Optically Active Polysilane Aggregates
A R T I C L E S
Table 1. Data for Polysilanes 1-4a
result in a longer wavelength UV λmax for 2. However, the UV
UVb
max ꢀ/fwhm
CDb
FL
λmax of polysilanes also depends on the global conformation of
6
λ
/
λ
max/∆ꢀ
gabsc/104
λmax
Mwd/10-
Mw/Mn yielde/%
the Si main chain. The UV characteristics of polydialkylsilanes
have been correlated with main chain extension and stiffness,
revealing a bathochromic shift in the wavelength of the
absorption maximum and an increase in molar absorptivity, ꢀ,
with a more rodlike nature.7,8 The UV molar absorptivity of 1
is about 1.4 times of that of 2, suggesting that 1 adopts a more
extended main chain conformation than 2. Thus, the more
extended main chain conformation of 1 appears to compensate
for the less effective electron donation from the m-butyl moiety,
resulting in a similar UV λmax to 2. The characteristics of 3 and
4, with p-propyl and p-ethyl substituents on the phenyl ring,
respectively, are similar to those of 2, indicating similar
conformations.
1
2
3
4
362/21600/18.2 357/3.20 1.48 378.6
360/15400/18.0 358/2.72 1.77 377.2
361/17500/18.0 359/2.82 1.61 377.9
361/17500/17.2 359/3.13 1.79 380.2
2.66
1.60
5.88
4.37
2.41
2.37
2.36
2.52
1.3
3.4
10.1
6.2
a UV and fluorescence data in THF at 20 °C. λmax units, nm; ꢀ units,
(Si repeat unit)-1 dm3 cm-1; fwhm ) full width (nm) at half-maximum of
λmax
c gabs: ratio of CD and UV molar absorptivities. d Molecular weights
b
.
determined by size exclusion chromatography (SEC) and relative to
polystyrene standards; eluant, THF. e Isolated yields of high molecular
weight fraction.
Aggregate State Chiroptical Properties of Polysilanes.
Aggregates were formed by mixing a poor solvent (methanol
or ethanol) with a solution of the polymers in a good solvent
(THF or isooctane), as previously described.3 The aggregate UV
absorptions occur at almost the same wavelength as in solution,
but due to particle size-dependent light scattering by the
aggregates, the intensities of the bands, though normalized to
the same scale to account for concentration differences, are less
consistent and tailing is observed on the long-wavelength side.
The aggregate CD spectra are bisignate, which is considered to
be characteristic of exciton coupling between closely situated
transition dipole moments on neighboring polymer segments
in chiral configurations.9 Although such effects may originate
from either intramolecular10 or intermolecular3 interactions, a
filtration experiment indicated that the size of the polymer
aggregates was greater than 0.5 µm, showing that the interactions
are largely intermolecular.
First, the results for polymers 1 and 2 (series A) will be
discussed. The CD spectra of 1 and 2 aggregates are shown in
Figure 2.
It is clear that the bisigned Cotton effects of 1 and 2 are
opposite in nature, indicating aggregates of opposite chirality,
despite the fact that they have the same positive CD signals in
solution, as was evident in Figure 1, and thus also the same
single molecule screw sense.
The opposite signs of the aggregate Cotton effects are not
dependent on solvent polarity (no solvent-dependent switching
is observed), solvent addition order, or good/poor solvent ratio,
as the CD spectra of the aggregates show the same profile under
conditions of normal or reverse order solvent addition and in
different cosolvent systems (see Supporting Information). These
results distinguish the present work from that reported in ref 3,
in which solvent effects are critical. The key development in
the present work is that the aggregate chirality can be controlled
by structural modification in the achiral side chain, i.e., whether
the n-butyl group is substituted para or meta on the phenyl ring.
We previously discussed a model of polysilane aggregate
chirality3 with reference to the cholesteric hard core model,11
in which the single molecule helical angle, φ, defined by the
helical pitch, p, and diameter, d, governs the handedness of the
Figure 1. CD and UV spectra of polymers 1-4 in THF.
Solution-State Optical and Chiroptical Properties of
Polysilanes. To discuss the structural dependence of the
aggregate chirality clearly, it is helpful to first consider the
solution-state characteristics. The polymers show relatively
narrow (ca. 18 nm) UV absorptions due to the conjugated Si
backbone σ-σ* transition, mirror image fluorescence emission
spectra and small Stokes’ shifts (ca. 16 nm), indicating regular
semiflexible polymer molecules with a long segment length.7
The solution UV and CD spectra of 1-4 in tetrahydrofuran
(THF) at 20 °C are shown in Figure 1.
All the polymers show positively signed Cotton effects in
their solution CD spectra, coincident with the absorptions due
to the Si σ-σ* transitions, which indicates that the backbones
of 1-4 have the same prevailing helical screw sense in the
solution state. The polymers have similar UV λmax, CD λmax
,
and dissymmetric ratio gabs (defined as the ratio of CD and UV
molar absorptivities). The similar UV λmax indicates that the
polymers have similar Si main chain σ-σ* transition energies.
These energies can be affected by steric and electronic effects
of the side groups and the Si main chain segment length.7 Since
groups in the para position can conjugate better with the silicon
atom, electron donation from the p-n-butyl moiety in 2 may
contribute more effectively to the σ-conjugating Si main chain
through the phenyl ring than can the m-butyl moiety in 1.
The difference of donor effects in 1 and 2 is also supported
by the 29Si NMR chemical shifts: the resonance for 2 (-35.01
ppm) is shifted about 0.4 ppm upfield compared with that for
1 (-34.64 ppm), indicating that electron donation from the
p-butyl moiety may occur more effectively and increase the
electron density in the Si main chain. Such donation should
(8) Fujiki, M.; Toyoda, S.; Yuan, C. H.; Takigawa, H. Chirality 1998, 10,
667-675.
(9) Peeters, E.; Delmotte, A.; Janssen, R. A.; Meijer, E. W. AdV. Mater. 1997,
9, 493-496.
(10) Obata, K.; Kira, M. Macromolecules 1998, 31, 4666-4668.
(11) Nakanishi, K.; Berova, N. Circular Dichroism: Principles and Applications;
Wiley-VCH: New York, 1994; Chapter 19.
(7) Fujiki, M. J. Am. Chem. Soc. 1994, 116, 11976-11981.
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J. AM. CHEM. SOC. VOL. 126, NO. 42, 2004 13823