Triarylamines designed to form molecular glasses. Derivatives of tris(p-terphenyl-4-yl)amine with multiple contiguous phenyl substituents

Article abstract of DOI:10.1021/ol902841c

[Chemical equation presented] The principles of crystal engineering can be used in a contrary way to help devise molecules that resist crystallization and form long-lived glasses. This can be achieved by making structural changes that thwart established patterns of crystallization. In using this strategy to block the crystallization of triarylamines, we have found that the introduction of methylpentaphenyl groups is particularly effective, presumably because they inhibit efficient molecular packing and normal intermolecular interactions.

Full text of DOI:10.1021/ol902841c

ORGANIC
LETTERS
2010
Vol. 12, No. 3
404-407
Triarylamines Designed to Form
Molecular Glasses. Derivatives of
Tris(p-terphenyl-4-yl)amine with Multiple
Contiguous Phenyl Substituents
Eric Gagnon,† Thierry Maris, and James D. Wuest*
De´partement de Chimie, UniVersite´ de Montre´al, Montre´al, Que´bec H3C 3J7 Canada
james.d.wuest@umontreal.ca
Received September 22, 2009
ABSTRACT
The principles of crystal engineering can be used in a contrary way to help devise molecules that resist crystallization and form long-lived
glasses. This can be achieved by making structural changes that thwart established patterns of crystallization. In using this strategy to block
the crystallization of triarylamines, we have found that the introduction of methylpentaphenyl groups is particularly effective, presumably
because they inhibit efficient molecular packing and normal intermolecular interactions.
The characteristic optoelectronic properties of triarylamines are
widely exploited in modern technology.1 Properties of particular
importance are the typically low oxidation potentials of triaryl-
amines, the stability of their delocalized radical cations, the
reversibility of their oxidation, high charge-carrier mobilities,
efficient fluorescence, and the ease of tuning behavior by making
rational structural alterations.1 As a result, triarylamines can serve
as critical components in a broad range of molecular optoelectronic
devices,1 including light-emitting diodes, field-effect transistors, and
solar cells, as well as in advanced materials for xerography, two-
photon absorption,2 and photorefraction.3
parency and flexibility, and avoid discontinuities created at the
intersection of individual crystalline domains. In principle,
crystallization can be suppressed by employing polymeric
triarylamines; however, this approach foregoes the advantages
of working with materials composed of small molecules,
including their well-defined compositions, increased volatility,
straightforward purification, and rigorous structural characteriza-
tion. For these reasons, learning how to engineer simple
triarylamines that resist crystallization is an important objective.
Small molecules tend to crystallize readily, but certain
compounds can form long-lived amorphous phases with high
glass-transition temperatures (Tg). Shirota and others have
presented valuable general guidelines for the design of mol-
Many of these applications require using triarylamines within
amorphous films. Such films simplify fabrication, favor trans-
† Fellow of the Natural Sciences and Engineering Research Council of
Canada (2003-2009).
(1) For recent reviews, see: Ning, Z.; Tian, H. Chem. Commun. 2009,
5483–5495. Mishra, A.; Fischer, M. K. R.; Ba¨uerle, P. Angew. Chem., Int.
Ed. 2009, 48, 2474. Roncali, J.; Leriche, P.; Cravino, A. AdV. Mater. 2007,
19, 2045. Thelakkat, M. Macromol. Mater. Eng. 2002, 287, 442.
(2) For recent references, see: Bordeau, G.; Lartia, R.; Metge, G.; Fiorini-
Debuisschert, C.; Charra, F.; Teulade-Fichou, M.-P. J. Am. Chem. Soc. 2008,
130, 16836. Bhaskar, A.; Ramakrishna, G.; Lu, Z.; Twieg, R.; Hales, J. M.;
Hagan, D. J.; Van Stryland, E.; Goodson, T., III. J. Am. Chem. Soc. 2006,
128, 11840.
(3) For a review, see: Zilker, S. J. ChemPhysChem 2000, 1, 72.
10.1021/ol902841c  2010 American Chemical Society
Published on Web 12/22/2009

