Photostability and Dynamic Helical Behavior in Chiral Poly(phenylacetylene)s with a Preferred Screw‐Sense

Abstract

Helical polymers such as poly(phenylacetylene)s (PPAs) are interesting materials due to the possibility of tuning their helical scaffold (sense and elongation) once they have been prepared and by the presence of external stimuli. The main limitation in the application of PPAs is their poor photostability. These polymers degrade under visible light exposure through a photochemical electrocyclization process. In this work, it was demonstrated, through a selected example, how the photochemical degradation in PPAs is directly related to their dynamic helical behavior. Thus, while PPAs with dynamic helical structures show poor photostability under UV/Vis light exposure, poly‐(R)‐1, bearing an enantiopure sulfoxide group as pendant group and designed to have a quasi‐static helical behavior, shows a large photostability due to the restricted conformational composition at the polyene backbone, needed to orient the conjugated double bonds prior to the photochemical electrocyclization process and the subsequent degradation of the material.

Thermoresponsive Helical Dendronized Poly(phenylacetylene)s: Remarkable Stabilization of Their Helicity via Photo-Dimerization of the Dendritic Pendants

“…Above findings are inconsistent with literature reports, where PPAs were found to decompose through UV irradiation and even formed low molar mass cyclic compounds such as 1,3,5-triphenylbenzene derivatives after extensive irradiation. − In order to understand the possible reason for the limited chain scission of these dendronized PPAs, their photoirradiation was conducted in organic solvents instead of water. As shown in Figure S35, Et=PPA in DMF continued to decompose with the extension of UV irradiation, in contrast to the photoirradiation in water.…”

“…It has been well-documented that high-dose photoirradiation can lead to photochemical electrocyclization and rearomatization of PPAs due to cis – trans isomerization of the polyene backbones, together with backbone scission. − Therefore, the possible photodegradation of these dendronized PPAs was checked. From the UV spectra in Figures a and S27a, only small changes in the absorption in the range of 400–450 nm corresponding to the PPA backbones (peak B) were observed during the irradiation.…”

Screw sense excess and reversals of helical polymers in solution

“…To determine an approximate value for ω 1 , the combination of the information obtained from different structural techniques —UV-vis, circular dichroism (CD) 34 – 36 , vibrational circular dichroism (VCD) 37 , Raman 38 , Raman optical activity (ROA) 39 , differential scanning calorimetry (DSC) 40 and atomic force microscopy (AFM)— 41 – 49 is needed. This fact is due to the complexity of the helical scaffolds found in PPAs, which are made by two coaxial helices: an internal helix described by the polyene backbone and an external one described by the pendants, which, depending on the stereoregularity of the polyene backbone, can rotate in the same (cis-cisoidal, ω 1 < 90°) or opposite directions (cis-transoidal, ω 1 > 90°) 50 – 52 . Recently, our group reported another approach to get information about the secondary structure of PPAs 53 .…”

Thermo/Light Dual-Responsive Helical Dendronized Poly(phenylacetylene)s

“…Photoresponsive polymers have been constructed through combination of photochromic units such as spiropyran, azobenzene, diarylethene, and coumarin, forming molecular photoswitches and optical devices, − and can be used for self-healing, aggregation-induced emission, light-induced wettability control, as well as light-responsive nanocarriers and drug delivery. − The combination of photochromic units and supramolecular chiral assembly constitutes a convenient route for manipulating supramolecular chirality and has led to the formation of intriguing chiral materials. − Therefore, the combination of photochromophores with helical polymers provides fascinating photoresponsiveness to the helical polymers, thereby mediating their helicities and allowing chiral applications. − …”