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

A series of azobenzene-modified polyamides fitted with main chain spirobiin dane turns and chiral binaphthyl bends was prepared from the solution polyc ondensation of trans-azobenzene-4,4'-dicarbonyl chloride with appropriate d iamine monomers. When evaluated in their all trans-azobenzene configuration s, these materials exhibited a good mix of physical properties suitable for high performance applications. Photoinduced trans --> cis isomerization re actions 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 op tical absorbance spectroscopy. Thermally induced cis --> trans reorganizati on within each polymer followed the first-order rate law. Activation energi es calculated for this process in DMAC all fell near 21-23 kcal mol(-1) and were not strongly correlated to backbone content. Polymers containing axia lly asymmetric S-(-)- or R-(+)-2,2'-binaphthyl main chain linkages exhibite d thermo- and photo-responsive chiroptical behavior when evaluated in dilut e 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 fo r the trans-polymers at the sodium D-line ranged into the hundreds of degre es and were dependent on the extent of binaphthyl loading along the polyami de backbone. The irradiation of the polymer samples to drive the trans --> cis isomerization reaction resulted in an immediate chiroptical response, w ith CD band intensities and optical rotations significantly diminished. The se effects were fully reversible and were attributed to the presence of put ative one-handed helical conformations in the ti-ans-azobenzene-modified po lymers that were severely disrupted following the trans --> cis isomerizati on reaction. (C) 2000 Elsevier Science Ltd. All rights reserved.