Siloxane-triarylamine hybrids: Discrete room temperature liquid triarylamines via the Piers-Rubinsztajn reaction

Article abstract of DOI:10.1021/ol102607v

A series of room temperature liquid siloxane-triarylamine hybrids were synthesized using the Piers-Rubinsztajn reaction. These materials displayed well behaved electrochemical oxidation and low T_g's and were free-flowing liquids. The interaction between the Lewis acidic tris(pentafluorophenyl)borane catalyst and the Lewis basic starting triarylamine substrate was also investigated by steady state UV-vis spectroscopy and ¹⁹F NMR.

Full text of DOI:10.1021/ol102607v

ORGANIC
LETTERS
2011
Vol. 13, No. 1
154-157
Siloxane-Triarylamine Hybrids: Discrete
Room Temperature Liquid Triarylamines
via the Piers-Rubinsztajn Reaction
Brett A. Kamino,† John B. Grande,§ Michael A. Brook,§ and
Timothy P. Bender*,†,‡
Department of Chemical Engineering and Applied Chemistry, The UniVersity of
Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5, Department of
Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario, Canada,
M5S 3H6, and Department of Chemistry and Chemical Biology, McMaster UniVersity,
1280 Main Street, Hamilton, ON, Canada, L8S 4M1
tim.bender@utoronto.ca
Received November 1, 2010
ABSTRACT
A series of room temperature liquid siloxane-triarylamine hybrids were synthesized using the Piers-Rubinsztajn reaction. These materials
displayed well behaved electrochemical oxidation and low Tg’s and were free-flowing liquids. The interaction between the Lewis acidic
tris(pentafluorophenyl)borane catalyst and the Lewis basic starting triarylamine substrate was also investigated by steady state UV-vis
spectroscopy and 19F NMR.
Triarylamines are well studied p-type semiconducting ma-
terials that have been used in many organic electronic devices
such as organic light emitting diodes (OLEDs), photovoltaics
(OPVs), organic field effect transistors (OFETs), and xero-
graphic photoreceptors (OPRs) among others.1 Their syn-
thesis is normally accomplished by palladium or copper-
catalyzed amination reactions starting from anilines or
diarylamines and aryl halides. These powerful synthetic tools
allow for the synthesis of many different complex triary-
lamine based structures that display a variety of interesting
and controllable properties and functions.2 Recently, the use
of liquid triarylamines has been explored for application in
OLEDs and dye sensitized solar cells (DSSCs). Specifically,
N-(2-ethylhexyl)carbazole has shown utility as a host material
having superior properties to its polymeric analog (polyvi-
nylcarbazole).3 Tris(4-methoxyethoxyphenyl)amine, also a
liquid, has been shown to be an alternative to the more widely
† Department of Chemical Engineering and Applied Chemistry, The
University of Toronto.
§ McMaster University.
‡ Department of Chemistry, University of Toronto.
(1) (a) Li, Z.; Meng, H. Organic Light Emitting Diodes Materials and
DeVices; Taylor and Francis Group: 2007. (b) Roncali, J.; Leriche, P.;
Cravino, A. AdV. Mater. 2007, 19, 2045. (c) Shirota, Y.; Kageyama, H.
Chem. ReV. 2007, 107, 953. (d) Weiss, D. S.; Abkowitz, M. Chem. ReV.
2010, 110, 479.
(2) (e) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for
Xerography; Marcel Dekker Inc.: New York, 1998. (a) Thelakkat, M.
Macromol. Mater. Eng. 2002, 287, 442. (b) Bender, T. P.; Graham, J. F.;
Duff, J. M. Chem. Mater. 2001, 13, 4105.
10.1021/ol102607v  2011 American Chemical Society
Published on Web 12/03/2010

