Chiral Silylene-Spaced Divinylarene Copolymers

There has been an ever burgeoning interest in synthetic chiral polymers because of their potential applications for optoelectronic devices,1 enantioselective sensors,2,3 and catalysis.4 It is known that the chirospectroscopic property is transferred from the chiral auxiliary to the polymeric backbone as witnessed by its circular dichroitic (CD) properties.5-15 Thus, in the presence of chiral substituents, conjugated polymers may adopt helical conformation and show characteristic induced CD curves.6-10 For random coil polymers, the CD curves appeared to be relatively weak because of the cancellation of the transition dipole moments.11 However, aggregation may play a pivotal role to enhance the CD properties.12-16 The chemistry of silylene-spaced conjugated copolymers is well-documented.17-28 We recently disclosed a series of silylene-spaced alternating donor-acceptor copolymers which exhibit efficient intrachain energy transfer between donor and acceptor chromophores.29 These copolymers may form a random coil so that intrachain chromophore-chromophore interaction may occur.30,31 It is envisaged that the introduction of chiral auxiliary into these copolymers would lead to the transfer of chiroptical properties through aggregation. We now wish to report the synthesis and photophysical sutdies of the first optically active silylene-spaced divinylarene copolymers 1 and 2 (see Chart 1). In the beginning of this research, we synthesized a pair of enantiomeric copolymers 1 (1a: 70% yield, Mn ) 15 900, PDI ) 2.15; 1b: 77% yield, Mn ) 16 300, PDI ) 2.27) by employing rhodium-catalyzed hydrosilylation of bis(alkyne) 4 with bis(vinylsilane) 5.29,30 The CD curves shown in Figure 1 suggested that 1a and 1b are mirror images. The UV-vis spectrum for 1 is also included in Figure 1 for comparison. Apparently, the chiroptical properties have been transferred from the chiral auxiliary to the aromatic chromophore in copolymers 1. The fluorescence spectra of 1a (λex ) 300 nm) at two different concentrations are shown in Figure 2. It is noteworthy that, at the concentration of the CD measurements, λem shifted to longer wavelength due to aggregation. At low concentration, the CD intensity was too weak to observe. Our next step was to synthesize a pair of enantiomeric copolymers 2 (2a: 87% yield, Mn ) 9900, PDI ) 2.56; 2b: 82% yield, Mn ) 9900, PDI ) 2.74) from 4 and 6 in a similar manner as that described above. Again, the CD curves of 2a and 2b, which matched very well with the absorption spectrum of 2, are also mirror images of each other as shown in Figure 3. In these cases, the CD curves were extended to the longer wavelength region due to the absorption of the second terphenylenetetravinylene chromophore in 2. The fluorescence profile of 2a showed a maximum at 448 nm which was ascribed to the emission of the terphenylene-tetravinylene chromophore. Apparently, energy transfer28 from the dialkoxydivinylbenzene moiety in 2 to this terphenylene† Department of Chemistry, National Taiwan University. ‡ Institute of Chemistry, Academia Sinica. § Institute of Polymer Science and Engineering, National Taiwan University. ⊥ On leave from Zhongshan Universtiy, Guangzhou, 1999-2000. Figure 1. CD curves of 1a (solid line) and 1b (dash line) (1.5 g/L in chloroform) and the UV-vis spectrum of 1a (dotted line).