Synthesis, thermal, electrochemical, and photophysical characterization of 1,5-bis(diarylamino)naphthalene derivatives as potential hole transport OLED materials

Article abstract of DOI:10.1139/v05-101

Novel 1,5-bis(diarylamino)naphthalene derivatives (1-9), which have potential as hole-transporting materials for electroluminescent devices, were obtained through palladium-catalyzed coupling of diarylamines and 1,5-dibromonaphthalene. The thermal, electrochemical, and photophysical properties of these compounds were examined and the effects of the N-aryl substituents on these properties were investigated. These materials possess glass transition temperatures (T_g) that range from 70-131 °C and these values are related to the identity of the aryl substituents. Cyclic voltammetric measurements demonstrated that these compounds possess two reversible oxidation processes and were further used to estimate the HOMO energy levels of these materials by comparison with the ferrocene/ferrocenium couple. The intramolecular charge mobility, as gauged by the difference between the two oxidation potentials, indicates that compounds 1-8 have a similar degree of delocalization as meta-diaminobenzene derivatives while that of 9 is similar to TPD. Compounds 1-9 emit in the blue-green region and optical absorption and emission data for these materials can be rationalized in terms of the electronic donating properties of the aryl substituents. This data when combined with the electrochemically determined HOMO energies allowed estimation of the LUMO energy levels.

Full text of DOI:10.1139/v05-101

958
Synthesis, thermal, electrochemical, and
photophysical characterization of 1,5-
bis(diarylamino)naphthalene derivatives as
potential hole transport OLED materials1
Jingning Shan, Glenn P.A. Yap, and Darrin S. Richeson
Abstract: Novel 1,5-bis(diarylamino)naphthalene derivatives (1–9), which have potential as hole-transporting materials
for electroluminescent devices, were obtained through palladium-catalyzed coupling of diarylamines and 1,5-
dibromonaphthalene. The thermal, electrochemical, and photophysical properties of these compounds were examined
and the effects of the N-aryl substituents on these properties were investigated. These materials possess glass transition
temperatures (Tg) that range from 70–131 °C and these values are related to the identity of the aryl substituents. Cyclic
voltammetric measurements demonstrated that these compounds possess two reversible oxidation processes and were
further used to estimate the HOMO energy levels of these materials by comparison with the ferrocene/ferrocenium cou-
ple. The intramolecular charge mobility, as gauged by the difference between the two oxidation potentials, indicates
that compounds 1–8 have a similar degree of delocalization as meta-diaminobenzene derivatives while that of 9 is simi-
lar to TPD. Compounds 1–9 emit in the blue-green region and optical absorption and emission data for these materials
can be rationalized in terms of the electronic donating properties of the aryl substituents. This data when combined
with the electrochemically determined HOMO energies allowed estimation of the LUMO energy levels.
Key words: hole transport materials, diaminonaphthalene, thermal analysis, palladium-catalyzed arylation.
Résumé : Faisant appel à un couplage catalysé par le palladium de diarylamines et de 1,5-dibromonaphtalène, on a
préparé des 1,5-bis(diarylamino)naphtalènes (1–9), qui ont le potentiel de transporter des trous pour les appareils élec-
troluminescents. On en a examiné les propriétés thermiques, électrochimiques et photophysiques et on a étudié les ef-
fets des substituants N-aryles sur ces propriétés. Ces matériaux possèdent des températures de transition de verre (Tg)
qui s’étalent de 70 à 131 °C et on a pu établir une corrélation entre cette propriété et l’identité des substituants aryles.
Des mesures de voltampérométrie cyclique démontrent que ces composés possèdent deux processus d’oxydations réver-
sibles qui ont, de plus, été utilisées pour évaluer les niveaux d’énergie de l’orbitale moléculaire haute occupée de ces
composés par comparaison avec le couple ferrocène/ferrocénium. La mobilité de la charge intramoléculaire, telle
qu’évaluée par la différence entre les deux potentiels d’oxydation, indique que les composés 1–8 possèdent un degré de
délocalisation semblable à celui des dérivés du méta-diaminobenzène alors que celle du composé 9 est semblable à
celle du TDP. Les composés 1–9 émettent dans la région du bleu-vert et les données d’absorption et émission optique
de ces composés peuvent être rationalisées en termes du caractère électrodonneur des substituants aryles. Lorsque ces
données sont combinées avec les énergies de l’orbitale moléculaire haute occupée déterminée électrochimiquement, il
est possible d’évaluer les niveaux d’énergie de l’orbitale moléculaire basse vacante.
Mots clés : matériaux pouvant transporter des trous, diaminonaphtalène, analyse thermique, arylation catalysée par le
palladium.
[Traduit par la Rédaction] Shan et al. 968
Introduction
(OLED) (1, 2). Generally the amine assumes the role of the
hole transport (HT) material in such devices (3, 4). Aromatic
diamine derivatives and in particular N,N,N ′,N ′-tetraaryl-
(1,1′-biphenyl)-4,4′-diamines remain synthetic targets for
HT materials because of the ease with which they form
Polyaromatic amines have been essential components in
molecular electroluminescent (EL) devices beginning with
the first report of a two-layer organic light-emitting diode
Received 25 November 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 20 August 2005.
Dedicated to Professor Howard Alper.
J. Shan, G.P.A. Yap, and D.S. Richeson.2 Department of Chemistry and the Center for Catalysis Research and Innovation,
University of Ottawa, Ottawa, ON K1N 6N5, Canada.
1This article is part of a Special Issue dedicated to Professor Howard Alper.
2Corresponding author (e-mail: darrin@science.uottawa.ca).
