C O M M U N I C A T I O N
Crystal Study of N,N
0
-diphenyl-N,N
0
-bis(1-naphthyl)-1,1
0
-biphenyl-4,4
0
-diamine
Jung-An Cheng•Pi-Ju Cheng
Received: 10 March 2008 / Accepted: 16 March 2010 / Published online: 26 March 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The structure of N,N0-diphenyl-N,N0 -bis(1-naphthyl)-1,10-biphenyl-4,40-diamine (NPB) with two molecules in the asymmetric unit, was solved and refined to a final R factor of 0.0434. The compound crystallized in the triclinic crystal system, space group P - 1, Z = 2, V = 1597.0(3) A˚3 with unit cell parameters a = 10.3077(10) A˚ , b = 11.3545(11) A˚, c = 14.4778(14) A˚, a = 82.339(2)°, b = 77.657(2)°, and c = 75.526(2)°. The two molecules in the asymmetric unit differed in the ori-entation of their triarylamino groups. The biphenyl group in the central structure is coplanar, and the angle between the least-square planes of the two six-carbon rings of the biphenyl moiety is zero degree. The packing of the mole-cules in the lattice is mainly due to the naphthyl-to-naphthyl p–p interaction with a distance of 3.861 A˚ . Keywords OLED NPB Single-crystal
Hole-transporting
Introduction
N,N0-diphenyl-N,N0-bis(1-naphthyl)-1,10-biphenyl-4,40 -dia-mine (NPB), shown in Fig.1, is a well-known hole-trans-porting material in organic light-emitting diodes (OLEDs), since the first high efficiency multi-layered OLEDs based on the small molecules were found [1]. Due to excellent hole mobility [2], high glass transition temperature (Tg),
and stable thin-film morphology during thermal deposition
[3], NPB has been widely utilized and applied in flat panel displays (FPDs). Despite the importance of the archetype hole transport molecule NPB in OLEDs, relatively few studies of its fundamental molecular properties have appeared in the literature [4–6], and little has been reported concerning the molecular geometry and molecular packing of NPB. In this study, we report the crystal and molecular structure of NPB, which was obtained by thermal deposi-tion in vacuum process. We will also summarize some results on the structures from the viewpoint of the steric influence on molecular conformation.
Experimental Section
NPB is prepared according to reference [7], and the crystal was grown by thermally evaporating NPB in the vacuum chamber at 523 K/36 mtorr.
Crystallography
A suitable colorless crystal with dimensions of 0.10 9 0.10 9 0.10 mm was selected for indexing and intensity data collection on a Siemens SMART CCD diffractometer equipped with a normal-focus, 3-kW sealed-tube X-ray source. Intensity data was collected at 295(2) K in 2359 frames with x scans (width 0.30° per frame). Multi-scan absorption corrections based on symmetry equivalents were applied (Tmin, max= 0.926, 0.986 ) [8] to the 19168
reflections collected (2hmax= 56.66°), 7920 unique
reflec-tions were observed [I [ 2r - (I)] after data reduction. The structure was solved by direct methods using SHEL-XS97 [9] and refined by SHELXL97 [10]: C and N atoms in the framework were found in difference Fourier maps. There are two NPB molecules per unit cell. The final cycles
J.-A. Cheng (&) P.-J. Cheng
Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC
e-mail: jacheng.ac89g@nctu.edu.tw
123
J Chem Crystallogr (2010) 40:557–560of least-squares refinement included atomic coordinates and anisotropic thermal parameters for all non-hydrogen atoms. The final difference Fourier maps were flat (4qmax, min= 0.13, -0.14 e/A˚3). All calculations were performed
using SHELXTL Version 5.1 software package [11]. Figure2 shows the final ORTEP plot. A summary of crystal data is given in Table1, fractional coordinates and equivalent isotropic thermal parameters in Table2.
Results and Discussion
The single-crystal structure of NPB molecules and related information in bond lengths and angles are important for understanding intermolecular interaction, fluorescence, and molecular film characteristics in OLEDs. Selected geo-metric parameters for NPB are given in Table 3. In the structure, the asymmetric unit consists of two independent molecules, A and B as shown in Fig.2, and the naphthyl groups are located trans to each other with respect to the biphenyl spacer. Besides, the individual aryl rings are essentially planar and resulting in co-planar p–p interac-tions across the C–C inter-ring bond. The magnitude of this twist is equivalent to the two independent molecules, such that the angle between the least-square planes of the two six-carbon rings of the biphenyl moiety is zero degree, in good agreement with the values obtained in a recent the-oretical study [12] and with the neutron single-crystal
Fig. 2 Two independent molecules, A and B, of NPB, showing the atom-numbering scheme. Displacement ellipsoids are shown at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (i) -x ? 1, -y ? 1, -z; (ii) -x, -y ? 2, -z ? 1.]