Scheme 1
ecules likely to yield long-lived glasses.4 Among the key
Analysis of the reported structure of triarylamine 1 shows
that (1) the triphenylamino core adopts a nonplanar, propel-
ler-shaped conformation and (2) multiple intermolecular
π···π and C-H···π aromatic interactions are present.8,9 As
summarized in a recent study of the structures of hexaphe-
nylbenzene, pentaphenylbenzene, methylpentaphenylben-
zene, and related compounds,10 simple arenes can be
modified by attaching multiple contiguous phenyl substituents
to create complex nonplanar topologies that disfavor exten-
sive π···π and C-H···π aromatic interactions. Applied to
triarylamines 1 and 2, such structural alterations should
weaken molecular association, impede molecular movement,
and hinder efficient packing,10 thereby enhancing the pre-
disposition to form glasses.4 Indeed, substituted triarylamine
3a has been shown to produce a long-lived glass (Tg ) 202
°C) and to serve as an effective hole transporter in elec-
troluminescent devices.11
To provide a deeper understanding of structural features
that can inhibit the crystallization of triarylamines, we
synthesized fully substituted derivatives 3b,c and studied their
behavior. As summarized in Scheme 1, triarylamine 3b was
prepared in 74% overall yield from tris(4-iodophenyl)amine
(4)12 by Sonogashira coupling with phenylethyne to give
intermediate 5, followed by heating with excess tetraphe-
nylcyclopentadienone to effect a 3-fold Diels-Alder reaction
and elimination of CO. Similarly, analogue 3c was obtained
in 75% overall yield from compound 4 by Negishi coupling
features are nonplanar molecular structures that adopt multiple
conformations and inhibit effective packing. In addition, recent
efforts to design molecular glasses have underscored the
effectiveness of turning to the “dark side” of crystal engineering
and making shrewd structural alterations that thwart established
patterns of crystallization, without necessarily changing other
properties of interest.5 To test these strategies in an area of
established practical importance, we have prepared new triaryl-
amines designed to form glasses with high values of Tg.
Triphenylamine itself is known to crystallize readily instead
of forming a molecular glass, even when molten samples are
cooled rapidly.6 In contrast, tris(biphenyl-4-yl)amine (1) and
tris(p-terphenyl-4-yl)amine (2) both form glasses (Tg ) 76 and
132 °C, respectively), possibly in part because they can adopt
more conformations than triphenylamine.6 This behavior allows
triarylamine 2 to serve as a modestly effective blue emitter in
thin-film molecular electroluminescent devices.7
`
(5) Lebel, O.; Maris, T.; Perron, M.-E.; Demers, E.; Wuest, J. D. J. Am.
Chem. Soc. 2006, 128, 10372.
(6) Higuchi, A.; Ohnishi, K.; Nomura, S.; Inada, H.; Shirota, Y. J. Mater.
Chem. 1992, 2, 1109.
(7) Kinoshita, M.; Kita, H.; Shirota, Y. AdV. Funct. Mater. 2002, 12,
780. Ogawa, H.; Ohnishi, K.; Shirota, Y. Synth. Met. 1997, 91, 243.
(8) Inada, H.; Ohnishi, K.; Nomura, S.; Higuchi, A.; Nakano, H.; Shirota,
Y. J. Mater. Chem. 1994, 4, 171.
(9) For a related study of the structure of the tris(biphenyl-4-yl)aminium
radical cation, see: Brown, G. M.; Freeman, G. R.; Walter, R. I. J. Am.
Chem. Soc. 1977, 99, 6910.
The principles of crystal engineering can be used to design
related triarylamines predisposed to form molecular glasses.
(10) Gagnon, E.; Maris, T.; Arseneault, P.-M.; Maly, K. E.; Wuest, J. D.
Cryst. Growth Des., in press (DOI: 10.1021/cg9010746).
(11) Tong, Q.-X.; Lai, S.-L.; Chan, M.-Y.; Lai, K.-H.; Tang, J.-X.;
Kwong, H.-L.; Lee, C.-S.; Lee, S.-T. Chem. Mater. 2007, 19, 5851.
(12) Varnavski, O. P.; Ostrowski, J. C.; Sukhomlinova, L.; Twieg, R. J.;
Bazan, G. C.; Goodson, T., III J. Am. Chem. Soc. 2002, 124, 1736.
(4) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953. Shirota, Y.
J. Mater. Chem. 2005, 15, 75. Shirota, Y. J. Mater. Chem. 2000, 10, 1.
Org. Lett., Vol. 12, No. 3, 2010
405

