Abstract
The chiroptical properties of amorphous chiral polymers functionalized with conjugated trans-azoaromatic chromophore linked to the backbone through a chiral cyclic pyrrolidine moiety of one single configuration at the solid state, as thin films, were investigated. For the first time nanometric thin films of amorphous polymers (not liquid crystals) showed a remarkable chiral amplification upon thermal treatment at a temperature close to their Tg. The side-chain azobenzene chromophores rearrangement driven by the enhanced chain mobility seems to favor the formation of nanodomains of conformationally ordered macromolecular chains with one prevailing helical handedness whose optical activity depends on the configuration of the intrinsic chirality of the monomeric units and which as a result are stable at room temperature for a long time.
Highlights
The Azobenzene derivatives are well known as photoresponsive materials: when subjected to UV light irradiation, the thermodynamically stable trans-isomer can be converted to the cis-one, which can back isomerize to the more stable trans-isomer either by photoexcitation or upon heating in the dark
The side-chain azobenzene chromophores rearrangement driven by the enhanced chain mobility seems to favor the formation of nanodomains of conformationally ordered macromolecular chains with one prevailing helical handedness whose optical activity depends on the configuration of the intrinsic chirality of the monomeric units and which as a result are stable at room temperature for a long time
When the trans-azoaromatic moieties belong to a polymer’s side chain, if the chromophores are linked to the polymeric backbone through chiral groups of one prevailing configuration [25], the whole macromolecule is brought to assume a conformational dissymmetry which can be revealed by Circular Dichroism (CD) measurements [17,26,27]
Summary
The Azobenzene derivatives are well known as photoresponsive materials: when subjected to UV light irradiation, the thermodynamically stable trans-isomer can be converted to the cis-one, which can back isomerize to the more stable trans-isomer either by photoexcitation or upon heating in the dark. The isomerization process appears, for this class of materials, highly effective and tunable, even when azobenzene moieties are bonded to polymeric structures [1,2,3,4,5,6,7]; this behavior makes them potential candidates for applications in a number of technological fields, such as devices for the optical storage of information [8,9], waveguides, holographic memories [10,11], nonlinear optical materials [12], optical input/electrical output memory devices [13], chemical photoreceptors [14,15], photoconductive and photorefractive materials [16,17] and, in general, as materials which, upon irradiation with light of appropriate intensity and frequency, exhibit photoresponsive properties [18,19,20,21,22,23,24]. For example Green and co-workers reported the possibility for inducing a helical conformation with a prevailing screw sense in polyisocyanates in dilute solution by functionalization of the macromolecules with chiral pendant groups having a small enantiomeric ratio [5,35] or by photoresolution [36]
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