N N
Fig. 1 The chemical structure of NPB
Table 1 Crystal data and structure refinement for NPB Empirical formula C44H32N2
Formula weight 588.72
Temperature (K) 295 (2)
Wavelength (A˚ ) 0.71073
Crystal system Triclinic
Space group P – 1
Unit cell dimensions, a = 10.3077 (10) A˚ , a = 82.339 (2)o b = 11.3545 (11) A˚ , b = 77.657 (2)o c = 14.4778 (14) A˚ , c = 75.526 (2)o Volume (A˚3) 1597.0 (3) Z 2 Density (Mg m-3) 1.224 Absorption coefficient (mm-1) 0.071 F(000) 620 Crystal size (mm3) 0.10 9 0.10 9 0.10 Theta range for data collection 1.44–28.33o
Index ranges -13 B h B 13, -15 B k B 15, -19 B l B 19
Reflections collected 19168
Independent reflections 7920 [R(int) = 0.0638] Completeness to theta = 28.33° 99.5%
Refinement method Full-matrix least-squares on F2
Data/restraints/parameters 7920/0/415 Goodness-of-fit on F2 0.713
Final R indices [I [ 2r(I)] R1= 0.0434, wR2= 0.0709
R indices (all data) R1= 0.1908, wR2= 0.0966
Largest diff. peak and hole (eA˚-3)
0.145 and -0.124
558 J Chem Crystallogr (2010) 40:557–560
diffraction data of the unsubstituted biphenyl structure [13].
The independent N atoms of molecules A and B are nominally sp3hybridized, but with some planarization of the molecule to take advantage of the resonance delocal-ization with the aryl rings, which leads to C–N–C bond
angles close to 120°, displaying features of sp2
hybridiza-tion. The angle sums around these N atoms are ca 359.83° for the atom N1 and ca 359.95° for the atom N2 (Table3). Molecular orbital calculations also revealed that the highest occupied molecular orbital (HOMO) is strongly delocal-ized across all aryl rings of the NPB molecule, with sig-nificant contributions from the lone pair electrons of the nitrogen [5,14]. This geometric feature of NPB molecules in the crystal lattice (similar to that of radical cation state, i.e. sp2hybridization and a planar conformation) provides favorable conditions for geometric reorganization during the charge-transfer process [15]. The N–C bond lengths vary from 1.405(2) to 1.431(2) A˚ , which are in agreement with the experimental crystal structure of triphenylamine [16]. The three aryl rings connected to each N atom adopt a propeller-like orientation. Their corresponding torsion angles (the plane of the rings with respect to the plane defined by the N atom and its three attached C atoms) for N1 and N2 are measured at 21.31–66.66° and 20.98– 65.64°, respectively, and summarized in Table3. The twisted conformation which results from the steric effect of the H atoms in the ortho positions of the phenyl and naphthyl rings can protect the N atoms from chemical attack, e.g. H-atom donors etc., to close the hole-transport function. As a result, NPB has an excellent lifetime when
Table 2 Atomic coordinates (9104) and equivalent isotropic displacement parameters (A˚29 103) for NPB
x y z U(eq) x y z U(eq) N(1) 3881(2) 1132(2) 1646(1) 59(1) C(22) 3254(3) -463(2) 1009(2) 76(1) N(2) 1262(2) 6302(2) 3206(1) 62(1) C(23) 904(2) 7336(2) 3722(2) 53(1) C(1) 4216(2) 2212(2) 1165(2) 50(1) C(24) 1020(2) 7256(2) 4661(2) 70(1) C(2) 4817(2) 2908(2) 1590(2) 56(1) C(25) 682(2) 8292(2) 5141(2) 70(1) C(3) 5124(2) 3977(2) 1127(2) 54(1) C(26) 189(2) 9449(2) 4740(2) 47(1) C(4) 4845(2) 4415(2) 235(2) 46(1) C(27) 98(2) 9507(2) 3790(2) 59(1) C(5) 4243(2) 3707(2) -175(1) 57(1) C(28) 447(2) 8487(2) 3297(2) 64(1) C(6) 3929(2) 2635(2) 276(2) 58(1) C(29) 708(2) 6282(2) 2393(2) 54(1) C(7) 4585(2) 449(2) 2349(2) 58(1) C(30) -674(2) 6725(2) 2406(2) 75(1) C(8) 3881(2) -7(2) 3176(2) 71(1) C(31) -1184(2) 6773(2) 1596(2) 89(1) C(9) 4586(3) -637(2) 3860(2) 98(1) C(32) -359(3) 6366(2) 777(2) 81(1) C(10) 5963(4) -791(3) 3741(3) 118(1) C(33) 996(3) 5894(2) 778(2) 79(1) C(11) 6655(3) -364(3) 2915(3) 113(1) C(34) 1528(2) 