Circularly polarized luminescence from bridged triarylamine helicenes

Article abstract of DOI:10.1021/ja035626e

The first examples of circularly polarized luminescence from chiral bridged triarylamine-based heterohelicene molecules are reported. The luminescence dissymmetry factors are ～ |0.001|. Copyright

Full text of DOI:10.1021/ja035626e

Published on Web 09/06/2003
Circularly Polarized Luminescence from Bridged Triarylamine Helicenes
Jason E. Field,† Gilles Muller,‡ James P. Riehl,‡ and D. Venkataraman*,†
Departments of Chemistry, UniVersity of Massachusetts Amherst, Massachusetts 01003, and
UniVersity of Minnesota Duluth, Minnesota 55812-3020
Received April 14, 2003; E-mail: dv@chem.umass.edu
Circularly polarized luminescence (CPL) is the differential
emission of right-circularly polarized light versus left-circularly
polarized light by chiral molecular systems. Applications of CPL
have primarily been used to obtain structural information about
molecular excited states, and the field has thus far been dominated
by studies involving chiral lanthanide complexes.1 In recent years,
there has been a growing interest in developing organic molecules
that are capable of emitting circularly polarized light because of
their potential applications in display devices,2 optical storage
devices,3 and in asymmetric photochemical synthesis.4 Although
all luminescent organic molecules that are chiral will display some
CPL, the extent of circular polarization is almost always less than
0.1%.1 Recent strategies that have been explored to increase the
circular polarization include attaching chiral side chains to conju-
gated polymers,5 embedding organic chromophores in a chiral
matrix,3 and the aggregation of chromophores.6 Here, we report
the first examples of CPL from two chiral heterohelicenes, 1 and
2, based on triarylaminessa moiety that has been widely exploited
in the OLED industry due to its electronic, photochemical, and
physical properties.
Figure 1. UV-vis electronic spectra of M-1 and P-1 in CHCl3. (A)
Absorption; (B) CD; (C) total luminescence; (D) CPL.
103 M-1‚cm-1). Similarly, M-2 and P-2 give identical absorption
We have recently reported the synthesis and properties of a series
of racemic bridged triarylamine helicenes.7 To study the properties
spectra with maxima at 442 nm (ꢀ ) 12 × 103 M-1‚cm-1). Further
evidence for the separation of the diastereomers is given by the
circular dichroism (CD) spectra of 1 (Figure 1, top left). The
preferential absorptions of left and right circularly polarized light
are identical in magnitude and opposite in sign for the two species,
indicative of chromophores with opposite chirality. Since the
chirality of the (1S)-camphanate is the same on each diastereomer,
the difference in the signs of the spectra can be attributed to the
intrinsic chirality of the helicene chromophore. As displayed in
Figure 2, the separated diastereomers of helicene 2 show CD spectra
similar to those observed for 1.
arising from the inherent chirality of these molecules, it was
necessary to separate them into their respective P- and M-
atropisomers. Recent work by Thongpanchang et al.8 has shown
that (1S)-camphanates are excellent resolving agents for function-
alized carbohelicenes. We thus synthesized the camphanate deriva-
tives, 1 and 2, and were then able to separate the diastereomeric
mixtures into their constituent P- and M-isomers using standard
column chromatography. Previous work8 suggests that the difference
in polarity arises from the preferred conformation of the camphanate
with respect to the helicene. Assuming that this holds true for the
present system, assignments can be made for the more polar
P-isomer (lower Rf) and the less polar M-isomer (higher Rf). These
assignments are also supported by NMR spectra (see Supporting
Information). The two constituent atropdiastereomers of 1 (M-1 and
P-1) give identical absorption spectra (Figure 1) in the UV-vis
region with the absorption maxima occurring at 434 nm (ꢀ ) 19 ×
The degree of circular polarization in the absorption is usually
expressed in terms of the absorption dissymmetry factor, gabs, which
is defined as
ꢀL - ꢀR
∆ꢀ
gabs
)
)
(1)
ꢀ
(1/2)(ꢀL + ꢀR)
† University of Massachusetts Amherst.
‡ University of Minnesota Duluth.
where ∆ꢀ is the difference in the molar absorptivity of left and
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J. AM. CHEM. SOC. 2003, 125, 11808-11809
10.1021/ja035626e CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
0.1% circularly polarized. It is somewhat surprising that the
measured dissymmetry factors for P-2 and M-2 are approximately
the same or even slightly less than that determined for 1. We had
anticipated that the increased helical overlap introduced by the
additional aromatic ring might lead to an increase in circular
polarization in the luminescence. It is well-known that interacting