Can. J. Chem. 83: 958–968 (2005)
doi: 10.1139/V05-101
© 2005 NRC Canada

Shan et al.
959
Chart 1.
amorphous thin films, their high hole mobility, and suitable
HOMO energy level (5–8).
Ar1
Ar2
Ar3
Ar4
Benchmark compounds in this field are represented by
N,N ′-diphenyl-N,N ′-di(m-tolyl)benzidine (TPD) and N,N ′-
bis(1-naphthyl)-N,N ′-diphenyl-benzidine (NPD). Establishing
relationships between the molecular structure of these and
related compounds and their physical characteristics such as
oxidation potential (9, 10), solid-state structures (11, 12), re-
organization energy upon charge transfer (13–15), and envi-
ronmental stability is of considerable interest, as each of
these properties will influence the hole transport perfor-
mance of the compound in a device.
Core
N
N
Ar = aromatic groups
the advantageous properties exhibited by compounds such as
TPD and NPD. A variety of approaches have been under-
taken to improve the performance features of the HT compo-
nent in EL devices, many of which center around core
modifications to the generic diamine represented in Chart 1
(3, 6, 18–25). In addition to variation of the core structure,
the aryl groups have been modified to produce symmetric
(17–20) and asymmetric compounds (21, 24). Among the
improved properties that have been observed in these exam-
ples are increased thermal stability and novel emission prop-
erties (20, 21, 26–29). Certainly, further development of
related species could have significant device implications.
Herein we report our results for potential HT materials de-
signed around the 1,5-diaminonaphthalene structure and an
examination of the effects of variations of the aromatic sub-
stituents in these species. Only one example of an HT mate-
rial designed around the naphthyl core has been reported and
this compound displayed improved thermal properties as
well as potential as a blue light-emitting material (30). In
this case, the implementation of a fused aromatic core will
remove the torsional freedom of the biphenyl core. In addi-
tion to characterizing the thermal behavior of these new
diamines, we present their electronic spectra and electro-
chemical properties for comparison with TPD. Our synthetic
methodology exploited the versatile and efficient palladium
catalyzed amine arylation reactions pioneered by Buchwald
and co-workers (31, 32) and Hartwig (33, 34).
N
N
TPD
N
N
NPD
The hole transport in these materials has associated with it
a reorganization energy (λ) that is related to the geometric
rearrangments between the neutral and radical cation states
(e.g., TPD and TPD+). In the case of diamines with a
biphenyl core such as TPD and NPD, the torsion angle be-
tween the two rings of the biphenyl bridges changes between
the neutral and the cation radical and this has been ascribed
to be the major barrier to electron exchange between these
two species (13). Minimizing this reorganization term is one
factor that may lead to improved hole mobility.
One of the chief failure modes for OLEDs is thermal in-
stability in the molecular thin films, and in most devices, the
HT component has the lowest thermal stability. Thermal
stresses are inherent in device operation or can arise in the
operating environment and can lead to significant expansion
of the material or crystallization of the HT material. Further-
more, poor lifetimes for devices operated near room temper-
ature have been attributed in part to thermal instabilities of
the amorphous organic layers, which has been correlated
with low glass transition temperatures (Tg) (2, 4, 16). For ex-
ample, the Tg of tris(8-hydroxyquinolinato)aluminum (Alq),
a common electron transport (ET) material, is 175 °C while
those for TPD and NPD are reported to be 65 and 95 °C, re-
spectively (17).
Experimental section
General information
All reactions were carried out under a nitrogen atmo-
sphere using standard Schlenk techniques. Aryl amines, aryl
bromides, diphenylamine, tris(dibenzylidineacetone)dipalla-
dium (Pd2(dba)3), rac-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl
(BINAP), and sodium tert-butoxide were purchased from the
Aldrich Chemical Company and used without further purifi-
cation. Tri-tert-butylphosphine was purchased from Strem
Chemicals. 1,5-Dibromonaphthalene was synthesized from
1,5-diaminonaphthalene according to literature procedures
(35). Toluene was purified by passage through a column of
activated alumina using an apparatus from Anhydrous Engi-
neering. Dichloromethane was freshly distilled under nitro-
gen using CaH2 as the dessicant.
Reactions were monitored using Silica gel 60 F254 TLC
aluminum sheets by UV lamp and (or) GC–MS using a mass
selective detector from Agilent GC systems 6890 series with
an HP-5 capillary column (cross-linked 5% PH ME siloxane).
1H NMR spectra were collected on a Varian Gemini-200
or a Bruker 300 MHz spectrometer using the residual pro-
tons of the deuterated solvent for reference. The overlapping
13C peaks were identified through 13C DEPT and H13C
1
A major objective in developing new HT materials is the
improvement of their thermal properties without sacrificing
HMQC analysis on a Bruker 300 MHz spectrometer. Elec-
tron impact mass spectra (EI-MS) were recorded on a Kratos
© 2005 NRC Canada

960
Can. J. Chem. Vol. 83, 2005
Scheme 1.
Concept IIH instrument using electron ionization at 70 eV.
