Synthesis and chiroptical properties of novel helical polyacetylenes containing fluorene pendant groups in the side chains

A series of novel substituted acetylene monomers bearing fluorene pendant groups (C13H9COCH2CH2CO-R-CH2CCH, R = NH (IV1), R = O (IV2), R = NHCH(CH3)CONH (IV3), R = NHCH(CH3)COO (IV4)) have been synthesized and subsequently polymerized with [Rh(nbd)Cl]2 as a catalyst to obtain the corresponding polyacetylenes (Poly(IV1-4)). The 1H NMR spectra demonstrated that all the obtained polyacetylenes had high cis-stereoregular structures. The results of CD (circular dichroism) and UV-vis spectra showed that Poly(IV3,4) took a tight helical structure, while Poly(IV1,2) did not. The chiral amino acid units in the side chains of Poly(IV3,4) induced the main chain to form the helical conformation. In addition, owing to the large steric repulsion and π-π interaction between the bulky fluorene groups, Poly(IV3,4) showed good helical stability at various temperatures (− 10-60 °C) and in strong polar solvents. Especially, the CD intensity of Poly(IV3) was enhanced with increasing the solvent polarity. Moreover, Poly(IV1-4) also showed good photoluminescent (PL) properties. The nonconjugated aliphatic spacer and the twisting of the polymer main chain were favorable to improve the PL efficiency.