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.