High resolution mass spectra (HR-MS) were obtained at the
University of Ottawa on a Kratos Concept IIH instrument
using electron ionization at 70 eV, acceleration 8 KV, and
resolution 7 000 – 10 000. The glass transition temperatures
and melting points were measured using a TA 2010 differen-
tial scanning calorimeter (DSC) instrument. Heating rates of
5–20 °C/min were employed for these measurements. UV
spectra were recorded in CH2Cl2 on a Varian Cary spectro-
photometer. Steady-state fluorescence spectroscopy was car-
ried out using a Photon Technology International version 1.2 X
luminescence spectrometer with the same cuvette used for
UV–vis spectra. Fluorescence quantum efficiency (Φf) of the
compounds was determined by comparing with that of
Coumarin-102 (Φf = 0.93 in ethanol) (36) in CH2Cl2 solu-
tion. Cyclic voltammograms (CV) were obtained using a
Hokuto Denko HA501 potentiostat connected to a Hokuto
Denko HB105 programmable function generator. A Nicolet
310 oscilloscope was used to follow the in situ CV scans. A
freshly polished Pt wire and Pt plate were used as working
and counter electrode, respectively. A silver wire was
employed as pseudo-reference electrode. All solutions were
prepared in millimolar concentrations in dry CH2Cl2, which
was freshly distilled from CaH2. Tetrabutyl ammonium
hexafluorophosphate (TBAHFP) was used as the supporting
electrolyte with a concentration of 0.1 mol/L. The electro-
chemical cell was degassed with nitrogen prior to analysis
and was maintained under a nitrogen atmosphere during the
potential scans. At the end of each set of CV scans, ferro-
cene (Cp2Fe) was added to the solution and another potential
scan was performed to provide the redox potential for the
Cp2Fe/Cp2Fe+ couple.
0.5-1% Pd2(dba)3/BINAP
Ar
NaOtBu
[1]
HN
Ar'Br
+
H2NAr
Toluene
reflux
Ar'
a-i
Ar = MeOC6H4
Ar' = C6H5, MeOC6H4, p-MeC6H4, p-FC6H4,
p-biphenyl,1-naphthyl
1-2% Pd(OAc)2/P(tBu)3
Ar'
Ar
Br
2 NaOtBu
Toluene
reflux overnight
N
+ 1/2
Br
HN
[2]
1/2
Ar
Ar
N
Ar'
Ar'
a-i
1-9
as the eluent. Collection of the product was monitored by
TLC and all fractions were combined at the end of the sepa-
ration procedure. The solvents were removed under vacuum
to yield pure compounds.
1,5-Bis(di-p-methoxyphenylamino)naphthalene (1)
1
Yield: 50%. H NMR (CDCl3) δ: 3.79 (s, 12H), 6.64–6.80
(m, 8H), 6.91 (d, 8H), 7.08–7.30 (m, 4H), 7.84 (d, 2H). 13C
NMR (CDCl3) δ: 55.48, 114.41, 121.98, 123.44, 126.12,
126.25, 132.82, 142.98, 144.92, 154.54. m/z: 582 (M+). HR-
MS (m/z) calcd. for C38H34N2O4: 582.2519; found: 582.2530.
1,5-Bis(p-methoxyphenylphenylamino)naphthalene (2)
Yield: 75%. 1H NMR (CDCl3) δ: 3.76 (s, 6H), 6.7–6.9 (m,
9H), 7.0–7.4 (m, 13H), 7.87 (d, 2H). 13C NMR (CDCl3) δ:
55.46, 114.55, 119.84, 120.26, 122.43, 125.38, 126.56,
126.95, 128.93, 133.06, 141.55, 144.30, 149.5, 155.49. m/z:
522 (M+). HR-MS (m/z) calcd. for C36H30N2O2: 522.2307;
found: 522.2328.
Compounds a–h were prepared via Pd catalyzed coupling
of arylbromides and arylamines according to eq. [1] in
Scheme 1. The general procedure for the synthesis of diaryl-
amines followed that reported in the literature with details
provided in the Electronic supplementary information (37,
38).3
1,5-Bis(p-methoxyphenyltolylamino)naphthalene (3)
Yield: 80%. H NMR (CDCl3) δ: 2.28 (s, 6H), 3.77 (s,
6H), 6.81 (t, 8H), 7.02 (t, 8H), 7.16–7.34 (m, 4H), 7.88 (d,
2H). 13C NMR (CDCl3) δ: 20.62, 55.46, 114.45, 120.77,
122.20, 124.59, 126.39, 126.58, 129.55, 130.11, 133.00,
142.18, 144.61, 147.05, 155.03. m/z: 550 (M+). HR-MS
(m/z) calcd. for C38H34N2O2: 550.2620; found: 550.2642.
1
General procedure for the synthesis of
diaminonaphthalene derivatives
The general procedure for the synthesis of 1,5-
diaminonaphthalene derivatives (1–9) according to eq. [2] in
Scheme 1 was modified from the literature (39). In a drybox,
a Schlenk flask was charged with toluene, 1,5-dibromon-
aphthalene, and 2.2 equiv. of the appropriate diarylamines
(a–i). To this solution was added 1% to 2% Pd(OAc)2/P(t-
Bu)3 (ligand/Pd = 1) and 2.8 equiv. of NaO-t-Bu. The reac-
tion mixture changed from orange to dark black over the
first few seconds. The flask was sealed, removed from the
drybox, and was heated at 120 °C overnight. The reaction
was then cooled to room temperature, filtered, and the resi-
due was washed with ethylacetate and combined with the
toluene. The solvents were then removed by vacuum evapo-
ration and the residue was dissolved in CHCl3. Column
chromatography was used to purify the product using CHCl3
1,5-Bis(p-methoxyphenyl-p-fluorophenylamino)naphtha-
lene (4)
1
Yield: 78%. H NMR (CDCl3) δ: 3.76 (s, 6H), 6.72–7.32
(m, 20H), 7.82 (d, 2H). 13C NMR (CDCl3) δ: 55.48, 114.64,
115.63 (JC-F = 22.6 Hz), 122.15 (JC-F = 5.0 Hz), 122.22,
124.60, 126.49, 126.54, 132.84, 142.11, 144.59, 145.70,
155.33, 157.58 (JC-F = 240.2 Hz). m/z: 558 (M+). HR-MS
(m/z) calcd. for C36H28F2N2O2: 558.2119; found: 558.2101.
