Stimuli-responsive polymers. 5. Azobenzene modified polyaramides containing atropisomeric binaphthyl linkages: Tuning chiroptical behavior with light and heat

Azobenzene modified polyaramides and several model compounds fitted with atropisomeric 2,2 ' -binaphthyl linkages exhibit thermo- and photoresponsive chiroptical behavior when evaluated in dilute solution environments. The trans-azobenzene modified polymers were characterized by CD spectra with intense molar ellipticities in the 300-400 nm spectral window. Specific rotation magnitudes at the sodium D-line ranged into the hundreds of degrees and were dependent upon the extent of binaphthyl loading along the polymer chain. The irradiation of the polymer samples to drive the trans --> cis isomerization process resulted in an immediate chiroptical response, with CD band intensities and optical rotations significantly diminished. These effects were fully reversible and were attributed to the presence of one-handed helical conformations in the trans-azobenzene modified polymers that were severely disrupted following the trans --> cis isomerization reaction.