Deuteration of molecules for neutron reflectometry on organic light-emitting diode thin films

Article abstract of DOI:10.1016/j.tetlet.2011.12.032

Deuterated forms of aromatic charge transporting heterocycles 2 and 3 used in organic light-emitting diodes have been produced by hydrothermal reactions, catalyzed by Pt/C or Pd/C. Comprehensive analysis by mass spectroscopy, ¹H, ²H

Full text of DOI:10.1016/j.tetlet.2011.12.032

Tetrahedron Letters 53 (2012) 931–935
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Deuteration of molecules for neutron reflectometry on organic
light-emitting diode thin films
Tamim A. Darwish a, , Arthur R.G. Smith b, Ian R. Gentle b,c, Paul L. Burn b, Emily Luks a, Greta Moraes a,
⇑
Marie Gillon a, Peter J. Holden a, Michael James a,d,
⇑
a National Deuteration Facility, Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia
b Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, QLD 4072, Australia
c School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia
d School of Chemistry, The University of New South Wales, Kensington, NSW 2052, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Deuterated forms of aromatic charge transporting heterocycles 2 and 3 used in organic light-emitting
diodes have been produced by hydrothermal reactions, catalyzed by Pt/C or Pd/C. Comprehensive anal-
ysis by mass spectroscopy, 1H, 2H and 13C NMR enables determination of the overall quantity of D atoms
present, as well as the level of deuteration at each molecular site. The roles of solubility and steric avail-
ability in deuteration are discussed in the light of these results. Neutron reflectometry indicates excellent
scattering contrast between protonated and deuterated forms of these molecules, with nanoscale thin
films showing the same density as in their bulk molecular forms. Although used for morphological stud-
ies of thin films typically used in OLEDs, the synthetic and analysis methods described here are generic
and suitable for deuteration of other conjugated aromatic heterocycles and other optoelectronic devices.
Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
Received 12 October 2011
Revised 27 November 2011
Accepted 9 December 2011
Available online 16 December 2011
Keywords:
Hydrothermal deuteration
Aromatic heterocycles
Steric availability
Organic light-emitting diode
Neutron reflectometry
Organic optoelectronics is a rapidly growing area of interdisci-
plinary science and technology, with investigations focused on both
small molecules,1,2 conjugated polymers,3a and dendrimers3b,c that
can be used as the light-emitting layers in organic light-emitting
diodes (OLEDs). A fundamental feature of these devices is that they
are comprised of more than one layer and hence rely on electron
transfer processes at interfaces. Conjugated aromatic heterocycles
such as 2 and 3 (Fig. 1) are widely used electron and hole transport
materials, respectively. The current generation of OLEDs, having
internal quantum efficiencies of ꢀ100%, are based on small molecule
phosphorescent iridium(III) complexes [e.g., fac-tris(2-phenylpyr-
idyl)iridium(III) (Ir(ppy)3] blended with an organic host material
[e.g., 4,40-bis(N-carbazolyl)biphenyl (CBP)] in the emissive layer.2
While efficiencies of these layered devices are very high, their life-
times depend on a range of factors including the morphological
stability of layers in the device.4 Investigations of functioning de-
vices with buried interfaces present numerous difficulties using
conventional surface science techniques and so neutron reflectome-
try along with selective deuteration has become the key method for
the study of morphology, diffusion, and interfacial behavior in or-
ganic thin-film semiconducting devices.5 Selective deuteration of
these aromatic and heterocyclic molecules leads to a substantial
Figure 1. OLED molecules 2 and 3 and precursor 1.
increase in neutron scattering length density compared to their
equivalent protonated forms. In addition to such morphological
studies, recent investigations have also centered on isotope effects
associated with deuterium that lead to substantial increases in the
quantum efficiency and high-voltage stability of Al-based5j and
Ir-based5k OLEDs.
There have been a number of studies detailing different methods
for synthesizing deuterated aromatic and heterocyclic compounds;
these being ably summarized by the early work of Hawthorne et al.6
and recent published works of Derdau, Atzrodt, and co-workers,7 as
well as numerous studies by Sajiki et al.8 This Letter reports the deu-
teration of molecules that form key components of the electron and
⇑
Corresponding authors.
E-mail address: tde@ansto.gov.au (T.A. Darwish).
0040-4039/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.032