1,5-Bis(p-methoxyphenyl-p-propylphenylamino)naphtha-
lene (5)
1
Yield: 66%. H NMR (CDCl3) δ: 0.92 (t, 6H), 1.57 (m,
4H), 2.47 (t, 4H), 3.76 (s, 6H), 6.75–6.83 (m, 8H), 6.95–
7.04 (M, 8H), 7.19–7.30 (m, 4H), 7.86 (d, 2H). 13C NMR
3 Supplementary data for this article are available on the Web site or may be purchased from the Depository of Unpublished Data, Document
Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0S2, Canada. DUD 3696. For more information on obtaining mate-
rial refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 242552 and 242553 contain the crystallographic data for this manu-
script. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2005 NRC Canada

Shan et al.
961
Table 1. Crystal data and structure refinement for compounds 3 and 5.
Compound
3
5
Empirical formula
C38H34N2O2
C42H42N2O2
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
550.67
203(2)
0.710 73
Monoclinic
P2(1)/c
606.78
203(2)
0.710 73
Monoclinic
P2(1)/c
8.114(2)
16.800(5)
11.090(3)
14.394 6(19)
11.688 2(16)
21.188(3)
b (Å)
c (Å)
β (°)
95.361(5)
1505.1(7)
109.706(3)
3356.1(8)
V (Å3)
Z
2
4
Final R indices (I > 2σ(I))
R1 = 0.085 8, wR2 = 0.197 0
R1 = 0.079 2, wR2 = 0.148 4
(CDCl3) δ: 13.91, 24.62, 37.30, 55.45, 114.45, 120.50,
122.27, 124.65, 126.40, 126.68, 128.89, 133.07, 134.99,
142.12, 144.59, 147.21, 155.03. m/z: 606 (M+). HR-MS
(m/z) calcd. for C42H42N2O2: 606.3246; found: 606.3229.
marized in Table 1. Data were collected on a Bruker AX
SMART 1 k CCD diffractometer using 0.3° ω scans at 0°,
90°, and 180° in φ. Unit-cell parameters were determined
from 60 data frames collected at different sections of the
Ewald sphere. Semi-empirical absorption corrections based
on equivalent reflections were applied. The structures were
solved by direct methods, completed with difference Fourier
syntheses, and refined with full-matrix least-squares proce-
dures based on F2. All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen at-
oms were treated as idealized contributions. All scattering
factors and anomalous dispersion factors are contained in
the SHELXTL 5.1 program library (40).
1,5-Bis(p-methoxyphenyl-p-biphenylamino)naphthalene (6)
1
Yield: 50%. H NMR (CDCl3) δ: 3.78 (s, 6H), 6.78–7.58
(m, 30H), 7.92 (d, 2H). 13C NMR (CDCl3) δ: 55.47, 114.64,
119.74, 122.58, 125.64, 126.45, 126.70 (two overlapping
singlets), 127.08, 127.55, 128.66, 132.82, 133.07, 140.79,
141.25, 144.14, 148.81, 155.51. m/z: 674 (M+). HR-MS
(m/z) calcd. for C48H38N2O2: 674.2933; found: 674.2908.
1,5-Bis(p-methoxyphenyl-1-naphthylamino)naphthalene
(7)
Results and discussion
Yield: 80%. 1H NMR (C6D6) δ: 3.20 (s, 6H), 6.54 (d, 4H),
6.72–7.22 (m, 16H), 7.44 (d, 2H), 7.63 (d, 2H), 8.25 (d, 2H),
8.41 (d, 2H). 13C NMR (C6D6) δ: 54.82, 114.88, 121.75,
123.85, 124.51, 124.92, 125.04, 125.16, 126.24, 126.43,
126.59, 127.12, 128.80, 130.35, 132.50, 135.83, 144.88,
146.34, 147.13, 155.29. m/z: 622 (M+). HR-MS (m/z) calcd.
for C44H34N2O2: 622.2620; found: 622.2647.
Synthesis of naphthalene diamines
Our strategy for preparing new aromatic amines possess-
ing a naphthyl core (1–9) is outlined with eqs. [1] and [2] in
Scheme 1. Diarylamines (a–i) were either first prepared via
Pd catalyzed coupling of arylbromide and arylamine (a–h)
(37, 38) or purchased (i). A Pd(OAc)2/P(t-Bu)3 catalyst sys-
tem was then employed for the coupling reactions between
1,5-dibromonaphthalene and diarylamines (39). We also
explored an alternative route based on the coupling of 1,5-
diaminonaphthalene with aryl halides. This proved to be un-
satisfactory and only led to products with a single arylation
of each nitrogen center as the major products. The synthesis
of a related diamine, 1,5-bis[N-(1-naphthyl)-N-phenyl]naph-
thalene diamine (NND) was recently reported via a modified
Ullman condensation of 1,5-diiodonaphthalene and naphthyl-
phenylamine, however, no details for this transformation
were reported (30).
1,5-Bis(di-m-methoxyphenylamino)naphthalene (8)
1
Yield: 50%. H NMR (CDCl3) δ: 3.61 (s, 3H), 3.68 (s,
3H), 6.15 (t, 1H), 6.18 (d, 1H), 6.33 (d, 1H), 6.82–7.33 (m,
14H), 7.96 (d, 2H). 13C NMR (CDCl3) δ: 54.99, 55.61,
103.82, 104.02, 110.55, 112.79, 121.26, 122.81, 125.99,
126.13, 126.26, 128.76, 129.10, 132.86, 135.95, 143.78,
150.78, 155.31, 160.12. m/z: 582 (M+). HR-MS (m/z) calcd.
for C38H34N2O4: 582.2519; found: 582.2509.