-
used I-/I3 redox couple in a DSSC. The resulting DSSC
reaction. In this study, pentamethyldisiloxane (-PMDS) was
chosen as a prototypical reactive silicone because it is a pure
and discrete compound, a liquid, inexpensive and readily
available. Para-methoxy functionalized triarylamines were
chosen as substrates due to their synthetic accessibility and
their known physical and electronic properties.2b
had a reported efficiency of 2.4% under AM1.5 illumination.4
However, in each of these cases, the arylamine structure is
not generally amenable to synthetic variation. As a result,
the physical (melting point, viscosity, solubility/dispersibility)
and electronic properties (oxidation potential) of the ary-
lamine cannot be systematically varied. Access to new
synthetic methods that lead to a broader range of liquid
arylamines is therefore necessary.
The Piers-Rubinsztajn reaction has been shown to be a
powerful way to construct complex discrete siloxane archi-
tectures in an efficient manner.5 This reaction uses the strong
Lewis acid tris(pentafluorophenyl)borane (B(C6F5)3) to cata-
lyze the reaction between Si-H and Si-O-R groups (where
R ) H, Me or other alkyl, R3SiH + R′OSiR′′3 f R3SiOSiR′′3
+ R′H, Scheme 1A).6 This chemistry typically occurs very
Each precursor para-methoxy-triarylamine can be syn-
thesized in a single step using well established Buchwald-
Hartwig coupling conditions9 by the reaction between
4-bromoanisole and bis(3,4-dimethylphenyl)amine, 3,4-dim-
ethylaniline, or p-anisidine giving triarylamines 1a-c,
respectively. Each methoxy functionalized triarylamine was
subsequently reacted with PMDS in the presence of tris(pen-
tafluorophenyl)borane. In a typical procedure, the triary-
lamine was dissolved in toluene (10 wt %) which contained
a catalytic amount of tris(pentafluorophenyl)borane (1 mol
%) at room temperature in an open vessel. To this, PMDS
was added dropwise. There is a short induction time
following which the rapid evolution of gas occurs (methane
in this case): Safety Note - the eVolution can be Vigorous,
and the addition rate of the silane should be adjusted
accordingly. Reactions were worked up by the addition of
∼0.5 g of basic alumina, which was allowed to stir for an
additional 20 min within the reaction vessel to capture the
borane catalyst. The reaction solution was filtered, and the
solvent and excess pentamethyldisiloxane were simply
removed by rotary evaporation. A general reaction is
illustrated in Scheme 2. The isolated yields for these reactions
Scheme 1. Partial Scope of the Piers-Rubinsztajn Reaction
rapidly and is done under nonaqueous conditions. Crucial
to the utility of the process is the fact that silicones do not
undergo metathesis/redistribution in the presence of this
Lewis acid.5b As well, the borane catalyst is generally easy
to remove and the byproduct is either hydrogen or volatile
hydrocarbon gases (such as methane) either of which rapidly
leave the solution during reaction or under gentle vacuum.
Using this chemistry, the synthesis of many complicated and
otherwise inaccessible siloxane structures and other chemical
derivatives can be achieved.7
Scheme 2. Synthesis of Siloxane-Triarylamine Materials
In addition to the synthesis of Si-O-Si bonds for discrete
siloxane architectures, this chemistry has also been shown
to work between Si-H bonds and aryl-hydroxyl groups and
aryl-methoxy groups to form aryl-O-Si bonds (Scheme
1B).8
In this communication we describe a series of free-flowing
room temperature liquid siloxane-triarylamine hybrid com-
pounds that were prepared using the Piers-Rubinsztajn
typically exceeded 90%. We found that no further purification
of these compounds was required after removal of excess
PMDS and boron catalyst (as shown by HPLC and 1H NMR
(3) (a) Xu, D.; Adachi, C. Appl. Phys. Lett. 2009, 95, 053304. (b)
Hendrickx, E.; Guenther, B. D.; Zhang, Y.; Wang, J. F.; Staub, K.; Zhang,
Q.; Marder, S. R.; Kippelen, B.l.; Peyghambarian, N. Chem. Phys. 1999,
245, 407. (d) Ribierre, J.; Aoyama, T.; Kobayashi, T.; Sassa, T.; Muto, T.;
Wada, T. J. Appl. Phys. 2007, 102, 033106. (e) Ribierre, J. C.; Aoyama,
T.; Muto, T.; Imase, Y.; Wada, T. Org. Electron. 2008, 9, 396.
(4) Snaith, H. J.; Zakeeruddin, S. M.; Wang, Q.; Pe´echy, P.; Gra¨tzel,
M. Nano Lett. 2006, 6, 2000.
1
analysis). The H NMR analysis is somewhat complicated
by the observation of long-range coupling. All three
siloxane-triarylamine hybrid compounds were isolated as
pale yellow, free-flowing liquids. Compounds 2b and 2c had
(5) (a) Grande, J. B.; Thompson, D. B.; Gonzaga, F.; Brook, M. A.
Chem. Commun. 2010, 46, 4988. (b) Brook, M. A.; Grand, J. B.; Ganachaud,
F. AdV. Polym. Sci. 2010, 1.
(9) (a) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.;
Alcazar Roman, L. M. J. Org. Chem. 1999, 64, 5575. (b) Jiang, L.; Stephan,
B. L. Palladium-Catalyzed Aromatic Carbon-Nitrogen Bond Formation.
Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere, A.,
Diederich, F., Eds.; Wiley-VCH: 2004; Chapter 13. (c) Bender, T. P.;
Coggan, J. A.; McGuire, G.; Murphy, L. D.; Toth, A. E. J. US Patent
7,408,085, 2008. (d) Bender, T. P.; Coggan, J. A. US Patent 7,402,700,
2008. (e) Bender, T. P.; Goodbrand, H. B.; Hu, N. X.; US Patent 7,402,699,
2008. (f) Bender, T. P.; Coggan, J. A.; US Patent 7,345,203, 2008. (g)
Coggan, J. A.; Bender, T. P. US Patent 7,332,630, 2008.
(6) (a) Piers, W. E. The chemistry of perfluoroaryl boranes. In AdVances
in Organometallic Chemistry; West, R., Hill, A. F., Eds.; Elsevier Academic
Press: San Diego, 2005; Vol. 52, 1. (b) Chojnowski, J.; Rubinsztajn, S.;
Cella, J. A.; Fortuniak, W.; Cypryk, M.; Kurjata, J.; Kazmierski, K.
Organometallics 2005, 24, 6077.
(7) Thompson, D. B.; Brook, M. A. J. Am. Chem. Soc. 2008, 130, 32.
(8) Cella, J.; Rubinsztajn, S. Macromolecules 2008, 41, 6965.
Org. Lett., Vol. 13, No. 1, 2011
155