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T. A. Darwish et al. / Tetrahedron Letters 53 (2012) 931–935
hole transport layers of an OLED device, and assesses the synthetic
outcomes in light of different deuteration conditions. The role of sol-
ubility and steric availability of proton sites is also discussed. Finally,
results from the application of these deuterated molecules in the
study of interfacial morphology of films typically applied in OLEDs
using neutron reflectometry are presented.
standard (i.e., MeOH). 2H NMR spectroscopy confirmed the deuter-
ation at these positions by showing reciprocal resonances for each
deuterated position (Supplementary data, Fig. S5), whereas 13C
NMR spectroscopy allowed observation of the deuterated and pro-
tonated extent at each position simultaneously, utilizing the sec-
ondary isotope effect on the observed 13C nucleus.9 The latter
was achieved by running an inverse gated 13C NMR experiment
with long delays between pulses (i.e., D1 = 20 s); observing 13C nu-
clei while decoupling both 1H and 2H nuclei. This technique is a
particularly useful analysis tool, as experiments can be designed
that discriminate between resonances of the various types of car-
bons present: fully deuterated groups (CD, CD2, CD3), partially deu-
terated groups (CHD, CHD2, CH2D), and non-deuterated groups (C,
CH, CH2, CH3). By comparing the 13C{1H, 2H} (decoupling both 1H
and 2H) spectrum to the normal 13C{1H} (proton decoupled only)
NMR, it was possible to differentiate between carbons with and
without deuterons. This allowed the determination of the percent
deuteration at specific sites in the molecule, as shown in Figure 3,
and confirmed the percent deuteration obtained from 1H NMR
analysis with an internal standard, the values agreeing to within
1%. Additionally, this technique allowed determination of the per-
cent deuteration at carbon positions where the corresponding
residual proton resonances overlap in the 1H NMR spectrum. For
example, in BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline),
the residual protons at the ortho, meta, and para positions of the
two phenyl groups at position C(3) (Fig. 3) have overlapping reso-
nances in the 1H NMR spectrum, while their three carbon reso-
nances in the 13C NMR spectra are well resolved (Supplementary
data, Fig. S8).
Of the 20 protons available within BCP, six distinct sites are
evident (Fig. 3) which were found to be deuterated to different
degrees depending on the steric hindrance and availability of these
sites for H/D exchange with the catalyst. Methyl protons at the C(1)
position showed the highest deuteration levels (90%), while
protons at the C(2) position showed only 40% deuteration due to
the steric shielding from the phenyl groups. The ortho-, meta-,
and para-positions on the two phenyl groups at position C(3) in
Figure 3 were found to be deuterated at 55%, 86%, and 90%, respec-
tively. The remaining two protons attached to the phenanthroline
moiety at position C(4) are strongly sterically shielded by the
pendant phenyl groups, such that the catalyst had restricted
access to these positions and hence they were deuterated at the
4% level.
Deuteration of the molecules presented in this study was
achieved either by H/D exchange reactions with deuterium oxide
catalyzed by Pt/C and Pd/C under hydrothermal conditions, or by
synthesis from deuterated precursors produced by the same
method. Sajiki et al. have demonstrated that platinum catalysts
generally have a higher tendency toward the deuteration of
aromatic positions, whereas palladium catalysts preferentially
deuterate aliphatic positions.8i The same authors also showed that
Pt/C and Pd/C when used together have a synergistic effect in
deuterating sterically hindered aromatic positions.8j It is known
that this method is dependent on both electronic and steric factors,
and it has been suggested that the high affinity of Pt or Pd metal
for a nitrogen lone pair increases the efficiency of the exchange
reaction.8e Lower incorporation of deuterium has been observed
on groups neighboring sterically encumbered nitrogen atoms. For
our study, the optimized conditions, overall yield and deuteration
outcomes are summarized in Table 1.
Carbazole-d8 (1) was the most efficiently produced molecule in
this study under hydrothermal conditions (240 °C), which was per-
formed on ca. 10 g scale and gave 80% yield and 94% deuteration of
the aromatic proton sites (Supplementary data, Figs. S1–S3). This is
attributed to the high solubility of carbazole in D2O at elevated
temperatures and the lack of steric crowding of its proton sites.
Although prepared at a lower temperature (180 °C) than 1, the
optimized hydrothermal deuteration of BCP-d20 (2) gave a poorer
yield (50%) and lower deuteration levels. The broad isotope cluster
shown in the ESI-MS spectrum of BCP-d20 (Fig. 2) indicates a distri-
bution of protonated and deuterated sites within the molecule
(incomplete deuteration). The analysis of the area under these
peaks in the spectrum suggests an average of 68% deuteration.
The presence of the signal m/z: 361, which corresponds to the com-
pletely non-deuterated species (i.e., d0), separate from the contin-
uous isotope cluster (d11–d20) suggests solubility issues. Material
that remained insoluble is expected to stay non-deuterated, while
the material that dissolved in the solution incorporated deuterium
atoms and formed a continuous mass distribution envelope (d11
–
d20). The combination of 1H, 2H and 13C NMR spectra (Supplemen-
tary data, Figs. S4–S8) allowed us to confirm the total deuterium
content for this molecule, but also importantly, gives a clear indi-
cation of the relative occurrence of hydrogen and deuterium atoms
at specific sites within the molecule.
Attempts at producing TCTA-d36 [tris(4-carbazoyl-9-ylphe-
nyl)amine-d36] using hydrothermal methods and a similar ap-
proach to that used for 1 produced either no deuteration at
240 °C, or substantial decomposition (30% yield) and poor deuter-
ation (15% D) at 265 °C (Supplementary data, Fig. S9). The lack of
deuteration at lower temperatures appears to be correlated to
the poor solubility of TCTA in D2O.
The percentage protonation of residual signals for individual 1H
resonances within the molecule was determined by 1H NMR spec-
troscopy (Supplementary data, Fig. S4) using a protonated internal
Table 1
Optimized hydrothermal reaction conditions for 1–3
Reagents/catalyst
Temp. (°C)
Time (h)
Composition/average % deuteration
Yield (%)
Carbazole (59.8 mmol), D2O (120 mL), Pt/C (2.5 g)
BCP (1.38 mmol), D2O (35 mL), Pt/C (0.25 g)
TCTA (0.67 mmol), D2O (30 mL), THF (10 mL), Pt/C (0.23 g), Pd/C (0.23 g), H2 (1 L)
240
180
140
48
C12D8HN (1) 94%
C26D14H6N2 (2) 68%
C54D15H21N4 (3) 42%
80
50
56
96
18 ꢁ 3a
a
Products were extracted and analyzed after each run, before returning to the Parr reactor with fresh reagents and catalysts.