with 1-propynylzinc bromide to give intermediate 6,13
followed again by heating with excess tetraphenylcyclopen-
tadienone. Thermogravimetric analysis (TGA) of triaryl-
amines 3b,c (Figure 1) confirmed their expected high thermal
To better understand these differences, we attempted to
grow crystals of triarylamines 3b,c suitable for structural
study by X-ray diffraction. Repeated attempts to crystallize
compound 3c were unsuccessful, but we found that crystals
of analogue 3b could be grown under carefully defined
conditions involving very slow evaporation of solutions in
THF. The crystals proved to belong to the monoclinic group
Pn and to have the approximate composition 3b·13 THF.14
A significant fraction of the volume of the crystals (49%) is
accessible to guests,15,16 which envelop molecules of triaryl-
amine 3b almost completely.14 The high percentage of guests
and the absence of π···π and C-H···π aromatic interactions
between molecules of compound 3b provide clear evidence
that normal crystallization has been thwarted.
As expected, the central triphenylamino core of compound
3b adopts a nonplanar, propeller-shaped conformation. In
addition, the average interaryl torsional angles between the
three fully substituted aromatic rings and their substituents
(66.5°, 73.5°, and 75.1°) are similar to the averages observed
in the structures of hexaphenylbenzene (79.3°) and related
compounds.10,17 In contrast, a much smaller average angle
is expected in partially substituted analogue 3a, as found in
two structures of pentaphenylbenzene itself (60.5° and
59.5°).10 Moreover, studies of compounds closely related to
pentaphenylbenzene have demonstrated rapid angular move-
ment around the bonds linking the peripheral phenyl sub-
stituents to the central aromatic ring, even in the solid state.18
As a result, significant conformational mobility is allowed,
whereas fully substituted analogues have less liberty and
more persistent shapes.19
Figure 1. Analyses of triarylamines 3b,c by TGA, conducted under
N2 at a heating rate of 10 °C/min.
stability, and no loss of weight was noted below 460 °C for
compound 3b or below 400 °C for analogue 3c. Study of
triarylamine 3c by differential scanning calorimetry (DSC)
revealed a well-defined glass transition (Tg ) 207 °C),
followed by an exotherm caused by crystallization and an
endotherm at 395 °C associated with melting (Figure 2).
Together, these structural data help rationalize the behavior
of triarylamines 3a-c and provide useful guidelines for
designing other molecules that can form long-lived glasses.
In particular, our observations show that interaryl torsional
angles in fully substituted aromatic rings with multiple
contiguous phenyl groups (such as those in triarylamines
3b,c) are normally much larger than those in analogues that
are only partially substituted (such as in triarylamine 3a). In
turn, larger torsional angles can promote the formation of
glasses in two ways: (1) by introducing a higher degree of
nonplanarity, thereby inhibiting efficient packing and (2) by
more effectively obstructing the formation of normal inter-
molecular π···π and C-H···π aromatic interactions, thereby
leading to lower degrees of association. Although triaryl-
amine 3b does not in fact show a glass transition, its structure
provides abundant signs of inefficient molecular packing,
Figure 2. Analysis of triarylamine 3c by DSC, using a heating rate
of 10 °C/min. A cycle of three heating runs is shown, with the
first, second, and third runs represented by solid, dashed, and dotted
lines, respectively.
Closely similar behavior was shown by partially substituted
derivative 3a.11 Unexpectedly, however, fully substituted
analogue 3b showed no glass transition upon heating, and
DSC revealed only a broad endotherm at 260 °C that may
be associated with a change of phase.14
Analysis by powder X-ray diffraction confirmed that
triarylamines 3b and 3c can form amorphous films.14
Moreover, methyl-substituted compound 3c proved to remain
amorphous even after being heated for 1 h at a temperature
(230 °C) above its Tg. In contrast, similar heating of
triarylamine 3b induced crystallization.
(15) We estimate the percentage of volume accessible to guests by using
the PLATON program.16 PLATON calculates the accessible volume by
allowing a spherical probe of variable radius to roll over the van der Waals
surface of the network. PLATON uses a default value of 1.20 Å for the
radius of the probe, which is an appropriate model for small guests such as
water.
(16) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2001. van der Sluis, P.; Spek,
A. L. Acta Crystallogr. 1990, A46, 194
(17) Maly, K. E.; Gagnon, E.; Maris, T.; Wuest, J. D. J. Am. Chem.
Soc. 2007, 129, 4306
.
.
(18) Wind, M.; Wiesler, U.-W.; Saalwa¨chter, K.; Mu¨llen, K.; Spiess,
H. W. AdV. Mater. 2001, 13, 752.
(13) For similar methodology, see: Yao, Y.-S.; Yao, Z.-J. J. Org. Chem.
2008, 73, 5221.
(19) Pascal, R. A., Jr.; Kraml, C. M.; Byrne, N.; Coughlin, F. J.
Tetrahedron 2007, 63, 11902. Komber, H.; Stumpe, K.; Voit, B. Tetrahedron
Lett. 2007, 48, 2655.
(14) See the Supporting Information for details.
406
Org. Lett., Vol. 12, No. 3, 2010