1,5-Bis(diphenylamino)naphthalene (9)
1
Yield: 52%. H NMR (CDCl3) δ: 6.90–7.31 (m, 24H),
The naphthyl core diamines presented in Table 2 were iso-
lated in good yields and were fully characterized by H and
7.88 (d, 2H). 13C NMR (CDCl3) δ: 121.71, 121.95, 122.69,
126.73, 127.52, 129.09, 133.24, 144.00, 148.50. m/z: 462
(M+). HR-MS (m/z): calcd. for C34H26N2: 462.2096; found:
462.2109.
1
13C NMR and HR-MS. The versatility of this method provides
an avenue to invoke a wide range of amine substituent varia-
tion including electron-donating and electron-withdrawing
groups. The molecular structures of two of these diamines,
compounds 3 and 5, were definitively established using sin-
gle crystal X-ray diffraction analysis (Table 1). The struc-
tures of 3 and 5 are presented in Figs. 1 and 2, respectively,
and selected bond distances and angles are provided in Ta-
Structural determination for compounds 3 and 5
Single crystals were mounted on thin glass fibers using
viscous oil and then cooled to the data collection tempera-
ture. Crystal data and details of the measurements are sum-
© 2005 NRC Canada

962
Can. J. Chem. Vol. 83, 2005
Table 2. Summary of the synthesis and thermal properties of HN(Ar)Ar′
and 1,5-bis(diarylamino)naphthalenes shown in Scheme 1.
Ar
Ar
Ar'
N
N
Ar'
Ar'
Ar
H
(% Yield)
N
(% Yield)
T /Tm/T
g
c
p
a
1
(50)
HN( -C6H4OMe)2
(78)
p
(Ar = Ar' = -C6H4OMe)
76/216/143 ºC
b
2
(75)
N(C6H4OMe)(C6H5)
(80)
p
(Ar = -C6H4OMe, Ar' = C6H5)
70/230/145 ºC
c
3
(80)
HN(C6H4OMe)(C6H4Me)
(70)
p
(Ar = -C6H4OMe, Ar' = C6H4Me)
82/249/103 ºC
d
4
(78)
HN(C6H4OMe)(C6H4F)
(84)
p
(Ar = -C6H4OMe, Ar' = C6H4F)
73/192/138 ºC
e
5
(66)
HN(C6H4OMe)(C6H4Pr)
(86)
p
(Ar = -C6H4OMe, Ar' = C6H4Pr)
51/187/126 ºC
f
6
(50)
HN(C6H4OMe)(4,4'-biphenyl)
(50)
p
(Ar = -C6H4OMe, Ar' = 4,4'-
bipheny)
116/283/200 ºC
g
7
(80)
HN(C6H4OMe)(1-naphthyl)
(50)
p
(Ar = -C6H4OMe, Ar' = 1-naphthyl)
131/220/NO ºCa
h
8
(50)
HN(m-MeOC6H4)2
(40)
m
(Ar = Ar' = -C6H4OMe)
72/220/NO ºCa
i
9
(52)
HN(C6H5)2
(Ar = Ar' = C6H5)
83/244/NO ºCa
65/175/167 ºC
TPD
aNO = not observed.
ble 3. Both species display planar nitrogen centers with
propeller-like arrangement of the three aromatic groups on
each nitrogen. In 3, the N—Cnaphthyl distance (N(1)—C(1) =
1.462(3) Å) is longer than the corresponding distances for the
outer aryl groups (N(1)—C(18) = 1.416(6) Å and N(1)—
C(11) = 1.449(6) Å). In contrast, although the longest N—C
distance in compound 5 is to the naphthyl core, the C—N
distances in this compound span a narrower range from
1.423(4) to 1.435(4) Å. These trends in C—N distance are
opposite to those observed for TPD and for N,N ′-diphenyl-
N,N ′-bis(2,4-dimethylphenyl)-(1,1′-biphenyl)-4,4′-diamine,
where the N—Cbiphenyl distances are the shortest distances
with values of 1.411(5) and 1.415(5) Å, respectively (7, 8).
There are no unusual C—C bond lengths within the periph-
eral or core aryl groups.
sociated with grain boundaries in polycrystalline materials
and provides for a high degree of uniformity within thin
films. Associated with this amorphous state is a characteris-
tic temperature (or temperature range) where the glassy and
amorphous state of the material begins to soften and flow,
which is referred to as the glass transition temperature (Tg).
Above Tg, materials tend to crystallize generating grain
boundaries and causing nonuniform thin films (17). The ki-
netics of the crystallization process will dictate whether or
not a Tg is observed. Generally, large, globular, and rigid
molecules or asymmetric molecules with high molecular
masses and without strong intermolecular forces have high
Tg values (17, 41). Such general features favor a high energy
for the reorganization – molecular reorientation of the amor-
phous phase into a crystalline material. Strong intermole-
cular forces may favor organization of the molecules as they
cool below their melting points, which, in turn, encourage
crystallization at the expense of glass formation.
Thermal properties
To be of practical consideration for OLED fabrication, the
diamines 1–9 should form stable glasses. This general fea-
ture of materials for OLED construction avoids problems as-
A typical DSC for the naphthalene diamines is shown in
Fig. 3. The initial temperature scan of compound 3 shows a
© 2005 NRC Canada

Shan et al.