T. A. Darwish et al. / Tetrahedron Letters 53 (2012) 931–935
933
Figure 2. ESI-mass spectrum (+ve ER) for partially deuterated BCP-d20 (2).
Figure 3. Site-specific deuteration levels for BCP-d20 (2) and TCTA-d36 (3).
Addition of THF as a co-solvent to the reaction solution im-
proved the solubility of TCTA and allowed significant deuteration
under hydrothermal conditions. A similar approach of using THF
as a co-solvent has also been shown to be effective by Sajiki and
co-workers8g to deuterate estrogen under conditions where solu-
bility in D2O was relatively poor. The isotopic distribution shown
in the ESI-MS spectrum of TCTA-d36 (Fig. 4) indicates a distribution
of protonated and deuterated sites, and analysis of the area under
the peaks shows increasing deuteration levels of 27%, 35%, and 42%
after reaction for 1, 2, and 3 days, respectively. The fully protonated
form of TCTA-h36 in this plot shows a peak at m/z = 740; while per-
deuterated TCTA-d36 gives an m/z peak of 776. The shift of the mass
distribution envelope to a higher mass (to the right) indicates high-
er deuterium content after each cycle. Characterization of TCTA-d36
(3; after 3 days) using NMR confirmed the overall deuteration level
(Supplementary data, Figs. S10–S13) and again allowed the assign-
ment of deuteration levels at specific sites (Fig. 3). 1H NMR spectro-
scopic resonances associated with positions C(7), C(8), and C(9) on
the carbazole pendants were well resolved, and showed deutera-
tion levels of 68%, 69%, and 31%, respectively (Fig. 5). 1H resonances
associated with positions C(1), C(2), and C(10) overlap so their
individual contributions could not be accurately assigned (Supple-
mentary data, Fig. S10), but as a group these three positions
showed an average deuteration level of 27%. Subsequently, 13C
NMR (Supplementary data, Figs. S12–S13) was used to help
discriminate between these three sites. Analysis of the data
showed that site C(2) was 75% deuterated, thus indicating that very
little deuteration (<3% on average) had taken place at the sterically
hindered sites C(1) and C(10).
Due to the poor yields and relatively low total deuteration level
of TCTA-d36 (3) by hydrothermal methods, another form of the
molecule (TCTA-d24 3b) was synthesized using the highly deuter-
ated carbazole-d8 (1) and protonated tris(4-bromophenyl)amine
via a modified Ullman condensation10 (Supplementary data, Sec-
tion 1.3). Analysis of the product 3b (80% yield) using ESI-MS
and NMR spectroscopy confirmed the expected 63% overall deuter-
ation level, with the protonated triphenylamine core and essen-
tially per-deuterated carbazole pendant groups (Scheme 1).
Compound 3b was subsequently used to produce thin film samples
for study using neutron reflectometry.
Thin films of the desired protonated and deuterated compounds
were prepared by thermal evaporation under high vacuum onto
silicon wafers, and neutron reflectivity measured using the Platy-
pus time-of-flight neutron reflectometer at the OPAL 20 MW re-
search reactor [Australian Nuclear Science and Technology
Organisation (ANSTO), Sydney, Australia].11 Further details of the
neutron reflectometry experiment are given in the Supplementary
data (Section 2). Very good fits to observed neutron reflectivity
data were obtained from films of TCTA-d24 (3b; Supplementary
data, Fig. S19) and BCP-d20 (2; Fig. S20) using the MOTOFIT analysis

