Synthesis of helical poly(N-propargylamides) carrying azobenzene moieties in side chains.: Reversible arrangement-disarrangement of helical side chain arrays upon photoirradiation keeping helical main chain intact

Optically active novel N-propargylamides bearing azobenzene, (R)-HC CCH2NHCOCH(CH3)O-1,4-C6H4-N=N-C6H5 (1a), (R)-HC CCH2NHCOCH(CH3)O-1,4-C6H4-N=N-1',4'-C6H5-n-hexy] (1b), (R)-HC CCH2NHCOCH(CH2C6H5)O-1,4-C6H4-N=N-C6H5 (2a), (R)-HC CCH2NHCOCH(CH2C6H5)O-1,4-C6H4-N=N-1',4'-C6H5-n-hexy] (2b), and (R)-HC CCH2NHCOCH(C6H5)0-1,4-C6H4-N=N-C6H5 (3) were synthesized and polymerized with [(nbd)RhCl](2) as a catalyst to obtain the corresponding polymers with moderate molecular weights (M-n = 6900-44 000) in 46-99% yields. Polarimetric, CD, UV-vis, NOE-NMR, and IR spectroscopic studies demonstrated that the resulting polymers took a helical structure with predominantly one-handed screw sense stabilized by intramolecular hydrogen bonding between the amide groups in the side chains in various solvents. The CD spectra simulated by molecular orbital calculation well agreed with the experimental ones, and indicated the arrangement of azobenzene moieties in a mutual chiral geometry of one-handed screw sense. The trans-azobenzene in the side chain isomerized into the cis form upon UV irradiation, while the helical structure of the main chain was not affected so much. The cis-azobenzene reisomerized into the trans form upon visible-light irradiation, which induced a recovery of chiral geometry of azobenzene moieties in the side chain. Poly(1b) formed a cholesteric liquid crystal.