963
Fig. 1. Molecular structure and atom numbering scheme for 3. Hydrogen atoms have been omitted for clarity.
Fig. 2. Molecular structure and atom numbering scheme for 5. Hydrogen atoms have been omitted for clarity.
crystalline phase transition (Ttr = 200 °C) and a melting
point (Tm) at 249 °C. This sample was then allowed to cool
to room temperature and heated a second time. This temper-
ature scan displayed the glass transition (Tg) at 82 °C. Con-
tinuing to heat this sample revealed an exotherm at 103 °C,
which was associated with crystallization. Finally, the sam-
© 2005 NRC Canada

964
Can. J. Chem. Vol. 83, 2005
Table 3. Selected bond lengths (Å) and angles (°) for compounds 3 and 5.
Compound 3
Compound 5
Bond distances (Å)
N(1)—C(18)
N(1)—C(11)
1.416(6)
1.449(6)
1.462(3)
N(1)—C(13)
N(1)—C(6)
N(1)—C(33)
N(2)—C(29)
N(2)—C(22)
N(2)—C(42)
1.423(4)
1.427(4)
1.429(4)
1.423(4)
1.429(4)
1.435(4)
N(1)—C(1)
Bond angles (°)
C(18)-N(1)-C(11)
C(18)-N(1)-C(1)
C(11)-N(1)-C(1)
123.3(4)
118.5(4)
116.5(4)
C(13)-N(1)-C(6)
C(13)-N(1)-C(33)
C(6)-N(1)-C(33)
C(29)-N(2)-C(22)
C(29)-N(2)-C(42)
C(22)-N(2)-C(42)
120.7(3)
119.1(3)
117.5(3)
119.0(3)
118.4(3)
116.7(3)
Torsion angles (°)
C(11)-N(1)-C(1)-C(2)
108.57
C(6)-N(1)-C(33)-C(34)
50.12
second run, the compound was allowed to cool under ambi-
ent conditions to room temperature, and a third heating cycle
was initiated. This run was stopped prior to melting. The
purpose of this heating cycle was to anneal the sample into a
crystalline phase. The fourth heating cycle confirmed that 6
was again a crystalline sample in that no Tg was observed
but Tm was clearly seen. The data presented in Fig. 4 dem-
onstrates that the rate of sample cooling of 6 has no effect
on the Tg values, but it will effect Tc. For example, Tc is ob-
served at 185 °C for the compound cooled in the liquid ni-
trogen, while Tc is 200 °C for the compound cooled under
ambient conditions.
Fig. 3. DSC of compound 3. Heating rate of 20 °C/min. Curve
(a) corresponds to first heating and curve (b) corresponds to sec-
ond heating.
Temperature (°C)
0
50
100
150
200
250
300
Ttr
Tm
(a)
(b)
As summarized in Table 4, compounds 1–9 possess melt-
ing points from 187–283 °C, which all exceed that of TPD
at 175 °C. With the exception of 5, diamines 1–9 have
higher Tg values than TPD and in two cases surpass this
value by more than 50 °C. We attribute the low Tg that was
observed for 5 to the fact that it possesses a n-propyl group,
a feature that makes it unique among these compounds and
likely reduces the overall rigidity of this compound. The
highest Tg values were observed for compounds 6 and 7,
which have extended aromatic substituents. The 4,4′-
biphenyl and 1-naphthyl groups increase both the molecular
weight and the rigidity of 6 and 7. The remaining com-
pounds have Tg values that fall within the narrow range of
70–83 °C. Compound 9 possesses N-substituents that are
simple phenyl rings and it exhibited a slightly higher Tg of
83 °C compared with the substituted aromatic analogues.
This suggests that there is little gain in thermal properties by
using simple methoxy, methyl, or fluoro substituents on the
aromatic groups in these systems.
Tg, 82 °C
Tc
ple was observed to melt at Tm. Similar DSC measurements
allowed the determination of the Tg and Tm values of
diamines 1–9 and these results are presented in Table 2. For
comparison, the corresponding values for TPD are included
in this table.
Figure 4 displays the DSC data for compound 6. In the
first run, compound 6 exhibits a coupled endothermic and
exothermic phase transition prior to melting at 283 °C. This
type of transition has been attributed to polymorphism of the
crystalline materials (42). After the compound was flash
cooled in liquid nitrogen to form the glassy state, the sample
was subjected to a second heating cycle that allowed obser-
vation of the Tg (116 °C) and Tc (185 °C). Following this
Electrochemistry
In a simple OLED, emission arises from the recombina-
tion of electrons originating in the ET layer with holes that
are transported in the HT material. This combination can oc-
cur in either of the two layers and for charge transfer across
the ET/HT interface some degree of HOMO and (or) LUMO
matching is required. Matching of the HOMO level of the
© 2005 NRC Canada

Shan et al.
965
Fig. 4. DSC curves for compound 6 under various heating and cooling conditions as described in the text. The five heating cycles are
sequentially labeled a–e. Scan rate of 20 °C/min.