934
T. A. Darwish et al. / Tetrahedron Letters 53 (2012) 931–935
Figure 4. ESI-mass spectra for TCTA-d36 as a function of reaction time with fresh THF/D2O mixture each cycle.
software12 and single layer models. Refined structural data indicate
high-quality films of thickness 168(1) Å and 185(1) Å, respectively,
surface roughness less than 1 nm, and scattering length density
(SLD) values of 4.83(3) ꢁ 10ꢂ6 Åꢂ2 and 5.02(4) ꢁ 10ꢂ6 Åꢂ2. These
values are important, as they indicate that films of TCTA-d24 and
BCP-d20 have the same mass density as their bulk protonated
forms.
Analysis of neutron reflectivity data (Fig. 6) from a bilayer film
formed by a 420(1) Å thick layer of protonated Ir(ppy)3:CBP (a typ-
ical light-emitting blend layer) deposited onto a 279(1) Å thick
base layer of TCTA-d24 (3b) shows excellent scattering contrast be-
tween
the
two
layers
(SLD = 2.46(3) ꢁ 10ꢂ6 Åꢂ2
and
4.83(4) ꢁ 10ꢂ6 Åꢂ2, respectively). The SLD values of each layer in
this bilayer film are essentially the same as those observed for
the individual single layer films. Excellent contrast was also ob-
served for the (TCTA-d24)/Ir(ppy)3:CBP/(BCP-d20) multilayer film
used in our study of interfacial diffusion.5h In that study, the three
layer structure was found to be stable at temperatures up to 60 °C.
At 100 °C however, the Ir(ppy)3:CBP and BCP-d20 layers were
found to intermix; while the (TCTA-d24)/Ir(ppy)3:CBP interface re-
mained stable with no diffusion taking place. Without the avail-
ability of BCP-d20 and TCTA-d24 such a study would not have
been possible. Protonated forms of BCP and TCTA in these thin-film
devices would not have provided adequate scattering contrast be-
tween the CBP emissive layer (SLD = 2.46 ꢁ 10ꢂ6 Åꢂ2); the BCP
Figure 5. 1H NMR (CD2Cl2, 400 MHz) spectra (from top to bottom) of TCTA-h36
TCTA-d24 (3) deuterated using a hydrothermal method and synthesized TCTA-d24
(3b) from deuterated carbazole and protonated tris(4-bromophenyl)amine.
,
Scheme 1. Synthesis of TCTA-d24 (3b).

T. A. Darwish et al. / Tetrahedron Letters 53 (2012) 931–935
935
Figure 6. Observed (points) and calculated (solid line) neutron reflectometry profiles for a TCTA-d24/Ir(ppy)3:CBP bilayer film on Si. Inset shows the SLD profile.
layer having an SLD of ꢀ2.3 ꢁ 10ꢂ6 Åꢂ2 and the TCTA layer with an
3. (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.;
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.
In conclusion, we have produced relatively large quantities of
deuterated materials typically used in OLEDs using hydrothermal
conditions with Pt/C and Pd/C as catalysts. In the case of BCP, the
availability of two nitrogen atoms which can coordinate to the me-
tal catalyst increases its accessibility and thus enhances the H/D
exchange process. The hydrothermal exchange reaction with insol-
uble TCTA was only achieved by using protonated THF as a co-sol-
vent. The extent of deuteration in each case was estimated using a
combination of ESI-MS, 1H, 2H, and 13C NMR analysis which
showed lower deuteration percentages at sites associated with ste-
ric hindrance and low accessibility for the catalyst. The use of in-
verse gated 13C NMR experiments were particularly useful in
decoupling overlapping 1H resonances from multiple sites, giving
unique assignment of site-specific deuteration levels. These syn-
thetic and characterization results show the ability for these tech-
niques to be used in the deuteration of other conjugated aromatic
heterocycles. Neutron reflectometry measurements indicate
smooth, fully dense molecular films with excellent scattering con-
trast between the protonated and deuterated components.
Acknowledgments
Synthesis of the molecules described here was conducted at the
National Deuteration Facility and was partly supported by the Na-
tional Collaborative Research Infrastructure Strategy—an initiative
of the Australian Government. A.R.G.S. would like to thank the Aus-
tralian Institute of Nuclear Science and Engineering for a postgrad-
uate research award. P.L.B. is a recipient of an Australian Research
Council Federation Fellowship (Project no. FF0668728). We
acknowledge funding from the University of Queensland (Strategic
Initiative-Centre for Organic Photonics & Electronics).
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Supplementary data
Supplementary data (synthesis, NMR and MS spectroscopy
characterization and spectra and the neutron reflectometry details)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2011.12.032.
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