Synthesis and chiroptical properties of optically active poly(ethynylcarbazole) derivatives: Substituent effect on the helix formation

Novel chiral acetylene monomers containing carbazole, 2‐ethynyl‐9‐[( S )‐2‐methylbutoxycarbonyl]carbazole ( 1 ), 3‐ethynyl‐9‐[( S )‐2‐methylbutoxycarbonyl]carbazole ( 2 ), 2‐ethynyl‐9‐[( S )‐2‐methylbutyl]carbazole ( 3 ), and 2‐ethynyl‐9‐[( S )‐4‐methylhexyl]carbazole ( 4 ) were synthesized and polymerized with [(nbd)RhCl] 2 Et 3 N. The corresponding polyacetylenes with number‐average molecular weights ranging from 68,700 to 310,000 were obtained in good yields. Poly( 1 ) exhibited a large specific rotation and an intense Cotton effect in toluene, indicating that it formed a helix with predominantly one‐handed screw sense, while the other three polymers showed no evidence for taking a helical structure. Poly( 1 ) largely decreased the CD intensity upon heating from −10 to 60 °C. Poly( 1 ) showed a Cotton effect in film state in a manner similar to solution state. No chiral amplification was observed in the copolymerization of 1 with achiral 2‐ethynyl‐9‐ tert ‐butoxycarbonylcarbazole ( 5 ).