Stimuli-responsive polymers. 4. Photo- and thermo-regulated chiroptical behavior in azobenzene-modified polymers fitted with main chain spirobiindane turns and chiral binaphthyl bends

Abstract

A series of azobenzene-modified polyamides fitted with main chain spirobiindane turns and chiral binaphthyl bends was prepared from the solution polycondensation of trans-azobenzene-4,4′-dicarbonyl chloride with appropriate diamine monomers. When evaluated in their all trans-azobenzene configurations, these materials exhibited a good mix of physical properties suitable for high performance applications. Photoinduced trans→ cis isomerization reactions were effected by irradiating polymer solutions with near UV light. Reverse cis→ trans isomerization of the backbone azobenzene segments was triggered by either photochemical or thermal means and was monitored by optical absorbance spectroscopy. Thermally induced cis→ trans reorganization within each polymer followed the first-order rate law. Activation energies calculated for this process in DMAC all fell near 21–23 kcal mol −1 and were not strongly correlated to backbone content. Polymers containing axially asymmetric S-(−)- or R-(+)-2,2′-binaphthyl main chain linkages exhibited thermo- and photo-responsive chiroptical behavior when evaluated in dilute THF solutions. Specifically, the trans-azobenzene-modified materials were all characterized by CD spectra showing intense molar ellipticities within the 300–400 nm spectral window. Specific rotation magnitudes determined for the trans-polymers at the sodium D-line ranged into the hundreds of degrees and were dependent on the extent of binaphthyl loading along the polyamide backbone. The irradiation of the polymer samples to drive the trans→ cis isomerization reaction 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 putative one-handed helical conformations in the trans-azobenzene-modified polymers that were severely disrupted following the trans→ cis isomerization reaction.