5850(2) 1575(2) 66(1) C(12) 5978(2) 233(2) 2215(2) 86(1) C(35) 2168(2) 5219(2) 3518(2) 60(1) C(13) 2879(2) 643(2) 1380(2) 59(1) C(36) 1716(2) 4178(2) 3818(2) 75(1) C(14) 1509(2) 1287(2) 1522(2) 59(1) C(37) 2579(3) 3115(2) 4133(2) 92(1) C(15) 1055(2) 2419(2) 1934(2) 66(1) C(38) 3876(3) 3105(2) 4154(2) 90(1) C(16) -287(3) 2986(2) 2070(2) 89(1) C(39) 4401(3) 4154(3) 3847(2) 75(1) C(17) -1237(3) 2482(4) 1812(2) 116(1) C(40) 5768(3) 4157(3) 3851(2) 104(1) C(18) -830(4) 1424(4) 1416(2) 108(1) C(41) 6240(3) 5162(4) 3531(3) 125(1) C(19) 533(3) 760(3) 1268(2) 71(1) C(42) 5416(3) 6209(3) 3189(2) 104(1) C(20) 981(4) -392(3) 885(2) 92(1) C(43) 4089(3) 6264(2) 3171(2) 80(1) C(21) 2312(4) -988(3) 766(2) 95(1) C(44) 3545(2) 5226(2) 3511(2) 59(1)
Table 3 Selected geometric parameters (A˚ ,˚)
N1–C1 1.412(2) N2–C23 1.405(2) N1–C7 1.408(2) N2–C(29 1.419(2) N1–C13 1.427(2) N2–C35 1.431(2) C4–C4a 1.483(3) C26–C26b 1.472(4) C1–N1–C7 121.81(18) C23–N2–C29 121.77(19) C1–N1–C13 120.48(18) C23–N2–C35 119.75(18) C7–N1–C13 117.54(17) C29–N2–C35 118.43(18) C1–N1–C7–C8 43.39(18) C23–N2–C29–C30 44.96(19) C1–N1–C13–C14 66.66(19) C23–N2–C35–C36 65.64(19) C7–N1–C1–C2 26.22(18) C29–N2–C23–C24 23.48(19) C7–N1–C13–C14 61.96(18) C29–N2–C35–C36 63.22(18) C13–N1–C1–C2 21.31(19) C35–N2–C23–C24 20.98(19) C13–N1–C7–C8 48.16(18) C35–N2–C29–C30 47.43(18) C3–C4–C4a–C5a 1.10(2) C25–C26–C26b–C27b 1.48(2) C5–C4–C4a–C3a 1.10(2) C27–C26–C26b–C25b 1.48(2) J Chem Crystallogr (2010) 40:557–560 559
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used as a hole-transporting material during the electrolu-minescence process.
In the crystal structure of the title compound, a major feature is the naphthyl-to-naphthyl stacking of molecules related to one another by a cell translation, thus forming stacks that propagate in the c direction (Fig.3). Taking account of the angle of the planes relative to the c cell edge, of the order of 72.48°, the repeat distance of 3.861 A˚ corresponds to an interplanar separation of *3.5 A˚ , which is consistent with the requirements of p–p stacking and clearly accommodates the distance of 3.4 A˚ required for overlapping p-aromatic ring systems [17].
In addition, molecules carrying triaryl groups, –NAr3,
often engage in multiple phenyl embraces (MPE), where the phenyl rings of two groups (Ar3N–) are interleaved
such that six and/or four edge-to-face aryl–aryl interactions are established [18, 19]. A prevalent MPE is either the sextuple phenyl embrace (SPE) or the quadruple phenyl embrace (QPE). The former has net attractive energies in the range from 60 to 85 kJ mol-1and NN separations in the range of 6.4–7.4 A˚ [13, 14, 20], while the latter involves two phenyl groups on each partner [15, 21]. Furthermore, QPEs frequently occur at relatively longer NN separations and have lower attractive energies. In Fig.3, the aryl rings of two molecules are interweaved in such a way that total of six edge-to-face aryl–aryl inter-action were established. The geometry and the NN
separation of 6.04–6.63 A˚ are similar to those of SPE. Each SPE dimer is in contact with two neighboring molecules at NN separation 8.73 A˚ and showed a weak QPE interaction.
Supplementary Material
CCDC 679993 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, United Kingdom; fax: (?44)-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgement The authors would like to thank Prof. Ju-Chun Wang, Department of Chemistry, Soochow University, Taiwan for fruitful discussion. We also thank Ms. Liao, Ms. Hung, and P.-L. Chen at National Tsing Hua University for single-crystal X-ray measurement. This work was supported by the National Science Council (NSC).
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560 J Chem Crystallogr (2010) 40:557–560
