Switching Helicity in Diarylpolysilylene
Michiya Fujiki, Julian R. Koe, and Hiroshi
Nakashima
Materials Science Laboratory
Functional polymers with potential for such
application devices as switches, data storage,
transmission and displays are attracting
much attention due to their relatively low
cost, ease of fabrication of large area thin
films, processability, and chemical tunability
of optoelectronic properties through choice
of substituents. Polymers adopting a preferential
screw sense helical conformation offer the
possibility of a two state, switchable material
if the opposite screw sense is energetically
accessible and the helix-helix transition
reversible. Such helical inversions remain
rare and for discrete molecules in solution
responding to a change in temperature, have
been reported only twice, firstly for a polypeptide
and very recently for a polyisocyanate.
Recently we demonstrated that poly(diarylsilylene)s
-a hybrid organic-inorganic chromophoric
and fluorophoric polymer- (of particular
importance due to their violet light electroluminescence
(EL) properties) with enantiopure chiral
side chains adopt preferential helical scew
sense backbone conformations in solution
[1]. Incorporation of the (S)-2- methylbutylphenyl group in polymers
of the type [(Ar')(Ar")Si]n (Ar', Ar" = para-n-butylphenyl, para-(S)-2-methylbutylphenyl or meta-(S)-2-methylbutylphenyl) afforded optically
active materials with positive or negative-sign
circular dichroism (CD) signal for polymers
with one or two chiral centers per silicon,
respectively. On the basis of understanding,
we anticipated that poly(diarylsilylene)
which could exhibit either positive or negative
Cotton effects, dependent upon the application
of an external stimulus. Such materials could
potentially find application in a polymer-based
chiroptical photonic device (eg. circularly
polarised EL device or semiconducting small
molecule chirality sensor). Now we found
that the poly[{bis-(meta-(S)-2-methylbutylphenyl)0.2-co-(bis-(para-n-butylphenyl}0.8-silylene], (1)(copolymer 1), undergoes a
reversible, thermo-driven helix-helix transition
in solution with a transition temperature
of -10C [2, 3].
[1] J. R. Koe, M. Fujiki, and H. Nakashima,
J. Am. Chem. Soc. 121 (1999) 9734.
[2] J. R. Koe et. al., Chem. Commun. 2000 (2000) 389.
[3] Science 287 (2000) 2117 (Editor's choice).
Fig. 1. CD and UV spectra of 1 at -70, -10
and 50C.
Fig.2. CD(De) and UV(e) absorptivities per
Si repeat unit for copolymer 1 as a function
of isooctane temperature.
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