Helical conformational dynamics and photoisomerism of alternating pyridinedicarboxamide/m-(phenylazo)azobenzene oligomers

Alternating sequences of pyridine-2,6-dicarboxamides and meta-(phenylazo)azobenzenes have been assembled into oligomers composed of four (8) and eight (9) azobenzene linkages. X-ray crystallography confirmed that oligomer 8 adopts a two-turn helical conformation with a helical pitch of approximately 3.4 angstrom in the solid state. The presence of a two- and four-turn helical conformation of 8 and 9, respectively, in polar and nonpolar solvents was elucidated by the anisotropic upfield shifting of protons located within the helices, NOE enhancements between protons oriented toward the helix interior, and the diastereotopicity of the terminal benzyloxycarbonyl (CBz) methylene protons. H-1 NMR line shape analysis of the CBz methylene hydrogens at the chain ends revealed a dynamic equilibria interconverting M and P helical conformations with energetic barriers (Delta G(double dagger)) of 11.1 (AS(double dagger) = -19.4 +/- 1.6 cal mol(-1) K-1; Delta H-double dagger = 6.5 +/- 0.4 kcal/mol) for 8 and 13.8 kcal/mol (Delta S-double dagger = -6.6 +/- 6.2 cal mol(-1) K-1; Delta H-double dagger = 11.8 +/- 1.8 kcal/mol) for 9. Irradiation of the oligomers with 350 nm light induces an E -> Z isomerization of the azo linkages that decreases in efficacy at longer helix lengths. The suppression of E -> Z isomerization is a consequence of the contrasting behavior of the azo linkages located at the helix termini, which afford Z/E ratios similar to those of model compound 7d, and the internal azo groups, which undergo significantly lower Z/E conversion ratios compared with 7e.