such as the inclusion of guests. Analogue 3c may be assisted
in its ability to form long-lived glasses by its lower
symmetry. In general, our results suggest that the introduction
of alkylpentaphenyl groups and related fully substituted
unsymmetric aryl units may be an especially effective
strategy for inducing the formation of glasses, primarily by
inhibiting efficient molecular packing and normal intermo-
lecular aromatic interactions. The low conformational mobil-
ity of fully substituted arenes with multiple contiguous phenyl
groups does not appear to make them inherently less apt to
form glasses than more mobile analogues with partial
substitution, as demonstrated by the similar behavior of
triarylamines 3a and 3c.
3b and 3c have maxima at 337 nm (3.0) and 324 nm (3.0),
respectively, whereas simpler triarylamines such as triphe-
nylamine, tris(biphenyl-4-yl)amine (1), and tris(p-terphenyl-
4-yl)amine (2) absorb at 299 nm (4.5), 345 nm (4.7), and
361 nm (4.9), respectively, in THF.6 Bandgaps calculated
from the absorption spectra of compounds 3a, 3b, and 3c
have similar values (3.1, 3.2, and 3.4 eV, respectively). Our
data reveal that compounds 3a-c achieve a degree of
π-conjugation, despite their twisted conformations. It is
noteworthy that the compound incorporating methylpen-
taphenyl groups (3c) shows the most blue-shifted absorption
and emission. Similarly, the absorption of methylpentaphe-
nylbenzene is blue-shifted relative to that of hexaphenyl-
benzene.14
Our results confirm that the principles of crystal engineer-
ing can be used in a contrary way to help guide the design
of molecules that resist crystallization and form long-lived
glasses. In particular, we have developed a detailed under-
standing of the crystallization of hexaphenylbenzene and
related arenes with multiple contiguous phenyl groups,10 and
we have applied this understanding to the problem of
devising triarylamines able to form amorphous thin layers
of potential use in molecular optoelectronic devices. Our
observations suggest that methylpentaphenyl substituents
may be particularly effective in inhibiting crystallization, both
by enforcing highly nonplanar topologies that pack poorly
and by interfering with normal molecular association induced
by π···π and C-H···π aromatic interactions.
Figure 3. Normalized absorption and emission spectra of solutions
of triarylamines 3a-c in CH2Cl2.
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada, the Ministe`re
de l′Education du Que´bec, the Canada Foundation for
Innovation, the Canada Research Chairs Program, and
Universite´ de Montre´al for financial support. In addition, we
thank Sylvain Essiembre of the Université de Montréal for
his help in analyzing compounds 3b,c by TGA and DSC.
It is important to note that altering the structure of tris(p-
terphenyl-4-yl)amine (2) to create highly substituted deriva-
tives 3a-c does not greatly change the optoelectronic
properties of individual molecules. Absorption and emission
spectra of compounds 3a-c are shown in Figure 3. In all
cases, two distinct absorption bands are observed, one
attributed to the π-π* transition of phenyl rings (<300 nm)
and the other assigned to absorption of the triphenylamino
core (300-400 nm). Among triarylamines 3a-c, conjugation
between the core and the highly substituted periphery appears
to be greatest in partially substituted compound 3a, which
is expected to have the smallest average interaryl torsional
angles and shows the most red-shifted absorption (λmax (log
ε) ) 345 nm (3.2) in CH2Cl2). Fully substituted analogues
´
Supporting Information Available: Experimental pro-
cedures, spectroscopic data for all new compounds, analysis
of compounds 3b,c by DSC, and additional crystallographic
details, including powder X-ray diffractograms, ORTEP
drawings, and structural data in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL902841C
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