Table 1. Characterization Data for Compounds 1a-ca and 2a-c
a Taken from ref 2b.
viscosities similar to that of a 5130 g/mol weight averaged
PDMS standard, whereas 2a was similar to a 24 800 g/mol
standard. Precise measurements are underway.
was used as an internal standard, and all data is corrected to
its published half wave potential.10 The results are sum-
marized in Table 1. Siloxane-triarylamine hybrids (2a-c)
have very similar electrochemical behavior to that of the
methoxy substituted triarylamines 1a-c with a single revers-
ible oxidation event. We observed that the halfwave oxidation
potential (E1/2) for compounds 2a-c decreases with increas-
ing PMDS substitution. This implies that PMDS is an
electron-donating group.11 However, based on the compari-
son to the E1/2 for compounds 1a-c we can conclude that
the PMDS moiety is a weaker electron-donating group than
methoxy.
The accepted mechanism for the Piers-Rubinsztajn reac-
tion initially involves the formation of a reversible complex
between the Si-H of the silane and the boron center of the
Lewis acid catalyst as a key intermediate step.6b It was
anticipated that the strong Lewis acid used herein12 could
competitively bind to the substrate triarylamine, which is a
weak Lewis base.13
Each of the compounds 2a-c was characterized by
differential scanning calorimetry (DSC) to establish the effect
of siloxane substitution on the arylamine and to observe any
thermal transitions which may exist (Table 1). DSC analysis
was performed by first rapidly cooling each liquid to -80
°C. Each sample was then subsequently heated to room
temperature, back down to -80 °C and finally back to room
temperature all at a rate of 10 °C/min. Sharp glass transitions
(Tg) were observed in all cases in each of the heating cycles.
No other thermal transitions were observed for these
compounds (complete DSC curves are provided in the
Supporting Information). It can be seen that addition of the
PMDS group has a strong effect on the physical properties
of the triarylamines. The normally crystalline starting meth-
oxy-triarylamines (1a-c) are converted to liquids with very
low glass transition temperatures ranging from -45 to -63
°C. It is observed that increasing the number of PMDS chains
decreases the glass transition temperature. In no cases were
other thermal transitions including crystalline transitions
observed.
Each siloxane-triarylamine hybrid was also characterized
by cyclic voltammetry (CV) to determine the effect of PMDS
substitution on the electrochemistry of the compounds. Each
was run under identical conditions to those for compounds
1a-c, and their results were compared.2b CV was performed
in dichloromethane with (Bu)4NClO4 as a supporting elec-
trolyte. Each compound was scanned from -300 to +1100
mV and back to -300 mV at 50 mV/s. Each was cycled
through this range four times in total. Decamethylferrocene
The efficiency of the process described herein suggests
there is little interaction between tris(pentafluorophenyl)bo-
rane and the triarylamine substrate. However, if a Lewis
acid|base interaction took place, it would likely involve the
removal of an electron from the triarylamine resulting in the
brightly colored radical cation of the triarylamine thereby
allowing for detection of even trace amounts of interac-
(10) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay,
P. A.; Masters, A. F.; Phillips, L. J. Phys. Chem. B 1999, 103, 6713.
(11) Brook, M. A. Silicon in Organic, Organometallic, and Polymer
Chemistry; John Wiley & Sons, Inc.: New York, 2000.
(12) Piers, W. E.; Chivers, T. Chem. Soc. ReV. 1997, 26, 345.
(13) Park, M. H.; Park, J. H.; Do, Y.; Lee, M. H. Polymer 2010, 51,
4735.
156
Org. Lett., Vol. 13, No. 1, 2011