independent chromophores exhibit maximum chirality when the
angle between the transition dipole directions is equal to 90°.
Although the additional aromatic ring in 2 increases the helical
twist, it must be the case that the net chirality is an inherently
complex function of the electronic structure of the twisted chro-
mophore. Previous studies on the CD of n-helicenes have also
shown that the prediction of net chirality of the observed transitions
is not a simple function of molecular geometry.9
It is important to compare the results presented here with previous
work by Phillips et al.6 In this study a 10-fold increase in the
luminescence dissymmetry ratio was observed (glum ) 0.01) for a
system of aggregating helicenes. However, the increased ordering
of these aggregates also results in a large degree of linear
polarization (P ) 0.39) which can greatly affect the CPL measure-
ment.10 In the measurements presented here, precautions have been
taken to ensure the complete removal of linearly polarized emission
in the direction of emission detection (see Supporting Information).
In summary, we have reported the first triarylamine-based
helicenes capable of preferentially absorbing and emitting circularly
polarized light. We believe that these systems are amenable to
device application, and we are thus pursuing derivatives of these
molecules to attain an increased CPL response that would allow
for the development of practical polarized organic light-emitting
diodes (POLEDs).
Figure 2. UV-vis electronic spectra of M-2 and P-2 in CHCl3. (A)
Absorption; (B) CD; (C) total luminescence; (D) CPL.
Acknowledgment. We thank Noah Tremblay for his contribu-
tions to the synthesis of these molecules, and Franc¸oise Muller for
assistance with the CPL measurements. D.V. gratefully acknowl-
edges the financial support of the National Science Foundation
CAREER Grant CHE-0134287, Bristol-Myers Squibb, and the
Camille and Henry Dreyfus New Faculty Award.
right circularly polarized light and ꢀ is defined as the average molar
absorptivity. Also plotted in Figures 1 and 2 are the total
luminescence spectrum (I) and the circularly polarized luminescence
spectra (∆I) for the diastereomers of 1 and 2. As expected, the
total emission spectra of M-1 and P-1 are virtually identical, as are
M-2 and P-2, and, therefore, only one is plotted. The emission
maximum occurs at 453 nm for helicene 1 and 478 nm for helicene
2.
Supporting Information Available: Synthetic procedures and
characterization data for all compounds, and experimental procedures
for the CPL measurements (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
In analogy with eq 1, the degree of circularly polarized
luminescence is given by the luminescence dissymmetry ratio
References
IL - IR
(1/2)I (1/2)(IL + IR)
∆I
glum
)
)
(2)
(1) Riehl, J. P.; Richardson, F. S. Chem. ReV. 1986, 86, 1-15.
(2) (a) Dyreklev, P.; Berggren, M.; Ingana¨s, O.; Andersson, M. R.; Wenner-
stro¨m, O.; Hjertberg, T. AdV. Mater. 1995, 7, 43-45. (b) Grell, M.;
Bradley, D. C. AdV. Mater. 1999, 11, 895-905. (c) Grell, M.; Oda, M.;
Whitehead, K. S.; Asimakis, A.; Neher, D.; Bradley, D. D. C. AdV. Mater.
2001, 13, 577-580.
(3) Chen, S. H.; Katsis, D.; Schmid, A. W.; Mastrangelo, J. C.; Tsutsui, T.;
Blanton, T. N. Nature 1999, 397, 506-508 and references therein.
(4) (a) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38,
3418-3438. (b) Sato, I.; Yamashima, R.; Kadowaki, K.; Yamamoto, J.;
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(10) For reviews of CPL measurement techniques see (a) Maupin, C. L.; Riehl,
J. P., Encyclopedia of Spectroscopy and Spectrometry; Lindon, J. C.,
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226, 539-553. (c) See ref 1.
where ∆I is the difference in the intensities of left and right
circularly polarized emissions and I is the total emission intensity.
In Figures 1 and 2, we plot ∆I at various wavelengths through the
luminescence transition. In Figure 1 each point is plotted with a
standard error bar, and in Figure 2 representative error bars are
given. The dashed lines in the CPL plot are presented to show the
luminescence spectral line shape. As can be seen in these figures,
for the same transition, gabs and glum have essentially the same value
for both helicenes. This is the result one expects in situations where
there is no significant geometry change upon population of the
emitting state. This is further corroborated by the small Stokes shifts
of the emission maxima. The dissymmetry factors at the peak
maxima for 1 and 2 are |0.001| and |0.0008|, respectively.
It should be noted that the results obtained show that even for
these very strained systems that the chiral discrimination in
absorption or the net circular polarization in emission is quite small.
A value of glum equal to 0.001 corresponds to light that is only
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