. (4) Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K. Jpn. J. Appl. Phys. 1993, 32, 514. (5) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys. 1996, 35, 5357. (6) Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S. R.; Cronin, J. A.; Thompson, M. E. J. Appl. Phys. 1996, 79, 7991. (7) Hopkins, T. A.; Meerholz, K.; Shaheen, S.; Anderson, M. L.; Schmidt, A.; Kippelen, B.; Padias, A. B.; Hall, H. K., Jr.; Peyghambarian, N.; Armstrong, N. R. Chem. Mater. 1996, 8, 344. (8) Deng, W. Q., Goddard, W. A.. III. J. Phys. Chem. B 2004, 108, 8614. (9) Yoneda, A.; Hakushi, T.; Newkome, G. R.; Matsushita, T. Trans1,2-Bis(8-hydroxy-2-quinolinyl)ethene: comparison with trans-stilbene. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1996, C52(1), 172. (10) Piguet, C.; Bu¨nzli, J.-C. G.; Bernardinelli, G.; Hopfgartner, G.; Williams, A. F. J. Am. Chem. Soc. 1993, 115, 8197. (11) Piguet, C.; Williams, A. F.; Bernardinelli, G.; Bunzli, J.-C. G. Inorg. Chem. 1993, 32, 4139. (12) Beche A. D. J. Chem. Phys. 1996, 104, 1040. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; AlLaham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (14) Wang, P.; Xie, Z.; Tong, S.; Wong, O.; Lee, C.-S.; Wong, N.; Hung, L.; Lee, S. Chem. Mater. 2003, 15, 1913. (15) Burin, A. L.; Berlin, Y. A.; Ratner, M. A. J. Phys. Chem. A 2001, 105, 2652. (16) Lin, B. C.; Cheng, C. P.; Lao, Z. P. M. J. Phys. Chem. A 2003, 107, 5241. (17) Kim, S.-H.; Cui, J.-Z.; Park, J.-Y.; Ryu, J.-H.; Han, E.-M.; Park, S.-M.; Jin, S.-H.; Koh, K., Ga, Y.-S. Dyes Pigments 2002, 55, 91. (18) SMART, Version 5.0; Bruker AXS, Madison, WI, 1998. (19) SAINT+, Version 6.0; Bruker AXS, Madison, WI, 1999. (20) Blessing, R. Acta Crystallogr., Sect. A 1995, 51, 33. (21) SHELXTL, Version 5.1; Bruker AXS, Madison, WI, 1998. (22) Sheldrick, G. M. SHELX97, Program for X-ray Crystal Structure Solution and Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
T1 dimer C-M dist (Å) VT1 (eV) T2 dimer CM dist (Å) VT2 (eV) T3 dimer CM dist (Å) VT3 (eV) T4 dimer CM dist (Å) VT4 (eV) diffusion coeff, D (cm2/s) drift mobility, µ (cm2/(V s)) this work exptl
TPD
2,7′-ethylenebis(8hydroxyquinoline)
9.6799 0.0022 7.3356 0.0068 6.0359 0.0088 8.7780 0.0039 1.257 × 10-4
10.3319 0.0742 6.9353 0.1377 12.5294 0.0485 14.8134 0.0336 0.0414
4.895 × 10-3 1.0 × 10-3
1.612
to afford compound 4. Purified compound 4 was obtained by recrystallization from toluene and had a melting point of 231-233 °C. Yield: 0.17 g, 89.5%. UV (in CHCl3; λmax, nm): 254, 377. IR (KBr; ν, cm-1): 3677.42, 1621.82, 1574.99, 1509.8. MS (EI; m/z): 314.0 [M+, 92.08%], 313.0 [(M - 1)+, 100%]. 1H NMR (CDCl3; δ, ppm): 7.188 (d, J ) 7.2 Hz, 1H), 7.311 (t, J ) 8 Hz, 2H), 7.412 (q, J ) 7.6, 8.0 Hz, 2H), 7.570 (q, J ) 3.2, 4.4 Hz, 1H), 7.662 (d, J ) 8.4 Hz, 1H), 7.936 (d, J ) 7.6 Hz, 1H), 8.155 (d, J ) 8.4 Hz, 1H), 8.389 (d, J ) 15.6 Hz, 1H), 8.673 (d, J ) 8.4 Hz, 1H), 8.833 (d, J ) 3.2 Hz, 1H). Anal. Found: C, 76.30; H, 4.50; N, 8.82. Calcd for C20H14N2O2: C, 76.43; H, 4.46; N, 8.92. Crystallographic Studies. Experimental details of the X-ray analyses are provided in Table 1. All diffraction data were collected on a Bruker Smart 1000 CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å) at room temperature using the program SMART18 and processed by SAINT+.19 Absorption corrections were applied by SADABS.20 Space groups of these compounds were determined from systematic absences and further justified by the refinement results. In all cases, the structures were solved by direct methods and refined using full-matrix least-squares/difference Fourier techniques using SHELX.21,22 All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms of the ligands were placed at idealized positions and refined as riding atoms with the relative isotropic parameters of the heavy atoms to which they are attached. The H atoms of water were located from the difference Fourier map in the final stage of refinement.
Acknowledgment. Financial support from the National Natural Science Foundation of China (Nos. 20231020, 20471020) is gratefully acknowledged. We are grateful to Dr. Ji Zhang of Chemical Research and Development, Global Research & Development, Pfizer, Inc., Ann Arbor Laboratories, for helpful discussions. Supporting Information Available: Text, tables, and CIF files giving general experimental methods and complete crystallographic data (collection, refinement, crystal parameters, atom coordinates and thermal parameters, bond lengths and angles). This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Tang, C. W. J. Appl. Phys. 1989, 65, 3610
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