Photochemical Electrocyclization of Poly(phenylacetylene)s: Unwinding Helices to Elucidate their 3D Structure in Solution

Abstract

Photochemical electrocyclization of poly(phenylacetylene)s (PPAs) is used for the structural elucidation of apolyene backbone.This method not only allows classification of PPAs in cis-cisoidal (w 1 < 908 8)o rc is-transoidal structures (w 1 > 908 8), but also approximating w 1 .APPAs olution is illuminated with visible light and monitoring the photochemical electrocyclization of the PPAhelix by measuring the ECD spectra at different times.P PAsw ith ac is-cisoidal structure show ar eduction of the ECD signal of at least 50 %b efore 30 min of irradiation, while cis-transoidal helices need much longer time because the transoidal bond must be isomerized. The different cis-cisoidal and cis-transoidal helices require different times to decrease their ECD signal by 50 %(t 1/2), depending on the degree of compression or stretching of the helix, establishing ar elationship between the secondary structure adopted by PPA(w 1)a nd the time required to lose the ECD vinylic signal by light irradiation.

Size Control of Chiral Nanospheres Obtained via Nanoprecipitation of Helical Poly(phenylacetylene)s in the Absence of Surfactants

“…From literature, it is known that poly(phenylacetylene)s are light-sensitive due to a photochemical electrocyclization of the polyene backbone under light exposure (Figure 7d). [14,28] The rate of this photochemical electrocyclization of PPAs is directly related to the secondary structure of the PPA (ω 1 ), being faster for compressed than for stretched helices. Therefore, as a proof of concept, we wanted to explore the release of encapsulated substances by irradiating a dispersion of polymer particles with UV/Vis light.…”

“…From these materials it is possible to create micelles, nanospheres or capsules, where the size and morphology control is a very important issue due to the properties associated to these parameters such as the surface area, diffusibility and mobility. Dynamic helical polymers [10–36] show interesting stimuli‐responsive properties [37] that allow controlling their helical sense and/or elongation. This fact makes these polymers attractive to be applied at the nanoscale level, where P / M dynamic chiral polymer particles can find potential applications in fields such as asymmetric catalyst, [38–39] CPL light‐emissive materials, [40–42] enantioseparation, [43] stimuli‐responsive particles, [44] crystallization, [45] chiral resolution, [46] drug delivery [47] and so on.…”

Size Control of Chiral Nanospheres Obtained via Nanoprecipitation of Helical Poly(phenylacetylene)s in the Absence of Surfactants

“…From literature, it is known that poly(phenylacetylene)s are light‐sensitive due to a photochemical electrocyclization of the polyene backbone under light exposure (Figure 7d). [14,28] The rate of this photochemical electrocyclization of PPAs is directly related to the secondary structure of the PPA (ω 1 ), being faster for compressed than for stretched helices. Therefore, as a proof of concept, we wanted to explore the release of encapsulated substances by irradiating a dispersion of polymer particles with UV/Vis light.…”

“…From these materials it is possible to create micelles, nanospheres or capsules, where the size and morphology control is a very important issue due to the properties associated to these parameters such as the surface area, diffusibility and mobility. Dynamic helical polymers [10–36] show interesting stimuli‐responsive properties [37] that allow controlling their helical sense and/or elongation. This fact makes these polymers attractive to be applied at the nanoscale level, where P / M dynamic chiral polymer particles can find potential applications in fields such as asymmetric catalyst, [38–39] CPL light‐emissive materials, [40–42] enantioseparation, [43] stimuli‐responsive particles, [44] crystallization, [45] chiral resolution, [46] drug delivery [47] and so on.…”

Control of One-Handed Helicity in Polyacetylenes: Impact of an Extremely Small Amount of Chiral Substituents

“…The high-resolution AFM observations of the 2D crystals of the helical polymers on HOPG combined with the XRD measurements of their oriented films enabled us to directly determine the helical structures including the helical pitch and absolute handedness. 80 We then first applied this AFM technique to visualize the helical structures of such PPA derivatives, i.e., the one-handed helical chiral/achiral copolymers of poly(1b 0.99 -co-(R)-3d 0.01 ) and poly(1b 0.99 -co-(S)-3d 0.01 ) by high-resolution AFM (Figure 4). As shown in the typical AFM images of poly(1b 0.99 -co-(R)-3d 0.01 ) (Figure 4a) and its enantiomeric counterpart, poly-(1b 0.99 -co-(S)-3d 0.01 ) (Figure 4b) deposited on HOPG from a dilute toluene solution (0.05 mg/mL), followed by toluene vapor exposure, the copolymers self-assembled to form 2D crystals with an average height of ca.…”