Temperature (°C)
0
50
100
150
200
250
300
350
Tm, 283 °C
a
Tc, 185 °C
b
c
d
Tc, 200 °C
e
70
90
110
130
150
-30.3
-30.8
Tg, 116 °C
Table 4. Summary of electrochemical and spectroscopic data for naphthyl core diamines.
a
b
∆Eoxid
(mV)
∆Ep
(V)
Absorption
(λmax, nm)
Emission
(λemi, nm)
Energy
HOMOe
(eV)
LUMO f
(eV)
(Φf)d
Compound
gapc (nm)
1
2
3
4
5
6
7
8
145
130
144
140
140
121
160
169
255
220
0.081
0.160
0.091
0.135
0.123
0.106
0.125
0.202
0.210
0.127
375
372
376
370
379
375
375
361
362
350
500
475
480
476
480
475
465
441
441
404
433
421
425
420
429
425
410
395
401
385
0.18
0.19
0.22
0.20
0.20
0.19
0.08
0.46
0.48
NAg
5.10
5.11
5.14
5.15
5.09
5.16
5.14
5.24
5.31
5.08
2.24
2.16
2.22
2.20
2.20
2.24
2.11
2.10
2.22
1.86
9
TPD
aPotential difference between first and second oxidations.
bPotential difference between the Cp2Fe+/0 and diamine+/0 couples.
cEstimated from the intersection of the normalized absorption and emission graph.
dRelative to Coumarin 102.
eHOMO = (4.8 + ∆Ep) eV.
fLUMO = HOMO – energy gap (eV).
gNA = not available.
HT layer and that of the cathode is also important in EL de-
vice engineering. An electrochemical method based on cy-
clic voltammetry was used to estimate the HOMO energies
of the naphthalene diamines 1–9 and to examine their elec-
trochemical stability (43, 44).
Cyclic voltammetric scans were recorded under a N2
atmosphere in CH2Cl2 solvent with a Ag wire pseudo-
reference electrode and with TBAHFP as the supporting
electrolyte. At the end of each set of scans, Cp2Fe was
added to the solution and another potential scan was per-
formed that provided a reference potential for the Cp2Fe/
Cp2Fe+ couple as a calibration standard.
Over the positive potential scan range (0 to ~1.5 V), com-
pounds 1–9 exhibited two reversible oxidation waves (exam-
ples are shown in Fig. 5). If the potential scan is extended to
more positive potentials (~2 V), irreversible oxidation pro-
cesses were also observed for all the compounds with the
exception of 8. The two reversible oxidation peaks in the CV
© 2005 NRC Canada

966
Can. J. Chem. Vol. 83, 2005
Fig. 5. CV scans of compounds 6, 8, and 9 at 100 mV/s in CH2Cl2 with 0.05 mol/L TBAHP under N2. Five scans are shown.
8.5
7.5
(offset 6 mA)
6.5
Compound 9
5.5
4.5
(offset 3 mA)
3.5
Compound 8
2.5
1.5
0.5
Compound 6
-0.5
-1.5
0
0.2
0.4
0.6
0.8
1
1.2
Potential (V) vs. Ag wire
curves can be ascribed to one-electron oxidation processes
of the two amine centers. The first oxidation process corre-
sponds to the HOMO energy level of the diamine and corre-
lates directly to the energy required for hole generation and
transport in the HT layer. The E1/2 value of the
diamine/diamine+ couple relative to the E1/2 value for the
Cp2Fe/Cp2Fe+ couple was used to estimate the HOMO en-
ergy levels for compounds 1–9. The established value of the
ferrocene HOMO energy level of 4.8 eV was added to the
difference in the two oxidation potentials to obtain the
diamine HOMO energy values and the results of these calcu-
lations are summarized in Table 4 (43, 44).
led to changes in the voltammograms. We attribute this in-
stability to the deposition of the oxidized species onto the
electrodes. In fact, during the CV scans for 9 a brown film
was observed on the working Pt electrode, while for com-
pound 8 a green precipitate was observed in the solution
near the working electrode. These precipitates were not ob-
served if the potential range did not extend beyond the sec-
ond oxidation peak. The identity of these new species that
are produced upon extensive oxidation of 8 and 9 has not
been established, but the fact that this observation is associ-
ated with N-substitutents that do not have para substitution
suggests that there may be a reaction of the oxidized species
on the N-phenyl ring.
The extent of intramolecular charge mobility can be esti-
mated by the difference between the first and second oxida-
tion events for each of the diamines (∆Eoxid). In the case of
a large degree of charge delocalization, the first positive
charge should have a pronounced effect on the second nitro-
gen center making it more difficult to oxidize and leading to
a large difference in the two oxidation potentials. On the
other hand, if the two charges are isolated from each other,
only one oxidation couple would be observed. For example,
TPD shows a potential gap of 220 mV indicating electronic
communication between the two nitrogens after the first oxi-
dation. Aromatic di- and triamines in which the N centers
are bridged by a single phenyl group with a para or ortho
disposition often exhibit even larger potential gaps in the
range 330–530 mV (45–48). In contrast, the corresponding
values for meta-diaminobenzene derivatives are smaller at
161–191 mV, presumably reflecting a low degree of conju-
gation between the two N centers (49). A similar lack of
charge delocalization is observed for compounds 1–8.
Except for compound 9 (∆Eoxid = 255 mV), the dia-
minonapthalene compounds 1–8 exhibit small ∆Eoxid values
(121–169 mV).
The reproducibility of the voltammetric behavior over
multiple potential scan cycles demonstrates the stability of
these materials to the repeated redox steps they would expe-
rience in a device. When only the first two oxidation waves
were scanned, compounds 1–9 showed reproducible CV be-
havior for at least five scan cycles as shown in the Fig. 5.