tions.14,15 Thus UV-vis spectra of triarylamine 1c were
taken between 300 and 2000 nm with varying equivalents
of tris(pentafluorophenyl)borane in dilute solutions of toluene
(0.0298 mol/L). Looking at the visible region of the
spectrum, a very weak absorbance centered at 742 nm can
be observed (Figure 1). This peak increased in intensity in
the interaction results in the formation of a radical cation|radical
anion pair. The absorbance at 742 nm was extremely weak
compared to its primary absorption band at 298 nm. Given
the extinction coefficient of a (tri)arylamine radical cation
is known to be very high16 relative to the neutral compound,
the spectra suggest that 1c and tris(pentafluorophenyl)borane
are in an equilibrium shifted far toward dissociation. To
confirm this, 19F NMR was performed on an equimolar
mixture of 1c and tris(pentafluorophenyl)borane.17 No change
in the 19F chemical shifts of tris(pentafluorophenyl)borane
was observed thereby confirming the equilibrium is shifted
far toward dissociation.
In summation, the functionalization of triarylamines with
discrete silioxane chains under Piers-Rubinsztajn conditions
(tris(pentafluorophenyl)borane catalysis) has been shown to
drastically change their physical properties. The result is a
sample of three free-flowing room temperature liquid
siloxane-triarylamine hybrids. Any Lewis acid/Lewis base
interaction between the arylamine and borane catalyst is weak
such that free catalyst is available to interact with the
hydrosilane and initiate the Piers-Rubinsztajn process. We
believe that this synthetic strategy will facilitate further
application of liquid arylamines in optoelectronic devices.
Figure 1. Steady state UV-vis absorbance spectra of 1c upon
addition of 0.25, 0.5, and 1 equiv of tris(pentafluorophenyl)borane.
Also shown is 1c oxidized with 0.5 equiv of SbCl5 for reference.
Acknowledgment. We would like to thank NSERC for
funding of this project through the Discovery Grant Program
(T.B.). The Ontario Graduate Scholarship program and the
Bert Wasmund Fellowship (UofT) for assistance (B.K.). We
would also like to thank Dr. Syed Abthagir and Dr. Ning
Yan (Faculty of Forestry, The University of Toronto) for
their assistance in acquiring the DSC results.
a nonlinear fashion with increasing amounts of tris(pen-
tafluorophenyl)borane. A chemically oxidized solution of
triarylamine 1c with 0.5 equiv of antimony(V) chloride
(SbCl5) was also prepared. It is well-known that the oxidative
action of SbCl5 on a (tri)arylamine produces the radical cation
via a single electron transfer.16 The UV-vis spectrum of
the mixture of 1c and SbCl5 overlaps with the position and
has the same peak shape observed for the mixtures of 1c
and tris(pentafluorophenyl)borane (Figure 1). It can thus be
concluded that there is some level of interaction between
tris(pentafluorophenyl)borane and the triarylamine and that
Supporting Information Available: Experimental data,
NMR spectral data, CV data and DSC data for compounds
1a-c and 2a-c. 19F NMR spectra for mixtures of 1c and
tris(pentafluorophenyl)borane. This material is available free
of charge via the Internet at http://pubs.acs.org.
(14) Blackwell, J. M.; Sonmer, E. R.; Scoccitti, T.; Piers, W. E. Org.
OL102607V
Lett. 2000, 2, 3921
.
(15) Amthor, S.; Noller, B.; Lambert, C. Chem. Phys. 2005, 316, 141–
152
.
(17) 19F NMR was performed in deuterated benzene (C6D6). All peaks
are referenced to an external standard of BF3·Et2O. Detailed 19F spectra are
included in the Supporting Information.
(16) Zhou, G.; Baumbarten, M.; Mu¨llen, K. J. Am. Chem. Soc. 2007,
129, 12211.
Org. Lett., Vol. 13, No. 1, 2011
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