With a wider potential range, compounds 1–7 continued to
exhibit superimposible CV curves for at least five scan cy-
cles. However, scanning of 8 and 9 over this wider potential
range that included the third, irreversible oxidation process
Electronic spectra
The naphthalene diamines 1–9 are white, yellow, or pale
orange solids and they possess well-separated absorption and
emission maxima with the later appearing in the blue to
green region of the spectrum. The UV–vis absorption and
emission spectra for these compounds is summarized in Ta-
ble 4, and typical spectra for compounds 1, 6, 8, and 9 are
shown in Fig. 6. Except for compound 6, these compounds
exhibit a single λmax in their absorption spectra. In the case
of 6, the value of λmax was taken from the shoulder at
375 nm (Fig. 6). The HOMO–LUMO gap energy was also
estimated from these spectra and is included in Table 4 (21,
22).
The absorption maximum for TPD at 350 nm has been as-
signed to an n → π* electronic transition that is due to elec-
tron transfer from a nitrogen lone pair to the π* orbital of
the biphenyl group (41). We assign the λmax absorption val-
ues for 1–9 as arising from a similar n → π* electron transi-
tion with the π* orbital residing on the naphthalene moiety.
In the case of 6, we propose that the rising absorption below
375 nm is a result of another n → π* electron transition in-
volving the nitrogen lone pair and the π* orbital of the
biphenyl group.
In comparison with compound 9, all of the para-
substituted phenyl compounds (1–7) exhibit a bathochromic
shift in both absorption and emission, which can be ascribed
to the electron-donating properties of the aromatic substitu-
ents. The electron-donating properties of the methoxy sub-
stituents appear to overwhelm the electron-withdrawing
effects of the fluoro (4), phenyl (6), and naphthyl groups (7).
© 2005 NRC Canada

Shan et al.
967
Fig. 6. UV–vis absorption (left) and emission (right) spectra for compounds 1 (ٗ), 6 (+), 8 (×), and 9 (᭺).
9
1
8
6
9
8
1
6
8 8
8888
88 88
8
8
8
8
8
8
8
88
88
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
88
8
8
8
8
8
8
8
8
8
88
8
8
8
8
8
88
8
88
8
8
8
8
88
88
888
88
88
8888
888
8888
8
8
8
8
88
8
888
88
888888 8888888888
8888888888888888888888888 8
350
400
450
500
550
600
Wavelength (nm)
As a result, compound 1 displays the largest red shift for
emission and Stokes shift.
electrochemically from the HOMO–LUMO energy gap that
was obtained from the absorption and emission spectra.
With this method we can then compare the relative energy
levels of the naphthyl core diamine compounds. From the
data, we can see that meta-substituted 8 and unsubstituted 9
have the largest HOMO values, while all of the para-
substituted compounds have similar HOMO values. These
two groupings of HOMO energies are consistent with a
HOMO that is predominantly the nonbonding electrons lo-
calized on the nitrogen centers. Furthermore, the very simi-
lar values of the LUMO energies for all of the compounds
are also in harmony with a naphthalene core localized π* or-
bital. The overall effects of the aryl substituents provides di-
rection for orbital level tuning of these and related materials
for the matching of the energy levels between the charge
transport layers that is required for successful OLED fabri-
cation.
Generally, the HT materials in an OLED are not used as
the source of emission. This is due to the fact the hole mo-
bility is much faster than electron mobility, and as a result,
the charge recombination usually occurs in the ET layer and
this is where the emission arises. However, if the difference
of HOMO energy levels between HT and ET is large enough
and the difference in LUMO energy levels is small, holes
will be blocked within the HT layer. As a result, emission
could arise in the HT layer and there are recent reports of
diamine-based HT materials functioning in the role of emit-
ter hosts and as emission sources (20, 26, 29, 50). The emis-
sion properties of HT materials, both the emission
wavelength (λ emi) and quantum efficiency (Φf) provide im-
portant performance criteria for OLED fabrication. The val-
ues of Φf for diamines 1–9 were measured in CH2Cl2
relative to Coumarin 102 as a standard (36) and the results
are presented Table 4.
Conclusions
Compounds 1–9 fall into two groups based on their Φ val-
f
ues. In particular, the Φ values for 8 and 9 are identical and
f
Pd catalyzed coupling reactions have been used to synthe-
size a series of naphthyl core diamines with the general for-
mula 1,5-[Ar(Ar′)N]2C10H6 (Ar = MeOC6H4; Ar′ = C6H5,
MeOC6H4, p-MeC6H4, p-FC6H4, p-PhC6H4, 1-naphthyl) and
the characterization of these species points to their potential
as optoelectronic materials. We have examined the thermal,
electrochemical, and photophysical properties of these mate-
rials. By varying the aryl groups, we have demonstrated the
features that lead to the best thermal stability. All of these
materials show enhanced glass forming ability and thermal
stability compared to TPD. The relative HOMO and LUMO
levels for these materials, important parameters for the de-
sign of efficient OLEDs, were assessed through electro-
chemical and spectroscopic measurements. Compounds 1–9
have the potential to function as hole transport materials and
they emit in the blue to green color range. We are currently
more than twice those of the other diamines. Furthermore, 8
and 9 have identical values for their absorption and emission
maxima. This phenomenon can be ascribed to the electron-
donating properties of the substituents on the N-aryl groups.
Since the transition is n → π* in the diamines, the electron-
rich amines that possess electron-donating groups at the para
position of the aryl substituents (i.e., 1–7) will display
higher energies for the nonbonding HOMO electrons relative
to those amines that do not have such groups (i.e., 8 and 9).
The result will be a reduced HOMO–LUMO gap and a
smaller energy difference between the singlet and triplet
states, which in turn allows for increased rate of intersystem
crossing and a reduction in the Φf (51).
Estimates for the diamine LUMO energy levels were ob-
tained by subtracting the HOMO energy values determined
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Can. J. Chem. Vol. 83, 2005
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their use as HT materials in OLEDs.
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